Controlled Orientations of Neighboring Tetracene Units by

8 hours ago - Read OnlinePDF (3 MB). Supporting Info ... The Supporting Information is available free of charge on the ACS Publications website at DOI...
0 downloads 0 Views 3MB Size
Download PDF
Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

pubs.acs.org/JACS

Controlled Orientations of Neighboring Tetracene Units by Mixed Self-Assembled Monolayers on Gold Nanoclusters for High-Yield and Long-Lived Triplet Excited States through Singlet Fission Toshiyuki Saegusa,† Hayato Sakai,† Hiroki Nagashima,‡ Yasuhiro Kobori,*,‡,◊ Nikolai V. Tkachenko,*,§ and Taku Hasobe*,†

Downloaded via CARLETON UNIV on September 6, 2019 at 17:47:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ‡ Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan ◊ Department of Chemistry, Graduate School of Science, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan § Chemistry and Advanced Materials Group, Faculty of Engineering and Natural Sciences, Tampere University, Korkeakoulunkatu 8, FI33720 Tampere, Finland S Supporting Information *

ABSTRACT: Although tetracene (Tc) is well-known as a good candidate for singlet fission (SF), the number of high-yield and long-lived triplet excited states through SF is extremely limited because of the relative acceleration of the reverse triplet−triplet annihilation (TTA) considering the energy matching between a singlet and two triplet states. Systematic control of electronic interactions between two neighboring units using conventional covalent linkages and molecular assembly methods to optimize these kinetic processes is quite difficult because of the complicated synthesis and random orientations. In this study, we propose a novel supramolecular strategy utilizing mixed self-assembled monolayers (SAMs) with two different chain lengths. Specifically, mixed Tc-SAMs on gold nanoclusters, which are prepared using Tc-modified heterodisulfides with two different chain lengths, attain high-yield SF (ΦSF ≈ 90%) and individual triplet yields (ΦΤ ≈ 160%). The obtained ΦSF is the highest value among Tc derivatives in homogeneous solution to the best of our knowledge.

■

INTRODUCTION

following energy matching condition between a singlet and two triplet states: [E(S1) ≥ 2E(T1)]. The typical molecule is a pentacene (Pc) [E(S1) = 2.1 eV, E(T1) = 0.8 eV]21 because of the exothermic reaction. In contrast, tetracene (Tc) is a good candidate for SF creating higher energy excitons regardless of the less exothermic process [i.e., E(S1) = 2.6 eV, E(T1) = 1.3 eV],36,37 which leads to the acceleration of the reverse triplet− triplet annihilation (TTA). For example, an energy transfer (EnT) process from 3Tc* to O2 through SF is promising for catalytic38 and biological applications.39,40 This is in sharp contrast to Pc because of the endothermic process from 3Pc* to O2 considering the energies of 3Pc* (0.8 eV) and 1O2 (1Δ) (0.98 eV).41 Thus, there seems to be various potential applications because of the higher E(T1), whereas the number of high-yield and long-lived 3Tc* through SF is quite limited as compared to Pc derivatives. Consequently, the precise control of orientation and distance between two neighboring Tc units is definitely required for long-lived individual triplets (T + T) through SF but not correlated triplet pair (TT). Especially, orientation

Construction of nanohybrid systems composed of organic− inorganic materials is promising for both basic and applied research. A useful synthetic strategy is to employ selfassembled monolayers (SAMs) utilizing organic molecules containing functional units such as a thiol group.1−4 Recently, detailed structures of metal nanoclusters capped by SAMs have been reported in addition to their molecular behaviors.5−11 On the other hand, one of the important features for SAMs is the ability to arrange alternately different types and lengths of ligands on metal surfaces (i.e., mixed SAMs).12 Therefore, we can precisely control the orientation and distance of two neighboring molecules on monolayer-protected clusters (MPCs). According to the previous reports of Brust’s methods,13−15 asymmetric disulfides favored the preparation of mixed SAMs based on heterologous pairs (e.g., Scheme 1). Singlet fission (SF) is a spin-allowed multiexciton generation process in which a singlet exciton is converted into two triplet excitons in two neighboring molecules.16−27 This peculiar photophysical process is highly promising for significant improvements in solar energy conversion properties (e.g., photovoltaics).28−31 Additionally, mechanistic studies are widely discussed.32−35 Efficient SF generally requires the © XXXX American Chemical Society

Received: June 20, 2019

A

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

because a mixture of (Tc-C11-S)2 and (Tc-C7-S)2 were expected to prepare homologous pairs [Tc-C(11,7)-HmMPC] in contrast with asymmetric disulfides (i.e., Tc-C11-SS-Cn-Tc).

