Control over excited state intramolecular proton transfer and photoinduced tautomerization: influence of the hydrogen-bond geometry

Exploring and Regulating Heteroatom-Electronegativity-Associated with Ring Aromaticity and Excited State Intramolecular Proton Transfer Mechanism for Benzothiazole-Based Fluorophore

A novel fluorophore 2-(2'-hydroxy-5'-benzaldehyde phenyl) benzothiazole (HBBT) with excited state intramolecular proton transfer (ESIPT) characteristics, showing good selectivity for Cu+/Cu2+ ions had been synthesized experimentally (Molecules 2022, 27, 7678). However, its ESIPT mechanism and fluorescent performance related to atomic substituents have not been investigated systematically. In this work, two HBBT derivatives, 2-(2'-hydroxy-5'-benzaldehyde phenyl)benzoimidazole (HBBI) and 2-(2'-hydroxy-5'-benzaldehyde phenyl)benzopyrrole (HBBP), were obtained by respectively using -NH and -CH2 groups in place of the sulfur atom in the thiazole ring. The absorption/emission spectra and ring aromaticity as well as ESIPT processes of HBBT and its derivatives were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT). The simulated absorption and fluorescence wavelengths of HBBT agreed well with the corresponding values obtained in the experiment. According to the analyses of geometry structures, electron densities, and infrared vibration frequencies, the intramolecular hydrogen bond becomes stronger upon light excitation. The frontier molecular orbitals were analyzed via establishing potential energy curves, and the ESIPT behavior was described deeply. Obviously, the NH-substitution makes ring 4 more aromatic, while the CH2-substitution changes ring 4 from aromatic to antiaromatic. The ESIPT process helps to alleviate the excited state antiaromaticity. The greater the antiaromaticity of the S1 state normal form, the higher the barrier of ESIPT.

When a Twist Makes a Difference: Exploring PCET and ESIPT on a Nonplanar Hydrogen-Bonded Donor-Acceptor System

Bioinspired benzimidazole-phenol constructs with an intramolecular hydrogen bond connecting the phenol and the benzimidazole have been synthesized to study both proton-coupled electron transfer (PCET) and excited-state intramolecular proton transfer (ESIPT) processes. Strategic incorporation of a methyl group disrupts the coplanarity between the aromatic units, causing a pronounced twist, weakening the intramolecular hydrogen bond, decreasing the phenol redox potential, reducing the chemical reversibility, and quenching the fluorescence emission. Infrared spectroelectrochemistry and transient absorption spectroscopy confirm the formation of the oxidized product upon PCET and probe excited-state relaxation mechanisms, respectively. Density functional theory calculations of redox potentials corroborate the experimental findings. Additionally, time-dependent density functional theory calculations uncover the fluorescence quenching mechanism, showing that the nonradiative twisted intramolecular charge transfer state responsible for fluorescence quenching is more energetically favorable in the methyl-substituted system. Incorporating groups causing steric hindrance expands the design of biomimetic systems capable of performing both PCET and ESIPT.

Fluorescence quenching of deprotonated phenylurea through twisting motion induced by an electron-donating substituent group

The excited-state proton transfer (ESPT) reaction between anthracen-2-yl-3-phenylurea (PUA) derivatives and tetrabutylammonium acetate (TBAAc) in dimethyl sulfoxide (DMSO) solvent was theoretically investigated using time-dependent density functional theory. The electron-donating methoxy group (OMe) and electron-withdrawing trifluoromethyl group (CF3) were bonded to 2PUA to form OMe-2PUA and CF3-2PUA, respectively. Two hydrogen bonds formed in the 1 : 1 hydrogen-bonded complexes between the 2PUA derivative and acetate ion (AcO-), namely N1-H1⋯O1 and N2-H2⋯O2. Strong charge transfer (CT) due to the electron-donating OMe group led to H1 transfer in the S1 state for the OMe-2PUA:AcO- hydrogen-bonded complex. On the contrary, weak CT due to the electron-withdrawing CF3 group led to H2 transfer in the S1 state for CF3-2PUA. After the ESPT reaction, the binding energies of the hydrogen-bonded complexes strongly decreased in both cases, and this promoted the separation of contact-ion pairs (CIPs*) and formed different types of anionic species. CF3-2PUA- could keep its nearly planar structure in the S1 state and emit "abnormal" fluorescence. On the other hand, the anionic OMe-2PUA- underwent a twisting motion to form a twisted structure in the S1 state with very low energy, and this led to a rapid internal conversion (IC) to quench long-wave fluorescence in the emission spectra.

