Electron-Transfer State of 9-Mesityl-10-methylacridinium Ion with a Much Longer Lifetime and Higher Energy Than That of the Natural Photosynthetic Reaction Center

Abstract

We have successfully achieved the electron-transfer (ET) state of 9-mesityl-10-methylacridinium ion, produced by a single step photoinduced electron transfer, which has a much longer lifetime (e.g., 2 h at 203 K) and higher energy (2.37 eV) than that of the natural system without loss of energy due to multistep electron-transfer processes.

Formation of a long-lived electron-transfer state in mesoporous silica-alumina composites enhances photocatalytic oxygenation reactivity

“…3A) is somewhat redshifted compared with the corresponding absorption band (λ ¼ 540 nm) of the electron-transfer state of Acr þ -Mes in deaerated MeCN (Fig. 3C) (18,30). The rate constant for formation of the electron-transfer state of Acr þ -Mes@tAlMCM-41 was determined to be 3.4 × 10 11 s −1 by monitoring the time profile of the absorbance change at λ ¼ 500 nm due to the Acr • moiety (Fig.…”

“…3 A and C, respectively. In both cases, the band at λ ¼ 490 nm, which is due to the singlet excited state of the Acr þ moiety is shifted to λ ¼ 500 nm, which is ascribed to the Acr • moiety (18). The transient absorption band at λ ¼ 560 nm due to the Mes •þ moiety of the electron-transfer state of Acr þ -Mes@tAlMCM-41 ( Fig.…”

“…The larger ΔH ‡ value may be the result of a larger driving force for the back electron transfer in the Marcus inverted region (32) because of a decreased solvation in the nanosized mesoporous silica-alumina. feasible because the free energy change for electron transfer from p-xylene (E ox ¼ 1.93 V vs SCE) to the Mes •þ moiety (E red ¼ 2.06 V vs SCE) is negative (18). The visible light irradiation using a xenon lamp and UV cut-off filter (λ > 390 nm) of the Acr þ -MesðClO 4 − Þ absorption band (7.5 × 10 −5 M) in oxygensaturated MeCN containing p-xylene (3.0 × 10 −2 M) results in the formation of p-tolualdehyde and hydrogen peroxide Eq.…”

“…copper complex catalyst | donor-acceptor dyad | nanosized silica-alumina | p-xylene | photoinduced electron transfer E xtensive efforts have been devoted to develop a variety of electron donor-acceptor linked molecules, which mimic charge-separation processes in the photosynthetic reaction center (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11); however, long-lived charge separation has been attained only in frozen media at low temperature (12)(13)(14)(15)(16)(17)(18) because intermolecular back electron transfer in solution is predominant as compared to the corresponding intramolecular process. For example, photoexcitation of a charge-shift type donor-acceptor dyad, 9-mesityl-10-methylacridinium ion (Acr þ -Mes) affords the electron-transfer (ET) state (Acr • -Mes •þ ) with an extremely long lifetime (e.g., 2 h at 203 K) in frozen media (18,19).…”

“…For example, photoexcitation of a charge-shift type donor-acceptor dyad, 9-mesityl-10-methylacridinium ion (Acr þ -Mes) affords the electron-transfer (ET) state (Acr • -Mes •þ ) with an extremely long lifetime (e.g., 2 h at 203 K) in frozen media (18,19). At room temperature in solution, however, Acr • -Mes •þ forms a π-dimer radical cation with Acr þ -Mes, which decays by diffusion-limited bimolecular back electron transfer (18,19). In order to avoid such bimolecular back electron transfer, electron donor-acceptor organic molecules must be isolated with respect to each other as in the natural photosynthetic centers where they are separated from each other in chloroplast thylakoid membrane protein environments.…”