Interaction of nitrogen dioxide with sulfonamide-substituted phthalocyanines: Towards NO2 gas sensor

Sensors and Actuators B 169 (2012) 1–9

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Interaction of nitrogen dioxide with sulfonamide-substituted phthalocyanines: Towards NO2 gas sensor a,b,2 ˚ Sergii Pochekailov a,b,∗ , Juraj Noˇzár a,c,1 , Stanislav Neˇspurek , Jan Rakuˇsan d,3 , Marie Karásková d,4 a

Institute of Macromolecular Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Heyrovsk´ y Sq. 2, 162 06 Prague 6, Czech Republic Brno University of Technology, Faculty of Chemistry, Purkynova 118, 162 00 Brno, Czech Republic ˇ Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Prague 2, Czech Republic d Center of Organic Chemistry Ltd. (COC s.r.o.), Rybitvi 296, 532 18 Pardubice, Czech Republic b c

a r t i c l e

i n f o

Article history: Received 23 June 2011 Received in revised form 1 December 2011 Accepted 23 December 2011 Available online 18 April 2012 Keywords: Phthalocyanine Sulfonamide Nitrogen dioxide Gas sensor Charge transfer complex

a b s t r a c t Much of life requires fast, cheap and reliable gas sensors. As nitrogen dioxide is a toxic and environmentally polluting gas, produced by humans in excess, to be able to monitor and detect it is necessary. One of the way to manufacture cheap and reliable gas sensors sensitive to nitrogen dioxide is to use soluble organic materials as the sensing media. In this paper, we study the properties of soluble sulfonamidesubstituted zinc and metal-free phthalocyanines. The sensing capabilities of these materials are discussed in detail. We have found that Zn sulfonamide-substituted phthalocyanine (Pc) forms a charge transfer complex with nitrogen dioxide, which results in a change of absorption spectrum and an increase of electrical conductivity as much as two orders of magnitude. On the other hand, metal-free Pc does not form the complex and its interaction with nitrogen dioxide is based on weak Van-der-Waals forces. The absorption spectrum does not change and the electrical conductivity slightly decreases during nitrogen dioxide exposure. We have also found that these materials, unlike literature reported derivatives, are reversibly sensitive to nitrogen dioxide at room temperature. This opens a great potential for the fabrication of cheap and reliable gas sensors. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Many human activities require qualitative and quantitative detection of gases. Detection of nitrogen dioxide (NO2 ) is important due to two main reasons: firstly, it is a gas which is harmful for human health [1–4] and which pollutes an environment [4–6], secondly, NO2 appears in many industrial processes as it is one of the product of combustion [7–9]. The known sensors for NO2 are of various operation features. Among them are chemiresistive [10–20], potentiometric [21], amperometric [22,23], optical [15,24–37] and gravimetric [38–42] detections.

∗ Corresponding author. Current address: Department of Electrical Engineering, University of Washington, 185 Stevens way, EE building - Room M254, Campus box 352500, Seattle, Washington 98115-2500, USA. Tel.: +1 617 259 9947; fax: +1 206 543 3842. E-mail addresses: [email protected], [email protected] (S. Pochekailov), ˚ [email protected] (J. Noˇzár), [email protected] (S. Neˇspurek), [email protected] (J. Rakuˇsan), [email protected] (M. Karásková). 1 Tel.: +420 222 511 696; fax: +420 222 516 969. 2 Tel.: +420 222 514 610; fax: +420 222 516 969. 3 Tel.: +420 466 822 373. 4 Tel.: +420 466 823 005. 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.12.087

