Synthesis and photophysics of platinum(II) complexes bearing 2-(7-(4-R-phenylethynyl)-9,9-dihexadecyl-fluoren-2-yl)-1,10-phenanthroline ligand

Article abstract of DOI:10.1016/j.ica.2012.03.033

A series of platinum(II) complexes bearing 2-(7-(4-R-phenylethynyl)-9,9- dihexadecyl-fluoren-2-yl)-1,10-phenanthroline ligand were synthesized and characterized. Their photophysical properties were systematically investigated by UV-Vis absorption, emission, and transient absorption spectroscopy. All complexes exhibit ¹π,π absorption bands below 410 nm and charge transfer transitions at 415-550 nm in CH₂Cl₂. They all emit at ca. 624 nm with vibronic structures. Both UV-Vis absorption and emission spectra show negative solvatochromic effect. All complexes exhibit broad and moderately strong triplet transient absorption from the near-UV to the near-IR spectral region. Reverse saturable absorption of these complexes in CH ₂Cl₂ for ns laser pulses at 532 nm was demonstrated.

Full text of DOI:10.1016/j.ica.2012.03.033

Inorganica Chimica Acta 388 (2012) 140–147
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Synthesis and photophysics of platinum(II) complexes bearing
2-(7-(4-R-phenylethynyl)-9,9-dihexadecyl-fluoren-2-yl)-1,10-phenanthroline
ligand
⇑
Xu-Guang Liu, Wenfang Sun
Department of Chemistry and Biochemistry, North Dokota State University, Fargo, ND 58108-6050, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of platinum(II) complexes bearing 2-(7-(4-R-phenylethynyl)-9,9-dihexadecyl-fluoren-2-yl)-1,10-
phenanthroline ligand were synthesized and characterized. Their photophysical properties were
systematically investigated by UV–Vis absorption, emission, and transient absorption spectroscopy.
Received 31 December 2011
Received in revised form 10 March 2012
Accepted 19 March 2012
All complexes exhibit 1p p⁄ absorption bands below 410 nm and charge transfer transitions at
,
Available online 27 March 2012
415–550 nm in CH2Cl2. They all emit at ca. 624 nm with vibronic structures. Both UV–Vis absorption
and emission spectra show negative solvatochromic effect. All complexes exhibit broad and moderately
strong triplet transient absorption from the near-UV to the near-IR spectral region. Reverse saturable
absorption of these complexes in CH2Cl2 for ns laser pulses at 532 nm was demonstrated.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Electronic absorption
Emission
2-(7-(4-R-Phenylethynyl)-9,9-dihexadecyl-
fluoren-2-yl)-1,10-phenanthroline
Platinum complexes
Transient absorption
1. Introduction
relatively intense emission at room temperature compared to the
platinum terpyridyl complexes [7]. This feature is attributed to
Square-planar platinum(II) terdentate complexes have attracted
great attention in the past two decades due to their unique photo-
physical properties, such as room-temperature phosphorescence
and broadband excited-state absorption. Most importantly, these
properties can be readily tuned via structural modifications to tailor
the specific requirements for different applications. It has been re-
ported that Pt(II) terdentate complexes can be potentially used in
light-emitting devices [1], chemosensors [2], chemical cells for
hydrogen generation [3], solar cells [4], and nonlinear optical
devices [5]. Among the Pt(II) terdentate complexes studied, Pt(II)
complexes containing aromatic N-donor (such as pyridine)
and/or cyclometalated (such as benzene) ligands are discovered to
display multiple charge-transfer excited states, including metal-
to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer
(LLCT), intraligand charge transfer (ILCT), and intra-/inter-molecu-
lar metal-metal-to-ligand charge transfer (MMLCT). A number of
Pt complexes exhibiting moderate charge-transfer absorption in
the visible spectral region and relatively long-lived charge-transfer
emission at room temperature in solutions have been reported [6].
Among which, platinum complexes bearing 6-phenyl-2,20-bipyri-
dine ligand (C^N^N) are particularly interesting because of their
the stronger p-donating ability of the phenyl ring, which admixes
the ILCT character into the lowest excited state, and to the reduced
distortion of the square-planar configuration around the Pt ion by
the C^N^N ligand, which decreases the nonradiative decay from
the excited state.
An earlier study by Che’s group revealed that by increasing the
p-conjugation of the C^N^N ligand, the structural distortion of the
triplet excited state is minimized; consequently the phosphores-
cence quantum yield was improved [1c,8]. Our group recently re-
ported the photophysics and nonlinear absorption of platinum(II)
complexes with 4-fluorenyl substituent on the C^N^N ligand,
which enhanced the triplet excited-state lifetime, improved the
emission quantum yield, and increased the ratios of the excited-
state absorption cross-section to that of the ground-state [9]. Che’s
group also reported that by incorporating fluorene component to
the C^N^N ligand, the emission quantum yield of Pt(II) complexes
could be increased up to 0.76 [10]. It appears that extending the p-
conjugation of the C^N^N ligand could significantly influence the
excited-state properties of the Pt(II) complexes.
1,10-Phenanthroline has been reported to be a useful chelating
bidentate ligand for transition metal ions [11]. Compared to the
2,20-bipyridine and 2,20;60,600-terpyridine ligands, the rigidity of
phenanthroline holds the nitrogen donor atoms juxtaposed and
thus favors metal binding. Recently Andraud’s group reported the
⇑
Corresponding author. Tel.: +1 701 231 6254; fax: +1 701 231 8831.
E-mail address: Wenfang.Sun@ndsu.edu (W. Sun).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.033

X.-G. Liu, W. Sun / Inorganica Chimica Acta 388 (2012) 140–147
141
two-photon induced excited-state absorption of novel 5-(oligoflu-
orenyl)-1,10-phenanthroline ligands and their Ru complexes [12].
However, the synthesis and utilization of 2-fluorenyl-1,10-
phenanthroline as a new type of C^N^N ligand for Pt(II) have not
been reported in the literature. Considering the rigidity of the
The synthetic route for platinum(II) complexes 5a–5d is out-
lined in Scheme 1, and the synthetic details and characterization
data for the ligands and the complexes are provided below. Precur-
sor 1 was prepared according to the literature procedures [14] via
iodination of fluorene, then alkylation of the 2-iodofluorene, and
acetylation of the alkylated 2-iodofluorene. After that, Sonogashira
cross-coupling reaction of 1 and 4-substituted phenylacetylene (2)
leads to compound 3. Ligands 4a–4d were then obtained through
the Friedländer condensation reaction between 4 and 8-amino-
quinoline-7-carbaldehyde. Finally, the platinum(II) complexes
were obtained by reacting K2PtCl4 with the corresponding ligands
under reflux in mixed acetic acid/chloroform (V/V = 10/1) solution
[10]. Due to the long alkyl chains at the fluorene motif, these cyclo-
metalated platinum(II) complexes exhibit good solubility in com-
mon organic solvents, which is important for the photophysical
studies and potential applications.
phenanthroline component and the possibility of extending the p-
conjugation via introducing substituted phenylethynyl group at
the 7-position of the fluorenyl ring, we anticipate that the emission
quantum yield and the lifetime of the triplet excited state of the Pt
complex incorporating this ligand could be increased. To verify this
hypothesis, we designed and synthesized a series of platinum(II)
complexes bearing 2-(7-(4-R-phenylethynyl)-9,9-dihexadecanyl-
fluoren-2-yl)-1,10-phenanthroline ligand (complexes 5a–5d in
Scheme 1). Different electron donating- or accepting-group was
introduced on the phenyl ring to understand how these auxiliary
substituents influence the photophysics of the complexes.
