First hyperpolarizabilities of hexa(ethynyl)benzene derivatives: effect of conjugation length
A variety of dipolar and octupolar molecules containing CC bonds as the conjugation bridge have been synthesized and the linear and nonlinear optical properties (β) were studied. The β(0) value of the dipole increases with conjugation length, whereas that of the octupole decreases from 2a to 2b with concomitant decrease in λmax. A further increase in the conjugation length to 3a–c increased β(0) because of the larger oscillator strength. Moreover, 3a with the most distorted structure shows the largest β(0), probably because of the dipolar contribution. On the other hand, βyyy/βzzz ratios are smaller than unity, probably because βzzz is overestimated by the errors involved in the resonance correction.