Design and synthesis of bipyridine platinum(II) bisalkynyl fullerene donor-chromophore-acceptor triads with ultrafast charge separation

Article abstract of DOI:10.1021/ja5040073

Donor-chromophore-acceptor triads, (PTZ)₂-Pt(bpy)-C₆₀ and (^tBuPTZ)₂-Pt(bpy)-C₆₀, along with their model compound, (Ph)₂-Pt(bpy)-C₆₀, have been synthesized and characterized; their photophysical and electrochemical properties have been studied, and the origin of the absorption and emission properties has been supported by computational studies. The photoinduced electron transfer reactions have been investigated using the femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, (Ph)₂-Pt(bpy)-C ₆₀ shows ultrafast triplet-triplet energy transfer from the ³MLCT/LLCT excited state within 4 ps to give the ³C ₆₀* state, while in (PTZ)₂-Pt(bpy)-C₆₀ and (^tBuPTZ)₂-Pt(bpy)-C₆₀, charge-separated state forms within 400 fs from the ³MLCT/LLCT excited state with efficiency of over 0.90, and the total efficiency with the contribution of ³C₆₀* is estimated to be 0.99. Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds of nanoseconds because of the difference in spin multiplicity between the charge-separated state and the ground state.

Full text of DOI:10.1021/ja5040073

Article
pubs.acs.org/JACS
Design and Synthesis of Bipyridine Platinum(II) Bisalkynyl Fullerene
Donor−Chromophore−Acceptor Triads with Ultrafast Charge
Separation
Sai-Ho Lee,† Chris Tsz-Leung Chan,‡ Keith Man-Chung Wong,† Wai Han Lam,† Wai-Ming Kwok,*,‡
and Vivian Wing-Wah Yam*,†
†Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China
‡Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong
Kong, P.R. China
S
* Supporting Information
ABSTRACT: Donor−chromophore−acceptor triads, (PTZ)2-Pt(bpy)-
C60 and (tBuPTZ)2-Pt(bpy)-C60, along with their model compound,
(Ph)2-Pt(bpy)-C60, have been synthesized and characterized; their
photophysical and electrochemical properties have been studied, and
the origin of the absorption and emission properties has been supported
by computational studies. The photoinduced electron transfer reactions
have been investigated using the femtosecond and nanosecond transient
absorption spectroscopy. In dichloromethane, (Ph)2-Pt(bpy)-C60 shows
ultrafast triplet−triplet energy transfer from the 3MLCT/LLCT excited state within 4 ps to give the 3C60* state, while in (PTZ)2-
Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60, charge-separated state forms within 400 fs from the 3MLCT/LLCT excited state with
3
efficiency of over 0.90, and the total efficiency with the contribution of C60* is estimated to be 0.99. Although the forward
electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds
of nanoseconds because of the difference in spin multiplicity between the charge-separated state and the ground state.
the poor solubility of the platinum(II) system as well as the
presence of a low-lying d-d ligand-field state that quenches the
3MLCT state at room temperature.10 It was only quite recently
that there has been a growing interest in the study of dyads and
triads using the platinum(II) polypyridine system given the
INTRODUCTION
■
Mimicking of the natural photosynthetic reaction center has
long been an active research area.1 The artificial models have
been constructed by connecting an electron donor to an
electron acceptor, which on light excitation will undergo
photoinduced electron transfer reaction, many of which have
proved their potential applications in photovoltaic devices.2
Transition metal-based chromophores, such as those of
Ru(II),3 Re(I),4 Os(II),5 Ir(III),2b,3a,b,5g,6 Cu(I)7 and Pt(II),1c,8
have been employed in donor−acceptor systems. Because of
the strong spin−orbit coupling imparted by the heavy metal
centers, rapid intersystem crossing (ISC) of the singlet metal-
to-ligand charge-transfer (1MLCT) excited state is expected.9
Subsequent charge transfer reactions would occur at the
3MLCT excited state to form the charge-separated (CS) state in
the triplet manifold. The triplet CS state should be much
longer-lived than that of the corresponding singlet CS state, as
the CS state and the ground state have different spin
multiplicities (i.e., charge-recombination (CR) from a triplet
CS state to a singlet ground state), where triplet−singlet
transition is spin-forbidden. The most commonly employed
metal−ligand chromophoric systems in the construction of
donor−acceptor dyads or triads are those of Ru(II),3 Re(I),4
Os(II),5 Ir(III),2b,3a,b,5g,6 and Cu(I),7 with MLCT excited
states. Corresponding studies with the d8 platinum(II)
polypyridine systems, which also show MLCT excited states,
are relatively less explored. Part of the reasons is associated with
3
enhanced solubility and MLCT excited state behavior of the
alkynyl platinum(II) system.9a,10e,f,11 In the current study, a
bipyridine Pt(II) bisalkynyl chromophore is utilized to
construct a series of donor−acceptor systems.
Recently, numerous studies have been made on transition
metal-based chromophore-fullerene dyads.2a,12 Fullerene, in
particular C60, is utilized as an electron acceptor due to its small
reorganization energy for electron transfer, which results from
the symmetrical shape, large size and π-electron delocalization
over the three-dimensional surface.13 In this regard, the CS
state lifetime of the transition metal-based chromophore-C60
dyads are expected to be long-lived. However, it is unfortunate
that instead of the desired long-lived CS state, the localized C60
triplet state (3C60*) has been observed in most of the dyads
2a,12
3
because of the lower-lying C60* (1.51 eV) energy.
In the present work, we extended the chromophore-C60 dyad
by the introduction of an additional electron donor to form a
donor−chromophore−C60 triad. With such modification, the
energy of the final CS state would be lowered and may become
Received: April 22, 2014
© XXXX American Chemical Society
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Chart 1
a
Scheme 1. Synthesis of Donor−Chromophore−Acceptor Triads, (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60
a
Reagents and conditions: (i) 4-bromoiodobenzene, K2CO3, CuSO4·5H2O, 220 °C; (ii) 4-bromoiodobenzene, K2CO3, 18-crown-6, Cu, DMF,
reflux; (iii) PdCl2(PPh3)2, CuI, PPh3, TMSA, Et3N, reflux; (iv) K2CO3, MeOH/THF, rt; (v) SeO2, dioxane, reflux; (vi) N-dodecyl glycine, C60,
PhMe, reflux; (vii) PtCl2(DMSO)2, CH2Cl2, rt; (viii) CuI, Et3N, CH2Cl2, rt.
3
lower than that of the C60* state. Moreover, the separation of
the charges may increase and may slow down the CR process.
Phenothiazine (PTZ)8b,14 along with its derivatives is a well-
known heteroarene with strong electron-donating ability;
therefore, in this study we developed donor−chromophore−
C60 triads employing the PTZ moiety.