2.1.2. Synthesis of precursor 1
2. Experimental
2.1.2.1. 2-Iodofluorene (S-2). Fluorene (0.10 mol, 16.60 g) was dis-
solved in 167 ml boiling solvent of AcOH/H2O/H2SO4 (V:V:V = 100:
20:3). After that, the reaction mixture was cooled down to 100 °C,
and periodic acid dehydrates (0.017 mol, 3.80 g) and iodine
(0.033 mol, 8.46 g) were added. The reaction mixture was further
cooled down to 70 °C and stirred at this temperature for 4 h until
the elementary iodine almost disappeared and precipitates were
formed. After cooling down to room temperature, the pale yellow
solid was collected by filtration and washed with saturated aque-
ous Na2CO3 and water several times. The crude product was puri-
fied by recrystallization from hexane to give 14.0 g pale yellow
solid (yield = 48%). 1H NMR (400 MHz, CDCl3): d 3.84 (s, 2H, CH2),
7.28–7.37 (m, 2H, Ar), 7.49–7.52 (m, 2H), 7.65–7.68 (m, 1H), 7.73
(d, J = 7.2 Hz, 1H, Ar), 7.85–7.86 (m, 1H, Ar).
2.1. Synthesis
2.1.1. General procedures and materials
All of the starting compounds (fluorene, 4-substituted phenyl-
acetylene (2), etc.) and solvents were purchased from Alfa Aesar
or Aldrich Chemical Co., and the solvents were used as is unless
otherwise stated. The precursor 8-amino-7-quinolinecarbaldehyde
was prepared according to the literature procedures [13]. The
intermediates were characterized by NMR, while the final plati-
num complexes were fully characterized by 1H NMR, 13C NMR,
electrospray ionization high resolution mass spectrometry (ESI-
HRMS), and elemental analyses.
NMR spectra were obtained on a Varian-Oxford 400 VNMR or a
Varian Oxford-500 VNMR spectrometer. ESI-HRMS analyses were
conducted on a Bruker Daltonics BioTOF III mass spectrometer.
Elemental analyses were carried out by NuMega Resonance Labo-
ratories, Inc., San Diego, California.
2.1.2.2. 9,9-Dihexadecyl-2-iodo-9H-fluorene (S-3). 2-Iodofluorene
(S-2, 10.0 mmol, 2.92 g) and tetra-(n-butyl)ammonium iodine
(1.0 mmol, 369 mg) were dissolved in 18 ml of DMSO and 7 ml of
Scheme 1. Synthetic route for platinum(II) complexes 5a–5d.

142
X.-G. Liu, W. Sun / Inorganica Chimica Acta 388 (2012) 140–147
50% aqueous NaOH. 1-Bromohexadecane (22 mmol, 6.71 g) was
then added in the solution. The reaction mixture was stirred at
50 °C under argon for 18 h; and was then diluted with ethyl ace-
tate. The organic layer was washed with dilute HCl twice and brine
twice, and was dried over anhydrous Na2SO4. The resultant oil was
purified by flash chromatography (silica gel) with hexane used as
the eluent. 1H NMR (400 MHz, CDCl3): d 0.85 (t, J = 7.2 Hz, 6H,
CH3), 1.01–1.29 (m, 56H, CH2), 1.81–1.93 (m, 4H, CH2), 7.28–7.31
(m, 3H, Ar), 7.41 (d, J = 8.4 Hz, 1H, Ar), 7.60–7.64 (m, 3H, Ar).
J = 8.8 Hz, 2H), 7.54 (d, J = 8.4 Hz, 2H), 7.94–7.96 (m, 2H), 8.22 (d,
J = 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): d 14.1, 22.7, 23.8,
26.9, 29.2, 29.3, 29.53, 29.59, 29.6, 29.9, 31.9, 40.1, 55.5, 88.3,
95.6, 120.0, 120.8, 121.6, 122.4, 123.7, 126.4, 128.3, 130.2, 131.2,
132.2, 136.4, 141.0, 144.9, 147.0, 151.5, 152.2, 197.9. ESI-MS: m/z
calcd for C55H79NO3 [M+2H+], 803.6211; found, 803.5492.
3d. 370 mg yellow solid, yield: 94%. 1H NMR (400 MHz, CDCl3):
d 0.56–0.61 (m, 4H, CH2), 0.87 (t, J = 6.8 Hz, 6H, CH3), 1.02–1.30 (m,
52H, CH2), 1.97–2.04 (m, 4H, CH2), 2.66 (s, 3H, CH3), 3.83 (s, 3H,
OCH3), 6.87–6.91 (m, 2H, Ar), 7.49–7.53 (m, 4H, Ar), 7.70–7.74
(m, 2H, Ar), 7.94–7.96 (m, 2H, Ar). 13C NMR (100 MHz, CDCl3): d
14.1, 22.7, 23.7, 26.8, 29.2, 29.3, 29.52, 29.57, 29.6, 29.9, 31.9,
40.2, 55.2, 55.4, 88.9, 90.2, 114.0, 115.3, 119.7, 120.6, 122.4,
123.3, 125.9, 128.2, 130.6, 133.0, 136.0, 139.6, 145.3, 151.3,
152.0, 159.7, 197.9. ESI-MS: m/z calcd for C56H82O2 [M+Na+],
809.6213; found, 809.6186.
2.1.2.3. Dihexadecyl-7-iodo-9H-fluoren-2-yl)ethanone (1) [14]. Acetyl
chloride (224 ll, 3.15 mmol, 247 mg) was slowly added at 0 °C to a
suspension of anhydrous AlCl3 (3.60 mmol, 480 mg) in dry CH2Cl2
(10 ml). A CH2Cl2 solution of 9,9-dihexadecyl-2-iodo-9H-fluorene
(3.00 mmol, 2.22 g) was added dropwise and the mixture was stir-
red at room temperature for 18 h. The reaction mixture was poured
onto ice and a concentrated HCl aqueous solution was added until
the precipitate of Al(OH)3 dissolved. The organic layer was sepa-
rated and the aqueous phase was extracted twice with CH2Cl2.
The organic phases were combined, washed with water, 2% NaOH
solution, water, brine, and dried over Na2SO4. After evaporation of
the solvent, the oil obtained was purified by flash chromatography
(silica gel, with hexane to hexane/ethyl acetate = 75:1–30:1–15:1
used as the eluent). 1.80 g white solid was obtained (yield = 77%).
