A derivative of 2-methylindole, namely 3-[2-(4-nitrophenyl)ethenyl]-1-(2-ethylhexyl)-2-metylindole (NPEMI-E) has
been synthesized. Materials obtained from this molecule have been studied as thin films between two ITO layers. The
study revealed that NPEMI-E collects in itself both photoconductivity and NLO characteristics. Differential Scanning
Calorimetry (DSC) measurements showed the formation of stable glass films characterized by a Tg temperature lower
than room temperature. Blends with the photoconductive poly-N-vinyl-2,3-dimethylindole (PVDMI) were also studied,
giving again stable glass films independently of the wt.% contents of NPEMI-E. Photorefractivity measurements on both
pure and blended NPEMI-E allowed to measure a value of the optical gain Γ = 627 cm-1 at an applied electric field E =
60 V/µm. This high value of Γ corresponds to a sharp maximum of the experimental trend of Γ as a function of the wt.%
content of NPEMI-E. The corresponding content was wt.% = 91.5. The presence of this maximum induced us to make
the hypothesis that, besides the well known reorientational contribution to the photorefractivity, a further mechanism
(recently theoretically studied) is active in our blends. This mechanism arises in the interactions among the NLO
polarized and polarizable moieties (cooperative effect). It can produce a rapid variation of some of the electrooptical
parameters conditioning the extent of the photorefractivity. This can happen at a well defined mean intermolecular
distance and hence at a well defined concentration of the NLO molecules.
The best conditions for the synthesis of poly(1-vinylindole) (PVI) and some of its methyl derivatives have been investigated. The aim of the research was to verify if PVI could be used instead of poly(1- vinylcarbazole) (PVK) in polymeric blends having photoconductive and/or photorefractive behavior. All synthesized polymers are characterized by a glass transition temperature that is lower than that of PVK. It has been verified that the indole ring system, inclusive of its methyl substituted derivatives, lacking of the symmetry characteristics of carbazole, possesses an electric dipole moment that is constantly higher than that of PVK. A higher dipole moment can be an advantageous feature to improve the solubility within the polymetric matrix of the optically non-linear molecule, necessary for photorefractivity. Charge-transfer complexes with a fluorenone-like photosensitizer are efficiently formed by both PVI and its derivatives. A direct current measuring apparatus has been assembled by which photocurrents of few picoamperes can be detected and recorded. Preliminary reported results clearly show that PVI is a photoconductive material. Its photoconductivity is compared with that of PVK and of hybrid materials containing mixtures of carbazole and indole moieties, present in the measuring cell as pendant groups on a macromolecular chain, as single molecules, or both.
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