Probing the electronic structure of platinum(II) chromophores: Crystal structures, NMR structures, and photophysical properties of six new bis- and di- phenolate/thiolate Pt(II)diimine chromophores

Article abstract of DOI:10.1021/ic051733t

A general route for synthesis of six structurally similar Pt(II) diimine thiolate/phenolates chromophores possessing bulky phenolate or thiolate ligands is reported. The Pt chromophores were characterized using an array of techniques including ¹H, ¹³C, and ¹⁹⁵Pt NMR, absorption, emission, (spectro)electrochemistry, and EPR spectroscopy. Systematic variation of the electronic structure of the Pt(II) chromophores studied was achieved by (i) changing solvent polarity; (ii) substituting oxygen for sulfur in the donor ligand; (iii) alternating donor ligands from bis- to di-coordination; and (iv) changing the electron donating/withdrawing properties of the ligand(s). The lowest excited state in these new chromophores was assigned to a [charge-transfer-to-diimine] transition from the HOMO of mixed Pt/S (or Pt/O) character on the basis of absorption and emission spectroscopy, UV/vis (spectro)electrochemistry, and EPR spectroscopy. One of the chromophores, Pt(dpphen)(3,5-di-tert-butyl-catecholate) represents an example of a Pt-(II) diimine phenolate chromophore that possesses a reversible oxidation centered predominantly on the donor ligand. Results from EPR spectroscopy indicate participation of the Pt(II) orbitals in the HOMO. There is a dramatic difference in the photophysical properties of carborane complexes compared to other mixed-ligand Pt(II) compounds, which includes room-temperature emission and photostability. The charge-transfer character of the lowest excited state in this series of chromophores is maintained throughout. Moreover, the absorption and emission energies and the redox properties of the excited state can be significantly tuned.

Full text of DOI:10.1021/ic051733t

Inorg. Chem. 2006, 45, 4544−4555
Probing the Electronic Structure of Platinum(II) Chromophores: Crystal
Structures, NMR Structures, and Photophysical Properties of Six New
Bis- and Di- Phenolate/Thiolate Pt(II)Diimine Chromophores
+
Julia A. Weinstein,*,†,‡,§ Mark T. Tierney,£, E. Stephen Davies,§ Karel Base,£,¶
Anthony A. Robeiro,| and Mark W. Grinstaff*,#
Department of Chemistry, The UniVersity of Sheffield, Sheffield S3 7HF, United Kingdom,
Department of Chemistry, Moscow LomonosoV State UniVersity, 119899, Moscow, Russia,
School of Chemistry, UniVersity of Nottingham, Nottingham NG7 2RD, United Kingdom,
Duke UniVersity, Durham, North Carolina 27708, Duke NMR Spectroscopy Center and
Department of Radiology, Duke UniVersity, Durham, North Carolina 27708, and
Departments of Chemistry and Biomedical Engineering, Metcalf Center for Science and
Engineering, Boston UniVersity, Boston, Massachusetts 02215
Received October 6, 2005
A general route for synthesis of six structurally similar Pt(II) diimine thiolate/phenolates chromophores possessing
bulky phenolate or thiolate ligands is reported. The Pt chromophores were characterized using an array of techniques
including 1H, 13C, and 195Pt NMR, absorption, emission, (spectro)electrochemistry, and EPR spectroscopy. Systematic
variation of the electronic structure of the Pt(II) chromophores studied was achieved by (i) changing solvent polarity;
(ii) substituting oxygen for sulfur in the donor ligand; (iii) alternating donor ligands from bis- to di-coordination; and
(iv) changing the electron donating/withdrawing properties of the ligand(s). The lowest excited state in these new
chromophores was assigned to a [charge-transfer-to-diimine] transition from the HOMO of mixed Pt/S (or Pt/O)
character on the basis of absorption and emission spectroscopy, UV/vis (spectro)electrochemistry, and EPR
spectroscopy. One of the chromophores, Pt(dpphen)(3,5-di-tert-butyl-catecholate) represents an example of a Pt-
(II) diimine phenolate chromophore that possesses a reversible oxidation centered predominantly on the donor
ligand. Results from EPR spectroscopy indicate participation of the Pt(II) orbitals in the HOMO. There is a dramatic
difference in the photophysical properties of carborane complexes compared to other mixed-ligand Pt(II) compounds,
which includes room-temperature emission and photostability. The charge-transfer character of the lowest excited
state in this series of chromophores is maintained throughout. Moreover, the absorption and emission energies
and the redox properties of the excited state can be significantly tuned.
Introduction
had concentrated primarily on d6 diimine complexes, with
Ru(bpy)3 and its derivatives being the most commonly
studied.5,9-11 The observation by McMillin12 and Eisenberg13
2+
New transition metal diimine complexes are actively
pursued as potential photocatalysts, artificial photosynthetic
receptors, photoluminescent nucleic acid probes, and mo-
lecular photonic devices.1-37 Previous reports in this area
(1) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949.
(2) Weinstein, J. A.; Blake, A. J.; Davies, E. S.; Davis, A. L.; George,
M. W.; Grills, D. C.; Lileev, I. V.; Maksimov, A. M.; Matousek, P.;
Mel′nikov, M. Y.; Parker, A. W.; Platonov, V. E.; Towrie, M.; Wilson,
C.; Zheligovskaya, N. N. Inorg. Chem. 2003, 42, 7077.
* To whom correspondence should be addressed. E-mail: mgrin@bu.edu
(M.W.G.); Julia.Weinstein@sheffield.ac.uk (J.A.W.).
† The University of Sheffield.
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‡ Moscow Lomonosov State University.
§ University of Nottingham.
£ Duke University.
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| Duke NMR Spectroscopy Center and Department of Radiology, Duke
University.
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(8) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects
in Molecules and Polymers; Wiley-Interscience: New York, 1991.
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ReV. 1996, 96, 759.
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C.; Balzani, V. Chem. ReV. 1994, 94, 993.
# Boston University.
+ Present address: Ohio University. Department of Chemistry and
Biochemistry, Athens, OH 45701.
¶ Present address: Duke University Medical Center, Department of
Radiology, Durham, NC 27710.
4544 Inorganic Chemistry, Vol. 45, No. 11, 2006
10.1021/ic051733t CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/06/2006

Probing the Electronic Structure of Platinum(II) Chromophores
of fluid-solution photoluminescence from Cu(I) bis-diimines
and Pt(II) diimine dithiolates has led to an interest in tuning
and optimizing the electronic structure and photophysical
properties of Cu(I) and Pt(II) complexes for specific
applications.1,13-25 Coordinately unsaturated, photolumines-
cent d8 metal chromophores, unlike their d6 counterparts,
possess open coordination sites for subsequent chemical
reactions such as self-quenching and cross-quenching,15,26-29
photoreactivity,30 and photocatalysis.31 Pt(II) chromophores
have also been explored as tunable materials for nonlinear
optical,32,33 light-emitting diode,34 and solar cells35 applica-
tions, as well as for sensitization of singlet oxygen.26,36,37
However, mixed-ligand complexes of Pt(II) with diimine
or triimine ligands are often nonemissive in solution at room
temperature, largely due to the presence of a population of
metal-centered (d-d) states close in energy, which leads to
efficient nonradiative deactivation. In recent years, two major
strategies have emerged which allowed for the development
of emissive Pt(II) complexes with diimine or triimine ligands.
The first strategy relies on raising the energy of the d-d
state(s) via introduction of very strong ligand field groups,
such as acetylide34,38-55 or cyanide56 co-ligands, while retain-
ing the predominantly ligand-to-ligand charge-transfer char-
acter of the emissive excited state. Alternatively, high
emission quantum yields and long lifetimes can be achieved
by changing the nature of the lowest excited state from
MLCT/LLCT to intraligand charge-transfer (ILCT) or π-π*
via modification of the terpyridine ligand in [Pt(tpy)Cl]+.57-62
The number of square-planar Pt(II) diimine thiolate/
phenolate complexes reported that luminesce in solution is
limited mainly to those containing chelating dithiolate ligands
such as 1,2-dithiolate1,13,20,22,23,25,63-67 or 1,1-dithiolates.68
There are even fewer reports of bis-monothiolate ligands in
the literature.2,32,33,69-71 The poor solubility of most Pt(II)
diimine thiolate chromophores (with the exception of some
1,1-dithiolates68) in common solvents hinders solution-phase
studies and characterization of their electronic properties. We
have synthesized Pt(II) chromophores possessing bulky
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Inorganic Chemistry, Vol. 45, No. 11, 2006 4545

