J. Phys. Chem. B 2001, 105, 3229-3234
3229
Enhancement of Second-Harmonic Response and Photocurrent Generation from a Benzothiazolium Styryl Dye LB Film through an Interfacial Self-Assembly Reaction Jie Zheng,* Fuyou Li, and Chun-Hui Huang* State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking UniVersity-The UniVersity of Hong Kong Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China
Ting-ting Liu, Xing-sheng Zhao, Xiaofen Yu, and Nianzhu Wu Department of Chemistry, Peking UniVersity, Beijing 100871, China ReceiVed: October 2, 2000; In Final Form: January 9, 2001
An interface self-assembly reaction between HgCl2 and an amphiphilic heteroaromatic styryl dye, 2-(4dihexadecylaminostyryl) benzothiazole methiodide (I), was studied. Its Langmuir-Blodgett films Ia, Ib, and Ic fabricated from different subphases (pure water, 5 × 10-8 M HgCl2, and 5 × 10-7 M HgCl2, respectively) were characterized by using UV-vis spectra and XPS measurements. Compared to those of film Ia, the seond-harmonic responses and photocurrent generation from film Ib and Ic were obviously enhanced with the addition of HgCl2 into the subphase. Under ambient conditions (0.5M KCl), the quantum yields of photoelectric conversion are 0.73%, 0.95%, and 2.45% for Ia, Ib, and Ic respectively. The effects of some factors such as bias voltage, methyl viologen (MV2+) and hydroquinone (H2Q) on the photocurrent generation have also been investigated.
Introduction
CHART 1
Nonlinear optical and photoelectric materials attract intense attention because of their prospective applications in the fields of information and energy resources.1,2 In recent years, our group has found that the Langmuir-Blodgett (LB) films of some organic dyes with donor-π-acceptor (D-π-A) structures such as those of hemicyanines have good photoelectric conversion properties (PEC), besides their well-known good secondharmonic generation (SHG) properties.3-8 Synthetic strategies such as changing donors, acceptors,3-5 or the number of the chromophores6-8 are generally used to enhance SHG and PEC properties of such type of organic dyes. However, there have been fewer attempts to change these two properties by using interfacial self-assembly reactions9 between soluble species in subphase with amphilphilic organic dye molecules on it. In this contribution, we studied an interaction between the HgCl2 subphase and molecules of an amphiphilic benzothiazolium styryl dye, 2-(4-dihexadecylaminostyryl) benzothiazolium styryl dye (I in Chart 1). The effects of HgCl2 subphase on SHG and PEC of the dye LB films were investigated as follows. Experimental Section Materials. The synthesis of 2-(4-dihexadecylaminostyryl) benzothiazole methiodide (I) was described in our past work.3 Hydroquinone (H2Q), KCl, methyl iodide, and other reagents are all analytical reagent grade and were used as received. Water (R ) 18 Mohm‚cm) used is purified by passing deionized water through an easy pure RT Compact ultrapure water system (Barnstead Co.). For methyl viologen diiodide (MV2+), 4,4bipyridyl was reacted with excess methyl iodide in refluxing ethanol for 6 h. The product was filtered and washed with ethanol at least four times. Its identity was confirmed by using 1H NMR spectra.
