Solvent Switching between Charge Transfer and Intraligand Excited States in a Multichromophoric Platinum(II) Complex

Article abstract of DOI:10.1021/jp049641x

Photophysical properties of a multichromophoric platinum (II) complex were investigated in different solvents of varying polarity. It was found that energy of the metal-to-charge transfer (MLCT) increased with the raise in solvent polarity. It was observed that metal-perturbed triplet intraligand (IL) state present within the molecule was invariant to the solvent polarity. The results show that decay of the excited state is directly influenced by the MLCT state.

Full text of DOI:10.1021/jp049641x

J. Phys. Chem. A 2004, 108, 3485-3492
3485
Solvent Switching between Charge Transfer and Intraligand Excited States in a
Multichromophoric Platinum(II) Complex
Irina E. Pomestchenko and Felix N. Castellano*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403
ReceiVed: January 26, 2004; In Final Form: February 18, 2004
The photophysical properties of a new multichromophoric platinum(II) complex, Pt(dbbpy)(C≡C-nap)2 (1)
(dbbpy ) 4,4′-(di-tert-butyl)-2,2′-bipyridine; C≡C-nap ) 1-ethynylnaphthalene), is reported in four different
solvent systems of varying polarity: toluene, 2-methyltetrahydrofuran, dichloromethane, and acetonitrile. Since
the low-energy dπ Pt f π* dbbpy metal-to-ligand charge transfer (MLCT) transitions in this chromophore
are negative solvatochromic in nature, increasing solvent polarity raises the energy of the MLCT state. At the
same time, the low-lying, metal-perturbed π,π* triplet intraligand (3IL) state present within the molecule on
the -C≡C-nap fragments are largely invariant to solvent polarity as evidenced by the absorption and emission
properties of the model chromophore lacking the low-energy charge transfer state, Pt(dppe)(C≡C-nap)2 (2)
(dppe ) 1,2-bis(diphenylphosphino)ethane). In solvents of low polarity, the static and dynamic absorption
and luminescence spectroscopy of 1 are largely consistent with a dπ Pt f π* dbbpy MLCT assignment. As
the solvent polarity is increased, the excited-state absorption and emission properties of 1 are significantly
altered, eventually resembling the spectral features seen in 2. However, the excited-state decay kinetics for
the structured emission measured in 2 is well beyond that observed for 1. The discrepancy in lifetime indicates
3
that the excited state of 1 in high polarity solvents is not composed of pure IL character. In fact it is very
3
3
likely that a significant electronic interaction exists between the MLCT and IL states in this molecule at
ambient temperature. To further probe this possibility, static and dynamic luminescence experiments were
performed on 1 and Pt(dbbpy)(C≡C-Ph)2 (3), C≡C-Ph is phenylacetylene, at 77 K as a function of matrix
3
polarity. In all glass matrixes at low temperature, the lowest-lying state in 1 is IL, whose energy is nearly
3
independent of the matrix. The reference molecule 3 possesses a low-lying MLCT state, and we believe it
3
reasonably models the MLCT energy in 1 at low temperature. In essence, variation in the solvent glass
changes the 3MLCT-3IL energy gap which modifies the observed excited-state lifetimes at 77 K, decreasing
as the apparent energy gap decreases. The experimental results suggest stronger mixing of the two triplet
states at room temperature relative to 77 K, but we cannot definitively rule out contributions from thermal
equilibrium. Unfortunately, the relative energy separation of the two states in 1 does not permit a quantitative
evaluation of potential configuration mixing of the two states. However, the close energetic proximity of the
two triplets along with the solvent-dependent nature of the charge transfer state permits a wide variety of
photophysical properties to be displayed by a single molecular system.
