Photochromic System among Boron Hydrides: The Hawthorne Rearrangement

Abstract

Photoswitchable molecules have attracted wide interest for many applications in chemistry, physics, and materials science. In this work, we revisit the reversible photochemical and thermal rearrangements of the two B20H18 2– isomers reported by Hawthorne and Pilling in 1966, whose mechanism had not been understood so far. We investigate the rearrangements by means of a joint experimental and computational study with the outcome that B20H18 2– represents the first boron-based photochromic system ever reported. Both photochemical and thermal isomerizations occur through the same intermediate and involve a diamond–square–diamond (DSD) mechanism. Given the absence within boron chemistry of named chemical reactions as opposed to organic chemistry, we propose to label the B20H18 2– photo- and thermal isomerization processes as the Hawthorne rearrangement.

Insights into the Photoinduced Isomerization Mechanisms of a N,C–Chelate Organoboron Compound: A Theoretical Study

“…We verified that the photoisomerization is thermally reversible and the triplet state is responsible for the long‐lived fsTAS signals . Our calculations have established a detailed mechanistic picture of the excited‐state dynamics of B(ppy)Mes 2 , which is expected to be useful for design of novel photoresponsive organoboron compounds and boron clusters …”

“…[41] Our calculations have established a detailed mechanistic picture of the excited-state dynamics of B(ppy)Mes 2 , which is expected to be useful for design of novel photoresponsive organoboron compounds and boron clusters. [31,70]…”

“…CIs are crossings between the potential energy surfaces of the same multiplicity, which act as efficient decay funnels from excited states to ground state . CIs have proven to play crucial roles in boron cluster chemistry and to be basic mechanistic elements for important photophysical and photochemical processes such as intramolecular electron transfer, photoisomerization, Wolff rearrangement, Curtius rearrangement, and aggregation‐induced emission …”

“…[23] CIs are crossings between the potential energy surfaces of the same multiplicity, which act as efficient decay funnels from excited states to ground state. [25][26][27] CIs have proven to play crucial roles in boron cluster chemistry [28][29][30][31] and to be basic mechanistic elements for important photophysical and photochemical processes such as intramolecular electron transfer, [32] photoisomerization, [33] Wolff rearrangement, [34,35] Curtius rearrangement, [36] and aggregationinduced emission. [37,38] Although the above mentioned computational studies have established that the phototransformations of B(ppy)Mes 2 -based compounds can be rationalized by the reaction pathways on the potential energy surface of electronic singlet states, there exist several experimental arguments about the involvement of photoactive triplet states in this class of reactions.…”

“On-The-Fly” Non-Adiabatic Dynamics Simulations on Photoinduced Ring-Closing Reaction of a Nucleoside-Based Diarylethene Photoswitch

“…The energies between the S1S0 structure and S0-TS are also very close to each other, namely, 40.8 vs. 44.0 kcal/mol, implying that the deactivation from the S1S0 conical intersection might directly lead to the transition structure in ground state. This similarity has also been found in some other photochromic systems, such as B 20 H 18 2− [ 93 ]. In addition, since the optimization from S0-O leads directly to such conical intersection, the relaxation from Franck–Condon region to the conical intersection region might be barrierless, which is crucial for the ultrafast non-adiabatic deactivation of the excited states.…”