Unveiling the photophysics of thiourea from CASPT2/CASSCF potential energy surfaces and singlet/triplet excited state molecular dynamics simulations

• Published in: Computational and theoretical chemistry Vol. 1151; pp. 36 - 42
• Main Authors: Aguilera-Porta, Neus, Granucci, Giovanni, Munoz-Muriedas, Jordi, Corral, Inés
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.2019
• Subjects: CASPT2 calculations; Internal conversion; Intersystem crossing; Molecular dynamics simulations; Thiourea; Thiourea; Molecular dynamics simulations; Intersystem crossing; Internal conversion; CASPT2 calculations
• ISSN: 2210-271X
• DOI: 10.1016/j.comptc.2019.01.026

[Display omitted] •Main decay pathway in thiourea: S2 → S1 → T2  →  T1 → S0.•In the gas phase, minor deactivation pathways are S2 → S0 and S2 → S1 → S0.•In solution, thermalization and H-bond networks hamper the decay to the ground state.•The heavy atom effect of S and the destabilization of the S1/S0 funnel favor intersystem crossing.•Heavy atom effect of sulfur. This work describes the decay mechanism of photoexcited thiourea, both in gas phase and in solution, from the information inferred from the topography of the excited and ground state potential energy surfaces and mixed singlet/triplet quantum classical molecular dynamics simulations. Our gas phase results reveal T1/S0 intersystem crossing as the dominant (49%) intrinsic decay channel to the ground state, which reaches a population of 0.28 at the final time of our simulations (10 ps). Population of the T1, would occur after internal conversion to the S1 from the spectroscopic S2 electronic state, followed by S1 → T2 intersystem crossing and T2 → T1 internal conversion processes. Minor decay channels occurring exclusively along the singlet manifold, i.e. S2 → S0 (33%) and S1 → S0 (18%), were also observed to play a role in the relaxation of photoexcited thiourea in the gas phase. The explicit incorporation of water-thiourea interactions in our simulations was found to provoke a very significant delay in the decay to the ground state of the system, with no transitions to the S0 being registered during the first 10 ps of our simulations. Intermolecular vibrational energy redistribution and explicit hydrogen bond interactions established between water molecules and the NH2 group of thiourea were found to structurally or energetically hamper the access to the intersystem crossing or internal conversion funnels with the S0.