Meng Zheng

Reversible piezochromic luminescence of 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes and the dependence on N-alkyl chain length

Aggregation-induced emission-active 9,10-bis[(N-alkylcarbazol-3-yl)vinyl]anthracenes (ACZn) with different length of N-alkyl chains have been designed and synthesized to systematically investigate the effect of chain length on the solid-state fluorescence properties. The results show that these materials exhibit not only reversibly pronounced red shift piezofluorochromism (PFC), but also chain length-dependent fluorescence properties. The solid-state fluorescence efficiencies are decreased upon grinding or with the increase of N-alkyl chain length. Meanwhile, the emission spectra of pristine and ground solids are respectively red-shifted and blue-shifted with the increase of N-alkyl chain length. The shorter the N-alkyl chains, the more remarkable the PFC behaviour of ACZn. Powder X-ray diffraction and DSC experiments reveal that the transformation between crystalline and amorphous states upon external stimuli is responsible for the PFC and restoration behaviours.

Solid-state fluorescence properties and reversible piezochromic luminescence of aggregation-induced emission-active 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]anthracenes

In this work, we have synthesized 9,10-bis[(9,9-dialkylfluorene-2-yl)vinyl]anthracene derivatives (FLA-Cnn) with propyl, pentyl, and dodecyl side chains to investigate their fluorescence properties. The results show that FLA-Cnn exhibit not only an aggregation-induced emission effect but also reversible piezofluorochromic (PFC) behaviour. Interestingly, the fluorescence emission and grinding-induced spectral shifts (ΔλPFC) of FLA-Cnn solids are alkyl length-dependent: the longer alkyl-containing FLA-Cnn solids show more blue-shifted emission and larger ΔλPFC. Moreover, the fluorescence emission of ground FLA-C12 solid can recover spontaneously at room temperature, Powder wide-angle X-ray diffraction and differential scanning calorimetry experiments reveal that the transformation between crystalline and amorphous states under various external stimuli is responsible for the PFC behaviour, and the spontaneous recovering emission of amorphized FLA-C12 solid is ascribed to its low cold-crystallization temperature. This work demonstrates once again the accessibility of tuning the solid-state optical properties of organic fluorophores by combining the simple alternation of molecular chemical structure and the physical change of aggregate morphology under external stimuli.

Cruciform 9,10-distyryl-2,6-bis(p-dialkylamino-styryl)anthracene homologues exhibiting alkyl length-tunable piezochromic luminescence and heat-recovery temperature of ground states

A series of 2,6-bis(p-dialkylaminostyryl)-9,10-distyrylanthracene (FCn) cruciforms with N-alkyl chains of different lengths have been synthesized, and their aggregation-enhanced fluorescence and piezochromic luminescence (PFC) behaviours are investigated. These 9,10-distyrylanthracene-containing cruciforms exhibit relatively low fluorescence quantum yields (Φ) in THF solution (Φ ≈ 10%) and moderate aggregation-enhanced emission in aqueous media (Φ ≈ 25%), but strong and chain length-dependent solid-state fluorescence emission. Grinding and pressing experiments indicate that they are all effective PFC materials in terms of mechanical stress-induced spectral shifts (ΔλPFC = 23–54 nm), moreover, the longer alkyl-containing FCn shows a larger ΔλPFC. Powder X-ray diffraction and differential scanning calorimetry measurements reveal that the transformation between the crystalline and amorphous states upon external stimuli is responsible for the reversible PFC behaviour. It is found that increasing the N-alkyl length could effectively decrease the cold-crystallization temperature of the ground states to render the PFC states with a tunable heat-recovering temperature, and ground FC10 and FC12 solids can recover spontaneously to their original states at room temperature.

N-Monoalkylated 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrroles as effective one- and two-photon fluorescence chemosensors for fluoride anions

We have synthesized three N-monoalkylated DPP (mDPP) derivatives, 3,6-bis(4-chloro-/methyl-/bromo-phenyl)-2-octyl-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-diones (mDPP-Cl/mDPP-Me/mDPP-Br), by facile one-step monoalkylation of commercially available or easy-prepared corresponding DPP derivatives. All the three compounds have high fluorescence quantum yields (Φ) and exhibit usable two-photon absorption cross-sections (δ = 58–87 GM). They are not only effective one-photon but also more effective two-photon fluorescence chemosensors selectively for fluoride anions. Moreover, the substituents at the benzene ring, even if simple methyl, chloride and bromide, could affect the intramolecular charge transfer from the lactam N to the DPP core, and the sensing performance of mDPP to fluoride ions is in the order, mDPP-Br ≥ mDPP-Cl > mDPP-Me. Proton NMR titrations indicate that the intermolecular proton transfer between the hydrogen atom on the lactam N and the fluoride anion is the sensing origin. The stable chemical structure, small molecular size and usable Φδ in terms of femto-second laser pulses make mDPP moieties suitable for the construction of new and effective fully aromatic two-photon materials applicable in two-photon science and technology.

Unusual mechanohypsochromic luminescence and unique bidirectional thermofluorochromism of long-alkylated simple DPP dyes

There have been hundreds of organic and metallorganic compounds found to exhibit mechanobathochromic luminescence within tens of nanometers. This paper presents the first example of unusual mechanohypsochromic luminescence and unique bidirectional thermofluorochromism based on 1,4-diketo-2,5-di(tetradecyl)-3,6-bis-(p-chlorophenyl)pyrrolo[3,4-c]pyrrole (TDPP). Pristine TDPP powder is a red emitting crystal (645 nm, R state) and can be changed into a yellow amorphous state (544 nm, Y state) by mechanical grinding or solution spin-coating. The Y state could revert to the R state upon annealing below 60 °C or solvent fuming, however, annealing the Y state above 70 °C produced a new green-emitting crystalline state (523 nm, G state). Furthermore, the G state can transform to the Y and R states upon grinding and solvent fuming, respectively. In contrast, those novel phenomena were not observed in the butylated DPP, demonstrating that the peripheral alkyl chain lengths of organic fluorophores can play diverse functional roles in determining the solid-state aggregation behavior and fluorescence properties. This work has improved some types of stimuli responsive materials and new optical properties of alkylated DPPs have been developed.