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

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The effect of steric bulk on electron delocalization in 4-arylpyridines 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 corresponding anions. These molecules are put forth as models for the ground and MLCT excited states of three polypyridyl ligands that were the subject of a recent report on the effects of sterics and delocalization on the photophysics of several Ru II complexes (Damrauer, et al. J. Am. Chem. Soc.1997, 119, 8253). The present study finds that, in the series 4-phenylpyridine, 4-(o-tolyl)pyridine, and 4-(2,6-dimethylphenyl)pyridine, the steric effect of the ortho-methyl groups serves to increase the dihedral angle between the pyridyl and phenyl rings of the neutral compounds from ca. 45° in the case of 4-phenylpyridine to ca. 65° and 90° 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 three model ligands reveal a preference for a coplanar structure, with the optimized geometries reflecting a balance between an energetic stabilization gained via conjugation in the planar form and unfavorable steric interactions between the methyl group(s) of the 4-aryl substituent and the pyridyl protons ortho to the central C-C bond. Calculated dihedral angles were 0°,25°, and45° for 4-phenyl-, 4-(o-tolyl)-, and 4-(2,6dimethyl)pyridine, respectively. 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 singlepoint energies of each compound as its monoanion in the optimized neutral geometry. Comparison of these energies with those of the fully optimized anions revealed effective reorganization energies of 4 -7 kcal/mol for 4-phenylpyridine, 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 pertain to ultrafast spectroscopic studies of MLCT excitedstate evolution in the corresponding Ru II bipyridyl complexes are discussed.