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Towards Broadband Reverse Saturable Absorption: Investigating the Impact of Cyclometalating Ligand #-Conjugation on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium Complexes Li Wang, Peng Cui, Svetlana V Kilina, and Wenfang Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12947 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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The Journal of Physical Chemistry

Towards Broadband Reverse Saturable Absorption: Investigating the Impact of Cyclometalating Ligand π-Conjugation on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium Complexes

Li Wang,a Peng Cui,a,b Svetlana Kilina,a Wenfang Suna,*

a

Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North

Dakota 58108-6050, United States b

Materials and Nanotechnology Program, North Dakota State University, Fargo, North

Dakota 58108-6050, United States

1

ACS Paragon Plus Environment


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ABSTRACT:

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The synthesis, photophysics and reverse saturable absorption of a series of

bis-cyclometalated

Ir(III)

complexes

Ir(C^N)2L•PF6,

where

L

=

3,8-bis[9,9-di(2-

ethylhexyl)-9H-fluoren-2-yl]-1,10-phenanthroline and C^N = 2-phenylpyridine (ppy, 1), 2-phenylquinoline (pqu, 2), 1-phenylisoquinoline (piq, 3), 2-phenylbenzo[g]quinoline (pbq, 4), 2,3-diphenylbenzo[g]quinoxaline (dpbq, 5), are reported. By gradually increasing the

π-conjugation along the pyridine or pyrazine ring of the C^N ligands, the energies of the lowest singlet (S1) and triplet (T1) excited states are significantly reduced, as reflected by the pronouncedly red-shifted charge transfer absorption bands at > 450 nm and the emission band(s) in their UV-vis absorption and emission spectra, respectively. Additionally, our density functional theory (DFT) calculations confirm that the natures of the S1 and T1 states vary with the increased π-conjugation, with the S1 state changing from the exclusive 1LLCT (ligand-to-ligand charge transfer) / 1MLCT (metal-to-ligand charge transfer) transitions in 1-3 to the predominant 1ILCT (intraligand charge transfer) / 1π,π* / 1MLCT / 1LLCT transitions in 4 and 5; while the T1 state being altered from the predominant ligand L based 3ILCT or 3

ILCT/3π,π* nature in 1 and 2, respectively, to the C^N ligand-localized 3π,π*/3MLCT/3ILCT

parentage in 3-5. All complexes exhibit broad and positive transient absorption (TA) in the visible to the near-IR region (ca. 430-800 nm) upon nanosecond laser excitation at 355 nm. However, the TA spectral features and the triplet lifetime vary dramatically from 1 to 5, reflecting the different natures of the T1 states when the degree of π-conjugation of the C^N ligands increases. Our nonlinear transmission experiments demonstrate moderate to strong reverse saturable absorption (RSA) for 1-5 for nanosecond laser pulses at 532 nm. The relative strength of the RSA follows the trend of 1 > 3 > 2 > 4 > 5. Our joined experimental 2


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and computational studies manifest that judicious choice of the C^N ligand with appropriate

π-conjugation is an effective approach to obtain Ir(III) complexes with desired photophysical properties for reverse saturable absorbers.

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INTRODUCTION Organometallic complexes possessing unique optical and electronic properties have attracted a great deal of interest from researchers worldwide in the past few decades.1 Iridium(III) complexes featuring d6 transition-metal center and octahedral coordination geometry are particularly promising because of their intriguing photophysical and photochemical properties.2,3,4,5 It is well known that iridium has one of the largest spin-orbit coupling constants among all transition metals, which facilitates the singlet to triplet intersystem crossing. Consequently, Ir(III) complexes typically possess high triplet quantum yields.6 Meanwhile, it is facile to tune the emission color of the Ir(III) complexes via ligand structural modifications.7,8,9 Based on these characteristics, Ir(III) complexes are among the widely studied organometallic complexes for applications in organic light-emitting diodes (OLEDs),1,10 luminescent biological reagents,11,12 molecular sensors,13,14 and light-emitting electrochemical cells (LEECs).15,16 In addition to the extensively studied emission properties of Ir(III) complexes targeting applications for OLEDs, LEECs, and bioimaging, reverse saturable absorption (RSA, defined as a nonlinear optical phenomenon in which the absorptivity of the materials increases with the increased incident fluence because of the stronger excited-state absorption than the ground-state absorption) of Ir(III) complexes is also an attractive feature for diverse photonic device applications, such as in optical switching,17 laser mode locking,18 optical pulse shaping,19 spatial light modulation,20 and laser beam compression and limiting21,22 etc. To develop ideal reverse saturable absorbers for these applications, the materials are required to have large ratios of the excited-state absorption cross section (σex) vs. the ground-state 4


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absorption cross section (σ0) at the wavelengths of interest, and possess highly populated, long-lived and strongly absorbing triplet excited state for nanosecond (ns) or longer pulse width laser excitation. In Ir(III) complexes, the rapid intersystem crossing induced by the heavy metal ion could facilitate the population of the triplet excited state and enhance the triplet excited-state absorption and thus RSA.23 In the past a few years, our group and other groups have demonstrated the RSA of Ir(III) complexes with varied degrees of π-conjugation or substituents on the diimine (N^N) and/or cyclometalating (C^N) ligands.5,6,24-33 These studies showed that extending the π-conjugation of the N^N ligands via attaching π-conjugated substituents on the bipyridine or phenanthroline ligand dramatically increased the lowest triplet excited-state (T1) lifetime by incorporating more N^N ligand-associated

