Photodynamic Behavior of Heteroleptic Ir(III) Complexes with

ARTICLE pubs.acs.org/JPCC

Photodynamic Behavior of Heteroleptic Ir(III) Complexes with Carbazole-Functionalized Dendrons Associated with Efficient Electron Transfer Processes Ah-Reum Hwang,† Won-Sik Han,† Kyung-Ryang Wee,† Hyun Young Kim,† Dae Won Cho,† Byoung Koun Min,‡ Suk Woo Nam,‡ Chyongjin Pac,†,* and Sang Ook Kang†,* † ‡

Department of Advanced Materials Chemistry, Korea University, Sejong Campus, Chungnam 339-700, Korea Korea Institute of Science and Technology, Hawolgok-dong 39-1, Seongbuk-gu, Seoul 136-791, Korea

bS Supporting Information ABSTRACT: We prepared dendrimers of heteroleptic iridium(III) complexes, [(dfppy
Cz1)2Ir(dpq)]+ (G1) and [(dfppy
Cz2)2Ir(dpq)]+ (G2), which have the dfppy ligand connected to carbazole-functionalized dendron Cz n (n = 1, 2) [dfppy
Cz n = 5-Cz n -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)]+ (G0) 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 (τp = 44 ns for G1 and 115 ns for G2) at room temperature. Upon exciting the carbazole chromophore of G1 and G2 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 G1 and 62 ps for G2, shorter by 2-orders of magnitude than that of free carbazole dendron Czn0
OH (τF = 6.1 ns). These observations demonstrate that both the excitedsinglet 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 G1 and G2 is ∼7 μs that is nearly identical to that of G0 (6.8 μs), and the carbazole fluorescence lifetime is ∼11.5 ( 0.5 ns, which is again almost the same as that of Czn0
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 G1 and G2 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 G1 and G2 by laser pulses.

’ INTRODUCTION A dendrimer is a hyperbranched macromolecule with a defined molecular structure having regular treelike branches that emanate from the central group.1 Following pioneering works on dendrimers,2 a number of dendritic molecules with different structures and particular functions have been developed,3 and various applications of dendrimers are currently under investigation, for example, catalysis,4 encapsulation and delivery of drugs,5 nano/ultrafiltration and phase transfer,6 and nanomaterial preparation.7 In particular, the dendrimer architecture has been effectively applied to light-harvesting antenna functions in artificial photosynthesis,8 solar cells,9 and organic light-emitting diodes.10 In dendrimer systems where the light-harvesting branch unit has a higher excitation energy than the core, energy transfer may occur from the excited-state antenna to the core following efficient light absorption by the peripheral antenna molecules in the dendrons, thus enhancing the photophysical and/or r 2011 American Chemical Society

photochemical processes of the core.8
10 Since this mimics the antenna function in photosynthesis, much attention has been paid to the development of light-harvesting dendrimer systems.11 Alternatively, intramolecular photoinduced electron transfer (PET) is another important process in dendrimer photochemistry mimicking the charge-separation function in photosynthesis.12 Among possible photofunctional molecules that can be used as dendrimer cores, emissive transition-metal complexes are of potential interest, because their electronic properties can be tuned with relative ease through suitable metal
ligand combinations and also because their triplet states may become highly emissive with a relatively long lifetime due to strong heavy-metal effects. In recent years, cyclometalated Ir(III) complexes have received a Received: September 27, 2011 Revised: December 6, 2011 Published: December 08, 2011 1973

dx.doi.org/10.1021/jp2093077 | J. Phys. Chem. C 2012, 116, 1973–1986

The Journal of Physical Chemistry C

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Chart 1. Molecular Structures of [(dfppy)2Ir(dpq)]+ and [(dfppy
Czn)2Ir(dpq)]+ (n = 1, 2)

great attention,13 particularly due to the immense applicability to phosphorescent organic-light emitting diodes (PHOLEDs)14 and photochemical hydrogen15 and/or oxygen evolution16 from water. Within the context of PHOLEDs, interesting papers have appeared on Ir(III) dendrimers with carbazole-functionalized branches connected to an ancillary ligand, which undergo rapid energy transfer from the excited-state carbazole to the Ir(III) complex core,17 providing good examples for the light-harvesting capabilities of dendrimers. While carbazole compounds are well-known as potential electron donors with low oxidation potentials,18 no PET was reported to occur in those systems, probably due to the lack of sufficient electron-accepting forces in the Ir(III) cores. Therefore, it is expected that PET would proceed in cases where the reduction potential of an appropriate Ir(III) complex core is positive enough to receive an electron from the carbazole dendron in either or both of the MLCT state of the core and the excited-singlet state of carbazole. However, it is foreseeable that rapid energy transfer may still participate as a dominant or competitive pathway in the relevant dendritic systems. In order to obtain insights into the possible participation of PET processes versus energy transfer in Ir(III) complex dendrimers containing carbazole units, we prepared [(dfppy)2Ir(dpq)]+ (G0) and [(dfppy
Czn)2Ir(dpq)]+ dendrimers G1 (n = 1) and G2 (n = 2) where dfppy
Czn is the 2-(40 ,60 -difluorophenyl)pyridinato-N,C20 ligand connected with carbazole-functionalized dendron Czn and dpq is 2,3-bis-(2-pyridyl)-qinoxaline, as shown in Chart 1. The present investigation was performed to explore details of their photophysical behavior by means of picosecond time-resolved emission analysis combined with nanosecond transient-absorption spectroscopy. It was found that the present dendrimers undergo PET in either the excited-singlet state of the Czn unit or the triplet MLCT state of the Ir(III) core, while no energy transfer event was observed.

’ EXPERIMENTAL SECTION Synthesis: General Procedures. All reactions were carried out in a dry N2 atmosphere. Solvents were distilled from calcium

