Synthesis and photoinduced electron transfer in platinum(ii) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes

Article abstract of DOI:10.1039/c4dt01397b

Platinum(ii) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes, (Cbz)₂-Pt(bpy)-C₆₀ and (^tBuCbz)₂-Pt(bpy)-C₆₀, were synthesized. Their photophysical properties were studied by electronic absorption and emission spectroscopy and the origin of the transitions was supported by computational studies. The electrochemical properties were also studied and the free energies for charge-separation and charge-recombination processes were evaluated. The photoinduced electron transfer reactions in the triads were investigated by femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, both triads undergo ultrafast charge separation from the ³MLCT/LLCT excited state within 300 fs to yield their respective triplet charge-separated (CS) states, namely (Cbz)₂⁺-Pt(bpy)-C₆₀^- and (^tBuCbz)₂⁺-Pt(bpy)-C₆₀^-, and the CS states would undergo charge recombination to give the ³C₆₀ state, which subsequently decays to the ground state in 22-28 μs. This journal is

Full text of DOI:10.1039/c4dt01397b

Dalton
Transactions
PAPER
Synthesis and photoinduced electron transfer in
platinum(II) bis(N-(4-ethynylphenyl)carbazole)-
bipyridine fullerene complexes†
Cite this: Dalton Trans., 2014, 43,
17624
Sai-Ho Lee,a Chris Tsz-Leung Chan,b Keith Man-Chung Wong,a Wai Han Lam,a
Wai-Ming Kwokb and Vivian Wing-Wah Yam*a
Platinum(II) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes, (Cbz)2–Pt(bpy)–C60 and
(tBuCbz)2–Pt(bpy)–C60, were synthesized. Their photophysical properties were studied by electronic
absorption and emission spectroscopy and the origin of the transitions was supported by computational
studies. The electrochemical properties were also studied and the free energies for charge-separation
and charge-recombination processes were evaluated. The photoinduced electron transfer reactions in
the triads were investigated by femtosecond and nanosecond transient absorption spectroscopy. In
dichloromethane, both triads undergo ultrafast charge separation from the 3MLCT/LLCT excited state
Received 12th May 2014,
Accepted 8th August 2014
within 300 fs to yield their respective triplet charge-separated (CS) states, namely (Cbz)2•+–Pt(bpy)–C60
•−
and (tBuCbz)2•+–Pt(bpy)–C60•−, and the CS states would undergo charge recombination to give the 3C60
*
DOI: 10.1039/c4dt01397b
www.rsc.org/dalton
state, which subsequently decays to the ground state in 22–28 μs.
longer than that of the singlet CS state generated via C60*,5b
as charge recombination (CR) from the triplet CS state to the
singlet ground state is a spin-forbidden process. One effective
approach to develop Cbz–C60 systems that are capable of gener-
ating the triplet CS state would be to utilize transition metal-
based chromophores for the development of donor–chromo-
phore–acceptor triads. Transition metal-based chromophores
which have low lying metal-to-ligand charge-transfer (MLCT)
excited states, such as Ru(II),6 Re(I),7 Os(II),8 Ir(III),1b,6a,b,8g,9
Cu(I)10 and Pt(II),11 have been extensively studied. Owing to the
ultrafast intersystem crossing (ISC) through the strong spin–
orbit coupling imparted by the heavy metal centres, charge
1
Introduction
The study of photoinduced electron transfer reactions in
donor–acceptor systems has been an active area of research
owing to their potential applications in artificial photo-
synthesis and photovoltaic devices.1 Donor–acceptor systems,
particularly those utilizing fullerene as an electron acceptor,
have attracted considerable attention in recent years.2 Fullerene
is known to be an excellent electron acceptor due to its sym-
metrical shape, large size, and π-electron delocalization over
the three-dimensional surface.2b,3 With small reorganization
energy, efficient charge separation and slow charge recombina-
tion were accomplished by utilizing C60 as an electron accep-
tor. On the other hand, carbazole (Cbz), an aromatic amine,
and its derivatives are known to be good electron donors4 and
have been employed to develop a number of Cbz–C60 donor–
acceptor systems with their photoinduced intramolecular elec-
tron transfer reactions well investigated.5 It has been demon-
strated that the lifetime of the triplet charge-separated (CS)
3
transfer processes generally occur at the MLCT excited state,
and hence the spin-multiplicity of the CS state can be
controlled.
