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Evanescent Wave Excited Fluorescence from Self-Assembled Phthalocyanine Monolayers Tim R. E. Simpson, David J. Revell, Michael J. Cook, and David A. Russell* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. Received August 2, 1996X Novel phthalocyanine (Pc) molecules have been designed and synthesized to incorporate a thiol moiety for the formation of monolayer films onto gold-coated optical waveguides via the technique of self-assembly. The thiol moiety is connected to the Pc macrocycle by either a (CH2)11 or (CH2)3 hydrocarbon chain. Both Pc SAMs have been characterized on the metal surface using reflection absorption infrared (RAIR) spectroscopy. The RAIR spectra suggest that the two Pc molecules lie in a different orientation with respect to the gold surface. This behavior can be rationalized with consideration of the different structures (the different alkyl connecting chain lengths and different macrocyclic peripheral side chains) of the two Pc molecules. The fluorescence emission spectrum of each of the Pc SAMs has been obtained by exciting each monolayer along the longitudinal axis of the optical waveguide via laser-induced evanescent wave stimulation. The use of the longer mercaptoalkyl connecting chain appears to inhibit quenching of the electronically excited state through energy transfer to the metal layer. It is thought that the presence of peripheral alkyl side chains (six hexyl or seven decyl chains, respectively) minimizes self-quenching in the single-component SAM film. It has also been shown that the fluorescence signal of the (CH2)11 mercaptoalkyl Pc SAM can be reversibly quenched by exposure to gaseous nitrogen dioxide (NO2). The decrease in fluorescence intensity has been related to increasing NO2 concentration by a Stern-Volmer relationship. The Pc SAM can detect concentrations of NO2 down to 10 ppmv with no interferent effect seen from other environmentally relevant gases: carbon monoxide and carbon dioxide. These results suggest that SAMs of specifically designed materials, such as Pc’s, may be used as sensitive receptor molecules to form the basis of a novel technology for the development of optical chemical sensors for gaseous species.

Introduction The deposition of self-assembled monolayers (SAMs) has become an established procedure for obtaining ultrathin films of organic compounds chemically bonded to the surface of a solid substrate.1 SAMs are commonly constructed by reacting organosilicon derivatives at hydroxylated glass or silicon surfaces2 and mercaptans or disulfides at gold3 or certain other metal surfaces. Much of the current research in this area is now directed toward developing applications of this deposition technology; e.g., in the area of chemical sensing, SAMs have been used as the recognition medium in electrochemical sensors,4 in surface acoustic wave devices,5 in piezoelectic devices,6 and in biosensing applications.7 We are currently interested in using the SA monolayer deposition technique for the formation of reproducible films of a series of phthalocyanine (Pc) derivatives and have obtained the first examples of Pc SAMs on hydroxylated

surfaces8 and on gold-coated glass substrates.9 In this paper we report for the first time the detection of fluorescence from single-component Pc SAMs on goldcoated optical waveguides, excited by means of the evanescent wave. SAMs were prepared using the Pc derivatives 1 and 2. The long mercaptoalkyl chain (C11)

* Author to whom correspondence should be addressed. E-mail: D. [email protected] X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (2) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674-676. (3) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (b) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560-6561. (d) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (4) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688-691. Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883-5884. (5) Moore, L. W.; Springer, K. N.; Shi, J.-X.; Yang, X.; Swanson, B. I.; Li, D.; Adv. Mater. 1995, 7, 729-731. (6) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Go¨pel, W. Science 1994, 265, 1413-1415. Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1994, 116, 3597-3598. (7) Go¨pel, W.; Heiduschka, P. Biosens. Bioelectron. 1995, 10, 853883.

on 1 was introduced to limit quenching by the metal surface, and the six hexyl chains were incorporated to (8) Cook, M. J.; Hersans, R.; McMurdo, J.; Russell, D. A. J. Mater. Chem. 1996, 6, 149-154. (9) (a) Simpson, T. R. E.; Russell, D. A.; Chambrier, I.; Cook, M. J.; Horn, A. B.; Thorpe, S. C. Sens. Actuators, B 1995, 29, 353-357. (b) Chambrier, I.; Cook, M. J.; Russell, D. A. Synthesis 1995, 1283-1286.

