NONLINEAR-OPTICAL CHARACTERIZATION OF CHROMOPHORE-MODIFIED POLY[L-GLUTAMATE] THIN-FILMS

The structural properties, molecular side-chain reorientation rates, a nd the magnitude of the second-order nonlinear tensor components were evaluated for poly[L-glutamate]modified with nonlinear optical chromop hores. Circular dichroism showed that the alpha-helical backbone confo rmation of the polypeptides remained intact after modification of the side chains with the polar aromatic chromophores. The (100) d spacing (WAXS) of the polypeptide thin films was indexed to a hexagonal unit c ell. The d spacing increased with the size of the side chain and the e xtent of modification. The onset temperature of side chain motion and chain libration, T(alpha), in these films increased with the size and polarity of the chromophore. The side-chain chromophores were aligned by electric field poling, and their reorientation was monitored by the decrease in second harmonic generation. A biexponential decay model w as used to describe the first 2 h of the second harmonic decay process . We found that polypeptides with higher T(alpha) values produced grea ter long-term side-chain alignment stability, as measured by the slowe r decay rate of the second harmonic signal. The second-order tensor co mponent, d33, was determined by the Maker Fringe technique to be ca. 7 pm/V at a fundamental of 1064 nm. Poly[L-glutamates]-based nonlinear optical materials have promise for electrooptical applications as they retain the nonlinear optical signal and alpha-helical secondary confo rmation.