Accidental degeneracy in the spiropyran radical cation: charge transfer between two orthogonal rings inducing ultra-efficient reactivity

Abstract

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 D electronic state, which rationalises the remarkably efficient visible light driven excited-state reactivity observed experimentally.

Electrochemical Ring-Opening and -Closing of a Spiropyran

“…The formation of the E -isomer through preparative oxidative and then reductive electrolysis indicates that Z – E isomerization is facile in the monocationic state ( tbMC +• ) with >25% yield. The yield is less than complete, partly due to the generation of protons at the working electrode “trapping” some of the E - tbMC in the thermally favored E - tbMCH + form (absorption band at 450 nm). , This effect limits the extent of oxidation that can be realized under the conditions employed. A second aspect to consider is the rapid equilibrium (low barriers) between tbSP +• and tbMC +• , and hence, upon switching to a reducing potential, under diffusion-controlled conditions, the ring-closing to tbSP +• followed by its subsequent reduction to tbSP competes with the desired reduction of tbMC +• to E - tbMC .…”

“…Electrochemical oxidation in these cases can, however, indirectly trigger spontaneous ring-opening through the protons generated at the electrode by protonation-driven ring-opening to the merocyanine Z - MCH + . 36 , 37 …”

“…Blocking of the para position of the indoline unit, as in a methyl-substituted spiropyran ( MeNSP ), disables dimerization, and the oxidation to the spiropyran radical cation becomes reversible (Figure ). Electrochemical oxidation in these cases can, however, indirectly trigger spontaneous ring-opening through the protons generated at the electrode by protonation-driven ring-opening to the merocyanine Z - MCH + . , …”

Photoisomerization-coupled electron transfer

“…A similar potential energy surface (including a conical intersection) can also be constructed for open-shell photochromic systems. 37,38 The Hamiltonian for the chargetransfer states (with a charged photochromic unit) can therefore similarly be written as:…”

A julolidine-fused anthracene derivative: synthesis, photophysical properties, and oxidative dimerization

“…We hypothesized that the dimeric compound 5 formed via oxidative coupling reaction, a mechanism well-documented for the dimerization of the dimethylaniline compounds. 8 The 1 H-NMR spectrum of 5 is distinct from that of J-A. All protons of the anthracene part (b′–d′) appear as a group of multiplet resonance signals (6.93–7.04 ppm) ( Scheme 2 ).…”