Scheme 1. Schematic Illustration of Organization Process of Tc Units on MPCsa

■

RESULTS AND DISCUSSION Synthesis and Structural Properties. A series of Tcmodified MPCs and reference materials were synthesized following the reported method.42,43 The synthetic details are shown in the experimental section, Schemes S1-S2 and Figures S1-S39 in Supporting Information (SI). TEM images of TcC(11,n)-Ht-MPCs and reference materials are shown in Figure 2A and Figures S40−S46. The mean diameters of all MPCs are

a

Red- and gray-colored units demonstrate Tc and linker chain units, respectively.

and distance-dependent SF by supramolecular methods is yet to be reported, so far. Herein we report the orientation control of neighboring Tc units by mixed SAMs on MPCs for highyield and long-lived 3Tc* through SF (Scheme 1 and Figure 1). In mixed Tc-SAMs prepared with Tc-hetero-disulfides: Tc-

Figure 2. (A) TEM image of Tc-C(11,7)-Ht-MPC. (B) MALDI-TOF MS spectrum of Tc-C(11,7)-Ht-MPC. (C) Absorption spectra of (a) Tc-C(11,7)-Ht-MPC (ca. 0.15 mg mL−1) and (b) Tc-Ref (11 μM) and (c) excitation spectrum of Tc-C(11,7)-Ht-MPC in toluene (monitored at 600 nm). Inset shows fluorescence spectra of (d) TcC(11,7)-Ht-MPC and (e) Tc-Ref in toluene (λex = 475 nm).

estimated to be ∼1.5 nm. MALDI-TOF mass data of TcC(11,n)-Ht-MPC (Figure 2B, Figures S33−S39) also demonstrated the mass range from ca. 74000 to 77000, which approximately corresponds to the compositions of Au144(SCnTc)30(SC11Tc)30 (Table S1).11,44 The structure (i.e., Au144 cluster) was also confirmed by XRD analysis according to the recent result (Figure S47).5 Absorption and fluorescence spectra of Tc-C(11,n)-HtMPCs demonstrated the presence of Tc on MPCs (Figure 2C and Figures S48−S52). In the absorption spectra, the spectral profile of Tc-C(11,7)-Ht-MPC (spectrum a in Figure 2) agrees with that of Tc-Ref (spectrum b). The small increased absorption of the baseline of Tc-C(11,7)-Ht-MPC is attributable to gold nanoclusters. The normalized fluorescence

Figure 1. Structures of Tc-C(11,n)-Ht-MPCs (n = 9, 7, 5) and reference materials in this study.

C11-S-S-Cn-Tc (n = 9, 7, 5) [denoted as Tc-C(11,n)-HtMPC], Tc-C(11,7)-Ht-MPC attains maximum correlated (ΦSF ≈ 90%) and individual (ΦΤ ≈ 160%) triplet yields. These values are much larger than those of single Tc-SAMs by a Tchomo-disulfide: (Tc-C11-S)2 [denoted as Tc-C(11,11)-MPC] and mixed SAMs by two different Tc-homo-disulfides: (TcC11-S)2 and (Tc-C7-S)2 [denoted as Tc-C(11,7)-Hm-MPC], B

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society spectrum of Tc-C(11,7)-Ht-MPC (spectrum d) agrees well with that of Tc-Ref (spectrum e), whereas the fluorescence intensity of Tc-C(11,7)-Ht-MPC is strongly quenched as compared to Tc-Ref because of the occurrence of SF (vide infra). Finally, the fluorescence excitation spectrum of TcC(11,7)-Ht-MPC (spectrum c) is in good agreement with the corresponding absorption spectrum (spectrum b), which indicates that the fluorescence emission arises from the only singlet excited states of Tc (1Tc*) units. The spectroscopic behaviors demonstrated no significant spectral shift of Tc units on MPCs relative to Tc-Ref because various orientations between two Tc units are possible in homogeneous solution (viz., relatively loose stacking structures of two nearby Tc units under the conditions of macroscopic measurement). The spectroscopic behaviors of the other MPCs were similarly observed (Figures S48−S52). Moreover, to examine the difference in the interaction of two Tc units in Tc-C(11,7)-Ht-MPC and Tc-C(11,7)-HmMPC, temperature-dependent 1H NMR experiments were performed (Figures S53 and S54). In Tc-C(11,7)-Hm-MPC, there is no significant change of the proton signals at two different temperatures (298 and 323 K) because of the relatively strong stacking structure between two neighboring Tc units. In contrast, relatively sharpened proton peaks of Tc units in Tc-C(11,7)-Ht-MPC were observed at high temperature (323 K) as compared to those at 298 K. These results suggested the stacked interaction of two Tc units was reduced in Tc-C(11,7)-Ht-MPC as compared to Tc-C(11,7)-HmMPC. Thus, we can conclude a smaller interaction of two Tc units in Tc-C(11,n)-Ht-MPCs (heterologous pairs in Scheme 1) as compared to Tc-C(11,7)-Hm-MPC (homologous pairs). Additionally, the stacking trend of Tc units in Tc-C(11,11)MPC (Figure S55) is similar to that in Tc-C(11,7)-Hm-MPC (Figure S53). Time-Resolved Spectroscopic Measurements. Femtosecond transient absorption (fsTA) spectra of Tc-C(11,n)-HtMPCs and reference compounds were measured (Figure 3 and Figures S56−S63) using 100 fs laser pulses at 350 nm in toluene. Note that the density of excitation was sufficiently reduced to the level where excitation of more than two Tc units adsorbed to a nanocluster can be ignored (Figure S61). First, triplet−triplet (T−T) absorption bands of Tc were examined using the EnT from anthracene to Tc-Ref (Figure S62). The T−T spectra of Tc-Ref were observed in the range from 450 to 550 nm. The singlet−singlet (S−S) absorption spectrum of Tc-Ref was separately confirmed in Figure S63 for comparison. The molar absorption coefficients (ε) of singlet and triplet excited states of Tc were accordingly assigned for evaluation of SF yields (vide infra). Figure 3A,B demonstrate the fsTA spectra of Tc-C(11,7)Ht-MPC and Tc-C(11,11)-MPC, respectively. The data of the other systems including Tc-C(11,7)-Hm-MPC are also shown in Figures S56−S58. Note that the T−T spectra heavily overlap with the corresponding S−S absorption in the range from ca. 450−600 nm and the ε values of T−T absorption are much smaller than those of S−S absorption (e.g., 1ε*(517 nm) = 29500 M−1 cm−1, 3ε*(517 nm) = 10100 M−1 cm−1, Figures S62 and S63). In contrast, we can simply analyze the kinetic process of 1Tc* in the long wavelength region (ca. 600−750 nm) because there is almost no T−T absorption in this spectral region. After laser pulse excitation of Tc-C(11,7)-HtMPC, a few broad S−S bands (e.g., 430 and 724 nm) were immediately seen, and the entire S−S absorption spectrum

