Study of Gas Sensing Performances of Langmuir−Blodgett Films

Langmuir 2001, 17, 8139-8144

8139

Study of Gas Sensing Performances of Langmuir-Blodgett Films Containinig an Alkyne-Linked Conjugated-Porphyrin Dimer A. Tepore,† A. Serra,† D. P. Arnold,‡ D. Manno,† G. Micocci,† A. Genga,† and L. Valli*,§ Dipartimento di Scienza dei Materiali, Universita` degli Studi di Lecce and Istituto Nazionale di Fisica della Materia (INFM), Via Arnesano-73100 Lecce, Italy; Dipartimento di Ingegneria dell’Innovazione, Universita` degli Studi di Lecce and INSTM, Via Monteroni, Edificio “La Stecca”-73100 Lecce, Italy; and School of Physical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane 4001, Australia Received May 16, 2001. In Final Form: August 16, 2001 meso,meso′-Buta-1,3-diyne-bridged Cu(II) octaethylporphyrin dimer thin films have been deposited by the Langmuir-Blodgett method and, for the first time, have been considered as the active layer in conductive gas sensors. In particular, the electrical conductivities of these films undergo a remarkable variation due to exposure to small concentrations of NO in air at an operating temperature of 90 °C. Moreover, such multilayers are substantially unaffected by the contemporary presence in the atmosphere of other gaseous species (NO2, CH4, C2H5OH, CO). To interpret the behavior of such organic films, a mechanism has been developed and proposed, and the conductivity variation as a function of test gas concentrations and time has been derived.

I. Introduction Interest in organic compounds as sensitive materials for gas sensors arises in many examples from the molecular architectures that give rise to interesting chemical and physical properties.1-3 Moreover, while conductivity processes in metallic oxides are greatly governed by volume or even more by surface defects, the reactivity and the related conductivity of organic counterparts are peculiar features of the molecules arranged in the thin layers. It is well-known that the reactivity of organic semiconductors to gases could be improved by modification of their molecular structure. For macrocycle molecules, such as metallophthalocyanines and metalloporphyrins, changing the macrocycle structure,4 the central metal ion,5 or the peripheral substituents6 could modify the gas-sensing features. In several previous papers, we analyzed Langmuir-Blodgett films of metallophthalocyanines7-10 and metalloporphyrins.11-13 † Dipartimento di Scienza dei Materiali, Universita ` degli Studi di Lecce and INSTM. ‡ Queensland University of Technology. § Dipartimento di Ingegneria dell’Innovazione, Universita ` degli Studi di Lecce and INSTM.

(1) Rothberg, L. J.; Lovinger, A. J. J. Mater. Res. 1996, 12, 3174. (2) Seki, K.; Tani, T.; Ishii, H. Thin Solid Films 1996, 273, 20. (3) Leray, I.; Vernieres, M. C.; Loucif-Saibi, R.; Bied-Charreton, C.; Faure, J. Sens. Actuators B 1996, 37, 67. (4) Leznoff, C. C.; Lever, A. B. P. Phthalocyanines, Properties and Applications; VCH: Weinheim, 1989. (5) Sadaoka, Y.; Jones, T. A.; Revell, G. S.; Gopel, W. J. Mater. Sci. 1990, 25, 5257. (6) Passard, M.; Blanc, J. P.; Maleysson, C. Thin Solid Films 1995, 271, 8. (7) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Langmuir 1997, 13, 6562. (8) Capone, S.; Mongelli, S.; Rella, R.; Siciliano, P.; Valli, L. Langmuir 1999, 15, 1748. (9) Wilde, J. N.; Petty, M. C.; Saffell, J.; Tepore, A.; Valli, L. Measurement + Control 1997, 30, 269. (10) Manno, D.; Rella, R.; Serra, A.; Siciliano, P.; Taurino, A.; Troisi, L.; Valli, L. Mater. Sci. Eng. C 1998, 5, 317. (11) Arnold, D. P.; Manno, D.; Micocci, G.; Serra, A.; Tepore, A.; Valli, L. Langmuir 1997, 13, 5951.

