Influence of Different Diimine (Nâ§ N) Ligands on the Photophysics

Article pubs.acs.org/JPCC

Influence of Different Diimine (N∧N) Ligands on the Photophysics and Reverse Saturable Absorption of Heteroleptic Cationic Iridium(III) Complexes Bearing Cyclometalating 2‑{3-[7(Benzothiazol-2-yl)fluoren-2-yl]phenyl}pyridine (C∧N) Ligands Rui Liu,†,§ Naveen Dandu,† Jinquan Chen,‡ Yuhao Li,† Zhongjing Li,† Shan Liu,† Chengzhe Wang,† Svetlana Kilina,† Bern Kohler,‡ and Wenfang Sun*,† †

Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States Department of Applied Chemistry, College of Sciences, Nanjing Tech University, Nanjing 211816, P.R. China ‡ Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States §

S Supporting Information *

ABSTRACT: Four heteroleptic cationic iridium(III) complexes containing cyclometalating 2-{3-[7-(benzothiazol-2-yl)fluoren-2-yl]phenyl}pyridine ligand and different diimine (N∧N) ligands (N∧N = 2-(pyridin-2-yl)quinoline (1), 1,10-phenanthroline (2), 2,2′-biquinoline (3), and 1,1′-biisoquinoline (4)) and a reference complex bearing 2-(pyridin-2-yl)quinoline and 2-phenylpyridine ligands (5) were synthesized and characterized. The influence of the diimine (N∧N) ligand on the photophysics of these complexes has been systematically investigated via spectroscopic methods and by time-dependent density functional theory (TDDFT). All complexes exhibit N∧N or C∧N ligand localized 1π,π* transitions below 400 nm, and broad and structureless metal-toligand and ligand-to-ligand charge transfer (1MLCT/1LLCT) absorption bands between 400 and 450 nm, and weak 3MLCT/3LLCT absorption above 450 nm. Increasing the π-conjugation of the N∧N ligand causes enhanced molar extinction coefficients of the absorption bands and a bathochromic shift of the 3 MLCT/3LLCT band. All complexes show orange to red phosphorescence at room temperature, with the emitting state being predominantly assigned to 3MLCT/3LLCT states for 1−5, but with some 3π,π* contributions for 3 and 5. Extending the πconjugation of the N∧N ligand induces a pronounced red-shift of the emission band and decreases the emission lifetime and quantum yield. Complexes 1−5 exhibit relatively strong singlet and triplet transient absorption from 450 to 800 nm, where the reverse saturable absorption (RSA) could occur. Nonlinear transmission experiments at 532 nm using nanosecond laser pulses demonstrate that complexes 1−5 are strong reverse saturable absorbers at 532 nm.

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and Huang’s group reported that by variation of the N∧N ligand with different degrees of π-conjugation, the emission energy and efficiency of the heteroleptic Ir(III) complexes containing 1phenylisoquinoline or 2-(2,4-difluorophenyl)pyridine ligands could be changed remarkably.8,19 Bryce’s group revealed that incorporation of fluorenyl units on the N∧N ligand (3,8-position of the phenanthroline) led to mixed 3MLCT (metal-to-ligand charge transfer) and 3π,π* character in the lowest triplet excited state (T1 state).21 Schanze’s group found that introducing a 4ethynyl-N,N-dihexylaniline substituent on the N∧N ligand (5,5′position of the 2,2′-bipyridine) increased the triplet excited-state lifetime and enhanced the triplet excited-state absorption, which gave rise to “dual-mode” nonlinear absorption features.13 Previous studies from our group have investigated how extending

INTRODUCTION Strong spin−orbit coupling in octahedral Ir(III) complexes efficiently generates triplet excited states, which are responsible for their strong room-temperature phosphorescence in fluid solution. In addition, the triplet states of Ir(III) complexes are readily tunable over a wide range of energies via ligand modification. Ir(III) complexes are thus of considerable interest for light-emitting devices (OLEDs),1−3 low-power upconversion,4−6 luminescent bioimaging,7−10 and nonlinear optics.11−16 Efficient tuning of the ground-state and excited-state properties of Ir(III) complexes for specific applications has been demonstrated in cationic heteroleptic Ir(III) complexes via modifications of either the cyclometalating ligands 2-phenylpyridine (C∧N ligand)17,18 or diimine ligand (N∧N ligand).19,20 For instance, implementation of different electron-donating/ withdrawing substituents, or extending the π-conjugation of the N∧N or C∧N ligands leads to significant changes in the photophysical properties of heteroleptic Ir(III) complexes. Li © 2014 American Chemical Society

Received: July 7, 2014 Revised: September 9, 2014 Published: September 11, 2014 23233

dx.doi.org/10.1021/jp506765k | J. Phys. Chem. C 2014, 118, 23233−23246

The Journal of Physical Chemistry C

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Chart 1. Structures of Heteroleptic Cationic Ir(III) Complexes 1−5

