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Dicyanopyrazine-linked porphyrin Langmuir–Blodgett films
Journal of Colloid and Interface Science 320 (2008) 548–554 www.elsevier.com/locate/jcis
Dicyanopyrazine-linked porphyrin Langmuir–Blodgett films Sung Taek Kang, Heejoon Ahn ∗ Department of Fiber and Polymer Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, Republic of Korea Received 2 November 2007; accepted 24 December 2007 Available online 11 February 2008
Abstract We investigated the influence of arachidic acid/cadmium dication (AA/Cd2+ ) as a transfer promoter for the deposition of dicyanopyrazinelinked porphyrin (2-DCPP) Langmuir–Blodgett (LB) films on both hydrophobic and hydrophilic substrates. In the case of LB deposition on a hydrophilic substrate, the presence of AA/Cd2+ does not improve 2-DCPP LB deposition. The poor transfer in the case of the hydrophilic surface is believed to be due to 2-DCPP not wetting the surface during the down-stroke deposition, and this is not improved by the transfer agent. However, on a hydrophobic substrate, deposition of 2-DCPP is significantly improved by the presence of AA/Cd2+ . Comparison of the UV–visible spectrum of a 2-DCPP LB film with that of 2-DCCP dissolved in chloroform reveals that the Soret and Q bands for the 2-DCPP LB film are broadened and red-shifted due to aggregation of porphyrin rings in the LB film. UV–visible spectral changes and ellipsometry as a function of the number of deposition layers suggest continuous transfer of 2-DCPP/AA onto the hydrophobic substrate and reproducibility in the deposition process. The Soret and Q bands of the 2-DCPP LB film upon acid vapor exposure have also been investigated, and these measurements may have chemical sensor applications. © 2007 Elsevier Inc. All rights reserved. Keywords: Acid vapor sensing; Dicyanopyrazine; Langmuir–Blodgett films; Porphyrin
1. Introduction Porphyrin is a heterocyclic macrocycle composed of four pyrrole subunits linked through methane bridges. Porphyrin and its derivatives have recently received much attention because of their potential applications in data storage, photonic and optoelectronic materials, photodynamic therapy, photovoltaic cells, and chemical sensors [1]. Bard et al. [2] developed an electrooptic data storage system by sandwiching solid thin films of zinc-octakiz(β-decoxyethyl)porphyrin between two optically transparent electrodes. A scanning tunneling microscope was used to trap charges (“write”) and release charges (“read”) in the storage device. Porphyrins are good candidates for use in optoelectronic materials due to their greater thermal stability compared to typical organic chromophores and their extended π -conjugated macrocyclic ring system. This extended π -conjugation system can induce large nonlinear optical (NLO) effects [3]. Suslick et al. [4] synthesized push–pull porphyrins * Corresponding author. Fax: +82 2 2220 4499.
E-mail address: [email protected] (H. Ahn). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.12.040
containing both electron donor and electron acceptor groups in the para position of 5, 10, 15, 20-substituted tetraphenylporphyrins. These researchers were the first to measure NLO properties of porphyrins [4]. Porphyrins also have applications in electronic devices such as electronic switches, wires, and transistors. Wasielewski et al. [5] designed a prototype molecular switch by utilizing photoinduced electron transfer in a donor–acceptor system. In this case, two porphyrins and N,N diphenyl-3,4,9,10-perylenebis(dicarboximide) work as electron donor and acceptor, respectively. When these molecules are exposed to a short light pulse, the porphyrins excite and induce single or double reduction of the acceptor. The singly and doubly reduced electron acceptors absorb light strongly at 713 and 546 nm, respectively [5]. The unique properties of a porphyrin, as discussed above, can be realized by the synthesis of new porphyrins containing desired functional groups, especially at meso-positions. A variety of synthetic methods have been developed to realize meso tetrasubstituted porphyrins [1]. Recently, J.Y. Jaung [6] synthesized new porphyrins bearing dicyanopyrazine moiety at mesopositions by the reaction of pyrrole and bis-styryl derivatives
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Fig. 1. Chemical structure of 5,15-di[5-{2-(4-alkoxyphenyl)-ethenyl}-6-{2-phenyl-ethenyl}-2,3-dicyano-pyrazino]-porphyrin (2-DCPP).
containing 2,3-dicyanopyrazine. The chromophoric systems of the dicyanopyrazine-linked porphyrin solutions were investigated to validate the acid/base sensing capability based on protonation/deprotonation of the porphyrins. An important consideration in applications of porphyrin molecules is the necessity of forming thin films. The Langmuir– Blodgett (LB) technique is one of the most popular methods for fabricating molecularly well-oriented and well-assembled thin films utilizing air–water interfaces [7]. The typical procedure to obtain the LB film is as follows. Amphiphilic molecules dissolved in an organic solvent are spread at the air–water interface. The solid-like monolayer can be formed by adjusting the surface pressure. Stable monolayers at the air–water interface can be transferred onto hydrophilic or hydrophobic solid substrates via dipping the substrates. However, difficulties in obtaining excellent quality for LB films of porphyrins have been found due to aggregation. Thus transfer promoters such as stearyl alcohol, stearamide, and arachidic acid (AA) have been incorporated with porphyrins or phthalocyanines to improve the quality of the LB films [8–13]. In addition, one may be able to control molecular aggregation and orientation in LB films by changing the mixing ratios of the transfer promoter and the porphyrin [9]. In this work, we fabricated dicyanopyrazine-linked porphyrin (2-DCPP) LB films and investigated the influence of arachidic acid as a transfer promoter for their deposition on hydrophilic and hydrophobic substrates. 2-DCPP is a particularly interesting molecule with potential sensor applications because of its charge transport properties [6]. Note that 2-DCPP films cannot be obtained by spin-coating and thermal evaporation methods because the molecule easily crystallizes during spincoating and degrades during thermal evaporation. We demonstrate that 2-DCPP LB films are almost perfectly transferred onto hydrophobic substrates in the presence of arachidic acid.
