Modulating electronic interactions between closely spaced complementary p surfaces with different outcomes: Regio- and diastereomerically pure subphthalocyanine-C60 tris adducts

Article abstract of DOI:10.1002/anie.200902767

Tightly coupled, regio- and diastereomerically pure subphthalocyanine- C₆₀ dyads (see spacefilling depiction; B orange, C light blue, H gray, N blue, O red), tethered by means of a trisaddition reaction, can follow different mechanisms on photo

Full text of DOI:10.1002/anie.200902767

Communications
DOI: 10.1002/anie.200902767
Electron Transfer
Modulating Electronic Interactions between Closely Spaced
Complementary p Surfaces with Different Outcomes: Regio- and
Diastereomerically Pure Subphthalocyanine–C60 Tris Adducts**
David Gonzꢀlez-Rodrꢁguez, Esther Carbonell, Dirk M. Guldi,* and Tomꢀs Torres*
The operation of many emerging organic/plastic optoelec-
tronic technologies,[1] such as solar-energy conversion devi-
ces,[2] relies ultimately on the ground- and excited-state
electronic interactions between donor (D) and acceptor (A)
components. The need to understand and control the primary
photophysical events occurring within the active layers, as
nature illustrates in the photosynthetic reaction center,[3] has
prompted chemists to design and study molecular D–A
models. In these, the yields and kinetics of energy and/or
electron transfer are related to the nature of the D and A
components[4] and their relative distance,[5] orientation,[6] or
electronic coupling.[7] Importantly, the knowledge gathered so
far has led to discrete molecular systems with improved
charge-separation performance in solution.[8] However, most
of these D–A models fail to reproduce a major characteristic
of solid-state devices: molecules are usually confined by
intimate van der Waals contacts, and conformational or
orientational motion is restricted. Natural photosynthetic
systems already demonstrate the importance of orbital over-
lap between embedded chromophores. In the so-called
special pair, for instance, strong electronic coupling between
two chlorophyll molecules held in close p–p contact causes a
red shift in the absorption that acts as a sink for all the energy
collected.[3,9]
Herein we report a model system in which a D–A pair is
forced to strongly interact through their p surfaces in a very
rigid and closely spaced structure.[10] We demonstrate how
small alterations in the distance between the two p surfaces,
and therefore in the degree of orbital overlap and electronic
coupling, influence the ground- and excited-state interactions.
To maximize the contact area, we exploited the complemen-
tarity between the concave aromatic surface of subphthalo-
cyanines (SubPcs),[11] versatile chromophores that have
shown outstanding, tunable properties in D–A systems,[12]
and C60.[13] At the same time, in order to hold the two units
in close contact and to limit the flexibility of the system,
threefold anchoring of the C3-symmetric macrocycle to C60 by
means of a Bingel tris-addition reaction[14] was envisaged.[15,16]
We found that, due to the semirigid nature of the tethers
employed, this key reaction proceeded with very high
regioselectivity and full diastereoselectivity.[17]
The three SubPc–C60 D–A systems prepared
(Scheme 1)[18] show only small differences in the connection
À
of the spacer to the SubPc macrocycle: a direct C C bond (C
series), an oxygen atom (O series), or a sulfur atom (S series).
Analysis of SubPc–C60 products 1C, 1O, and 1S by 1H NMR
spectroscopy and HPLC revealed that the regioselectivity of
the final tris-addition process is very sensitive to the length
and flexibility of the spacer in C3-symmetric SubPc precursors
2C, 2O, and 2S. For instance, compound 2O, having a
phenoxy spacer, meets all the requirements for fully regiose-
lective tris-addition to C60 to yield a single regioisomer with C3
symmetry (1O; Figure 1 and Figures S6–S9, Supporting
Information). In contrast, the reaction of SubPc 2C at 208C
yielded a 5:95 mixture of two regioisomers (1Ca and 1Cb;
Figure 1 and Figures S1–S5, Supporting Information) that
could be separated by column chromatography. The minor
component (1Ca) clearly retains the original C3 symmetry of
the precursor SubPc, whereas 1Cb has C1 symmetry. The
shorter nature of the biphenyl linker seems to restrict
formation of a C3-symmetric tris-addition product, and the
tether prefers to anchor in a less symmetric arrangement to
release strain.[18] These triple addition reactions are not only
highly regioselective, but also totally diastereoselective; each
SubPc enantiomer generates only one enantiomeric addition
pattern. Such selectivities are lost, however, in the formation
of 1S from SubPc 2S. Analyses by NMR and HPLC revealed
the presence of a complex mixture of isomers that was
difficult to separate (Figure S10, Supporting Information).
