Theoretical study of excited-state proton transfer of 2,7-diazaindole·(H2O)2 cluster via hydrogen bonding dynamics

Effect of concentration and wavelength of excitation on the photophysics of salicylate anion in acetonitrile and water

Sodium salicylate (NaSA) molecule exists as salicylate anion in acetonitrile (ACN) and water solvents, and exhibits large Stokes shifted fluorescence due to excited state intramolecular proton transfer (ESIPT), with decay times of ∼5 ns in ACN and ∼3.9 ns in water by 300 nm (absorption maxima) excitation. Previous studies report both ground and excited state enol-keto tautomerization in ACN, but only excited state tautomerization in water at 10-4 M. However, the current work explores the effect of concentration and excitation wavelengths on the photoinduced dynamics of ESIPT in the salicylate anion. On increasing the concentration of NaSA, no change in absorption spectra appears in both the solvents, and emission spectra of NaSA in water remain unaffected by changes in concentration or excitation wavelength, whereas, a slight red shift and decrease in FWHM appears in ACN. Time-domain fluorescence measurements show predominantly single-exponential decay throughout the emission profile by 300 nm excitation above the 10-5 M concentration in both the solvents, while by 375 nm excitation, multi-exponential fluorescence decay is observed at low concentrations, and as the concentration of NaSA increases, this decay behaviour tends to converge towards a single exponential decay. These results suggest that solute-solvent interactions stabilize the ground-state intermolecular hydrogen-bonded species at low concentrations, while higher concentrations weaken these interactions, leading to emission solely from the salicylate anion. Peak fit analysis of excitation spectra confirms enol-keto tautomerization in both the solvents, with the keto form being more stabilized in ACN. These findings underscore that in ACN, behaviour of NaSA is influenced by both concentration and excitation wavelength and contrary to previous reports, the keto form of the molecule is also present in water, though at a very low concentration and an increase in non-radiative transitions in water cause fluorescence quenching.

Competing Excited-State Hydrogen and Proton-Transfer Processes in 6-Azaindole-S3,4 and 2,6-Diazaindole-S3,4 Clusters (S=H2 O, NH3 )

Excited state hydrogen (ESHT) and proton (ESPT) transfer reaction pathways in the three and four solvent clusters of 6-azaindole (6AI-S3,4 ) and 2,6-diazaindole (26DAI-S3,4 )(S=H2 O, NH3 ) were computationally investigated to understand the fate of photo-excited biomolecules. The ESHT energy barriers in (H2 O)3 complexes (39.6-41.3 kJmol-1 ) were decreased in (H2 O)4 complexes (23.1-20.2 kJmol-1 ). Lengthening the solvent chain lowered the barrier because of the relaxed transition states geometries with reduced angular strains. Replacing the water molecule with ammonia drastically decreased the energy barriers to 21.4-21.3 kJmol-1 in (NH3 )3 complexes and 8.1-9.5 kJ mol-1 in (NH3 )4 complexes. The transition states were identified as Ha atom attached to the first solvent molecule. The formation of stronger hydrogen bonds in (NH3 )3,4 complexes resulted in facile ESHT reaction than that in the (H2 O)3,4 complexes. The ESPT energy barriers in 6AI-S3,4 and 26DAI-S3,4 were found to range between 40-73 kJmol-1 . The above values were significantly higher than that of the ESHT processes and hence are considered as a minor channel in the process. The effect of N(2) insertion was explored for the very first time in the isolated solvent clusters using local vibrational mode analysis. In DAI-S4 , the higher Ka (Ha ⋯Sa ) values depicted the increased photoacidity of the N(1)-Ha group which may facilitate the hydrogen transfer reaction. However, the increased N(6)⋯Hb bond length elevated the reaction barriers. Therefore, in the ESHT reaction channel, the co-existence of two competing factors led to a marginal/no change in the overall energy barriers due to the N(2) insertion. In the ESPT reaction pathway, the energy barriers showed notable increase upon N(2) insertion because of the increased N(6)⋯Hb bond length.

