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

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

Revisiting photocyclization of the donor-acceptor stenhouse adduct: missing pieces in the mechanistic jigsaw discovered.

Donor-acceptor Stenhouse adducts (DASA) have recently emerged as a class of visible-light-induced photochromic molecular switches, but their photocyclization mechanism remains puzzling and incomplete. In this work, we carried out MS-CASPT2//SA-CASSCF calculations to reveal the complete mechanism of the dominant channels and possible side reactions. We found that a new thermal-then-photo isomerization channel, i.e., EEZ → EZZ → EZE, other than the commonly accepted EEZ → EEE → EZE channel, is dominant in the initial step. Besides, our calculations rationalized why the expected byproducts ZEZ and ZEE are unobserved and proposed a competitive stepwise channel for the final ring-closure step. The findings here redraw the mechanistic picture of the DASA reaction by better accounting for experimental observations, and more importantly, provide critical physical insight in understanding the interplay between thermal- and photo-induced processes widely present in photochemical synthesis and reactions.

Nonadiabatic molecular dynamics simulations for ultrafast photo-induced ring-opening and isomerization reactions of 2,2-diphenyl-2H-chromene.

Nonadiabatic molecular dynamics simulations with a global switching algorithm have been performed at the TD-CAM-B3LYP-D3/def2-SVP level of theory for ultrafast photo-induced ring-opening and isomerization reactions upon S1 excitation for 2,2-diphenyl-2H-chromene (DPC). Both DPC-T and DPC-C conformers undergo ring-opening relaxation and isomerization pathways accompanied with pyran conformation conserved and converted on the S1 or S0 states via competition and cooperation between C-O bond dissociation and pyran inversion motions. Upon S1 excitation, the DPC-T mainly relaxes to the T-type conical intersection region and thus yields a higher ring-opening efficiency with a faster S1 decay and intermediate formation than those of the DPC-C mainly relaxing to C-type conical intersection. The simulated ring-opening quantum yield for DPC-T (DPC-C) is 0.91 (0.76), which is in good agreement with the experimental value of 0.7-0.9, and the thermal weight averaged lifetimes are estimated as 182.0 fs, 228.6 fs, and 1262.4 fs for the excited-state decay, intermediate formation, and ring-opening product, respectively. These time constants are in good agreement with the experimentally measured τ1 time constant of 190-450 fs and τ2 time constant of 1000-1800 fs. The present work could be a valuable reference for understanding the nature of the photorelaxation mechanisms of DPC, and could help to develop DPC-based photoresponsive materials.

Fluorescence Excitation and Dispersed Fluorescence Spectra of the 1-Hydronaphthyl Radical (1-C10H9) in Solid para-Hydrogen.

Matrix isolation spectroscopy with para-hydrogen (p-H2) has previously been employed to record IR absorption spectra of hydrogenated and protonated polycyclic aromatic hydrocarbons (PAHs), prospective carriers of unidentified infrared and diffuse interstellar bands. Despite the promising prospects of p-H2 as matrix host, especially the rather weak interaction with the guest molecules and the resulting small matrix shifts, p-H2 matrix isolation spectroscopy has rarely been applied to study electronic transitions of guest molecules. Here, we present the dispersed fluorescence and fluorescence excitation spectrum of the 1-hydronaphthyl radical (1-C10H9) isolated in solid p-H2. We observed a strong 000 band associated with the electronic transition to the first excited electronic state at 18881 cm-1, red-shifted by ∼68 cm-1 relative to a value reported for jet-cooled 1-C10H9. From a comparison of our experimental results to simulated vibrationally resolved electronic absorption and emission spectra computed on the basis of (TD-)DFT geometry optimizations and scaled harmonic vibration calculations using the FCclasses code, we derived assignments for observed vibronic transitions. The dispersed fluorescence spectrum of 1-C10H9 is new; it complements the infrared spectrum and identified many vibrational modes unidentifiable with infrared. The excitation spectrum covers a much wider spectral range than previous reports. We compare the excitation spectrum in solid p-H2 to the reported electronic absorption spectrum of jet-cooled gaseous 1-C10H9 and that of 1-C10H9 isolated in solid Ne to assess the influence of p-H2 as a matrix host on the electronic transition of 1-C10H9 and discuss a potential contribution of 1-C10H9 to the diffuse interstellar bands.

Trajectory surface hopping molecular dynamics simulations for retinal protonated Schiff-base photoisomerization.