Scheme 1. Synthetic Routes for Complexes 1−5

the π-conjugation of the N∧N ligand via attaching the benzothiazolylfluorenyl substituents affects the photophysical properties of a series of heteroleptic Ir(III) complexes.14−16 These work demonstrated that the extended π-conjugation of the N∧N ligand via introducing the benzothiazolylfluorenyl substituents to the 5,5′- or 4,4′-position of the N∧N ligand drastically increased the lifetime of the T1 state due to its mixed 3π,π* and 3 MLCT/3LLCT (ligand-to-ligand charge transfer) characters. In contrast, incorporation of the benzothiazolylfluorene motif on the C∧N ligands caused more 3MLCT/3LLCT character to be admixed into the T1 state, leading to a significant decrease in the triplet excited-state lifetime. Despite the progress in understanding the ground- and excited-state properties of Ir(III) complexes upon varying ligand π-conjugation, reports on the effect of extending ligand πconjugation on the nonlinear optical properties, such as reverse saturable absorption (RSA) of the heteroleptic cationic Ir(III) complexes are still scarce. Understanding how tuning of the degree of π-conjugation of the N∧N ligand affects the

photophysical properties of heteroleptic Ir(III) complexes is essential for the rational design of efficient reverse saturable absorbers. In this work, we synthesized four cationic iridium(III) complexes bearing benzothiazolylfluorenyl substituted phenylpyridine (C∧N) ligands and different diimine ligands (N∧N = 2(pyridine-2-yl)quinoline (1), 1,10-phenanthroline (2), 2,2′biquinoline (3), and 1,1′-biisoquinoline (4)) with a variable degree of π-conjugation (structures shown in Chart 1). Although the diimine ligands used have been employed in other Ir(III) complexes containing the 1-phenylisoquinoline (C∧N) ligand,8 the effect of the extent of ligand conjugation on nonlinear absorption has never been explored. In addition, we replaced the 1-phenylisoquinoline ligands by benzothiazolylfluorenyl substituted phenylpyridine ligands because our previous studies revealed that the Ir(III) complexes bearing benzothiazolylfluorenyl substituted phenylpyridine ligands exhibited large ratios of the excited-state absorption cross section to the ground-state absorption cross section at 532 nm, which gave rise to strong RSA.14,16 To further understand the effect of this C∧N ligand on 23234



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Figure 1. Experimental (a) and calculated (b) UV−vis absorption spectra of complexes 1−5 in CH2Cl2. The inset in part a is the expansion of the spectra in the region of 450−720 nm.

Table 1. Photophysical Properties of Ir(III) Complexes 1−5 λabs/nm (ε/104·L/mol·cm) a

1 2 3 4 5

272 (7.09), 361 (11.8), 460 (0.50), 533 (0.11) 267 (5.89), 361 (9.33), 462 (0.33), 507 (0.065) 269 (8.43), 362 (13.6), 463 (0.47), 539 (0.17) 274 (6.96), 362 (13.8), 461 (0.60), 551 (0.10) 269 (6.52), 340 (2.04), 378 (0.93), 499 (0.098)

λem/nm (τem/ns, Φem)b room temp 630 (180, 0.022) 584 (630, 0.15)

h

656 (110, 0.017) 672 (80, 0.004) 619 (245, 0.037)

λem/nm (τem/μs)c 77 K

S1/nmd

T1/nme

578 (4.3), 626 (4.1) 547 (12.5), 596 (9.3)h 608 (6.2)

670

695

608 (2.1), 650 (1.9) 559 (5.8)

h

h

λS1−Sn/nm (τS/ps)f

λT1‑Tn/nm (τT/ns, εT1‑Tn /104·L/mol·cm, ΦT)g

530 (12 ± 2)

500 (30, −, −)

600 (140 ± 30)

590 (230, 4.87, 0.13)h

586

597

704

748

500 (8.3 ± 5)

i

752

810

560 (5.0 ± 2)

i

625

668

430 (26 ± 15)

430 (100, 2.83, 0.36)

a

Room temperature electronic absorption band maxima and molar extinction coefficients in CH2Cl2. bRoom temperature emission energy, lifetime, and quantum yield in CH2Cl2. c = 1 × 10−5 mol/L. The reference used for the quantum yield measurement was [Ru(bpy)3]Cl2 in degassed aqueous solution (Φem = 0.042, λex = 436 nm). cThe emission energy and lifetime at 77 K measured in BuCN glassy matrix, c = 1 × 10−5 mol/L. dThe singlet emission energy in CH2Cl2 calculated by TDDFT via optimization of the singlet excited state geometry. eThe triplet emission energy in CH2Cl2 calculated by the delta-SCF TDDFT-based method via optimization of the triplet ground state geometry. ffs TA band maximum and singlet excitedstate lifetime in CH3CN. gns TA band maximum, triplet extinction coefficient, triplet excited-state lifetime and quantum yield in toluene. The reference used was SiNc in C6H6 (ε590 = 70,000 L·mol−1·cm−1, ΦT = 0.20). hFrom ref 16. iToo weak to be measured.

