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

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 a naphthalene diimide (NDI) acceptor (T4PE4NDI) 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 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 hole over the T4 segment. On the contrary, electron transfer is suppressed with excitation at longer wavelengths (≥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 × 1011 to 1.0 × 109 s-1 with increasing bridge length between the T4 and NDI components (T4NDI to T4PE4NDI). Furthermore, wavelength-dependent photoinduced electron transfer was not observed in either T4NDI or T4PE2NDI due to an insufficient PEn bridge length. This work demonstrates the ability to use optical wavelength to control photoinduced electron transfer in a fully π-conjugated oligomer.