Thermally activated triplet exciton release for highly efficient tri-mode organic afterglow

Abstract

Developing high-efficient afterglow from metal-free organic molecules remains a formidable challenge due to the intrinsically spin-forbidden phosphorescence emission nature of organic afterglow, and only a few examples exhibit afterglow efficiency over 10%. Here, we demonstrate that the organic afterglow can be enhanced dramatically by thermally activated processes to release the excitons on the stabilized triplet state (T₁^*) to the lowest triplet state (T₁) and to the singlet excited state (S₁) for spin-allowed emission. Designed in a twisted donor-acceptor architecture with small singlet-triplet splitting energy and shallow exciton trapping depth, the thermally activated organic afterglow shows an efficiency up to 45%. This afterglow is an extraordinary tri-mode emission at room temperature from the radiative decays of S₁, T₁, and T₁^*. With the highest afterglow efficiency reported so far, the tri-mode afterglow represents an important concept advance in designing high-efficient organic afterglow materials through facilitating thermally activated release of stabilized triplet excitons.

Fig. 1. A mechanism for improving the organic afterglow efficiency.

a Mechanism of OURTP by constructing T₁^* in organic aggregates. The molecule in the ground state (S₀) is excited (Exc.) to the lowest singlet excited state (S₁) for fluorescence (Fluo.) with lifetime of several nanoseconds (ns) and ISC (step 1) to populate the lowest triplet excited state (T₁) for weak phosphorescence (Phos.); when T₁ is stabilized in T₁^* (step 2), the radiative deactivation of T₁^* results in OURTP with lifetime up to seconds (s). b TADF mechanism with facile RISC process (step 3) for delayed fluorescence (DF) with lifetime around several microseconds (μs). c TAA emission realized by thermally activated exciton release (TAER) (step 4) and RISC (step 3) processes. Firstly, the trapped excitons in T₁^* were released by thermal perturbation to T₁ at a small E_TD for delayed phosphorescence (DP). Secondly, the released T₁ exciton was transformed to S₁ exciton for the spin-allowed DF at a small ΔE_ST. d Design of TAA molecules based on difluoroboron β-diketonate and carbazole in a twisted D–A–D architecture.

Fig. 2. Photophysical properties of DCzB.

a Absorption and steady-state PL spectra of DCzB in dilute toluene solution and film states. b Plot of PL intensity ratio (I/I₀) and PL lifetime versus water fraction in the THF solvent, where I₀ is the PL intensity in pure THF and the insets are the photographs of DCzB solution with 0 and 95% water under 365 nm excitation at room temperature. c Lifetime decay profiles (510 nm) of DCzB solution in air and argon. d Steady-state PL (black line), afterglow (red line, delay 100 ms), and phosphorescent spectra (blue line, delay 5 ms) of DCzB crystal at 300 and 77 K with corresponding photographs taken after the removal of the 365 nm excition source. The upper inset shows the transient PL decay image of the sample recorded at 300 K. e Fluorescence decay profiles of DCzB crystal at 300 and 77 K. f Afterglow decay profiles of the 475, 495, and 525 nm emission bands of DCzB crystal excited at 380 nm at room temperature. Source data are provided as a “Source Data file”.

Fig. 3. Proposed mechanism for the thermally activated organic afterglow.

a Temperature-dependent afterglow spectra from 80 to 300 K of DCzB crystal. b Lifetimes of 475, 495, and 525 nm afterglow at different temperatures. c CIE 1931 coordinates of steady-state PL and afterglow emission from 80 to 300 K. d HOMO and LUMO isosurfaces of DCzB in single molecular and dimer states. e TD-DFT calculated energy level diagram and the corresponding SOC constants. f QM/MM model of DCzB molecule, the hydrogen atoms were ignored for clarity. g Molecular packing and inter/intra-molecular interactions in DCzB crystal. h Proposed mechanism of the highly efficient tri-mode afterglow. The excitation wavelength is at 380 nm. Source data are provided as a “Source Data file”.

Fig. 4. Control experiments for confirming the mechanism of TAA.

a, b Steady-state PL and phosphorescent (delay 5 ms) spectra (a) and lifetime decay curves (b) of DNPhB crystal excited at 330 nm at room temperature. c, d Temperature-dependent phosphorescent (delay 10 ms) spectra from 140 to 300 K (c) and room temperature lifetime decay curves (d) of CzBNPh crystal excited at 390 nm. Source data are provided as a “Source Data file”.

Fig. 5. Applications in afterglow cell imaging and visual temperature detection.

a Bottom-up preparation of DCzB NPs using F127. b, c Particle size distribution revealed by dynamic light scattering (b) and transmission electron microscope images (c). d, e Absorption (black curve), steady-state PL (red curve), room-temperature phosphorescent spectra (blue curve, delay 5 ms) (d) and phosphorescence decay curve (e) of the DCzB NPs excited at 380 nm. The inset is the photographs taken under daylight and 365 nm light irradiation (UV on) and after the removal of the excitation (UV off). f, g Confocal fluorescence images (f), PLIM and time-gated images (delayed 100 µs) (g) of Hela cells incubated with DCzB nanoparticles at 37°C for 2 h. The collection range is 450–550 nm and the excitation wavelength is 405 nm. h Photographs of the pattern before (UV on) and after (UV off) the turning off of the 365 nm UV lamp at 77, 195, 273, and 300 K. i Temperature-dependent color chart with corresponding CIE coordinate showing the ability of DCzB crystals in visual sensing of temperatures. Source data are provided as a “Source Data file”.