In this review article, we summarize our recent progress in rational structure modification and spectroscopic studies of artificial porphyrinoids. Structural modifications, including meso aryl substitution, peripheral aromatic ring fusion, ring contraction and expansion, and core modification with heterocyclic subunits other than pyrrole, perturb the electronic structures and aromaticity of porphyrinoids, thus regulating their electronic properties. The relationship between structure and spectroscopic properties as well as the applications of these porphyrinoids are also reviewed.

Zwei stabile, kernmodifizierte Rubyrine mit einer oder zwei Diethienylethen(DTE)-Einheiten (1 und 2) wurden hergestellt. Das Rubyrin 1 mit einer “geschlossenen” DTE-Einheit zeigt 26 π-Aromatizität, während bei Einführung einer “offenen” DTE-Einheit in den Makrocyclus 2 dieser sich als nichtaromatische Verbindung verhält.

Two stable core-modified rubyrins bearing one and two dithienylethene (DTE) units (1 and 2) have been synthesized. With one “closed-form” DTE unit, 1 shows aromaticity associated with its conjugated circuit of 26 π-electrons. In contrast, rubyrin 2 containing one “open-form” DTE unit has nonaromatic properties.

A 2,6-distyryl-substituted boradiazaindacene (BODIPY) dye and a new series of 2,6-p-dimethylaminostyrene isomers containing both α- and β-position styryl substituents were synthesized by reacting styrene and p-dimethylaminostyrene with an electron-rich diiodo-BODIPY. The dyes were characterized by X-ray crystallography and NMR spectroscopy and their photophysical properties were investigated and analyzed by carrying out a series of theoretical calculations. The absorption spectra contain markedly redshifted absorbance bands due to conjugation between the styryl moieties and the main BODIPY fluorophore. Very low fluorescence quantum yields and significant Stokes shifts are observed for 2,6-distyryl-substituted BODIPYs, relative to analogous 3,5-distyryl- and 1,7-distyryl-substituted BODIPYs. Although the fluorescence of the compound with β-position styryl substituents on both pyrrole moieties and one with both β- and α-position substituents was completely quenched, the compound with only α-position substituents exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane. Protonation studies have demonstrated that these 2,6-p-dimethylaminostyrene isomers can be used as sensors for changes in pH. Theoretical calculations provide strong evidence that styryl rotation and the formation of non-emissive charge-separated S₁ states play a pivotal role in shaping the fluorescence properties of these dyes. Molecular orbital theory is used as a conceptual framework to describe the electronic structures of the BODIPY core and an analysis of the angular nodal patterns provides a reasonable explanation for why the introduction of substituents at different positions on the BODIPY core has markedly differing effects.

Boron dipyrromethenes (BODIPYs) with bulky triphenylsilylphenyl(ethynyl) and triphenylsilylphenyl substituents on pyrrole sites were prepared via Hagihara–Sonogashira and Suzuki–Miyaura cross-coupling with ethynyl-terminated tetraphenylsilane and boronic acid-terminated tetraphenylsilane. The chromophores are designed to prevent intermolecular π–π stacking interaction and enhance fluorescence in the solid state. Single crystals of 1 a and 2 b for X-ray structural analysis were obtained, and weak π–π stacking interactions of the neighboring BODIPY molecules were observed. Spectroscopic properties of all of the dyes in various solvents and in films were investigated. Triphenylsilylphenyl-substituted BODIPYs generally show more pronounced increases in solid-state emission than triphenylsilylphenyl(ethynyl)-substituted BODIPYs. Although the simple BODIPYs do not exhibit any fluorescence in the solid state (Φ=0), arylsilyl-substituted BODIPYs exhibit weak to moderate solid-state fluorescence with quantum yields of 0.03, 0.07, 0.10, and 0.25. The structure–property relationships were analyzed on the basis of X-ray crystallography, optical spectroscopy, cyclic voltammetry, and theoretical calculations.

The synthesis of a series of novel core-modified and fused-ring-expanded tetraphenylporphyrins is reported. Theoretical calculations and magnetic circular dichroism (MCD) and fluorescence spectroscopic measurements were used to analyze the effect of core modification with Group 16 oxygen, sulfur, selenium, and tellurium atoms on the optical properties and electronic structures of the porphyrins. Marked redshifts of the Q and B bands and accelerated intersystem-crossing rates were observed, thus making these compounds potentially suitable for use in a variety of applications. The scope for further fine-tuning of these optical properties based on additional structural modifications, such as the incorporation of fused benzene rings to form ABAB structures by using a thiophene precursor with a fused bicyclo[2.2.2]octadiene ring and the introduction of various substituents onto the meso-phenyl rings, is also examined.

