Sumwzary. The kinetics of the formation of the photochromic forms B and C of tetramethyltlianthrone has been studied in triacetin and methylcyclohexane/Z-methylpentane at several temperatures. A flash spectroscopy method was developed, which allows the recording of the transicnt behaviour of the sample as function of both -wavelength and time -using a single photoflash for excitation. The results show that the photochromic forms B and C are directly populated from the excited singlet state. I n addition a new triplet-triplet absorption band in the near infrared region was detected.Introduction. -During the last two decades the reversible photochemical behaviour of the dianthrone (I) system has been the subject of many investigations. Even though many questions have been answered in the course of time there still remain unresolved aspects. Fischer gave an excellent survey with a large number of references at the 1970 IUPAC Photochemistry Symposium [l]. Our group applied the methods of flash spectroscopy t o the tetramethyldianthrone TMD (11) systems and in an earlier paper [Z] we reported

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