Unconditionally stable indole-derived glass blends having very high photorefractive gain: the role of intermolecular interactions

Abstract

The photorefractivity of an indole derivative and of its polymer blends has been studied at room temperature. The indole derivative 3-[2-(4-nitrophenyl)ethenyl]-1-(2-ethylhexyl)-2-methylindole (NPEMI-E) is a typical low-molecular-weight glass-forming molecule having peculiar nonlinear optics characteristics. It is unconditionally soluble in the photoconductive poly-(N-vinyl-2,3-dimethylindole) so that all the possible blends can be studied for a weight percent (wt. %) content of NPEMI-E ranging from zero to 100. A very high and sharp maximum of the photorefractive optical gain Gamma(2) approximately 2000 cm(-1) was obtained for a NPEMI-E wt. % content of about 90. On the basis of recently published theoretical calculations, we have made the hypothesis that the rapid change of Gamma(2) can also be ascribed to a correspondingly quick variation of the molecular electro-optic parameters of the dissolved chromophore for some well distinguished values of its concentration in the polymer matrix. Differential scanning calorimetry measurements were made and the results carefully analyzed with the aim of obtaining information on the intermolecular interactions. These last measurements also allowed rationalizing the unconditionally stable glass appearance of the obtained blends. Measurements of spectroscopic ellipsometry were also made on blends with different NPEMI-E content.

The Relevance of the Collaborative Effect in Determining the Performances of Photorefractive Polymer Materials

“…The results are compared with those previously obtained for NPEMI-E [7] by rationalizing the different values of T g of the two blends. The value of T g , strictly connected to the value of the "free volume" and of the electrostatic interactions, has been found to be perhaps the most important factor conditioning both the value of G 2 and the shelf lifetime of the studied blend.…”

“…These values have to be compared with the corresponding values found for NPEMI-E that are T m = 68.0 8C and T g = À2.5 8C. [7] Figure 2 reports the trend of the experimental T g values as a function of the wt. % composition of the blends.…”

“…The first one is a high value of the photorefractive gain G 2 assuring that the recorded data are represented by a very high contrast hologram. The second property refers to the time durability of the cell (usually defined as shelf lifetime) [6,7] assuring that it maintains indefinitely its transparency without segregation of the chromophore all along its utilization time. Both properties can be rationalized, besides the already mentioned electro-optical parameters that give the chromophores their characteristics of optical non-linearity, also by thermodynamical considerations that take into account the so-called "free volume" of the polymer blend.…”

“…The syntheses of both NPEMI-E and PVDMI have already been reported. [7,11] The synthesis of NPEMI-A follows a very similar route and will be published elsewhere. A third component, 2,4,7-trinitrofluorenylidenemalononitrile (TNFM) is always present in each blend at a fixed wt.…”

Organic Photorefractive Materials and Applications

“…[ 196 ] A feature article on this matter can be found in. [ 197 ] An interesting variation of weight contents of low molecular weight glass forming multifunctional chromophore and a poly meric hole-conductor and thus a transfer from composite to LMW-material was recently described for NPEMI-A and NPEMI-E in [ 174 ] and, [ 175 ] respectively. Here, as both 2-methylindole-derivatives show a high mutual solubility in PVDMI ( 3b ,see Figure 4 ) due to their structural similarity, composite samples of chromophore content between 0% and 99% chromophore (each material was doped by 1 wt.% TNFM sensitizer) were fabricated.…”