Photoinduced anisotropy of push-pull chromophores incorporating 5-(1,3-benzodithiol-2-ylidene)-1-(4-nitrophenyl) penta-1,3-diene embedded into photopolymer oligoetheracrylate matrices

Photoinduced (PI) birefringence (Delta n) and phototransparency (T) have be en studied for a wavelength of 633 nm (He-Ne laser) under photoillumination by doubled-frequency YAG:Nd pulse laser polarized beam (lambda = 0.53 mu m ; pulse duration tau = 10-50 ps). The push-pull chromophore 5-(1,3-benzodit hiol-2-ylidene)-1-(4-nitrophenyl) penta-1,3-diene was incorporated into an oligoetheracrylate photopolymer matrix in a concentration of about 2.5 wt.% . Special properties of the molecule are connected with substantial role pl ayed by electron-vibration interactions in the observed PI effects. The Del ta n decreases with decreasing temperature. Optical density D depends non-l inearly on the PI beam power. With increasing photoinducing power (up to 0. 41 GW/cm(2)), the PI birefringence Delta n and the optical density D approa ches (1.4-1.7) x 10(-3) and 1.5%, respectively. The intensity dependence of intramolecular (669 cm(-1)) and intermolecular (89.5 cm(-1)) vibration mod e has a maximum and a broad minimum (at a PI power of about 0.45 GW/cm(2)), respectively. Such behavior correlates well with the PI dependencies of th e Delta n and the optical density D. Description of the obtained results is based on quantum chemical calculations with taking into account an electro n-vibration anharmonicity of the chromophore. Essential role of the PI elec trostatic potential redistribution under influence of the PI polarized ligh t beam has been demonstrated. Possible mechanisms of the observed phenomena are discussed within a phenomenological as well as microscopic approach. ( C) 2000 Elsevier Science B.V. All rights reserved.