Influence of Molecular Organization of Asymmetrically Substituted

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Influence of Molecular Organization of Asymmetrically Substituted Porphyrins on Their Response to NO2 Gas Jose´ M. Pedrosa,*,† Colin M. Dooling,‡ Tim H. Richardson,‡ Robert K. Hyde,§ Chris A. Hunter,§ Marıá T. Martıń,† and Luis Camacho† Departamento de Quı´mica Fı´sica y Termodina´ mica Aplicada, Universidad de Co´ rdoba, Campus Universitario de Rabanales, E-14014 Co´ rdoba, Spain, Department of Physics & Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, U.K., and Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. Received May 28, 2002 Two asymmetrically substituted porphyrins (5,15-bis(4-nitrophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)-21H,23H-porphine, CAH3; 5,15-bis(4-aminophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)21H,23H-porphine, CAH4) have been used in this work. Good Langmuir monolayers of these compounds have been prepared on water and transferred successfully to glass slides at much faster than conventional deposition rates (500 mm min-1). The monolayer behavior has been investigated by measuring surface pressure-area isotherms as well as by Brewster angle microscopy (BAM) and reflection spectroscopy. For both porphyrins, the intermolecular π-π interaction is sufficiently strong that significant preaggregation occurs prior to compression of the film. However, as the monolayers are compressed, changes in the molecular association and orientation are observed for CAH4 while no molecular reorganization is appreciated for CAH3. The different molecular packing of the Langmuir monolayers is maintained during the transfer. The UV-vis spectra of CAH3 and CAH4 solutions, respectively, were found to be sensitive to NO2 gas. However, only the Langmuir-Blodgett (LB) assemblies of CAH4 showed a response in the presence of the toxic gas. This different behavior has been explained in terms of the different molecular organization of the two porphyrins.

Introduction Porphyrins and related compounds, when deposited as thin films, have attracted increasing attention in recent years, owing to their potential use in many fields of technological interest such as photoconductors, optical actuators, and chemical sensors.1-5 This interest arises from the electronic structure of these assemblies and the two-dimensional geometry of the porphyrin macrocycle that facilitate processes such as fast, vectorial electron transfer or gas-surface interactions. In particular, recent developments based on spectral changes in the optical properties of Langmuir-Blodgett (LB) films of porphyrins have shown these materials as promising gas sensors.6,7 Of utmost importance with respect to gas sensor application is the sensitivity and response of the material to the target gas in the working environment and its reversibility. Since the interaction of a film with the gas molecules is primarily a surface effect, a large surface area-to-volume ratio of the thin film sensor is advantageous. The ultrafast deposition method6 has been proved to produce inhomogeneous, perforated film structures. However, this un* To whom correspondence should be addressed. E-mail: [email protected]. † Universidad de Co ´ rdoba. ‡ Department of Physics, University of Sheffield. § Department of Chemistry, University of Sheffield. (1) Kampas, F.; Yamashita, K.; Fajer, J. Nature 1980, 284, 40. (2) Jones, R.; Tredgold, R. H.; Hoorfar, A. Thin Solid Films 1985, 123, 307. (3) Beswick, R. B.; Pitt, C. W. Chem. Phys. Lett. 1988, 143, 589. (4) Vandevyver, M. Thin Solid Films 1992, 210, 240. (5) Rakow, N. A.; Suslick, K. S. Nature 2000, 406, 710. (6) Dooling, C. M.; Worsfold, O.; Richardson, T. H.; Tregonning, R.; Vysotsky, M. O.; Hunter, C. A.; Kato, K.; Shinbo, K.; Kaneko, F. J. Mater. Chem. 2001, 11, 392. (7) Richardson, T.; Smith, V. C.; Johnstone, R. A. W.; Sobral, A. J. F. N.; Rocha-Gonsalves, A. M. D’A. Thin Solid Films 1998, 327-329, 315.

