Development of molecular photoswitch with very fast photoresponse based on asymmetrical bis-azospiropyran

• Synthesis of novel asymmetrical bifunctional bis-azo spiropyran photochromic dye is reported.
• Mc form with largest ε and Sp with quickest response to light, highest φf were achieved and showed a superior FSP/FMC ratio.
• We have novel photoswitch with improved light sensitivity and superior discrimination power between OFF and ON forms.

To study the effects of an extended bis-azo conjugated bridge with two different photochemical functions on a molecule in photochromic responses, a novel asymmetrical bifunctional bis-azo spiropyran photochromic dye was designed and synthesized. The obtained photoresponses were compared with symmetrical bifunctional bis-azo spiropyran analogues, and relative mono-azo and simple spiropyrans. Colourimetric behaviour, luminescence, and switching kinetics of all the dyes were studied. The largest molar absorption coefficient in merocyanine form, quickest response to light, and highest fluorescence quantum yield of the spiropyran form with a superior ratio of emission intensities of spiropyran to merocyanine form were achieved for the asymmetric bis-azospiropyran. Solvatochromic effect was studied to observe the solvent effects on non-irradiation colouration of the photochromic dyes. Furthermore, The molecular energy levels for optimized geometries of the synthesized bis-azospiropyrans and their probable photochemical products were obtained at the B3LYP/6-31G(d) level of theory.

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