Synthesis, photophysics, and reverse saturable absorption of 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9: H -fluoren-2-yl tethered [Ir(bpy)(ppy)2]PF6 and Ir(ppy)3 complexes (bpy = 2,2′-bipyridine, ppy = 2-phenylpyridine)

Article abstract of DOI:10.1039/c5ra20084a

We report the synthesis, photophysics, and reverse saturable absorption of five iridium(iii) complexes 1-5 with 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl (BTF) pendant attached to the 2-phenylpyridine ligand (1: [Ir(bpy)(BTF-ppy)₂]PF₆; 2: [Ir(bpy)(BTF - ppy)₂]PF₆; 3: Ir(ppy)₂(BTF-ppy); 4: Ir(ppy)(BTF-ppy)₂; 5: (BTF-ppy)₃, where bpy = 2,2′-bipyridine and ppy = 2-phenylpyridine). The effects of the extended π-conjugation of the ppy ligand and the increased number of BTF-ppy ligand, as well as the effects of neutral complex vs. cationic complex were evaluated. All complexes exhibit predominantly ligand-localized ¹π,π? transitions below 430 nm and charge-transfer transitions between 430 and 550 nm. They all emit at room temperature and at 77 K, mainly from the metal-to-ligand charge transfer (³MLCT)/ligand-to-ligand charge transfer (³LLCT) states for 1 and 2, and from the BTF-ppy ligand-centered ³π,π? excited states with significant contributions from the ³MLCT states for 3-5. The triplet excited states of 1-5 also manifest broad transient absorption (TA) in the visible to the near-IR spectral region. The electronic absorption, emission, and ns transient absorption are all red-shifted by extending the π-conjugation of the ppy ligand or increasing the number of BTF-ppy ligand. The energies of the lowest singlet and triplet excited states of the neutral complex 4 are lowered compared to those of its cationic counterpart 1; while the transient absorbing triplet excited state of 4 is much longer lived than that of 1. These complexes all exhibit strong reverse saturable absorption (RSA) for ns laser pulses at 532 nm, with a trend of 5 < 4 < 1 ≈ 3 < 2. This trend is primarily determined by the ratio of the excited-state absorption cross section to that of the ground state (σ_ex/σ₀) at 532 nm with the triplet quantum yield also playing a role for complexes 3-5. It appears that the increased number of BTF-ppy ligand reduces the RSA of the neutral complexes while the increased π-conjugation of the ppy ligand in the cationic complexes improves the RSA. However, the neutral complex 4 exhibits a weaker RSA at 532 nm than its cationic counterpart 1.

Full text of DOI:10.1039/c5ra20084a

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PAPER
Synthesis, photophysics, and reverse saturable
absorption of 7-(benzothiazol-2-yl)-9,9-di(2-
ethylhexyl)-9H-fluoren-2-yl tethered
Cite this: RSC Adv., 2016, 6, 41214
[
2
Ir(bpy)(ppy) ]PF and Ir(ppy) complexes (bpy ¼
2
6
3
0
,2 -bipyridine, ppy ¼ 2-phenylpyridine)†
Zhongjing Li,‡ Hui Li,‡ Brendan J. Gifford, Wadumesthrige D. N. Peiris, Svetlana Kilina
and Wenfang Sun*
We report the synthesis, photophysics, and reverse saturable absorption of five iridium(III) complexes 1–5
with 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl (BTF) pendant attached to the 2-
phenylpyridine ligand (1: [Ir(bpy)(BTF-ppy)
2
6 2 6 2
]PF ; 2: [Ir(bpy)(BTF–^–ppy) ]PF ; 3: Ir(ppy) (BTF-ppy); 4:
0
Ir(ppy)(BTF-ppy) ; 5: (BTF-ppy)
2
3
, where bpy ¼ 2,2 -bipyridine and ppy ¼ 2-phenylpyridine). The effects of
the extended p-conjugation of the ppy ligand and the increased number of BTF-ppy ligand, as well as
the effects of neutral complex vs. cationic complex were evaluated. All complexes exhibit predominantly
1
ligand-localized p,p* transitions below 430 nm and charge-transfer transitions between 430 and 550
nm. They all emit at room temperature and at 77 K, mainly from the metal-to-ligand charge transfer
3
3
(
MLCT)/ligand-to-ligand charge transfer ( LLCT) states for 1 and 2, and from the BTF-ppy ligand-
3
3
centered p,p* excited states with significant contributions from the MLCT states for 3–5. The triplet
excited states of 1–5 also manifest broad transient absorption (TA) in the visible to the near-IR spectral
region. The electronic absorption, emission, and ns transient absorption are all red-shifted by extending
the p-conjugation of the ppy ligand or increasing the number of BTF-ppy ligand. The energies of the
lowest singlet and triplet excited states of the neutral complex 4 are lowered compared to those of its
cationic counterpart 1; while the transient absorbing triplet excited state of 4 is much longer lived than
that of 1. These complexes all exhibit strong reverse saturable absorption (RSA) for ns laser pulses at 532
nm, with a trend of 5 < 4 < 1 z 3 < 2. This trend is primarily determined by the ratio of the excited-state
0
absorption cross section to that of the ground state (sex/s ) at 532 nm with the triplet quantum yield
Received 29th September 2015
Accepted 15th April 2016
also playing a role for complexes 3–5. It appears that the increased number of BTF-ppy ligand reduces
the RSA of the neutral complexes while the increased p-conjugation of the ppy ligand in the cationic
complexes improves the RSA. However, the neutral complex 4 exhibits a weaker RSA at 532 nm than its
cationic counterpart 1.
DOI: 10.1039/c5ra20084a
www.rsc.org/advances
efficiency for iridium(III) complexes. Extensive research has
been carried out in utilizing the photoluminescence properties
Introduction
The spin–orbit coupling constant of iridium is among the of iridium(III) complexes as dopant for organic light-emitting
1
2–4
largest in transition metals, which leads to fast intersystem diodes (OLEDs)
and light-emitting electrochemical cells
5
–10
crossing to the triplet excited state and high phosphorescence (LEECs).
Other applications of Ir(III) complexes, including
therapeutic and bioimaging,
lytic water splitting, photoredox catalysis, and solution
electroluminescence are also explored.
11,12
13
oxygen sensing, photo cata-
14
15
Department of Chemistry and Biochemistry, North Dakota State University, Fargo, ND
16
5
8108-6050, USA. E-mail: wenfang.sun@ndsu.edu; Tel: +1-701-231-6254
1
It is well known that the electronic structure of Ir(III)
†
Electronic supplementary information (ESI) available: The H NMR spectra for
3
–5. The electronic absorption and emission spectra in different solvents, complexes can be efficiently controlled by deliberate structural
14,17
time-resolved transient absorption spectra in toluene for 1–5, 1-L and 2-L. The modications.
By using different types of diimine or cyclo-
normalized emission spectra of 3–5 in toluene at different excitation
18
19
metalating ligands, or varying substituent on the ligands, the
nature of the lowest-energy singlet and triplet excited states can
be signicantly adjusted. For instance, density functional
theory (DFT) calculations and experimental studies carried out
wavelengths. The normalized excitation spectra of 5 in toluene when monitored
at different emission wavelengths. The calculated emission state diagram and
NTOs for 1–5. See DOI: 10.1039/c5ra20084a
‡
These two authors contribute equally to this project.