conventional structure is very useful in gas sensing due to the enhanced surface area. Characterization of the film macrostructure and the molecular organization of the active element can provide valuable information to understand and control the film formation as well as its interaction with the gas molecules leading to the fabrication of improved gas-sensing devices. One of the most reliable way to investigate molecular association and orientation in monolayers at the airwater interface as well as in systems assembled on solids is the measurement of spectra (UV-vis or IR) of such monolayers.8-11 In particular, evaluation of the molecular orientation can be achieved by using polarized radiation incident on the system under well-defined angles.12,13 An alternative possibility, used here, of detecting such an orientation using unpolarized light under normal incidence is based on the comparison of the spectrum measured at the air-water interface with that obtained in solution. In this paper we report on the preparation and characterization of Langmuir and Langmuir-Blodgett films of two asymmetrically substituted porphyrins (CAH3 and CAH4) by reflection spectroscopy and Brewster angle microscopy (BAM) as well as UV-vis absorption spectroscopy. The optical response to NO2 gas of these (8) Bu¨cher, H.; Drexhage, K. H.; Fleck, M.; Kuhn, H.; Mo¨bius, D.; Scha¨fer, F. P.; Sondermann, J.; Sperling, W.; Tillmann, P.; Wiegand, J. Mol. Cryst. 1967, 2, 199. (9) Takenaka, T.; Nogami, K.; Gotoh, H.; Gotoh, R. J. Colloid Interface Sci. 1971, 35, 395. (10) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (11) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993, 97, 12869. (12) Orrit, M.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986, 85, 4966. (13) Vandevyver, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571.

10.1021/la026004u CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002

Response to NO2 Gas of Porphyrin LB Films

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Chart 1. Chemical Structures of Porphyrins CAH3 and CAH4

Figure 1. Surface pressure-area (π-A) isotherms for CAH3 and CAH4 on water.

porphyrins and its relationship with the molecular organization in the LB film assemblies have been also studied. Materials and Methods The synthesis of 5,15-bis(4-nitrophenyl)-10,20-bis(3,4-bis(2ethylhexyloxy)phenyl)-21H,23H-porphine (CAH3) and 5,15-bis(4aminophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)-21H,23Hporphine (CAH4), whose chemical structures are shown in Chart 1, has been be published elsewhere.14 UV-visible electronic absorption spectra of CAH3 and CAH4 in chloroform solution were recorded on a Cary 100 Bio UVvisible spectrophotometer using quartz cuvettes. The absorbance of the Soret band (λmax ) 427 nm) as a function of the porphyrin concentration was found to be linear in the concentration range 2 × 10-6-2 × 10-4 M. Monolayers of CAH3 and CAH4, respectively, were prepared by spreading a chloroform solution (2 × 10-4 M) onto a clean water surface (Milli-Q water at pH ∼ 6.2, T ) 20 °C) on a NIMA 601BAM trough for isotherms, reflection spectroscopy,15 and Brewster angle microscopy16 measurements and on a constant perimeter Joyce-Loebl minitrough for ultrafast deposition.6 Both troughs were provided with a filter paper Wilhemy plate.17 After evaporation of the organic solvent, the floating Langmuir film was compressed at a rate of 0.1 nm2 molecule-1 min-1. The difference in reflectivity, ∆R, of the monolayer-covered water surface and the bare water surface is determined. Details on the reflection spectrometer have been described elsewhere.15 The reflection spectra were normalized to the same surface density of porphyrin by multiplying ∆R by the surface area, i.e., ∆Rnorm) ∆RA, where A (nm2/porphyrin molecule) is taken from the surface pressure-area (π-A) isotherm. Brewster angle microscopy (BAM) has been used to obtain additional information on the molecular organization of CAH3 and CAH4 at the air-water interface. Details of this technique have been published elsewhere.16 A NFT BAM2plus (lateral resolution: 2 µm) has been used in this study. Monolayers of CAH4 were compressed to 19 mN m-1 and transferred onto glass plates (rendered hydrophobic by immersion in 1,1,1,3,3,3-hexamethyldisilazane for 24 h) at a rate of 500 mm min-1. Although not compatible with all materials, this unconventional high transfer rate was found to be successful for the (14) Pedrosa, J. M.; Dooling, C. M.; Richardson, T. H.; Hyde, R. K.; Hunter, C. A.; Martıń, M. T.; Camacho, L. J. Mater. Chem., in press. (15) Gru¨niger, H.; Mo¨bius, D.; Meyer, H. J. Chem. Phys. 1983, 79, 3701. (16) Ho¨nig, D.; Overbeck, A.; Mo¨bius, D. Adv. Mater. 1992, 4, 419. (17) Fromherz, P. Rev. Sci. Instrum. 1975, 46, 1380.

porphyrins used here. Notes on ultrafast LB deposition have been published elsewhere.6 A purpose-built gas testing chamber6 was used to assess the gas-sensitive optical properties of CAH3 and CAH4 assemblies. The gas mixture (NO2 obtained at 4.6 ppm in dry nitrogen, BOC, Guildford, U.K., and dry nitrogen as a further diluent) was directed into the gas testing chamber that held the CAH4 samples. A WPI Spectromate optical fiber spectrophotometer incorporating a multichannel photodiode array detector was used to record visible absorption spectra of the sample over the wavelength range 350-850 nm. Data were collected every 3 s during the gas exposure/recovery cycles. The exposure occurred at 293 K, and the recovery phase (dry nitrogen only), at 350 K.

