A theoretical assignment on excited‐state intramolecular proton transfer mechanism for quercetin

In the present work, we investigate a new chromophore (ie, quercetin) (Simkovitch et al J Phys Chem B 119 [2015] 10244) about its complex excited‐state intramolecular proton transfer (ESIPT) process based on density functional theory and time‐dependent density functional theory methods. On the basis of the calculation of electron density ρ( r ) and Laplacian ?²ρ( r ) at the bond critical point using atoms‐in‐molecule theory, the intramolecular hydrogen bonds (O₁‐H₂?O₅ and O₃‐H₄?O₅) have been supported to be formed in the S₀ state. Comparing the prime structural variations of quercetin involved in its 2 intramolecular hydrogen bonds, we find that these 2 hydrogen bonds should be strengthened in the S₁ state, which is a fundamental precondition for facilitating the ESIPT process. Concomitantly, infrared vibrational spectra analysis further verifies this viewpoint. In good agreement with previous experimental spectra results, we find that quercetin reveals 2 kinds of excited‐state structures (quercetin* and quercetin‐PT1*) in the S₁ state. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. Our scanned potential energy curves according to variational O₁‐H₂ and O₃‐H₄ coordinates demonstrate that the proton transfer process should be more likely to occur in the S₁ state via hydrogen bond wire O₁‐H₂?O₅ rather than O₃‐H₄?O₅ because of the lower potential energy barrier 2.3 kcal/mol. Our present work explains previous experimental result and makes up the deficiency of mechanism in previous experiment. In the end, we make a reasonable assignment for ESIPT process of quercetin.     

Theoretical insight into the excited‐state proton transfer process: Role of the substituent ‐CN on HBT system

In this work, using density functional theory and time‐dependent density functional theory methods, we theoretically studied the excited‐state behaviors of 3 novel 2‐(2‐hydroxyphenyl)benzothiazole (HBT) derivatives (HBT‐H‐H, HBT‐CN‐H, and HBT‐CN‐CN). Analyses about primary chemical structures such as bond lengths and bond angles, we found that all the intramolecular hydrogen bonds in these 3 structures should be strengthened in the S₁ state upon the photoexcitation. Exploring the infrared vibrational spectra at the hydrogen bonds groups, we confirmed that nonsubstitutional HBT‐H‐H structure might play more important roles in the excited‐state intramolecular proton transfer (ESIPT) reaction than HBT‐CN‐H and HBT‐CN‐CN. Further, investigating vertical excitation process, it can be revealed that charge redistribution involved in hydrogen bonding moieties could facilitate the ESIPT reaction. Based on constructing potential energy curves of both S₀ and S₁ states, we confirmed that the substituents on HBT systems can reasonably regulate and control the ESIPT processes because of the different potential energy barriers. We deem that this present work not only elaborates the different excited‐state behaviors of HBT‐H‐H, HBT‐CN‐H, and HBT‐CN‐CN but also may play important roles in designing and developing new materials and applications involved in HBT systems in future.     

Theoretical investigation on the excited‐state intramolecular proton transfer mechanism of 2‐(2′‐benzofuryl)‐3‐hydroxychromone

Spectroscopic studies on excited‐state proton transfer of a new chromophore 2‐(2′‐benzofuryl)‐3‐hydroxychromone (BFHC) have been reported recently. In the present work, based on the time‐dependent density functional theory (TD‐DFT), the excited‐state intramolecular proton transfer (ESIPT) of BFHC is investigated theoretically. The calculated primary bond lengths and angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared (IR) vibrational spectra as well as the calculated hydrogen bonding energies. Further, hydrogen bonding strengthening manifests the tendency of excited state proton transfer. Our calculated results reproduced absorbance and fluorescence emission spectra of experiment, which verifies that the TD‐DFT theory we used is reasonable and effective. The calculated Frontier Molecular Orbitals (MOs) further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O―H coordinate, the potential energy barrier of about 14.5 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 5.4 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Elaborating the excited state behavior of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine coupling with methanol solvent

