Synthesis, photophysics, and reverse saturable absorption of platinum complexes bearing extended π-conjugated C^N^N ligands

Article abstract of DOI:10.1039/c3dt51430g

The synthesis of ligands 1-L-6-L that feature the 6-[9,9-di(2-ethylhexyl)- 7-R-9H-fluoren-2-yl]-2,2′-bipyridine (C^N^N) core (R = 4-R′-phenylethynyl with R′ = NO₂, benzothiazol-2-yl (BTZ), H and OCH₃ or R = 4′-BTZ-phenyl or BTZ) and their platinum complexes 1-6 were reported in this paper. The photophysical properties of these ligands and the Pt(ii) complexes, including the UV-vis absorption spectra, emission characteristics at room temperature and at 77 K, and the triplet transient difference absorption spectra, were systematically investigated in order to understand the effects of the substituent at the 4-position of the 1-ethynylphenyl component and the extension of π-conjugation between the C^N^N core and the BTZ substituent. Reverse saturable absorption (RSA) of complexes 1-6 was demonstrated at 532 nm using 4.1 ns laser pulses. The UV-vis absorption spectra of 1-L-6-L are featured by strong ¹π,π* transitions in the blue spectral region, and the absorption bands are effectively red-shifted by substitution at the 4-position of the ethynylphenyl motif and by the extended π-conjugation of the linkage. A similar effect was observed for the fluorescence spectra of these ligands in CH₂Cl₂ at room temperature, but the nature of the fluorescence varies from ¹π,π* fluorescence in 3-L and 6-L, to intraligand charge transfer (¹ILCT) fluorescence in 1-L, 2-L and 5-L; while 4-L possesses mixed ¹π,π*/ ¹ILCT characters. All ligands exhibit moderate triplet transient absorption (TA) in the visible spectral region, with substitution at the 4-position of the ethynylphenyl component broadening of the TA bands, while extended π-conjugation of the linkage inducing red-shifts of the TA bands. For Pt(ii) complexes 1-6, their UV-vis absorption spectra constitute red-shifted ¹π,π* transitions and low-energy metal-to-ligand charge transfer (¹MLCT/¹ILCT) tails. The emission of these complexes at room temperature in CH₂Cl₂ predominantly originates from the C^N^N core localized ³π,π* state, probably mixed with minor ³MLCT character. 4-Position substitution and extended π-conjugation on the ligands exert a negligible effect on the shape and energy of the emission spectra. Similar to their respective ligands, 1-6 all exhibit broader and red-shifted TA spectra with respect to their ligands and both the 4-position substitution and extended π-conjugation bathochromically shift the TA band maxima. The nonlinear transmission experiments carried out for 1-6 at 532 nm reveal that all complexes exhibit strong reverse saturable absorption (RSA), and the degree of RSA follows this trend: 6 <4 <5 <2 ≤3 <1. The RSA performance is efficiently improved by electron-withdrawing substituents (NO₂ and BTZ) and by extending the π-conjugation; while electron-donating substituent (OCH ₃ in 4) decreases the RSA at 532 nm.

Full text of DOI:10.1039/c3dt51430g

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Synthesis, photophysics, and reverse saturable
absorption of platinum complexes bearing extended
Cite this: Dalton Trans., 2013, 42, 14021
π-conjugated C^N^N ligands†
Zhongjing Li and Wenfang Sun*
The synthesis of ligands 1-L–6-L that feature the 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2’-bipyri-
2 3
dine (C^N^N) core (R = 4-R’-phenylethynyl with R’ = NO , benzothiazol-2-yl (BTZ), H and OCH or R =
4
’-BTZ-phenyl or BTZ) and their platinum complexes 1–6 were reported in this paper. The photophysical
properties of these ligands and the Pt(II) complexes, including the UV-vis absorption spectra, emission
characteristics at room temperature and at 77 K, and the triplet transient difference absorption spectra,
were systematically investigated in order to understand the effects of the substituent at the 4-position of
the 1-ethynylphenyl component and the extension of π-conjugation between the C^N^N core and the
BTZ substituent. Reverse saturable absorption (RSA) of complexes 1–6 was demonstrated at 532 nm
1
using 4.1 ns laser pulses. The UV-vis absorption spectra of 1-L–6-L are featured by strong π,π* transitions
in the blue spectral region, and the absorption bands are effectively red-shifted by substitution at the
4
-position of the ethynylphenyl motif and by the extended π-conjugation of the linkage. A similar effect
was observed for the fluorescence spectra of these ligands in CH
2
Cl
2
at room temperature, but the
1
nature of the fluorescence varies from π,π* fluorescence in 3-L and 6-L, to intraligand charge transfer
1
1
1
(
ILCT) fluorescence in 1-L, 2-L and 5-L; while 4-L possesses mixed π,π*/ ILCT characters. All ligands
exhibit moderate triplet transient absorption (TA) in the visible spectral region, with substitution at the
-position of the ethynylphenyl component broadening of the TA bands, while extended π-conjugation
4
of the linkage inducing red-shifts of the TA bands. For Pt(II) complexes 1–6, their UV-vis absorption
1
spectra constitute red-shifted π,π* transitions and low-energy metal-to-ligand charge transfer
1
1
(
2 2
MLCT/ ILCT) tails. The emission of these complexes at room temperature in CH Cl predominantly orig-
3
3
inates from the C^N^N core localized π,π* state, probably mixed with minor MLCT character. 4-Position
substitution and extended π-conjugation on the ligands exert a negligible effect on the shape and
energy of the emission spectra. Similar to their respective ligands, 1–6 all exhibit broader and red-shifted
TA spectra with respect to their ligands and both the 4-position substitution and extended π-conjugation
bathochromically shift the TA band maxima. The nonlinear transmission experiments carried out for 1–6
at 532 nm reveal that all complexes exhibit strong reverse saturable absorption (RSA), and the degree of
RSA follows this trend: 6 <4 <5 <2 ≤3 <1. The RSA performance is efficiently improved by electron-with-
Received 1st June 2013,
Accepted 17th July 2013
DOI: 10.1039/c3dt51430g
2
drawing substituents (NO and BTZ) and by extending the π-conjugation; while electron-donating substi-
www.rsc.org/dalton
tuent (OCH in 4) decreases the RSA at 532 nm.
