Belt-Shaped π-Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring

Title: Belt-Shaped π-Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring

Authors:  J. K. Sprafke, D. V. Kondratuk, M. Wykes, A. L. Thompson, M. Hoffmann, R. Drevinskas, W.H. Chen, C. K. Yong, J. K€arnbratt, J. E. Bullock, M. Malfois, M. R. Wasielewski, B. Albinsson, L. M. Herz, D. Zigmantas, D. Beljonne, H. L. Anderson

Journal: Journal of the American Chemical Society

Affiliation: University of Oxford (UK), University of Mons (Belgium), Lund University (Sweden), Chalmers University of Technology (Sweden), Northwestern University (USA), Diamond Light Source Ltd. (UK), Clarendon Laboratory (UK)

Take Home Importance: A cyclic π-conjugated hexamer of porphyrin rings connected by butadiyne spacers was prepared and found to exhibit totally ring-delocalized excited states upon excitation as well as emission in the near infrared (NIR).

Figure 1. Crystal structure of the templated porphyrin hexamer (left) and solution absorption and emission spectra (right).
Summary: It is known that conjugation can permit excited state delocalization and interesting material properties. However, conjugated systems are typically finite, do not have special symmetry, and….have ends (reminds of that book Where the Sidewalk Ends). Here the authors (of many countries and institutions!) describe detailed characterization and synthesis of a cyclic fully π-conjugated oligomer. The ring itself is constructed from six porphyrin rings  (each coordinating a zinc ion at the center) connected by six butadiyne spacers. Net result? A highly symmetric fully-conjugated ring structure. Now, it is very interesting to ask how one would go about preparing this ring molecule because one could envision that upon mixing the butadiyne spacers with the porphyrin centers you could end up with just a long chain of n porphyrins connected by butadiyne spacers. To avoid this problem, the authors turned to a clever technique called templating: by introducing into the solution a substrate which could coordinate six and only six zinc ions at the proper geometries, the porphyrin ring could feasibly construct itself around the substrate! Specfically, the pyridul nitrogens in the template molecule T6 below each coordinated the center of one porphyrin ring, effectively holding it in the correct position. This method actually worked quite well, as you can see in the synthesis laid out in Figure 2 below.
Figure 2. Detailed synthetic route for the preparation of the porphyrin ring hexamer. See how T6 was just of the right size with the correct angles to permit templating of the porphyrin molecules about it by binding to the porphyrin zinc centers.
     While there are quite a few steps all told, what is rather remarkable about this synthesis is the final ring construction occurs in a single reaction! The ring structure itself was determined by single-crystal X-ray diffraction (a difficult feat considering the large number of atoms present in the structure). To confirm that this template-bound ring structure was also present in solution, Solution-Phase Small-Angle X-ray Scattering (SAXS) was utilized. SAXS permits interrogation of short-range order within a sample when tied to a pair-distribution function (PDF) analysis, which yields information on the average distances between centers of electron densities. If we were to sit at one of the six zinc atoms in the ring and measure the distances to the other zinc centers, we would expect to see a signal corresponding to the electron density of the other zinc ions in the ring. Using this sort of PDF analysis, the authors confirmed that in solution the ring system structure remains intact, even after the original template molecule T6 was removed.
     Having prepared this gorgeous conjugated ring macromolecule, the authors investigated it with detailed photophysical, electrochemical, and computational study. Comparison with a separately prepared linear chain of six porphyrins connected by butadiyne linkers revealed that the ring system possessed a smaller HOMO-LUMO gap (both with and without the template T6 present) as well as red-shifted (lower energy) fluorescence. Additionally, the ring system and linear chain were found to have a substantially different lifetimes, 250 and 650 picoseconds, respectively. The shorter lifetime observed in the ring system provides strong evidence that the excited state is indeed delocalized over the whole ring and is not confined to a sole porphyrin monomer. In fact, there are two primary excited states: S1, which is delocalized over all six porphyrin centers and is lowest in energy; and S2, which is delocalized over four of the six and is higher in energy. The unique sharp nature of the near infrared emission (see Figure 1 above) was interpreted to result from the Herzberg-Teller (HT) coupling of the S1 and S2 excited states. Herzberg-Teller coupling can be understood as a breaking of the excited state symmetry which then permits an otherwise symmetry-forbidden transition to occur. In essence, the unique symmetry and endless-nature of this ring molecule resulted in some fascinating photophysical effects. Also, while the authors don’t mention this, the ring looks awfully like the ring structures in biological light-harvesting centers!

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