Time-Resolved Photoelectron Studies of Thiophene and 2,5-Dimethylthiophene.

The photoinduced dynamics of thiophene and 2,5-dimethylthiophene (2,5-DMT) were investigated upon excitation at 200 and 255 nm (2,5-DMT only) using time-resolved photoelectron spectroscopy and compared with results from ab initio coupled cluster calculations. For thiophene, depopulation of the initially excited B2(π3π4*) state to the lower-lying A1(π2π4*) state occurs within 25 ± 20 fs, with a subsequent bifurcation into a ring-puckering channel and a ring-opening channel with lifetimes of 80 ± 20 and 450 ± 50 fs, respectively. For 2,5-DMT, the dynamics following excitation at 200 nm is described by a monoexponential decay with a time constant of 120 ± 20 fs, while that following excitation at 255 nm is best fit by a biexponential decay with time constants of 115 ± 20 fs and 15 ± 3 ps, respectively. The fast signal observed after excitation of 2,5-DMT is assigned to the ring-opening channel, which is favored with respect to thiophene due to a lower excited-state barrier along the ring-opening coordinate and an increased inertia toward the ring-puckering channel. Coupled cluster calculations have been undertaken to compare the relaxation dynamics of thiophene to thiazole and isothiazole. For the latter two molecules, we find a strong gradient along the ring-opening coordinate in the Franck-Condon region of the initially populated ππ* state and predict that ring-opening is the dominating relaxation channel after photoexcitation. We use the extracted information for a comparison of the thiophene dynamics with the light-induced processes observed in other five-membered heterocyclic molecules.

Solvent-dependent dual fluorescence of the push-pull system 2-diethylamino-7-nitrofluorene.

The solvent-dependent excited state behavior of the molecular push-pull system 2-diethylamino-7-nitrofluorene has been explored using femtosecond transient absorption spectroscopy in combination with density functional theory calculations. Several excited state minima have been identified computationally, all possessing significant intramolecular charge transfer character. The experimentally observed dual fluorescence is suggested to arise from a planar excited state minimum and another minimum reached by twisting of the aryl-nitrogen bond of the amino group. The majority of the excited state population, however, undergo non-radiative transitions and potential excited state deactivation pathways are assessed in the computational investigation. A third excited state conformer, characterized by twisting around the aryl-nitrogen bond of the nitro group, is reasoned to be responsible for the majority of the non-radiative decays and a crossing between the excited state and ground state is localized. Additionally, ultrafast intersystem crossing is observed in the apolar solvent cyclohexane and rationalized to occur via an El-Sayed assisted transition from one of the identified excited state minima. The solvent thus determines more than just the fluorescence lifetime and shapes the potential energy landscape, thereby dictating the available excited state pathways.

The involvement of triplet receiver states in the ultrafast excited state processes of small esters.

The photoinduced processes of methyl formate and methyl acetate have been probed by femtosecond time-resolved mass spectrometry and photoelectron spectroscopy experiments supported by quantum chemical calculations. Upon excitation to a vibrationally hot S1 state both molecules were found to relax away from the Franck-Condon geometry faster than the cross-correlation (≈170 fs). During relaxation of the S1 surface intersystem crossing to the triplet manifold is possible via the T2 state which enables an El-Sayed allowed transition. The time-resolved photoelectron spectra show indications of a triplet state formed on an ultrafast timescale. We suggest that the ultrafast timescale of the intersystem crossing process is possible due to the shape of the potential energy surface directing and restricting the dynamics along the carbonyl group de-planarization coordinate, where an El-Sayed allowed intersystem crossing is possible at all times, and where the S1 and T2 states are nearly iso-energetic all the way along the reaction coordinate.

Isomerization of the protonated acetone dimer in the gas phase.

Mass spectrometry-based methods have been employed in order to study the reactions of non- (h(6)/h(6)), half (d(6)/h(6)), and fully (d(6)/d(6)) deuterium labeled protonated dimers of acetone in the gas phase. Neither kinetic nor thermodynamic isotope effects were found. From MIKES experiments (both spontaneous and collision-induced dissociations), it was found that the relative ion yield (m/z 65 vs m/z 59) from the dissociation reaction of half deuterium labeled (d(6)/h(6)) protonated dimer of acetone is dependent on the internal energy. A relative ion yield (m/z 65 vs m/z 59) close to unity is observed for cold, nonactivated, metastable ions, whereas the ion yield is observed to increase (favoring m/z 65) when the pressure of the collision gas is increased. This is in striking contrast to what would be expected if a kinetic isotope effect were present. A combined study of the kinetics and the thermodynamics of the association reaction between acetone and protonated acetone implicates the presence of at least two isomeric adducts. We have employed G3(MP2) theory to map the potential energy surface leading from the reactants, acetone and protonated acetone, to the various isomeric adducts. The proton-bound dimer of acetone was found to be the lowest-energy isomer, and protonated diacetone alcohol the next lowest-energy isomer. Protonated diacetone alcohol, even though it is an isomer hidden behind many barriers, can possibly account for the observed relative ion yield and its dependence on the mode of activation.

A G2 study of SH+ exchange reactions involving lone-pair donors and unsaturated hydrocarbons.

The ligand exchange reactions between mono-adducts of the sulfenium ion ([SH-X]-) and either unsaturated hydrocarbons or lone-pair donors have been investigated computationally at the G2 level. The mono-adducts react with acetylene or ethylene to form a thiiranium or a thiirenium ion, in most cases without an overall barrier. In the reactions involving lone-pair donors, the original lone-pair donor is expelled from the [SH-X]- mono-adduct with the formation of a new mono-adduct. The reaction proceeds in this case via an intermediate di-adduct. Both the hydrocarbon and the lone-pair donor attack the mono-adduct with the relevant orbitals aligned in a near-collinear fashion, as was also the case for previously investigated reactions involving PH2+ and Cl+. The reaction energies and the binding energies of the intermediate complexes in the exchange reactions are primarily determined by the electronegativities of the lone-pair donors. The thermochemical data can be rationalized within the framework of qualitative molecular orbital theory, and the results are compared with our previous findings for the corresponding reactions involving PH2+ and Cl+.