3
Introduction
Square-planar platinum(II) complexes have gained intense
interest in the past two decades due to their potential appli-
Department of Chemistry and Biochemistry, North Dakota State University, Fargo,
ND 58108-6050, USA. E-mail: wenfang.sun@ndsu.edu; Fax: +1 701 231 8831;
Tel: +1 701 231 6254
1
–4
cations as antitumor reagents,
in organic light emitting
5
–7
8–10
†
Electronic supplementary information (ESI) available: The synthetic procedures diodes (OLED),
photon upconversion,
and as nonlinear
1
11–14
and H NMR data of 6, 7b, and 9, the absorption and emission spectra of 1-L,
-L–6-L and 1–6 in different solvents, the emission spectra at different concen-
trations in CH Cl and Stern–Volmer plots of 1, 2, 4–6, the nanosecond time-
absorbing materials,
etc. Most of these applications are
4
intrigued by their rich photophysical properties that are
closely associated with their long-lived triplet excited states
due to the heavy-atom enhanced spin–orbit coupling. It has
been reported that the photophysical properties of the Pt(II)
2
2
resolved transient difference absorption spectra of 2-L–6-L and 2–6, and the
emission quantum yields of 1-L–6-L and 1–6 in different solvents. See DOI:
10.1039/c3dt51430g
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complexes can be readily tuned by structural modifications of complexes 2, 5, and 6, the linkage between the C^N^N core
the ligands. Therefore, understanding the structure–property and the benzothiazol-2-yl (BTZ) component was varied from
correlations is critical for designing platinum complexes with single bond (6) to phenyl (5) to ethynylphenyl (2). Comparison
a predetermined application. It is found that variation of sub- of the photophysics and RSA of 1–4 would allow the terminal
stituent on the ligand from electron-withdrawing to electron- substituent effect to be evaluated; while investigation of 2, 5
1
5
donating groups, tuning the degree of π-conjugation in the and 6 would help us to better understand the linkage effect
1
6
ligand, alternation of different linkages between components and thus the extended π-conjugation effect.
1
7
in ligand, and changing configuration of the Pt(II) complexes
1
8
(
cis or trans) could all influence the photophysics of the plati-
Experimental section
num complexes.
Our group has extensively studied the photophysics and Synthesis and characterization
nonlinear absorption of platinum complexes bearing polypyri-
The synthetic routes for ligands 1-L–5-L and complexes 1–5 are
dine ligands (terpyridine or diimine) or cyclometalating
illustrated in Scheme 2. The synthesis of ligand 6-L and
1
1–14
C^N^N ligands.
Most of the complexes exhibited excellent
19
complex 6 has been reported previously. 1-Ethynyl-4-nitro-
reverse saturable absorption (RSA, i.e. an increase of absorpti-
vity of molecules with the increased incident energy, and conse-
quently a decrease of transmission with increased incident
energy) for 532 nm ns laser pulses, which is typically related to
the large ratios of the excited-state absorption cross section to
that of the ground state of the molecule. Studies by our group
benzene (7a), phenylacetylene (7c), and 1-ethynyl-4-methoxy-
benzene (7d) were purchased from Aldrich Co. All of the other
reagents and solvents were purchased from Aldrich Chemical
Co. or Alfa Aesar and used as is unless otherwise stated. Silica
gel for column chromatography was purchased from Sorbent
2
−1
Technology (60 Å, 230–400 mesh, 500–600 m
g
, pH:
1
4
on platinum bipyridine bisacetylide complexes revealed that
the triplet excited states and RSA can be efficiently tuned by
the auxiliary substituent on the acetylide ligands. Recently, we
studied the effects of the substituent at the cyclometalating
C^N^N ligand on the photophysics and RSA of a series of Pt(II)
complexes and discovered that the electron-donating or with-
drawing substituents can significantly influence the excited-
21
22
6.5–7.5). The synthesis of precursors 8 and 11 followed the
literature procedures. The synthesis and characterization of
precursors 6, 7b, and 9 were described in the ESI.† The syn-
thetic procedures and characterization data for the key pre-
cursors 10a–10d, 12, ligands 1-L–5-L and complexes 1–5
are provided below. Sonogashira coupling reactions between
7a–7d and 9 respectively yielded 10a–10d. Suzuki coupling reac-
1
9
state characteristics and RSA. Our studies on Pt(II) bipyridine
acetylide complexes discovered that extending the π-conju-
gation on the bipyridine or/and the acetylide ligands can sig-
nificantly increase the triplet excited-state lifetime and
tion between 6 and 11 formed compound 12. 10a–10d and 12
then reacted with 8 respectively under Suzuki coupling reac-
tion conditions to afford 1-L–5-L. The ligands 1-L–5-L reacted
respectively with K PtCl in refluxing acetic acid under argon
2 4
for 24 hours to afford complexes 1–5. All intermediates,
2
0
dramatically improve the RSA at 532 nm for ns laser pulses.
Intrigued by these studies, we wonder if the extended π-conju-
gation on the cyclometalating C^N^N ligand could also
prolong the triplet excited-state lifetime and enhance the RSA
of the Pt(II) complexes; and whether the terminal substituent
at the extended π-conjugated C^N^N ligand would influence
the photophysics and RSA of the complexes.
To answer these questions, five Pt(II) complexes containing
a 6-[9,9-di(2-ethylhexyl)-7-R-fluoren-2-yl]-2,2′-bipyridine ligand
or a 6-[9,9-di(2-ethylhexyl)-7-(4-(benzothiazol-2-yl)phenyl)-fluoren-
2-yl]-2,2′-bipyridine ligand (complexes 1–5 in Scheme 1) were syn-
thesized and their photophysics and RSA were systematically
investigated. For comparison of the effect of extended π-conju-
1
9
gation, complex 6 that was reported previously was also
included in this paper. In complexes 1–4, different substituents
were introduced at the 4-position of the ethynylphenyl unit. In
Scheme 1 Structures for complexes 1–6 (C
8
H
17 = 2-ethylhexyl).
Scheme 2 Synthetic routes for ligands 1-L–5-L and complexes 1–5.
1
4022 | Dalton Trans., 2013, 42, 14021–14029
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1
ligands and the Pt(II) complexes were characterized by
H
24 hours. After reaction, the solid was removed by filtration
NMR. Ligands 1-L–5-L and complexes 1–5 were also character- and the organic phase was collected and washed with brine
ized by electrospray ionization high-resolution mass spec- and dried over MgSO . After removal of the solvent, the
trometry (ESI-HRMS), and elemental analyses. residue was purified by running a silica gel column eluted
H NMR spectra were obtained on Varian Oxford-VNMR with dichloromethane.
spectrometers (300 MHz, 400 MHz, or 500 MHz). ESI-HRMS 1-L. 0.10 g yellow oil was collected as the product (yield:
analyses were performed on a Bruker BioTOF III mass spectro- 81%). H NMR (CDCl
meter. Elemental analyses were conducted by NuMega Reson- J = 7.6 Hz, 1H), 8.37 (d, J = 7.6 Hz, 1H), 8.20–8.22 (m, 2H),
4
1
1
3
, 400 MHz): 8.68–8.69 (m, 1H), 8.64 (d,
ance Laboratories, Inc. in San Diego, California.
