Control of photoinduced electron transfer through selective excitation of a π-conjugated diblock oligomeric system featuring tetrathiophene (T4) and tetra(phenylene ethynylene) (PE4) donor blocks capped with an naphthalene diimide (NDI) acceptor (T4PE4NDI) is demonstrated. Each π-conjugated oligomeric segment has its own discrete ionization potential, electron affinity and optical bandgap which provides an absorption profile that has specific wavelengths that offer selective excitation of the PE4 and T4 blocks. Therefore, T4PE4NDI can be selectively excited to form a charge-separated state via ultrafast photoinduced electron transfer from the PE4 segment to NDI when excited at 370 nm, but it does not produce a charge-separated state when excited at 420 nm (T4). Picosecond transient absorption techniques were performed to probe the excited-state dynamics, revealing ultrafast charge separation (~ 4 ps) occurring from the PE4 segment to NDI when excited at 370 nm, followed by delocalization of the cation over the T4 segment. On the contrary, electron transfer is suppressed with excitation at longer wavelength (≥ 420 nm) where the spectrum is dominated by the T4 unit. The rate of electron transfer and charge recombination was investigated versus the length of the PE bridge unit in oligomers featuring zero and two PE units (T4NDI and T4PE2NDI). The rate of charge recombination decreases from 1.2 x 1011 to 1.0 x 109 s-1 with increasing bridge length between the T4 and NDI components (T4NDI to T4PE4NDI). Furthermore, there wavelength dependent photoinduced electron transfer was not observed in either T4NDI or T4PE2NDI due to insufficient PEn bridge length. This work demonstrates the ability to use optical wavelength to control photoinduced charge electron transfer in a fully π-conjugated oligomer.

中文翻译：

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π-共轭二嵌段低聚物中激发波长相关的光致电子转移

演示了通过选择性激发具有四噻吩 (T4) 和四(亚苯基乙炔) (PE4) 供体块的 π 共轭二嵌段低聚体系统来控制光致电子转移，该供体块用萘二亚胺 (NDI) 受体 (T4PE4NDI) 封盖。每个 π 共轭寡聚链段都有自己的离散电离电位、电子亲和性和光学带隙，这提供了具有特定波长的吸收曲线，可选择性激发 PE4 和 T4 块。因此，当在 370 nm 激发时，T4PE4NDI 可以通过从 PE4 段到 NDI 的超快光致电子转移被选择性激发以形成电荷分离态，但在 420 nm (T4) 激发时不会产生电荷分离态。使用皮秒瞬态吸收技术来探测激发态动力学，揭示了在 370 nm 激发时从 PE4 段到 NDI 发生的超快电荷分离（~4 ps），然后是 T4 段上的阳离子离域。相反，在较长波长 (≥ 420 nm) 的激发下，电子转移受到抑制，其中光谱由 T4 单元主导。研究了电子转移和电荷复合的速率与具有零个和两个 PE 单元（T4NDI 和 T4PE2NDI）的低聚物中 PE 桥单元的长度之间的关系。随着 T4 和 NDI 组件（T4NDI 到 T4PE4NDI）之间桥长度的增加，电荷复合率从 1.2 x 1011 降低到 1.0 x 109 s-1。此外，由于 PEn 桥长度不足，在 T4NDI 或 T4PE2NDI 中均未观察到波长依赖性光致电子转移。这项工作证明了使用光波长来控制完全 π 共轭低聚物中光致电荷电子转移的能力。