Synthesis and characterization of photoresponsive diphenylaminofluorene chromophore adducts of [60]fullerene

Article abstract of DOI:10.1039/b515055h

A class of acceptor-keto-donor structures as hindered 9,9-di(3,5,5- trimethylhexyl)-2-diphenylaminofluoreno-methano[60]fullerene C ₆₀(>DPAF-C₉) and the related bisadducts C ₆₀(>DPAF-C₉)₂ and C_60<

Full text of DOI:10.1039/b515055h

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Synthesis and characterization of photoresponsive diphenylaminofluorene
chromophore adducts of [60]fullerene{
Prashant A. Padmawar,a Taizoon Canteenwala,a Sarika Verma,a Loon-Seng Tan*b and Long Y. Chiang*a
Received 24th October 2005, Accepted 15th December 2005
First published as an Advance Article on the web 23rd January 2006
DOI: 10.1039/b515055h
A class of acceptor–keto–donor structures as hindered 9,9-di(3,5,5-trimethylhexyl)-2-
diphenylaminofluoreno-methano[60]fullerene C60(>DPAF-C9) and the related bisadducts
C60(>DPAF-C9)2 and C60(>DPAF-C2)2 were synthesized. They are derivatives of multiphoton
absorptive C60(>DPAF-C2) showing enhanced cross-sections of simultaneous two-photon
absorption under laser excitation at 800 nm in nanosecond region. Molecular synthesis of these
C60–DPAF conjugates involved the covalent attachment of a diphenylaminofluorene moiety to
methano[60]fullerene via a keto linkage for increasing molecular acceptor–donor polarization
of the chromophore in conjunction with the fullerene cage. Preparation of 7-(1,2-dihydro-1,2-
methanofullerene[60]-61-carbonyl)-9,9-dialkyl-2-diphenylaminofluorene C60(>DPAF-Cn)
involved cyclopropanation of C60 with a key synthon 7-a-bromoacetyl-9,9-dialkyl-2-
diphenylaminofluorene. Synthesis of this synthon was achieved by a three-steps procedure
starting from 2-bromofluorene via dialkylation at C9 of the fluorene ring, attachment of a
diphenylamino group at C2 of dialkylfluorene, and Friedel–Craft acylation of the a-bromoacetyl
group at C7 of diphenylaminofluorene. All C60–DPAF derivatives were fully characterized
with the chemical structures confirmed by various spectroscopic analyses and validated by the
single-crystal structural analysis data of C60(>DPAF-C2). Strong solvent-sensitive fluorescence
quenching phenomena of C60(>DPAF-C2), C60(>DPAF-C9), and C60(>DPAF-C9)2 were noticed,
showing no fluorescence band above 700 nm in more polar solvents, such as DMF, PhCN,
and THF, while in less polar solvents (toluene, CHCl3, and CS2) a fullerenyl fluorescence band at
700–710 nm was observed. It was attributed to the occurrence of electron transfer via the singlet
excited state of the fullerene moiety 1C60*(>DPAF-Cn) in the former group of the solvents. On the
contrary, energy transfer processes from DPAF-Cn moiety to the fullerene cage are favored in
the latter group of the solvents.
separation phenomena in the composite or nanospheres5 of
Introduction
C60 dyads and in the ordered structures of bis- and tris-
fullerenes,6 such as elongated wires and entangled spheres,
Molecular design of photoresponsive chromophores via
covalent linkage of electron donors onto the C60 acceptor
enhances their potential applications in the area of artificial
photosynthetic materials,1 molecular electronic devices,7 and
cage stimulates much interest in exploration of photosynthesis
photovoltaic cells.8 In the case of the energy-transfer processes
mimicking involving sequential intramolecular and inter-
molecular electron- or energy-transfer events.1 A large number
taking place from the photoactivated donor moieties to the
of the acceptor–donor (A–D) type C60–D dyads2,3 have been
C60 cage resulting in generation of the triplet C60 excited state
(3C60*) or its derivatives (3C60*–D), formation of singlet
synthesized using C60 as the acceptor component for this
oxygen (1O2) becomes possible via the triplet energy transfer
purpose. Many photoinduced transient processes engage
mainly one-photon excitation of the C60 cage and donor
chromophore moieties followed by either electron- or energy-
transfer between these A–D components at the excited state.1,4
Occurrence of the photoinduced charge generation and
3
from the C60* moiety to molecular oxygen. Examples of
efficient singlet oxygen production in biological medium
allowed investigation of its cytotoxicity in single-photon
excitation based photodynamic therapeutic (PDT) treatment
against the tumor and cancer cells in vitro and in vivo using
hydrophilic molecular micelle-like C60 derivatives (FC4S) as
photosensitizers.9 High efficiency of singlet oxygen generation
of FC4S was substantiated in its nanosphere structures.9c In
several closely related in vitro studies, singlet oxygen and
oxygenated radicals generated from hydrophilic fullerene
derivatives or composites have been implicated in direct or
indirect oxidative damage of biomolecules, including selective
DNA cleavage,10 membrane lipid peroxidation,11 and virus
inactivation.12
aDepartment of Chemistry, University of Massachusetts Lowell, Lowell,
MA 01854, USA. E-mail: Long_Chiang@uml.edu; Fax: +1 978934 3013;
Tel: +1 978 934 3663
bAFRL/ML, Air Force Research Laboratory, Wright-Patterson Air
Force Base, Dayton, OH 45433, USA.
E-mail: Loon-Seng.Tan@wpafb.af.mil; Fax: +1 937 255 9157;
Tel: +1 937 255 9141
{ Electronic supplementary information (ESI) available: 1H NMR
spectra of C60(>DPAF-C9) 1b and C60(>DPAF-C9)2 2b, 13C
NMR spectra of 1b and 2b, and X-ray crystallographic data of
C60(>DPAF-C2) 1a in CIF format. See DOI: 10.1039/b515055h
1366 | J. Mater. Chem., 2006, 16, 1366–1378
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Simultaneous two-photon absorption (2PA) excitation
processes allow the photodynamic applications using a laser
light source with the photon energy in the near-infrared range
equivalent to nearly a half of HOMO–LUMO energy gap of
the photosensitizer. Combination of 2PA and reverse saturable
absorption (RSA) at the excited states of complex organic dyes
with appreciably large cross-sections was proposed for the
potential optical power limiting application of the materials.13
To achieve multiphoton absorption (MPA)-based applica-
tions, enhancement of MPA cross-sections becomes a pre-
requisite. Observation of simultaneous two-photon excitation
of a C60–keto–donor dyad system was reported recently
for 7-(1,2-dihydro-1,2-methanofullerene[60]-61-carbonyl)-9,9-
diethyl-2-diphenylaminofluorene, denoted C60(>DPAF-C2)
dyad 1a, synthesized by the attachment of 9,9-diethyl-2-
diphenylaminofluorene (DPAF-C2) donor subunit at the C7
carbon position on the C60 cage.3g The compound 1a exhibits
large effective molecular two-photon absorption (2PA) cross-
sections s29 = 196 6 10248 cm4 s. Two-photon excitation has
also been demonstrated in the practice of photodynamic
therapy-related biomedical investigation with the use of highly
penetrating, nondamaging near infrared light in a spectral
transparent window of 800–1100 nm in mammalian tissue.14
The approach has potential to provide a means for improving
localization of the PDT-related antibacteria therapy or
obtain high-resolution, three-dimensional images of the tissue
by depth-resolved contrast using two-photon fluorescence
microscopy.15
1a can be modified to include multiple two-photon
active chromophore arms on a single C60 cage to afford a
starburst structure for further enhancing 2PA cross-sections.
The concept of multi-dimensional conjugation as an appro-
ach to improve cross-sections of 2PA molecules was
proposed recently.17 Our synthesis of monoadducts 1a and
C60(>DPAF-C9) dyad 1b was also accompanied with a minor
yield of the corresponding bisadducts C60(>DPAF-C2)2 2a
and C60(>DPAF-C9)2 2b. That allowed us to investigate
this concept by using the C60 cage as a molecular core for
attaching multiple chromophore donor moieties, acting as
antenna components for light harvesting at visible wave-
lengths. Accordingly, we report the synthesis and structural
characterization of the dyads C60(>DPAF-Cn) with the
isolation of the first series of the triads C60(>DPAF-Cn)2,
containing two DPAF arms by the use of hindered alkyl
side-chain to increase solubility of chromophore conjugates
and minimize the molecular aggregation of DPAF moieties
in solution.
Materials and methods
General
Pure C60 (99.5%) was purchased from NeoTech Product
Company, Russia. Further purification of C60 was made by
thin-layer chromatography (TLC, SiO2, toluene). Reagents
of tris(dibenzylideneacetone)dipalladium(0), rac-2,29-bis(di-
phenylphosphino)-1,19-binaphthyl (BINAP), and 2-bromo-
fluorene were purchased from Aldrich Chemicals. All other
chemicals were purchased from Acros Ltd. Toluene and
benzene were dried and distilled over sodium.
