Dimers of crown-porphyrins as a possible model of statistical pairs responsible for the concentration quenching of fluorescence of tetrapyrrole molecules

Photophysics of the Zn-meso-5,10,15,20-tetrakis(benzo-15-crown)porphyrin (Z nTCP) dimer, which is formed in solutions in the presence of K+ ions, and h as the (ZnTCP)(2)(K+)(4) composition and the "face-to-face" geometry struct ure, is studied. It is shown that dimerization of ZnTCP enhances the rate c onstants of intersystem crossing and internal conversion more than a hundre d times, these rate constants amounting to 7 x 10(10) and 3 x 10(10) s(-1), respectively. The quantum yield of the triplet formation and the tripler-s tate lifetime vary in this case only slightly. Because such behavior of the photophysical parameters is also typical for the so-called diffusion stati stical pairs (SPs) responsible for the concentration quenching of fluoresce nce in solutions of tetrapyrrole molecules and dyes, the dimer under study can be considered as a model for such SPs. Possible mechanisms of the enhan cement of nonradiative channels of relaxation from the singlet S-1 state in the (ZnTCP)(2)(K+)(4) dimer are discussed.