A Spectroscopic Investigation into the Photophysics of Donor-Acceptor Emitters Intended for Use in Thermally Activated Delayed Fluorescence Based Optoelectronic Systems: A Comparison of Excited State Dynamics in Solution and Solid-State

Abstract

Optoelectronic systems have become an integral part of day-to-day life for a significant portion of the world. While the past generations of organic light emitting diodes (OLEDs) have achieved success utilizing phosphorescent compounds, there is still significant room for improvement. Namely, the phosphorescent compounds that are used as emitters have heavy rare metals (such as iridium and platinum) incorporated into their molecular design. This limits the molecular structures available, makes them relatively expensive to produce, and causes competition with other fields that utilize the same metals such as catalysis. Beyond this, there are still OLED stability issues that plague these systems such as triplet-triplet annihilation. This is especially true for blue OLEDs, where the high energy need to excite the blue emitters causes device degradation faster than the red and green OLEDs. Thus, there is a need for a cheaper and more stable alternative that can reach the same or better device efficiencies as the phosphorescent based OLEDs. One promising class of emitters is known as thermally activated delayed fluorescence (TADF). The TADF mechanism is the idea that non-emissive triplet excited states can be converted into emissive singlet excited states via a reverse intersystem crossing process. This process is energized by the thermal energy of the surroundings and can occur at room temperature. Additionally, TADF emitters can be purely organic making them relatively cheaper than the previous generation of emitters. It's still unclear what molecular design best lends itself to TADF and how the emitter photophysics are influenced by guest-host interactions that occur in solid-state films. These factors are critical if the current OLED efficiencies are to be improved. Understanding these factors would also aid other areas of optoelectronics, particularly those that utilize solid-state host materials. As a result, the focus of this thesis is to utilize both time-resolved and nonlinear optical techniques to: 1) investigate and improve the understanding of the structure-function relationships that govern the TADF mechanism, and 2) determine what guest-host interactions result in changes in emitter photophysics. It was found that emitters that incorporate linkers into their molecular design are not well suited for TADF-activity. This is due to the linker inhibiting sufficient HOMO/LUMO separation which results in large singlet-triplet energy gaps (EST) and prevents spin-orbit coupling (SOC) between the singlet and triplet manifolds. However, emitters that have multiple identical donors directly attached to the acceptor unit were found to be better suited for TADF-activity. This is due to sufficient HOMO/LUMO separation being achieved with this molecular design, allowing for a significantly smaller EST, greater SOC, and a sufficing rate of reverse intersystem crossing (krISC). Further investigation revealed that utilizing the linker and multiple identical donors does enhance SOC, but results in phosphorescence and not TADF.

It was also determined that guest-host interactions play a significant role in determining the emissive mechanism of the guest chromophore. Specifically, results indicate that hydrogen bonding interactions can induce TADF-activity for the BCC-TPTA emitter while non-bonding interactions between the aromatic moieties of the guest and host results in triplet-triplet annihilation up-conversion. Overall, this work utilized established spectroscopic techniques of the Goodson lab and required the development of a new addition to the nanosecond transient absorption system. Each of these techniques were utilized to elucidate which molecular structure is better suited for TADF-activity and the relevant guest-host interactions within solid-state films.