3

π,π*/3ILCT (intraligand charge transfer)

characters into the T1 state.24,25 Meanwhile, the predominant 3π,π*/3ILCT nature of the T1 state greatly extended the triplet excited-state absorption to the near-IR region. In contrast, the similar modification on the C^N ligands reduced the T1 lifetime and weakened the T1 absorption by introducing more 3MLCT/3LLCT characters into the T1 state.24,26 Interestingly, fusing aromatic rings to the C^N ligand increased the T1 lifetime by switching the T1 state from the 3MLCT/3LLCT nature to the C^N ligand localized 3π,π*/3ILCT nature.5 Although these studies demonstrated the feasibility and promise of utilizing Ir(III) complexes as reverse saturable absorbers, most of the reported complexes have the drawback of either being lack of weak broad ground-state absorption in the 600-900 nm region to populate the excited states via one-photon absorption,5,24,26-29,32 which is a prerequisite for RSA; or having no

5



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excited-state absorption in the near-IR region.5,24,26 Both aspects prohibit the potential application of Ir(III) complexes as broadband (400-900 nm) reverse saturable absorbers. To broaden the ground-state absorption to the near-IR region, we fused the aromatic rings to the bipyridine ligand to red-shift the charge transfer bands in their ground-state absorption spectra.25,31 In particular, the tails of the absorption spectra were bathochromically shifted to 750 nm for complexes with 2-(quinolin-2-yl)quinoxaline (quqo) ligand.31 However, the lifetimes of the lower-lying T1 state were dramatically reduced in these complexes,25,31 which weakened their triplet excited-state absorption and limited their application as broad temporal responsive reverse saturable absorbers. As such, it remains to be a challenge in developing ideal broadband reverse saturable absorbers that contain weak ground-state absorption (ε ≤ 3,000 L.mol-1.cm-1) from 450 to 900 nm, while retaining long triplet lifetime and the strong and broad triplet excited-state absorption in this spectral region. In this paper, we address this challenge by synthesizing and characterizing, both experimentally and computationally, five bis-cyclometalated Ir(III) complexes Ir(C^N)2L•PF6, with L = 3,8-bis[9,9-di(2-ethylhexyl)-9H-fluoren-2-yl]-1,10- phenanthroline and C^N = 2-phenylpyridine

(1),

2-phenylquinoline

(2),

1-phenylisoquinoline

(3),

2-phenylbenzo[g]quinoline (4), 2,3-diphenylbenzo[g]quinoxaline (5). The structures of complexes 1-5 are shown in Chart 1. We choose 3,8-difluorenylphenanthroline (L) as the N^N ligand because we have previously demonstrated that cationic Ir(III) complexes bearing this ligand gave long triplet excited-state lifetime (several microseconds) and broad triplet transient absorption.26 The C^N ligands with varied degrees of π-conjugation were chosen because our group and other groups have manifested that increasing the π-conjugation of the 6


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C^N ligands by fusing aromatic ring(s) to pyridine or pyrazine can bathochromically shift the weak charge-transfer ground-state absorption band.30,35 Meanwhile, such an approach has been proven to be an effective way to red-shift the emission of the Ir(III) complexes to the near-IR region.30,35-37 Thus, in this work, we intend to utilize this approach to find out the most appropriate C^N ligand that matches the ligand L in order to develop Ir(III) complexes that contain weak ground-state absorption from 450 to 900 nm, while retaining the long triplet lifetime and strong and broad triplet excited-state absorption in this spectral region.

Chart 1.

Molecular structures of complexes 1-5.

EXPERIMENTAL SECTION Synthesis and characterization. All reagents and solvents were purchased from commercial sources and used as is unless otherwise mentioned. 1H NMR spectra were recorded on a Bruker-400 spectrometer in CDCl3 with tetramethylsilane (TMS) as the internal standard. High resolution mass (HRMS) analyses were performed on Waters Synapt G2-Si Mass Spectrometer. Elemental analyses were conducted by NuMega Resonance Laboratories, Inc. in San Diego, California. The cyclometalating ligands 2-phenylbenzo[g]quinoline (pbq)38 and 7



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2,3-diphenylbenzo[g]quinoxaline (dpbq)35 were prepared according to the reported procedures while the other ligands (ppy, pqu, and piq) were commercially available. The diimine ligand 3,8-difluorenylphenanthroline (L) was prepared by Suzuki coupling reaction according to the procedure reported previously.26,34 All Ir(III) dimers [Ir(C^N)2Cl]2 were prepared following the reported Nonoyama method.39 Complex 1 was reported before by our group.26 Complexes 2-5 were synthesized by reactions of the diimine ligand L with the corresponding [Ir(C^N)2Cl]2 dimer as outlined in Scheme 1. The synthetic details and the characterization data for complexes 2-5 are provided below. The general procedure for the preparation of complexes 2-5.

The diimine ligand L

(65 mg, 0.068 mmol), corresponding [Ir(C^N)2Cl]2 dimer (0.034 mmol) and AgSO3CF3 (18 mg, 0.068 mmol) were added to a mixture of CH2Cl2 and CH3OH (v/v = 20/10 mL) in a 100-mL round bottle flask. The reaction mixture was heated to reflux under argon atmosphere for 22 hours. After the mixture was cooled to room temprature, NH4PF6 (80 mg, 0.50 mmol) was added and the mixture was stirred at r.t. for another 3 hours. Then the solvent was removed, and the residue was purified by column chromatography (silica gel, CH2Cl2/CH3OH = 50/1 (v/v) as the eluent) to afford the corresponding complexes. Complex 2. Orange-red solid of 66 mg was afforded (yield: 57%). 1H NMR (400 MHz, CDCl3): δ 8.80-8.93 (m, 2H), 8.72-8.78 (m, 2H), 8.09-8.35 (m, 8H), 7.90-7.92 (d, 2H), 7.63-7.81 (m, 6H), 7.36-7.48 (m, 12H), 7.24 (t, 2H), 6.95-6.98 (m, 2H), 6.78-6.84 (m, 4H), 2.10-2.22 (m, 8H), 0.23-0.96 (m, 60H). HRMS (ESI) calcd for [C100H108IrN4]+: 1557.8214, found: 1557.8250. Anal calcd (%) for C100H108IrN4PF6•2H2O: C, 69.06; H, 6.49; N, 3.22. Found: C, 68.83; H, 6.09; N 3.10. 8


