Comparative studies on energy relaxation dynamics of directly linked Zn^II porphyrin dimers with different dihedral angles

Photophysical properties of the strapped Zn^II porphyrin dimers (Sn, the strap chain between the porphyrins changes; -O-(CH₂)_n-O-, n = 1, 2, 3, 4, 8, and 10) and directly linked Zn^II porphyrin dimer (Z2) have been investigated by the steady-state and time-resolved spectroscopic methods. The reduced dihedral angle in Sn gives rise to additional absorption bands around 400 nm as well as broad fluorescence in the near-infrared region. The fluorescence lifetimes of Sn become gradually, but not so significantly, shorter as the strap length becomes shorter, indicating that the lifetimes of the lowest excited singlet states of Sn are not affected much despite their structural distortions such as mean plane deviations and the existence of two conformers. Fluorescence excitation and viscosity-dependent fluorescence spectra suggest that S2 exists as two conformers. A and B conformer, which has been also confirmed by X-ray data. The B conformer with a shorter interporphyrin distance has stronger interporphyrin interactions than the A conformer, which was revealed by the fluorescence excitation and fluorescence spectra. The A conformer is responsible for the shorter wavelength fluorescence at ca. 660 nm, whereas the B conformer gives rise to the near-infrared fluorescence. The photoinduced near-infrared absorption band was suggested to be characteristic of S2, which was not observed in Z2. The conformational dynamics of S2 with a time constant of 15 ps was observed in the transient absorption and fluorescence upconversion measurements. From the comparative studies of Z2 and Sn, we have demonstrated that the orientation of the two porphyrin macrocycles has a considerable effect on their photophysical properties.