Homoconjugation in diporphyrins: Excitonic behaviors in singly and doubly linked Zn(ii)porphyrin dimers

We have comparatively investigated excitonic features between singly and doubly alkyl bridged Zn(ii)porphyrin dimers (SLZn and DLZn, respectively) with Zn(ii)tetraphenylporphyrin (ZnTPP) as a reference by using various time-resolved anisotropy measurements. Time-resolved fluorescence anisotropy decay of the B-state of ZnTPP in toluene showed ultrafast dephasing times of 83 and 185 fs with an initial anisotropy value (r₀) of ∼0.7 being consistent with the theoretically proposed one. On the other hand, the Q-state of ZnTPP exhibited a depolarization time of 120 fs with a smaller r₀of 0.25 than that of the B-state because of probing photo-induced absorption in transient absorption anisotropy measurements. Spectroscopic features in absorption, fluorescence, and excitation anisotropy spectra of SLZn are similar to those of ZnTPP indicating a relatively weak exciton coupling in the Q-state. As a result, the observed depolarization time with a time-constant of 1.4 ps can be explained by a Förster-type incoherent energy transfer process between two constituent subunits. When compared to ZnTPP and SLZn however, DLZn exhibited very different optical characteristics such as more red-shifted absorption and emission spectra, three times larger fluorescence excitation anisotropy values, and ultrafast sign inversion in transient absorption anisotropy. According to these results, it can be inferred that the overall π-electron densities of the electronic excited states in DLZn are fully delocalized in a whole dimer and that DLZn can be regarded as a coherently coupled single quantum system not as separate individual chromophores. Frontier π-molecular orbital structures as well as electron density difference maps between occupied and unoccupied molecular orbitals involved in the electron promotions for underlying excited states clearly represent weakly and strongly exciton-coupled natures of SLZn and DLZn, respectively. Accordingly, the extended conjugation of π-electrons over the whole DLZn molecule demonstrates an example for homoconjugation between two aromatic porphyrins.