8.12–8.17 (m, 2H), 7.78–7.90 (m, 4H), 7.74 (d, J = 7.6 Hz, 1H),
7
7
.65–7.69 (m, 2H), 7.57–7.60 (m, 1H), 7.54–7.56 (m, 1H),
.30–7.33 (m, 1H), 2.00–2.16 (m, 4H), 0.51–0.95 (m, 30H).
General synthetic procedure for 10a–10d
The mixture of 7a–7d (1.0 mmol), 9 (0.94 mmol), CuI (40 mg), Elemental analysis calculated for C47
Pd(PPh Cl (40 mg), PPh (80 mg), and Et N (20 mL) was 0.25C 14: C, 80.88; H, 7.64; N, 5.82. Found: C, 81.27; H, 8.07;
heated to reflux under argon for 24 hours. After reaction, the N, 5.80. ESI-HRMS: m/z calc. for [C H N O + H] : 690.4054;
51 3 2 2 2
H N O ·0.125CH Cl ·
3
)
2
2
3
3
6
H
+
4
7
51 3 2
solid was removed by filtration and Et
lation. Purification was carried out by column chromatography
3
N was removed by distil- Found: 690.4035.
2-L. 0.12 g yellow solid was collected as the product (yield:
1
(silica gel) eluted with mixed solvent of hexane and dichloro- 86%). H NMR (CDCl , 400 MHz): 8.68–8.70 (m, 1H), 8.64–8.66
3
methane (v/v = 2/1 for 10a and 10b; v/v = 5/1 for 10c and 10d).
(m, 1H), 8.35–8.38 (m, 1H), 8.11–8.17 (m, 1H), 8.06–8.10 (m,
1
0a. 90 mg yellow oil was collected as the product (yield: 3H), 7.77–7.91 (m, 5H), 7.72–7.74 (d, J = 8.0 Hz, 1H), 7.64–7.69
1
1
4%). H NMR (CDCl , 500 MHz): 8.19–8.22 (m, 2H), 7.77–7.84 (m, 2H), 7.58–7.60 (m, 1H), 7.54–7.57 (m, 1H), 7.47–7.51 (m,
3
(
4
m, 2H), 7.64–7.71 (m, 4H), 7.49–7.55 (m, 2H), 1.93–2.06 (m, 1H), 7.36–7.40 (m, 1H), 7.28–7.33 (m, 2H), 2.01–2.16 (m, 4H),
H), 1.34 (m, 12H), 0.45–0.95 (m, 30H). 0.52–0.97 (m, 30H). Elemental analysis calculated for
0b. 0.16 g light yellow oil was collected as the product C H N S·0.25CH Cl : C, 81.52; H, 7.00; N, 5.26. Found: C,
1
5
4
55
3
2
2
1
(
yield: 22%). H NMR (CDCl
.93–7.94 (d, J = 8.0 Hz, 1H), 7.81–7.88 (m, 2H), 7.68–7.73 (m, H] : 778.4189; Found: 778.4162.
3
, 500 MHz): 8.09–8.12 (m, 3H), 81.12; H, 7.35; N, 5.37. ESI-HRMS: m/z calc. for [C54
H
55
N
3
S +
+
7
5
1
H), 7.51–7.60 (m, 3H), 7.36–7.43 (m, 3H), 1.97–2.10 (m, 4H),
.39 (s, 12H), 0.50–0.99 (m, 30H).
3-L. 0.10 g yellow oil was collected as the product (yield:
86%). H NMR (CDCl , 400 MHz): 8.65–8.71 (m, 2H), 8.37–8.39
3
1
1
0c. 0.54 g light yellow oil was collected as the product (m, 1H), 8.12–8.18 (m, 2H), 7.71–7.91 (m, 5H), 7.52–7.57 (m,
1
(
yield: 93%). H NMR (CDCl , 400 MHz): 7.80–7.87 (m, 2H), 4H), 7.31–7.38 (m, 4H), 2.01–2.17 (m, 4H), 0.50–0.95 (m, 30H).
3
7
1
.68–7.70 (d, J = 11.2 Hz, 2H), 7.50–7.59 (m, 3H), 7.29–7.38 (m, Elemental analysis calculated for C47
0H), 1.95–2.08 (m, 4H), 1.37 (s, 11H), 0.49–0.94 (m, 30H). 86.58; H, 8.05; N, 4.29. Found: C, 86.89; H, 8.37; N, 4.30.
0d. 0.25 g light yellow oil was collected as the product ESI-HRMS: m/z calc. for [C H N + H] : 645.4203; Found:
52 2 2 2
H N ·0.1CH Cl : C,
+
1
4
7
52 2
1
(
7
(
yield: 41%). H NMR (CDCl
.65–7.67 (m, 2H), 7.45–7.51 (m, 4H), 6.86–6.88 (m, 2H), 3.82
s, 3H), 1.93–2.05 (m, 4H), 1.35 (s, 12H), 0.46–0.92 (m, 30H).
3
, 400 MHz): 7.77–7.83 (m, 2H), 645.4172.
4-L. 0.10 g yellow oil was collected as the product (yield:
1
82%). H NMR (CDCl , 400 MHz): 8.65–8.71 (m, 2H), 8.36–8.38
3
(
2
m, 1H), 8.11–8.18 (m, 2H), 7.83–7.91 (m, 2H), 7.79–7.81 (m,
H), 7.70–7.71 (m, 1H), 7.48–7.56 (m, 4H), 7.31–7.34 (m, 1H),
6.88–6.90 (m, 2H), 3.83 (s, 3H), 2.00–2.16 (m, 4H), 0.52–0.97
O·
14: C, 83.25; H, 8.15; N, 3.90. Found: C,
Synthesis of 12
The mixture of 6 (0.3 g, 1 mmol), 11 (0.65 g, 1 mmol), K CO
2
3
(
0.52 g), Pd(PPh
and water (10 mL) was heated to reflux under argon for 0.25CH
0 hours. After reaction, the solid was removed by filtration 83.21; H, 8.43; N, 4.02. ESI-HRMS: m/z calc. for [C H N O + H] :
3
)
2
Cl
2
(48 mg), PPh
3
(78 mg), toluene (15 mL), (m, 30H). Elemental analysis calculated for
48 54 2
C H N
2
2
Cl ·0.25C
6
H
+
3
48 54 2
and the organic phase was washed with brine and dried over 675.4309; Found: 675.4283.
