Optical and infrared spectroscopic studies of chemical sensing by copper phthalocyanine thin films

Materials Chemistry and Physics 112 (2008) 793–797

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Optical and infrared spectroscopic studies of chemical sensing by copper phthalocyanine thin films Sukhwinder Singh, S.K. Tripathi, G.S.S. Saini ∗ Department of Physics, Panjab University, Chandigarh 160014, India

a r t i c l e

i n f o

Article history: Received 25 August 2007 Received in revised form 5 June 2008 Accepted 21 June 2008 Keywords: Copper phthalocyanine thin film Chemical sensing Infrared spectra XRD spectrum Optical absorption

a b s t r a c t Thin films of copper phthalocyanine have been deposited on KBr and glass substrates by thermal evaporation method and characterized by the X-ray diffraction and optical absorption techniques. The observed X-ray pattern suggests the presence of ␣ crystalline phase of copper phthalocyanine in the as-deposited thin films. Infrared spectra of thin films on the KBr pallet before and after exposure to the vapours of ammonia and methanol have been recorded in the wavenumber region of 400–1650 cm−1 . The observed infrared bands also confirm the ␣ crystalline phase. On exposure, change in the intensity of some bands is observed. A new band at 1385 cm−1 , forbidden under ideal D4h point group symmetry, is also observed in the spectra of exposed thin films. These changes in the spectra are interpreted in terms of the lowering of molecular symmetry from D4h to C4v . Axial ligation of the vapour molecules on fifth coordination site of the metal ion is responsible for lowering of the molecular symmetry. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The phthalocyanine (Pc) belongs to a class of chemically and thermally stable organic semiconductors. These materials are being studied due to their several applications as photosensitizers [1], chemical sensors [2–6], photoconducting agents [7], non-linear optics [8], photovoltaic cell elements [9], liquid crystals [10] and optical data storage materials [11]. Adsorption of oxidizing or reducing gases on the surface of Pc thin films induces change in the electrical conductivity of Pc films [6]. Hence, sensing properties of Pcs have been extensively studied by the electrical conductivity measurements. Detection of NO2 gas down to 25 ppb concentration with a lead phthalocyanine film has been attained [12]. Very recently these compounds have been successfully tested for the detection of volatile organic compounds and sedative drugs by optical techniques also [13,14]. In literature, there are few reports of Raman and infrared (IR) spectral studies of Pc thin films sensing [2–5,15–17]. However, vibrational spectroscopic techniques are not frequently used for the study of sensing properties of Pcs, though these techniques provide valuable data that can be used in the investigation of interaction between chemicals and Pc on molecular level. The vibrational frequency positions are sensitive to the sub-Å changes in bond length. Intensities of vibrational bands

provide information about the orientational changes due to distortions in molecules. Moreover, IR absorption spectroscopy has also been used to identify the crystalline nature for both powder and thin films of Pc [18,19]. Despite being an important tool to study the material at molecular level, one probable reason for the lesser use of vibrational spectroscopy in the study of sensing properties of Pc thin films is that a large number of bands due to different fundamental, overtone and combination vibrations are, generally, present in spectra. This makes deciphering information from spectra really difficult. However, recently reported density functional theory (DFT)-based normal coordinate analysis of copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) [20,21] have made it possible to assign the observed IR bands accurately and gain insight into the bonding arrangements. Chemical vapours induced changes in the structure of Pc molecules, therefore, can be monitored with the help of IR and Raman spectroscopy. These changes depend on the nature of interacting chemical vapours/gases. In the present work, we analyze the IR spectra of CuPc thin films, before and after exposure to the vapours of ammonia and methanol to investigate the effect of vapours on CuPc molecule by monitoring the changes in the spectra. Thin films are also characterized by X-ray diffraction (XRD) and optical absorption techniques. 2. Experimental

∗ Corresponding author. Tel.: +91 172 2534454; fax: +91 172 2783336. E-mail addresses: [email protected] (S. Singh), [email protected] (S.K. Tripathi), [email protected] (G.S.S. Saini). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.06.044

In the present study, CuPc powder (sublimed grade, dye content 99%) from Aldrich, is used without further purification. Methanol and ammonia from Qualigens Fine Chemicals, India, are used without further purification. Thin films of CuPc have been deposited on KBr and Corning 7059 glass substrates by vacuum

