Influence of the cavity size of cyclodextrins on the photochromism of azoimidazoles

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1-Alkyl-2-(arylazo)imidazole (RaaiR/) exists in trans configuration about the –NN- bond. Upon optical excitation in UV region the trans-configuration isomerises to cis-RaaiR/. The photochromism is very susceptible to internal substituents and external environment like solvent polarity, viscosity and presence of innocent foreign species. In this work, the trans-to-cis photoisomerisation of RaaiR/has been studied in DMF solution of cyclodextrin (α/β/γ-CD). The rate of trans-to-cis isomerisation is decreased in presence of CD compared to native form of RaaiR/. The quantum yield of the photoisomerisation is decreased by 35–55% in inclusion phase, CD@RaaiR/, than free photochrome and follows the rate sequence: free state > γ-cyclodextrin > β-cyclodextrin > α-cyclodextrin. The cis-to-trans isomerisation proceeds slowly in visible light irradiation while it is appreciably fast with increasing temperature. The activation energy (Ea) of cis → trans thermal isomerisation is also diminished compared to free state of photochrome. The absorption spectral studies have been used in case of Pai-C18H37 with β-CD and inclusion constant is Kb ​= ​0. 121 M−1. The 1H NMR spectral measurement also suggests interaction of aryl protons with cavity protons of β-CD. DFT computation is also attempted to examine the inclusion of RaaiR/with CD and the negative values of binding energy show that the process of inclusion is spontaneous and complexes formed are stable.