Art Bragg

Structure, Dynamics, and Disorder in Excited-States of Photoresponsive Conjugated Materials

Conjugation is a key ingredient in photoresponsive organic materials (e.g. photovoltaics and photoswitches) and there is continued interest in tailoring conjugated moieties and their interactions in order to control material behaviors.  A critical step in this direction is to understand relationships between structure at the intra- and intermolecular levels and photoinduced behaviors (dynamics).  In this talk we present our efforts to illuminate structural properties and dynamics of excited states in conjugated materials as well as structure-dynamics relationships that underlie their photoresponsive behaviors.  Experimentally we utilize femtosecond stimulated Raman spectroscopy in combination with time-resolved electronic spectroscopies to interrogate variations in local structure and delocalization and to probe nuclear dynamics induced by excitation. We present work with poly- and oligothiophenes demonstrating that ultrafast evolution in excited-state conformation and exciton localization can be tracked through time-dependence in Raman frequencies and in mode-specific resonance enhancements (bottom left).  We also demonstrate that resonance enhancement enables photoselectivity for interrogating the nature of various structural motifs that support localized excited and charge-separated states in aggregated materials (center).  Our work with novel photoswitches demonstrates how the photophysics of oligothiophene-based materials enable multiplicity-selective switching mechanisms (right).