Novel nonplanar and rigid fluorophores with intensive emission in water and the application in two-photon imaging of live cells

Bright and photostable fluorophores play a key role in the imaging of specific intracellular molecules. Here we report a facile approach to improve the brightness of naphthalimide fluorophores in water. We synthesized a series of nonplanar and rigid compounds (FM1, FM2, and FM3) by fusing a seven-membered heterocycle onto a naphthalimide skeleton. Compared with classic naphthalimide (FM0), these novel fluorophores exhibited much higher quantum yield values (Φ = 0.60–0.66), and higher molar extinction coefficients in aqueous solution, which elicited superior brightness to FM0. They also showed good photostability, pH-independence, and two-photon microscope (TPM) imaging ability. This new strategy of structural nonplanarity and rigidification provides an efficient way to improve the quantum efficiencies of traditional dyes.