MgSO . After removal of the solvent, the residue was purified 5-L. 0.12 g yellow solid was collected as the product (yield:
4
1
by running a silica gel column eluted with a mixed solvent of 88%). H NMR (CDCl , 400 MHz): 8.66–8.70 (m, 2H), 8.36–8.38
3
dichloromethane and hexane (v/v = 1/3). 0.24 g colorless oil (m, 1H), 8.13–8.20 (m, 4H), 8.09 (d, J = 8.4 Hz, 1H), 7.75–7.91
1
was collected as the product (yield: 33%). H NMR (CDCl
3
,
(m, 8H), 7.68 (m, 1H), 7.62–7.65 (dt, J = 8.0, 1.6 Hz, 1H),
5
00 MHz): 8.21 (d, J = 8.0 Hz, 1H), 8.12 (d, J = 8.0 Hz, 1H), 7.94 7.47–7.51 (m, 1H), 7.36–7.40 (m, 1H), 7.30–7.33 (m, 1H),
(d, J = 8.0 Hz, 1H), 7.89 (d, J = 9.0 Hz, 1H), 7.81–7.84 (m, 2H), 2.07–2.21 (m, 4H), 0.51–0.92 (m, 30H). Elemental analysis cal-
7
7
1
55 3
.78 (d, J = 8.0 Hz, 2H), 7.74 (d, J = 7.5 Hz, 1H), 7.67 (m, 1H), culated for C52H N S: C, 82.82; H, 7.35; N, 5.57. Found: C,
+
.63–7.65 (m, 1H), 7.52 (t, J = 7.5 Hz, 1H), 7.41 (t, J = 7.8 Hz, 82.42; H, 7.44; N, 5.52. ESI-HRMS: m/z calc. for [C H N S + H] :
52
55
3
H), 2.05–2.14 (m, 4H), 1.39 (s, 12H), 0.52–0.91 (m, 30H).
754.4189; Found: 754.4167.
General synthetic procedure for 1-L–5-L
General synthetic procedure for 1–5
The mixture of 8 (0.20 mmol), 10a–10d or 12 (0.18 mmol), The mixture of 1-L–5-L (0.17 mmol),
CO (0.60 g), Pd(PPh Cl (40 mg), PPh
10 mL), and water (5 mL) was heated to reflux under argon for for 24 hours. After reaction, the solvent was removed by
K
2
PtCl
4
(72 mg,
K
2
3
3
)
2
2
3
(60 mg), toluene 0.17 mmol), and acetic acid (10 mL) was refluxed under argon
(
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distillation. The red solid was purified by column chromato- in degassed solutions). A degassed aqueous solution of
2
4
graphy on silica gel eluted with dichloromethane. Further [Ru(bpy) ]Cl (Φ = 0.042, λex = 436 nm) was used as the
3
2
em
purification was carried out by recrystallization from dichloro- reference for complexes 1–6, and a 1 N sulfuric acid solution
2
5
methane and hexane.
of quinine bisulfate (Φem = 0.546, λex = 347.5 nm) was used
1
. 62 mg red powder was collected as the product (yield: as the reference for ligands 1-L–6-L.
1
4
0%). H NMR (CDCl
3
, 400 MHz): 8.93–9.00 (m, 1H), 8.19–8.22
The nanosecond transient difference absorption (TA)
(m, 2H), 8.11–8.12 (m, 1H), 7.90–7.95 (m, 2H), 7.81–7.84 (m, spectra and decays of 1-L–6-L and 1–6 were measured in
1
7
H), 7.72–7.79 (m, 1H), 7.64–7.69 (m, 2H), 7.46–7.54 (m, 5H), degassed CH CN solutions on an Edinburgh LP920 laser flash
.30–7.31 (m, 1H), 1.94–2.03 (m, 4H), 0.48–0.94 (m, 30H). photolysis spectrometer. The third harmonic output (355 nm)
3
Elemental analysis calculated for C47
H
3 2
50ClN O Pt: C, 61.40; H, of a Nd:YAG laser (Quantel Brilliant, pulse width: 4.1 ns, rep-
5
.48; N, 4.57. Found: C, 61.34; H, 5.72; N, 4.51. ESI-HRMS: m/z etition rate was set at 1 Hz) was used as the excitation source.
+
calc. for [C47
H
50
N
3
O
2
PtCl + H] : 920.3317; Found: 920.3355.
Each sample was purged with argon for 30 min prior to
2
. 52 mg yellow powder was collected as the product (yield: measurement. The triplet excited-state absorption coefficient
1
3
1
7
0%). H NMR (CDCl , 400 MHz): 9.17–9.19 (m, 1H), 8.14 (s, (ε ) at the TA band maximum was determined by the singlet
H), 8.04–8.09 (m, 4H), 7.89–7.92 (m, 2H), 7.81–7.83 (m, 2H), depletion method. The triplet quantum yields (Φ
3
T
2
6
T
) of 1–6
.64–7.66 (m, 3H), 7.47–7.56 (m, 5H), 7.36–7.40 (m, 1H), 7.33 were determined by the relative actinometry using SiNc in
m, 1H), 1.98–1.99 (m, 4H), 0.48–0.92 (m, 30H). Elemental ana- benzene as the reference (ε590 = 70 000 M− cm , Φ = 0.20).
1
−1
27
(
T
3 2 2
lysis calculated for C54H54ClN PtS·0.5CH Cl : C, 62.34; H, 5.28;
Nonlinear transmission experiment
The reverse saturable absorption of complexes 1–6 was charac-
N, 4.00. Found: C, 62.48; H, 5.54; N, 4.00. ESI-HRMS: m/z calc.
+
for [C H N SPtCl + H] : 1008.3453; Found: 1008.3501.
5
4
54 3
3
. 30 mg yellow powder was obtained as the product (yield: terized by nonlinear transmission experiment at 532 nm using
1
2
3
0%). H NMR (CDCl , 500 MHz): 9.04–9.08 (m, 1H), 8.14 (s, a Quantel Brilliant laser as the light source. The pulse width of
1
H), 7.93–8.01 (m, 2H), 7.77–7.84 (m, 2H), 7.52–7.58 (m, 7H), the laser was 4.1 ns and the repetition rate was set at 10 Hz.
