Article pubs.acs.org/IC
Impact of Substituents on Excited-State and Photosensitizing Properties in Cationic Iridium(III) Complexes with Ligands of Coumarin 6 Shin-ya Takizawa,*,† Naoya Ikuta,† Fanyang Zeng,‡ Shohei Komaru,† Shinogu Sebata,† and Shigeru Murata*,† †
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan ‡ Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan S Supporting Information *
ABSTRACT: A series of bis-cyclometalated cationic iridium (Ir) complexes were synthesized employing two coumarin 6 ligands and a 2,2′-bipyridine (bpy) with various substituents as new sensitizers, realizing both features of strong visible-light absorption and long-lived excited state. Complexes 2−4, with electron-donating methyl and methoxy groups, absorbed visible light strongly (ε: 126 000−132 000 M−1 cm−1) and exhibited room-temperature phosphorescence with remarkably long lifetimes (21−23 μs) in dichloromethane. In contrast, the excited state of prototype complex 1 without any substituents was short-lived, particularly in highly polar acetonitrile. Phosphorescence of complex 5 with the strong electronwithdrawing CF3 groups was too weak to be detected at room temperature even in less polar dichloromethane. The triplet energies of their coumarin ligandcentered (3LC) phosphorescent states were almost invariable, demonstrating that selective tuning of the excited-state lifetime is possible through this “simple chemical modification of the bpy ligand” (we name it the “SCMB” method). The spectroscopic and computational investigations in this study suggest that a potential source of the nonradiative deactivation is a triplet ligand-to-ligand charge-transfer state (3LLCT state, coumarin 6 → bpy) and lead us to conclude that the energy level of this dark 3LLCT state, as well as its thermal population, is largely dependent on the substituents and solvent polarity. In addition, the significant difference in excited-state lifetime was reflected in the photosensitizing ability of complexes 1−5 in visible-lightdriven hydrogen generation using sodium ascorbate and a cobalt(III) diglyoxime complex as an electron donor and a waterreduction catalyst, respectively. This study suggests that the SCMB method should be generally effective in controlling the excited state of other bis-cyclometalated cationic Ir(III) complexes.
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INTRODUCTION In the past decade a great deal of research has been devoted to the photochemistry of bis-cyclometalated cationic iridium(III) complexes because they can serve as luminophores in lightemitting electrochemical cells,1 bioimaging,2 sensing,3 and oxygen probes for living cells.4 Many cationic iridium (Ir) complexes display phosphorescence at room temperature with long lifetimes as a result of enhancing intersystem crossing (ISC) from their singlet excited states to the triplet states. The tunability of the phosphorescence color as well as the feasible functionalization has also facilitated the use of these complexes in the above-mentioned wide variety of applications through rational design, synthesis, and the selection of their cyclometalating and diimine ligands. In addition, a number of cationic Ir(III) complexes have emerged as useful sensitizers for photoreactions including singlet-oxygen generation,5 photoinduced hydrogen (H2) generation from water,6 photochemical CO2 reduction,7 and triplet−triplet annihilation-based photon upconversion (TTA-UC).8 However, reported Ir(III) sensi© XXXX American Chemical Society
tizers are mainly confined to iridium(III) bis(2-phenylpyridinato-N,C2′)-2,2′-bipyridine [Ir(ppy)2(bpy)]+ and its derivatives. Their relatively poor visible-light-harvesting ability has also limited further application of the Ir(III) sensitizers, while some research groups have developed other types of Ir(III) complex having different cyclometalating ligands with moderate to high absorptivity in the visible region.9 Other important features crucial to rendering the complexes effective sensitizers include long-lived triplet excited state and high photochemical durability in a certain solvent required for each photoreaction. We recently reported on cationic Ir(III) complexes with ligands of 2-(2-pyridyl)benzo[b]thiophene (btp) and a 2,2′bipyridine (bpy) bearing various substituents as our previous contribution to this journal.10 Systematic investigations have revealed that phosphorescence lifetimes of complexes having Received: May 27, 2016
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DOI: 10.1021/acs.inorgchem.6b01279 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Article
