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.

One of the fundamental design paradigms in organic photovoltaic device engineering is based on the idea that charge separation is an extrinsically driven process requiring an interface for exciton fission. This idea has driven an enormous materials science engineering effort focused on construction of domain sizes commensurate with a nominal exciton diffusion length of order 10 nm. Here, we show that polarized optical excitation of isolated pristine crystalline nanowires of a small molecule n-type organic semiconductor, 7,8,15,16-tetraazaterrylene, generates a significant population of charge-separated polaron pairs along the π-stacking direction. Charge separation was signalled by pronounced power-law photoluminescence decay polarized along the same axis. In the transverse direction, we observed exponential decay associated with excitons localized on individual monomers. We propose that this effect derives from an intrinsic directional charge-transfer interaction that can ultimately be programmed by molecular packing geometry.

A versatile technique to measure two-dimensional (2D) coherent electronic spectra involves coherent excitation of a nonlinear optical response and time- and phase-resolved measurement of a resulting population observable, such as photoluminescence intensity, photocurrent, or photoinduced absorption. This coherent spectroscopy is a unique probe in materials research in that it resolves energy pathways of ultrafast processes by addressing the coupling between electronic states in semiconductors in the complex dielectric environment of operating devices. However, we have previously demonstrated that an \emph{incoherent} nonlinear response due to pairs of photoexcitations produced via linear excitation pathways contribute to the measured signal as unexpected background [Gr\'egoire et al., J.\ Chem.\ Phys. \textbf{147}, 114201 (2017)]. Here, we simulate the effect of such incoherent population mixing in the photocurrent signal collected from a GaAs solar cell by acting externally on the transimpedance amplifier circuit used for phase-sensitive detection. We identify an effective strategy to recognize the presence of incoherent population mixing in 2D data, and we discuss a possible way of isolating the coherent response.

Supercritical fluids, exhibiting a combination of liquid-like solvation power and gas-like diffusivity, are a relatively unexplored medium for processing and crystallization of oligomer and polymeric semiconductors whose optoelectronic properties critically depend on the microstructure. Here we report oligomer crystallization from the polymer organic semiconductor, poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) in supercritical hexane, yielding needle-like single crystals up to several microns in length. We characterize the crystals' photophysical properties by time- and polarization-resolved photoluminescence (TPRPL) spectroscopy. These techniques reveal two-dimensional interchromophore coupling facilitated by the high degree of π-stacking order within the crystal. Furthermore, the crystals obtained from supercritical fluid were found to be similar photophysically as the crystallites found in solution-cast thin films and distinct from solution-grown crystals that exhibited spectroscopic signatures indicative of different packing geometries.

In this work, we used solvent vapor annealing as a route toward controlled cluster growth of a nitrogen-substituted terrylene molecule (7,8,15,16-tetrazaterrylene or TAT) to probe the underlying molecular packing behind the exciton band curvature (EBC) inversion leading to a so-called HJ aggregates. Using a suite of photoluminescence (PL) imaging tools, we show that the EBCas signaled by PL spectral signaturesare correlated with distinct polarization and defocused emission patterns suggestive of a high degree of chromophore alignment. Selected area electron diffraction showed that the small aggregates adopt nominally the same crystal packing structure as the extended crystals though with a different plane orientation ([010] vs [011]) with respect to the surface normal. These findings demonstrate experimentally that the EBC inversion is not due to a crystal polymorphism but, rather subtle differences in molecular registration that profoundly impacts interchromophore coupling.