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

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.


Single crystal analysis
DCzB exhibits good solubility in common organic solvents; thus, highly pure material was obtained by column chromatography and its single crystals can be grown by slow evaporation of a mixed CH2Cl2 and PE solution at room temperature. The data of the single crystal structure were collected on a Bruker SMART APEX (II)-CCD at 298 K. Crystal structures were analyzed by Mercury 1.4 software. The CCDC number for DCzB is 1941925. The crystallographic data were summarized in Supplementary Table 1

Thermal properties
Thermogravimetric analysis (TGA) was conducted on a DTG-60 Shimadzu thermal analyst system under a heating rate of 10ºC/min and a nitrogen flow rate of 50 cm 3 /min. TGA reveals that DCzB has a good thermal stability among boron-containing compounds with a high decomposition temperature (Td = 251℃), owing to its stable four-coordinate boron structure 28 . Melting point of 220℃ is obtained from melting point apparatus. Thermogravimetric analysis reveals that this D-A-D molecule has a decomposition temperature over 250℃, which is high thermal stabilities among the boron-containing optoelectronic materials 28,29 . Figure 9. TGA curve of DCzB.

Photophysical properties
Ultraviolet/visible (UV/Vis) and photoluminescence (PL) spectra of DCzB in dilute toluene, CH2Cl2, tetrahydrofuran (THF), trichloromethane (CHCl3) and solid film were measured on a Lambda 650 S Perkin Elmer UV/VIS spectrophotometer and Edinburgh FLS 980 fluorescence spectrophotometer, respectively. For fluorescence lifetime measurements, a picosecond pulsed light emitting diode (EPLED-380, wavelength: 377 nm; pulse width: 947.7 ps) was used. The phosphorescence spectrum of DCzB in dilute toluene was obtained using an Edinburgh FLS 980 fluorescence spectrophotometer at 77 K in a dewar vessel with 5 ms delay time after excitation using a microsecond (μs) flash lamp. The microsecond flash lamp produces short, typically a few microsecond, and high irradiance optical pulses for room temperature phosphorescence measurements in the range from microseconds to seconds. The afterglow spectra, kinetic measurements, lifetime (τ) and time-resolved emission spectra of DCzB crystal were also measured on an Edinburgh FLS 980 fluorescence spectrophotometer. The absolute photoluminescence quantum yield (PLQY) was obtained using an integrating sphere. The lifetimes of the luminescence were figured out by fitting the luminescent intensity decay curve (I(t)) with a multi-exponential decay function in Supplementary Equation (1): where Ai and τi represent the amplitudes and lifetimes of the individual PL components in multiexponential decay profiles, respectively. The temperature-dependent afterglow spectra and lifetime decay profiles were measured using a closed cycle cryostat (oxford-instruments Optistat DN2). The photographs were taken by a Nikon D90 camera.

Absorption and PL properties at room temperature
Absorption and steady-state PL spectra of DCzB in different solvents (Supplementary Figure  10) exhibit a maximum absorption around at 380 nm, which should be ascribed to intermolecular charge transfer (ICT) transition from the donor of carbazole group to the acceptor of difluoroboron β-diketonate unit, since the absorption bands of both the donor and acceptor units are significantly blue-shifted (around 295 nm) 30 . This ICT characteristic can be confirmed by the remarkable positive solvatochromism in PL emission, showing gradually red-shifted PL peaks with the polarity increase of the solvents. Thus, DCzB should be a typical ICT type molecule due to its D-A electronic structure. Figure 10. UV-vis absorption (solid symbols) and photoluminescence spectra (open symbols) of DCzB in toluene, CH2Cl2, CHCl3 and THF at room temperature.

Aggregation induced emission (AIE) property of DCzB
AIE properties of DCzB were observed in THF/water mixed solvents with different water fraction (fw) (Supplementary Figure 11). DCzB exhibits weak emission in pure THF solution, but its PL enhances gradually with the increase of fw, demonstrating a typical AIE characteristic. In addition, when the value of fw increases, the PL lifetime also increases, suggesting a turn of fluorescence in THF to phosphorescence of aggregates at high fw values. Therefore, DCzB also has the aggregation induced phosphorescence (AIP) property (Supplementary Figure 12). Figure 11. Photoluminescence spectra of DCzB in THF/water mixtures (~10 -5 mol/L) with different water fractions (fw). The excitation wavelength is at 380 nm. Figure 12. Lifetime decay profiles of DCzB in THF/water mixtures with different water fraction (fw). The excitation wavelength is at 380 nm.

Thermally activated delayed fluorescence (TADF) property of DCzB
By comparing the PL spectra of DCzB in aerated and argon-degassed toluene solutions, the PL enhances significantly in the absence of triplet-quenching oxygen (Supplementary Figure 13), indicating the emission is related to the triplet state. Through the fluorescence and phosphorescence spectra in dilute toluene at 77 K (Supplementary Figure 14a), the singlet-triplet splitting energy (ΔEST) were identified to be 0.15 eV, which is small enough for the typical TADF molecules. Nevertheless, efficient phosphorescent emission was also observed with lifetime of 398.65 ms in dilute solution at 77 K (Supplementary Figure 14b).

