Room‐Temperature Phosphorescence of Cocrystals of Aromatic Bisimides and Triplet Sensitizer Pt(acac)₂

Abstract Four sterically shielded aromatic bisimides are synthesized and their ability to form cocrystals with the triplet sensitizer platinum(II) acetylacetonate (Pt(acac)₂) is investigated. Single crystals of the individual electron‐deficient bisimides are studied first, two showing bright blue fluorescence with quantum yields of up to 28%, while the other two barely emit at all. After successful cocrystallization of all four aromatic bisimides with the electron‐rich Pt(II) complex, two different stoichiometries, either 1:1 or 1:2, are found in the crystalline solid state, whose absorbance and photoluminescence exhibit distinct bathochromic shifts. While two naphthalene bisimides show strong and long‐lived room‐temperature phosphorescence (RTP) with quantum yield exceeding 50%, the other two aromatic bisimides show only minor photoluminescence at RT, but a bright low‐temperature phosphorescence (LTP) at 80 K. Theoretical calculations reveal the importance of changing the magnetic quantum number m_l of the involved d‐orbitals at the platinum center as well as the alignment of the Pt(II) center above an aromatic C─C bond for an efficient intersystem crossing in these hybrid systems.