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Effect of pyridine on infrared absorption spectra of copper phthalocyanine
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Spectrochimica Acta Part A 69 (2008) 619–623
Effect of pyridine on infrared absorption spectra of copper phthalocyanine Sukhwinder Singh, S.K. Tripathi, G.S.S. Saini ∗ Department of Physics, Panjab University, Chandigarh 160 014, India Received 13 November 2006; received in revised form 28 April 2007; accepted 5 May 2007
Abstract Infrared absorption spectra of copper phthalocyanine in KBr pellet and pyridine solution in 400–1625 and 2900–3200 cm−1 regions are reported. In the IR spectra of solid sample, presence of weak bands, which are forbidden according to the selection rules of D4h point group, is explained on the basis of distortion in the copper phthalocyanine molecule caused by the crystal packing effects. Observation of a new band at 1511 cm−1 and change in intensity of some other bands in pyridine are interpreted on the basis of coordination of the solvent molecule with the central copper ion. © 2007 Elsevier B.V. All rights reserved. PACS: 33.20.Ea Keywords: Infrared spectra; Copper phthalocyanine; Pyridine
1. Introduction Despite a number of studies and applications of phthalocyanine (Pc) during last 4–5 decades, the scientific research on these organic semiconductors is still being undertaken owing to their unique properties such as thermal stability, chemical inertness and biocompitability. These compounds are being used for various applications, which include chemical sensor [1], photoconducting agents [2], photovoltaic cell elements [3], non-linear optics [4], electrocatalysis [5], colorants in chemical industry [6], liquid crystals [7], optical data storage [8] and photosensitizers [9]. Gas sensing properties of these compounds have mainly been studied by electrical conductivity measurements, since the conductivity of thin films of these complexes changes when exposed to reducing or oxidizing gases such as ammonia, halogens and NOx . Thin films of lead phthalocyanine can detect NO2 gas down to 25 ppb concentration [10]. Very recently, optical techniques have also been successfully applied for the detection of volatile organic compounds by Pcs [11]. It has been shown from optical absorption spectroscopy that copper phthalocyanine (CuPc) can also be used to detect
∗
Corresponding author. Tel.: +91 172 2534454; fax: +91 172 2783336. E-mail addresses: [email protected] (S.K. Tripathi), [email protected] (G.S.S. Saini). 1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.05.012
sedative drugs [12]. However, studies of chemical sensing properties of Pc by optical methods are rare. This is particularly true for vibrational spectroscopic techniques such as infrared (IR) absorption and Raman scattering, though these techniques provide valuable data that can be used in investigation of interaction between chemical vapor/gas and the Pc on molecular level. The wavenumbers of IR bands are affected by structural changes induced by the interaction. This facilitates the determination of structural changes on a sub-armstrong scale from the IR spectroscopy. One reason for rare use of vibrational spectroscopy is that a large number of bands corresponding to different fundamental, overtone and combination vibrations of molecule are, generally, present in vibrational spectra. This makes deciphering information from spectra really difficult. However, recently reported DFT based normal coordinate analysis of copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) [13,14] have made it possible to assign the observed IR bands accurately. Therefore, it is now possible to gain insight into the bonding arrangement and changes induced by chemical vapors in Pc molecules from the study of IR spectra. These changes depend upon the nature (acidic or basic) of interacting chemical vapors/gases. In the present work, we have studied the IR spectra of CuPc in powder form (KBr pallet) and in pyridine in order to investigate the effect of pyridine on CuPc molecule by observing changes in the spectra. This study, therefore, provides the deeper understanding of the pyridine sensing
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properties of CuPc in particular and Pc in general on molecular level. 2. Experimental The CuPc from M/S Alfa Aesar was used without further purification. However, its purity was checked with electronic absorption spectroscopy. Pyridine from M/S Qualigens Fine Chemicals, India, was also used without any purification. IR spectra of CuPc powder were obtained in KBr pellet. For recording of IR spectra of the sample in pyridine, few drops of solution of CuPc were put on a KBr window. Solvent was allowed to evaporate, before taking the measurements, in order to remove the IR absorption peaks of the solvent. The IR spectra were determined on a Perkin-Elmer PE-R × 1 FTIR Spectrophotometer. The spectral resolution of the IR spectrometer was 2 cm−1 throughout the experiment. The UV-visible spectra were recorded on a HITACHI 330 UV-VIS-NIR spectrophotometer. 3. Results and discussion Structure and atom labeling scheme of CuPc are given in Fig. 1. We observe the B (Soret) band at 354 nm in UV region of its absorption spectrum in the pyridine (figure not shown here). The other well-known bands of the Pc molecule, namely the Q-bands appear at 610, 642 (both weak) and 670 nm in the visible region. We have also recorded optical absorption spectrum of annealed thin film of CuPc deposited on KBr substrate. Observed band electronic pattern of the CuPc in pyridine correlate well with the bands observed for thin film except red shifts of nearly 10 nm in the B band and 2–3 nm in the Q bands in the solution spectrum. These band positions for the B and Qbands also match well with the reported values in the literature [12,15–17]. The observed pattern of electronic absorption bands of the CuPc can be explained on the basis of – ∗ excitation between bonding and anti-bonding molecular orbital. The pyridine molecule contains electron withdrawing N atom, therefore, it may interact through the N atom with other molecules such as CuPc. Hence, due to possible interaction between the pyridine and CuPc molecules, some electron density from the latter molecule shifts to the former molecule. The observed red shift
Fig. 1. Structure and atom labeling scheme of copper phthalocyanine.
Fig. 2. IR spectra of copper phthalocyanine: (a) in KBr pellet and (b) in pyridine, in 400–1025 cm−1 region.
of electronic transitions reflects the change in electron density on the molecule. Here, it is relevant to note that optical absorption bands of this molecule shift towards longer wavelength due to interaction with some drugs [12]. Further, we also observe a new band at 730 nm in the pyridine solution. This band may arise due to charge transfer between CuPc and pyridine. The IR spectra of CuPc in 400–1025, 1025–1625 cm−1 , and C H stretching regions are shown in Figs. 2–4 . As can be seen from these figures, we observe a large number of IR bands in the spectra. In order to simplify the assignment of observed IR bands of this molecule, we assume that the molecule is planar with an overall symmetry D4h , which can also be inferred from the inspection of Fig. 1. Moreover, DFT calculations of CuPc also resulted in the idealized D4h point group symmetry [13]. The molecule has 57 atoms and, therefore, 165 normal modes of vibrations. These vibrations can be broadly divided into two groups: one consisting of in-plane vibrations of symmetry A1g , A2g , B1g , B2g , Eu and other consisting of out-of-plane vibrations of A1u , A2u , B1u , B2u , Eg symmetry. Out of these, only the vibrational modes of symmetry species A2u and Eu show IR activity. Therefore, its spectra consist of a few intense out-of-plane bands
Fig. 3. IR spectra of copper phthalocyanine: (a) in KBr pellet and (b) in pyridine, in 1025–1625 cm−1 region.