Global switching trajectory surface hopping molecular dynamics simulations are performed using accurate on-the-fly (TD)CAM-B3LYP/6-31G potential energy surfaces to study retinal protonated Schiff-base photoisomerization up to S1 excitation. The simulations detected two-layer conical intersection networks: one is at an energy as high as 8 eV and the other is in the energy range around 3-4 eV. Six conical intersections within the low-layer energy region that correspond to active conical intersections under experimental conditions are found via the use of pairwise isomers, within which nonadiabatic molecular dynamics simulations are performed. Eight isomer products are populated with simulated sampling trajectories from which the simulated quantum yield in the gas phase is estimated to be 0.11 (0.08) moving from the all-trans isomer to the 11-cis (11-cis to all-trans) isomer in comparison with an experimental value of 0.09 (0.2) in the solution phase. Each conical intersection is related to one specific twist angle accompanying a related CC double bond motion during photoisomerization. Nonplanar distortion of the entire dynamic process has a significant role in the formation of the relevant photoisomerization products. The present simulation indicates that all hopping points show well-behaved potential energy surface topology, as calculated via the conventional TDDFT method, at conical intersections between S1 and S0 states. Therefore, the present nonadiabatic dynamics simulations with the TDDFT method are very encouraging for simulating various large systems related to retinal Schiff-base photoisomerization in the real world.

Nonadiabatic molecular dynamics simulation for the ultrafast photoisomerization of dMe-OMe-NAIP based on TDDFT on-the-fly potential energy surfaces.

Global switching on-the-fly trajectory surface hopping molecular dynamics simulation was performed on the accurate TD-B3LYP/6-31G* potential energy surfaces for E-to-Z and Z-to-E photoisomerization of dMe-OMe-NAIP up to S1(ππ*) excitation. The present TD(DFT) simulation provides accurate calculation for conical intersections between the first-excited and ground states. Thus, simulated quantum yield and lifetime of 0.23 and 620 fs (0.15 and 600 fs) for E-to-Z (Z-to-E) isomerization are in good (relatively good) agreement with experimental observation of 0.25 and 480 fs (0.24 and 430 fs), respectively. Simulated results reveal that photoisomerization pathways are initially uphill to conical intersection zones on the S1 potential energy surface and then downhill to product zones. Three types of representative conical intersections are found for determining photoisomerization mechanisms: one is the rotation type responsible for reactive isomerization and the other two are close to E and Z configurations, respectively, only for nonreactive isomerization. The present conclusions can be held in general for similar large NAIP systems of photoinduced isomerization based on E and Z configurations.

Photoisomerization-mechanism-associated excited-state hydrogen transfer in 2'-hydroxychalcone revealed by on-the-fly trajectory surface-hopping molecular dynamics simulation.

By performing global-switching on-the-fly trajectory surface-hopping molecular dynamics simulation at the OM2/MRCI (14,15) quantum level, we probed the S3(ππ*) photoisomerization mechanisms associated with excited-state intramolecular hydrogen transfer for 2'-hydroxychalcone (2HC) within the interwoven conical intersection networks from four singlet electronic states (S3, S2, S1, and S0). The simulated quantum yields of 0.03 for cis-to-trans and zero for trans-to-cis photoisomerization were due to almost all the conical intersections being localized either in the cis-2HC or in trans-2HC region, and there was little chance for sampling trajectories to reach the rotation conical intersection (S1/S0) in between cis-2HC and trans-2HC that is key for reactive isomerization. The potential energy well on the S1 state in the trans-2HC region prevents trajectories from trans-to-cis photoisomerization, while the fact there is no well on S1 state in cis-2HC region opens a few chances for trajectories to reach the rotation conical intersections. The present simulation found that excited-state intramolecular hydrogen transfers in 2HC have a negative impact for reactive isomerization, and that hydrogen transfers take place on the S1 state, while back-transfer on the S0 state prevents sampling trajectories reaching rotational conical intersections. It was realized that it could be possible to enhance the quantum yield of 2HC photoisomerization by suppressing the hydrogen transfer (such as by changing an electron-donating substitution or adjusting the substitution position to decrease the acidity of the phenol group). From a perspective view of the potential energy surfaces, the theoretical design of such 2HC derivatives could enhance (control) the quantum yield by shifting the conical intersections away from the cis- and trans-region.

Functional and Basis Set Dependence for Time-Dependent Density Functional Theory Trajectory Surface Hopping Molecular Dynamics: Cis-Azobenzene Photoisomerization.

