Accidental degeneracy in the spiropyran radical cation: charge transfer between two orthogonal rings inducing ultra-efficient reactivity.
Photochromism of the spiropyran radical cation to the corresponding merocyanine form is investigated by a combination of electrochemical oxidation, UV/vis absorption spectroscopy, spectroelectrochemistry and first-principles calculations (TD-DFT, CAS-SCF and CAS-PT2). First, we demonstrate that the ring-opening of mono-spiropyrans occurs upon one-electron oxidation and that it can be driven photochemically as well as thermally, with trapping of the merocyanine by protonation. Second, in order to explain this experimentally observed spectroelectrochemical behaviour we suggest a theoretical mechanism based on the reactivity of the two lowest electronic excited-states, which promotes effective electron transfer from the indoline (nitrogen-ring) to the pyran (oxygen-ring) moieties (and vice versa) through a conical intersection seam of degeneracy. Characterisation of the minimum energy conical intersection on this crossing revealed that it presents a rare diabatic trapping topology. The excited state molecule cannot escape from crossing the intersection seam due to the presence of only one degeneracy-lifting coordinate that efficiently channels into the formation of the merocyanine photoproduct, so giving rise to a "kitchen sink" funnel-like effect. Therefore, assuming rapid relaxation after vertical excitation to a higher electronic state, photoconversion cannot be avoided in the D1electronic state, which rationalises the remarkably efficient visible light driven excited-state reactivity observed experimentally.
Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM-ENSCM, CTMM, Université Montpellier, CC 15001, Place Eugène Bataillon, 34095 Montpellier, France. david.mendive-tapia@univ-montp2.fr and Chimie Et Interdisciplinarité, Synthèse, Analyse, Modélisation (CEISAM), UMR CNRS no. 6230, BP 92208, Université de Nantes, 2, Rue de la Houssinière, 44322 Nantes Cedex 3, France. denis.jacquemin@univ-nantes.fr.
David
D
Mendive-Tapia
Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. w.r.browne@rug.nl.
Luuk
L
Kortekaas
Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. w.r.browne@rug.nl.
Jorn D
JD
Steen
Université Paris Diderot Sorbonne Paris Cité, 5 rue Thomas Mann, F-75205 Paris Cedex 13, France and Institut de Recherche de Chimie Paris, PSL Research University, CNRS - Chimie Paris Tech, 11 Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France.
Aurélie
A
Perrier
Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM-ENSCM, CTMM, Université Montpellier, CC 15001, Place Eugène Bataillon, 34095 Montpellier, France. david.mendive-tapia@univ-montp2.fr.
Benjamin
B
Lasorne
Molecular Inorganic Chemistry, Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands. w.r.browne@rug.nl.
Wesley R
WR
Browne
Chimie Et Interdisciplinarité, Synthèse, Analyse, Modélisation (CEISAM), UMR CNRS no. 6230, BP 92208, Université de Nantes, 2, Rue de la Houssinière, 44322 Nantes Cedex 3, France. denis.jacquemin@univ-nantes.fr and Institut Universitaire de France, 1 rue Descartes, 75005 Paris Cedex 5, France.
Denis
D
Jacquemin