Energy Transfer Processes in the Excited States of an {[Ir(N C) 2 (N N)] + ‐Rhodamine} Dyad: An Experimental and Theoretical Study

Abstract

Herein we report synthesis, characterization and photophysical study of the luminescent dyad Ir-RhB containing two emissive centres: fluorescent (RhB = rhodamine B) and phosphorescent (Ir = orthometalated iridium complex), bound each other by aliphatic bridge. The absence or very weak electronic conjugation of these chromophores makes them nearly independent but their excited triplet states proved to be very close in energy that allows for reversible energy transfer between them. The dynamics of this process and variation in excited states population have been studied by using pump-probe technique in a wide range of lifetimes, from picosecond to microsecond domains. The observed transient absorption spectra and their time dependent variations were interpreted in terms of relatively simple kinetic model, where selective excitation of iridium-based chromophore affords the corresponding emissive triplet, followed by its equilibration with the rhodamine based triplet and simultaneous relaxation to the dyad ground state through phosphorescent emission. This model allows for analytical solution to give key characteristics of the dyad excited state dynamics, including the energy gap between the Ir and RhB triplets (ΔE=0.017 eV) and the rate constant of energy transfer (k Ir-RhB =6.0x10 8 s -1 ) between these excited states. The data derived from experimental study of the transient absorption kinetics were also verified by quantum chemical DFT and TD DFT calculations.