A synthetic route towards homodiselenacalix[4]arene macrocycles is presented, based on the dynamic covalent chemistry of diselenides. The calixarene inner rim is decorated with either alkoxy or tert-butyl ester groups. Single-crystal X-ray analysis of two THF solvates with methoxy and ethoxy substituents reveals the high similarity of their molecular structures and alterations on the supramolecular level. In both crystal structures, solvent channels are present and differ in both shape and capacity. Furthermore, the methoxy-substituted macrocycle undergoes a single-crystal-to-single-crystal transformation during which the molecular structure changes its conformation from 1,3-alternate (loaded with THF/water) to 1,2-alternate (apohost form). Molecular modelling techniques were applied to explore the conformational and energetic behaviour of the macrocycles.

Functionalized 1,2,3-triazole heterocycles have been known for a long time and hold an extraordinary potential in diverse research areas ranging from medicinal chemistry to material science. However, the scope of therapeutically important 1-substituted 4-acyl-1H-1,2,3-triazoles is much less explored, probably due to the lack of synthetic methodologies of good scope and practicality. Here, we describe a practical and efficient one-pot multicomponent reaction for the synthesis of α-ketotriazoles from readily available building blocks such as methyl ketones, N,N-dimethylformamide dimethyl acetal, and organic azides with 100 % regioselectivity. This reaction is enabled by the in situ formation of an enaminone intermediate followed by its 1,3-dipolar cycloaddition reaction with an organic azide. We effectively utilized the developed strategy for the derivatization of various heterocycles and natural products, a protocol which is difficult or impossible to realize by other means.

Facile synthesis of fused 1,2,3-triazoles by a proline-catalyzed reaction of an azido aldehyde and a nitroalkane is elaborated. The present tandem protocol proceeds via an organocatalytic Knoevenagel condensation of the azido aldehyde and nitroalkane followed by intramolecular azide–nitroalkene cycloaddition. The functionalized bicyclic triazole is obtained by elimination of HNO₂ from the cycloadduct. Application of this strategy enabled us to synthesize a range of functionalised 5–7 membered ring fused triazoles. The reaction calls for mild conditions, affords high yields, and results in good regiospecificity while displaying excellent substrate scope.

We describe herein the first radical CH arylation of BODIPY dyes. This novel, general, one-step synthetic procedure uses ferrocene to generate aryl radical species from aryldiazonium salts and allows the straightforward synthesis of brightly fluorescent (Φ>0.85) 3,5-diarylated and 3-monoarylated boron dipyrrins in up to 86 % yield for a broad range of aryl substituents. In this way, new and complex dyes with red-shifted spectra can be easily prepared.

We describe herein the first radical CH arylation of BODIPY dyes. This novel, general, one-step synthetic procedure uses ferrocene to generate aryl radical species from aryldiazonium salts and allows the straightforward synthesis of brightly fluorescent (Φ>0.85) 3,5-diarylated and 3-monoarylated boron dipyrrins in up to 86 % yield for a broad range of aryl substituents. In this way, new and complex dyes with red-shifted spectra can be easily prepared.

A one-step synthetic procedure for the radical CH alkylation of BODIPY dyes has been developed. This new reaction generates alkyl radicals through the oxidation of boronic acids or potassium trifluoroborates and allows the synthesis of mono-, di-, tri-, and tetraalkylated fluorophores in a good to excellent yield for a broad range of organoboron compounds. Using this protocol, multiple bulky alkyl groups can be introduced onto the BODIPY core thus creating solid-state emissive BODIPY dyes.

In the present study, we have investigated different strategies for diastereoselective synthesis of thia[n]helicenes. We describe the introduction of different chiral auxiliaries at various positions and investigated their effect in the photocyclization reaction. Different chiral groups were placed at the sterically hindered position of the helical core and their interactions with various solvents and metals like copper were investigated. The use of Cu^I salts has led to high diastereoselectivity in the photocyclization process and we were successful in obtaining the thia[5]helicene in enantiomerically pure form in good yield. The single diastereomer obtained was characterized by X-ray crystallography. From the study of the barrier of racemization of these thia[5]helicenes, the stability was found to be comparable to unsubstituted tetrathia[7]helicenes and substituted diazadithia[7]helicenes. This approach provides an easy access to enantiopure helicenes.

Several dendritic zinc(II)-porphyrins bearing carbazole units at the terminals have been prepared through click reaction of azide-substituted Zn-porphyrin precursors and carbazole-based alkynes under [Cu(NCCH₃)₄][PF₆] catalysis. This family of new dendritic metalloporphyrins shows dual luminescence from both the upper S₂ and the lowest S₁ singlet states. The observed trends in the spectroscopic data and the photophysical properties of the dendrimers have been rationalized in terms of the type of meso-spacer between the macrocycle and dendritic shell. The key feature of the meso-spacer is proposed to be the degree of hindrance towards aryl ring rotation relative to the mean porphyrin plane. Based on the observed differences in the S₁S₀ fluorescence quantum yield on going from four- to five-coordinate dendrimers, it was shown that dendrimer architectures with all four meso-aryl spacers sterically hindered, are most appropriate to monitor processes related to axial ligation of the dendrimer core, since they provide larger luminescence response differences between the four-coordinate and five-coordinate forms. The blue S₂S₀ fluorescence quantum yield has been measured and the observed trend has been rationalized in terms of the S₂S₁ energy gap law. No significant differences were observed between the compounds either with different rotational degree of freedom of the meso-spacers or with different ligation states of the dendrimer core.

