LIGAND-BRIDGED RHENIUM(I) COMPLEXES - AN ELECTROCHEMICAL AND PHOTOPHYSICAL STUDY

The synthesis and properties of a series of rhenium(I) carbonyls incorporating the bidentate bridging ligands 4',7'-phenanthrolino-5',6':5,6-pyrazine (ppz), 2,3-bis(2-pyridyl)pyrazine (bppz), and 6,7-dimethyl-2,3-bis(2-pyridyl)quinoxaline (ddpq) have been investigated. All mononuclear complexes are emissive in room-temperature fluid solution; the luminescence originates from an MLCT excited state. On the other hand, emission was detected for only one of the binuclear complexes, [(ppz)(Re-(CO)3(CH3CN))2]2+. The lack of emission from the remaining binuclear species is attributed to the perturbation of the pi* orbital on the bridging ligand that results from the attachment of the second metal center. The magnitude of this effect has been probed by electrochemical and spectroscopic experiments. Lifetime measurements confirm the existence of a correlation between MLCT energy and the nonradiative decay rate, and the observed trend is in at least qualitative agreement with the energy gap law. Variations in the quantum yield for emission between similar complexes possessing different bridging ligands are attributed to changes in ligand rigidity, and complexes of ppz exhibit the largest phi(em) values. Not only are the corresponding bppz and ddpq derivatives much weaker emitters, but steric interactions lead to the thermal decomposition of binuclear complexes based on these ligands.