Ellipsometric thickness measurements reveal continuous transfer of 2-DCPP/AA LB layers onto hydrophobic substrates and reproducibility in the deposition process. The spectral changes of 2-DCPP LB films upon nitric acid vapor exposure demonstrate the acid vapor sensing capability of this system. 2. Materials and methods 2.1. Materials 5,15-Di[5-{2-(4-alkoxyphenyl)-ethenyl}-6-{2-phenyl-ethenyl}-2,3-dicyano-pyrazino]-porphyrin (referred to as 2-DCPP) was synthesized by J.Y. Jaung at Hanyang University. Fig. 1 shows the chemical structure of this porphyrin. As seen in the figure, two meso-positions of the porphyrin ring are substituted with the dicyanopyrazine-containing moiety. Chloroform, arachidic acid, cadmium chloride, and potassium hydrogencarbonate were purchased from Sigma–Aldrich and used without additional purification. Tridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane was obtained from Gelest. Double-side polished Si(111) wafers were purchased from Empco, Inc., Korea. 2.2. Substrate preparation To prepare hydrophilic substrates, quartz slides and silicon wafers were cleaned in “piranha” solution (7:3 volume ratio of concentrated 95% H2 SO4 and 30% H2 O2 ) for 30 min, rinsed several times by sonicating them in pure water and acetone for 10 min, and dried under pure nitrogen. To prepare hydrophobic substrates, the cleaned hydrophilic quartz slide and silicon wafer substrates were exposed to tridecafluoro-1,1,2,2tetrahydrooctyl trichlorosilane in vacuum. Quartz slides were used only for UV–visible spectroscopy experiments.
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2.3. Langmuir–Blodgett film preparation Measurement of the surface pressure–area (π–A) isotherms and the LB films deposition experiment were performed with a computer-controlled KSV Minitrough II Instrument (KSV Instruments Ltd., Finland) at room temperature. Ultrapure water with a resistivity of ca. 18.2 M and pH 5.8, purified by a Milli-Q ultrapure water system, was used as the subphase for pure 2-DCPP LB film deposition. For 2-DCPP/AA LB film deposition, the aqueous subphase contained 3.0 × 10−4 M CdCl2 buffered with 1.0 × 10−5 M KHCO3 . The 1:4 molar mixture of 2-DCPP and arachidic acid was dissolved in chloroform as the spreading solvent. The concentrations of 2-DCPP and arachidic acid solutions in chloroform were 7.7×10−5 and 3.1×10−4 M, respectively. The chloroform solution of 2-DCPP/AA (200 µl) was spread onto the air–water interface using a glass syringe (Hamilton Co., USA). After complete evaporation of the chloroform, the monolayer floating above the water surface was compressed at a speed of 10 mm/min. The LB films were transferred onto substrates by dipping them at a constant surface pressure of 25 mN/m with a dipping speed of 7 mm/min.
Fig. 2. Surface pressure–molecular area (π –A) isotherms of pure 2-DCPP and 2-DCPP/AA monolayers at the air–water interface at 25 ◦ C.
2.4. Characterization of LB films UV–visible spectra of the LB films on quartz were recorded with a SCINCO S-4100 UV–visible spectrophotometer. The thickness of the LB film was measured using an ellipsometer (Rudolph Research AutoEL-II) equipped with a He–Ne laser, a rotating polarizer, an analyzer, and a multichannel detection system. Ellipsometric measurements were taken at 60◦ with respect to the surface normal on the substrates before and after LB film deposition. An average of five measurements was obtained for each film. 3. Results and discussion Fig. 2 shows surface pressure–molecular area (π−A) isotherms of pure 2-DCPP and 2-DCPP/arachidic acid monolayers on the water subphase. For pure 2-DCPP, the surface pressure starts rising at an area of 230 Å2 and gradually increases upon compression until the onset of a plateau-like region at about 45 mN/m. In the case of 2-DCPP/AA, the onset of a plateaulike region appears at about 27 mN/m. Upon further compression, the surface pressure rises up to nearly 45 mN/m when collapse of the monolayer occurs. The LB multilayers were prepared by accumulation of up-strokes and down-strokes at a pressure of 25 mN/m on hydrophilic and hydrophobic substrates. The extrapolated area per molecule at 25 mN/m is 162 and 202 Å2 for pure 2-DCPP and 2-DCPP/AA, respectively. These values are analogous to the typical molecular area of the porphyrins bearing long chains at meso-positions [14,15]. The quantity and quality of the deposited monolayer on a substrate could be evaluated by a so-called transfer ratio. This is defined as the ratio between the decrease in monolayer area during a deposition stroke and the area of film deposited on the substrate. Thus, for ideal transfer the transfer ratio is equal to 1. Fig. 3 shows the transfer ratio of pure 2-DCPP and 2-DCPP/
Fig. 3. Transfer ratio versus number of deposited layers of 2-DCPP/AA (") and pure 2-DCPP (!) on a hydrophilic Si(111) substrate at a surface pressure of 25 mN/m.