Both C60 acceptor and DPAF donor are involved in
simultaneous 2PA excitation processes. We found that an
approach using the C60–keto–D assembly structure to a
composition of C60(>DPAF-Cn) gave significant improvement
of 2PA cross-sections from that of DPAF derivative alone.3g,3h
In this analog of compounds, the DPAF moiety is linked
on methano[60]fullerene by a keto functional group with
only one sp3 carbon between two conjugative chromophores.
DPAF chromophores containing various electron donating
and withdrawing substituents exhibit high fluorescent photo-
emission quantum yield, two-photon absorptivity, and good
resistance to thermal and photooxidative degradation.
Physical measurements
Infrared spectra were recorded as KBr pellets on a Nicolet 750
1
series FT-IR spectrometer. H NMR, 13C NMR, and COSY
spectra were recorded on either a Bruker Avance Spectrospin-
400 or Bruker AC-300 spectrometer. HMQC spectra were
recorded on Bruker-500 SB FT-NMR spectrometer. UV–vis
spectra were recorded on a Hitachi U-3410 UV spectrometer.
Fluorescence spectra were recorded on a FLUOROLOG (ISA
Instruments) spectrofluorometer. Mass spectroscopic studies
were performed by the use of either negative ion desorption
chemical ionization (DCI2) technique with a direct probe on a
JEOL JMS-SX 102A mass spectrometer or positive ion fast
atom bombardment (FAB+) technique with a direct probe
on a JEOL SX-102A mass spectrometer. X-Ray single crystal
analysis on NONIUS KAPPA CCD diffractometer and
elemental analyses of fullerene derivatives were carried out at
National Taiwan University, Taiwan.
Its close linkage to the fullerene cage by a short separation
˚
distance of only roughly 2 A leads to observation of ultrafast
photoresponse within 10 ps after a 150 fs laser pulse.16
Subsequent intramolecular electron-transfer processes of
C60(>DPAF-C2) proceeded with a charge-separation quantum
yield of 0.96 in PhCN and a long-lived charge separated
states of 150 ns in this donor–acceptor assembly. In non-
polar solvent, such as toluene, no charge-transfer states
were revealed. Triplet state of the fullerene cage in
3C60*(>DPAF-C2) was detected, instead, with the lifetime
of 33 ms.16
Synthesis of 2-bromo-9,9-diethylfluorene (5a)
Combination of two-photon absorptivity and intramole-
cular electron- or energy-transfer phenomena of C60–keto–
DPAF assemblies may allow us to explore two-photon
excitation based photonics. Evidently, the phenomenon of
intramolecular two-photon energy transfer from the DPAF
moiety to the C60 cage was detected upon the pulsed laser
excitation at 800 nm that significantly decreases the DPAF-
derived 2PA emission at 450 nm.3g Synthetically, the molecule
To a solution of 2-bromofluorene 4 (2.5 g, 10.2 mmol) in
toluene (40 ml) was added tetra-n-butylammonium bromide
(1.83 g, 5.7 mmol) as a phase transfer agent. A freshly prepared
solution of aqueous sodium hydroxide (25 ml, 50% w/w) was
added in one portion to the solution. The mixture turned
orange and became viscous. To this bilayer solution was added
iodoethane (2.4 ml, 30 mmol). The mixture was stirred at 60 uC
This journal is ß The Royal Society of Chemistry 2006
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for overnight. The reaction mixture was then diluted with
ethyl acetate (25 ml) and washed several times with water. The
organic layer was dried over magnesium sulfate and the
solution concentrated to give the crude product which was
purified by column chromatography (silica gel, hexane as
eluent). A chromatographic fraction corresponding to Rf = 0.8
on thin-layer chromatography (TLC, SiO2, hexane as eluent)
was isolated to afford colorless oil of 2-bromo-9,9-diethyl-
fluorene 5a (2.5 g) in 81% yield. The compound 5a gave
water in sequence. The resulting organic layer was dried over
sodium sulfate and concentrated in vacuo to give the crude
product. It was purified by column chromatography (silica gel)
using a mixture of hexane–toluene, 9 : 1, as eluent. A
chromatographic fraction corresponding to Rf = 0.6 on TLC
(SiO2, hexane–toluene, 4 : 1, as eluent) was isolated to afford
9,9-diethyl-2-diphenylaminofluorene 6a as white amorphous
low melting solids in a yield of 98% (3.1 g). Spectroscopic data
of 6a: FAB+-MS calcd for
12
C
29
1H2714N1 m/z 389; found, m/z
crystalline white solids after standing. Spectroscopic data of
5a: FAB+-MS calcd for
389; FT-IR (KBr) umax 3058 (m), 3030 (m), 2956 (s), 2923 (s),
12
C
1H1779.9Br1 m/z 301; found, m/z
2868 (m), 1584 (vs), 1485 (vs), 1448 (s), 1326 (s), 1300 (s), 1268
(s), 751 (s), 733 (s), 698 (s), 694 (m), and 512 (m) cm21 1H
;
17
301; FT-IR (KBr) umax 3060 (w), 2964 (vs), 2919 (s), 2876 (s),
2852 (s), 1598 (w), 1570 (w), 1443 (vs), 1404 (s), 1378 (s), 1255
(m), 1132 (m), 1061 (m), 1004 (m), 876 (m), 823 (s), 781 (s), 767
(vs), 733 (vs), 569 (m), and 418 (s) cm21; 1H NMR (400 MHz,
CDCl3, ppm) d 7.61–7.59 (m, 1H), 7.49–7.46 (m, 1H), 7.38–
7.36 (m, 2H), 7.25–7.23 (m, 3H), 1.94–1.91 (m, 4H), and 0.24
(t, J = 8Hz, 6H); 13C NMR (400 MHz, CDCl3, ppm) d 152.0,
149.2, 140.3, 140.16, 129.9, 127.3, 126.9, 126.0, 122.7, 120.9,
120.9, 119.6, 56.1, 32.5, and 8.3.
NMR (400 MHz, CDCl3, ppm) d 7.64–7.57 (m, 2H), 7.33–7.23
(m, 7H), 7.15–7.12 (m, 5H), 7.06–6.99 (m, 3H), 1.99–1.87 (m,
4H), and 0.36 (t, J = 7.3 Hz, 6H); 13C NMR (400 MHz,
CDCl3, ppm) d 151.2, 149.8, 148.0, 147.1, 141.2, 136.7, 129.1,
126.8, 126.3, 123.7, 123.7, 122.7, 122.4, 120.3, 119.4, 119.1,
56.0, 32.6, and 8.5.
Synthesis of 9,9-diethyl-2-diphenylaminofluorene (6a) via the
step vc of Scheme 1
Synthesis of 2-bromo-9,9-di(3,5,5-trimethylhexyl)fluorene (5b)
To a cooled solution of 7-bromo-2-diphenylamino-9,9-diethyl-
fluorene 7a (1.0 g, 2.1 mmol) in dry THF (20 ml) at 278 uC
under nitrogen atmospheric pressure was added n-BuLi in
hexane (0.53 ml, 1.6 M, 8.5 mmol) over a period of 5 min. The
reaction mixture was stirred for an additional period of 4.0 h
while the bath temperature was raised slowly to 220 uC. It
was quenched with saturated ammonium chloride solution
To a solution of 2-bromofluorene 4 (4.0 g, 16.3 mmol) in dry
tetrahydrofuran (150 ml) was added potassium t-butoxide
(5.5 g, 49.0 mmol) resulting in orange suspension. The mixture
was stirred at room temperature for a period of 20 min.
The suspension was cooled below 10 uC and added a tetra-
hydrofuran solution of 3,5,5-trimethylhexyl mesylate (11.0 g,
49.0 mmol) dropwise over a period of 30 min. Gradual color
change of the reaction mixture from deep red to deep purple
was observed by stirring overnight. At the end of the reaction,
it was quenched by addition of water and extracted with ethyl
acetate, dried over sodium sulfate, and concentrated in vacuo
to give the crude product which was purified by column
chromatography (silica gel, hexane as eluent). A chromato-
graphic fraction corresponding to Rf = 0.8 on TLC (SiO2,
hexane as eluent) was isolated to afford 2-bromo-9,9-di(3,5,5-
trimethylhexyl)fluorene 5b (7.6 g) as colorless oil in a yield
of 94%. Spectroscopic data of 5b: FAB+-MS calcd for
12
C
31
1H4579.9Br1 m/z 497; found, m/z 496 and 498; FT-IR
(KBr) similar to that of 5a; 1H NMR (400 MHz, CDCl3, ppm)
d 7.65–7.62 (m, 1H), 7.55–7.51 (m, 1H), 7.46–7.41 (m, 2H),
7.35–7.27 (m, 3H), 2.1–1.8 (m, 4H), and 1.35–0.38 (m, 34H);
13C NMR (400 MHz, CDCl3, ppm) d 152.8, 150.2, 140.2,
140.1, 129.8, 127.5, 126.9, 126.1, 122.8, 121.0, 120.9, 119.7,
55.1, 50.7, 50.7, 37.7, 32.8, 32.7, 30.9, 29.9, 29.5, 29.4, 22.4,
and 22.4.
