Interaction of triplet C60 with p-tert-butyl-calixarenes and their complexes with pyridine derivatives

Article abstract of DOI:10.1039/b301633a

The interaction of triplet excited C₆₀ with p-tert-butyl-calix[n]arenes (BCXn, n = 4, 6 or 8) and with their 2,4,6-trimethylpyridine (TMP) and pyridine complexes has been studied with laser flash photolysis experiments. It has been found that in polar solvents ³C₆₀ is quenched by BCX6 via electron transfer, producing C₆₀^-? radical anion with a yield of 0.45 ± 0.07 in benzonitrile. In contrast, no reaction occurs in nonpolar media or with BCX4 and BCX8. Upon the addition of TMP, the rate of quenching is enhanced considerably due to the formation of TMP-BCXn complexes. The significant deuterium isotope effect indicates that the electron movement from the calixarene to the triplet excited C₆₀ is coupled to the proton displacement from the calixarene to the base. In the presence of pyridine, which has weaker hydrogen-bonding power, the enrichment of the solvate shell in the proton acceptor molecules may play an important role in promoting triplet C₆₀ quenching.

Full text of DOI:10.1039/b301633a

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Interaction of triplet C60 with p-tert-butyl-calixarenes and their
complexes with pyridine derivatives
a
Benedek Poor, Laszlo Biczok* and Miklos Kubinyi
a
ab
´
´
´
´
´
a
Chemical Research Center, Hungarian Academy of Sciences, PO Box 17, 1525 Budapest,
Hungary. E-mail: biczok@chemres.hu
Department of Physical Chemistry, Budapest University of Technology and Economics,
1521 Budapest, Hungary
b
Received 12th February 2003, Accepted 8th April 2003
First published as an Advance Article on the web 22nd April 2003
The interaction of triplet excited C60 with p-tert-butyl-calix[n]arenes (BCXn, n ¼ 4, 6 or 8) and with their 2,4,6-
trimethylpyridine (TMP) and pyridine complexes has been studied with laser flash photolysis experiments. It
ꢁ
ꢀ
3
has been found that in polar solvents C60 is quenched by BCX6 via electron transfer, producing C60 radical
anion with a yield of 0.45 ꢂ 0.07 in benzonitrile. In contrast, no reaction occurs in nonpolar media or with
BCX4 and BCX8. Upon the addition of TMP, the rate of quenching is enhanced considerably due to the
formation of TMP-BCXn complexes. The significant deuterium isotope effect indicates that the electron
movement from the calixarene to the triplet excited C60 is coupled to the proton displacement from the
calixarene to the base. In the presence of pyridine, which has weaker hydrogen-bonding power, the enrichment
of the solvate shell in the proton acceptor molecules may play an important role in promoting triplet C60
quenching.
Introduction
Calixarenes are macrocyclic phenolic compounds employed
widely as building blocks in supramolecular chemistry.1 Their
host–guest complex formation can be exploited in the large-
scale separation and purification of fullerenes.2,3 Moreover,
fullerenes can be solubilized in water by encapsulating them
in the bowl-shaped cavity of water-soluble calixarenes, which
is the prerequisite of many applications.4 Ikeda et al. demon-
strated that calixarenes with preorganized cone conformation
and a proper inclination of the benzene rings can interact with
C
60 , even without a deep inclusion of the fullerene host in the
calixarene cavity.5 The thermodynamics of the complex forma-
tion between fullerene and various calixarene derivatives indi-
cate that both solvophobic effect and p–p interactions are
major driving forces for the association process.6a–e
Scheme 1
The main goal of the present work has been to elucidate how
calixarenes influence the kinetics of the photoinduced processes
of C60 . The formula of the investigated substances is presented
in Scheme 1. The calixarenes applied consist of 4, 6 or 8 pheno-
lic units, with tert-butyl substituents attached in their para
positions to reduce the flexibility of the macrocyclic systems.
