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

The structural properties, molecular side-chain reorientation rates, and the magnitude of the second-order nonlinear tensor components were evaluated for poly[L-glutamate]modified with nonlinear optical chromophores. Circular dichroism showed that the alpha-helical backbone conformation 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 cell. The d spacing increased with the size of the side chain and the extent 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 was used to describe the first 2 h of the second harmonic decay process. We found that polypeptides with higher T(alpha) values produced greater long-term side-chain alignment stability, as measured by the slower decay rate of the second harmonic signal. The second-order tensor component, 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 conformation.