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evaporation technique by keeping substrate at room temperature and base pressure ≈ 2 × 10−5 mbar, using a molybdenum boat. The films have been kept in the deposition chamber in dark for 24 h to attain thermodynamic equilibrium [22]. The thickness of the film, used in the present study, is found ≈ 750 nm with a Surface Profiler Decktak 3030 ST. The IR spectra of the film deposited on the KBr substrate are recorded with a Perkin Elmer PE-Rx1 FTIR Spectrophotometer having spectral resolution of 1 cm−1 . The UV–vis spectra were recorded on a HITACHI 330 UV-VIS-NIR Spectrophotometer having resolution of 0.07 nm. The crystalline nature of the thin films is characterized by using X-ray diffraction technique (Model: Philips PW 1610, Goniometer: Philips 1710, geometry configuration: – , detector: Cu K ␣). Exposure of the thin films with each analyte vapour was done for 20 min in a desiccator under vacuum.

3. Results and discussion The molecular structure of CuPc has a square-planar configuration with metal ion in its centre as shown in Fig. 1. The figure also shows the atom labeling scheme of the molecule. The molecule has an overall symmetry of D4h , which can be inferred from the figure. Moreover, recent DFT calculations also resulted in the idealized D4h point group symmetry for this molecule [20]. In present study, the CuPc thin films are characterized by optical, IR absorption and XRD spectroscopic techniques, as discussed below. 3.1. Optical absorption characterization We observe the B (Soret) band at 365 nm in UV region of absorption spectrum of the as-deposited thin film of CuPc (Fig. 2a). The other well-known bands of the Pc molecule, namely the Qbands appear at 609 (broad), 673 (shoulder) and 720 nm in the visible region. On exposure of the thin films to the methanol vapours, absorption bands do not show any appreciable shift in their positions, except the 720 nm band, which shifts down to 715 nm (Fig. 2c). The intensity of the lowest energy band increases, whereas the intensity of the other bands in the Q region and the Soret band decreases. Observed band pattern correlate well with the observed spectra of thin films of ␣-crystalline CuPc reported in the literature [23,24]. Sharp and Abkowitz [25], however, reported lower energy Q band for ␣-CuPc thin films at 694 nm. The origin of electronic absorption bands of the CuPc can be explained on the basis of ␲– ␲∗ excitation from HOMO, a1u and a2u , to LUMO, 2eg . Although metal dx2−y2 orbital of b1g symmetry is half filled and lies in between the HOMO, a1u , and LUMO orbitals of the Pc ring [26], yet the dx2−y2 orbital does not take part in electronic transitions since transition from dx2−y2 to LUMO is forbidden. However, metal d-orbital can take part in non-radiative processes such as electrical conductivity.

Fig. 2. Optical absorption spectrum of the CuPc thin film in 350–800 nm region: (a) as-deposited; (b) exposed to ammonia; (c) exposed to methanol vapours.

The broadening of the absorption bands is caused by the aggregation of the molecules [27]. Observation of more than one band in Q spectral region is due to the Davydov splitting [28]. The amount of Davydov splitting and, therefore, positions of the bands in Q region depend on the interactions between the transition dipole moments and relative orientations from adjacent molecules [29]. On exposure, changes in the intensity of the bands without much change in their positions suggest no appreciable change in the strength of interaction between the dipole moments of adjacent molecules. However, number of such interacting dipole moments decreases, which can be understood on the basis of adsorption of vapours at the film surface and subsequent formation of five coordinated molecular species. Similar changes in the optical absorption but a to lesser extent are observed with the ammonia vapours (Fig. 2b). 3.2. XRD study Fig. 3 shows the XRD patten of 750 nm thick as-deposited CuPc film on a glass substrate at room temperature. All the observed values of diffraction angles (2) and interplanar distances (d) are given in Table 1. In the XRD spectrum, peaks at d = 12.8563, 3.4836, 3.3783, 3.3154, 3.2122, 3.1445, 3.0864, 3.0125 and 2. 8515 Å arise due to ¯ (3 1 2), (8 0 0), (3 1 3), (5 1 0) and (5 1 3) planes (2 0 0), (3 1 0), (1 1 3), respectively of ␣-crystallites belonging to the monoclinic symmetry [30]. Our observed d values and calculated planes match well with previously reported values in the literature [30–33]. Observed XRD pattern, therefore, suggests the presence of ␣-crystallites in the as-deposited CuPc thin film. Further evidence of the presence Table 1 Observed diffraction angles, interplanar spacings and Miller indices of the asdeposited ␣-crystalline CuPc thin film

Fig. 1. Structure and atom labeling scheme of copper phthalocyanine.

