Determining the quantum yield of photochemical reactions in crystals from simultaneous effects of photothermal and photochemical bending of needle-shaped crystals

Photoinduced bending of needle crystals caused by photochemical transformation can be used as an extremely sensitive method for studying the kinetics of the transformation. However, the determination of the absolute value of the quantum yield of the reaction requires an accurate value of the intensity of light penetrating the crystal, in contrast to reactions in solutions where only the value of the total absorbed irradiation dose is sufficient. To address this problem, this study utilizes the effect of photothermal bending of a crystal due to its heating by light, occurring simultaneously with the bending due to transformation and proportional to the same value of light intensity. The ratio of the amplitudes of the two effects is independent of the light intensity, which allows the quantum yield to be determined without knowledge of the intensity value. In addition, the method allows the light intensity and thermal conductivity of the crystal to be estimated. The method is applied to measure wavelength dependence of the quantum yield of nitro-to-nitrito photoisomerization in [Co(NH3)5NO2]Cl(NO3) crystals. A monotonically decreasing value of the quantum yield phi from 0.19 to 0.04 in the range of lambda from 403 to 523 nm was obtained. This result indicates the qualitative differences in the transformation mechanism in crystals and in solutions, where phi = 0.03 independent of lambda in the same wavelength range. Photoinduced bending of needle crystals caused by photochemical transformation can be used as an extremely sensitive method for studying the kinetics of the transformation.