The synthesis of [14]triphyrin(2.1.1) compounds is described. In contrast with conventional subporphyrins, which consistently contain a central boron atom, free-base heteroaromatic compounds can be formed. A modified Lindsey method was used to prepare a range of different [14]triphyrins(2.1.1) in yields of up to 35 % based on the reaction of diethylpyrrole (1 a) and fused pyrroles of bicyclo[2.2.2]octadiene (BCOD) (2 a–e) and dihydroethanonaphthalene (4 a) with various aryl aldehydes. The concentration of BF₃⋅OEt₂ catalyst plays the key role in determining the yield of the [14]triphyrin(2.1.1) macrocycle relative to the conventional tetrapyrrole porphyrin product. Retro-Diels–Alder reactions of 2 a–e and 4 a result in the formation of [14]tribenzotriphyrin (2.1.1) (3 a–e) and [14]trinaphthotriphyrin(2.1.1) (5 a). The effects of exocyclic ring annulation on the electronic structure are examined in detail based on optical spectroscopy, theoretical calculations, and electrochemical measurements. The availability of free-base compounds enables the formation of [Re^I(CO)₃(triphyrin)] (6 a) and [Ru^II(CO)₂Cl(triphyrin)] (7 a) complexes based on a modified retro-Diels–Alder reaction. X-ray structures are reported for 4 a and 6 a.

The successful synthesis of tetraphenyltetraphenanthroporphyrin (TPTPhenP; 5 a) in 2006 under modified Rothemund–Lindsey conditions yielded a tetraphenyl porphyrinoid with a B band redshifted to an unprecedented 576 nm. Radially symmetric fused-ring expansion of tetraphenylporphyrin with phenanthrene moieties results in very deep saddling due to steric crowding and very marked redshifts of the Q and B (or Soret) porphyrinoid absorption bands. The extent to which the TPTPhenP structure can be further modified is explored, and the optical properties of TPTPhenPs are analyzed based on a perimeter model approach that makes use of time-dependent DFT calculations and magnetic circular dichroism spectroscopy and also based on a detailed analysis of the fluorescence emission. Attempts to introduce substituents at the ortho and meta positions of the meso-phenyl groups and to insert a central metal proved unsuccessful. The synthesis of a series of TPTPhenPs with strong electron-withdrawing (CN, NO₂) and -donating (CH₃, N(CH₃)₂) substituents at the para positions of the meso-phenyl rings is reported. Marked redshifts of the main spectral bands were consistently observed. The most pronounced spectral changes were observed with N(CH₃)₂ groups (5 i) due to a marked destabilization of the HOMO, which has large MO coefficients on the meso-carbon atoms. Protonation of 5 i at both the ligand core and at the N(CH₃)₂ groups resulted in unprecedented Q₀₀ band absorption at wavelengths greater than 1200 nm.

A series of fused-ring-expanded aza-boradiazaindacene (aza-BODIPY) dyes have been synthesized by reacting arylmagnesium bromides with phthalonitriles or naphthalenedicarbonitriles. An analysis of the structure–property relationships has been carried out based on X-ray crystallography, optical spectroscopy, and theoretical calculations. Benzo and 1,2-naphtho-fused 3,5-diaryl aza-BODIPY dyes display markedly red shifted absorption and emission bands in the near-IR region (>700 nm) due to changes in the energies of the frontier MOs relative to those of 1,3,5,7-tetraaryl aza-BODIPYs. Only one 1,2-naphtho-fused aza-BODIPY of the three possible isomers is formed due to steric effects, and 2,3-naphtho-fused compounds could not be characterized because the final BF₂ complexes are unstable in solution. The incorporation of a N(CH₃)₂ group at the para-positions of a benzo-fused 3,5-diaryl aza-BODIPY quenches the fluorescence in polar solvents and results in a ratiometric pH response, which could be used in future practical applications as an NIR “turn-on” fluorescence sensor.

A new series of boron–dipyrromethene (BDP, BODIPY) dyes with dihydronaphthalene units fused to the β-pyrrole positions (1 a–d, 2) has been synthesised and spectroscopically investigated. All the dyes, except pH-responsive 1 d in polar solvents, display intense emission between 550–700 nm. Compounds 1 a and 1 b with a hydrogen atom and a methyl group in the meso position of the BODIPY core show spectroscopic properties that are similar to those of rhodamine 101, thus rendering them potent alternatives to the positively charged rhodamine dyes as stains and labels for less polar environments or for the dyeing of latex beads. Compound 1 d, which carries an electron-donating 4-(dimethylamino)phenyl group in the meso position, shows dual fluorescence in solvents more polar than dibutyl ether and can act as a pH-responsive “light-up” probe for acidic pH. Correlation of the pK_a data of 1 d and several other meso-(4-dimethylanilino)-substituted BODIPY derivatives allowed us to draw conclusions on the influence of steric crowding at the meso position on the acidity of the aniline nitrogen atom. Preparation and investigation of 2, which carries a nitrogen instead of a carbon as the meso-bridgehead atom, suggests that the rules of colour tuning of BODIPYs as established so far have to be reassessed; for all the reported couples of meso-C- and meso-N-substituted BODIPYs, the exchange leads to pronounced redshifts of the spectra and reduced fluorescence quantum yields. For 2, when compared with 1 a, the opposite is found: negligible spectral shifts and enhanced fluorescence. Additional X-ray crystallographic analysis of 1 a and quantum chemical modelling of the title and related compounds employing density functional theory granted further insight into the features of such sterically crowded chromophores.