Hyperpolarizability studies of some nonconjugated twin donor-acceptor molecules

Extensive theoretical calculation on the effects of spacer length enhancement on the second-order NLO properties of twin donor acceptor molecules having two amide units bridged by the CH2 spacers was performed. The role of such aliphatic bridges on the Donor-Acceptor groups was computed by ZINDO/CV quantum chemical formalism. The odd-even effects were observed in twin donor acceptor systems (with two aliphatic units) linked by an alkyl spacer of varying length from n = 1 to n = 12. The system considered for the present study was N,N′-alkane-(1, n) diyl bis-4-hydroxy hexanamides. For an odd number of CH2 spacers, the β value was an order of magnitude higher than that for the even number of CH2 spacers. The origin for such oscillation is attributed to the similar oscillations in the dipole moment difference between the ground state and the dipole allowed state and to some extent on the variation in the oscillator strength.