2 (◦ )

dh k l (Å) observed

hkl

06.870 25.550 26.360 26.870 27.750 28.360 28.905 29.630 31.345 32.490

12.8563 3.4836 3.3783 3.3154 3.2122 3.1445 3.0864 3.0125 2.8515 2.7536

200 310 1 1 3¯ 312 800 313 510 512 513 516

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Fig. 4. IR spectra of the CuPc thin films: (a) as-deposited; (b) exposed to ammonia; (c) exposed to methanol in 400–1650 cm−1 region.

Fig. 3. XRD spectrum of the as-deposited CuPc thin film.

of ␣ crystalline CuPc in thin film is provided by the IR absorption study as discussed in the next subsection. The presence of large number of peaks in the observed XRD spectrum is also supported by the fact that more than one diffraction peak, corresponding to the lower d values, are generally seen in the XRD spectrum of relatively thicker films [34]. It may be noted here that, the thin films used in the present studies are ≈ 750 nm thick. The observation of large number of peaks, corresponding to the different d values, also indicates that the thin films consist of randomly oriented ␣-crystallites, as there is no preferential orientation of the crystals. However, the peaks corresponding to d = 3.3154 and 3.1445 Å are much more intense than the other peaks. This suggests that majority of the crystals in the films are predominantly oriented in such a way that none of the crystal axis a, b or c lies in the plane of substrate. 3.3. IR spectroscopic study The IR spectra of CuPc thin film in 400–1650 cm−1 region under different experimental conditions are shown in Fig. 4. As can be seen from traces in the above figure, we observe a large number of IR bands in the spectra. In order to simplify the assignment of observed IR bands, we assume that the film consists of CuPc molecules since CuPc crystals are molecular solids with two molecules of CuPc per unit cell. 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 of the A2u symmetry and in-plane bands of the Eu symmetry. A large number of weak/very weak bands including symmetry forbidden, which becomes allowed due to distortions in the molecule can also be seen in the spectra. We have utilized a simpler, qualitative approach to interpret the IR data of CuPc thin film on the basis of recently reported normal coordinate calculations [20,21]. These calculations

along with the reported resonance Raman and IR studies of these complexes [18,19,35–38] 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 [20,21]. 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 2. The IR bands in 700–800 cm−1 region of the spectra of Pcs are often used to identify different polymorphs, ␣, ␤, ␧, etc., because of their sensitivity to the crystal packing arrangements in thin films [39–41]. In the spectrum of as-deposited CuPc thin film before exposure (Fig. 4a), we observe bands at 722, 754 and 770 cm−1 with a shoulder at 780 cm−1 . The 722 cm−1 and 770 cm−1 bands can be assigned to out-of-plane wagging motion of Cc –H bonds. The former band also has contribution from Ca –N out-of-plane vibrations [20]. Band at 754 cm−1 has been assigned to heaving motion of Ni –Ni atoms and Ca –Ni –Ca in-plane bending [20]. The observed wavenumbers of these IR bands are in complete agreement with the reported positions of the bands of ␣-crystallites. Therefore, it is clear that ␣ crystalline form is present in the thin film. The presence of ␣ crystallite in the thin film is also supported by the appearance of unresolved bands at 864 and 870 cm−1 , and at 942 and 950 cm−1 . Thus the observed IR band positions are in tandem with the XRD data. In the IR spectra of CuPc thin film exposed to different chemical vapours (Fig. 4b and c), we observe C–H bending modes at 722, 754 and 769 cm−1 with a shoulder at 779 cm−1 . These band positions match well with the corresponding bands in the IR spectrum of as-deposited CuPc thin film of ␣-phase. Hence, it is clear from the observed band positions that crystalline form of the thin film does not change on exposure to vapours. Therefore, it can be inferred from this observation that bond orders are nearly identical in the ␣-CuPc films before and after exposure to the chemical vapours. However, in the presence of vapors, relative intensity of almost all the in-plane bending and stretching modes, e.g. at 1091, 1121, 1167, 1288, 1334, 1508 and 1612 cm−1 is decreased with respect to the out-of-plane bending vibrations at 722 cm−1 . It seems that the symmetry of CuPc molecules in the exposed thin film is changed from D4h . Since the vapours used in the present study are well known coordinating chemicals, therefore, it is possible that vapour molecule attaches to the fifth coordination site of the Cu ion. In fact, there are evidences of coordination of ammonia molecules with the central copper ion of the phthalocyanine thin films in the literature

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Table 2 Position and assignment of observed IR bands of CuPc thin film before and after exposure Band a (cm−1 ) Thin film on KBr

