Preparation and Characterization of 5,5′-Bis(4′-Cyanobiphenyl-4-Yl)-2,2′-Bithiophene Nanocrystals Using an Ionic Liquid

A novel preparation method using an ionic liquid (IL) (1-hexadecyl-3-methylimidazolium chloride) is developed to control size and prevent agglomeration during the fabrication of nanocrystals (NCs) of 5,5′-bis(4′-cyanobiphenyl-4-yl)-2,2′-bithiophene (BP2T-CN), a type of thiophene/phenylene co-oligomers (TPCOs). BP2T-CN NCs with sizes ranging from 86 to 550 nm are prepared by changing the concentration of the IL in an improved reprecipitation technique. Photoluminescence (PL) and absorption measurements reveal blue-shifts in PL and absorption bands with decreasing NC size, attributing to the site-shift effect. The energies of PL and absorption bands vary between the energies of bulk crystal and monomer state. PL spectra of BP2T-CN NCs (550, 260, and 86 nm) measured under atmospheric and vacuum conditions show small peak energy changes (9.61–12.8 meV) due to surface effects. Under vacuum conditions, NCs of 550 nm (without IL) exhibits 1.6–2.5 times longer PL decay times, while NCs of 260 nm (with 11 mM IL) and 86 nm (with 27 mM IL) show 0.8–1.1 times longer constants. These smaller changes in decay times under vacuum, caused by IL surface passivation, highlight that the method effectively suppresses NC aggregation and reduces surface effects.