The most promising nowadays are chemiresistive and optical sensors. Former are exceptionally simple in preparation and usage, but they suffer lack of selectivity, which is usually overcome by using arrays of different sensing elements [43–48]. Optical sensors could be as simple as chemiresistive ones. They are potentially more selective, because one can use the changes over all spectrum under the influence of an analyte. NO2 sensors based on inorganic materials usually operate in the concentration range from 1 ppb to 100 ppm. However, very often high temperature (200–400 ◦ C) is necessary for the operation, which limits the field of applications [49,50]. Gas sensors based on organic sensing layers overcome the problem, as they can operate very often at room temperature. Manufacturing of the sensing films is potentially cheap because one can use cold technologies, such as spin-coating, drop-casting, “doctor blading” and dipping. However, sometimes the problems with sensor stability and signal recovery take place [8]. Among the goals of currently running researches of NO2 detection methods is to find a material, which will have good sensitivity to NO2 , fast response and recovery time, long term stability and which operates at room temperature. Phthalocyanines (Pcs) are promising materials to be applied as sensing materials for NO2 detection. They are thermally and chemically stable, environmentally friendly and cheap in production. They absorb light in UV and visible regions. Optical absorption is the

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result of the transitions of electrons in ␲-conjugated Pc skeleton; the orbitals of the central metal atoms are also included [51,52]. The absorption of visible light and photovoltaic properties makes it possible to use Pcs for the construction of gas sensors with optical detection. The influence of NO2 leads to the decrease of the intensity of Q-band (550–700 nm) in the absorption spectrum and the appearance of new bands with maxima at about 500 and over 700 nm. Similar changes were observed in the spectrum during the formation of donor–acceptor complex between Pcs and electron acceptors [53]. This effect was reported in literature for copper Pc [54–56], cobalt Pc [55], nickel Pc [55], lead Pc [15,55,57], titanium diphthalocyanine [26,58], iron(II) Pc [29,55] and lanthanide diphthalocyanines [56]. In the last case, the changes in the spectrum are higher due to the higher polarizability of ␲-conjugated system [8]. Battisti et al. reported that for metal-free Pc (in particular, tetratert-butyl substituted derivative) there are much weaker change in the spectrum than for metallophthalocyanines [56]. The desorption of NO2 appear to be low, in many cases absorption spectrum does not completely recover. Capobianchi et al. proposed two-step mechanism of NO2 interaction with Pc [26,58]. The first step consists of transferring one electron from ionized Pc to NO2 . During the second step, another electron is transferred from Pc to the gas molecule, thus forming bication of Pc complexed with two NO2 molecules: •

MePc + NO2 •  (MePc)+ NO2 •

•

(MePc)+ NO2 + NO2 •  (MePc)2+ 2NO2 The equilibrium of the first step is shifted in the direction of reaction products. The second step reaction is equilibrated closer to the initial reagents. This explains the partial reversibility of NO2 sensors reported in literature. The formation of a charge transfer complex usually changes the electrical conductivity of the material [59–62]. Literature sources confirm the decrease of electrical resistivity in the presence of nitrogen dioxide. Non-substituted Pcs are not soluble, the preparation of sensing layers requires expensive vacuum evaporation. The use of substituted Pcs solves the problem with solubility, and potentially can improve the recovery rate. Alkyl-substituted Pcs show similar sensing properties as non-substituted derivatives [63]. However, an optimal material with suitable response rate and sensitivity was not still found. In this article, we present the results of the studies concerning optical and electrical sensitivities of novel metal-free and Zn 3-diethylamino-1-propylsulfonamide substituted phthalocyanines to NO2 . We discuss the mechanism of NO2 interaction with Pcs under study, which is supported with quantum chemical calculations.

Fig. 1. Structural formula of Pcs under study. M = Zn and two hydrogens for ZnPcSu and H2 PcSu, respectively.

plug, which contained thin tubes for gas inlet and outlet. Optical absorption spectra were measured using Perkin-Elmer Lambda 950 UV-VIS-NIR spectrometer. Electrical gas sensitivity measurements were performed using ceramic substrates with an interdigital platinum electrode system. The distance between the electrodes was 20 ␮m. We used spincoating technique for Pc thin film deposition on these substrates. Electrical measurements were performed in DC regime using Keithley 6517A electrometer. The solutions for all types of wet depositions were prepared by dissolving Pc in chloroform of spectroscopic grade purity. The concentration for spin-coating was 0.025 g/ml and for drop-casting was 0.01 g/ml. Gas exposure was realized using the continuous flow of carrier gas mixed with analyte in defined proportion and constant flow rate. Atmospheric air was used as a carrier gas, 100 ppm mixture of NO2 with synthetic air was used as an analyte gas. The