41214 | RSC Adv., 2016, 6, 41214–41228
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20
by Hay revealed that the contribution of Ir(III) d orbital to the nonlinear absorption of the neutral tris-cyclometalating Ir(III)
28
low-lying metal-to-ligand charge transfer (MLCT) state varied complexes to the best of our knowledge.
from 45% to 60% in replacing one 2-phenylpyridine (ppy) ligand In this paper, ve cationic or neutral Ir(III) complexes 1–5
in Ir(ppy) with acetylacetonate or benzoylacetonate. In another were synthesized and studied. The structures of these
3
work reported by Watts and Houten, the energy level and life- complexes are shown in Chart 1, which feature BTF pendant at
3
time of the emitting d,d excited state of the iridium 1,10-phe- the para-position of the cyclometalating carbon of the phenyl
nathroline complexes were found to be readily tuned through ring. The aim of our research is to illustrate the structure–
modication with different substituents, such as electron property correlations in neutral and cationic Ir(III) complexes for
2
withdrawing groups (–NO and –Cl) or electron donating group rational design of nonlinear absorbing materials. Complexes 1
(
–CH ) on the 4, 5, 6, or 7 position of the 1,10-phenathroline and 2 contain one bipyridine ligand and two BTF substituted 2-
3
21
ligand.
phenylpyridine ligands and thus are cationic, while complexes
Further structure–property correlation studies were con- 3–5 bear three 2-phenylpyridine ligands and are neutral. Instead
ducted in more “bulky” molecules with molecular weight up to of single bond connection in 1, the BTF unit is connected to the
3000. In recent reports by Zhao and co-workers, the photo- 2-phenylpyridine via a triple bond in 2. In this way, the effect of
physics of tris-cyclometalated iridium complexes with varied extended p-conjugation length on the cyclometalating ligands
positions or repeating units of the alkylated uorenyl oligomer can be evaluated. From 3 to 5, the number of the BTF pendant
2
2,23
on the 2-phenylpyridine ligand was studied.
Spectroscopic increases. This variation possibly allows us to evaluate the
3
studies and DFT calculations revealed that the triplet lifetime interplay or mixing between the ligand-centered p,p* state
increased but the phosphorescence quantum yield decreased predominantly from the BTF substituted cyclometalating ligand
3
3
with increased chain length of the oligouorene, due to locali- and the MLCT/ LLCT excited states related to the coordination
zation of the triplet excitons. Meanwhile, the contribution of the core. Comparison of the cationic complexes vs. neutral
MLCT state to the emissive excited state is signicantly affected complexes can be realized by assessing the photophysics of 1
by the attachment position of the oligouorene. Particularly, and 4, which contain the same cyclometalating ligands but 1
rd
attaching the triuorene at the para-position of the cyclo- has bipyridine as the 3 ligand, while 4 has 2-phenylpyridine as
rd
metalating carbon of the phenyl ring produced a ligand- the 3 ligand. The much stronger ligand eld of 2-phenyl-
centered triplet excited state with long lifetime and moderate pyridine can induce different emission and absorption charac-
quantum yield.
teristics. It should be noted that although complexes 1–5 all
Unlike the extensive study on the light-emitting applications, contain the BTF substituted 2-phenylpyridine ligands, which
research on nonlinear absorption (which has potential appli- appears to be similar to some of the cationic Ir(III) complexes
25–27
cations in all-optical switching, laser beam stabilization or previously reported by our group,
either the diimine ligand
compression, optical signal rectication, photodynamic is different from the reported complexes (such as bipyridine in
therapy, bioimaging, optical data storage, etc.) of Ir(III) complexes 1 and 2 vs. phenanthroline or the substituted
complexes is quite limited. Schanze and co-workers rst re- bipyridine or phenanthroline in ref. 25–27) or the triplet bond
ported the dual mechanism nonlinear absorption of an Ir(III) position is different in the BTF substituted 2-phenylpyridine
0
complex bearing two 2-phenylpyridine ligands and one 4,4 - ligand (such as the case in complex 2) or the formal charges of
0
0
(
2,2 -bipyridine-5,5 -diylbis(ethyne-2,1-diyl))bis(N,N-dihexylani-
the complexes are different (i.e. 0 charge in complexes 3–5 vs. +1
24
line) ligand. Strong excited-state absorption in ca. 560–1500 charge in the cationic Ir(III) complexes reported in ref. 25–27).
nm region was observed. Meanwhile, the absorbing excited
state can be populated by 5 ns, 1064 nm laser via two-photon
absorption. Therefore, both one-photon process and two-
photon absorption can induce nonlinear transmission by this
complex. Our group also reported the photophysics and reverse
saturable absorptions of heteroleptic cationic Ir(III) complexes
with extended p-conjugation on the diimine and/or the phe-
25–27
nylpyridine ligands.
It was found that extending the p-
conjugation of the diimine ligand via attaching the BTF motifs
can signicantly increase the triplet lifetime by admixing the
3
substituted bipyridine-localized p,p character into the lowest
triplet excited state; while the extended p-conjugation of the
phenylpyridine ligand via attaching the BTF motif decreased
3
3
the triplet lifetime due to predominantly MLCT/ LLCT char-
25,26
acters in the lowest triplet excited state.
All complexes
exhibited very strong RSA at 532 nm for nanosecond laser pul-
ses. These results are quite promising. However, all reported
nonlinear absorption work are on the cationic heteroleptic Ir(III)
complexes. To date, there has been only one report on the
Chart 1 Structures of complexes 1–5.
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2
ꢀ1
Finally, the reverse saturable absorption of these complexes is 600 m g , pH: 6.5–7.5). Complexes 1–5 and ligands 1-L and 2-L
1
investigated at 532 nm using ns laser pulses, aiming to evaluate were characterized by H NMR, electrospray ionization high-
the effect of structural variation on the nonlinear absorption of resolution mass spectrometry (ESI-HRMS), and elemental
1
the Ir(III) complexes. The nature of transitions is characterized analyses. Other intermediates were characterized by H NMR.
1
computationally using time-dependent density functional
H NMR was obtained on Varian Oxford-VNMR spectrome-
theory (TDDFT), thereby supporting the aforementioned effects ters (400 or 500 MHz). ESI-HRMS analyses were performed on
on the optical properties of the Ir(III) complexes.
a Bruker BioTOF III mass spectrometer. Elemental analyses
were conducted by NuMega Resonance Laboratories, Inc. in San
Diego, California.
Experimental section
Synthesis and characterization
Br-ppy. To the argon protected THF (30 mL) solution of 2-
ꢁ
bromopyridine (1 mL, 10.5 mmol) at ꢀ78 C, BuLi in hexane (6
The synthetic routes for 1–5 are illustrated in Scheme 1. Br-F8- mL, 15 mmol) was added dropwise. The solution was stirred
29
ꢁ
BTZ was synthesized according to literature procedure. The under argon at ꢀ78 C for 90 minutes. Bu
3
SnCl (3 mL, 11 mmol)
ꢁ
synthetic details and characterization data for Br-ppy, B-ppy, 1- was added dropwise at ꢀ78 C aerwards. The mixture was then
L, 2-L, and 1–5 are reported herewith. All the chemicals and allowed to gradually warm up to room temperature. Aer 10
solvents were purchased from Aldrich Chemical Co. or Alfa hours, the reaction was stopped, and the solution was poured
4
Aesar and used as received. Silica gel for chromatography was into saturated aqueous NH Cl solution (50 mL). The organic
˚
layer was collected and dried over MgSO . The solvent was
purchased from Sorbent Technology (60 A, 230–400 mesh, 500–
4
Scheme 1 Synthetic routes for complexes 1–5.
41216 | RSC Adv., 2016, 6, 41214–41228
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removed and the residue was puried by a silica gel column distillation of Et
3
N was used directly for the next step without
eluted with hexane and CH Cl (v/v ¼ 8/1). The dark oil collected further purication.
2
2
was dried in vacuum oven for 24 hours and was used for next
step reaction directly without further purication.
