Nonadiabatic Coupling Dictates the Site-Specific Excited-State Decay Pathways of Fluorophenols

In this paper, a combined photophysical and electronic structure theory study demonstrating a remarkable site-specific fluorine substitution effect on the excited-state dynamics of monofluorophenols has been presented. The S1 ← S0 electronic origin band of phenol is shifted to a longer wavelength for para substitution, but to shorter wavelengths for ortho and meta substitutions. The observed sequence of excitation wavelengths of 2-fluorophenol (2FP) < 3-fluorophenol (3FP) < phenol < 4-fluorophenol (4FP) is consistent with the transition energies predicted by TDDFT/CAMB3LYP/6-311++G(d,p) and CASSCF(8,8)/Dunning cc-pVDZ theoretical methods. The most notable contrast of excited-state dynamics is revealed in the different features of the fluorescence spectra; the fluorescence yield of 4FP is almost 6 times larger compared to that of 3FP and the spectral bandwidth of 2FP is nearly 1.5 times larger than that of 4FP. Electronic structure calculation predicts a low-energy S1/S0 conical intersection (CI) near the 1ππ* minimum with respect to the prefulvenic vibronic mode of the aromatic ring, and the energetic location of this CI is altered with the substitution site of the fluorine atom. The predicted energy barrier to this prefulvenic CI is smallest for 3FP but largest for 4FP, leading to a facilitated nonradiative electronic relaxation of the former (3FP), and emission occurs with a much diminished fluorescence intensity.

Chlorine Substitution Effect on the S1 Relaxation Dynamics of Chlorobenzene and Chlorophenols

The S1 state relaxation dynamics of chlorobenzene (CB), 3-chlorophenol (3-CP), 3-CP·H2O, and 2-chlorophenol·H2O (2-CP·H2O) have been investigated by means of picosecond time-resolved pump-probe spectroscopy in a state-specific manner. For CB, the S1 state relaxes via the S1-S0 internal conversion in the low internal energy region (<2000 cm-1), whereas the direct C-Cl bond dissociation channel mediated by the upper-lying repulsive πσCCl* state is opened to give the rather sharp increase of the S1 relaxation rate in the high internal energy region (>2000 cm-1). A similar dynamic feature has been observed for 3-CP in terms of the lifetime behavior with an increase in the S1 internal energy, suggesting that the H atom tunneling dissociation reaction from OH might contribute less compared to the internal conversion, although it is not clear at the present time whether or not the sharp increase of the S1 relaxation rate in the high internal energy region of 3-CP (>1500 cm-1) is entirely due to that of the internal conversion. The fact that the internal conversion is facilitated by the Cl substitution implies that the energetic location of the S1/S0 conical intersection should have been strongly influenced by chlorine substitution on the aromatic ring. The approximate energetic location of the saddle point of the S1(ππ*)/πσCCl* conical intersection along the seam coordinate for CB or 3-CP could be inferred from the energy-dependent S1 lifetime measurements. It is discussed in comparison with the dynamic role of the S1(ππ*)/πσCCl* conical intersection, which is strongly influenced by the O-H···Cl intramolecular hydrogen bond in the rather complicated yet ultrafast S1 relaxation dynamics of the cis-2-CP. The S1 lifetimes of 3-CP·H2O and 2-CP·H2O reveal the importance of the conformational structures, especially in terms of the intramolecular hydrogen bonding.

Photoinduced hydrogen dissociation in thymine predicted by coupled cluster theory

The fate of thymine upon excitation by ultraviolet radiation has been the subject of intense debate. Today, it is widely believed that its ultrafast excited state gas phase decay stems from a radiationless transition from the bright ππ* state to a dark nπ* state. However, conflicting theoretical predictions have made the experimental data difficult to interpret. Here we simulate the early gas phase ultrafast dynamics in thymine at the highest level of theory to date. This is made possible by performing wavepacket dynamics with a recently developed coupled cluster method. Our simulation confirms an ultrafast ππ* to nπ* transition (τ = 41 ± 14 fs). Furthermore, the predicted oxygen-edge X-ray absorption spectra agree quantitatively with experiment. We also predict an as-yet uncharacterized πσ* channel that leads to hydrogen dissociation at one of the two N-H bonds. Similar behavior has been identified in other heteroaromatic compounds, including adenine, and several authors have speculated that a similar pathway may exist in thymine. However, this was never confirmed theoretically or experimentally. This prediction calls for renewed efforts to experimentally identify or exclude the presence of this channel.

