# Naturally Derived Donor-π-Acceptor Compounds for Efficient Long-Wavelength LEDs/Sunlight-Induced Polymerization and High-Precision Multiple 3D Printing

Ji Feng, Tong Gao, Fabrice Morlet-Savary, Michael Schmitt, Céline Dietlin, Jing Zhang, Pu Xiao, Frédéric Dumur, Jean-patrick Joly, Jacques Lalevée

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## To cite this version:

Ji Feng, Tong Gao, Fabrice Morlet-Savary, Michael Schmitt, Céline Dietlin, et al., Naturally Derived Donor-π-Acceptor Compounds for Efficient Long-Wavelength LEDs/Sunlight-Induced Polymerization and High-Precision Multiple 3D Printing. Advanced Functional Materials, 2025, 10.1002/adfm.202518942. [DOI: 10.1002/adfm.202518942](https://doi.org/10.1002/adfm.202518942)

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# Naturally Derived Donor-π-Acceptor Compounds for Efficient Long-Wavelength LEDs/Sunlight-Induced Polymerization and High-Precision Multiple 3D Printing

Ji Feng, Tong Gao, Fabrice Morlet-Savary, Michael Schmitt, Céline Dietlin, Jing Zhang, Pu Xiao,* Frédéric Dumur, Jean-Patrick Joly,* and Jacques Lalevée*

Photopolymerization driven by long-wavelength visible LEDs offers a promising approach for biocompatible, low-energy 3D printing, minimizing light scattering and photodamage compared to conventional ultraviolet (UV) and near-UV systems. However, the inherently slow polymerization kinetics of long-wavelength photoinitiators (PIs) typically necessitate high light intensities, extended curing times, or excessive PI loadings, limiting their practical applications. To address these challenges, the design and synthesis of two novel donor-π-acceptor (D-π-A) PIs are reported, incorporating 2-phenylnaphtho[2,3-d]thiazole-4,9-dione as an electron acceptor and two naturally derived aldehydes (furfural and butyraldehyde) as electron donors. These compounds enable exceptionally rapid polymerization under blue and green light LEDs, as well as natural sunlight, even at low light intensities and minimal PI concentrations. The unprecedented curing speed and efficiency are further demonstrated through the fabrication of intricate 3D structures using three distinct printing techniques—direct laser writing (DLW), digital light processing (DLP), and liquid crystal display (LCD) printing. This work expands the frontiers of long-wavelength photopolymerization by combining high efficiency with eco-friendly, low-energy activation, paving the way for advanced applications in biocompatible additive manufacturing.

## 1. Introduction

Photocuring is a technology that uses light as an energy source to rapidly convert liquid resins into solid objects[1]. The process

of photocuring allows an efficient spa-
tiotemporal control and has enabled
transformative technological advances in
imaging, lithography, adhesives, coatings,
and 3D printing.[1,2,3,4] Particularly, lessons
learned from 2D light patterning have
been effectively extended to the rapid
preparation of 3D objects with customized
shapes through additive manufacturing
(i.e., 3D printing), driven by the digital
revolution.[1] Compared to high-energy UV,
visible LEDs offer 1) low cost and milder
reaction conditions, 2) deeper light pen-
tration due to reduced light scattering, and
3) wavelength selectivity (i.e., use of differ-
ent wavelengths to activate different chemi-
cal pathways).[5,6,7,8] Therefore, visible light-
induced photocuring is more suitable for
the preparation of biocompatible materials,
as it avoids the risk of cell damage asso-
ciated with high-energy UV absorption.[9]
In the dental field, the preparation of pho-
tocurable coatings has shifted to blue light
irradiation to reduce the risks associated
with UV exposure while maintaining

effective curing through visible light.[10,11] However, research on photocuring with longer-wavelength light (e.g., green light) remains limited, making the development of green-light sensitive photoinitiating systems (PISs) an intriguing area of study.

