Absorption and Fluorescence Features of an Amphiphilic meso-Pyrimidinylcorrole: Experimental Study and Quantum Chemical Calculations.

Corroles are emerging as an important class of macrocycles with numerous applications because of their peculiar photophysical and metal chelating properties. meso-Pyrimidinylcorroles are easily deprotonated in certain solvents, which changes their absorption and emission spectra as well as their accessible supramolecular structures. To enable control over the formation of supramolecular structures, the dominant corrole species, i.e., the deprotonated form or one of the two NH-tautomers, needs to be identified. Therefore, we focus in the present article on the determination of the UV-vis spectroscopic properties of the free-base NH-tautomers and the deprotonated form of a new amphiphilic meso-pyrimidinylcorrole that can assemble to supramolecular structures at heterointerfaces as utilized in the Langmuir-Blodgett and liquid-liquid interface precipitation techniques. After quantification of the polarities of the free-base NH-tautomers and the deprotonated form by means of quantum chemically derived electrostatic potential distributions at the corroles' van der Waals surfaces, the preferential stabilization of (some of) the considered species in solvents of different polarity is identified by means of absorption spectroscopy. For the solutions with complex mixtures of species, we applied fluorescence excitation spectroscopy to estimate the relative weights of the individual corrole species. This technique might also be applied to identify dominating species in molecularly thin films directly on the subphase' surface of Langmuir-Blodgett troughs. Supported by quantum chemical calculations we were able to differentiate between the spectral signatures of the individual NH-tautomers by means of fluorescence excitation spectroscopy.

Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin-Copper Corrole Donor-Acceptor Conjugates.

An electron-deficient copper(III) corrole was utilized for the construction of donor-acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump-probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge-separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron-deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10(10)  s(-1) and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems.

Surface-assisted dehydrogenative homocoupling of porphine molecules.

The templated synthesis of porphyrin dimers, oligomers, and tapes has recently attracted considerable interest. Here, we introduce a clean, temperature-induced covalent dehydrogenative coupling mechanism between unsubstituted free-base porphine units yielding dimers, trimers, and larger oligomers directly on a Ag(111) support under ultrahigh-vacuum conditions. Our multitechnique approach, including scanning tunneling microscopy, near-edge X-ray absorption fine structure and photoelectron spectroscopy complemented by theoretical modeling, allows a comprehensive characterization of the resulting nanostructures and sheds light on the coupling mechanism. We identify distinct coupling motifs and report a decrease of the electronic gap and a modification of the frontier orbitals directly associated with the formation of triply fused dimeric species. This new on-surface homocoupling protocol yields covalent porphyrin nanostructures addressable with submolecular resolution and provides prospective model systems towards the exploration of extended oligomers with tailored chemical and physical properties.

Molecular structures and absorption spectra assignment of corrole NH tautomers.

The individual absorption spectra of the two NH tautomers of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole are assigned on the basis of the Gouterman four-orbital model and a quantum chemical TD-DFT study. The assignment indicates that the red-shifted T1 tautomer is the one with protonated pyrrole nitrogen atoms N(21), N(22) and N(23), whereas the blue-shifted T2 tautomer has pyrrole nitrogen atoms N(21), N(22) and N(24) protonated. A wave-like nonplanar distortion of the macrocycle in the ground state is found for both NH tautomers, with the wave axis going through the pyrroles containing N(22) and N(24). The 7C plane determined by the least-squares distances to the carbon atoms C1, C4, C5, C6, C9, C16, and C19 is suggested as a mean corrole macrocycle plane for the analysis of out-of-plane distortions. The magnitude of these distortions is distinctly different for the two NH tautomers, leading to substantial perturbations of their acid-base properties, which are rationalized by the interplay of the degree of out-of-plane distortion of the macrocycle as a whole and the tendency of the pyrrole nitrogen atoms toward pyramidalization, with the former leading to a basicity increase whereas the latter enhances the acidity.

Linear and cyclic hybrids of alternating thiophene-amino acid units: synthesis and effects of chirality on conformation and molecular packing.