A combined spectroscopic and computational investigation on the solvent-to-chromophore excited-state proton transfer in the 2,2'-pyridylbenzimidazole-methanol complex

This article demonstrates experimental proof of excited state 'solvent-to-chromophore' proton transfer (ESPT) in the isolated gas phase PBI (2,2'-pyridylbenzimidazole)-CH₃OH complex, aided by computational calculations. The binary complexes of PBI with CH₃OH/CH₃OD were produced in a supersonic jet-cooled molecular beam and the energy barrier of the photo-excited process was determined using resonant two-colour two-photon ionization spectroscopy (R2PI). The ESPT process in the PBI-CH₃OH complex was confirmed by the disappearance of the Franck-Condon active vibrational transitions above 000 + 390 cm^-1. In the PBI-CH₃OD complex, the reappearance of the Franck-Condon transitions till 000 + 800 cm^-1 confirmed the elevation of the ESPT barrier upon isotopic substitution due to the lowering of the zero-point vibrational energy. The ESPT energy barrier in PBI-CH₃OH was bracketed as 410 ± 20 cm^-1 (4.91 ± 0.23 kJ mol^-1) by comparing the spectra of PBI-CH₃OH and PBI-CH₃OD. The solvent-to-chromophore proton transfer was confirmed based on the significantly decreased quantum tunnelling of the solvent proton in the PBI-CH₃OD complex. The computational investigation resulted in an energy barrier of 6.0 kJ mol^-1 for the ESPT reaction in the PBI-CH₃OH complex, showing excellent agreement with the experimental value. Overall, the excited state reaction progressed through an intersection of ππ* and nπ* states before being deactivated to the ground state via internal conversion. The present investigation reveals a novel reaction pathway for the deactivation mechanism of the photo-excited N-containing biomolecules in the presence of protic-solvents.

Solvent assisted excited-state deactivation pathways in isolated 2,7-diazaindole-S1-3 (S = Water and Ammonia) complexes

The role of solvent molecules in the deactivation of photo-excited 2,7-diazaindole (DAI) - (H₂O)_1-3 and DAI - (NH₃)_1-3 complexes were computationally investigated. An excited-state proton transfer (ESPT) path from the solvent to the DAI molecule was followed using the TD-DFT-D4 (B3LYP) level of theory. The computed potential energy profile of ESPT process has shown intersection between ππ* and nπ* states facilitated via relative stabilization of the nπ* state with decreasing N(7)-H_b bond length. The ESPT process, starting from the DAI-S_n (ππ*) state, crosses through a barrier ranging from 27 to 36 kJmol^-1 for water complexes and 26-30 kJmol^-1 for ammonia complexes. The energy of the excited state was rapidly decreased with a shorter N(7)-H_b bond length. Subsequently, a significant trend of finding a second intersection between the ground and the excited state was observed for all the complexes. The results firmly suggested a significant deactivation channel of excited azaindole derivatives. In the present system, two competing channels, ESPT and ESHT, were found to be energetically accessible. The energy barriers associated with the ESPT barriers for DAI-(H₂O)_1-3 complexes are similar to the ESHT barrier, depicting equal dominance of both processes. The increased basicity of the N(7) atom in the excited state resulted a facile ESPT process from the water to N(7) of the DAI molecule. However, DAI-(NH₃)_1-3 complexes show clear preference for ESHT over ESPT process owing to its higher gas-phase pK_a value making it a poor proton donor. The above systems can be used as a model to computationally and experimentally investigate the competing radiative and deactivation pathways of photo-excited solvated complexes of N-H-bearing bio-relevant molecules via proton and hydrogen transfer reactions.

