Direct and oxygen-mediated triplet-triplet annihilation of tetraphenylporphyrin in multilayers of decanol on the external surface of NaA zeolite

The kinetics of triplet state decay and concomitant delayed fluorescence (DF) of meso-tetraphenylporphyrin (TPP) adsorbed on the external surface of NaA zeolite covered by coadsorbed n-decanol (DEC) were studied by the laser flash photolysis. In deoxygenated solid samples of NaA with coadsorbed DEC, the diffusion-controlled bimolecular triplet-triplet annihilation (TTA) dominates in contrast to what is observed on the dry NaA surface. The TTA which results in DF can be described in terms of two reaction channels: (a) an energy transfer pathway with singlet excited state formation; (b) enhanced intersystem crossing in the triplet encounter complex. Whilst the TTA in multilayers occurs in DEC microphase, the reaction parameters change significantly when adsorbed ground-state molecules are involved, A fast TTA rate constant found at large TPP concentration in solid samples with DEC was attributed to triplet energy migration in TPP assemblies. Singlet oxygen works as an efficient mobile energy carrier between TPP triplets in both solid and liquid aerated samples. The parameters of this reaction may also be modified by changing the ratio [TPP]/[DEC] in multilayers, which in general act as microreactor systems with supramolecular organization of reactants. (C) 2001 Elsevier Science B.V. All rights reserved.