Afterglow properties of DCzB crystal
In crystal state, DCzB exhibits high PLQY of 55.0% and large afterglow component of 81.6% in avarage (Supplementary Table 2). Therefore, we can conclude that the afterglow efficiency of DCzB crystal is up to 44.9%. This efficiency is the highest afterglow efficiency among the single component organic afterglow molecules reported so far.  The influence of the excitation strength on afterglow emission was investigated by monitoring the afterglow emission intensity with different Iris of the excitation source at 380 nm. Iris is a parameter in Edinburgh FLS 980 spectrophotometer used to adjust the excitation light intensity. Larger Iris means the stronger excitation intensity. By increasing Iris from 10% to 100%, the excitation intensity can be controlled to increase linearly. From Supplementary Figures 17a-b, the excitation light at 380 nm with varied Iris was turned on at 5 s, maintained for 10 s, and then turned off. The resultant afterglow emission intensity remains stable during the excitation for 10 s, then decays in exponential manner after the turn-off of the excitation, while the steady-state emission and afterglow luminescence intensity increases linearly. Moreover, the influences of the excitation duration on the afterglow emission were investigated by monitoring its emission band with different irradiating time of the excitation (Supplementary Figures 17c-d). The steady-state and afterglow emission intensities become almost constant when the excitation duration extends from 0.4 to 10 s, indicating that the afterglow can be effectively excited with low intensity and short duration of light under ambient conditions.

S18
In order to uncover the mechanism of the thermally activated tri-mode afterglow, the temperature-dependent steady-state PL and afterglow emission were investigated by changing the temperature from 77 K to 300 K. The steady-state PL enhances with the reduced temperature owing to the suppressed nonradiative transition (Supplementary Figure 21). In afterglow decay profiles, due to the both suppressed nonradiative transition and thermally activated triplet exciton release processes, the lifetimes of 475 and 495 nm emission reduce firstly then elongate, when the temperature drops from 300 K to 77 K. This changing trend is in consistent with TADF, since these two emission peaks are corresponding to the fluorescence and phosphorescence ( Supplementary  Figures 22a-b) 31 . The afterglow lifetime of 525 nm emission is due to the radiative decay of the stable triplet excited state of T1 * ; thus, it rises gradually at reduced temperature owing to the suppressed nonradiative transition (Supplementary Figure 22c) 32 . Therefore, these measurements give clear evidences for that the tri-mode afterglow is realized by populating S1 and T1 with the long-lifetime excitons on T1 * through thermally activated exciton release and reverse intersystem crossing processes. Figure 21. Temperature-dependent photoluminescence spectra of DCzB crystal at different temperatures from 80 K to 280 K. The excitation wavelength is at 380 nm.

Theoretical calculations
Density functional theory (DFT) and time-dependent DFT (TD-DFT) computations were carried out using Gaussian 09 package. The ground state geometry was optimized by DFT method of B3LYP/6-31G(d); the optimized stationary point was further characterized by harmonic vibrational frequency analysis to ensure that real local minima were reached. TD-DFT calculations were performed to predict the excitation energies in the n-th singlet (Sn) and n-th triplet (Tn) states on the basis of the optimized ground structure via spin-restricted formalism using B3LYP/6-31G(d) (Supplementary Table 3). Spin-orbit coupling (SOC) matrix elements between the singlet excited states and triplet excited states were calculated with quadratic response function methods using the Dalton program 34,35 . The SOC of DCzB was performed at the optimized geometry of the lowest triplet excited state (T1) using B3LYP functional and cc-PVTZ basis set (Supplementary Table 4).
The quantum mechanics/molecular mechanics (QM/MM) model was built from the single crystal structure and was implemented to evaluate the electronic properties of the active QM molecule embedded in the aggregated crystal state, while the surrounding molecules were defined as rigid MM part to model the effect of solid-state environment. The high layer for QM is calculated by the TD-DFT method of B3LYP/6-31G(d), and the low layer for MM is described by the universal force field (UFF) enhanced by Coulomb interactions which are in-line with the quantum method. The dimer was set up to high layer for QM calculated by the TD-DFT method of B3LYP/6-31G(d).
The longer π-π interaction distance on the excited states compared to that on the ground state was observed, suggesting that photoexcitation can disturb the π-π interactions significantly.
In the ground state, the dihedral angle between donor and acceptor reduces from 32.6˚ in single molecular state to 19.9˚ in crystal, which may be related to the enhanced local-excited (LE) state emission feature in film state. When DCzB was photo-excited to either S1 or T1 states, much larger torsion angles were resulted, suggesting that the photoexcitation may disturb the π-π stacking of the molecules for a shallow exciton trapping depth.

