Stereoisomerism-controlled Packing in Ladder-type Indacenodithieno[3,2-b]thiophene Crystals

Stereochemistry is a potent way to direct the molecular packing in condensed phases. Here, we synthesized the enantiopure, racemic and achiral isomers of p-type small molecule i.e. indacenodithieno[3,2-b]thiophene (IT) to understand the impact of stereochemistry on molecular packing and solid state properties. In the solution state, the optical properties remain nearly identical among the isomers, however significant difference was observed in the condensed phase. X-ray diffraction pattern revealed enantiopure isomers were more tightly packed and exhibited strong π-stacking relative to their racemic and achiral counterparts. Two dimensional arrangement of the stacked enantiopure isomers gives more dense-packed crystalline phases than an achiral analog, which is unusual anti-Wallach type condensed phases. The enantiopure one exhibited two times higher photoconductivity than its achiral or racemic analogues, as well as showing high electron spin polarizability (~ 75%) with electrical current throughput (100 nA). The result highlights the role of stereochemistry as a key strategy to direct the condensed phase packing and properties in conjugated crystals.