BINOL-like atropisomeric chiral nanographene

Abstract

Interest in making chiral polycyclic aromatic hydrocarbons (PAHs) or nanographenes (NGs) has greatly increased recently. To date, a majority of chiral nanocarbons have been designed based on helical chirality. Here, we describe a novel atropisomeric chiral oxa-NG 1 by the selective dimerization of naphthalene-containing, hexa-<i>peri</i>-hexabenzocoronene (HBC)-based PAH 6. The photophysical properties of the oxa-NG 1 and monomer 6 were investigated, including UV-vis absorption (<i>λ</i> _max = 358 nm for 1 and 6), fluorescence emission (<i>λ</i> _em = 475 nm for 1 and 6), fluorescence decay (15 <i>vs.</i> 16 ns), and fluorescence quantum yield, and it was found that the photophysical properties of the monomer are nearly maintained in the NG dimer due to its perpendicular conformation. Single-crystal X-ray diffraction analysis shows that both enantiomers cocrystallize in a single crystal, and the racemic mixture can be resolved by chiral high-performance liquid chromatography (HPLC). The circular dichroism (CD) spectra and circularly polarized luminescence (CPL) of the enantiomers of 1-<i>S</i> and 1-<i>R</i> were studied and the CD and CPL spectra exhibited opposite Cotton effects and fluorescence signals. Density functional theory (DFT) calculations and HPLC-based thermal isomerization results showed that the racemic barrier is as high as 35 kcal mol^-1, suggesting a rigid chiral nanographene structure. Meanwhile, <i>in vitro</i> studies indicated that oxa-NG 1 is an efficient photosensitizer for white-light-induced singlet oxygen generation.