Previous studies on the reaction of triplet excited C60 with
hydrogen-bonded phenol-base pairs have given evidences
for the occurrence of concerted electron and proton move-
ment.9–11 As an extension of these investigations, we now
devote special attention to the role of calixarene-pyridine type
Despite the fundamental scientific importance and poten-
tial applications of fullerene-calixarene supramolecular sys-
tems, very few information is available on the interaction
between their constituents in the excited state. Islam et al.
have studied the excited state properties of C60 incorporated
into a cationic homooxacalix[3]arene in aqueous solution.7
They observed a large blue shift in the triplet-triplet absorp-
tion maximum and a significantly accelerated triplet decay rate
for the complex compared to those of bare C60 . Prat
et al. found sharp difference in the properties of triplet C60–
calix[8]arene complexes.8 The encapsulation of C60 in the cavity
of p-tert.-butyl-calix[8]arene had only little effect on the triplet
yield, the triplet lifetime and the efficiency of oxygen sensiti-
sation. In contrast, in case of the other calixarene derivative,
in which the phenolic OH-groups were substituted with
propylsulfonate moieties, the absorption spectrum indicated
a strong charge-transfer interaction between the components.
In addition, light absorption gave rise to the formation
of a long-lived unidentified transient, whose decay kinetics
was insensitive to oxygen.
complex formation in the reductive quenching of triplet C60
.
Experimental section
C60(99.9%, SES Research Co.) and p-tert-butyl-calix[n]arenes
(BCXn) ( > 99%, FLUKA) were used without further purifica-
tion. 2,4,6-Trimethylpyridine (TMP) and pyridine (Py) (Aldrich)
were distilled before use. Solvents (HPLC grade, Aldrich) were
dried over molecular sieve 4A (Aldrich). Samples containing
ca. 10ꢀ4 M C60 were purged with nitrogen.
DOI: 10.1039/b301633a
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The UV-visible absorption spectra were obtained with a
GBC Cintra 10E apparatus. Flash photolysis experiments were
carried out with a Nd-YAG laser (Continuum Surelight) giv-
ing 5 ns pulses at 532 nm. Absorption was detected perpendi-
cular to the laser light using a 450 W xenon lamp, an Oriel 0.25
m monochromator and two detectors: a photomultiplier
(Hamamatsu) for the 300–850 nm range and an InGaAs
photodiode (Oriel 71898) for the near-IR. Transient absorp-
tion was digitised with a Hitachi VC-6041(Z) storage oscillo-
scope interfaced to a computer. The laser energy was reduced
to ꢃ10 mJ to minimize the contribution of the triplet-triplet
annihilation.
The time evolution of the triplet C60 absorbance at 750 nm
was strictly single exponential if triplet lifetime was shorter
than ca. 4 ms in the presence of quencher. The longer-lived
traces (A(t)) were analysed taking into account the second-
order disappearance of the triplet, fitting the following func-
tion to the experimental data:12
Fig. 1 Pseudo-first-order rate constants for triplet C60 quenching vs.
BCX6 concentration in benzonitrile (S), CH2Cl2 (L), chlorobenzene
(/) and toluene (;).
A0 expðꢀk0tÞ
AðtÞ ¼
ð1Þ
nitrile, CH2Cl2 , chlorobenzene and toluene. Immediately after
the laser flash the well-known characteristic absorption of tri-
plet C60 appeared,13 whose decay was followed at 750 nm, the
maximum of the triplet-triplet absorption band. Addition of
1 þ A0½1 ꢀ expðꢀk0tÞꢄkTT=ðk0eTlÞ
where A0 is the initial absorbance, l is the optical path, eT
represents the molar absorption coefficient of triplet C60 , k0
and kTT denote the rate constants of the first- and second-
order decay processes, respectively. When absorbance of
long-lived radicals had significant contribution, the signals
were fitted with the numerical solution of the set of the differ-
ential equations describing first-order followed by second-
order kinetics.
3
BCX4 or BCX8 did not alter the C60 lifetime because no
complex formation occurs for the former compound, and the
binding of BCX8 does not affect the energy dissipation proces-
ses from the triplet state. In contrast, increasing amounts of
BCX6 gradually accelerated the triplet deactivation in benzo-
nitrile and CH2Cl2 .
The variation of the pseudo-first-order decay rate constant
(k) as a function of BCX6 concentration is shown in Fig. 1.
