Spectroscopic and electrochemical properties of thin solid films of yttrium bisphthalocyanine

05~8539193 $6.00+0.00 0 1993 Pqamon Press Ltd

Vol. 49A. No. 7. pp. 965-973. 1993 Printed in Great Britain

Spectrochimico Ac~a.

Spectroscopic and electrochemical properties of thin solid fdms of yttrium bisphthalocyanine M. L. RODRIGUEZ-MENDEZand R. AROCA* Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, N9B 3P4 Canada

and J. Department

A. DESAJA

of Condensed Matters Physics, University of Valladolid, 47011 Valladolid, Spain (Received 22 May 1992; in final form and accepted 19 September 1992)

Abstract-Langmuir-Blodgett (LB) and evaporated thin solid films of the yttrium bisphthalocyanine complex (YP%) have been prepared on various substrates. Cyclic voltammograms of films are discussed and the electrochromic effect on LB films is reported. A detailed spectroscopic characterization of the YPQ material is given using resonance Raman scattering (RRS), surface-enhanced resonance Raman scattering (SERRS), transmission and reflection absorption FZ-IR spectroscopy and UV-vis spectra. The spectroscopic characterization of the chemical and electrochemical oxidations products of YPc2 films and solutions was carried out by in situ UV-vis spectroscopy. Potential applications are discussed.

BISPHTHALOCYANINE dyes are of interest due to their electrochromic properties and potential applications in thin film devices [l, 21. Yttrium (III) bisphthalocyanine is an important model compound for the understanding of the electrochromic properties of bisphthalocyanine complexes. The preparation of YPc, was initially reported by MOSKALEV and KIRIN [3]. The authors proposed a YHPcz formula where PC denotes the [C,,H,&#ligand. The reported electronic absorption spectrum of the YPc2 solution presented a Q-band maximum at 623 nm which is characteristic of the blue YPc, material. The corresponding IR spectrum of blue form in nujol was recorded in the 700-1500 cm-’ spectral region, with three intense bands at 728,1110 and 1321 cm-‘. The Raman spectra and the vibrational interpretation for YPc, frequencies have not yet been reported. Two oxidation potentials for tetra(n-butyl)ammonium YPQ: 157 and 598 mV, against the Ag/AgCl reference electrode were reported by KONAMI et al. [4]. Notably, the difference between the first and the second oxidation potential is 441 mV, a difference value that seems to be characteristic in all substituted bisphthalocyanine of the lanthanide series [3-61. KASUGAet al. [7] studied the oxidation of LnPq, NdPc, and YPc, withp-benzoquinone and determined that the oxidation reaction of YPc, had the highest rate constant. The present work attempts a spectroscopic characterization of the YPc, and products of oxidation-reduction. It is the first report on the fabrication of LB films of the YPc, material. The electrochromic behaviour of LB layers deposited onto conducting glass was studied and colour changes in LB films due to gas adsorption are reported.

EXPERIMENTAL

The sample of YPc2 was kindly provided by Professor A. Tzivadze. Elemental analysis of the sample confirmed that the original material, green in colour, was a bisphthalocyanine complex. The presence of the H (as in YHPcJ cannot be confirmed or rejected on the basis of elemental analysis. Smooth Ag and Au island films were prepared using a Balkers evaporating system, and * Author to whom correspondence

