Theoretical and experimental investigation on photophysical properties of the π-conjugated extension dependent fluorene based oligomers

• Two fluorine (F) based oligomers are synthesized and investigated in detail.
• F units modulate the steady-state spectra of oligomers.
• DFT calculations analyze molecular structure and the electronic transition.
• F units increase the TPF efficiency and the TPA cross-section of oligomers.
• F units shorten the lifetime of excited oligomers.

Two fluorene based linear conjugated oligomers (P-F-P and F-P-F-P-F) were synthesized where fluorene and phenothiazine groups arrange alternately. In comparison with P-F-P, two fluorene units are introduced into F-P-F-P-F, which could further enhance the π-conjugated system. Spectral measurements exhibit that the absorption and emission spectra of oligomers apparently red shift and the bandgap becomes narrow, due to the extension of π-conjugated system. The quantum chemical calculation is used to analyze their molecular structures and the electronic transition behaviors, indicating that they do not have intramolecular charge transfer character after photoexcitation. Moreover, the increasing of the two-photon absorption cross-section and the shortening of the excited state lifetime would be both attributed to the enhancement of π-conjugated system. The intensity-dependent dynamic measurement shows that no intermolecular interaction occurs in the solution, even though their π-conjugated systems are obviously enhanced.