Results and Discussion Surface Pressure-Area Isotherms. Figure 1 shows the surface pressure-area (π-A) isotherms of CAH3 and CAH4 at the air-water interface. The extrapolated area per molecule at 19 mN m-1 is similar for both systems, 0.73 nm2 for CAH3 and 0.75 nm2 for CAH4. However, at low surface pressures some differences are clearly observed. First, the takeoff of the isotherm for CAH4 occurs at an area 0.2 nm2 larger than that of CAH3. Also, in the case of the aminophenyl derivative (CAH4) a transition takes place in the surface pressure range 4-9 mN m-1 while for the nitrophenyl derivative (CAH3) no such changes in slope are observed during the compression process. Modeling, using CPK (Corey, Pauling, Koltun) spacefilling models of tetraphenylporphyrins, yields a molecular area of the porphyrin core of ∼2.25 nm2 in flat orientation. However, the measured areas for both porphyrins at high surface pressure are significantly smaller than this value and similar to those proposed for other porphyrins in a edge-on orientation (0.70-0.90 nm2).18-20 This means that either the porphyrin cores are tilted with respect to the plane of the water surface or nonmonolayer aggregate regions are formed within a monolayer host. The different molecular areas at low surface pressure and the different phase behavior suggest that the organization of CAH4 molecules is not the same as for the CAH3 molecules, at least in that pressure range. This is surprising, taking into account the similar chemical structure of both porphyrins (see Chart 1). Extracting structural information about monolayers from isotherm data can be, however, rather error prone (18) Scheidt, W. R. Acc. Chem. Res. 1977, 10, 39. (19) Bull, R. A.; Bulkowski, J. E. J. Colloid Interface Sci. 1983, 92 (1), 1. (20) Chou, H.; Chen, C. T.; Stork, K. F.; Bohn, P. W.; Suslick, K. S. J. Phys. Chem. 1994, 98, 383.

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Figure 2. Brewster angle microscope (BAM) images of monolayers of CAH3 (left, images a-d) and CAH4 (right, images e-h) at different areas/molecule, A, on water. Area, A, and surface pressure, π, as indicated in the images.

as previously discussed by other authors.20,21 In particular, special attention should be paid when dealing with molecular aggregates, since mean molecular areas obtained for such systems can be very different from those of the insulated monomer. In this way, molecular orientation and association of amphiphiles containing extended π-systems can be investigated more directly by spectroscopy rather than by interpreting isotherms. Also, the visualization of film structure by means of microscopy usually offers valuable information that may help to elucidate more clearly the molecular organization at the air-water interface. Brewster Angle Microscopy. Direct observation of the morphology of the monolayers at the air-water interface was obtained by Brewster angle microscopy (BAM). In this technique, the reflectivity of the monolayer is determined by the layer thickness and its refractive index.22 BAM images of CAH3 and CAH4 monolayers at different surface pressures are shown in Figure 2 (a-d, 0, 0.1, 10, 19 mN m-1, respectively, for CAH3; e-h, 0, 0.1, 5, 19 mN m-1, respectively, for CAH4). As can be seen in Figure 2a (CAH3, area/molecule, A ) 1.50 nm2, π ) 0 mN (21) Kroon, J. M.; Sudho¨lter, E. J. R.; Schenning, A. P. H. J.; Nolte, R. J. M. Langmuir 1995, 11, 214. (22) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.

