Syntheses, Structures And Properties Of Conjugated Molecules Based On Sulfur-Rich Heterocycles

1,3-dithiol-2-ylidene derivatives with unique structures and many excellent physical and chemical properties, as a building block of TTF molecule, have been widely explored for their potential use in nonlinear optics, molecular electronics and life sciences etc.Here, we designed and synthesized four novel organic molecules of sulfur-rich heterocycles provided with conjugated structure in the main task of this paper: 3-(4,5-bis(3-picolylthio)-1,3-dithiol-2-ylidene)naphthopyranone 1, 3-(4,5-bis (carbomethoxy)-1,3-dithiol-2-ylidene)naphthopyranone 2, 4',5'-diaza-9'-(4,5-bis(3-picolylthio)-1, 3-dithiole-2-ylidene)-fluorene 3 and 4,4',5,5'-Tetrakis(benzylsulfanyl)tetrathiafulvalene 4, which were characterized and identified by 1H NMR, TOF-MS, FT-IR, UV-Vis spectra and Elemental Analyses. We fostered crystals by chemical method and diffusion process. The structures of 1, 3 and 4 have been determined by single crystal X-ray diffraction. The crystal structure of 1 belongs to orthorhombic space group Pbcn. Crystal structure shows that 1 is non-planar conformation. Both the two targets adopt chair-like conformation, and the pyridine rings of moieties are severely twisted from plane of 1,3-dithiole ring. The molecule with cyanogens group exhibits a three-dimensional supramolecular architecture constructed through hydrogen bonds. The crystal structure of 3 belongs to monoclinic space group C2/c. Two pyridine rings are twisted from the planarity of 1,3-dithiole ring with the two targets adopting half chair-like conformation. The crystal structure of 4 belongs to triclinic space group Pī. The plane determinated by two benzene rings and TTF displays half chair-like conformation.We investigated thermogravimetric property (TG-DTA), electrochemistry (CV) and fluorescent property.TG-DTA analyses of 1, 2, 3 and 4 exhibit that organic framework start collapse until the temperature rising up to 300°C, 390°C, 350°C and 290°C, respectively, showing the better thermal stability.Cyclic voltammetry measurements show two separate one-electron oxidation waves (Eox1 = -0.68 V and Eox2 = 1.00 V) for 1 and one irreversible oxidation wave. (Eox = 1.21 V) for 2. Experimentally estimated energy levels of the frontier orbital are EHOMO =–5.16 eV 1 and EHOMO =–5.42 eV 2. Fluorescence results indicate that dichloromethane solution of 1, 2 and 3 exhibit emission bands at 577 nm for 1, 410 nm,460 nm for 2 and 465 nm for 3, respectively, which may be attributed to intraligand charge transfer. Compound 4 is no fluorescence emission peak.We performed theoretical calculations of the compounds 1, 2 and 3 at DFT/B3LYP/6-31+g(d,p) level, employing the Gaussian 03 program package. On the basis of the calculating results we studied their electronic structure and frontier molecular orbital. The energy levels obtained from DFT calculations (EHOMO =–5.12 eV 1 and EHOMO =–5.43 eV 2) are in good agreement with those (EHOMO =–5.16 eV 1 and EHOMO =–5.42 eV 2) experimentally estimated energy levels of the frontier orbital. The results indicate that charge transfer transition is easier realized from excited state to ground state due to the decrease of the values of energy gaps (Eg = 2.38 eV 1, 2.35 eV 2) between HOMO and LUMO which will be beneficial to generate fluorescence emission.