Photophysical properties of polypyridyl carbonyl complexes of rhenium(I)

Abstract

The photophysical properties of the metal to ligand charge transfer (m.l.c.t.) excited states of the complexes [Re(4,4′-X₂-bipy)(CO)₃Cl](X = NH₂, NEt₂, NHCOCH₃, OCH₃, CH₃, H, Ph, Cl, CO₂Et or NO₂; bipy = bipyridine) vary systematically as the substituent X is varied. For the cases where m.l.c.t. states are lowest lying a quantitative correlation exists between ln (k_nr× 1 s)(k_nr is the rate constant for non-radiative decay) and the Franck–Condon factor calculated from parameters obtained by emission spectral fitting. The solvent reorganizational energy for [Re(bipy)(CO)₃Cl] has been determined to be 1100 cm^–1 in EtOH–MeOH (4:1 v/v) and 650 cm^–1 in 2-methyltetrahydrofuran by a temperature dependent bandwidth study. Based on a comparative analysis of properties with related polypyridyl complexes of Ru^II and Os^II it has been concluded that: (1) the extent of distortion at the 4,4′-X₂-bipy acceptor ligand correlates with the energy gap between the excited and ground states; these results are in agreement with an earlier correlation found for polypyridyl complexes of Os^II; (2) the unusually large Stokes shift and the broadening of the vibronic components in absorption and emission spectra arise from a combination of increased solvent reorganizational energies and greater distortions in the low-frequency modes between the excited and ground states; and (3) the relatively short lifetimes for the complexes of Re^I have as a major contributing factor the participation of a ν(CO) mode at ca. 2020–2040 cm^–1 as an energy acceptor in non-radiative decay.