Organic Long‐Persistent Luminescence from a Thermally Activated Delayed Fluorescence Electron Donor‐Acceptor Binary System

Organic long‐persistent luminescence (OLPL) based on long‐lived charge‐separated (CS) states shows considerable emission persistence time owing to the ability to store absorbed photon energy. Herein, a novel electron donor‐acceptor (D–A) binary system consisting of electron donor 4,4,4‐tris[3methylphenyl(phenyl)amino]triphenylamine and electron acceptor tris‐[3‐(3‐pyridyl)mesityl]borane is designed and investigated for outstanding OLPL performance. It is found that the proposed binary system exhibits effective OLPL emission that lasted for more than 100 s after 1 min of ultraviolet light irradiation at room temperature (292 K). Moreover, this electron D–A binary system also exhibits thermally activated delayed fluorescence emission prior to OLPL emission. Both emission types are found to be temperature‐sensitive, which are believed to result from the occurrence of reverse intersystem crossing and the formation of a CS state, respectively. The findings enrich the electron D–A system OLPL materials and clarify the rich excited‐state transitions during OLPL emission of electron D–A systems.


Introduction
Long-persistent luminescence (LPL), also colloquially known as the "glow-in-the-dark," refers to the phenomenon by which a material continues to emit light for more than 0.1 s when the excitation light stops. [1]LPL materials can store the absorbed photon energy in excited states and slowly release this energy as light, [2] making them particularly useful in optoelectronics, including emergency lighting, optical recording, bioimaging, and solar cell applications. [3]At present, almost all highefficiency LPL emissions are generated from inorganic materials doped with rare earth elements such as Nd, Eu, and Dy. [4]owever, high temperatures and complex steps are generally required for the fabrication of inorganic LPL materials. [5]Organic LPL (OLPL) materials are promising alternatives to inorganic materials because they are flexible, transparent, rare-element-free, and structurally tunable. [6]Currently, OLPL is mainly based on long-lived phosphorescence by stabilizing triplet excited states, [1,7] such as small-molecule crystalline materials, [8] H-aggregation materials, [9] metal-organic framework (MOF) materials, [10] and polymer materials. [11]However, stabilized triplet excited states are easily deactivated by ground-state triplet oxygen [12] and nonradiative deactivation processes, such as the vibration and rotation of molecules. [13]Therefore, it is typically necessary to ensure spatial isolation between the phosphor and quenching species, such as oxygen and moisture, as well as to rigidify molecular conformations to reduce the nonradiative loss. [14]In addition, owing to the lack of an energy storage mechanism, the persistence time of triplet-state phosphorescence is independent of the excitation parameters and typically exhibits an exponential decay on the order of microseconds, which is much shorter than the emission duration of inorganic LPL. [7,15]Therefore, there is still an urgent need to develop OLPL materials that are independent of the triplet state and capable of storing the absorbed photon energy for a long duration.
In 2017, Kabe and Adachi [16] developed a host-guest doping system with efficient OLPL emission irradiated with a conventional light source at room temperature, composed of an organic electron donor (D) and an acceptor (A) molecule.The electron donor-acceptor (D-A)-based OLPL emission, which lasted for more than 1 h, originated from the slow charge recombination (CR) of the photo-induced long-lived charge-separated (CS) states.The slow diffusion of free electron carriers generated by the separation of charge-transfer (CT) excitons enabled energy storage, which caused the OLPL emission to follow a power-law decay.In their electron D-A binary system, N,N,N 0 ,N 0 -tetramethylbenzidine (TMB), which has a very stable radical cation, was selected as the guest and electron donor, whereas 2,8-bis(diphenylphosphoryl)dibenzo[b,d] thiophene (PPT), with its high triplet energy and rigid structure, was selected as the host and electron acceptor.It should be emphasized that the PPT matrix, which provides a rigid amorphous environment for free electron carriers diffusion, achieves a long-lived CS state to store the absorbed photon energy and leads to a long duration of OLPL.In addition, some D-A combinations based on other organic acceptor molecules such as m-MTDATA/TPBi, m-MTDATA/Bpy-OXD, and m-MTDATA/DPEPO have also been used for OLPL research. [17]uch OLPL electron D-A systems are ideal candidates to replace inorganic LPL materials, but are limited by the fact that a few rigid molecules are available for constituting efficient OLPL systems.Moreover, the transition process of the luminescent species in organic electron D-A binary systems is unclear.
In this work, we report a novel OLPL electron D-A binary system consisting of 4,4,4-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) as a donor dopant and tris-[3-(3-pyridyl) mesityl]borane (3TPYMB) as an acceptor host matrix.In this electron D-A binary system, 3TPYMB, which provides a rigid amorphous environment to suppress nonradiative energy loss and store the absorbed photon energy, was doped with m-MTDATA at a molar concentration of 1% to form a thick blend film by a conventional melt-casting method.After one minute of ultraviolet (UV) light irradiation at room temperature (292 K), the electron D-A binary system exhibited effective OLPL emission which lasted for more than 100 s.The OLPL emission, generated from the electron D-A exciplex excited state, exhibited a power-law decay with a m-value of 0.8 in a log-log plot due to the slow CR from the CS states.The positive feedback of the OLPL emission persistence time on the excitation power and irradiation time confirmed the existence of CS states.Interestingly, the electron D-A binary system exhibited thermally activated delayed fluorescence (TADF) emission prior to OLPL emission.Both emission types were temperature-sensitive radiative recombination processes, which we believe to result from the occurrence of reverse intersystem crossing (RISC) and the formation of a CS state, respectively.Our results provide a promising acceptor candidate for OLPL electron D-A systems and clarify the excited-state transitions during OLPL emission.