Excited-State Proton Transfer in Luminescent Dyes: From Theoretical Insight to Experimental Evidence

The effective utilization of luminescent dyes often relies on a comprehensive understanding of their excitation and relaxation pathways. One such pathway, Excited-State Proton Transfer (ESPT), involves the tautomerization of the dye in its excited state, resulting in a new structure that exhibits distinct emission properties, such as a very large Stokes' shift or dual-emission. Although the ESPT phenomenon is well-explained theoretically, its experimental demonstration can be challenging due to the presence of numerous other phenomena that can yield similar experimental observations. In this review, we propose that an all-encompassing methodology, integrating experimental findings, computational analyses, and a thorough evaluation of diverse mechanisms, is essential for verifying the occurrence of ESPT in luminescent dyes. Investigations have offered significant understanding of the elements impacting the ESPT process and the array of approaches that can be used to validate the existence of ESPT. These discoveries hold crucial ramifications for the advancement of molecular probes, sensors, and other applications that depend on ESPT as a detection mechanism.

Effects of solvent polarity on the novel excited-state intramolecular thiol proton transfer and photophysical property compared with the oxygen proton transfer

Recently, the dual-fluorescent phenomena of excited state intramolecular thiol proton transfer (ESIPT) for 3-thiolflavone derivative (3NTF) were reported by Chou and coworkers for the first time [J. Am. Chem. Soc. 143 (2021) 12715-12724], which opened a new chapter in the field of ESIPT. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT), the proton transfer processes of 3NTF in toluene, dichloromethane and acetonitrile were studied. By optimizing the structure of the ground (S0) state and first excited (S1) state of 3NTF in different solvents, the hydrogen-bond parameters and proton-transfer potential energy curves were calculated. It was shown that although photo-excitation enhanced the intramolecular hydrogen bonding strength and thus promoted the occurrence of ESIPT, the solvent polarities inhibited the enhancement of the hydrogen bond of S1 state, which was not conducive to ESIPT. The electron spectra analyses were consistent with experimental data, which confirmed the rationality of molecular configurations. The time-evolved excited state dynamics simulation was performed based on the optimized structure of 3NTF, indicating that the ESIPT was an ultrafast photochemical reaction less than 180 fs. Moreover, we compared the potential energy surfaces of ESIPT, electronic structures based on natural transition orbitals (NTOs) method and electron-hole isosurfaces for the 3NTF and the traditional flavone molecule (3NHF), concluded that the unusually large Stokes shift fluorescence of 3NTF was mainly caused by the coupling of ESIPT and twisting intramolecular charge transfer (TICT), and the 3NTF isomer had the more nπ* character in the electron transition process. The nπ* ICT significantly increased with the decrease of solvent polarities, affecting the molecular photophysical properties, this made it more widely used in biomedical, photochemical, materials science and other fields.

ESIPT in the pyrrol pyridine molecule: mechanism, timescale and yield revealed using dynamics simulations

Excited State Intramolecular Proton Transfer in pyrrol pyridine is theoretically investigated using non-adiabatic dynamics simulations. The photochemical process is completely characterised: the reaction time, the total yield and the accessibility of the conical intersection are evaluated. Finally, new mechanistic interpretation are extracted: the proton transfer reaction in this molecule is shown to be driven by two complementary mechanisms.

Fluorescence monitoring of binding of a Zn (II) complex of a Schiff base with human serum albumin

Zn (II) complexes of Schiff bases have potential applications in biomedical sciences as imaging agents, cancer therapeutics and diagnostics. Thus, it is important to understand their interaction with carrier proteins, like serum albumins. The present paper focuses on the binding interactions between Human serum albumin (HSA) and Znsalampy, making use of fluorescence spectroscopic techniques at ensemble as well as at single molecular level. An idea about the binding constant is obtained from the quenching of the single Trp (Tryptophan) residue of HSA by Znsalampy. Fluorescence correlation spectroscopy (FCS) has also been used to monitor the protein-ligand binding. The location of Znsalampy in its complex with HSA is determined by competitive binding experiments and molecular docking calculations. The binding constant obtained from the Znsalampy-HSA interaction falls in the ideal range for biological applications and the location is found to be in the proximity of Sudlow's site I. The esterase activity of HSA is retained in the presence of the Znsalampy. Hence, it is concluded that this Znsalampy may be a potential probe and biomarker in biomedical applications.

