Assignment of metal-ligand modes in Pt(II) diimine complexes relevant to solar energy conversion.

This work describes a comprehensive assignment of the vibrational spectra of the platinum(II) diimine bisthiolate and chloride complexes as a prototype structure for a diversity of Pt(II) diimine chromophores. The dynamics and energy dissipation pathways in excited states of light harvesting molecules relies largely on the coupling between the high frequency and the low frequency modes. As such, the assignment of the vibrational spectrum of the chromophore is of utmost importance, especially in the low-frequency region, below 500 cm(-1), where the key metal-ligand framework modes occur. This region is experimentally difficult to access with infrared spectroscopy and hence frequently remains elusive. However, this region is easily accessible with Raman and inelastic neutron scattering (INS) spectroscopies. Accordingly, a combination of inelastic neutron scattering and Raman spectroscopy with the aid of computational results from periodic-DFT and the mode visualizations, as well as isotopic substitution, allowed for an identification of the modes that contain significant contributions from Pt-Cl, Pt-S, and Pt-N stretch modes. The results also demonstrate that it is not possible to assign transition energies to "pure", localized modes in the low frequency region, as a consequence of the anticipated severe coupling that occurs among the skeletal modes. The use of INS has proved invaluable in identifying and assigning the modes in the lowest frequency region, and overall the results will be of assistance in analyzing the structure of the electronic excited state in the families of chromophores containing a Pt(diimine) core.

Sterically hindered Re- and Mn-CO2 reduction catalysts for solar energy conversion.

Novel molecular Re and Mn tricarbonyl complexes bearing a bipyridyl ligand functionalised with sterically hindering substituents in the 6,6'-position, [M(HPEAB)(CO)3(X)] (M/X = Re/Cl, Mn/Br; HPEAB = 6,6'-{N-(4-hexylphenyl)-N(ethyl)-amido}-2,2'-bipyridine) have been synthesised, fully characterised including by single crystal X-ray crystallography, and their propensity to act as catalysts for the electrochemical and photochemical reduction of CO2 has been established. Controlled potential electrolysis showed that the catalysts are effective for electrochemical CO2-reduction, yielding CO as the product (in MeCN for the Re-complex, in 95 : 5 (v/v) MeCN : H2O mixture for the Mn-complex). The recyclability of the catalysts was demonstrated through replenishment of CO2 within solution. The novel catalysts had similar reduction potentials to previously reported complexes of similar structure, and results of the foot-of-the-wave analysis showed comparable maximum turnover rates, too. The tentative mechanisms for activation of the pre-catalysts were proposed on the basis of IR-spectroelectrochemical data aided by DFT calculations. It is shown that the typical dimerisation of the Mn-catalyst was prevented by incorporation of sterically hindering groups, whilst the Re-catalyst undergoes the usual mechanism following chloride ion loss. No photochemical CO2 reduction was observed for the rhenium complex in the presence of a sacrificial donor (triethylamine), which was attributed to the short triplet excited state lifetime (3.6 ns), insufficient for diffusion-controlled electron transfer. Importantly, [Mn(HPEAB)(CO)3Br] can act as a CO2 reduction catalyst when photosensitised by a zinc porphyrin under red light irradiation (λ > 600 nm) in MeCN : H2O (95 : 5); there has been only one reported example of photoactivating Mn-catalysts with porphyrins in this manner. Thus, this work demonstrates the wide utility of sterically protected Re- and Mn-diimine carbonyl catalysts, where the rate and yield of CO-production can be adjusted based on the metal centre and catalytic conditions, with the advantage of suppressing unwanted side-reactions through steric protection of the vacant coordination site.

Pt(II) mono-carbonyl complexes of a cyclometallating 2-(2'-thienyl)-(pyridinato-C,3N') ligand: nature and dynamics of the lowest excited state of the chloro- and thiolato-complexes.

The synthesis of a cyclometallated Pt(II) thiolate carbonyl complex Pt(thpy)(CO)(mts), (thpy = 2-(2'-thienyl)-pyridinate, mts = methylthiosalicylate) is reported. A combination of emission and time-resolved infrared (TRIR) techniques revealed for both Pt(thpy)(CO)(mts) and its chloride analogue Pt(thpy)(CO)Cl the predominant intra 2-(2'-thienyl)-pyridinate 3pi pi* character of the lowest electronic excited state. The unusually short lifetime (780 ps) of the intraligand 3pi pi* lowest excited state of Pt(thpy)(CO)(mts) indicates that this electronic state is influenced by another close-lying excited state, probably charge-transfer in origin.

PtII diimine chromophores with perfluorinated thiolate ligands: nature and dynamics of the charge-transfer-to-diimine lowest excited state.

The synthesis of new Pt(II) diimine complexes bearing perfluorinated thiolate ligands, Pt(II)(NN)(4-X-C(6)F(4)-S)(2), where NN = 2,2'-bipyridine or 1,10-phenanthroline and X = F or CN, is reported, together with an investigation of the nature and dynamics of their lowest excited states. A combined UV-vis, (spectro)electrochemical, resonance Raman, and time-resolved infrared (TRIR) study has suggested that the HOMO is mainly composed of thiolate(pi)/S(p)/Pt(d) orbitals and that the LUMO is largely localized on the pi*(diimine) orbital, thus revealing the [charge-transfer-to-diimine] nature of the lowest excited state. An enhancement of the thiolate ring vibrations, C-F vibrations, and the vibration of the CN-substituent on the thiolate moiety was observed in the resonance Raman spectra, whereas no such enhancement was seen for the nonfluorinated analogues. Thus, the introduction of fluorine substituents on the thiolate moiety probably leads to a more pronounced contribution of the intrathiolate modes to the HOMO compared to the analogous complexes with nonfluorinated thiolates. Furthermore, the introduction of the p-CN group into the thiolate moiety has allowed the dynamics of the lowest excited state of Pt(bpy)(4-CN-C(6)F(4)-S)(2) to be monitored by picosecond TRIR spectroscopy. The dynamics of the lowest [charge-transfer-to-diimine] excited state are governed by ca. 2-ps vibrational cooling and 35-ps back electron transfer.