A Tetrapyrrole Macrocycle Displaying a Multielectron Redox Chemistry and Tunable Absorbance Profile

Abstract

Porphyrins are attractive chromophores for incorporation into light harvesting devices. Some of the most efficient porphyrin derivatives in this regard are synthetically complex platforms with specially tailored electronic properties. This work details the unique geometric and electronic structure of the phlorin framework. X-ray crystallography and NMR spectroscopy demonstrate that unlike typical tetrapyrrole macrocycles, the phlorin is not aromatic. These unusual electronics are manifest in distinct photophysical and redox properties, as the phlorin displays a rich multielectron redox chemistry. The phlorin also displays an intriguing supramolecular chemistry and can reversibly bind up to two equivalents of fluoride in cooperative fashion. Accordingly, this synthetically accessible sensitizer displays a rich multielectron redox chemistry, excellent spectral coverage and an intriguing anion binding chemistry that distinguishes this system from more commonly studied porphyrinoids.

Figure 1

Solid-state structure of 3H(Phl^F) shown from (a) above the plane of the macrocycle and (b) side on. The reduced side of the phlorin is displaced 0.60 Å above the rest of the macrocycle. A molecule of cocrystallized CHCl₃ and all non-nitrogen bound hydrogen atoms have been omitted for clarity.

Figure 2

UV-vis absorption (green) and normalized emission (red) spectra recorded for a 10 μM solution of Phl^F in THF.

Figure 3

Cyclic voltammograms recorded using a scan rate of 50 mV/s for 3H(Phl^F) (green), 3H(TpFPC) (maroon) and 2H(TpFPP) (purple) in CH₂Cl₂ containing 0.1 M TBAPF₆ with an internal decamethylferrocene standard (Fc*).

Figure 4

Changes in the UV-vis absorption spectrum of Phl^F upon titration with TBAF. Inset: Hill plot constructed for fluoride binding to generate 3H(Phl^F) ·2F^–.

Figure 5

(a) Comparison of the UV-vis absorption profiles of 2H(TFPP) and 3H(Phl^F) μ superimposed onto the SPS; (b) Superposition of the SPS onto the absorption profile of 10 M 3H(Phl^F) + 0.6 equiv. of TBAF in THF (green) and the calculated spectrum obtained for a · solution of 70% 3H(Phl^F) and 30% 3H(Phl^F) 2F^– (dashed orange).

Scheme 1

Strategy adapted for development of the phlorin architecture.