Conjugated triarylboryl donor-acceptor systems supported by 2,2'-bipyridine: Metal chelation impact on intraligand charger transfer emission, electron accepting ability, and "Turn-on" fluoride sensing

Article abstract of DOI:10.1021/ic9000335

To investigate the impact of metal-chelation on intraligand charge transfer emission involving a triarylboron group, three 2,2'-bipyridine derivative molecules, 5,5'-bis(BMes₂)-2,2'-bipy (B2bpy), 5-(BMes ₂)-5'-(NPh₂)-2,2'-

Full text of DOI:10.1021/ic9000335

Inorg. Chem. 2009, 48, 3755-3767
Conjugated Triarylboryl Donor-Acceptor Systems Supported by
2,2′-Bipyridine: Metal Chelation Impact on Intraligand Charger Transfer
Emission, Electron Accepting Ability, and “Turn-on” Fluoride Sensing
Yi Sun and Suning Wang*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada
Received January 8, 2009
To investigate the impact of metal-chelation on intraligand charge transfer emission involving a triarylboron group,
three 2,2′-bipyridine derivative molecules, 5,5′-bis(BMes2)-2,2′-bipy (B2bpy), 5-(BMes2)-5′-(NPh2)-2,2′-bipy (BNbpy),
and 5,5′-bis(NPh2)-2,2′-bipy (N2bpy) have been synthesized, which can be described as donor-only, donor-acceptor,
and acceptor-only systems. Each of these molecules displays distinctive electrochemical and photophysical properties
with BNbpy and N2bpy being bright emitters and B2bpy being a strong electron acceptor. In addition, BNbpy
displays a “turn-on” fluorescent response while B2bpy has a “turn-off” response upon binding with fluoride ions.
These molecules can readily chelate to a PtPh2 or a PtCl2 group, producing square planar complexes Pt(B2bpy)Ph2
(Pt-1), Pt(B2bpy)Cl2 (Pt-1a), Pt(BNbpy)Ph2 (Pt-2), Pt(BNbpy)Cl2 (Pt-2a), and Pt(N2bpy)Ph2 (Pt-3) that have significantly
altered electrochemical and photophysical properties from those of the free ligands. Metal chelation has been
found to greatly enhance the electron accepting ability of the three ligands, especially B2bpy and BNbpy. The Ph
and Cl auxiliary ligands have also been found to have a significant impact on the electrochemical and photophysical
properties of the complexes. B2bpy complexes Pt-1 and Pt-1a are not luminescent at ambient temperature while
BNbpy complexes Pt-2 and Pt-2a display room temperature phosphorescence in solution under air that has a
similar “turn-on” response toward fluoride ions as the free BNbpy does but with a much more dramatic color switch
(orange or red to blue-green). The persistent intraligand NfB charge transfer transition in the BNbpy complexes
is believed to play a key role in their unique phosphorescent response toward fluorides. The complex Pt-3 displays
a bright blue-green phosphorescence in solution at ambient temperature. Density functional theory computations
established that the lowest electronic transition in the Pt(II) complex is from the Pt(II) d orbital and the auxiliary
ligand to the π* orbital of the 2,2′-bipy derivative ligand.
Introduction
of materials for various photonic and optoelectronic applica-
tion including nonlinear optical materials,1 charge-transport
materials, and emitters in organic light emitting devices
(OLED).2 Furthermore, three-coordinate organoboron com-
pounds have also been shown to be effective colorimetric,
fluorescent, or ratiometric sensors for the selective detection
Conjugated three-coordinate organoboron compounds have
recently emerged as an important and very promising class
* To whom correspondence should be addressed. E-mail: wangs@
chem.queensu.ca.
(1) (a) Yuan, Z.; Taylor, N. J.; Ramachandran, R.; Marder, T. B. Appl.
Organomet. Chem. 1996, 10, 305. (b) Yuan, Z.; Entwistle, C. D.;
Collings, J. C.; Albesa-Jove´, D.; Batsanov, A. S.; Howard, J. A. K.;
Kaiser, H. M.; Kaufmann, D. E.; Poon, S.-Y; Wong, W.-Y; Jardin,
C.; Fathallah, S.; Boucekkine, A.; Halet, J.-F.; Taylor, N. J.; Marder,
T. B. Chem.sEur. J. 2006, 12, 2758. (c) Entwistle, C. D.; Marder,
T. B. Angew. Chem., Int. Ed. 2002, 41, 2927. (d) Entwistle, C. D.;
Marder, T. B. Chem. Mater. 2004, 16, 4574. (e) Stahl, R.; Lambert,
C.; Kaiser, C.; Wortmann, R.; Jakober, R. Chem.sEur. J. 2006, 12,
2358. (f) Matsumi, N.; Chujo, Y. Polymer J. (Tokyo, Jpn.) 2008, 40,
77. (g) Lequan, M.; Lequan, R. M.; Ching, K. C. J. Mater. Chem.
1991, 1, 997. (h) Liu, Z. Q.; Fang, Q.; Cao, D. X.; Wang, D.; Xu,
G. B. Org. Lett. 2004, 6, 2933. (i) Tao, L. M.; Guo, Y. H.; Huang,
X. M.; Wang, C. K. Chem. Phys. Lett. 2006, 425, 10.
(2) (a) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714. (b)
Shirota, Y. J. Mater. Chem. 2005, 15, 75. (c) Noda, T.; Ogawa, H.;
Shirota, Y. AdV. Mater. 1999, 11, 283. (d) Shirota, Y.; Kinoshita, M.;
Noda, T.; Okumoto, K.; Ohara, T. J. Am. Chem. Soc. 2000, 122, 1102.
(e) Jia, W. L.; Bai, D. R.; McCormick, T.; Liu, Q. D.; Motala, M.;
Wang, R.; Seward, C.; Tao, Y.; Wang, S. Chem.sEur. J. 2004, 10,
994. (f) Jia, W. L.; Moran, M. J.; Yuan, Y. Y.; Lu, Z. H.; Wang, S.
J. Mater. Chem. 2005, 15, 3326. (g) Jia, W. L.; Feng, X. D.; Bai,
D. R.; Lu, Z. H.; Wang, S.; Vamvounis, G. Chem. Mater. 2005, 17,
164. (h) Li, F. H.; Jia, W. L.; Wang, S.; Zhao, Y. Q.; Lu, Z. H. J. Appl.
Phys. 2008, 103, 034509/1–034509/6. (i) Wakamiya, A.; Mori, K.;
Yamaguchi, S. Angew. Chem., Int. Ed. 2007, 46, 4237.
10.1021/ic9000335 CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 8, 2009 3755
Published on Web 03/03/2009

Sun and Wang
of anions such as fluorides and cyanides3,4 and for the
activation of molecular hydrogen molecules5 by utilizing the
empty pπ orbital on the boron center. To be effective for
either OLEDs or sensor applications, it is necessary for the
triarylboron compound to have high emission quantum
efficiency and strong electron accepting ability or Lewis
acidity. Donor-acceptor systems involving a triarylboron
group have been shown to be particularly effective as either
bright emitters for OLEDs or as fluorescent sensors for
anions.5 Previous studies have established that the direct
conjugation of a donor and an acceptor via an aromatic linker
in a triarylboron compound can greatly enhance the quantum
efficiency of the donor to acceptor charge transfer
emission.4d-f Although many π-conjugated donor-acceptor
triarylboron compounds have been reported recently, they
typically involve non-coordinating aromatic linkers such as
phenyl, biphenyl, and thienyl with an electron accepting
ability/Lewis acidity comparable to that of a typical triaryl-
bis(BMes2)-2,2′-bipy (B2bpy) molecule has a high electron
accepting ability and a strong binding constant with F-.8
Furthermore, we have shown that by chelating to either a
PtIIR2 (R ) Me, Ph) or a [CuI(PPh3)2]+ unit, the Lewis
acidity/electron accepting ability of the boron centers can
be further enhanced.8 Most significant is that the Pt(II) or
Cu(I) complexes of B2bpy display distinct MLCT absorption
bands in the visible region that can be quenched by the
addition of fluoride ions, thus allowing the detection of
fluoride ions via visual color change. Nonetheless, the
acceptor-only B2bpy molecule has a very low fluorescent
quantum efficiency (1%) and its Pt(II) and Cu(I) complexes
are not emissive in solution at ambient temperature; hence,
they are not suitable as emitters for OLEDs and not very
effective for sensing fluorides in the fluorescent mode. To
enhance the luminescent efficiency and to examine the impact
of metal chelation on Lewis acidity and electron accepting
ability of donor-acceptor chelate molecules, we have
investigated a new donor-acceptor molecule BNbpy shown
in Scheme 1. To fully understand the impact of the three-
coordinate boron center on the photophysical properties of
B2bpy and BNbpy and their metal complexes, we also
investigated the donor-only molecule, N2bpy. The results
of our comprehensive investigation on B2bpy, BNbpy,
N2bpy, and their corresponding Pt(II) complexes are pre-
sented herein.
