Spectroscopy and dynamics of photoexcited π-conjugated materials



Poly-conjugated molecules are ubiquitous components in molecular-based materials, including novel conductors and photovoltaic devices (i.e., solar cells). The structural properties and dynamics of conjugated moities dictate the courses of energy- and charge-transfer processes that underscore greater material properties and function, and should be well-understood in order to optimize material design. Additionally, excited states of poly-conjugated systems exhibit energy-transfer behaviors and structural characteristics that are of great fundamental chemical interest. Towards developing a molecular-level understanding of these features, we are interrogating the dynamics and spectroscopy of various poly-conjugated molecular systems that have potential material applications.

One focus of our group’s research aims to illuminate the properties and dynamics of excited and charge-separated states in conjugated oligomers, polymers and their materials, which are dependent on local conformation and material microstructure or morphology. Resonantly enhanced time-resolved Raman spectroscopy is central to our work and is used to (1) directly examine conformational and structural variations that underlie broad electronic absorption features of transient states, (2) identify spectroscopic signatures of structural distortions that demarcate boundaries between excited (or charged) and unexcited (or neutral) regions of the polymer, thereby defining the spatial extent of localized states, and (3) directly probe nuclear dynamics following polymer excitation. Our work to date with poly-(3-hexylthiophene) illustrates that resonance-enhanced Raman is sensitive to structural and conformational disorder of polymer excited states; this is evident from variation in the excited-state resonant-Raman spectrum with changes to Raman-excitation energy. We have also demonstrated that evolution in excited-state conformation can be tracked according to the mode-specific time-dependence in the resonance enhancement of Raman features (JPCL 3, 1321-29 (2012)). Our research aims to fill a gap in the molecular-level understanding of the nature of localized states in extended, one-dimensional π-conjugated systems. Current work in our group is examining excited-state properties and dynamics of thiophene polymers and oligomers with the goal of assessing structure-dynamics relationships for these localized states. We have also begun to use polymer nanoparticles and films to explore how these localized states are influenced by the intermolecular interactions.