Conformational studies of molecules in the excited triplet state by time-resolved Fourier-transform infrared spectroscopy

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To perform this research project, a Fourier-transform infrared spectro-photometer with an asynchronous time-resolving capability has been used in combination with an infrared microscope. The use of an infrared microscope is mecessary to observe laser-induced spectral changes from a small spot in the solid sample irradiated by loosely focused laser pulses. The fumdamental line at 1064 nm of a continuous-wave Nd : YAG laser and its second harmonic at 532 nm are mixed in a nonlinear crystal (BBO) to generate the third harmonic at 355 nm. The highest power of the third harmonic is 15 mW under a Q-switched operation which gives pulses with 70 ns duration at a repetition rate of 2 kHz. The third harmonic thus obtained is used to excite molecules in a KBr disk placed on the sample stage of the infrared microscope. In measurements by using the above spectrophotometer at room temperature, some infrared bands assignable to acridine in the lowest excited triplet state have been observed. For other organic compounds tested, however, effects due to chemical reaction (presumably oxidation), local heating, and evaporation of the sample are sensitively detected. To suppress these side-effects, attempts have been made to cool the sample down to about 120 K in vacuum with a miniature cryostat, which can be placed on the sample stage of the infrared microscope. Either dry nitrogen of argon gas of high pressure is used as the coolant. By using this apparatus, laser-induced infrared spectral changes have been observed for various aromatic compounds such as benzophenone, 4-phenylbenzophenone, and benzylideneaniline. Even at 120 K,effects of local heating are observed, whereas evaporation of the sample and chemical reaction seem to be suppressed to a satisfactory degree. It is necessary to separate effects of the local heating from spectral changes arising from genuine phtophysical and photochemical processes.