Probing photochromic properties by correlation of UV-visible and infra-red absorption spectroscopy: a case study with cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene

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 CCl 4 supported by quantum chemical computations lead to the unambiguous estimation of molar absorption coefficients of the closed isomer ( ε CF = 4650 L mol −1 cm −1 at 512 nm). It facilitates the first determination of absolute quantum yields of this reversible photochromic reaction in CCl 4 by fitting the UV-visible spectral data ( Φ OF→CF = 0.41 ± 0.05 and Φ CF→OF = 0.12 ± 0.02 at 405 nm and 546 nm, respectively).