Protic solvent effects on the photophysical properties of OTiIVTSPP: photoinduced electron transfer

Abstract

The photophysical properties of oxotitanium(IV) meso-tetra(4-sulfonatophenyl) porphyrin (OTi^IVTSPP) have been investigated in water and methanol by laser spectroscopic techniques. The fluorescence emission spectrum of OTi^IVTSPP in methanol exhibits two strong emission bands at 610 and 670 nm at room temperature with the decay time of ca. 310 ± 10 ps and the rise time shorter than 30ps, in contrast to the extremely weak emission with the decay time of ca. 27 ± 4 ps in water, indicating that the fluorescence emissive states are different in the two solvents as supported by the solvent dependences of the excitation spectrum. The transient Raman spectra of OTi^IVTSPP in water has been observed to exhibit a remarkable enhancement of phenyl-related mode at 1599 cm⁻¹, while in methanol, the Raman frequencies of the porphyrin skeletal modes (υ₂ and υ₄) are down-shifted without any apparent enhancement of the phenyl-related mode, indicating different interactions of the two solvents with the excited OTi^IVTSPP. These Raman studies reveal that methanol molecule interacts with the photoexcited OTi^IVTSPP more strongly than water, forming the exciplex, OTi^IVTSPP(MeOH)*, suggesting that the two different emissive states are the singlet Franck–Condon state and the exciplex state in methanol and water, respectively. A broad triplet transient absorption of OTi^IVTSPP has been also observed at 480 nm in water as well as in methanol, which is decreased upon addition of methyl viologen (MV²⁺) with appearance of a new absorption band at 620 nm. This indicates that the photoinduced electron transfer (PET) takes place from the porphyrin to MV²⁺in both solvents. The kinetic analysis of the transient absorption band exhibits the PET rate constants of 4.76 × 10⁵ s⁻¹ and 3,03 × 10⁴ s⁻¹ in methanol and water, respectively. All these results infer that the PET takes place from the (d,π) CT state and the triplet state of the excited porphyrin in methanol and water, respectively.