Ultrafast Raman Spectroscopy as a Probe of Local Structure and


Sep 15, 2016 - Biography. Art Bragg received his Ph.D. from UC Berkeley, where he worked with Professor Daniel M. Neumark studying the electronic rela...
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Ultrafast Raman Spectroscopy as a Probe of Local Structure and Dynamics in Photoexcited Conjugated Materials Arthur E Bragg, Wenjian Yu, Jiawang Zhou, and Timothy J. Magnanelli J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01060 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 16, 2016

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“An important challenge with light-responsive conjugated organic materials is to relate the properties of transient states directly with local structure, such as molecular conformation and material microstructure or morphology.”

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“Given that steady-state Raman spectroscopy of conjugated polymers and oligomers exhibit sensitivities to changes in charge distribution, electron delocalization, and microstructure or morphology, transient Raman spectroscopy holds promise for interrogating properties of transient excited and charge-separated states and their dependence on local structural characteristics.”

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“However, electronic spectroscopy provides limited, indirect insight on structural evolution in conjugated system, whereas time-resolved vibrational spectroscopy can serve as a more sensitive probe of ultrafast structural evolution.”

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“Experimental techniques with vibrational sensitivity have promise for interrogating local structure and structural dynamics associated with trapping and relaxation of excited and chargeseparated states in motifs along the extended macromolecular structure.”

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“Conjugated materials as used in device applications are aggregated or assembled; this raises the question about how local intermolecular interactions impact the nature of transient and chargeseparated states in these materials.”

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Ultrafast Raman Spectroscopy as a Probe of Local Structure and Dynamics in Photoexcited Conjugated Materials

Arthur E. Bragg,* Wenjian Yu, Jiawang Zhou, Timothy Magnanelli Department of Chemistry, Johns Hopkins University 3400 N. Charles St., Baltimore, MD 21218 *corresponding author: [email protected], 410-516-5616

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Abstract: An important challenge in the study of conjugated organic materials is to relate the properties of transient states underlying macroscopic material responses directly with intra- and intermolecular structure. We discuss recent efforts using the vibrational sensitivity of timeresolved Raman spectroscopy that have interrogated structural properties of transient excited and charge-separated states in conjugated oligomers and polymers in order to relate them to molecular conformation and material microstructure. We focus on our recent work on excitedstate Raman spectroscopy that provides mode-specific signatures of structural relaxation in oligo- and polythiophenes; examination of structural heterogeneities associated with exciton localization in different structural motifs of amorphous polymer; and interrogation of correlations between microstructure and properties of charge-separated states within polymer aggregates. Based on these and related work from other labs, we provide an outlook for further applying this method to elucidate relationships between structure, properties of transient states, and the photoresponses of conjugated materials.

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Conjugated polymers and oligomers have become a common semiconducting component in organic electronics, including photovoltaics1-3 and light-emitting diodes4,5 as well as organic field-effect transistors and other organic semiconductor devices.6,7 Interest in conjugated organic materials arises from their inherent processing advantages and flexibility relative to their solidstate, inorganic counterparts. However, these benefits come with a catch: the properties and responses of organic semiconducting materials are intimately linked with their structure and interactions at the molecular and intermolecular level; for light-responsive applications in particular, local structure determines the nature of transient electronic and charge-separated states and their dynamics that underlie material responses.3,8,9 An important challenge with lightresponsive conjugated organic materials is to relate the properties of transient states directly with local structure, such as molecular conformation and material microstructure or morphology. Transient electronic spectroscopies have been valuable for elucidating mechanisms and timescales for relaxation and kinetics of transient states,10-16 but provide limited explicit information on relationships with local structure or structural dynamics. On the other hand, steady-state vibrational spectroscopies have been used to establish valuable correlations between material structure and macroscopic material responses, but with limited insights about transient states that underlie these responses.17-19 This perspective discusses recent interest and efforts aimed at using the structural sensitivity of time-resolved Raman spectroscopy to interrogate transient excited and charge-separated states in conjugated organic materials in connection with their local molecular or material structure.

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Figure 1: Variation (or “dispersion”) in symmetric C=C stretching frequency of oligofurans (OF, Ref. 31), oligopyrroles (OP, Ref. 32), and oligothiophenes (OT, Ref. 30) with number of monomer units (n). Best-fit trend lines (vs. 1/n) are plotted for comparison, as are the C=C symmetric stretching frequencies for annealed and amorphous films of poly(3-hexylthiophene) (Refs. 17,19). The symmetric stretching mode is illustrated in the inset image of septithiophene (7T). Raman Spectroscopy and the Steady-State Characterization of Conjugated Materials Raman spectroscopy has a long history as a probe of conjugated oligomeric and polymeric materials, used initially for structural fingerprinting of conductive and conjugated polymers first synthesized in the 1980s and 90s.20-23 Raman cross-sections of many vibrational modes of conjugated oligomers and homo-polymers are unusually large due to the fact that these modes induce large changes in polarizability that is associated with significant electron delocalization of excited electronic states; under resonant conditions many of these modes are coincident with the structural displacement between nominally benzoidal ground- and quinoidal excited-state geometries (i.e. large electron-phonon coupling occurs along this structural coordinate).24,25 In particular the Raman spectra of conjugated homo-oligomers and polymers

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are dominated by symmetric C=C stretching, and to lesser degree C-C (intra- and inter-ring) and in-plane CH bending. As oxidized conjugated polymers and oligomers take on a delocalized, quinoidal character, steady-state Raman spectroscopy has been a valuable tool for interrogating charge doping of conjugated materials.21,26,27 In addition to resonant enhancements driven by strong electron-phonon coupling, the intensities and frequencies of Raman-active modes are strongly affected by the length of a conjugated chain.24,25,28-32 Figure 1 illustrates specifically that the symmetric C=C stretching frequencies of various conjugated oligomers shift with length, an effect termed Raman-frequency dispersion.24,29,32

Frequency dispersion can be recognized as a quantum-mechanical effect

associated with electron delocalization for two reasons: firstly, Raman frequency dispersion generally appears as a frequency decrease with oligomer length, whereas pure mechanical coupling between monomers to form a vibrational exciton would give rise to frequency increases with increasing oligomer length;33 secondly, the degree of dispersion with oligomer length is anticorrelated with monomer aromaticity (e.g. thiophene, phenyl, phenylene vinylene < pyrrole, furan