Organic semiconductors have attracted considerable attention due to their applications in low-cost, solution-processable (opto)electronic devices. An important class of high-performance organic semiconductors is pentacene derivatives, which exhibit high charger carrier mobilities in field-effect transistors and ultrafast singlet fission in photovoltaic devices. These derivatives have served as benchmark materials for systematic studies of exciton and charge carrier dynamics, and they are the focus of this thesis. Photo-induced degradation of organic semiconductors is one of the bottlenecks preventing their wide-spread use in optoelectronic devices. As such, it is important to understand the underlying processes and develop strategies for their mitigation. For functionalized pentacene (Pn) two photodegradation processes are known to dominate: endoperoxide formation (EPO), which occurs in the presence of oxygen, and photodimerization, which occurs regardless of the presence of oxygen. The rate of decay is dependent on the specific Pn derivative and the local environment. This work explores the effects of environmental factors and specific molecular characteristics that affect the photostability and photodegradation reversibility of functionalized fluorinated pentacene (Pn-R-F8) derivatives, where R is a variable side group, and their non-fluorinated counterparts (Pn-R). Experiments are done in solutions and in films, from the bulk level (typically utilized in optoelectronic devices) to the single molecule level.
In solutions, degradation of Pn molecules (monitored via changes in optical absorption under continuous illumination in air) and their partial recovery after thermolysis were quantified for various derivatives depending on the solvent, Pn concentration, side group (R), and fluorination. Fluorinated molecules (Pn-R-F8) were more stable than their non-fluorinated counterparts (Pn-R) and larger side groups (R) also protected the molecule from degradation. More concentrated solutions were considerably more stable as compared to dilute solutions. The nature of the solvent was also a factor; for example, molecules in chlorobenzene decay much faster than those in benzene under the same illumination conditions. The freshly made and photobleached solutions were analyzed using nuclear magnetic resonance (NMR) to identify the types of products formed. NMR spectra enabled identifi�cation of multiple products indicating that both EPO formation and dimerization are occurring simultaneously.
In guest-host polymer films, where Pn molecules (guests) were dispersed at various concentrations in a host polymer, the photo-degradation was measured using photoluminescene spectroscopy (PL). In agreement with the experiments from solution, the thin-films showed that polymer host had a significant impact on photostability of Pn in films. For example, for the same concentration of Pn molecules, the films with a polymethylmethacrylate (PMMA) host exhibited considerably slower photodegradation as compared to those with the polyvinylidene fluoride (PVDF) host. Furthermore, an improvement in photostability was observed on the photodegradation and recovery rates when Pn is functionalized with the side group TCHS, than when using the side group TIPS. The effects of temperature of the samples were also measured using thin-film PL. Thermally activated behavior for the photodegradation processes was observed, with faster decay at higher temperatures as the added energy acted as a catalyst for the photo-decay reactions. However, added energy did not increase the amount of PL recovery in the samples until the temperature reached a high enough threshold, which in this case is between 350-370 K. The last parameter tested was concentration of Pn in films. At higher concentrations, the rate of photo-decay decreased, which indicates that in aggregate, the molecules are more protected from the causes of photo-degradation as compared to isolated molecules. The enhanced protection is enabled by concentration-dependent changes in the excited state dynamics and associated populations of reactive states.
In order to understand and isolate the photodegradation processes on the molecular level, studies were performed on guest-host films with ultra-low concentrations of the Pn guest molecules. The Pn molecules were imaged in a variety of polymer matrices at 633 nm excitation at room temperature in air using wide-field fluorescence microscopy. Fluorescence time trajectories were collected and statistically analyzed to quantify blinking due to reversible EPO formation depending on the host matrix. This was also compared to single Pn donor (D) molecules that were imaged in PMMA in the presence of acceptor (A) molecules at various concentra�tions, which modified the local environment. Both changes to nanoenvironment affected the fluorescence of the molecules. For example, the PMMA host promoted the photostability of Pn molecules as compared to other polymer hosts studied, whereas addition of acceptor molecules reduced the photostability of the Pn donor molecules.
To understand the physical changes of the molecular system, a Monte Carlo method was used to create a multi-level simulation, which enabled us to relate the change in the molecular transition rates to the experimentally measured parame�ters in our single-molecule fluorescence spectroscopy experiments. These compre�hensive studies provide insight into the synergistic effect of the local environment and molecular characteristics on the photodegradation and subsequent recovery of functionalized pentacene, which is important for development of next-generation materials with enhanced stability for organic electronic devices