Apparatus. UV-vis spectra were recorded by using Shimadzu UV-3100 spectrometer. Films were formed and deposited on ITO electrodes by using a Langmuir trough (Nima Technology Model 622). X-ray photoelectron spectra (XPS) were measured by a VG ESCA LAB 5 Multitechniques photoelectron spectrometer (VG Co., UK), and Al was used as the target of X-ray source. LB Film Formation. A Sample of I in chloroform solution (0.3 mg ml-1) was spread onto water and HgCl2 solution
10.1021/jp003605q CCC: $20.00 © 2001 American Chemical Society Published on Web 03/28/2001
3230 J. Phys. Chem. B, Vol. 105, No. 16, 2001 subphase at 20 ( 1 °C, respectively. After the evaporation of the solvent, the monolayer was compressed at the rate of 25 cm2 min-1 and then transferred at a rate of 5 mm min-1 (Vertical dipping) under a surface pressure of 30 mN m-1 onto transparent electrodes of indium-tin oxide (ITO)-coated borosilicate glass (the resistance was 250 Ω). To ensure the formation of the hydrophilic surface, we immsersed the plate for 2 days in a saturated sodium methanol solution and then thoroughly rinsed it with pure water under ultrasonication for several times. Films with a transfer ratio of ca. 1.0 ( 0.1 were used in the experiments. SHG Measurements. SHG measurements were performed in transmission geometry using a Y-type quartz plate (d11 ) 0.5 pm V-1) as a reference and with a Q-switched Nd:YAG laser (1.064 µm). A 1/2λ plate and a Glan-Taylor polarizer were used to vary the polarization direction of the laser beam. The laser light, linearly polarized either parallel (p) or perpendicular(s) to the plane of incidence, was directed at an incidence angle of 45° onto vertically mounted samples. A set of 1.064 µm filters and a monochromator were used to ensure that the signal detected by the photomuliplier was due to second-harmonic radiation generated by film. The average output signal was recorded on a digital storage oscilloscope (HP545100). All the reported SHG data are averaged values of at least three separate measurements. Photoelectrochemical and Electrochemical Measurements. A 500 W Xe arc lamp was used as the light source in the photoelectrochemical studies, and various filters (ca. 300-800 nm) were used to obtain different wavelengths. The intensity of light was measured with an energy and power meter (Scientech, USA). A conventional glass three-electrode cell with the film-fabricated ITO electrode as the working electrode, a polished Pt wire as the counter-electrode, and saturated calomel electrode (SCE) as the reference electrode were used in the measurements. The supporting electrolyte was an aqueous solution of 0.5 M KCl in the photoelectrochemical studied. All experimental data were recorded using a model CH 600voltammetric analyzer controlled by a computer. In the electrochemical measurements, the solution was deoxygenated with bubbling nitrogen for at least 15 min. The effective illuminated area of the working electrode was 0.5 cm2 in all photoelectrochemistry experiments. Results and Discussion Characterization of LB Films. 1. Formation of LangmuirBlodgett Film. Surface pressure versus area isotherms of I LB films were distinctive with changes in the subphases used. As shown in Figure 1, the limiting area of an isotherm (a) obtained from pure water subphase is 53 Å2, and the corresponding collapse pressure is 48 mN m-1. When HgCl2 was added into the subphase and reached 5 × 10-8 M, the limiting area increased to 57 Å2, and the collapse pressure decreased to about 40 mN m-1 (isotherm b). Enhancement of HgCl2 concentration to 5 × 10-7 M leads to the increase of limiting area to 61 Å2 and the decrease of collapse pressure to about 35 mN m-1 (isotherm c). However, further enhancement of HgCl2 concentration to 5 × 10-6 M in subphase does not have obvious effects on the limiting area and collapse pressure (isotherm d). These experimental results indicate that addition of HgCl2 into the water subphase could affect the dye film properties in a certain range. 2. UV-Vis Spectra of the Films. To further understand effect of HgCl2 subphase on the molecular arrangement, we studied UV-vis spectra of the films fabricated from different subphase,
Zheng et al.
Figure 1. Surface pressure-area isotherms of dye I on pure water and HgCl2 subphases with different concentrations (20 ( 1 °C).
Figure 2. Absorption spectra of the films Ia, Ib, and Ic on the ITO substrate.
and they are shown in Figure 2. Several conclusions can be obtained from experimental data. First, the maximum absorption of I film fabricated from pure water subphase is at 464 nm (Ia). Compared to the maximum wavelength (564 nm) of I in chloroform,3 the large blue shift (100 nm) suggests that H aggregates10 were formed on ITO electrodes.10 Second, when HgCl2 was added into subphase to 5 × 10-8 M, the maximum wavelength of film absorption spectrum is red-shifted to 504 nm (Ib). Third, when the HgCl2 concentration is further increased to 5 × 10-7 M, the maximum wavelength of the film reaches 525 nm (Ic), but higher concentration cannot make the maximum wavelength red-shift any more, in agreement with film compression data. These results indicated that addition of HgCl2 into the subphase could affect the dye molecular alignment on ITO substrates. For comparison, the isotherms and UV-vis spectra of some other dyes II and III (shown in Chart 1) with structures similar to that of I were also studied under the same conditions; however, no obvious changes were observed in the limiting areas (53 Å2 and 57 Å2), and in maximum wavelengths of LB monolayers (476 nm, and 529 nm) for II and III respectively, with change of subphase. These experimental data are in agreement with the reported values in the past work.11,12 Replacement of HgCl2 with KCl and NaCl salts cannot change the isotherms or UV-vis spectra under the same conditions
Benzothiazolium Styryl Dye LB Film
Figure 3. XPS spectra of Hg4f7/2 in HgCl2 on ITO electrode and films Ib and Ic.