AA LB films with respect to the number of layers deposited on hydrophilic Si(111) substrates at a constant surface pressure of 25 mN/m. For 2-DCPP/AA LB deposition, cadmium cations are added in the subphase. Note that the first deposition is up-stroke and the second deposition is down-stroke. Thus the transfer ratios for the odd number and even number of depositions are for up-stroke and down-stroke depositions, respectively. As shown in the figure, both pure 2-DCPP and 2-DCPP/AA display transfer ratios close to 1 for up-stroke depositions. To the contrary, the transfer ratios are negative values for even deposition numbers, corresponding to the down-stroke depositions. These results indicate that the pure 2-DCPP and 2-DCPP/AA LB films are almost perfectly transferred during the up-stroke deposition but that the deposited LB films are stripped off and respread onto the water surface during the
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subsequent down-stroke deposition. During the up-stroke deposition, the aqueous subphase wets the hydrophilic substrate well and, thus, the direction of the movement of the substrate and that of the surface of the subphase near the substrate form an obtuse angle. Subsequently, a monolayer on the subphase can touch and attach to the hydrophilic substrate, resulting in transfer of the LB film. However, during the down-stroke deposition, the direction of the movement of the substrate and that of the subphase near the substrate form an acute angle. Therefore, the monolayer cannot attach to the substrate. In addition, part of the LB film deposited on the previous up-stroke deposition was peeled from the substrate and respread onto the air–water interface. For the deposition of LB films, contact angles and contact lines have been recognized to play important roles [16,17]. The role of contact angles in LB film deposition has been defined by several scientists [16,18–24]. Even though the specific contact angle values are slightly different, the general conclusion is that LB film deposition is possible if the dynamic contact angle is greater than 90◦ during the down-stroke deposition or smaller than 90◦ during the up-stroke deposition. In the current study, the measured contact angles of the hydrophilic substrate, 2-DCPP, and 2-DCPP/AA monolayer films are 5, 80, and 81◦ , respectively. Therefore, in accord with the aforementioned role of contact angle in LB film formation, the 2-DCPP, and 2-DCPP/AA cannot form LB films on the hydrophilic substrate during the down-stroke deposition. In addition, the observed negative values of transfer ratio during down-stroke depositions indicate respreading of the LB film from the substrate onto the water surface. This respreading of the LB film implies weak or essentially no interaction between the 2-DCPP and the hydrophilic substrate. Comparison of the transfer ratios between pure 2-DCPP and 2-DCPP/AA illustrates the effect of the presence of arachidic acid as a transfer promoter for LB film deposition. As shown in Fig. 3, in the case of LB deposition on a hydrophilic substrate, arachidic acid does not play a role as a transfer promoter to improve the LB deposition. However, in the case of 2-DCPP LB deposition on hydrophobic substrates, the presence of the arachidic acid and cadmium cations remarkably improves the quality of the LB film. Fig. 4 displays the transfer ratio of pure 2-DCPP and 2-DCPP/AA LB films with respect to the number of layers deposited on hydrophobic Si(111) substrates at a constant surface pressure of 25 mN/m. Note that the first deposition on a hydrophobic substrate is down-stroke deposition. In the case of 2-DCPP/AA LB deposition, cadmium cations are added to the water subphase. For pure 2-DCPP LB deposition, the transfer ratio for the first down-stroke deposition is only 0.42, and it gradually decreases with increasing number of depositions. In some cases, the transfer ratios are negative values. In the case of 2-DCPP/AA, the transfer ratios are close to 1 (0.94 ± 0.17) up to 18 multilayer depositions, which indicates improvement in the deposition quality due to the presence of arachidic acid. It has been reported that incorporation of transfer promoters such as arachidic acid, stearyl alcohol, and stearamide can produce high-quality LB multilayers of porphyrins or phthalocyanines [8–13]. Ohigashi et al. [25] proposed that the transfer promoter
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Fig. 4. Transfer ratio versus number of deposited layers of 2-DCPP/AA (") and pure 2-DCPP (!) on a hydrophobic Si(111) substrate at a surface pressure of 25 mN/m.
played a role as stiffening poles for the stack of phthalocyanines in LB multilayers. In addition, stabilization of the mixed 2-DCPP/AA LB films can be enhanced by the addition of divalent ions in the subphase. It has also been reported that the divalent cations dramatically improve the film stability during deposition because they interact with arachidic acid to form a “di-soap,” which enhances the film stability [17,26–30]. Chen and Liu [17] studied the effects of the addition of cations in AA LB films. It is reported that the addition of Cd2+ or Ba2+ improves the AA LB film quality. Therefore, when AA is used as a transfer promoter, divalent cations are suggested to be added with AA. However, the authors also reported that the AA LB films with a transfer ratio over 0.9 could be obtained without CdCl2 . The results of Fig. 4 clearly show the effect of the addition of transfer promoter and cations on the deposition of the 2-DCPP LB film. Comparison between Figs. 3 and 4 indicates the effect of hydrophobic and hydrophilic substrates on the deposition of the 2-DCPP LB film. In the case of the hydrophilic substrate, as shown in Fig. 3, the presence of the transfer promoter and cations does not improve the LB film deposition. In contrast, LB deposition of 2-DCPP with transfer promoter and cations can be significantly improved when the LB film is deposited on a hydrophobic substrate (Fig. 4). Fig. 5 presents UV-visible spectra of 2-DCPP/AA dissolved in chloroform and an 18-layered LB film on a hydrophobic quartz slide. In general, porphyrins exhibit absorption spectra with a strong Soret band at around 450 nm and relatively weak Q bands at higher wavelengths (ca. 690 nm) [31]. These bands are due to the π–π ∗ transition in the porphyrin moiety [32]. The UV–visible spectrum of a 2-DCPP chloroform solution shows a sharp and strong Soret band at 425 nm and four weaker Q bands at 519, 561, 589, and 651 nm. In the case of 2-DCPP LB film, the Soret band is broadened and red-shifted to 437 nm compared with that in the chloroform solution. The Q bands
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Fig. 5. UV–visible spectra of 2-DCPP solution in chloroform and an 18-layered LB film on quartz slide.
