Synthesis of Bis([2,2′-bithiophen]-5-yl)-Substituted Oligothiadiazoles: Effect of the Number of Acceptor Units on Electrochemical and Spectroscopic Properties

1,3,4-thiadiazole, 2,2'-bi(1,3,4-thiadiazole), 2,2':5',2''-ter(1,3,4-thiadiazole) and 2,2':5',2'':5'',2'''-quater(1,3,4-thiadiazole) symmetrically disubstituted with 3-alkyl-(2,2'-bithiophen)-5-yl were synthesized by new procedures, using readily available ethyl 3-alkyl-(2,2'-bithiophene)-5-carboxylate as a convenient substrate. These new compounds with fixed number of donor rings and increasing number of acceptor rings showed very interesting, tunable redox properties. In particular, they exhibited electron affinities (EAs) ranging from -3.06 eV to -3.83 eV reaching EA values desired for air operating n-type organic semiconductors. Their electrochemically determined ionization potentials (IPs) were only moderately dependent on the number of thiadiazole rings, varying from 5.83 eV to 6.01 eV. Emission spectra of these compounds could also be tuned in a wide range (from 470 nm to 600 nm). Spectroscopic and electrochemical data were confirmed by DFT calculations demonstrating full consistency.