Synthesis of 9,9-diethyl-2-diphenylaminofluorene (6a)
A
mixture of 2-bromo-9,9-diethylfluorene 5a (2.44 g,
8.1 mmol), diphenylamine (1.43 g, 8.5 mmol), tris(dibenzyl-
ideneacetone)dipalladium(0) (18 mg, 0.25 mmol%), rac-
2,29-bis(diphenylphosphino)-1,19-binaphthyl (BINAP, 37 mg,
0.75 mmol%), and sodium t-butoxide (1.08 g, 11.3 mmol) in
dry toluene (100 ml) was heated to refluxing temperatures for
a period of 8.0 to 10.0 h under nitrogen atmosphere. The
reaction mixture was then cooled to room temperature, diluted
with diethyl ether (60 ml), and washed with brine (40 ml) and
Scheme 1 Reagents and conditions: i. Et–I, NaOH, TBA–Br, toluene,
60 uC, 8.0 h; ii. diphenylamine, tris(dibenzylideneacetone)dipalla-
dium(0) (cat.), rac-BINAP (cat.), t-BuONa, toluene, 110 uC, 8.0 h; iii.
bromoacetyl bromide, AlCl3, ClCH2CH2Cl, 0 uC to rt, 4.0 h; iv. C60
,
DBU, toluene, rt, 5.0 h; va, Mg, a-bromoacetyl bromide, THF, D; vb.
n-butyllithium, a-bromoacetyl bromide, THF, 278 uC to 230uC; vc.
n-butyllithium, THF, 278 uC, NH4Cl.
1368 | J. Mater. Chem., 2006, 16, 1366–1378
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(NH4+Cl2). The products were extracted by chloroform
(2 6 20 ml) and the combined organic extracts were washed
with aqueous saturated ammonium chloride followed by
water. The organic layer was dried over anhydrous sodium
sulfate, concentrated in vacuo, and purified on thin-layer
chromatographic plates (TLC, Rf = 0.6 using hexane–toluene,
4 : 1, as eluent) to give 9,9-diethyl-2-diphenylaminofluorene 6a
as viscous colorless oil, which solidified upon standing, in 95%
yield (780 mg).
72.94; H, 5.49; N, 2.74; found: C, 73.22; H, 5.62; N, 2.54;
UV–vis (CHCl3, 2.0 6 1025 M) lmax (e) 299 (2.4 6 104) and
406 nm (2.7 6 104 L mol21 cm21); FT-IR (KBr) umax 3037 (w),
2966 (s), 2928 (m), 2878 (w), 1674 (s), 1595 (vs), 1491 (s), 1281
1
(vs), 754 (s), and 698 (s) cm21; H NMR (400 MHz, CDCl3,
ppm) d 7.95 (dd, J = 8 Hz, J = 1.6 Hz 1H), 7.92 (d, J = 1.4 Hz,
1H), 7.65 (d, J = 8 Hz, 1H), 7.60 (d, J = 8 Hz, 1H), 7.28–7.09
(m, 10H), 7.05–7.02 (m, 2H), 4.49 (s, 2H), 2.05–1.84 (m, 4H),
and 0.35 (t, J = 7.3 Hz, 6H); 13C NMR (400 MHz, CDCl3,
ppm) d 191.0, 152.8, 150.3, 148.9, 147.3, 134.3, 131.6, 129.3,
129.2, 128.9, 124.4, 123.1, 122.8, 121.6, 118.8, 118.1, 56.2, 32.4,
31.2, and 8.47.
Synthesis of 9,9-di(3,5,5-trimethylhexyl)-2-diphenylamino-
fluorene (6b)
A
mixture of 9,9-di(3,5,5-trimethylhexyl)-2-bromofluorene
7-a-Bromoacetyl-9,9-di(3,5,5-trimethylhexyl)-2-diphenyl-
5b (5.9 g, 11.9 mmol), diphenylamine (2.4 g, 14.2 mmol)
aminofluorene (3b)
tris(dibenzylideneacetone)dipalladium(0) (27 mg, 0.25 mmol%),
rac-2,29-bis(diphenylphosphino)-1,19-binaphthyl
(BINAP,
To a suspension of aluminium chloride (792 mg, 6.0 mmol)
in 1,2-dichloroethane (30 ml) at 0 uC was added a solution
of 9,9-di(3,5,5-trimethylhexyl)-2-diphenylaminofluorene 6b
(3.17 g, 5.4 mmol) in 1,2-dichloroethane (30 ml). It was then
added by a-bromoacetyl bromide (0.57 ml, 6.5 mmol) over
10 min while maintaining the temperature of reaction mixture
between 0–10 uC. The mixture was warmed to ambient
temperature and stirred for an additional 4.0 h. At the end
of the reaction, it was quenched by slow addition of water
(100 ml) while maintaining the temperature below 45 uC. The
organic layer was separated and washed sequentially with dil.
HCl (1.0 N, 50 ml) and water (50 ml 6 2). It was dried over
sodium sulfate and concentrated in vacuo to get the crude
product as crystalline yellow solids. It was purified by column
chromatography (silica gel) using hexane–EtOAc, 9 : 1,
as eluent. A chromatographic fraction corresponding to
Rf = 0.7 on TLC (SiO2, hexane–EtOAc, 9 : 1, as eluent) was
isolated to afford 7-a-bromoacetyl-9,9-di(3,5,5-trimethyl-
hexyl)-2-diphenylaminofluorene 3b as yellow viscous oil in
89 mg, 0.75 mmol%) and sodium t-butoxide (1.6 g, 16.6 mmol)
in dry toluene (150 ml) was heated to refluxing temperatures
for a period of 8.0 to 10.0 h under nitrogen atmosphere. After
cooling the reaction mixture to room temperature, it was
diluted with diethyl ether (60 ml), washed with brine (80 ml),
and further washed with water. The diethyl ether layer was
dried over sodium sulfate and concentrated in vacuo to give the
crude product, which was purified by column chromatography
(silica gel) using hexane as eluent. A chromatographic fraction
corresponding to Rf = 0.6 on TLC (SiO2, hexane as eluent)
was isolated to afford 9,9-di(3,5,5-trimethylhexyl)-2-diphenyl-
aminofluorene 6b as colorless oil in 97.5% yield (6.8 g).
12
Spectroscopic data of 6b: FAB+-MS calcd for
C
1H5514N1
43
m/z 585; found, m/z 585; 1H NMR (400 MHz, CDCl3, ppm) d
7.6–7.52 (m, 2H), 7.3–7.19 (m, 7H), 7.15–7.05 (m, 5H), 7.05–
6.94 (m, 3H), 1.99–1.75 (m, 4H), and 1.29–0.38 (m, 34H); 13C
NMR (400 MHz, CDCl3, ppm) d 151.9, 150.4, 148.0, 147.0,
140.9, 136.6, 129.2, 129.1, 126.7, 126.3, 124.1, 123.7, 122.6,
122.4, 120.3, 119.4, 119.1, 54.8, 50.9, 37.9, 33.1, 34.0, 30.9,
29.9, 29.4, 22.5, and 22.5.
67% yield (2.6 g). Spectroscopic data of 3b: FAB+-MS calcd
12
for
C
1H5679.9Br114N116O1 m/z 706; found, m/z 705 and
45
707; UV–vis (CHCl3, 2.0 6 1025 M) lmax (e) 407 nm (3.8 6
1
104 L mol21 cm21); H NMR (400 MHz, CDCl3, ppm) d 7.94
7-a-Bromoacetyl-9,9-diethyl-2-diphenylaminofluorene (3a)
(m, 2H), 7.63 (d, J = 8Hz, 1H), 7.58 (d, J = 8 Hz, 1H), 7.27–
6.97 (m, 12H), 4.46 (t, J = 4 Hz, 2H), 2.08–1.79 (m, 4H), and
1.31–0.39 (m, 34H); 13C NMR (400 MHz, CDCl3, ppm) d
190.9, 153.4, 150.8, 148.8, 147.6, 146.9, 134.1, 131.5, 129.2,
128.7, 124.3, 124.3, 123.0, 123.0, 122.9, 121.6, 118.8, 118.0,
55.0, 50.8, 50.7, 37.6, 33.0, 32.9, 31.5, 30.9, 29.8, 29.3, 29.3,
22.4, and 22.4.
