Generation of second harmonics and photoinduced birefringence in comb-like polymers with nonlinear optical (nlo) chromophores in side chains

摘要

A series of comblike (co)polymers of esters of methacrylic acid with photochromic chromophores exhibiting nonlinear optical activity in the side chain were synthesized. The processes taking place in polymer films due to the effect of external fields (laser beam and electric field) and nonlinear optical activity of these films were investigated. It was shown that the chemical structure and the composition of (co)polymers are related to the processes of increase and decrease in photoinduced birefringence as well as to the stability of signal of second harmonic generation. Owing to the development and improvements of photon technologies as the most progressive in optical information processing, at present the interest is growing in polymer materials the properties of which can be varied with the aid of light. The most popular chromophores making polymers photosensitive are azo- and azomethine dyes with a developed system of delocalized π-electrons and polar donor-acceptor substituents. As a rule, these dyes have high values of non-resonance nonlinear optical (NLO). Susceptibilities and are also able to undergo reversible trans-cis photoisomerization which leads to a considerable change in physical and chemical properties of polymers. The rapidity of response permits us to hope that these chromophores are promising for developing fast- responding information processing systems. The efficiency and stability of induced processes in polymers depend both on the chromophore and the polymer structure and on the procedure of introducing the chromophore into the polymer (composition or covalent introduction, introduction into the main chain or into the sidechain, through a flexible spacer or "rigidly"). To carry out the investigation, a series of comblike (co)polymers of methacrylic acid esters with nonlinear optical (NLO)chromophores covalently bound to main chains was synthesized. The chemical structures and designations of these chromophores are given in the table. The polymers differ in the structure of the second comonomer alkyl-(CP3 and CP6) or fluoroalkyl (CP2, CP4, and CP5) side chain, by the introduction of the third comonomer increasing the free volume (CP2 -CP6), by changing the structure of the NLO fragment introducing into the conjugation chain the azo (CP5 and CP6) or azomethene (P1, CP2 -CP4) group or a heterocyclic fragment: benzothiazole (CP6), or by the structure of the spacer binding the chromophore to the main chain (side chain is the sequence of -CH2 (CP2-CP6) or -CF2(P1) groups). The chosen structures make it possible to follow the relationship of the dynamics of photoinduced processes to the chemical structure of the (co)polymers. Note that so far the effect of the presence of fluorinated chains in the polymer structure and that of the presence of heterocyclic fragments in the chromophore structure on photoinduced birefringence have not been considered in the literature.