Excited-State Behavior of a Fluorescent and Photochromic

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J. Phys. Chem. C 2007, 111, 3853-3862

3853

Excited-State Behavior of a Fluorescent and Photochromic Diarylethene on Silver Nanoparticles Hidehiro Yamaguchi,† Kenji Matsuda,*,†,‡ and Masahiro Irie*,† Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity, and PRESTO, JST, 744 Motooka, Nishi-ku, Fukuoka 812-8581, Japan ReceiVed: September 8, 2006; In Final Form: December 22, 2006

Silver (Ag) nanoparticles covered with a photochromic diarylethene derivative that has an anthracene unit were synthesized by Brust’s method and their photochromic and fluorescent performance were studied by using absorption and fluorescence spectroscopy. The diarylethene ligand showed a reversible photochromic reaction and the open-ring isomer emitted fluorescence (Φf ) 0.009). On the surface of the Ag nanoparticles, the diarylethene showed normal reversible photochromic reactions; however, the conversion from the opento the closed-ring isomer decreased from 81 to 16%. Besides, fluorescence from the anthracene unit was quenched to 7.6% of the unbound state. Quantum yield analysis showed that the cyclization reaction was efficiently quenched while the cycloreversion reaction was scarcely affected. The kinetic analysis on the excited states of the open- and the closed-ring isomers was carried out. The cycloreversion reaction was quenched by the Ag nanoparticles by the rate of 2.8 × 1010 s-1, while the quenching rate constant of the cyclization was 9.1 × 1011 s-1. These results indicate that the quenching of the excited-state is due to the excited energy transfer from the diarylethene to the Ag nanoparticles, and that the overlap between the fluorescence of diarylethene and the plasmon absorption of Ag nanoparticles regulates the quenching of the excited-state of organic molecules on the Ag nanoparticles.

Introduction Photochromism is defined as a reversible color change between two chemical species triggered by irradiation with UV and visible light.1 Recently, a lot of photochromic organic molecules such as fulgide,2 azobenzene,3 and diarylethene4 have been synthesized and studied because of their potential applications for molecular switches and memories. Among these photochromic molecules, diarylethene derivatives have attracted increasing attention because of their high fatigue-resistant5 and thermal irreversible6 properties. One of the important steps to fabricate optoelectronic devices is the realization of the metalorganic junctions.7 A well-known technique to attach organic molecules to metal surfaces is the formation of the selfassembled monolayer (SAM) by using thiol groups.8 However, the excited-state of the organic molecules on noble metal surfaces is readily quenched by the surface plasmon resonance.9 Therefore, it is required to develop technology to avoid the surface plasmon quenching. Nanometer-sized metal and semiconductor nanoparticles have shown characteristic spectroscopic,10 electrical,11 and magnetic12 properties depending on their composition, size, and shape.13 Noble metal nanoparticles have vivid colors in the visible region and have potential applications in optoelectronics and sensors.14 These colors are due to plasmon absorption, which is caused by harmonic oscillation of the electrons triggered by alternating electric field of the irradiated light.13,10b Typical plasmon absorption of gold nanoparticle exists around 520 nm and that of Ag nanoparticle exists around 400 nm.15 * To whom correspondence should be addressed. E-mail: (K.M.) [email protected]; (M.I.) [email protected]. Phone and fax: +81-92-802-2927. † Kyushu University. ‡ PRESTO, JST.

