Single-Molecule Spectroscopy of Polyfluorene Chains Reveals β-Phase Content and Phase Reversibility in Organic Solvents

Abstract

Conjugated polymers are an important class of carbon-based materials, combining semiconducting properties with simple fabrication of devices from solution. In solution, each polymer chain can adopt different shapes-so-called conformations-with remarkably different photoluminescence (PL) properties. We have developed a microscopy-based technique to study the shape and PL features on a chain-by-chain basis. Using this method, we monitored the formation of planarized b-phase conformers in poly-dialkylfluorene chains and observed single chains reversibly exchanging between b phase and disordered conformations in real time.

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

“…We note that even in the chains showing the pure β-phase spectra, most of the chain probably stays in the glassy phase and the energy absorbed by the glassy segments is transferred to the one or a few β-phase segments from where it is emitted. 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 β-phase, consistent with other recent single-molecule studies. , …”

“…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.…”

“…For example, cryogenic temperature experiments on oligofluorenes dispersed and solvent annealed in inert polymer films showed that a minimum of 9 monomer units are necessary to efficiently form b-phase [28]. Very recent study on PFO immobilized in solution confirmed reversibility of the b-phase upon cycling between good and poor solvents [30].…”

“…In dilute solutions, the first step of β-phase formation was found to be driven by the side-chain interactions, because these interactions change upon cooling due to varying conformational and rotational freedom. , Apart from the side-chain length, the overall molecular weight of PFO also affects the efficiency of β-phase formation, but different approaches provide conflicting results. − Some of the questions on reversibility of the β-phase or on the role of aggregation can be unveiled by studying individual PFO chains on a truly single-molecular level. − For example, cryogenic temperature experiments on oligofluorenes dispersed and solvent annealed in inert polymer films showed that a minimum of nine monomer units are necessary to efficiently form a β-phase . A very recent study on PFO immobilized in solution confirmed reversibility of the β-phase upon cycling between good and poor solvents …”

“…29 A very recent study on PFO immobilized in solution confirmed reversibility of the β-phase upon cycling between good and poor solvents. 31 Apart from the glassy and β-phases, an intermediate γ-phase (also referred to as a crystalline phase) is occasionally observed. Spectrally, this form of PFO appears as a longwavelength shoulder in absorption of the glassy phase; 13 its emission is located between the glassy and β-phases 8 and is identifiable in Raman spectra.…”