Qingbao Song

Polymorphic crystals and their luminescence switching of triphenylacrylonitrile derivatives upon solvent vapour, mechanical, and thermal stimuli

For piezo-, vapo-, and thermochromic materials, it remains a challenge to figure out the underlying reason for fluorescence color changes upon external stimulation and determine why only some fluorophores reveal emission switching. A novel triphenylacrylonitrile derivative (TPAN-MeO) with remarkably twisted conformations has been carefully prepared via the Suzuki coupling reaction. The fluorescence of TPAN-MeO in the aggregate state depends on the polymorphic forms: three crystalline forms BCrys, SCrys and YCrys exhibit bright blue, sky-blue and yellow emission, respectively; meanwhile the amorphous powders are also strongly fluorescent with green emission. The crystals BCrys and SCrys exhibit mechano- and piezochromism in that grinding and high pressure could alter the emission colour, respectively. In addition, the amorphous film exhibits vapo- and thermochromic behaviour in that organic vapour and heating could change the green colour into sky-blue. Interestingly, the solvent vapour and heating stimuli can trigger a crystal-to-crystal transformation between SCrys form and YCrys form.

Effect of stacking mode on the mechanofluorochromic properties of 3-aryl-2-cyano acrylamide derivatives

Three structurally simple 3-aryl-2-cyano acrylamide derivatives, 2-cyano-3-(2-methoxyphenyl)-2-propenamide (1), 2-cyano-3-(3-methoxyphenyl)-2-propenamide (2) and 2-cyano-3-(4-methoxyphenyl)-2-propenamide (3) were synthesized. They exhibited different optical properties due to their distinct face-to-face stacking mode. The as-prepared crystals of 1 exhibited green luminescence and the emission peak did not change after grinding treatment. However, the emission peaks of 2 (Φf = 12%) and 3 (Φf = 16%) exhibited an obvious red-shift upon grinding, and their corresponding quantum yields decreased to 8% and 10%, respectively. Differential scanning calorimetry and powder X-ray diffractometry data indicated that the optical properties of 2 and 3 could be attributed to the transformation from the crystalline phase to the amorphous phase. X-ray crystal structures, infrared spectroscopy and data from fluorescence lifetime experiments further validated the relationship between fluorescence switching, stacking mode and molecular interactions.

Ratiometric pressure sensors based on cyano-substituted oligo(p-phenylene vinylene) derivatives in the hybridized local and charge-transfer excited state

Ratiometric sensors for visual monitoring of pressure environments are highly valuable in various fields, such as security inks and optoelectronic devices. However, ratiometric pressure sensors remain inadequate due to the lack of piezo-chromic luminescent materials with high-contrast color change and high emission efficiency. A donor–acceptor cyano-substituted oligo(p-phenylene vinylene) derivative mF-TPA exhibits a high luminescence efficiency (Φf = 82.6%), which is related to the hybridized local and charge-transfer (HLCT) states. Moreover, its crystalline particles can sense exact hydrostatic pressure as high as 10 GPa, accompanied by visible color changes (λPL = 146 nm, from light-green to deep-red). Interestingly, the photo-luminescence peak wavelengths have a linear relationship with the external pressure, enabling its use as a ratiometric pressure sensor. In situ Raman spectroscopy confirms that the intermolecular interaction is obviously enhanced due to the closer packing at high pressure, inducing a red-shift of the emission peak and a decrease of the fluorescence intensity.

Efficient Protocol for the Synthesis of Novel Spiro[acenaphthylene-1,2′-pyrrolidin]-2-one Compounds

Abstract An efficient catalyst-free synthesis of 3′-benzoyl-4′,5′-diphenyl-2H-spiro[acenaphthylene-1,2′-pyrrolidin]-2-one derivatives via one-pot 1,3-dipolar cycloaddition of acenaphthenequinone, arylmethyl amines, and chalcones with high regioselectivity is described. The structure of the cycloadducts were characterized by infrared, high-resolution mass spectrometry (electrospray ionization), 1H NMR, and 13C NMR spectra, and the structure of 4a was confirmed using x-ray single-crystal structure analysis. GRAPHICAL ABSTRACT

Highly efficient luminescent E- and Z-isomers with stable configurations under photoirradiation induced by their charge transfer excited states

