A solution of spiroindolinonaphthoxazine in a common organic solvent shows profound photochromic effect. The colorless form in ethanol displays three absorption bands (~320, 235, 203 nm) in the ultraviolet region of the spectrum. The absorption spectrum of the colored metastable form shows two prominent absorption peaks at 612 and 578 nm. The triplet state energy level of the colorless form is estimated to be in the range from 2.4 × 104 to 2.1 × 104 cm−1 by the triplet–triplet energy transfer method. There exists a thermal equilibrium between the colorless and colored forms. From the measurement of absorbance of the colored form at different temperatures, the standard enthalpy of equilibrium is obtained (ΔH0 = 1500 cm−1). The thermal decay of the colored form to the colorless form depends strongly on temperature. The decay rate increases three times for every 10 °C increase in temperature. From these temperature dependence studies of the thermal decay rate, the thermal energy barrier for the colored metastable state to decay to the colorless form is determined to be 6.9 × 103 cm−1.
Two ways for finding the thermal decay rate at weak friction