As can be seen, there are linear dependencies in benzonitrile
and dichloromethane, clearly demonstrating that triplet C60
readily reacts with BCX6 in a dynamic quenching process,
whereas there is no observable quenching in chlorobenzene
and toluene. The rate constants, taken from the slopes of the
plots, are k1 ¼ (1.7 ꢂ 0.3) ꢅ 109 Mꢀ1 sꢀ1 and (3.1 ꢂ 0.4) ꢅ 107
Mꢀ1 sꢀ1 in benzonitrile and CH2Cl2 , respectively. The pro-
nounced diminishing trend of k1 with decreasing solvent
polarity suggests that electron transfer is the predominant
quenching mechanism.
Results and discussion
1. Photoinduced electron transfer from calixarenes to C60
An overview of the processes taken into account in the analysis
of the experimental results is presented in Scheme 2. A fraction
of the triplet C60 molecules returns to the ground state without
interacting with the quencher in a simple first-order transition
with rate constant k0 , or in a second order, annihilation type
process with rate constants kTT . The presence of calixarene
(BCXn) in the system may cause a static quenching, via form-
ing a supramolecular complex with ground state C60 . The
equilibrium constant of this reaction is K1 . In addition, BCXn
quenches triplet C60 dynamically, producing radicals with yield
FR , besides ground state C60 . Photoinduced processes follow-
ing light absorption of the C60-BCXn complex is not included
in Scheme 2 because under the majority of our experimental
conditions negligible or small fraction of C60 is bound to calixar-
enes in the ground state. The few cases, when the excitation
of the C60-BCXn complex has to be considered, will be dis-
cussed below.
Effect of complexation in the ground state
The flash photolysis measurements on C60-BCX6 system,
applied to the study of dynamic quenching, have also shown
that in dichloromethane the initial absorbance of triplet C60
gradually decreases with increasing calixarene concentration.
E.g. a reduction of ca. 40% has been observed in case of the
solution containing 7.5 mM BCX6. This effect is attributed
to the change of the triplet yield upon complexation in the
ground state. In the other solvents the triplet yield is invariant
within the limits of experimental errors.
Quenching of triplet C60
Triplet C60 quenching by p-tert-butyl-calix[n]arenes (BCXn)
has been studied in four solvents of various polarities: benzo-
The interaction between the ground state components has
been studied via recording the steady-state absorption spectra.
The variation of the absorption spectrum of C60 upon the
addition of BCX6 is shown in Fig. 2. The upper plot clearly
demonstrates that the change of the absorbance at a particular
wavelength (A) can be described well assuming the formation
of a complex with 1:1 stoichometry.
The data have been analysed using the following expression:
A ¼ A0ð1 þ K1½BCX6ꢄeC=eFÞ=ð1 þ K1½BCX6ꢄÞ
ð2Þ
where A0 is the absorbance in the absence of BCX6, K1 the
association constant for the C60-BCX6 complex, eC/eF denotes
the ratio of the molar absorption coefficients for the complexed
and free forms of C60 . The non-linear least-squares fit of this
function to the experimental data has yielded K1 ¼ 110 ꢂ 10
Mꢀ1 in CH2Cl2 . A comparison of this value to the one
Scheme 2
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Fig. 3 Time-resolved absorption spectra in benzonitrile solution of
10ꢀ4 M C60 + 0.14 mM BCX6 immediately (L) and 9 ms after the laser
ꢁ
ꢀ
flash (/). Insert: decay of C60 anion radical absorption at 1080 nm.
decay of the triplet state, a strong IR band emerges with a
maximum at 1080 nm together with some weaker bands in
the 460ꢀ970 nꢀm range. These features are unambiguously
ꢁ
assigned to C60 anion radical on the basis of general agree-
ment with previous spectroscopic results.15
It is well-established that the radical cations of phenols are
strongly acidic, e.g. the pKa value of the 4-tert-butyl derivative
is ꢀ7 in DMSO.16 The incipient cation radicals, formed in the
electron transfer from phenols to triplet excited molecule, lose
a proton, rapidly leading to phenoxyl radicals.17 Results of
laser flash photolysis and time-resolved EPR experiments have
Fig. 2 (upper part) Variation of the absorbance at 445 nm vs. BCX6
concentration; line represents the best fit of eqn. 2. (lower part) Altera-
tion of the absorption spectrum of C60 on addition of BCX6 in
CH2Cl2 . Insert: details of the spectral evolution in the 395–435 nm
range.
