Conjugated polymers (CP) have become a common component in organic electronic materials and there is continued interest in characterizing the electronic states that underlie their material properties. At a fundamental level, CPs present highly interesting questions regarding the nature of localized states in highly inhomogeneous systems and the structural features along an extended p-conjugated framework that are responsible for localizing transient states. We present our experimental and computational efforts to illuminate the structural properties and dynamics of localized states in disordered CPs. Experimentally we utilize resonantly enhanced and time-resolved Raman spectroscopy to (1) examine variations in structure and delocalization amongst configurations contributing to broad electronic absorption bands, (2) identify vibrational signatures associated with localization at specific motifs along the polymer, and (3) probe nuclear dynamics that follow excitation. Raman spectroscopy is a natural tool for examining variations in structure in conjugated oligomers and polymers, as C=C stretching frequencies are highly sensitive to the extent of delocalization along a conjugated chain (an effect termed �Raman dispersion�). We demonstrate that resonance enhancement enables photoselectivity for interrogating different environments that underlie inhomogenously broadened electronic polymer absorption bands, as shown through correlations between excited-state resonant-Raman spectra and Raman-excitation energy. We also present work with poly-(3-hexylthiophene) (P3HT) and poly-(3-cyclohexyl-4-methylthiophene) (PCMT) demonstrating that evolution in excited-state conformation can be tracked through time-dependence in Raman frequencies and in mode-specific resonance enhancement. Computational efforts support our assignments to variations and evolution in delocalization length of localized states and also enable characterization of excited-state vibrational features.

Structure, dynamics, and delocalization in conjugated polymers revealed with Raman spectroscopy

248th National Meeting (2014)

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