Figure 3. fsTA spectra of (A) Tc-C(11,7)-Ht-MPC and (B) TcC(11,11)-MPC in toluene. The excitation wavelength is 350 nm.

monotonously decreased with increasing time. In contrast, an absorption band at 517 nm remains relatively intense as compared to the above wavelengths because of the overlapped S−S and T−T absorptions. The difference is clearly seen in the range from ca. 20 to 1000 ps (insets in Figure 3A,B) because of more efficient SF of Tc-C(11,7)-Ht-MPC relative to TcC(11,11)-MPC. This is in sharp contrast with the monotonous decreased band at 525 nm in Tc-C(11,11)-MPC (Figure 3B). Since the determination of singlet and triplet species was still unclear, decay-associated spectra (DAS) of Tc-C(11,n)-HtMPC and Tc-C(11,11)-MPC were further analyzed in Figures S64−S67 and Table S2. In all systems, there is a fast component with the deep well at around 610 nm, which arises from stimulated emission at the second fluorescence band. In this range, the samples have no ground state absorption, which would allow attribution of this deep well to ground state bleaching. This feature is a good indication of the relaxation of the singlet excited state of Tc responsible for the stimulated emission and quenched by the SF. These fast lifetime species are also dependent on the alkyl chain (n) in Tc-C(11,n)-HtMPC. The lifetimes decreased with decreasing length of alkyl chains (n) (vide infra). On the other hand, the longer C

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

D

Determined by fluorescence-lifetime of Tc/C11-MPC (Figure S69). bDetermined by fsTA. cCalculated by kDISS = (kDISS + kREC)(ΦT/(2ΦSF)). kDISS + kREC was estimated by the kinetic model. Calculated by kREC = (kDISS + kREC)[ΦSF − ΦT/(2ΦSF)]. eDetermined by the longer component of τT. fDetermined by nsTA. gDetermined by fsTA (see SI). hDetermined by 1O2 emission (Figure 4). Note that no 1O2 emission was observed in Tc/C11-MPC and C11-MPC. iDetermined by nsTA. d

a

20 74 158 92 23 3 6 4 4 1 ± ± ± ± ± 22 69 89 53 24 0.25 (24%), 2.7 (76%) 1.9 (17%), 7.7 (83%) 9.3 2.6 0.10 (14%), 3.2 (86%) 5.4 3.7 1.3 1.1 3.8 3.1 1.9 31 22 3.0 1.1 41 (0.14) (0.46) (1.08) (0.39) (0.21) 1.9 3.6 4.8 2.4 2.1 26 26 27 7.0 37 57 48 30 8.1 78 370 200 120 44 370 Tc-C(11,11) Tc-C(11,9)-Ht Tc-C(11,7)-Ht Tc-C(11,5)-Ht Tc-C(11,7)-Hm Tc-Ref

4.2a 4.2a 4.2a 4.2a 4.2a

190 56 25 18 180

(kDISS + kREC) × 10−9,b s−1 kTTA× 10−9,b s−1 kSF × 10−9,b s−1 kEnT × 10−9, s−1 MPC

Table 1. Summarized Rate Constants, Lifetimes, and Quantum Yields

kDISS × 10−9,c s−1

kSF/kTTA (kDISS/kTTA)