should be addressed. %5

966

M. L. RODRIGUEZ-MENDEZ etal.

the thickness was monitored by an XTC Inficon quartz crystal oscillator. The 110 nm Ag films were made by evaporation of silver at a rate of 5 A/s onto a glass substrate reviously kept at 200°C. The 4 nm Au island films were prepared by evaporation of gold at 0.5 8: Is onto glass kept at 200°C. Evaporated films of YPcZ (150 nm thick) were prepared in a separate identical evaporator to avoid contamination of the metals. The evaporation was done at 5 A/s onto a different substrate as Coming glass, IT0 glass and a KBr disk. LB monolayers were prepared in a Lauda Langmuir film balance equipped with an electronically controlled dipping device, Lauda Filmlift FL-l. A chloroform solution 3.55 X lo-’ M of YPc2 was prepared by stirring the solution in an ultrasonic bath for 3 h. The trough was filled with deionized water (Milli Q) and the solution was then spread onto the subphase. The temperature of the subphase was 15°C for both the pressure-area (isotherms) studies and the monolayer transfer. For isotherm studies, the monolayers were compressed at 0.3 x l@ nme2 mol-’ s-‘. The monolayers were transferred to Coming 7059 glass slides previously treated for 4 h with hot chromosulphuric acid at a constant speed of 2.5 mm/min and a pressure of 25 mN/m. The same conditions were used to transfer the molecules to other substrates as Ca.F* polished disks (Wildmad Glass Co. Inc.), metal coated glass slides and IT0 coated glass used in electrochemical experiments. Cast films were prepared by covering the surface of the IT0 electrode with a concentrated solution of YPc. The solvent evaporated within a few minutes. Cyclic voltammetry (CV) was recorded in the conventional three electrode cell and using an EG&G Princeton Applied Research Model 173 Potentiostat Galvanostat interfaced to an IBM-PC microcomputer. A platinum wire and Ag/AgCI electrode were used as the counter and reference electrodes. The electrolyte used in the thin solid films experiments was a 0.1 M solution of KCI04. The electrolyte was degassed with high purity argon prior to any electrochemical measurement. A two window cuvette was used to obtain the in situ electronic absorption spectra. The EPR spectra

were recorded in a Varian E-12 instrument for toluene and chloroform solutions of YPc2 as well as for a solid sample. Electronic absorption spectra were recorded on a Response UV-vis spectrophotometer interfaced to an IBM-PC computer. Spectra-Physics model 2020Kr+ ion laser was used to obtain Raman spectra. Raman shifts were measured with a Spex-1403 double spectrometer. Infrared spectra were measured on a BOMEM DA3 FT-IR spectrometer. For data analysis, all files were imported to Spectra Calc?’ software available from Galactic Industries Corp.

RESULTS AND DISCUSSION

Characterization of the starting material The elemental analysis confirmed a diphthalocyanine (PcZ) complex.

the fact that the starting

material

of green colour was

EPR spectra at room temperature showed a single signal with a g factor of 2.002. This value is close to the free electron value. The slope-toslope linewidth was about 3G. However, no signal splitting was observed by running the spectrum at liquid nitrogen temperature or after adding pyridine to the solution. The results are consistent with the existence of at least one unpaired electron in the structure. It would be reasonable to postulate a stable free radical with YPc, molecular formula as has been established for LuPcl [8,9]. The electronic spectrum of the starting green material (YPc,) in chloroform is shown in Fig. 1. The absorption spectrum of an LB film obtained with this material is also given in Fig. 1. The spectrum is in full agreement with reported spectra for lanthanide complexes such as TmPcz [9]. The Q band in solution was observed at 666 nm and the Soret band has a maximum at 320 nm. Weak bands were also seen at 600, 582 and 458 nm. The molar extinction coefficient calculated for YPq solutions at different concentrations was 1 X 10s M-l cm-’ at 666 nm. UV-vis spectra of evaporated and LB films of the green material showed Q bands at 672 and 674 nm, respectively. The Soret bands were observed at 322 nm for both films. The green colour of the starting material in chloroform solution changed to blue by adding hydrazine. The spectrum of the blue product has a maximum of the Q band at 623 nm, as reported by MOSKALEV and KIRIN [3], with a second absorption peak at 680 nm. The addition of a dilute solution of HNOs to the green YPc, solution produced a spectrum characteristic of the oxidized form (red in colour). The oxidation seems to be completed by adding concentrated HNOs as illustrated in Fig. 2. The oxidation of YPc, increases the intensity

%?