Weinstein et al.
Scheme 1. Synthesis of Pt(II)
4,7-diphenyl-1,10-phenanthroline-thiolates and Phenolates, 1-6
phenolate or thiolate ligands as a salient feature. This imparts
solubility over a wide range of solvent polarities, enabling
one to systematically probe the electronic structure. Our
strategy is also amenable to determining the affect of donor
atom (S vs O), ligand coordination (di- vs bis-mono), and
electron donating/withdrawing properties of the ligand on
the electronic structure of square-planar d8 chromophores.
Specifically, we are exploiting two routes to increase the
solubility of Pt(II) chromophores while retaining structural
homology about the platinum center. First, bulky tert-butyl
groups are introduced to the periphery of the phenolate/
thiolate ligands to prevent molecular stacking from either
metal-to-metal or ligand-to-ligand interactions, a reoccurring
observation with platinum(II) diimine complexes.15,19,28,72,73
Bulky functional groups such as tert-butyl, isopropyl, or
phenyl-tert-butyl have been used successfully to prevent self-
association or dimerization of metal complexes designed for
enhanced catalysis and luminescence.12,19,74 The second route
capitalizes on the large 3-dimensional structure of thiocar-
boranes. The bulky electron-withdrawing closo-boranes are
particularly suited for this use since these structures possess
high chemical and thermal stability, and once bound to a
metal center, thiocarboranes are expected to act as strong
field ligands, thus opening a possibility for enhanced
emission properties in solution.
Herein we report a series of structurally similar Pt(II)
diimine chromophores 1-6 (Scheme 1), characterized by
X-ray crystallography and multinuclear (1H, 13C, and 195Pt)
NMR, as well as absorption, emission, (spectro)electrochem-
istry, and EPR spectroscopy. The electronic structure of these
chromophores was altered by varying the (i) oxygen and
sulfur donor ligands; (ii) electron-donating properties of the
thiolate/phenolate ligands; and (iii) bis- and di-coordination
modes. The unique reversible nature of the oxidation process
of Pt(dpphen)(3,5-di-tert-butyl-cathecolate), 5, has allowed
for a direct probing of Pt(II) orbital contribution into the
HOMO of this chromophore by EPR spectroscopy. Absorp-
tion, emission, UV/vis (spectro)electrochemical studies have
revealed the same nature of the lowest excited state in this
series of chromophores while allowing for significant tuning
of absorption and emission energies and redox properties in
the excited state.
yl-1,10-phenanthroline 3,5-di-tert-butylbenzene-1,2-dithiolate
platinum(II), [Pt(dpphen)(dtbdt)], 1; 4,7-diphenyl-1,10-
phenanthroline bis-4-tert-butylbenzenethiolate platinum(II),
[Pt(dpphen)(tbt)2], 2; 4,7-diphenyl-1,10-phenanthroline-[1,2-
dithiolato-1,2-dicarba-closo-dodecaborane(12)] platinum(II),
[Pt(dpphen)(dtoc)], 3; 4,7-diphenyl-1,10-phenanthroline-bis-
[1-thiolato-1,2-dicarba-closo-dodecaborane(12)] platinum(II),
[Pt(dpphen)(toc)2], 4; 4,7-diphenyl-1,10-phenanthroline 3,5-
di-tert-butylbenzene-1,2-catecholate platinum(II), [Pt(dp-
phen)(dtbc)], 5; and 4,7-diphenyl-1,10-phenanthroline bis-
4-tert-butylphenolate platinum(II) [Pt(dpphen)(tbp)2], 6. The
phenolate and catecholate ligands, 4-tert-butyl-phenolate (13)
and 3,5-di-tert-butyl-catecholate (12) described are com-
mercially available. The thiolate ligands 3,5-di-tert-butyl-
benzene dithiolate (8) and 4-tert-butyl-thiophenol (9) were
synthesized in accordance with their respective literature
procedures.75,76 The synthetic procedure for carboranes 10
and 11 are reported elsewhere.77
Results and Discussion
X-ray Crystal Structure of Pt(dpphen)(dtbdt), 1. Slow
evaporation of a benzene solution of 1 gave dark blue crystals
(0.40 × 0.08 × 0.01 mm3) suitable for X-ray analysis. The
crystal-structure data for 1, along with the data for the
monodentate analogue, 4, reported previously69 and included
for comparison, are summarized in Table 1. The ORTEPII
thermal ellipsoid plot of 1, drawn at 50% probability, is
shown in Figure 1. The key bond lengths and angles around
the central platinum atom in 1 and 4 are displayed in Figure
2. The geometry about the central platinum metal in both 1
Synthesis. The general synthetic route to the sterically
crowded Pt(II) diimine chromophores containing both oxygen
or sulfur donor ligands is shown in Scheme 1. Treatment of
the phenolate or thiolate ligand with base in the presence of
Pt(dpphen)Cl2 affords the following complexes: 4,7-diphen-
(69) Base, K.; Grinstaff, M. W. Inorg. Chem. 1998, 37, 1432.
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Mel′nikov, M. Y. Russ. J. Inorg. Chem. (Transl. of Zh. Neorg. Khim.)
1998, 43, 1361.
(71) Weinstein, J. A.; Zheligovskaya, N. N.; Mel′nikov, M. Y.; Hartl, F.
J. Chem. Soc., Dalton Trans. 1998, 2459.
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369.
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(74) McMillin, D. R.; Kirchhoff, J. R.; Goodwin, K. V. Coord. Chem. ReV.
1985, 64, 83.
(75) Sellmann, D.; Prechtel, W.; Knock, F.; Moll, M. Z. Naturforsch. 1992,
47b, 1411.
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1013.
4546 Inorganic Chemistry, Vol. 45, No. 11, 2006

