THEORETICAL-STUDIES OF STERIC EFFECTS ON INTRALIGAND ELECTRON DELOCALIZATION - IMPLICATIONS FOR THE TEMPORAL EVOLUTION OF MLCT EXCITED-STATES

The effect of steric bulk on electron delocalization in 4-arylpyridine s has been studied by computational methods. Ab initio (HF, UHF, ROHF, MP2, UMP2, and ROMP2) as well as density functional theory (USVWN and UB-LYP) approaches were applied to a series of molecules and their co rresponding anions. These molecules are put forth as models for the gr ound and MLCT excited states of three polypyridyl ligands that were th e subject of a recent report on the effects of sterics and delocalizat ion on the photophysics of several Run complexes (Damrauer, et al. J. Am. Chem. Sec. 1997, 119, 8253). The present study finds that, in the series 4-phenylpyridine, 4-(o-tolyl)pyridine, and 4-(2,6-dimethylpheny l)pyridine, the steric effect of the ortho-methyl groups serves to inc rease the dihedral angle between the pyridyl and phenyl rings of the n eutral compounds from ca. 45 degrees in the case of 4-phenylpyridine t o ca. 65 degrees and 90 degrees in the mono-and dimethylated compounds , respectively. These results are generally consistent with the single -crystal X-ray structures of the three corresponding bipyridines, also reported herein. Upon one-electron reduction, calculations on all thr ee model ligands reveal a preference for a coplanar structure, with th e optimized geometries reflecting a balance between an energetic stabi lization gained via conjugation in the planar form and unfavorable ste ric interactions between the methyl group(s) of the 4-aryl substituent and the pyridyl protons ortho to the central C-C bond. Calculated dih edral angles were 0 degrees, similar to 25 degrees, and similar to 45 degrees for 4-phenyl-, 4-(o-tolyl)-, and 4-(2,6-dimethyl)pyridine, res pectively. Finally, a simulation of the Franck-Condon state evolution of MLCT states of molecules containing the bipyridyl analogues of the three models was carried out by computing single-point energies of eac h compound as its monoanion in the optimized neutral geometry. Compari son of these energies with those of the fully optimized anions reveale d effective reorganization energies of 4-7 kcal/mol for 4-phenylpyridi ne, 4-7 kcal/mol for 4-(o-tolyl)pyridine, and ca. 6 kcal/mol for 4-(2, 6-dimethylphenyl)pyridine. The implications of these results as they p ertain to ultrafast spectroscopic studies of MLCT excited-state evolut ion in the corresponding Ru-II bipyridyl complexes are discussed.