506 573 722 754 770 780 802 864 870 (sh) 900 942 950 (sh) 1002 1069 1091 1121 1167 1191 1262 1288 1334 1342 (sh) 1422 1459 (sh w) 1466 1479 1500 (sh w) 1508 1543 1560 (w) 1590 1612

Mode no. and assignment b, c Thin film exposed to NH3

CH3 OH

474 506 541 (vw) 572 722 754 769 779 802 865 871 (sh) 900 942 949 (sh)

476 507 541 (vw) 573 722 754 769 779 802 864 872 (sh) 900 942 (vw) 949 (sh)

1069 1091 1120 1167 1191 (vw)

1070 1091 1120 1167

1286 1333 1342 (sh) 1385 1422 1459 (sh vw) 1463

1288 1331 1342 (sh) 1385 1422 1459 (w) 1466 1480 (vw) 1500 (vw) 1508 1545 1560 (w)

1507 1540 (vw) 1558 (vw) 1613

Eg (E); [w (Cb –Cb )s , w (Cc –Cc )s ]as + w (Ca –Nb ) Eu (E);  (c ring) +  (Ca –Ni –Ca ) + (Nb –Nb ) B1g (B1 ); (b ring) + (Cu–Ni ) Eu (E); (b ring) + (Cc –Cc ) +  (Ca –Ni –Ca ) A2u (A2 ); w (Ca –Ni )s + w (Cc –H)s Eu (E); (Ni –Ni ) + ı(Ca –Ni –Ca )as +  (Cc –Cc )as Eu (E); (Ni –Ni ) + ı(Ca –Ni –Ca )as +  (Cc –Cc )as A2u (A2 ); w (Cc –H)s Eu (E); (Ni –Ni ) + [(b ring,c ring)]as +  (Cc –Cc )as Eg (E); w (Cc –H)as Eu (E); (Ni –Ni ) + [(b ring,c ring)]as +  (Cc –Cc )as A2u (A2 ); w (Cc –H)s B2g (B2 ); ıs (Ni –Cu–Ni ) Eu (E); ı(Cc –H) + (Ni –Ni ) Eu (E); [(Ni –Ni ) + (Cc –Cc )]as Eu (E); ı(Cc –H) + (Ni –Ni ) Eu (E); ı(Cc –H) B1g (B1 ); ı(Cc –H)as Eu (E); (Ni –Ni ) +  (Cc –H)as A2g (A2 );  (Cb –Cb ) Eu (E); ı(Cb –Cb ) +  (Cc –H)as Eu (E); (Cb –Cb , Cc –Cc )as + (Ni –Ni ) + ı(Ca –Ni –Ca )as B1g (B1 ); s (Cb –Cb , Cc –Cc ) A1g (A1 ); (Cu–Ni )s + ı(Ca –Nb –Ca ) Eu (E); [s (Cb –Cb ), s (Cc –Cc )]as + ı(Cc –H) B2g (B2 ); (Ca –Nb )as + ı (Ca –Ni –Ca ) + ı(Ni –Cu–Ni ) Eu (E);  (Cb –Cb , Cc –Cc )as Eu (E); ı(Ca –Ni –Ca ) +  (Cb –Cb , Cc –Cc )as Eu (E); ı (Ca –Ni –Ca )as +  (Cb –Cb , Cc –Cc )as Eu (E); (Ca –Nb )as + ı(Ca –Nb –Ca )as B1g (B1 ); (Ca –Nb )as + ı(Ca –Ni –Ca )as B1g (B1 ); (Cb –Cb , Cc –Cc )as Eu (E); (Cb –Cb , Cc –Cc )as Eu (E); ı (Cb –Cb , Cc –Cc )as

a

sh, w and vw denote shoulder to a band, weak band and very weak band respectively. b Assignments are based on references [20,21]. c The , ı,  and  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.

[15,42]. 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 the doming of Pc macrocycle with decrease in overall molecular symmetry from D4h to C4v . On doming of the macrocycle, some bands may gain intensity due to relaxation of the selection rules for C4v point group. For example, bands of the A2g species in D4h become IR active as they correspond to A1 species in C4v . On the other hand, some bands may loose their intensity also as observed by us. One interesting observation is that on exposure the position of most of the bands do not show any significant shift except a small and negligible shift only in few bands. It is also possible that chemical vapours are not able to diffuse through the entire thickness of the film. However, intensity of the band observed at 1385 cm−1 (discussed below) only in the spectra of exposed films suggests that diffusion of the vapours in the films below the exposed surface cannot be completely ruled out. In fact, Wang and Lando [43] have also explained the two stages of conductivity change in the Pc film in terms of initial adsorption of gas molecules on the film surface and subsequent diffusion into the film. Adsorption of vapours at the surface and subsequent diffusion into the thickness up to certain depth may give rise to five coordinated CuPc at the surface and just below it and four coordinated CuPc at the interface between film and substrate or inside the film near this interface. Hence, exposed thin film may simultaneously contains five and