The slightly higher flexibility and diameter of the tether in 2S
must be responsible for this effect.
[*] Dr. E. Carbonell, Prof. Dr. D. M. Guldi
Department of Chemistry and Pharmacy and Interdisciplinary
Center for Molecular Materials (ICMM)
Friedrich-Alexander-Universitꢀt Erlangen-Nꢁrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
Fax: (+49)9131-852-8307
E-mail: dirk.guldi@chemie.uni-erlangen.de
Dr. D. Gonzꢂlez-Rodrꢃguez, Prof. T. Torres
Departamento de Quꢃmica Orgꢂnica (C-I)
Facultad de Ciencias, Universidad Autꢄnoma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Fax: (+34)91-497-3966
E-mail: tomas.torres@uam.es
Homepage: http://www.uam.es/gruposinv/ftalo/
[**] Funding from MEC (CTQ2008-00418/BQU, CONSOLIDER-
INGENIO 2010 CDS2007-00010 NANOCIENCIA MOLECULAR),
ESF-MEC (MAT2006-28180-E, SOHYDS), COST Action D35, and
CAM (S-0505/PPQ/000225) is acknowledged. This work was also
supported by the Deutsche Forschungsgemeinschaft through
SFB583, DFG (GU 517/4-1), FCI, and the Office of Basic Energy
Sciences of the U.S. Department of Energy. We would like to
acknowledge Dr. Carmen Atienza Castellanos for the electrochem-
istry studies.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902767.
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Chemie
is consistent with the exceptional upfield shift experienced
by proton a in compound 1O since, due to the conforma-
tion adopted by the spacer, it is affected by the aromatic
ring current of the nearby phenyl group (Figure 1). The
NOESY and 13C NMR spectra are also in accordance with
this assignment. On the other hand, a t3,t3,t4 regioisomeric
binding pattern was assigned to C1-symmetric compound
1Cb.[18,21]
We have therefore in hand two C3-symmetric, t3,t3,t3
SubPc–C60 tris-adducts (1Ca and 1O) that basically differ
in the spacing between the two complementary p surfaces,
as imposed by the nature of the tether. In fact, the DFT-
optimized structures show that, in 1Ca, the concave face of
the SubPc is kept in tight van der Waals contact (3.25–
3.30 ꢀ) with the C60 sphere (which explains the low yield of
this compound), while in compound 1O the distance
increases to 3.5–3.6 ꢀ.[18,22,23] We reasoned that these
small p–p distances in such rigid structures must influence
the molecular orbitals and the electronic interaction
between the two redox- and photoactive units. This is
clearly reflected in ground-state electronic absorption and
cyclic voltammetry measurements. For instance, the UV/
Vis spectra of 1Ca and 1O show broadening, a bath-
ochromic shift, and tailing of the SubPc Q band, more
significant for 1Ca, compared to 2C and 2O, respectively
(Figure 2a). The features of this transition did not change
with changing solvent polarity (i.e., toluene, CHCl3, THF,
or benzonitrile) or concentration. Similarly, substantial
shifts in the redox features of the two electron donor–
acceptor conjugates relative to the references indicate
appreciable electronic interactions between the active
moieties (i.e., electron-donating SubPc and electron-
accepting C60; see Table 1). In particular, SubPc oxidation
reveals the extent of electronic change. For example, the
closer SubPc–C60 separation in 1Ca leads to larger differ-
ences in the first oxidation step (190 mV) relative to 1O
(70 mV). Additionally, fullerene reduction becomes appre-
ciably harder, by approximately 60–90 mV.
Table 1: Half-wave potentials of the first redox processes and some photo-
physical parameters of a reference tris(diethylmalonate)–C60 (6),[18] SubPcs 2C
and 2O, and SubPc–C60 dyads 1Ca and 1O.
Scheme 1. Synthesis of SubPc–C60 dyads 1C, 1O, and 1S. DBU=1,8-
diazabicyclo[5.4.0]undec-7-ene, DMAP=4-dimethylaminopyridine.