Probing photochemical dynamics using electronic vs vibrational sum-frequency spectroscopy: The case of the hydrated electron at the water/air interface

Photo-dynamics can proceed differently at the water/air interface compared to in the respective bulk phases. Second-order non-linear spectroscopy is capable of selectively probing the dynamics of species in such an environment. However, certain conclusions drawn from vibrational and electronic sum-frequency generation spectroscopies do not agree as is the case for the formation and structure of hydrated electrons at the interface. This Perspective aims to highlight these apparent discrepancies, how they can be reconciled, suggests how the two techniques complement one another, and outline the value of performing both techniques on the same system.

The tautomer-specific excited state dynamics of 2,6-diaminopurine using resonance-enhanced multiphoton ionization and quantum chemical calculations

2,6-Diaminopurine (2,6-dAP) is an alternative nucleobase that potentially played a role in prebiotic chemistry. We studied its excited state dynamics in the gas phase by REMPI, IR-UV hole burning, and ps pump-probe spectroscopy and performed quantum chemical calculations at the SCS-ADC(2) level of theory to interpret the experimental results. We found the 9H tautomer to have a small barrier to ultrafast relaxation via puckering of its 6-membered ring. The 7H tautomer has a larger barrier to reach a conical intersection and also has a sizable triplet yield. These results are discussed relative to other purines, for which 9H tautomerization appears to be more photostable than 7H and homosubstituted purines appear to be less photostable than heterosubstituted or singly substituted purines.

Excitation Energy-Dependent Decay Dynamics of the S1 State of N-Methyl-2-pyridone

The UV-induced decay dynamics of N-methyl-2-pyridone is investigated using a femtosecond time-resolved photoelectron spectroscopy method. Irradiation in the wavelength range of 339.3-258.9 nm prepares N-methyl-2-pyridone molecules with very different vibrational levels of the S1(11ππ*) state. For v' = 0 (origin) and a few low-energy vibrational levels slightly above the S1 state origin, the radiative decay channel is in operation for some specific vibrations. This is revealed by the excited-state lifetime of ≫1 ns. In addition, some other nearby S1 vibronic states have a much shorter lifetime in the range of several picoseconds to a few tens of picoseconds, indicating that the radiation-less decay to the ground state (S0) via internal conversion is the dominant channel for them. As the pump wavelength slightly decreases, the radiative decay is suddenly not important at all, and the deactivation rate of the S1 state becomes faster. At shorter pump wavelengths, the lifetime of highly excited vibrational states of the S1 state further decreases with the increase in the vibrational excess energy. This study provides quantitative information about the excitation energy-dependent decay dynamics of the S1 state of N-methyl-2-pyridone. Methyl substitution effects on the excited-state dynamics of 2-pyridone are also discussed.

Dynamic Role of the Intramolecular Hydrogen Bonding in the S1 State Relaxation Dynamics Revealed by the Direct Measurement of the Mode-Dependent Internal Conversion Rate of 2-Chlorophenol and 2-Chlorothiophenol

The dynamic role of the intramolecular hydrogen bond in the S1 relaxation of cis-2-chlorophenol (2-CP) or cis-2-chlorothiophenol (2-CTP) has been investigated in a state-specific manner. Whereas ultrafast internal conversion is dominant for 2-CP, the H-tunneling competes with internal conversion for 2-CTP even at the S1 origin. The S0-S1 internal conversion rate of 2-CTP could be directly measured from the S1 lifetimes of 2-CTP-d1 (Cl-C6H4-SD) as the D-tunneling is kinetically blocked, allowing distinct estimations of tunneling and internal conversion rates with increasing the energy. The internal conversion rate of 2-CTP increases by two times at the out-of-plane torsional mode excitation, suggesting that the internal conversion is facilitated at the nonplanar geometry. It then sharply increases at ∼600 cm-1, indicating that the S1/S0 conical intersection is readily accessible at the extended C-Cl bond length. The strength of the intramolecular hydrogen bond should be responsible for the distinct dynamic behaviors of 2-CP and 2-CTP.