To a suspension of aluminium chloride (375 mg, 2.8 mmol) in
1,2-dichloroethane (15 ml) at 0 uC was added a solution of
2-diphenylamino-9,9-diethylfluorene 6a (1.0 g, 2.6 mmol) in
1,2-dichloroethane (15 ml). It was added by a-bromoacetyl
bromide (0.27 ml, 3.1 mmol) over 10 min while maintaining the
temperature of reaction mixture between 0–10 uC. The mixture
was warmed to ambient temperature, stirred for an additional
4.0 h, and then quenched by slow addition of water (50 ml)
while maintaining the temperature below 45 uC. The organic
layer was isolated and washed sequentially with dil. HCl (1.0 N,
50 ml) and water (50 ml, 26). It was dried over sodium sulfate
and concentrated in vacuo to give a crystalline yellow solid.
The crude solid product was purified by column chromato-
graphy (silica gel) using hexane–toluene, 3 : 2, as eluent. A
chromatographic fraction corresponding to Rf = 0.3 on TLC
(SiO2, hexane–toluene, 3 : 2, as eluent) was isolated to afford
7-bromoacetyl-9,9-diethyl-2-diphenylaminofluorene 3a as
Synthesis of 7-(1,2-dihydro-1,2-methanofullerene[60]-61-
carbonyl)-9,9-diethyl-2-diphenyl aminofluorene, the monoadduct
C60(>DPAF-C2) 1a, and the bisadduct, C60[methanocarbonyl-7-
(9,9-diethyl-2-diphenylaminofluorene)]2, C60(>DPAF-C2)2 2a
C60 (1.0 g, 1.4 mmol) and 7-bromoacetyl-9,9-diethyl-2-
diphenylaminofluorene 3a (0.7 g, 1.4 mmol) were dissolved
in toluene (700 ml) under an atmospheric pressure of nitrogen.
To this was added by 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU, 0.2 ml, 1.4 mmol) and stirred at room temperature
for a period of 5.0 h. At the end of stirring, suspending solids
of the reaction mixture were filtered off and the filtrate was
concentrated to a 10% volume. Methanol (100 ml) was then
yellow crystalline solids in 66% yield (865 mg). Spectroscopic
data of 3a: FAB+-MS calcd for
12
C
31
1H2879.9Br114N116O1 m/z
510; found, m/z 509 and 511; Anal. Calcd for C31H28BrNO: C,
This journal is ß The Royal Society of Chemistry 2006
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added to effect precipitation of the crude product, which was
isolated by centrifugation. The isolated solid was a mixture of
the monoadduct 1a and bisadduct 2a. Separation of 1a and 2a
were made by column chromatography (silica gel) using a
solvent mixture of hexane–toluene (3 : 2) as eluent. The first
chromatographic band corresponding to Rf = 0.6 on the thin-
layer chromatographic plate (TLC, SiO2, hexane–toluene, 3 : 2)
afford 7-(1,2-dihydro-1,2-methanofullerene[60]-61-carbonyl)-
9,9-diethyl-2-diphenylaminofluorene 1a, the monoadduct
C60(>DPAF-C2), as greenish brown solid (780 mg, 71% based
filtered off and the filtrate was concentrated to a 10% volume.
Methanol (100 ml) was then added to cause precipitation of
the crude product, which was isolated by centrifugation. The
isolated solid was a mixture of the monoadduct 1b and
bisadduct 2b. Separation of 1b and 2b were made by column
chromatography on silica gel using a solvent mixture of
hexane–toluene, 3 : 2, as eluent. The first chromatographic
band at Rf = 0.45 on the thin-layer chromatographic (TLC,
SiO2) plate using hexane–toluene, 3 : 2, as eluent afford
C60(>DPAF-C9) monoadduct 7-(1,2-dihydro-1,2-methanoful-
lerene[60]-61-carbonyl)-9,9-di(3,5,5-trimethylhexyl)-2-diphenyl-
aminofluorene 1b, as greenish brown solids (960 mg, 70%
on recovered C60). Spectroscopic data of 1a: FAB+-MS calcd
1H2714N116O1 m/z 1149; found, m/z 1150; DCI2-MS
12
for
C
91
based on recovered C60). Spectroscopic data of 1b: FAB+-
1H2714N116O1 m/z 1149; found, m/z 1149; Anal.
12
calcd for
C
91
12
MS calcd for
C
1H5514N16O m/z 1345; found, m/z 1345
Calcd for C91H27NO: C, 95.03; H, 2.34; N, 1.2; found: C,
94.58; H, 2.63; N, 1.00; UV–vis (CHCl3, 2.0 6 1025 M)
lmax (e) 260 (1.5 6 105), 328 (6.4 6 104), and 410 nm (4.7 6
104 L mol21 cm21); FT-IR (KBr) umax 3029 (w), 2963 (s), 2921
(m), 2875 (w), 2853 (w), 1677 (s), 1591 (vs), 1492 (s), 1276 (s),
750 (s), 695 (s), and 524 (s) cm21; 1H NMR (400 MHz, CDCl3,
ppm) d 8.48 (dd, J = 8 Hz, J = 1.6 Hz, 1H), 8.32 (d, J = 1.6 Hz,
1H), 7.83 (d, J = 8 Hz, 1H), 7.66 (d, J = 8 Hz, 1H), 7.29–7.11
(m, 10H), 7.07–7.03 (m, 2H), 5.69 (s, 1H), 2.13–1.89 (m, 4H),
and 0.40 (t, J = 8 Hz, 6H); 13C NMR (400 MHz, CDCl3, ppm)
d 189.6, 153.0, 150.8, 149.1, 148.2, 147.9, 147.6, 146.9, 145.6,
145.4, 145.3, 145.1, 145.0, 144.9, 144.7, 144.6, 144.6, 144.3,
143.9, 143.7, 143.3, 143.1, 143.0, 143.0, 142.9, 142.8, 142.5,
142.2, 142.2, 142.1, 141.2, 140.9, 139.7, 136.6, 134.2, 133.5,
129.3, 129.0, 124.5, 123.2, 123.1, 122.8, 121.8, 119.2, 118.1,
72.7, 56.4, 44.4, 32.5, and 8.7.
105
and 1346; Anal. Calcd for C105H55NO: C, 93.68; H, 4.08; N,
1.04; found: C, 93.52; H, 3.88; N, 0.95; UV-vis (CHCl3,
2.0 6 1025 M) lmax (e) 263 (1.7 6 105), 326 (5.7 6 104),
and 410 nm (4.4 6 104 L mol21 cm21); FT-IR (KBr) umax
3029 (w), 2946 (s), 2860 (m), 1677 (s), 1591 (vs), 1490 (s),
1463 (s), 1423 (s), 1273 (s), 1198 (s), 750 (s), 695 (s), and
525 (s) cm21 1H NMR (400 MHz, CDCl3, ppm) d 8.48 (d,
;
J = 8 Hz, 1H), 8.34 (s, 1H), 7.83 (d, J = 8 Hz, 1H), 7.67 (d,
J = 8 Hz, 1H), 7.35–7.02 (m, 12H), 5.69 (t, J = 4 Hz, 1H),
2.2–1.8 (m, 4H), and 1.28-0.48 (m, 34H). 13C NMR
(400 MHz, CDCl3, ppm) d 189.3, 153.7, 151.4, 149.1,
148.2, 148.20, 147.6, 147.6, 146.9, 145.6, 145.4, 145.3, 145.1,
145.0, 144.9, 144.7, 144.6, 144.6, 144.5, 143.9, 143.7, 143.3,
143.1, 143.1, 143.0, 142.9, 142.9, 142.8, 142.5, 142.2, 142.1,
141.2, 140.9, 139.6, 136.6, 134.0, 133.5, 129.4, 128.9, 124.5,
123.2, 122.8, 121.8, 119.3, 118.0, 72.6, 55.2, 50.9, 44.6, 38.2,
33.2, 30.9, 30.0, 29.5, and 22.6.
The second chromatographic band corresponding to Rf =
0.3 on the thin-layer chromatographic plate (TLC, SiO2,
hexane–toluene, 3 : 2) gave the bisadduct C60[methanocarbo-
nyl-7-(9,9-diethyl-2-diphenylaminofluorene)]2, C60(>DPAF-
C2)2 2a, as brownish solids in a minor quantity of roughly
The second chromatographic band corresponding to Rf = 0.25
on TLC gave the bisadduct C60(methanocarbonyl-9,9-di(3,5,5-
trimethylhexyl)-2-diphenylaminofluorene)2, C60(>DPAF)2 2b,
11% yield (170 mg). Spectroscopic data of 2a: FAB+-MS calcd
as brownish solids in 14% yield (190 mg). Spectroscopic
data of 2b: FAB+-MS calcd for
1H11014N216O2 m/z
12
C
12
1H5414N216O2 m/z 1578; found, m/z 1579; Anal.
