Synthesis and spectroscopic properties of platinum(II) terpyridine complexes having an arylborane charge transfer unit

Article abstract of DOI:10.1021/ic061435b

Synthesis, redox, spectroscopic, and photophysical properties of a new class of Pt(II) complexes of the type [PtL_nCl]⁺ are reported, where L_n is 4′-phenyl(dimesitylboryl)-2,2′: 6′,2″-terpyridine (L₁) or 4′-duryl(dimesitylboryl)- 2,2′: 6′,2″-terpyridine (L₂). The free L ₁ or L₂ ligand in CH₃CN shows the absorption band responsible for intramolecular charge transfer (CT) from the π-orbital of the aryl group in L₁ or L₂ (π(aryl)) to the vacant p-orbital on the boron atom (p(B)), in addition to ππ* absorption in the 2,2′:6′,2″-terpyridine (tpy) unit. In particular, the L₁ ligand shows an intense CT absorption band as compared with L ₂. Such intramolecular π(aryl)-p(B) CT interactions in L ₁ give rise to large influences on the redox, spectroscopic, and photophysical properties of [PtL₁Cl]⁺. In practice, [PtL₁Cl]⁺ shows strong room-temperature emission in CHCl₃ with the quantum yield and lifetime of 0.011 and 0.6 μs, respectively, which has been explained by synergetic effects of Pt(II)-to-L ₁ MLCT and π(aryl)-p(B) CT interactions on the electronic structures of the complex. In the case of [PtL₂Cl]⁺, the dihedral angle between the planes produced by the tpy and duryl(dimesitylborane) groups is very large (84°) as compared with that between the tpy and phenyl-(dimesitylborane) units in [PtL₁Cl]⁺ (26-39°), which disturbs electron communication between the Pt(II)-tpy and arylborane units in [PtL₂Cl]⁺. Thus, [PtL₂Cl]⁺ is nonemissive at room temperature. The important roles of the synergetic CT interactions in the excited-state properties of the [PtL₁Cl] ⁺ complex are shown clearly by emission quenching of the complex by a fluoride ion. The X-ray crystal structure of [PtL₁Cl]⁺ is also reported.

Full text of DOI:10.1021/ic061435b

Inorg. Chem. 2006, 45, 10670−10677
Synthesis and Spectroscopic Properties of Platinum(II) Terpyridine
Complexes Having an Arylborane Charge Transfer Unit
Eri Sakuda, Akiko Funahashi, and Noboru Kitamura*
DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810,
Japan
Received July 31, 2006
Synthesis, redox, spectroscopic, and photophysical properties of a new class of Pt(II) complexes of the type [PtLn-
Cl]+ are reported, where Ln is 4
-phenyl(dimesitylboryl)-2,2 :6 ,2′′-terpyridine (L1) or 4 -duryl(dimesitylboryl)-2,2
,2′′-terpyridine (L2). The free L1 or L2 ligand in CH3CN shows the absorption band responsible for intramolecular
charge transfer (CT) from the -orbital of the aryl group in L1 or L2 (aryl)) to the vacant p-orbital on the boron
atom (p(B)), in addition to ππ* absorption in the 2,2 :6 ,2′′-terpyridine (tpy) unit. In particular, the L1 ligand shows
an intense CT absorption band as compared with L2. Such intramolecular (aryl) p(B) CT interactions in L1 give
rise to large influences on the redox, spectroscopic, and photophysical properties of [PtL1Cl]+. In practice, [PtL1Cl]+
shows strong room-temperature emission in CHCl3 with the quantum yield and lifetime of 0.011 and 0.6 s,
respectively, which has been explained by synergetic effects of Pt(II)-to-L1 MLCT and (aryl) p(B) CT interactions
on the electronic structures of the complex. In the case of [PtL2Cl]+, the dihedral angle between the planes produced
by the tpy and duryl(dimesitylborane) groups is very large (84 ) as compared with that between the tpy and phenyl-
(dimesitylborane) units in [PtL1Cl]+ (26
39 ), which disturbs electron communication between the Pt(II) tpy and
′
′
′
′
′:
6
′
π
(π
′
′
π
−
µ
π
−
°
−
°
−
arylborane units in [PtL2Cl]+. Thus, [PtL2Cl]+ is nonemissive at room temperature. The important roles of the synergetic
CT interactions in the excited-state properties of the [PtL1Cl]+ complex are shown clearly by emission quenching
of the complex by a fluoride ion. The X-ray crystal structure of [PtL1Cl]+ is also reported.
Introduction
In practice, the dipole moment change accompanied by the
CT transition in TAB is as large as ∼8.0 D.5-7 Beside TAB,
it has been reported that various triarylborane derivatives
show unique spectroscopic properties. Therefore, an ap-
propriate choice of an aryl group(s) in a triarylborane
derivative would provide a new class of novel functional
materials.
The vacant p-orbital on the boron atom (p(B)) in a
triarylborane derivative plays crucial roles in determining
the electronic structure of the compound. As a typical
example, tris(9-anthryl)borane (TAB) first reported by
Yamaguchi et al. shows broad and structureless absorption
and fluorescence bands in the visible region, which are
different totally from those of anthracene itself.1-4 Our recent
studies on the spectroscopic and excited-state properties of
TAB indicated that the characteristic absorption and fluor-
escence bands mentioned above were ascribed essentially
to the charge transfer (CT) transition between the π-orbital
of the anthryl group (π(aryl)) and p(B): π(aryl)-p(B) CT.