(tBuPTZ)2-Pt(bpy)-C60 as well as their relevant model
compounds (Chart 1) using a wide variety of experimental
techniques including electronic absorption and emission
spectroscopy, cyclic voltammetry, femtosecond and nano-
second time-resolved transient absorption spectroscopy, and
the photochemical processes are identified and rate constants
are determined. Density functional theory (DFT) and time-
dependent density functional theory (TDDFT) have been
Herein, we report a detailed studies of a series of donor−
chromophore−acceptor triads, (PTZ)2-Pt(bpy)-C60 and
B
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performed to provide further insights into the origin of the
excited states.
(Ph)2-Pt(bpy)-C60 and (PTZ)2-Pt(bpy)-C60, the dodecyl
group of N-dodecylpyrrolidine on C60 was replaced by a
methyl group, and the corresponding complexes were labeled
as (Ph)2-Pt(bpy)-C60′ and (PTZ)2-Pt(bpy)-C60′ (see Compu-
tational Details).
The highest occupied molecular orbital (HOMO) and
HOMO−1 of (Ph)2-Pt(bpy) are the π orbitals of the
phenylalkynyl ligands mixed with the platinum dπ orbitals,
while the HOMO and HOMO−1 of (PTZ)2-Pt(bpy)
correspond to the π orbitals mainly localized on the
phenothiazine entities, owing to the excellent electron-donating
ability of phenothiazine (Figures S1 and S2, Supporting
Information). For (Ph)2-Pt(bpy)-C60′ (Figure S3, Supporting
Information) and (PTZ)2-Pt(bpy)-C60′ (Figure 2), the
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of donor−chromophore−acceptor
triads, (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60, are
outlined in Scheme 1. Various heteroarylacetylenes were first
prepared utilizing Ullmann-type reaction to form the N-(4-
bromophenyl)-heteroaryl compounds. This was followed by
palladium-catalyzed Sonogashira coupling reactions with
(trimethylsilyl)acetylene (TMSCCH) to yield the TMS-
terminated heteroarylacetylenes. The TMS group was sub-
sequently removed by stirring with potassium carbonate in
tetrahydrofuran-methanol solution. On the other hand, 4,4′-
dimethyl-2,2′-bipyridine was oxidized with selenium oxide to
give 4-methyl-2,2′-bipyridine-4′-carboxaldehyde, and the incor-
poration of fullerene was achieved by the cycloaddition of the
azomethine ylide formed upon condensation of N-dodecyl
glycine. The resulting fullerene-appended bipyridine compound
was reacted with [PtCl2(DMSO)2] to afford the
dichloroplatinum(II) precursor, which was then coordinated
with the alkynyls using copper-catalyzed coupling reaction.
Electronic Absorption and Emission Spectroscopy.
The electronic absorption spectra of (Ph)2-Pt(bpy), (PTZ)2-
Pt(bpy) and (tBuPTZ)2-Pt(bpy) in dichloromethane show an
intense absorption band at ca. 280−330 nm and a moderately
intense band at ca. 360−450 nm with λmax at ca. 400 nm
(Figure 1). The high-energy intense absorption band is
Figure 2. Spatial plots (isovalue = 0.03) of selected molecular orbitals
of (PTZ)2-Pt(bpy)-C60′ obtained from PBE0/CPCM calculation.
HOMO and HOMO−1 are localized on the PtCCPh
units and phenothiazine entities, respectively. On the other
hand, the lowest unoccupied molecular orbital (LUMO) of
(Ph)2-Pt(bpy) and (PTZ)2-Pt(bpy) correspond to the π*
orbital of the bipyridine ligand, whereas the LUMO for (Ph)2-
Pt(bpy)-C60′ and (PTZ)2-Pt(bpy)-C60′ is the π* orbital
localized on the C60 moiety.
Figure 1. UV−visible absorption spectra of (PTZ)2-Pt(bpy)-C60 and
(tBuPTZ)2-Pt(bpy)-C60 as well as their model compounds of (PTZ)2-
Pt(bpy), (tBuPTZ)2-Pt(bpy), (Ph)2-Pt(bpy)-C60, (Ph)2-Pt(bpy) and
di-tBuPhC60 in CH2Cl2.
The first two transitions computed at 432 and 475 nm for
(Ph)2-Pt(bpy) are the LLCT/MLCT [π(CCPh)/dπ(Pt)
→ π*(bpy)] transitions (Table S1, Supporting Information).
Selected singlet−singlet transitions for the other complexes are
also listed in Table S1 (Supporting Information). For (PTZ)2-
Pt(bpy), the first two transitions contain the LLCT [π(PTZ)
→ π*(bpy)] character and are computed to be less intense,
while the more intense LLCT/MLCT [π(CCPhPTZ)/
dπ(Pt) → π*(bpy)] transitions are computed at 416 and 455
nm. These results further support the assignment of an origin
of LLCT/MLCT character of the low-energy absorption band
in the visible region. Several transitions (from S0 → S1 to S0 →
S4) at 575−624 nm for (Ph)2-Pt(bpy)-C60′ are the IL π−π*
transitions of the C60, which is in agreement with the low-
energy absorption tail observed in the electronic absorption
spectrum. Different from (Ph)2-Pt(bpy)-C60′, the S1 and S2
excited states for (PTZ)2-Pt(bpy)-C60′ has LLCT [π(PTZ) →
π*(C60)] character. When compared with (Ph)2-Pt(bpy) and
(PTZ)2-Pt(bpy), the LLCT[π(CCPhPTZ/Ph) →
π*(bpy)]/MLCT[dπ(Pt) → π*(bpy)] transitions for (Ph)2-
assigned as the π−π* transitions of the phenothiazine,
bipyridine and alkynyl moieties. On the other hand, the
absorption band in the visible region is attributed to an
admixture of platinum-to-bipyridine metal-to-ligand charge
transfer (MLCT) and alkynyl-to-bipyridine ligand-to-ligand
charge transfer (LLCT) transitions.9a,11a,b With the presence of
the C60 moiety in (Ph)2-Pt(bpy)-C60, (PTZ)2-Pt(bpy)-C60 and
(tBuPTZ)2-Pt(bpy)-C60, an additional absorption tail extend-
ing to 600 nm is observed, which can also be found in the
electronic absorption spectrum of di-tBuPhC60 and is
tentatively assigned as the π−π* transitions of C60.
To gain further insight into the nature of the electronic
transitions for the complexes, computational studies were
performed on selected complexes, (Ph)2-Pt(bpy), (PTZ)2-
Pt(bpy), (Ph)2-Pt(bpy)-C60 and (PTZ)2-Pt(bpy)-C60. For
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Pt(bpy)-C60′ and (PTZ)2-Pt(bpy)-C60′ are found to be red-
shifted (Table S1, Supporting Information).
