Synthesis and photophysical characterization of a subphthalocyanine fused dimer-C60 dyad

Article abstract of DOI:10.1039/b502041g

Photophysical studies of a newly synthesized fused subphthalocyanine dimer-C₆₀ I revealed a complex cascade of energy transfer events to succeed the initial SubPc dimer photoexcitation. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502041g

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Synthesis and photophysical characterization of a subphthalocyanine
fused dimer–C60 dyad{
Rodrigo S. Iglesias,a Christian G. Claessens,a Tomas Torres,*a G. M. Aminur Rahmanb and Dirk M. Guldi*b
Received (in Cambridge, UK) 11th February 2005, Accepted 16th March 2005
First published as an Advance Article on the web 29th March 2005
DOI: 10.1039/b502041g
arousing great interest both theoretically6 and experimentally.7 For
Photophysical studies of a newly synthesized fused subphtha-
example, SubPc dimers 2a and 2b were found to present very
different features from those of their corresponding SubPc
monomers—both in terms of optical and magnetic properties.
To enhance the understanding of the nature of this new p-extended
surface, we have studied their photophysical properties and their
ability to promote energy or charge transfer processes within a
newly synthesised covalent SubPc dimer–C60 complex, 1.
locyanine dimer–C60
1 revealed a complex cascade of
energy transfer events to succeed the initial SubPc dimer
photoexcitation.
Subphthalocyanines (SubPcs)1—phthalocyanine analogues—are
intriguing compounds made of three diiminoisoindoline units
N-fused around a boron atom. Their 14 p-electron aromatic core
associated with their curved structures renders them appealing
building blocks for the construction of multicomponent photo- or
electroactive assemblies.2 Recently, the development of an
improved synthetic methodology for SubPc preparation3 allowed
the efficient synthesis of fused subphthalocyanine dimers4 (2 in
Fig. 1) and trimers, and the separation of their syn and anti
topoisomers (2a and 2b, respectively).5 Such nonplanar conjugated
and aromatic systems are fascinating structures that have been
SubPc dimers 2 were prepared as a mixture of topoisomers syn
2a and anti 2b in 18% yield by statistical condensation of 1,2,4,5-
tetracyanobenzene with an excess of tetrafluorophthalonitrile
(1–10 M) in the presence of BCl3 in p-xylene at 140 uC for 2–3 h.
2a and 2b were separated by column chromatography on silica gel;
the syn : anti ratio being 1 in all cases. Changing the
tetracyanobenzene : tetrafluorophthalonitrile ratio from 1 : 1 to
1 : 20 did not improve the overall yield of SubPc dimer formation.
Following a successful two-step procedure developed in the case
of monomeric subphthalocyanines,2a–c C60 was grafted to the
SubPc dimers. Thus, the syn dialdehyde precursor 3a was prepared
by axial chlorine substitution in 40% yield by reacting SubPc dimer
2a with 3-hydroxybenzaldehyde in toluene at reflux for 20 h.3
Surprisingly, the anti isomer 2b gave rise to decomposition
products in the same reaction conditions instead of yielding the
corresponding dialdehyde 3b. No rational explanation for this fact
has been found so far. The syn dialdehyde 3a was then reacted
with N-methylglycine and C60, to yield the C60 bisadduct8 SubPc
dimer–C60 ensemble 1 in 14% yield. 1 was unambiguously
characterized by 1H NMR spectroscopy, MALDI TOF mass
spectrometry, and elemental analysis. The 1H NMR spectrum of 1
showed an impressive number of broad signals as a consequence of
both the presence of a mixture of up to 20 stereoisomers9 and the
dynamic processes occurring within the dyad, i.e. slow motion on
the NMR time scale of the C60 moiety with respect to the SubPc
dimer—see supplementary information.{
The extended p-electronic structure of the SubPc dimer core
impacts ground and excited state features relative to the monomeric
dodecafluorinated SubPc.2b In the ground state, drastic red-shifts
(573 to 692 nm) parallel those seen in the excited state fluorescence
(585 to 708 nm). Other physico-chemical features that reveal
appreciable changes are ground state extinction (e692nm
5
79 500 M21 cm21), fluorescence quantum yield (W 5 0.24 ¡
0.01) and fluorescence lifetime (t 5 2.5 ¡ 0.1 ns—see Fig. S1{).
