Fluorenylethynylpyrene derivatives with strong two-photon absorption: influence of substituents on optical properties

Abstract

The one- and two-photon absorption properties of five newly synthesized pyrene-based chromophores with donor–π–acceptor, donor–π–acceptor–π–donor and acceptor–π–donor–π–acceptor structural motifs and with mono-, di- and tetra-branched fluorenyl groups linked to pyrene through an acetylene π-bridge are systematically studied both experimentally and theoretically. Analysis of the photophysical properties of the materials has been performed to derive structure–property relationships and to formulate guidelines for both spectral tuning and enhancement of molecular two-photon absorption cross-sections in the near-infrared spectral range. The influence of donor/acceptor strength, π-conjugation length, and molecular symmetry on the cross-sections of these new molecules has been studied. These dyes have high fluorescence quantum yields with good two-photon cross-sections ranging between 250 and 2500 GM. All of the tetra-substituted chromophores showed relatively strong two-photon absorption in the NIR region with cross-sections in the range of 1500–2500 GM at excitation wavelengths between 700 and 900 nm. Cross-sections as high as 9000 GM can be accessed at shorter wavelengths. The calculated two-photon absorption cross-section values and trends are in good agreement with the experimental results. It was found that the presence of donor or acceptor groups increases the 2PA cross-sections relative to molecules with simple aryl substituents. Increasing the donor strength is conducive to attain large two-photon cross-sections. Moreover, the combination of large two-photon absorption cross-sections and high fluorescence quantum yields makes these dyes good two-photon chromophores for a variety of applications.