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

Abstract

Although 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran (DCM)h as been knownf or many decades as ab right and photostable fluorophore, used for aw ide varietyo fa pplications in chemistry,b iology and physics, only little attention has been paid so far to the presence of multiple isomersa nd conformers, namely strans-(E), s-cis-(E), s-trans-(Z), and s-cis-(Z). In particular, lightinduced E-Z isomerization playsagreat role on the overall photophysical properties of DCM. Herein, we give af ull description of ap hotoswitchableD CM derivativeb yacombination of structural, theoretical and spectroscopic methods. The main s-trans-(E)i somer is responsible for most of the fluorescence features, whereas the s-cis-(E)c onformer only contributes marginally.T he non-emitting Z isomers are generated in large conversion yields upon illumination with visible light (e.g.,4 85 or 514 nm) and converted back to the E forms by UV irradiation (e.g.,3 65 nm). Such photoswitching is efficient andr eversible, with high fatigue resistance. The E!Z and Z!E photoisomerization quantum yields were determined in differents olvents and at different irradiation wavelengths. Interestingly,t he fluorescencea nd photoisomerization properties are strongly influenced by the solvent polarity:t he fluorescencei sp redominanta th igherp olarity, whereas photoisomerization becomesm ore efficient at lower polarity.I ntermediate medium (THF) represents an optimizeds ituation with ag ood balance between these two features.

Photoisomerization of a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran analog dye: a combined photophysical and theoretical investigation

“…The results from Table 1 fully confirm the interpretation reported in our previous paper. 14 From fluorescence decays collected at different excitation wavelengths, with or without irradiation, we can conclude that the Z-forms of DCM-azide are completely non-emissive, and the fluorescence components (t 1 and t 2 ) identified by fluorescence decays correspond to s-trans-(E) and s-cis-(E) conformers.…”

“…However, the photoisomerization properties of DCM have been ignored in the vast majority of reports, with a few notable exceptions. 14,[23][24][25][26][27][28][29] The E 2 Z photoisomerization studies and the related conversion yield quantification of a DCM analog (DCM-o-alkyne) have been reported by our group. 14 In our previous article, we described steady-state spectroscopy, HPLC analysis, conversion yields, photostationary states (PSSs), and characterization of the E-form and Z-form, under illumination, at different wavelengths and in different solvents.…”

“…The fluorescence spectrum of DCM-azide in the E-form is broad and intense around 590 nm, and its emission quantum yield is appreciable (F f = 0.25) but lower than that of the DCMo-alkyne (F f = 0.32), 14 due to its azido lateral part. Fluorescence decay curves of DCM-azide were recorded at different excitation wavelengths using the time-correlated single-photon counting (TCSPC) technique (Fig.…”

“…These geometrical parameters are fully in line with the previously described DCM-o-alkyne derivative. 14 In a previous study on DCM-o-alkyne, the THF solvent (medium polarity) was revealed to be an ideal trade-off between the fluorescence quantum yield and the E -Z photoisomerization conversion yield, outperforming acetonitrile and toluene in terms of fluorescence photoswitching benchmark. Therefore, we decided to employ THF as the main solvent in the present study of DCM-azide.…”

“…Previous studies on DCM, using spectroscopic or quantum chemical calculations, indicated the existence of 4 stable isomers (E-and Z-forms) and conformers (s-cis-and s-trans-forms) and their possible interconversions. 14,29 In the present paper, we focus our attention on the DCMazide, 30 an azido variant of the previously studied DCM-oalkyne, allowing further functionalization. The molecular structures of the four isomers/conformers of DCM-azide are shown in Fig.…”