Switching between H- and J-type electronic coupling in single conjugated polymer aggregates

Abstract

The aggregation of conjugated polymers and electronic coupling of chromophores play a central role in the fundamental understanding of light and charge generation processes. Here we report that the predominant coupling in isolated aggregates of conjugated polymers can be switched reversibly between H-type and J-type coupling by partially swelling and drying the aggregates. Aggregation is identified by shifts in photoluminescence energy, changes in vibronic peak ratio, and photoluminescence lifetime. This experiment unravels the internal electronic structure of the aggregate and highlights the importance of the drying process in the final spectroscopic properties. The electronic coupling after drying is tuned between H-type and J-type by changing the side chains of the conjugated polymer, but can also be entirely suppressed. The types of electronic coupling correlate with chain morphology, which is quantified by excitation polarization spectroscopy and the efficiency of interchromophoric energy transfer that is revealed by the degree of single-photon emission.

Control of Intrachain Morphology in the Formation of Polyfluorene Aggregates on the Single‐Molecule Level

“…We note that interchain coupling, i. e. H‐type aggregation, can be ruled out in this case, as opposed to other conjugated polymer materials aggregated by SVA . H‐type aggregation leads to a red‐shift of fluorescence spectra with an additional increase of the 0‐1/0‐0 peak ratio . The transition dipole moments of the individual monomers of the β‐phase effectively add up like in a J‐aggregate .…”

“…One way to obtain such a direct connection is by avoiding ensemble averaging and measuring discrete mesoscopic regions of the PFO film. Such an approach can be experimentally achieved by bottom‐up assembly of mesoscopic aggregates, built from one molecule at a time, and investigating these particles with single molecule spectroscopic (SMS) techniques …”

“…Interestingly, by applying predominantly the good solvent chloroform (5/95) as an annealing vapor, 61 % of the aggregates emerge in the γ‐phase or in a mixture of β‐ and γ‐phases. Changing the vapor ratio therefore offers a unique handle to manipulate the intrachain phase while potentially also controlling interchain ordering . However, further exploration of the SVA induced aggregation and its impact on interchain ordering is necessary so that a correlation with the degree and duration of swelling can lead to further handles of morphological control …”

Printing 2D Conjugated Polymer Monolayers and Their Distinct Electronic Properties

“…In contrast, coupling between chromophoric units that belong to different polymer chains and lie side‐by‐side gives rise to H‐coupling. The resulting changes in spectral signatures due to H‐aggregation compared to isolated polymer chains is diametrically opposed to J‐coupling, i.e., A 0–0 / A 0–1 decreases with enhanced H‐coupling 22a,23. We observed that the A 0–0 / A 0–1 value increases by almost 65% from 0.61 in monolayer to 1.00 in 7.5 nm film, reaches the highest value of 1.09 at ≈14 nm and gradually decreases on further thickness increase (Figure 6b).…”

Interplay Between J‐ and H‐Type Coupling in Aggregates of π‐Conjugated Polymers: A Single‐Molecule Perspective

“…In any ensemble, static and dynamic disorder control the interaction distances and the degree of electronic resonance between molecules, amplifying the effect of disorder broadening, which masks electronic coherences even in ultrafast experiments . Single‐molecule techniques have emerged as a route to study aggregation . With methods of controlled solvent‐vapor annealing, a few single molecules can be assembled to one single aggregate, probing the realm of single‐molecular mesoscopic structures with minimal disorder .…”