Photoluminescence Properties of [Core+exo]-Type Au6 Clusters

Subscriber access provided by WEBSTER UNIV

C: Physical Processes in Nanomaterials and Nanostructures 6

Photoluminescence Properties of [Core+exo]-Type Au Clusters: Insights into the Effect of Ligand Environments on the Excitation Dynamics Yukatsu Shichibu, Mingzhe Zhang, Takeshi Iwasa, Yuriko Ono, Tetsuya Taketsugu, Shun Omagari, Takayuki Nakanishi, Yasuchika Hasegawa, and Katsuaki Konishi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01810 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Photoluminescence Properties of [Core+exo]-Type Au6 Clusters: Insights into the Effect of Ligand Environments on the Excitation Dynamics Yukatsu Shichibu,*,†,‡ Mingzhe Zhang,† Takeshi Iwasa,# Yuriko Ono,# Tetsuya Taketsugu,#,∆ Shun Omagari,§,‖ Takayuki Nakanishi,§,¶,┴ Yasuchika Hasegawa,§,¶,∆ and Katsuaki Konishi*,†,‡ †Graduate

School of Environmental Science and ‡Faculty of Environmental Earth Science,

Hokkaido University, North 10 West 5, Sapporo 060-0810, Japan. #Department of Chemistry, Faculty of Science, Hokkaido University, North 10 West 8, Sapporo 060-0810, Japan. ∆Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, North 21 West 10, Sapporo 001-0021, Japan. §Graduate School of Engineering and ¶Faculty of Engineering, Hokkaido University, North 13 West 8, Sapporo 060-8628, Japan.

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

ABSTRACT: The use of multidentate chelating ligands enriches the geometric diversity of coordinated gold clusters, offering much knowledge to benefit the understanding of their unique electronic structures and optical properties. Herein we report different behaviors of [core+exo]type Au6 clusters bearing C3- and C4-bridged diphosphines (1 and 2), highlighting profound effects of the steric constraints of the ligand environments on the excited-state structural dynamics. Although the Au6 geometries of 1 and 2 in the crystalline states were somewhat different, their absorption spectra in solution were almost similar. However, marked differences were found in the photoluminescence properties; phosphorescent-type emission was dominantly observed for 1, whereas 2 gave both fluorescence- and phosphorescence-type emissions. Theoretical calculations showed that the bridging chains influence the geometries of the Au6 unit in the excited states, leading to the observed differences in emission behaviors.


2

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Atomically precise gold clusters protected by organic ligands have currently received increasing attention due to their fascinating physicochemical properties that set them apart from the corresponding gold nanoparticles and complexes.1-2 Detailed structural studies based on singlecrystal X-ray diffractions coupled with theoretical inspections have revealed their prominent molecular-like features associated with discrete electronic structures that give rise to unique optical absorption properties.3-4 It is also known that some clusters are photoluminescence (PL) active, which expands the scope of the fundamental interest and practical applications of this class of compounds.5-8 However, thus far, the nature of the excited states has been elucidated limitedly, so further studies on the photodynamic processes are still needed. Phosphines are known to serve as suitable capping ligands for the generation of small gold clusters with nuclearity of ~10.9 Recently, it has been shown that the use of multidentate phosphines results in the generation of exceptional geometric structures that cannot be obtained with conventional monophosphine ligands.10-14 Among them, [core+exo]-type clusters having a polyhedral core and additional Au atom(s) form an intriguing family.15-18 Structurally, the atoms at the polyhedral peripheries are coordinated by one ligand (on-top coordination), whereas the exo Au atoms accommodate two ligands to form a “staple-like” substructure found in thiolateligated gold clusters.1 One of the distinctive features of these core+exo type gold clusters is their optical properties. Unlike conventional polyhedral-only clusters, they exhibit isolated visible absorption bands due to the dominant oscillator strengths of the HOMO-LUMO transitions, which were verified by the theoretical studies.19 In addition, the behaviors of the excited states are interesting since they give characteristic PL associated with the HOMO-LUMO transitions. We have recently reported that the PL properties of core+exo type Au8 clusters with auxiliary


