Synthesis, Aromaticity and Photophysical Behaviour of Ferrocene‐ and Ruthenocene‐Appended Semisynthetic Chlorin Derivatives

Abstract

Two novel synthetic strategies to covalently link a metallocene electron-donor unit to a chlorin ring are presented. In one approach, pyropheophorbide a is readily converted into its 13(1) -ferrocenyl dehydro derivative by nucleophilic addition of the ferrocenyl anion to the 13(1) -carbonyl group. In another approach, the corresponding 13(1) -pentamethylruthenocenyl derivative is synthesised from 13(1) -fulvenylchlorin by a facile ligand exchange/deprotonation reaction with the [RuCp*(cod)Cl] (Cp*=pentamethylcyclopentadienyl; cod=1,5-cyclooctadiene) complex. The resulting metallocene-chlorins exhibit reduced aromaticity, which was unequivocally supported by ring-current calculations based on the gauge-including magnetically induced current (GIMIC) method and by calculated nucleus-independent chemical shift (NICS) values. The negative ring current in the isocyclic E ring suggests the antiaromatic character of this moiety and also clarifies the spontaneous reactivity of the complexes with oxygen. The oxidation products were isolated and their electrochemical and photophysical properties were studied. The ruthenocene derivatives turned out to be stable under light irradiation and showed photoinduced charge transfer with charge-separation lifetimes of 152-1029 ps.

Similar, Yet Different: Long-Range Metal–Metal Coupling and Electron-Transfer Processes in Metal-Free 5,10,15,20-Tetra(ruthenocenyl)porphyrin

“…Transient absorption spectroscopy of ruthenocelyn-containing chlorin 2 and several similar compounds was indicative of the formation of a charge-separated state. 101 The ruthenocene− porphyrin interaction was dramatically improved in β,β′-fused ruthenocenyl porphyrin 3, as reported by Smith and coworkers (Figure 1). 102 In this system, a ruthenocenyl fragment is directly conjugated to the porphyrin core, which ensured a direct orbital overlap between porphyrin's π-system and ruthenocene.…”

“… This system, prepared with the aim of investigating the electronic coupling between ruthenocene and porphyrin, proved unsatisfactory since the two fragments behaved as electronically decoupled entities. Transient absorption spectroscopy of ruthenocelyn-containing chlorin 2 and several similar compounds was indicative of the formation of a charge-separated state . The ruthenocene–porphyrin interaction was dramatically improved in β,β′-fused ruthenocenyl porphyrin 3 , as reported by Smith and co-workers (Figure ).…”

A Study of the Reaction between Methylpyropheophorbide a and Hydrazine Hydrate

“…[12][13][14] Chemical modification of the 13-carbonyl group has been also reported. [15][16][17][18] Previously we described an efficient approach towards the 3-vinyl group substitution involving cross-coupling reactions. [19] In continuation of our research for synthesis of the naturally derived chlorins with the enhanced optical properties we focused on the modification of the 13-carboxy group of the chlorophyll a derivatives.…”

Synthesis and Characterization of Ferrocenyl Chlorins, 1,1′-Ferrocene-Linked Chlorin Dimers, and their BODIPY Analogues

“…(a) Possible porphyrin structures carrying directly linked or fused metallocenes. (b) Chlorins (pheophorbides) covalently attached to Fc units reported to date . (c) Access to 10-substituted ferrocenyl-chlorin and meso -Fc-BODIPY from the same Fc-dipyrromethane precursor.…”

“…Chlorins constitute the cores of the chlorophylls and as such are responsible for light harvesting and photosynthesis on Earth. To date, there has been a single report on covalently Fc-modified chlorins (Figure 1b), 32 which used naturally derived pyropheophorbide functionalized though its isocyclic ring. Most of the products were unstable and underwent oxidative ring opening upon exposure to air.…”