Within three functionals (TD-B3LYP, TD-BHandHLYP, and TD-CAM-B3LYP) in combination with four basis sets (3-21g, 6-31g, 6-31g(d), and cc-pvdz), global switching (GS) trajectory surface hopping molecular dynamics has been performed for cis-to-trans azobenzene photoisomerization up to the S1 (nπ*) excitation. Although all the combinations show artificial double-cone structure of conical intersection between ground and first excited states, simulated quantum yields and lifetimes are in good agreement with one another; 0.6 (±5%) and 40.5 fs (±10%) by TD-B3LYP, 0.5 (±10%) and 35.5 fs (±4%) by TD-BHandHLYP, and 0.44 (±9%) and 35.2 fs (±10%) by TD-CAM-B3LYP. By analyzing distributions of excited-state population decays, hopping spots, and typical trajectories with performance of 12 functional/basis set combinations, it has been concluded that functional dependence for given basis set is slightly more sensitive than basis set dependence for given functional. The present GS on-the-fly time-dependent density functional theory (TDDFT) trajectory surface hopping simulation can provide practical benchmark guidelines for conical intersection driven excited-state molecular dynamics simulation involving in large complex system within ordinary TDDFT framework. © 2019 Wiley Periodicals, Inc.

Quantum yields of singlet and triplet chemiexcitation of dimethyl 1,2-dioxetane: ab initio nonadiabatic molecular dynamic simulations.

The global nonadiabatic switching on-the-fly trajectory surface hopping simulation at the 8SA-CASSCF quantum level has been performed to estimate the quantum yield of chemiexcitation for the uncatalyzed decomposition reaction of the open-shell biradical trans-3,4-dimethyl-1,2-dioxetane system. The present ab initio nonadiabatic molecular dynamic simulation involving both conical intersection and intersystem crossing is to compute for the first time the population evolution of quantum yields at the four lowest singlet and four lowest triplet states. The simulated results demonstrate not only the stepwise dissociation of O-O and C-C bond breaking, but also confirm the existence of a biradical entropic trap which is responsible for chemiexcitation. The simulated quantum yield of the triplet chemiexcitation ΦT1 = 0.266 ± 0.096 agrees with the experimental value of 0.20 ± 0.04 very well. The present nonadiabatic molecular dynamic simulation of dimethyl 1,2-dioxetanes provides a further advanced understanding and stepping stone for future studies on chemi- and bio-luminescence.

The absorption and fluorescence spectra of 4-(3-methoxybenzylidene)-2-methyl-oxazalone interpreted by Franck-Condon simulation in various pH solvent environments.

The absorption and fluorescence spectra of 4-(3-methoxybenzylidene)-2-methyl-oxazalone (m-MeOBDI) dissolved in neutral, acidic, and basic solvent environments have been investigated and assigned by using Franck-Condon (FC) simulations at the quantum TDDFT level. Four different structures of m-MeOBDI in the ground and excited states are optimized and are found to be responsible for the observed absorption and fluorescence spectra. The (absorption) fluorescence of m-MeOBDI in pure methanol and neutral/basic methanol/water (1/9 vol) mixed solvent is found to arise from the (ground neutral N-I) excited neutral N-I* and cationic C-III* species, respectively. In acidic solvent, the absorption is found to arise from ground acidic C-II species, and the excited divalent cation DC-IV* is found to be formed in its excited state due to the excess H+ in the solution, and then it emits ∼560 nm fluorescence. FC simulations have also been employed to confirm our assignments as well as interpret the vibronic band profiles. As expected, the simulated emission spectrum of the divalent cationic species is in good agreement with the experimental observation. Therefore, within the present FC simulation, the observed absorption and fluorescence spectra have been reasonably interpreted and novel fluorescence mechanisms of m-MeOBDI in various pH solvent environments have been proposed.

IR-VUV spectroscopy of pyridine dimers, trimers and pyridine-ammonia complexes in a supersonic jet.