Pedrosa et al.

m-1) and Figure 2e (CAH4, A ) 1.55 nm2, π ) 0 mN m-1), domains with higher reflectivity than surroundings are formed already before compression. When the analyzer was turned, no optical anisotropy in the plane was observed, and hence, these domains can be attributed to an early self-aggregation of the porphyrin molecules that are not uniformly distributed but clustered in domains of different shapes and sizes. It is easily observed that the size of the domains for the CAH3 molecules is bigger than that for CAH4, indicating that the former has a higher tendency to aggregate than the latter. In the range of the horizontal part of the isotherms (π ) 0 mN m-1) the domains grow and come together [see Figure 2b (CAH3, A ) 1.0 nm2, π ) 0.1 mN m-1) and Figure 2f (CAH4, A ) 1.20 nm2, π ) 0.1 mN m-1)], until the takeoff of the surface pressure where the darker area vanishes. At this point, images not shown, a relatively homogeneous film is observed for the CAH4 molecules. However, in the CAH3 monolayer small bright points appear in the boundaries of the domains. These points remain in the CAH3 monolayer during the rest of the compression process (Figure 2c,d) delimiting the domains formed at the beginning. At the moment, this phenomenon is not well understood and is a subject of further research. As the surface pressure is increased, areas with different brightness appear in the CAH4 monolayer (Figure 2g, CAH4, A ) 0.88 nm2, π ) 5 mN m-1). This small difference in brightness can be explained in terms of different molecular organization, i.e., either a different tilt angle or a change in the molecular packing. However, when the monolayer is compressed to higher pressures, the contrast nearly disappears (Figure 2h, CAH4, A ) 0.53 nm2, π ) 19 mN m-1), indicating the absence of domains (or domains of lateral dimensions below the lateral resolution of the BAM used here) in the dense and uniform film in its solid state. Reflection Spectroscopy at the Air-Water Interface. This technique15 has been shown to be a valuable tool to infer the molecular organization of Langmuir monolayers containing dyes molecules.23,24 Figure 3 shows the reflection spectra (∆R) of monolayers of CAH3 and CAH4, respectively, on water at different surface pressures. Also represented in this figure are the spectra of the two porphyrins in solution (dotted line). These consist of a strong Soret absorption band centered at 427 nm together with four additional Q-bands arising between 500 and 700 nm. The CAH3 spectrum in solution is broader than that obtained for CAH4. However, the absorbance of the Soret band as a function of the concentration of the two porphyrins was found to be linear between 2 × 10-6 and 2 × 10-4 M, indicating that these porphyrins exist as monomers in that concentration range. The ∆R spectra of CAH3 and CAH4 at the air-water interface also show the typical morphology of the porphyrin spectrum. However, some remarkable differences between these spectra and those measured in solution are clearly observed. First, the Soret band is significantly broader for the reflection spectra; the full width at half-maximum (fwhm) is 38 nm for CAH3 and 44 nm for CAH4 compared with 25 and 17 nm, respectively, for the solution spectra. Also, the peak wavelength of the Soret band of the Langmuir film spectra, around 460 nm for CAH3 and 440 nm for CAH4, is red shifted with respect to the solution (427 nm). Furthermore, the CAH4 reflection spectra show (23) Ahuja, R. C.; Caruso, P. L.; Mo¨bius, D.; Wildburg, G.; Ringsdorf, H.; Philp, D. J.; Preece, A.; Stoddart, J. F. Langmuir 1993, 9, 1534. (24) Martıń, M. T.; Prieto, I.; Camacho, L.; Mo¨bius, D. Langmuir 1996, 12, 6554.


Figure 3. Reflection spectra ∆R of monolayers of CAH3 (top) and CAH4 (bottom) on water taken at different surface pressures (π ) 0, 5, 10, 15, and 19 mN m-1). For comparison, the spectra measured in solution (solvent, chloroform; concentration, 10-5 M; length of the light path, 0.2 cm) are also plotted, dotted curve.

a shoulder around 460 nm, which is present at every surface pressure. The observed red-shift of the Soret band even at low surface pressures indicates that the intermolecular π-π interaction between porphyrin rings is sufficiently strong that significant preaggregation can occur prior to compression of the film. This is strongly supported by the morphology observed by BAM (see Figure 2a,e), where the attractive interaction between molecules leads to the formation of islands of self-oriented domains. This phenomenon has been also observed in other similar porphyrins.20,21 The shifting and broadening of the absorption spectra of aggregated species in thin films, relative to the monomeric spectrum in solution, has been interpreted by application of exciton models such as the point dipole model proposed by McRae and Kasha25,26 or the extended dipole model proposed by Kuhn and collaborators.27,28 Excellent discussions and comparisons between them can be found in the literature.29-31 Examples of the application of these models to aggregated porphyrins when deposited as LB films are provided elsewhere.32,33 According to the point dipole approximation, and assuming that the transition moments of the chromophores are lying parallel to each (25) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (26) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology; Academic Press: New York, 1964; p 23. (27) Czikkely, V.; Fo¨sterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (28) Kuhn, H.; Fo¨sterling, H. D. Principles Of Physical Chemistry; John Wiley & Sons Inc.: New York, 2000. (29) Czikkely, V.; Fo¨sterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (30) Evans, C. E.; Song, Q.; Bohn, P. W. J. Phys. Chem. 1993, 97, 12302. (31) Kuhn, H. Colloids Surf., A 2000, 171, 3. (32) Schick, G. A.; Schreimann, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (33) Prieto, I.; Pedrosa, J. M.; Martıń-Romero, M. T.; Mo¨bius, D.; Camacho, L. J. Phys. Chem. B 2000, 104, 9966.