The dipeptide isostere 5-aminothiophene carboxylic acid has been combined with L-phenylalanine moieties to provide linear and cyclic hybrid oligopeptides. A suitable protecting group strategy and appropriate coupling methods have been developed to guarantee a high degree of enantiopurity of the resulting amides. Cyclic tetraamides have been efficiently obtained by macrocyclization of the linear derivatives. In the case of racemized cyclization precursors, two diastereomeric macrocycles (S,S/R,R and meso) have been isolated. Their crystal structures show clear effects of the stereogenic centers on the ring conformations and molecular packing.

Linear and cyclic amides with a thiophene backbone: ultrasound-promoted synthesis and crystal structures.

A full synthetic study of linear and cyclic thiophene oligoamides has been carried out. The combination of an ultrasonic technique to diminish the intramolecular backfolding of longer oligoamide chains, therefore enhancing the accessibility of the carboxylic acid, and T3P as coupling reagent led to shorter reaction time and higher yields for both linear and cyclic oligoamides. By controlling the degree of dilution, macrocyclic amides with different sizes can selectively be prepared. Different crystal structures of cyclic thiophene oligoamides were also analyzed.

Unraveling the fluorescence features of individual corrole NH tautomers.

The fluorescence spectra of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole have been studied in the temperature range from 4.2 to 332 K. For the first time, the individual fluorescence profiles of the two corrole NH tautomers have been assigned over the whole temperature range. The pronounced temperature dependence of the fluorescence spectra of the meso-pyrimidinylcorrole under study was found to originate from switching between the fluorescence emissions of the two tautomers due to a reduced NH tautomerization rate with decreasing temperature. As a result, the long wavelength tautomer dominates the total emission spectrum at room temperature, whereas at low temperatures, the majority of the emission comes from the short wavelength tautomer. Energy level diagrams (involving the two NH tautomers) explaining the excitation energy deactivation channels in the meso-pyrimidinylcorrole at room temperature and below are presented. A significant H/D isotope effect on the NH tautomerization rate has been observed, resulting in an enhanced contribution of the short wavelength tautomer to the total fluorescence spectrum at the expense of that of the long wavelength tautomer. Substantially different fluorescence quantum yields have been determined for the individual NH tautomers, leading to a pronounced temperature dependence of the overall fluorescence quantum yield. The obtained results allow the unambiguous statement that the two NH tautomers of corroles coexist in fluid and solid solutions in a wide range of temperatures, with the proportion depending on the corrole substitution pattern. Moreover, this study shows that the (future) interpretation of the fluorescence properties of meso-pyrimidinylcorroles and all other corrole materials should be done (more) carefully, taking into account the coexistence of NH tautomers with individual spectral signatures.

Solvent-dependent deprotonation of meso-pyrimidinylcorroles: absorption and fluorescence studies.

The absorption spectra of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole have been studied in 15 solvents. The formation of deprotonated corrole species was found to account for the dramatic changes in the absorption spectra in several solvents. Careful analysis of the relationship between the formation of deprotonated species and solvent properties results in the conclusion that there is no single solvent parameter correlation, and either multiparameter correlations or specific solute-solvent interactions (preferential solvation of the most acidic NH tautomer or perturbation of intramolecular hydrogen bonding in the macrocycle core) should be considered. The fluorescence properties of the deprotonated pyrimidinylcorrole are also reported for the first time and compared to those of free-base and protonated species.

Corrole NH tautomers: spectral features and individual protonation.

Protonation of a free-base meso-pyrimidinyl-substituted AB(2)-corrole (H(3)AB(2)) in ethanol solution by stepwise addition of sulfuric acid has been studied in the temperature range from 293 to 333 K. The formation rate of protonated species was found to depend profoundly on the temperature at which the titration was undertaken. Two steps in the titration curve were identified at temperatures around 293-298 K, whereas one-step formation of protonated species was found to occur at temperatures above 308 K. The protonation product was the same in both cases, i.e., H(4)AB(2)(+) corrole, protonated at the macrocycle core nitrogen atoms. The two steps in the protonation kinetics at lower temperatures were attributed to protonation of individual tautomers of the free-base H(3)AB(2) corrole. To the best of our knowledge, this is the first well-illustrated (spectrophotometric) observation of individual properties of corrole NH tautomers in fluid solution. Concomitant increase in the NH tautomerization rate with increasing temperature is proposed to account for the one-step protonation. Evidences for the role of individual corrole NH tautomers in the protonation process as well as their optical features are discussed based on spectroscopic results and simulation data.

