Ground State Intramolecular Proton Transfer Research Articles

Exploring electron donor and acceptor effects: DFT analysis of ESIPT/GSIPT in 2-(oxazolinyl)-phenols for photophysical and luminophore enhancement.

Understanding excited-state intramolecular proton transfer (ESIPT) is essential for designing organic molecules to enhance photophysical and luminophore properties in the development of optoelectronic devices. In this context, an attempt has been made to understand the impact of substituents on the ESIPT process of 2-(oxazolinyl)-phenol. Electron donating (EDG: -NH2, -OCH3, and -CH3) and electron withdrawing (EWG: -Cl, -Br, -COOH, -CF3, -CN, and -NO2) substitutions have been computationally designed and screened through density functional theory (DFT) and time-dependent density-functional theory (TDDFT) calculations. Furthermore, the ground state intramolecular proton transfer and ESIPT mechanisms of these designed luminophores are explored using the transition state theory. The results reveal that molecules with EDG show higher absorption and emission peaks than molecules with EWG and also indicate that the mobility of charge carriers in 2-(oxazolinyl)-phenol derivatives is significantly influenced by substituents. We found that the EWGs decrease the reorganization energy and increase the vertical ionization potential and electron affinity values, as well as the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, compared to the EDG substituted molecules. Significantly, the excited state (S1) of the keto emission (K) form shows notably larger values for the EDG substitutions. The intersystem crossing pathway efficiency weakens with reduced spin-orbit coupling matrix element in the enol form with electron-donating substituents and vice versa in the keto form during S1-T3 transitions. Our research links intramolecular proton transfers and triplet generation, making these substituted molecules appealing for optoelectronic devices. Introducing EDGs, such as -NH2, boosts the ESIPT reaction in 2-(oxazolinyl)-phenol. This study guides designing ESIPT emitters with unique photophysical properties.

TD-DFT analysis of the photo-ring-opening reaction of the flav-3-en-2-ol in acetonitrile solution: Functional choice and effect of solvated hydrogen bond

Of the forty hybrid functionals supported by Gaussian 16, only fourteen functionals with small contributions from the exact Hartree-Fock exchange demonstrated the breaking of the C2-O1 bond of flav-3-en-2-ol: TPSSh, O3LYP, τHCTHhyb, B3LYP, B3P86, B3PW91, B971, B972, X3LYP, B98, APF, APFD, mPW1LYP, and mPW3PBE. TPSSh and O3LYP gave rise to a cis-2-hydroxychalcone transition accompanied by ground-state intramolecular proton transfer (GSIPT), and the remaining twelve functionals selected showed a return to the flav-3-en-2-ol with the recovery of the C2-O1 bond. The strong H-bond of acetonitrile with flav-3-en-2-ol has a striking effect on the consequences of the excitation in comparison with a single flav-3-en-2-ol molecule: optimization of the S1 excited state of the solvated complex leads to the breaking of the C2-O1 bond by O3LYP functional only. However, upon relaxation of the excited enol form, this bond was recovered, and a return to the flav-3-en-2-ol occurred. Nevertheless, the O3LYP functional gives a theoretical model consistent with the experiment: the H-bond of flav-3-en-2-ol with the acetonitrile molecule is a switch in the reaction path. If this H-bond is present at the excitation moment, then the flav-3-en-2-ol molecule undergoes a C2-O1 bond cleavage but recovers upon relaxation. If this H-bond is absent, then the flav-3-en-2-ol molecule transforms into the enol form which goes further into cis-2-hydroxychalcone. The excitation of the solvated complex causes a pronounced strengthening of this intermolecular H-bond. GSIPT is caused by a change in the polarization of the enol form during its electronic relaxation, as a result of which the proton begins to experience a stronger attraction to the opposite part of the molecule.