Fig. 7. Ellipsometric thickness of 2-DCPP/AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic Si(111) substrates.
exhibit similar red-shifts with a loss of resolution. Note that the 2-DCPP LB film shows only two Q bands. This phenomenon is due to aggregation of porphyrin rings as the LB film assembles or a conformational change of the porphyrin macrocycle in the film [13,15,33,34]. Fig. 6 shows the electronic absorption spectra of 2-DCPP/ AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic quartz slides. The inset displays the dependence of absorbance measured at 437, 576, and 663 nm upon the number of deposition layers. All curves increase almost linearly with increasing number of layers, indicating constant transfer of 2-DCPP/AA LB films on hydrophobic substrates during
sequential up/down-strokes. This result also suggests that the 2-DCPP/AA LB film transfer on the hydrophobic substrate is reasonably reproducible. It is interesting to note that the band positions are not dependent on the number of layers, suggesting the similarity in the structure of the aggregates formed in the 2-DCPP LB films with a different number of deposited layers. Fig. 7 presents ellipsometric thickness of 2-DCPP/AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic Si(111) substrates. The observed ellipsometric thicknesses for 2-DCPP/ AA LB films with 2, 6, 10, 14, and 18 layers are 6.5, 18.8, 31.1,
Fig. 6. UV–visible spectra of 2-DCPP/AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic quartz slides. Inset: plot of absorbance at 437, 576, and 663 nm as a function of number of layers.
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Fig. 8. Evolution of the UV–visible spectrum of an 18-layered 2-DCPP/AA LB film upon exposure to nitric acid vapor.
45.4, and 57.5 nm, respectively. As shown in the figure, the ellipsometric thickness of 2-DCPP/AA LB film is proportional to the number of deposited layers, which confirms continual transfer of the LB film and reproducibility in the deposition process. This is consistent with the results of the UV–visible spectra in Fig. 6. The mean thickness value per monolayer is estimated to be 3.2 nm. Free base porphyrins contain two slightly acidic NH groups and two basic imine-type nitrogen atoms. The imine-type nitrogen atoms are protonated to give the monocation and dication species. The formation of the monocation and dication depends upon the inductive effects of the substituent groups at the macrocyclic periphery [1]. Electronic transitions of these forms are quite characteristic and distinct. In general, due to the small differences in electronic transition between the monoand the dication, only two forms of porphyrin (free base and dication) are observed by spectral methods. Fig. 8 shows the evolution of the UV–visible absorbance spectrum of an 18layered 2-DCPP/AA LB film upon nitric acid vapor exposure. The concentration of the nitric acid for the present experiment is that of vaporized nitric acid under atmosphere at room temperature. A decay of the Soret band (437 nm) coupled to the growth of a new band (476 nm) is observed upon acid vapor exposure. Simultaneously, Q bands at 576 and 663 nm diminish in intensity, and a new strong band appears at 704 nm. These acid-vapor-induced spectral changes, especially the formation of new bands, can be explained by partial charge transfer from the nitrogen atom to the porphyrin π -electron system under the formation of a stabilized dication by protonation, which can be represented by different resonance structures [35,36].
4. Conclusions We studied the effect of the presence of arachidic acid as a transfer promoter on the deposition quality of a dicyanopyrazine-linked porphyrin LB film on hydrophobic and hydrophilic substrates. The transfer ratio for the deposition of pure 2-DCPP and 2-DCPP/AA on a hydrophilic substrate significantly depends on the relationship between the liquid contact line on the substrate and the direction of movement of the substrate. The transfer of pure 2-DCPP on a hydrophilic substrate is almost perfect during the up-stroke deposition. However, for the down-stroke deposition, part of the previously deposited films are peeled off and respread onto the water surface. No improvement is found in the deposition quality with the presence of arachidic acid. In the case of LB transfer on a hydrophobic substrate, the transfer ratio of pure 2-DCPP is close to 0.4 for the first up-stroke deposition and decreases as the number of strokes increases, indicating the lack of reproducibility in the deposition process. With the addition of arachidic acid, however, the transfer ratio of 2-DCPP is close to 1 and essentially stable for both up- and down-stroke depositions. Thus deposition of 2-DCPP on hydrophobic substrates is significantly improved by the presence of arachidic acid. The linear increase in absorbance at 437, 576, and 663 nm and ellipsometric thickness with increasing number of layers suggests that a constant amount of 2-DCPP/AA is deposited on the hydrophobic substrate in each stroke, and the LB transfer is reasonably reproducible. The Soret and Q bands for 2-DCPP LB films are red-shifted upon acid vapor exposure, which is due to protonation of imine-type nitrogens in the porphyrin rings. This result demonstrates the acid-vapor-sensing capability of the 2-DCPP LB films.
