Ligand localized triplet excited states in platinum(II) bipyridyl and terpyridyl peryleneacetylides

Article abstract of DOI:10.1021/ic702454k

An investigation of the photophysics of two complexes, [Pt( ^tBu₃tpy)(C≡C-perylene)]BF₄ (1) and Pt(^tBu₂bpy)(C≡C-perylene)₂ (2), where ^tBu₃tpy is 4,4′,4″-tri(tert-butyl)

Full text of DOI:10.1021/ic702454k

Inorg. Chem. 2008, 47, 4348-4355
Ligand Localized Triplet Excited States in Platinum(II) Bipyridyl and
Terpyridyl Peryleneacetylides
Aaron A. Rachford,† Se´bastien Goeb,† Raymond Ziessel,*,‡ and Felix N. Castellano*,†
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403, and Laboratoire de Chimie Mole´culaire associe´ au Centre National
de la Recherche Scientifique (LCM-CNRS), Ecole de Chimie, Polyme`res, Mate´riaux (ECPM),
25 rue Becquerel, 67087 Strasbourg Cedex, France
Received December 19, 2007
An investigation of the photophysics of two complexes, [Pt(tBu3tpy)(Ct C-perylene)]BF4 (1) and Pt(tBu2bpy)(C≡C-
perylene)2 (2), where tBu3tpy is 4,4′,4′′-tri(tert-butyl)-2,2′:6′,2′′-terpyridine, tBu2bpy is 4,4′-di(tert-butyl)-2,2′-bipyridine,
and C≡C-perylene is 3-ethynylperylene, reveals that they both exhibit perylene-centered ligand localized excited
triplet states (3IL) upon excitation with visible light. These complexes do not display any significant photoluminescence
at room temperature but readily sensitize 1O2 in aerated CH2Cl2 solutions, as evidenced by its characteristic emission
near 1270 nm. The transient absorption difference spectra were compared to bi- and tridentate phosphine
peryleneacetylides intended to model the 3IL peryleneacetylide excited states in addition to the related phenylacetylide-
bearing polyimine analogues, with the latter model being the respective triplet charge-transfer (3CT) excited states.
The transient difference spectra of the two title compounds display excited-state absorptions largely attributable to
perylene localized 3IL states yet exhibit somewhat attenuated excited-state lifetimes relative to those of the phosphine
model chromophores. The abbreviated lifetimes in 1 and 2 may suggest the involvement of the energetically proximate
3CT triplet state exerting an influence on excited-state decay, and the effect appears to be stronger in 1 relative
3
to 2, consistent with the energies of their respective CT states.
Introduction
acetylide oligomers and polymers, where excitation of the
lowest singlet state produces the lower lying triplet via
intersystem crossing.12–14 In Pt(II) bipyridyl and terpyridyl
structures, the opportunity presents itself to utilize charge-
transfer (CT) transitions in the visible to sensitize acetylide-
based ligand-localized triplet states.9,13,15,16 While this
strategy is conspicuous, there are only a handful of examples
The photochemistry and photophysics of platinum(II)
bipyridyl and terpyridyl acetylide complexes continue to
inspire fundamental and applied research interest.1–11 Triplet
states are commonly accessed in trans-disposed (PBu3)2Pt(II)
* To whom correspondence should be addressed. E-mail: castell@bgsu.edu
(F.N.C.), ziessel@chimie.u-strasbg.fr (R.Z.). Fax: (419) 372-9809.
†
Bowling Green State University.
CNRS.
(9) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro,
M. L; Rajapakse, N. Coord. Chem. ReV. 2006, 250, 1819–1828.
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2006, 45, 4304–4306.
(11) Ziessel, R.; Diring, S.; Retailleau, P. J. Chem. Soc., Dalton Trans.
2006, 3285–3290.
‡
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4348 Inorganic Chemistry, Vol. 47, No. 10, 2008
10.1021/ic702454k CCC: $40.75  2008 American Chemical Society
Published on Web 04/19/2008

3IL Excited States in Pt(II) CT Complexes
Chart 1. Structures of Complexes 1-6
were freshly distilled over CaH2. All other reagents from com-
mercial sources were used as received. 1H NMR and 13C-{1H} NMR
spectra were recorded on a Bruker Avance 300 (300 MHz)
spectrometer. All chemical shifts are referenced to the residual
solvent signals previously referenced to TMS, and splitting patterns
are designated as s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet), and br (broad). 31P NMR spectra were run on a Varian
VXR-400 spectrometer, and chemical shifts were related to external
85% H3PO4. EI mass spectra (70 eV) were measured in-house by
using a direct insertion probe in a Shimadzu QP5050A spectrometer.
