Proton transfer induced excited-state aromaticity gain for chromophores with maximal Stokes shifts

Excited state aromaticity (ESA) offers a fascinating route for driving photophysical and photochemical processes but is challenging to harness fully due to its inherent association with unstable antiaromatic ground states. Here, we propose to circumvent this problem via the introduction of a new class of photophysical processes, the generation of ESA via an excited-state intramolecular proton transfer. We select twelve candidate molecules based on the cyclobutadiene and pentalene scaffolds and investigate their ground and excited state properties using computation. The study highlights the feasibility of proton transfer induced ESA gain and shows that it gives rise to pronounced excited-state relaxation producing Stokes shifts in excess of 2 eV. The underlying electronic structure properties are analysed in terms of the orbitals involved as well as aromaticity descriptors illustrating the pronounced changes these molecules undergo upon both excitation and proton transfer. In summary, we believe that the present work will pave the way toward a new class of chromophores with maximal Stokes shifts and excited-state relaxation.

Unveiling the Reaction Mechanisms of the Synthesis and the Excited State Intramolecular Proton Transfer of 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine

Given its wide variety of applications in the pharmaceutical industry, the synthesis of imidazo[1,2-a]pyridines has been extensively studied since the beginning of the last century. Here, we disclose the mechanism for the synthesis of imidazo[1,2-a]pyridines by means of the Ortoleva-King reaction. We also reveal the reaction pathway leading to the formation of a iodinated byproduct, demonstrating the challenge of preventing the formation of such a byproduct because of the low energy barrier to access it. Moreover, quantum chemistry tools were employed to investigate the mechanism of intramolecular proton transfer in the excited state, and connections with aromaticity were explored.

Salicylideneaniline/Dithienylethene Hybrid Molecular Switches: Design, Synthesis, and Photochromism

A hybrid molecular switch comprising salicylideneaniline (SA) and dithienylethene (DTE) moieties around a single benzene ring is reported. Due to an interplay between solvent-assisted enol-keto tautomerization in the former moiety and photochromic electrocyclization in the latter, this dithienylbenzene derivative was found to be photoresponsive at room temperature with a thermally stable closed form. The main photoproduct featuring ring-closed DTE and keto-enamine SA structures could be isolated and converted back to the starting material by irradiation with visible light. The optical properties of the potential structures involved in the overall process were characterized by using density functional theory (DFT) calculations in good agreement with the measured data. The reversibility of the conversion could be tuned by the presence of donor and acceptor substituents, while the introduction of the imine in the form of a benzothiazole moiety enabled photochemistry even in nonprotic solvents.

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.

Calculation of the ELF in the excited state with single-determinant methods

Since its first definition, back in 1990, the electron localization function (ELF) has settled as one of the most commonly employed techniques to characterize the nature of the chemical bond in real space. Although most of the work using the ELF has focused on the study of ground-state chemical reactivity, a growing interest has blossomed to apply these techniques to the nearly unexplored realm of excited states and photochemistry. Since accurate excited electronic states usually require to account appropriately for electron correlation, the standard single-determinant ELF formulation cannot be blindly applied to them, and it is necessary to turn to correlated ELF descriptions based on the two-particle density matrix (2-PDM). The latter requires costly wavefunction approaches, unaffordable for most of the systems of current photochemical interest. Here, we compare the exact, 2-PDM-based ELF results with those of approximate 2-PDM reconstructions taken from reduced density matrix functional theory. Our approach is put to the test in a wide variety of representative scenarios, such as those provided by the lowest-lying excited electronic states of simple diatomic and polyatomic molecules. Altogether, our results suggest that even approximate 2-PDMs are able to accurately reproduce, on a general basis, the topological and statistical features of the ELF scalar field, paving the way toward the application of cost-effective methodologies, such as time-dependent-Hartree-Fock or time-dependent density functional theory, in the accurate description of the chemical bonding in excited states of photochemical relevance.

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.

