Reticulating node-linker-modulator chemical spaces for modular design of alkoxide-based glasses and liquids

Domain: physics
Published: 2026-01-16
Editorial outcome: major_revision

Abstract
Glasses linked by molecular building blocks are emerging materials promising for combining the processability of glasses and designability of reticular frameworks. Achieving modular chemical space construction is key to promoting structural diversity for these glasses, which requires systematically understanding their structures and properties. Herein, we combine rheological, thermal, compositional and total scattering techniques to study glasses and liquids based on titanium, zirconium and boron nodes, multi-dentate alcohol linkers and monodentate alcohol modulators and elucidate their structure-property relationships. With decreasing modulator, the node-linker-modulator series show increased connectivity characterized by increased viscosity and diminished heat capacity jump at glass transition, eventually giving glassy coordinative networks. By systematically tuning their compositions, we show the glass transition temperatures of these materials are affected by the interplay between non-covalent interactions and covalent network formation. The modular designability in alkoxide-based glasses is exploited for synthesizing fluorescent molecular network-forming glasses with high quantum yield, which can be integrated into electroluminescent devices. The current development represents an important expansion of reticular chemistry into the regime of glassy materials.

Introduction
One key essence of reticular chemistry is modular designability, where molecular building blocks can be linked into crystalline frameworks in rational and predictable manners1,2. Such modular designability requires that molecular building blocks with suitable linkages can be combined almost arbitrarily to give crystalline frameworks that form a chemical space. For metal-organic frameworks (MOFs), such combinatorial chemical spaces construction has been observed for a rather large set of linkers and metal building units, which provides an enormous library of MOFs with vast structural diversities. In stark contrast to the great structural diversities of crystalline frameworks, only a few of them can be vitrified through melt-quenching or perturbative methods3–12. Thus, expansion of reticular chemistry into the regime of glassy materials requires establishing molecular network-forming glasses chemical spaces with modular designability and systematic understanding of their glass-forming abilities.

Glasses can be defined as nonequilibrium and non-crystalline condensed matters that can undergo glass transition, which is a continuous solid-to-liquid transition characterized by heat capacity jump and rheological behaviors due to the emergence of collective configurational motions13,14. However, achieving such configurational degree-of-freedom in covalently linked networks are challenging due to the constraint posed by strong and rigid covalent linkages between molecular building blocks. The conventional melt-quenching method essentially uses high temperature to break these covalent linkages and achieve liquification, which often causes the thermally labile organic linker to decompose and thus has seen very limited success in producing network-forming glasses from crystalline frameworks incorporating organic building blocks3.

Although the linkages between building blocks are strong and directional, they can also be reversible and dynamic. Harnessing the intrinsic reversibility and dynamics of these bonds represent a viable method to achieve covalently linked networks with suitable connectivity and thus preserve configurational degree of freedom necessary for a glass transition. Introducing monodentate modulators represent a generalizable approach to facilitate linkage reversibility through modulator-linker exchange15–17. The competition between linkers and modulators can result in missing-linker defects that reduces network connectivity, which also alleviate the constraint on configurational motion posed by the covalent linkage18–20. Increasing linker flexibility is also an intuitive way to promote network motion, which may add to the degree-of-freedom provided by network modulator to give collective configurational motion necessary for glass transition.

Alkoxide linked networks are promising candidates for constructing chemical spaces of molecular network-forming glasses, where the charged alkoxide linkage is adequate to give high thermal stability and its non-chelating feature can enable facile ligand exchange. Although several examples of monoliths and glasses are known to be made of metal-oxo clusters and linkers with hydroxyl functional groups21,22, the systematic understandings necessary for modular tunability and rational design has yet to be achieved. Specifically, a potentially generalizable method to produce glassy alkoxide networks involves evaporating alcohol solvents from a solution of the node and linker. As the alcohol solvents also play the role of network modulator, its removal would cause increase in network connectivity and viscosity. Comprehensive understanding of such continuous evolution from solutions to viscous liquids and glasses is vital for achieving processability and rational synthesis for these glasses, which requires elucidating the relationship between the composition of the node-linker-modulator tertiary mixtures and their glass transition behavior and viscosity. To achieve modular design, it is also important to expand the types of metal nodes that can be incorporated into the alkoxide network-forming glasses and demonstrate their effect on framework properties.

In this report, we show that node-linker-modulator represents a generic formula to construct glasses and liquids based on alkoxide linkages, where nodes and linkers with suitable flexibility can form covalently linked networks and the modulator would affect configurational motion through reduction in network connectivity and modulating noncovalent interactions between building blocks. The dynamic exchange between alcohol-based linker and monodentate alcohol modulators enables a generic evaporative synthetic method to produce these liquids and glasses from solutions of volatile alcohol solvents. We demonstrate such chemical space construction with titanium- and zirconium- oxo cluster nodes, multi-dentate alcohol linkers with various flexibility and monodentate alcohol modulators with various molar ratio (Supplementary Table 1). Systematic compositional, rheological calorimetric, spectroscopic and total scattering studies are carried out to elucidate their structure and properties. The node-linker-modulator strategy is then applied to networking forming glasses with organic composition to produce glassy borate ester networks, which echoes the synthesis of first covalent organic framework using the boronic ester linkage23. The modular tunability achieved in the node-linker-modulator chemical spaces enables the incorporation of fluorescent molecular motifs into network-forming glasses, which is then incorporated in a proof-of-concept electroluminescent device. We believe this research would expand the library of molecular network-forming glasses and represent an important step to expand reticular chemistry into glassy materials.

Results
Although reticular design of crystalline frameworks and glasses all targeted networks linked with strong bonds, their design strategy is different: crystalline frameworks employ rigid building blocks with well-defined connectivity that can direct the formation of ordered structures; glassy networks would prefer flexible building blocks and versatile connectivity that can increase configurational entropy. Thus, we chose a series of titanium alkoxide networks with flexible linkers as the model system to study the effect of modulator in the node-linker-modulator chemical space, which incorporates in situ formed titanium-oxo cluster as the metal nodes, 2,2’,2”-((ethane-1,1,1-triyltris(benzene-4,1-diyl))tris(oxy))tris(ethan-1-ol) (TB) as the linker and 3,4,5-trimethoxybenzyl alcohol (TMBA) as modulator (Fig. 1a, Supplementary Figs. 1–6): (1) titanium-oxo clusters produced by heating titanium isopropoxide (TTIP) in alcohol solvents have high solubility, multiple coordination sites for alcohol linkers and are known to undergo facile coordinative ligand exchange24; (2) TB linker is highly flexible and has alcohol functional groups that can form dynamic titanium-alkoxide bonds with titanium-oxo clusters and its tri-dentate rigid core would facilitate network formation and preservation of pores (3) TMBA modulator has the alcohol functional group to coordinatively exchange with the TB linker and can dissolve TB and titanium-oxo cluster without phase separation. TMBA also has high boiling point and would not evaporate at room temperature, thus allowing the construction of node-linker-modulator tertiary systems with well-defined compositions.

The Ti-TB-TMBA series can be made by heating an ethanol solution of TTIP, TB and TMBA to reflux for overnight followed by evaporating ethanol solvent at elevated temperature (Methods). Under the synthetic condition, TTIP is known to form titanium-oxo clusters through partial hydrolysis, which often have a distribution of size and composition at a given condition25. The molar ratio between Ti and TB is initially chosen to be 3:2, giving a titanium-to-alkoxide ratio of 1:2, same as a prominent Ti16O16(OEt)32 cluster, and the amount of TMBA can be varied systematically to give a series of Ti-TB0.67TMBAx (x=18–0.22) with x denoting the molar ratio of TMBA compared to titanium. The Ti-TB0.67TMBAx materials have the appearance of liquid (x=18, 8, 6, 4), viscoelastic material (x=3, 2, 1.33), and glassy solid (x=0.86, 0.5, 0.22) with different amount of modulator (Supplementary Fig. 1), which all shown on-crystalline feature in X-ray diffraction (XRD, Supplementary Fig. 2). The molar ratio of TB and TMBA can be experimentally verified through digested NMR (Supplementary Figs. 3, 4, Supplementary Table 2), and the titanium contents are determined by inductively coupled plasma mass spectrometry (ICP-MS, Supplementary Text 1). The Ti-TB-TMBA samples are also characterized by infrared spectroscopy (IR, Supplementary Fig. 5), which shows the preservation of TB linker. Notably, with low content of TMBA (x smaller than 4), residue ethanol is found in the products by digested NMR even after prolonged heating at 85 °C (Supplementary Fig. 3), which indicates these ethanol molecules, along with TMBA and TB, are deprotonated and exist as ethoxide ligand coordinated to the titanium-oxo cluster. Consequently, the Ti-TB0.67TMBAx series all have thermal stability above 120 °C as evidenced by TGA (Supplementary Fig. 6).