FT-IR spectra were measured with a ThermoNicolet IR 200
spectrometer.
where ligand-localized triplet states have been sensitized to
visible light in these structures.9,17–19
In the present work, we report two new Pt(II) CT complexes,
[Pt(tBu3tpy)(Ct C-perylene)]BF4 (1) and Pt(tBu2bpy)(Ct C-
t
perylene)2(2), where Bu3tpy is 4,4′,4′′-tri(tert-butyl)-2,2′:
t
6′,2′′-terpyridine, Bu2bpy is 4,4′-di(tert-butyl)-2,2′-bipyri-
dine, and Ct C-perylene is 3-ethynylperylene. The phosphine
analogues, [Pt(P3)(Ct C-perylene)]BF4 (3) and [Pt(P2)(Ct C-
perylene)2] (4), where P3 is bis[(2-diphenylphosphino)eth-
yl]phenylphosphine and P2 is 1,2-bis(dicyclohexylphosphi-
no)ethane, have been prepared and studied in order to
examine the nature of localized peryleneacetylide intraligand
excited states (3IL). In conjunction with these complexes,
the respective charge-transfer model complexes, [Pt(tBu3tpy)-
(Ct C-Ph)]ClO4 (5) and Pt(tBu2bpy)(Ct C-Ph)2 (6), are also
examined to compare the characteristics of molecules
exhibiting low lying triplet charge-transfer excited states
(3CT). The structures of complexes 1-6 are shown in Chart
1. Complexes 1-4 readily sensitize 1O2 emission in aerated
solutions upon visible excitation, implying that the lowest
energy excited state in these complexes is triplet in nature.
Time-resolved spectroscopic studies of all six complexes
indicate that the excited states of the title compounds are
Preparations. [Pt(tBu3tpy)Cl]BF4,20 Pt(tBu2bpy)Cl2,21 [bis[2-
(diphenylphosphino)ethyl]phenylphosphine platinum triflate]tri-
flate,22 [1,2-bis(dicyclohexylphosphino)ethane platinum dichlo-
ride],23 3-ethynylperylene,24 complex 5,25 and complex 63,26 were
synthesized according to the literature procedures and yielded
1
satisfactory masses and H NMR spectra.
General Synthetic Procedure. A Schlenk flask was charged with
the appropriate platinum(II) chloride precursor, 3-ethynylperylene,
and CH2Cl2/i-Pr2NH for 1 and 3 or DMF/Et3N for 2. The solution
was degassed for 30 min. CuI (10% mol) was added, the solution
was stirred at room temperature for 12 h, and then the solvent was
evaporated under vacuum. The residue was purified by column
chromatography, followed by recrystallization in a mixture of
CH2Cl2/cyclohexane.
3
most consistent with a IL excited state localized on the
peryleneacetylide(s) rather than the 3CT photophysics more
commonly observed in Pt(II) terpyridyl and bipyridyl acetyl-
ide chromophores.
(20) Lai, S.; Chan, M. C. W.; Cheung, K.; Che, C. Inorg. Chem. 1990, 29,
4262–4267.
(21) Hodges, K. D.; Rund, J. V. Inorg. Chem. 1975, 14, 525–528.
(22) Annibale, G.; Bergamini, P.; Cattabriga, M. Inorg. Chim. Acta 2001,
316, 25–32.
Experimental Section
(23) ALQaisi, S. M.; Galat, K. J.; Chai, M.; Ray, D. G.; Rinaldi, P. L.;
Tessier, C. A.; Youngs, W. J. J. Am. Chem. Soc. 1998, 120, 12149–
12150.
General Methods. All reactions were carried out under an inert
and dry argon atmosphere by using standard techniques. Anhydrous
CH2Cl2, diisopropylamine, and triethylamine suitable for synthesis
(24) Inouye, M.; Hyodo, Y.; Nakazumi, H. J. Org. Chem. 1999, 64, 2704–
2710.
(25) Shikhova, E.; Danilov, E. O.; Kinayyigit, S.; Pomestchenko, I. E.;
Tregubov, A. D.; Camerel, F.; Retailleau, P.; Ziessel, R.; Castellano,
F. N. Inorg. Chem. 2007, 46, 3038–3048.
(18) Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano,
F. N. Inorg. Chem. 2003, 42, 1394–1396.
(19) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Che, C.-M.; Zhu, N.;
Lee, S.-T. J. Am. Chem. Soc. 2004, 126, 4958–4971.
(26) Chan, S.; Chan, W.; Wang, Y.; Che, C.; Cheung, K.; Zhu, N.
Chem.sEur. J. 2001, 7, 4180–4190.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4349

Rachford et al.
Complex 1 was prepared using the general synthetic procedure
from [Pt(tBu3tpy)Cl]BF4 (100 mg, 0.15 mmol), 3-ethynylperylene
(41 mg, 0.15 mmol), CuI (1.9 mg, 0.01 mmol), DMF (3 mL), and
i-Pr2NH (1 mL); chromatography was performed on alumina,
eluting with 99/1 CH2Cl2/methanol (v/v) to 98/2 CH2Cl2/methanol
(v/v) to give 104 mg (73%) of 1 as a red solid after recrystallization.
1H NMR (400.1 MHz, d3-acetonitrile/d6-dimethylsulfoxide (v/v
1237, 1100, 1050, 812. ES-MS m/z (nature of the peak, relative
intensity): 1004.4 ([M - BF4]+, 100). Anal. Calcd for C60H53BF4-
P3Pt: C, 62.73; H, 4.65. Found: C, 62.51; H, 4.42.
