Solvent-Dependent Aromatic versus Antiaromatic Conformational Switching in meso-(Heptakis)pentafluorophenyl [32]Heptaphyrin

• Published in: Chemistry : a European journal 2011-06, Vol.17 (24), p.6707-6715
• Main Authors: Yoon, Min-Chul, Shin, Jae-Yoon, Lim, Jong Min, Saito, Shohei, Yoneda, Tomoki, Osuka, Atsuhiro, Kim, Dongho
• Format: Article
• Language: English
• Subjects: aromaticity; Chemistry; expanded porphyrins; Möbius confomation; porphyrinoids; ultrafast spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201003736

We have investigated conformational switching dynamics of meso‐heptakis(pentafluorophenyl) [32]heptaphyrin(1.1.1.1.1.1.1) in various solvents using steady‐state, time‐resolved, and temperature‐dependent spectroscopy. Absorption and fluorescence spectra of [32]heptaphyrin are quite sensitive to solvent environments. In nonpolar toluene, the antiaromatic figure‐of‐eight conformation is dominant, as seen in the X‐ray crystallography, based on broad and weak absorption bands without any fluorescence and moderate paratropic ring current. On the other hand, a well‐resolved sharp absorption spectrum, strong fluorescence, and diatropic ring current in the 1H NMR spectrum in slightly polar THF indicate that most of [32]heptaphyrin molecules take significantly distorted Möbius conformation with aromatic character. By using transient absorption (TA) spectroscopy, the lowest singlet excited‐state lifetimes have been revealed to decay biexponentially with the time constants of 5 and 65 ps for figure‐of‐eight and Möbius conformations, respectively. Based on these results along with vertical excitation energy calculations, we are able to assign two conformers as Hückel antiaromatic and Möbius aromatic species, respectively; it shoulf be noted that the aromaticity/antiaromaticity does not change with temperature variation. Interestingly, in moderately polar solvent, ethyl ether, we find out that these two conformational isomers coexist with a dynamic equilibrium, as revealed by excitation‐wavelength‐dependent TA, temperature‐dependent absorption and 1H NMR spectra. Through our findings, we have demonstrated that the conformational switching dynamics between Hückel antiaromatic and Möbius aromatic conformers in [32]heptaphyrin(1.1.1.1.1.1.1) are strongly affected by solvent medium as well as temperature. Between Hückel and Möbius: Switching dynamics of [32]heptaphyrin between Hückel antiaromatic and Möbius aromatic conformers are strongly affected by solvent polarity as well as temperature. Existence of conformers different in structure and aromaticity has been revealed mainly by time‐resolved spectroscopic measurements along with theoretical calculations (see graphic).