A metal-free three-component reaction to synthesize 1,4,5-trisubstituted 1,2,3-triazoles from readily available building blocks, such as aldehydes, nitroalkanes, and organic azides, is described. The process is enabled by an organocatalyzed Knoevenagel condensation of the formyl group with the nitro compound, which is followed by the 1,3-dipolar cycloaddition of the azide to the activated alkene. The reaction features an excellent substrate scope, and the products are obtained with high yield and regioselectivity. This method can be utilized for the synthesis of fused triazole heterocycles and materials with several triazole moieties.

A metal-free three-component reaction to synthesize 1,4,5-trisubstituted 1,2,3-triazoles from readily available building blocks, such as aldehydes, nitroalkanes, and organic azides, is described. The process is enabled by an organocatalyzed Knoevenagel condensation of the formyl group with the nitro compound, which is followed by the 1,3-dipolar cycloaddition of the azide to the activated alkene. The reaction features an excellent substrate scope, and the products are obtained with high yield and regioselectivity. This method can be utilized for the synthesis of fused triazole heterocycles and materials with several triazole moieties.

Selenacalix[3]triazines, cyclotrimeric metacyclophanes with direct Se linkages between the heteroaryl constituents, were shown to associate with various guest species. The preorganization of three electron deficient triazine rings allows for anions to bind through anion-π interactions, and alignment of the central nitrogen lone pairs and the well-defined size of the macroring enable association with a single proton. Extended UV/Vis titration studies indicated a clear difference in complexation behavior depending on the outer-rim substitution pattern. The host–guest properties of analogous selena- and thiacalix[3]triazines were found to be notably different.

Diazadithia[7]helicenes were synthesized from the readily available building block ethyl 7-chloro-8-formylthieno[3,2-f]quinoline-2-carboxylate by a Wittig reaction–photocyclization strategy. The helicene core was functionalized by nucleophilic aromatic substitution with a variety of nucleophiles (e.g., O-, N-, and C-centered) and palladium-catalyzed reactions such as Suzuki coupling and Buchwald–Hartwig amination. Racemization studies confirmed that the enantiopure forms of these [7]helicenes are conformationally stable compared to their lower analogues. The solid-state structures of the novel diazadithia[7]helicenes were determined by single-crystal X-ray diffraction. The crystal structures of these azathia[7]helicenes show columnar stacking in antiparallel fashion. The HOMO–LUMO gaps of the new compounds were determined on the basis of electrochemical and optical measurements.

Starting from appropriately halogenated acylpyrroles, a wide range of reactive 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives can be obtained. The fluorescent dyes display excellent reactivity in nucleophilic aromatic substitution, transition-metal-catalyzed cross-coupling and copper-catalyzed cycloaddition to azides (click chemistry). The spectral properties of the starting and final difluoroboron dipyrromethenes are discussed as a function of their structure.

The synthesis and energy-transfer properties of a series of oligo(p-phenylene ethynylene)–BODIPY (OPEB) cassettes are reported. A series of oligo(p-phenylene ethynylene)s (OPEs) with different conjugated chain lengths as energy donor subunit in the energy-transfer system were capped at both ends with BODIPY chromophores as energy-acceptor subunits. The effect of the conjugated chain of OPEs on energy transfer in the OPEB cassettes was investigated by UV/Vis and fluorescence spectroscopy and modeling. With increasing number n of phenyl acetylene units (n=1–7), the absorption and emission maxima of OPEn are bathochromically shifted. In the OPEBn analogues, the absorption maximum assigned to the BODIPY moieties is independent of the length of the OPE spacer. However, the relative absorption intensity of the BODIPY band decreases when the number of phenyl acetylene units is increased. The emission spectra of OPEBn are dominated by a band peaking at 613 nm, corresponding to emission of the BODIPY moieties, regardless of whether excitation is at 420 or 550 nm. Furthermore, a very small band is observed with a maximum between 450 and 500 nm, and its intensity relative to that of the BODIPY emission increases with increasing n, that is, the excited state of OPE subunits is efficiently quenched in OPEBn by energy transfer to the BODIPY moieties. Energy transfer (ET) from OPEn to BODIPY in OPEBn is very efficient (all Φ_ET values are greater than 98 %) and only slightly decreases with increasing length of the OPE units. These results are supported by theoretical studies that show very high energy transfer efficiency (Φ_ET>75 %) from the OPE spacer to the BODIPY end-groups for chains with up to 15–20 units.

