Michael Sekita

25th anniversary article: 25 years of fullerene research in electron transfer chemistry.

The past 25 years have served as a test bed for exploring the chemistry and physics, in general, and the electron transfer chemistry, in particular, of low-dimensional carbon. Nevertheless, the new realm started with the advent of fullerenes, followed in chronological order by carbon nanotubes, and, more recently, by graphene. The major thrust of this Review article is to historically recap the versatility of fullerenes regarding the design, the synthesis, and the tests as an electroactive building block in photosynthetic reaction mimics, photovoltaics, and catalysis.

Intense Ground-State Charge-Transfer Interactions in Low-Bandgap, Panchromatic Phthalocyanine–Tetracyanobuta-1,3-diene Conjugates

A cycloaddition-retroelectrocyclization reaction between tetracyanoethylene and two zinc phthalocyanines (Zn(II) Pcs) bearing one or four anilino-substituted alkynes has been used to install a strong, electron-accepting tetracyanobuta-1,3-diene (TCBD) between the electron-rich Zn(II) Pc and aniline moieties. A combination of photophysical, electrochemical, and spectroelectrochemical investigations with the Zn(II) Pc-TCBD-aniline conjugates, which present panchromatic absorptions in the visible region extending all the way to the near infrared, show that the formal replacement of the triple bond by TCBD has a dramatic effect on their ground- and excited-state features. In particular, the formation of extremely intense, ground-state charge-transfer interactions between Zn(II) Pc and the electron-accepting TCBD were observed, something unprecedented not only in Pc chemistry but also in TCBD-based porphyrinoid systems.

Assembling a Phthalocyanine and Perylenediimide Donor–Acceptor Hybrid through a Platinum(II) Diacetylide Linker

In a novel electron-donor-acceptor conjugate, phthalocyanine (Pc) and perylenediimide (PDI) are connected through a trans-platinum(II) diacetylide linker to yield Pc-Pt-PDI 1. In the ground state, the presence of Pt(II) disrupts the electronic communication between the two electroactive components, as revealed by UV/Vis spectroscopy and electrochemical studies. The photophysical behavior of 1 is compared with that of the corresponding Pc-PDI electron-donor-acceptor conjugate 2 in terms of charge separation and charge recombination. The insertion of Pt(II) between Pc and PDI impacts the results in a longer-lived Pc(.) (+) /PDI(.) (-) radical ion-pair state. In addition, the intermediately formed Pc triplet excited state is formed with higher quantum yields in 1 than in 2.

Porphyrin Donor and Tunable Push-Pull Acceptor Conjugates-Experimental Investigation of Marcus Theory

We report on a series of electron donor-acceptor conjugates incorporating a ZnII -porphyrin-based electron donor and a variety of non-conjugated rigid linkers connecting to push-pull chromophores as electron acceptors. The electron acceptors comprize multicyanobutadienes or extended tetracyanoquinodimethane analogues with first reduction potentials ranging from -1.67 to -0.23 V vs. Fc+ /Fc in CH2 Cl2 , which are accessible through a final-step cycloaddition-retroelectrocyclization (CA-RE) reaction. Characterization of the conjugates includes electrochemistry, spectroelectrochemistry, DFT calculations, and photophysical measurements in a range of solvents. The collected data allows for the construction of multiple Marcus curves that consider electron-acceptor strength, linker length, and solvent, with data points extending well into the inverted region. The enhancement of electron-vibration couplings, resulting from the rigid spacers and, in particular, multicyano-groups in the conformationally highly fixed push-pull acceptor chromophores affects the charge-recombination kinetics in the inverted region drastically.

Tuning the Electron Acceptor in Phthalocyanine-Based Electron Donor-Acceptor Conjugates.

Zinc phthalocyanines (ZnPc) have been attached to the peri-position of a perylenemonoimide (PMI) and a perylenemonoanhydride (PMA), affording electron donor-acceptor conjugates 1 and 2, respectively. In addition, a perylene-monoimide-monoanhydride (PMIMA) has been connected to a ZnPc through its imido position to yield the ZnPc-PMIMA conjugate 10. The three conjugates have been studied for photoinduced electron transfer. For ZnPc-PMIMA 10, electron transfer occurs upon both ZnPc and PMIMA excitation, giving rise to a long-lived (340 ps) charge-separated state. For ZnPc-PMI 1 and ZnPc-PMA 2, stabilization of the radical ion pair states by using polar media is necessary. In THF, photoexcitation of either ZnPc or PMI/PMA produces charge-separated states with lifetimes of 375 and 163 ps, respectively.

Ground and Excited State Electronic Interactions in Push–Pull-Chromophore–[60]Fullerene Conjugates

A variety of push–pull-chromophore–[60]fullerene conjugates connected with different spacers was successfully synthesized by applying “click chemistry” to the corresponding acetylene-appended fullerene precursors. Direct connection of the fullerene cages to push–pull motifs gives rise to ground state electronic interactions, which were characterized by electrochemical studies. On the other hand, when the two moieties were linked through pyrrolidine rings, no interactions occurred between the C60 units and the push–pull motifs in the ground states. Instead, an electron transfer proceeded upon light exitation, giving the charge-separated states, which was corroborated by time-resolved transient absorption measurements.