Efficient Enhancement of the Visible-Light Absorption of Cyclometalated Ir(III) Complexes Triplet Photosensitizers with Bodipy and Applications in Photooxidation and Triplet–Triplet Annihilation Upconversion

Abstract

We report molecular designing strategies to enhance the effective visible-light absorption of cyclometalated Ir(III) complexes. Cationic cyclometalated Ir(III) complexes were prepared in which boron-dipyrromethene (Bodipy) units were attached to the 2,2'-bipyridine (bpy) ligand via -C≡C- bonds at either the meso-phenyl (Ir-2) or 2 position of the π core of Bodipy (Ir-3). For the first time the effect of π conjugating (Ir-3) or tethering (Ir-2) of a light-harvesting chromophore to the coordination center on the photophysical properties was compared in detail. Ir(ppy)2(bpy) (Ir-1; ppy = 2-phenylpyridine) was used as model complex, which gives the typical weak absorption in visible range (ε < 4790 M(-1) cm(-1) in region > 400 nm). Ir-2 and Ir-3 showed much stronger absorption in the visible range (ε = 71,400 M(-1) cm(-1) at 499 nm and 83,000 M(-1) cm(-1) at 527 nm, respectively). Room-temperature phosphorescence was only observed for Ir-1 (λ(em) = 590 nm) and Ir-3 (λ(em) = 742 nm). Ir-3 gives RT phosphorescence of the Bodipy unit. On the basis of the 77 K emission spectra, nanosecond transient absorption spectra, and spin density analysis, we proposed that Bodipy-localized long-lived triplet excited states were populated for Ir-2 (τT = 23.7 μs) and Ir-3 (87.2 μs). Ir-1 gives a much shorter triplet-state lifetime (0.35 μs). Complexes were used as singlet oxygen ((1)O2) photosensitizers in photooxidation. The (1)O2 quantum yield of Ir-3 (ΦΔ = 0.97) is ca. 2-fold of Ir-2 (ΦΔ = 0.52). Complexes were also used as triplet photosensitizer for TTA upconversion; upconversion quantum yields of 1.2% and 2.8% were observed for Ir-2 and Ir-3, respectively. Our results proved that the strong absorption of visible light of Ir-2 failed to enhance production of a triplet excited state. These results are useful for designing transition metal complexes that show effective strong visible-light absorption and long-lived triplet excited states, which can be used as ideal triplet photosensitizers in photocatalysis and TTA upconversion.

Influence of a Naphthaldiimide Substituent at the Diimine Ligand on the Photophysics and Reverse Saturable Absorption of PtII Diimine Complexes and Cationic IrIII Complexes

“…Additionally, each complex possesses a broad, structureless tail in the region of ca. 425 nm to 550 nm, which primarily arises from the 1 MLCT [metal-to- III complexes reported previously, [7][8][9] and the assignments are supported by the TDDFT calculated natural transition orbitals (NTOs) contributing to the respective high-energy (displayed in Table 2) and low-energy (displayed in Table 3) major absorption bands of complexes 1-4. The different natures of the high-energy main absorption bands and the low-energy broad tail are further supported by the solvatochromic effect in complexes 1-4.…”

“…Organometallic complexes with heavy transition-metal ions, such as square-planar d 8 Pt II complexes [1] and octahedral d 6 Ir III complexes, [2] have attracted much interest in the past decade due to the heavy-atom-induced strong spinorbit coupling, which gives rise to efficient intersystem crossing, a high triplet quantum yield, and room-temperature phosphorescence. These characteristics make them 5241 states for complex 4.…”

“…promising candidates for applications in organic light-emitting devices (OLEDs), [3][4][5][6] low-power upconversion, [7][8][9] luminescent bioimaging, [10][11][12][13] nonlinear optical devices, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] etc. In addition, the chemical structures of Pt II and Ir III complexes are readily modified to tune the photophysical properties in order to tailor the specific requirements for each application.…”

“…Efficient tuning of the ground-state and excited-state properties of Pt II /Ir III complexes for specific applications has been demonstrated in neutral Pt II diimine acetylide complexes and cationic heteroleptic Ir III complexes by modifications of either the diimine ligand (N ∧ N ligand), the acetylide ligands (for Pt II complexes), or the cyclometalating C ∧ N ligands (for Ir III complexes). [7][8][9] It has been revealed that either introducing electron-donating or -withdrawing substituents, or extending the π-conjugation of the N ∧ N, acetylide, or C ∧ N ligands can drastically change the energy of the lowest singlet and triplet excited states, or even alter the nature of the lowest triplet excited state in some cases. Li and Huang's group revealed that by varying the π-conjugation of the N ∧ N ligand by the fusion of aromatic rings, the emission energy and efficiency of the heteroleptic Ir III complexes containing 1-phenylisoquinoline or 2-(2,4-difluorophenyl)pyridine (C ∧ N) ligands could be altered dramatically.…”

Photochemical upconversion: present status and prospects for its application to solar energy conversion

“…However, efficient TTA-UC is not restricted to these classes of molecules, as others can also satisfy the abovementioned conditions: Transition metal complexes such as Ir(ppy) 3 28 or Ru(dmb) 3 29 also facilitate efficient ISC by incorporating a heavy atom and can act as triplet sensitizers. Borondipyrromethene derivatives (so-called BODIPY dyes) 30 were found to function as rare-metal-free triplet sensitizers at reasonable efficiency, and can also be combined with Ir complexes 31 to increase their absorption strength. Regarding emitter species, most studies have focussed on PAHs, with BODIPY-and conjugated polymer-based emitters being notable exceptions.…”