Efficient Light Harvesting and Energy Transfer in a Red Phosphorescent Iridium Dendrimer

A series of red phosphorescent iridium dendrimers of the type [Ir­(btp)₂(pic-PC_<i>n</i>)] (<b>Ir-G</b>_{<b><i>n</i></b>}; <i>n</i> = 0, 1, 2, and 3) with two 2-(benzo­[<i>b</i>]­thiophen-2-yl)­pyridines (btp) and 3-hydroxypicolinate (pic) as the cyclometalating and ancillary ligands were prepared in good yields. Dendritic generation was grown at the 3 position of the pic ligand with 4-(9<i>H</i>-carbazolyl)­phenyl dendrons connected to 3,5-bis­(methyleneoxy)­benzyloxy branches (PC_<i>n</i>; <i>n</i> = 0, 2, 4, and 8). The harvesting photons on the PC_<i>n</i> dendrons followed by efficient energy transfer to the iridium center resulted in high red emissions at ∼600 nm by metal-to-ligand charge transfer. The intensity of the phosphorescence gradually increased with increasing dendrimer generation. Steady-state and time-resolved spectroscopy were used to investigate the energy-transfer mechanism. On the basis of the fluorescence quenching rate constants of the PC_<i>n</i> dendrons, the energy-transfer efficiencies for <b>Ir-G</b>_<b>1</b>, <b>Ir-G</b>_<b>2</b>, and <b>Ir-G</b>_<b>3</b> were 99, 98, and 96%, respectively. The energy-transfer efficiency for higher-generation dendrimers decreased slightly because of the longer distance between the PC dendrons and the core iridium­(III) complex, indicating that energy transfer in <b>Ir-G</b>_{<b><i>n</i></b>} is a Förster-type energy transfer. Finally, the light-harvesting efficiencies for <b>Ir-G</b>_<b>1</b>, <b>Ir-G</b>_<b>2</b>, and <b>Ir-G</b>_<b>3</b> were determined to be 162, 223, and 334%, respectively

Photodynamic Behavior of Heteroleptic Ir(III) Complexes with Carbazole-Functionalized Dendrons Associated with Efficient Electron Transfer Processes

We prepared dendrimers of heteroleptic iridium(III) complexes, [(dfppy–Cz₁)₂Ir(dpq)]⁺ (<b>G1</b>) and [(dfppy–Cz₂)₂Ir(dpq)]⁺ (<b>G2</b>), which have the dfppy ligand connected to carbazole-functionalized dendron Cz_<i>n</i> (<i>n</i> = 1, 2) [dfppy–Cz<i>_n</i> = 5-Cz<i>_n</i>-2-(4,6-difluorophenyl)pyridine, dpq = 2,3-bis-(2-pyridyl)-qinoxaline, Cz₁ = 4-(9-carbazolyl)benzyloxymethyl, and Cz₂ = 4-[1,3-bis(9-carbazolyl)benzyloxy]benzyloxymethyl]. While parent complex [(dfppy)₂Ir(dpq)]⁺ (<b>G0</b>) shows an intense emission at ∼635 nm with a lifetime of 1 μs assigned to dpq-based metal-to-ligand charge-transfer (MLCT) phosphorescence, excitation of the dendrimers at either carbazole (309 nm) or MLCT band (355 nm) resulted in markedly weaker and much shorter-lived MLCT emission (τ_p = 44 ns for <b>G1</b> and 115 ns for <b>G2</b>) at room temperature. Upon exciting the carbazole chromophore of <b>G1</b> and <b>G2</b> at 309 nm, furthermore, both the carbazole fluorescence and the MLCT emission were very weak at room temperature. It was found that the lifetime of carbazole fluorescence is 20 ps for <b>G1</b> and 62 ps for <b>G2</b>, shorter by 2-orders of magnitude than that of free carbazole dendron Cz_<i>n</i>′–OH (τ_F = 6.1 ns). These observations demonstrate that both the excited-singlet state of carbazole and the triplet MLCT state of the Ir(dpq) core are efficiently quenched in the dendrimers. At 77 K, however, the MLCT emission lifetime for both <b>G1</b> and <b>G2</b> is ∼7 μs that is nearly identical to that of <b>G0</b> (6.8 μs), and the carbazole fluorescence lifetime is ∼11.5 ± 0.5 ns, which is again almost the same as that of Cz_<i>n</i>′–OH (11.5 ns). Since the apparent quenching of either carbazole fluorescence or MLCT emission observed at room temperature does not occur at 77 K, the temperature-dependent emission behavior of <b>G1</b> and <b>G2</b> for both the carbazole fluorescence and the MLCT phosphorescence was attributed to the participation of activated processes, that is, electron transfer from excited-singlet carbazole to the Ir(dpq) core as well as from the ground-state carbazole unit to the triplet MLCT Ir(dpq) core. This mechanism was supported by transient-absorption spectroscopic experiments that demonstrate the generation of the carbazole radical cation after exciting <b>G1</b> and <b>G2</b> by laser pulses

Electronic Alteration on Oligothiophenes by <i>o</i>‑Carborane: Electron Acceptor Character of <i>o</i>‑Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group

