The dissociative photoionization of vinyl chloride (C2H3Cl) in the 11.0–14.2 eV photon energy range was investigated using threshold photoelectron photoion coincidence (TPEPICO) velocity map imaging. Three electronic states, namely, A2A′, B2A″, and C2A′, of the C2H3Cl+ cation were prepared, and their dissociation dynamics were investigated. A unique fragment ion, C2H3+, was observed within the excitation energy range. TPEPICO three-dimensional time-sliced velocity map images of C2H3+ provided the kinetic energy release distributions (KERD) and anisotropy parameters in dissociation of internal-energy-selected C2H3Cl+ cations. At 13.14 eV, the total KERD showed a bimodal distribution consisting of Boltzmann- and Gaussian-type components, indicating a competition between statistical and non-statistical dissociation mechanisms. An additional Gaussian-type component was found in the KERD at 13.65 eV, a center of which was located at a lower kinetic energy. The overall dissociative photoionization mechanisms of C2H3Cl+ in the B2A″ and C2A′ states are proposed based on time-dependent density functional theory calculations of the Cl-loss potential energy curves. Our results highlight the inconsistency of previous conclusions on the dissociation mechanism of C2H3Cl+.

Three new triplet photosensitizers consisting of a bodipy derivative and C60 moieties were synthesized for triplet–triplet annihilation upconversion of perylene. With the extension of the π-conjugated structure of the bodipy derivative, the three photosensitizers exhibited different absorption wavelengths, e.g. 517 nm for B-2, 532 nm for B-4, and 557 nm for B-6. The steady-state and transient absorption, steady-state fluorescence, and upconverted fluorescence emission were investigated to reveal step-by-step the dynamic processes of the above systems. The quantum yields of intramolecular energy transfer, intersystem crossing, and triplet–triplet energy transfer were measured. From the upconverted fluorescence emission spectra, the overall quantum yield of the triplet–triplet annihilation upconversion, ΦUC, was determined to be 5.80% for B-2, 7.95% for B-4, and 4.99% for B-6.

Solvent effects play a very important role in photochemical reactions and energy transfer processes in solution; however, these effects are rarely mentioned in the triplet–triplet annihilation (TTA) upconversion fluorescence experiments. In a typical TTA upconversion system of a photosensitizer of diiodo-Bodipy (I2-Bodipy) and a triplet acceptor of perylene, five common inert solvents, hexane, heptane, toluene, 1,4-dioxane, and dimethyl sulfoxide (DMSO), were used to investigate the solvent effects on the overall quantum yield of upconversion fluorescence. Femtosecond and nanosecond time-resolved transient difference absorption spectra were obtained to study the efficiencies of intersystem crossing (ISC) and triplet–triplet energy transfer (TTET). From the obtained upconversion fluorescence emission spectra, the overall TTA upconversion fluorescence quantum yield was derived. Among the five solvents, the upconversion quantum yield in dioxane is the highest at 19.16%, more than twice that that in toluene (8.75%). For the solvents hexane, heptane, toluene, and dioxane, the yields generally follow the sequences of polarity and viscosity. However, a very low upconversion quantum yield (1.51%) was observed in DMSO although the TTET process and fluorescence quantum yield of perylene in DMSO were almost as efficient as in dioxane. Based on density functional theory calculations, a reasonable explanation for these solvent effects was proposed.

•The unknown D2Π state was confessedly identified in threshold photoelectron spectrum of N2O at ∼20 eV.•Kinetic energy distributions of NO+ fragments dissociated from D2Π and C2Σ+ states were observed very similar.•Dissociation mechanisms of N2O+ in D2Π(v2+) and C2Σ+(0,0,0) states were proposed respectively.Dissociative photoionization (DPI) of N2O at ∼20 eV has been reinvestigated with threshold photoelectron–photoion coincidence (TPEPICO) velocity imaging. In threshold photoelectron spectrum, a shoulder peak at 20.045 eV is observed close to the ground vibrational level of C2Σ+ state at 20.100 eV. Through comparing the coincident mass spectra recorded at 20.045 and 20.100 eV, the assignment of the shoulder band is corrected to a vibrational excited D2Π ionic state from the previous conclusions of the vibrationless level of b4Π or hot band of C2Σ+ state. For the dominant photofragment of NO+ at 20.045 eV, TPEPICO time-sliced velocity image is measured to obtain the corresponding total kinetic energy and angular distributions. Interestingly, both the bimodal vibrational population and angular distribution of NO+ fragment from dissociation of N2O+(D2Π) are very similar to those of N2O+(C2Σ+) ions. With the aid of potential energy curves, the DPI mechanisms of N2O via D2Π ionic state at 20.045 eV along the NO+(X1Σ+) + N(2D) and NO+(X1Σ+) + N(2P) dissociation channels are clarified, in which the internal conversion from D2Π to B2Π state is the rate-determined step.

