Unraveling ultrafast dynamics of the photoswitchable bridged dithienylethenes under structural constraints

Photorelease Reaction Mechanism Study of a Diarylethene Caged Dual Functions Compound

Femtosecond transient absorption, nanosecond transient absorption, and nanosecond resonance Raman spectroscopy techniques coupled with density functional theory calculations were performed to unravel the photocyclization and photorelease mechanisms of a diarylethene based compound (1o) containing two caged groups (OMe and OAc). Since the ground state parallel (P) conformer of 1o with a large dipole moment is stable in DMSO, the fs-TA transformations observed for 1o in DMSO mainly have contributions from the P conformer, which undergoes an intersystem crossing to generate a corresponding triplet state species. In a less polar solvent like 1,4-dioxane, in addition to the P path behavior of 1o, an antiparallel (AP) conformer can also take place to give a photocyclization reaction from the Franck-Condon state and finally give a deprotection via this pathway. This work provides a deeper understanding for these reactions, which not only facilitate the applications of diarylethene compounds but also help for the future design of functionalized diarylethene derivatives for particular applications.

Photo-induced 6π-electrocyclisation and cycloreversion of isolated dithienylethene anions

The diarylethene chromophore is commonly used in light-triggered molecular switches. The chromophore undergoes reversible 6π-electrocyclisation (ring closing) and cycloreversion (ring opening) reactions upon exposure to UV and visible light, respectively, providing bidirectional photoswitching. Here, we investigate the gas-phase photoisomerisation of meta- (m) and para- (p) substituted dithienylethene carboxylate anions (DTE^-) using tandem ion mobility mass spectrometry coupled with laser excitation. The ring-closed forms of p-DTE^- and m-DTE^- are found to undergo cycloreversion in the gas phase with maximum responses associated with bands in the visible (λ_max ≈ 600 nm) and the ultraviolet (λ_max ≈ 360 nm). The ring-open p-DTE^- isomer undergoes 6π-electrocyclisation in the ultraviolet region at wavelengths shorter than 350 nm, whereas no evidence is found for the corresponding electrocyclisation of ring-open m-DTE^-, a situation attributed to the fact that the antiparallel geometry required for electrocyclisation of m-DTE^- is energetically disfavoured. This highlights the influence of the carboxylate substitution position on the photochemical properties of DTE molecules. We find no evidence for the formation in the gas phase of the undesirable cyclic byproduct, which causes fatigue of DTE photoswitches in solution.

"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.

Thermodynamic driving forces of guest confinement in a photoswitchable cage

Photoswitchable cages that confine small guest molecules inside their cavities offer a way to control the binding/unbinding process through irradiation with light of different wavelengths. However, detailed characterization of the structural and thermodynamic consequences of photoswitching is very challenging to achieve by experiments alone. Thus, all-atom molecular dynamics (MD) simulations were carried out to gain insight into the relationship between the structure and binding affinity. Binding free energies of the B₁₂F₁₂^2- guest were obtained for all photochemically accessible forms of a photoswitchable dithienylethene (DTE) based coordination cage. The MD simulations show that successive photo-induced closure of the four individual DTE ligands that form the cage gradually decreases the binding affinity. Closure of the first ligand significantly lowers the unbinding barrier and the binding free energy, and therefore favours guest unbinding both kinetically and thermodynamically. The analysis of different enthalpy contributions to the free energy shows that binding is enthalpically unfavourable and thus is an entropy-driven process, in agreement with the experimental data. Separating the enthalpy into the contributions from electrostatic, van der Waals, and bonded interactions in the force field shows that the unfavourable binding enthalpy is due to the bonded interactions being more favourable in the dissociated state, suggesting the presence of structural strain in the bound complex. Thus, the simulations provide microscopic explanations for the experimental findings and provide a possible route towards the targeted design of switchable nanocontainers with modified binding properties.

Cyclization Dynamics and Competitive Processes of Photochromic Perfluorocyclopentene Dithienylethylene in Solution

Time-resolved absorption spectroscopy measurements were performed to study the dynamics of photochromic 1,2-Bis(2,4-dimethylthiophene-3-yl)perfluoro-cyclopentene (DMTPF) in chloroform, including antiparallel conformer ring-closure reaction and parallel conformer photophysics. All characteristic times are given, discussed and compared to a previous publication concerning the close molecule substituted with phenyl rings. (Hamdi et al., PCCP, 2016). Apart from the expected photocyclization process, condensed ring by-product formation is observed and hypotheses concerning the origin of this by-product are presented.