Synthesis and effect of substituent position, metal type on the electrochemical properties of (3-morpholin-4-ylpropoxy) groups substituted cobalt, manganese phthalocyanines.

In this work, 4-(3-morpholin-4-ylpropoxy)phthalonitrile 2, 3-(3-morpholin-4-ylpropoxy)phthalonitrile 3, Co(II)Pc and Mn(III)Pcs containing (3-morpholin-4-ylpropoxy) groups at peripheral and nonperipheral positions were synthesized. Phthalonitrile derivatives (2 and 3), Co(II)Pc and Mn(III)Pcs (2a, 2b, 3a, 3b) were characterized by using FT-IR, NMR (only for 2 and 3), mass and UV-Vis (except 2 and 3) spectral data techniques. Also, electrochemistry of (3-morpholin-4-ylpropoxy) group substituted Co(II)Pc and Mn(III)Pcs were inspected by using cyclic voltammetry. Electrochemical studies show that (3-morpholin-4-ylpropoxy) group substituted Co(II)Pc and Mn(III)Pcs electropolymerized on the Pt working electrode.

1. Introduction

Peripheral or nonperipheral tetra-substituted phthalocyanines have been investigated in different areas owing to their chemical and thermal stability that possess physical and chemical properties [1,2]. Phthalocyanines and their derivatives have been used in many applications such as chemical and biosensor [3], solar cell, [4,5], catalyst [6], nonlinear optic [7], liquid crystal [8], catalyst [9,10], photosensitizers in photodynamic therapy (PDT) [11,12], because they have intense blue-green colour owing to the electronic delocalization of their 18-π electrons. On the other hand, electrochemical characterization of phthalocyanines is important for the electrocatalyst [13], electrosensing [14], electropolymerization [15,16], electrochromic fields [17].

Phthalocyanines bearing redox active metals (Co, Fe and Mn) have been investigated owing to their electrocatalytic properties [18]. The usage of cobalt phthalocyanines as an electrochemical sensor is of interest [19,20]. Also, Mn(III)Pcs exhibit exciting electrochemical response owing to different oxidation states of manganese ion [21]. In this work, we have synthesized Co(II)Pc and Mn(III)Pcs containing (3-morpholin-4-ylpropoxy) groups at peripheral and nonperipheral positions. It has been found that the attachment of -(3-morpholin-4-ylpropoxy) group to the phthalocyanine molecule in either a peripheral or nonperipheral position has a great effect on the electrochemical properties.

2. Experimental design

All information about the used equipment, materials, synthesis, electrochemistry experiments is given in the Supplementary Information.

