Effects of π-spacers on the linear and nonlinear optical properties of novel fluorenone-based D–π–A–π–D type conjugated oligomers with different donors

We compare the effects of fluorene–ethylene (FE) spacers on the photo-physical properties of fluorenone-based conjugated oligomers. The introduction of FE spacer could modulate the steady-state spectra and weaken the difference between donor-dependent spectral features. Meanwhile, the quantum chemical calculations exhibit that the electron transition mechanism modulated by FE unit is variable with the donor of oligomers. The FE π-spacer mainly rises the HOMO and facilitates the electron delocalization in the fluorene end-capped oligomer, while it lowers the LUMO and enhances the donor effect in the triphenylamine end-capped one. The time-resolved fluorescence measurement exhibits that the difference between donor-dependent excited state lifetimes of oligomers becomes less obvious after the introduction of FE units. Moreover, the nonlinear optical measurements show that the FE spacer is able to improve the two-photon fluorescence efficiency and enhance the two-photon absorption cross-section of oligomers simultaneously, but the difference in two-photon characteristics between oligomers with different donors is also decreased.

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► π-Spacer dependent optical properties of fluorenone-based oligomers are studied.
► Fluorene–ethylene (FE) spacer could modulate the steady-state spectra of oligomers.
► FE spacer could increase the TPF efficiency and TPA cross-section of oligomers.
► FE spacer could weaken the difference of donor-dependent optical properties.
► DFT calculations show different modulating mechanisms of FE π-spacer in oligomers.