TRANSIENT ABSORPTION-SPECTROSCOPY AND ANGULAR REORIENTATION OF AZOBENZENE MOLECULES IN A DR1-DOPED PMMA POLYMER MATRIX

The reorientational motions during trans <-> cis photoisomerization cy cles of an azobenzene derivative (Disperse Red 1, DR1) in a polymer ma trix (PMMA) have been investigated using visible absorption spectrosco py. Transient absorption measurements perpendicular (A(perpendicular t o)) and parallel (A(//)) to a linearly polarized pump beam (lambda(0)= 514.5 nm) were recorded during both orientation and relaxation process es. From these data the time variations of the average absorption (T-0 (t) approximate to 2A(perpendicular to) + A(//)) and of the linear dic hroism (T-2(t) approximate to A(//) - A(perpendicular to)) were extrac ted. Similar experiments were performed for various pump irradiances ( 0.8 mW/cm(2) to 400 mW/cm(2)). in most cases it is shown that T-0(t) a nd T-2(t) are very well accounted for, at least as a sum of two first- order kinetics, a fast one and a slower one. The kinetic results are d iscussed according to theoretical models already proposed to describe the photoisomerization-induced reorientations of azobenzene molecules in a polymer environment. The lifetime of the cis metastable state and rotational diffusion mechanisms are taken into account. For the relax ation processes over a 3600-s period the T-0(t) and T-2(t) variations are particularly well-fitted by a sum of two monoexponential functions : on the one hand, for T-0(t) the two rate constants are associated wi th the cis thermal back-relaxations, i.e. with a short lifetime (appro ximate to 10 s) for molecules in sufficient free volumes and a longer lifetime (approximate to 300 s) for the cis molecules in a constrained geometry; on the other hand, for T-2(t) another set of slow and fast rate constants is also effective, but the values are then modulated by the rotational diffusion constants. Finally, the estimated correspond ing rate constants and lifetimes of the cis isomer are compared with l iterature data in related systems.