Synthesis, Photophysics, and Reverse Saturable Absorption of trans-Bis-cyclometalated Iridium(III) Complexes (CNC)Ir(R-tpy)+ (tpy = 2,2′:6′,2″-Terpyridine) with Broadband Excited-State Absorption

Article abstract of DOI:10.1021/acs.inorgchem.0c00961

Extending the bandwidth of triplet excited-state absorption in transition-metal complexes is appealing for developing broadband reverse saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes (Ir1-Ir5) bearing trans-bis-cyclometalating (CNC) and 4′-R-2,2′:6′,2″-terpyridine (4′-R-tpy) ligands were synthesized. The effects of the structural variation in cyclometalating ligands and substituents at the tpy ligand on the photophysics of these complexes have been systematically explored using spectroscopic methods (i.e., UV-vis absorption, emission, and transient absorption spectroscopy) and time-dependent density functional theory (TDDFT) calculations. All complexes exhibited intensely structured ¹π,π? absorption bands at <400 nm and broad charge transfer (¹CT)/¹π,π? transitions at 400-600 nm. Ligand structural variations exerted a very small effect on the energies of the ¹CT/¹π,π? transitions; however, they had a significant effect on the molar extinction coefficients of these absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broad-band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the ³π,π?/³CT states. Although alteration of the ligand structures influenced the emission energies slightly, these changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption (RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3. The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm for these complexes.

Full text of DOI:10.1021/acs.inorgchem.0c00961

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Article
Synthesis, Photophysics, and Reverse Saturable Absorption of trans-
Bis-cyclometalated Iridium(III) Complexes (C^N^C)Ir(R-tpy)+ (tpy =
2,2′:6′,2″-Terpyridine) with Broadband Excited-State Absorption
Bingqing Liu, Mohammed A. Jabed, Svetlana Kilina, and Wenfang Sun*
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ABSTRACT: Extending the bandwidth of triplet excited-state absorption in
transition-metal complexes is appealing for developing broadband reverse
saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes
(Ir1-Ir5) bearing trans-bis-cyclometalating (C^N^C) and 4′-R-2,2′:6′,2″-terpyr-
idine (4′-R-tpy) ligands were synthesized. The effects of the structural variation in
cyclometalating ligands and substituents at the tpy ligand on the photophysics of
these complexes have been systematically explored using spectroscopic methods
(i.e., UV−vis absorption, emission, and transient absorption spectroscopy) and
time-dependent density functional theory (TDDFT) calculations. All complexes
1
exhibited intensely structured π,π* absorption bands at <400 nm and broad
charge transfer (1CT)/1π,π* transitions at 400−600 nm. Ligand structural
variations exerted a very small effect on the energies of the 1CT/1π,π* transitions;
however, they had a significant effect on the molar extinction coefficients of these
absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broad-
band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both
3
originating from the π,π*/3CT states. Although alteration of the ligand structures influenced the emission energies slightly, these
changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet
excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption
(RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3.
The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm
for these complexes.
most widely studied prototypes.3,25,26 2-Phenylpyridine (ppy)
INTRODUCTION
■
and its derivatives have been the most utilized cyclometalating
In the past 20 years, octahedral d6 iridium(III) complexes have
ligands owing to their ease of synthesis and determinable
gained intense interest because of their rich photophysical
photophysical properties for the resultant Ir(III) complexes.
properties, such as long-lived triplet excited states, high
Optimization of the absorption and emission characteristics of
phosphorescence quantum yields, excellent emission color
these complexes can be realized through introducing electron-
tunability, and good chemical and thermal stability.1−4 Various
donating/-withdrawing substituents or fusing aromatic rings on
Ir(III) complexes have been explored for a wide range of
the C^N and/or N^N ligands.17,21−28
applications, including luminescent bioimaging,5−11 photo-
In contrast to the tris-bidentate cyclometalated Ir(III)
dynamic therapy (PDT),8−17 low-power upconversion,18,19
complexes, bis-terdentate Ir(III) complexes endow important
organic light-emitting diodes (OLED),20,21 light-emitting
advantages due to their axial symmetry that precludes
electrochemical cells (LEECs),22,23 nonlinear optics,24−27 etc.
geometrical isomers and facile tuning of intramolecular charge
Efficient tuning of the photophysical properties of the Ir(III)
transfer.29 However, most of the reported bis-terdentate Ir(III)
complexes for specific applications can be realized via
complexes are the bis-terpyridine (tpy) complexes Ir-
structural modifications of the coordinating/cyclometalating
ligands,26−28 while understanding the structure−property
correlations holds the key for developing Ir(III) complexes
Received: March 31, 2020
with predetermined properties.
Among the various Ir(III) complexes, tris-bidentate Ir(III)
complexes bearing cyclometalating ligands, such as Ir(C^N)3
and (C^N)2Ir(N^N)+ (where C^N denotes the cyclometalat-
ing ligands and N^N represents the diimine ligands), are the
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Scheme 1. Structures and Synthetic Route for (C^N^C)Ir(tpy)+ Complexes Ir1−Ir5
3+
(tpy)2
.