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the nonlinear absorption of the Ir(III) complexes reported in this work, complex 5 with phenylpyridine ligands was prepared as a reference complex. In this paper, we report the synthesis and characterization of the Ir(III) complexes 1 and 3−5. The photophysical properties and RSA of these complexes were systematically investigated in order to evaluate the effect of the extended π-conjugation of the N∧N ligand. The synthesis, photophysics and RSA of complex 2 have been reported previously;16 however, for comparison purpose the corresponding data are included in this paper as well. In addition, absorption and emission properties of the complexes were calculated using time-dependent density functional theory (TDDFT) in order to gain insight into the nature of the singlet and triplet electronic transitions. This combined experimentalcomputational approach reveals important structure−property correlations that facilitate the rational design of nonlinear absorbing materials with improved RSA.

RESULTS AND DISCUSSION

Synthesis and Characterization. The synthetic scheme for complexes 1−5 is illustrated in Scheme 1. The N∧N ligands, including 2-pyridylquinoline, 2,2′-biquinoline and 1,1′-biisoquinoline, were prepared according to the procedures reported by Li and Huang’s group.19 The synthesis and characterization of ligand 6 have been reported by our group before.14 The cyclometalating ligands were first reacted with IrCl3·3H2O to obtain the dinuclear Ir(III) dichloro-bridged complexes. Next, the dinuclear Ir(III) complexes were coordinated with the N∧N ligand via breaking the Cl-bridge to obtain the heteroleptic complexes. All complexes are stable in air and can dissolve in many organic solvents such as CH2Cl2, CHCl3, acetone, tetrahydrofuran, toluene, DMF and DMSO. The structures of the complexes were confirmed by their 1H NMR and high resolution mass spectra (HRMS), as well as elemental analyses. 23235



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Table 2. Natural Transition Orbitals (NTOs) Contributing to the Lowest-Energy Singlet Transitions of Complexes 1−5 in CH2Cl2

Geometry optimization. The geometries of all five Ir(III) complexes were optimized in dichloromethane by density functional theory (DFT) calculations (see Table S1 of Supporting Information). All the Ir(III) complexes bearing the benzothiazolylfluorene motif were initially assumed to be planar with respect to the phenylpyridine (C∧N) ligand. However, after performing the optimization, benzothiazolylfluorene motifs were twisted from phenylpyridine (C∧N) plane at a dihedral angle of ∼36° in all of the complexes. Such a dihedral angle is typical in conjugated polymers with fluorene units.22 Similarly, the diimine (N∧N) ligand was assumed to be coplanar between the two Ncontaining rings connected via a single C−C bond; but after optimization, there was a slight twisting between the rings. In complex 1, the dihedral angle between the pyridine and the quinoline rings was about 11°, while the two quinoline rings in complex 3 had a twist of about 18° and the two isoquinoline rings in complex 4 had a twist of about 40°. These variations in the twisting angles along with the electronic differences of the diimine ligands define the difference in their photophysical properties and nonlinear absorption, as we will discuss in the following sections. UV−Vis Absorption. The UV−vis absorption spectra of complexes 1−5 were recorded in CH2Cl2 solutions with concentration varying from 1 × 10−6 to 1 × 10−4 mol/L. The absorption obeys Beer’s law in this concentration range, indicating the absence of aggregation. This is accounted for by the presence of branched alkyl chains on the fluorene components and the octahedral geometry of the Ir(III) complexes, which reduce the intermolecular stacking. Figure 1 displays the experimental and calculated absorption spectra of complexes 1−5 in CH2Cl2. The absorption band maxima and

extinction coefficients are compiled in Table 1. Although the calculated absorption bands are somewhat red-shifted compared to experiment, the overall agreement is satisfactory. Figure 1 shows that complex 5 has strong absorption below 330 nm, which arises from the 1π,π* transitions within the N∧N ligand with reference to the natural transition orbitals (NTOs) shown in Table 4 for complex 5. The weak tail between 330 and 500 nm is due to the mixture of 1LLCT (∼340 nm), 1MLCT (∼378 nm), and 3LLCT/3MLCT (∼500 nm), which are in line with literature reports for other Ir(III) complexes.23,24 In contrast to complex 5, complexes 1−4 exhibit more intense absorption bands between 330 and 400 nm, and broad tails above 400 nm. In Table 2, we list the natural transition orbitals (NTOs) that contribute to the lowest-energy singlet transitions of complexes 1−5. All complexes exhibit 1LLCT (ligand-to-ligand charge transfer)/1MLCT (metal-to-ligand charge transfer) nature in their lowest-energy transitions. Note that the oscillator strength of these transitions is negligibly weak and, therefore, the lowest-energy absorption band is much less pronounced in the calculated spectra than in the experimental absorption spectra. This discrepancy is likely due to the lack of triplet contribution to the absorption in our calculations, while experimental spectra, indeed, have a significant mixture between singlet and triplet transitions due to strong spin−orbit couplings in Ir(III) complexes. The inset in Figure 1a clearly evidences the weak low-energy transitions between 500 and 650 nm for these complexes, presumably of 1,3LLCT/1,3MLCT character. The spin-forbidden 3LLCT/3MLCT or 3π,π* transitions have already been reported for other Ir(III) complexes in the literature.14,19,23,24 23236



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Table 3. Natural Transition Orbitals (NTOs) Representing Singlet Transitions Responsible for the Main Absorption Bands of Complexes 1−5 in CH2Cl2