Homothiacalix[n]arenes have been largely underexposed compared with related (homo)heteracalixarenes, although their inherent structural features are particularly attractive for supramolecular host–guest chemistry. In this contribution, the synthetic macrocyclization protocols that afford homothiacalix[n]arenes have been reinvestigated and optimized, providing straightforward access to the parent homothiacalix[4]arene skeleton. Moreover, inner-rim (bis and tetrakis) ester functionalization and dimethylenethia bridge oxidation were successfully performed as well. Solution-phase (variable-temperature) NMR spectroscopy studies and solid-state X-ray structures provided complementary information on the conformational features of the novel macrocycles.

Oxacalix[2]arene[2]quinazoline macrocycles were prepared in good yields, as mixtures of syn and anti isomers, through nucleophilic aromatic substitution cyclocondensation reactions of 2,4-dichloroquinazolines and m-dihydroxybenzenes. The macrocyclization conditions were optimized and the isomeric ratio was investigated by means of one-step and fragment-coupling approaches. The oxacalixarene substitution pattern could easily be varied by altering the dichloroquinazolinyl biselectrophilic and dihydroxyaryl bisnucleophilic building blocks. The solid-state (1,3-alternate) conformational behavior and the oxacalix[4]arene cavity size were explored by X-ray diffraction studies.

meso-Indolocarbazolylporphyrins endowed with a different number of indolocarbazole units have been synthesized via condensation of an appropriately substituted monoformylated 5,11-dihydroindolo[3,2-b]carbazole precursor and mesityldipyrromethane. Under specific conditions, analogous meso-indolocarbazolylcorroles could also be prepared. The photophysical features of the novel luminescent free-base and Zn-porphyrin derivatives were investigated. The introduction of indolocarbazole substituents results in progressive bathochromic shifts of the porphyrin absorbance and fluorescence bands due to the rising energy of the a_2u orbital. The excitation energy is efficiently transferred from the meso-indolocarbazole units to the porphyrin macrocycle. An increased number of indolocarbazole moieties does not lead to porphyrin fluorescence quenching; on the contrary, a small increase of the fluorescence quantum yield is observed. The main route for excitation energy deactivation of all the studied porphyrins is intersystem S₁T₁ crossing, with the intersystem crossing quantum yield, as determined by the photosensitized formation of singlet molecular oxygen, being as high as about 70 % for the free-bases and more than 80 % for the Zn complexes. The intersystem crossing quantum yield seems to be barely affected by meso-indolocarbazole substitution. A noticeable part of the excitation energy was found to deactivate through radiationless internal S₁S₀ conversion.

meso-Pyrimidinyl-substituted AB₂-corroles were efficiently synthesized starting from 5-mesityldipyrromethane and various 2-substituted 4,6-dichloropyrimidine-5-carbaldehydes. The corrole yield was significantly enhanced by optimization of the amount of Lewis acid catalyst (BF₃⋅OEt₂). The main advantage of pyrimidinylcorroles over other meso-triarylcorroles is their wide range of functionalization possibilities, which has been explored by nucleophilic and electrophilic aromatic substitution, and Pd-catalyzed cross-coupling reactions. Stepwise substitution of the chlorine functions afforded asymmetrically substituted pyrimidinylcorroles. Due to the lability of the free-base corrole macrocycles, functionalization of the corrole periphery was preferentially performed on the Cu-metalated counterparts. Functionalized free-base AB₂-pyrimidinylcorroles were, however, readily accessible by the reversible sequence Cu insertion and subsequent reductive demetalation. AB₂-pyrimidinylcorroles can hence be regarded as highly versatile platforms towards more sophisticated corrole systems. X-ray analysis of a bis(4-tert-butylphenoxy)-substituted Cu–pyrimidinylcorrole showed the typical features of a Cu–corrole: short NCu distances and a saddled corrole plane. The absorption spectra and photophysical properties of some representative free-base AB₂-pyrimidinylcorroles were examined in depth. The absorption spectra displayed typical corrole features: intense spin-allowed π–π* bands, which can be classified as Soret- and Q-type bands. The photophysical properties, investigated both in fluid solution at room temperature and in rigid matrix at 77 K, were governed by the lowest-lying π–π* singlet state; however, in most cases, a state with partial charge-transfer character (from the corrole ring to the pyrimidinyl group) was proposed to contribute to the dynamic properties of the emissive level.

This Microreview presents a concise overview of the different synthetic approaches that have been applied to porphyrin dendrimers over the years, with the porphyrin macrocycle(s) being embedded variously as the dendritic cores, at the peripheries, within the branching units, or spread throughout entire dendritic architectures. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

meso-Dichloropyrimidyl-BODIPY dyes are readily substituted by nucleophiles or through palladium-catalysed coupling reactions, while retaining excellent quantum yields of fluorescence. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)