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Complex 3. Orange-red solid of 60 mg was afforded (yield: 55%). 1H NMR (400 MHz, CDCl3): δ 9.05 (m, 2H), 8.90-8.92 (m, 2H), 8.41-8.44 (m, 4H), 7.64-7.90 (m, 12H), 7.24-7.41 (m, 16H), 7.06-7.11 (m, 2H), 6.57-6.60 (m, 2H), 1.92-2.09 (m, 8H), 0.25-0.98 (m, 60H). HRMS (ESI) calcd for [C100H108IrN4]+: 1557.8214, found: 1557.8270. Anal calcd (%) for C100H108IrN4PF6•2H2O: C, 69.06; H, 6.49; N, 3.22. Found: C, 68.94; H, 6.38; N 3.19. Complex 4. Orange-red solid of 90 mg was afforded (yield: 75%). 1H NMR (400 MHz, CDCl3): δ 9.03-9.10 (m, 2H), 8.74-8.87 (m, 2H), 8.17-8.41 (m, 8H), 7.76-7.98 (m, 10H), 7.30-7.54 (m, 14H), 7.17-7.19 (m, 2H), 6.92-6.95 (m, 4H), 6.56-6.62 (m, 2H), 2.11-2.23 (m, 8H), 0.20-0.98 (m, 60H). HRMS (ESI) calcd for [C108H112IrN4]+: 1657.8529, found: 1657.8539. Anal calcd (%) for C108H112IrN4PF6•2H2O: C, 70.84; H, 6.56; N, 2.98. Found: C, 70.87; H, 6.78; N 3.23. Complex 5. Red solid of 60 mg was afforded (yield: 51%). 1H NMR (400 MHz, CDCl3): δ 9.34-9.41 (m, 2H), 8.95-8.98 (m, 2H), 8.56-8.60 (d, 2H), 8.22-8.24 (m, 2H), 7.86-8.04 (m, 8H), 7.65-7.84 (m, 6H), 7.31-7.50 (m, 22H), 6.81-6.90 (m, 6H), 2.09-2.16 (m, 8H), 0.49-1.10 (m, 56 H), 0.17-0.21 (m, 4H). HRMS (ESI) calcd for [C118H118IrN6]+: 1811.9061, found: 1811.9073. Anal calcd (%) for C118H118IrN6PF6•2H2O: C, 72.40; H, 6.08; N, 4.29. Found: C, 72.00; H, 6.30; N 4.63. Photophysical measurements. The solvents used for photophysical studies were spectroscopic grade and were purchased from VWR International and used without further purification. The ultraviolet-visible (UV-vis) absorption spectra were recorded on a Varian Cary® 50 Spectrophotometer. Steady-state emission spectra were obtained on a Jobin-Yvon FluoroMax-4 fluorometer/phosphorometer. The emission quantum yields were determined by 9



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relative actinometry method in degassed solvents, in which [Ru(bpy)3]Cl2 in degassed CH3CN (λex = 436 nm, Φem = 0.097) was used as reference for the complexes.40 The nanosecond transient difference absorption (TA) spectra and decay profiles were measured in degassed toluene solutions on an Edinburgh LP920 laser flash photolysis spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant, pulse width = 4.1 ns, repetition rate = 1 Hz) was used as the excitation source. Each sample was purged with argon for 45 min prior to measurement. The triplet excited-state absorption coefficients (εT) at the TA band maximums were determined by the singlet depletion method.41 The RSA of complexes 1-5 were demonstrated by nonlinear transmission experiment at 532 nm using a Quantel Brilliant laser as the light source. The pulse width of the laser was 4.1 ns, and the repetition rate was set to 10 Hz. All the complexes were dissolved in toluene and the concentrations of the sample solutions were adjusted to make the linear transmission of 80% at 532 nm in a 2-mm cuvette. The experimental setup and details are the same as those reported previously.24 The radius of the beam waist at the focal plane was approximately 96 µm. Computational methods. Singlet geometry optimizations of complexes 1-5 and their ground state electronic structure were calculated using density functional theory (DFT), as implemented in Gaussian 09 software package.42 The hybrid PBE1 functional43-45 with 30% Hartree-Fork (HF) exchange was used and LANL2DZ basis set46-48 was applied for Ir, while 6-31G* basis set49-53 was applied for the remaining atoms. This approach has been shown