The statistically average connectivity of Ti-TB0.67TMBAx series can be analyzed based on the stoichiometry of the titanium node and the linkers (Supplementary Text 2). Notably, various titanium oxo clusters can be produced by partial hydrolysis of simple titanium alkoxides depending on the reaction conditions and the alkoxide ligands25. Under the synthetic conditions of Ti-TB-TMBA where mixed alcohols are used, there would be a complex mixture of titanium oxo clusters in the final product, which would have a distribution in size and structure. The Ti16O16(OEt)32 cluster is used as the representative node structure for this analysis, considering its synthetic conditions (i.e. temperature, solvent) are similar to that of Ti16O16(OEt)32. In addition, most known titanium oxo clusters produced in relevant conditions have similar structure features and stoichiometry as the Ti16O16(OEt)32 cluster. However, we note that the following analysis based on Ti16 cluster is only meant to convey a semi-quantitative picture of how network connectivity changes with the amount of modulator. While the analysis shows certain consistency between structure speculations and physical characterizations, due to the inherent complexity of the materials and uncertainty associated with the detailed structure of inorganic node, molecularly precise chemical structures or rigorously quantitative analysis on network connectivity are not feasible. For simplicity, hereafter we denote the Ti16O16(OEt)32 cluster as Ti16 cluster as an abbreviation. For the Ti16 cluster, there are two alkoxide ligands per titanium atom. With excess amount of TMBA (x larger than 4), on average the TB has less than one hydroxyl group coordinated to the Ti16 cluster, giving a TMBA solution of Ti16 clusters and non-coordinating TB, consistent with their liquid like appearance (Supplementary Text 2). Decreasing the amount of TMBA would have TB coordinate to Ti16 cluster, initially giving TB and TMBA coordinated Ti16 clusters that form supramolecular assembly (Ti-TB0.67TMBA3). Further reducing TMBA to x lower than 2 would allow substantial amount of TB to bridge Ti16 clusters, which would give oligomer, linear coordination polymer and eventually network structure, corresponding to the change of physical form from viscous liquid to viscoelastic solid and brittle monoliths.

To give a chemically intuitive picture of Ti-TB0.67Mx series (M denotes modulator), Polymatic program is used to model their structures in a supercell. In a preliminary analysis, 5 Ti16 clusters and 50 TB linkers are put in a cubic cell 3.4 nm in length (Fig. 1b), and the presence of modulator can be accounted for by randomly capping a given number of coordinative sites while allowing the remaining sites to link with TB. The structure modeling for Ti-TB0.67M1.33 give a network oligomer with 4 TB bridging 5 Ti16 clusters and the rest of TB molecules coordinating with one hydroxyl group (Fig. 1b, Supplementary Text 3, Supplementary Fig. 7). As the modulator content of Ti-TB0.67M1.33 is in the middle of this series, this structure model is a corroborating evidence that the connectivity of the Ti-TB0.67M series can span from coordination oligomer to coordination network. Comparison between pair-distribution function (PDF) simulated from the structure model and the experimental pattern shows qualitative consistency (Fig. 1c), where the short-range features can be allocated to atom pairs in the Ti16 cluster and organic linker (Supplementary Fig. 8). Notably, due to inherent disordering in the structure, the PDF mostly reflects short-range structure corresponding to C-C, C-O, Ti-O bonds, phenyl rings and Ti-O-Ti motifs of the titanium-oxo cluster. Conversely, information on the intermediate size regime, such as the overall structure of the titanium oxo cluster extending several Ti-O-Ti motifs and the connection mode of TB linker (bridging/dangling), are hidden in the broad features of the pattern and cannot be directly obtained from the PDF analysis. Similarly, long-range structure regime reflecting the network connectivity is also unavailable from total scattering analysis. Nevertheless, based on well-known chemical knowledge on titanium alkoxide chemistry, the combination of above-mentioned stoichiometric analysis, structure modeling and short-range ordering obtained from PDF provides a plausible picture for the tunable network connectivity of Ti-TB0.67TMBAx, which show consistency with physical property characterizations and compositional analysis in the sections below.

Rheology measurement is employed to quantitatively study the change of physical form of Ti-TB0.67TMBAx from liquid to viscoelastic solid with decreasing TMBA (x from 18 to 2), which can be viewed as TMBA solution of TB and titanium-oxo cluster with increasing concentration. the viscosity of Ti-TB0.67TMBAx series increase exponentially with decreased amount of TMBA, indicating the percolation of Ti-TB coordination polymer with increasing molecular weight at higher concentration (Fig. 1d). In comparison, the viscosity of regular polymer solution would increase in a polynomial manner with increased polymer concentration26,27. The network formation can be more clearly shown by its transition from viscous liquid to elastic solid as evidenced by the relative value of storage and loss modulus of Ti-TB0.67TMBAx with x changing from 4 to 3 and 2. For Ti-TB0.67TMBA2 (Fig. 1e), its storage modulus is larger than loss modulus, indicating it behaves as elastic solid. The frequency dependent rheology measurement at room temperature finds that the Ti-TB0.67TMBA3 and Ti-TB0.67TMBA2 samples show shear thinning (Supplementary Fig. 9), which is often allocated to polymer chain entanglement and orientation change, thus indicating the presence of polymeric coordinative networks. Although the rheology data does not provide the full temperature range to quantitatively extract fragility, from the slope of the Angell plot (Fig. 1f) it can be seen that higher linker/modulator ratio tend to give less fragile liquid behavior. Considering that many network-forming glasses such as SiO2 and low-density liquid phase of ZIF-4 show strong liquid behavior28, the decrease of Angell plot slope for Ti-TB0.67TMBAx with smaller x value may be viewed as a corroborating evidence supporting the increase in network connectivity with lower modulator content. For Ti-TB0.67TMBAx networks with x lower than 2, their elasticity become rather low and are thus not suitable for rheological measurement. From a practical perspective, these observation in rheological measurements convey information on the initial stage of continuous transformation from solution to glass triggered by modulator evaporation in the synthesis of glassy alkoxides with volatile modulators, where the viscosity change strongly affects the processability and shaping of the final product.

The glass transition behavior of the Ti-TB-TMBA series is studied by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) to track their evolution from solution to supramolecular assembly, coordination polymer and glassy network with decreased TMBA (Fig. 1g–i, Supplementary Figs. 10–13). With increased network ratio, the glass transition temperature of Ti-TB0.67TMBAx gradually increase from the Tg of TMBA (i.e. −42 °C, Supplementary Fig. 12) to −16 °C for Ti-TB0.67TMBA2 accompanied by monotonic decrease of the heat capacity jump associated with glass transition (Fig. 1h, i). The trend in heat capacity jump is an indication that the configurational degree-of-freedom of the corresponding super-cooled liquid becomes larger when the amount of TMBA is increased. DMA shows material softening at Tg, which indicates the configurational motion associated with glass transition involves the entirety of the tertiary system and not solely originate from the motion of TMBA (Fig. 1g). For Ti-TB0.67TMBAx (x<2), these samples also do not possess adequate mechanical strength for direct DMA measurement but are also not suitable for powder measurement kit used for samples with higher TMBA content. The glass transitions of Ti-TB0.67TMBAx (x<2) start to show significant broadening in the DSC curve that prevents reliable extraction of delta Cp values (Supplementary Fig. 13). Due to the broadening, although the Tg values can still be nominally extracted from the DSC curve using inflection point method, these values are used only for comparison within the Ti-TB-modulator series to show general trends. Such broadenings in DSC curve are typically observed for asymmetric mixtures composed of materials with distinctly different Tg and indicate the presence of dynamic decoupling and heterogeneity driven by strong concentration fluctuation29. In the Ti-TB0.67TMBAx (x<2), such dynamic heterogeneity would arise from the presence of coordination networks with different connectivity and conformations. Notably, for the Ti-TB0.67TMBAx series, the viscosity increases to solidification and the broadening of DSC curve both occur at around x=2, which is the point where the connectivity analysis starts to give substantial bridging of TB linker and form coordination polymer (Supplementary Text 2), thus providing a rather consistent picture of the chemical space.

