FLUORESCENCE LINE NARROWING STUDY OF CYCLOPENTANEPORPHYRIN CHEMICAL DIMERS AT 4.2 K

The monomers of Zn-cyclopentanporphyrins (ZnOEP-cycle and ZnOEP-cycle= CH2) and their chemical dimers covalently linked via isocycles (Zn-cyc lodimers) have been studied by the method of fluorescence line narrowi ng (FLN) in tetrahydrofurane-toluene (3:1) glassy matrixes at 4.2 K. W ell-resolved fluorescence spectra upon laser excitation into the S-0-- >S-1 absorption band have been observed. In contrast to this, it has b een shown that the excitation into the region of S-0-->S-2 absorption band leads to FLN disappearance for individual monomers. On the basis of this effect positions of the S-0-->S-2 electronic transitions in th e energy scale have been determined. The normal coordinate treatment o f FLN spectra of ZnOEP-cycle=CH2 and those for the Zn-cyclodimers perm itted us to determine the normal modes which are connected with the fo rmation of the dimeric species. The weak interaction of Q-transitions of donor (D, ZnOEP-cycle) and acceptor (A, ZnOEP-cycle=CH2) subunits i n Zn-cyclodimers manifests itself in the strong fluorescence quenching of D. Under excitation into the S-0-->S-1 transition of D the FLN spe ctra of the dimers (belonging to their A subunits) were not observed. These facts are connected with the effective non-radiative singlet-sin glet energy transfer in conditions of essential spectra inhomogeneity.