3


Page 4 of 21

anionic ligands can be altered by ligand-mediated interactions20-21 or cluster aggregation events.22 Herein we report results of studies focusing on related Au6 clusters protected solely by alkyl-bridged diphosphines and discuss the effect of the subtle difference of the bridge lengths on the structures and optical properties. Based on the photoluminescence profiles combined with density functional theory (DFT) and time-dependent DFT (TDDFT) analyses, we demonstrate that the bridging moieties sterically affect the structural dynamics of the excited states. 2. EXPERIMENTAL AND THEORETICAL METHODS Materials. Methanol (99.5%), dichloromethane (99%), and diethyl ether (99%) were purchased from Kanto Chemical Co., Inc. 1,3-Bis(diphenylphosphino)propane (dppp, 97%) and 1,4-bis(diphenylphosphino)butane (dppb, 97%) were obtained from Sigma-Aldrich. All reagents were used as received. [Au9(PPh3)8](NO3)3 and [Au6(dppp)4](NO3)2 (1·(NO3)2) were prepared according to previously reported procedures.23-24 Syntheses of the Au6 clusters. [Au6(dppp)4](BPh4)2 (1·(BPh4)2) was obtained by the anion exchange reaction of 1·(NO3)2 with excess NaBPh4 in methanol. [Au6(dppb)4](BPh4)2 (2·(BPh4)2) was obtained by the ligand exchange reaction of a precursor [Au9(PPh3)8]3+ with dppb followed by the above mentioned anion exchange method. To a dichloromethane solution (5 mL) of [Au9(PPh3)8](NO3)3 (100 mg, 24.6 μmol) was added a dichloromethane solution (2 mL) of dppb (210.4 mg, 493.4 μmol) and the mixture was stirred for 15 min. After the resulting mixture was dried in vacuo, the residue redissolved in methanol was treated with excess NaBPh4 to give a brown precipitate, which was collected by filtration, washed with diethyl ether and methanol, and dried to give 2·(BPh4)2 as aqua-blue solids (53 mg, 41 % based on Au). Crystals of 1·(BPh4)2 and 2·(BPh4)2 suitable for X-ray analyses were grown by the vapor diffusion of diethyl ether into methanol/dichloromethane (1:1) solutions of the clusters.


4

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Measurements. Optical absorption spectra were recorded using a JASCO V-670 double-beam spectrometer. Photoluminescence spectra were measured using a SPEX Fluorolog-3ps (Horiba Scientific) measurement system equipped with photomultiplier tubes (R928S and R5509, Hamamatsu Photonics). Luminescence lifetimes (within the range of 200 ns to 10 μs) were determined using a Horiba Scientific FluoroCube instrument equipped with a NanoLED-375LH laser diode. For picosecond-order lifetime measurement of 2, a Hamamatsu Photonics C11200 streak camera system with the lifetime range of 100 ps to 10 ms was used. Quantum yields were determined relative to rhodamine 6G in ethanol. Crystal structure data were collected on a Bruker SMART Apex II CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). All crystal structures were solved by direct methods and refined by full-matrix least-squares methods on F2 using APEX 2 software. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were located at calculated positions and refined isotropically. ESI mass spectrometry was performed using a Bruker micrOTOF-HS mass spectrometer.