Upon photoexcitation, (Ph)2-Pt(bpy) displays observable
emission in deaerated CH2Cl2 (Figure 3). The emission band
(tBuPTZ)2-Pt(bpy)-C60 when compared to that of
di-tBuPhC60, while the Pt(bpy)/Pt(bpy•−) reduction is shifted
anodically by 20−50 mV compared to that of (Ph)2-Pt(bpy).
The reduction potentials of the investigated compounds are
summarized in Table 1 and the cyclic voltammograms of
(tBuPTZ)2-Pt(bpy) and (tBuPTZ)2-Pt(bpy)-C60 are shown in
Figure 4.
Figure 3. Normalized emission spectrum of (Ph)2-Pt(bpy) in
deaerated CH2Cl2.
Figure 4. Cyclic voltammograms of (tBuPTZ)2-Pt(bpy) (top) and
(tBuPTZ)2-Pt(bpy)-C60 (bottom) in CH2Cl2 containing 0.10 M n-
Bu4NPF6 at 298 K.
11a,b
3
observed in (Ph)2-Pt(bpy) is of MLCT/LLCT origin.
However, the other platinum(II) complexes, (PTZ)2-Pt(bpy)-
C60, (tBuPTZ)2-Pt(bpy)-C60, (PTZ)2-Pt(bpy), (tBuPTZ)2-
Pt(bpy) and (Ph)2-Pt(bpy)-C60, are nonemissive under
identical conditions. The nonemissive nature of the phenothia-
zine- and/or fullerene-containing complexes may possibly be
due to the occurrence of a quenching process in the excited
state. A possible interpretation of this is that a competitive
process of photoinduced electron transfer that leads to
photochemical charge separation has taken place.
From the observed reduction potentials given in Table 1, the
free-energy changes associated with the CS and CR processes
for (Ph)2-Pt(bpy)-C60, (PTZ)2-Pt(bpy), (tBuPTZ)2-Pt(bpy),
(PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60 can be
estimated and are given in Table 2. These values include the
corrections of the Coulombic interactions between the centers
of positive and negative charges, with the distances R obtained
from the optimized structure of (PTZ)2-Pt(bpy)-C60′ by DFT
calculation (Table 3). It is interesting to note that two center-
to-center distances between the donor and C60 (RD−C60) are
obtained, with the donor group farther away from the C60
referred to as trans- while the other as cis-. As RD−C60, in each
form, differs by ca. 4 Å, the Coulombic interaction of the ion-
pair is very different, and the driving forces for charge
separation in the trans- form are lowered by 0.03 eV, i.e.,
ΔG(CS) less negative by 0.03 eV. The energy difference of the
3MLCT/LLCT excited state and the ground state (E00) is
estimated from the intersection of the emission and excitation
spectra of (Ph)2-Pt(bpy) measured at room temperature in
deaerated dichloromethane, and is estimated to be 2.55 eV.
Electrochemistry. Cyclic voltammetry was carried out in
0.1 M n-Bu4NPF6 solution in deaerated dichloromethane. As
expected, the first oxidation couple observed is the oxidation of
t
the donor moiety (i.e., PTZ or BuPTZ) and the introduction
of the di-tert-butyl groups onto the 3,7-positions of
phenothiazine would lower the potentials for oxidation by as
much as 180 mV. On the other hand, the first and second
reduction couples are the reductions of C60, and the third
reduction wave is the reduction of bipyridine ligand. The
attachment of the C60 group to a platinum(II) bipyridine has
been found to significantly affect the unoccupied orbitals of
•−
both groups. The C60/C60 reduction is shifted anodically by
50−60 mV in (Ph)2-Pt(bpy)-C60, (PTZ)2-Pt(bpy)-C60 and
a
Table 1. Standard Potentials (vs Fc+/Fc, in V) of (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60, and Their Model
t
Compounds PTZC6H4CCPh, BuPTZC6H4CCPh, (Ph)2-Pt(bpy), (PTZ)2-Pt(bpy), (tBuPTZ)2-Pt(bpy), (Ph)2-
Pt(bpy)-C60 and di-tBuPhC60, in CH2Cl2
E°/ V (vs Fc+/Fc) in CH2Cl2
D•+/D
C60/C60
C60•−/C60
Pt(bpy)/Pt(bpy•−
)
•−
2−
molecule
PTZC6H4CCPh
tBuPTZC6H4CCPh
(Ph)2-Pt(bpy)
+0.31
+0.18
−
−
−
−
−1.13
−
−
−
−
−1.51
−
−
−
−1.85
−
−1.83
−1.83
−1.82
−1.80
−1.80
di-tBuPhC60
−
(PTZ)2-Pt(bpy)
+0.27
+0.13
(+1.40)
+0.31
+0.13
(tBuPTZ)2-Pt(bpy)
(Ph)2-Pt(bpy)-C60
(PTZ)2-Pt(bpy)-C60
(tBuPTZ)2-Pt(bpy)-C60
−
−
bc
,
−1.08
−1.07
−1.07
−1.45
−1.45
−1.44
a
The standard potentials were determined as E° = (E°pa + E°pc)/2 by cyclic voltammetry in CH2Cl2 using n-Bu4NPF6 (0.1 M) as supporting
b
c
electrolyte; scan rate = 100 mV s−1. Irreversible wave; E°pa is reported. A mixed platinum(II)/alkynyl-centered oxidation process.
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Table 2. Free-Energy Changes ΔG (eV) for Electron Transfer Reactions in CH2Cl2 Calculated from the Electrochemical
a
Potentials after Correction for the Coulomb Interaction between the Charges
reaction
ΔG(CS)/eV
−0.23
−0.61
−0.75
−0.60
−0.73 (−0.70)
−0.25
−1.08 (−1.05)
−0.78
−0.68 (−0.65)
−0.24
−1.22 (−1.19)
ΔG(CR)/eV
−2.32
−1.94
−1.80
−1.95
−1.22 (−1.25)
−2.30
−1.22 (−1.25)
−1.77
−1.09 (−1.12)
−2.31
−1.09 (−1.12)
3[(Ph)2-Pt(bpy)]*-C60 → [(Ph)2-Pt]•+(bpy)-C60
(PTZ)2-3Pt(bpy)* → (PTZ)2•+-Pt(bpy)•−
•−
(tBuPTZ)2-3Pt(bpy)* → (tBuPTZ)2•+-Pt(bpy)•−
(PTZ)2-3Pt(bpy)*-C60 → (PTZ)2•+-Pt(bpy)•−-C60
(PTZ)2•+-Pt(bpy)•− -C60 → (PTZ)2•+-Pt(bpy)-C60
b
b
b
b
b
b
b
b
•−
(PTZ)2-3Pt(bpy)*-C60 → (PTZ)2-Pt(bpy)•+-C60
•−
(PTZ)2-Pt(bpy)•+-C60 → (PTZ)2•+-Pt(bpy)-C60
•−
•−
(tBuPTZ)2-3Pt(bpy)*-C60 → (tBuPTZ)2•+-Pt(bpy)•−-C60
(tBuPTZ)2•+-Pt(bpy)•−-C60 → (tBuPTZ)2•+-Pt(bpy)-C60
•−
(tBuPTZ)2-3Pt(bpy)*-C60 → (tBuPTZ)2-Pt(bpy)•+-C60
•−
(tBuPTZ)2-Pt(bpy)•+-C60 → (tBuPTZ)2•+-Pt(bpy)-C60
•−
•−
a
ΔG(CR) = E°(anion red.) − E°(cation ox.) + e/4πε0εsRDA is for charge-recombination where RDA is the charge-separation distance determined
from DFT (Table 3); ΔG(CS) = −ΔG(CR) − E00 is the driving force for charge-separation where E00 is estimated from the intersection of the
emission and excitation spectra of (Ph)2-Pt(bpy) measured at room temperature in deaerated dichloromethane, and is estimated to be 2.55 eV. The
parameters used are ε0 = 8.85, εs = 8.93 for CH2Cl2. Driving forces for radical ion-pairs in trans-position.
