Negligible diradical character for the ultralong C–C bond in 1,1,2,2-tetraarylpyracene derivatives at room temperature

There is evidence that the crystalline-state thermochromic behavior of dispiropyracene acr-1 with an ultralong C–C bond [1.791(3) Å at 413 K, 1.771(3) Å at 93 K] might be due to thermal generation of a bond-dissociated triplet diradical. The C1–C2 bond lengths (d) in the newly prepared 1,1,2,2-tetraarylpyracenes 1a–e [1.717(4)–1.761(4) Å at 113/123/153 K] are also much larger than the standard value for C(sp3)–C(sp3) (1.54 Å). The fact that there is no correlation between d and the radical-stabilizing parameter (σ•) provides evidence that there is no contribution from a crystallographic artifact caused by contamination of the diradical in the crystal of 1 at room temperature or below. Further indication for the covalent nature of the ultralong C–C bond in tetraphenylpyracene 1c was attained by Raman spectroscopy, showing the far red-shifted C1–C2 stretching vibration (638 cm−1) and by the theoretical prediction of very large vertical singlet–triplet energy gap (58.3 kcal mol−1 at the UB3LYP/6-31G∗ level). However, at elevated temperatures bond dissociation might occur forming a triplet diradical.