J. Feng, T. Gao, F. Morlet-Savary, M. Schmitt, C. Dietlin, J. Lalevée
Université de Haute-Alsace
CNRS, IS2M UMR7361
Mulhouse F-68100, France
E-mail: jacques.lalevee@uha.fr

J. Feng, T. Gao, F. Morlet-Savary, M. Schmitt, C. Dietlin, J. Lalevée
Université de Strasbourg
France

J. Zhang
Future Industries Institute
University of South Australia
Mawson Lakes, SA 5095, Australia

P. Xiao
State Key Laboratory of High Performance Ceramics
Shanghai Institute of Ceramics
Chinese Academy of Sciences
Shanghai 200050, P. R. China
E-mail: p.xiao@mail.sic.ac.cn

F. Dumur, J.-P. Joly
Aix Marseille Univ
CNRS, ICR, UMR 7273
Marseille F-13397, France
E-mail: jean-patrick.joly@univ-amu.fr

ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202518942

© 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adfm.202518942
Long-wavelength light can effectively reduce light scattering and photodamage. To tackle the challenge of efficient photocuring with long-wavelength light (≥500 nm, e.g., green light), researchers have explored the potential of various visible light systems to induce polymerization.[12,13,14] These PISs have been classified into two categories.[15] Specifically, Type I PISs do not require the addition of co-initiators, the active fragments that initiate polymerization can be produced after exposure to light. However, most Type I PISs are typically driven by shorter wavelengths of light (<420 nm).[16,17] In contrast to the Type I PISs,[16] Type II PISs can work via $\pi \rightarrow \pi^*$ excitation and have stronger molecular absorptivity at longer light wavelengths (≥500 nm).[1] However, compared to UV-driven polymerization, polymerization initiated by long-wavelength light is slower and less efficient, requiring prolonged exposure time, higher light intensities, and/or increased PI concentrations.[16,18,19] Therefore, the development of Type II PIs for three-component systems that enable rapid initiation with low PI concentrations and low-intensity green LED light is of significant interest.

Meanwhile, sunlight, with its broad emission spectrum and abundant availability, serves as an excellent energy source for photopolymerization. Its potential in photopolymerization reactions is vast, making it a highly active area of research.[20,21,22] Although Type I PIs can undergo direct homolytic bond cleavage to generate active species for photopolymerization, they suffer from stability issues under sunlight. In contrast, Type II PIs are more suitable for sunlight-induced polymerization due to their inherent stability when exposed to sunlight.[23] Interestingly, previous studies in our group developed some three-component systems based on 2-phenylnaphtho[2,3-d]thiazole-4,9-diones derivatives, suggesting good photoinitiation ability under sunlight.[23]

Considering the global trend towards sustainable development and green chemistry, the chemical industry is shifting towards the utilization of bio-based products.[24] Over time, two main approaches have emerged for utilizing natural products in photopolymerization. The first strategy involves the direct use of isolated and purified natural products as PIs. However, natural PIs, such as polyphenols[25] and naphthoquinones,[26] are highly light-sensitive and prone to instability in environmental conditions. For example, in our previous study, natural naphthoquinone vitamin K1 was successfully used as a PI for free radical polymerization.[26] However, its instability presented challenges during operation. The second strategy is to simply chemically modify natural products and use it as PIs. This strategy retains the high reactivity of the natural products while increasing their stability, which is a very promising approach for the future development of PIs.

To enhance the efficiency of visible light photocuring, we develop an innovative three-component PISs based on 2-phenylnaphtho[2,3-d]thiazole-4,9-dione derivatives. This system addresses key challenges in photopolymerization by integrating a uniquely synthesized electron acceptor (2-phenylnaphtho[2,3-d]thiazole-4,9-dione) with two natural electron donors—Furfural and Syringaldehyde—through a straightforward one-step synthesis of photocatalysts with D-π-A structures (See Scheme 1a). The system is further enhanced by incorporating ethyl 4-dimethylaminobenzoate (EDB) and bis(4-tert-butylphenyl)iodonium hexafluorophosphate (iodonium salt, Iod) to complete the 3-component system. Remarkably, this sys-