As standard DSC up scan curve give broad features, the glass transition of Ti-TB0.67TMBAx (x<2) is further elucidated by measuring the enthalpy relaxation, which is a Cp overshoot in the glass transition region during heating that reflects the recovery of the enthalpy lost during prior annealing13. Theoretically, the enhancement of this overshoot is the direct consequence of the nonexponentiality and nonlinearity of the glass relaxation process30, and the extent of the overshoot increases with extending the prior annealing time around Tg. For enthalpy relaxation measurements, the Ti-TB0.67TMBA0.5 and Ti-TB0.67TMBA0.22 are annealed for an hour at different temperatures near their Tg. The subsequent DSC up scan (exo up) show enthalpy relaxation as a depression with convex shape, which correspond to an overshoot compared to the broad feature in standard DSC up scan and confirm presence of glass transition (Supplementary Figs. 14, 15). Notably, for different annealing temperatures, the overshoot peaks corresponding to enthalpy relaxations appear at slightly different temperatures, which is also a manifestation of dynamic heterogeneity. For Ti-TB0.67TMBA0.5, annealing at 30 °C for an hour prior to up scan give enthalpy relaxation peak at around 45 °C. For Ti-TB0.67TMBA0.22, annealing at 45 °C for an hour prior to up scan give enthalpy relaxation peak at around 70 °C. These enthalpy relaxation measurements provide important evidence supporting the presence of glass transitions for Ti-TB0.67TMBAx (x<2) and the increase in Tg with decreased TMBA content, consistent with increase in coordinative network connectivity.

The presence of coordinative network for Ti-TB-TMBA series with low TMBA content can also be quantitatively shown by comprehensive composition analysis (Supplementary Text 1). For Ti-TB0.67TMBA0.22, its empirical formula is determined by a combination of elemental analysis, digested NMR and ICP analysis to be Ti16O16(TB)10.6(TMBA)3.5(OEt)7.4, where TB to modulator (ethoxide and TMBA) ratio reaches 1:1. Consequently, from the perspective of network connectivity, Ti-TB0.67TMBA0.22 correspond to Ti-TB0.67M0.67 (M includes TMBA and ethoxide) and would have higher connectivity than Ti-TB0.67M1.33, which is shown to have a oligomeric coordination network (Supplementary Text 3). Thus Ti-TB0.67TMBA0.22 would have a network structure. Without TMBA, Ti-TB0.67 has white precipitate and cannot be made into transparent monolithic network. Decreasing the Ti to TB ratio to 1.2 give Ti-TB0.82 as a glassy network without TMBA, which shows high thermal stability in TGA, non-crystalline feature in XRD and enthalpy relaxation in DSC (Supplementary Figs. 16–19). Thus, we have shown that by tuning the amount of modulator, the node-linker-modulator tertiary system can give a series of glasses and liquids with different connectivity and glass transition behavior, which could be a generalizable formula for designing molecular network-forming glasses.

The modular designability of the node-linker-modulator chemical space for network-forming glass can be established by changing the molecular building blocks and studying the properties of the resulting materials to unravel general design rules and handles for tunability (Fig. 2a). Glasses incorporating titanium-oxo cluster node and different modulators and linkers are obtained in similar synthetic condition as the Ti-TB-TMBA series, which shows similarity in the short and medium range in PDF consistent with Ti-O bonds and Ti…Ti correlations of the titanium-oxo clusters (Fig. 2b, Supplementary Fig. 8). Specifically, both 2.1 and 3.8 Å atom pairs are characteristic length between two titanium atoms in Ti-O-Ti motifs of Ti16 cluster. As is stated above, the titanium oxo clusters formed by titanium alkoxide partial hydrolysis could have slight differences in the intermediate size regime beyond 4 Å25, and the flexible TB linkers would also give almost random features in this regime, thus resulting in the lack of clear features in pair distribution functions beyond 4 Å.

To shed light on the effect of modulator on the node-linker-modulator chemical space, a solid-state modulator methyl 4-(hydroxymethyl)benzoate (BAMH, Supplementary Fig. 20) is chosen to produce Ti-TB0.67BAMHx series (x=4.7, 3, 2, 1.33, 0.86, 0.22), which are also characterized by digested NMR to give the relative ratio between TB, BAMH and residue ethoxide ligands corroborated by IR spectra (Supplementary Figs. 21–23, Supplementary Table 3). The samples in this series show non-crystalline features in XRD (Supplementary Fig. 24) and similar thermal stability as the Ti-TB0.67TMBAx series (Supplementary Fig. 25). Changing from the liquid modulator TMBA to a solid-state modulator would affect the energy landscape of the Ti-TB-modulator chemical space and unravel the factors affecting the glass transition. While BAMH and TB and commonly known titanium-oxo clusters are all solids at room temperature, the Ti-TB0.67BAMH4.7 and Ti-TB0.67BAMH3 have the appearance of liquids at room temperature. Ti-TB0.67BAMH4.7 shows glass transition temperature of −46 °C (Supplementary Fig. 26), which is close to 2/3 of the BAMH melting point of 50 °C. Considering the empirical rule that material Tg would be close to 2/3 of the melting point, such agreement indicates that the Tg of node-linker-modulator system with large excess of modulator is primarily determined by the noncovalent interactions of the modulators, which is consistent with the physical picture that Ti-TB0.67Mx (x>4) can be viewed as solutions with modulator as solvent and the linker and node as solute. With lower amount of BAMH, the Tg of Ti-TB0.67BAMHx increases and start to converge with that of Ti-TB0.67TMBAx with x reaches 2, indicating the glass transition in this region is dominated by the property of the coordination network (Fig. 2c, Supplementary Fig. 26). This is also the x value where polymeric networks start to form according to the analysis above (Supplementary Text 2). Ti-TB0.67BAMH0.22 show similar enthalpy relaxation as Ti-TB0.67TMBA0.22 (Supplementary Fig. 27), and its composition analysis also indicates the presence of coordinative network (Supplementary Text 1). The consistent increase in Tg for Ti-TB0.67TMBAx and Ti-TB0.67BAMHx for x smaller than 2 indicate the presence of modulator promotes configurational motion, which echoes the evaporation induced vitrification process observed for metal alkoxides and phenolate glasses. Notably, the alkoxide ligand exchange on Ti16 cluster can take place at rather low temperature, far below the boiling point of ethanol (Supplementary Fig. 28). From a molecular perspective, the modulator affects the material energy landscape through reduction in network connectivity, maintaining free volume and modulating the noncovalent interactions between linkers and nodes.

To demonstrate the modular design of linker in the node-linker-modulator chemical space, we synthesized Ti-GDPA1BAMHx (x=3, 1.3, 0.5, 0) series using a di-topic, diethylene glycol functionalized diphenylanthracene linker (denoted as GDPA), which also has a fluorescent aromatic core and flexible terminal motifs similar to TB. Ti-GDPA1BAMHx series are synthesized with a similar method as Ti-TB0.67BAMHx, with the ratio between titanium and linker hydroxyl groups kept at 1:2 and an ethanol/tetrahydrofuran solution mixture. The Ti-GDPA1BAMHx series are also characterized by digested NMR and IR spectra (Supplementary Figs. 29–31, Supplementary Table 4). The samples in this series show non-crystalline features in XRD (Supplementary Fig. 32) and no weight loss in TGA until 120 °C (Supplementary Fig. 33). Ti-GDPA1BAMH3 can also be viewed as a BAMH solution of titanium-oxo cluster and linker, and its Tg of −27 °C coincide with Ti-TB0.67BAMH3, which is a manifestation of the dominant role of BAMH. With increased linker-to-modulator ratio, the Tg of Ti-GDPA1BAMHx series increases (Fig. 2c, Supplementary Figs. 34, 35). Without BAMH modulator, the Ti-GDPA1 glassy network also shows broad DSC curve. With annealing temperature of 65 °C, Ti-GDPA1 show enthalpy relaxation peak above 80 °C, which is a manifestation of glass transition in this temperature regime (Supplementary Fig. 36).