We present a theoretical investigation about the excited state dynamical mechanism of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine (DMAPIP‐c). Within the framework of density functional theory and time‐dependent density functional theory methods, we reasonably repeat the experimental electronic spectra, which further confirm the theoretical level used in this work is feasible. Given the best complex model, 3 methanol (MeOH) solvent molecules should be connected with DMAPIP‐c forming DMAPIP‐c‐MeOH complex in both ground state and excited state. Exploring the changes about bond lengths and bond angles involved in hydrogen bond wires, we find the O7‐H8···N9 one should be largely strengthened in the S₁ state, which plays an important role in facilitating the excited state intermolecular proton transfer (ESIPT) process. In addition, the analyses about infrared vibrational spectra also confirm this conclusion. The redistribution about charges distinguished via frontier molecular orbitals based on the photoexcitation, we do find tendency of ESIPT reaction due to the most charges located around N9 atom in the lowest unoccupied molecular orbital. Based on constructing the potential energy curves of both S₀ and S₁ states, we not only confirm that the ESIPT process should firstly occur along with hydrogen bond wire O7‐H8···N9, but also find a low potential energy barrier 8.898 kcal/mol supports the ESIPT reaction in the S₁ state forming DMAPIP‐c‐MeOH‐PT configuration. Subsequently, DMAPIP‐c‐MeOH‐PT could twist its dimethylamino moiety with a lower barrier 3.475 kcal/mol forming DMAPIP‐c‐MeOH‐PT‐TICT structure. Our work not only successfully explains previous experimental work but also paves the way for the further applications about DMAPIP‐c sensor in future.     

A theoretical study on the ESPT mechanism for a novel Bis‐HPBT fluorophore

In this work, based on the density functional theory and time‐dependent density functional theory methods, the properties of the 2 intramolecular hydrogen bonds (O1‐H2···N3 and O4‐H5···N6) of a new photochemical sensor 4‐(3‐(benzo[d]thiazol‐2‐yl)‐5‐tert‐butyl‐4‐hydroxybenzyl)‐2‐(benzo[d]thiazol‐2‐yl)‐6‐tert‐butyl phenol (Bis‐HPBT) have been investigated in detail. The calculated dominating bond lengths and bond angles about these 2 hydrogen bonds (O1‐H2···N3 and O4‐H5···N6) demonstrate that the intramolecular hydrogen bonds should be strengthened in the S₁ state. In addition, the variations of hydrogen bonds of Bis‐HPBT have been also testified based on infrared vibrational spectra. Our theoretical results reproduced absorption and emission spectra of the experiment, which verifies that the theoretical level we used is reasonable and effective in this work. Further, hydrogen bonding strengthening manifests the tendency of excited state intramolecular proton transfer (ESIPT) process. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. According to the calculated results of potential energy curves along stepwise and synergetic O1‐H2 and O4‐H5 coordinates, the potential energy barrier of approximately 1.399 kcal/mol is discovered in the S₁ state, which supports the single ESIPT process along with 1 hydrogen bond of Bis‐HPBT. In other words, the proton transfer reaction can be facilitated based on the electronic excitation effectively. In turn, through the process of radiative transition, the proton‐transfer Bis‐HPBT‐SPT form regresses to the ground state with the fluorescence of 539 nm.     

A detailed DFT/TDDFT study on excited‐state intramolecular hydrogen bonding dynamics and proton‐transfer mechanism of 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol

In this present work, using density functional theory and time‐dependent density functional theory methods, we theoretically study the excited‐state hydrogen bonding dynamics and the excited state intramolecular proton transfer mechanism of a new 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol (2PYP) system. Via exploring the reduced density gradient versus sign(λ₂(r))ρ(r), we affirm that the intramolecular hydrogen bond O1‐H2?N3 is formed in the ground state. Based on photoexcitation, comparing bond lengths, bond angles, and infrared vibrational spectra involved in hydrogen bond, we confirm that the hydrogen bond O1‐H2?N3 of 2PYP should be strengthened in the S₁ state. Analyses about frontier molecular orbitals prove that charge redistribution of 2PYP facilitates excited state intramolecular proton transfer process. Via constructing potential energy curves and searching transition state structure, we clarify the excited state intramolecular proton transfer mechanism of 2PYP in detail, which may make contributions for the applications of such kinds of system in future.     

