Probing UV Photoreaction Pathways of 2-Iodothiophene with Transient X-ray Absorption Spectroscopy*

Photoinduced ring-opening mechanisms play a critical role in many chemical and biological processes such as vitamin D3 synthesis, DNA repair, and degradation of organic electronic materials. Thiophene derivatives, in particular, exhibit promising properties for use in solar cells and light-emitting diodes. However, further understanding of the underlying photophysics is crucial to overcome challenges with undesired photodegradation. 2-iodothiophene exhibits multiple energetically accessible reaction pathways, including C-I bond fission and C-S ring- opening. While iodine dissociation has been extensively studied, the predicted ring-opening reaction remains to be observed. Ultrafast transient X-ray absorption spectroscopy (TXA), an element-specific technique, enables detailed probing of the transient chemical environment in the vicinity of local reporter atoms, facilitating the study of even minor reaction channels. Using TXA at the iodine N-edge, the UV (268 nm) induced C-I bond fission has been shown to occur within approximately 160 fs.[1] This dissociation is slower compared to, for example, halogen abstraction in alkyl halides, which is attributed to a 2-step photoreaction where an initial ππ* excitation couples nonadiabatically to a repulsive πσ* state. To investigate the ring-opening reaction, a different reporter atom is required, ideally one directly involved in bond fission, such as sulfur or carbon. Here, we employ a femtosecond high-harmonic generation (HHG) X-ray source to probe ultrafast dynamics in UV-excited 2-iodothiophene at the sulfur 2p, 2s, and carbon 1s edges. Preliminary results of 268 nm excited 2-iodothiophene focused on the sulfur L-edge are presented.

[1] B. W. Toulson et al., J. Chem. Phys. 159, 034304 (2023).