1412
Langmuir 2005, 21, 1412-1415
Diels-Alder Chemistry on Alkene Functionalized Films Vincent Roucoules, Corinne A. Fail, Wayne C. E. Schofield, Declan O. H. Teare, and Jas Pal S. Badyal* Department of Chemistry, Science Laboratories, Durham University, Durham DH1 3LE, England Received August 13, 2004. In Final Form: November 17, 2004 A substrate-independent method has been devised for ring formation at solid surfaces. This entails the aminolysis reaction of allylamine with maleic anhydride pulsed plasma polymer films to yield terminal alkene groups at the surface. Subsequent exposure to 1,3-cyclohexadiene leads to a Diels-Alder type (4 + 2) cycloaddition reaction to give a mixture of endo- and exo-bicyclo[2.2.2]oct-2-ene rings.
1. Introduction The Diels-Alder reaction is a well-established solutionphase method for making complex ring molecules in a single step.1 Recent studies have demonstrated that this type of cycloaddition reaction can also take place at a solid surface, thereby making it of potential interest for a number of technological applications. For instance, the (100) planar faces of silicon,2 germanium,3 and diamond4,5 exhibit dienophile behavior which can facilitate controlled growth of ordered organic layers onto these surfaces. Other examples of surface Diels-Alder chemistry include selfassembled monolayers (SAMs),6,7 the immobilization of dyes,8 and direct reaction with polymer substrates containing polymer repeat units which participate in the Diels-Alder reaction.9-11 However in each case, a specific preparative chemical procedure is required which limits wide-scale applicability. In this paper we describe a simple, multisubstrate approach based on pulsed plasma deposited maleic anhydride thin films. Pulsed plasmachemical surface functionalization comprises two distinct reaction regimes corresponding to monomer activation and the generation of surface sites during the duty cycle on-period (via UV irradiation, ion, or electron bombardment) followed by conventional polymerization during the subsequent offtime (in the absence of any UV-, ion-, or electron-induced damage). Advantages associated with this method include the fact that the gaseous and reactive nature of the electrical discharge means that it is applicable to a whole host of materials and complex geometries (e.g. microspheres, fibers, tubes, etc.). Examples devised in the past * To whom correspondence should be addressed. E-mail:
[email protected]. (1) March, J. Advanced Organic Chemistry, 4th ed.; Willey/Interscience: New York, 1992. (2) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 1998, 108, 6803. (3) Lee, S. W.; Nelen, L. N.; Ihm, H.; Scoggins, T.; Greenlief, C. M. Surf. Sci. 1998, 410, L773. (4) Wang, G. T.; Bent, S. F.; Russell, J. N. Jr.; Butler, J. E.; D’Evelyn, M. P. J. Am. Chem. Soc. 2000, 122, 744. (5) Hossain, M. Z.; Aruga, T.; Takagi, N.; Tsuno, T.; Fujimori, N.; Ando, T.; Nishijima, N. Jpn. J. Appl. Phys. 1999, 38, L1496. (6) Yousaf, M. N.; Mrksich, M. J. Am. Chem. Soc. 1999, 121, 4286. (7) Yousaf, M. N.; Chan, E. W. L.; Mrksich, M. Angew. Chem., Int. Ed. 2000, 39, 1943. (8) Tanaka, M. Eur. Pat. Appl. 655345, A1 1995. (9) Patel, H. S.; Patel, V. C. Eur. Polym. J. 2001, 37, 2263. (10) Liaw, D.-J.; Huang, C.-C.; Wu, P.-L. Polymer 2001, 42, 9371. (11) Hodge, P.; Rhodes, C. M.; Uddin, R. Polymer 2001, 42, 8549.