S. Singh et al. / Spectrochimica Acta Part A 69 (2008) 619–623
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Table 1 Position and assignment of observed IR bands of CuPc under different experimental conditions Band (cm−1 ) In KBr 406 414 433
Fig. 4. IR spectra of copper phthalocyanine: (a) in KBr pellet and (b) in pyridine, in 2900–3200 cm−1 region.
of the A2u symmetry and in-plane bands of the Eu symmetry. A large number of weak bands including symmetry forbidden, which becomes allowed due to distortions in the molecule can also be seen in the spectra of CuPc both in pellet and in pyridine. The assignment of the observed bands in the IR spectra is difficult because of its extreme complexity due to appearance of a large number of allowed and forbidden bands. We have utilized a simpler, qualitative approach to interpret the IR data of CuPc on the basis of recently reported normal coordinate calculations [13,14]. These calculations along with the reported resonance Raman and IR studies of these complexes [18–25] considerably simplify the assignment of IR bands of CuPc. Most of the bands are straight forward to assign, since their positions are close to the reported values [13,14]. We expect that the mode compositions obtained in this way to be reasonably accurate and the major contributing motions should be correct. The observed IR bands and their assignments are listed in Table 1 . The IR bands in 700–800 cm−1 in the CuPc are often used to identify different polymorphs α, β, , etc., since these bands are sensitive to crystal packing arrangements present in the crystalline thin films. Out of these, the β form is most stable. It has marker bands due to C H bending at 730, 754 and 780 cm−1 [25]. In the spectrum of CuPc powder in the KBr pellet (Fig. 2a), we observe bands at 729, 753 and 779 cm−1 . Therefore, it is clear that β crystalline form is predominantly present in the solid sample, with a very small percentage of the α, which can be inferred from the asymmetry in the 729 cm−1 band on lower wavenumber side. We also observe a large number of weak bands almost in entire wavenumber region of our study. These extra bands other than the allowed bands under D4h symmetry, may arise due to one or more of the following reasons: (1) formation of dimer or higher order aggregates; (2) crystal packing effect in solid powder of crystalline sample; (3) co-existence of polymorphs of CuPc; (4) lowering of symmetry from D4h to C4v due to attachment of a molecule at the fifth coordination site of the central metal ion. In the solid sample, observation of extra bands, which are forbidden under D4h point group symmetry, may be due to second and/or third possibilities. In the molecular stacking of the β modification, the Cu atom at the
454 467 480 489 505 515 523 532 544 571 588 598 612 630 637 668 678 689 716 724 729 753 772 779 801 870 876 899 948 955 983 1002 1066 1088 1100 1119 1173 1188 1202 1286 1310 1332
In pyridine 405 410 433 450 454 465 479 489 505 523 531 543 561 572 588 597 612 620 630 638 679 689 724 729 754 772 780 801
876 899 949 955 982 1002 1067 1089 1099 1119 1191 1202 1217 1287 1310 1332 1351
1359 1371 1385 1393 1404 1418 1442
Symmetry and assignmenta ,b
1369 1383 1391 1420 1443
Eg (E); ρw (Cb –Cc –H)as Eg (E); ρw (Cb –Cc –H)as A2u (A2 ); ρw (Cb –Cc –H)s B2g (B2 ); ρτ (c ring)+ δ (Ni –Cu–Ni ) Eg (E); [ρw (Cb –Cb ), ρw (Cc –Cc )]as + ρw (Ca –Nb ) Eu (E); ρτ (c ring)+ ρτ (Ca –Ni –Ca )+ η (Nb –Nb ) B2g (B2 ); ρτ (c ring)+ ρτ (Ca –Ni –Ca ) B1g (B1 ); η (b ring, Cc –Cc )s A2g (A2 ); ρτ (Ca –Ni –Ca )+ ρτ (Cb –Cb , Cc –Cc )s Eu (E); η (b ring)+ η (Cc –Cc )+ ρτ (Ca –Ni –Ca ) A1g (A1 ); η (a)+ η (Cb –Cb , Cc –Cc )as A2g (A2 ); ρτ (Ni –Cu–Ni )+ ρτ (c ring) Eg (E); ρw (Ca –Ni –Ca )+ ρw (Cc –Cc ) A1g (A1 ); η (Nb –Nb )+ η (Cb –Cb )as Eg (E); ρw (Ca –Ni –Ca )+ ρw (Cc –H) A2u (A2 ); ρw (Ca –Ni )s + ρw (Cc –H) Eu (E); η (Ni –Ni )+ δas (Ca –Ni –Ca )+ ρτ (Cc –Cc )as B1g (B1 ); ν (Cu–Ni )+[ν (b ring, c ring)]as A2u (A2 ); ρw (Cc –H)s Eu (E); η (Ni –Ni )+[ν (b ring, c ring)]as + ρτ (Cc –Cc )as Eg (E); [ρw (Cc –H)]as Eu (E); η (Ni –Ni )+[ν (b ring, c ring)]as + ρτ (Cc –Cc )as A2u (A2 ); ρw (Cc –H) Eg (E); ρw (Cc –H) B1g (B1 ); δ (Cc –H) Eu (E); δ (Cc –H)+ νas (Cc –Cc ) Eu (E); [η (Ni –Ni )+ η (Cc –Cc )]as Eu (E); δ (Cc –H)+ η (Ni –Ni ) B2g (B2 ); ρτ (Cc –H) Eu (E); δ (Cc –H) B1g (B1 ); [δ (Cc –H)]as A2g (A2 ); ρτ (Cc –H)+ ρτ (Ni –Cu–Ni )s Eu (E); η (Ni –Ni )+[ρτ (Cc –H)]as B2g (B2 ); ρτ (Cc –H)+ δs (Ni –Cu–Ni ) Eu (E); δ (Cb –Cb )+[ρτ (Cc –H)]as Eu (E); η (Ni –Ni )+ δas (Ca –Ni –Ca )+ νas (Cb –Cb ) Eu (E); νas (Cb –Cb , Cc –Cc )+ η (Ni –Ni )+ δas (Ca –Ni –Ca ) Eg (E); [νs (Cb –Cb , Cc –Cc )]as + ηas (Cc –Cc ) A1g (A1 ); νs (Cu–Ni )+ δ (Ca –Nb –Ca ) B2g (B2 ); [νs (Cb –Cb ), νs (Cc –Cc )]as + δ (Cc –H) Eu (E); [νs (Cb –Cb ), νs (Cc –Cc )]as + δ (Cc –H) B1g (B1 ); [ν (Cb –Cb )]as + δ (Cc –H)
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Table 1 (Continued ) Band (cm−1 ) In KBr
In pyridine
1446 1455 1463 1469 1480 1487 1496 1505 1532 1538 1548 1557 1564 1568 1574 1586 1608 .. . 3009 3025 3046 3072
Symmetry and assignmenta ,b
1451 1463
B2g (B2 ); ν (Ca –Nb )as + δτ (Ca –Ni –Ca )+ δ (Ni –Cu–Ni ) A2g (A2 ); ρτ (Cb –Cb , Cc –Cc ) Eu (E); ρτ (Cb –Cb , Cc –Cc )as B2g (B2 ); ρτ (Cb –Cb , Cc –Cc )
1479 Eu (E); δτ (Ca –Ni –Ca )as +[ρτ (Cb –Cb , Cc –Cc )]as 1505 1511 1530 1536 1549
Eu (E); [νs (Ca –Nb )]as + δas (Ca –Ni –Ca ) (B1 ); δ (Ca –Nb –Ca ) A1g (A1 ); [νs (Ca –Nb )]s +[δs (Ca –Ni –Ca )]s
1565
B1g (B1 ); [νs (Cb –Cb , Cc –Cc )]as
1572 1587 1608 .. . 3009 3025 3048 3075
A1g (A1 ); [νs (Cb –Cb , Cc –Cc )]s Eu (E); [νs (Cb –Cb , Cc –Cc )]as Eu (E); [δτ (Cb –Cb , Cc –Cc )]as .. . ν (C H) ν (C H) ν (C H) ν (C H)
a