Effect of push–Pull functional group on the ESIPT behaviour and photophysical property of thiolflavone-based compound: a TD-DFT investigation

ABSTRACT The excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of three –CN-substituted (2-(4-(diethylamino)phenyl)- 3-mercapto-4H-chromen-4-one (3-NTF) compounds (3NTF-1, 3NTF-2, 3NTF-3) have been studied systematically based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. We analysed the structural parameters, infrared frequencies and electron densities of the studied compounds, and found that the intramolecular hydrogen bonds (IHBs) at tautomer form are weakened in the excited (S1) state. The weaker IHB at tautomer form is beneficial to ESIPT process. The potential energy curves (PECs) were established and showed that the ESIPT processes of 3NTF-1 and 3NTF-2 are barrierless. The ESIPT process of 3NTF-3 needs to overcome a very low energy barrier. The electron-withdrawing group (–CN) increases the energy barrier of ground state intramolecular proton transfer (GSIPT) process in 3NTF. The order of GSIPT barrier is consistent with the order of their IHB strengths of normal forms. When the electron-withdrawing group (–CN) was introduced, the absorption and fluorescence peaks red-shifted. Two –CN groups resulted in the fluorescence peak of 3NTF red-shift more than one –CN group did.

CASPT2//CASSCF studies on mechanistic photophysics of 3-hydroxyflavone

3-Hydroxyflavone (3-HF) as the backbone of flavonols has been extensively studied on the dual fluorescence and excited-state intramolecular proton transfer (ESIPT) after photoexcitation. However, its detailed excited-state relaxation mechanism is still unclear. Herein, the minimum-energy structures, conical intersections and singlet–triplet crossing points of 3-HF in the 1ππ∗, 1nπ∗, 3ππ∗, 3nπ∗, and S0 states in the gas phase were first optimized by the CASPT2//CASSCF method. Then, the minimum-energy proton transfer (MEPT) and excited-state decay (ESD) pathways were calculated as well. Based on the calculation results, we proposed several possible ESD pathways from the initially populated bright S2(ππ*) state. The major channel is singlet-involved and stretch-torsion coupled ESIPT pathway. 3-HF in normal (N) form first undergoes efficient 1ππ* ESIPT with a small barrier of 2.2 kcal/mol to yield the 1ππ* proton-transferred tautomer (T). Subsequently, the T tautomer approaches the nearby accessible S1/S0 conical intersection and then occurs internal conversion (IC) to fall back to the S0 state. Finally, the S0 species quickly transfers into the N conformer via the reverse ground state intramolecular proton transfer (GSIPT) to realize the ground-state recovery. In addition, the minor ESD pathways are involved less efficient intersystem crossing (ISC) processes. The present work could provide insights into understanding the photophysics of flavonols and their derivatives.

Regulating excited‐state intramolecular proton transfer behavior and fluorescent property of 3‐thiolflavone by electron‐donating ability: A time‐dependent density functional theory study

AbstractBackgroundSince the excited‐state intramolecular proton transfer (ESIPT) process involving sulfur is extremely fast, its mechanism is still unclear, and its derivatives are rarely studied, we have performed a theoretical study the ESIPT process of 3‐thiolflavone (3TF) and its derivatives.ObjectiveThe ESIPT mechanism and photophysicalproperties of 3‐thiolflavone (3TF) and its derivatives 3NTF, 3ATF and 3OTF modified by ‐N(C2H5)2, ‐NH2 and ‐OCH3 groups were studied theoretically.MethodsDensity functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) with PBE0 functional and 6‐31G(d, p) basis set.ResultsFor 3TF, 3NTF, 3ATF and 3OTF, the ESIPT processes are barrier‐less, and the barrier of ground‐state intramolecular proton transfer (GSIPT) reduces in the order of 3TF > 3OTF > 3ATF > 3NTF, as the electron‐donating ability of the substituent increases. When the electron‐donating groups are introduced, the absorption and fluorescence wavelengths of 3TF red‐shift and blue‐shift, respectively.ConclusionSubstituents with electron‐donating ability can regulate the ground‐state intramolecular proton transfer process and photophysical properties of 3TF.

