The development of the 5,5-dimethylphlorin and 10,10-dimethylbiladiene tetrapyrrole platforms
Date
2015
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Tetrapyrrole scaffolds containing a dimethyl sp 3 -meso center were explored. The 5,5-dimethylphlorin was explored as X-ray crystallography and NMR spectroscopy were used to determine that the phlorin, unlike the common porphyrin is nonaromatic. Several phlorin derivatives were synthesized through variation of the aromatic group at the 15-meso-position including pentafluorophenyl (3H(Phl F )), mesityl (3H(Phl Mes )), 4-nitrophenyl (3H(PhlNO2 )), 4-tert-butylcarboxyphenyl (3H(PhlCO2tBu )), 2,6-bismethoxyphenyl (3H(PhlOMe )), 3,5-bistrifluoromethylphenyl (3H(PhlCF3 )), and 3,5-bis-tert-butylphenyl (3H(Phl3,5-tBu )). The phlorin macrocycles were found to display unusual multielectron redox chemistry and photophysical properties. The phlorins were able to be oxidized up to three times at modest potentials and reduced two times. The substitution at the 15-meso-position was found to have the ability to attenuate the redox potentials of the phlorins. The electron donating substituents were easier to oxidize and more difficult to reduce as the electron withdrawing groups were easier to reduce and required more positive potentials to be oxidized. The photophysics of the phlorins were also influenced through the substitution at the 15-meso-position with the electron rich derivatives having higher quantum yields and longer fluorescence lifetimes. The phlorin also displays solvatochroism in the Soret absorption, where the polarized LUMO+1 state observed from the DFT calculations is involved. The phlorin macrocycle also displays unique supramolecular activity with the ability to cooperatively bind two equivalents of F- anions but not bind any other halogens such as Cl- and Br- . Carboxylate anions were also found to form hydrogen bonding assemblies, but in a 1:1 ratio with the phlorin. The binding of anions was found to significantly attenuate both the redox and photophysical properties of the phlorin. The formation of the hydrogen binding assemblies was probed using carboxylates of different size and basicity. The addition of a transition metal to the center of the phlorin core was also explored and two novel gold(III) phlorin derivatives were synthesized. The coordination of the gold(III) to the phlorin core was found to have a significant impact on the redox and photophysical properties. The crystal structure obtained for the gold(III) phlorin displays significant flattening of the macrocycle and the absorption profile in the Q band region red shifts by approximately 100 nm. The gold(III) phlorin derivatives that were synthesized were found to display solvatochroism, similar to that observed with the free base phlorin. The redox properties from the metalated derivative were significantly different from the free base. The gold(III) phlorins displayed two oxidations and two reductions, all of which were fully reversible. Both the gold(III) and free base phlorin derivatives show the ability to react with (dichloroiodo)benzene to chlorinate the phlorin backbone at β-pyrrole positions. Another tetrapyrrole scaffold containing a dimethyl sp3 -meso center, the 10,10-dimethylbiladiene was investigated for its use as a ligand system. The dimethylbiladiene (DMBil) is a non-macrocyclic system that is able to bind Zn2+ and Cu2+ centers. X-ray crystallography shows that the open ligand design of the DMBil binds a single metal center at the core. The DMBil, Zn(DMBil) and Cu(DMBil) all display interesting redox chemistry with the ability to be both oxidized and reduced by two electrons. The biladiene derivatives also display strong absorption in the UV-vis region and are weakly emissive. The sensitization of 1 O2 was investigated using the free base DMBil and the Zn(DMBil) as photosensitizers with 500 nm wavelength light. Both DMBil and Zn(DMBil) were found to sensitize 1 O2 with modest efficiencies and may provide a potential platform for the use in photodynamic therapy.