Assignments are based on Refs. [13,14]. The ν, δ, ρ and η denote stretching, in-plane bending, out-of-plane bending and heaving motion, respectively. The subscript ‘s’ and ‘as’ represent the symmetric and asymmetric modes, respectively. Mode symmetries under D4h and C4v points groups are given out of the bracket and within the bracket, respectively. b
center of each molecule is coordinated with the N atoms of the adjacent molecules. This forms distorted octahedron geometry, which is very common and favored in copper complexes. No such coordination is possible in α-type crystals. Because of crystal packing arrangement in β, the electrons in one molecule and the peripheral hydrogen atoms of the molecules in adjacent stacks are in close proximity. This contributes to a strong association between various molecular stacks inside a crystal. Therefore, crystal-packing effects, which reduce the molecular symmetry of CuPc in solid powder, are responsible for the observation of these IR forbidden modes under ideal D4h symmetry group. The IR spectra of CuPc in pyridine show some interesting changes compared to the powder spectra. As observed for the crystalline sample, the spectra of CuPc in the pyridine also show bands due to IR forbidden modes. In the pyridine solution of CuPc, we observe C H bending modes at 729, 754 and 780 cm−1 . These band positions match well with the corresponding bands in the IR spectrum of the β crystalline form. Therefore, it can be inferred from this observation that the CuPc molecule in solution has nearly identical bond order as in the β form. Relative intensity of 753 and 779 cm−1 bands is changed with respect to the 729 cm−1 band in the pyridine. This change arises due to change in conformation of the molecule in the presence of the pyridine as discussed below. Since pyridine is well known coordinating solvent, therefore, it is possible that the
pyridine molecule attaches to the fifth coordination site of the Cu ion. Due to this interaction, the Cu ion moves out of the mean Pc plane. The displacement of the Cu metal ion from the mean plane of the Pc leads to doming of Pc macrocycle with decrease in overall symmetry of the molecule from D4h to C4v . Due to doming of the core, some bands may gain intensity, for example, bands of the A2g symmetry become IR active, while some other bands may loose their intensity as observed by us. A change in conformation is more likely to affect the bands due to both out-of-plane (oop) modes, viz. 729 and 780 cm−1 , and in-plane (ip) mode, viz. 753 cm−1 . One interesting observation is that almost all the bands with changed intensity remain at the same wave number positions as in the powder sample. In addition the Pc ring in solution can also flex about the Nb atoms, which are bridging the isoindole groups. The flexing, which is constrained in the solid form, of the Pc ring may in part account for the variable vibrational structure in solution compared to powder. It is also possible that two pyridine molecules may also simultaneously attached to fifth and sixth coordination site of the Cu metal ion, which keeps the Cu ion in the mean phthalocyanine plane with approximate symmetry of the D4h . Therefore, two CuPc molecules in pyridine may exist simultaneously in the solution as five and six coordinated species with C4v and D4h point group symmetries, respectively. If both of these species coexist, one expects more number of vibrational modes, which are allowed according to the selection rules for these point groups. Moreover, due to possible change in the bonding arrangement in two species, corresponding frequencies may also differ by few wavenumbers. Change in the intensity of some of the low wavenumber bands at 505, 571, 638 and 689 cm−1 also indicates towards the structural distortion of the CuPc molecule in pyridine. These bands arise due to oop torsion/bending motion of the molecule. As discussed earlier, occupation of fifth and/or sixth coordination site of the Cu metal ion by the pyridine produces out-of-plane distortion. This distortion permits the mixing of eigenvectors of oop modes with the ip modes due to which some vibrational energy of allowed bands might be transferred to the ip modes. In low wavenumber region, intensity of the 899 cm−1 band which is assigned to η (Ni –Ni ) and asymmetric isoindole ring vibration, is also reduced in pyridine solution. This observation also suggests the conformational change in the CuPc molecule in presence of the pyridine molecules. In high wavenumber region, the IR spectrum of CuPc in pyridine shows a band with enhanced intensity at 1383 cm−1 , which is very weak in the spectrum of KBr pellet at 1385 cm−1 (Fig. 3). This band is assigned to the symmetric Cu–Ni stretching vibration with a contribution from in-plane Ca –Nb –Ca bending motion [13,14]. The vibrational symmetry of this mode under D4h group is A1g , which reduces to A1 when the molecular symmetry of CuPc is decreased to C4v on coordination with the solvent molecule. Hence, the mode becomes IR allowed for coordinated species. Therefore, presence of this band with the enhanced intensity in solution relative to the pellet also indicates the coordination of solvent to the Cu ion at out of the Pc plane. Apart from the intensity changes in a number of bands, the most important change in solution spectra compared to powder spectra is the observation of a new band at 1511 cm−1 . This
S. Singh et al. / Spectrochimica Acta Part A 69 (2008) 619–623
