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. 2025 Apr 16;10(16):16539-16547.
doi: 10.1021/acsomega.4c11627. eCollection 2025 Apr 29.

Nonlinear Investigation of Fluorene-Benzothiadiazole Copolymers with Multiphoton Absorption and Highlights as Optical Limiters

Leandro H Zucolotto Cocca  1   2 , João V P Valverde  1 , Elisa B de Brito  3 , Jilian Nei de Freitas  3 , Maria de F Vmarques  4 , Cleber R Mendonça  1 , Leonardo De Boni  1
Affiliations

Nonlinear Investigation of Fluorene-Benzothiadiazole Copolymers with Multiphoton Absorption and Highlights as Optical Limiters

Leandro H Zucolotto Cocca et al. ACS Omega. .

Erratum in

Abstract

In recent years, conjugated polymers have garnered significant interest due to their versatile optical and electronic properties, including low band gaps and strong absorption in the visible and near-infrared regions. These features, combined with high molar absorptivity and notable photoluminescence and electroluminescence quantum yields, make these materials highly suitable for applications in optoelectronics, nonlinear optics, and other advanced photonic technologies. This study investigates the linear and nonlinear optical properties of three fluorene-benzothiadiazole-based copolymers-PFDTBT, PFDTBT-M24, and F8BT-differentiated by their electron-accepting units and polymer chain lengths. Through comprehensive spectroscopic analysis, including one-photon absorption, fluorescence emission, and multiphoton absorption studies, as well as quantum chemical calculations, the research provides insights into how molecular design can be optimized for nonlinear optical performance. The results reveal significant two-photon absorption cross sections and demonstrate the potential of these materials for multiphoton-excited fluorescence and optical limiting applications.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
–Representation of chemical structure of D–A polyfluorene-based copolymers. The copolymers have a 9,9-dioctyl-9H-fluorene moiety (blue) acting as an electron-donating group, a 4,7-dithiophen-2-yl-benzo[1,2,5]thiadiazole moiety (red) acting as an electron acceptor for PFDTBT and -M24, and benzo[1,2,5]thiadiazole (red) unit acting as an electron acceptor for F8BT.
Figure 2
Figure 2
–One-photon absorption (black line: left axis) and normalized fluorescence emission (red lines: right axis) spectra of the copolymers in chloroform.
Figure 3
Figure 3
–Molecular orbitals involved in the two lowest-energy electronic transitions, with their respective percentage contributions. Calculations performed at the TD-PCM-M06/6–311++G(d,p) level for PFBTBT (a) and F8BT (b). The percentages values represent the percentage contributions of HOMO–LUMO and HOMO–LUMO+1 excitations.
Figure 4
Figure 4
–One-photon absorption (black line) and two-photon absorption (red circles) spectra of the copolymers in chloroform. The dashed red line is a visual guide. The estimated error is approximately 20%.
Figure 5
Figure 5
–Fluorescence signal versus laser pulse energy, plotted on a log–log scale, for the copolymers. The excitation wavelengths are 900 and 1500 nm for PFDTBT and -M24, and 720 and 1100 nm for F8BT. The slope (α) of the fitted line indicates the order of the multiphoton absorption process: values close to 2.0 ± 0.2 and 3.0 ± 0.3 correspond to 2PA and 3PA, respectively.
Figure 6
Figure 6
–Optical limiting curves of the copolymers: output power versus input power. The dashed black line represents the ideal case of a material without two-photon absorption, with no optical limiting effect.
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