RESULTS AND DISCUSSION Synthesis and Characterization. A detailed synthetic route of new Ir(III) complexes 2−5 is given in the Supporting Information (Scheme S1), which is similar to that in our previous report of complex 1.9b These complexes were successfully synthesized by the reaction of a di-μ-chlorobridged Ir(III) dimer with the corresponding 2,2′-bipyridine ligand in hot ethylene glycol, followed by anion metathesis reaction of the chloride salts with ammonium hexafluorophosphate. For complex 6, we followed the procedure reported by Talarico et al. in the synthesis of a bis(2-phenylpyridine)ethylenediamine complex with some minor modifications.14 Complexes 2−6 were fully characterized by 1H NMR spectroscopy, mass spectrometry, and elemental analysis. Since the 1H NMR spectra showed one set of proton signals corresponding to the coumarin 6 and the pyridyl moiety of the bpy ligand, complexes 1−5 were found to have C2-symmetric structure. The coumarin ligands are basically coordinated with the iridium cation with the octahedral geometry, where the nitrogen atoms of the coumarin ligands are in trans positions, as illustrated in Scheme 1. Electronic Absorption Properties. UV−vis absorption spectra of complexes 1−6 were measured in dichloromethane (CH2Cl2) at room temperature, as depicted in Figure 1a. The
electron-donating methyl or methoxy groups on the bpy ligand are significantly longer than that of the parent complex without any substituents. Such a “simple chemical modification of the bpy ligand” (SCMB) was very effective in controlling the excited-state lifetime and photosensitizing property while leaving the absorption and emission spectral features unchanged. We tentatively proposed that the increased σdonating ability of the ancillary ligand substituted with the electron-donating groups destabilizes a short-lived, nonemissive triplet metal-centered (3MC) state and increases the energy separation between the 3MC state and emissive triplet ligandcentered (3LC) state based on the btp ligand. However, more work is needed to elucidate whether our proposed scheme of the excited-state deactivation is reliable and applicable to other Ir(III) complexes. In the present work, the SCMB method was tested for another cationic Ir(III) complex comprising two coumarin 6 ligands and one bpy ligand (depicted as complex 1 in Scheme 1). Complex 1 has already been reported by us as a Scheme 1. Chemical Structure of Ir Complexes 1−6
new promising sensitizer for visible-light-driven H2 generation.9b This Ir(III) dye exhibits a high molar absorption coefficient reaching 129 000 M−1 cm−1 at 483 nm, which is markedly larger than those of the Ir complexes with the btp ligands (7740−9400 M−1 cm−1 at 428−437 nm)10 and a frequently used sensitizer, Ru(bpy)32+ (13 000 M−1 cm−1 at 452 nm).11 Subsequent to our report, the Elliot and Gibson groups preliminarily evaluated this type of visible-light-absorbing coumarin complex as a sensitizer in dye-sensitized solar cells.12 Here we synthesized new coumarin-based complexes 2−5, bearing different substituents on the bpy ligand, to finely tune the excited-state property of prototype complex 1 (Scheme 1). Complexes 2 and 3 both possess methyl groups and enable us to investigate their positional effect on fundamental properties. Ir(III)-N,N′-dimethylethylenediamine complex 6 was prepared to include one example of bpy-free cationic Ir(III) complexes; it allows us to understand the effect of the bpy ligand on the absorption property and excited-state dynamics by comparing with the properties of complexes 1−5. Furthermore, the photosensitizing ability of complexes 1−5 was investigated using a visible-light-driven H2 generation system that incorporates ascorbate as an electron donor and a cobalt complex as a water reduction catalyst. This work is also intended to pursue new effective Ir(III) sensitizers realizing both features of strong visible-light absorption and long-lived excited state without employing more synthetically challenging dyads that, for example, couple a Ir(III) moiety with a visiblelight-harvesting organic dye.13
Figure 1. (a) UV−vis absorption and (b) phosphorescence spectra of complexes 1−6 in CH2Cl2 at room temperature.
absorption maximum wavelengths are collected in Table 1 along with molar extinction coefficients. As clearly seen in Figure 1a and Table 1, complexes 1−5 were found to absorb light strongly in the visible region (400−550 nm). For example, the molar absorption coefficient of complex 5 reached 134 000 M−1 cm−1 at 488 nm, which is the highest value among those of other Ir(III)-based dyes reported so far.5c,15 The fact that complex 6 without the bpy ligand equally showed intense absorption in this region, accompanied by a similar spectral profile, suggests that these absorption bands of complexes 1−6 are dominated by spin-allowed ligand-centered charge transfer in the coumarin ligand with a minor contribution from the bpy ligands. This interpretation is also consistent with the following observation. Introduction of the substituents on the bpy ligand B
DOI: 10.1021/acs.inorgchem.6b01279 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Photophysical Properties of Complexes 1−6 at Room Temperature solvent 1 2 3 4 5g 6 1g 2 3 4 5g 6
CH2Cl2
CH3CN
λabs/nm (ε/M−1 cm−1)a 484 483 483 482 488 489 479 478 479 477 480 480
(127 000) (126 000) (132 000) (131 000) (134 000) (126 000) (110 000) (117 000) (117 000) (112 000) (109 000) (104 000)
λem/nmb 592, 587, 588, 586,
Φc
636(sh) 630(sh) 631(sh) 629(sh)
0.134 0.262 0.207 0.271
