Syntheses and Energy Transfer in Multiporphyrinic Arrays Self‐Assembled with Hydrogen‐Bonding Recognition Groups and Comparison with Covalent Steroidal Models

Abstract

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. ...

Self-Organized Porphyrinic Materials

“…This strategy allows the construction of materials with two different porphyrins, thereby increasing the diversity and the potential properties of the resultant materials 70,89. For example, the carboxy groups on two Zn(II)5-(4-carboxyphenyl)-10,15,20-(triphenyl)porphyrins combine with the amino groups of trans 1,2-diaminocyclohexane to form a ternary system organized by H-bonding and Coulombic interactions, which can accept various diamine guests96 When the recognition motifs are rigidly attached to the porphyrin, the formation of the linear or cyclic arrays depends on the positions of these motifs, but when flexible spacers tether the recognition motives, the topology is largely directed by the nature of the spacer 97. Triazines are a versatile scaffold for porphyrin arrays 45,69,98.…”

“…However, the synthesis of many of the porphyrins described in this section is synthetically challenging for several reasons, such as difficulties in preparing the precursor aldehydes and separation of statistical mixtures of compounds and isomers after condensation with the pyrrole. The synthesis of porphyrins with meso aryl groups appended with a saturated tether terminated in similar recognition motifs is significantly easier, but the dynamics of the tether result in the formation of non-discrete aggregates of varying sizes 97. Nonetheless, self-assembly of these systems into dimers and self-organization into nanoaggregates results in systems where electron transfer rates can be measured.…”

Self-assembly of iron TCPP (meso-tetra(4-carboxyphenyl)porphyrin) into a chiral 2D coordination polymer

“…This is the reason why synthetic metalloporphyrin complexes are often used as analogues of natural systems found in photosynthesis, oxygen carriers, and catalysts [5] [9][10][11][12][13][14]. For instance, these systems are used to understand the mechanism of electron transfer through intermolecular interactions, and some works show that efficient energy transfer may occur across the intermolecular bonds [15][16][17][18][19]. From the point of view of coordination chemistry, the dimensionality of metalloporphyrin frameworks is defined by the extension of a structural unit in one, two or the three dimensions of the space via coordination bonds.…”

Interfacially polymerized metalloporphyrin thin films for colorimetric sensing of organic vapors

“…Several methods of thin film assembly rely on electrostatic attraction of monomers to a substrate including alternating deposition of anionic and cationic species [8], reliance upon hydrogen bonding of monomers [9] and Langmuir-Blodgett film deposition [4,6,10,11]. While electrostatically deposited thin films have demonstrated predictable and selective sensing upon the initial exposure of the sensor to an analyte, electrostatic attractions are easily overcome by exposing films to a suitable solvent, potentially resulting in film dissolution or reduced efficacy over repeated use, and a limited choice of analytes due to solubility concerns.…”