Effects of Edge Functionalization of Nanographenes with Small Aromatic Systems

Regulation of the physical properties of nanographenes (NGs) by edge functionalization is an active research area. We conducted a computational study of the effects of edge functionalization on the physical properties of NGs. The computed NGs were models of experimentally obtained NGs and composed of a C₁₇₄ carbon framework with one to four 3,5-dimethylnaphthalene units on the edge. The effects were assessed structurally, magnetically, and electronically by the least square planarity index, harmonic oscillator model of aromaticity, nucleus-independent chemical shift, and HOMO-LUMO (H-L) gaps. Density functional theory calculations indicate that although the structures of the model NGs are not very sensitive to edge functionalization, but the magnetic and electronic properties are. The installed substituents narrowed the H-L gap and induced a redshift of the photoluminescence (PL) band by the π conjugation between NG and the substituent. These results are consistent with the extension of the absorption band and the redshift of the PL bands of the experimentally modified NGs. Furthermore, the calculations confirmed the contribution of the charge transfer character to the absorption spectra.