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 substituting different electron‐withdrawing groups (CF 3 , CN, CNCl, NO 2 , (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 ${1 \over 2}$ A max , the rate of SRG formation increases at longer λ ex .