Self-assembly of non-linear optical chromophores through ionic interactions Joong Ho Moon, Jung Up Choi, Jin Ho Kim, Hoon Chung, Jong Hoon Hahn, Seung Bin Kim and Joon Won Park* Department of Chemistry, Pohang University of Science and Technology, Center for Biofunctional Molecules, San 32 Hyoja-dong, Pohang, 790-784, Korea Ionic attractions were applied to the construction of non-linear optical (NLO) chromophoric monolayers. In order to fully utilize such an interaction, stilbene-type NLO chromophores having a long alkyl chain and an anionic group at one end of the chain were designed; of this family of chromophores, sodium 11-[4-(trans-4’-pyridylstyryl)oxy]undecan-1-yl sulfate (3) was synthesized. By allowing a few minutes, this anionic chromophore self-assembles successfully on a cationically charged surface, which is prepared by treating clean fused silica with 3-aminopropyltriethoxysilane and then with iodomethane.The pyridine moiety of the self- assembled chromophore can be methylated to augment the molecular hyperpolarizability, p. The characteristics of the monolayer were examined via contact angle measurements, UV-VIS spectroscopy, grazing-angle FTIR spectroscopy, and NLO property measurements. Materials with desirable non-linear optical (NLO) properties have attracted enormous interest in recent years owing to their wide applicability.’ In addition to inorganic materials (e.g. LiNbO,, KH2P04, P-BaB204), organic materials with conju- gated n-electron systems offer great promise. The advantage of organic materials over inorganic ones lies in their inherent tunability, fast non-linear responses in the VIS-IR region, and the possibility of better fabrication, processing, and molecular architecture.In addition, organic materials have larger hyper- polarizabilities, lower relative permittivities, and higher laser- damage thresholds. Second-order NLO phenomena include processes such as second harmonic generation (SHG), linear electro-optic (Pockels) effect, optical rectification, and sum and difference frequency mixing. These processes find many poten- tial applications in optical processing and the storage of data or images. Highly conjugated aromatic organic molecules bearing an electron-donating group at one end and an electron-accepting group at the other end exhibit large values of the first molecular hyperpolarizability, p.However, these molecules should be stacked non-centrosymmetrically in bulk media to gain second order non-linear susceptibility, x(’). Most organic materials stack randomly in bulky media to cancel out dipole moments. In order to asymmetrically align NLO molecules on the surface of a substrate, several methods have been developed. Recent studies’ have demonstrated that self-assembly (SA) is a very useful method for generating ordered ultra-thin films, especially for NLO materials. Thin NLO films have been reported in which the sequential construction of covalently self-assembled chromophores forms a multilayer ~tructure.~Alternatively, transition-metal ions, e.g.zirconium(Iv), can be positioned between layers to stabilize the ~tructure.~Decher and co-workers demonstrated that the application of ionic attraction is an attractive alternative method with inherent advantages.’ Very recently, this approach was successfully applied to the building of multilayers of porphyrin derivatives,6 and a monolayer of an NLO chr~rnophore.~ We report here that the chromophore 3, which contains an anionic functional group and long alkyl chain, forms a mono- layer on a cationically charged surface within minutes at room temperature. 3 Experimental All chemicals used were reagent grade from Aldrich Chemical Co. All solvents for the self-assembly processes were of HPLC grade from Mallinckrodt Chemical Co.Elemental analyses were performed at Galbraith Laboratories, Inc. The fused quartz plates were purchased from Wale Apparatus Co. ‘H and I3C NMR spectra were recorded on a Bruker AM 300 spectrometer operating at 300 and 75.4 MHz, respectively. IR spectra were recorded on a Bomem MB-102 FTIR spec-trometer. UV-VIS spectra were recorded on a Hewlett-Packard diode-array 8452A spectrophotometer. Melting points were measured with a Thomas Hoover capillary melting-point apparatus and were uncorrected. Contact angles were meas- ured with face contact angle meters (Model CA-D and CA-DT) from the Kyowa Interface Science Co. Synthesis Compound 1 was prepared using methods described elsewhere.