tem enables rapid polymerization upon exposure to a range of light sources, including LEDs at 405, 450, 470 nm (blue), 530 nm (green), and even direct sunlight. In this work, we systematically investigate the photoinduced electron/energy transfer (PET) mechanism within this three-component system and its impact on photocuring efficiency. Theoretical calculations revealed that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the synthesized D-π-A compounds are completely separated, facilitating an efficient electron transfer. To further elucidate the underlying design principles, we employed a comprehensive set of characterization techniques, including UV-vis absorption spectroscopy, fluorescence spectroscopy, Electron Spin Resonance Spin Trapping (ESR-ST), and Real-Time Fourier Transform Infrared Spectroscopy (RT-FTIR). These insights provide a crucial framework for advancing long-wavelength photopolymerization, including applications in sunlight-driven polymerization. Beyond fundamental studies, we successfully applied our three-component system to 3D printing, demonstrating its practical potential. Notably, high-resolution, smooth-surfaced macroscopic 3D patterns were achieved across three different printing technologies: DLW, DLP, and LCD printing. This breakthrough highlights the effectiveness of our system in improving photocuring efficiency. We anticipate that our design approach will further expand the capabilities of long-wavelength (>500 nm) visible light photocuring, paving the way for broader industrial applications.

## 2. Results and Discussion

### 2.1. Photoinitiators B1 and B2: Molecular Design, Synthesis and Theoretical Calculations

The two PIs B1 and B2 were synthesized directly from the natural products furfural and syringaldehyde in one step by using previously reported synthetic routes.[23] Scheme 1a outlines the specific synthetic route. The synthesized compounds have a simple D-π-A structure with the natural product moiety as an electron donor group and 2-phenylnaphtho[2,3-d]thiazole-4,9-dione moiety as an electron acceptor group. Notably, the D-π-A conjugated extends the absorption wavelength of the PIs into the visible range.

Density functional theory (DFT) calculations of B1 and B2 were performed to assist in the understanding of the electronic leaps associated with their absorption bands. As expected, as shown in Scheme 1b, the excited singlet states obtained indicate a complete separation of the electron-donating and electron-attracting units, implying some extent of conjugation between the two units. The calculations also indicate that the HOMO were all located in the natural product group with low contribution of the 2-phenylnaphtho[2,3-d]thiazole-4,9-diones group, and the LUMO were completely shifted to the 2-phenylnaphtho[2,3-d]thiazole-4,9-dione groups. The clear charge transfer features correspond to the electron leaving domains of the photoinduced HOMO-LUMO leaps ($\pi \rightarrow \pi^*$ lowest energy leaps),[27] which were elucidated by DFT calculations.

Molecular simulations also demonstrated the possibility of electron transfer between dyes and additives in the three-component system (dye/EDB/Iod). As shown in Scheme 1c, firstly, the dye was excited to an excited state by light irradiation,
<figure><img src="image_4.png" /><figcaption>**Scheme 1.** a) Synthetic routes to B1 and B2; b) calculated frontier orbitals of B1 and B2. DFT geometries optimization was performed using the B3LYP/6-31G* functional, frontier orbital energies have then been computed in a single point at a MPW1PW91/6-31g(d) level of theory; c) chemical mechanism of three-component (dye/EDB/iod) electron transfer reaction.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** UV-visible absorption spectra a); fluorescence spectra b); fluorescence lifetime of B1 c); singlet state energy determination of B1 d). The concentration of B1 and B2 was set to 2.5 × 10⁻⁵ M and dissolved in DCM.</figcaption></figure>

and the electrons of the dye transfer from HOMO to LUMO. Then, the electrons in the EDB HOMO level can be easily possibly transferred to the HOMO level of the dye. Similarly, the LUMO level electrons of dyes can be possibly transferred to the LUMO level of Iod. Thus, the feasibility of electron transfer between the three components was theoretically demonstrated from the energetic positions of the molecular orbitals involved.

## 2.2. Light Absorption and Fluorescence Emission Properties

The absorption spectra and associated extinction coefficients of the dyes were characterized. As shown in **Figure 1a**, the UV-visible spectra in dilute solution (2.5 × 10⁻⁵ M, dissolved in dichloromethane (DCM)) showed that the two dyes have absorption around 400–550 nm, which may be determined by their intrinsic rigid aromatic structures and D-π-A conjugated structures. Specifically, B1 and B2 have λmax of 318 nm and 327 nm, respectively (See **Table 1**), and the molar extinction coefficients (εmax) at λmax were quite high, ranging from 27 080–41 520 M⁻¹cm⁻¹ (See **Table 1**). In the visible range, λ were 415 and 441 nm, respectively, with ε ranging from 6480–7200 M⁻¹cm⁻¹.

