Syntheses, Spectroscopic and DFT Studies of Two New D‐π‐A Compounds

Two novel 1,3‐dithiole‐2‐ylidene derivatives with a push–pull structures, 3‐(4,5‐dicarbomethoxy‐1,3‐dithiol‐2‐ylidene)naphthopyranone 1 and 3‐(4,5‐dimethylthio‐1,3‐dithiol‐2‐ylidene)naphthopyranone 2, have been synthesized and characterized by 1H NMR, IR, MS. The UV–vis spectra of 1, 2 in CH2Cl2, the lowest‐energy absorption bands, are centered at 280, 316, and 430 nm for 1 and 284, 317, and 450 nm for 2, respectively, which are caused by the HOMO → LUMO single electron promotion. Furthermore, the steady‐state fluorescence originating states of 1, 2 from the excited charge‐transfer were observed. To estimate the position and energies of frontier orbitals for 1, 2, DFT calculations were performed using the Gaussian 03 program at the B3LYP/6‐31 G* level. The calculated vertical excitation energies are in good agreement with the experimental data. The high HOMO–LUMO gaps of 1 (3.08 eV) and 2 (3.00 eV) indicate high kinetic stability of the title compounds.