Exploring excited‐state proton transfer mechanism for 9,10‐dihydroxybenzo[h]quinolone

In this work, we mainly focus on the excited‐state intramolecular proton transfer mechanism of a new molecule 9,10‐dihydroxybenzo[h]quinoline (9‐10‐HBQ). Within the framework of density functional theory and time‐dependent density functional theory methods, we have theoretically investigated its excited‐state dynamical process and our theoretical results successfully reappeared previous experimental electronic spectra. The ultrafast excited‐state intramolecular proton transfer process occurs in the first excited state (S₁ state) forming 9‐10‐HBQ‐PT1 structure without potential energy barrier along with hydrogen bond (O₃–H₄···N₅). Then the second proton may transfer via another intramolecular hydrogen bonded wire (O₁–H₂···N₃) with a moderate potential energy barrier (about 7.69 kcal/mol) in the S₁ state forming 9‐10‐HBQ‐PT2 configuration. After completing excited‐state dynamical process, the molecule on the first excited electronic state would come back to the ground state. We not only clarify the excited‐state dynamical process for 9‐10‐HBQ but also put forward new predictions and successfully explain previous experimental results.     

The ESIPT mechanism of dibenzimidazolo diimine sensor: a detailed TDDFT study

Spectroscopic investigations on excited state proton transfer of a new dibenzimidazolo diimine sensor (DDS) were reported by Goswami et al. recently. In our present work, based on the time‐dependent density functional theory (TDDFT), the excited‐state intramolecular proton transfer (ESIPT) mechanism of DDS is studied theoretically. Our calculated results reproduced absorption and fluorescence emission spectra of the previous experiment, which verifies that the TDDFT method we adopted is reasonable and effective. The calculated dominating bond lengths and bond angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared vibrational spectra. Further, hydrogen bonding strengthening manifests the tendency of ESIPT process. The calculated frontier molecular orbitals further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O–H coordinate, the potential energy barrier of about 5.02 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 0.195 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical investigation on the excited state intramolecular proton coupled charge transfer phenomenon for a novel fluorophore

We theoretically investigate the excited state behaviors of the novel fluorophore tetraphenylethene‐2‐(2′‐hydroxyphenyl)benzothiazole (TPE‐HBT), which was designed based on the intersection of TPE and HBT, using density functional theory and time‐dependent density functional theory methods. Compared with previous experimental results about fluorescence peaks, our calculated results are in good agreement with experimental data, which further confirms that the theoretical level we used is reasonable. Furthermore, our results confirm that the excited state intramolecular proton transfer (ESIPT) process happens upon photoexcitation, which is distinctly monitored by the infrared spectra and the potential energy curves. In addition, the calculation of highest occupied molecular orbital and lowest unoccupied molecular orbital reveals that the electron density change of proton acceptor because of the intramolecular charge transfer (ICT) process in the S₁ state induces the ESIPT. Moreover, the transition density matrix is worked out to facilitate deeper insight into the ESIPT coupled ICT process. It is hoped that the present work not only elaborates the ESIPT coupled ICT phenomenon and corresponding mechanisms for the TPE‐HBT but also may be helpful to design and develop new materials and applications involved in TPE‐HBT systems in future.     

