Solid-state Effects in the Cyclovoltammetric HOMO-LUMO Determination: The case of Dinitrophenyl-hydrazone a,v-substituted Oligothiophenes

New acceptor-donor-acceptor (A-D-A) small molecules consisting of a central thiophene-based donor building block, i.e. 3-dodecyl-thiophene (for NTN) and 3,30'-dihexyl-terthiophene (for NT3N), and dinitrophenyl-hydrazone acceptor ends were synthesized. The materials display intense optical absorption spectra with absorption edge down to 1.65 eV. Cyclic voltammetry in solution shows oxidation and reduction processes with HOMO and LUMO energy levels at -5.6 and -3.5 eV. Cyclic voltammetry of NT3N cast films displays an unusual splitting of both oxidation and reduction peaks into two components with the same potential separation (0.45 V). Films of tetrahexyl-sexithiophene, selected as a model compound of NT3N without polar ends, show an analogous albeit much lower (0.13 V) splitting of the first one-electron oxidation process. The co-presence of a crystalline and an amorphous phase in the cast films is believed to be responsible for the splitting, which corresponds to the crystallization energy.