hydride for dichloromethane (CH2Cl2), acetonitrile (CH3CN), and dimethylformamide (DMF) or from sodium/benzophenone for tetrahydrofuran (THF) under a nitrogen atmosphere and stored over molecular sieves. Other reagents were obtained commercially and used without further purification. Glassware, syringes, magnetic stirring bars, and needles were dried in a convection oven for >4 h. Reactions were monitored with thin layer chromatography (TLC). Commercial TLC plates (Merck Co.) were developed, and the spots were identified under UV light at 254 and 365 nm. Column chromatography was done on silica gel 60 G (particle size 5
40 μm, Merck Co.). All synthesized compounds were characterized by 1H NMR, 13C NMR, and HRMS (FAB) or MALDI-TOF mass analysis. The 1H and 13C NMR spectra were recorded on a Varian Mercury 300 spectrometer operating at 300.1 and 75.4 MHz, respectively. The elemental analyses (Carlo Erba Instruments CHNS-O EA 1108 analyzer), high-resolution tandem mass spectrometry (Jeol LTD JMS-HX 110/110A), and MALDI-TOF mass spectroscopy (ABI Voyager STR) were performed by the Ochang branch of the Korean Basic Science Institute. 4-(Carbazol-9-yl)-benzaldehyde (1). A mixture of 4-fluorobenzaldehyde (10.6 mL, 100 mmol), K2CO3 (41.5 g, 300 mmol), and carbazole (16.7 g, 100 mmol) in DMF (150 mL) was stirred at 150 °C for 12 h, cooled to room temperature, and poured into ice water. The resulting precipitate was collected by filtration and washed with water, methanol, ethanol, and n-hexane in a consecutive order. A yellowish white solid was obtained after drying under vacuum. Yield: 23 g (82%). mp: 163
164 °C; 1H NMR (300.1 MHz, CDCl3, ppm) δ 10.11 (s, 1H), 8.11
8.17 (dd, 4H), 7.80 (d, 2H), 7.51 (d, 2H), 7.44 (d, 2H), 7.34 (d, 2H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 191.1, 143.5, 140.1, 134.7, 131.5, 126.9, 126.3, 124.1, 121.1, 120.9, 109.9; HRMS (FAB): m/z = 271.1004 [M]+. Anal. calcd for C19H13NO: C, 84.11; H, 4.83; N, 5.13. Found: C, 83.98; H, 4.86; N, 5.15. (4-(Carbazol-9-yl)-phenyl)methanol (Cz10
OH). To a suspension of LiAlH4 (3.4 g, 89 mmol) in dry THF (150 mL) under ice cooling was added 4-(carbazol-9-yl)benzaldehyde 1 (20.0 g, 74 mmol). The suspension was then allowed to warm to room 1974

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The Journal of Physical Chemistry C temperature and stirred until the reaction was completed, as indicated by TLC (for about 10
12 h). After that, the reaction mixture was poured into cold water under ice cooling, which was then stirred at room temperature for a further 15 min before being filtered. The filtrate was extracted with CH2Cl2, and the organic layer was dried over MgSO4. After evaporation under reduced pressure, the residue was purified by silica gel column chromatography using CH2Cl2 as the eluent to afford pure Cz1
OH as a white solid. Yield: 17.2 g (86%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.20 (d, 2H), 7.58 (dd, 4H), 7.44 (dd, 4H), 7.32
7.35 (m, 2H), 4.78 (d, 2H), 2.38 (s, 1H,); 13 C NMR (75.4 MHz, CDCl3, ppm) δ 141.1, 140.3, 137.3, 128.7, 127.4, 126.3, 123.7, 120.6, 120.3, 110.0, 65.0; HRMS (FAB): m/z = 273.1156 [M]+. Anal. calcd for C19H15NO: C, 83.49; H, 5.53; N, 5.12. Found: C, 83.33; H, 5.51; N, 5.11. 9-(4-(((6-Chloropyridin-3-yl)methoxy)methyl)phenyl)9H-carbazole (2). NaH (1.3 g, 5 mmol) was slowly added to a solution of Cz10
OH (5.0 g, 18 mmol) in 1:1 CH3CN/THF (40 mL). After stirring for 30 min, a solution of 2-chloro5-(chloromethyl)pyridine (2.9 g, 18 mmol) in dry THF (20 mL) was added dropwise to the mixture at 0 °C under an ice bath. The mixture was then allowed to warm to room temperature and stirred overnight. After that, the reaction was quenched by adding cold water, and then, the mixture was stirred at room temperature for 15 min. The organic layer was extracted with CH2Cl2 and dried over MgSO4. After evaporation under reduced pressure, silica gel column chromatography of the residue using ethyl acetate/n-hexane (v/v = 1:4) eluent afforded pure 2 as a colorless oil. Yield: 5.3 g (73%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.45 (s, 1H), 8.19 (d, 2H), 7.70 (d, 1H), 7.59 (s, 4H), 7.46 (d, 4H), 7.33
7.36 (m, 4H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 151.1, 149.2, 141.0, 138.6, 137.6, 137.1, 132.9, 129.5, 127.3, 126.3, 124.5, 123.7, 120.6, 120.3, 120.3, 110.21, 72.5, 69.4; HRMS (FAB): m/z = 398.1190 [M]+. Anal. calcd for C25H19ClN2O: C, 75.28; H, 4.80; N, 7.02. Found: C, 75.10; H, 4.78; N, 7.03. 9-(4-((6-(2,4-Difluorophenyl)pyridin-3-yl)methoxymethyl)phenyl)-9H-carbazole (dfppy
Cz1). A solution of 2 (5.0 g, 12.5 mmol), 2,4-difluorophenylboronic acid (2.0 g, 12.5 mmol), Pd(OAc)2 (0.4 g, 0.6 mmol), K3PO4 (8.7 g, 37.6 mmol), and tris(3-methoxyphenyl)phosphine (0.7 g, 1.9 mmol) in dimethylether (DME) (90 mL) and H2O (30 mL) were stirred at 50 °C for 12 h. After quenching with water followed by stirring at room temperature for 15 min, the organic layer was extracted with CH2Cl2 and dried over MgSO4. After evaporation under reduced pressure, silica gel column chromatography of the residue using ethyl acetate/n-hexane (v/v = 1:2) eluent afforded pure dfppy
Cz1 as colorless oil. Yield: 3.9 g (65%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.76 (s, 1H), 8.16 (d, 2H), 8.03 (q, 1H), 7.86 (d, 1H), 7.81 (d, 1H), 7.63 (d, 2H), 7.58 (d, 2H), 7.41
7.43 (m, 4H), 7.29
7.32 (m, 2H), 7.03 (td, 1H), 6.93 (td, 1H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 164.3, 161.7, 162.7, 159.9, 152.3, 149.5, 141.0, 138.3, 137.5, 137.3, 136.4, 132.5, 132.3, 129.5, 127.4, 126.2, 124.2, 120.6, 120.2, 112.2, 104.6, 72.4, 70.0; HRMS (FAB): m/z = 476.1705 [M]+. Anal. calcd for C31H22F2N2O: C, 78.14; H, 4.65; N, 5.88. Found: C, 78.22; H, 4.62; N, 5.90. 9-(4-(Bromomethyl)phenyl)-carbazole (3). Br2 (4 mL, 80 mmol) was added to a solution of PPh3 (21.0 g, 80 mmol) in a minimum amount of dry CH2Cl2 at 0 °C, and the solution was stirred for 15 min. The resulting salt was added to a solution of Cz10
OH (10.9 g, 40 mmol) in dry CH2Cl2 (150 mL) via a cannula. The solution was allowed to warm to room temperature

