Near-Infrared S2 Fluorescence from Deprotonated Möbius Aromatic [32]Heptaphyrin.

This study revealed S2 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 S2 fluorescence from chromophores with twisted Möbius topology, and the observation of S2 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 S2 fluorescence origin. In the antiaromatic [32]heptaphyrin, a fast time component of 65 fs was assigned as an internal conversion process from the SB state to the SQ state, which occurs prior to relaxation to the optically dark, lowest electronic state (SD). Therefore, the SQ state of the antiaromatic [32]heptaphyrin acts as a trap state intervening radiative transitions from the SB state to the S0 state. In deprotonated [32]heptaphyrin, the internal conversion from the SB state to the SQ state proceeds with a slower time constant of 150 fs for owing to its rigid structure, helping the observation of its S2 fluorescence.