boron compound such as BMes3 (Mes ) mesityl).2-4
A
number of Ir(III) complexes with N,C-chelating phenylpy-
ridine (ppy) and derivative ligands functionalized by a BMes2
group have been shown recently to be effective phospho-
rescent emitters in OLEDs6a and sensors for fluoride ions,6b,c
although the role of the B center in phosphorescence is not
clear because of the highly emissive nature of the IrIII-ppy
chromophore. By investigating Pt(II) and Cu(I) complexes,
we have shown that a BMes2 functionalized N,N-chelate
ligand (N-2-(5-BMes2)pyridyl)-7-azaindole, BNPA) can greatly
facilitate metal to ligand charge transfer (MLCT) emission
in the complexes, compared to the non-functionalized NPA
ligand.7 We have also demonstrated that the 2,2′-bipy ligand
is an excellent linker for two BMes2, and the resulting 5,5′-
Experimental Section
All reactions were performed under an inert atmosphere of dry
N2 with standard Schlenk techniques unless otherwise noted. All
starting materials were purchased from Aldrich Chemical Co. and
used without further purification. Tetrahydrofuran (THF), toluene,
and CH2Cl2 were purified using the solvent purification system
(Innovation Technologies Co.). Deuterated solvents from Cambridge
Isotopes were used as received without further drying. NMR spectra
were recorded on a Bruker Avance 400 spectrometer (400.13 MHz
for 1H, 100.62 MHz for 13C, 376.50 MHz for 19F), chemical shifts
are referenced to the residual solvent peaks and have been reported
in parts per million (ppm) relative to TMS (1H and 13C) and CFCl3
(19F). UV-vis spectra were recorded on an Ocean Optics USB2000+
spectrophotometer. Cyclic voltammetry was performed using a BAS
CV-50W analyzer with a scan rate of 100 mV/s to 1 V/s and a
typical concentration of 5 mg of the compounds in 3 mL of
dimethylformamide (DMF). The electrolytic cell used was a
conventional three-compartment cell, in which a Pt working
electrode, a Pt auxiliary electrode, and a Ag/AgCl reference
electrode were employed. The CV measurements were performed
at room temperature using 0.10 M NBu4PF6 as the supporting
electrolyte and DMF as the solvent. The ferrocenium/ferrocene
couple was used as the standard (E0 ) 0.55 V). Elemental analyses
were performed at Canadian Microanalytical Service, Delta, British
Columbia. 5,5′-Dibromo-2,2′-bipyridine,9 5,5′-bis(BMes2)-2,2′-bi-
pyridine (B2bpy),8 [PtPh2(SMe2)]n (n ) 2 or 3),10 and Pt(B2bpy)Ph2
(1)8 were prepared by modified methods described in the literature.
(3) (a) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am.
Chem. Soc. 2002, 124, 8816. (b) Yamaguchi, S.; Akiyama, S.; Tamao,
K. J. Am. Chem. Soc. 2001, 123, 11372. (c) Sole´, S.; Gabba¨ı, F. P.
Chem. Commun. 2004, 1284. (d) Melaimi, M.; Gabba¨ı, F. P. J. Am.
Chem. Soc. 2005, 127, 9680. (e) Chiu, C. W.; Gabba¨ı, F. P. J. Am.
Chem. Soc. 2006, 128, 14248. (f) Hudnall, T. W.; Melaimi, M.; Gabba¨ı,
F. P. Org. Lett. 2006, 8, 2747. (g) Lee, M. H.; Agou, T.; Kobayashi,
J.; Kawashima, T.; Gabba¨ı, F. P. Chem. Commun. 2007, 1133. (h)
Lee, M. H.; Gabba¨ı, F. P. Inorg. Chem. 2007, 46, 8132. (i) Hudnall,
T. W.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2007, 129, 11978. (k) Dorsey,
C. L.; Jewula, P.; Hudnall, T. W.; Hoefelmeyer, J. D.; Taylor, T. J.;
Honesty, N. R.; Chiu, C.-W.; Schulte, M.; Gabba¨ı, F. P. Dalton Trans.
2008, 4442. (l) Hudnall, T. W.; Kim, Y.-M.; Bebbington, M. W. P.;
Bourissou, D.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2008, 130, 10890.
(4) (a) Sundararaman, A.; Venkatasubbaiah, K.; Victor, M.; Zakharov,
L. N.; Rheingold, A. L.; Ja¨kle, F. J. Am. Chem. Soc. 2006, 128, 16554.
(b) Parab, K.; Venkatasubbaiah, K.; Ja¨kle, F. J. Am. Chem. Soc. 2006,
128, 12879. (c) Ja¨kle, F. Coord. Chem. ReV. 2006, 250, 1107. (d)
Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int. Ed. 2006, 45,
5475. (e) Bai, D. R.; Liu, X. Y.; Wang, S. Chem.sEur. J. 2007, 13,
5713. (f) Zhao, S. B.; Wucher, P.; Hudson, Z. M; Mc Cormick, T. M.;
Liu, X. Y.; Wang, S.; Feng, X. D.; Lu, Z. H. Organometallics 2008,
27, 6446. (g) Zhou, G.; Baumgarten, M.; Mu¨llen, K. J. Am. Chem.
Soc. 2008, 130, 12477.
(5) (a) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124. (b) Welch, G. C.; Stephan, D. W. J. Am.
Chem. Soc. 2007, 129, 1880. (c) Chase, P. A.; Welch, G. C.; Jurca,
T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 8050. (d) Geier,
S. J.; Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008, 130,
12632.
(6) (a) Zhou, G. J.; Ho, C. L.; Wong, W. Y.; Wang, Q.; Ma, D. G.; Wang,
L. X.; Lin, Z. Y.; Marder, T. B.; Beeby, A. AdV. Funct. Mater. 2008,
18, 499. (b) You, Y. M.; Park, S. Y. AdV. Mater. 2008, 20, 3820. (c)
Zhao, Q.; Li, F. Y.; Liu, S. J.; Yu, M. X.; Liu, Z. Q.; Yi, T.; Huang,
C. H. Inorg. Chem. 2008, 47, 9256.
(8) Sun, Y.; Ross, N.; Zhao, S.-B.; Huszarik, K.; Jia, W. L.; Wang, R. Y.;
Wang, S. J. Am. Chem. Soc. 2007, 129, 7510.
(9) Liu, S.-J.; Zhao, Q.; Chen, R.-F.; Deng, Y.; Fan, Q.-L.; Li, F.-Y.;
Wang, L.-H.; Huang, C.-H.; Huang, W. Chem.sEur. J. 2006, 12, 4351.
(10) (a) Song, D.; Wang, S. J. Organomet. Chem. 2002, 648, 302. (b) Scott,
J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643.
(7) Zhao, S. B.; McCormick, T.; Wang, S. Inorg. Chem. 2007, 46, 10965.