Achieving single-molecule-based multi-color emissions by the regulations of intramolecular proton transfer and interphase transformation

Endowing fluorescent materials with multi-color tunable properties in a single-component system remains challenging. Herein, a series of salicylaldehyde Schiff base derivatives with different lengths of alkoxy chains (NPA-n, n = 1, 4, 8, 12) are synthesized. All the derivatives exhibit wide-range tunable emissions through intramolecular proton transfer and interphase transformation. In the solution state, NPA-n derivatives emit blue (E*) and green (K*) emissions, produced by the excited-state intramolecular proton transfer (ESIPT) effect by controlling the polarity of solvents. In the solid state, J-type aggregation results in red-shifted emissions. Polymorphic emissions of NPA-12 with yellow and red, produced by ground-state intramolecular proton transfer (GSIPT) effect, are realized by controlling the aggregated degree. Therefore, NPA-12 presents color tunable emissions covering fully visible spectrum (about 183 nm), including blue, green, yellow and red. The as-obtained system is further applied to thermochromic and mechanochromic aspects. The research provides a strategy to construct multi-color fluorophores for diverse applications.

Coexistence of ground and excited state intramolecular proton transfer of the Schiff base 2-((E)-(naphthalene-3-ylimino)-methyl)phenol: A combined experimental and computational study

Prototropic behavior of the Schiff base compound, 2-((E)-(naphthalene-3-ylimino)-methyl)phenol (NMP), has been explored by photophysical/photochemical means and reported in this paper in a comprehensive manner. The study, comprising of absorption, steady state and time resolved fluorescence, 1H NMR, together with quantum chemical calculations, establishes that the probe undergoes intramolecular proton transfer (IPT) process in both ground and lowest excited singlet state. Resolution of the low energy absorption band of NMP in different solvents and observation of two different emission bands upon excitation at two different positions within the low energy absorption band indicates co-existence of two distinct species of the probe in the ground state, implying occurrence of the ground state intramolecular proton transfer (GSIPT) process. Large downshifted 1H NMR peak at δ > 13 ppm suggests a strong intramolecular H-bonding in the ground state and goes in favor of the GSIPT process. Excited state intramolecular proton transfer (ESIPT) of the probe has been confirmed by the existence of the large Stokes shifted emission band of the tautomer along with the normal (enol) emission band, especially in non-polar media. Observation of the single isoemissive point in the time resolved area normalized emission spectra (TRANES) in nonpolar solvents confirms existence of two distinct species (enol (N*) and tautomer (T*)) in the photoexcited state. Quantum chemical calculations using DFT/TDDFT have been performed to provide support to the experimental findings.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution.

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the 1ππ*, 1nπ*, 3ππ*, 3nπ*, and S0 states, we have identified several feasible excited-state relaxation pathways for the initially populated S2(1ππ*) state to decay to the initial enol isomer' S0 state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless 1ππ* ESIPT process to generate the 1ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S1 → S0 internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S2(1ππ*) minimum, an S2(1ππ*)/S1(1nπ*)/T2(3nπ*) three-state intersection region helps the S2 system branch into the T1 state through a S2 → S1 → T1 or S2 → T2 → T1 process. Once it has reached the T1 state, the system may relax to the S0 state via direct ISC or via subsequent nearly barrierless 3ππ* ESIPT to yield the T1 keto tautomer and ISC. The resultant S0 keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T1/S0 crossing points, the ISC to S0 state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S1(1ππ*) state and the lower-lying T1 and T2 states. In the major relaxation pathway, the photoexcited S1 keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S1 enol minimum, near which a favorable S1/S0 conical intersection decays the system to the S0 state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S0 keto species. In the minor one, an S1/T2/T1 three-state intersection in the keto region makes the T1 state populated via direct and T2-mediated intersystem crossing (ISC) processes. In the T1 state, an ESIPT occurs, which is followed by ISC near a T1/S0 crossing point in the enol region to the S0 state and finally back to the S0 keto species. In addition, a T1/S0 crossing point near the T1 keto minimum can also help the system decay to the S0 keto species. However, small spin-orbit couplings between T1 and S0 at these T1/S0 crossing points make ISC to the S0 state very slow and make the system trapped in the T1 state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.

Substituent Effect on the Photophysics and ESIPT Mechanism of N,N'-Bis(salicylidene)-p-phenylenediamine: A DFT/TD-DFT Analysis.

Excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes are widely exploited in the designing of organic materials for multifarious applications. This work explores the aftereffects of combining both ESIPT and ICT events in a single molecule, namely, N,N'-bis(salicylidene)-p-phenylenediamine (BSP) exploiting DFT and TD-DFT formalisms. The PBE0 functional employed in the present study is found to yield results with better accuracy for excited-state calculations. The results reveal that introduction of electron donor (-NH2) and electron acceptor (-NO2) substituents on BSP produces a strikingly red-shifted emission with respect to the corresponding emission from the unsubstituted analogue in polar solvents. This red-shifted emission originated due to the coupled effect of ESIPT and planar-ICT (PICT) processes from the coplanar geometry adopted by the substituted molecule (s-BSP). Based on the computed potential energy curves, the ground-state intramolecular proton transfer (GSIPT) was found to take place more favorably in s-BSP than in BSP under all solvent conditions. In the case of ESIPT, the barrier and relative energies of the phototautomers of s-BSP were slightly higher than BSP, which shows that simultaneous substitution of -NH2 and -NO2 groups causes slight perturbation to the ESIPT process. Overall, the computed results show that simultaneous substitution of suitable electron donor and acceptor substituents provides profitable changes in the photophysical properties of ESIPT molecules like BSP. These molecular-level insights will pave way for designing better materials for diverse applications.

Ground-state intramolecular proton transfer and observation of high energy tautomer in 1,4-Dihydroxyanthraquinone

Temperature dependent steady state UV/vis absorption technique is combined with ab initio calculations to investigate the ground-state intramolecular proton transfer (GSIPT) process in 1,4-dihydroxy-9,10-anthraquinone (quinizarin). The relative energy for various tautomers is calculated at MP2/6-311++G(d,p) level of theory. The next higher energy tautomer for quinizarin is identified as 9,10-dihydroxy-1,4-anthraquinone which is around 7.7 kcal/mol higher in energy. The experimental Gibbs free energy of the equilibrium between two tautomers is evaluated to be 4.5±1.5 kcal/mol. The transition state for GSIPT process leading to the higher energy tautomer is theoretically characterized which lies ~11.5 kcal/mol above the ground state of quinizarin.

Chromogenic properties of 2-(2-carbomethoxy-3,4-dichloro-6-hydroxyphenyl)benzoxazole and its Zn(II) and Cd(II) complexes

The chromogenic properties of 2-(2-carbomethoxy-3,4-dichloro-6-hydroxyphenyl)benzoxazole (1) were studied. Solvatochromism of 1 was observed at keto-enol equilibrium due to the ground state intramolecular proton transfer (GSIPT). It was found that solvents with a good hydrogen bond acceptor (β) ability were able to stabilise the keto form of 1. The excited-state intra molecular proton transfer (ESIPT) in molecules of 1 resulted in intense fluorescence with an anomalous Stokes shift of up to 8493 cm−1 and quantum yields of 0.08–0.19. The keto-form of 1 exhibits negative T-type photochromism with a thermally reversible photoconversion to the enol form. The complexes of ligand 1 with Zn(II) and Cd(II) were synthesised. According to X-ray structural analysis, the zinc complex is the neutral complex, which is formed by two oxazole-N and two phenol O donors of ligand 1 which crystallise in a centrosymmetric group with the zinc ion located on the inversion centres. The synthesised complexes possess effective blue fluorescence and resistance to photodegradation.

Experimental and theoretical investigation of ground state intramolecular proton transfer (GSIPT) in salicylideneaniline Schiff base derivatives in polar protic medium.

Ground state intramolecular proton transfer process has been comprehensively investigated in three salicylideneaniline Schiff base derivatives (SB1, SB2, and SB3) using experimental and theoretical methods. It has been confirmed that all the three Schiff base molecules in the ground electronic state exist in the enol form in non-polar and polar aprotic solvents. Keto form is being populated by the polar protic solvent through ground state intramolecular proton transfer (GSIPT) process. Ground state equilibrium between the enol and keto tautomers for SB1 and SB3 is mainly governed by the proton donating ability of the solvent. Ground state equilibria between the enol and keto tautomers of SB2 which is a positional isomer of SB3 is governed by the polarity and proton donating ability of the solvents. Excited state intramolecular proton transfer (ESIPT) process is also evidenced in all the three Schiff base molecules. Theoretical calculations at the B3LYP/cc-pVDZ level in the gas phase and in different solvents using polarisable continuum model (PCM) have failed to establish the GSIPT process. Microsolvation of individual enol and keto conformers has been investigated considering upto three solvent molecules. The energetics of the individual conformers together with the corresponding transition state have been calculated. It has been confirmed that the keto conformer is more stable compared to the enol conformer in microsolvated cluster of three methanol molecules. Lowering of activation energy for the enol to keto tautomerisation in the presence of methanol also supports the experimental observation for GSIPT process. TDDFT/B3LYP/cc-pVDZ single point calculations for microsolvated clusters of enol and keto form of the Schiff base molecules exhibit an excellent agreement with the experimentally obtained absorption spectra. Difference in spectral nature of the Schiff base molecules has been explained using natural bond orbital (NBO) analysis. Quantum theory of atoms in molecules (QTAIM) has also been utilised to understand the GSIPT process in detail.

