Full-Dimensional Excited-State Intramolecular Proton Transfer Dynamics of Salicylic Acid

Molecular Dynamics of Artificially Pair-Decoupled Systems: An Accurate Tool for Investigating the Importance of Intramolecular Couplings

We propose a numerical technique to accurately simulate the vibrations of organic molecules in the gas phase, when pairs of atoms (or, in general, groups of degrees of freedom) are artificially decoupled, so that their motion is instantaneously decorrelated. The numerical technique we have developed is a symplectic integration algorithm that never requires computation of the force but requires estimates of the Hessian matrix. The theory we present to support our technique postulates a pair-decoupling Hamiltonian function, which parametrically depends on a decoupling coefficient α ∈ [0, 1]. The closer α is to 0, the more decoupled the selected atoms. We test the correctness of our numerical method on small molecular systems, and we apply it to study the vibrational spectroscopic features of salicylic acid at the Density Functional Theory ab initio level on a fitted potential. Our pair-decoupled simulations of salicylic acid show that decoupling hydrogen-bonded atoms do not significantly influence the frequencies of stretching modes, but enhance enormously the out-of-plane wagging and twisting motions of the hydroxyl and carboxyl groups to the point that the carboxyl and hydroxyl groups may overcome high potential energy barriers and change the salicylic acid conformation after a short simulation time. In addition, we found that the acidity of salicylic acid is more influenced by the dynamical couplings of the proton of the carboxylic group with the carbon ring than with the hydroxyl group.

Excitation Energy-Dependent, Excited-State Intramolecular Proton Transfer-Based Dual Emission in Poor Hydrogen-Bonding Solvents

2-(2'-Hydroxyphenyl)benzoxazole (HBO) substituted at the 5'-position with bipyridylvinylene phenylenevinylene (compound 2) produces both normal and, via an excited-state intramolecular proton transfer (ESIPT) reaction, tautomer emissions in solvents that preserve intramolecular hydrogen bonds. The abundance of the tautomer emission from compound 2 in a poor hydrogen-bonding solvent increases in response to the application of a higher excitation energy. Based on quantum chemical calculations, the excitation-dependent dual emission is consistent with a model in which the ESIPT reaction is more favored in the S₂ than in the S₁ electronically excited state.

Excited State Intramolecular Proton Transfer (ESIPT) from -NH2 to the Carbon Atom of a Naphthyl Ring

Excited state intramolecular proton transfer (ESIPT) has been documented from an amino NH₂ group to a carbon atom of an adjacent aromatic ring. This finding changes the paradigm, as hitherto such processes have not been considered as plausible due to slow protonation of carbon and low (photo)acidity of the NH₂ group. The ESIPT was studied by irradiation of 2-(2-aminophenyl)naphthalene in CH₃CN-D₂O, whereupon regiospecific incorporation of deuterium takes place at the naphthalene position 1, with a quantum yield of Φ = 0.11. A synergy of experimental and computational investigations completely unraveled the mechanism of this important photochemical reaction. Upon excitation to the photoreactive S₂(L_a) state, a favorable redistribution of charge sets the stage for ESIPT to the carbon atom in naphthalene position 1. H₂O molecules are needed, as they increase the excitation energy and oscillator strength for the population of the S₂(L_a) state. The gain in energy is used to surmount a small energy barrier on the pathway from the Franck-Condon geometry to the conical intersection with the S₀, delivering aza-quinone methide.

Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry

We present a conspectus of recent joint spectroscopic and computational studies that provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario that unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.

Spectroscopic and theoretical studies of fluorescence effects induced by the ESIPT process in a new derivative 2-Hydroxy-N-(2-phenylethyl)benzamide - Study on the effects of pH and medium polarity changes

The paper presents the results of studies conducted with the use of stationary and time-resolved fluorescence spectroscopy for the new derivative 2-Hydroxy-N-(2-phenylethyl)benzamide (SAL-3) in aqueous solutions with various concentrations of hydrogen ions as well as in solvent mixtures (i.e. media with changing polarity/polarizability). For the compound selected for the study placed in aqueous solutions with varying concentrations of hydrogen ions, the fluorescence emission spectra revealed a single emission band within most of the pH range, however, at low pH (pH<3) a significant broadening (noticeable effect of dual fluorescence) and shifting of the band was observed. Whereas, for water and polar (protic) solvents, we observed a very interesting phenomenon of dual fluorescence never before reported for this particular group of analogues (with the specific substituent system). Based on the results of the experiments, it was observed that the presented effects may be related both with conformational effects (related to the possible positioning of the-OH group on the side of the carbonyl system, which facilitates the possibility of proton transfer) as well as, most importantly, the effects of excited state intramolecular proton transfer (ESIPT-Excited State Intramolecular Proton Transfer) related in this case with the necessary (new/previously unobserved in published literature) presence of ionic and non-ionic forms of the compound). Both the conducted quantum-mechanical [TD]DFT-Time-Dependent Density Functional Theory) calculations and excited state dipole moment change calculations for the analyzed molecule in solvents with varying pH confirmed the association between the observed fluorescence phenomena and the two aforementioned effects.