The correlation between the plasmon absorption and the photochemical property of the photochromic diarylethene ligands is an attractive subject. Ag nanoparticles have their plasmon absorption around 400 nm, whose overlap with the absorption of the ligands is larger than that of gold nanoparticles.15 To study the excited-state behavior of the organic molecule on the metal nanoparticles, it has been common to use fluorescent organic dye bound by a thiol group.16 For example, Gittins et al. reported the particle-size dependence of the fluorescence behavior of lissamine dye molecules chemically attached on the gold nanoparticles by measuring the timeresolved fluorescence17 and Thomas et al. reported the fluorescent property of a pyrene derivative on the gold nanoparticles.16d The effective quenching by energy transfer and electron transfer to metal nanoparticles deactivates the fluorescence from organic dyes on the metal nanoparticles.16,17 The quenching takes place in several picoseconds; fluorescence from organic dye is almost completely quenched because of their relatively long lifetimes (ordinary several nanoseconds).18 Recently, our group reported photochromism of diarylethenes on the gold and silver nanoparticles.19 Photochromic reactions of the diarylethene derivatives are reported to take place within 10 picoseconds,20 which is much faster than fluorescence. The nanoparticles underwent photochromic reactions despite the existence of the surface plasmon absorption. Although several studies on gold nanoparticles capped with other photochromic molecules were reported,21 there is no example of quantitative discussion on the excited-state behavior of organic dyes attached on the noble metal nanoparticles. In this paper, we present the results of the measurement on the excited-state behavior and the relationship between the plasmon absorption and the photochemical property of the ligand. A photochromic diarylethene which has a thiol group

10.1021/jp065856q CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007

3854 J. Phys. Chem. C, Vol. 111, No. 10, 2007 and an anthracene group was synthesized and characterized using UV-visible and fluorescence spectroscopy. We prepared Ag nanoparticles capped with the open-ring isomer and the closed-ring isomer by Kim’s method, which is a modification of the Brust’s protocol.22,23 The correlation between the plasmon absorption of the Ag nanoparticles and the fluorescence of the diarylethene will be discussed. Experimental A. Nomenclature. The suffix “a” means the open-ring isomer and the suffix “b” means the closed-ring isomer. “Ag-1” is the diarylethene-capped Ag nanoparticle prepared from the openring isomer and “Ag-1′” is the diarylethene-capped Ag nanoparticle prepared from the closed-ring isomer. Therefore, “Ag-1′a” means the sample in which the closed-ring isomer on the Ag nanoparticles was completely photoisomerized to the open-ring isomers by irradiation with visible light. B. Materials. 