Spectroscopic characterization of supramolecular chiral porphyrin homoassociates at the air-water interface

The water-soluble 4-sulfonatophenyl meso-substituted porphyrin (TPPS) dye exhibits a transformation to a chiral self-aggregate from the non-aggregated species (diprotonated H4TPPS(2-)) at low concentration (no more than 1 × 10(-5) M). Immobilization of supramolecular chiral porphyrin homoassociates was mediated by the electrostatic interaction between the anionic TPPS molecule and cationic surfactant monolayer at the air-water interface. With the immobilization, a reversible transformation from monomeric TPPS to J-aggregate (M↔J) could be changed into an irreversible (M→J), which is desirable for stabilization of aggregation structure for a long period. The novel finding was achieved using a fine-tuned specialized solid-state circular dichroism (CD) spectrophotometer and derived analytical procedure to obtain artifact-free CD signals. To our knowledge, this is the first report achieving the chiral control of a homoassociate induced by a chiral surfactant at the air-water interface, indicating that the handedness of the formed homoassociate could be determined.

Pyranine-induced self-assembly of colloidal structures using poly(allylamine-hydrochloride)

Complexes of dyes and polyelectrolytes have found widespread use in a variety of functional materials and interfaces. Here it is found that upon mixing the anionic dye pyranine and a cationic polyelectrolyte, poly(allylamine-hydrochloride), two different colloidal structures may form. Above a certain concentration of anionic dye, crosslinking of the polyelectrolyte is initiated, and the formation of sheet-like colloidal structures was observed. Addition of hydroxyl ions resulted in the formation of micron-sized spherical colloids. It was also found that the colloidal shape transition was accompanied by a significant red-shift in the fluorescence emission. Combining fluorescence measurements with studies of the particle size with time, it was found that red-shift was related to the crosslinking of the dye and the polyelectrolyte, and was not influenced significantly by the aggregation and particle growth. Further information about the colloidal behavior and stability was obtained by letting droplets dry and follow the kinetics of this process. It was found that the particles collapsed near the contact line and formed a ring deposit, in agreement with previous studies. However, unlike previous studies, the thickness of the ring deposit did not grow significantly with time, due to the peculiar process of formation found here.

Spectral resolution of molecular ensembles under ambient conditions using surface plasmon coupled fluorescence emission

We report the ability to spectrally resolve excited-state ensembles of pyranine (Py) utilizing nanometer-thick metal films as a low-cost analytical tool. Surface plasmon coupling allows to mitigate the effect of spectral broadening that is responsible for blurring the emission spectrum at room temperature, a situation common in conventional fluorescence spectroscopy. The approach is especially useful in the case when several excited-state species are present. Fluorescence emission from closely located protonated, deprotonated, and excimer species of Py couple into surface plasmons and are easily separated and observed with 11-14 fold intensity enhancements. Furthermore, the ultranarrowband photon-sorting of emission from microenvironments in a multispecies system is performed in this study using instruments that are readily available in most laboratories without employing any deconvolution procedure and/or additional dispersive optics.