Unusual Photoinduced Electron Transfer from a Zinc Porphyrin to a Tetrapyridyl Free‐Base Porphyrin in a Noncovalent Multiporphyrin Array

Abstract

Excitation of the peripheral Zn porphyrin units in a noncovalent five-porphyrin array, formed by gable-like zinc(II) bisporphyrins and a central free-base meso-tetrakis(4-pyridyl)porphyrin in a 2:1 ratio, (ZnP(2))(2).(TPyP), does not lead to a quantitative sensitization of the luminescence of the free-base porphyrin acceptor, even though there is an effective energy transfer. Time resolution of the luminescence evidences a quenching of TPyP upon sensitization by the peripheral ZnP(2). The time evolution of the TPyP fluorescence in the complex can be described by a bi-exponential fitting with a major component of 180 ps and a minor one of 5 ns, compared to an isolated TPyP lifetime of 9.4 ns. The two quenched lifetimes are shown to be correlated to the presence of 2:1 and 1:1 complexes, respectively. No quenching of TPyP fluorescence occurs in (ZnP(2))(2).(TPyP) at 77 K in a rigid solvent for which only an energy-transfer process (tau=150+/-10 ps) from peripheral ZnP(2) to the central TPyP is observed. An unusual HOMO-HOMO electron-transfer reaction from ZnP(2) to the excited TPyP units, responsible for the observed phenomena, is detected. The resulting charge-separated state, (ZnP(2))(+)(2).(TPyP)(-) is found to recombine to the ground state with a lifetime of 11 ns.

Coordination Chemistry‐Assembled Multicomponent Systems Built from a Gable‐Like Bis‐Porphyrin: Synthesis and Photophysical Properties

“…Therefore, we decided to examine in more detail the system to support electron transfer and find evidence of the presence of radicals by making use of time‐resolved transient absorption techniques. In the picosecond range, as reported elsewhere in detail, we could measure small spectral variations around 490 nm, consistent with the biphasic formation of a zinc porphyrin radical cation ( ZnP 2 + ) with lifetimes of 160 ps and several nanoseconds, respectively, in agreement with the two lifetimes found for the luminescence quenching of TPyP . However, the extensive spectral overlap of the Zn porphyrin cation bands with the singlet and triplet excited state absorption of ZnP 2 and TPyP makes this data not conclusive .…”

“…In the picosecond range, as reported elsewhere in detail, we could measure small spectral variations around 490 nm, consistent with the biphasic formation of a zinc porphyrin radical cation ( ZnP 2 + ) with lifetimes of 160 ps and several nanoseconds, respectively, in agreement with the two lifetimes found for the luminescence quenching of TPyP . However, the extensive spectral overlap of the Zn porphyrin cation bands with the singlet and triplet excited state absorption of ZnP 2 and TPyP makes this data not conclusive . More evidence comes from a nanosecond flash photolysis experiment where a decay with a lifetime of 11 ns around 480 nm can be positively ascribed to ZnP 2 + and represents the charge recombination process to the ground state (Fig.…”

“…Not only TPyP singlet excited state is found to be reactive in (ZnP 2 ) 2 •(TPyP) , at variance with what was found for 4′‐ cis DPyP in (ZnP 2 )•(4′‐ cis DPyP) , but the biexponential behavior (180 ps and 5 ns) indicates two different populations of complexes nonequilibrating during the lifetime of the species. We were able to exclude different conformations for the same stoichiometry (ZnP 2 ) 2 •(TPyP) and proved the existence of complexes with different stoichiometry, associating the lifetime of 180 ps to (ZnP 2 ) 2 •(TPyP) and the 5 ns to (ZnP 2 )•(TPyP) . Both electronic and steric factors could account for the difference in reactivity between these two different complexes.…”

“…More evidence comes from a nanosecond flash photolysis experiment where a decay with a lifetime of 11 ns around 480 nm can be positively ascribed to ZnP 2 + and represents the charge recombination process to the ground state (Fig. ) .…”

“…This process, generally endoergonic for simple zinc and free‐base porphyrins, is made possible by the fact that pyridyl‐substituted porphyrins can be reduced more easily than the corresponding tolyl derivatives and that Zn porphyrins can be more easily oxidized when involved in axial bonds with pyridyl residues . A more detailed discussion can be found in reference .…”