44138-AC3
Design, Synthesis, and Photophysics of Platinum(II) Sensitizers for Solar Energy Conversion
The scientific explorations initiated either as a direct or indirect result of ACS-PRF AC funding was rather extensive. While the project initiated as an exploration of Pt(II) complexes surface bound to titania for solar energy conversion, many other avenues of study were generated. In terms of the primary project objectives, preliminary results generated by this funding were crucial in obtaining major NSF funding for my research group’s solar hydrogen program. Here, TiO2-surface anchored Pt(dcbpy)Cl2 was demonstrated as a rather efficient hydrogen-evolving species in aqueous solutions under UV-irradiation. This was not a result of molecular-based photocatalysis but rather the Pt(II) complex served as a photochemical precursor for the production of Pt(0) nanoparticles on the TiO2 surface. This was an important finding as it suggested that many of the so-called homogeneous catalysis systems using Pt(II) complexes required closer examination. One of the postdoctoral researchers associated with this project was partially funded by the ACS-PRF in this endeavor.
Operating the above system with visible light generated a novel dye sensitization system for the photodegradation of aqueous organic pollutants under visible light irradiation. The experimental results suggest that injected conduction band electrons initiate the reduction of dioxygen which promotes hydroxyl radical formation. The presence of OH· radicals in the present system was verified by three independent scavenging experiments. The hydroxyl radicals produced from the reductive route leads to the photooxidation of 4-chlorophenol (4-CP) along several concurrent reaction pathways, leading to rapid decarboxylation and dechlorination. Given the positive oxidation peak potential exhibited by Pt(dcbpy)Cl2 in this study, 4-CP is thermodynamically poised for irreversible oxidation by these surface bound structures, opening a second oxidative degradation pathway. The experimental data supports 4-CP serving the role of sacrificial electron donor which regenerates the resting oxidation state of the PtII dye sensitizer. We believe the general concept of using organic pollutant rich wastewaters in solar hydrogen generation schemes will be relevant for large scale photochemical energy production utilizing molecular charge transfer complexes. A doctoral graduate student’s stipend was supported by the ACS-PRF in this study.
Other Pt(II) structures relevant to solar energy conversion were also explored during this funding period. We presented a strategy designed to permit access to the perylenediimide (PDI) triplet manifold that preserves the desirable colorfastness and visible light-absorption properties associated with these dyes. To this end, three new Pt(II) complexes each bearing two PDI moieties tethered to the metal center via acetylide linkages emanating from one of the PDI bay positions was synthesized, structurally characterized, and thoroughly examined by nanosecond laser flash photolysis and ultrafast transient absorption spectroscopy. Upon ligation, the bright singlet-state fluorescence of the PDI chromophore is quantitatively quenched and no long wavelength photoluminescence is observed from the Pt(II)-PDI complexes in deaerated solutions. In each of the Pt-PDI chromophores, quantitatively similar transient absorption difference spectra were obtained; the only distinguishing characteristic is in their single exponential lifetimes (t = 246 ns, 1.0 ms, and 710 ns). Triplet-state sensitization experiments of “free” PDI-CCH using thioxanthone confirmed the PDI triplet state assignments in each of the Pt-PDI structures as did singlet oxygen generation experiments, observed by its characteristic photoluminescence in the near-IR.
A separate study provided clear-cut experimental evidence for solvent-induced configuration mixing and complete triplet state inversion at room temperature in a Pt(II) charge transfer complex bearing a combination of energetically proximate charge transfer and intraligand triplet excited states. This was the first time that such a phenomenon has been experimentally observed in a metal-organic chromophore.
During collaborative efforts with Prof. G.J. Meyer’s group at JHU on Pt(II) structures, we initiated a study on Ru(II) dyes on titania surfaces. Again, a graduate student from my group was able to visit JHU, facilitated by PRF funding, while another’s stipend was partially supported by the PRF to develop microwave synthetic procedures for a variety of Pt(II) and Ru(II)-based dyes. This work led to a study which demonstrated for the first time that following fast photo-induced electron injection into TiO2 and iodide regeneration, sensitizers are present in an environment distinctly different from that prior to light absorption. Significantly, the newly-generated sensitizer is in an environment that is known to be less favorable for excited-state electron injection. Under air mass 1.5, 1 sun irradiation, the slow (ms – ms) cation transfer discovered is not expected to limit the efficiency of Grätzel solar cells as a typical Ru(II) sensitizer absorbs light approximately every second. However, at higher irradiances or at planar TiO2 surfaces this effect may limit light-to-electrical power conversion efficiency. In all cases, the operative sensitization mechanism put forth in review-type articles, where a Ru(II) sensitizer is regenerated to its initial state within 10 ns, needs to be modified. The oxidized sensitizer may be reduced in ~10 ns but it is not brought back to the environment prior to light absorption until slow (ms – ms) cation transfer.