10


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being feasible in describing both the ground and excited state properties of Ir(III) complexes.24-32 For calculations of absorption spectra, the time-dependent DFT (TDDFT)54,55 was applied by using the same density functionals and basis set for the geometry optimization. TDDFT framework provides the density matrix from which the excitation energies and oscillator strength of optical transition could be extracted by iteratively solving the eigenvalue equation based on Davidson algorithm.56-58 Forty excited states were calculated to simulate the experimental UV-vis absorption spectra. To obtain the fluorescence energies, the lowest singlet excited-state geometry was optimized using analytical gradient TDDFT59 as implemented in Gaussian 09 software package. To obtain the phosphorescence energies, the lowest triplet ground-state geometry was optimized using unrestricted DFT method (∆SCF approach). Then, the lowest triplet excitation energy was calculated through the combined scalar relativistic ZORA60 and TDDFT approach using NWChem software package.61 Natural transition orbitals (NTOs)62,63 were used to visualize the molecular orbitals that correspond to the coupled hole-electron pairs upon photoexcitation by utilizing Gaussian 09 software package. A hole-electron transition from a ground state to an excited state could be realized through unitary transformation of transition density matrix of a specific excited state obtained from TDDFT calculations.62,63 For visualizing the lowest-energy emitting state, we plot the dominant molecular orbitals by performing the eigenvector analysis on the lowest excited state.64 Chemcraft-1.7 software65 was used for plotting excited charge densities by setting the value as 0.02. 11



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Both geometry optimization and optical absorption calculations were performed via conductor-like polarizable continuum model (CPCM)66,67 as implemented in Gaussian 09, whereas the fluorescence and phosphorescence calculations were performed via COSMO continuum solvation68,69 as implemented in NWChem software package.61 Toluene was chosen as the solvent media to be consistent with the experiments.

RESULTS AND DISCUSSIONS Synthesis. The synthetic route for complexes 1-5 is outlined in Scheme 1. Suzuki coupling reaction

between

2-(9,9-di(2-ethylhexyl)-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-

dioxaborolane and 3,8-dibromo-1,10-phenanthroline (phen-2Br) gave the diimine ligand L in a moderate yield of 44%. The challenge of this critical step lies in the separation of the target bisubstituted product from the monosubstituted byproduct. We have tried different catalysis systems (such as Pd2(dba)3/P(o-Tol)3 or Pd(PPh3)4], different solvent combinations like toluene/H2O or toluene/C2H5OH/H2O), different bases (Cs2CO3 or K2CO3), and different reaction times from 24 hours to 72 hours. It was found that both the catalyst and the solvent had minor influence on the reaction yield. However, short reaction time (24 hours) appeared to give more monosubstituted byproduct. Finally, we chose Pd(PPh3)4 in toluene/H2O (v/v = 10/1) with the aid of K2CO3 and extended the reaction time to 72 hours in the absence of light to carry out the reaction. After column separation, the ligand L was collected as light yellow oil.

12


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i) phen-2Br, Pd(PPh3)4, K2CO3, toluene/H2O, reflux; ii) IrCl3•3H2O, 2-ethoxylethanol/H2O, reflux; iii) AgSO3CF3, CH2Cl2/CH3OH, reflux; then NH4PF6, r.t. Scheme 1.

synthetic routes for complexes 1 – 5.

For the synthesis of [(Ir(C^N)2Cl]2 dimers, the Nonoyama method39 was followed by refluxing the cyclometalating ligand (C^N) with IrCl3•3H2O in a mixture of ethoxyethanol and water (v/v = 3/1). The formed precipitates were collected by filtration and washed with hexane and ether to give the [(Ir(C^N)2Cl]2 dimers with satisfactory purity. At last, the complexation reactions were carried out in a mixture of CH2Cl2 and CH3OH (v/v = 2/1) and the crude products were purified by column chromatography on silica gel to get the desired pure complexes. The obtained complexes were characterized by 1H NMR, HRMS and elemental analyses. The 1H NMR and HRMS spectra are provided in Figures S1-S4 of the Supporting Information. All complexes showed good solubility in chlorinated solvents (chloroform and CH2Cl2), toluene, THF, DMSO and acetonitrile and partially dissolved in acetone and hexane. The good stability of the complexes was demonstrated by monitoring the

13



CDCl3 solution left in a NMR tube via TLC and by measuring the UV-vis spectra before and after more than 1000 times 355 nm laser pulse irradition. The UV-vis absorption spectra of complexes 1-5 were

Electronic absorption.

measured in toluene and compared to the calculated spectra in the same solvent (see Figure 1). The absorption band maximum and molar extinction coefficients are compiled in Table 1. No ground state aggregation was detected for these complexes in the studied concentration range (1×10-6 - 1×10-4 mol/L) based on the obedience of the Beer’s law, which is a common feature for Ir(III) complexes due to the prevention of intermolecular stacking by the octahedral geometry of the complexes and the presence of branched alkyl chains on the fluorene motifs. 5

-1

)

ε (L.mol-1.cm-1)

1.2x10

4

9.0x10

-1

ε (L.mol .cm

4

6.0x10

500

1 2 3 4 5

400 300 200 100 0 500

600

700

800

900

Wavelength (nm)

4

3.0x10

0.0

Abs. (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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a 1

1.6

2

1.2

3 4

0.8

5

0.4

b

0.0 300

400

500

600

700

Wavelength (nm)

Figure 1. (a) Experimental and (b) calculated absorption spectra of complexes 1-5 in toluene. The inset in (a) shows the expansion of the absorption spectra between 500 and 900 nm measured at a concentration of 1×10-4 mol/L. Table 1.