Such a strong absorption cross-section was advantageous for photopolymers, as it allows for the use of lower initiator concentrations and light intensities. Furthermore, the fluorescence emission spectrum of B1 at the same concentration is demonstrated in Figure 1b, while its fluorescence lifetime is presented in Figure 1c, indicating that dye B1 exhibits a long fluorescence lifetime (See **Table 1**). Compared to B1, the fluorescence intensity of B2 is relatively low when the concentration was 2.5 × 10⁻⁵ M. The fluorescence spectrum of B2 at a higher concentration (8 × 10⁻⁵ M) is illustrated in Figure S1 (Supporting Information). In particular, the singlet energy (E$_{S1}$) can be calculated from the intersection of the normalized of the UV-visible absorption and fluorescence spectra (See **Figure 1d**).

## 2.3. Fluorescence Quenching Experiments, Steady State Photolysis, and ESR Experiments

Figure 2a,b show the process of fluorescence quenching of B1/EDB and B1/Iod systems, and the quenching can be more intuitively perceived by the Stern–Volmer plot in Figure 2c,d, represented as an increase in the ratio of the initial fluorescence

<table><caption>**Table 1.** Light absorption characteristics.</caption><thead><tr><th>PIs</th><th>λ<sub>max</sub> [nm]</th><th>ε<sub>max</sub> [M<sup>-1</sup>cm<sup>-1</sup>]</th><th>λ<sub>max</sub> in visible range [nm]</th><th>ε<sub>max</sub> in visible range [M<sup>-1</sup>cm<sup>-1</sup>]</th><th>ε<sub>405</sub> [M<sup>-1</sup>cm<sup>-1</sup>]</th><th>λ<sub>em</sub> [nm]</th><th>Fluorescence lifetime [ns]</th></tr></thead><tbody><tr><td>B1</td><td>318</td><td>41 520</td><td>415</td><td>6480</td><td>6200</td><td>550</td><td>6.81</td></tr><tr><td>B2</td><td>327</td><td>27 080</td><td>441</td><td>7200</td><td>3280</td><td>460</td><td>—</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Fluorescence quenching of a) B1/EDB, b) B1/Iod in DCM. Stern – Volmer treatment for fluorescence quenching of c) B1/EDB, d) B1/Iod.</figcaption></figure>

intensity ($I_0$) to the intensity (I) of a specific concentration of addi-
tive (EDB/Iod) (i.e., $I_0$/I). This suggests that the singlet photoex-
cited state of B1 makes some contribution to the polymerization
process. In addition, we observed that consecutive additions of
EDB or Iod can lead to the fluorescence quenching of B1, sug-
gesting that dye B1 can accept electrons from EDB or provide
electrons to Iod. This provides the basis for the construction of
the 3-component system. Thus, the main mechanistic pathway
of this system is thought to follow a redox quenching process.
In particular, the Stern-Volmer plot shows that successive addi-
tions of EDB lead to a steeper increase compared to the addi-
tion of Iod, indicating a higher electron transfer quantum yield.
Specifically, the electron transfer quantum yield of the B1/EDB
system reached 0.88, compared to only 0.52 for the B1/Iod sys-
tem. This suggests that B1 was more prone to accepting electrons
from EDB, a hypothesis further confirmed by subsequent poly-
merization experiments, where the B1/EDB system exhibited a
higher polymerization rate and a greater monomer conversion
than the B1/Iod system. As shown in Figure S2 (Supporting In-
formation), the monomer conversion of B1/EDB system reached
approximately 75%, significantly higher than the 50% conver-
sion of B1/Iod system. Meanwhile, compared with the long ex-
cited state lifetime of B1, the excited state lifetime of B2 was very
short. Longer excited lifetimes are beneficial, which means that
these excited molecules have a higher chance of experiencing col-
lisions before electronic relaxation.[28,29] Specifically, the Stern-
Volmer coefficient ($K_{sv}$) between the dye and EDB or Iod was
determined by the slope of the fluorescence quenching Stern-
Volmer treatment, and the electron transfer quantum yield was
calculated from $K_{sv}$ (See Table S1, Supporting Information). In
addition, the Stern-Volmer analysis described above was also ver-

ified again by cyclic voltammetry (See Figure S3, Supporting In-
formation). The free energy change ($\Delta G^{S1}$) for the electron trans-
fer between the dye and the additives (EDB and Iod) was cal-
culated according to the Rehm-Weller equation,[30] and the val-
ues are listed in Table S1 (Supporting Information). From the
table, the $\Delta G^{S1}$ for the dye is negative, which indicates that the
electron transfer reaction between the dye and the additives was
feasible.

