Formation and photophysics of a stable concave-convex supramolecular complex of C60 and a substituted s-triazine derivative

Article abstract of DOI:10.1039/b207661f

Spectroscopic, electrochemical and computational data (show that C₆₀ and a highly phenylated s-triazine derivative form a stable supramolecular complex at micromolar concentrations in solution at ambient temperatures, due to strong van der Waals attraction between their complementary surfaces.

Full text of DOI:10.1039/b207661f

Formation and photophysics of a stable concave–convex supramolecular
complex of C60 and a substituted s-triazine derivative
David I. Schuster,*a Joel Rosenthal,a Shaun MacMahon,a Peter D. Jarowski,a Christopher A. Alabia
and Dirk M. Guldi*b
a Department of Chemistry, New York University, New York, NY, 10003, USA.
E-mail: david.schuster@nyu.edu
b Radiation Laboratory, University of Notre Dame, Notre Dame, IN, 46556, USA. E-mail: guldi.1@nd.edu
Received (in Corvallis, OR, USA) 5th August 2002, Accepted 16th September 2002
First published as an Advance Article on the web 1st October 2002
Spectroscopic, electrochemical and computational data
show that C60 and a highly phenylated s-triazine derivative
form a stable supramolecular complex at micromolar
concentrations in solution at ambient temperatures, due to
strong van der Waals attraction between their complemen-
tary surfaces.
of 1b with cyanuric chloride in the solid state.9 All products
were characterized by 1H and 13C NMR, UV-Vis and
fluorescence emission spectroscopy as well as mass spectrome-
try. There is no obvious difference between the UV-Vis
spectrum of a 1+1 mixture of 1 and C60 in toluene or o-
dichlorobenzene (ODCB) versus the sum of the spectra for the
two independent moieties. However, evidence for ground state
complexation was provided by electrochemical data obtained in
ODCB vs. Ag+/Ag. The first three reduction potentials of free
C60 (21.09 V, 21.52 V and 22.04 V) are substantially
increased in the presence of one equivalent of 1 (both 8.3 3
1024 M) to 20.95 V, 21.39 V and 21.90 V, respectively,
indicating that a new molecular species is formed in which the
C60 moiety is more easily reduced. In the presence of C60, the
oxidation potential of triazine 1 in ODCB increases from 0.28 V
to 0.31 V.
While innumerable cases of intramolecular photoinduced
electron transfer (PET) between redox centres with covalent and
even mechanical linkages have been described in the lit-
erature,1–3 far fewer cases of well-defined, reversible inter-
molecular PET have been observed. This is hardly surprising,
since intermolecular quenching interactions in solution, which
optimally are diffusion controlled processes, are not able to
efficiently compete with deactivation of photoexcited electron
donors on the pico-/nanosecond lifetime scale, when the
acceptor is present at micromolar concentrations. Conversely,
intermolecular donor–acceptor interactions involving C60 are
well known in the solid state.4
Formation of supramolecular complexes is a simple and
powerful means to engineer ultrafast electron transfer in
solution. Since the formation of such ensembles involves either
entropically or enthalpically driven organization of individual
components, the yields of such complexes frequently exceed
those obtained for analogous covalently linked systems.
An important incentive of our work on donor–acceptor
electronic interactions is to develop synthetic host systems that
would show easily detectable, but differentiable, output signals
upon associating with guests of varying shape and size. The
unique structures and electronic features of fullerenes make
them ideal candidates for such studies.5 Therefore, electron
donors whose shape might allow strong association in solution
with the curved surface of C60 have been sought. It has been
observed that fullerenes fit nicely into the preorganized concave
cavities of some host molecules in solution, in particular
cyclotriveratrylenes (CTVs),6 calix[5]arenes7 and substituted
corannulenes.8 Association constants with C60 and C70 in these
systems range between 102–103 M21, with higher values
reported for some acylated CTV derivatives. However, these
systems are either not capable of undergoing PET or such data
were not reported.
Upon excitation of a 2.5 3 1025 M solution of 1 in toluene or
ODCB at 300 nm, which is close to the ground-state absorption
maximum of 1, strong fluorescence emission was observed at
409 nm. Under these conditions, competitive light absorption by
C60 was minimal. The fluorescence emission intensity de-
creased in the presence of C60 in the concentration range
1.25–7.5 3 1025 M, eventually reaching a plateau value (see
Fig. 1). Inspection of this figure reveals that simultaneously
with the quenched emission of 1 at 409 nm, a new lower-energy
emission band appears, centred at 423 nm. We tentatively
attribute this new emission to a charge-transfer (CT) state, i.e.,
(1·+–C602).11 In ODCB, which is a more polar solvent, this
maximum shifts to 427 nm, consistent with improved solvation
of the CT state.
The fluorescence of 1 is well fitted by a mono-exponential
decay, for which a relatively short lifetime of 0.22 ns was
determined in deoxygenated toluene. Upon addition of C60 to
this solution, a double-exponential fluorescence decay was
observed at 425 nm with lifetimes of 0.19 ns and 1.9 ns. The two
lifetimes are maintained throughout the titration assay, with an
increasing contribution of the long-lived component as the
Highly phenylated triamino-s-triazines are good electron