ꢁ
ꢀ
confirmed the formation of phenoxyl radical and C60 anion
radical also in the photoreduction of C60 by hydroquinone.13,18
Similar behaviour is expected when a macrocyclic phenol,
BCX6 is used as electron donor. Based on the absorption
spectra of phenoxyl,19 4-methylphenoxyl20 and 4-tert-butyl-
phenoxyl21 radicals, the absorption of the phenoxyl type radical
formed from the calixarene (BCX6ꢁ) is expected to be fairly
weak and located below 450 nm. Hence, the band in the
360–450 nm region is ascribed to the superposition of the
reported in toluene14 (K1 ¼ 230 Mꢀ1) indicates that the
ground state complex is more stable in apolar solvent. It
should be noted that no significant change in the absorption
spectrum of C60 is detected on the addition of BCX6 in benzo-
nitrile. This confirms the decrease of K1 with increasing solvent
polarity. It can be calculated from the equilibrium constant
that in the dichloromethane solution with ꢆ10ꢀ4 M C60 and
7.5 mM BCX6 concentrations about 45% of C60 is complexed
by the calixarene. This value agrees with the extent of the tri-
plet yield diminution suggesting that the excitation of C60-
BCX6 complex does not lead to triplet formation. The electron
transfer within the singlet-excited complex is so rapid in
CH2Cl2 that the transition to the triplet state is not able to
compete with it. The larger energy of the excited singlet relative
to the triplet state enhances the driving force and expedites
electron transfer. The electron back-transfer in the incipient
singlet radical ion pair leads predominantly to rapid deactiva-
tion re-forming the ground state reactants.
In benzonitrile no change in the initial triplet absorbance
was observed because the rapid triplet quenching required
the use of low BCX6 concentrations, where ground state com-
plexation is precluded. C60 readily binds to BCX6 in toluene,
but the triplet yield for the complex remains the same as that
of the free C60 . The nonpolar character of the solvent slows
down the electron transfer significantly in the singlet excited
complex and the intersystem crossing becomes the dominant
deactivation process.
ꢁ
ꢀ
absorption of C60 and BCX6ꢁ.
Yield of radical formation
The insert in Fig. 3 presents a typical transient absorption of
C60 anion radical recorded at 1080 nm and the result of
the nonlinear least-squares fit assuming second-order decay
ꢁ
ꢀ
ꢁ
ꢀ
kinetics. Using15a e(C60 ) ¼ 18 300 Mꢀ1cmꢀ1 for the molar
ꢁ
ꢀ
absorption coefficient of C60 at 1080 nm, k2 ¼ 5.9 ꢅ 109
M
ꢀ1sꢀ1 is obtained for the second-order rate constant of
ꢁ
ꢀ
C60 consumption, very close to the diffusion-controlled limit.
The absorbance at 1080 nm grows at the same rate as triplet
decay (Fig. 4A) indicating that the triplet state is the precursor
ꢁ
ꢀ
of C60 . The time evolution of the signals has been fitted with
the numerical solution of the set of the differential equations
describing first-order followed by second-orderꢁ processesꢁ.
ꢀ
ꢀ
Thus, we obtained the total absorbance (DA(C60 )) of C60
corrected for reaction during the formation period, the
pseudo-first-order (k1[BCX6]) and the second order (k2) rate
constants.
