"N-substituted Open-shell Materials: Synthesis, Optoelectronic Properties and Device Characterization"
Open-shell compounds that possess unpaired spin demonstrate unique optoelectronic properties that are not accessible to their closed-shell counterparts. A novel open-shell N-substituted bisphenalenyl π-radical cation has been synthesized and its optoelectronic properties have been thoroughly investigated. The ambient stable π-radical cation represents a new molecular motif for emissive organic radicals as it demonstrates fluorescence through doublet emission from a higher excited state (anti-Kasha), which is the first demonstration of anti-Kasha emission from air-stable organic radicals. Single crystal devices show electrical conductivity (0.013 S/cm) supported by closer π-stacking evidenced in single crystal XRD structure analysis. Additionally, organic field effect transistor (OFET) devices demonstrate the first example of a small molecular material capable of antiambipolar conduction, a phenomenon previously only observed in p-n heterojunction devices.1 A detailed experimental and computational study has been conducted on the photophysics of a π-conjugated dication (DC) and a radical cation (RC) that differ in their redox states by one electron. The redox pair of DC and RC have significantly different electronic structure, absorption spectrum and magnetic properties. Regardless, steady-state and transient absorption spectroscopic investigations show that DC and RC display quite similar excited-state properties. TDDFT studies revealed that excited-state reorganization occurs by hole relaxation in singlet DC, while doublet RC undergoes a Jahn-Teller distortion by bending its π-backbone in order to facilitate spin-pairing between singly-occupied molecular orbitals. Overall, this study illustrates a potential strategy for modulating the emission properties of π-conjugated radicals.2
We have also discovered that a closed-shell precursor (PQPL) has self-sensitized reversible reactivity with molecular oxygen and changes color from red to colorless upon photooxygenation. Three derivatives of PQPL with different substituents (R = OCH3, H, CF3) have been synthesized, and their oxygen binding and release reactivities have been shown to be modulated by electronic effects. Furthermore, polymer-supported thin films have been developed in which PQPL retains the oxygen binding and release properties demonstrating a simple strategy for colorimetric oxygen sensing. In short, these findings for the first time explicitly demonstrate how endoperoxide formation of higher acenes can be used for simple and cost-effective colorimetric oxygen sensing with potential applications in modified atmospheric packaging and oxygen storage.3 Further investigations are underway with our collaborators for studying the long-lived triplet excited state of PQPL and utilizing acid-triggered singlet-oxygen release of PQPL endoperoxides for antimicrobial applications.
1) M. Imran, C. M. Wehrmann, and M. S. Chen, J. Am. Chem. Soc. 2020, 142, 1, 38–43.
2) C. M. Wehrmann, M. Imran, C. Pointer, L. A. Fredin, E. R. Young and M. S. Chen, Chem. Sci., 2020, 11, 10212-10219.
3) M. Imran and M. S. Chen (manuscript submitted).