Introduction
Pt(diimine)L2, where L is a monodentate ligand such as halide,
nitrile, thiolate, or acetylide.49-63 Several recent papers have
The coupling of transition metals or transition-metal com-
plexes with selected organic chromophores has generated new
molecules and materials that exhibit a wide range of funda-
mentally interesting and potentially useful excited-state proper-
ties.1-22 A significant amount of experimental work in this area
has largely focused on d6 metal polypyridine complexes of Ru-
(II), Os(II), and Re(I) which display metal-to-ligand charge
transfer (MLCT) excited states. The general motivation for using
these molecules as the “metal-containing” fragment in more
complex systems lies in the widespread experience and knowl-
edge gained in the field over the past 45 years.23-41 In more
recent history, MLCT chromophores have become appealing
for a variety of technologies, including photovoltaics,42 optical
data storage,9,11 luminescent sensors,43,44 and electrolumines-
cence displays.45-48
appeared describing structure-photophysical property relation-
ships in systems containing a diimine ligand such as 2,2′-
bipyridine or 1,10-phenanthroline (and structurally related
derivatives), where L is typically a substituted phenylacetyl-
ide.53,56,57,62,63 In most cases, these molecules possess long-lived
excited states with photophysical properties consistent with a
dπ Pt f π* diimine MLCT assignment. However, there are
numerous examples within the work from several laboratories
where the photophysical behavior of certain Pt(diimine)(C≡CR)2
molecules is inconsistent with the MLCT assignment, which
has indeed been recognized in several instances.53,57 In those
3
cases, it is believed that a low-lying intraligand π-π* (3IL)
excited state based within the diimine or acetylide ligand(s)
contributes to the excited-state decay. Our group has recently
recognized that the 3MLCT excited state can be used to
internally sensitize the formation of a low-lying 3IL excited state
on an appended pyrenylacetylide ligand, leading to the observa-
tion of extremely long-lived room-temperature phosphorescence
Over the past 15 years, there has been increasing interest in
the properties of d8 Pt(II) complexes of the general formulation
* Corresponding author. Tel: (419) 372-7513. Fax: (419) 372-9809.
E-mail: castell@bgnet.bgsu.edu.
10.1021/jp049641x CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/30/2004

3486 J. Phys. Chem. A, Vol. 108, No. 16, 2004
Pomestchenko and Castellano
in the red.17 Clearly, the photophysical processes available in
Pt(diimine)(C≡CR)2 complexes warrant their further investiga-
tion. We postulate that proper selection of ligands and/or solvent
medium in Pt(diimine)(C≡CR)2 complexes may be able to
produce either “pure” 3MLCT or 3π-π* excited states in
addition to equilibrated or configuration mixed excited states
composed of varying degrees of 3MLCT and 3π-π* character.
This suite of accessible photophysical behavior can be expected
to yield a wide range of excited-state absorptions, emissions,
and associated dynamics. In related work, it has been shown
that energy flow in [Ru(Pyrnbpy)(CN)4]2- (n ) 1,2, Pyr )
pyrene) is readily modulated with solvent, yielding a variety of
excited-state decay behavior.19
Pt(dbbpy)(C≡C-Nap)2 (1). Pt(dbbpy)Cl2 (0.25 g, 0.46
mmol) and CuI (8.8 mg, 0.046 mmol) were placed in the
sealable reaction vessel, equipped with a Teflon screw-cap and
O-ring. A solution of dichloromethane and diisopropylamine
(3:1) (200 mL) was degassed for 15 min and added to the flask.
1-Ethynylnaphthalene (0.213 g, 1.39 mmol) was dissolved in a
small volume of dichloromethane (10 mL), degassed for 15 min,
and then added to the reaction mixture. The reaction vessel was
sealed under argon with a Teflon screw-cap fitted with an
O-ring, and stirred at room temperature for 3 days in the dark.
The solvent was removed under vacuum and the residue was
chromatographed on silica gel using dichloromethane as eluent.
Evaporation of the solvent yielded the title compound as an
orange solid (0.194 g, 54%). Rf ) 0.52 (dichloromethane, silica
gel).
1H NMR (CDCl3): δ 9.86 (d, 2H), 8.92 (d, 2H), 7.98 (d,
2H), 7.73-7.80 (m, 4H), 7.66 (d, 2H), 7.59 (m, 2H), 7.34-
7.44 (br m, 6H), 1.46 (s, 18H). EI-MS (70 eV): m/z 766. APCI-
MS (CHCl3/MeOH): m/z 766. Anal. Calcd for C42H38N2Pt‚1.5
CH2Cl2: C, 58.44; H, 4.59; N, 3.13. Found: C, 58.95; H, 4.67;
N, 3.23.
In the present work, we set out to design a new molecule
3
where the IL states on the arylacetylide ligands would be
3
energetically proximate to the luminescent MLCT state. This
motif features the possibility of observing radiative decay from
two distinct triplet states at room temperature and permits the
evaluation of potential excited triplet state interactions. Through
well-established synthetic protocols, we were able to assemble
the multichromophoric system, Pt(dbbpy)(C≡C-nap)2 (1)
(dbbpy ) 4,4′-(di-tert-butyl)-2,2′-bipyridine; C≡C-nap )
1-ethynylnaphthalene), whose triplet energy levels were coarse-
adjusted by design and synthetically imparted into the structure.