The radical ion yield in the photoinduced electron transfer
reaction from BCX6 to triplet C60 can be expressed as
Transient absorption spectra
3
DA0ðC60ꢀꢁÞ eð C60Þ
k1½BCX6ꢄ
To provide a direct proof of electron transfer from BCX6 to
triplet C60 , time-resolved transient absorption spectra have
been recorded in benzonitrile (Fig. 3). The spectrum appearing
concomitant to laser pulse corresponds to the triplet-triplet
absorption of C60 . In the spectrum obtained following the
FR
¼
ð3Þ
3
eðC60ꢀꢁÞ ðk0 þ k1½BCX6ꢄÞ
DA0ð C60Þ
where k0 denotes the rate constant of triplet deactivation in the
ꢁ
absence of additive. DA0(3C60) and DA0(C60 ) represent
ꢀ
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Scheme 3
2. Effect of the addition of pyridine derivatives
The addition of a pyridine base, like TMP, to C60–calixarene
mixtures opens a new channel for the quenching of triplet
C
60 , which is shown in Scheme 3. The base and the calixarene
form a hydrogen-bonded adduct, the stability of which is char-
acterized with the equilibrium constant k2 . Triplet C60 is
quenched with this adduct with rate constant kq . Based on
the negligible change of the absorption spectrum, we conclude
that no complexation occurs between C60 and the pyridine
derivatives used in this study. In the absence of calixarenes
pyridine derivatives do not quench triplet C60
.
Interaction of calixarenes with 2,4,6-trimethylpyridine
(TMP) has been found to promote electron transfer to triplet
C
60 . Representative data are provided in Fig. 5, where the
pseudo-first-order triplet decay rate enhancement, k0 ꢀ k00 ,
normalized with respect to total calixarene concentration,
[BCXn]0 , is plotted against TMP concentration. It is worth
noting that the observed reaction rate enhancements are inde-
pendent of BCXn concentration in the presence of a large
excess of TMP. As can be seen, the reaction rate acceleration
depends markedly on the size of the calixarene ring. The extent
of reaction rate increase is similar for BCX6 and BCX8, the
compounds that readily form ground state inclusion com-
plex2,3,14 with C60 . However, the small size of BCX4 macro-
cycle prevents the close contact of the BCX-TMP complex
with triplet C60 . The larger distance of the reactants in the
collision complex decelerates the electron transfer. These
results are in agreement with the ground state complex
formation ability of BCXn. Molecular dynamic simulations
and UV absorption spectroscopic experiments carried out by
Schlachter et al.14 led to the conclusion that the equilibrium
constant of C60–BCX8 and C60–BCX6 complex formation
has similar values, but C60 is not bound to BCX4.
Fig. 4 Absorption–time profiles in the presence of 0.5 mM BCX6 in
benzonitrile A, at 750 and 1080 nm; B, at 500 nm.
the initial absorbances of the respective species, e(3C60) and
ꢁ
ꢀ
e(C60 ) are their molar absorption coefficients. The absor-
bance changes have been measured on the same solutions
applying the same laser energies.
The value of the radical yield has been determined from two
sets of data:
(i) from the initial transient absorbances at 750 and 1080
nm, where the triplet and the radical anion forms of C60 can
be detected selectively (see Fig. 3). Taking 16 100 Mꢀ1cmꢀ1
and 18 300 Mꢀ1cmꢀ1 for the molar absorption coefficient of
ꢁ
ꢀ
3C60 and C60 , respectively at the above wavelengths,13,15a
the results of several experiments have provided FR
0.48 ꢂ 0.04 for the radical yield.
¼
(ii) The transient absorbance at 500 nm, where the contribu-
tions of the two species are commensurable, allowed the
determination of FR from a single decay curve. The molar
absorption coefficients atꢁ this wavelength13 are e(3C60) ¼
It is well established that although calixarene–pyridine type
complexes can have various structures, hydrogen bonding
between the phenolic OH-group and the heterocyclic nitrogen
is always the predominant interaction.23 The hydrogen bond-
ing expedites electron transfer because of two main reasons:
ꢀ
3200 Mꢀ1cmꢀ1 and e(C60 ) ¼ 2600 Mꢀ1cmꢀ1. As an average
of the values calculated from several decay curves,
FR ¼ 0.42 ꢂ 0.04 has been obtained, in a fair agreement with
the result given above. Based on the two types of independent
measurements, we suggest FR ¼ 0.45 ꢂ 0.07 for the radical
yield from triplet reaction in benzonitrile.
