Photobehavior of (α-Diimine)dimesitylplatinum(II) complexes

Article abstract of DOI:10.1021/ic000268r

The photobehavior of complexes of the type Pt(diimine)(mes)₂ is investigated (where diimine = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp), 2,9-dimethyl-1,10-phenanthroline (2,9-dmp), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp), and 4,7-diphenyl-1,10-phenanthroline (dpp) and mes = the mesityl (2,4,6-trimethylphenyl) anion). For all compounds studied, solution RT emission is observed to be weak and excited-state lifetimes are found to be short (≤ 20 ns) regardless of solvent choice. Evidence is presented for energy-transfer quenching of Pt(dpp)(mes)₂ luminescence in toluene by dissolved O₂ (primarily producing singlet oxygen) with an observed quenching rate constant of k(q) ≥ 1.3 x 10⁹ M^-1 s^-1. Electron-transfer quenching is also observed in the presence of 3,5-dinitrobenzonitrile, yielding a quenching rate constant of k(q) ≥ 1.6 x 10⁹ M^-1 s^-1. The latter observation suggests that these Pt(II) systems may have future value as excited-state reductants. All of the complexes display a much more intense and longer-lived luminescence in the solid state at room temperature. Several possible explanations for this dependence on phase are proposed, with the most probable mechanism involving radiationless deactivation in solution via rotation of the o-methyl groups of the mesityl ligands.

Full text of DOI:10.1021/ic000268r

5192
Inorg. Chem. 2000, 39, 5192-5196
Ar ticles
Photobehavior of (r-Diimine)dimesitylplatinum(II) Complexes
Keenan E. Dungey, Brian D. Thompson, Noel A. P. Kane-Maguire,* and Laura L. Wright*
Department of Chemistry, Furman University, Greenville, South Carolina 29613-1120
ReceiVed March 9, 2000
The photobehavior of complexes of the type Pt(diimine)(mes)2 is investigated (where diimine ) 2,2′-bipyridine
(bpy), 1,10-phenanthroline (phen), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp), 2,9-dimethyl-1,10-phenanthroline
(2,9-dmp), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp), and 4,7-diphenyl-1,10-phenanthroline (dpp) and mes )
the mesityl (2,4,6-trimethylphenyl) anion). For all compounds studied, solution RT emission is observed to be
weak and excited-state lifetimes are found to be short (e20 ns) regardless of solvent choice. Evidence is presented
for energy-transfer quenching of Pt(dpp)(mes)2 luminescence in toluene by dissolved O2 (primarily producing
singlet oxygen) with an observed quenching rate constant of kq g 1.3 × 109 M-1 s-1. Electron-transfer quenching
is also observed in the presence of 3,5-dinitrobenzonitrile, yielding a quenching rate constant of kq g 1.6 × 109
M-1 s-1. The latter observation suggests that these Pt(II) systems may have future value as excited-state reductants.
All of the complexes display a much more intense and longer-lived luminescence in the solid state at room
temperature. Several possible explanations for this dependence on phase are proposed, with the most probable
mechanism involving radiationless deactivation in solution via rotation of the o-methyl groups of the mesityl
ligands.
Introduction
often strongly reflect the presence of this coordinatively
unsaturated square planar geometry.3-6,12,13 Interesting excep-
tions are complexes of the type Pt(R-diimine)(mes)2 recently
described by Klein and Kaim, where mes is the mesityl (2,4,6-
trimethylphenyl) anion and R-diimine is a bidentate polypyridyl
ligand based on the 2,2′-bipyridine or 1,10-phenanthroline
framework.16-18 Their electrochemical studies show that the
presence of two mesityl ligands is a necessary condition for
reversible oxidation. The unusual stability of the Pt(III) oxidation
state was proposed to be due to blocking of the potential axial
coordination sites by the o-methyl groups on the two mesityl
ligands.17 Klein et al. also reported that the complexes have
low-lying MLCT excited states and exhibit room temperature
(RT) emission. These observations suggest that these species
may be of value as donors in excited-state energy-transfer and
electron-transfer studiessan area as yet not extensively explored
for Pt(II) diimine complexes.6,11,14,15
The spectroscopy of platinum(II) complexes containing
polypyridyl ligands has been the focus of considerable current
attention.1-15 For these square planar d8 molecules the axial
positions are relatively open and vulnerable to both nucleophilic
attack and intermolecular Pt-Pt stacking. Thus, the correspond-
ing spectral behavior and electrochemistry of these Pt(II) species
* To whom correspondence should be addressed. Phone: (864) 294-
3374 (N.A.P.K.-M.); (864) 294-3375 (L.L.W.). Fax: (864) 294-3559.
E-mail: noel.kane-maguire@furman.edu; laura.wright@furman.edu.
(1) Ballardio, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg. Chem.
1986, 25, 3858.
(2) Barigelletti, F.; Sandrini, D.; Maestri, M.; Balzani, V.; von Zelewsky,
A.; Chassot, L.; Jolliet, P.; Maeder, U. Inorg. Chem. 1988, 27, 3644.
(3) Che, C.-M.; He, L.-Y.; Poon, C.-K.; Mak, T. C. W. Inorg. Chem.
1989, 28, 3081-3083.
(4) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529-
1533.
We have therefore initiated a study of the photobehavior of
six Pt(diimine)(mes)2 complexes. The diimines selected for study
(see Figure 1) included 2,2′-bipyridine (bpy), 1,10-phenanthro-
line (phen), 3,4,7,8-tetramethylphenanthroline (tmp), 2,9-di-
methylphenanthroline (2,9-dmp), 5,6-dimethylphenanthroline
(5,6-dmp), and 4,7-diphenylphenanthroline (dpp). The com-
plexes include three previously reported by Kaim et al.16-18 and
three new phenanthroline-based complexes whose syntheses are
described in the present investigation. Examples of excited-state
electron transfer and excited-state energy transfer producing
singlet oxygen are reported for Pt(dpp)(mes)2 in RT toluene
(5) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625-5627.
(6) Wan, K.-T.; Che, C.-M.; Cho, K.-C. J. Chem. Soc., Dalton Trans.
1991, 1077-1080.
(7) Miskowski, V. M.; Houlding, V. H.; Che, C.-M.; Wang, Y. Inorg.
Chem. 1993, 32, 2518-2524.
(8) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722-727.
(9) Bevilacqua, J. M.; Eisenberg, R. Inorg. Chem. 1994, 33, 2913-2923.
(10) Zhang, Y.; Ley, K. D.; Schanze, K. S. Inorg. Chem. 1996, 35, 7102-
7110.
(11) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949-
1960.
(12) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119, 11620-
11627.
(13) Connick, W. B.; Geiger, D.; Eisenberg, R. Inorg. Chem. 1999, 38,
3264-3265.
(16) Klein, A.; Hausen, H.-D.; Kaim, W. J. Organomet. Chem. 1992, 440,
207-217.
(17) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176-1186.
(18) Klein, A.; Kaim, W.; Waldho¨r, E.; Hausen, H.-D. J. Chem. Soc., Perkin
Trans. 2 1995, 2121-2126.
(14) Lai, S.-W.; Chan, M. C.-W.; Cheung, T.-C.; Peng, S.-M.; Che, C.-M.
Inorg. Chem. 1999, 38, 4046-4055.
(15) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-457.
10.1021/ic000268r CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/07/2000

(R-Diimine)dimesitylplatinum(II) Complexes
Inorganic Chemistry, Vol. 39, No. 23, 2000 5193
Figure 2. Structure of the Pt(dpp)(mes)2 complex.
electrode were used, and ferrocene was utilized as an added calibrant
in the solvent system employed. Observed potentials were converted
to voltages vs NHE by assuming E° (Cp2Fe+/0) ) +0.40 V vs NHE.25
General Synthetic Procedure for Pt(diimine)(mes)2 Complexes.
In analogy to the literature procedure for the synthesis of Pt(bpy)-
(mes)2,17 a solution of bis(dimethyl sulfoxide)-dimesitylplatinum(II) (40
mg, 0.068 mmol) and an R-diimine ligand (0.074 mmol) in 11 mL of
toluene was prepared and heated to reflux for 3 days. During the course
of the reaction, the solution turned from colorless to deep red. Upon
cooling, the solvent was removed to yield a yellow-orange solid, which
was then dissolved in hot 1,2-dichloroethane. The resulting solution
was filtered through a fine frit to remove colloidal platinum; then cold
hexanes were added to induce recrystallization.
(a) Pt(2,9-dmp)(mes)2. Yield: 81%. 1H NMR (CDCl3): δ 8.29 (d,
1H, 4,7-H), 7.77 (s, 1H, 5,6-H), 7.40 (d, 1H, 3,8-H), 6.44 (s, 2H, mes
H), 2.36 (s, 6H, o-CH3), 2.15 (s, 3H, 2,9-CH3), 2.12 (s, 3H, p-CH3).
Anal. Calcd for C32H34N2Pt: C, 59.89; H, 5.34; N, 4.37. Found: C,
59.56; H, 5.25; N, 4.38.
Figure 1. R-Diimine ligands selected for study.
solution. The complexes are weakly emissive in RT solution
but exhibit intense luminescence and long excited-state lifetimes
in the solid state.
Experimental Section
General Details. Unless noted otherwise, all reactions were per-
formed under inert atmosphere (prepurified N2) using standard Schlenk
techniques. Toluene was predried over Na/benzophenone and distilled
under N2. Pt(mes)2(dmso)2 (dmso ) dimethyl sulfoxide) was prepared
according to a literature procedure.19 Other chemicals were purchased
from Aldrich or GFS Chemicals and used as received. CHN analyses
were performed by Midwest Microlab.
(b) Pt(5,6-dmp)(mes)2. Yield: 45%. 1H NMR (CDCl3): δ 8.69 (m,
2H, 2,4,-H, 7,9-H), 7.66 (dd, 1H, 3,8-H), 6.70 (s, 2H, mes H), 2.74 (s,
3H, 5,6-CH3), 2.48 (s, 6H, o-CH3), 2.24 (s, 3H, p-CH3). Anal. Calcd
for C32H34N2Pt: C, 59.89; H, 5.34; N, 4.37. Found: C, 59.56; H, 5.25;
N, 4.36.
Instrumentation and Methods. NMR spectra were recorded on
Varian VXR-300s and Varian Inova 500 spectrometers. A Perkin-Elmer
Paragon 500 spectrometer was employed for IR spectral measurements,
and UV-visible absorption spectra were obtained on an HP 8452 diode
array spectrophotometer. Solution and solid-state steady-state emission
spectra for the Pt(diimine)(mes)2 complexes were collected on an SPEX
Fluorolog-2 spectrofluorometer employing a red-sensitive Hamamatsu
R928 photomultiplier tube. The emission quantum yield (QPt) for an
air-saturated toluene solution of Pt(tmp)(mes)2 was estimated from a
comparison of the integrated emission signal with that of an absorbance-
matched, N2-saturated, aqueous solution of Ru(bpy)32+ under identical
1
(c) Pt(4,7-dpp)(mes)2. Yield: 43%. H NMR (CDCl3): δ 8.80 (d,
1H, 2,9), 7.98 (s, 1H, 5,6-H), 7.61 (d, 1H, 3,8-H), 7.56 (m, 5H, 4,7-
C6H5), 6.73 (s, 2H, mes H), 3.73 (s, 1H, CH2Cl2), 2.55 (s, 6H, o-CH3),
2.26 (s, 3H, p-CH3). Anal. Calcd for C42H38N2Pt‚C2H4Cl2: C, 61.09;
H, 4.90; N, 3.24. Found: C, 60.24; H, 4.80; N, 3.33.
Results and Discussion
slit conditions. QPt is given by the relationship20 QPt ) QRu (ntoluene
/
n
H O)2(DPt/DRu), where QRu ) absolute quantum yield for Ru(bpy)3
2+
Syntheses. Six complexes with the general formula Pt-
(diimine)(mes)2 were synthesized. The bpy, phen, and tmp
complexes had previously been reported by Klein et al., and
the new 2,9-dmp, 5,6-dmp, and dpp systems were synthesized
using their general method.17,18 The procedure involved refluxing
a toluene solution of bis(dimethyl sulfoxide)dimesitylplatinum-
(II) and the appropriate R-diimine for 3 days. Upon cooling
and concentration of the resulting solution, yellow-orange solids
were isolated. A typical structure for the Pt(diimine)(mes)2
complexes is shown in Figure 2. Each complex was recrystal-
lized from dichloroethane/hexane and characterized by 1H NMR
spectroscopy and elemental analysis to confirm purity.
Solution Emission Studies. Preliminary studies revealed that
the Pt(diimine)(mes)2 systems were remarkably stable in the
solvent toluene. For example, solution absorption spectra of the
complexes in this noncoordinating solvent remained unchanged
after several weeks of standing at RT. For this reason, most
solution steady-state emission data were collected in toluene.
Representative UV-vis absorption and emission spectra are
presented in Figure 3 for the case of Pt(dpp)(mes)2, and emission
) 20.042,21 n ) solvent refractive index, and D ) area under the relevant
emission spectrum. Corresponding steady-state emission studies of the
1270 nm singlet oxygen, O2(1∆g), phosphorescence signal were
undertaken using the near-IR accessory of the Fluorolog-2 instrument
(which incorporates a germanium photodiode cooled to 77 K). The
1
quantum yield for O2 generation by Pt(dpp)(mes)2 was measured by
comparing the integrated O2 emission signal sensitized by the Pt(II)
1
complex with that of a perinaphthenone standard under identical
experimental conditions.22 Excited-state lifetime studies of Pt(diimine)-
(mes)2 species utilized a N2/dye laser system described elsewhere.23
Cyclic voltammetry studies employed a BAS100b potentiostat and were
carried out in toluene solution at approximately 50 °C using 0.2 M
tetrahexylammonium perchlorate as the supporting electrolyte.24 Pt disk
working and auxiliary electrodes and a Ag wire quasi-reference
(19) Eaborn, C.; Kundu, K.; Pidcock, A. J. Chem. Soc., Dalton Trans. 1981,
933-938.
(20) Demas, J. N.; Crosby, G. A. J. Am. Chem. Soc. 1970, 92, 7262.
(21) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1975, 97, 3843.
(22) Schmidt, R.; Tanielian, C.; Dunsbach, R.; Wolff, C. J. Photochem.
Photobiol., A 1994, 79, 11-17; and references therein.
(23) Watson, R. T.; Desai, N.; Wildsmith, J.; Wheeler, J. F.; Kane-Maguire,
N. A. P. Inorg. Chem. 1999, 38, 2683-2687.
(24) Geng, L.; Ewing, A. G.; Jernigan, J. C.; Murray, R. W. Anal. Chem.
1986, 58, 852-860.
(25) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854-2855.

5194 Inorganic Chemistry, Vol. 39, No. 23, 2000
Dungey et al.
Figure 4. Steady-state RT emission spectra of Pt(tmp)(mes)2 in the
solid state (s) and for a 4.0 × 10-5 M toluene solution (‚‚‚).
Wavelength of excitation ) 440 nm.
Figure 3. UV-visible absorption spectrum (‚‚‚) and emission spectrum
(s) at RT for a 3.8 × 10-5 M toluene solution of Pt(dpp)(mes)2.
Wavelength of excitation for emission ) 450 nm (1 cm cell).
Table 2. Photoluminescence Lifetimes (τ) of Pt(diimine)(mes)2
Table 1. Absorption and Emission Maxima for Pt(diimine)(mes)2
Complexes in RT Toluene Solutions
Complexes in the Solid Statea
RT
77 K
em (nm)
rel
λem
emission
diimine
λ
em (nm)
τ (µs)
λ
τ (µs)
diimine
λ
abs (nm)
(nm)a
intensb
bpy
610
600
575
580
0.15
0.30
0.62
0.38
590
600
560
570
620
610
0.9
1.8
2.1
1.7
1.2
1.3
bpy
phen
tmp
352 (sh), 406, 432, 459, 504, 570 (sh) 650
0.21
0.67
1.00c
0.54
phen
338 (sh), 418, 450, 500, 565 (sh)
344, 398, 444, 468 (sh), 522 (sh)
640
600
640
none
tmp
5,6-dmp
2,9-dmp
dpp
5,6-dmp 416, 454, 496, 570 (sh)
2,9-dmp 418, 494
dpp
<20 ns
620
0.30
402 (sh), 426, 460 (sh), 508, 570 (sh) 660
0.75
a λex ) 450 nm. b Absorbance-matched solutions at the excitation
wavelength. No corrections have been made for wavelength variations
in PMT response. Manufacturer data for the R928 detector indicate a
25% decrease in detector quantum efficiency between 600 and 660
nm. c The emission quantum yield for Pt(tmp)(mes)2, QPt, is ap-
proximately 5 × 10-4, from a comparison of the integrated emission
signal with that of an absorbance-matched Ru(bpy)32+ aqueous solution
(see Experimental Section for details).
a λex ) 440 nm.
earlier. A representative comparison between solution and solid-
state emission spectra is provided in Figure 4 for the case of
Pt(tmp)(mes)2, while Table 2 summarizes the steady-state
emission maxima and emission lifetimes obtained for the solids
upon 440 nm excitation. Marked increases in RT emission decay
times were also apparent in the solid state, with solids in general
exhibiting excited-state lifetimes g0.15 µs. The emission
spectral band shapes were similar to the corresponding solution
values, although emission maxima were consistently at shorter
wavelengths (see Tables 1 and 2). This hypsochromic shift may
be a reflection of restricted excited-state distortion in the solid-
matrix environment. The corresponding solid-state data at 77
K are also provided in Table 2, which reveals 3-6-fold increases
in emission lifetimes relative to the RT values.
Origin of Weak Solution Emission. Although RT solution
emission was uniformly weak and emission lifetimes were short
(e20 ns), at 77 K in 2-methyltetrahydrofuran (2-MeTHF) glass
the compound Pt(tmp)(mes)2 yielded an excited-state lifetime
of 3.7 µs. A thermally activated excited-state decay process is
therefore assumed to be responsible for the weak emission
signals in RT solutions. The relative likelihood of a variety of
possible deactivation pathways is discussed below.
(a) MLCT f dd Thermal Conversion. It has been sug-
gested that the absence of solution emission for the majority of
known mononuclear Pt(II) complexes is due to the presence of
low-lying, thermally accessible dd states.2,5 Such a pathway in
the present systems does not appear probable in view of the
strong σ-donating ability of the mesityl carbanion ligands. This
should increase the energy gap between the MLCT and dd states
and is in accord with the intense, long-lived solution lumines-
cence recently reported for related Pt(diimine)(CN)2 and Pt-
(diimine)(acetylide)2 systems.3,5,15
and absorption maxima in toluene solution for the six complexes
studied are collated in Table 1. In all cases, emission decay
times were immeasurably short (e20 ns). The emission spectral
band shapes and wavelength maxima are very similar to solution
values reported earlier by Klein and Kaim et al.17 and are thus
assigned as π*(diimine) f dπ(Pt) charge transfer (MLCT) in
nature.
Although RT solution emission was invariably weak, a
comparison of the luminescence signals of absorbance-matched
toluene solutions of Pt(phen)(mes)2 and Pt(bpy)(mes)2 revealed
an approximately 3-fold greater emission intensity for the
phenanthroline-based systems. Using the complex that had the
strongest emission signal, Pt(tmp)(mes)2, steady-state emission
spectra were also obtained in acetonitrile, chloroform, deuterated
chloroform, dimethylformamide, benzene, benzene-d6, toluene-
d8, mesitylene, 2-methyltetrahydrofuran (2-MeTHF), and carbon
tetrachloride. Only minor variations in emission efficiencies
were detected over this range of solvents. A very close
agreement between the solution absorption and excitation spectra
was observed for these complexes (Figure S1, Supporting
Information). This observation is in accord with the emission
being associated with the title compounds and not sample
impurities.
Solid-State Emission Studies. All six complexes displayed
an almost iridescent glow when solid samples were exposed to
excitation light of λ g300 nm at RT. This observation represents
a dramatic increase in emission intensity (about 2 orders of
magnitude) relative to the corresponding solution data noted
(b) Solvent Deactivation. Excited-state deactivation via
solvent nucleophilic attack or compound to solvent vibrational

(R-Diimine)dimesitylplatinum(II) Complexes
Inorganic Chemistry, Vol. 39, No. 23, 2000 5195
and thus enhance the probability of excited-state decay via
luminescence.26
Energy-Transfer Studies. Quenching by O2. Despite the
weak emission signatures of Pt(diimine)(mes)2 complexes in
RT solutions, their luminescent excited states are sufficiently
long-lived to be intercepted by dissolved oxygen. A reproducible
20% decrease in the steady-state emission intensity is observed
on progressing from an N2-saturated to an O2-saturated solution
(Figure S3, Supporting Information). The linear Stern-Volmer
plot of Io/I vs [O2] yields a Stern-Volmer slope, KSV, of 26.7
M-1 (where Io and I are the steady-state emission intensities in
the absence and presence of oxygen, respectively).27 Since KSV
) τokq and τo e 20 ns (where τo is the emission lifetime in the
absence of O2), the bimolecular quenching rate constant, kq, is
g1.3 × 109 M-1 s-1 (i.e., kq g (1/20)kd, where kd is the
diffusion-controlled rate constant obtained in toluene solu-
tion).28,29
Figure 5. Singlet oxygen, O2(1∆g), phosphorescence spectra obtained
at RT for absorbance-matched (A400 ) 0.57) oxygen-saturated toluene
solutions of Pt(dpp)(mes)2 (‚‚‚) and perinaphthenone (s). Wavelength
of excitation ) 400 nm.
Oxygen quenching of the 3MLCT state of Pt(dpp)(mes)2 may
proceed by an energy transfer mechanism producing singlet
oxygen, 1O2, and/or an electron-transfer mechanism, as depicted
in eqs 1 and 2, respectively. For the present systems, both
energy transfer is considered unlikely for the title complexes,
owing the small variations in emission efficiencies for solvents
with markedly different ligating abilities and the absence of
solvent deuterium isotope effects. This conclusion is also
consistent with steric constraints on solvent access to axial
coordination sites.
3MLCT Pt2+ + 3O2 f Pt2+ + O2 (1∆g)
(1)
(2)
-
3MLCT Pt2+ + O2 f Pt3+ + O2
processes are thermodynamically feasible, but in this report, we
provide direct experimental confirmation that oxygen quenching
proceeds primarily via the 1O2 pathway.30,31 When an oxygen-
saturated toluene solution of Pt(dpp)(mes)2 was excited at 440
nm, the diagnostic O2(1∆g) f 3O2 phosphorescence signal was
detected at 1270 nm.22 An 8-fold increase in the intensity of
the 1270 nm emission was observed in an analogous study of
an absorbance-matched solution of Pt(dpp)(mes)2 in toluene-d8
(Figure S4, Supporting Information). This observation is
consistent with the much longer singlet-oxygen lifetime in the
deuterated solvent.32 The quantum yield for 1O2 production, Φ∆-
(Pt), was determined by comparing the steady-state 1270 nm
signal upon 400 nm excitation of Pt(dpp)(mes)2, Io(Pt), with
that of an absorbance-matched solution of the reference standard
perinaphthenone, Io(PN). Inspection of Figure 5 reveals an Io(Pt)/
Io(PN) ratio of 1/8, which is then inserted into eq 3 for the
(c) Agostic Interactions. On the basis of X-ray crystal-
lographic data reported by Kaim et al. for Pt(bpy)(mes)2 and
Pt(phen)(mes)2,16,18 an alternative relaxation pathway might
involve an agostic interaction between hydrogens on the mesityl
methyls and the platinum center. This type of interaction could
deactivate the excited state by utilizing the C-H bond stretching
motion of the methyl groups. However, this type of interaction
may be more prevalent in the solid state than in fluid solution,
which would result in a reversal of the relative luminescence
intensity from what is actually observed.
(d) Complex Self-Quenching. Recent studies have demon-
strated that excited-state self-quenching is a common phenom-
enon in RT solutions for strongly emissive Pt(II) diimine
complexes.13,14 In some cases, such as that of the phenylacetylide
complex Pt(phen)(CCPh)2,13 quenching of monometallic lumi-
nescence is accompanied by excimer emission. A bimolecular
process of this type is unlikely for our systems due to steric
constrictions along the Pt‚‚‚Pt axis and the very short solution
emission lifetimes (even under very dilute conditions). In
addition, we observed no evidence for excimer formation as
solution concentrations were increased. An alternative self-
quenching pathway involving overlap of the diimine π clouds
on separate complex molecules is also improbable on the basis
of our observation of no emission quenching upon the addition
of excess free phenanthroline to toluene solutions of Pt(dpp)-
(mes)2 complexes.13
Φ∆(Pt) )
[Io(Pt)/Io(PN)][Φ∆(PN)/ΦISC(Pt)][f(PN)/f(Pt)] (3)
calculation of Φ∆(Pt). Here Φ∆(PN) ) 1.0,22 f(PN) ) 1.0 is
the fraction of triplet excited state of perinaphthenone quenched
by O2,22 f(Pt) ) 0.20 is the fraction of the 3MLCT excited state
1
of Pt(dpp)(mes)2 quenched by O2, and ΦISC(Pt) is the MLCT
f 3MLCT intersystem crossing yield for Pt(dpp)(mes)2. Since
the value for ΦISC(Pt) is 1.0 or less, the quantum yield for
singlet-oxygen production, Φ∆(Pt), is g0.63. We conclude
therefore that the majority of oxygen quenching occurs via the
(e) Mesityl o-Methyl Group Rotation. We suggest that the
most likely solution-phase excited-state-deactivation pathway
may be rotation of the o-methyl groups on the mesityl ligands.
This rotation would be hindered in the solid state at room
temperature and would be largely eliminated at 77 K. Modest
support for this hypothesis comes from the 35% increase
observed in the emission intensity of Pt(dpp)(mes)2 in mol wt
400 poly(ethylene glycol) solvent compared with that for an
absorbance matched solution in toluene (Figure S2, Supporting
Information). A more viscous, higher molecular weight solvent
should slow the rotation of the methyl groups on the complex
(26) Sauerwein, B.; Murphy, S.; Schuster, G. B. J. Am. Chem. Soc. 1992,
114, 7920-7922.
(27) Oxygen concentrations were determined using Bunsen solubility
coefficient data in toluene solution.28,29
(28) Grewer, C.; Brauer, H.-D. J. Phys. Chem. 1993, 97, 5001-5002.
(29) Grewer, C.; Brauer, H.-D. J. Phys. Chem. 1994, 98, 4230-4235.
(30) Srivastasa and co-workers31 have demonstrated the generation of singlet
oxygen by a range of other Pt(II) complexes utilizing the specific
singlet oxygen chemical scavenger 2,2,6,6-tetramethyl-4-piperidinol.