150
for
C
122
1971; found, m/z 1971 ; UV–vis (CHCl3, 2.0 61025 M)
lmax (e) 255 (1.9 6 105), 310 (1.1 6 105), and 406 nm
(8.5 6 104 L mol21 cm21); FT-IR (KBr) umax 3029 (w),
2946 (s), 2860 (m), 1677 (s), 1591 (vs), 1490 (s), 1463 (s),
Calcd for C122H54N2O2: C, 92.77; H, 3.42; N, 1.77; found: C,
92.60; H, 3.05; N, 1.94; UV–vis (CHCl3, 2.0 6 1025 M)
lmax (e) 254 (1.2 6 105), 308 (8.4 6 104), and 406 (7.8 6
104 L mol21 cm21); FT-IR (KBr) umax 3029 (w), 2959 (s), 2920
(m), 2850 (w), 1677 (s), 1592 (vs), 1490 (s), 1277 (s), 752 (s), 696
(s), and 525 (s) cm21; 1H NMR (400 MHz, CDCl3, ppm) d 8.7–
8.2 (m, 4H), 7.9–7.7 (m, 2H), 7.7–7.5 (m, 2H), 7.4–7.2 (m,
20H), 7.2–7.0 (m, 4H), 5.6–5.3 (m, 2H), 2.1–1.9 (m, 8H), and
0.40 (m, broad, 12H).
1423 (s), 1273 (s), 1198 (s), 750 (s), 695 (s), and 525 (s) cm21
;
1H NMR (400 MHz, CDCl3, ppm) d 8.6–8.4 (m, 4H), 7.9–
7.4 (m, 4H), 7.32–7.04 (m, 24H), 5.7–5.4 (m, 2H), 2.2–1.6
(m, 8H), and 1.3–0.2 (m, 68H). 13C NMR (200 MHz,
CDCl3, ppm) d 189.8, 154.1, 151.8, 151.7, 149.4, 148.1,
148.0, 147.7, 147.5, 146.9, 146.8, 146.6, 146.2, 146.0, 145.9,
145.7, 145.5, 145.0, 145.0, 144.9, 144.8, 144.7, 144.5, 144.3,
144.0, 143.9, 143.7, 143.6, 143.3, 143.3, 143.1, 143.0, 143.0,
142.9, 142.6, 142.5, 142.3, 142.3, 142.2, 142.1, 140.9, 140.5,
140.0, 139.8, 139.7, 139.5, 134.5, 134.0, 129.8, 129.5, 129.3,
128.65, 124.9, 123.6, 122.3, 119.6, 118.5, 73.4, 73.2, 55.6,
51.3, 51.2, 44.4, 44.4, 42.9, 38.2, 38.2, 38.1, 33.6, 33.5, 33.5,
31.4, 30.4, 29.9, 27.7, 23.1, and 22.9.
Synthesis of 7-(1,2-dihydro-1,2-methanofullerene[60]-61-
carbonyl)-9,9-di(3,5,5-trimethylhexyl)-2-diphenylaminofluorene,
the monoadduct C60(>DPAF-C9) 1b, and the bisadduct,
C60[methanocarbonyl-7-(9,9-di(3,5,5-trimethylhexyl)-2-
diphenylaminofluorene)]2, C60(>DPAF-C9)2 2b
C60 (1.0 g, 1.38 mmol) and 7-bromoacetyl-9,9-di(3,5,5-
trimethylhexyl)-2-diphenylaminofluorene
3b
(970
mg,
1.38 mmol) were dissolved in toluene (700 ml) under an
atmospheric pressure of nitrogen. To this was added
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 0.2 ml, 1.38 mmol)
and stirred at room temperature for a period of 5.0 h. At the
end of stirring, suspending solids of the reaction mixture were
Results and discussion
Molecular conjugation of C60, which exhibits the reverse-
saturable absorption capability, and the diphenylaminodi-
alkylfluorene (DPAF-Cn) component with relatively broad
1370 | J. Mater. Chem., 2006, 16, 1366–1378
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optical absorption in the visible wavelength region is con-
sidered in our design of new photonic materials. The conjuga-
tion may lead possibly to, for example, the light- harvesting
efficiency enhancement of fullerene-chromophore derivatives
in the visible region. Accordingly, covalent coupling of
fullerenyl p-orbitals with electron-accepting (A) characteristics
and p-electron-donating DPAF-Cn (D) moieties produces a
three-dimensional C60–(D)n molecular system. Aside from its
strongly electron-deficient properties with the capability of
accepting multiple electrons, C60 is attractive owing to its
multiple reactive sites available for attaching up to six or more
photoactive arms. We propose a direct chemical bonding of
multiple DPAF derivatives on a C60 cage in close vicinity,
forming a spherical star-like structure, for enhancing the
overall cross-section of two-photon absorption and reducing
the ability of 2PA chromophores to aggregate and order.
In our approach using C60–keto–donor assembly structure
for the construction of photoresponsive C60-dyads, a fluorene
moiety was chosen because of its inherent thermal and
photochemical stability. Aromatic C2 and C7 positions of
the fluorene ring can be modified with either electron-rich or
electron-withdrawing substituents for the alteration of its
electronic properties. Simultaneous attachment of alkyl
groups at C9 position should increase the solubility of the
resulting C60-fluorene dyad. The first diphenylaminofluoreno-
methano[60]fullerene dyad C60(>DPAF-C2) 1a was prepared
by a synthetic procedure depicted in Scheme 1. It involved
synthesis of the key intermediate 7-bromoacetyl-9,9-diethyl-
2-diphenylaminofluorene 3a, starting from commercially
available 2-bromofluorene 4, prior to attachment of the
fluorene moiety to the C60 cage by a keto linker. The
linkage places the diphenylaminofluorene moiety in a close
neighborhood of the C60 cage.
spectrum of 6a showed a main peak at m/z 389 consisting of its
molecular ion mass. Additional structural confirmation was
1
made by H NMR spectroscopic analysis with new resonance
observed for the aromatic protons of diphenylamino moiety
at d 6.99 to 7.64 along with the proton signals of two ethyl
groups at d 0.36 (t, terminal methyl, 6H) and two groups of
multiplet centered at d 1.90 and 2.01 for four methylene
protons at C9 position.
Friedel–Crafts acylation19 of 6a at C7 position of diphenyl-
aminofluorene moiety was achieved with a-bromoacetyl
bromide and aluminium chloride in 1,2-dichloroethane at
0 uC to ambient temperature for 4.0 h to afford 7-a-bromo-
acetyl-9,9-diethyl-2-diphenylaminofluorene 3a as yellow semi-
crystalline solids in 66% yield after column chromatography
(silica gel, hexane–toluene, 3 : 2, Rf = 0.3 on TLC). As
anticipated, positive ion FAB mass spectrum of 3a showed a
main peak at m/z 510 consisting with its molecular ion
mass. Infrared spectrum of 3a displayed a strong carbonyl
stretching absorption band centered at 1674 cm21 indicating
the presence of a-bromoacetyl group. Characteristic signal of
two methylene protons (Ha) of a-bromoacetyl group appeared
1
as a singlet at d 4.50 in H NMR spectrum of 3a. Attachment
assignment of the a-bromoacetyl group at C7 position of
the fluorene moiety was made by a new carbonyl carbon
peak at d 191.0 of 13C NMR spectrum and clear downfield
shift of phenyl protons at C5, C6, and C8 of fluorene unit to d
7.65 (d, J = 8 Hz), 7.95 (dd, J = 8 Hz, J = 1.6 Hz) and 7.92 (d,
J = 1.4 Hz), respectively, from those of 6a in 1H NMR
spectrum. In addition, elemental analyses of C, H, and N
atoms were found to agree well with theoretically calculated
weight percentage values.
Similar reaction conditions were used for the synthesis of the
key intermediate 7-a-bromoacetyl-9,9-di(3,5,5-trimethylhexyl)-
2-diphenylaminofluorene 3b from 9,9-di(3,5,5-trimethylhexyl)-
2-diphenylaminofluorene 6b with a-bromoacetyl bromide
(1.2 equiv.) in the presence of aluminium chloride (1.1 equiv.)
at 0 uC to ambient temperature for 4.0 h. The product was
obtained in roughly 67% yield. Positive ion FAB mass (FAB+-
MS) spectrum of 3b showed a main peak at m/z 705 and 707
consisting with its molecular ion mass. The chemical shift of
two Ha protons of 3b appeared as a triplet at d 4.46 in 1H
NMR spectrum (Fig. 1a). The doublet peak at d 7.63 was
assigned to the chemical shift of phenyl proton (H5) at C5.