Unlike Ru(II),6 Re(I),7 Os(II),8 Ir(III),1b,6a,b,8g,9 and Cu(I),10
square-planar d8 Pt(II) based donor–acceptor systems are rela-
tively less extensively studied. It was only until quite recently
that Pt(II) based donor–acceptor systems have been explored
for their photoinduced electron transfer properties,11 mainly
encouraged by the development of the Pt(II) polypyridine
alkynyl chromophores that display enhanced 3MLCT excited
state behavior through the enlargement of the d–d orbital
splitting by the incorporation of an alkynyl ligand that leads to
a higher-lying d–d ligand-field state.12 In the present study, we
develop a series of donor–chromophore–acceptor systems
based on the bipyridine Pt(II) bisalkynyl chromophore. Herein,
we report the synthesis, electrochemical and photophysical
3
state in a Cbz–C60 dyad generated via C60* is significantly
aDepartment of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong,
P.R. China
bDepartment of Applied Biology and Chemical Technology, The Hong Kong
Polytechnic University, Hung Hom, Kowloon, Hong Kong, P.R. China.
E-mail: wwyam@hku.hk
†Electronic supplementary information (ESI) available: The selected singlet excited
states (Sn) computed by TDDFT/CPCM calculation, selected molecular orbitals and
Cartesian coordinates of the optimized structures. See DOI: 10.1039/c4dt01397b
17624 | Dalton Trans., 2014, 43, 17624–17634
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Paper
Electronic absorption and emission spectroscopy
The electronic absorption spectra of (Cbz)2–Pt(bpy) and
(tBuCbz)2–Pt(bpy) exhibit an intense absorption band at ca.
280–360 nm and a moderately intense band at ca. 360–450 nm
with a λmax at ca. 400 nm in dichloromethane (Fig. 1). The
high-energy intense absorptions are the π–π* transitions of the
carbazole, bipyridine and alkynyl moieties, whereas the less
intense absorptions in the visible region are 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.12b,i,j With the C60 moiety
appended in (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60,
an absorption tail extending to 700 nm, which can also be
observed in the electronic absorption spectrum of N-methylful-
leropyrrolidine,3b is tentatively assigned to the π–π* transitions
of C60. TDDFT calculations were performed on selected com-
plexes (Cbz)2–Pt(bpy) and (Cbz)2–Pt(bpy)–C60 to provide
further insight into the nature of the electronic transitions for
the complexes. To reduce the computational cost, the dodecyl
group of N-dodecylpyrrolidine on C60 for (Cbz)2–Pt(bpy)–C60
was replaced by a methyl group and was labeled as (Cbz)2–Pt-
(bpy)–C60′ (see Computational details). Selected singlet–singlet
transitions for the complexes are listed in Table S1† and the
molecular orbitals involved in the transitions are shown in
Fig. 2 and S1.† The first two transitions computed at 484 and
451 nm for (Cbz)2–Pt(bpy) are predominantly the LLCT
[π(CuC–Ph–Cbz)→π*(bpy)] transitions, slightly mixed with the
MLCT[dπ(Pt)→π*(bpy)] character (Table S1†). For (Cbz)2–Pt-
(bpy)–C60′, the several transitions computed to be less intense
in the lower-energy region are the IL π–π* transitions of the
C60. The LLCT [π(CuC–Ph–Cbz)→π*(C60)] excited state is the
S2 state which is close in energy to the lowest-energy IL(C60)
state. When compared with (Cbz)2–Pt(bpy), the LLCT
[π(CuC–Ph–Cbz)→π*(bpy)]/MLCT[dπ(Pt)→π*(bpy)] transitions
for (Cbz)2–Pt(bpy)–C60′ are found to be red-shifted.