Excited state hydrogen atom transfer pathways in 2,7-diazaindole - S1-3 (S = H2O and NH3) clusters

The photoinduced tautomerization reactions via hydrogen atom transfer in the excited electronic state (ESHT) have been computationally investigated in 2,7-diazaindole (27DAI) - (H₂O)_1-3 and 27DAI - (NH₃)_1-3 isolated clusters to understand the role of various solvent wires. Two competing ESHT reaction pathways originating from the N(1)-H group to the neighbouring N(7) (R_(1H-Sn-7H)) and N(2) (R_(1H-Sn-2H)) atoms were rigorously examined for each system. Both one- and two-dimensional potential energy surfaces have been calculated in the excited state to investigate the pathways. The R_(1H-Sn-7H) was found to be the dominant route with reaction barriers ranging from 26-40 kJmol^-1 for water clusters, and 14-26 kJmol^-1 for ammonia clusters. The barrier heights for ammonia clusters were found to be nearly half of the that observed for the water systems. The lengthening of the solvent chain up to two molecules resulted in a drastic decrease in the barrier heights for R_(1H-Sn-7H). The barriers of the competing reaction channel R_(1H-Sn-2H) were found to be significantly higher (31-127 kJmol^-1) but were observed to be decreasing with the lengthening of the solvent wire as in the R_(1H-Sn-7H) pathway. In both the reactions, the angle strain present in the transition state structures was dependent upon the solvent chain's length and was most likely the governing factor for the barrier heights in each solvent cluster. The results have also affirmed that the ammonia molecule is a better candidate for hydrogen transfer than water because of its higher gas-phase basicity. The results delineated from this investigation can pave the way to unravel the excited-state hydrogen atom transfer pathways in novel N-H bearing molecules.

Theoretical Insights into Excited-State Intermolecular Proton Transfers of 2,7-Diazaindole in Water Using a Microsolvation Approach

The detailed excited-state intermolecular proton transfer (ESInterPT) mechanism of 2,7-diazaindole with water wires consisting of either one or two shells [2,7-DAI(H₂O)_n; n = 1-5] has been theoretically explored by time-dependent density functional theory using microsolvation with an implicit solvent model. On the basis of the excited-state potential energy surfaces along the proton transfer (PT) coordinates, among all 2,7-DAI(H₂O)_n, the multiple ESInterPT of 2,7-DAI(H₂O)₂₊₃ through the first hydration shell (inner circuit) is the most easy process to occur with the lowest PT barrier and a highly exothermic reaction. The lowest PT barrier resulted from the outer three waters pushing the inner circuit waters to be much closer to 2,7-DAI, leading to the enhanced intermolecular hydrogen-bonding strength of the inner two waters. Moreover, on-the-fly dynamic simulations show that the multiple ESInterPT mechanism of 2,7-DAI(H₂O)₂₊₃ is the triple PT in a stepwise mechanism with the highest PT probability. This solvation effect using microsolvation and dynamic simulation is a cost-effect approach to reveal the solvent-assisted multiple proton relay of chromophores based on excited-state proton transfer.

Sensing mechanism of a fluorescent probe for thiophenols: Invalidity of excited-state intramolecular proton transfer mechanism

Simple and effective detection of thiophenols has attracted great attention. A fluorescent probe 1 with high selectivity and sensitivity is designed and synthesized based on the excited-state intramolecular proton transfer (ESIPT) in experiment. However, we conclude that the ESIPT process fails to happen actually based on the calculation results. In the present work, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to investigate the real sensing mechanism. The calculated absorption and emission spectra agree well with the experimental results. By comparing the energy of enol and keto configurations and the constructed potential energy surfaces (PESs) in the ground (S₀) and excited (S₁) states of 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2), the ESIPT process is confirmed impossible because of the relatively high keto form energy and potential energy barrier. Besides, the transition state of dye 2 is optimized to offer the accurate potential energy barrier. The results of calculated frontier molecular orbitals (FMOs) and spectra indicate that it is the photoinduced electron transfer (PET) process that results in the fluorescence quenching of probe 1. After adding thiophenols, the thiolysis of 2,4-dinitrophenyl ether bond is triggered and dye 2, which emits strong fluorescence because of the absence of PET process, is obtained. Consequently, our study has demonstrated that probe 1 can act as a fluorescent probe to detect thiophenols through the off-on fluorescence variation based on the PET mechanism but not the ESIPT process.

