Self-assembled monolayers of fluorene- and nitrofluorene-terminated thiols on polycrystalline gold electrode: Electrochemical and optical properties

Journal of Electroanalytical Chemistry 654 (2011) 20–28

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Self-assembled monolayers of fluorene- and nitrofluorene-terminated thiols on polycrystalline gold electrode: Electrochemical and optical properties Faruk Pak, Kadem Meral, Ramazan Altundasß, Duygu Ekinci ⇑ Department of Chemistry, Faculty of Sciences, Atatürk University, 25240 Erzurum, Turkey

a r t i c l e

i n f o

Article history: Received 5 May 2010 Received in revised form 26 January 2011 Accepted 31 January 2011 Available online 23 February 2011 Keywords: Fluorene-terminated thiols Self-assembled monolayers Cyclic voltammetry Fluorescence spectroscopy Spectroelectrochemical measurements

a b s t r a c t In this paper, two new thiols, [4-(9H-fluoren-9-ylmethyl)-phenyl]-methanethiol (6a) and [4-(2-nitro-9Hfluoren-9-ylmethyl)-phenyl]-methanethiol (6b), were synthesized, and self-assembled monolayers (SAMs) of these thiols were formed on gold electrodes. The structure and surface properties of molecular films were investigated by contact angle measurements and attenuated total reflectance infrared spectroscopy (ATR-FTIR). The blocking behavior of Au-6a and Au-6b SAMs was examined with cyclic voltammetry in the presence of redox probes such as K3Fe(CN)6, Ru(NH3)6Cl3 and ferrocene. Electrochemical measurements revealed that the voltammetric behavior of the redox probes was dependent on the nature of the probe molecules, the electrolytic solution composition and the monolayer structure. The optical properties of the SAMs were studied by steady-state and time-resolved fluorescence spectroscopy. It was obtained that the fluorescence emission bands of Au-6a and Au-6b monolayers were red-shifted and broadened compared to those of free thiols in solution as well as significant reduction at their emission intensities. The fluorescence decay profiles of 6a and 6b monolayers were described by a monoexponential function. In order to determine the possible deactivation mechanism between the metal support and photoexcited molecules, spectroelectrochemical steady-state fluorescence and lifetime measurements on the Au-fluorophore electrodes were also performed as a function of the applied potential. These results indicate that the fluorescence quenching occurs via an energy transfer mechanism from the excited molecules to the gold substrate. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, self-assembled monolayers (SAMs) of surfaceactive organic molecules on various substrates have attracted considerable attention due to their potential applications in important technological fields such as molecular electronics [1,2], optical switches [3], chemical sensors [4], and corrosion inhibitors [5]. There are many more possible systems to prepare such organized assemblies like silanes on silicon surfaces [6], fatty acids on metal oxides [7,8], and thiols [9–11] or disulfides [12,13] on noble metals. Among them, long-chain alkanethiols on gold have proven to be excellent model systems to form monolayers with controllable thickness and desirable function. These stable monolayers can be easily composed by soaking an Au substrate in the solution of a suitable alkanethiol without the requirement of expensive equipment. Both experimental and theoretical studies have demonstrated that the propulsive force in their formation process is the chemical bond formation between the substrate and sulfur atoms, and the hydrophobic interactions between the alkyl chains [14,15].

⇑ Corresponding author. Tel.: +90 442 231 4387; fax: +90 442 236 0948. E-mail address: [email protected] (D. Ekinci). 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.01.041

The surface properties of thiol monolayers for various purposes can be easily tailored by changing the chemical nature of the terminal groups attached to the other end of alkyl chains [16–18]. In this sense, the modification of the metal surface with photoactive molecules positioned at the outside of SAM is of fundamental importance in development of artificial optoelectronic devices. It is well known that, under ambient conditions, the photophysical and photochemical properties of probe molecules transferred onto a solid substrate are affected by many factors such as rigid environment, molecular orientation, and surface irregularities [19]. Furthermore, the excited states of the fluorophores in SAM can be quenched by the metal support either via energy transfer or electron transfer processes, and their emission intensity is, thus, partially decreased [20]. Therefore, the length and chemical structure of the molecular bridge separating the terminal fluorophore from the metal surface is a major factor for quantum yields of photoactive group [21,22]. The photophysical (or photochemical) behavior and quenching process of the excited state observed upon irradiation of the SAMs which contain the photoactive groups such as fluorene [23–25], stilbene [26], anthracene [27] and coumarin [28] as a terminal group at the end of a long alkyl spacer have been investigated in previous studies. However, no report has so far been published on the photoresponsiveness of the thiol