2. Experimental There were two 3-diethylamino-1-propylsulfonamide substituted phthalocyanines studied in this work: one with zinc atom (ZnPcSu) and the other with two hydrogen atoms (H2 PcSu) in the center of the Pc skeleton (Fig. 1). Their synthesis, as well as detailed description of the experimental setup, is presented in our previous work [64]. For optical absorption measurements, we prepared the samples by the deposition of Pc on the fused silica glass substrate using spin-coating technique. Optical gas detection was based on the measuring of light absorbance change in transmittance mode. For the measurements we used UV-grade fused silica spectroscopic cuvettes (Fig. 2) with thin films of Pc deposited on the inner side of it. In this case, drop-casting was used as deposition technique. After thin film preparation, the cuvette was closed with modified

Fig. 2. Modified cuvette with Pc layer for optical gas sensitivity measurements.

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9 Mass absorption coefficient, ε (10 m /mol)

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The phthalocyanines under study absorb visible light, as do their analogues reported in literature [65–69]. Light absorption is influenced by the 18 ␲-electron system and orbitals of the central atom [51,52]. The absorption spectra of both ZnPcSu and H2 PcSu are similar. Their spectra in visible and near-UV region consists of two main bands: the Q-band at 600–700 nm and the Soret band with the maximum at about 350 nm. The Q-band is split into a doublet [52,67,68] (Fig. 4). The spectra in solution and thin films have no significant shift relative to each other, proving that some aggregates are already formed in the solution. The peak at 600 nm is called “vibronic band”, and is caused by internal molecular

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3.1. Pcs light absorption

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In this article, we discuss optical properties and the results of quantum chemical simulations for sulfonamide-substituted phthalocyanines (PcSu). Then, we explain the changes of these properties under NO2 exposure. We compare this influence with classical effect of charge transfer complex formation, which is known from literature and has been confirmed in our experiments. In the last subsection, we present a model of the interaction of PcSu with NO2 .

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3. Results and discussion

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presence of oxygen in the mixture prevents the transformation of NO2 to nitric oxide NO. Quantum chemical calculations were performed using M05 method with 6-31G(*) basis (program Gaussian 03). In order to simplify the calculations, we have used the Pcs with methylsulfonamide substituents as a model molecules (Fig. 3).

α

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Fig. 3. Structural formula of Pc molecule, which was used for quantum chemical calculations. M = Zn and two hydrogens for ZnPcSu and H2 PcSu, respectively.

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Fig. 4. Absorption spectra of ZnPcSu (a) and H2 PcSu (b). Blue curves (˛) represent the absorption of Pc thin films, red curves () represent spectra of chloroform solutions, and green lines (s) represent calculated spectra by B3LYP method. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

vibrations. The dimeric structures, which occurs in Pc thin films, result in the weak absorption band at 620 nm.

3.2. Influence of NO2 on light absorption In order to study the influence of NO2 on the absorption spectrum, we have performed the following experiment. Thin film of Pc was repeatedly exposed to air and to the mixture of air with NO2 analyte gas. The absorption spectrum was measured for each concentration of NO2 . We found, that the spectrum of ZnPcSu is slightly changed in the presence of the analyte. The changes, however, are too small to be seen in the classical absorption spectrum thus differential spectrum representation had to be used (Fig. 5). The main change in the spectrum observed, is the appearance of a new band at about 500 nm (strong decrease of the differential transmittance T). The intensity of the Q-band decreased (increase of T at 600–750 nm). We suspect that a charge transfer complex is formed between ZnPcSu (donor) and NO2 (acceptor). This complex has an absorption maximum near 500 nm. At the same time, the intensity of the Q-band decreases. We were not able to observe

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Concentration of NO2, ppm: 0.125 0.25

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400 500 600 Wavelength, λ (nm)

700

800

0.0

Fig. 5. Differential optical transmittance spectra of ZnPcSu film: the difference between the spectrum measured in air and in air with a certain concentration of NO2 . There is also optical absorption spectrum of ZnPcSu film presented (dashed line), in order to visualize the origin of the absorption changes.

the same effect in the case of H2 PcSu; in fact, the spectrum did not change. The kinetic measurement during NO2 exposure followed by relaxation in clean air is shown in Supplementary information (Figs. S13 and S14). The kinetics show the saturation of the spectrum changes during NO2 exposure, and complete recovery under the clean air conditions.