To further demonstrate the modular designability of tertiary node-linker-modulator series and shed light on the how the interplay between covalent linkages and noncovalent interactions affect their properties, a more flexible di-topic linker, pentaethylene glycol (PEG5), is employed to synthesize a series of Ti-PEG5-TMBA glasses. PEG5 is a liquid at ambient temperature and has a Tg lower than −75 °C (Supplementary Fig. 37). The binary mixtures of PEG5 with TMBA have increasing Tg with higher TMBA content, which is a manifestation of noncovalent interactions between TMBA and PEG molecules (Fig. 2e, Supplementary Fig. 38). With the addition of TTIP, in situ formed titanium node can connect PEG5 into networks, where reducing TMBA modulator would cause increased network connectivity and Tg. Consequently, in Ti-PEG5-TMBA series, the decrease of TMBA modulator would have a negative effect on Tg from the perspective of noncovalent interactions and a positive effect on Tg from the perspective of increased network connectivity, and the overall effect on Tg depend on the interplay between covalent linkage and noncovalent interactions. To study such interplay, we synthesized three series of Ti-PEG5-TMBA glasses with different titanium content: Ti-PEG51TMBAx, Ti-PEG50.75TMBAx, and Ti-PEG50.5TMBAx series, which are all characterized by digested NMR and IR spectra to confirm the ratio between linker and modulators (Supplementary Figs. 39–45, Supplementary Table 5). All samples show non-crystalline features in XRD (Supplementary Figs. 46–49) and TGA of representative samples show thermal stability up to 120 °C (Supplementary Figs. 50, 51). The PDF of a representative sample, Ti-PEG51TMBA0.22 show consistency with the titanium oxo cluster linked network (Fig. 2b, Supplementary Fig. 8). Among the three series, the Ti-PEG51TMBAx series has lowest titanium content and thus lowest connectivity with same linker-to-modulator ratio. For this series, the sample viscosity also increases exponentially with reduced amount of TMBA similar to the trend observed for Ti-TB0.67TMBAx series, showing the gradual formation of coordination network (Supplementary Fig. 52). However, the Tg of this series decrease with lower TMBA modulator content, which aligns with the TMBA-PEG5 binary system and indicate the inter-molecular interaction play relatively dominant role (Fig. 2e, Supplementary Fig. 53). For Ti-PEG50.75TMBAx series, the sample Tg first decrease then increase with reduced TMBA, indicating the increase in coordinative connectivity start to prevail against noncovalent interactions in the energy landscape at low modulator regime where coordinative cross-linking start to occur (Fig. 2d). For Ti-PEG50.5TMBAx, the Tg increase more profoundly with decreased modulator from x=1.5, indicating the presence of higher network connectivity dominate the energy landscape (Supplementary Fig. 54). Thus, the trend of Tg of the Ti-PEG5-TMBA series has comprehensively show the interplay between intermolecular interactions and coordinative network formation for affecting the glass transition, which is a manifestation of collective configurational motions of the network. Ti-PEG51 is chosen to be the representative modulator-free sample for full compositional analysis (Supplementary Text 1). For Ti-PEG50.75, its high connectivity lead to rather broad DSC curve, and the glass transition feature can be analyzed by enthalpy relaxation (Supplementary Fig. 55).

To expand the chemical space of alkoxide glassy networks and achieve modular designability in terms of the metal node, we synthesized a series of alkoxide glasses based on TB and GDPA linker using zirconium butoxide precursor (Fig. 3). While known zirconium alkoxide clusters with well-defined crystal structure often incorporates mixed carboxylate and alkoxide ligand, the Zr3O(OR)5(OMc)5 (OMc denotes methacrylate) type of cluster is known to form with comparably small amount of acid from zirconium butoxide precursor and can undergo facile exchange with monodentate alcohol, which is a rather plausible structure model for the metal node in these glasses. Similar to the titanium alkoxide glass series, the exact structure of the zirconium cluster in this synthesis cannot be rigorously determined. The cluster presence would have a finite distribution, and the Zr3O(OR)5(OMc)5 is only used as an over-simplified model for zirconium node to give an plausible structure that is also chemically intuitive31. The PDF pattern of Zr-GDPA1BAMH2 and Zr-TB0.67TMBA2 show Zr-O bond distance of 2.1 Å and Zr…Zr correlation of 3.5 Å consistent with the simulated PDF of the Zr3O(OR)5(OMc)5 cluster, supporting the presence of zirconium oxo clusters (Fig. 3a, b, Supplementary Fig. 56). The Zr-TB0.67TMBAx series, Zr-GDPA1 and Zr-GDPA1BAMH2 are characterized by digested NMR, IR, TGA and XRD (Supplementary Figs. 57–64, Supplementary Tables 6, 7). The Tg of Zr-TB0.67TMBAx series increase from −43 °C to −30 °C with x decrease from 8 to 2. However, it has high tendency towards crystallization or phase separation into unknown white precipitate especially at lower TMBA content range, which prevents synthesizing samples with x<2 and the study of glassy Zr-TB network with higher connectivity. The Zr-TB0.67TMBA2 glass, which has the highest connectivity in the series, have an empirical formula of Zr3O(TB)2.2(TMBA)6.9, consistent with a network structure (Supplementary Text 1). With GDPA linker and BAMH modulator, Zr-GDPA1BAMH2 can also be obtained as monolithic glass with well-defined Tg of 20 °C. Without non-volatile modulator, Zr-GDPA1 can be synthesized in similar way as Ti-GDPA1, which also show enthalpy relaxation in DSC heating curve at around 60 °C when annealed in the cooling run, indicating the presence of glass transition type transformation (Supplementary Fig. 65). The empirical formula for the Zr-GDPA1 glass is determined to be Zr3O(GDPA)2.65(OBu)1.5(OH)3.6, which would have most OH group of GDPA deprotonated and link to zirconium (Supplementary Text 1).

In the materials described above, we have demonstrated the chemical space construction for metal-organic network-forming glasses by varying the linker, metal and modulator. The node-linker-modulator formula to construct network-forming glass can be further generalized to covalent organic networks. We demonstrate such possibility by reacting GDPA with trimethyl borate to construct borate ester networks with dynamic B-O bonds as linkage (Fig. 4)32,33. Both B-GDPA1 and B-GDPA1POE0.65 (POE denotes 2-phenoxyethanol) are characterized by digested NMR, XRD, IR and TGA analysis (Supplementary Figs. 66–70, Supplementary Table 8). B-GDPA1 and B-GDPA1POE0.65 network show well-defined Tg at 15 and −2 °C, respectively. The lower Tg of B-GDPA1POE0.65 can be rationalized in similar ways as the node-linker-modulator metal-organic network-forming glasses reported above, where the POE modulator, with Tg near −60 °C, promotes configurational motion by reducing network connectivity and altering the energy landscape of noncovalent interactions. Notably, the B-GDPA1 network without non-volatile modulator can be made into uniform transparent plates (Fig. 4a) and show a much more pronounced glass transition in DSC curve compare to Ti-GDPA1 and Zr-GDPA1 (Fig. 4b). PDF analysis mainly show features of the GDPA linker, which is also consistent with a structure model that links GDPA to boron atom with the hydroxyl group (Fig. 4c, Supplementary Figs. 72–73). Combined NMR, elemental analysis and ICP give an empirical formula of B(GDPA)(EtO)0.53(OH)0.47 based on structure model of B(OR)332–34. The B NMR of DMSO solution of B(OMe)3 and GDPA confirms the presence of boron ice esters in the B-GDPA series (Supplementary Fig. 74)35.