The yellow oil collected from the previous step, combined
with K CO (3.7 g, 27 mmol) and MeOH (50 mL), were stirred at
2
3
The dried oily product from the previous step, 1,3-dibro- room temperature for 12 hours. Aer reaction, water (100 mL)
mobenzene (2.5 g, 10.5 mmol), Pd(PPh
(30 mg), and toluene was added and the mixture was extracted with hexane (50 mL ꢂ
30 mL) were added together. The reaction mixture was 3). The combined hexane solution was then dried over MgSO
3 4
)
(
4
.
degassed with argon and heated to reux for 24 hours. Aer Aer the solvent was removed, the red oil was puried by
reaction, the precipitate was removed by ltration. The solvent running a silica gel column eluted rst with hexane and then
of the ltrate was then removed by distillation. The resultant with the mixed solvent of hexane and CH Cl (v/v ¼ 3/1). 2.65 g
2
2
dark oil was puried by a silica gel column eluted with hexane yellow oil was collected as the nal product (yield: 91% from Br-
1
and ethyl acetate (v/v ¼ 10/1). 1.7 g yellow oil was collected as F8-BTZ). H NMR (CDCl , 400 MHz): 8.06–8.13 (m, 3H), 7.88–
3
1
the nal product (yield: 61% from 2-bromopyridine). H NMR 7.91 (m, 1H), 7.76–7.78 (m, 1H), 7.67–7.69 (m, 1H), 7.46–7.54
(
CDCl
Hz, 1H), 7.73–7.77 (m, 1H), 7.69 (d, J ¼ 8.0 Hz, 1H), 7.54 (d, J ¼ 4H), 0.48–0.96 (m, 30H).
.0 Hz, 1H), 7.32–7.35 (m, 1H), 7.24–7.28 (m, 1H). 2-L. The mixture of Br-ppy (0.60 g, 2.6 mmol), BTZ-F8-C^CH
3
, 500 MHz): 8.69–8.70 (m, 1H), 8.19 (s, 1H), 7.91 (d, J ¼ 8.4 (m, 3H), 7.35–7.39 (m, 1H), 3.13 (t, J ¼ 2.4 Hz, 1H), 1.97–2.12 (m,
8
B-ppy. To the argon protected THF (30 mL) solution of 2-(3- (1.4 g, 2.6 mmol), Pd(PPh
3
)
2
Cl
2
(20 mg, 0.028 mmol), PPh
3
(50
ꢁ
bromophenyl)pyridine (1.5 g, 6.4 mmol) at ꢀ78 C, BuLi in mg, 0.19 mmol), CuI (5 mg, 0.025), and Et N (30 mL) was heated
3
hexane solution (2.5 M, 4 mL, 10 mmol) was added dropwise. to reux under argon for 24 hours. Aer reaction, the precipitate
Aer stirring for 2 hours, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2- was removed by ltration. The solvent was removed by distil-
dioxaborolane (2 mL, 11.8 mmol) was added dropwise to the lation and the resultant brown oil was puried by running
solution. The system was allowed to warm up to room temper- a silica gel column. The column was rst eluted with CH
ature aerwards, and continued stirring for 10 hours. Then the hexane (v/v ¼ 1/1) then with ethyl acetate/hexane (v/v ¼ 1/3).
mixture was poured into saturated aqueous NH Cl solution (100 0.95 g yellow oil was collected as the nal product (yield:
mL). The organic phase was collected and dried over MgSO
2 2
Cl /
4
1
4
.
3
52%). H NMR (CDCl , 400 MHz): 8.70–8.72 (m, 1H), 8.22–8.23
Aer removal of the solvent, purication of the residue was (m, 1H), 8.07–8.14 (m, 3H), 7.95–7.98 (m, 1H), 7.88–7.91 (m,
carried out by running a silica gel column eluted with mixed 1H), 7.71–7.79 (m, 4H), 7.54–7.62 (m, 3H), 7.44–7.50 (m, 2H),
solvent of hexane and ethyl acetate (v/v ¼ 7.5/1). 1.04 g colorless 7.34–7.38 (m, 1H), 7.22–7.25 (m, 1H), 2.03–2.16 (m, 4H), 0.50–
1
+
52 2
oil was collected as the nal product (yield: 58%). H NMR 0.96 (m, 30H). HRMS: m/z calc. for [C49H N S + H] : 701.3924;
(
1
7
CDCl
3
, 500 MHz): 8.70–8.71 (m, 1H), 8.41 (s, 1H), 8.14–8.16 (m, found: 701.3924. Anal. calc. for C49
H), 7.87–7.89 (m, 1H), 7.79–7.80 (m, 1H), 7.72–7.75 (m, 1H), 83.15; H, 8.30; N, 3.60. Found: C, 82.79; H, 8.76; N, 4.06.
.49–7.52 (m, 1H), 7.20–7.23 (m, 1H), 1.37 (s, 12H). 1. The mixture of 1-L (136 mg, 0.20 mmol), IrCl $4H
H
52
N
2
S$0.4H
2
O$0.8C
6
H
14: C,
3
2
O (37
1-L. The mixture of Br-F8-BTZ (1.78 g, 3.0 mmol), B-ppy (0.87 mg, 0.1 mmol), 2-ethoxyethanol (9 mL) and water (3 mL) was
g, 3.0 mmol), K CO (2.11 g, 15.3 mmol), Pd(PPh ) (55 mg, heated to reux under argon for 24 hours. Aer the mixture was
2
3
3 4
0
.048 mmol), PPh (150 mg, 0.57 mmol), toluene (30 mL), and cooled to room temperature, 50 mL water was added and the
3
H O (15 mL) was heated to reux under argon for 36 hours. yellow precipitate was collected and washed with hexane and
2
Aer reaction, the organic phase was collected, washed with water. The resultant yellow powder was dried in vacuum oven
brine (50 mL ꢂ 2) and water (50 mL ꢂ 2), and dried over MgSO
4
.
for 24 hours and directly used for the following reaction.
0
Aer toluene was removed by distillation, the sticky oil was
The yellow powder collected, 2,2 -bipyridine (16 mg, 0.1
3 3
puried by silica gel column eluted with mixed solvent of mmol), AgCF SO (26 mg, 0.1 mmol) and 2-ethoxyethanol (10
hexane and ethyl acetate (v/v ¼ 10/1). 1.11 g colorless oil was mL) were heated to reux under argon for 48 hours. Aer the
1
collected as the product (yield: 55%). H NMR (CDCl , 500 mixture was cooled to room temperature, NH PF (17 mg, 0.1
3
4
6
MHz): 8.76 (d, J ¼ 4.5 Hz, 1H), 8.29–8.31 (m, 1H), 8.15–8.19 (m, mmol) was added and the mixture was stirred at room
H), 8.11–8.14 (m, 2H), 8.01 (d, J ¼ 7.5 Hz, 1H), 7.93 (d, J ¼ 8.0 temperature for 3 hours. Aer the reaction was nished, 50 mL
Hz, 1H), 7.78–7.86 (m, 4H), 7.70–7.74 (m, 3H), 7.59–7.62 (m, ethyl acetate was added. The combined organic solution was
1
1
H), 7.50–7.53 (m, 1H), 7.40 (t, J ¼ 7.5 Hz, 1H), 7.27–7.30 (m, washed with brine (50 mL ꢂ 3). Aer drying over MgSO
4
and
1H), 2.11–2.21 (m, 4H), 0.54–0.99 (m, 30H). HRMS: m/z calc. for ltered, the solvent was removed by distillation. Purication
+
[C
47
H
52
N
2
S + H] : 677.3924; found: 677.3918. Anal. calc. for was conducted by a ash silica gel column eluted with
C
47
H
52
N
2
S$0.5H
2
O$0.5C
6
H14: C, 82.37; H, 8.30; N, 3.84. Found: dichloromethane followed by recrystallization from dichloro-
C, 82.24; H, 8.52; N, 4.21.
methane and hexane. 69 mg orange powder was collected as the
1
BTZ-F8-C^CH. The mixture of Br-F8-BTZ (3.2 g, 5.3 mmol), nal product (yield: 37% for two steps). H NMR (CDCl , 500
3
(
trimethylsilyl)acetylene (1 mL, 7.1 mmol), Pd(PPh ) Cl (10 mg, MHz): 8.70–8.71 (m, 2H), 8.19–8.23 (m, 2H), 8.07–8.16 (m, 10H),
3
2
2
0
.014 mmol), PPh
N (50 mL) was heated to reux under argon for 24 hours. 7.52 (m, 4H), 7.39 (t, J ¼ 7.5 Hz, 2H), 7.30–7.33 (m, 2H), 7.17 (t, J
Aer reaction, the precipitate was removed by ltration. Aer ¼ 6.5 Hz, 2H), 6.50–6.53 (m, 2H), 2.07–2.20 (m, 8H), 0.51–0.99
3
(20 mg, 0.076 mmol), CuI (2 mg, 0.01 mmol), 7.99–8.00 (m, 2H), 7.79–7.93 (m, 8H), 7.62–7.65 (m, 6H), 7.48–
and Et
3
+
being conrmed by HRMS, the yellow oil obtained aer (m, 60H). HRMS: m/z calc. for [C104
110 6 2
H N S Ir] : 1699.7866;
2
+
found: 1699.7893. m/z calc. for [C104 N S Ir + H] : 850.3969;
H
110 6 2
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found: 850.3998. Anal. calc. for C104
H
110
N
6
S
2
IrPF
6
: C, 67.69; H,
C
105
H
110
N
5
S
2
Ir$2CH
2
Cl
2
$0.75C
6
H14$2H
2
O: C, 68.02; H, 6.58; N,
6
.01; N, 4.55; S, 3.48. Found: C, 67.63; H, 6.32; N, 4.62; S, 3.80. 3.56. Found: C, 67.91; H, 6.31; N, 3.26.
