Staircase Alignment of Phosphacoumarin Dimers as Alternate n- and p‑Type Organic Semiconductors for Photocurrent Generation

Three distinct types of dimers of phosphacoumarin (1) within the crystal packing make it a unique photocurrent generator. Weak C–H···O interactions within the antiparallel ester–ester dimer (EED) participate in the staircase arrangement of the antiparallel π-stack dimer (ππD). In addition to these dimers, crystal packing contains an ester-π dimer (EπD), which is perpendicular to the staircase orientations. Solid-state intermolecular charge transfer (iCT), which is responsible for red-shifted emission of 1 in comparison to intramolecular charge transfer (ICT)-induced solution-state emission, has been assessed by temperature-dependent EPR and magnetic measurements. A thin film of 1 generates photocurrent upon applying voltage in a vertical device without any external electrolyte. Computational studies on the mobilities (μ) for hole (h+) and electron (e–) suggest n- and p-type semiconductor behavior in ππD and EED, respectively. Significantly poor mobilities of h+ and e– in EπD suggest an insulator environment around the staircase alignment of n- and p-type semiconductors for photocurrent generation.