Photophysical data for complexes 1-5 in toluene. 14


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λabs/nm (ε/104 L mol-1 cm-1)a

λem/nm (τ0/µs); Φemb

theor

λphosc/nm λT1-Tn /nm (τT/µs, εT1-Tn/104 L mol-1 cm-1); ΦTd

330 (4.51), 387 (4.86), 406 598 (5.0); 0.30 604 720 (4.9, 4.09); (4.41), 470 (0.12) 0.37 336 (5.55), 358 (4.62), 385 560 (2.8); 0.16 551 590 (2.7, 4.90); 2 (4.50), 406 (4.00), 470 (0.40) 0.16 334 (5.50), 359 (4.76), 386 590 (3.1); 0.46 571 532 (3.2, -); -e 3 (5.15), 405 (4.35), 470 (0.53), 570 (0.02) 313 (10.45), 346 (4.31), 390 700 (2.5); 0.07 733 660 (2.6, -); - e 4 (5.09), 411 (4.66), 502 (0.47), 626 (0.008) 325 (7.64), 411 (5.03), 490 788 (0.40), 907 805 532 (0.44, -); - e 5 (7.07), 558 (0.42), 679 (0.015), (-), 965 (-); 748 (0.005) 0.004 a Absorption band maxima and molar extinction coefficients at room temperature. bRoom temperature emission band maxima, intrinsic lifetimes and emission quantum yields. A degassed [Ru(bpy)3]Cl2 in acetonitrile solution was used as reference (Φem = 0.097, λex = 436 nm). cCalculated by PBE1PBE for optimized triplet geometry. dNanosecond transient absorption band maxima, triplet extinction coefficients, triplet excited-state lifetimes and quantum yields measured at room temperature. SiNc in benzene was used as the reference (ε590 nm = 70,000 L mol-1 cm-1, Φem = 0.20). eNo bleaching bands were detected, thus the εT1-Tn values were unable to be estimated using the singlet depletion method. 1

As shown in Figure 1a, the absorption spectra of complexes 1-5 are composed of three major bands: the intense, structured bands below 360 nm, the broad, featureless band(s) between 360 and 450 nm, and the weak, broad tails beyond 450 nm. Based on the structured feature and the large molar extinction coefficients (4.5×104 - 1.0×105 L.mol-1.cm-1, Table 1), we assign the bands below 360 nm to the ligand based 1π,π* transitions. The broad band(s) at 360-450 nm possess the similar molar extinction coefficients and the similar energies, implying that these band(s) are likely originated from the same structural component, i.e. the diimine ligand L. The broad tails beyond 450 nm have very weak molar extinction coefficients, with reference to the other reported Ir(III) complexes,3,5-7,24-32 we ascribe these 15



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bands to the

1,3

MLCT/1,3LLCT transitions. As expected, when the π-conjugation of the

cyclometalating ligands increases, these low-energy band(s) are gradually red-shifted and the molar extinction coefficients increase. Especially for complexes 4 and 5, these bands extend to 710 and 800 nm, respectively. This feature clearly manifests the impact of increased

π-conjugation of the C^N ligands on the low-energy electronic transitions. Comparison of the low-energy bands of complexes 2 and 3 reveals that although the isomeric C^N ligands in these two complexes have the same number of π-electrons, the different positions of benzannulation on the pyridine ring induce a salient difference in the region of 560-600 nm for these two complexes. Complex 3 bearing the piq ligands possesses an absorption band at ca. 570 nm, which is absent in complex 2 containing the pqu ligands. To better understand the effect of structural variations on the optical transitions, we need to unambiguously assign the optical transitions to the absorption bands in each complex. Thus, TDDFT calculations were carried out for complexes 1-5 in toluene. The calculated UV-vis absorption spectra of 1-5 are displayed in Figure 1b, and the comparison of the experimental and calculated spectra for each complex with the discrete oscillation strengths of each transition is given in Figure S5. Because the TDDFT calculations do not include the spin-orbit coupling effect, the spin-forbidden absorption band(s) in these complexes beyond 600 nm are not represented in the calculated spectra. Except for this disparity, both the shape and energy of the calculated spectra well match the corresponding experiment spectra. The NTOs corresponding to the low-energy singlet transitions shown in Table 2 indicate that the absorption band(s) at 450 – 600 nm in complexes 1 – 5 has/have the charge transfer nature. For complexes 1-3, the 1MLCT/1LLCT characters dominate the lowest-energy transition. 16


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


When the π-conjugation of the C^N ligand increases, the C^N ligand-localized 1ILCT (intraligand charge transfer) becomes the major contributor to the lowest-energy transition in complexes 4 and 5, admixing with some 1π,π* (C^N ligand based) / 1MLCT/1LLCT characters. These transitions give rise to the additional bands between 480 and 600 nm in 4 and 5 in addition to the 1MLCT/1LLCT transitions at ca. 450 nm.

Table 2. Natural transition orbitals (NTOs) representing the low energy transitions of complexes 1-5 in toluene.

1

2

3

4

Excited state and properties S1 440 nm f = 0.0018 S1 445 nm f = 0.008 S2 430 nm f = 0.002 S1 453 nm f = 0.004 S1 462 nm f = 0.01

Holes

S2 453 nm f = 0.08 S3 451 nm f = 0.004 5

S1 500 nm f = 0.05

17


Electrons


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

S2 487 nm f = 0.05 S3 452 nm f = 0.09 S5 442 nm f = 0.006

For the absorption band at 360-450 nm, the main contribution arises from the N^N ligand-associated 1ILCT/1π,π* transitions (see the NTOs of S2 for 1, S4 for 2, S3 for 3, S7 for 4, and S9 for 5, shown in Table 3), which have the largest oscillator strength and similar energies for all complexes. The large oscillator strength of these transitions is a clear reflection of the 1π,π* contribution. In addition, contributions from the C^N ligand-associated 1

π,π*/1ILCT transitions to this absorption band are non-negligible, especially for complexes 4

and 5, although such transitions have significantly smaller oscillator strengths than the N^N ligand-associated 1ILCT/1π,π* transitions. As for the high energy bands at 300-360 nm, the major transitions are 1π,π* and 1LLCT, mixed with 1ILCT/1MLCT transitions (see Table S1 in Supporting Information). As the π-conjugation of the C^N ligand being extended from 1 to 5, the contribution from the N^N ligand-based 1ILCT transition to these high-energy bands decreases.

