Photochemistry and Excited-states Dynamics of Tetraarylborates and Tetraarylborate-pyridinium complexes.

• Grant number: 15/13756-1
• Support type: Scholarships abroad - Research Internship - Post-doctor
• Effective date (Start): September 30, 2015
• Effective date (End): September 29, 2016
• Field of knowledge: Physical Sciences and Mathematics - Chemistry - Physical-Chemistry
• Principal Investigator: Daniel Rodrigues Cardoso
• Grantee: Willy Glen Santos
• Supervisor abroad: Malcolm Donald Elder Forbes
• Home Institution: Instituto de Química de São Carlos (IQSC). Universidade de São Paulo (USP). São Carlos , SP, Brazil
• Local de pesquisa : Bowling Green State University (BGSU), United States
• Associated to the scholarship: 12/19823-4 - Phototransients of organoboranes generated by photolysis and Flavin sensitization., BP.PD

Abstract

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.

Scientific publications

(References retrieved automatically from Web of Science and SciELO through information on FAPESP grants and their corresponding numbers as mentioned in the publications by the authors)

SANTOS, WILLY G.; BUDKINA, DARYA S.; DEFLON, VICTOR M.; TARNOVSKY, ALEXANDER N.; CARDOSO, DANIEL R.; FORBES, MALCOLM D. E. Photoinduced Charge Shifts and Electron Transfer in Viologen-Tetraphenylborate Complexes: Push-Pull Character of the Exciplex. Journal of the American Chemical Society, v. 139, n. 23, p. 7681-7684, JUN 14 2017. Web of Science Citations: 16.

TOMAZETT, VINICIUS K.; SANTOS, WILLY G.; LIMA-NETO, BENEDITO S. Infrared spectroscopy as an effective tool in ring-opening metathesis polymerization: monitoring the polymerization kinetics of norbornene with amine-based Ru catalysts in real time. Reaction Kinetics Mechanisms and Catalysis, v. 120, n. 2, p. 663-672, APR 2017. Web of Science Citations: 4.

SANTOS, WILLY G.; PINA, JOAO; BURROWS, HUGH D.; FORBES, MALCOLM D. E.; CARDOSO, DANIEL R. New insight into the photophysics and reactivity of trigonal and tetrahedral arylboron compounds. PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES, v. 15, n. 9, p. 1124-1137, 2016. Web of Science Citations: 4.