Chiral microneedles from an achiral bis(boron dipyrromethene): spontaneous mirror symmetry breaking leading to a promising photoluminescent organic material

Supramolecular self-assembly of a highly flexible and achiral meso bis(boron dipyrromethene) [bis-(BODIPY)] dye straightforwardly yields fluorescent micro-fibers, exhibiting an intriguing anisotropic photonic behavior. This performance includes the generation of chiroptical activity owing to spontaneous mirror symmetry breaking (SMSB). Repetition of several self-assembly experiments demonstrates that the involved SMSB is not stochastic but quasi deterministic in the direction of the induced chiral asymmetry. The origin of these intriguing (chiro)photonic properties is revealed by fluorescent microspectroscopy studies of individual micrometric objects, combined with X-ray diffraction elucidation of microcrystals. Such a study demonstrates that J-like excitonic coupling between bis(BODIPY) units plays a fundamental role in their supramolecular organization, leading to axial chirality. Interestingly, the photonic behavior of the obtained fibers is ruled by inherent nonradiative pathways from the involved push-pull chromophores, and mainly by the complex excitonic interactions induced by their anisotropic supramolecular organization

Förster Resonance Energy Transfer and Laser Efficiency in Colloidal Suspensions of Dye-Doped Nanoparticles: Concentration Effects

The donor and acceptor concentration effects on Förster resonance energy transfer (FRET) and laser properties of polymer nanoparticles (NPs) highly doped with two dyes are comprehensively analyzed. Rhodamine 6G (Rh6G, donor) and Nile Blue (NB, acceptor) are incorporated into anionic methacrylic NPs ∼40 nm in size, in concentrations [Rh6G] = 1–9 mM and [NB] = 0.5–11 mM. The FRET efficiency is mostly influenced by the acceptor concentration due to the presence of more available energy traps and a reduction in the average donor/acceptor distances. We show that the presence of homo-FRET among donors may give rise to an enhancement on the net hetero-FRET efficiency mainly when the concentration of donors exceeds that of the acceptors. When the concentration of both dyes is raised beyond a given value, the FRET efficiency is reduced due to the influence of competing quenching processes. Carefully selected mixtures of Rh6G/NB allow achieving FRET efficiencies as high as 88% and efficient laser emission in which the excitation/pumping light has been fully transferred from Rh6G (∼575 nm) to NB (∼700 nm). Finally, it is shown that, although a higher FRET efficiency does not guarantee higher acceptor laser efficiencies, both are mostly affected by the acceptor concentration and the total amount of dye molecules inside the NPs. This study acquires special relevance since the use of NPs not only allows achieving FRET efficiencies much higher than those attainable in liquid solution (88% vs 57%) but also opens the door to the study of FRET dynamics at concentrations beyond the solubility limit in liquid solutions and without the undesirable effects of reabsorption/re-emission processes (at least for the Rh6G/NB pair)