* 11-[ 4-(tvans-4-Pyridylstyryl)oxy] undecan-1-01 2.Ethanol (32 cm3) was added to a mixture of trans-4-hydroxystyryl- pyridine 1 ( 1.00 g, 5.05 mmol), potassium carbonate (0.84 g, 6.06 mmol) and a trace amount of potassium iodide; then 11- bromoundecan-1-01 (1.90 g, 7.57 mmol) in ethanol (10 cm3) was added slowly at room temperature. The solution was heated for 16 h using an oil bath, the temperature of which was maintained at 80 “C. The resulting solution was evaporated to dryness, redissolved in THF, and filtered off. The product was eluted through a column packed with silica gel (eluent chloroform-acetone, 7 : 1-4 :1 v/v). The crude product could be purified further by recrystallization from ethyl acetate to give 1.06 g (57%) of a yellow powder (2).‘H NMR [(CD,),SOj 6: 8.50 (d, 2 H), 7.58 (d, 2 H), 7.50 (d, 2 H), 7.47 (d, 1 H, J= 16.8 Hz, vinylic), 7.06 (d, 1 H, J=16.8 Hz, vinylic), 6.96 (d, 2 H), 4.40 [t, 2 H, CH,(CH2),,OH], 4.27 (t, 1 H, OH), 3.37 [m, 2 H, (CH,),,CH,OH], 1.71, 1.40, 1.26 [m, 18 H, CH,(CH,),CH,OH]; I3C NMR [(CD,),SO] 6: 159, 150, 145, 133, 128.5, 128.4, 123, 120, 115, 67.4, 60.6, 32.41, 32.37, 28.94, 28.87, 28.82, 28.76, 28.64, 28.52, 25.38; UV-VIS (methanol)A,,, 330 nm; IR (KBr) vlcm-’: 3366br, 3029,2921,2851, 1594, 1512, 1472, 1419, 1289, 1259, 1176, 1057, 1014, 970, 832; mp 128-131 “C (decomp). Elemental analysis, Calc.: C, 78.43; H, 9.05; N, 3.81.Found: C, 78.45; H, 9.25; N, 3.81%. J. Muter. Chem., 1996, 6(3), 365-368 365 Sodium 11-[ 4-( trans-4 -Pyridylstyryl) oxy] undecan-1-yl sulfate 3' Compound 2 (0 76 g, 2 07 mmol) was dissolved in anhydrous pyridine (4 0 cm3) under a nitrogen atmosphere To the solution cooled in an ice bath, chlorosulfonic acid (0 156 cm3, 2 351 mmol) was slowly added After 3 h at room temperature, the resulting solution was neutralized by adding sodium carbonate (3 g) and water (5 cm3) Filtration and evaporation to dryness gave a yellow powder Diethyl ether was added cautiously onto a methanolic solution (30 cm3) of this powder to produce a double layer Placing in an freezer (-20 "C) gave the analytically pure product ( 100 g, 97%) [Corrosive chlorosulfonic acid can be replaced by dicyclohexyl- carbodiimide (DCC) and sulfuric acid,5b but this results in longer, laborious purification steps ] 'H NMR [(CD,),SO] 6 850 (d, 2 H), 758 (d, 2 H), 751 (d, 2 H), 747 (d, 1 H, J= 16 2 Hz, vinylic), 7 07 (d, 1 H, J= 16 2 Hz, vinylic), 6 96 (d, 2 H), 399 [t, 2 H, CH2(CH,),oS0,Na], 368 [t, 2 H, (CH,)loCH,S04Na], 170, 145, 126 [m, 18 H, CH,(CH2)9CH,S04Na], I3C(lH} NMR [(CD,),SO] 6 159, 150, 145, 132, 129, 128, 123, 120, 115, 674, 654, 2898, 2888, 28 85, 28 82, 28 66, 28 63, 28 52, 25 41, 25 35, UV-VIS (meth-anol) Amax/nm 236 (&/drn3 mol-' cm-l 117 x lo4), 330 (E 2 82 x lo4), (DMF) E.,,,/nm 328 (c/dm3 mol-' cm-' 2 81 x lo4), IR (KBr) v/cm-l 3366br, 3026, 2923, 2852, 1597, 1512, 1473, 1242, 1175, 1069, 1041, 999, 820, mp 210°C (decomp ) Elemental analysis, Calc C, 61 38, H, 6 87, N, 2 98 Found C, 61 03, H, 6 93, N, 2 93% Self-assem bl y Clean plates of fused silica were sonicated in piranha solution" (H,SO,-30% H202, 7 3 v/v) for 1h For further cleaning, the H20-30% H,O,-conc NH3 (5 1 1) and H,O-30% H,O,-conc HCl (6 1 1) steps of the RCA (SC-1 and SC-2) procedure" were applied The substrates were washed with copious amounts of deionized water and subsequently with acetone, and dried in a vacuum The thus-prepared substrates were immersed into a toluene solution (24 cm3) containing 3- aminopropyltriethoxysilane'2 (4 cm3) under a nitrogen atmos- phere for 24 h at 25 "C The substrates were then washed with toluene, sonicated in a tolueneacetone mixture (1 1 v/v) for 2 min Finally, the substrates were sonicated again in acetone for 2min, washed with fresh acetone, and air-dried The aminosilylated surfaces were quaternized at room temperature by reaction with iodomethane (1 cm3) dissolved in toluene (16 cm3) for 24 h in the presence of 1,8-bis(dimethylamino)- naphthalene (34 mg) After this, the substrates were washed thoroughly with toluene and air-dried The