The modular designability of the node-linker-strut chemical space provide unique opportunity to develop fluorescent molecular network-forming glasses that can be used for optoelectronic devices. As a proof-of-concept demonstration, the fluorescent GDPA linker is used to synthesize molecular network-forming glasses with Ti, Zr and B based precursors and ethanol solvent, which is then evaporated under elevated temperature. The Ti-, Zr-, B-GDPA1 show blue fluorescence with 350 nm excitation with quantum yield of 3.5%, 43% and 66%, respectively (Fig. 5a, Supplementary Fig. 75, Supplementary Table 9). The B-GDPA1 glass with high brightness is used to fabricate a proof-of-concept electroluminescence device (Fig. 5b). In this demonstration, a simple light emitting capacitor is constructed with carbon nanotube network as source contact, gold as current collector, Si as back gate and SiO2 as gate oxide36. The B-GDPA1 is drop casted on the top CNT network as the emitter layer (Fig. 5d). With alternating gate voltage at 400 kHz, oppositely charged carriers are injected to B-GDPA1 to give electroluminescence starting from 25 V (Fig. 5c, e, f). It is worth noting that such device is a simple demonstration of the potential of B-GDPA1 in constructing electroluminescence device, and it only takes relatively routine engineering optimization to lower the operating voltage and increase brightness for these device37,38. We believe the incorporation of fluorescent linker and screening of the suitable metal node for high quantum yield successfully demonstrate the power of modular and reticular design for molecular network-forming glasses.

In summary, we have reported a general strategy to construct alkoxide-based molecular network-forming glass with node-linker-modulator formula, which can achieve modular designability and substantially expand the material scope for glassy materials. A series of alkoxide-based glasses can be constructed in a combinatorial manner similar to vector direct product of node, linker and modulators, thus enabling modular designability and structural diversity. The modular design has allowed for incorporation of functional molecular building blocks, which we demonstrate by designing glasses with high fluorescent quantum yield that can be used for electroluminescent devices. We believe the current work represent an important step to expand the concept of reticular material design into glasses.

Methods
Synthesis
All the samples were synthesized based on the stoichiometry in Supplementary Table 1. Ti-TB0.67TMBA18 is obtained by dissolving TB (109.5 mg, 0.25 mmol) and TMBA (1337.8 mg, 6.75 mmol) in 3 mL ethanol followed by adding TTIP (106.5 mg, 0.375 mmol) to this solution. The reaction was carried out at 80 °C for 24 h. The mixture was poured into a petri dish at 80 °C for 1 h to give the product as a liquid. Ti-TB0.67TMBAx (x=8, 6), are synthesized using the same method with different amount of TMBA. Ti-TB0.67TMBAx (x<6) are viscous liquids and solids, thus are synthesized with a smaller scale to ensure uniformity during evaporation (Supplementary Table S1). For these samples, the evaporation is carried out at 85 °C for 4 h to ensure the complete removal of unbounded EtOH. Ti-TB0.67BAMH, Ti-PEG5-TMBA series are synthesized with similar procedures. For Ti-GDPA1BAMHx, a mixed solvent of tetrahydrofuran/ethanol (1:1) was used to improve solubility of GDPA. For Zr based liquids and glasses are synthesized with the same procedure using zirconium butoxide butanol solution (80 wt%) as the zirconium source, and the Zr-GDPA1BAMHx is also synthesized with a mixed solvent of tetrahydrofuran/ethanol (1:1). B-GDPA1POEx is synthesized with B(OMe)3 as boron precursor and tetrahydrofuran/ethanol (9:1) as solvent.

AC electroluminescence device
SiO2/p++Si substrate with 100 nm oxide layer was washed by piranha solution, followed by O2 plasma treatment for 3 min. Poly-l-lysine solution was dropped on the clean substrate and then washed with Ultrapure water. Afterwards, semiconducting carbon nanotubes solution was used to soak the substrate for 15 min followed by washing with ultrapure water. The substrates were annealed in a mixed gas (5% H2 in Ar) atmosphere at 250 °C for 1 h. After deposition of the source electrodes (Au) using Ebeam (Electron Beam Evaporation), the metal electrodes were patterned to form the fork finger electrodes using ML3 (laser direct writing technique). Finally, carbon nanotubes outside the device area were etched with O2 plasma using UV lithography. The B-GDPA1 solution was stirred for 24 h, dropped on the CNT top layer of the device and then heated at 80 °C for 1 h. The square wave AC signal to drive the device is generated from the ATG-2021H power signal generator of Aigtek Electronic Technology Co., Ltd.

X-ray diffraction and total scattering
Powder X-ray diffraction data were collected on a Rigaku MiniFlex instrument using CuKα (λ = 1.5418 Å) radiation, which are specified in the Supplementary Figs. The X-ray total scattering measurements were performed at room temperature at BL13SSW of the Shanghai Synchrotron Radiation Facility with wavelength of 0.247 Å. The raw data is processed with Dioptas software and pdfgetX3 program.

Differential scanning calorimetry and Thermogravimetric analysis
The glass temperature transition of samples was measured using a TA Instruments DSC250 differential scanning calorimeter with RCS 90 cooler. The samples of ~5 mg were loaded in Tzero Aluminium hermetic pans. The DSC curves are measured with ramp rate of 10 °C/min unless otherwise noted. The glass transition temperatures and delta Cp are determined in TA Instruments Trios software with the midpoint of the transition determined by inflection point method and the onset and end of the transition determined by tangent method. For enthalpy relaxation measurements, the samples were first annealed at a given temperature before subsequent heating run. Thermogravimetric analysis (TGA) was conducted using a TA Instrument TGA55 instrument under an Nitrogen atmosphere with a heating rate of 10 °C/min in the temperature range from 30 °C to 300 °C.

Dynamic mechanical analysis
DMA was carried out by a PerkinElmer DMA8000 dynamic mechanical analysis machine with powder kits with 0.02 mm strain at 1 Hz. The Tan delta was defined as the ratio of the loss modulus E” to the energy storage modulus E′.

Rheology measurements
The rheology measurements were performed on a TA Instrument ARES-G2 with a 25 mm aluminum parallel plate, the low viscosity sample was measured by slow-sweep mode; otherwise samples with high modulus was measured by small amplitude oscillatory shear mode. all the samples were tested at 25 °C expect for varied temperature measurements. For slow sweep mode: shear rate 1–100 1/s; points per decad: 5; off equilibration time 10.0 s; averaging time 30.0 s. In small amplitude oscillatory shear mode, active mode in axial force adjustment was used for compression, the samples were applied to 0.01% strain rate, and oscillated at a logarithmic sweep angular frequency of 100 to 0.1 rad/s with 6 points per decade.

Photoluminescence and infrared spectroscopy
PL spectra were obtained with a MetaTest Corporation equipped with mLaser_C03. Infrared spectroscopy was measured with a PerkinElmer FT-IR Spectrometer Frontier instrument.