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hinder close overlap of adjacent molecules and hence minimize self-quenching. The methyl group was introduced for both synthetic convenience and to facilitate structural characterization by 1H NMR spectroscopy. The second Pc derivative, 2, incorporates a shorter mercaptoalkyl chain (C3) and seven decyl peripheral side chains. The fluorescence characteristics of 1 and 2 self-assembled on the gold surface have been compared and related to their structural properties. In further aspects of this research, the quenching of the fluorescence signal when the Pc SAM of 1 was exposed to nitrogen dioxide (NO2) gas and its subsequent recovery after purging with nitrogen gas have been investigated. Experimental Section Reagents. The synthesis and characterization of the two Pc molecules used in this study, i.e., 1,4,8,11,15,18,-hexahexyl-22methyl-25-(11-mercaptoundecyl)phthalocyanine (1) and 1,4,8,11,15,18,22-heptadecyl-25-(3-mercaptopropyl)phthalocyanine (2), have been reported elsewhere.9b All other chemicals were obtained from the Aldrich Chemical Company, unless otherwise stated, and used as received. Solution Electronic Spectroscopy Data. UV-visible absorption spectra and fluorescence excitation and emission spectra were obtained from a 1.56 × 10-6 mol dm-3 cyclohexane solution of 1 and a 1.72 × 10-6 mol dm-3 cyclohexane solution of 2 using an Hitachi U3000 spectrophotometer and an Hitachi F4500 spectrofluorimeter, respectively. Formation of Pc Self-Assembled Monolayers. Glass microscope slides (BDH Ltd.) were used as the substrate for the SAMs. The slides were cleaned using a solution of aqueous KOH in methanol (100 g of KOH was dissolved in 100 mL of Millipore water and then diluted with methanol to a total volume of 250 mL). The slides were rinsed with fresh Millipore water and then dried in a stream of refluxing propan-2-ol vapor. The cleaned, dry slides were then coated with a gold/chromium layer, the thickness of which was dependent upon the spectroscopic technique employed. The Pc SAM was then formed on the freshly prepared substrate. For reflection absorption infrared (RAIR) spectroscopic characterization, ≈500 nm of gold was deposited by thermal evaporation under vacuum onto the glass slides. The Pc SAMs were formed by immersion of the gold-coated glass microscope slides in a 1.02 × 10-4 mol dm-3 solution of 1 or 2.60 × 10-4 mol dm-3 solution of 2 in cyclohexane for a period of 24 h. Reference substrates, with no SAM, were immersed in cyclohexane, again for 24 h. The gold substrates were then washed in fresh cyclohexane and dried at 25 °C for 2 h. For the fluorescence experiments, ≈5 nm of chromium was deposited on the cleaned optical slide (to ensure adhesion of the gold onto the glass) followed by an ≈20 nm gold layer. Both metals were deposited by thermal evaporation under vacuum using an Edwards 306 vacuum evaporator. The Pc SAMs were formed by immersion of gold-coated glass microscope slides in a 1.02 × 10-4 mol dm-3 solution of 1 or a 2.60 × 10-4 mol dm-3 solution of 2 in cyclohexane for a period of 24 h. Reference substrates, with no SAM, were immersed in cyclohexane for 24 h. The gold substrates were then washed in fresh cyclohexane and dried at 25 °C for 2 h. RAIR Spectroscopic Characterization of the Pc SAMs. RAIR spectra were obtained at an incidence angle of 85° using p-polarized light using a Bio-Rad FTS 40 FT infrared spectrometer coupled with a Spectra-Tech FT85 specular reflectance unit. Spectra were obtained from the co-addition of 1024 scans of 4096 data points, giving a resolution of 4 cm-1. Fluorescence Spectroscopy of the Pc SAMs. The instrumentation used to obtain fluorescence data from the Pc SAMs is shown in Figure 1. The fluorescence data were obtained by exciting the Pc SAM using a 3 mW p-polarized laser diode at 670 nm (Model VLM2-3L, Applied Laser Systems). The glass substrate was used as the optical waveguide, and the laser was positioned so that the beam entered the glass slide along one edge at an angle of approximately 30° to the edge normal. The laser radiation was transmitted along the slide by total internal reflection. Fluorescence from the SAM was excited by means of