16,30−34 Studies on other types of bis-terdentate Ir(III)
ground-state absorption at the longer visible to NIR regions
and the insufficient ESA intensity in the NIR regions hamper
their potential as broadband reverse saturable absorbers. To
date, the RSA of bis-terdentate Ir(III) complexes has not been
investigated. Because the broad and strong ESA in the red
complexes, especially those with trans-bis-cyclometalating
ligands, have been quite limited.29,30,34−37 The challenges
probably arise from the harsh reaction conditions in synthesiz-
ing these complexes, including high reaction temperature
(>180 °C), restrictions to light and oxygen, and low product
yield. The groundbreaking work on using 2,6-diphenylpyridine
(dppy) derivatives as the trans-bis-cyclometalating ligand
(C^N^C) was reported by Scandola and co-workers.35,36 It
was revealed that the heteroleptic linearly arranged (C^N^C)-
Ir(tpy)+ complexes exhibited bathochromically shifted charge
transfer absorption and emission in comparison to their bis-
3+
spectral regions in some of the reported Ir(tpy)2 and
(C^N^C)Ir(tpy)+ complexes16,34−36 (in some cases extending
to the NIR regions) and their red-shifted ground-state
absorption via appropriate substitution on the terpyridine
ligand16 are desirable features for RSA, the bis-terdentate
Ir(III) complexes could exhibit a strong RSA in the long visible
spectral regions. Moreover, the stronger σ-donating ability of
the C^N^C ligand could further bathochromically shift the
charge-transfer ground-state absorption bands in the
(C^N^C)Ir(tpy)+ type of complexes with respect to their
corresponding Ir(tpy)23+ types,37 which would further broaden
the RSA spectral region.
35−37
3+
terpyridine counterparts Ir(tpy)2
.
These changes were
rationalized by the stronger σ-donating ability of the C^N^C
ligand, which raised the C^N^C- and metal-based highest
occupied molecular orbital (HOMO) and the lowest singly
occupied molecular orbital (l-SOMO), consequently lowering
the ligand-to-ligand charge transfer (LLCT)/metal-to-ligand
charge transfer (MLCT) absorption and emitting states.35−37
The substituents (CH3 or Br) on the 4′-phenyl group of the
tpy ligand manifest some effects on the emission energies,
while substitution on the 4-phenyl ring of the C^N^C ligand
had a negligible effect on the emission energies of the
complexes. Nonetheless, the types of substituents investigated
have been quite limited, and exploration of the potential
applications of this type of complexes is scarce.35−37 To the
extent of our knowledge, the only reported work on utilizing a
(C^N^C)Ir(tpy)+ type complex is the selective photooxidation
of DNA purine base.37
Since 2013, our group has systematically investigated the
reverse saturable absorption (RSA, a nonlinear absorption in
which the stronger excited-state absorption over the ground-
state absorption enhances the absorptivity of a material when
the incident light fluence increases) of cyclometalated tris-
bidentate Ir(III) complexes.17,24−27,38−50 Our ultimate goal has
been to develop reverse saturable absorbers with broadband
absorption in the visible to the NIR regions for optical limiting
applications.51 It is known that the predominant parameter
determining the RSA strength is the ratio of the excited-state
absorption (ESA) cross section (σex) with respect to that of the
ground state (σ0) at the defined wavelength; a broadband
reverse saturable absorber is anticipated to possess a very weak
ground-state absorption but much stronger ESA at 400−900
nm. Although many of our studied tris-bidentate Ir(III)
complexes display very strong RSA at 532 nm, the lack of
To verify this prediction, we have designed and synthesized
a series of (C^N^C)Ir(tpy)+ complexes bearing the trans-
cyclometalating ligand dppy (or its derivative) and tpy ligand
with different substituents: i.e., [(2-phenylbenzo[h]quinoline)-
Ir(4′-phenyl-tpy)]+ (Ir1), [(2,6-diphenylpyridine)Ir(4′-phe-
nyl-tpy)]+ (Ir2), [(2,6-diphenylpyridine)Ir(4′-(4-dimethylami-
nophenyl)-tpy)]+ (Ir3), [(2,6-diphenylpyridine)Ir(4′-dimethy-
lamino-tpy)]+ (Ir4), [(2,6-diphenylpyridine)Ir(4′-(4-
HOCH2CH2OOC-phenyl)-tpy)]+ (Ir5) (the structures are
shown in Scheme 1). A series of spectroscopic techniques and
theoretical simulations were utilized to systematically study
their photophysical properties. Their ESAs and RSAs at 532
nm were manifested as well. In comparison to Ir2, Ir1
containing a C^N^C ligand with a fused additional phenyl ring
allows for demonstration of the effects of expansive π
conjugation in the C^N^C ligand. Ir3−Ir5 have either strongly
electron donating or withdrawing substituents on the tpy
ligand to tune the photophysical properties and RSAs of these
complexes.
EXPERIMENTAL SECTION
■
Synthesis and Characterization. All solvents and reagents were
obtained from VWR Scientific and used as received unless otherwise
mentioned. Silica gels (230−400 mesh) and Al2O3 gels (activated,
neutral, Brockmann I) for column chromatography were purchased
from Sorbent Technology. The ligand 2,6-diphenylpyridine (dppy)
was obtained from Alfa Aesar, while 2-phenylbenzo[h]quinoline was
synthesized according to the literature procedure.52 The N^N^N
ligands 4′-phenyl-2,2′:6′,2″-terpyridine,53 4′-(4-dimethylaminophen-
B
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yl)-2,2′:6′,2″-terpyridine,54 4′-dimethylamino-2,2′:6′,2″-terpyridine,55
4′-(4-COOH-phenyl)-2,2′:6′,2″-terpyridine,56 (4′-phenyl-tpy)IrCl3,57
and (4′-(4-COOH-phenyl)-tpy)IrCl3/(4′-(4-HOCH2CH2OOC-phe-
1.2CH2Cl2·0.8CH3CN: C, 49.69; H, 3.00; N, 6.50. Found: 49.65;
H, 3.24; N, 6.77.
Ir2. With (4′-phenyl-tpy)IrCl3 (121 mg, 0.2 mmol) and 2,6-
58
nyl)-tpy)IrCl3 were prepared according to the procedures reported
diphenylpyridine (46 mg, 0.2 mmol) as the starting materials, an
1
in the literature. The synthesis and characterization of (4′-(4-
dimethylaminophenyl)-tpy)IrCl3, (4′-dimethylamino-tpy)IrCl3, and
Ir1−Ir5 are reported below. 1H NMR spectroscopy, electrospray
ionization high-resolution mass spectrometry (ESI-MS), and
elemental analysis were used to characterize the structures of Ir1−
Ir5. The NMR spectra were collected on a Bruker-400 spectrometer.
