How To Reach Intense Luminescence for Compounds Capable of Excited‐State Intramolecular Proton Transfer?

Abstract

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine and imidazo[1,2-f]phenanthridine skeletons, which mediate excited-state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five-membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady-state and time-resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited-state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited-state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT-capable compounds originates from their enol excited states.

Electronically‐tuned 2‐(2′‐Hydroxyphenyl)‐4‐pyrenylthiazole through Bond Energy Transfer Donor–Acceptor Couples: Sensing and Biological Applications

“…Accordingly, such emission bands showed vibronic bands in nonpolar media, as visualized in the solvent‐dependent emission spectra (Table ). However, there were extensive variations in the quantum yields between polar protic (almost 0) and polar aprotic solvents, indicating the presence of factors besides the radiative enol emission for the solution of 1 in DMSO.…”

Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

“…[46][47] Photoexcitation that leads to opposite shifts in the pK a characteristics of two neighboring protonatable sites can lead to excited state proton transfer (ESPT). [48][49] ET occurs between an electron donor (D) and an electron acceptor (A). Transferring n electrons from a donor to an acceptor causes a positive shift in the charge of the donor and a negative shift in the charge of the acceptor:…”

Bioinspired approach toward molecular electrets: synthetic proteome for materials

“…Time-correlated single photon counting (TCSPC) measurements were conducted with a FluoroLog-3 spectrofluorometer equipped with a UV LED source (λ ex = 276 nm; FWHM = 900 ps), and a TBX detector run in single-photon-counting mode. Placing selected reflection neutral-density filters in front of the LED controls the intensity of the excitation light [181][182][183]. All samples for steady-state and time-resolved emission measurements were purged with argon for 5-10 min per 1 mL of sample.…”