Quantification of the relative composition of the isomers in a photochromic system at any irradiation time interval is a critical issue in determining absolute quantum yields. For this purpose, we have developed a simple and convenient protocol involving combination of UV-visible and infra-red absorption spectroscopy. Photochromic cyclization reaction of cis-l,2-dicyano-l,2-bis(2,4,5-trimethyl-3-thieny1)ethene (CMTE) is analyzed to demonstrate the efficiency of the proposed methodology. This approach is based on the fact that the two isomers show distinctive infra-red bands. Detailed investigations of the UV-visible and infra-red spectra of the mixture obtained at different irradiation times in CCl4 supported by quantum chemical computations lead to the unambiguous estimation of molar absorption coefficients of the closed isomer (εCF = 4650 L mol−1 cm−1 at 512 nm). It facilitates the first determination of absolute quantum yields of this reversible photochromic reaction in CCl4 by fitting the UV-visible spectral data (ΦOF→CF = 0.41 ± 0.05 and ΦCF→OF = 0.12 ± 0.02 at 405 nm and 546 nm, respectively).
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