Article Cite This: Langmuir 2019, 35, 9740−9746
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Triplet−Triplet Annihilation-Based Upconversion Sensitized by a Reverse Micellar Assembly of Amphiphilic Ruthenium Complexes Keisuke Fujimoto,† Kyosuke Kawai,† Shota Masuda,† Toshihiro Mori,† Takumi Aizawa,§ Toshiyasu Inuzuka,‡ Takashi Karatsu,§ Masami Sakamoto,§ Shiki Yagai,§,∥ Tetsuya Sengoku,† Masaki Takahashi,*,† and Hidemi Yoda†
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Department of Applied Chemistry, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8561, Japan ‡ Division of Instrumental Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan § Department of Applied Chemistry and Biotechnology, Graduate School of Engineering and ∥Institute for Global Prominent Research (IGPR), Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan S Supporting Information *
ABSTRACT: We have developed a new photon upconversion (UC) system utilizing a new amphiphilic sensitizer 1a that comprises a hydrophilic ruthenium complex and a lipophilic bisanthracene appendage. At concentrations higher than 5 μM in toluene, the sensitizer 1a formed a reverse micellar assembly which facilitated the triplet sensitization of 9,10diphenylanthracene (DPA) more efficiently than homogeneously dispersed solutions to enhance the UC efficiency up to 38.2%. The Stern−Volmer analyses revealed the stepwise triplet−triplet energy transfers (TTET): (1) intramicellar energy transfer from the ruthenium core to the bisanthracene surface and (2) diffusion-dependent energy transfer from the surface to DPA. On these bases, it can be assumed that the reverse micellar assemblies accelerate the former TTET process to enhance the UC efficiency.
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INTRODUCTION Triplet−triplet annihilation (TTA)-based photon upconversion (UC) has emerged as a highly topical research area over the past decade because of potential possible applications1−3 for photovoltaic,4 photocatalytic,5 and bioimaging6 technologies by virtue of prospective advantages of low power excitation with noncoherent light sources, such as sunlight. Early studies have utilized homogeneous organic liquid media to promote diffusion-controlled collisional triplet−triplet energy transfer (TTET) and TTA between electronically excited triplet components.7 Recently, researchers have developed nonvolatile liquid, solid-state, and self-assembled systems, as exemplified by solid polymer films,8 liquid molecular networks,9 host−guest associations,10 gel nanofibers,11 and metal−organic frameworks,12 aiming for practical applications. Along this line of consideration, a reverse micellar system can be regarded as an attractive platform to investigate the TTA-UC phenomena in the colloidal assembly.13 Nevertheless, to the best of our knowledge, there have been no reports of using a triplet sensitizer that forms reverse micellar assemblies to promote the TTA-UC process so far. As a © 2019 American Chemical Society
pioneering work, Castellano and co-workers utilized a ruthenium complex (e.g., [Ru(dmb) 3 ] 2+ ) as a triplet sensitizer.7 After this finding, a number of chemically modified ruthenium sensitizers have been explored by Zhao and coworkers to realize more elaborated photon-UC systems.14−17 Here, we paid attention to the intrinsic hydrophilicity of the ruthenium complex which enables us to construct an amphiphilic dyad with a lipophilic organic functionality. In this work, we designed and synthesized a new ruthenium complex covalently coupled with lipophilic bisanthracene appendages (i.e., 1a in Figure 1).18 The bisanthracene moiety would function as a triplet energy harvester capturing triplet excitons from the clustered ruthenium core to form a longlived triplet excited state.19 Here, we evaluated a selfassembling nature, a triplet-sensitizing efficiency, and a characteristic TTET mechanism of the newly developed amphiphilic sensitizer. Received: May 14, 2019 Revised: July 3, 2019 Published: July 4, 2019 9740
DOI: 10.1021/acs.langmuir.9b01433 Langmuir 2019, 35, 9740−9746
Article
Langmuir
Figure 1. Structures of the amphiphilic ruthenium sensitizers 1a, 1b, and 2a (R = 2-ethylhexyl) and description of a formation of a reverse micellar assembly.
Scheme 1. Synthesis of Ruthenium Sensitizers 1a, 1b, and 2a (R = 2-Ethylhexyl); (i) Pd(PPh3)4, Na2CO3, Toluene/EtOH/ H2O, 80 °C, and Then (ii) TBAF, THF, RT, 77% in Two Steps; (iii) 5, NaH, DMF, RT, 85% for L1 and 93% for L2; (iv) Ru(bpy)2Cl2·H2O, Acetone/EtOH/H2O, Reflux, 88% for 1a and 92% for 2a; and (v) KPF6, EtOH, RT, 74%
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Dynamic light scattering (DLS) measurements were conducted on Zetasizer Nano (Malvern Instruments). The temperature for measurements was kept at 20 °C. All the UC measurements were performed at room temperature on the JASCO FP-6200 equipped with a diode-pumped solid-state continuous laser (type MBL-III-473 nm to 10 mW-12080907, Changchun New Industries Optoelectronics Tech. Co., Ltd., China) that delivers a maximum output power of 24 mW at 473 nm. A
EXPERIMENTAL SECTION
UV−vis absorption and emission spectra were recorded on a JASCO V-630 spectrophotometer and a JASCO FP-6200 spectrofluorometer, respectively. Atomic force microscopy (AFM) images were obtained under ambient conditions using MultiMode 8 NanoScope V (Bruker Instrument) in Peak Force Tapping (ScanAsyst) mode. Silicon cantilevers (SCANASYST-AIR) with a spring constant of 0.4 N/m and a frequency of 70 kHz (nominal value; Bruker) were used. 9741
DOI: 10.1021/acs.langmuir.9b01433 Langmuir 2019, 35, 9740−9746
Article
Langmuir neutral density (ND) filter (type S73-51-50, Suruga Seiki) with an optical transparency of 50% in the visible region was placed in front of the sample. The solutions were excited by a 473 nm CW laser, and UC emission was detected at 90° relative to the excitation beam. Because of relatively high susceptibility of the bisanthracene chromophores of 1a to oxidation under visible light irradiation, the UC measurements were repeated at least twice on identical fresh samples obtained from the same preparation batch. The quantum efficiency was determined by a relative method employing fluorescein in 0.1 M NaOHaq (ΦF 0.925) as a standard.20 The ΦUC was evaluated by the following equation: ΦUC = 2Φstd(Astd/ Aunk)(Iunk/Istd)(ηunk/ηstd), where A, I, and η represent absorbance, integrated UC emission intensity, and the refractive index at the excitation wavelength, respectively; “unk” and “std” denote the UC sample and the standard, respectively. The equation is multiplied by factor 2. A reviewer suggested a possibility that self-quenching caused by the chromophore aggregation in the reverse micellar system would reduce intrinsic UC efficiency. However, for the present TTA-UC system, TTA must occur outside the reverse micelles via collision of two excited triplets of freely diffusing DPA as described below. In this situation, the aggregation behavior of the amphiphilic sensitizers in reverse micellar forms should have essentially no influence on the intensity of UC emission, enabling us to obtain the intrinsic UC efficiency. All solvents and reagents were of reagent grade quality from Wako Pure Chemicals and Tokyo Chemical Industry (TCI) used without further purification. For the reverse micelle preparation, spectroscopic grade toluene (water content
