Mechanofluorochromism of carbazole-type D–π–A fluorescent dyes

Mechanofluorochromism of (D-π-)2A-type azine-based fluorescent dyes

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC or h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes OUY-2, OUK-2, and OUJ-2 possessing intramolecular charge-transfer (ICT) characteristics from two (diphenylamino)carbazole–thiophene units as D (electron-donating group)–π (π-conjugated bridge) moieties to a pyridine, pyrazine, or triazine ring as A (electron-withdrawing group): grinding of the recrystallized dyes induced red or blue shifts of the fluorescent colors, that is, bathochromic or hypsochromic shifts of the fluorescence maximum wavelengths (λ^fl-solid_max). The degrees of MFC evaluated by the absolute value of differences (Δλ^fl-solid_max) in λ^fl-solid_max before and after grinding of the recrystallized dyes increased in the order of OUY-2 (+7 nm) < OUK-2 (−17 nm) < OUJ-2 (+45 nm), so that OUJ-2 exhibits obvious b-MFC, but OUK-2 exhibits h-MFC. X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) demonstrated that the recrystallized dyes were in the crystalline state but the ground dyes were in the amorphous state. When the ground solids were heated above their crystallization temperatures (T_c), the colors and fluorescent colors recovered to the original ones before grinding or converted to other ones, that is, heating the ground solids in the amorphous state induced the recrystallization to recover the original microcrystals or to form other microcrystals due to polymorph transformation. However, (D–π–)₂Ph-type fluorescent dye OTK-2 having a phenyl group as a substitute for the azine rings exhibited non-obvious MFC. Molecular orbital (MO) calculations indicated that the values of the dipole moments (μ_g) in the ground state were 4.0 debye, 1.4 debye, 3.2 debye, and 2.9 debye for OTK-2, OUY-2, OUK-2, and OUJ-2, respectively. Consequently, on the basis of experimental results and MO calculations, we have demonstrated that the MFC of the (D–π–)₂A-type azine-based fluorescent dyes is attributed to reversible switching between the crystalline state of the recrystallized dyes and the amorphous state of the ground dyes with changes in the intermolecular dipole–dipole and π–π interactions before and after grinding. Moreover, this work reveals that (D–π–)₂A fluorescent dyes possessing dipole moments of ca. 3 debye as well as moderate or intense ICT characteristics make it possible to activate the MFC.

Bathochromic or hypsochromic shift-type mechanofluorochromism (b-MFC of h-MFC) was found for (D–π–)₂A-type azine-based fluorescent dyes: grinding of the recrystallized dyes induced bathochromic or hypsochromic shifts of the fluorescence bands.

Integrated prodrug micelles with two-photon bioimaging and pH-triggered drug delivery for cancer theranostics

Nanodrug carriers with fluorescence radiation are widely used in cancer diagnosis and therapy due to their real-time imaging, less side effect, better drug utilization as well as the good bioimaging ability. However, traditional nanocarriers still suffer from unexpectable drug leakage, unsatisfactory tumor-targeted drug delivery and shallow imaging depth, which limit their further application in cancer theranostics. In this study, an integrated nanoplatform is constructed by polymeric prodrug micelles with two-photon and aggregation-induced emission bioimaging, charge reversal and drug delivery triggered by acidic pH. The prodrug micelles can be self-assembled by the TP-PEI (DA/DOX)-PEG prodrug polymer, which consists of the two-photon fluorophore (TP), dimethylmaleic anhydride (DA) grafted polyethyleneimine (PEI) and polyethylene glycol (PEG). The PEG segment, DOX and DA are bridged to polymer by acid cleavable bonds, which provides the micelles a 'stealth' property and a satisfactory stability during blood circulation, while the outside PEG segment is abandoned along with the DA protection in the tumor acidic microenvironment, thus leading to charge reversal-mediated accelerated endocytosis and tumor-targeted drug delivery. The great antitumor efficacy and reduced side effect of these pH-sensitive prodrug micelles are confirmed by antitumor assays in vitro and in vivo. Meanwhile, these micelles exhibited great deep-tissue two-photon bioimaging ability up to 150 μm in depth. The great antitumor efficacy, reduced side effect and deep two-photon tissue imaging make the TP-PEI (DA/DOX)-PEG prodrug micelles would be an efficient strategy for theranostic nanoplatform in cancer treatment.

Aggregation-Induced Fluorescence of Carbazole and o-Carborane Based Organic Fluorophore

Carbazole based fluorophores 9-butyl-3,6-bis-(phenylethynyl)-9H-carbazole (1) and 9-butyl-3,6-bis-(2-phenyl-o-carborane)-9H-carbazole (2) were synthesized via Sonogashira type cross-coupling reaction and followed by insertion with decaborane. Compound 1 exhibited far more intense fluorescence than 2 in THF solution, while in solid state 2 exerted stronger fluorescence than 1. The fluorescence quenching behavior of 2 in THF solution could be attributed to the intramolecular charge transfer of donor-acceptor system in 2, which was confirmed by electrochemical experiments and DFT calculations. The fluorescence enhancement of 2 in solid state can be ascribed to aggregational induced packing which was evidenced by crystallographic study. In addition, compound 2 showed typical aggregation induced emission (AIE) behavior.

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

Synergy between twisted conformation and effective intermolecular interactions: strategy for efficient mechanochromic luminogens with high contrast

A strategy towards efficient mechanochromic luminogens with high contrast is developed. The twisted propeller-like conformations and effective intermolecular interactions not only endow the luminogens with AIE characteristics and high efficiency in the crystalline state, but also render them to undergo conformational planarization and disruption in intermolecular interactions upon mechanical stimuli, resulting in remarkable changes in emission wavelength and efficiency.