Optimal control of photodissociation of phenol using genetic algorithm

Photodissociation dynamics of the OH bond of phenol is studied with an optimally shaped laser pulse. The theoretical model consists of three electronic states (the ground electronic state, ππ* state, and πσ* state) in two nuclear coordinates (the OH stretching coordinate as a reaction coordinate, r, and the CCOH dihedral angle as a coupling coordinate, θ). The optimal UV laser pulse is designed using the genetic algorithm, which optimizes the total dissociative flux of the wave packet. The latter is calculated in the adiabatic asymptotes of the S0 and S1 electronic states of phenol. The initial state corresponds to the vibrational levels of the electronic ground state and is defined as | n r, n θ⟩, where n r and n θ represent the number of nodes along r and θ, respectively. The optimal UV field excites the system to the optically dark πσ* state predominantly over the optically bright ππ* state with the intensity borrowing effect for the |0, 0⟩ and |0, 1⟩ initial states. For the |0, 0⟩ initial condition, the photodissociation to the S1 asymptotic channel is favored slightly over the S0 asymptotic channel. Addition of one quantum of energy along the coupling coordinate increases the dissociation probability in the S1 channel. This is because the wave packet spreads along the coupling coordinate on the πσ* state and follows the adiabatic path. Hence, the S1 asymptotic channel gets more ([Formula: see text]11%) dissociative flux as compared to the S0 asymptotic channel for the |0, 1⟩ initial condition. The |1, 0⟩ and |1, 1⟩ states are initially excited to both the ππ* and πσ* states in the presence of the optimal UV pulse. For these initial conditions, the S1 channel gets more dissociative flux as compared to the S0 channel. This is because the high energy components of the wave packet readily reach the S1 channel. The central frequency of the optimal UV pulse for the |0, 0⟩ and |0, 1⟩ initial states has a higher value as compared to the |1, 0⟩ and |1, 1⟩ initial states. This is explained with the help of an excitation mechanism of a given initial state in relation to its energy.

Revealing the role of excited state proton transfer (ESPT) in excited state hydrogen transfer (ESHT): systematic study in phenol-(NH3) n clusters

Excited State Hydrogen Transfer (ESHT), proposed at the end of the 20th century by the corresponding authors, has been observed in many neutral or protonated molecules and become a new paradigm to understand excited state dynamics/photochemistry of aromatic molecules. For example, a significant number of photoinduced proton-transfer reactions from X–H bonds have been re-defined as ESHT, including those of phenol, indole, tryptophan, aromatic amino acid cations and so on. Photo-protection mechanisms of biomolecules, such as isolated nucleic acids of DNA, are also discussed in terms of ESHT. Therefore, a systematic and up-to-date description of ESHT mechanism is important for researchers in chemistry, biology and related fields. In this review, we will present a general model of ESHT which unifies the excited state proton transfer (ESPT) and the ESHT mechanisms and reveals the hidden role of ESPT in controlling the reaction rate of ESHT. For this purpose, we give an overview of experimental and theoretical work on the excited state dynamics of phenol–(NH₃)_n clusters and related molecular systems. The dynamics has a significant dependence on the number of solvent molecules in the molecular cluster. Three-color picosecond time-resolved IR/near IR spectroscopy has revealed that ESHT becomes an electron transfer followed by a proton transfer in highly solvated clusters. The systematic change from ESHT to decoupled electron/proton transfer according to the number of solvent molecules is rationalized by a general model of ESHT including the role of ESPT.

A general model of excited state hydrogen transfer (ESHT) which unifies ESHT and the excited state proton transfer (ESPT) is presented from experimental and theoretical works on phenol–(NH₃)_n. The hidden role of ESPT is revealed.

Proton Transfer Reaction Rates in Phenol-Ammonia Cluster Cation

Proton transfer (PT) in an interaction system of a hydroxyl-amino group (OH-NH) plays a crucial role in photoinduced DNA and enzyme damage. A phenol-ammonia cluster is a prototype of an OH-NH interaction and is sometimes used as a DNA model. In the present study, the reaction dynamics of phenol-ammonia cluster cations, [PhOH-(NH₃)_n]⁺ (n = 1-5), following ionization of the neutral parent clusters, were investigated using a direct ab initio molecular dynamics (AIMD) method. In all clusters, PTs from PhOH⁺ to (NH₃)_n were found postionization, the reaction of which is expressed as PhOH⁺-(NH₃)_n → PhO-H⁺(NH₃)_n. The time of the PT was calculated as 43 (n = 1), 26 (n = 2), and 13 fs (n = 3-5), suggesting that the rate of PT increases with an increase in n and is saturated at n = 3-5. The difference in the PT rate originates strongly from the proton affinity of the (NH₃)_n cluster. In the case of n = 3-5, a second PT was found, the reaction of which is expressed as PhO-H⁺(NH₃)_n → PhO-NH₃-H⁺(NH₃)_n-1, and a third PT occurred at n = 4 and 5. The time of the PT was calculated as 10-13 (first PT), 80-100 (second PT), and 150-200 fs (third PT) in the case of larger clusters (n = 4 and 5). The reaction mechanism based on the theoretical results is discussed herein.