1H NMR (400 MHz, CDCl3): d 0.52–0.54 (m, 4H, CH2), 0.84 (t,
J = 6.4 Hz, 6H, CH3), 0.86–1.27 (m, 52H, CH2), 1.89–1.98 (m, 4H,
CH2), 2.63 (s, 3H, CH3), 7.45–7.48 (m, 1H, Ar), 7.65–7.71 (m, 3H,
Ar), 7.90–7.93 (m, 2H, Ar). 13C NMR (100 MHz, CDCl3): d 14.1,
22.7, 23.7, 26.8, 29.2, 29.3, 29.5, 29.57, 29.63, 29.8, 31.9, 40.0,
55.5, 94.3, 119.6, 122.2, 122.4, 128.2, 132.3, 136.1, 136.3, 139.4,
144.9, 150.5, 154.2, 197.8.
2.1.4. Synthesis of ligands 4a–4d
2.1.4.1. Typical procedure for Friedländer condensation [15]. A mix-
ture of 8-aminoquinoline-7-carbaldehyde (1.0 equiv.), 3 (1.1 equiv.)
and KOH (2.5 equiv.) in absolute EtOH was refluxed for 18–24 h
under argon and cooled to room temperature. The solvent was
evaporated under vacuum and the residue was extracted with
ethyl acetate. The organic phase was washed with water for three
times and dried over anhydrous Na2SO4. After removal of the sol-
vent, the residue was purified by column chromatography (neutral
Al2O3 gel) with hexane/ethyl acetate used as the eluent.
Ligand 4a: 266 mg yellow oil, yield: 78%. 1H NMR (400 MHz,
CDCl3): d 0.67–0.70 (m, 4H, CH2), 0.86 (t, J = 6.8 Hz, 6H, CH3),
1.05–1.29 (m, 52H, CH2), 2.02–2.16 (m, 4H, CH2), 7.32–7.38 (m,
3H, Ar), 7.55–7.60 (m, 4H, Ar), 7.61–7.64 (dd, J1 = 4.4 Hz,
J2 = 8.0 Hz, 1H, Ar), 7.74–7.82 (m, 3H, Ar), 7.87 (d, J = 8.0 Hz, 1H,
Ar), 8.13–8.16 (m, 2H, Ar), 8.24 (dd, J1 = 1.6 Hz, J2 = 8.0 Hz, 1H,
Ar), 8.30 (dd, J1 = 4.4 Hz, J2 = 8.4 Hz, 1H, Ar), 8.45 (dd, J1 = 1.6 Hz,
J2 = 8.0 Hz, 1H, Ar), 9.25 (dd, J1 = 1.6 Hz, J2 = 4.0 Hz, 1H, Ar). 13C
NMR (100 MHz, CDCl3): d 14.1, 22.7, 23.8, 29.3, 29.56, 29.59,
29.6, 30.0, 31.9, 40.5, 55.4, 89.5, 09.6, 120.1, 120.5, 121.1, 121.7,
121.9, 122.9, 123.5, 126.0, 126.2, 126.4, 127.5, 127.8, 128.1,
128.3, 129.1, 130.7, 131.6, 136.0, 136.7, 139.3, 141.0, 141.7,
146.2, 146.4, 150.4, 151.4, 151.5, 158.1. ESI-MS: m/z calcd for
2.1.3. Synthesis of 3 using Sonogashira cross-coupling reaction
2.1.3.1. General procedure for Sonogashira cross-coupling reac-
tion. Compounds
1
(1.0 equiv.),
2
(1.0 equiv.), Pd(PPh3)Cl2
(0.10 equiv.), PPh3 (0.20 equiv.), and CuI (0.20 equiv.) were added
to a round-bottom flask. The flask was evacuated and backfilled
with argon. Et3N (15 ml) was then added. The reaction mixture
was heated at reflux for 24 h; and then was allowed to cool down
to room temperature. After removal of the solvent, the residue was
extracted with CH2Cl2, and the CH2Cl2 layer was washed with
water and brine and dried over anhydrous Na2SO4. The solvent
was then removed and the crude product was purified by column
chromatography (silica gel) with hexane/ethyl acetate used as
eluent.
C
C
65H84N2 [M+H+], 893.6713; found, 893.6718. Anal. Calc. for
65H84N2 + H2O: C, 85.66; H, 9.51; N, 3.07. Found: C, 85.72; H,
9.10; N, 3.17%.
Ligand 4b: 238 mg yellow solid, yield: 78%. 1H NMR (400 MHz,
CDCl3): d 0.67–0.70 (m, 4H, CH2), 0.86 (t, J = 6.4 Hz, 6H, CH3),
1.05–1.27 (m, 52H, CH2), 2.05–2.17 (m, 4H, CH2), 7.42–7.44 (m,
2H, Ar), 7.48–7.51 (m, 2H, Ar), 7.53–7.55 (m, 2H, Ar), 7.64 (dd,
J1 = 4.4 Hz, J2 = 8.0 Hz, 1H, Ar), 7.74–7.83 (m, 3H, Ar), 7.87 (d,
J = 8.0 Hz, 1H, Ar), 8.14–8.16 (m, 2H, Ar), 8.25 (d, J = 8.0 Hz, 1H,
Ar), 8.32 (d, J = 8.4 Hz, 1H, Ar), 8.46 (d, J = 8.0 Hz, 1H, Ar), 9.25–
9.26 (m, 1H, Ar). 13C NMR (100 MHz, CDCl3): d 14.1, 22.6, 23.8,
29.3, 29.5, 29.6, 30.0, 31.9, 40.4, 55.4, 88.5, 91.8, 120.1, 120.5,
121.1, 121.3, 121.9, 122.3, 122.4, 122.9, 125.9, 126.2, 126.3,
127.4, 127.8, 129.0, 130.7, 131.6, 132.9, 136.0, 136.7, 139.4,
141.3, 141.6, 146.2, 146.4, 150.4, 151.4, 151.5, 158.0. ESI-MS: m/z
calcd for C65H83Br79N2 [M+H+], 971.5818; found, 971.6057. Anal.
Calc. for C65H83BrN2: C, 80.30; H, 8.60; N, 2.88. Found: C, 80.13;
H, 8.38; N, 2.86%.
3a. 320 mg yellow solid, yield: 85%. 1H NMR (400 MHz, CDCl3):
d 0.55–0.57 (m, 4H, CH2), 0.85 (t, J = 6.0 Hz, 6H, CH3), 1.01–1.27 (m,
52H, CH2), 1.93–2.05 (m, 4H, CH2), 2.64 (s, 3H, CH3), 7.32–7.34 (m,
3H, Ar), 7.52–7.56 (m, 4H, Ar), 7.71 (t, J = 6.8 Hz, 2H, Ar), 7.93–7.95
(m, 2H, Ar). 13C NMR (100 MHz, CDCl3): d 14.1, 22.7, 23.7, 26.8,
29.2, 29.3, 29.52, 29.57, 29.6, 29.9, 31.9, 40.2, 55.4, 90.2, 119.7,
120.6, 122.4, 123.0, 123.2, 126.1, 128.2, 128.3, 128.34, 130.8,
131.6, 136.1, 139.9, 145.2, 151.3, 152.0, 197.9. ESI-MS: m/z calcd
for C55H80O [M+Na+], 779.6107; found, 779.6094.
