Synthesis and characterization of acetylene-linked bisphenalenyl and metallic-like behavior in its charge-transfer...

We prepared and isolated a phenalenyl-based neutral hydrocarbon (1 b) with a biradical index of 14%, as well as its charge-transfer (CT) complex 1 b-F(4)-TCNQ. The crystal structure and the small HOMO-LUMO gap assessed by electrochemical and optical methods support the singlet-biradical contribution to the ground state of the neutral 1 b. This biradical character suggests that 1 b has the electronic structure of phenalenyl radicals coupled weakly through an acetylene linker, that is, some independence of the two phenalenyl moieties. The monocationic species 1 b*+ was obtained by reaction with the organic electron acceptor F4-TCNQ. The cationic species has a small disproportionation energy deltaE for the reaction 2 x 1 b*+ <==> 1 b + 1 b2+, which presumably originates from the independence of the phenalenyl moieties. The small deltaE led to a small on-site Coulombic repulsion U(eff) = 0.61 eV in the CT complex. Moreover, a very effective orbital overlap of the phenalenyl rings between molecules afforded a relatively large transfer integral t = 0.09 eV. The small U(eff)/4t ratio (= 1.7) resulted in a metallic-like conductive behavior at around room temperature. Below 280 K, the CT complex showed a transition into a semiconductive state as a result of bond formation between phenalenyl and F4-TCNQ carbon atoms.