Supramolecular Lattice Deformation and Exciton Trapping in Nanotubular J-Aggregates

Abstract

Interactions between excitons and molecular vibrational modes limit the extent of exciton delocalization and rate of energy transport in organic molecular aggregates, diminishing their performance in many optical device applications. This coupling leads to exciton self-trapping and subsequently changes their emission behavior. Certain amphiphilic cyanine dyes form nanotubular aggregates that demonstrate high exciton transport rates and show no such coupling between excitons and molecular vibrational modes. However, under sustained illumination these aggregates undergo photobrightening (PB) and can show a doubling in quantum yield. We investigate this reversible PB process through spectral- and time-resolved photoluminescence (PL) measurements under low illumination intensities. We observe lengthening exciton lifetimes with no corresponding spectral change. Furthermore, wide-angle X-ray scattering measurements show a change in the aggregate structure following PB. We propose a model of PB through large polaron formation, leading to trapping or shielding of these long coherence length excitons through interactions with supramolecular vibrations rather than the intramolecular vibrations typically observed in other aggregates. These excitons are then less able to access nonradiative recombination sites, which leads to the observed increase in quantum yield. The lattice deformations persist after emission and accumulate over time, resulting in brightening of the aggregate under sustained illumination. We support this model through temperature-, color-, and matrix-dependent photoluminescence measurements and show that the model correctly predicts the changes observed upon addition of a FRET quencher. Finally, we demonstrate control over PB behavior through rigidification of the aggregate with a silica shell, potentially enabling the development of long-term-photobrightened devices utilizing molecular aggregates with significantly higher photoluminescence quantum yields.

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

“…We next make use of exciton self-trapping to effectively shield Frenkel excitons from accessing nonradiative recombination centers in TDBC J-aggregates. Under sustained illumination, TDBC supramolecular assemblies show a lengthening of the fluorescence lifetime and a significant increase in the QY without any corresponding spectral changes, consistent with the supramolecular lattice deformation and exciton selftrapping model of Klein et al [50] As the concentration of nonradiative recombination centers is reduced during self-annealing, exciton shielding has an increasingly lower impact on the QY. As a consequence, aged supramolecular assemblies show minimal photo-brightening behavior under sustained illumination.…”

“…Under sustained low‐intensity illumination, supramolecular assemblies of C8S3 show an interesting behavior in their superradiant emission. [ 22,50 ] Specifically, their PL QY and lifetime increase substantially with no corresponding change in their absorption or emission spectra. Recently, Klein et al.…”

“…Recently, Klein et al investigated this process using spectral-and time-resolved PL techniques and attributed the photobrightening behavior to coupling of delocalized Frenkel excitons with supramolecular lattice deformations. [50] The formation of excitons results in structural changes in the molecular aggregate, which in turn leads to the self-trapping and localization of Frenkel excitons. These localized Frenkel excitons are then less likely to access nonradiative recombination centers through long-range transport.…”

“…Under sustained low-intensity illumination, supramolecular assemblies of C8S3 show an interesting behavior in their superradiant emission. [22,50] Specifically, their PL QY and lifetime increase substantially with no corresponding change in their absorption or emission spectra. Recently, Klein et al investigated this process using spectral-and time-resolved PL techniques and attributed the photobrightening behavior to coupling of delocalized Frenkel excitons with supramolecular lattice deformations.…”

White light emission from helically stacked humin-mimic based H-aggregates in heteroatom free carbon dots

“…Whereas, due to the vanished oscillator strength at the bottom of the excitonic band formed by face-to-face stacked molecules, the transition dipoles interact destructively or resonant electronic (excitonic) coupling is positive (higher energy), resulting in non-fluorescent H-aggregates. [33][34][35] Nevertheless, despite the non-fluorescent behaviour being a prevalent characteristic of H-aggregates, several instances of their red-shifted fluorescence through the formation of excimer and excitonic trap states have been documented in the literature. [36][37][38] This unexpected fluorescence from H-aggregates is believed to arise from ground state symmetry breaking due to the imperfect face-to-face arrangement of molecular fluorophores, which makes the transition feasible from the lower-energy exciton state.…”