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

Abstract

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.

Excited state dynamics of thiophene and bithiophene: new insights into theoretically challenging systems

“…The rest of the trajectories resists up several hundred femtoseconds. This observation is consistent with the earlier CASSCF dynamics 20 where a time constant of 65 AE 5 fs was obtained for 80% of the trajectories. However, a non-negligible portion of the trajectories terminates with the ring puckering, which is represented by the black dots in the lower part of the graphs.…”

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.…”

Ground‐state dissociation pathways for the molecular cation of 2‐chlorothiophene: A time‐of‐flight mass spectrometry and computational study

“…To cite a few, the experimental work includes studies related to its UV absorption spectrum in the gas phase, its excited‐state dynamics and resonance Raman spectroscopy . On the theoretical side, studies include the electronic structure calculations in the Franck−Condon region to understand its UV spectrum, its ultrafast internal conversion and photodissociation dynamics . The gas‐phase UV absorption spectrum of thiophene shows an A‐band located at 225 nm (5.5 eV) in the lowest valence state attributed to the 2 1 A 1 ← 1 1 A 1 or simply a π* ← π transition.…”

“…15 On the theoretical side, studies include the electronic structure calculations in the Franck−Condon region to understand its UV spectrum, its ultrafast internal conversion and photodissociation dynamics. [16][17][18] The gas-phase UV absorption spectrum of thiophene shows an A-band located at 225 nm (5.5 eV) in the lowest valence state attributed to the 2 1 A 1 ← 1 1 A 1 or simply a π* ← π transition.…”