Modulating the Photocyclization Reactivity of Diarylethenes through Changes in the Excited-State Aromaticity of the π-Linker

Quantum chemical calculations are performed to explore if the reactivity of diarylethene switches toward photocyclization can be controlled by the excited-state aromaticity of their bridging π-linker. Using an archetypal diarylethene with a non-aromatic π-linker as a reference, completely different outcomes are found when the π-linker is allowed to become either aromatic (no reaction) or antiaromatic (fast reaction) upon photoexcitation. The results demonstrate a possibility to use the excited-state aromaticity concept for actual modulation of photochemical reactivity.

Correlation between Excited-State Intramolecular Proton Transfer and Electron Population on Proton Donor/Acceptor in 2-(2'-Hydroxyphenyl)oxazole Derivatives

Modulating the excited-state intramolecular proton transfer (ESIPT) reaction to achieve anticipant performance is always fascinating for chemists. However, feasible methods and a definite mechanism for tuning the ESIPT reaction remain insufficient. In this work, we reported the feasibility of continuously modulating the ESIPT dynamics in 2-(2'-hydroxyphenyl)oxazole (HPO) derivatives with different substitutions on the positions 5 and 5' of the core HPO through steady-state/transient spectroscopy and theoretical calculations. We found that the main factor affecting the tendency of the ESIPT reaction is the variation of electron population on proton donor and acceptor. An index Δp_dif was proposed to evaluate the overall promotion effect on proton transfer caused by the variation of electron population on proton donor and acceptor, which shows high reliability in interpreting the ESIPT tendency. This method, for its capacity to quickly estimate the tendency of ESIPT, shows great potential in ESIPT molecular design with chemical substitution of electron-donating/withdrawing moieties.

Transient changes in aromaticity and their effect on excited-state proton transfer reactions

The common approach to investigate the impact of aromaticity on excited-state proton transfer by probing the (anti)aromatic character of reactants and products alone is scrutinized by modelling such reactions involving 2-pyridone. Thereby, it is found that energy barriers can be strongly influenced by transient changes in aromaticity unaccounted for by this approach, particularly when the photoexcited state interacts with a second excited state. Overall, the modelling identifies a pronounced effect overlooked by most studies on this topic.

On the nature of bonding in the photochemical addition of two ethylenes: C-C bond formation in the excited state?

In this work, the 2s + 2s (face-to-face) prototypical example of a photochemical reaction has been re-examined to characterize the evolution of chemical bonding. The analysis of the electron localization function (as an indirect measure of the Pauli principle) along the minimum energy path provides strong evidence supporting that CC bond formation occurs not in the excited state but in the ground electronic state after crossing the rhombohedral S₁/S₀ conical intersection.

Revised the excited-state intramolecular proton transfer direction of the BTHMB molecule: A theoretical study

The spectroscopic properties of 3-(benzo[d]-thiazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde molecule were investigated [J. Phys. Chem. A 2019, 123, 10246-10253]. The result shows that the excited-state intramolecular proton transfer (ESIPT) was driven toward the N center over the O center. In this research, the density functional theory and time-dependent density functional theory method were used to calculate molecule structures. Through our calculations, the ESIPT process toward N atom is proved to be feasible. Moreover, the emission peak we obtained of ESIPT process from the OH proton to aldehyde O atom is located at 564 nm, which is attributed to 500 nm in previous research. From the potential energy curves, the 0.35 kcal/mol energy barrier indicates the ESIPT process could occur when excited to S1 state from the OH proton to aldehyde O atom. In addition, the frontier molecular orbitals analysis and IR spectrum were also calculated. Finally, we revise the direction of BTHMB molecule, the two directions of ESIPT are both feasible when excited to S₁ state.