Water-assisted ground state intra-molecular proton transfer in 2,5-dihydroxy-substituted azobenzenes: experimental and computational studies

The phenomenon of ground-state proton transfer in a series of 2,5-dihydroxy azobenzene derivatives has been studied. In addition, the effect of the substitutions and water molecules has been investigated by the theoretical calculation.

Time-Resolved Spectroscopic and Density Functional Theory Investigation of the Photogeneration of a Bifunctional Quinone Methide in Neutral and Basic Aqueous Solutions.

Binol quinone methides (BQMs) can be generated from 1,1′-(2,2′-dihydroxy-1,1′-binaphthyl-6,6′-diyl)bis(N,N,N-trimethylmethanamiuium) bromide (BQMP-b) in a 1:1 MeCN:H2O mixed solution via a ground state intramolecular proton transfer (GSIPT), as mentioned in our previously reported studies. Here, the photoreaction of BQMP-b in neutral and basic aqueous solution (pH = 7, 10, 12) was investigated to explore the possible mechanisms and the key intermediates produced in the process of the photoreaction and to examine whether they are different from those in a neutral mild-mixed MeCN:H2O solution. The studies were conducted using femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3) in conjunction with results from density functional theory (DFT) computations. The results showed that BQMP-b was deprotonated initially and produced BQMs species more effectively through an E1bc elimination reaction in a strong basic aqueous condition (pH = 12), which differed from the reaction pathway that took place in the solution with pH = 7 or 10. A related single naphthol ring molecule 1-(6-hydroxynaphthalen-2-yl)-N,N,N-trimethylmethanaminium bromide (QMP-b) that did not contain a second naphthol ring was also investigated. The related reaction mechanisms are elucidated in this work, and it is briefly discussed how the mechanisms vary as a function of aqueous solution pH conditions.

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy.

4'- N, N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump-dump-probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump-probe and pump-dump-probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.

Influence of Water in the Photogeneration and Properties of a Bifunctional Quinone Methide.

Quinone methides (QM) are crucial reactive species in molecular biology and organic chemistry, with little known regarding the mechanism(s) for the generation of short-lived reactive QM intermediates from relevant precursors in aqueous solutions. In this study, several time-resolved spectroscopy methods were used to directly examine the photophysics and photochemical pathways of 1,1'-(2,2'-dihydroxy-1,1'-binaphthyl-6,6'-diyl)bis(N,N,N-trimethylmethanaminium) bromide (BQMP-b) from initial photoexcitation to the generation of the key reactive binol QM intermediate (BQM) in aqueous solution. The fluorescence of BQMP-b is effectively quenched with a small amount of water, which suggests an excited state intramolecular proton transfer (ESIPT) occurs. The kinetics isotope effects observed in femtosecond and nanosecond time-resolved transient absorption experiments provide evidence for the participation of water molecules in the BQMP-b singlet excited state ESIPT process and in the subsequent -HNMe3+ group release and ground state intramolecular proton transfer that give rise to production of the reactive BQM intermediate. Nanosecond time-resolved resonance Raman (ns-TR3) measurements were also employed to investigate the structure and properties of several intermediates, including the key reactive BQM in aqueous solution. The ns-TR3 and density functional theory (DFT) computational results were compared, and this indicates the binol moiety and water molecules both have important roles in the characteristics and structure of the key reactive BQM intermediate produced from BQMP-b. The results presented here also provide new benchmark characterization of bifunctional quinone methide intermediates that can be utilized to guide direct time-resolved spectroscopic study of the alkylation and interstrand cross-linking reactions of quinone methides with DNA in the future.

