Water-soluble paracyclophane fluorophores with large two-photon action cross sections

A series of tetradonor substituted [2,2]paracyclophane-based two-photon absorption (TPA) fluorophores were designed, synthesized and characterized. Different substituents were chosen to modulate the conjugation and the donor strength and to allow the molecules to be neutral (N series, organic-soluble) or charged (C series, water-soluble), The overall set of compounds series allows us to examine the correlation among the solvent polarity, donor ability of the substituents, and TPA action cross section (ηδ, where δ is the TPA cross section and η is the quantum yield). In water, a significant drop of both δ and η is observed, approximately one-third relative to those in toluene. Wenker donors and/or weaker acceptors with enhanced conjugation are a better choice due to a minimized quenching related to strong charge transfer (CT) upon excitation. One thus needs to Tine-tune the magnitude of CT and find an optimal balance among CT, δ and η.