Theoretical and Experimental Chemistry, Vol. 44, No. 3, 2008
SPECTROSCOPIC, ELECTROCATALYTIC, AND PHOTOELECTROCHEMICAL CHARACTERISTICS OF MIXED-LIGAND BIS(b-DICARBONYLATO)PHTHALOCYANINE COMPLEXES OF ZIRCONIUM(IV) AND HAFNIUM(IV)
L. A. Tomachinskaya, I. N. Tret’yakova, V. Ya. Chernii, G. Ya. Kolbasov, V. S. Vorobets, and S. V. Volkov
UDC 546.(831+832):667.287
The electronic absorption (EAS) and fluorescence spectra and electrocatalytic and photoelectrochemical characteristics of mixed-ligand complexes of zirconium(IV) and hafnium(IV) based on phthalocyanines in solutions and in thin films were investigated. It was established that a significant bathochromic shift of the Q band and redistribution of the intensities in the absorption bands are observed in the electronic absorption spectra. It was shown that films of the synthesized compounds are promising for use as photosensitizers and electrochemical sensors for oxygen in solution. Key words: zirconium(IV) and hafnium(IV) mixed-ligand complexes, phthalocyanines, electronic spectroscopy, photosensitizers, electrochemical oxygen sensitizers.
Organic dyes, the molecular structures of which are p-conjugated systems, attract the attention of researchers in the field of the creation of novel materials for photonics [1]. Natural representatives of such organic dyes are porphyrins, whose electronic structure and optical, spectroelectrochemical, photoelectric, and other properties make it possible to use them as photoelectric “light–signal” converters. While being derivatives of porphyrins, phthalocyanines differ from the latter in the structure of the molecule and, thereby, the number of p electrons in the molecular skeleton, bringing about changes in the spectral, electrochemical, photoelectric, and other properties. The nature of the central metal atom, substituents introduced at the periphery of the macrocycle, and out-of-plane extra ligands added directly to the central atom has a significant effect on the physicochemical characteristics of the phthalocyanine system as a whole [1-3]. Earlier we reported on the synthesis of the bis(b-dicarbonylato)phthalocyanine complexes of zirconium(IV) and hafnium(IV) and investigation of their electrochemical characteristics in nonaqueous media [4, 5]. In this article we present data from physicochemical investigations (spectroscopic, electrocatalytic, and photoelectrochemical) of the properties of a series of mixed-ligand complexes of zirconium(IV) and hafnium(IV) based on phthalocyanine in solutions and in thin films with a view to creating new photosensitive and photoconducting materials for photonics. As subjects for investigation we chose the following complexes that we synthesized: Zirconium(IV) bis(2,4-pentanedionato)phthalocyanine (I), zirconium(IV) bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)phthalocyanine (II), zirconium(IV) and hafnium(IV) bis(1-phenyl-1,3-butanedionato)phthalocyanines (III) and (IV) respectively, and zirconium(IV) bis(4-benzoyl-3-methyl-1-phenyl-2-pyrazolin5-onato)phthalocyanine (V). _______________________________________________________________________________________________ V. I. Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Prospekt Akademika Palladina, 32/34, Kyiv 03680, Ukraine. E-mail:
[email protected]. Translated from Teoreticheskaya i Éksperimental’naya Khimiya, Vol. 44, No. 3, pp. 133-137, May-June, 2008. Original article submitted May 15, 2008.
0040-5760/08/4403-0139 ©2008 Springer Science+Business Media, Inc.
