Fluorescence line narrowing study of cyclopentaneporphyrin chemical dimers at 4.2 K

Abstract

The monomers of Zn-cyclopentanporphyrins (ZnOEP-cycle and ZnOEP-cycle=CH2) and their chemical dimers covalently linked via isocycles (Zn-cyclodimers) have been studied by the method of fluorescence line narrowing (FLN) in tetrahydrofurane-toluene (3:1) glassy matrixes at 4.2 K. Well-resolved fluorescence spectra upon laser excitation into the S₀→S₁ absorption band have been observed. In contrast to this, it has been shown that the excitation into the region of S₀→S₂ absorption band leads to FLN disappearance for individual monomers. On the basis of this effect positions of the S₀→S₂ electronic transitions in the energy scale have been determined. The normal coordinate treatment of FLN spectra of ZnOEP-cycle=CH2 and those for the Zn-cyclodimers permitted us to determine the normal modes which are connected with the formation of the dimeric species. The weak interaction of Q-transitions of donor (D, ZnOEP-cycle) and acceptor (A, ZnOEP-cycle=CH2) subunits in Zn-cyclodimers manifests itself in the strong fluorescence quenching of D. Under excitation into the S₀→S₁ transition of D the FLN spectra of the dimers (belonging to their A subunits) were not observed. These facts are connected with the effective non-radiative singlet-singlet energy transfer in conditions of essential spectra inhomogeneity.