Highly efficient luminescent E-/Z-isomers based on double bonds with stable E/Z configurations in the excited state under photoirradiated conditions are of great significance to organic photochemistry and photoelectronics. Herein we report the highly efficient luminescent E-/Z-isomers of a donor–acceptor triphenylamine–cyanostilbene molecule, TPNCF, which were synthesized simultaneously via a Knoevenagel reaction, with their E/Z configurations confirmed by the obtained single crystal structure. The NMR spectra showed that both E-TPNCF and Z-TPNCF maintained their stable E/Z configurations in DMSO-d6 solvent and films under photoirradiation conditions. The in situ UV spectra in different polar solvents showed that polarity played a positive role in stabilizing the E/Z isomers in their configurations. PL solvatochromism experiment and theoretical calculation results indicated that the S1 state has charge-transfer (CT) character in both E-TPNCF and Z-TPNCF. As the solvent polarity increased, the energy of the S1 state gradually decreased, and the E/Z isomerization reaction at the higher energy S2 state was effectively suppressed by a rapid internal conversion process from the S2 to the S1 state. This was supposed to be the main reason for the observed stable E-/Z-isomers under photoirradiated conditions. Photophysical investigations demonstrated that both E-TPNCF and Z-TPNCF exhibited an obvious aggregation-induced emission (AIE) with high fluorescence quantum efficiencies of 70% and 50% in films respectively. Further research revealed that the AIE phenomenon in the TPNCF molecules was mainly dominated by their CT excited state characteristic but not the E/Z isomerization or the restricted intramolecular rotation effect. Our work will contribute considerably to the basic photochemistry studies on E/Z isomerization reactions and the material design of stable E/Z isomers for organic photo-electronic applications.

Highly Twisted Isomers of Triphenylacrylonitrile Derivatives with High Emission Efficiency and Mechanochromic Behavior

Two novel triphenylacrylonitrile derivatives (TPAN-C2 and TPAN-C3) with highly twisted conformations have been successfully prepared by Suzuki coupling reaction. Both of them show mechanochromic (MC) fluorescent behaviour and exhibit remarkable fluorescence redshifts from sky-blue to light green upon fully grinding them. Interestingly, the ground powders recover their original states upon heating at 100 °C for 1 min or fuming with solvents vapour. Such intriguing MC properties of TPAN-C2 and TPAN-C3 are attributed to their phase transitions between crystalline and amorphous states, which are confirmed by powder X-ray diffraction (XRD) data. Moreover, the behaviour of both TPAN-C2 and TPAN-C3 indicates that the aggregation-induced emission (AIE) gives rise to rather high fluorescence quantum efficiencies (Φf ) of 0.78 and 0.97, respectively. The influences of stacking mode and molecular conformation on MC properties and the high Φf of TPAN-C2 and TPAN-C3 are discussed.

Photo-irradiated E/Z Isomerization Reaction of Star-shaped Isomers Containing Two Cyanostilbene Arms with Charge Transfer Excited States

The E/Z isomerization reaction of the multi-cyanostilbene molecule is still not clear. Herein, we have designed and synthesized three star-shaped molecular isomers with a triphenylamine core linked to two cyanostilbene groups with E/Z isomerization, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, possessing three different isomeric molecular configurations, to investigate the specific E/Z isomerization reaction of the cyanostilbene groups in the two molecular arms. The in situ UV, 1H NMR and HPLC spectra under UV-irradiation clearly showed that the E/Z isomerization reactions of both E,E-TPDCF and Z,Z-TPDCF firstly turned them into Z,E-TPDCF, and the Z,E-TPDCF was almost simultaneously turned into more E,E-TPDCF and less Z,Z-TPDCF due to the calculated lowest unoccupied molecular orbitals of Z,E-TPDCF on the cyanostilbene arm with the Z-configuration. In general, Z,E-TPDCF exhibited a relatively better configurational stability than Z,Z-TPDCF or E,E-TPDCF under the photo-irradiation conditions. Further research demonstrated that all three isomers exhibited excellent aggregation-induced emission (AIE) properties.

p-TSA-catalyzed one-pot synthesis of novel 7-aryl-6H-benzo[h][1,3]dioxolo[4,5-b]xanthene-5,6(7H)-diones in ethanol

ABSTRACT A one-pot, three-component reaction of aromatic aldehydes, 2-hydroxy-1,4-naphthoquinone, and 3,4-methylenedioxyphenol has been reported for the synthesis of novel 7-aryl-6H-benzo[h][1,3]dioxolo[4,5-b]xanthene-5,6(7H)-diones using environmentally benevolent para-toluene sulfonic acid (p-TSA) as the catalyst. This methodology provides a mild and fast route to diverse pyran derivatives in moderate to good yields. GRAPHICAL ABSTRACT