Excitation-Wavelength-Dependent Photoinduced Electron Transfer in a π-Conjugated Diblock Oligomer

摘要

Control of photoinduced electron transfer through selective excitation of a π-conjugated diblock oligomeric system featuring tetrathiophene (T_4) and tetra(phenylene ethynylene) (PE_4) donor blocks capped with a naphthalene diimide (NDI) acceptor (T_4PE_4NDI) is demonstrated. Each π-conjugated oligomeric segment has its own discrete ionization potential, electron affinity, and optical band gap which provides an absorption profile that has specific wavelengths that offer selective excitation of the PE_4 and T_4 blocks. Therefore, T_4PE_4NDI can be selectively excited to form a charge-separated state via ultrafast photoinduced electron transfer from the PE_4 segment to NDI when excited at 370 nm, but it does not produce a charge-separated state when excited at 420 nm (T_4). Picosecond transient absorption techniques were performed to probe the excited-state dynamics, revealing ultrafast charge separation (～4 ps) occurring from the PE_4 segment to NDI when excited at 370 nm, followed by delocalization of the hole over the T_4 segment. On the contrary, electron transfer is suppressed with excitation at longer wavelengths (＞420 nm), where the spectrum is dominated by the T_4 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 (T_4NDI and T_4PE_2NDI). The rate of charge recombination decreases from 1.2 × 10~(11) to 1.0 × 10~9 s~(-1) with increasing bridge length between the T_4 and NDI components (T_4NDI to T_4PE_4NDI). Furthermore, wavelength-dependent photoinduced electron transfer was not observed in either T_4NDI or T_4PE_2NDI due to an insufficient PE_n bridge length. This work demonstrates the ability to use optical wavelength to control photoinduced electron transfer in a fully π-conjugated oligomer.