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 from 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 indicated that although the structures of the model NGs were not very sensitive to edge functionalization, the magnetic and electronic properties were. The installed substituents narrowed the H-L gap and induced a redshift of the PL band by π conjugation between NG and the substituent. These results were 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.