Molecular packing and aggregation-induced emission of 4-dicyanomethylene-2,6-distyryl-4H-pyran derivatives

What Leads to Aggregation-Induced Emission?

Aggregation-induced emission (AIE), usually referring to the phenomenon in which molecules emit more strongly in the aggregate state than in the solution state, is intriguing and promising in various optoelectronic and biosensing applications. In this Perspective, the basic principles that can lead to AIE and experimental evidence to reveal the AIE mechanism of tetraphenyl ethylene (TPE)-type molecules are discussed. AIE is the consequence of two factors: (1) the fast energy dissipation by crossing a conical intersection (CI) in solutions but not in solids results in low luminescence efficiencies in the solutions, and (2) the weak intermolecular coupling and thus slow intermolecular energy/charge transfers in the AIE solids effectively prevent quenching and result in relatively high luminescence efficiencies. The key to AIE is that the luminescence efficiency is tuned by controlling molecules to cross or not to cross a CI by changing the phase of molecules. How fast a molecule can cross a CI is dependent on the energy barrier of isomerization, which can be tuned in many ways, including mechanical or electrical stimuli, in addition to changing phases. Barrier-dependent crossing CI also results in a very important consequence: excitation-wavelength-dependent fluorescence yield within one electronic excited state, an anti-Vavilov's rule phenomenon. In principle, there can be an alternative way to tune luminescence efficiency by manipulating the formation of CIs instead of crossing or not crossing them. This approach relies on the fact that the electronic ground state and the excited state have many different properties, e.g., dipole moment. By tuning the environment, e.g., dielectric constant, to favor or disfavor one state, one may be able to lift or lower the potential surface of one state so that the potential surfaces of two states can vary between intersected and not contacted.

An Unexpected 4,5-Diphenyl-2,7-naphthyridine Derivative with Aggregation-Induced Emission and Mechanofluorochromic Properties Obtained from a 3,5-Diphenyl-4H-pyran Derivative

For a specific fluorescent molecule, the increase of molecular conformation distortion is beneficial to endow it with aggregation-induced emission (AIE) and mechanofluorochromic (MFC) properties. Herein, 3,5-diphenyl-4H-pyran derivative 5 and 4,5-diphenyl-2,7-naphthidine derivative 7 with highly twisted conformations were synthesized. For compound 5, although the introduction of phenyl rings with large steric hindrance at 3 and 5 positions of the 4H-pyran skeleton realized the transformation from aggregation-induced quenching (ACQ)-active molecule to AIE-active molecule, it only showed a low-contrast MFC activity. Compound 7 was accidentally obtained from compound 5 and n-butylamine via a ring-opening and subsequent intramolecular ring-closing mechanism. Compound 7 was confirmed to have a highly twisted molecular conformation by the crystal structural analysis and exhibited AIE activity originated from the restriction of intramolecular rotation. Furthermore, compound 7 exhibited reversible high-contrast MFC activity. Upon grinding, the change of solid-state fluorescence color from orange to yellow was confirmed to be due to the partial destruction of crystal structure. This work provides new ideas for the design and synthesis of novel AIE-active and MFC-active fluorescent molecules based on ACQ-active parent molecules.

Aggregation-Induced Emission of Tetraphenylethene-Conjugated Phenanthrene Derivatives and Their Bio-Imaging Applications

In this study, a series of rationally designed emissive phenanthrene derivatives were synthesized and their aggregation-induced emission (AIE) properties in tetrahydrofuran (THF)/water mixtures were investigated. Two tetraphenylethene (TPE) segments were conjugated to both ends of the phenanthrene core at the para-positions and meta-positions, resulting in pTPEP and mTPEP derivatives, respectively. While the TPE-conjugated phenanthrene derivatives did not show any emission when dissolved in pure THF, they showed strong sky-blue emissions in water-THF mixtures, which is attributed to the restriction of intramolecular motions of TPE segments by aggregation. Furthermore, silica nanoparticles loaded with these AIE-active compounds were prepared and proved to be promising intracellular imaging agents.

Hydrogen bond-assisted aggregation-induced emission and application in the detection of the Zn(ii) ion

The compounds of 3-aminopyridine-2-carboxylic acid with K(+) (1) and Zn(2+) (2) were found to be AIE-active. The AIE behaviours could be attributed to the restriction of intramolecular rotation (RIR) and vibration (RIV) via hydrogen bonds, resulting in rigidity enhancement of the molecules. An AIE-based fluorescence turn on chemosensor for the Zn(ii) ion has been developed in aqueous media with high selectivity and sensitivity.

