Elucidating excited-state dynamics in organic functional materials using steady-state and time-resolved spectroscopies

Abstract

The chemical and structural tunability of organic functional materials (OFMs) makes them attractive alternatives to inorganic analogs for industrial applications. However, optimizing these organic systems can be difficult due to heterogeneous electronic structures, which can be traced to two main contributors. First, electronic states in organic materials exhibit intermediate delocalization, occupying a regime between molecular and semiconductor limits. Secondly, even at low concentrations, chemical and structural defects can affect the electronic structure enough to alter the measured photophysical properties of bulk materials. The variability in electronic structures from heterogeneous structural and chemical properties makes elucidating the photophysical properties of organic semiconductors extremely challenging. Nevertheless, combining steady-state and time-resolved optical techniques can help resolve the electronic characteristics and excited-state behavior in challenging OFM systems. This work utilizes steady-state and time-resolved spectroscopies and microscopies to resolve the electronic structures and excited-state dynamics in two OFMs: graphitic carbon nitride (gCN) and a zinc porphyrin (ZnP) molecular aggregate. Chapter one introduces materials and relevant considerations. Chapter two describes the theory and the optical components of time-resolved spectroscopies used in this work. Chapters 3-5 strive towards the same goal: resolving intrinsic photophysical pathways in OFM systems before and after structural or chemical perturbation. Chapter three highlights the difficulties of extracting structure-function relationships in heterogeneous gCN photocatalysts by comparing photocatalytic degradation rates of rhodamine B to chemical, structural, and optical properties of gCN photocatalysts. Chapter four resolves ultrafast electron transfer in Ag co-catalyzed gCN by monitoring a spectral shift of Ag surface plasmon resonances. Chapter five combines ultrafast broadband transient absorption microscopy and spectroscopy to probe ZnP aggregates in solutions and thin films. These works highlight several optical approaches for deconvolving photophysics in OFMs with complicated electronic structures. We have found that gCN and ZnP systems, like many OFMs, experience variable and often rapid excited-state relaxation due to strong nuclear coupling and structural heterogeneity, which can lead to unfavorable photophysical processes. However, the photophysical limitations arising from strong nuclear coupling can be mitigated by exploiting the exquisite tunability of OFMs.