band is forbidden under the D4h symmetry in IR spectra and arises due to large displacement of Ca –Nb –Ca bridge bonds of the Pc ring. From the width of the 1505 cm−1 band in powder spectrum, it seems that the 1511 and 1502 cm−1 bands are degenerate. These bands are resolved in pyridine due to upward shift in wavenumber of the former band while the latter band shows small down shift. In Raman spectra of metal-Pc, this band has been identified as highly sensitive to the metal ion present [26]. The variation for the position of this band correlates well with the metal ion size and structural effects identified from the crystal structure [27]. This band is observed at higher positions for the Pc, which have more distorted macrocycle. It is observed around 1545 and 1511 cm−1 in nickel phthalocyanine (NiPc) and CuPc, respectively [13,14,27]. The size of Cu ion exactly fits into the macrocycle core to such an extent that in the absence of any external perturbations, there is no deviation from the planar structure. On the other hand, in NiPc due to the small size of the Ni ion, four isoindole rings are pulled in towards the Ni ion. This gives not only small core diameter but also has an effect on Ca –Nb –Ca bridge bonds. Owing to this, Pc macrocycle is significantly deformed in NiPc. Similarly, when pyridine attaches to the oop coordination site of the Cu ion, the Cu ion is forced out of the mean Pc plane resulting in the pulling of Ni atoms inside the core with concomitant contraction of the Pc macrocyle. This situation is quite similar to the NiPc. Hence, CuPc in pyridine should also show upward shift in the Ca –Nb –Ca related vibration by few wavenumbers. In fact, corresponding band shifts by 6 cm−1 from 1505 to 1511 cm−1 in the solution spectrum (Fig. 3b). Therefore, upshift in this band also supports the formation of pyridine coordinated CuPc complex in the solvent. The vibrational band corresponding to this mode in water soluble cationic copper 4,4 ,4 ,4 -tetrasulfanato phthalocyanine also splits into isotropic and anisotropic components at 1527 and 1538 cm−1 , respectively [28,29]. The higher wavenumber band arises due to aggregated species, whereas low wavenumber band originate from the monomeric phthalocyanine. Dimer or higher order aggregates considerably deform the macrocycle, which is indicated by the presence of higher wavenumber band at 1538 cm−1 band in their spectra. In the present study, we have used non-ionic non-substituted CuPc, which is soluble only in a few organic solvents. Hence, formation of the aggregated CuPc can be ruled out in the pyridine solution [29]. Therefore, observation a band at 1511 cm−1 in the IR spectra can be explained only, if we consider the out-of-plane displacement of Cu ion from the mean Pc plane due to coordination of pyridine. The IR spectra of CuPc in high frequency region are shown in Fig. 4. Fundamental C H stretching vibrations of isoindole rings, and overtone and combination modes dominate this region. In this region, both the spectra are almost similar in band positions expect small change of intensity of these bands. Intensity change can be explained based on the perturbation caused by the solvent molecule as discussed above. Positions of C H stretching bands indicate that the C H bonding arrangement in the solution remains almost identical to the solid. Weak bands
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in this region may arise due to overtone and combination of fundamental bands, hence these are not considered in the present study. 4. Conclusions From the above discussion, it is clear that crystal-packing effects are responsible for observation of some weak bands in the spectra of solid CuPc. Observed spectral changes in the IR and optical spectra of CuPc in the pyridine solvent indicate towards the coordination of solvent molecule with metal ion. Acknowledgements This work is financially supported by CSIR, India. Spectra were recorded at SAIF, Punjab University. References [1] T. Miyata, S. Kawaguchi, M. Ishii, T. Minami, Thin Solid Films 425 (2003) 255. [2] K.-Y. Law, Chem. Rev. 93 (1993) 449. [3] T.D. Anthopoulos, T.S. Shafai, Thin Solid Films 441 (2003) 207. [4] G. de la Torre, P. Vazquez, F. Agullo-Lopez, T. Torres, J. Mater. Chem. 8 (1998) 1671. [5] B. Bottger, U. Schindewolf, J.L. Avila, R. Rodrigues-Amaro, J. Electroanal. Chem. 432 (1997) 139. [6] B.D. Bezerin, Coordination Compounds of Porphyrins and Phthalocyanines, Wiley, New York, 1981. [7] J. Simon, C. Sirlin, Pure Appl. Chem. 61 (1989) 1625. [8] J.E. Kudler, J. Imag. Sci. 32 (1988) 51. [9] K. Kato, Y. Nishioka, K. Kaifu, K. Kawamura, S. Ohno, Appl. Chem. Lett. 86 (1985) 196. [10] T.A. Tomofonte, K.F. Schoch, J. Appl. Phys. 65 (1989) 1350. [11] J. Spadavecchia, G. Ciccarella, G. Vasapollo, P. Siciliano, R. Rella, Sens. Actuators B 100 (2004) 135. [12] M. Safarikova, I. Safarik, Eur. Cells Mater. 3 (2002) 188. [13] D. Li, Z. Peng, L. Deng, Y. Shen, Y. Zhou, Vib. Spectrosc. 39 (2005) 191. [14] D.R. Tackley, G. Dent, W.E. Smith, Phys. Chem. Chem. Phys. 2 (2000) 3949. [15] M. Wojdyla, B. Derkowska, M. Rebarz, A. Bratkowski, W. Bala, J. Opt. A: Pure Appl. Opt. 7 (2005) 463. [16] M.M. El-Nahass, F.S. Bahabri, R. Al-Harbi, Egypt. J. Sol. 24 (2001) 11. [17] N.-S. Cho, K.-H. Kim, C.-H. Han, J. Korean Chem. Soc. 16 (1971) 378. [18] W. Su, M. Bao, J. Jiang, Vib. Spectrosc. 39 (2005) 186. [19] X. Sun, M. Bao, N. Pan, X. Cui, D.P. Arnold, J. Jiang, Aust. J. Chem. 55 (2002) 587. [20] M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.A. Darwish, Mater. Chem. Phys. 92 (2005) 185. [21] F. Lu, L. Zhang, H. Liu, X. Yan, Vib. Spectrosc. 39 (2005) 139. [22] R. Aroca, E. Johnson, Langmuir 8 (1992) 3137. [23] R. Aroca, A. Thedchanamoorthy, Chem. Mater. 7 (1995) 69. [24] A.J. Bovill, A.A. McConnell, J.A. Nimmo, W.E. Smith, J. Phys. Chem. 90 (1986) 569. [25] G. Maggioni, A. Quaranta, S. Carturan, A. Patelli, M. Tonezzer, R. Ceccato, G.D. Mea, Chem. Mater. 17 (2005) 1895. [26] G. Dent, F. Farrell, Spectrochim. Acta A 53 (1997) 21. [27] D.R. Tackley, G. Dent, W.E. Smith, Phys. Chem. Chem. Phys. 3 (2001) 1419. [28] H. Abramczyk, I. Szymczyk, J. Mol. Liq. 110 (2004) 51. [29] I. Szymczyk, H. Abramczyk, Pure Appl. Chem. 76 (2004) 183.
