Large negative hyperpolarizabilities (β) of the protonated Schiff bases of the azulenic retinal analogues

The molecular first hyperpolarizabilities (beta) and electronic properties of some azulenic retinal analogues and their derivatives have been investigated theoretically by employing semiempirical approaches. The results indicate that the protonated Schiff bases (PSB) of the 2-substituted azulenic retinal analogues possess extremely large negative beta values and very good transparency. These can be attributed to the large difference between the ground state dipole moment and the first excited state dipole moment according to the electronic property analysis. The characteristic blueshifted absorption in polar solvents of the 2-substituted PSB chromophores can be well explained by the negative solvatochromic effects. The largest calculated \ mu beta \ value can reach the magnitude of 10(-44) esu, which is close to the highest reported values of synthesized chromophores.