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SAMs formed from a short aromatic linker and a terminal fluorophore group. In this study, we report the first synthesis and photophysical properties of fluorene- and nitrofluorene-terminated thiols with a benzene linker. When compared to those of linear thiols, aromatic thiols can form more densely packed and well-ordered films as a result of p–p stacking interactions [29]. Additionally, monolayers containing aromatic rings display a high electrical conductivity due to the easy delocalization of electrons in the ring while the saturated hydrocarbon chains act as effective insulating layers [30]. Thus, the electrons are transported through these layers by the tunneling mechanism. This behavior can be emerged as a significant disadvantage for us because, with increasing tunneling rate, the lifetimes of the excited molecules bound to the metal can be decreased via electron transfer process. To avoid this problem, we purposely added a methylene unit in between the benzene and the fluorene rings. Furthermore, a second methylene unit was also inserted to increase the degree of order in between the aryl and sulfur head group [31]. The radiative deactivation of the resultant molecular fluorophores (6a and 6b in Scheme 1) on gold was investigated by steady-state fluorescence emission spectroscopy, and their fluorescence lifetimes were measured with time-resolved fluorescence spectroscopy. Cyclic voltammetry experiments were used to examine the relationship between the packing quality and the electron transfer behavior of the films in the presence of electroactive redox probes. Additionally, spectroelectrochemical experiments were performed as a combination of electrochemistry and emission spectroscopy. These measurements provided quite useful information to better understand the effect of the short benzene ring in the absence and presence of electron acceptor nitro substituent on the mechanism of molecular dynamics following photoexcitation. The molecular structure and surface properties of these films were also characterized using horizontal attenuated total reflectance infrared (HATR-FTIR) spectroscopy and contact angle measurements, respectively.

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2. Experimental section 2.1. Synthesis of thiol compounds The synthesis of 6a and 6b was achieved as outlined in Scheme 1 by our group. 2.1.1. Synthesis of 1,4-Bis(bromomethyl)benzene (2) To a magnetically stirred solution of p-xylene (1) (25 g, 0.24 mol) in 75 mL of dry CH2Cl2 was added dropwise a solution of bromine (56 g, 0.35 mol) in 5 mL of CH2Cl2. The reaction mixture was irritated with a 500-W lamp for 5 min at room temperature. The solvent was evaporated. The residue was purified by column chromatography on silica gel eluting with n-hexane to yield 2 (17 g, 40%) as a white crystal (mp:141–144 °C, Lit. [32] 142– 144 °C). 1H NMR (400 MHz, CDCl3) d 4.48 (s, 4H), 7.37 (s, 4H); 13 C NMR (100 MHz, CDCl3) d 33.1, 129.7, 138.2; FTIR (KBr pellet, cm1) 3046, 3024, 3008, 2972, 2855, 1923, 1804, 1689, 1508, 1437, 1419, 1297, 1254, 1228, 1198, 1126, 1085, 857, 848. 2.1.2. Synthesis of 9-(4-Bromomethyl-benzyl)-9H-fluorene (5a) To a solution of 4a (0.79 mg, 4.76 mmol) in 30 mL of dry hexane under nitrogen atmosphere was added n-butyl lithium (1.6 M, 6.09 mmol, 3.8 mL). The mixture was heated at 60 °C for 3 h. After allowing to cool to room temperature, it was treated with a solution of 2 (2.76 g, 10.50 mmol) in CH2Cl2 (5 mL). The resulting mixture was stirred for 2 h at the same temperature. 50 mL of water was added and organic layer was separated, dried over magnesium sulfate, filtered, and concentrated under reduced pressure. The residue was purified by column chromatograph on silica gel eluting with CH2Cl2/hexanes (10%) to afford 5a (0.29 mg, 50%) as a white crystal (mp: 98–102 °C). 1H NMR (400 MHz, CDCl3) d 3.12 (d, 2H, J = 7.3 Hz), 4.22 (t, 1H, J = 7.3 Hz), 4.52 (2H, s), 7.18–7.38 (m, 10H), 7.75 (d, 2H, J = 7.3); 13C NMR (100 MHz, CDCl3) d 33.8, 39.9, 48.8, 120.1, 125.0, 126.9, 127.4, 129.2, 130.2, 136.1, 140.4, 141.1, 146.8; FTIR (KBr pellet, cm1) 3036, 3018, 2946, 2926,

Scheme 1. Synthesis of 6a and 6b thiols.

