Synthesis, Structural and Photophysical Properties of Pentacene

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Synthesis, Structural and Photophysical Properties of Pentacene Alkanethiolate Monolayer-Protected Gold Nanoclusters and Nanorods: Supramolecular Intercalation and Photoinduced Electron Transfer with C

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Daiki Kato, Hayato Sakai, Toshiyuki Saegusa, Nikolai V. Tkachenko, and Taku Hasobe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01164 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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The Journal of Physical Chemistry

Synthesis, Structural and Photophysical Properties of Pentacene Alkanethiolate Monolayer-Protected Gold Nanoclusters and Nanorods: Supramolecular Intercalation and Photoinduced Electron Transfer with C60 Daiki Kato, a Hayato Sakai, a Toshiyuki Saegusa, a Nokolai V. Tkachenko*,b and Taku Hasobe*,a a

Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama, 223-8522, Japan ｂ

Laboratory of Chemistry and Bioengineering, Tampere University of Technology, 33720 Tampere, Finland

Corresponding Author *E-mail: [email protected] (T.H.). *E-mail: [email protected] (N.T.)

ACS Paragon Plus Environment

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ABSTRACT

6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene: TP) alkanethiolate monolayerprotected gold nanoclusters (TP-Cn-X-MPCs: X stands for small (S) and large (L) nanocluter sizes) and nanorods (TP-Cn-MPRs) with different alkyl chain lengths (n = 7, 11) were synthesized to examine the structural and photophysical properties as well as intercalation trends with C 60 . The synthesis of TP-Cn-X-MPCs and TP-Cn-MPRs were successfully performed using two different precursors: TP disulfides and TP alkanethiols. The detail structural properties were confirmed by 1H NMR, elemental analyses and transmission electron micrograph (TEM). In the spectroscopic absorption and fluorescence excitation measurements, spectra shapes of TP units on the gold surface were clearly observed, whereas fluorescence intensities of TP units were strongly quenched as compared to the corresponding reference monomer (TP-Ref). Then, fluorescence quenching titration experiments to determine the association constants (K app ) between C 60 and TP assemblies (TP-Cn-X-MPCs and TP-Cn-MPRs) were performed by adding C 60 in toluene. The K app values were largely dependent on the sizes of nanoclusters and alkyl chain lengths in TP-Cn-X-MPC. For example, the K app value of TP-C7-S-MPC (73,800 M–1) was much larger than those of TP-C11-S-MPC (37,800 M–1) and TP-C7-L-MPC (5,350 M–1). This trend is in sharp contrast with the similar K a values (~66,000 M–1) in TP-Cn-MPR (n = 7, 11). These results suggest that the intercalation behaviors are dependent on the surface structures (nanocluster vs. nanorod). Such fluorescence quenching processes by photoinduced electron transfer (PET) in the complex between TP-C7-S-MPC and C 60 were directly observed by femtosecond transient absorption measurements, monitoring the TP radical cation and C 60 radical anion.


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INTRODUCTION

Metal nanomaterials such as spherical metal nanoparticles are significantly fascinating in the fields of material science and photochemistry because of tunable optical and electronic properties.1-4 Particularly, the absorption of gold nanoclusters with size of less than 2 nm takes place via the transition of electrons from the ground state to excited state. This molecule-like behavior is in sharp contrast with the light absorption derived from combined oscillation of charge in larger gold nanoparticles.3 Therefore, gold nanoclusters have been widely used in various research areas such as photovoltaics,5-7 photocatalysis,8, 9 sensor,10, 11 clinical diagnosis and theraphy.12-14 As compared to the isotropic gold nanoclusters and nanoparticles, anisotropic gold nanorods are also attractive materials due to the unusual size- and shape-dependent properties.15,

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Gold nanorods exhibit characteristic surface plasmon resonance (SPR) in the

broad region from visible to near-infrared (NIR).17 Consequently, gold nanorods provide various photophysical insights, which results in various applications in optics,18 photocatalysis,19, 20 and biological applications.21, 22 Design and synthesis of organic-inorganic hybrid materials have attracted widespread interest in terms of fundamental and applied researches. Many different kinds of chromophores such as pyrene,23, 24 porphyrin,11, 25, 26 phthalocyanine27-29 and fullerene30, 31 have been used to assemble on the surface of gold nanoclusters by self-assembled monolayers (SAMs). In most cases, the excited states of chromophores are significantly quenched by surface-dipole energy transfer (SET) to the gold surface.32 Actually, the fluorescence emission decreases by more than 90% and generation of triplet states through intersystem crossing is basically negligible. The rate of SET is proportional to the inverse fourth power of the distance between the chromophore and gold surface in point-to-surface approximation.32, 33 Accordingly, SET is highly related to Förster