Wavelength-dependent photochemistry of a salicylimine derivative studied with cryogenic and ultrafast spectroscopy approaches

Salicylimines are versatile compounds in which an excited-state intramolecular proton transfer and torsional motions may set in upon photoexcitation. Here, we study N-(α-phenylethyl)salicylimine (PESA) to elucidate how the photochemical reaction pathways depend on the excitation wavelength and to what extent the relative photoproduct distribution can be steered towards a desired species. DFT structure and potential energy calculations disclose that the most stable ground-state conformer is an enol species and that the photodynamics may proceed differently depending on the excited state that is reached. With matrix isolation infrared spectroscopy, the predominance of the enol conformer of PESA is confirmed. Illumination of the cryogenic sample with different wavelengths shifts the ratio of enol and keto products, and by sequential irradiation a selective re- and depopulation is possible. Femtosecond transient absorption spectroscopy further reveals that also at room temperature, the outcome of the photoreaction depends on excitation wavelength, and in combination with the calculations, it can be rationalized that the decisive step occurs within the first hundred femtoseconds. Since the ultrafast dynamics mostly match those of similar salicylimines, our findings might also apply to those systems and provide additional insight into their reported sensitivity on excitation energy.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal₂ with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λ_max = 600-650 nm). The coordination of HLq by Zn²⁺ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S₁ state, leading to the only possible fluorescence channel in the tautomeric form (T), S₁^T → S₀^T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH₂Cl₂ solutions, the compounds demonstrate dual emission in the UV (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁^N → S₀^N and S₁^T → S₀^T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S₀^T, resulting in the possibility of the S₀^T → S₁^T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part I: Dual Emission from the Neutral Species

The effects of 5'-(para-R-phenylene)vinylene (PV) substituents on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO) are analyzed using steady-state and time-resolved absorption and emission spectroscopies in addition to quantum chemical calculations. All members in the series of PVHBOs are capable of excited-state intramolecular proton transfer (ESIPT) with a solvent sensitivity that is typical of a HBO derivative to produce a normal (aka enol) emission and an excited-state tautomer (aka keto) emission. These two emission bands of the neutral dyes are discussed in the current paper. The intermolecular proton transfer, i.e., the deprotonation, of a PVHBO results in the third band of the triple emission, which is described in the succeeding paper. The placement of an electron-withdrawing substituent R on the PVHBO scaffold increases the intensity of the keto emission relative to the enol emission in hydrogen-bonding solvents. The R substituents do not significantly alter the wavelengths of the enol and keto emission bands, which are located in the blue and green regions, respectively, of the visible spectrum. The ultrafast time-resolved spectroscopies and quantum chemical calculations offer explanations on how the R group and the solvent affect the enol and keto emission properties (i.e., wavelength, lifetime, fluorescence quantum yield, and relative ratio of their emissions). The key findings include the following: (1) the emission energies of both enol and keto forms are not sensitively dependent on the R substituent and (2) the solvent-engaged enol excited state is quenched more efficiently as the R substituent becomes more electron-withdrawing. A PVHBO acts as a fusion of HBO and stilbenoid that intersect at the hydroxyphenyl moiety. Depending on the solvent and other environmental conditions, PVHBOs may exhibit the ESIPT property of HBO or the substituent-dependent emission of stilbenoid. This paper and the succeeding article provide a photophysical model of PVHBOs to explain the wavelengths and relative abundances of the three emission bands (enol, keto, and anion) that these compounds are able to produce. Judicial selection of the environmental factors may drive the emission of a PVHBO into the spectral regions of blue, green, and, in a couple of cases, orange or red.

Lack of the ESIPT band of aromatic ortho-aminoaldehyde derivatives triggered by N-H vibration

The excited state intramolecular proton transfer (ESIPT) reactions and the fluorescence emission spectra of o-aminoaldehyde and o-aminoketone derivatives were systematically studied with density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results suggest that the ESIPT process can be characterized as an ultra-fast process and that N-H vibration plays an important role in fluorescence emissions. The minimum energy pathways (MEP) on the excited states along the proton transfer coordinates (N-H vibration) were constructed for both o-aminoaldehyde and o-aminoketone derivatives, respectively, which showed a small barrier between the normal and tautomer (ESIPT) structures. By comparing the proton transfer barriers (E_b) and the N-H reorganization energies (λeleNH), we find that λeleNH is less than E_b in o-aminoketone derivatives, while λeleNH is greater than E_b in o-aminoaldehyde derivatives. It is clear that protons could move freely in o-aminoaldehyde derivatives, and thus only one normal emission band could be observed. Subsequently, the fluorescence emission spectra upon introduction of the N-H vibration effect can further confirm this mechanism, and the simulated spectra are in good agreement with the experimental observations, in which the o-aminoaldehyde derivatives have only one normal emission band while the o-aminoketone derivatives have two emission bands corresponding to the normal and tautomer structures. Consequently, this work can elucidate the lack of the ESIPT band in o-aminoaldehyde derivatives and also offer new insight into ESIPT by considering the vibronic effect.

