Summary: Electron donor-acceptor (D-A)-type molecules are the most widely employed two-photon scaffolds. Unfortunately, current D-A-type fluorophores suffer from serious solvent-dependent emission wavelength and brightness, thwarting their use for high-fidelity imaging in complicated biological systems. Herein, with single-atom replacement and acceptor fusing in acedan (6-acetyl-2-(dimethylamino)naphthalene), we devise a new class of D-A type fluorophore (TPQL dyes). Our design results in TPQL fluorophores with a slight emission redshift (ΔλTPQL1 < 32 nm versus ΔλAcedan-NH2 = 91 nm) and high brightness (εΦTPQL1 = 7,600 to 6,923 versus εΦAcedan-NH2 = 7,372 to 1,314) in different solvent systems. Such features not only provide bright and unbiased fluorescent images of TPQL fluorophores in the cell but also bring deeper tissue penetration and greater signal-to-background ratios in tissue. As a proof-of-concept demonstration, using probe TPQL-N3 and TPQL-APN, we monitor the dynamic changes of H2S and APN during zebrafish liver development with high fidelity to the best of our knowledge.
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