3b. 370 mg yellow solid, yield: 86%. 1H NMR (400 MHz, CDCl3):
d 0.56–0.58 (m, 4H, CH2), 0.87 (t, J = 6.8 Hz, 6H, CH3), 1.02–1.28 (m,
52H, CH2), 1.94–2.05 (m, 4H, CH2), 2.66 (s, 3H, CH3), 7.40–7.43 (m,
2H, Ar), 7.47–7.54 (m, 4H, Ar), 7.71–7.75 (m, 2H, Ar), 7.94–7.97 (m,
2H, Ar). 13C NMR (100 MHz, CDCl3): d 14.1, 22.7, 23.7, 26.8, 29.2,
29.3, 29.5, 29.58, 29.63, 29.9, 31.9, 40.1, 55.4, 89.1, 91.3, 119.8,
120.7, 122.2, 122.4, 122.6, 126.1, 128.2, 130.8, 131.7, 133.0,
136.2, 140.2 145.1, 151.3, 152.1, 197.9. ESI-MS: m/z calcd for
Ligand 4c: 186 mg yellow solid, yield: 60%. 1H NMR (400 MHz,
CDCl3): d 0.65–0.67 (m, 4H, CH2), 0.82 (t, J = 6.8 Hz, 6H, CH3),
1.02–1.23 (m, 52H, CH2), 2.01–2.17 (m, 4H, CH2), 7.53–7.55 (m,
2H, Ar), 7.60 (dd, J1 = 4.0 Hz, J2 = 8.0 Hz, 1H, Ar), 7.65 (d,
J = 8.8 Hz, 2H, Ar), 7.71–7.78 (m, 3H, Ar), 7.86 (d, J = 8.0 Hz, 1H,
Ar), 8.12 (d, J = 8.4 Hz, 1H, Ar), 8.17–8.29 (m, 4H, Ar), 8.28 (d,
J = 8.4 Hz, 1H, Ar), 8.42 (d, J = 8.4 Hz, 1H, Ar), 9.20–9.21 (m, 1H).
13C NMR (100 MHz, CDCl3): d 14.0, 22.6, 23.8, 29.2, 29.5, 29.6,
C
55H79OBr81 [M+Na+], 859.5368; found, 859.6055.
3c. 290 mg yellow solid, yield: 72%. 1H NMR (500 MHz, CDCl3):
d 0.54–0.55 (m, 4H), 0.82–0.86 (t, J = 6.4 Hz, 6H), 1.00–1.23 (m,
52H), 1.96–2.02 (m, 4H), 2.65 (s, 3H), 7.53–7.56 (m, 2H), 7.68 (d,

X.-G. Liu, W. Sun / Inorganica Chimica Acta 388 (2012) 140–147
143
29.9, 31.8, 40.3, 55.5, 87.8, 96.1, 120.2, 120.4, 120.6, 120.9, 121.9,
122.8, 123.5, 126.12, 126.16, 126.3, 127.4, 127.7, 129.0, 130.4,
131.0, 132.0, 136.0, 136.7, 139.6, 141.3, 142.0, 146.0, 146.3,
146.7, 150.2, 151.5, 151.6, 157.8, ESI-MS: m/z calcd for
Anal. Calc. for C65H82ClN3O2Pt: C, 66.85; H, 7.08; N, 3.60. Found:
C, 66.38; H, 6.88; N, 3.51%.
Complex 5d: 80 mg orange solid, yield: 50%. 1H NMR (500 MHz,
CDCl3): d 0.78–0.87 (m, 10H, CH2 + CH3), 1.10–1.27 (m, 52H, CH2),
2.00–2.08 (m, 4H, CH2), 3.86 (s, 3H, OCH3), 6.92 (d, J = 7.0 Hz, 2H,
Ar), 7.32 (s, 1H, Ar), 7.51–7.68 (m, 8H, Ar), 7.85 (d, J = 8.0 Hz, 1H,
Ar), 8.00–8.05 (m, 2H, Ar), 8.32 (d, J = 8.0 Hz, 1H, Ar), 8.90 (d,
J = 3.0 Hz, 1H, Ar). 13C NMR (100 MHz, CDCl3): d 14.1, 22.6, 24.0,
29.3, 29.5, 29.6, 30.2, 31.9, 40.6, 54.8, 55.3, 89.3, 89.8, 114.0,
115.6, 118.1, 119.4, 121.0, 122.5, 125.5, 125.6, 125.9, 126.5,
126.6, 126.7, 130.6, 130.9, 133.0, 136.2, 137.0, 141.0, 140.9,
144.3, 145.1, 145.9, 146.7, 147.5, 149.2, 151.5, 159.6, 165.3. ESI-
MS: m/z calcd for C66H85ClN2OPt, [M+Na+] 1174.5896; found,
1174.5884. Anal. Calc. for C66H85ClN2OPt: C, 68.76; H, 7.43; N,
2.43. Found: C, 68.38; H, 7.06; N, 2.47%.
C
C
65H83N3O2, [M+H+] 938.6558; found, 938.6519. Anal. Calc. for
65H83N3O2: C, 83.20; H, 8.92; N, 4.48. Found: C, 82.72; H, 8.67;
N, 4.43%.
Ligand 4d: 191 mg yellow solid, yield: 59%. 1H NMR (400 MHz,
CDCl3): d 0.66–0.68 (m, 4H, CH2), 0.86 (t, J = 6.8 Hz, 6H, CH3),
1.04–1.29 (m, 52H, CH2), 1.99–2.16 (m, 4H, CH2), 3.83 (s, 3H,
OCH3), 6.90 (d, J = 8.8 Hz, 2H, Ar), 7.51–7.55 (m, 4H, Ar), 7.64 (dd,
J1 = 4.4 Hz, J2 = 8.0 Hz, 1H, Ar), 7.73–7.87 (m, 4H, Ar), 8.13–8.17
(m, 2H, Ar), 8.25 (dd, J1 = 2.0 Hz, J2 = 8.0 Hz, 1H, Ar), 8.32 (d,
J = 8.4 Hz, 2H, Ar), 8.45 (dd, J1 = 1.6 Hz, J2 = 8.0 Hz, 1H, Ar), 9.25
(dd, J1 = 1.6 Hz, J2 = 4.4 Hz, 1H, Ar). 13C NMR (100 MHz, CDCl3): d
14.1, 22.6, 23.8, 29.3, 29.5, 29.6, 30.0, 31.9, 40.5, 55.2, 55.4, 89.3,
89.5, 114.0, 115.5, 120.0, 120.4, 121.1, 121.8, 122.1, 122.9, 125.8,
126.1, 126.3, 127.4, 127.8, 129.1, 130.5, 133.0, 136.0, 136.7,
139.2, 140.7, 141.8, 146.2, 146.4, 150.4, 151.4, 151.5, 158.1,
159.6. ESI-MS: m/z calcd for C66H86N2O, [M+H+] 923.6818; found,
923.6839. Anal. Calc. for C66H86N2O: C, 85.85; H, 9.39; N, 3.03.
Found: C, 85.48; H, 9.02; N, 3.10%.
