No abstract

Directly meso-meso linked porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four, six, and eight porphyrins have been synthesized in a stepwise manner from a 5,10-diaryl zinc(II) porphyrin building block. Symmetric cyclic structures have been indicated by their very simple (1)H NMR spectra that exhibit only a single set of porphyrin and their absorption spectra that display a characteristic broad nonsplit Soret band around 460 nm. Energy minimized structures calculated at the B3LYP/6-31G* level indicate that a dihedral angle between neighboring porphyrins decreases in order of CZ6 > CZ8 > CZ4, which is consistent with the (1)H NMR data. Photophysical properties of these molecules have been examined by the steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related with the excitation energy migration processes within the porphyrin rings, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Forster-type incoherent energy hopping model. Consequently, the excitation energy hopping rates have been estimated for CZ4 (119 +/- 2 fs)(-)(1), CZ6 (342 +/- 59 fs)(-)(1), and CZ8 (236 +/- 31 fs)(-)(1), which reflect the magnitude of the electronic coupling between the neighboring porphyrins. Overall, these porphyrin rings serve as a well-defined wheel-shaped light harvesting antenna model in light of very efficient excitation energy hopping along the ring.

A dodecameric porphyrin wheel was prepared by Ag(I)-promoted intramolecular coupling of a linear porphyrin dodecamer and was observed by scanning tunneling microscopy (STM). Efficient energy hopping along the array was revealed by femtosecond transient anisotropy measurements.

Intramolecular excitation energy hopping (EEH) time within a dodecameric porphyrin wheel C6ZA, in which six meso-meso linked zinc(II) diporphyrin (Z2) subunits are bridged by 1,3-phenylene spacers, is deduced by a Förster energy hopping model based on S(1)-S(1) exciton-exciton annihilation and anisotropy depolarization. Under the assumption that the energy hopping sites are six Z2 subunits, two different observables (e.g., exciton-exciton annihilation and anisotropy depolarization times) consistently give the EEH time of 4.0 +/- 0.4 ps via 1,3-phenylene spacer of C6ZA, which is faster than 9.4 ps of linear 2Z2 (1,3-phenylene-linked zinc(II) tetraporphyrin). As a consequence, C6ZA serves as a well-defined two-dimensional model for a light-harvesting complex.

The excitation-energy-hopping (EEH) times within two-dimensional cyclic zinc(II)-porphyrin arrays 5 and 6, which were prepared by intermolecular coordination and ring-closing metathesis reaction of olefins, were deduced by modeling the EEH process based on the anisotropy depolarization as well as the exciton-exciton annihilation dynamics. Assuming the number of energy-hopping sites N = 5 and 6, the two different experimental observables, that is, anisotropy depolarization and exciton-excition annihilation times, consistently give the EEH times of 8.0 +/- 0.5 and 5.3 +/- 0.6 ps through the 1,3-phenylene linkages of 5 and 6, respectively. Accordingly, the self-assembled cyclic porphyrin arrays have proven to be well-defined two-dimensional models for natural light-harvesting complexes.

We report the femtosecond transient absorption dynamics of the gold and gold-polypyrrole nanoparticles by photoexcitation at various wavelengths. The bleach recovery dynamics of the surface plasmon band for the gold and gold-polypyrrole nanoparticles exhibit different responses to the pump beam energy and intensity in thermal energy transfer from the gold nanoparticles to the surrounding medium or embedding material. This indicates that directly attached polypyrroles provide fast thermal energy transfer pathways for the core gold nanoparticles.

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.

Conjugated rod molecules bearing flexible coil polymer moieties on both ends self‐organize into 1D or 2D structures, and eventually into a 3D tetragonal superlattice (see Figure), depending on the number of monomeric units in the coils. The 3D ordered rods exhibit a highly extended fluorescence lifetime over the 1D or 2D arrays, making synthetic control of the rod‐to‐coil volume fraction a viable strategy to regulate photophysical properties.