One possible approach to the research along the line
mentioned above is to combine a π(aryl)-p(B) system with
other electronic system of a molecule.8 Among various
molecular systems, a metal-to-ligand charge transfer (MLCT)
system of a transition metal complex has received broad
research interests, and its applications to functional materials
* To whom correspondence should be addressed. E-mail: kitamura@
sci.hokudai.ac.jp.
(1) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
(5) Kitamura, N.; Sakuda, E. J. Phys. Chem. A 2005, 109, 7429.
(6) Kitamura, N.; Sakuda, E.; Yoshizawa, T.; Iimori, T.; Ohta, N. J. Phys.
Chem. A 2005, 109, 7435.
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2005, 109, 22326.
(2) Yamaguchi, S.; Shirasaka, T.; Tamao, K. Org. Lett. 2000, 2, 4129.
(3) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001,
(8) (a) Jia, W.-L.; Song, D.-R.; Wang, S. J. Org. Chem. 2003, 68, 701.
(b) Jia, W.-L.; Bai, D.-R.; McCormick, T.; Liu, Q.-D.; Motala, M.;
Wang, R.-Y.; Sewawd, C.; Tao, Y.; Wang, S. Chem. Eur. J. 2004,
10, 994.
123, 11372.
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652, 3.
10670 Inorganic Chemistry, Vol. 45, No. 26, 2006
10.1021/ic061435b CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/29/2006

Platinum(II) Terpyridine Complexes
Scheme 1. Synthesis of [PtL1Cl]+ and [PtL2Cl]+ Complexesa
such as light-emitting diodes and nonlinear optical devices
are expanding research areas.9 Synthetic control of the redox,
spectroscopic, and excited-state properties of a transition
metal complex is thus the fundamental basis for further
advances in the relevant researches, and various studies have
been explored by employing a variety of π-chromophoric
ligands and transition metal ions: Ru(II);10-16 Re(I);17-19
Pt(II);20-22 Ir(III).23-24 Among these metal complexes, a
square-planar Pt(II) complex is quite interesting, since it
shows MLCT, ligand-centered (LC), and/or metal-metal-to-
ligand CT (MMLCT) emission, depending on a ligand
structure, temperature, medium, or concentration.25-34 This
indicates that the orbital energies involved in the ground and
excited states of the complex respond very sensitively to
these factors and, thus, synthetic control of redox, spectro-
scopic, and excited-state properties of a Pt(II) complex is
worth exploring.
Our strategy toward tuning the redox, spectroscopic, and
excited-state properties of a Pt(II) complex is to combine
the π(aryl)-p(B) CT interaction in a triarylborane derivative
with the MLCT state of the complex. In the case of a [Pt-
a Key: (i) 2, Pd(PPh3)4, and Na2CO3 in a CH3CN/THF/EtOH mixture;
(ii) [Pt(COD)Cl2] in MeOH, where COD ) 1,5-cyclooctadiene.
(ph-tpy)Cl]+ complex (ph-tpy ) 4′-phenyl-2,2′:6′,2′′-terpy-
ridine), it has been reported that an introduction of an electron
donor or acceptor substituent on the phenyl ring in ph-tpy
influences the excited-state properties of the complex through
variationsofbothMLCTandd-dexcited-stateenergies.20-22,29,31,33
An introduction of a triarylborane chromophore to the ligand
of a Pt(II) complex would also influence the excited-state
properties. We expect, therefore, that π(aryl)-p(B) CT
interactions in a triarylborane ligand unit influence the MLCT
excited state of a Pt(II) complex, and this provides new and
novel characteristics of the complex.
As a new ligand, we designed a tpy derivative having a
phenyl(dimesitylborane) (L1) or duryl(dimesitylborane) unit
(L2) at the 4′-position of tpy: the structures are shown in
Scheme 1. Since an organic borane derivative is generally
moisture sensitive and the periphery of the boron atom in
the compound should be surrounded by bulky groups, we
introduced mesityl groups on the boron atom in L1 or L2.
On the basis of such ligand design, L1/L2 and their Pt(II)
complexes prepared are stable in solution and air. In the
following, we report synthesis of the L1 and L2 ligands, and
their Pt(II) complexes: [PtL1Cl]+ and [PtL2Cl]+. Character-
istic redox, spectroscopic (absorption and emission), and
excited-state properties of [PtL1Cl]+ and [PtL2Cl]+ in CH3-
CN or CHCl3 are reported. On the basis of the present results,
we discuss the roles of the π(aryl)-p(B) CT interactions in
the MLCT excited state of the complex. The X-ray crystal
structure of [PtL1Cl]+ is also reported.
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Experimental Section
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Chemicals. Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4,
Tokyo Kasei Kogyo Co., Ltd.), potassium tetrachloroplatinate(II)
(K2[PtIICl4], Wako Pure Chemicals Ind), and 1,5-cyclooctadiene
(COD, Wako Pure Chemicals Ind) were used as supplied. Aceto-
nitrile (Wako Pure Chemicals Ind), N,N-dimethylformamide (DMF,
Kanto Chemical Co. Inc.), and tetra-n-butylammonium perchlorate
(Wako Pure Chemicals Ind) for spectroscopic and/or electrochemi-
Inorganic Chemistry, Vol. 45, No. 26, 2006 10671

Sakuda et al.
cal measurements were purified by the accepted procedures prior
to use.35 Tetra-n-butylammonium fluoride (TBAF, Wako Pure
Chemicals Ind) was purified by repeated recrystallizations from
an acetone/diethyl ether mixture. Other chemicals used in this study
were the highest grade available from domestic suppliers and used
without further purification.