ESI-MS data were obtained on a Waters Synapt G2-Si mass
spectrometer. Elemental analyses were carried out by NuMega
Resonance Laboratories, Inc. (San Diego, CA).
orange powder was obtained as the product (10 mg, 6%). H NMR
(400 MHz, CD3OD): δ 9.19 (s, 2H), 8.84 (d, J = 7.8 Hz, 2H), 8.28−
8.22 (m, 2H), 8.09 (s, 3H), 8.04−7.97 (m, 2H), 7.87 (d, J = 7.3 Hz,
2H), 7.80 (d, J = 5.2 Hz, 2H), 7.75 (t, J = 7.5 Hz, 2H), 7.66 (t, J = 7.4
Hz, 1H), 7.32 (ddd, J = 7.3, 5.8, 1.3 Hz, 2H), 6.96 (td, J = 7.8, 1.2 Hz,
2H), 6.74 (td, J = 7.3, 1.1 Hz, 2H), 6.23 (d, J = 6.6 Hz, 2H). ESI-
HRMS (m/z): calcd for [C38H26IrN4]+, 731.1789; found, 731.1795.
Anal. Calcd for C38H26F6IrN4P·0.2CH2Cl2: C, 51.39; H, 2.98; N, 6.28.
Found: C, 51.05; H, 3.26; N, 6.25.
Ir3. With (4′-(4-dimethylaminophenyl)-tpy)IrCl3 (130 mg, 0.2
mmol) and 2,6-diphenylpyridine (46 mg, 0.2 mmol) as the starting
materials, a red powder was obtained as the product (18 mg, 10%).
1H NMR (400 MHz, d6-DMSO): δ 9.28 (s, 2H), 9.00 (d, J = 8.1 Hz,
2H), 8.26 (d, J = 9.1 Hz, 2H), 8.16 (d, J = 7.6 Hz, 2H), 8.08 (d, J =
7.1 Hz, 1H), 8.06−8.00 (m, 2H), 7.89 (d, J = 7.3 Hz, 2H), 7.67 (d, J
= 5.7 Hz, 2H), 7.37−7.30 (m, 2H), 6.96 (d, J = 9.0 Hz, 2H), 6.90 (t, J
= 7.5 Hz, 2H), 6.67 (t, J = 7.3 Hz, 2H), 6.17 (d, J = 6.2 Hz, 2H), 3.08
(s, 6H). ESI-HRMS (m/z): calcd for [C40H31IrN5]+, 774.2211; found,
774.2201. Anal. Calcd for C40H31F6IrN5P·1.5H2O: C, 50.79; H, 3.62;
N, 7.40. Found: C, 50.78; H, 3.99; N, 7.24.
Ir4. With (4′-dimethylamino-tpy)IrCl3 (115 mg, 0.2 mmol) and
2,6-diphenylpyridine (46 mg, 0.2 mmol) as the starting materials, a
red powder was obtained as the product (6.7 mg, 4%). 1H NMR (400
MHz, d6-DMSO): δ 8.84 (d, J = 8.1 Hz, 2H), 8.26 (s, 2H), 8.16 (d, J
= 7.8 Hz, 2H), 8.08−7.97 (m, 3H), 7.91 (d, J = 7.7 Hz, 2H), 7.60 (d,
J = 5.9 Hz, 2H), 7.28 (t, J = 6.0 Hz, 2H), 6.92 (t, J = 6.9 Hz, 2H),
6.73 (t, J = 6.7 Hz, 2H), 6.28 (d, J = 6.5 Hz, 2H), 3.48 (s, 6H). ESI-
HRMS (m/z): calcd for [C34H27IrN5]+, 698.1898; found, 698.1898.
Anal. Calcd for C34H27F6IrN5P·2H2O: C, 46.47; H, 3.56; N, 7.79.
Found: C, 46.23; H, 3.61; N, 7.62.
General Synthetic Procedure for (4′-R-tpy)IrCl3. The ligand
4′-R-tpy (0.1 mmol, R = phenyl, 4-dimethylaminophenyl, dimethy-
lamino, 4-COOH-phenyl) and IrCl3·3H2O (0.1 mmol) were added to
10 mL of ethylene glycol. The suspension was purged with N2 and
heated to 160 °C for 15 min in dark. After the mixture was cooled to
room temperature, the precipitate was collected, washed with water (2
× 10 mL) and ethanol (2 × 10 mL), and then dried under vacuum to
yield the target complex, which was used for the following reaction
step without further purification.
It should be noted that when 4′-(4-COOH-phenyl)-tpy reacted
with IrCl3·3H2O, the resultant complex was presumed to be a mixture
of (4′-(4-COOH-phenyl)-tpy)IrCl3 and its ester (4′-(4-
HOCH2CH2OOC-phenyl)-tpy)IrCl3 due to the possible reaction
between the carboxyl group and ethylene glycol at a high reaction
temperature, as reported previously in the literature.58 However,
because of the poor solubility of the complexes, they were not
separated and were used directly for the following reaction step, in
which the higher reaction temperature (196 °C) and longer reaction
time (24 h) could convert the remaining COOH group into an ester
group and thus the pure complex (4′-(4-HOCH2CH2OOC-phenyl)-
tpy)Ir(dppy)PF6 (Ir5) was separated.