The infrared spectra of the C-H stretching vibrations of (pyridine)m, m = 1-3, and the N-H stretching vibrations of (pyridine)m-(NH3)n, m = 1, 2; n = 1-4, complexes were investigated by infrared (IR)-vacuum ultraviolet (VUV) spectroscopy under jet-cooled conditions. The ionization potential (IP0) of the pyridine monomer was determined to be 74 546 cm-1 (9.242 eV), while its complexes showed only smooth curves of the ionization thresholds at ∼9 eV, indicating large structural changes in the ionic form. The pyridine monomer exhibits five main features with several satellite bands in the C-H stretching region at 3000-3200 cm-1. Anharmonic calculations including Fermi-resonance were carried out to analyze the candidates of the overtone and combination bands which can couple to the C-H stretching fundamentals. For (pyridine)2 and (pyridine)3, most C-H bands are blue-shifted by 3-5 cm-1 from those of the monomer. The structures revealed by random searching algorithms with density functional methods indicate that the π-stacked structure is most stable for (pyridine)2, while (pyridine)3 prefers the structures stabilized by dipole-dipole and C-Hπ interactions. For the (pyridine)m-(NH3)n complexes, the mass spectrum exhibited a wide range distribution of the complexes. The observed IR spectra in the N-H stretching vibrations of the complexes showed four main bands in the 3200-3450 cm-1 region. These features are very similar to those of (NH3)n complexes, and the bands are assigned to the anti-symmetric N-H stretching band (ν3), the symmetric N-H stretching (ν1) band, and the first overtone bands of the N-H bending vibrations (2ν4). The anharmonic calculations including the Fermi-resonance between ν1 and 2ν4 well reproduced the observed spectra.

Insight into the Expanded Mislinked Porphyrins with High Second Order Nonlinear Optical Response.

Materials with outstanding nonlinear optical (NLO) response exhibit excellent prospects in electrooptic devices. Thus, it is essential to find a high-performance NLO material to meet the growing demand for high-speed data transmission. In this study, theoretical investigations on the second order NLO properties of the novel expanded mislinked thia-norhexaphyrin, sulfur-free pentaphyrin, and their substituted derivatives were performed using density functional theory. Theoretical calculations display that the approximate planar structures of sulfur-free pentaphyrin embedded with two five-membered rings exhibits a remarkable NLO response and holds large dipolar contribution (ΦJ=1 = 63.5%) to the first hyperpolarizability among four parent expanded mislinked porphyrins. The static first hyperpolarizability values of these expanded porphyrins were found to range from 3490 to 14 229 au. In addition, the second order NLO response of these porphyrins has greatly improved except for minority electron-releasing- and electron-withdrawing-group substituted cases, and the static first hyperpolarizability value has increased to 47 950 au after installing the donor and acceptor groups. Unambiguous evidence reveals that expanded mislinked porphyrin can serve as a potential candidate for NLO materials.

Electronic States and Nonradiative Decay of Cold Gas-Phase Cinnamic Acid Derivatives Studied by Laser Spectroscopy with a Laser-Ablation Technique.

We performed UV spectroscopy for p-coumaric acid (pCA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S1(1ππ*)-S0 absorption in the 31 500-33 500 cm-1 region. Both pCA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the 1ππ* state was measured by picosecond pump-probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from 1ππ* and its time profile were measured by nanosecond UV-deep UV pump-probe spectroscopy. The transient state was observed for pCA and FA and assigned to the T1 state, and we concluded that the NRD process of 1ππ* is S1(1ππ*) → 1nπ* → T1(3ππ*), similar to those of methyl cinnamate and para-substituted cinnamates such as p-hydroxy and p-methoxy methyl cinnamate. On the other hand, the transient T1 state was not detected in CafA, and its NRD route is suggested to be S1(1ππ*) → 1πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the 1ππ* energy and suppresses the NRD process for all the species.

Electronic State and Photophysics of 2-Ethylhexyl-4-methoxycinnamate as UV-B Sunscreen under Jet-Cooled Condition.

The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans-cis isomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S1-S0 transition; UV-UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (-OCH3) groups orient in opposite directions to each other. Picosecond UV-UV pump-probe spectroscopy revealed the NRD process from the excited singlet (S1 (1ππ*)) state occurs at a rate constant of ∼1010-1011 s-1, attributed to internal conversion (IC) to the 1nπ* state. Nanosecond UV-deep UV (DUV) pump-probe spectroscopy identified a transient triplet (T1 (3ππ*)) state, whose energy (from S0) and lifetime are 18 400 cm-1 and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S1 (trans, 1ππ*) → 1nπ* → T1 (3ππ*) → S0 (cis)".

Extremely solvent-enhanced absorbance and fluorescence of carbazole interpreted using a damped Franck-Condon simulation.