Synthesis of Boron-Containing Terpyridine Ligands and
Their Pt(II) Complexes. 4′-Phenyl(dimesitylboryl)-2,2′;6′,2′′-terpy-
ridine (L1), 4′-duryl(dimesitylboryl)-2,2′;6′,2′′-terpyridine (L2), and
their platinum(II) complexes were synthesized according to Scheme
1. Detailed synthetic procedures are as follows.
4′-Phenyl(dimesitylboryl)-2,2′:6′,2′′-terpyridine (L1). A tet-
rahydrofuran (THF, 3 mL) solution of (iodophenyl)dimesitylborane
(1, 450 mg, 1.0 mM)2 and Pd(PPh3)4 (45 mg) was deaerated by
purging an N2-gas stream for 10 min under stirring. Into the solution,
4′-((neopentyl glycolato)boron)-2,2′:6′,2′′-terpyridine (2, 0.17 g, 0.5
mM)36 dissolved in a CH3CN (10 mL)/THF (15 mL)/EtOH (15
mL) mixture and an aqueous Na2CO3 solution (2 M, 0.62 mL) were
added.37 The mixture was deaerated by purging an N2-gas stream
for 10 min and, then, heated at reflux (85 °C) for 6 h under N2
atmosphere. After complete consumption of 2 as confirmed by thin-
layer chromatography, the solvents were removed under reduced
pressure. The residues dissolved in CH2Cl2 (25 mL) were washed
successively with H2O (25 mL) and a dilute aqueous NaOH solution
(0.1 M, 100 mL) for three times and dried over anhydrous Na2-
SO4. Removal of the solvents gave colorless solids. The crude
product was purified by column chromatography on deactivated
alumina with an ethyl acetate/n-hexane (1:9, v/v %) mixture as an
eluent: colorless powder (yield ) 29%). 1H NMR (270 MHz,
CDCl3): δ 8.74 (s, 4H), 8.72 (m, 2H), 8.68 (d, 2H, J ) 8 Hz),
7.89 (t, 2H, J ) 8 Hz, 8 Hz), 7.83 (d, 2H, J ) 8 Hz) 7.64 (d, 2H,
J ) 8 Hz), 7.36 (m, 2H), 6.85 (s, 4H), 2.33 (s, 6H), 2.04 (s, 12H).
MS (FD-MS): m/z 557 (M+).
oughly with diethyl ether and then dissolved in a small amount of
methanol. Into the solution, an excess amount of a methanol solution
of NH4PF6 was added. The PF6 salt precipitated was collected by
suction filtration and recrystallized from acetone, affording a pure
product: yellowcrystal([PtL1Cl]+PF6-)orredpowder([PtL2Cl]+PF6-),
yield ) 53 or 73%, respectively.
1
(i) [PtL1]+PF6-. H NMR (270 MHz, DMSO-d6): δ 9.04 (s,
2H), 8.99 (br, 2H), 8.87 (d, 2H, J ) 8 Hz), 8.57 (m, 2H), 8.15 (d,
2H, J ) 8 Hz), 8.00 (m, 2H), 7.61 (d, 2H, J ) 9 Hz), 6.88 (s, 4H),
2.29 (s, 6H), 1.98 (s, 12H). MS (ESI-HRMS): m/z 787.23 (M+-
PF6). Anal. Calcd for C39H36BN3PtClPF6: C, 50.20; H, 3.89; N,
4.50. Found: C, 50.28; H, 4.13; N, 4.30.
(ii) [PtL2]+PF6- 40 1H NMR (DMSO-d6, 270 MHz): δ 9.02 (d,
.
2H, J ) 4 Hz), 8.79 (d, 2H, J ) 8 Hz), 8.57 (s, 2H), 8.52 (t, 2H,
J ) 7 Hz, 8 Hz), 7.99 (t, 2H, J ) 6 Hz, 6 Hz), 6.82 (s, 4H), 2.25
(s, 6H), 2.03 (s, 12H), 1.95 (s, 12Η). MS (ESI-HRMS): m/z 843.96
(M+ - PF6).
Spectroscopic and Electrochemical Measurements. Absorption
and corrected emission spectra of the complexes at 298 K were
recorded on a Hitachi UV-3300 spectrophotometer and a Hitachi
F-4500 spectrofluorometer, respectively. The emission quantum
yields (Φem) of the Pt(II) complexes were determined by using [Ru-
(bpy)3](PF6)2 in CH3CN as a standard (0.061).41 Emission lifetime
measurements were conducted by using a streak camera (Hamamat-
su Photonics, C4334) as a photodetector at 355 nm excitation
(Continuum Surelite-II, 355 nm, 6 ns pulse width). For emission
spectroscopy, sample solutions were deaerated by purging an Ar-
gas stream over 30 min. NMR spectra were recorded on a JEOL
JME-EX270 FT-NMR SYSTEM (270 MHz).
Cyclic voltammetry was conducted by using an electrochemical
analyzer (BAS, ALS-701A). The concentration of the Pt complex
in DMF was set at 5 × 10-4 M, and tetra-n-butylammonium
perchlorate (TBAClO4, 0.1 M) was used as a supporting electrolyte.
The solution was deaerated by purging an Ar-gas stream over 20
min prior to the experiments. A three-electrode system was
employed by using platinum working, platinum auxiliary, and SCE
reference electrodes.