kREC × 10−9,d s−1

kIC × 10−7,e s−1

τT,f μs

ΦSF,g %

ΦT,h %

component is the closest match to the T−T spectrum in Figure S62A. The time-dependent concentrations of the singlet (S1) and triplet (T1) states in Tc-C(11,7)-Ht-MPC are quantitatively estimated using the above-mentioned ε values. The timedependent concentrations of S1 and T1 states are shown in Figure S68, which resulted in the determination of the maximum concentration of T1 relative to S1 at ∼5.6 ns. The quantum yield of the correlated triplet pair, TT, namely SFyield (ΦSF), was accordingly calculated to be ΦSF = 89% (maximum ΦSF = 100%). This is much larger than those of the other MPCs including Tc-C(11,11)-MPC (ΦSF = 22%), TcC(11,9)-Ht-MPC (ΦSF = 69%) and Tc-C(11,5)-Ht-MPC (ΦSF = 53%) [the calculation process is shown in SI (Tables S3−S8)]. The obtained ΦSF (∼90%) is the highest value among Tc derivatives in homogeneous solution to the best of our knowledge,45 whereas quantitative SF processes were already reported in thin films.46,47 Moreover, the ΦSF of TcC(11,7)-Hm-MPC (24%) is much smaller than that of TcC(11,7)-Ht-MPC (89%) because of the formation of homologous Tc pairs on the MPC. These results ensure that the appropriate control of orientations and distances between two Tc nearest neighbors plays an important role in the improved ΦSF value. Additionally, to compare the triplet state yields of TcC(11,n)-Ht-MPCs and Tc-C(11,11)-MPC, we demonstrated normalized spectra at 1.8 ps and at 5 ns delay using the same normalization coefficients (Figures S70 and S71). Figure S71 indicates the comparison of spectra at 5 ns relative to 1.8 ps. The most essential differences are observed for the relative spectral intensities at longer delay time (5 ns). On the qualitative level, the relative spectral intensities agree with the above-mentioned SF yield (ΦSF) as shown in Table 1. Observation of Individual Tc Triplet Excited States. To further examine the individual triplet (3Tc*) yield (maximum ΦΤ = 200%), we employed 1O2 phosphorescence measurements utilizing EnT from 3Tc* to O2 (Figure 4 and Table S9) in oxygen-saturated toluene solution.41 By utilizing EnT from the individual triplet excited state of Tc to O2, we can detect the 1O2 phosphorescence at ca. 1270 nm, assuming negligible quenching processes from the singlet excited states and TT states.41 The analyzed values are shown in Table 1. Note that no 1O2 emission was observed in Tc/C11-MPC and C11-MPC because no SF occurs. This indicates that the heavy atom effect (i.e., gold nanoclusters) does not contribute to the singlet−triplet conversion. Especially, Tc-C(11,7)-Ht-MPC demonstrated the largest ΦΤ (158%), which suggested that the dissociation process from TT to T + T is a dominant pathway considering the following equation of the quantitative comparison: ΦΤ ≈ 2ΦSF. The trend is also similar to that for Tc-C(11,5)-Ht-MPC (ΦSF = 53%, ΦΤ = 92%). These are in sharp contrast to those in Tc-C(11,11)-MPC (ΦSF = 22%, ΦT = 20%) and Tc-C(11,9)-Ht-MPC (ΦSF = 69%, ΦT = 74%). These results revealed that the smaller interaction between two Tc units on MPC resulted in the relatively larger ΦΤ values. These differences between ΦSF and ΦT in Tc-C(11,n)-HtMPC are further associated with the lifetimes of T−T absorption evaluated by nanosecond transient absorption (nsTA). To further observe the long-lived triplet excited states, nsTA spectra of Tc-C(11,n)-Ht-MPC and reference materials were measured using excitation at 532 nm. The typical transient spectra, such as that of Tc-C(11,7)-Ht-MPC, is shown in

±1 ±5 ±5 ±3 ±1 27i

Journal of the American Chemical Society

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

Time-Resolved Electron Spin Resonance (TR-ESR) Measurements. To confirm SF directly, the quintet TT state, 5(TT), was observed using time-resolved electron spin resonance (TR-ESR). Signals of quintets 5(TT) and triplets (3T) with spin polarization of emission (E)/absorption (A) polarized patterns were obviously observed at 480 ns after photoirradiation (Figure 6). These peak positions are

Figure 4. Singlet-oxygen emission spectra of (a) Tc-C(11,11)-MPC (blue line), (b) Tc-C(11,9)-Ht-MPC (pink line), (c) Tc-C(11,7)-HtMPC (red line), (d) Tc-C(11,5)-Ht-MPC (green line), and (e) TcRef (black line) in toluene (λex = 350 nm).

Figure 5. The other systems are also shown in Figures S72− S76. The obtained transient absorption bands of Tc-C(11,7)-

Figure 6. TR-ESR spectra of Tc-C(11,7)-Ht-MPC in methylcyclohexane at 120 K (λex = 532 nm). A and E denote microwave absorption and emission, respectively. Simulated spectra were obtained by setting spin sublevel populations to be 0.455, 0.242, 0.303, 0, 0 for the quintet in mS = −2, −1, 0, +1, and +2, respectively.48 The individual triplet (3T) EPR spectra were computed and added to the spectrum of the quintet ESR spectrum as previously reported.48

coincident with the reported peaks in SF systems.48,49 The absorptive quintet TR-ESR signal was explained by chemically induced dynamics electron polarization (CIDEP)48 in which the quintet sublevel populations in ms = −1 and −2 were caused by the singlet−quintet mixings due to level-crossings in the presence of the exchange interaction (J), together with mixing at ms = 0 in the absence of J of the separated T−T pairs generated by SF. This CIDEP was reported to be induced by sub-nanosecond exciton diffusions in disorderly aggregated regions.48 It is thus expected that after SF at the highly ordered hot spot in Figure 7A (vide infra), the quintet multiexcitons were generated in the disorder area. Such disorder is suggested by the sharp 1H NMR peaks denoting mobile Tc moieties in Figure S54 obtained at room temperature. Because the exergonic quintet generation was explained by entropy enhancement by exciton migrations to the disorder regions, the high ΦT and small kREC values in Table 1 for Tc-C(11,7)Ht-MPC are coincident with the level-crossing CIDEP. Kinetic Discussion. The rate constants and quantum yields are analyzed following the kinetic model (Figure S77, Scheme 2, and Tables S3−S11). The photoinduced process is initiated by relaxation pathways such as EnT (kEnT), SF (kSF), and TTA (kTTA). The subsequent TT is able to generate the T + T state by the dissociation process (kDISS) in addition to the direct recombination from TT (kREC) to the ground state. In

Figure 5. Nanosecond transient absorption spectra of Tc-C(11,7)-HtMPC in toluene. Inset shows the corresponding time profile at 525 nm. Excitation wavelength is 532 nm. The lifetime of the triplet state is determined to be 9.3 μs.

