In our previous report (FAPESP proc. 2012/19823-4), tetrahedral arylborates were shown by ultrafast transient absorption spectroscopy to form a p-orbital coupling between two adjacent aryl groups by the direct observation of a transient absorption band, centered at 680 nm, with a lifetime of 6.7 ns in hexanol at 298 K, attributed to the twisted intramolecular charge transfer state (TICT). The TICT excited state did not form with trigonal arylboron compounds, in agreement with ab initio density functional theory (DFT) calculations, which shows lack of orbital symmetry in the ground and excited-state. The TICT and S1 excited states from the tetrahedral and trigonal boron compounds, respectively, pass nonradiatively to the corresponding triplet-states by different pathways. The triplet-states were revealed to yield aryl and diphenylboron radicals by nonradiative decay. In the presence of methyl viologen (MV2+&B(aryl)4-) or other pyridinium salt, tetrahedral arylborates generates an intermolecular ion-pair complex exhibiting a charge-transfer band with extended photochromism to the visible region of the absorption spectrum. This charge-transfer band may arise from the interaction of p-orbitals of the aryl-groups in the boron compound with the positively charged nitrogen atom in the pyridinium moiety. The electronic absorption and fluorescence emission properties of the ion-pair complex may be modulated by different substituents in the aromatic group of the tetrahedral arylboron compounds leading to different luminescent materials and/or photo-electronic devices. Photoexcitation of (MV2+&B(aryl)4-) ion-pair complex in the charge transfer band produce MV+* and the radical photoproducts of B(Ph)4-. However, the early steps and the mechanism leading to radical formation still elusive, and time resolved EPR may contribute to better understand the early photochemical events and provide a mechanism insight on the reaction. The knowledge of the mechanism is relevant for design new complex with desired photochemical and photophysical properties for photopolymerization, photodynamic therapy and for luminescent materials.
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