b
a
spectra (EADS) and with a parallel model to yield decay-
associated difference spectra (DADS).15 In general, laser-
irradiation of complexes at 400 nm in dichloromethane initially
Table 3. Center-to-Center Distances (Å) between Various
Moieties of (PTZ)2-Pt(bpy)-C60′
center-to-center distance/Å
1
populates ca. 60% of the MLCT/LLCT excited state and ca.
1
RPt−C60
RD−Pt
RD−C60 (trans) RD−C60 (cis)
19.16 15.00
40% of the C60 excited state, based on the absorption of the
related model compounds, (Ph)2-Pt(bpy) and di-tBuPhC60
(PTZ)2-Pt(bpy)-C60′
9.80
10.19
a
1
(Figure 1). Intersystem crossing (ISC) of the MLCT/LLCT
The center-to-center distance is measured from the centroid of the
moieties.
excited state occurs subsequently within the instrumental
response time with a time constant of τ < 150 fs to produce the
3MLCT/LLCT excited state, consistent with that of a related
Pt(II) bipyridine complex.11a,b The first resolved EADS
component for all complexes therefore is the combination of
the 3MLCT/LLCT and 1C60 excited states, characterized by the
unstructured absorption between 550 and 700 nm (λmax = 650
Transient Absorption Spectroscopy. Femtosecond
transient absorption spectroscopy was used to investigate the
photoinduced electron transfer reactions in (Ph)2-Pt(bpy)-C60,
(PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60 and the
resulting spectra have been subjected to global fitting with a
sequential model to yield evolution-associated difference
11a,b
3
nm) associated with the MLCT/LLCT excited state,
and
Figure 5. Transient absorption difference spectra of (Ph)2-Pt(bpy)-C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the
spectroscopic responses following femtosecond laser pulse irradiation; (b) EADS (top) and DADS (bottom) derived from the global analysis; (c)
time decay profiles of optical density at 510 and 690 nm; and (d) taken at 5 μs after nanosecond laser pulse excitation at 355 nm in deaerated
CH2Cl2, with time-decay profile of optical density at 680 nm (inset).
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Figure 6. Transient absorption difference spectra of (PTZ)2-Pt(bpy)-C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the
spectroscopic responses following femtosecond laser pulse irradiation; (b) EADS (top) and DADS (bottom) derived from the global analysis; (c)
time decay profiles of optical density at 520, 650 and 1000 nm; and (d) taken at 50 ns after nanosecond laser pulse excitation at 355 nm in deaerated
CH2Cl2, with time-decay profile of optical density at 520 nm (inset).
Figure 7. Transient absorption difference spectra of (tBuPTZ)2-Pt(bpy)-C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the
spectroscopic responses following femtosecond laser pulse irradiation; (b) EADS (top) and DADS (bottom) derived from the global analysis; (c)
time decay profiles of optical density at 560, 650 and 1000 nm; and (d) taken at 5 ns after nanosecond laser pulse excitation at 355 nm in deaerated
CH2Cl2, with time-decay profile of optical density at 555 nm (inset).
the absorption at λmax = 510 and 890 nm of the localized 1C60*
(Figures 5−7).13b
1C60* which decays after 724 ps through ISC to produce the
3
final component of C60* with characteristic absorption at λmax
For (Ph)2-Pt(bpy)-C60, the 3MLCT/LLCT excited state
undergoes triplet−triplet (T−T) energy transfer to C60 at τ1 =
= 690 nm.12c,e,13b,16 The ISC rate of 1C60* is close to that found
for di-tBuPhC60 of ∼1000 ps (Figure S4, Supporting
3
4 ps, confirmed by the presence of C60* with characteristic
3
absorption at λmax = 690 nm12c,e,13b,16 in the second EASD
Information). The remaining long-lived C60* state eventually
component. This component also shows the coexistence of
decays with a lifetime τ = 26 μs (Figure 5d), in good agreement
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Scheme 2. Schematic Energy Diagrams for Photoinduced Energy and Charge Transfer of (a) (Ph)2-Pt(bpy)-C60, (b) (PTZ)2-
a b
,
Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60 in CH2Cl2
a
b
1
3
Energies of C60* and C60* excited states are obtained from ref 12e. Energy for radical ion-pairs in trans-position.
1
Table 4. Observed Reaction Rates of Energy-Transfer (kEnT), Intersystem Crossing of C60* (kISC), Charge-Separation (kCS),
3
Vibrational Relaxation (kVR), Charge-Recombination (kCR) and Relaxation of C60* (kT) for di-tBuC60, (Ph)2-Pt(bpy)-C60,
(PTZ)2-Pt(bpy), (tBuPTZ)2-Pt(bpy), (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60
molecule
di-tBuC60
kEnT/s−1
kISC/s−1
kCS/s−1
kVR/s−1
kCR/s−1
kT/s−1
4.2 × 104
3.8 × 104
−
−
−
−
−
9.1 × 109
1.4 × 109
−
−
−
−
−
−
−
(Ph)2-Pt(bpy)-C60
2.5 × 1011
(PTZ)2-Pt(bpy)
−
−
−
−
6.3 × 1011
1.2 × 1012
2.5 × 1012
3.6 × 1012
2.1 × 1010
4.4 × 1010
1.4 × 1011
3.0 × 1011
4.8 × 108
4.4 × 108
5.6 × 106
1.0 × 107
(tBuPTZ)2-Pt(bpy)
(PTZ)2-Pt(bpy)-C60
(tBuPTZ)2-Pt(bpy)-C60
−
1.9 × 109
2.3 × 109
3
with that of di-tBuPhC60 (τ = 24 μs, Figure S5, Supporting
Information).
the charge transfer from a vibrationally unrelaxed MLCT/
LLCT excited state is possible18 and the subsequent vibrational
relaxation of the CS state, identified by the blue-shifting of peak
maxima as well as the narrowing of the spectral bands,19 occurs
at a time τ2 = 7 ps for (PTZ)2-Pt(bpy)-C60 and 3 ps for
(tBuPTZ)2-Pt(bpy)-C60. As the charge transfer process is
exceptionally fast, much faster than that of the T−T energy
transfer process (τ = 4 ps for (Ph)2-Pt(bpy)-C60), the
It should be noted that charge separation through an electron
3
transfer reaction from the MLCT/LLCT excited state to C60
•−
generating [(Ph)2-Pt]•+(bpy)-C60 is energetically accessible
(ΔG(CS) = −0.23 eV, Table 2), but this component with the
well-established C60•− absorption at about 1000 nm2a,13b,16 was
not found, indicating that the CS state is not populated.
