Monomer and polymer indole-derived systems having NLO properties for very high optical gain photorefractive materials. The role of the electrostatic interactions.

Object of the present research is the study of some indole-based photorefractive (PR) materials. Novel indole-based NLO chromophores have been synthesized and their photoconductive (PC) and PR behaviour have been investigated when employed both alone (as low molecular weight glass forming multifunctional PR moieties) as well as in blends with a PC indolyl polymer, namely poly-(N-vinyl-2,3-dimethylindole). The aim was to get more insight in the onset of PR effect and to evaluate various contributions to it related to intermolecular interactions occurring among the components of a PR material. The introduction of increasing contents of polymer counterpart in PR blends containing the novel NLO chromophores permitted to follow these interactions in their onset and the establishment of new supramolecular arrangements, expected to be responsible for dramatic changes of electrooptic properties. In this way, it has been also possible to look for the proper formulation in order to take advantage of such interactions for the achievement of the best PC and PR performances. The joint analysis of the results of PC, PR and thermal (DSC) behaviour of the investigated materials revealed itself as particularly useful, due to the complexity of the many processes occurring simultaneously in organic PR materials. A careful comparison with numerous theoretical treatments permitted to rationalize the obtained results and to put in evidence the prominent role played by intermolecular interactions. On the other hand, the obtaining of long-term phase stability of amorphous PR materials, desirable for their practical use in a number of applications, was also a challenge. Many PR materials described in the past, indeed, although having good photorefractive properties, showed the undesirable drawback of fast recrystallization of one or more of their molecular components, leading to opacization of films and to the rapid loss of PR properties. Such aspect has been faced during this research obtaining indefinitely stable materials. As concerns PR, very large values of the photorefractive optical gain Γ (a main PR figure of merit) have been achieved in many cases, classifying the materials object of this research among the most efficient PR materials.