The most downfield-shifted multiplet peak at d 7.94 with an
integration intensity corresponding to two protons (H6 and
H8) is the chemical shift of phenyl protons at C6 and C8 of the
fluorene unit.
Diethylation of 4 was carried out with ethyl iodide in
toluene in the presence of aqueous sodium hydroxide and
a phase transfer agent, tetra-n-butylammonium bromide, at
60 uC for a period of 8.0 h. The resulting product 2-bromo-9,9-
diethylfluorene 5a was obtained as colorless semisolid in
81% yield after thin-layer chromatographic (TLC) purification
(SiO2, Rf = 0.8, hexane) or column chromatography. Its
structure was confirmed by detection of the molecular ion
mass at m/z 301 in positive ion fast atom bombardment mass
spectrum (FAB+-MS) and 1H NMR spectrum. The latter
displayed the characteristic resonance of ethyl groups with six
terminal methyl protons appearing as a triplet at d 0.24
and a broad multiplet between d 1.94–1.91 corresponding
to four methylene protons at a position to fluorenyl benzylic
carbon. Amination of 5a was performed by Buchwald’s
Addition of a substituted DPAF unit to C60 leading to
the formation of dyads 1a and 1b was accomplished by the
treatment of a-bromoacetylfluorene derivative 3a and 3b,
respectively, with C60 in toluene in the presence of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU, 1.0 equiv.) at ambient
temperature for 5.0 h.20 Systematic evaluation of the reaction
mixture by TLC technique revealed the need of excess 3a and
3b for full utilization of C60 in the cyclopropanation reaction.
However, a higher quantity of a-bromoacetyl fluorine than
1.0 equiv. used in the reaction resulted in an increased yield of
bisaddition and trisaddition products of C60 detectable by a
lower Rf value on an analytical TLC plate. Formation of
procedure18 using diphenylamine as
presence of tris(dibenzylideneacetone)dipalladium(0) catalyst,
rac-2,29-bis(diphenylphosphino)-1,19-binaphthyl (BINAP)
a reagent in the
ligand, and sodium t-butoxide in toluene at refluxing
temperatures for 8.0 to 10 h under nitrogen atmosphere. The
product 9,9-diethyl-2-diphenylaminofluorene 6a was isolated
as a white amorphous solid in 98% yield after column
chromatographic purification (silica gel, hexane–toluene, 9 : 1,
Rf = 0.6 on TLC using hexane–toluene, 4 : 1, as eluent).
Complete disappearance of C–Br stretching vibration was
revealed in the infrared spectrum of 6a. Positive ion FAB mass
This journal is ß The Royal Society of Chemistry 2006
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evidence of the composition mass of fullerene–fluorene dyad
1a. In a separate measurement, the molecular ion mass of
bisadduct C60(>DPAF-C2)2 2a was detected in FAB+-MS at
m/z 1578 and 1579 (M+). It was followed by a group of mass
peaks centered at m/z 1149 matching well with that of the
monoadduct 1a with the loss of one 7-aceto-9,9-diethyl-2-
diphenylaminofluorene subunit from the molecular mass. In
the case of 1b and 2b, positive ion FAB mass spectrum
displayed a group of sharp mass peaks at m/z 1345 and 1346,
corresponding to the molecular ion mass of C60(>DPAF-C9)
1b, with no mass fragmentation peaks in the range of m/z 800–
1330 following the molecular ion peak, as shown in Fig. 2a.
The next major group of mass fragmentation peaks occurred
at m/z 721 corresponding to the ion mass of the C60 cage. The
relatively simple spectrum in the higher mass region revealed
high stability of aromatic diphenylaminofluorene moiety
under FAB+-MS measurement conditions. That led to the
bond cleavage occurring only at the cyclopropanyl carbon
conjunction bonds bridging fullerene and DPAF moieties.
Similar FAB+-MS spectrum profile (Fig. 2b) was observed
for the bisadduct C60(>DPAF-C9)2 2b showing the corre-
sponding molecular ion mass at m/z 1971, followed by no mass
fragmentation peaks until m/z 720 corresponding to the ion
mass of C60.
Fig. 1 1H NMR spectra of (a) the parent 7-a-bromoacetyl-9,9-
di(3,5,5-trimethylhexyl)-2-diphenylaminofluorene 3b, (b) the mono-
adduct C60(>DPAF-C9) 1b, and (c) the bisadduct C60(>DPAF-C9)2 2b
in CDCl3.
fullerenyl bisadduct persists even with only one equivalent of
a-bromoacetylfluorene applied. Therefore, chromatographic
separation of [6,6]-cyclopropanyl fullerene bisadducts from the
main monoadduct product became necessary. This separation
procedure was made by column chromatography (silica gel)
using a solvent mixture of hexane–toluene, 3 : 2, as eluent.
The first chromatographic band corresponding to Rf = 0.6
on a TLC (SiO2) plate afforded the monoadduct 1a, 7-(1,2-
dihydro-1,2-methanofullerene[60]-61-carbonyl)-9,9-diethyl-2-
The molecular structure of C60(>DPAF-C2) was confirmed
by the X-ray crystallographic analysis of a single crystal grown
in ClCH2CH2Cl. Perspective 3-D ORTEP view of molecular
unit cell packing is displayed in Fig. 3. A highly ordered array
of the fullerene cages along the a-axis of the unit cell was
observed with the planar diphenylaminofluorene (DPAF)
diphenylaminofluorene [C60(>DPAF-C2)], or 1b (Rf
=
0.45, TLC, SiO2), 7-(1,2-dihydro-1,2-methanofullerene[60]-61-
carbonyl)-9,9-di(3,5,5-trimethylhexyl)-2-diphenylaminofluorene
[C60(>DPAF-C9)], both as greenish brown solids in
a
similar yield of more than 50% (or 71% based on recovered
C60). The second chromatographic band corresponding
to Rf = 0.3 on TLC gave the corresponding bisadduct
2a,
C60[methanocarbonyl-7-(9,9-diethyl-2-diphenylamino-
fluorene)]2 [C60(>DPAF-C2)2], as brownish solids in a minor
yield of roughly 11%. Similarly, the bisadduct 2b (Rf = 0.25,
TLC, SiO2), C60[methanocarbonyl-7-(9,9-di(3,5,5-trimethyl-
hexyl)-2-diphenylaminofluorene)]2 [C60(>DPAF-C9)2], was
isolated in a yield of 14%.
Evidence of the DPAF moiety bridging on the methano[60]-
fullerene cage by a keto group can be seen in the infrared
spectra of 1a and 1b as both showing a strong optical
absorption band of carbonyl stretching centered at 1677 cm21
with three typical fullerene derivative signals at 750, 695 and
524 cm21. The former absorption band matches well with that
of 3a and 3b, however, in a roughly 50 cm21 shift from the
normal carbonyl stretching band at 1720–1750 cm21 due to
influence of the fluorine ring structure. The molecular ion mass
of 1a was clearly detected in both negative ion desorption
chemical ionization mass spectrum (DCI2-MS), by a group of
mass peaks with a maximum peak intensity centered at m/z
1149 (M+), and positive ion fast atom bombardment mass
spectrum (FAB+-MS) at m/z 1150 (M+). That clearly provides
Fig. 2 Positive ion FAB mass spectrum (direct probe) of (a)
C60(>DPAF-C9) 1b and (b) C60(>DPAF-C9)2 2b, showing the
corresponding molecular ion mass at m/z 1345 and 1971, respectively.
1372 | J. Mater. Chem., 2006, 16, 1366–1378
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Fig. 3 Molecular packing of C60(>DPAF-C2) 1a in a single crystal unit cell showing a highly ordered array of the fullerene cages along the a-axis.
Interchange among different views of (a), (b), and (c) can be made by rotation along the axis indicated.
rings located next to each other in the a–b plane of the unit cell,
showing no direct p–p interaction between DPAF rings
(Fig. 3c). That revealed the intermolecular forces involved in
the molecular assembly of C60(>DPAF-C2) being dominated
by the strong fullerenyl p-interactions. The closest distance
with Z = 4; Rint = 0.0396; abs. coefficient 0.089 mm21; R1 =
0.0524 and wR2 = 0.1279 for I > 2s(I), R1 = 0.0786 and wR2 =
0.1510 for all data.{
With the molecular structure of 1a resolved, we are able to
assign all proton peaks in its 1H NMR spectrum. The a-proton
(Ha, next to the carbonyl group) peak of 1a and 1b appeared as
a singlet and triplet, respectively, at d 5.69 (Fig. 1b) with a
large downfield shift of roughly 1.2 ppm from that of 3a and
3b. It was also accompanied with downfield shift of all phenyl
proton at C5, C6, and C8 of the fluorene moiety to 7.83 (d, J =
8 Hz), 8.48 (dd, J = 8 Hz, J = 1.6 Hz) and 8.32 (d, J = 1.6 Hz)
for 1a and to 7.83 (d, J = 8 Hz), 8.48 (d, J = 8 Hz), and 8.34 (s)
for 1b, as shown in Fig. 1b. In the case of the bisadducts
C60(>DPAF-C2)2 2a and C60(>DPAF-C9)2 2b, comparable 1H
NMR spectra were obtained. For example, using the spectrum
of 1b as the reference, we assigned the multiplet peaks at d 7.7–
7.9 (2H), 8.2–8.35 (2H), and 8.35–8.6 (2H) in Fig. 1c for the
chemical shift of H5, H8, and H6 protons of 2b, respectively.
