Effects of central metal ion (Mg, Zn) and solvent on singlet excited-state energy flow in porphyrin-based nanostructures

Abstract

Zinc porphyrins have been widely used as surrogates for chlorophyll (which contains magnesium) in photosynthetic model systems and molecular photonic devices. In order to compare the photodynamic behaviour of Mg- and Zn-porphyrins, dimeric and star-shaped pentameric arrays comprised of free-base (Fb) and Mg- or Zn-porphyrins with intervening diarylethyne linkers have been prepared. A modular building block approach is used to couple ethynyl- or iodo-substituted porphyrins in defined metallation states (Fb, Mg or Zn)via a Pd-catalysed reaction in 2–6 h. The resulting arrays are purified in 45–80% overall yields by combinations of size exclusion chromatography and adsorption chromatography (≥95% purity). High solubility of the arrays in organic solvents facilitates chemical and spectroscopic characterization. The star-shaped Mg ₄ Fb- and Zn ₄ Fb-pentamers, where the Fb-porphyrin is at the core of the array, have pairwise interactions similar to those of dimeric MgFb- and ZnFb-arrays. The arrays have been investigated by static and time-resolved absorption and fluorescence spectroscopy, as well as resonance Raman spectroscopy. The major findings include the following. (1) The rate of singlet excited-state energy transfer from the Mg-porphyrin to the Fb-porphyrin [(31 ps) ^-1 ] is comparable to that from the Zn-porphyrin to the Fb-porphyrin [(26 ps) ^-1 ] in the dimeric arrays. Qualitatively similar results are obtained for the star-shaped pentamers. The similar rates of energy transfer for the Mg- and Zn-containing arrays are attributed to the fact that the electronic coupling between the metalloporphyrin and Fb-porphyrin is approximately the same for Mg- vs. Zn-containing arrays. (2) The quantum yield of energy transfer is slightly higher in the Mg-arrays (99.7%) than in the Zn-arrays (99.0%) due to the longer inherent lifetime of Mg-porphyrins (10 ns) compared with Zn-porphyrins (2.5 ns). (3) The rate of energy transfer and the magnitude of the electronic coupling are essentially independent of the solvent polarity and the coordination geometry of the metalloporphyrin (four- or five-coordinate for Zn-porphyrins, five- or six-coordinate for Mg-porphyrins). (4) Polar solvents diminish the fluorescence yield and lifetime of the excited Fb-porphyrin in arrays containing either Mg- or Zn-porphyrins. These effects are attributed to charge-transfer quenching of the Fb-porphyrin by the adjacent metalloporphyrin rather than to changes in electronic coupling. The magnitude of the diminution is greater for the Mg-containing arrays, which is due to the greater driving force for charge separation. (5) The Zn-containing arrays are quite robust while the Mg-containing arrays are slightly labile toward demetallation and photooxidation. Taken together, these results indicate that porphyrin-based nanostructures having high energy-transfer efficiencies can be constructed from either Mg- or Zn-porphyrins. However, Mg-containing arrays may be superior in situations where a succession of energy-transfer steps occurs (due to a slightly higher yield per step) or where charge transfer is a desirable feature. On the other hand, Zn-porphyrins are better suited when it is desirable to avoid charge transfer quenching reactions. Accordingly, the merits of constructing a device from Mg- vs. Zn-containing porphyrins will be determined by the interplay of all of the above factors.