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theory, where the rate constant of energy transfer is proportional to the spectral overlap between the donor (D) emission and acceptor (A) absorption and to the inverse sixth power of the distance between the D and A in point-to-point approximation.34 In contrast to the abovementioned gold nanoclusters modified by chromophores, the number of reported examples on chromophore-modified gold nanorods is extremely limited.35-37 Little attention has been accordingly drawn toward the comparison of spectroscopic and photophysical properties of chromophore-modified gold nanomaterials with different sizes and shapes (i. e., chromophoremodified gold nanorods and nanoclusters in this study). Supramolecular organization between host and guest molecules utilizing the surface of metal nanomaterials is an interesting research topic in various fields such as optical, energy conversion, biological and medical applications.38-47 In particular, one of the important concepts of the above-mentioned chromophore-modified metal nanomaterials is utilization of supramolecular cavity between two nearby chromophores to accommodate the guest molecules.37, 44, 46, 48 For example, we have previously reported supramolecular photovoltaic cells composed porphyrin alkanethiolate monolayer-protected gold nanoclusters (MPCs) (host) and fullerenes (guest).49 In this system, a long alkyl spacer of porphyrin MPCs affords sufficient cavity space for fullerenes to insert these guest molecules (fullerenes) between the nearest two porphyrin units effectively as compared to those with shorter alkyl spacers, leading to the efficient solar energy conversion. However, structural size- and shape-dependent properties (e.g., nanoclusters vs. nanorods) have yet to be reported, so far. We have recently reported the synthesis and photophysical properties of 6,13bis(triisopropylsilylethynyl) (TIPS)-pentacene-alkanethiolate MPCs [denoted as TP-Cn-X-MPC] with different chain lengths and particle sizes.50 However, supramolecular intercalation of TP-


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modified MPCs with guest molecules has yet to be examined. Based on the above points, our focus in this study is to newly synthesize TP-alkanethiolate monolayer-protected gold nanorods [denoted as TP-Cn-MPRs] and compare the spectroscopic and structural properties with the TPCn-MPCs (Chart 1). The details on the structural and photophysical properties of these nanomaterials with different metal cluster sizes and shapes as well as intercalation trends with C 60 will be discussed here.


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Chart 1. Chemical Structures of Pentacene-Modified Nanorods and Nanoclusters in This Study.

EXPERIMENTAL METHODS Synthesis of TP-C7-SH: Sodium methoxide (23.3 mg, 0.33 mmol) in methanol solution (4.70 mL) was added dropwise to compound 1 (300 mg, 0.33 mmol) in dry THF solution (47 mL) at 0 °C. After keeping the solution at 0 °C for 10 min, the mixture was stirred at room temperature for 1 h. Then, the organic solution was evaporated. The crude was purified by silica gel column


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chromatography eluting with hexane/toluene (7/1, v/v), TP-C7-SH (yield: 18%, 52.2 mg, 0.0604 mmol) as a black green powder afforded. 1H NMR (400 MHz, CDCl 3 ): δ (ppm): 9.29 (s, 4H), 8.00 (m, 4H), 7.70 (m, 3H), 7.41 (dd, J = 6.8, 3.2 Hz, 2H), 7.03 (d, J = 8.8 Hz, 2 H), 4.01 (t, J = 6.4 Hz, 2H), 2.53 (m, 2H), 1.81 (m, 2H), 1.63 (m, 3H) 1.38 (m, 48H);

