Molecular orbital and resonance Raman studies of the structures of N,N’-disubstituted indigo dyes.

Molecular orbital calculations have shown that the structures of both trans and cis isomers of N,N’-disubstituted indigo dyes are considerably twisted about the central C=C bond. The twisting angle of the cis isomer is not always larger than that of the corresponding trans isomer, indicating that the large hypsochromic shift on the trans→cis isomerization of N,N’-disubstituted indigo dyes cannot be explained by the twisted structure of the cis isomer. It is shown that the hypsochromic shift can be ascribed to a larger contribution of the zwitterionic form to the resonance hybrid structure in the lowest excited singlet state S1 than in the ground state S0 .This makes the repulsion between the negative charges on the carbonyl oxygen atoms enhanced and the instability of the cis configuration compared to the trans configuration much larger in the S1 state than in the S0 state and leads to a larger vertical S1 ←S0 transition energy for the cis isomer than for the trans isomer. [ABSTRACT FROM AUTHOR]