Excited-state intramolecular proton transfer with and without the assistance of vibronic-transition-induced skeletal deformation in phenol-quinoline

The excited-state intramolecular proton transfer (ESIPT) reaction of two phenol-quinoline molecules (namely PQ-1 and PQ-2) were investigated using time-dependent density functional theory. The five-(six-) membered-ring carbocycle between the phenol and quinolone moieties in PQ-1 (PQ-2) actually causes a relatively loose (tight) hydrogen bond, which results in a small-barrier (barrier-less) on an excited-state potential energy surface with a slow (fast) ESIPT process with (without) involving the skeletal deformation motion up to the electronic excitation. The skeletal deformation motion that is induced from the largest vibronic excitation with low frequency can assist in decreasing the donor-acceptor distance and lowering the reaction barrier in the excited-state potential energy surface, and thus effectively enhance the ESIPT reaction for PQ-1. The Franck-Condon simulation indicated that the low-frequency mode with vibronic excitation 0 → 1' is an original source of the skeletal deformation vibration. The present simulation presents physical insights for phenol-quinoline molecules in which relatively tight or loose hydrogen bonds can influence the ESIPT reaction process with and without the assistance of the skeletal deformation motion.

H2O-Bridged Proton-Transfer Channel in Emitter Species Formation in Obelin Bioluminescence

Bioluminescence of a number of marine organisms is conditioned by Ca²⁺-regulated photoprotein (CaRP) with coelenterazine as the reaction substrate. The reaction product, coelenteramide, at the first singlet excited state (S₁) is the emitter of CaRP. The S₁-state coelenteramide is produced via the decomposition of coelenterazine dioxetanone. Experiments suggested that the neutral S₁-coelenteramide is the primary emitter species. This supposition contradicts with theoretical calculations showing that the anionic S₁-coelenteramide is a primary product of the decomposition of coelenterazine dioxetanone. In this study, applying molecular dynamic (MD) simulations and the hybrid quantum mechanics/molecular mechanics (QM/MM) method, we investigated a proton-transfer (PT) process taking place in CaRP obelin from Obelia longissima for emitter formation. Our calculations demonstrate a concerted PT process with a water molecule as a bridge between anionic S₁-coelenteramide and the nearest histidine residue. The low activation barrier as well as the strong hydrogen-bond network between the proton donor and the proton acceptor suggests a fast PT process comparable with that of the lifetime of excited anionic S₁-coelenteramide. The existence of the PT process eliminates the discrepancy between experimental and theoretical studies. The fast PT process at emitter formation can also take place in other CaRPs.

From fluorogens to fluorophores by elucidation and suppression of ultrafast excited state processes of a Schiff base

Strategies have been explored for developing strongly fluorescent species out of a weakly fluorescent Schiff base, 2-(((pyridin-2-ylmethyl)imino)methyl)phenol (salampy). The locally excited enolic state of salampy undergoes an intramolecular proton transfer with a time constant of ca. 200 fs. The emissive cis keto state thus formed decays completely within 50 ps. Its fast decay and miniscule fluorescence quantum yield are attributed to efficient non-radiative channels associated with conformational relaxation. The anionic form, salampy^-, has a significantly longer fluorescence lifetime of 800 ps. Its emissive state evolves in tens of picoseconds, from the locally excited state, by solvent and conformational relaxation. Both the neutral and anionic forms have a fluorescence lifetime of about 6 ns at 77 K, a temperature at which all activated nonradiative channels are blocked. This lifetime is similar to that obtained at room temperature, upon rigidification of the anion by complexation with Zn²⁺. Two such complexes have been studied. The first is binuclear, with acetate bridge between the two Zn²⁺ ions. The second, with ClO₄^- as the counterion, is mononuclear with two salampy ligands ligating the metal ion. Unlike a previous report on a different Schiff base, in which the ligands are π-stacked in its dimeric Zn²⁺ complex, no additional nonradiative deactivation pathway opens up in the Zn complexes of salampy, which are devoid of such stacking. The complex of salampy with Al³⁺ has an even longer fluorescence lifetime of 9 ns, indicating a greater degree of rigidification and consequent suppression of nonradiative processes.