(4′-(4-Dimethylaminophenyl)-tpy)IrCl3. With 4′-(4-dimethyla-
minophenyl)-tpy (35.2 mg, 0.1 mmol) and IrCl3·3H2O (35.2 mg, 0.1
mmol) as the starting materials, a dark red powder was obtained as
Ir5. With the mixed (4′-(4-COOH-phenyl)-tpy)IrCl3/(4′-(4-
HOCH2CH2OOC-phenyl)-tpy)IrCl3 (139 mg, 0.2 mmol if it was
pure (4′-(4-HOCH2CH2OOC-phenyl)-tpy)IrCl3) and 2,6-diphenyl-
pyridine (46 mg, 0.2 mmol) as the starting materials, an orange
1
the product (41 mg, 63%). H NMR (400 MHz, d6-DMSO): δ 9.21
1
(dd, J = 5.6, 1.0 Hz, 2H), 8.97 (s, 2H), 8.89 (d, J = 8.1 Hz, 2H), 8.27
(td, J = 7.9, 1.5 Hz, 2H), 8.13 (d, J = 9.0 Hz, 2H), 7.97−7.89 (m,
2H), 6.93 (d, J = 9.1 Hz, 2H), 3.10 (s, 6H).
powder was obtained as the product (11 mg, 6%). H NMR (400
MHz, d6-DMSO): δ 9.49 (s, 2H), 9.09 (d, J = 6.9 Hz, 2H), 8.55 (d, J
= 8.3 Hz, 2H), 8.33 (d, J = 8.8 Hz, 2H), 8.23 (d, J = 8.4 Hz, 2H), 8.13
(ddd, J = 16.5, 9.3, 3.0 Hz, 3H), 7.95 (d, J = 7.5 Hz, 2H), 7.77 (d, J =
4.9 Hz, 2H), 7.46−7.38 (m, 2H), 6.98−6.90 (m, 2H), 6.76−6.68 (m,
2H), 6.18 (d, J = 7.6 Hz, 2H), 5.05 (s, 1H), 4.44−4.37 (m, 2H),
3.83−3.76 (m, 2H). ESI-HRMS (m/z): calcd for [C41H30IrN4O3]+,
819.1949; found, 819.1953. Anal. Calcd for C41H30F6IrN4O3P: C,
51.09; H, 3.14; N, 5.81. Found: C, 51.46; H, 3.33; N, 6.12.
(4′-Dimethylamino-tpy)IrCl3. With 4′-dimethylamino-tpy (27.6
mg, 0.1 mmol) and IrCl3·3H2O (35.2 mg, 0.1 mmol) as the starting
materials, a dark red powder was obtained as the product (45 mg,
1
78%). H NMR (400 MHz, d6-DMSO): δ 9.19 (d, J = 5.7 Hz, 2H),
8.72 (d, J = 7.4 Hz, 2H), 8.21 (t, J = 7.1 Hz, 2H), 7.90 (s, 2H), 7.89−
7.83 (m, 2H), 3.46 (s, 6H).
General Procedure for the Synthesis of Ir1−Ir5. (4′-R-
tpy)IrCl3 (0.2 mmol), the C^N^C ligand (0.2 mmol), and AgOTf
(154 mg, 0.6 mmol) in ethylene glycol (10 mL) were heated to reflux
under an N2 atmosphere at 196 °C for 24 h in the dark. After the
mixture was cooled to rt, NH4PF6 (163 mg, 1.0 mmol) was added to
the mixture and then stirring was continued at rt for 2 h. The formed
AgCl precipitate and other black byproducts were removed using a
flash silica gel column with methanol as eluent. After removal of
methanol, deionized water was added to precipitate the crude
product. The pure product was isolated by silica gel chromatography
with CH2Cl2/acetonitrile as eluent (from 1/0 to 50/1, v/v). The
orange or orange-red band was collected and dried under vacuum to
afford the target complex.
Ir1. With (4′-phenyl-tpy)IrCl3 (121 mg, 0.2 mmol) and 2-
phenylbenzo[h]quinoline (51 mg, 0.2 mmol) as the starting materials,
an orange powder was obtained as the product (11 mg, 6%). 1H NMR
(400 MHz, CDCl3): δ 8.89 (s, 2H), 8.65 (d, J = 7.9 Hz, 2H), 8.40 (d,
J = 8.6 Hz, 1H), 8.18 (dd, J = 8.0, 3.5 Hz, 3H), 7.87 (dd, J = 14.0, 7.8
Hz, 4H), 7.77 (d, J = 8.8 Hz, 1H), 7.71 (t, J = 7.8 Hz, 2H), 7.57 (t, J =
7.5 Hz, 1H), 7.47 (t, J = 7.1 Hz, 3H), 7.17−7.11 (m, 1H), 7.04 (dd, J
= 11.6, 5.7 Hz, 3H), 6.83 (t, J = 6.8 Hz, 1H), 6.52 (d, J = 6.4 Hz, 1H),
6.36 (d, J = 6.7 Hz, 1H). ESI-HRMS (m/z): calcd for [C40H26IrN4]+,
755.1789; found, 755.1782. Anal. Calcd for C40H26F6IrN4P·
Photophysical Study. The solvents used for the photophysical
studies were spectroscopic grade and were obtained from Alfa Aesar.
A Varian Cary 50 spectrophotometer was used for UV−vis spectral
measurements. The emission spectra were recorded on a HORIBA
FluoroMax-4 fluorometer/phosphorometer. The emission quantum
yields of Ir1−Ir5 in deaerated solutions were determined by the
relative actinometry method,59 in which a deaerated acetonitrile
solution of [Ru(bpy)3]Cl2 (Φem = 0.097 in, λex = 436 nm)60 was
utilized as the reference for all complexes. The nanosecond transient
difference absorption (TA) spectra and triplet lifetimes of Ir1−Ir5 in
degassed acetonitrile solutions were studied on an Edinburgh LP920
laser flash photolysis spectrometer using the third-harmonic output
(355 nm) of a Nd:YAG laser (Quantel Brilliant, 4.1 ns, 1 Hz) as the
excitation light. The triplet excited-state molar extinction coefficients
at the TA band maxima were estimated using the singlet depletion
method.61 To determine the triplet excited-state quantum yields of
Ir1−Ir5, the relative actinometry method62 was applied and SiNc
(ε590 = 70000 M−1 cm−1, ΦT = 0.20) in benzene was used as the
standard.63
Nonlinear Transmission Measurements. The RSA strengths of
Ir1−Ir5 were evaluated by nonlinear transmission measurements
using a Quantel Brilliant 4.1 ns laser (the repetition rate was set to 10
Hz) as the light source. The concentrations of the acetonitrile
C
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solutions were adjusted to reach a linear transmission of 80% at 532
nm in the cuvette with a 2 mm path length. The experimental setup
and details resembled those reported by our group previously.64 With
an f = 40 cm plano-convex lens, the beam radius was ∼96 μm at the
focal point.