. The synthesis of 2 followed the same procedure as for 1. 2- fac-5. The mixture of 1-L (136 mg, 0.20 mmol), IrCl $4H O
L (140 mg, 0.20 mmol) instead of 1-L was used for the reaction. (37 mg, 0.1 mmol), 2-ethoxyethanol (9 mL) and water (3 mL) was
2
3
2
4
2
3 mg yellow powder was collected as the nal product (yield: heated to reux under argon for 24 hours. Aer the mixture was
3% for two steps). H NMR (CDCl , 400 MHz): 8.67–8.69 (m, cooled to room temperature, 50 mL water was added and the
3
1
2H), 7.69–8.18 (m, 24H), 7.35–7.56 (m, 12H), 7.10–7.13 (m, 2H), yellow precipitate was collected and washed with hexane and
6
.30–6.32 (m, 2H), 1.98–2.12 (m, 8H), 0.46–0.90 (m, 60H). water. The resultant yellow powder was dried in vacuum oven
2
+
110 6 2
HRMS: m/z calc. for [C108H N S Ir + H] : 874.3970; found: for 24 hours and used for the following reaction directly. The
8
6
74.3999. Anal. calc. for C108H110N S IrPF $4H O: C, 66.00; H, yellow powder collected, 1-L (68 mg, 0.10 mmol), AgCF SO (26
6 2 6 2 3 3
.05; N, 4.28. Found: C, 65.93; H, 6.29; N, 4.03.
fac-3. The mixture of 2-phenylpyridine (32 mg, 0.20 mmol), reux under argon for 48 hours. Aer the reaction was nished,
IrCl $4H O (37 mg, 0.1 mmol), 2-ethoxyethanol (9 mL) and 50 mL ethyl acetate was added. The combined organic solution
water (3 mL) was heated to reux under argon for 24 hours. Aer was washed with brine (50 mL ꢂ 3). Aer drying over MgSO
the mixture was cooled to room temperature, 50 mL water was the solvent was removed by distillation. Purication was carried
added and the yellow precipitate was collected and washed with out by a ash silica gel column eluted with dichloromethane (R
mg, 0.1 mmol) and 2-ethoxyethanol (10 mL) were heated to
3
2
4
,
f
hexane and water. The resultant yellow powder was dried in ¼ 0.37) followed by recrystallization from dichloromethane and
vacuum oven for 24 hours and used for the following reaction hexane. 9 mg orange powder was collected as the nal product
1
directly. The yellow powder collected, 1-L (68 mg, 0.1 mmol), (yield: 4% for two steps). H NMR (CDCl , 500 MHz): 8.07–8.14
3
AgCF SO (26 mg, 0.1 mmol) and 2-ethoxyethanol (10 mL) were (m, 12H), 7.98 (m, 3H), 7.92 (d, J ¼ 8.0 Hz, 3H), 7.78–7.81 (m,
3
3
heated to reux under argon for 48 hours. Aer the reaction was 6H), 7.66–7.71 (m, 12H), 7.48–7.52 (m, 3H), 7.39 (t, J ¼ 8.0 Hz,

nished, 50 mL ethyl acetate was added. The combined organic 3H), 7.25–7.31 (m, 3H), 7.10 (m, 3H), 6.98–7.00 (m, 3H), 2.05–
solution was washed with brine (50 mL ꢂ 3). Aer drying over 2.19 (m, 12H), 0.51–0.99 (m, 90H). Anal. calc. for C141
MgSO , the solvent was removed by distillation. Purication was Ir$0.5C 14$5H O: C, 73.49; H, 7.28; N, 3.57. Found: C, 73.45;
carried out by a ash silica gel column eluted with dichloro- H, 7.38; N, 3.61.
It is worth mentioning that during the column purication
H N -
153 6
4
S
3
6
H
2
methane (Rf ¼ 0.76) followed by recrystallization from
dichloromethane and hexane. 15 mg yellow powder was of complexes 4 and 5, the mer-isomers of 4 (R ¼ 0.17 eluted with
f
1
collected as the nal product (yield: 13% for two steps). H NMR CH Cl ) and 5 (R ¼ 0.27 eluted with CH Cl ) that have different
2
2
f
2
2
1
(CDCl
3
, 500 MHz): 7.88–8.14 (m, 8H), 7.19–7.81 (m, 13H), 7.37–
H-NMR spectra and much shorter emission lifetimes were also
7
.41 (m, 1H), 7.18–7.20 (m, 1H), 6.85–6.99 (m, 10H), 2.06–2.16 obtained. However, the mer-isomer of complex 3 was unable to
SIr + be obtained under the aforementioned experimental condi-
tions. Because the mer-isomers are not the focus of this work,
O: C, 70.15; H, 5.97; N, 4.64. Found: C, their characterization data and photophysical properties are not
67 4
(m, 4H), 0.51–0.90 (m, 30H). HRMS: m/z calc. for [C69H N
+
H] : 1177.4793; found: 1177.4806. Anal. calc. for C69
SIr$0.25C 14$0.5H
9.98; H, 6.22; N, 4.27.
fac-4. The mixture of 1-L (136 mg, 0.20 mmol), IrCl $4H O
67 4
H N -
6
H
2
6
included in this paper and will be reported later.
3
2
(
37 mg, 0.1 mmol), 2-ethoxyethanol (9 mL) and water (3 mL) was
Photophysical and nonlinear transmission measurements
heated to reux under argon for 24 hours. Aer the mixture was
cooled to room temperature, 50 mL water was added and the The solvents used for photophysical experiments were spec-
yellow precipitate was collected and washed with hexane and troscopic grade, and were purchased from VWR International
water. The resultant yellow powder was dried in vacuum oven and used as is without further purication. UV-vis absorption
for 24 hours and used for the following reaction directly. The spectra were recorded on a Shimadzu UV-2501 spectropho-
yellow powder collected, 2-phenylpyridine (16 mg, 0.1 mmol), tometer. A FluoMax-4 uorometer/phosphorometer was used to
AgCF SO (26 mg, 0.1 mmol) and 2-ethoxyethanol (10 mL) were measure the steady-state emission spectra in different solvents.
3
3
heated to reux under argon for 48 hours. Aer the reaction was The emission quantum yields were determined by the relative
30

nished, 50 mL ethyl acetate was added. The combined organic actinometry in degassed solutions, in which the degassed
solution was washed with brine (50 mL ꢂ 3). Aer drying over acetonitrile solution of [Ru(bpy) ]Cl (F ¼ 0.097, l ¼ 436
3
2
em
ex
31
MgSO
4
, the solvent was removed by distillation. Purication was nm) was used as the reference for complexes 1–5, and
carried out by a ash silica gel column eluted with dichloro- a degassed 1 N sulfuric acid solution of quinine bisulfate (Fem
¼
32
methane (R
f
¼ 0.55) followed by recrystallization from 0.546, lex ¼ 347.5 nm) was used as the reference for ligands 1-L
dichloromethane and hexane. 11 mg orange powder was and 2-L.
