Porphyrin Core Star Polymers: Synthesis, Modification, and Implication for Site Isolation

During the last years, efforts have been made to encapsulate porphyrins1 within dendrimers to tune their photophysical,2 electrochemical,3 and chemical4 properties. Work carried out in several laboratories has demonstrated the profound ability of a dendritic shell to isolate the porphyrin core moiety facilitating the design of hemeprotein mimics. Unfortunately, dendrimer synthesis is usually time-consuming, and a more rapid route to encapsulated porphyrins that preserves the architectural and functional control afforded by a dendritic shell is desired. Recent advances in the area of “living” ring opening polymerization (ROP) of -caprolactone5 have enabled the accurate construction of star-shaped polymers.6 We sought to take advantage of this methodology coupled with the space filling branching approach of dendrimer synthesis to achieve site isolation of porphyrins. Our approach is based on a highly branched porphyrin core that acts as a multisite initiator for the ROP of -caprolactone. Versatile modification of the core and the end-group functionalities renders the synthesis flexible, allowing one to tailor-design a variety of functional star polymers. Such materials may be of general applicability in the design of oxidation catalysts7 as well as energyharvesting8 and optoelectronic devices.9 Starting from tetrakis(4-hydroxyphenyl)porphyrin 1a and tetrakis(3,5-dihydroxyphenyl)porphyrin 1b, respectively, the initiators 2a,b were prepared via esterification employing acetonide-protected 2,2-bis(hydroxymethyl)-propionic acid10 followed by deprotection of the diol functionalities under acidic conditions (Scheme 1). By using the methodology developed by Trollsas and Hedrick,6a-c we obtained star polymers 3a,b possessing either 8 or 16 arms by bulk polymerization of -caprolactone with initiators 2a,b and tin(II) 2-ethylhexanoate (Sn(Oct)2) as the catalyst. By adjusting the monomer-to-initiator ratio, we could prepare polymers with varying chain lengths and low polydispersities in almost quantitative yields (Table 1). To tune the properties of these materials, we accomplished further modification of the dendritic stars by metalation of the core moiety as well as by derivatization of the hydroxyl-functional chain ends (Scheme 2). Insertion of zinc(II) into the free base porphyrin gave 4b, which is easily identified by its characteristic electronic absorption spectrum in the Q-band region11 (Figure 1a). Introduction of the dye coumarin at each of the 16 hydroxyl chain ends of 3b was easily accomplished by esterification with coumarin-3-carboxylic acid chloride affording 5b. The additivity of the individual coumarin chromophore units contributes to the large absorption in the UV region of the spectrum (Figure 1a). The proton NMR spectra of 3b and 5b (Figure 1b) reveal a significant downfield shift of the signal associated with the end groups of 3b (CH2OH) by conversion to the coumarin esters in 5b. Due to the energetic match between the emission of the coumarin donors and the absorption of the porphyrin acceptor, compounds such as 5b offer the opportunity of efficient light harvesting and resonance energy transfer.12 Further studies of these processes are currently underway. A significant advantage of the high yield processes used is that the only purification required after polymerization, metalation, (1) For an introduction to porphyrin chemistry, see: Milgrom, L. R. The Colours of Life; Oxford University Press: Oxford, U.K., 1997, and references therein. (2) (a) Jin, R.-H.; Aida, T.; Inoue S. J. Chem. Soc., Chem. Commun. 1993, 1260. (b) Tomoyose, Y.; Jiang, D.-L.; Jin, R.-H.; Aida, T.; Yamashita, T.; Horie, K.; Yashima, E.; Okamoto, Y. Macromolecules 1996, 29, 5236. (c) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996, 118, 3978. (d) Pollak, K. W.; Leon, J. W.; Frechet, J. M. J.; Maskus, M.; Abruna, H. D. Chem. Mater. 1998, 10, 30. (e) Pollak, K. W.; Sanford, E. M.; Frechet, J. M. J. J. Mater. Chem. 1998, 8, 519. (f) Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1998, 120, 10895. (3) (a) Dandliker, P. J.; Diederich, F.; Gross, M.; Knobler, C. B.; Louati, A.; Sanford, E. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1739. (b) Dandliker, P. J.; Diederich, F.; Gisselbrecht, J.-P.; Louati, A.; Gross, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 2725. (c) Dandliker, P. J.; Diederich, F.; Zingg, A.; Gisselbrecht, J.-P.; Gross, M.; Louati, A.; Sanford, E. M. HelV. Chim. Acta 1997, 80, 1773. (4) (a) Jiang, D.-L.; Aida, T. Chem. Commun. 1996, 1523. (b) Collman, J. P.; Fu, L.; Zingg, A.; Diederich, F. Chem. Commun. 1997, 193. (c) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708. (d) Bhyrappa, P.; Vaijayanthimala, G.; Suslick, K. S. J. Am. Chem. Soc. 1999, 121, 262. (5) For a recent review, consult Lofgren, A.; Albertsson, A.-C.; Dubois, P.; Jerome, R. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1995, C35, 379. (6) (a) Trollsas, M.; Hedrick, J. L.; Mecerreyes, D.; Dubois, P.; Jerome, R.; Ihre, H.; Hult, A. Macromolecules 1997, 30, 8508. (b) Trollsas, M.; Hedrick, J. L.; Mecerreyes, D.; Dubois, P.; Jerome, R.; Ihre, H.; Hult, A. Macromolecules 1998, 31, 2756. (c) Trollsas, M.; Hedrick, J. L. J. Am. Chem. Soc. 1998, 120, 4644, and references therein. (7) Detailed review are given in (a) Sheldon, R. A., Ed. Metalloporphyrins in Catalytic Oxidations; Marcel Dekker: New York, 1994. (b) Montanari, F., Casella, L., Eds. Metalloporphyrin Catalyzed Oxidations; Kluwer: London, 1995. (8) For a recent discussion, see Freemantle, M. Chem. Eng. News 1998, (10/26), 37. (9) An enhancement of the luminescence properties of lanthanide ions using the concept of site isolation has recently been reported: Kawa, M.; Frechet, J. M. J. Chem. Mater. 1998, 10, 286. (10) Ihre, H.; Hult, A.; Frechet, J. M. J.; Gitsov, I. Macromolecules 1998, 31, 4061. (11) (a) Buchler, J. W. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. I, Part A, p 390. (b) Smith, K. M. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: New York, 1976; p 3 and appendix p 871. (12) For a related study, see Gilat, S. L.; Adronov, A.; Frechet, J. M. J. Angew. Chem., Int. Ed. Engl. 1999, 38, 1422. Scheme 1