Molecular Structures and Luminescence Mechanism
In our OLPL electron D-A binary system, 3TPYMB was selected as the acceptor host matrix and m-MTDATA was selected as the donor guest dopant.As shown in Figure 1a, m-MTDATA, a π-conjugated starburst molecule, is an electron donor material. [18]3TPYMB is a π-conjugated multibranched molecule that is an electron-acceptor material. [19]In m-MTDATA, the large π-conjugated electronic structure stabilizes the radical cation state. [20]In 3TPYMB, a weak inelastic scattering caused by weak vibronic coupling (electron-molecular vibration interaction) leads to good electron transmission and mobility. [21]These structural characteristics are important for generating OLPL emissions.
Another important factor to ensure OLPL emission is the relative energy level of the donor and acceptor.Figure 1b shows the energy-level diagrams of m-MTDATA and 3TPYMB.The energy levels for the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), local excited state (LE), and CT excited state are taken from the literature. [22]This D-A combination has been demonstrated to form an efficient exciplex via intermolecular CT interactions when m-MTDATA molecules and 3TPYMB molecules are mixed and excited owing to their suitable energy levels.The energy offset between the HOMO (1.7 eV) and LUMO (1.3 eV) of the donor and acceptor is much larger than the excitonic binding energy (approximately 0.5 eV [23] ); hence, it allows the conversion of excitons from the LE state to the CT state upon formation in the donor or acceptor molecules.The energy gap (ΔE ST ) between the CT singlet excited state ( 1 CT) and CT triplet excited state ( 3 CT) is rather small (0.051 eV [22a] ) and comparable to the thermal energy, leading to the occurrence of efficient RISC from 3 CT to 1 CT followed by a TADF emission. [24]Furthermore, the local triplet-state ( 3 LE) energy levels of m-MTDATA (2.70 eV) and 3TPYMB (2.98 eV) are higher than the CT-state energy levels (2.30 eV) of the formed exciplex, preventing the nonradiative energy loss of CT excitons through the 3 LE state, and thereby resulting in reduced exciton loss and efficient fluorescence emission. [25]he m-MTDATA/3TPYMB combination has been widely used in organic light-emitting diodes (OLEDs) and has exhibited excellent luminescence efficiency. [23,26]igure 1c illustrates the OLPL processes originating from the CT exciplex generated by the slow CR of long-lived intermediate CS states, first proposed by Kabe and Adachi in 2017. [16]The OLPL system investigated in this study is an m-MTDATA/ 3TPYMB blend.The donor, m-MTDATA, was doped at low concentrations to reduce the concentration at the CR interface.In this electron D-A binary system, when an electron is photoexcited from the HOMO to the LUMO of the m-MTDATA molecule, it quickly hops from the LUMO of m-MTDATA onto the LUMO of 3TPYMB to form a CT state because of the large energy offset between the LUMO levels.It has been proven that the CT excitons formed between organic electron donors and acceptors can be spontaneously dissociated into free charge carriers, [27] which is called the CS state (D þ þ A À ); this mechanism is also the core mechanism of organic photovoltaic devices. [28]herefore, the free electron carriers formed by CT exciton dissociation will diffuse in the 3TPYMB matrix owing to the carrier concentration gradient.Once the free electron carriers diffuse to the next m-MTDATA/3TPYMB interface, it recombines with the positive charge on m-MTDATA to form new a CT exciton, which is then inactivated by fluorescence radiation.It should be emphasized that the free electron carriers diffusion caused by concentration is a slow process, which determines that the luminescence through the CS and CR processes must be LPL emission, [29] and the emission follows a power-law decay. [30]