Bend-to-Break: Curvilinear Proton Transfer in Phenol-Ammonia Clusters

The electric field experienced by the OH group of phenol embedded in the cluster of ammonia molecules depends on the relative orientation of the ammonia molecules, and a critical field of 236 MV cm^-1 is essential for the transfer of a proton from phenol to the surrounding ammonia cluster. However, exceptions to this rule were observed, which indicates that the projection of the solvent electric field over the O-H bond is not a definite descriptor of the proton transfer reaction. Therefore, a critical electric field is necessary, but it is not a sufficient condition for the proton abstraction. This, in combination with an adequate solvation of the acceptor ammonia molecule in a triple donor motif that energetically favors the proton transfer process, constitutes necessary and sufficient conditions for the spontaneous proton abstraction. The proton transfer process in phenol-(ammonia)_n clusters is statistically favored to occur away from the plane of the phenyl ring and follows a curvilinear path which includes the O-H bond elongation and out-of-plane movement of the proton. Colloquially, this proton transfer can be referred to as a "bend-to-break" process.

Excited state hydrogen transfer dynamics in phenol-(NH3)2 studied by picosecond UV-near IR-UV time-resolved spectroscopy

Time-evolutions of excited state hydrogen transfer (ESHT) in phenol (PhOH)-(NH₃)₂ clusters have been measured by three-color picosecond (ps) ultraviolet (UV)-near infrared (NIR)-UV pump-probe ion dip spectroscopy. The formation of a reaction product, ˙NH₄NH₃, is detected by its NIR absorption due to a 3p-3s Rydberg transition. The ESHT reactions from all of the vibronic levels show biexponential time-evolutions, even from the S₁ origin. Based on the biexponential time-evolution, it is suggested that there is a second reaction path via the triplet πσ* state, which gives the slow component. The fast time-evolution of the ESHT reaction from the S₁ origin is measured to be 268 ps, which is 10-times slower than that in PhOH-(NH₃)₃, and a higher barrier between the ππ* and reactive πσ* states is suggested. The size dependence of the ESHT reaction rates is discussed based on a potential distortion due to the proton transferred state in the ππ* potential surface.

Identification of an Emitting Metastable State of p-Fluorophenol-Ammonia 1:2 Complex by Laser-Induced Fluorescence Spectroscopy

We have demonstrated here, for the first time to our knowledge, the formation of an emitting metastable species upon lowest electronic excitation (S₁) of a hydrogen-bonded 1:2 complex of para-fluorophenol (pFP) with ammonia (NH₃), which is known to be one of the smallest reactive complexes to undergo excited state H-atom transfer (HAT) reaction to produce ^•NH₄(NH₃) radical fragment. The emission spectrum of the species is characterized to be red-shifted, broad, and structureless. From the viewpoint of energy balance, an excited state proton transfer (ESPT) is unfavorable, but according to predicted electronic structure parameters, the metastable state species could be stabilized by charge transfer (CT) interaction at the hydrogen-bonded geometry of the complex. We propose that this species could act as an intermediate to the HAT process in the excited state. The observation of such a state could be valuable to understand the complex dynamics of similar events in biologically relevant systems.

Influence of the solvent in the electronic excitation of aromatic alcohols: Excited state IR-UV of propofol(H2O)8

It is well known that water plays an important role in the reactivity and dynamics in a solution of molecules in electronic excited states. For example, electronic excitation is usually accompanied by a solvent rearrangement that may also influence the redistribution of the excitation energy. However, there is a lack of experimental data on such processes. Here, we explore the structural changes that follow electronic excitation in aggregates of propofol (2,6-diisopropylphenol) with up to eight water molecules, using a combination of mass-resolved excitation spectroscopy and density functional theory calculations. The molecules of water form a polyhedron around the hydroxyl group of propofol, also interacting with the π cloud of the aromatic ring. Electronic excitation produces a strong structural change in the water superstructure, which moves to an interaction with one of the carbon atoms of the aromatic ring, producing its distortion into a prefulvenic structure. Such deformation is not observed in smaller water clusters or in propofol-phenol aggregates highlighting the decisive role played by the solvent.

