Excitonic Energy Migration in Conjugated Polymers: The Critical Role of Interchain Morphology

Abstract

Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in regioregular poly(3-hexylthiophene) (rr-P3HT) enables long-range interchain energy migration, while disordered packing in regiorandom poly(3-hexylthiophene) (rra-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to rr-P3HT, interchain energy migration in poly(3-(2'-methoxy-5'-octylphenyl)thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes.

Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes

“…To examine how fluorination affects the morphology of single-polymer chains and aggregates, fluorescence excitation polarization analysis was performed (see SI Materials and Methods for details). In these experiments, the fluorescence intensity traces were modulated with a rotating linearly polarized excitation light and then fitted with IðαÞ ∝ 1 + M cos2ðα − ϕÞ, where α is the excitation polarization angle and ϕ is the polarization angle at maximum absorption (2,26,27). The modulation depth, M, represents the anisotropy of the absorption (excitation) tensor projected on the x−y plane of the laboratory frame and is related to the morphological order of individual molecules or aggregates.…”

“…1A) (see SI Materials and Methods for details). To avoid the complexity of intramolecular folding-induced interchain interactions, we use polymers with a low number average molecular mass of 13 kDa, for which there is no detectable folding of the polyalkylthiophene chain according to our previous studies (2,21,22). Fluorescence excitation polarization experiments and molecular dynamics (MD) simulations reveal that backbone fluorination dramatically changes the polymer chain conformation from disordered coil-like for P3EHT into extended rod-like for F-P3EHT.…”

“…Nevertheless, fundamental and critical knowledge about the influence of fluorination on single-chain conformation, intermolecular morphology, and excitonic coupling on a molecular basis is still lacking. Singlemolecule/aggregate spectroscopy in conjunction with nanoscale aggregates fabricated via a solvent-vapor-annealing (SVA) technique has proven to be an unprecedented and elegant approach in elucidating complex relations between morphology and photophysics in CPs (2, 19), while circumventing ensemble averaging effects due to bulk measurements and morphological heterogeneity in bulk samples (2,19,20). …”

“…orphology and excitonic coupling are among the most important factors dictating the functions and performance of conjugated polymers (CPs) in a variety of optoelectronic applications (1)(2)(3)(4). To tune and optimize the morphological and optoelectronic properties of CPs, chemical structure modification of the backbone and side chains is one of the main and most effective approaches (5)(6)(7).…”

Excitonic coupling dominates the homogeneous photoluminescence excitation linewidth in semicrystalline polymeric semiconductors

“…For example, Raithel et al show that subtle aggregation of P3HT chains produces strong redshifts and broadening in comparison to spectra from isolated chains [22]. Hu et al observed that long-range exciton transport depends strongly on aggregate order [26], further highlighting the importance of interchain interactions on exciton dynamics. In a further study, Hu et al demonstrated that dielectric-induced stabilization of nonradiative charge-transfer states very sensitively controls PL quantum efficiencies in suspended P3HT aggregates [27], consistent with the role proposed by Yamagata et al of charge-transfer components of excitonic coupling [25], indicating once again the key role the environment plays in dictating photophysics in aggregates.…”