New insight into the photophysics and reactivity of trigonal and tetrahedral arylboron compounds

Abstract

The photophysics and reactivity of two tetraphenylborate salts and triphenylborane have been studied using ultrafast transient absorption, steady-state fluorescence, electron paramagnetic resonance with spin trapping, and DFT calculations. The singlet excited state of tetraarylborates exhibit extended π-orbital coupling between two adjacent aryl groups. The maximum fluorescence band, as well as the transient absorption bands centered at 560 nm (τ = 1.05 ns) and 680 nm (τ = 4.35 ns) are influenced by solvent viscosity and polarity, indicative of a twisted intramolecular charge transfer (TICT) state. Orbital contour plots of the HOMO and LUMO orbitals of the tetraarylboron compounds support the existence of electron delocalization between two aryl groups in the LUMO. This TICT-state and aryl-aryl electron extension is not observed for the trigonal arylboron compound, in which excited π-orbital coupling only occurs between the boron atom and one aryl group, which restricts the twist motion of the aryl-boron bond. The excited triplet state is deactivated primarily through aryl-boron bond cleavage, yielding aryl and diphenylboryl radicals. In the presence of oxygen, this photochemistry results in phenoxyl and diphenylboroxyl radicals, as confirmed by EPR spectroscopy of spin trapped radical adducts. The TICT transition and radical generation is not expected for BoDIPY molecules where the rotational vibration of the B-aryl bond is rigid, restricting changes in the geometric structure. In this sense, this work contributes to the development of new BoDIPY derivatives where the TICT transition may be observed for aryl ligands with free rotational vibrations in the BoDIPY structure.

Experimental Evidence for a Triplet Biradical Excited-State Mechanism in the Photoreactivity of N,C-Chelate Organoboron Compounds

“…On the basis of its solvent-dependent lifetime (vide infra), diffusion controlled oxygen quenching (see Supporting Information), and previous studies, 35 we assign the emissivestate of 1 to a triplet state (T 1 ) resulting from the aforementioned CT from π-Mes to π*-ppy. This is in agreement with the results of tetraarylborates, where photoexcitation results in the population of their T 1 state that has CT character, 18 as well as reports on donor-appended N,Cchelate organoborates, which show that their singlet/triplet CT states are close in energy and in that case display thermally activated delayed fluorescence (TADF). 36−38 The excited state properties herein must involve a triplet state as well.…”

“…On the basis of these findings, Williams et al proposed that photoexcitation leads to a di-π-borate rearrangement, analogous to the well-known di-π-methane (or Zimmerman) rearrangement, where two of the neighboring benzene rings are coupled via their ipso -C atoms, resulting in the formation of a biradical intermediate with a borirane ring (three-membered boracycle; I* ) that is responsible for releasing the organic products and borinate salt. This hypothesis was later supported by Schuster et al, − who were able to crystallographically characterize the very first example of a boratanorcaradiene (e.g., IIa ; Scheme b), the product of the aforementioned biradical intermediate undergoing two subsequent “walk” rearrangements. − Following these initial investigations, the di-π-borate rearrangement was found to be a general phenomenon for tetraarylborates with adjacent unsaturated or strained bonds (alkenyl, ethynyl, cyclopropyl), , as well as triarylboranes complexed with Lewis bases. , Despite this growing interest in the photoreactivity of four-coordinated organoboron compounds, the underlying excited-state dynamics of these reactions remained completely unknown until recently, when a combination of pump–probe experiments and TD-DFT calculations were able to reveal the involvement of a triplet charge-transfer (CT) excited-state in the C–C coupling event. On the basis of the results of spin trapping studies, this state was further proposed to be the precursor that ultimately leads to B–C Ph bond cleavage and the formation of the organic products.…”

Inverse photochromism in viologen–tetraarylborate ion-pair complexes: optical write/microwave erase switching in polymer matrices

“…The visible transient absorption spectra around 400−650 nm are attributed to absorption from the twisted intramolecular charge transfer (TICT) state as previously proposed by us for the sodium tetraphenylborate salt (2a) in polar solvents (i.e., acetonitrile, methanol, and ethanol). 19 The results observed for the sodium 2a, 2b or 2c samples show that the TICT absorption band decreases with a concomitant increase of the triplet-triplet absorption band around 350 -450 nm. The latter has a long decay lifetime (> 1.2 ns).…”

Polymerization Rate Modulated by Tetraarylborate Anion Structure: Direct Correlation of Hammett Substituent Constant with Polymerization Kinetics of 2-Hydroxyethyl Methacrylate

“…Steady and Dynamic Processes of Transient Species. As previously investigated by Santos and co-workers, 40,46 during irradiation in the UV-absorption band (λ < 290 nm) of the tetra(phenyl)borate, the 3 π,π* transition is excited with subsequent B−C(aryl) bond cleavage, generating a reactive phenyl radical and a low reactive boranyl radical species.…”

“…Electron transfer reactions from the photoinduced excited state of riboflavin represent an important route of free radical formation that in the presence of an electron donor or co-initiator (i.e., amine) may initiate vinyl monomer polymerization. − , Alkylboron derivatives have been used as co-initiators of photopolymerization reactions − as well. The irradiation of the two-component system made by a dye and an alkylboron compound release a free and reactive alkyl radical with subsequent reaction with monomer units, initiating the polymerization process. ,,− …”