Organic Long-Persistent Luminescence Performance
The electron D-A binary system was fabricated into an amorphous film using a conventional melt-casting method [31] after mixing the donor and acceptor at a molar ratio of 1:99, and exhibited cracks that formed during rapid cooling.Figure 2a shows photographs of the 1 mol% m-MTDATA/3TPYMB thick film at room temperature (292 K) under ambient light, during excitation by a 365 nm UV lamp, and at various times after turning off the excitation.The low-concentration donor-doped thick film appears slightly yellowish under ambient light.During irradiation with a 365 nm UV lamp, the prepared thick film emits yellow-green light and maintains the luminescence state for several seconds after the UV lamp is removed, demonstrating OLPL emission from this electron D-A binary system.In the meantime, PLQYs of the D-A blend, pristine D, and A were also measured, and the results show that the PLQY of the D-A blend (21.13%) is higher than that of pristine D (5.34%) and A (7.76%).This demonstrates the existence of a RISC energy harvesting mechanism in the D-A blend.
To characterize the significant OLPL emission from the 1 mol% m-MTDATA/3TPYMB film, the kinetic curves of luminescence detected at 525 nm after 60 s of excitation at 380 nm using a Xe light source are shown in Figure 2b,c.As shown in Figure 2b, after turning off the photoexcitation, the electron D-A binary system continues to glow for over 100 s, which is an extra-long afterglow phenomenon.The LPL duration is defined as the time for the luminous intensity to decay to the lower limit of intensity that the FLS1000 can measure.More details are discussed in the Supporting Information.Because the LPL emission is not an exponential decay phenomenon in a semilog plot, we cannot quantify its characteristics with a lifetime parameter.In fact, the LPL emission of the 1 mol% m-MTDATA/3TPYMB binary system follows an inverse-power function of time given by I(t) = t Àm and exhibits a linear decay in a log-log plot with a slope of À0.8, as shown in Figure 2b.This decay law can be directly expressed as an order of magnitude of decay every time the emission intensity increases by an order of magnitude.The slope value we measured conforms to that of the electron diffusion model proposed by Debye and Edwards, [32] which describes long-range electron diffusion and recombination.In this model, LPL decay follows the power-law Equation t -m , in which m depends on both the materials and the concentration, and its value should be between 0.5 and 2. [33] Steady-state and transient spectra were characterized to investigate the excited state of the OLPL. Figure 3a shows the normalized optical absorption spectra of the films of the 1 mol% m-MTDATA/3TPYMB D-A blend, pristine m-MTDATA, and pristine 3TPYMB.The absorption onset for pristine m-MTDATA is at 400 nm and that for pristine 3TPYMB is at 383 nm, while that for the blended film is extended up to 420 nm with a tail of   500 nm, which agrees well with the excitation spectra of the exciplex emissions shown in Figure S1, Supporting Information.The long-wavelength excitation component becomes significantly stronger as the concentration of the CT state increases, suggesting that the long-wavelength excitation may be attributed to CT-state absorption. [16]17b] Electron D-A mixtures tend to exhibit CT emission rather than LE emission.To investigate the fluorescence characteristics of the samples, their photoluminescence (PL) spectra were measured at room temperature (292 K), and are shown in Figure 3b.The fluorescence emission range of the 1 mol% m-MTDATA/ 3TPYMB mixture was significantly broadened compared to the spectral range of the individual components.In addition, the emission peak wavelength of the 1 mol% m-MTDATA/ 3TPYMB film was located at 525 nm, representing a significant red shift compared to those of m-MTDATA (435 nm) and 3TPYMB (389 nm).The broadened emission wavelength range and red-shifted emission peak are typical characteristics of CT excited-state radiation, [34] which is strong evidence for the formation of the CT excited state in the 1 mol% m-MTDATA/3TPYMB binary system.In fact, our previous research [35] showed that even with a low D-A molar ratio (1:26), the emission of the m-MTDATA/3TPYMB system mainly originated from the radiation decay of the CT state, rather than from the LE state.This is due to the fact that at low-doping concentrations, electrons, and holes can be coupled to form CT excitons over a long distance, rather than being confined to the D-A interface.The long-range coupling [36] ability of electrons and holes ensures that CT states are formed, followed by fluorescent emission from the 1 CT state.
To estimate the phosphorescence emission associated with the triplet state, the PL spectra of the films were measured at 77 K (Figure 3c).The two emission peaks of the m-MTDATA luminescence spectrum measured at 77 K are located at 434 and 487 nm, corresponding to the fluorescence and phosphorescence emission peaks. [26]For 3TPYMB, the emission peak at 388 nm is attributed to fluorescence emission.In contrast, the emission spectrum of the mixture has only one emission peak at 525 nm, which is the same as the spectral position measured at room temperature.The spectral profile of the mixture at 77 K completely overlaps with that measured at room temperature, proving that the spectrum at 77 K is derived from the same excited state as the room temperature spectrum. [37]In fact, the spin-1 3 CT states are forbidden from recombining to the ground state, which is typically a spin-0 singlet.The luminescence originates from the radiation recombination of the 1 CT excitons.The small energy difference between the 1 CT state and triplet state ( 3 CT) of the formed exciplex excitons leads to an efficient RISC process, even at low temperatures, which guarantees efficient fluorescence emission. [38]o investigate the luminescence emission at different timescales, time-resolved PL spectra were obtained at room temperature (292 K), and collected from 410 to 650 nm (Figure 3d).The emission of the sample has similar profiles and peaks in the microsecond scale to the second scale, which indicates that the emission of the D-A binary system originates from the same excited state over the entire time range. [39]As will be discussed in more detail below, the luminescence emission of the 1 mol% m-MTDATA/3TPYMB system consists of three components distributed over different timescales: prompt fluorescence (PF) emission, delayed fluorescence (DF) emission, and LPL emission.As shown in Figure 6d, the time range of 0-2s is sufficient to cover all three types of emission.In this context, it can be concluded that the multitimescale exciplex luminescence emission of 1 mol% m-MTDATA/3TPYMB originates from the radiative recombination of 1 CT excitons.

