Optical properties of oxotitanium (IV) meso-tetrakis(4-sulfonatophenyl)porphyrin intercalated into the layered double hydroxides (LDH) studied by laser spectroscopy

Abstract

Some new nanohybrid materials have been synthesized by intercalating the oxotitanium(IV) meso-tetrakis(4- sulfonatophenyl) porphyrin(O=Ti(IV) TSPP) into the Zn/Al layered double hydroxides (LDHs), and their structures and photophysical properties have been investigated by various laser spectroscopic techniques. According to the XRD pattern of the synthesized nanohybrid materials, the macrocycle plane of O=Ti(IV) TSPP are grafted perpendicular to the LDH layers. The O=Ti(IV) TSPP-intercalated LDH exhibits band broadening of the absorption spectrum and a blue shift of Q-band as compared to that observed in solution. Resonance Raman spectral measurements demonstrate that the positively charged LDHs give rise to a slight decrease of the electronic density of the porphyrin ring accompanying a small change of the electronic distribution of the O=Ti(IV) TSPP. Consequently the LDH environment affects the energies of the two highest occupied molecular orbitals (HOMOs) of the O=Ti(IV) TSPP, a1u and a2u, producing a mixed orbital character. Being consistent with these electronic structural changes of O=Ti(IV) TSPP in LDH, both the fluorescence spectral change and the fs-diffuse reflectance transient measurements imply that the photoexcitation of the O=Ti(IV) TSPP intercalated into LDH undergoes fast relaxation to the O=Ti(IV) TSPP+-LDH- charge transfer (CT) state within a few picoseconds, followed by a photoinduced electron transfer between the O=Ti(IV) TSPP and LDHs with a rate constant greater than 1 × 1010 S-1. No evidence is found for back electron transfer. In conclusion, the O=Ti(IV) TSPP intercalated LDH seems to be a possible candidate for an artificial reaction center for an efficient solar energy conversion system.