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.