Crystallography.TheX-raystructuraldataforsingle[PtL1Cl]+PF6-
crystals were collected on Mercury CCD area detectors coupled
with Rigaku AFC-7R diffractometers, by using CrystalClear
(Rigaku Co.) with graphite-monochromated Mo KR radiation
(0.7107 Å). The structures were solved by CrystalStructure 3.6.042
and SHELX97.43 Full-matrix least-square refinements were em-
ployed against F2. Crystallographic data (excluding structure factors)
for the structure reported in this paper have been deposited with
Cambridge Crystallographic Data Center as supplementary publica-
tion no. CCDC-170237. Copies of the data can be obtained free of
chargeonapplicationtoCCDC,12UnionRoad,CambridgeCB21EZ,
U.K. (fax, (+44) 1223-336-033; e-mail, deposit@ccdc.cam.ac.uk).
4′-Duryl(dimesitylboryl)-2,2′:6′,2′′-terpyridine (L2). The L2
ligand was synthesized by the procedures analogous to those of
L1. A THF solution (3 mL) of 1 (380 mg, 8.3 mM)2 and Pd(PPh3)4
(45 mg) was deaerated by purging an N2-gas stream for 10 min
under stirring. Into the solution, 2 (0.17 g, 0.5 mM) dissolved in a
CH3CN (10 mL)/THF (15 mL)/EtOH (10 mL) mixture and an
aqueous Na2CO3 solution (2 M, 0.62 mL) were added. The mixture
was deaerated by purging an N2-gas stream for 10 min and then
heated at reflux (85 °C) for 8 h under N2 atmosphere. Workup
procedures analogous to those for preparation of L1 gave a dark-
1
yellow powder (yield ) 48%). H NMR (270 MHz, DMSO-d6):
δ 8.71 (d, 2H), 8.66 (br, 2H), 8.59 (d, 2H), 7.87 (dt, 2H), 7.36 (dt,
2H), 6.73 (s, 4H), 2.44 (br, 6H), 2.26 (br, 6Η), 2.09 (br, 6Η), 1.98
(br, 12H).38 MS (FD-MS): m/z 613 (M+).
[PtL1Cl]+PF6-/[PtL2Cl]+PF6-. Into a suspension of [Pt(COD)-
Cl2] (0.15 g, 0.4 mmol)39 in methanol (25 mL), solid L1 or L2 (0.45
mM) was added under stirring and the mixture was heated at 60
°C. After 2 h reaction, all of the solid reactants were dissolved and
the mixture turned to a clear yellow solution. After cooling of the
sample to room temperature, unreacted [Pt(COD)Cl2] precipitated
was removed by filtration. The filtrate was then evaporated to
dryness, giving red-orange solids. The solids were washed thor-
Results and Discussion
X-ray Crystal Structure of [PtL1Cl](PF6). Recrystalli-
zations of [PtL1Cl]+ from acetone gave yellow single crystals,
and we conducted an X-ray crystal structure analysis of the
(40) Elemental analysis suggests that the complex includes a small amount
of an impurity, which cannot be detected by the NMR/mass spectra
and the cyclic voltammogram of the complex. However, this does
not influence the main conclusions of this study: participation of the
synergetic MLCT/π(aryl)-p(B) CT interactions in [PtL1Cl]+.
(41) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583.
(42) Crystal Structure Analysis Package; Rigaku and Rigaku/MSC: 9009
NewTrails Dr., The Woodlands, TX 77381, 2000-2004.
(35) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980.
(36) Aspley, C. J.; Williams, J. A. G. New J. Chem. 2001, 25, 1136.
(37) Goodall, W.; Wild, K.; Arm, K. J.; Williams, J. A. G. J. Chem. Soc.,
Perkin Trans. 2 2002, 1669.
(38) Since the NMR spectrum was very complicated, we have not
determined the J values.
(39) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem. Soc.
1976, 98, 6521.
(43) Sheldrick, G. M. A program for crystal structure determination and
refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
10672 Inorganic Chemistry, Vol. 45, No. 26, 2006

Platinum(II) Terpyridine Complexes
Table 1. Crystallographic Data for [PtL1Cl]+ Crystalsa
formula
fw
C42H42N3OPtBPF6Cl
991.13
size, mm
T, K
cryst syst
space group
cell constants
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
0.13 × 0.13 × 0.03
153.1
triclinic
P1h
9.220(3)
12.734(4)
17.939(5)
101.335(3)
90.097(4)
101.769(4)
2019.8(10)
2
1.630
36.30
0.7107
0.041
Z
F
calc, g/cm3
µ, cm-1
λ(Mo KR), Å
R1, %
wR2, %
0.105
2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|, wR2 ) [Σ(|Fo| - |Fc|)2/Σw|Fo| ]1/2
.
Table 2. Selected Bond Distances/angles for [PtL1Cl]+
atoms
distance (Å)
atoms
distance (Å)
Pt(1)-Cl(1)
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-N(3)
2.290(2)
2.020(4)
1.948(4)
2.017(4)
C(8)-C(16)
C(19)-B(1)
C(22)-B(1)
C(31)-B(1)
1.482(8)
1.572(9)
1.569(9)
1.596(7)
atoms
angle (deg)
atoms
angle (deg)
N(1)-Pt(1)-Cl(1)
N(2)-Pt(1)-Cl(1)
N(3)-Pt(1)-Cl(1)
N(2)-Pt(1)-N(1)
N(3)-Pt(1)-N(2)
N(3)-Pt(1)-N(1)
C(7)-C(8)-C(16)
C(16)-C(8)-C(9)
C(8)-C(16)-C(17)
C(8)-C(16)-C(21)
98.9(1)
178.9(1)
99.1(1)
81.1(2)
80.9(2)
162.0(2)
120.4(5)
121.3(4)
120.8(4)
120.0(5)
C(18)-C(19)-B(1)
B(1)-C(19)-C(20)
C(19)-B(1)-C(22)
C(19)-B(1)-C(31)
B(1)-C(22)-C(23)
B(1)-C(22)-C(27)
C(22)-B(1)-C(31)
B(1)-C(31)-C(32)
C(31)-C(32)-C(33)
B(1)-C(31)-C(36)
121.8(5)
121.7(4)
120.0(4)
116.5(5)
122.0(6)
120.8(5)
123.4(5)
119.8(5)
119.7(5)
121.5(5)
Figure 1. X-ray crystal structures of [PtL1Cl]+.