Subsequently, steady-state photolysis experiments were per-
formed at 405 nm LED. One-component, two-component,
and three-component systems were investigated: dye-alone,
dye/EDB, dye/Iod, dye/EDB/Iod, respectively, with the concen-
tration of the dye set to 2.5 × 10⁻⁵ M and the concentrations
of the additives set to 5 × 10⁻⁵ M. **Figure 3a–c**, and **d** show
the photolysis curves of the 1-component, 2-component, and
3-component systems for B1, respectively, while Figure S4a–d
(Supporting Information) show the photolysis curves of the 1-
component, 2-component, and 3-component systems for B2, re-
spectively. The photolysis rate of dye B1 was faster than that of
B2, indicating that B1-based systems respond more effectively
to light. We also observed that the 3-component system exhib-
ited the fastest photolysis rate, followed by the 2-component sys-
tem, with the 1-component system showing the slowest rate.
In addition, we observed that the absorption intensities of the
two-component B1/EDB and B2/EDB systems were higher than
those of the 1-component dyes. In contrast, the absorption inten-
sities of the 2-component B1/Iod and B2/Iod systems showed no
significant change compared to the 1-component dyes. This dif-
ference may be attributed to the absorption peak of EDB, which
occurs at approximately 310 nm—nearly overlapping with that
of the dyes, as shown in Figure S5 (Supporting Information). In
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Steady state photolysis profile of a) B1-alone, b) B1/EDB, c) B1/Iod, d) B1/EDB/Iod at LED@405 nm in DCM. The concentration of B1 was set to 2.5 × 10⁻⁵ M and the concentrations of EDB and Iod were set to 5 × 10⁻⁵ M.</figcaption></figure>

comparison, the absorption peak of Iod is around 250 nm, which does not coincide with the dyes' absorption peaks and therefore does not influence them. In particular, the results of the photolysis experiments for the B1-based two-component system were closely aligned with the fluorescence quenching experiments. The photolysis rate of the B1/EDB system was faster than that of the B1/Iod system.

The ESR spectrum of the B1/EDB system is shown in **Figure 4a,b** with the hyperfine coupling constants $\alpha_N = 14.3$ G, and $\alpha_H = 2.4$ G (accuracy ± 0.1 G), while the B2/Iod system is shown in Figure 4c,d with the ultrafine coupling constants $\alpha_N = 14.2$ G, $\alpha_H = 2.1$ G (accuracy ± 0.1 G), corresponding to aminoalkyl and aryl radicals (Figure 4e,f), respectively. It is further shown that the electron transfer between the dye and the additive can generate radicals to initiate the polymerization of the monomer.

## 2.4. Photochemical Mechanism in Electron Transfer Reaction for Dye

Based on the theoretical calculations and the experimental results discussed above, we proposed the photochemical mechanism for the radical generation of the three-component system, as summarized in **Scheme 2**. Firstly, the molecular simulation calculations determined that the electron transfer reaction between the dye and the additives could take place at the theoretical

level. Then the chemical mechanism was further verified by combining photolysis, fluorescence quenching, and ESR experiments that the chemical mechanism was mainly a result of the redox reaction between the excited state of the dye and additives (EDB/Iod). As shown in Scheme 2, the dye is excited to the excited state (dye*) upon light irradiation. The dye* then undergoes redox reactions with EDB and $Ar_2I^+$, generating aminoalkyl radicals and aryl radicals. Interestingly, this process also produces $H^+$. Throughout the reaction, radical polymerization can be initiated by free radicals, while cationic polymerization can be initiated by cations.

