Platinum chloride complexes containing 6-[9,9-Di(2-ethylhexyl)-7-R-9 H-fluoren-2-yl]-2,2′-bipyridine ligand (R = NO2, CHO, benzothiazol-2-yl, n-Bu, carbazol-9-yl, NPh2): Tunable photophysics and reverse saturable absorption

Article abstract of DOI:10.1021/ic400683u

Six new platinum(II) chloride complexes 1-6 containing a 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine (R = NO ₂, CHO, benzothiazol-2-yl (BTZ), n-Bu, carbazol-9-yl (CBZ), NPh ₂) ligand were synthesized and characterize

Full text of DOI:10.1021/ic400683u

Article
pubs.acs.org/IC
Platinum Chloride Complexes Containing 6‑[9,9-Di(2-ethylhexyl)-7-
R‑9H‑fluoren-2-yl]-2,2′-bipyridine Ligand (R = NO2, CHO,
Benzothiazol-2-yl, n‑Bu, Carbazol-9-yl, NPh2): Tunable Photophysics
and Reverse Saturable Absorption
Zhongjing Li, Ekaterina Badaeva, Angel Ugrinov, Svetlana Kilina, and Wenfang Sun*
Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States
S
* Supporting Information
ABSTRACT: Six new platinum(II) chloride complexes 1−6 containing a
6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine (R = NO2,
CHO, benzothiazol-2-yl (BTZ), n-Bu, carbazol-9-yl (CBZ), NPh2) ligand
were synthesized and characterized. The influence of the electron-
donating or electron-withdrawing substituent at the 7-position of the
fluorenyl component on the photophysics of these complexes was
systematically investigated by spectroscopic methods and simulated by
time-dependent density functional theory (TDDFT). Electron-with-
drawing or -donating substituents exert distinct effects on the photo-
physics of the complexes. All complexes feature a low-energy, broad
1MLCT (metal-to-ligand charge transfer)/1ILCT (intraligand charge
transfer)/1π,π* absorption band (tail) above ca. 430 nm and a major
1
absorption band(s) between 320 and 430 nm, which admix MLCT,
1π,π*, 1ILCT, and/or 1LLCT (ligand-to-ligand charge transfer) characters.
The contributions of different configurations to the major absorption band(s) vary depending on the nature of the substituent.
Strong electron-donating or -withdrawing substituents (NPh2 and NO2) and the aromatic substituent BTZ cause a pronounced
red-shift of the absorption spectra of 1, 3, and 6. All complexes are emissive at room temperature and at 77 K. The emitting
3
3
excited state is dominated by π,π* character in 1−3, with some contributions from MLCT in 1 and 2, while the emission is
3
3
3
predominantly from the MLCT state for 4 and 5 but with some π,π* character. For 6, the emitting state is ILCT in nature.
With the increased electron-donating ability of the substituent, the π,π* character diminishes while charge transfer character
3
increases. All complexes exhibit broad and strong triplet excited-state absorption (TA) from the near-UV to the near-IR spectral
region. The TA band maxima are red-shifted for complexes 1−3 (which possess the electron-withdrawing substituents)
compared to those of 4−6 (which contain electron-donating substituents). All complexes manifest strong reverse saturable
absorption (RSA) for a nanosecond laser pulse at 532 nm, which originates from the much stronger triplet excited-state
absorption than the ground-state absorption of 1−6 in the visible spectral region. The strength of RSA follows this trend: 4 ≈ 5 <
1 ≈ 3 < 2 < 6, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the
ground-state absorption (σex/σ0) at 532 nm. The σex/σ0 ratios (116−261) of 1−6 at 532 nm are much larger than those of most
of the reverse saturable absorbers reported in the literature, with the ratio of 6 (σex/σ0 = 261) being among the largest values
reported to date. This makes complexes 1−6, especially 6, very promising reverse saturable absorbers.
recent years, our group and Schanze’s group have also
extensively explored the application of various Pt(II) complexes
as nonlinear transmission materials.8
Compared to bidentate ligands, terdentate ligands are more
resistant to distortion toward D2d conformations when
coordinated with d8 transition metals, and thus disfavor
nonradiative processes.9 Terpyridine ligands stand out for
their preference to adopt planar geometries. Unfortunately, the
binding angle of terpyridine is not ideal when forming a square
planar configuration,10 which reduces the ligand field and thus
INTRODUCTION
■
Platinum(II) complexes have been fascinating for decades due
to their rich photophysics and potential applications in various
photonic devices and in cancer treatment.1 For instance,
Lippard’s group has extensively investigated the application of
platinum complexes as anticancer reagents;2 Che and co-
workers employed platinum(II) diimine arylacetylide com-
plexes and cyclometalated 6-aryl-2,2′-bipyridine arylacetylide
platinum complexes in organic light emitting devices;3 The
Eisenberg,4 Schanze,5 and Yam6 groups independently studied
the application of platinum complexes in dye sensitized solar
cells; our group and several other groups reported the
utilization of platinum complexes as chemical sensors.7 In
Received: March 20, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Scheme and Structures for Complexes 1−6 (C8H17 = 2-Ethylhexyl)
facilitates the accessibility of the deactivating metal-centered d−
d state. In contrast, terdentate cyclometalating ligands with a
C∧N∧N framework, by incorporating an hydrocarbon aryl unit
at the 6-position of 2,2′-bipyridine, have shown several merits
over terpyridine ligands. First, cyclometalation makes it possible
to construct a neutral inner coordination sphere. Second, the
strong σ donating ability of the ligating C− atom and good π
accepting capability of the bipyridine motif offer the metal
center a strong ligand field, which increases the d−d excited
state and consequently reduces the deactivation of the lowest-
energy charge transfer excited state via the d−d excited state.
Third, the C∧N∧N type ligand would form a less-distorted
square-planar geometry with the d8 transition metals, which
reduces the radiationless decay as well. Fourth, the stronger π-
donating ability of the 6-aryl ring could also introduce some
intraligand charge transfer character into the lowest excited
states, resulting in a long-lived triplet excited state.
Pt(II) complexes.8c As a result, stronger reverse saturable
absorption was obtained. Inspired by this investigation and by
the merit of C∧N∧N ligand vs terpyridine ligand, we recently
designed a complex with the C∧N∧N framework that
incorporates fluorene at the 6-position of the bipyridine.11
The complex features large ratios of excited-state absorption
cross section to ground-state absorption cross section in the
visible spectral region and strong two-photon absorption in the
near-IR region. Both of them account for the strong nonlinear
transmission of the complex. These results are quite exciting;
however, the structure−property correlations in Pt(II)
complexes with this type of C∧N∧N ligand have not been
explored yet. In addition, in order to further improve the
reverse saturable absorption of this type of Pt(II) complexes, it
is critical to reduce the ground-state absorption of the
complexes in the visible spectral region. This could be realized
by replacing the 4-tolylacetylide coligand by chloride coligand,
which would reduce the contribution of the ligand-to-ligand
charge transfer (LLCT) to the lowest excited state and cause
hyposochromic shift of the charge transfer absorption band.
Taking these factors into account, we designed and synthesized
six new Pt(II) complexes (structures of the new complexes 1−6
are illustrated in Scheme 1) with different electron-donating or
In one of our previous studies, we found that incorporation
of fluorenyl substituent to the 4-position of the center pyridine
ring of the terpyridine ligand could efficiently increase the
emission efficiency, prolong the triplet excited-state lifetime,
and increase the ratio of the excited-state absorption cross
section relative to the ground-state absorption cross section in
B
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Elemental analyses were conducted by NuMega Resonance Labo-
ratories, Inc. in San Diego, California.
-withdrawing substituents at the 7-position of the fluorene
component. In order to increase the solubility of the Pt(II)
complexes, branched alkyl chains (2-ethylhexyl) were intro-
duced at the 9-positon of the fluorene. Here we report the
synthesis, photophysics, and reverse saturable absorption of
these complexes. Insights into the nature of optical transitions
have also been obtained from the time-dependent density
functional theory (TDDFT) calculations.
