Photophysical Properties of a Three-Dimensional Zinc(II) Porphyrin Box

Abstract

Photophysical properties of a three-dimensional zinc(II) porphyrin box ((PyZ2)4) and its constituent unit (orthogonally linked porphyrin dimers such as PyZ2 and Z2) have been comparatively investigated by absorption, fluorescence, picosecond time-resolved fluorescence/fluorescence anisotropy, resonance Raman, and femtosecond transient absorption measurements. The negligible temperature dependence on the fluorescence and fluorescence excitation spectra of PyZ2 in CH2Cl2 indicates that the porphyrin box is not easily disorganized in the temperature range of ∼10−40 °C. The enhancement of a short-lived (∼1 ns) fluorescence decay component, as well as the shifts of the ν2 and ν4 Raman bands, indicates that the porphyrin ring planarity is somewhat perturbed, because of the five-coordination of the zinc(II) porphyrin moiety in the porphyrin box. The relatively slow rotational diffusion time of ∼4.7 ns and r∞ = 0 in the time-resolved fluorescence anisotropy decay measurement illustrates that the porphyrin box rotates more slowly than the porphyrin dimers (PyZ2 and Z2), because of its large molecular volume. In addition, the fast anisotropy rise time of 12 ± 3 ps in the femtosecond transient absorption anisotropy decay measurement reflects an excitation energy-transfer process in the porphyrin box. The red-shifted high-energy Soret band and the blue-shifted low-energy Soret band of the porphyrin box are indicative of excitonic dipole−dipole interactions between four parallel transition dipole moments along the x- or y-axis and between eight parallel transition dipole moments along the z-axis of the box. This process is regarded as a dipole redistribution process within the three-dimensional zinc(II) porphyrin box.