## 2.5. Free Radical Polymerization (FRP) Under LED@405, 450, 470, 530 nm, and sunlight

The polymerizations were carried out using trimethylolpropane triacrylate (TMPTA, Figure S6, Supporting Information) as the monomer, selected for its low volatility and commercial viability. The monomer-to-polymer conversion of thick samples (about 2 mm) was determined by monitoring the FTIR peak at 6150 cm⁻¹ during LED irradiation; the monomer-to-polymer conversion of thin samples (about 25 µm) was determined by monitoring the peak at 1650 cm⁻¹. Specifically, to compare the difference in photoinitiation capability between one-component (dye-alone), two-component (dye/EDB or dye/Iod), and three-component (dye/EDB/Iod, See Figure S6
<figure><img src="image_8.png" /><figcaption>**Figure 4.** ESR-ST spectra of the PBN radical adducts (in tert-butylbenzene under nitrogen atmosphere). Irradiated by a) B1/EDB for 0 s, 50, 100, and 150 s; b) B1/EDB for 150 s and simulated; c) B2/Iod for 0, 50, 100 and 150 s; d) B2/Iod for 50 s and simulated under LED@405 nm irradiation; e) aminoalkyl radicals; f) aryl radicals.</figcaption></figure>

(Supporting Information) for dye-B1, dye-B2, EDB and Iod structures) systems, the initial experiments were performed using a concentration of 0.01 wt.% of the dye, a concentration of 1 wt.% of the additive (EDB/Iod), and an LED at 405 nm as the source of irradiation. As shown in Figures S2 and S7a (Supporting Information), a comparison revealed that the three-component

system had the highest conversion and the best photoinitiation capability (See Figure S7a, Supporting Information), and that all dye-initiated resin polymerizations showed excellent temporal response and good stability, as no measurable conversion occurred in the dark, corresponding to RT-FTIR spectroscopic measurements for the first 10 s without the light on, indicating

<figure><img src="image_9.png" /><figcaption>**Scheme 2.** The photoinitiation mechanisms of the prepared dye as Pl.</figcaption></figure>
that the resins were stable in the dark and that light was necessary to drive polymerization. In particular, the dye B1 3-component system (B1/EDB/Iod) exhibited photoinitiation capabilities similar to those of commercial 2-isopropylthioxanthone (ITX, Figure S6, Supporting Information), with conversions all around 82%, which was much higher than the 63% of the blank control system (EDB/Iod). The photoinitiation efficiency of the 3-component system was higher than that of the 2-component system and the 1-component system. This phenomenon may be attributed to the ability of the dyes to undergo light-induced electron transfer between its intrinsic donor and acceptor groups. As illustrated in Scheme 1c, the introduction of additives facilitates electron transfer between the dyes and the additives, thereby generating free radicals essential for polymerization. The experimental findings unequivocally supported this assertion. In a one-component system, polymerization either failed to occur or proceeded with significantly low efficiency. However, in a multi-component system with additives, the polymerization efficiency was markedly enhanced.

In view of the excellent photoinitiation capability of the three-component system, we further optimized the concentrations of the dye. As shown in Figure S7 (Supporting Information), the concentration of the additive (EDB/Iod) was fixed 1 wt.%, and four different dye concentrations were examined, and the dye concentrations of Figure S7a-d (Supporting Information) were set to 0.01, 0.05, 0.1, and 0.5 wt.%, respectively. By comparison, the conversions decrease significantly by increasing the dye concentration. This can be attributed to the increase in dye concentration, which can deepen the color of the system. The high molar extinction coefficients of the dyes can lead to an internal filtering effect, limiting the light penetration into greater depths.[31] Impressively, B1 showed excellent photoinitiation capability at ultra-low concentrations, reaching a maximum conversion of 82% after 10 s with the light on at concentrations of 0.01 and 0.05 wt.%.

Thus, for further exploration, we focused on the 3-component system B1/EDB/Iod, which demonstrated the highest photoinitiation capability. From the UV spectrum, we can see that the absorption of B1 in the visible range is from 400 nm to about 550 nm. In addition to the light source of 405 nm LED, we also selected 3 long-wavelength LEDs 450, 470 nm (blue light), and 530 nm (green light) as the irradiation source and investigated the photoinitiation capability of different B1 concentrations at these three wavelengths. As shown in Figure 5a-c, the PISs still demonstrated a high photoinitiation ability under blue light 450 and 470 nm (See Figure 5a,b), even under green light 530 nm (See Figure 5c), with the highest conversions of TMPTA close to 50%. Notably, the commercial initiator ITX did not effectively initiate the polymerization under these three light sources. Furthermore, an intriguing phenomenon was observed: the dye B1 concentration that achieved maximum monomer conversion increased with increasing LED wavelength. Specifically, the optimal B1 concentration was 0.01 wt.% at 450 nm, 0.05 wt.% at 470 nm, and 0.1 wt.% at 530 nm. This trend may be attributed to the fact that as the LED wavelength increased, the photon energy decreased, necessitating a higher PI concentration to maintain efficient radical generation and polymerization initiation. A higher PI concentration can lead to an increased number of initiating radicals, thereby enhancing double-bond conversion. However, as the PI concentration rose, the dye's high molar extinction co-

