DELAYED FLUORESCENCE INDUCED BY MOLECULAR-OXYGEN QUENCHING OF ZINC TETRAPHENYLPORPHYRIN TRIPLETS AT GAS SOLID INTERFACES OF SILICA AND ZEOLITE/

The quenching of Zn(II) tetraphenylporphyrin (ZnTPP) triplets adsorbed onto porous silica or NaA zeolite by molecular oxygen in the gas phas e was studied at different temperatures by the diffuse-reflectance las er flash photolysis technique. The quenching is enhanced with decrease in temperature (apparent activation energies are within the range -1. 5 to -2.0 kcal/mol) and occurs with high rate constants (2.1 x 10(5) t o 2.4 x 10(5) Torr(-1) s(-1)). While the ZnTPP delayed fluorescence (D F) is undetectable in evacuated samples, it appears in the presence of O-2. The yield of this oxygen-induced DF of ZnTPP corresponds to 10-2 0% of the prompt fluorescence in a wide range of O-2 concentration (pa rtial O-2 pressure from 0.001 up to 10 Torr). DF kinetics exhibits a d istinct rise part at relatively high O-2 pressure. Experimental result s are described in terms of a singlet oxygen feedback mechanism which includes the efficient triplet energy transfer from (ZnTPP)-Zn-3 to O- 3(2) and formation of O-1(2) followed by a much more efficient energy transfer from O-1(2) to the neighbor 3ZnTPP with formation of ZnTPP in the excited singlet state. A homogeneous kinetic treatment predicts s ome of the unique DF peculiarities (namely the apparent quadratic depe ndence of DF yield on initial amount of 3ZnTPP, obtained by varying th e laser pulse fluence, the growth of DF with a first-order rate consta nt 2 times greater than that of DF decay at high oxygen concentration, and DF decaying at the same rate as 3ZnTPP at high O-2 content or fas ter at low O-2 content) except the lack of variation of DF efficiency with ZnTPP content on the surface, strongly suggesting the existence o f a specific arrangement of ZnTPP molecules. The yield of ZnTPP triple ts on the surface depends essentially on the oxygen amount in the narr ow pressure range from 0.01 to 0.1 Torr which is ascribed to the speci ficity of ZnTPP adsorption sites on the surface based on oxygen vacanc ies type defects.