Tuning the photophysics and reverse saturable absorption of heteroleptic cationic iridium(III) complexes via substituents on the 6,6′-bis(fluoren-2-yl)-2,2′-biquinoline ligand

Article abstract of DOI:10.1021/acs.inorgchem.6b02028

To understand the effects of the terminal substituent at the diimine ligand on the photophysics of heteroleptic cationic Ir(III) complexes and to obtain Ir(III) complexes with extended ground-state absorption to the near-IR region while retaining the long-lived and broadly absorbing triplet excited state, we synthesized three heteroleptic cationic iridium(III) complexes bearing cyclometalating 1-phenylisoquinoline (C^N) ligands and substituted 6,6′-bis(7-R-fluoren-2-yl)-2,2′-biquinoline (N^N) ligand (R = H, NO₂, or NPh₂). The photophysics of these complexes was systematically investigated via spectroscopic methods and time-dependent density functional theory. All complexes possess strong ligand-localized ¹π,π? transitions mixed with ligand-to-ligand charge transfer (¹LLCT)/metal-to-ligand charge transfer (¹MLCT) transitions below 400 nm, and a broad and featureless absorption band above 400 nm that arises from the N^N ligand-localized ¹π,π?/¹ILCT (intraligand charge transfer) transitions as well as the very weak ^1,3LLCT/^1,3MLCT transitions at longer wavelengths. The electron-withdrawing NO₂ substituent on the N^N ligand leads to a blue-shift of the ¹π,π?/¹ILCT absorption band, while the electron-donating NPh₂ substituent causes a pronounced red-shift of this band. The unsubstituted and NO₂-substituted complexes (complexes 1 and 2, respectively) are moderately emissive at room temperature (RT) in solution as well as at 77 K in the glassy matrix, while the NPh₂-substituted complex (3) is weakly emissive at RT, but the emission becomes much brighter at 77 K. Complexes 1 and 2 show very broad and strong triplet excited-state absorption from 460 to 800 nm with moderately long lifetimes, while complex 3 exhibits weak but broad absorption bands from 384 to 800 nm with a longer lifetime than those of 1 and 2. The nonlinear transmission experiment manifests that complexes 1 and 2 are strong reverse saturable absorbers (RSA) at 532 nm, while 3 shows weaker RSA at this wavelength. These results clearly demonstrate that it is feasible to tune the ground-state and excited-state properties of the Ir(III) complexes via the terminal substituents at the diimine ligand. By introducing the fluoren-2-yl groups to the 2,2′-biquinoline ligand to extend the diimine ligand π-conjugation, we can obtain Ir(III) complexes with reasonably long-lived and strongly absorbing triplet excited state while red-shifting their ^1,3LLCT/^1,3MLCT absorption band into the near-IR region. These features are critical in developing visible to near-IR broadband reverse saturable absorbers.

Full text of DOI:10.1021/acs.inorgchem.6b02028

Article
pubs.acs.org/IC
Tuning the Photophysics and Reverse Saturable Absorption of
Heteroleptic Cationic Iridium(III) Complexes via Substituents on the
6,6′-Bis(fluoren-2-yl)-2,2′-biquinoline Ligand
Xiaolin Zhu, Levi Lystrom, Svetlana Kilina, and Wenfang Sun*
Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States
S
* Supporting Information
ABSTRACT: To understand the effects of the terminal substituent at the
diimine ligand on the photophysics of heteroleptic cationic Ir(III) complexes
and to obtain Ir(III) complexes with extended ground-state absorption to the
near-IR region while retaining the long-lived and broadly absorbing triplet
excited state, we synthesized three heteroleptic cationic iridium(III) complexes
bearing cyclometalating 1-phenylisoquinoline (C^N) ligands and substituted
6,6′-bis(7-R-fluoren-2-yl)-2,2′-biquinoline (N^N) ligand (R = H, NO2, or
NPh2). The photophysics of these complexes was systematically investigated
via spectroscopic methods and time-dependent density functional theory. All
1
complexes possess strong ligand-localized π,π* transitions mixed with ligand-
to-ligand charge transfer (1LLCT)/metal-to-ligand charge transfer (1MLCT)
transitions below 400 nm, and a broad and featureless absorption band above
1
400 nm that arises from the N^N ligand-localized π,π*/1ILCT (intraligand
charge transfer) transitions as well as the very weak 1,3LLCT/1,3MLCT transitions at longer wavelengths. The electron-
1
withdrawing NO2 substituent on the N^N ligand leads to a blue-shift of the π,π*/1ILCT absorption band, while the electron-
donating NPh2 substituent causes a pronounced red-shift of this band. The unsubstituted and NO2-substituted complexes
(complexes 1 and 2, respectively) are moderately emissive at room temperature (RT) in solution as well as at 77 K in the glassy
matrix, while the NPh2-substituted complex (3) is weakly emissive at RT, but the emission becomes much brighter at 77 K.
Complexes 1 and 2 show very broad and strong triplet excited-state absorption from 460 to 800 nm with moderately long
lifetimes, while complex 3 exhibits weak but broad absorption bands from 384 to 800 nm with a longer lifetime than those of 1
and 2. The nonlinear transmission experiment manifests that complexes 1 and 2 are strong reverse saturable absorbers (RSA) at
532 nm, while 3 shows weaker RSA at this wavelength. These results clearly demonstrate that it is feasible to tune the ground-
state and excited-state properties of the Ir(III) complexes via the terminal substituents at the diimine ligand. By introducing the
fluoren-2-yl groups to the 2,2′-biquinoline ligand to extend the diimine ligand π-conjugation, we can obtain Ir(III) complexes
with reasonably long-lived and strongly absorbing triplet excited state while red-shifting their 1,3LLCT/1,3MLCT absorption band
into the near-IR region. These features are critical in developing visible to near-IR broadband reverse saturable absorbers.
readily tuned by utilizing different functionalized cyclometalat-
ing (C^N) ligands or diimine (N^N) ligands.27−30
INTRODUCTION
■
Octahedral Ir(III) complexes have attracted intense interest in
the past decade because of their versatile applications in organic
light-emitting devices (OLEDs),1−3 light-emitting electro-
chemical cells,4−6 photocatalysis,7 biosensing,8−12 nonlinear
optics,13−26 etc. These applications are intrinsically based on
the efficient intersystem crossing induced by the heavy Ir(III)
ion in these complexes that results in high phosphorescence
efficiency, the presence of multiple charge transfer transitions
that lead to broad charge-transfer absorption in the visible
spectral region, their electrochemical characteristics, and/or
high chemical and thermal stabilities. Recently, structural
modifications of Ir(III) complexes to obtain desirable optical
or electronic properties have been extensively developed,
especially for OLEDs or light-emitting electrochemical cell
applications. Among these Ir(III) complexes, cationic hetero-
leptic Ir(III) complexes have been the subject of interest
because their ground-state and excited-state properties can be
In addition to the extensive investigations for OLEDs or
light-emitting electrochemical cell applications, a growing
interest in utilizing Ir(III) complexes as nonlinear optical
materials has emerged. Roberto, Angelis, and co-workers
studied the second-order nonlinear optical properties of some
cationic Ir(III) complexes with substituted 1,10-phenanthroline
ligand and revealed that the substituents on the phenanthroline
ligand and the π-conjugation of the cyclometalating C^N ligand
influenced the second-order nonlinearities of the Ir(III)
complexes.13,14 Guerchais, Roberto, and co-workers also
reported that substitution of neutral Ir(ppy)3 (ppy = 2-
phenylpyridine) complexes with vinyl-aryl substituents at the
para-position of the pyridine rings could readily tune the
Received: August 20, 2016
© XXXX American Chemical Society
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second-order nonlinear optical response of these complexes.15
Schanze’s group discovered that a cationic Ir(III) complex with
4-ethynyl-N,N-dihexylaniline substituents on the N^N ligand
exhibited “dual mechanism” nonlinear absorption (i.e., reverse
saturable absorption (RSA) at 532 nm and two-photon initiated
excited-state absorption at 1064 nm).16 Zhao’s group
demonstrated the two-photon and three-photon absorption of
oligofluorene-substituted neutral Ir(ppy)3 complexes.17 In
recent years, our group has systematically investigated the
photophysics and reverse saturable absorption of cationic and
neutral Ir(III) complexes with π-conjugated substituents at the
N^N and/or C^N ligands.19−24 We also extended the π-
conjugation of the N^N or C^N ligands via benzannula-
tion.24−26 We found that incorporation of the π-conjugated
substituent to the N^N ligand via a single bond or triple bond
linkage significantly prolonged the triplet excited-state life-
time,19,21 while extending the π-conjugation of the N^N ligands
via benzannulation dramatically decreased the triplet lifetime,
although such a change red-shifted the ground-state charge
transfer absorption band(s) to the near-IR region.24,25 In
contrast, attaching the π-conjugated substituent to the C^N
ligand via a single or triple bond linkage induced more charge
transfer characters to the lowest triplet excited state (T1) and
thus decreased its lifetime,19,21 while benzannulation of the
C^N ligands could switch the T1 state to the C^N ligand
physics and reverse saturable absorption. We aim to not only
understand the effects of terminal substituent at the diimine
ligand on the photophysics of heteroleptic cationic Ir(III)
complexes but also obtain Ir(III) complexes with extended
ground-state absorption to the near-IR region while retaining
the long-lived and broadly absorbing triplet excited state. 2,2′-
Biquinoline (bqu) was chosen as the coordinating diimine
ligand because our previous work demonstrated that the Ir(III)
complex containing this ligand exhibited much red-shifted
charge transfer ground-state absorption band extending to
approximately 650 nm.25 1-Phenylisoquinoline (piq) was
selected as the C^N ligand because we have shown that the
Ir(III) complex bearing this ligand possessed relatively long-
lived triplet excited state and strong reverse saturable
absorption.26 Fluoren-2-yl motif was introduced to the 6,6′-
position of biquinoline ligand based on its proven effect on
increasing the triplet lifetime and broadening the triplet excited-
state absorption to the near-IR region due to the intraligand
charge transfer transition induced by its π-donating ability.21 To
explore the effect of the terminal substituent at the N^N ligands
on the photophysics and RSA, an electron-withdrawing NO2
substituent or electron-donating NPh2 substituent was
introduced to the 7-position of the fluoren-2-yl motif. The
systematic photophysical studies were carried out for 1−3 to
understand the structure−property correlations to develop
effective broadband reverse saturable absorbing materials.