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Acknowledgment This work was financially supported by the Korea Science and Engineering Foundation (Grant No. R01-2006-00010489-0). References [1] K.M. Kadish, The Porphyrin Handbook, Academic Press, USA, 2000. [2] C.-Y. Liu, H.-L. Pan, M.A. Fox, A.J. Bard, Science 261 (1993) 897. [3] Z.S. Yoon, J.H. Kwon, M.-C. Yoon, M.K. Koh, S.B. Noh, J.L. Sessler, J.T. Lee, D. Seidel, A. Aguilar, S. Shimizu, M. Suzuki, A. Osuka, D. Kim, J. Am. Chem. Soc. 128 (2006) 14128. [4] K.S. Suslick, C.-T. Chen, G.R. Meredith, L.-T. Cheng, J. Am. Chem. Soc. 114 (1992) 6928. [5] M.P. O’Neil, M.P. Niemczyk, W.A. Svec, D. Gosztola, G.L. Gaines III, M.R. Wasielewski, Science 257 (1992) 63. [6] J.Y. Jaung, Dyes Pigments 72 (2007) 315. [7] A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir– Blodgett to Self-Assembly, Academic Press, USA, 1991. [8] W.R. Barger, A.W. Snow, H. Wohltjen, N.L. Jarvis, Thin Solid Films 133 (1985) 197. [9] S.M. Baker, J. Danzer, H. Desaire, G. Credo, R. Flitton, Langmuir 14 (1998) 5267. [10] I.L. Emelianov, V.V. Khatko, Thin Solid Films 354 (1999) 237. [11] I.-H. Ku, Y.-L. Lee, C.-H. Chang, Y.-M. Yang, J.-R. Maa, Colloids Surf. A Physicochem. Eng. Aspects 191 (2001) 223. [12] C.-W. Sheu, K.-M. Lin, I.-H. Ku, C.-H. Chang, Y.-L. Lee, Y.-M. Yang, J.-R. Maa, Colloids Surf. A Physicochem. Eng. Aspects 207 (2002) 81. [13] M.I. Viseu, A.M. Gonçalves da Silva, P. Antunes, S.M.B. Costa, Langmuir 18 (2002) 5772. [14] A. Ruaudel-Teixier, A. Barraud, B. Belbeoch, M. Roulliay, Thin Solid Films 99 (1983) 33. [15] H.-G. Liu, X.-S. Feng, Q.-B. Xue, L. Wang, K.-Z. Yang, Thin Solid Films 340 (1999) 265.
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Dicyanopyrazine-linked porphyrin Langmuir–Blodgett films Sung Taek Kang, Heejoon Ahn ∗ Department of Fiber and Polymer Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, Republic of Korea Received 2 November 2007; accepted 24 December 2007 Available online 11 February 2008
Abstract We investigated the influence of arachidic acid/cadmium dication (AA/Cd2+ ) as a transfer promoter for the deposition of dicyanopyrazinelinked porphyrin (2-DCPP) Langmuir–Blodgett (LB) films on both hydrophobic and hydrophilic substrates. In the case of LB deposition on a hydrophilic substrate, the presence of AA/Cd2+ does not improve 2-DCPP LB deposition. The poor transfer in the case of the hydrophilic surface is believed to be due to 2-DCPP not wetting the surface during the down-stroke deposition, and this is not improved by the transfer agent. However, on a hydrophobic substrate, deposition of 2-DCPP is significantly improved by the presence of AA/Cd2+ . Comparison of the UV–visible spectrum of a 2-DCPP LB film with that of 2-DCCP dissolved in chloroform reveals that the Soret and Q bands for the 2-DCPP LB film are broadened and red-shifted due to aggregation of porphyrin rings in the LB film. UV–visible spectral changes and ellipsometry as a function of the number of deposition layers suggest continuous transfer of 2-DCPP/AA onto the hydrophobic substrate and reproducibility in the deposition process. The Soret and Q bands of the 2-DCPP LB film upon acid vapor exposure have also been investigated, and these measurements may have chemical sensor applications. © 2007 Elsevier Inc. All rights reserved. Keywords: Acid vapor sensing; Dicyanopyrazine; Langmuir–Blodgett films; Porphyrin
1. Introduction Porphyrin is a heterocyclic macrocycle composed of four pyrrole subunits linked through methane bridges. Porphyrin and its derivatives have recently received much attention because of their potential applications in data storage, photonic and optoelectronic materials, photodynamic therapy, photovoltaic cells, and chemical sensors [1]. Bard et al. [2] developed an electrooptic data storage system by sandwiching solid thin films of zinc-octakiz(β-decoxyethyl)porphyrin between two optically transparent electrodes. A scanning tunneling microscope was used to trap charges (“write”) and release charges (“read”) in the storage device. Porphyrins are good candidates for use in optoelectronic materials due to their greater thermal stability compared to typical organic chromophores and their extended π -conjugated macrocyclic ring system. This extended π -conjugation system can induce large nonlinear optical (NLO) effects [3]. Suslick et al. [4] synthesized push–pull porphyrins * Corresponding author. Fax: +82 2 2220 4499.
E-mail address: [email protected] (H. Ahn). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.12.040
containing both electron donor and electron acceptor groups in the para position of 5, 10, 15, 20-substituted tetraphenylporphyrins. These researchers were the first to measure NLO properties of porphyrins [4]. Porphyrins also have applications in electronic devices such as electronic switches, wires, and transistors. Wasielewski et al. [5] designed a prototype molecular switch by utilizing photoinduced electron transfer in a donor–acceptor system. In this case, two porphyrins and N,N diphenyl-3,4,9,10-perylenebis(dicarboximide) work as electron donor and acceptor, respectively. When these molecules are exposed to a short light pulse, the porphyrins excite and induce single or double reduction of the acceptor. The singly and doubly reduced electron acceptors absorb light strongly at 713 and 546 nm, respectively [5]. The unique properties of a porphyrin, as discussed above, can be realized by the synthesis of new porphyrins containing desired functional groups, especially at meso-positions. A variety of synthetic methods have been developed to realize meso tetrasubstituted porphyrins [1]. Recently, J.Y. Jaung [6] synthesized new porphyrins bearing dicyanopyrazine moiety at mesopositions by the reaction of pyrrole and bis-styryl derivatives
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Fig. 1. Chemical structure of 5,15-di[5-{2-(4-alkoxyphenyl)-ethenyl}-6-{2-phenyl-ethenyl}-2,3-dicyano-pyrazino]-porphyrin (2-DCPP).
