Synthesis, electrochemistry and ab initio molecular orbital calculations on some norbornane- and bicyclo[2.2.2]octane-fused 5,12-bis(dicyanomethylidene)naphthalene systems

Abstract

A series of norbornane- and bicyclo[2.2.2]octane-fused tetracyanonaphthoquinodimethane (benzoTCNQ) derivatives, 4,6,8,10 and 12 has been synthesized. Cyclic voltammetry measurements revealed two one-electron reduction waves for the benzo-TCNQ systems 4, 8 and 10 which possess either a cyclobutane or a norbornane moiety fused to the quinonoid ring, but a single (presumably two-electron) reduction wave for 6 and 12 which possess a bicyclo[2.2.2]octane group fused to the gmnonoid ring. The benzo-TCNQ systems were found to be potent electron acceptors. The results of (U)HF/3-21G calculations carried out on representative benzo-TCNQ derivatives suggest that steric interactions between the quinonoid ring and its adjacent fused alicyclic group are largely responsible for the observed decrease in the electron affinities along the series: 8 > 4 > 6. The MO calculations further suggest that LUMO energies of neutral TCNQ derivatives are qualitatively reliable predictors of electron affinities. The calculated (gas phase) internal reorganization energies, λ_i, for the benzo-TCNQ systems are ca. 54 kJ mol^–1 and are comparable to that calculated for the dicyanovinyl (DCV) system, 27. The similarity between the λ_i values for the benzo-TCNQ systems and 27, together with the experimental finding that benzo-TCNQ systems are reduced at ca. 1.7 V less negative potentials than 27, should make the norbornane- and bicyclo [2.2.2] octane-benzo-TCNQ systems superior to the DCV moiety as acceptor chromophores in giant multichromophoric systems.