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.

一类全新的π延伸四硫富瓦烯（π-extended tetrathiafulvalene，exTTF）给体-受体发色团家族，可通过四氰基乙烯（tetracyanoethylene，TCNE）与exTTF取代炔烃的[2+2]环加成反应及后续环逆转反应合成得到。X射线衍射数据与基于B3LYP/6-31G**理论级别开展的理论计算结果显示，该类新型发色团具有高度扭曲的非平面分子结构，其中1,1,4,4-四氰基丁-1,3-二烯（1,1,4,4-tetracyanobuta-1,3-diene，TCBD）单元发生了大幅扭转。通过紫外-可见光谱法与电化学测试对其电子与光学性质进行研究后发现，当TCBD受体单元被给体苯基取代时，体系性质会发生显著改变：该取代会进一步增强TCBD单元的扭曲程度，并削弱两个二氰乙烯亚单元之间的共轭作用。苯基取代基的引入不仅抑制了氧化还原过程，同时还大幅提升了体系的光学带隙。尽管受扭曲的分子几何结构限制，给体与受体单元间仍存在有效的电子耦合，这一现象在基态与激发电子态中均得到了证实。该类化合物的电子吸收光谱以延伸至近红外区域的中低强度电荷转移吸收带为典型特征。