and the hydrogen atoms in tpy. Unfortunately, we have not
obtained single crystals of [PtL2Cl]+ and, thus, the crystal
structure of the complex is unknown at the present stage of
the investigation. Nonetheless, our molecular orbital calcula-
tions (WinMOPAC/AM 1) on the L1 and L2 ligands (not Pt
complexes) indicate that the dihedral angle between the tpy
and phenyl (L1) or duryl planes (L2) is 39 or 84°, respectively.
The dihedral angle calculated for L1 (39°) is comparable to
that determined by X-ray crystallography for [PtL1Cl]+: 26°.
These discussions indicate that the dihedral angle between
the tpy and duryl planes in [PtL2Cl]+ could be much larger
than that of [PtL1Cl]+, which should reflect on other physical
properties of the complexes. In practice, such structural
differences reflected on the redox, spectroscopic, and excited-
state properties of the [PtL1Cl]+ and [PtL2Cl]+ complexes
as described in the following sections.
Redox Properties of [PtL1Cl]+ and [PtL2Cl]+. Figure 2
shows cyclic voltammograms of the Pt(II) complexes in
DMF. An irreversible oxidation peak is observed for [Pt-
(tpy)Cl]+ at around 1.0 V (vs SCE), while oxidation of [PtL2-
Cl]+ is ambiguous and no obvious oxidation peak is observed
for [PtL1Cl]+. On the other hand, each complex shows
reversible two reduction peaks as the data are summarized
in Table 3. Although both the first (Ered(1)) and second
reduction potentials (Ered(2)) of [PtL2Cl]+ are almost com-
parable to those of [Pt(tpy)Cl]+ at -0.76 to -0.78 and -1.31
complex. Tables 1 and 2 summarize the crystallographic data
and selected bond distances/angles of [PtL1Cl]+, respectively.
Figure 1 also shows the molecular structure of the complex
determined by X-ray crystallography.
The carbon (C)-boron (B)-carbon (C) bond angles (angle
C-B-C) of the phenyl(dimesitylborane) unit in the complex
are ca. 120° (116.5(5), 120.0(4), and 123.4(5)°). This is
readily understood by the sp2-like configuration of the boron
atom. On the other hand, the bond distances and angles of
the Pt-tpy unit are comparable to those of [Pt(tpy)Cl]+27
and the Cl-Pt(II)-tpy plane is almost planar similar to that
of [Pt(ph-tpy)Cl]+.21,31 Therefore, an introduction of the
phenyl(dimesitylborane) group to the 4′-position of tpy does
not influence the coordination structure of the complex
around the Pt ion. X-ray structure analysis of the complex
demonstrates, however, that the dihedral angle between the
Cl-Pt(II)-tpy and phenyl planes is 26°: see Figure 1. Such
a geometrical distortion (i.e., non-coplanarity) has been also
reported for [Pt(ph-tpy)Cl]+ and is responsible for steric
hindrance between the hydrogen atoms in the tpy and phenyl
groups in the complex. In the case of [PtL2Cl]+, this indicates
that the dihedral angle between the tpy and duryl planes
should be much larger than that of [PtL1Cl]+, owing to large
steric hindrance between the methyl groups in the duryl unit
Inorganic Chemistry, Vol. 45, No. 26, 2006 10673

Sakuda et al.
Figure 3. Absorption spectra of the Pt(II) complexes (upper panel) and
the free ligands in CH3CN at 298 K (lower panel).
and Ered(2) values of [PtL1Cl]+ as compared with the relevant
value of [PtL2Cl]+. The Ered(1,2) values of [PtL2Cl]+ compa-
rable to those of [Pt(tpy)Cl]+ will also support insufficient
electron delocalization over the L2 ligand in the complex. It
is worth emphasizing, furthermore, that the vacant p-orbital
on the boron atom in a triarylborane derivative shows an
electron-withdrawing ability. Therefore, we suppose that the
positive shifts of Ered(1) and Ered(2) observed for [PtL1Cl]+ as
compared with those of [PtL2Cl]+ and [Pt(tpy)Cl]+ are
responsible essentially for the presence of the phenyl-
(dimesitylborane) unit in the L1 ligand and, thus, for the
contribution of π(aryl)-p(B) CT interactions to the electronic
structure of the [PtL1Cl]+ complex.
Electronic Absorption Characteristics of [PtL1Cl]+ and
[PtL2Cl]+. Aborption spectra of [PtL1Cl]+ and [PtL2Cl]+ in
CH3CN at 298 K are shown in Figure 3a, together with that
of [Pt(tpy)Cl]+. Figure 3b also shows absorption spectra of
L1, L2, and tpy in CH3CN as references, and the spectroscopic
data are summarized in Table 4. The absorption bands
observed for [PtL1Cl]+ or [PtL2Cl]+ at around 400 and 330-
290 nm are assigned to the MLCT and LC(ππ*) transitions,
respectively, as judged from the assignment of the relevant
transition in [Pt(tpy)Cl]+. The MLCT absorption maximum
wavelength (λabs) of [PtL2Cl]+ (404 nm) is almost comparable
to that of [Pt(tpy)Cl]+ (402 nm), while that of [PtL1Cl]+ is
shifted to the longer wavelength: 408 nm. It is worth
pointing out that the difference in the absorption energy
between [PtL1Cl]+ and [Pt(tpy)Cl]+ (∼60 meV) agrees very
well with that in Ered(1) between the two complexes (i.e., 50
meV). Therefore, the transition observed at around 400 nm
can be assigned confidently to the MLCT transition.