1H NMR spectra were recorded on a Varian Gemini 200 (200 MHz) and a Bruker AVANCE 400 (400 MHz) spectrometer. IR spectra were recorded on a Perkin-Elmer Spectrum One instrument by attenuated total reflection (ATR) method. Mass spectra were obtained on a JEOL JMS-GCmate II. All reactions were monitored by thin-layer chromatography carried out on 0.2 mm E. Merck silica gel plates (60F-254). Column chromatography was performed on silica gel (Kanto, 63-210 mesh). 1-{3-Bromo-5-(4-methoxymethoxy-phenyl)-2,4-dimethylthiophenyl}perfluorocylopentene (3). To a solution of 3-bromo5-(4-methoxymethoxy-phenyl)-2,4-dimethylthiophene 224 (3.17 g, 9.7 mmol) in dry THF (40 mL) was added n-BuLi (1.6 M, 6.5 mL, 10.4 mmol) under Ar atmosphere at -78 °C, and then the mixture was stirred for 15 min. After addition of perfluorocyclopentene (5 mL) to the reaction mixture all at once at -95 °C, the reaction vessel was allowed to warm slowly to room temperature and then water was added to quench the reaction. The reaction mixture was extracted with ether, washed by brine for 3 times, dried over MgSO4, and then purified by silica gel column chromatography (hexane/AcOEt ) 4:1) to give compound 3 (2.77 g, 65%). 1H NMR (200 MHz, CDCl3, TMS) δ ) 2.08 (s, 3H), 2.37 (s, 3H), 3.50 (s, 3H), 5.21 (s, 2H), 7.08 (d, 2H, J ) 7 Hz), 7.33(d, 2H, J ) 9 Hz); FAB-HRMS: m/z calcd for C19H15F7O2S [M]+, 440.0681; found, 440.0691. 2-Anthracen-9-yl-4-bromo-3,5-dimethyl-thiophene (5). To a solution of 2,4-dibromo-3,5-dimethylthiophene 4 (5.0 g, 18.5 mmol) in dry ether (40 mL) was added n-BuLi (1.6 M, 10 mL, 16 mmol) under Ar atmosphere at -78 °C, and then the mixture was stirred for 15 min. A solution of anthrone (2.91 g, 15 mmol) in toluene (100 mL) was added to the reaction mixture for 60 min. After the solution was stirred for 1h, the reaction vessel was allowed to warm slowly to room temperature, and then the reaction mixture was poured into ice and water. The mixture was extracted with ether, washed three times with brine, and concentrated. The residue was dissolved in ethanol (25 mL) and toluene (25 mL) and then hydrochloric acid (35%, 8 mL) was added. After refluxed for 3 h, the mixture was extracted with ether, washed three times with brine, dried over MgSO4, and concentrated. The residue was purified by silica gel column chromatography (hexane) to give compound 5 (3.63 g, 65%). 1H NMR (200 MHz, CDCl , TMS) δ ) 1.84 (s, 3H), 2.55 (s, 3 3H), 7.37-7.52 (m, 4H), 7.70-7.82 (m, 2H), 8.02-8.10 (m, 2H), 8.55 (s, 1H); FAB-HRMS: m/z calcd for C20H15BrS [M]+, 366.0078; found, 366.0082. 1-{5-(4-Methoxymethoxy-phenyl)-2,4-dimethyl-thiophen-3yl}-2-{5-anthracen-9-yl-2,4-dimethyl-thiophenyl}-perfluorocy-