Chart 1
studies of a series of donor–chromophore–acceptor triads
(Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 as well as their
relevant model compounds, (Cbz)2–Pt(bpy), (tBuCbz)2–Pt(bpy),
t
CbzC6H4–CuC–Ph and BuCbzC6H4–CuC–Ph (Chart 1), using
a variety of experimental techniques including electronic
absorption and emission spectroscopy, cyclic voltammetry,
femtosecond and nanosecond time-resolved transient absorption
spectroscopy, and the photochemical processes are identified
and the rate constants are determined. Density functional theory
(DFT) and time-dependent density functional theory (TDDFT)
calculations were performed to provide further insights into the
origin of the electronic absorption and emission properties.
Results and discussion
Synthesis
On photoexcitation, (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy)
The synthesis of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 display observable emission in dichloromethane (Fig. 3), while
is shown in Scheme 1. N-(4-Ethynylphenyl)carbazole and N- fullerene-appended complexes (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–
(4-ethynylphenyl)-3,6-di-tert-butyl-carbazole were first prepared Pt(bpy)–C60 are non-emissive under identical conditions. The
by the Ullmann-type reaction between carbazoles and emission is red-shifted after the introduction of the di-tert-
1-bromo-4-iodobenzene, followed by the palladium-catalyzed butyl groups at the 3,6-positions of carbazole in (tBuCbz)2–Pt-
Sonagashira coupling with (trimethylsilyl)acetylene (TMSA), (bpy). The emission of (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy)
and the subsequent removal of the TMS group with potassium was found to be strongly quenched with their luminescence
carbonate in tetrahydrofuran–methanol solution. On the other quantum yields determined to be 0.03 and 0.01, respectively,
hand, according to literature procedures,13 the fullerene- much lower than 0.50 for a platinum(II) diimine bis(alkynyl)
appended bipyridine derivative was obtained by the oxidation chromophore.12a The weak emissive nature of the carbazole-
of 4,4′-dimethyl-2,2′-bipyridine with selenium oxide yielding containing complexes may possibly be due to the occurrence
4-methyl-2,2′-bipyridine-4′-carboxaldehyde, followed by the of a quenching process in the excited state. A possible
incorporation of fullerene with the Prato reaction.14 The fuller- interpretation of this is that a competitive process of photo-
ene-appended bipyridine derivative was reacted with induced electron transfer between the carbazole moiety and
[PtCl2(DMSO)2] to afford the dichloroplatinum(II) precursor, the Pt(II) bipyridine chromophore that leads to photochemical
which was then reacted with alkynes using copper-catalyzed charge separation has taken place. Similarly, the non-
coupling reaction to yield (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2– emissive nature of the fullerene-containing complexes
Pt(bpy)–C60, with their identities confirmed by 1H NMR could again be ascribed to photoinduced electron transfer
spectroscopy, FAB mass spectrometry and elemental analysis.
processes.
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Scheme 1 Synthesis of donor–chromophore–acceptor triads, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60. Reagents and conditions: (i)
4-bromoiodobenzene, K2CO3, 18-crown-6, Cu, DMF, reflux; (ii) PdCl2(PPh3)2, CuI, PPh3, TMSA, Et3N, reflux; (iii) K2CO3, MeOH–THF, r.t.; (iv) SeO2,
dioxane, reflux; (v) N-dodecyl glycine, C60, PhMe, reflux; (vi) PtCl2(DMSO)2, CH2Cl2, r.t.; (vii) CuI, Et3N, CH2Cl2, r.t.
Electrochemistry
tials to +1.24 V in (tBuCbz)2–Pt(bpy)–C60. The redox potentials
of the investigated compounds are summarized in Table 1.
