The effects of ligand substitution and deuteriation on the spectroscopic and photophysical properties of [Ru(LL)(CN)4]2− complexes

Abstract

The spectroscopic characterisation of a series of [Ru(LL)(CN)₄]²⁻ complexes, where LL = 1,10-phenanthroline (phen) and its methyl- and phenyl-substituted derivatives and several deuteriated isotopologues are reported. The optical and vibrational properties of these complexes are compared with that of the series of 2,2′-bipyridine (bipy) derivatives and analogous [Ru(LL)₃]²⁺ complexes. It has been demonstrated that substitution at the 4,4′ positions of bipy and 4,7-positions of phen by electron donating (CH₃) and withdrawing (C₆H₅, COO⁻) groups induces a pronounced blue and red shift, respectively, in the lowest energy ¹MLCT absorption band of [Ru(LL)(CN)₄]²⁻. The energy of the emission originating from the ³MLCT excited state is found to be dependant on the nature of the vibrational modes of the aromatic rings and the electron donating and/or withdrawing properties of the substituents. Single-mode Franck–Condon analysis indicates that methyl substitution leads to a significant increase in the Huang–Rhys factor (S_M), while phenyl substitution results in a decrease in S_M for both series (bipy and phen) of complexes. The rate of non-radiative (k_nr) and radiative decay (k_ph) to the ground state and the parameters of thermally activated deactivation pathways (A_4th, ΔE_4th and A_dd, ΔE_dd) were estimated from the temperature dependence of luminescence quantum yields and lifetimes. It has been demonstrated that the non-radiative decay rate and the temperature dependent decay processes are more efficient for bipy complexes than for phen derivatives due to the rigidity of the latter ligand.