Electronic structure and linear and nonlinear optical properties of symmetrical and unsymmetrical squaraine dyes

In this theoretical study, we investigate the molecular and electronic structures of symmetrical and unsymmetrical squaraine dyes. Such dyes can be represented by the formula D-A-D, where D is a donor group and A an acceptor moiety. We analyze the evolution in geometric structure that results from changing both donor substituents simultaneously or from varying only one donor group to produce an asymmetrical system. The changes in geometric and electronic structures are compared and found to be consistent. The trends in linear and nonlinear optical properties, and in particular in second-order polarizabilities, are investigated in several ways. The two-state model appears to be inadequate in describing the second-order polarizability, β. Consequently, we are unable to deduce simple structure/property relationships that might help in the design of quadrupolar compounds for nonlinear optics. Finally, a series of unsymmetrical squaraines with OH substitution and enol-ketone isomerism are investigated; the calculated nonlinear properties follow a similar trend to the experimental results: the OH substitution and isomerization contribute to increasing the ground-state polarization.