3756 Inorganic Chemistry, Vol. 48, No. 8, 2009

Conjugated Triarylboryl Donor-Acceptor Systems
Scheme 1
Synthesis of 5-Bromo-5′-diphenylamino-bipyridine (Br-bpy-
NPh2). A mixture of 5,5′-dibromo-2,2′-bipyridine (314 mg, 1.0
mmol) and diphenylamine (115 mg, 0.67 mmol) was dissolved in
toluene (10 mL). Sodium-tert-butoxide (90 mg, 0.90 mmol),
tri(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (12 mg, 0.013
mmol), and 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl ((BINAP)
(17 mg, 0.026 mmol) were added to the solution. The reaction
mixture was stirred at 100 °C for 16 h under nitrogen. After cooling
to room temperature, the mixture was quenched by the addition of
an aqueous solution of EDTA, and the product was extracted with
CH2Cl2. The crude product was purified by column chromatography
on silica gel (hexane/ether acetyl, 30/1) to afford light yellow solid
of the product, 81 mg in 30% yield. 1H NMR (400 MHz, CDCl3):
8.66 (d, J ) 1.5 Hz, 1H, py), 8.37 (d, J ) 1.5 Hz, 1H, py), 8.20
(dd, J ) 2.1 Hz, J ) 6.3 Hz, 2H, py), 7.87 (dd, J ) 1.5 Hz, J )
6.3 Hz, 1H, py), 7.44 (dd, J ) 1.5 Hz, J ) 6.3 Hz, 1H, py), 7.31
Synthesis of N2bpy. A mixture of 5,5′-dibromo-2,2′-bipyridine
(157 mg, 0.50 mmol) and diphenylamine (203 mg, 1.20 mmol)
was dissolved in toluene (10 mL). Sodium-tert-butoxide (115 mg,
1.20 mmol), Pd2(dba)3 (9.5 mg, 0.010 mmol) and (()BINAP (13
mg, 0.020 mmol) were added to the solution. The reaction mixture
was stirred at 100 °C for 24 h under the nitrogen. After cooling to
room temperature, the mixture was quenched by the addition of an
aqueous solution of EDTA, and the product was extracted with
CH2Cl2. The crude product was purified by column chromatography
on silica gel (hexane/THF, 10/1) to afford light yellow solid of
1
N2bpy, 113 mg in 46% yield. Mp 125 °C. H NMR (400 MHz,
CDCl3): 8.37 (d, J ) 2.4 Hz, 2H, py), 8.15 (d, J ) 6.9 Hz, 2H,
py), 7.45 (dd, J ) 2.7 Hz, J ) 8.7 Hz, 2H, py), 7.30 (m, 8H, Ph),
13
7.10 (m, 12H, Ph). C NMR (100 MHz, CDCl3): 146.7, 143.8,
129.9, 129.6, 124.6, 123.8, 120.6 (aryl C). HRMS: calcd for
C34H26N4, [M]+: 490.2157; found: 490.2148.
13
(t, J ) 5.7 Hz, 4H, ph), 7.13 (m, 6H, ph). C NMR (100 MHz,
Synthesis of Pt(BNbpy)Ph2 (2). BNbpy (57 mg, 0.10 mmol)
and [PtPh2(µ-SMe2)]n (n ) 2 or 3, 41 mg, 0.10 mmol) were mixed
in 20 mL of CH2Cl2 and stirred for 12 h under nitrogen at room
temperature. After removal of solvent, the red residue was washed
by Et2O (10 mL) and dried under vacuum to afford 2 in 87% yield.
1H NMR (400 MHz, CD2Cl2): 8.33 (s, 1H, py), 8.20 (d, J ) 2.5
Hz, 1H, py), 7.98 (dd, J ) 1.5 Hz, J ) 8.0 Hz, 1H, py), 7.86 (d,
J ) 9.0 Hz, 1H, py), 7.84 (d, J ) 8.0 Hz, 1H, py), 7.50 (dd, J )
2.5 Hz, J ) 9.0 Hz, 1H, py), 7.34 (t, J ) 8.0 Hz, 4H, N-Ph), 7.21
(t, J ) 7.5 Hz, 4H, N-Ph), 7.14 (m, 2H, N-Ph), 7.08 (d, satellite,
J ) 7.5 Hz, JPt-H ) 62.5 Hz, 4H, Pt-Ph), 6.80 (s, 4H, Mes), 6.63
(m, 4H, Pt-Ph), 6.56 (m, 2H, Pt-Ph), 2.32 (s, 6H, Me), 1.94 (s,
12H, Me). 13C NMR (100 MHz, CD2Cl2): 158.1, 156.7, 145.1,
144.6, 140.8, 140.0, 138.9, 138.1, 138.0, 130.5, 128.9, 127.2, 126.2,
125.2, 123.7, 121.7, 120.5 (aryl C), 23.7, 21.3 (Me). Anal. Calcd
for C52H48BN3Pt: C, 67.82; H, 5.25; N, 4.56. Found: C, 67.75; H,
4.92; N, 4.10.
Synthesis of Pt(N2bpy)Ph2 (3). N2bpy (49 mg, 0.10 mmol) and
[PtPh2(µ-SMe2)]n (n ) 2 or 3, 41 mg, 0.10 mmol) were mixed in
20 mL of CH2Cl2 and stirred for 12 h under the nitrogen at room
temperature. After removal of solvent, the yellow residue was
washed by Et2O (10 mL) and dried under vacuum to afford 3 in
82% yield. 1H NMR (400 MHz, CD2Cl2): 8.11 (d, J ) 2.4 Hz, 2H,
py), 7.65 (d, J ) 8.8 Hz, 2H, py), 7.51 (dd, J ) 2.4 Hz, J ) 8.8
CDCl3): 154.9, 150.3, 148.1, 146.9, 145.2, 143.4, 139.6, 131.3,
130.0, 129.2, 125.3, 124.6, 121.9, 121.3, 120.3 (aryl C). HRMS:
calcd for C22H16N3Br, [M]+: 401.0528; found: 401.0518.
Synthesis of BNbpy. To a solution of Br-bpy-NPh2 (100 mg,
0.25 mmol) in THF (50 mL) was added a hexane solution of n-BuLi
(1.6 M, 0.17 mL, 0.28 mmol) at -100 °C, and the mixture was
stirred for 1 h at this temperature. Then a solution of dimesitylboron
fluoride (82 mg, 90%, 0.28 mmol) in THF (10 mL) was added.
The reaction mixture was stirred for another 1 h at -100 °C,
allowed to slowly reach room temperature, and kept stirring
overnight. Solvents were removed under reduced pressure. The
residue was subjected to column chromatography on silica gel
(hexane/ether acetyl, 25:1) to afford BNbpy (91 mg, 64% yield).
1
Mp 80 °C. H NMR (400 MHz, CD2Cl2): 8.60 (m, 1H, py), 8.35
(d, J ) 1.5 Hz, 1H, py), 8.32 (dd, J ) 0.8 Hz, J ) 8.8 Hz, 1H,
py), 8.29 (d, J ) 1.2 Hz, J ) 8.0 Hz, 1H, py), 7.80 (dd, J ) 1.5
Hz, J ) 8.0 Hz, 1H, py), 7.42 (d, J ) 2.1 Hz, J ) 8.8 Hz, 1H, py),
7.33 (t, J ) 5.7 Hz, 4H, Ph), 7.13 (m, 6H, Ph), 6.85 (s, 4H, Mes),
2.29 (s, 6H, Me), 2.03 (s, 12H, Me). 13C NMR (100 MHz, CD2Cl2):
159.7, 158.6, 156.0, 150.1, 148.9, 146.9, 145.4, 144.7, 143.5, 141.1,
139.6, 130.0, 128.9, 128.7, 125.4, 124.6, 122.1, 119.9 (aryl C), 23.6,
21.3 (CH3 of Mes). HRMS: calcd for C40H38BN3, [M]+: 571.3159;
found: 571.3166.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3757

Sun and Wang
Chart 1
Hz, 2H, py), 7.32 (t, J ) 7.8 Hz, 4H, N-Ph), 7.21 (m, 4H, N-Ph),
7.13 (m, 2H, N-Ph), 7.06 (dd, satellite, J ) 0.8 Hz, J ) 7.5 Hz,
JPt-H ) 49.6 Hz, 4H, Pt-Ph), 6.61 (m, 4H, Pt-Ph), 6.53 (m, 2H,
Pt-Ph). 13C NMR (100 MHz, CD2Cl2): 150.7, 149.5, 145.7, 143.9,
143.1, 142.6, 142.5, 138.4, 138.2, 131.4, 130.3, 127.2, 126.8, 125.8,
121.6 (aryl C). Anal. Calcd for C36H36N4Pt: C, 65.78; H, 4.32; N,
6.67. Found: C, 65.77; H, 4.41; N, 6.16.
Synthesis of Pt(B2bpy)Cl2 (1a). To a stirred solution of K2PtCl4
(41.5 mg, 0.10 mmol) in mixed solvent H2O (10 mL) and DMSO
(0.5 mL) was added B2bpy (65.3 mg, 0.10 mmol) in CH2Cl2 (5
mL). The mixture was stirred at room temperature for 6 h. When
the color of the water phase disappeared, the yellow organic phase
was separated. The product was precipitated by hexane (20 mL)
and washed by diethyl ether to afford yellow powder of 1a, 77 mg
1
in 84% yield. H NMR (400 MHz, CD2Cl2): 9.73 (d, J ) 1.2 Hz,
2H, py), 8.14 (dd, J ) 1.2 Hz, J ) 8.0 Hz, 2H, py), 7.96 (d, J )
8.0 Hz, 2H, py), 6.91 (s, 8H, Mes), 2.34 (s, 6H, Me), 2.07 (s, 12H,
Me). 13C NMR (100 MHz, CD2Cl2): 162.7, 162.3, 158.3, 156.0,
147.1, 145.3, 141.5, 141.2, 140.0, 138.0, 129.3, 127.2, 123.1, 116.4,
113.6 (aryl C), 23.8, 21.3 (Me). Anal. Calcd for C46H50B2Cl2N2Pt:
C, 60.15; H, 5.49; N, 3.05. Found: C, 60.35; H, 5.66; N, 2.75.