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EXPERIMENTAL The mixed-ligand complexes of zirconium(IV) and hafnium(IV) based on phthalocyanines were synthesized by the previously developed methods [4]. The electronic absorption spectra (EAS) were recorded on a Specord M-40 spectrophotometer in quartz cuvettes with an absorbing layer thickness of 10 mm [the concentration of the compounds amounted to 2·10–6 M, with dimethyl sulfoxide (DMSO)], toluene, tetrahydrofuran (THF) as solvents] and also in thin films on a substrate of quartz glass. The fluorescence spectra were recorded on a Hitachi MPF-4 fluorescence spectrometer with a 150 W xenon lamp in the region of 200-900 nm in quartz cuvettes with an absorbing layer thickness of 10 mm and solutions at concentrations of 2·10–6 M. The quantum yields of fluorescence of the complexes were calculated by the comparison method described in [6, 7] on the basis of the electronic absorption spectra and fluorescence spectra using the equation Φ = ΦR
n 2 SA R
(1)
n R2 S R A
where FR is the fluorescence quantum yield of the standard; S and SR are the areas under the fluorescence curves of the sample and standard respectively; A and AR are the absorption intensity of the sample and standard respectively; n and nR are the refraction coefficients of the sample and standard respectively. A solution of zinc phthalocyanine (PcZn) in DMSO (FR = 0.20) [8] was used as standard. The natural lifetime of the phthalocyanine complexes was determined by data from the electronic absorption spectra and the fluorescence spectra [6, 7], using the equation
∫
F (λ )
dλ ε (λ ) 1 λ2 = 2.288⋅ 10−9 n 2 ∫ dλ τ λ ∫ F ( λ )λ dλ
(2)
where n is the coefficient of refraction; the integrals of F(l) and e(l) are the areas under the peaks in the EAS and fluorescence spectra respectively. Films of the synthesized complexes were produced on a Ti substrate or on films of nanodisperse TiO2 deposited on a Ti substrate by the procedure in [9] and also on a quartz plate by repeated immersion of the substrate in a solution containing the complexes with concentrations of 10–4-10–5 M and drying the substrate after each immersion. The electrocatalytic activity was studied for the reduction of oxygen under potentiostatic conditions using a specially developed electrochemical bench based on a personal computer with the following characteristics: Measurement currents 2·10–9-10–10 A; potential sweep rate 0.01-50 mV/s; variation range of working electrode potential –4 to +4 V. The electrochemical measurements were made in a cell with separate cathode and anode compartments, the auxiliary electrode was a platinum plate, and the reference electrode was a silver chloride electrode (SCE). The measurements were made in an isotonic solution (0.9%) of NaCl. The photoelectrochemical measurements were conducted with an MDR-2 monochromator and a high-pressure DKSSh-500 xenon lamp [10].
RESULTS AND DISCUSSION The introduction of ligands containing various types of substituent at the central atom of the macrocycle has a significant effect on the electronic structure of the whole macromolecule. In particular it can alter the electronic structure of the phthalocyanine macromolecule, the spatial bonds between the neighboring molecules (through steric effects), and thereby the strength of intermolecular interaction. Each of these effects affects the optical, photoelectric, and other characteristics. Mixed-ligand bis(b-dicarbonylato)zirconium(IV) and hafnium(IV) phthalocyanine complexes are readily soluble in most organic solvents. This makes it possible to investigate their luminescence spectral characteristics in solutions and also to produce thin films of them. 140
TABLE 1. The Data from the Spectral Investigations of PcML2 (lA absorption maximum; e molar extinction coefficient; lF fluorescence maximum; FF fluorescence quantum yield; Dl Stokes shift; t lifetime of excited state) and the E1/2 Potentials for the Films lA, nm
e×105
lF, nm
FF
Dl, nm
t, ns
E1/2, V
Toluene
670
3.4209
676
0.07
6
2.41
–
DMSO
673
1.5900
686
0.2
13
6.76
–
THF
666
2.3305
678
0.23
12
5.36
–
PcZrCl2
DMSO
688
1.1646
697
0.006
11
14.17
–
I
Toluene
688
0.8276
703
0.024
15
1.66
–
DMSO
684