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and stirred for 1 h. Then, the reaction was quenched with cold water under an ice bath, and the organic layer was extracted with CH2Cl2 and dried over MgSO4. After evaporation under reduced pressure, silica gel column chromatography of the residue using ethyl acetate/n-hexane (v/v = 1:10) eluent afforded pure 3 as a white solid. Yield: 12.2 g (91%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.14 (d, 2H), 7.63 (d, 2H), 7.56 (d, 2H), 7.42 (t, 2H), 7.41 (d, 2H), 7.30 (t, 2H), 4.62 (s, 2H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 140.8, 138.0, 137.0, 130.8, 127.5, 126.2, 123.7, 120.6, 120.3, 110.0, 33.0; HRMS (FAB): m/z = 335.0312 [M]+. Anal. calcd for C19H14BrN: C, 67.87; H, 4.20; N, 4.17. Found: C, 67.74; H, 4.19; N, 4.18. Methyl 3,5-Bis(4-(9H-carbazol-9-yl)benzyloxy)benzoate (4). A mixture of 3 (8.0 g, 23.8 mmol), methyl-3,5-dihydroxybenzoate19 (2.0 g, 12 mmol), and K2CO3 (18.6 g, 134 mmol) was dissolved in dry CH3CN (150 mL). The solution was stirred at 85 °C for 12 h, cooled to room temperature, and poured into ice water. The organic layer was extracted with CH2Cl2 and dried over MgSO4. After evaporation under reduced pressure, silica gel column chromatography of the residue using CH2Cl2/n-hexane (v/v = 3:1) eluent afforded pure 4 as a white solid. Yield: 11.5 g (71%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.19 (d, 4H), 7.71 (d, 4H), 7.64 (d, 4H), 7.42
7.48 (m, 14H), 6.99 (s, 1H), 5.23 (s, 4H), 4.00 (s, 3H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 167.0, 160.0, 141.0, 137.8, 135.9, 132.6, 129.4, 127.4, 126.3, 123.7, 120.7, 120.3, 110.0, 108.8, 107.6, 70.1, 52.6; HRMS (FAB): m/z = 678.2515 [M]+. Anal. calcd for C46H34N2O4: C, 81.40; H, 5.05; N, 4.13. Found: C, 81.28; H, 5.03; N, 4.14. (3,5-Bis(4-(9H-carbazol-9-yl)benzyloxy)phenyl)methanol (Cz20
OH). This compound was prepared by LiAlH4 reduction of 4 in a way similar to that used for Cz10
OH and obtained as a white solid after column chromatography using CH2Cl2/ n-hexane (v/v = 5:1) eluent. Yield: 9.8 g (89%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.15 (d, 4H), 7.69 (d, 4H), 7.61 (d, 4H), 7.38
7.45 (m, 8H), 7.27
7.32 (m, 4H), 6.75 (s, 2H), 6.70 (s, 1H), 5.30 (s, 1H), 5.20 (s, 4H), 4.73 (d, 2H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 160.3, 143.8, 140.8, 137.6, 136.0, 129.2, 127.5, 127.3, 126.4, 126.1, 123.5, 120.4, 109.9, 105.9, 69.8, 65.4; HRMS (FAB): m/z = 650.2572 [M]+. Anal. calcd for C45H34N2O3: C, 83.05; H, 5.27; N, 4.30. Found: C, 82.92; H, 5.25; N, 4.31. 9,90 -(4,4 0 -(5-((6-Chloropyridin-3-yl)methoxymethyl)-1, 3-phenylene)bis(oxy)bis(methylene)bis(4,1-phenylene))bis(9H-carbazole) (5). This compound was prepared from Cz20
OH by a procedure similar to that used for 2. The mixture was purified by silica gel column chromatography using ethyl acetate/n-hexane (v/v = 1:2) eluent afforded 5 as a colorless oil. Yield: 3.7 g (62%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.41 (s, 1H), 8.17 (d, 4H), 7.69 (d, 4H), 7.61 (d, 4H), 7.37
7.41 (m, 8H), 7.25
7.30 (m, 6H), 6.72 (s, 3H), 5.19 (s, 4H), 4.59 (d, 4H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 160.4, 151.1, 149.2, 141.0, 140.5, 138.5, 137.7, 136.1, 129.3, 127.3, 126.2, 124.4, 123.3, 120.6, 120.2, 110.0, 107.0, 101.8, 72.8, 69.9, 69.0, 60.6; HRMS (FAB): m/z = 775.2608 [M]+. Anal. calcd for C51H38ClN3O3: C, 78.90; H, 4.93; N, 5.41. Found: C, 78.78; H, 4.91; N, 5.42. 9,90 -(4,40 -(5-((6-(2,4-Difluorophenyl)pyridin-3-yl)methoxymethyl)-1,3-phenylene)bis(oxy)bis (methylene)bis(4,1-phenylene))bis(9H-carbazole) (dfppy
Cz2). This compound was prepared from 5 by a procedure similar to that used for dfppy
Cz1 and obtained as colorless oil after the purification by silica gel column chromatography using ethyl acetate/n-hexane (v/v = 1:2) eluent. Yield: 2.1 g (63%). 1H NMR (300.1 MHz, 1975