include the following: perfluoroalkyl,12 epoxide,13 anhydride,14 carboxylic acid,15 cyano,16 hydroxyl,17 and amine18 functionalized surfaces. The overall approach in the present study entails allylamine undergoing aminolysis with maleic anhydride pulsed plasma polymer films to yield alkene functionalized surfaces,19-21 Scheme 1 (step A). The generated covalent amide linkages are subsequently converted into cyclic imide groups upon heating, Scheme 1 (step B).13,22 These surfaces are then shown to be suitable dienophiles for the Diels-Alder (4 + 2) cycloaddition reaction with 1,3cyclohexadiene to give mixtures of endo- and exo-bicyclo[2.2.2]oct-2-ene groups, Scheme 1 (steps C or D). 2. Experimental Section Briquettes of maleic anhydride (Aldrich 99%) were ground into a fine powder and loaded into a sealed glass tube. Plasma polymerization experiments were carried out in an electrodeless cylindrical glass reactor (4.5 cm diameter, 460 cm3 volume, base pressure of 5.2 × 10-3 mbar, with a leak rate below 1.0 × 10-10 kg s-1) enclosed in a Faraday cage. The reactor was fitted with an externally wound copper coil (4 mm diameter, 9 turns, spanning 8-15 cm from the gas inlet), a thermocouple pressure gauge, and a 30 L min-1 two-stage rotary pump attached via a liquid nitrogen cold trap. All joints were grease-free. An L-C matching network was used to match the output impedance of the RF generator (13.56 MHz) to that of the partially ionized gas load by minimizing the standing wave ratio (SWR) of the transmitted power. Pulsed plasma polymerization experiments entailed triggering the RF power supply from a signal generator. The pulse width and amplitude were monitored with an oscil(12) Coulson, S. R.; Woodward, I. S.; Brewer, S. A.; Willis, C.; Badyal, J. P. S. Chem. Mater. 2000, 12, 2031. (13) Tarducci, C.; Brewer, S. A.; Willis, C.; Badyal, J. P. S. Chem. Mater. 2000,12, 1884. (14) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S. Chem. Mater. 1996, 8, 37. (15) Hutton, S. J.; Crowther, J. M.; Badyal, J. P. S. Chem. Mater. 2000, 12, 2282 (16) Tarducci, C.; Schofield, W. C. E.; Brewer, S.; Willis, C.; Badyal, J. P. S. Chem. Mater. 2001, 13, 1800. (17) Tarducci, C.; Schofield, W. C. E.; Brewer, S. A.; Willis, C.; Badyal, J. P. S. Chem. Mater. 2002, 14, 2541. (18) Rimsch, C. L.; Chem, X. L.; Panchalingam, V.; Savage, C. R.; Wang, J. H.; Eberhart, R. C.; Timmons, R. B. Polym. Prepr. 1995, 209, 141. (19) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S. Chem. Mater. 1996, 8, 37. (20) Evenson, S. A.; Fail, C. A.; Badyal, J. P. S. Chem. Mater. 2000, 12, 3038. (21) Schiller, S.; Hu, J.; Jenkins, A. T. A.; Timmons, R. B.; SanchezEstrada, F. S.; Knoll, W.; Fo¨rch, R. Chem. Mater. 2002, 14, 235. (22) Sutherland, I.; Sheng, E.-S.; Brewis, M.; Heath, R. J. J. J. Mater. Chem. 1994, 4, 683.
10.1021/la0479657 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/14/2005
Diels-Alder Chemistry on Alkene Films
Langmuir, Vol. 21, No. 4, 2005 1413
Scheme 1. Reaction of Maleic Anhydride Plasma Polymer with Allylamine and 1,3-Cyclohexadiene.
