Enhanced two-photon brightness in molecular-based organic nanoparticles built from articulated-dipoles

New dyes built from two articulated push-pull dipolar moieties have been prepared and characterized as building blocks for the design of hyper-bright fluorescent organic nanoparticles (FONs). The two dipolar modules are coupled via a fluorene moiety that allows free rotation resulting in two remarkable conformations displaying dipolar and/or quadrupolar behavior. Whereas the dyes show little or no fluorescence in solution, a major enhancement of their luminescent properties as well as of their two-photon absorption (2 PA) response in the NIR region is achieved upon their self-aggregation in water. In contrast, earlier linear dipolar dyes yielding FONs upon self-aggregation in water evidenced a reduction of both reduced emission and peak 2 PA as compared to molecular dyes dissolved in low to medium polarity organic solvents. This striking difference demonstrates the potential of inner control of dipolar interactions within symmetrical and flexible molecular structures to promote combined emission and 2 PA enhancement. This work opens a fruitful bottom-up approach to complementary green and red-emitting molecular-based nanoparticles combining giant two-photon brightness (over 300 000 and 400 000 GM) as well as excellent chemical and colloidal stability. The remarkable chemical and colloidal stability of these molecular-based nanoparticles coupled with their brightness make them highly promising candidates for bioimaging applications.