Moreover, the UV−vis absorption and emission spectra of
1−3 were calculated by time-dependent density functional
theory (TDDFT) to better understand the nature of the optical
transitions of these complexes.
26
3
localized π,π* state and thus increased the T1 lifetime.
Because long-lived triplet excited state and broadband
ground-state absorption to the near-IR region are desirable
features for ideal broadband reverse saturable absorbers and
new generation photosensitizers for photodynamic therapy, it is
important to maintain the long-lived triplet excited state while
red-shifting the charge-transfer absorption bands. Our previous
studies found that benzannulation of the N^N ligand red-
shifted the ground-state charge transfer absorption band(s) to
the near-IR region but shortened the triplet lifetime,24,25 while
the triplet lifetime was increased by attaching π-conjugated
substituent to the N^N ligand.19,21 In addition, it has been
reported that substitution of the bipyridine ligand at its 4, 5, or
6 positions all influenced their triplet energies and lifetimes.29,30
However, there has been no report on the effect of the terminal
substituents at the π-conjugated substituents attached on the
N^N ligand.
EXPERIMENTAL SECTION
■
Synthesis and Characterization. All reagents were purchased
from Sigma-Aldrich or Alfa Aesar Co. Ltd. Tetrahydrofuran was
distilled under N2 over sodium benzophenone ketyl. All other reagents
were used as received. Silica gels (230−400 mesh) used for
chromatography were purchased from Sorbent Technology. The
1
intermediate compounds were characterized by H NMR spectrosco-
1
py, while the ligands and Ir(III) complexes were characterized by H
NMR, HRMS, and elemental analyses. 1H NMR spectra were obtained
on a Bruker 400 MHz spectrometer using CDCl3 as the solvent with
tetramethylsilane as internal standard. ESI-HRMS analyses were
performed on a Bruker BioTOF III mass spectrometer. Elemental
analyses were carried out by NuMega Resonance Laboratories, Inc. in
San Diego, California.
Therefore, in this work, we synthesized three heteroleptic
cationic Ir(III) complexes containing the substituted 6,6′-
bis(fluoren-2-yl)-2,2′-biquinoline ligand (1−3, structures
shown in Chart 1) and systematically studied their photo-
Scheme 1 illustrates the synthetic routes for ligands L1−L3 and
complexes 1−3. The synthesis of the precursors 8a−8c and 10
followed the literature procedures.26,31 The synthetic procedures and
characterization data for compounds 4−7, 9a−9c, L1−L3, and
complexes 1−3 are reported herewith.
2-Amino-5-bromobenzaldehyde (4). To a stirred solution of 2-
aminobenzaldehyde (363 mg, 3.0 mmol) in DMF (60 mL) was added
NBS (534 mg, 3.0 mmol). The solution was stirred at RT for 1 h. The
yellow solution was poured into water (200 mL) and then extracted
with CH2Cl2 (3 × 100 mL). The organic layer was washed with water
and then dried over MgSO4. The solvent was then removed under
reduced pressure to obtain yellow powder as the crude product, which
was used for the synthesis of compound 7 immediately without further
purification.
Chart 1. Molecular Structures of Complexes 1−3
6-Bromo-N-methoxy-N-methylquinoline-2-carboxamide (5). A
DMF (1 mL) solution of 6-bromoquinoline-2-carboxylic acid (2.0 g,
7.95 mmol), triethylamine (3.32 mL, 23.8 mmol), N,O-dimethylhy-
droxyamine hydrochloride (0.93 g, 9.50 mmol), and HBTU (3.31 g,
8.75 mmol) was stirred at RT for 2 h. The mixture was poured into
water, and the precipitate was collected by filtration. The crude
product was further purified by column chromatography (silica gel,
B
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Scheme 1. Synthetic Routes for L1−L3 and 1−3
hexane/ethyl acetate = 4/1, v/v) to obtain 2.1 g of orange solid (yield:
90%). 1H NMR (400 MHz, CDCl3) δ 8.18 (d, J = 8.1 Hz, 1H), 8.07−
8.01 (m, 2H), 7.86−7.82 (m, 1H), 7.80−7.69 (br, 1H), 3.81 (s, 3H),
3.48 (s, 3H).
diethyl ether. The organic layer was washed with brine and dried over
MgSO4. After removal of the solvent, the crude product was purified
by silica gel column chromatography to afford a pure product.
9a: Hexane/ethyl acetate (100/1, v/v) was used as the eluent to
afford a colorless oil (330 mg, yield: 80%). 1H NMR (400 MHz,
CDCl3) δ 7.80 (t, J = 7.2 Hz, 1H), 7.75 (d, J = 8 Hz, 1H), 7.68−7.63
(m, 2H), 7.35−7.33 (m, 1H), 7.30−7.23 (m, 2H), 2.02−1.91 (m, 4H),
1.34 (s, 12H), 0.88−0.63 (m, 22H), 0.49−0.41 (m, 8H).
9b: Hexane/ethyl acetate (100/1, v/v) was used as the eluent to
afford a yellow oil (242 mg, yield: 54%). 1H NMR (400 MHz, CDCl3)
δ 8.28 (d, J = 6.9 Hz, 2H), 7.90 (t, J = 8.3 Hz, 1H), 7.88−7.83 (m,
2H), 7.79 (d, J = 7.6 Hz, 1H), 2.14−2.01 (m, 4H), 1.39 (s, 12H),
0.91−0.67 (m, 22H), 0.57−0.40 (m, 8H).
9c: Hexanes/toluene (5/1, v/v) was used as the eluent to obtain a
light yellow oil (366 mg, yield: 66%). 1H NMR (400 MHz, CDCl3) δ
7.79 (t, J = 8.5 Hz, 2H), 7.63−7.61 (m, 2H), 7.27−7.23 (m, 4H),
7.13−7.05 (m, 6H), 7.03−6.99 (m, 2H), 2.04−1.98 (m, 2H), 1.87−
1.77 (m, 2H), 1.37 (s, 12H), 0.94−0.65 (m, 22H), 0.60−0.48 (m, 8H).
General Synthetic Procedure for L1−L3. Compounds 7 (124
mg, 0.30 mmol) and 9a−9c (0.66 mmol), Pd(PPh3)4 (35 mg, 0.03
mmol), and K2CO3 (248 mg, 1.80 mmol) were added to 18 mL of
mixed solvent toluene/ethanol/water (4/1/1, v/v/v). The mixture was
heated to reflux under nitrogen for 48 h. The reaction mixture was
then cooled to RT and extracted with CH2Cl2. The organic layer was
washed with brine and dried over Na2SO4. After removal of the
solvent, the crude product was purified by column chromatography
(silica gel) to afford pure product.
1-(6-Bromoquinolin-2-yl)ethanone (6). To a solution of 6-bromo-
N-methoxy-N-methylquinoline-2-carboxamide (2.14 g, 7.28 mmol) in
THF (60 mL) was added methyl magnesium bromide (9.1 mL, 8.7
mmol, 1 M in THF solution) at 0 °C dropwise, and the mixture was
stirred at 0 °C for 1 h. The mixture was then poured into water and
extracted with ethyl acetate. The organic layer was washed with brine
and dried over MgSO4. After removal of the solvent, the crude product
was purified by column chromatography (silica gel, hexane/ethyl
1
acetate = 4/1, v/v) to obtain a white solid (1.7 g, yield: 94%). H
NMR (400 MHz, CDCl3) δ 8.21−8.02 (m, 4H), 7.89−7.83 (m, 1H),
2.67 (s, 3H).
6,6′-Dibromo-2,2′-biquinoline (7). Compounds 4 and 6 (700 mg,
2.8 mmol) were dissolved in ethanol (20 mL), and the solution was
purged with N2. Saturated ethanolic KOH (20 mL) was then added
dropwise, and the mixture was heated to reflux overnight. After being
cooled to RT, the mixture was poured into water. The precipitate was
collected by filtration and washed with CH2Cl2 (50 mL) to obtain the
crude product. The crude product was further purified by column
chromatography (silica gel, hexane/ethyl acetate = 8/1, v/v) to obtain
1
a pale yellow powder (670 mg, yield: 64%). H NMR (400 MHz,
CDCl3) δ 8.87 (d, J = 8.5 Hz, 2H), 8.26 (d, J = 8.5 Hz, 2H), 8.11 (d, J
= 9.0 Hz, 2H), 8.08 (s, 2H), 7.85 (d, J = 8.5 Hz, 2H).