General Procedure for the Synthesis of Ligands 1-L−6-L.
The mixture of R-F8-B (1 mmol), 6-bromo-2,2′-bipyridine (235 mg, 1
mmol), K2CO3 (410 mg, 3 mmol), Pd(PPh3)2Cl2 (30 mg), and PPh3
(60 mg) in toluene (20 mL) and water (10 mL) was heated to reflux
under argon for 40 h. After reaction, the organic layer was collected,
washed with brine (30 mL × 3), and dried over MgSO4. Then the
solvent was removed to obtain the crude product. The crude product
was purified using column chromatography.
EXPERIMENTAL SECTION
Synthesis and Characterization. The synthetic routes for 1−6
are illustrated in Scheme 1. All of the reagents and solvents were
1-L. Purified by silica gel column chromatography eluting with
■
dichloromethane. 0.53 g of pale yellow solid was obtained (yield:
1
90%). H NMR (CDCl3, 400 MHz): 8.70−8.71 (m, 1H), 8.63−8.65
(m, 1H), 8.40−8.42 (m, 1H), 8.27−8.29 (m, 2H), 8.19−8.22 (m, 2H),
7.80−7.94 (m, 5H), 7.32−7.35 (m, 1H), 2.08−2.18 (m, 4H), 0.48−
0.83 (m, 30H). ESI-HRMS: m/z calc for [C39H47N3O2 + H]+:
590.3741. Found: 590.3734. Anal. Calc for C39H47N3O2: C, 79.42; H,
8.03; N, 7.12. Found: C, 79.13; H, 8.06; N, 7.15.
Table 1. Single Crystal X-ray Parameters and Refinement
Data for 1
T (K)
100(2)
2-L. Purified by silica gel column chromatography eluting with
λ (Å)
0.71073
1
dichloromethane. 0.49 g of yellow oil was obtained (yield: 86%). H
formula
MW
C39H46ClN3O2Pt·CH2Cl2
904.25
NMR (CDCl3, 400 MHz): 10.07 (s, 1H), 8.69−8.70 (m, 1H), 8.64−
8.66 (m, 1H), 8.40 (d, J = 8.0 Hz, 1H), 8.16−8.21 (m, 2H), 7.94−7.97
(m, 1H), 7.80−7.92 (m, 6H), 7.31−7.34 (m, 1H), 2.06−2.19 (m, 4H),
0.46−0.86 (m, 30H). ESI-HRMS: m/z calc for [C40H48N2O + H]+:
573.3839. Found: 573.3823. Anal. Calc for C40H48N2O: C, 83.87; H,
8.45; N, 4.89. Found: C, 83.49; H, 8.80; N, 4.82.
3-L. Purified by silica gel column chromatography eluting with
hexane/ethyl acetate (10:1 v/v). 0.55 g of yellow oil was obtained
(yield: 81%). 1H NMR (CDCl3, 400 MHz): 8.64−8.70 (m, 2H),
8.36−8.39 (m, 1H), 8.07−8.19 (m, 5H), 7.80−7.91 (m, 6H), 7.46−
7.50 (m, 1H), 7.30−7.38 (m, 2H), 2.15−2.17 (m, 4H), 0.48−0.95 (m,
30H). ESI-HRMS: m/z calc for [C46H51N3S + H]+: 678.3876. Found:
678.3884. Anal. Calc for C46H51N3S·0.33DMF·0.33CH2Cl2: C, 77.81;
H, 7.45; N, 6.39. Found: C, 77.51; H, 7.50; N, 6.46.
crystal size (mm)
crystal system
space group
a (Å)
0.33 × 0.12 × 0.09
monoclinic
P2(1)/n
12.7116(9)
17.5310(11)
17.1311(11)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
90.489(1)
90
3817.5(4)
4
Z
ρcalc (g/cm3)
1.573
4-L. Purified by silica gel column chromatography eluting first with
μ (cm−1
)
3.924
hexane/dichloromethane (2:1 v/v) and then with hexane/ethyl acetate
1
(5:1 v/v). 0.47 g of yellow oil was obtained (yield: 79%). H NMR
radiation type
F(000)
Mo
(CDCl3, 400 MHz): 8.65−8.69 (m, 2H), 8.35 (dd, J = 7.6, 0.8 Hz,
1H), 8.08−8.13 (m, 2H), 7.73−7.88 (m, 4H), 7.62 (d, J = 7.6 Hz,
1H), 7.28−7.32 (m, 1H), 7.20 (s, 1H), 7.13 (dd, J = 7.8, 1.4 Hz, 1H),
2.67 (t, J = 7.6 Hz, 2H), 1.96−2.12 (m, 4H), 1.59−1.64 (m, 2H),
1.31−1.39 (m, 2H), 0.47−0.96 (m, 33H). ESI-HRMS: m/z calc for
[C43H56N2 + H]+: 601.4516. Found: 601.4514. Anal. Calc for
C43H56N2·0.67toluene: C, 86.44; H, 9.33; N, 4.23. Found: C, 86.80;
H, 9.70; N, 4.60.
5-L. Purified by silica gel column chromatography eluting with
dichloromethane. 0.78 g of yellow oil was collected as the product
(yield: 75%). 1H NMR (CDCl3, 400 MHz): 8.68−8.72 (m, 2H), 8.40
(d, J = 7.6 Hz, 1H), 8.16−8.24 (m, 4H), 7.97 (d, J = 7.6 Hz, 1H),
7.83−7.94 (m, 4H), 7.51−7.61 (m, 2H), 7.28−7.43 (m, 7H), 1.98−
2.25 (m, 4H), 0.52−0.98 (m, 30H). ESI-HRMS: m/z calc for
[C51H55N3 + H]+: 710.4469. Found: 710.4475. Anal. Calc for
C51H55N3·0.17CH2Cl2: C, 84.86; H, 7.70; N, 5.80. Found: C, 85.08;
H, 7.98; N, 5.74.
1816
reflns collected
unique reflns
no. of reflns (I ≥ 2σ)
resolution (Å)
gof on F2
65292
11451
8722
0.7
1.022
a
R1/wR2 (I ≥ 2σ(I))
R1/wR2 (all data)
0.0317/0.0732
a
0.0529/0.0827
a
R1 = ∑||Fo| − |Fc||/∑|Fo|, wR22= [∑(w(Fo − Fc2)2)/∑w(Fo )2]1/2
2
2
2
2
2
for Fo > 2σ(Fo ), w = 1/[σ2(Fo ) + (AP)2 + BP], where P = (Fo
+
2Fc2)/3; A (B) = 0.0370 (5.4894).
purchased from Aldrich Chemical Co. or Alfa Aesar and used as is
unless otherwise stated. Silica gel for column chromatography was
purchased from Sorbent Technology (60 Å, 230−400 mesh, 500−600
́
m2/g, pH: 6.5−7.5). The synthesis of precursors Br-F8-I,12 Br-F8-
CHO,13 CHO-F8-B,14 Br-F8,15 NO2-F8-Br,16 NO2-F8-B,16 BTZ-F8-
Br,17 CBZ-F8-B,18 NPh2-F8-B,8d C4H9-F8-B,19 and BPY-Br20
followed the literature procedures. BTZ-F8-B was obtained by
reacting BTZ-F8-Br in THF solution at −78 °C with 1 equiv of
BuLi followed by addition of 1 equiv of 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane. All intermediates were characterized
by 1H NMR. The synthetic details and 1H NMR data of these
intermediates are provided in the Supporting Information. The general
procedures for the synthesis of ligands 1-L−6-L and for complexes 1−
6 are provided below. Ligands 1-L−6-L and complexes 1−6 were all
characterized by 1H NMR, electrospray ionization high-resolution
mass spectrometry (ESI-HRMS), and elemental analyses.
