Accounts of Chemical Research 2012-11-20

Electrochemistry and electrogenerated chemiluminescence of BODIPY dyes.

Alexander B Nepomnyashchii, Allen J Bard

Index: Acc. Chem. Res. 45(11) , 1844-1853, (2012)

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Abstract

BODIPY (boron dipyrromethene) dyes are unique materials with spectroscopic and electrochemical properties comparable to those of aromatic hydrocarbons. Electrochemical studies are useful in understanding the redox properties of these materials and finding structure-stability relations for the radical ions; along with spectroscopy, these studies help researchers design novel compounds with desired properties. This Account represents our attempt at a full description of the electrochemical and electrogenerated chemiluminescence (ECL) properties of the BODIPY dyes. When the dyes are completely substituted with alkyl or other groups, the radical ions of BODIPY dyes are highly stable. But if they include unsubstituted positions, the radical ions can undergo dimerization or other reactions. BODIPY dyes also show unusually large separations, ~1.0 V, between the first and second cyclic voltammetric (CV) waves for both oxidation and reduction half-reactions. Alkyl-substituted BODIPY dyes show good photoluminescence (PL) quantum efficiencies, and radical ion electron transfer annihilation in these molecules produces electrogenerated chemiluminescence (ECL), the intensity of which depends on the structure of the dye. The large separation between waves and the presence of strong ECL signals are both important in the design of stable ECL-based materials. The ECL spectra provide a fast method of monitoring the electrochemical formation of dimers and aggregates from the monomers. BODIPY dyes are particularly good systems for studying stepwise electron transfer in their chemically synthesized oligomers and polymers because of the small separation between the first oxidation and first reduction waves, generally about 2.0-2.4 V, and their relative ease of reduction compared with many other aromatic compounds. The larger separation between consecutive waves for oxidation compared with reduction is noticeable for all BODIPY dimers and trimers. We also observe a more difficult addition or extraction of a third electron compared with the second for the trimers, signaling the importance of electrostatic interactions. In general, BODIPY dyes combine interesting electrochemical and spectroscopic properties that suggest useful analytical applications.