The solvatochromism and aggregation-induced enhanced emission based on triphenylamine-propenone

The weak intermolecular interactions and the suppression of the intramolecular charge transfer result in the efficient aggregation-induced enhanced emission characteristic of PhO-TPA and PyO-TPA.

Single-layer transition metal dichalcogenide nanosheet-assisted assembly of aggregation-induced emission molecules to form organic nanosheets with enhanced fluorescence

Single-layer transition metal dichalcogenide nanosheets including MoS2, TiS2 and TaS2 can be used as novel platforms for the fluorescence enhancement of aggregation-induced emission fluorophores. The small organic AIE unit can be assembled into a two-dimensional sheet structure via a simple precipitation technique assisted by these monolayers. The resultant organic sheets possess a size of 0.2-2 μm and a thickness of 9-20 nm.

Click synthesis, aggregation-induced emission, E/Z isomerization, self-organization, and multiple chromisms of pure stereoisomers of a tetraphenylethene-cored luminogen

It has been difficult to decipher the mechanistic issue whether E/Z isomerization is involved in the aggregation-induced emission (AIE) process of a tetraphenylethene (TPE) derivative, due to the difficulty in the synthesis of its pure E and Z conformers. In this work, pure stereoisomers of a TPE derivative named 1,2-bis{4-[1-(6-phenoxyhexyl)-4-(1,2,3-triazol)yl]phenyl}-1,2-diphenylethene (BPHTATPE) are successfully synthesized. Both isomers show remarkable AIE effect (α(AIE) ≥ 322) and high fluorescence quantum yield in the solid state (Φ(F) 100%). The conformers readily undergo E/Z isomerization upon exposure to a powerful UV light and treatment at a high temperature (>200 °C). Such conformational change, however, is not observed under normal fluorescence spectrum measurement conditions, excluding the involvement of the E/Z isomerization in the AIE process of the TPE-based luminogen. The molecules of (E)-BPHTATPE self-organize into ordered one-dimensional nanostructures such as microfibers and nanorods that show obvious optical waveguide effect. BPHTATPE shows rich chromic effects, including mechano-, piezo-, thermo-, vapo-, and chronochromisms. Its emission peak is bathochromically shifted by simple grinding and pressurization and the spectral change is reversed by fuming with a polar solvent, heating at a high temperature, or storing at room temperature for some time. The multiple chromic processes are all associated with changes in the modes of molecular packing.

Quinoacridine derivatives with one-dimensional aggregation-induced red emission property

A new series of acceptor-donor-acceptor (A-D-A) type quinoacridine derivatives (1-3) with aggregation-induced red emission properties were designed and synthesized. In these compounds, the electron-withdrawing 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile groups act as electron-accepting units, while the alkyl-substituted conjugated core acts as electron-donating units. The restriction of intramolecular rotation was responsible for the AIE behavior of compounds 1-3. All compounds were employed as building blocks to fabricate one-dimensional (1-D) organic luminescent nano- or microwires based on reprecipitation or slow evaporation approaches. Morphological transition from zero-dimensional (0-D) hollow nanospheres to 1-D nanotubes has been observed by recording SEM and TEM images of aggregated sates of compound 2 in THF/H(2)O mixtures at different aging time. It was demonstrated that the synthesized compounds with different lengths of alkyl chains displayed different wire formation properties. The single-crystal X-ray analysis of compound 2 provided reasonable explanation for the formation of 1-D nano- or microstructures.

Aggregation-induced emission enhancement of aryl-substituted pyrrole derivatives

The relationship between the structures and light emission properties of five aryl-substituted pyrrole derivatives was studied during aggregation in THF-water mixtures. Only pentaphenylpyrrole clearly shows, however, an aggregation-induced emission enhancement (AIEE) phenomenon. On comparison of the optical properties and single-crystal structures of these pyrrole derivatives, it is suggested that the more twisted configuration which prevented parallel orientation of conjugated chromophores combined with the restricted intramolecular rotation (RIR) effect was the main cause of the AIEE phenomenon.