efficient intensified the system's coloration, resulting in an internal filtering effect that attenuated incident radiation and limited light penetration into deeper regions.[31] Notably, longer wavelengths were less affected by scattering and absorption, allowing for deeper light penetration. Consequently, increasing the excitation wavelength from 450 to 530 nm can significantly reduce the light scattering effects, which enhances the photopolymerization efficiency in highly concentrated PI systems.[32,33] As a control, we also tested the polymerization of another monomer (ethoxylated trimethylolpropane triacrylate, SR454, Figure S6, Supporting Information) under blue and green light under the same conditions. As shown in Figure 5d-f, the polymerization of SR454 under LED@450, 470, and 530 nm demonstrated that the concentration of dye B1 at 0.1 wt.% increased the conversion across all three LEDs. Notably, under green light, the conversion reached nearly 60%. However, further increasing the concentration to 0.2 wt.% resulted in decreased photopolymerization efficiency (Figure 5d-f). To identify the formulation with the optimal photoinitiation ability under green light, we refined the composition ratios to achieve the polymerization results for thick samples as depicted in Figure 6a. The formulation B1/EDB/Iod at 0.1/0.5/0.5wt.% led to a conversion rate of 62% for SR454. For thin samples, this conversion rate increased to 76% within 30 s, as shown in Figure 6b.

The formulations developed in this study were also utilized for sunlight-induced polymerization. This was conducted in Mulhouse (+77 43' E, 47 75' N) in eastern France on October 6th, 2024, between 1 pm and 3 pm. The weather was sunny, with temperatures around 18 °C. As shown in Figure 6c, the dye B1-based formulation achieved 65% conversion of TMPTA under sunlight within 60 s, which was much higher than the 52% of the commercial ITX-based one. The extraordinary photoinitiation capability of B1 highlighted its great potential for long-wavelength LEDs and sunlight-induced polymerization.

## 2.6. Cationic Polymerization (CP) Under LED@405 and 450 nm Irradiation

According to our proposed photochemical mechanism, it can be concluded that the three-component system can initiate both FRP and CP. Therefore, we tested the photoinitiation capability of the 3-component system in epoxy resin ((3,4-epoxycyclohexane)methyl 3,4-epoxycyclo-hexylcarboxylate (EPOX), Figure S6, Supporting Information), and the results are shown in Figure S8a,b (Supporting Information). The irradiation sources were LED@405 and 450 nm, respectively. Thin samples (about 25 µm) of the formulation dye/EDB/Iod 0.1 / 1/1 wt.% were investigated. Notably, the conversion of EPOX under LED@405 nm irradiation can reach as high as 96%, which was much higher than that initiated by the ITX-based system (75%). When LED@450 nm irradiation source was employed, only the B1-based system was able to initiate the polymerization, whereas ITX-based system was ineffective. In addition, we also carried out the control experiments of 1-component and two-component, and the results are shown in Figure S8c (Supporting Information). No polymerization was detected in all formulations. It indicates that dye-B1 is pivotal in the process of CP.
<figure><img src="image_10.png" /><figcaption>**Figure 5.** Photopolymerization profiles of TMPTA a,b,c) and SR454 d,e,f) for thick sample (about 2 mm, three-component B1/EDB/Iod) upon exposure to LED@450 nm (a,d), 470 nm (b,e), and 530 nm (c,f) irradiation. The irradiation starts for $t = 10$ s.</figcaption></figure>