Photoluminescence quantum yield (PLQY)
HORIBA Fluorolog-3 is used to measure PLQY. The sample is evenly dispersed on a quartz glass substrate while taking a blank quartz glass sheet of the same size as the reference. The prepared sample is placed in the integrating sphere, and the monochromatic light of 350 nm is introduced into the sample in the integrating sphere through the optical fiber. By keeping the excitation wavelength unchanged at 350 nm, the number of photons absorbed by the sample can be determined by integrating the fluorescence spectrum of the sample and the blank reference at 330–370 nm, and the number of photons emitted by the sample can be determined by integrating the fluorescence spectrum between 370 and 600 nm with the same method.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, Liu et al. present an approach to the design of network-forming glasses by introducing a versatile node-linker-modulator network, showcasing examples involving titanium, zirconium, and boron-based systems. While the authors present a diverse array of node-linker-modulator combinations, the structural frameworks proposed are largely conceptual. The paper is quite superficial and does not provide an in-depth scientific discussion of the experimental observations. Without the support of NMR and elemental analysis, the structural claims remain highly speculative, raising concerns regarding the actual compositions and identities of these materials. Furthermore, the thermal analysis, primarily conducted via DSC, requires a more thorough and nuanced interpretation to provide clarity on the role of the modulators in influencing glass transition behaviours. Similarly, the X-ray total scattering data analysis does not offer sufficient insight into the network connectivity or long-range structural order, which strongly limits the reliability of the presented conclusions. In this regard, we do not recommend this manuscript for publication. Below are the specific comments to address the primary issues:\n1. Scheme 1 is overstating the science reporting in the paper. The matrix view suggests that most of the possible combinations of nodes, linkers and modulators have been made experimentally. However, only some of those are included in the paper. The authors should clearly visualize which combinations have been done.\n2. We note that approximately 3 mL of ethanol was used in each synthesis, and although the samples were placed in a petri dish at 80 °C for an hour, it remains uncertain whether the ethanol, if acting as a modulator, was fully removed. Without NMR analysis, this is difficult to confirm. Even though TGA for the three networks without additional modulators shows stability up to 150 °C, this does not conclusively verify ethanol removal. It is further requested that TGA is conducted on all samples to provide comprehensive and sufficient evidence supporting sample stability. 1H NMR spectroscopy and elemental analysis of digested samples should also be performed in order to provide important insights into the chemical identity and composition of the materials. TGA under an oxygen atmosphere (burning the materials to TiO2, ZrO2 or B2O3 to get information on the metal/boron content) is also important to get the full composition of the materials. The trends discussed in the manuscript can only be understood and rationalised if the exact chemical compositions of the materials are known.\n3. The definition of the molar ratio used in the manuscript is very confusing. It took us a long time to figure out that a low molar ratio correlates with a high modifier content. Since all the data in the text are discussed in terms of modulator concentration, it would be more convenient if the data were plotted as a function of modulator content so that the degree of network connectivity decreases from left to right.\n4. All DSC traces lack clear indicators of endothermic or exothermic signal direction, which must be added for clarity. The origin of the signals assigned to a low-temperature glass transition is unclear. Given the low temperature of the glass transition, it is unreasonable to assume that the Tg is associated with coordination dynamics involving modulator-linker exchange at the metal nodes. The metal-oxygen and boron-oxygen bonds are much too strong to be broken at temperatures below or around room temperature. Thus, the Tg signal must be assigned to the configuration dynamics of the linker and modulator molecules. The current experimental data does not prove whether these molecules are part of a coordination network. Further, it is important to compare the DSC data with reference measurements on the pure modulators and the amorphous materials without any modulator. If the modulator-free materials do not exhibit a glass transition, the chemical origin of the glass transition in these materials is very different to that of typical melt-quenched glasses, such as ZIF glasses. These differences should be clarified and discussed in detail. In addition, the authors claim that more than 50% of the samples have only a weak Tg, so it is mandatory to report DSC data of the modulator-free networks as well. Also, the authors, citing the elimination of thermal history, only show DSC data from the second heating, and what happened in the first heating is also a matter of considerable interest. There are also tests where the DSCs were all performed below 0 °C, and we think it is necessary to provide DSC data up to pre-decomposition temperatures.\n5. Panel a and b in Fig. 3 appear to be labelled the other way around in the figure caption. Moreover, the DSC data shown in the supplementary information is inconsistent with the data shown in Figure 3b. This points towards some problems with data evaluation and representation in the manuscript. The opposite trend in the change in Tg of the Ti-PEG5-TMBA-x% versus the rest of the samples is of interest, but only the use of PEG5-TMBA alone to illustrate this does not seem to be sufficient, and the fact that more modifiers have higher Tg values requires explanation. This hints towards a wrong interpretation of the glass network structure and the dynamics of the framework at the glass transition. Furthermore, the Ti-PEG5-EtOH sample cannot be found in Fig. 3c as described. The readers must have access to these details, such as the thermal analysis data of this mixed-metal sample is missing.\n6. Page 4: “The trend in heat capacity jump is an indication that the configurational degree-of-freedom of the corresponding super-cooled liquid becomes larger when the amount of TMBA is increased“. It is highly likely that the thermal events observed in the DSC are not properties of a hybrid network but properties of individual molecules present in the amorphous materials.\n7. One of the most substantial issues with this paper is the lack of structural models of the inorganic building units. The authors show structural models of the Ti- and Zr-oxo clusters in Scheme 1. However, these models are not used to assign peaks in the PDF analysis. Are the chemical structures shown in Scheme 1 and Fig. 1a just cartoons? The lack of structural models for the inorganic units means that the results of this work are difficult to transfer to other systems. PDFs are inadequately assigned and analysed. Supplementary Fig. 3 only assigns peaks below 3.3 Å. We can clearly find a Ti-O-Ti distance of about 3.8 Å in the provided reference cluster structure. A systematic simulation of the PDF for each atom-atom distance and overall PDFs is necessary. Long-range PDF analysis (beyond 30 Å) and differential PDF with various modulators are also recommended to provide additional structural information. It must be mentioned that both XRD and X-ray total scattering seem to be tested at room temperature, which characterises what should be a supercooled liquid or liquid state, NOT a characterisation of the vitrified glass. If the glass is to be elaborated on, scattering data should be recorded below Tg. Finally, the corresponding scattering factor S(Q) data must be provided, as they contain valuable reciprocal space information of the materials.\n8. In addition, attempts were made to find some structural correlation evidence in the FTIR, but the FTIR spectra also only investigated the Ti-TB network. Individual monomers, as well as the rest of the network, were not investigated, and no inferences could be made. A detailed FTIR analysis, including spectra of the pure modifier, linker and modifier-free amorphous network, would shine light on the chemical structures of these amorphous materials.\n9. PXRD data must be reported for all compounds under study to clearly demonstrate their amorphous nature.\n10. For the XRD data of B-GDPA-TMBA-x% in Supplementary Fig.15. There seems to be a certain amount of crystallinity. If these tests were all at room temperature (i.e. (supercooled) liquid phase), it is reasonable to suspect that the so-called “glassy” phase at lower temperatures may be even more crystalline.\n11. Given their alkoxide-based chemistry, it is to be expected that all of these amorphous materials are sensitive to water and hydration. It is known that titanium and zirconium alkoxides hydrolyse under a humid atmosphere and eventually form crystalline or amorphous zirconium and titanium oxides. Similarly, boron alkoxides hydrolyse to borates. The authors should provide data (DSC, TGA, XRD, FTIR) on the water stability of these amorphous materials.\n12. The porosity of coordination polymer glass or MOF glasses has long been an important property, as in the authors' previous work Nat Commun 15, 7339 (2024). This raised the natural question of whether the different modulators can be removed and the effect on porosity after their removal.\n13. There are several typographic errors in the Supplementary Information PDF file, which make it difficult to evaluate the Supplementary Information fully.", "reviewer_2": "This manuscript presents an impressive and groundbreaking study on molecular network-forming glasses, addressing a highly relevant challenge in the field of reticular and glassy materials. The innovative approach of introducing network modulators to enhance configurational motion and enable vitrification in strong-bonded network systems is both novel and impactful. The proposed node-linker-modulator design offers a flexible, universal framework that opens the door for designing diverse network-forming glasses with tunable properties and significant potential applications. The systematic demonstration of this approach with different node types, linkers, and modulators further enhances the value of the study, highlighting its robustness and versatility.\nThe authors effectively show the scalability and practical benefits of this modular approach, especially in terms of controlling viscosity and glass transition properties. This work not only advances our fundamental understanding of glass formation but also brings an exciting expansion of reticular chemistry into the realm of functional glass materials. The successful synthesis of fluorescent molecular network-forming glasses and their integration into electroluminescent devices is particularly noteworthy, showcasing the translational potential of the research. This study will likely inspire new directions in the design of molecular network materials.