Figure 1. Infrared spectra: (a) reflection-absorption spectra of maleic anhydride plasma polymer; (b) absorbance spectra of allylamine; (c) reflection-absorbance spectra of maleic anhydride plasma polymer functionalized with allylamine; (d) reflection-absorbance spectra of maleic anhydride plasma polymer functionalized with allylamine heated at 120 °C; (e) absorbance spectra of 1,3-cyclohexadiene; (f) reflection-absorbance spectra of d reacted with 1,3-cyclohexadiene; and (g) reflection-absorbance spectra of c reacted with 1,3-cyclohexadiene. loscope. Prior to each experiment, the reactor was submitted to a 30 min high-power (50 W) air plasma cleaning treatment. Next, the chamber was vented to air and a piece of silicon substrate (MEMC Electronic Materials, S.p.a., +99.99%) placed into the center, followed by evacuation back down to base pressure. Maleic anhydride vapor was then introduced into the reactor at a constant pressure of 2.6 × 10-1 mbar and a flow rate of approximately 1.6 × 10-9 kg s-1. At this stage, the electrical discharge was ignited using previously optimized conditions12 (power output (PCW) ) 5 W, pulse on-time (TON) ) 20 µs, pulse off-time (TOFF) ) 1200 µs, and total deposition time ) 30 min). Upon completion of deposition, the RF supply was switched off and the monomer allowed to continue to flow through the system for a further 5 min prior to pumping down to base pressure. Next the deposited maleic anhydride plasma polymer films were reacted with allylamine (Aldrich, +99%). This was carried out under vacuum without exposure to air (in order to avoid hydrolysis of the surface anhydride groups or reaction of amine groups with atmospheric CO223-25), by isolating the vacuum pump, and filling the chamber with 1 mbar of allylamine vapor at ambient temperature (∼20 °C). At this stage, timing of the surface functionalization reaction commenced. Upon termination of exposure, the allylamine reservoir was isolated and the whole apparatus pumped back down to the system base pressure. The alkene functionalized surface was then removed from the reactor (in the case of imide formation, it was placed into an oven at 120 °C for 2 h, Scheme 1 (step B)). Next, the sample was immersed in a flask containing a 5% vol 1,3-cyclohexadiene (Aldrich, 97%) in toluene (BDH, minimum 99.5%) solution and heated to 90 °C (reflux) for 15 h. Afterward, the functionalized surface was (23) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1991, 13, 6704. (24) Chakraborty, A. K.; Bischoff, K. B.; Astarita, G.; Damewood, J. R. J. Am. Chem. Soc. 1988, 110, 6947. (25) Penny, D. E.; Ritter, T. J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2103.
washed with toluene and dried under a stream of nitrogen prior to characterization. Film thickness measurements were carried out using a spectrophotometer (nkd-6000, Aquila Instruments Ltd.). Transmittance-reflectance curves (350-1000 nm wavelength range) were fitted to the Cauchy model for dielectric materials using a modified Levenburg-Marquardt algorithm. The deposited plasma polymer film thickness was estimated by reflectometry to be 95 ( 5 nm. A Perkin-Elmer FTIR spectrometer equipped with a variable angle accessory (Specac) and a liquid nitrogen cooled MCT detector was used for reflection-absorption (RAIRS) measurements (p-mode polarization) with the incident infrared radiation beam at an angle of 66°. All spectra were obtained at 4 cm-1 resolution over 256 scans. A VG ESCALAB MKII electron spectrometer equipped with a nonmonochromated Mg KR X-ray source (1253.6 eV) and a hemispherical analyzer operating in CAE mode (20 eV pass energy) was used for X-ray photoelectron spectroscopy (XPS). XPS core level spectra were fitted using Marquardt minimization computer software assuming a linear background and equal full width at half-maximum (fwhm). Elemental compositions were calculated using instrument sensitivity (multiplication) factors determined from chemical standards, C(1s):O(1s):N(1s) ) 1.00: 0.36:0.57.
3. Results and Discussion Infrared analysis of 95 ( 5 nm nm thick maleic anhydride pulsed plasma polymer films confirmed a high degree of anhydride group incorporation, Figure 1.12,26 The following characteristic infrared absorption features of cyclic anhydride groups were identified: asymmetric and (26) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: Singapore, 1991; pp 91 and 123.
1414
Langmuir, Vol. 21, No. 4, 2005
Roucoules et al.
