Donor-substituted octacyano[4]dendralenes: a new class of cyano-rich non-planar organic acceptors

Double [2+2] cycloaddition/retro-electrocyclisation reactions between tetracyanoethene (TCNE) and various anilino-capped buta-1,3-diynes furnished a series of octacyano[4]dendralene derivatives featuring intense, low-energy intramolecular charge-transfer absorptions. These novel chromophores are strong electron acceptors and undergo facile one-electron reductions at potentials (-0.09 to -0.17 eV vs. Fc(+)/Fc, in CH2Cl2-0.1 M (Bu4NPF6)-Bu-n) lower than those reported for the benchmark organic acceptors, such as TCNE (-0.32 eV) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) (-0.25 eV). The electron-accepting power of one octacyano[4]dendralene, as expressed by the computed adiabatic electron affinity (EA), compares to that of the reference acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F-4-TCNQ) used as a p-type dopant in organic light-emitting diodes (OLEDs) and solar cells. Gas-phase density functional theory (DFT) calculations predict a stretched-out conformation as the global energy minimum for octacyano[4]dendralenes. In the solid state however, folded conformations were observed for two structures by X-ray analysis. Taking the solid state environment approximately into account calculations predict a energetical degeneracy between the stretched-out and folded conformation. Therefore conformational preference probably is a result of supramolecular dimer formation, mediated by two pairs of intermolecular, antiparallel dipolar CN center dot center dot center dot CN interactions.