Wei-Hai Fang

Color-Tuning Mechanism of Firefly Investigated by Multi-Configurational Perturbation Method

This is the first report on a multiconfigurational reference second-order perturbation theory-molecular mechanics study of the color modulation of the observed bioluminescence of the oxyluciferin-luciferase complex of the Japanese genji-botaru firefly using structures according to recent X-ray data. Our theoretical results do not support the experimentally deduced conclusion that the color modulation of the emitted light primarily depends on the size of the compact luciferase protein cavity embedding the excited oxyluciferin molecule. Rather, we find, in agreement with recent experimental observations, that the wavelength of the emitted light depends on the polarity of the microenvironment at the phenol/phenolate terminal of the benzothiazole fragment in oxyluciferin.

State-specific heavy-atom effect on intersystem crossing processes in 2-thiothymine: A potential photodynamic therapy photosensitizer

Thiothymidine has a potential application as a photosensitizer in cancer photodynamic therapy (PDT). As the chromophore of thiothymidine, 2-thiothymine exhibits ultrahigh quantum yield of intersystem crossing to the lowest triplet state T(1) (ca. 100%), which contrasts with the excited-state behavior of the natural thymine that dissipates excess electronic energy via ultrafast internal conversion to the ground state. In this work, we employed high-level complete-active space self-consistent field and its second-order perturbation methods to explore the photophysical mechanism of a 2-thiothymine model. We have optimized the minimum energy structures in the low-lying seven electronic states, as well as ten intersection points. On the basis of the computed potential energy profiles and spin-orbit couplings, we proposed three competitive, efficient nonadiabatic pathways to the lowest triplet state T(1) from the initially populated singlet state S(2). The suggested mechanistic scenario explains well the recent experimental phenomena. The origin responsible for the distinct photophysical behaviors between thymine and 2-thiothymine is ascribed to the heavy-atom effect, which is significantly enhanced in the latter. Additionally, this heavy-atom effect is found to be state-specific, which could in principle be used to tune the photophysics of 2-thiothymine. The present high-level electronic structure calculations also contribute to understand the working mechanism of thiothymidine in PDT.

Systematic theoretical investigation on the light emitter of firefly

This is a systematic theoretical investigation on all the possible light emitters of firefly using a multireference method. Six chemical forms of oxyluciferin (OxyLH2) molecules/anions were studied by a multistate complete active space second-order perturbation (MS-CASPT2) method in vacuum and dimethyl sulfoxide. The calculated results and subsequent analysis excluded enol-OxyLH2, keto-OxyLH2, and enolate-OxyLH(-) as possible light emitters. The remaining three candidates, phenolate-enol-OxyLH(-), phenolate-keto-OxyLH(-), and OxyL(2-), were further investigated in protein by a MS-CASPT2/molecular mechanics (MM) study to explain the natural bioluminescence of firefly. By comparison of the MS-CASPT2/MM calculated results of phenolate-enol-OxyLH(-), phenolate-keto-OxyLH(-), and OxyL(2-) with the experimental observation and detailed analysis, we concluded that the direct decomposition excited-state product of firefly dioxetanone in vivo and the only light emitter of firefly in natural bioluminescence is the first singlet excited state (S1) of phenolate-keto-OxyLH(-).

Photodissociation of formic acid

The photodissociation of formic acid has been studied experimentally and theoretically. Ab initio calculations were performed to study the dissociative profiles of five reaction channels on the S0, S1, and T1 potential energy surfaces. The vibrationally excited nascent products were detected using a time-resolved Fourier transform infrared spectrometer after laser photolysis at 248 or 193 nm. In the 248 nm photolysis, the HCOOH molecule was first excited to the S1 state, but it was found that the dissociation takes place on the S0 surface after internal conversion. The products of the vibrationally excited CO, CO2(v3) and H2O(v1) were detected. During the dissociation process the vibrationally energized molecule is geometrically memorized and dynamically controlled, with the yield preference of CO and H2O over that of CO2 and H2. The ratio of CO(v⩾1)/CO2(v⩾1) is estimated as <7.5. Vibrationally excited CO (v) and CO2(v3) are also found in the 193 nm photolysis but the CO/CO2 ratio increases to 11. Most of...

Theoretical characterization of the excited-state structures and properties of phenol and its one-water complex

The structures and properties of phenol and its complex were characterized at the Hartree–Fock (HF), the second-order Moller–Plesset perturbation theory (MP2), and complete active space self-consistent field (CASSCF) levels for the ground state (S0) and at the configuration interaction with single excitation (CIS) and CASSCF levels for the excited electronic state (S1). The intermolecular interaction has little influence on the structures of phenol and water. However, a significant change is found in the properties upon complex, and this has been discussed in detail. A comparison with the experimental findings shows that the present calculations provide a good description of the nature of phenol and its complex in S0 and S1.

Mechanistic insight into the chemiluminescent decomposition of firefly dioxetanone.