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Figure 4. Plots of the corresponding values of ∆Rnorm ) ∆RA for CAH3 (top) and CAH4 (bottom) against wavelength λ, from those represented in Figure 3. The arrows inside indicate increasing surface pressure, π.

other, a red shift is expected when the displacement angle between the transition moments and the line connecting their centers is smaller than 54° (J aggregate), while a larger displacement angle will produce a blue shift (H aggregate). In general, a Scheibe aggregate34,35 (J aggregate) is formed when Coulomb attraction forces between the interacting dipoles outweigh repulsion. Often, this situation is predicted more accurately by the extended dipole approximation, especially at direct contact of the molecules. In the case of the porphyrins studied here, the observed red-shifts in the reflection spectra reveal that the two dyes form J aggregates at the air-water interface. However, the amount of wavelength shift from the monomer band depends on the mutual orientation, the distance, and the aggregation number of the chromophores. Hence, the different red-shifts observed in the two porphyrin spectra can be attributed to different states of J aggregation, where the number of chromophores associated in each aggregate and/or the mutual orientation and the distance between their transition moments are different. This behavior reveals the influence of the side groups in the molecular organization of porphyrins thin films. As the monolayer is compressed, the reflection ∆R increases as a result of the increasing surface density and the shape of the spectra seems to remain unaltered. However, the normalization of the reflection spectra (∆R) at every molecular area of porphyrin (A) could show more clearly changes in the spectral morphology. Figure 4 shows the corresponding product ∆RA ) ∆Rnorm from those represented in Figure 3. A constant value of ∆Rnorm at different surface pressures is expected if all molecules of porphyrin remain at the interface in constant orientation and association. As can be observed, the normalized reflection spectra of CAH3 (Figure 4) do not experience (34) Scheibe, G. Angew. Chem. 1937, 50, 51. (35) Kuhn, H.; Kuhn C. In J-Aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996; pp 1-40.

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any change during the compression process. This supports the view that, after the initial preaggregation, the selforganized domains of aggregated CAH3 molecules are driven together and come into close contact without further reorganization during the rest of the compression process. However, as the CAH4 monolayer is compressed, some changes in the normalized reflection spectra (∆Rnorm) are observed. At 440 nm, ∆Rnorm is nearly constant until the pressure range 5-10 mN m-1, where a decrease of the normalized reflection at this wavelength occurs. Above 10 mN m-1 no further reduction of ∆Rnorm at 440 nm is observed. On the other hand, ∆Rnorm at 460 nm invariably increases as the surface pressure is increased. This behavior allows us to attribute the main peak around 440 nm as well as the shoulder at about 460 nm to two different aggregation states of the CAH4 molecules. This also suggests that the band at 460 nm represents the same aggregation state for both porphyrins. From now on, we will refer to these aggregation states as state 1 (band at 440 nm) and state 2 (band at 460 nm), independently of the type of porphyrin. In the case of CAH4, the decrease of the band at 440 nm and the increase of that at 460 nm indicates a partial change from the aggregation state 1 to the state 2 when the monolayer is compressed. Moreover, this change mainly occurs between 5 and 10 mN m-1 in agreement with the transition observed in the isotherm (see Figure 1). The average orientation of transition moments in monolayers on water can be evaluated from measurements of ∆R with polarized light incident under various welldefined angles.12 However, in this work we have measured the reflection spectra with unpolarized light under normal incidence. The used method, described below, is based on the comparison of the spectrum obtained in solution (where the molecules are oriented at random) with that measured at the air-water interface (where the molecules can be oriented with a certain angle with respect to the incident light). For low values of absorption, the reflection ∆R is given in a reasonable approximation by15