Ht-MPC agree with the T−T absorption of Tc-Ref (Figure S62). The lifetime of Tc-C(11,7)-Ht-MPC was determined to be τΤ = 9.3 μs from the single-exponential fitting at 525 nm (Similarly, τΤ = 2.6 μs in Tc-C(11,5)-Ht-MPC). In contrast, Tc-C(11,11)-MPC and Tc-C(11,9)-Ht-MPC have two different lifetime components (Table 1). The initial and faster one is due to the TT pair, whereas the second and longer component is attributable to the T + T state according to the previous report.21 This is in sharp contrast with Tc-C(11,7)-Ht-MPC and Tc-C(11,5)-Ht-MPC with single lifetime component of T + T. Such differences of lifetime components are probably associated with the differences between ΦSF and ΦT in TcC(11,n)-Ht-MPCs (vide supra). E

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

model (Figure 7A) and (ii) equally spaced model (Figure 7B) on MPCs for simple discussions. The detail calculation processes are described in SI (Figures S78−S80 and Table S12). In the case of model ii, equally spaced model, the centerto-center distances in two Tc nearest neighbors approximately remain constant in the range from ca. 12 to 13 Å even though the alkyl chain lengths (n) are changed. In contrast, the distances systematically increase with decreasing length (n) of alkyl chains in model i, close-stacking model. Considering the above-mentioned rate constants of SF (kSF) in Tc-C(11,n)-HtMPCs, the increase trend of kSF with an increase in alkyl chain number (n) is in good agreement with the close-stacking model, which may be associated with the established electron transfer theory, that is, linker distance-dependent rate constants of electron transfer (see the analyzed figure in Figure S81).50 Such a dimeric formation of Tc units may be associated with the formation of charge-transfer (CT) states for occurrence of SF according to the recent mechanistic discussions.16,33,51,52

■

CONCLUSION In conclusion, we have newly synthesized a series of mixed TcSAMs on gold nanoclusters prepared from a Tc-modified heterodisulfide with two different chain lengths (Tc-C11-S-SCn-Tc (n = 9, 7, 5)) [Tc-C(11,n)-Ht-MPC]. The detailed structures of these Tc-modified MPCs were successfully assigned by steady-state spectroscopy, MALDI-TOF mass, TEM and XRD. Temperature-dependent 1H NMR measurements suggested smaller interaction of two Tc units in TcC(11,7)-Ht-MPC as compared to mixed SAMs by two different Tc-homo-disulfides: (Tc-C11-S)2 and (Tc-C7-S)2 [Tc-C(11,7)-Hm-MPC]. Such control of electronic interaction between two neighboring Tc units in C(11,7)-Ht-MPC resulted in the highest ΦSF ≈ 90% among Tc derivatives in homogeneous solution (together with ΦΤ ≈ 160%). This study provides a novel supramolecular technique for future applications of MPCs such as solar energy conversion, electronics, and biological fields.

Figure 7. Proposed dimeric structures of Tc units in Tc-C(11,n)-HtMPCs. (A) Close-stacking models for n = 11 (ΔD11 = 3.0 Å), n = 9 (ΔD9 = 4.2 Å), n = 7 (ΔD7 = 5.8 Å), and n = 5 (ΔD5 = 8.5 Å) and (B) equally spaced models for n = 11 (ΔD′11 = 12.5 Å), n = 9 (ΔD′9 = 12.3 Å), n = 7 (ΔD7′ = 12.7 Å), and n = 5 (ΔD5′ = 13.4 Å). The diameter of the gold nanocluster (7.7 Å) was estimated by TEM measurements (Figure 2A and Table S1). The detailed estimation processes are shown in SI (Figures S78−S80).

Scheme 2. A Kinetic Model

■

Tc-C(11,n)-Ht-MPCs, the rate constants of kSF and kTTA monotonously decreased with decreasing length of alkyl chains (n). The trend of kSF values agrees with the time constants of SF evaluated from the global data fitting (Table S2 in SI). Note that these rate constants are much larger than the corresponding kEnT. This suggested that the smaller interaction between two Tc nearest neighbors resulted in the decrease of kSF and kTTA, which is similar to the trend of kREC. In contrast, the relative ratios of kSF/kTTA and kDISS/kTTA in Tc-C(11,7)Ht-MPC attain the largest values, which resulted in the maximum ΦSF and ΦT. Proposed Structures of Tc Units on MPCs for SF. As discussed earlier, no aggregation effect on the spectroscopic behaviors was observed in homogeneous solution because of the relatively loose stacking structures of two nearby Tc units under the condition of macroscopic measurement. In contrast, temperature-dependent 1H NMR measurements revealed the smaller interaction of two Tc units in Tc-C(11,n)-Ht-MPCs (heterologous pairs in Scheme 1) as compared to Tc-C(11,7)Hm-MPC (homologous pairs). Occurrence of SF is definitely required for the appropriate distance and orientation of two nearby Tc units (i.e., a certain kind of complex formation in the excited state) at the molecular level. Therefore, we propose two different dimeric structures such as (i) close-stacking