General Synthetic Procedure for 9a−9c. Compounds 8a−8c
(0.8 mmol), bis(pinacolato)diboron (304 mg, 1.2 mmol), Pd(dppf)Cl2
(32 mg, 0.04 mmol), and KOAc (196 mg, 2.0 mmol) were added to 15
mL of dry DMF. The mixture was heated to 90 °C under nitrogen
overnight. Then, the mixture was poured into water and extracted with
L1: Hexane/ethyl acetate (200/1, v/v) was used as the eluent to
obtain a light yellow solid (114 mg, yield: 46%). 1H NMR (400 MHz,
CDCl3) δ 8.94 (d, J = 8.6 Hz, 2H), 8.44 (d, J = 8.6 Hz, 2H), 8.35 (d, J
= 8.6 Hz, 2H), 8.13−8.10 (m, 4H), 7.85 (d, J = 7.8 Hz, 2H), 7.79−
C
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7.75 (m, 6H), 7.45 (d, J = 7.1 Hz, 2H), 7.37 (t, J = 7.3 Hz, 2H), 7.33
(t, J = 7.3 Hz, 2H), 2.14−2.06 (m, 8H), 1.00−0.57 (m, 60H).
Elemental Analysis Calcd (%) for C76H92N2: C, 88.32; H, 8.97; N,
2.71. Found: C, 88.64; H, 8.60; N, 2.92.
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.
L2: Hexane/ethyl acetate (100/1, v/v) was used as the eluent to
The singlet depletion method35 was used to determine the triplet
excited-state molar extinction coefficients (εT1−Tn) at the TA band
maximum. After the εT1−Tn value was obtained, the triplet excited-state
quantum yield was calculated by the relative actinometry36 in which
SiNc in benzene was used as the reference (ε590 = 70 000 M−1 cm−1,
ΦT = 0.20).37
1
obtain 97 mg of a yellow powder (yield: 40%). H NMR (400 MHz,
CDCl3) δ 8.97 (d, J = 8.6 Hz, 2H), 8.45 (d, J = 8.6 Hz, 2H), 8.38−8.33
(m, 6H), 8.15 (s, 2H), 8.10 (d, J = 8.7 Hz, 2H), 7.95 (d, J = 7.8 Hz,
2H), 7.90−7.83 (m, 6H), 2.24−2.13 (m, 8H), 0.92−0.70 (m, 32H),
0.68−0.55 (m, 28H). Elemental Analysis Calcd (%) for C76H90N4O4·
0.6CH2Cl2: C, 78.33; H, 7.83; N, 4.77. Found: C, 78.01; H, 7.73; N,
4.94.
Computational Methods. All calculations, including ground state
geometry optimization and excited state calculations, were performed
using Gaussian 09 software package.38 The ideal octahedral geometries
of complexes 1−3 were optimized in their singlet and triplet spin
configurations by DFT using hybrid Perdew, Burke, and Ernzerhof
functional (PBE1PBE)39−41 and LANL2DZ basis set42−44 assigned for
Ir(III) ion and 6-31g* basis set45−49 for all the remaining atoms. All
calculations were done in dichloromethane solvent within the
conductor-like polarizable continuous model (CPCM).50,51 To reduce
the computational cost, the 2-ethylhexyl substituents on the fluorenes
were reduced to methyl groups. This change does not affect the
electronic levels contributing to absorption and emission in the visible
spectral range.
Absorption spectra of ligands L1−L3 and complexes 1−3 were
obtained using TDDFT52,53 utilizing the same functional and basis set
used in the ground state calculations. To obtain the spectrum, the
lowest 60 optical transitions were calculated and broadened by a
Gaussian distribution with the line width of 0.1 eV, corresponding to
the thermal broadening of the experimental spectra. Although the
calculated absorption spectra qualitatively agree with the experimental
ones, they are systematically red-shifted (see Figure S7 in the
Supporting Information). To better match the experimental data, the
percentage of Hartree−Fock (HF) exchange in the PBE1PBE
functional was increased from 25 to 40%. It is well-known that the
portion of the orbital exchange used in the density functional changes
the optical gap, i.e., the larger the HF exchange portion, the larger the
energy gap. However, the spectral features remain nearly the same,
independent of the hybrid functionals.54 Such a tunability of
functionals allows for matching the calculated and experimental
spectra better. The calculated spectra with 32% HF were found to
match the experimental spectra the best.
Calculations of the emission energies were carried out using
analytical TDDFT gradients55,56 by optimizing either the lowest
singlet (fluorescence) or triplet (phosphorescence) state as
implemented in the Gaussian09 software. The same functional, basis
set, and solvent model used for the ground state TDDFT calculations
were used for the emission calculations as well. For optimization of the
excited state, we started with several different initial guesses for the
wave function, choosing either the very lowest triplet state (root = 1)
or the second lowest triplet state (root = 2). Both guessing states were
initially taken at the ground state geometry with the triplet spin
configuration, where their electronic wave functions were calculated by
regular TDDFT procedure. By starting with different initial wave
functions, we were able to converge to the triplet states that are in a
better agreement with experimental emission energies.
The nature of optical transitions was classified based on analysis of
natural transition orbitals (NTOs)57 implemented in Gaussian09
software. NTOs redistribute the electron density obtained from the
transition density matrix for the hole (occupied) and electron
(unoccupied) while presenting many-body excited states. To visualize
these NTOs, the isosurface of 0.02 was used to obtain a better
representation of excited orbitals.
L3: Hexane/ethyl acetate (98/2, v/v) was used as the eluent to
afford a yellow oil that slowly became solidified into a yellow solid (48
1
mg, yield: 33%). H NMR (400 MHz, CDCl3) δ 8.93 (d, J = 8.6 Hz,
2H), 8.42 (d, J = 8.6 Hz, 2H), 8.33 (d, J = 8.6 Hz, 2H), 8.11−8.09 (m,
4H), 7.78−7.74 (m, 6H), 7.68−7.65 (m, 2H), 7.30−7.26 (m, 8H),
7.18−7.11 (m, 12H), 7.05−7.02 (m, 4H), 2.12−1.87 (m, 8H), 1.28−
0.82 (m, 40H), 0.80−0.63 (m, 20H). Elemental Analysis Calcd (%) for
C100H110F6N4·3.5CH2Cl2: C, 74.65; H, 7.08; N, 3.36. Found: C, 74.95;
H, 6.79; N, 3.62.
General Synthetic Procedure for Complexes 1−3. To a stirred
solution of L1−L3 (0.078 mmol) and 10 (50 mg, 0.039 mmol) in
degassed CH2Cl2 (30 mL) and methanol (15 mL) was added
AgSO3CF3 (20 mg, 0.078 mmol). The mixture was refluxed overnight
under nitrogen. After being cooled to RT, 10-fold NH4PF6 was added.
The suspension was stirred at RT for 2 h. After removal of the solvent,
the crude product was purified by silica gel column chromatography.
Complex 1. CH2Cl2/ethyl acetate (50/1, v/v) was used as the
eluent to afford a red solid (65 mg, yield: 50%). 1H NMR (400 MHz,
CDCl3) δ 8.90−8.84 (m, 4H), 8.73 (d, J = 8.8 Hz, 2H), 8.26 (d, J = 8.1
Hz, 2H), 8.07 (s, 2H), 7.92−7.73 (m, 14H), 7.60 (s, 2H), 7.55 (d, J =
7.9 Hz, 2H), 7.42−7.32 (m, 10H), 7.14−7.12 (m, 2H), 6.93 (t, J = 7.5
Hz, 2H), 6.52 (d, J = 7.7 Hz, 2H), 2.07−2.04 (m, 8H), 0.93−0.89 (m,
60H). ESI-HRMS (m/z): calcd for [C106H112IrN4]+, 1633.8529;
found, 1633.8558. Elemental Analysis Calcd (%) for C106H112F6IrN4P:
C, 71.56; H, 6.34; N, 3.15. Found: C, 71.26; H, 6.32; N, 3.01.
Complex 2. CH2Cl2/ethyl acetate (50/1, v/v) was used as the
eluent to afford a red solid (61 mg, yield: 45%). 1H NMR (400 MHz,
CDCl3) δ 8.92−8.85 (m, 4H), 8.77 (dd, J = 8.6 and 3.5 Hz, 2H),
8.33−8.26 (m, 6H), 8.11 (s, 2H), 7.95−7.81 (m, 8H), 7.80−7.72 (m,
6H), 7.71−7.66 (m, 4H), 7.40 (d, J = 7.5 Hz, 2H), 7.34 (d, J = 6.5 Hz,
2H), 7.17−7.12 (m, 2H), 6.94 (t, J = 7.5 Hz, 2H), 6.51 (d, J = 7.7 Hz,
2H), 2.14−2.07 (m, 8H), 0.92−0.66 (m, 40H), 0.55−0.45 (m, 20H).
ESI-HRMS (m/z): calcd for [C106H110IrN6O4]+, 1723.8230; found,
1723.8232. Elemental Analysis Calcd (%) for C106H110F6IrN6O4P: C,
68.11; H, 5.93; N, 4.50. Found: C, 67.95; H, 5.67; N, 4.58.
