The interaction of transition metal phthalocyanines with organic molecules: a quartz-microbalance study

SEgJRS

mums B Sensors and Actuators

B 2425

(1995) 69-71

CHEMICAL

The interaction of transition metal phthalocyanines with organic molecules: a quartz-microbalance study K.-D. Schierbaum a, R. Zhou a, S. Knecht b, R. Dieing b, M. Hanack b, W. GGpel a ’ Imtifufe of Physical and

Theoretical Chemistry, Universityof Tiibingen, Auf der Mmgemtelle 8, D-72076 Tiibingen, Gemany bln.stituteof Organic Chemistry, Universityof Tiibingen, Auf der Morgenstelle 18, D72076 Ttibingen, Germany

Abstract

Results are presented on the interaction of monomeric transition metal phthalocyanines (PcMs) with organic solvent molecules. For this purpose, quartz-microbalance oscillators are coated with thin PcM films and exposed to different concentrations of organic solvent vapours in air. Frequency changes, which result from an increase of mass during the thin-film deposition and subsequent molcculeiPcM interaction, are determined and discussed comparatively for a variety of phthalocyanines with cobalt, iron, nickel, and copper as central metal atoms and with different ligands. Keywordr: Metal

phthalocyanines;

Organic solvent detection; Quartz-crystal microbalance

1. Introduction Phthalocyanines (PcMs) are known to act as suitable chemically sensitive coatings of electronic conductance devices for the detection of halogens like FZ and of nitrogen dioxide (NO,) [l]. In the context of their electrical, non-linear optical or liquid-crystalline properties, a variety of bisaxially coordinated complexes PcML, and bridged systems [PcM(L)], have also been prepared and investigated in the past with different ligands (L) coordinated to the central metal atom (M) [2]. In the present paper, monomeric soluble transition metal phthalocyanines R,PcM (with different side groups R=2,2-dimethyl-3-phenyl-propoxy=PPO or R=rert-butyl coupled to the aromatic rings) and R,PcM (with R=heptyl=C!,H,,) are now used as chemically sensitive coatings for quartz-microbalance transducers to monitor organic solvent molecules by mass changes. The chemical structures of the different phthalocyanines investigated here are shown in Fig. 1. Their preparation has been given in detail in Refs. [3,4].

2. Experimental Microbalance oscillators (10 MHz) with gold electrodes (Kristallverarbeitung Neckarbischofsheim, Germany) are coated with 0.1 wt.% solutions of phthalocyanines in chloroform by means of dipping. The masses of the R,PcM and R,PcM thin films were determined 0 1995 Elsevier Science S.A. All rights resewed S.sDl 0925-4005(94)01318-c

09254MWW$O9.50

from the shift Af, of oscillation frequencies before and after coating the quartz-microbalance oscillators. The sensors were then exposed to different concentrations (c < 3000 ml m-‘) of various organic solvent molecules including acetone, benzene, toluene, dichloromethane, chloroform, tetrachloromethane, trichlorethylene, tetrachlorocthylene, methanol, ethanol, n-propanol, n-hexane and n-octane in air at room temperature. Frequency changes Af of the oscillators were determined during gas exposure in a gas-flow mixing system as a function of time. Experimental details have been published earlier [5].

3. Results and discussion Typical results on the time dependence of frequency changes Afduring molecule/phthalocyanine interactions are shown in Fig. 2 for (t-Bu),PcNi exposed to different concentrations of chloroform. Evidently, thermodynamically controlled reversible signals are found for this system under equilibrium conditions that result from the mass increase. Rise and decay times t, and t&, of frequency changes Af are in the order of minutes and show characteristic differences between the different phthalocyanines (Fig. 3). This new class of modifiedphthalocyanines with larger substituted organic side groups and hence higher solubilities in organic solvents show far better sensor

70

H,C.

M = H2. Co, Ni

Fig. 2. qpical examples of frequency changes 4 of quartz-microbalance sensors. Here, (t-Bu),PcNi films were exposed to different concentrations of tetrachloroethylene (51X2.500 ml m-‘) in air at T-298 K Between the gas-exposure steps, the films were exposed to synthetic air in order to detcrminc characteristic rise and decay times.

50 ml m-” C2Cl4

(b)

I

I

30 min

’

I+

,

Fig. 3. Rise (1,“) and decay (&,J of frequency changes af of quartzmicrobalance transducers coated with (t-Bu)lPcNi, (C,H&PcNi and (PPO),PcNi. The sensors are exposed to a concentration of 50 ml m-’ (i.e., 50 ppm) of tetrachloroethylene in air at T=298 K. Values fYo and f&, relate to 90% changes of corresponding steady-state signals.