Ab initio investigation of sulfur monofluoride and its singly charged cation and anion in their ground electronic state

The SF radical and its singly charged cation and anion, SF+and SF-, have been investigated on the MRCI/aug-ccp VX Z(X = Q, 5, 6) levels of theory with Davidson correction. Both the core–valence correlation and the relativistic effect are considered. The extrapolating to the complete basis set(CBS) limit is adopted to remove the basis set truncation error.Geometrical parameters, potential energy curves(PECs), vibrational energy levels, spectroscopic constants, ionization potentials, and electron affinities of the ground electronic state for all these species are obtained. The information with respect to molecular characteristics of the SFn(n =-1, 0, +1) systems derived in this work will help to extend our knowledge and to guide further experimental or theoretical researches.     

Solvent controlling excited state proton transfer reaction in quinoline/isoquinoline‐pyrazole isomer QP‐I: A theoretical study

In this present work, we theoretically study the excited state intramolecular proton transfer (ESIPT) mechanism about a quinoline/isoquinoline‐pyrazole isomer QP‐I system. Compared with previous experimental results, our calculated results reappear previous data, which further confirm the theoretical level we used is reasonable. We mainly adopt 2 kinds of solvents (nonpolar cyclohexane and polar acetonitrile) to explore solvents effects on this system. Through reduced density gradient (RDG) function, the intramolecular hydrogen bond N1─H2···N3 has been confirmed existing in both S₀ and S₁ states, although the distance between H2 and N3 is not short. In addition, the strengthening N1─H2···N3 in the S₁ state provides possibility for ESIPT. Explorations about charge redistribution reveal the trend of ESIPT, and frontier orbital gap reflects the reactivity in polar and nonpolar solvents. The constructing potential energy curves reveal that potential energy barriers could be controlled and regulated by solvent polarity.     

Excited‐state hydrogen bond strengthening of coumarin 153 in ethanol solvent: a TDDFT study

So far, coumarin dyes have been extensively studied with various means to understand their photophysical behaviors and photochemical properties. Here, our performing time‐dependent density functional theory calculation is aimed at exploring the excited‐state hydrogen bonding dynamics of coumarin 153 (C153) in protic ethanol (EtOH) solvent. The calculated results suggest that the excited‐state hydrogen bond C?O?H?O between C?O group and O?H group in the C153‐EtOH complex is strengthened, and the S₀ → S₁ transition of the complex corresponds to the highest occupied molecular orbital (HOMO) hopping to the lowest unoccupied molecular orbital (LUMO). The excited‐state hydrogen bond strengthening has been further confirmed by its larger binding energy in the S₁ state than in the S₀ state. In addition, because of the formation of the hydrogen bond C?O?H?O, a red shift of about 7 nm occurs in the electronic spectra of the C153‐EtOH complex, which is in good accordance with the experiment result. Copyright © 2016 John Wiley & Sons, Ltd.     

Vibrational investigation of 1-cyclopentylpiperazine: A combined experimental and theoretical study

FT-IR and Raman spectra of 1-cyclopentylpiperazine(1cppp)have been experimentally examined in the region of 4000–200cm-1.The optimized geometric parameters,conformational equilibria,normal mode frequencies and corresponding vibrational assignments of 1cppp(C9H18N2)are theoretically examined by means of B3LYP hybrid density functional theory(DFT)method together with 6-31++G(d,p)basis set.On the basis of potential energy distribution(PED)reliable vibrational assignments have been made and the thermodynamics functions,highest occupied and lowest unoccupied molecular orbitals(HOMO and LUMO)of 1cppp have been predicted.Calculations are employed for four different conformations in C1 and Cs point groups of 1cppp in gas phase.Comparison between the experimental and theoretical results indicates that B3LYP method is able to provide satisfactory results for predicting vibrational frequencies and the structural parameters,vibrational frequencies and assignments.Furthermore,C1(equatorial-axial)point group has been found as the most stable conformer of 1cppp.     