7
.33–7.39 (m, 4H), 1.97–2.05 (m, 4H), 0.52–0.96 (m, 30H). The complexes were dissolved in CH Cl . The concentration of
2
2
2
Elemental analysis calculated for C47H51ClN Pt: C, 64.56; H, the sample solution was adjusted to obtain a linear trans-
5
.88; N, 3.20. Found: C, 64.10; H, 6.21; N, 3.13. ESI-HRMS: m/z mission of 90% at 532 nm in a 2 mm-thick cuvette. The experi-
+
28
calc. for [C47
H
51
N
2
PtCl + H] : 875.3466; Found: 875.3432.
mental setup is similar to that reported previously, with a
4
. 46 mg yellow powder was obtained as the product (yield: 40 cm plano-convex lens being used to focus the beam to the
1
3
1
2
2
0%). H NMR (CDCl , 400 MHz): 9.08–9.11 (m, 3H), 8.10 (s, center of the 2 mm-thick sample cuvette. The radius of the
3
H), 7.96–7.99 (m, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.75–7.80 (m, beam waist at the focal point was approximately 96 μm.
H), 7.45–7.59 (m, 7H), 7.29 (d, J = 1.6 Hz, 1H), 6.85–6.87 (m,
H), 3.81 (s, 3H), 1.96–1.97 (m, 4H), 0.47–0.94 (m, 30H).
Elemental analysis calculated for C48
H
53ClN
2
OPt: C, 63.74; H, Results and discussion
5
.91; N, 3.10. Found: C, 63.55; H, 5.91; N, 3.05. ESI-HRMS: m/z
Electronic absorption
+
calc. for [C H N OPtCl + H] : 905.3572; Found: 905.3592.
4
8
53 2
The electronic absorption of 1-L–6-L and 1–6 were measured in
5
. 53 mg yellow powder was obtained as the product (yield:
−
6
1
dichloromethane in the concentration range of 1 × 10 –1 ×
3
2%). H NMR (CDCl
3
, 400 MHz): 9.24–9.26 (m, 1H), 8.19–8.21
−
4
−1
10
mol L , and the spectra are shown in Fig. 1a for the
(m, 3H), 8.08–8.11 (m, 2H), 7.92–7.94 (m, 3H), 7.83–7.86 (m,
1
7
H), 7.78–7.80 (m, 2H), 7.65–7.67 (m, 2H), 7.49–7.60 (m, 6),
.38–7.42 (m, 2H), 2.00–2.10 (m, 4H), 0.49–0.94 (m, 30H).
Elemental analysis calculated for C H ClN PtS·0.33CH Cl :
5
2
54
3
2
2
C, 62.12; H, 5.45; N, 4.15. Found: C, 61.86; H, 5.49; N, 4.14.
+
54 3
ESI-HRMS: m/z calc. for [C52H N SPtCl + H] : 984.3452;
Found: 984.3462.
Photophysical measurements
The spectroscopic grade solvents used for the photophysical
experiments were purchased from VWR International and
used as is without further purification. A Shimadzu UV-2501
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 emis-
sion spectra. The emission spectra of 1–6 were recorded after
their solutions were purged with argon for 30 minutes. The
emission quantum yields of 1-L–6-L and 1–6 were determined
Fig. 1 (a) UV-vis absorption spectra of ligands 1-L–6-L measured in CH
Cl
2 2
Cl ;
2
3
by the relative actinometry method (1–6 were measured (b) UV-vis absorption spectra of complexes 1–6 measured in CH
2
2
.
1
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Table 1 Electronic absorption, emission (room temperature and 77 K), and excited-state absorption parameters for complexes 1–6 and ligands 1-L–6-L
λ
k
em/nm (τ
sq/L mol
0
/μs;
λ
em/nmc
λ
ε
T
T1−Tn/nm (τ /μs;
−1
abs/nm (ε/L mol− cm
1
−1 a
)
−1
s
−1); Φem R. T.
b
(τem/μs) 77 K
T1−Tn/L mol cm−1; Φ
)
d
λ
T
9
1
2
3
4
5
6
450 (9120), 410 (47 670), 393 (53 650), 338 (29 500)
450 (5490), 410 (55 380), 389 (72 130), 366 (63 380)
450 (3270), 405 (28 100), 381 (41 910), 336 (41 630)
450 (3030), 400 (28 300), 385 (33 170), 335 (27 980)
450 (4650), 405 (46 110), 382 (62 880), 348 (55 090)
450 (4310), 404 (37 320), 385 (49 820), 343 (40 510)
593 (7.5; 1.53 × 10 ),
6
583 (26.1), 632 (23.9)
585 (26.1), 633 (28.1)
577 (22.4), 626 (20.9)
578 (22.9), 627 (23.3)
576 (24.7), 623 (23.0)
581 (24.0), 630 (28.0)
560 (5.1; 26 530; 0.37)
675 (8.1; 90 010; 0.19)
610 (5.7; 37 510; 0.43)
640 (7.1; 37 190; 0.25)
650 (7.3; 60 730; 0.12)
625 (8.2; 56 980; 0.33)
9
38 (7.0; 1.60 × 10 ); 0.13
9
592 (6.2; 1.63 × 10 ),
9
6
36 (6.2; 1.26 × 10 ); 0.053
9
589 (4.7; 1.66 × 10 ),
9
6
30 (4.5; 1.21 × 10 ); 0.084
9
591 (4.6; 1.69 × 10 ),
9
6
35 (4.5; 1.31 × 10 ); 0.051
9
590 (3.4; 1.79 × 10 ),
9
6
40 (3.4; 1.26 × 10 ); 0.069
9
588 (6.1; 1.44 × 10 ),
8
6
34 (6.1; 9.33 × 10 ); 0.092
1
2
3
4
5
6
-L
-L
-L
-L
-L
-L
400 (29 080), 377 (39 530)
384 (63 900), 366 (79 400)
357 (52 000), 344 (60 230)
363 (55 150), 348 (68 850)
371 (56 100), 356 (71 580)
373 (44 200), 358 (58 250)
571 (–); 0.032
428 (–); 0.92
393 (–); 0.96
400 (–); 0.99
427 (–); 0.92
410 (–); 0.80
485
430, 461
378, 392, 413
383, 399, 416
402, 421, 462
395, 410, 437
510 (26.7; –; –)
625 (12.3; –; –)
555 (14.0; –; –)
530 (29.6; –; –)
610 (15.3; –; –)
580 (24.0; –; –)
a
b
Absorption band maximum (λabs) and molar extinction coefficient (εmax) in CH
Cl
bisulfate for the ligands. In BuCN glassy matrix. Triplet excited-state absorption band maximum (λT1−Tn), molar extinction coefficient (εT1−Tn),
quantum yield (Φ ) and lifetime (τ ) measured in CH CN.