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The Journal of Physical Chemistry C CDCl3, ppm) δ 8.74 (s, 1H), 8.17 (d, 4H), 7.97 (q, 1H), 7.81 (d, 1H), 7.75 (d, 1H), 7.71 (d, 4H), 7.63 (d, 4H), 7.39
7.44 (m, 8H), 7.31 (t, 4H), 7.00 (td, 1H), 6.92 (td, 1H), 6.78 (s, 2H), 6.74 (s, 2H), 5.21 (s, 4H), 4.67 (s, 2H), 4.65 (s, 2H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 164.2, 161.6, 162.7, 160.4, 159.9, 151.1, 149.3, 141.0, 140.8, 137.7, 136.4, 136.1, 132.7, 129.3, 127.4, 126.2, 124.3, 123.6, 120.6, 120.3, 110.0, 107.0, 101.8, 72.7, 70.0, 69.7, 60.7; HRMS (FAB): m/z = 853.3120 [M]+. Anal. calcd for C57H41F2N3O3: C, 80.17; H, 4.84; N, 4.92. Found: C, 80.02; H, 4.82; N, 4.93. [(dfppy)2Ir(dpq)](PF6). The cyclometalated iridium(III) chlorobridged dimer [Ir(dfppy)2(μ-Cl)]2 was prepared according to methods described in the literature.20 A solution of [Ir(dfppy)2(μ-Cl)]2 (1.0 g, 0.8 mmol) and 2.2 equiv of ancillary ligand 2,3-bis(2-pyridyl)-qinoxaline (dpq) (0.5 g, 1.8 mmol) in CH2Cl2/MeOH (v/v = 2:1, 60 mL) was refluxed under nitrogen in the dark for 6 h. The resulting red solution was cooled to room temperature, and then, a 10-fold excess of KPF6 (1.5 g, 8.2 mmol) was added to the solution. After which, the suspension was stirred for 2 h and then filtered to remove the insoluble inorganic salts. After evaporation of the filtrate followed by silica gel column chromatography using acetone/CH2Cl2 (v/v = 1:20) eluent, recrystallization from CH2Cl2/n-hexane afforded [(dfppy)2Ir(dpq)](PF6) as a red to orange solid. Yield: 0.71 g (86%). 1H NMR (300.1 MHz, acetone-d6, ppm) δ 8.70 (d, 1H), 8.58 (d, 1H), 8.53 (d, 1H), 8.42
8.44 (m, 2H), 8.15
8.26 (m, 6H), 7.93
8.01 (m, 3H), 7.62
7.92 (m, 4H), 7.36 (t, 1H), 7.23 (t, 1H), 6.84 (t, 1H), 6.74 (t, 1H), 6.05 (d, 1H), 5.75 (d, 1H); 13 C NMR (75.4 MHz, CDCl3, ppm) δ 158.1, 156.2, 153.8, 153.7, 152.3, 150.3, 149.9, 142.6, 140.3, 140.0, 138.7, 133.5, 133.0, 131.0, 130.2, 128.6, 127.6, 125.9, 124.7, 124.5, 124.3, 123.5, 123.4, 114.2, 113.6, 99.6, 99.0; MALDI-TOF MS: m/z = 857.174 ([M
PF6]+; C40H24F4IrN6+; calcd 857.163); Anal. calcd for C40H24F10 IrN6P: C, 47.95; H, 2.41; N, 8.39. Found: C, 47.86; H, 2.41; N, 8.42. Dendritic Cyclometalated Iridium Chlorobridged Dimer, [Ir(dfppy
Cz1)2(μ-Cl)]2. This compound was prepared from dfppy
Cz1 by a procedure similar to that used for [Ir(dfppy)2(μ-Cl)]2. Recrystallization of the crude product from n-hexane/ toluene gave [Ir(dfppy
Cz1)2(μ-Cl)]2 as a yellow solid. Yield: 3.6 g (72%). HRMS (FAB): Calcd 2356.5123. Found: 2356.5115 [M]+. Dendritic Ir(III) Complex, [(dfppy
Cz1)2Ir(dpq)](PF6). This compound was prepared by a procedure similar to that used for [(dfppy)2Ir(dpq)](PF6) using [Ir(dfppy
Cz1)2(μ-Cl)]2. After the purification by silica gel column chromatography using acetone/CH2Cl2 (v/v = 1:20) eluent, recrystallization of the crude product from CH2Cl2/n-hexane gave the complex as a red solid. Yield: 0.86 g (73%). 1H NMR (300.1 MHz, acetone-d6, ppm) δ 8.65 (t, 1H), 8.55 (d, 1H), 8.42 (s, 1H), 8.35 (s, 1H), 8.18
8.20 (m, 8H), 8.15 (t, 2H), 8.10 (d, 1H), 7.99 (d, 1H), 7.94 (t, 1H), 7.83 (t, 1H), 7.57
7.65 (m, 4H), 7.43
7.49 (m, 8H), 7.39 (t, 2H), 7.26
7.35 (m, 10H), 6.85 (t, 1H), 6.76 (t, 1H), 6.11 (d, 1H), 5.81 (d, 1H), 4.77 (d, 1H), 4.71 (d, 1H), 4.61 (t, 2H), 4.51 (s, 4H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 58.1, 156.1, 153.9, 153.6, 150.6, 149.9, 147.9, 142.5, 140.9, 140.1, 139.4, 139.3, 138.8, 138.6, 137.6, 137.0, 136.9, 133.6, 133.0, 131.0, 130.1, 129.1, 128.6, 127.5, 127.0, 126.8, 126.3, 126.2, 126.1, 124.6, 123.6, 123.5, 120.6, 120.5, 120.3, 109.8, 72.1, 71.8, 68.7, 68.3; MALDI-TOF MS: m/z = 1428.460 ([M
PF6]+; C80H54F4IrN8O2+; calcd 1427.394). Anal. calcd for C80H54F10IrN8O2P: C, 61.10; H, 3.46; N, 7.13. Found: C, 61.01; H, 3.45; N, 7.14.

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Dendritic Cyclometalated Iridium(III) Chlorobridged Dimer, [Ir(dfppy
Cz2)2(μ-Cl)]2. This compound was prepared by

a procedure similar to that used for [Ir(dfppy)2(μ-Cl)]2 using dfppy
Cz2. After purification by silica gel column chromatography using CH2Cl2 eluent, recrystallization of the crude product from n-hexane/toluene gave the complex as a yellow solid. Yield: 1.1 g (51%). HRMS (FAB) calculated M+ 3865.0786; observed M+ 3865.0779. Dendritic Ir(III) Complex, [(dfppy
Cz2)2Ir(dpq)](PF6). This compound was prepared from [Ir(dfppy
Cz2)2(μ-Cl)]2 by a procedure similar to that used for [(dfppy)2Ir(dpq)](PF6). After purification by silica gel column chromatography using acetone/ CH2Cl2 (v/v = 1:20) eluent, recrystallization of the crude product from CH2Cl2/n-hexane gave the complex as a red solid. Yield: 0.5 g (63%). 1H NMR (300.1 MHz, CDCl3, ppm) δ 8.46 (s, 1H), 8.37 (d, 1H), 8.25 (m, 4H), 8.12 (d, 1H), 8.09 (d, 6H), 8.02
8.06 (t, 4H), 7.95
7.98 (m, 2H), 7.83 (d, 1H), 7.67
7.70 (m, 12H), 7.56
7.58 (m, 2H), 7.52
7.54 (m, 8H), 7.37 (d, 2H), 7.23
7.28 (m, 18H), 7.13
7.16 (m, 6H), 6.48 (d, 2H), 6.36 (d, 2H), 5.93 (dd, 1H), 5.66 (dd, 1H), 5.12 (d, 6H), 5.53
5.59 (m, 2H), 4.42
4.45 (m, 2H), 4.36 (d, 2H), 4.27 (d, 2H), 4.21 (s, 4H); 13C NMR (75.4 MHz, CDCl3, ppm) δ 162.3, 160.3, 154.5, 153.6, 150.9, 149.9, 149.0, 147.8, 147.3, 145.1, 144.3, 143.9, 140.5, 138.5, 137.4, 137.1, 136.7, 135.7, 134.0, 133.3, 133.0, 129.8, 129.5, 128.7, 128.3, 127.2, 126.8, 126.3, 125.4, 123.6, 121.3, 120.6, 120.3, 115.6, 113.7, 110.3, 109.8, 107.4, 106.8, 102.7, 101.1, 72.3, 72.0, 69.4, 68.2, 68.0; MALDI-TOF MS: m/z =2183.743 ([M
PF6]+; C132H92F4IrN10O6+; calcd 2181.677). Anal. calcd for C132H92F10IrN10O6P: C, 68.12; H, 3.98; N, 6.02. Found: C, 68.25; H, 3.96; N, 6.04. Density Functional Calculations. The ground-state geometry of [(dfppy)2Ir(dpq)]+ has been optimized at the density function theory (DFT) level. Characterization of the low-lying excited singlet and triplet states relies on time-dependent DFT (TD-DFT) calculation that is performed on the basis of groundstate geometry by B3LYP density functional theory (DFT), using a LanL2DZ basis set.21 All calculations were performed with the Gaussian 03 package.22 Cyclic Voltametry (CV). CV measurements were performed on a BAS 100 electrochemical analyzer equipped with a threeelectrode cell system (a glassy carbon working electrode, a platinum wire counter electrode, and an Ag/AgNO3 reference electrode). Potentials were initially referenced to an internal ferrocene redox (Fc+/Fc) standard. All sample solutions in CH2Cl2 containing 0.1 M tetrabutylammonium tetrafluoroborate as the supporting electrolyte were deoxygenated with argon for at least 10 min before measurements. Measurements of UV
Vis Absorption and Luminescence Spectra. Steady-state UV
vis absorption spectra were recorded on a Shimadzu (UV-3101PC) scanning spectrophotometer. Emission and excitation spectra were taken on a Varian fluorescence spectrophotometer (Cary Eclipse) at room temperature. Sample solutions were degassed with argon for 20 min before measurements. Luminescence quantum yields (Φ)23 were determined by using Ru(bpy)32+ emission (Φ = 0.029 in CH2Cl2)24 as standard. Lifetime and Transient Absorption Measurements. Nanosecond transient absorption measurements were carried out by a laser-flash photolysis method using the third harmonic (THG, 355 nm) of a Q-switched Nd/YAG laser (Continuum, Surelite II, pulse width of 4.5 ns) as the excitation light source and a Xenon lamp (ILC Technology, PS 300-1) as the probe light. Temporal 1976