2.2. Photophysical measurements
The UV–Vis absorption spectra were acquired on an Agilent
8453 spectrophotometer in different HPLC-grade solvents. The
steady-state emission spectra were recorded on a SPEX flurolog-3
fluorometer/phosphorometer in different solvents.
The emission quantum yields of the platinum complexes were
determined by a comparative method in degassed solutions [16],
2.1.5. Synthesis of platinum complexes 5a–5d
in which a degassed aqueous solution of [Ru(bpy)3]Cl2 (Uem =
2.1.5.1. General procedure for complexation [10]. A mixture of 4
(1.00 equiv.) and K2PtCl4 (1.25 equiv.) in chloroform and acetic acid
mixture (V:V = 1:10) was refluxed for 24 h under argon. The orange
solid was filtered, washed with water and diethyl ether several
times.
0.042, kex = 436 nm) was used as the reference [17]. The excited-
state lifetimes, the triplet excited-state quantum yields, and the
triplet transient difference absorption spectra were measured in
degassed solutions on an Edinburgh LP920 laser flash photolysis
spectrometer. The third harmonic output (355 nm) of a Nd:YAG la-
ser (Quantel Brilliant, pulsewidth ꢀ4.1 ns, repetition rate was set
at 1 Hz) was used as the excitation source. Each sample was purged
with argon for at least 30 min before measurement.
Complex 5a: 90 mg orange solid, yield: 43%. 1H NMR (500 MHz,
CDCl3): d 0.77–0.87 (m, 10H, CH2 + CH3), 1.11–1.27 (m, 52H, CH2),
2.01–2.09 (m, 4H, CH2), 7.33–7.40 (m, 4H, Ar), 7.53–7.69 (m, 8H,
Ar), 7.86 (d, J = 8.0 Hz, 1H, Ar), 8.00–8.05 (m, 2H, Ar), 8.32 (d,
J = 8.0 Hz, 1H, Ar), 8.89–8.91 (m, 1H, Ar). 13C NMR (100 MHz,
CDCl3): d 14.1, 22.6, 24.0, 29.3, 29.5, 29.6, 30.2, 31.9, 40.7, 54.9,
89.8, 90.6, 118.1, 119.4, 121.0, 122.1, 123.5, 125.6, 125.7, 125.9,
126.6, 126.7, 126.8, 128.2, 128.4, 130.8, 130.9, 131.6, 136.2,
137.0, 141.0, 141.3, 144.2, 145.2, 145.9, 146.7, 147.5, 149.2,
The self-quenching rate constants (kq) in dichloromethane were
deduced following the Stern–Volmer equation,
kobs ¼ kq½cꢁ þ k0
ð1Þ
where kobs is the measured radiative decay rate constant of the
emission (kobs = 1/ em) at a given concentration, kq is the self-
quenching rate constant, [c] is the concentration of the complex
in mol/L, and k0 (k0 = 1/ 0) is the decay rate constant of the excited
s
151.5, 165.2. ESI-MS: m/z calcd for
C
65H83ClN2Pt, [M+Na+]
1144.5790; found, 1144.5787. Anal. Calc. for C65H83ClN2Pt: C,
69.53; H, 7.45; N, 2.49. Found: C, 69.89; H, 7.34; N, 2.50%.
s
state at infinite dilute solution. A plot of the observed decay rate
constant versus concentration should give rise to a straight line.
The slope of the straight line corresponds to kq and the intercept
corresponds to k0.
The triplet excited-state absorption coefficient (eT) at the
absorption band maximum was measured by the singlet depletion
Complex 5b: 72 mg orange solid, yield: 33%. 1H NMR (400 MHz,
CDCl3):d 0.71–0.87 (m, 10H, CH2 + CH3), 1.10–1.28 (m, 52H, CH2),
1.99–2.08 (m, 4H, CH2), 7.32 (s, 1H, Ar), 7.44 (d, J = 8.4 Hz, 2H,
Ar), 7.50–7.52 (m, 3H, Ar), 7.55–7.68 (m, 5H, Ar), 7.85 (d,
J = 7.6 Hz, 1H, Ar), 8.00 (s, 1H, Ar), 8.04 (d, J = 8.0 Hz, 1H, Ar), 8.31
(d, J = 7.6 Hz, 1H, Ar), 8.88 (d, J = 4.4 Hz, 1H, Ar). 13C NMR
(100 MHz, CDCl3): d 14.1, 22.6, 24.0, 29.3, 29.4, 29.6, 30.2, 31.9,
40.5, 54.9, 88.8, 91.8, 118.2, 119.4, 121.1, 121.7, 122.3, 122.5,
125.7, 125.9, 126.6, 126.7, 130.8, 130.9, 131.6, 132.9, 136.3,
137.0, 140.9, 141.5, 144.1, 145.3, 145.9, 146.7, 147.6, 149.2,
151.6, 165.2. ESI-MS: m/z calcd for C65H82BrClN2Pt, [M+Na+]
1222.4895; found, 1222.4891. Anal. Calc. for C65H82BrClN2Pt: C,
64.96; H, 6.88; N, 2.33. Found: C, 64.92; H, 6.72; N, 2.39%.
method [18], in which the optical density changes during transient
absorption at the minimum of the bleaching band (
maximum of the positive band ( ODT) were monitored. eT was
then calculated using the following equation [18]:
DODS) and the
D
es
ꢂ
D
ODT
ODS
eS is the ground-state molar extinction coefficient at the elec-
eT
¼
ð2Þ
D
where
Complex 5c: 83 mg orange solid, yield: 44%. 1H NMR (500 MHz,
CDCl3): d 0.76–0.86 (m, 10H, CH2 + CH3), 1.11–1.25 (m, 52H, CH2),
2.05–2.07 (m, 4H, CH2), 7.33 (s, 1H, Ar), 7.55–7.65 (m, 6H, Ar), 7.72
(d, J = 8.5 Hz, 2H, Ar), 7.87 (d, J = 7.5 Hz, 1H, Ar), 7.99–8.02 (m, 2H,
Ar), 8.25 (d, J = 8.5 Hz, 2H, Ar), 8.30 (d, J = 8.0 Hz, 1H, Ar), 8.83 (d,
J = 4.5 Hz, 1H, Ar). 13C NMR (100 MHz, CDCl3): d 14.1, 22.6, 24.1,
29.3, 29.4, 29.5, 29.6, 30.2, 31.8, 40.5, 54.9, 88.1, 96.2, 118.2,
119.4, 120.8, 121.1, 123.7, 125.8, 125.9, 126.6, 126.7, 126.8,
130.5, 130.9, 131.2, 132.2, 136.3, 137.1, 141.1, 142.3, 143.7,
145.6, 145.8, 146.7, 146.8, 147.5, 149.1, 151.7, 164.9. ESI-MS: m/z
calcd for C65H82ClN3O2Pt, [M+Na+] 1189.5641; found, 1189.5648.
tronic absorption band maximum.
The quantum yield of the triplet excited state formation (UT) was
determined using the comparative method [19]. SiNc in benzene
(
UT = 0.20, e
T,590 = 70000 Mꢃ1 cmꢃ1) [20] was used as the reference.