Calixarenes have many conformational isomers due to the
rotational modes of the individual arene units. The flexibility
of the macrocycle is highly dependent on the temperature, sub-
stituent, solvent and ring size. Temperature-dependent NMR
measurements demonstrated22 that BCX6 is more flexible than
BCX8 and BCX4. The easier interconversion among the con-
formers in BCX6 helps to reach the molecular structure, which
transfers most effectively electron to triplet C60 . The intramo-
lecular hydrogen bonds among the phenolic HO-groups
appended on the lower rim plays a decisive role not only in
the stabilization of the conformers but it also may decrease
the oxidation potential. Previous studies have established that
hydrogen bonded phenols are much better electron donors.9–11
The lack of effects on the yield and lifetime of triplet C60 upon
addition of BCX4 and BCX8 arise probably primarily from
the higher oxidation potential of these compounds compared
with that of BCX6.
3
Fig. 5 Effect of calixarene structure on the TMP induced C60
quenching in CH2Cl2 (/) 2.5 ꢅ 10ꢀ4M BCX8, (S) 6 ꢅ 10ꢀ4M BCX6,
(L) 2 ꢅ 10ꢀ3M BCX4.
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Table 1 Equilibrium constants of complex formation and rate constants of triplet C60 quenching by BCXn–pyridine type complexes
BCX8
BCX6
BCX4
10ꢀ8kq/
Mꢀ1 sꢀ1
10ꢀ8kq/
Mꢀ1 sꢀ1
10ꢀ8kq/
Mꢀ1 sꢀ1
Solvent
PhCN
Base
Kk2in/Mꢀ1
Ka2bs/Mꢀ1
Kk2in/Mꢀ1
Ka2bs/Mꢀ1
Kk2in/Mꢀ1
Ka2bs/Mꢀ1
TMP
Py
25 ꢂ 3
11 ꢂ 3
6 ꢂ 3
Quenched without additive
Low solubility
19 ꢂ 3
0.3 ꢂ 0.2
1.1 ꢂ 0.4
0.2 ꢂ 0.1
0.5 ꢂ 0.2
a
CH2Cl2
PhCl
TMP
Py
21 ꢂ 3
22 ꢂ 3
1.5 ꢂ 0.3
1.3 ꢂ 0.2
3.8 ꢂ 0.5
1.0 ꢂ 0.3
1.3 ꢂ 0.2
3.7 ꢂ 0.9
6.1 ꢂ 0.6
1.1 ꢂ 0.3
a
a
Concave
1.5 ꢂ 0.9
Concave
TMP
Py
0.5 ꢂ 0.2
0.8 ꢂ 0.3
16 ꢂ 2
Low solubility
Low solubility
Concave
9.1 ꢂ 0.8
Concave
PhCH3
TMP
Py
Low solubility
a
Effect is too small to determine.
(i) it stabilizes the cone conformation of calixarenes and (ii)
decreases their oxidation potential. Earlier studies proved that
hydrogen-bonding with pyridine derivatives lowers the oxida-
tion potential and consequently, increases the reducing power
of phenols.11
The saturation character of the curves in Fig. 5 is attributa-
ble to the much higher reactivity of the calixarene-TMP com-
plexes relative to the free calixarenes. After the steep initial rise
the rate of each reaction tends to level off at high TMP concen-
trations, where most of BCXn is complexed. For the relative
enhancement of the decay rate constant of triplet C60 , in the
presence of calixarene BCXn and TMP, the latter in large
excess, the formula
librated with 0.5 M CH3OH or CH3OD. NMR and mass spec-
tra proved that fast isotope exchange takes place between the
alcoholic and phenolic hydroxy-groups and the lower rim of
BCX8 is fully deuterated in the presence of the ca. 100-fold
excess of CH3OD. The non-linear curve fitting of the data
shown in Fig. 7 gives kq ¼ (1.3 ꢂ 0.1) ꢅ 109
M
ꢀ1sꢀ1 and
K2 ¼ 0.5 ꢂ 0.2 Mꢀ1 for the undeuterated BCX8-TMP complex,
whereas these quantities are kq ¼ (0.9 ꢂ 0.1) ꢅ 109
M
ꢀ1sꢀ1
and K2 ¼ 0.4 ꢂ 0.2 Mꢀ1 for the deuterated complex. Thus,
we can conclude that the deuteration of the hydroxy groups
in BCX8 influences predominantly the rate of electron-transfer
from the complex to triplet C60 . Based on the marked isotope
effect on this process, we suggest that the reaction is undergone
via a trimolecular exciplex, in which the electron movement
from the calixarene to the triplet excited C60 is coupled to
the proton displacement from the calixarene to the base. A
similar reaction mechanism was found when hydrogen-bonded
k0 ꢀ k00
K2½TMPꢄ0
¼ kq
ð4Þ
½BCXnꢄ0
1 þ K2½TMPꢄ0
can be derived. In eqn. (4) k0 and k00 denote the pseudo-first-
order decay rate constants measured in the presence and
in the absence of TMP, respectively, kq is the rate constant
of the triplet C60 quenching by the TMP-BCXn complex, k2
is the equilibrium constant for the formation of the TMP-
BCXn complex. A non-linear least squares fit of this function
to the kinetic results provides the values for kq and k2 . The cal-
culated parameters are compiled in Table 1. As expected for
electron transfer, kq diminishes considerably with decreasing
solvent polarity.