## 2.7. Application of 3D Printing

In this study, three 3D printing techniques were demonstrated: DLW, DLP, and LCD, and the formulations were B1/EDB/Iod 0.05/1/1wt.% dissolved in the monomer TMPTA. Figure S9 (Supporting Information) demonstrates the DLW-printed 3D monogram “XZZ”, which was observed by electron microscopy to have clear outlines of the letters and a flat surface. Figure 7 demonstrates a 3D “Square Mesh” (Figure 7a) and a “Decorative Shoe” (Figure 7b) model printed by DLP. The resolution of the printed complex 3D objects was investigated using SEM. The square mesh structure showed regular edges and corners at the micrometer size (See Figure 7a), and the “Decorative Shoe” was structurally intact (See Figure 7b), indicating the fidelity of the structure. Figure 8 demonstrates the 3D “Cobra” (Figure 8a) and “Cube” (Figure 8b) models printed by LCD. In the SEM image, we can see that the beautifully detailed structure of the cobra was perfectly reproduced by the LCD printer with excellent spatial resolution, and all items showed satisfac-

tory accuracy at all sizes. The results showed that the B1 can be adapted to a variety of 3D printing techniques at the same time, and has unlimited potential applications in the field of additive manufacturing.

## 3. Conclusion

In summary, incorporating bio-derived electron-donating groups into 2-phenylnaphtho[2,3-d]thiazole-4,9-dione has proven to be an effective strategy for enhancing long-wavelength, visible light-induced photopolymerization efficiency without requiring extensive $\pi$-conjugation. In this study, we designed and synthesized two donor-$\pi$-acceptor (D-$\pi$-A) dyes using natural products as electron donors, forming a highly efficient three-component photoinitiation system with additives. Through a combination of theoretical calculations and experimental studies, we proposed a detailed photochemical mechanism for the electron transfer reaction in this system. These strategies enable exceptionally fast and efficient polymerization under blue (405, 450, 470 nm) and
<figure><img src="image_11.png" /><figcaption>**Figure 6.** Photopolymerization profiles for a) thick sample (about 2 mm) and b) thin sample (about 25 µm, three-component B1/EDB/Iod) upon exposure to LED@530 nm irradiation. Photopolymerization profiles of TMPTA for c) thick (about 2 mm, three-component PI/EDB/Iod) under sunlight. The irradiation starts for $t = 10$ s.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 7.** Demonstration of high-precision structure construction of DLP printing technologies. a) The “Square Mesh” was printed and SEM characterization. b) The “Decorative Shoe” was printed and SEM characterization. The formulation is dye-B1/EDB/Iod 0.05/1/1wt.% in TMPTA. DLP 3D printer-Anycubic Photon D2, China. Printing parameters were set as follows: a curing time of 70 s per layer, a layer thickness of 20 µm, five bottom layers with a curing time of 100 s, and a light-off time of 0.5 s.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure 8.** Demonstration of high-precision structure construction of LCD printing technologies. a) The "Cobra" printed and SEM characterization. b) The "Cube" was printed and SEM characterization. The formulation is dye-B1/EDB/Iod 0.05/1/1wt.% in TMPTA. LCD 3D printer-Anycubic Photon Mono, China. Printing parameters were set as follows: a curing time of 30 s per layer, a layer thickness of 100 µm, three bottom layers with a curing time of 200 s, a light off time of 2 s.</figcaption></figure>

green (530 nm) LEDs, as well as natural sunlight, achieving ultra-fast polymerization within 30 seconds at remarkably low catalyst loadings (0.01 wt.% for blue light, 0.1 wt.% for green light). Fluorescence lifetime spectroscopy further revealed a direct correlation between the excited-state lifetime of the dye and its photoinitiation efficiency, indicating that longer excited-state lifetimes enhanced photoredox activity for polymerization initiation. Notably, the unprecedented photoinitiation ability of dye B1 was successfully leveraged for high-precision 3D printing using multiple techniques, including DLW, DLP, and LCD printing. These results not only validated the excellent photoinitiation capability of the dye but also highlighted its potential for advancing low-energy, high-resolution 3D printing with green LEDs. Looking ahead, this design approach holds promise for expanding photopolymerization-based 3D printing in biomedical applications and beyond.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This research project is supported by the China Scholarship Council (CSC) (No. 202208220049). The authors acknowledge funding from the Science and Technology Commission of Shanghai Municipality (24520750100). This work was performed using HPC resources of the Mesocentre of the University of Strasbourg and the HPC resources from GENCI-IDRIS (Grant 2023-AD010812313R3/Jean_Zay).

## Conflict of Interest

The authors declare no conflict of interest.
## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

3D printing, LEDs, Long wavelength visible light, photopolymerization, sunlight

Received: July 22, 2025

Revised: September 9, 2025

Published online:

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