Conformational dynamics and excitation wavelength dependent photoluminescence of decameric organic nanoparticles

We report a new type of fluorescent organic nanoparticles (16-EDFONP) which are composed of ten bis(3,4,5-tris(16-alkyloxy) monobenzoylglyceride) bisphenol A molecules. The nanoparticles are only found in high concentration solution and exhibit the excitation-dependent fluorescence (EDF). The 16-EDFONP shows clear spectral relaxation on the nanosecond time scale. We have observed similar spectral shifts in cyclohexane, and thus attribute the time-dependent Stokes shift to the emission from multiple conformations of 16-EDFONP. With the employment of steady state and time-resolved fluorescence anisotropy measurements, the hydrodynamic radius of 16-EDFONP is estimated to be 3.13 nm, which is consistent with the size measured using the dynamic light scattering and high-resolution transmission electron microscopy techniques. The time-resolved anisotropy reveals the change in fundamental anisotropy upon different excitation wavelengths, arising from the structural heterogeneity of hydrogen-bonded monoacylglycerol clusters of the 16-EDFONP. Our findings indicate that incomplete spectral relaxation and the size distribution of nanoparticles are not the source of the observed EDF. The EDF comes from the selective excitation of the 16-EDFONP with different monoacylglycerol hydrogen-bond conformations.