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 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 hexaphyrin, which maintains a planar conformation in solution. Spectacles-shaped vinylene-bridged - and 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 hexaphyrin (8.6 ps) is 30 times shorter than that of the aromatic congener 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 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.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry