A novel parachute-shaped C60-porphyrin dyad

Article abstract of DOI:10.1039/a807214k

A novel covalently linked C₆₀-porphyrin dyad has been prepared by cyclopropanation of C₆₀ with a strapped porphyrin malonate; its fluorescence spectrum shows strong quenching of the porphyrin singlet excited state by the attached C

Full text of DOI:10.1039/a807214k

A novel parachute-shaped C –porphyrin dyad
6
0
Peng Cheng, Stephen R. Wilson and David I. Schuster*
Department of Chemistry, New York University, 100 Washington Square East, New York, NY 10003, USA.
E-mail: david.schuster@nyu.edu
Received (in Corvallis. OR, USA) 14th September 1998, Accepted 23rd November 1998
A novel covalently linked C60–porphyrin dyad has been
prepared by cyclopropanation of C60 with a strapped
porphyrin malonate; its fluorescence spectrum shows strong
quenching of the porphyrin singlet excited state by the
malonyl dichloride gave compound 2. Condensation of the
dialdehyde with 4 equiv. of benzyl 3,4-diethylpyrrole-2-carbox-
ylate5 gave compound 3. Hydrogenation of 3 followed by
reaction with methyl orthoformate catalyzed by trichloroacetic
acid led to strapped porphyrin 4, which was separated in 12%
attached C60
.
2 2
yield by flash column chromatography (silica gel, CH Cl –
Photosynthetic model systems rely on spatially organized units
with suitable photochemical and electronic properties. Recently
much attention has been focused on using C60 as the electron
acceptor in donor–acceptor systems due to its unique shape and
redox properties.1 To continue a project aimed at studying
interactions between two chromophores in covalently linked
MeOH 20:1). The porphyrin malonate was then attached to C60
6
via Bingel cyclopropanation accomplished by the action of
7
CBr
4
and DBU. The dyad 5† was isolated in 25% yield by
2 2
preparative TLC (silica gel, CH Cl –MeOH 25:1).
–3
The structure of dyad 5 was confirmed by spectral data
including 1H MMR, C NMR, He NMR, UV–VIS and
13
3
C
60–porphyrin dyads with flexible and rigid tethers,3 we
MALDI-TOF mass spectra.‡ The H NMR spectrum in CDCl
1
3
became interested in the synthesis of a novel parachute-shaped
dyad in which the two p-systems are forced into a face-to-face
arrangement.
exhibits the expected features with correct integration ratios,
which is similar to the spectrum of porphyrin 4 except for the
disappearance of the two protons on the methylene carbon
between the two carbonyl groups. The 13C NMR spectrum of 5
We synthesized a key intermediate porphyrin 4 using a
4
3
strapped porphyrin strategy in order to avoid tedious chromato-
shows eight of the nine resonances for the sp carbon atoms, 24
graphic separation of two ortho-linked atropisomers (Scheme
of the 28 resonances in the region d 97.0–160.0 arising from the
sp carbon atoms of the C60 moiety and the porphyrin, as well
2
1). Esterification of 2-(3-hydroxypropyl)benzaldehyde with
as a resonance (d 161.53) for the malonate carbonyl groups,
clearly demonstrating that dyad 5 has C2v symmetry. Inter-
3
estingly, the dyad made from He@C60 shows a peak at d 28.5
3
3
relative to He gas via He NMR spectroscopy, whereas typical
3
methano-1,2-dihydrofullerenes-C60 like 6 have a He NMR
8
resonance at d 28.1. The upfield shift is attributable to the
shielding effect of the porphyrin ring current. Calculations
indicate that the average distance between the center of the four
3
pyrrole N atoms and the center of the two sp carbons on the C60
is 6.4 Å in the CVFF minimized structure (InsightII 97.2,
Discover 3). The two tethers make the system relatively rigid.
The UV–VIS spectrum of 5 displays strong absorption bands
due to both the porphyrin and fullerene moieties. Fig. 1 shows
the UV–VIS spectrum of 5 together with that of porphyrin 4 and
6
for comparison. Since the porphyrin dominates the visible
region whereas C60 dominates the UV region, the electronic
spectrum of 5 is a virtual superimposition of the two
independent chromophores present in the molecule, indicating
no appreciable ground state interaction between the two p-
systems.
2 2
In CH Cl , porphyrin 4 displays fluorescence maxima at 635
and 698 nm (Fig. 2). The dyad 5, excited at 580 nm, shows an
emission spectrum with characteristic features of the porphyrin.
However, the porphyrin emission is efficiently quenched by the
attached C60 by a factor of 40. It remains to be clarified from
time-dependent measurements whether the quenching of the
Scheme 1 Reagents and conditions: i, malonyl dichloride, Et
C; ii, benzyl 3,4-diethylpyrrole-2-carboxylate, HCl, EtOH, reflux, 5 h; iii,
, 10% Pd/C, Et N, THF, room temp., 5 h, then CH(OCH , Cl CCO H,
CH Cl , room temp., 48 h; iv, C60, CBr , DBU, toluene, room temp.
3 2 2
N, CH Cl , 0
°
H
2
3
3
)
3
3
2
2
2
4
Chem. Commun., 1999, 89–90
89

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Notes and references
†
Detailed procedures for the synthesis and spectroscopic data of 4 and 5
will be reported elsewhere.
Selected data for 5: d (200 MHz, CDCl
d), 7.80 (2H, t), 7.46 (2H, t), 7.13 (2H, d), 4.09 (8H, q), 3.80 (4H, t), 3.18
4H, m), 2.72 (4H, m), 2.16 (4H, t), 1.97 (12H, t), 1.22 (12H, t), 1.11 (4H,
m), 21.90 (2H, br s); d (75 MHz, CDCl , 25 °C) 161.53,159.83, 145.44,
45.06, 144.98, 144.90, 144.53, 144.39, 144.29, 143.58, 142.85, 142.75,
‡
H
3
, 25 °C) 10.33 (2H,s), 8.57 (2H,
(
C
3
1
1
1
1
42.67, 141.95, 141.61, 141.09, 140.62, 138.06, 134.03, 131.75, 130.45,
20.21, 113.46, 112.83, 97.27, 65.01, 63.37, 32.35, 27.89, 20.78, 19.91,
8.47, 17.48;
d
He(500 MHz, 1-methylnaphthalene–CD
2
21
Cl
2
)
26.5
3
3
3
21
(
(
He@C60), 28.5 ( He@ 5); lmax(CH
115600), 328 (44400), 412 (138000), 510 (10000), 544 (6400), 577 (7600);
Cl )/nm 698, 635; m/z (MALDI-
2 2
Cl )/nm (e/dm mol cm ) 259
fluorescence (lexc = 580 nm) lmax(CH
2
2
+
+
TOF) 1622.3 (M + H ), calc. 1622.8 (M + H ).
(2.5 3 102 mol
6
Fig. 1 UV–VIS spectra of (a) 4, (b) 5 and (c) 6 in CH
dm ).
2
Cl
2
1
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Fig. 2 Fluorescence spectra of (a) 4 and (b) 5 in CH
2
Cl
2
23
dm
)
2
62.
4
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porphyrin fluorescence results from photoinduced energy
transfer and/or electron transfer.
We thank Dr Anthony Khong (Yale University) for the He
NMR measurement. Financial support of this research by the
US National Science Foundation is gratefully acknowledged.
We also thank Andreas Hirsch (Erlangen) and François
Diederich (ETH, Zürich) for communication prior to publica-
tion of independent synthesis of rigid C60–porphyrin cyclo-
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Communication 8/07214K
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Chem. Commun., 1999, 89–90