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C NMR (100 MHz,

CDCl 3 ): δ (ppm): 158.99, 137.85, 133.00, 132.52, 132.26, 132.20, 131.33, 130.93, 130.72, 130.63, 130.52, 128.16, 129.19, 128.64, 128.25, 126.42, 126.27, 126.12, 125.99, 124.66, 118.37, 118.10, 119.95, 107.11, 107.03, 104.72, 104.67, 67.99, 33.96, 29.70, 29.22, 28.87, 28.28, 25.97, 24.64, 19.01, 11.68; MS (MALDI-TOF) m/z : 863 [M+2H] Synthesis of TP-C11-SH: (TP-C11-S) 2 (164 mg, 1.8 mmol), diispropylethylamine (167 µL, 0.9 mmol) and dithiothreitol (283 mg, 1.8 mmol) were dissolved in mixture solution (THF/DMF = 1/1, v/v) (120 ml) under nitrogen atmosphere. The mixture was stirred at room temperature for 6 h. Next, the solvent was evaporated. The crude was purified by column chromatography on silica gel with eluting hexane/toluene (7/1, v/v), TP-C11-SH (yield: 54%, 88 mg, 0.97 mmol) was afford as a brack green powder. 1H NMR (400 MHz, CDCl 3 ): δ (ppm): 9.30 (s, 4H), 8.01 (m, 4H), 7.71 (m, 3H), 7.41 (dd, J = 3.6, 7.2 Hz, 2H), 7.06 (d, J = 8.8 Hz, 2 H), 4.04 (t, J = 6.4 Hz, 2H), 2.53 (m, 2H), 1.83 (m, 2H), 1.61 (m, 3H) 1.38 (m, 56H); 13C NMR (100 MHz, CDCl 3 ): δ (ppm): 7.89, 132.97, 132.52, 132.25, 132.17, 131.31, 130.93, 130.67, 130.61, 130.50, 129.18, 129.00, 128.65, 128.24, 126.42, 126.22, 16.13, 125.99, 124.67, 118.38, 118.09, 114.97, 107.10, 107.05, 104.72, 104.65, 68.13, 34.06, 29.52, 29.42, 29.31, 29.09, 28.40, 26.09, 24.70, 19.03, 11.70; MS (MALDI-TOF) m/z : 917 [M+H] Synthesis of TP-Cn-MPR: 0.01 M HAuCl 4 aqueous solution (0.5 mL) was added to 0.1 M cetyltrimethylammonium bromide (CTAB) aqueous solution (15 mL), the color of solution changed to a bright brown-yellow. Next, 0.01 M sodium borohydride aqueous solution (1.20


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mL) was added to the mixture and shook at room temperature for 2 min, the color changed to pale brown-yellow. Then, the mixture was kept at 25 °C for 30 min. From here, we indicate the mixture solution as seed-solution. Next, the other solution was prepared by mixing 0.1 M CTAB aqueous solution (9.5 mL), 0.01 M HAuCl 4 aqueous solution (0.40 mL) and 0.01 M AgNO 3 aqueous solution (0.06 mL). By adding 0.1 M ascorbic acid aqueous solution (0.06 mL) to the mixture, the color of solution changed to colorless. Then, seed-solution (0.02 mL) was added to the prepared colorless mixture. After shaking it very slowly for 10 s, CTAB-GNR solution was obtained by keeping the mixture solution at 25 °C for 3 h. CTAB-GNR solution was centrifuged at 9000 rpm per 90 min. After removing the solution, GNRs aqueous solution was obtained by adding water (1.5 mL). Next, GNRs aqueous solution was added dropwise to TP-Cn-SH (0.061 mmol) in THF (40 mL), stirred at room temperature for 3 days under nitrogen atmosphere. After centrifuging the mixture and removing the solution, the residue was dispersed into CHCl 3 and sonicated. TP-Cn-SH (0.012 mmol) was added to the mixture, stirred at room temperature for 24 h. The solvent was removed by centrifuging the mixture, the residue was dispersed into CHCl 3 , sonicated and added TP-Cn-SH. This procedure carried out three times over. After centrifuging the residue, the solvent was removed. For improving purity, the procedure was carried out several times over until an observation of no signal in an absorption spectrum of the top layer solution. Finally, preparation of TP-Cn-MPR was identified by observing which the component of the shortest lifetime (τ > 10 ns) was less than 1% in the measurement of fluorescence lifetime.

RESULTS AND DISCUSSION


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Synthesis of TP-Cn-MPRs and TP-Cn-X-MPCs. The chemical structures of TP-Cn-XMPCs, TP-Cn-MPRs, TP-Ref (reference monomer), TP-disulfides [(TP-Cn-S) 2 ] and TPalkanethiols (TP-Cn-SH) are shown in Chart 1. (TP-Cn-S) 2 (n = 7, 11) were synthesized by our reported synthetic route.50 On the other hand, according to the reported procedure for synthesis of MPRs,37 chromophre-modified-alkanethiols but not disulfides were required. Therefore, (TPCn-S) 2 were

reduced

to

the

corresponding

thiols

by

use

of

dithiothreitol

and

diisopropylethylamine. The detail characterizations of TP-Cn-SH are shown in Supporting Information (SI) Figures S1-S6. TP-Cn-X-MPCs were synthesized following the reported method (Scheme 1).50 Concerning the synthesis of TP-Cn-MPRs, cetyltrimethylammonium bromide (CTAB)-coated gold nanorods (CTAB-GNR) were freshly prepared by the seedmediated growth method.15 Next, CTAB-GNR was washed by water for removing CTAB on the surface. Finally, TP-Cn-GNRs were obtained by reaction of the GNR with TP-Cn-SH (Scheme 2).