Probing the Electronic Structure of Platinum(II) Chromophores
Figure 2. The key bond lengths and angles around the central platinum
atom in Pt(dpphen)(dtbdt), 1, and Pt(dpphen)(toc)2, 4, complexes.
Figure 1. a. An ORTEPII diagram (50% probability ellipsoids) of Pt-
(dpphen)(dtbdt), 1, showing the atomic numbering system. H atoms are
not shown. b. An ORTEPII diagram (50% probability ellipsoids) of Pt-
(dpphen)(toc)2, 4. H atoms are not shown.69
Table 1. X-ray Crystallographic Data for 1 and 4
compound
1
4
formula
PtC48.5H46.5N2S2
919.62
dark blue
plates
PtC28H40B20N2S2Cl2
960.94
Yellow
fw (g mol-1
color
)
Figure 3. An ORTEPII diagram (50% probability ellipsoids) of Pt-
(dpphen)(dtbdt), 1, showing the upward bend in the molecule about the
two sulfur atoms. H atoms are not shown.
shape
plates
cryst syst
space group
a, (Å)
b, (Å)
c, (Å)
monoclinic
C2/c
monoclinic
P21/a
36.080(14)
14.323(9)
36.672(17)
112.063(1)
17563(15)
1.387
16
3.31
173
29 487
16.9705(8)
12.9105(6)
18.9109(9)
99.5730(10)
4085.6(3)
1.526
4
3.68
173
21 213
previously for other Pt(II) dithiolates which feature five-
membered [C-S-Pt-S-C] rings. For example, the dihedral
angle between the [NN-Pt-SS] plane and the [S-C-C-
S] plane of the thiolate ligand is 3° for (4,4′-dimethyl-2,2′-
bpy)(1,2-dimethoxycarbonylethylene-1,1-dithiolato)Pt, 14 4.5°
for Pt(bpy)(benzene-1,2-dithiol),26 and 7° for (bpy)(6,7-
dihydro-5H-1,4-dithiepin-2,3-dithiolato)Pt.78
As summarized in Figure 2, the Pt-S and Pt-N diimine
bond lengths in Pt(dpphen)(dtbdt) are 2.251(4), 2.244(4),
2.066(9), and 2.055(10) Å, respectively, and in the range of
bond lengths typical for other Pt diimine dithiolate struc-
tures.14,78 In the Pt(II) bis-monothiolate complex 4, the Pt-S
carborane and Pt-N diimine bond lengths are slightly
longer: 2.313(3), 2.286(2), 2.082(8), and 2.078(7) Å,
respectively. The Pt-S distances of Pt(dpphen)(toc)2 are in
agreement with previously reported bis-monothiolate struc-
â (deg)
V, (Å3)
F
calcd (g cm-3
)
Z
λ, Å
temp (K)
total no. of reflns
measured
no. of unique reflns
no. of reflns with
Inet > 2.5F(Inet
GOF
R
14 492
8244
7240
5470
)
1.50
0.053
0.058
1.46
0.058
0.077
Rw
and 4 is square planar with very little torsional deformation.
The structure of 1 reveals an asymmetric curvature of the
molecule with regard to the plane formed by the metal center
and chelating atoms. As shown in Figure 3, there is an up-
ward bend in the molecule about the two sulfur atoms. This
most likely arises from the slightly strained five-membered-
ring structure formed. The same observation has been made
(78) Zuo, J. L.; Xiong, R. G.; You, X. Z.; Huang, X. Y. Inorg. Chim. Acta
1995, 237, 177.
Inorganic Chemistry, Vol. 45, No. 11, 2006 4547

Weinstein et al.
Table 2. 1H NMR Assignments of Pt(dpphen)D Complexesa
3 (dtoc)b
δ, (mult, J/Hz)
4 (toc)2
5 (dtbc)
δ, (mult, J/Hz)
6 (tbp)2
δ, (mult, J/Hz)
b,c
complex
D ) position
1 (dtbdt)
δ, (mult, J/Hz)
2 (tbt)2
δ, (mult, J/Hz)
δ, (mult, J/Hz)
9
2
8
3
9.41 (d, 5.5)
9.31 (d, 5.5)
7.62 (d)
10.13 (d,5.5)
8.97
(d, 5.5)
7.78
(d, 5.5)
8.07 (s)
10.21(d,5.5)
9.46 (d, 5.5)
9.41 (d, 5.5)
7.68 (d, 5.5)
7.66 (d, 5.5)
7.96 (s)
9.33
(d, 5.5)
7.80
(d, 5.5)
8.05 (s)
7.82
(d, 5.5)
8.06 (s)
8.08 (d, 5.5)
8.13 (s)
7.65 (d)
5, 6
7.99 (m)
7.96 (m)
7.46 (m)
7.53 (m)
7.51 (m)
7.57 (m)
7.55 (m)
2′,6′ 2′′,6′′
3′,5′ 3′′,5′′
7.61 (m)
7.61 (m)
∼7.59 (m]
∼7.61 [m]
∼7.60 [m]
∼7.65 (m)
∼7.65 (m)
∼7.65 (m)
∼7.58 (m)
∼7.58 (m)
∼7.59 (m)
∼7.61 (m)
∼7.61 (m)
4′, 4′′
7.61 (m)
7.57
∼7.61 (m)
D2, D6d
7.22
(d, 8.4)
7.00
(d, 8.4)
(d, 8.8)
7.07
(d, 8.8)
D3e
6.93 (m)
6.48 (m)
D5
7.23 (m)
1.35 (s)
1.67 (s)
6.68 (m)
1.33 (s)
1.55 (s)
4tBu Me
2tBu Me
1.23 (s)
1.25 (s)
b
a 1H NMR chemical shift relative to internal TMS taken from 600 MHz NMR data at 25 °C. B10H10 protons appear as broad signal near 2.53 ppm in
3 and 2.50 ppm in 4. c Carborane CH appears at 4.65 ppm. d In symmetrical complexes 2 and 6, the D-2′/D-6′ signals are isochronous with D-2 and D-6.
e In symmetrical complexes 2 and 6, the D-5, D-3′, and D-5′ signals are isochronous with D-3.
tures, Pt(dbbpy)(S-4-py)2,79 and Pt(bpy)(4-CN-C6F4-S)2
(2.282(3)/2.296(3) Å)2 but are longer than those reported for
the dithiolate14,80 systems, indicating less covalent character
of the Pt-S bond in the former case. However, the slight
lengthening of the Ccarborane-thiol bond from 1.729(4) Å in
carborane-thiolate81 to 1.806(10) Å in Pt(dpphen)(toc)2
indicates good conjugation in the Pt-S-Ccarborane system.
Structural studies on o-carborane and its derivatives show
that skeletal C-C bonds in the closo icosahedron are in the
range of 1.63-1.74 Å for the neutral trisulfide 1,1′-S3-(2-
C6H5-1,2-C2B10H10). The skeletal C-C bonds in Pt(dpphen)-
(toc)2 are in the range of a closo icosahedron (i.e., 1.606(13)
and 1.637(14)Å for C(51)-C(52) and C(71)-C(72), respec-
tively), whereas the parent [1-S-2-C6H5-1,2-C2B10H11]- anion
has a skeletal C-C distance of 1.835 Å.
Figure 4. Schematic representation of (left panel) Pt(II) complexes 2 and
6 with the symmetry axis indicated and (right panel) complexes 1 and 5
with asymmetric tBu groups on the donor ring. The atom numbering scheme
for the NMR assignment are as indicated. “D” represents atoms from the
donor thiolate or catecholate rings.
Both Pt chromophores are not closely packed in the unit
cell, and this is a likely consequence of the bulky tert-butyl
groups or carborane cage. There are 16 molecules in the large
unit cell of 1 (see the Supporting Information for detail),
while there are only four in the unit cell of 4.69 The lack of
molecular stacking observed in the crystal structure is
noteworthy since Pt(II) complexes are well-known to stack
in the solid state, with short Pt-Pt or π-to-π (3.24 and 3.51
Å, respectively) distances.80,82,83 The Pt-Pt distances for 1
and 4 are 4.25 and 8.48 Å, respectively, clearly indicating
that the bulky substituents on the ligands are preventing
stacking of the chromophores in the crystal.
by the nature of the donor ligand (designated D). Complexes
2-4 and 6 are symmetric about an axis which bisects the
phenanthroline ring, the platinum atom, and the thiolate (or
phenolate) ligand(s). The proton assignments are summarized
in Table 2, and a complete description of the analysis is found
in the Supporting Information. Of particular note is the proton
spectra obtained from chromophores 1 and 5. The lack of
symmetry for 1 and 5 results in distinct environments for
H-2, H-9, H-3, H-8, H-5, and H-6 (Table 2) and their
corresponding carbon atoms (Figure 5 and Supporting
Information).
NMR Spectroscopy. To gain insight into the structures
Lack of symmetry in chromophores 1 and 5 also leads to
separate 13C NMR signals from the “top” and “bottom”
portions of the phenanthroline ring (Figure 5). Steady-state
1-dimensional (1D) NOE experiments on chromophore 5 at
1
of all six Pt(II) chromophores, 1-6, in solution, H, 13C,
and 195Pt NMR studies84-90 have been performed (see the
Supporting Information for complete assignment of the
resonances). These six Pt(II) complexes possess a common
substituted phenanthroline acceptor group (dpphen) and differ
(84) Bax, A.; Freeman, R.; Morris, G. A. J. Magn. Reson. 1981, 42, 164.
(85) Bax, A.; Subramanian, S. J. Magn. Reson. 1986, 67, 656.
(86) Bax, A.; Summers, M. F. J. Am. Chem. Soc. 1986, 108, 2093.
(87) Garbow, J. R.; Wietekamp, D. P.; Pines, A. Chem. Phys. Lett. 1982,
93, 504.
(79) Paw, W.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 1998, 37, 4139.
(80) Connick, W. B.; Marsh, R. E.; Schaefer, W. P.; Gray, H., B. Inorg.
Chem. 1997, 36, 913.
(81) Coult, R.; Fox, M. A.; Gill, W. R.; Wade, K. Polyhedron 1992, 11,
2717.
(82) Connick, W. B.; Henling, L. M.; Marsh, R. E.; Gray, H. B. Inorg.
Chem. 1996, 35, 6261.
(88) Pregosin, P. S. Coord. Chem. ReV. 1982, 44, 247.
(89) Shaka, A. J.; Barker, P. S.; Freeman, R. J. Magn. Reson. 1985, 64,
547.
(90) Summers, M. F.; Marzilli, L. G.; Bax, A. J. Am. Chem. Soc. 1986,
108, 4285.
(83) Osborn, R. S.; Rogers, D. J. Chem. Soc., Dalton. Trans. 1974, 1002.
4548 Inorganic Chemistry, Vol. 45, No. 11, 2006