Yamaguchi et al. lopentene (6). To a solution of compound 5 (240 mg, 0.654 mmol) in dry THF (10 mL) was added n-BuLi (1.6 M, 0.5 mL, 0.8 mmol) under argon atmosphere at -78 °C, and then the mixture was stirred for 15 min. A solution of compound 3 (350 mg, 0.80 mmol) in dry THF (3 mL) was added to the mixture. The reaction vessel was allowed to warm slowly to room temperature, and water was added to quench the reaction. The reaction mixture was extracted with ether, washed three times with brine, dried over MgSO4, and concentrated. The residue was purified by silica gel column chromatography (hexane/AcOEt ) 4:1) to give compound 6 (300 mg, 64%). 1H NMR (200 MHz, CDCl3, TMS) δ ) 1.25 (s, 2H), 2.05 (s, 2H), 2.09 (s, 2H), 2.36 (s, 1.5H), 2.41(s, 1.5H), 2.49 (s, 1.5H), 2.54 (s, 1.5H), 3.50 (s, 3H), 5.21 (s, 2H), 7.08-7.15 (m, 3H), 7,28-7.46 (m, 6H), 7.84-8.01 (m, 3H), 8.50 (s, 1H); FABHRMS: m/z calcd for C39H30F6O2S2 [M]+, 708.1591; found, 708.1591. 1-{5-(4-hydroxyphenyl)-2,4-dimethyl-thiophen-3-yl}-2-{5-anthracen-9-yl-2,4-dimethyl-thiophen-3-yl}-perfluorocyclopentene (7). To a solution of compound 6 (490 mg, 0.69 mmol) in THF (8 mL) was added hydrochloric acid (35%, 2 mL) , and then the mixture was stirred overnight at room temperature. The reaction mixture was extracted with ether, washed three times with brine, dried over MgSO4, and concentrated. The residue was purified by silica gel column chromatography (hexane/ AcOEt ) 3:1) to give compound 7 (439 mg, 95%). 1H NMR (400 MHz, CDCl3, TMS): δ ) 1.56 (s, 1.5H), 1.61 (s, 1.5H), 2.03 (s, 1.5H), 2.07 (s, 1.5H), 2.33 (s, 1.5H), 2.42 (s, 1.5H), 2.49 (s, 1.5H), 2.55 (s, 1.5H), 5.22 (s, 1H), 6.84-6.89 (m, 2H), 7.03-7.48 (m, 7H), 7.84-8.02 (m, 3H), 8.49 (d, 1H, J ) 8 Hz); FAB-HRMS: m/z calcd for C37H26F6OS2 [M]+, 664.1329; found, 664.1334. 1-{5-(4-Bromopentyloxyphenyl)-2,4-dimethyl-thiophen-3-yl}2-{5-anthracen-9-yl-2,4-dimethyl-thiophen-3-yl}-perfluorocyclopentene (8). To a solution of compound 7 (439 mg, 0.62 mmol) and K2CO3 (200 mg) in acetone (5 mL) was added 1,5dibromopentane (400 µL, 3.0 mmol), and then the reaction mixture was refluxed for 48 h at 80 °C. The solid residue was removed by filtration and the organic layer was concentrated. The reaction mixture was extracted with ether, washed three times with brine, dried over MgSO4, filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane/CHCl3 ) 2:1) to give compound 8 (332 mg, 66%). 1H NMR (400 MHz, CDCl3, TMS) δ ) 1.56 (s, 1.5H), 1.62-1.66 (m, 3.5H), 1.83 (quart, 2H, J ) 8 Hz), 1.94 (quart, 2H, J ) 8 Hz), 2.36 (s, 1.5H), 2.41 (s, 1.5H), 2.49 (s, 1.5H), 2.55 (s, 1.5H), 3.44 (t, 2H, J ) 7 Hz), 4.00 (t, 2H, J ) 6 Hz), 6.89 -7.18 (m, 3H), 7.30-7.47 (m, 6H), 7.85-8.05 (m, 3H), 8.48 (d, 1H, J ) 8 Hz); FAB-HRMS: m/z calcd for C42H35BrF6OS2 [M]+, 812.1217; found, 812.1252. 1-{5-(4-Mercaptopentyloxyphenyl)-2,4-dimethyl-thiophen-3yl}-2-{5-Anthracen-9-yl-2,4-dimethyl-thiophen-3-yl}-perfluorocyclopentene (1a). To a solution of compound 8 (80 mg, 0.102 mmol) in THF (2 mL) was added the mixture of bis(trimethylsilyl) sulfide (30 µL, 0.12 mmol) and tetrabutylammonium fluoride (40 µL, 0.125 mmol) at -10 °C, and then the reaction mixture was stirred for 12 h at room temperature. The product was extracted with CH2Cl2 and washed three times by aqueous ammonium chloride and brine. The organic layer was dried over MgSO4, filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane/AcOEt ) 8:1) to give 35 mg of the mixture of 1a and the disulfide byproduct. According to the NMR analysis, the ratio between 1a and the disulfide byproduct was 2:1. Then, the product was

