Tin(IV)-Centered 9,9’(10 <i>H</i> ,10’ <i>H</i> )-Spirobiacridine: Synthesis and Electronic Structure

Spirobiacridine derivatives with an N-donor spiro skeleton exhibit unique electronic structures and have attracted attention for applications in molecular magnetism and organic semiconductors. In this study, we synthesized a novel tin(IV)-centered spirobiacridine compound, 2,2’,7,7’-tetra-tert-butyl-9,9’(10H,10’H)-spirobistannaacridine. Despite its relatively long Sn–C bond lengths compared with those of the C-, Si-, and Ge-centered derivatives, the Sn compound exhibits two types of orbital interactions: (i) spiroconjugation and (ii) σ*(Sn–C)−π* interactions. These interactions were elucidated through a combination of electrochemical and spectroscopic measurements and theoretical studies. The results reveal that spiroconjugation is progressively weakened (C > Si > Ge > Sn), whereas stabilization of the lowest unoccupied molecular orbital (LUMO) is more pronounced for Sn than for Ge (Si > Sn > Ge).