Complex 3. Purified by column chromatography twice (CH2Cl2/
ethyl acetate = 20/1, v/v) to afford a red solid (23 mg, yield: 25%). 1H
NMR (400 MHz, CDCl3) δ 8.86 (t, J = 8.9 Hz, 4H), 8.71 (d, J = 8.1
Hz, 2H), 8.25 (d, J = 7.2 Hz, 2H), 8.05 (s, 2H), 7.92−7.52 (m, 20H),
7.39 (d, J = 9.0 Hz, 2H), 7.32−7.25 (m, 10H), 7.13−6.91 (m, 18H),
6.51 (d, J = 7.7 Hz, 2H), 2.04−1.29 (m, 8H), 0.97−0.54 (m, 60H).
ESI-HRMS (m/z): calcd for [C130H130IrN6]+, 1967.9986; found,
1967.9932. Elemental Analysis Calcd (%) for C130H130F6IrN6P·
0.5CH2Cl2: C, 72.70; H, 6.12; N, 3.90. Found: C, 72.61; H, 6.13;
N, 3.81.
Photophysical Measurements. All of the solvents used for
photophysical studies were spectrophotometric grade and purchased
from Alfa Aesar Co. Ltd. A Shimadzu UV-2501 spectrophotometer
and an HORIBA Fluoro-Max 4 fluorometer/phosphorometer were
used to measure the UV−vis absorption spectra and emission spectra,
respectively, in different solvents. The emission quantum yields were
determined by the relative actinometry method32 in degassed solutions
in which a degassed CH3CN solution of [Ru(bpy)3]Cl2 (Φem = 0.097,
λex = 436 nm)33 was used as the reference for 1−3 and a 1 N sulfuric
acid solution of quinine bisulfate (Φem = 0.546, λex = 347.5 nm)34 was
used as the reference for ligands L1−L3. The nanosecond transient
difference absorption (TA) spectra and decays were measured in
degassed toluene solutions on an Edinburgh LP920 laser flash
Nonlinear Transmission Experiment. The nonlinear trans-
mission experiments at 532 nm for complexes 1−3 were carried out
in toluene solutions in a 2 mm cuvette using 4.1 ns laser pulses. The
linear transmission of 1−3 in toluene was adjusted to 80% in the 2 mm
cuvette at 532 nm. A Quantel Brilliant ns laser with a repetition rate of
10 Hz was used as the light source. The experimental setup and details
were described previously.58,59 The beam radius at the focal point was
approximately 96 μm.
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absorption spectra of L1−L3 in CH2Cl2 possess two intense
absorption bands at 270−320 nm and 350−450 nm. On the
basis of the large molar extinction coefficients of these
absorption bands, the minor solvatochromic effect (Supporting
Information, Figure S12), and the calculated natural transition
orbitals (NTOs) (Supporting Information, Table S1), these
RESULTS AND DISCUSSION
■
Geometry Optimization. The optimized geometries of 1−
3 in CH2Cl2 are shown in Figure S8 of the Supporting
Information. After optimization, the fluorenyl substituents
attached at the 6,6′-position of 2,2′-biquinoline are twisted
from the quinoline rings by an angle of ca. 35° (which are
similar to the angles in complexes with fluorenyl-substituted
phenanthroline ligand21), while the angles between the two
quinoline rings are around 24−25°. Upon geometry
optimization, the propeller shaped diphenylamino substituents
at the 7-position of fluorene twist from the fluorene plane at a
dihedral angle of ca. 42° in 3. In contrast, the nitro groups in 2
are coplanar with the fluorene groups. For the C^N ligand, the
isoquinoline rings twist from the phenyl rings at ca. 17° for all
three complexes.
Electronic Absorption. The UV−vis absorption of L1−L3
and 1−3 follows Beer’s law in CH2Cl2 in the concentration
range studied (5 × 10−6 to 1 × 10−4 mol L−1), suggesting that
no aggregation occurs within this concentration range. The
experimental and calculated UV−vis absorption spectra of L1−
L3 and 1−3 in CH2Cl2 are presented in Figure 1 and Figures
1
bands are characterized as the predominant π,π* transitions.
For L3, which contains the strong electron-donating NPh2
substituents, intramolecular charge transfer transition from the
NPh2-fluorenyl components to the biquinoline component also
makes some contributions (see NTO of S1 for L3 in Table S1).
In comparison to L1, which contains no terminal substituent,
both the electron-withdrawing substituent (NO2) in L2 and
electron-donating substituent (NPh2) in L3 cause a red-shift of
the 350−450 nm absorption band due to the delocalization of
the electron density to these substituents, which extends the π-
conjugation in these molecules and thus induces the red-shift.
As depicted in Figures 1b and 1c, both the experimental
spectra and TDDFT calculations show that complexes 1−3
possess 2−3 intense absorption bands from 300 to 500 nm (the
band extends to 600 nm for 3) as well as very weak and broad
tails at the longer wavelengths. The overall trends revealed in
the experimental data are well reproduced in the theoretical
spectra. In comparison to the absorption spectra of L1−L3, the
major absorption bands of 1−3 become broader and red-
shifted. Considering the large molar extinction coefficients and
the NTOs representing the major transitions contributing to
these bands (Tables 2 and 3), we attribute the intense high-
energy absorption band(s) between 300 and 400 nm for 1−3
1
to the N^N and/or C^N ligand-localized π,π* transitions
mixed with some ligand-to-ligand charge transfer (1LLCT)/
metal-to-ligand charge transfer (1MLCT) characters (see Table
3). For complex 3, intraligand charge transfer (1ILCT,
π(diphenylaminofluorene) → π*(bqu)) transition also makes
a major contribution to this band in addition to the
1LLCT/1MLCT/1π,π* transitions. In contrast, the broad
band at 400−500 nm for 1 and 2 mainly arises from the
N^N ligand-based 1π,π* transitions mixed with 1ILCT
(π(fluorene) → π*(bqu)) transitions (see NTOs for S2 state
of 1 and S3 state of 2 in Table 2). For complex 3, the
corresponding band at 450−600 nm is dominated by the
1ILCT (π(diphenylaminofluorene) → π*(bqu)) transition with
a minor contribution from the bqu 1π,π* transition (see NTOs
for S1 state of 3 in Table 2).
Figure 1. Experimental UV−vis absorption spectra of L1−L3 (a) and
1−3 (b) and the calculated spectra of 1−3 with 32% HF (c) in
CH2Cl2. The inset in panel b shows the expanded experimental spectra
between 500 and 750 nm.
S9−S11 of the Supporting Information, and their photophysical
parameters are listed in Table 1. As depicted in Figure 1a, the
Table 1. Experimental Photophysical Data for L1−L3 and 1−3
b
c
d
λem (nm) [τem (μs)], Φem
λem (nm) [τem (μs)]
λT1‑Tn (nm) [εT1‑Tn (104 L mol−1 cm−1), τT (μs)],
a
λ (nm) [ε (104 L mol−1 cm−1)]
RT
77 K
ΦT
L1 285 (7.97), 370 (10.5)
437 (-), 0.94
501 (-), 0.05
560 (-), 0.16
657 (1.02), 0.13
697 (8.99, 62), 0.048
730 (9.34, 41), 0.24
520 (11.4, 12), 0.072
L2 275 (6.89), 385 (13.1)
L3 315 (7.96), 383 (7.19)
1
2
3
319 (11.6), 439 (5.07), 550 (0.167)
633 (2.85), 693 (2.78) 349 (1.46, 0.73), 514 (3.63, 0.88), 730 (3.00, 0.88),
0.44
296 (7.95), 355 (7.79), 420 (7.45),
550 (0.165)
658 (0.72), 0.10
608 (-), 0.002
629 (3.49), 688 (3.25) 555 (3.63, 0.39), 755 (1.21, 0.39), 0.17
296 (7.71), 366 (7.52), 515 (2.19),
600 (0.390)
644 (3.08), 701 (3.02) 585 (0.72, 3.96), 0.11
a
b
Room temperature electronic absorption band maxima and molar extinction coefficients in CH2Cl2. Room temperature emission energy, lifetime,
c
d
and quantum yield in CH2Cl2. Emission energy at 77 K measured in 2-methyltetrahydrofuran glassy matrix, c = 1 × 10−5 mol/L. Nanosecond 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).
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Table 2. NTOs) Representing Transitions Contributing to the Low-Energy Absorption Bands of 1−3 in CH2Cl2
Table 3. NTOs Representing Transitions Contributing to the Main Absorption Bands at 300−400 nm for 1−3 in CH2Cl2
In addition to these major absorption bands, all complexes
exhibit weak absorption in the region of 500−650 nm for 1 and
2, extending to 700 nm for 3. With reference to the other
Ir(III) complexes that exhibit such tails24,25,27,60,61 and in view
of the NTOs of S1 states for 1 and 2 in Table 2, we ascribe
these tails to 1,3LLCT/1,3MLCT transitions.
