Tuning the excited-state intramolecular proton transfer (ESIPT) process of indole–pyrrole systems by π-conjugation and substitution effects: experimental and computational studies

A series of amino (NH)-type hydrogen-bonding (H-bonding) compounds, BNDAB-1–4, containing π-enlarged indole and β-ethoxycarbonyl-substituted pyrrole units were designed and synthesized. BNDAB-1 and BNDAB-3 exhibited dual emission and BNDAB-2 and BNDAB-4 exhibited a single emission with a large Stokes shift in dichloromethane, methanol, DMSO and toluene except for a dual emission for BNDAB-4 in toluene. Inspired by their photophysical properties, the ESIPT process was speculated and further investigated by theoretical calculations including geometry and thermodynamic analyses. The results showed that the ester substitution on the proton donor unit and π-conjugation on the proton acceptor unit by structural modification can regulate the ESIPT behaviors of these compounds. First, a strong electron-withdrawing group promoted the ESIPT process according to the comparison of the ESIPT processes of NDAB-H and NDAB-6, BNDAB-1 and BNDAB-2, and BNDAB-3 and BNDAB-4. Second, π-conjugation in different positions ([g]- and [e]-position) of the indole unit decreased the speed of the ESIPT process irrespective of whether ethoxycarbonyl was substituted on the pyrrole ring based on the ESIPT process of Series 1 and 2. Finally, this work elucidated that the ESIPT process can be rationally tuned by π-conjugation and substitution, which is in good agreement with the experimental results.