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

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