We studied electronic change in oligothiophenes by employing <i>o</i>-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the <i>o</i>-carborane, a series of linear A₁-D-A₂ (<b>1a</b>–<b>1c</b>) or V-shaped A₁-D-A₂-D-A₁ <b>(2a</b>–<b>2c</b>) oligothiophene chain structures of variable length were prepared; A₁, D, and A₂, represent dicyano-vinyl, oligothiophenyl, and <i>o</i>-carboranyl groups, respectively. Among this series, <b>2a</b> shows strong electron-acceptor capability of <i>o</i>-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*−π* interaction. As a result, electronic communications between <i>o</i>-carborane and dicyano-vinyl groups are successfully achieved in <b>2a</b>

Electronic Alteration on Oligothiophenes by <i>o</i>‑Carborane: Electron Acceptor Character of <i>o</i>‑Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group

We studied electronic change in oligothiophenes by employing <i>o</i>-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the <i>o</i>-carborane, a series of linear A₁-D-A₂ (<b>1a</b>–<b>1c</b>) or V-shaped A₁-D-A₂-D-A₁ <b>(2a</b>–<b>2c</b>) oligothiophene chain structures of variable length were prepared; A₁, D, and A₂, represent dicyano-vinyl, oligothiophenyl, and <i>o</i>-carboranyl groups, respectively. Among this series, <b>2a</b> shows strong electron-acceptor capability of <i>o</i>-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*−π* interaction. As a result, electronic communications between <i>o</i>-carborane and dicyano-vinyl groups are successfully achieved in <b>2a</b>

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir­(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir­(III) complexes is designed and prepared: Ir­(dfppy)₂­(pic-OH) (<b>1a</b>), Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>), Ir­(ppy)₂­(pic-OH) (<b>2a</b>), and Ir­(ppy)₂­(pic-OMe) (<b>2b</b>). The emission lifetime of Ir­(dfppy)₂­(pic-OH) (<b>1a</b>) (τ_em = 3.19 μs) in dichloromethane solution was found to be significantly longer than that of Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>) (τ_em = 0.94 μs), because of a substantial difference in the nonradiative decay rate (<i>k</i>_nr = 0.28 × 10⁵ s^–1 for (<b>1a</b>) vs 2.99 × 10⁵ s^–1 for (<b>1b</b>)). These results were attributed to the intramolecular OH···OC hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when <b>1a</b> was used as the dopant compared to <b>FIrpic</b>, a well-known blue dopant. Device <b>III</b> (<b>1a</b> as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device <b>IV</b> (<b>FIrpic</b> as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir­(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir­(III) complexes is designed and prepared: Ir­(dfppy)₂­(pic-OH) (<b>1a</b>), Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>), Ir­(ppy)₂­(pic-OH) (<b>2a</b>), and Ir­(ppy)₂­(pic-OMe) (<b>2b</b>). The emission lifetime of Ir­(dfppy)₂­(pic-OH) (<b>1a</b>) (τ_em = 3.19 μs) in dichloromethane solution was found to be significantly longer than that of Ir­(dfppy)₂­(pic-OMe) (<b>1b</b>) (τ_em = 0.94 μs), because of a substantial difference in the nonradiative decay rate (<i>k</i>_nr = 0.28 × 10⁵ s^–1 for (<b>1a</b>) vs 2.99 × 10⁵ s^–1 for (<b>1b</b>)). These results were attributed to the intramolecular OH···OC hydrogen bond of the 3-hydroxy-picolinato ligand. Finally, device lifetime was significantly improved when <b>1a</b> was used as the dopant compared to <b>FIrpic</b>, a well-known blue dopant. Device <b>III</b> (<b>1a</b> as dopant) achieved an operational lifetime of 34.3 h for an initial luminance of 400 nits compared to that of device <b>IV</b> (<b>FIrpic</b> as dopant), a value of 20.1 h, indicating that the intramolecular hydrogen bond in ancillary ligand is playing an important role in device stability

Flexible and Micropatternable Triplet–Triplet Annihilation Upconversion Thin Films for Photonic Device Integration and Anticounterfeiting Applications

Triplet–triplet annihilation upconversion (TTA-UC) has recently drawn widespread interest for its capacity to harvest low-energy photons and to broaden the absorption spectra of photonic devices, such as solar cells. Although conceptually promising, effective integration of TTA-UC materials into practical devices has been difficult due to the diffusive and anoxic conditions required in TTA-UC host media. Of the solid-state host materials investigated, rubbery polymers facilitate the highest TTA-UC efficiency. To date, however, their need for long-term oxygen protection has limited rubbery polymers to rigid film architectures that forfeit their intrinsic flexibility. This study introduces a new multilayer thin-film architecture, in which scalable solution processing techniques are employed to fabricate flexible, photostable, and efficient TTA-UC thin films containing layers of oxygen barrier and host polymers. This breakthrough material design marks a crucial advance toward TTA-UC integration within rigid and flexible devices alike. Moreover, it introduces new opportunities in unexplored applications such as anticounterfeiting. Soft lithography is incorporated into the film fabrication process to pattern TTA-UC host layers with a broad range of high-resolution microscale designs, and superimposing host layers with customized absorption, emission, and patterning ultimately produces proof-of-concept anticounterfeiting labels with advanced excitation-dependent photoluminescent security features