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2855, 1611, 1574, 1513, 1476, 1447, 1418, 1376, 1339, 1301, 1264, 1227, 1203, 1152, 1099, 1021, 1004, 962, 937, 889, 858. 2.1.3. Synthesis of 9-(4-bromomethyl-benzyl)-2-nitro-9H-fluorene (5b) To mixture of 4b (1.10 g, 5.20 mmol) and NaH (0.63 g, 15.60 mmol) in THF (25 mL) under nitrogen atmosphere was added slowly a solution of 2, 3.50 g (13.00 mmol) in 50 mL THF at room temperature. The reaction mixture was stirred for 2 h and then quenched with 20 mL water. The mixture was extracted with ethylacetate (50 mL), washed with water (50 mL). The combined organic layers were dried over magnesium sulfate, filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel eluting with CH2Cl2–hexane (10%) to yield 5b (0.36 mg, 41%) as a yellow crystal (mp: 162–163 °C). 1H NMR (400 MHz, CDCl3) d 3.16 (dd, 1H, b part of AB system, JAB = 13.9, JBX = 7.7 Hz), 3.18 (dd, 1H, a part of AB system, JAB = 13.9, JAX = 7.7 Hz), 4.29 (t, 1H, J = 7.3 Hz), 4.50 (s, 2H), 7.12 (benzene, 2H, AA0 part of AA0 BB0 system, quasid, J = 8.1 Hz), 7.27 (benzene, 2H, BB0 part of AA0 BB0 system, quasid, J = 7.3 Hz), 7.32 (d, 1H, HI, JIH = 8.1 Hz), 7.36 (t, 1H, HH, JGH = JHI = 7.5 Hz), 7.43 (t, 1H, HG, JFG = JGH = 7.3 Hz), 7.79 (d, 1H, HE, JDE = 8.3 Hz), 7.81 (d, 1H, HF, JFG = 7.3 Hz), 8.03 (d, 1H, HC, JCD = 1.8 Hz), 8.26 (dd, 1H, HD, JCD = 2.0, JDE = 8.4 Hz); 13C NMR (100 MHz, CDCl3) d 33.5, 39.4, 48.9, 120.1, 120.5, 121.6, 123.8, 125.4, 128.1, 128.9, 129.1, 129.4, 130.1, 136.6, 138.8, 139.1, 146.9, 147.5, 148.4; FTIR (KBr pellet, cm1) 3052, 3027, 2963, 2924, 2852, 1613, 1590, 1519, 1481, 1469, 1448, 1418, 1337, 1264, 1228, 1205, 1170, 1153, 1123, 1079, 1019, 962, 946, 894, 859. 2.1.4. The general procedure for the synthesis of 6a and 6b 1.0 equiv of starting material (5a or 5b) and thiourea (1.3 equiv) in dry DMF (20 mL) was heated for 4 h at 80 °C under nitrogen atmosphere. Then, the reaction mixture was treated with 0.1 M NaOH (10 mL) and stirred for 2 h at 80 °C. After allowing to room temperature, it was acidified with conc. H2SO4 (10 mL), extracted with ethylacetate (50 mL) and washed with water (50 mL). The combined organic layers were dried over magnesium sulfate, filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel eluting with CH2Cl2–hexane (20%) to yield 6a and 6b. 2.1.5. Synthesis of [4-(9H-Fluoren-9-ylmethyl)-phenyl]-methanethiol (6a) The synthesis of 6a (0.55 g, 48%, mp: 92–93 °C) was achieved as mentioned in general procedure by using 5a (0.38 mmol) and thiourea (0.50 mmol). 1H NMR (400 MHz, CDCl3) d 1.79 (t, 1H, J = 7.5 Hz), 3.11 (d, 2H, J = 7.6 Hz), 3.76 (d, 2H, J = 7.3 Hz), 4.22 (t, 1H, J = 7.5 Hz), 7.11–7.22 (m, 6H), 7.24 (t, 2H, J = 7.3 Hz,), 7.36 (t, 2H, J = 7.3 Hz), 7.75 (d, 2H, J = 7.6 Hz); 13C NMR (100 MHz, CDCl3) d 28.9, 39.9, 48.9, 120.1, 125.1, 126.9, 127.4, 128.2, 130.0, 138.9, 139.4, 141.1, 146.9; FTIR (KBr pellet, cm1) 3058, 3036, 3019, 2926, 2851, 2564, 1608, 1575, 1512, 1476, 1447, 1415, 1339, 1301, 1272, 1252, 1195, 1166, 1150, 1105, 1029, 1021, 1004, 959, 939, 887, 861, 823. 2.1.6. Synthesis of [4-(2-Nitro-9H-fluoren-9-ylmethyl)-phenyl]methanethiol (6b) The synthesis of 6b (0.62 g, 45%, yellow crystal, mp: 143– 144 °C) was achieved as mentioned in general procedure by using 5b (0.38 mmol) and thiourea (0.50 mmol). 1H NMR (400 MHz, CDCl3) d 1.78 (t, 1H, J = 7.6), 3.11 (dd, 1H, b part of AB system, JAB = 13.9, JBX = 7.7 Hz), 3.16 (dd, 1H, a part of AB system, JAB = 13.9, JAX = 7.7 Hz), 3.75 (d, 2H, J = 7.4), 4.29 (t, 1H, J = 7.4 Hz), 7.11 (benzene, 2H, AA0 part of AA0 BB0 system, quasid, J = 8.1 Hz), 7.27 (benzene, 2H, BB0 part of AA0 BB0 system, quasid, J = 7.3 Hz),

