Ultrafast photoisomerization mechanism of azaborine revealed by nonadiabatic molecular dynamics simulations

Abstract

1,2-dihydro-1,2-azaborine is an isoelectronic analog of benzene with a B–N substitution, and its unique photoisomerization behavior, which is distinct from that of benzene, has drawn significant attention. To understand the...

Photochemical formation of the elusive Dewar isomers of aromatic systems: why are substituted azaborines different?

“…Furthermore, from the corresponding oscillator strengths, it is clear that whereas the S 0 → S 1 transition is strongly allowed for the C s -symmetric azaborines, it is forbidden for the benzenes ( D 6h and C s ). Notably, the S 0 → S 1 energy of 5.18 eV obtained for the plain azaborine agrees very well with previous high-level, ab initio calculations, which have yielded values of 5.15 (using the multireference configuration interaction method 36 ) and 5.16 eV (using the equation-of-motion coupled cluster singles and doubles method 37 ) for this transition.…”

“…This CI is connected to a prefulvene-like S 0 minimum (at 3.69 eV) in one direction, and to the S 0 parent species in the other. The prefulvene-like minimum, which was also found in previous studies, 36,37 can evolve through a small energy barrier to either the S 0 Dewar isomer (barrier of 0.06 eV) or the S 0 parent species (0.12 eV). Accordingly, this minimum is a sort of pivotal point steering the competition between these two pathways.…”

“…36 Recently, these results were corroborated by means of non-adiabatic molecular dynamics (NAMD) simulations, from which it was concluded that the prefulvene-like structure is indeed a key mechanistic element for the photochemistry of azaborine, mediating both the regeneration of the parent species and the formation of the Dewar isomer. 37 However, the quantum yield for Dewar formation extractable from these simulations appears much too low (only a few percent 37 ) compared to the experimental observations. 27…”

A BN-Benzvalene

“…By shortening the residence time, we were able to monitor the formation of deuterium-labeled BN-Dewar benzenes 9a and 10a . Consistent with the previously reported mechanism for BN-Dewar benzene formation, , no N–C6 and B–C3 bond cleavage was observed (eqs 4 and 5). The findings illustrated in Scheme indicate that the B–C3 bond must be cleaved during the transformation from BN-Dewar benzene to BN-benzvalene.…”

“…First, the starting 1,2-azaborine undergoes a photoinduced formal 4π disrotatory electrocyclization to generate the BN-Dewar benzene intermediate. 29,34 Then, the styrene chromophore in BN-Dewar benzene is excited by UV-light to generate a biradical species. The radical at the C4 position subsequently undergoes a 1,2-boron shift, 35 36,37 and phospha-12 and sila-benzene 14,15a analogues to yield their respective benzvalenes.…”