Fluorescent Organic Nanoparticles (FONs) are light-emitting, molecular-based nanoparticles that can be obtained from the nanoprecipitation of dedicated hydrophobic organic dyes in water. They represent a versatile family of water-soluble fluorescent nanotools whose luminescent properties can be tuned by bottom-up molecular engineering of their composing dyes. We recently reported on a quadrupolar red-emitting dye that yields spontaneously stealthy bare FONs which do not require coating to hinder interactions with cell membranes. Its quadrupolar design also hints to the possibility that it may be a strong two-photon absorber for bioimaging purposes. In this paper, we further investigate the two-photon absorption (2PA) of this dye and resulting FONs and report on a structure-related series of extended dyes engineered to yield NIR-emitting FONs. All dyes lead to stable, small (12–20 nm in diameter), and bright FONs. The experimental study reveals that molecular confinement strongly influences the fluorescence and 2PA properties of these dyes depending on the nature of the π-extended system. As expected, extension of the π-conjugated system induces a red-shift of the absorption and emission bands as well as an increase and spectral broadening of the 2PA responses in solution. Upon aggregation of the dyes within nanoparticles, a reduction of the fluorescence quantum yield is observed whose amplitude depends strongly on the nature of the π-conjugated systems. Interestingly, the peak 2PA cross sections increase upon confinement of the shortest dye, while a spectral broadening and slight red shift of the 2PA bands of the most extended dyes is observed. Taken together, these properties allow the most extended dye to yield very bright NIR-emitting FONs (εmaxΦ = 7 × 106 M–1 cm–1, σ2maxΦ = 4 × 105 GM). Interestingly, the nature of the π-conjugated system was also found to modulate the stealthiness of the resulting nanoparticles toward biological membranes. As a result, by fine-tuning the molecular design of the quadrupolar FON-composing dyes, we achieved NIR-absorbing, NIR-emitting, spontaneously stealthy small nanoparticles having record one- and two-photon brightness. Finally, we demonstrate that these FONs can be noncovalently surface-coated with a polycationic polymer, thanks to their highly negative surface potentials. This induces a reversal of their surface potential which in turn triggers their internalization within cells.Read less <