Computational Methodology. All complexes of this series were
optimized at the singlet ground state at the level of density functional
theory (DFT)65 using the B3LYP functional.66 Ir was treated with the
LANL2DZ67 basis set that includes the effective core potential. All
other atoms were treated with the 6-31G* basis set.68,69 Dichloro-
methane was used as the solvent within the conductor-like polarizable
continuum model (CPCM)70 reaction field method.
Linear response time-dependent density functional theory (TD-
DFT)71 was used to calculate the singlet excited-state energy and
oscillation strength of the transitions. For calculations of the excited
states, the same functional, basis sets, and CPCM solvation method
were used as those used for the ground-state calculations. Absorption
spectra were plotted using the Gaussian function with the line-
broadening parameter of 0.10 eV, which produced an acceptable
agreement to the thermal broadening of experimental absorption
spectra.
absorption bands at 400−600 nm are tentatively assigned to
1
charge-transfer (1CT) transitions mixed with π,π* characters.
The assignment of the optical transitions making contribu-
tions to the absorption bands can be confirmed by the TDDFT
calculations. As shown in Figure S2 in the Supporting
Information, the calculated spectra matched well with the
experimental spectra. The natural transition orbitals (NTOs)
in Table 2 indicate that the lowest-energy absorption bands at
490−600 nm are dominated by the ligand-to-ligand charge
transfer (1LLCT)/metal-to-ligand charge transfer (1MLCT)
transitions in Ir1, Ir2, and Ir5, while the corresponding bands
in Ir3 and Ir4 that bear the strongly electron donating
dimethylaminophenyl or dimethylamino substituents have
1
predominant R-tpy ligand localized ILCT (intraligand charge
transfer)/1MLCT/1π,π* characters. In contrast, the absorption
bands at 400−490 nm in Ir1, Ir2, and Ir5 are dominated by
1
the R-tpy-based π,π*/1ILCT transitions admixing with some
1MLCT/1LLCT contributions, whereas these bands in Ir3 and
1
Ir4 have predominant LLCT/1MLCT characters. For the
To calculate the emission energies, the analytical gradient TDDFT
method was applied to optimize the lowest triplet state,72 with the
same functional and basis sets as in the ground-state calculations. A
qualitative description of excited states was presented using natural
transition orbitals (NTOs),73 which represent special distribution of
the excited electron/hole wave function. The Gaussian16 software
package was used for all calculations.74 The VMD software packages75
were used to visualize the NTOs with 0.02 isovalue.
absorption bands at <400 nm, the NTOs in Tables S1−S5 in
1
the Supporting Information manifest the predominant π,π*
nature, admixing with some charge transfer configurations.
Similar to those reported for (C^N^C)Ir(tpy)+ com-
plexes,29,35−37 Ir1−Ir5 exhibited greatly red-shifted and
distinguishable 1LLCT/1MLCT absorption bands in the
3+
regions of 400−600 nm in comparison to that of the Ir(tpy)2
prototype16,33 because of the stronger σ-donating capability of
the trans-bis-cyclometalating C^N^C ligand. Fusing an addi-
tional phenyl ring in the C^N^C ligand in Ir1 slightly
increased the molar extinction coefficient of the
1π,π*/1LLCT/1MLCT absorption band at 434 nm owing to
the extended π conjugation of the C^N^C ligand in
comparison to that in Ir2. However, the intensity of the
lowest-energy absorption band at 517 nm in Ir1 is the lowest
among these five complexes. A careful examination of the
NTOs corresponding to the S1 transitions in these complexes
(see Table 2) revealed that the S1 transition in Ir1 had the least
RESULTS AND DISCUSSION
■
Electronic Absorption. The absorption spectra of Ir1−Ir5
in CH2Cl2 and in other solvents (acetonitrile, THF, toluene)
are presented in Figure 1 and in Figure S1 in the Supporting
1
contribution from the tpy-localized π,π* configuration, which
accounted for the lower absorptivity of this absorption band in
Ir1 with respect to those in Ir2−Ir4. Considering the effect of
the 4′-R substituents at the tpy ligand on the low-energy
absorption bands, we can clearly see that the strongly electron
donating 4-dimethylaminophenyl substituent drastically in-
creased the molar extinction coefficients of these bands at
400−600 nm in Ir3; however, it did not change the transition
energies of these bands. In contrast, the electron-withdrawing
ester substituent evoked minor effects on either the energies or
the molar extinction coefficients of the absorption bands in Ir5
in comparison to those in Ir2. A comparison of the spectra of
Ir3 and Ir4 found that the 4′-phenyl group at the tpy ligand
Figure 1. Experimental UV−vis absorption spectra of Ir1−Ir5 at
room temperature in CH2Cl2. The expanded spectra in the 450−650
nm regions are displayed as inset.
1
1
played a distinct role in enhancing both the π,π* and CT
transitions associated with the tpy ligand but did not alter the
transition energies.
Information, respectively. The associated absorption parame-
ters, i.e. band maxima and molar extinction coefficients, are
given in Table 1. In the studied concentration range between 1
× 10−6 and 1 × 10−4 mol L−1, the absorption of Ir1−Ir5
followed Beer’s law, indicating the absence of ground-state
aggregation below the concentration of 1 × 10−4 mol L−1. The
absorption spectra of all complexes shared similar features,
with strong absorption bands below 400 nm and broad,
moderately strong bands between 400 and 600 nm.