There are two groups of proton peaks at d 5.35–5.6 and 5.6–
5.85 in a different integration intensity accounted for a total of
two Ha protons. That revealed two possible regioisomers of
the bisadduct in a molecular ratio of roughly 1 : 2.8 in the
˚
between two neighboring C60 cages was found to be 2.72 A
along the a-axis (Fig. 3b). Two single-bond lengths connecting
the methanocarbon and two fullerenyl carbons in the
˚
˚
cyclopropanyl ring were found to be 1.498 A and 1.487 A.
The distance between two fullerenyl carbons in the cyclopro-
˚
panyl ring of 1a is 1.628 A, longer than the normal single C–C
bond length, that leads to three angles of cyclopropanyl ring as
66.13u, 56.61u, and 57.26u with the former largest angle located
at the methanocarbon. The crystallographic data collection
and structural refinement information are as follows: empirical
formula, C91H27NO; formula weight, 1150.14; crystal system,
monoclinic; space group, P21/n; T = 295(2) K; unit cell dimen-
˚
sions a = 10.00100(10) A, b = 19.5790(2) A, c = 25.7150 (3) A,
˚
˚
3
˚
a = 90u, b = 93.0510(10)u, c = 90u; volume V = 5028.11(9) A
{ CCDC reference numbers 183726. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b515055h
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axis along the plane of two sp3 fullerene carbons, as shown in
Fig. 5a. These characteristic carbon signals confirmed the
overall chemical structure of dyad C60(>DPAF-C2). In 13C
NMR spectrum of the dyad C60(>DPAF-C9), the correspond-
ing key signals of C9, carbonyl carbon, a-carbon, and
fullerenyl sp3 carbons were recorded at d 55.2, 189.3, 44.6,
and 72.6, respectively. Confirmation of a C2 molecular sym-
metry for dyad 1b as a DPAF monoadduct of C60 was made by
the close resemblance of its fullerenyl sp2 carbon peak pattern
to that of 1a, showing a total of identical 29 peaks (Fig. 5b).
Optical absorption of 1a gave three major bands at 260
(e = 1.5 6 105), 328 (e = 6.4 6 104), and 410 nm (e = 4.7 6
104 L mol21 cm21), whereas 1b displayed similar three absorp-
tion bands at 263 (e = 1.7 6 105), 326 (e = 5.7 6 104), and
410 nm (e = 4.4 6 104 L mol21 cm21) in their UV–vis spectra
(Fig. 6). The former two bands match approximately with
those of the model compounds C60(>COPh) (Fig. 6Ac) and
C60[>(CO2Et)2] (Fig. 6Ad) containing mainly an optical active
fullerene moiety that confirms these absorption bands
being attributed to the C60 cage. The latter band fits well with
the main optical absorption of 7-bromoacetyl-9,9-diethyl-2-
diphenylaminofluorene (DPAF-C2, 3a, Fig. 6Aa) centered at
Fig. 4 13C-DEPT NMR spectrum of the dyad C60(>DPAF-C2) 1a in
CDCl3 showing (a) upward signals for CH and CH3 carbons and
downward signals for CH2 carbons, (b) the upward CH carbon signal,
and (c) the rest of non-protonated fullerenyl carbon signals.
isolated, chromatographically non-separable fraction, as a
result of the large DPAF moiety size and a close similarity of
the molecular polarity between regioisomers.
DEPT (distortionless enhancement by polarization transfer)
measurement of 13C NMR spectrum of C60(>DPAF-C2)
(Fig. 4) provides a good method for identification of carbon
groups in the structures of 1a and 1b. Accordingly,
1H-decoupled 13C signals appear upward for the carbon atoms
1
directly bonded with one or three H nuclei or downward for
the carbon atoms directly bonded with two 1H nuclei.
Combination of Fig. 4a and 4b allowed differentiation of
terminal methyl and methylene carbon peaks at d 8.7 and 32.5,
respectively, corresponding to ethyl groups attached at C9 of
1a. Most importantly, the chemical shift of cyclopropanyl
carbon next to the keto group was found to be d 44.4 showing
an upward carbon peak in Fig. 4b due to bonding with one Ha
atom. The remaining two fullerenyl sp3 carbon and C9 peaks
appeared at d 72.7 and 56.4, respectively. The most downfield
peak at d 189.6 was assigned to the bridging carbonyl carbon
(Fig. 4c). All other fullerenyl carbons appeared as a total of
29 peaks in the region of d 136 to 147 accounted for 58 sp2
carbons of the fullerene cage, indicating clearly a C2 symmetry
Fig. 6 UV–vis spectra of (A) and (B) as (a) DPAF-C2 3a, (b) DPAF-
C9 3b, (c) C60(>COPh), (d) C60[>(CO2Et)2], (e) C60(>DPAF-C2) 1a, (f)
C
60(>DPAF-C9) 1b, and (g) C60(>DPAF-C2)2 2a in chloroform (2.0 6
1025 M). Very weak characteristic steady state absorption band of the
methano[60]fullerene moiety of C60(>DPAF-C2) became visible at
roughly 690 nm at a higher concentration of 4.4 6 1024 M, as shown
in the insert of (A).
Fig. 5 13C NMR spectra of (a) C60(>DPAF-C2) 1a and (b)
C
60(>DPAF-C9) 1b showing close resemblance of a fullerenyl carbon
peak pattern consisting of 29 carbon signals.
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corresponding intermolecular process. In the case of DMF, the
reduction of fluorescence at 475 nm to a band intensity of
less than 30% was measured upon the same photoexcitation
at 400 nm, as shown in steady-state fluorescence spectrum of
C60(>DPAF-C2) (Fig. 8a). This 475 nm fluorescence indicates
that the lowest excited singlet energy of C60[>1(DPAF-C2)*]
is 2.61 eV in DMF. Strong solvent-sensitive fluorescence
emission was observed in these C60–DPAF conjugates.
For example, multiple fluorescence emission bands of the
DPAF-C2 moiety with the peak maxima appearing at 425, 450,
and 475 nm were detected in benzonitrile, as shown in Fig. 8b.
We propose that both the heterogeneous molecular aggrega-
tion of C60(>DPAF-C2) and aggregate–solvent interactions
contribute to the variation in the shape and intensity of the
fluorescence band in polar solvents.
Fig. 7 Comparison of intermolecular and intramolecular fluores-
cence quenching efficiency of the system (a) DPAF-C2 control, (b) an
equimolar mixture of C60 and DPAF-C2, and (c) C60(>DPAF-C2)
under 400 nm excitation in CHCl3 (2.0 6 1025 M).
Interestingly, this one-photon excited emission of 1a
was highly suppressed in less polar and non-polar solvents,
including CHCl3, CS2, THF, o-dichlorobenzene (o-DCB),
nitrobenzene and ethyl acetate, showing only a very weak
broad band centered at 475 nm. Instead, a low intensity of the
fluorescence band with the peak at 700–710 nm was recorded
in chloroform, CS2, o-dichlorobenzene and THF, as shown in
the insert of Fig. 8, with appreciable emission intensity in the
first two solvents (Figs. 8c and 8d). No fluorescence band of 1a
at either 475 or 700 nm was detectable in nitrobenzene and
ethyl acetate. Disappearance of all fluorescence emission may
299 (e = 2.4 6 104) and 406 nm (e = 2.7 6 104 L mol21 cm21
and DPAF-C9 (3b, Fig. 6Ab) at 407 nm (3.8
)
6
104 L mol21 cm21) that is attributed to the corresponding
diphenylaminofluorene moiety. Nearly absorptive superimpo-
sition of Figs. 6Ae and 6Af with the combined spectra of two
independent chromophores containing either the fullerene cage
or diphenylaminofluorene revealed no ground-state interac-
tion between these two moieties present in the molecule of 1a
and 1b in chloroform (2.0 6 1025 M). In addition, a very
weak characteristic steady state absorption band of the
methano[60]fullerene moiety of C60(>DPAF-C2) became
visible only at a higher concentration than 4.4 6 1024
M
with the peak maximum appearing at roughly 690 nm, as
shown in Fig. 6A(insert). As the number of DPAF addends
increases to two in the structure of C60(>DPAF-C2)2 2a,
the absorption intensity of the DPAF components becomes
significant and dominates the spectrum with two DPAF
bands centered at 308 (8.4 6 104) and 406 nm (7.8 6
104 L mol21 cm21) clearly shown in Fig. 6Bg. Intensity of
the latter band is almost 1.8-fold higher than that of the
monoadduct 1a (Fig. 6Be), consistent with the composition of
the bisadduct 2a.