Although several publications have commented on the strong
negative solvatochromism displayed by the low-energy charge-
transfer absorptions in Pt(diimine)(C≡CR)2 chromophores,53,62
to our knowledge nobody has attempted to use this dependence
to systematically modulate the excited-state properties in this
class of molecules. In this study we chose four different solvents
to test this hypothesis: acetonitrile (CH3CN), dichloromethane
(CH2Cl2), 2-methyltetrahydrofuran (MTHF), and toluene. We
postulated that the solvent polarity dependence could be used
Pt(dppe)(C≡C-Nap)2 (2). Pt(dppe)Cl2 (1.0 g, 1.505 mmol)
was placed in a sealable reaction vessel, equipped with a Teflon
screw-cap and O-ring. A solution of freshly distilled diethyl-
amine (300 mL) and dichloromethane (250 mL) was degassed
for 15 min and added to the flask. 1-Ethynylnaphthalene (0.473
g, 3.311 mmol) was dissolved in diethylamine (10 mL) in a
separate flask, degassed, and transferred to the reaction vessel.
Copper iodide (15 mg, 0.150 mmol) was added and the reaction
vessel was sealed under argon and stirred at room temperature
for 20 h in the dark. The reaction mixture was filtered to remove
unreacted Pt(dppe)Cl2, and then solvents were removed under
reduced pressure. Crude product was purified by column
chromatography (silica gel) using 25 vol % hexane in dichlo-
romethane as eluent. Evaporation of the solvents yielded the
title compound as a yellow crystalline solid (0.153 g, 11%). Rf
) 0.65 (25 vol % hexane in dichloromethane).
3
3
to fine-adjust the MLCT energy relative to the π-π* levels
in 1, potentially accessing a variety of excited-state behaviors
in a single molecular system. Indeed this is the case as the
solvent polarity increases the nature of the excited-state becomes
more 3IL-like, as revealed by static and dynamic absorption and
luminescence spectroscopy.
1H NMR (CDCl3): δ 8.31 (d, 2H), 7.98-8.07 (br m, 8H),
7.69 (d, 2H), 7.57 (d, 2H), 7.35-7.48 (br m, 14H), 7.30 (m,
4H), 7.09 (m, 2H), 2.46 (m, 4H). 31P{1H} NMR (162 MHz,
CDCl3):δ 41.826 (JPt-P ) 2286.5 Hz). MALDI-MS: m/z 896.60
[M + H]+. Anal. Calcd for C50H38P2Pt: C, 67.03; H, 4.28.
Found: C, 66.37; H, 4.31.
Experimental Section
General. All synthetic manipulations were performed under
an inert and dry argon atmosphere using standard techniques.
Anhydrous CH2Cl2, diisopropylamine, and diethylamine were
obtained by distillation over CaH2. All other reagents and
materials from commercial sources were used as received. Silica
gel used in chromatographic separations was obtained from EM
Physical Measurements. UV-Vis absorption spectra were
measured with a Hewlett-Packard 8453 Diode array spectro-
photometer, accurate to (2 nm. Uncorrected steady-state
photoluminescence spectra were obtained with a single photon
counting spectrofluorimeter from Edinburgh Analytical Instru-
ments (FL/FS 900). The excitation was accomplished with a
450 W Xe lamp optically coupled to a monochromator ((2 nm)
and the emission was gathered at 90° and passed through a
second monochromator ((2 nm). The luminescence was
measured with a Peltier-cooled (-30°C), R955 red-sensitive
photomultiplier tube (PMT). Excitation spectra were corrected
with a photodiode mounted inside the fluorimeter that continu-
ously measures the Xe lamp output. All photophysical experi-
ments used optically dilute solutions (OD ) 0.09-0.11)
prepared in spectroscopic grade solvents unless otherwise stated.
All luminescence samples in 1 cm2 anaerobic quartz cells (Starna
Cells) were deoxygenated with solvent-saturated argon for at
least 30 min prior to measurement. Photoluminescence quantum
yields were determined as previously described,5,66 using [Ru-
(bpy)3]2+ in CH3CN (Φem ) 0.062) as the quantum counter.32
Frozen glass emission samples at 77 K were prepared by
1
Science (Silica Gel 60, 230-400 mesh). H and 31P NMR
spectra were recorded on a Varian Unity 400 (400 MHz)
spectrometer. All chemical shifts are referenced to 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). In the case of 31P{1H}
NMR, chemical shifts are reported relative to 85% H3PO4, used
as an external standard. Atmospheric pressure chemical ioniza-
tion (APCI) mass spectra were measured at the University of
Toledo using an Esquire-LC spectrometer. EI mass spectra were
measured in-house using a Shimadzu QP5050A spectrometer.
MALDI mass spectra were measured in-house using a Bruker-
Daltonics Omniflex spectrometer. Elemental analyses were
performed by Atlantic Microlab, Norcross, GA.
Preparations. Pt(dbbpy)Cl2,64 Pt(dppe)Cl2,65 and Pt(dbbpy)-
53,56
(C≡C-Ph)2
(3) were prepared according to literature
procedures and yielded satisfactory mass and 1H NMR spectra.

Multichromophoric Platinum(II) Complex
J. Phys. Chem. A, Vol. 108, No. 16, 2004 3487
TABLE 1: Photophysical Data at Room Temperaturea
d
compound
solvent
CH3CN
λ
abs, nm
392
λ
em, nm
τ
em,b µs
τ
TA,c µs
Φem
kr, ×10-5 s-1 e
knr, ×10-5 s-1 f
1
548m
591
562m
591
2.8
3.5
0.046
0.16
3.41
CH2Cl2
404
3.0
2.7g
0.11
0.36
2.97
toluene
MTHF
427
422
591
593
0.87
0.54
0.75
0.52
0.168
0.023
1.93
0.43
9.56
18.09
3
2
CH3CN
CH2Cl2
toluene
MTHF
386
396
425
416
561
562
569
573
0.69
1.36
1.05
0.16
0.60
0.15
0.34
0.307
0.047
2.20
2.50
2.92
2.94
12.29
4.85
6.60
1.02
0.15
59.56
MTHF
330
350
542m
587
100
69
a Argon-degassed solutions. b Emission intensity decay lifetime, (5%. c Absorption transient lifetime, (10%. d Quantum yield of photoluminescence
relative to [Ru(bpy)3]2+ in CH3CN (Φem ) 0.062). e Radiative decay rate, calculated by kr ) Φem/τem f Nonradiative decay rate, calculated by knr
) (1 - Φem)/τem
g Measured in 1,2-dichloroethane.
.
.
inserting a 5 mm inner diameter NMR tube containing a 10-5
M solution of the appropriate compound into a quartz-tipped
finger dewar containing liquid nitrogen.
Emission lifetimes were measured by means of a nitrogen-
pumped broadband dye laser (2-3 nm fwhm) from PTI (GL-
3300 N2 laser, GL-301 dye laser). BPBD (360-395 nm) and
Coumarin 460 (440-480 nm) laser dyes were used to tune the
excitation. Pulse energies were typically attenuated to ∼50 µJ/
pulse, measured with a Molectron Joulemeter (J4-05). The
luminescence was collected at 90° through a long pass optical
filter, focused through a lens system, and passed through a
monochromator ((4 nm). The emission was detected with a
Hamamatsu R928 PMT. The PMT signal was terminated
through a 50 ohm resistor to a Tektronix TDS 300 digital
oscilloscope (400 MHz). The data from the oscilloscope
represented an average of 128 laser shots collected at 2-3 Hz,
were transferred to a computer, and processed by Origin 6.1
software.
The experimental apparatus for nanosecond transient absorp-
tion spectroscopy, operating under the control of LabView, has
been described previously.5,7 Transient absorption spectra and
decay kinetics were obtained using the unfocused second or
third harmonic provided by a Continuum Surelite I Nd:YAG
laser (532 or 355 nm, 5-7 ns fwhm). Alternatively, the third
harmonic was used to pump an in-house constructed high-
pressure Raman shifter filled with H2 gas (400 psi) and the first
Stokes line isolated (λex ) 416 nm, 3 mJ/pulse). The data
consisting of a 32-shot average were analyzed by Origin 6.1
software. All flash photolysis measurements were conducted
at the ambient temperature 22 ( 2 °C. All samples were
thoroughly degassed prior to measurements with high purity
argon and kept under argon atmosphere throughout the experi-
ment.
3
model the IL manifold resident on the naphthaleneacetylide
ligands, whereas compound 3 is intended to represent the
3MLCT state in 1.
Room-Temperature Absorption and Photoluminescence.
The room-temperature spectroscopic and photophysical proper-
ties of all the complexes investigated in this study are collected
in Table 1. Figure 1 displays the absorption spectra of Pt(dbbpy)-
(C≡C-nap)2 measured in different solvents. It is clear that the
low-energy MLCT absorption (ꢀCH CN ) 7900 M-1 cm-1
@
3
386 nm) blue shifts with increasing solvent polarity. This is
not surprising as negative solvatochromism has been previously
observed for Pt(dbbpy)(C≡C-Ph)2 (and structurally related
molecules),53,62 resulting from a large ground-state dipole
moment. We have performed density functional theory (DFT)
calculations on the ground states of 1 and 3 to estimate each
dipole moment.67 Using optimized structures in Gaussian 98
(BP86 functional with a double-ú basis set using an effective
core potential for Pt),67 the calculated values of the ground-
state dipole moments are 10.8 and 10.6 D, respectively,
consistent with the transient DC photoconductivity study
performed by Hupp and co-workers in a structurally related Pt-
(II) diimine dithiolate complex.68 In both cases the ground-state
dipole moment vector directly opposes that produced by MLCT
excitation. The higher-energy ligand-localized π-π* transitions
between 300 and 350 nm resulting from electronic transitions
within the naphthaleneacetylide fragments are not significantly
perturbed by solvent polarity. These observations are easily
confirmed by the absorption spectra in Figure 1 as well as that
displayed by the structural model 2. In essence, solvent polarity
can be used to tune the MLCT energy in relation to the π-π*
energy on the C≡C-nap chromophores in 1.
Results and Discussion
Structures. All of the Pt(II) complexes used in this study
were prepared in accordance with standard procedures with final
purification achieved by chromatography over silica. All
complexes are stable solids at room temperature and are readily
soluble in a variety of organic solvents. Compounds 1-3 were
structurally characterized by NMR, mass spectrometry, and
elemental analysis. Multichromophoric 1 possesses a combina-
tion of low-lying 3MLCT and 3IL excited states, whereas 2 is a
reference compound containing the metal-perturbed naphtha-
leneacetylide chromophores arranged in a cis-geometry around
the Pt(II) center, enforced by the bidentate dppe ligand. Structure
2 lacks a low-lying charge transfer state and will be used to

3488 J. Phys. Chem. A, Vol. 108, No. 16, 2004
Pomestchenko and Castellano
Figure 1. UV-Vis absorption spectra of 1 in CH3CN, MTHF, CH2-
Cl2, and toluene.
The luminescence spectra of 1 obtained in various solvents
are displayed in Figure 2a. The spectra observed in MTHF and
toluene are broad and structureless, closely resembling the
breadth and shape measured for 3 in the same solvents, Figure
2b. Therefore, we tentatively assign the photoluminescence of
1 in those solvents as MLCT-based. The photoluminescence
spectrum of 1 measured in CH3CN is qualitatively similar to
the spectrum observed for 2, Figure 2c. In this case, the emission
appears to be most consistent with 3IL phosphorescence
emanating from a naphthaleneacetylide moiety. The lumines-
cence spectrum of 1 measured in CH2Cl2 is intriguing as it lies
somewhat between that observed in CH3CN and MTHF, Figure
2a. The static luminescence data at room temperature strongly
3
suggest that the 3MLCT and IL states cross over in energy as
the solvent polarity is increased from MTHF to CH3CN. In low
polarity solvents the emission is dominated by the MLCT state,
whereas in higher polarity media the emission appears to be
3
dictated by the IL state residing on one of the C≡C-nap
ligands. With the notable exception of MTHF, the photolumi-
nescence quantum yields observed for 1 are significantly lower
than those measured for 3, Table 1. When the quantum yields
and excited-state lifetimes (see below) are taken into account
in both molecules, the calculated values of kr and knr (Table 1)
provide evidence of the proposed state inversion in 1. The
radiative decay rates observed for 1 in CH3CN and CH2Cl2 are
an order of magnitude smaller than those measured for 3 in the
same solvents, while the knr values are respectively smaller. This
3
drop in kr and knr may reflect more of a IL character in the
excited-state decay.57 In toluene, where the 3MLCT is lower in
energy, the kr and knr values measured for 1 are of the same
3
magnitude as those seen for 3, suggesting that the MLCT
manifold is largely responsible for the radiative decay. The data
obtained in MTHF cannot be readily interpreted as the emission
spectrum and lifetime are consistent with a 3MLCT excited state
but values of kr and knr are both smaller than the values obtained
for 3 in the same solvent. We note that the emission spectrum
of 1 is symmetrically quenched in all solvents investigated when
the solution environment changes from argon-purged to air-
saturated. This seemingly simple result is important as it suggests
that either the emission manifold is comprised of one state or
two closely lying states in rapid thermal equilibrium.
Figure 2. Static photoluminescence spectra of (a) 1 and (b) 3 in CH3-
CN, MTHF, CH2Cl2, and toluene. (c) Photoluminescence spectra of 1
(solid line) and 2 (dashed line) in CH3CN.
