Molecular and electronic structure in the metal-to-ligand charge-transfer excited states of d6 transition-metal complexes in solution

Abstract

The ground and metal-to-ligand (MLCT) excited electronic states of the complexes O~(bpy),,(P~)~-?+ (bpy = 2,2'-bipyridine, P2 = C~~-( C~H~)~P C H~H P ( C~H~)~; n = 1-3) have been studied by resonance Raman spectroscopy, time-resolved resonance Raman spectroscopy, and emission spectroscopy. The time-resolved resonance Raman evidence confirms that the charge-transfer electron density is localized in the lowest x * orbital of one bpy ligand rather than delocalized over the P* orbitals of all of the available bpy ligands on the vibrational time scale. The amount of charge transferred from the metal to bpy T * in the MLCT state has been determined. Application of Badger's rule to the Raman data and Franck-Condon analysis of the emission data lead to two independent determinations of average bond length displacements in the MLCT state. The two approaches yield displacement values that agree within 0.00 1-0.003 8, for the three complexes studied, suggesting that it may be possible to determine excited-state structures in solution with a precision similar to that of a good X-ray crystal structure. Analysis of time-resolved resonance Raman and emission data for Ru(bpy)?+, Ru(bpy),(en)2+, andfac-Re(bpy)(CO),CI by these methods yields results in agreement with the osmium data. The results suggest a general prescription for the determination of the molecular and electronic structures of electronically excited states in solution.As photochemical systems in solution, the metal-to-ligand charge-transfer (MLCT) excited states of d6 transition-metal complexes are rapidly becoming a paradigm. Information has appeared detailing both the photochemical and photophysical properties of many such systems.'-8 However, to date, essentially no quantitative information has emerged concerning molecular structure in this class of excited state in fluid solution. In this report we describe a study of this kind on the emitting MLCT excited states for the series of complexes Os(bpy),,(P2)3-;+ (bpy = 2,2'-bipyridine, P, = cis-l,2-bis(diphenylphosphino)ethylene (cis-Ph2PCH=CHPPh2); n = 1-3), which appear to be largely triplet in character but which contain considerable single character via spin-orbit coupling. The bases for our studies include both emission bandshape analyses and time-resolved resonance Raman spectroscopy. The results obtained allow us to infer quantitative information concerning both electronic and molecular structures for the M L C T excited states.Previously, time-resolved resonance Raman studies9-' I have conclusively established that the realistic formulation of the 600-ns-lived MLCT state of Ru(bpy)32+ is Ru"'(bpy), (bpy--),+; Le., the MLCT electron density is localized on a single bpy ligand on the time scale of molecular vibrations. This possibility had been suggested previously1as2 and has since been supported by several lines of evidence from a number of l a b o r a t~r i e s .~~~~~~J~J 'The experimental approaches employed in this study extend the conclusions concerning localization in MLCT excited states to the Os comp...

Characterization of excited electronic and vibronic states of platinum metal compounds with chelate ligands by highly frequency-resolved and time-resolved spectra

“…3.8). The present investigation strongly supports a delocalized and covalent characterization, while the other two models predict a localization due to a strong distortion [268,302] and, in contrast, a delocalization due to a weak exciton coupling between (Os-bpy) subunits [310], respectively. These two models will be briefly compared to the experimental situation.…”

“…)-vibrational energies. The values given in the third column are taken in refs [268,302] as a confirmation of this model. These experimens were carried out with [Os(bpy)3] 2+ dissolved in polar and fluid solutions at room temperature.…”

“…The higher members of the corresponding progressions are hidden below the experimental noise level due to ~9 A number of vibrational satellites, which are HT active with respect to the unperturbed state ] I i (e.g., at 210, 442, and 479 cm-1), are still relatively intense in the emission from the perturbed state I I~) even at B = 6 T, while other HT satellites have strongly reduced intensities (e.g., 1242 and 1446 cm-~; Fig. 24 (Table 9; see [302]). This behavior also supports the classification of these modes to FC active ones, if the RR enhancements result from a so-called Albrecht's A-Term scattering (FC-like) and if a B-term scattering (vibronic term) can be excluded for these ground state modes [104,211,[303][304][305][306][307][308].…”

“…Table 11 summarizes the results from [268,302]. The first column gives a number of ground state vibrational energies, while the second column shows values obtained by time-resolved resonance Raman scattering of largely saturated excited state(s).…”

“…These shifts are assigned to result from the localization of nearly one electron charge on one bpy-ligand. Thus, it is assumed in this model [268,302] that these excited state vibrational energies should compare well to (bpy. )-vibrational energies.…”