Polyazine-Bridged Ru(II),Pt(II) Trimetallic and Tetrametallic Supramolecular Complexes Exhibiting Unusual Excited State Dynamics Important in Catalysis and PDT Drug Development

Field: Chemistry (Physical Sciences)

The goal of this research was to develop structurally diverse polyazine-bridged Ru(II),Pt(II) trimetallic and tetrametallic supramolecular complexes and study the impact of component variation on the redox, spectroscopic, and excited state properties that influence photoinduced charged separation and multielectron reduction. These complexes are active photocatalysts for H2O reduction to H2. Tetrametallic complexes with the supramolecular architecture [{(TL)2Ru(dpp)}2Ru(BL")PtCl2]6+ (Ru3Pt; TL = phen = 1,10-phenanthroline or Ph2phen = 4,7-diphenyl-1,10-phenanthroline; BL" = dpp = 2,3-bis(2-pyridyl)pyrazine or dpq = 2,3-bis(2-pyridyl)quinoxaline) feature a trimetallic dpp-bridged Ru(II) light absorber coupled to a cis-PtCl2 reactive metal center. Trimetallic complexes with one less Ru-based light absorbing unit, [{(Ph2phen)2Ru(dpp)Ru(bpy)(BL")PtCl2]4+ (Ru2Pt; BL" = dpp or dpq), represent a new supramolecular architecture that was designed and synthesized to provide less complex systems for excited state analysis. Both the Ru3Pt and Ru2Pt systems have a remote Ru center separated from the reactive Pt site designed to provide extended 3MLCT lifetimes relative to directly coupled [(TL)2Ru(BL)PtCl2]2+ systems.\n\nThe building block synthetic method used in constructing supramolecules provides the ability to purify and analyze the properties following each synthetic step, allowing sub-unit variation and structural diversity. Building a knowledge base about the properties of smaller, less complicated structures is critical in understanding the electrochemistry, spectroscopy, and excited state dynamics of multi-metallic, multi-ligand complexes. Electrochemical analysis of the [{(TL)2Ru(dpp)}2Ru(BL")PtCl2]6+ complexes suggests a HOMO localized on the terminal Ru centers (E1/2 (RuII/III) = 1.56-1.63 V vs. Ag/AgCl) and a LUMO localized on the remote BL" coordinated to the reactive Pt site, with the energy dictated by the BL" identity (E1/2 = "0.32 or "0.33 V for BL" = dpp or E1/2 = "0.02 or "0.05 V for BL" = dpq). This provides spatially separated HOMOs and LUMOs which predict lowest-lying charge separated states. The complexes are robust UV and visible light absorbers due to multiple broad, overlapping ligand centered and metal-to-ligand charge transfer (MLCT) transitions. The lowest energy 1MLCT absorption is centered around 540-550 nm for the four tetrametallic complexes with high molar absorptivity (31,000-42,000 M"1cm"1). BL" variation has only a small impact on the electronic absorption spectroscopy, while the TL variation greatly enhances the absorptivity between 350 nm and 500 nm from 29,000 to 42,000 M"1cm"1 for complexes with TL = phen and Ph2phen, respectively. \n\nThe tetrametallic [{(TL)2Ru(dpp)}2Ru(BL")PtCl2]6+ complexes exhibit unusual excited state dynamics as well as spatially separated HOMOs and LUMOs. The lowest lying optically populated state is a terminal Ru\\'¼-dpp MLCT in all the Ru3Pt and Ru2Pt systems reported herein. The terminal Ru(dÀ) based HOMO and BL"(À*) based LUMO suggests a lower lying terminal Ru\\'BL" CS state in all systems. Because the lowest lying terminal Ru(dÀ)\\'BL"(À*) 3CS (charge separated) state is optically inaccessible, indirect population occurs. The tetrametallic [{(TL)2Ru(dpp)}2Ru(BL")PtCl2]6+ complexes have similar excited state lifetimes (Ä) of 75-83 ns, and they exhibit quantum yields of emission ("em) of 7.1 x 10"4 (when BL" = dpp) and 3.2-3.7 x 10"4 (when BL" = dpq). The lifetimes are shortened and the emission quantum yields are quenched in comparison to the [{(TL)2Ru(dpp)}2Ru(BL")]6+ models which possess the same emissive terminal Ru\\'¼-dpp 3MLCT state with Ä = 110 ns and "em = 1.0-1.1 x 10"3. \n\nIn marked contrast to the large number of Ru polyazine systems studied, both monometallic and supramolecular complexes, the lowest lying 3MLCT state of the Ru3Pt complexes is not populated with unit efficiency due to 3CS state population from the emissive terminal Ru(dÀ)\\'dpp(À*) 3CT state and a higher energy 3MLCT state, likely the central Ru(dÀ)\\'BL"(À*) 3CT state. The degree of population of this state is strongly dependent on the LUMO energy or driving force for population. Stabilization of the BL" = dpq(À*) compared to higher energy dpp(À*) provides a larger driving force for intramolecular electron transfer to populate the 3CS state, resulting in ca. 40 % and 95 % population of the emissive state when BL" = dpq and dpp, respectively. This suggests ca. 60 % and 5 % indirect population of the non-emissive 3CS state via a higher-lying 3MLCT state and the terminal Ru\\'dpp 3MLCT emissive state when BL" = dpq and dpp, respectively. These complexes violate Kasha\\'s rule, an unusual occurrence for Ru(II) polyazine complexes, as the emissive state is not populated with unit efficiency. Instead, the degree of population of the emissive state is dependent on the excitation wavelength.