Transient resonance Raman spectroscopic and ab initio MO investigation of substituent effects on the T-1 triplet states of halobiphenyls

Nanosecond transient resonance Raman spectra of the T-1 states and Raman sp ectra of the S-0 states of nine halogenated biphenyl compounds were obtaine d and ab initio computations were performed to determine the optimized geom etry and vibrational wavenumbers of the T-1 and S-0 states of 10 halogenate d biphenyl compounds. Substitution of halogen atoms at meta or para positio ns relative to the interring C - C bond leads to little change in the struc ture of the halogenated biphenyl compounds relative to the corresponding S- 0 and T-1 states of biphenyl. Substitution of halogen atoms at ortho positi ons results in significant increases in the twisting between the two phenyl rings of the S-0 and T-1 states of the halogenated biphenyl compounds rela tive to the S-0 and T-1 states of biphenyl. The degree of twisting between the two phenyl rings in ortho-substituted halogenated biphenyls appears to depend on the steric, electronegativity and/or polarizability properties of the halogen atom. Copyright (C) 2001 John Wiley & Sons, Ltd.