Figure 4. XPS spectra of S2p in films Ia, Ib, and Ic.
either. These results suggest the change in isotherms and the spectra of I could be attributed to the interaction between the S atom of I and the Hg atom in HgCl2. 3. X-ray Photoelectron Spectroscopy (XPS) Measurements. To further clarify this interaction, XPS technique was used to study the binding energies of S and Hg atoms under the different conditions. Several conclusions can be obtained from the data.
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3231 First, it can be seen from Figure 3 that organic films Ib and Ic contain characteristic peaks of Hg 4f7/2, but the binding energy of Hg 4f7/2 is shifted from 98.6 eV for pure HgCl2 (Curve HgCl2) on ITO electrode to 100.1 eV for both films Ib and Ic, which suggests that the coordination surrounding of Hg atom has been changed. Second, the same values of binding energy of Hg atom in films Ib and Ic indicated that the coordination surrounding of Hg atom in the films is independent of the concentration of HgCl2 subphase. Third, it can seen from Figure 4 that the binding energy of S2p is shifted from 164.4 eV for Ic to 163.9 eV in film Ib and 162.9 eV in film Ic, which also suggests that the HgCl2 and I molecules interact through S and Hg atoms and form coordination interaction in the films. Fourth, the atom ratio of S and Hg for film Ic is about 1.94:1, suggesting that coordination ratio of I and HgCl2 is 2:1. Because HgCl2 does not ionize and exists in molecule form in water,13 a possible scheme of alignment for dye I molecules on HgCl2 subphase (5 × 10-7 M) is proposed (Scheme 1), in which the Hg atom is in four-coordinated with two Cl atoms and two dye molecules. Fifth, the atom ratio of S and Hg in film Ib is 4.5:1, indicating that not all S atoms of the dye coordinate with Hg atom at the concentration of 5 × 10-8 M HgCl2 subphase, leading to binding energy of S atom for film Ib (Figure 4) sites between 164.4 eV and 162.9 eV. Second-Order Susceptibilities of the Dye Films. On the assumption that the chromophores have a common tilt angle φ° with respect to the normal of the film surface, with a random azimuthal distribution, a monolayer thickness of (3 nm) for all the films is calculated from the length of the molecules. Considering the partical resonance absorption of all the films with second-harmonic wavelength (λ2ω ) 532 nm), we take the refractive indexes of the films at the fundamental (nω) and seond-harmonic frequencies (n2ω) is to be 1.5 and 1.8 from the films. Data of SHG from the LB films were analyzed by the general procedures described by Ashwell et al.14 Seond-harmonic generation properties of the films were shown in Table 1. Several conclusions can be obtained from the results. First, it can be seen that common tilt angle φ° changes from 43° for film Ia to 34° for film Ic, suggesting chromophores of the dyes become more and more perpendicular to substrates with the addition of HgCl2 into the subphase(as shown in Scheme 2). The tilt angle for film Ib is 37° because just part of dye molecules coordinate with HgCl2, leading to the existence of different φ°’s of 43° and 34° at the same time in the same film. Second, the addition of HgCl2 into subphase
SCHEME 1: Possible Arrangement of I Dye Molecules on 5 × 10-7 M HgCl2 Subphase
3232 J. Phys. Chem. B, Vol. 105, No. 16, 2001
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Figure 5. Photocurrent generation from the IC-ITO electrode upon irradiation with white light at 113 mWcm-2 in the 0.5 M KCl solution (20 ( 1 °C).