The bathochromic shift of the absorption spectra of calixar-
enes upon the addition of TMP offers further evidence for
complexation. The equilibrium constants determined from
UV-spectra (Ka2bs), also given in Table 1, agree closely with
the corresponding values obtained from kinetic measurements
(Kk2in).
When we use a less basic additive, pyridine (Py), which pos-
sesses weaker hydrogen-bonding power,24 both the binding
equilibrium constant and the reaction rate enhancement is
much smaller compared with the effect of TMP. It is apparent
from Fig. 6 that even the form of the pyridine concentration
dependence is entirely different in CH2Cl2 and chlorobenzene.
Instead of approaching to a plateau, the reaction rate steadily
rises with increasing slope. This behaviour cannot be attribu-
ted merely to the local polarity augmentation because BCX8
does not quench triplet C60 even in a strongly polar solvent,
like benzonitrile. Hence, also the enrichment of the solvate
shell in the proton acceptor pyridine and the conformation
change of the macrocycle caused thereby may facilitate the
oxidation of calixarenes.
In order to gain deeper insight into the quenching mechan-
ism, we have studied the effect of the deuteration of the
hydroxy moieties in the lower rim of calixarenes, comparing
the reaction rate enhancement in 10ꢀ3 M BCX8 solutions equi-
Fig. 6 Reaction rate enhancement on addition of TMP (A) and pyr-
idine (B) in benzonitrile (S), CH2Cl2 (L) and chlorobenzene (/).
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Fig. 7 Deuterium isotope effect on the reaction rate increase 10ꢀ4 M
C60 + 10ꢀ3 M BCX8 in benzonitrile containing 0.5 M CH3OH (S) or
0.5 M CH3OD (/).
7
8
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Conclusion
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Our steady-state spectroscopic and laser flash photolysis
experiments revealed how the size of the calixarene macro-
cycle, solvent polarity and addition of pyridine derivatives
affect the photoinduced reactions with C60 . We established
that in benzonitrile triplet C60 is ꢁreadily quenched by BCX6
´
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´
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ꢀ
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of 0.45 ꢂ 0.07. The rate constant of this process was found
to be strongly dependent on the solvent polarity. In CH2Cl2 ,
excitation of the C60–BCX6 complex induced electron transfer
in the singlet excited state, which was followed by fast recom-
bination of the incipient radical ion pair leading to energy dis-
sipation. In apolar solvents, intersystem crossing proved to be
the dominant transition of the singlet excited C60–BCX6 com-
plex. Calixarenes consisting of 4 or 8 phenolic units (BCX4,
BCX8) were unreactive with both the singlet and triplet
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The addition of bases (trimethylpyridine or pyridine) form-
ing hydrogen bonded adducts with calixarenes accelerated
significantly the electron transfer to triplet C60 . The sensitivity
of the rate of this process to deuteration indicates that the tri-
plet quenching occurs via a trimolecular exciplex, in which
coupled electron proton movement plays an important role.
The concave dependence of the reaction rate enhancement
on pyridine concentration was attributed to the enrichment
of the solvate shell in the proton acceptor molecules.
Acknowledgements
The authors very much appreciate the support of this work
by the Hungarian Scientific Research Fund (OTKA,
Grant T34990 and T25561) and the 1/047 NKFP Medichem
Project.
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Phys. Chem. Chem. Phys., 2003, 5, 2047–2052