7.28 (d, 1H, HI, JIH = 7.3 Hz), 7.36 (t, 1H, HH, JGH = JHI = 7.3 Hz), 7.43 (t, 1H, HG, JFG = JGH = 7.3 Hz), 7.79 (d, 1H, HE, JDE = 8.3 Hz), 7.81 (d, 1H, HF, JFG = 7.3 Hz), 7.97 (d, 1H, HC, JCD = 2.0), 8.26 (dd, 1H, HD, JCD = 2.0, JDE = 8.3 Hz); 13C NMR (100 MHz, CDCl3) d 28.8, 39.3, 48.9, 119.9, 120.5, 121.4, 123.6, 125.3, 127.9, 128.3, 128.9, 129.8, 137.5, 138.7, 139.9, 146.8, 147.4, 147.5, 148.5; FTIR (KBr pellet, cm1) 3053, 3024, 2925, 2850, 2569, 1613, 1590, 1518, 1481, 1469, 1448, 1419, 1338, 1264, 1250, 1171, 1155, 1121, 1106, 1079, 1021, 1008, 959, 907, 858, 838. 2.2. Chemical and reagents Absolute ethanol was purchased from Fluka and used without further purification. Acetonitrile (Fluka for HPLC analysis) was purified by drying with calcium hydride, followed by distillation from phosphorus pentoxide. It was kept over molecular sieves (3 Å, Merck) in order to eliminate its water content as much as possible. Potassium chloride (KCl), potassium ferricyanide (K3Fe(CN)6), hexaammineruthenium(III) chloride (Ru(NH3)6Cl3), ferrocene (Fc) and tetrabutylammonium perchlorate (TBAClO4) were obtained from Fluka and Aldrich. All chemicals were of analytical reagent grade. Au wire (0.762 mm diameter, 99.999% purity) and Au foil (0.127 mm diameter, 99.99% purity) were purchased from Alfa Aesar. 2.3. Monolayer formation The gold electrodes used for electrochemical measurements were prepared as described earlier by Hamelin [33]. First of all, Au wire was cleaned by sequential rinsing with distilled water, absolute ethanol, piranha solution (3:1, H2SO4, 30% H2O2), distilled water, and absolute ethanol. Caution: Piranha is a vigorous oxidant and should be used with extreme caution! After being dried in nitrogen gas stream, the clean Au wire was melted in a H2/O2 flame to form a 1.5–2.5 mm diameter droplet at the end of the wire, and then the droplet was annealed in the flame. Finally, the surface of Au electrode containing some elliptical (1 1 1) facets was tested electrochemically in 1 M H2SO4 solution [34]. The gold plates used for spectroscopic experiments were cleaned by immersion in piranha solution for 10 min, followed by a 1-min rinse in water. Afterwards, the Au substrates were electrochemically cleaned by cycling the electrode potential between 0.0 and 1.6 V vs. Ag/AgCl in 1 M H2SO4 solution until stable voltammograms corresponding to an unmodified gold electrode surface were obtained, and then washed with distilled water and ethanol, and dried in nitrogen gas stream. Monolayers of 6a and of 6b were prepared by dipping freshly cleaned gold substrates in a 1 mM ethanol solution of each aromatic thiol for at least 72 h at 298 K in dark. After being removed from the solution, the modified surfaces were rinsed with ethanol several times to remove weakly adsorbed molecules from the SAM surface, and were then dried with a flow of nitrogen gas. 2.4. Electrochemistry Electrochemical experiments were performed on a Bioanalytical Systems BAS 100B/W electrochemical analyzer. All measurements were carried out in a single-compartment electrochemical cell with a standard three-electrode arrangement. The gold electrode modified with aromatic thiols and a spiral Pt wire were used as the working and the counter electrodes, respectively. All potentials were reported versus Ag/AgCl/KCl (3.0 M) reference electrode at room temperature. For faradaic electrochemical measurements, the analytes were 1.0 mM K3Fe(CN)6 (in 0.1 M KCl), 1.0 mM Ru(NH3)6Cl3 (in 0.1 M KCl) and 1.0 mM ferrocene (in acetonitrile containing 0.1 M TBAClO4). The electrolyte solutions were

