1
2
2
2
1
1 Faculty of Chemistry, UMCS, M. Curie-Sklodowska Sq. 2, 20-031 Lublin, Poland, e-mail: radzki@hermes.umcs.lublin.pl; http://hermes.umcs.lublin.pl/users/radzki
2
Institute of General and Inorganic Chemistry, Palladina Av. 32/34, 03680 Kyiv,
Ukraine.
Introduction.
The biological and technological importance of porphyrins and phthalocyanines makes them a widely studied class of compounds because of their particular photophysical and photochemical properties. Porphyrins and phthalocyanines are organic dyes of increasingly diverse application: from industrial (catalysis, laser dyes, photoconductors, fluorescent probes) [1], through a system which can mimic photochemical reaction in photosynthesis, to the biomedical use (photodynamic therapy) [2]. The optical spectra of the porphyrins and phthalocyanines are dominated by the bands associated with the heteroaromatic, 16 atom, 18 π-electron inner perimeter cyclic polyene. Despite experimental measurements spanning over 60 years, no model exists that completely accounts for all bands in the UV–VIS–near IR regions in each of the accessible redox states of many of these ring compounds. Spectral data for many neutral porphyrin and phthalocyanine compounds with a range of metals, axial and peripheral substituents have been reported from absorption and magnetic circular dichroism (MCD) techniques [1].
In this paper we present the optical properties of twelve new complexes of phthalocyanines of Zr(IV) and Hf(IV) without axial ligands, and with gallic, 5sulfosalicylic, oxalic acids, catechol and methyl ether of gallic acid as axial ligands directly bonded to the metal located out-of-plane of the N
8
moiety. Axial substitution of pthalocyanines caused several effects: first of all, including of axially substituted ligand to metal atom makes that phthalocyanine complexes becomes water and DMSO soluble, what is not characteristic of metal phthalocyanine complexes. Also, electronic structure of the phthalocyanine ring N
8
moiety could be altered; additional perpendicular to the macrocycle plane dipole moment can be observed; new axial ligands vary the spatial relationships between neighboring molecules via steric effects and also change the magnitude of the intermolecular interactions; large axially coordinated ligands are able to alter the packing of the molecules in the solid state and their tendency to aggregate in solutions. Each of these effects can influence the photo-conductive and non-liner optical properties of phthalocyanines [3-5,14].
Materials and methods.
All investigated complexes (Fig. 1) have been synthesized by methods described earlier [3-5]. The structures and purity of all synthesized complexes were confirmed by elemental analysis, IR and NMR
1
H (Bruker, 300MHz) spectroscopy [5].
Preparation of the solutions and spectral investigations: Starting solutions of complexes were prepared by dissolving 5·10
-6
M of phthalocyanine in 25ml of DMSO
(Sigma-Aldrich). For the absorbance investigation of phtalocyaninato metal complexes
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in various solvents we added 100 μl of starting solution to 2 ml of solvent in 1cm
Hellma quartz cell. Absorption measurements were carried out using a Carl Zeiss-Jena
M42 spectrophotometer. The spectra were recorded between 300 and 900 nm at 21±1
°C. In the Lambert–Beer low linearity experiments, the solvent was titrated by 10 μl portions of starting solution in 2, 1, 0.5 and 0.1 cm Hellma quartz cells for EtOH and by
20 μl portions for H
2
O. The spectra were recorded between 550 and 750 nm at 21±1° C.
M
OH
OH
PcM(IV)(OH)
2
M
O
O
PcM(IV)Gallic
OH
O
OH
O
M
O
O
S
O
OH
O
PcM(IV)5-Sulfosalicylic
O
O O O
O
CH
3
O
M
M M
O O
O
O
H O
PcM(IV)Oxalic
PcM(IV)GallicMethylEther
PcM(IV)Catechol
Fig.1 Molecular structures of phthalocyanines with metal-coordinated ligands. M=Zr(IV) or Hf(IV).
Fluorescence measurements were carried out using a Fluoromax-2 spectrofluorometer (Jobin Yvon-Spex, Horiba Group), with the detector oriented at 90° relative to the light source and using 1-cm quartz cell. All the fluorescent spectra were excited at the wavelength of the Soret band and recorded at 21±1 °C in the range of
300-850 nm. Absorption and emission spectra were recorded digitally and the Sigma
Plot (Jandel Corp.) program was used in data processing.
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Solutions of phthalocyaninato metal complexes were freshly prepared in the spectral purity solvents at the concentration range about 10
-5
M. We have tried to dissolve the expatiated phthalocyanines in less polar solvents, but they were insoluble or soluble in such small amounts that it was impossible to register a high quality spectrum.
Solutions were kept in the dark to prevent photo degradation. All the fluorescence and absorption spectra were recorded for the same samples.
Results and discussion.
The absorbance spectra of all axially substituted Zr(IV) and Hf(IV) phthalocyanines are characterized by presence of ultraviolet Soret band (λ max
in the range of 335 to 350 nm) and visible Q band (λ max
in the range of 675 to 701 nm), as it was described in the literature for similar compounds [6-9]. The position of λ max
in the
Soret region and in Q region depends on the solvent polarity - the lower is Reichardt’s empirical parameter of solvent polarity, the higher red shift of λ max is observed.
Furthermore, this shift is bigger in the Q region [10, 13]. The red shift of absorbance band maxima in the Q region for the solvents with relatively high polarity (MeOH,
EtOH) and for the solvents with relatively low polarity (CH
2
Cl
2
, CHCl
3
) is up to 10 nm
(Tab. 1), as it was observed for other phthalocyanines [11, 12].