Scheme 1. Synthetic Scheme of TP-Terminated Gold Nanoclusters.

HAuCl4 3H2O / water

(i) TOAB / toluene (ii) (TP-Cn-S)2 (iii) NaBH4 / water

TP-Cn-X-MPC

Scheme 2. Synthetic Schemes of TP-Terminated Gold Nanoclusters.


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(i) CTAB / water (ii) Seed-solution HAuCl4 4H2O / water

CTAB-GNR (i) TP-Cn-SH / THF (ii) TP-Cn-SH / CHCl3

CTAB-GNR

TP-Cn-MPR

Structural Characterizations of TP-Cn-X-MPCs and TP-Cn-MPRs. The TEM images and corresponding size-distributions of TP-Cn-X-MPCs and TP-Cn-MPRs are shown in Figures 1 and 2 and SI Figures S7 and S8. Although the related results were shown in our previous work,50 the brief discussions are as follows. The mean diameters (2R CORE ) of TP-C7-S-MPC and TP-C7L-MPC are 1.62 ± 0.27 nm and 2.20 ± 0.32 nm, respectively (Figure 1). The reported spherical model51 predicted that the average cores of TP-C7-S-MPC and TP-C7-L-MPC contains 149 and 372 Au atoms, respectively. Additionally, based on the values of elemental analysis (SI Table S1) there are 53 (TP-C7-S-MPC) and 87 (TP-C7-L-MPC) TP molecules on one nanocluster. TPC11-X-MPCs are also similar to these values as shown in Table 1.50


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Figure 1. TEM images and corresponding size-distributions of (A) TP-C7-S-MPC and (B) TPC7-L-MPC.

The MALDI-TOF mass spectra also suggested the formations of TP-Cn-X-MPCs as shown in SI Figures S9 and S10 in addition to our previous reported 1H NMR analysis.50 In contrast to the above-mentioned spherical nanoclusters, TP-Cn-MPR demonstrates anisotropic bar-shaped structures, the structures of TP-C7-MPR are 12.3 ± 1.80 nm in diameter and 35.7 ± 6.75 nm in length (Figure 2). These values are very similar to the structure of TP-C11-MPR (12.4 ± 1.95 nm


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in diameter and 37.6 ± 4.57 nm in length). The relative molecular ratios in TP-Cn-MPR, which are estimated by elemental analysis (See: SI Table S1), are much smaller as compared to those of TP-Cn-X-MPCs because the relative structural sizes of gold nanorods are much larger and bulkier than those of gold nanoclusters.

Figure 2. TEM images and corresponding size-distributions of (A) TP-C7-MPR and (B) TPC11-MPR.


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Table 1. Summarized Structural Parameters of TP-Cn-X-MPCs.

TP-C7-S-MPC

Diameter, nma (Standard Deviation, nm) 1.62 (0.27)

Number of Au Atomsb 149

Number of TP Unitsb 53

TP Coverage, %b 68

TP-C7-L-MPC

2.20 (0.32)

372

87

55

TP-C11-S-MPC

1.65 (0.30)

158

51

63

TP-C11-L-MPC

2.13 (0.33)

338

75

51

Compound

a

Analyzed by TEM. bEstimated by TEM and Elemental Analysis.

Spectroscopic Measurements of TP-Cn-X-MPCs and TP-Cn-MPRs. Measurements of absorption and fluorescence spectra were performed to investigate the electric structures of TPCn-X-MPCs, TP-Cn-MPRs and a corresponding reference compound (TP-Ref) in toluene as shown in Figures 3, 4 and Figures S11 and S12 in SI. In absorption spectral measurements (Figure 3A), the spectra shapes of TP-C7-S-MPC (spectrum a) and TP-C7-L-MPC (spectrum b) match well with that of TP-Ref (spectrum c), whereas the increased baseline of TP-C7-L-MPC relative to TP-C7-S-MPC is due to the absorption of gold nanocluster including surface plasmon resonance. This may be attributable to the increased absorption coefficient of gold nanoclusters. Additionally, the small peak of localized surface plasmon resonance (LSPR) at ca. 500 nm was seen in TP-C7-S-MPC. However, as compared to the reported results,52,