Probing the Electronic Structure of Platinum(II) Chromophores
Figure 7. Electronic absorption spectra of 1-6 in CHCl3 at room
temperature.
NOE and the D-3 proton is broadened from steric collisions
with both tBu methyl groups.
The 195Pt NMR data are shown in Table 4. The shift in
the 195Pt resonance to less negative values from compound
1 to compound 6 reflects an increase in the electron-
withdrawing ability of the thiolate/phenolate ligand along
this row. As will be discussed below, the trend in the 195Pt
resonances also correlates with the trend in oxidation peak
potentials of 1-6. Of particular note is the 300 ppm upfield
shift in the 195Pt NMR signal of complex 3 compared to that
of complex 4. This shift gives evidence for the 195Pt atom to
be more electronically shielded in 4 likely due to a better
overlap of platinum d orbitals with sulfur p orbitals.
However, the H-2, H-9 and H-3, H-8 NMR signals of the
phenanthroline ring of complex 4 are shifted to lower field
compared to those of complex 3, showing that there is
aconcomitant decrease in the electronic shielding in the
phenanthroline ring with the increase in electronic shielding
at the Pt atom. We believe that the differences in shielding
of phenanthroline hydrogens can be explained by the
enhanced electron-withdrawing effect of two o-carborane
cages in 4.
Figure 5. Partial 150 MHz 13C NMR spectra at 25 °C showing aromatic
13C NMR resonances of thiolate Pt complexes 2 (top panel) and 1 (bottom
panel). Due to the axis of symmetry in complex 2, the 36 aromatic carbons
are simplified to 14 carbon resonances. Lack of symmetry in complex 1
leads to separate phenanthroline C-2/C-9, C-3/C-8, and C-5/C-6 carbon
signals. The six carbons of the donor dithiolate ring yield six broad and
low-amplitude 13C signals (range of line widths 4-15 Hz for D-1 to D-6).
The D-1 and D-6 signals in complex 5 have similar line widths to the D-1
and D-6 signals of 1 (7-8 Hz), but the D-2 to D-5 resonances are
considerably broader (30-40 Hz).
Visible Absorption Spectroscopy. Solutions of complexes
1-6 exhibit an intense absorption at 290 nm due to the π-to-
π* transition of diphenylphenanthroline and a second
absorption at longer wavelength with extinction coefficient
values of the order of 103 dm3 mol-1 cm-1 (Table 3, Figure
7). The energy and the extinction coefficient of this visible
absorption band depends on the electron withdrawing/
donating ability of the ligand, the coordination geometry (di
vs bis ligation), and the nature of donor atom, as shown in
Table 3. The energy of the transition increases on going from
electron donating tert-butyl derivatives to the strongly
electron-withdrawing carboranes (e.g, 1 to 3 or 2 to 4). In
all cases, the absorption shifts to lower energy from bis- to
di-coordination. Compound 4 possesses the highest-energy
visible absorption band, which is partly obscured by π-to-
π* transition of the diphenyl-phenanthroline, and 5 possesses
the lowest-energy absorption. These observations demon-
strate thiolate/phenolate ligand orbital participation in the
occupied molecular orbital responsible for this electronic
transition. Within the same structural motif (5 vs 1), changing
from oxygen to sulfur slightly (550 cm-1) shifts the absorp-
tion to higher energy.
Figure 6. 500 MHz NMR spectra of Pt(II) complex 5 at 25 °C. (A-B)
difference NOE spectra: (A) irradiation of the 2-tBu methyl gives selective
NOE enhancements at the D-3 proton of the donor phenolate ring and one
proton of the acceptor phenanthroline ring which corresponds to H-9 in
the numbering scheme used. (B) Excitation of the 4-tBu methyl reveals
enhancements at D-5 and D-3 of the donor phenolate ring. (C) 1H NMR
spectrum.
25 °C (Figure 6) show lack of a mutual NOE between the
2-tBu and 4-tBu methyl groups, implying that those are
oriented away from each other and possibly located on
opposite faces of the donor ring resulting from steric
hindrance. Taking into account the generally larger 13C line
widths of D-2 to D-5 compared to D-1 and D-6 (attached to
S- or O-), the specific line broadening of D-3, and the
observation of a specific NOE indicating proximity of the
donor 2-tBu group to the acceptor H-9 phenanthroline proton,
the overall NMR data suggest a working model with distorted
or bent donor ring where a 2-tBu group on the “top” face
approaches H-9 of the phenanthroline ring to yield a distinct
The extinction coefficients for the lowest absorption band
are much higher for the complexes with bidentate thiolate
ligands than for those with the monodentate ligands (8000-
Inorganic Chemistry, Vol. 45, No. 11, 2006 4549

