Features of exciton dynamics in molecular nanoclusters (J-aggregates): Exciton self-trapping (Review Article)

Abstract

We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = εLR/2β, wherein εLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g < 1) and a strong (g > 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model.

Fluorescent J-aggregates of cyanine dyes: basic research and applications review

“…In formal description of their electronic properties an analogy is often used with molecular crystals (actually, their 1D models). In this description, such exciton characteristics as the width of the exciton band, exciton migration parameters, exciton decay time and extinction coefficients of exciton absorption are applied [48]. Accordingly, in dye aggregates the excitons can be delocalized across many chromophores [49].…”

“…As opposed to molecular crystals, for J-aggregates there is the possibility of performing the estimates of N c from the measurements of two independent parameters: the ratio of the spectral widths of the absorption bands and the radiative lifetimes comparing these data with that for monomer molecules [48].…”

“…One of the most important effects determining the fluorescence properties of J-aggregates is the exciton self-trapping [48], which becomes significant in the structures with a high degree of static disorder. The narrow fluorescence bands with slight Stokes shifts observed in the majority of experiments are commonly attributed to the emission of free excitons.…”

“…Since the charge density distribution along the dye polymethine chain changes on excitation, this must result in specific molecular chain deformation. It was suggested that this leads to generation of self-trapped excitons resulting in quenching [48].…”

Enhanced Room‐Temperature Photoluminescence Quantum Yield in Morphology Controlled J‐Aggregates

“…The generally observed low PL quantum yield (PLQY) in J‐aggregates at room temperature was explained by static disorder, vibronic coupling, or non‐radiative relaxation of self‐trapped excitons. [ 12 ] Among all organic J‐aggregate forming dyes, cyanine dyes stick out with characteristic features: high ground state polarizability, small bond‐length alternation, and charge inversion upon photoexcitation give the aggregates high structural stability and their unique spectral properties. Another valuable property of cyanine dyes is the possibility to extend the absorption range to the near‐infrared (NIR).…”