A theoretical approach to the formation mechanism of diphenyldithieno[3,2-b:2′,3′-d]thiophene from 1,8-diketone, 4,5-bis(benzoylmethylthio)thiophene: a DFT study

The mechanism of formation of dithieno[3,2-b:2′,3′-d]thiophene (DTT) through the reaction of 1,8-diketone, 4,5-bis(benzoylmethylthio)thiophene with P4S10 was examined in detail by employing DFT method at B3LYP/6-311+G(d,p) level. Two mechanisms were considered. The first one included two parts (i) transformation of the 1,8-diketone, 4,5-bis(benzoylmethylthio)thiophene to the dithione by the reaction of P4S10 with the carbonyl groups and (ii) cyclization of the dithione to the final product, DTT, through an intramolecular reaction of the thiophene with thiones. The second mechanism consists of an initial attack of the carbonyl oxygen to the phosphorus atom of P4S10 followed by cyclization via an intramolecular attack from the thiophene ring to the highly electrophilic carbons connected to the oxygens to form DTT. According to the calculated Gibbs free energies of the studied paths, the second mechanism is more favorable than the first one and both pathways proceed in a stepwise manner.

The possible reaction mechanisms of 1,8-diketone, 4,5-bis(benzoylmethylthio)thiophene with P4S10 yielding organic semiconducting materials, dithieno[3,2-b:2′,3′-d]thiophene (DTT) and its derivatives, have been investigated through DFT method at B3LYP/6-311+G(d,p) level in accordance with experimental findings. Calculations showed that the mechanism, which involves direct cyclization of ketone reactant in the presence of P2S5 is the most favorable mechanism and it proceeds in a stepwise manner.