Mode-dependent H atom tunneling dynamics of the S1 phenol is resolved by the simple topographic view of the potential energy surfaces along the conical intersection seam

Mode-dependent H atom tunneling dynamics of the O-H bond predissociation of the S₁ phenol has been theoretically analyzed. As the tunneling is governed by the complicated multi-dimensional potential energy surfaces that are dynamically shaped by the upper-lying S₁(ππ*)/S₂(πσ*) conical intersection, the mode-specific tunneling dynamics of phenol (S₁) has been quite formidable to be understood. Herein, we have examined the topography of the potential energy surface along the particular S₁ vibrational mode of interest at the nuclear configurations of the S₁ minimum and S₁/S₂ conical intersection. The effective adiabatic tunneling barrier experienced by the reactive flux at the particular S₁ vibrational mode excitation is then uniquely determined by the topographic shape of the potential energy surface extended along the conical intersection seam coordinate associated with the particular vibrational mode. The resultant multi-dimensional coupling of the specific vibrational mode to the tunneling coordinate is then reflected in the mode-dependent tunneling rate as well as nonadiabatic transition probability. Remarkably, the mode-specific experimental result of the S₁ phenol tunneling reaction [K. C. Woo and S. K. Kim, J. Phys. Chem. A 123, 1529-1537 (2019)] (in terms of the qualitative and relative mode-dependent dynamic behavior) could be well rationalized by semi-classical calculations based on the mode-specific topography of the effective tunneling barrier, providing the clear conceptual insight that the skewed potential energy surfaces along the conical intersection seam (strongly or weakly coupled to the tunneling reaction coordinate) may dictate the tunneling dynamics in the proximity of the conical intersection.

Radiationless deactivation pathways versus H-atom elimination from the N-H bond photodissociation in PhNH2-(Py)n (n = 1,2) complexes

Minimum energy structures of the ground and lowest excited states of aniline (PhNH₂) solvated by pyridine (Py) show that the clusters formed are stabilized by hydrogen bonds in which only one or both hydrogen atoms of the NH₂ group take part. Two different N-H bonds photodissociation in PhNH₂-(Py)_n (n = 1,2) complexes, free and hydrogen bonded have been studied by analyzing excited state potential energy surfaces. In the first one, only N-H bonds engaged in hydrogen bonding in these complexes are considered. RICC2 calculations of potential energy (PE) profiles indicate that all photochemical reaction paths along N-H stretching occur mainly via the proton-coupled electron transfer (PCET) mechanism. The repulsive charge transfer ¹ππ*(CT) state dominates the PE profiles, leading to low-lying ¹ππ*(CT)/S₀ conical intersections and thus provide channels for ultrafast radiationless deactivation of the electronic excitation or stabilization to biradical complexes. The second photoreaction consists of a direct dissociation along the free N-H bond of the NH₂ group. It has been shown that this process is played by excited singlet states of ¹πσ* character having repulsive potential energy profiles with respect to the stretching of N-H bond, which dissociates over an exit barrier about 0.5 eV giving rise to the formation of a ¹πσ*/S₀ conical intersection. This may cause an internal conversion to the ground state or may lead to H-atom elimination. This photophysical process is the same in both planar and T-shaped conformers of the PhNH₂-Py monomer complex. Our findings reveal that there is no single dominating path in the photodissociation of N-H bonds in PhNH₂-(Py)_n complexes, but rather a variety of paths involving H-atom elimination and several quenching mechanisms.

πσ*-Mediated Nonadiabatic Tunneling Dynamics of Thiophenols in S1: The Semiclassical Approaches

The S-H bond tunneling predissociation dynamics of thiophenol and its ortho-substituted derivatives (2-fluorothiophenol, 2-methoxythiophenol, and 2-chlorothiphenol) in S₁ (ππ*) where the H atom tunneling is mediated by the nearby S₂ (πσ*) state (which is repulsive along the S-H bond extension coordinate) have been investigated in a state-specific way using the picosecond time-resolved pump-probe spectroscopy for the jet-cooled molecules. The effects of the specific vibrational mode excitations and the SH/SD substitutions on the S-H(D) bond rupture tunneling dynamics have been interrogated, giving deep insights into the multidimensional aspects of the S₁/S₂ conical intersection, which also shapes the underlying adiabatic tunneling potential energy surfaces (PESs). The semiclassical tunneling rate calculations based on the Wentzel-Kramers-Brillouin (WKB) approximation or Zhu-Nakamura (ZN) theory have been carried out based on the ab initio PESs calculated in the (one, two, or three) reduced dimensions to be compared with the experiment. Though the quantitative experimental results could not be reproduced satisfactorily by the present calculations, the qualitative trends among different molecules in terms of the behavior of the tunneling rate versus the (adiabatic) barrier height or the number of PES dimensions could be rationalized. Most interestingly, the H/D kinetic isotope effect observed in the tunneling rate could be much better explained by the ZN theory compared to the WKB approximation, indicating that the nonadiabatic coupling matrix elements should be invoked for understanding the tunneling dynamics taking place in the proximity of the conical intersection.