Table 3. Natural transition orbitals (NTOs) representing the major absorption bands of complexes 1-5 in toluene. Excited states and properties

Holes

18


Electrons

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2

3

4

S2 401 nm f = 1.40 S3 414 nm f = 0.09 S4 400 nm f = 1.32 S2 419 nm f = 0.16 S3 406 nm f = 1.32 S4 413 nm f = 0.14 74%

74%

24%

24%

S7 398 nm f = 1.16 S9 394 nm f = 0.11 5

S9 405 nm f = 0.85 S14 389 nm f = 0.30 S15 385 nm f = 0.36 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Photoluminescence. The room-temperature emission of 1-5 was studied in toluene and the spectra are shown in Figure 2 and the emission data are summarized in Table 1. All of the observed emission can be assigned to phosphorescence considering the facts that the emission displayed a significant red-shift in comparison to their corresponding excitation wavelengths, and the long lifetime (varying from hundreds of nanoseconds to more than one microsecond). The emitting color gradually changed from yellow (1-3) to orange-red (4) to dark-red (5), indicating the decrease of the emission energy as the π-conjugation of the cyclometalating ligands (C^N) increases. Meanwhile, the emission became more structured, the quantum yields decreased, and the lifetimes became shorter from 1 to 5. All these changes reflect the impacts of extending π-conjugation of the cyclometalating ligands (C^N), which likely changed the nature of the emitting state as the C^N ligands became more π-expansive. The structureless feature of the emission for complexes 1 and 2 implies the charge transfer nature of the emitting state; while the structured emission of 3-5 suggests more 3π,π* transition nature of the emission. However, the much shorter emission lifetimes of 3-5 than a pure 3π,π* emission imply the involvement of charge transfer characters. Because the emission energies, lifetimes and quantum yields of 3-5 are drastically different, it is likely that the emission of these complexes emanates from the C^N ligand-associated excited state(s). This notion is partially supported by the comparison of the emission of these complexes to their corresponding [Ir(C^N)2Cl]2 dimer. As shown in Supporting Information S7, the emission energies and lifetimes are nearly the same in complexes 4 and 5 compared to those of their corresponding [Ir(C^N)2Cl]2 dimers. Interestingly, for complexes 2 and 3 that differ only in 20


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the site of benzannulation in their C^N ligands, while minor differences were observed in their absorption spectra, the emission spectrum of 3 was ca. 30 nm red-shifted compared to that of 2, and the structured feature became more salient in 3. On the other hand, the lifetime of 3 (3.1 µs) was slightly longer than that of 2 (2.8 µs), but the emission quantum yield of 3 was much higher than that of 2, which reflects the increased 3π,π* character in the emission of 3.

Normalized Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2

1 2 3 4 5

1.0 0.8 0.6 0.4 0.2 0.0 500

600

700

800

900

1000 1100 1200 1300

Wavelength (nm)

Figure 2. Normalized emission spectra for complexes 1 (λex = 420 nm), 2 (λex = 384 nm), 3 (λex = 468 nm), 4 (λex = 500 nm) and 5 (λex = 550 nm) in deaerated toluene at room temperature. The emission of 1-4 was monitored using a Hamamatsu R928 PMT as the detector, while an InGaAs sensor was used for monitoring the emission of 5. The emission of all the complexes showed minor solvent dependency in the solvents used (toluene, CH2Cl2 and CH3CN, Figure S8 and Table S2), which is typical for Ir(III) complexes due to their octahedral configuration that prevents the approaching of the solvent molecules to the metal center.24-32 To unambiguously assign the nature of the excited states involved in emission and gain better understanding of the aforementioned phenomena, TDDFT calculations of the triplet emission using the optimized triplet excited state geometries were conducted. The results are 21



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Page 22 of 41

depicted in Table 4. The calculated phosphorescence energies matched the experiment results very well despite the fact that the spin-orbit couplings were not included in the calculations. According to the calculated NTOs contributing to the T1 state, both the electrons and holes are localized on the ligand L in complexes 1 and 2. However, in complex 1, the T1 state has predominant 3ILCT character; while it has mixed 3ILCT/3π,π∗ characters in 2.

The

increased 3π,π∗ contribution in 2 resulted in the blue-shift of the emission band in 2 compared to that in 1. In contrast, the nature of the emission for complexes 3-5 switched to the C^N ligand-localized

3

π,π∗ transition mixed with

3

MLCT/3ILCT configurations, which is

consistent with the structured emission from these complexes. In complex 5, some 3LLCT characters are involved in the emission.

Table 4. Natural transition orbitals (NTOs) representing transitions contributing to the singlet and triplet emission of 1-5 in toluene. The emission was calculated via optimization of the singlet and triplet excited state geometries, respectively. Singlet / Triplet Emission

Electrons

S1 578 nm 1 T1 604 nm S1 490 nm 2 T1 551 nm 3

S1 492 nm

22


Holes

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Singlet / Triplet Emission

Electrons

T1 571 nm

Holes

30%

30%

21%

21%

22%

22%

S1 534 nm

4

T1 773 nm

30%

30%

11%

11%

38%

38%

20%

20%

19%

19%

S1 504 nm

T1 805 nm 5

Transient absorption (TA). The triplet excited-state characteristics of complexes 1-5 were further studied by nanosecond transient absorption (TA) spectroscopy, which could

23



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provide information on the excited-state absorption spectra, the decay characteristics of the triplet excited-state, and the triplet excited-state quantum yield. All of these properties would influence the reverse saturable absorption. The TA spectra of 1-5 at zero-delay in deaerated toluene solution are depicted in Figure 3. The absorption band maxima, triplet excited-state absorption coefficients, and the triplet lifetimes deduced from the decay of the TA signals are summarized in Table 1. All complexes possessed broad and positive TA signals in the range of 450-820 nm. But the features of these TA bands are drastically different. Complexes 1 and 2 exhibited bleaching band at 400 nm and 387 nm, respectively, which are in line with their respective 1

ILCT/1π,π* bands in their UV-vis absorption spectra. Thus the transient absorbing triplet

excited state could be related to the ligand L.