53

LSPR absorption

bands were not clearly observed in TP-Cn-X-MPC. This may results from the overlapped absorption with pentacene derivatives. The fluorescence intensities of TP-C7-S-MPC and TP-C7-L-MPC were strongly quenched relative to TP-Ref because of energy transfer from the singlet excited state of TP (1TP*) to gold surface and singlet fission between neighboring two TP units as reported


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previously.50 To further compare the emission spectra shapes, the fluorescence spectra of TP-C7S-MPC (spectrum a), TP-C7-L-MPC (spectrum b) and TP-Ref (spectrum c) were normalized in Figure 3B. The shapes of fluorescence spectra of TP-C7-S-MPC and TP-C7-L-MPC are approximately similar to that of TP-Ref. The fluorescence excitation spectrum was also observed (spectrum c in Figure 3A) to carefully identify the fluorescence species of TP-C7-S-MPC. The excitation spectrum is identical with the corresponding absorption spectrum (spectrum a in Figure 3A), which suggested that the emission species arises from 1TP* units only. Finally, the energy of the singlet excited state for TP was determined to be ~1.9 eV based on these measurements (vide infra).

Figure 3. (A) Absorption spectra of (a) TP-C7-S-MPC (0.05 mg/mL), (b) TP-C7-L-MPC (0.05 mg/mL) and (c) TP-Ref (10 μM) and (d) excitation spectrum of TP-C11-S-MPC in toluene (observed at 722 nm). (B) Normalized fluorescence spectra of (a) TP-C7-S-MPC, (b) TP-C7-LMPC and (c) TP-Ref in toluene (λ ex = 600 nm).

We have also measured absorption spectra of TP-C7-MPR as shown in Figure 4A. The absorption features of TP-C7-MPR (spectrum a in Figure 4A) are in sharp contrast with those of TP-C7-S-MPC (spectrum a in Figure 3A) and TP- C7-L-MPC (spectrum b in Figure 3A) because


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the absorption of TP units is not seen. This is certainly due to the significant increase of amount of gold units toward TP molecules as compared to TP-C7-S-MPC and TP-C7-L-MPC (See: SI Table S1). According to the reported result,54 gold nanorods possess two absorption bands, one at a shorter wavelength (ca. 500 nm) and the other at a longer wavelength which undergoes a bathochromic shift with increasing aspect ratios. In this system, two different peaks at around 520 nm and 750 nm were observed (Spectra a and b in Figure 4A). The fluorescence intensity of TP in TP-C7-MPR (spectrum a in Figure 4B) was strongly quenched as compared to TP-Ref (spectrum b in Figure 4B). In contrast with the absorption spectral trends, the fluorescence excitation spectrum of TP-C7-MPR (spectrum d in Figure 4A) approximately agrees with the corresponding absorption spectrum of TP-Ref (spectrum c in Figure 4A). This result demonstrates that TP units are successfully attached onto the gold nanorod surface. The similar result was also observed for TP-C11-MPR (SI Figure S12). Additionally, to further examine the excited-state dynamics of TP units on the gold nanorod surface, picosecond fluorescence lifetime measurement of TP-C11-MPR was performed in degassed toluene solution (SI Figure S13) by using a pulsed laser light (λ ex = 404 nm). As compared to the TP-Ref, strong fluorescence quenching of TP in TP-C11-MPR was observed because of SET from 1TP* to the gold surface. It should also be noted that the fluorescence lifetime measurements of TP-Cn-X-MPCs demonstrate the ultrafast quenching process of 1TP* because of singlet fission between two neighboring TP units.50


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Figure 4. (A) Absorption spectra of (a) TP-C7-MPR (0.05 mg/mL), (b) C7-GNR (0.05 mg/mL), (c) TP-Ref (10 μM), and (d) excitation spectrum of TP-C7-MPR in toluene (Observed at 722 nm). (B) Normalized fluorescence spectra of (a) TP-C7-GNR and (b) TP-Ref in toluene (λ ex = 600 nm).

