Excited-State Dynamic in Thiophene/Phenylene Co-Oligomer Nanocrystals

Thiophene/phenylene co-oligomer (TPCO) nanocrystals (NCs) hold strong promise for nano-environment photoluminescence probes, qubits, and quantum light source. Their optical properties show considerable versatility and flexibility depending on the size of the nanocrystals. This adaptability enhances light emission efficiency, making them a promising choice for improving organic light-emitting diodes (OLED) technology, especially as active layers. To extend the potential applications of these nanocrystals, it is essential to investigate how the size of nanocrystals is related with the excited state relaxation and photoluminescence (PL) dynamics. For example, we expect the excited state lifetime for smaller NCs should be affected by the enhanced spatial overlap of electron and hole states. In the present study, we observe the transient absorption spectrum in NCs of 5,5’-bis(4’-cyanobiphenyl-4-yl)-2,2’-bithiophene (BP2T-CN). BP2T-CN NCs with an average size of 75 nm were prepared by the reprecipitation method, which show strong yellow emission centered around 555 nm. The size-dependent steady-state optical properties were elucidated through the measurement of absorption and PL spectra. In comparison to vapor-deposited films, the BP2T-CN NCs water dispersion displayed shifts in the 0-1 and 0-2 absorption bands, attributed to lattice softening (surface effect) and quantum effects. Additionally, PL lifetime measurements yielded a decay time of 1.33 ns at an excitation wavelength of 400 nm. The transient absorption signal in the yellow-emitting BP2T-CN NCs is characterized by three exponential decays with 0.65 ps, 2.3 ps, and 75 ps components. These time constants shed light on relaxation processes, including electron trapping, intra-band relaxation, and non-radiative decay. Photoinduced absorption bands (PA, Δ<i>T</i>/<i>T</i> &lt; 0) appeared at 670-700 nm from the lowest excited state. Our findings will give a clue on the photophysical processes within these co-oligomer NCs, providing valuable insights for potential applications in optoelectronic devices and photonics, especially in OLED development.