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degassed with purified nitrogen for 10 min before each experiment and bubbled with nitrogen during the experiment. The water used for electrochemical experiments was purified using a Barnstead Nanopure purification system and had a resistance of 18 MX cm. 2.5. Spectroscopic measurements Transmission infrared spectra of both thiols were measured as a KBr pellet using a Perkin–Elmer Spectrum-1 spectrometer. The reflectance IR measurements of the monolayers on gold were recorded on the horizontal attenuated total reflection cell of a Perkin–Elmer Spectrum-1 system. The samples were placed directly on the ZnSe window (45° angle of incidence), and the spectra were recorded with a resolution of 2 cm1 and averaged 1200 scans. Similarly, the spectrum for the bare gold substrate was also collected. The resulting spectra of the films were obtained by subtracting the bare Au spectrum from the corresponding AuSAM spectrum. Steady-state fluorescence spectra were obtained by using a Shimadzu RF-5301 PC spectrofluorometer. Lifetime measurements for modified gold surfaces were made on a LaserStrobe Timemaster fluorescence lifetime spectrometer. The pulsed nitrogen laser/tunable dye laser combination was used as the excitation source. The pulse width of the laser is about 800 ps with a repetition rate of up to 20 pulses per second. The emission slits were kept at 3 nm. Each lifetime decay was measured using two averages of five shots per point, collected randomly in time over 200 channels. Data were acquired according to an arithmetic data collection method, and analyzed using the commercial Timemaster software with both single exponential and double exponential fits. The quality of fit was evaluated by v2 values and visual inspection of the residuals. Spectroelectrochemical fluorescence measurements for modified gold surfaces in 0.1 M KCl solution were carried out in an emission cuvette used as the electrochemical cell. A platinum wire counter electrode and a silver wire as a pseudo-reference electrode were placed in this cell, and these electrodes were kept away from the path of the light. The working electrodes were the gold plate

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electrodes modified with thiols. The steady-state and timeresolved fluorescence measurements were recorded as mentioned above under different potentials applied to the working electrodes. 1 H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Varian spectrometer at room temperature. Static contact angles were measured with deionized water using a CAM 101 contact angle goniometer. Drops were placed on the surface in the ambient environment and were reported as the average of three measurements.

3. Results and discussion 3.1. Characterization of thiol SAMs Contact angle measurements are known to be a simple and useful method to characterize the surface properties of organic thin films [35]. If a clean gold surface, which should result in a very small contact angle with water, is covered with self-assembled monolayers, the wettability of the surface can be changed. However, before immersion in the thiol solution, a gold surface will be relatively more hydrophobic due to the adsorption of the organic contaminants from ambient environment [36]. In our laboratory atmosphere, we determined a water contact angle of 62° for the fresh gold surface. The contact angle values for the monolayers of 6a and 6b were also measured as ha = 74° ± 2° and ha = 71° ± 1°, respectively, indicating that the chemical and structural properties of the surfaces were similar. Infrared spectroscopy is another useful technique for studying the presence and order of molecules adsorbed on gold surface [37]. In this regard; the Au-6a and Au-6b monolayers were investigated by ATR-FTIR spectroscopy by comparison of the infrared group frequencies of the bulk compound and monolayer. Fig. 1 displays the reflection spectra for both monolayers prepared from 6a and 6b with the transmission spectra of bulk thiols in KBr. The resemblance of transmission spectra for thiol 6a and 6b (Fig. 1a and b) is not surprising, except for NO2 asymmetric (mas(NO2) at

Fig. 1. Infrared spectra of 6a and 6b (a,b) in KBr pellet and (c,d) on Au (ATR-FTIR).

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1518 cm1), symmetric (ms(NO2) at 1337 cm1) and wagging (mw(NO2) at 838 cm1) modes. The high energy absorptions between 2800 and 3100 cm1 for thiols (Fig. 1a) are related to the symmetric-CH2 (ms(CH2)), asymmetric-CH2 (mas(CH2)) and aromatic-CH vibrations. The peaks around 2564–2569 cm1 are also the characteristic-SH stretching vibrations. In the low-frequency region for thiols (Fig. 1b), the peaks in the vicinity of 1450 and 1600 cm1 can be assigned to the C@C stretching modes of aromatic rings. Also, the observed peaks at 1608 and 1612 cm1 for 6a and 6b, respectively, are typically the characteristic bands of the para-substituted phenyl rings [38]. Additionally, it should be noted that the peaks at 1446 and 1476 cm1 in 6a spectrum correspond to the fluorene ring modes. However, in this region, 6b displays the ring modes shifted to higher energy (1448 and 1481 cm1) and a new band at 1468 cm1 as a consequence of the electron-withdrawing power of the nitro substituent on the fluorene ring. After modification, the spectra in the carbon–hydrogen region of the films are depicted in Fig. 1c. The peak positions of the symmetric and asymmetric methylene stretching bands are red-shifted by 2 cm1 compared to those of the KBr samples (2850 and 2920 cm1) [39]. On the other hand, the bands between 3000 and 3060 cm1 corresponding to the aromatic-CH stretch are not visible in SAMs. The reason for this is not clear, however, our results are very similar to the previous reports on the fluoreneterminated alkylthiols on gold substrate or gold clusters [23,24]. Fig. 1d shows the spectra of the SAMs in midfrequency region. As expected, the nitro peaks are not seen in the spectrum of the Au-6a, however, in the spectrum of the Au-6b both symmetric and asymmetric NO2 peaks are observed at 1335 and 1513 cm1, respectively.

