White Light Induced E/Z-Photoisomerization of Diphenylamine-Tethered Fluorescent Stilbene Derivatives: Synthesis, Photophysical, and Electrochemical Investigation.

A facile synthesis and detailed photophysical investigation of E/Z-isomerization of fluorescent diphenylamine tethered stilbene derivatives (DPASs) under white light exposure have been carried out to understand the effect on fluorescence, electrochemical properties, and photostability under various activation/deactivation pathways. In solution state, in the dark, the E-isomer of DPASs (6a-d) exhibited high fluorescence quantum yields (Φfl ≈ 53% to 60% in DMSO). However, on white light exposure, 1 H NMR and HPLC studies revealed that pure E-isomer of the DPAS 6a (∼9.5 mM) started converting into its Z-form by photoisomerization until it reaches to nearly equilibrium. At low concentrations (∼10 μM), the absorption band of the pure E-isomer in the range of 350-450 nm gradually decreased to adopt Z-conformation 6a' until a photostationary state was reached. The structure of the E-isomer 6a was unequivocally confirmed by X-ray diffraction analysis. The synthesized DPAS compounds 6a-d possessed positive solvatochromic properties, two photon absorption properties, and good thermal stability. The electrochemical investigations using DPASs showed reversible oxidation resulting in formation of a stable radical cation. Owing to useful photophysical, electrochemical and thermal properties, these DPAS derivatives are suitable for their application in biomedical imaging as well as in fabrication of electroluminescent materials.