Ultrafast C_Spiro–O Dissociation via a Conical Intersection Drives Spiropyran to Merocyanine Photoswitching

Abstract

The mechanism of the photochemical conversion of spiropyran to merocyanine is investigated theoretically. Calculations were performed at TD-DFT/ωB97XD/cc-pVDZ level of theory, which shows good agreement with the reference RI-CC2 method. A two-dimensional scan of the potential energy surface has been performed along the C-O distance and the central torsion angle in the ground state and in the first excited state, where the reaction takes place. Starting at the Franck-Condon geometry, the energy of the first excited state decreases in the direction of the C-O dissociation while the ground-state energy increases. This leads to a barrierless C-O bond dissociation in the first excited state. While relaxing on the S1 PES toward longer C-O distances, the torsion angle hardly changes, but other coordinates start to vary, leading to a conical intersection of the ground state and the first excited state at a C-O distance of about 3.4 Å. Passing the conical intersection, the reaction continues on the ground-state PES. At these large C-O distances, either barrierless Cspiro-O rebinding occurs that quenches spiropyran isomerization or rotation around the central torsion angle occurs that leads to merocyanine. For the latter an energy barrier of 0.1 eV must be overcome explaining the low quantum yield of spiropyran to merocyanine photoswitching.

Electron Injection and Energy-Transfer Properties of Spiropyran–Cyclodextrin Complexes Coated onto Metal Oxide Nanoparticles: Toward Photochromic Light Harvesting

“…The TD-DFT results are consistent with those reported for similar spiropyrans. 29 Excitation of the MC optical absorption band at 532 nm yields an emission band centered at 635 nm, assigned to the S 0 ← S 1 transition.…”

Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

“…Indeed, such photo-induced ring-openings have now been explored theoretically in several heterocyclic systems, including furan, [117][118][119] pyrrole, 120 imidazole, 121 2-aminooxazole, 122 adenine, 123 spiropyran, 124,125 and -glucose.…”

Near ultraviolet photochemistry of 2-bromo- and 2-iodothiophene: Revealing photoinduced ring opening in the gas phase?

“…This work adds to the growing literature highlighting the importance of (n/π)σ * excited states in facilitating both emergent and complete ring opening in heterocyclic molecules. [20][21][22][23][24] …”