Wave length Fig. 6. Absorption spectra of phthalocyanine–tetracyanoquinodimethane (Pc–TCNQ) complexes. Blue lines represent optical absorption spectra of ZnPcSu (lines a and b) and red curves are the spectra of H2 PcSu (lines c and d). Dashed lines are the spectra of Pcs without TCNQ (lines a and c) and full lines are the spectra of Pcs with TCNQ (lines b and d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

result was observed with other Pc derivatives [53]. This confirms the possibility of CTC formation, and also suggests the approximate position of a CTC band in the spectrum; it is similar to the band appeared in the case of Pc–NO2 interaction (Fig. 5).

3.3. Charge transfer complex involving Pcs

3.4. Simulation of the Pcs interaction with NO2

In order to confirm the hypothesis regarding charge transfer complex (CTC) formation between Pc and NO2 , we tried to use another strong electron acceptor to see how it influences the absorption spectrum of Pc. Tetracyano-1,4-quinodimethane (TCNQ) and Pc solutions in dimethyl formamide were mixed together in the molar stoichiometry 1:1 and a thin film was prepared. The spectra of the resulted film were measured and compared with ones of pure Pc (Fig. 6). The two-component spectrum shows the presence of an additional peak in the region 450–500 nm, which is not present in the spectrum of neither Pc nor TCNQ. Similar

In this section, the results of the model calculations of the interactions of ZnPcSu and H2 PcSu with NO2 are discussed. Two types of interactions between Pc and NO2 are possible. First, the interaction of the gas with central atom(s) of the Pc ring. The second, the interaction of gas molecules with the Pc side-group. We have found that the interaction with the ␲-conjugated skeleton is unprobable. In the case of ZnPcSu, NO2 tends to interact with Zn atom. There is no energetic minimum showing that NO2 molecule interacts with sulfonamide substituent. There are two ways that an NO2 molecule can interact with Pc ring (Fig. 7):

Fig. 7. Formation of “Zn O” (a) and “Zn N” (b) coordination bonds according to quantum chemical calculations.

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Table 1 HOMO and LUMO orbitals of the ZnPcSu NO2 complex. Complex type

HOMO

LUMO

Zn O

Zn N

1. Oxygen from NO2 is the closest to Zn atom, forming thus coordination bond Zn · · · O N O, or “Zn O” complex. The distance a ˚ between Zn and O is 1.95 A. 2. Nitrogen from NO2 is oriented closest to Zn, so the coordination bond Zn · · · NO2 is formed, we will call it “Zn N”complex. The ˚ distance a between Zn and N is 2.05 A.

Quantum chemical calculations show, that both complexes have similar nature and properties. Electron density of HOMO is shifted to NO2 , confirming the CTC formation. Electrons on LUMO are delocalized on Pc and NO2 molecules, forming common ␲-conjugated system (Table 1). The presence of the hole at HOMO boosts the p-type conductivity of the Pc layer. Additionally, more than one molecule in the Pc stack may participate in the complex formation, increasing the conductivity of the layer (though, it would be challenge to prove the last statement experimentally). In order to answer the question, which complex is more probably formed, we have calculated the stabilization energies for both systems (Table 2). After correcting the basis set superposition error (BSSE), one can see, that complex “Zn N” has a negative stabilization energy, thus, it is unlikely to be formed. Complex “Zn O” is stable; the energy, which is required to break the bond, is of the same order of magnitude as thermal oscillations under normal conditions, simplifying the desorption process.

