White light emission from a single organic molecule with dual phosphorescence at room temperature

The development of single molecule white light emitters is extremely challenging for pure phosphorescent metal-free system at room temperature. Here we report a single pure organic phosphor, namely 4-chlorobenzoyldibenzothiophene, emitting white room temperature phosphorescence with Commission Internationale de l’Éclair-age coordinates of (0.33, 0.35). Experimental and theoretical investigations reveal that the white light emission is emerged from dual phosphorescence, which emit from the first and second excited triplet states. We also demonstrate the validity of the strategy to achieve metal-free pure phosphorescent single molecule white light emitters by intrasystem mixing dual room temperature phosphorescence arising from the low- and high-lying triplet states.


Supplementary Methods
All the chemicals and reagents were purchased from Aldrich and used as received without further purification. All the molecules synthesized were purified by column chromatography and recrystallization from dichloromethane and hexane for two times, and fully characterized by 1 H NMR, 13 C NMR and high resolution mass spectroscopies and elementary analysis. 1 H and 13 C NMR spectra were recorded on a Bruker AV 400 Spectrometer at 400 and 100 MHz in CDCl 3 , respectively. Tetramethylsilane was used as the internal standard. High-resolution mass spectra (HRMS) were recorded on a GCT premier CAB048 mass spectrometer operating in MALDI-TOF mode. Elementary analysis was performed on a Thermo Finnigan Flash EA1112. Gel filtration chromatography was performed using a ZORBAX SB-C18 column (Agilent) conjugated to an Agilent 1260 Infinite HPLC system. Before running, each sample was purified via 0.22 µm filter to remove any aggregates. The flow rate was fixed at 1.0 mL/min, the injection volume was 20 µL and each sample was run for 6 min. The absorption wavelength used was set at 330 nm. 100 % percent of acetonitrile was used as the running buffer. The photoluminescence spectra were measured on a PerkinElmer LS 55 spectrophotometer. The lifetime, time-resolved excitation spectra, steady state and time-resolved emission spectra, temperature dependent photoluminescence spectra and absolute luminescence quantum yield were measured on a Edinburgh FLSP 920 fluorescence spectrophotometer equipped with a xenon arc lamp (Xe900), a microsecond flash-lamp (uF900), a picosecond pulsed diode laser (EPL-375), a closed cycle cryostate (CS202*I-DMX-1SS, Advanced Research Systems) and an integrating sphere (0.1 nm step size, 0.3 second integration time, 5 repeats), respectively. Mean decay times (τ P ) were obtained from individual lifetimes τ i and amplitudes a i of multi-exponential evaluation. Powder X-Ray diffraction patterns were performed on an X'Pert PRO MPD diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 25 °C (scan range: 4.5−50°). Single crystal data was collected on a Bruker Smart APEXII CCD diffractometer using graphite monochromated Cu Kα radiation (λ = 1.54178 Å). The photos and videos were recorded by a Cannon EOS 60D.
The amorphous solids of the phosphors were prepared by heating the samples to melt with a heating gun and quenching the melt with liquid nitrogen.
All the crystalline samples were obtained from slowly evaporative crystallization using hexane/chloroform mixture (3:1, v/v). To further check the purity of the solid samples, all the solid samples were dissolved in 100 % percent of acetonitrile and got sample solutions (50 µM), then run the HPLC.
To check the optical stability of powder samples, a pile of powder was exposed to a 365nm UV light (the power of UV tube is 8W) for 30 min to 12h, then the solid samples were dissolved in 100 % percent of acetonitrile and then sample solutions were prepared (50 µM), finally run the HPLC.
Mean decay times (τ P ) were obtained from individual lifetimes τ i and amplitudes a i of multi-exponential evaluation through:

Synthesis
Synthetic route for BDBT, FBDBT, ClBDBT, BrBDBT and BCBP.  was used as the light source. The impulse response function (IRF) (green) was from the output of pulsed diode laser which has a typical pulse width of less than 100 ps. All the decay profiles almost coincide with the IRF, suggesting that no obvious nanosecond fluorescence decay was detected. As persistent lifetimes were detected in all the emission bands, the photoluminescence feature of BDBT, FBDBT, ClBDBT, BrBDBT and BCBP was proved to be pure phosphorescence.

Computational methods and results
The computational models were built from the crystal structure shown below. The quantum mechanics/molecular mechanics (QM/MM) method was implemented to deal with the electronic structures in crystal by virtue of ChemShell 3.5. 1 , interfacing Turbomole 6.5 2 for QM and DL_POLY 3 with the general Amber force field (GAFF) 4 for MM. The atomic partial charges were generated by the restrained electrostatic potential (RESP) 5 method. Molecular geometry optimizations were performed for the ground state (S 0 ) at the level of B3LYP/6-31G(d) and for the triplet state (T 1 and T 2 ) at the TDDFT/B3LYP/6-31G(d) level. The excitation energies were calculated by using TDDFT for electronic excited singlet and triplet states. We further calculated the vibrational frequencies at S 0 , T 1 and T 2 states at (TD)B3LYP/6-31G(d) level in order to determine the vibration emission spectra of T 1 →S 0 and T 2 →S 0 . At the same level, the oscillator strength of triplet states is given by Beijing Density Function (BDF) program. [6][7][8] The emission spectra were calculated by MOMAP program 9 with detailed formulation discribed in our previous work. 10  Because we failed to obtain the single crystal structure of FBDBT, no QM/MM model calculations thus could be carried out. For BDBT and BrBDBT, they have different energy gaps, electronic transition characters for T 1 and T 2 states and the corresponding frontier molecular orbitals. Briefly, BDBT has a large energy gap between T 1 and T 2 state. BrBDBT has similar energy gaps as ClBDBT, but its T 2 state show smaller (n,π*) transition.