Tab 1. Dependence of λmax [nm] from Reichardt empirical parameter of solvent polarity (E
N
T
).
Solvent E N
T
PcZr(OH)
2
λmax [nm]
PcHf(OH)
2
λmax [nm]
PcZr(IV) gallic
λmax
[nm]
PcHf(IV) gallic
λmax
[nm]
PcZr(IV) sulfosalicylic
λmax [nm]
PcHf(IV) sulfosalicylic
λmax [nm]
S Q S Q S Q S Q S Q S Q
H
2
O 1 345 699 345 697 337 690 346 701 343 696 341 689
MeOH 0.762 348 684 351 683 340 682 346 684 350 684 350 683
EtOH 0.654 348 685 349 684 339 682 349 686 349 685 340 685
DMSO 0.444 350 690 347 686 342 685 349 690 348 688 347 687
DMF 0.404 346 685 346 687 341 681 347 688 347 684 346 681
Aceton 0.355 347 683 345 682 344 683 348 687 348 693 347 685
CH
2
Cl
2
0.309 350 689 348 696 341 688 349 696 348 686 347 689
CHCl
3
0.259 349 692 348 693 341 689 348 693 349 693 346 691
PcZr(IV) PcHf(IV) PcZr(IV) PcHf(IV) PcZr(IV) PcHf(IV)
Solvent E N
T oxalic
λmax[nm] oxalic
λmax[nm] catechol
λmax[nm] catechol
λmax[nm]
Gal-Me-Ether
λmax[nm]
Gal-Me-Ether
λmax[nm]
S Q S Q S Q S Q S Q S Q
H
2
O 1 338 345 345 682 345 696 343 699 343 700 344 695
MeOH 0.762 341 347 347 676 347 685 346 685 348 684 345 685
EtOH 0.654 342 347 347 678 347 686 346 686 349 684 347 686
DMSO 0.444 343 349 349 678 349 689 349 691 349 688 347 688
DMF 0.404 340 350 350 675 350 682 348 687 446 682 346 686
Aceton 0.355 341 349 349 676 349 687 347 685 346 684 347 685
CH
2
Cl
2
0.309 342 348 348 683 348 692 349 692 348 690 347 692
CHCl
3
0.259 342 350 350 686 350 687 349 695 349 692 349 693
Unusual phenomenon could be observed in water solutions. Although water has the highest value of empirical parameter of solvent polarity among the used solvents, in the contrast to other solvents, Q band for water is not shifted into red region. Among the studied phthalocyanines, it is the biggest deviation from observed interdependence of
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empirical solvent parameter polarity and absorption wavelengths. Also the shape of Qband in water solutions varies from the Q bands for the other solvents. These effects are result of the complexes dimerization in aqueous solution, what is additionally confirmed by Lambert-Beer low linearity investigations. Deviation from Lambert-Beer low linearity was observed in water solution for all investigated complexes, also by other authors for the similar phthalocyanines [12]. For the DMSO and ethanol solutions such a deviation from Lambert-Beer law linearity was not observed.
The investigation of the fluorescence spectra confirms that position of characteristic emission maxima (in the range from 725 to 737 nm) is dependent on the type of axial ligand present in the complex, and on the type of metal in N
8
moiety (Tab.
2). The additional emission maxima in 420-550 nm region were also observed. Such a maxima were not observed for complexes of metallophthalocyanines without axially coordinated ligand (PcM(OH)
2
) and were not earlier described in the literature neither for phthalocyanines nor for the metallophthalocyanines. These additional bands depend on the type of axial ligand and on the nature of metal.
Tab 2. Dependence of the phthalocyanine absorbance and emission band maxima in the
DMSO solution on the type of central metal atom and on the nature of axial ligand (ΔS [nm] = Stokes shift).
Investigated complexes
Absorption λmax
[nm] Excitation
λ [nm]
Emission λmax [nm]
ΔS
[nm]
S
PcZr(IV)(OH)
2
PcHf(IV)(OH)
2
PcZr(IV)gallic
350
347
PcHf(IV)gallic
342
349
PcZr(IV)sulfosalicylic 348
PcHf(IV)sulfosalicylic 347
PcZr(IV)oxalic 343
PcHf(IV)oxalic
PcZr(IV)GallicMeEther
341
349
PcHf(IV)GallicMeEther 349
PcZr(IV)pirocatechol 347
PcHf(IV)pirocatechol 349
Q
690
686
685
690
688
687
681
678
688
691
688
689
420
420
420
420
420
420
410
410
410
410
420
418
420-550nm region
493
489
499
474
539
525
496
478
498
475
499
475
600-750nm region
728
735
730
735
725
732
726
737
735
737
724
726
Conclusion.
In the absorption spectra of the axially substituted Zr(IV) and Hf(IV) phthalocyanines in various solvents, the red shift of the Q spectral band with the decreasing of solvent polarity was observed. The phenomena observed in the water solutions of complexes, can be explained by the dimerization of complexes in water solution. The conclusion is additionally confirmed by the deviation from the Lambert-
Beer law linearity, observed in water solution for all investigated complexes.
The analysis of the fluorescence spectra reveals that position of characteristic emission maxima (in the range from 725 to 737 nm) depends from the type of axial ligand coordinated directly to the metal. Also, additional emission maxima in the 420-
550 nm region were observed. The presence of this band in the emission spectra of phthalocyanines and metallophthalocyanines was not earlier reported.
37
45
45
59
38
49
45
45
49
44
36
35
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