Aggregation-induced delayed fluorescence luminogens: the innovation of purely organic emitters for aqueous electrochemiluminescence

Abstract

Due to overcoming the limitation of aggregation caused quenching (ACQ) of solid-state emitters, aggregation-induced emission (AIE) organic luminogens have become a promising candidate in aqueous electrochemiluminescence (ECL). However, restricted by the physical nature of fluorescence, current organic AIE luminogen-based ECL (AIECL) faces the bottleneck of low ECL efficiency. Here, we propose to construct de novo aqueous ECL based on aggregation-induced delayed fluorescence (AIDF) luminogens, called AIDF-ECL. Compared with the previous organic AIE luminogens, purely organic AIDF luminogens integrate the superiorities of both AIE and the utilization of dark triplets via thermal-activated spin up-conversion properties, thereby possessing the capability of close-to-unity exciton utilization for ECL. The results show that the ECL characteristics using AIDF luminogens are directly related to their AIDF properties. Compared with an AIECL control sample based on a tetraphenylethylene AIE moiety, the ECL efficiency of our AIDF-ECL model system is improved by 5.4 times, confirming the excellent effectiveness of this innovative strategy.

Scheme 1. (A) Schematic illustration of the fluorescent-type ECL (FL-ECL), aggregation-induced emission-type ECL (AIE-ECL) and aggregation-induced delayed fluorescence (AIDF)-type ECL (AIDF-ECL) and their mechanisms, in which S₁, T₁, S₀, PF-ECL, DF-ECL, IS, RISC and NR represent the lowest singlet state, the lowest triplet state, the ground singlet state, prompt fluorescent ECL, delayed fluorescent ECL, intersystem crossing, reverse intersystem crossing, and nonradiative deactivation, respectively. (B) Preparation of the AIDF aggregated luminogens by self-assembly in a mixed solvent. Inside the dashed box are the schematic diagrams of photophysical transitions in the molecular state in THF solution or the aggregated solid state in a THF/H₂O mixed solvent. (C) Chemical structures of AIE (left) and AIDF (right) molecules used in this report.

Fig. 1. PL spectra of the (A) mCP-BP-PXZ and (B) TPE-TAPBI molecule in THF/water mixtures with different water fractions (f_w). Inset: the corresponding photographs of the luminogens in THF/water mixtures under UV irradiation (λ = 365 nm). The transient PL decay curves of the (C) mCP-BP-PXZ molecule with different f_w (excitation wavelength: 363 nm, and detection wavelength: 553 nm, in air) at 300 K, and the (D) TPE-TAPBI molecule with different f_w (excitation wavelength: 363 nm, and detection wavelength: 454 nm, in air) at 300 K.

Fig. 2. (A) Anodic cyclic voltammograms and (B) the oxidative-reduction ECL responses of the AIDF molecule (red line, f_w: 95%) and AIE molecule (blue line, f_w: 95%). (C) The oxidative-reduction ECL responses of the AIDF luminogen-modified GCE under the conditions of a potential window ranging from 0 V to 1.3 V (vs. Ag/AgCl), scan rate of 0.5 V s⁻¹, 0.1 M PBS containing 0.1 M KCl and 40 mM TPrA, and pH 7.44. PMT: 850 V. Inset: PL and ECL spectrum of the AIDF luminogen in the solid state (f_w: 95%). (D) PL and ECL trends of the AIDF luminogen with different f_w.

Fig. 3. (A) The schematic subprocesses of oxidative-reduction ECL of the AIDF-luminogen-modified GCE/TPrA couple and (B) the proposed ECL mechanisms, in which 1AIDF gen* and 3AIDF gen* represent singlet and triplet excited states that were electrochemically generated on AIDF luminogens, respectively.