Fine Tuning the Energetics of Excited-State Intramolecular Proton Transfer (ESIPT): White Light Generation in A Single ESIPT System

Using 7-hydroxy-1-indanone as a prototype (I), which exhibits excited-state intramolecular proton transfer (ESIPT), chemical modification has been performed at C(2)–C(3) positions by fusing benzene (molecule II) and naphthalene rings, (molecule III). I undergoes an ultrafast rate of ESIPT, resulting in a unique tautomer emission (λmax ∼530 nm), whereas excited-state equilibrium is established for both II and III, as supported by the dual emission and the associated relaxation dynamics. The forward ESIPT (normal to proton-transfer tautomer species) rates for II and III are deduced to be (30 ps)−1 and (22 ps)−1, respectively, while the backward ESIPT rates are (11 ps)−1 and (48 ps)−1. The ESIPT equilibrium constants are thus calculated to be 0.37 and 2.2 for II and III, respectively, giving a corresponding free energy change of 0.59 and −0.47 kcal/mol between normal and tautomer species. For III, normal and tautomer emissions in solid are maximized at 435 and 580 nm, respectively, achieving a white light generation with Commission Internationale de l’Eclairage (CIE) (0.30, 0.27). An organic light-emitting diode based on III is also successfully fabricated with maximum brightness of 665 cd m–2 at 20 V (885 mA cm–2) and the CIE coordinates of (0.26, 0.35). The results provide the proof of concept that the white light generation can be achieved in a single ESIPT system.