Relaxation Behavior of Twin Nonlinear Optical Chromophores: Effect of the Spacer Length

Abstract

A new series of twin nonlinear optical (NLO) molecules, having two 4-nitrophenol chromophores that are linked via a flexible polymethylene spacer of varying length [(CH2)n, n = 1−12], were synthesized. Powder second harmonic generation measurements of these twin samples indicated a pronounced odd−even oscillation, with the odd twins exhibiting a high SHG value while the even ones gave no measurable SH signal. This behavior reflects the crystal packing preferences in such twin NLO systems that have odd and even numbers of atoms linking themthe even ones appear to prefer a centrosymmetric packing arrangement. The orientational/disordering dynamics of these twin NLO molecules, doped in a polymer (poly(methyl methacrylate)) matrix, has also been studied using SHG in electric field poled samples. Interestingly, the maximum attainable SH signal, χ(2), in the poled samples also showed an odd−even oscillation; the odd ones again having a higher value of χ(2). This unprecedented odd−even oscillation in such molecu...