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Additionally, the longer O(1)-O(2) distance in 1 than that (O(1)-O(2) < 2.7 [Angstrom]) in most other ESIPT molecules [36,37] supports that compound 1 has a weaker intramolecular hydrogen bond.
Accordingly, the assignment of a 574 nm emission for 1 in cyclohexane to a proton-transfer tautomer emission is unambiguous, and ESIPT takes place from the phenolic proton (O(1)-H) to the O(2) oxygen, forming the keto-tautomer species depicted in Figure 4.
Therefore, there is no doubt that the decrease of intramolecular hydrogen bond lengths from E ([K.sup.*]) to [E.sup.*] (K) is a very important positive factor for the ESIPT (GSIPT: ground state intramolecular proton transfer) reaction.
The results clearly demonstrate that, upon electronic excitation of 1, the hydroxyl proton (O(1)-H) is expected to be more acidic, whereas the carbonyl oxygen O(2) is more basic with respect to their ground state, driving the proton transfer reaction (forward ESIPT).
On the other hand (for the first singlet excited state), one can clearly see that the potential energy barriers of the forward (8.0 kcal/mol) and the backward (2.7 kcal/mol) ESIPT are in the same order of magnitude, which is in good agreement with previous theoretical results of 2 .
Analysis of the geometric structures clearly indicates that the intramolecular hydrogen bond length is shortened upon the photoexcitation, which is regarded as a very important factor for ESIPT. Furthermore, the potential energy curves suggest that the forward ESIPT and backward ESIPT may happen on the same timescale and leads to the rapidly established excited-state equilibrium.
Yang, "Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials," Physical Chemistry Chemical Physics, vol.
Stefani, "Novel ESIPT fluorescent benzazolyl-4-quinolones: synthesis, spectroscopic characterization and photophysical properties," Dyes and Pigments, vol.
Park, "Advanced organic optoelectronic materials: harnessing excited-state intramolecular proton transfer (ESIPT) process," Advanced Materials, vol.
Lin et al., "Fine tuning the energetics of excited-state intramolecular proton transfer (ESIPT): white light generation in a single ESIPT system," Journal of the American Chemical Society, vol.
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