SCHEME 2: Different Arrangement of I Dye Molecules in the Different Subphase
Figure 6. Action spectra of the cathodic photocurrent forfilms Ia, Ib, and Ic. The intensities of photocurrent at the different wavelengths were all normalized.
TABLE 2: Photoelectric Conversion Property Data for Films Ia, Ib, and Ica Ib (nA/cm2)
φb (%)
films
KClc
biasd
MV2+ e
H2Qf
KClc
biasd
MV2+ e
H2Qf
Ia Ib Ic
83 100 108
137 188 233
257 272 134
-139 -217 -223
0.73 0.95 2.45
1.36 1.79 5.29
2.25 2.62 3.04
1.22 2.10 5.07
a Ia: Film fabricated from pure water subphase. Ib: Film fabricated from 5 × 10-8 M HgCl2 subphase. Ic: Film fabricated from 5 × 10-7 M HgCl2 subphase. b Under the irradiation of 464 nm monochromated light with an intensuity of 3.24 × 1015 photon/cm2 in 0.5 M KCl solution; absorptions are 0.0097, 0.009, and 0.0036 for Ia, Ib, and Ic respectively. c Ambient conditions. d -200mV bias voltage. e 1 mg/mL MV2+. f 0.1 mg/mL H2Q in 0.5 M KCl solution.
TABLE 1: Seond-Harmonic Generation for Films Ia, Ib, and Ic films
χzzz (pm V-1)
χzxx (pm V-1)
φ (deg)
Ia Ib Ic
215 282 397
93 80 90
43 37 34
also leads to the enhancement of χzzz from 215 pmV-1 for film Ia to 397 pmV-1 for film Ic, suggesting that the introduction of HgCl2 into the subphase is favorable to increasing the SHG property of the dye I LB film. Third, little change of χzxx values with the addition of HgCl2 into the subphase could be attributed to more perpendicular chromophores in film Ic to the substrates, which is unfavorable to enhancement of seond-harmonic response in zxx direction. Photocurrent Generation from the Dye-ITO Electrodes. To understand effect of the HgCl2 subphase on photoinduced electron transfer in dye I LB films, we studied photocurrent generation responses in the films Ia, Ib, and Ic. Steady cathodic photocurrents flowed instantly when the different film-modified ITO electrodes were illuminated by the white light of 113 mW cm-2 and fell as soon as irradiation was terminated. When O2 was removed form the electrolyte solution by bubbling N2, the cathodic photocurrents decrease greatly, suggesting that the existence of O2 as electron acceptors in solution is responsible for the cathodic photocurrent generation in the ambient condition.15 The photoelectric responses were unchanged after repeated on-off cycling. Figure 5 shows a typical example of the experiments. The average values (obtained form eight samples for every type of film) are 618, 986, and 1580 nA cm-2 for film Ia, Ib, and Ic respectively. The photocurrent action
spectra for the films were obtained by illuminating the dyefabricated ITO electrodes respectively under the different wavelengths of light (shown in Figure 6). The action spectra for the films are coincident with their absorption spectra on ITO electrodes (Figure 2), and the maximum wavelengths of the action spectra are also red-shifted with a change in the concentration of HgCl2 in the subphase, which is in agreement with the observance in their electronic spectra. An anodic photocurrent of about 10 nA was generated when a blank ITO electrode was irradiated. These results indicated that photoinduced electron transfer took place and that the aggregates on the different films were responsible for the generation of photocurrents. Table 2 shows the photoelectric properties for the films. It can be clearly seen that under the illumination of monochromated 464 nm light with intensity of 3.24 × 1015 photo cm-2 s-1, the efficiency of photoelectric conversion are 0.73%, 0.95%, and 2.45% for films Ia, Ib, and Ic, respectively. Compared to that for film Ia, the quantum efficiencies can be enhanced about 1.3 times and 3.36 times for Ib and Ic by just the addition of HgCl2 into pure water subphase to 5 × 10-8 and 5 × 10-7 M, respectively. The observed cathodic photocurrents indicated that the photogenerated electron flows from the films to solution. To explore this process in detail, we investigated the different factors on these processes as follows. Effect of Bias Voltage. To study electron transfer between the ITO electrode and LB films, we investigated the effect of bias voltage. Figure 7 shows that there are good linear relationships between the potentials and photocurrents for the dyes within the range from -200 to 200 mV. The cathodic photocurrents increase when the potential of the working electrode becomes more negative to the SCE electrode and vice
Benzothiazolium Styryl Dye LB Film
Figure 7. Photocurrent vs electrode potential for films Ia, Ib, and Ic under ambient condition (113 mW cm-2 white light, 0.5 M KCl solution, 20 ( 1 °C).