Photochromic Diarylethene on Ag Nanoparticles

J. Phys. Chem. C, Vol. 111, No. 10, 2007 3855

SCHEME 1: Photochromism of Diarylethene 1

dissolved into the mixture of THF (10 mL) and distilled water (1 mL). Tri-n-butylphosphine (40 µL, 0.17 mmol) was added and the mixture was stirred for 48 h at room temperature. The reaction mixture was extracted with ether, washed three times with brine, dried over MgSO4, filtrated, and concentrated. The residue was purified by silica gel column chromatography (hexane/AcOEt ) 8:1). The product was further purified by GPC and HPLC (normal phase, Hex/AcOEt ) 9:1) to give compound 1a (10 mg, 28%). 1H NMR (400 MHz, CDCl3, TMS) δ ) 1.35 (t, 1H, J ) 8 Hz), 1.53-1.62 (m, 5H), 1.69 (quart, 2H, J ) 7 Hz), 1.81 (quart, 2H, J ) 8 Hz), 2.04 (s, 1.5H), 2.08 (s, 1.5H), 2.36 (s, 1.5H), 2.41 (s, 1.5H), 2.49 (s, 1.5H), 2.53-2.59 (m, 3.5H), 3.99 (t, 2H, J ) 6 Hz), 6.90-7.18 (m, 3H), 7.26-7.48 (m, 6H), 7.85-8.02 (m, 3H), 8.49 (d, 1H, J ) 8 Hz); FAB-HRMS: m/z calcd for C42H36F6OS3 [M]+, 766.1832; found, 766.1830; UV-vis (EtOAC) λmax () 351 (7000), 369 (10300), 388 (9400) nm; IR (Ge ATR) 2933, 2867, 1608, 1513, 1441, 1340, 1273, 1246, 1145, 1112, 1050, 983, 757, 735, cm-1. Cyclic voltammetry (dichloromethane; electrolyte, NBu4ClO4; working electrode, Pt; counter electrode, Pt, vs Ag/Ag+) +1.19 V (irreversible), -1.53 V (irreversible). Separation Condition of Closed-Ring Isomer 1b. Condition for the separation of the closed-ring isomer 1b was as follows: pump, Hitachi L-2420; detector, Hitachi L-2130 (detection wavelength, 313 nm); column, Mightysil (Kanto Chemical Co., Inc.) 250-4.6 mm; eluent, hexane/ethyl acetate ) 92:8. UV-vis (EtOAC) λmax () 569 (10300) nm. Cyclic voltammetry (dichloromethane; electrolyte, NBu4ClO4; working electrode, Pt; counter electrode, Pt, vs Ag/Ag+) +0.60 V (irreversible), -1.29 V (irreversible). Synthesis of Ag-1a. A solution of silver nitrate (99.999%, 82 mg, 0.483 mmol) in ultrapure water (18.2 MΩ, 10 mL) was prepared. To a portion of the solution (1 mL) were added CHCl3 (5 mL) and tetraoctylammonium bromide (105 mg, 0.19 mmol), and then the mixture was stirred heavily for 30 min. A solution of compound 1a (12.34 mg, 0.0161 mmol) in CHCl3 (1 mL) was added to the reaction vessel, and then the mixture was further stirred for 30 min. A solution of NaBH4 (18 mg, 0.48 mmol) in ultrapure water (1 mL) was added slowly to the solution, and the mixture was stirred for 3 h. The organic layer was evaporated, and the residue was poured into ethanol (50 mL). Formed precipitate was collected by membrane filter and dissolved into CHCl3 (0.5 mL). After evaporation, the same processes were repeated using methanol and hexane instead of ethanol. The final product Ag-1a was stored in CHCl3. Synthesis of Ag-1′b. The closed-ring isomer 1b was separated in the dark by HPLC from the mixture of 1a and 1b in the photostationary state. Ag nanoparticles were prepared starting from the closed-ring isomer 1b. The experimental process was similar to that of 1a using the ratio of Ag+ ions/1b ) 3:1. C. Photochemical Measurement. Absorption spectra were measured on a spectrophotometer (Hitachi U-3500). Photoirradiation was carried out using a USHIO 500 W super highpressure mercury lamp or a USHIO 500 W xenon lamp. Mercury lines of 313 and 578 nm were isolated by passing the