Analogous results have been obtained in the related Ru-
(bpy)32+-C60 complexes.12d
formation of (PTZ)2•+-Pt(bpy)-C60 and (tBuPTZ)2•+-Pt-
•−
•−
(bpy)-C60
is very efficient with quantum yields
(PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60 behave
differently when compared with their related dyad (Ph)2-
Pt(bpy)-C60 upon photoexcitation at 400 nm in dichloro-
methane. The EADS and DADS of the four resolved
components are shown in Figures 6b and 7b. The first
(ΦCS(MLCT/LLCT)) estimated to be 0.91 and 0.94, respectively
(Supporting Information Section S3). From the decay profiles
at 520 nm for (PTZ)2-Pt(bpy)-C60 and 555 nm for
(tBuPTZ)2-Pt(bpy)-C60, the lifetimes of the CS states are
determined to be 178 and 98 ns (Figures 6d and 7d). Notably,
1C60* is also observed in the resolved component spectra,
3
component depicts the initial formation of the MLCT/LLCT
3
excited state. This is followed by charge transfer process to
which undergoes ISC to form C60* with time constants τ3 =
•−
produce the CS species, namely, (PTZ)2•+-Pt(bpy)-C60 and
523 ps for (PTZ)2-Pt(bpy)-C60 and τ3 = 429 ps for
(tBuPTZ)2•+-Pt(bpy)-C60•−, with rise time τ1 = 394 and 275 fs,
respectively (vide infra). This component is characterized by
strong absorption features at ca. 520 nm for PTZ•+ or ca. 560
(tBuPTZ)2-Pt(bpy)-C60. The absorption of C60* is also
3
observed in the transient absorption difference spectrum of
(PTZ)2-Pt(bpy)-C60 taken at 50 ns (Figure 6d), and the time
decay profile at 680 nm (not presented) obeys the first-order
kinetics with a lifetime of 178 ns, identical to that of the 520 nm
8b,14a,d,16,17
t
•−
nm for BuPTZ•+,
as well as the C60 absorption at
ca. 1000 nm.2a,13b,16 As vibrational relaxation of the MLCT/
3
LLCT excited state has not yet been completed until ∼4 ps,11b
absorption. Unambiguously, the C60* state eventually decays
3
G
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to the ground state via the energetically lower-lying CS state,
(PTZ)2•+-Pt(bpy)-C60•−. Furthermore, due to the large energy
separated at a distance of 10 Å (see RD−Pt for (PTZ)2-Pt(bpy)-
C60′ in Table 3) and are entirely localized onto the
phenothiazine and bipyridine moieties (see HOMO and
LUMO in Figure S2, Supporting Information). Therefore, the
donor−acceptor electronic coupling is sufficiently weak, and
hence the CR process has a lifetime of 2 ns which is comparable
to the 5 ns obtained for the nonconjugated system.8b The slow
CR process is also attributed to the difference in spin
multiplicity between the CS state and the ground state.
3
gap between the CS state and the C60* state (ΔG(CS) −
ΔG(3C60*) = 0.29 eV), from the Boltzmann distribution, the
CS state is largely populated at the equilibrium and the direct
relaxation rate of the CS state is equal to the observed CR rate
(τCR = 178 ns or kCR = 5.6 × 106 s−1), with essentially no delay
due to the slow relaxation of the 3C60* state.20 By assuming the
ISC of 1MLCT/LLCT excited state is unity (i.e.,
ΦISC(MLCT/LLCT) = 1) and the quantum yield for the ISC of
1C60* (ΦISC(C60)) is 0.98,21 and the total efficiencies for the
Moreover, the charge-separation of (PTZ)2-Pt(bpy)-C60 and
3
(tBuPTZ)2-Pt(bpy)-C60 from the MLCT/LLCT excited state
formation of (PTZ)2•+-Pt(bpy)-C60 and (tBuPTZ)2•+-Pt-
•−
can, in principle, occur via two alternative mechanisms, namely,
reductive quenching (eq 2) and oxidative quenching (eq 3):
•−
(bpy)-C60
(ΦCS(total)) are both estimated to be 0.99
(Supporting Information Section S3).
(PTZ)2‐3Pt(bpy)*‐C60
The energetic and kinetic data for (Ph)2-Pt(bpy)-C60,
(PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60 are sum-
marized in Scheme 2, and the observed reaction rate constants
are listed in Table 4. The reaction profiles of the triads clearly
illustrate that the charge-separation process is in the Marcus
normal region22 as greater driving forces in (tBuPTZ)2-
Pt(bpy)-C60 lead to faster reactions, while the charge-
recombination process is in the inverted region22 with slower
reaction observed for (PTZ)2-Pt(bpy)-C60.
→ (PTZ)2•+‐Pt(bpy)•−‐C60
•−
→ (PTZ)2•+‐Pt(bpy)‐C60
(2)
(PTZ)2‐3Pt(bpy)*‐C60
•+
•−
•−
→ (PTZ)2‐Pt (bpy)‐C60
→ (PTZ)2•+‐Pt(bpy)‐C60
(3)
2
⎡
⎤
−(λ + ΔG°)
4λkBT
As reductive quenching (eq 2) and oxidative quenching (eq
3) are energetically accessible, i.e., ΔG < 0 eV (Table 2), both
pathways are expected to be observed. However, the associated
dyad (Ph)2-Pt(bpy)-C60 illustrates no electron transfer from
the 3MLCT/LLCT excited state to the C60 unit, suggesting the
oxidative quenching mechanism (eq 3) is not likely. On the
other hand, reductive quenching has been observed in (PTZ)2-
Pt(bpy) and (tBuPTZ)2-Pt(bpy), and thus charge separation
2π
ℏ
kET
=
V2(4πλkBT)−1/2exp
⎢
⎣
⎥
⎦
(1)
Ultrafast charge transfer reactions are also observed in the
associated dyads, (PTZ)2-Pt(bpy) and (tBuPTZ)2-Pt(bpy),
and their observed femtosecond transient absorption spectra
and EADS are shown in Figures S6 and S7 (Supporting
3
Information). The revealed CS process from MLCT/LLCT
in the triads should occur via the reductive quenching
excited state occurred at τ = 2 ps for (PTZ)2-Pt(bpy) and τ =
814 fs for (tBuPTZ)2-Pt(bpy). According to the rate equation
for nonadiabatic electron transfer (eq 1),22 the rate for the
reaction kET depends on the electronic coupling term (V), the
driving force (ΔG°) and the total reorganization energy (λ).