Table 4. Natural Transition Orbitals (NTOs) for High-Energy Transitions of Complexes 1−5 in CH2Cl2

centered near 360 nm arises from a C∧N ligand-localized 1π,π* state with a little admixture of 1MLCT character. The 1π,π*

Table 3 depicts the NTOs that contribute to the main absorption bands of complexes 1−4. The absorption band 23237



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those for 2−5 are provided in Supporting Information Figures S1−S4. It is clearly evident that the low-energy absorption bands are red-shifted in hexane and toluene (i.e., solvents with lower polarity) with respect to those in CH3CN and CH2Cl2 (i.e., solvents with higher polarity), as exemplified in Figure 2 for complex 1. This negative solvatochromic effect is a characteristic of LLCT/MLCT transitions, because the dipole moment of the ground state is larger than those of the LLCT/MLCT excited states. This supports our assignments of the nature of these absorption bands based on the TDDFT calculation results. On the contrary, the solvatochromic effect of the major absorption band (∼325−400 nm) is quite minor, which is in line with the 1 π,π* transition nature of this band. Photoluminescence. Complexes 1−5 are emissive in fluid solutions at room temperature upon excitation at their respective charge-transfer band. The normalized emission spectra of 1−5 in CH2Cl2 at a concentration of 1 × 10−5 mol/L are shown in Figure 3 and the emission parameters are compiled in Table 1. The

character provides strong optical intensity to the main band in complexes 1−4, in contrast to that of the complex 5 with predominant 1LLCT/1MLCT nature of transitions in this energy range. The high-energy band (at wavelengths 1 > 3 > 4, implying that extended π-conjugation of the N∧N ligand reduces the lifetime of the singlet excited state. The ns component is attributed to the decay of the triplet excited states. The ns TA measurements were carried out for complexes 1−5. However, the ns TA of complexes 3 and 4 are too weak to be

Figure 4. Normalized room temperature emission spectra of complex 1 with 436 nm excitation in different solvents.

in Supporting Information, Figures S5−S8, for the other complexes. The polarity of the solvent negligibly affects the emission energy of complexes 1−5, but the relative intensity of the vibronic peaks varied in different solvents, which is more prominent in toluene solutions for complexes 1, 3, and 4. The different spectral features in toluene imply that the emitting state has a different degree of 3MLCT/3LLCT and 3π,π* character compared to those in other solvents. Table 6 summarizes the emission parameters (i.e., energies, lifetimes and quantum yields) for complexes 1−5 in different solvents. The emission spectra of complexes 1−5 in BuCN glassy matrix at 77 K (Figure 5 and Figures S5−S8) exhibit a hypsochromic shift and become narrower and more structured compared to the respective room temperature spectra of these complexes in CH3CN. The lifetimes become much longer with respect to the room temperature lifetimes. The thermally induced Stokes shifts are in the range of 1203 cm−1 to 1734 cm−1, indicating that the

Table 6. Room Temperature Emission Parameters (Energies, Lifetimes and Quantum Yields) of Complexes 1−5 in Different Solvents λem/nm (τem/ns, Φema) 1 2c 3 4 5

hexaneb

toluene

THF

CH2Cl2

CH3CN

623 (120, 0.024) 584 (330, 0.02) 654 (140, 0.006) 629 (d, 0.003) 618 (150, 0.024)

585 (35, 0.013) 586 (220, 0.04) 654 (170, 0.007) 630 (30, 0.003) 619 (100, 0.016)

625 (130, 0.014) 587 (380, 0.08) 656 (160, 0.01) 674 (80, 0.002) 620 (140, 0.02)

626 (180, 0.022) 584 (630, 0.15) 656 (110, 0.017) 672 (80, 0.004) 621 (245, 0.037)

629 (190, 0.009) 584 (630, 0.06) 659 (160, 0.006) 632 (d, 0.002) 625 (140, 0.016)

The reference used for the quantum yield measurement was [Ru(bpy)3]Cl2 in degassed aqueous solution (Φem = 0.042, λex = 436 nm). bWith ∼10−20% CH2Cl2. cFrom ref 16. dToo weak to be measured.

a

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Figure 6. Time-resolved fs TA spectra of complexes 1−5 and ligand 6 in CH3CN. Samples were pumped at 475 nm for complexes 1−5 and 266 nm for ligand 6. Because of scattering from the pump pulse, transient spectra are not shown near 475 nm for 1−5.