Solvent reorganization triggers photo-induced solvated electron generation in phenol

Charge-transfer states with large electron–hole separation, correlating to the formation of solvated electrons, are found below the maximum of the absorbing ππ* band of solvated phenol.

Stereochemistry-dependent structure of hydrogen-bonded protonated dimers: the case of 1-amino-2-indanol

Stereochemistry effects on the structure of molecular aggregates are studied in the prototypical 1-amino-2-indanol. Conformer-selective IR-UV double resonance spectroscopy reveals how stereochemistry shapes its dimers.

Electron-Proton Transfer Mechanism of Excited-State Hydrogen Transfer in Phenol-(NH3 )n (n=3 and 5)

Excited-state hydrogen transfer (ESHT) is responsible for various photochemical processes of aromatics, including photoprotection of nuclear basis. Its mechanism is explained by internal conversion from the aromatic ππ* to πσ* states via conical intersection. This means that the electron is transferred to a diffuse Rydberg-like σ* orbital apart from proton migration. This picture means the electron and the proton do not move together and the dynamics are different in principle. Here, we have applied picosecond time-resolved near-infrared (NIR) and infrared (IR) spectroscopy to the phenol-(NH₃ )₅ cluster, the benchmark system of ESHT, and monitored the electron transfer and proton motion independently. The electron transfer monitored by the NIR transition rises within 3 ps, while the overall H transfer detected by the IR absorption of NH vibration appears with a lifetime of about 20 ps. This clearly proves that the electron motion and proton migration are decoupled. Such a difference of the time-evolutions between the NIR absorption and the IR transition has not been detected in a cluster with three ammonia molecules. We will report our full observation together with theoretical calculations of the potential energy surfaces of the ππ* and πσ* states, and will discuss the ESHT mechanism and its cluster size-dependence between n=3 and 5. It is suggested that the presence and absence of a barrier in the proton transfer coordinate cause the different dynamics.

Watching proton transfer in real time: Ultrafast photoionization-induced proton transfer in phenol-ammonia complex cation

In this paper, we give a full account of our previous work [C. C. Shen et al., J. Chem. Phys. 141, 171103 (2014)] on the study of an ultrafast photoionization-induced proton transfer (PT) reaction in the phenol-ammonia (PhOH-NH₃) complex using ultrafast time-resolved ion photofragmentation spectroscopy implemented by the photoionization-photofragmentation pump-probe detection scheme. Neutral PhOH-NH₃ complexes prepared in a free jet are photoionized by femtosecond 1 + 1 resonance-enhanced multiphoton ionization via the S₁ state. The evolving cations are then probed by delayed pulses that result in ion fragmentation, and the ionic dynamics is followed by measuring the parent-ion depletion as a function of the pump-probe delay time. By comparing with systems in which PT is not feasible and the steady-state ion photofragmentation spectra, we concluded that the observed temporal evolutions of the transient ion photofragmentation spectra are consistent with an intracomplex PT reaction after photoionization from the initial non-PT to the final PT structures. Our experiments revealed that PT in [PhOH-NH₃]⁺ cation proceeds in two distinct steps: an initial impulsive wave-packet motion in ∼70 fs followed by a slower relaxation of about 1 ps that stabilizes the system into the final PT configuration. These results indicate that for a barrierless PT system, even though the initial PT motions are impulsive and ultrafast, the time scale to complete the reaction can be much slower and is determined by the rate of energy dissipation into other modes.

Photofragmentation mechanisms in protonated chiral cinchona alkaloids

Photo-fragmentation of protonated alkaloids results in C₈–C₉ cleavage accompanied or not by hydrogen migration, with a stereochemistry-dependent branching ratio.

Trapped Hydronium Radical Produced by Ultraviolet Excitation of Substituted Aromatic Molecule

The gas phase structure and excited state dynamics of o-aminophenol-H2O complex have been investigated using REMPI, IR-UV hole-burning spectroscopy, and pump-probe experiments with picoseconds laser pulses. The IR-UV spectroscopy indicates that the isomer responsible for the excitation spectrum corresponds to an orientation of the OH bond away from the NH2 group. The water molecule acts as H-bond acceptor of the OH group of the chromophore. The complexation of o-aminophenol with one water molecule induced an enhancement in the excited state lifetime on the band origin. The variation of the excited state lifetime of the complex with the excess energy from 1.4 ± 0.1 ns for the 0-0 band to 0.24 ± 0.3 ns for the band at 0-0 + 120 cm(-1) is very similar to the variation observed in the phenol-NH3 system. This experimental result suggests that the excited state hydrogen transfer reaction is the dominant channel for the non radiative pathway. Indeed, excited state ab initio calculations demonstrate that H transfer leading to the formation of the H3O(•) radical within the complex is the main reactive pathway.