Spectrochimica Acta Part A 69 (2008) 619–623
Effect of pyridine on infrared absorption spectra of copper phthalocyanine Sukhwinder Singh, S.K. Tripathi, G.S.S. Saini ∗ Department of Physics, Panjab University, Chandigarh 160 014, India Received 13 November 2006; received in revised form 28 April 2007; accepted 5 May 2007
Abstract Infrared absorption spectra of copper phthalocyanine in KBr pellet and pyridine solution in 400–1625 and 2900–3200 cm−1 regions are reported. In the IR spectra of solid sample, presence of weak bands, which are forbidden according to the selection rules of D4h point group, is explained on the basis of distortion in the copper phthalocyanine molecule caused by the crystal packing effects. Observation of a new band at 1511 cm−1 and change in intensity of some other bands in pyridine are interpreted on the basis of coordination of the solvent molecule with the central copper ion. © 2007 Elsevier B.V. All rights reserved. PACS: 33.20.Ea Keywords: Infrared spectra; Copper phthalocyanine; Pyridine
1. Introduction Despite a number of studies and applications of phthalocyanine (Pc) during last 4–5 decades, the scientific research on these organic semiconductors is still being undertaken owing to their unique properties such as thermal stability, chemical inertness and biocompitability. These compounds are being used for various applications, which include chemical sensor [1], photoconducting agents [2], photovoltaic cell elements [3], non-linear optics [4], electrocatalysis [5], colorants in chemical industry [6], liquid crystals [7], optical data storage [8] and photosensitizers [9]. Gas sensing properties of these compounds have mainly been studied by electrical conductivity measurements, since the conductivity of thin films of these complexes changes when exposed to reducing or oxidizing gases such as ammonia, halogens and NOx . Thin films of lead phthalocyanine can detect NO2 gas down to 25 ppb concentration [10]. Very recently, optical techniques have also been successfully applied for the detection of volatile organic compounds by Pcs [11]. It has been shown from optical absorption spectroscopy that copper phthalocyanine (CuPc) can also be used to detect
∗
Corresponding author. Tel.: +91 172 2534454; fax: +91 172 2783336. E-mail addresses: [email protected] (S.K. Tripathi), [email protected] (G.S.S. Saini). 1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.05.012
sedative drugs [12]. However, studies of chemical sensing properties of Pc by optical methods are rare. This is particularly true for vibrational spectroscopic techniques such as infrared (IR) absorption and Raman scattering, though these techniques provide valuable data that can be used in investigation of interaction between chemical vapor/gas and the Pc on molecular level. The wavenumbers of IR bands are affected by structural changes induced by the interaction. This facilitates the determination of structural changes on a sub-armstrong scale from the IR spectroscopy. One reason for rare use of vibrational spectroscopy is that a large number of bands corresponding to different fundamental, overtone and combination vibrations of molecule are, generally, present in vibrational spectra. This makes deciphering information from spectra really difficult. However, recently reported DFT based normal coordinate analysis of copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) [13,14] have made it possible to assign the observed IR bands accurately. Therefore, it is now possible to gain insight into the bonding arrangement and changes induced by chemical vapors in Pc molecules from the study of IR spectra. These changes depend upon the nature (acidic or basic) of interacting chemical vapors/gases. In the present work, we have studied the IR spectra of CuPc in powder form (KBr pallet) and in pyridine in order to investigate the effect of pyridine on CuPc molecule by observing changes in the spectra. This study, therefore, provides the deeper understanding of the pyridine sensing
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properties of CuPc in particular and Pc in general on molecular level. 2. Experimental The CuPc from M/S Alfa Aesar was used without further purification. However, its purity was checked with electronic absorption spectroscopy. Pyridine from M/S Qualigens Fine Chemicals, India, was also used without any purification. IR spectra of CuPc powder were obtained in KBr pellet. For recording of IR spectra of the sample in pyridine, few drops of solution of CuPc were put on a KBr window. Solvent was allowed to evaporate, before taking the measurements, in order to remove the IR absorption peaks of the solvent. The IR spectra were determined on a Perkin-Elmer PE-R × 1 FTIR Spectrophotometer. The spectral resolution of the IR spectrometer was 2 cm−1 throughout the experiment. The UV-visible spectra were recorded on a HITACHI 330 UV-VIS-NIR spectrophotometer. 3. Results and discussion Structure and atom labeling scheme of CuPc are given in Fig. 1. We observe the B (Soret) band at 354 nm in UV region of its absorption spectrum in the pyridine (figure not shown here). The other well-known bands of the Pc molecule, namely the Q-bands appear at 610, 642 (both weak) and 670 nm in the visible region. We have also recorded optical absorption spectrum of annealed thin film of CuPc deposited on KBr substrate. Observed band electronic pattern of the CuPc in pyridine correlate well with the bands observed for thin film except red shifts of nearly 10 nm in the B band and 2–3 nm in the Q bands in the solution spectrum. These band positions for the B and Qbands also match well with the reported values in the literature [12,15–17]. The observed pattern of electronic absorption bands of the CuPc can be explained on the basis of – ∗ excitation between bonding and anti-bonding molecular orbital. The pyridine molecule contains electron withdrawing N atom, therefore, it may interact through the N atom with other molecules such as CuPc. Hence, due to possible interaction between the pyridine and CuPc molecules, some electron density from the latter molecule shifts to the former molecule. The observed red shift
Fig. 1. Structure and atom labeling scheme of copper phthalocyanine.
Fig. 2. IR spectra of copper phthalocyanine: (a) in KBr pellet and (b) in pyridine, in 400–1025 cm−1 region.
of electronic transitions reflects the change in electron density on the molecule. Here, it is relevant to note that optical absorption bands of this molecule shift towards longer wavelength due to interaction with some drugs [12]. Further, we also observe a new band at 730 nm in the pyridine solution. This band may arise due to charge transfer between CuPc and pyridine. The IR spectra of CuPc in 400–1025, 1025–1625 cm−1 , and C H stretching regions are shown in Figs. 2–4 . As can be seen from these figures, we observe a large number of IR bands in the spectra. In order to simplify the assignment of observed IR bands of this molecule, we assume that the molecule is planar with an overall symmetry D4h , which can also be inferred from the inspection of Fig. 1. Moreover, DFT calculations of CuPc also resulted in the idealized D4h point group symmetry [13]. The molecule has 57 atoms and, therefore, 165 normal modes of vibrations. These vibrations can be broadly divided into two groups: one consisting of in-plane vibrations of symmetry A1g , A2g , B1g , B2g , Eu and other consisting of out-of-plane vibrations of A1u , A2u , B1u , B2u , Eg symmetry. Out of these, only the vibrational modes of symmetry species A2u and Eu show IR activity. Therefore, its spectra consist of a few intense out-of-plane bands
Fig. 3. IR spectra of copper phthalocyanine: (a) in KBr pellet and (b) in pyridine, in 1025–1625 cm−1 region.
