Exciton interactions in a self-assembled phthalocyanine dimer

Photophysical properties of tri-tert-butylpyridinotribenzotetraazaporphinatozinc (ZnPcPy) complexes, a self-assembled dimer ((ZnPcPy)₂) and a monomer (ZnPcPy-Py) in a non-polar solvent and with pyridine, respectively, have been studied. The lowest excited singlet (S₁) state is investigated by the combined use of electronic absorption, magnetic circular dichroism and fluorescence spectroscopy and the S₁ properties are analyzed by configuration interaction calculations including exciton interactions between ZnPcPy constituents. The lowest excited triplet state is investigated by time-resolved electron paramagnetic resonance (TREPR). TREPR spectra of (ZnPcPy)₂ exhibit a dramatic temperature dependence at 10-220 K, in contrast to a negligible change for ZnPcPy-Py. The spectral changes are reasonably attributed to triplet energy transfer between two ZnPcPy constituents, and are well reproduced using energy transfer rates, k = 3 x 10⁸ s^-1 (100 K), 7 x 10⁸ s^-1 (140 K), and 2 x 10⁹ s^-1 (180 K), respectively. The activation energy of the energy transfer process is evaluated as 2.7 x 10² cm^-1 from the Arrhenius plot. This study demonstrates the utility of TREPR for investigating energy transfer processes among these macrocycles.