Photochemical carbon-sulfur bond cleavage in thioethers mediated via excited state Rydberg-to-valence evolution

Time-resolved photoelectron imaging and supporting ab initio quantum chemistry calculations were used to investigate non-adiabatic excess energy redistribution dynamics operating in the saturated thioethers diethylsulfide, tetrahydrothiophene and thietane. In all cases, 200 nm excitation leads to molecular fragmentation on an ultrafast (<100 fs) timescale, driven by the evolution of Rydberg-to-valence orbital character along the S-C stretching coordinate. The C-S-C bending angle was also found to be a key coordinate driving initial internal conversion through the excited state Rydberg manifold, although only small angular displacements away from the ground state equilibrium geometry are required. Conformational constraints imposed by the cyclic ring structures of tetrahydrothiophene and thietane do not therefore influence dynamical timescales to any significant extent. Through use of a high-intensity 267 nm probe, we were also able to detect the presence of some transient (bi)radical species. These are extremely short lived, but they appear to confirm the presence of two competing excited state fragmentation channels - one proceeding directly from the initially prepared 4p manifold, and one involving non-adiabatic population of the 4s state. This is in addition to a decay pathway leading back to the S₀ electronic ground state, which shows an enhanced propensity in the 5-membered ring system tetrahydrothiophene over the other two species investigated.

Vibrational-state dependent decay dynamics of 2-pyridone excited to the S1 electronic state

The S₁(¹ππ*) state decay dynamics of 2-pyridone excited around the 000 band origin is investigated using femtosecond time-resolved photoelectron imaging technique. At a pump wavelength of 334.0 nm, the vibrational ground state and a few low energy vibrational states covered by the bandwidth of the pump laser pulses are excited. The lifetimes of the vibrational states show strong dependence on the vibrational energy and mode. A quantum beat between two lowest energy vibrational states is also observed. This study provides quantitative information about the vibrational-state dependent lifetime of the S₁ state of 2-pyridone.

The ultrafast vibronic dynamics of ammonia's D̃ state

Using vacuum-ultraviolet time-resolved velocity map imaging of photoelectrons, we study ultrafast coupled electronic and nuclear dynamics in low-lying Rydberg states of ammonia. Vibrationally-resolved internal vibrational relaxation (IVR) is observed in a progression of the e' bending modes. This vibrational progression is only observed in the D̃ state, and is lost upon ultrafast internal conversion to the C̃ and B̃ electronic states. Due to the ultrashort time scale of the internal conversion (ca. 64 fs), and the vibronic resolution, the non-adiabatic coupling vectors are identified and verified with ab initio calculations. The time-scale of this IVR process is highly surprising and significant because IVR is usually treated as an incoherent process that proceeds statistically, according to a "Fermi's Golden Rule"-like model, where the process scales with the available degrees of freedom. Here, we show that it can be highly non-statistical, restricted to only a very small subset of vibrational motions.

IR absorption spectra of aniline cation, anilino radical, and phenylnitrene isolated in solid argon

Electron bombardment of aniline (PhNH₂) in an Ar matrix mainly generated the aniline cation (PhNH₂⁺), anilino (PhNH) and phenyl (Ph) radicals, and phenylnitrene (PhN). Further irradiation of the electron-bombarded matrix sample at 365 nm depleted PhNH₂⁺ and PhN, and resulted in the formation of PhNH₂, PhNH, and Ph. In separate experiments, irradiation of the PhNH₂/Ar matrix samples at 265 or 160 nm mainly generated PhNH and Ph radicals, but without the formation of PhNH₂⁺ and PhN. According to the observed photochemical behaviors, quantum-chemically predicted harmonic vibrational wavenumbers of each species, and the information reported in previous photodissociation studies, we unambiguously characterized the IR features of the aromatic species. The information of the vibrational fundamentals of PhNH is new and the formation mechanism is discussed.

Deep learning study of tyrosine reveals that roaming can lead to photodamage

Amino acids are among the building blocks of life, forming peptides and proteins, and have been carefully 'selected' to prevent harmful reactions caused by light. To prevent photodamage, molecules relax from electronic excited states to the ground state faster than the harmful reactions can occur; however, such photochemistry is not fully understood, in part because theoretical simulations of such systems are extremely expensive-with only smaller chromophores accessible. Here, we study the excited-state dynamics of tyrosine using a method based on deep neural networks that leverages the physics underlying quantum chemical data and combines different levels of theory. We reveal unconventional and dynamically controlled 'roaming' dynamics in excited tyrosine that are beyond chemical intuition and compete with other ultrafast deactivation mechanisms. Our findings suggest that the roaming atoms are radicals that can lead to photodamage, offering a new perspective on the photostability and photodamage of biological systems.