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Scheme 1. Synthetic Route to the Dendritic Ligand, dfppy
Czn (n = 1 and 2)a

(a) K2CO3, DMF, 150 °C, 12 h; (b) LiAlH4, THF, 0 °C, 4 h; (c) NaH, THF, CH3CN, rt, 12 h; (d) 2,4-difluorophenylboronic acid, Pd(OAc)2, P(p-PhOMe)3, K3PO4, DME/H2O (= 3:1), 50 °C, 12 h; (e) PPh3, Br2, CH2Cl2, 0 °C, 1 h; (f) K2CO3, CH3CN, 85 °C, 12 h. a

profiles were measured by a photomultiplier (Zolix Instruments Co., CR 131) and a digital oscilloscope (Tektronix, TDS-784D) equipped with a monochromator (DongWoo Optron, Monora 500i). Reported signals were obtained as averages of 500 events. All sample solutions were Ar-saturated. Time-resolved fluorescence spectra were measured by a single photon counting method using a streakscope (Hamamatsu Photonics, C4334-01) equipped with a polychromator (Acton Research, SpectraPro150). Ultrashort laser pulses were generated from a Ti/sapphire laser (Spectra-Physics, Tsunami 3941-M1BB, fwhm 100 fs) pumped with a diode solid-state laser (Spectra-Physics, Millennia VIIIs). For excitation of the samples, the output of the Ti/sapphire laser was converted to THG (300 nm) with a harmonic generator (SpectraPhysics, GWU-23FL). The instrument response function was also determined by measuring the scattered laser light to analyze a temporal profile. This method gave a time resolution of approximately 20 ps after deconvolution. The observed temporal emission profiles were well fitted into a single- or double-exponential function. The residuals were less than 1.1% for each system.

’ RESULTS Synthesis. The carbazole-functionalized dendrons were prepared according to Scheme 1. The carbazole dendron precursor (4-(carbazol-9-yl)-phenylmethanol, Cz10
OH) was prepared in 86% yield by LiAlH4 reduction of 4-(9-carbazolyl)benzaldehyde 1 and was then converted to the corresponding benzyl bromide 3 in 80% yield by treating with PPh3 and Br2 in CH2Cl2. Similarly, 3,5-bis(4-(9-carbazolyl)benzyloxy)benzoic acid methyl ester 4

prepared from 3 and methyl 3,5-dihydroxybenzoate was reduced by LiAlH4 to (3,5-bis(4-(9H-carbazol-9-yl)benzyloxy)phenyl)methanol (Cz20
OH). The dendritic dfppy ligands denoted as dfppy
Czn (n = 1, 2) were synthesized in a sequence involving the initial substitution reaction of 2-chloro-5-(chloromethyl)pyridine with Czn0
OH and the subsequent palladium-catalyzed direct Suzuki cross coupling of the Czn0 -linked 2-chloropyridine derivative with 2,4-difluorophenyl boronic acid. Since the dendritic Ir(III) complexes consist of the dendritic dfppy
Czn ligands and the ancillary dpq ligand, the synthesis of [(dfppy
Czn)2Ir(dpq)]+ follows the two-step method reported by Nonoyama (Scheme 2).20 The cyclometalated iridium(III) μ-chloro-bridged dimers [(dfppy)2Ir(μ-Cl)2Ir(dfppy)2] were initially prepared by heating iridium chloride hydrate (IrCl3 3 3H2O) with an excess amount of dfppy
Czn in aqueous 2-ethoxyethanol and then converted to [(dfppy
Czn)2Ir(dpq)]+ by the reaction with the ancillary dpq ligand. Electrochemistry. Figure 1 shows CV traces of G0, G1, and G2 in CH2Cl2, from which their oxidation and reduction potentials versus Fc+/Fc were estimated (Table 1). All the complexes exhibit two quasireversible reduction waves at approximately
1.2 and
2.0 V. It is known that electrochemical reduction processes of the related Ir(III) complexes proceed on the ligand-centered orbitals and the redox potential from the diimine ligand is less negative than that of fluorinated phenylpyridine ligand.24,25 Based on the TD-DFT calculation results (vide infra), therefore, the first and second potentials were attributed, respectively, to the reductions of the dpq and dfppy ligands. On the other hand, the oxidation of G0 gives an irreversible wave at 1.34 V, revealing significantly different behavior from 1977

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Scheme 2. Synthetic Route to the Ir Complex, [(dfppy)2Ir(dpq)](PF6) and [(dfppy
Czn)2Ir(dpq)](PF6) (n = 1 and 2)a

a

(g) 2-ethoxyethanol/H2O (= 3:1), 120 °C, 12 h; (h) CH2Cl2/MeOH (= 2:1), 160 °C, 6 h.

Figure 1. Cyclic voltammograms of G0, G1, and G2 taken at a sweep rate of 0.1 V/s for 0.5 mM CH2Cl2 solution containing 0.1 M TBAPF.

the pseudoreversible oxidation of similar complex (dfppy)2Ir(2-picolinato) occurring at a less positive potential (∼0.87 V). The electrochemical oxidation of the latter compound and the related Ir(III) heteroleptic complexes has been assigned to the metal-centered IrIII/IrIV redox process.26 Compared to such Ir(III) complexes, the irreversible oxidation behavior of G0 at a more positive potential might be attributable to the increasing contribution to the HOMOs of covalent σ (Ir
C) character.26b,27 In the case of G1 and G2, the CV showed an additional oxidation peak at ∼0.9 V upon scanning toward positive potential and two cathodic peaks at ∼0.8 and ∼0.5 V upon reverse scan. This electrochemical behavior can be attributed to the oxidation of the carbazole units, since identical CV behavior was reported for N-alkylated carbazoles.28 In accord with this

