Evolution of HJ Coupling in Nanoscale Molecular Self-Assemblies

Abstract

We investigated the evolution of combined Frenkel exciton and charge-transfer chromophore coupling in nanoscale self-assembled clusters of 7,8,15,16-tetraazaterrylene. Using spatially and wavelength-resolved fluorescence imaging, we observed significant changes in the relative intensities of vibronic transitions (J-like to H-like) in isolated clusters with increasing size (along the crystal growth axis), suggestive of a change in the interference of the combined short-and long-range interactions upon assembly. Large clusters and nanowires showed spectral signatures that appear superficially like Haggregates with a diminished origin (0−0) intensity relative to the higher vibronic sidebands, whereas small nanoclustersappearing as diffraction limited spotshad distinct J-aggregate spectral signatures (enhancement of the origin intensity) suggestive of different packing structures in small clusters. The isolation of the different nanoscale assemblies allows for the opportunity to understand changes in molecular registration upon solution-phase crystallization.

Unraveling the Contribution of Residual Monomer to the Emission Spectra of Poly(3-hexylthiophene) Aggregates: Implications for Identifying H- and J-type Coupling

“…Results similar to those in Figure have been previously reported in P3HT ,,− and several other organic conjugated systems ,,, and were ascribed to evolution of the most favorable aggregate structure from H-like to J-like due to solvent-induced forces. Changing pressure or temperature produces similar effects that have been attributed to alterations in the chain packing and therefore the electronic coupling between the chains. ,, Next, we examine various existing models to fit the rr-P3HT emission spectra and to explain its evolution with increasing poor solvent and then describe experiments that support an alternative approach.…”

“…Within this picture, the evolution of the emission spectra with decreasing solvent quality (Figure a) results from a decrease in the percentage of monomer present and the emergence of an aggregate species with an electronic origin at 650 nm and a J-like vibronic structure. In contrast, the HJ model would attribute this spectral evolution to a change from a weakly coupled H-aggregate to a J-aggregate with the change in solvent quality. ,,, …”

“…rr-P3HT and other conjugated polymers are considered to form weakly coupled H-aggregates under most common conditions. ,− Their absorption and emission spectra are weakly red-shifted from that of the corresponding monomer due to the planarized chain morphology . Interestingly, while H-type aggregation predominates in P3HT, conditions have been found that produce predominantly J-type, suggesting that the aggregate type can be controlled by the processing conditions. ,, …”

“…Next, we consider the WCH model, described earlier, that has been frequently applied to P3HT. ,− ,, In this fitting approach (eq ) the 0–0 band intensity is varied independently while the vibronic transitions are fit using the expected Franck–Condon intensity distribution assuming a single S for the CC mode. Here, n is the refractive index, ℏω is the photon energy, α is a scaling factor that is allowed to vary, Γ is the Gaussian line width function, E 0 is the 0–0 transition energy, and E d is the energy of the CC vibrational mode (set to 0.175 eV) . The S values of these systems were determined via a nonlinear least-squares fit of eq .…”

“…More recently, the WCH model has been expanded to explain the type of spectral evolution seen in Figure a that is commonly seen in conjugated organics with decreasing solvent quality or changes in temperature or pressure. ,,,− This has been ascribed to a change from H- to J-type packing driven by the environment. ,,,, Therefore, these systems are referred to as HJ-aggregates as their spectral properties are governed by the relative magnitudes of the H- and J-type coupling. Applying this model to our results, the spectra at 50% MeCN and above would be assigned to a J-aggregate whereas all the vibronic features of the aggregate at lower MeCN percentages would be ascribed to the H-aggregate form.…”