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b06567. Additional experimental, spectroscopic, and calculation data of Tc-modified gold nanoclusters and reference materials (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *nikolai.tkachenko@tut.fi ORCID

Hiroki Nagashima: 0000-0003-1162-6669 Yasuhiro Kobori: 0000-0001-8370-9362 Nikolai V. Tkachenko: 0000-0002-8504-2335 Taku Hasobe: 0000-0002-4728-9767 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

■

K.; Sfeir, M. Y.; Bawendi, M. G.; Swager, T. M.; Friend, R. H.; Baldo, M. A.; Van Voorhis, T. A Transferable Model for Singlet-Fission Kinetics. Nat. Chem. 2014, 6, 492−497. (18) Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, 6891−6936. (19) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M. T.; Choi, B.; Xia, J.; Taffet, E. J.; Low, J. Z.; Miller, J. R.; Roy, X.; Zhu, X. Y.; Steigerwald, M. L.; Sfeir, M. Y.; Campos, L. M. Quantitative Intramolecular Singlet Fission in Bipentacenes. J. Am. Chem. Soc. 2015, 137, 8965−8972. (20) Kato, D.; Sakai, H.; Tkachenko, N. V.; Hasobe, T. High-Yield Excited Triplet States in Pentacene Self-Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission. Angew. Chem., Int. Ed. 2016, 55, 5230−5234. (21) Sakai, H.; Inaya, R.; Nagashima, H.; Nakamura, S.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. Multiexciton Dynamics Depending on Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs. J. Phys. Chem. Lett. 2018, 9, 3354−3360. (22) Garoni, E.; Zirzlmeier, J.; Basel, B. S.; Hetzer, C.; Kamada, K.; Guldi, D. M.; Tykwinski, R. R. Two-Photon Absorption in Pentacene Dimers: The Importance of the Spacer Using Upconversion as an Indirect Route to Singlet Fission. J. Am. Chem. Soc. 2017, 139, 14017−14020. (23) Wasielewski, M. R. Dynamic Duos. Nat. Phys. 2017, 13, 114. (24) Pensack, R. D.; Tilley, A. J.; Parkin, S. R.; Lee, T. S.; Payne, M. M.; Gao, D.; Jahnke, A. A.; Oblinsky, D. G.; Li, P.-F.; Anthony, J. E.; Seferos, D. S.; Scholes, G. D. Exciton Delocalization Drives Rapid Singlet Fission in Nanoparticles of Acene Derivatives. J. Am. Chem. Soc. 2015, 137, 6790−6803. (25) Sakuma, T.; Sakai, H.; Araki, Y.; Mori, T.; Wada, T.; Tkachenko, N. V.; Hasobe, T. Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission. J. Phys. Chem. A 2016, 120, 1867−1875. (26) Sakai, H.; Inaya, R.; Tkachenko, N. V.; Hasobe, T. High-Yield Generation of Triplet Excited States by an Efficient Sequential Photoinduced Process from Energy Transfer to Singlet Fission in Pentacene-Modified Cdse/Zns Quantum Dots. Chem. - Eur. J. 2018, 24, 17062−17071. (27) Pensack, R. D.; Ostroumov, E. E.; Tilley, A. J.; Mazza, S.; Grieco, C.; Thorley, K. J.; Asbury, J. B.; Seferos, D. S.; Anthony, J. E.; Scholes, G. D. Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission. J. Phys. Chem. Lett. 2016, 7, 2370−2375. (28) Krishnapriya, K. C.; Musser, A. J.; Patil, S. Molecular Design Strategies for Efficient Intramolecular Singlet Exciton Fission. ACS Energy Lett. 2019, 4, 192−202. (29) Kunzmann, A.; Gruber, M.; Casillas, R.; Zirzlmeier, J.; Stanzel, M.; Peukert, W.; Tykwinski, R. R.; Guldi, D. M. Singlet Fission for Photovoltaics with 130% Injection Efficiency. Angew. Chem., Int. Ed. 2018, 57, 10742−10747. (30) Lee, J.; Jadhav, P.; Reusswig, P. D.; Yost, S. R.; Thompson, N. J.; Congreve, D. N.; Hontz, E.; Van Voorhis, T.; Baldo, M. A. Singlet Exciton Fission Photovoltaics. Acc. Chem. Res. 2013, 46, 1300−1311. (31) Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A. External Quantum Efficiency above 100% in a SingletExciton-Fission-Based Organic Photovoltaic Cell. Science 2013, 340, 334−337. (32) Le, A. K.; Bender, J. A.; Arias, D. H.; Cotton, D. E.; Johnson, J. C.; Roberts, S. T. Singlet Fission Involves an Interplay between Energetic Driving Force and Electronic Coupling in Perylenediimide Films. J. Am. Chem. Soc. 2018, 140, 814−826. (33) Basel, B. S.; Zirzlmeier, J.; Hetzer, C.; Reddy, S. R.; Phelan, B. T.; Krzyaniak, M. D.; Volland, M. K.; Coto, P. B.; Young, R. M.; Clark, T.; Thoss, M.; Tykwinski, R. R.; Wasielewski, M. R.; Guldi, D. M. Evidence for Charge-Transfer Mediation in the Primary Events of Singlet Fission in a Weakly Coupled Pentacene Dimer. Chem. 2018, 4, 1092−1111.

ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Numbers JP18H01957, 18K19063, 17H05270, and 17H05162 to T.H., 19H00888, 17K19105, and 16H04097 to Y.K., 17J01125 to H.N., and 17K14476 to H.S. This work was carried out by the joint research program of Molecular Photoscience Research Center, Kobe University. We also thank Prof. H. Tsunoyama (Keio Univ.) for useful suggestions and discussion regarding XRD measurements.

■

REFERENCES

(1) Thomas, K. G.; Kamat, P. V. Chromophore-Functionalized Gold Nanoparticles. Acc. Chem. Res. 2003, 36, 888−898. (2) Jin, R.; Zeng, C.; Zhou, M.; Chen, Y. Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities. Chem. Rev. 2016, 116, 10346−10413. (3) Abbas, M. A.; Kamat, P. V.; Bang, J. H. Thiolated Gold Nanoclusters for Light Energy Conversion. ACS Energy Lett. 2018, 3, 840−854. (4) Zeng, C.; Chen, Y.; Kirschbaum, K.; Lambright, K. J.; Jin, R. Emergence of Hierarchical Structural Complexities in Nanoparticles and Their Assembly. Science 2016, 354, 1580−1584. (5) Yan, N.; Xia, N.; Liao, L.; Zhu, M.; Jin, F.; Jin, R.; Wu, Z. Unraveling the Long-Pursued Au144 Structure by X-Ray Crystallography. Sci. Adv. 2018, 4, No. eaat7259. (6) Zeng, C.; Chen, Y.; Kirschbaum, K.; Appavoo, K.; Sfeir, M. Y.; Jin, R. Structural Patterns at All Scales in a Nonmetallic Chiral Au133(Sr)52 Nanoparticle. Sci. Adv. 2015, 1, No. e1500045. (7) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution. Science 2007, 318, 430−433. (8) Desireddy, A.; Conn, B. E.; Guo, J.; Yoon, B.; Barnett, R. N.; Monahan, B. M.; Kirschbaum, K.; Griffith, W. P.; Whetten, R. L.; Landman, U.; Bigioni, T. P. Ultrastable Silver Nanoparticles. Nature 2013, 501, 399−402. (9) Li, Q.; Zhou, M.; So, W. Y.; Huang, J.; Li, M.; Kauffman, D. R.; Cotlet, M.; Higaki, T.; Peteanu, L. A.; Shao, Z.; Jin, R. A MonoCuboctahedral Series of Gold Nanoclusters: Photoluminescence Origin, Large Enhancement, Wide Tunability, and Structure-Property Correlation. J. Am. Chem. Soc. 2019, 141, 5314−5325. (10) Stamplecoskie, K. G.; Kamat, P. V. Size-Dependent Excited State Behavior of Glutathione-Capped Gold Clusters and Their LightHarvesting Capacity. J. Am. Chem. Soc. 2014, 136, 11093−11099. (11) Negishi, Y.; Nakazaki, T.; Malola, S.; Takano, S.; Niihori, Y.; Kurashige, W.; Yamazoe, S.; Tsukuda, T.; Häkkinen, H. A Critical Size for Emergence of Nonbulk Electronic and Geometric Structures in Dodecanethiolate-Protected Au Clusters. J. Am. Chem. Soc. 2015, 137, 1206−1212. (12) Kato, D.; Sakai, H.; Araki, Y.; Wada, T.; Tkachenko, N. V.; Hasobe, T. Concentration-Dependent Photophysical Switching in Mixed Self-Assembled Monolayers of Pentacene and Perylenediimide on Gold Nanoclusters. Phys. Chem. Chem. Phys. 2018, 20, 8695−8706. (13) Noh, J.; Hara, M. Nanoscopic Evidence for Dissociative Adsorption of Asymmetric Disulfide Self-Assembled Monolayers on Au(111). Langmuir 2000, 16, 2045−2048. (14) Manna, A.; Chen, P.-L.; Akiyama, H.; Wei, T.-X.; Tamada, K.; Knoll, W. Optimized Photoisomerization on Gold Nanoparticles Capped by Unsymmetrical Azobenzene Disulfides. Chem. Mater. 2003, 15, 20−28. (15) Shon, Y.-S.; Mazzitelli, C.; Murray, R. W. Unsymmetrical Disulfides and Thiol Mixtures Produce Different Mixed MonolayerProtected Gold Clusters. Langmuir 2001, 17, 7735−7741. (16) Miyata, K.; Conrad-Burton, F. S.; Geyer, F. L.; Zhu, X. Y. Triplet Pair States in Singlet Fission. Chem. Rev. 2019, 119, 4261− 4292. (17) Yost, S. R.; Lee, J.; Wilson, M. W. B.; Wu, T.; McMahon, D. P.; Parkhurst, R. R.; Thompson, N. J.; Congreve, D. N.; Rao, A.; Johnson, G

