Theoretical investigation of the origin of the large non-linear optical response in acceptor-substituted carotenoids

Correlated semiempirical quantum-chemical calculations are performed to rationalize the origin of the large first- (α), second- (β), and third- (γ) order molecular polarizabilities found in carotenoid-like molecules capped at a single end by an acceptor group. The ground state polarization in the apocarotenal H(-CH double bond CH-)₁₀CH double bond C(CN)₂, is made to vary via the application of an external homogeneous electric field. The important structural characteristics found for this long polymethine are (i) the relative instability of the cyanine-like (equal-bond length) structures and (ii) the double-step evolution of the dipole moment caused by the transfer of a first and then a second electron towards the acceptor end of the molecule as the electric field increases in the range 10⁷-10⁸ V/cm. The abruptness of these steps results in very large values of the molecular polarizabilities. The longitudinal components of the molecular polarizabilities are analyzed as a function of the bond-order alternation (BOA) parameter.