Complex 4 was prepared using the same procedure as that for
complex 1 from [1,2-bis(dicyclohexylphosphino)ethane platinum
dichloride] (50 mg, 0.07 mmol), 3-ethynylperylene (42 mg, 0.15
mmol), CuI (1 mg, 7.26 × 10-3 mmol), CH2Cl2 (10 mL), and
i-Pr2NH (2 mL); chromatography was performed on alumina,
eluting with 15/85 CH2Cl2/petroleum ether (v/v) to 70/30 CH2Cl2/
petroleum ether (v/v) to give 75 mg (88%) of 4 as an orange solid
3
1/1)): δ 8.56 (d, J ) 6 Hz, 1H), 8.27 (m, 1H), 8.44 (s, 2H), 8.12
3
(d, J ) 8 Hz, 1H), 7.99 (s, 2H), 7.84 (s, 2H), 7.75 (m, 1H), 7.62
(m, 2H), 7.48 (d, 3J ) 7 Hz, 1H), 7.43 (t, 3J ) 8 Hz, 1H), 7.36 (m,
1
after recrystallization. H NMR (300.0 MHz, CDCl3): δ 8.75 (d,
3
3
1H), 7.25 (d, J ) 7 Hz, 1H), 7.15 (m, 1H), 7.09 (t, J ) 7 Hz,
1H), 6.91 (t, 3J ) 8 Hz, 1H), 1.60 (s, 18H), 1.47 (s, 9H). 13C{1H}
NMR (100.6 MHz, d3-acetonitrile/d6-dimethylsulfoxide (v/v 1/1)):
δ 168.6, 167.4, 159.7, 154.7, 151.7, 151.2, 151.0, 137.2, 131.9,
127.5, 125.9, 125.3, 123.8, 122.9, 122.3, 122.0, 119.9, 102.9, 94.8,
37.9, 36.6, 30.1, 29.6. FT-IR (KBr, cm-1): 3053, 2960, 2901, 2866,
2103 (νCt C), 1617, 568, 1481, 1419, 1402, 1387, 1370, 1257, 1052
(νBsF), 917, 808, 767. ES-MS m/z (nature of the peak, relative
intensity): 871.2 ([M - BF4]+, 100). Anal. Calcd for C49H46N3-
PtBF4: C, 61.38; H, 4.84; N, 4.38. Found: C, 61.22; H, 4.19; N,
4.18.
3J ) 8 Hz, 2H), 8.17-8.09 (m, 8H), 7.67-7.60 (m, 6H), 7.48-7.38
(m, 6H), 2.50-2.31 (m, 8H), 1.96-1.26 (m, 40H). 31P{1H} NMR
1
(161.9 MHz, CDCl3): δ 62.5 (s), 62.5 (d, J ) 1988 Hz). FT-IR
(solid, cm-1): 3046, 2923, 2849, 2088 (νCt C), 1567, 1496, 1443,
1384, 1265, 808, 764. ES-MS m/z (nature of the peak, relative
intensity): 1167.5 ([M + H]+, 100). Anal. Calcd for C70H70P2Pt:
C, 71.96; H, 6.04. Found: C, 71.72; H, 5.82.
Photophysical Measurements. Static UV-vis absorption spectra
were measured with a Varian 50 Bio spectrophotometer. Corrected
steady-state luminescence spectra were obtained with a PTI
Instruments spectrofluorimeter equipped with a Peltier cooled
InGaAs detector using lock-in detection. This fluorimeter operates
under the control of FeliX32 software from PTI. A Coherent Innova
300 argon-ion laser was used as the excitation source where the
488 nm output (38.2 mW) was isolated using a diffraction grating
in concert with several optics including a 488.0 nm band-pass filter.
The laser power was measured with a Molectron Power Max 5200
power meter. All photophysical measurements were conducted at
ambient temperature, 22 ( 2 °C. All luminescence samples were
prepared with spectroscopic grade CH2Cl2 in 1 cm2 anaerobic quartz
cells (Starna Cells), degassed by solvent-saturated high purity argon
for at least 35 min prior to the measurements, and maintained under
argon atmosphere throughout the experiments. Fluorescence lifetime
measurements were obtained by time-correlated single-photon
counting on an Edinburgh Analytical Instruments (FLSP 920)
spectrofluorimeter equipped with a pulsed H2 flash lamp (nF900).
The instrument response function was collected using a dilute
solution of Ludox at the detection wavelength. Reconvolution of
the fluorescence decay and instrument response function was
performed within the Edinburgh software in conjunction with least-
squares lifetime fitting. Finally, the data was exported and plotted
using Origin 7.5. The fluorescence quantum yields of 3 and 4 in
dichloromethane were determined relative to perylene in ethanol
(Φfl ) 0.92),27 and values reported herein represent an average of
at least three independent measurements. In some instances, single
photon counting experiments in the near-IR were performed at the
University of Victoria, Victoria, BC, using a NIR-PMT detector
(R5509, Hamamatsu) cooled to -80 °C.
Complex 2 was prepared using the general synthetic procedure
from Pt(tBu2bpy)Cl2 (96 mg, 0.18 mmol), 3-ethynylperylene (124
mg, 0.45 mmol), CuI (3 mg, 0.016 mmol), CH2Cl2 (10 mL), and
i-Pr2NH (5 mL); chromatography was performed on alumina,
eluting with 80/20 CH2Cl2/cycloxehane (v/v) to 99/1 CH2Cl2/
cyclohexane (v/v) to give 113 mg (62%) of 2 as a deep-carmin
1
solid after recrystallization. H NMR (400.1 MHz, d6-dimethyl-
sulfoxide/d4-methanol (v/v, 9/1)): δ 9.86 (d, 3J ) 6 Hz, 2H), 8.84
3
3
(d, J ) 8 Hz, 2H), 8.16 (m, 10H), 8.01 (s, 2H), 7.77 (d, J ) 8
3
4
Hz, 2H), 7.64 (m, 6H), 7.46 (td, J ) 8 Hz, J ) 2 Hz, 2H), 7.40
(t, 3J ) 8 Hz, 2H), 1.49 (s, 18H). 13C{1H} NMR (100.6 MHz, d6-
dimethylsulfoxide/d4-methanol (v/v 9/1)): δ 162.5, 157.8, 149.9,
124.2,123.6,122.4,121.9-121.7(multiplepeaks),121.2,119.2-118.8
(multiple peaks), 118.3, 118.0, 117.6, 117.4, 117.0, 113.9, 112.7,
112.6, 111.7, 103.9, 96.8, 39.1, 31.8. FT-IR (KBr, cm-1): 3040,
2963, 2094 (νCt C), 1617, 1576, 1495, 1416, 1386, 1240, 807, 766.
ES-MS m/z (nature of the peak, relative intensity): 1014.2 ([M +
H]+, 100). Anal. Calcd for C62H46N2Pt: C, 73.43; H, 4.57; N, 2.76.
Found: C, 73.04; H, 4.22; N, 2.55.
Complex 3. A Schlenk flask was charged with [bis[2-(diphe-
nylphosphino)ethyl]phenylphosphine platinum triflate]triflate (50
mg, 4.86 × 10-2 mmol), 3-ethynylperylene (14 mg, 5.10 × 10-2
mmol), and CH2Cl2. The solution was stirred for 30 min, poured
into 10 mL of a saturated solution of NaBF4, and stirred again for
10 min. The organic extracts were washed with water and dried
over cotton absorbent. The solvent was removed by rotary evapora-
tion. The residue was purified by column chromatography on
alumina, eluting with CH2Cl2/petroleum ether (15/85 v/v) to CH2Cl2/
MeOH (99/1 v/v) to give 43 mg (80%) of 3 as an orange solid
after recrystallization. 1H NMR (300.0 MHz, CDCl3): δ 8.16-7.86
(m, 14H), 7.35 (d, 3J ) 8 Hz, 1H), 7.65 (dd, 3J ) 8 Hz, 4J ) 2 Hz,
Transient absorption spectra were collected on a Proteus
spectrometer (Ultrafast Systems) equipped with a 150 W Xe-arc
lamp (Newport), a Bruker Optics monochromator equipped with
two diffraction gratings blazed for visible and near-IR dispersion,
respectively, and Si or InGaAs photodiode detectors (DET 10A
and DET 10C, Thorlabs) optically coupled to the exit slit of the
monochromator. Excitation at 475 nm (2 mJ/pulse) from a
computer-controlled Nd:YAG laser/OPO system from Opotek
(Vibrant LD 355 II) operating at 10 Hz was directed to the sample
with an optical absorbance of 0.4 at the excitation wavelength. The
data consisting of a 128-shot average were analyzed by Origin 7.5
3
3
2H), 7.56-7.41 (m, 17H), 7.27 (d, J ) 8 Hz, 1H), 7.10 (t, J )
8 Hz, 1H), 3.52-3.33 (m, 4H), 2.38-2.27 (m, 4H). 13C{1H} NMR
(100.6 MHz, CDCl3): δ 134.8, 134.7, 134.1, 133.9, 133.6, 133.5,
133.4, 133.3, 133.2, 133.1, 133.0, 132.2, 131.3, 131.2, 131.1, 130.0,
129.9, 129.8, 129.6, 129.3, 128.6, 128.5, 128.2, 127.9, 127.8, 127.5,
126.8, 126.6, 126.4, 125.7, 124.1, 122.1, 120.3, 120.2, 119.8, 112.8,
85.6, 27.0, 26.5. 31P{1H} NMR (161.9 MHz, CDCl3): δ 92.2 (s),
1
1
92.2 (d, J ) 1988 Hz), 39.2 (s), 39.2 (d, J ) 2508 Hz). FT-IR
(27) Montalti, M.; Credi, A.; Prodi, L.; Gandolfini, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Press LLC: Boca Raton, FL, 2005.
(solid, cm-1): 3052, 2922, 2095 (νCt C), 1570, 1483, 1435, 1386,
4350 Inorganic Chemistry, Vol. 47, No. 10, 2008

3IL Excited States in Pt(II) CT Complexes
to the sharp and more distinct vibronic features exhibited
by 1, 3, and 4. At first glance, the spectral broadening in 2
might potentially indicate ground-state intramolecular inter-
actions between the neighboring, cis-disposed acetylides or
even perhaps intermolecular π-stacking between perylene
moieties. Since the structurally similar diphos model system
(4) does not exhibit similar broadening, we conclude that
intramolecular interactions are not responsible for the shape
of the ground-state absorption spectrum in 2. Intermolecular
π-stacking events potentially contributing to the breadth and
distortion of the bands are ruled out by the fact that the entire
electronic spectrum of 2 strictly obeys the Beer-Lambert
law over all concentration ranges used in the present optical
experiments, which have an upper limit of 6.4 × 10-4
M
(see Figure S1 of the Supporting Information), and the
normalized spectra can be completely superimposed in the
spectral region of interest. Since the highest occupied
molecular orbital (HOMO) in metal-acetylide chromophores
exhibiting CT character significantly combines metal and
acetylide ligand electron density,28,29 most absorption fea-
tures characteristic of the model structures (free acetylene
ligand and appropriate Pt(II) phenylacetylide complex) do
not simply add together.9,13,16,18,25,30,31 In our experience,
the bands associated with both ligand-localized and CT
transitions red shift relative to the corresponding model
chromophores in all instances. Additionally, the use of the
diphos (4) and triphos (3) models yields reasonable estimates
of how linkage to Pt(II) affects the ligand-localized transitions
in the acetylide relative to the free acetylene. As stated above,
significant red shifts and slight spectral broadening in the
π-π* transitions are observed in peryleneacetylene upon
bonding with Pt(II). In the CT phenylacetylide model
complexes 5 and 6, broad low energy CT absorptions are
readily observed and are not obscured by low energy ligand-
localized π-π* transitions. Comparison of the absorption
spectra of 1 and 5 shows how the attachment of the
peryleneacetylide chromophore substantially affects the posi-
tion of the CT transitions relative to the corresponding
phenylacetylide structure. Although there are a combination
of responsible electronic perturbations on both the metal and
the ligand, which induce such a significant red shift in 1,
the important result is that the CT transitions are red shifted
on the order of several thousand wavenumbers.32 Assuming
that an identical shift occurs in the CT bands of 2, this would
superimpose a broad low energy absorption band similar to
that observed in 6, at approximately 440 nm in the spectrum
of 4, qualitatively accounting for the spectral broadening seen
in 2.
Figure 1. Absorption spectra of complexes 1-6 in CH2Cl2 at room
temperature.
software. Solution concentrations were OD ∼ 0.15 and OD ∼ 0.4
for photoluminescence and transient absorption measurements,
respectively.
Results and Discussion
Absorption and 1O2 Emission Sensitization. The absorp-
tion spectra of compounds 1-4 in CH2Cl2 are displayed in
Figure 1, plotted as extinction coefficient versus wavelength.
The peryleneacetylide chromophore(s) in complexes 1-4 are
largely responsible for the structured low energy π-π*
transitions between 400-500 nm. The lowest energy bands
are substantially red shifted relative to the free perylene-
Ct CsH species (456 nm in CH2Cl2, ε ) 41000 M-1 cm-1).
In 1, the lowest energy transitions with small relative
extinction coefficients are assigned as charge-transfer (CT)
in nature, and the strong, structured perylene-localized π-π*
transitions are positioned at slightly higher energy. The CT
assignment is supported by the fact that, in the analogous
triphos model chromophore (3), there are no corresponding
long wavelength absorption bands. However, in the bipyridyl
peryleneacetylide system (2), the anticipated CT absorption
bands in the visible are obscured by the strong perylene-
localized π-π* transitions, which appear broadened relative
The emission profile typical of platinum complexes with
a 3CT excited state is quantitatively quenched in complexes
(28) Zhou, X.; Zhang, H.-X.; Pan, Q.-J.; Xia, B.-H.; Tang, A.-C. J. Phys.
Chem. A 2005, 109, 8809–8818.
(29) Liu, X.-J.; Feng, J.-K.; Meng, J.; Pan, Q.-J.; Ren, A.-M.; Zhou, X.;
Zhang, H.-X. Eur. J. Inorg. Chem. 2005, 1856–1866.
(30) Goeb, S.; Rachford, A. A.; Castellano, F. N. Chem. Commun. 2008,
814–816.
(31) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Angew. Chem., Int. Ed.
2003, 42, 1400–1403.
(32) This assumes that an identical energy shift is observed in the CT bands
in the bipyridyl complex in relation to the terpyridyl complex.
Inorganic Chemistry, Vol. 47, No. 10, 2008 4351

Rachford et al.
Figure 2. Steady-state 1O2 luminescence spectra of complexes 1 and 2
measured in aerated (solid lines) and degassed (dashed lines) CH2Cl2
solutions and measured using 488 nm laser excitation at 32.8 mW incident
power.
1 and 2. This suggests that the lowest energy excited state
is likely perylene-based in nature. These complexes do not
exhibit the singlet fluorescence characteristic of the free
perylene-CCH species. However, their model phosphine
analogues (3 and 4) exhibit weak fluorescence at 505 and
493 nm with fluorescence quantum yields (Φfl) of 0.013 and
0.034, respectively (see Figure S2 of the Supporting Infor-
mation). Time-correlated single-photon counting measure-
ments performed on these chromophores at short wavelengths
yield fluorescence lifetimes of 4.17 and 2.5 ns for 3 and 4,
respectively (see Figures S3-S5 of the Supporting Informa-
tion). The triplet nature of these excited states can be
1
indirectly identified by their ability to sensitize O2 photo-
luminescence, which has a characteristic narrow spectrum
near 1270 nm.33 The data presented in Figure 2 clearly
demonstrates that, upon air equilibration of the CH2Cl2
solutions, excitation of 1 and 2 with visible light results in
the sensitization of 1O2 emission. This sensitization does not
occur under the same conditions when the same solutions
are thoroughly degassed (Figure 2). Complexes 3 and 4
display the analogous behavior; these combined static
photophysical observations strongly suggest that the lowest
energy excited state in 1-4 is most likely triplet perylene-
acetylide in nature. Hence, nanosecond transient absorption
spectroscopy was employed to definitively characterize the
low lying triplet state present in these chromophores.
Transient Absorption Spectroscopy. The transient ab-
sorption difference spectra of complexes 1-6 measured at
select delay times following nanosecond laser pulses are
presented in Figures 3 and 4. The current data provides
emission-corrected spectral difference profiles for compounds
5 and 6 between 400 and 1400 nm. To our knowledge, the
near-IR difference spectra of these two chromophores have
not been reported to date. The difference spectra acquired
for the remaining chromophores (1-4) are not emission
corrected, as these samples do not render any long-lived
photoluminescence in degassed solutions. Single wavelength
kinetic traces and their respective fits with residuals, in
addition to the lifetimes determined from these fits, are
collected in each panel. The spectrum observed in each case
Figure 3. Selected transient absorption difference spectra of 1 (A), 3 (B),
and 5 (C) measured in deaerated CH2Cl2 following a 475 nm, a 5-7 ns
fwhm, and a 2 mJ laser pulse. Single wavelength kinetic traces (black),
single exponential fits (red), and residuals (green) are inset in each panel.
appears promptly within the ∼10 ns instrument response,
so no information could be obtained on the spectral evolution
of the transients. All of the transients are long-lived,
consistent with a triplet excited-state parentage, and in every
case, the excited-state absorptions and bleaching features self-
consistently recover to the baseline exhibiting single expo-
nential kinetics.
The transient absorption difference spectrum of 1 (Figure 3A)
does not resemble what is typically observed for the charge-
transfer excited state in structurally related molecules (Figure
3C).4,9,13,18,25,34 The difference spectrum in 1 exhibits two
intense positive OD bands at 430 and 545 nm with a high
energy shoulder at 510 nm and a single intense ground-state
bleach centered at 470 nm. The bleach coincides with the strong
ground-state absorption associated with singlet π-π* transitions
of the peryleneacetylide chromophore. The fact that this
absorption is bleached on a long time scale and kinetically
recovers with the positive OD transients indicates that the
peryleneacetylide chromophore has likely crossed to the
(34) Guo, F.; Sun, W.; Liu, Y.; Schanze, K. S. Inorg. Chem. 2005, 44,
(33) Zhang, X.; Rodgers, M. A. J. J. Phys. Chem. 1995, 99, 12797–12803.
4352 Inorganic Chemistry, Vol. 47, No. 10, 2008
4055–4065.

3IL Excited States in Pt(II) CT Complexes
excited-state dynamics are quite disparate. In the case of 3,
all transients possess a lifetime of 12.6 µs in degassed
CH2Cl2, which is more than 4 times that of 1 (τ ) 3.0 µs)
measured under the same conditions. If exactly the same
excited state is sensitized in both molecules, one would
anticipate a closer match in lifetime. Interestingly, 5,
which was selected as a model chromophore whose
transient absorption features would best describe that of
3
a CT excited state, possesses a lifetime of 3.0 µs, in
agreement with our previous measurements.25 It appears
that, while the spectral features of 1 seem to be associated
3
with a IL peryleneacetylide localized excited state as
observed in 3, its lifetime corresponds precisely to that
of the analogous CT chromophore 5. Since we cannot
make comparisons using photoluminescence spectroscopy,
we cannot precisely affirm with confidence that the
lifetime data results from configuration mixing of ener-
getically proximate 3CT and 3IL states present in 1, which
would indeed yield excited-state properties in 1 that are
essentially a superposition of those exhibited by 3 and
5.16,30,35 Since the ground-state CT absorption bands are
present at low energy in 1, it seems reasonable to speculate
that the 3IL(peryleneacetylide) state may lie close in
3
energy to the CT state, while, in the bipyridine complex
(2), the 3CT states should lie much higher in energy
relative to the 3IL state. This assumes similar singlet-triplet
energy gaps present in both chromophores. If this is indeed
the case, one would expect stronger configuration mixing
in 1 relative to 2, which would be most evident in the
observed excited-state lifetime. As shown below, com-
pound 2 is much longer lived than 1, which implies weaker
configuration mixing in the excited-state manifold in the
3
Figure 4. Selected transient absorption difference spectra of 2 (A), 4 (B),
and 6 (C) measured in deaerated CH2Cl2 following a 475 nm, a 5-7 ns
fwhm, and a 2 mJ laser pulse. Single wavelength kinetic traces (black),
single exponential fits (red), and residuals (green) are inset in each panel.