•Intramolecular proton transfer was identified for the photoexcited DHAQ in transition absorption spectra.•Electron transfer and hydrogen abstraction processes were clarified for photochemical reactions between triplet DHAQ and three pyrimidines, C, T and U.•Electron transfer rates of these reactions were determined.•Structural differences of photosensitizers were suggested to alter the trend of electron-transfer reactivity of nucleobases from their redox potential sequence.Electron transfer (ET) and hydrogen abstraction (HA) reactions between a photosensitizer, 1,8-dihydroxyanthraquinone (DHAQ), and three pyrimidines, cytosine (C), thymine (T) and uracil (U), have been investigated with a method of nanosecond time-resolved laser flash photolysis. Under photo-irradiation at 355 nm, both the triplet DHAQ of normal structure and tautomer structure are identified via intersystem crossing (ISC) in pure acetonitrile and CH3CN/H2O solvent, and they have the very similar behavior in the reaction with nucleobases. With the aid of a complete spectral assignment, decay dynamics of various intermediates have been measured and discussed. A photo-induced ET process followed a HA reaction is confirmed for the reaction between DHAQ and C, while there is no distinct evidence for ET and HA between 3DHAQ* and T (or U). Interestingly, the quenching rate of triplet DHAQ by three pyrimidines is contrary to the redox potential (Eox) order of these DNA bases. By comparing structural difference of two quinones and the ET efficiency from these pyrimidines to DHAQ with the case of menadione (MQ), we can infinitely demonstrate an alternation in trend in reactivity of these bases caused by substituent group on pyrimidine ring.

Direct experimental evidence for dissociative photoionization of oxygen molecule via the 2Σu– ionic optical dark state is presented by an investigation using the method of threshold photoelectron–photoion coincidence (TPEPICO) velocity imaging. Besides vibrational progress of the B2Σg– state, several weak vibrational bands of the 2Σu– ionic optical dark state are observed concomitantly in an excitation energy range of 20.2–21.1 eV. Only O+ fragments are detected in the whole excitation energy range; therefore, all vibrational bands are completely predissociative. TPEPICO three-dimensional time-sliced velocity images of O+ fragments dissociated from vibrational state-selected O2+(2Σu–,v+) ions are recorded. For the 2Σu–(v+=0–3) vibrational states, only the lowest dissociation channel of O+(4S) + O(3P) is observed. Once the photon energy is slightly increased to the 2Σu–(v+=4) level, a new concentric doughnut appears in the image, indicating that the second dissociation channel of O+(4S) + O(1D) is identified indeed. With the aid of potential energy curves, the dissociative mechanism of O2+ in the 2Σu–(v+) state is proposed.

The bimolecular nucleophilic substitution (SN2) reactions of hydroxide anion (OH−) with fluoroamine (NH2F) and chloramine (NH2Cl) have been investigated with ab initio molecular dynamics simulations. For the SN2 reaction of OH− with NH2F, there are two main dynamic reaction pathways after passing the [HO···NH2···F]− barrier. The first one is that the [HO···NH2···F]− transition state directly dissociates to the products of F− and NH2OH without involving any dynamic intermediate complex, and on the contrary, the other one involves the dynamic hydrogen bond F−···H−NH−OH and/or F−···H−O−NH2 intermediate complexes. As to the SN2 reaction of OH− with NH2Cl, there is only one dominant dynamic reaction pathway, which leads to the products of Cl− and NH2OH directly. According to our calculations, the statistical theories including the Rice–Ramsperger–Kassel–Marcus (RRKM) theory and transition state (TS) theory cannot be utilized to model the reaction kinetics for these two SN2 reactions.Graphical abstractHighlights► Dynamics simulations were performed for two SN2 reactions of OH− + NH2F and NH2Cl. ► A direct SN2 reaction mechanism does not involve any intermediate. ► An indirect SN2 mechanism involves the dynamic hydrogen bond intermediates.

Using the recently developed threshold photoelectron–photoion coincidence (TPEPICO) velocity imaging mass spectrometer (Tang et al. Rev. Sci. Instrum.2009, 80, 113101), dissociation of vibrational state-selected O2+(B2Σg¯, v+ = 0–6) ions was investigated. Both the speed and angular distributions of the O+ fragments dissociated from individually vibronic levels of the B2Σg¯ state were obtained directly from the three-dimensional time-sliced TPEPICO velocity images. Two dissociation channels, O+(4S) + O(3P) and O+(4S) + O(1D), were respectively observed, and their branching ratios were found to be heavily dependent on the vibrational states. A new intersection mechanism was suggested for the predissociation of O2+(B2Σg¯) ions, especially for dissociation at the energy of the v+ = 4 level. In addition, the anisotropic parameters for O+ fragments from different dissociative pathways were determined to be close to zero, indicating that the v+ = 0–6 levels of B2Σg¯ predissociate on a time scale that is much slower than that of molecular rotation.