Ab Initio Electronic Structure Study of the Photoinduced Reduction of Carbon Dioxide with the Heptazinyl Radical

The photocatalytic conversion of carbon dioxide to liquid fuels with electrons taken from water with solar photons is one of the grand goals of renewable energy research. Polymeric carbon nitrides recently emerged as metal-free materials with promising functionalities for hydrogen evolution from water as well as the activation of carbon dioxide. Molecular heptazine (Hz), the building block of polymeric carbon nitrides, is one the strongest known organic photo-oxidants and has been shown to be able to photo-oxidize water with near-visible light, resulting in reduced (hydrogenated) heptazine (HzH) and OH radicals. In the present work, we explored with ab initio computational methods whether the HzH chromophore is able to reduce carbon dioxide to the hydroxy-formyl (HOCO) radical in hydrogen-bonded HzH-CO₂ complexes by the absorption of a photon. In remarkable contrast to the high barrier for carbon dioxide activation in the electronic ground state, the excited-state proton-coupled electron transfer (PCET) reaction is nearly barrierless, but requires the diabatic passage of three conical intersections. The possibility of barrierless carbon dioxide activation by excited-state PCET has so far not been taken into consideration in the interpretation of photocatalytic carbon dioxide reduction on carbon nitride materials.

Exciton interactions in helical crystals of a hydrogen-bonded eumelanin monomer

Eumelanin, a naturally occurring group of heterogeneous polymers/aggregates providing photoprotection to living organisms, consist of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) building blocks. Despite their prevalence in the animal world, the structure and therefore the mechanism behind the photoprotective broadband absorption and non-radiative decay of eumelanin remain largely unknown. As a small step towards solving the incessant mystery, DHI is crystallized in a non-protic solvent environment to obtain DHI crystals having a helical packing motif. The present approach reflects the solitary directional effect of hydrogen bonds between the DHI chromophores for generating the crystalline assembly and filters out any involvement of the surrounding solvent environment. The DHI single crystals having an atypical chiral packing motif (P212121 Sohncke space group) incorporate enantiomeric zig-zag helical stacks arranged in a herringbone fashion with respect to each other. Each of the zig-zag helical stacks originates from a bifurcated hydrogen bonding interaction between the hydroxyl substituents in adjacent DHI chromophores which act as the backbone structure for the helical assembly. Fragment-based excited state analysis performed on the DHI crystalline assembly demonstrates exciton delocalization along the DHI units that connect each enantiomeric helical stack while, within each stack, the excitons remain localized. Fascinatingly, over the time evolution for generation of single-crystals of the DHI-monomer, mesoscopic double-helical crystals are formed, possibly attributed to the presence of covalently connected DHI trimers in chloroform solution. The oligomeric DHI (in line with the chemical disorder model) along with the characteristic crystalline packing observed for DHI provides insights into the broadband absorption feature exhibited by the chromophore.

Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation

Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S₁(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S₁(ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S₁(ππ*) lifetime for the s-cis conformer of para-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S₁(ππ*) state lifetime of s-cis para-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [¹ππ*→¹nπ*→³ππ*→S₀] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate.

S1-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters

The S₁-state decaying rates of the three different benzenediols, catechol, resorcinol, and hydroquinone, and their 1:1 water clusters have been state-specifically measured using the picosecond time-resolved parent ion transients obtained by the pump (excitation) and probe (ionization) scheme. The S₁ lifetime of catechol is found to be short, giving τ ∼ 5.9 ps at the zero-point level. This is ascribed to the H-atom detachment from the free OH moiety of the molecule. Consistent with a previous report (J. Phys. Chem. Lett. 2013, 4, 3819-3823), the S₁ lifetime gets lengthened with low-frequency vibrational mode excitations, giving τ ∼ 9.0 ps for the 116 cm^-1 band. The S₁ lifetimes at the additional vibronic modes of catechol are newly measured, showing the nonnegligible mode-dependent fluctuations of the tunneling rate. When catechol is complexed with water, the S₁ lifetime is enormously increased to τ ∼ 1.80 ns at the zero-point level while it shows an unusual dip at the intermolecular stretching mode excitation (τ ∼ 1.03 ns at 146 cm^-1). Otherwise, it is shortened monotonically with increasing the internal energy, giving τ ∼ 0.67 ns for the 856 cm^-1 band. Two different asymmetric or symmetric conformers of resorcinol give the respective S₁ lifetimes of 4.5 or 6.3 ns at their zero-point levels according to the estimation from our transients taken within the temporal window of 0-2.7 ns. When resorcinol is 1:1 complexed with H₂O, the S₁ decaying rate is slightly accelerated for both conformers. The S₁ lifetimes of trans and cis forms of hydroquinone are measured to be more or less same, giving τ ∼ 2.8 ns at the zero-point level. When H₂O is complexed with hydroquinone, the S₁ decaying process is facilitated for both conformers, slightly more efficiently for the cis conformer.