The free energy changes for CS and CR processes in (Cbz)2–
Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 are evaluated with the
corrections of the Coulombic interactions between the centres
of positive and negative charges using the Weller equations15
In order to evaluate the free energy changes associated with
the CS and CR processes in (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–
Pt(bpy)–C60, cyclic voltammetry was carried out in 0.1 M
n-Bu4NPF6 solution in deaerated dichloromethane. The first
and second reduction couples in the triads are assigned to the
reduction of the C60 moiety, with the observed reduction
potentials E°(C60/C60•−) and E°(C60•−/C602−), at −0.59 V and
−0.98 V (vs. SCE). Furthermore, the third reduction couple,
which is assigned as the bipyridine ligand-centred reduction,
E°[Pt(bpy)/Pt(bpy)•−], is observed at −1.32 V, anodically shifted
by 50–60 mV compared to that of (Cbz)2–Pt(bpy) and
(tBuCbz)2–Pt(bpy). On the other hand, the first oxidation
couple in the triads is found to be associated with the oxi-
dation of the carbazole unit with the potential for oxidation of
+1.36 V for (Cbz)2–Pt(bpy)–C60. The introduction of the di-tert-
butyl group at the 3,6-positions of carbazole shifts the poten-
ꢀ
ꢁ
e
ΔGCR
¼
E°ðA=A•ꢀÞ ꢀ E°ðD•þ=DÞ þ
eV
ð1Þ
4π ε0εSRDA
with the distances RPt–C = 9.80 Å, RD–Pt = 10.49 Å, RD–C (cis-)
60
60
= 15.08 Å and RD–C (trans-) = 19.51 Å obtained from the opti-
60
mized structure of (Cbz)2–Pt(bpy)–C60′. The calculated free
energy changes for the CS and CR processes are listed in
Table 2. The energy differences of the 3MLCT/LLCT excited
state and the ground state (E00) are estimated from the inter-
section of the emission and excitation spectra of (Cbz)2–
Pt(bpy) and (tBuCbz)2–Pt(bpy) measured at room temperature
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Paper
in deaerated dichloromethane, and are estimated to be 2.51 eV
and 2.42 eV, respectively.
Transient absorption spectroscopy
The photoinduced electron transfer reactions in (Cbz)2–
Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 were studied by femto-
second transient absorption spectroscopy and the resulting
spectra were subjected to global fitting with a sequential
model16 to yield evolution-associated difference spectra (EADS)
and with a parallel model to yield decay-associated difference
spectra (DADS) (Fig.
4 and 5). (Cbz)2–Pt(bpy)–C60 and
(tBuCbz)2–Pt(bpy)–C60 behave similarly upon photo-excitation
at 400 nm in dichloromethane with the first resolved com-
ponent depicting the formation of the 3MLCT/LLCT excited
3
state (Fig. 4b and 5b). The MLCT/LLCT excited state is gene-
Fig. 1 UV-Visible absorption spectra of (Cbz)2–Pt(bpy)–C60 and
(tBuCbz)2–Pt(bpy)–C60 as well as their model compounds (Cbz)2–
Pt(bpy) and (tBuCbz)2–Pt(bpy) in CH2Cl2.
rated within the instrumental response time with a time con-
stant of τ < 150 fs via ISC of the initially populated 1MLCT/
LLCT excited state upon laser irradiation. The first EADS
Fig. 2 Spatial plots (isovalue = 0.03) of selected molecular orbitals of (Cbz)2–Pt(bpy)–C60’ obtained from PBE0/CPCM calculation.
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These absorptions are different from the characteristic signal
of Cbz•+ (λmax ∼ 750 nm).17 This may be due to the charge de-
localization across the Pt–CuC–Ph and the Cbz/tBuCbz units
in the 3LLCT excited state, (Cbz)2•+–Pt(bpy)•−–C60 and
(tBuCbz)2•+–Pt(bpy)•−–C60 (see HOMO and HOMO−1 of (Cbz)2–
Pt(bpy) in Fig. S1†), leading to absorptions quite similar to
•+ 17
that observed for the cation radical of a Cbz dimer, (Cbz)2
.
