Multicomponent Syntheses of Fluorophores Initiated by Metal Catalysis

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.

Heterocycles by Consecutive Multicomponent Syntheses via Catalytically Generated Alkynoyl Intermediates

Multicomponent processes are beneficial tools for the synthesis of heterocycles. As densely substituted bifunctional electrophiles, ynones are essential intermediates by applying cyclocondensations or cycloadditions in numerous heterocycle syntheses. The respective alkynoyl intermediates are generally accessible by palladium-, copper- and palladium/copper-catalyzed alkynylation. In turn, the mild reaction conditions allow for a fast and versatile entry to functional heterocycles in the sense of consecutive multicomponent processes. This review collates and presents recent advances in accessing thirteen heterocycle classes and their applications by virtue of catalytic alkynoyl generation in diversity-oriented multicomponent syntheses in a one-pot fashion.

Three-Component Suzuki-Knoevenagel Synthesis of Merocyanine Libraries and Correlation Analyses of Their Oxidation Potentials and Optical Band Gaps

The Suzuki coupling Knoevenagel condensation one-pot synthesis of boronic acids/esters, (hetero)aromatic bromo aldehydes and methylene active compounds is a highly practical consecutive three-component process to provide substance libraries with 60 donor-π-bridge-acceptor molecules, i.e., merocyanines in a broader sense, in moderate to excellent yield. As already seen with the naked eye, a broad variation of the optical properties becomes accessible using this practical synthetic tool. More systematically, correlation analyses upon plotting the optical band gaps against the first oxidation potentials of redox active systems of consanguineous series furnishes linear correlations and, by extension, two parameter plots (oxidation potential and emission maximum) planar correlations with the optical band gaps.

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.

Unimolecular Exciplexes by Ugi Four-Component Reaction

Exciplex or excited complex emission is an excited state process, arising from considerable charge transfer of an excited energy donor to an acceptor, which can be identified by the occurrence of a redshifted emission band that is absent in the individual constituents. Particularly interesting are exciplexes that are formed by intramolecular excited state interaction, which are inherently concentration independent. Based upon our previous experience in the Ugi-4CR syntheses of donor-acceptor conjugates capable of photo-induced intramolecular electron transfer (PIET), that is, generation of light-induced charge separation, we now disclose the diversity-oriented approach on unimolecular exciplex emitters and their reference systems by Ugi-4CR. The photophysics is studied by absorption and emission spectroscopy and accompanied by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations.

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.

3-Formylpyrazolo[1,5- a]pyrimidines as Key Intermediates for the Preparation of Functional Fluorophores

A one-pot route for the regioselective synthesis of 3-formylpyrazolo[1,5- a]pyrimidines 4a-k in good yields through a microwave-assisted process is provided. The synthesis proceeds via a cyclocondensation reaction between β-enaminones 1 with NH-3-aminopyrazoles 2, followed by formylation with an iminium salt moiety (Vilsmeyer-Haack reagent). These N-heteroaryl aldehydes 4 were successfully used as strategic intermediates for the preparation of novel functional fluorophores with yields up to 98%. The structures of the products obtained and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction analysis. Since pyrazolo[1,5- a]pyrimidines (PPs) 3 have shown an important fluorescence, photophysical properties of four 2-methylderivatives substituted at position 7 with different acceptor (A) or donor (D) groups were investigated. The compounds evaluated exhibited large Stokes shift in different solvents, but only the substituted p-methoxyphenyl (4-An) showed a strong fluorescence intensity with quantum yields up to 44% due to its greater ICT. Therefore, hybrid systems based on pyrazolo[1,5- a]pyrimidines could be used as fluorescent probes to detect biologically or environmentally relevant species.

De Novo Ring-Forming Consecutive Four-Component Syntheses of 4-Pyrazolyl-1,2,3-triazoles from (Triisopropylsilyl)butadiyne as a C4 Building Block

4-Pyrazolyl-1,2,3-triazoles can be readily synthesized in a one-pot fashion and moderate yield by employing a consecutive four-component process consisting of a sequentially Pd-Cu-catalyzed alkynylation-cyclocondensation-desilylation-CuAAC process. Most distinctly, (triisopropylsilyl)butadiyne is employed as a four-carbon building block in this one-pot de novo formation of both heterocyclic moieties.

One-pot syntheses of blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones by T3P® activation of 3-arylpropiolic acids

In situ activation of 3-arylpropiolic acids with T3P^® (n-propylphosphonic acid anhydride) initiates a domino reaction furnishing 4-arylnaphtho[2,3-c]furan-1,3-diones in excellent yields. Upon employing these anhydrides as reactive intermediates blue-luminescent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones are formed by consecutive pseudo three-component syntheses in a one-pot fashion. The Stokes shifts correlate excellently with the Hammett-Taft σ_R parameter indicating an extended degree of resonance stabilization in the vibrationally relaxed excited singlet state.