Weinstein et al.
voltammetry and UV/vis (spectro)electrochemistry to elu-
cidate the nature of the frontier orbitals in these chro-
mophores. The redox potentials of compounds 1-6 are
summarized in Table 4. Representative cyclic voltammo-
grams are shown in Figure 9 for 2, 5, and 3. The absorption
maxima of the electrochemically generated radical anions
of compounds 1-6 and Pt(dpphen)Cl2 and that of the radical
cation of 5 registered in DMF at 273 K are summarized in
Table 5.
Reduction. All the complexes investigated exhibit a
reversible one-electron first reduction process in the region
between -1.49 and -1.68 V. The complexes with bidentate
thiolate or phenolate ligands display the half-wave potential
of the first reduction at ca. 60 mV more negative than their
monodentate analogues (5 vs 6 and 1 vs 2). There is only a
small (0.02 V) negative shift in the first reduction potential
when an S-containing ligand is replaced by an O-containing
analogue (1 vs 5, 2 vs 6). Thus, neither the mode of
coordination (bis vs di), nor the type of donor atom (S vs
O), nor the electron withdrawing/donating properties of the
ligand substantially effect the magnitude of the first reduction
potential of the complexes, which is close to that of the first
reduction potential of Pt(dpphen)Cl2 (-1.55 V). We therefore
conclude that the first reduction is predominantly localized
on the diimine ligand and that the LUMO is of mainly
diimine character. The small gradual positive shift in the
reduction potential from 1 to the complex containing two
thiocarboranes (4) is due to a decrease in the electron-
donating capability of the thiolate/phenolate ligand, which
increases diimine-to-platinum σ-donation.
Figure 8. Electronic absorption spectra of 2 in acetonitrile (thick solid
line), CHCl3 (dashed line), and benzene (thin solid line). The plot of
absorption energy (in cm-1) vs solvent parameter1 is shown in the insert.
11000 vs 2250-4000 L mol-1 cm-1). This is likely due to
better mixing with the metal d orbitals for the former.
Interestingly, solutions of 3 and 4 in DMF, CHCl3, CH2Cl2,
and C6H6 are stable to irradiation into the long-wavelength
absorption band which can be contrasted with the previously
reported reactivity of Pt(diimine)(dithiolate) complexes such
as Pt(bpy)(tdt).65,91 The lowest-energy absorption bands in
1-6 show negative solvatochromic behavior, that is, the band
position shifts to lower energies with an increase of solvent
polarity. Such behavior is indicative of a significant differ-
ence between the ground- and excited-state dipole moments
and is typical for MLCT/LLCT transitions.92,93 A representa-
tive example of the solvatochromic behavior of these
chromophores is shown in Figure 8 for 2.
A linear correlation has been observed between the energy
of this lowest absorption band and the solvent polarity
parameter value based on the energy of the [charge-transfer-
to-diimine] transition in Pt(dbbpy)(tdt) (dbbpy ) 4,4′-di-tert-
butyl-2,2′-bipyridine, tdt ) toluene-3,4-dithiolate).1 The slope
of the linear correlation (Figure 8) gives a so-called solva-
tochromic shift (Table 3); the values obtained for 1-6 are
within the range observed for charge-transfer transitions. The
pronounced solvatochromism, as well as thiolate/phenolate
ligand effect on the energy of the lowest absorption band,
indicates the charge-transfer character of the corresponding
electronic transition.
Most of the compounds studied also exhibit a shoulder at
3000-3900 cm-1 lower in energy than the main transition
(Table 3), which is characterized by an oscillator strength
several times smaller than that of the main charge-transfer
band discussed above. It has been shown for other Pt(II)
compounds that this singlet-triplet transition can be of
considerable intensity due to the presence of the heavy atom
effect induced by the Pt(II) center.36 We hence tentatively
assign the low-energy absorbance to a singlet-triple satellite
to singlet-singlet charge-transfer transition described above.
(Spectro)electrochemistry. The redox behavior of the PtII
4,7-diphenyl-1,10-phenanthroline complexes with thiolate
and phenolate ligands, 1-6, was studied in DMF by cyclic
The second reduction process, detected in the region from
-2.05 to -2.28 V, is virtually insensitive to the nature of
the donor ligand and very similar to the second reduction
process for Pt(dpphen)Cl2 (-2.17 V). Hence, it is proposed
that the second cathodic step is also localized on the diimine
ligand.
To obtain better insight in the nature of the LUMO,
(spectro)electrochemical experiments were performed for
1-6 and Pt(dpphen)Cl2. For all the compounds investigated,
the first reduction process is accompanied by a gradual
change in the absorption spectrum, with the formation of
new bands across the visible region (Table 5). As a typical
example, the spectral changes accompanying the first reduc-
tion process of 4 are shown in Figure 10. The position and
extinction coefficients of these bands remain virtually
unaffected by the nature of the thiolate/phenolate ligand: the
spectral profiles of all the [Pt(dpphen)(RS/RO)]-• are re-
markably similar to each other (Table 5, Figure 10, insert),
and are comparable to that of [Pt(dpphen)Cl2]-•. This
establishes that the first reduction process is directed to the
π1*(dpphen), thus supporting the assignment of LUMO as
predominately diimine-based for 1-6.
Oxidation. So far, direct spectroscopic probing of the
orbital parentage of the HOMO in Pt(II) diimine thiolate/
phenolates has not been possible due to the chemical
irreversibility of the oxidation process. In line with this fact,
all the chromophores studied except the catecholate 5 display
an irreversible oxidation wave in DMF solution at 298 K.
(91) Williams, R.; Billig, E.; Waters, J. H.; Gray, H. B. J. Am. Chem. Soc.
1966, 88, 43.
(92) Lever, A. B. P. Inorganic electronic spectroscopy, 2nd ed.; Elsevier:
Amsterdam; Oxford, 1984.
(93) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1986, 25, 3212.
4550 Inorganic Chemistry, Vol. 45, No. 11, 2006

Probing the Electronic Structure of Platinum(II) Chromophores
Table 3. Absorption and Emission Data for the Platinum Chromophores 1-6
abs
em
platinum chromophore
λmax
ꢀ
solvatochromic
λmax
a
(CHCl3 at 25 °C)
(nm)
(mol-1 L cm-1
)
shift, (eV-1
0.42
)
(nm)c
710
æb
1 Pt(dpphen)(dtbdt)
2 Pt(dpphen)(tbdt)2
3 Pt(dpphen)(dtoc)
4 Pt(dpphen)(toc)2
634
8300
1400
4000
1450
8080
1400
3460
940
2.2 × 10-3
1.1 × 10-3
ca.800 sh
510
ca.625 sh
450
ca.520 sh
414
0.35
0.34
660
550
[640]
530
0.13
[7 × 10-6
]
0.09
0.25
0.46
0.26
0.05
475
657
470
[635]
[1.6 × 10-4
3.2 × 10-2
]
d
5 Pt(dpphen)(dtbc)
6 Pt(dpphen)(tbp)2
11 000
2250
750
556
ca. 575
a A gradient of the plot of νmax (eV) vs solvent parameter1 in 2-MeTHF, 77 K. b (20%, measured relative to [Ru(bpy)3]Cl2 (æ ) 0.376 at 77 K in
MeOH glass94). c Values in square brackets correspond to studies at 298 K in aerated CH2Cl2, æ measured relative to [Ru(bpy)3]Cl2 (æ ) 0.028 in aerated
H2O). d No emission was observed between 500 and 800 nm.
abs
Table 4. Electrochemical Data for the Platinum Chromophores 1-6
∆E (Fc/Fc+) mV
δ
195Pt
∆Eredox, V
Eemis, 77 K, cm-1 (eV)
0/-
-/2-
a
metal chromophore
E1/2
E1/2
Ep (ox)
b
5 Pt(dpphen)(dtbc)
1 Pt(dpphen)(dtbdt)
2 Pt(dpphen)(tbt)2
6 Pt(dpphen)(tbp)2
3 Pt(dpphen)(dtoc)
-1.66 (70)
-1.68 (70)
-1.62 (70)
-1.60 (70)
-1.54 (70)
-1.61
-1.49 (70)
-1.57 (70)
-1.55 (70)
-2.28 (70)
-2.28 (70)
-2.22 (70)
-2.22 (70)
-2.14 (70)
-0.07 (70) and 0.56
70
70
70
70
70
-1992.7
-3517
1.63
1.73
1.81
2.00
2.49
0.05
13790 (1.71)
15150 (1.88)
17990 (2.23)
18180 (2.25)
0.19
0.40 and 0.73
0.95
0.99
1.05
1.09
0.89
-3444.2
-1816.7
-3437.9
-3215
c
Pt(phen)(C6F5-S)2
4 Pt(dpphen)(toc)2
4d Pt(dpphen)(toc)2
Pt(dpphen)Cl2
-2.06 (80)
-2.05 (90), qr
-2.17 (70)
70
70
70
-3142.1
2.54
2.65
20040 (2.48)
20040 (2.48)
a
a The first and second half-wave reduction potentials, E1/20/- and E1/2-/2-, oxidation potential E1/20/+ or oxidation wave, Ep , in V, vs Fc/Fc+, with anodic/
cathodic peak separation given in parentheses. ∆Eredox ) Epa - E1/20/-, Eemis ) emission maximum at 77 K is given to be compared with ∆Eredox b Emission
.
not detected. b From ref 2. c In dichloromethane.
Table 5. Electronic Absorption Spectra of the Radical Anions of PtII
4,7-Diphenyl-1,10-phenanthroline Chromophores 1-6 (at 273 K) and of
the Radical Cation of [Pt(dpphen)(dtbc)], 5, (at 243 K)
Electrochemically Generated at an OTE, in DMF Solution
metal chromophore
λmax/nm
1 [Pt(dpphen)(dtbdt)]-• 272, 283sh, 317sh, 384, 453, 501, 605, 677, 734, 817
2 [Pt(dpphen)(tbt)2]-• 289, 338sh, 426, 497, 610, 660, 716sh, 794
3 [Pt(dpphen)(dtoc)]-• 281, 308sh, 334, 369sh, 402, 472sh, 595, 643, 716, 799
4 [Pt(dpphen)(toc)2]-• 268, 295, 317, 336sh, 426, 458sh, 513, 578, 624, 692,
770, 861
5 [Pt(dpphen)(dtbc)]-• 284, 307, 365sh, 444sh, 502, 616, 664, 733, 814
6 [Pt(dpphen)(tbp)2]-• 290, 322 sh, 425, 502 sh, 604 sh, 656, 812, 792
[Pt(dpphen)Cl2]-•
325, 357, 441, 475, 591(sh), 635, 709(sh), 790
5 [Pt(dpphen)(dtbc)]+• 324sh, 372sh, 478, 608, 660sh
Figure 10. Spectral changes accompanying one-electron reduction process
of [Pt(dpphen)(toc)2], 4, in DMF at 0°C, in the presence of 0.2 M [nBu4N]-
[BF4]. Insert: Spectra of radical anions Pt(dpphen)(dtbc)2, 5-• (thick line),
and Pt(dpphen)(tbp)2, 6-• (thin line), generated electrochemically in an OTE
cell under the same conditions.
while remaining bound to the metal center.95-98 Reversible
thiolate/phenolate-based oxidation has been reported for Pd-
(2,2′-bipyridine)L (L ) 3,5-di-tert-butylbenzene-1,2-dithi-
olate or 1,2-dicathecolate).99 To the best of our knowledge,
5 represents the first example of such behavior in a Pt(II)
diimine thiolate/phenolate complex. Thus, we initiated studies
to determine the orbital parentage of the HOMO for this class
of complexes.
Figure 9. Cyclic voltamograms of 10-3 M solution of (a) [Pt(dpphen)-
(tbt)2], (2); (b) [Pt(dpphen)(dtbtc)], (5); (c) [Pt(dpphen)(dtoc)], (3). DMF,
0.2 M [nBu4N][BF4] at 293 K, glassy carbon disk electrode, 100 mV scan
rate; x axis shows potential vs Fc+/Fc couple.
Remarkably, the first oxidation of the catecholate chro-
mophore 5 is a reversible one-electron process. o-Semi-
quinone radical anions are known to form stable radical
complexes with transition metals and undergo redox reaction
(96) Piepont, C. G.; Buchanan, R. M. Coord. Chem. ReV. 1981, 38, 45.
(97) Piepont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(98) Rall, J.; Kaim, W. J. Chem. Soc., Faraday Trans. 1994, 90, 2905.
(99) Ghosh, P.; Begum, A.; Herebian, D.; Bothe, E.; Hildenbrand, K.;
Weyhermuller, T.; Weighardt, K. Angew. Chem., Int. Ed. 2003, 42,
563.
(94) Demas, J. N.; Crosby, G. A. J. Am. Chem. Soc. 1971, 93, 2481.
(95) Hartl, F.; Vlcek, A. Inorg. Chem. 1996, 35, 1257.
Inorganic Chemistry, Vol. 45, No. 11, 2006 4551