\nMinor Revisions:\nMore discussion on the possible size of the Ti-O cluster would be beneficial. Pair Distribution functon(PDF) analysis should be able to shed light on this aspect.\nFigure 1 is slightly confusing as some panels refer to the molar ratio. Please specify the component to which the molar ratio refers.\nFor Figure 1e, please indicate the endo/exo direction.\nRegarding the statement on “the presence of polymeric coordinative networks” with high TMBA loading, it would be valuable to discuss the possible polymeric network structure. Would it include multiple clusters and linkers entangled together?\nAs a benchmark, if possible, present the Differential Scanning Calorimetry (DSC) data for the modulators in the Supplementary Information (SI).\nIf available, include UV-Vis absorption results for the GDPA-containing glasses.\nFor PLQY, consider including the original data in the SI.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "Authors Ying Liu et al reported their recent study on modular-design strategy for meltable MOF and glass by use reticulating the node-linker chemical spaces of coordination network. They showed that introducing network modulators can facilitate configurational motion by dynamic exchange and reduction of network connectivity, which represent a highly generic strategy to facilitate the vitrification of coordination networks, and then to synthesize their glasses derived from parent metal-organic networks and covalent organic networks. It is interesting and exciting for peoples whose work focused on MOFs or COFs’ realm. However, some substantive issues should be addressed as following.In this regard, the manuscript may be considered for publication in Nature Communications after major revisions.\n1) They stated that the modulators, i.e. various monodentate molecules, undergo dynamic exchange with the muti-dentate linker, where the dynamic exchange between network modulators and linkers can promote configurational notion for networks approaching liquid state as temperature increased. So, they claimed and ascribed the vitrifaction of composites of Ti, Zr and B-O based networks to the activation of molecular and the dynamic exchange between modulator and linker. That may be true, but further or crucial experiment evidence is needed. What would happen if a metal-linker product mix or react with a modulator, such as the Ti-TB coordination polymer mixed with TMBA? Alternatively, would the order of adding TB and TMBA to the reaction affect the monolith or final products? Furthermore, what is the difference between this family and supermolecular glasses (Nature Communications, 2024, 15, 3929, Nature Communications, 2023, 14, 1990)?\n2) We would strongly advise authors to cite and clearly discuss their previous work in this area to indicate the advances of the present study over that previous study in Nature Communications, 2024, 15, 7339.\n3) We noted the molar ratio of metal compound and linker was fixed to be 3:2 in this study, which is promising for the produce of vitrified monolith. But this ratio of metal to linker is much different from Ti16O16(OEt)16 or Zr9O6(OBu)18, the crucial node structure claimed for the CRN glass network vitrified. Furthermore evidences are necessary for supporting such conclusion, except for those EXAFS spectra and PDF analysis presented.\n4) The Delta Cp of vitrifactions are around 0.6 Jg-1oC-1, which are higher than those commonly observed ~0.2 Jg-1oC-1 for MOF-based glasses.\n5) Statement ‘Ti…Ti distance is over the standard 3.1~3.3 Å value…’ for Fig 1c and Fig S3 is not appropriate/exact. It much deviates from 3.8 Å reported.\n6）‘……as the Tg of the Ti-TB-TMBA series is close to that of TMBA, the activation barrier of the network configurational motion is mainly associated with TMBA’ , what is about the Tg of TMBA?\n7) Successive up-scan plots (second, third …) of DSC for those monoliths are necessary. Are they stable enough?\n8）Supplementary Fig. 13 showed a different trend of Tg variation with ratio of linker from others. Would a much distinct structure such as one ordered dimensional polymer rather than that deducted monolith random node-linker network is produced? Because PEG5 linker is much longer and flexible compared to other two linker.\n9) ‘……as the Tg of the Ti-TB-TMBA series is close to that of TMBA, the activation barrier of the network configurational motion is mainly associated with TMBA’ what is about the Tg value of TMBA?\n10) The activation of modulator motion contributes to the dynamic exchange between TMBA and TB linker, also it provides configurational degree-of-freedom for glass transition by reducing network connectivity and dynamically exchanging with the linkers. It would be better and more clear if related theory computer study is performed.", "reviewer_5": "This is a high-quality manuscript reporting on a novel node-linker-modulator strategy for constructing molecular network-forming glasses. By controlling the introduction of network modulators, which can induce dynamic exchange and reduction in network connectivity during the formation of networks, the authors prepared a series of new metal-organic networks and covalent organic network glasses with various nodes and linkers. The effect of the addition of modulators on the formation of glasses was also investigated. This manuscript is of novelty and is well written. I suggest that authors should address the following issues:\n1. The heat-capacity jump is an essential indicator for the glass transition in the studied glasses. As shown in the method section, the DSC heat flow was measured. But how was the heat-capacity values obtained? Please describe this in the manuscript.\n2. In fact, their MOF glasses were obtained by a melt-quenching route despite the temperature was relatively low during synthesize. This is why the Tg can be observed.\n3. The mass change curves should be given, which helps understand the DSC response of the synthesized materials during heating.\n4. In the Figs 1i and 2f, the Angell plots are used to show the change in fragility with increased network ratio, but the result of decrease in fragility derived from visual observation is not convincing. Since both Tg and viscosity data have been obtained, the fragility index should be calculated and given.\n5. The method and data for confirming the chemical structure of the synthesized network glasses should be added.\n6. The endothermic or exothermic direction should be marked in the DSC figures.\n7. As shown in Figs. 1, 2 and 4, the increase of the amount of modulator causes an increase of Tg and decrease of heat-capacity jump. The authors ascribe such trend to the reduction of network connectivity and the increase of configurational degree-of-freedom. Generally, the degree of network connectivity (so called “polymerization”), has a major impact on the glass transition temperature. Why does the reduction of polymerization induce an increase of Tg in this work? Please clarify it.\n8. The method section should be added to the end of the main text."}, "author_response": "We thank the reviewer for their constructive comments. We have now selected the most representative material systems (Ti-TB-TMBA, Ti-PEG5-TMBA, Zr-TB-TMBA, Zr-GDPA, B-GDPA) to carried out in-depth characterization and quantitative analysis over their compositions, structures, thermal analysis and glass transition behaviors. The network connectivity is analyzed based on comprehensive information on their compositions, thermal stabilities and short-to-medium range order revealed by PDF analysis. With these information and quantitative analysis, the role of the modulator and be clarified in different scenarios, which mainly reduces the network connectivity and affect the non-covalent interactions between the building blocks. Notably, as glass transition is the manifestation of collectively coupled configurational motions, we have revised the statement that attribute the configurational motion at glass transition primarily to the dynamic exchange between modulator and linker (see response to question 4)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have improved their manuscript substantially compared to the previous version. In particular, they have focused their study on fewer compounds and now provide more in-depth structural and compositional characterisation for at least some of the materials. This is an improvement, as understanding the composition is essential to rationalise the observed physicochemical properties.\nNevertheless, several substantial issues remain that, in my view, significantly compromise some of the hypotheses put forward in the manuscript.\nThe following points must be fully addressed before I am able to recommend publication of this work:\n1. The authors now report 1H NMR data to quantify the ratios of the different organic building blocks. However, ICP and EA results are only provided for eight samples. Given the importance of the inorganic nodes (Ti, Zr, B) and oxygen content in determining the average connectivity of the proposed clusters, this dataset is too limited. Without a robust assessment of metal, oxygen and carbon stoichiometry across the series, including the variations with increasing modulator content, the claim of systematic tuning of connectivity remains insufficiently substantiated.\n2. The authors interpret increasing viscosity and reduced heat capacity jump at the glass transition as evidence for higher connectivity. However, they do not discuss the possibility that the “bridging” organic units (e.g., TB, GDPA, PEG-5) might simply coordinate both ends to the same inorganic cluster, yielding intramolecular chelation rather than extended network formation. This might be relevant for all these materials, as it would also contribute to relatively low Tg values. The manuscript would benefit from an explicit discussion of this possibility and, ideally, experimental approaches to distinguish between true network connectivity and localised chelation. However, I understand that the latter is very difficult to achieve.\n3. The authors use an experiment mixing TMBA and TTIP, observing isopropanol release, to argue for dynamic exchange of modulator and linker. However, this experiment does not convincingly demonstrate exchange in the final glass materials. In the TMBA-TTIP experiment, the system begins out of equilibrium. Isopropanol evolution results from condensation and evaporative shifting of the equilibrium. There is also water in the system, which shifts the equilibrium. In contrast, the glassy materials are prepared such that isopropoxide and water (likely the source of the oxide anions proposed to be present in the Ti16 and Zr3 clusters) have already completely reacted and isopropanol release is complete. At room temperature or below, alkoxide exchange at Ti, Zr, or B centres is expected to be extremely slow, given the high bond strengths and activation energies. The authors have not provided experimental evidence for such exchange in the glasses, particularly above Tg, where some mobility might occur. This remains an important open question.