In the literature, different models have been proposed to explain the gas-sensing mechanism of some inorganic metal oxide based gas sensors.14-16 In the case of metal oxide materials, the sensing mechanism is based on oxidation and reduction phenomena. On the contrary, the interaction between organic films and surrounding atmosphere is a more complex phenomenon; namely, the electrical resistance variation is probably connected to charge delocalization. On the other hand, the transport properties are expected to be strongly affected by the presence of a suitable ligand that allows extensive charge delocalization and only weakly by the presence of a central metal ion. Therefore, the conjugated meso,meso′-buta-1,3diyne-bridged Cu(II) octaethylporphyrin dimer has been used (from here on this derivative will be labeled “porphyrin”). Such a derivative has unconventional properties and we have investigated its application as the active layer in selective NO gas sensors. The butadiyne bridge offers some interesting advantages, in that it is rigid, linear, and sterically nondemanding and allows the formation of an extensively conjugated molecular structure. To the best of our knowledge, only in Richardson’s group was an analogous derivative investigated in the form of thin films in order to study their optical characteristics.17 Concerning the LB film formation, possibly since our derivative has no relevant amphiphilic character but is merely surface active, a multilayer film can be fabricated (12) Arnold, D. P.; Manno, D.; Micocci, G.; Serra, A.; Tepore, A.; Valli, L. Thin Solid Films 1998, 327-329, 341. (13) Valli, L.; Manno, D.; Micocci, G.; Serra, A.; Tepore, A.; Arnold, D. P. Mater. Sci. Eng. C 1999, 8-9, 107. (14) Clifford, P. K. In Proceedings of the International Meeting on Chemical Sensors; Seiyama, T., Fueki, K., Shiokawa, J., Suzuki, S., Eds.; 1983; p 135. (15) Lalauze, R.; Pijolat, C.; Vincent, S.; Bruno, L. Sens. Actuators B 1992, 8, 237. (16) Huang, X. J.; Scoonman, J.; Chen, L. Q. Sens. Actuators B 1994, 22, 227. (17) Grieve, M. B.; Richardson, T.; Anderson, H. L.; Bradley, D. D. C. Thin Solid Films 1996, 284-285, 648.

10.1021/la0107289 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/28/2001

8140

Langmuir, Vol. 17, No. 26, 2001

Tepore et al.

Figure 1. Structure of the meso,meso′-buta-1,3-diyne-bridged Cu(II) octaethylporphyrin dimer.

