Multicomponent and Domino Syntheses of AIE Chromophores

Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.

Fluorescent Donor-Acceptor Psoralen Cruciforms by Consecutive Suzuki-Suzuki and Sonogashira-Sonogashira One-Pot Syntheses

Two novel donor-acceptor cruciform topologies are efficiently synthesized by site-selective Suzuki-Suzuki and Sonogashira-Sonogashira multicomponent reactions starting from a bromo-triflato-functionalized psoralen scaffold. In addition to tunability of photophysical properties, such as absorption and emission, many derivatives possess partially high relative fluorescence quantum yields in solution and fluoresce strongly in the solid state. Additionally, the promising compounds show solvatochromism and acidochromic effects. In addition, 8-p-anisyl-5-p-cyanophenyl-substituted psoralen exhibits aggregation-induced emission properties. Experimentally (applying the Lippert-Mataga model) and computationally (TD-DFT calculations), the pronounced charge transfer character of the longest wavelength absorption band was confirmed.

Solid-State Emissive Aroyl-S,N-Ketene Acetals with Tunable Aggregation-Induced Emission Characteristics

N-Benzyl aroyl-S,N-ketene acetals can be readily synthesized by condensation of aroyl chlorides and N-benzyl 2-methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid-state emission and aggregation-induced emission characteristics. Varying the substituent from electron-donating to electron-withdrawing enables the tuning of the solid-state emission color from deep blue to red.

Emission solvatochromic, solid-state and aggregation-induced emissive α-pyrones and emission-tuneable 1H-pyridines by Michael addition-cyclocondensation sequences

Starting from substituted alkynones, α-pyrones and/or 1H-pyridines were generated in a Michael addition-cyclocondensation with ethyl cyanoacetate. The peculiar product formation depends on the reaction conditions as well as on the electronic substitution pattern of the alkynone. While electron-donating groups furnish α-pyrones as main products, electron-withdrawing groups predominantly give the corresponding 1H-pyridines. Both heterocycle classes fluoresce in solution and in the solid state. In particular, dimethylamino-substituted α-pyrones, as donor-acceptor systems, display remarkable photophysical properties, such as strongly red-shifted absorption and emission maxima with daylight fluorescence and fluorescence quantum yields up to 99% in solution and around 11% in the solid state, as well as pronounced emission solvatochromism. Also a donor-substituted α-pyrone shows pronounced aggregation-induced emission enhancement.

Diversity-Oriented Synthesis and Optical Properties of Bichromophoric Pyrrole-Fluorophore Conjugates

The mild reaction conditions of the palladium-copper coupling-isomerization reaction open a highly convergent, chromogenic route to blue emissive pyrroles in the sense of a consecutive four-component reaction. By virtue of this strategy a phenol derivative can be readily accessed, which can be transformed in a level-2 transformation to a library of bichromophoric pyrrol-fluorophore conjugates by facile alkylation with fluorophore halides. The photophysics of the underlying blue emitter derivative and the conjugates is studied by absorption and emission spectroscopy, furnishing intramolecular energy transfer at short distances as well as competing fluorescence quenching. In some cases partial energy transfer results in the occurrence of dual emission, for instance seen as magenta-rose emission arising from blue and red orange luminescence. The experimental photophysical studies are rationalized by DFT and TD-DFT calculations.

Three-Component Activation/Alkynylation/Cyclocondensation (AACC) Synthesis of Enhanced Emission Solvatochromic 3-Ethynylquinoxalines

2-Substituted 3-ethynylquinoxaline chromophores can be readily synthesized by a consecutive activation-alkynylation-cyclocondensation (AACC) one-pot sequence in a three-component manner. In comparison with the previously published four-component glyoxylation starting from electron-rich π-nucleophiles, the direct activation of (hetero)aryl glyoxylic acids allows the introduction of substituents that cannot be directly accessed by glyoxylation. By introducing N,N-dimethylaniline as a strong donor in the 2-position, the emission solvatochromicity of 3-ethynylquinoxalines can be considerably enhanced to cover the spectral range from blue-green to deep red-orange with a single chromophore in a relatively narrow polarity window. The diversity-oriented nature of the synthetic multicomponent reaction concept enables comprehensive investigations of structure-property relationships by Hammett correlations and Lippert-Mataga analysis, as well as the elucidation of the electronic structure of the emission solvatochromic π-conjugated donor-acceptor systems by DFT and time-dependent DFT calculations with the PBEh1PBE functional for a better reproduction of the dominant charge-transfer character of the longest wavelength absorption band.

Boron Complexes of Glyoxal-Derived Ugi Adducts as a New Class of Aggregation-Induced Emission Photoluminescent Materials

A series of O,O-chelated boron complexes was prepared through a four-component Ugi reaction followed by complexation of the resulting 1,3-dicarbonyl compounds with boron trifluoride diethyl etherate. The optical properties of these novel luminophores were investigated by UV/Vis spectroscopy and spectrofluorometry, revealing pronounced aggregation-induced emission (AIE) features.