Fig. 2. Cyclic voltammograms of: (a) bare, (b) 6a and (c) 6b modified gold electrodes in 1 mM K3Fe(CN)6 and 0.1 M KCl.

3.2. Blocking behavior of thiol SAMs Cyclic voltammetry experiments would provide detailed information about the integrality and the packing quality of a SAM on gold by probing the blocking degree of the electrode reactions of the redox active probes. Hence, the relationship between the structural order and electrochemical behavior of Au-6a and Au-6b SAMs was investigated by cyclic voltammetry in the presence of various 3þ redox probes such as FeðCNÞ3 6 , RuðNH3 Þ6 and ferrocene (Fc). Figs. 2 and 3, respectively, show the cyclic voltammograms of 1 mM K3Fe(CN)6 and 1 mM Ru(NH3)6Cl3 in 0.1 M KCl solution for bare Au and SAM-modified Au electrodes at a potential scan rate of 100 mV s1. As expected, both of the redox probes on bare gold electrodes exhibit a reversible voltammogram corresponding to the diffusion-limited one-electron transfer process (Figs. 2a and 3a). In contrast, the modified gold electrodes, in the presence of FeðCNÞ3 6 , do not present any faradaic current associated with the reduction and reoxidation of FeðCNÞ3 6 (Fig. 2b and c). On the other hand, when performing the experiments in Ru(NH3)6Cl3 solution, the well-defined reduction and oxidation peaks of redox probe on the Au-6b electrode (Fig. 3c) is still seen with attenuated conductivity and increased peak-to-peak separation (DEp, from 70 to 135 mV) compared to that on the bare Au electrode. However, under the same conditions, the redox reaction of RuðNH3 Þ3þ on the 6 Au-6a electrode is almost inhibited with an increase in capacitive current (Fig. 3b). In order to probe the reason of this remarkable difference between the electrochemical behaviors of the assemblies, the voltammetric studies were performed in an aprotic solvent like acetonitrile using the Fc redox probe. As shown in Fig. 4, the modified surfaces exhibit poor blocking ability towards this redox probe. The shape of cyclic voltammogram and current response at modified electrodes depend on a lot of variables such as the structural order and thickness of the SAM [37,40–42], the size and

Fig. 3. Cyclic voltammograms of: (a) bare, (b) 6a and (c) 6b modified gold electrodes in 1 mM Ru(NH3)6Cl3 and 0.1 M KCl.

chemical nature of probe molecules [43,44], and the solvent media containing monolayer and the probe molecules [43]. When these factors are considered, the cyclic voltammetric characteristics of 6a and 6b SAMs can be explained as follows: As a first conclusion, the fact that the peak-shaped electrochemical response of FeðCNÞ3 is almost completely suppressed on both 6 films (Fig. 2b and c) indicates the absence of large surface defects in the SAMs. If there were extremely disordered regions within the SAMs, the electroactive species could directly approach to the electrode from these regions and easily undergo the electron transfer reaction with the underlying Au-surface [45]. On the other hand, it is apparent that the films are permeable to the Fc/Fc+ redox species in acetonitrile due to the observed faradaic currents (Fig. 4b and c). This result can be explained by the interactions between the monolayer and contacting solvent media [46,47]. In aqueous media, the repulsion between hydrophobic monolayer and water molecules forms an impermeable and compact monolayer. On the other hand, an organic solvent like

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Fig. 4. Cyclic voltammograms of: (a) bare, (b) 6a and (c) 6b modified gold electrodes in acetonitrile solution containing 1 mM ferrocene and 0.1 M TBAClO4, scan rate is 100 mV s1.

acetonitrile has a tendency to loosen the monolayer packing due to its ability to solvate the aromatic rings. As a result of this effect, the existing micro-pinholes in the SAMs will arise and these pinholes will act as microelectrode arrays [48–50]. Meanwhile, if the density of micro-pinholes within the SAM is insignificant the resulting cyclic voltammogram will appear as sigmoidal-shaped due to radial-diffusion limited electron transfer. On the contrary, if the monolayer has numerous microelectrode arrays, the diffusion spheres of these microelectrodes overlap, and the electrodes display peak-shaped voltammograms similar to those of lineardiffusion controlled cyclic voltammograms. In acetonitrile the obtained current–potential curves for modified electrodes (Fig. 4), especially that of Au-6b, have pointed out the presence of such a behavior with a very small bend of the sigmoidal curve. In addition, it can be concluded that, on the basis of peak-to-peak separation and current density for the films, Au-6a has more micro-pinhole density than Au-6b. To confirm the more compact nature of the Au-6b monolayer, the capacitance of monolayers were measured at 0 V in 0.1 M KCl using cyclic voltammetry, and the capacitance values for Au-6a and Au-6b electrodes were found to be 8.7 lF cm2 and 6.9 lF cm2, respectively. The results of these experiments also confirm that 6b forms a closely packed layer in comparison to 6a [51,52]. However, unexpectedly, the redox reaction of Ru(NH3)6Cl3 is quite facile in the case of the compact Au-6b monolayer. This behavior can result from many factors such as electron tunneling process, the presence of the redox-active NO2 substituent on the fluorene ring and the nature of the redox probe molecules in solution. We have carried out several experiments to examine the impacts of these factors on the selectivity of Au-6b layer. Primarily, the electrochemical behavior of the films was investigated in 0.1 M KCl solution at 100 mV s1 over the potential range of +0.2 to 0.8 V. The cyclic voltammograms are presented in Fig. 5. No faradaic process is observed over this potential range for Au-6a electrode (Fig. 5a). On the other hand, for Au-6b, a large irreversible reduction peak at Ep = 735 mV and a reversible couple at E ° = 260 mV are seen (Fig. 5b). This voltammetric behavior is in good agreement with the well-known electrochemistry of nitroaromatic compounds [53,54]. We also studied the voltammetric behavior of hydrophilic FeðCNÞ3 and RuðNH3 Þ3þ redox probes on the modified Au elec6 6 trodes at large overpotentials. Cyclic voltammograms were recorded for both redox probes in 0.1 M KCl solution at a scan rate

