Designing Highly Luminescent Molecular Aggregates via Bottom-Up Nanoscale Engineering

Abstract

Coupling of excitations between organic fluorophores in J-aggregates leads to coherent delocalization of excitons across multiple molecules, resulting in materials with high extinction coefficients, long-range exciton transport, and, in particular, short radiative lifetimes. Despite these favorable optical properties, uses of J-aggregates as high-speed light sources have been hindered by their low photoluminescence quantum yields. Here, we take a bottom-up approach to design a novel J-aggregate system with a large extinction coefficient, a high quantum yield and a short lifetime. To achieve this goal, we first select a J-aggregating cyanine chromophore and reduce its nonradiative pathways by rigidifying the backbone of the cyanine dye. The resulting conformationally-restrained cyanine dye exhibits strong absorbance at 530 nm and fluorescence at 550 nm with 90% quantum yield and 2.3 ns lifetime. We develop optimal conditions for the self-assembly of highly emissive J-aggregates. Cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS) reveal micron-scale extended structures with 2D sheet-like morphology, indicating long-range structural order. These novel J-aggregates have a strong red-shifted absorption at 600 nm, resonant fluorescence with no Stokes shift, 50% quantum yield, and 220 ps lifetime at room temperature. We further stabilize these aggregates in a glassy sugar matrix and study their excitonic behavior using 2 temperature-dependent absorption and fluorescence spectroscopy. These temperaturedependent studies confirm J-type excitonic coupling and superradiance. Our results have implications for the development of a new generation of organic fluorophores that combine high speed, high quantum yield and solution processing.

Design rules for obtaining narrow luminescence from semiconductors made in solution

“…598 As an example, the left portion of Figure 31B shows the structure of a recently reported J-aggregate-forming dye, Cy3UB. 599 Like many such aggregate-forming dyes, this is a cyanine dye, the planarity and large polarizability of which favors aggregate formation in poor solvents, and even the isolated molecule shows a fairly narrow emission spectrum (Figure 31B center), with the aggregate (Figure 31B right) clearly showing the three characteristics of J-aggregate spectra noted above, notably a narrower 0,0 emission.…”

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

“…It was previously confirmed that the concentrations of rhodamine 6G used in 0.1 mm cuvettes do not lead to the quenching of the dye, retaining the same high QY. [ 44 ] Fluorescence spectra were collected using the front facing geometry, where rhodamine 6G and J‐aggregates were excited using 500 and 565 nm excitation sources, respectively. The QY of TDBC J‐aggregates was calculated using the ratio of the slopes of the calibration plots, taking into account differences in refractive indices and the power of excitation sources.…”

“…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.…”

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

“…The increased relative intensity of the 0,0 subpeak can been explained using quantum-mechanical analysis of the vibronic structure, while the 0,0 peak is narrowed due to rapid motion of the exciton between different sites . As an example, the left portion of Figure B shows the structure of a recently reported J-aggregate-forming dye, Cy3UB . Like many such aggregate-forming dyes, this is a cyanine dye, the planarity and large polarizability of which favors aggregate formation in poor solvents, and even the isolated molecule shows a fairly narrow emission spectrum (Figure B, center), with the aggregate (Figure B, right) clearly showing the three characteristics of J-aggregate spectra noted above, notably a narrower 0,0 emission.…”

“…(A) Schematic showing how a side-by-side (H-aggregate) alignment of chromophore transition dipoles leads to a low-energy nonemissive state but how end-to-end (J-aggregate) alignment leads to an emissive state lower in energy than that of the monomer. (B) Structure of Cy3UB, an example of a cyanine dye reported to form J aggregates, along with absorption and emission spectra of monomeric Cy3UB in MeOH and of Cy3UB J aggregates in 9:1 MeOH/H 2 O. Spectra reproduced with permission from ref . Copyright 2022 American Chemical Society.…”