Figure 7. Time-resolved ns TA spectra of complexes 2 and 5 in toluene. λex = 355 nm, A355 = 0.4 in a 1 cm cuvette. 23241



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4.1 ns pulse duration used in our RSA study. Therefore, strong RSA is observed from all complexes. It appears that complexes 1−4 bearing benzothiazolylfluorenyl component on the C∧N ligands exhibit significantly reduced transmission in comparison to complex 5, especially when we compare the nonlinear transmission signals of 1 to 5 (both of them contain the same diimine ligand). The trend of the nonlinear transmission signal of 1−5 appears to roughly parallel the strength of the fs TA signal at 1 ps delay after the excitation at 532 nm (i.e., 0.0288 for 1, 0.0203 for 2, 0.0143 for 3, 0.0160 for 4, and 0.0054 for 5), and is inversely correlated to the ground-state absorption of these complexes at 532 nm (see the inset in Figure 1a). As the π-conjugation is extended in the N∧N ligand, the ground-state absorption in the visible spectral region increases, as reflected by the UV−vis absorption spectra of 3 and 4, while the excited-state absorption decreases. Both changes cause the ratio of the excited-state absorption cross section to that of the ground state (σex/σ0) to decrease at 532 nm for 3 and 4. Our previous studies have revealed that the key factor that determines the strength of RSA is the σex/σ0 ratio. The reduced ratios of σex/σ0 for 3 and 4 compared to those of 1 and 2 result in reduced RSA for 3 and 4.

detected. The time-resolved TA spectra of 2 and 5 in toluene are provided in Figure 7 and the associated triplet excited-state parameters for these two complexes, i.e., the triplet−triplet absorption band maxima, the triplet excited-state absorption molar extinction coefficients, and the triplet excited-state lifetimes and quantum yields, are compiled in Table 1. The time-resolved ns TA spectra of complex 1 in different solvents are shown in the Supporting Information Figure S9. The triplet lifetimes obtained from the decay profiles of TA for complexes 1, 2 and 5 in toluene resemble those deduced from the decay of emission in toluene (Table 6), implying that the observed TA originates from the same triplet excited state that emits, i.e. predominantly the 3MLCT/3LLCT states for 1 and 2 and 3 π,π*/3MLCT/3LLCT states for 5. The unobservable ns TA for complexes 3 and 4 could be attributed to the rapid decay of the triplet excited states in these two complexes that exceeds the instrument’s time resolution. Reverse Saturable Absorption (RSA). Although the ns transient absorption of complexes 1, 3 and 4 is very weak or unobservable using our ns TA instrument, the fs TA spectra at longer decay times (i.e., 3 ns) clearly indicate that all complexes possess broad positive TA bands in the visible and extend to the near-IR wavelengths. This implies that excited-state absorption of these complexes exceeds that of the ground state in this spectral region from ps to ns time scales; therefore, RSA (i.e., increasing absorption of the complex with increasing incident energy) is anticipated from these complexes upon ns laser excitation. Nonlinear transmission experiments were carried out for complexes 1−5 with 532 nm laser pulses that are 4.1 ns in duration. All complex solutions were adjusted to get 80% linear transmission at 532 in a 2 mm cuvette in order to excite the same fraction of molecules to the singlet excited state upon laser irradiation. Under this condition, the strength of the nonlinear transmission signal is a measure of the strength of the excitedstate absorption. Figure 8 shows the transmission vs incident

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CONCLUSIONS A series of new cationic iridium(III) complexes (1−5) bearing different diimine (N∧N) ligands were synthesized and studied via steady-state and time-resolved spectroscopic techniques as well as theoretical calculations. All complexes exhibit N∧N or C∧N ligand based 1π,π* transitions below 400 nm, broad and structureless 1MLCT/1LLCT transitions between 400 and 450 nm, and weak 3MLCT/3LLCT tails above 450 nm. These assignments are supported by TDDFT calculations. The increased π-conjugation of the N∧N ligand causes an increased molar extinction coefficient of the absorption bands and a redshift of the 3MLCT/3LLCT band. All complexes exhibit room temperature phosphorescence from 500 to 800 nm, which can be attributed to predominantly 3MLCT/3LLCT emission. The emission energy, lifetime and quantum yield decrease when the π-conjugation of the N∧N ligand increases. The femtosecond transient absorption measurements reveal that all complexes possess broad excited-state absorption bands that span the visible to near-IR region. In addition, these studies demonstrate that these complexes exhibit ultrafast intersystem crossing. Because of the stronger excited-state absorption than the ground-state absorption in the visible spectral region, all complexes exhibit strong RSA at 532 nm under ns laser pulses excitation, which follows the trend of 1 ≈ 2 > 4 > 3 > 5. This trend is roughly correlated to the ratios of the excited-state absorption cross section with respect to that of the ground state for these complexes at 532 nm. Benzothiazolylfluorenyl substitution at the phenylpyridine ligands increases the RSA of complex 1 significantly compared to that of 5 (which has the same N∧N ligand as 1 but with phenylpyridine ligands).

Figure 8. Nonlinear transmission curves for complexes 1−5 in toluene at 532 nm in a 2 mm cuvette. The pulse duration was 4.1 ns, and the linear transmission of all solutions was 80% at 532 nm in a 2 mm cuvette.