Thin solid films of yttrium bisphthalocyanine 666

YPc, LB film

YPc,

400

(6 layem)

Solution

600

800

WAVELENGTH (MI) Fig. 1. Electronic absorption spectrum of the starting green material of YPc, in CHC& solution (lower trace) and of an LB tilm on glass (upper trace).

of the 470 nm band. The vibrational characterization given here, the LB fabrication, and the electrochemical work were carried out on samples of the green form of YPe. Langmuir- Blodgett jilms The surface pressure vs area isotherm for the floating monolayer on water subphase obtained at 15°C is shown in Fig. 3. The limiting area per molecule obtained by extrapolation of the slope of the low compressibility region to zero pressure was 0.76 nm’. Using the approximate molecular geometry obtained by energy minimixation calculations (PC model program), the minimum surface area for packing of YPQ molecules in an edge-on organization would be ca 0.5 nm*. Similarly, the face-on organization would occupy an area per molecule of about 1.44 nm2. The isotherm indicates that the YPq molecules are tilted and assembled with the main molecular axis

4io

060

WAVELENGTH (nm)

0iO

/ Fig. 2. Electronic absorption spectrum of the YPo, green material in dimethylformamide (a), after treatment with diluted HNO, (b) and concentrated HN03 (c).

968

M. L.

RODRIGUEZ-MENDEZ et al.

y._.

,

I

120

100

80

Area/Molecule

(nm’.

102)

Fig. 3. Surface pressure-area isotherm for YPc2 on water subphase at 15°C and molecular model of YP+

parallel to the water surface. A single monolayer was transferred to a 4 nm thick Au island film for SERRS experiments. Monolayers of the neat green material were also transferable to glass slides and CaFz substrate to form multilayers using a Z deposition geometry. However, to fabricate LB films on Ag coated glass and IT0 glass it was necessary to prepare mixed layers of YPc2 with arachidic acid in a 1: 3 ratio. Electrochemistry

Cyclic voltammetry of YPc_, (green material) in non-aqueous solution was performed using 0.1 M tetrabuthylammonium hexalluorophosphate (from Aldrich) in dimethylformamide (DMF) (Aldrich HPLC grade). Pt wire was used as counter and working electrode. Ag/AgCl non-aqueous reference electrode filled with 0.1 M AgNOj in DMF was used. The YPc, solution in DMF has a blue colour which corresponds to the first reduction of the green material, Scanning at 100 mV/s, the blue-green-red cycle in YPc, Cyclic Voltammetry 0.06

Potential

(mV)

Fig. 4. Cyclic voltammogram of YPcz LB film on IT0 glass. Scan rate 100 mV/s.

Thin solid films of yttrium bisphthalocyanine

YPc,RRS

(KBr Pellet)

YPc,FTIR

(Evaporated

ldO0

5bO

Wavenumber

969

Film) 1500

(cm-

1)

Fig. 5. Resonant Raman scattering and transmission FT-IR of YPI+ evaporated film.

solution was recorded between Ellz values of 0.14 and - 0.21 V. The separation between the two peaks (340mV) comparable with the 400mV separation value reported by KONAMI et al. [4]. The differences could be attributed to the dissimilar selection of solvent. For cyclic voltammetry of films, the working electrode was an IT0 glass coated with a cast film, an evaporated film or an LB film. Pt wire was used as counter electrode, and the reference electrode was Ag/AgCl. 0.1 M KC104 in deionized water was used as the electrolyte. CV experiments were carried out after bubbling argon for 1 h and keeping the system under argon atmosphere.

SERRS of YPc, on 40;

Au

500 Wavenumber

1000 (cm-l)

Fig. 6. SERRS of a single LB monolayer of YPc2 on Au island film. Polarized RRS spectra are shown in the inset.

M. L. RODRIGUEZ-MENDEZet al.

970

Table 1. Observed vibrational fundamentals of YPc, IR film

IR pellet

(6) (6)

499 563

(8) (6)

627 (3) 678 (4) 727 (100) 740 (20)

627 678 727 740

(5) (5) (60) (40)

780 812 884

(6) (6) (8)

779 811 883

(10) (6) (16)

1062

(8)

1061

(14

499 563

1114

1282

(40)

1114

RRS (SS)

SERRS

159 235 282 352 477

(5) (4) (7) (8) (21)

157 239 286 353 478

(13) (4) (8) (12) (26)

576

(23)

577

(20)

Benz. radial

678 (100)

680 (100)

740 769 782 816

(59) (14) (20) (26)

742 775 783 817

(45) (9) (13) (21)

PC breathing C-H wag PC ring

938

(8)

940

(5)

1102

(13)

1104

(6)

1140 1218

(15) (17)

1142 1217

(8) (6)

1298 1301 1330

(15) (21) (14)

1302 1333

(4) (10)

1421

(28)

1422

(6)

1501 1519

(25) (34)

1524

(10)

(4)

(64) (8)

1321 (100) 1368 (12)

1449

(20)

1486 1503

(5) (8)

1448 1485 1502

(37) (7) (7)

1595

(3)

3046 3067

(4) (4)

1595 1605 3047 3076

(7) (7) (5) (5)

Metal-N st.

