Excited-State Energy Transfer Processes in Phenylene- and Biphenylene-Linked and Directly-Linked Zinc(II) and Free-Base Hybrid Diporphyrins

Abstract

The photoinduced energy transfer processes in 1,4-phenylene-, 1,3-phenylene, 1,2-phenylene, and 4,4‘-biphenylene-linked and directly-linked Zn(II)-free base porphyrin heterodimers in THF were investigated by femtosecond transient absorption spectroscopy. The energy transfer rates were compared between TPP-type and OEP-type heterodimers respectively as A2u-HOMO and A1u-HOMO subunits, for evaluating the relative contribution of the through-bond and through-space interactions. The rate difference becomes smaller with a decrease of spacer, more than 10 for 1,4-bis(phenylethynyl)phenylene and 1,4-diphenylethynylene, 4 for 4,4‘-biphenylene-linked heterodimer, and 3 for 1,3- and 1,4-phenylene-linked spacers. In the meso−meso directly-linked case, the energy transfer rates are the same ((0.55 ps)-1) for 5,5,15,15-tetrakis(3,5-bis(octyloxy)phenyl)-substituted and 5,5,15,15-tetrakis(pentafluorophenyl)-substituted heterodimers, featuring only a minor influence of the frontier orbital characteristics on the energy transfer rate. The energy transfer rates are identical (0.55 ps)-1 for the directly-linked meso−meso heterodimers substituted with 3,5-bis(octyloxy)phenyl and pentafluorophenyl groups regardless of the difference in the HOMO orbital symmetry characteristics, suggesting the predominant Coulombic interaction for the energy transfer in these close proximity porphyrin dimers. In the case of 1,2-phenylene-linked heterodimers, the choice of the peripheral substituents can lead to a state-to-state rapid energy transfer with a rate of (0.55 ps-1) for the TPP-type model or a delocalized excimer-like diporphyrin excited state for the OEP-type model. Collectively, these results indicate that even for the covalently-linked models the relative contribution of the through-space Coulombic interaction becomes increasingly important upon the decrease of the center-to-center separation. Especially, the fast and efficient energy transfer occurring in the directly-linked heterodimer illustrates that this porphyrin unit can be utilized as a good candidate for energy transfer functional arrays in molecular photonic devices.

Meso-Linked Multiporphyrins as Model for Light Harvesting Systems: Review

“…There are two mechanisms for excitation energy transfer, Forster-type (through-space, TS) energy transfer by coulombic interaction between transition dipole moments and Dexter-type (through-bond, TB) energy transfer via electron-exchange interaction through direct or indirect overlap of the molecular orbitals. The importance of the orbital interaction on the TB-EET rate was exhibited by the comparison of tetraphenylporphyrin (TPP)-type diporphyrins versus octaethylporphyrin (OEP)-type diporphyrins, both of which have the same center-to center distance between the two porphyrin units [91]. The energy transfer rates in the TPP-type diporphyrins are distinctly larger as compared to their OEP-type counterparts but such transfer rate enhancement decreases on decreasing the distance between the two porphyrins [92].…”

“…The enhanced contribution of the Forster mechanism for excitation energy transfer has been accounted when the bridging group between the two porphyrins becomes shorter, as the energy transfer rate enhancement decreased on decreasing the distance between the porphyrins as in case of TPP-type diporphyrins. The Forster excitation energy transfer is highly operative for a donor-acceptor (D-A) model with quite a short D-A separation [91]. On the contrary, the Forster energy transfer rate decreases quickly with increasing the distance between two porphyrin units.…”

“…This change in symmetry alters the photophysical properties of porphyrinic arrays depending on the degree of electronic interactions between the porphyrins [97]. The directly linked orthogonal porphyrin arrays provide the prospects as artificial light harvesting arrays and molecular photonic wires because the unique photophysical aspect of these molecular arrays arising from substantial interchromophoric electronic interactions mimics the facile energy migration processes in biological light harvesting assemblies, where electronic delocalization is negligible due to a lack of direct bond linkage between individual pigment molecules [91].…”

A Recent Review on Chemiluminescence Reaction, Principle and Application on Pharmaceutical Analysis

“…Resonance energy transfer, on the other hand, is not diffusion-controlled, does not depend upon solvent viscosity and may be observed at the lowest concentrations of acceptor species. Energy transfer from the initially chemiexcited species to a suitable acceptor flowed by fluorescence from the acceptors in an important process in chemiluminescence [45,46].…”

Manipulation of supramolecular 2D assembly of functional dyes toward artificial light-harvesting systems

“…Since each dye has a specific absorption region and is different from other dyes, appropriate combinations of these dyes and/or with other molecules have been investigated. The method for (ii) involves dye assembly, which can be done by (a) covalent bonds [13][14][15], (b) supramolecular interaction between dyes [16][17][18][19], or (c) supramolecular interaction between dyes and inorganic materials [20][21][22].…”