Ultrafast decay dynamics of electronically excited 2-ethylpyrrole

The excited-state decay dynamics of 2-ethylpyrrole following UV excitation in the wavelength range of 254.8-218.0 nm is investigated in detail using the femtosecond time-resolved photoelectron imaging method. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analysed and the following picture is derived: at the longest pump wavelengths (254.8, 248.3 and 246.1 nm), 2-ethylpyrrole is excited to the S₁(¹πσ*) state having a lifetime of about 50 fs. At 248.3, 246.1 and 237.4 nm, another excited state of Rydberg character is excited. The lifetime of this state is ∼570 fs at 237.4 nm and becomes slightly longer at other two pump wavelengths. At the shortest pump wavelengths (230.8 and 218.0 nm), 2-ethylpyrrole is excited to a state which is tentatively assigned to the 1¹ππ* state, having a lifetime of 75 ± 15 and 48 ± 10 fs for the longer and shorter pump wavelengths, respectively. Internal conversion to the S₁(¹πσ*) state might be one of the decay mechanisms of the 1¹ππ* state.

Role of Triplet States in the Photodynamics of Aniline

The dynamics of excited heteroaromatic molecules is a key to understanding the photoprotective properties of many biologically relevant chromophores that dissipate their excitation energy nonreactively and thereby prevent the detrimental effects of ultraviolet radiation. Despite their structural variability, most substituted aromatic compounds share a common feature of a repulsive ¹πσ* potential energy surface. This surface can lead to photoproducts, and it can also facilitate the population transfer back to the ground electronic state by means of a ¹πσ*/S₀ conical intersection. Here, we explore a hidden relaxation route involving the triplet electronic state of aniline, which has recently been discovered by means of time-selected photofragment translational spectroscopy [J. Chem. Phys. 2019, 151, 141101]. By using the recently available analytical gradients for multiconfiguration pair-density functional theory, it is now possible to locate the minimum-energy crossing points between states of different spin and therefore compute the intersystem crossing rates with a multireference method, rather than with the less reliable single-reference methods. Using such calculations, we demonstrate that the population loss of aniline in the T₁(³ππ*) state is dominated by C₆H₅NH₂ → C₆H₅NH· + H· dissociation, and we explain the long nonradiative lifetimes of the T₁(³ππ*) state at the excitation wavelengths of 294-264 nm.

Nonadiabatic dynamics simulation of photoinduced ring-opening reaction of 2(5H)-thiophenone with internal conversion and intersystem crossing

In the present work, the quantum trajectory mean-field approach, which is able to overcome the overcoherence problem, was generalized to simulate internal conversion and intersystem crossing processes simultaneously. The photoinduced ring-opening and subsequent rearrangement reactions of isolated 2(5H)-thiophenone were studied based on geometry optimizations on critical structures and nonadiabatic dynamics simulations using this method. Upon 267 nm irradiation, the molecule is initially populated in the 1ππ* state. After a sudden rupture of one C-S bond within 100 fs in this state, the lowest two singlet excited states and the lowest two triplet excited states become quasi-degenerated, and then the intersystem crossing processes between singlet and triplet states accompanied by rearrangement reactions can be observed several times. Compared with our previous nonadiabatic simulations in the absence of intersystem crossing (ChemPhotoChem, 2019, 3, 897-906), some new nonadiabatic relaxation pathways involving triplet states and different ring-opening products were identified. The present work provides new mechanistic insights into the photoinduced ring-opening of thio-substituted heterocyclic molecules and reveals the importance of nonadiabatic dynamics simulation that is able to deal with multiple electronic states with different spin multiplicities.

Improved insights in time-resolved photoelectron imaging

We review new light source developments and data analysis considerations relevant to the time-resolved photoelectron imaging technique. Case studies illustrate how these themes may enhance understanding in studies of excited state molecular dynamics.