2
Cl
2
.
0
Emission wavelength (λem), intrinsic lifetime (τ ), self-
quenching rate constant (ksq), and emission quantum yield measured in CH
2
2 3 2
with Ru(bpy) Cl as the standard for the complexes and quinine
c
d
T
T
3
ligands and Fig. 1b for the complexes. The absorption band with respect to the other ligands. This implies that there could
maxima and molar extinction coefficients for the ligands and be strong mixing of the intramolecular charge transfer charac-
1
1
the complexes are summarized in Table 1. No ground-state ter with the π,π* transition in 1-L, which decreases the π,π*
aggregation was evident in the concentration range studied, as transition oscillation strength but increases the intramolecular
supported by the obedience of the absorption to the Beer’s law. charge transfer transition oscillation strength. In contrast, the
1
The major absorption bands of 1-L–6-L appear at the 300 nm– absorption of 2-L–4-L should be dominated by the π,π* tran-
4
50 nm region with the molar extinction coefficients being in sitions. Comparison of the UV-vis absorption spectra of 2-L,
4
−1
−1
the order of 10 L mol cm . Considering the large molar 5-L and 6-L reveals that with the extended π-conjugation between
extinction coefficients and the minor solvent effect (see Fig. S1– the BTZ unit and the C^N^N core, the UV-vis absorption
S6 of the ESI†), these bands are ascribed to the π,π* tran- spectra of 2-L and 5-L are pronouncedly red-shifted and their
1
sitions. The UV-vis absorption spectra of the Pt(II) complexes molar extinction coefficients are dramatically increased.
1
(
5
–6 are featured by strong absorption bands below ca. 430 nm
450 nm for 1) and a broad tail from 440 nm (450 nm for 1) to variation on the π,π* transitions are significant for the
00 nm. Comparing to the UV-vis absorption spectra of their ligands, these effects become much weaker in complexes 1–6.
Although the effects of terminal substituent and linkage
1
1
respective ligands and with reference to the other Pt(II) C^N^N The ligand centered π,π* transitions of complexes 3–6 are
chloride complexes with similar structures, the major absorp- essentially at the same energy except for that they are obviously
2
9
tion bands can be attributed to predominantly the ligand cen- red-shifted in 1 and 2. However, the molar extinction coeffi-
1
tered π,π* transitions; while the broad tail is believed to be cient still increases with the terminal substitution and
1
dominated by the MLCT transition, probably mixed with some extended π-linkage except for 4. In contrast, the effect of the
intraligand charge transfer (from the ethynylfluorene com- π-linkage on the charge-transfer tails of 2, 5 and 6 is more pro-
1
ponent to the bipyridine component) characters. The π,π* tran- nounced. The tail in 2 and 5 are clearly more red-shifted and
sitions in the complexes are significantly red-shifted compared stronger than that in 6.
to those in their respective ligands, indicating the electron delo-
Photoluminescence
calization induced by the interactions with the metal ion.
Comparing the UV-vis absorption spectra of 1-L–4-L, it is
very clear that substitution at the 4-position of the ethynylphe-
nyl (regardless electron-donating or electron-withdrawing sub-
stituent) causes pronounced red-shifts of the absorption
bands and/or increased molar extinction coefficients, as evi-
denced by 1-L, 2-L and 4-L compared to 3-L, which suggests
electron delocalization induced by the substituents. Notably,
the absorption spectrum of 1-L is much broader and red-
shifted, but with significantly low molar extinction coefficients
The emission spectra of 1-L–6-L and 1–6 in dichloromethane
at room temperature and in butyronitrile glassy matrix at 77 K
were measured and shown in Fig. 2. The emission band
maxima, lifetimes and quantum yields for the ligands and
complexes are summarized in Table 1.
As shown in Fig. 2a, the emission of the ligands 3-L and 6-L
2 2
in CH Cl is featured by distinct vibronic structures, while
the emission from 4-L is slightly structured. In contrast, the
emission from 1-L, 2-L and 5-L is broad and structureless. The
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trend follows the similar trend as the UV-vis absorptions,
namely, both substitution and extended π-conjugation of the
linkage between the BTZ substituent and the C^N^N core
effectively lower the fluorescence energy. In addition, both the
substitution and extended π-conjugation lead to significant
charge transfer characters in their respective fluorescence
spectra in polar solvents. It is worth noting that the strong
electron withdrawing group like NO in 1-L plays the most sig-
2
nificant role in switching the emission into charge transfer
fluorescence, testified by the solvatochromic study shown in
Fig. S13,† in which the emission was only observed in dichloro-
methane and THF but diminished in nonpolar solvents such
3
as hexane and toluene and in the more polar solvent CH CN.
The emission spectra of 1-L–6-L in butyronitrile glassy
matrix at 77 K are shown in Fig. 2b. The spectra become
Fig. 2 (a) Normalized emission spectra of ligands 1-L–6-L (λex = 398 nm for
1
-L, 365 nm for 2-L, 345 nm for 3-L, 350 nm for 4-L, 356 nm for 5-L, and 358 nm narrower and more structured (except 1-L) in comparison to
−5 −1 −6 −1
for 6-L; c = 4 × 10 mol L for 1-L and 3-L, 5 × 10 mol L for 2-L, 5-L, and
those at room temperature but essentially remain of similar
energy. Therefore, they are attributed to the low-temperature
fluorescence.
-L, and 1 × 10 mol L−1 for 4-L). (b) Normalized emission spectra of 1-L–6-L at
−6
6
7
3
5
3
3
−
5
−1
7 K in glassy BuCN matrix at the concentration of 1 × 10 mol L (λex
98 nm for 1-L, 365 nm for 2-L, 345 nm for 3-L, 350 nm for 4-L, 356 nm for
-L, and 358 nm for 6-L). (c) Normalized emission spectra of complexes 1–6 (λex
=
The emission spectra of the Pt(II) complexes 1–6 in CH Cl
2
=
2
93 nm for 1, 383 nm for 2, 381 nm for 3, 385 nm for 4, 383 nm for 5, and at room temperature are displayed in Fig. 2c. Despite the
83 nm for 6) at the concentration of 2 × 10− mol L . (d) Normalized emission
5
−1
different substituents and π-linkage, the emission spectra of
these complexes all resemble each other, with a remarkable
Stokes shift, distinct vibronic structure, long lifetime
−5
−1
spectra of complexes 1–6 at 77 K in glassy BuCN matrix, c = 2 × 10 mol L
ex = 393 nm for 1, 383 nm for 2, 381 nm for 3, 385 nm for 4, 383 nm for 5,
(
λ
and 383 nm for 6).