1
collected as the nal product (yield: 6% for two steps). H NMR
The nanosecond transient difference absorption (TA) spectra
(
CDCl , 400 MHz): 7.88–8.12 (m, 13H), 7.73–7.78 (m, 4H), 7.57– and decays were measured in degassed toluene solutions on an
3
7.70 (m, 11H), 7.45–7.50 (m, 2H), 7.34–7.37 (m, 2H), 7.17–7.20 Edinburgh LP920 laser ash photolysis spectrometer. The third
(
6
m, 2H), 6.85–7.04 (m, 8H), 2.01–2.13 (m, 8H), 0.47–0.86 (m, harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant,
2
+
0H). HRMS: m/z calc. for [C105
110 5 2
H N S Ir + 2H] : 849.8993; pulse width: 4.1 ns, repetition rate was set to 1 Hz) was used as
found:
849.8991.
Anal.
calc.
for the excitation source. Each sample was purged with argon for 30
min prior to measurement. The triplet excited-state absorption
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coefficient (3
T
) at the TA band maximum was determined by the energy of the lowest triplet state, independent on the initial
33
singlet depletion method. Aer obtaining the 3 value, the F
wavefunction conguration.
T
T
could be determined by the relative actinometry using SiNc in
ꢀ
1
ꢀ1
34
benzene as the reference (3590 ¼ 70 000 M cm , F ¼ 0.20).
T
Results and discussion
Geometries of complexes 1–5
The reverse saturable absorption of complexes 1–5 was
characterized by nonlinear transmission experiment at 532 nm
using a Quantel Brilliant laser as the light source. The pulse It has been widely reported in the literature that the generally
width of the laser was 4.1 ns and the repetition rate was set to 10 formed conguration for the heteroleptic cationic Ir(III)
+
Hz. The complexes were dissolved in CH
2
Cl
2
. The concentration complexes Ir(ppy) (bpy) is the one with the two cyclometalating
2
of the sample solutions was adjusted to obtain a linear trans- carbon atoms cis to each other and lying in the same plane with
mission of 90% at 532 nm in a 2 mm-thick cuvette. The exper- the two nitrogen atoms from the bpy ligand.4
8–52
Therefore, we
imental setup and details are similar to that reported believe our two cationic complexes 1 and 2 also adopt this
35
previously. A 40 cm plano-convex lens was used to focus the conguration (see the conguration drawn for complexes 1 and
beam to the center of the 2 mm thick sample cuvette. The radius 2 in Chart 1). For tris-cyclometalating Ir(C^N) complexes, fac
3
49,53,54
of the beam at the focal plane was approximately 96 mm.
and mer isomers are possible.
The fac isomers have much
longer emission lifetimes and higher emission quantum yields
53,55
in comparison to their corresponding mer isomers.
It has
Computational methodology
been reported that when the reaction temperature to convert the
All calculations were performed using Gaussian-09 soware Cl-bridged Ir dimmer into the mononuclear Ir complex is below
36
ꢁ
53,54
package. Density functional theory (DFT) was utilized to 160 C, the mer isomer is the dominant product.
determine the optimized singlet ground state geometry of each isomers can be the major product when AgCF SO or AgCF CO
2
However, fac
3
3
3
complex. These calculations were performed using the is used as the catalyst and the reaction temperature is below 160
3
7,38
39,40
41–43
ꢁ
55,56
PBE1PBE
functional with LANL2DZ
and 6-31G(d)
C.
Because our reactions for synthesis of complexes 3–5 were
ꢁ
basis set for the metal and ligand atoms respectively. The carried out at ꢃ130 C in the presence of AgCF SO , and the
3
3
44,45
Conductor Polarized Continuum Model (CPCM)
was utilized emission lifetimes of complexes 3–5 are all on the order of tens
to simulate the solvent toluene. Time-dependent density func- of ms (will be discussed in the photoluminescence section), it is
tional theory (TDDFT) calculations were performed from the reasonable to believe that these complexes adopt the fac geom-
optimized geometries using the same functional, basis set, and etry although all of our attempts to obtain single crystals to
CPCM solvent model. Our previous studies have demonstrated conrm the geometry of these complexes failed due to the
that this methodology reasonably reproduced the absorption presence of branched long alkyl chains in their ligands. The fac
and emission spectra of the heteroleptic cationic Ir(III) geometry is used in all of our DFT calculations for the electronic
complexes with extended p-conjugation on the diimine and/or absorption and emission. It should also be noted that although
25–27
the phenylpyridine ligands.
As such, it is expected to be the C^N ligand contains two cyclometalating units, only the 2-
suitable for complexes 1–5. For calculations of absorption phenylpyridine unit coordinates with the Ir(III) ion.
spectra, 70 excited states were calculated. Spectra were gener-
ated by broadening the resultant optical transitions with
a Gaussian function with line-width of 0.1 eV to t the experi-
mental spectra. Natural transition orbitals (NTOs) were gener-
Electronic absorption
The electronic absorption of 1-L, 2-L, and 1–5 in toluene obey
ꢀ
6
ꢀ4
Beer's law in the concentration range of 5 ꢂ 10 to 1 ꢂ 10
ated to characterize the nature of the optical transitions in two
different energetic regions: 413–600 nm and 250–413 nm.
Calculations of the emission energies were carried out using
ꢀ
1
mol L , suggesting the absence of ground-state aggregation in
46,47
analytical TDDFT gradients,
by optimizing either the lowest
singlet (uorescence) or triplet (phosphorescence) state as
implemented in the Gaussian-09 soware. 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 wavefunction choosing either
the very lowest triplet state (root ¼ 1) or the second lowest triplet
state (root ¼ 2). Both guessing states were initially taken at the
ground state geometry with the triplet spin conguration, where
their electronic wavefunctions were calculated by regular
TDDFT procedure. By starting with different initial wave-
functions, we were able to converge to different triplet states
3
having similar p,p* character but very different energies for
complex 3. In contrast, other complexes did not recover
Fig. 1 Electronic absorption spectra of 1-L, 2-L, and 1–5 in toluene at
different states, resulting in the same type and nearly the same room temperature.
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Table 1 Electronic absorption, emission, and excited-state absorption parameters for complexes 1–5 and ligands 1-L and 2-L
a
b
a
l
abs /nm
l
em/nm
l
em /nm
l
T1–Tn /nm
ꢀ
1
ꢀ1
a
ꢀ1
ꢀ1
(
log 3/L mol cm
)
(s/ms); Fem , R.T.
(s/ms), 77 K
(s /ms; log 3T1–Tn/L mol cm ; F )
T
T
1
2
3
350 (sh, 5.06), 361 (5.13),
80 (sh, 5.03), 420 (3.85)
350 (sh, 5.07), 369 (5.15),
92 (sh, 4.97), 425 (4.03)
363 (sh, 4.68), 390 (4.73),
60 (3.66), 484 (3.34)
583 (6.8), 633 (7.6); 0.095
592 (8.5), 644 (6.0); 0.13
546 (1447), 594 (966)
558 (933), 598 (—)
548 (857), 592 (622)
510 (0.06; 5.14; 0.042)
568 (7.9; 5.04; 0.063)
524 (27.5; 4.73; 0.28)
3
3
518 (29.0 (83%), 1.15 (17%)),
556 (27.9 (89%), 1.17 (11%)),
4
6
0
04 (25.6 (85%), 1.53 (15%));
.22
4
5
365 (sh, 4.96), 388 (4.98),
64 (4.81), 495 (3.40)
520 (36.0 (92%), 1.25 (8%)),
555 (34.1 (96%), 1.31 (4%)),
552 (426), 590 (—)
532 (35.3; 5.03; 0.14)
534 (37.7; 5.14; 0.11)
4
5
6
0
95 (34.2 (78%), 2.40 (22%)),
48 (24.3 (56%), 1.69 (44%));
.38
366 (sh, 5.15), 389 (5.18),
65 (4.81), 498 (3.43)
519 (35.1), 555 (33.1), 594
(33.1 (85%), 2.9 (15%)),
556 (502), 586 (411)
4
6
0
48 (34.8 (68%), 2.6 (32%));
.45
1
2
-L
-L
353 (4.63), 371 (4.49)
362 (4.68), 377 (4.57)
389 (—), 409 (—), 429 (—); 0.76
392 (—), 415 (—), 438 (—); 0.67
580 (67.7; —; —)
610 (7.1; —; —)
a
b
In toluene. In butyronitrile.
