Unveiling the molecular structure and two-photon absorption properties relationship of branched oligofluorenes

Organic molecules have been intensively studied during the last few decades because of their photonics and biological applications. In this material class, the fluorene molecules present outstanding optical features, for example, high values of two-photon absorption (2PA) cross-sections, visible transparency, and high fluorescence quantum yield. Also, it is possible to improve the nonlinear optical response by modifying the fluorene molecular structure. In this context, herein, we have synthesized V and Y-shaped branching oligofluorenes containing two and three fluorene moieties in each branch. Such a molecular strategy may exponentially enhance the nonlinear optical response due to the coherent coupling among the molecular arms. Thus, we combined the use of femtosecond Z-scan spectroscopy and white light transient absorption spectroscopy (TAS) to understand the molecular structure and 2PA property relationship of branching oligofluorenes. The results show that there is a universal relationship between the 2PA cross-section and the effective pi-electron number (N-eff) given by sigma(2PA)(GM) = (079 +/- 0.03)N-eff(2), which is independent of the molecular shape (linear, V or Y-shaped). Therefore, the intramolecular charge transfer responsible for the cooperative effect among the branches does not occur. This statement is corroborated by the results of the femtosecond TAS technique.

Automated summary

Organic molecules, especially fluorene molecules, have been extensively studied for their photonics and biological applications due to their outstanding optical features. In this study, V and Y-shaped branching oligofluorenes containing multiple fluorene moieties were synthesized to enhance the nonlinear optical response. The relationship between the two-photon absorption (2PA) cross-section and the effective pi-electron number was investigated using femtosecond Z-scan spectroscopy and white light transient absorption spectroscopy (TAS).