containing 2,3-dicyanopyrazine. The chromophoric systems of the dicyanopyrazine-linked porphyrin solutions were investigated to validate the acid/base sensing capability based on protonation/deprotonation of the porphyrins. An important consideration in applications of porphyrin molecules is the necessity of forming thin films. The Langmuir– Blodgett (LB) technique is one of the most popular methods for fabricating molecularly well-oriented and well-assembled thin films utilizing air–water interfaces [7]. The typical procedure to obtain the LB film is as follows. Amphiphilic molecules dissolved in an organic solvent are spread at the air–water interface. The solid-like monolayer can be formed by adjusting the surface pressure. Stable monolayers at the air–water interface can be transferred onto hydrophilic or hydrophobic solid substrates via dipping the substrates. However, difficulties in obtaining excellent quality for LB films of porphyrins have been found due to aggregation. Thus transfer promoters such as stearyl alcohol, stearamide, and arachidic acid (AA) have been incorporated with porphyrins or phthalocyanines to improve the quality of the LB films [8–13]. In addition, one may be able to control molecular aggregation and orientation in LB films by changing the mixing ratios of the transfer promoter and the porphyrin [9]. In this work, we fabricated dicyanopyrazine-linked porphyrin (2-DCPP) LB films and investigated the influence of arachidic acid as a transfer promoter for their deposition on hydrophilic and hydrophobic substrates. 2-DCPP is a particularly interesting molecule with potential sensor applications because of its charge transport properties [6]. Note that 2-DCPP films cannot be obtained by spin-coating and thermal evaporation methods because the molecule easily crystallizes during spincoating and degrades during thermal evaporation. We demonstrate that 2-DCPP LB films are almost perfectly transferred onto hydrophobic substrates in the presence of arachidic acid.
Ellipsometric thickness measurements reveal continuous transfer of 2-DCPP/AA LB layers onto hydrophobic substrates and reproducibility in the deposition process. The spectral changes of 2-DCPP LB films upon nitric acid vapor exposure demonstrate the acid vapor sensing capability of this system. 2. Materials and methods 2.1. Materials 5,15-Di[5-{2-(4-alkoxyphenyl)-ethenyl}-6-{2-phenyl-ethenyl}-2,3-dicyano-pyrazino]-porphyrin (referred to as 2-DCPP) was synthesized by J.Y. Jaung at Hanyang University. Fig. 1 shows the chemical structure of this porphyrin. As seen in the figure, two meso-positions of the porphyrin ring are substituted with the dicyanopyrazine-containing moiety. Chloroform, arachidic acid, cadmium chloride, and potassium hydrogencarbonate were purchased from Sigma–Aldrich and used without additional purification. Tridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane was obtained from Gelest. Double-side polished Si(111) wafers were purchased from Empco, Inc., Korea. 2.2. Substrate preparation To prepare hydrophilic substrates, quartz slides and silicon wafers were cleaned in “piranha” solution (7:3 volume ratio of concentrated 95% H2 SO4 and 30% H2 O2 ) for 30 min, rinsed several times by sonicating them in pure water and acetone for 10 min, and dried under pure nitrogen. To prepare hydrophobic substrates, the cleaned hydrophilic quartz slide and silicon wafer substrates were exposed to tridecafluoro-1,1,2,2tetrahydrooctyl trichlorosilane in vacuum. Quartz slides were used only for UV–visible spectroscopy experiments.
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2.3. Langmuir–Blodgett film preparation Measurement of the surface pressure–area (π–A) isotherms and the LB films deposition experiment were performed with a computer-controlled KSV Minitrough II Instrument (KSV Instruments Ltd., Finland) at room temperature. Ultrapure water with a resistivity of ca. 18.2 M and pH 5.8, purified by a Milli-Q ultrapure water system, was used as the subphase for pure 2-DCPP LB film deposition. For 2-DCPP/AA LB film deposition, the aqueous subphase contained 3.0 × 10−4 M CdCl2 buffered with 1.0 × 10−5 M KHCO3 . The 1:4 molar mixture of 2-DCPP and arachidic acid was dissolved in chloroform as the spreading solvent. The concentrations of 2-DCPP and arachidic acid solutions in chloroform were 7.7×10−5 and 3.1×10−4 M, respectively. The chloroform solution of 2-DCPP/AA (200 µl) was spread onto the air–water interface using a glass syringe (Hamilton Co., USA). After complete evaporation of the chloroform, the monolayer floating above the water surface was compressed at a speed of 10 mm/min. The LB films were transferred onto substrates by dipping them at a constant surface pressure of 25 mN/m with a dipping speed of 7 mm/min.
Fig. 2. Surface pressure–molecular area (π –A) isotherms of pure 2-DCPP and 2-DCPP/AA monolayers at the air–water interface at 25 ◦ C.
2.4. Characterization of LB films UV–visible spectra of the LB films on quartz were recorded with a SCINCO S-4100 UV–visible spectrophotometer. The thickness of the LB film was measured using an ellipsometer (Rudolph Research AutoEL-II) equipped with a He–Ne laser, a rotating polarizer, an analyzer, and a multichannel detection system. Ellipsometric measurements were taken at 60◦ with respect to the surface normal on the substrates before and after LB film deposition. An average of five measurements was obtained for each film. 3. Results and discussion Fig. 2 shows surface pressure–molecular area (π−A) isotherms of pure 2-DCPP and 2-DCPP/arachidic acid monolayers on the water subphase. For pure 2-DCPP, the surface pressure starts rising at an area of 230 Å2 and gradually increases upon compression until the onset of a plateau-like region at about 45 mN/m. In the case of 2-DCPP/AA, the onset of a plateaulike region appears at about 27 mN/m. Upon further compression, the surface pressure rises up to nearly 45 mN/m when collapse of the monolayer occurs. The LB multilayers were prepared by accumulation of up-strokes and down-strokes at a pressure of 25 mN/m on hydrophilic and hydrophobic substrates. The extrapolated area per molecule at 25 mN/m is 162 and 202 Å2 for pure 2-DCPP and 2-DCPP/AA, respectively. These values are analogous to the typical molecular area of the porphyrins bearing long chains at meso-positions [14,15]. The quantity and quality of the deposited monolayer on a substrate could be evaluated by a so-called transfer ratio. This is defined as the ratio between the decrease in monolayer area during a deposition stroke and the area of film deposited on the substrate. Thus, for ideal transfer the transfer ratio is equal to 1. Fig. 3 shows the transfer ratio of pure 2-DCPP and 2-DCPP/
Fig. 3. Transfer ratio versus number of deposited layers of 2-DCPP/AA (") and pure 2-DCPP (!) on a hydrophilic Si(111) substrate at a surface pressure of 25 mN/m.
