New 1,3,4-oxadiazole containing materials with the effective leading substituents: The electrochemical properties,optical absorptions,and the electronic structures

The electrochemical and optical absorption properties as well as the thermal stabilities of a series of 1,3,4-oxadiazole dimers 1,3-bis2-(4-methylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-X) and its derivatives with the different alkoxy substituents on the central benzene ring such as O(CH₂)_n?1CH₃ (OXD-An, n = 3, 7, 10, 16), O(CH₂)_nOC₆H₅CH₃ (OXD-Bn, n = 6), and O(CH₂)_nOC₆H₄NNC₆H₄OCH₃ (OXD-Cn, n = 3, 6, 10) are studied. The DSC measurements exhibit dramatically elevated glass transition temperatures for OXD-X and OXD-An (120–245 °C) in contrast to the well-known PBD (～60 °C), indicating the better thermal stabilities. From OXD-X to OXD-An, OXD-Bn or OXD-Cn, the cyclic voltammograms and UV–vis absorption spectroscopy display significant variation, in which the later three species show additional lower energy absorptions at λ > 330 nm compared with OXD-X and particularly, both of OXD-Bn and OXD-Cn display an oxidation peak at ～+1.0 V and two successive redox reactions occur for OXD-Cn. Theoretically, the B3LYP/6-31g calculations explore that these extraordinary properties are due to the influence of the substituents on the benzene ring to the frontier molecular orbital distribution, especially the O(CH₂)_nOC₆H₄NNC₆H₄OCH₃ groups in OXD-Cn deduce the new pictures of the frontier molecular orbitals, causing the electron-transporting behavior originally happening on the molecular skeleton transferred to the side chain.