Structurally-thrifty and visible-absorbing fluorophores

Fluorophores with a minimal push-pull backbone are actively pursued due to their potentials in biological labelling. Herein a series of structurally-thrifty and visible-absorbing fluorophores (SDXs) were successfully constructed following the D′D-π-A design strategy, in which a secondary donor (D′) was introduced in conjugation with the donor (D) to enhance its electron donating capability. For a very small scaffold, SDXs exhibit a surprisingly long-wavelength absorption band in the visible spectral range (λ_abs = 420 nm) and a strong green fluorescence emission (λ_em = 530 nm) with a fluorescence quantum yield up to 0.84. Notably, fluorescence of SDXs was quenched in hydrogen-bonding solvents, e.g. MeOH and H₂O. This phenomenon renders SDXs feasibility for imaging of cellular non-hydrogen-bonding microenvironment, as demonstrated with BEAS-2B cells. These results proved that the D′D-π-A is a powerful design strategy to construct novel structurally-thrifty fluorophores.