A very efficient and stable supramolecular organic blend having a very high value of the optical gain for photorefractivity applications

A derivative of 2-methylindole, 3-[2-(4-nitrophenyl)ethenyl]-1-allyl-2-methylindole, NPEMI-A, has been studied for its photoconductivity and photorefractivity behaviour. The neat material was studied together with its blends with the organic polymer poly-(2,3-dimethyl-N-vinylindole), PVDMI. It was possible to carry out measurements with the content of the chromophore NPEMI-A changing from zero to 100 wt. %. No opacity hint was observed. The photorefractive optical gain Γ2 was obtained as a function of the chromophore content inside the different films. Differential Scanning Calorimetry measurements (DSC) were also carried out to obtain the whole change of the glass transition temperature Tg as a function of the amount of chromophore contained in the blends. From the experimental trend of Tg it was possible to gather a meaningful quantitative estimate of the value of the electrostatic interactions inside the blends. The importance of the value of Tg, and hence of the electrostatic interactions, in determining the extent of the photorefractivity has been clearly put in evidence. This was done by comparing the results obtained for NPEMI-A (Γ2 = 210.1 cm-1) with other ones previously discussed for a molecule similar to NPEMI-A but having a 2-ethylhexyl group, NPEMI-E (Γ2 = 2,027 cm-1), instead of an allyl group. Three different contributions to the photorefractivity of NPEMI-A, namely the Pockels, the Kerr and the Collaborative ones, could be distinguished from the peculiar trend of Γ2 as a function of the composition of the blends.