M) deaerated solutions of MTHF, toluene, CH2Cl2, and CH3-
CN are 0.54, 0.87, 3.0, and 2.8 µs, respectively. The recovered
lifetimes were invariant at the low sample concentrations used
in these experiments. In all cases the emission intensity decays
were well modeled by single exponential kinetics, and lifetimes
were constant as a function of the emission monitoring
Photoluminescence Dynamics. The excited-state lumines-
cence lifetimes of 1 measured in optically dilute (10-6-10-5

Multichromophoric Platinum(II) Complex
J. Phys. Chem. A, Vol. 108, No. 16, 2004 3489
wavelength. This suggests that in every solvent there is only
one type of emitting state, consistent with the static data
described above. However, all of our results are also compatible
with a solvent-dependent fast equilibrium (relative to excited-
state decay) between 3MLCT and 3IL excited states. In MTHF
and toluene, we believe that the lifetimes are most consistent
with a lowest excited state composed primarily of 3MLCT
character. Indeed, these lifetimes are in the range of values
typically observed in Pt(diimine)(C≡CR)2 complexes, whose
excited states are derived from MLCT composition.52,53,56,57,63
In CH2Cl2 and CH3CN the lifetimes of the emissions increase
3
to ∼3.0 µs, suggesting that significant IL character is present
in the excited-state manifold. The lifetime of 2 measured in
CH3CN with 337 nm excitation is 100 µs. This value is ∼30
times larger than the similar radiative decay process in 1.
Although the static emission spectrum appears to be consistent
with a 3IL assignment, the observed dynamics suggest that there
is some degree of MLCT character in the excited-state decay
of 1, which substantially shortens the lifetime relative to a purely
3IL excited state. This is quite different than what we have
observed previously in Pt(dbbpy)(C≡C-pyrene)2, where the
3
excited state could be readily interpreted as largely IL (pyre-
nylacetylide) in nature.17 The major difference is that in the
latter multichromophoric system, the pyrenylacetylide triplet
states were substantially lower in energy relative to the 3MLCT
manifold, preventing any mixing or thermal equilibration
between the two triplets. On the basis of the luminescence data,
it appears that the 3MLCT and 3IL states in 1 are interacting in
some manner at room temperature and that the solvent medium
dictates the extent of the interaction.
Transient Absorption. The transient absorption spectra of
1 following visible excitation at 416 nm in toluene and CH3-
CN are displayed in Figure 3. This excitation wavelength was
chosen in order to preferentially pump the low-energy MLCT
transitions. The flash photolysis data obtained in the other
solvents are available as Supporting Information. In these
experiments, the concentrations used were slightly larger than
in the luminescence experiments, and minor variations in
excited-state lifetime were observed, Table 1.55 It is apparent
that the relative intensities in the observed absorption transients
markedly change with solvent, while the positions of the same
bands do not shift dramatically, Figure 3. In toluene, the excited-
state absorptions occur at ∼380 and ∼485 nm, with the 380
nm band possessing slightly more intensity, Figure 3a. The
absorption signatures obtained for 1 in CH3CN tell a different
story as the band positions are slightly shifted ∼365 and ∼510
nm, and most of the transient absorption intensity can be found
in the low-energy band at ∼510 nm, Figure 3b. It is now
worthwhile to make direct spectroscopic comparisons to the
reference chromophores 2 and 3. Compound 3 in toluene,
believed to possess a low-energy MLCT excited state, features
excited-state absorptions at ∼360 and ∼485 nm, with most of
the intensity present in the high-energy 360 nm band, Figure
4a. These data are consistent with the excited-state difference
spectra measured by Schanze and co-workers in structurally
related Pt(II) systems.57 The first-order kinetics displayed by
the absorption transients as well as the bleach recoveries for 1
and 3 in deaerated toluene were in quantitative agreement within
experimental error of their respective luminescence intensity
decays, Table 1. If the solvent is changed to CH3CN, the
absorption transients observed in 3 both experience a ∼10-15
nm blue shift relative to toluene, with peak band positions at
∼350 and ∼470 nm, Figure 4b. However, the relative intensities
of the bands are comparable to that observed for 3 in toluene.
Figure 3. Excited-state absorption difference spectra of 1 in (a) toluene
and (b) CH3CN following 416 nm excitation. The delay times are
specified on each spectrum.