All the solutions were optically matched at the excitation wave-
length (355 nm), and the UT was calculated using equation [19]:
D
ODsT erTef
UTs
¼
UrTef
ꢂ
ꢂ
ð3Þ
eTs
D
ODrTef
where
during transient absorption due to triplet-triplet transition of the
D D
ODsT and ODrTef are the maximum optical density change

144
X.-G. Liu, W. Sun / Inorganica Chimica Acta 388 (2012) 140–147
sample and the reference, respectively; erTef and eTs are the molar
extinction coefficients of this transition at the wavelength where
intense absorption bands in the visible region, i.e. 450–550 nm,
likely arise from the metal-to-ligand charge transfer (1MLCT) tran-
sitions. For all complexes, there appears to be two distinct bands in
this region, indicating two orbitally distinct MLCT transitions due
to unequal interaction of the ligand field with the participating
Pt d-orbitals of different symmetry. Similar feature has been re-
ported by Schanze’s group for diimine Pt(II) bis-acetylide com-
plexes [22] and by our group for Pt(II) terpyridyl acetylide
complexes [23]. It is interesting to note that although the auxiliary
substituent at the phenylethynyl motif exhibits a minor effect on
the transition energy of the Pt(II) complexes, the molar extinction
coefficients of these complexes are influenced significantly by the
nature of the substituent. Electron-withdrawing substituent, such
as NO2, significant enhances the molar extinction coefficient of
the complex; while electron-donating substituent like OCH3 de-
D
ODT is measured for the reference and the sample, respectively.
UrTef is the triplet excited state quantum yield of the reference.
2.3. Nonlinear transmission measurements
The experimental setup was similar to what had been described
previously [21]. The light source was the second harmonic output
(k = 532 nm) of
a 4.1 ns (fwhm), 10 Hz, Q-switched Quantel
Brilliant Nd:YAG laser. A f = 30 cm plano-convex lens was used to
focus the laser beam to the center of a 2-mm-thick sample cuvette.
The radius of the beam waist was ꢀ75
lm. Two Molectron J4-09
pyroelectric probes and an EPM2000 energy/power meter were
used to monitor the incident and output energies.
creases the
e values remarkably.
The effect of solvent polarity on the UV–Vis absorption spectra
of complexes 5a–5c was investigated and the results are displayed
in Fig. 2 and Supporting information Figs. S1–S3. As exemplified in
Fig. 2 for complex 5d, the absorption bands are bathochromically
shifted in less polar solvents, such as in toluene, compared to those
in more polar solvents, i.e. CH3CN and CH2Cl2. This negative solva-
tochromic effect indicates that the dipole moment of the ground
state is larger than that of the excited state, which is in line with
many other Pt(II) C^N^N complexes [1a,1,9,24] and Pt (II) terpyr-
idyl complexes reported in the literature [23]. For Pt(II) chloride
complexes bearing C^N^N ligand, the largest dipoles are located
around the Pt–Cl and Pt–C bonds due to the negative charges on
Cl and C. When the molecule is excited, ligand-to-ligand charge
transfer from Cl to phenanthroline and intraligand charge transfer
from fluorenyl component to phenanthroline occur. The transition
dipoles correlated would cancel the Pt–Cl and Pt–C bond dipoles,
which reduces the dipole moment of the excited state. In polar sol-
vent, the more polar ground state is stabilized more than the less
polar excited state, therefore the energy gap between the excited
state and ground state increases. This causes the blue-shift of the
UV–Vis absorption spectrum.
3. Results and discussion
3.1. Electronic absorption
The UV–Vis absorption of 5a–5d were studied in CH2Cl2 solu-
tions at different concentrations (1 ꢂ 10ꢃ6–5 ꢂ 10ꢃ4 mol/L). The
absorption of all complexes obeys Beer’s law in the concentration
range studied, indicating that no ground-state aggregation occurs
in this concentration range (see inset in Fig. 1b) due to the
presence of long alkyl chains at the fluorenyl component, which
prevents aggregation. The UV–Vis absorption spectra of 5a–5d
are shown in Fig. 1b and the band maxima and molar extinction
coefficients are listed in Table 1. All complexes exhibit intense
structured absorption bands between 350 and 415 nm, which are
red-shifted with respect to other reported cyclometalated C^N^N
platinum (II) complexes [5a,9,10], and to the major absorption
band of their respective ligand (Fig. 1a). Considering the large
extinction coefficients of these bands and the similar energy to
those of their corresponding ligands, we attribute these absorption
bands to the 1p p⁄ transitions within the C^N^N ligand. The red-
,
shift of these bands with respect to those of their ligands implies
the interactions of the ligand-centered molecular orbitals with
the platinum dp orbitals, which cause delocalization of the molec-
3.2. Emission
ular orbitals. The shoulder around 420–430 nm could be tenta-
tively assigned to the intraligand charge transfer transition
(1ILCT) from the phenylethynylfluorenyl component to the phe-
nanthroline component. Similar spectral feature and assignment
have been reported by Che’s and our groups for platinum(II)
complexes containing fluorene unit [9,10]. The broad, moderately
The emission characteristics of complexes 5a–5d in a variety of
solvents, at different concentrations in CH2Cl2 at room tempera-
ture, and in butyronitrile glassy matrix at 77 K were investigated.
All complexes are emissive in solutions at room temperature and
in glassy matrix at 77 K. The typical emission spectra in different
solvents are provided in Fig. 3a and in Supporting information Figs.
S4–S6. The emission lifetimes and quantum yields are summarized
in Table 1.
300
350
400
450
500
550
600
8.0x104
As exemplified in Fig. 3a for complex 5a, upon excitation at the
MLCT band in CH2Cl2, a structured emission appears at 624 and
680 nm. The vibronic spacing is approximately 1320 cmꢃ1, which
falls in the ring breathing mode of the aromatic rings. The lifetime
a
4a
4b
4c
4d
6.0x104
4.0x104
2.0x104
of the emission is approximately 7.0 ls. In view of the large Stokes
shift of the emission and the long lifetime, the emission from 5a at
room temperature should originate from a triplet excited state. The
vibronic structure in the emission spectrum and the long lifetime
0.0
5
4
3
2
1
0
5a
5b
5c
5d
8.0x104
b
5a
6.0x104
4.0x104
2.0x104
5b
5c
5d
suggest that the emission should originate from the 3p p⁄ state,
,
ε
but possibly admixes some 3MLCT character in view of the nega-
tive solvatochromic effect shown in Fig. 3a. The emission spectra
of the other three complexes are exactly the same as that for 5a
(Supporting information Fig. S7), with a slightly different emission
lifetime and quantum yield (Table 1). The minor effect of the
auxiliary substituent at the phenylethynyl motif on the emission
suggests that the emitting excited state should be localized on
the 2-fluorenyl-1,10-phenanthroline motif.
0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006
Concentration (mol.L-1
)
0.0
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. (a) UV–Vis absorption of ligands 4a–4d in CH2Cl2. (b) UV–Vis absorption
spectra of complexes 5a–5d in CH2Cl2, inset shows the linear fitting of the
absorbance vs. concentration at 491 nm for 5a–5d.

