Control of photodynamic motions of azobenzene-derivative polymers by laser excitation wavelength

Abstract

The dynamic properties of photoinduced alignment and surface relief grating (SRG) formation as a function of excitation wavelength (lambda(ex) = 454,488, 514, 532, 568, and 647 nm) are examined using five azobenzene-derivative polymers, whose absorption spectra are tuned by substituting different electron-withdrawing groups (CF3, CN, CNCl, NO2, (CN)(2)). The dynamics sensitively depend on ex. The fastest rate of photoinduced alignment is observed when lambda(ex) is between lambda(max) of the trans pi-pi* transition and that of the cis n-pi* transition. A model of trans-cis photoisomerization has been developed based on competition between photoexcitation and relaxation, in which the n-pi* transition has a faster relaxation pathway than the pi-pi* transition. Under conditions of adequate absorbance of the trans form at lambda(ex), i.e., greater than 1/2A(max), the rate of SRG formation increases at longer lambda(ex)