assignment, Czn0
OH revealed an anodic peak and two cathodic peaks at very similar potentials, and current efficiencies are higher with the greater number of the carbazole unit in the dendrimers and Czn0
OH. It is well-established that N-substituted carbazoles exhibit unique behavior upon electrochemical oxidation that has been interpreted in terms of the initial one-electron oxidation followed by oxidative coupling to generate dimers and polymers susceptible of oxidation at lower potentials.29 TD-DFT Calculations. It is now widely accepted that TD-DFT calculations are potentially useful for obtaining insights into different electronic states related to the relevant absorption and emission processes. Following the accurate geometry optimization, we modeled the electronic structure of [(dfppy)2Ir(dpq)]+, G0. As shown in Figure 2, the highest occupied molecular orbital (HOMO) consists principally of Ir(III) d orbitals with a substantial contribution of the dfppy π orbitals, in line with the published result for (dfppy)2Ir(2-picolinato), so-called FIrpic.30 Similar features can be observed for the other occupied MOs, HOMO
1 ∼ HOMO
3, whereas substantial populations of HOMO
4 and HOMO
5 on the Ir center can be found. However, the dpq ancillary ligand has little population among all the occupied MOs. By contrast, the lowest unoccupied molecular orbital (LUMO) is largely localized on the dpq ligand with little metal orbital character, and this is again true for LUMO+1. On the other hand, the higher unoccupied MOs (LUMO+2 and LUMO+3) are mainly populated on the (dfppy)Ir part. Table 2 lists typical electronic transitions with significant oscillator strengths (f > 0.01) produced by TD-DFT calculations. Although the HOMO f LUMO transition calculated from TDDFT was expected to occur at 573 and 583 nm, the oscillator strength is nil or negligibly small (f e 7  10
4). Similarly, calculations predict that the HOMO
1 f LUMO transition 1978

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Table 1. Electrochemical Data of [Ir(dfppy)2(dpq)]+ and [Ir(dfppy
Czn)2(dpq)]+ oxidation peaks (V)a Epa1

compd [(dfppy)2Ir(dpq)]

+

Epa2b

reduction peaks (V)a

Epc1

Epc2

Epc3b

0.80

0.54

0.87

0.78

0.48

0.86

1.44

[(dfppy
Cz1)2Ir(dpq)]+

0.94

[(dfppy
Cz2)2Ir(dpq)]+

0.94

1.45

Eox

Epab

Epc1

Epc2b

1.34


1.10


1.17


2.02


1.14

1.34


1.07


1.17


2.05


1.12


1.98


1.10


1.16


2.05


1.13


2.0

Ered

a

Potentials calibrated with an internal ferrocene redox reference (Fc+|Fc) using 0.1 M TBAPF6 electrolyte in CH2Cl2. b Epa, Epa1, and Epa2 = anodic peak potentials. Epc, Epc1, Epc2, and Epc3 = cathodic peak potentials. Eox = half-wave oxidation potential between anodic and cathodic peaks. Ered = half-wave reduction potential between anodic and cathodic peaks.

Figure 2. Selected frontier orbitals for [(dfppy)2Ir(dpq)]+.

Table 2. Calculated Electronic Transitions and Oscillator Strength of [(dfppy)2Ir(dpq)]+ λ (nm) f (oscillator strength) 416

0.0175

transition (contribution)

assignment

HOMO
3 f LUMO (82%)

L0 LCT

HOMO
5 f LUMO (11%)

MLCT

HOMO
4 f LUMO (2%)

MLCT

393

0.0408

HOMO
5 f LUMO (83%) HOMO
3 f LUMO (13%)

MLCT L0 LCT

378

0.0127

HOMO f LUMO+1 (78%)

L0 LCT

HOMO f LUMO+2 (11%)

L0 LCT

HOMO f LUMO+3 (7%)

L0 LCT

HOMO f LUMO+2 (84%)

L0 LCT

HOMO f LUMO+1 (11%)

L0 LCT

374

0.0261

occurs at 463 nm with a very low transition probability (f = 1.6  10
3). The calculated results in Table 2 imply that the observed absorption band of G0 at 330
450 nm involve the transitions of

HOMO
3 f LUMO, HOMO
5 f LUMO, and HOMO f LUMO+1 to substantial degrees together with a significant contribution from the HOMO f LUMO+2 transition. The transitions from the occupied MOs to LUMO and from HOMO to LUMO+1 apparently possess mixed characters of [dπ (Ir) f π*dpq] MLCT and [πdfppy f π*dpq] ligand-to-ligand charge transfer (LLCT). Among them, the transition from HOMO
5 to LUMO with a dominant MLCT character is expected to have a major impact on the 330
450 nm band. On comparison, the HOMO f LUMO+2 transition has a [dπ (Ir) f π*dfppy] MLCT character, being expected to occur at a shorter wavelength region in the range of 330
450 nm. Photophysical Properties. Steady-State Absorption and Emission Spectra. The absorption spectra of G0, G1, and G2 in CH2Cl2 are presented in Figure 3(a), and the corresponding characteristics are summarized in Table 3. The absorption spectrum of G0 appears to consist of three regions, (1) intense bands at 350 nm are essentially identical to that of G0, indicating that the dynamics of MLCT emissions can be conveniently investigated by excitation at >350 nm with no participation of the absorption and fluorescence of the Czn unit (Figure 3b). As shown in Figure 3b, G0 shows a broad red phosphorescence peak at ∼635 nm with a lifetime of 1.18 μs at room temperature. The long lifetime indicates that the emission is phosphorescence in nature. The emission wavelength is much longer than that of the related heteroleptic complexes, typically FIrpic which emits the blue phosphorescence at 470 nm from the dfppy-based triplet MLCT state (3MLCT).33 The red-emission 1980