Double linking C60 to 2a leads, in the ground state, to minute
perturbations of the p-electronic structure of the SubPc dimer core.
When photoexciting 1, the fluorescence quantum yields, however,
decrease strongly. In toluene, THF and benzonitrile quantum
yields are as low as 0.02 ¡ 0.01—see Fig. 2. Despite the
Fig. 1 Synthesis of SubPc dimer–C60 1.
{ Electronic supplementary information (ESI) available: experimental
procedures, NMR, IR, UV, and MS data for 3a and 1. Time resolved
fluorescence decay and time–absorption profiles of singlet excited state of
2a and 1. See http://www.rsc.org/suppdata/cc/b5/b502041g/
*tomas.torres@uam.es (Tomas Torres)
dirk.guldi@chemie.uni-erlangen.de (Dirk M. Guldi)
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Fig. 2 Absorption and emission of 1 in toluene—exciting at 550 nm.
Fig. 3 Differential absorption spectrum (visible and near-IR) obtained
upon nanosecond flash photolysis (532 nm) of y5.0 6 1026 M solutions
of 1 in nitrogen saturated toluene with a time delay of 50 ns.
quenching, the fluorescence spectra of 1 and 2a are strict
superimpositions, that is, no quenching products can be assigned
based on these experiments. The C60 reference fluoresces in the
same spectral region with quantum yields of ca. 1024
.
are identical to those depicted in Fig. 3. We determined for this
oxygen-sensitive triplet intermediate (i.e., koxygen y 109 M21 s21
Fluorescence decay measurements shed light onto the SubPc
dimer deactivation and also onto the photoproducts. Fig. S1
illustrates this for toluene.{ The fluorescence–time profiles for 1 are
best fitted—with x2 values of at least 1—by bi-exponential
expressions, while 2a shows best fits with a mono-exponential
expression. In all solvents, a short-lived (i.e., 0.27 ¡ 0.02 ns) and a
long-lived (i.e., 1.2 ¡ 0.05 ns) component was found. The short
lifetime contribution reflects the actual intramolecular deactivation
of the SubPc singlet excited state, while the longer-lived
contribution resembles that of the C60 reference. This observation
leads to the postulation that a transduction of singlet excited state
energy between nearly isoenergetic states, that is, from SubPc
(1.77 eV) to C60 (1.76 eV) occurs. Once the singlet excited state
energy is funneled to the C60 core, intersystem crossing populates
the triplet manifold.
)
lifetimes of around 90 ¡ 10 ms. Then 1 was studied—a spectrum is
shown in Fig. 3—and compared to 2a. The differential absorption
changes are absolutely identical. They indicate that the only
detectable photoproduct is that of the SubPc dimer triplet excited
state with a 1080 nm peak, a long lifetime (107 ms) and a high
triplet quantum yield (95% of that seen for 2a).
In summary, a cascade of energy transfer events succeeds the
initial SubPc dimer photoexcitation. The intriguing molecular
geometry of 1 opens the possibility to explore the properties of
other photo and electroactive SubPc dimer-based ensembles.
This work was supported by CICYT (Spain), Comunidad de
Madrid (Spain), SFB 583, the Office of Basic Energy Sciences of
the US Department of Energy, the European Union through
grants BQU2002-04697, 07N/0030/2002 and HPRN-CT-2000-
00020, respectively. RSI would like to thank the Coordenac¸a˜o de
Aperfeic¸oamento de Pessoal de N´ıvel Superior (CAPES, Brazil)
for a grant conceded to accomplish this work. CGC would like to
thank the CICYT for a ‘‘Ramon y Cajal’’ contract.