The peroxide decomposition that generates the excited-state carbonyl compound is the key step in most organic chemiluminescence, and chemically initiated electron exchange luminescence (CIEEL) has been widely accepted for decades as the general mechanism for this decomposition. The firefly dioxetanone, which is a peroxide, is the intermediate in firefly bioluminescence, and its decomposition is the most important step leading to the emission of visible light by a firefly. However, the firefly dioxetanone decomposition mechanism has never been explored at a reliable theoretical level, because the decomposition process includes biradical, charge-transfer (CT) and several nearly degenerate states. Herein, we have investigated the thermolysis of firefly dioxetanone in its neutral (FDOH) and anionic (FDO(-)) forms using second-order multiconfigurational perturbation theories in combination with the ground-state intrinsic reaction coordinate calculated via the combined hybrid functional with Coulomb attenuated exchange-correlation, and considered the solvent effect on the ground-state reaction path using the combined hybrid functional with Coulomb attenuated exchange-correlation. The calculated results indicate that the chemiluminescent decomposition of FDOH or FDO(-) does not take place via the CIEEL mechanism. An entropic trap was found to lead to an excited-state carbonyl compound for FDOH, and a gradually reversible CT initiated luminescence (GRCTIL) was proposed as a new mechanism for the decomposition of FDO(-).

An ab initio study on photodissociation of acetone

Abstract CH3COCH3 photodissociation was investigated using the CASSCF energy gradient techniques. After the acetone molecules are populated in the S1 state by photoexcitation at 193 nm, the intersystem crossing to the T1 surface is the most probable pathway for CH3COCH3 (S1) deactivation. Relaxing to the T1 state, CH3COCH3 (T1) first dissociates into CH3CO ( 2 A ′ ) and CH3 ( 2 A 2 ″ ) products, and then the CH3CO ( 2 A ′ ) formed further easily dissociates into CH3 and CO. This stepwise mechanism is consistent with numerous experiments.

Ab Initio Trajectory Surface-Hopping Study on Ultrafast Deactivation Process of Thiophene

The ultrafast S(1)((1)ππ*) → S(0) deactivation process of thiophene in the gas phase has been simulated with the complete active space self-consistent field (CASSCF) based fewest switch surface hopping method. It was found that most of the calculated trajectories (∼80%) decay to the ground state (S(0)) with an averaged time constant of 65 ± 5 fs. This is in good agreement with the experimental value of about 80 fs. Two conical intersections were determined to be responsible for the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. After thiophene is excited to the S(1)((1)ππ*) state in the Franck-Condon region, it quickly relaxes to the minimum of the S(1)((1)ππ*) state, then overcomes a small barrier near the conical intersection (CI((1)ππ*/(1)πσ*)), and eventually arrives at the minimum of one C-S bond fission (S(1)((1)πσ*)). In the vicinity of this minimum, the conical intersection (CI((1)πσ*/S(0))) funnels the electron population to the ground state (S(0)), completing the ultrafast S(1)((1)ππ*) → S(0) internal conversion process. This decay mechanism matches well with previous experimental and theoretical studies.

A resonance Raman spectroscopic and CASSCF investigation of the Franck–Condon region structural dynamics and conical intersections of thiophene

Resonance Raman spectra were acquired for thiophene in cyclohexane solution with 239.5 and 266 nm excitation wavelengths that were in resonance with ∼240 nm first intense absorption band. The spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mostly along the reaction coordinates of six totally symmetry modes and three nontotally symmetry modes. The appearance of the nontotally symmetry modes, the C-S antisymmetry stretch +C-C=C bend mode ν(21)(B(2)) at 754 cm(-1) and the H(7)C(3)-C(4)H(8) twist ν(9)(A(2)) at 906 cm(-1), suggests the existence of two different types of vibronic-couplings or curve-crossings among the excited states in the Franck-Condon region. The electronic transition energies, the excited state structures, and the conical intersection points (1)B(1)/(1)A(1) and (1)B(2)/(1)A(1) between 2 (1)A(1) and 1 (1)B(2) or 1 (1)B(1) potential energy surfaces of thiophene were determined by using complete active space self-consistent field theory computations. These computational results were correlated with the Franck-Condon region structural dynamics of thiophene. The ring opening photodissociation reaction pathway through cleavage of one of the C-S bonds and via the conical intersection point (1)B(1)/(1)A(1) was revealed to be the predominant ultrafast reaction channel for thiophene in the lowest singlet excited state potential energy hypersurface, while the internal conversion pathway via the conical intersection point (1)B(2)/(1)A(1) was found to be the minor decay channel in the lowest singlet excited state potential energy hypersurface.

Ultrafast asynchronous concerted excited-state intramolecular proton transfer and photodecarboxylation of o-acetylphenylacetic acid explored by combined CASPT2 and CASSCF studies.

Photodecarboxylation was found to be an ultrafast process for o-acetylphenylacetic acid, which is triggered by excited-state intramolecular proton transfer. The reaction starts from the charge-transfer pipi* singlet state and passes through the conical intersection to the ground state. Subsequent electron transfer and proton transfer in the ground state lead to formation of the final products. This represents a completely new mechanism of photoinduced decarboxylation for various arylcarboxylic acids.