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Table 1 Position and assignment of observed IR bands of CuPc under different experimental conditions Band (cm−1 ) In KBr 406 414 433
Fig. 4. IR spectra of copper phthalocyanine: (a) in KBr pellet and (b) in pyridine, in 2900–3200 cm−1 region.
of the A2u symmetry and in-plane bands of the Eu symmetry. A large number of weak bands including symmetry forbidden, which becomes allowed due to distortions in the molecule can also be seen in the spectra of CuPc both in pellet and in pyridine. The assignment of the observed bands in the IR spectra is difficult because of its extreme complexity due to appearance of a large number of allowed and forbidden bands. We have utilized a simpler, qualitative approach to interpret the IR data of CuPc on the basis of recently reported normal coordinate calculations [13,14]. These calculations along with the reported resonance Raman and IR studies of these complexes [18–25] considerably simplify the assignment of IR bands of CuPc. Most of the bands are straight forward to assign, since their positions are close to the reported values [13,14]. We expect that the mode compositions obtained in this way to be reasonably accurate and the major contributing motions should be correct. The observed IR bands and their assignments are listed in Table 1 . The IR bands in 700–800 cm−1 in the CuPc are often used to identify different polymorphs α, β, , etc., since these bands are sensitive to crystal packing arrangements present in the crystalline thin films. Out of these, the β form is most stable. It has marker bands due to C H bending at 730, 754 and 780 cm−1 [25]. In the spectrum of CuPc powder in the KBr pellet (Fig. 2a), we observe bands at 729, 753 and 779 cm−1 . Therefore, it is clear that β crystalline form is predominantly present in the solid sample, with a very small percentage of the α, which can be inferred from the asymmetry in the 729 cm−1 band on lower wavenumber side. We also observe a large number of weak bands almost in entire wavenumber region of our study. These extra bands other than the allowed bands under D4h symmetry, may arise due to one or more of the following reasons: (1) formation of dimer or higher order aggregates; (2) crystal packing effect in solid powder of crystalline sample; (3) co-existence of polymorphs of CuPc; (4) lowering of symmetry from D4h to C4v due to attachment of a molecule at the fifth coordination site of the central metal ion. In the solid sample, observation of extra bands, which are forbidden under D4h point group symmetry, may be due to second and/or third possibilities. In the molecular stacking of the β modification, the Cu atom at the
454 467 480 489 505 515 523 532 544 571 588 598 612 630 637 668 678 689 716 724 729 753 772 779 801 870 876 899 948 955 983 1002 1066 1088 1100 1119 1173 1188 1202 1286 1310 1332
In pyridine 405 410 433 450 454 465 479 489 505 523 531 543 561 572 588 597 612 620 630 638 679 689 724 729 754 772 780 801
876 899 949 955 982 1002 1067 1089 1099 1119 1191 1202 1217 1287 1310 1332 1351
1359 1371 1385 1393 1404 1418 1442
Symmetry and assignmenta ,b
1369 1383 1391 1420 1443
Eg (E); ρw (Cb –Cc –H)as Eg (E); ρw (Cb –Cc –H)as A2u (A2 ); ρw (Cb –Cc –H)s B2g (B2 ); ρτ (c ring)+ δ (Ni –Cu–Ni ) Eg (E); [ρw (Cb –Cb ), ρw (Cc –Cc )]as + ρw (Ca –Nb ) Eu (E); ρτ (c ring)+ ρτ (Ca –Ni –Ca )+ η (Nb –Nb ) B2g (B2 ); ρτ (c ring)+ ρτ (Ca –Ni –Ca ) B1g (B1 ); η (b ring, Cc –Cc )s A2g (A2 ); ρτ (Ca –Ni –Ca )+ ρτ (Cb –Cb , Cc –Cc )s Eu (E); η (b ring)+ η (Cc –Cc )+ ρτ (Ca –Ni –Ca ) A1g (A1 ); η (a)+ η (Cb –Cb , Cc –Cc )as A2g (A2 ); ρτ (Ni –Cu–Ni )+ ρτ (c ring) Eg (E); ρw (Ca –Ni –Ca )+ ρw (Cc –Cc ) A1g (A1 ); η (Nb –Nb )+ η (Cb –Cb )as Eg (E); ρw (Ca –Ni –Ca )+ ρw (Cc –H) A2u (A2 ); ρw (Ca –Ni )s + ρw (Cc –H) Eu (E); η (Ni –Ni )+ δas (Ca –Ni –Ca )+ ρτ (Cc –Cc )as B1g (B1 ); ν (Cu–Ni )+[ν (b ring, c ring)]as A2u (A2 ); ρw (Cc –H)s Eu (E); η (Ni –Ni )+[ν (b ring, c ring)]as + ρτ (Cc –Cc )as Eg (E); [ρw (Cc –H)]as Eu (E); η (Ni –Ni )+[ν (b ring, c ring)]as + ρτ (Cc –Cc )as A2u (A2 ); ρw (Cc –H) Eg (E); ρw (Cc –H) B1g (B1 ); δ (Cc –H) Eu (E); δ (Cc –H)+ νas (Cc –Cc ) Eu (E); [η (Ni –Ni )+ η (Cc –Cc )]as Eu (E); δ (Cc –H)+ η (Ni –Ni ) B2g (B2 ); ρτ (Cc –H) Eu (E); δ (Cc –H) B1g (B1 ); [δ (Cc –H)]as A2g (A2 ); ρτ (Cc –H)+ ρτ (Ni –Cu–Ni )s Eu (E); η (Ni –Ni )+[ρτ (Cc –H)]as B2g (B2 ); ρτ (Cc –H)+ δs (Ni –Cu–Ni ) Eu (E); δ (Cb –Cb )+[ρτ (Cc –H)]as Eu (E); η (Ni –Ni )+ δas (Ca –Ni –Ca )+ νas (Cb –Cb ) Eu (E); νas (Cb –Cb , Cc –Cc )+ η (Ni –Ni )+ δas (Ca –Ni –Ca ) Eg (E); [νs (Cb –Cb , Cc –Cc )]as + ηas (Cc –Cc ) A1g (A1 ); νs (Cu–Ni )+ δ (Ca –Nb –Ca ) B2g (B2 ); [νs (Cb –Cb ), νs (Cc –Cc )]as + δ (Cc –H) Eu (E); [νs (Cb –Cb ), νs (Cc –Cc )]as + δ (Cc –H) B1g (B1 ); [ν (Cb –Cb )]as + δ (Cc –H)
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Table 1 (Continued ) Band (cm−1 ) In KBr
In pyridine
1446 1455 1463 1469 1480 1487 1496 1505 1532 1538 1548 1557 1564 1568 1574 1586 1608 .. . 3009 3025 3046 3072
Symmetry and assignmenta ,b
1451 1463
B2g (B2 ); ν (Ca –Nb )as + δτ (Ca –Ni –Ca )+ δ (Ni –Cu–Ni ) A2g (A2 ); ρτ (Cb –Cb , Cc –Cc ) Eu (E); ρτ (Cb –Cb , Cc –Cc )as B2g (B2 ); ρτ (Cb –Cb , Cc –Cc )
1479 Eu (E); δτ (Ca –Ni –Ca )as +[ρτ (Cb –Cb , Cc –Cc )]as 1505 1511 1530 1536 1549
Eu (E); [νs (Ca –Nb )]as + δas (Ca –Ni –Ca ) (B1 ); δ (Ca –Nb –Ca ) A1g (A1 ); [νs (Ca –Nb )]s +[δs (Ca –Ni –Ca )]s
1565
B1g (B1 ); [νs (Cb –Cb , Cc –Cc )]as
1572 1587 1608 .. . 3009 3025 3048 3075
A1g (A1 ); [νs (Cb –Cb , Cc –Cc )]s Eu (E); [νs (Cb –Cb , Cc –Cc )]as Eu (E); [δτ (Cb –Cb , Cc –Cc )]as .. . ν (C H) ν (C H) ν (C H) ν (C H)
a