Full-Dimensional Photodynamics of Bistable Proton Transfer Switches

Excited-state intramolecular proton transfers (ESIPT) are one of the fastest reactions in chemistry (<100 fs) which – among other features like high photostability – makes them an important reaction class for molecular switches. ESIPTs can be coupled with double bond rotation/isomerization, so that molecules can act as “molecular cranes”, facilitating long-range proton transfer. A versatile model system is 7-hydroxy-4-methylquinoline-8-carbaldehyde (HMQCA): it features two proton-accepting sites, two stable ground-state isomers and should allow for easy derivatization. There is also experimental and theoretical reference data available, however, only for static properties, e.g. ground-state IR spectra or potential energy surface scans. In this contribution we show the results of full-dimensional surface-hopping molecular dynamics (MD) of HMQCA after photo-excitation, employing semiempirical quantum mechanics coupled to floating-occupation configuration interaction. The results support the potential of HMQCA as prototype system for directed proton transport by ESIPT.

QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model

Methyl 2-cyano-3,3-diphenylacrylate (MCDPA) shares the same molecular skeleton with octocrylene (OCR) that is one of the most common molecules used in commercially available sunscreens. However, its excited-state relaxation mechanism is unclear. Herein, we have used the QM(CASPT2//CASSCF)/MM method to explore spectroscopic properties, geometric and electronic structures, relevant conical intersections and crossing points, and excited-state relaxation paths of MCDPA in methanol solution. We found that in the Franck-Condon (FC) region, the V(¹ππ*) state is energetically lower than the V'(¹ππ*) state only by 2.8 kcal/mol and is assigned to experimentally observed maximum absorption band. From these two initially populated singlet states, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, when the V(¹ππ*) state is populated in the FC region, the system diabatically evolves along the V(¹ππ*) state into its minimum where the internal conversion to S₀ occurs. In the second one, the V'(¹ππ*) state is populated in the FC region and the system adiabatically overcomes a barrier of ca. 3.0 kcal/mol to approach the V(¹ππ*) minimum eventually leading to a V(¹ππ*)-to-S₀ internal conversion. In the third one, the V'(¹ππ*) state first hops via the intersystem crossing to the T₂ state, which then decays through the internal conversion to the T₁ state. The T₁ state is finally converted to the S₀ state via the T₁/S₀ crossing point. Our present work contributes to understanding the photophysics of OCR and its variants.

Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state

Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV-UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA-H₂O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S₀-S₁ transition that begins in the UV-A region, with the electronic origin at 28 852 cm^-1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NHO[double bond, length as m-dash]C H-bond, which shifts the ³nπ* state well above the ¹ππ* S₁ state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck-Condon activity due to three in-plane vibrations that modulate the distance between the NH₂ and CO₂Me groups, ν₃₃ (421 cm^-1), ν₃₄ (366 cm^-1), and ν₃₆ (179 cm^-1). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NHO[double bond, length as m-dash]C H-bond. This leads to a shortening of the NHO[double bond, length as m-dash]C H-bond distance from 1.926 Å to 1.723 Å, equivalent to about a 25% reduction in the HO distance compared to full H-atom transfer. As a result, the excited state process near the S₁ origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 Å increase in the NH bond length relative to its ground state value.

Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2'-Hydroxyphenyl)benzoxazole

Excitation-dependent multiple fluorescence of a 2-(2'-hydroxyphenyl)benzoxazole (HBO) derivative (1) is described. Compound 1 contains the structure of a charge-transfer (CT) 4-hydroxyphenylvinylenebipy fluorophore and an excited-state intramolecular proton transfer capable (ESIPT-capable) HBO component that intersect at the hydroxyphenyl moiety. Therefore, both CT and ESIPT pathways, while spatially mostly separated, are available to the excited state of 1. The ESIPT process offers two emissive isomeric structures (enol and keto) of 1 in the excited state, while the susceptibility of 1 to a base adds another option to tune the composite emission color. In addition to the ground-state acid-base equilibrium that can be harnessed for the control of emission color by excitation energy, compound 1 exhibits excitation-dependent emission that is attributed to solvent-affected ground-state structural changes. Therefore, depending on the medium and excitation wavelength, the emission from the enol, keto, and anion forms could occur simultaneously, which are in the color ranges of blue, green, and orange/red, respectively. A composite color of white with CIE coordinates of (0.33, 0.33) can be materialized through judicious choices of medium and excitation wavelength.

Fluorescence of Hydroxyphenyl-Substituted "Click" Triazoles

The structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole molecules ("click" triazoles) are described. "Click" triazoles are prepared from the copper(I)-catalyzed azide-alkyne cycloaddition reactions. The alkyne-derived C4 substituent of a "click" triazole engages in electronic conjugation more effectively with the triazolyl core than the azide-derived N1 substituent. Furthermore, triazolyl group exerts a stronger electron-withdrawing effect on the N1 than the C4 substituent. Therefore, the placement of an electron-donating group at either C4 or N1 position and the presence or the absence of an intramolecular hydrogen bond (HB) have profound influences on the optical properties of these compounds. The reported "click" triazoles have fluorescence quantum yields in the range of 0.1-0.3 and large apparent Stokes shifts (8000-13 000 cm^-1) in all tested solvents. Deprotonation of "click" triazoles with a C4 hydroxyphenyl group increases their Stokes shifts; while the opposite (or quenching) occurs to the triazoles with an N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular HBs, neither experimental nor computational results support a model of excited state intramolecular proton transfer (ESIPT). Rather, the excited state internal (or intramolecular) charge transfer (ICT) mechanism is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted "click" triazoles; specifically, the large Stokes shifts of these compounds.

Charge transfer dynamics at the boron subphthalocyanine chloride/C60 interface: non-adiabatic dynamics study with Libra-X

The Libra-X software for non-adiabatic molecular dynamics is reported. It is used to comprehensively study the charge transfer dynamics at the boron subphtalocyanine chloride (SubPc)/fullerene (C₆₀) interface.