A Photochromic Benzothiadiazole-Diarylethene System with Tunable On/Off Fluorescence Modulation

Organic molecules with photoswitchable emission properties are of interest in the optical devices field. We present herein in-solution studies for two dimethoxydithienylethene-fluorophore (DTE-fluorophore) conjugates with a carboxylic acid anchor group attachable to semiconductor surfaces (DTE-BODIPY and BTD-DTE, where BTD=benzothiadiazole). Both conjugates show almost quantitative reversible photoisomerization between open (OF) and closed forms (CF). With excitation of the fluorophores, both conjugates emit fluorescence with highly solvent-dependent OFs (BTD-DTE>DTE-BODIPY) and minor solvent-dependent CFs. In the polar solvent MeCN for BTD-DTE a TICT state is clearly observed, which will be located in the twisted conjugated structure of the benzoic acid-BTD-fluorophore-methoxy-thiophene unit. BTD-DTE shows positive solvatofluorochromism with a decrease of fluorescence quantum yields by increasing the Lippert orientation polarizability function (Delta f) verified in Lippert-Mataga plots. This leads to solvent dependent on-off/off-on (CF-OF) emission modulation by Forster resonance energy transfer (FRET) with well-separated emission bands for BTD-DTE.