6-L. Purified by silica gel column chromatography eluting with
1
dichloromethane. 0.50 g of yellow oil was obtained (yield: 70%). H
NMR (CDCl3, 400 MHz): 8.64−8.69 (m, 2H), 8.34 (d, J = 7.6 Hz,
1H), 8.12 (dd, J = 8.2, 1.4 Hz, 1H), 8.06 (m, 1H), 7.83−7.89 (m, 2H),
7.78 (d, J = 7.6 Hz, 1H), 7.71 (m, 1H), 7.60−7.63 (m, 1H), 7.29−7.33
(m, 1H), 7.19−7.24 (m, 4H), 7.05−7.13 (m, 6H), 6.67−7.01 (m, 2H),
1.84−2.08 (m, 4H), 0.51−1.02 (m, 30H). ESI-HRMS: m/z calc for
[C51H57N3 + H]+: 712.4625. Found: 712.4623. Anal. Calc for
C51H57N3: C, 86.03; H, 8.07; N, 5.90. Found: C, 86.29; H, 8.20; N,
5.71.
General Procedure for the Synthesis of Complexes 1−6. The
mixture of ligand (0.5 mmol), K2PtCl4 (0.5 mmol), and acetic acid (10
mL) was heated to reflux under argon for 24 h. After reaction, the
solvent was removed. The red residual was dissolved in dichloro-
methane, washed with brine (10 mL × 3), and dried over MgSO4.
Purification was carried out by running flash silica gel column
1H NMR spectra were obtained on Varian Oxford-VNMR
spectrometers (300 MHz, 400 MHz, or 500 MHz). ESI-HRMS
analyses were performed on a Bruker BioTOF III mass spectrometer.
C
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Figure 1. (a) Structure of the diastereomer in which C25’s configuration is R; the percentage of this diastereomer is 52%. (b) Structure of the
diastereomer in which C25’s configuration is S; the percentage of this diastereomer is 48%. (c) 3D packing of complex 1 viewed along the a axis.
chromatography eluting with dichloromethane, and then recrystalliza-
tion from dichloromethane and hexane.
1. 237 mg of orange powder was obtained (yield: 58%). H NMR
(CDCl3, 400 MHz): 9.15−9.19 (m, 1H), 8.21−8.29 (m, 3H), 8.03−
8.08 (m, 1H), 7.85−7.95 (m, 3H), 7.58−7.67 (m, 3H), 7.38 (m, 1H),
1.98−2.08 (m, 4H), 0.44−0.93 (m, 30H). ESI-HRMS: m/z calc for
[C39H46N3O2ClPt + H]+: 820.3001. Found: 820.3003. Anal. Calc for
C39H46N3O2ClPt: C, 57.17; H, 5.66; N, 5.13. Found: C, 56.98; H,
5.68; N, 5.11.
1H), 2.68 (t, J = 7.5 Hz, 2H), 1.93−2.03 (m, 4H), 1.60−1.67 (m, 2H),
1.32−1.42 (m, 2H), 0.48−1.00 (m, 33H). ESI-HRMS: m/z calc for
[C43H55ClN2Pt + H]+: 831.3778. Found: 831.3796. Anal. Calc for
C43H55ClN2Pt·0.25toluene: C, 62.98; H, 6.73; N, 3.28. Found: C,
62.68; H, 6.93; N, 3.37.
1
1
5. 287 mg yellow powder was obtained (yield: 61%). H NMR
(CDCl3, 400 MHz): 9.16 − 9.19 (m, 1H), 8.24 (m, 1H), 8.16 (d, J =
7.6 Hz, 2H), 8.02 − 8.07 (m, 2H), 7.95 (d, J = 8.0 Hz, 1H), 7.83 −
7.87 (m, 1H), 7.49 − 7.66 (m, 5H), 7.34 − 7.44 (m, 5H), 7.26 − 7.31
(m, 2H), 1.94 − 2.10 (m, 4H), 0.54 − 0.97 (m, 30H). ESI-HRMS: m/
z calc. for [C51H54ClN3Pt+H]+: 940.3733; Found, 940.3729. Anal.
Calc. for C51H54ClN3Pt: C, 65.20; H, 5.79; N, 4.47; Found: C, 64.94;
H, 6.08; N, 4.44.
2. 40 mg of red powder was obtained (yield: 10%). 1H NMR
(CDCl3, 400 MHz): 10.05 (s, 1H), 8.88−8.93 (m, 1H), 8.12−8.23 (m,
1H), 7.84−7.92 (m, 5H), 7.73−7.79 (m, 1H), 7.56 (d, J = 8.0 Hz,
1H), 7.43−7.50 (m, 2H), 7.33 (d, J = 2.8 Hz, 1H), 1.96−2.09 (m,
4H), 0.42−0.93 (m, 30H). ESI-HRMS: m/z calc for [C40H47N2ClOPt
+ H]+: 803.3100. Found: 803.3072. Anal. Calc for C40H47ClN2OPt: C,
59.88; H, 5.90; N, 3.49. Found: C, 59.88; H, 6.28; N, 3.37.
1
6. 287 mg of red powder was obtained (yield: 61%). H NMR
(CDCl3, 400 MHz): 9.19−9.22 (m, 1H), 8.02−8.06 (m, 2H), 7.89−
7.91 (d, J = 8.0 Hz, 1H), 7.77−7.82 (td, J = 8.0, 2.0 Hz, 1H), 7.68−
7.71 (m, 1H), 7.62−7.66 (m, 1H), 7.48−7.52 (t, J = 8.4 Hz, 2H),
7.19−7.26 (m, 5H), 6.96−7.10 (m, 8H), 1.78−1.93 (m, 4H), 0.49−
1.02 (m, 30H). ESI-HRMS: m/z calc for [C51H56ClN3Pt + H]+:
942.3889. Found: 942.3878. Anal. Calc for C51H56ClN3Pt: C, 65.06;
H, 5.99; N, 4.46. Found: C, 64.71; H, 6.25; N, 4.43.
Crystallographic Analysis. Single crystals of complex 1 were
obtained by slow diffusion of hexane into the dilute dichloromethane
solution of 1. Single crystal X-ray diffraction data of 1 were collected
on a Bruker Apex Duo diffractometer with an Apex 2 CCD area
detector at T = 100 K. Mo radiation was used. All structures were
processed with an Apex 2 v2010.9-1 software package (SAINT v.
7.68A, XSHELL v. 6.3.1).21 A direct method was used to solve the
1
3. 86 mg of yellow powder was obtained (yield: 19%). H NMR
(CDCl3, 400 MHz): 9.14−9.17 (m, 1H), 8.18 (s, 1H), 8.01−8.11 (m,
4H), 7.88−7.94 (m, 3H), 7.80−7.85 (m, 1H), 7.60−7.64 (m, 1H),
7.53−7.57 (t, J = 7.8 Hz, 2H), 7.45−7.49 (m, 1H), 7.33−7.37 (m,
2H), 1.98−2.13 (m, 4H), 0.45−0.93 (m, 30H). ESI-HRMS: m/z calc
for [C46H50N3ClSPt + H]+: 908.3137. Found: 908.3132. Anal. Calc for
C46H50ClN3PtS: C, 60.88; H, 5.55; N, 4.63; S, 3.53. Found: C, 60.73;
H, 5.90; N, 4.47; S, 3.92.
