ELECTROCHEMICAL OLIGOMERIZATION OF ARENEMETHYLIDENES - ATTEMPTS TO SYNTHESIZE LOW BANDGAP SYSTEMS

The electrochemical oligomerization of the arenemethylidenes, 1,3-dihydro-bis-1,3-(2-thienylmethylidene)-isothianaphthene (1b), 1,3-dihydro-bis-1,3-[2-(N-methylpyrryl)methylidene]isothianaphthene (4b), 1,3-dihydro-bis-1,3-(2-thienylmethylidene)thieno[3,4-c]thiophene (5b), 2,5-dihydro-bis-2,5-(2-thienylmethylidene)thiophene (6b), 1,3-dihydro-bis-1,3-(2-thienylmethylidene)isothianaphthene-2-oxide (1a), 1,3-dihydro-bis-1,3-[2-(N-methylpyr-ryl)methylidene]isothianaphthene-2-oxide (4a), 1,3-dihydro-bis-1,3-(2-thienylmethylidene)thieno[3,4-c]thiophene-1-oxide (5a) and 2,5-dihydro-bis-2,5-(2-thienylmethylidene)thiophene-1-oxide (6a), is investigated by cyclovoltammetry, coulometry, spectroelectrochemistry (UV-Vis-NIR, EPR) and a.c. impedance spectroscopy. Detailed spectroelectrochemical investigation of the oxidation process of compounds la, lb, 4a, 4b, 5a, 5b, 6a and 6b reveals red-shifted-absorptions after oxidation indicating the formation of larger pi-electron systems. The oxidatively formed layers are electroactive as shown by independent cyclovoltammetry and spectroelectrochemistry measurements; the layers show a reversible switching behaviour between the oxidized and reduced forms. The reactive side of the arenemethylidene precursors in electrochemical oligomerization is determined by blocking possible linking positions with methyl groups. The 4,4'-disubstituted 1,3-dihydro-bis-1,3-(4-methyl-2-thienylmethyli-dene)isothianaphthene (2b) is oxidatively oligomerizable. The 5,5'-disubstituted 1,3-dihydro-bis-1,3-(5-methyl-2-thienylmethylidene)isothianaphthene (3b) does not form oligomer layers on the electrode due to the methyl-blocked 5,5'-(alpha,alpha') positions and 3b exhibits hyper-fine structure in the EPR spectrum. From this it is concluded that the monomer units within an oligomer chain of polyarenemethylidenes are alpha-alpha linked. The bandgap values of the polyarenemethylidenes are determined from UV-Vis-NIR spectra to be in the range 1.8-2.1 eV. The conductivity of the oxidized oligomer layers is higher than the reduced layers as shown by a.c. impedance spectroscopy. 2,5-Bis-(di-2-thienylmethylidene)-2,5-dihydrothiophene (7) is not oxidatively oligomerizable due to the formation of stable triheteroaryl methyl cationic structures.