1.3140
702
0.073
18
6.33
–
THF
684
1.6225
697
0.032
13
5.34
–
In film
706
–
–
–
–
–
–0.81
Toluene
686
2.871
702
0.063
16
2.58
–
DMSO
682
0.8995
700
0.075
18
8.87
–
THF
680
1.1300
692
0.117
12
7.78
–
In film
704
–
–
–
–
–
–0.83
Toluene
688
3.2571
701
0.042
13
1.66
–
DMSO
686
1.4809
703
0.072
17
5.42
–
THF
682
2.1950
696
0.069
12
4.09
–
In film
706
–
–
–
–
–
–0.93
Toluene
686
3.3942
–
–
–
–
–
DMSO
686
1.4360
–
–
–
–
–
THF
686
2.1725
–
–
–
–
–
In film
707
–
–
–
–
–
–0.85
Toluene
691
4.323
708
0.073
17
1.99
–
DMSO
694
1.4995
709
0.063
15
5.79
–
THF
690
2.3065
701
0.061
11
4.12
–
In film
727
–
–
–
–
–
–0.70
Complex
PcZn
II
III
IV
V
Solvent
The electronic absorption spectra of the initial PcMCl2 and of the synthesized mixed-ligand complexes were recorded in various organic solvents: DMSO, toluene, and THF. Two bands are observed in the electronic absorption spectra: a Soret band in the region of 330-350 nm and a Q band in the region of 680-700 nm. Here it should be noted that the introduction of two b-dicarbonyl fragments at the central atom of the macrocycle leads to a slight shift of the Q band into the blue region of the spectrum for complexes I-IV and to a bathochromic shift for complex V; also, as a rule, it leads to an increase in the molar extinction coefficient (Table 1). In solutions, irrespective of the solvent, all the investigated complexes exist in monomeric form. At the same time the electronic absorption spectra of the investigated complexes show broadening of the absorption bands, a significant bathochromic shift of the Q band, and redistribution of the intensities among the absorption bands: an increase in the intensity of the Soret band and a vibrational satellite in the Q band (Fig. 1). Such changes in the electronic absorption spectrum can be explained by intermolecular interactions arising on account of Van der Waals forces in the thin films of the dye [11].
141
Fig. 1. The electronic absorption spectra of hafnium(IV) bis(1-phenyl-1,3-butanedionato)phthalocyanine [complex (IV)] in solution (solid line) and in a thin film (dashed line).
Fig. 2. The spectra of the quantum yield of photoelectrochemical current for a film of dispersed TiO2 (dashed line) and a film of dispersed TiO2 modified with zirconium(IV) bis(4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-onato)phthalocyanine [complex (V)] (solid line).
The presence of the “rigid” p-conjugated phthalocyanine system in the complexes suggests the display of luminescent characteristics by compounds of this type. We investigated the fluorescence characteristics of the synthesized mixed-ligand phthalocyanine complexes of zirconium(IV) and hafnium(IV) in three different solvents: toluene, DMSO, THF. In order to avoid secondary emission processes the excitation and fluorescence spectra were recorded in solutions with the complexes at a concentration of 2·10–6 M. Investigation of the luminescence characteristics of the phthalocyanine complexes of zirconium(IV) and hafnium(IV) in organic solvents showed that the presence of the heavy central atom reduces the fluorescence intensity of the phthalocyanine complexes. Thus, the fluorescence intensity of the phthalocyanine complexes of zirconium is lower than for the phthalocyanine complexes of zinc, while the complex (IV) containing hafnium as central atom does not fluorescence at all (Table 1). This agrees with published data to the effect that the heavy ions of metals intensify the intercombination conversion of the excited state to the triplet state, thereby reducing the quantum yield of fluorescence [12, 13]. At the same time the substitution of the two chlorine atoms by two b-dicarbonyl ligands in the phthalocyanine complex of zirconium leads to a significant increase in the quantum yield of fluorescence (Table 1). An effect from the solvent on the fluorescence characteristics of compounds of this type was not observed. This may be due to the “rigidity” of the molecule as a whole – a combination of the p-conjugated system of the phthalocyanine macrocycle and the out-of-plane b-dicarbonyl ligands. The electrocatalytic activity of films of the phthalocyanines deposited on a Ti substrate in the electroreduction of oxygen was studied; this process lies at the basis of amperometric sensors for dissolved oxygen [9]. During the electroreduction of oxygen one reduction wave was observed, similar to films based on nanodisperse TiO2 [9]. It was shown that the value of the
