Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution

Abstract

We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (3TF), namely, 2-(4-(diethylamino)phenyl)-3mercapto-4H-chromen-4-one (3NTF), which possesses an SH•••O intramolecular H-bond (denoted by the dashed line) and has an S 1 absorption at 383 nm. Upon photoexcitation, 3NTF exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm −1 . Upon methylation on the thiol group, 3MeNTF, lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for 3NTF, followed by a tautomer emission lifetime of 120 ps. In sharp contrast to 3NTF, both 3TF and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4Hchromen-4-one (3FTF) are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings nonnegligible nπ* contribution to the S 1 ′ state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For 3NTF, the extension of π-electron delocalization by the diethylamino electron-donating group endows the S 1 ′ state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.

Recent Progress of Novel Organic Near‐Infrared‐Emitting Materials

“…f 2 ) The absorption (circle and line) and emission (solid line) spectra of 3NTF (black) and 3MeNTF (orange) in cyclohexane at 298 K. Reproduced with permission. [ 69 ] Copyright 2021, American Chemical Society.…”

“…Similarly, the ESIP molecules show NIR emission, such as 4‐dimethylamino‐2′‐hydroxychalcone with deep‐red and NIR emission at 600–750 nm, [ 67 ] ( E )‐3‐(4‐(dimethylamino)‐2‐methoxyphenyl)‐1‐(1‐hydroxynaphthalen‐2‐yl)prop‐2‐en‐1‐one (DMHP) with NIR emission at 720 nm, [ 68 ] 2‐(4‐(diethylamino)phenyl)‐3‐mercapto‐4H‐chromen‐4‐one with the NIR emission at 710 nm in cyclohexane. [ 69 ] The cascaded occurrence of the ESIPT process is regarded as a promising approach to obtain a more narrow bandgap for NIR emission. Inspired by this molecular design, our group further finely prepared the cascaded ESIPT‐active molecule of ( E )‐1‐(1,8‐dihydroxynaphthalen‐2‐yl)‐3‐(4‐(dimethylamino) −2‐methoxyphenyl)prop‐2‐en‐1‐one (DDMP) [9a] .…”

“…In additional, the proton transfers based on the intramolecular hydrogen bonding between sulfhydryl groups and carbonyl are also favorable for the ESIPT process based on 2‐(4‐(diethylamino)phenyl)‐3‐mercapto‐4H‐chromen‐4‐one (3NTF). [ 69 ] Upon photoexcitation, 3NTF exhibits charming NIR emission with a maximized PL peak at 710 nm and an anomalously large Stokes shift of 12 230 cm −1 in cyclohexane (Figure 6f).…”

Organic Near‐Infrared Luminescent Materials Based on Excited State Intramolecular Proton Transfer Process†

“…Very recently, a thiol group participated ESIPT process was demonstrated. [ 48 ] So, the engineering of hydrogen bond donors and acceptors is one of the most important strategies in the design of ESIPT‐based NIR luminescent materials. The representative progress will be discussed in detail according to the type of hydrogen bond donor groups.…”

Photoinduced and ground state conversions in a cyclic β-thioxoketone

“…β-Thioxoketones have good chelating properties that are widely used in the organic synthesis 1–3 and in the development of new drugs. 4 They exhibit photochromic properties and are good models for investigation of photochemical phenomena such as excited-state intramolecular proton transfer (ESIPT), both experimentally 5 and theoretically. 6,7 The photochemistry revealed in the present paper leads to new unusual compounds and may shed more light on the photoreactions in β-thioxoketones, not least on the influence of the environment on these particular photoprocesses.…”