The mechanism of excited-state proton transfer in 1-naphthol–piperidine clusters

Photoexcitation directly triggers proton transfer in 1-naphthol–(piperidine)_n. This mechanism is essentially different from 1-naphthol–(NH₃)_n in which the internal conversion process is required to promote excited-state proton transfer.

Photophysics and photochemistry of cis- and trans-hydroquinone, catechol and their ammonia clusters: a theoretical study

Excited state hydrogen transfer in hydroquinone- and catechol-ammonia clusters has been extensively investigated by high level ab initio methods. The potential energy profiles of the title systems at different electronic states have been determined at the MP2/CC2 levels of theory. It has been predicted that double hydrogen transfer (DHT) takes place as the main consequence of photoexcited tetra-ammoniated systems. Consequently, the DHT processes lead the excited systems to the (1)πσ*-S0 conical intersections, which is responsible for the ultrafast non-radiative relaxation of UV-excited clusters to their ground states. Moreover, according to our calculated results, the single hydrogen detachment or hydrogen transfer process essentially governs the relaxation dynamics of smaller sized clustered systems (mono- and di-ammoniated).

Microhydration effects on the electronic properties of protonated phenol: a theoretical study

The CC2 (second-order approximate coupled cluster method) has been employed to investigate microhydration effect on electronic properties of protonated phenol (PhH(+)) According to the CC2 calculation results on electronic excited states of microhydrated PhH(+), for the S1 and S2 electronic states, which are of (1)ππ* nature and belong to the A' representation of molecular Cs point group, a significant blue shift effect on the S1 and S2 electronic states, which are of 1ππ* nature and belong to the A' representation of molecular Cs point group, in comparison to corresponding transitions on bare cation (PhH(+)), has been predicted. Nevertheless, for the S3-S0 (1A'', 1σπ*) transition, a large red shift effect has been predicted. Furthermore, it has been found that the lowest (1)σπ* state plays a prominent role in the photochemistry of these systems. In the bare protonated phenol, the (1)σπ* state is a bound state with a broad potential curve along the OH stretching coordinate, while it is dissociative in microhydrated species. This indicates to a predissociation of the S1((1)ππ*) state by a low-lying (1)σπ* state, which leads the excited system to a concerted proton-transfer reaction from protonated chromophore to the solvent. The dissociative (1)σπ* state in monohydrated PhH(+) has small barrier, while increasing the solvent molecules up to three removes the barrier and consequently expedites the proton-transfer reaction dynamics.

Hole-burning spectra of m-fluorophenol/ammonia (1:3) clusters and their excited state hydrogen transfer dynamics

Hole-burning spectra of m-fluorophenol/ammonia (1:3) clusters are measured by four-color UV-near IR-UV-UV hole-burning spectroscopy. Cis and trans isomers of the cluster are clearly distinguished in the (1:3) cluster. Picosecond time evolutions of the excited state hydrogen transfer (ESHT) reaction in the (1:3) clusters are measured by the ion depletion due to 3p-3s Rydberg transition of reaction products ⋅NH(4)(NH(3))(2) lying in the near infrared region. From the wavelength dependence of the time evolution, we have concluded 1) the initial formation of a metastable ⋅NH(4)-NH(3)-NH(3) radical and 2) successive isomerization to the most stable NH(3)-⋅NH(4) -NH(3) radical in both cis and trans isomers. The reaction lifetimes of ESHT are determined by the rate equation analysis as 32.4 and 31.8 ps for the cis and trans isomer, respectively, and the isomerization and its back-reaction lifetime of both isomers are determined to be 3.3 ps and 11.2 ps. The almost same reaction rates are consistent with the similarity of the hydrogen bond networks in both clusters.