3
Subsequent to the formation of the MLCT/LLCT excited state
is the electron transfer reaction generating the fully CS
•−
species, namely (Cbz)2•+–Pt(bpy)–C60
and (tBuCbz)2•+–Pt-
(bpy)–C60•−, with a rise time τ1 = 185 fs and 221 fs, respectively.
The formation of the CS states is characterized by the strong
transient absorption at ca. 1000 nm, which is the characteristic
•− 1a,3b,5e
signal of C60
.
It should be noted that the formation of
the CS states is very fast, much faster than that of the
vibrational relaxation of the 3MLCT/LLCT excited state which
was determined to be ∼4 ps.12i As a result, the photoinduced
electron transfer reaction is expected to occur at a vibrationally
unrelaxed 3MLCT/LLCT excited state,18 and the subsequent
vibrational relaxation of the resulting CS state occurs at a time
constant τ2 = 1.7 ps for (Cbz)2–Pt(bpy)–C60 and 1.5 ps for
(tBuCbz)2–Pt(bpy)–C60. The lifetime of the CS states is deter-
mined to be τ3 = 56 ps for (Cbz)2–Pt(bpy)–C60 and τ3 = 331 ps
for (tBuCbz)2–Pt(bpy)–C60. On the other hand, laser irradiation
Fig. 3 Normalized emission spectra of (Cbz)2–Pt(bpy) and (tBuCbz)2–
Pt(bpy) in deaerated CH2Cl2.
Table 1 Standard potentials (vs. SCE, in V)a of (Cbz)2–Pt(bpy)–C60 and
(tBuCbz)2–Pt(bpy)–C60, and their model compounds CbzC6H4–CuC–
Ph, tBuCbzC6H4–CuC–Ph, (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) in
CH2Cl2
E°/V (vs. SCE) in CH2Cl2
1
at 400 nm also populates the C60 excited state, where the
Pt(bpy)/
extinction coefficient (ε) of N-methylfulleropyrrolidine at
•−
2−
Molecule
D•+/D
C60/C60
C60•−/C60
Pt(bpy)•−
400 nm is ca. 5000 dm3 mol−1 cm−1 3b
Therefore, the first four
.
CbzC6H4–CuC–Ph
+1.35b
—
—
—
—
—
—
—
—
−0.98
−0.98
—
—
−1.37
−1.38
−1.32
−1.32
resolved EADS components of both complexes depict the pres-
tBuCbzC6H4–CuC–Ph +1.21
ence of the 1C60 excited state with absorption features at λmax
=
(Cbz)2–Pt(bpy)
+1.33b
+1.24
1
510 and 890 nm.3b The C60* eventually proceeds to the
(tBuCbz)2–Pt(bpy)
(Cbz)2–Pt(bpy)–C60
3C60* through ISC at time constants of τ4 = 896 ps and τ4
=
+1.36b −0.59
(tBuCbz)2–Pt(bpy)–C60 +1.24
−0.59
943 ps for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60,
a The standard potentials were determined as E° = (E°pa + Ep°c)/2 by
respectively. The final resolved component with strong transi-
cyclic voltammetry in CH2Cl2 using n-Bu4NPF6 (0.1 M) as the ent absorption between 550 and 750 nm (λmax = 680 nm) is
supporting electrolyte; scan rate = 100 mV s−1
couple.
.