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The Journal of Physical Chemistry C of G0 should arise from a triplet state with a dominant dpq-based MLCT character, as suggested by the electrochemical and theoretical results for G0 that the LUMO is largely populated on dpq. A support of this assignment is given by the emission reported for [Ir(dpq)2Cl2]+ that appears at 634 nm, almost the same as the emission wavelength for G0.36 In 2-methyltetrahydrofuran (2-MeTHF) glass at 77 K, moreover, the emission maximum of G0 appears at ∼570 nm with a vibrational structure, illustrating a considerable blue shift by ∼1730 cm
1 from that taken for a fluid solution at room temperature (Figure 3b). This shift falls into an usual shift range of 1000
2000 cm
1 reported for MLCT phosphorescence of Ir(III) cyclometalated complexes.37 The temperature-dependent behavior of phosphorescence is typical for MLCT emissions, being interpreted in terms of a change from solvation stabilization of the relevant MLCT emissive states in a fluid solution to the freezing of solvent reorganization at 77 K. It is of crucial significance to note that the red emission from G1 and G2 is remarkably weaker compared to that for G0 when exciting the 1MLCT band at ∼416 nm, an observation indicating the occurrence of efficient 3MLCT quenching in the dendrimers. The quantum yield of G0 is 0.12 and that of Ir dendrimer is almost zero.23 The phosphorescence quenching was attributed to PET from the carbazole unit to 3MLCT (vide infra). Another notable observation is that both the MLCT phosphorescence at 634 nm and the carbazole fluorescence at 350 nm are very weak or negligible upon exciting G1 and G2 at 292 nm where the Czn dendrons mainly or dominantly absorb the incident light, as seen in Figure 3b. In particular, G1 with a shorter dendron size compared with G2 showed negligible carbazole fluorescence and weaker MLCT phosphorescence. The emission behavior of the present dendrimers are quite different from that reported for a related carbazole-linked Ir(III)-dendrimer system (dfppy)2Ir(III)(L0
(Den)2) (L0 = 3-Den-substituted 2-picolinato, Den = 3,5-bis-(9-carbazolyl)bezyloxy-based dendrons).17 In this system, excitation of the carbazole unit results in efficient quenching of the carbazole fluorescence accompanied by enhanced appearance of the blue-green phosphorescence from the FIrpic core. This occurs through efficient F€orster-type energy transfer from the excited-singlet carbazole chromophore to the (dfppy)2Ir(III) core followed by fast intersystem crossing to 3 MLCT; energy transfer occurs at ∼4  109 s
1 to 1010 s
1 depending on the dendrimer generation. Another difference of this system from our dendrimers is the lack of 3MLCT quenching, as indicated by the enhancement of phosphorescence due to energy transfer. In our dendrimer systems, it may also be possible that F€orster energy transfer from the excited-singlet state of carbazole (1Cz*) to the (dfppy)2Ir(dpq) core would occur to result in quenching of the carbazole fluorescence, considering a good spectral overlap can be found between the carbazole fluorescence and the MLCT absorption band. If this was the case, the quenching of both carbazole fluorescence and MLCT emission observed upon exciting the carbazole unit of G1 and G2 would be attributed to the sequential events involving initial energy transfer from 1 Cz* to the Ir(III) core and subsequent quenching of resulting 3 MLCT by the Czn dendron through PET. In order to obtain further insights into the anomalous emission behavior of dendrimers, we measured the emission spectra of the dendrimers in 2-MeTHF glass at 77 K. Interestingly, all the complexes studied here exhibited strong MLCT emission with similar intensities at ∼570 nm when excited at 416 nm in

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2-MeTHF matrix. Such an observation demonstrates the lack of phosphorescence quenching in G1 and G2 at 77 K, which is distinctively different to the efficient quenching at room temperature. Furthermore, the dendrimers revealed the carbazole fluorescence with intensities comparable to, but slightly weaker than, that of Czn0
OH at 77 K when excited at 309 nm in 2-MeTHF matrix (see the Supporting Information, Figure S3). This temperature-dependent behavior was further confirmed by emission lifetime measurements at 77 K (vide infra). The lack of quenching for the carbazole fluorescence at 77 K indicates that the weak emission properties of the dendrimers at room temperature cannot be attributed solely to energy transfer from 1Cz* to the Ir(III) core, because F€orster energy transfer can occur even in rigid media at 77 K. In order to explain this temperaturedependent behavior, we assume the participation of an activated mechanism, that is, PET from the Cz dendron to the Ir(III) core via either 1Cz* or 3MLCT. Figure 4 shows the excitation spectra for the MLCT phosphorescence monitored at 630 nm. While the excitation spectrum of G0 exactly traces the absorption spectrum involving the weak absorption band at 500 nm, the dendrimers reveal considerable discrepancies between the excitation and absorption spectra, particularly at 1010 s
1) and kqp (∼107 s
1) can be reasonably interpreted in terms of the large ΔGPET difference between the two PET processes. The large kfq values might arise from the high driving force (ΔGPET =
1.5 eV) close to the solvent reorganization energy. The kqf and kqp values are significantly different between G1 and G2, probably reflecting the distance dependences of the PET processes. According to the energy minimized geometries of G1

Figure 7. Schematic energy diagram for [(dfppy
Czn)2Ir(dpq)]+ complexes. CR indicates the charge recombination.

and G2 obtained by the semiempirical PM3 method, the average distances between the carbazole and Ir(dpq) parts were estimated to be ∼12 Å for G1 and ∼16 Å for G2, a difference that can give an appreciable effect on the PET processes. However, it should be stressed that the estimated distances are only intuitive for an understanding of the distance dependences of kfq and kpq. It appears that the kfq values are unusually large for the “longrange” PET with a distance exceeding 10 Å. In fact, the benzyloxy spacer is not rigid but more or less flexible to allow closer proximity of the carbazole unit to the Ir(dpq) core in fluid solution, though the two chromophores are incapable of being in contact. An alternative mechanism that may explain the observed results would be provided by assuming F€orster-type energy transfer from 1Cz* to the (dfppy)2IrIII(dpq) core followed by PET from carbazole to 3MLCT of the core after fast intersystem crossing from 1MLCT (see Figure 7.). However, this mechanism cannot rationalize the lack of fluorescence quenching at 77K, since F€orster energy transfer can occur even at 77 K via nonactivated dipole
dipole resonance mechanism. It was reported for a related Ir(III)
dendrimer system, (dfppy)2IrIII(L0
(Den)2) (L0 = 3-Den-substituted 2-picolinate, Den = 3,5-bis-(9-carbazolyl)bezyloxy-based dendrons), that energy transfer from 1Cz* to the (dfppy)2IrIII core occurs with rate constants of ∼(4  109
1010) s
1,17 significantly smaller than the present quenching rate constants, kfq’s. Therefore, it is reasonable to assume that the PET in 1Cz* might proceed fast enough to overcome the F€orster energy transfer, even though the latter would occur at room temperature. However, it seems puzzling why F€orster energy transfer in our dendrimers was not observed at 77 K. The fluorescence of Czn0
OH appears in the region of 340
410 nm with the maxima at 350 and 365 nm, well overlapped with the intense MLCT absorption bands of G0 (Figure 3 and Table 3). This spectral situation would be apparently optimum for efficient resonance energy transfer to occur from 1Cz* to the (dfppy)2IrIII(dpq) core in the dendrimers. Compared with MLCT absorption bands of related FIrpic in which the LUMO is located on dfppy,30 the 370 nm band of G0 reveals different spectral features with the longer end absorption reaching to ∼550 nm as well as with the high absorption coefficient, several fold higher or higher even by an order of magnitude than those of FIrpic. This means that the MLCT transition based on dpq should have a major contribution to the 370 nm band, that is, that the major transition 1984

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The Journal of Physical Chemistry C dipole should occur along the Ir
dpq segment. We speculated that the transition dipole of 1Cz* of the dendrimers with stable conformations frozen at 77 K would be unfavorable for resonance energy transfer, for example, perpendicularly or nearly so with respect to that of the IrIII
dpq MLCT transition. This would also happen, to a more or less extent, in the case of the IrIII
dfppy MLCT transition. As the consequence, energy transfer would be much slower than the decay of 1Cz* at 77 K. We are now under more detailed investigation to explore the photophysical behavior of the present IrIII complexes and their related compounds.