Nature of Fast Relaxation Processes and Spectroscopy of a Membrane-Active Peptide Modified with Fluorescent Amino Acid Exhibiting Excited State Intramolecular Proton Transfer and Efficient Stimulated Emission

A fluorescently labeled peptide that exhibited fast excited state intramolecular proton transfer (ESIPT) was synthesized, and the nature of its electronic properties was comprehensively investigated, including linear photophysical and photochemical characterization, specific relaxation processes in the excited state, and its stimulated emission ability. The steady-state absorption, fluorescence, and excitation anisotropy spectra, along with fluorescence lifetimes and emission quantum yields, were obtained in liquid media and analyzed based on density functional theory quantum-chemical calculations. The nature of ESIPT processes of the peptide's chromophore moiety was explored using a femtosecond transient absorption pump-probe technique, revealing relatively fast ESIPT velocity (∼10 ps) in protic MeOH at room temperature. Efficient superluminescence properties of the peptide were realized upon femtosecond excitation in the main long-wavelength absorption band with a corresponding threshold of the pump pulse energy of ∼1.5 μJ. Quantum-chemical analysis of the electronic structure of the peptide was performed using the density functional theory/time-dependent density functional theory level of theory, affording good agreement with experimental data.

Excited-State Intramolecular Proton Transfer: A Short Introductory Review

In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller’s discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT.

Role of Skeletal and O-H Vibrational Motions in the Ultrafast Excited-State Relaxation Dynamics of Alizarin

The role of two skeletal (C═C and C═O stretch) and O-H vibrational motions in the internal conversion dynamics associated with the coupled S₁(ππ*, A') -S₂(nπ*, A″) potential energy surfaces of alizarin are investigated theoretically. Quantum wavepacket dynamics simulations reveal a nonadiabatic population transfer from the "bright" S₁(ππ*, A') to "dark" S₂(nπ*, A″) state on a time scale of 10 fs. A detailed analysis of computed structural parameters, energetics, and time-dependent observables suggest that these vibrations promote the nonadiabatic dynamics before initiating the proton transfer process. We also discuss how the simultaneous evolution of multidimensional dynamics involving several vibrational degrees of freedom would increase the complexity, while analyzing the spectral and kinetic data of time-resolved spectroscopy measurements.

Excited-state intramolecular proton transfer in the kinetic-control regime

A new series of molecules bearing a 2,11-dihydro-1H-cyclopenta[de]indeno[1,2-b]quinoline (CPIQ) chromophore with the N-HN type of intramolecular hydrogen bond are strategically designed and synthesized, among which CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs in solution exhibit a single emission band with an anomalously large Stokes shift, whereas CPIQ-NH2 and CPIQ-NHMe show apparent dual-emission property. This, in combination with time-resolved spectroscopy and the computational approach, leads us to conclude that CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs undergo ultrafast, highly exergonic excited-state intramolecular proton transfer (ESIPT), while a finite rate of ESIPT is observed for CPIQ-NH2 and CPIQ-NHMe with a time constant of 117 ps and 39 ps, respectively, in acetonitrile at room-temperature. Further temperature-dependent studies deduce an appreciable ESIPT barrier for CPIQ-NH2 and CPIQ-NHMe. Different from most of the barrier associated ESIPT molecules that are commonly in the thermodynamic-control regime, i.e. found in the thermal pre-equilibrium between excited normal and proton-transfer tautomer states, CPIQ-NH2 and CPIQ-NHMe cases are in the kinetic-control regime where ESIPT is irreversible with a significant barrier. The barrier is able to be tuned by the electronic properties of the -R group in the NR-H proton donor site, resulting in ratiometric fluorescence for normal versus tautomer emission.

Effect of Moderate Hydrogen Bonding on Tautomer Formation via Excited-State Intermolecular Proton-Transfer Reactions in an Aromatic Urea Compound with a Steric Base

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), which forms weak hydrogen bonds despite the high basicity caused by its hindered structure, was used to investigate tautomer formation via excited-state intermolecular proton-transfer (ESPT) reactions. The kinetics of the ESPT reactions of anthracen-2-yl-3-phenylurea (2PUA) in the presence of DBU were compared to that observed for the acetate anion (Ac) using time-resolved fluorescence measurement. Based on the association constants in the ground state, the intermolecular hydrogen bond between 2PUA and DBU was less stable than the bond between 2PUA and Ac due to steric hindrance and the geometry of the hydrogen bond. In the fluorescence spectra, 2PUA-DBU displayed prominent tautomeric emission in chloroform (CHCl₃), whereas 2PUA-Ac exhibited distinct tautomeric emissions in dimethyl sulfoxide (DMSO). Kinetic analysis revealed that the rate constant of the ESPT reaction of 2PUA-DBU remarkably decreased when the proton-accepting ability of the solvent increased whereas the reaction of 2PUA-Ac was linked to the solvent polarity rather than proton-accepting ability. These results indicated that moderate hydrogen bonds due to steric hindrance were influenced by the type of solvent present, particularly if the solvents exhibited proton-accepting capabilities like DMSO. This, in turn, affected the rate constant of tautomer formation.