1
,3
1,3
the concentration range used. The electronic absorption spectra the charge transfer transitions including MLCT, LLCT or
1
,3
1
of 1-L, 2-L, and 1–5 in toluene are shown in Fig. 1, and their
ILCT. Similar to the trend observed from the p,p* transi-
absorption band maxima and molar extinction coefficients are tions, the charge-transfer absorption bands in the neutral
presented in Table 1.
For ligands 1-L and 2-L, well-resolved vibronic structures are
observed in the 300–400 nm region. Considering the large
4
ꢀ1
ꢀ1
molar extinction coefficients (on the order of 10 L mol cm
)
and the well-resolved vibronic structures, these absorption
1
bands are attributed to the p,p* transitions. The absorption
bands of 2-L are red-shied and have larger extinction coeffi-
cients compared to those of 1-L, which is in line with the
extended p-conjugation via incorporating the triple bond in 2-L.
Compared to the absorption spectra of ligands 1-L and 2-L,
the absorption spectra of 1–5 are broader and red-shied, and
the molar extinction coefficients of the major absorption bands
are much larger than their corresponding C^N ligands. The
bathochromic shi indicates an electron delocalization
induced by the interactions with the metal center. The broad-
1
ening and red-shi of the ligand centered p,p* transitions are
more signicant for the neutral complexes 3–5 compared to the
cationic complexes 1 and 2, suggesting a stronger interaction
between the ligand and the metal center in the neutral
complexes than in the cationic complexes. This can be
explained by the stronger ligand eld strength of the 2-phe-
nylpyridine ligand than the bipyridine ligand. It is worth noting
that complexes 3–5 exhibit the similar shape and energy of the
major absorption bands, and the molar extinction coefficients
of the band at ca. 390 nm are roughly proportional to the
number of BTF-ppy ligands. This conrms that the major
1
absorption bands should arise mainly from the p,p* transi-
tions localized on the BTF-ppy ligand.
1
In addition to the ligand centered p,p* transitions, lower
energy absorption bands are observed for all of the ve
Fig. 2 Computed transitions and associated absorption spectra in
complexes above 430 nm, which can be tentatively attributed to toluene for complexes 1–5.
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Table 2 Natural transition orbitals for complexes 1–5 representing the major transitions
Excited state and properties Hole
Electron
Excited state and properties Hole
Electron
1
S
1
, 548 nm, f ¼ 0.0004
2
S10, 388 nm, f ¼ 1.726
S
S
4
, 413 nm, f ¼ 0.138
10, 368 nm, f ¼ 2.385
3 S
1
, 405 nm, f ¼ 1.016
, 400 nm, f ¼ 0.465
S
2
S
11, 365 nm, f ¼ 1.203
S15, 331 nm, f ¼ 0.650
4
S
1
, 404 nm, f ¼ 2.024
, 395 nm, f ¼ 0.176
S
S
4
S
15, 361 nm, f ¼ 0.383
19, 330 nm, f ¼ 0.820
2
S
S
1
, 574 nm, f ¼ 0.0008
, 431 nm, f ¼ 0.316
5 S
1
, 404 nm, f ¼ 0.010
, 402 nm, f ¼ 2.136
4
S
3
S
6
, 403 nm, f ¼ 0.404
S
6
, 392 nm, f ¼ 0.989
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Table 2 (Contd.)
Excited state and properties Hole
Electron
Excited state and properties Hole
Electron
S
S
8
, 395 nm, f ¼ 0.303
, 394 nm, f ¼ 2.525
S
22, 329 nm, f ¼ 0.904
24, 328 nm, f ¼ 0.488
9
S
complexes 3–5 are more red-shied and stronger in comparison room temperature solution and in butyronitrile glassy matrix at
to the cationic complexes 1 and 2. With the increased number of 77 K. The emission spectra of 1–5 at room temperature are
the BTF-ppy ligands, the charge-transfer bands become shown in Fig. 3(a), and the emission spectra of 1–5 at 77 K are
stronger.
shown in Fig. 3(b). The emission spectra of 1-L and 2-L are
Comparison of the spectra of complexes 1 and 4 reveals that included in Fig. 3(a) for comparison purpose. The emission
the neutral Ir(III) complex 4 exhibits weaker but much broader band maxima, emission lifetimes, and quantum yields are
major absorption bands, while its charge transfer bands are summarized in Table 1.
much broader and stronger.
The ligands 1-L and 2-L show strong emission with well-
The aforementioned assignments for each of the absorption resolved vibronic structures and short lifetimes (<5 ns, too
band are supported by the TDDFT calculation results (Fig. 2). short to be measured on our spectrometer), which can be
1
Table 2 displays the NTOs for the major transitions contributing assigned to the p,p* uorescence. Similar to the trend
to the absorption bands of 325–600 nm. The major absorption observed from their electronic absorption spectra, the uores-
bands in the range of 325–425 nm predominantly arise from the cence spectrum of 2-L is red-shied compared to that of 1-L,
1
ligand-localized p,p* transitions, with minor contributions indicating the electron delocalization induced by the more
1
1
from the MLCT/ LLCT transitions. In complexes 3–5, contri- extended p-conjugation in 2-L with the C^C bond.
1
butions from the ILCT transition are also evident. For the two
cationic complexes 1 and 2, the lowest-energy transition in the
1
1
energy range of 425–600 nm is MLCT/ LLCT in nature; and it
separates well from the major absorption bands. However, for
the neutral complexes 3–5, the low-energy transitions essen-
1
tially have the p,p* character, with some contributions from
1
1
the MLCT/ ILCT transitions, which makes the transitions in
the 400 nm region having much higher intensity.
The ascription of the absorption bands in complexes 1–5 is
further supported by the solvent-dependent study. As illustrated
in ESI Fig. S2–S6,† these complexes display a minor sol-
vatochromic effect for both the major absorption bands and the
low-energy absorption tails. This supports the assignment of the
1
predominant p,p* transitions for the major absorption bands.
The insensitivity of the low-energy transitions to the polarity of
solvent could be explained by the shielding of the central metal
from the solvent due to the octahedral geometry of the Ir(III)
complexes. Alternatively, the different directions of the various
charge transfer transitions could partially cancel out the dipole
moment of the lowest-energy transition, which in turn reduces
the solvent inuence. Moreover, mixing of the charge transfer
Fig. 3 (a) Normalized emission spectra of complexes 1–5 and ligands
-L and 2-L at room temperature in toluene at the concentration of 1
1
1
transitions with the p,p* transitions could also play a role.
ꢀ5
ꢀ1
ꢂ 10 mol L (lex: 353 nm for 1-L, 360 nm for 2-L, 363 nm for 1, 400
nm for 2, and 390 nm for 3–5). (b) Normalized emission spectra of 1–5
ꢀ
5
at 77 K in butyronitrile glassy matrix at the concentration of 1 ꢂ 10
Photoluminescence
ꢀ1
mol L (lex: 363 nm for 1, and 400 nm for 2–5). The calculated triplet
emission energies corresponding to the optimized excited states at T
Complexes 1–5 all exhibit strong emission with relatively long
lifetimes (in the scale of several to tens of microseconds) both in ¼ 0 K are shown as droplines in panel (b).
