Spectroscopic Probes of Charge Separation, Energy Transfer, and Their Dependencies on Local Structure in Assembled Organic Materials

Ultrafast, spectroscopic techniques were utilized to interrogate the properties and dynamics of excitonic and charge-separated states in aggregates and assemblies of conjugated polymeric and oligomeric materials relevant to organic electronics. Dependencies between photophysical dynamics and local morphology, intermolecular interactions, and chemical composition were explored. Novel, advanced experimental set-ups were developed and optimized for transient absorption (TA), femtosecond stimulated Raman (FSRS), and broadband transient polarization (BPA) spectroscopies. FSRS measurements yielded direct structural evidence that coupled charge transfer transients localize on morphologically well-ordered sites and clarified photophysical dynamics in poly-thiophene nanoaggregates. Transient absorption and broadband polarization measurements were used to characterize exciton dynamics in nonplanar thiophene-methanoannulene polymers and indicate limited intermolecular interaction with polymer aggregation. Evidence of prolonged charge separation and long-range energy transfer was identified in assemblies of peptidic charge transfer dyads and conjugated polyelectrolyte complexes, respectively. A unique experimental set-up for conducting ultrafast measurements of broadband electronic polarization anisotropy (BPA) was developed and used to characterize the directionality and relaxation dynamics between excited states of 9,10-bis(phenylethynyl)anthracene and applied as a spectroscopic tool for examining other conjugated materials