Femtosecond laser pulses were employed to visualize the
different products. Typical differential absorption, as recorded
with several time delays—0 to 4 ps—following photoexcitation of
2a, are shown in Fig. S2.{ A particular strong transient bleach is
seen in the Q-band region. The underlying singlet excited state
intersystem crosses slowly to the triplet manifold—see Fig. S2.10
Photoexciting 1 led at early times—10 ps—to the same absorption
changes. This confirms the formation of the SubPc dimer singlet
excited state. The SubPc dimer singlet–singlet features decay,
however, not via intersystem crossing, as Fig. S3 indicates a much
faster reaction takes place, which lies within the first 250 ps.{
Interestingly, simultaneous to the rapid SubPc dimer singlet–
singlet decay, the fullerene singlet–singlet features grow-in above
800 nm.
Rodrigo S. Iglesias,a Christian G. Claessens,a Tomas Torres,*a
G. M. Aminur Rahmanb and Dirk M. Guldi*b
aUniversidad Auto´noma de Madrid, Departamento de Qu´ımica
Orga´nica, Campus de Cantoblanco, Madrid, Spain.
E-mail: tomas.torres@uam.es; Fax: +34 91 497 3966; Tel: +34 497 4151
bUniversita¨t Erlangen, Institute for Physical and Theoretical Chemistry,
Egerlandstr. 3, 91058 Erlangen, Germany.
E-mail: dirk.guldi@chemie.uni-erlangen.de; Fax: +49 9131 8528307;
Tel: +49 9131 8527340
While in the C60 reference intersystem crossing (5.0 6 108 s21
)
Notes and references
dominates the deactivation of the singlet excited state, this
transition decays in 1 with slightly faster kinetics (y8.5 6
108 s21). Instead of forming the C60 triplet, attributes of the SubPc
dimer triplet excited state were recorded (Fig. 3). Similar kinetics at
the 700 nm minimum—Fig. S2—allowed us to follow the SubPc
dimer triplet generation. From this we conclude that the rate-
determining step is the C60 intersystem crossing.
1 C. G. Claessens, D. Gonza´lez-Rodr´ıguez and T. Torres, Chem. Rev.,
2002, 102, 835.
2 (a) D. Gonza´lez-Rodr´ıguez, T. Torres, D. M. Guldi, J. Rivera and
L. Echegoyen, Org. Lett., 2002, 4, 335; (b) D. Gonza´lez-Rodr´ıguez,
T. Torres, D. M. Guldi, J. Rivera, M. A. Herranz and L. Echegoyen,
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Complementary nanosecond experiments further corroborated
the femtosecond experiments. First, we probed 2a in toluene, THF
and benzonitrile and obtained differential absorption changes that
2114 | Chem. Commun., 2005, 2113–2115
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3 C. G. Claessens, D. Gonza´lez-Rodr´ıguez, B. del Rey, T. Torres,
G. Mark, H.-P. Schuchmann, C. von Sonntag, J. G. MacDonald and
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4 An axially linked SubPc dimer was described recently: T. Fukuda,
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5 C. G. Claessens and T. Torres, Angew. Chem., Int. Ed., 2002, 41, 2561;
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9 The bisaddition to C60 may result in up to 8 isomers if considering only
the addition to the [6,6] double bonds. Elementary molecular modeling
indicates that trans-1, trans-2 and trans-3 are the only ones attainable.
Regiosisomers trans-2 and trans-3 are racemic mixtures of two
enantiomers. Finally, these 5 stereoisomers become 20 as a consequence
of the chirality of the two pyrrolidine rings.
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10 Apparently the deactivation is slower than our experimental time
window of 1500 ps, but it is safe to assume that the fluorescence lifetime
of 2.5 ¡ 0.1 ns is a good estimate for the singlet lifetime.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2113–2115 | 2115