Electrochemical Measurements. Cyclic voltammograms of TP-C7-S-MPC and a reference TPRef were also measured to examine the electrochemical behaviors. It should be noted that we could not obtain voltammograms of TP-Cn-MPR because of the poor solubility. The voltammogram of TP-C7-S-MPC was compared with TP-ref (5 mM based on the number of the TP units) in CH2Cl2 containing 0.1 M n-Bu 4 NPF 6 with a sweep rate of 100 mVs–1. The first oxidation and reduction potentials of TP-C7-S-MPC are E OX = 0.86 V and E RED = –0.95 V vs SCE, respectively. These values are very comparable to those of TP-Ref (E OX = 0.88 V and E RED = –1.0 V vs SCE) as shown in Figure 5.


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Figure 5. Cyclic voltammograms of (a) TP-C7-S-MPC (5 mM based on the number of TP units) and (b) TP-Ref (10 mM) in CH 2 Cl 2 with 0.1 M nBu 4 NPF 6 as supporting electrolyte. Scan rate: 100 mV/s-1.

Evaluation of Association Constants between C 60 and TP Assemblies Utilizing Photoinduced Electron Transfer (PET). As discussed above, TP-Cn-X-MPCs and TP-CnMPRs are expected to have sufficient reaction cavities for accommodation of guest molecules. Therefore, fluorescence quenching measurements were carried out to investigate and compare the association constants between fullerenes: C 60 (guest) and TP assemblies (TP-Cn-X-MPCs and TP-Cn-MPRs). We have chosen C 60 as an electron acceptor (guest molecule) because the first one-electron reduction potential of C 60 is E RED = –0.44 V vs SCE.55 The energy level of the charge separated state of the C 60 radical anion and TP radical cation was approximately estimated from the difference between the first one-electron oxidation potential of TP (E OX = 0.86 V vs SCE) in TP-Cn-MPCs and reduction potential of C 60 to be ~1.3 eV.


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The excitation energy of singlet for TP were determined to be ~1.9 eV from the spectroscopic measurements (vide supra). Because the free energy change of PET is always negative, PET from the excited singlet state of TP (1TP*) to C 60 is energetically favorable. Therefore, we have performed fluorescence quenching experiments by adding C 60 following the reported methods under aggregate conditions.49, 56, 57 As shown in Figure 6A and SI Figure S14, significant fluorescence quenching processes of TP-Cn-X-MPCs and TP-Cn-MPRs were observed by addition of C 60 (0-0.1 mM), which are attributable to PET from 1TP* to C 60 . The appearance association constants (K app ) between TP and C 60 (equation 1) on gold surface were also estimated by the following equation.49, 56, 57

The observed fluorescence intensities of TP moieties (I obsd ) can be associated with those of uncomplexed (I 0 ) and complexed (I f ) TP moieties. Thus, the equation is defined as

I obsd = (1–α)I 0 + αI f

(2)

where α is the degree of the association between TP and C 60 on gold surface. The equation 2 can be recalculated to the following equation 3:

I 0 – I obsd = α(I 0 – I f )

(3)

At relatively high concentrations of C 60 >> [TP], α is given by


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(4)

From equations 3 and 4, the following equation 5 can be finally obtained. Namely, the observed decrease in the emission intensity (I 0 – I obsd ) is related to the concentrations of C 60 .

(5)

The K app values were determined by a linear correlation between (I 0 – I obsd )–1 and [C 60 ]–1 in equation 5, wherein I 0 and I obsd are the intensities of TP-Cn-X-MPCs and TP-Cn-MPRs in the absence and presence of C 60 . The association constants were determined by the linear dependence of (I 0 – I obsd )–1 on [C 60 ] –1 as shown in Figure 6B. The K app values are summarized in Table 2. The K app value between TP-C7-S-MPC and C 60 was obtained to be 73,800 M–1, which is much larger than those of TP-C7-L-MPC (5,350 M–1). Similarly, the estimated K app values of TP-C11-S-MPC and TP-C11-L-MPC are 37,800 M–1 and 1,210 M–1, respectively (Table 2). These results indicate that the association constants on gold nanoclusters are largely dependent on the particle sizes and alkyl chain lengths. The detail internal structures of supramolecular formation between TP and C 60 on gold nanoclusters was estimated following our reported method.49 In brief, the structures of TPalkanethiol (n = 7 and 11) on gold nanoclusters are calculated by molecular mechanics calculations (SI Figure S15). Considering the radius of the covalent bond of sulfur (S): 1.04 Å, the distance between the centers of TP units to the gold surface are 23.0 Å (TP-C7-X-MPC) and