Photoinduced Changes in Aromaticity Facilitate Electrocyclization of Dithienylbenzene Switches

The concepts of excited-state aromaticity and antiaromaticity have in recent years with increasing frequency been invoked to rationalize the photochemistry of cyclic conjugated organic compounds, with the long-term goal of using these concepts to improve the reactivities of such compounds toward different photochemical transformations. In this regard, it is of particular interest to assess how the presence of a benzene motif affects photochemical reactivity, as benzene is well-known to completely change its aromatic character in its lowest excited states. Here, we investigate how a benzene motif influences the photoinduced electrocyclization of dithienylethenes, a major class of molecular switches. Specifically, we report on the synthesis of a dithienylbenzene switch where the typical nonaromatic, ethene-like motif bridging the two thienyl units is replaced by a benzene motif, and show that this compound undergoes electrocyclization upon irradiation with UV-light. Furthermore, through a detailed quantum chemical analysis, we demonstrate that the electrocyclization is driven jointly and synergistically by the loss of aromaticity in this motif from the formation of a reactive, antiaromatic excited state during the initial photoexcitation, and by the subsequent relief of this antiaromaticity as the reaction progresses from the Franck–Condon region. Overall, we conclude that photoinduced changes in aromaticity facilitate the electrocyclization of dithienylbenzene switches.

Photophysical transformations induced by chemical substitution to salicylaldimines

The role of electron-acceptor strength and microenvironment polarity on the photophysical properties of salicylaldimines.

Is the origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals single or double proton transfer?

The origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals (BCLCs) is not understood clearly.

Effect of number and different types of proton donors on excited-state intramolecular single and double proton transfer in bipyridine derivatives: theoretical insights

Intra-HBs are strengthened upon photoexcitation, confirmed by red-shift in vibrational mode and topology analysis. Number and type of donors result in difference in photophysical properties. Occurrence of ESIPT depends on barrier and reaction energy.

Excited-state proton transfer relieves antiaromaticity in molecules

Baird's rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4n + 2] π-aromatic in the ground state, become [4n + 2] π-antiaromatic in the first ¹ππ* states, and proton transfer (either inter- or intramolecularly) helps relieve excited-state antiaromaticity. Computed nucleus-independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. o-Salicylic acid undergoes ESPT only in the "antiaromatic" S₁ (¹ππ*) state, but not in the "aromatic" S₂ (¹ππ*) state. Stokes' shifts of structurally related compounds [e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with protic substrates] vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird's rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation.

Demonstration of Baird's rule complementarity in the singlet state with implications for excited-state intramolecular proton transfer

Greater aromaticity in the ground state leads to greater antiaromaticity in the excited state (and vice versa) which helps rationalize previously unexplained behavior of ESIPT fluorophores.

Chemical bonding analysis of excited states using the adaptive natural density partitioning method

A novel approach to chemical bond analysis for excited states has been developed. Using an extended adaptive natural density partitioning method (AdNDP) as implemented in AdNDP 2.0 code, we obtained chemically intuitive bonding patterns for the excited states of H2O, B5+, and C2H4+ molecules. The deformation pathway in the excited states could be easily predicted based on the analysis of the chemical bond pattern. We expect that this new method of chemical bonding analysis would be very helpful for photochemistry, photoelectron spectroscopy, electron spectroscopy and other chemical applications that involved excited states.

Partition of electronic excitation energies: the IQA/EOM-CCSD method

Different developments in chemistry and emerging technologies have generated a renewed interest in the properties of molecular excited states. We present herein the partition of black-box, size-consistent equation-of-motion coupled cluster singles and doubles (EOM-CCSD) excitation energies within the framework of the interacting quantum atoms (IQA) formalism. We denote this method as IQA/EOM-CCSD. We illustrate this approach by considering small molecules used often in the study of excited states. This investigation shows how the combination of IQA and EOM-CCSD may provide valuable insights into the molecular changes induced by electron excitation via the real space distribution of the energy of an absorbed photon in a molecular system. Our results reveal (i) the most energetically deformed atomic basins and (ii) the most affected covalent and non-covalent interactions within a molecule due to a given photoexcitation. In other words, this kind of analysis provides insights into the spatial energetic redistribution accompanying an electronic excitation, with interesting foreseeable applications in the rational design of photoexcitations with tailored chemical effects. Altogether, we expect that the IQA/EOM-CCSD excitation energy partition will prove useful in the understanding of systems and processes of interest in photophysics and photochemistry.

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.

Unravelling the solvent polarity effect on the excited state intramolecular proton transfer mechanism of the 1- and 2-salicylideneanthrylamine. A TD-DFT case study

TD-DFT has been used to investigate the relaxation pathways of 1- and 2-salicylideneanthrylamine in different solvent environments.

