Structure and Polymorphism of Crystalline 4,5-Iminophenanthrene

4,5‐Iminophenanthrene (4H‐benzo[def]carbazole, C₁₄H₉N) crystallizes in space group ${\text{I4}}_1\mathrm{cd}$ of the tetragonal system with cell dimensions of $\text{a\hspace{0.17em}=\hspace{0.17em}b\hspace{0.17em}=\hspace{0.17em}7.428(5)}$ and $\text{c\hspace{0.17em}=\hspace{0.17em}35.58(3) Å}$ . The molecular packing is reported. Tetragonal crystals can undergo a sluggish transformation to a triclinic form with cell dimensions of $\text{a\hspace{0.17em}=\hspace{0.17em}12.70(4), b\hspace{0.17em}=\hspace{0.17em}12.96(3), c\hspace{0.17em}=\hspace{0.17em}11.88(3) Å, α\hspace{0.17em}=\hspace{0.17em}90.0(1), β\hspace{0.17em}=\hspace{0.17em}99.0(1), γ\hspace{0.17em}=\hspace{0.17em}98.3(1)°}$ . The new form exhibits a red‐shifted, broad, structureless fluorescence spectrum characteristic of excimer emission. Tetragonal form and monomerlike fluorescence are recovered at a well‐defined transition at 73°C. Delayed fluorescence due to triplet–triplet exciton annihilation has been observed in both forms of the material.