Figure 2. Cyclic voltammograms of the Pt(II) complexes (5 × 10-4 M)
in DMF in the presence of 0.1 M (TBA)ClO4 (scan rate ) 100 mV/s).
Table 3. Reduction Potentials of the Pt(II) Complexes
redn potential/Va (vs SCE)
compd
Ered(1)
Ered(2)
[Pt(tpy)Cl]+
[PtL1Cl]+
-0.76
-0.71
-0.78
(-1.22)
-1.31
-1.19
-1.33
(-1.76)
[PtL2Cl]+
[Pt-4′-ph-tpyCl]+ b
a In DMF at 298 K. b Data taken from ref 21 reported vs Fc+/0 in DMF
(0.1 M TBAH).
to -1.33 V (vs SCE), respectively, the potentials of [PtL1-
Cl]+ are shifted to the positive direction by 50-70 (-0.71
V) and 120-140 mV (-1.19 V), respectively. In the case
of [Pt(tpy)Cl]+, since the lowest excited state is the MLCT
state, the first reduction peak of the complex is ascribed to
reduction of the tpy ligand. Therefore, the first reduction
potential of [PtL1Cl]+ or [PtL2Cl]+ could be also assigned
to the reduction of the L1 or L2 ligand in the complex,
respectively. The presence of the four methyl groups in L2
contributes partly to the negative potential shifts of [PtL2-
Cl]+ as compared with the Ered(1) and Ered(2) values of [PtL1-
Cl]+. As discussed in the preceding section, on the other
hand, the dihedral angle between the tpy and phenyl planes
in [PtL1Cl]+ is ∼26°. Therefore, electron delocalization
would take place more or less over the L1 ligand in [PtL1-
Cl]+, while the large dihedral angle between the tpy and duryl
planes (predicted to be 84°) would disturb electron delocali-
zation over the L2 ligand in [PtL2Cl]+. We suppose that this
will be the primary reason for the positive shifts of the Ered(1)
The most important aspect into the absorption character-
istics of the present Pt(II) complexes is the very large
absorption intensity (molar absorptivity: ꢀ) of [PtL1Cl]+ as
compared with that of [PtL2Cl]+ or [Pt(tpy)Cl]+ in the entire
wavelength region observed. As seen in Figure 3b, tpy is
transparent in the wavelength region longer than 330 nm,
10674 Inorganic Chemistry, Vol. 45, No. 26, 2006

Platinum(II) Terpyridine Complexes
Table 4. Spectroscopic and Excited-state Properties of the Pt(II) Complexes
emission (298 K)
em/µs
compd
abs:a λabs/nm (ꢀ/M-1 cm-1
)
λ
em/nm
Φem
τ
kr/s-1
knr/s-1
[Pt(tpy)Cl]+
[PtL1Cl]+
[PtL2Cl]+
L1
402 (1800), 381, 349, 334, 318, 304, 283
408 (16 000), 386, 335, 288
404 (3500), 385, 333, 320 sh, 305 sh, 284
318, 282
Nd
550b
Nd
0.011b
0.6b
1.8 × 105
1.5 × 106
L2
332 sh, 315, 280
402 (5200), 380 sh
[Pt-4'-ph-tpyCl]+ c
535d
0.0021d
0.085d
2.5 × 104
1.2 × 107
a In acetonitrile. b In CHCl3. c Data taken from ref 21. d In dichloromethane.
while the free L1 and L2 ligands show relatively strong
absorption above 300 nm. Our previous studies on the
spectroscopic properties of triarylborane derivatives indicate
that the absorption band observed for L1 or L2 at around
320-350 nm is attributed to the electronic transition from
the π(aryl) orbital to the vacant p-orbital on the boron atom
(p(B)); π(aryl)-p(B) charge transfer,5,6 where “aryl” would
be mainly the phenyl-tpy unit in L1 or L2. Since the dihedral
angle between the tpy and phenyl planes in L1 is much
smaller than that between the tpy and duryl planes as
discussed above, the larger absorption intensity of the π-
(aryl)-p(B) CT transition observed for L1 as compared with
that of L2 will be the reasonable consequence in respect to
the extent of electron delocalization in the ligand. Owing to
such absorption characteristics of the free L1 or L2 ligand,
both intraligand ππ*(tpy) and π(aryl)-p(B) CT transitions
are superimposed in the absorption spectrum of [PtL1Cl]+
or [PtL2Cl]+ in the wavelength region shorter than 350 nm.
Thus, the absorption intensities of [PtL1Cl]+ and [PtL2Cl]+
in this wavelength region can be explained roughly by those
of the L1 and L2 ligands, respectively.
Figure 4. Emission spectrum of [PtL1Cl]+ (2 × 10-6 M) in CHCl3 at
298 K (excitation wavelength ) 400 nm). The inserted is the emission decay
profile of the complex (monitored at 550 nm).
in [PtL1Cl]+: synergetic effects of Pt(II)-to-L1 MLCT and
π(aryl)-p(B) CT (discussed later again). These results and
discussions are consistent with the differences in the dihedral
angle between the tpy and arylborane (phenyl or duryl) planes
in the (L1 or L2) ligand as well as in the redox properties
between [PtL1Cl]+ and [PtL2Cl]+ as described above.