1
4. 120 mg of orange powder was obtained (yield: 29%). H NMR
(CDCl3, 500 MHz): 8.80 (dd, J = 22, 5.3 Hz, 1H), 8.03−8.10 (m,
1H), 7.83−7.92 (m, 2H), 7.66−7.73 (m, 2H), 7.50−7.53 (m, 1H),
7.36−7.45 (m, 2H), 7.25−7.30 (m, 1H), 7.17 (s, 1H), 7.13−7.15 (m,
D
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structure after multiscan absorption corrections. Details of data
collection and refinement are given in Table 1.
compact representation of the transition density between the ground
and excited states in terms of an expansion into single-particle
transitions (hole and electron for each given excited state). Here we
refer to the unoccupied NTO as the “electron” transition orbital and
the occupied NTO as the “hole” transition orbital. NTOs shown in
this paper were produced with the isovalue of 0.02 and visualized with
the GaussView 5.1 graphical software.35
Photophysical Measurements. The solvents used for photo-
physical experiments were spectroscopic grade and were purchased
from VWR International and used as is without further purification. An
Agilent 8453 spectrophotometer was used to record the UV−vis
absorption spectra in different solvents. A SPEX fluorolog-3
fluorometer/phosphorometer was used to measure the steady-state
emission spectra in different solvents. The emission quantum yields
were determined by the relative actinometry method22 in degassed
solutions, in which a degassed aqueous solution of [Ru(bpy)3]Cl2
(Φem = 0.042, λex = 436 nm)23 was used as the reference for complexes
1−6, and a degassed 1 N sulfuric acid solution of quinine bisulfate
(Φem = 0.546, λex = 347.5 nm)24 was used as the reference for ligands
1-L−6-L.
RESULTS AND DISCUSSION
Crystal Structure of 1. Single crystals of complex 1 were
obtained by slow diffusion of hexane into the dichloromethane
■
Table 2. Selected Bond Lengths, Bond Angles, and Torsion
Angles of Complex 1
The nanosecond transient difference absorption (TA) spectra and
decays were measured in degassed solutions on an Edinburgh LP920
laser flash photolysis spectrometer. The third harmonic output (355
nm) of a Nd:YAG laser (Quantel Brilliant, pulse width: 4.1 ns,
repetition rate was set at 1 Hz) was used as the excitation source. Each
sample was purged with argon for 30 min prior to measurement. The
triplet excited-state absorption coefficient (εT) at the TA band
maximum was determined by the singlet depletion method.25 After
obtaining the εT value, the ΦT could be determined by the relative
actinometry using SiNc in benzene as the reference (ε590 = 70000 M−1
cm−1, ΦT = 0.20).26
Nonlinear Transmission Experiment. The reverse saturable
absorption of complexes 1−6 was characterized by a nonlinear
transmission experiment at 532 nm using a Quantel Brilliant laser as
the light source. The pulse width of the laser was 4.1 ns, and the
repetition rate was set at 10 Hz. The complexes were dissolved in
CH2Cl2. The concentration of the sample solutions was adjusted to
obtain a linear transmission of 90% at 532 nm in a 2-mm-thick cuvette.
The experimental setup and details are similar to those reported
previously.8b A 40-cm plano-convex lens was used to focus the beam to
the center of the 2-mm-thick sample cuvette. The radius of the beam
waist at the focal point was approximately 96 μm.
Computational Details. The ground-state properties of com-
plexes 1−6 were studied using density functional theory (DFT), while
the excited states were simulated using linear response time-dependent
DFT (TDDFT). All calculationsthe geometry optimization, the
ground-state and excited-state electronic structures, and optical
spectrawere performed using the Gaussian 09 quantum chemistry
software package.27 All procedures were done utilizing the long-rang
corrected functional CAM-B3LYP.28 The LANL08 basis set29 was
used for the heavier Pt atom, while the remaining atoms were modeled
with the 6-31G* basis set. The chosen method represents one of the
currently most accurate DFT functionals and basis sets that have
already shown good agreement with experimental data for different
organometallic complexes.30 All calculations have been performed in
solvent using the conductor polarized continuum model (CPCM),31
as implemented in Gaussian 09. Dichloromethane (CH2Cl2, εr = 9.08)
was chosen as the solvent for consistency with the experimental
studies. Implementation of solvent also helps to avoid any unnatural
charge transfer states within the energy gap of these complexes.32
The excited-state energies and oscillator strengths have been studied
using the linear response TDDFT formalism,33 in which the adiabatic
approximation for the exchange-correlation kernel was used. These
calculations were done using the same basis sets and functional as used
in the ground-state DFT calculations. For absorption spectra, the 40
lowest singlet optical transitions were considered to reach the
transition energies of ∼5 eV (250 nm). Each spectral line obtained
from the TDDFT calculation was broadened by a Gaussian function
with the line width of 0.1 eV to match the experimentally observed
homogeneous broadening. The phosphorescence energies were
calculated by first optimizing the lowest triplet state geometry
followed by the vertical triplet excitations calculated via TDDFT.
In order to analyze the nature of the singlet and triplet excited
states, natural transition orbital (NTO) analysis was performed based
on the calculated transition densities.34 This method offers the most
atom 1 atom 2 atom 3 atom 4 bond length/Å or bond angle/deg
Pt
Cl
2.296(1)
1.944(3)
2.101(3)
1.986(3)
1.364(4)
1.349(4)
1.359(5)
1.342(4)
1.484(5)
1.467(5)
178.49(9)
98.42(8)
98.97(11)
80.14(11)
82.48(13)
162.56(13)
117.8(2)
118.7(2)
123.5(3)
112.2(2)
128.3(2)
119.5(3)
112.3(3)
129.5(3)
115.0(3)
−164(3)
179.1(3)
0.5(2)
Pt
N1
N2
C23
C10
C6
C5
C1
C6
C11
Pt
Pt
Pt
N1
N1
N2
N2
C5
C10
Cl
N1
Cl
Pt
N2
Cl
Pt
C23
N2
N1
N1
N2
Pt
Pt
Pt
C23
C23
C10
C6
Pt
N1
N1
N1
N2
N2
N2
C23
C23
C11
Pt
Pt
C10
Pt
C6
C5
Pt
C1
C5
Pt
C1
C11
C22
C10
N1
Pt
C23
Cl
C10
C10
C10
N2
N2
C23
N1
C23
Pt
N1
Pt
N1
C6
C11
C5
0.6(4)
C10
N1
−0.6(4)
solution of 1. The crystal structure and packing pattern are
illustrated in Figure 1, and selected bond lengths, bond angles,
and torsion angles are compiled in Table 2. As shown in Figure
1, two diastereomers (Figure 1a: R configuration at C25, 52%;
Figure 1b: S configuration at C25, 48%) were observed in the
crystal, which could arise from the racemic starting material 2-
ethylhexyl bromide. In contrast, only the S configuration of C33
was observed. Nevertheless, the configuration of C25 or C33
plays a negligible role on the photophysics and nonlinear
absorption of the Pt(II) complexes and thus is not the focus of
this study. As a result, we only pick the diastereomer with R
configuration at C25 (Figure 1a) for structural and packing
discussion. The packing pattern is illustrated by two single cells
viewed along the b axis in Figure 1c. Regardless of the two alkyl
chains at the 9-position of the fluorene motif, the complex
basically has a planar structure, in which bipyridine, Pt−Cl,
fluorene, and nitro sit roughly in the same plane. The geometry
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Figure 2. (a) UV−vis absorption spectra of ligands 1-L−6-L measured in CH2Cl2. (b) UV−vis absorption spectra of complexes 1−6 measured in
CH2Cl2. (c) Calculated absorption spectra for complexes 1−6; vertical lines resemble excited states and the corresponding oscillator strength.
optimization of the molecule performed by DFT calculations
also results in the planar configurations (Supporting
Information Table S1).
the Pt···Pt association range of 3.09−3.71 Å.36 Therefore, no
intermolecular Pt···Pt interactions are present even if in the
crystal form.