light through a combination of band-pass filter (UV-D33S) or sharp-cut filter (Y-50) and monochromator (Ritsu MC-20L). Determination of quantum yield of 1 and Ag-1′. Cyclization and cycloreversion quantum yields of diarylethene 1 were measured by the previously reported method using fulgide and 1,2-bis(2-methyl-benzothiophen-3-yl)hexafluorocyclopentene as a reference, respectively.25,26 Absorbance of sample solution and reference solution were adjusted to be the same at the excitation wavelength. For the measurement of the quantum yield of Ag-1′, there was a problem that the absorption at the excitation wavelength (365 nm) contains the absorption derived from diarylethene and Ag nanoparticles. To avoid this problem, the amount of diarylethene was estimated based on the hypothesis that the absorption coefficient () of diarylethene (10 300 at 569 nm) was unaltered on the Ag nanoparticles, and cycloreversion quantum yield of Ag-1′b was measured using the sample in which all of the diarylethenes on the Ag nanoparticles were the closed-ring isomers. Cyclization quantum yield of Ag-1′a was measured using the sample in which the closedring isomer on the Ag nanoparticles was photoisomerized to the open-ring isomers by irradiation with visible light. D. Transmission Electron Microscopy (TEM) Measurement. TEM measurement was performed on a HITACHI H-7500 instrument. The measurement was performed at 100 kV. TEM samples were prepared by a drop of ethyl acetate solution of Ag-1a and Ag-1′b onto carbon-coated copper grid. E. Electrochemical Measurement. Cyclic voltammograms were measured by ALS-600 electrochemical analyzer. The measurement was performed in dichloromethane. Bu4NClO4 was used for the electrolyte. The working and counter electrode were Pt and the reference electrode was Ag/Ag+. Results A. Molecular Design and Synthesis of Ligands 1a. As mentioned in Introduction, it is well-known that the excited states of organic molecules are readily quenched on the noble metal surface. Previous researches showed that energy transfer and electron transfer to the metal surfaces are the main cause of the quenching.17,27 These quenching processes are faster than fluorescence process; the time constant of energy transfer and electron transfer on the metal surfaces is normally around several picoseconds; on the other hand, fluorescence lifetime is normally around several nanoseconds. In previous researches,28 to avoid the surface plasmon quenching the distance from the surface to the fluorescence dye should be longer than ∼5 nm. In the previous paper,19 we have reported that the diarylethene derivative attached on the gold nanoparticle with pentamethylene linkage undergoes photochromic reaction without severe quenching. The reason of the retained reactivity was attributed to the fast reaction of diarylethene compared with fluorescence.20 To investigate the detailed excited-state behavior of the organic dye molecule, diarylethene 1a was synthesized (Scheme 1). Compound 1a has two independent properties: photochromic and fluorescent properties (Figure 1). The compound 1a has a terminal thiol group and a pentamethylene alkyl

3856 J. Phys. Chem. C, Vol. 111, No. 10, 2007

Figure 1. Schematic illustration of Ag-1a.

chain to form a self-assembled monolayer on the Ag nanoparticles. The anthracene unit is introduced at the 5-position of the other thiophene ring. The fluorescent and photochromic behaviors enables us to study the quenching mechanism in detail. The synthetic procedure leading to 1a is shown in Scheme 2. 3-Bromo-5-(4-methoxymethoxyphenyl)-2,4-dimethylthiophene (2) and 2,4-dibromo-3,5-dimethylthiophene (4) were synthesized according to the previously reported method.24 After the anthracene group was introduced into 5-position of the thiophene ring of 4 using anthrone,29 coupling reaction was carried out with perfluorocyclopentene derivative 3. After the cleavage of the methoxymethyl group, the alkyl chain was introduced and the terminal bromo group then was converted to the thiol group by SN2 reaction. The disulfide byproduct was reduced to the desired thiol 1a. The structure of 1a was

Yamaguchi et al. confirmed by high-resolution mass, UV-vis, IR, and NMR spectroscopies. B. Characterization of Diarylethene Ligands. The compound 1a underwent a photochromic reaction in ethyl acetate solution. The colorless solution turned bluish purple upon irradiation with 365 nm UV light, and the colored solution turned colorless upon irradiation with 578 nm light. The color change is due to the formation of the closed-ring isomer 1b. Figure 2 shows the absorption spectra of 1a, 1b, and the photostationary state (PSS) under irradiation with 365 nm light. The population of 1b estimated from the absorbance in the PSS is 81%. (Open-ring isomers exist 19%.) The conversion ratio is theoretically expressed by the following equation and is used as a benchmark for the reactivity of photochromic reaction because it correlates to the reaction quantum yields.30

conVersionOfC )

ΦOfCO ΦOfCO + ΦCfOC

(1)

The cyclization quantum yield (irradiated with 365 nm light) was measured using fulgide, and the cycloreversion quantum yield (irradiated with 517 nm light) was measured using 1,2-bis(2-methyl-benzothiophen-3-yl)hexafluorocyclopentene as references. The obtained cyclization quantum yield was 0.18 and the cycloreversion quantum yield was 0.019. Figure 3 shows the fluorescence spectrum of 1a in ethyl acetate solution. Emission maximum exists at 421 nm, and the fluorescence quantum yield is 0.009 ( 0.001 using anthracene as a reference. When the solution was irradiated with 365 nm