Photoluminescence. The emission characteristics of L1−
L3 and 1−3 were investigated in different solvents at room
temperature, and the normalized emission spectra in CH2Cl2
are shown in Figure 2. The emission data are summarized in
Tables 1 and 4, and in Table S3 (Supporting Information). As
we anticipated, incorporation of different functionalized
fluorene motifs to the biquinoline core prominently tuned
the emission of L1−L3 from the blue emission (437 nm in L1)
to the yellow emission (560 nm in L3) in CH2Cl2. As depicted
in Figure 3 for L1 and L3 and in Figure S13 for L2, the
featureless emission and the positive solvatochromic effect
suggest that the emitting state in CH2Cl2 can be assigned to the
1π,π*/1ILCT state for L1 and L2 and predominantly the 1ILCT
state for L3. This character of transition is confirmed by NTOs
corresponding to the emitting S1 states (Table S4 of the
Supporting Information). However, the emission of L1 and L2
in less polar solvents such as hexane and toluene exhibited
salient vibronic structures (Figures 3 and S13), suggesting
It is worthy to note that introducing electron-withdrawing
NO2 substituent to the fluorenyl motif at the N^N ligand leads
1
to a blue-shift of the ILCT/1π,π* absorption band in 2 with
respect to that in 1, whereas the electron-donating NPh2
1
substituent causes a pronounced red-shift of the ILCT band
in 3. This trend clearly reflects the charge-transfer nature of this
band. The NO2 group reduces the electron-donating ability of
the fluorene component and thus induces the blue-shift of the
1ILCT/1π,π* band, while NPh2 group increases the electron
density on fluorene and thus its electron-donating ability,
resulting in a stronger ILCT and a red-shift of the 1ILCT band
in comparison to that in 1.
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Figure 2. Normalized emission spectra of L1−L3 (λex = 347.5 nm)
and 1−3 (λex = 436 nm) in CH2Cl2 solutions at RT.
Table 4. Emission Data for Complexes 1−3 in Different
a
Solvents at RT
hexane/
toluene
(1/1)
λem (nm)
b
[τem(ns), Φem ],
toluene
CH2Cl2
acetone
acetonitrile
1
2
3
628 (810,
0.11)
634 (930, 657 (1000, 0.13) 658 (360, 658 (600,
0.12)
0.076)
0.065)
631 (170,
0.063)
636 (370, 658 (600, 0.10)
0.083)
660 (360, 659 (480,
0.044)
0.037)
c
603 (-,
0.002)
608 (-,
0.009)
608 (-, 0.002)
611 (-,
0.001)
624 (-,
0.001)
a
b
The excitation wavelength was 436 nm. The reference used for the
Figure 3. Normalized emission spectra of L1 and L3 in different
solvents at RT (λex = 347.5 nm).
quantum yield measurement was [Ru(bpy)3]Cl2 in degassed CH3CN
solution (Φem = 0.097, λex = 436 nm). The emission signals were too
weak to allow for the lifetimes to be accurately measured.
c
Complexes 1−3 are emissive at 77 K. Comparison of the
emission spectra in the 2-methyltetrahydrofuran (MTHF)
matrix at 77 K and at RT is shown in Figures 4 and S14. At 77
dominant 1π,π* characters. The increased charge transfer
character in more polar solvents is also reflected by the
drastically decreased emission quantum efficiency, as being
represented by L3 (see Table S3 of the Supporting
1
Information), in which ILCT is the dominant feature in its
emitting state. With the increased electron-donating ability of
the substituent on fluorene motifs, the 1ILCT character
obviously increases.
Compared to that of the three ligands, the emission of the
complexes is significantly red-shifted to ca. 660 nm for 1 and 2
and ca. 610 nm for 3 in degassed CH2Cl2. Considering the large
red-shifts of the emission with respect to their corresponding
excitation wavelengths, the relatively long emission lifetimes
(Table 1), and that it is prone to oxygen quenching, we assign
the emission of complexes 1−3 as phosphorescence. As shown
in Figure 2 and Table 1, the emission energy of 1 and 2 are
almost the same, along with similar emission lifetimes (ca. 1000
ns for 1 and 720 ns for 2). These features imply that the
emission of 1 and 2 originates from the same excited state that
is independent of the terminal substituent at the N^N ligand.
Taking into account the minor negative solvatochromic effect
(Figure S13) and the calculated NTOs (see Table S4 of the
Supporting Information), we assign the emission of 1 and 2 as
3π,π*/3MLCT/3ILCT states. In contrast, complex 3 containing
the electron-donating NPh2 substituents exhibits a very weak
emission with a nonmeasurable lifetime. These features suggest
the predominant charge transfer nature of the emitting state for
3. With reference to the NTOs, such a state could be assigned
Figure 4. Emission spectra of 3 in deaerated MTHF at RT and 77 K.
The inset includes the photographic images of 3 in aerated MTHF
under UV light (365 nm) irradiation at (a) RT and (b) 77 K.
K, the emission spectra of these complexes become structured
and narrower, and the emission is brighter and longer-lived
(Figure 4 and Table 1). The thermally induced Stokes shifts for
complexes 1−3 are 484, 537, and 865 cm−1, respectively, which
is in line with their 3π,π*/3CT nature of the emitting states (for
complex 3, the larger shift indicates a major 3CT contribution).
In comparison to the reported Ir(III) complex that contains
the unsubstituted bqu and piq ligands (i.e., complex 4 in ref
27), introduction of the π-conjugated fluorenyl substituents to
3
as the ILCT/3π,π* state.
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Figure 5. Time-resolved nanosecond TA spectra of L1−L3 and 1−3 in toluene solution. λex = 355 nm; A355 = 0.4 in a 1 cm cuvette.
the bqu ligand causes a significant increase in the triplet lifetime
(1000 ns for complex 1 in this work vs 490 ns for complex 4 in
ref 27) and emission quantum yield (0.13 for complex 1 in this
work vs 0.018 for complex 4 in ref 27) enhancement. Such
changes are related to the alternation of the T1 state parentage
much but only slightly decreases the emission lifetime and
quantum yield. In contrast, electron-donating NPh2 substituent
changes the nature of the emitting state and thus affects the
emission energy, lifetime, and quantum yield distinctively.
Transient Absorption (TA). To further understand the
substituent effect on the triplet excited-state characteristics, the
excited-state absorption of 1−3 and L1−L3 was investigated.
The time-resolved nanosecond TA spectra recorded upon
excitation at 355 nm at room temperature in degassed toluene
solution are shown in Figure 5.
3
upon fluorenyl substitution, i.e., changing the MLCT/3LLCT
3
parentage for complex 4 in ref 27 to the π,π*/3CT parentage
of the T1 state for complex 1 in this work. This is consistent
with that observed in other Ir(III) complexes that contain π-
conjugated substituents at the bipyridine or phenanthroline
ligand.19,21 It is worthy of noting that electron-withdrawing
NO2 substituent does not influence the emission energy very
As shown in Figure 5, all three ligands exhibit positive TA
band at 420−800 nm and bleaching bands below 400 nm that
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are in line with their 1π,π* absorption bands. The TA spectrum
of L1 features broad absorption bands between 407 and 772
nm, while L2 possesses a more intense and red-shifted band
from 414 to 800 nm. For L3, its TA band maximum appears at
516 nm, and its intensity is weaker and shape is different from
those of the other two ligands. Considering the very long TA
lifetime for L1 (62 μs) and L2 (41 μs) and their similar spectral
feature, we attribute the observed TA of these two ligands to
3π,π* state absorption. L3 has a much more blue-shifted TA
and a much shorter lifetime (12 μs), suggesting the
3
involvement of significant ILCT contribution.
Resembling the similarity of the TA spectra of their
corresponding ligands L1 and L2, the spectra of 1 and 2 are
similar in that they both exhibit a bleaching band between 380
and 460 nm and two broad and strong absorption bands at
470−800 nm for 1 and 460−800 nm for 2. Especially for
complex 1, its TA band extends beyond 800 nm. The lifetimes
obtained from the decay of their TA spectra (Table 1) are
consistent with those obtained from the decay of emission
(Table 4), suggesting that the TA of 1 and 2 likely emanates
from the same excited state that emits. Thus, the observed TA
Figure 6. Nonlinear transmission curves for complexes 1−3 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. The radius of the beam waist at the linear focal plane was
approximately 96 μm.
3
from 1 and 2 is tentatively ascribed to π,π*/3CT in nature.
displays the nonlinear transmission vs incident energy plots for
complexes 1−3 at 532 nm. When the incident energy was
increased, the transmission of the solutions decreased. This
clearly indicates the occurrence of RSA with a trend of 1 ≈ 2 ≫
3. The transmission of 1 and 2 decreased to ∼18% at the
incident energy of ∼460 μJ, which is among the strongest
organometallic reverse saturable absorbers for ns laser pulses at
532 nm.20−26,31,58,59,68,69 This trend clearly manifests that the
strong electron-donating NPh2 substituent on the biquinoline
ligand in complex 3 dramatically decreases the RSA at 532 nm.
Such a decrease should be attributed to the significantly
increased ground-state absorption cross section at 532 nm for 3
compared to that of the other two complexes. Because the
strength of RSA is primarily determined by the σex /σ0 ratio,
these key parameters at 532 nm were estimated based on the
measured UV−vis absorption spectra, the nanosecond TA
spectra at zero delay, and the procedure described by our group
before.70 The values of the σex/σ0 ratio in Table 5 correlate with
Complex 3 exhibits a bleaching band at ca. 366 nm and weak
but broad absorption bands from 384 to 800 nm. Interestingly,
the lifetime deduced from the decay of TA for 3 is much longer
than those of 1 and 2. Although the emission lifetime of 3 was
unable to be reliably measured due to the very weak signal,
which prevents us from determining whether the transient
absorbing excited state in 3 is the excited state that emits, the
long-lived transient absorption suggests that the observed TA
3
from 3 likely emanates from its π,π*/3CT state(s). It appears
that both the terminal electron-withdrawing NO2 group and
electron-donating NPh2 substituent on the diimine ligand
reduce the TA intensity, and especially the effect of NPh2 is
more dramatic.