donors (IP1 ~ 0.9 V vs. SCE)9 and are also highly fluorescent,
which makes them ideal for studies of intermolecular electron
transfer interactions. Our approach is to use conformationally
flexible substituted triazines which are able to adapt their shape
to provide a concave binding site for fullerenes, in which
ultrafast and efficient electron transfer can take place. We
present evidence that remarkably strong intermolecular associa-
tion occurs between fullerenes and triazine 1 at micromolar
concentrations in condensed media at room temperature, and
that this complexation controls the ground and excited state
dynamics of these binary systems.
Triazine 1 was synthesized following the procedure outlined
in Scheme 1. The triazine arms were prepared using Pd-
catalysed aryl-amination chemistry10 to yield 1a and 1b. The
final product was prepared by condensation of three equivalents
Scheme 1 Synthesis of tris-p-methoxyphenyltriamino-s-triazine 1.
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both absorb at much shorter wavelengths than 1. Computation-
ally, the complex of C60 with tris-diphenylamino-s-triazine was
unstable, dissociating without a barrier to the individual
moieties.
In conclusion, the convex shape of C60 is well suited for
efficient and selective association with the concave shape of
triazine 1 at concentrations as low as 1025 M, which allows
efficient intermolecular PET to take place on the nanosecond
time scale. The association constant, greater than 105 M21, is
unprecedented for binding between pristine C60 and a simple
monomeric host chromophore in solution. These effects are
directly attributable to van der Waals attraction between the
complementary surfaces of the two moieties. Complex forma-
tion is reversible and pH dependent. Photoexcitation of the
complex leads to a new strongly emissive species, possibly a
charge-transfer state, which does not dissociate to give free
fullerene radical anions. One can easily envisage generation of
more stable complexes as well as larger molecular assemblies
incorporating fullerenes and triazines using this novel type of
supramolecular complexation. Experiments along these lines
are in progress.
Fig. 1 Fluorescence spectra of 1 (2.5 3 1025 M) and increasing
concentrations of C60 (1.25–9.0 3 1025 M), proceeding from top to bottom,
in toluene at 27 °C, with laser excitation at 300 nm. Insert: Relative
fluorescence intensities, I/Io, vs. C60 concentration. Curve shown was used
to determine K (see text).
The authors are thankful to the National Science Foundation
(Grants CHE 9712735 and 0097089) and the Office of Basic
Energy Sciences of the US Department of Energy for support of
this research. This is contribution NDRL-4413 from the
Radiation Laboratory. J. R. is thankful to Pfizer Central
Research and Development, Groton, CT, for an undergraduate
summer research fellowship, and S. M. is grateful to the Organic
Division of the American Chemical Society for a graduate
fellowship sponsored by Glaxo-SmithKline. We also acknowl-
edge helpful discussions with Professor Stephen R. Wilson.
concentration of C60 is increased. At all concentrations, the pre-
exponential factors were in good agreement with the ratio of
complexed versus uncomplexed forms. Upon addition of
trifluoroacetic acid, the original short-lived fluorescence com-
ponent reappears, concomitant with disappearance of the long-
lived species, demonstrating that complex formation is reversi-
ble. While C70 also quenches 1, the association constant is lower
by about an order of magnitude. These data all point to a static
quenching event occurring inside a well-defined supramo-
lecular complex of 1 and C60.
To verify the proposed concave–convex geometry of the
1–C60 complex, in vacuo computations were performed using
Insight II.12 The resultant structure (Fig. 2a) depicts the C60
sphere nestled within the arms of the triazine. The centre-to-
centre distance between the triazine ring and C60 is only 7.07 Å,
while the distance between the closest atoms of the two moieties
is 3.79 Å. The binding energy of the complex is 26.7 kcal
mol21, relative to the separate molecules. In contrast, much
looser association is predicted computationally for the complex
of 1 with C70, in which the side arms of the triazine do not
appear to interact with the graphitic equatorial belt of C70 (see
Fig. 2b). The computed binding energy in this case is only 4.6
Notes and references
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11 Transient absorption measurements indicate that this emissive state
( ~ 1.4 eV above So) decays quantitatively to the ground state without
generating free fullerene radical anions or fullerene triplet excited
states.
12 D. I. Schuster, P. D. Jarowski, A. N. Kirschner and S. R. Wilson, J.
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Fig. 2 Computed minimum energy structure of (a) supramolecular complex
of 1 and C60, and (b) the 1–C70 complex.
Fitting the titration curve in the inset of Fig. 1 according to an
established formulation,13 values of 1.8 ± 0.08 3 105 and 3.4 ±
0.9 3 105 M21 were obtained for the association constants of 1
and C60 in toluene and ODCB, respectively. In control studies,
no quenching of triazine fluorescence beyond that attributable
to competitive light absorption by C60 was observed using tris-
diphenylamino- or tris-diisopropylamino-s-triazines, which
13 L. Famigni and M. B. Johnston, New. J. Chem., 2001, 25, 1368.
CHEM. COMMUN., 2002, 2538–2539
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