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EXPERIMENTAL SECTION Synthesis and Characterization. All solvents and reagents were obtained from Aldrich or Alfa-Aesar and used as received. The intermediates 6 − 8 were prepared according to the procedures reported by our group previously.14 Ligand 1,10phenanthroline was purchased from Alfa Aesar, and the other diimine ligands 2-(pyridin-2-yl)quinoline, 2,2′-biquinoline, and 1,1′-biisoquinoline were synthesized following the literature

energy curves for complexes 1−5. It is very clear that as the incident energy increases, all of the complexes exhibit a dramatic decrease in transmission, suggesting that strong RSA occurs in these complexes. The trend of the RSA for complexes 1−5 is 1 ≈ 2 > 4 > 3 > 5. Although the shorter triplet lifetimes of complexes 3 and 4 prevent us from observing the ns TA spectra on our ns TA spectrometer, the triplet lifetimes should be longer than the 23242



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procedures.19 Silica gels (230−400 mesh) and Al2O3 gels (activated, neutral, Brockmann I) for chromatography were obtained from Sorbent Technology and Aldrich, respectively. The Ir(III) complexes 1−5 were characterized by 1H NMR, HRMS and elemental analyses. A Varian Oxford-400 or Oxford500 VNMR spectrometer was used to record the 1H NMR spectra. ESI−HRMS analyses were conducted on a Bruker BioTOF III mass spectrometer. NuMega Resonance Laboratories, Inc. in San Diego, California carried out the elemental analyses. Complex 1. Compounds 7 (82.0 mg, 0.026 mmol) and 2(pyridin-2-yl)quinoline (10.7 mg, 0.052 mmol) were suspended in 15 mL of 2-ethoxyethanol, then the mixture was heated to reflux for 24 h under Ar atmosphere. The red solution was allowed to cool down to room temperature, and then a 10-fold potassium hexafluorophosphate was added. The suspension was stirred at room temperature for 2 h. After removal of the solvent, the residue was extracted with CH2Cl2. The CH2Cl2 layer was washed with water and dried with MgSO4. Then the solvent was removed, and the crude product was purified by column chromatography (silica gel, hexane:ethyl acetate = 10:1 (v/v) was used as the eluent) and then recrystallized from CH2Cl2/ EtOH to afford red solid 30 mg (yield: 60%). 1H NMR (400 MHz, CDCl3): δ 8.91 (d, J = 7.6 Hz, 1H), 8.78 (d, J = 9.2 Hz, 1H), 8.65 (d, J = 8.8 Hz, 1H), 8.32−8.24 (m, 2H), 8.12−8.06 (m, 6H), 8.01−7.96 (m, 2H), 7.91−7.83 (m, 6H), 7.81−7.77 (m, 3H), 7.72 (t, J = 7.6 Hz, 1H), 7.61−7.58 (m, 4H), 7.53−7.43 (m, 5H), 7.38−7.31 (m, 3H), 7.25−7.24 (m, 5H), 7.07 (t, J = 6.6 Hz, 1H), 7.00 (t, J = 6.6 Hz, 1H), 6.57−6.53 (m, 1H), 6.28−6.25 (m, 1H), 2.18−2.01 (m, 8H), 0.96−0.76 (m, 44H), 0.57−0.48 (m, 16H). ESI−HRMS (m/z): calcd for [C108H 112 N6S 2Ir]+, 1749.8023; found, 1749.8027. Anal. Calcd (%) for C108H112IrN6S2PF6·2CH3CN·2H2O: C, 66.80; H, 6.12; N, 5.57. Found: C, 66.62; H, 5.73; N, 5.66. Complex 2. Compounds 7 (82 mg, 0.026 mmol) and 1,10phenanthroline (9.4 mg, 0.052 mmol) were suspended in 15 mL of 2-ethoxyethanol, then heated to 110 °C for 24 h under Ar atmosphere. The red solution was allowed to cool down to room temperature, and then a 10-fold potassium hexafluorophosphate was added. The suspension was stirred at room temperature for 2 h. After removal of the solvent, the residue was extracted with CH2Cl2. The CH2Cl2 layer was washed with water and dried with MgSO4. Then the solvent was removed, and the crude product was purified by column chromatography (silica gel, CH2Cl2 was used as the eluent). Recrystallization from CH2Cl2/hexane afforded red solid 20 mg (yield: 44%). The characterization data (1H NMR, HRMS, and elemental analysis) are the same as those reported in ref 16 for this complex. Complex 3. The synthetic procedure was the same as that for complex 1 except for that 100 mg (0.032 mmol) compound 7 and 16.2 mg (0.063 mmol) 2,2′-biquinoline were used for the reaction and the reaction mixture was refluxed for 24 h under the Ar atmosphere. A 20 mg yellow solid was obtained as the product (yield: 32%). 1H NMR (400 MHz, CDCl3): δ 9.59 (s, 2H), 8.87 (s, 2H), 8.13−7.74 (m, 24H), 7.51−7.38 (m, 12H), 7.11 (t, J = 7.2 Hz, 2H), 6.98 (t, J = 6.2 Hz, 2H), 6.41−6.38 (m, 2H), 2.18− 2.00 (m, 8H), 0.96−0.75 (m, 44H), 0.57−0.48 (m, 16H). ESI− HRMS (m/z): calcd for [C112H114IrN6S2]+, 1799.8176; found, 1799.8192. Anal. Calcd (%) for C112H114IrN6S2PF6·2C6H14: C, 70.32; H, 6.77; N, 3.97. Found: C, 70.59; H, 6.73; N, 4.27. Complex 4. The synthetic procedure was the same as that for complex 1 except for that 100 mg (0.032 mmol) compound 7 and 16.2 mg (0.063 mmol) 2,2′-biisoquinoline were used for the

