Aromatic versus antiaromatic effect on photophysical properties of conformationally locked trans-vinylene-bridged hexaphyrins

Min Chul Yoon, Sung Cho, Masaaki Suzuki, Atsuhiro Osuka, Dongho Kim

Research output: Contribution to journalArticle

74 Citations (Scopus)

Abstract

We have investigated the electronic structures and energy relaxation dynamics of vinylene-bridged hexaphyrins using steady-state and time-resolved spectroscopies along with theoretical calculations in order to reveal their aromaticity-dependent electronic and magnetic properties. Ethynyl-TIPS- substituted planar and rectangular [28]hexaphyrin, regarded as a Hü ckel antiaromatic compound, tends to adopt a twisted Möbius aromatic topology via structural distortion in order to reduce the total internal energy, in contrast to aromatic [26]hexaphyrin, which maintains a planar conformation in solution. Spectacles-shaped vinylene-bridged [26]- and [28]hexaphyrins represent highly Hückel aromatic and antiaromatic natures, respectively, as revealed by NMR spectroscopy, giving rise to remarkable differences in NICS(0) and HOMA values and shapes of steady-state absorption and emission spectra. In particular, lifetime of the lowest singlet excited state of [28]hexaphyrin (8.6 ps) is 30 times shorter than that of the aromatic congener [26]hexaphyrin (282 ps), as measured by the femtosecond transient absorption technique. Both frontier molecular orbital analyses and vertical excitation energy calculations suggest that vinylene-bridged [28]hexaphyrin has an optically dark lowest singlet state in the NIR region, as observed in the absorption spectrum with a very low oscillator strength, which might act as a ladder state in the excited-state energy relaxation dynamics. Our findings provide further insight into the aromaticity-driven electronic properties of various porphyrinoids as well as of aromatic/antiaromatic hydrocarbon systems.

Original languageEnglish
Pages (from-to)7360-7367
Number of pages8
JournalJournal of the American Chemical Society
Volume131
Issue number21
DOIs
Publication statusPublished - 2009 Jun 3

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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