•+
mechanism (eq 2). As the primary CS state, (PTZ)2
-
Pt(bpy)•−-C60 and (tBuPTZ)2•+-Pt(bpy)•−-C60, cannot be
observed in both triads, the subsequent electron transfer
reaction should be enormously fast such that the detection of
the primary CS state becomes impossible. The ultrafast electron
transfer from the bipyridine unit to the C60 unit is attributed to
the strong coupling between the bipyridine and the C60
moieties, in which the two units are connected in close
proximity, and such a strong coupling is also observed in
3
The electron delocalization in the MLCT/LLCT excited state
in fact reduces the reorganization energy and enhances the
electronic coupling, both of which would lead to a more
efficient charge transfer.23 Forward charge transfer reactions in
(PTZ)2-Pt(bpy) and (tBuPTZ)2-Pt(bpy) are ∼250−500 times
faster than that of the structurally similar but nonconjugated
platinum(II) bipyridine bis(phenothiazine alkynyl) system with
a methylene bridge between the PtCCPh and the PTZ
units,8b for which the observed lifetime is on the order of
hundreds of picoseconds. The driving forces for the charge-
transfer reaction are determined to be −0.61 eV for (PTZ)2-
Pt(bpy) and −0.75 eV for (tBuPTZ)2-Pt(bpy) (Table 2),
much larger than that of the nonconjugated system (ΔG(CS) =
−0.34 eV).8b Therefore, the electron transfer reaction for a
system in the Marcus normal region22 should be accelerated.
More importantly, the conjugation is preserved after the
removal of the methylene bridge between the PtCCPh
and the PTZ units, and therefore enhanced electronic coupling
with the adjacent PTZ donor is provided to mediate the charge-
transfer process. Similarly, a slower charge transfer rate is
observed in a PTZ-Pt(bpy)-MNDI complex, in which the
electronic coupling between the PTZ and the PtCCPh
units is minimized by restricting the rotation of the additional
phenyl ring adjacent to the PTZ and the phenyl ring on the
PtCCPh unit.14d On the other hand, in the CS state of
(PTZ)2-Pt(bpy) and (tBuPTZ)2-Pt(bpy), the charges are well
24
directly linked porphyrin/phytochlorin-C60 and ferrocene-
porphyrin systems,25 which also lead to efficient charge
transfers. Because of the strong coupling, the electron transfer
reaction is likely to be mediated by tunneling via the through-
space mechanism or by superexchange in the through-bond
mechanism.26 On the other hand, with regard to the ultrafast
CS process in the triads, the CR process is relatively slow, with
5-orders of magnitude slower than the CS process (Table 4).
As the charges are separated by 15 Å for the cis- or 20 Å for the
trans- system, the donor−acceptor coupling is significantly
reduced, which slows down the CR process. The difference in
spin multiplicity between the CS state and the ground state
would also slow down the CR process significantly.
CONCLUSION
■
The photoinduced electron transfer reactions of donor−
chromophore−C60 triads, (PTZ)2-Pt(bpy)-C60 and
(tBuPTZ)2-Pt(bpy)-C60, along with their associated dyad
(Ph)2-Pt(bpy)-C60, were studied using the femtosecond and
nanosecond transient absorption spectroscopy. (Ph)2-Pt(bpy)-
C60 shows ultrafast picosecond T−T energy transfer from the
H
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3MLCT/LLCT excited state to C60, and the resulting long-lived
3C60* state decays to the ground state in microsecond time
scale. Through the direct attachment of the electron donor to
the chromophore and short spatial distances between the
chromophore and the donor/acceptor, both triads show
ultrafast femtosecond charge separation with quantum yields
of over 90% in which the charges are 15−20 Å apart; also,
controlling the spin multiplicity of the CS state with the
cooperation of a transition metal−ligand chromophore slows
down the CR process.
corrections have been done to eliminate the wavelength-dependent
time shift caused by the group velocity dispersion.
The fs-TA spectra were analyzed by global analysis15 with the
graphical interface program Glotaran28 (http://glotaran.org/). The
global analysis is based upon the fact that the observed TA data are a
superposition of the spectroscopic properties of ncomp different
components weighted by their concentration, and the time-resolved
spectrum ψ(t,λ) can be described as15,28
ncomp
ψ(t, λ) =
c(t)ε(λ)
∑
i
i
(4)
i=1
where ci(t) and εi(λ) are the concentration and spectrum of
component i. The TA spectra are globally fitted with two different
models: a parallel model and a sequential model. In the parallel model,
which constitutes the decay-associated difference spectra (DADS),
ncomp noninteracting components monoexponentially decay in parallel
and can be described as15,28
EXPERIMENTAL SECTION
■
1
General Methods. H NMR spectra were recorded on a Bruker
AVANCE 300 (300 MHz) or Bruker AVANCE 400 (400 MHz)
Fourier-transform NMR spectrometers with chemical shifts reported
relative to tetramethylsilane (Me4Si). Positive-ion FAB mass
spectrometry were recorded on a Thermo Scientific DFS high-
resolution magnetic sector mass spectrometer. Elemental analyses of
the newly synthesized complexes were performed using a Flash EA
1112 elemental analyzer at the Institute of Chemistry, Chinese
Academy of Sciences, Beijing. Ultraviolet−visible spectra were
recorded on a Varian Cary 50 UV−vis spectrophotometer in
spectroscopic grade CH2Cl2. Steady-state emission spectra were
recorded on a Spex Fluorolog-3 Model FL3-211 spectrofluorometer
equipped with a R2658P PMT detector. Measurements of the
butyronitrile glass at 77 K were conducted with samples contained in a
quartz tube inside a quartz-walled Dewar flask filled with liquid
nitrogen. The concentrations of the complex solutions in butyronitrile
for glass-emission measurements were in the range of 10−6 mol dm−3.
All solutions for photophysical studies were deaerated on a high-
vacuum line in a two-compartment cell that consisted of a 10 mL
Pyrex bulb and a 1 cm path length quartz cuvette and sealed against
the atmosphere by a Bibby Rotaflo HP6 Teflon stopper. The solutions
were rigorously degassed with at least four successive freeze−pump−
thaw cycles.