\n4. The manuscript proposes that a Ti16 cluster is the main inorganic building unit in the Ti-based glasses. However, the reported PDFs do not convincingly support this hypothesis. The experimental PDFs lack clear features beyond ~7 Å. Given the expected dimension of a Ti16O16 cluster of about 1 nm, distinct interatomic correlations should be visible to at least 8-10 Å. Moreover, the simulated PDFs presented appear unusually damped, with no discernible signal above 4 Å. The authors should clarify the modelling parameters applied (e.g. damping functions) and justify why such strong damping is appropriate. As it stands, the mismatch between experimental and simulated PDFs, particularly at intermediate distances (the distances in the experimental ones appear to be significantly longer than the simulated ones), suggests either substantial structural disorder within the clusters or that the real inorganic building units differ from the proposed Ti16 structure.\n5. A critical issue concerns the presentation of the X-ray total scattering data. The authors only provide the derived PDFs, but not the corresponding total scattering functions S(Q) in the Supporting Information. The total scattering functions contain important reciprocal-space information and allow readers to assess the data quality, including the reliability of background subtraction, normalisation, and the presence of artefacts. Moreover, the S(Q) data shown in the rebuttal letter only extend to Q = 12 Å-1. This Q-range is insufficient to generate reliable high-resolution PDFs. Too narrow a Q-range can lead to strong termination ripples and rapidly decaying correlations in r-space. This issue may well explain why the experimental PDFs exhibit such a rapid loss of features beyond nearest neighbours. Without access to the full S(Q) data, preferably extending to at least 20-25 Å-1, the quality of the PDFs, and hence the validity of the structural conclusions drawn from them, remains questionable. The authors must therefore include the full measured S(Q) data in the Supporting Information, along with details of the Q-range used in the Fourier transform.\n6. The various XRD patterns now presented in the Supporting Information show highly unsystematic variations in diffuse scattering features with increasing modulator content. Such large, non-systematic changes raise serious doubts about the structural similarity of the polymeric networks across the series. The authors should address this point in detail and discuss whether these variations imply significant differences in the underlying connectivity, which would complicate the interpretation of the trends in viscosity, Tg, and other properties.\n7. Since it is so difficult to characterize the structure of the Ti and Zr inorganic building units, their structures (compositions) might be amenable to characterisation via mass spectrometry. The authors could attempt to dissolve the materials in excess ethanol at elevated temperature (which would solvate and stabilise the clusters) and collect mass spectra to see whether signals corresponding to the proposed Ti16 cluster, or other cluster sizes, can be detected. Such an experiment would provide important additional evidence.\n8. The authors claim that the materials are stable in water, but both XRD and FTIR data show substantial changes upon water addition. Such changes are thermodynamically expected, given the reactivity of metal alkoxides with water. It is important to clarify whether these changes are reversible. For example, does the Tg return to its original value after drying? Do the XRD and FTIR spectra revert to their original forms? Such tests would help to assess the true stability of these glasses.", "reviewer_2": "The authors have addressed my comments from my previous review. now the paper can be accepted for publication in its current form", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "Authors Ying Liu et al. have carefully responded all comments arose by referees, and made corresponding revisions in the main text as well as the supporting information with more added/extended experiment data. So, I am pleaed to say this work meets the demanded standard of the Journal,and suggest to publish in Nat. Commun.", "reviewer_5": "The authors have very carefully addressed the reviewers' comments and made substantial improvements to the manuscript. Therefore, I recommend accepting the manuscript."}, "author_response": "We thank the reviewers for their positive feedback, and we now address the remaining issues in this revision."}, {"round": 3, "reviewer_comments": {"reviewer_1": "First, I would like to apologise for the delay in providing my comments on this second revision. I have been away on summer vacation and was unfortunately unable to address the manuscript earlier.\nThe authors have made additional efforts to strengthen their work and have included new compositional analyses. While some of these partially support the hypotheses, the fundamental structural assumptions remain insufficiently substantiated, and several major issues persist.\nThe central concern continues to be the reliance on Ti16O16 clusters as the representative inorganic building unit of the reported coordination polymers. This assumption is not convincingly supported by the data. The Ti contents measured by ICP deviate substantially for some materials, suggesting variations in oxygen content and implying the presence of either a distribution of Ti-oxo clusters with different compositions and (very importantly) connectivities or the presence of a very different kind of cluster type. Literature evidence highlights that Ti-oxo clusters can vary widely in alkoxide/Ti ratios (e.g. only about ~1.2 for Ti17O24 clusters versus 2 for the envisaged Ti16O32). These differences have major implications for network structure and network connectivity and, therefore, for any composition-structure-property relationships. Mass spectrometry indeed suggests the presence of an ensemble of different clusters, some of them very likely lighter ones, such as Ti17O24. By contrast, the PDFs cannot resolve the nature of the inorganic clusters, as the low Ti concentration in the samples means that the signal is dominated by the organic units. For example, the PDF shown in Figure 1c is from Ti-TB0.67TMBA1.33. The Ti content, according to ICP, is only 4%! Thus, the correlations in the PDF are strongly dominated by the organic building blocks of the material and identification of the Ti16O32 cluster is not possible. I suspect that a PDF of the organic building units TB and TMBA, without the addition of Ti(OiPr)4, will have similar features to the ones shown for the Ti-oxo-based materials. Consequently, the PDFs (similar to the MS and DLS data) do not provide sufficient evidence for the predominant existence of Ti16O16 clusters (or Zr3O clusters for the Zr-based materials). The correlations at 2.1 Å and 3.5 Å (page 9) may also be assigned to C···C distances in the organic units.\nThe DLS data are unable to substantiate that the Ti-oxo clusters present in the materials are of the type Ti16O16, since clusters of different composition and connectivity (alkoxide groups) but similar size are well documented. Hence, the discussion of the network connectivity based on the sole presence of Ti16O16 clusters is not supported by the experimental data. The corresponding text in the manuscript, including the discussion of the computational model presented in Figure 1b, requires thorough revision. The authors should explicitly acknowledge and discuss the limitations of the currently available experimental evidence regarding the structural building units of the materials.\nThe newly introduced control experiment involving ligand exchange on a pre-synthesised Ti16O16 cluster is also not informative. Similar to the control experiment in the previous revision, the new experiment also starts out of equilibrium. The significantly higher acidity of benzyl alcohol compared to ethanol is the thermodynamic origin of the ligand exchange. The experiment does not provide evidence for kinetic ligand exchange at the very low Tg values reported in the manuscript, and as such, is not considered evidence for the proposed dissociative processes in the glasses.\nOverall, while I acknowledge the authors’ efforts and agree that the reported systems are intrinsically complex, the evidence for the proposed Ti16O16 cluster model remains weak (the same is true for the Zr3O cluster model of the Zr-based materials). As such, the structure-property relationships discussed in the manuscript are mainly composition-property relationships; the structural and connectivity parts are not sufficiently reliable, and the conclusions risk overstating the level of structural understanding. Nevertheless, I believe the work could still be suitable for Nature Communications if the authors explicitly and openly acknowledge the limitations of their structural model, reframe their conclusions accordingly, and present their findings primarily in a phenomenological manner. Substantial revisions would therefore still be required before the manuscript could be considered for publication.\nThere are also some other issues remaining:\n- The new PXRD measurements (Bruker instrument) show essentially no scattering signal from the sample. Even for materials composed of light elements, one would expect diffuse scattering representative of the amorphous structure, its absence is concerning and suggests that the data quality is insufficient. The blue PXRD pattern shown in Figure R11a features a very strange line shape in the region from 20° to 35° 2theta, suggesting substantial instrumental problems.\n- The vertical axis in Figure 1i is incorrectly labelled.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We thank the reviewers for their time and input, especially the perspective from (likely) a more physical community. From a material chemistry perspective, the main message of this paper is the modular synthesis of a series of glassy coordinative and covalent networks. Thus the structural and connectivity analysis is mostly meant to support: (1) the presence of coordinative network at low modulator content; (2) the gradual reduction of modulator content give increasing network connectivity and viscosity, enabling the evaporative synthetic method. Thus, the fundamental structural assumption is not the predominant presence of the Ti cluster. Instead, the key assumption that the structural analysis is trying to support is that the present titanium node is similar to the Ti cluster in terms of size and stoichiometric concerns. And this “similarity” is defined with respect to the tunability range of the modulator content. For instance, even in extreme cases where the Ti O (OEt) is a dominant species, the currently founded modulator content, in the lower limit, still support the presence of a coordinative network. Thus, we are not trying to propose a molecularly precise metal node or quantitatively precise connectivity, although in the quantitative analysis, it is found that the Ti based analysis give a relatively good match with physical properties. However, we also agree that the current narration may mislead certain authors from different background, and we revise the paper to clearly state the limitations."}, {"round": 4, "reviewer_comments": {"reviewer_1": "The authors have revised the manuscript once again and have appropriately moderated those statements that are not supported by experimental data. As previously indicated, the findings of the paper are undoubtedly valuable. The manuscript can now be accepted for publication.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter