DOI: 10.5061/dryad.5x69p8dbg
title: Charge trapping for controllable persistent luminescence in organics

description: Persistent luminescence, long-lived emission from inorganic or organic
 materials after the cessation of excitation, receives considerable
 attention in the field of optoelectronics. Despite great achievements in
 the past decades, the performance of organic materials still lags behind
 their inorganic counterparts, which have thousands of years of history.
 This is largely caused by the limited understanding of the mechanisms
 involved in organic materials. Here we report trap-induced persistent
 luminescence (TIP) in organic host-guest materials, with controllable trap
 depths from 0.11 to 0.56 eV and tunable afterglow emission at wavelengths
 from 507 to 669 nm via energy level engineering. The TIP phenomenon in a
 typical TN@TPBi film lasts for more than 24 h, with additional energy
 stored at room temperature for over 1 week. It is found that the trap
 depth in TIP is probably determined by the energy gap between the lowest
 unoccupied molecular orbitals of the radical anions of the host and guest
 molecules, matching well with density functional theory calculations. TIP
 was also observed after electrical excitation, demonstrating the potential
 of exploiting the semiconductor features of the organic hosts. These
 results provide a fundamental principle to design metal-free organic
 emitters of persistent luminescence, thereby expanding their applications
 in fields such as medical delivery identification, semiconductor devices,
 and imaging techniques.
description: <strong>Materials and characterizations</strong>: TN
 was synthesized in two steps including dehydration reaction and palladium
 catalyzed Suzuki cross-coupling reaction, followed by comprehensive
 characterizations of nuclear magnetic resonance (NMR) spectroscopy
 (Advance III 500 MHz NMR, Bruker), mass spectrometry (FTICR MS, Bruker),
 and element analysis (Vario EL III, Elementar).
 <strong>TN@TPBi film fabrication.</strong> A mixture
 of TN and TPBi (total 1 mmol, around 60 mg) was heated up to 350 °C on a
 quartz substrate in a glovebox. After melting, the molten liquid of
 materials was stirred thoroughly, then cooled rapidly to RT and
 encapsulated under a nitrogen atmosphere using ultraviolet-cured epoxy
 resin and glass covers. The other blend films were fabricated with the
 same method. <strong>Photocurrent device
 fabrication</strong>: Si-SiO<sub>2</sub> substrate was
 washed with deionized water, acetone, and isopropanol sequentially, and
 dried in drying oven at 100 °C. The gold as parallel electrodes were
 evaporated on the substrate at 3.5 Å/s when the vacuum degree of the
 evaporating cave reached 1×10<sup>−4</sup> Pa. Then the gold
 electrode deposited substrate was washed with above solutions again and
 finally treated by oxygen plasma for 30 min. 1 wt% TN@TPBi melt-casting
 film was prepared via a melt-cooling assisted crystallization method on
 the substrate and encapsulated with glass covers in a glovebox.
 <strong>Photophysical properties
 measurement.</strong> Emission spectra were collected in a
 fluorescence spectrometer (FLS980, Edinburgh and QE-Pro, Ocean Optics).
 The absorption spectra were measured by an
 ultraviolet/visible/near-infrared spectrophotometer (UV-3600Plus,
 Shimadzu). The cyclic voltammetry curves and electrochemical deposition
 operation were carried out by electrochemical workstation (CHI660E, CH
 Instruments). The radical cation and anion of host/guest were obtained
 from electrical oxidation or reduction reaction when the host/guest were
 dissolved in dichloromethane containing 0.1 M
 TBAPF<sub>6</sub>. We obtained ESR spectra using the X-band
 EPR spectrometer (EMX-10/12, Bruker). A device was fabricated by
 depositing two Au electrodes on a glass substrate and then melting the
 organic emitter to cover electrodes, as shown in the inset of Fig. 3g. The
 photocurrents of the device with ultraviolet light (365 nm) on and off (at
 zero bias) were measured using a high-precision electrometer (6517B,
 Keithley). For persistent luminescence photocurrent measurements, the
 device was illuminated under 365 nm light for 300 s, and after the
 illumination removal, the photocurrent decay versus time (300 s) was
 recorded. Five cycles were applied. For dark current recording, the device
 was first thermally bleached at 400 K to fully eliminate trapped charge
 carriers. Photographs and Videos of the samples were taken with a digital
 camera (EOS 5D Mark II, Canon and α7SIII, Sony).