Weinstein et al.
Figure 12. EPR spectra of radical anions of Pt(dpphen)(dtbc)2, 5,
electrochemically generated in DMF at 273 K, recorded at 77 K. The system
of coordinates used is shown as an insert. Experimental (thick line) and
Figure 11. Spectral changes accompanying one-electron oxidation process
of [Pt(dpphen)(dtbc)], 5, in DMF at 243 K, in the presence of 0.2 M [n-
Bu4N][BF4].
simulated spectra (thin line) are offset for clarity. a. Radical anion, 5-•
,
modulation amplitude 10 Gs. Parameters used in simulation (A ) hyperfine
splitting constant, 10-4 cm-1; line width in parentheses, 10-4 cm-1): Axx,
-86 (11), Ayy, -64 (11); Azz, -18 (48); gyy ) 2.039, gxx 1.993, gzz 1.848.
b. Radical cation 5+•, modulation amplitude 1 Gs. Parameters used in
simulation: Axx, 56 (7), Ayy, 35 (17), Azz, 22 (11); gxx, 2.002, gyy, 2.038, gzz,
1.948.
a
The oxidation peak potential Ep (Table 4) is remarkably
sensitive to the nature and binding mode of the S- or O-
donor ligand. The Ep value shifts considerably to negative
a
potentials with increasing donor capacity of the thiolate/
phenolate ligand, providing a 1.1 V difference within the
series of the chromophores studied. For instance, compounds
3 and 4 possessing thiocarborane derivatives exhibit the most
positive oxidation potentials. A change from sulfur to oxygen
donor atom alters the electrochemical behavior significantly,
shifting the oxidation potentials by ca. 200 mV toward more
positive values. The change from mono- to bidentate binding
mode leads to a decrease of the oxidation potential. These
observations suggest that the HOMO has significant contri-
bution from the orbitals centered at the donor ligand.
The reversible nature of this first oxidation processes
observed for the catecholate chromophore 5 allowed for a
(spectro)electrochemical investigation of the nature of the
HOMO. The formation of a mono-oxidized species (Figure
11) is accompanied by the depletion of the absorption band
at 601 nm and the formation of an intense new absorption
band at ca. 478 nm. The position of this band is very close
to that reported for the semiquinone 3,5-di-tert-butylcathecol
radical coordinated to the Ni(II) center (472 nm in acetoni-
trile).100 This electronic transition has been assigned previ-
ously to a d(π)-π* MLCT transition,95 where the π* orbital
of the catecholate semiquinone ligand has 3b1 symmetry
assuming a C2V point group. These results suggest that the
oxidation is centered on the catecholate ligand, thus further
supporting the assignment of the HOMO as predominantly
phenolate/thiolate ligand based.
Pt nucleus (nuclear spin I ) 0 (66.2%) and I ) 1/2 (33.8%),
101
as manifested by the spectral shape and the values of
gxx,yy,zz obtained.
The EPR spectrum of radical anion 5-• at 77 K is a
superposition of two rhombic EPR spectra, corresponding
to the anionic species containing a Pt nucleus with I ) 0
and 1/2. The overall shape of the EPR spectrum of 5-• is
similar to those observed previously102,103 for several
[Pt(diimine)Cl2]-• complexes. The results of the simulation
of the EPR spectra are presented in Figure 12, with the
parameters used summarized in the figure caption.
It has been noted previously that in the complexes of the
type Pt(diimine)Cl2 the main contribution of the Pt center
to the SOMO of the radical anion comes from 5dyz and 6pz
orbitals.102,103 McInnes and Yellowlees102,103 described an
approach which allowed contributions from 5d and 6p Pt
orbitals in the SOMO of radical anions [Pt(diimine)Cl2]-•
to be calculated from metal hyperfine splitting constants
extracted from the EPR spectra of these species.
To extract contributions of 5dyz (a2) and 6pz (b2) orbitals
in the SOMO of 5-•, the following equations were used.102,103
Axx ) As - (4/7)Pda2 - (2/5)Ppb2; Ayy ) As + (2/7)Pda2 -
(2/5)Ppb2; Azz ) As + (2/7)Pda2 + (4/5)Ppb2. As is the
isotropic Fermi constant term. Assuming 5d8 configuration
of the central atom, the electron nuclear dipolar coupling
parameters for 5d and 6p Pt electrons, Pd and Pp, were taken
as 549 × 10-4 and 402 × 10-4 cm-1 (calculated in refs 102
and 103 on the basis of ref 101).
EPR Spectroscopy. The nature of the electrochemically
generated radical anion and radical cation of 5 was further
studied by EPR spectroscopy (Figure 12) to evaluate the
extent of Pt orbitals contribution into the frontier orbitals.
The EPR spectra of electrochemically generated 5-• and 5+•
were recorded in frozen solution at 77 K. There is a clear
difference between the EPR spectrum obtained for the radical
anion and radical cation of 5 (Figure 12), although both
spectra reveal an interaction of the unpaired electron with
We extended this treatment to the catecholate complex 5,
assuming 5d8 configuration of the central atom and that the
symmetry of the LUMO of the neutral molecule is not
(101) Rieger, P. H. J. Magn. Res. 1997, 124, 140.
(102) McInnes, E. J. L.; Farley, R. D.; Macgregor, S. A.; Taylor, K. J.;
Yellowlees, L. J.; Rowlands, C. C. J. Chem. Soc., Faraday Trans.
1998, 94, 2985.
(103) McInnes, E. J. L.; Farley, R. D.; Rowlands, C. C.; Welch, A. J.;
Rovatti, L.; Yellowlees, L. J. J. Chem. Soc., Dalton Trans. 1999,
4203.
(100) Benelli, C.; Dei, A.; Gatteschi, D.; Pardi, L. Inorg. Chem. 1989, 28,
1476.
4552 Inorganic Chemistry, Vol. 45, No. 11, 2006