through the use of film-promoting substances; in this case, arachidic acid has been used. The availability of an appropriate model able to foresee the electrical behavior of a sensitive material could represent a powerful tool to plan neural sensor arrays. For this reason, it is of fundamental importance to relate the gas adsorption mechanism of conductive gas sensors to basic kinetic parameters (e.g., adsorption and desorption probabilities, response and recovery time, number of adsorbed molecules). Models previously proposed gave only a qualitative description of organic film performance. In an earlier paper18 we began to investigate the behavior of meso,meso′-buta-1,3-diyne-bridged metalated octaethylporphyrin dimer Langmuir-Blodgett films in a controlled atmosphere. In this work the interaction of porphyrin LangmuirBlodgett films with NO gas has been analyzed. The molecular structure of this derivative is reported in Figure 1. The analysis of conductivity variation as a function of NO concentration and time, together with a fitting of experimental and theoretical behavior, allows us to determine response and recovery times, average number of adsorbed molecules, and adsorption and desorption probabilities. The results strongly support the reliability of the proposed mechanism over a large range of gas concentrations. II. Experimental Section meso,meso′-Buta-1,3-diyne-bridged Cu(II) octaethylporphyrin dimer was prepared by Arnold’s group through a synthetic procedure already described.19 Langmuir experiments were carried out using a KSV5000 System 3 LB apparatus (850 cm2). A 1:4 molar mixture of porphyrin and arachidic acid was dissolved in chloroform (7.5 × 10-5 M and 3.1 × 10-4 M). In our depositions 200 µL of the spreading solution was spread onto the subphase, whose temperature was regulated at 20 °C by a Haake GH D8 apparatus. The subphase was ultrapure water (Millipore, MilliRO, MilliQ, resistivity 18.2 MΩ cm) containing 3 × 10-4 M CdCl2 buffered with 10-5 M KHCO3 to pH ) 6.0. After the solvent evaporated off, the floating film was compressed at a speed of 10 mm/min. The transfer on the substrates was performed at a surface pressure of 25 mN/m and at a speed of 6 mm/min. Y-type deposition up to 100 layers was obtained onto hydrophobic quartz: the deposition ratio for both withdrawing and lowering the substrate through the monolayer onto water was always close to unity. To test all obtained thin films as gas-sensing material, two ohmic gold contacts in the gap configuration were sputtered onto the film surface. The gas effect on electrical conductivity was measured in a dynamic flow system implemented in our laboratory as already described.20 Dry air and argon at ambient pressure were used as the carrier and the reference gas, respectively. Test measurements, carried out in the 0.5-1000 ppm gas concentration range, consisted of sets of cycles in which (18) Tepore, A.; Serra, A.; Manno, D.; Valli, L.; Micocci, G.; Arnold, D. P. J. Appl. Phys. 1998, 84, 1416. (19) Arnold, D. P.; Heath, G. A. J. Am. Chem. Soc. 1993, 115, 12197. (20) Manno, D.; Serra, A.; Di Giulio, M.; Micocci, G.; Tepore, A. Thin Solid Films 1998, 324, 44.

Figure 2. Relative resistance variations, ∆R/Rgas, after exposure to 20 ppm of NO, NO2, CO, CH4, and C2H5OH at 90 °C working temperature. increasing gas concentrations were separated by exposures back to the reference gas. No different conduction regimes were observed at various operating temperatures and atmospheres, and the current-voltage characteristics of the porphyrin dimer thin films exhibit a linear response in the 1-100 V voltage range. Therefore, in the present study, all the experiments are performed using 5 V polarization voltage to minimize the metal/material contact effect and to work primarily on gas/material interactions. Hall effect measurements carried out on meso,meso′-buta-1,3diyne-bridged Cu(II) octaethylporphyrin dimer LB films showed a p-type semiconductor behavior, like similar macrocyclic complexes,21,22 and the measured Hall mobility was about 0.20 cm2/Vs.

III. Discussion Gas-Sensing Properties. Electrical measurements in controlled atmospheres were performed in the 20-130 °C temperature range. The highest reactivity to gases was obtained at 90 °C. Above 130 °C, irreversible structural and morphological modifications can be thermally induced,23,24 while at temperatures below 20 °C, the rate of the adsorption process is too low to be analyzed. To utilize the sensing films in practical applications, the selectivity of the film must be characterized. With the intention of analyzing the resistance variation induced by other gaseous species and thus determining the porphyrin dimer selectivity, our films were exposed to NO, NO2, CO, CH4, and C2H5OH. First, resistance versus time experiments were performed for every gas mentioned above at different sample temperatures ranging from 20 to 130 °C. These experiments showed very low film response to any of the examined gases, except nitrogen monoxide. The results are summarized in Figure 2. The relative resistance variations, ∆R/Rgas, of a 64-layer LB film to 20 ppm of NO, NO2, CO, CH4, and C2H5OH, respectively, at 90 °C working temperature are compared. It can be seen that the film resistance variation induced by 20 ppm of NO at 90 °C is at least 100 times the corresponding resistance variation induced by other gases. Figure 3 reports typical response transients for different NO concentrations at a temperature of 90 °C. The carrier gas was dry synthetic air. It is clear that the injection of NO leads to a drastic drop in the resistance. After 1500 s the NO gas flux is shut off. Then, the film resistance approaches its initial value, and this proves that the NO adsorption process is reversible. The relative resistance variation, following NO adsorption, ranges between 90% for 0.5 ppm of NO and 1.85 · 105 % for 1000 ppm of NO. (21) Petty, M. C. Langmuir-Blodgett Films; Cambridge University Press: United Kingdom, 1996. (22) Honeybourne, C. L. J. Phys. Chem. Solids 1987, 48, 109. (23) Naselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D. Thin Solid Films 1985, 134, 173. (24) Riegler, J. E. J. Phys. Chem. 1989, 93, 6475.