Considering the large molar extinction coefficients, the intense
absorption bands below 400 nm can be attributed to ligand-
Photoluminescence. The emission of Ir1−Ir5 was
investigated in CH3CN, THF, CH2Cl2, and toluene + 5%
CH2Cl2 at rt. The normalized emission spectra in CH2Cl2 are
displayed in Figure 2, and the emission parameters are given in
Table 1. The emission spectra and parameters in the other
solvents are presented in Figure S3 and Table S6 in the
Supporting Information. Upon excitation at the corresponding
charge transfer bands, all complexes exhibited deep red
emission, with the emission energies varying from 661 nm in
Ir1 to 688 nm in Ir5 and the emission lifetimes falling into the
1
localized spin-allowed π,π* transitions, while the moderate
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Table 1. Photophysical Properties of Complexes Ir1−Ir5
λem/nm
(τem/μs); Φem
a
b
c
d
e
λabs/nm (log ε)
kr/s−1
knr/s−1
λT1‑Tn/nm (τTA/μs; log εT1−Tn); ΦT
Ir1 286 (4.86), 434 (4.05), 517 (3.57)
661 (2.08); 0.072
678 (2.05); 0.033
679 (3.12); 0.049
676 (1.46); 0.015
688 (1.66); 0.022
5.17 × 104
2.06 × 104
3.27 × 104
6.04 × 104
5.30 × 104
6.66 × 105 375 (2.11; −), 625 (2.15; 4.34); 0.67
6.05 × 105 360 (1.61; −), 612 (1.64; 4.64); 0.78
6.35 × 105 399 (2.39; −), 768 (2.46; 4.52); 0.48
Ir2 280 (4.77), 308 (4.61), 445 (4.01), 527 (3.82)
Ir3 280 (4.80), 311 (4.76), 461 (4.36), 528 (4.26)
Ir4 280 (4.73), 324 (4.23), 437 (3.85), 528 (3.74)
Ir5 283 (4.81), 309 (4.65), 450 (4.08), 529 (3.87)
3.97 × 106 366 (1.12; −), 550 (1.16; -), 750 (1.14; 4.54); 0.17
2.36 × 106 360 (1.88; −), 639 (1.84; 4.88); 0.25
a
b
Electronic absorption band maxima (λabs) and molar extinction coefficients (log ε) in CH2Cl2 at room temperature. Room-temperature emission
band maxima (λem) and lifetimes (τem) for Ir1−Ir5 measured in CH2Cl2 (c = 1 × 10−5 mol L−1). The emission quantum yields were obtained using
c
[Ru(bpy)3]Cl2 (Φem = 0.097, λex = 436 nm) in a deaerated acetonitrile solution as the reference. Radiative decay rates (kr) calculated by kr = Φem/
d
e
(ΦTτem). Nonradiative decay rates (knr) calculated by knr = (1 − Φem)/(ΦTτem). Nanosecond TA band maxima (λT1‑Tn), triplet excited-state
lifetimes (τTA), and quantum yield (ΦT) measured in CH3CN at room temperature.
Table 2. Natural Transition Orbitals (NTOs) for the Major Transitions Making Contributions to the Low-Energy Absorption
Bands of Ir1−Ir5 in CH2Cl2
range of 1.46−3.12 μs in degassed CH2Cl2 solutions. The
emission was subject to oxygen quenching, exhibited large red
shifts in comparison to the excitation wavelengths, and
possessed long-lived lifetimes. All of these features suggest a
phosphorescent nature of the emission, which is in line with
that reported for the other (C^N^C)Ir(tpy)+ complexes in the
literature.29,35−37 In addition, the markedly red shifted but
poorly resolved emission spectra are also in accordance with
those observed for the other reported (C^N^C)Ir(tpy)+
complexes,29,35−37 while they are quite distinct from those
16,33,37
3+
reported for the Ir(tpy)2 complexes.
The featureless emission bands and the 1.46−3.12 μs
lifetimes of Ir1−Ir5 imply a charge transfer character of the
emitting state, which has been reported for the other
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that can be understood in terms of the ground-state triplet
instability problem.76 It has been reported that, in cases where
the Hartree−Fock (HF) stability is less than ∼2 eV, the
inclusion of exact HF exchange in the functional (e.g., 25% in
B3LYP) leads to an artificial decrease in excitation energy of
the triplet state. The lower degree of HF stability in Ir4 in
comparison to that in the other complexes is presumably
rationalized by the lack of a phenyl ring in the substituent on
the tpy ligand, which places the electron-donating N(CH3)2
group closer to the tpy ligand, resulting in a higher order of
electron density redistribution between the substituent and the
tpy ligand. However, this discrepancy in the excitation energy
is expected to negligibly affect the special localization/
delocalization properties of the excited wave function. The
NTOs corresponding to the T1 states of Ir1−Ir5 illustrate that
both the electrons and holes are predominantly distributed on
the N^N^N ligands and the metal d orbitals, with the holes
also containing slightly more distribution on the C^N^C
ligand. Therefore, the nature of the emitting states in these
complexes can be ascribed to the 4′-R-tpy ligand localized
Figure 2. Normalized emission spectra of Ir1 (λex = 500 nm), Ir2 (λex
= 460 nm), Ir3 (λex = 460 nm), Ir4 (λex = 460 nm), and Ir5 (λex
=
450 nm) in the N2-purged CH2Cl2 solution at room temperature (c =
1 × 10−5 mol L−1). The sticks indicate the emission energies
calculated by TDDFT using the B3LYP functional in CH2Cl2.
(C^N^C)Ir(tpy)+ complexes in the literature.29,35,36 However,
in view of the very minor solvatochromic effects in different
solvents with varied polarities (see Figure S3 in the Supporting
Information), the emitting states should have significant
ligand-centered 3π,π* configurations. This attribution is
consistent with the recent report on the (CH3OPh-C^N^C)-
Ir(HO2CPh-tpy)+ complex.37
Further evidence supporting this assignment comes from the
TDDFT calculation results via optimizing the lowest triplet
excited states (T1) of Ir1−Ir5. As shown in Table 3 and Figure
2, except for Ir4, the trend in the calculated emission energies
for the other four complexes qualitatively matches the
experimental emission energy trend. The off-trend of Ir4
could arise from the underestimated lowest triplet-state energy
3
3π,π* states admixing with some MLCT/3LLCT characters.