Figure 8. The relationship between MV2+ concentration and photocurrents under ambient conditions (113 mW cm-2 white light, 0.5 M KCl solution, 20 ( 1 °C) for films Ia, Ib, and Ic.
versa, indicating that photocurrents flow in the same direction as the applied negative voltage. Such negative voltage on the ITO electrode can form a strong electric field within the LB films (ca. 3 nm), which can accelerate the rate of charge migration within dye aggregates and the rate of electron transfer from the ITO electrode to the film, leading to enhancement of cathodic photocurrents.15 If -200 mV is applied on an ITO electrode, quantum efficiencies of photocurrent conversion can reach 1.36%, 1.79%, and 5.29% for films Ia, Ib, and Ic, respectively. Effect of Electron Acceptor and Donor. To investigate the electron transfer between the excited aggregates and the electron donor or acceptor in the solution, we selected some typical electron donor and acceptor for photocurrent generation studies. Figure 8 shows that cathodic photocurrents increase with the addition of MV2+ in the solution up to 0.8 mg mL-1, indicating that MV2+ is a favorable factor to cathodic photocurrent generation. The enhancement of the photocurrents is due to the high electron affinity of MV2+.15 MV2+ accepts electrons from the film and accelerates the rate of electrons transfer from the film to the solution, consequently increasing the cathodic photocurrent. When the concentration of MV2+ reaches 1 mg mL-1, quantum yields can reach 2.25%, 2.62%, and 3.04%, for films Ia, Ib, and Ic, respectively. When H2Q was added into the solution, the photocurrents quickly reverse direction. The
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3233
Figure 9. Dependence of the photocurrent on the concentration of H2Q under ambient conditions (113 mW cm-2 white light, 0.5 M KCl solution, 20 ( 1 °C) for films Ia, Ib, and Ic.
resulting anodic photocurrent levels off when H2Q concentration is larger than 0.08 mg mL-1 in the solution (Figure 9). When the concentration of H2Q reaches 0.1 mg mL-1, quantum yields become 1.22%, 2.10%, and 5.07% for Ia, Ib, and Ic respectively. The H2Q in the solution donates electrons to quench the excited aggregates, resulting in the formation dye of anion radicals. These radicals inject electrons into the conduction band of ITO, leading to the anodic photocurrent generation. The experimental results indicated that the existence of the strong electron donor is unfavorable to the generation of the cathodic photocurrent.16 Although the electron acceptor or donor has different effects on the electron-transfer process from the films to the solution, they do not affect the formation of photogenerated electron-hole pairs within a dye aggregates, which is relative to energy levels of ground and excited states of organic dyes.17 For example, cathodic photocurrent is generated through the following process: the dye aggregates are excited by absorbing the light energy, and electrons transfer from the excited aggregates to the electron acceptor such as O2 or MV2+ in the solution; subsequently, electrons from the conduction band of the ITO electrode inject to the hole residing in the dyes aggregates.15,16 Although much work is still needed in clarifying the reason for enhancement of photocurrent with the addition of HgCl2, one possible reason is that addition of HgCl2 into subphase can the decrease energy levels of the excited states of the dye molecules