Reverse Saturable Absorption (RSA). RSA is a nonlinear
absorption phenomenon in which the absorptivity of the
material increases with the increased incident light fluence. RSA
has important applications in optical switching,62 laser mode
locking,63 optical pulse shaping,64 spatial light modulation,64,65
and laser beam compression and limiting,66,67 etc. To realize
RSA, the material must have weak ground-state absorption at
the interested wavelength to populate the excited state via one-
photon absorption; meanwhile, the excited-state absorption at
the same wavelength should be stronger than that of the
ground state, i.e., the ratio of the excited-state absorption cross
section (σex) vs ground-state absorption cross section (σ0) at
the RSA wavelength must be larger than 1. The larger the σex/
σ0 ratio is, the stronger the RSA appears. For broadband RSA, it
is expected that the σex/σ0 ratios should be larger than 1 in the
spectral range of 400−900 nm. In addition, for RSA of
nanosecond (ns) and longer pulse width laser beams, triplet
excited-state absorption plays the dominant role. Thus, a high
triplet excited-state quantum yield and long triplet lifetime
would enhance the RSA.
Table 5. Ground-State and Excited-State Absorption Cross
Sections of Complexes 1−3 in Toluene
complex
1
2
3
σ0 (10−18 cm2)
σex (10−18 cm2)
σex/σ0
7.53
174
23.1
6.86
195
28.4
70.1
221
3.15
the observed RSA trend very well. It appears that introducing
an electron-withdrawing NO2 substituent on N^N ligands leads
to a slightly decreased ground-state absorption and a slightly
increased excited-state absorption at 532 nm compared to the
that of the unsubstituted complex 1, while the electron-
donating NPh2 substituent dramatically decreases the σex/σ0
ratio and results in a weaker RSA at 532 nm.
On the basis of the positive TA signals of 1−3 in the region
of 470−800 nm and their triplet lifetimes, we envision that
complexes 1−3 can exhibit RSA under ns laser irradiation. To
demonstrate this, nonlinear transmission experiments were
conducted at 532 nm using 4.1 ns laser pulses for 1−3 in
toluene solutions. For convenience of comparison, the
solutions of all complexes were prepared to obtain an identical
80% linear transmission in a 2 mm cuvette at 532 nm. Figure 6
CONCLUSIONS
■
We synthesized and investigated the photophysics of three
heteroleptic cationic iridium(III) complexes (1−3) bearing
cyclometalating 1-phenylisoquinoline (C^N) ligands and
terminally substituted 6,6′-bis(7-R-fluoren-2-yl)-2,2′-biquino-
line (N^N) ligands (R = H, NO2, or NPh2) to understand
the effects of terminal substituent at the diimine ligand. We also
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obtained Ir(III) complexes with extended ground-state
absorption to the near-IR region while retaining the long-
lived and broadly absorbing triplet excited state. We found that
introducing the 7-R-fluoren-2-yl motifs to the 2,2′-biquinoline
AUTHOR INFORMATION
Corresponding Author
*E-mail: Wenfang.Sun@ndsu.edu; Telephone: 701-231-6254;
Fax: 701-231-8831.
■
1
ligand caused a new π,π*/1ILCT band between 400 and 500
Notes
nm for 1 and 2 and between 450 and 600 nm for 3 in addition
to the 1π,π*/1LLCT/1MLCT transitions below 400 nm and the
very weak but broad 1,3LLCT/1,3MLCT band at 500−650 nm
for 1 and 2 and 600−700 nm for 3. The electron-withdrawing
terminal substituent NO2 induced a blue-shift of the
1π,π*/1ILCT absorption band without affecting the
1,3LLCT/1,3MLCT band distinctively, while the electron-
donating terminal substituent NPh2 enhanced the 1ILCT
transition and caused a significant red-shift of the
1π,π*/1ILCT and 1,3LLCT/1,3MLCT bands in comparison to
those of complex 1 without the terminal substituents.
Complexes 1−3 were weakly or moderately emissive at RT
in deaerated solution. However, the emission became much
stronger at 77 K in the glassy matrix. The NO2 substituent did
not affect the emission energy but slightly decreased the
emission lifetime, while the NPh2 substituent drastically
quenched the emission at RT due to the increased charge
transfer character in its emitting state. The nanosecond TA
spectroscopic study demonstrated that 1 and 2 possessed very
broad and strong absorption bands from the visible spectral
region, extending into the near-IR region and originating from a
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the Army Research
Laboratory (Grant W911NF-10-2-0055) for the materials
synthesis and characterization to W.S. and was also partially
supported by the National Science Foundation (Grants CNS-
1229316 and DMR-1411086) for the computational part of the
work to W.S. and S.K. S.K. acknowledges the U.S. Department
of Energy Early-Career Grant DESC008446 and the Alfred P.
Sloan Foundation (Research Fellowship BR2014-073) for
partial financial support and the Center for Computationally
Assisted Science and Technology (CCAST) at North Dakota
State University for computer access and administrative
support.
REFERENCES
■
(1) Lee, S.; Kim, S.-O.; Shin, H.; Yun, H.-J.; Yang, K.; Kwon, S.-K.;
Kim, J.-J.; Kim, Y.-H. Deep-Blue Phosphorescence from Perfluoro
Carbonyl-Substituted Iridium Complexes. J. Am. Chem. Soc. 2013, 135,
14321−14328.
3
moderately long-lived π,π*/3CT state. In contrast, the TA of
(2) Fan, C.; Zhu, L.; Jiang, B.; Li, Y.; Zhao, F.; Ma, D.; Qin, J.; Yang,
C. High Power Efficiency Yellow Phosphorescent OLEDs by Using
New Iridium Complexes with Halogen-Substituted 2-Phenylbenzo-
[d]thiazole Ligands. J. Phys. Chem. C 2013, 117, 19134−19141.
(3) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.;
Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.;
Okada, S.; Hoshino, M.; Ueno, K. Homoleptic Cyclometalated Iridium
Complexes with Highly Efficient Red Phosphorescence and
Application to Organic Light-Emitting Diode. J. Am. Chem. Soc.
2003, 125, 12971−12979.
(4) Sun, L.; Galan, A.; Ladouceur, S.; Slinker, J. D.; Zysman-Colman,
E. High Stability Light-Emitting Electrochemical Cells from Cationic
Iridium Complexes with Bulky 5,5′ Substituents. J. Mater. Chem. 2011,
21, 18083−18088.
(5) Zysman-Colman, E.; Slinker, J.-D.; Parker, J.-B.; Malliaras, G. G.;
Bernhard, S. Improved Turn-On Times of Light-Emitting Electro-
chemical Cells. Chem. Mater. 2008, 20, 388−396.
(6) Bolink, H.-J.; Cappelli, L.; Coronado, E.; Gratzel, M.; Orti, E.;
Costa, R.-D.; Viruela, P.-M.; Nazeeruddin, M. K. Stable Single-Layer
Light-Emitting Electrochemical Cell Using 4,7-Diphenyl-1,10-phenan-
throline-bis(2-phenylpyridine)iridium(III) Hexafluorophosphate. J.
Am. Chem. Soc. 2006, 128, 14786−14787.
(7) Somen, M.; Sourav, K.-S.; Parna, G.; Pradipta, P. Ultrafast
Photoinduced Electron Transfer between Carbon Nanoparticles and
Cyclometalated Rhodium and Iridium Complexes. J. Phys. Chem. C
2015, 119, 25122−25128.
complex 3 was much weaker, although the absorbing excited
state was much longer lived compared to those of 1 and 2.
Owing to the much stronger excited-state absorption with
respect to that of the ground state, complexes 1 and 2 exhibited
quite strong RSA at 532 nm for ns laser pulses, while the
significantly increased ground-state absorption at 532 nm for
complex 3 distinctively weakened the RSA of 3 at this
wavelength. These results clearly demonstrate that the terminal
substituents at the diimine ligand can significantly tune the
ground-state and excited-state properties of the Ir(III)
complexes, and it is feasible to obtain Ir(III) complexes with
reasonably long-lived, strongly absorbing triplet excited state
while red-shifting their ground-state charge transfer absorption
band into the near-IR region. These features are critical in
developing broadband reverse saturable absorbers that cover
the visible to the near-IR region.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02028.
(8) Zhang, K.-Y.; Zhang, J.; Liu, Y.-H.; Liu, S.-J.; Zhang, P.-L.; Zhao,
Q.; Tang, Y.; Huang, W. Core−shell Structured Phosphorescent
Nanoparticles for Detection Of Exogenous and Endogenous
Hypochlorite in Live Cells via Ratiometric Imaging and Photo-
luminescence Lifetime Imaging Microscopy. Chem. Sci. 2015, 6, 301−
307.
(9) Shi, H.-F.; Sun, H.-B.; Yang, H.-R.; Liu, S.-J.; Jenkins, G.; Feng,
W.; Li, F.-Y.; Zhao, Q.; Liu, B.; Huang, W. Cationic Polyfluorenes with
Phosphorescent Iridium(III) Complexes for Time-Resolved Lumines-
cent Biosensing and Fluorescence Lifetime Imaging. Adv. Funct. Mater.