Synthesis of Pt(BNbpy)Cl2 (2a). To a stirred solution of K2PtCl4
(20.6 mg, 0.05 mmol) in mixed solvent H2O (5 mL) and DMSO
(0.2 mL) was added BNbpy (28.6 mg, 0.05 mmol) in CH2Cl2 (5
mL). The mixture was stirred at room temperature for 6 h. When
the color of water phase disappeared, the orange organic phase was
separated. The product was precipitated by hexane (20 mL) and
washed by diethyl ether to afford orange powder of 2a, 33 mg in
B2bpy is a white solid while BNbpy and N2bpy are both
yellow solids. Like 2,2′-bpy, B2bpy, BNbpy and N2bpy are
all effective chelate ligands for metal ions, and their
corresponding PtPh2 complexes Pt(B2bpy)Ph2 (Pt-1), Pt(B-
Nbpy)Ph2 (Pt-2), and Pt(N2bpy)Ph2 (Pt-3) were obtained
readily by the reaction of the chelate ligand with
[PtPh2(SMe2)]n in high yields. The PtCl2 complexes
Pt(B2bpy)Cl2 (Pt-1a) and Pt(BNbpy)Cl2 (Pt-2a) were ob-
tained by the reaction of the chelate ligand with K2PtCl4.
The structures of all Pt(II) complexes are shown in Chart 1.
The choice of the Pt(II) ion is based on the consideration
that Pt(II) diimine complexes are well-known to display
metal-to-ligand (diimine) charge transfer (MLCT) transi-
tions,11 thus allowing us to examine the influence of the
donor and acceptor groups on MLCT transitions. The three
PtPh2 complexes display distinct colors: Pt-1, red, Pt-2,
orange, and Pt-3, yellow. They are all air stable in both
solution and the solid state. The ligands and their Pt(II)
complexes have been fully characterized by NMR spectros-
copy and HR-MS or element analyses. The crystal structures
of B2bpy, Pt-1 were presented in a preliminary communica-
tion article.8 The structure of Pt-1a determined by X-ray
diffraction is shown in Figure 1. The Pt-N bond lengths of
Pt-1a are much shorter (1.988(4), 1.990(14) Å) than those
in Pt-1 (2.095(2) Å) because of the much greater trans effect
of the phenyl group. In addition, the two boron atoms in
Pt-1a are much less out of linearity with the two pyridine
rings than those in Pt-1, as evidenced by the B · · ·B-Cpy
angle, 6.13°, 7.98° in Pt-1a, 8.84° in Pt-1, attributable to
the much reduced steric congestion in Pt-1a. All other Pt(II)
complexes did not produce single crystals that are appropriate
for X-ray diffraction analysis.
1
78% yield. H NMR (400 MHz, CD2Cl2): 9.58 (s, 1H, py), 9.36
(d, J ) 3.0 Hz, 1H, py), 8.02 (dd, J ) 1.8 Hz, J ) 7.8 Hz, 1H,
py), 7.69 (d, J ) 9.0 Hz, 1H, py), 7.67 (d, J ) 7.8 Hz, 1H, py),
7.50 (dd, J ) 2.4 Hz, J ) 9.0 Hz, 1H, py), 7.47 (t, J ) 7.8 Hz, 4H,
N-Ph), 7.32 (t, J ) 7.2 Hz, 4H, N-Ph), 7.25 (m, 2H, N-Ph), 6.90
(s, 4H, Mes), 2.32 (s, 6H, Me), 2.02 (s, 12H, Me). 13C NMR (100
MHz, CD2Cl2): 164.0, 159.3, 156.2, 147.8, 146.2, 144.4, 141.3,
140.7, 130.7, 129,1 127.2, 126.6, 125.2, 124.0, 120.5 (aryl C), 23.9,
21.4 (Me).
Results and Discussion
Syntheses of Ligands and Their Pt(II) Complexes. The
synthetic procedures for B2bpy, BNbpy, and N2bpy are
shown in Scheme 1. The starting material 5-bromo-5′-
diphenylamino-bipyridine (Br-bpy-NPh2) and N2bpy were
synthesized by Pd-catalyzed C-N bond formation reaction
of 5,5′-dibromo-2,2′-bipyridine (Br2-bpy) with HNPh2 in a
1:0.67 and a 1:2.4 ratio, respectively. The yields of Br-bpy-
NPh2 and N2bpy were found to be highly dependent on the
ratio of Br2-bpy and HNPh2 and the catalytic systems for
performing aryl amination. For the synthesis of Br-bpy-NPh2,
the combination of Br2-bpy and HNPh2 in 1.0:0.67 ratio
significantly reduces the amount of the byproduct N2bpy,
compared to the reaction with a 1:1 ratio. For the synthesis
of N2bpy, the 1.0:2.4 ratio of Br2-bpy and HNPh2 gave the
best yield. The mixture of Pd2(dba)3 and (()BINAP (1:2)
was found to be the most effective catalytic system for the
synthesis of Br-bpy-NPh2 and N2bpy. The effectiveness of
the chelating BINAP ligand was presumably due to the fact
that the chelating ligand inhibits Pd(II) binding to the bpy
site. The synthesis of B2bpy and BNbpy were accomplished
by the reaction of BMes2F with Li2(2,2′-bpy) and Li(5-NPh2-
2,2′-bpy), respectively, in THF at -100 °C in ∼60% yields.
Absorption Spectra. Ligands. The absorption spectra of
the free ligands B2bpy, BNbpy, and N2bpy are shown in
(11) (a) Wadas, T. J.; Chakraborty, S.; Lachicotte, R. J.; Wang, Q.;
Eisenberg, R. Inorg. Chem. 2005, 44, 2628. (b) Chakraborty, S.;
Wadas, T. J.; Hester, H.; Schmehl, R.; Eisenberg, R. Inorg. Chem.
2005, 44, 6865. (c) Chakraborty, S.; Wadas, T. J.; Hester, H.;
Flaschenreim, C.; Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005, 44,
6284.
3758 Inorganic Chemistry, Vol. 48, No. 8, 2009

Conjugated Triarylboryl Donor-Acceptor Systems
chelate ligand’s energy levels. Compared to the free ligands,
the Pt(II) complexes have much more intense colors: red (Pt-
1), light yellow (Pt-1a), yellow (Pt-2), yellow-orange (Pt-
2a), yellow (Pt-3) in solution (CH2Cl2). While the distinct
red color of Pt-1 is clearly caused by the weak but well-
resolved MLCT absorption band at 525 nm (ε ) 1390 M-1
cm-1), the color displayed by the other Pt(II) complexes may
be attributed to both πfπ* (or NfB) absorption band of
the chelate ligand and the MLCT band because these two
bands overlap and are not resolved in the absorption spectra.
Nonetheless, the absorption spectral data support unambigu-
ously that the MLCT energy of complex Pt-1 is much lower
than all other Pt(II) complexes among this group. The two
boron centers and the phenyl auxiliary ligands in Pt-1 clearly
play a key role in promoting such a low energy MLCT
transition by stabilizing the π* level and enhancing the Pt(II)
d energy level (HOMO). The PtMe2 analogue8 of Pt-1
displays a similar MLCT band at λ ) 550 nm. The red shift
of the MLCT energy from Pt(B2bpy)Ph2 to Pt(B2bpy)Me2
is consistent with the greater σ donating ability of methyl
that increases the d (HOMO) level, relative to phenyl, thus
decreasing the MLCT energy. It is noteworthy that the
MLCT band of Pt(B2bpy)Ph2 and Pt(B2bpy)Me2 experiences
a blue shift with increasing solvent polarity, consistent with
the excited-state of these Pt(II) complex being less polarized
than the ground state.11 To verify the impact of Pt(II)
chelation and the auxiliary ligands on the π* (LUMO) energy
level and the electron accepting ability of the boron center
in the complexes, the electrochemical properties of the free
ligands and their complexes were examined.
