DOI: 10.1002/cphc.200900619 The Relevance of the Collaborative Effect in Determining the Performances of Photorefractive Polymer Materials Rocco Angelone, [b] Francesco Ciardelli, [b, c] Arturo Colligiani,* [a, c] Francesco Greco, [b, c] Paolo Masi, [a] Annalisa Romano, [a] and Giacomo Ruggeri [b, c] 1. Introduction Electro-optical potentialities of photorefractive (PR) materials are very promising tools in direct light amplification, reversible image and data recording, as well as many other valuable and specialized applications. [1, 2] In more recent years, the photore- fractive effect, initially discovered in inorganic materials, has been particularly studied in organic molecular substrates both as neat material (low molecular weight organic glass formers, LMWG) and as blends in which the PR chromophore is dis- solved in different organic matrices or polymers. [3–5] Organic PR materials offer the advantage that their molecular components can be largely selected and modified to obtain specific electro- optical characteristics. The electro-optical parameters of a given selected chromophore—the electric dipole moment (edm), the polarizabilities a and b, and the first- and second- order susceptibilities—that determine the efficiency of the photorefractivity, can today be computed. [4] The values of the computed parameters dictate suitable chemical syntheses fol- lowed by tests on the PR behaviour. Many organic molecules and blends have been synthesized and tested, but totally satis- factory results have been seldom recorded. Indeed, a PR mate- rial, if it must be employed as a useful device (the PR cell) for technological purposes, usually as a thin glass film, must pos- sess at least two essential properties. 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 indefi- nitely its transparency without segregation of the chromo- phore all along its utilization time. Both properties can be ra- tionalized, besides the already mentioned electro-optical pa- rameters 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 poly- mer blend. [8] The chromophores, that can be allocated inside the microvoids forming the “free volume”, are allowed to reor- ient under the effect of applied electric fields and of their mutual electrostatic interactions. The possible reorientation is the origin of the “birefringence contribution” (BR) to the pho- torefractivity. [9, 10] The simultaneous presence of electrostatic in- teractions can also induce, as we will see, the sudden appear- ance of a “collaborative contribution” (COLL) that can strongly influence the PR behaviour. In this paper dealing with the NPEMI-A blends, we have for the first time quantitatively evalu- ated the different contributions to the photorefractivity and confirmed the essential role played by the COLL contribution. A derivative of 2-methylindole, 3-[2-(4-nitrophenyl)ethenyl]-1- allyl-2-methylindole, NPEMI-A, is studied for its photoconduc- tivity and photorefractivity behaviour. Its blends with the or- ganic polymer poly-(2,3-dimethyl-N-vinylindole), PVDMI, are also investigated. Due to the expected and devised mutual sol- ubility of the two components of the blends, it is possible to carry out measurements with the weight percent of the chro- mophore NPEMI-A changing from zero to 100. Films were pro- duced by a squeezing process between two ITO-covered glass sheets. No opacity phenomena, that are so common for many other organic blends due to the segregation of the dissolved chromophore, are observed. The photorefractive optical gain G 2 is obtained as a function of the chromophore content. Dif- ferential scanning calorimetry measurements (DSC) are also carried out to obtain the whole change of the glass transition temperature T g as a function of the amount of chromophore contained in the blends. From the experimental trend of T g a meaningful quantitative estimate of the value of the electro- static interactions acting in the studied blends, is obtained. The importance of the value of T g , and of the electrostatic in- teractions, in determining the extent of the photorefractivity is clearly evident. The results are compared for NPEMI-A (G 2 = 210 cm 1 ) and for NPEMI-E (G 2  2000 cm 1 ) that has a N-2- ethylhexyl group instead of a N-allyl group. The Pockels and Kerr contributions and—for the first time—a “collaborative effect” of the photorefractivity of NPEMI-A are distinguished and quantitatively evaluated. [a] Prof. A. Colligiani, Prof. P. Masi, Dr. A. Romano Department of Food Science University of Napoli “Federico II” Via Università 100, 80055 Portici (Italy) Fax: (+ 39)-050-2219260 E-mail : colligia@dcci.unipi.it [b] Dr. R. Angelone, Prof. F. Ciardelli, Dr. F. Greco, Prof. G. Ruggeri Department of Chemistry and Industrial Chemistry University of Pisa, Via Risorgimento 35, 56100 Pisa (Italy) [c] Prof. F. Ciardelli, Prof. A. Colligiani, Dr. F. Greco, Prof. G. Ruggeri Polylab - Istituto Nazionale per la Fisica della Materia Largo B. Pontecorvo 3, 56127 Pisa (Italy) 460  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2010, 11, 460 – 465