Fig. 1. Structure and optical properties of the organic samll molecule fluorescent dyes. (a) Molecular structures of P-BDP and MP-BDP; (b) absorption and photoluminescence (PL) spectra of P-BDP and MP-BDP in THF (the concentrations of both P-BDP and MP-BDP in THF are 10^-6 mol/L); (c) photoluminescence quantum yield (PLQY) of P-BDP and MP-BDP in THF (the concentrations of both P-BDP and MP-BDP in THF are 10^-6 mol/L); (d) fluorescence lifetime of P-BDP and MP-BDP in THF (the concentrations of both P-BDP and MP-BDP in THF are 10^-6 mol/L)

Fig. 2. Quantum-chemical calculations of P-BDP and MP-BDP. (a) Optimized ground-state (S₀) and first excited-state (S₁) conformations of P-BDP and MP-BD; (b) natural transition orbitals (NTO) analysis of P-BDP and MP-BDP at S₁→S₀ electron transition process; (c) root-mean-square deviation (RMSD) of P-BDP and MP-BDP obtained based on the difference between S₀ and S₁ optimized conformations

Fig. 3. Excited-state dynamics of P-BDP and MP-BDP in THF. (a)‒(b) Femtosecond transient absorption spectra of P-BDP and MP-BDP in THF; (c) evolution-associated difference spectra; (d) schematic illustration of excited-state dynamics (VR: vibration relaxation; PL: photoluminescence)

Fig. 4. Optical properties of MP-BDP NPs. (a) Absorption and photoluminescence (PL) spectra of MP-BDP NPs (insets: TEM image of MP-BDP NPs, scale bar:100 nm); (b) fluorescence lifetime of MP-BDP in THF and water; (c) two-photon-induced PL intensity versus input power intensity (excitation: 800 nm)

Fig. 5. Potential of MP-BDP NPs for applications inside and outside organisms. (a) Cell viability of 4T1 cells incubated with different mass concentrations of MP-BDP NPs; (b) in vitro two-photon-confocal fluorescence imaging of 4T1 cells stained with MP-BDP NPs (scale bar: 20 μm; DIC: differential interference contrast microimaging); (c) in vivo two-photon-confocal fluorescence lifetime imaging of zebrafish stained with MP-BDP NPs (excitation: 800 nm, 80 MHz, 120 fs)