Most of C60–DPAF conjugates exhibit either intramolecular
electron- or energy-transfer phenomena at its photoexcited
nanosecond transient states from the DPAF donor moiety to
the fullerene acceptor moiety. Diphenylaminofluorene sub-
units DPAF-C2 and DPAF-C9 are highly fluorescent upon
photoexcitation at 400 nm showing fluorescence emission with
the peak maximum centered at 500 nm. In CHCl3, a good
solvent for DPAF-C2 and C60(>DPAF-C2), intermolecular
partial quenching of DPAF-C2 fluorescence (Fig. 7b) by C60
was detected at a concentration of 2.0 6 1025 M that reduced
its intensity to less than a half of the control value (Fig. 7a).
Covalent attachment of the fullerene cage to diphenylamino-
fluorene moiety in close vicinity forming the dyad structure of
1a quenched the fluorescence emission of DPAF-C2 moiety
in a near quantitative yield (Fig. 7c), indicating that the
intramolecular energy transfer process from the DPAF-C2
moiety to the fullerene cage is much more efficient than the
Fig. 8 Fluorescence spectra of C60(>DPAF-C2) 1a upon 400 nm
excitation in the solvents of (a) DMF, (b) PhCN, (c) CHCl3, (d) CS2,
(e) o-dichlorobenzene, (f) THF, (g) EtOAc, and (h) nitrobenzene in the
presence of air at a concentration of 2.0 6 1025 M.
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be the result of severe molecular aggregation and the related
quenching effect, due to low solubility of 1a in these solvents.
The observed fluorescence band at >700 nm was compared to
that of methanofullerene C60(>CH2) at 717 nm in toluene,16
revealing its source arising from the fullerene cage moiety.
Therefore, this fluorescence band can also be correlated to
difficulty of both fullerene cage and fluorene ring. This
decreases the aggregation-induced fluorescence quenching
effect making the band comparatively visible.
In the case of the bisadduct C60(>DPAF-C9)2 2b containing
one more DPAF-C9 subunit than 1b, its fluorescence spectra in
all solvents, except CS2, exhibited the DPAF-C9 fluorescence
band in intensity stronger than those of 1b, as shown in Fig. 10.
Specifically, a visible increase of peak intensity profile in
CHCl3 (Fig. 10d) and o-dichlorobenzene (Fig. 10f) revealed
incomplete fluorescence quenching of the DPAF-C9 moieties
by the fullerene cage, via energy transfer, in these solvents. We
assume the cause of full fullerenyl fluorescence suppression
of 2b in DMF, PhCN, and THF, similar to 1a, arising
1
the lowest excited singlet energy of C60*(>DPAF-C2) and
estimated to be 1.74–1.75 eV in CHCl3. The fullerenyl
fluorescence intensity of 1a was fully quenched in polar
solvents including DMF and benzonitrile shown as the profiles
(a) and (b) in the insert of Fig. 8. Large decrease in
fluorescence of the fullerene moiety may indicate the
occurrence of charge-separation due to electron transfer from
DPAF-C2 moiety to the fullerene cage via the singlet excited
state 1C60*(>DPAF-C2) in polar solvents as determined by the
transient spectroscopic measurements in nanosecond region.16
Contrary phenomena were detected in non-polar solvent, such
as toluene, using the same measurement demonstrating
favorable energy transfer processes occurring from photo-
excited DPAF-C2 moiety to C60. Accordingly, we assume
similar energy transfer processes occurring during photo-
excitation of 1a in CHCl3 and CS2 for the detection of
extended fluorescence emission above 720 nm corresponding
to the energy of excited fullerene state. Comparable solvent-
dependent fluorescence emission was recorded in fluorescence
spectra of C60(>DPAF-C9) 1b upon 420 nm excitation in
various solvents of CHCl3, CS2, toluene, DMF, o-DCB, THF,
and PhCN at a concentration of 2.0 6 1025 M, as shown in
Fig. 9. No fluorescence band above 700 nm was observed in
DMF, PhCN, and THF indicating that these three solvents
as a group exhibit similar fluorescence behavior of 1b while
toluene, CHCl3, and CS2 representing the other group of the
less polar solvents showing a fluorescence band centered at
704, 701, and 709 nm (the insert of Figs. 9c, 9d, and 9e),
respectively, in an intensity relatively higher than those in
Fig. 8 for 1a. One explanation is made by the sufficient
steric hindrance induced by two 3,5,5-trimethylhexyl groups
attached at C9 carbon of the fluorene unit in the structure of 1b
resulting in aggregation inhibition and molecular stacking
2
?
from the formation of charge-separated states C60
and
(>DPAF-C9)+ . A noticeable red shift of the peak maximum
to 500–560 nm in all solvents, except toluene, was detected.
That can be correlated to the decreased lowest excited singlet
energy of C60[>1(DPAF-C2)*]2 at 2.48–2.21 eV, slightly lower
than that of 1b. Attachment of multiple hindered 3,5,5-
trimethylhexyl groups in the structure of 2b making direct
intermolecular stacking contact of fullerene cages and diphe-
nylaminofluorene rings more difficult. That may give better
presentation of solvent-dependent molecular fluorescence
characteristics. These results provide a better understanding
on the use of highly fluorescent DPAF-C9 addends as the
antenna components for the construction of C60(>DPAF-C9)x
starburst fullerene multiads to harvest light energy. As the
photoinduced electron- and energy-transfer processes
become crucial to the therapeutic method for photodynamic
treatments, further investigation of these intramolecular
phenomena in the molecular system of C60(>DPAF-Cn)x
facilitates their uses as one-photon or two-photon absorp-
tion-based photosensitizers.
?
Conclusion
Novel C60–keto–acceptor structures as hindered 9,9-di(3,5,5-
trimethylhexyl)-2-diphenylaminofluorene monoadduct of
methano[60]fullerene C60(>DPAF-C9) and the related bis-
adducts C60(>DPAF-C9)2 and C60(>DPAF-C2)2 were synthe-
sized. All C60-DPAF derivatives were fully characterized with
their chemical structure confirmed by various spectroscopic
analyses. X-Ray structural analysis on a single crystal of
C60(>DPAF-C2) validated its structural composition and the
basic molecular structure of similar dyads. Strong solvent-
sensitive fluorescence quenching phenomena of C60(>DPAF-
C2), C60(>DPAF-C9), and C60(>DPAF-C9)2 were observed,
showing no C60-based fluorescence band above 700 nm in
DMF, PhCN, and THF as a group of the solvents exhibiting
similar fluorescence behavior while toluene, CHCl3, and CS2
representing the other group of the less polar solvents showing
a fullerenyl fluorescence band at 700–710 nm. It was attri-
butable to the occurrence of electron transfer via the singlet
1
excited state of the fullerene moiety as C60*(>DPAF-Cn) in
the former group of the solvents. Contrarily, energy transfer
processeses from DPAF-Cn moiety to the fullerene cage are
favored in the latter group of the solvents. These results appear
to support our design rationale for these C60-DPAF conjugates
with the covalent attachment of diphenylaminofluorene
Fig. 9 Fluorescence spectra of C60(>DPAF-C9) 1b upon 420 nm
excitation in N2-saturated solutions of (a) DMF, (b) PhCN, (c)
toluene, (d) CHCl3, (e) CS2, (f) o-dichlorobenzene, and (g) THF in a
concentration of 2.0 6 1025 M.
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Fig. 10 Fluorescence spectra of C60(>DPAF-C9)2 2b upon 420 nm excitation in N2-saturated solutions of (a) DMF, (b) PhCN, (c) toluene, (d)
CHCl3, (e) CS2, (f) o-dichlorobenzene, and (g) THF in a concentration of 2.0 6 1025 M.
moiety to methano[60]fullerene via a keto linkage in terms of
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Acknowledgements
We thank the Air Force Office of Scientific Research under
the contract number F49620-03-1-0334 and the Materials &
Manufacturing Directorate, Air Force Research Laboratory
for partial financial support of this work. We also thank the
National Science Council, Taiwan for partial financial
support, Yi-Hung Liu and Shu-Yun Sun of Taiwan
University Instrumentation Center for X-ray structural
analysis and FAB+-MS measurements, respectively, and
Shih-Jen Wang of Tsing Hwa University Instrumentation
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