Excited-state dynamics of normal and doubly N-confused type hexaphyrin derivatives studied by time-resolved fluorescence measurements

The excited-state dynamics of two expanded porphyrin derivatives which have six pyrrole rings of normal and doubly N-confused types was studied by time-resolved fluorescence and transient absorption measurements. We have found that their dynamics is very different due to four substituents in the doubly N-confused hexaphyrin derivative. The bleaching recovery of the free-base doubly N-confused hexaphyrin shows the double exponential fiinction with time constants of tau = 80 +/- 10 ps and 47 +/- 10 ps. The fluorescence decay curve, on the other hand, shows single-exponential function. To explain such behavior, we propose a model that the recovery process originates from two conformers in the S-1 state. It is plausible that the two excited-state conformers are deactivated independently to the ground state with radiative and nonradiative processes, respectively. (C) 2007 Elsevier B.V. All rights reserved.