Excited states of thiophene: ring opening as deactivation mechanism

Abstract

(Time-dependent) Kohn-Sham density functional theory and a combined density functional/multi-reference configuration interaction method (DFT/MRCI) were employed to explore the ground and low-lying electronically excited states of thiophene. Spin-orbit coupling was taken into account using an efficient, nonempirical mean-field Hamiltonian. Phosphorescence lifetimes were calculated by means of spock.ci, a selecting direct multi-reference spin-orbit configuration interaction program. Throughout this paper, we use the following nomenclature: S1, S2,..., T1, T2,..., denominate electronic structures in their energetic order at the ground state minimum geometry, whereas S1, S2,..., T1, T2,..., refers to the actual order of electronic states at a given nuclear geometry. Multiple minima were found on the first excited singlet (S1) potential energy hypersurface with electronic structures S1 (piHOMO-1-->pi+piHOMO-->pi), S2 (piHOMO-->pi), and S3 (piHOMO-->sigma*) corresponding to the 2 1A1 (S1), 1 1B2 (S2), and 1 1B1 (S3) states in the vertical absorption spectrum, respectively. The S1 and S2 minimum geometries show out-of-plane deformations of the ring. The S3 electronic structure yields the global minimum on the S1 surface with an adiabatic excitation energy of merely 3.81 eV. It exhibits an asymmetric planar nuclear arrangement with one significantly elongated C-S bond. A constrained minimum energy path calculation connecting the S1 and S3 minima suggests that even low-lying vibrational levels of the S1 potential well can access the global minimum of the S1 surface. Nonradiative decay of the electronically excited singlet population to the electronic ground state via a close-by conical intersection will be fast. According to our work, this ring opening mechanism is most likely responsible for the lack of fluorescence in thiophene and the ultrafast decay of the S1 vibrational levels, as observed in time-resolved pump-probe femtosecond multiphoton ionization experiments. An alternative relaxation pathway leads from the S1 minimum via vibronic coupling to the S2 potential well followed by fast inter-system crossing to the T2 state. For an estimate of individual rate constants a quantum dynamical treatment will be required. The global minimum of the T1 surface has a chair-like nuclear conformation and corresponds to the T1 (1 3B2, piHOMO-->pi) electronic structure. Phosphorescence is weak here with a calculated radiative lifetime of 0.59 s. For the second potential well on the T1 surface with T3 (1 3B1, piHOMO-->sigma*) electronic structure, nonradiative processes are predicted to dominate the triplet decay.

Fragmentation of isoxazole molecules by electron impact in the energy range 10–85eV

“…Such molecular processes can be uncovered by a combination of experimental spectroscopic measurements and ab initio theoretical investigations. In recent years, much attention has been paid [1][2][3][4][5][6] to studies of nonradiative deactivation of lower lying excited states of five-membered heterocyclic aromatic molecules, furan (C 4 H 4 O), pyrrole (C 4 H 4 NH), thiophene (C 4 H 4 S) (see Fig. 1) and their derivatives.…”

Near ultraviolet photochemistry of 2-bromo- and 2-iodothiophene: Revealing photoinduced ring opening in the gas phase?

“…This finding served to reinforce speculations offered to account for various of the products observed in earlier experimental studies of thiophene photolysis under both bulk 12 and molecular beam 13,14 conditions and accords with conclusions from several recent theoretical studies of the ultrafast deactivation of gas phase thiophene molecules. [15][16][17][18] Finding direct evidence for the operation of such pathways experimentally is a challenge, as any primary ringopened products will almost inevitably be formed with sufficiently high levels of vibrational excitation to defy definitive spectroscopic characterisation and, in many cases, will decay further to smaller secondary products. One way of circumventing these difficulties is to study such photoinduced ring-opening reactions in a weakly interacting solvent, as illustrated by our recent ultrafast pump-(broadband infrared (IR) absorption) probe studies of the 267 nm photolysis of thiophenone in solution in acetonitrile.…”

Electrochemistry and time dependent DFT study of a (vinylenedithio)-TTF derivative in different oxidation states

“…[61]/6-31+G(d) and BMK/def2-TZVPP+R [62,63] level of theory employing the CPCM [64,65] solvent model and dichloromethane as the solvent [66]. Full convergence of the SCF procedure was requested with the SCF = tight keyword.…”

“…But the higher energy transitions could still not be satisfactorily modelled. Additionally, the large def2-TZVPP+R basis was applied where 1s1p1d diffuse Gaussian Rydberg functions are added to sulphur [62,63]. This basis turned out to be superior over TZVP and TZVPP for The low energy bands of the experimental spectra of 1 and 2 are similar in energy.…”