Time-Dependent Density Functional Theory Investigation of Excited State Intramolecular Proton Transfer in Tris(2-hydroxyphenyl)triazasumanene

Previously, our group reported dual-emission spectra for tris(2-hydroxyphenyl)triazasumanene (OHPhTAS), comprising three OH···N-type intramolecular hydrogen bonds from three phenolic rings connected to the nitrogen-doped buckybowl skeleton, corresponding to the excited state intramolecular proton transfer (ESIPT) in the solid state. However, the dual emission is not observed in a nonpolar solution. In this study, the mechanism and multiplicity of potentially photoinduced dynamic ESIPT were investigated both in ground (S₀) and in excited states (S₁) by time-dependent density functional theory calculations. Different pathways, concerted and stepwise (single, double, or triple) PT processes, are considered. The calculated vertical emission energies (S₁ → S₀ states) and adiabatic total energies at S₀ and S₁ states of OHPhTAS and its tautomers revealed that a single PT, trienol (EEE) → monoketo (KEE), is the main contribution in OHPhTAS with an ultrasmall PT energy barrier. The nonradiative decay of OHPhTAS was analyzed by the potential energy curve (PEC) at the S₁ state along EEE* to KEE*. The results indicated that nonradiative decay was prohibited in the solid state but significantly stabilized in nonpolar solutions. The nonradiative routes in the solution state were confirmed by the minimum energy crossing point of the T₁/S₀ pathway, wherein the dihedral angle φ between the phenolic ring and pyridine moiety on the buckybowl structure relaxed to 123°.

Substituent Effects on Photochemistry of Anthracene-Phenol-Pyridine Triads Revealed by Multireference Calculations

Inverted region behavior for concerted proton-coupled electron transfer (PCET) was recently demonstrated for biomimetic anthracene-phenol-pyridine molecular triads. Photoexcitation of the anthracene to a locally excited state (LES) is followed by concerted electron transfer from the phenol to the anthracene and proton transfer from the phenol to the pyridine, forming a relatively long-lived charge separated state (CSS). The long-lived CSS and the inverted region behavior associated with the decay from the CSS to the ground state through charge recombination were experimentally observed only for triads with certain substituents on the anthracene and the pyridine. To explain this distinction, we computed the proton potential energy curves in four substituted triads using the complete active space self-consistent-field method and multireference perturbation theory, including solvent effects with a dielectric continuum model. The calculations revealed a local electron-proton transfer (LEPT) state, in which both the electron and proton transfer from the phenol to the pyridine. When the LEPT state is lower in energy than the CSS, it may provide an alternative pathway for fast decay from the LES to the ground state and thereby preclude detection of the CSS and the inverted region behavior. These calculations predict that substituents stabilizing negative charge on the pyridine and destabilizing negative charge on the anthracene will favor the LEPT pathway, while substituents with the reverse effects will favor the CSS pathway, which could exhibit inverted region behavior. These insights about the stabilization of energy-storing charge-separated states have implications for designing and controlling PCET reactions in artificial photosynthetic systems and other energy conversion processes.

Fine-tuned dual fluorescence behavior of N-substituted aniline-imidazopyridine based switches: Mechanistic understanding, substituent and solvent effects

In order to understand the fine-tuned photo-physical behaviors of the N(X)-H⋯N systems, the excited state intramolecular proton transfer (ESIPT) switching in the N-substituted X_1-5-NHIPA molecules (NHIPA = 2-(imidazo[1,2-a]pyridin-2-yl)aniline and X = H, COCH₃, CH₃C₆H₄SO₂, C₆F₅SO₂, and COCF₃) were investigated in detail in gas phase and three solvent media at TD-PBE0/6-311++G(d,p) and TD-M06-2X/6-311++G(d,p) levels of theory. ESIPT reactions at S₁ state were approximately without energy barrier, exergonic processes and were quantitatively demonstrated to be mainly sensitive to substituents and solvent media. The X-NHIPA (X = CH₃C₆H₄SO₂, C₆F₅SO₂ and CF₃CO) compounds were predicted to undergo fast, irreversible proton transfer at S₁ state and, in turn, exhibit tautomer emission with anomalous large Stokes shift in the gas phase. In the toluene solvent, except for C₆F₅SO₂-NHIPA that showed exclusively a tautomer emission with a long wavelength, all other X-NHIPA molecules were predicted to involve in the reversible ESIPT and hence exhibit a dual normal and tautomer emissions behavior, in good agreement with the experimental observations. In polar solvents, it is expected that all compounds show dual normal and tautomer emissions. The nearly equal intensities of the normal and tautomer emissions can lead to the generation of the white lights with the potential in lighting applications.

Photophysics of proton transfer in hydrazides: a combined theoretical and experimental analysis towards OLED device application

Effect of conjugation to support ESIPT with impossible double proton transfer in structurally favored species.