 <strong>TIP measurements.</strong> TL spectra,
 persistent luminescence spectra, and decay profiles were obtained using
 the TIP measurement system shown in Supplementary Fig. 24. The samples
 were placed in a cryostat controlled by a cooling-heating stage (THMS600E,
 Linkam Scientific Instruments), and excited by a 365-nm LED with a 5
 mW/cm<sup>2</sup> excitation power density for 5 min. A
 filter-attached photomultiplier tube (PMT, R928P, Hamamatsu photonics), a
 luminance meter (LM-5, Evenfine), a multimeter (2400, Keithley), and a
 high voltage power supply (HVC1800, Zolix) simultaneously monitored the
 persistent luminescence intensity (including TL emission and TIP decay
 profile) after ceasing the excitation source. The TIP emission spectra
 were recorded at the same time by using a multichannel spectrometer
 (QE-Pro, Ocean Optics) during TL measurements. In a typical TL
 measurement, the sample was firstly cooled to 100 K and irradiated by the
 excitation source for 5 min. Twenty seconds after creasing the 365-nm LED
 source, the sample was heated to 400 K at a certain heating rate (50, 20,
 10, 5, and 2 K/min) to record the emission intensity. The above-mentioned
 homemade measurement systems were driven by LabVIEW-based PC programs. An
 NIR laser at 980 nm (~200 mW/cm<sup>2</sup>) was projected
 onto the surface of the sample as a photo-stimulation source.
 <strong>Computational details.</strong> All molecules
 were optimized at the B3LYP/def2-SVP level for the equilibrium geometries
 of ground state (S<sub>0</sub>) using Gaussian 09 program, and
 subsequently, the frontier molecular orbitals based on the
 S<sub>0</sub>-geometry were depicted for the close-shell and
 open-shell systems.
description: # Charge trapping for controllable persistent luminescence in organics
 The source data involved
 in the main text. ## Description of the data and file structure **Figure 2
 |  Photophysical properties of the 1 wt% TN@TPBi phosphor. b**,
 Steady-state photoluminescence (PL), phosphorescence (delay time: 25 ms),
 and TIP spectra (delay time: 1 h and 9 h) at RT. **c**, TIP decay curve of
 the TN@TPBi phosphor after excitation by a 365 nm UV lamp for 300 s at RT.
 The noise signal is given for comparison (grey dots). 0.32 mcd/m2 is the
 minimum standard value for inorganic TIP phosphors (100 times higher than
 the limit of the sensitivity of the human eye in a dark-adapted
 condition). **d**, TIP decay curves under different thermal simulation
 conditions. Notably, the excited TN@TPBi phosphor was kept at RT for 10
 min, and then heated to 300 (blue), 350 (green), and 400 K (red) with a
 heating rate of 50 K/min. **e**, TIP decay curves with 980 nm NIR
 photo-stimulation in a continuous output mode (green), NIR
 photo-stimulation in a pulsed mode (repeating on and off every 30 s, red),
 and without NIR photo-stimulation (blue). **f**, TL curves of the phosphor
 after a 365 nm-light excitation and kept at RT in the dark for different
 lengths of time. The storage time ranged from 0.5 to 168 h. TL curves were
 collected at temperatures ranging from 300 to 400 K with a heating rate of
 50 K/min. **Figure 3 | Trap profiles for TIP of the TN@TPBi phosphor. a**,
 Temperature-dependent TL glow contour mapping with a heating rate of 50
 K/min. **b**, TL glow curves with different heating rates of 2, 5, 10, 20,
 and 50 K/min. The phosphor was firstly excited by UV light for 5 min at
 100 K. **c**, Estimation of trap depth with the Hoogenstraaten method.
 **d**, Self-absorption spectra (grey line, top) obtained by the
 differences in absorptions of TN@TPBi after and before the 365 nm
 excitation. The self-absorption spectra could be decomposed into several
 bands (blue, green, orange, and red areas in the top figure). The middle
 and bottom figures are the reference absorption spectra of TN radical
 cation (TN•+, green curve), TN radical anions (TN•−, orange curve), TPBi
 radical cation (TPBi•+, blue curve), and TPBi radical anions (TPBi•−, red
 curve) in dichloromethane. **f**, ESR spectra recorded after UV
 irradiation at 110 K (green curve) and 300 K (orange curve). The grey
 curve was obtained after UV irradiation at 110 K and kept at RT for 1 week
 (grey line). **g**, Persistent photocurrent (orange curve) after ceasing
 365 nm UV irradiation. Dark current (green curve) was recorded after
 bleaching the trapped charge carriers by holding the sample at 400 K for
 10 min for comparison. The inset shows the diagram of the TN@TPBi film
 based photocurrent device. The area of the TN@TPBi film was 1×1 cm2 and
 the size of the substrate was 3×2 cm2. **Figure 4 | Versatile molecules
 for TIP with different trap depths and tunable emission colors. b**,
 Heating rate plots of PPO/DT/NPBP/DN/TAQ@TPBi and TN@TMPyPB phosphors. The
 heating rates were 2, 5, 10, 20, and 50 K/min for each case. **c**, TIP
 spectra of PPO/DT/NPBP/DN/TAQ@TPBi and TN@TMPyPB phosphors obtained by TL
 measurements. **e**, TIP decay curve of an OLED device after repeated
 electrical charging with a voltage of 25 V at 150 K. The TIP lasted for 1
 min after ceasing the electronic charging. The right figure shows the
 device structure. The TN@TPBi was used as the emitter layer. **f**, TIP
 decay curve of the OLED at 150 K, followed by higher temperatures with a
 heating rate of 150 K/min. The OLED was electrically charged at 150 K
 before decay recording. ## Sharing/Access information Links to other
 publicly accessible locations of the data: *