3. Results and discussion

3.1. Synthesis and characterization

The synthesis of Co(II)Pc and Mn(III)Pcs bearing (3-morpholin-4-ylpropoxy) groups was shown in Figure 1. Firstly, 4-(3-morpholin-4-ylpropoxy)phthalonitrile (2), 3-(3-morpholin-4-ylpropoxy)phthalonitrile (3) were synthesized from 3-morpholin-4-yl-propan-1-ol in the presence of available phthalonitrile by using K2 CO3 in dry DMF [22,23]. Then, peripheral, nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted Co(II)Pc and Mn(III)Pcs (2a, 2b, 3a, 3b) were synthesized by cyclotetramerization from 2, 3. In the IR spectrum of 2, 3, stretching vibrations of C≡N groups at 2230 (for 2), 2228 (for 3) cm-1 occurred at expected frequencies, respectively. In 1H-NMR spectrum of 2 and 3 in CDCl3 , aromatic protons appeared at 7.72–7.20 and 7.66–7.30 ppm. In the 13C-NMR spectrum of 2 and 3, nitrile carbon atoms were resonated at δ 115.75, 115.32 ppm (for 2) and 116.98, 113.00 ppm (for 3). The molecular ion peak of 4-(3-morpholin-4-ylpropoxy)phthalonitrile (2), 3-(3-morpholin-4-ylpropoxy)phthalonitrile (3) were found at m/z 272 [M+H]+. The absence of the C≡N stretches at 2230 (for 2) and 2228 cm-1 (for 3) in the IR spectra of the Co(II)Pc and Mn(III)Pcs bearing (3-morpholin-4-ylpropoxy) groups confirms that the cyclotetramerization consisted. The IR spectra of the Co(II)Pc and Mn(III)Pcs bearing (3-morpholin-4-ylpropoxy) groups are very similar. 1H-NMR and 13C-NMR evaluations of the Co(II)Pc and Mn(III)Pcs were inhibited because of their paramagnetic characteristic [24]. MALDI-TOF mass spectra of Co(II)Pc and Mn(III)Pcs (2a, 2b, 3a, 3b) confirmed the structures, with the molecular ion being easily identified at 1144.09 [M]+, 1140.42 [M-Cl]+ (Figure 2), 1144.12 [M]+ and 1140.63 [M-Cl]+, respectively. The UV-Vis spectra of Co(II)Pc and Mn(III)Pcs (2a, 2b, 3a, 3b) in CHCl3 are shown in Figure 3. UV-Vis spectra of Co(II)Pc and Mn(III)Pcs (2a, 2b, 3a, 3b) showed single Q band absorption of π → π∗ transitions at 673, 735, 694, 764 nm, respectively. B bands of Co(II)Pc and Mn(III)Pcs (2a, 2b, 3a, 3b) were appeared in the UV region at 324, 384, 309, (355, 332) nm, respectively. On the other hand, 2b and 3b have an absorption band at 530 nm for 2b and 542 nm for 3b, interpreted as a charge transfer absorbtion [25].

The synthesis of Co(II)Pc and Mn(III)Pcs bearing (3-morpholin-4-ylpropoxy) groups. (i) dry DMF, K2CO3, 60 °C, 96 h.

MALDI-TOF MS spectrum of 2b.

UV-Vis spectra of 2a, 2b, 3a, 3b in CHCl3. (Concentration: 1.00 ×10-5 M)

3.2. Electrochemical studies

The electrochemistry of Co(II)Pc and Mn(III)Pcs (2a, 2b, 3a, 3b) were obtained in DCM using a (DCM)/(TBAP) electrolyte system on a Pt working electrode. The electrochemical data were listed in Table. Figure 4a and Figure 4b show the CV responses of 2a, 3a in DCM/TBAP electrolyte system. 2a and 3a exhibited 2 reduction labelled as R1 (E1/2 = –0.39 V for 2a, E1/2 = –0.40 V for 3a) and R2 (E1/2 = –1.54 V for 2a, E1/2 = –1.57 V for 3a) in DCM/TBAP electrolyte system. CoII can reduce before Pc ring, thus the R1 process of 2a and 3a at E1/2 = –0.39 V and –0.40 V is easily assigned to the CoII /CoI reduction reaction of the complexes [26]. Figure 5a and Figure 5b show the CV responses of 2b, 3b in DCM/TBAP electrolyte system. 2b and 3b exhibited 2 reduction labelled as R1 (E1/2 = –0.25 V for 2b, E1/2 = –0.21 V for 3b) and R2 (E1/2 = –1.29 V for 2b, E1/2 = –1.09 V for 3b) in DCM/TBAP electrolyte system. The first reduction can be assigned to [Cl-MnIII Pc-2 ] / [Cl-MnII Pc-2 ]-1 because of the redox active manganese metal ion [27]. Then second reduction can be assigned to [MnII Pc-2 ] / [MnI Pc-2 ]-1 couple [28]. After first reduction, [Cl-MnII Pc-2 ]-1 species release axial chloride ion. Similar results were appeared for Co(II)Pc and Mn(III)Pcs in literature [29,30].

(a) CV graph of 2a. (b) CV graph of 3a.

(a) CV graph of 2b. (b) CV graph of 3b.