Computational studies on electron and proton transfer in phenol-imidazole-base triads

The electron and proton transfer in phenol-imidazole-base systems (base = NH(2)(-) or OH(-)) were investigated by density-functional theory calculations. In particular, the role of bridge imidazole on the electron and proton transfer was discussed in comparison with the phenol-base systems (base = imidazole, H(2)O, NH(3), OH(-), and NH(2)(-)). In the gas phase phenol-imidazole-base system, the hydrogen bonding between the phenol and the imidazole is classified as short strong hydrogen bonding, whereas that between the imidazole and the base is a conventional hydrogen bonding. The n value in sp(n) hybridization of the oxygen and carbon atoms of the phenolic CO sigma bond was found to be closely related to the CO bond length. From the potential energy surfaces without and with zero point energy correction, it can be concluded that the separated electron and proton transfer mechanism is suitable for the gas-phase phenol-imidazole-base triads, in which the low-barrier hydrogen bond is found and the delocalized phenolic proton can move freely in the single-well potential. For the gas-phase oxidized systems and all of the triads in water solvent, the homogeneous proton-coupled electron transfer mechanism prevails.

Photodissociation dynamics of thiophenol-d1: the nature of excited electronic states along the S-D bond dissociation coordinate

The S-D bond dissociation dynamics of thiophenol-d1 (C6H5SD) pumped at 266, 243, and 224 nm are examined using the velocity map ion imaging technique. At both 266 and 243 nm, distinct peaks associated with X and A states of the phenylthiyl radical (C6H5S*) are observed in the D+ image at high and low kinetic energy regions, respectively. The partitioning of the available energy into the vibrational energy of the phenylthiyl radical is found to be enhanced much more strongly at 266 nm compared to that at 243 nm. This indicates that the pipi* electronic excitation at 266 nm is accompanied by significant vibrational excitation. Given the relatively large anisotropy parameter of -0.6, the S-D dissociation at 266 nm is prompt and should involve the efficient coupling to the upper-lying n(pi)sigma* repulsive potential energy surface. The optical excitation of thiophenol at 224 nm is tentatively assigned to the pisigma* transition, which leads to the fast dissociation on the repulsive potential energy surface along the S-D coordinate. The nature of the electronic transitions associated with UV absorption bands is investigated with high-level ab initio calculations. Excitations to different electronic states of thiophenol result in unique branching ratios and vibrational excitations for the fragment of the phenylthiyl radical in the two lowest electronic states.

Biradicalic excited states of zwitterionic phenol-ammonia clusters

Phenol-ammonia clusters with more than five ammonia molecules are proton transferred species in the ground state. In the present work, the excited states of these zwitterionic clusters have been studied experimentally with two-color pump probe methods on the nanosecond time scale and by ab initio electronic-structure calculations. The experiments reveal the existence of a long-lived excited electronic state with a lifetime in the 50-100 ns range, much longer than the excited state lifetime of bare phenol and small clusters of phenol with ammonia. The ab initio calculations indicate that this long-lived excited state corresponds to a biradicalic system, consisting of a phenoxy radical that is hydrogen bonded to a hydrogenated ammonia cluster. The biradical is formed from the locally excited state of the phenolate anion via an electron transfer process, which neutralizes the charge separation of the ground state zwitterion.

Computational studies of the photophysics of neutral and zwitterionic amino acids in an aqueous environment: tyrosine-(H(2)O)(2) and tryptophan-(H(2)O)(2) clusters

Tyrosine-(H(2)O)(2) and tryptophan-(H(2)O)(2) clusters have been considered as models for the study of the photochemistry of neutral and zwitterionic tyrosine and tryptophan in an aqueous environment. It has been found that the detachment of neutral NH(3) in the S(1) state of the zwitterionic clusters leads to a low-lying conical intersection of the S(1) and S(0) energy surfaces. This conical intersection can provide the mechanism for efficient radiationless deactivation of the excited state back to the ground state or, alternatively, deamination (loss of ammonia). These results provide a mechanistic explanation of the efficient fluorescence quenching and the high quantum yield of ammonia in the UV photolysis of tyrosine and tryptophan in aqueous solution.

Direct versus indirect H atom elimination from photoexcited phenol molecules

The active role of the optically dark pi sigma* state, following UV absorption, has been implicated in the photochemistry of a number of biomolecules. This work focuses on the role of the pi sigma* state in the photochemistry of phenol upon excitation at 200 nm. By probing the neutral hydrogen following UV excitation, we show that hydrogen elimination along the dissociative pi sigma* potential energy surface occurs within 103 +/- 30 fs, indicating efficient coupling at the S1/S2 and S0/S2 conical intersections, with no identifiable role of statistical unimolecular decay of vibronically excited (S0) phenol in the timeframe of our measurements.