Donor-Acceptor-Donor Small-Molecular Dots are Brighter than Polymer Dots of Similar Chromophore Units

Organic nanoparticles exhibiting bright fluorescence are promising for bioimaging applications. Both small-molecule organic fluorophores and conjugated polymers have been used to fabricate fluorescent nanoparticles. However, factors controlling the fluorescence characteristics of the two types of nanoparticles are not well understood. Here, we synthesized a series of donor-acceptor-donor (D-A-D) small molecules consisting of structural units similar to their polymer counterparts that showed fluorescence in the visible, near-infrared, and shortwave infrared spectral regions, and systematically characterized their spectroscopic properties in the solid state (i.e., nanodots). The nanodots consisting of the D-A-D small molecules consistently exhibited brighter fluorescence than the (D-A)_n conjugated polymers. We separated the contributions of light absorption and emission to the fluorescence brightness using single-particle fluorescence microscopy, which revealed that the enhancement of both light absorption efficiency (i.e., molar extinction coefficient) and light-emitting efficiency (i.e., fluorescence quantum yield) resulted in the observed brighter fluorescence of the small-molecular dots compared with the polymer dots. Density functional theory (DFT) calculation and other spectroscopic studies suggested that a less twisted conformation (i.e., more planar conformation) of the molecules in the small-molecular dots directly caused the enhanced molar extinction coefficient. Steady-state and time-resolved fluorescence measurements suggested that this planarization upon particle formation indirectly resulted in the improved fluorescence quantum yield through the enhancement of the radiative rate. The single-particle excitation polarization modulation experiment demonstrated a more ordered overall orientation of the individual molecules in the small-molecular dots. This might explain their planar conformation and less efficient excited-state quenching due to intermolecular interactions. Our findings pave the way for the rational design and development of organic nanoparticles with bright fluorescence for bioimaging applications.