Films of Alkyne-Linked Conjugated-Porphyrin Dimer

Langmuir, Vol. 17, No. 26, 2001 8141

Figure 3. Typical response transients for different NO concentrations at 90 °C (film thickness of 64 layers).

Figure 4. Conductance variation induced by 20 ppm of NO at 90 °C.

Using LB films containing a number of layers ranging between 6 and 100, we observed no correlation between the film thickness and the magnitude and rate of response to NO. A possible rationale of this phenomenon is that bulk diffusion of NO into the LB films occurs so slowly that the penetration depth is less than the thickness (about 17 nm)8 of the thinnest film used in this work. Similar conclusions are reported for metal-phthalocyanine LB films by Archer et al.25 The response of our films to NO was analyzed also in argon flow. The change of carrier gas does not affect the response to nitrogen monoxide, so the adsorption mechanism of NO gas does not involve the participation of different molecules (e.g., oxygen). Porphyrin Dimer Film-NO Gas Interaction. It is well-known that metal-porphyrin molecules are aromatic compounds with an extended π electron system. In our porphyrin dimer molecules the possibility of coplanarity of the two rings allowed by the conjugating butadiyne bridge ensures significant overlap of the porphyrin π systems. In particular, the presence of the butadiyne bridge reduces the gap between the highest occupied and lowest unoccupied ligand-based orbitals.26,27 In fact, the π orbitals of the butadyine ligand interact with the porphyrin π system, destabilizing the ligand HOMO (highest occupied molecular orbital) and stabilizing one component of the LUMO (lowest unoccupied molecular orbital). In this way one obtains a reduction of the ligandbased HOMO-LUMO gap and, therefore, a red-shift in π-π* transitions.28-30 Charge delocalization processes are promoted with low activation energies. These features make these metal-porphyrin dimer molecules good electron donors (and acceptors). So, the adsorption of electron-acceptor gas molecules, such as NO, favors a charge-transfer interaction: electrons are transferred from the π electron system of the macrocycle molecule to the electron acceptor gas molecules. The following processes can represent the interaction between the porphyrin dimer unit in the LB film surface and the Y test gas:31,32 (I) m molecules of Y test gas are physisorbed per time unit by a porphyrin dimer molecule M in the LB film

surface and placed in the adsorption sites.

(25) Archer, P. B. M.; Chadwick, A. V.; Miasik, J. J.; Tamizi, M.; Wright, J. Sens. Actuators 1989, 16, 379. (26) Tanaka, K.; Kosai, N.; Maruyama, H.; Kobayashi, H. Synth. Met. 1998, 92, 253. (27) Stranger, R.; McGrady, J. E.; Arnold, D. P.; Lane, I.; Heath, G. A. Inorg. Chem. 1996, 35, 7791. (28) Anderson, H. L. Inorg. Chem. 1994, 33, 972. (29) Anderson, H. L. Chem. Commun. 1999, 2323. (30) Arnold, D. P.; Heath, G. A.; James, D. A. J. Porphyrins Phthalocyanines 1999, 3, 5. (31) Passard, M.; Pauly, A.; Germain, J. P.; Maleysson, C. Synth. Met. 1996, 80, 25. (32) Passard, M.; Pauly, A.; Blanc, J. P.; Dogo, S.; Germain, J. P.; Maleysson, C. Thin Solid Films 1994, 237, 272.

