Two-Photon-Absorption Properties of Pyrene-Based Dipolar D-π-A Fluorophores

Two-photon absorption (TPA) properties of pyrene-based derivatives are rare as a result of the limited number of controllable synthetic methods to prepare them. However, these materials are in great demand given their potential practical application in photics and biological imaging. Herein, we present a set of pyrene-based dipolar donor-π-acceptor (D-π-A) fluorophores with a wide-range of color tuning and large TPA cross-sections (up to 2200 GM at 780 nm) by regioselective substitution at the 1,3- and 6,8-positions under the perspective of theoretical analysis. The linear and nonlinear optical properties of these compounds have been studied. The near-identical emission wavelengths between the two-photon-excited fluorescence (TPEF) and one-photon excited fluorescence (OPEF) indicated that they are generated from the same fluorescent excited state by either one- or two-photon excitation. With the exception of the strong donor [N(CH₃)₂] appended fluorophore, the TPEF exhibited a large red-shift compared with the OPEF spectra as a result of the complicated working mechanisms in operation, including intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT). All fluorophores exhibit high two-photon cross-sections (or two-photon brightness, δΦ), especially for 1348 GM, which indicated that these materials can be used as colorants for probe and bioimaging applications.