∆R ) 2.303 × 103ΓforientxRi

(1)

where Γ is the surface concentration in mol cm-2, Ri ≈ 0.02 is the reflectivity of the air-water interface at normal incidence, is the extinction coefficient given as L mol-1 cm-1, and forient is a numerical factor that takes into account the different average orientation of the porphyrin chromophore in solution as compared to the monolayer at the air-water interface. In porphyrins the excited state is 2-fold degenerate, with the transition moments aligned along the x-axis and the y-axis, respectively. In solution, where the orientation is at random, the absorption must be proportional to a factor 2/3, because only two of the three components of the transition moments of the porphyrin are interacting with the incident unpolarized radiation. However, at the air-water interface, if the porphyrin molecules are lying flat on the water surface, this factor is 1, since all the transition moments interact with the incident unpolarized radiation. Therefore, in this case forient ) 1/(2/3) ) 3/2. For a general case where only the in-plane components of the transition moments are parallel with respect to the water surface while the other can be tilted, this factor is (1 + sin2(θ))/2, and the orientation factor is

3 forient ) (1 + sin2(θ)) 4

(2)

where θ is the angle between the plane of the transition

Table 1. Average Tilt Angle θ (deg) for the Porphyrins CAH3 and CAH4 at Different Surfaces Pressures π on Water π (mN m-1) porphyrin

1

5

10

15

19

CAH3 CAH4

8 51

8 50

10 27

11 24

9 26

moments and the normal to the air-water interface. Equation 2 is valid only under the assumption of in-plane isotropic distribution and identical orientation with respect to the surface normal for all transition moments. On the other hand, the oscillator strength is defined as28

f)

402.303mec0 NAe

2

∫band dυ ) 1.44 × 10-19∫band dυ (3)

where 0 is the permittivity of vacuum, me the electron mass, e the elementary charge, c0 the speed of light in a vacuum, and NA the Avogadro constant. In eq 3 the numerical factor 1.44 × 10-19 is expressed in mol L-1 cm s. An integration of the Soret band in solution yields the oscillator strength f ) 2.18 for CAH3 and f ) 1.83 for CAH4. Combining eqs 1 and 3, taking into account the relationship between the surface concentration, Γ, in mol cm-2, and the area/molecule, A, in nm2 (Γ ) 1014/ANA) and introducing the value Ri ) 0.02, we obtain

forient )

2.69 × 10-12 f

∫band A∆R dυ ) 2.69 × 10-12 f

∫band ∆Rnorm dυ

(4)

where the numeric factor 2.69 × 10-12 is expressed in nm-2 s. Thus, forient is obtained by the integration of the Soret band in the normalized reflection spectra. In any case, eq 4 is applicable only if the oscillator strength is the same in solution and at the air-water interface. Nevertheless, this latter assumption has been proved to be valid for other porphyrins.24,36 The tilt angle θ can be calculated for any forient by using eq 2. The values thus obtained for CAH3 and CAH4 at different surface pressures are shown in Table 1. These results show that the CAH3 molecules are oriented nearly perpendicular with respect to the water surface, in good agreement with the molecular area from the isotherm (0.73 nm2), and the tilt angle is irrespective of the surface pressure. However, in the case of CAH4 the molecules are less tilted until about 5 mN m-1, where the average orientation starts to become more perpendicular with respect to water surface. From about 10 mN m-1, the tilt angle is again constant until the collapse of the film. Therefore, we can conclude that the reduction of the tilt angle for the CAH4 molecules mainly takes place during the transition observed in the isotherm. In this case, the main molecular area also corresponds to the calculated average orientation at high surfaces pressures. However, it is worth pointing to the fact that the simple determination of the tilt angle from the measured area/molecule can be error prone since the porphyrin molecules selfaggregate just after spreading adopting a certain orientation. This orientation is maintained during the whole compression process for CAH3 and only changes for CAH4 during the transition between 4 and 9 mN m-1. (36) Martıń, M. T.; Mo¨bius, D. Thin Solid Films 1996, 284-285, 663.


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Figure 5. Schematic representation of the possible arrangements for the aggregation states 1 and 2.