For Ir3, the electron distribution of the holes is mainly on the
4′-(4-dimethylaminophenyl)pyridine motif and the metal d
orbital, while the electron density is delocalized on the entire
N^N^N ligand. Thus, the emitting state of Ir3 has a significant
3ILCT configuration in addition to the aforementioned
3π,π*/3MLCT/3LLCT characters.
In comparison to the emission of Ir2, the emission energy of
Ir1 is increased, concomitant with a higher emission quantum
yield but a similar emission lifetime. The increased emission
quantum yield should be attributed to the much higher
radiative decay rate (kr) in Ir1. A closer examination of the
holes of the T1 states revealed an increased 3LLCT character in
the emitting state of Ir1 in comparison to that of Ir2 due to the
π-extended C^N^C ligand in Ir1, which likely accounted for
the increased kr value and consequently a higher emission
quantum yield in Ir1 while the nonradiative decay rates
remained similarly. Different from the effect of alteration of the
cyclometalating ligand, incorporating a strongly electron
donating dimethylamino substituent either on the 4′-phenyl
ring or directly on the tpy ligand exerted a minor effect on the
emission energies of Ir3 and Ir4. However, these structural
variations pronouncedly affected the emission lifetimes and
quantum yields in Ir3 and Ir4 in comparison to those in Ir2.
Complex Ir3 with the dimethylamino substituent on the
phenyl ring exhibited a longer emission lifetime and higher
emission quantum yield, with the latter stemming from a 59%
higher kr value and a merely 5.0% higher knr value with respect
to those of Ir2. In contrast, directly attaching the
dimethylamino substituent to the tpy ligand in Ir4 resulted
in a 2.9-fold increase in kr but a 6.6-fold increase in knr in
comparison to those of Ir2, which shortened the emission
lifetime and lowered the emission quantum yield for Ir4. On
consideration of the effects of the electron-withdrawing ester
group in Ir5, it caused a 10 nm bathochromic shift of the
emission spectrum in comparison to that of Ir2 due to the
stabilization of the electron in the T1 state by the electron-
withdrawing substituent. Meanwhile, the kr value was altered to
2.6-fold and knr to 3.9-fold increases with respect to those of
Ir2. Consequently, the emission lifetime was shortened, and
the quantum yield was reduced in Ir5 in comparison to those
in Ir2.
Table 3. NTOs Representing the Transitions That
Contribute to the T1 States of Ir1−Ir5 in CH2Cl2 Calculated
by the TDDFT ΔSCF Method, Which Are Optimized with
the B3LYP Functional and LANL2DZ/6-31G* Basis
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Transient Absorption (TA). TA spectroscopy is a
powerful technique to provide further information on the
triplet excited-state absorption and decay characteristics. To
better understand the triplet excited-state properties, especially
to figure out the spectral regions where the excited-state
absorption is stronger than that of the ground state, the
nanosecond TA of Ir1−Ir5 was studied in acetonitrile
solutions at room temperature. Figure 3 displays the TA
Reverse Saturable Absorption (RSA). We have man-
ifested that many Ir(III) tris-bidentate complexes exhibited
strong RSA and potentially could be utilized as optical limiting
materials.17,24−27,38−50 Generally, RSA requires a stronger
excited-state absorption in comparison to the ground-state
absorption at the same wavelength. The TA spectra in Figure 3
illustrated that all complexes, except for Ir3, hold positive
signals at 532 nm, implying that the excited-state absorption at
this wavelength was stronger than the ground-state absorption.
This feature along with their much longer triplet excited-state
lifetimes with respect to the pulse width of the laser beams (4.1
ns) indicate that RSA could occur at 532 nm from these
complexes upon 532 nm nanosecond laser irradiation. To
manifest this assumption, the nonlinear transmission measure-
ments of Ir1−Ir5 in a 2 mm cuvette in acetonitrile solutions
were carried out using 4.1 ns laser pulses at 532 nm. The
transmittance vs incident energy curves for Ir1−Ir5 are
presented in Figure 4. For comparison purposes, solutions
with the same linear transmission of 80% at 532 nm in a 2 mm
cuvette were employed.
Figure 3. TA spectra of Ir1−Ir5 in acetonitrile immediately after laser
excitation (λex = 355 nm). A355 = 0.4 in a 1 cm cuvette.
spectra immediately after 355 nm laser excitation, and the
time-resolved TA spectra are provided in Figure S4 in the
Supporting Information. The TA parameters such as the band
maxima, triplet excited-state lifetimes deduced from the decay
profiles of TA, and the T1 state extinction coefficients and
quantum yields are compiled in Table 1. The deduced TA
lifetimes for Ir1−Ir5 in acetonitrile are in line with their
respective emission lifetimes in the same solution, indicating
that the transient absorbing and emitting species originate
from the same state. Considering the natures of the emitting
states discussed in the section on emission, we can assign the
observed TA signals from these complexes to the
3π,π*/3MLCT/3LLCT T1 states in these complexes, with the
Figure 4. Nonlinear transmission plots of Ir1−Ir5 in acetonitrile
solutions with a linear transmittance of 80% in a 2 mm cuvette at 532
nm. The laser pulse duration was 4.1 ns.
3
Except for Ir3, the transmission of all the other complexes
decreased with an increase in incident energy, suggesting that
RSA took place. The strength of the RSA follows the trend Ir2
≈ Ir4 > Ir1 > Ir5 > Ir3, which is essentially in accordance with
their corresponding ΔOD values at 532 nm (i.e., 0.012 for Ir1,
0.024 for Ir2, −0.028 for Ir3, 0.026 for Ir4, and 0.008 for Ir5).