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27.6 Å (TP-C11-X-MPC), respectively. Because there are 53 TP-alkanethiolate chains on the gold surface for TP-C7-S-MPC, the structure of C 60 (60 carbons and 7.1 Å in diameter) can be utilized for the estimation of the structure of TP-C7-S-MPC (16.2 Å in diameter) as the gold core (See: TEM results in Figure 1). There are two types of C-C bonds of C60: i.e., 30 short six-six ring fusions (1.404 Å) and 60 long six-five ring fusions (1.448 Å).49 The average distance of two gold atoms, to which nearest TP-alkanethiols are anchored, is estimated to be 3.27 Å, multiplying the average C-C bond length (1.433 Å) by the ratio of the diameter of the gold nanocluster (16 Å) to the C60 diameter (7.1 Å) as shown in Figure 7. The schematic insertion structure (Figure 7) provides the center-to-center distance between two TP units in TP-C7-SMPC as 12.6 Å, which is smaller than that in TP-C11-S-MPC (14.5 Å) (K app : 37,800 M–1). According to the reported calculation of C 60 /pentacene interface,58 the closest distance between a carbon of C 60 and a hydrogen of TP has been estimated to be 1.7 Å. Thus, the smallest center-tocenter distance of two nearby TP units on gold nanoclusters which can accommodate C 60 between two nearby TP units is estimated as 10.5 Å by adding the diameter of C 60 (7.1 Å) to twice the above-mentioned closest distance (1.7 Å) between a carbon of C 60 and the center of the TP unit. Additionally, this value (12.6 Å) is approximately similar to our reported case between porphyrin and C 60 (12.8 Å).49 The decreased trends of K app values in TP-C7-L-MPC is due to the increased distances between two TP units which hampers the effective complex formations between TP and C 60 because of smaller surface coverages and large particle sizes (See: Table 2). In contrast, the associate constants of TP-C7-MPR and TP-C11-MPR with C 60 were determined as 66,100 M–1 and 66,300 M–1, respectively (Table 2). The comparison of these values may suggest the similar distances between two nearby TP units in TP-Cn-MPRs.


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Figure 6. (A) Fluorescence emission spectra of (a) TP-C7-S-MPC, (b) TP-C7-L-MPC and (c) TP-C7-MPR in toluene at various concentrations of C 60 . The excitation wavelength is 325 nm. The concentration of TP is 0.1~1.0 µM. (B) Dependence of I 0 /(I 0 – I obsd ) on the reciprocal concentrations of C 60 .

Table 2. Summarized Associate Constants between TP and C 60 onto the Gold Surface. System

TP-C7-SMPC

TP-C7-LMPC

TP-C11-SMPC

TP-C11-LMPC

TP-C7MPR

TP-C11MPR

K app (M–1)

73,800

5,350

37,800

1,210

66,100

66,300


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Figure 7. Insertion of C 60 (guest) between two nearby TP units in TP-C7-S-MPC.

Ultrafast Photoinduced Electron Transfer Analyzed by Femtosecond Transient Absorption Spectra. The excited-state dynamics of pristine TP-Cn-X-MPC was already reported. Briefly, the relaxation dynamics of plasmon band of gold nanoclusters and singlet fission between two neighboring TP units can be modeled by two components with time constants ~2 and ~20 ps.50 The transient absorption spectra of TP-Cn-MPR were also measured and shown in SI Figure S16. The characteristic plasmon breach recovery of TP-Cn-MPR was immediately observed at ca. 700 nm after laser pulse excitation.59 However, the excited-state dynamics of pentacene units on MPR was not observed because this is certainly attributable to the significant increase of amount of gold units toward TP molecules as compared to TP-Cn-X-MPC.


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Figure 8. Femtosecond transient absorption spectra of (A) TP-C7-S-MPC (50 µM) and C 60 (0.7 mM) at 265 ps in toluene. (B) Absorption spectral changes observed in electron-transfer oxidation of TP-Ref by increasing the electrochemical voltages in CH 2 Cl 2 . (C) Femtosecond transient absorption spectra of TP-Ref at 265 ps in toluene. (D) Time profiles of TP•+ at 1040 nm and C 60 •— at 1090 nm.

