The triplet excited state of ruthenium(ii) bis(2,2′:6′,2″-terpyridine): Comparison between experiment and theory

The emission spectrum of ruthenium(II) bis(2,2′:6′,2″-terpyridine) has been recorded as a function of temperature over the range 77–290 K. Analysis of the spectrum allows calculation of the triplet energy and of the total reorganization energy accompanying deactivation of the metal-to-ligand, charge-transfer (MLCT) triplet state in both frozen glasses and fluid solutions. Emission quantum yields and triplet lifetimes decrease markedly with increasing temperature above 140 K and can be explained satisfactorily in terms of a 4-state model wherein the lowest-energy MLCT triplet interacts with two other triplets and with the ground state. The barrier for reaching the highest-energy triplet state is 1,700 cm−1 but, although this upper-lying triplet is usually described as being of metal-centred (MC) character, the experimental work does not help identify the nature of the interacting states. The same parameters were calculated by quantum chemical methods. It is seen that agreement between experiment and theory is rather good. Although the quantum chemical calculations indicate that the vertical energy difference between MLCT and MC triplets is about 3,000 cm−1, the latter species is characterised by a relatively large reorganization energy. When the fully optimised structures are taken into account, the energy difference between these two triplets is computed to be less than 1,300 cm−1.