Investigation of Thermally Activated Delayed Fluorescence in Donor-Acceptor Organic Emitters with Time-Resolved Absorption Spectroscopy

In this contribution, we utilize multiple time-resolved and nonlinear optical measurements and quantum chemical simulations to investigate the excited-state dynamics of organic chromophores with thermally activated delayed fluorescent (TADF) characteristics. We are most interested in probing the influence of a phenylene linker on the photophysical properties of emitters composed of carbazole-based donors linked to either a phthalonitrile (PN) or diphenyltriazine (Trz) acceptor. The PN-acceptor compounds display a near double-fluorescence quantum yield (Phi F) enhancement in oxygen-free conditions. The fluorescent lifetime measurements indicate that the Trz-acceptor compounds are more efficient fluorescence emitters and quickly go from a delocalized to localized state. They also reveal that only the PN-acceptor compounds display a long-lived emissive lifetime component associated with TADF activity. Analysis of the nanosecond transient absorption spectra and kinetics reveals long-lived excited-state absorption (ESA) bands associated with triplet states for the PN-acceptor compounds. No ESA bands were observed for the Trz-acceptor compounds, despite observing a quantum yield enhancement for the Trz-acceptor compounds after oxygen purging. From the transient absorption measurements, it was determined that the PN-acceptor compounds have reverse intersystem crossing rates (krISC) that are able to compete with other triplet decay pathways. From quantum chemical calculations, it is proposed that inclusion of the phenylene linker prevents sufficient highest occupied molecular orbital/lowest unoccupied molecular orbital separation and suppresses TADF activity and that directly linking multiple donors to the acceptor will aid in achieving TADF activity.