Extremely solvent-enhanced absorption and fluorescence spectra of carbazole were investigated by performing a generalized multi-set damped Franck-Condon spectral simulation. Experimental absorption and fluorescence spectra of carbazole in the gas phase were first well reproduced by performing an un-damped Franck-Condon simulation, but a one-set scaling damped Franck-Condon simulation severely underestimated the intensities of the peaks of experimental absorption and fluorescence spectra of carbazole in n-hexane. Then, a multi-set scaling damped Franck-Condon simulation was proposed and carried out for simulating the extremely solvent-enhanced absorbance and fluorescence, and here, the simulated spectra agreed well with the experimental ones. Five (four) representative solvent-enhanced normal modes corresponding to the combination of ring stretching and ring breathing vibrational motions were determined to be responsible for enhanced absorbance (fluorescence) in n-hexane solution. Furthermore, different scalings were applied to the ground and first-excited states, resulting in different enhancement of absorbance and fluorescence, and this analysis revealed atoms in the carbazole interacting with n-hexane solvent molecules and, hence, leading to different normal-mode vibrational vector patterns in the ground and first-excited states, respectively. Basically, the same conclusion was drawn from a simulation with HF-CIS and the three functionals (TD)B3LYP, (TD)B3LYP-35, and (TD)BHandHLYP. The present multi-set scaling damped Franck-Condon simulation scheme was demonstrated to successfully interpret extremely solvent-enhanced absorbance and fluorescence of carbazole in n-hexane-solvent.

Photorelaxation Pathways of 4-(N,N-Dimethylamino)-4'-nitrostilbene Upon S1 Excitation Revealed by Conical Intersection and Intersystem Crossing Networks.

Multi-state n-electron valence state second order perturbation theory (MS-NEVPT2) was utilized to reveal the photorelaxation pathways of 4-(N,N-dimethylamino)-4'-nitrostilbene (DANS) upon S1 excitation. Within the interwoven networks of five S1/S0 and three T2/T1 conical intersections (CIs), and three S1/T2, one S1/T1 and one S0/T1 intersystem crossings (ISCs), those competing nonadiabatic decay pathways play different roles in trans-to-cis and cis-to-trans processes, respectively. After being excited to the Franck-Condon (FC) region of the S1 state, trans-S1-FC firstly encounters an ultrafast conversion to quinoid form. Subsequently, the relaxation mainly proceeds along the triplet pathway, trans-S1-FC → ISC-S1/T2-trans → CI-T2/T1-trans → ISC-S0/T1-twist → trans- or cis-S0. The singlet relaxation pathway mediated by CI-S1/S0-twist-c is hindered by the prominent energy barrier on S1 surface and by the reason that CI-S1/S0-trans and CI-S1/S0-twist-t are both not energetically accessible upon S1 excitation. On the other hand, the cis-S1-FC lies at the top of steeply decreasing potential energy surfaces (PESs) towards the CI-S1/S0-twist-c and CI-S1/S0-DHP regions; therefore, the initial twisting directions of DN and DAP moieties determine the branching ratio between αC=C twisting (cis-S1-FC → CI-S1/S0-twist-c → trans- or cis-S0) and DHP formation relaxation pathways (cis-S1-FC → CI-S1/S0-DHP → DHP-S0) on the S1 surface. Moreover, the DHP formation could also take place via the triplet relaxation pathway, cis-S1-FC → ISC-S1/T1-cis → DHP-T1 → DHP-S0, however, which may be hindered by insufficient spin-orbit coupling (SOC) strength. The other triplet pathways for cis-S1-FC mediated by ISC-S1/T2-cis are negligible due to the energy or geometry incompatibility of possible consecutive stepwise S1 → T2 → T1 or S1 → T2 → S1 processes. The present study reveals photoisomerization dynamic pathways via conical intersection and intersystem crossing networks and provides nice physical insight into experimental investigation of DANS.

A Density Functional Theory Study on Nonlinear Optical Properties of Double Cage Excess Electron Compounds: Theoretically Design M[Cu(Ag)@(NH3 )n ](M = Be, Mg and Ca; n = 1-3).

In this work, we investigated the nonlinear optical (NLO) properties of excess electron electride molecules of M[Cu(Ag)@(NH3 )n ](M = Be, Mg and Ca; n = 1-3) using density functional theory (DFT). This electride molecules consist of an alkaline-earth (Be, Mg and Ca) together with transition metal (Cu and Ag) doped in NH3 cluster. The natural population analysis of charge and their highest occupied molecular orbital suggests that the M[Cu(Ag)@(NH3 )n ] compound has excess electron like alkaline-earth metal form double cage electrides molecules, which exhibit a large static first hyperpolarizability ( ß 0 e ) (electron contribution part) and one of which owns a peak value of ß 0 e 216,938 (a.u.) for Be[Ag@(NH3 )2 ] and vibrational harmonic first hyperpolarizability ( ß z z z nr ) (nuclear contribution part) values and the ratio of ß z z z nr / ß z z z e , namely, η values from 0.02 for Be[Ag@(NH3 )] to 0.757 for Mg[Ag@(NH3 )3 ]. The electron density contribution in different regions on ß z z z e values mainly come from alkaline-earth and transition metal atoms by first hyperpolarizability density analysis, and also explains the reason why ß z z z e values are positive and negative. Moreover, the frequency-dependent values ß(-2ω,ω,ω) are also estimated to make a comparison with experimental measures. © 2018 Wiley Periodicals, Inc.

