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

Abstract

ordered molecules in a wide variety of morphologies, such as fibers, sheets, and nanotubes. [1,2] Since their independent discovery by Jelley and Scheibe in 1936, [3,4] there has been keen interest in understanding supramolecular assemblies for their unique exciton properties which are distinctly different from those of the single constituting molecules. Close and highly ordered packing allows strong long-range excitonic interaction between the chromophores, resulting in optical properties that strongly depend on the precise alignment of transition dipole moments in the aggregate. In particular, the head-to-tail arrangement of transition dipole moments in J-aggregates leads to the formation of new molecular excitons with redshifted and narrowed spectra. [5] As in natural lightharvesting complexes, molecular excitons in supramolecular assemblies, also called Frenkel excitons, are delocalized over several chromophores in the aggregates. At low temperatures, exciton delocalization lengths of up to 200 molecules have been observed. [6,7] The collective nature of excitations in supramolecular assemblies is responsible for their unique optical properties, such as remarkably narrow and intense absorption, long-range exciton transport, and short radiative lifetimes.As a result of coherent exciton coupling, the oscillator strength for certain excitonic transitions in J-aggregates increases nonlinearly with the number of coupled molecules, leading to strong absorption. In fact, J-aggregates are some of the strongest absorbers known, with measured absorption constants as high as 10 6 cm −1 and absorption strengths reaching 30% at their resonance wavelengths even for a single monolayer. [8,9] Because of their absorption and transport properties, J-aggregates have been widely used as spectral sensitizers in the photographic film industry. [1,2,10] Coherent exciton coupling also leads to a cooperative emission, known as superradiance, which occurs at much faster timescales since the radiative rate is proportional to the oscillator strength. [1,11,12] Even at room temperature, radiative lifetimes as low as 27 ps have been observed for high-quality 2D monolayers of perylene bisimide J-aggregates in the solid state. [9] Because of their strong absorption and superradiant emission, molecular J-aggregates hold great promise towards efficient high-speed light sources, which are important for many applications including high-speed Three general effective strategies are shown to mitigate nonradiative losses in the superradiant emission from supramolecular assemblies. J-aggregates of 5,5′,6,6′-tetrachloro-1,1′-diethyl-3,3′-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) are used to elucidate the nature of nonradiative processes. Selfannealing at room temperature (RT), photo-brightening, and purification of the dye monomers are shown to all lead to substantial increases in emission quantum yields (QYs) and a concomitant lengthening of the emission lifetime, with purification having the largest effect. Structural and optical measuremen...

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“…TDBC Jaggregates were formed from monomers that were purified prior to their self-assembly following a previously established method. 25 The QY enhancement was achieved by removing impurity sites that act as nonradiative recombination centers. Increasing the number of purification steps to three did not result in a substantial improvement in the QY of bare Jaggregates, moving from 82 to 88% (Figure 4D).…”

“…Purification of TDBC Monomers. Unless otherwise noted, all Jaggregate samples were prepared using purified TDBC monomers, following a slightly modified purification procedure provided by Ulugbek et al 25 Initially, 150 mg of TDBC, as received from the vendor (FEW Chemicals, S0046), was dispersed in 25 mL methanol (>99.9%, VWR, EM-MX0488-1), and the resulting mixture was heated for 2 h at 60 °C. Afterward, the mixture was slowly cooled to room temperature overnight in the dark.…”

“…38−40 However, we recently demonstrated a QY of 82% with a 174 ps emissive lifetime through the purification of monomers prior to their self-assembly. 25 Purification of monomers enables successful removal of impurities embedded in the densely packed J-aggregate supramolecular lattice, which can trap excitons and function as nonradiative recombination centers. 25 While the optical performance of TDBC J-aggregates has improved, their persistent structural and chemical instability restricts their practical applications.…”

“…25 Purification of monomers enables successful removal of impurities embedded in the densely packed J-aggregate supramolecular lattice, which can trap excitons and function as nonradiative recombination centers. 25 While the optical performance of TDBC J-aggregates has improved, their persistent structural and chemical instability restricts their practical applications. This limitation also leaves room for further enhancement of their optical properties, including their fluorescence QY.…”

“…Superradiance occurs when the ensemble of aggregated chromophores collectively emits light coherently . This cooperative emission causes red-shifted and narrowed spectra, extended exciton delocalization, long-range exciton transport, − and notably subnanosecond emissive lifetimes. ,− Leveraging these exceptionally distinct properties, J-aggregates have potential applications in sensing and imaging − and are proposed as bright, fast, and ideal light sources for high-speed free-space optical , and quantum communication . However, despite their merits, J-aggregates still suffer from limitations that have hindered their use in devices and other applications.…”