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society (34) Fuemmeler, E. G.; Sanders, S. N.; Pun, A. B.; Kumarasamy, E.; Zeng, T.; Miyata, K.; Steigerwald, M. L.; Zhu, X. Y.; Sfeir, M. Y.; Campos, L. M.; Ananth, N. A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers. ACS Cent. Sci. 2016, 2, 316−324. (35) Mirjani, F.; Renaud, N.; Gorczak, N.; Grozema, F. C. Theoretical Investigation of Singlet Fission in Molecular Dimers: The Role of Charge Transfer States and Quantum Interference. J. Phys. Chem. C 2014, 118, 14192−14199. (36) Nakamura, S.; Sakai, H.; Nagashima, H.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. Quantitative Sequential Photoenergy Conversion Process from Singlet Fission to Intermolecular TwoElectron Transfers Utilizing Tetracene Dimer. ACS Energy Lett. 2019, 4, 26−31. (37) Korovina, N. V.; Joy, J.; Feng, X.; Feltenberger, C.; Krylov, A. I.; Bradforth, S. E.; Thompson, M. E. Linker-Dependent Singlet Fission in Tetracene Dimers. J. Am. Chem. Soc. 2018, 140, 10179− 10190. (38) Hoffmann, N. Photochemical Reactions as Key Steps in Organic Synthesis. Chem. Rev. 2008, 108, 1052−1103. (39) Almeida-Marrero, V.; van de Winckel, E.; Anaya-Plaza, E.; Torres, T.; de la Escosura, A. Porphyrinoid Biohybrid Materials as an Emerging Toolbox for Biomedical Light Management. Chem. Soc. Rev. 2018, 47, 7369−7400. (40) Lucky, S. S.; Soo, K. C.; Zhang, Y. Nanoparticles in Photodynamic Therapy. Chem. Rev. 2015, 115, 1990−2042. (41) Tokuo, K.; Sakai, H.; Sakanoue, T.; Takenobu, T.; Araki, Y.; Wada, T.; Hasobe, T. Control of the Electrochemical and Photophysical Properties of N-Substituted Benzo[ghi]perylene Derivatives. Mater. Chem. Front. 2017, 1, 2299−2308. (42) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-Derivatised Gold Nanoparticles in a Two-Phase Liquid-Liquid System. J. Chem. Soc., Chem. Commun. 1994, 0, 801− 802. (43) Kato, D.; Sakai, H.; Saegusa, T.; Tkachenko, N. V.; Hasobe, T. Synthesis, Structural and Photophysical Properties of Pentacene Alkanethiolate Monolayer-Protected Gold Nanoclusters and Nanorods: Supramolecular Intercalation and Photoinduced Electron Transfer with C60. J. Phys. Chem. C 2017, 121, 9043−9052. (44) Qian, H.; Jin, R. Controlling Nanoparticles with Atomic Precision: The Case of Au144(SCH2CH2Ph)60. Nano Lett. 2009, 9, 4083−4087. (45) Yamakado, T.; Takahashi, S.; Watanabe, K.; Matsumoto, Y.; Osuka, A.; Saito, S. Conformational Planarization Versus Singlet Fission: Distinct Excited-State Dynamics of Cyclooctatetraene-Fused Acene Dimers. Angew. Chem., Int. Ed. 2018, 57, 5438−5443. (46) Alagna, N.; Han, J.; Wollscheid, N.; Lustres, J. L. P.; Herz, J.; Hahn, S.; Koser, S.; Paulus, F.; Bunz, U. H. F.; Dreuw, A.; Buckup, T.; Motzkus, M. Tailoring Ultrafast Singlet Fission by the Chemical Modification of Phenazinothiadiazoles. J. Am. Chem. Soc. 2019, 141, 8834−8845. (47) Margulies, E. A.; Kerisit, N.; Gawel, P.; Mauck, C. M.; Ma, L.; Miller, C. E.; Young, R. M.; Trapp, N.; Wu, Y.-L.; Diederich, F.; Wasielewski, M. R. Substituent Effects on Singlet Exciton Fission in Polycrystalline Thin Films of Cyano-Substituted Diaryltetracenes. J. Phys. Chem. C 2017, 121, 21262−21271. (48) Nagashima, H.; Kawaoka, S.; Akimoto, S.; Tachikawa, T.; Matsui, Y.; Ikeda, H.; Kobori, Y. Singlet-Fission-Born Quintet State: Sublevel Selections and Trapping by Multiexciton Thermodynamics. J. Phys. Chem. Lett. 2018, 9, 5855−5861. (49) Weiss, L. R.; Bayliss, S. L.; Kraffert, F.; Thorley, K. J.; Anthony, J. E.; Bittl, R.; Friend, R. H.; Rao, A.; Greenham, N. C.; Behrends, J. Strongly Exchange-Coupled Triplet Pairs in an Organic Semiconductor. Nat. Phys. 2017, 13, 176−181. (50) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen, L.; Tomé, J. P. C.; Fazio, M. A.; Schuster, D. I. Energy and Electron Transfer in Polyacetylene-Linked Zinc-Porphyrin-[60]Fullerene Molecular Wires. Chem. - Eur. J. 2005, 11, 3375−3388.

(51) Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E. T.; Casillas, R.; Basel, B. S.; Thoss, M.; Tykwinski, R. R.; Guldi, D. M. Singlet Fission in Pentacene Dimers. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5325−5330. (52) Lukman, S.; Chen, K.; Hodgkiss, J. M.; Turban, D. H. P.; Hine, N. D. M.; Dong, S.; Wu, J.; Greenham, N. C.; Musser, A. J. Tuning the Role of Charge-Transfer States in Intramolecular Singlet Exciton Fission through Side-Group Engineering. Nat. Commun. 2016, 7, 13622.

H

DOI: 10.1021/jacs.9b06567 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX