Kinetics of Spontaneous Formation of Chiral J-Aggregate of N-Sulfobutyl Oxacarbocyanine

Structural and optical variation of pseudoisocyanine aggregates nucleated on DNA substrates

Coherently coupled pseudoisocyanine (PIC) dye aggregates have demonstrated the ability to delocalize electronic excitations and ultimately migrate excitons with much higher efficiency than similar designs where excitations are isolated to individual chromophores. Here, we report initial evidence of a new type of PIC aggregate, formed through heterogeneous nucleation on DNA oligonucleotides, displaying photophysical properties that differ significantly from previously reported aggregates. This new aggregate, which we call the super aggregate (SA) due to the need for elevated dye excess to form it, is clearly differentiated from previously reported aggregates by spectroscopic and biophysical characterization. In emission spectra, the SA exhibits peak narrowing and, in some cases, significant quantum yield variation, indicative of stronger coupling in cyanine dyes. The SA was further characterized with circular dichroism and atomic force microscopy observing unique features depending on the DNA substrate. Then by integrating an AlexaFluor^TM647 (AF) dye as an energy transfer acceptor into the system, we observed mixed energy transfer characteristics using the different DNA. For example, SA formed with a rigid DNA double crossover tile (DX-tile) substrate resulted in AF emission sensitization. While SA formed with more flexible non-DX-tile DNA (i.e. duplex and single strand DNA) resulted in AF emission quenching. These combined characterizations strongly imply that DNA-based PIC aggregate properties can be controlled through simple modifications to the DNA substrate's sequence and geometry. Ultimately, we aim to inform rational design principles for future device prototyping. For example, one key conclusion of the study is that the high absorbance cross-section and efficient energy transfer observed with rigid substrates made for better photonic antennae, compared to flexible DNA substrates.

Reorganization of self-assembled dipeptide porphyrin J-aggregates in water-ethanol mixtures

The self-assembly of a neutral meso-methoxyphenylporphyrin functionalized with a dipeptide glycilglycine substituent (MGG) in water and in water-ethanol mixtures was studied by absorption and fluorescence spectroscopy. In water, hydrophobic interactions and the noncovalent intermolecular hydrogen bonding between the terminal carboxylate group of one porphyrin and the hydrogen atoms of the pyrrolic nitrogens of another porphyrin originate nonspecific disorganized H- and J-aggregates. The addition of ethanol (0.1-25% v/v) to the water creates small clusters within which porphyrin J-aggregates reorganize as revealed by a narrow intense band detected by the Rayleigh light scattering (RLS) at 443 nm. Similar phenomenology is detected in SDS premicellar aggregates. Computational DFT calculations of a model dimer formation stabilized via intermolecular hydrogen bonding estimate an energy gain of -22 kJ mol(-1) and a center-to-center and interplane distances between porphyrin moieties of 16.8 and 3.7 Å, respectively. The kinetics of the J-aggregate formation could be fitted with a time-dependent model, and an activation energy of 96 kJ mol(-1) was estimated. The aggregate's morphology of MGG was followed by transmission electron microscopy (TEM) which showed rod-type structures of 5-8 μm evolving to spherical particles with increased ethanol content. Similar images and sizes were obtained in analogous samples using fluorescence lifetime imaging microscopy (FLIM) and dynamic light scattering (DLS). The formation of excitonically coupled supramolecular MGG structures of brickwork or staircase types is proposed in these water-ethanol mixtures.