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.

Ultrafast exciton dynamics after Soret- or Q-band excitation of a directly β,β′-linked bisporphyrin

“…32 Leading examples are dimers and polymers, 11,19,33 linear arrays of various lengths, 12,34 thin films, 35 rings, 17 and boxes. 36,37 In early work, Hochstrasser and coworkers investigated an ethyne bridged meso,meso 0linked and a b,b 0 -linked dimer. 11 The latter dimer differs from the dimer that we report on here by the presence of an ethyne bridge rather than a direct linkage.…”

Axial, Helical, and Planar Chirality in Directly Linked Basket-Handle Porphyrin Arrays

“…The intensively studied meso - meso coupled dimers are achiral as long as the meso substituents orthogonal to the axis are identical. ,,,,− Nonidentical substituents lead to the loss of all mirror planes within the dimer and therefore to an unsymmetric and thus axially chiral structure (Figure A). We have discovered several examples of such meso-meso linked systems in the literature, where, however, the axial chirality was not recognized at all, and only one case where enantiomers had been resolved but absolute configurations were not assigned. − The same chirality criteria apply for β-meso linked bisporphyrins: If the meso -coupled building block comprises two identical meso substituents orthogonal to the axis, the resulting bisporphyrin is achiral . In all cases with nonidentical meso substituents, the resulting structure is chiral (Figure B).…”

Delocalization and Migration of Excitation Energy and Charge in Supramolecular Systems

“…28,29 For porphyrin boxes, the energy transfer rate from porphyrin to coordinated porphyrin was on the order of several tens of picoseconds, depending on the length of the phenyl ring linking porphyrin and the zinc center. For prisms, the energy transfer rate depends on the distance.…”