A theoretical study of the photodynamics of salicylidene-2-anthrylamine in acetonitrile solution

The ultrafast photoinduced processes of salicylidene-2-anthrylamine (2-AntSA) in acetonitrile solution have been investigated using DFT/TD-DFT static electronic structure calculations and excited state ab initio molecular dynamics simulations. Two different isoenergetic enol ground-state structures with suitable geometry for excited state intramolecular proton transfer (ESIPT) where chosen for the excited-state dynamics. The S1 relaxed potential energy profiles for the excited state intramolecular proton transfer and the N[double bond, length as m-dash]C double bond isomerization reactions predict that both reactions occur over an energy barrier and that they are competitive processes in the deactivation of the Franck-Condon state. The photodynamic simulations show that the ESIPT occurs in the femtosecond time scale for both conformers (77 and 213 fs for IA and IIA, respectively) and that the speed is modulated by the ability of the conformers to evolve toward a planar conformation in the S1 state. The trajectories predict two conical intersections which provide nonradiative relaxation pathways to the S0 state. The first one is located in the twisted enol region, where the proton transfer process is unlikely, and only occurs for the conformer IIA in a time scale ≥600 fs. The second conical intersection is located in the cis-keto region, and represents an effective depopulation channel toward the trans-keto form. All our results are in remarkably good agreement with experiments.

Dynamics of Oxygen-Independent Photocleavage of Blebbistatin as a One-Photon Blue or Two-Photon Near-Infrared Light-Gated Hydroxyl Radical Photocage

Development of versatile, chemically tunable photocages for photoactivated chemotherapy (PACT) represents an excellent opportunity to address the technical drawbacks of conventional photodynamic therapy (PDT) whose oxygen-dependent nature renders it inadequate in certain therapy contexts such as hypoxic tumors. As an alternative to PDT, oxygen free mechanisms to generate cytotoxic reactive oxygen species (ROS) by visible light cleavable photocages are in demand. Here, we report the detailed mechanisms by which the small molecule blebbistatin acts as a one-photon blue light-gated or two-photon near-infrared light-gated photocage to directly release a hydroxyl radical (^•OH) in the absence of oxygen. By using femtosecond transient absorption spectroscopy and chemoselective ROS fluorescent probes, we analyze the dynamics and fate of blebbistatin during photolysis under blue light. Water-dependent photochemistry reveals a critical process of water-assisted protonation and excited state intramolecular proton transfer (ESIPT) that drives the formation of short-lived intermediates, which surprisingly culminates in the release of ^•OH but not superoxide or singlet oxygen from blebbistatin. CASPT2//CASSCF calculations confirm that hydrogen bonding between water and blebbistatin underpins this process. We further determine that blue light enables blebbistatin to induce mitochondria-dependent apoptosis, an attribute conducive to PACT development. Our work demonstrates blebbistatin as a controllable photocage for ^•OH generation and provides insight into the potential development of novel PACT agents.

Molecular Photoswitching Aided by Excited-State Aromaticity

Central to the development of optoelectronic devices is the availability of efficient synthetic molecular photoswitches, the design of which is an arena where the evolving concept of excited-state aromaticity (ESA) is yet to make a big impact. The aim of this minireview is to illustrate the potential of this concept to become a key tool for the future design of photoswitches. The paper starts with a discussion of challenges facing the use of photoswitches for applications and continues with an account of how the ESA concept has progressed since its inception. Then, following some brief remarks on computational modeling of photoswitches and ESA, the paper describes two different approaches to improve the quantum yields and response times of switches driven by E/Z photoisomerization or photoinduced H-atom/proton transfer reactions through simple ESA considerations. It is our hope that these approaches, verified by quantum chemical calculations and molecular dynamics simulations, will help stimulate the application of the ESA concept as a general tool for designing more efficient photoswitches and other functional molecules used in optoelectronic devices.

Open-Resonance-Assisted Hydrogen Bonds and Competing Quasiaromaticity

The delocalization of electron density upon tautomerization of a proton across a conjugated bridge can alter the strength of hydrogen bonds. This effect has been dubbed resonance-assisted hydrogen bonding (RAHB) and plays a major role in the energetics of the tautomeric equilibrium. The goal of this work was to investigate the role that π-delocalization plays in the stability of RAHBs by engaging other isomerization processes. Similarly, acid-base chemistry has received little experimental attention in studies of RAHB, and we address the role that acid-base effects play in the tautomeric equilibrium. We find that π-delocalization and the disruption of adjacent aromatic rings is the dominant effect in determining the stability of a RAHB.

Cooperative and anticooperative effects in resonance assisted hydrogen bonds in merged structures of malondialdehyde

We analyzed non-additive effects in resonance assisted hydrogen bonds (RAHBs) in different β-enolones, which are archetypal compounds of these types of interactions. For this purpose, we used (i) potential energy curves to compute the formation energy, ΔE, of the RAHBs of interest in different circumstances along with (ii) tools offered by quantum chemical topology, namely, the Quantum Theory of Atoms In Molecules (QTAIM) and the Interacting Quantum Atoms (IQA) electronic energy partition. We established the effect that a given H-bond exerts over ΔE associated with another RAHB, determining in this way the cooperativity or the anticooperativity of these interactions. The mesomeric structures and the QTAIM delocalisation indices are consistent with the determined cooperative or anticooperative character of two given RAHBs. The HB cooperativity and anticooperativity studied herein are directly reflected in the IQA interaction energy E, but they are modulated by the surrounding hydrocarbon chain. The IQA decomposition of ΔE_coop, a measure of the cooperativity between a pair of interacting RAHBs, indicates that the analyzed H-bond cooperative/anticooperative effects are associated with greater/smaller (i) strengthening of the pseudo-bicyclic structure of the compounds of interest and (ii) electron localisations with its corresponding changes in the intra and intermolecular exchange-correlation contributions to ΔE. Overall, we expect that this investigation will provide valuable insights into the interplay among hydrogen bonded atoms and the π system in RAHBs contributing in this way to the understanding of the general features of H-bonds.

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.

Chemical bonding in excited states: Energy transfer and charge redistribution from a real space perspective

This work provides a novel interpretation of elementary processes of photophysical relevance from the standpoint of the electron density using simple model reactions. These include excited states of H₂ taken as a prototype for a covalent bond, excimer formation of He₂ to analyze non-covalent interactions, charge transfer by an avoided crossing of electronic states in LiF and conical interesections involved in the intramolecular scrambling in C₂ H₄ . The changes of the atomic and interaction energy components along the potential energy profiles are described by the interacting quantum atoms approach and the quantum theory of atoms in molecules. Additionally, the topological analysis of one- and two-electron density functions is used to explore basic reaction mechanisms involving excited and degenerate states in connection with the virial theorem. This real space approach allows to describe these processes in a unified way, showing its versatility and utility in the study of chemical systems in excited states. © 2017 Wiley Periodicals, Inc.

Synthesis of thermo- and photo-responsive polysiloxanes with tunable phase separationviaaza-Michael addition

Two kinds of thermo- and photo-dual-responsive polysiloxanes were synthesized through a facile, effective, and catalyst-free aza-Michael addition.

Enhanced fluorescence with nanosecond dynamics in the solid state of metal ion complexes of alkoxy salophens

Incorporation of an alkoxy moiety dramatically alters the photophysics and molecular packing of aluminum complexes of salophen leading to enhanced emission in the solid state.

How Electronic Excitation Can be Used to Inhibit Some Mechanisms Associated to Substituent Effects

Despite the fact that transferability and chemistry go hand in hand, transferability studies in electronically excited states (EESs) are normally omitted, although these states are becoming extremely important in modern processes and applications. In this work, it is shown that this kind of studies can be used to understand how substituent effects can be modified in EESs. Thus, for example, the analysis of the carbonyl oxygen transferability in different HCO-R molecules allowed us to find that the nO→πCO* excitation can be used to break the π conjugation associated to the resonance substituent effect. Moreover, as a direct consequence, the oxygen transferability is enhanced in the first electronically excited state.