The observation of the normalized reflection spectra for CAH3 and CAH4 (Figure 4) along with the calculated tilt angle at different surface pressures (Table 1) suggest that the two aggregation states mentioned above can be related to different molecular orientations with respect to the water surface. In particular, the porphyrin molecules in the state 2 (band at 460 nm) would be close to vertical orientation, while those in the state 1 (band at 440 nm) would be oriented with θ above 50 degrees. In this way, the reduction of the measured tilt angle for CAH4 between 5 and 10 mN m-1 would be in good agreement with the proposed partial change of aggregation state in the same surface pressure range. Figure 5 outlines possible arrangements of the porphyrin molecules in the aggregation states 1 and 2. To explain the observed red-shift in the reflection spectra, the molecules have been drawn slipped with respect to each other. However, the magnitude of this displacement is only qualitative, since no theoretical calculations have been performed. NO2 Gas Sensitivity. The temporal evolution of the UV-visible spectra of two chloroform solutions of CAH3 and CAH4, respectively, when bubbled with an NO2 gas stream (4.6 ppm), is shown in Figure 6. As can be seen, the response for the two porphyrins is similar. A progressive reduction of the Soret band (427 nm) intensity occurs coupled to the growth of two new bands around 471 and 684 nm for CAH3 and 464 and 711 nm for CAH4. A possible mechanism for such spectral changes could imply protonation of the porphyrin ring or a charge-transfer process. However, no strong evidence to discriminate between them has been obtained to date. Further research concerning this subject is currently in progress. The speed of response was also found to be similar in both cases, with saturation times of 100 and 80 s for CAH3 and CAH4, respectively. The absorbance spectrum reverses (not shown) to its original form several hours after switching off the gas stream. It should be noted that as mentioned above, in chloroform solution the porphyrin molecules are in their monomeric form. To investigate the gas-sensing properties of the two porphyrins when deposited as thin films, monolayers of CAH3 and CAH4 were compressed to 19 mN m-1 and

Figure 6. Temporal evolution (t) of the UV-visible spectrum of chloroform solutions of CAH3 (top) and CAH4 (bottom) during exposure to 4.6 ppm NO2 gas (time interval ) 10 s).

transferred to hydrophobic glass substrates by vertical ultrafast deposition.6 The high deposition rate employed (500 mm min-1) by this method leads to inhomogeneous perforated film structures consisting of isolated micrometer-size domains, as evidenced by atomic force microscopy and imaging ellipsometry, data shown elsewhere.6,14 This morphology possesses higher surface area to bulk volume ratios than conventional LB film structures leading to a faster interaction of the analyte gas molecules with the sensing material. LB assemblies (3 excursions) thus prepared of CAH3 and CAH4, respectively, were exposed to 4.6 ppm NO2 (sample temperature ) 293 K). One excursion means two passages of the substrate through the floating monolayer, one downward followed by its subsequent withdrawal. The evolution of the Soret band intensity was followed as a function of time and is represented in Figure 7. Similar changes than in solution (Figure 6) where found in the absorption spectrum of CAH4 film assemblies, but surprisingly, no changes where observed in those of CAH3. Further analysis of the absorption spectra of the LB films during the gas exposure, and its influence in the different response of the two porphyrins to NO2, will be discussed shortly. The kinetics of the gas exposure of the CAH4 assemblies (Figure 7) is characterized by a fast reduction of the Soret band intensity immediately after the NO2 gas stream is switched on, eventually saturating at an absorbance intensity 60% lower than the original level. As the NO2 gas stream is switched off, a dry nitrogen flush is activated while the sample is quickly heated to 353 K. This procedure initiates the rapid recovery, and after a few seconds the original Soret band intensity has been restored. Repeated cycles of gas exposure and recovery do not result in any loss of response. It should be pointed out that conventional deposition rates (0.1-10 mm min-1) or other deposition methods, as solution casting, did not improve the NO2 gas-sensing characteristics of any of the two porphyrins studied, showing slower rates of response and recovery. The stability of the gas sensing properties of

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Figure 7. Absorbance of the Soret band of LB films (3 excursions) of CAH3 and CAH4 as a function of time during exposure to 4.6 ppm NO2.

Figure 8. Absorption spectra of an LB film of CAH4 (3 excursions) before the first exposure to 4.6 ppm NO2, solid line, and after recovery, dashed line. For comparison the reflection spectrum on water (surface pressure π ) 19 mN m-1) is also plotted, dotted line.