Figure 1. Diagram showing the structure of Pt-1a with 35% thermal
ellipsoids and labeling schemes.
Figure 2 along with those of the Pt(II) complexes. The
absorption bands at λ > 300 nm are listed in Table 1. The
λmax of the low energy absorption band for the free ligands
follows the order of B2bpy (350 nm) < N2bpy (381 nm) <
BNbpy (400 nm) in CH2Cl2, which accounts for their color
difference (colorless, pale yellow, yellow). Among the three
ligands, BNbpy has the lowest absorption energy that arises
from the NfB charge transfer transition. The absorption
spectra of BNbpy show a small red shift with increasing
solvent polarity (λmax ) 393 nm in hexanes, 400 nm in
CH2Cl2) that is consistent with the excited-state being more
polarized than the ground state. The trend of the absorption
energy for the B2bpy and N2bpy molecules is consistent with
πfπ* transitions where electron accepting boron centers in
B2bpy clearly widens the π-π* gap by greatly stabilizing
the highest occupied molecular orbital (HOMO) level while
the electron donating nitrogen atoms in N2bpy narrows the
π-π* gap by mostly destabilizing the HOMO level.
Electrochemical Properties. Ligands. The reduction
potentials of the free ligands and the Pt(II) metal complexes
were recorded by cyclic voltammetry. For comparison, 2,2′-
bpy and Pt(bpy)Ph2 were also examined by CV. The
reduction potentials are shown in Table 2 and Figure 3.
N2bpy and 2,2′-bpy do not display any reversible reduction
peaks under the experimental conditions we used (DMF as
the solvent, NBu4PF6 as the electrolyte). The BNbpy
molecule has one reversible reduction peak at -1.99 V (vs
FeCp20/+), which is much more positive than that of B(Mes)2-
biphenyl-NPh(1-naph) (BNPB, -2.36 V). The greater elec-
tron accepting ability of BNbpy, compared with that of
BNPB, can be attributed to the greater π conjugation, caused
by the electronegative nitrogen atom and the greater copla-
narity of the two py rings in BNbpy. For the diboron B2bpy
molecule, two reversible reduction peaks at -1.69 and -2.04
V, respectively, were observed, which may be attributed to
the sequential reduction of the two boron centers. The
E1/2red1 of B2bpy is about 0.30 V more positive than that of
BNbpy, supporting that the second boron center in B2bpy
can greatly enhance the electron accepting ability of the
molecule via π conjugation with the first boron center
through the bpy linker. Again, the two reduction potentials,
Complexes. Each Pt(II) complex has an absorption band
similar to the low energy absorption band of the correspond-
ing free ligand except that the λmax of this band in the
complex is red-shifted, an indication that N,N-chelation to
the Pt(II) center decreases the πfπ* (or NfB in case of
BNbpy) energy, which can be attributed to the enhanced π
conjugation of the ligand and the σ donation. Nonetheless,
the λmax of this ligand centered band of the PtPh2 complexes
follows a different order from that of the free ligands with
Pt(B2bpy)Ph2 (Pt-1, 355 nm) < Pt(BNbpy)Ph2 (Pt-2, 430
nm) ≈ Pt(N2bpy)Ph2 (Pt-3, 431 nm) in CH2Cl2. This change
of ordering is mainly caused by the dramatic red shift of the
πfπ* transition band of Pt(N2bpy)Ph2 (50 nm), that is much
greater than that of the πfπ* transition band of
Pt(B2bpy)Ph2 (5 nm) and the NfB charge transfer transition
of Pt(BNbpy)Ph2 (30 nm), relative to the corresponding free
ligand. The energy of the same ligand-based band of the
PtCl2 complexes Pt(B2bpy)Cl2, (Pt-1a, 369 nm) and Pt(B-
Nbpy)Cl2, (Pt-2a, 455 nm) is further red-shifted, relative to
the PtPh2 analogues, supporting that the auxiliary ligands
on the Pt(II) center have a significant impact on the N,N-
red1
red2
E1/2
and E1/2
of B2bpy are much more positive than
those of B(Mes)2-biphenyl-BMes2, further supporting the
effectiveness of the bpy linker in enhancing the electron
accepting ability of the boron center. Notably neither BNbpy
nor N2bpy display reversible oxidation peaks, which is in
(12) (a) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985,
107, 1431. (b) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-
Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.;
Thompson, M. E. Inorg. Chem. 2001, 40, 1740.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3759

Sun and Wang
Figure 2. UV-vis absorption spectra of free ligands and their Pt(II) complexes recorded in CH2Cl2.
Table 1. Absorption and Luminescence Data
compd
UV-vis, nm (ε, M-1 cm-1
)
λ
em, nm/ τc, µs CH2Cl2, rt
λem, nm /τ, ms CH2Cl2, 77 K
φa Neat Film
φb CH2Cl2
B2bpy
BNbpy
N2bpy
Pt-1
Pt-1a
Pt-2
350 (41,518)
400 (23,378)
381 (35,870)
355 (28,000),525 (1,390)
369 (19,387)
356 (11,968), 430 (19,687)
360 (9,371), 455 (13,455)
363 (16,903),431(21,967)
430
501
428
N/A
489/2.7(2)
538/5.9(1)
548/2.6(1)
515/0.49(1)
535/0.0253(1) 0.238(1)
597/0.52(1)
634/0.0258(1)
565/0.40(1)
<0.01
0.15(2)
0.05(2)
N/A
N/A
<0.01
N/A
0.01
0.95
0.76
N/A
N/A
N/A
565/6.80(1)
591/8.40(1)
510/2.3(1), 7.8(1)
0.0007
0.0002
0.0058
Pt-2a
Pt-3
<0.01
a Quantum efficiency obtained using an integration sphere spectrometer. b The quantum efficiency for the free ligands was obtained under air using
9,10-diphenylanthracene as the standard. For the Pt(II) complexes, the quantum efficiency was measured under N2 using Ir(ppy)3 as the standard (Φ )
0.40).12 The relative quantum efficiencies of complexes Pt-2, Pt-2a, and Pt-3 versus Ir(ppy)3 under air are 0.22, 0.022 and 1.07, respectively. c The decay
lifetime at r.t. for the Pt(II) complexes was measured under N2 and air, respectively. Similar lifetimes were observed under either N2 or air. The numbers
in the table were obtained under N2.
Table 2. Electrochemical Dataa
red1
red2
E1/2 (V)
E1/2
(V)
∆E (V)
B2bpy
Pt-1
Pt(B2bpy)Me2
Pt-1a
BNbpy
Pt-2
Pt-2a
-1.69
-1.34
-1.38
-1.17
-1.99
-1.62
-1.40
-1.99
-1.93
-2.04
-1.73
-1.76
-1.54
0.35
0.39
0.38
0.37
-2.10
-1.84
0.48
0.44
Pt-3
Pt(bpy)Ph2
a All potentials are relatively to FeCp20/+, measured in DMF, using
NBu4PF6 as the electrolyte with scan rates 100 mV to 1000 mV.
sharp contrast to BNPB and NPB ((1-naph)PhN-biphenyl-
NPh(1-naph)) molecules, both of which display reversible
ox
ox1
oxidation peaks (E1/2 ) 0.55 V for BNPB, E1/2 ) 0.25
V for NPB),2f,g that are typical of triarylamino groups. Hence,
the bpy linker decreases the electrochemical stability of the
amino group in BNbpy and N2bpy toward oxidation, which
is not surprising since its electron withdrawing nature
destabilizes the radical cation, relative to a phenyl or
biphenyl.
Figure 3. CV diagrams for the Pt(II) complexes recorded in DMF and the
free ligands B2bpy and BNbpy. Note: Pt(BNbpy)Cl2 has a poor solubility
and its CV spectrum was recorded at a very low concentration, as a
consequence, the impurity peak at ∼-1.5 V from the electrolyte became
significant.
to the donation of the lone pair electrons of B2bpy to the
metal center that reduces the electron density on the ligand,
thus increasing its electron accepting ability, and the
increased π-conjugation that effectively lowers the π* level.