substrates were immersed into a DMF solution of the chromophore ( low3mol dm-3) at 0°C Typically 1 min was allowed for the self- assembly of the chromophore on the surface Finally, the substrates were immersed for 3 h at 25°C in toluene solution (16 cm3) containing iodomethane (1cm3) for quaternization of the pyridine moiety Grazing-angle FTIR spectroscopy All grazing-angle IR measurements were made with an evacu- able BOMEM 2 26 FTIR interferometer equipped with an MCT wide-band detector, and at a grazing incidence angle of 80-85" using the grazing incidence reflection technique (SPECAC P/N 19650 monolayer/grazing angle accessory) The spectra were recorded at a resolution of 4 cm-' with 7000 co- added scans Reference spectra for the films were obtained on the freshly cleaned fused silica substrates Second harmonic generation (SHG) The 1064nm, 6ns output of a Q-switched Nd YAG laser, operating with a repetition rate of 10 Hz, was attenuated to ca 25 mJ by using a half-wave plate and a polarizer A visible- 366 J Muter Chem , 1996,6(3), 365-368 blocking filter was employed to eliminate any visible radiation emitted by the laser flashlamps The horizontally polarized laser beam was focused onto a sample with a lens of focal length 20 cm The beam diameter on the sample was approxi- mately 0 5 mm The sample was mounted on a rotation stage, of which the resolution was ca 05" The p-polarization was selected for the light to be detected by a Glan-Thompson linear polarizer The second harmonic radiation was separated from the fundamental by using IR-blocking filters and band pass filters (532 O& 1 5 nm), and was detected by a photomul- tiplier tube, the output of which was amplified and integrated by a boxcar averager with a 30ns gate-width The level of output signal from the boxcar integrator was monitored by an oscilloscope A 3 mm thick, y-cut quartz single crystal (dll = 1 1 x lo-' esu) was used as a reference to determine the NLO coefficients of the sample films Results and Discussion A chromophore with a sulfonate anion on the long alkyl chain can be synthesized as shown in Scheme 1 The adopted syn- thetic scheme allows easy variation of the alkyl chain length, and this advantage will be helpful in the study of the effect of the chain length on the self-assembly process The chromo- phore itself shows a relatively small molecular hyperpolariz- ability, p,but quaternization of the pyridine moiety increases the p value dramatically For successful self-assembly, the substrate was treated as shown in Scheme 2 Treatment with 3-aminopropyltriethoxysil- ane (3-APTS) gave substrates coated with organic layers of various thicknesses, depending upon the reaction conditions OH 6-or H2SOJDCC 1 2 3 Scheme 1 Synthesis of the chromophore 3-m MeIfuseddamsubstrate d -3, A B C 3 w 4-Scheme 2 Self-assembly of the NLO chromophore on the cationally charged surface Table 1 The measured advancing contact angles of water after each step of the chemical modification substrate contact angle, BJdegrees A clean fused silica 8-13 B aminosilylated surface 46-5 1 C quaternized surface 33-38 D self-assembled surface 59-63 E pyridinium surface 28-33 E '1 b',03 d4 \ -00 .. 2 00 308 4 00 500 A /nm Fig. 1 UV VIS absorption spectra of self-assembled monolayers -, before, after methylation Upon methylation of the pyridine moiety, the absorption maximum shifts from 308 to 364nm Both values are shifted to shorter wavelengths in comparison to those in solution, and confirm the interaction between the neighbouring chromophores Through ellipsometric measuremepts, it was found that the thickness of the layer was ca 100 A when the substrates were treated with 3-APTS in toluene solution for 1 day The primary amine groups on the surface were methylated to give quaternary amine groups, in which the presence of the cation is independent of the pH of the environment Adsorption studies showed that the self-assembly of the chromophore through ionic exchange was successful and complete in only a few minutes under ambient conditions Only the methylattd substrates with the appropriate thickness (in this case ca 100A) produced a self-assembled chromophoric layer of high surface density The self-assembled chromophore was further exposed to methyl