Large two-photon absorption of terpyridine-based quadrupolar derivatives: towards their applications in optical limiting and biological imaging.

Developing organic chromophores with large two-photon absorption (TPA) in both organic solvents and aqueous media is crucial owing to their applications in solid-state photonic devices and biological imaging. Herein, a series of novel terpyridine-based quadrupolar derivatives have been synthesized. The influences of electron-donating group, type of conjugated bridge, as well as solvent polarity on the molecular TPA properties have been investigated in detail. In contrast to the case in organic solvents, bis(thienyl)-benzothiadiazole as a rigid conjugated bridge will completely quench molecular two-photon emission in aqueous media. However, the combination of alkylcarbazole as the donor and bis(styryl)benzene as a conjugation bridge can enlarge molecular TPA cross-sections in both organic solvent and aqueous media. The reasonable two-photon emission brightness for the organic nanoparticles of chromophores 3-5 in the aqueous media, prepared by the reprecipitation method, enables them to be used as probes for in vivo biological imaging.