Single-Acetylene Linked Porphyrin Nanorings

Abstract

The synthesis of ethyne-linked porphyrin nanorings has been achieved by template-directed Sonogashira coupling. The cyclic hexamer and octamer are predicted by density functional theory to adopt low symmetry conformations, due to dihedral twists between neighboring porphyrin units, but their symmetries are effectively D_6h and D_8h, respectively, in solution by ¹H NMR. The fluorescence spectra indicate that the singlet excited states of these nanorings are highly delocalized.

Figure 1

(a) Types of linear porphyrin–porphyrin connection. Zn–Zn distances and dihedral angles θ (measured from the carbon atoms indicated by blue circles from density functional theory (DFT)-optimized geometries). (b–e) Calculated geometries of c-P6, c-P6·T6, c-P8 and c-P8·(T4)₂. Two orthogonal views are shown for each structure. Solubilizing aryl groups and hydrogen atoms are omitted for clarity. (B3LYP/6-31G*; see SI for details.).

Scheme 1. Synthesis of c-P8 and c-P6

Reaction conditions: (a) 2, Pd₂dba₃, PPh₃, CuI, PhMe/i-Pr₂NH (2:1), 50 °C, 15 h, 13%; (b) TBAF, CH₂Cl₂, 25 °C, 10 min, 79%; (c) T4, Pd₂dba₃, PPh₃, CuI, PhMe/i-Pr₂NH (10:1), 40 °C, 2 d, 32%; (d) SEC pyridine/PhMe (1:1), 99%; (e) 1, Pd(PPh₃)₄, PPh₃, CuI, PhMe/i-Pr₂NH (2:1), 50 °C, 15 h, 30%; (f) TBAF, CH₂Cl₂, 25 °C, 10 min, 95%; (g) T6, Pd(PPh₃)₄, PPh₃, CuI, PhMe/i-Pr₂NH (2:1), 40 °C, 16 h, 15%; (h) SEC (pyridine/PhMe 1:1), 99%. Ar: 1,3-bis(trihexylsilyl)phenyl. Syntheses of 1, 2, 3, T4 and T6 are outlined in the SI.

Figure 2

Steady-state absorption (continuous lines) and fluorescence (dashed) spectra at 295 K of l-P6 in toluene/1% pyridine, c-P6·T6 in toluene and c-P6 in toluene/1% pyridine. Quantum yields are given in %. The apparent dip in the fluorescence spectra at 1140 nm is due to absorption by the solvent.