Figure 1. Instrumental configuration for the excitation of fluorescence from the Pc SAMs including the gas flow-throughcell. the evanescent wave along the length of the optical waveguide. The emitted fluorescence was passed into the entrance slit of a 600 mm monochromator (Monospek 600; 600 lines/mm plane grating blazed at 750 nm, Hilger Watts Ltd.). The signal was detected by a red-enhanced (germanium diode) photomultiplier tube (C31304A; RCA/TE-182TS-RF, Products for Research) cooled to -20 °C and a photon counter (Model 9511; Brookdeal Electronics). Fluorescence Quenching by NO2 Gas. A gas cell with a glass optical window was clamped onto the surface of the optical waveguide (shown in Figure 1). The optical window was placed in contact with the monochromator entrance slit. A gas blender (Signal Corps., Model 852) was used to provide the various concentrations of NO2 in nitrogen (oxygen free nitrogen, OFN grade, BOC Ltd.) which were passed over the surface of the Pc SAM on the gold-coated optical waveguide. The NO2 gas was supplied at a concentration of 1000 ppmv (part per million per volume) diluted in oxygen-free nitrogen. In practice the gas would, therefore, be a mixture of nitrogen oxides, with nitrogen dioxide the major component. Other possible interferent gases, e.g. carbon dioxide, were blended in nitrogen and passed over the SAM optical waveguide. Variations of fluorescence intensity were monitored as a function of gas concentration.

Results and Discussion Electronic Spectral Properties. The UV-visible absorption spectrum of 1 in cyclohexane solution was typical of a nonmetalated highly substituted Pc molecule; i.e., it has a split Q-band with maxima at 692 and 728 nm. The emission spectrum, using 728 nm excitation, gave a maximum at 740 nm. Similar absorption and fluorescence excitation and emission spectra were obtained for 2. The molar decadic absorption coefficient (), in the cyclohexane solution, at 670 nm (the wavelength of light used to excite the Pc SAMs) was ca. 2.8 × 104 mol-1 dm3 cm-1 and ca. 3.2 × 104 mol-1 dm3 cm-1 for 1 and 2, respectively. RAIR Characterization of the Pc SAMs. The characterization of both Pc SAMs was performed using reflection-absorption infrared spectroscopy (RAIRS) on the gold-coated glass optical waveguides. The RAIR spectrum of 1 is shown in Figure 2. Figure 2a shows the strong absorption bands of3b CH3 (νas) at 2959 cm-1, CH2 (νas) at 2925 cm-1, CH3 (νs) at 2877 cm-1, and CH2 (νs) at 2852 cm-1. Additionally, there is a weak band at 3300

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Figure 3. Partial reflection absorption infrared (RAIR) spectrum of the SAM of 2 on a gold-coated optical waveguide.

Figure 2. Reflection absorption infrared (RAIR) spectrum of the SAM of 1 on a gold-coated optical waveguide.

cm-1 which has been previously assigned as the NH stretch.10a In Figure 2b the following bands have been assigned:10b 1512 cm-1, NH (δip); 1482 cm-1 (s), ring stretching; 1461 cm-1, δ CH2; 1414 cm-1, ring stretch; 1303 cm-1; ring stretch; 1022 cm-1, skeletal mode of the central ring. The bands at 1146 and 1095 cm-1 have both been assigned to the CdN stretch involving the azo nitrogens;10c however, the band at 1146 cm-1 has also been assigned to C-H (δip).10d Figure 3 shows the partial RAIR spectrum of 2. The spectrum shows the absorption bands of CH3 (νas) at 2966 cm-1, CH2 (νas) at 2920 cm-1, CH3 (νs) at 2877 cm-1, and CH2 (νs) at 2851 cm-1. However, the band at 3300 cm-1 assigned to the NH stretch is not observed for the Pc SAM of 2. Additionally, while the RAIR spectrum of 2 between 900 and 1600 cm-1 shows the δ CH2 band at 1465 cm-1, the bands associated with the Pc macrocycle are not present in the spectrum (data not shown). A definitive assignment of the orientation of the Pc molecules on the surface of the polycrystalline gold cannot be made from the RAIR spectra. However, the RAIR data shown in Figures 2 and 3 suggests that the two Pc molecules may have self-assembled in a somewhat different orientation with respect to the gold surface. With consideration of the metal surface selection rule, the presence of the aromatic ring stretching modes and the NH stretching band indicates that the Pc macrocycle of 1 does not lie parallel to the gold surface. However, the absence of these bands in the RAIR spectum of 2 suggests that this molecule may lie parallel to the gold surface. The presence of both symmetric and asymmetric CH2 stretches of both Pc SAMs also suggests that the peripheral (10) (a) Siderov, A. N.; Kotlyar, I. P. Opt. Spectrosc. 1961, 11, 92-96. (b) Stymne, B.; Sauvage, F. X.; Wettermark, G. Spectrochim. Acta 1979, 35A, 1195-1201. (c) Shurvell, H. F.; Pinzuti, L. Can. J. Chem. 1966, 44, 125-136. (d) Starke, M.; Wagner, H. Z. Chem. 1969, 9, 193-195.

