Mechano-induced persistent room-temperature phosphorescence from purely organic molecules

Interesting purely organic persistent room-temperature phosphorescence induced by not only photoirradiation but also mechanical action has been presented.

Ultra-pure water was used during the work-up procedures in the experiments.
All other reagents and solvents (analytical grade) were purchased from Guangzhou Jincheng Company (China) and used without further purification.
Crystals of CX49 and CX11 were grown from a dichloromethane (DCM)/nhexane mixture by slowly solvent evaporation.

Characterization
Proton and carbon NMR ( 1 H NMR and 13 C NMR) spectra were obtained on a Fourier transform nuclear magnetic spectrometer (Bruker AVANCE 400) (CDCl 3 , tetramethylsilane (δ = 0) as the internal standard). The mass spectra were measured using thermo-spectrometers (DSQ & MAT95XP-HRMS). The elemental analysis was conducted using an Elementar Vario EL analyzer. The high performance liquid chromatography (HPLC) was measured by a Thremo LCQ DECA XP liquid chromatography spectrometry. The UV-visible absorption spectrum was determined on a Hitachi U-3900 spectrophotometer. The photoluminescence (PL) spectra were collected with a spectrometer (FluoroLog-3) from HORIBA Instruments or a spectrometer system (Maya Pro2000) form Ocean Optics with a 310 nm Rhinospectrum RhinoLED as the excitation source. The persistent phosphorescent spectra were detected at a delayed time of 8 ms after the excitation source turn off. The measurements of mechanoluminescent spectra were carried out on a spectrometer of Acton SP2750 with a liquid-nitrogen-cooled CCD (SPEC-10, Princeton) as a power detector. The mechanoluminescent photos were taken using a Canon EOS 700D camera (18000k pixel) in a REC mode, which frame number per second is 30. The first spot in the frame was set as the beginning of the ML for each scratching. Powder wide-angle X-ray diffraction (PXRD) measurements were performed on an X-ray diffractometer (Smartlab) using an X-ray source of Cu K (λ = 0.15406 nm) at 40 kV and 30 mA, at a scan rate of 4° (2θ) per min. The quantum chemistry calculations were performed at the B3LYP/6-31G (d) level of theory using the time-dependent density functional theory (TD-DFT) method with the Gaussian 09 software.
The single-crystal X-ray diffraction data were collected using a Gemini A Ultra system (Agilent Technologies) or a Smart 1000 CCD (Bruker), with Cu-K radiation ( = 1.54178 Å). Both structures were solved using direct methods following the different Fourier syntheses. By using the SHELXTL program suite, all non-hydrogen atoms were anisotropically refined through least-squares on F 2 and assigned with the anisotropic thermal parameters. Meanwhile, the hydrogen atoms attached to carbon were placed in idealized positions and refined using a riding model. CCDC 1581141 contains the crystallographic data of crystal CX11 in this paper, respectively.

Synthesis of 2-fluoro-9H-xanthen-9-one (FX)
Cuprous iodide (0.18 g, 0.96 mmol) and pyridine (1.54 mL, 19.16 mmol) were added to a solution of 2-chlorobenzoic acid (3.00 g, 19.16 mmol), and 4fluorophenol (4.30 g, 38.32 mmol) in N,N-dimethylformamide (DMF, 50 ml). The resulting solution was heated at 150 o C for 5h. The reaction mixture was cooled to room temperature and poured into a diluted HCl solution. The precipitate was filtered, and washed several times with water in order to remove any leftover acid. The crude product was collected without further purification.
The above product was dissolved in concentrated sulfuric acid (50 ml). The resulting suspension was stirred for 6 h at 100 o C. After cooling to room temperature, the mixture was poured slowly over ice. The precipitate was filtered, and washed several times with water in order to remove any leftover sulfuric acid. The collected solid was then re-dissolved in dichloromethane and washed with water (20 ml × 3). The combined water layers were extracted with additional dichloromethane. Then the combined organic extracts were dried over anhydrous sodium sulfate. After filtration and solvent evaporation under reduced pressure, a white solid was obtained in 78.1% yield (2.53 g). 1

Time-Dependent Density Functional Theory (TD-DFT) Calculations
The Gaussian 09 1 program was utilized to perform the TD-DFT calculations similar with the previous literature. 2 In order to maintain the specific molecular configurations and corresponding intermolecular locations, the ground state (S 0 ) geometries of the CX crystal were obtained from the CX11 single crystal structure and no further geometry optimization was conducted. The ground state (S 0 ) geometries of free CX molecule in vacuum was obtained by geometry optimization based on B3LYP 3 /6-31G (d). The exciton energies of the n-th singlet (S n ) and n-th triplet states (T n ) were obtained on the corresponding ground state geometries using the combination of TD 4 -B3LYP/6-31G (d). Kohn-Sham frontier orbital analysis was obtained for elucidating the mechanisms of possible singlet-triplet intersystem crossings (ISC). The possible S 1 to T n ISC channels are believed to share part of the same transition orbital compositions, and the energy levels of T n are considered to lie within the range of E S1 ± 0.3 eV. Especially, the major ISC channels are mainly determined based on two elements: (1) the ratio of the same transition configuration in S 1 and T n states should be large in all the transition orbital compositions, and (2) the energy gap between S 1 and the specific T n state should be small. When the energy of T n is lower than S 1 , the first element is considered to be more important. The determination of minor ISC channels is vice versa. and triplet (T n ) states. The notations H and L refer to HOMO and LUMO, respectively.
The plain arrows refer to the major ISC channel.

Fig. S12
Photo of mechano-induced phosphorescence of CX49 at room temperature.
The sample was simply scratched by a scraper.  from reference at room temperature. 5 No any light spot can be seen after the scratch stopping. The sample was simply scratched by a scraper. ML-1 means that the sample was scratched for the first time, and so on.