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Cofacial Boron Dipyrromethene (Bodipy) Dimers: Synthesis, Charge Delocalization, and Exciton Coupling

Lookup NU author(s): Professor Andrew Benniston, Graeme Copley, Emeritus Professor Anthony Harriman, Dr Ross Harrington, Professor William Clegg

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Abstract

A series of compounds containing two boron dipyrromethene (Bodipy) units has been synthesized and fully characterized in which the spacer between the two Bodipy groups is varied from dibenzothiophene (BD1), to dibenzofuran (BD2), to 9,9-dimethylxanthene (BD3), and finally to diphenyl ether (BD4 and BD5). For BD1-BD4 the Bodipy units adopt, to varying degrees, cofacial conformations that allow for systematic variations of both the mutual orientation and the mean separation of the two Bodipy residues. In the remaining dimer, BD5, the Bodipy units are well-separated and cannot conic into close proximity. Single-crystal X-ray structures have been determined for BD1-BD3 and reveal that the "bite angle" between the two Bodipy residues decreases progressively along the series with individual values of 41.33(5)degrees, 36.95(6)degrees, and 8.57(3)degrees. Detailed H-1 and F-19 NMR studies for BD3 and BD4 show the methylene protons to be diastereotopic due to restricted rotation of the two Bodipy groups. For BD4 conformational rocking is invoked to explain the variable-temperature NMR spectra, whereby the methyl and methylene groups become inequivalent. Cyclic voltammetry indicates reversible oxidation and reduction of the Bodipy groups. However, the close proximity of the Bodipy groups in BD3 and BD4 results in two well-resolved waves in the anodic region, and slight splitting of the cathodic wave. Peak splitting is attributed to charge delocalization. Spectroelectrochemical measurements at a fixed oxidative potential reveal an optical intervalence charge-transfer (IVCT) absorption band. This IVCT band is attributed to electron exchange between the cofacially arranged neutral and mono-oxidized Bodipy units. Various levels of exciton coupling are observed for BD1-BD4, but not BD5 since here the Bodipy groups remain isolated.

Publication metadata

Author(s): Benniston AC, Copley G, Harriman A, Howgego D, Harrington RW, Clegg W

Publication type: Article

Publication status: Published

Journal: Journal of Organic Chemistry

Year: 2010

Volume: 75

Issue: 6

Pages: 2018-2027

Print publication date: 15/02/2010

ISSN (print): 0022-3263

ISSN (electronic): 1520-6904

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/jo1000803

DOI: 10.1021/jo1000803

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