Functional Group-Directed Photochemical Reactions of Aromatic Alcohols, Amines, and Thiols Triggered by Excited-State Hydrogen Detachment: Additive-free Oligomerization, Disulfidation, and C(sp2)-H Carboxylation with CO2

Exploring new types of photochemical reactions is of great interest in the field of synthetic chemistry. Although excited-state hydrogen detachment (ESHD) represents a promising prospective template for additive-free photochemical reactions, applications of ESHD in a synthetic context remains scarce. Herein, we demonstrate the expansion of this photochemical reaction toward oligomerization, disulfidation, and regioselective C(sp²)-H carboxylation of aromatic alcohols, thiols, and amines. In the absence of any radical initiators in tetrahydrofuran upon irradiation with UV light (λ = 280 or 300 nm) under an atmosphere of N₂ or CO₂, thiols and catechol afforded disulfides and oligomers, respectively, as main products. Especially, the photochemical disulfidation proceeded highly selectively with the NMR and quantum yields of up to 69 and 0.46%, respectively. In stark contrast, the photolysis of phenylenediamines and aminophenols results in photocarboxylation in the presence of CO₂ (1 atm). p-Aminophenol was quantitatively carboxylated by photolysis for 17 h with a quantum yield of 0.45%. Furthermore, the photocarboxylation of phenylenediamines and aminophenols proceeds in a highly selective fashion on the aromatic C(sp²)-H bond next to a functional group, which is directed by the site-selective ESHD of the functional groups for the formation of aminyl and hydroxyl radicals.

The Role of Norrish Type-I Chemistry in Photoactive Drugs: An ab initio Study of a Cyclopropenone-Enediyne Drug Precursor

We present a contemporary mechanistic description of the light-driven conversion of cyclopropenone containing enediyne (CPE) precusors to ring-opened species amenable to further Bergman cyclization and formation of stable biradical species that have been proposed for use in light-induced cancer treatment. The transformation is rationalized in terms of (purely singlet state) Norrish type-I chemistry, wherein photoinduced opening of one C-C bond in the cyclopropenone ring facilitates non-adiabatic coupling to high levels of the ground state, subsequent loss of CO and Bergman cyclization of the enediyne intermediate to the cytotoxic target biradical species. Limited investigations of substituent effects on the ensuing photochemistry serve to vindicate the experimental choices of Popik and coworkers (J. Org. Chem., 2005, 70, 1297-1305). Specifically, replacing the phenyl moiety in the chosen model CPE by a 1,4-benzoquinone unit leads to a stronger, red-shifted parent absorption, and increases the exoergicity of the parent → biradical conversion.

Excited-state hydrogen detachment from a tris-(o-phenylenediamine) iron(ii) complex in THF at room temperature

We previously reported that a tris-(o-phenylenediamine) iron(ii) complex promotes photochemical H₂ generation and C-H carboxylation of o-phenylenediamine without any additives under N₂ and CO₂ atmospheres, respectively, in tetrahydrofuran at room temperature. Herein, the key mechanistic process, namely, excited-state hydrogen detachment from the o-phenylendiamine moiety, is demonstrated under an N₂ atmosphere.

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.

Exploring the Photochemistry of an Ethyl Sinapate Dimer: An Attempt Toward a Better Ultraviolet Filter

The photochemistry and photostability of a potential ultraviolet (UV) radiation filter, dehydrodiethylsinapate, with a broad absorption in the UVA region, is explored utilizing a combination of femtosecond time-resolved spectroscopy and steady-state irradiation studies. The time-resolved measurements show that this UV filter candidate undergoes excited state relaxation after UV absorption on a timescale of ~10 picoseconds, suggesting efficient relaxation. However, steady-state irradiation measurements show degradation under prolonged UV exposure. From a photochemical standpoint, this highlights the importance of considering both the ultrafast and “ultraslow” timescales when designing new potential UV filters.

Cation, Anion, and Radical Isomers of C4H4N: Computational Characterization and Implications for Astrophysical and Planetary Environments

Nitrogen-containing ions and molecules in the gas phase have been detected in non-Earth environments such as dark molecular clouds and more recently in the atmosphere of Saturn's moon Titan. These molecules may serve as precursors to larger heterocyclic structures that provide the foundation of complex biological molecules. On Titan, molecules of m/z 66 have been detected by the Cassini mission, and species of the empirical formula C₄H₄N may contribute to this signature. We have characterized seven isomers of C₄H₄N in anionic, neutral radical, and cationic states using density functional theory. Structures were optimized using the range-separated hybrid ωB97X-V with the cc-pVTZ and aug-cc-pVTZ basis sets. Anionic and radical C₄H₄N favor cyclic structures with aromatic and quasi-aromatic electron arrangements, respectively. Interestingly, ionization from the radical surface to the cation induces significant changes in structural stability, and the global minimum for positively charged isomers is CH₂CCHCNH⁺, a pseudo-linear species reminiscent of cyanoallene. Select formation pathways to these structures from Titan's existing or postulated gas-phase species, reactions that are also relevant for other astrophysical environments, are discussed. By characterizing C₄H₄N isomers, we have identified energetically stable anionic, radical, and cationic structures that may be present in Titan's atmosphere and dark molecular clouds.

