Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles

Abstract

Conjugated polymers offer potential for many diverse applications, but we still lack a fundamental microscopic understanding of their electronic structure. Elementary photoexcitations (excitons) span only a few nanometres of a molecule, which itself can extend over microns, and how their behaviour is affected by molecular dimensions is not immediately obvious. For example, where is the exciton formed within a conjugated segment and is it always situated on the same repeat units? Here, we introduce structurally rigid molecular spoked wheels, 6 nm in diameter, as a model of extended π conjugation. Single-molecule fluorescence reveals random exciton localization, which leads to temporally varying emission polarization. Initially, this random localization arises after every photon absorption event because of temperature-independent spontaneous symmetry breaking. These fast fluctuations are slowed to millisecond timescales after prolonged illumination. Intramolecular heterogeneity is revealed in cryogenic spectroscopy by jumps in transition energy, but emission polarization can also switch without a spectral jump occurring, which implies long-range homogeneity in the local dielectric environment.

Ultrafast delocalization of excitation in synthetic light-harvesting nanorings

“…3). Ultrafast PL depolarization has previously been observed in other π-conjugated macrocycles based on thiophene 54 or phenylcarbazole 24 units, although none of these rings are as large as c -P24 (diameter 10 nm). Furthermore, the previously studied systems do not exhibit indications of temporally sustained delocalization – they do not show the reduced radiative rates or thermally activated superradiance expected for emission from a circularly delocalized state.…”

Using coherence to enhance function in chemical and biophysical systems

“…Energetic disorder disrupts these perfect wavelike properties underpinning exciton states, thereby diminishing their delocalization through space 12,58 . Delocalization competing with localization is seen in experimental data and is well illustrated by recent studies of various supramolecular systems [59][60][61] as well as in natural ring-shaped light-harvesting complexes from purple bacteria 62 . Superradiance is the collective fluorescence emission from two or more interacting chromophores, which lead to shorter radiative rates (analogously to coherent backscattering).…”

Chromophores in Molecular Nanorings: When Is a Ring a Ring?

“…Interestingly, the model underestimates the anisotropy for these intermediate cases; it is likely that while such randomisation occurs, certain sites are preferred due to the local energetic enviroment as discussed by Aggarwal and colleagues. 3 The present model does not take into account quantum coherence, 13,41 static distortion 42 or breaks in conjugation, 31 due to the computational challenges arising from a full quantum chemical treatment.…”

“…2,3,30 Finally, molecules that support complete, three-dimensional reorientation from the originally excited dipole transition ought to display γ = 0. Molecules such as the smaller nanoring systems presented typically have γ ≤ 0.1,…”