The reaction mechanism of atomic oxygen radical anion (O−) with pyridine (C5H5N) has been investigated at the G3MP2B3 level of theory. Three different entrance potential energy surfaces are explored, respectively, as atomic oxygen radical anion attacks γ-, β- and α-H atoms of pyridine. Possible thermodynamic product channels are examined subsequently. Based on the calculated G3MP2B3 energies and optimized geometries of all species for the title reaction, it has been demonstrated that the oxide anion formation channel is dominant, and the C5H3N− + H2O channel is also favorable in thermodynamics, whereas the H-abstraction and H+-abstraction channels are inaccessible at room temperature. The present conclusions are consistent qualitatively with the previous experimental results. The secondary reactions of the anionic products are expected to be responsible for the contradiction of branching ratios between present calculation and previous experiments.

The dynamic reaction pathways after passing the initial barrier for the reaction of atomic oxygen radical anion (O−) with ethylene (CH2CH2) have been investigated with Born–Oppenheimer molecular dynamics (BOMD) simulations. The BOMD simulations initiated at this [O⋯H⋯CHCH2]− barrier on the exit-channel potential energy surface (PES) reveal several different types of dynamic reaction pathways leading to various anionic products. In particular, as the energy added on the transition vector of the [O⋯H⋯CHCH2]− transition state increases remarkably, the OH− and CH2CH become the dominant products instead of the CH2CHO− and H. As a result, animated images are displayed and more extensive reaction mechanisms are illuminated for the title reaction from the perspective of the dynamic reaction pathways.

The static and dynamic reaction pathways involved in the reaction between O− and CH3F have been investigated. A special attention has been paid to the SN2 reaction channel, that the intrinsic reaction coordinate (IRC) calculation, one-dimensional relaxed potential energy scan, and Born–Oppenheimer molecular dynamics (BOMD) simulations have been performed, respectively. Both the forward IRC calculation and the relaxed potential energy scan from the [O⋯CH3⋯F]− barrier show that the static reaction products are HF and CH2O−. However, the BOMD simulations initiated at this SN2 barrier reveal two major dynamic reaction processes, which correspond to the products of HF + CH2O− and the SN2 reaction products of F− + CH3O, respectively. Although only 104 dynamic trajectories are calculated, the HF + CH2O− production channel seems to be more dominant than the SN2 pathway. However, the electron detachment process of CH2O− potentially causes its vanishing in experiments as an anionic product.

•The unknown D2Π state was confessedly identified in threshold photoelectron spectrum of N2O at ∼20 eV.•Kinetic energy distributions of NO+ fragments dissociated from D2Π and C2Σ+ states were observed very similar.•Dissociation mechanisms of N2O+ in D2Π(v2+) and C2Σ+(0,0,0) states were proposed respectively.Dissociative photoionization (DPI) of N2O at ∼20 eV has been reinvestigated with threshold photoelectron–photoion coincidence (TPEPICO) velocity imaging. In threshold photoelectron spectrum, a shoulder peak at 20.045 eV is observed close to the ground vibrational level of C2Σ+ state at 20.100 eV. Through comparing the coincident mass spectra recorded at 20.045 and 20.100 eV, the assignment of the shoulder band is corrected to a vibrational excited D2Π ionic state from the previous conclusions of the vibrationless level of b4Π or hot band of C2Σ+ state. For the dominant photofragment of NO+ at 20.045 eV, TPEPICO time-sliced velocity image is measured to obtain the corresponding total kinetic energy and angular distributions. Interestingly, both the bimodal vibrational population and angular distribution of NO+ fragment from dissociation of N2O+(D2Π) are very similar to those of N2O+(C2Σ+) ions. With the aid of potential energy curves, the DPI mechanisms of N2O via D2Π ionic state at 20.045 eV along the NO+(X1Σ+) + N(2D) and NO+(X1Σ+) + N(2P) dissociation channels are clarified, in which the internal conversion from D2Π to B2Π state is the rate-determined step.

Solvent effects play a very important role in photochemical reactions and energy transfer processes in solution; however, these effects are rarely mentioned in the triplet–triplet annihilation (TTA) upconversion fluorescence experiments. In a typical TTA upconversion system of a photosensitizer of diiodo-Bodipy (I2-Bodipy) and a triplet acceptor of perylene, five common inert solvents, hexane, heptane, toluene, 1,4-dioxane, and dimethyl sulfoxide (DMSO), were used to investigate the solvent effects on the overall quantum yield of upconversion fluorescence. Femtosecond and nanosecond time-resolved transient difference absorption spectra were obtained to study the efficiencies of intersystem crossing (ISC) and triplet–triplet energy transfer (TTET). From the obtained upconversion fluorescence emission spectra, the overall TTA upconversion fluorescence quantum yield was derived. Among the five solvents, the upconversion quantum yield in dioxane is the highest at 19.16%, more than twice that that in toluene (8.75%). For the solvents hexane, heptane, toluene, and dioxane, the yields generally follow the sequences of polarity and viscosity. However, a very low upconversion quantum yield (1.51%) was observed in DMSO although the TTET process and fluorescence quantum yield of perylene in DMSO were almost as efficient as in dioxane. Based on density functional theory calculations, a reasonable explanation for these solvent effects was proposed.