The bond lengths of Pt−N1 (1.944(3) Å), Pt−N2 (2.101(3)
Å), and Pt−C23 (1.986(3) Å) resemble our previously
published data for a similar core structure in ref 11, with the
corresponding values for complex 1 in ref 11 being 1.980(7) Å,
2.112(8) Å, and 1.997(8) Å, respectively.11 The shorter bond
length of Pt−N1 in this work is probably due to the weaker
trans effect of chloride ligand compared to 4-tolylacetylide
coligand in ref 11. The bond angles around the platinum center,
Cl−Pt−C23 (98.97(11)), Cl−Pt−N2 (98.42(8)), N1−Pt−
C23 (82.48(13)), and N1−Pt−N2 (80.14(11)), reveal
deviation of the structure from an ideal square planar
configuration around the metal center. The torsion angle
between the fluorene and bipyridine components is approx-
imately 0.6 deg (revealed by the torsion angle of C23−C11−
C10−N1), while the NO2 plane deviates from the fluorene
plane by an angle of 6.5 deg (see the torsion angle of O1−N3−
C17−C18).
Four molecules of 1 form a single centrosymmetric cell in the
crystal matrix. In every single cell, two molecules adopt a head-
to-tail packing with their π-conjugated planes opposite to each
other; while the other two are in the perpendicular direction of
the two packing molecules. The average distance between the
two packing π-conjugated planes, defined by the distance of the
two planes averaging the 26 atoms within the ring system of
each molecule, is 3.306 Å, indicating the presence of π−π
interactions between the two molecules in the crystal matrix.
The nearest Pt···Pt separation is 4.902 Å, which is longer than
Electronic Absorption. The electronic absorption spectra
of ligands 1L−6L and complexes 1−6 at different concen-
trations are recorded in dichloromethane solutions. The
absorptions of these compounds obey Beer’s Law in the
concentration range of 1 × 10−6 to 1 × 10−4 mol/L, indicating
the absence of ground-state aggregation in the concentration
range studied in dichloromethane. As shown in Figure 2a, 1L−
6L display intense absorption bands from 300 to 400 nm,
which predominantly arise from the 1π,π* transitions. However,
contribution from intraligand charge transfer cannot be
1
completely ruled out. The predominant π,π* character of
these absorption bands is supported by the minor solvatochro-
mic effect in these ligands, as demonstrated in Supporting
Information Figures S1−S6. Comparing to 4-L, the major
absorption bands of all other ligands are red-shifted, which is
likely the sign of electron delocalization induced by the
substituent. A stronger electron-withdrawing substituent (NO2
in 1-L) and stronger electron-donating substituent (NPh2 in 6-
L) cause the largest red-shift of the absorption band. Extended
π-conjugation from the BTZ substituent in 3-L also induces a
significant red-shift and enhanced extinction coefficient.
For complexes 1−6, the UV−vis absorption spectra consist
of intense absorption bands below 420 nm and a broad tail
above 420 nm (see spectra in Figure 2b). The intense
absorption bands below 420 nm in complexes 1−3 (which
contain electron-withdrawing substituents at the 7-position of
fluorene) exhibit more featured spectra with a few well-distinct
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main absorption band of 6, compared to those in complexes 4
and 5, in which the electron is primarily localized on the bpy
component, while the hole is spread over the fluorene motif.
Note that although, for all complexes except 6, the substituents
are not directly involved in the NTOs contributing to the
lower-energy edge of the main absorption band (380−420 nm),
they play an important role in overall delocalization of the
charge density over fluorene. Thus, the large red-shift of
complex 3 is also associated with the increase in π-conjugation
due to the substituent, which tends to partially spread both the
hole and electron charge density over fluorene toward the BTZ
substituent. The same trend is also noticeable (but less
pronounced) in complex 1 with the strong electron-with-
drawing substituent pulling the charge density to the fluorene,
which explains its red-shift with respect to complex 2.
Table 3. Experimental and Calculated Electronic Absorption
Parameters for Complexes 1−6 and Experimental
Absorption Data for Ligands 1-L−6-L in CH2Cl2
λabs/nm (εmax
/
a
b
Theor
M−1·cm−1
)
λ /nm (Sn; fosc)
abs
1
2
3
409 (23430),
375 (34850),
339 (23530)
394 (S1; 0.0563), 341 (S2; 0.0094), 340 (S3;
0.228), 327 (S4; 0.679), 308 (S6; 0.710)
402 (15400),
361 (28200),
334 (32530)
395 (S1; 0.0416), 340 (S2; 0.0091), 339 (S3;
0.103), 323 (S4; 0.649), 305 (S7; 0.914)
404 (37320),
385 (49820),
343 (40510)
398 (S1; 0.096), 343 (S2, 0.692), 341 (S3;
0.009), 329 (S4; 1.047), 313 (S6; 0.572)
4
5
6
376 (24930),
329 (28780)
400 (S1; 0.0280), 340 (S2; 0.0096), 339 (S3;
0.0575), 321 (S4; 0.420), 306 (S6; 0.8470)
379 (31980),
328 (32330)
398 (S1; 0.0382), 340 (S2; 0.0097), 339 (S3;
0.0966), 321 (S4; 0.682), 308 (S6; 0.863)
In addition to the major absorption bands, a well
pronounced tail appears at the low-energy end of the major
absorption bands and extends to 550 nm in the absorption
spectra of complexes 1−6, as seen in Figure 2b and c. These
tails become more distinguishable from the major absorption
bands in MTHF, toluene, and hexane (see Supporting
Information Figures S7− S12). The calculated NTOs
contributing to the lowest-energy, low-intensity transition
411 (40130),
310 (35550)
406 (S1; 0.104), 348 (S2; 0.782), 340 (S3;
0.0103), 331 (S4; 0.525)
1-L 362 (30540),
276 (20490)
2-L 341 (43250)
3-L 373 (44200),
358 (58250)
4-L 326 (37730)
5-L 340 (40650)
1
(Table 5) reveal that this tail features MLCT/1π,π*/1ILCT
characters for all complexes. However, in accordance with the
trend observed from the major absorption bands, this tail
bathochromically shifts to longer wavelengths in complexes 1,
3, and 6 compared to those in 2, 4, and 5, with the most
pronounced red-shift and the highest intensity in complex 6,
due to the strongest delocalization of NTOs over the fluorene
promoted by the strongest electron-donating NPh2 group
(Table 5).
6-L 370 (32500),
285 (25000)
a
λabs is absorption wavelength, εmax is molar extinction coefficient.
b
Theor
λ
is calculated wavelength corresponding to the transition
abs
between the ground and excited states of interest (number of excited
states is shown in parentheses); fosc is the calculated oscillator strength
for the corresponding excitations.