Assignments are based on Refs. [13,14]. The ν, δ, ρ and η denote stretching, in-plane bending, out-of-plane bending and heaving motion, respectively. The subscript ‘s’ and ‘as’ represent the symmetric and asymmetric modes, respectively. Mode symmetries under D4h and C4v points groups are given out of the bracket and within the bracket, respectively. b
center of each molecule is coordinated with the N atoms of the adjacent molecules. This forms distorted octahedron geometry, which is very common and favored in copper complexes. No such coordination is possible in α-type crystals. Because of crystal packing arrangement in β, the electrons in one molecule and the peripheral hydrogen atoms of the molecules in adjacent stacks are in close proximity. This contributes to a strong association between various molecular stacks inside a crystal. Therefore, crystal-packing effects, which reduce the molecular symmetry of CuPc in solid powder, are responsible for the observation of these IR forbidden modes under ideal D4h symmetry group. The IR spectra of CuPc in pyridine show some interesting changes compared to the powder spectra. As observed for the crystalline sample, the spectra of CuPc in the pyridine also show bands due to IR forbidden modes. In the pyridine solution of CuPc, we observe C H bending modes at 729, 754 and 780 cm−1 . These band positions match well with the corresponding bands in the IR spectrum of the β crystalline form. Therefore, it can be inferred from this observation that the CuPc molecule in solution has nearly identical bond order as in the β form. Relative intensity of 753 and 779 cm−1 bands is changed with respect to the 729 cm−1 band in the pyridine. This change arises due to change in conformation of the molecule in the presence of the pyridine as discussed below. Since pyridine is well known coordinating solvent, therefore, it is possible that the
pyridine molecule attaches to the fifth coordination site of the Cu ion. Due to this interaction, the Cu ion moves out of the mean Pc plane. The displacement of the Cu metal ion from the mean plane of the Pc leads to doming of Pc macrocycle with decrease in overall symmetry of the molecule from D4h to C4v . Due to doming of the core, some bands may gain intensity, for example, bands of the A2g symmetry become IR active, while some other bands may loose their intensity as observed by us. A change in conformation is more likely to affect the bands due to both out-of-plane (oop) modes, viz. 729 and 780 cm−1 , and in-plane (ip) mode, viz. 753 cm−1 . One interesting observation is that almost all the bands with changed intensity remain at the same wave number positions as in the powder sample. In addition the Pc ring in solution can also flex about the Nb atoms, which are bridging the isoindole groups. The flexing, which is constrained in the solid form, of the Pc ring may in part account for the variable vibrational structure in solution compared to powder. It is also possible that two pyridine molecules may also simultaneously attached to fifth and sixth coordination site of the Cu metal ion, which keeps the Cu ion in the mean phthalocyanine plane with approximate symmetry of the D4h . Therefore, two CuPc molecules in pyridine may exist simultaneously in the solution as five and six coordinated species with C4v and D4h point group symmetries, respectively. If both of these species coexist, one expects more number of vibrational modes, which are allowed according to the selection rules for these point groups. Moreover, due to possible change in the bonding arrangement in two species, corresponding frequencies may also differ by few wavenumbers. Change in the intensity of some of the low wavenumber bands at 505, 571, 638 and 689 cm−1 also indicates towards the structural distortion of the CuPc molecule in pyridine. These bands arise due to oop torsion/bending motion of the molecule. As discussed earlier, occupation of fifth and/or sixth coordination site of the Cu metal ion by the pyridine produces out-of-plane distortion. This distortion permits the mixing of eigenvectors of oop modes with the ip modes due to which some vibrational energy of allowed bands might be transferred to the ip modes. In low wavenumber region, intensity of the 899 cm−1 band which is assigned to η (Ni –Ni ) and asymmetric isoindole ring vibration, is also reduced in pyridine solution. This observation also suggests the conformational change in the CuPc molecule in presence of the pyridine molecules. In high wavenumber region, the IR spectrum of CuPc in pyridine shows a band with enhanced intensity at 1383 cm−1 , which is very weak in the spectrum of KBr pellet at 1385 cm−1 (Fig. 3). This band is assigned to the symmetric Cu–Ni stretching vibration with a contribution from in-plane Ca –Nb –Ca bending motion [13,14]. The vibrational symmetry of this mode under D4h group is A1g , which reduces to A1 when the molecular symmetry of CuPc is decreased to C4v on coordination with the solvent molecule. Hence, the mode becomes IR allowed for coordinated species. Therefore, presence of this band with the enhanced intensity in solution relative to the pellet also indicates the coordination of solvent to the Cu ion at out of the Pc plane. Apart from the intensity changes in a number of bands, the most important change in solution spectra compared to powder spectra is the observation of a new band at 1511 cm−1 . This
S. Singh et al. / Spectrochimica Acta Part A 69 (2008) 619–623
