Kaxiras's Porphyrin: DFT Modeling of Redox-Tuned Optical and Electronic Properties in a Theoretically Designed Catechol-Based Bioinspired Platform

Abstract

A detailed computational investigation of the 5,6-dihydroxyindole (DHI)-based porphyrin-type tetramer first described by Kaxiras as a theoretical structural model for eumelanin biopolymers is reported herein, with a view to predicting the technological potential of this unique bioinspired tetracatechol system. All possible tautomers/conformers, as well as alternative protonation states, were explored for the species at various degrees of oxidation and all structures were geometry optimized at the density functional theory (DFT) level. Comparison of energy levels for each oxidized species indicated a marked instability of most oxidation states except the six-electron level, and an unexpected resilience to disproportionation of the one-electron oxidation free radical species. Changes in the highest energy occupied molecular orbital (HOMO)⁻lowest energy unoccupied molecular orbital (LUMO) gaps with oxidation state and tautomerism were determined along with the main electronic transitions: more or less intense absorption in the visible region is predicted for most oxidized species. Data indicated that the peculiar symmetry of the oxygenation pattern pertaining to the four catechol/quinone/quinone methide moieties, in concert with the NH centers, fine-tunes the optical and electronic properties of the porphyrin system. For several oxidation levels, conjugated systems extending over two or more indole units play a major role in determining the preferred tautomeric state: thus, the highest stability of the six-electron oxidation state reflects porphyrin-type aromaticity. These results provide new clues for the design of innovative bioinspired optoelectronic materials.

Keywords: DFT; ab initio calculation; eumelanin; porphyrin.

Scheme 1

Formation of a porphyrin-like tetramer (Kaxiras’s porphyrin, KP) by oxidation of 5,6-dihydroxyindole (DHI).

Figure 1

Computed ultraviolet–visible (UV–Vis) spectra of the most significant tautomers/conformers in the reduced state. (a) Neutral form in vacuo, a0_b0_c0_d0, S₄. (b) Neutral form in water, a0_b0_c0_d0, S₄.

Figure 2

Computed infrared (IR) spectra of the most significant tautomers/conformers in the reduced state. (a) Neutral form in vacuo, a0_b0_c0_d0, S₄. (b) Neutral form in water, a0_b0_c0_d0, S₄.

Figure 3

Computed UV–Vis spectra of the most significant tautomers/conformers in the one-electron oxidation state. (a) Neutral form in vacuo: black line, a5_b0_c0_d0, C₁, conf1; red line, a6_b0_c0_d0, C₁, conf1. (b) Neutral form in water: black line, a5_b0_c0_d0, C₁, conf1; red line, a6_b0_c0_d0 C₁, conf1.

Figure 4

Building blocks for generation of staring structures of closed-shell tautomers of Kaxiras’s porphyrin. (a) Fully reduced units. (b–e) One-electron oxidized units. (f–i) Two-electron oxidized units.

Figure 5

Computed UV–Vis spectra of the most significant tautomers/conformers in the two-electrons oxidation state. (a) Neutral form in vacuo: black line, a16_b0_c0_d0, C₁, conf1; red line, a6_b6_c0_d0, C₁, conf1. (b) Neutral form in water: black line, a16_b0_c0_d0, C₁, conf1; red line, a6_b6_c0_d0, C₁, conf1.

Figure 6

Computed IR spectra of the most significant tautomers/conformers in the two-electrons oxidation state. (a) Neutral form in vacuo: black line, a16_b0_c0_d0, C₁, conf1; red line, a6_b6_c0_d0, C₁, conf1. (b) Neutral form in water: black line, a16_b0_c0_d0, C₁, conf1; red line, a6_b6_c0_d0, C₁, conf1.

Figure 7

Computed UV–Vis spectra of the most significant tautomers/conformers in the four-electrons oxidation state. (a) Neutral form in vacuo: black line, a16_b6_c6_d0, C₁, conf1; red line, a6_b16_c6_d0, C₁, conf1; green line, a6_b6_c16_d0, C₁, conf1; blue line, a6_b6_c6_d6, C₂, conf1. (b) Neutral form in water: black line, a16_b0_c16_d0, C₂, conf1; red line, a16_b6_c6_d0, C₁, conf1; green line, a6_b16_c6_d0, C₁, conf1; blue line, a6_b6_c16_d0, C₁, conf1; magenta line, a6_b6_c6_d6, C₂, conf1.

Figure 8

Computed IR spectra of the most significant tautomers/conformers in the four-electrons oxidation state. (a) Neutral form in vacuo: black line, a16_b6_c6_d0, C₁, conf1; red line, a6_b16_c6_d0, C₁, conf1; green line, a6_b6_c16_d0, C₁, conf1; blue line, a6_b6_c6_d6, C₂, conf1. (b) Neutral form in water: black line, a16_b0_c16_d0, C₂, conf1; red line, a16_b6_c6_d0, C₁, conf1; green line, a6_b16_c6_d0, C₁, conf1; blue line, a6_b6_c16_d0, C₁, conf1; magenta line, a6_b6_c6_d6, C₂, conf1.

Scheme 2

a16_b6_c16_d6 as a hybrid between two equivalent contributing structures. The pattern of conjugated double bonds forming an 18-electron aromatic system is highlighted in bold.

Figure 9

Computed UV–Vis spectra of the most significant tautomers/conformers in the six-electron oxidation state. (a) Neutral form in vacuo, a16_b6_c16_d6, C₂, conf1. (b) Neutral form in water, a16_b6_c16_d6, C₂, conf1.

Figure 10

Computed IR spectra of the most significant tautomers/conformers in the six-electrons oxidation state. (a) Neutral form in vacuo, a16_b6_c16_d6, C₂, conf1. (b) Neutral form in water, a16_b6_c16_d6, C₂, conf1.

Figure 11

Computed UV–Vis spectra of the most significant tautomers/conformers in the eight-electrons oxidation state. (a) Neutral form in vacuo, a16_b56_c16_d56, C₂, conf1. (b) Neutral form in water, a16_b56_c16_d56, C₂, conf1.

Figure 12

Computed IR spectra of the most significant tautomers/conformers in the eight-electrons oxidation state. (a) Neutral form in vacuo, a16_b56_c16_d56, C₂, conf1. (b) Neutral form in water, a16_b56_c16_d56, C₂, conf1.

Figure 13

A three-electron oxidized building block, which would be needed for the generation of closed-shell tautomers at oxidation levels above KP-8e.