Electrochemical Measurements. The cyclic voltammetry
measurements were performed on a CHI 620A electrochemical
analyzer in deaerated dichloromethane containing 0.10 M n-Bu4NPF6
as supporting electrolyte at 298 K. A glassy carbon working electrode
was polished with Linde polishing alumina suspension and rinsed with
deionized water followed by dichloromethane before use. A platinum
wire was used as a counter electrode and an Ag/AgNO3 (0.1 M in
acetonitrile) was used as a reference electrode. All experiments were
followed by the addition of ferrocene, and the ferrocene/ferricenium
redox potential was used as an internal reference.
Transient Absorption (TA) Measurements. The fs time-
resolved experiments were performed based on a commercial
Ti:sapphire regenerative amplifier laser system and home-built TA
spectrometers.27 The sample solutions were excited by a 400 nm
pump laser; the subsequent excited state processes taking place on fs
to several ns time-scale were detected by a second probe pulse induced
by an 800 nm laser in the TA method. The 400 nm pump laser was the
second harmonic of the 800 nm fundamental (1 kHz repetition rate,
∼35 fs duration) produced with a BBO crystal. In the fs-TA
experiments, the excited states were probed by a white light continuum
(WLC) pulse produced by focusing part of the 800 nm laser onto a
rotating quartz plate. The TA signals were collected by a
monochromator and detected with a thermoelectric cooled CCD
detector. The spectral window for the TA covers 450−740 nm and
600−1050 nm regions. TA signals at wavelength from 740 to 880 nm
were masked by the 800 nm fundamental and thus were not shown in
the fs-TA in Figures 5−7 and Figures S4−S7 (Supporting
Information). Temporal delay of the probe relative to pump pulse
was controlled by an optical delay line in the fs-TA. The instrument
response function (IRF) of our ultrafast system is wavelength-
dependent. As the detection wavelength varies from 1050 to 450 nm,
the IRF change from ∼150 to 250 fs. For the fs-TA spectra,
ncomp
ψ(t, λ) =
cDADS(t)DADSi(λ)
∑
i
(5)
i=1
where cDi ADS(t) = exp(−kit) ⊕ i(t) is the exponentially decaying
concentration of component i convolved with the IRF i(t). In the
sequential model, the associated spectra are called evolution-associated
difference spectra (EADS) and can be described as15,28
ncomp
ψ(t, λ) =
cEADS(t)EADSi(λ)
∑
i
(6)
i=1
with ciEADS(t) = ∑ij=1 bji exp(−kjt) ⊕ i(t) and bji = ∏im=1 km/∏ni =1,n≠j
(kn − kj), where kj is the decay rate of component j and the amplitudes
bji of the exponential decays are defined for j ≤ i assuming b11 = 1.15,28
The DADS are the change of absorption with a certain lifetime, while
the EADS represent the spectral evolution with successively increasing
lifetime.
Nanosecond transient absorption spectroscopy experiments were
performed using an LP920-KS Laser Flash Photolysis Spectrometer
(Edinburgh Instruments, Ltd., Livingston, UK) at ambient temper-
ature. The excitation source was the 355 nm output (third harmonic)
of an Nd:YAG laser (Spectra-Physics Quanta-Ray Lab-130 Pulsed
Nd:YAG Laser) and the probe light source was a Xe900 450 W xenon
arc lamp. The transient absorption spectra were detected using an
image-intensified CCD camera (Andor) with PC plug-in controller,
fully operated by L900 spectrometer software. The absorption kinetics
were detected using a Hamamatsu R928 photomultiplier tube and
recorded using a Tektronix Model TDS3012B (100 MHz, 1.25 GS
s−1) digital oscilloscope. Samples were freshly prepared and degassed
with at least four freeze−pump−thaw cycles on a high-vacuum line in a
two compartment cell that consisted of a 10 mL Pyrex bulb and a 1 cm
path length quartz cuvette.
Computational Details. Calculations were carried out using
Gaussian 09 software package.29 The ground-state geometries of
(Ph)2-Pt(bpy), (PTZ)2-Pt(bpy), (Ph)2-Pt(bpy)-C60′ and (PTZ)2-
Pt(bpy)-C60′ were fully optimized by using the density functional
theory (DFT) with hybrid Perdew, Burke, and Ernzerhof functional
(PBE0).30 Vibrational frequency calculations were performed for all
stationary points to verify that each was a minimum (NIMAG = 0) on
the potential energy surface. On the basis of the ground state
optimized geometries, the time-dependent density functional theory
(TDDFT)31 at the same level of theory associated with the conductor-
like polarizable continuum model (CPCM)32 using CH2Cl2 as the
solvent were employed to compute the singlet−singlet transitions for
the complexes. For all the calculations, the Stuttgart effective core
potentials (ECPs) and the associated basis set were applied to describe
Pt33 with f-type polarization functions (ζ = 0.993),34 while the 6-
31G(d,p) basis set35 was used for all other atoms. The DFT and
TDDFT calculations were performed with a pruned (99,590) grid.
Synthesis. 4-Methyl-2,2′-bipyridine-4′-carboxaldehyde,36 N-dodec-
yl glycine,37 and 3,7-di-tert-butylphenothiazine38 were prepared
I
dx.doi.org/10.1021/ja5040073 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Article
according to literature procedures. The synthetic details of organic
ligands and organic model compounds, PTZC6H4CCPh,
tBuPTZC6H4CCPh and di-tBuPhC60, are described in the
Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
The selected singlet excited states (Sn) computed by TDDFT/
CPCM, selected molecular orbitals, transient absorption
difference spectra of di-tBuPhC60, (PTZ)2-Pt(bpy) and
(tBuPTZ)2-Pt(bpy) in CH2Cl2, estimation of quantum yields,
Cartesian coordinates of the optimized structures, synthesis and
characterization of organic ligands and model compounds, and
the full form of ref 29. This material is available free of charge
via the Internet at http://pubs.acs.org.
General Procedure of Platinum Complexation. 4,4′-Dimethyl-
2,2′-byipyridine/fullerene-appended bipyridine and [PtCl2(DMSO)2]
were stirred in dichloromethane, under N2, for 15 h. The solvent was
then reduced and diethyl ether was added. The resulting precipitate,
dichloroplatinum(II) precursor, was filtered and dried.
Dichloroplatinum(II) precursor and arylacetylene (2.5 equiv) were
added in deaerated dichloromethane. Triethylamine (∼1 mL) was
added, followed by the addition of copper iodide (∼2 mg). The
reaction mixture was stirred at room temperature, under N2, for 15 h.
The reaction was filtered and the solvent was removed under reduced
pressure. The product was purified by column chromatography on
silica gel using chloroform-1% methanol as eluent, and then
recrystallized from chloroform/diethyl ether.