(
3.4–7.5 μs), and minor solvatochromic effect (see Fig. S17–S22
of ESI†). Taking these features into account, we can assign the
observed emission of 1–6 to the phosphorescence predomi-
lifetimes of the emission from the ligands are too short to be
measured on our spectrometer. This suggests that the emis-
sion of the ligands predominantly originates from the π,π*
3
nantly from the C^N^N core localized π,π* state. However,
1
3
contribution from the MLCT state is possible in view of the
state for 3-L and 6-L, but predominantly emanates from the
ILCT (intraligand or intramolecular charge transfer state) for
several microsecond lifetime, which is relatively shorter than a
pure π,π* phosphorescence, and the minor but clear negative
solvatochromic effect. This assignment is also supported by
1
3
1
2 2
-L, 2-L and 5-L in CH Cl . For 4-L, the emission exhibits
1
1
mixed characters of π,π*/ ILCT. This assignment is supported
by the different solvatochromic effects for these ligands. As
demonstrated in Fig. 3 for 2-L and 3-L and in ESI Fig. S13–
S16† for the other four ligands, 3-L and 6-L exhibit minor sol-
vatochromic effects, with the similar shape of spectra in both
polar and nonpolar solvents and a slight red-shift when the
polarity of solvent increases. These features are characteristics
the small thermally induced Stokes shifts for these complexes,
−
1
−1
−1
−1
i.e. 289 cm for 1, 202 cm for 2, 353 cm for 3, 381 cm
for 4, 412 cm− for 5, and 205 cm for 6. This phenomenon is
1
−1
also consistent with that observed from the Pt(II) complexes
with
,10-phenanthroline ligand.
A concentration-dependent emission study of 1–6 in CH
a 2-(9,9-dihexadecyl-7-(4-R-phenylethynyl)-fluoren-2-yl)-
2
9
1
2
Cl
2
1
of π,π* fluorescence. However, for the other four ligands, the
was conducted. As exemplified in Fig. 4 for complex 3, the
fluorescence spectra become structureless and drastically red-
emission intensity increases with increased concentration up
to 2 × 10− mol L . Above this concentration, the emission
intensity decreases when the concentration gets higher. Con-
sidering the remarkable ground-state absorption of this
complex at the excitation wavelength (381 nm), the primary
inner-filter effect is clearly a cause. Contribution from the
ground-state aggregation can be excluded because our concen-
tration-dependent UV-vis absorption study described earlier
confirms that no ground-state aggregation occurs in the con-
centration range used in our study. However, lifetime measure-
ments reveal that the lifetime of the solution keeps decreasing
with increased concentration, which clearly indicates the pres-
ence of self-quenching. The self-quenching rate constant
5
−1
2 2 3
shifted in polar solvents (such as CH Cl and CH CN), reflect-
1
ing the transition from π,π* character in less polar solvents to
1
ILCT character in more polar solvents. Comparing the emis-
sion spectra of 1-L–6-L, it is clear that the fluorescence energy
9
deduced for 3 at 589 nm emission wavelength is 1.66 × 10 L
−
1 −1
mol
s , which is in line with those reported for Pt(II) terpyri-
Fig. 3 Normalized emission spectra of (a) 2-L and (b) 3-L in different solvents.
1
6,30
λex = 347.5 nm.
dine, Pt(C^N^N) or Pt(II) diimine complexes.
Similar
1
4026 | Dalton Trans., 2013, 42, 14021–14029
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2 2
Fig. 4 Concentration-dependent emission spectra of complex 3 in CH Cl .
Fig. 5 Nanosecond transient difference absorption spectra of (a) ligands 1-L–
λex = 381 nm. The inset shows the Stern–Volmer plot monitored at 589 nm.
6-L at zero delay after excitation; (b) time-resolved TA spectra of 1-L; (c) com-
plexes 1–6 at zero delay after excitation; (d) time-resolved TA spectra of 1. All of
the spectra were measured in CH
cuvette.
3
CN, λex = 355 nm, A355 = 0.4 in a 1 cm
phenomena were observed from the other five complexes, and
the results are provided in the ESI Fig. S23–S28† and in
Table 1.
The TA spectra of 1–6 at zero delay after excitation are illus-
trated in Fig. 5c. The triplet excited-state lifetimes, extinction
coefficients, and quantum yields are deduced or calculated
and the results are summarized in Table 1. The TA spectra of
all complexes possess three characteristic bands: the bleaching
band in the blue spectral region corresponding to their
The emission spectra of 1–6 at 77 K BuCN glassy matrix are
presented in Fig. 2d. These spectra are somewhat blue-shifted
and become more structured and narrower with respect to
those at room temperature, which are ascribed to the rigido-
3
1
chromic effect. The vibronic progressions for 1–6 are in the
−
1
range of 1212–1357 cm , and the thermally induced Stokes
1
−1
respective π,π* absorption band, the moderate and narrow TA
shifts for these complexes are approximately 200–410 cm . All
3
band in the wavelength range of 400–500 nm, and the broad
and strong TA band in the wavelength range of 500–700 nm.
The energy of the 500–700 nm TA band varies with the term-
inal substituent and the extent of π-conjugation between the
these features indicate the π,π* nature of the emission at 77 K
for these complexes.
Transient absorption
The nanosecond transient difference absorption (TA) measure- C^N^N core and the BTZ unit: both electron-withdrawing and
ments were conducted for ligands 1-L–6-L and complexes 1–6 electron-donating substituents (NO in 1, BTZ in 2, and OCH
2
3
in acetonitrile to understand the triplet excited-state character- in 4) cause a red-shift of the major TA band, extending the
istics. By performing the transient absorption experiments, the π-conjugation in the linkage also effectively red-shifts the
spectral feature of the triplet excited-state absorption can be major TA bands in 2 and 5 compared to that in 6. From the
obtained, and the spectral region where the excited state decay of the TA, the triplet lifetimes were deduced to be
absorbs stronger than the ground state can be identified from 5–8 μs, suggesting that the TA spectra likely arise from the
3
the positive absorption band(s) of the TA spectrum. In ligand based π,π* state as well, which is supported by the red-
addition, the excited-state lifetime and the triplet excited-state shift of the major TA bands in 1–6 compared to the TA bands
quantum yield can be deduced from the decay of the TA and in their corresponding ligands. The red-shift reflects the inter-
determined by the relative actinometry, respectively.
actions between the ligand π orbital and the metal d orbital.
