MQ-Fullerene Donor-Acceptor Complexes
A R T I C L E S
1H), 7.45 (dd, J ) 8.46 Hz, 1H), 7.15 (d, J ) 7.13 Hz, 1H), 5.58
(s, 1H), 5.09 (d, J ) 9.15 Hz, 1H), 4.39 (d, J ) 9.15 Hz, 1H), 2.81
ppm (s, 3H). 13C NMR (CDCl3:CS2): 40.53 (N-CH3); 66.57 (N-
CH2); 83.25 (N-CH) 109.21, 120.01, 124.28, 124.51, 132.85,
137.81, 139.97, 148.88, 158.10 (hydroxyquinoline), 70.58, 73.58,
136.00, 136.75, 137.50, 139.86, 139.75, 139.70, 141.22, 141.28,
141.73, 141.85, 141.92, 141.98, 142.08, 142.10, 142.20, 142.56,
142.60, 143.00, 144.35, 144.49, 144.60, 145.19, 145.25, 145.38,
145.40, 145.44, 145.50, 145.60, 145.63, 145.75, 145.98, 146.01,
146.18, 146.19, 146.20, 146.30, 147.20, 147.28, 150.82, 153.76,
154.85 (fullerene moiety), UV-vis (PhCN): λmax ) 330, 422.5,
701 nm. Mass (APCI mode in CH2Cl2): calcd, 921.0; found, 921.3.
Instrumentation. 1H NMR spectra were obtained from chloro-
form-d1 solutions using a Varian 400 MHz NMR spectrometer with
tetramethylsilane as internal standard. The UV-vis spectral mea-
surements were carried out with a Shimadzu model 1600 UV-visible
spectrophotometer. The fluorescence emission was monitored by
using a Varian Eclipse spectrometer. Cyclic voltammograms were
recorded on a EG&G PARSTAT electrochemical analyzer with a
three-electrode system. A platinum button electrode was used as
the working electrode. A platinum wire served as the counterelec-
trode and an Ag/AgCl was used as the reference electrode.
Ferrocene/ferrocenium redox couple was used as an internal
standard. All the solutions were purged prior to electrochemical
and spectral measurements using argon gas. The mass spectra were
recorded on a Varian 1200 L Quadruple MS using APCI mode in
dry CH2Cl2. Matrix-assisted laser desorption/ionization time-of-
flight mass spectra (MALDI-TOF-MS) were measured on a Kratos
Compact MALDI I (Shimadzu) for metal complex in PhCN with
dithranol used as a matrix.
decaying to the ground-state via a charge recombination process.
Although the phosphorescence of C60 is too weak for any OLED
application, the first detailed mechanistic study presented herein
on populating the triplet state of the acceptor via a charge-
separated intermediate provides an alternate approach for
excitation energy transfer of MQn.
Experimental Section
Chemicals. Tetra-n-butylammonium perchlorate, (n-Bu)4NClO4
was obtained from Fluka Chemicals. 8-Hydroxyquinoline and
2-formyl-8-hydroxyquinoline were procured from Aldrich Chemi-
cals. All other reagents and solvents were procured from Fisher
Chemicals. Benzonitrile used in spectral studies was freshly distilled
over P2O5 in vacuo to remove the impurity.21
The syntheses of 8-hydroxyquinoline appended fullerene deriva-
tives 1 and 2 were accomplished by reacting 2- or 5-formyl-8-
hydroxyquinoline with C60 and sarcosine using the standard Prato
method of fulleropyrrolidine synthesis.22 The structural integrity
of 1 and 2, and the subsequent metal complexes, were established
from UV-vis, ESI-mass, and NMR (1H and 13C) spectroscopic as
well as electrochemical methods.
Synthesis. 5-formyl-8-hydroxyquinoline. 8-Hydroxyquinoline
(5 g, 34.4 mmol) was dissolved in a solution of aqueous NaOH
and ethanol and the reaction mixture was stirred at 40 °C. The pale
opalescent mixture was heated and 8 mL of chloroform was added
over 40 min. The resulting black mixture was heated at reflux for
12 h. Subsequently, the solvent was evaporated under reduced
pressure, and the semisolid mass was poured into 200 mL of cold
water and pH was adjusted to 2.0 with conc. HCl. The brown
precipitate was filtered off, crushed, and dried. The crude compound
was purified by column chromatography (silica gel, ethyl acetate)
to give the product (495 mg, 8.3%). 1H NMR (300 MHz, CDCl3):
δ ) 10.14 (s, 1H), 9.69 (dd, J ) 8.65 Hz, 1H), 8.87 (dd, J ) 4.26
Hz, 1H), 8.00 (d, J ) 8.03, 1H), 7.66 (q, 4.31 Hz, 1H), 7.28 ppm
(d, J ) 7.89 Hz, 1H). UV-vis (PhCN): λmax ) 330.0, 388.2 nm.