On the other hand, the molar absorption coefficient of the
MLCT transition observed for [PtL1Cl]+ at λabs is 5 times
larger than that of [PtL2Cl]+ (see Table 4), while the ratio
of the ꢀ value of the π(aryl)-p(B) CT transition in the free
L1 ligand to that in L2 at around 320 nm is ∼1.5. It is well-
known that the absorption strength of a molecule is deter-
mined by an oscillator strength (f) and the f value is given
by
Emission Characteristics of [PtL1Cl]+. Among three Pt-
(II) complexes studied, only the [PtL1Cl]+ complex is
emissive in solution at room temperature. The emission
spectrum of [PtL1Cl]+ in CHCl3 at 298 K is shown in Figure
4, in which the inserted figure displays the relevant emission
decay profile. The emission data are included in Table 4.
The [PtL1Cl]+ complex shows emission at the maximum
wavelength (λem) of 550 nm with the lifetime (τem) and
quantum yield (Φem) of 0.6 µs and 0.011, respectively. As
an analogous complex to [PtL1Cl]+, it has been reported that
[Pt(ph-tpy)Cl]+ also shows room-temperature emission: λem
) 535 nm, τem ) 0.085 µs, and Φem ) 0.0021 in CH2Cl2.21
Thus, the Φem and τem values of [PtL1Cl]+ are larger and
longer than the relevant value of [Pt(ph-tpy)Cl]+ by factors
of 7 and 5, respectively. The data indicate that the radiative
(kr) and nonradiative decay rate constants (knr) of [PtL1Cl]+
are 1.8 × 105 and 1.5 × 106 s-1, respectively, while those
of [Pt(ph-tpy)Cl]+ are 2.5 × 104 and 1.2 × 107 s-1,
respectively (see Table 4). These values demonstrate that
an introduction of the dimesitylborane group to the ph-tpy
ligand gives rise to an increase in kr by 7 times, while the
8πmeν˜
f )
µe2
(1)
(
)
3he2
where me is the mass of an electron, ν˜ is the energy of the
transition, h is Planck’s constant, and µe is the transition
dipole moment.44 For a given transition, a large absorption
strength (ꢀ) corresponds to a large transition dipole moment
through the relation44
f ≡ 4.3 × 10-9 ꢀ dν˜
(2)
∫
The very large ꢀ value for the MLCT transition of [PtL1-
Cl]+ as compared with that of [PtL2Cl]+ thus demonstrates
the larger transition dipole moment of the former complex
than that of the latter complex. This will be due to the
contribution of the π(aryl)-p(B) CT to the MLCT transition
(44) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings
Publishing: Menlo Park, NJ, 1978.
Inorganic Chemistry, Vol. 45, No. 26, 2006 10675

Sakuda et al.
knr value of [PtL1Cl]+ becomes almost 1/10 of the value for
[Pt(ph-tpy)Cl]+.
It is known that kr of a molecule is related to the oscillator
strength of the relevant absorption transition and, thus, to
the ꢀ value:
k ≈ ν˜2f ≈ ν˜ ꢀ dν˜
(3)
∫
r
The large kr value observed for [PtL1Cl]+ agrees very well
with the larger ꢀ (MLCT) value of [PtL1Cl]+ (1.6 × 104 M-1
cm-1) as compared with that of [Pt(ph-tpy)Cl]+ (5.2 × 103
M-1 cm-1), which is ascribed essentially to the enhanced
absorption-transition dipole moment by the synergetic effects
between the Pt(II)-to-L1 MLCT and π(aryl)-p(B) CT
interactions. Such synergetic interactions in the complex
should stabilize the MLCT excited-state energy. In practice,
the emission spectrum of [PtL1Cl]+ (λem ) 550 nm) is shifted
to the longer wavelength as compared with that of [Pt(ph-
tpy)Cl]+ (λem ) 535 nm). The X-ray crystal structure of
[PtL1Cl]+ is very similar to that of [Pt(ph-tpy)Cl]+, except
for the presence or absence of the dimesitylborane group.
This indicates that the coordination structures around the Pt-
(II) ions in these complexes are very similar and, thus, the
d-d excited-state energy will be comparable between the
two complexes. Since nonradiative decay from the excited
state of a Pt(II) complex proceeds via thermal population
from the emissive MLCT excited-triplet state to the higher
lying excited d-d triplet state, the lower MLCT excited-
state energy of [PtL1Cl]+ as compared with that of [Pt(ph-
tpy)Cl]+ results in the decrease in the knr value of the former
complex. The strong and long-lifetime emission of [PtL1-
Cl]+ is thus explained reasonably by the synergetic MLCT
and π(aryl)-p(B) CT interactions.
It should be noted that clear evidence for the participation
of the π(aryl)-p(B) CT interactions in the MLCT excited
state of [PtL1Cl]+ has been obtained. It has been reported
that the vacant p-orbital on the boron atom in a triarylborane
derivative coordinates selectively with a fluoride ion and this
gives rise to the changes in both absorption and emission
spectra of the derivative.1,3,4 For example, the binding
constant of TAB with a fluoride ion in THF has been reported
to be 105 M-1, while other halide ion such as Cl- or Br-
cannot bind with TAB. Similar to such observations on TAB,
an addition of a fluoride ion (i.e., tetra-n-butylammonium
fluoride: (TBA)F) in a CHCl3 solution of [PtL1Cl]+ results
in quenching of the MLCT emission without any appreciable
change in the spectral band shape: Figure 5. Furthermore,
emission quenching of [PtL1Cl]+ by F- accompanies a
double exponential emission decay: the short- (τs) and long-
lifetime components (τl) of 40-50 ns and ∼0.5 µs, respec-
tively. The τs value is very similar to the emission lifetime
of [Pt(ph-tpy)Cl]+ (80 ns),21 while the τl value is comparable
to that of [PtL1Cl]+ in the absence of F- (0.6 µs). It is worth
nothing that although the τs and τl values are almost constant
irrespective of the concentration of a fluoride ion ([F-]), the
contribution of the τs component to the overall decay profile
increases with increasing [F-] (data are not shown here).