b Quasi-reversible
3
attributed to the C60* with a lifetime τ5 > 6 ns.3b,5e,19 In order
to investigate reactions at times over 6 ns, nanosecond transi-
ent absorption spectroscopy was carried out on both triads
and their observed absorption difference spectra taken at t =
shows a broad absorption between 550 and 700 nm, which is 5 μs after laser irradiation at 355 nm in dichloromethane are
attributed to the 3MLCT excited state.12b,i A positive absorption shown in Fig. 4d and 5d. These spectra match the EADS of the
at wavelengths shorter than 475 nm as well as a broad absorp- final component of both complexes with strong transient
tion in the near-infrared region (λ > 900 nm) are also observed. absorption between 550 and 750 nm (λmax = 680 nm), which
Table 2 Free energy changes ΔG (eV) for electron-transfer reactions in CH2Cl2 calculated from the electrochemical potentials after correction for
the Coulomb interaction between the chargesa
Reaction
CS
CR
(Cbz)2–Pt(bpy)–C60 → (Cbz)2•+–Pt(bpy)•−–C60
(Cbz)2•+–Pt(bpy)•−–C60 → (Cbz)2•+–Pt(bpy)–C60
—
−2.53
−0.69 (−0.66)b
—
−1.84 (−1.87)b
−2.41
•−
(tBuCbz)2–Pt(bpy)–C60 → (tBuCbz)2•+–Pt(bpy)•−–C60
(tBuCbz)2•+–Pt(bpy)•−–C60 → (tBuCbz)2•+–Pt(bpy)–C60
−0.69 (−0.66)b
−1.72 (−1.75)b
•−
a ΔGCR = E°(A/A•−) − E°(D•+/D) + e/4πε0εsRDA is for charge-recombination where RDA is the charge-separation distance determined from DFT; ΔGCS
is the driving force for charge-separation. The parameters used are ε0 = 8.85, εs = 8.93 for CH2Cl2, RPt–C = 9.80 Å, RD–Pt = 10.49 Å, RD–C (cis-) =
60
15.08 Å, RD–C (trans-) = 19.51 Å with the distances taken from the DFT optimized structure of (Cbz)2–Pt(6b0py)–C60′. b Driving forces for radical ion
60
pairs in the trans-position.
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Fig. 4 Transient absorption difference spectra of (Cbz)2–Pt(bpy)–C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the spectro-
scopic 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 460 nm, 680 nm and 1020 nm; and (d) taken at 5 μs after nanosecond laser pulse excitation at 355 nm in deaerated
CH2Cl2, with the time-decay profile of optical density at 690 nm (inset).
3
3
is the characteristic signal of the C60*.3b,5e,19 The C60* the CS transient absorption is accompanied by the rise of the
eventually decays to the ground state at time constants of τ = 3C60* absorption at a similar rate (Fig. 4c and 5c).
28 and 22 μs, respectively, for (Cbz)2–Pt(bpy)–C60 and
(tBuCbz)2–Pt(bpy)–C60 (Fig. 4d and 5d), close to that found for
a functionalized C60 of 25 μs.20 The energetic and kinetic data
Conclusions
for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 are summar-
ized in Scheme 2.
Donor–chromophore–acceptor triads, (Cbz)2–Pt(bpy)–C60 and
(tBuCbz)2–Pt(bpy)–C60, were synthesized and their photo-
•−
It is noteworthy that the CS states, (Cbz)2•+–Pt(bpy)–C60
and (tBuCbz)2•+–Pt(bpy)–C60•−, are produced via photoinduced physical and electrochemical properties were studied. The
electron transfer from the 3MLCT/LLCT excited state, and photoinduced electron transfer processes were investigated by
therefore are in the triplet manifold. As the CR processes occur femtosecond and nanosecond transient absorption spec-
in a few hundred picoseconds, CR should therefore proceed troscopy. Laser irradiation at 400 nm predominantly excites
through a spin-allowed process to the lower-lying C60* state the MLCT/LLCT transition, followed by an ultrafast ISC to
instead of a much slower spin-forbidden direct recombination generate the MLCT/LLCT excited state, from which CS occurs
3
3
process to the singlet ground state. In addition, with large within a picosecond. The triplet CS state decays to the energe-
3
driving forces (ΔG = −1.84 eV for (Cbz)2–Pt(bpy)–C60 and tically lower-lying spin-allowed C60* state in a few hundred
−1.72 eV for (tBuCbz)2–Pt(bpy)–C60) and the small reorganiz- picoseconds instead of the spin-forbidden singlet ground
ation energy in C60-containing complexes,14,15 CR processes state. The resulting long-lived C60* state decays to the ground
3
are expected to be in the Marcus inverted region,21 and hence state in the microsecond timescale. The ultrafast ISC in the
the direct recombination process to the ground state should be triads through the strong spin–orbit coupling mediated by
3
slow. The recombination to the lower-lying C60* state is the platinum(II) centre and the subsequent ultrafast electron
further supported experimentally in that the diminishing of transfer, which eventually generates the CS state in the triplet
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Fig. 5 Transient absorption difference spectra of (tBuCbz)2–Pt(bpy)–C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the spectro-
scopic 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 460 nm, 680 nm and 1020 nm; and (d) taken at 5 μs after nanosecond laser pulse excitation at 355 nm in deaerated
CH2Cl2, with the time-decay profile of optical density at 680 nm (inset).