Photoluminescence. Photoluminescence of ligands 1-L−
6-L and Pt(II) complexes 1−6 was studied at room
temperature in different solvents and at 77 K in a butyronitrile
glassy matrix. The spectra are presented in Figures 3 and 4 and
in Supporting Information Figures S13−S20. The emission
parameters are summarized in Table 6 and in Supporting
Information Tables S2 and S3. As shown in Figure 3a and in
Tables 6 and S2, ligands 3-L−6-L exhibit intense fluorescence
in all solvents in the region of 350−550 nm with the emission
quantum yield in the range of 0.50−0.91. The emissions of 1-L
and 2-L at room temperature are much weaker in comparison
to those of 3-L−6-L. Ligand 1-L exhibits dual emission in
CH2Cl2 solution when excited at 325 nm, with a slightly
peaks than those in complexes 4−6, whic contain an electron-
donating substituent. Similar to the trend observed in their
corresponding ligands, the UV−vis absorption spectra for
complexes with stronger electron-withdrawing or -donating
substituents (complexes 1 and 6) and with a BTZ substituent
(complex 3) are red-shifted compared to the other three
complexes. This trend is well reproduced by our TDDFT
calculations illustrated in Figure 2c and listed in Table 3,
although the energies of all complexes are blue-shifted with
respect to experimental data due to a high portion of the
Hartree−Fock exchange in the CAM-B3LYP functional. The
natural transition orbitals (NTOs) listed in Table 4 clearly
indicate that the transitions contributing to the absorption
bands around 400−410 nm and 360−380 nm for 1−3 are
featured by mixed 1MLCT (dπ(Pt) → π*(bpy)), 1LLCT
1
structured π,π* fluorescence at ca. 414 nm and a structureless
1ILCT fluorescence at ca. 480 nm. When excited at 400 nm,
1
1
1
only the ILCT fluorescence is observed. The charge transfer
(π(Cl) → π*(bpy)), π,π*, and ILCT (intraligand charge
transfer) characters, while the band at ca. 340 nm primarily
arises from the mixed 1π,π*/1ML′CT (dπ(Pt) → π*(fluorene))
transitions. In contrast, the major absorption bands for 4 and 5
(with weak or moderate electron-donating substituents at the 7-
nature of the 480 nm band is supported by the positive
solvatochromic effect of this emission band and by the fact that
this emission band is only observed in polar solvents such as
1
CH2Cl2 and CH3CN (Figure 4a). For 2-L, only the π,π*
1
position of fluorene) originate from the MLCT/1π,π*/1ILCT
fluorescence was observed in all of the solvents when excited
1
below 330 nm. However, ILCT was detected in CH3CN
transitions, while complex 6, with a strong electron-donating
substituent, is featured predominantly with 1π,π*/1MLCT
transitions in its major absorption band at ca. 410 nm, with a
minor contribution from 1LLCT/1ILCT in its high-energy
shoulder around 380 nm (see NTOs in Table 4). Such a
pronounced 1π,π* character of optical transitions at ∼410 nm is
a result of the NPh2 substituent in complex 6 that strongly
delocalizes both the hole and electron orbitals via spreading the
charge density toward the NPh2 group. Increase in π-
conjugation provides the most pronounced red-shift in the
solution when excited at 374 nm. The absence of charge
transfer emission in solvents other than CH3CN implies a
1
weaker admixture of the ILCT character in the lowest singlet
excited state in 2-L compared to 1-L. This is reasonable
because of the weaker electron-withdrawing ability of the
formyl substituent with respect to the nitro substituent. In
contrast, 3-L only displays 1π,π* fluorescence with clear
vibronic structure independent of the solvent polarity and the
excitation wavelength. This feature can be attributed to the
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a
Table 4. Natural Transition Orbitals (NTOs) Representing Transitions That Correspond to the Main Absorption Bands for
Complexes 1−6
a
Note that excited-state NTOs differ from the ground-state MOs, and rather can be considered as the linear combination of the ground-state MOs
that contribute to a given excited state.
Table 5. Natural Transition Orbitals Contributing to the Lowest-Energy Absorption “Tail” for Complexes 1−6
H
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Figure 3. (a) Normalized emission spectra of ligands 1-L−6-L (λex was 325 nm for 1-L, 320 nm for 2-L, 348 nm for 3-L, 326 nm for 4-L, 340 nm for
5-L, and 370 nm for 6-L) in CH2Cl2 at the concentration of 1 × 10−5 mol/L for 2-L−6-L and 5 × 10−4 mol/L for 1-L. (b) Normalized emission
spectra of complexes 1−6 (λex was 372 nm for 1, 360 nm for 2, 383 nm for 3, 375 nm for 4, 380 nm for 5, and 410 nm for 6) in CH2Cl2 at the
concentration of 1 × 10−5 mol/L. (c) Normalized emission spectra of 1-L−6-L at 77 K in a glassy BuCN matrix (λex was 405 nm for 1-L, 350 nm for
2-L, 358 nm for 3-L, 326 nm for 4-L, 340 nm for 5-L, and 375 nm for 6-L). (d) Normalized emission spectra of complexes 1−6 at 77 K in a glassy
BuCN matrix at the concentration of 1 × 10−5 mol/L (λex was 372 nm for 1, 360 nm for 2, 383 nm for 3, 375 nm for 4, 380 nm for 5, and 410 nm
for 6).
extended π-conjugation induced by the BTZ substituent and
the weaker electron-withdrawing ability of the BTZ substituent.
The emission characteristics of 4-L−6-L (all with electron-
donating substituents) at room temperature are quite similar.
As exemplified in Figure 4b for 6-L, the fluorescence spectrum
becomes significantly red-shifted and featureless going from
nonpolar solvents (hexane and toluene) to polar solvents
579, and 588 nm, respectively, which show somewhat negative
solvatochromic effect (as shown in Figure 4c for 2 and Figures
S17 and S18 for 1 and 3, respectively, in the Supporting
Information). The vibronic progressions are 1208, 1220, and
1234 cm−1 for 1, 2, and 3, respectively, and the emission
lifetimes of 1−3 are in the range of 5.3−6.1 μs. These features
suggest that the emissions of complexes 1, 2, and 3 emanate
predominantly from the ligand-localized 3π,π* state. This
assignment is confirmed by the calculated NTOs contributing
to the lowest-energy triplet transition, as shown in Table 7.
However, the calculated NTOs suggest that the emission of 1
1
(THF, CH2Cl2, CH3CN), indicating a transition from π,π*
1
fluorescence in nonpolar solvents to ILCT fluorescence in
polar solvents. The positive solvatochromic effect is more
pronounced in 6-L than in 5-L and 4-L, and the 1ILCT
emission energy apparently decreases from 4-L to 5-L to 6-L.
This trend is consistent with the strength of the electron-
donating ability of the substituent in these compounds. With
the increased electron-donating ability (i.e., NPh2 > CBZ > n-
Bu), the degree of intraligand (intramolecular) charge transfer
is enhanced, which consequently induces the red-shift of the
charge transfer emission.
The emission of 1-L−6-L at 77 K in a butyronitrile glassy
matrix was investigated in order to understand the phosphor-
escence from these compounds. As illustrated in Figure 3c,
structured phosphorescence between 500 and 700 nm was
observed only in 1-L and 2-L. The emission detected below
500 nm for 2-L−6-L is low-temperature fluorescence, which
becomes narrower, structured, and slightly blue-shifted due to
the rigidochromic effect.37
3
and 2 has some contributions from the MLCT states. In
contrast, the emission spectra of 4, 5, and 6 are broad and less
structured (or structureless for 6) in CH2Cl2 solution at room
temperature (as shown in Figure 3b), with apparently shorter
lifetimes (i.e., 1.33 μs for 4 and 2.10 μs for 5; and the lifetime
for 6 was unable to be measured due to very weak signal) and
lower quantum yields (see Table 6). The emission of 4 and 5
shows a negative solvatochromic effect in different solvents, as
shown in Figure 4d for 5 and in Supporting Information Figure
S19 for 4. The emission of complex 6 was substantially
quenched in polar solvents such as CH2Cl2 and CH3CN.
Taking all these facts into account, we conclude that the
emission of 4, 5, and 6 has significant contribution from a
triplet charge transfer state. Our TDDFT calculations confirm
that the triplet transitions of complexes 4 and 5 admix
3MLCT/3π,π* characters, while the triplet transition of 6
Complexes 1, 2, and 3 exhibit well-structured emission at
room temperature (Figure 3b) with the band maximum at 582,
3
primarily has ILCT character, which is quite distinct from the
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Figure 4. Normalized emission spectra of (a) 1-L, (b) 6-L, (c) 2, and (d) 5 in different solvents. λex = 360 nm for 6-L, and λex = 436 nm for 2 and 5.
3π,π* dominated character of the triplet transitions in
The moderate Stokes shifts in 4 and 5 also reflect the mixed
3MLCT/3π,π* characters in their emitting states.
complexes 1−3, as illustrated in Table 7.
The emission of 1−6 at room temperature is concentration-
dependent. As exemplified in Figure 5 for complex 1, the
emission intensity increases in the concentration range of 1 ×
10−6 to 2 × 10−5 mol/L but decreases when the concentration
is higher than 2 × 10−5 mol/L. Meanwhile, the emission
lifetime keeps decreasing with increased concentration. Both
facts suggest the occurrence of self-quenching in these
complexes. The self-quenching rate constants deduced for 1−
6 are provided in Table 6, which are on the order of 109
L·mol−1·s−1 and are in line with those reported in the literature
for other Pt(II) terdentate or diimine complexes.8b,c,38 On the
other hand, in view of the strong absorption at the respective
excitation wavelength of each complex, the reduced emission
intensity at high concentrations should also have contribution
from a primary inner-filter effect.