M + rYgas T M + mYads + (r - m)Ygas

(1)

where r is the number of inlet Y test gas molecules for a porphyrin dimer molecule. The subscripts “gas” and “ads” denote the gaseous and adsorbed state of the Y molecules. (II) Then a charge-transfer process occurs, whereby an electron is transferred from the π system of the dimer unit to the m gas molecules Y already adsorbed.

M + mYads T M+ + (mYads)-

(2)

(III) This charge transfer gives rise to a hole, h, that is delocalized according to the following relation

M+ + (mYads)- T M + (mYads)- + h

(3)

In this context, m represents the number of test gas molecules that a porphyrin dimer adsorbs to generate a delocalized carrier per time unit and surface unit. So, the delocalized hole density, h, is given by the relation

h)

[Yads] m

(4)

Consequently, the film electrical conductance variation ∆G ) Ah is given by the following law:

∆G ) A

[Yads] m

(5)

where A is a parameter that depends on the hole mobility, the electron charge, and the arrangements of electrical connections on the porphyrin film surface.18 Gas Adsorption-Desorption. In Figure 4, a typical conductance variation ∆G induced by gas adsorption on a 32-layer LB film is illustrated. It is apparent that the interaction between the LB film and the adsorbed gas is a dynamic process. When the films are exposed to gas, the adsorption and desorption processes will occur simultaneously. In dynamic equilibrium, the number of adsorbed gas molecules will be equal to the number of the desorbed gas molecules. Then, the film conductivity attains a constant value. This is called the response process. Now, if the gas flux is cutoff, only a desorption process will occur and the film conductivity will return to its original value. Remembering relations 2 and 3, m molecules of a test gas Y have to be adsorbed to generate one delocalized free electrical charge. If [Yads] is the test gas molecule concentration trapped in the adsorption sites, by assuming that in the film there are S adsorption surface sites, [Yads]/

8142

Langmuir, Vol. 17, No. 26, 2001

Tepore et al.

m of which are occupied by test gas molecules, the response and recovery kinetics can be described by the following equations:33

(

)

[Yads] d[Yads] ) R[Ygas] S - P[Yads] dt m

(6)

in the response process d[Yads] ) -P[Yads] dt

(7)

in the recovery process where [Ygas] is the total adsorbed test gas concentration on the film surface, R is the adsorption probability of gas molecules, P ) ν exp(-Ed/kT), where ν represents an attempt-to-escape frequency for adsorbed gas molecules bound to the adsorption sites, and Ed is the desorption activation energy.34 The first term on the right-hand side of eq 6 represents the adsorption rate, while the second term represents the desorption rate. Then, from eq 6 one obtains the adsorption relation:

[Yads] )

R[Ygas]S (1 - exp(-t/τa)) R [Ygas] + P m

(8)

where we define

τa )

1 R [Y ] + P m gas

(9)

as the response time. From eq 7 the following desorption relation is obtained:

[Yads] )

R[Ygas]S

R [Y ] + P m gas

(exp(-t/τd))

(10)

1 P

(11)

( )

(13)

The preexponential factor of eqs 12 and 13

as the recovery time. In eq 10, t ) 0 means that the test gas flux has been switched off and this corresponds to t ) ∞ in eq 8. In the response process, remembering that the conductance variation is given by ∆G ) A[Yads]/m, from eq 8 it is obtained that

R [Y ]S m gas [1 - exp(-t/τa)] ∆G ) A R [Ygas] + P m

( )

R [Y ]S m gas ∆G ) A [exp(-t/τd)] R [Ygas] + P m

where we have defined

τd )

Figure 5. (a) Normalized conductivity variation ∆G/∆Gmax induced by 9 ppm of NO at 90 °C. (b) ∆G/∆Gmax versus [1exp(-t/τa)], for response transient analysis. (c) ∆G/∆Gmax versus exp(-t/τd) for recovery transient.

