Tuning the electronic properties of nonplanar exTTF-based push-pull chromophores by aryl substitution

A new family of π-extended tetrathiafulvalene (exTTF) donor-acceptor chromophores has been synthesized by [2 + 2] cycloaddition of TCNE with exTTF-substituted alkynes and subsequent cycloreversion. X-ray data and theoretical calculations, performed at the B3LYP/6-31G** level, show that the new chromophores exhibit highly distorted nonplanar molecular structures with largely twisted 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) units. The electronic and optical properties, investigated by UV/vis spectroscopy and electrochemical measurements, are significantly modified when the TCBD acceptor unit is substituted with a donor phenyl group, which increases the twisting of the TCBD units and reduces the conjugation between the two dicyanovinyl subunits. The introduction of phenyl substituents hampers the oxidation and reduction processes and, at the same time, largely increases the optical band gap. An effective electronic communication between the donor and acceptor units, although limited by the distorted molecular geometry, is evidenced both in the ground and in the excited electronic states. The electronic absorption spectra are characterized by low- to medium-intense charge-transfer bands that extend to the near-infrared.