reaction and the reaction mixture was refluxed for 24 h under the Ar atmosphere. A 32 mg yellow solid was obtained as the product (yield: 52%). 1H NMR (400 MHz, CDCl3): δ 8.32 (s, 2H), 8.11−8.04 (m, 8H), 7.96−7.89 (m, 8H), 7.82−7.78 (m, 8H), 7.72−7.69 (m, 2H), 7.63−7.62 (m, 4H), 7.48 (t, J = 7.2 Hz, 2H), 7.36 (t, J = 6.2 Hz, 2H), 7.28−7.21 (m, 4H), 7.16−7.14 (m, 2H), 7.08−6.98 (m, 2H), 6.51−6.45 (m, 2H), 2.17−2.06 (m, 8H), 0.96−0.75 (m, 44H), 0.58−0.48 (m, 16H). ESI−HRMS (m/z): calcd for [C112H114IrN6S2]+, 1799.8176; found, 1799.8191. Anal. Calcd (%) for C112H114IrN6S2PF6·CH2Cl2·H2O: C, 66.25; H, 5.83; N, 4.10. Found: C, 66.09; H, 6.06; N, 3.94. Complex 5. Compounds 8 (40 mg, 0.037 mmol) and 2(pyridin-2-yl)quinoline (15.4 mg, 0.075 mmol) were suspended in 15 mL of 2-ethoxyethanol, then the mixture was heated to reflux under Ar atmosphere for 24 h. The red solution was allowed to cool down to room temperature, and a 10-fold potassium hexafluorophosphate was added. The suspension was stirred at room temperature for 2 h. After that the solvent was removed, and the residue was extracted with CH2Cl2. The CH2Cl2 layer was washed with water and dried with MgSO4. After removal of the solvent, the crude product was recrystallized from hexane/CH2Cl2/EtOH to afford red solid 20 mg (yield: 63%). 1H NMR (400 MHz, CDCl3): δ 8.84 (d, J = 8.4 Hz, 1H), 8.73 (d, J = 8.8 Hz, 1H), 8.61 (d, J = 8.8 Hz, 1H), 8.24−8.18 (m, 2H), 7.96−7.94 (m, 1H), 7.88−7.85 (m, 2H), 7.77−7.73 (m, 3H), 7.65−7.60 (m, 3H), 7.50 (t, J = 7.4 Hz, 1H), 7.41−7.37 (m, 1H), 7.33 (d, J = 5.2 Hz, 1H), 7.21−7.16 (m, 1H), 7.05−6.85 (m, 6H), 6.36 (d, J = 7.6 Hz, 1H), 6.08 (d, J = 7.2 Hz, 1H). ESI− HRMS (m/z): calcd for [C36H26IrN4]+, 707.1783; found, 707.1770. Anal. Calcd (%) for C36H26IrN4PF6: C, 50.76; H, 3.08; N, 6.58. Found: C, 50.28; H, 3.15; N, 6.37. DFT Calculations. Complexes 1−5 were optimized in their ground singlet and triplet states using hybrid Perdew, Burke and Ernzerh functional (PBE1PBE method).26−28 The basis sets used for these calculations include 6-31G* set29−33 for all the lighter atoms and the LANL2DZ set34−36 for the Ir atom. For all complexes, full equilibrium geometry optimizations were carried out using the conductor polarized continuum model (CPCM).37,38 Dichloromethane (CH2Cl2) was chosen as the solvent for consistency with the experimental studies. Instead of the 2-ethylhexyl groups, methyl groups were used in the Ir complexes to simplify the calculations. Linear response TDDFT calculations were carried out for 50 lowest singlet transitions to obtain absorption spectra of complexes 1−5. The functional, basis sets, and solvent model used for TDDFT calculations were the same as those used for the geometry optimization, and a broadening of 0.1 eV was applied to spectral transitions to match with the experimental absorption spectra. Analytic TDDFT39 was used to calculate the emission energies of the complexes via optimization of the geometries of the complexes at their singlet and triplet excited states. For phosphorescence, we also used an additional method based on the delta-SCF approach25 that is computationally less expensive than the optimization of the excited state, which still provides relatively accurate results.25 It involvs a geometry optimization on the ground triplet state using unrestricted DFT, and then a TDDFT calculation of a few lowest triplet excited states using that ground-state triplet geometry.24 All calculations were carried out using Gaussian 09 software package.40 Optical transitions were analyzed by natural transition orbitals (NTOs) implemented in the Gaussian 09 software package.41 NTOs introduce the occupied (hole) and unoccupied (electron) excited orbitals and characterize redistribution of the electronic 23243