2013, 23, 3268−3276.
(10) Law, W. H.-T.; Lee, L. C.-C.; Louie, M.-W.; Liu, H.-W.; Ang, T.
W.-H.; Lo, K. K.-W. Phosphorescent Cellular Probes and Uptake
Indicators Derived from Cyclometalated Iridium(III) Bipyridine
1H NMR spectra of L1−L3 and 1−3, optimized ground-
state geometries of 1−3, comparison of the experimental
and theoretical UV−vis absorption spectra of L1−L3 and
1−3 in CH2Cl2, normalized UV−vis absorption spectra
of L1−L3 and 1−3 in different solvents, normalized
emission spectra of L2 and 1−3 at room temperature
and of 1 and 2 at 77 K in MTHF glassy matrix, NTOs
representing transitions contributing to the main
absorption bands of L1−L3 and 1−3 in CH2Cl2,
NTOs corresponding to the emitting excited states of
L1−L3 and 1−3 in CH2Cl2, and the full author list for
ref 38 (PDF)
J
DOI: 10.1021/acs.inorgchem.6b02028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Complexes Appended with a Glucose or Galactose Entity. Inorg. Chem.
2013, 52, 13029−13041.
(11) Zhao, Q.; Yu, M.; Shi, L.; Liu, S.; Li, C.; Shi, M.; Zhou, Z.;
Huang, C.; Li, F. Cationic Iridium(III) Complexes with Tunable
Emission Color as Phosphorescent Dyes for Live Cell Imaging.
Organometallics 2010, 29, 1085−1091.
(12) Liu, J.; Liu, Y.; Liu, Q.; Li, C.; Sun, L.; Li, F. Iridium(III)
Complex-Coated Nanosystem for Ratiometric Upconversion Lumi-
nescence Bioimaging of Cyanide Anions. J. Am. Chem. Soc. 2011, 133,
15276−15279.
(13) Dragonetti, C.; Righetto, S.; Roberto, D.; Ugo, R.; Valore, A.;
Fantacci, S.; Sgamellotti, A.; De Angelis, F. Cyclometallated Iridium-
(III) Complexes with Substituted 1,10-Phenanthrolines: A New Class
of Highly Active Organometallic Second Order NLO-Phores with
Excellent Transparency with Respect to Second Harmonic Emission.
Chem. Commun. 2007, 4116−4118.
(14) Valore, A.; Cariati, E.; Dragonetti, C.; Righetto, S.; Roberto, D.;
Ugo, R.; De Angelis, F.; Fantacci, S.; Sgamellotti, A.; Macchioni, A.;
Zuccaccia, D. Cyclometalated IrIII Complexes with Substituted 1,10-
Phenanthrolines: A New Class of Efficient Cationic Organometallic
Second-Order NLO Chromophores. Chem. - Eur. J. 2010, 16, 4814−
4825.
(15) Zaarour, M.; Singh, A.; Latouche, C.; Williams, J. A. G.; Ledoux-
Rak, I.; Zyss, J.; Boucekkine, A.; Bozec, H. L.; Guerchais, V.;
Dragonetti, C.; et al. Linear and Nonlinear Optical Properties of Tris-
cyclometalated Phenylpyridine Ir(III) Complexes Incorporating π-
Conjugated Substituents. Inorg. Chem. 2013, 52, 7987−7994.
(16) Kim, K.-Y.; Farley, R. T.; Schanze, K. S. An Iridium(III)
Complex That Exhibits Dual Mechanism Nonlinear Absorption. J.
Phys. Chem. B 2006, 110, 17302−17304.
(17) Fan, Y.; Ding, D.; Zhao, D. Two- and Three-Photon Absorption
and Excitation Phosphorescence of Oligofluorene-Substituted Ir-
(ppy)3. Chem. Commun. 2015, 51, 3446−3449.
(18) Massue, J.; Olesiak-Banska, J.; Jeanneau, E.; Aronica, C.;
Matczyszyn, K.; Samoc, M.; Monnereau, C.; Andraud, C. Remarkable
Effect of Iridium Cyclometalation on the Nonlinear Absorption
Properties of a Quadrupolar Imine Ligand. Inorg. Chem. 2013, 52,
10705−10707.
(19) Li, Y.; Dandu, N.; Liu, R.; Hu, L.; Kilina, S.; Sun, W. Nonlinear
Absorbing Cationic Iridium(III) Complexes Bearing Benzothiazolyl-
fluorene Motif on the Bipyridine (N^N) Ligand: Synthesis, Photo-
physics and Reverse Saturable Absorption. ACS Appl. Mater. Interfaces
2013, 5, 6556−6570.
(20) Li, Y.; Dandu, N.; Liu, R.; Kilina, S.; Sun, W. Synthesis and
Photophysics of Reverse Saturable Absorbing Heteroleptic Iridium-
(III) Complexes Bearing 2-(7-R-Fluoren-2′-yl)pyridine Ligands.
Dalton Trans. 2014, 43, 1724−1735.
(21) Li, Y.; Dandu, N.; Liu, R.; Li, Z.; Kilina, S.; Sun, W. Effects of
Extended π-Conjugation in Phenanthroline (N^N) and Phenyl-
pyridine (C^N) Ligands on the Photophysics and Reverse Saturable
Absorption of Cationic Heteroleptic Iridium(III) Complexes. J. Phys.
Chem. C 2014, 118, 6372−6384.
(22) Pei, C.; Cui, P.; McCleese, C.; Kilina, S.; Burda, C.; Sun, W.
Heteroleptic Cationic Iridium(III) Complexes Bearing Naphthalimidyl
Substituents: Synthesis, Photophysics and Reverse Saturable Absorp-
tion. Dalton Trans. 2015, 44, 2176−2190.
(23) Li, Z.; Li, H.; Gifford, B. J.; Peiris, W. D. N.; Kilina, S.; Sun, W.
Synthesis, Photophysics, and Reverse Saturable Absorption of 7-
(Benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl Tethered [Ir-
(bpy) (ppy)2]PF6 and Ir(ppy)3 Complexes (bpy = 2,2′-bipyridine, ppy
= 2-phenylpyridine). RSC Adv. 2016, 6, 41214−41228.
(24) Sun, W.; Pei, C.; Lu, T.; Cui, P.; Li, Z.; McCleese, C.; Fang, Y.;
Kilina, S.; Song, Y.; Burda, C. Reverse Saturable Absorbing Cationic
Iridium(III) Complexes Bearing the 2-(2-Quinolinyl)quinoxaline
Ligand: Effects of Different Cyclometalating Ligands on Linear and
Nonlinear Absorption. J. Mater. Chem. C 2016, 4, 5059−5072.
(25) Liu, R.; Dandu, N.; Chen, J.; Li, Y.; Li, Z.; Liu, S.; Wang, C.;
Kilina, S.; Kohler, B.; Sun, W. Influence of Different Diimine (N^N)
Ligands on the Photophysics and Reverse Saturable Absorption of
Heteroleptic Cationic Iridium(III) Complexes Bearing Cyclometalat-
ing 2-{3-[7-(Benzothiazol-2-yl)fluoren-2-yl]phenyl} pyridine (C^N)
Ligands. J. Phys. Chem. C 2014, 118, 23233−23246.
(26) Li, Z.; Cui, P.; Wang, C.; Kilina, K.; Sun, W. Nonlinear
Absorbing Cationic Bipyridyl Iridium(III) Complexes Bearing Cyclo-
metalating Ligands with Different Degrees of π-Conjugation: Syn-
thesis, Photophysics, and Reverse Saturable Absorption. J. Phys. Chem.
C 2014, 118, 28764−28775.
(27) Zhao, Q.; Liu, S.; Shi, M.; Wang, C.; Yu, M.; Li, L.; Li, F.; Yi, T.;
Huang, C. Series of New Cationic Iridium(III) Complexes with
Tunable Emission Wavelength and Excited State Properties:
Structures, Theoretical Calculations, and Photophysical and Electro-
chemical Properties. Inorg. Chem. 2006, 45, 6152−6160.
(28) Ladouceur, S.; Fortin, D.; Zysman-Colman, E. Role of
Substitution on the Photophysical Properties of 5,5′-Diaryl-2,2′-
bipyridine (bpy*) in [Ir(ppy)2(bpy*)]PF6 Complexes: A Combined
Experimental and Theoretical Study. Inorg. Chem. 2010, 49, 5625−
5641.
(29) Neve, F.; La Deda, M.; Crispini, A.; Bellusci, A.; Puntoriero, F.;
Campagna, S. Cationic Cyclometalated Iridium Luminophores:
Photophysical, Redox, and Structural Characterization. Organometallics
2004, 23, 5856−5863.
(30) Costa, R.-D.; Monti, F.; Accorsi, G.; Barbieri, A.; Bolink, H.-J.;
Orti, E.; Armaroli, N. Photophysical Properties of Charged Cyclo-
metalated Ir(III) Complexes: A Joint Theoretical and Experimental
Study. Inorg. Chem. 2011, 50, 7229−7238.
(31) Liu, R.; Li, Y.; Chang, J.; Liu, S.; Waclawik, E.; Sun, W. Pt(II)
Bipyridyl Complexes Bearing Substituted Fluorenyl Motif on the
Bipyridyl and Acetylide Ligands: Synthesis, Photophysics, and Reverse
Saturable Absorption. Inorg. Chem. 2014, 53, 9516−9530.