Excited state proton transfer coupled with twisted intermolecular charge transfer for N,N-dimethylanilino-1,3-diketone in high polar acetonitrile solvent

In the present work, N,N-dimethylanilino-1,3-diketone (DMADK), a new chromophore of the unsymmetrically substituted 1,3-diketone [R. Ghosh and D. K. Palit, Photochem. Photobiol. Sci. 12 (2013) 987–995], has been investigated about the excited state proton transfer (ESPT) based on the time-dependent density functional theory (TDDFT) method. The experimental UV/Vis and emission spectra are well reproduced by the calculated vertical excitation energies in the S0 and S1 states. For the optimized Enol-B* structure, the twisted intermolecular charge transfer (TICT) process can be confirmed in the S1 state. Hydrogen bond strengthening has been testified in the S1 state based on comparing primary bond lengths and bond angles involved in the intramolecular hydrogen bond between the S0 state and the S1 state. Furthermore, infrared spectra (IR) at the OH stretching vibrational region and Molecular electrostatic potential surface (MEPS) based on our calculation also declare the phenomenon of hydrogen bond strengthening. The frontier molecular orbitals (MOs) analysis, MEPS, Mulliken's charge distribution analysis, Hirshfeld charge distribution analysis and Natural bond orbital (NBO) analysis methods manifest the intramolecular charge transfer, which reveals the tendency of excited state intramolecular proton transfer (ESIPT) process. The constructed PESs of both S0 and S1 states demonstrate that ground state intramolecular proton transfer (GSIPT) as well as reversed GSIPT processes exists in the S0 state, and the ESIPT coupled with the TICT process can occur in the S1 state rather than sequential ESIPT and TICT processes.

Polarizability, hardness and electrophilicity as global descriptors for intramolecular proton transfer reaction path

Potential energy (PE) curves for intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states of 1-hydroxy-2-acetonaphthone (1H2AN) is studied using DFT-B3LYP/6-31+G(d,p) and TD-DFT/6-31+G(d,p) level of theory respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer in 1H2AN. Excited states PE calculations support the viability of ESIPT process in 1H2AN. Here, for the first time, polarizability, chemical hardness and electrophilicity are being used as global reactivity descriptors to locate the transition state for intermolecular proton transfer process. Both the minimum polarizability principle (MPP) and maximum hardness principle (MHP) are being obeyed along the intrinsic reaction co-ordinate (IRC) for intramolecular proton transfer process. We have also raised the decade old issue i.e. the use of O–H distance of enol tautomer as proton transfer co-ordinate instead of IRC. Our computation of these global reactivity descriptors along the proton transfer co-ordinate support intrinsic reaction coordinate (IRC) as the effective proton transfer co-ordinate, instead of variation of O–H distance of enol tautomer.

Excited state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)benzoxazole and its naphthalene-fused analogs: A TD-DFT quantum chemical study

The intramolecular proton transfer reactions in 2-(2′-hydroxyphenyl)benzoxazole (HBO) and its naphthalene-fused analogs, (HNB1–3) in both S0 and S1 states at the PBE1PBE/6-311++G(2d,2p) level of theory in the gas phase and water have been investigated to find the effects of extension of aromaticity on the intramolecular proton transfer and photophysical properties. The results show that the ground state intramolecular proton transfer (GSIPT) in the studied species is impossible. Excited states potential energy surface calculations support the existence of ESIPT process. Structural parameters, relative energy of isomers, H-bonding energy, adsorption and emission bands, vertical excitation and emission energies, oscillator strength, fluorescence rate constant, dipole moment, atomic charges and electron density at critical points were calculated. Orbital analysis shows that vertical S0→S1 transition in the studied molecules corresponds essentially to the excitation from HOMO (π) to LUMO (π*). The potential of HNB2 molecule as an emissive and electron transport material in designing improved organic white light emitting diodes is predicted in this work. Our calculations are also supported by the experimental observations.