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.本研究以具有激发态分子内质子转移（excited-state intramolecular proton transfer, ESIPT）特性的7-羟基-1-茚酮（原型分子I）为对象，通过在其C(2)-C(3)位点分别稠合苯环与萘环，得到分子II与分子III并对其开展化学修饰。分子I展现出超快的ESIPT速率，进而产生独特的互变异构体发射（最大波长约530 nm）；而分子II与III均建立了激发态平衡，该结论得到双发射行为与相关弛豫动力学的佐证。经推导，分子II与III的正向ESIPT（从常态物种到质子转移互变异构体物种）速率分别为(30 ps)⁻¹与(22 ps)⁻¹，反向ESIPT速率则分别为(11 ps)⁻¹与(48 ps)⁻¹。据此计算得到分子II与III的ESIPT平衡常数分别为0.37与2.2，对应常态与互变异构体物种间的自由能变化分别为0.59 kcal/mol与-0.47 kcal/mol。对于分子III，其固态下的常态发射与互变异构体发射峰值分别位于435 nm与580 nm，可实现国际照明委员会（Commission Internationale de l’Eclairage, CIE）坐标为(0.30, 0.27)的白光发射。基于分子III的有机发光二极管也已成功制备，在20 V（电流密度885 mA·cm⁻²）下实现了665 cd·m⁻²的最大亮度，CIE坐标为(0.26, 0.35)。本研究结果证实了在单一ESIPT体系中可实现白光发射的概念可行性。