Table 1. Infrared Assignments of Maleic Anhydride Plasma Polymer (MAPP), Allylamine (AA) Plasma Polymer Functionalized with Allylamine (MAPP + AA), Plasma Polymer Functionalized with Allylamine and Heated to 120 °C (MAPP + AA + 120 °C), 1,3-cyclohexadiene (C), Cycloaddition Product with Imide Linkage (BCI), and Cycloaddition Product with Amide Linkage (BCA)
assignment asymmetric CdO anhydride stretching symmetric CdO anhydride stretching imide bands carboxylic acid stretching amide I CdC stretching amide II CH2 scissoring dCH2 scissoring + dCH scissoring C-C-H deformation CNC asymmetric stretching CN stretching (amide) in-plane CH deformation cyclic conjugated anhydride stretching CNC symmetric stretching (imide stretching) CH2 rocking CH2 twisting CH2 wagging ring vibrations COC stretching CN stretching (amine) H-Cd stretching cyclic unconjugated anhydride stretching (*overlap with the CdC stretch and dCH2 wag band) Ring stretching CdC stretching + dCH2 in phase wagging (*overlap with the cyclic unconjugated anhydride stretch bands) NH2 wagging N-H wagging C-C-C deformations
MAPP
AA
1860 1796
MA + AA
MA + AA + 120 °C
C
BCI imide linkage
1796
1638 1453 1422
1716 1658 1638 1510-1550 1453 1422
1796 1775, 1710 1716 1658 1638 1510-1550 1453 1422
1776, 1710 1716 1658 1638, 1618 1510-1550
1716 1658 1638, 1618 1510-1550
1430-1470 1409 1338 1262
1430-1470 1408 1338 1262
1179 1164
1179 1164
1070
1070
986
984
930, 940 949
949
744
739
1702 1430
1410 1337 1370 1241, 1196
1241, 1196 1160 1131 1104
1097, 1062
1160 1131 1104
BCA amide linkage
1241, 1196 1140-1210 1131 1104
1240 1160 1060
1097, 1062 1046 995
964, 938, 906
918
995 995 964, 938, 906* 964, 938, 906*
918*
918*
750-820
750-820
750-875
symmetric CdO stretching (1860 and 1796 cm-1), cyclic conjugated anhydride group stretching (1241 and 1196 cm-1), C-O-C stretching vibrations (1097 and 1062 cm-1), and cyclic unconjugated anhydride group stretching (964, 938, and 906 cm-1), Figure 1a and Table 1. The infrared absorbance spectrum of the allylamine molecule (molecule I in Scheme 1) displayed the following strong characteristic bands: CdC stretching (1638 cm-1), CH2 scissoring (1453 cm-1), dCH2 scissoring and dCH scissoring (1422 cm-1), CH2 rocking (1160 cm-1), CH2 twisting (1131 cm-1), CH2 wagging (1104 cm-1), CN stretching (1046 cm-1), H-Cd stretching (995 cm-1), Cd C stretching and dCH2 in-phase wagging (918 cm-1), and NH2 wagging (875-750 cm-1), Figure 1b and Table 1.27 Reaction of allylamine vapor with the deposited maleic anhydride pulsed plasma polymer layer resulted in ring opening of the cyclic anhydride centers to yield amide (amide I at 1658 cm-1 and amide II at 1550-1510 cm-1), carboxylic acid stretching (1716 cm-1), and NH wagging (820-750 cm-1) infrared absorbances, Figure 1c.28,29 Furthermore, the presence of some allylamine (1638, 1453, 1422, 1160, 1131, 1104, 995, and 918 cm-1) and maleic anhydride pulsed plasma polymer (1796, 1241, 1196, 1097, 1062, 964, 938, and 906 cm-1) spectral features were also observed. The broad band centered at 931 cm-1 can be attributed to the overlap of the cyclic unconjugated (27) Silvi, B.; Perchard, J. P. Spectrochim. Acta 1976, 32 A, 23. (28) Lin-Vien, C.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991; pp 119, 142, and 146. (29) Zhao, M.; Liu, Y.; Crooks, R. M.; Bergbreiter, D. E. J. Am. Chem. Soc. 1999, 121, 923.