The same is true in the other solvents investigated (see
Supporting Information), illustrating that 3 possesses a lowest
3
energy MLCT state in all solvents investigated, in agreement
with the literature.53,56,57 The excited-state absorption difference
spectrum obtained for 2 in MTHF (λex ) 355 nm) exhibits a
strong absorption transient near 530 nm along with a strong
ground-state bleaching signal below 375 nm, Figure 4c. We
note that there is no significant solvent dependence for the
transient absorption spectrum of 2, so only the data obtained in
MTHF will be discussed. The absorption at 530 nm is assigned
to the triplet-to-triplet absorption of the naphthaleneacetylide
chromophore, as no other absorption transients are expected for
this model system. The single-exponential lifetime of the
absorption transients measured for 2 are well beyond that
observed in 1, mirroring the kinetic behavior seen in the time-
resolved photoluminescence experiments, Table 1.
The entire collection of transient absorption data yields
significant insight into the proposed solvent-induced reordering
of the lowest excited state in 1. Similar to that observed in the
luminescence intensity decays, in all instances the transients
self-consistently exhibit similar lifetimes throughout the absorp-
tion envelope, illustrating that a single type of excited state is
probably responsible for the experimental observations. We note
that the nanosecond flash photolysis data do not yield any insight

3490 J. Phys. Chem. A, Vol. 108, No. 16, 2004
Pomestchenko and Castellano
major difference in the observed transients is in the relative
magnitude of their absorption intensities, the band in the red
being greatly enhanced as the solvent polarity is increased. The
enhancement of this low-energy band is most consistent with
3
the formation of IL states in the naphthaleneacetylide chro-
mophore(s), as can be easily ascertained by the data provided
in the model system 2, Figure 4c. What is interesting is that in
all solvents investigated, the relative intensities of the two
transient absorption bands vary but they never approach the ratio
seen in 3, which is expected for a “pure” 3MLCT excited state,57
see Figures 3 and 4 and Supporting Information. These data
imply that in all solvents, the excited-state manifold in 1 can
best be described as something intermediate of 3MLCT and 3IL.
However, we do believe that the transient absorption data reveal
the proposed reordering of the lowest excited state in 1 from
predominately 3MLCT to 3IL as the solvent polarity is increased.
Since the absorption bands characteristic of both triplet states
severely overlap, we cannot definitively say that one state or
the other (or a combination of the two) solely exist in any given
solvent at room temperature. However, the independently
measured excited-state dynamics support the notion that both
triplet states interact at room temperature and the photophysical
properties observed as a function of solvent likely represent
different composites of the two states.
Low-Temperature Photoluminescence. Luminescence spec-
tra obtained at 77 K also yield some insight into the nature of
the lowest excited state in 1. All low-temperature luminescence
data are cataloged in Table 2. Figure 5 presents the room
temperature and 77 K spectra of compounds 1-3 measured in
MTHF, selected because of its low polarity and capacity to
readily form high-quality, low-temperature glasses. Immediately
one notices that the thermally induced Stokes shifts (∆Es )
E00(77 K) - E00(298 K)) observed for 3 are extraordinarily large
in the nonpolar MTHF solvent (∆Es ) 2470 cm-1), consistent
with previous observations from the Schanze laboratory in
related complexes (Figure 5a).57 This indicates the likelihood
of a charge-transfer excited state in 3, as there are clearly large
differences in ground- and excited-state dipole moments.35 In
the model system 2, ∆Es plummets to ∼170 cm-1 (Figure 5b),
signaling the low level of charge redistribution typically
associated with π-π*-based intraligand transitions, such as
those expected from the phosphorescence of the naphthalene-
acetylide ligands. Therefore, the magnitude of ∆Es serves as
an indicator for the nature of the luminescence from the lowest
excited state. Interestingly, the ∆Es values are very distinct in
complex 1, varying from 1490 cm-1 in MTHF (Figure 5c) to
230 cm-1 in BuCN (see Supporting Information). We interpret
this result in much the same manner as all previous data in the
manuscript. In more polar solvents, the ∆Es values in 1 are small
and within experimental error of that measured in reference
system 2, more consistent with a 3IL naphthaleneacetylide-based
lowest excited state. In low-polarity solvents, the ∆Es values
in 1 are large, but not nearly as large as the values obtained in
model system 3. This latter result suggests that the excited state
Figure 4. Excited-state absorption difference spectra of 3 in (a) toluene
and (b) CH3CN following 416 nm excitation. (c) Excited-state absorp-
tion difference spectrum of 2 in MTHF following a 355 nm laser flash.
The delay times are indicated on each spectrum.
3
is composed mainly of MLCT character, but there are likely
3
contributions from the IL manifold as well. In any case, the
∆Es values are much smaller in 1 relative to 3 and the emission
profiles at low temperature are markedly different, signaling a
fundamental difference in the excited-state composition of the
two Pt(II) chromophores.
into the dynamics occurring within the 7 ns laser pulse so it is
possible that these data reflect transients associated with the
lowest of two states connected together via thermal equilibrium.
