Aromatic versus Antiaromatic Effect on Photophysical Properties of Conformationally Locked trans-Vinylene-B ridged Hexaphyrins

作者：Yoon Min Chul; Cho Sung; Suzuki Masaaki; Osuka Atsuhiro^*; Kim Dongho

来源：Journal of the American Chemical Society, 2009, 131(21): 7360-7367.

DOI：10.1021/ja9000536

We have investigated the electronic structures and energy relaxation dynamics of vinylenebridged 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 HOckel antiaromatic compound, tends to adopt a twisted Mobius 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 HOckel aromatic and antiaromatic natures, respectively, as revealed by NMR spectroscopy, giving rise to remarkable differences in NICS(O) 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..

• 出版日期2009-6-3