EFFECTS OF CONFORMATION, SUBSTITUENTS AND SOLVENT ON MOLECULAR HYPERPOLARIZABILITIES OF PUSH-PULL DIARYL-ALKENES AND DIARYL-DIENES - A COMPUTATIONAL STUDY

The effects of substituents [donor strength (NMe(2) vs. MeO), number a s well as positional isomerism], conformation (E/Z isomerism with resp ect to formal single bonds) and solvent;on molecular first-order hyper polarizabilities, beta, of a series of differently substituted push-pu ll dicyanomkthylenediaryl-alkenes (1-21) and -dienes (23-25) is discus sed with the aid of semiempirical molecular orbital calculations (AM1 for structures, INDO/S for spectroscopic properties and hyperpolarizab ilities). Without exception E-isomers are calculated to be slightly mo re stable and to have higher hyperpolarizabilities. The effect on both excitation energies and beta of a fairly polar solvent (acetone) is f ound to be rather pronounced. Methoxy substitution of the arylalkenyl moiety leads to higher hyperpolarizabilities than the isomeric substit ution of the aryl group (1-8 vs. 9-21). Extension of the chromophore b y a second arylalkenyl group (10 vs. 22; 20 vs. 23) or introduction of an additional donor (22 vs: 24; 23 us. 25) decreases beta.