Luminescence of meso-pyrimidinylcorroles: relationship with substitution pattern and heavy atom effects.

The luminescence properties of a series of corroles containing multiple meso-pyrimidinyl groups have been studied. In particular, nine corroles containing two pyrimidinyl moieties, four corroles containing three pyrimidinyl groups and one corrole carrying a single pyrimidinyl substituent have been investigated, and their properties have been compared with some model species. The results indicate that the energy of the emissive π-π* corrole-core-based state is not significantly perturbed by the various substituents, whereas the emission lifetimes and quantum yields depend on the number of appended meso-dichloropyrimidinyl substituents. In particular, both emission lifetime and quantum yield decrease with increasing the number of meso-dichloropyrimidinyl substituents, whereas pyrimidinyl substituents which do not carry further electron withdrawing groups such as the chlorine atoms do not affect the corrole emission properties. Two hypotheses are taken into consideration to rationalize the results: (i) the presence of meso-dichloropyrimidinyl substituents could introduce low-lying CT states which mix with the corrole π-π* emissive level, so reducing emission efficiency; (ii) the ortho,ortho'-chlorine groups of the meso-pyrimidinyl substituents, which lie in the proximity of the corrole macrocycle, can increase the intersystem crossing rate constant of the corrole-based fluorescent state via the heavy-atom effect. A comparison of the results of the studied corrole compounds with formerly investigated species and the linear dependence of the radiationless decay rate constants and emission quantum yields on the squared spin-orbit coupling constants, calculated considering the number of chlorine atoms in ortho-position of the corrole meso-substituents, suggest that hypothesis (ii) is most likely the valid one. These results are of particular interest for the design of (a) corrole compounds featuring highly efficient triplet formation and (b) multicomponent systems containing photo-active corrole subunits and pyrimidinyl spacers.

Synthetic, structural, and photophysical exploration of meso-pyrimidinyl-substituted AB2-corroles.

meso-Pyrimidinyl-substituted AB(2)-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 (BF(3).OEt(2)). 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 AB(2)-pyrimidinylcorroles were, however, readily accessible by the reversible sequence Cu insertion and subsequent reductive demetalation. AB(2)-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 N-Cu distances and a saddled corrole plane. The absorption spectra and photophysical properties of some representative free-base AB(2)-pyrimidinylcorroles were examined in depth. The absorption spectra displayed typical corrole features: intense spin-allowed pi-pi* 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 pi-pi* 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.

meso-Pyrimidinyl-substituted A2B- and A3-corroles.

A variety of meso-pyrimidinyl-substituted A(2)B- and A(3)-corroles (A = 4,6-dichloropyrimidin-5-yl) have been synthesized by careful optimization of the macrocyclization conditions. meso-Pyrimidinylcorroles offer the distinct advantage of an unprecedented broad scope of functionalization options. Highly sterically encumbered triarylcorroles were readily prepared via efficient nucleophilic aromatic substitution and Suzuki cross-coupling procedures.

meso-pyrimidinyl-substituted A2B-corroles.

meso-Pyrimidinyl-substituted A2B-corroles were synthesized in good yields by condensation of 5-mesityldipyrromethane and 2-substituted 4,6-dichloropyrimidine-5-carbaldehydes. A simple reduction of the amount of Lewis acid (BF3.OEt2) resulted in the formation of A2B-corroles, which was optimized to maximize the corrole yield. Nucleophilic aromatic substitution, Suzuki, and Stille cross-coupling reactions were performed on the Cu-metalated pyrimidinylcorroles to obtain sterically encumbered triarylcorroles, while the substitution pattern at the 2-position of the pyrimidinyl substituent was altered through Liebeskind-Srogl cross-couplings.