Theoretical design of solvatochromism switching by photochromic reactions using donor-acceptor disubstituted diarylethene derivatives with oxidized thiophene rings

"On-The-Fly" Non-Adiabatic Dynamics Simulations on Photoinduced Ring-Closing Reaction of a Nucleoside-Based Diarylethene Photoswitch

Nucleoside-based diarylethenes are emerging as an especial class of photochromic compounds that have potential applications in regulating biological systems using noninvasive light with high spatio-temporal resolution. However, relevant microscopic photochromic mechanisms at atomic level of these novel diarylethenes remain to be explored. Herein, we have employed static electronic structure calculations (MS-CASPT2//M06-2X, MS-CASPT2//SA-CASSCF) in combination with non-adiabatic dynamics simulations to explore the related photoinduced ring-closing reaction of a typical nucleoside-based diarylethene photoswitch, namely, PS-IV. Upon excitation with UV light, the open form PS-IV can be excited to a spectroscopically bright S₁ state. After that, the molecule relaxes to the conical intersection region within 150 fs according to the barrierless relaxed scan of the C1–C6 bond, which is followed by an immediate deactivation to the ground state. The conical intersection structure is very similar to the ground state transition state structure which connects the open and closed forms of PS-IV, and therefore plays a crucial role in the photochromism of PS-IV. Besides, after analyzing the hopping structures, we conclude that the ring closing reaction cannot complete in the S₁ state alone since all the C1–C6 distances of the hopping structures are larger than 2.00 Å. Once hopping to the ground state, the molecules either return to the original open form of PS-IV or produce the closed form of PS-IV within 100 fs, and the ring closing quantum yield is estimated to be 56%. Our present work not only elucidates the ultrafast photoinduced pericyclic reaction of the nucleoside-based diarylethene PS-IV, but can also be helpful for the future design of novel nucleoside-based diarylethenes with better performance.

A computationally-derived model for the solvatochromism of p-phenolates with high predictive power

We report that the positive, reverse or negative solvatochromism of p-phenolate-based dyes is highly correlated with the multireferential (MR) character of their ground-state wave function, with negative compounds presenting the highest degeneracy. CASSCF/NEVPT2 calculations show that the high MR character of the wave-function in negative dyes allows those systems to increase the dipole moment of the ground state by breaking the degeneracy as a response to the field of a polar solvent. The resulting stabilization of the ground-state with increasing solvent polarity leads to the observed negative solvatochromic behavior. A computational indicator based on our results has been successfully used for determining the direction of the solvatochromic shift of 24 dyes. Thus, our work sheds light on the physical-chemical basis for solvatochromism while providing experimental chemists with a practical tool for the design of novel negative, positive or reverse solvatochromic dyes.