Photochemical and photophysical primary processes of solid poly(ethy1ene terephthalate) powder were investigated with nanosecond diffuse reflectance laser photolysis and conventional steady-state luminescence techniques. The existence of four transient species was confirmed. To assign these transients, dependencies of the transient absorption spectra and decay kinetics on foreign gas, dopant aromatic compounds which act as a photosensitizer or quencher, and crystallinity were investigated. In addition, the temperature effect was examined. As a result, the transient species, observed only under an aerated condition, was assigned to the precursor species leading to the photooxidation or photodegradation reaction. Other transient species were assigned to three different triplet states, which correspond to three different local structures: monomer site, dimer site, and a site which forms an excimer in the excited singlet state. The generation of three different triplet states is ascribed to an inhomogeneity of the aggregation and relative orientation of the monomer unit. It is discussed that crystalline and amorphous regions, and their boundary region, have a key role in photochemical and photophysical processes in polymer powder.
IntrodllctionFor elucidating a photochemical reaction mechanism, it is very important and indispensable to measure directly the dynamic behavior of excited molecules and chemical intermediates by laser photolysis. This technique has made great contributions to studies on physical, chemical, and biological systems.' Rather simple systems such as molecules in dilute solutions and in the gas phase have been frequently examined by the technique, while it is also powerful for photochemical studies of molecular aggregates, molecular organizates, and polymers. A typical example of the application of laser photolysis to polymer systems is investigation of photochemical reactions such as a-(Norrish type 1) and 8-scissions (Norrish type 11) of carbonyl-containing compounds.2 Another interesting topic is the photoinduced charge separation process of polymers, since this process is closely related to photoconductivity and solar energy conversion. Interchromophoric interactions and configurational and conformational structures in the excited singlet, triplet, cationic, and anionic states were elucidated on the basis of absorption spectral data. Since the excitation intensity in laser photolysis is relatively high, nonlinear photochemical behavior characteristic of polymers is easily induced and has received much attention as a new topic. Mutual interactions between excited states in one polymer chain, electron transfer from the higher excited states to the neighboring chromophores, and transient polyelectrolyte formation are such special photochemical beha~ior.~ Most of these investigations were carried out in solution, and measuring transient absorption spectra of polymers in the solid state has been an important subject for a long time. However, simple applicatioin of the conventional transmittance la...