alkyl chains are tilted in relationship to the parallel and perpendicular planes of the metal surface. Fluorescence Spectroscopy of the Pc SAMs. The fluorescence spectra of the Pc SAMs could not be obtained using the available conventional spectrofluorimeters, presumably due to the thickness of the film. Instead, the fluorescence spectra of both Pc SAMs on gold-coated waveguides were obtained by exciting fluorescence, using a 670 nm diode laser, via the evanescent wave, along the longitudinal axis of the optical waveguide (Figure 1). The fluorescence emission spectra obtained (maxima at 793 nm) are shown in Figure 4a. It is clear that there is a large “red-shift” of the fluorescence maxima of both Pc SAMs as compared to their solution spectra. Such shifts have been previously observed in the UV-visible absorption spectra of highly ordered Pc Langmuir-Blodgett (LB) films11 and SAMs containing naphthalene chromophores.12 Monolayers containing naphthoyl chromophores selfassembled in a disordered manner gave spectra similar to those obtained from solution.13 These results imply that the Pc SAMs formed in this work are packed in an ordered manner, which leads to the red-shifted fluorescence. While a fluorescence spectrum could be obtained from both Pc molecules formed as SAMs, it is clear that the fluorescence intensity of 1 is far greater than that of 2. A number of studies have shown that the excited state of a fluorophore positioned near a metal surface is quenched via nonradiative electronic energy transfer processes.14 However, it has been previously shown that fluorescence information can be obtained from molecules self-assembled onto a metal surface through the use of a C10 hydrocarbon spacer between the thiol moiety and the fluorophore.15 (11) Cook, M. J. J. Mater. Chem. 1996, 6, 677-689. (12) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3002-3008. (13) Durfor, C. N.; Turner, D. C.; Georger, J. H.; Peek, B. M.; Stenger, D. A. Langmuir 1994, 10, 148-152. (14) (a) Kuhn, H. J. Chem. Phys. 1970, 53, 101-108. (b) Chance, R. R.; Prock, A.; Silbey, R. J. Chem. Phys. 1975, 62, 2245-2253. (c) Chance, R. R.; Prock, A.; Silbey, R. In Advances in Chemical Physics; Prigogine, I., Rice, S. A., Eds.; Wiley: New York, 1978; Vol. 37, pp 1-65. (d) Campion, A.; Gallo, A. R.; Harris, C. B.; Robota, H. J.; Whitmore, P. M. Chem. Phys. Lett. 1980, 73, 447-450. (e) Adams, A.; Rendell, R. W.; West, W. P.; Broida, H. P.; Hansma, P. K.; Metiu, H. Phys. Rev. B 1980, 21, 5565-5571. (15) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1994, 116, 74137414.