Solvent
6
2C
2O
1Ca
1O
SubPc oxi- THF
+1.03 +1.13 +1.22
+1.20
To ascertain the binding patterns to the fullerene in 1Ca,
1Cb, and 1O, we performed molecular modeling studies using
a combination of semiempirical (PM3) and DFT (B3LYP/6-
31G) methods.[18] Due to the rigid nature of the SubPc core
and the restricted flexibility of the spacers, the number of
possible tris-addition patterns is quite limited. So, among the
four possible C3-symmetric tris-addition patterns to C60
(c1,c1,c1, e,e,e, t3,t3,t3, and t4,t4,t4),[14,19] only the t3,t3,t3
isomer can be expected for compounds 1Ca and 1O, while
the other three isomers have rather strained structures (quite
obvious for the c1,c1,c1 and e,e,e patterns) due to the smaller
spacing between the cyclopropane rings.[20] This assignment
was further supported by some of the features found in the
dation[a]
C60 reduc- THF
À0.54
À0.60
À0.63
tion[a]
FSubPc[b] toluene
THF
0.08
0.081 0.08
0.079
5.8ꢅ10À4 8.5ꢅ10À4
5.2ꢅ10À4 7.3ꢅ10À4
3.9ꢅ10À4 6.2ꢅ10À4
8.0ꢅ10À4
benzonitrile
toluene
THF
benzonitrile
THF
THF
THF
FC60
8.0ꢅ10À4
1.7
6.2ꢅ10À4
[b]
[c]
[d]
5.7ꢅ10À4
tSubPc
[c]
tC60
1.6
1.7
<0.1
<0.1
1.7
ksinglet
9.5ꢅ1010 6.7ꢅ1010
[a] Determined by cyclic voltammetry. Data in Volts versus Ag/Ag+. [b] SubPc or
C60 fluorescence yield. [c] SubPc or C60 fluorescence lifetime [ns]. [d] Singlet-
NMR spectra. For instance, only a t3,t3,t3 tris-addition pattern excited-state deactivation rates [sÀ1].
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Figure 1. 1H NMR spectra (CDCl3, 298 K, 500 MHz) of SubPc–C60 dyads 1C and 1O compared to those of SubPc precursors 2C and 2O. In the
case of 1C, formation of a 5:95 mixture of two isomers 1Ca (one set of signals; solid lines) and 1Cb (three sets of signals; dashed lines) was
observed. Formation of a 1:1 mixture of two enantiomers is evidenced in the splitting of the diastereotopic methylene protons (f) on tris-adduct
formation. On top, the optimized structural models of 1Ca, 1Cb, and 1O are shown, together with a magnification showing the conformation of
the spacers in 1Cb and 1O, which may explain the extraordinary downfield (for 1Cb)[21] or upfield (for 1O) shift of the signal of proton a.
When comparing the fluorescence spectra of 1Ca/1O
with those of 2C/2O, strong SubPc fluorescence quenching
becomes evident (see Table 1).[24] A closer analysis of the
fluorescence spectra for 1O reveals, besides the strongly
quenched SubPc fluorescence in the 580–650 nm range, the
familiar C60 fluorescence in the red (i.e., 650–850 nm;
Figure 2b). The C60 fluorescence quantum yields, which are
about (8.0 Æ 0.2) ꢁ 10À4 in toluene, THF, and benzonitrile,
suggest quantitative transduction of singlet-excited-state
energy from SubPc (i.e., 2.0 eV) to C60 (i.e., ca. 1.7 eV).[25]
On the contrary, for 1Ca, the lack of C60 fluorescence implies
a different reactivity, namely, charge separation to form the
one-electron-reduced C60 radical anion and the one-electron-
oxidized SubPc radical cation (see below).
Transient absorption measurements shed light onto the
photoreactivity of 2C/2O and 1Ca/1O. On 550 nm excitation
of 2C/2O we observed the singlet-excited-state character-
istics of SubPc (Figure S17, Supporting Information). In both
cases, bleaching of the ground state dominates the transient
absorption spectrum, accompanied by a new transition that
develops in the red. For 2C/2O, maxima and minima evolve
around 460, 635, and 560 nm, respectively. The rate of
intersystem crossing converting the strongly emitting singlet
excited state to the corresponding triplet manifold is (6.3 Æ
0.2) ꢁ 108 sÀ1. In the nanosecond regime the triplet features,
which involve transient bleaching of the ground-state max-
imum and a broad transient maximum between 600 and
900 nm, are monitored.[12a] The triplet lifetimes in deoxygen-
Figure 2. a) Electronic absorption spectra (CHCl3, c=10À5 m) of
SubPc–C60 dyads 1Ca and 1O compared to those of SubPcs 2C and
2O. b) Steady-state fluorescence spectra of 2C (attenuated by a factor
of 10), 1O, and 1Ca in toluene solutions exhibiting the same
ated THF are 28 ms and involve quantitative recovery of the
singlet ground state.
The photoreactivity of 1O is different from those of 2C
and 2O, although on the nanosecond timescale the only
detectable product is the long-lived SubPc triplet excited state
absorption of 0.1 at the excitation wavelength of 560 nm. See also
Figure S16 (Supporting Information).