Is excited state intramolecular proton transfer frustrated in 10-hydroxy-11H-benzo[b]fluoren-11-one?

Recently, Piechowska and coworker found that hydroxybenzofluorenone 10-hydroxy11H-benzo[b]fluoren-11-one (10-HHBF) does not show dual fluorescence, which is in contrast to its well-known analogue 1-hydroxy-11H-benzo[b]fluoren-11-one (1-HHBF) [Dyes Pigm. 2019, 165, 346-353.]. Based on the increased donor-acceptor distance and the lower stability of the excited state tautomer of former, they believe that different from 1-HHBF, ESIPT is not occurring in 10-HHBF. In the preset work, in order to clarify whether ESIPT would take place in 10-HHBF, we have optimized the four-state geometrical structures (ground state S₀, first singlet excited state S₁, transition state S_1-TS and after proton transfer S_1-PT), carried out the Natural Population Analysis and scanned the ground-state and excited-state potential energy curves of 1-HHBF and 10-HHBF at TD-CAM-B3LYP/6-311 + g(2d,2p)/IEFPCM (cyclohexane) theory level. It is found that ESIPT should take place in both 1-HHBF and 10-HHBF and the Gibbs free energy diagram further indicates that the ESIPT process is more favorable in 10-HHBF than in 1-HHBF.

Halogen substituent effect on the water-assisted excited-state tautomerization of 2, 7-diazaindole-H2O complex in aqueous solution: A theoretical study

We studied the ESPT process in the 2-DAI-H₂O complex theoretically for the first time, and compared the kinetics of 2-DAI-H₂O with those features of 7-DAI-H₂O. The substituted effect on the dynamics of excited-state double proton transfer in 2-DAI-H₂O and 7-DAI-H₂O clusters in water were also investigated at the TD-M06-2X/6-311+G(d, p) level. In this work, 2,7-DAI-H₂O is also expressed as 2-DAI-H₂O and 7-DAI-H₂O, in which correspond to different ESPT reactions and generate two tautomers (N₂H form and N₇H form). In both the 2-DAI-H₂O and 7-DAI-H₂O complexes, ESPT processes happened in a concertedly but asynchronously protolysis pathway. The ESPT process preferred to occur in the 7-DAI-H₂O complex due to its lower barrier height. For the 3-X-2-DAI-H₂O and 3-X-7-DAI-H₂O (X = H, F, Cl, Br) complexes, the replacement of halogen atom did not influence the ESPT mechanism. However, the replacement of halogen atom changed the structural parameters evidently, reduced the barrier height up to 4-5 kcal/mol, and enlarged the asynchronicity of ESPT apparently. ∆(R₁+R₂) values in the 3X-2-DAI-H₂O and 3X-7-DAI-H₂O complexes have linear correlation to the ZPE-corrected ESPT barrier height linearly.

Theoretical insights into excited-state intramolecular and multiple intermolecular hydrogen bonds in 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone

The photophysical properties and photochemistry reactions of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ) system in different solutions are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Our theoretical investigation explores that an ultrafast barrier-free excited state intramolecular proton transfer (ESIPT) process occurs and the configuration twisting is found in the electronic excited state. In the polar protic methanol solution, the hydrogen-bonded complex composed by HPQ and two methanol molecules (HPQ-2M) could exist stably in the ground state. Upon photoexcitation the isolated HPQ is initially excited to the first excited state, while the HPQ-2M system is firstly excited to the S₃ state and undergoes internal conversion (IC) to the S₁ state. The intermolecular hydrogen bonds are strengthened in the excited state. The simulated electronic spectra agree well with the experimental results. The strengthening of the intermolecular hydrogen bonds is also confirmed by the calculated vibrational spectra. In addition, the intramolecular charge transfer happens in both HPQ and HPQ-2M systems from the frontier molecular orbital analysis.