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peripheral C10H21 substituent side chains on 2) sterically hindered the overlap of the Pc macrocycles and thereby prevented self-quenching of the fluorescence signal. Exposure of the Pc SAM of 1 to NO2 Gas. The intensity of the fluorescent signal of a SAM of 1 was shown to be quenched as a function of exposure to the environmentally important NO2 gas. Figure 4b shows the decrease of the fluorescence signal when 200 ppmv of NO2 was passed over the Pc SAM surface. Quantifiable decreases in the fluorescence signal could be detected for concentrations of NO2 gas down to a level of ca. 10 ppmv. The data were shown to fit the Stern-Volmer (SV) quenching model when measured between 0 and 200 ppmv. The SV relationship obtained in this study can be expressed by the following equation:

I0/IQ ) 1 + KSV[NO2]

Figure 4. (a) Fluorescence spectra of the SAM of 1, 2, and background (gold-coated waveguide) and (b) fluorescence spectra obtained from a SAM of 1 before and after interaction with 200 ppmv of NO2 gas. The fluorescence emission spectra were obtained using evanescent wave excitation at 670 nm.

Such a spacer would separate the fluorophore from the metal surface by ca. 14 Å. In practice the distance between fluorophore and metal surface is probably less than 14 Å, as it is known that the hydrocarbon chain of alkanethiols self-assembled onto polycrystalline gold (the type of metal surface used in this study) is tilted between 20 and 30° from the normal.3b,16 This would suggest that the distance between fluorophore and metal surface would be ca. 1213 Å. A tilt from the normal of between 20 and 30° is consistent with the RAIR spectroscopic data obtained from the Pc SAMs. With such a tilt, the Pc macrocycle would be ca. 13 or 4 Å from the gold surface for 1 and 2, respectively. Therefore, it is probable that the metal surface more efficiently quenches the excited state energy of 2 as compared with that of 1. Indeed, while we have data for only two molecules with different chain length spacer groups self-assembled to the gold surface, it would appear that the fluorescence emission from the Pc molecules is quenched in an approximate d-3 relationship (where d is the distance in centimeters between the fluorophore and the metal surface), as predicted by Chance et al.14b In previously reported studies it has been necessary to “dilute” the fluorescent molecule on the surface of the metal by the formation of a mixed SAM12,15,17 to prevent selfquenching. While a characteristic of SAMs is their high degree of dense packing,18 such dilution proved unnecessary in this study as fluorescence emission spectra were obtained from both Pc SAMs. It is thought that the six peripheral C6H13 substituent side chains of 1 (seven (16) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141-149. (17) Chen, S. H.; Frank, C. W. Langmuir 1991, 7, 1719-1726. (18) Bain, C. D.; Evans, S. D. Chem. Br. 1995, 31, 46-48.

where I0 is the intensity of fluorescence at zero concentration of quencher, IQ is the intensity of fluorescence at concentration Q of the NO2 quencher, and KSV is the SternVolmer coefficient. By plotting I0/IQ versus [NO2], a value of 0.004 834 mg-1 dm3 was obtained for KSV. While we have yet to measure the fluorescence lifetime (τf) of a SAM of 1 on the gold surface, a value of τf for a metal-free Pc molecule in solution has been reported19 as 6.5 ns. Therefore a value of the quenching rate coefficient (kq) can be calculated as 7.44 × 105 mg-1 dm3 s-1. This value of kq would represent a lower limit, as the value of τf of the SAM of 1 on the gold surface would be expected to be shorter than the corresponding solution lifetime. The response time for the equilibrium fluorescence intensity response for 200 ppmv of NO2 was 20 min. The interaction of the NO2 could be reversed by flushing the SAM with nitrogen gas with a consequent increase in the fluorescence signal. The time taken for recovery of the original signal was dependent on the time of SAM exposure to the NO2 gas; e.g., for a 20 min exposure of 200 ppmv of NO2 a 75% fluorescence intensity signal recovery took 15.5 h, while a 7 min exposure of the same gas concentration took 1.5 h for a 75% signal recovery. With continual use it was found that a gradual deterioration of response was observed. Typically each SAM could be reused at least 15 times before deterioration was complete. After such use, one Pc SAM was subjected to a high NO2 concentration, viz. 1000 ppmv. The fluorescence signal of this Pc SAM could not be regenerated. In related studies, using LB films of Pc molecules, it has been shown that high concentrations of NO2 irreversibly bleach the Pc molecule.20 When NO2 gas (1000 ppmv diluted in nitrogen) was bubbled through a cyclohexane solution of 1 (1.23 × 10-3 mol dm-3) for 1 h, the Pc was bleached. UV-visible and infrared evidence confirmed that the Pc had been oxidized to phthalimide derivatives, similar to the oxidation of a structurally related Pc molecule.21 However, for the typical exposure time of the SAM of 1 to the NO2 gas (0-20 min) at a concentration of 0-200 ppmv, there was no evidence of the characteristic carbonyl absorption band of the imide moiety oxidation product, as determined by RAIR spectroscopy. This would suggest that at the lower NO2 concentrations investigated in this study, i.e. between 0 and 200 ppmv, the electron acceptor gas quenches the fluorescence intensity of the Pc mono(19) Chahraoui, D.; Valat, P.; Kossanyi, J. Res. Chem. Intermed. 1992, 17, 219-232. (20) Zhu, D. G.; Petty, M. C.; Harris, M. Sens. Actuators, B 1990, 2, 265-269. Souto, J.; de Saja, J. A.; Gobernado-Mitre, M. I.; Rodriguez, M. L.; Aroca, R. Sens. Actuators, B 1993, 15, 306-311. (21) Cook, M. J.; Chambrier, I.; Cracknell, S. J.; Mayes, D. M.; Russell, D. A. Photochem. Photobiol 1995, 62, 542-545.

