Abstract
Single molecule photoluminescence (PL) spectroscopy of conjugated polymers has shed new light on the complex structure–function relationships of these materials. Although extensive work has been carried out using polarization and excitation intensity modulated experiments to elucidate conformation-dependent photophysics, surprisingly little attention has been given to information contained in the PL spectral line shapes. We investigate single molecule PL spectra of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) which exists in at least two emissive conformers and can only be observed at dilute levels. Using a model based on the well-known “Missing Mode Effect” (MIME), we show that vibronic progression intervals for MEH-PPV conformers can be explained by relative contributions from particular skeletal vibrational modes. Here, observed progression intervals do not match any ground state Raman active vibrational frequency and instead represent a coalescence of multiple modes in the frequency domain. For example, the higher energy emitting “blue” MEH-PPV form exhibits PL maxima at ~18,200 cm−1 with characteristic MIME progression intervals of ~1200–1350 cm−1, whereas the lower energy emitting “red” form peaks at ~17,100 cm−1 with intervals in the range of ~1350–1450 cm−1. The main differences in blue and red MEH-PPV chromophores lie in the intra-chain order, or, planarity of monomers within a chromophore segment. We demonstrate that the Raman-active out-of-plane C–H wag of the MEH-PPV vinylene group (~966 cm−1) has the greatest influence in determining the observed vibronic progression MIME interval. Namely, larger displacements (intensities)—indicating lower intra-chain order—lower the effective MIME interval. This simple model provides useful insights into the conformational characteristics of the heterogeneous chromophore landscape without requiring costly and time-consuming low temperature or single molecule Raman capabilities.
Highlights
The existence of multiple conformers in conjugated polymers has important implications for material performance and can be revealed most straightforwardly from optical absorption and photoluminescence (PL) spectra [1,2,3,4,5,6,7,8]
We show that the vibronic progression intervals vary depending on the emitting form and can be explained by the Missing Mode Effect” (MIME) model
Assignment of emissive conformers was based on the PL energy of the electronic origin (E0-0 ) and sub-ensemble PL spectra were generated by averaging spectra within their respective distributions
Summary
The existence of multiple conformers in conjugated polymers has important implications for material performance and can be revealed most straightforwardly from optical absorption and photoluminescence (PL) spectra [1,2,3,4,5,6,7,8]. Despite the extensive studies of the bimodality and apparent accidental vibronic interval energy separation phenomena in MEH-PPV and many other polymer systems, investigations into the structural attributes of these emitters and the extent of geometric rearrangement between ground and emitting states has received much less attention In their detailed study of transformations between emitting species, Köhler et al reported a second order phase transition between disordered (blue) and ordered (red) forms at ca. We report the emergence of a minority lower energy emitter separate from the typical blue and red species in low MW polystyrene hosts These “ultra-red” species overlap energetically with excimer-like transitions in thermally annealed bulk films they possess resolved vibronic structure with PL line shapes resembling blue forms. Adapting the simple MIME line shape analysis to MEH-PPV single molecules in different environments sheds new light on the structural qualities of emitter chromophores beyond chain conformational characteristics determined from polarization studies
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