(32) Demas, J. N.; Crosby, G. A. The Measurement of Photo-
luminescence Quantum Yields. A Review. J. Phys. Chem. 1971, 75,
991−1024.
(33) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara,
T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Reevaluation of Absolute
Luminescence Quantum Yields of Standard Solutions Using a
Spectrometer with an Integrating Sphere and a Back-Thinned CCD
Detector. Phys. Chem. Chem. Phys. 2009, 11, 9850−9860.
(34) Melhuish, W. H. Quantum Efficiencies of Fluorescence of
Organic Substances: Effect of Solvent and Concentration of the
Fluorescent Solute. J. Phys. Chem. 1961, 65, 229−235.
(35) Carmichael, I.; Hug, G. L. Triplet-Triplet Absorption Spectra of
Organic Molecules in Condensed Phases. J. Phys. Chem. Ref. Data
1986, 15, 1−250.
(36) Kumar, C. V.; Qin, L.; Das, P. K. Aromatic Thioketone Triplets
and Their Quenching Behaviour Towards Oxygen and di-t-
Butylnitroxy Radical. A Laser-Flash-Photolysis Study. J. Chem. Soc.,
Faraday Trans. 2 1984, 80, 783−793.
(37) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kenney, M. E.; Rodgers,
M. A. J. Silicon Naphthalocyanine Triplet State and Oxygen. A
Reversible Energy-Transfer Reaction. J. Am. Chem. Soc. 1988, 110,
7626−7630.
(38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Peterson, G. A.; et al. Gaussian 09, revision A.1; Gaussian, Inc.:
Wallingford, CT, 2009.
(39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient
Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.
(40) Ernzerhof, M.; Scuseria, G.-E. Assessment of the Perdew−
Burke−Ernzerhof Exchange-Correlation Functional. J. Chem. Phys.
1999, 110, 5029−5036.
(41) Adamo, C.; Barone, V. Toward Reliable Density Functional
Methods without Adjustable Parameters: The PBE0Model. J. Chem.
Phys. 1999, 110, 6158−6170.
(42) Hay, P. J.; Wadt, W. R. Ab initio Effective Core Potentials for
Molecular Calculations. Potentials for K to Au including the
Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299−310.
K
DOI: 10.1021/acs.inorgchem.6b02028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(43) Hay, P. J.; Wadt, W. R. Ab initio Effective Core Potentials for
Molecular Calculations. Potentials for the Transition Metal Atoms Sc
to Hg. J. Chem. Phys. 1985, 82, 270−283.
(44) Wadt, W. R.; Hay, P. J. Ab initio Effective Core Potentials for
Molecular Calculations. Potentials for Main Group Elements Na to Bi.
J. Chem. Phys. 1985, 82, 284−298.
(45) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V.
R. Efficient Diffuse Function-Augmented Basis Sets for Anion
Calculations. III. The 3-21+G Basis Set for First-Row Elements, Li-
F. J. Comput. Chem. 1983, 4, 294−301.
(46) Francl, M.-M.; Pietro, W.-J.; Hehre, W.-J.; Binkley, S.; Gordon,
M.-S.; DeFrees, D.-J.; Pople, J.-A. Self-Consistent Molecular Orbital
Methods. XXIII. A Polarization-Type Basis Set for Second-Row
Elements. J. Chem. Phys. 1982, 77, 3654−3665.
(47) Gill, P.-M.-W.; Johnson, B.-G.; Pople, J.-A.; Frisch, M.-J. The
Performance of the Becke-Lee-Yang-Parr (B-LYP) Density Functional
Theory with Various Basis Sets. Chem. Phys. Lett. 1992, 197, 499−505.
(48) Hariharan, P.-C.; Pople, J.-A. The Influence of Polarization
Functions on Molecular Orbital Hydrogenation Energies. Theor. Chim.
Acta. 1973, 28, 213−222.
(49) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-
Consistent Molecular Orbital Methods. XX. A Basis Set for Correlated
Wave Functions. J. Chem. Phys. 1980, 72, 650−654.
(65) Speiser, S.; Orenstein, M. Spatial Light Modulation via Optically
Induced Absorption Changes in Molecules. Appl. Opt. 1988, 27,
2944−2948.
(66) Band, Y. B.; Harter, D. J.; Bavli, R. Optical Pulse Compressor
Composed of Saturable and Reverse Saturable Absorbers. Chem. Phys.
Lett. 1986, 126, 280−284.
(67) Perry, J. W.; Alvarez, D.; Choong, I.; Mansour, K.; Marder, S. R.;
Perry, K. J. Enhanced Reverse Saturable Absorption and Optical
Limiting in Heavy-Atom-Substituted Phthalocyanines. Opt. Lett. 1994,
19, 625−627.
(68) Liu, R.; Li, Y.; Li, Y.; Zhu, H.; Sun, W. Photophysics and
Nonlinear Absorption of Cyclometalated 4,6-Diphenyl-2,2′-bipyridyl
Platinum(II) Complexes with Different Acetylide Ligands. J. Phys.
Chem. A 2010, 114, 12639−12645.
(69) Liu, R.; Dandu, N.; Li, Y.; Kilina, S.; Sun, W. Synthesis,
Photophysics and Reverse Saturable Absorption of Bipyridyl Platinum-
(II) Bis(Arylfluorenylacetylide) Complexes. Dalton Trans. 2013, 42,
4398−4409.
(70) Li, Y.; Liu, R.; Badaeva, E.; Kilina, S.; Sun, W. Long-lived π-
Shape Platinum (II) Diimine Complexes Bearing 7-Benzothiazoly-
fluoren-2-yl Motif on the Bipyridine and Acetylide Ligands: Admixing
π,π* and Charge Transfer Configurations. J. Phys. Chem. C 2013, 117,
5908−5918.
(50) Barone, V.; Cossi, M.; Tomasi, J. Geometry Optimization of
Molecular Structures in Solution by the Polarizable Continuum
Model. J. Comput. Chem. 1998, 19, 404−417.
(51) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies,
Structures, and Electronic Properties of Molecules in Solution with the
C-PCM Solvation Model. J. Comput. Chem. 2003, 24, 669−681.
(52) Marques, M. A. L.; Gross, E. K. U. Time-Dependent Density
Functional Theory. Annu. Rev. Phys. Chem. 2004, 55, 427−455.
(53) Ullrich, C. A. Time-Dependent Density-Functional Theory:
Concepts and Applications; Oxford University Press: New York, 2012.
(54) Kilina, S.; Kilin, D.; Tretiak, S. Light-Driven and Phonon-
Assisted Dynamics in Organic and Semiconductor Nanostructures.
Chem. Rev. 2015, 115, 5929−5978.
(55) Furche, F.; Ahlrichs, R. Adiabatic Time-Dependent Density
Functional Methods for Excited State Properties. J. Chem. Phys. 2002,
117, 7433−7447.
(56) Van Caillie, C.; Amos, R. D. Geometric Derivatives of Density
Functional Theory Excitation Energies Using Gradient-Corrected
Functionals. Chem. Phys. Lett. 2000, 317, 159−164.
(57) Martin, R. L. Natural Transition Orbitals. J. Chem. Phys. 2003,
118, 4775−4777.
(58) Guo, F.; Sun, W.; Liu, Y.; Schanze, K. Synthesis, Photophysics,
and Optical Limiting of Platinum(II) 4′-Tolylterpyridyl Arylacetylide
Complexes. Inorg. Chem. 2005, 44, 4055−4065.
(59) Liu, R.; Azenkeng, A.; Zhou, D.; Li, Y.; Glusac, K.-D.; Sun, W.
Tuning Photophysical Properties and Improving Nonlinear Absorp-
tion of Pt(II) Diimine Complexes with Extended π-Conjugation in the
Acetylide Ligands. J. Phys. Chem. A 2013, 117, 1907−1917.
(60) Wu, S.-H.; Ling, J.-W.; Lai, S.-H.; Huang, M.-J.; Cheng, C. H.;
Chen, I.-C. Dynamics of the Excited States of [Ir(ppy)2bpy]+ with
Triple Phosphorescence. J. Phys. Chem. A 2010, 114, 10339−10344.
(61) Chen, H.-Y.; Yang, C.-H.; Chi, Y.; Cheng, Y.-M.; Yeh, Y.-S.;
Chou, P.-T.; Hsieh, H.-Y.; Liu, C.-S.; Peng, S.-M.; Lee, G.-H. Room-
Temperature NIR Phosphorescence of New Iridium (III) Complexes
with Ligands Derived from Benzoquinoxaline. Can. J. Chem. 2006, 84,
309−318.
(62) Li, C.; Zhang, L.; Wang, R.; Song, Y.; Wang, Y. Dynamics Of
Reverse Saturable Absorption and All-Optical Switching in C60. J. Opt.
Soc. Am. B 1994, 11, 1356−1360.
(63) Penzkofer, A. Passive Q-Switching and Mode-Locking for the
Generation of Nanosecond to Femtosecond Pulses. Appl. Phys. B:
Photophys. Laser Chem. 1988, 46, 43−60.
(64) Reddy, K. P. J. Applications of Reverse Saturable Absorbers in
Laser Science. Curr. Sci. 1991, 61, 520−526.
L
DOI: 10.1021/acs.inorgchem.6b02028
Inorg. Chem. XXXX, XXX, XXX−XXX