former. Without question, the IL excited state in peryle-
neacetylide is indeed accessed in both 1 and 3, and this
state dominates the excited-state absorption spectroscopy
in these chromophores.
The transient absorption difference spectrum of 2 (Figure
4A) exhibits intense transient absorptions and ground-state
bleaching features attributable to the peryleneacetylide
moieties analogous to those observed above in 1 and 3
(Figure 3). In 2, excited-state absorptions are revealed at 420
and 550 nm; the latter is substantially stronger in intensity
and closely matches the features observed in 1 and 3. Beyond
the excited-state absorption at 550 nm, the spectrum tails
off into a broad featureless absorption past 1000 nm until
the observation of a third low-energy absorption feature at
1140 nm. This peak was not readily observable in complex
1. However, it was observed in 3 and is again present in 4
(see below). The characteristic ground-state bleach of the
1perylene π-π* transition occurs at 470 nm. Again, the
remaining transient absorption features in 2 do not resemble
the spectrum obtained from one electron reductive spectro-
electrochemistry of the bipyridine complex and are not
accounted for in the CT model complex (Figure 4C).36 The
triplet excited state, in essence depleting the ground-state
singlet transitions. The remaining transient features do not
agree with the one electron reduction of the terpyridine
moiety, as we previously determined in the case of 5, nor
do they agree with the visible to near-IR difference spectrum
of 5 (Figure 3C).25 A broad transient featuring multiple
distinguishable bands is also observed between 600 and 1000
nm in 1; beyond this is a continuous region of low excited-
state absorption that extends across the near-IR throughout
our remaining detection range. The weak low-energy features
do indeed correspond with some of the strong features
observed in the analogous experiment performed on CT
model 5 (Figure 3C). However, given the significant differ-
ences between the data obtained in Figure 3A (1) and C (5)
and the strong spectral coincidences between Figure 3A (1)
and B (3), we conclude that the triplet excited-state composi-
tions in 1 and 3 are identical, assigned as 3IL in nature, and
localized on the peryleneacetylide subunit. The triphos model
3 notably lacks a low lying CT state, so the long-lived
spectroscopic features in this complex must be associated
(35) Liu, S.; Schanze, K. S. Chem. Commun. 2004, 1510–1511.
(36) Hua, F.; Kinayyigit, S.; Rachford, A. A.; Shikhova, E. A.; Goeb, S.;
Cable, J. R.; Adams, C. J.; Kirschbaum, K.; Pinkerton, A. A.;
Castellano, F. N. Inorg. Chem. 2007, 46, 8771–8783.
3
exclusively with the IL excited state of peryleneacetylide.
While the spectral features in 1 and 3 are quite similar, their
Inorganic Chemistry, Vol. 47, No. 10, 2008 4353

Rachford et al.
intense transient absorption features of 2 are in good
agreement with those of the respective diphos analogue 4
(Figure 4B), which serves as a model chromophore contain-
ing only perylene-centered excited states and effectively
reproduces the cis geometry in 2. Excited-state absorptions
are observed at 420, 560, and 1140 nm in 4; the 1140 nm
feature is clearly discernible in this molecule and must be
characteristic of one of the peryleneacetylide triplet-to-triplet
absorptions. Ground-state bleaching is again observed at 470
nm, consistent with the production of the 3IL peryleneacetylide
triplet state, which effectively nulls the corresponding singlet
ground-state absorptions on the microseconds time scale. The
most disparate differences between 2 and 4 are the excited-
state lifetimes and the relative intensities of the low-energy
feature at 1140 nm. The measured lifetime of 23.4 µs in
complex 4 is significantly longer than that of complex 2 (5.7
µs). Given that the excited-state lifetimes of both 2 and 4
greatly exceed that of 6 (τ ) 1.2 µs) and the transient
difference spectra of 2 and 4 are essentially superimposed,
Supporting Information). All of the quencher triplet-state
energies are conveniently collected in a single reference.27
On the basis of all of the data presented above, we believe
that the 3IL energy in 4 will be representative of the lowest
triplet state in molecules 1-4. The most efficient diffusion
controlled dynamic quenching only occurs when the lower
triplet-energy quenchers such as tetracene (10 240 cm-1) and
O2 are utilized; therefore, the 3IL state lies above 10 240 cm-1
yet is likely close in energy to perylene (12 260 cm-1), as
log kq in the latter instance is 7.59. No dynamic quenching
was observed whatsoever using 9,10-diphenylanthracene
(DPA, 13 710 cm-1) or p-terphenyl. Therefore, we conclude
3
that the IL state in molecules 1-4 lies somewhere near
∼12 000 cm-1. Unfortunately, not enough energetically
appropriate quenchers could be identified for the construction
of a proper Sandros plot.43,44
Conclusions
Compounds 1 and 2 represent simple molecular designs
that impart long lifetime excited states into synthetically
facile metal-organic systems, important for applications in
photonics and excited-state chemistry schemes. Excited-state
absorption data measured for 1 and 2 were collected
throughout the visible and near-IR and were directly
compared to the transient absorption difference spectra
3
the lowest excited state in 2 is assigned as IL in nature,
localized on one of the peryleneacetylide chromophores. The
lifetime differences are again significant between 2 and 4,
which may imply that some excited-state configuration
mixing takes place in 2 but is not likely as strong an
interaction as that seen in 1 (see above). This trend would
be anticipated based on the relative energetic differences
3
obtained for the analogous CT (5 and 6) and IL (3 and 4)
3
3
between the CT and IL states implied by the differences
in the ground-state electronic spectra in 1 and 2.