To further investigate quenching process of excited states of TP assemblies (TP-Cn-XMPC) by C 60 , femtosecond transient absorption measurements were performed. It should be emphasized that the concentration of C 60 (0.7 mM) is in excess of one TP unit (50 µM) in toluene to clearly form supramolecular donor-acceptor (D-A) complex and observe the chargeseparated state. Figure 8A shows the time-resolved transient absorption spectra of TP-C7-S-MPC and C 60 composites in toluene. The appearance of two different absorption features at 1040 nm


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and 1090 nm were clearly monitored. The transient absorption band at around 1090 nm is characteristic of C 60 radical anion (C 60 •—),60-65 whereas the TP radical cation (TP•+) has an absorption maximum at ca. 1040 nm as shown in Figure 8B. These measurements are evidence to support mainly ultrafast PET from 1TP* to C 60 . The above-mentioned absorption features of TP-C7-S-MPC are totally different from those of TP-Ref (Figure 8C) with no absorption maximum in the range from 1000 to 1120 nm because of singlet-singlet absorption of TP-Ref. Moreover, Figure 8D shows the time profiles at 1040 and 1090 nm, indicating the radical species of TP•+ and C 60 •—, respectively. The dynamics were fit to double exponential functions with time constants of 1.5 ps and 850 ps (See: the experimental section in SI). The initial and faster component is a mainly transient species of gold nanoclusters because the similar species was observed previously.66 The slower component was assigned as a charge recombination process between C 60 •— and TP•+. The kinetic of the charge recombination is calculated to be 1.2×109 s-1. In the case of TP-C7-MPR and C 60 composites, the PET process could not be observed because we can mainly see transient absorption of gold nanorods as discussed above.

CONCLUSION In conclusion, TP-alkanethiolate monolayer-protected gold nanoclusters and nanorods were successfully synthesized to examine the chain length-, structural-size and surface shapedependent structural and photophysical properties. In TEM measurements, the two different mean diameters of TP-Cn-S-MPC and TP-Cn-L-MPC are approximately ~1.7 nm and ~2.1 nm, respectively, whereas the structure of TP-Cn-MPR is assigned as ~12 nm in diameter and ~36 nm in length. The elemental analysis of these materials indicated the appropriate surface coverage of TP units on gold nanoclusters. In contrast, the relative TP molecular ratios in TP-Cn-


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MPR estimated by elemental analysis are much smaller as compared to those of TP-Cn-X-MPCs because the relative structural sizes of gold nanorods are much larger than those of gold nanoclusters. In absorption and fluorescence excitation spectral measurements, the species of TP units on gold surfaces were observed, whereas the fluorescence intensities of TP units were strongly quenched as compared to the reference monomer. Then, fluorescence quenching measurements were performed to determine the association constants between C 60 and TP assemblies (TP-Cn-X-MPC and TP-Cn-MPR) by adding C 60 in toluene. The associate constants (K app ) are largely dependent on the sizes of nanoclusters and alkyl chain lengths in TP-Cn-SMPC, whereas the K a values approximately remain constant in TP-C7-MPR and TP-C11-MPR. These results suggested that the molecular capsule formation for intercalation, which is composed of two neighboring TP units onto the gold surface, depends on the surface structures (i.e., nanocluster vs. nanorod). Such a quenching process was also confirmed by femtosecond transient absorption measurements in the complex between TP-C7-S-MPC and C 60 . Thus, our synthetic and supramolecular strategies may provide a new perspective for construction of future organic-inorganic hybrid materials.

ASSOCIATED CONTENT Supporting Information Experimental section, 1H, 13C NMR and MALDI-TOF MS spectra, elemental analyses of TPCn-X-MPC and TP-Cn-MPR, TEM images and corresponding size distribution of TP-C11-XMPC, UV−vis absorption and fluorescence spectra of TP-C11-X-MPC and TP-C11-MPR, fluorescence decay profiles of TP-C11-GNR, fluorescence emission spectra of TP-C11-X-MPC,


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TP-C11-L-MPC and TP-C11-MPR at various concentrations of C 60 , and optimized structural calculations of TP-Cn-SH by molecular mechanics and femtosecond transient absorption spectra of TP-C11-GNR. The material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by Grant-in-Aid for Scientific Research (Nos. 26286017, 16 Ｋ14067, 15H01003 “π-System Figuration”, and 15H01094 “Photosynergetics” to T.H.). This work was performed under the Cooperative Research Program of "Network Joint Research Center for Materials and Devices". This work was also performed under the Research Program for Next Generation Young Scientists of "Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials" (D. K.). We are grateful to Dr. Hironori Tsunoyama (Keio University) for useful discussion and suggestion regarding the structural analysis.


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Synthesis, Structural and Photophysical Properties of Pentacene

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