band is forbidden under the D4h symmetry in IR spectra and arises due to large displacement of Ca –Nb –Ca bridge bonds of the Pc ring. From the width of the 1505 cm−1 band in powder spectrum, it seems that the 1511 and 1502 cm−1 bands are degenerate. These bands are resolved in pyridine due to upward shift in wavenumber of the former band while the latter band shows small down shift. In Raman spectra of metal-Pc, this band has been identified as highly sensitive to the metal ion present [26]. The variation for the position of this band correlates well with the metal ion size and structural effects identified from the crystal structure [27]. This band is observed at higher positions for the Pc, which have more distorted macrocycle. It is observed around 1545 and 1511 cm−1 in nickel phthalocyanine (NiPc) and CuPc, respectively [13,14,27]. The size of Cu ion exactly fits into the macrocycle core to such an extent that in the absence of any external perturbations, there is no deviation from the planar structure. On the other hand, in NiPc due to the small size of the Ni ion, four isoindole rings are pulled in towards the Ni ion. This gives not only small core diameter but also has an effect on Ca –Nb –Ca bridge bonds. Owing to this, Pc macrocycle is significantly deformed in NiPc. Similarly, when pyridine attaches to the oop coordination site of the Cu ion, the Cu ion is forced out of the mean Pc plane resulting in the pulling of Ni atoms inside the core with concomitant contraction of the Pc macrocyle. This situation is quite similar to the NiPc. Hence, CuPc in pyridine should also show upward shift in the Ca –Nb –Ca related vibration by few wavenumbers. In fact, corresponding band shifts by 6 cm−1 from 1505 to 1511 cm−1 in the solution spectrum (Fig. 3b). Therefore, upshift in this band also supports the formation of pyridine coordinated CuPc complex in the solvent. The vibrational band corresponding to this mode in water soluble cationic copper 4,4 ,4 ,4 -tetrasulfanato phthalocyanine also splits into isotropic and anisotropic components at 1527 and 1538 cm−1 , respectively [28,29]. The higher wavenumber band arises due to aggregated species, whereas low wavenumber band originate from the monomeric phthalocyanine. Dimer or higher order aggregates considerably deform the macrocycle, which is indicated by the presence of higher wavenumber band at 1538 cm−1 band in their spectra. In the present study, we have used non-ionic non-substituted CuPc, which is soluble only in a few organic solvents. Hence, formation of the aggregated CuPc can be ruled out in the pyridine solution [29]. Therefore, observation a band at 1511 cm−1 in the IR spectra can be explained only, if we consider the out-of-plane displacement of Cu ion from the mean Pc plane due to coordination of pyridine. The IR spectra of CuPc in high frequency region are shown in Fig. 4. Fundamental C H stretching vibrations of isoindole rings, and overtone and combination modes dominate this region. In this region, both the spectra are almost similar in band positions expect small change of intensity of these bands. Intensity change can be explained based on the perturbation caused by the solvent molecule as discussed above. Positions of C H stretching bands indicate that the C H bonding arrangement in the solution remains almost identical to the solid. Weak bands
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in this region may arise due to overtone and combination of fundamental bands, hence these are not considered in the present study. 4. Conclusions From the above discussion, it is clear that crystal-packing effects are responsible for observation of some weak bands in the spectra of solid CuPc. Observed spectral changes in the IR and optical spectra of CuPc in the pyridine solvent indicate towards the coordination of solvent molecule with metal ion. Acknowledgements This work is financially supported by CSIR, India. Spectra were recorded at SAIF, Punjab University. References [1] T. Miyata, S. Kawaguchi, M. Ishii, T. Minami, Thin Solid Films 425 (2003) 255. [2] K.-Y. Law, Chem. Rev. 93 (1993) 449. [3] T.D. Anthopoulos, T.S. Shafai, Thin Solid Films 441 (2003) 207. [4] G. de la Torre, P. Vazquez, F. Agullo-Lopez, T. Torres, J. Mater. Chem. 8 (1998) 1671. [5] B. Bottger, U. Schindewolf, J.L. Avila, R. Rodrigues-Amaro, J. Electroanal. Chem. 432 (1997) 139. [6] B.D. Bezerin, Coordination Compounds of Porphyrins and Phthalocyanines, Wiley, New York, 1981. [7] J. Simon, C. Sirlin, Pure Appl. Chem. 61 (1989) 1625. [8] J.E. Kudler, J. Imag. Sci. 32 (1988) 51. [9] K. Kato, Y. Nishioka, K. Kaifu, K. Kawamura, S. Ohno, Appl. Chem. Lett. 86 (1985) 196. [10] T.A. Tomofonte, K.F. Schoch, J. Appl. Phys. 65 (1989) 1350. [11] J. Spadavecchia, G. Ciccarella, G. Vasapollo, P. Siciliano, R. Rella, Sens. Actuators B 100 (2004) 135. [12] M. Safarikova, I. Safarik, Eur. Cells Mater. 3 (2002) 188. [13] D. Li, Z. Peng, L. Deng, Y. Shen, Y. Zhou, Vib. Spectrosc. 39 (2005) 191. [14] D.R. Tackley, G. Dent, W.E. Smith, Phys. Chem. Chem. Phys. 2 (2000) 3949. [15] M. Wojdyla, B. Derkowska, M. Rebarz, A. Bratkowski, W. Bala, J. Opt. A: Pure Appl. Opt. 7 (2005) 463. [16] M.M. El-Nahass, F.S. Bahabri, R. Al-Harbi, Egypt. J. Sol. 24 (2001) 11. [17] N.-S. Cho, K.-H. Kim, C.-H. Han, J. Korean Chem. Soc. 16 (1971) 378. [18] W. Su, M. Bao, J. Jiang, Vib. Spectrosc. 39 (2005) 186. [19] X. Sun, M. Bao, N. Pan, X. Cui, D.P. Arnold, J. Jiang, Aust. J. Chem. 55 (2002) 587. [20] M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.A. Darwish, Mater. Chem. Phys. 92 (2005) 185. [21] F. Lu, L. Zhang, H. Liu, X. Yan, Vib. Spectrosc. 39 (2005) 139. [22] R. Aroca, E. Johnson, Langmuir 8 (1992) 3137. [23] R. Aroca, A. Thedchanamoorthy, Chem. Mater. 7 (1995) 69. [24] A.J. Bovill, A.A. McConnell, J.A. Nimmo, W.E. Smith, J. Phys. Chem. 90 (1986) 569. [25] G. Maggioni, A. Quaranta, S. Carturan, A. Patelli, M. Tonezzer, R. Ceccato, G.D. Mea, Chem. Mater. 17 (2005) 1895. [26] G. Dent, F. Farrell, Spectrochim. Acta A 53 (1997) 21. [27] D.R. Tackley, G. Dent, W.E. Smith, Phys. Chem. Chem. Phys. 3 (2001) 1419. [28] H. Abramczyk, I. Szymczyk, J. Mol. Liq. 110 (2004) 51. [29] I. Szymczyk, H. Abramczyk, Pure Appl. Chem. 76 (2004) 183.