One step Pd(0)-catalyzed synthesis, x-ray analysis, and photophysical properties of cyclopent[hi]aceanthrylene: Fullerene-like properties in a nonalternant cyclopentafused aromatic hydrocarbon

A simple procedure for the synthesis of cyclopentafused polycyclic aromatic hydrocarbons (CP-PAH) with Pd(PPh₃)₂Cl₂ catalyst has been applied to the one-pot palladium(0)-catalyzed coupling of 9,10-dibromoanthracene (1) with 2-methyl-3-butyn-2-ol. Reactions carried out in refluxing benzene in the presence of CuSO₄/Al₂O₃ yielded 9,10-dialkynylanthracene 2a, alkynyl aceanthrylene 2b, and 2,7-disubstituted cyclopent[hi]aceanthrylene 2c in 13%, 23%, and 19% purified yields, respectively, with total conversions of 80-90%. Sealed tube reactions without copper at 110 °C improved the yield of 2c up to >75%. Single-crystal X-ray analyses of 2a and 2c reveal a three-dimensional hydrogen bonding network, producing a unique crystal packing. The packing structure of 2b is dominated by π-π stacking interactions between two aceanthrylene molecules. CP-PAHs 2b and 2c have potentially interesting fullerene-like photophysics. While the UV-vis and fluorescence spectra of 2a (Φ_F = 0.87) show the characteristic vibronic structure of anthracene, the UV-vis spectra of ruby-red aceanthrylene 2b and greenish-black cyclopent[hi]aceanthrylene 2c extend well into the visible range. Isomers 2b and 2c showed no detectable fluorescence emission. Unlike fullerenes, compounds 2b and 2c are poor singlet oxygen sensitizers with measured ¹O₂ quantum yields of 0.02 and 0.06, respectively. As expected from a simple Hückel analysis, 2c has relatively low two-electron reduction potentials as determined by cyclic voltammetry.