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Polymers 2016, 8(11), 388; doi:10.3390/polym8110388

Understanding the Structural Evolution of Single Conjugated Polymer Chain Conformers

Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM 87131, USA
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Academic Editors: Ping Xu and Hsing-Lin Wang
Received: 16 September 2016 / Revised: 23 October 2016 / Accepted: 31 October 2016 / Published: 3 November 2016
(This article belongs to the Special Issue Conjugated Polymers 2016)
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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. View Full-Text
Keywords: single molecule spectroscopy; conjugated polymers; emissive conformers single molecule spectroscopy; conjugated polymers; emissive conformers
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Wise, A.J.; Grey, J.K. Understanding the Structural Evolution of Single Conjugated Polymer Chain Conformers. Polymers 2016, 8, 388.

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