\n\nThe Ru3Pt complexes are active photocatalysts for H2O reduction to H2. In the presence of light and the sacrificial electron donor, DMA (N,N-dimethylaniline), the tetrametallic complexes collect electrons on the ligand À* orbitals of the central Ru to serve as photoinitiated electron collectors. The photocatalytic activity in H2 production is drastically impacted by BL" identity, consistent with the enhanced driving force for charge separation to move electrons toward the cis-PtCl2 moiety. After photolysis for 10 h, 15 ± 1 ¼mol (66 ± 4 TON) and 4 ± 1 ¼mol (18 ± 1 TON) of H2 were produced using [{(phen)2Ru(dpp)}2Ru(dpq)PtCl2]6+ and [{(phen)2Ru(dpp)}2Ru(dpp)PtCl2]6+, respectively. Varying TL to Ph2phen serves to enhance light absorptivity and subsequently increase H2 production to 21 ± 1 ¼mol (94 ± 6 TON) and 5 ± 1 ¼mol (23 ± 2 TON) using [{(Ph2phen)2Ru(dpp)}2Ru(dpq)PtCl2]6+ and [{(Ph2phen)2Ru(dpp)}2Ru(dpp)PtCl2]6+, respectively. The identity of BL" greatly influences the ability to direct the flow of charge through the supramolecular architecture impacting photocatalysis, while the identity of TL serves to fine tune the light absorbing and excited state properties.\n\nDue to the complicated excited state properties imparted by the large number of MLCT transitions in the [{(TL)2Ru(dpp)}2Ru(BL")PtCl2]6+ complexes, the analogous [(Ph2phen)2Ru(dpp)Ru(bpy)(BL")PtCl2]4+ complexes were designed and a synthetic scheme developed. The trimetallics possess similar electronic absorption spectroscopy with fewer terminal metal-based transitions due to removal of one (TL)2RuII(dpp) sub-unit and allow for distinguishable spectroscopic shifts resulting from perturbation of specific sub-units and the related molecular orbitals. The trimetallic complexes exhibit similar redox properties with the HOMO localized on the terminal Ru and the LUMO localized on the remote BL", providing a low lying 3CS state. Similar degrees of emission quenching are observed with the trimetallic complexes and their [(Ph2phen)2Ru(dpp)Ru(bpy)(BL")]4+ models as was observed in the tetrametallic complexes. The values of Ä and "em measured for [(Ph2phen)2Ru(dpp)Ru(bpy)(dpp)PtCl2]4+ were 90 ns and 1.1 x 10"3, respectively, and these values were 100 ns and 5.2 x 10"4 for [(Ph2phen)2Ru(dpp)Ru(bpy)(dpq)PtCl2]4+. Similar to the Ru3Pt systems, the lifetimes are shortened and the emission is quenched compared to the [(Ph2phen)2"Ru(dpp)Ru(bpy)(BL")]4+ models (Ä = 120 ns and "em = 1.50 x 10"3, regardless of BL" identity). These values provide ca. 98 % and 45 % population of the emissive state in the Ru2Pt systems for BL" = dpp and dpq, respectively, suggesting ca. 2 % and 55 % population of the non-emissive 3CS state for BL" = dpp and dpq, respectively. This supports the use of this new Ru2Pt motif as models to study the excited state dynamics. A substantial difference was observed between the excitation and absorption spectra for [(Ph2phen)2Ru(dpp)Ru(bpy)(dpq)PtCl2]4+, consistent with non-unity population of the emissive 3MLCT excited state. The simplified electronic absorption spectroscopy allowed the use of nanosecond transient absorption (TA) spectroscopy to analyze the excited state, and strong evidence of violation of Kasha\\'s rule through partial population of the terminal Ru(À)\\'BL"(À*) 3CS state (Ä = 25 ns) in addition to the emissive terminal Ru(dÀ)\\'dpp(À*) 3MLCT state (Ä = 120 ns) was observed through excitation-based emission spectroscopy and time-resolved TA spectroscopy. \n\nThe synthesis, electrochemistry, electronic absorption spectroscopy, and steady-state and time-resolved emission spectroscopy for the [{(TL)2Ru(dpp)}2Ru(BL")PtCl2]6+ and [(Ph2phen)2Ru(dpp)Ru(bpy)(BL")PtCl2]4+ complexes as well as photocatalytic H2 production with [{(TL)2Ru(dpp)}2Ru(BL")PtCl2]6+ and transient absorption spectroscopy of [(Ph2phen)2Ru(dpp)Ru(bpy)(BL")PtCl2]4+, are reported herein. The work discussed within this dissertation provides in depth knowledge about the effects of component modification on the excited state dynamics and photocatalytic activity of structurally diverse Ru3Pt and Ru2Pt supramolecular complexes that is important in developing photochemical molecular devices. Unusual excited state dynamics make it clear that much remains to be uncovered about structure-property relationship in these complex mixed-metal, mixed-ligand supramolecular motifs. These supramolecular motifs also have applications in photodynamic therapy drug development and are currently under investigation by members of the Brewer research group.