Multiple-Porphyrin Functionalized Hexabenzocoronenes

Abstract

Porphyrin-hexabenzocoronene architectures serve as good model compounds to study light-harvesting systems. Herein, the synthesis of porphyrin functionalized hexa-peri-hexabenzocoronenes (HBCs), in which one or more porphyrins are covalently linked to a central HBC core, is presented. A series of hexaphenylbenzenes (HPBs) was prepared and reacted under oxidative coupling conditions. The transformation to the respective HBC derivatives worked well with mono- and tri-porphyrin-substituted HPBs. However, if more porphyrins are attached to the HPB core, Scholl oxidations are hampered or completely suppressed. Hence, a change of the synthetic strategy was necessary to first preform the HBC core, followed by the introduction of the porphyrins. All products were fully characterized, including, if possible, single-crystal XRD. UV/Vis absorption spectra of porphyrin-HBCs showed, depending on the number of porphyrins as well as with respect to the substitution pattern, variations in their spectral features with strong distortions of the porphyrins' B-band.

Keywords: Scholl oxidation; hexabenzocoronene; nanographene; nanostructures; porphyrinoids.

Figure 1

HBC as a model system for nanographenes with porphyrins directly attached to the periphery. Influence of the geometrical alignment (previous work, left side) and the amount (this work, right side) of porphyrins on the HBC core.

Scheme 1

Synthesis of mono‐ and tri‐porphyrin‐HBCs 6, 7, and 8. Molecules 3, 4, and 6 are depicted as X‐ray crystal structures (for details see the Supporting Information).45 a) Ph₂O, 260 °C, μW; b) Ni(acac)₂, toluene, 110 °C; c) Co₂(CO)₈, toluene, 110 °C; d) H₂SO₄, CH₂Cl₂, 0 °C; e) FeCl₃, CH₃NO₂, CH₂Cl₂, 0 °C. Excess amounts of FeCl₃ (29–41 equiv) were used for the Scholl oxidation as these conditions form the respective HBC derivatives efficiently within a relatively short amount of time (<24 h).

Scheme 2

Planned synthetic route towards hexa‐porphyrin‐HBC 11 through cyclotrimerization and Scholl reaction. a) Co₂(CO)₈, toluene, 110 °C; b) H₂SO₄, CH₂Cl₂, 0 °C.

Scheme 3

Suggested reaction products of Scholl oxidation with hexa‐nickel‐porphyrin‐HPB 10⋅Ni₆. The reaction is simplified by showing only one porphyrin of 10⋅Ni₆. For a single‐bond formation, up to three different isomers per porphyrin are feasible.

Scheme 4

Successful synthesis of hexa‐porphyrin‐HBC 11 through Suzuki reaction. a) FeCl₃, CH₃NO₂, CH₂Cl₂, 0 °C; b) Pd(PPh₃)₄, Cs₂CO₃, toluene, DMF, 80 °C; c) trifluoroacetic acid, CHCl₃.

Figure 2

¹H NMR (400 MHz, CDCl₃, rt) of hexa‐porphyrin‐HPB 10 (top) and hexa‐porphyrin‐HBC 11 (bottom). Calculated structures (semi empirical, PM6) are depicted; hydrogen atoms are omitted for clarity.

Figure 3

a) Structural motif of hexa‐zinc‐porphyrin‐HBC 11⋅Zn₆ with MeOH coordinated to each central zinc atom; ^[32] b) top view of eight molecules of 11⋅Zn₆; c) side view of four molecules of 11⋅Zn₆; b), c) 3,5‐di‐tert‐butylphenyl groups and hydrogen atoms as well as disordered groups are omitted for clarity.

Figure 4

a) Structural motif of hexa‐free‐base‐porphyrin‐HBC 11; b) top view of eight molecules of 11; c) side view of four molecules of 11; b), c) 3,5‐di‐tert‐butylphenyl groups and hydrogen atoms as well as disordered groups are omitted for clarity.

Figure 5

UV/Vis absorption spectra63 of molecules 3, 6, 7, 8, and 11 in THF. Inserts show magnifications of the β‐ (left side) and Q‐band (right side) absorptions.