M = Co. Ni

(cl

Fig. 1. Chemical structures of the different soluble phthalocyanines used in this study: (a) tetra-leti-butyl phthalocyanine (t-Bu).,PcM with M=H*, Co and Ni; (b) tetra(2,2-dimethyl-3-phenyl-propoxy) phthalocyanine (PPO)IPcM with M=H>, Co, Ni, Fe and Cu, (c) 1,4,8,11,15,18,22,25-octa-heptyl phthalocyanine (GH,,), with M=Co and Ni.

properties with respect to sensitivities, response and decay times and stabilities than insoluble phthalocyanines [6]. The latter show pronounced longer response

times. Hence for these compounds, the kinetically controlled slopes dAf/dt can be determined from the initial decrease of Af under chopped-flow conditions and are correlated with concentrations of organic molecules in air. In the following, only results for soluble phthalocyanines will be discussed. Results of sensitivities S= (Af,))‘(Af/c) of (t-Bu),PcH,, (t-Bu),PcCo and (tBu),PcNi towards different organic molecules are given in Fig. 4. Here, the sensitivity is defined as the frequency change Af divided by the concentration c with respect to the difference Af, of the oscillation frequencies of uncoated and coated quartz oscillators. This definition of the sensitivity is related to the mass of the phthalocyanine films and hence to the number of PC macromolecules (‘recognition sites’). The differences in the molecular weights of the different phthalocyanines may be neglected here to a first approximation.

K.-D. Schierhaum et al / Sensors and Actuators B 24-25 (1995) 69-71

f

80

2 5 60 b =. .r: x 40 .$ ._ a 2 20

Fig. 4. Sensitivities of (t-Bu),PcH*, (t-Bu),PcCo and (t-Bu)iPcNi for different solvent molecules in air at T-298 K. For the definition of sensitivities, see text.

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metal atoms (i.e., for M=H,) and with different central transition metal atoms. This is illustrated in Fig. 5 for (PPO),PcCo, (PPO),PcFe, (PPO),PcNi, (PPO),PcCu and (PPO),PcH, Similar results were obtained for phthalocyanines R,PcM with R=2,2-dimethyl-ethyl. No evidence can be derived from our experiments for possible metal/molecules interactions involving, e.g., metallic electronic states and r-bonds of the molecules, since the sensitivities are also different for non-polar and non-polarizable molecules like n-hexane and noctane. Therefore, it is concluded that the central metal atom affects the electronic polarizabilities of the aromatic rings of the phthalocyanines. This changes the van der Waals interaction energies and hence the sensitivities of the PcM-coated sensors. Of particular relevance for practical applications is the extremely high sensitivity of (PPO),PcCo towards tetrachloroethylene, which is one order of magnitude larger than the sensitivities of ‘simple’ polymers like polydimethylsiloxane [S]. 5. Conclusions

40

Different soluble transition metal phthalocyanines (PcMs) were prepared and used as coatings for quartzmicrobalance sensors to determine organic solvent molecules in air. Several such PcMs show reversible interactions and large sensitivities, in particular, for organic solvents with high boiling temperatures. References W. Snow and W.R. Bqer, Phthalocyanine films in chemical sensors, in C.C. Lenznoff and A.B.P. Lever (eds.), Phfhalocyanines Pmpetiiesand Applicafions,VCH, Weinheim, 1992, Ch. 5, p. 341; Y. Sadaoka,T.A. Jones and W. Giipel, Fast NO2 detection at roomtemperature with optimized lead phthalocyanine thin-film structures, Sensors and Actuators, BZ (1990) 148. VI H. Schultz, H. Lehmann, M. Rein and M. Hanack, Phthalocyaninatometal and related complexes with special electrical and optical properties, in Smrcr~re and Bonding 74, Springer, Heidelberg, 1991, pp. 41-146 and refs. therein. I31 S. Knecht, Synthese und Eigenschaften tetra- und octa-alI+ substituierter Phthalocyaninatoiibergangsmetallkomplexe,Ph.D. T?zesis,Tiibingen, 1994. 141 R. Dicing, Synthese und Eigenschaften van 2,3Anthracenocyaninen, Ph.D. Thesis, Tiibingen, 1994. I51 K.D. Schierbaum, A. Gerlach, M. Haug and W. Gijpel, Selective detection of organic molecules with polymers and supramolecular compounds: application of capacitance, quartz. microbalance and calorimetric transducers, Sensors and Actuators A, 32 (1992) 13&137. der metallkomple161 R. Zhou, Synthese und Untersuchung xierenden und chemisch-sensitiven Eigenschaften van funktionellen Polymeren, Ph.D. Thesis, Tiibingen, 1994. [71 K.D. Schierbaum and W. Gtipel, Functional polymers and supramolecular compounds for chemical sensors, Synfh. Met, 61 (1993) 37-45.

[ll

Fig. 5. Sensitivities of (PPO),PcM for different solvent molecules in air at T=298 K. Further explanations are given in the text.

The pronounced changes in sensitivities may be explained in termsof differences in the boiling temperature of the different organic molecules, as was usually found earlier in simple polymeric systems such as polydimethylsiloxane layers [7]. From this finding, it may be concluded that molecule/PcM interactions are mainly determined by van der Waals bond energies between the polarizable aromatic rings of the phthalocyanine and the molecules. Specific interactions involving electronic states of the central metal may play only a minor role. However, the strong sensitivity of the metal-free (t-Bu),PcH, towards alcohols with their hydroxyl group (OH) is strongly suppressed if the phthalocyanine ring contains a central metal atom (set Fig. 4). Drastic differences of the sensitivities were found between phthalocyanines (PPO),PcM without central