Multi-state Energy Landscape for Photoreaction of Stilbene and Dimethyl-stilbene

4a,4b-Dihydrophenanthrene → cis-stilbene photoconversion: TD-DFT/DFT study

CONTEXT

DHP → CS photoconversion was analyzed in terms of electron density redistribution for the first time. The following explanation for the non-recovery of the C4a-C4b bond upon CS relaxation is proposed: during this process, the Coulomb repulsion energy between these pairs of atoms increases by almost one and a half times, and their bonding by an electron at LUMO is insufficient to recover the C4a-C4b bond. According to calculations, upon CS relaxation, the linker connecting the benzene rings undergoes significant structural changes. In this case, the distance between the C4a and C4b atoms increases from 3.00 Å to 3.28 Å. Calculations showed that the C4a-C4b vibration of the DHP bond has a very low intensity. Therefore, thermal motion does not contribute to the rupture of this bond.

METHODS

All calculations were performed using the Gaussian16 software package at the B3LYP/6-311 +  + G(d,p)/IEFPCM theory level. B3LYP was the only hybrid functional supported by Gaussian16, which ensured the cleavage of the C4a-C4b bond of DHP while optimizing its S1 excited state. A quantitative description of the redistribution of electron density in the studied conformers was carried out using the analysis of the NPA of atomic charges. Cyclohexane was used as an implicitly specified non-polar solvent. Visualization of molecular orbitals, and electron densities, as well as plotting of calculated IR spectra, were performed using the Gaussview6 software package.

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene

Ultrafast excited-state dynamics of the simplest nitrostilbenes, namely trans-4-nitrostilbene (t-NSB), was studied in solvents of various polarities with ultrafast broadband time-resolved fluorescence and transient absorption spectroscopies, and by quantum-chemical computations. The results revealed that the initially excited S1(ππ*) state deactivation dynamics is strongly influenced by the solvent polarity. Specifically, the t-NSB S1-state lifetime decreases by three orders of magnitude from ∼60 ps in high-polarity solvents to ∼60 fs in nonpolar solvents. The strong solvent-polarity dependence arises from the differences in dipole moments among the S1 and relevant states, including the major intersystem crossing (ISC) receiver triplet states, and therefore, the solvent polarity can modulate their relative energies and ISC rates. In nonpolar solvents, the sub-100 fs lifetime is due to a combination of efficient ISC and internal conversion. In medium-polarity solvents, the S1-state population decays via a competing ISC relaxation mechanism in a biphasic manner, and the ISC rates are found to obey the inverse energy gap law of the strong coupling case. In high-polarity solvents, the S1 state is stabilized to a much lower energy such that ISC becomes energetically infeasible, and the S1 state decays via barrier crossing along the torsion angle of the central ethylenic bond to the nonfluorescent perpendicular configuration. Regardless of the initial S1-state deactivation pathways in various solvents, the excited-state population is ultimately trapped in the metastable T1-state perpendicular configuration, at which a slower ISC occurs to bring the system to the ground state and bifurcate into either trans or cis form of NSB.