Considering the fact that the triplet lifetimes

obtained from the decay of TA are consistent with those from the decay of the emission in the same toluene solution, we attribute the transient absorbing excited state to the same excited states that emit. In view of the nature of the emitting states discussed in the previous section, the observed transient absorption for 1 and 2 should emanate from the diimine ligand L based 3

ILCT/3π,π* state, with more 3π,π* contribution in 2. Such attributions are supported by the

similar feature of these TA spectra to those of the ligand L with Zn2+ being added, which generated the 3ILCT/3π,π* absorption (the UV-vis absorption and TA spectra of L and L+Zn2+ are given in Supporting Information Figures S9 and the comparison of the TA spectra of 1 and 2 to those of L and L+Zn2+ are shown in S10).

24


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Page 25 of 41

0.04

0.04

a

b

0.02

0.00

∆ OD

∆ OD

0.02

1 2 3 4 5

-0.02 -0.04

0.00

0 µs 1.6 µs 3.2 µs 4.8 µs 6.4 µs

-0.02 -0.04

-0.06

-0.06

400

500

600

700

800

400

500

600

700

800

Wavelength (nm)

Wavelength (nm) 0.03 0.02

c

0 µs 1 µs 2 µs 3 µs 4 µs

d ∆ OD

∆ OD

0.02 0.00

0 µs 1 µs 2 µs 3 µs 4 µs

-0.02

400

500

600

700

0.01

0.00

-0.01

800

400

Wavelength (nm)

500

600

700

800

Wavelength (nm)

0 ns 800 ns 1600 ns 2400 ns 3200 ns

e

0.02

∆ OD

0.02

∆ OD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.01

0.00

0 ns 120 ns 240 ns 360 ns 480 ns

f

0.01

0.00 400

500

600

700

800

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

Figure 3. TA spectra at zero-time delay (a) and the time-resolved triplet TA spectra of 1-5 (b-f) in toluene solution. λex = 355 nm, A355 = 0.4 in a 1-cm cuvette.

For complexes 3-5, the TA signals were all positive from 360 nm to 800 nm. The spectral features and lifetimes resembled those of their corresponding dimers [Ir(C^N)2Cl]2 (The comparison of the TA spectra of the complexes and their respective dimers and the time-resolved TA spectra of the corresponding dimers for 3-5 are provided in Supporting Information Figures S10 and S11, respectively.). Thus, the transient absorbing excited states 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in 3-5 can be ascribed to the coordinated C^N ligand associated 3π,π*/3ILCT/3MLCT states. This assignment is further supported by the similar lifetimes obtained from the decay of the TA to those measured from the emission for 3-5, which suggests the TA and emission are from the same excited state and have the same nature. It is clear that the increased

π-conjugation of the C^N ligand shifted the lowest triplet excited state from the coordinated diimine ligand L based 3ILCT/3π,π* characters in 1 and 2 to the coordinated C^N ligand localized 3π,π*/3ILCT/3MLCT characters in 3-5. Reverse saturable absorption. As discussed in the previous section, complexes 1-5 all

possess stronger excited-state absorption than the ground-state absorption in the visible to the near-IR region (from ca. 430 nm to 800 nm), as reflected by the positive TA bands in this spectral region. Therefore, reverse saturable absorption (RSA) is expected to occur from these complexes under the nanosecond laser irradiation.

To demonstrate this, nonlinear

transmission experiment was carried out for complexes 1-5 in toluene solution in a 2-mm cuvette using 4.1 ns, 532 nm laser pulses. To ensure the same numbers of molecules being excited to the singlet excited state upon laser irradiation, the linear transmission of all solutions was adjusted to 80% at 532 nm in the 2-mm cuvette. Under such an identical excitation condition, the observed RSA difference should reflect the difference in the excited-state absorption, which is a result of the excited-state population and the excited–state absorption cross section. The transmission vs. incident energy curves for 1-5 are depicted in Figure 4. All of the complexes exhibited remarkable transmission decrease when the incident energy was increased, clearly indicating the occurrence of strong RSA. The strength of the RSA followed this trend: 1 > 3 > 2 > 4 > 5. 26


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0.8

Transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

1 2 3 4 5 1E-5

1E-4

1E-3

Incident Energy (J)

Figure 4. Transmittance vs incident energy curves for 1-5 in toluene for 4.1 ns laser pulses at 532 nm in a 2-mm cuvette. The linear transmission was adjusted to 80% for each sample in a 2-mm cuvette. The radius of the beam waist at the focal plane was approximately 96 µm.

Because the key parameter determining the strength of RSA is the ratio of the excited-state absorption cross section (σex) vs. the ground-state absorption cross section (σ0), it is important to estimate these parameters from the transient absorption spectra and the UV-vis absorption spectra of these complexes, respectively.