Among the three B2bpy complexes, the PtMe2 compound
has the most negative reduction potentials while the PtCl2
Pt(II) Complexes. Like B2bpy, Pt(B2bpy)Ph2, Pt(B2bpy)-
Me2, and Pt(B2bpy)Cl2 display two reversible reduction
peaks that are more positive than those of B2bpy by
0.30-0.52 V. The great enhancement of the electron
accepting ability of the Pt(II) complexes can be attributed
3760 Inorganic Chemistry, Vol. 48, No. 8, 2009

Conjugated Triarylboryl Donor-Acceptor Systems
Table 3. HOMO and LUMO Energy and Gap of Ligands and
Complexes
experimental energy/eV
calculated energy/eV
energy
energy
gap
HOMOc LUMOa
gapb
HOMO LUMO
B2Bbpy
BNbpy
N2bpy
Pt-1
Pt-2
Pt-3
Pt-1a
Pt-2a
Pt(bpy)Ph2 -5.21
-6.22
-5.39
-5.46d
-5.46
-5.58
-5.27
-6.11
-5.70
-3.11
-2.81
-2.58e
-3.46
-3.18
-2.81
-3.63
-3.40
-2.87
3.11
2.58
2.88
2.00
2.40
2.46
2.48
2.30
2.34
-6.25
-5.37
-5.10
-5.11
-5.06
-5.02
-5.91
-5.80
-2.44
-2.15
-1.48
-2.98
-2.62
-2.15
-3.37
-2.98
3.81
3.22
3.62
2.13
2.44
2.87
2.54
2.82
a From the reduction potential in DMF. b From absorption edge of the
UV-vis spectrum. c Calculated from experimental LUMO and the
HOMO-LUMO gap. d From the oxidation potential in DMF. e From the
experimental HOMO and the HOMO-LUMO gap.
one has the most positive reduction potential (E1/2red1 ) -1.17
red2
V, E1/2
) -1.54 V), which is consistent with the much
weaker trans effect of the chloride ligand, relative to the
phenyl or methyl ligand, that leads a stronger B2bpy binding
to the Pt(II) center in Pt-1a, as supported by the crystal
Figure 4. HOMO and LUMO energy levels of free ligands and Pt(II)
complexes obtained from CV and UV-vis data (see Table 3).
red2
structural data. It is noteworthy that both E1/2red1 and E1/2
and BNbpy by metal chelation, and the significant impact
of the different bpy chelate ligands and the auxiliary ligands
on the HOMO energy level. For the chloride auxiliary ligand,
the B2bpy complex Pt-1a has a much lower HOMO than
that of the BNbpy complex Pt-2a, while the opposite is
observed for Pt-1 and Pt-2 that contain the phenyl ligands.
For the B2bpy chelate ligand, the HOMO level of the
complexes increases in the order of Cl- < Ph < Me, which
is consistent with the HOMO level being dominated by the
Pt(II) d orbital that is destabilized by strong σ auxiliary
ligands such as a methyl. Also notable is that the HOMO
energy difference between the BNbpy complexes Pt-2 and
Pt-2a is much less pronounced than that of the B2bpy
complexes Pt-1 and Pt-2.
Luminescence. Ligands. The luminescent properties of
the free ligands and their complexes were examined by
fluorescent and phosphorescent spectra. Under UV irradia-
tion, all three free ligands display luminescence in solution
(CH2Cl2) at ambient temperature: a weak blue emission for
B2bpy (λem ) 430 nm, Φ ) 0.01), a bright green emission
for BNbpy (λem ) 501 nm, Φ ) 0.95), and a bright blue
emission for N2bpy (λem ) 427 nm, Φ ) 0.76), with the
donor-acceptor compound BNbpy having the lowest emis-
sion energy and the highest emission quantum efficiency.
The emission band of BNbpy has a strong dependence on
the solvent polarity with λmax shifting from 434 nm in
hexanes to 552 nm in CH3CN (Figure 5), confirming the
presence of a highly polarized excited-state and the charge
transfer nature of the emission. In the solid state, B2bpy has
no visible emission while BNbpy and N2bpy emit a bright
blue-green color at 483 nm (Φ ) 0.15, measured by
integration sphere methods) and 495 nm (Φ ) 0.05),
respectively. The ∼65 nm red shift by N2bpy from solution
to the solid state is likely caused by intermolecular π stacking
interactions in the solid state.
of the B2bpy complexes are much more positive than that
of Pt(bpy)Ph2 (one reduction peak at -1.93 V). For Pt-1a,
a third reduction peak at ∼1.8 V is also observed, which
may be attributed to the reduction of the bpy ring.
Pt(N2bpy)Ph2, (Pt-3) displays a reversible reduction peak at
-1.99 V, that is somewhat more negative than that of
Pt(bpy)Ph2 and attributed to the reduction of the bpy ring,
supporting that the electron donating NPh2 group of N2bpy
indeed destabilizes the lowest unoccupied molecular orbital
(LUMO) level of the Pt-3 complex, relative to the bpy
complex. Both Pt-2 and Pt-2a display two reversible
reduction peaks that are far apart (∆E ) 0.48 and 0.44 V,
respectively). On the basis of the CV data of the B2bpy and
N2bpy complexes, the two reduction peaks of the BNbpy
complexes Pt-2 and Pt-2a can be assigned to the reduction
of the boron center and the bpy ring, respectively. Again,
the reduction potentials of the chloride complex Pt-2a are
much more positive than those of the phenyl analogue Pt-2,
consistent with the trend observed for the B2bpy complexes
Pt-1 and Pt-1a. The E1/2red1 potentials of the BNbpy cmplexes
are all much more positive than the free ligand, again
supporting that chelation to the Pt(II) center enhances the
electron accepting ability of the boron center. Interestingly,
red1
the magnitude of the E1/2
potential shift from the free
ligands to the Pt(II) complexes is nearly identical for both
B2bpy and BNbpy systems. On the basis of the CV data,
the electron accepting ability of the complexes and the
ligands follows the order of Pt-1a > Pt-1 > Pt(B2bpy)Me2
> Pt-2a > Pt-2 > B2bpy > Pt(bpy)Ph2 > Pt-3 ≈ BNbpy.
Hence, the ability of the boron center in the metal complexes
and the free ligands to bind to anions such as fluoride is
expected to follow the order of Pt-1a > Pt-1 > Pt(B2bpy)Me2
> Pt-2a > Pt-2 > B2bpy > BNbpy.
Using the CV and UV-vis data, the HOMO and LUMO
energies of the ligands and their complexes were obtained
and shown in Table 3 and Figure 4. The data show
consistently the stabilization of the LUMO level of B2bpy
Complexes. The luminescent property of Pt(B2bpy)Me2
was not investigated because of its poor stability in solution.
Pt-1 and Pt-1a have no detectable luminescence in solution
Inorganic Chemistry, Vol. 48, No. 8, 2009 3761

Sun and Wang
emission while Pt-2 and Pt-3 emit a weak red and a red-
orange color with λmax at 647 and 594 nm, respectively. To
verify that the solid state emission is from intramolecular
transitions, we measured the emission spectra of Pt-2 and
Pt-3 doped in a PMMA matrix (10% by weight, PMMA )
poly(methyl methacrylate) at ambient temperature. The
PMMA film of Pt-3 displays a phosphorescent peak with
λmax ) 591 nm (τ ) 9.61(3) µs) that resembles the solid
state emission spectrum. The Pt-2 PMMA film displays two
emission peaks at λ ) 498 and 585 nm, respectively, with
the former being fluorescent, attributable to BfN charge
transfer, and the latter being phosphorescent (τ ) 11.29(4)
µs), similar to that of the frozen solution at 77 K, but much
blue-shifted, compared to that of the solid at ambient
temperature. Hence, the solid state emission spectrum of Pt-2
can be attributed to intermolecular interactions.
Figure 5. Emission spectra of BNbpy in different solvents, [BNbpy] )
∼1.0 × 10-5 M.
Density Functional Theory (DFT) Calculations. To
better understand the photophysical properties of the com-
plexes, DFT calculations for the three free ligands and their
PtPh2 complexes were performed. The geometric parameters
from X-ray diffraction analysis were used initially and further
optimized for the calculations for B2bpy, Pt-1, and Pt-1a.
For all other compounds, geometric parameters were obtained
by molecular modeling and geometry optimization. The
Gaussian suite of programs13 (Gaussian 03, basis set B3LYP-
6-311G) was employed for the calculations. The calculated
HOMO and LUMO energy levels and the energy gaps for
the ligands and the complexes are provided in Table 3. The
HOMO and LUMO orbitals of the free ligands and the PtPh2
complexes and Pt-2a are shown in Figures 7 and 8 (the
diagrams for Pt-1a are provided in Supporting Information),
respectively.