manifold, establishes another strategy for the development of tions for photophysical studies were deaerated on a high-
new donor–acceptor systems that are capable of effectively gen- vacuum line in a two-compartment cell that consisted of a
erating triplet CS states with long CS lifetimes.
10 mL Pyrex bulb and a 1 cm path length quartz cuvette and
sealed against the atmosphere with a Bibby Rotaflo HP6 Teflon
stopper. The solutions were rigorously degassed with at least
four successive freeze–pump–thaw cycles. The luminescence
quantum yield was measured at room temperature using
[Ru(bpy)3]Cl2 as a standard (Φem = 0.06).22
Experimental section
General
1H NMR spectra were recorded on a Bruker AVANCE 300
(300 MHz) or Bruker AVANCE 400 (400 MHz) Fourier-transform
Electrochemical measurements
NMR spectrometer with chemical shifts reported relative to The cyclic voltammetry measurements were performed on a
tetramethylsilane (Me4Si). Positive-ion FAB mass spectra were CHI 620A electrochemical analyzer in deaerated dichloro-
recorded on a Thermo Scientific DFS high-resolution magnetic methane containing 0.10 M n-Bu4NPF6 as the supporting elec-
sector mass spectrometer. Elemental analyses of the newly syn- trolyte at 298 K. A glassy carbon working electrode was
thesized complexes were performed using a Flash EA 1112 polished with Linde polishing alumina suspension and rinsed
elemental analyzer at the Institute of Chemistry, Chinese with deionized water followed by dichloromethane before use.
Academy of Sciences, Beijing. Ultraviolet-visible spectra were A platinum wire was used as a counter electrode and an Ag/
recorded on a Varian Cary 50 UV-vis spectrophotometer in AgNO3 (0.1 M in acetonitrile) was used as a reference electrode.
spectroscopic grade CH2Cl2. Steady-state emission spectra All experiments were followed by the addition of ferrocene and
were recorded on a Spex Fluorolog-3 Model FL3-211 spectro- the ferrocene/ferricenium redox potential was used as an
fluorometer equipped with a R2658P PMT detector. All solu- internal reference.
17630 | Dalton Trans., 2014, 43, 17624–17634
This journal is © The Royal Society of Chemistry 2014

Dalton Transactions
Paper
by global analysis16 with the graphical interface program
Glotaran23 (http://glotaran.org/).
Nanosecond transient absorption spectroscopy experiments
were performed using an LP920-KS Laser Flash Photolysis
Spectrometer (Edinburgh Instruments Ltd, Livingston, UK) at
ambient temperature. 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 recorded using an image-intensified
CCD camera (Andor) with a PC plug-in controller, fully
operated by the L900 spectrometer software. The absorption
kinetics were determined using a Hamamatsu R928 photo-
multiplier tube and recorded using
TDS3012B (100 MHz, 1.25 GS s−1
a
Tektronix Model
)
digital oscilloscope.
Samples were freshly prepared and degassed with at least four
freeze–pump–thaw cycles on a high-vacuum line in a two com-
partment cell that consisted of a 10 mL Pyrex bulb and a 1 cm
path length quartz cuvette.