iodide for methylation, which enhanced the efficiency for SHG Contact-angle measurement IS one of the most sensitive methods for examining the chemical nature of surfaces Changes in hydrophilicity (or hydrophobicity), which is greatly influenced by the kinds of functional groups present on the surface, can be monitored easily using the method Advancing contact angles between water and the surfaces were measured after each chemical treatment (Table 1) The observed values agreed with the expected ones, and the observation partly confirms the success of each building step Self-assembly of the chromophore can also be monitored by UV-VIS absorption spectroscopy (Fig 1) The measured absorption maximum of the chromophoric layer is 308 nm, this shifted to 364 nm by derivatizing the pyridine moiety with iodomethane These values are blue-shifted in comparison with the values for the chromophore (A,,, =330 nm in MeOH, 328 nm in DMF) and the methylated chromophore (A,,,= 292, 386 nm in MeOH, 370 nm in DMF) in the solution The blue shift of the absorption confirms that the chromophore is packed in a parallel polar arrangement l3 The surface density of the chromophore is calculated from the absorption coefficient (~=2 8 x lo4dm3 mol-l cm-I in MeOH and DMF) and measured absorbance (A=0 04 +_ 0 01) The calculated t In Me,SO solution the chromophore 3 can be completely methylated by the addition of an excess of methyl iodide at room temperature I 1 3000 2950 2900 2850 2800 wavenum berkm-' Fig.2 Grazing-angle FTIR spectra (a) aminosilylated substrate (state C in Scheme 2, 6 0 cm resolution, 7000 scans), (b) methylated chromophoric layer (state E in Scheme 2, 8= 83" 4 0 cm resolution 7000 scans) surface density (4k1 molecules per 100 A') is comparable to those of similar self-assembled molecules 1 Grazing-angle FTIR spectroscopy is a very useful analytical method that provides valuable information about the direction and packing mode of chromophores in the molecular layers For alkyl chains on the surface, the tilt angle of the chain can be deduced from the intensity of the methylene vibrations [v,,,(CH,), vasym(CH2)]in the spectra It is also observed that C-H stretching frequency of the methylene group for the crystalline samples shows a red-shift relative to that for liquid- like samples [2918 us 2924cm-' for vaSym(CH2), 2851 us 2855 cm-' for v,,,(CH,)] ''As shown in Fig 2, the IR spec-trum of the self-assembled layer shows C-H stretches at 2919 and 2851 cm-l The stretching frequency of the methylene group shows that the packing of the alkyl chains is more like that of crystalline states Significant SHG intensity was detected when the methylated chromophoric layer was irradiated with light (1 064 pm) from the Nd YAG laser The measured macroscopic hyperpolariz- ability [x'')] of esu is comparable to those obtained from other SA and Langmuir-Blodgett (LB) methods Meanwhile, the value is about 10 times larger than that for the same type of chromophore containing a short alkyl chain Therefore, it can be said that the long alkyl chain assists the polar orien- tation of the chromophore in the layer Conclusion The ionic interaction has been successfully applied for the self- assembly of an NLO chromophore with a long dkyl chain and a sulfonate functional group at the end of the chain Because of the short time required for the assembly, this method may be an attractive alternative for the assembly of non-centrosymmetric NLO chromophoric layers This work is supported in part by the Basic Science Research Institute Program, Ministry of Education, 1995, Project No BSRI-95-3436 $ Self-assembled monolayers of 6-( 4-pheny1azophenoxy)hexane 1 thiol on gold have been studied with AFM and STM, an4 it was found that the unit cell, with lattice dimensions of 6 1 and 7 9 A, is populated with two molecules See H Wolf, H Ringsdorf, E Delamarche, T Takami, H Kang, B Michel, C Gerber, M Jaschke, H-J Butt and E Bamberg, J Phys Chem, 1995, 99, 7102 J Mater Chem , 1996, 6(3), 365-368 367 References 1 (a) P N Prasad and D J Williams, Introduction to Nonlinear Optical 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Meyer, J Chem Phys , 1986,85,4966 Paper 5/02982A, Received 10th May, 1995 368 J Muter Chem , 1996,6(3), 365-368