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

Abstract

Controlling the morphology of π-conjugated polymers for organic optoelectronic devices has long been a goal in the field of materials science. Since the morphology of a polymer chain is closely intertwined with its photophysical properties, it is desirable to be able to change the arrangement of the polymers at will. We investigate the π-conjugated polymer poly(9,9dioctylfluorene) (PFO), which can exist in three distinctly different structural phases: the α-, β-, and γ-phase. Every phase has a different chain structure and a unique photoluminescence (PL) spectrum. Due to its unique properties and the pronounced spectral structure-property relations, PFO can be used as a model system to study the morphology of π-conjugated polymers. To avoid ensemble averaging, we examine the PL spectrum of single PFO chains embedded in a non-fluorescent matrix. With single-molecule spectroscopy the structural phase of every single chain can be determined, and changes can be monitored very easily. To manipulate the morphology, solvent vapor annealing (SVA) was applied, which leads to a diffusion of the polymer chains in the matrix. The βand γ-phases appear during the self-assembly of single α-phase PFO chains into mesoscopic aggregates. The extent of βand γ-phase formation is directed by the solvent-swelling protocol used for aggregation. Aggregation unequivocally promotes formation of the more planar βand γ-phases. Once these lower-energy more ordered structural phases are formed, SVA cannot return the polymer chain to the less ordered phase by aggregate swelling.

Real-Time Monitoring of Formation and Dynamics of Intra- and Interchain Phases in Single Molecules of Polyfluorene

“…1, within the same location of the film we observe chains with purely b-phase emission, purely glassy phase emission and occasionally also spectra with mixed glassy and b-phase characteristics. Distribution of the fluorescence 0-0 peak wavelengths obtained from the fitting of 63 single PFO chains shows a peak around 422 nm corresponding to the glassy phase and a peak around 436 nm corresponding to the b-phase, consistent with other recent singlemolecule studies [30,34]. The spectra presented in Fig.…”

“…S1 shows that even though the SVA leads to diffusion of considerable fraction of the single PFO chains, the average numbers of emitting spots (PFO chains), as well as the emission intensity distributions are similar in all three cases. This result indicates that, unlike the recent study on PFO aggregates [34], we can analyze and interpret our results as phenomena happening on the true single-chain level.…”

Brush Printing Creates Polarized Green Fluorescence: 3D Orientation Mapping and Stochastic Analysis of Conductive Polymer Films

“…63,64 In fact, fluorene polymers show red-shifted PL by forming aggregates, e.g., refs. 65,66 The fluorescence spectra of F8BT have spectral components at longer wavelengths, which might be due to aggregate species. However, the species of longer wavelength have not been attributed to date, to the best of our knowledge.…”

“…To consider the alignment of polymer molecules that do not involve crystalline aggregates of F8BT, S f was evaluated from the fluorescence spectra using a shorter wavelength (500–525 nm). This is because the component of the fluorescence spectra in a longer wavelength has been characterized as a component of crystalline aggregates for π-conjugated polymers in refs. , In fact, fluorene polymers show red-shifted PL by forming aggregates, e.g., refs. , The fluorescence spectra of F8BT have spectral components at longer wavelengths, which might be due to aggregate species. However, the species of longer wavelength have not been attributed to date, to the best of our knowledge.…”

Photon correlations probe the quantized nature of light emission from optoelectronic materials

“…Regular spin coating tends to promote formation of the kinetically trapped glassy phase, which can then be annealed to the -phase by solvent-vapor exposure. 245 While the differences between the two phases in room-temperature PL spectra in Figure 28 may not appear to be overly dramatic, the effect on the photon statistics, shown in panels c, d, is: the -phase chain shows substantial photon antibunching whereas the glassyphase chain does not. The 𝑔 Δ𝑡 histograms under pulsed excitation indicate the level of the 𝑔 Δ𝑡 0 dip expected for one (*) or two (**) emitters given the signal-to-noise level.…”