Theoretical study of the SrLi+ molecular ion: structural,electronic and dipolar properties

ABSTRACT

The structural and electronic properties of (SrLi)⁺ molecular ion have been determined by the use of ab initio approaches. Potential energy curves (PECs) with their spectroscopic constants (R_e, D_e, ω_e, T_e, B_e and ω_eχ_e) have been calculated. Also, the vibrational properties and the electric dipole moments, either permanent (PDM) or transition (TDM) ones, have been investigated and analysed. Large Gaussian basis sets, the full valence configuration interaction (FCI) and the formalism of non-empirical pseudo-potential including the two approaches, effective core potential (ECP) and core polarisation potential (CPP), are analysed. Therefore, (SrLi)⁺ is considered as a two effective electrons system. Numerous excited states of symmetries ^1,3Σ⁺, ^1,3Π and ^1,3Δ dissociating below the ionic limit Sr²⁺Li^? have been investigated. This study shows interesting behaviours around the avoided crossing related to charge transfer involving important processes in physics and astrophysics such as dynamics, pre-dissociation and inelastic transitions.     

Determining the excited‐state substituent constants of meta‐substituent from 3,4'‐disubstituted stilbenes

In this paper, 61 samples of 3,4'‐disubstituted stilbenes and 18 samples of 3,3'‐disubstituted stilbenes were synthesized, and their UV data were measured in anhydrous ethanol. Based on the UV absorption energy (wavenumber) of 3,4'‐disubstituted stilbenes, the excited‐state substituent constants of meta‐substituent were determined by means of curve‐fitting. The availability of was confirmed by the good correlation with the UV absorption energy of 3,4'‐disubstituted stilbenes and 4,4'‐disubstituted stilbenes. Further, using the obtained constants and the correlation equation, we calculated the UV wavenumbers of 3,3'‐disubstituted stilbenes, and the calculated wavenumbers are in good agreement with the experimental values. These results verified that the excited‐state substituent constants of meta‐substituent are reliable parameter to scale the effect of meta‐substituent on the UV absorption energy. Copyright © 2012 John Wiley & Sons, Ltd.     

Kr-CS2体系势能面及束缚态能级的理论研究

本文采用单双迭代(包括非迭代三重激发)耦合簇CCSD(T)方法,对C、S原子采用aug-cc-PVTZ基组,对Kr原子采用cc-PVTZ –DK基组,并且加上中心键函数(3s3p2d2f1g),计算得到Kr-CS2体系的势能面。该势能面为T型结构,存在一个全局极小值和两个等价的局域极小值。全局极小值位于R =7.05 a0,θ= 90°处,势能值为-396.194 cm-1。两个局域极小值分别位于R = 10.15 a0,θ= 0°和180°处,势能为-243.647 cm-1。利用该势能面,通过数值求解相应的薛定谔方程,计算得出体系J≤10的束缚态能级及微波谱跃迁频率,并通过跃迁频率拟合得到相应的光谱常数。     

Dimethyl sulfoxide/deep eutectic solvents mixtures as media in the reaction of 1‐fluoro‐2,4‐dinitrobenzene with piperidine: A solvent effect study

Aromatic nucleophilic substitution reaction of 1‐fluoro‐2,4‐dinitrobenzene with piperidine was kinetically investigated in ethylene glycol‐choline chloride and glycerol‐choline chloride as 2 deep eutectic solvents (DESs) mixed with dimethyl sulfoxide, in whole mole fractions, at room temperature. The investigation of the reaction in different concentrations of the piperidine shows that the reaction follows the base‐catalyzed mechanism. The measured rate coefficients of the reaction demonstrated a sharp decreasing in all mixtures with the increasing mole fraction of DESs. Linear free energy relationship investigations confirm that hydrogen bond donor ability in addition to polarity‐polarizability of the media has a major effect on the reaction rate. The decrease in the rate coefficient is attributed to not only hydrogen‐bonding donor interactions of the media with piperidine as both reactant and catalyst but also the preferential solvation of reactants by DES compared with the intermediate of the reaction.