Probing the Electronic Structure of Platinum(II) Chromophores
affected by the replacement of the Cl ligand with the
catecholate (b2 symmetry of π*-bpy LUMO, C2V point
group). The contribution of 5dyz and 6pz Pt orbitals in the
SOMO of 5-• obtained from the parameters of the EPR
spectrum at 77 K was 0.047 and 0.095, respectively. The
larger contribution of 6pz orbital and the overall contribution
of 14% are consistent with the data reported previously103
for the SOMO of [Pt(diimine)Cl2]-•. These data, along with
the values of g factors, strongly support Pt orbital participa-
tion into the SOMO of the radical anion of 5-• and perhaps
in the LUMO of the neutral 5.
Figure 13. Emission spectra of 1-4, and 6 in tetrahydro-2-methylfuran
at 77 K. The spectrum of Pt(dpphen)Cl2 is shown as a dotted line for
comparison (æ ) 0.07, emission lifetime ) 11 µs).
Oxidation of 5 yields the radical cation 5+•. Similar to
5-•, a characteristic rhombic EPR spectral profile is observed
in the spectrum obtained in the frozen matrix at 77 K (Figure
12). The smaller absolute values of metal hyperfine splitting,
A, and the g values obtained for the radical cation 5+• if
compared to the radical anion 5-• (Figure 12 caption) suggest
smaller contribution of the Pt orbitals into the SOMO of 5+•.
We attempted to extract the contribution of Pt orbitals into
the SOMO of 5+• more quantitatively by extending the
treatment described above for the radical anion of Pt-
(dpphen)(dtbc) to the radical cation 5+•. The symmetry of
the SOMO in the semiquinone catecholate radical is b1 if
one assumes C2V point group and axes as specified in Figure
12 (xy plane is the plane of the molecule; please see ref 104
for the detailed description of the orbitals). Thus, as for 5-•,
the 5dyz and 6pz orbitals are expected to make the most
significant contribution of the metal center into the SOMO.
This assumption allowed the assignment of the sign for all
components of A values for 5+• as opposite to those
experimentally determined previously102,103 for 5-•. The
hyperfine splitting constants obtained from simulating the
EPR spectrum of 5+• (Figure 12, caption) were used to
deduce Pt d orbitals contribution in the SOMO of 5+•. A
5d8 configuration for the metal center in 5+• was assumed
since the value of the oxidation potential of 5 and the UV/
vis spectrum of 5+• were consistent with the oxidation of
neutral 5 being primarily localized on the catecholate ligand.
The contribution of 5dyz and 6pz Pt d orbitals in the SOMO
of 5+• was determined to be -0.045 and -0.027, respec-
tively. This translates to an overall 7% Pt orbital contribution
into the delocalization of the SOMO. The positive A values,
and consequently “negative” values obtained for electron
density on Pt orbitals, reflect the deficiency105 of the spin
density on the Pt nucleus in 5+• if compared to the neutral
species.
of spin density at the Pt center and positive if there is a deficit
of electron density on Pt center. For the anisotropic spectrum,
the sign of the individual components of A shall be
determined individually, and this cannot be done from a
conventionalEPRspectrum.Fortheradicalcations[Pt(diimine)-
Cl2]-•, it has been determined previously that Axx, Ayy, and
Azz are negative. We hence assumed that Axx, Ayy, and Azz for
5-• are negative, since a replacement of a Cl ligand by a
more donating catecholate in 5 is unlikely to affect the
reduction process (see Electrochemical section). As dis-
cussed, the metal orbitals involved in redistribution of
electron density for the radical cation 5+• are likely to be
the same as for the radical anion. Then the signs of Axx, Ayy,
and Azz for the radical cation will be opposite to that for the
radical anion; i.e., Axx, Ayy, and Azz for 5+• are assumed to be
positive.
A smaller contribution of the 6pz orbital in the SOMO of
5+• if compared with the SOMO of 5-• is consistent with
the generally accepted localized character of the SOMO in
semiquinone radicals bound to the metal center. The par-
ticipation of metal orbitals in the SOMO of 5+• is very
interesting despite the presence of the absorption band at
478 nm in 5+• which matches the energy of the MLCT
transition in a valence-localized (3,5-ditertbutyl-semiqunone)-
Ni(II) complex. The SOMO in 5+• has a somewhat delocal-
ized character. This result extends to Pt(II) complexes the
previous findings on the electronic structure of Re and Mn
complexes of 3,5-ditertbutyl-semiqunone.95 In those studies,
the ancillary ligand was shown to tune the extent of electron
delocalization within the [MI(semiquinone)] chelate ring,
leading to a change of metal contribution in the SOMO,
which in turn was tuning the energy of the corresponding
MLCT transition.
The sign of the hyperfine splitting constant105 can be
derived on the basis of the expression for A ) (2µ0/3)geµBγQ.
Here, ge is the free electron g factor, µB is the Bohr magneton,
γ is the nucleus magnetogyric ratio which is characteristic
for the isotope, and Q is the spin density at the nucleus. For
an isotropic EPR spectrum, the sign of Aiso is determined by
the product of the sign of the nuclear magnetogyric ratio
and that of the spin density at the nucleus. Since γ for 195Pt
is positive, Aiso is expected to be negative if there is an excess
Emission Spectroscopy. Compounds 1-4 and 6 are
emissive at 77 K in the frozen glass matrix of 2-Me-THF
upon excitation into the lowest absorption band (Figure 13).
Emission energy is sensitive to the nature of the thiolate or
phenolate ligand and increases when the coordination
geometry changes from di to bis (e.g., 1 vs 2) or upon
introduction of a more electron-withdrawing thiolate/
phenolate ligands (1 vs 3 or 2 vs 4). Within the same
structural motif, a change from sulfur to oxygen also
increases emission energy. No emission was observed for
the catecholate complex 5. In contrast to the other Pt(II)
(104) Gordon, D. J.; Fenske, R. F. Inorg. Chem. 1982, 21, 2907.
(105) Atherton, N. M. Principles of Electron Spin Resonance; Ellis
Horwood and PTR Prentice Hall: New York, 1993.
Inorganic Chemistry, Vol. 45, No. 11, 2006 4553