While peripheral, nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted Co(II)Pc and Mn(III) Pcs (2a, 2b, 3a, 3b) illustrate widespread reduction reactions during the cathodic scans, Co(II)Pc and Mn(III)Pcs were electropolymerized on the working electrode during the anodic scans. Figure 6 shows the CV responses of peripheral, nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted cobalt(II) phthalocyanines (2a and 3a) during repetitive CV cycles. When Figure 6a is examined, the onset oxidation potential of peripheral tetra-(3-morpholin-4-ylpropoxy) group substituted cobalt(II) phthalocyanine 2a at around 1.32 V is observed, whereas the onset potential of the nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted cobalt(II) phthalocyanine 3a has been determined at around 1.36 V (Figure 6b). In the following cycles, the oxidation peak currents raised and there was a small shift in the oxidation peaks. This suggests that the Co(II)Pc and Mn(III)Pcs incur polymerization in each scan and deposit onto the electrode surface. Figure 7 shows the CV responses of peripheral, nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted manganese(III) chloride phthalocyanines (2b and 3b) during repetitive CV cycles. When Figure 7a is examined, the onset oxidation potential of peripheral tetra-(3-morpholin-4-ylpropoxy) group substituted manganese(III) chloride phthalocyanine 2b at around 1.38 V is observed, whereas the onset potential of the nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted manganese(III) chloride phthalocyanine 3b has been determined at around 1.33 V (Figure 7b). Similar to 2a and 3a, peripheral, nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted manganese(III) chloride phthalocyanines (2b and 3b) show the electropolymerization process. Morpholine derivatives generally polymerize during the oxidation reaction. For example, cobalt, titanium, manganese phthalocyanines bearing [(5-{[(1E)-(4-morpholin-4-ylphenyl)methylene]amino}-1-naphthyl)oxy] substituent was electropolymerized on GCE [20]. When compared with [(5-{[(1E)-(4-morpholin-4-ylphenyl)methylene]amino}-1-naphthyl)oxy] cobalt, titanium, manganese phthalocyanines, redox processes of the studied phthalocyanines in present work, are generally compatible with a small potential difference. Because of the electropolymerization properties, peripheral, nonperipheral tetra-(3-morpholin-4-ylpropoxy) group substituted cobalt(II), manganese(III), phthalocyanines (2a, 2b, 3a, 3b) may be a good nominee in electrochemical fields such as electrocatalysts, electrochromic applications.

(a) Repetitive CVs of 2a. (b) Repetitive CVs of 3a.

(a) Repetitive CVs of 2b. (b) Repetitive CVs of 3b.

Conclusion

As a conclusion, synthesis and electrochemistry of Co(II)Pc and Mn(III)Pcs containing (3-morpholin-4-ylpropoxy) groups at peripheral and nonperipheral positions have been presented in this study. Cyclic voltammetry was used in order to determine electrochemistry of Co(II)Pc and Mn(III)Pcs containing (3-morpholin-4-ylpropoxy) groups at peripheral and nonperipheral positions. According to the electrochemical results, Co(II)Pc and Mn(III)Pcs gave common reduction reactions. On the other hand, Co(II)Pc and Mn(III)Pcs containing (3-morpholin-4-ylpropoxy) groups at peripheral and nonperipheral positions were plated on Pt electrodes with the oxidation for polymerizable morpholino groups. Electropolymerization renders phthalocyanine, a valuable material for the production of different electrochemical applications, for example electrocatalytic, electrochromic, and electrosensing applications.

Table

Voltammetric data of the Pcs. All voltammetric data were given versus SCE

Pcs		Oxidations	Reductions
2a	aE1/2	1.32c	–0.39	–1.54
	bΔEp (mV)	-	113	140
2b	aE1/2	1.38c	–0.25	–1.29
	bΔEp (mV)	-	175	166
3a	aE1/2	1.36c	–0.40	–1.57
	bΔEp (mV)	-	96	131
3b	aE1/2	1.33c	–0.21	–1.09
	bΔEp (mV)	-	122	151

a: E1/2 values ((Epa + Epc)/2) were given versus SCE at 0.100 Vs-1 scan rate. b: ΔEb = Epa-Epc. c:Epa of first CV cycle.