Photophysics and Electrochemistry of Conjugated Oligothiophenes Prepared by Using Azomethine Connections

Novel conjugated azomethines consisting of 1 to 5 thiophenes and up to 4 azomethine bonds prepared
from a stable diaminothiophene are presented. The effect of the number of thiophene and azomethines
bonds on the photophysics and electrochemistry was examined. A high degree of conjugation was
confirmed by bathochromic shifts upward of 120 and 210 nm for the absorbance and fluorescence,
respectively, relative to the diaminothiophene precursor. Acid doping with methanesulfonic acid resulted
in further bathochromic shifts along with lowering of the HOMO-LUMO energy gaps to 1.3 eV. Moreover,
the compounds are extremely stable as evidenced by the absence of decomposition products under acid
conditions. The resulting heteroatomic covalent bonds are furthermore reductively and hydrolytically
resistant. Increasing the degree of conjugation shifts the nonradiative mode of singlet excited state energy
dissipation from internal conversion (IC) to intersystem crossing (ISC). The resulting triplet manifold
produced by ISC was efficiently deactivated by intramolecular self-quenching from the azomethine bond
leading to a nonemissive triplet. Cyclic voltammetry revealed unprecedented reversible radical cation
formation of the azomethines. Both one-electron oxidations and reductions were found by electrochemical
measurements demonstrating the azomethines' capacity to be mutually p- and n-doped. One of the
azomethines exhibited reversible electrochromic behavior with the electrochemically generated radical
cation absorbing in the NIR at 1630 and 792 nm. X-ray crystallography confirmed the thermodynamically
stable E isomer was formed uniquely and that the thiophenes are coplanar adopting an antiparallel
arrangement.

本论文展示了由稳定的二氨基噻吩制备的，由1至5个噻吩和最多4个亚甲基亚胺键组成的新型共轭亚甲基亚胺。研究了噻吩和亚甲基亚胺键的数量对光物理性质和电化学性质的影响。通过吸光度和荧光的 bathochromic shifts 分别向上移动了120 nm和210 nm，相对于二氨基噻吩前体，证实了高度共轭的存在。使用甲磺酸进行酸掺杂，进一步导致了 bathochromic shifts，并降低了HOMO-LUMO能隙至1.3 eV。此外，化合物表现出极高的稳定性，这在酸性条件下无分解产物的存在得到了证实。由此产生的杂原子共价键进一步表现出对还原性和水解性的抗性。增加共轭程度导致单重激发态能量耗散的非辐射模式从内部转换（IC）转变为系间窜越（ISC）。通过亚甲基亚胺键的分子内自猝灭有效地钝化了由ISC产生的三重态，从而形成了非发射性三重态。循环伏安法揭示了亚甲基亚胺前所未有的可逆自由基阳离子形成。电化学测量发现，既有一电子氧化反应也有一电子还原反应，证明了亚甲基亚胺具有相互p型和n型掺杂的能力。其中一种亚甲基亚胺表现出可逆的电致变色行为，电化学生成的自由基阳离子在近红外区域（1630 nm和792 nm）吸收。X射线晶体学证实了热力学稳定的E异构体独特形成，并且噻吩采取共平面排列，呈现反平行结构。