Spectroscopic Properties of a Self-Assembled Zinc Porphyrin Tetramer II. Time-Resolved Fluorescence Spectroscopy

Abstract

Excited-state kinetics of complexes of a functionalized zinc tetraphenylporphyrin (ZnTPP) derivative, zinc mono(4-pyridyl)triphenylporphyrin (ZnPyP) in toluene and polystyrene/toluene mixtures have been investigated by time-resolved fluorescence spectroscopy. In addition to the ∼2.0 ns monomer fluorescence lifetime, a ≈ 1.5 ns component was found by applying global analysis to the time-resolved fluorescence decay. The 1.5 ns component is assigned to a cyclic porphyrin tetramer [Part I], with a ≈ 1 ns rotational correlation time at 10°C. The initial fluorescence anisotropy of the monomer is found to be 0.1. In the tetramer an additional depolarization process occurs with a correlation time of ∼31 ps, resulting in a further decrease of the anisotropy from 0.1 to 0.025. This additional depolarization is ascribed to singlet energy transfer between the porphyrin units that constitute the tetramer. The intramolecular energy transfer processes have been simulated using the Monte Carlo method, yielding rate constants of (26 ( 4 ps) -1 and e (180 ps) -1 for energy transfer between nearest neighbor and next nearest neighbor porphyrins in the tetramer.

Folding‐Induced Modulation of Excited‐State Dynamics in an Oligophenylene–Ethynylene‐Tethered Spiral Perylene Bisimide Aggregate

“…Polarization‐dependent TA measurements, which employ different polarizations of the pump and probe pulses, provide valuable insights into the previously mentioned ultrafast depolarization channel;20b, 24, 27 this channel is too fast to be elucidated by the TCSPC technique with the time resolution of approximately 50 ps. As opposed to PBI J‐aggregate systems, in which the transition dipole moments are placed parallel to each other, the helically organized nature of our aggregate system allows for a fast decay component in the time‐dependent transient absorption anisotropy (TAA) profiles, which is indicative of an EET process between the constituent units.…”

Supramolecular Coordination: Self-Assembly of Finite Two- and Three-Dimensional Ensembles

“…276 The electron transfer rate was found to be ~8 times faster than the analogous Zn(II)-porphyrin tetramers. 717,718 A self-assembled cyclic zinc chlorophyll tetramer exhibited an ultrafast intramolecular energy transfer rate as determined from femtosecond singlet-singlet annihilation and transient absorption anisotropy studies. 719 The cyclic tetramer upon excitation with 120 fs laser pulses yielded its lowest singlet excited state in which the rate of energy transfer was ~20 times faster than the tetramer in the ground state.…”

Supramolecular Organization of Porphyrins and Phthalocyanines by Use of Biomimetic Coordination Methodology

“…Schaafsma and coworkers observed photoexcited energy migration among the neighboring porphyrin units with a rate of (26 ps) −1 in the porphyrin square array 26b 4 . 160,161 On the other hand, Wasielewski et al observed the photoexcited energy migration in a similar square array of pyridyl-appended chlorophyll 27. 85 The rate of energy-hopping in 27 4 was (1.3 ps) −1 , 20 times faster than that in 26b 4 (Scheme 56).…”

“…Photoexcited energy migration in porphyrin square array 26b 4 and chlorophyll square array 27 4 . 85,160,161 Scheme 57. Macrocyclic wheels 42 and 114, synthesized using the template-directed method, incorporating tetrapyridylporphyrin as an electron acceptor and 115 as an energy acceptor.…”