Thermally activated delayed fluorescence (TADF) emitters: sensing and boosting spin-flipping by aggregation

Abstract

Metal-free organic emitters with thermally activated delayed fluorescence (TADF) characteristics are emerging due to the potential applications in optoelectronic devices, time-resolved luminescence imaging, and solid-phase sensing. Herein, we synthesized two (4-bromobenzoyl)pyridine (BPy)-based donor-acceptor (D-A) compounds with varying donor size and strength: the emitter BPy-pTC with tert-butylcarbazole (TC) as the donor and BPy-p3C with bulky tricarbazole (3C) as the donor unit. Both BPy-pTC and BPy-p3C exhibited prominent emission with TADF properties in solution and in the solid phase. The stronger excited-state charge transfer was obtained for BPy-p3C due to the bulkier donor, leading to a more twisted D-A geometry than that of BPy-pTC. Hence, BPy-p3C exhibited aggregation-induced enhanced emission (AIEE) in a THF/water mixture. Interestingly, the singlet-triplet energy gap (ΔE _ST) was reduced for both compounds in the aggregated state as compared to toluene solution. Consequently, a faster reverse intersystem crossing rate (k _RISC) was obtained in the aggregated state, facilitating photon upconversion, leading to enhanced delayed fluorescence. Further, the lone-pair electrons of the pyridinyl nitrogen atom were found to be sensitive to acidic protons. Hence, the exposure to acid and base vapors using trifluoroacetic acid (TFA) and triethylamine (TEA) led to solid-phase fluorescence switching with fatigue resistance. The current study demonstrates the role of the donor strength and size in tuning ΔE _ST in the aggregated state as well as the relevance for fluorescence-based acid-base sensing.

Keywords: intramolecular charge transfer; molecular aggregates; sensing; thermally activated delayed fluorescence (TADF).

Scheme 1

Synthetic schemes of BPy-pTC and BPy-p3C.

Figure 1

(a) Normalized absorption spectra of BPy-pTC and BPy-p3C in toluene at room temperature; (b) normalized fluorescence (λ_ex = 375 nm, 10 µM in toluene) spectra at ambient temperature and phosphorescence spectra (λ_ex = 375 nm, 10 µM) of BPy-pTC and BPy-p3C at 77 K; emission spectra (λ_ex = 375 nm) of BPy-pTC (c) and BPy-p3C (d) in various polar solvents. (TL: toluene, DIO: 1,4-dioxane, THF: tetrahydrofuran, DCM: dichloromethane).

Figure 2

Transient photoluminescence decay (λ_ex = 375 nm) of (a) BPy-pTC and (b) BPy-p3C in degassed THF (10 µM) at room temperature.

Figure 3

AIEE studies: Emission spectra (λ_ex = 375 nm, 10 µM) of (a) BPy-pTC and (d) BPy-p3C in THF with increasing water fraction (in vol %) at room temperature. Digital photographs of (c) BPy-pTC and (d) BPy-p3C in THF/water solutions under exposure to a UV lamp (λ_ex = 365 nm). (e) Plot of PL intensity vs water fraction of BPy-pTC and BPy-p3C depicting the AIEE phenomenon in BPy-p3C. (f) SEM image of BPy-pTC aggregates formed in a 90 vol % water/THF mixture.

Figure 4

Normalized fluorescence at room temperature and phosphorescence spectra at 77 K (λ_ex = 375 nm, 10 µM) of (a) BPy-pTC and (b) BPy-p3C aggregates in 90 vol % water/THF mixture.

Figure 5

Transient photoluminescence decay (λ_ex = 375 nm, 20 µM) of (a) BPy-pTC and (b) BPy-p3C aggregates in 90 vol % water/THF mixture.

Figure 6

Fluorescence switching by acid and base fumes exposure: Emission spectra (λ_ex = 375 nm) of (a) BPy-pTC and (c) BPy-p3C in neat film upon exposure to TFA and TEA vapor at room temperature. Fluorescence switching by addition of TFA and KOH to (b) BPy-pTC and (d) BPy-p3C in toluene solution.

Figure 7

Fluorescence intensity vs number of exposures for (a) BPy-p3C and (b) BPy-pTC thin films upon exposure to TFA and TEA vapors.