C-H bend

(54)

1321 1369

Interpretation

C-H bend Pyrrole st. C-H bend C-H bend C-H bend C-H bend Pyrrole st. Isoindole st. Isoindole st. Isoindole st. Pyrrole st. Aza group st . Benzene st. Benzene st. C-H stretch C-H stretch

The cyclic voltammograms for films showed one oxidation and one reduction peak of the green material as illustrated in Fig. 4. The results obtained, scanning at 100 mV/s, for the formal potential of the cast film were: Er,* = -0.38 and 0.78 V. In evaporated films &= -0.34 and 0.52 V. For LB films E,,2=-0.45and 0.57 V. Important differences were observed between the LB films and the microcrystallized films. In cast and evaporated films the first cyclic voltammogram is different from the one recorded after repetitive scans. In LB films there were only minor differences between the first and subsequent sweeps. The two-dimensional organization in LB films seems to allow a better response to potential change. Consequently, a higher electrochemical stability was observed for LB films. A linear dependence, for the anodic and the cathodic peak current of the El,* = 0.78 wave, was observed for scan rates between 5 and 400 mV/s. A similar behaviour has been observed for LB films of substituted rare earth PC, [lo]. The electrochromic behaviour of YPcZ LB films was followed by in situ UV-vis spectroscopy. The application of a negative potential, below -0.38 V (see Fig. 4), produced a blue colour film and its electronic absorption spectrum showed a blue shifted Q band at 623 nm. Similar changes in the electronic spectrum were recorded for cast films and for evaporated films. The colour of the film was green for electric potentials lying between the reduction and the oxidation peak. At more positive potentials the films turned red. In summary, three distinct colours were observed: blue, green and red, within a potential range of 1 V.

Thin solid films of yttrium bisphthalocyanine

971

Vibrational spectra The staggered configuration of YP% belong to the point group symmetry Dw There are 333 vibrational modes. However, only 23 b2 and 42 e, frequencies would be IR active. Similarly, 21 a, totally symmetric vibrations as well as 42 e2 and 41 e3 degenerate vibrations could be observed in Raman spectrum. The transmission IR spectrum of the evaporated flhn and the resonant Raman spectrum are shown in Fig. 5. The simplicity of the FT-IR spectrum is advantageous for analytical applications and is very similar to the corresponding spectra of LuPq [ll], HoPcz and DyPc2 [12]. The RRS displays the characteristic spectrum of the PC chromophore. For a well dispersed KBr pellet of YPq, polarization measurements in the RRS spectra (SP/SS ratio) showed that the 678 cm-’ was polarized, while the 74Ocm-’ band was depolarized. Similarly, the 576 and 133Ocm-’ bands were found to be polarized. To illustrate the difference in polarized spectra a section of the SS and SP spectra are given as an inset in Fig. 6. The RRS and the SERRS excited with S-polarized light were identical and the SERRS can be seen in Fig. 6. Infrared and Raman frequencies are listed in Table 1. There are several vibrational frequencies that were observed in both the FT-IR and Raman spectra. Therefore, the molecular model could not be assigned to the highly symmetric structure Du (staggered) or the De (eclipsed). A local C,,, symmetry for each PC ligand could be used for the interpretation of observed frequencies [111.In this case the totally symmetric aI and the e vibrations are active in IR and Raman. The latter symmetry could also be realized for the complex when the metal-ring distances are different. Assignment of observed fundamentals based on characteristic group frequencies of MPc, complexes [ 11,121 is given in Table 1. Molecular organization in LB and evaporated jilms The assignment of the 727 cm-’ band in the FT-IR spectrum to the wagging vibration is particularly important for the discussion of molecular orientation. The C-H wag vibration was observed with strong relative intensity in the spectrum of YPc, in the KBr pellet (random microcrystal distribution). However, the C-H wag is the most intense band in the FT’-IR of the evaporated film (see Fig. 5), indicating that the axis of stacking of YPQ molecules would be parallel to the film surface (edge-on orientation). The transition dipole moment for the wagging vibration would also be parallel to the film’s surface. In order to probe the molecular orientation of an LB layer, the reflection-

YPC, -Tobacco

\

smoke I

\r.

