2010
Langmuir 1991, 7, 2010-2012
Photomodification of Surfaces Using Heterocyclic Azides Mark A. Harmer Du Pont, Central Research and Development, Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328 Received May 20, 1991. I n Final Form: August 6, 1991
A novel route for the photomodification of surfaces using heterocyclic azides has been discovered. Photoactivation of heterocyclic azides, to generate highly reactive nitrenes, has been demonstrated to be very effective for the photochemical modification of a wide range of surfaces (from glass, polyethylene, polyimide, polyester, tin oxide, silicon to aluminum). The basic idea is to introduce organic functional groups onto the surface via reaction of highly reactive intermediates. For example, 3-azidopyridine,when photoactivated, reacts with and adds pyridine-type organic functional groups onto the surface. A model for surface modification is proposed. The modified surfaces have been characterized by ellipsometry and X-ray photoelectron spectroscopy. Introduction The ability to tailor surface properties has become a very important and also challenging area of science.' In many applications, for example with polymeric materials, it is necessary to obtain surface properties which are quite different from the inherent nature of the polymer. The aim of the present work, reported herein, has been directed at developing a very general modification procedure that may be applicable to a wide range of surfaces. A novel method for the photochemical modification of a range of surfaces has been discovered. In addition the method lends itself to lithographic applications. The basic idea of the work was to test whether highly reactive intermediates generated at or near a surface would provide a novel and facile route for reacting with and modifying the surface. Heterocyclic azides were chosen since these molecules contain both the photochemically activated azide group and also the organic functional group, namely the heterocycle, which may be grafted on to the surface giving rise to a modified surface with essentially heterocyclic character. In the case of organic azides (upon photolysis with ultraviolet light) a nitrene radical is generated. The nitrene radical formed is extremely reactive and can undergo a multitude of reactions, for example, insertion into C-H, N-H, and 0-H bonds, addition to olefins, proton abstraction reactions to give the corresponding amine, and in the case of aryl azides a number of ring expansion reactions have been observedq2 Herein we describe a general surface modification method which is applicable to a wide range of surfaces such as glass, polyethylene, and polyimides.
Experimental Section The heterocyclic azides 3-azidopyridine,34-azidopyridine,' and 3-azidothiophene6were prepared according to published procedures. Materials were shown to be pure by chemical analysis, mass spectrometry, and ultraviolet-visible absorption spectra. Irradiationof the heterocyclicazides with ultravioletlight yields the corresponding nitrene as the primary product. These types of reactions are very efficient with quantum yields close to one. The light sourcewas a Rayonet photochemical reactor using 254nm bulbs. In all cases the azides were irradiated neat, either in the gas phase or by applying the solution to the surface of the (1) Tazuke, S.; Matoba, T.; Kimura, H.; Oada, T. In Modification of Polymers; Carraher, C. E., Tsuda, M., Eds.; ACS Symposium Series 121; American Chemical Society: Washington, DC, 1979; pp 217-242. (2) Scriven, E. F. V. Azides and Nitrenes;Acedemic Press: New York,
1984. (3)Sawanishi, H.;Tsuchiya, T. Chem. Pharm. Bull. 1987, 35,4101. (4)Dyall, L. K.; Wah, W. M. A u t . J. Chem. 1983,38, 1043. ( 6 ) Spagnolo, P.;Zanirato, P. J. Org. Chem. 1978, 43, 3539.
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substrate and subsequently photolyzing the surface, the system being kept under a nitrogen atmosphere. The gas-phase experiment could be performed by applying a vacuum (about 1W Torr) to a photolysis cell which contains the azide frozen using liquid nitrogen, sealing the cell, and then letting the cell warm up. Upon warming the vapor pressure generated by the azide is sufficient for surface modification. Commerciallyavailable quartz glass, polyethylene, polyimide (Kapton),and poly(tetrafluoroethy1ene)was used as substrates in this study,typically 2 cm2and cleaned via sonication in toluene and acetone. In a typical experiment, 50 pL of the azide was placed in a quartz cell containing the substrate and outgassed as described above. The sample was then irradiated with a Rayonet light source. The resultant modified films were characterized by ultraviolet-visible, infrared, X-ray photoelectron spectroscopy and a Rudolf AutoEL ellipsometer, with a He-Ne laser using a refractive index of 1.48. Results and Discussion We have found that the heterocyclic azides, 3-azidopyridine, 4-azidopyridine, and 3-azidothiophene, upon photolysis are very effective surface modifiers forming films on a wide variety of surfaces. Films can be formed from either the gas phase or surface coating with the liquid followed by photolysis. Very similar results were found for all of the azides investigated; however for brevity the following discussion will depict the use of 3-azidopyridine. Films can be formed within a few seconds and the film thickness can be varied by varying the photolysis time. The film thickness and uniformity have been measured by ellipsometry. In the case of the thicker films these appear as transparent light brown, shown in Figure 1, for both photomodified polyethylene and glass substrates. The films were strongly absorbing below 320 nm with a small shoulder at 500 nm. Surfaces which have been modified include polyethylene, glass,tin oxide, silicon, aluminum, and polyimide forming adherent films which are visually homogeneous. These surfaces were stable to extended washing with a range of solvents and also stable to mechanical abrasion and were resistant to peel testa. In the absence of light or the azide (for example photolysis of pyridine or 3-aminopyridine)no modification occurred. Fluoropolymers (poly(tetrafluoroethy1ene)) have also been photomodified. There is however, a large difference between photomodified fluoropolymer surfaces and the modified surfaces described above. The fluoropolymer modified surfaces were very unstable and simple washing with solvent removed the film. This difference may indicate that in the case of glass and polyethylene surfaces, for example, direct covalent bonding of the heterocycle 0 1991 American Chemical Society
Letters
Langmuir, Vol. 7, No.10, 1991 2011 Scheme I
R Activation
Step.
