Control of the Reversibility of Excited-State Intramolecular Proton Transfer (ESIPT) Reaction: Host-Polarity Tuning White Organic Light Emitting Diode on a New Thiazolo[5,4-d]thiazole ESIPT System

By using the thiazolo[5,4-d]thiazole (TzTz) moiety as the core of a proton acceptor, compounds 2,2′-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(4-tert-butylphenol) (t-HTTH) and 4-tert-butyl-2-(5-(5-tert-butyl-2-methoxyphenyl)thiazolo[5,4-d]thiazol-2-yl)phenol (t-MTTH) have been strategically designed and synthesized. Upon photoexcitation, both t-HTTH and t-MTTH undergo a reversible type excited-state intramolecular proton transfer (ESIPT), the underlying mechanism of which has been verified by femtosecond early relaxation dynamics in various solvents. The pre-equilibrium in the excited state leads to both normal (∼440 nm) and proton-transfer tautomer (∼560 nm) emissions, for which the intensity ratio is dependent on both the molecular structure and the polarity of surrounding media. As a result, the emission can be widely tuned from blue to yellow via white-light luminescence. On the basis of t-MTTH, a white organic light emitting diode (WOLED) was successfully fabricated, which achieved external quantum efficiency (ηext) of 1.70% with Commission Internationale de L'Eclairage coordinates of (0.29, 0.33). More importantly, the electroluminescent spectra show superior color stability that is independent of luminance. The result demonstrates for the first time a credible WOLED based on a unimolecular ESIPT reaction, which may have far-reaching implications for practical application.