Factors Influencing the Duration of Long-Persistent Luminescence
The duration of LPL depends on the m-MTDATA doping concentration, excitation power, irradiation time, and sample temperature. [16]In an electron diffusion system, the donorto-acceptor ratio is the key factor for long-term charge accumulation. [40]To characterize the LPL duration of the binary systems at different donor doping concentrations, 5, 20, and 50 mol% m-MTDATA-doped 3TPYMB binary systems were also prepared.The LPL emission decay characteristics are shown in Figure 4a and their PL spectra are shown in Figure S2, Supporting Information.There was no noticeable LPL emission for the 50 and 20 mol% systems, whereas the 5 mol% system exhibits a shorter LPL than the 1 mol% system.Although high concentrations of m-MTDATA doping can provide more donor-acceptor interfaces to facilitate charge separation, excessively high donor concentrations lead to a speedy CR process.Therefore, to obtain long-duration LPL emission, the optimal donor concentration was 1 mol%.
The accumulation of charge carriers by weak photo-irradiation depends on the excitation power and irradiation time.Figure 4b,c shows the LPL emission decay characteristics of the 1 mol% m-MTDATA/3TPYMB film under excitation at different irradiation time intervals and powers, respectively.The LPL duration of the film at room temperature (292 K) excited by a Xe lamp at 380 nm increased as the excitation time increased from 1 to 60 s.The dependence of the LPL duration on the excitation time demonstrates the formation of CS states and the generation of charge accumulation processes. [16]Meanwhile, the LPL duration of the film at room temperature, excited by a Nd:YAG nanosecond laser at 355 nm, increased as the excitation intensity increased from 580 to 610 V.With an increase in the excitation power, more CS states were generated in the film and charge accumulation was enhanced.The dependence of LPL emission on the excitation time and power proves that there is a free charge diffusion process in the doped film, which is also the main ratedetermining step of LPL emission. [16]It should be noted that the timescale used in the power-dependent measurement ranged from 1 to 1 ms.Excited by a Nd:YAG nanosecond laser, the LPL emission profiles exhibited power-law decays with a slope of approximately À1.6 in the log-log plots, indicating that the main emission of the film at this timescale is LPL emission rather than PF emission or TADF emission with exponential decay.
The LPL persistence time from the 1 mol% m-MTDATA/ 3TPYMB film depends on the sample temperature because of the nonradiative deactivation caused by molecular vibration.The temperature dependence of the LPL emission decay profile is shown in Figure 4d.The LPL emission duration becomes slightly longer with an increase in temperature in the hightemperature region above 200 K.In fact, it is almost independent of the temperature in the low-temperature region of 100-200 K (Figure S3, Supporting Information) because of the suppression of nonradiative deactivation by molecular vibration at low temperatures.Although the nonradiative deactivation caused by molecular vibration (which is positively correlated with the temperature) leads to energy loss, [41] the CS and electron diffusion processes that favor LPL are also intensified by an increase in temperature. [42]During continuous excitation for up to 60 s, the samples at higher temperatures exhibited more efficient charge accumulation through rapid CS and electron diffusion processes.Thus, as the temperature increased from 200 to 340 K, the m-value of the PL emission decay increased from 0.65 to 0.81.Although the more rapid nonradiative decay results in a steeper slope (À0.81) for the 340 K sample, the highconcentration energy storage allows it to emit with a higher intensity and longer emission duration.A comparison of Figure 4c,d shows that the LPL emission decay curves in time ranges of the order of microseconds have a steeper slope (À1.6) compared to that in the order of seconds (À0.65 to À0.81).This is because the LPL emission decay in the shorter time range is influenced by the tail of the fast exponential decay fluorescence process.We will continue to discuss this in the next section.
To show the actual stability of the 1 mol% m-MTDATA/ 3TPYMB thick film at the high charge-carrier concentrations, we measured the emission spectra and LPL decay curves after multiple repeated of 60 s illumination.The emission spectra and LPL decay curves were measured after the samples were irradiated by a 380 nm Xe lamp light source for 60 s.The results obtained after 20 uninterrupted repetitions of the measurements are shown in Figure 5.It can be seen from Figure 5a,b that there is a continuous decrease in the emission intensity at consistently high photogenerated charge-carrier concentrations.Similarly, in Figure 5c,d, it can also be found that the LPL emission duration becomes shorter at consistently high photogenerated chargecarrier concentrations.

Temperature-Dependent Emission Decay Curves
It is important to note that, as a common exciplex D-A combination, the 1 mol% m-MTDATA/3TPYMB binary system also produces efficient TADF emission.In fact, the exciplex luminescence from the 1 CT state of 1 mol% m-MTDATA/3TPYMB contains three components distributed over different timescales: PF, TADF, and LPL emissions. [32]The PF emission originates from 1 CT states formed by LE exciton dissociation, the TADF emission originates from 1 CT states formed through the RISC of 3 CT excitons, and the LPL emission originates from 1 CT states formed through the CR process of the CS states.The conversion of these three emissions can be clearly demonstrated by timeresolved transient PL decay curves.Figure 6 shows the room-temperature time-resolved transient PL decay profiles of pristine m-MTDATA, 3TPYMB, and 1 mol% m-MTDATA/3TPYMB, measured at their PL emission peaks.As shown in Figure 6a, the m-MTDATA and 3TPYMB samples exhibit a monoexponential decay in the PL intensity with short lifetimes of τ = 1.24 and 1.44 ns, respectively, which follow the typical decay law of radiation from 1 LE states. [34]Figure 6b shows the exponential decay of m-MTDATA and 3TPYMB on logarithmic plots.In contrast, as shown in Figure 6c, the 1 mol% m-MTDATA/3TPYMB film exhibits a longer decay process of the order of milliseconds, which is significantly longer than the decay of an equimolar mixture. [23]Figure 6d shows the decay profile of the 1 mol% m-MTDATA/3TPYMB film in a log-log plot.The time distribution of the different emission components can be clearly observed by curve fitting, which shows two exponential decays at the 0.25-100 μs timescale and a longer timescale of 100-1000 μs.As discussed above, the emission of the 1 mol% m-MTDATA/3TPYMB film at different timescales originates from the same 1 CT state.The different decay trends represent different exciton transitions.LPL is a slow emission process.Although the CS state can be formed within the timescale of CT-state formation, [43] the decay of the LPL becomes more apparent in the late stage of the decay of PF and DF.
It is known that RISC is associated with TADF emission.CS states are associated with LPL emission, and nonradiative recombination processes of the electron D-A system are temperature-sensitive. [44]Therefore, changes in the sample temperature will inevitably affect the temporal distribution of the three luminescent components, as well as the luminescence duration.To study the temperature-dependent excited-state behavior of the electron D-A system, we further measured the transient decay curves at various temperatures from 80 to 360 K collected at the emission peak (525 nm) with 380 nm excitation.As shown in Figure 7a,b, in the log-log plots, the PL decay curves show two exponential components, followed by a powerlaw component at different timescales.
In the timescale of less than 4 μs, the PF emission decay curves are almost independent of temperature because of the rapid decay of the 1 CT excitons formed directly from the 1 LE states.From 4 to 100 μs, the DF emission decays exhibit a clear dependence on the temperature, which is a characteristic of the TADF mechanism. [45]In the low-temperature range of 80-280 K, with an increase in the thermal activation energy, higher intensities are obtained at higher temperatures, and the emission intensity reaches its maximum at 280 K.This is due to the fact that the thermally activated RISC process converts more of the dark 3 CT excitons into bright 1 CT excitons as the temperature rises.In the high-temperature range, the TADF emission intensities decrease as the temperature increases from 280 to 360 K.At this point, the nonradiative energy loss becomes the main temperature response factor, which becomes more significant with increasing temperature.To visualize the temperature dependence of the TADF emission decay region, we integrated the emission intensities over time and plotted the results in    .With an increase in temperature, the DF emission intensities are enhanced by the TADF effect of collecting 3 CT energy until it reaches 280 K, where the nonradiative energy loss overtakes the TADF energy collection and the DF emission intensity starts to decrease. [41]n the 100 μs to 1 ms time range, as shown in Figure 7a,b, the LPL emission decay accelerates with increasing temperature from 80 to 360 K. Figure 7d shows the slope values of the emission attenuation in the log-log plot.The slope value of the LPL emission decay changes from À1.45 to À2.36 as the temperature increases from 80 to 360 K.The lower CT-state distribution after efficient TADF radiation compounding and greater nonradiative energy loss at higher temperatures limit the CS process.As described in the literature, [46] power-law emission decay occurs because of the recombination of separated carriers, which leads to the formation of 25% 1 CT and 75% 3 CT. [47]A lower CS-state concentration leads to less long-lived 1 CT and 3 CT state distributions, which results in an even lower percentage of LPL emission and leads to a faster decay of PL with a steeper slope following TADF emission.It should be noted that the LPL emission decay on the 100 μs timescale overlaps with the trailing TADF emission decay, and therefore has a steeper slope compared to the decay on the second scale.
The TADF energy recovery mechanism and nonradiative energy loss mechanism in the organic D-A binary system are both positively correlated with temperature.As shown in Figure 8, to clearly demonstrate the competitive relationship between the TADF emission promoting and the nonradiative emission quenching mechanisms, we studied the temperaturedependent emission spectra of the 1 mol% m-MTDATA/ 3TPYMB thick film.As shown in Figure 8a, the organic D-A binary system has similar emission profiles at temperatures ranging from 80 to 360 K.The emission peaks at 523 nm at a temperature of 80 K and at 532 nm at a temperature of 360 K are due to the red-shift of the emission caused by the increase in the thermal vibration of the molecule as the temperature increases.More importantly, as shown in Figure 8b, the emission intensity first increases (from 140 to 220 K) and then decreases (from 240 to 360 K) with the increase in temperature.The increase in emission intensity can be attributed to the gradual enhancement of TADF with increasing temperature.And it can be seen that at a temperature of 160 K, a significant TADF emission starts to appear.However, when the temperature goes above 240 K, the nonradiative energy loss reverses the effectiveness of TADF and the emission intensity starts to gradually decrease.

Conclusion
In summary, we investigated a new OLPL electron D-A binary system, a 1 mol% m-MTDATA/3TPYMB thick film.The hostguest system was fabricated by melt-casting and exhibited excellent OLPL performance, lasting for >100 s after 1 min of UV light irradiation at room temperature (292 K).Importantly, the electron acceptor 3TPYMB was first used as a host matrix, which successfully suppressed nonradiative energy losses and formed long-lived CS states to store the absorbed photon energy owing to the rigid molecular structure.The electron donor, m-MTDATA, was doped into the 3TPYMB matrix at a molar ratio of 1:99 to generate exciplex luminescence.The positive feedback of the OLPL emission persistence time on the excitation power and irradiation time confirmed the existence of CS states.More importantly, the exciplex luminescence exhibited PF, TADF, and LPL emissions.Temperature-sensitive RISC, CS, and nonradiative recombination processes influence the excited-state transitions during OLPL emission.This study will facilitate photophysical research and industrial applications of OLPL materials based on CS states.
Film Fabrication: Films for optical measurements were fabricated using a melt-casting method.The donor molecules m-MTDATA and acceptor molecules 3TPYMB in a molar ratio of 1:99 were dissolved in tetrahydrofuran.The solvent was removed by evaporation under reduced pressure in the dark to obtain a powder.In a nitrogen-filled glove box, the powdered blend was placed on a cleaned quartz substrate with a 100 mm 2 surface area and 0.5 mm depth and heated to melt for 10 s.Immediately after melting, the quartz substrate was cooled to room temperature and encapsulated using acrylic AB glue and quartz covers under a nitrogen atmosphere.

Figure 1 .
Figure 1.a) Molecular structures of the electron donor, m-MTDATA, and the electron acceptor, 3TPYMB.b) Scheme of the TADF emission from the exciplex formed between m-MTDATA and 3TPYMB.The distribution of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), local-excited (LE) state, and charge-transfer (CT) state energy level is depicted according to data in the literature, where ISC refers to the intersystem crossing, and RISC refers to the reverse intersystem crossing.c) Schematic of the OLPL emission from the m-MTDATA/3TPYMB electron D-A binary system, where CS refers to the charge separation, and CR refers to the charge recombination.

Figure 2 .
Figure 2. a) Photographs of 1 mol% m-MTDATA/3TPYMB thick film fabricated by melt-casting (0.5 mm) at room temperature under ambient light, during excitation by a 365 nm UV lamp, and at various times after turning off the excitation.b) Semilogarithmic plot of the LPL emission decay profile of 1 mol% m-MTDATA/3TPYMB thick film excited by a Xe lamp at 380 nm for 60 s at room temperature.The kinetic curve of luminescence is detected at 525 nm.c) Logarithmic plot of the LPL emission decay profile of a 1 mol% m-MTDATA/3TPYMB thick film excited by a Xe lamp at 380 nm for 60 s at room temperature.

Figure 3 .
Figure 3. a) Normalized absorption spectra of pristine 3TPYMB, m-MTDATA, and the 1 mol% m-MTDATA/3TPYMB thick film.b) Normalized photoluminescence spectra excited at 340 nm at room temperature.c) Normalized photoluminescence spectra excited at 340 nm at 77 K. d) Time-resolved normalized emission spectra of the 1 mol% m-MTDATA/3TPYMB thick film at room temperature.

Figure 4 .
Figure 4. a) LPL emission decay profiles of the m-MTDATA/3TPYMB thick film at various doping concentrations excited by a Xe lamp at 380 nm.b) LPL emission decay profiles of the 1 mol% m-MTDATA/3TPYMB thick film excited by a Xe lamp at 380 nm for various times.c) LPL emission decay profiles of the 1 mol% m-MTDATA/3TPYMB thick film excited by an Nd:YAG nanosecond laser at 355 nm at various excitation powers.The sample size was larger than the excitation beam.d) LPL emission decay profiles of the 1 mol% m-MTDATA/3TPYMB thick film excited by a Xe lamp at 380 nm at various temperatures.

Figure 5 .
Figure 5. a) The emission spectra of the 1 mol% m-MTDATA-3TPYMB binary system after being irradiated by a 380 nm Xe lamp light source for 60 s in 20 consecutive repetitions.b) The integrated values of the emission spectra of the 20 consecutive irradiations.c) Semilogarithmic plot of the LPL emission decay profiles of the 1 mol% m-MTDATA/3TPYMB binary system after being irradiated by a 380 nm Xe lamp light source for 60 s in 20 consecutive repetitions.d) Logarithmic plot of the LPL emission decay profiles.

Figure 6 .
Figure 6.a) Semilogarithmic plots of the emission decay profiles of the pristine m-MTDATA and 3TPYMB excited by an EPLED-320 pulsed laser.b) Logarithmic plots of the emission decay profiles of the pristine m-MTDATA and 3TPYMB excited by an EPLED-320 pulsed laser.c) Semilogarithmic plot of the emission decay profile of the 1 mol% m-MTDATA/3TPYMB thick film excited by a microsecond flash lamp at 380 nm.d) Logarithmic plot of the emission decay profile of the 1 mol% m-MTDATA/3TPYMB thick film excited by a microsecond flash lamp at 380 nm.

Figure 7 .
Figure 7. a,b) Logarithmic plots of the emission decay profiles of the 1 mol% m-MTDATA/3TPYMB thick film excited by a microsecond flash lamp at 380 nm at various temperatures.c) Integral values of the TADF emission decay intensity against time.d) LPL emission decay slopes in logarithmic plots.

Figure 7c
Figure 7c.With an increase in temperature, the DF emission intensities are enhanced by the TADF effect of collecting 3 CT energy until it reaches 280 K, where the nonradiative energy loss overtakes the TADF energy collection and the DF emission intensity starts to decrease.[41]In the 100 μs to 1 ms time range, as shown in Figure7a,b, the LPL emission decay accelerates with increasing temperature from 80 to 360 K. Figure7dshows the slope values of the emission attenuation in the log-log plot.The slope value of the LPL emission decay changes from À1.45 to À2.36 as the temperature increases from 80 to 360 K.The lower CT-state distribution after efficient TADF radiation compounding and greater nonradiative energy loss at higher temperatures limit the CS process.As described in the literature,[46] power-law emission decay occurs because of the recombination of separated carriers, which leads to the formation of 25% 1 CT and 75% 3 CT.[47]A lower CS-state concentration leads to less long-lived1 CT and 3 CT state distributions, which results in an even lower percentage of LPL emission and leads to a faster decay of PL with a steeper slope following TADF emission.It should be noted that the LPL emission decay on the 100 μs timescale overlaps with the trailing TADF emission decay, and therefore has a steeper slope compared to the decay on the second scale.The TADF energy recovery mechanism and nonradiative energy loss mechanism in the organic D-A binary system are both positively correlated with temperature.As shown in Figure8, to clearly demonstrate the competitive relationship between the TADF emission promoting and the nonradiative emission quenching mechanisms, we studied the temperaturedependent emission spectra of the 1 mol% m-MTDATA/ 3TPYMB thick film.As shown in Figure8a, the organic D-A binary system has similar emission profiles at temperatures ranging from 80 to 360 K.The emission peaks at 523 nm at a temperature of 80 K and at 532 nm at a temperature of 360 K are due to the red-shift of the emission caused by the increase in the thermal vibration of the molecule as the temperature increases.More importantly, as shown in Figure8b, the emission intensity first increases (from 140 to 220 K) and then decreases (from 240 to 360 K) with the increase in temperature.The increase in emission intensity can be attributed to the gradual enhancement of TADF with increasing temperature.And it can be seen that at a temperature of 160 K, a significant TADF

Figure 8 .
Figure 8. a) Temperature-dependent emission spectra of 1 mol% m-MTDATA-3TPYMB thick film excited at 380 nm at various temperatures from 80 to 360 K. b) Emission profiles integration values.