The obtained theoretical results are in agreement with practical observations. Theoretical absorption lines of the complex “Zn O” were found in the same spectral region where new experimental band appeared during NO2 exposure (Fig. 8). At the same time, the oscillator strengths of the Pc absorption lines decreased. Electrical properties follow the optical observations. Nitrogen dioxide causes the increase of the conductivity of ZnPcSu thin films as much as two orders of magnitude (Fig. 9). This confirms the hypothesis about CTC formation. Apart from literature data [8], the response and recovery speed of this compound are much faster. In addition, the recovery rate is improved. Based on these results, we were able to create a prototype for the alarm sensor for NO2 . The behavior of the sensing layer in terms of commercial application is described in Supplementary information (Figs. S15–S17). The electrical response of H2 PcSu to NO2 was negligible and it is not presented here. Table 2 Calculated stabilization energies for “Zn O” and “Zn N” complexes. Level of theory M05/6-31G*. Here Estab is the stabilization energy of the complex; Estab [BSSE] is the value of energy after BSSE correction. Complex type

Estab (kcal/mol)

Estab [BSSE] (kcal/mol)

Zn O Zn N

12.6 6.8

5.2 −0.6

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8 (I)

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Wavelength, λ (nm) Fig. 8. Comparison of the absorption spectra of undoped ZnPcSu thin film (curve I), differential transmittance spectrum measured in the presence of 2.5 ppm of NO2 (see also Fig. 5) (curve II) and calculated absorption lines of “Zn O” complex (curve III).

Simulation of H2 PcSu interaction with NO2 shows completely different mechanism. It appears, that no charge transfer occur, but weak Van-der-Waals (NO2 with the center) and hydrogen (NO2 in the side-group) bonds are formed (Fig. 10). In the first case, the distance between Pc and NO2 is 2.95 A˚ and stabilization energy of such system is negative (Table 3). In the other case, very probably the classical hydrogen bond is formed. The intermolecular distance ˚ The stabilization energy between NO2 and the substituent is 2.33 A. is positive in this case.

Table 3 Calculated stabilization energy of the systems H2 PcSu NO2 . Level of theory M05/631G*. Here Estab is the stabilization energy of the complex; Estab [BSSE] is the value of energy after BSSE correction. Complex type

Estab (kcal/mol)

Estab [BSSE] (kcal/mol)

Center Edge

3.6 4.8

1.1 2.7

Fig. 9. Influences of the various concentrations of nitrogen dioxide (green dashed line) on electric current of ZnPcSu (black solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

There is no delocalization of electrons occur during interaction both with the center and side groups. Here we show the form of the molecular orbitals only for side-group-adsorbed NO2 on Pc (Fig. 11). The calculated spectrum does not differ from that of pure phthalocyanine, which agrees with the data of optical gas sensing measurements (Fig. 12). The electrical conductivity of such material should not significantly change, as it was found experimentally. H2 PcSu seem to be promising material for gas sensing application due to very weak interaction with analyte NO2 and therefore fast desorption. However, the sensitivity is quite low.

Fig. 10. Formation of Van-der-Waals interactions with the central atoms of Pc ring (a) and hydrogen bonds with the sulfonamide side-groups (b).

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Fig. 11. Molecular orbitals of H2 PcSu · · · NO2 system: HOMO orbital is localized on Pc ring (a), while LUMO orbital location is on NO2 molecule (b).

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Acknowledgements

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Authors thank to Dr. Jennifer Brookes for reviewing the manuscript and scrutinizing the language. Financial support from the Academy of Sciences of the Czech Republic (grant KAN 400720701) and Ministry of Industry and Trade of the Czech Republic (grant FR-TI1/144) are gratefully appreciated. The access to the MetaCentrum supercomputing facilities provided under the research intent MSM6383917201 is highly appreciated.

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Fig. 12. Comparison of the absorption spectra of H2 PcSu thin film (I) and calculated absorption for H2 PcSu · · · NO2 structure adsorbed on the side-group (II).

4. Conclusions The comparison of Zn and metal-free sulfonamide-substituted phthalocyanines reveal, that both optical and electrical responses for interaction with NO2 are different for these two similar materials. While sulfonamide-substituted Zn phthalocyanine is optically and electrically sensitive to NO2 , its metal-free analogue does not show such sensitivity. The investigation of their interaction with NO2 , using optical and electrical methods as well as quantum chemical simulations reveal, that ZnPcSu forms charge transfer complex with NO2 with electron density shifted from phthalocyanine to NO2 . On the contrary, metal-free phthalocyanine forms only weak complexes hold by hydrogen bonds and Van-der-Waals interactions between Pc and NO2 . Based on the results, the prototype for NO2 electrical gas sensor is proposed.