The nanosecond transient absorption spectra of 1-L–6-L at
zero delay after excitation are displayed in Fig. 5a. In addition
Reverse saturable absorption (RSA)
to the bleaching bands in the UV region that correspond to the As manifested by the positive absorption bands in the visible
ground-state absorption bands, all the ligands show moderate spectral region of the TA spectra, complexes 1–6 all exhibit
to strong transient absorption in the visible spectral region. stronger excited-state absorption than that of the ground state
Compared to 3-L, the TA bands of 1-L, 2-L and 4-L are much at 532 nm. Meanwhile, the triplet-state lifetimes are signifi-
broader. For the three ligands with different π-conjugated lin- cantly longer than the ns laser pulse width (4.1 ns). Therefore,
kages, i.e. 2-L, 5-L, and 6-L, the major TA band is noticeably it is expected that these complexes would exhibit reverse satur-
red-shifted from 6-L, to 5-L, and to 2-L although the TA bands able absorption (decrease of transmission with increased inci-
of 2-L and 5-L are weaker. The TAs of all of the ligands show dent energy) for ns laser pulses at 532 nm. To verify this
well time-resolved decay as illustrated for 1-L in Fig. 5b and in assumption, nonlinear transmission experiment was carried
2 2
Fig. S29–S33† for 2-L–6-L. The lifetimes deduced from the out for complexes 1–6 in CH Cl solutions at a linear transmit-
decay of TA are all tens of μs, implying that the excited state tance of 90% in a 2 mm cuvette using the 532 nm ns laser
3
giving rise to the observed TA spectra should be the π,π*.
pulses. Fig. 6 displays the nonlinear transmission results. The
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R′ = NO
2
, BTZ, H and OCH
3
or R = 4′-BTZ-phen-1-yl). The ligand
6-L and its Pt(II) complex reported earlier were also included in
this paper for comparison purpose. The effects of terminal
substituents and the different π-conjugated linkages between
the BTZ component and the C^N^N core on the photophysics
of these ligands and complexes and on the RSA of the com-
plexes were systematically investigated. It is found that both
the terminal substitution and extended π-conjugation of the
linkage cause pronounced red-shift of the UV-vis absorption
bands and the fluorescence bands in the ligands; and induces
significant intraligand charge transfer character in their fluo-
rescence in polar solvents like CH
2 2 3
Cl and CH CN. 1-L–6-L all
exhibit moderate triplet transient absorption in the visible
Fig. 6 Nonlinear transmission curves for 1–6 at 532 nm for 4.1 ns laser pulses. spectral region. Terminal substitution causes a broadening of
The radius of the beam waist at the focal point was approximately 96 μm. The the TA bands in 1-L, 2-L and 4-L; while extended π-conjugation
linear transmission for all sample solutions was adjusted to 90% in a 2 mm
in the linkage induces a red-shift of the TA bands in 2-L and
cuvette.
5-L compared to that in 6-L. After complexation with the Pt(II)
ion, the major absorption bands in the formed complexes 1–6
all bathochromically shift compared to those of their respect-
ive ligands, accompanied by a low-energy charge transfer tail.
Terminal substitution and extending π-conjugation exert
similar effects to those observed in the ligands. In contrast,
the emission spectra and energies are essentially independent
of the variation of the substituent and degree of π-conjugation
Table 2 Ground-state (σ
0
) and excited-state (σex) absorption cross-sections of
CN
complexes 1–6 at 532 nm in CH
3
1
2
3
4
5
6
σ
σ
σ
0
/10− cm
18
0.35
65
186
0.59
84
142
0.38
59
155
0.57
74
130
0.57
80
140
0.67
80
125
−18
ex/10
cm
3
for 1–6, suggesting a C^N^N core localized π,π* phosphor-
ex/σ
0
escence in nature. Similar to the trend observed in the UV-vis
absorption spectra, the TA bands in 1–6 also red-shift with
terminal substitution and extended π-conjugation. These
bands also notably shift to lower energy compared to those in
their corresponding ligands. The ground-state absorption and
transient absorption studies demonstrate that the absorption
properties of both the ground state and the excited state can
be efficiently tuned by structural modifications. As a result,
complexes 1–6 exhibit strong RSA for ns laser pulses at
transmissions of all of the complexes decrease remarkably
with increased incident energy, which clearly demonstrates a
strong RSA from these complexes. The strength of the RSA
follows this trend: 6 <4 <5 <2 ≤3 <1. It appears that electron-
2
withdrawing substituents (NO and BTZ) enhance the RSA of 1
and 2, while electron-donating substituent (OCH3 in 4)
decreases the RSA at 532 nm. Extending the π-conjugation of
the linkage also results in improved RSA, as evident by the
trend of 6 <5 <2.
532 nm. The degree of RSA (6 <4 <5 <2 ≤3 <1) is also affected
by the substituents and the π-conjugation on the ligand. The
RSA performance is efficiently improved by electron-withdraw-
To rationalize the trend of RSA performance for these com-
plexes, the ratios of the excited-state absorption cross section
with respect to that of the ground state (σ /σ ) at 532 nm,
ing substituents (NO and BTZ) and by extending the π-conju-
2
gation; while electron-donating substituent (OCH
3
in 4)
ex
0
decreases the RSA at 532 nm. The broadband excited-state
absorption, long triplet lifetime and strong RSA of 1–6 at
which is the key parameter determining the RSA, have to be
evaluated. According to the ground-state absorption at 532 nm
and at the wavelength of the TA band maximum, the optical
density changes (ΔOD) at 532 nm and at the TA band
maximum, the molar extinction coefficient at the TA band
maximum (εT1−Tn), and the method described by our group
532 nm suggest these complexes could be promising nonlinear
absorbing materials for photonic device applications.
2
0b
Acknowledgements
previously,
for 1–6 at 532 nm were estimated and the results are shown in
Table 2. The calculated ratios of σ /σ follow this trend: 6 <4
the excited-state absorption cross sections (σex)
This work is partially supported by the National Science Foun-
dation (CAREER CHE-0449598) and partially by the Army
Research Laboratory (W911NF-06-2-0032 and W911NF-10-2-
ex
0
<
5 <2 <3 <1, which matches the observed RSA trend very well.
0055) to W. Sun.
Conclusions
Notes and references
We have synthesized and characterized five new 6-[9,9-di-
(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine ligands (1-L–5-L)
1 B. Rosenberg, L. VanCamp, J. E. Trosko and V. H. Mansour,
and their Pt(II) complexes (1–5) (R = 4-R′-phenylethynyl with
Nature, 1969, 222, 385.
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4028 | Dalton Trans., 2013, 42, 14021–14029
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