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Table 3 NTOs contributing to the singlet and triplet emissions at T ¼ 0 K
S
1
(nm)
Hole
Electron
T
1
(nm)
Hole
Electron
1
2
685, f ¼ 0.001
720, f ¼ 0.001
691
725
3
4
472, f ¼ 1.831
470, f ¼ 1.992
738
738
5
482, f ¼ 0.045
738
The emission spectra of complexes 1–5 are drastically red-
For the cationic complexes 1 and 2, the room temperature
shied in comparison to their corresponding ligands. They all emission lifetimes are shorter than 10 ms, and the emission
possess well-resolved vibronic structures, with the lifetimes on quantum yields are ca. 10%. The solvent-dependency study (see
the order of several to tens of microseconds. They are all readily ESI Figs. S7 and S8†) reveals a minor negative solvatochromic
quenched by oxygen as well. Considering these facts, we assign effect. These features suggest a predominant charge transfer
the observed emission for these complexes to phosphorescence. nature of the emitting states. In addition, as shown in Fig. 3(b),
However, the emission characteristics (such as the lifetime and the emission spectra of complexes 1 and 2 in butyronitrile
temperature dependence) of the cationic complexes 1 and 2 and matrix at 77 K exhibit well-resolved peaks that are pronouncedly
the neutral complexes 3–5 appear to be quite different, implying blue-shied and narrower compared to the room temperature
a different nature of the emitting triplet excited state for the emission spectra due to rigidochromic effect in the solid
5
7
cationic and neutral complexes.
matrix. The thermally induced shis are on the order of ꢃ1200
ꢀ1
cm , which is a characteristics of the charge transfer emitting
state.
The charge transfer nature of the emitting states for
complexes 1 and 2 is further supported by the TDDFT calcula-
tion results. As shown in Table 3, the holes of complexes 1 and 2
are located on the cyclometalating C^N ligand and the d-
orbitals of the Ir(III) ion; while the electrons are exclusively on
the bipyridine ligand. Therefore, the emitting triplet excited
3
3
states (T ) are the MLCT/ LLCT states for these two complexes.
1
The NTOs in Table 3 also show that the calculated singlet states
responsible for uorescence (S ) have predominate
1
1
1
MLCT/ LLCT character for these two complexes and the
similar energies to their corresponding triplet excited states. It
was previously shown that the stronger the MLCT character
involved in both the singlet and triplet states, the stronger the
spin–orbit coupling (SOC) governing the rate of intersystem
58
crossing (ISC) is. However, identical orbitals with different
spin congurations result in negligible SOC and slow ISC,
Fig. 4 Emission spectra of 5 in toluene under different deaeratation
conditions. lex ¼ 436 nm, A436nm ¼ 0.08 in a 1 cm cuvette.
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which is referred to as the “organometallics version of El- nm, which are absent in the excitation spectra monitored at the
Sayed's rule” in ref. 58. Because the singlet states (S ) are similar high-energy emission bands of 519 and 555 nm. Another piece
1
to the triplet (T ) states in nature, involving a high degree of of evidence conrming the different natures of the high-energy
1
MLCT and identical character of d-orbitals, the SOC in and low-energy emission bands is the different sensitivities of
complexes 1 and 2 should be negligible. Therefore, we conclude these bands to oxygen quenching. As shown in Fig. 4 and ESI
that the observed emission for these two complexes is governed Fig. S12† for complex 5 in toluene, the relative intensities of the
3
3
by the MLCT/ LLCT states.
high-energy emission bands increase more pronouncedly than
The emission energy of 2 is red-shied compared to that of the low-energy emission bands upon purging nitrogen for
1
both at room temperature and 77 K, manifesting the extended longer period of time, indicating the higher sensitivity of the
electron delocalization due to the triple bond in the cyclo- high-energy emission bands to oxygen than the low-energy
metalating ligand in 2. The same trend is reproduced by TDDFT bands due to the different lifetimes of the excited states
calculations, although the calculated energies of triplet emis- predominantly contributing to these bands. This phenomenon
sion are noticeably underestimated compared to the experi- is consistent with that reported in the literature for a neutral
3
mental data. This is a typical error for such calculations due to heteroleptic Ir(III) complex exhibiting high-energy p,p* and
3
60
the higher inaccuracy in the performance of the functional in an low-energy CT dual emission. The different natures of the
excited state optimization restricted to triplet spin congura- emission bands are also reected by the relative intensity of
tion and a lack of consideration of the SOC, which is important these bands in different solvents. As shown in ESI Fig. S13–S15†
for Ir(III) complexes. Nonetheless, the qualitative trend is for complexes 3–5, the relative intensity of the 519 nm band
reproduced by our calculations and agrees with experimental gradually decreases when the polarity of the solvent increases,
emission spectra (see Fig. 3).
while the relative intensities of the 595 and 640 nm bands
In contrast to the emission of the two cationic complexes 1 increase with increased solvent polarity. This trend coincides
3
3
and 2, the emission of these fac-isomers of the neutral with the proposed p,p* and CT nature for the respective high-
complexes 3–5 is drastically blue-shied and more structured. energy and low-energy bands, in which the polar solvents favor
Most importantly, they exhibit dual emission (see the biexpo- the charge transfer emission. TDDFT optimization of the lowest
3
nential decay lifetimes in Table 1), with a long-lived emission of triplet exited state also points to the involvement of MLCT
2
5–35 ms and a short-lived one of 1–2 ms. Although the dual transition in complexes 3–5 (see NTOs presented in Table 3) in
emission is rarely encountered in the Ir(III) complexes, it has the lowest triplet excited state.
been reported in the literature for both the neutral and cationic Because the mer-isomers of complexes 4 and 5 both exhibit
It was generally regarded that the long- structured emission at ca. 595 and 650 (sh) nm, and the emis-
59–61
Ir(III) complexes.
lived high-energy emission emanates from an excited state sion lifetimes are similar to the shorter-lived emission from the
3
enriched with p,p* character, while the short-lived low-energy fac-isomers, it is reasonable to doubt whether the short-lived
3
emitting state has more charge transfer ( CT) character.
emission arises from the trace amount of mer-isomers mixed
The presence of dual emission is partially supported by the in the fac-isomers. However, this possibility can be excluded by
1
different excitation spectra when monitored at the high-energy the absence of the unique H-NMR peaks at 6.0–6.2 ppm and
and low-energy emission bands. As exemplied in ESI Fig. S11† the peaks above 9.0 ppm corresponding to the mer-isomers (see
1
for complex 5, the excitation spectra monitored at the low- the H-NMR spectra for the mer-isomers of 4 and 5 in ESI
energy emission bands of 595 and 647 nm clearly show the
charge-transfer absorption bands at 435–475 nm and 475–550
Fig. 6 Nonlinear transmission of 1–5 at the linear transmission of 90%
2 2
Fig. 5 Nanosecond transient difference absorption spectra of 1–5 in in CH Cl solution in a 2 mm cuvette for 532 nm 4.1 ns laser. The radius
toluene at zero time decay (lex ¼ 355 nm, A355 ¼ 0.4 in a 1 cm cuvette). of the beam waist at the focal point was approximately 96 mm.
41224 | RSC Adv., 2016, 6, 41214–41228
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Fig. S1†) and the absence of the corresponding spot to the mer- shape of the TA spectra, the DODs at the TA band maxima, and
isomers during the TLC analysis. Even if there are undetectable the triplet lifetimes of 3–5 are all similar, indicating that the TA
mer-isomers present, their emission quantum yields are one likely arises from the similar structural component, i.e. the BTF-
order of magnitude lower than those for the fac-isomers (ꢃ4– ppy ligand. Considering the aforementioned feature, the long
5
% for the mer-isomers vs. ꢃ40% for the fac-isomers). Consid- lifetimes (tens of microseconds) deduced from the decays of TA
ering the undetectable amount, we believe that it is impossible (see Table 1), the similarity and difference of the TA spectra with
for the short-lived emission in the fac-isomers of complexes 3–5 that of 1-L (ESI Fig. S26†), and the similar lifetimes to those
being arising from the undetectable amount of the mer-isomers. obtained from the decay of the long-lived emitting species, we
Comparison of the emission of the cationic complex 1 and its conclude that the observed TA of 3–5 originates from the BTF-
neutral counterpart 4 reveals that the neutral complex exhibits ppy ligand centered p,p* state, likely mixed with some
a more structured blue-shied emission with biexponential
3
3
MLCT character in view of the broad TA band in the near-IR
decays. These differences reect the different natures of the region that is lacking in the TA spectrum of the correspond-
emitting states in these two complexes. Except for the presence ing ligand 1-L.