’ CONCLUSIONS We synthesized new dendrimers of heteroleptic Ir complexes, [(dfppy
Czn)2IrIII(dpq)]+, and characterized their structures. The electronic spectra of [(dfppy)2Ir(dpq)]+ (G0) have been discussed in terms of dpq-based 1MLCT transition mixed with minor contributions from dfppy-based MLCT, dfppy-to-dpq CT, and dpq-localized π,π* characters for the intense absorption band at 370 nm, a transition to dpq-based 3MLCT for the weak absorption at 500 nm, and dpq-based MLCT phosphorescence for the intense red emission at 635 nm. We have investigated details of the photophysical and photodynamic behavior of the newly synthesized carbazole-functionalized dendrimers [(dfppy
Czn)2IrIII(dpq)]+ (G1 and G2) compared with G0 using steadystate and time-resolved spectroscopic methods and have shown that both the carbazole fluorescence and the MLCT phosphorescence of G1 and G2 are largely quenched at room temperature compared with that of G0 but not at all in frozen glass at 77K. The lack of emission quenching at 77 K has proved that activated processes are involved in the emission quenching at room temperature and that energy transfer from the excited-singlet carbazole chromophore to the Ir(dpq) core should be much slower than the activated process at room temperature as well as than the carbazole-fluorescence lifetime at 77 K (∼11 ns). We have also confirmed that the carbazole radical cation and the Ir(dpq) radical anion are formed upon the excitation of the 1 MLCT band by laser pulses. The present results have indicated that electron transfer occurs at room temperature from the carbazole unit to 3MLCT of the Ir(dpq) core at 2.3  107 s
1 in G1 and at 8.7  106 s
1 in G2 or from the excited-singlet carbazole chromophore to the ground-state Ir(dpq) core at 5.0  1010 s
1 in G1 and at 1.3  1010 s
1 in G2. The rate constant differences have been attributed to the different driving forces of the excited-singlet and 3MLCT electron-transfer processes and to a difference between the donor
acceptor distances of G1 and G2. The present investigation implies that Ir(III) complex dendrimers with suitable ligand-dendron combinations will provide potential systems capable of undergoing efficient charge separation induced by visible light, one of the essential issues in artificial photosynthesis. ’ ASSOCIATED CONTENT

bS

Supporting Information. UV
vis spectra of dfppy and dpq ligands, the fluorescence lifetime at 77 K and the component rate at room temperature of carbazole for [(dfppy
Czn)2Ir(dpq)]+, the phosphorescence lifetime of Ir(dpq) core at room temperature and 77 K, the transient absorption spectra of G2, and the optimized structures of G1 and G2. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: (C.P.) [email protected], (S.O.K.) [email protected]; Phone: +82-41-860-1334; Fax: +82-41-867-5396.

’ ACKNOWLEDGMENT This work is supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 20110018595), the Center for Next Generation Dye-sensitized Solar Cells (No. 2011-0001055), and the Fundamental Technology Development Programs for the Future through the Korea Institute of Science and Technology. We also thank Prof. Tetsuro Majima and Mamoru Fujistuka in Institute of Scientific and Industrial research (SAKEN) for providing access to an fs laser analyser. ’ REFERENCES (1) (a) Ardoin, N.; Astruc, D. Bull. Soc. Chim. Fr. 1995, 132, 875. (b) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (2) (a) Buhleier, E.; Wehner, W.; Voegtle, F. Synthesis 1978, 155. (b) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K. J. Org. Chem. 1985, 50, 2003. (c) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M. E.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (3) (a) Hecht, S.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74. (b) Momotake, A.; Arai, T. Polymer 2004, 45, 5369. (c) Miller, L. L.; Duan, R. G.; Tully, D. C.; Tomalia, D. A. J. Am. Chem. Soc. 1997, 119, 1005. (4) (a) Ropartz, L.; Morris, R. E.; Foster, D. F.; Cole-Hamilton, D. J. Chem. Commun. 2001, 361. (b) Twyman, L. J.; King, A. S. H.; Martin, I. K. Chem. Soc. Rev. 2002, 31, 69. (c) Ropartz, L.; Haxton, K. J.; Foster, D. F.; Morris, R. E.; Slawin, A. M. Z.; Cole-Hamilton, D. J. J. Chem. Soc., Dalton Trans. 2002, 4323. (d) Liang, C.; Frechet, J. M. J. Prog. Polym. Sci. 2005, 30, 385. (e) Astruc, D. Pure Appl. Chem. 2003, 75, 461. (f) van Koten, G.; Jastrzebski, J. T. B. H. J. Mol. Catal. A: Chem. 1999, 146, 317. (5) (a) Flomenbom, O.; Amir, R. J.; Shabat, D.; Klafter, J. J. Lumin. 2005, 111, 315. (b) Khopade, A. J.; Caruso, F. Biomacromolecules 2002, 3, 1154. (c) Furuta, P.; Frechet, J. M. J. J. Am. Chem. Soc. 2003, 125, 13173. (d) Svenson, S.; Tomalia, D. A. Adv. Drug Delivery Rev. 2005, 57, 2106. (e) Najlah, M.; D’Emanuele, A. Curr. Opin. Pharmacol. 2006, 6, 522. (6) (a) Goetheer, E. L. V.; Baars, M.; van den Broeke, L. J. P.; Meijer, E. W.; Keurentjes, J. T. F. Ind. Eng. Chem. Res. 2000, 39, 4634. (b) Chen, G. H.; Guan, Z. B. J. Am. Chem. Soc. 2004, 126, 2662. (7) (a) Cornelissen, J. J. L. M.; van Heerbeek, R.; Kamer, P. C. J.; Reek, J. N. H.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Adv. Mater. 2002, 14, 489. (b) Yeung, L. K.; Crooks, R. M. Nano Lett. 2001, 1, 14. (8) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26. (9) (a) Yin, J.-F.; Chen, J.-G.; Lu, Z.-Z.; Ho, K.-C.; Lin, H.-C.; Lu, K.-L. Chem. Mater. 2010, 22, 4392. (b) Sp€anig, F.; Lopez-Duarte, I.; Fischer, M. K. R.; Martínez-Diaz, M. V.; B€auerle, P.; Torres, T.; Guldi, D. M. J. Mater. Chem. 2011, 21, 1395. (10) (a) Hwang, S.-H.; Moorefield, C. N.; Newkome, G. R. Chem. Soc. Rev. 2008, 37, 2543. (b) Shirota, Y. J. Mater. Chem. 2000, 10, 1. (c) Markham, J. P. J.; Lo, S. C.; Magennis, S. W.; Burn, P. L.; Samuel, I. D. W. Appl. Phys. Lett. 2002, 80, 2645. (11) Cho, S.; Li, W.-S.; Yoon, M.-C.; Ahn, T. K.; Jiang, D.-L.; Kim, J.; Aida, T.; Kim, D. Chem.—Eur. J. 2006, 12, 7576. (12) (a) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996, 118, 3978. (b) Capitosti, G. J.; Cramer, S. J.; Rajesh, C. S.; Modarelli, D. A. Org. Lett. 2001, 3, 1645. (c) Rajesh, C. S.; Capitosti, G. J.; Cramer, S. J.; Modarelli, D. A. J.Phys. Chem. B 2001, 105, 10175. (d) Capitosti, G. J.; Guerrero, C. D.; Binkley, D. E., Jr.; Rajesh, C. S.; Modarelli, D. A. J. Org. Chem. 2003, 68, 247. 1985

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