Transient Absorption. The nanosecond transient absorp-
tion (TA) measurements were conducted for ligands 1-L−6-L
and complexes 1−6. The following information can be
obtained through the TA experiments: (1) the spectral feature
of the triplet excited-state absorption; (2) the spectral region
where the excited state absorbs more strongly than the ground
state (can be identified from the positive absorption band(s) of
the TA spectrum); (3) the excited-state lifetime (can be
deduced from the decay of the TA); and (4) the triplet excited-
state quantum yield (can be determined by the relative
actinometry).
The nanosecond TA spectra of ligands 1-L−6-L at zero delay
after excitation are illustrated in Figure 6a. All compounds
exhibit moderate to strong transient absorption in the visible to
the NIR spectral region. Two trends emerge by examining the
shape of the TA spectra of these compounds. First, the TA
band maxima of the compounds with electron-withdrawing
substituents (1-L−3-L) are obviously red-shifted compared to
those of 4-L−6-L, which contain electron-donating substitu-
ents. Second, the TA spectra of 1-L, 5-L, and 6-L consist of two
major absorption bands with a relatively narrower band
between 400 and 500 nm, and a broader band above 500 nm
(see Figures S21, S25, and S26 of the Supporting Information).
Considering these different features, we tentatively attribute the
observed TA to the 3π,π* states for these compounds.
The emission of 1−6 at 77 K was investigated in a
butyronitrile glassy matrix, and the results are provided in
Figure 3d and in Table 6. The emission spectra at 77 K for all
complexes are blue-shifted, are more structured, and become
narrower with respect to their corresponding emission spectra
at room temperature, reflecting the rigidochromic effect.37 The
thermally induced Stokes shifts are 270 cm−1 for 1, 304 cm−1
for 2, 205 cm−1 for 3, 892 cm−1 for 4, 607 cm−1 for 5, and 1479
cm−1 for 6. The much smaller thermally induced Stokes shifts
3
for 1−3 are consistent with the π,π* dominated nature of the
3
emission, and the large Stokes shift for 6 is in accordance with
However, for 1-L, 5-L, and 6-L, ILCT could also contribute
the charge transfer character in the emitting triplet excited state.
significantly to the observed TA in view of the facile intraligand
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charge transfer nature of the triplet excited state that gives rise
to the observed TA. Considering the nature of the emitting
state of 6 discussed earlier, we believe the TA of 6
Table 6. Emission and Excited-State Absorption Parameters
for Complexes 1−6 and Ligands 1-L−6-L
λem/nm (τ0/μs; ksq/
a
λem/nm
b
3
L·mol−1·s−1); Φem
(τem/μs)
predominantly arises from the ILCT state.
Reverse Saturable Absorption. The TA spectra of 1−6
indicate that all complexes exhibit positive absorption at 532
nm, suggesting stronger excited-state absorption than that of
the ground state. In addition, the triplet excited-state lifetimes
for these complexes are much longer than the nanosecond laser
pulse width (4.1 ns). These provide the necessary conditions
for reverse saturable absorption (RSA, defined as a decreased
transmission upon increase of incident energy) to occur for a
nanosecond laser pulse at 532 nm. To demonstrate this, a
nonlinear transmission experiment using complexes 1−6 at 532
nm for nanosecond laser pulses was carried out in CH2Cl2
solution at a linear transmittance of 90% in a 2-mm cuvette. For
comparison purposes, the RSA of complex 1 in ref 11, which
has a similar structure to that of complex 3 in this work but
with a tolylacetylide coligand instead of a Cl coligand, was also
measured under the same experimental conditions. The results
are displayed in Figure 7. The transmission of the complexes
drastically decreases with increased incident energy, a clear
indication of strong RSA. The degree of RSA of these
complexes increases in the order of 4 ≈ 5 < 1 ≈ 3 < 2 < 6,
and the RSA of 3 is slightly better than that of complex 1 in ref
11. It appears that the RSA is generally stronger in complexes
with electron-withdrawing substituents (1−3) than in com-
plexes with electron-donating substituents, except for 6, which
contains the strongest electron-donating substituent. The
strongest RSA from 6 should be related to its stronger
excited-state absorption at 532 nm, which is evident by its TA
spectrum, in which one of the absorption bands occurs at ca.
530 nm.
λT −T /nm (τT/μs;
1
n
εT −T /M−1 cm−1
;
theor
λ
/
1
n
phos
c
RT
77 K
ΦT)
nm
1
2
3
582 (6.0; 2.04 × 109), 573 (15.7),
655 (6.1; 18530;
0.56)
621
626 (6.2;
621 (16.0)
1.83 × 109); 0.077
579 (5.3; 1.46 × 109), 569 (19.3),
580 (5.5; 27910;
0.40)
621
634
623 (5.5;
617 (19.3)
1.62 × 109); 0.11
588 (6.1; 1.44 × 109), 581 (24.0),
625 (8.2; 56980;
0.33)
634 (6.1;
630 (28.0)
9.33 × 108); 0.092
4
5
6
595 (1.3;
565 (22.1),
610 (20.0)
555 (2.1; 14870;
0.50)
600
601
618
3.49 × 109); 0.031
584 (2.1;
564 (21.5),
609 (21.2)
560 (3.8; 13170;
0.32)
2.00 × 109); 0.06
638 (−); 0.003
583 (24.9),
629 (20.9)
530 (1.4; 35680;
0.38), 755 (1.1;
20990; 0.38)
1-L 414 (−), 483 (−); − 530, 572
2-L 383 (−); 0.066
655 (18.6; −; −)
535 (4.9; −; −)
378, 515,
549
3-L 410 (−); 0.80
4-L 378 (−); 0.69
5-L 401 (−); 0.91
6-L 444 (−); 0.84
410
385
388
425
580 (5.4; −; −)
520 (10.1; −; −)
445 (10.0; −; −)
460 (−; −; −)
a
Emission wavelength (λem), intrinsic lifetime (τ0), self-quenching rate
constant (ksq), and emission quantum yield measured in CH2Cl2. In
BuCN glassy matrix. Triplet excited-state absorption band maximum
b
c
(λT −T ), molar extinction coefficient (εT −T ), quantum yield (ΦT),
1
n
1
n
and lifetime (τT) measured in CH3CN for 1−5; 6 was measured in
toluene.
To better rationalize the observed RSA trend of these
complexes at 532 nm for nanosecond laser irradiation, the
ratios of the excited-state absorption cross section (σex) relative
to the ground-state absorption cross section (σ0) (which is a
key parameter for RSA) at 532 nm for 1−6 have to be
evaluated. The σex values at 532 nm can be estimated from the
ΔODs of the nanosecond TA at zero delay at 532 nm and at
the TA band maximum (λT −T ), from the ground-state
3
charge transfer that could admix the ILCT character into the
3π,π* states.
Figure 6b shows the TA spectra of complexes 1−6 at zero
delay after excitation. The time-resolved TA spectrum of 1 is
illustrated in Figure 6c, while the others are provided in Figures
S27−S31 of the Supporting Information. The TA spectra for all
complexes feature two major absorption bands, a narrower one
between 400 and 500 nm and a broader one above 500 nm.
The low-energy TA bands are stronger than the 400−500 nm
absorption bands in 1−3, but the relative intensity of these two
bands is opposite in 4−6. Compared to the TA spectra of their
respective ligands, the low-energy absorption bands of the TA
spectra for these complexes are red-shifted, indicating electron
delocalization induced by the interaction of the Pt(II) center
with the ligand. For all complexes, bleaching occurs at λ < 430
nm, which is consistent with the position of the major
absorption band in their respective UV−vis absorption spectra.