AUTHOR INFORMATION
Corresponding Authors
wwyam@hku.hk
■
wm.kwok@polyu.edu.hk
(Ph)2-Pt(bpy). (Yield: 85%). 1H NMR (400 MHz, C2D2Cl4, 90 °C):
δ/ppm 9.63 (d, J = 5.2 Hz, 2H), 7.87 (s, 2H), 7.54 (d, J = 7.2 Hz, 4H),
7.40 (d, J = 4.9 Hz, 2H), 7.31 (t, J = 7.2 Hz, 4H), 7.20 (t, J = 7.2 Hz,
2H), 2.54 (s, 6H). FAB-MS (m/z): [M + H]+ 582.2. Elemental
analysis calcd (%) for C28H22N2Pt·H2O: C 56.09, H 4.03, N 4.67.
Found: C 56.12, H 3.81, N 4.75.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.W.-W.Y. acknowledges support from The University of Hong
Kong and the URC Strategic Research Theme on New
Materials. This work has been supported by a grant from the
Theme-Based Research Scheme (TBRS) of the Research
Grants Council of the Hong Kong Special Administrative
Region, China (Project No. T23-713/11). We are grateful to
the Information Technology Services of The University of
Hong Kong for providing computational resources.
(PTZ)2-Pt(bpy). (Yield: 82%). 1H NMR (400 MHz, CDCl3): δ/ppm
9.64 (d, J = 5.7 Hz, 2H), 8.14 (d, J = 7.7 Hz, 4H), 7.92 (s, 2H), 7.76
(d, J = 8.5 Hz, 4H), 7.43 (m, 14H), 7.28 (m, 4H), 2.55 (s, 6H). FAB-
MS (m/z): [M]+ 975.9. Elemental analysis calcd (%) for
C52H36N4S2Pt·2H2O: C 61.71, H 3.98, N 5.54. Found: C 61.59, H
3.71, N 5.46.
1
(tBuPTZ)2-Pt(bpy). (Yield: 50%). H NMR (400 MHz, CDCl3): δ/
ppm 9.57 (d, J = 5.7 Hz, 2H), 7.94 (s, 2H), 7.72 (d, J = 8.4 Hz, 4H),
7.38 (d, J = 5.1 Hz, 2H), 7.25 (d, J = 8.6 Hz, 4H), 7.04 (d, J = 2.2 Hz,
4H), 6.86 (dd, J = 8.6, 2.2 Hz, 4H), 6.22 (d, J = 8.6 Hz, 4H), 2.55 (s,
6H), 1.23 (s, 36H). FAB-MS (m/z): [M]+ 1200.1. Elemental analysis
calcd (%) for C68H68N4S2Pt·2H2O: C 66.05, H 5.87, N 4.53. Found: C
66.30, H 5.62, N 4.59.
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1
(Ph)2-Pt(bpy)-C60. (Yield: 36%). H NMR (400 MHz, C2D2Cl4, 90
°C): δ/ppm 9.94 (d, J = 5.9 Hz, 1H), 9.70 (d, J = 5.5 Hz, 1H), 8.53 (s,
1H), 8.12 (bs, 1H), 7.91 (s, 1H), 7.52 (m, 4H), 7.46 (d, J = 5.8 Hz,
1H), 7.29 (m, 4H), 7.19 (m, 2H), 5.32 (s, 1H), 5.24 (d, J = 9.6 Hz,
1H), 4.33 (d, J = 9.6 Hz, 1H), 3.21 (m, 1H), 2.80 (m, 1H), 2.56 (s,
3H), 2.51 (m, 1H), 2.05 (m, 1H), 1.98 (m, 1H), 1.74 (m, 1H), 1.60
(m, 4H), 1.35 (bs, 12H), 0.95 (bs, 3H). FAB-MS (m/z): [M − 2H]+
1494.2. Elemental analysis calcd (%) for C101H47N3Pt·3H2O: C 78.18,
H 3.44, N 2.71. Found: C 78.03, H 3.22, N 2.88.
1
(PTZ)2-Pt(bpy)-C60. (Yield: 30%). H NMR (400 MHz, CDCl3, 50
°C): δ/ppm 9.95 (d, J = 5.7 Hz, 1H), 9.73 (d, J = 5.7 Hz, 1H), 8.54
(bs, 1H), 8.11 (bs, 1H), 7.95 (s, 1H), 7.74 (d, J = 8.2 Hz, 2H), 7.71 (d,
J = 8.2 Hz, 2H), 7.46 (d, J = 5.2 Hz, 1H), 7.26 (d, J = 1.9 Hz, 2H),
7.24 (d, J = 1.9 Hz, 2H), 6.98 (d, J = 1.9 Hz, 2H), 6.97 (d, J = 1.9 Hz,
2H), 6.83 (m, 4H), 6.77 (m, 4H), 6.29 (dd, J = 3.4, 1.2 Hz, 2H), 6.27
(dd, J = 3.4, 1.2 Hz, 2H), 5.27 (s, 1H), 5.20 (d, J = 9.6 Hz, 1H), 4.26
(d, J = 9.6 Hz, 1H), 3.15 (m, 1H), 2.73 (m, 1H), 2.58 (s, 3H), 2.04
(m, 1H), 1.95 (m, 1H), 1.74 (m, 1H), 1.56 (m, 1H), 1.43 (m, 4H),
1.29 (bs, 12H), 0.89 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): [M]+ 1890.4.
Elemental analysis calcd (%) for C125H61N5S2Pt·H2O: C 78.60, H 3.32,
N 3.67. Found: C 78.58, H 3.31, N 3.72.
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(tBu-PTZ)2-Pt(bpy)-C60. (Yield: 14%). H NMR (400 MHz, CDCl3,
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50 °C): δ/ppm 9.95 (d, J = 5.7 Hz, 1H), 9.72 (d, J = 5.7 Hz, 1H), 8.59
(bs, 1H), 8.12 (bs, 1H), 8.00 (s, 1H), 7.67 (t, J = 7.7 Hz, 4H), 7.45 (d,
J = 5.7 Hz, 1H), 7.22 (d, J = 8.3 Hz, 4H), 7.02 (t, J = 1.8 Hz, 4H), 6.85
(d, J = 8.6 Hz, 4H), 6.23 (dd, J = 8.6, 2.5 Hz, 4H), 5.33 (s, 1H), 5.17
(d, J = 9.7 Hz, 1H), 4.22 (d, J = 9.7 Hz, 1H), 3.13 (m, 1H), 2.73 (m,
1H), 2.56 (s, 3H), 2.03 (m, 1H), 1.93 (m, 1H), 1.72 (m, 1H), 1.48 (m,
5H), 1.29 (bs, 12H), 0.89 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): [M −
4H]+ 2112.1. Elemental analysis calcd (%) for C141H93N5S2Pt·2H2O:
C 78.68, H 4.54, N 3.25. Found: C 78.61, H 4.61, N 3.44.
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