Excited-State Proton Transfer via Hydrogen-Bonded Acetic Acid (AcOH) Wire for 6-Hydroxyquinoline

Abstract

Spectroscopic studies on excited-state proton transfer (ESPT) of hydroxyquinoline (6HQ) have been performed in a previous paper. And a hydrogen-bonded network formed between 6HQ and acetic acid (AcOH) in nonpolar solvents has been characterized. In this work, a time-dependent density functional theory (TDDFT) method at the def-TZVP/B3LYP level was employed to investigate the excited-state proton transfer via hydrogenbonded AcOH wire for 6HQ. A hydrogen-bonded wire containing three AcOH molecules at least for connecting the phenolic and quinolinic -N-group in 6HQ has been confirmed. The excited-state proton transfer via a hydrogen-bonded wire could result in a keto tautomer of 6HQ and lead to a large Stokes shift in the emission spectra. According to the results of calculated potential energy (PE) curves along different coordinates, a stepwise excited-state proton transfer has been proposed with two steps: first, an anionic hydrogen-bonded wire is generated by the protonation of -N-group in 6HQ upon excitation to the S 1 state, which increases the proton-capture ability of the AcOH wire; then, the proton of the phenolic group transfers via the anionic hydrogen-bonded wire, by an overall "concerted" process. Additionally, the formation of the anionic hydrogenbonded wire as a preliminary step has been confirmed by the hydrogen-bonded parameters analysis of the ESPT process of 6HQ in several protic solvents. Therefore, the formation of anionic hydrogen-bonded wire due to the protonation of the -N-group is essential to strengthen the hydrogen bonding acceptance ability and capture the phenolic proton in the 6HQ chromophore.

Excited-state hydrogen bonding effect on dynamic fluorescence of coumarin 102 chromophore in solution: A time-resolved fluorescence and theoretical study

“…We simulated the inner solvation shell by hydrogen-bonded complexes with C102 and methanol molecule. This model has been widely used in our previous works [24][25][26][27][28][29][30][31][32][33][34][35][36], and obtained good agreements with experiments results including C102 in phenol [33]. The bulk effect of the outer solvation shells was considered using the COSMO package (with e¼32.7).…”

“…Therefore, it is significant to investigate the hydrogenbonded thermodynamic equilibrium in electronic excited states, since the equilibrium shift caused by excited-state hydrogen bond plays an important role in photochemical behavior [44,46]. According to the excited-state hydrogen bond strengthening we demonstrated in previous works [24][25][26][27][28][29][30][31][32][33][34][35][36] that the free energy of the system should be changed following hydrogen bond strengthening, which would lead to the breaking of ground-state equilibrium distribution after an excitation to the excited state. Therefore, the pK a values of hydrogen-bonded thermodynamics in different electronic states are important constants.…”

“…However, little is known about electronic excited-state hydrogen bond. In our previous works [24][25][26][27][28][29][30][31][32][33][34][35][36], we have successfully simulated the hydrogen-bonded dynamics and theoretically demonstrated that intermolecular hydrogen bond between solute and alcoholic molecules can be significantly strengthened in the electronic excited state upon photoexcitation [33,34], which plays an important role in photochemistry, such as intermolecular charge transfer [33], fluorescence quenching [34], excited-state proton transfer [35] and tuning effects on photochemistry [36].…”

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

“…Recently, Zhao and Han [12][13][14][15][16][17][18][19] have determined theoretically that intermolecular hydrogen bonds between solute and alcoholic molecules can be significantly strengthened in the electronic excited state upon photoexcitation. In previous works [20][21][22][23], we have demonstrated that, the excited-state hydrogen bonding behavior would play an important role in many photochemical reactions such as fluorescence quenching [20], excited-state proton transfer [21], tuning effects on photochemistry [22]. Novel mechanisms have been proposed and the important roles played by hydrogen bonds in these dynamic processes were demonstrated.…”

A DFT/TDDFT investigation of the excited state proton transfer reaction of fisetin chromophore

“…Up to now, many different sensing mechanisms, such as intramolecular charge transfer (ICT), photo-induced electron transfer (PET), fluorescence resonance energy transfer (FRET), and excited state proton transfer (ESPT) and so forth [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38], are relevant with hydrogen bonding. Particularly, the excited state interand intra-molecular proton transfer (ESIPT) reactions have been drawing great attention due to their unique photo-physical and photo-chemical properties.…”