The triplet lifetimes deduced from the decay of the TA (Table
6) for these complexes are similar to those obtained from the
decay of emission, implying that the excited state giving rise to
the observed TA could be the same excited state that emits. On
the basis of these features, we tentatively ascribe the TA of 1−5
1
n
absorbance (A) at 532 nm and at the TA band maximum of
the same solution for the TA measurement, and from the molar
extinction coefficient (εT −T ) at the TA band maximum, as well
1
n
as from the conversion equation σ = 3.82 × 10−21ε. The
detailed procedure for the estimation of these values follows
that described by our group previously.8m The σ0 values at 532
nm can be deduced from the ground-state absorption molar
extinction coefficients using σ = 3.82 × 10−21ε. The resultant
σex and σ0 values as well as the ratios of σex/σ0 are compiled in
Table 8. It is obvious that the σex/σ0 values follow the trend of 4
< 5 < 1 < 3 < 2 < 6, which correlates very well with the trend of
the RSA plots of these complexes. The correspondence
between the σex/σ0 values and RSA performances indicates
that the RSA mainly arises from the triplet excited states of the
complexes. This notion is quite reasonable because the
femtosecond TA study on complex 1 in ref 11 found that
intersystem crossing from the singlet to triplet excited states
occurs in ∼88 ps (τisc = τs/ΦT); and during the nanosecond
excitation, the triplet excited state is the dominant contributor
based on the calculated fractional populations of the affected
excited states.11 Considering the similar core structure of
3
as being predominantly from their π,π* states, with possible
contributions from the MLCT states for 1, 2, 4, and 5. For
complex 6, the TA in CH3CN was too weak to be detected.
However, a moderately strong TA spectrum was observed in
toluene. The absence of TA in a polar solvent such as CH3CN
and the shorter lifetime (τT = 1.4 μs in toluene) suggest the
3
K
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Inorganic Chemistry
Article
Table 7. Natural Transition Orbitals Representing the Lowest-Energy Triplet Transitions for Complexes 1−6
that the σex/σ0 ratios for 1−6 are much larger than those of
most of the reverse saturable absorbers reported in the
literature;8b,c,h,i,f,11,38b,c,39 and the σex/σ0 value for 6 is among
the largest values reported to date.
CONCLUSION
■
Six new Pt(II) complexes (1−6) containing 6-[9,9-di(2-
ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine (R = NO2,
CHO, benzothiazol-2-yl, n-Bu, carbazol-9-yl, NPh2) ligands
were synthesized, and their photophysics were systematically
investigated by spectroscopic techniques and TDDFT theoreti-
cal calculations. The absorption and emission characteristics of
1−6 can be substantially adjusted by the 7-substituent. It is
found that electron-withdrawing substituents (NO2, CHO,
BTZ) and electron-donating substituents (n-Bu, CBZ, NPh2)
exert distinct effects on the photophysics of the complexes. The
lowest-energy absorption bands of 1−6 all feature
1MLCT/1ILCT/1π,π* characters, while the contributions of
1MLCT, 1LLCT, 1π,π*, and 1ILCT configurations to the major
absorption band(s) vary depending on the nature of the
substituent. The different substituents influence the nature of
the lowest triplet excited states of 1−6 more distinctively,
reflected by the fact that the emitting excited state of complexes
1−3 is dominated by the 3π,π* characters with a little
Figure 5. Concentration-dependent emission spectra of 1 in CH2Cl2.
λex= 372 nm.
complexes 1−6 to that of complex 1 in ref 11, it is reasonable
to assume that the dominant contributing excited state to the
RSA within the nanosecond pulse width should be the triplet
excited states for 1−6 as well. Complex 6 possesses the
strongest triplet excited-state absorption at 532 nm and thus
the strongest RSA. The σex/σ0 value for complex 3 is also much
larger than that for its counterpart with the tolylacetylide ligand
(i.e. complex 1 in ref 11), which has σT/σ0 = 69.6.11 The much
larger ratio for complex 3 should be mainly due to the
significantly reduced ground-state absorption cross section of 3
at 532 nm (σ0 = 4.3 × 10−19 cm2) with respect to complex 1 in
ref 11 (σ0 = 1.48 × 10−18 cm2). This confirms our initial design
concept that replacing the tolylacetylide coligand with Cl
coligand could reduce the ground-state absorption in the visible
spectral region and thus increase the σex/σ0 ratio and enhance
the RSA in the visible spectral region. It is also worth noting
3
contribution from MLCT in 1 and 2, while the emission of
3
4 and 5 admixes MLCT/3π,π* characters, and 6 primarily
3
3
exhibits very weak ILCT emission. It appears that the π,π*
character diminishes while charge transfer character increases
when the strength of the electron-donating substituent
increases. All complexes exhibit broad and strong triplet
excited-state absorption from the near-UV to the near-IR
spectral region, which originates from the same triplet excited
L
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Table 8. Ground-State (σ0) and Excited-State (σex)
Absorption Cross Sections for Complexes 1−6 at 532 nm
1
2
3
4
5
6
a
σ0/10−18 cm2
0.27
46
0.27
67
0.43
80
0.38
44
0.38
46
0.88
230
261
b
σex/10−18 cm2
σex/σ0
170
248
186
116
121
a
b
In CH2Cl2. 1−5 in CH3CN and 6 in toluene.
substituents) compared to those of 4−6 (which contain
electron-donating substituents). Due to the much stronger
triplet excited-state absorption than the ground-state absorp-
tion of 1−6 in the visible spectral region, all complexes exhibit
strong reverse saturable absorption for a nanosecond laser pulse
at 532 nm, with complex 6 showing the strongest RSA due to
its strongest excited-state absorption at 532 nm and thus the
largest ratio of σex/σ0. The σex/σ0 ratios for the other five
complexes are also much larger than most of the reverse
saturable absorbers reported in the literature at 532 nm. This
makes complexes 1−6, especially 6, very promising candidates
for devices that require strong RSA.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data for complex 1 in CIF format, the
synthetic procedures and 1H NMR data of all the intermediates,
the absorption spectra of 1-L−6-L and 1−6 in different
solvents, the emission spectra of 2-L−5-L and 1, 3, 4, and 6 in
different solvents, the nanosecond time-resolved transient
absorption spectra of 1-L−6-L and 2−6, the emission quantum
yields of 1-L−6-L and 1−6 in different solvents, and the
optimized geometries of 1−6 via DFT calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 6. Nanosecond transient difference absorption spectra of (a)
ligands 1-L − 6-L and (b) complexes 1 − 6 at zero delay after
excitation; (c) time-resolved TA spectra of 1. 1-L − 5-L and 1 − 5
AUTHOR INFORMATION
Corresponding Author
*E-mail: Wenfang.Sun@ndsu.edu. Phone: 701-231-6254. Fax:
■
were measured in CH3CN, 6-L and 6 were measured in toluene. λex
355 nm, A355 = 0.4 in a 1-cm cuvette.
=
701-231-8831.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is partially supported by the National Science
Foundation (CAREER CHE-0449598 and CHEM-0946990)
and partially by the Army Research Laboratory (W911NF-06-2-
0032 and W911NF-10-2-0055) to W. Sun. S. Kilina acknowl-
edges the U.S. Department of Energy Early-Career Grant DE-
SC008446 for financial support and North Dakota State
University and the Center for Integrated Nanotechnology
(CINT) at Los Alamos National Laboratory for computer
access and administrative support.
Figure 7. Nonlinear transmission curves for 1−6 and complex 1 in ref
11 in CH2Cl2 in a 2 mm cuvette for 4.1 ns laser pulses at 532 nm. The
radius of the beam waist at the focal point was approximately 96 μm.
The linear transmission for all samples was adjusted to 90% in a 2 mm
cuvette.
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