Figure 6. Emission spectra of B2bpy, BNbpy, Pt-2, Pt-2a, and Pt-3 in
CH2Cl2, 1.0 × 10-5 M, r.t. under air.
The HOMO level for B2bpy is mainly from the mesityls
while the LUMO level is a π* orbital dominated by the two
boron atoms and the bpy unit; hence, the lowest electronic
transition of B2bpy can be considered as mesityls to the
B-bpy-B unit charge transfer. The HOMO level of BNbpy
involves the bpy unit and the NPh2 group while the LUMO
is dominated by the B atom with significant contributions
from the bpy, supporting that the lowest electronic transition
of BNbpy is charge transfer from NPh2 to the boron center,
mediated by the bpy unit via π-conjugation. For N2bpy, the
HOMO level is a π orbital involving the entire molecule
while the LUMO level is a π* orbital dominated by the bpy
unit. Hence, the lowest electronic transition of N2bpy is a
πfπ* transition. The general trend is that both HOMO and
LUMO levels increase in the order of B2bpy < BNbpy <
N2bpy with B2bpy displaying the most pronounced HOMO
stabilization while the N2bpy displaying the most LUMO
destabilization, consistent with the experimental data shown
in Figure 3. The overall calculated HOMO-LUMO energy
gap follows the order of BNbpy < N2bpy < B2bpy, in
agreement with the trend observed experimentally.
at ambient temperature while Pt-2 and Pt2a display an
orange and a red-orange phosphorescent emission with λmax
) 565 nm, τ ) 6.80 (2) µs; and 591 nm, τ ) 7.70(2) µs,
respectively, in CH2Cl2 under air (Figure 6). The order of
the emission energy of Pt-2 and Pt-2a follows that of the
low energy absorption band of the two complexes and may
be attributed to either a BNbpy centered BfN charge transfer
transition or MLCT transitions. However, since the auxiliary
ligands phenyl and chloride have no significant impact on
the HOMO level of Pt-2 and Pt-2a, as shown in Figure 4
and Table 3, the HOMO level of these two complexes is
likely dominated by the π orbital of BNbpy. Consequently,
the emission of Pt-2 and Pt-2a is most likely from ligand
centered BfN charge transfer transitions. The N2bpy
complex Pt-3 emits a blue color with λmax ) 510 nm, τ )
2.3(1) µs, 7.8(1) µs in CH2Cl2 at ambient temperature, which
is likely from a ligand-based transition as well. At 77 K, in
frozen CH2Cl2, all Pt(II) complexes display phosphorescence
with long decay lifetimes as shown in Table 1. Compared
to the 77 K emission spectra of the corresponding free
ligands, the spectra of the complexes are all red-shifted
considerably (see Table 1). The phosphorescent decay
lifetimes of the complexes at 77 K are all much shorter than
those of the free ligands (2.6-5.9 ms), consistent with the
heavy atom effects in the complexes. In the solid state, at
ambient temperature, Pt-1 and Pt-1a have no detectable
As shown in Figure 8, the HOMO level for the three PtPh2
complexes consists of π orbitals of the two phenyl group
and the d orbital of the Pt(II) center, with the energy varying
slightly (Pt-1 < Pt-2 < Pt-3). In contrast, the LUMO level
of each complex is quite distinctive and closely resembles
3762 Inorganic Chemistry, Vol. 48, No. 8, 2009

Conjugated Triarylboryl Donor-Acceptor Systems
Figure 7. HOMO and LUMO orbitals of B2bpy, BNbpy, and N2bpy plotted with a 0.02 iso-contour value.
that of the corresponding free ligand. For example, the
LUMO of Pt-1 is a π* orbital of the B2bpy ligand with some
contributions of a d orbital from the Pt(II) center as part of
the π-conjugation. The LUMO levels of Pt-2 and Pt-3 also
exhibit similar d orbital contributions from the Pt(II) center.
Hence, the lowest electronic transition of the three PtPh2
complexes can be indeed considered as charge transfer from
PtPh2 to the π* orbital of B2bpy, BNbpy, and N2bpy,
respectively. The DFT results also show that the HOMO
energy does not change significantly among the three PtPh2
complexes while the LUMO energy varies quite dramatically
with Pt-1 having the deepest LUMO (-2.98 eV) and Pt-3
the highest LUMO (-2.15 eV), and as a consequence, the
HOMO-LUMO gap follows the order of Pt-1 < Pt-2 < Pt-
3, which agrees well with the experimental data. For the two
PtCl2 complexes, Pt-1a and Pt-2a, the HOMO level consists
of the Pt d orbital and the lone pair orbitals of the Cl- ligands
while the LUMO level resembles that of the PtPh2 analogues.
The computational results show that the HOMO and LUMO
energies of Pt-1a and Pt-2a are much lower than those of
Pt-1 and Pt-2 and that the impact of the auxiliary ligand
phenyl and chloride is most pronounced at the HOMO level,
a trend again consistent with the experimental data.
Response to Fluorides. One unique feature of the B2bpy
and BNbpy Pt(II) complexes that distinguishes them from
previously known push-pull Pt(II) complexes is the presence
of a three-coordinate boron center capable of selective
binding to anions such as fluorides and cyanides.3,4 This
feature can not only be exploited for anion sensing but more
importantly can be used to probe the various electronic
transition pathways such as MLCT and the Lewis acidity of
the boron center in the complexes. We therefore examined
and compared the response of B2bpy, BNbpy, and their Pt(II)
complexes toward fluoride ions in both UV-vis and
fluorescent modes.
B2bpy and BNbpy. As demonstrated previously, B2bpy
undergoes sequential addition of F- with the formation of
B2bpyF- being quantitative and the need of ∼2 equiv of F-
for the formation of B2bpyF22- with K1 and K2 being g108
M-1 and ∼106 M, respectively. The binding strength of the
BNbpy to F- is similar to that of B2bpyF-, as supported by
the similar UV-vis spectral change and the Stern-Volmer
plots (Figure 9 and Supporting Information). The most
interesting contrast between B2bpy and BNbpy is their
fluorescent spectral change upon binding with fluorides as
shown in Figure 9. For B2bpy, the addition of F- causes a
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Sun and Wang
Figure 8. HOMO and LUMO orbitals of Pt-1, Pt-2, Pt-3, and Pt-2a plotted with a 0.02 iso-contour value.
partial quenching and a small red shift of the emission peak.
For BNbpy, the addition of F- causes a complete quenching
of the emission peak at 501 nm, accompanied by the
appearance and a rapid increase of a new emission peak at
400 nm, resulting in the emission color switch from blue-
green to deep blue. Therefore, BNbpy can be described as a
fluorescent turn-on sensor toward fluorides while B2bpy a
turn-off sensor. The new emission band at 400 nm of
BNbpyF- may be attributed to a πfπ* transition of the
adduct. The “turn-on” response of BNbpy is rare for
conjugated donor-acceptor triarylboron systems3,4 and is in
sharp contrast to the biphenyl analogue, BNPB, which has
the NfB charge transfer emission energy at ∼440 nm about
60 nm blue-shifted, compared to that of BNbpy and
undergoes a straight fluorescent quenching upon the addition
of fluoride ions.4d,e The emission quenching of BNPB by
fluoride can be explained by the fact that the πfπ* emission
energy of the fluoride adduct BNPBF- is in the same region
as that of the BNPB charge transfer but has a much lower
quantum efficiency because of the lack of coplanarity of the
biphenyl group. In contrast, the unusual “turn-on” behavior
of BNbpy can be explained by the large energy separation
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Conjugated Triarylboryl Donor-Acceptor Systems
Figure 9. UV-vis (left) and fluorescent (right) spectral change of BNbpy (∼1.0 × 10-5 M) with the addition of NBu4F (λex ) 350 nm). Inset: Photographs
of the BNbpy solution before (left) and after (right) F- under UV irradiation (lamp wavelength 365 nm).
Figure 10. UV-vis spectral change of Pt-1 (left) and Pt-1a (right) with the addition of NBu4F in CH2Cl2. The red region shows the spectral change after
1 equiv of F-. Inset: photographs showing the color change before (left) and after (right) NBu4F.
between the NfB charge transfer band of BNbpy and the
πfπ* band of BNbpyF-, and the relatively high emission
quantum efficiency of BNbpyF-, for which the bpy linker
in BNbpy is responsible by lowering the LUMO level and
enhancing the coplanarity of BNbpy via hydrogen bonding
between the two pyridyl rings. For B2bpy, its emission band
is in the same region as those of B2bpyF- and B2bpyF2-,
with the latter still having a fairly high emission intensity;
thus, the addition of F- only led to partial quenching of the
B2bpy emission peak.