Short-wavelength probes in time-resolved photoelectron spectroscopy: an extended view of the excited state dynamics in acetylacetone

Time-resolved photoelectron spectroscopy using a vacuum ultraviolet probe brings new insight to the excited state dynamics operating in acetylacetone.

Unraveling the Mechanisms of the Excited-State Intermolecular Proton Transfer (ESPT) for a D-π-A Molecular Architecture

Precise revealing the mechanisms of excited-state intermolecular proton transfer (ESPT) and the corresponding geometrical relaxation upon photoexcitation and photoionization remains a formidable challenge. In this work, the compound (E)-4-(((4H-1,2,4-triazol-4-yl)imino)methyl)-2,6-dimethoxyphenol (TIMDP) adopting a D-π-A molecular architecture featuring a significant intramolecular charge transfer (ICT) effect has been designed. With the presence of perchloric acid (35 %), TIMDP can be dissolved through the formation of a HClO₄ -H₂ O-OH(TIMDP)-N(TIMDP) hydrogen-bonding bridge. At the ground state, the ICT effect is dominant, giving birth to crystals of TIMDP. Upon external stimuli (e.g., UV light irradiation, electro field), the excited state is achieved, which weakens the ICT effect, and significantly promotes the ESPT effect along the hydrogen-bonding bridge, resulting in crystals of [HTIMDP]⁺ ⋅[H₂ O]⋅[ClO₄ ]^- . As a consequence, the mechanisms of the ESPT can be investigated, which distorted the D-π-A molecular architecture, tuned the emission color with the largest Stokes shift of 242 nm, and finally, high photoluminescence quantum yields (12 %) and long fluorescence lifetimes (8.6 μs) have achieved. These results not only provide new insight into ESPT mechanisms, but also open a new avenue for the design of efficient ESPT emitters.

Experimental observation of nonadiabatic bifurcation dynamics at resonances in the continuum

The surface crossing of bound and unbound electronic states in multidimensional space often gives rise to resonances in the continuum. This situation happens in the πσ*-mediated photodissociation reaction of 2-fluorothioanisole; optically-bright bound S1 (ππ*) vibrational states of 2-fluorothioanisole are strongly coupled to the optically-dark S2 (πσ*) state, which is repulsive along the S-CH3 elongation coordinate. It is revealed here that the reactive flux prepared at such resonances in the continuum bifurcates into two distinct reaction pathways with totally different dynamics in terms of energy disposal and nonadiabatic transition probability. This indicates that the reactive flux in the Franck-Condon region may either undergo nonadiabatic transition funneling through the conical intersection from the upper adiabat, or follow a low-lying adiabatic path, along which multiple dynamic saddle points may be located. Since 2-fluorothioanisole adopts a nonplanar geometry in the S1 minimum energy, the quasi-degenerate S1/S2 crossing seam in the nonplanar geometry, which lies well below the planar S1/S2 conical intersection, is likely responsible for the efficient vibronic coupling, especially in the low S1 internal energy region. As the excitation energy increases, bound-to-continuum coupling is facilitated with the aid of intramolecular vibrational redistribution, along many degrees of freedom spanning the large structural volume. This leads to the rapid domination of the continuum character of the reactive flux. This work reports direct and robust experimental observations of the nonadiabatic bifurcation dynamics of the reactive flux occurring at resonances in the continuum of polyatomic molecules.

Exploring Norrish type I and type II reactions: an ab initio mechanistic study highlighting singlet-state mediated chemistry

Norrish reactions are important photo-induced reactions in mainstream organic chemistry and are implicated in many industrially and biologically relevant processes and in the processing of carbonyl molecules in the atmosphere.

Ultrafast photodissociation dynamics of pyrazole, imidazole and their deuterated derivatives using ab initio multiple cloning

Fully quantum nonadiabatic dynamics calculation of photodissociation of azoles shows good agreement with experiment and foreshadows the predictive ability of the method.

Ultrafast photodissociation dynamics of 2-ethylpyrrole: adding insight to experiment with ab initio multiple cloning

Fully quantum nonadiabatic dynamics calculation of 2-ethylpyrrole photodissociation shows the experimentally obtained ultrafast time-constant has two components.

New insights into the dissociation dynamics of methylated anilines

Combined time-resolved photoelectron spectroscopy and photofragment imaging supports a possible valence-to-Rydberg decay mechanism in methylated anilines.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

We survey and assess current knowledge regarding the primary photochemistry of hydrocarbon molecules and radicals.