Computational details
Calculations were carried out using the Gaussian 09 software
package.24 The ground-state geometries of (Cbz)2–Pt(bpy) and
(Cbz)2–Pt(bpy)–C60′ were fully optimized by the density func-
tional theory (DFT) with the hybrid Perdew, Burke, and Ernzer-
hof functional (PBE0).25 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)26 calculation at
the same level of theory associated with the conductor-like
polarizable continuum model (CPCM)27 using CH2Cl2 as the
solvent was employed to compute the singlet–singlet transi-
tions for the complexes. For all the calculations, the Stuttgart
effective core potentials (ECPs) and the associated basis set
were applied to describe Pt28 with f-type polarization functions
(ζ = 0.993),29 while the 6-31G(d,p) basis set30 was used for all
other atoms. The DFT and TDDFT calculations were performed
with a pruned (99 590) grid.
Scheme 2 Schematic energy diagram for photoinduced electron trans-
fer of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 in CH2Cl2. aEner-
gies of C60* and C60* excited states are obtained from ref. 19a.
bEnergy for radical ion pairs in the trans-position.
1
3
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. The sample solutions were
excited using a 400 nm pump laser; the subsequent excited
state processes taking place on the 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 thermo-
electric cooled CCD detector. The spectral window for the TA
covers 450–740 nm and 600–1050 nm regions. TA signals at
wavelengths from 740 nm to 880 nm were masked by the
800 nm fundamental and thus were not shown in the fs-TA in
Fig. 4 and 5. Temporal delay of the probe relative to the pump
pulse was controlled by an optical delay line in the fs-TA
system. The instrument response function (IRF) of the ultrafast
system is wavelength-dependent. As the detection wavelength
varies from 1050 to 450 nm, the IRF changes from ∼150 to
250 fs. For the fs-TA spectra, corrections have been made to
eliminate the wavelength-dependent time shift caused by the
group velocity dispersion. The fs-TA spectra were analyzed
Synthesis
3,6-Di-tert-butylcarbazole,31
N-(4-bromophenyl)-3,6-di-tert-butylcarbazole,32
phenyl)carbazole,33 N-(4-ethynylphenyl)-3,6-di-tert-butylcarba-
N-(4-bromophenyl)carbazole,32
N-(4-ethynyl-
zole,33
9-(4-(2-phenylethynyl)phenyl)carbazole
(CbzC6H4–
CuC–Ph),34 3,6-di-tert-butyl-9-(4-(2-phenylethynyl)phenyl)car-
bazole (tBuCbzC6H4–CuC–Ph)34 and fullerene-appended
bipyridine13 were prepared according to literature procedures.
General procedure for platinum(II) complexation
4,4′-Dimethyl-2,2′-bipyridine/fullerene-appended
bipyridine
and [PtCl2(DMSO)2] were stirred in dichloromethane under N2
for 15 h. The solvent was then reduced in volume and diethyl
ether was added. The resulting precipitate, dichloroplatinum(II)
precursor, was filtered and dried. Dichloroplatinum(II) pre-
cursor and arylacetylene (2.5 equiv.) were added to deaerated
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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 reac-
tion 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 the eluent,
and then recrystallized from chloroform–diethyl ether or
chloroform–methanol.
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(tBuCbz)2–Pt(bpy). Yield: 62%. H NMR (400 MHz, CDCl3):
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9.6 Hz, 1H), 4.26 (d, J = 9.6 Hz, 1H), 3.19 (m, 1H), 2.78 (m, 1H),
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1.50 (s, 39H), 1.34 (bs, 12H), 0.94 (t, J = 6.8 Hz, 3H). FAB-MS
(m/z): [M − 2H]+ 2049.5. Elemental analysis calcd (%) for
C141H93N5Pt·CH3OH·1/2CHCl3: C 79.83, H 4.58, N 3.27; found:
C 79.89, H 4.95, N 3.55.
Acknowledgements
V.W.-W.Y. acknowledges support from The University of Hong
Kong and the URC Strategic Research Theme on New
Materials. This work was 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 Infor-
mation Technology Services of The University of Hong Kong
for providing computational resources.
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