SCHEME 2: Synthesis of Diarylethene Ligand 1a

(a) n-BuLi, octafluorocyclopentene, dry THF, 65%; (b) n-BuLi, anthrone, dry Et2O, HCl, ethanol, toluene, 65%; (c) n-BuLi, dry THF, 64%; (d) HCl, THF, 95%; (e) 1,5-dibromopentane, K2CO3, acetone, 66%; (f) bis(trimethylsilyl) sulfide, tetrabutylammonium fluoride, THF, tri-n-butylphosphine, THF, water, 28%.

Photochromic Diarylethene on Ag Nanoparticles

J. Phys. Chem. C, Vol. 111, No. 10, 2007 3857 SCHEME 3: Parallel Conformer and Antiparallel Conformer

Figure 2. Absorption spectra of 1 in ethyl acetate solution: the open ring isomer (solid line); the closed ring isomer (dashed line); in the photostationary state under irradiation with 365 nm light (dotted line).

Figure 3. Fluorescence spectrum (excited at 350 nm) of 1a in ethyl acetate solution.

Figure 4. Fluorescence excitation spectrum (excited at 420 nm) of 1a in ethyl acetate solution. Inset: absorption spectrum of 1a.

light, the fluorescence intensity decreased. This is because 1a isomerized to the nonfluorescent closed-ring isomer 1b.4c, 31 As seen in the spectra, there is no vibrational structures from the anthracene group, but the excitation spectrum of 1a is similar to the absorption spectrum of anthracene (Figure 4). The fluorescence lifetime of 1a was 0.41 ns, which is much shorter than that of nonsubstituted anthracene. Because the antiparallel conformer of 1a has a photochromic reactivity and the cyclization reaction is much faster than the fluorescence process, the observed fluorescence lifetime originates from photoinactive parallel conformer (Scheme 3). The decrease of fluorescence quantum yield and lifetime of 1a is attributed to the increase of the radiationless process induced by the vibrational and rotational motion of the substituted diarylethene unit.31 C. Characterization of Ag Nanoparticles Ag-1a and Ag-1′b. Ag nanoparticles capped with 1a (Ag-1a) were prepared by Kim’s method, which is a modification of the Brust and Shiffrin protocol.23 When NaBH4 was added to the reaction vessel, the color of the solution suddenly turned black. This is attributed to the formation of Ag nanoparticles.

The IR spectrum was measured to confirm that 1a covered the Ag surface. Figure 5 shows the IR spectrum of 1a and Ag-1a. The spectrum of Ag-1a was similar to that of 1a itself. Figure 5a clearly indicates that 1a covered the Ag surface. The peaks at 2933 and 2867 cm-1, which were assigned to C-H alkyl symmetric and asymmetric mode, were shifted to a shorter wavenumber when 1a was located on the Ag surface. (Ag-1a showed these peaks at 2920 and 2852 cm-1.) This shift suggested that alkyl groups of diarylethenes on the Ag nanoparticles adopt trans conformation on the Ag surface.15a, 32 The size of Ag nanoparticles is an important factor for plasmon absorption spectroscopic property.10-15,33 To measure the average diameter of Ag nanoparticles, TEM measurement was carried out. Figure 6a shows core size distribution and Figure 6b shows the TEM image of Ag-1a. The distribution of the particle size was moderately monodisperse. The space of each particle is distinct reflecting the diarylethene monolayer. According to the core size histogram, the average diameter of Ag-1a is 3.9 ( 0.9 nm. To investigate the photochromic and fluorescent properties in detail, Ag nanoparticles capped with the closed-ring isomer (Ag-1′b) were prepared. The closed-ring isomer was separated by HPLC in the dark (see Experimental). Core size histogram and TEM image were shown in Figure 6c,d. The average core diameter of Ag-1′b was 3.7 ( 0.6 nm, and this value was very close to that of Ag-1a. The molar ratio between the ligand and the metal source was adjusted to be similar in the preparation of Ag-1a and Ag-1′b. D. Photochromic and Fluorescent Properties of Ag-1a and Ag-1′b. Figure 7a illustrates the absorption spectral change of the ethyl acetate solution of Ag-1 by photoirradiation. The spectrum of Ag-1a has a plasmon absorption peak at 465 nm and some vibrational structures around 350 nm originating from the anthracene unit. The peaks show a slight bathochromic shift by 3 nm. This suggests that diarylethene was tightly attached and densely packed to the surface of Ag nanoparticles. However, the interaction between each diarylethene on the surface of Ag nanoparticle is considered to be weak because the vibrational structures from anthracene group remained and the bathochromic shift was small. Upon UV (365 nm) irradiation, the absorbance around 570 nm increased. This is due to the photochromic reaction of diarylethene on the Ag nanoparticles. The absorption spectrum returned back to original spectrum by irradiation with 578 nm light. Although Ag-1a has a strong plasmon absorption, diarylethene on the Ag nanoparticles showed normal reversible photochromic reaction. However, the change in absorption spectrum was small. This suggests that

