Theoretical Investigation and Molecular Design of Some Azulene Derivatives with Large Hyperpolarizabilities

The molecular hyperpolarizabilities and electronic properties of some azulene-containing chromophores have been investigated by employing AMI/FF and ZINDO/S-CI approaches. The calculation results show good agreement with the experimental values. It is found that the nonalternant azulene ring can be regarded as a more efficient conjugation bridge compared with benzene and thiophene because of its lower delocalization energy. Enhanced β values can be obtained by extending the conjugation length, altering substituted positions, or choosing appropriate substituents. The 1-donor-5-acceptor-azulenes are found to have large β values and small dipole moments (e.g., the 1-dithioylidenemethyl-5-[2,2-dicyanovinylbutadienyl]azulene has a high β₀ value of 181.7 ′ 10^-30 esu, which is close to the highest reported values and a relatively small dipole μ of 5.3 D), and the latter properties may be helpful in reducing chromophore-chromophore electrostatic interactions and in optimizing the chromophore loading level and the order parameter while incorporated into the polymer matrix.