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Biographies Sergii Pochekailov has received his PhD in chemistry in Brno University of Technology, in 2009. The topic of his work concerns the properties of soluble phthalocyanines and their application in the field of organic semiconductors. The topics of his papers are connected with the design and characterization of phthalocyanine-based solar cells, light detectors and gas sensors. He obtains his master’s degree in chemistry (2005) in the National Technical University of Ukraine, with specialization of organic chemistry and technology of organic compounds. His current research interests include photovoltaics based on organic and bioorganic compounds, the concept of an electronic nose and molecular logic elements. Juraj Noˇzár is presently completing his Ph.D. studies at Charles University in Prague. The topic of his thesis is charge carrier transport in molecular systems and influence of the surroundings. Mainly it is concerned with transport of polaron quasi-particles over sigmaconjugated silane polymers, the influence of electron transfer on polymer degradation and utilization of charge transfer complex formation for detection purposes. He obtained his master’s degree in physics (2006) also at Charles University in Prague, with specialization in macromolecular physics and electron phenomena in polymers.

˚ was born in Ivanˇcice (Czech Republic) Stanislav Neˇspurek in 1940. He received his M.Sc. (1962) degree in electronics and physics of semiconductors from the University of Brno, Ph.D. (1969) degree in chemical and solid state physics from Charles University of Prague. Since 1969, he has been working at the Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences (currently Academy of Sciences of the Czech Republic), Prague. At present, he is working at the same Institute as a Chief Research Fellow of the Department of Optoelectronic Phenomena and Materials and a project leader. He is Professor of Material Science in the Institute of Chemistry, Technical University of Brno. In 1978 he was a Senior Research

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Fellow at the Chemistry Department, University of Queensland, Australia, in 1990 a visiting professor at the Institute of Molecular Science, Japan. His research interests include physics and chemistry of molecular materials, and in particular molecular electronics and electronic and optical properties of polymers and molecular crystals. Jan Rakuˇsan was born in Olomouc (Czech Republic) in 1938. He received his M.Sc. degree in organic chemistry and technology from University of Pardubice in 1962 and Ph.D. degree in organic chemistry from Institute of Technology Prague in 1967. Since 1963 until 2008 he was working at the Research Institute of Organic Syntheses (currently VÚOS a.s.) Rybitví, Czech Republic. Since 2009 he is working as a senior researcher and project leader at Center of Organic Chemistry Ltd. in Rybitví, a daughter organization of VÚOS a.s. During his career has been interested mostly in the technology of phthalocyanine dyes and pigments, the last ten years in chemistry of phthalocyanine functional dyes. He was giving lectures in EEDP at Ho Chi Minh City (Vietnam) in 1990 as a specialist of UNIDO and in 1991 at Shenyang Research Institute and Dalian University (China) after he had been invited there by Ministry of Chemistry of China. In cooperation with Draiswerke Mannheim Co. (Germany) in 2001 he designed and put into operation a production unit, producing 1500 MT of copper phthalocyanine per year in China. His research interests include phthalocyanine derivatives suitable for microelectronic and phthalocyanine photosensitizers for photodynamic therapy of cancerous tumors.

Marie Karásková. Marie Karásková was born in Pardubice (Czech Republic) in 1946. She received her M.Sc. degree in organic chemistry and technology from University of Pardubice. Since 1967 until 2009 she was working at the Research Institute of Organic Syntheses (currently VÚOS a.s.) Rybitví, Czech Republic. Since 2010 is she working as a Chief Research Fellow of the phthalocyanine group and project leader at Center of Organic Chemistry Ltd. in Rybitví, a daughter organization of VÚOS a.s. During her career she has been interested in technology of dyes and intermediates, the last ten years mostly in chemistry of phthalocyanine functional dyes. Her research interests include phthalocyanine photosensitizers for photodynamic therapy both of cancerous tumors and bacterial infections and also phthalocyanine derivatives suitable for microelectronic.