Pt(B2bpy)Ph2 and Pt(B2bpy)Cl2. As described in the
preliminary communication, the addition of F- to the solution
of Pt-1 in CH2Cl2 causes a sequential quenching of the
MLCT absorption because of the stepwise formation of the
1:1 and 2:1 fluoride adducts with K1 g 109 M-1 and K2 )
∼106 M-1 in CH2Cl2 and visual stepwise color switching
from red to orange to light yellow. The response of Pt-1a to
fluoride ions is similar to that of Pt-1 except that the color
change is from yellow to colorless because of the blue shift
of the MLCT band, as shown in Figure 10 (for comparison,
the spectrum of Pt-1 is also shown). The CV data indicate
that Pt-1a should have a stronger binding with fluorides than
Pt-1 does. However, because of the strong binding of both
complexes with F-, accurate binding constants that allow
direct comparison could not be obtained. Nonetheless, both
complexes are capable of binding to one F- in either THF/
ethanol or DMF/H2O solvent mixtures (see Supporting
Information). Although the color change of Pt-1a with F-
is not as striking as Pt-1, for practical applications, it does
have the advantage for being much more stable in alcohol
or aqueous solvents than Pt-1 which slowly decomposes over
time. The binding of F- with Pt-1 and Pt-1a was also
1
confirmed by H and 19F NMR spectra.
Pt(BNbpy)Ph2 and Pt(BNbpy)Cl2. As shown in Figure
11, the absorption spectral change of Pt-2 and Pt-2a with
the addition of F- resembles that of [(Pt-1)F-] and [(Pt-
1a)F-], respectively, with quenching and a blue shift of the
low energy absorption band. The color of these two
complexes changes from light yellow and yellow to colorless
and light yellow, respectively, with fluoride. The binding
constants with fluoride for these two complexes were
estimated to be ∼7.0 × 105 M-1 and 5.0 × 106 M-1,
respectively, with Pt-2a having a stronger binding constant.
The key advantage of the BNbpy complexes over the
B2bpy ones is their luminescence at ambient temperature
under air, enabling the detection of the fluoride binding event
by fluorescent spectroscopy. As shown by the photographs
and the fluoride titration diagrams in Figure 12, the addition
of fluoride ions to the solution of Pt-2 or Pt-2a causes the
emission color switching from orange or red-orange to bright
whitish blue-green, with the emission λmax change from 565
to 485 nm for Pt-2, and 591 to 510 nm for Pt-2a and a great
gain of emission intensity (This “turn-on” phenomenon is
persistent with λex ) 400-430 nm for Pt-2, 430-470 nm
for Pt-2a.). The saturation point is reached after the addition
of ∼1.5 equiv of F- for both compounds. The “turn-on”
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Sun and Wang
Figure 11. UV-vis spectral change of Pt-2 (left) and Pt-2a (right) with the addition of NBu4F in CH2Cl2. Inset: photographs showing the absorption color
change before (left) and after (right) F-.
Figure 12. Fluorescent spectral change of Pt-2 (left, λex ) 400 nm) and Pt-2a (right, λex ) 430 nm) (∼1.0 × 10-5 M) with the addition of NBu4F in
CH2Cl2. Inset: photographs of the solution of the metal complexes at the titration concentration before (left) and after (right) F- under UV irradiation (lamp
wavelength 365 nm).
luminescent spectral change of Pt-2 and Pt-2a resembles
that of the free BNbpy ligand except that compared to
BNbpy, the emission peaks of Pt-2, Pt-2a, and their fluoride
adducts are all at much longer wavelengths. Hence, the “turn-
on” response of Pt-2 and Pt-2a toward fluoride may have
the same origin as the BNbpy free ligand, namely, a
switching from a NfB charge transfer (with possible MLCT
contributions) to a πfπ* transition in the fluoride adduct.
The role of the Pt(II) center is shifting the emission band to
a lower energy by enhancing the π-conjugation of the BNbpy
ligand. Metal complexes that respond to fluoride ions in the
luminescent mode at ambient temperature under air are rare,
and only a few examples of Ir(III) complexes based the ppy
ligand have been reported recently.6b,c What distinguish the
Pt(II) BNbpy complexes from other complexes are their
distinct color switch and the great emission intensity gain
upon the addition of fluoride.
toward fluoride ions mediated by the bipy unit. The 2,2′-
bipy unit greatly enhances the electron-accepting ability
of the triarylboron center by stabilizing the radical anion,
compared to a biphenyl unit. For the donor-only molecule,
the 2,2′-bipy unit reduces the electron donating ability of
the amino center by destabilizing the radical cation.
Chelation by a Pt(II) center to these new ligands has been
found to greatly enhance the electron accepting ability
via both σ donation and increased π-conjugation. The
auxiliary ligands on the Pt(II) center have also been found
to have a significant impact on the electron affinity of the
2,2′-bipy derivative ligands with the chloride complexes
having the deepest LUMO level, and thus the strongest
electron acceptor. Furthermore, chelation by a Pt(II) ion
reduces the HOMO-LUMO gap dramatically, compared
to the free ligand, because of the participation of the Pt(II)
d orbital and the auxiliary ligand at the HOMO level and
the enhanced π conjugation of the ligand. As a result, the
Pt(II) complexes can detect fluoride ions with a distinct
color change by either absorption or emission in the visible
region. Significantly the Pt(II) complexes of BNbpy and
N2bpy all display room-temperature phosphorescence in
solution under air that is most likely dominated by BNbpy
or N2bpy ligand-centered transitions. The phosphorescent
emission of Pt(BNbpy)Ph2 and Pt(BNbpy)Cl2 has a
dramatic “turn-on” response toward fluoride ions with
color switch from orange or red-orange to whitish blue-
Conclusions
A new class of 2,2′-bipyridyl supported conjugated
system that contains either two BMes2 acceptor groups,
or two NPh2 donor groups, or a BMes2 and a NPh2
donor-acceptor groups has been developed. The acceptor-
only molecule B2bpy has the highest electron affinity
while the donor-acceptor molecule BNbpy has the lowest
HOMO-LUMO gap and the highest emission quantum
efficiency with a unique fluorescent “turn-on” response
3766 Inorganic Chemistry, Vol. 48, No. 8, 2009

Conjugated Triarylboryl Donor-Acceptor Systems
green, attributed to switching from a NfB charge transfer
transition to a πfπ* transition on the BNbpy ligand. The
persistent “turn-on” response toward fluoride ions by the
free BNbpy ligand and its Pt(II) complexes can be
explained by the large emission energy difference between
the fluoride-bound and the non-bound states of the
molecule and the high degree of π-conjugation of the
BNbpy ligand promoted by the bipy unit and metal
chelation. Thus, the new 2,2′-bipy supported conjugated
ligand system is not only useful in achieving potentially
highly emissive ambient temperature phosphorescent metal
complexes but also very promising as a new class of
highly effective “turn-on” visual color sensors for fluoride
ions in both absorption and emission modes.
support and Dr. Rui-Yao Wang for his assistance in the
structural refinement of Pt-1a.
Supporting Information Available: UV-vis or fluorescent
spectra in different solvents (BNbpy, Pt-2, Pt-3), low temperature
and solid state emission spectra of ligands and complexes, the
emission spectra of Pt-2, Pt-2a, and Pt-3 recorded under N2 and
air, respectively, UV-vis and fluorescent titration spectra of B2bpy
by NBu4F in CH2Cl2, UV-vis titration spectra of Pt-1a in 1:1
CH2Cl2/MeOH by NBu4F, luminescent titration spectra of Pt-2 and
Pt-2a by NBu4F using 430 and 450 nm excitation energy,
respectively, the binding constant fitting data for BNbpy, Pt-2, and
Pt-2a with fluorides, HOMO and LUMO diagrams of Pt-1a,
complete crystal structural data of Pt-1a. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC9000335
Acknowledgment. We thank the Natural Sciences and
(13) Frisch, M. J. et al. Gaussian 03, Revision C.02; Gaussian, Inc.:
Engineering Research Council of Canada for financial
Wallingford, CT, 2004.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3767