Photochromism of 2-Chloro-N-salicylideneaniline in Amorphous Polymer Solids

Abstract

ABSTRACT:Photochromism of 2-chloro-N-salicylideneaniline, Sa(H,2-Cl), molecularly dispersed in amorphous polymer matrices was investigated under conventional steady-state irradiation. The final conversion of photochromic reaction in amorphous polymers was found to decrease with temperature due to notable influence on a thermal back reaction. The effect of polymer matrix on the thermal decay reaction was explained by the half-life (l/t, 12 ) of the colored species and concept of free volume of polymer matrices: the values at 20°C for Sa(H,2-Cl) in poly(methyl methacrylate) (PMMA), polystyrene (PS), and polycarbonate (PC) were I I.I, 3.0, and 2.5 min, respectively. This indicates that the size of free volume of PMMA is the smallest in the three polymers at room temperature. Arrhenius plots of l/t 112 for PC and PS broke obviously at 20 and 14°C in contrast to a linear relationship for PMMA, and apparent activation energies for the thermal decay reactions in both polymers were approximately half above the break points.KEY WORDS 2-Chloro-N-salicylideneaniline / Photochromism / Thermal Decay Reaction/ Half-Life of the Colored Species/ Photochromic Probe/ Free Volume/ Photochromism in polymer matrices has aroused much interest in connection with applications to EDRA W (erasable and directly reading and writing) memories, 1 and it has also been regarded as a molecular probe method for obtaining information of molecular motion of polymers in glassy state. 2 Reactions in amorphous polymer solids are generally governed by mobility and heterogeneity of reaction sites, so that photochromic processes may be retarded by polymer matrices compared with the liquid phase, and processes in amorphous polymers often deviate from first-order kinetics even if apparently elementary first-order reactions. 3 Salicylideneaniline and its derivatives (anils) are well-known organic photochromic compounds and their photochromism has been investigated in pure crystalline states and various matrices. 4 -6 In general, the photochrom-136 ism of anils in liquid solutions does not appear under conventional steady-state irradiation, because the photo-reaction yield of anils is relatively low and thermal reversion is extremely fast compared with other photochromic compounds. Therefore, the viscosity effect of solutions as well as other matrix effects on photochromism have been measured by laser-flash photolysis. 5 • 7 In this paper, the photochromism of 2-chloro-N-salicylideneaniline which is molecularly dispersed in amorphous polymer matrices was investigated under conventional steadystate irradiation at room temperature, and the matrix effects of polymer on the photochromism are discussed by comparing the thermal decay reactions in poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC), and others. The effect of polymer matrices on the thermal decay reactions was elu-

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