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

Herein we report the synthesis, characterization, photophysical and theoretical study of the luminescent dyad Ir-RhB containing two emissive centers: The first fluorescent (RhB = rhodamine B) and the second phosphorescent (Ir = orthometalated iridium complex), bound with an aliphatic bridge. These nearly independent chromophores have triplet excited states which are very close in terms of energy, allowing for reversible energy transfer between them. The observed transient absorption spectra and their time dependent variations were interpreted in terms of a relatively simple kinetic model, where selective excitation of the iridium-based chromophore affords the corresponding emissive triplet state, followed by its equilibration with the rhodamine based triplet state and simultaneous emissive relaxation to the dyad ground state. This model allows for analytical solution to give key characteristics of the dyad excited-state dynamics, including the energy gap between these triplets (Delta E=0.017 eV) and the rate constant of energy transfer (k((Ir-RhB)) = 6.0 . 10(8) s(-1)). These data were also verified by quantum chemical DFT and TD-DFT calculations.