NOR1 Y. C. CHU. Can. J. Chem. 61, 300 (1983). 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 X lo4 to 2.1 x 10' cm-' by the triplet-triplet energytransfer 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 (AH" = 1500 cm-I). 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 x 10' cm-I.NORI Y. C. CHU. Can. J. Chem. 61, 300 (1983). La spiroindolinonaphthoxazine en solution dans un solvant organique ordinaire montre un effet photochimique marqui. La forme incolore dans l'ithanol prtsente trois bandes d'absorption (-320, 235, 203 nm) dans l'ultraviolet. Le spectre d'absorption de la forme metastable colorie montre deux pics de forte intensiti a 612 et 578 nm. On Cvalue que le niveau d'tnergie de l'itat triplet de la forme incolore est de l'ordre de 2,4 X 10' a 2,l X lo4 cm-' selon la mtthode de transfert d'tnergie triplet-triplet. I1 existe un Cquilibre thermique entre les formes incolore et colorCe. A partir des mesures d'absorptirn de la forme colorCe a diffirentes tempiratures, on obtient I'enthalpie standard d'iquilibre (AH0 = 1500 cm-I). I I disintegration thermique de la forme colorie en la forme incolore dipend fortement de la tempirature. La vitesse de disintegration augmente par un facteur de 3 pour chaque augmentation de tempirature de 10°C. A partir des etudes de l'influence de la temptrature sur la vitesse de disintegration thermique, on a ditermini la barriere d'inergie thermique de la disintegration de la forme mitastable colorie en la forme incolore. Cette barrikre est de 6,9 X lo3 cm-I.[Traduit par le journal]

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In this paper we have examined the fluorescence emission spectra, intensity, and decay characteristics and the excitation spectra of pure pyrene crystals and of perylene-doped pyrene crystals at temperatures down to 1.7°K. From these observations we conclude that energy transfer in pyrene crystals occurs by four different mechanisms with the following characteristics: (i) Reabsorption of emitted photons: important at high guest concentrations or with thick samples. (ii) Monomer exciton migration: Operates at all temperatures with a transfer rate constant on the order of ∼1012 sec−1. (iii) Excimer exciton migration: important only at temperatures above about 130°K, thermally activated with an apparent activation energy of 112 cm−1 and a transfer rate constant of 4×106 sec−1 at room temperature. (iv) Forster transfer from excimers to perylene guest molecules: important only with perylene concentrations greater than about 10−3 mole/mole, or at slightly lower concentrations at reduced temperatures. In addition to obtaining information about the nature of energy-transfer processes which take place in pyrene crystals, we have also examined the crystal absorption spectrum on the long-wavelength side of the first major absorption band. The low-temperature excimer excitation spectra of highly purified pyrene crystals exhibits a tail which extends almost to 425 nm and appears to overlap with the origin of the excimer emission. This tail absorption is attributed to direct excitation to the excimer state.