3858 J. Phys. Chem. C, Vol. 111, No. 10, 2007

Yamaguchi et al.

Figure 5. (a) IR spectrum of 1a and (b) IR spectrum of Ag-1a. The measurement was performed by ATR-IR method.

Figure 6. (a) Core size histogram and (b) TEM image of Ag-1a. (c) Core size histogram and (d) TEM image of Ag-1′b. Samples were prepared by placing a drop of the ethyl acetate solution on the copper grid.

Figure 7. Absorption spectra of (a) Ag-1 and (b) Ag-1′ in ethyl acetate solution: the open ring isomer (solid line); the closed ring isomer (dashed line); in the photostationary state under irradiated with UV light (dotted line). (c) Differential absorption spectrum of Ag-1′ (the closed-ring isomer-the open-ring isomer).

the cyclization reaction of diarylethene was heavily quenched by Ag nanoparticles. Figure 7b shows the spectral change of Ag-1′ upon irradiation with UV or visible light. After the original spectrum of Ag-1′b had been recorded, the sample was irradiated with

visible (578 nm) light. The decrease of absorbance in the visible region was observed. The decrease is due to the cycloreversion reaction of diarylethene ligands. When the sample was irradiated with UV (365 nm) light, regaining of the absorbance in the visible region resulting from cyclization reaction of the ligand

Photochromic Diarylethene on Ag Nanoparticles

Figure 8. Fluorescence spectra of Ag-1′a (dashed line) and 1a (9.2 µM, solid line) in ethyl acetate solution. The concentration of diarylethene was adjusted to be the same between two samples.

TABLE 1: Cyclization and Cycloreversion Quantum Yields of 1 and Ag-1′ in Ethyl Acetate Solution compound

cyclization reaction

cycloreversion reaction

1 Ag-1′

0.18 0.015

0.019 0.018

was observed. According to these spectra, the conversion in the photostationary state was estimated to be 16%. This is much lower than that of free 1 (80%), suggesting the effective quenching process is introduced to the excited-state of 1a on the Ag nanoparticles, and then the cyclization reaction is influenced by the Ag nanoparticles. The detail will be discussed in the discussion section. Provided that the absorption coefficient does not change between the free and Ag-bound diarylethene, the amount of 1a on the Ag nanoparticles in Ag-1′b can be estimated from the decrease with absorbance in the visible region under irradiation of visible light.34 Fluorescence quantum yield was obtained based on the above treatment and compared with that of unbound state. The fluorescence spectra of Ag-1′a (ligands are the open-ring isomers) were shown in Figure 8 (dashed line) along with the spectrum of free 1a of the same concentration (solid line). Fluorescence quantum yield of Ag-1′a was obtained to be 6.84 × 10-4, which is 7.6% of free 1a. The fluorescent lifetime of Ag-1′a was too short to detect with the setup used in this study (