The σ0 values could be

deduced from the ε values at 532 nm obtained from the UV-vis absorption spectra using the conversion equation σ = 2303ε/NA (where NA is the Avogadro’s constant), which gave 1.53×10-19 cm2, 2.02×10-18 cm2, 2.02×10-18 cm2, 8.40×10-18 cm2, and 1.68×10-17 cm2 for complexes 1-5, respectively. The σex values of 1 and 2 could be estimated using the method described by our group previously.70 However, for complexes 3-5, there were no detectable bleaching bands on their TA spectra. Thus the excited-state molar extinction coefficients (εT) at 532 nm were unable to be obtained by the singlet depletion method41 and the procedure described by our group before.70 Nevertheless, the ∆OD values at 532 nm represent the absorptivity difference between the excited state and the ground state. Therefore, it is 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reasonable to use these values, i.e. 0.016 for 1, 0.012 for 2, 0.021 for 3, 0.011 for 4, and 0.017 for 5, along with their ground-state absorption cross sections at 532 nm to rationalize the observed RSA trend. Complex 1 has a moderate ∆OD value of 0.016 at 532 nm but the smallest σ0 value (1.53×10-19 cm2) at this wavelength, which is 1-2 orders of magnitude smaller than those of the other four complexes. Therefore, it is expected that this complex has the largest ratio of σex/σ0 and the strongest RSA at 532 nm. For complexes 2 and 3, although their σ0 values (2.02×10-18 cm2) at 532 nm are the same, complex 3 has the largest ∆OD value (0.021) among these five complexes and thus the comparable RSA to that of 1. With the increased π-conjugation of the C^N ligands in complexes 4 and 5, the red-shifted absorption tails caused a dramatic increase of the σ0 values (8.40×10-18 cm2 and 1.68×10-17 cm2 for 4 and 5, respectively) at 532 nm. On the other hand, their ∆OD values were comparable or smaller than the other three complexes. This would reduce the σex/σ0 values for these two complexes at 532 nm and weaken the RSA of these two complexes at 532 nm. Although the RSA of 4 and 5 at 532 nm is not very strong, considering their much broader ground-state absorption to the NIR region, it is reasonable to predict that these two complexes, especially complex 5, could potentially be a broadband reverse saturable absorber in the NIR region. Confirmation of this prediction would be conducted in the near future when the multiple wavelength laser source becomes available in our group.

CONCLUSIONS We reported the synthesis, photophysics and reverse saturable absorption of five cationic iridium(III) complexes with different π-conjugated C^N ligands. By gradually increasing the 28


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π-conjugation of the C^N ligands, the low-energy absorption bands in the UV-vis absorption spectra of complexes 1-5 were red-shifted. Especially the tails corresponding to the spin-forbidden 3CT transitions in 4 and 5 bathochromically shifted to the NIR region. More importantly, the extended π-conjugation of the C^N ligands changed the nature of the lowest-energy singlet transitions from the N^N ligand associated

1

MLCT/1LLCT

configurations to the C^N ligand based 1ILCT/1π,π*/1MLCT/1LLCT character. This switch also holds for the lowest triplet excited state (T1) for these complexes, as reflected by the drastically different characteristics of the emission and transient absorption (such as the shapes of the emission and TA spectra, the emission energies, lifetimes and quantum yields) for complexes 1 and 2 compared to those of complexes 3-5. The results from these photophysical studies clearly demonstrated that the nature of the lowest singlet and triplet excited states in the cationic Ir(III) complexes is controlled by the degree of π-conjugation of the coordinating core ligands. When the degree of π-conjugation of the core C^N ligands is larger than that of the core N^N ligand, the natures of the lowest singlet and triplet excited states could be switched to the C^N ligand based

1,3

CT/1,3π,π* transitions. This would alter

the photophysical properties of the Ir(III) complexes dramatically. As a consequence of the changes in the ground-state and excited-state absorption cross sections, the RSA strength of 1-5 under 532 nm nanosecond laser irradiation differed drastically with a trend of 1 > 3 > 2 > 4 > 5. Although it appeared that the increased π-conjugation of the C^N ligands in 4 and 5 decreased their RSA at 532 nm, the much red-shifted spin-forbidden 3CT transitions in 4 and 5 could potentially make these two complexes broadband reverse saturable absorbers. Further

29



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experiments to demonstrate the broadband RSA in the visible to the NIR region for complexes 4 and 5 are underway.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 1

H-NMR and high-resolution mass spectra (HRMS). Experimental and calculated UV-vis

absorption spectra. Natural transition orbitals (NTOs). Normalized UV-vis absorption and emission spectra in different solvents. Comparison of emission spectra and time-resolved TA spectra of 1-5 and dimers [Ir(C^N)2Cl]2. Time-resolved TA spectra of ligand L and L+Zn2+. Full author lists for References 38 and 42.

AUTHOR INFORMATION Corresponding Author *E-mail: Wenfang Sun ([email protected]); Phone: 701-231-6254

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS W. Sun acknowledges the financial support from the US Army Research Laboratory (W911NF-14-2-0081) for all of the experimental work and National Science Foundation (DMR-1411086 and CNS-1229316) for the computational work. S. Kilina acknowledges the 30


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US Department of Energy (DE-SC008446) for financial support and the Center for Computationally Assisted Science and Technology (CCAST) at North Dakota State University for computer access and administrative support.

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Table of Content Graphic

1 2 3 4 5

0.8

Transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

+

0.4

N C 8H17

C8H17 N

C

0.2

N

Ir

C

=

PF6-

N C 8H17

C8H17

N C N

N 1 (ppy)

N 2 (pqu)

N

N 3 (piq)

N

4 (pbq)

1E-5

5 (dpbq)

1E-4

Incident Energy (J)

41


1E-3

Toward Broadband Reverse Saturable Absorption - ACS Publications

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