2-(1-Methylfulleropyrrolidin-2-yl)-quinolin-8-ol, 1. A solution
of C60 (100 mg, 0.138 mmol), N-methylglycine (24 mg, 0.277
mmol), and 2-formyl-8-hydroxyquinoline (72 mg, 0.416 mmol) in
toluene (100 mL) was heated at reflux for 4 h before the solvent
was evaporated. The crude product was purified by column
chromatography (silica gel, hexane/toluene 5:95) to give the product
The computational calculations were performed by DFT B3LYP/
3-21G(*) methods with Gaussian 03 software package23 on high
speed computers. The graphics of frontier orbitals were generated
using the GaussView software.
Time-Resolved Transient Absorption Measurements. Fem-
tosecond laser flash photolysis was conducted using a Integra-C
laser system and an optical detection system provided by Ultrafast
Systems (Helios). The source for the pump and probe pulses were
derived from the fundamental output of Integra-C laser system (780
nm, 2 mJ/pulse, and fwhm ) 130 fs) at a repetition rate of 1 kHz.
A second harmonic generator introduced in the path of the laser
beam provided 410 nm laser pulses for excitation; 95% of the
fundamental output of the laser was used to generate the second
harmonic, while 5% of the deflected output was used for white
light generation. Prior to generating the probe continuum, the laser
pulse was fed to a delay line that provided an experimental time
window of 3.2 ns with a maximum step resolution of 7 fs. The
pump beam was attenuated at 5 µJ/pulse with a spot size of 2 mm
diameter at the sample cell where it was merged with the white
probe pulse in a close angle (<10°). The probe beam after passing
through the 2 mm sample cell was focused on a 200 µm fiber optic
cable which was connected to a CCD spectrograph. Typically, 5000
excitation pulses were averaged to obtain the transient spectrum at
a set delay time. The kinetic traces at appropriate wavelengths were
assembled from the time-resolved spectral data.
Measurements of nanosecond transient absorption spectrum were
performed according to the following procedure. A deaerated
solution containing a dyad was excited by a Panther OPO pumped
by Nd:YAG laser (Continuum, SLII-10, 4-6 ns fwhm) at λ ) 430
nm. The photodynamics were monitored by continuous exposure
to a xenon lamp (150 W) as a probe light and a photomultiplier
tube (Hamamatsu 2949) as a detector. Transient spectra were
recorded using fresh solutions in each laser excitation. The solution
wasdeoxygenatedbyargonpurgingfor15minpriortomeasurements.
Time-resolved fluorescence spectra were measured by a Photon
Technology International GL-3300 with a Photon Technology
1
(29 mg, 22.7%). H NMR (300 MHz, CDCl3): δ ) 8.28 (d, J )
8.12 Hz, 1H), 8.19 (d, J ) 8.24 Hz, 1H), 7.44 (t, J ) 7.95, 1H),
7.33 (dd, J ) 8.42 Hz, 1H), 7.15 (d, J ) 7.59 Hz, 1H), 5.38 (s,
1H), 5.05 (d, J ) 9.51 Hz, 1H), 4.38 (d, J ) 9.51 Hz, 1H), 2.87
ppm (s, 3H); 13C NMR (CDCl3:CS2): 42.83 (N-CH3); 67.01 (N-
CH2); 84.56 (N-CH), 110.01, 118.42, 118.44, 130.01, 129.05,
136.77, 137.54, 146.71, 153.01 (hydroxyquinoline ring), 72.58,
75.58, 136.04, 136.85, 137.50, 139.50, 139.85, 139.70, 141.22,
141.27, 141.73, 141.85, 141.90, 141.98, 142.08, 142.10, 142.20,
142.58, 142.62, 143.00, 144.35, 144.49, 144.60, 145.19, 145.25,
145.38, 145.40, 145.44, 145.50, 145.60, 145.63, 145.75, 145.98,
146.01, 146.18, 146.19, 146.20, 146.30, 147.20, 147.28, 150.80,
153.66, 154.80 (fullerene moiety). UV-vis (PhCN): λmax ) 325.2,
431.5, 702.0 nm. Mass (APCI mode in CH2Cl2): calcd, 921.0; found,
921.2.
5-(1-Methylfulleropyrrolidin-2-yl)-quinolin-8-ol, 2. A solution
of C60 (190 mg, 0.269 mmol), N-methylglycine (48 mg, 0.538
mmol), and 5-formyl-8-hydroxyquinoline (140 mg, 0.808 mmol)
in toluene (100 mL) was heated at reflux for 15 h before the solvent
was evaporated. The crude product was purified by column
chromatography (silica gel, toluene/ethyl acetate 96:4) to give the
product (35 mg, 14%). 1H NMR (300 MHz, CDCl3): δ ) 8.31 (d,
J ) 8.23 Hz, 1H), 8.17 (t, J ) 8.60 Hz, 1H), 7.74 (d, J ) 8.05,
(21) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. In Purification of
Laboratory Chemicals, 4th ed.; Pergamon Press: Elmsford, NY, 1996.
(22) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798–9799.
(23) Gaussian 03, Frisch, M. J.; et al. Gaussian, Inc., Pittsburgh PA,
2003.
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J. AM. CHEM. SOC. VOL. 130, NO. 50, 2008 16961