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.晶体堆积中存在磷杂香豆素(phosphacoumarin，1)的三种不同二聚体，使其成为独特的光电流发生器。反平行酯-酯二聚体(EED)内部的弱C–H···O相互作用，参与了反平行π堆积二聚体(ππD)的阶梯状排布。除上述二聚体外，晶体堆积中还存在一种与该阶梯取向垂直的酯-π二聚体(EπD)。相较于由分子内电荷转移(ICT)诱导的溶液态发射，磷杂香豆素1的固态发射发生红移，其背后的固态分子间电荷转移(iCT)机制已通过温度依赖性电子顺磁共振(EPR)与磁性测量得到表征。将磷杂香豆素1制备为薄膜后，在无需外部电解质的垂直器件中施加电压即可产生光电流。针对空穴(h+)与电子(e–)的迁移率(μ)开展的计算研究表明，ππD与EED分别呈现n型与p型半导体特性。EπD内空穴与电子的迁移率显著偏低，这表明用于光电流生成的n型与p型半导体的阶梯排列周围存在绝缘体环境。