in film, leading to more effective electron transfer between the different energy levels.15-19 Conclusions HgCl2 in the subphase interacts with an organic styryl amphiphilic dye I, 2-(4-dihexadecylaminostyryl) benzothiazole methiodide, through an interfacial self-assembly reaction. Evidence suggests that HgCl2 acts as a bridge to connect two dye molecules through coordination bonding between S and Hg atoms. Marked increases in the quantum yield of photoelectric conversion and seond-harmonic generation are observed. The experimental results indicated that modification of intermolecular action among chromophore molecules in a LB film may provide a new way to enhance these two properties for dyes with D-π-A structures. The study on the reason for this interesting phenomenon is in progress. Acknowledgment. The authors thank the State Key Program of Basic Research (G1998061300) and the National Natural Science Foundation of China for financial support on this
3234 J. Phys. Chem. B, Vol. 105, No. 16, 2001 project. J.Z. sincerely thanks Dr. Greg. Van. Patten for very helpful discussions and suggestions. References and Notes (1) (a)Mattay, J., Ed. In Topics in Current Chemisrty; SpringerVerlag: New York, 1991; Vol. 159. (b) Pfannschmidt, T.; Nilsson, A.; Allen, J. F. Nature 1999, 397, 625. (c) Steinberg-Yfrach, G.; Rigaud, J.-L.; Durantini, E. N.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1998, 392, 479. (d) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissortel, F.; Salbeck, J.; Spreiitzer, H.; Gratzel, M. Nature 1998, 395, 583. (2) (a) Mirikin, C. A.; Ratner, M. A. Annu. ReV. Phys. Chem. 1992, 43, 719. (b) Albert, I. D. L.; Marks, T. J.; Ratner, M. A. J. Am. Chem. Soc. 1997, 119, 6575. (c) Varanasi, P. R.; Jen, A. K.-Y.; Chandrasekhar, J.; Namboothiri, I. N. N.; Rathna, A. J. Am. Chem. Soc. 1996, 118, 12443. (3) Zheng, J.; Huang, C.-H.; Wei, T.-X.; Huang, Y.-Y.; Gan, L.-B. J. Mater. Chem. 2000, 10, 921. (4) Xia, W. S.; Huang, C. H.; Gan, L. B.; Luo, C. P. J. Phys. Chem. 1996, 100, 15525. (5) Liang, A. D.; Zhai, J.; Huang, C. H.; Gan, L. B.; Zhao, Y. L.; Zhou, D. J.; Chen Z. D. J. Phys. Chem. B 1998, 102, 1424. (6) Wu, D. G.; Huang, C. H.; Gan, L-B; Zheng, J. Langmuir 1999, 15, 7276.
Zheng et al. (7) Wu, D. G.; Huang, C. H.; Gan, L.-B.; Zhang, W.; Zheng, J.; Luo, H. X.; Li, N. Q. J. Phys. Chem. B 1999, 103, 4377. (8) Wu, D. G.; Huang, C. H.; Huang, Y.-Y.; Gan, L. B. J. Phys. Chem. B 1999, 103, 7130. (9) Weissbuch, I.; Baxter, P. N. W.; Kuzmenko, I.; Cohen, H.; Cohen, S.; Kjaer, K.; Howes, P. B.; Als-Neilsen, J.; Leiserowitz, L.; Lahav, M. Chem. Eur. J. 2000, 6, 4, 725, and reference therein. (10) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502. (11) Bubeck, C.; Laschewsky, A.; Lupo, D.; Neher, D.; Ottenbreit, P.; Prass, W.; Ringsdorf, H.; Wenger, G. AdV. Mater. 1991, 3, 54. (12) Li, F. Y.; Zheng, J.; Jin, L. P.; Guo, J. Q.; Huang, C. H. Unpublished results. (13) Cao, X. Z. Inorganic Chemistry; University Education Press: Beijing, 1990. (14) Ashwell, G. J.; Jackson, P. D.; Crossland, W. A. Nature 1994, 368, 438. (15) Kim, Y. S.; Liang, K.; Law, Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 984. (16) Auweraer Vaes, A.; Bosmans, M. V. D. P.; Schryver, F. C. D. J Phys. Chem. B 1998, 102, 5451. (17) Forrest, S. R. Chem. ReV. 1997, 97, 1973, and reference therein. (18) Law, K.-Y. Chem. ReV. 1993, 93, 449, and reference therein. (19) Shammai, S. Chem. ReV. 1996, 969, 1953, and reference therein.
