Synthesis and photophysics of diaza-crown ether-based bisporphyrins

Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores

So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. , We show here that thanks to of fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Incorporation of redox-inactive cations promotes iron catalyzed aerobic C-H oxidation at mild potentials

The synthesis and characterization of the Schiff base complexes Fe(ii) (2M) and Fe(iii)Cl (3M), where M is a K+ or Ba2+ ion incorporated into the ligand, are reported. The Fe(iii/ii) redox potentials are positively shifted by 440 mV (2K) and 640 mV (2Ba) compared to Fe(salen) (salen = N,N'-bis(salicylidene)ethylenediamine), and by 70 mV (3K) and 230 mV (3Ba) compared to Fe(Cl)(salen), which is likely due to an electrostatic effect (electric field) from the cation. The catalytic activity of 3M towards the aerobic oxidation of allylic C-H bonds was explored. Prior studies on iron salen complexes modified through conventional electron-donating or withdrawing substituents found that only the most oxidizing derivatives were competent catalysts. In contrast, the 3M complexes, which are significantly less oxidizing, are both active. Mechanistic studies comparing 3M to Fe(salen) derivatives indicate that the proximal cation contributes to the overall reactivity in the rate determining step. The cationic charge also inhibits oxidative deactivation through formation of the corresponding Fe2-μ-oxo complexes, which were isolated and characterized. This study demonstrates how non-redox active Lewis acidic cations in the secondary coordination sphere can be used to modify redox catalysts in order to operate at milder potentials with a minimal impact on the reactivity, an effect that was unattainable by tuning the catalyst through traditional substituent effects on the ligand.

Introducing the Staudinger phosphazene reaction to porphyrin chemistry

The syntheses, characterizations and transformations of three tetraphenylporphyrins derived from methoxymethylated benzaldehyde 3 are described. Benzaldehyde 3 reacted with pyrrole under Lewis acid catalysis to give dipyrromethane 4 which was used as precursor in porphyrin syntheses. Porphyrins 6, αα-7 and αβ-7 were obtained using conditions for sterically encumbered benzaldehydes, with αα-7 and αβ-7 being atropisomers. The methoxymethyl groups of 6, αα-7 and αβ-7 were transformed into bromomethyl substituents (porphyrins 8, αα-9 and αβ-9) which were easily modified by nucleophilic reaction with the azide anion. Porphyrin azide 10 was subjected to a Staudinger phosphazene formation with triphenylphosphine. Subsequent reaction of the porphyrin phosphazene 12 with carboxylic acids gave acetamide 13, benzamide 14, and ferrocene carboxamide 15, respectively. Kornblum oxidation of monobromomethyl porphyrin 8 gave the formyl derivative 16.

Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates

A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.

Syntheses and energy transfer in multiporphyrinic arrays self-assembled with hydrogen-bonding recognition groups and comparison with covalent steroidal models

A number of new porphyrins equipped with complementary triple hydrogen-bonding groups were synthesized in good yields. Self-assembly was investigated by NMR spectroscopy, dynamic light scattering (DLS), and atomic force microscopy (AFM). These artificial antenna systems were further characterized by stationary and time-resolved fluorescence techniques to investigate several yet unsolved questions on the mechanism of excitation energy transfer (EET) in supramolecular systems. For example, the photophysics of a simple D--U[triple chemical bond]P--A dyad was studied, in which donor D and acceptor A are ZnII- metalated and free-base porphyrins, respectively, and U (uracyl) and P (2,6-diacetamidopyridyl) are complementary hydrogen-bonding groups linked by flexible spacers. In this dyad, the EET occurs with about 20 % efficiency with a lifetime of 14 ps. Reversal of the nonsymmetric triple hydrogen-bonding groups to give a A--U[triple chemical bond]P--D construct results in an EET efficiency of about 25 % and a lifetime of 19 ps. Thus, there is a slight directionality of EET mediated by these asymmetric triple hydrogen-bonding units tethered to flexible spacers. In polymeric systems of the type P-D-P[triple chemical bond]U-A-U[triple chemical bond]P-D-P, or U-D-U[triple chemical bond]P-A-P[triple chemical bond]U-D-U, the EET efficiency doubles as each donor is flanked by two acceptors. Because doubling the probability of photon capture doubles the EET efficiency, there is no energy amplification, which is consistent with the "antenna effect". For these polymeric systems, AFM images and DLS data indicate large rodlike assemblies of a few hundred nanometers, whereas the components form much smaller aggregates under the same conditions. To understand the importance of the flexible hydrogen-bonding zipper, three different covalently bridged D-B-A molecules were synthesized in which the bridge B is a rigid steroidal system and the same ester chemistry was used to link the porphyrins to each end of the steroid. The geometry inferred from molecular modeling of D-B-A indicates geometric similarities between B and some conformations of the --P[triple chemical bond]U-- supramolecular bridge. Although the EET efficiency is a factor of two greater for the steroidal systems relative to the supramolecular dyads, the rate is 50-80 times slower, but still slightly faster than that predicted by Förster-type mechanisms. Circular dichrosim (CD) spectra provide a conformational sampling of the porphyrin groups appended on the steroidal skeleton, thus allowing an estimation of the orientation factor kappa for the transition dipole moments, which significantly affects the EET rate. We conclude that the flexible hydrogen-bonded linked systems are adaptive and have variable geometries with foldamers in which the D and A groups can approach well under 1 nm. In these folded conformations, a rapid EET process occurs, probably also involving a Dexter-type exchange mechanism, thus explaining the fast EET relative to the rigid steroidal compounds. This study predicts that it is indeed possible to build large supramolecular antennas and the component design and supramolecular dynamics are essential features that dictate EET rates and efficiencies.