Near-Infrared S Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin.

This study revealed S fluorescence from deprotonated meso-pentafluorophenyl-substituted Möbius aromatic [32]heptaphyrin(1.1.1.1.1.1.1) that was formed upon treatment of neutral antiaromatic [32]heptephyrin with tetrabutylammonium fluoride. Higher excited-state dynamics and emission were studied by fs-transient absorption spectroscopy and a broad-band fluorescence upconversion technique. This is the first S fluorescence from chromophores with twisted Möbius topology, and the observation of S fluorescence in the near-infrared region has been unprecedented. The higher excited-state dynamics of neutral and deprotonated [32]heptaphyrins were compared by ultrafast transient absorption spectroscopy to understand the S fluorescence origin. In the antiaromatic [32]heptaphyrin, a fast time component of 65 fs was assigned as an internal conversion process from the S state to the S state, which occurs prior to relaxation to the optically dark, lowest electronic state (S). Therefore, the S state of the antiaromatic [32]heptaphyrin acts as a trap state intervening radiative transitions from the S state to the S state. In deprotonated [32]heptaphyrin, the internal conversion from the S state to the S state proceeds with a slower time constant of 150 fs for owing to its rigid structure, helping the observation of its S fluorescence.