Twisted intramolecular charge transfer in a carbazole-based chromophore: the stable [(4-N-carbazolyl)-2,3,5,6-tetrachlorophenyl]bis(2,3,5,6-tetrachlorophenyl)methyl radical

Abstract

A neutral stable organic radical adduct, 1, composed of a donor-acceptor dyad is reported. The electron-donor part is the carbazolyl ring directly linked to the electron-acceptor polychlorotriphenylmethyl radical through the para position of a phenyl ring. In the synthetic procedure a C(sp2)-H bond is transformed into a C(sp2)-N bond through the radical-radical cross-coupling process. Theoretical calculations predict that the tetrachlorophenyl bridge moiety lies perpendicular to the carbazolyl group to minimize the repulsion with the chlorine atoms in the ortho position. The electron paramagnetic resonance (epr) spectrum of 1 exhibits a small coupling of the electronic spin with the carbazolyl nitrogen (0.32 G), the spin density being mainly located in the central sp2 carbon atom (30.22 G). The radical adduct presents a charge transfer band (l = 598-640 nm) showing a hypsochromic shift with solvent polarity. In DMF solution, 1 exhibits a new weak band (l = 493 nm) which is tentatively attributed to a zwitterionic structure of the molecule resulting from a net electron transfer from the nitrogen to the central sp2 carbon atom. Cyclic voltammetry of 1 confirms the amphoteric character of the molecule. Computed values of ionization potential (IP) and electron affinity (EA) are in good agreement with the experimental values.