Ultrafast Energy Relaxation Dynamics of Directly Linked Porphyrin Arrays

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Abstract

A variety of porphyrin arrays connected together with different linkage were devised for possible applications to molecular optoelectronic devices such as wires, logic gates, and artificial light-harvesting arrays, etc. It has been relatively well established that the light signal transmission in these molecular assemblies is based on exciton migration process, which possibly gives rise to the structural changes during the exciton delocalization process. Zinc(II) 5,15-di(3,5-di-tert-butylphenyl)porphyrin (Z1), its directly meso,meso-linked porphyrin dimer (Z2), trimer (Z3), and tetramer (Z4) were synthesized with the goal to elucidate the relationship between exciton migration and structural change upon photoexcitation. One of the most important factors in structural changes for these porphyrin arrays is mainly determined by the dihedral angle between adjacent porphyrin moieties. For a systematic approach toward the investigation of the exciton coupling dynamics influenced by the relative orientation between neighboring porphyrin molecules, various time-resolved spectroscopic techniques such as fluorescence decay and transient absorption measurements with different polarization in pump/probe beams have been utilized. The steady-state excitation anisotropy spectra of Z2, Z3, and Z4 porphyrin arrays show that the photoexcitation of the high-energy exciton Soret band induces a large angle change between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. In the order of Z1, Z2, Z3, and Z4, their S1 states decay faster because of the increasing energy dissipation processes into a larger number of accessible states. In contrast, the rotational diffusion rates become slower in the same order because the overall molecular shape is elongated along the long axis of the molecular arrays, which experiences a large displacement of solvent molecules in rotational diffusion motion. Ultrafast fluorescence decay measurements show that the S2 → S1 internal conversion process occurs in less than 1 ps in Z2, Z3, and Z4 due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (∼1.6 ps). Femtosecond transient absorption experiments with magic angle and different polarization in probe beam were performed to find the relationship between energy relaxation and anisotropy dynamics upon photoexcitation. The internal conversion in Z2, Z3, and Z4 is likely to be accompanied by the incoherent energy hopping processes occurring in less than ∼200 fs judging from a large change in the anisotropy value in the transient absorption decay. In addition, the decay components with approximately 8 ps time constant were observed in both fluorescence up-conversion and femtosecond transient absorption decays. These components are believed to arise from the conformational change in the excited states, because the dihedral angle distribution in these arrays was estimated to be 90° ± 20° at ambient temperature from the AM1 calculation.

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