This possibility is currently being explored with ultrafast
transient absorption spectroscopy. Figure 3 shows that in the
two extremes of solvent polarity (toluene versus CH3CN), the
To further investigate potential triplet state electronic interac-
tions in 1, we measured the static and dynamic luminescence
properties of this complex at 77 K and compared these data to
those measured for 3.31,69-71 Table 2 presents the static and

Multichromophoric Platinum(II) Complex
J. Phys. Chem. A, Vol. 108, No. 16, 2004 3491
TABLE 2: Luminescence Data at 77 K.
λem.
,
τ,
3MLCT-3IL gap,
cm-1 d
b
compound solvent matrixa nm (cm-1
)
µsc
1
3
2
EtOH/MeOH 542 (18450) 100
1960
2050
1570
1640
BuCN
MTHF
3MP
541 (18480)
545 (18350)
546 (18310)
95
65
61
EtOH/MeOH 490 (20410)
3.51
BuCN
MTHF
3MP
487 (20530)e
502 (19920)
501 (19960)
3.20
3.35
2.58
MTHF
537 (18620) 470
a EtOH/MeOH ) 4:1 EtOH/MeOH; BuCN ) butyronitrile; 3MP )
b
3-methylpentane. Emission maximum, (2 nm.
cEmission decay lifetime, (10%.
d3MLCT/3IL gap, calculated by the difference in the E00 energies of 3
and 1 at 77 K in each matrix, (∼200 cm-1
.
eHigh-energy shoulder.
independent of monitoring wavelength, implying the presence
of one type of emitting state. The 77 K spectral data obtained
for 3 vary with the nature of the matrix, blue-shifting with
increasing polarity. Therefore, we rationalized that the only
energy levels that significantly shift with solvent matrix in 1
are those associated with dπ Pt f π* dbbpy MLCT states.
Interestingly, the excited-state lifetimes measured at 77 K for
1 are much longer and vary as a function of matrix, whereas
those measured for 3 are nearly constant and are relatively short-
3
lived in comparison. Qualitatively, as the MLCT/3IL energy
gap becomes larger (see Table 2), the lifetimes measured for 1
increase, suggesting that there is indeed a perturbation induced
on the 3IL level by the 3MLCT state. We believe that the extent
of this interaction dictates the lifetime observed from the lowest
excited state.69-71 Unfortunately, the relative energy levels
available in the present case prevent a true test of this model
because the two relevant states at 77 K never really approach
“resonance”. In MTHF, the solvent with the smallest measured
triplet energy gap, the states are separated by ∼1570 cm-1, well
beyond that required for significant configuration interaction.31
In any case, our data appear to be qualitatively consistent with
the first-order perturbation model proposed by Crosby and co-
workers over 30 years ago, used to describe electronic interac-
tions between charge transfer and ligand localized triplet states
in Ir(III) diimine complexes at low temperature.31,69-71
Conclusion
The photophysical properties of 1 have been explored and
compared to the model chromophores 2 and 3 as a function of
solvent and two extremes of temperature. The excited-state
properties of 1 at room temperature can best be described as
possessing a combination of charge transfer and intraligand
character, where the lower-lying state dominates the behavior
and solvent medium allows transitioning between these two
limits. The data obtained in solvents of low polarity are
consistent with an excited state composed of primarily 3MLCT
character, whereas the situation becomes inverted in solvents
of higher polarity. In the latter cases, the static and dynamic
optical data are most consistent with an excited state whose
Figure 5. Static photoluminescence spectra of (a) 3, (b) 2, and (c) 1
in MTHF at room temperature (solid lines) and 77 K (dashed lines).
3
dynamic photophysical data obtained for these complexes at
77 K in each solvent glass. In all glass matrixes studied, the
emission at 77 K in 1 is derived exclusively from the C≡C-
nap fragment(s) and its spectrum is largely invariant to the
polarity of each matrix. In other words, the apparent state
reordering seen at room temperature is no longer observed at
low temperature. The measured low-temperature lifetimes are
photophysics is largely determined by the IL states resident
on the naphthaleneacetylide unit(s). Interestingly, in solvents
of higher polarity, excited-state decay is not exclusively
determined by the 3IL level(s), as the lifetimes are significantly
3
shorter than one would expect for a pure π-π* intraligand
state. These results suggest that the 3MLCT state actively
influences the decay of the excited state at room temperature,

3492 J. Phys. Chem. A, Vol. 108, No. 16, 2004
Pomestchenko and Castellano
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Supporting Information Available: Additional transient
absorption and low-temperature luminescence spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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