3
of p,p* enriched high-energy emission bands in complex 4, the
It should be noted that although the emission of the fac-
two low-energy emission bands in 4 are slightly red-shied with isomers of 3–5 exhibit biexponential decay, the TA signals decay
respect to those of 1. This can be rationalized by the extended monoexponentially, with the TA lifetimes being consistent with
electron delocalization on the BTF-ppy ligand in 4 versus the the longer emission lifetimes. In addition, the TA decay kinetics
bipyridine ligand in 1 (see NTOs in Table 3), which reduces the monitored at various visible and near-IR wavelengths remain
3
energy of the emitting MLCT states and gives rise to the slightly the same monoexponential decay character (see the similar TA
red-shied emission bands and a shorter lifetime according to lifetimes monitored at different wavelengths in ESI Table S8†).
the energy gap law.
The disparity between the emission and TA kinetics for 3–5 is
At 77 K, the emission spectra of 3–5 are noticeably different uncommon but not unprecedented because the simultaneous
3
from those at room temperature, with the disappearance of the population of two triplet excited states (such as the p,p* state
3
high-energy emission band at 520 nm. Because this band and and the MLCT state) but with only one state emitting and the
the 550 nm band have the similar lifetime and the separation other state showing transient absorption has been reported for
ꢀ
1
62
63,64
between these two bands is approximately 1100 cm , it is some Ru(II) and Pt(II)
reasonable to consider this band as a vibronic structure of the appears that both states emit, but only the high-lying state
50 nm band. At low temperature, this high-energy vibronic exhibits transient absorption. The different emission and TA
state can't be reached, resulting in the disappearance of this kinetics was also found in complex 2.
band at 77 K. The similar phenomenon has been reported in Comparison of the TA features of 4 to those of 1 reveals that
complexes. For complexes 3–5, it
5
60
literature for another neutral Ir(III) complex.
replacing the bipyridine ligand with the 2-phenylpyridine ligand
results in a broadening of the TA spectrum of 4, and switching
3
3
3
the TA excited state from CT in 1 to p,p*/ MLCT in 4.
Transient absorption
Nanosecond transient absorption (TA) of complexes 1–5 in
toluene was investigated to further understand their triplet
Reverse saturable absorption
excited-state characteristics. The spectra for 1–5 at zero delay Complexes 1–5 all possess stronger excited-state absorption
aer excitation are displayed in Fig. 5. The time-resolved TA than that of the ground state at 532 nm noticing the positive
spectra of 1–5 are shown in ESI Figs. S17–S21.† The triplet absorption bands in the visible spectral region of their TA
excited-state lifetimes, extinction coefficients, and quantum spectra. Meanwhile, the triplet-state lifetimes are signicantly
yields are summarized in Table 1.
longer than the ns laser pulse width (4.1 ns). Therefore, it is
The TA spectra of all complexes feature a bleaching band in expected that these complexes would exhibit reverse saturable
1
the blue spectral region corresponding to their respective p,p* absorption (RSA) for ns laser pulses at 532 nm. To verify this
absorption band. Complexes 1 and 2 show moderate and broad assumption, nonlinear transmission experiment was carried
TA band spreading the wavelength range of 400–700 nm for 1 out for complexes 1–5 in CH Cl solutions with a linear trans-
2 2
and 400–800 nm for 2, and the TA of 2 is red-shied in mission of 90% in a 2 mm cuvette using the 532 nm ns laser
comparison to that of 1 due to the more extended p-conjugation pulses and the results are shown in Fig. 6. The transmissions of
in the C^N ligand. The triplet excited-state lifetimes deduced all of the complexes decrease remarkably with increased inci-
from the decay of the TA is ꢃ60 ns for 1, suggesting the triplet dent energy, which clearly demonstrates a strong RSA from
3
excited-state absorption is likely from a short-lived CT state. In these complexes. The strength of the RSA follows this trend:
contrast, the TA lifetime of 2 is several microseconds, which is 5 < 4 < 1 z 3 < 2. It appears that the increased number of BTF-
in line with the lifetime deduced from the emission. Therefore, ppy ligand reduces the RSA of the neutral complexes while the
the observed TA for 2 is tentatively attributed to the increased p-conjugation of the ppy ligand in the cationic
3
3
MLCT/ LLCT state.
The positive TA region of 3–5 features two parts: a stronger exhibits a weaker RSA at 532 nm than its cationic counterpart 1.
absorption band in the wavelength range of 450–610 nm and The ratios of the excited-state absorption cross section to
a weaker but broader band in the range of 610–800 nm. The that of the ground state (sex/s ) at 532 nm were evaluated to
complexes improves the RSA. However, the neutral complex 4
0
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3
3
Table 4 Ground-state (s ) and excited-state (sex) absorption cross- states for 1 and 2, and from p,p*/ MLCT states for 3–5. The
0
sections of complexes 1–5 at 532 nm in toluene
neutral complex 4 exhibits a much broader, stronger, and
longer-lived TA in comparison to the cationic complex 1.
Reverse saturable absorption of these complexes at 532 nm were
Complex
1
2
3
4
5
ꢀ
18
2
s
s
s
0
/10
cm
0.63
298
473
0.65
394
606
0.31
204
658
0.75
413
551
0.80 evaluated for nanosecond laser pulses. The results demonstrate
ꢀ
18
2
ex/10
ex/s
cm
530
663
that these complexes all exhibit strong exhibit strong RSA for ns
laser pulses at 532 nm, with a trend of 5 < 4 < 1 z 3 < 2. It
appears that the increased number of BTF-ppy ligand reduces
the RSA of the neutral complexes while the increased p-conju-
gation of the ppy ligand in the cationic complexes improves the
RSA. However, the neutral complex 4 exhibits a weaker RSA at
0
rationalize the RSA trend of these complexes. According to the
ground-state molar extinction coefficients at 532 nm and at the
wavelength of the TA band maximum, the optical density
changes (DOD) at 532 nm and at the TA band maximum, the
molar extinction coefficient at the TA band maximum (3T1–Tn),
5
32 nm than its cationic counterpart 1. The trend is primarily
determined by the ratio of the excited-state absorption cross
section to that of the ground state (sex/s ) at 532 nm, but the
0
65
and the method described by our group previously, the
excited-state absorption cross sections (sex) for 1–5 at 532 nm
were estimated and the results are shown in Table 4. The trend
of the s /s ratio roughly matches the observed RSA trend. It is
triplet quantum yield also plays a role in complexes 3–5.
Acknowledgements
ex
0
clear that the two cationic complexes 1 and 2 have the similar
ground-state absorption cross section at 532 nm, but their
triplet excited-state absorption cross section is drastically
different at 532 nm. Complex 2 that has the triple bond to
connect the BTF substituent to the phenylpyridine ligand
exhibits much stronger excited-state absorption and thus
a stronger RSA. For the three neutral complexes 3–5, both the
ground-state absorption and excited-state absorption cross
sections at 532 nm increase when the number of BTF-ppy ligand
The synthesis and photophysical studies of this work was sup-
ported by the Army Research Laboratory (W911NF-10-2-0055) to
W. Sun. The DFT calculations were supported by the National
Science Foundation (NSF DMR-1411086 and CNS-1229316) to
W. Sun and S. Kilina. In the frame of computational method
development, S. Kilina acknowledges support from the NDSU
Advance FORWARD program sponsored by NSF HRD-0811239
joined and ND EPSCoR through the NSF grant EPS-0814442
and nancial support of the Alfred P. Sloan Research Fellow-
ship BR2014-073. For computational resources and adminis-
trative support, we thank the Center for Computationally
Assisted Science and Technology (CCAST) at North Dakota State
University.
0
increases, the sex/s ratio remains quite similar from complex 3
to complex 5. However, the triplet quantum yield decreases
from complex 3 to complex 5. The combined parameter F
corresponds to the observed RSA trend for 3–5.
T 0
sex/s
Conclusions
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