Bridging the Gap between H- and J-Aggregates: Classification and Supramolecular Tunability for Excitonic Band Structures in 2-Dimensional Molecular Aggregates

Abstract

Molecular aggregates with long-range excitonic couplings have drastically different photophysical properties compared to their monomer counterparts. From Kasha’s model for 1-dimensional systems, positive or negative excitonic couplings lead to blue or red shifted optical spectra with respect to the monomers, labelled H-and J-aggregates respectively. The overall excitonic couplings in higher dimensional systems are much more complicated and cannot be simply classified from their spectral shifts alone. Here, we provide a unified classification for extended 2D aggregates using temperature dependent peak shifts, thermal broadening and quantum yields. We discuss the examples of six 2D aggregates with J-like absorption spectra but quite drastic changes quantum yields and superradiance. We find the origin of the differences is, in fact, a different excitonic band structure where the bright state is lower energy than the monomer but still away from the band edge. We call this an ‘I-aggregate’. Our results provide a description of the complex excitonic behaviors that cannot be explained solely on Kasha’s model. Further, such properties can be tuned with the packing geometries within the aggregates providing supramolecular pathways for controlling them. This will allow for precise optimizations of aggregate properties in their applications across the areas of optoelectronics, photonics, excitonic energy transfer, and shortwave infrared technologies.

Near‐Unity Superradiant Emission from Delocalized Frenkel Excitons in a Two‐Dimensional Supramolecular Assembly

“…The methanol solution of TDBC has a relatively low QY of 3%. [ 42,44 ] This is because of the flexible trimethine chain, where excited‐state C–C rotations act as a major nonradiative recombination pathway reducing the PL QY. [ 45,46 ] It has been shown that rigidification of the polymethine chain in many cyanine dyes using fused ring systems leads to a significantly improved PL QY.…”

“…In order to study the temporal dynamics of fluorescence, lifetime traces were obtained by integrating the emission across the wavelength range from 570–600 nm. The time‐resolved fluorescence decay of the monomer in methanol follows a biexponential curve with an 80 ps average lifetime, [ 42 ] whereas that of the freshly prepared J‐aggregates in water follows a monoexponential curve with a 130 ps lifetime (Figure 1G). Even though the monomer has a shorter lifetime than the J‐aggregate, most of the fluorescent decay in the monomer is because of nonradiative recombinations as evidenced by the low 3% QY.…”

“…This explains highly variable reported PL QY values ranging from 5% to 49% for the J‐aggregates of TDBC in solution. [ 23,27,42 ] We previously reported that even a small concentration of impurities diminishes the QY of Cy3UB J‐aggregates considerably. [ 44 ] In order to obtain highly emissive Cy3UB J‐aggregates, it was crucial to use an extensively purified monomer.…”

“…Despite the latter, reported PL QY values for TDBC J‐aggregate are low, ranging from <5% to 49% in solution. [ 23,27,41,42 ] Incorporation of small molecules and surfactants has been shown to influence nanoscale packing of J‐aggregates, further improving the PL QY to 60%. [ 23,27 ] In this paper, we perform an in‐depth investigation of fundamental exciton behavior and nanoscale structural dynamics in supramolecular assemblies of TDBC.…”