Nitrene Covalent Attachment to the Surface.
(a)
(c) (d) Figure 1. Photomodification of surfaces using 3-azidopyridine shown for varying photolysis times on polyethylene of (a) 30 s, (b) 1 min, and (c) 10min. Figure Id shows the photomodication of a quartz surface using 3-azidopyridine for a total photolysis time of 10 min. Light source was a Rayonet photochemical reactor.
k
(b)
140
-
0
N
C
-
Insertion
Polymer
Prod; ct
Formation.
Pyridine type modified surface
-~@lu8urbo,
z A
20550 500 450 400 350 300 250 200 150 100 50 Binding Energy / eV
0
Figure 2. XPS spectrum of a quartz surface which has been photochemically modified by the gas-phase photolysis of 3-azidopyridine, photolysis time 2 min.
(via reaction of the nitrene) may occur; however in the case of the fluoropolymer with very inert C-F bonds, no such interaction with the nitrene occurs, as might be expected. The surface-modified film on glass and polyethylene is bonded strongly and covalent attachment is implied due to the criteria of good adhesion and solvent resistance. X-ray photoelectron spectroscopy,XPS, was carried out on films formed via the photolysis of 3-azidopyridine in the gas phase on quartz glass. The data confirm the deposition of nitrogen and carbon onto the surface. This can be seen from the XPS of the modified glass surface which shows the loss of the silicon peaks and gain of the C and N peaks, Figure 2. Some oxidation of the film is evident from an estimated oxygen content of 4% (after correcting for the silica substrate). The atomic percents of C and N were 64.3 and 22, respectively (in addition to Si and 0 at 5.4 and 8.3, respectively). The surface stability with respect to solvent washing was reflected in the XPS measurements where the XPS spectra and signal count did not change when glass and polyethylene-modified surfaceswere washed with solvents (hexane,diethyl ether, dichloromethane, acetone, and water). In the case of the fluoropolymer most of the film was removed. Film thickness and uniformity were measured by use of ellipsometry which had been calibrated using a silicon wafer. Photochemically deposited films of 3-azidopyridine were deposited on the wafer for times of 1, 15,and 60 s. The measured film thickness varied from about 8 to 270 to 750 A for the increasing times. The uniformity appeared quite good and in the case of the 270-L%film varied by about 10% by sampling the film at different regions. Longer photolysis times (typically 30 min) resulted in thick films of about 1 pm as measured directly by scanning electron microscopy. The surfaces appeared smooth and uniform. It appears that the films can be readily varied from the region of monolayer to multilayer by varying the photolysis time.