The excited-state intramolecular proton transfer in NH-type dye molecules with a seven-membered-ring intramolecular hydrogen bond: A theoretical insight

Excited-state intramolecular proton transfer (ESIPT) reactions of a series of N(R)H⋯N-type seven-membered-ring hydrogen-bonding compounds were explored by employing density functional theory/time-dependent density functional theory calculations with the PBE0 functional. Our results indicate that the absorption and emission spectra predicted theoretically match very well the experimental findings. Additionally, as the electron-withdrawing strength of R increases, the intramolecular H-bond of the NS₁ form gradually enhances, and the forward energy barrier along the ESIPT reaction gradually decreases. For compound 4, its ESIPT reaction is found to be a barrierless process due to the involvement of a strong electron-withdrawing COCF₃ group. It is therefore a reasonable presumption that the ESIPT efficiency of these N(R)H⋯N-type seven-membered-ring H-bonding systems can be improved when a strong electron-withdrawing group in R is introduced.

Dynamics of excited state proton transfer in nitro substituted 10-hydroxybenzo[h]quinolines

The ground state tautomerism and excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo[h]quinoline (HBQ) and its nitro derivatives, 7-nitrobenzo[h]quinolin-10-ol (2) and 7,9-dinitrobenzo[h]quinolin-10-ol (3), have been studied in acetonitrile using steady state as well as time dependent spectroscopy and quantum-chemical calculations. In addition to the enol form absorbance in the range 360-390 nm, the absorption spectra of 2 and 3 exhibit a red shifted band at ∼450 nm. Chemometric data processing, based on individual band decomposition, allowed us to estimate the position of the ground state enol-keto tautomeric equilibrium (ΔG values of 1.03 and 0.62 kcal mol^-1 respectively for 2 and 3). The fluorescence stems from the keto form even if the enol form is optically excited as proven by the shape of the excitation spectra indicating that ESIPT takes place. The Stokes shift of the substituted compounds is substantially lower compared to HBQ, which follows from the fact that the substitution occurs in the formal cyclohexa-2,4-dienone moiety and leads to a decrease of the HOMO level of the keto tautomer. The pump-probe experiments show that in the nitro substituted HBQs 2 and 3 ESIPT occurs with a time constant of 0.89 ps and 0.68 ps, respectively. In both cases a mixture of the enol and proton transfer forms is optically excited. The enol form exhibits then the ESIPT and subsequently both fractions take the same relaxation path. We propose that in 2 and 3 the ESIPT path exhibits a potential energy barrier resulting in an incoherent rate governed process while in HBQ the ESIPT proceeds as a ballistic wavepacket motion along a path without significant barriers. The theoretical calculations (M06-2X/TZVP) confirm the existence of a barrier in the ground and excited states as result of the substitution.

The Quest of Excited-State Intramolecular Proton Transfer via Eight-Membered Ring π-Conjugated Hydrogen Bonding System

Searching for eight-membered ring π-conjugated hydrogen bonding (8-MR H-bonding) systems with excited-state intramolecular proton transfer (ESIPT) property is seminal and synthetically challenging. In this work, a series of π-conjugated molecules (8-HB-1, 8-HB-L1 and 8-HB-2) potentially possessing 8-MR H-bonding are strategically designed, synthesized and characterized. The configurations of these three potential molecules are checked by their X-ray structures, among which 8-HB-L1 (a structurally locked 8-HB-1 core chromophore) is proved to be an 8-MR H-bonding system, whereas 8-HB-1 and 8-HB-2 are too sterically hindered to form the 8-MR intramolecular H-bond. The ESIPT property of 8-HB-L1 is confirmed by the dual fluorescence consisting of normal and proton-transfer tautomer emissions. The insight into the ESIPT process of 8-HB-L1 is provided by femtosecond fluorescence upconversion measurements together with computational simulation. The results demonstrate for the first time a successful synthetic route to attain the 8-MR H-bonding molecule 8-HB-L1 with ESIPT property.