31P

NMR spectra were collected in (CD3)2CO at ambient temperature on a JEOL EX-400 NMR spectrometer, and the chemical shifts (in ppm) were referenced to 85% H3PO4 (external standard). Computational details. Ground- and excited-state calculations were performed by DFT and TDDFT methods, respectively, with the B3LYP functional25-26 and the def-SV(P) basis set27 implemented in a TURBOMOLE 6.4 program package. The crystallographically determined structures of 1 and 2, followed by replacement of the phenyl groups with hydrogen atoms, were adopted as the initial structures for ground-state geometry-optimization calculations. The default 60-electron relativistic effective core potential (ECP)28 was employed for the Au atoms, and the


5


Page 6 of 21

resolution of identity (RI) approximation for Coulomb interactions29 was used to speed up the calculations. 3. RESULTS AND DISCUSSION It is already known that the reaction of [Au9(PPh3)8](NO3)3 with C3-bridged diphosphine ligand (dppp = Ph2P(CH2)3PPh2) affords [core+exo]-type Au6 cluster ([Au6(dppp)4]2+, 1).23 The use of the one-methylene longer diphosphine (dppb = Ph2P(CH2)4PPh2) also results in the formation of a similar Au6 cluster decorated by four ligands ([Au6(dppb)4]2+, 2). Crystal structures of the tetraphenylborate salts revealed that they had apparently similar Au6 frameworks, but a certain degree of distortion was observed for 2. For example, as shown in Figure 1, two exo-containing Au3 planes of 1 were oriented almost orthogonally (89.7°), while those of 2 were tilted with an

Figure 1. Bird views of the crystal structures of (a) 1 and (b) 2. Au-Au bonds >3.0 Å in length (i.e., Auedge1-Auedge3 and Auedge2-Auedge4 bonds in (b)) are shown as dotted lines. Phenyl rings and hydrogens are omitted for clarity. In parentheses, the Au6 units viewed along the longitudinal direction are provided with the aqua (Auedge1-Auedge2) and pink (Auedge3-Auedge4) lines to guide the eye.


6

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


angle of 75.5°. Accordingly, for two edges of the central tetrahedra along the longitudinal direction (Auedge1-Auedge3 and Auedge2-Auedge4 in Figure 1), 2 gave longer distances than 1 (3.2101 and 3.0643 Å versus 2.8491 and 2.8578 Å, respectively). Such a distortion of the gold frameworks in 2 seems associated with the steric requirement of the longer C4 bridges. These structural features seemed essentially retained in solution since the chemical shifts of the

31P-

NMR signals of 1·BPh4 and 2·BPh4 in (CD3)2CO were evidently different from each other (Figure S2). We next compared the optical properties of the two clusters in order to assess the effect of the distortion of the Au6 units. As shown in Figure 2a, the absorption spectra of 1·BPh4 and 2·BPh4 in acetone at 20 °C exhibited similar spectral profiles to give intense absorption bands at 586 nm due to the HOMO-LUMO transitions,19 indicating that the distortion of the Au6 frameworks has marginal effects on their frontier orbitals. This was further supported by theoretical absorption data of simplified-ligand models of 1 and 2 (vide infra) in Figure S6. Thus, the nature of frontier orbitals on gold clusters is rather governed by the basic arrangement patterns of the Au atoms, as we have shown in the isomeric Au11 clusters.17 In contrast, a clear difference was observed in the PL spectra upon excitation of the absorption bands. We have recently demonstrated that related [core+exo]-type Au7 and Au8 clusters in the monomeric form give only fluorescence emissions with stokes shifts of ~0.3 eV.16 Unlike them, the present Au6 clusters (1 and 2) have dual PL activity to give fluorescence- and phosphorescence-type emissions. At 20 °C, 1 showed an emission band at 834 nm with a quantum efficiency of 0.051%, which accompanied a negligibly small emission at a higher energy (~650 nm) (Figure 2b (i), black line). In contrast, 2 showed a clear dual-emission activity to give two relatively larger emission bands at 655 and 876 nm with quantum efficiencies of 0.074 and 0.16%,


7


Page 8 of 21

respectively (Figure 2 (ii), black line). Based on the time-resolved PL decay experiments (Figure S5), the near-IR emissions gave microsecond-order lifetimes of 0.48 and 3.04 s for 1 (834 nm) and 2 (900 nm), respectively, whereas the lifetime of the short-wavelength emissions of 2 (655 nm) was immeasurably fast (

Photoluminescence Properties of [Core+exo]-Type Au6 Clusters

Recommend Documents