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 RuII are reported. The static and time-resolved emission properties of [Ru(dpb)3](PF6)2, where dpb is 4,4‘-diphenyl-2,2‘-bipyridine, have been examined and are contrasted with those of [Ru(dmb)3](PF6)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](PF6)2 in CH3CN 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 3MLCT 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](PF6)2 (dptb = 4,4‘-di-p-tolyl-2,2‘-bipyridine), [Ru(dotb)3](PF6)2 (dotb = 4,4‘-di-o-tolyl-2,2‘-bipyridine), and [Ru(dmesb)3](PF6)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)](PF6)2. Finally, consideration of how this system evolves from the Franck−Condon state to the planar thermalized 3MLCT 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.

Theoretical Studies of Steric Effects on Intraligand Electron Delocalization: Implications for the Temporal Evolution of MLCT Excited States

“…As was alluded to above, we have recently detailed the photophysics of [Ru(dpb) 3 ](PF 6 ) 2 and related compounds in the context of intraligand delocalization. 7 Synthetic manipulations of ligand structure provided compelling evidence that the photophysical properties of these compounds are dictated to a large extent by intraligand delocalization in the 3 MLCT excited state. The ligands, comprising what we refer to as the "dpb series", are illustrated below.…”

Static and Time-Resolved Spectroscopic Studies of Low-Symmetry Ru(II) Polypyridyl Complexes

“…The details of this analysis are described elsewhere, 31,33,34 but essentially, the emission envelope is modeled as a sum of Gaussians spaced by the average vibrational mode coupled to the transition. E 0 then corresponds to the highest energy Gaussian and gives the relative vertical positions of the excited-and ground-state potential surfaces, i.e., the 3 MLCT/ 1 A 1 gap.…”

“…Instrumentation used for obtaining nanosecond time-resolved data has been described elsewhere. 31 Excited-state difference absorption spectra were obtained with constant laser excitation power (∼3 mJ at 480 nm) and pump-probe cross section, and were normalized for sample absorbance at the excitation wavelength for comparative purposes. The ∆OD values at the bleach minimum were very similar (∼0.11), and the spectra presented below are normalized to one at this minimum for comparison of energetic trends.…”

Ultrafast Dynamics in the Metal-to-Ligand Charge Transfer Excited-State Evolution of [Ru(4,4‘-diphenyl-2,2‘-bipyridine)3]2+

“…2 It was observed that the [Ru(dpb) 3 ] 2+ complex has a substantially smaller rate of nonradiative 3 MLCT f 1 A 1 relaxation following 1 MLCT r 1 A 1 absorption than what is seen for [Ru(dmb) 3 ] 2+ . Using a combination of synthetic and spectroscopic methods, 2 we demonstrated that the drop in nonradiative decay (k nr ) for [Ru-(dpb) 3 ] 2+ was linked to a decrease in excited-state geometric distortion relative to the ground state because of extensive conjugation between the bipyridyl fragment and the aryl substituents in the excited state. Theoretical studies of model ligand systems further supported the assignment of a fully conjugated reduced ligand in the excited state.…”