Effects of Intraligand Electron Delocalization, Steric Tuning, and Excited-State Vibronic Coupling on the Photophysics of Aryl-Substituted Bipyridyl Complexes of Ru(II)

Abstract

The synthesis and photophysical characterization of a series of aryl-substituted 2,2‘-bipyridyl complexes of Ru II are reported. The static and time-resolved emission properties of [Ru(dpb) 3 ](PF 6 ) 2, where dpb is 4,4‘-diphenyl-2,2‘-bipyridine, have been examined and are contrasted with those of [Ru(dmb) 3 ](PF 6 ) 2 (dmb = 4,4‘-dimethyl-2,2‘-bipyridine). It is shown through analysis of electrochemical data and detailed fitting of the emission spectrum that the unusually large radiative quantum yield for [Ru(dpb) 3 ](PF 6 ) 2 in CH 3 CN solution at room temperature is due to reduction of the degree of geometric distortion along primarily ring-stretch acceptor mode coordinates relative to other molecules in this class. It is proposed that the 3 MLCT excited state of [Ru(dpb) 3 ] 2+ is characterized by a ligand conformation in which the 4,4‘-phenyl substituents are coplanar with the bipyridyl fragment, leading to extended intraligand electron delocalization and a smaller average change in the C−C bond length upon formation of the excited state as compared to [Ru(dmb) 3 ] 2+ . These conclusions are further supported by photophysical data on several new molecules, [Ru(dptb) 3 ](PF 6 ) 2 (dptb = 4,4‘-di- p -tolyl-2,2‘-bipyridine), [Ru(dotb) 3 ](PF 6 ) 2 (dotb = 4,4‘-di- o -tolyl-2,2‘-bipyridine), and [Ru(dmesb) 3 ](PF 6 ) 2 (dmesb = 4,4‘-dimesityl-2,2‘-bipyridine). The systematic increase in steric bulk provided by this ligand series results in clear trends in k r, k nr, and S M (the Huang−Rhys factor), consistent with the delocalization model. In addition, time-resolved resonance Raman data reveal frequency shifts in ring-stretch modes across the series supporting the notion that, as the steric bulk of the ligand increases, the ability for the peripheral phenyl rings to become coplanar with the bipyridyl fragment is hindered. Ab initio calculations employing Hartree−Fock and second-order perturbation theory on neutral and anionic 4-phenylpyridine, put forth as a model for the ground and excited states of [Ru(dpb) 3 ] 2+, are also reported. These calculations suggest a canted geometry for the ground state, but a considerable thermodynamic driving force for achieving planarity upon reduction of the ligand. The canted ground-state geometry is also observed in the single-crystal X-ray structure of the mixed-ligand complex [Ru(dmb) 2 (dpb)](PF 6 ) 2 . Finally, consideration of how this system evolves from the Franck−Condon state to the planar thermalized 3 MLCT state is discussed with regard to the possibility of time-resolving the onset of extended electron delocalization in the excited state by using ultrafast spectroscopy.