model chromophores, leaving little room for ambiguity in
the spectroscopic assignments. The transients produced in 1
and 2 are quite consistent with the transient spectra obtained
in the analogous phosphine structures, strongly supporting
a 3IL peryleneacetylide localized lowest triplet excited-state
in both instances. Specifically, the ground-state bleaches
observed are consistent with several peak wavelengths
associated with ground-state absorptions in the perylene-
acetylide systems, and the absorption transients largely match
the intensity and positions of the analogous peaks in 3 and
4. The peryleneacetylide localized triplet state assignment
is also consistent with the singlet O2 sensitization studies
and the long-lived absorption transients measured for these
structures. Transient absorption-based Stern-Volmer quench-
Another hypothesis regarding the excited states present
in 1 and 2 is a thermal equilibrium between the 3CT and the
3IL states.37–42 Although convenient for the explanation of
the observed excited-state lifetimes, it becomes difficult to
assign such an interaction due to the complete lack of
emission in these chromophores. In addition, one would
expect to observe transient absorption features attributable
to both types of excited states in the difference spectra. The
3
sheer intensity of the IL absorptions and the apparent lack
of significant contributions from the 3CT state, as ascertained
by comparison to model chromophores 5 and 6, argues
against an excited-state equilibrium model governing excited-
state decay in 1 and 2.
3
ing studies reveal that the IL state in compounds 1-4 lies
In the absence of detectable phosphorescence emanating
from all of the Pt-acetylide complexes in this study, an
alternative method for estimating the triplet-state energies
within these complexes becomes necessary. Therefore, a
series of triplet-state quenchers ranging from high (p-
terphenyl, 20 160 cm-1) to low (O2, 7690 cm-1) triplet energy
were used to dynamically quench the excited-state absorption
feature present at 560 nm in 4 (see Table S1 of the
at approximately 12 000 cm-1 in CH2Cl2. The complexes in
this study are unique in that long-lived excited states are
produced without generating any corresponding photolumi-
nescence, which may be desirable under certain circum-
stances.
Acknowledgment. F.N.C. acknowledges support from the
Air Force Office of Scientific Research (FA9550-05-1-0276),
the National Science Foundation (CHE-0719050), the ACS-
PRF (44138-AC3), and the BGSU Research Enhancement
Initiative. R.Z. acknowledges financial support from the
CNRS and the ANR FCP-OLEDs 05-BLAN-0004-01 and
the experimental assistance of Ste´phane Diring. R.Z. also
(37) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2917–2920.
(38) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak,
P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012–
11022.
(39) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955–
10960.
(40) Tyson, D. S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J. Phys.
Chem. A 2001, 105, 8154–8161.
(41) Tyson, D. S.; Luman, C. R.; Zhou, X.; Castellano, F. N. Inorg. Chem.
2001, 40, 4063–4071.
(43) Sandros, K. Acta Chem. Scand. 1964, 18, 2355–2374.
(44) Walters, K. A.; Ley, K. D.; Schanze, K. S. Chem. Commun. 1998,
1115–1116.
(42) McClenaghan, N. D.; Barigelletti, F.; Maubert, B.; Campagna, S.
Chem. Commun. 2002, 602–603.
4354 Inorganic Chemistry, Vol. 47, No. 10, 2008

3IL Excited States in Pt(II) CT Complexes
Supporting Information Available: Concentration-dependent
electronic spectra of 2, fluorescence spectra of 3 and 4, TCSPC
intensity decays, and TA-based Stern-Volmer quenching data. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
warmly thanks Johnson Matthey PLC for the loan of precious
metals and F.N.C. thanks Prof. F. C. J. M. van Veggel and
Dr. J.-C. Boyer at the University of Victoria in Canada for
verifying the lack of near-IR phosphorescence from our
samples.
IC702454K
Inorganic Chemistry, Vol. 47, No. 10, 2008 4355