(12)

Likewise, in the recovery process from eq 10 one obtains that (33) Chen, R.; Kirsh, Y. Analysis of thermally stimulated processes; Pergamon Press: Oxford, UK, 1981. (34) Redhead, P. A. J. Vac. Sci. Technol. 1995, A13, 467.

( )

R [Y ]S m gas ∆Gmax ) A R [Y ] + P m gas

(14)

represents in stationary conditions the maximum value of conductivity variation, that is, t ) 0 in eq 13 or t ) ∞ in eq 12. Conductance response and recovery transients have been analyzed according to eqs 12 and 13, respectively. Figure 5a illustrates the normalized conductance variation ∆G/∆Gmax induced by 9 ppm of NO, and Figure 6a shows the normalized conductance variation induced by 20 ppm of NO at 90 °C in a 32-layer LB film. Concerning the response transients, ∆G/∆Gmax versus [1 - exp(-t/τa)] has been fitted by a high-convergence procedure with τa as free parameter. These plots, as reported in Figures 5b and 6b, result in straight lines, according to eq 12, only

Films of Alkyne-Linked Conjugated-Porphyrin Dimer

Langmuir, Vol. 17, No. 26, 2001 8143

Figure 7. Response times as a function of NO gas concentrations. The solid line represents the best fit.

Figure 8. ∆Gmax as a function of NO gas concentration. The solid line represents the best fit.

Figure 6. (a) Normalized conductivity variation ∆G/∆Gmax induced by 20 ppm of NO at 90 °C. (b) ∆G/∆Gmax versus [1exp(-t/τa)], for response transient analysis. (c) ∆G/∆Gmax versus exp(-t/τd) for recovery transient. Table 1. Values Obtained from Conductance Transient Measurements Carried Out at Different NO Concentrations NO gas (ppm)

R/m (10-6) (s-1 ppm-1)

τa (s)

τd (s)

0.5 3 6 9 20 100 200 500 1000

5.1 ( 0.3 5.1 ( 0.3 5.2 ( 0.3 4.9 ( 0.3 5.0 ( 0.3 4.8 ( 0.4 5.1 ( 0.3 5.0 ( 0.3 5.0 ( 0.3

430 ( 40 420 ( 40 400 ( 40 380 ( 40 300 ( 30 140 ( 20 85 ( 5 65 ( 5 30 ( 2

435 ( 35 415 ( 35 420 ( 35 418 ( 35 435 ( 35 420 ( 35 430 ( 35 425 ( 35 422 ( 35

for response times τa ) 380 ( 40 and 300 ( 30 s for 9 and 20 ppm of NO, respectively. Similarly, for the recovery transients, eq 13 allows us to obtain the recovery time τd from the experimental data by fitting ∆G/∆Gmax versus exp(-t/τd) with τd as parameter. As reported in Figures 5c and 6c, the recovery transients analyses for 9 and 20 ppm of NO result in straight lines only for τd ) 418 ( 35 and 435 ( 35 s, respectively. Similar analyses were performed for all investigated NO gas concentrations. Results are summarized in Table 1. It is worth stressing that recovery times are totally unaffected by gas concentrations, according to eq 11.

