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 (π_HOMO–1 → π^*_LUMO + π_HOMO → π^*_LUMO+1), S2 (π_HOMO → π^*_LUMO), and S3 (π_HOMO → σ*) 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, π_HOMO → π^*_LUMO) 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, π_HOMO → σ^*) electronic structure, nonradiative processes are predicted to dominate the triplet decay.