Photophysical Properties of 4‐Dicyanomethylene‐2‐methyl‐6‐( p ‐dimethylamino‐styryl)‐4 H ‐pyran Revisited: Fluorescence versus Photoisomerization

Although 4‐dicyanomethylene‐2‐methyl‐6‐( p ‐dimethylamino‐styryl)‐4 H ‐pyran (DCM) has been known for many decades as a bright and photostable fluorophore, used for a wide variety of applications in chemistry, biology and physics, only little attention has been paid so far to the presence of multiple isomers and conformers, namely s‐trans ‐( E ), s‐cis ‐( E ), s‐trans ‐( Z ), and s‐cis ‐( Z ). In particular, light‐induced E – Z isomerization plays a great role on the overall photophysical properties of DCM. Herein, we give a full description of a photoswitchable DCM derivative by a combination of structural, theoretical and spectroscopic methods. The main s‐trans ‐( E ) isomer is responsible for most of the fluorescence features, whereas the s‐cis ‐( E ) conformer only contributes marginally. The non‐emitting Z isomers are generated in large conversion yields upon illumination with visible light (e.g., 485 or 514 nm) and converted back to the E forms by UV irradiation (e.g., 365 nm). Such photoswitching is efficient and reversible, with high fatigue resistance. The E → Z and Z → E photoisomerization quantum yields were determined in different solvents and at different irradiation wavelengths. Interestingly, the fluorescence and photoisomerization properties are strongly influenced by the solvent polarity: the fluorescence is predominant at higher polarity, whereas photoisomerization becomes more efficient at lower polarity. Intermediate medium (THF) represents an optimized situation with a good balance between these two features.