CAH4 LB films is being investigated by aging experiments. Future work also concerns the study of different kinetics models as well as gas concentration and temperature dependence. Figure 8 shows the absorption spectra of a 3-excursion LB film of CAH4 before exposure to NO2 (solid line) and after the first exposure-recovery cycle (dashed line). Also shown in this figure is the reflection spectrum of a CAH4 monolayer at the air-water interface, at the same surface pressure as that chosen for the LB transfer (dotted line). As can be seen, the initial preexposure spectrum shows the maximum intensity of the Soret band at the same wavelength (440 nm) as the reflection spectrum of the CAH4 monolayer before deposition (19 mN m-1) and hence implies the same red-shift compared to the solution. However, closer inspection of the spectra reveals that the shoulder at 460 nm detected in the reflection spectrum is reduced in the transferred film spectrum, indicating that the aggregation state 2 assigned to this band is partially lost during the transfer process. However, the molecular packing responsible for the band at 440 nm is maintained during and after the transfer. The spectra of the CAH3 films were also recorded before and after the transfer (data not shown), and a perfect coincidence between them was found.

Pedrosa et al.

Of special interest is, however, the difference observed in the absorption spectrum of the CAH4 assemblies before and after the first exposure-recovery cycle to NO2. As shown in Figure 8, the spectrum measured after the first cycle (dashed line) is narrower, higher, and 4 nm blueshifted as compared to the preexposure spectrum. These differences are attributed to the complete loss of the band at 460 nm during the first exposure to the toxic gas. Successive cycles did not result in further changes of the recovered spectrum. Moreover, the speed of response measured during the first cycle is lower than that measured in subsequent cycles. From the second cycle on, that speed was found to be constant. This behavior led us to propose that the gas-porphyrin interaction for the molecules in the aggregation state 2 (band at 460 nm), is strongly prevented by the closed molecular packing of this arrangement. However, those porphyrin molecules in the aggregation state 1 (band at 440) can quickly interact with the incoming gas molecules allowing relaxation and reorganization within the film (see Figure 5). This could result in a slow penetration and interaction of the gas molecules with the porphyrins in the aggregation state 2. When the recovery phase starts and the gas molecules leave the film, the aggregation state 2 is not recovered, since no external forces are present in the solid film. Only the spontaneously formed aggregation state 1 remains unaltered, and hence, no spectral changes are observed between the subsequent recovered spectra. However, for the CAH3 assemblies, where no response was observed, all the porphyrin molecules are in the aggregation state 2 resulting in no interaction with the gas molecules or too slow to be detected in normal operation times. Also, other factors such as the influence of the electron-withdrawing nature of the nitrophenyl side groups of the CAH3 molecule, over the gas-porphyrin interaction, can be considered. Assuming a charge-transfer interaction mechanism of the electrophilic NO2 gas with the active porphyrin sites, the nature of the nitrophenyl side groups would also be expected to lead to a much reduced gas response. However, this work shows that, in contrast with the behavior observed in solution, the interaction of CAH3 and CAH4 LB films with the NO2 molecules is mainly directed by the molecular aggregation in these assemblies, where a close molecular packing can prevent the gas-porphyrin interaction. These results reveal that special interest should be paid to the molecular organization when designing LB films gas sensors. Conclusions Monolayers of CAH3 and CAH4, respectively, have been prepared at the air-water interface and characterized by reflection spectroscopy and Brewster angle microscopy. The results show that intermolecular interaction between phorphyrins rings directs the molecular organization even at submonolayer coverage. Changes in molecular association and tilt angle of the porphyrin molecules were evaluated by analyzing the reflection spectra at different surface pressures. The porphyrin macrocycles were found to lay nearly perpendicular with respect to the water surface. The monolayers were transferred to hydrophobic glass slides by vertical ultrafast (500 mm min-1) deposition; however, the different degree of aggregation observed for the two porphyrins at the air-water interface was maintained during and after the transfer. The optical absorbance spectrum of chloroform solutions of CAH3 and CAH4, respectively, has been shown to be sensitive to a low concentration of NO2. However, only the LB assemblies of CAH4 showed a response to NO2 gas. This behavior has been attributed to the different molecular organization in


the LB film, where the molecular packing in a highly aggregated state can prevent the gas-porphyrin interaction. Acknowledgment. The authors thank Dr. I. Prieto and Dr. N. Vila-Romeu (University of Vigo, Vigo, Spain)

Langmuir, Vol. 18, No. 20, 2002 7601

for the BAM images and Dr. F. Romero (University of Co´rdoba, Co´rdoba, Spain) for his useful comments. This work was supported by the Spanish DGICyT (Project BQ42001-1792). LA026004U

Influence of Molecular Organization of Asymmetrically Substituted

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