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Fig. 5. Cyclic voltammograms of: (a) Au-6a and (b) Au-6b electrodes in 0.1 M KCl solution, scan rate is 100 mV s1.

of 100 mV s1. Fig. 6A and B shows the voltammograms obtained for FeðCNÞ3 and RuðNH3 Þ3þ on the modified electrodes, respec6 6 tively. On the Au-6a electrode, only very low currents are observed for both redox probes at large overpotentials. In contrast, Au-6b electrode displays significant currents associated with the reduction and reoxidation of the redox probes. These experiments clearly indicate that the facile nature of electron transfer between RuðNH3 Þ3þ 6 redox probe and the Au-6b electrode cannot be attributed to the tunneling process because 6b film is structurally similar to 6a. On the other hand, electrochemical chemical ECcat mechanism is an important process for the electrochemical behavior of the redox probes in the presence of the monolayers containing redox centers [55–57]. For the ECcat mechanism, the chemical step is thermodynamically promoted only for probes having a standard electrode potential less than that of the redox centers immobilized in the film. In addition, it is known that RuðNH3 Þ3þ 6 is a simple outer-sphere redox species with fast surface-insensitive electron transfer kinetics [58,59]. Hence, the access of the RuðNH3 Þ3þ to 6 the electrode surface is not necessary for the electron transfer to occur. In contrast, since FeðCNÞ3 6 is an inner-sphere redox species, its electron transfer process depends on the electrode surface, and the access of the redox probe to the electrode surface is required for the electron transfer to occur. Consequently, we believe that the redox reaction of RuðNH3 Þ3þ 6 on Au-6b originates from an ECcat mechanism. 3.3. Optical properties of thiol SAMs Fig. 7 shows steady-state fluorescence spectra of 6a and 6b in acetonitrile and in the solid state as monolayer. In homogeneous solution, upon excitation at 280 nm, 6a shows a strong emission band at 310 nm (curve a), whereas 6b displays a maximum at 300 nm (curve b). These spectral profiles obtained for thiols are similar to those of their parent fluorene and 2-nitrofluorene compounds with a slight red shift (3–5 nm) as a result of the presence of the benzene ring. On the other hand, the emission spectra of 6a and 6b films on gold (curve c and d) are markedly different from those recorded in acetonitril solution. In this case, they present significantly red-shifted (50–60 nm) and broadened emission bands. Furthermore, the peak emission intensities of Au-6a and Au-6b SAMs are strongly quenched. A large red shift in the

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F. Pak et al. / Journal of Electroanalytical Chemistry 654 (2011) 20–28

Fig. 7. Steady-state fluorescence spectrum of: (a) 6a and (b) 6b in acetonitrile solution and front-face reflectance fluorescence spectrum of (c) Au-6a and (d) Au6b SAMs.

Fig. 8. Fluorescence decay curves for 6a and 6b adsorbed on gold.

Fig. 6. Cyclic voltammograms recorded for (a) Au-6a and (b) Au-6b electrodes in (A) 1 mM K3Fe(CN)6 and 0.1 M KCl and (B) 1 mM Ru(NH3)6Cl3 and 0.1 M KCl solutions.

emission spectrum can be ascribed to the order and close packing of the molecules in the films leads to very strong intermolecular interactions [23]. The decreased fluorescence intensity for monolayers is also caused by the efficient quenching of the excited chromophores by the gold support. To gain further insight into the excited-state dynamics of Au-6a and Au-6b films, the fluorescence decay profiles of the SAMs were analyzed by using time-resolved fluorescence spectroscopy. Emission decay traces were acquired at 337 nm, near the steadystate emission maximum. Typical fluorescence decay curves of the films are shown in Fig. 8. Unlike alkylfluorenyl thiols on gold (biexponential decay curves, s1  260 ps and s2  3.2 ns) [23], the fluorescence decays of our samples were best described by a monoexponential function, and the fluorescence decay lifetimes were determined to be s = 1.90 ± 0.04 ns (v2 = 1.10) for Au-6a and s = 1.68 ± 0.02 ns (v2 = 0.95) for Au-6b. These relative lifetimes for both fluorophores attached to the gold are shorter than that obtained for the free 6a in acetonitrile solution (s = 4.60 ± 0.04 ns and v2 = 1.10).