H-aggregation for stabilizing triplet excitons
H-aggregation has been proved to be critical for stabilizing triplet exciton, resulting the efficient OURTP emission at the solid states since it is highly effective in trapping the excited states in low-lying aggregation-split energy level 36  where M is the electric dipole transition moment, ruv represents the intermolecular distance between the molecular pair, α is the angle between the transition dipole moments of the two molecules, and θ1 and θ2 are angles between transition dipole moments of the two molecules and the interconnection of their molecular centers, respectively. Based on this criterion, large and positive Δε values for strong H-aggregation were observed in DCzB.

Bioimaging
Preparation of nanoparticles of DCzB: The DCzB nanoparticles in aqueous solution were prepared via a well-documented matrix-encapsulation method in a bottom-up approach (nanoprecipitation) with appropriate optimization. PEG-b-PPG-b-PEG (F127) was chosen as the encapsulation matrix due to its good encapsulation performance and high biocompatibility. Briefly, F127 (10 mg) and DCzB (1 mg) were dissolved into a THF (1 mL) solution. The mixture was then rapidly injected into Milli-Q water (10 mL) under continuous sonication in a sonicator bath (Branson) for 5 min. Then, THF was evaporated with a gentle nitrogen flow. Finally, the aqueous solution was filtered through a 0.22 µm PVDF syringe driven filter (Millipore). The obtained nanoparticle solution was stored in dark at 4 °C. Characterization method: The particle size and morphology of the DCzB nanoparticles were characterized by dynamic light scattering (DLS) and transmission electron microscope (TEM). DLS was performed using a quartz cuvette at 25 °C. Average particle sizes were determined by laser light scattering with a particle size analyzer (90 Plus, Brookhaven Instruments Co., Holtsville, NY, USA) at a fixed angle of 90° at room temperature. Samples for TEM measurements were prepared by drop casting the DCzB nanoparticle dispersion onto copper grids. The samples were allowed to dry at room temperature, then the TEM images were obtained using a Hitachi H-H7500 microscope operated at 120 kV. The steady-state PL and phosphorescence spectra of DCzB nanoparticles were obtained on Edinburgh FLS 980 fluorescence spectrophotometer with xenon lamp and microsecond lash lamp. Confocal luminescence imaging was carried out on an Olympus FV1000 laser scanning confocal microscope equipped with a 40 immersion objective len. The HeLa cells were incubated with the DCzB nanoparticles (10 μM) for 3 h at 37 °C. Then the cells were washed with PBS (phosphate buffer saline) for three times and transferred into Live Cell Imaging System for confocal luminescence imaging. Five samples of thus treated cells were randomly selected for imaging tests and typical results were illustrated. Under the excitation of 405 nm semiconductor laser, the emission was collected from 450 to 550 nm. Phosphorescence lifetime imaging (PLIM) was carried out on the Olympus IX81 laser scanning confocal microscope. The photoluminescence signal was detected by the system of the confocal microscope and correlative calculation of the data was performed by professional software which was provided by PicoQuant Company. The light from the pulse diode laser head (PicoQuant, PDL 800-D) with excitation wavelength of 405 nm with a 40×/NA 0.95 objective lens for single-photon excitation. These experiments were repeated three times and similar results were observed.

Multicolour display and visual temperature detection
The character "8" is fabricated by using DCzB powders. Photographs under the excitation of the 365 nm UV light were taken at 300 K, 273 K, 195 K and 77 K respectively to illustrate the potential applications in anti-countefeiting, multicolour display and visual detection of temperature. On the excitation, the "8" pattern shows blue emission at different temperature with negligible color difference. After the cease the excitation, the pattern exhibit obvious color difference from the bluegreen afterglow at 300 K to green-yellow long-live emission at 77 K, exhibiting great potential in multicolour display. From the temperature colour chart obtained through the CIE 1931 coordinates of the afterglow spectra at various temperatures from 77 K to 300 K, the afterglow color gradually changes from green-yellow to blue-green, demonstrating the naked eyes visible detection of temperature in a wide range. The function was fitted for temperature detection based on the Commission International de l'Eclairage (CIE) coordinate diagram of DCzB crystal's afterglow colour under different temperature. Good fitting can be achieved by the fourth order polynomial equation in Supplementary Equation (3) and (4) where T represents the temperature from 77 to 300 K. x, y represents the x-coordinate and ycoordinate of afterglow emission colour, respectively. A and B are constants (A1 = 0.41722, A2 = -0.00119, A3 = 1.1422*10 -5 , A4 = -3.07067*10 -8 , A5 = 3.54869*10 -12 , R 2 = 0.99; B1 = 0.55159, B2 = -2.79942*10 -5 , B3 = -3.34644*10 -7 , B4 = 1.56719*10 -8 , B5 = -5.95813*10 -11 , R 2 = 0.97). R 2 represents the coefficient of association. Therefore, we can quantitatively identify the visual detection of temperature.
Supplementary Figure 23. The fitting function based on DCzB afterglow emission upon the change of the temperature from 77 to 300 K.