Ultrafast energy relaxation dynamics of directly linked porphyrin arrays

A variety of porphyrin arrays connected together with different linkage wer e devised for possible applications to molecular optoelectronic devices suc h as wires, logic gates, and artificial light-harvesting arrays, etc. It ha s been relatively well established that the light signal transmission in th ese molecular assemblies is based on exciton migration process, which possi bly gives rise to the structural changes during the exciton delocalization process. Zinc(II) 5,15-di(3,5-di-tert-butylphenyl)porphyrin (Z1), its direc tly meso,meso-linked porphyrin dimer (Z2), trimer (Z3), and tetramer (Z4) w ere synthesized with the goal to elucidate the relationship between exciton migration and structural change upon photoexcitation. One of the most impo rtant factors in structural changes for these porphyrin arrays is mainly de termined by the dihedral angle between adjacent porphyrin moieties. For a s ystematic approach toward the investigation of the exciton coupling dynamic s influenced by the relative orientation between neighboring porphyrin mole cules, 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 o f the high-energy exciton Soret band induces a large angle change between a bsorption 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 S-1 states decay faster because of the increasing energy dissipa tion processes into a larger number of accessible states. In contrast, the rotational diffusion rates become slower in the same order because the over all molecular shape is elongated along the long axis of the molecular array s, which experiences a large displacement of solvent molecules in rotationa l diffusion motion. Ultrafast fluorescence decay measurements show that the S-2 --> S-1 infernal 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 deacti vation channel, while this process is relatively slow in Z1 (similar to 1.6 ps). Femtosecond transient absorption experiments with magic angle and dif ferent polarization in probe beam were performed to find the relationship b etween energy relaxation and anisotropy dynamics upon photoexcitation. The internal conversion in Z2, Z3, and Z4 is likely to be accompanied by the in coherent energy hopping processes occurring in less than similar to 200 fs judging from a large change in the anisotropy value in the transient absorp tion decay. In addition, the decay components with approximately 8 ps rime constant were observed in both fluorescence up-conversion and femtosecond t ransient absorption decays. These components are believed to arise from the conformational change in the excited states, because the dihedral angle di stribution in these arrays was estimated to be 90 degrees +/- 20 degrees at ambient temperature from the AMI calculation.