Direct binding of a fluorophore to the metal surface often results in a similar quenching of the excited state life time, and this quenching is attributed to energy or electron transfer processes considered to be major deactivation pathways for excited fluorophores on metal surfaces [60–62]. An alternative explanation for the decreased fluorescence lifetime is also intersystem crossing from the excited singlet to the corresponding triplet. However, nanosecond flash photolysis studies of SAMs containing the fluorene terminal group at the end of a long alkyl spacer have revealed that deactivation by the gold support is more favorable than intersystem crossing [24]. Moreover, the deactivation rate is known to depend strongly on distance between the fluorophore and surface and the nature of the chemical bonds present in the spacer [23,63]. Therefore, we believe that intersystem crossing for fluorene and nitrofluorene molecules bound to the gold via a short benzene spacer is effectively suppressed. In order to estimate the possible contribution of photoinduced energy and/or electron transfer processes into the solid state photophysical properties of 6a and 6b SAMs and the dominant process, we performed spectroelectrochemical fluorescence and lifetime measurements in a similar way to the spectroelectrochemical measurements made by Kamat et al. [64]. Then, the gold working

F. Pak et al. / Journal of Electroanalytical Chemistry 654 (2011) 20–28 Table 1 Fluorescence lifetimes obtained for Au-6a and Au-6b in 0.1 M KCl solution as a function of potential. Potential (V)

No bias 0 0.1 0.2 0.4 0.6 0.8 1 1.2

Au-1

Au-2

s (ns) ± 0.02

v2

s (ns) ± 0.02

v2

1.30 1.88 1.55 1.77 1.33 1.64 1.35 1.50 1.25

1.00 0.99 0.95 1.02 1.02 0.97 0.98 1.1 1.12

1.18 1.22 1.19 1.19 1.18 1.18 1.13 s1 = s2 = 1.26 s1 = s2 = 1.32

1.02 0.95 0.98 0.98 1.07 0.96 0.96 1.2 1.2

electrodes modified with 6a and 6b were placed at an angle of about 45° in the emission cuvette containing 0.1 M KCl solution, and the steady-state fluorescence spectra and decay curves were recorded as a function of the potential applied to the working electrodes. The typical fitting results of the fluorescence decay curves are listed in Table 1. When the potential applied to the gold electrodes was tuned to negative values (from 0.0 V to 0.8 V), a significant change in the lifetime responses was not observed. Additionally, it was obtained that the emission band intensities for both films remained almost constant. These results clearly indicate that the excited-state deactivation of the excited molecules on the metal surfaces takes place mainly by energy transfer process. Conversely, if electron transfer was a dominant pathway for the deactivation the electron transfer reaction between the excited fluorophore and negatively charged gold would be prevented due to the externally applied electrochemical bias, and thus, an enhancement in the emission intensities and life times of the surface-bound fluorophores could be observed [20]. On the other hand, when the potential reached to more negative values (1.0 V), the fluorescence decay of the 6b monolayer was described by a biexponential function. This change can be also attributed to the new species (hydroxylamine –NHOH or nitroso –NO functionalized molecules) formed by the reduction of the nitro group in aqueous media [53].

4. Conclusions In this study, self-assembled monolayers of fluoreneterminated thiols, 6a and 6b, on gold substrates were successfully prepared and the SAMs were analyzed by contact angle measurements and HATR-FTIR spectroscopy. In order to obtain the integrity of the films and to assess their passivating behavior, we performed cyclic voltammetry experiments in the presence of different redox probes. According to the results of these experiments, the monolayers act as a kinetic barrier toward the electrochemistry of FeðCNÞ3 6 redox species. In contrast, they are partially permeable toward hydrophobic ferrocene in an organic solvent like acetonitrile as a result of solvation effect. Additionally, the insertion of nitro group into the fluorene terminated thiol was found to facilitate the redox reaction of RuðNH3 Þ6 Cl3 because of redox mediation. The steady-state fluorescence experiments showed that the emission bands of SAMs were red-shifted and broadened as well as their diminishing emission intensities. Furthermore, it was obtained that the lifetimes of the fluorophores attached to the gold are shorter than that of the thiol 6a in solution. The analysis of the data obtained by a combination of potential-controlled electrolysis and fluorescence measurements demonstrate that the excited molecules are quenched by the gold surface via energy transfer process.

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