Investigation on the linear and nonlinear optical properties of fluorenone-based linear conjugated oligomers: The influence of π-spacer

Effect of π-spacer on the optical properties of two novel D-π-A-π-D type fluorenone-based linear conjugated oligomers 2,7-di((E)-2-(10-octyl-10H- phenothiazin-3-yl)vinyl)-9-fluorenone (P-FO-P) and 2,7-di((E)-2-(9,9-dioctyl-7-((E)-2-(10-octyl-10H-phenothiazin-3-yl)vinyl)-9H-fluoren-2-yl)vinyl)-9-fluorenone (P-F-FO-F-P) is investigated, where fluorenone and phenothiazine units act as the electron acceptor and donor moieties, respectively. We observe that the fluorene–ethylene (FE) spacer could modulate the steady-state spectral features. Quantum chemical calculations offer complementary information regarding the frontier orbitals and molecular structures, which confirm that the intramolecular charge transfer (ICT) transition exists in the HOMO–LUMO transitions. Nonlinear optical measurements show that the FE spacer could improve the two-photon photoluminescence yield and enhance the two-photon absorption cross-section. The results of transient absorption measurements exhibit that an ICT state exists in the relaxation process of excited state, and its generation process is obviously accelerated with the introduction of π-spacer.