142
half-wave potential E1/2 for the reduction of oxygen was least negative in films based on the complex (V) (Table 1) and amounted to E1/2 = –0.70 V with reference to a silver chloride electrode. In the series of complexes (I)–(II)–(IV)–(III) the half-wave potential moved toward the cathodic side (Table 1). The reproducibility of the polarization characteristics for the obtained films during cyclic variation of the potential was preserved over 15-20 cycles. Such characteristics show that these films can be used in amperometric sensors for oxygen in solutions. In addition, the photoelectrochemical characteristics of films of the synthesized mixed-ligand phthalocyanine complexes, deposited on a layer of dispersed titanium dioxide, were investigated (Fig. 2). It was established that the films were photosensitive in the visible region of the spectrum; the quantum yield of the photocurrent amounted to h = 0.1-0.2 (Fig. 2). The potentials of the flat zones of the TiO2 electrodes, the surface of which was coated with films based on the synthesized mixed-ligand complexes [PcZr(Hf)L2/TiO2], were determined. In contrast to films of TiO2, for which the potential of the flat zones amounted to Efz = –0.3 V, the potentials of the flat zones in the modified electrodes PcZr(Hf)L2/TiO2 had values of Efz = –0.8 V. This gives rise to an increase in the intensity of the catalytic processes, due to the accumulation or release of hydrogen, in photoelectric systems for the conversion of solar energy [14]. Thus, thin films on a layer of dispersed titanium dioxide were produced on the basis of the synthesized mixed-ligand b-dicarbonyl phthalocyanine complexes of zirconium(IV) and hafnium(IV), and the spectral characteristics of the synthesized complexes both in nonaqueous media and in thin films were also investigated. It was established that a significant bathochromic shift (about 20-30 nm) of the Q band, broadening of the bands, and redistribution of the intensities among the bands are observed in the electronic absorption spectra for thin films compared with the electronic absorbtion spectra for solutions. During investigation of electrocatalytic and photoelectrochemical characteristics of the synthesized complexes it was shown that the films produced from them can be used in electrochemical sensors for oxygen in solutions. It was also established that the obtained films are photosensitive in the visible region of the spectrum. This makes it possible to hope for their subsequent use in photoelectrochemical converters.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
N. B. Mckeown, Phthalocyanine materials, Cambridge Univ. Press, Cambridge (1998). K. Lang, J. Mosinger, and D. M. Wagnerova, Coord. Chem. Rev., 248, 321-350 (2004). T.-H. Tran-Thi, Coord. Chem. Rev., 160, 53-91 (1997). L. A. Tomachynski, V. Ya. Chernii, and S. V. Volkov, J. Porphyr. Phthaloc., 6, No. 2, 114-121 (2002). L. A. Tomachinskaya, Yu. Yu. Kolotilova, V. Ya. Chernii, and S. V. Volkov, Teor. Éksp. Khim., 39, No. 2, 96-100 (2003). [Theor. Experim. Chem., 39, No. 2, 104-108 (2003) (Engl. Transl.).] D. Wrobel, C. r. Chimie, 6, 417-429 (2003). D. Wrobel and A. Boguta, J. Photochem. Photobiol. A, 150, 67-76 (2002). A. Ogunsipe, D. Maree, and T. Nyokong, J. Mol. Struct., No. 650, 131-140 (2003). G. Ya. Kolbasov, V. S. Vorobets, A. M. Korduban, 79, No. 4, 605-610 (2006). N. Smirnova, Yu. Gnatyuk, A. Eremenko, et al., Int. J. Photoenergy, No. 1, 224-229 (2006). R. M. Hochstrasser, Molecular Aspects of Symmetry, W. A. Benjamin (1966). J. Lakovich, Fundamentals of Fluorescence Spectroscopy [Russian translation], Mir, Moscow (1986). M. O. Liu, C.-H. Tai, and A. T. Hua, J. Photochem. Photobiol. A, 165, 193-200 (2004). Yu. M. Solonin, G. Ya. Kolbasov, L. G. Shcherbakova, et al., Hydrogen Materials Science and Chemistry of Carbon Nanomaterials, D. V. Schur (ed.), ICHMS’2007, Kiev, AHEU (2007), pp. 350-353.
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