Spectroscopic and optical characterization of a series of azobenzene-containing side-chain liquid crystalline polymers

A series of azobenzene-containing side chain liquid crystalline polymers wi th various spacer lengths (pXMAN, where X, the number of methylene units, v aries from 4 to 12) have been synthesized and characterized. Phases and agg regation have been studied by UV-visible spectroscopy as a function of temp erature. Polarized spectra were measured before and after irradiation with a resonant pump light at 488 nm. The annealing process on polymers with sho rt spacers (p4MAN-p8MAN) induces a hypsochromic shift while, for longer spa cers (p9MAN-p12MAN), it induces a large bathochromic shift. These effects m ay be due to the strong dipolar interaction and interdigitation of the side chain chromophores in an antiparallel orientation. The irradiation of such organized films with either circularly or linearly polarized light induces a bathochromic shift by breaking the antiparallel organization. This is a consequence of the angular reorientation of the azobenzene moieties as a re sult of trans-cis photoisomerization. To estimate the orientation of the si de chain azobenzene photoactive group, of the aliphatic spacer and of the m ain chain, linear dichroism in the infrared spectral range was measured usi ng the polarization-modulation technique with in situ irradiation. From the se experiments, a high and stable value of photoinduced linear dichroism re veals a strong interaction and self-organization of azobenzene side chain m olecules during the relaxation process. The anisotropic reorientation of th e polymer main chain and of the spacer is small when compared to the reorie ntation of the azobenzene side chains, which become perpendicular to the li nearly polarized pump beam. Additional birefringence measurements were carr ied out with in situ irradiation followed by a relaxation cycle for the who le series of polymers in various phases. The stability and the rate of the induced birefringence depend on the initial degree of organization within t he thin film and on its thermal history. It is noteworthy that annealing ca n lead to a significant improvement of the birefringence level (+44%). The results obtained on photoinduced orientation and phase organization are cor related with the polymer properties.