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

Takashi Takeda, Hidetoshi Kawai, Rainer Herges, Eva Mucke, Yoshitaka Sawai, Kei Murakoshi, Kenshu Fujiwara, Takanori Suzuki

Research output: Contribution to journalArticlepeer-review

27 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)3693-3697
Number of pages5
JournalTetrahedron Letters
Volume50
Issue number26
DOIs
Publication statusPublished - 2009 Jul 1
Externally publishedYes

Keywords

  • Diradical
  • Steric repulsion
  • Strained molecule
  • Thermochromism
  • Ultralong bond
  • X-ray analysis

ASJC Scopus subject areas

  • Biochemistry
  • Drug Discovery
  • Organic Chemistry

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