Synthesis and Optical Properties of Excited-State Intramolecular Proton Transfer Active π-Conjugated Benzimidazole Compounds: Influence of Structural Rigidification by Ring Fusion

Two excited-state intramolecular proton transfer (ESIPT) active benzimidazole derivatives (1 and 2) were synthesized by acid-catalyzed intramolecular cyclization. The steady-state fluorescence spectrum in THF revealed that ring-fused derivative 1 exhibits a dual emission, namely, the major emission was from the K* (keto) form (ESIPT emission) at 515 nm with a large Stokes shift of 11 100 cm^-1 and the minor emission was from the E* (enol) form at below 400 nm. In contrast, the normal emission from the E* form was dominant and the fluorescence quantum yield was very low (Φ ∼ 0.002) for nonfused derivative 2. The time-resolved fluorescence spectroscopy of 1 suggested that ESIPT effectively occurs due to the restricted conformational transition to the S₁-T_ICT state, and the averaged radiative and nonradiative decay rate constants were estimated as ⟨k_f⟩ = 0.15 ns^-1 and ⟨k_nr⟩ = 0.60 ns^-1, respectively. The fluorescence emission of 1 was influenced by the measurement conditions, such as solvent polarity and basicity, as well as the presence of Lewis base. The ESIPT process and solvatochromic behavior were nicely reproduced by the DFT/TDDFT calculation using the PCM model. In the single-crystal fluorescent spectra, the ESIPT emissions were exclusively observed for both fused and nonfused compounds as a result of hydrogen-bonding interactions.

A quantum-chemical insight into the tunable fluorescence color and distinct photoisomerization mechanisms between a novel ESIPT fluorophore and its protonated form

Enol-keto proton tautomerization and cis-trans isomerization reactions of a novel excited-state intramolecular proton transfer (ESIPT) fluorophore of BTImP and its protonated form (BTImP⁺) were explored using density functional theory/time-dependent density functional theory (DFT/TD-DFT) computational methods with a B3LYP hybrid functional and the 6-31+G(d,p) basis set. In addition, the absorption and fluorescence spectra were calculated at the TD-B3LYP/6-31+G(d,p) level of theory. Our results reveal that both BTImP and BTImP⁺ can undergo an ultrafast ESIPT reaction, giving rise to the single fluorescence emission with different fluorescence colors, which are nicely consistent with the experimental findings. Calculations also show that following the ultrafast ESIPT, BTImP and BTImP⁺ can experience the distinctly different cis-trans isomerization processes. The intersystem crossing between the first excited singlet S₁ state and triplet T₁ state is found to play an important role in the photoisomerization process of BTImP⁺. In addition, the energy barrier of the trans-keto→cis-keto isomerization in the ground state of BTImP⁺ is calculated to be 10.49kcalmol^-1, which implies that there may exist a long-lived trans-keto species in the ground state for BTImP⁺.

Theoretical insight into the excited-state intramolecular proton transfer mechanisms of three amino-type hydrogen-bonding molecules

Excited-state intramolecular proton transfer (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) and 2-(5'-cyano-2'-tosylaminophenyl)benzothiazole (CN-PBT-NHTs) were studied by the time-dependent density functional theory (TD-DFT) approach with the B3LYP density functional, 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 and 0.12eV, respectively, showing the trend of gradual decrease, which 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 dye 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 0.34eV, 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.

How To Reach Intense Luminescence for Compounds Capable of Excited-State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine and imidazo[1,2-f]phenanthridine skeletons, which mediate excited-state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five-membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady-state and time-resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited-state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited-state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT-capable compounds originates from their enol excited states.

Excited-state dynamics and electron transfer process of 1,3,5-triamino-2,4,6-trinitrobenzene

Insights into the excited-state dynamics and electron transfer processes of nitro explosives offer an efficient tool for unravelling ultrafast and complex detonation physics.

Optically Triggered Stepwise Double-Proton Transfer in an Intramolecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde

1,8-Dihydroxy-2-naphthaldehyde (DHNA), having doubly intramolecular hydrogen bonds, was strategically designed and synthesized in an aim to probe a long-standing fundamental issue regarding synchronous versus asynchronous double-proton transfer in the excited state. In cyclohexane, DHNA shows the lowest lying S0 →S1 (π-π*) absorption at ∼400 nm. Upon excitation, two large Stokes shifted emission bands maximized at 520 and 650 nm are resolved, which are ascribed to the tautomer emission resulting from the first and second proton-transfer products, denoted by TA* and TB*, respectively. The first proton transfer (DHNA* → TA*) is ultrafast (< system response of 150 fs), whereas the second proton transfer is reversible, for which the rates of forward (TA* → TB*) and backward (TA* ← TB*) proton transfer were determined to be (1.7 ps)(-1) and (3.6 ps)(-1), respectively. The fast equilibrium leads to identical population lifetimes of ∼54 ps for both TA* and TB* tautomers. Similar excited-state double-proton transfer takes place for DHNA in a single crystal, resulting in TA* (560 nm) and TB* (650 nm) dual-tautomer emission. A comprehensive 2D plot of reaction potential energy surface further proves that the sequential two-step proton motion is along the minimum energetic pathway firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unambiguously a stepwise, proton-relay type of intramolecular double-proton transfer reaction in the excited state, which should gain fundamental understanding of the multiple proton transfer reactions.