Weinstein et al.
Figure 14. Plot of emission energy at 77 K (Eemis) vs the difference
between the ground-state oxidation and reduction potentials for the previous
diimine dithiolate platinum(II) complexes1 (O) and complexes 1-4 and 6
(b).
Figure 15. Correlation between anodic peak potential (V, vs Fc/Fc+) for
the thiolates 1-4 (b) and for the previously studied Pt(II) diimines with
perfluorinated thiolate ligands, featuring bipyridine (O) or phenanthroline
(]). Compound 3, Pt(dpphen)(dtoc), does not follow this trend.
diimine thiolate/phenolate compounds, the emission spectra
for Pt(II) complexes 3 and 4 at 77 K are structured. The
920 cm-1 vibrational spacing observed in the emission
spectrum of 4 roughly matches the energy of the ν(CS) in
12. This observation suggests some involvement of intra-
carborane vibrations in the excitation process and in the
process of redistribution of the excitation energy. The
emission spectrum of 3 possesses well-defined vibrational
progression of about 1350 cm-1, which corresponds to the
CdC stretch of the diimine ligand. At 77 K, the quantum
yields for Pt(II) chromophores 1, 2, and 6 are approximately
10-3, whereas the quantum yields for the thiocarborane
derivatives 3 and 4 are 100 times larger.
state are of the same origin, and hence that emission
originates from a (charge-transfer-to-diimine) excited state.
Electronic Structure. The nature of the frontier orbitals
in 1-6 has been determined by a combination of absorption,
emission, and (spectro)electrochemical techniques. The
charge-transfer nature of the lowest electronic transition is
supported by (i) the strong dependence of the absorption
energy on the electronic structure of the thiolate or phenolate
ligand; (ii) the large solvatochromism; and (iii) the correlation
between low-temperature emission energies and the differ-
ence between oxidation and reduction potentials of the
ground state. The values of the first reduction potential for
all the compounds investigated (Table 4) are virtually
independent of the donor ligand and are close to the value
of the first reduction potential of Pt(dpphen)Cl2, indicating
that the LUMO is mostly localized on the π* orbital of the
4,7-diphenyl-1,10-phenanthroline. This conclusion is further
supported by the UV/vis (spectro)electrochemistry results,
which show the formation of the diimine-localized absorben-
cies in the course of the first reversible reduction process.
The strong influence of the thiolate/phenolate ligand on the
value of the oxidation peak potential and on the absorption
energy is indicative of the HOMO being mainly centered
on the thiolate/phenolate ligand. For 5, the EPR spectroscopic
data suggest that Pt(II) orbitals participate both in the LUMO
and HOMO.
Complexes 3 and 4 are emissive in fluid solution, featuring
structureless emission spectrum which is shifted to the lower
energies when compared to the spectra at 77 K (Table 3).
Emission lifetime of 3 and 4 are 6 and 26 ns, respectively,
in aerated CH2Cl2 solution at room temperature. Thus,
compounds 3 and 4 join the family of relatively rare
examples14,15,22,25 of solution-emitting Pt(II) diimine com-
plexes with donor ligand other than acetylide. One possibility
for the enhancement of room-temperature emission is an
increase in the energy of the d-d state. Perhaps a strongly
electron-withdrawing carborane cage acts as strong-field
ligand, making the d-d state less thermally accessible, as
with Pt(II) imine acetylides34,38-55 or cyanides.56 This ob-
servation is also consistent with the higher photostability
observed for the carborane derivatives 3 and 4 in comparison
to previously known Pt(diimine)(arylthiolates).65,71 Other
factors which might account for carborane complexes being
emissive in solution include higher energy of the lowest
excited state if compared to the other chromophores studied
and vibrational progression observed in the emission spectra
which could be indicating smaller intraligand distortion upon
excitation for the carborane complexes if compared to their
non-carborane counterparts. (We are grateful to the referee
for bringing the latter two possibilities to our attention.)
The large increase of the extinction coefficient of the
lowest absorption band (from 2250-4000 to 8000-11000
mol-1 dm3 cm-1) upon changing from mono- to bidentate
coordination can also be rationalized in terms of increased
contribution of Pt orbitals in the HOMO of the latter. Thus,
the lowest electronic transition in 1-6 studied is charge-
transfer from Pt/thiolate(phenolate)-based HOMO to the
diimine/Pt-based LUMO.
A correlation is also observed between the 195Pt NMR data
and oxidation potentials. The trend in the values of chemical
shifts of 195Pt resonances correlates with the trend in the
oxidation peak potentials with the thiolate (Figure 15) and
phenolate subgroups of complexes. An increase in the anodic
peak potential coincides with a less-negative shift in the 195Pt
NMR spectrum. Even though there is a rather limited data
set available for other thiolate compounds,2 the data fit this
correlation (Table 4, Figure 15). This observation further
The trend in emission energies of 1-4 and 6 follows the
trend in absorption energies and the trend established for
Pt(diimine)(1,2-dithiolate) complexes (Figure 14) between
emission energies and the difference between the oxidation
and reduction potentials of the ground state. These data
support the picture that the emissive state and the absorbing
4554 Inorganic Chemistry, Vol. 45, No. 11, 2006

Probing the Electronic Structure of Platinum(II) Chromophores
supports the conclusion about participation of metal orbitals
in the HOMO.
mono-coordinated donor ligands. The lower extinction coef-
ficient observed for the corresponding transition for the
phenolate in comparison to the thiolate chromophores reflects
larger mixing of Pt d orbitals with the more donating sulfur
atom. Consistent with the energy gap law, the 77 K emission
energy decreases along with a decrease in the transition
energy, with the lowest emission energy detected for the
compound with the most donating and rigid dithiolate ligand.
There is a dramatic difference in the photophysical properties
of carborane complexes 3 and 4 compared to other mixed-
ligand Pt(II) compounds, which includes the presence of
room-temperature emission and photostability. Hence, thio-
carboranes offer an alternative to the acetylide or cyanide
ligands with respect to enabling emission of Pt(II) complexes
in solution, which does not require a carbon anion directly
bound to the metal center. The reversible oxidation exhibited
by Pt(dpphen)(dtbc) allowed for direct probing of the
parentage of the HOMO by UV/vis (spectro)electrochemistry
and EPR spectroscopy, which confirmed Pt(II) orbital
involvement in the HOMO. These results provide additional
guidelines and insight to prepare Pt(II) chromophores with
specific photophysical properties. Given the broad interest
in metal chromophores for applications ranging from elec-
tronics to catalysis, continued basic studies to elucidate the
underlying chemical/physical phenomena and to create
structure-property-function relationships are critical for
long-term success.
Interestingly, EPR results indicate larger Pt d orbital
contribution in the SOMO of the radical anion than the
radical cation of 5. In the neutral species, the situation is
likely to be reversed, as evidenced by the large metal center
contribution in the HOMO obtained by EH calculations for
23
Pt(bpy)(PhS)2 and by a shift in energy of the lowest
electronic transition on changing the central atom from Pt-
(II) to Pd(II). The photophysical properties of Pt(II) thio-
carborane compounds, 3 and 4, differ from those of the other
phenolate/thiolate Pt(II) diimine chromophores, despite the
same nature of the lowest excited state. Chromophores 3 and
4 exhibit structured emission in fluid solution, which can be
rationalized in terms of the strong ligand field of the
thiocarboranes, leading to an increase in the energy of the
d-d state, thus creating conditions for fluid solution emission
of the charge transfer state. For the case of arylthiolate Pt-
(II) diimine compounds, it has been shown by resonance
Raman spectroscopy71 that none of the intra-thiolate vibra-
tions are significantly affected upon excitation of the MLCT/
LLCT band. The pronounced vibrational structure of the
emission spectra of 4, which is different from that observed
for Pt(dpphen)Cl2 emission, supports involvement of the
orbitals localized on the carborane moiety in the emissive
electronic state. Thus, thiocarboranes cannot be considered
as direct analogues of arylthiols due to delocalization of
HOMO over the carborane cage while in the case of
arylthiols the HOMO is more localized on the S atoms.
Experimental Section
Complete details of the synthesis and experiments can be found
in the Supporting Information.
Conclusion
In the present study, a general route for synthesis of Pt-
(II) diimine thiolate/phenolates chromophores possessing
bulky phenolate or thiolate ligands is reported. The lowest
excited state in these new chromophores is assigned to the
(charge-transfer-to-diimine) transition from the HOMO of
mixed Pt/S (or Pt/O) character. The decrease in energy of
the lowest detectable transition on going from bis-mono to
di-coordinated donor ligands confirms participation of the
donor ligand orbitals in the HOMO and larger Pt d orbital
contribution in the HOMO of the complexes with di- vs
Acknowledgment. We thank Duke University and Bos-
ton University, Universities of Nottingham and Sheffield,
Engineering and Physical Sciences Research Council, and
Russian Fund for Basic Research for support, as well as Dr.
Celine Taboy for experimental assistance.
Supporting Information Available: Crystallographic data;
experimental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC051733T
Inorganic Chemistry, Vol. 45, No. 11, 2006 4555