Theoretical Insight Into The Excited-state Intramolecular Proton Transfer Of Small Organic Molecules

Excited state proton transfer?ESPT?is one of the most basic reactions in photochemical and photobiological processes.Therein,an excited state intramolecular proton transfer?ESIPT?molecule commonly incorporates a proton transfer from a proton-donating group to a proton-accepting group in the potential energy surface of the excited state,through forming a five-,six-or even seven-membered ring with an intramolecular hydrogen-bond?H-bond?.Proton donors are usually hydroxyl and amino,and proton acceptors are usually carbonyl and nitrogen atoms.Since the protonic acidity of the amino group is much weaker than that of the hydroxyl group,ESIPT rarely takes place in N-H H-bonding systems unless the acidity of the N-H proton can be enhanced via suitable chemical modification.However,compared with O-H,it is possible to modulate ESIPT reaction by substituting a proton in NH₂ with a group of different properties.Therefore,amino-type ESIPT molecules have very important potential applications.In this paper,density functional theory?DFT?and time-dependent density functional theory?TD-DFT?were used to study the mechanism of intramolecular proton transfer in the ground and excited states of two kinds of amino hydrogen bonding systems and one kind of double proton transfer system.The main results are as follows:?1?ESIPT dynamics of the amino-type hydrogen-bonding compound 2-?2?-aminophenyl?benzothiazole?PBT-NH₂?as well as its two derivatives 2-?5?-cyano-2?-aminophenyl?benzothiazole?CN-PBT-NH₂?and2-?5?-cyano-2?-tosylaminophenyl?benzothiazole?CN-PBTNHTs?were studied by the TD-DFT approach,and their absorption and emission spectra were also explored at the same level of theory.A good agreement is observed between the theoretical simulations and experimental spectra,indicating that the present calculations are reasonably reliable.In addition,it is also found that the energy barriers of the first excited singlet state of the three targeted molecules along the ESIPT reaction are computed to be 0.38,0.34 and0.12 eV,respectively,showing the trend of gradual decrease.This implies that the introduction of the electron-withdrawing cyano or tosyl group can facilitate the occurrence of the ESIPT reaction of these amino-type H-bonding systems.Following the ESIPT,both CN-PBT-NH₂ and CN-PBT-NHTs molecules can undergo the cis-trans isomerization reactions in the ground-state and excited-state potential energy curves along the C2-C3 bond between benzothiazole and phenyl moieties,where the energy barriers of the trans-tautomer?cis-tautomer isomerizations in the ground states are calculated to be 0.83 and 0.34 eV,respectively.According to our calculations,it is plausible that there may exist the long-lived trans-tautomer species in the ground states of CN-PBT-NH₂ and CN-PBT-NHTs.It is expected that these computational results can provide a theoretical guide for designing the new amino-type ESIPT molecules to achieve the color tunability in lighting applications.?2?The geometries of a series of amide-based N-H···N hydrogen-bonding compounds in their ground S₀ states and first excited singlet S₁ states were optimized with DFT and TD-DFT approaches,respectively.The results show that the normal form is more stable in the S₀ state while the tautomer form is more stable in the S₁ state.Both topological analysis and noncovalent interactions analysis show strong intramolecular hydrogen-bonds in the studied five systems and the electron density function??r?exhibits good linear relationship with the distance of H???N2.The potential energy curves of the S₀ and S₁ states were scanned by using of DFT and TD-DFT to elucidate the ESIPT process.It reveals that all the systems considered here can undergo an ultrafast ESIPT reaction,with energy barrier of less than 0.05 eV,giving rise to the single fluorescence emission from the proton-transfer tautomer.It is also found that the shorter the hydrogen bond in the normal-form S₁ state,the easier the ESIPT takes place.Furthermore,our new designed compound is expected to be a good ESIPT molecule,with nearly barrierless?0.01 eV?in the first excited state along the ESIPT potential energy surface.?3?The single and double proton transfer dynamic process in the excited state of2,5-bis?2-benzothiazolyl?hydroquinone?BBTHQ?were theoretically investigated based on a TDDFT method.The calculations of primary bond lengths,angles,infrared vibrational spectra and non-covalent interaction analysis prove that the intramolecular hydrogen bonds of BBTHQ?O1…H1-N1 and O2…H2-N2?are strengthened in the first excited state,which provides a driving force for the ESIPT process.The potential energy surfaces?PESs?of the S₀ and S₁ states were constructed to further elucidate the mechanism of intramolecular proton transfer of BBTHQ.In addition,our prediction of the fluorescence peak at 1020 nm for BBTHQ-DT was not found in previous experiment,which needs to be further verified by the experimental work.