High Solid Fluorescence of a Pyrazoline Derivative through Hydrogen Bonding

Pyrazoline and its derivatives often exhibit strong emissions in dilute solutions, but their emission intensity is often dramatically reduced in the solid state due to strong intermolecular interactions between neighboring molecules. In this report, we successfully synthesized a new pyrazoline 4-(3-(4-(decyloxy)phenyl)-1-(2,3,5,6-tetrafluoro-4-(trifluoromethyl)phenyl)-4,5-dihydro-1H-pyrazol-5-yl)-N,N-diethylaniline (PPDPD), into which seven fluorine (F) atoms were incorporated. In the solid state, PPDPD emits a strong blue light at λmax 430 nm with a fluorescence quantum yield of up to 41.3%. Single-crystal analysis showed the presence of intra/intermolecular C-H···F bonds that may impede molecular motion and block the non-radiative decay channel. Compound PPDPD therefore shows high emission efficiency in the solid state.

MALDI-TOF mass spectrometric measurements were finished on Shimadzu Biotech AXIMA-TOF 2TM . The UV-vis and fluorescence spectra were carried out on Shimadzu UV-2501 and RF-5301 spectrophotometer, respectively. Cyclic voltammetry measurements were conducted on a CHI 604E electrochemical analyser with glassy carbon (diameter: 1.6 mm; area: 0.02 cm 2 ) as a working electrode, and platinum wires as a counter electrode and a reference electrode, respectively. Fc + /Fc was used as an internal standard. Potentials were recorded versus Fc + /Fc in a solution of anhydrous acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as a supporting electrolyte at a scan rate of 100 mV s -1 , by empirical formulas, E LUMO/HOMO = -[4.8-E Fc + E re/ox onset ] eV, where E Fc = 0.062 eV (measured in our setup).
Fluorescence microscope measurement. Fluorescence image was obtained using a Nikon Ti-U Inverted Microscope System equipped with a Nikon C-SHG 1 mercury lamp. The exposure time to acquire a bright photo on a fluorescence microscope is 400 ms, with four attenuators (1/4, 1/8, 1/16, and 1/32).
The fluorescent quantum yields in dilute solutions were determined using quinine bisulfate (ϕ = 54.6% in 0.1 N H 2 SO 4 ) as a standard using the equation: In this equation, subscripts s and r represent the sample and reference, respectively. F is the integral area of the fluorescence spectra, n is the refractive index of the solution, A is the absorbance, ϕ is the fluorescence quantum yield.
Photoluminescence spectra (PL) and fluorescence quantum yield for solid state were collected on an Edinburgh Instruments FLS 980 system spectrofluorimeter equipped with Xe-900 and integrating sphere.
Single-crystal X-ray diffraction. The suitable single-crystals were selected for single-crystal X-ray data collection with a Bruker SMART APEX-II CCD area detector on a D8 goniometer. The data were collected using graphite-monochromated and 0.5 mm-Mono Cap-collimated Mo-Kα radiation (λ = 0.71073 Å) with the ω scan method at room temperature (Form I and Form II) and 100 K (Form III). The data were processed with the SAINT program of the APEX2 software for reduction and cell refinement. Multi-scan absorption corrections were applied by using the SADABS program for area detector. All structures were solved by the direct method and refined by the full-matrix least-squares method on F 2 . All non-H atoms were refined anisotropically.
Hydrogen atoms were placed in idealized positions and included as riding with Uiso (H) = 1.2 Ueq (C). Crystallographic data and structural refinements are summarized in Table S1.
The thermal property of compound PPDPD was evaluated by TGA under nitrogen atmosphere. As shown in Figure S8, compound PPDPD exhibits very good thermal stability with an onset decomposition temperature of ~ 295 o C (considering the 5% weight loss temperature).