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Chemie
reduced C60 radical anion around 1090 nm is seen.[26] This
+
confirms formation of the SubPcC –C60CÀ radical-ion pair.
From the time–absorption profiles a charge-separation rate
constant of 1.4 ꢁ 1011 sÀ1 was derived in THF, which is in good
agreement with the steady-state fluorescence experiments. As
Figure 3b demonstrates, the SubPc·+–C60CÀ pair is surprisingly
stable on our femtosecond timescale (i.e., up to 1500 ps) and
starts to decay on the nanosecond timescale (i.e., starting at
8 ns). In THF, a lifetime of 97 ns (1.0 ꢁ 107 sÀ1) was found. A
likely explanation for this remarkably long-lived charge-
transfer state may be—besides the low reorganization ener-
gies of C60 and SubPc—stabilization of the SubPc·+ species by
partial charge shift to the axial electron-rich phenoxy
group.[27]
Our results shed light on the role of electron donor–
acceptor spacing and orbital overlap on the subtle interplay
between photoinduced energy- and charge-transfer mecha-
nisms. The main attributes of our capped SubPc–C60 systems,
compared to similar systems studied previously,[15,16] are:
1) the low conformational flexibility owing to threefold
tethering with rigid spacers, 2) a high degree of orbital
overlap due to the complementarity of the p surfaces, and
3) the possibility of tailoring the distance between electron
donor and acceptor. Remarkably, the short SubPc–C60
distance and high orbital overlap in 1Ca leads to notable
perturbation of the electronic structure of both components in
the ground state, as evidenced, for example, in the absorption
spectra and redox potentials. A reasonable rationale implies a
partial shift of electron transfer density. In this pre-activated
state, charge separation is favored over energy transfer. In the
case of slightly larger interchromophore distances and/or
higher flexibility, as in 1O, weaker ground-state interactions
result in dominant energy-transfer deactivation, despite the
similar HOMO(SubPc)–LUMO(C60) gap (ca. 1.82 eV;[28] see
Figure S15, Supporting Information) of both systems.
Figure 3. Differential absorption spectra (visible and near-infrared)
obtained on femtosecond flash photolysis (550 nm, 150 nJ) of 1O/
1Ca in THF with several time delays between 0 (black line) and
3000 ps (gray line) at room temperature. a) 1O. Inset: time–absorp-
tion profiles of the spectra at 570 and 650 nm, monitoring the decay of
the SubPc singlet excited state. b) 1Ca. Inset: time–absorption profiles
of the spectra at 520 and 630 nm, monitoring the decay of the SubPc
singlet excited state and formation of the radical-ion pair state. See
also Figure S18 (Supporting Information).
Received: May 24, 2009
Revised: July 21, 2009
Published online: September 18, 2009
(1.45 eV). The mechanism for converting the initially formed
SubPc singlet excited state (see Figure 3a) into the final
SubPc triplet excited state differs from that seen in 2C and
2O: It is a cascade of energy-transfer processes with rate
constants of 1.5 ꢁ 1011 sÀ1 (i.e., singlet–singlet energy transfer),
6.3 ꢁ 108 sÀ1 (i.e., intersystem crossing), and @ 6.3 ꢁ 108 sÀ1
(i.e., triplet–triplet energy transfer). Similar reactivity was
reported for several weakly coupled SubPc–C60 conjugates, in
which the SubPc, unless substituted with strongly electron-
donating groups (i.e., amines), usually behaved as an excited-
state energy donor.[12a,d]
Keywords: electron transfer · energy transfer · fullerenes ·
.
subphthalocyanines · p interactions
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The transient absorption changes monitored for 1Ca are
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peaks at 440 and 625 nm, which evolve as the initially formed
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[24] The corresponding rate constants of SubPc singlet excited-state
deactivation are 9.5 ꢁ 1010 sÀ1 (1Ca) and 6.7 ꢁ 1010 sÀ1 (1O) in
THF.
[25] In fluorescence lifetime measurements a lifetime of 1.7 ns was
registered for C60 fluorescence.
[26] D. M. Guldi, M. Prato, Acc. Chem. Res. 2000, 33, 695 – 703.
[27] In fact, DFT calculations localize the phenoxyl HOMO very
close in energy to the SubPc HOMO (see Figure S15, Supporting
Information).
[28] Calculated as the sum of the absolute values of the first oxidation
potentials of the SubPc and the first reduction potentials of C60
(see Table 1).
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[18] See Supporting Information for further details.
[19] For the sake of simplicity and clarity, we use the terminology: c
for cis, e for equatorial, and t for trans: L. Pasimeni, A. Hirsch, I.
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Angew. Chem. Int. Ed. 2009, 48, 8032 –8036