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layer through a partially reversible chemisorption interaction rather than an oxidative breakdown of the parent molecule. There is a large amount of literature which discusses the formation of Pc LB films for the development of an NO2 gas chemical sensor. This literature, which has been the subject of a recent review,11 has assessed the selectivity of such sensors in relation to the structure of the Pc molecule. It is apparent that the sensitivity of a particular Pc molecule toward NO2 can be enhanced through the insertion of metals, such as lead and copper, into the center of the Pc macrocycle and that some degree of selectivity can be achieved by the use of highly substituted Pc molecules, such as 1 and 2. While it was not the intention of this study to repeat the previously reported exhaustive literature on the effect of interferents on structurally related highly substituted Pc molecules (formed as LB films), a preliminary interferent characterization was conducted. It was established that the fluorescence signal of the Pc SAM was not affected by concentrations of the atmospherically relevant gases carbon dioxide or carbon monoxide up to a level of 1000 ppmv. Other studies have shown that structurally related Pc’s, while sensitive to NO2, were not adversely affected by other electron acceptor gases (SO2 or Cl2) or electron donor gases such as NH3 at occupational health levels.11 Therefore, it is clear that the nature of the peripheral groups on the Pc macrocycle can impart a degree of selectivity while the sensitivity of the SAM could be tuned to the analyte of interest through the further design and synthesis of derivative molecules. Conclusions In summary, we have shown that phthalocyanine molecules can be formulated as self-assembled monolayers

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on gold-coated optical waveguides. RAIR spectroscopic characterization of two such molecules suggests that the structural properties of the Pc have a profound bearing on the manner in which the molecule self-assembles on the gold surface. Fluorescence emission spectra from both of the Pc SAMs were obtained by exciting the monolayer via laser-induced evanescent wave stimulation. The fluorescence emission of the Pc SAM of 1 was of greater intensity as compared with that of 2. The intensity difference between the two Pc SAMs is probably due to the close proximity of the Pc nucleus of 2 to the metal surface which would quench the excited state energy. Quenching of the fluorescence signal of the Pc SAM of 1 on interaction with gaseous NO2 was probed and related to the concentration of the target analyte. These results suggest that self-assembled monolayers of phthalocyanines can be used as sensitive recognition surfaces and could form the basis of a novel technology for the development of optical chemical sensors for gaseous species. Acknowledgment. The authors would like to thank Dr. I. Chambrier (supported by EU HCM contract CHRX CT94-0558) for the gift of the Pc materials used in this work, the EPSRC (formerly SERC) for the provision of studentships to T.R.E.S. and D.J.R., and Dr. S. C. Thorpe of the Health and Safety Executive, Sheffield, U.K. for partial financial support of this work. Additionally, Dr. A. B. Horn (University of York) is acknowledged for his helpful discussions throughout this work. LA960763T