Quenching of meso-tetramethylpyridyl porphyrin excited triplet state by inorganic salts: Exciplex formation

Abstract

The effects of LiCl, LiBr, LiI, NaCl, NaBr, NaI, KCl, KBr and KI salts on the excited triplet state characteristics of meso-tetramethylpyridyl porphyrin (TMPyP) in aqueous solutions were studied. It was demonstrated that inorganic salts in concentrations below 0.1 M effectively quench the TMPyP T1* triplet state. The logarithm of the quenching constant decreases linearly with the increase of the salt anion ionization potential, the effect of the cation nature being negligible. Triplet state quenching was explained by partial electron transfer from the salt anion to TMPyP in its triplet state (exciplex formation). This hypothesis was confirmed by the phosphorescence detection. The analysis of the singlet oxygen phosphorescence at λ = 1270 nm demonstrates that the quantum yield of the singlet oxygen increases in the presence of salts. Besides, the characteristic time of the singlet oxygen formation and its lifetime increase in the salt presence. These effects occur, probably, via energy transfer from the exciplex to the oxygen molecule. Moreover, a new phosphorescence at λ = 1200 nm appeared in the presence of salts has been observed, which we associate with that of the exciplex.The exciplex formation is accompanied by reversible short-lived optical absorption. On the other hand, salts in concentrations below 0.1 M do not induce any significant changes in the TMPyP S0→S1* absorption spectrum. Moreover, due to complete dissipation of the exciplex the salts in these concentrations do not influence the TMPyP photostability.