four coordinated species of CuPc belonging to the C4v and D4h point groups, respectively. The coexistence of two types of species in the exposed thin film leads to more number of IR bands, which are allowed according to the selection rules for these point groups. Moreover, due to small/negligible changes in the bonding arrangement in two species of the exposed films, the wavenumbers of a given band in these species may also not differ much from each other. A change in the conformation is likely to affect the intensity of bands both due to out-of-plane and in-plane modes in the low wavenumber as well as high wavenumber regions. Change in the intensity of some of the low wavenumber bands, which arise due to out-of-plane torsion/bending motion of the molecule, hence indicates towards the structural distortion of molecules in the exposed thin film. In low wavenumber region, intensity of the 900 cm−1 band which is assigned to the heaving motion of Ni –Ni and asymmetric isoindole ring vibration [20], is also reduced in the spectra of exposed film. This observation also suggests the conformational change in the CuPc molecule in presence of the vapours. Similarly, in the high frequency region almost all the bands, e.g. at 1091, 1120, 1167, 1286/1288, 1422, 1507/1508 cm−1 , etc. show reduced intensity on exposure of the thin films to the vapours due to reduction in the molecular symmetry. Similar observations of intensity change in the surface enhanced Raman spectra (SERS) of NO2 exposed Langmuir–Blodgett films of cerium bisphthalocyanine (CePc2 ) are reported by Aroca et al. [2]. These authors have

S. Singh et al. / Materials Chemistry and Physics 112 (2008) 793–797

also not observed any detectable shift in the SERS bands of exposed CePc2 films. Apart from the change of intensity of a number of bands, the most visible change in the IR spectra of exposed thin films is the appearance of a band at 1385 cm−1 , which is not present in the spectrum of as-deposited film. In the spectra of exposed films, it is present with large intensity. In fact its intensity is more than the intensity of the band at 1334 cm−1 , which is one of the most intense band in the spectrum of as-deposited thin film. This band arises due to symmetric Cu–Ni stretching vibration with a contribution from in-plane Ca –Nb –Ca bending motion. 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 vapours. Therefore, presence of this band also indicates the coordination of the vapours to the Cu ion at out of the Pc plane. The non-presence of other bands of this symmetry species also suggests that the distortion in the CuPc molecules of the exposed film is limited to the core or inner part of the ring, since vibrations of this symmetry arising from the motion of the outer part of the ring are either not present or very weak in the spectra. Moreover, a band of E symmetry at 1335 cm−1 , which also has contribution from asymmetric deformation from the Ca –Ni –Ca bonds, is observed with reduced intensity in the spectra of exposed thin films. This observation further lends support to the lowering of molecular symmetry of CuPc molecules of the exposed film. It is well known that CuPc is a p-type organic semiconductor. Methanol and ammonia molecules have lone pair of electrons, which can be transferred to other electron accepting molecules such as CuPc. When methanol or ammonia is attached to the fifth coordination site of Cu ion, some electron density from the coordinating molecule is transferred to the empty d-orbital of the metal. The increased electron density reduces the number of holes in the sample by occupying the vacant sites with concomitant decrease in the electrical conductivity of the material as reported by us elsewhere [44]. This further justifies our point that these chemical vapors attach at the out-of-plane axial coordination site of Cu ion. However, Cu–Ni bond length remains unaffected. Therefore, on one hand, a decrease in the conductivity of the CuPc thin film is observed on exposure [44], on the other hand due to negligible change in the bond orders and bond lengths, IR bands do not show much shift in their positions but show quite appreciable change in their intensities. Apart from these common features in the spectra of exposed film, some differences are also present, e.g. the relative intensity of 1333 and 1385 cm−1 bands is quite different. Hence, relative intensity of these bands can be used to differentiate the effect of different vapours. 4. Conclusions From the above discussion, it is clear that the intensity of some IR bands of CuPc thin film changes on exposure to chemical vapours. Relative intensity change and observation of new bands in the spectra of exposed films may be explained on the basis of axial ligation of vapours at the fifth coordination site of the metal ion of the CuPc thin film.

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