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.
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 SH•••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.
Finding arelationship between kinetics and thermodynamics maybedifficult. However,semi-empirical rules exist to compensate for this shortcoming,a mong which the Bell-Evans-Polanyi (B-E-P)p rinciple is an example for reactions involving bond breakage and reformation. We expand the BE -P principle to an ew territory by probing photoinduced structure planarization (PISP) of as eries of dibenz-[b,f]azepine derivatives incorporating bent-to-planar and rotation motion. The latter involves twisting of the partial double bond character,t hereby inducing ab arrier that is substituent dependent at the para N-phenyl position. The transition-state structure and frequency data satisfy and broaden the BE -P principle to PISP reactions without bond rearrangement. Together with dual emissions during PISP,this makes possible harnessing of the kinetics/thermodynamics relationship and hence ratiometric luminescence properties for excited-state structural transformations.
A series of compounds containing 5-(2-aminobenzylidene)-2,3-dimethyl-3,5-dihydro-4H-imidazol-4-one (o-ABDI) as the core chromophore with a seven-membered-ring N-H-type intramolecular hydrogen bond have been synthesized and characterized. The acidity of the N-H proton and thus the hydrogen-bond strength can be fine-tuned by replacing one of the amino hydrogen atoms by a substituent R, the acidity increasing with increasing electron-withdrawing strength of R, that is, in the order H
3‐Hydroxythioflavone (3‐HTF) was synthesized upon oxidation with selenium dioxide, in contrast to 3‐hydroxyflavone, a prototype molecule undergoing excited‐state intramolecular proton transfer (ESIPT) reaction. Also achieved were the syntheses of the 3‐HTF sulfone analogue and an interesting sulfonyl selane intermediate. Their structures were fully characterized. 3‐HTF showed an ultrafast ESIPT property, as evidenced by >150 fs−1 reaction rate constant and it solely exhibited a proton‐transfer tautomer emission (590 nm) in cyclohexane at room temperature and in a 77 K solid matrix. The lower electronegativity of the sulfur atom in 3‐HTF leads to a significant redshift of the tautomer emission compared with that of 3‐hydroxyflavone. The sulfone analogue showed a lack of ESIPT, mainly owing to the decrease in carbonyl basicity.
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