X.-G. Liu, W. Sun / Inorganica Chimica Acta 388 (2012) 140–147
145
Table 1
Photophysical parameters for 5a–5d.
a
b
c
d
kabs/nm
(e
/103 L molꢃ1 cmꢃ1
)
kem/nm
(s0/
l
s; kq/L molꢃ1 sꢃ1); Uem R.T.
kem/nm
(
sem
/
ls) 77 K
kT1–Tn/nm
(sTA/ls; e ; UT)
T1–Tn/L molꢃ1 cmꢃ1
5a
5b
5c
5d
347 (34.2), 396 (57.7), 424 (25.1),
491 (6.39), 524 (4.3)
348 (39.2), 396 (62.4), 424 (26.9),
490 (6.84), 522 (4.73)
347 (30.5), 400 (71.8), 424 (44.4),
487 (9.10), 522 (5.82)
34.8 (23.1), 399 (41.9), 424 (24.8),
492 (5.04), 523 (3.32)
624 (7.18; 7.24 ꢂ 108),
610 (10.9), 668 (10.9)
610 (10.7), 668 (11.2)
612 (11.2), 670 (11.6)
612 (9.20), 670 (9.24)
435 (5.64; 26270; 0.39), 590 (5.46; 48960; 0.39)
450 (4.55; 24170; 0.44), 620 (6.26; 37840; 0.44)
462 (5.10; 29310; 0.53), 666 (5.01, 29312, 0.53)
454 (6.42, 29207, 0.51), 654 (5.94, 32858, 0.51)
680 (6.98; 6.80 ꢂ 108); 0.091
624 (7.19; 8.28 ꢂ 108),
680 (7.10; 8.00 ꢂ 108); 0.052
624 (5.63; 8.89 ꢂ 108),
680 (5.72; 7.48 ꢂ 108); 0.079
624 (7.26; 7.48 ꢂ 108),
680 (7.44; 6.83 ꢂ 108); 0.059
a
b
c
In CH2Cl2 solution.
Measured at a concentration 1 ꢂ 10ꢃ5 mol/L in CH2Cl2 solution. s0 is the intrinsic lifetime at infinite dilute concentration, kq is the self-quenching rate constant.
Measured at a concentration 1 ꢂ 10ꢃ5 mol/L in CH3CH2CH2CN glassy matrix.
d
In degassed toluene solution.
intrinsic lifetime (s0) of complex 5a is thus obtained to be
s, and kq to be 7.24 ꢂ 108 L molꢃ1 sꢃ1 when monitored at
CH3CN
CH2Cl2
Toluene
ꢀ7.2
l
1.0
0.8
0.6
0.4
0.2
0.0
624 nm. The s0 and kq for the other three complexes are listed in
Table 1. The kq values for these complexes are in line with the val-
ues reported for other C^N^N Pt complexes [9] and for Pt terpyridyl
complexes [25].
It should be noted that at the lowest concentration of
1 ꢂ 10ꢃ6 mol/L, a broad, weak emission band at ca. 490 nm was ob-
served when excited around 400 nm. This band is more salient for
5a (Fig. 3b) than for the other complexes. Considering the short
lifetime of this emission band (<5 ns), we attribute it to the fluores-
cence of the coordinated ligand. The possibility of this band being
from the free ligand impurity can be ruled out because the respec-
tive free ligand exhibits a structured fluorescence at ꢀ400–415 nm
in dichloromethane (Supporting information Fig. S12) with a life-
time of 960 ps for 4a. The red-shift of this band with respect to that
of the free ligand should be the result of electron delocalization
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 2. Normalized absorption spectra of 5d in different solvents at room
temperature.
through interaction with the Pt dp orbital. The disappearance of
this band at higher concentrations is caused by the inner filter ef-
fect considering the remarkable electronic absoprtion at the excita-
tion wavelength.
The concentration-dependency emission at room temperature
was studied for complexes 5a–5d in CH2Cl2, the results are shown
in Fig. 3b for complex 5a and in Supporting information Figs. S8–
S10 for the other three complexes. The emission intensity of each
In addition, the emission spectra of the ligands and complexes
in butyronitrile matrix at 77 K were investigated (Supporting infor-
mation Figs. S13 and S14). For all of the ligands, the emission spec-
tra at 77 K exhibit clear vibronic structures but remain the same
energy as those at room temperature, thus the observed emission
is considered as low-temperature fluorescence. When excess
amount of iodoethane (CH3CH2I) was added to the glassy solution,
very weak phosphorescence from 525 to 650 nm was observed in
addition to the fluorescence signal. The emission spectra of com-
plexes 5a–5d at 77 K become narrower and slightly blue-shifted
compared to those at room temperature (as exemplified in
complex increases in the concentration range of 1 ꢂ 10ꢃ6
–
1 ꢂ 10ꢃ5 mol/L, but decreases when the concentration is higher
than 1 ꢂ 10ꢃ5 mol/L. However, the lifetime keeps decreasing with
increased concentration. Therefore, self-quenching clearly occurs
for all complexes in the concentration range studied. The self-
quenching rate constant and the intrinsic lifetime can be deduced
from the slope and the intercept of the Stern–Volmer plot (exem-
plified in inset of Fig. 3b and in Supporting information Fig. S11
for 5a), which was described in the experimental section. The
(a)
(b)
4x105
5x105
at 624 nm
CH CN
3
1.0
0.8
0.6
0.4
0.2
0.0
4x105
CH Cl
2
2
3x105
5E-4
Toluene
Hexane
3x105
2x105
2x105
k
1E-4
5E-5
1E-5
5E-6
1E-6
1x105
0
1.0x10-4 2.0x10-4 3.0x10-4 4.0x10-4 5.0x10-4
Concentration (mol.L-1
0.0
)
1x105
0
500
600
700
800
400 450 500 550 600 650 700 750 800
Wavelength (nm)
Wavelength (nm)
Fig. 3. (a) Normalized emission spectra of 5a in different solvents at room temperature. kex = 436 nm; (b) Concentration-dependency emission of 5a in CH2Cl2 at room
temperature. kex = 396 nm. The inset shows the Stern–Volmer fitting result at k = 624 nm.

146
X.-G. Liu, W. Sun / Inorganica Chimica Acta 388 (2012) 140–147
Fig. S14 for 5a), which is due to the rigidochromic effect [26]. The
vibronic spacing is approximately 1420 cmꢃ1. Considering the
small thermally induced Stokes shift, the similar shape and vibron-
ic spacing of the spectra at 77 K and at room temperature for com-
plexes 5a–5d, the emission of these Pt complexes at 77 K is
in the positive absorption spectral region. To demonstrate this,
nonlinear transmission measurement was conducted for 5a–5d
in CH2Cl2 solution at 532 nm using 4.1 ns laser pulses. To compare
the degree of RSA under the same excitation condition, all the com-
plex solutions were adjusted to an identical linear transmission of
80% at 532 nm in a 2-mm cuvette. As shown in Fig. 5, complexes
5a–5d exhibit very weak transmission decrease, i.e. very weak
RSA, with increased incident fluence. The degree of RSA for these
four complexes differs slightly, with 5c that bears nitro substituent
exhibiting the strongest RSA. It seems that electron-withdrawing
substituent slightly enhances the RSA, while electron-donating
substituent reduces the RSA. As discovered from our previous stud-
ies on other platinum complexes, the degree of RSA is mainly
determined by the ratio of the excited-state absorption cross-sec-
assigned to the intraligand 3p p⁄ as well.
