Donor-Acceptor Based Solvent-free Organic Liquid Hybrids with Exciplex Emission and Room Temperature Phosphorescence

Solvent-free organic liquids have been known for their excellent luminescent features. Hence, the recent developments in this area have marked them as potential emitters with high quantum yield and enhanced...

Modeling and characterization of exciplexes in photoredox CO2 reduction: Insights from quantum chemistry and fluorescence spectroscopy

Interactions between excited state arenes and amines can lead to the formation of structures with distinct emission behavior. These excited state complexes or exciplexes can reduce the ability of the arene to participate in other reactions, such as CO2 reduction, or increase the likelihood of degradation via Birch reduction. Exciplex geometries are necessary to understand photophysical behavior and probe degradation pathways but are challenging to calculate. We establish a detailed computational protocol for calculation, verification, and characterization of exciplexes. Using fluorescence spectroscopy, we first demonstrate the formation of exciplexes between excited state oligo-(p-phenylene) (OPP), shown to successfully carry out CO2 reduction, and triethylamine (TEA). Time-dependent density functional theory (TDDFT) is employed to optimize the geometries of these exciplexes, which are validated by comparing both emission energies and their solvatochromism with experiment. Excited state energy decomposition analysis confirms the predominant role played by charge transfer interactions in the red-shift of emissions relative to the isolated excited state OPP*. We find that although the exciplex emission frequency depends strongly on solvent dielectric, the extent of charge separation in an exciplex does not. Our results also suggest that the formation of solvent-separated ionic radical states upon complete electron transfer competes with exciplex formation in higher dielectric solvents, thereby leading to reduced exciplex emission intensities in fluorescence experiments.

Efficient Exciplex-Based Deep-Blue Organic Light-Emitting Diodes Employing a Bis(4-fluorophenyl)amine-Substituted Heptazine Acceptor

The realization of a deep-blue-emitting exciplex system is a herculean task in the field of organic light-emitting diodes (OLEDs) on account of a large red-shifted and broadened exciplex emission spectrum in comparison to those of the corresponding single compounds. Herein, 2,5,8-tris(di(4-fluorophenyl)amine)-1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3FDPA) was designed as an electron acceptor by integrating three bis(4-fluorophenyl)amine groups into a heptazine core, while 1,3-di(9H-carbazol-9-yl)benzene (mCP) possessing two electron-donating carbazole moieties was chosen as the electron donor. Excitingly, the exciplex system of 8 wt% HAP-3FDPA:mCP exhibited deep-blue emission and a high photoluminescence quantum yield of 53.2%. More importantly, an OLED containing this exciplex system as an emitting layer showed deep-blue emission with Commission Internationale de l’Eclairage coordinates of (0.16, 0.12), a peak luminance of 15,148 cd m−2, and a rather high maximum external quantum efficiency of 10.2% along with a low roll-off. This study not only reports an efficient exciplex-based deep-blue emitter but also presents a feasible pathway to construct highly efficient deep-blue OLEDs based on exciplex systems.

Perylene Based Solution Processed Single Layer WOLED with Adjustable CCT and CRI

In solution processed single layer white organic light emitting diode (WOLED) applications, the choice of host matrix and optimization of dopant levels represent two crucial parameters to consider. In this work, poly(N-vinylcarbazole) (PVK): 2-(4-Biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD) and PVK:1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (OXD-7) matrices are used as hosts for perylene based devices. PVK:PBD presented better compatibility and lower turn-on voltages compared to PVK:OXD-7. Benefiting from the exciplex emission observed at 630 nm, a color rendering index (CRI) value of 90 is reached with the device containing PVK:PBD as the host and 0.1 wt.% of an orange emitting perylene derivative, i.e., PDI. Introduction of the perylene based green emitter, i.e., PTE, in this emitting layer not only caused a fading in the exciplex emission, but also resulted in disappearance of the electroplex peak at 535 nm, which is detected between PVK:PBD and PTE in bare PTE containing devices. Full visible range coverage is achieved by optimizing the PDI:PTE ratio. WOLED containing PVK:PBD:0.06 wt.% PDI:0.03 wt.% PTE presented high CRI (≥95) and adjustable correlated color temperatures (CCT, 3800 K-5100 K).

Lock-and-Key Exciplexes for Thermally Activated Delayed Fluorescence

We combine synthetic supramolecular chemistry and materials science to develop novel exciplexes for thermally activated delayed fluorescence. Our approach starts from a bowl-shaped acceptor molecule for which we synthesize tailor-made donors that bind in a lock-and-key fashion. The donor design is guided by extensive density functional theory calculations of three independent donor families. The investigation of a large number of custom-synthesized donors allows us to derive empirical relationships for the prediction of the exciplex emission color. Incorporated within organic light-emitting devices, the lock-and-key exciplexes yield external quantum efficiencies of up to 5.4%, with potentially tunable emission color across the blue and green visible spectrum.