AA LB films with respect to the number of layers deposited on hydrophilic Si(111) substrates at a constant surface pressure of 25 mN/m. For 2-DCPP/AA LB deposition, cadmium cations are added in the subphase. Note that the first deposition is up-stroke and the second deposition is down-stroke. Thus the transfer ratios for the odd number and even number of depositions are for up-stroke and down-stroke depositions, respectively. As shown in the figure, both pure 2-DCPP and 2-DCPP/AA display transfer ratios close to 1 for up-stroke depositions. To the contrary, the transfer ratios are negative values for even deposition numbers, corresponding to the down-stroke depositions. These results indicate that the pure 2-DCPP and 2-DCPP/AA LB films are almost perfectly transferred during the up-stroke deposition but that the deposited LB films are stripped off and respread onto the water surface during the
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subsequent down-stroke deposition. During the up-stroke deposition, the aqueous subphase wets the hydrophilic substrate well and, thus, the direction of the movement of the substrate and that of the surface of the subphase near the substrate form an obtuse angle. Subsequently, a monolayer on the subphase can touch and attach to the hydrophilic substrate, resulting in transfer of the LB film. However, during the down-stroke deposition, the direction of the movement of the substrate and that of the subphase near the substrate form an acute angle. Therefore, the monolayer cannot attach to the substrate. In addition, part of the LB film deposited on the previous up-stroke deposition was peeled from the substrate and respread onto the air–water interface. For the deposition of LB films, contact angles and contact lines have been recognized to play important roles [16,17]. The role of contact angles in LB film deposition has been defined by several scientists [16,18–24]. Even though the specific contact angle values are slightly different, the general conclusion is that LB film deposition is possible if the dynamic contact angle is greater than 90◦ during the down-stroke deposition or smaller than 90◦ during the up-stroke deposition. In the current study, the measured contact angles of the hydrophilic substrate, 2-DCPP, and 2-DCPP/AA monolayer films are 5, 80, and 81◦ , respectively. Therefore, in accord with the aforementioned role of contact angle in LB film formation, the 2-DCPP, and 2-DCPP/AA cannot form LB films on the hydrophilic substrate during the down-stroke deposition. In addition, the observed negative values of transfer ratio during down-stroke depositions indicate respreading of the LB film from the substrate onto the water surface. This respreading of the LB film implies weak or essentially no interaction between the 2-DCPP and the hydrophilic substrate. Comparison of the transfer ratios between pure 2-DCPP and 2-DCPP/AA illustrates the effect of the presence of arachidic acid as a transfer promoter for LB film deposition. As shown in Fig. 3, in the case of LB deposition on a hydrophilic substrate, arachidic acid does not play a role as a transfer promoter to improve the LB deposition. However, in the case of 2-DCPP LB deposition on hydrophobic substrates, the presence of the arachidic acid and cadmium cations remarkably improves the quality of the LB film. Fig. 4 displays the transfer ratio of pure 2-DCPP and 2-DCPP/AA LB films with respect to the number of layers deposited on hydrophobic Si(111) substrates at a constant surface pressure of 25 mN/m. Note that the first deposition on a hydrophobic substrate is down-stroke deposition. In the case of 2-DCPP/AA LB deposition, cadmium cations are added to the water subphase. For pure 2-DCPP LB deposition, the transfer ratio for the first down-stroke deposition is only 0.42, and it gradually decreases with increasing number of depositions. In some cases, the transfer ratios are negative values. In the case of 2-DCPP/AA, the transfer ratios are close to 1 (0.94 ± 0.17) up to 18 multilayer depositions, which indicates improvement in the deposition quality due to the presence of arachidic acid. It has been reported that incorporation of transfer promoters such as arachidic acid, stearyl alcohol, and stearamide can produce high-quality LB multilayers of porphyrins or phthalocyanines [8–13]. Ohigashi et al. [25] proposed that the transfer promoter
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Fig. 4. Transfer ratio versus number of deposited layers of 2-DCPP/AA (") and pure 2-DCPP (!) on a hydrophobic Si(111) substrate at a surface pressure of 25 mN/m.
played a role as stiffening poles for the stack of phthalocyanines in LB multilayers. In addition, stabilization of the mixed 2-DCPP/AA LB films can be enhanced by the addition of divalent ions in the subphase. It has also been reported that the divalent cations dramatically improve the film stability during deposition because they interact with arachidic acid to form a “di-soap,” which enhances the film stability [17,26–30]. Chen and Liu [17] studied the effects of the addition of cations in AA LB films. It is reported that the addition of Cd2+ or Ba2+ improves the AA LB film quality. Therefore, when AA is used as a transfer promoter, divalent cations are suggested to be added with AA. However, the authors also reported that the AA LB films with a transfer ratio over 0.9 could be obtained without CdCl2 . The results of Fig. 4 clearly show the effect of the addition of transfer promoter and cations on the deposition of the 2-DCPP LB film. Comparison between Figs. 3 and 4 indicates the effect of hydrophobic and hydrophilic substrates on the deposition of the 2-DCPP LB film. In the case of the hydrophilic substrate, as shown in Fig. 3, the presence of the transfer promoter and cations does not improve the LB film deposition. In contrast, LB deposition of 2-DCPP with transfer promoter and cations can be significantly improved when the LB film is deposited on a hydrophobic substrate (Fig. 4). Fig. 5 presents UV-visible spectra of 2-DCPP/AA dissolved in chloroform and an 18-layered LB film on a hydrophobic quartz slide. In general, porphyrins exhibit absorption spectra with a strong Soret band at around 450 nm and relatively weak Q bands at higher wavelengths (ca. 690 nm) [31]. These bands are due to the π–π ∗ transition in the porphyrin moiety [32]. The UV–visible spectrum of a 2-DCPP chloroform solution shows a sharp and strong Soret band at 425 nm and four weaker Q bands at 519, 561, 589, and 651 nm. In the case of 2-DCPP LB film, the Soret band is broadened and red-shifted to 437 nm compared with that in the chloroform solution. The Q bands
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Fig. 5. UV–visible spectra of 2-DCPP solution in chloroform and an 18-layered LB film on quartz slide.
