Tuning Reductive and Oxidative Photoinduced Electron Transfer in Amide‐Linked Anthraquinone–Porphyrin–Ferrocene Architectures

Abstract

Porphyrin amino acids 3a–3h with meso substituents Ar of tunable electron‐donating power (Ar = 4‐C6H4OnBu, 4‐C6H4OMe, 4‐C6H4Me, Mes, C6H5, 4‐C6H4F, 4‐C6H4CF3, C6F5) have been linked at the N terminus to anthraquinone Q as electron acceptor through amide bonds to give Q‐PAr dyads 4a–4h. These were conjugated to ferrocene Fc at the C terminus as electron donor to give the acceptor‐chromophore‐donor Q‐PAr‐Fc triads 6a–6h. To further modify the energies of the electronically excited and charge‐separated states, the triads 6a–6h were metallated with zinc(II) to give the corresponding Q‐(Zn)PAr‐Fc triads Zn‐6a–Zn‐6h. The Q‐PAr1 dyad (Ar1 = C6H5) was further extended with a second porphyrin PAr2 (Ar2 = 4‐C6H4Me) as well as appended to a ferrocene to give the tetrad Q‐PAr1‐PAr2‐Fc 9. Almost all the conjugates show strongly reduced fluorescence quantum yields and excited‐state lifetimes, which has been interpreted as photoinduced electron transfer (PET) either from the excited porphyrin to the quinone (oxidative PET) or from the ferrocene to the excited porphyrin (reductive PET). Electrochemical data, absorption spectroscopy, steady‐state emission, time‐resolved fluorescence, transient absorption pump‐probe spectroscopy as well as DFT calculations have been used to elaborate the preferred PET pathway (reductive vs. oxidative PET) in these architectures with systematically varied electron‐donating substituents at the central chromophore.

Gold(II) Porphyrins in Photoinduced Electron Transfer Reactions

“…In gold(III) porphyrins, the initial gold‐based reduction is followed by a porphyrin‐based reduction . The individual redox potentials of the dyads [1][PF 6 ] – [3][PF 6 ] are close to those of the respective reference zinc and gold porphyrins Zn‐Ac‐Ia‐ ‐ Zn‐Ac‐IIIa and [Au‐Ac‐Ib][PF 6 ] – [Au‐Ac‐IIb][PF 6 ] (Figure ; Supporting Information, Figures S17–S19) indicating the absence of ground‐state interactions between the zinc(II)‐porphyrin chromophores and the attached gold(III)‐porphyrin acceptors (Table ) . Furthermore, the negligible shift of the first reduction potential of the mononuclear gold(III) porphyrins and the corresponding dyads, is in agreement with a metal‐based redox mechanism that is only weakly affected by the appended chromophore .…”

“…The free‐base and aurated building blocks, Ia–IIIa and [Au‐Ib][PF 6 ] – [Au‐IIIb][PF 6 ] , as well as N ‐acetyl‐ or ester‐protected zinc(II) and gold(III) reference compounds Zn‐Ac‐Ia – Zn‐Ac‐IIIa and [Au‐Ac‐Ib][PF 6 ] – [Au‐Ac‐IIIb][PF 6 ] were prepared according to literature procedures . The cationic gold(III) porphyrin acids were easily prepared by metalation of free‐base porphyrins Ib – IIIb with potassium tetrachlorido aurate(III) in the presence of HOAc/NaOAc according to “Fleischer's route” .…”

“…Herein we present the synthesis and properties of three novel donor‐bridge‐acceptor dyads [Zn II (P)–Au III (P)][PF 6 ] [1][PF 6 ] – [3][PF 6 ] , which were designed to undergo rapid photoinduced electron transfer using zinc(II)‐porphyrin amino‐acid derivatives as chromophores and electron donors and gold(III)‐porphyrin amino‐acid derivatives as electron acceptors connected by a π‐conjugated amide bridge with porphyrin‐aryl torsion angles around 50–80° in the solid state . To tune the driving forces of the forward PET and recombination reactions, the individual building blocks bear electron‐donating and electron‐withdrawing meso ‐aryl groups (Ar=4‐O n Bu‐C 6 H 4 , 4‐CF 3 ‐C 6 H 4 ).…”

Redox and Photochemistry of Bis(terpyridine)ruthenium(II) Amino Acids and Their Amide Conjugates – from Understanding to Applications

“…0.27 eV, Table 1). Hence, reductive quenching seems to be a feasible deactivation pathway in [45] 2+ and [46] 2+ in addition to triplet-triplet energy transfer to give ferrocene triplet states similar to amide-linked porphyrin-ferrocenyl dyads [122,123] and alkynyl-bridged ferrocenyl ruthenium(II) complexes [ Intermolecular, concentration-dependent phosphorescence quenching is also found for [6] 2+ and ferrocene (E 1/2 = 0.0 V), ferrocenecarboxylic acid methyl ester (E 1/2 = 0.27 V [87] ) and ferrocene-1,1Ј-dicarboxylic acid dimethyl ester (E 1/2 = 0.49 V [87] ). For the first two ferrocenes reductive quenching is exergonic (ΔG ET Ͻ 0), while it is estimated slightly endergonic for the latter (ΔG ET Ͼ 0).…”

Otto Bayer Award: F. Merkt / Hoechst Dozentenpreis: F. Laquai / Bunsen–Kirchhoff Prize: O. Reich / Robert Burwell Lectureship: C. T. Campbell

“…Laquai’s research is focused on investigating the dynamics of excited states in conjugated organic materials by ultrafast transient‐absorption spectroscopy and various other time‐resolved optical techniques. He has reported in Angewandte Chemie on two‐dimensional conjugated microporous polymers,2a and in the European Journal of Inorganic Chemistry on reductive and oxidative photoinduced electron transfer 2b…”