Relationship between Dipole Moment of Chromophores and Photoinduced Birefringence

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In this paper, we describe the result of studies on photoinduced birefringence ($\Delta n$) properties of amorphous azobenzene copolymer consisting of nonpolar long $\pi$-conjugated chromophore and polar chromophores. In particular, we studied the relationship between the dipole moment balance of chromophores and the $\Delta n$ properties that can be used for holographic data storage. The polymer films were obtained by spin coating ($d=0.8$ μm) and were irradiated with a linear polarized Ar+ ion laser beam ($\lambda=488$ nm, irradiance = 1 W/cm2). Then the $\Delta n$ was probed simultaneously with a He–Ne ($\lambda=633$ nm) laser using a polarimeter and calibrated in relation to film thickness. From the result, the observed $\Delta n$ value decreased as the difference in the dipole moment between the two chromophores increased. As far as the investigation in this experiment are concerned, the largest photoinduced birefringence value, 0.244, was achieved with the poly{4-[2-(methacryloyloxy)ethyloxy]-2-methyl-4-(phenylazo)azobenzene-co-4-[2-(methacryloyloxy)ethyloxy]-4$'$-cyanoazobenzene}. This might be explained from the viewpoint of the dipole matching that will contribute molecular cooperative motion between two azo chromophores in the copolymer.
2006-01-15