Fig. 7. Ellipsometric thickness of 2-DCPP/AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic Si(111) substrates.
exhibit similar red-shifts with a loss of resolution. Note that the 2-DCPP LB film shows only two Q bands. This phenomenon is due to aggregation of porphyrin rings as the LB film assembles or a conformational change of the porphyrin macrocycle in the film [13,15,33,34]. Fig. 6 shows the electronic absorption spectra of 2-DCPP/ AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic quartz slides. The inset displays the dependence of absorbance measured at 437, 576, and 663 nm upon the number of deposition layers. All curves increase almost linearly with increasing number of layers, indicating constant transfer of 2-DCPP/AA LB films on hydrophobic substrates during
sequential up/down-strokes. This result also suggests that the 2-DCPP/AA LB film transfer on the hydrophobic substrate is reasonably reproducible. It is interesting to note that the band positions are not dependent on the number of layers, suggesting the similarity in the structure of the aggregates formed in the 2-DCPP LB films with a different number of deposited layers. Fig. 7 presents ellipsometric thickness of 2-DCPP/AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic Si(111) substrates. The observed ellipsometric thicknesses for 2-DCPP/ AA LB films with 2, 6, 10, 14, and 18 layers are 6.5, 18.8, 31.1,
Fig. 6. UV–visible spectra of 2-DCPP/AA LB films with 2, 6, 10, 14, and 18 layers on hydrophobic quartz slides. Inset: plot of absorbance at 437, 576, and 663 nm as a function of number of layers.
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Fig. 8. Evolution of the UV–visible spectrum of an 18-layered 2-DCPP/AA LB film upon exposure to nitric acid vapor.
45.4, and 57.5 nm, respectively. As shown in the figure, the ellipsometric thickness of 2-DCPP/AA LB film is proportional to the number of deposited layers, which confirms continual transfer of the LB film and reproducibility in the deposition process. This is consistent with the results of the UV–visible spectra in Fig. 6. The mean thickness value per monolayer is estimated to be 3.2 nm. Free base porphyrins contain two slightly acidic NH groups and two basic imine-type nitrogen atoms. The imine-type nitrogen atoms are protonated to give the monocation and dication species. The formation of the monocation and dication depends upon the inductive effects of the substituent groups at the macrocyclic periphery [1]. Electronic transitions of these forms are quite characteristic and distinct. In general, due to the small differences in electronic transition between the monoand the dication, only two forms of porphyrin (free base and dication) are observed by spectral methods. Fig. 8 shows the evolution of the UV–visible absorbance spectrum of an 18layered 2-DCPP/AA LB film upon nitric acid vapor exposure. The concentration of the nitric acid for the present experiment is that of vaporized nitric acid under atmosphere at room temperature. A decay of the Soret band (437 nm) coupled to the growth of a new band (476 nm) is observed upon acid vapor exposure. Simultaneously, Q bands at 576 and 663 nm diminish in intensity, and a new strong band appears at 704 nm. These acid-vapor-induced spectral changes, especially the formation of new bands, can be explained by partial charge transfer from the nitrogen atom to the porphyrin π -electron system under the formation of a stabilized dication by protonation, which can be represented by different resonance structures [35,36].
4. Conclusions We studied the effect of the presence of arachidic acid as a transfer promoter on the deposition quality of a dicyanopyrazine-linked porphyrin LB film on hydrophobic and hydrophilic substrates. The transfer ratio for the deposition of pure 2-DCPP and 2-DCPP/AA on a hydrophilic substrate significantly depends on the relationship between the liquid contact line on the substrate and the direction of movement of the substrate. The transfer of pure 2-DCPP on a hydrophilic substrate is almost perfect during the up-stroke deposition. However, for the down-stroke deposition, part of the previously deposited films are peeled off and respread onto the water surface. No improvement is found in the deposition quality with the presence of arachidic acid. In the case of LB transfer on a hydrophobic substrate, the transfer ratio of pure 2-DCPP is close to 0.4 for the first up-stroke deposition and decreases as the number of strokes increases, indicating the lack of reproducibility in the deposition process. With the addition of arachidic acid, however, the transfer ratio of 2-DCPP is close to 1 and essentially stable for both up- and down-stroke depositions. Thus deposition of 2-DCPP on hydrophobic substrates is significantly improved by the presence of arachidic acid. The linear increase in absorbance at 437, 576, and 663 nm and ellipsometric thickness with increasing number of layers suggests that a constant amount of 2-DCPP/AA is deposited on the hydrophobic substrate in each stroke, and the LB transfer is reasonably reproducible. The Soret and Q bands for 2-DCPP LB films are red-shifted upon acid vapor exposure, which is due to protonation of imine-type nitrogens in the porphyrin rings. This result demonstrates the acid-vapor-sensing capability of the 2-DCPP LB films.
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