Factors Controlling the Spectroscopic Properties and Supramolecular Chemistry of an Electron Deficient 5,5-Dimethylphlorin Architecture

A new 5,5-dimethylphlorin derivative (3H­(PhlCF3)) was prepared and studied through a combination of redox, photophysical, and computational experiments. The phlorin macrocycle is significantly distorted from planarity compared to more traditional tetrapyrrole architectures and displays solvatochroism in the soret region of the UV–vis spectrum (∼370–420 nm). DFT calculations indicate that this solvatochromic behavior stems from the polarized nature of the frontier orbital (LUMO+1) that is most heavily involved in these transitions. Compound 3H­(PhlCF3) also displays an intriguing supramolecular chemistry with certain anions; this phlorin can cooperatively hydrogen-bond two equivalents of fluoride to form 3H­(PhlCF3)·2F but does not bind larger halides such as Cl or Br. Analogous studies revealed that the phlorin can hydrogen-bond with carboxylate anions such as acetate to form 1:1 complexes such as 3H­(PhlCF3)·OAc. These supramolecular assemblies are robust and form even in relatively polar solvents such as MeCN. Hydrogen-bonding of fluoride and acetate anions to the phlorin N–H residues significantly attenuates the redox and photophysical properties of the phlorin. Moreover, The ability to independently vary the size and pKa of a series of carboxylate hydrogen-bond acceptors has allowed us to probe how phlorin–anion association is controlled by the anion’s size and/or basicity. These studies elucidate the physical properties and the electronic effects that shape the supramolecular chemistry displayed by the phlorin platform.