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of toluene absorption at the 266 nm excitation wavelength for the kinetic study. The ns TA measurements (spectra, triplet lifetimes and quantum yields) were conducted on a laser flash photolysis spectrometer (Edinburgh LP920) excited by the third-harmonic output of a Quantel Brilliant Nd:YAG laser with a pulse duration of 4.1 ns and a repetition rate of 1 Hz. Before measurement, all sample solutions were purged with Ar for 30 min. The triplet excited-state molar extinction coefficients (εT) were determined by the singlet depletion method.47 After the εT value was obtained, the triplet excited-state quantum yield was calculated via the relative actinometry,48 using SiNc in benzene as the reference (ε590 = 70 000 M−1 cm−1, ΦT = 0.20).49 Toluene was selected as the solvent because of the better stability of the organometallic complexes in nonchlorinated solvents and the considerably good solubility of complexes 1 − 5 in toluene, and the relatively high boiling point of toluene. Nonlinear Transmission Measurement. The experimental setup and details were described previously.14−16,50 The linear transmission of the toluene solutions of complexes 1−5 was 80% at 532 nm in the 2 mm cuvette. The beam waist (radius) at the focal plane was ∼96 μm, focused by an f = 40 cm plano-convex lens.

density upon photoexcitation. In order to provide a better representation of excited orbitals, the isosurface of 0.02 was used to visualize the NTOs. It sould be noted that the counterion PF6− was not taking into account in our DFT calculations because it is a standard practice to neglect the counterion effect in modeling electronic and optical properties of organic and metal−organic molecules in solvents. The main reason for this practice is that the position of counterion in solvents is unknown and cannot be detected from experiments. This is particularly unclear for the complexes studied in this project, in which the positive charge originated from the metal ion is surrounded by the ligands, which prevents this charge from the direct interaction with the PF6− counterion. Therefore, placing the counterion at some random positions in the model would unrealistically perturb the electronic structure of the complex, rather than improve the accuracy of DFT calculations. In addition, investigations of the effect of the counterion on the ground state of some solid-state molecules have demonstrated that stable conformations seem to be unaffected by the counterion, as validated from chemical shifts of compounds calculated with and without counterions.42,43 Our previous studies on the Ru(II)25 and Ir(III)14−16 complexes also confirmed the validity of neglecting the counterion in the DFT calculations of ionic organometallic complexes, in which the DFT calculations showed very good agreement (in some cases not only qualitatively but also quantitatively) between the calculated and experimental absorption spectra and emission energies. Photophysical Studies. The UV−vis absorption spectra of complexes 1−5 were recorded using a UV-2501 spectrophotometer (Shimadzu) in 1 cm quartz cuvettes. The steady-state emission spectra were measured on a fluorometer/phosphorometer (HORIBA FluoroMax 4). The emission quantum yields in degassed solutions were obtained using the relative actinometry method.44 The reference used for complexes 1−5 was a degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem = 0.042, λex = 436 nm).45 The fs TA spectra and the singlet excited-state lifetimes were studied using a previously described pump−probe spectrometer.46 Briefly, a 1 kHz amplified Ti:sapphire (Libra-HE, Coherent Inc.) laser system was used to generate 85 fs pulses centered at 800 nm. Pump pulses with a central wavelength of 266 nm were generated from the third harmonic of a small portion (20%) of the fundamental. Pump pulses at 475 nm were generated using an optical parametric amplifier (OPerA solo from Coherent Inc.) pumped by ∼40% of the fundamental. The 0.75 μJ pump pulses were chopped at 100 Hz by a mechanical chopper (New Focus) positioned in front of the sample. The white light continuum probe pulses were generated using a CaF2 crystal. A motorized IMS600PP optical delay stage (Newport Inc.) with a maximum time delay of 4 ns was used to delay the pump pulses. The angle between the linear polarizations of pump and probe pulses was set to the magic angle (54.7°). Pump and probe spot diameters were measured to be 0.48 mm and 0.15 mm by knife-edge scan at the sample position, respectively. After the sample, the probe pulses were detected by a multichannel spectrometer (Ocean optics USB 2000+). Detector signals were recorded using Labview-based data collection software. The sample solution was held in a 1 mm path length quartz cuvette (Starna Cell). CH3CN was chosen as the solvent for the fs TA studies because the complexes are more stable in nonchlorinated solvents upon UV laser excitation. On the other hand, CH3CN instead of toluene was used as the solvent due to the interference

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ASSOCIATED CONTENT

S Supporting Information *

Optimized ground-state geometries of complexes 1−5, normalized UV−vis absorption spectra of 2−5 in different solvents, normalized emission spectra of 2−5 in different solvents at room temperature and at 77 K in BuCN, the timeresolved ns TA spectra of complex 1 in different solvents, UV− vis absorption spectrum of ligand 6 in CH3CN, femtosecond TA fitting parameters for ligand 6 in CH3CN with 266 nm pump, and full author list for refs 11, 24, and 40. 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: [email protected]. Telephone: 701-231-6254. Fax: 701-231-8831. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The synthetic work and the photophysical and RSA studies were supported by the US Army Research Laboratory (W911NF-102-0055) to W.S. She is also grateful to the National Science Foundation (NSF CNS-1229316) for supporting the DFT calculations. S. Kilina acknowledges the US Department of Energy (DE-SC008446) for partial support and the Center for Computationally Assisted Science and Technology (CCAST) at North Dakota State University for computer access and administrative assistance. The work conducted at Montana State University was supported in part by the Department of Energy (DE-FG02-07ER46477) to B. Kohler.

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REFERENCES

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