The stronger RSAs of Ir2 and Ir4 are obviously related to their
stronger excited-state absorption at 532 nm with respect to
those of the other complexes, whereas the weaker excited-state
absorption of Ir3 in comparison to its ground-state absorption
(as reflected by the negative ΔOD value) at 532 nm prevents
the RSA in Ir3. However, in view of the much stronger TA
signals at >585 nm for Ir1−Ir3 and Ir5, the RSA of these
complexes could be much stronger than that of Ir4 at
wavelengths longer than 585 nm. Nonetheless, Ir4 holds the
advantage of a broader excited-state absorption than the other
complexes, making it a potential broadband reverse saturable
absorber in the visible to the NIR spectral region.
T1 state in Ir3 admixing with a significant ILCT character.
As demonstrated in Figure 3, all complexes exhibited a
positive TA band at 350−420 nm and broad and strong
positive TA band(s) from the visible region extending to the
NIR region (i.e., 496−800 nm for Ir1, 517−800 for Ir2 and
Ir5, 550−800 nm for Ir3, and 456−800 nm for Ir4). Bleaching
occurred at the wavelengths essentially in accordance with the
1CT bands in their respective UV−vis absorption spectra. The
TA spectral features of Ir1, Ir2, and Ir5 resembled each other;
however, the TA signals were much stronger in Ir2 with
respect to those in Ir1 and Ir5. This could be related to the
highest triplet excited-state formation quantum yield of Ir2
among these complexes. For Ir3, its broad TA band kept
increasing in the NIR regions, which could be ascribed to the
3ILCT character in its T1 state. In contrast, the TA spectrum of
Ir4 was quite distinct from those of Ir1−Ir3 and Ir5 in that the
positive TA bands were much broader with two band maxima
at 550 and 750 nm, respectively. However, the TA intensities
of Ir4 were weaker in the entire TA spectrum in comparison to
those of the other complexes. This reflects the absence of the
phenyl ring in the N^N^N ligand in Ir4, confirming the
involvement of the 4′-R-tpy ligand in the transient absorbing
T1 state in these complexes.
CONCLUSIONS
■
Five bis-terdentate Ir(III) complexes bearing a trans-bis-
cyclometalating ligand and a 4′-R-tpy ligand, i.e. (C^N^C)Ir-
(R-tpy)+, were synthesized to explore the effect of C^N^C
ligand π conjugation and the 4′-R substituent at the tpy ligand
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on the photophysics and excited-state absorption of these
complexes to assess the feasibility of these types of complexes
as reverse saturable absorbers. In comparison to the Ir(tpy)2
611X; Phone: 701-231-6254; Email: Wenfang.Sun@
ndsu.edu; Fax: 701-231-8831
3+
Authors
complexes, the stronger σ-donating ability of the C^N^C
1
ligand caused a distinct red shift of the CT/1π,π* bands to
Bingqing Liu − Department of Chemistry and Biochemistry,
North Dakota State University, Fargo, North Dakota
58108−6050, United States; orcid.org/0000-0002-1540-
2235
Mohammed A. Jabed − Department of Chemistry and
Biochemistry, North Dakota State University, Fargo, North
Dakota 58108−6050, United States; orcid.org/0000-
0001-8552-0301
Svetlana Kilina − Department of Chemistry and Biochemistry,
North Dakota State University, Fargo, North Dakota
58108−6050, United States; orcid.org/0000-0003-1350-
2790
400−600 nm. Although structural variations in either the
cyclometalating or the tpy ligand exerted a minor effect on the
1
energies of the CT/1π,π* bands, these changes altered the
molar extinction coefficients of these absorption bands
drastically, especially that with the strongly electron donating
4′-(4-dimethylaminophenyl) substituent (Ir3). All complexes
displayed deep red room-temperature phosphorescence (661−
693 nm) in a variety of organic solvents (CH2Cl2, CH3CN,
THF, and toluene (with 5% CH2Cl2)) at room temperature.
The emission spectrum of Ir1 with a more π expansive C^N^C
ligand was blue-shifted in all solvents in comparison to those of
the other complexes bearing the 2,6-diphenylpyridine ligand,
concomitant with the highest emission quantum yield. While
introducing an electron-withdrawing ester substituent on the
4′-phenyl group of the tpy ligand caused a red shift of the
emission spectrum for Ir5 with a reduced emission quantum
yield in comparison to those of Ir2, electron-donating 4′-(4-
dimethylaminophenyl) and 4′-dimethylamino substituents on
the tpy ligand had a minor effect on the emission energies of
Ir3 and Ir4 with respect to that of Ir2. However, Ir3 possessed
a higher emission quantum yield with respect to that of Ir2,
whereas the emission quantum yield was lower in Ir4 in
comparison to that of Ir2. All complexes exhibited broad and
strong nanosecond TA bands in the visible to the NIR region
originating from their 3π,π*/3CT T1 states. Structural
alteration affected the spectral features and the TA signal
intensities pronouncedly, with Ir2 giving the strongest TA
signals peaking at 612 nm, Ir3 showing the highest TA signals
in the NIR, and Ir4 having the broadest TA bands at 456−800
nm. These complexes exhibited varying degrees of RSA at 532
nm, i.e. Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3, which primarily
corresponded to their ΔOD values at 532 nm. In view of the
strongest RSAs of Ir2 and Ir4 at 532 nm and their strong
broadband TA signals, these two complexes hold the potential
as broadband reverse saturable absorbers for optical limiting of
nanosecond laser pulses.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00961
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the financial support of the National
Science Foundation (NSF DMR-1411086 and CHE-1800476)
for this work. S.K. also thanks the Center for Computationally
Assisted Science and Technology (CCAST) at North Dakota
State University for computational resources and adminis-
trative support.
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■
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AUTHOR INFORMATION
Corresponding Author
Wenfang Sun − Department of Chemistry and Biochemistry,
North Dakota State University, Fargo, North Dakota
58108−6050, United States; orcid.org/0000-0003-3608-
■
H
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