Redox and Photophysical Properties of Four Subphthalocyanines Containing Ferrocenylcarboxylic Acid as Axial Ligands

Abstract

Four new ferrocenylsubphthalocyanines Y-BSubPc, with ferrocenylcarboxylic acid YH = Fc-CH 2 −CH 2 −COOH (1) or Fc-CHCH−COOH (2) in the axial Y position, were synthesized with a 40% yield. The axial ferrocenylcarboxylic acid moiety did not have a significant influence on the position of the Q bands maxima of the UV/vis spectra of the ferrocenylsubphthalocyanines or on the 1 H NMR position of the ring proton peaks of Y-BSubPc(H 12 ) or on the 19 F NMR position of the ring F peaks of YSubPc(F 12 ), relative to their respective parent compounds Cl-BSubPc(H 12 ) 3 and Cl-BSubPc(F 12 ) 4, containing axial chlorine. Very weak metal-to-ligand-charge-transfer bands (MLCT) in the near-IR region were observed. An electrochemical study utilizing cyclic voltammetry showed that electronic communication exists between ferrocene on the axial ferrocenylcarboxylic acid and the π-electrons of the macrocycle of the ferrocenylsubphthalocyanine (Fc(CH 2 ) 2 COO)-BSubPc(H) 12 ,5, (Fc(CH) 2 COO)-BSubPc(H) 12 , 6, (Fc(CH 2 ) 2 COO)-BSubPc(F) 12 , 7, and (Fc(CH) 2 COO)-BSubPc(F) 12 , 8. The Fe group of the ferrocenyl-containing axial ligand is involved in the first reversible oxidation process, followed by a second oxidation localized on the subphthalocyanine ligand. The fluorine ring substituents in SubPcs 7 and 8 caused the ferrocenyl oxidation to shift more positive by ca. 0.1 V, compared to SubPcs 5 and 6 without fluorine. Density functional theory (DFT) calculations provided further insight into the properties of these novel ferrocenylsubphthalocyanines. The neutral species of SubPcs 5−8 have LUMOs with mainly π-ring character and HOMOs with mainly iron-d character, confirming ring-based reduction, metal-based Fe(II) to Fe(III) oxidation, as well as weak MLCT in the near-IR region. Further DFT optimization of the cation (oxidized) species was essential to verify the second ring-based oxidation.

Synthesis, Spectroscopy, Electrochemistry and DFT of Electron-Rich Ferrocenylsubphthalocyanines

“…The oxidation of the Fe group (Fe II/III ) of the ferrocenyl moiety on the axial ligand is the first observed redox process for all five SubPc 7-11. The assignment that the Fe group is oxidised first, is supported by DFT calculations (see computational analysis below) and is in agreement with literature [21,22,24]. Both oxidation and the first reduction peaks are chemically reversible with peak ratios approaching 1 and peak current separations, ΔEp, of 0.074-0.076 (ferrocenyl oxidation), 0.080-0.084 V (wave I in Figure 3) and 0.082-0.086 10 (× 10 −3 M), obtained with a 1 cm pathlength cuvette with tetrahydrofuran (THF) as solvent.…”

“…Both oxidation and the first reduction peaks are chemically reversible with peak ratios approaching 1 and peak current separations, ΔEp, of 0.074-0.076 (ferrocenyl oxidation), 0.080-0.084 V (wave I in Figure 3) and 0.082-0.086 10 (× 10 −3 M), obtained with a 1 cm pathlength cuvette with tetrahydrofuran (THF) as solvent. UV/vis spectra of SubPc 9 can be found in reference [24]. Insert: The Beer-Lambert correlation between the absorbance A and concentration of FcCO 2 BSubPc(H) 12 , 7 (ε = 176547 dm 3…”

“…The CVs and the LSVs of SubPc 7, 8, 10 and 11, performed at 25 • C in dichloromethane (DCM) at a scan rate of 0.100 Vs −1 are shown in Figure 3 with the relevant electrochemical data summarised in Table 2. Data of the free ferrocenylcarboxylic acids 1-5 [14] and SubPcs 6 [20] and 9 [24] are added for comparative reasons in Table 2. The CVs of SubPc 7, 8, 10 and 11 ( Figure 3) showed two oxidation and two reduction peaks in the experimental solvent window of DCM.…”

“…This trend was similar as observed for the free ferrocenylcarboxylic acids 1-4 [14], see Figure 4a. [14], data of 9 from reference [24]. (b) Relationship between E°'(wave II) and Epc (wave III) and n, of the first and second ring-based reductions respectively, of ferrocenylsubphthalocyanine dyads 7-10.…”

“…The appropriate ferrocenyl carboxylic acids 2, 4 and 5 was directly bound to the boron atom in the axial position of Cl-BSubPc(H)12, 6, to form the three new ferrocenylsubphthalocyanine dyads, FcCH2CO2BSubPc(H)12, 8, Fc(CH2)3CO2BSubPc(H)12, 10, and FcCO(CH2)2CO2BSubPc(H)12, 11 (see Scheme 1 [23] and Figure 1). For comparative purposes, the two known ferrocenylsubphthalocyanine dyads 7 [21] and 9 [24] were included in this study to systematically evaluate (i) the effect of the electron rich macrocycle of SubPcs 7-11 on the formal reduction potential of Fe of the ferrocenyl group of the ferrocenylcarboxylic acid moieties of 7-11, and (ii) the influence of the axially ferrocenylcarboxylic acid 1-5 on the UV/vis maxima and the ringbased oxidations and reductions of the ferrocenylsubphthalocyanine dyads 7-11. [24], Fc(CH2)3CO2BSubPc(H)12, 10 (novel) and FcCO(CH2)2CO2BSubPc(H)12, 11 (novel), containing Cl or the ferrocenylcarboxylic acids FcCO2H (1), FcCH2CO2H (2), Fc(CH2)2CO2H (3), Fc(CH2)3CO2H (4) and FcCO(CH2)2CO2H (5) in the axial position.…”