This indicates that the τs component observed in the presence
Figure 5. Emission quenching of [PtL1Cl]+ by a fluoride ion ((TBA)F)
in CHCl3 at 298 K (excitation wavelength ) 400 nm). The insert is the
relevant absorption spectrum changes of the complex. [PtL1Cl]+ ) 1.0 ×
10-6 M, and [TBAF] ) 0 (solid line), 1.1 × 10-6 (dashed line), and 2.2 ×
10-6 M (dotted line).
of F- is attributed to the excited [PtL1Cl]+ complex
coordinated with a fluoride ion. Although the effect of a
fluoride ion on the absorption spectrum of [PtL1Cl]+ is rather
small, MLCT emission quenching of the complex by F- in
Figure 5 demonstrates clearly the participation of the
π(aryl)-p(B) CT interactions in the MLCT excited state of
[PtL1Cl]+. Therefore, we conclude that the excited state of
the [PtL1Cl]+ complex is best characterized by the MLCT
and π(aryl)-p(B) CT interactions.
All of the experimental results on the redox, spectroscopic,
and excited-state properties of [PtL1Cl]+ are explained
reasonably by a single context of the participation of the
π(aryl)-p(B) CT interaction in the MLCT excited state.
Also, the non- or very weak emissive nature of the [PtL2-
Cl]+ complex at ambient temperature can be explained by
weak coupling between the MLCT and π(aryl)-p(B) CT
states.
Conclusions
This study demonstrated that the synergetic Pt(II)-to-L1
MLCT and π(aryl ) L1)-p(B) CT interactions determined
the redox, spectroscopic, and excited-state properties of [PtL1-
Cl]+. Owing to such interactions, [PtL1Cl]+ shows strong
room-temperature emission with Φem ) 0.011 and τem
)
0.6 µs, which are considerably larger and longer, respectively,
than the relevant value of other Pt(II) complex having a tpy-
type ligand. The importance of the electron communication
between the Pt(II) ion and p(B) through the MLCT/π(aryl)-
p(B) CT interactions in determining the excited-state proper-
ties of the Pt(II) complex is supported by almost nonemissive
nature of the [PtL2Cl]+ complex, in which the large dihedral
angle between the tpy and duryl(dimesitylborane) groups
disturbs electron delocalization over the ligand. Phenom-
enologically, the present results on [PtL1Cl]+ are very similar
to the effects of π-conjugation in the tpy ligand on the
photophysical properties of a Ru(II) complex having a 4′-
10676 Inorganic Chemistry, Vol. 45, No. 26, 2006

Platinum(II) Terpyridine Complexes
(2-pyrimidyl)terpyridine ligand.45-49 Another important as-
pect of this study is the enhanced transition dipole moment
of MLCT absorption, as revealed by the large ꢀ value of the
MLCT absorption band of [PtL1Cl]+ as compared with that
of [Pt(tpy)Cl]+ or [Pt(ph-tpy)Cl]+. Although we have not
determined the dipole moment in the MLCT excited state
of [PtL1Cl]+, the value should be much larger than that of
[Pt(tpy)Cl]+ or [Pt(ph-tpy)Cl]+. Since all of the present
results can be explained reasonably by the participation of
the synergetic MLCT/π(aryl)-p(B) CT interactions in [PtL1-
Cl]+, strongly emitting transition metal complexes could be
realized through appropriate design of a boron-conjugated
π-chromophore ligand. It is worth noting, finally, that the
effects of synergetic MLCT and π(aryl)-p(B) CT interac-
tions on the redox, spectroscopic, and excited-state properties
of a transition metal complex will not be limited to those of
a Pt(II) complex and the present L1/L2 ligands. The present
idea of the synergetic MLCT/π(aryl)-p(B) CT effects on
the electronic structure of a compound could be applied to
various transition metal complexes with a variety of π-chro-
mophoric ligands.
(45) Fang, Y.-Q.; Taylor, N. J.; Hanan, G. S.; Loiseau, F.; Passalacqua,
R.; Campagna, S.; Nierengarten, H.; Dorsselaer, A. V. J. Am. Chem.
Soc. 2002, 124, 7912.
(46) Polson, M. I. J.; Taylor, N. J.; Hanan, G. S. Chem. Commun. 2002,
1356.
(47) Polson, M. I. J.; Medlycott, E. A.; Hanan, G. S.; Mikelsons, L.; Taylor,
N. J.; Watanabe, M.; Tanaka, Y.; Loiseau, F.; Passalacqua, R.;
Campagna, S. Chem.sEur. J. 2004, 10, 3640.
(48) Passalacqua, R.; Loiseau, F.; Campagna, S.; Fang, Y.-Q.; Hanan, G.
S. Angew. Chem., Int. Ed. 2003, 42, 1607.
Acknowledgment. We thank Prof. Y. Sasaki and Dr. K.
Tsuge at the Division of Chemistry, Hokkaido University,
for X-ray crystallography experiments.
(49) Medlycott, E. A.; Hanan, G. S. Chem. Soc. ReV. 2005, 34, 133.
IC061435B
Inorganic Chemistry, Vol. 45, No. 26, 2006 10677