460

600

WAVELENGTH

800

(nm)

Fig. 7. Electronic spectra of a VP% LB film (six monolayers), after NOX exposure, and its reaction to tobacco smoke.

912

M. L. RODRIGUEZ-MENDEZ et al.

absorption FT-IR spectrum (RAIRS) of an LB (six monolayers) on Ag was recorded. In the RAIR spectrum, the intensity of fundamental vibrations with transition dipole component perpendicular to the surface are enhanced. If the edge-on organization observed for the Langmuir layer were preserved during transfer, the C-H wagging should be very weak in the RAIR spectrum. In effect, the 727cm-’ frequency was observed with a very low relative intensity. However, “in-plane” vibrational frequencies at 1114 and 1321 cm-’ with a z component of the transition moment (z perpendicular to the surface) were seen with strong relative intensity in RAIRS. In particular, the 1448cm-’ band was favourably enhanced in the RAIR spectrum, and its relative intensity was equal to that of the 1321 cm-’ band. Additional support for the edge-on organization in the LB film was obtained from the FI’-IR transmission spectrum of 72 monolayers on CaF,. The observed spectrum (spectral region = 1100-3500 cm-‘) was similar to that of the evaporated film. A 4 cm-’ resolution and 3000 scans were used to obtain IR spectra of the evaporated film, LB on CaF2 and the RAIR spectra.

Potential applications Langmuir-Blodgett films of YPc2 could be of interest in terms of at least two potential applications: thin film electrochromic devices and gas sensors. In a recent report [15], we have shown the reversible adsorption of N02/N204 (NO,) on single LB monolayer. Similar studies were carried out with YPc2 LB using six monolayers LB films on glass and one LB monolayer on 4 nm Au island film. The LB films were exposed to NH3, NO, and cigarette smoke in a low vacuum chamber. The gas adsorption was monitored using SERRS and UV-vis spectroscopy and the UV-vis results are illustrated in Fig. 7. The desorption of NO, was monitored after leaving the samples under low vacuum for 2 h. The electron donor NH3 gas did not produce any noticeable change in the SERRS and UV-vis spectra with respect to the untreated reference sample. The electron acceptor NO, produced the typical oxidized spectrum of YPc2 with a red-shifted Q-band at 710 nm and a broad free radical band at 512 nm. Correspondingly, the SERRS spectrum showed a different pattern of relative intensities compared with the original sample. Notably, the exposure to NO, changes the colour of the LB film, a green+red change that can be seen with the naked eye. The exposure of the LB film to tobacco smoke leads to a partial reduction of the YPc, shifting the Q band to the blue. The latter green + blue change can also be followed visually. However, the film in air rapidly loses the blue colour of reduction and recovers the original green colour after a few minutes. In summary, a reversible adsorption of NO, and the smoke in LB films of YPc, was determined. Qualitatively, the adsorption of electron acceptors (NO,) and electron donors (smoke) can be directly detected by a change in the colour of the LB film. Acknowledgemenrs-Financial assistance from NSERC of Canada is gratefully acknowledged. One of us (M.L.R.) would like to thank the Ministry of Education and Science of Spain for a Research Fellowship.

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[12] J. Souto, R. Aroca and J. A. DeSaja, J. Raman Spectrosc. 22,349 (1991). [13] E. B. Hayden, in Vibrational Spectroscopy of Molecules on Surfaces (Edited by J. T. Yates, Jr and T. E. Madey), p. 267. Plenum, N.Y. (1987). [14] J. Umemura, T. Kamata, T. Kawai and T. Takenaka, 1. Phys. C/rem. 94.62 (1990). [15] D. Battisti and R. Aroca, J. Am. Chem. Sot. 114, 1201 (1992).