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

• Published in: Chemistry : a European journal 2007-01, Vol.13 (30), p.8411-8427
• Main Authors: Balaban, Teodor Silviu, Berova, Nina, Drain, Charles Michael, Hauschild, Robert, Huang, Xuefei, Kalt, Heinz, Lebedkin, Sergei, Lehn, Jean-Marie, Nifaitis, Fotis, Pescitelli, Gennaro, Prokhorenko, Valentyn I., Riedel, Gernot, Smeureanu, Gabriela, Zeller, Joachim
• Format: Article
• Language: English
• Subjects: Energy Transfer; Hydrogen Bonding; hydrogen bonds; Light; Microscopy, Atomic Force; Models, Molecular; porphyrins; Porphyrins - chemistry; Scattering, Radiation; self-assembly; Spectrometry, Fluorescence; supramolecular chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200601691

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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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U‐A‐UP‐D‐P⋅⋅⋅, or ⋅⋅⋅U‐D‐UP‐A‐P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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 κ for the transition