(31) Anbalagan, V.; Srivastasa, T. S. J. Photochem. Photobiol., A. 1995,
89, 113-119 and references therein.
(32) Jenny, T. A.; Turro, N. J. Tetrahedron Lett. 1982, 23, 2923-2926.

5196 Inorganic Chemistry, Vol. 39, No. 23, 2000
Dungey et al.
formation of 1O2, although a significant electron-transfer
component (eq 2) may also be operative.
(e20 ns). Despite these short solution lifetimes, evidence has
been presented for energy-transfer quenching by dissolved O2
(primarily producing singlet oxygen) and electron-transfer
quenching by nitroaromatics. The latter observation suggests
that these Pt(II) systems may have future value as excited-state
reductants. Interestingly, all of the complexes displayed a much
more intense and longer-lived luminescence in the solid state
at RT. Several possible explanations for this marked phase
difference were proposed, with the most probable mechanism
involving radiationless deactivation in solution via mesityl
o-methyl group rotation.
Electron-Transfer Studies. Nitroaromatics have been widely
employed as test-case substrates for evaluating transition metal
complexes as excited-state reducing agents.33 We utilized
quenching of the Pt(dpp)(mes)2 emission signal by added
nitroaromatics as a probe of photoinitiated electron transfer. The
3
reductive power of the MLCT excited state of Pt(dpp)(mes)2
in toluene solution was estimated from a combination of cyclic
voltammetry (CV) and luminescence data obtained in toluene
solution. A semireversible Pt2+/3+ redox couple was observed
by CV at 0.89 V vs NHE, while the 0-0 transition energy for
3
the MLCT excited state was assessed as 2.07 V from the
position of overlap of the emission and excitation spectra.14 The
complex Pt(dpp)(mes)2 is thus a potentially strong photoreduc-
tant (E°(*Pt2+/Pt3+) ≈ 1.18 V vs NHE), which should be
thermodynamically capable of reducing 3,5-dinitrobenzonitrile
(E°(Q/Q-) ) -0.86 V vs NHE), whereas the corresponding
reduction of nitrobenzene (E°(Q/Q-) ) -1.24 V vs NHE) is
marginally disfavored.
Acknowledgment. We wish to thank the National Science
Foundation, the Camille and Henry Dreyfus Scholar/Fellow
Program for Undergraduate Institutions, the Council on Under-
graduate Research, the Duke Endowment, and the Furman
Advantage Program for financial support of this research.
Supporting Information Available: For Pt(dpp)(mes)2, UV-vis
absorption and excitation spectra (Figure S1), Emission spectra in
toluene vs poly(ethylene glycol) (Figure S2), emission spectra in the
presence and absence of dissolved O2 (Figure S3), singlet-oxygen
phosphorescence spectra in toluene and toluene-d8 (Figure S4), and
Stern-Volmer plots of quenching by nitroaromatics (Figure S5). This
information is available free of charge via the Internet at http://
pubs.acs.org.
These expectations have been borne out experimentally, as
evident from inspection of the Stern-Volmer quenching plots
for the two aromatics (Figure S5, Supporting Information). Only
in the case of 3,5-dinitrobenzonitrile is strong emission quench-
ing apparent, with a slope, KSV, of 32.5 M-1. Since KSV ) τ0kq
and τ0 e 20 ns, the bimolecular quenching rate constant, kq, is
g1.6 × 109 M-1 s-1 34
.
Conclusions
IC000268R
For all Pt(diimine)(mes)2 compounds studied, we found
solution RT emission to be weak and excited-state lifetimes short
(34) Extended irradiation of Pt(dpp)(mes)2/dinitrobenzonitrile solutions
using the 488 nm line of an Ar ion laser (125 mW, 20 min) revealed
only minimal photodecomposition, suggestive of a rapid thermal back-
reaction.
(33) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten,
D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101,
4815-4824.