On the contrary, it is apparent that response times are strongly dependent on NO gas concentration, according to eq 9. In Figure 7 response times τa are reported as a function of NO gas concentrations. In this case, three different regions can be discriminated: (1) for very low NO gas concentrations, the response time is almost constant; (2) for intermediate NO gas concentrations, the response time decreases quickly with increasing concentration; and (3) on the contrary, for higher gas concentrations, the response time is weakly affected by gas concentration variations. So, the experimental data have been fitted (solid line in Figure 7) by eq 9, with R/m and P as free parameters. The optimal values

R ) (4.85 ( 0.30) × 10-6 s-1 ppm-1, m P ) (2.3 ( 0.2) × 10-3 s-1 accurately fitted the theoretical prediction. It is apparent that for low NO concentrations the gas adsorption mechanism is negligible with respect to gas desorption; in the 10-100 ppm gas concentration range, NO adsorption and desorption are competitive phenomena; for high gas concentration values, NO adsorption prevails over gas desorption. In addition, according to eq 14, we can plot ∆Gmax as a function of NO gas concentration, as reported in Figure 8. The experimental data are in good agreement with the theoretical prediction (solid line) with the parameters

R ) (5.0 ( 0.3) × 10-6 s-1 ppm-1, m P ) (2.19 ( 0.27) × 10-3 s-1, S ) (1.35 ( 0.34) × 1016 cm-2 Therefore, we can discriminate between two extreme regions:

8144

Langmuir, Vol. 17, No. 26, 2001

Tepore et al.

Conclusions

Figure 9. Recovery time as a function of NO gas concentration. The solid line represents the best fit. Table 2. Values Obtained from Conductance Transient Measurements Carried Out at 20 ppm NO Concentration and at Different Temperatures T (°C)

τa (s)

τd (s)

T (°C)

τa (s)

τd (s)

20 50 75

8500 ( 450 2950 ( 350 1150 ( 150

>105 7000 ( 500 1935 ( 170

90 110 130

300 ( 30 140 ( 20 65 ( 10

435 ( 35 200 ( 20 91 ( 15

(1) the first one, at low gas concentration values, where P is very large with respect to the other quantities, and ∆Gmax is a power law of gas concentration; and (2) the second, which can be observed at high values of gas concentration. In this region P is negligible and ∆Gmax attains a constant value. The above analyses were also performed in the whole investigated temperature range for different NO gas concentrations. In Table 2 the recovery and response times vs working temperature, for a concentration of 20 ppm, are reported. It is apparent that both the recovery and response time decrease while the temperature increases. In addition, according to eq 11, the plot of τd vs. 1/(kT), shown in Figure 9, allows us to determine the desorption activation energy Ed ) 0.62 ( 0.05 eV.

In this paper we have confirmed that meso,meso′-buta1,3-diyne-bridged Cu(II) octaethylporphyrin dimer deposited as thin films by Langmuir-Blodgett technique is a novel gas-sensitive material of considerable promise. In particular, we have observed that the porphyrin seems to be suitable for the detection of relatively low concentrations of NO gas at an operating temperature of 90 °C, and the resistance variation induced by adsorption of 20 ppm of NO at 90 °C is at least 100 times the corresponding resistance variation induced by other gases, such as NO2, CO, CH4, and C2H5OH. Charge delocalization in the large π system makes metal-porphyrin molecules good electron donors. So the adsorption of electron acceptor gas molecules, such as NO, favors a charge-transfer interaction: electrons are transferred from the π electron system of metal-porphyrin dimer molecule to electron acceptor gas molecule. In addition, we have proposed a model to describe the adsorption kinetics in our LB films. The fitting of theoretical and experimental behavior allows us to determine the basic kinetic parameters (number of adsorbed gas molecules per porphyrin dimer unit, response and recovery times, adsorption and desorption probabilities, and adsorption site density). Besides, the model takes into account the response of the organic material in a controlled atmosphere at both low and high gas concentrations and even when dynamic equilibrium between adsorption and desorption phenomena is achieved. Acknowledgment. The authors are grateful to L. Dimo, A. R. De Bartolomeo, and G. D’Elia for technical assistance. Note Added after ASAP Posting. This article was released ASAP on 11/28/2001 with errors in eqs 13 and 14. The correct version was posted on 12/04/01. LA0107289