First-principles study on sum-frequency generation spectroscopy of methanol adsorbed on TiO2(110) surface: Effects of substrate and molecular coverages.

In this work, starting from the general theory of sum-frequency generation (SFG), we proposed a computational strategy utilizing density functional theory with periodic boundary conditions to simulate the vibrational SFG of molecules/solid surface adsorption system. The method has been applied to the CH3OH/TiO2(110) system successfully. Compared with the isolated molecule model, our theoretical calculations showed that the TiO2 substrate can significantly alter the second-order susceptibilities of a methanol molecule which is directly related to the SFG intensity. In addition, the SFG spectra have obvious changes while the methanol coverage increases, especially for the OH vibration peaks. Our theoretical spectra agree reasonably well with experimental measurements at 1 ML coverage, and an interesting peak which is absent in the theoretical spectra is tentatively assigned to some CH3 stretch vibration of methanol adsorbed on the oxygen vacancy of TiO2.

An excited-state Wolff rearrangement reaction of 5-diazo Meldrum's acid: an ab initio on-the-fly nonadiabatic dynamics simulation.

A global switching on-the-fly trajectory surface hopping dynamics simulation at the 3SA-CASSCF(12,11)/6-31G* quantum level has been employed to probe the photo-induced Wolff rearrangement (WR) reaction of 5-diazo Meldrum's acid (DMA) within three low-lying electronic excited states. The present simulation predicted that the branching ratios for relaxing back to the ground state, isomerizing to diazirine, and reaction to ketene I via carbene I are 69% ± 0.1, 3% ± 0.4, and 28% ± 0.1, which are in excellent agreement with those obtained by the femtosecond spectroscopy experiment, 67%, 3% and 30%, respectively. In particular, the present simulation revealed that the major WR reaction to ketene I pathway is stepwise via the excited-state to carbene I (17.8% ± 0.2) and via the ground-state to carbene I (8.7% ± 0.2), and the minor pathway is concerted synchronous (1.5% ± 0.6). The photo-induced WR reaction of DMA has been quantitatively interpreted in terms of the distribution of extended seam surfaces as a function of CN dissociation bonds for two important conical intersections within three low-lying electronic excited states. Ultrafast dynamic time constants have been estimated to be about 500 fs ± 120 fs and 180 fs ± 80 fs for the stepwise and the concerted WR reaction to ketene I which are also in good agreement with those determined by the experiment. Therefore, the photo-induced excited-state WR reaction mechanism has been quantitatively revealed by the present real-time dynamics simulation.

Ultrafast intersystem crossing for nitrophenols: ab initio nonadiabatic molecular dynamics simulation.

Ultrafast intersystem crossing mechanisms for two p- and m-nitrophenol groups (PNP and MNP) have been investigated using ab initio nonadiabatic molecular dynamics simulations at the 6SA-CASSCF level of theory. Trajectory surface hopping simulation has been performed within an intersystem crossing network constructed from two low-lying singlets (S0 and S1) and two low-lying triplets (T1 and T2). It is found that the dominant relaxation S1 → T2 pathway accounts for 65.4% (85.0%) of the quantum yield with a time constant of 13.4 fs (22 fs) and the S1 → T2 → S0 pathway accounts for 33.1% (13.5%) with a time constant of 275 fs (375 fs) for PNP (MNP). In comparison with the previously studied excited-state proton transfer process for ONP, the dominant relaxation S1 → T2 → T1 pathway accounts for 49.3% with a time constant of 40 fs and the S1 → T2 → T1 → S0 pathway accounts for 47.5% with a time constant of 300 fs. The relaxation mechanisms and electronic structures of the intersystem crossings are in close relation with the relative motion between the torsion motions of the nitro-group and the hydroxyl group. The present simulation provides new physical insight for understanding ultrafast photochemical intersystem crossing dynamics.