,
3.3. Transient absorption
Transient difference absorption (TA) spectroscopy is a useful
technique to understand the triplet excited state characteristics.
It measures the absorption difference between the ground state
and the excited state. A negative band, namely bleaching band,
indicates that the ground-state absorption is stronger than that
of the excited state, while a positive transient absorption band is
vise versa. To reveal whether 5a–5d exhibit triplet excited-state
absorption, the triplet TA spectra of these four complexes have
been measured using Edinburgh LP920 laser flash photolysis spec-
trometer, in which the excitation beam is 355 nm from a Quantum
Brilliant 4.1 ns laser.
tion (rex) relative to that of the ground state (rg). Although com-
plexes 5a–5d exhibit a broad and moderately strong triplet
transient absorption in the visible to the near-IR region, their
ground-state absorption cross sections at 532 nm are relatively
large, which are between 1.07 ꢂ 10ꢃ17 and 1.58 ꢂ 10ꢃ17 cm2. On
the other hand, 532 nm lies almost at the valley of the TA spectra,
suggesting that the excited-state absoprtion of these complexes at
Fig. 4a illustrates the time-resolved TA spectra of complex 5a in
degassed toluene solution. The other three complexes exhibit sim-
ilar TA spectra (Fig. 4b). All complexes exhibit two broad positive
absorption bands from 425 nm to 820 nm, with a bleaching band
occurring between 370 and 425 nm, which coincides with the
532 nm is quite weak. Both factors lead to reduced ratios of rex/rg.
As a result, the RSA of these complexes becomes very weak. How-
ever, the RSA of these complexes could possibly become stronger
at longer wavelengths (>550 nm), where the ground-state absorp-
tion decreases but the excited-state absorption increases. This
needs to be studied in the future when a tunable ns laser is avail-
able in our laboratory.
1p p⁄ transition band in the UV–Vis absorption spectrum. This sug-
,
gests that the transient absorption likely originates from the 3p p⁄
,
excited state. All the TA decays monoexponentially throughout the
whole spectral range, indicating that the transient absorption
arises from the same excited state. Additionally, the lifetime de-
duced from the decay of the TA and from the decay of the emission
are essentially the same, suggesting that the TA could arise from
the same excited state that emits, or a state that is in equilibrium
with the emitting state.
0.9
5a
5b
5c
0.8
5d
0.7
It should be note that the TA signals of 5a at wavelengths above
750 nm go negative at longer decay time. Two factors could be ac-
counted for this. First, the TA signal at longer decay time is very
weak, the accuracy of the measurement becomes problematic.
Thus the observed negative signal could be an artifact of the exper-
iment. Secondly, the sample solution exhibits minor degradation
after several hundreds of laser excitation, which could also contrib-
ute to the negative signal.
0.6
0.5
0.4
0.3
0.1
1
3.4. Reverse saturable absorption (RSA)
Incident Fluence (J/cm2)
Fig. 5. Nonlinear transmission of complexes 5a–5d in CH2Cl2 solution for 4.1 ns
laser pulses at 532 nm. The linear transmission is 80%, and the path length of the
cuvette is 2 mm. The solution concentration in this study is 1.46 ꢂ 10ꢃ4 mol/L for
5a, 1.36 ꢂ 10ꢃ4 mol/L for 5b, 1.18 ꢂ 10ꢃ4 mol/L for 5c, and 1.73 ꢂ 10ꢃ4 mol/L for 5d.
As discussed above, complexes 5a–5d possess long-lived triplet
excited states and exhibit broad positive triplet transient absorp-
tion. Therefore, reverse saturable absorption is expected to occur
0.05
(a)
(b)
0.04
0.04
0.03
0.02
0.01
0.00
0.02
0.00
-0.02
-0.01
-0.02
-0.03
-0.04
0 μs
5a
1.6
3.2
4.8
7.2
μ
μ
μ
μ
s
s
s
s
5b
-0.04
5c
5d
-0.06
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 4. (a) Time-resolved transient difference absorption spectra of 5a in degassed toluene solution. kex = 355 nm. (b) Triplet transient different absorption spectra of 5a–5d in
degassed toluene solution at zero time delay following 355 nm excitation. For all measurements, A355nm = 0.4 in a 1-cm cuvette.

X.-G. Liu, W. Sun / Inorganica Chimica Acta 388 (2012) 140–147
147
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Another point worthy of mention is that the nonlinear transmission
experiments were conducted at high concentration solutions, i.e.
1.46 ꢂ 10ꢃ4 mol/L for 5a, 1.36 ꢂ 10ꢃ4 mol/L for 5b, 1.18 ꢂ 10ꢃ4 mol/L
for 5c, and 1.73 ꢂ 10ꢃ4 mol/L for 5d. According to the concentration-
dependency emission study discussed before,
a significant self-
(d) S. Masaoka, Y. Mukawa, K. Sakai, Dalton Trans. (2010) 5868;
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quenching occurs at such a high concentration, which reduces the triplet
excited-state lifetime drastically. Using the intrinsic lifetime ( 0) and self-
quenching rate constant (kq) listed in Table 1 and the Stern–Volmer Eq.
(1), the triplet excited-state lifetime is estimated to be 4.09 s for 5a,
3.97 s for 5b, 3.54 s for 5c, and 3.74 s for 5d at the respective concen-
s
l
l
l
l
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tration used for the nonlinear transmission measurement. Although
these lifetimes are shorter than the intrinsic lifetimes, they are still much
longer than the laser pulse width (4.1 ns) used for the nonlinear transmis-
sion study. Therefore, the effect of self-quenching on the excited-state
absorption should not be significant. Consequently, the weak RSA ob-
served for these complexes should be predominantly attributed to the
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6155.
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4. Conclusion
The platinum(II) complexes bearing 2-(7-(4-R-phenylethynyl)-
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9,9-dihexadecyl-fluoren-2-yl)-1,10-phenanthroline ligand exhibit
ligand-centered 1 p⁄ in the UV and blue region, a 1ILCT absorption
p,
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3p p⁄ character, possibly mixed with some 3MLCT character. The
,
auxiliary substituent on the phenylethynyl motif exhibits negligible
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oline component. All complexes possess long-lived 3p p⁄ excited
,
state, which gives rise to moderate emission and broadband
transient absorption. However, due to their relatively strong
ground-state absorption at 532 nm, the RSA of these complexes
for nanosecond laser pulse is very weak at 532 nm. Nonetheless,
the RSA of these complexes could be stronger at wavelengths longer
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Acknowledgments
This work is partially supported by National Science Foundation
(CAREER CHE-0449598) and partially by the Army Research Labo-
ratory (W911NF-06-2-0032 and W911NF-10-2-0055). We are also
grateful to Barry Pemberton and Professor Sivaguru Jayaraman at
North Dakota State University for measuring the fluorescence life-
times for ligands 4a–4d.
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