The precise mechanism of surface modification is not fully understood; however one proposed mechanism which is consistent with known nitrene chemistry2is shown in Scheme I for the photomodification of polyethylene. The initial step involves the photoactivation of the heterocyclic azide to give the nitrene radical. These radicals are generally short-lived (microseconds or less) and will react with a range of nearby functional groups. A likely mechanism for anchoring to the surface may arise due to the insertion or addition of the highly reactive nitrene to surface -CH (polyethylenesurface) and C = C (polyimide surface) bonds. In the case of the more nucleophilic hydroxyl surface it is also possible that the nitrene inserts on to the -OH group. An additional possibility is that the nitrene undergoes rearrangement and ring expansion to give an intermediatecyclic ketenimine. This intermediate would then rapidly react with the surface -OH to give the addition product, containing the ring expanded diazepine. This type of addition chemistry, to nucleophilic groups such as -NH and -OH, is well documented in the phoDue to the diverse tolysis of aryl and heteroryl range of reactions that nitrenes can undergo, other mechanisms cannot be ruled out. From the XPS and ellipsometry the films are multilayer in nature and it is likely that polymeric species are present on the surface. Polymer formation is characteristic of nitrene type reactions. Upon continued photolysis further nitrene radicals could approach the surface and undergo C-H bond insertion (or addition to a double bond on the ring) by reaction with the first layer of molecules which have been photografted on to the surface. The process could be repeated giving rise to a polymeric structure on the surface, Scheme I. Measurement of the infrared spectra of the thicker modified films showed a very broad band in the region of 1600 cm-l with no sharp features. This implies that the films are not made up of a single aromatic group (which would show sharp bands) and may therefore contain both six- and possibly sevenmembered rings (for example diazepines)linked in varying positions. The formation of seven-membered diazepines from the thermolysis of heterocyclic azides has been previously reported.8 Upon photomodification, the resultant surface properties change in a manor which is consistent with the attachment of the heterocycle or a polymeric form of the (6) Doering, W.E.;Mum,R. A. Tetrahedron 1966,22,81. (7) Scriven, E. F. V.; Turnbull, K.Chem. Rev. 1988,88,297. (8) Ohba, Y.;Mataukura,I.; Fukazawa,Y.;Nishiwaki,T.Heterocycles 1985,23, 287.
2012 Langmuir, VoZ. 7, No. 10,1991
Figure3. Photomodifiedaluminumsurfaceto showlithographic potential of the photolytic technique. The azide (3-azidopyridine) was applied to the surface and the surface was photolyzed through a photomask.
heterocycle (in the case of thicker films). A number of interesting properties are found. Hydrophobic surfaces, for example polyethylene, when photomodified with pyridine type functionalities become more hydrophilic (the water contact angle of the treated surface is 60’ as compared to the untreated surface of 95O). In the case of polyethylene-and polyimide-modifiedsurfaces sputtered gold films show much improved adhesion as measured by simple peel tests. Thus metal adhesion is improved. In this case it is likely that gold binds more strongly to the basic pyridine. We have also found that these surfaces display acid-base character. The photomodified surfaces from 3-azidopyridinebehave somewhat like polypyridine on the surface. The modified surfaces can be protonated with acids, and a range of anions can then be exchanged in to these protonated films. Thus, a modified glass surface when treated with dilute acid (0.1 M HCl) was found to pick up anionic dyes, for example bromophenol blue in the anionic form. Without the protonation step, no uptake of anions occurs. The presence of the dye was confirmed by UV-visible spectroscopywhich showed a characteristic absorbance at 580 nm, with a coverage of about lo7 mol cm-2. A similar approach to this has also been used to develop
Letters chemically modified electrodes containing redox active molecules. In particular, a conducting tin oxide electrode has been photomodified with 3-azidopyridine giving rise to a polymeric pyridine type surface which can be protonated by treatment with acid as described above. Redox active anions, for example ferricyanide and hexachloroiridate, can then be anion exchanged onto the modified electrode surface and become attached to the surface. The electrode is extensively washed with water. The resultant electrochemistry shows surface bound peaks of the redox active groups, measured by cyclic voltammetry. Ferricyanide modified electrodes show well-defined oxidation and reduction peaks with a redox potential of about 0.31 V versus the silver/ silver chloride reference electrode, with a peak to peak separation of about 80 mV. Surface coverage, estimated from the current flow, corresponds to about 107-108 mol cm-2 consistent with multilayer coverage and agrees quite well with the adsorbed dye results. This anion exchange behavior is similar to previous reports on anion exchanged redox couples on poly(viny1pyridine)modified electrodes. Further details on the applications of this technique will be published separately. Photomodification of surfaces using heterocyclic azides can also be performed imagewise, indicating that this method may be used for lithographic applications. This is shown in Figure 3, for an aluminum surface. The heterocycle, 3-azidopyridine, was applied to a surface and the resultant surface was photolyzed with a photomask. After the film was washed, a well-defined image was obtained. Scanning electron microscopy has shown that resolution between 1and 5 pm has been obtained. In summary we have shown that generating highly reactive intermediates, in this case nitrenes from the corresponding heterocyclic azides, at or near a surface, provides a simple and rapid route for surface modification. A key point to note here is the generality of the reaction whereby various organic groups (in this case pyridine and thiophene) have been bonded to a range of surfaces, from glass with hydroxyl groups to polymers with surface CH groups. The modified surface displays properties consistent with that of the bound heterocycle.
Acknowledgment. The technical assistance of D. Wipf is appreciated. Further acknowledgment is due to L. Firment and D. Prior for XPS measurements.