739
anhydride groups stretching bands with the CdC stretching and CH2 in-phase wagging absorbances associated with the allylamine molecule. Heating of these surfaces to 120 °C gave rise to ring closure and the formation of cyclic imides, Figure 1d and Scheme 1 (step B),13,18 as seen by the drop in intensity of strong amide, NH wagging, and CdO acid bands, while two imide stretching vibration bands appear at 1775 and 1710 cm-1.13,17 Other new features included CN stretching vibrations at 1337 cm-1 15 and two new bands at 11401210 and 1410 cm-1 characteristic of CNC symmetric stretching (imide stretching) and CNC asymmetric stretching, respectively.17 However, no change was observed in the CdC stretching (1638 cm-1) and the H-Cd stretching (995 cm-1) bands associated with the terminal alkene group. The infrared spectrum of 1,3-cyclohexadiene (molecule II in Scheme 1) exhibits the following strong absorbance bands:3 ring C-C stretching (1702 cm-1), CH2 scissoring and ring C-C stretching (1430 cm-1), CH in plane (1370 cm-1), CH2 rocking (1240 cm-1), CH2 twisting (1160 cm-1), CH2 wagging (1060 cm-1), ring stretching (930 and 940 cm-1), and C-C-C deformations (744 cm-1), Figure 1e. The aforementioned plasma polymer surface functionalized with cyclic imide groups was exposed to 1,3cyclohexadiene solution in toluene at 90 °C for 15 h. This gave rise to a loss in intensity of the CdC stretching band (1638 cm-1) belonging to the surface alkene in conjunction with the appearance of a new band at 1618 cm-1, Figure 1f. The latter assignment is consistent with the CdC ring stretching mode of bicyclo[2.2.2]oct-2-ene previously re-
Diels-Alder Chemistry on Alkene Films
ported in the literature at 1618 cm-1.30 The accompanying shift of the H-Cd stretching band from 995 to 986 cm-1 confirmed that a structural rearrangement had taken place at the surface. Other new absorbances include the broad band centered at 739 cm-1 attributable to the characteristic C-C-C deformation bands at 867, 805, and 693 cm-1 of bicyclo[2.2.2]oct-2-ene;19 a broad band centered at 949 cm-1 assigned to ring stretching vibrations at 959 and 946 cm-1; and a strong band at 1070 cm-1 due to overlap of two ring-deformation modes at 1095 and 1042 cm-1; while the 1430-1470 cm-1 region correlates to the C-C-H deformation vibrations reported for bicyclo[2.2.2]oct-2-ene at 1467, 1454, and 1449 cm-1.19 The band at 1409 cm-1 is associated with CNC stretching, the two bands at 1179 and 1164 cm-1 belong to the CH2 deformation modes of bicyclo[2.2.2]oct-2-ene, and the 1262 cm-1 peak correlates to the in-plane deformation of the unconjugated cis double bonds present in bicyclo[2.2.2]oct2-ene. A control experiment entailed immersion of the cyclic imide functionalized plasma polymer surface into toluene at 90 °C for 15 h. No change in the infrared spectrum was observed. The intermediate amide functionalized plasma polymer surface was also exposed to 1,3-cyclohexadiene solution in toluene at 90 °C, for 15 h, Scheme 1, (step D). Infrared analysis showed spectral features similar to those seen previously for the imide surface, Figure 1g, the main (30) Kawai, N. T.; Butler, I. S.; Gilson, D. F. R. J. Raman Spectrosc. 1992, 23, 29.
Langmuir, Vol. 21, No. 4, 2005 1415
difference being the peak between 1800 and 1600 cm-1 indicating the presence of amide rather than imide linkages at the surface. Angle-resolved XPS analysis for both of the aforementioned routes showed that there was no compositional variation within the sampling depth (∼2 nm) and conversion of the anhydride groups reached 86 ( 3%. These reactions were then repeated on maleic anhydride pulsed plasma polymer films deposited onto steel plates, polyethylene film, polystyrene microspheres, and paper in order to demonstrate the general applicability of this approach. 4. Conclusions Bicyclo[2.2.2]oct-2-ene groups can be introduced onto solid substrates via the Diels-Alder (4 + 2) cycloaddition reaction by reacting 1,3-cyclohexadiene with a welladhered maleic anhydride pulsed plasma polymer layer which has been functionalized with dienophile groups. This approach offers great flexibility in that the density of such cyclic rings at the surface can be easily tailored by simply programming the pulsed plasma duty cycle.12 Furthermore, compared to previous examples of DielsAlder chemistry at solid surfaces (which suffer from the drawback of being restricted to specific substrate materials), this method can be easily adapted to a variety of surfaces. LA0479657
