Effects of conformation, substituents and solvent on molecular hyperpolarizabilities of push–pull diaryl-alkenes and -dienes: a computational study

The effects of substituents [donor strength (NMe2vs. MeO), number as well as positional isomerism], conformation (E/Z isomerism with respect to formal single bonds) and solvent on molecular first-order hyperpolarizabilities, β, of a series of differently substituted push–pull dicyanomethylenediaryl-alkenes (1–21) and -dienes (23–25) is discussed with the aid of semiempirical molecular orbital calculations (AM1 for structures, INDO/S for spectroscopic properties and hyperpolarizabilities). Without exception E-isomers are calculated to be slightly more stable and to have higher hyperpolarizabilities. The effect on both excitation energies and β of a fairly polar solvent (acetone) is found to be rather pronounced. Methoxy substitution of the arylalkenyl moiety leads to higher hyperpolarizabilities than the isomeric substitution of the aryl group (1–8vs. 9–21). Extension of the chromophore by a second arylalkenyl group (10vs. 22; 20vs. 23) or introduction of an additional donor (22vs. 24; 23vs. 25) decreases β.