Theory of excited state decays and optical spectra: application to polyatomic molecules

General formalism of absorption and emission spectra, and of radiative and nonradiative decay rates are derived using a thermal vibration correlation function formalism for the transition between two adiabatic electronic states in polyatomic molecules. Displacements, distortions, and Duschinsky rotation of potential energy surfaces are included within the framework of a multidimensional harmonic oscillator model. The Herzberg-Teller (HT) effect is also taken into account. This formalism gives a reliable description of the Q(x) spectral band of free-base porphyrin with weakly electric dipole-allowed transitions. For the strongly dipole-allowed transitions, e.g., S(1) --> S(0) and S(0) --> S(1) of linear polyacenes, anthracene, tetracene, and pentacene, the HT effect is found to enhance the radiative decay rates by approximately 10% compared to those without the HT effect. For nonradiative transition processes, a general formalism is presented to extend the application scope of the internal conversion theory by going beyond the promoting-mode approximation. Numerical calculations for the nonradiative S(1) --> S(0) decay rate of azulene well explain the origin of the violation of Kasha's rule. When coupled with first-principles density functional theory (DFT) calculations, the present approach appears to be an effective tool to obtain a quantitative description and detailed understanding of spectra and photophysical processes in polyatomic molecules.

Hydrogen-bonded dimer stacking induced emission of aminobenzoic acid compounds

A series of aminobenzoic acid crystals with stacking-induced emission properties has been achieved and the packing structures of the hydrogen-bonded acid dimers provided an explanation for the emission characteristics of the crystals.

Morphology transition and aggregation-induced emission of an intramolecular charge-transfer compound

An intramolecular charge-transfer (ICT) compound, (TCBD)2OPV3, has been synthesized and fabricated into one-dimensional nanotubes by a reprecipitation method. The observation of SEM and TEM showed the nanotubes were formed from their zero-dimensional precursors of hollow nanospheres. Reconstruction was found to happen during the morphology transition progress. The morphology transition and reconstruction are proposed to be a "curvature strain releasing" process driven by donor-acceptor dipole-dipole interactions. An aggregation-induced emission (AIE) effect was observed for (TCBD)2OPV3. Both aggregates of vesicles and nanotubes were observed to be good red emitters with near-infrared end emission of 750-850 nm, which endows the material with potential applications in the fields of optical devices, biosensors, and biolabels.

Protein detection and quantitation by tetraphenylethene-based fluorescent probes with aggregation-induced emission characteristics

Three functionalized derivatives of tetraphenylethylene (TPE), namely, 1,2-bis(4-methoxyphenyl)-1,2-diphenylethene (1), 1,2-bis(4-hydroxyphenyl)-1,2-diphenylethene (2), and 1,2-bis[4-(3-sulfonatopropoxyl)phenyl]-1,2-diphenylethene sodium salt (3), were synthesized and their fluorescence properties were investigated. All the TPE molecules are nonluminescent in the solution state but are induced to emit efficiently by aggregate formation. This novel process of aggregation-induced emission (AIE) is rationalized to be caused by the restriction of intramolecular rotations of the dye molecules in the aggregate state. The possibility of utilizing the AIE effect for protein detection and quantification is explored using bovine serum albumin (BSA) as a model protein, with salt 3 being found to perform as a stable, sensitive, and selective bioprobe.

Color-Tunable, Aggregation-Induced Emission of a Butterfly-Shaped Molecule Comprising a Pyran Skeleton and Two Cholesteryl Wings

A chiral pyran derivative containing two cholesteryl groups (1) is synthesized, and its optical properties are investigated. Whereas the isolated molecule of 1 is virtually nonluminescent in dilute solutions, it becomes highly emissive with a 2 orders of magnitude increase in fluorescence quantum yield upon aggregation in poor solvents or in solid state, showing a novel phenomenon of aggregation-induced emission (AIE). The color and efficiency of the AIE of 1 can be tuned by varying the morphology of its aggregates: photoluminescence of its aggregates formed in a tetrahydrofuran/water mixture progressively red-shifts (green --> yellow --> red) with increasing water content of the mixture, with the crystalline aggregates emitting bluer lights in higher efficiencies than their amorphous counterparts.

Switching the light emission of (4-biphenylyl)phenyldibenzofulvene by morphological modulation: crystallization-induced emission enhancement

(4-Biphenylyl)phenyldibenzofulvene is weakly luminescent in the amorphous phase but becomes highly emissive upon crystallization; this unusual crystallization-induced emission enhancement effect allows its emission to be repeatedly switched between dark and bright states by fuming-heating and heating-cooling processes.

Fluorescence enhancements of benzene-cored luminophors by restricted intramolecular rotations: AIE and AIEE effects

Photoluminescence of simple arylbenzenes with ready synthetic accessibility is enhanced by two orders of magnitude through aggregate formation; viscosity and temperature effects indicate that the emission enhancement is due to the restriction of their intramolecular rotations in the solid state.