Ultrafast Photoinduced Electron Transfer in Directly Linked Porphyrin−Ferrocene Dyads

Abstract

The ultrafast electron transfer occurring upon Soret excitation of three new porphyrin-ferrocene (XP-Fc) dyads has been studied by femtosecond up-conversion and pump-probe techniques. In the XP-Fc dyads (XP-Fcs) designed in this study, the ferrocene moiety is covalently bonded to the meso positions of 3,5-di-tert-butylphenyl zinc porphyrin (BPZnP-Fc), pentafluorophenyl zinc porphyrin (FPZnP-Fc), and 3,5-di-tert-butylphenyl free-base porphyrin (BPH2P-Fc). Charge separation and recombination in the XP-Fcs were confirmed by transient absorption spectra, and the lifetimes of the charge-separated states were estimated from the decay rate of the porphyrin radical anion band to be approximately 20 ps. The charge-separation rates of the XP-Fcs were found to be >10(13) s-1 from the S2 state and 6.3x10(12) s-1 from the S1 state. Charge separation from the S2 state was particularly efficient for BPZnP-Fc, whereas the main reaction pathway was from the S1 state for BPH2P-Fc. Charge separation from the S2 and S1 states occurred at virtually the same rate in benzene and tetrahydrofuran and was much faster than their solvation times. Analysis of these results using semiquantum Marcus theory indicates that the magnitude of the electronic-tunneling matrix element is rather large and far outside the range of nonadiabatic approximation. The pump-probe data show the presence of vibrational coherence during the reactions, suggesting that wavepacket dynamics on the adiabatic potential energy surface might regulate the ultrafast reactions.

Field-Induced Fluorescence Quenching and Enhancement of Porphyrin Sensitizers on TiO2 Films and in PMMA Films

“…We observed a quenching of the fieldinduced fluorescence of both YD2 and YD2-oC8, i.e., a negative value of A, which is attributed to a relaxation to a CS state that is nonfluorescent in nature. 32 This model is consistent with our observation that for push−pull porphyrins such as YD2 and YD2-oC8 the nonfluorescent CS state was involved. We Table 2 imply that the electric-dipole moment in the excited state is larger for YD2 than that for YD2-o-C8, resulting in a separation of charge for YD2 more facilitated than for YD2-o-C8 from the emitting state in PMMA.…”

π-Interactions as a tool for an easy deposition of meso-tetraferrocenylporphyrin on surfaces

“…13 The overwhelming majority of reported metallocenyl-substituted porphyrins are composed of redox-active ferrocenyl moieties appended to the meso or to the b positions of the tetrapyrrolic macrocycle. Besides the well-known potentialities brought about by these substituents (redox-driven fluorescence switches 14 or sensors, 15 photo-induced electron transfer processes 16 for solar energy conversion 17 and molecular-based multibit information storage devices 18 ), additional exciting perspectives are expected if ferrocenyl groups are efficiently conjugated through the porphyrin core. Indeed, these compounds exhibit strong metal-metal coupling which is responsible for multiredox processes, magnetic coupling and unpaired electron density migration.…”

Synthesis and Characterization of Peripherally Ferrocene-modified Zinc Phthalocyanine for Dye-sensitized Solar Cell

“…Note that the excited state absorption signals at 450 -600 and 750 -850 nm appeared from the formation of charge separated (CS) state of phthalocyanine's anion. 15,16 The transient signal probed at 542 nm ( Figure 4b) features a quick rise, limited to the laser pulse (<300 fs) and a double exponential decay. The fast decay (τ 1 < 1 ps) at 542 nm was due to the decreasing S1 state or vibrational relaxation process.…”

“…The fast decay (τ 1 < 1 ps) at 542 nm was due to the decreasing S1 state or vibrational relaxation process. 16 The lifetime of the charge separate state was determined to be 170 ± 8 ps (τ 2 ).…”