Elucidation of an Aggregate Excited State in the Electrochemiluminescence and Chemiluminescence of a Thermally Activated Delayed Fluorescence (TADF) Emitter

Abstract

The electrochemistry, electrochemiluminescence (ECL), and chemiluminescence (CL) properties of a thermally activated delayed fluorescence (TADF) emitter 4,4′-(1,2-dihydroacenaphthylene-5,6-diyl)bis(N,N-diphenylaniline) (TPA-ace-TRZ) and three of its analogues were investigated. TPA-ace-TRZ exhibits both (a) delayed onset of ECL and (b) long-persistent luminescence, which we have attributed to the formation of an aggregate excited state in excimer or exciplex form. The evidence of this aggregate excited state was consistent across ECL annihilation and coreactant pathways as well as in CL. The absolute ECL efficiency of TPA-ace-TRZ using benzoyl peroxide (BPO) as a coreactant was found to be 0.028%, which was 9-fold stronger than the [Ru(bpy) 3 ] 2+ /BPO reference coereactant system. Furthermore, the absolute CL quantum efficiency of TPA-ace-TRZ was determined to be 0.92%. The performance and flexibility of the TADF emitter TPA-ace-TRZ under these various emissive pathways are highly desirable toward applications in sensing, imaging, and light-emitting devices.

Highly Efficient Electrochemiluminescence from Imidazole‐Based Thermally Activated Delayed Fluorescence Emitters

“…In addition, a build‐up profile in the ECL signal is always observed independently from the solvent. Such behaviour has been already observed in literature and attributed to the formation of excimer of exciples even leading to organic long‐persistent ECL (OLECL), [13,14,31,32] as uncompensated solution resistance, triplet‐triplet annihilation and thermally activated delayed fluorescence could be ruled out as contributing mechanism to ECL.…”

Thermally Activated Delayed Fluorescent Polymer Dots for Electrochemiluminescent Sensing of Cu2+ Ions

“…The PL emission of PABPC5-Pdots is confirmed to be TADF emission in photophysics (mentioned above). Thus, it implies that the ECL emission of the coreactant ECL system originates from the TADF emission of PABPC5-Pdots in nature, rather than other possibilities, such as exciplex, excimer, long-persistent phosphorescence mentioned elsewhere. − …”

“…These T 1 states (ca. 75% in total) are capable of complete up-conversion into S 1 states under the assistance of environmental thermal energy via the efficient reverse ISC (RISC) process, followed by ECL via the delayed fluorescent (DF) route (DF-ECL). − Approximately 100% of Φ ECL can be realized in theory for the purely organic TADF-ECL system compared to ∼25% of Φ ECL for the conventional organic fluorescent (FL) ECL (FL-ECL) system. , The all-exciton-harvesting potential of TADF-ECL in organic media has been well documented, regardless of annihilation ECL or coreactant ECL. − Presently, the Φ ECL of TADF-ECL emitters using modified TADF molecules or polymers outperforms those of Ru­(bpy) 3 2+ and FL-ECL emitters. Furthermore, our group developed a few aqueous TADF-ECL biosensors.…”

“…Despite the progress of aqueous ECL systems using TADF-Pdots, the corresponding Φ ECL was inferior to that of Ru­(bpy) 3 2+ , owing to complicated mechanisms and functions. Exciplex, excimer, long-persistent room phosphorescence, or other routes occur between the ECL chromophore themselves and/or between ECL chromophores and coreactants. − If these above mechanisms happened, then TADF-ECL emitters would not emit via the TADF mechanism during electrochemical operation. Therefore, aqueous ECL using TADF-Pdots does not certainly emit via the TADF mechanism.…”

Materials for Electrochemiluminescence: TADF, Hydrogen‐Bonding, and Aggregation‐ and Crystallization‐Induced Emission Luminophores

“…We also investigated the ECL behaviour of the TSCT TADF molecule TPA‐ace‐TRZ (Figure 3A). [15] This emitter produced ECL emission by both the annihilation and co‐reactant pathways that was significantly red‐shifted compared to the PL emission (Figure 3B). A significant delay in the ECL onset was seen (Figure 3C), which as observed with the inefficient D‐A TADF emitters in Figure 2, hints at the formation of an exciplex.…”