Excited States Relaxation in Covalently Linked Dyads and Triads Based on Tryptophan, Deuteroporphyrin and Naphthoquinone

Abstract

Steady-state and time-resolved spectral-fluorescent characteristics have been measured for covalently linked dyads and triads consisting of deuteroporphyrin IX being attached via b-positions either to naphthoquinone or to one or two tryptophan residues in solutions of various polarity at 293 K. For the tryptophan-porphyrin dyad it was shown that experimental (F ET exper =0.75) and theoretical (F ET theor =0.87) values of the energy transfer efficiency are in a reasonable agreement and the Foerster theory of inductive resonance is still applicable to weakly interacting donor-acceptor systems at intercenter distances R DA ≈19÷25 Å. For the porphyrin-quinone dyad and tryptophan-porphyrin-quinone triad in dimethylformamide at 293 K, it was argued that the porphyrin fluorescence quenching may be appropriately described by the semi-classical Marcus theory as an endergonic or moderately exergonic non-adiabatic photoinduced electron transfer occurring within the "normal" region with the rate constant k PET =2.7⋅10 8 s-1. The quantitative experimental and theoretical analysis of both energy and photoinduced electron transfer processes for the systems under study leads to the conclusion that the formation of folded conformations is hardly realized for the dyads and triads in liquid solvents at ambient temperature.