[2+2] Photocycloaddition in a bichromophoric dyad: photochemical concerted forward reaction following Woodward–Hoffmann rules and photoinduced stepwise reverse reaction of the ring openingviapredissociation

Abstract

A novel biphotochromic dyad with styrylbenzo[f]quinoline photochromes was designed and synthesized to study the [2+2] photocycloaddition (PCA) reaction leading to cyclobutane with two benzo[f]quinoline (BQ) substituents and the reverse four-membered...

Density functional theory study of the styrylbenzoquinoline dyad and the related dibenzoquinolylcyclobutane formed in the [2 + 2] photocycloaddition reaction

“…The folded conformer B is 18.5 kcal/mol lower in energy than the extended conformer; the difference slightly decreases to 17.87 kcal/mol with the zero point correction. It is noteworthy that the folded conformer has an asymmetric parallel-displaced structure, in full accordance with the conclusion made upon analysis of the 1 H NMR spectrum [51]. The two SBQ units are located one above the other in parallel planes, and the distance between the reactive double bonds is 4.06 Å.…”

“…Thus, according to DFT calculations, already in the ground S 0 state, the dyad D44B can form folded conformers stabilized by π‐stacking interactions. Excitation of these conformers can lead to the formation of excimers, which was observed experimentally [51]. Due to the flexibility of the tetramethylene chains in the dyad, different conformers can be formed in the ground S 0 state and, accordingly, different excimers in the excited S 1 state.…”

“…Consequently, when the D44B dyad is excited, one of the ethylene groups (in one of the SBQ photochromes) is excited, while the other remains in the ground S 0 state, as required by the Woodward–Hoffmann rules. This explains the formation of cyclobutane CB44B as a result of the PCA reaction; see Scheme 1 [51].…”

“…We have recently found that, as a result of the PCA reaction, supramolecular systems with two covalently linked styrylbenzoquinoline (SBQ) photochromes produce cyclobutanes with two benzoquinoline (BQ) substituents [48][49][50][51]. For example, the dyad D44B produces the rctt-cyclobutane stereoisomer CB44B with two vicinal BQ substituents in a syn-position; this isomer is formed from the folded head-to-head dyad conformer, Scheme 1.…”

“…Under irradiation within the absorption band of the BQ substituents, the cyclobutane CB44B undergoes ring opening. Consequently, the reverse retro-PCA reaction cannot proceed in concert according to the Woodward-Hoffmann rules (since it requires direct excitation to the σσ* state) but inevitably involves a step of excitation energy transfer from BQ to cyclobutane, which means the predissociation mechanism [51].…”