Anodic Coupling of Diphenylbenzo[k]fluoranthene
J . Org. Chem., Vol. 62, No. 3, 1997 531
Voltammograms recorded for kinetic analysis utilized a Ag/
Ag+ reference electrode (0.1 mM AgClO4, 0.3 M TBAPF6)
separated from the bulk solution by a fine frit. This reference
electrode was also calibrated by the addition of ferrocene
following most experiments. Approximately 500 mg of acti-
vated alumina (ICN Biomedicals, Super I grade) was added
to each solution used for kinetic measurements to scavenge
any residual water. Data was gathered over a range of
concentrations from 0.45 to 3.0 mM in 1 and over a range of
scan rates from 75 to 17 000 mV/s.
3 using chemical oxidizing reagents. Because the product
dimer could also be oxidized and reduced at the potentials
used to produce ECL from 1, red light from this dimer
was visible along with the blue emission from the
monomer. The conversion of 1 (RH2) to 3 (R R) involves
the removal of two electrons and two protons per mol-
ecule (eq 2). We were interested in the details of the
reaction mechanism and the possibility of finding reac-
tion intermediates.
Digital simulations were performed using Digisim 2.01 for
Windows (Bioanalytical Systems, Inc.). All electron-transfer
reactions were considered fast (k0 ) 10 000 s-1), all dispro-
portionations were assumed to be diffusion-controlled, and all
R values were taken to be 0.5. The intermolecular coupling
reactions were considered irreversible because of the following
fast deprotonations and thus were assigned large equilibrium
constants. Diffusion coefficients for 1 and 2 were measured
using chronoamperometry,8 and ions were assigned the same
coefficients as their neutral parents: 1.0 × 10-5 cm2/s for 1
2RH2 - 4e- f R R + 4H+
(2)
Here we describe this radical cation coupling reaction
in greater detail, including the observation, isolation, and
independent synthesis of a stable reaction intermediate
possessing only one bond between two monomer units
(HR-RH). The electrochemical properties and ECL of
this intermediate species are discussed. Cyclic voltam-
metry affords a convenient means to follow the oxidative
coupling reaction, and we present a study of this process.
Digital simulation of these voltammograms was carried
out to establish the mechanism of the reaction and to
determine the second-order rate constant of the dimer-
ization process.
and 1•+, and 5.0 × 10-6 cm2/s for 2, 2•+, and 22+
. Other
parameters and the mechanisms used for the digital simula-
tions are given in the text. Experimental cyclic voltammo-
grams used for comparison to simulated voltammograms had
the background current subtracted and were corrected for
ohmic drop.
Bu lk Electr olysis. Bulk electrolysis experiments were
performed using a Princeton Applied Research Model 173/175
potentiostat/universal programmer. The working electrode
consisted of a large platinum screen with a surface area of
about 4 cm2. A silver wire in a fritted compartment was used
as a quasi-reference electrode (QRE). A platinum screen
counter electrode was located in a compartment separated
from the bulk solution by a fine frit.
In a typical experiment, 1 (15 mg), Na2CO3 (∼300 mg), and
alumina (∼500 mg) were added to the working electrode
compartment, and solvent containing electrolyte was added
(benzene/acetonitrile 1:1; 0.1 M TBAPF6). The potential of the
working electrode was increased until a current of 1 mA began
to flow and was then held at that potential. The reaction was
deemed complete when a cyclic voltammogram of the reaction
solution contained no waves attributable to 1. Following
electrolysis, the product was isolated by removing the solvent
under vacuum and extracting with benzene. The extracts were
filtered and the benzene removed under vacuum to leave a
yellow solid that was then chromatographed on a silica column.
Hexanes were used to elute any unreacted 1 (blue fluorescence)
followed by CH2Cl2/hexanes (1:5) to remove the product, 2
(blue-green fluorescence, 24% yield).
Exp er im en ta l Section
Tetra-n-butylammonium hexafluorophosphate (TBAPF6)
(SACHEM, Inc.) was recrystallized from EtOH/H2O (4:1) three
times and dried at 100 °C before use. Benzene (Aldrich, ACS
grade) and CH3CN (Burdick and J ackson, UV grade) were used
as received after being transported unopened into an inert
atmosphere drybox (Vacuum Atmospheres Corp.). UV, fluo-
rescence, electrochemical, and ECL solutions were prepared
in a drybox and sealed in air-tight vessels for measurements
completed outside the drybox.
1H-NMR spectra were recorded on a General Electric QE-
300 (300 MHz) spectrometer as solutions in deuteriochloroform
(CDCl3). Chemical shifts are expressed in parts per million
(ppm, δ) downfield from tetramethylsilane (TMS) and are
referenced to CDCl3 (7.24 ppm) as internal standard. 13C-NMR
spectra were recorded on a General Electric QE-300 (75 MHz)
instrument as solutions in CDCl3. Exact mass determinations
were obtained on a VG analytical ZAB2-E instrument.
The relative fluorescence efficiency of 2 was measured5 using
5 µM solutions in benzene and with diphenylanthracene as a
standard (λexc ) 400 nm; φDPA ) 0.98 in benzene6 ). Fluores-
cence spectra were recorded on a SLM Aminco SPF-500
spectrofluorometer, and UV spectra were recorded on a Milton
Roy Spectronic 3000 array spectrophotometer.
ECL measurements were performed as previously reported7
using a charged couple device (CCD) camera (Photometrics
CH260, Photometrics, Tucson, AZ) cooled to -130 °C. ECL
solutions were similar in composition to those used for cyclic
voltammetric measurements.
Cyclic voltammograms were recorded on a Bioanalytical
Systems 100A electrochemical analyzer. The working elec-
trode in all cases consisted of an inlaid platinum disk (1.3 mm
diameter) that was polished on a felt pad with 0.05 µm
alumina (Buehler, Ltd.) and sonicated in absolute EtOH for 1
min prior to each experiment. Platinum gauze served as a
counter electrode. For standard cyclic voltammetric measure-
ments, a silver wire was utilized as a quasi-reference electrode,
and potentials were calibrated versus SCE by the addition of
ferrocene as an internal standard using E°(Fc/Fc+) ) 0.424 V
vs SCE.
Electrolyses of 2 and 3 were performed similarly, although
in these cases the products were not isolated, but were
identified by thin-layer chromatography.
4-Br om o-(7,12-d ip h en yl)ben zo[k]flu or a n th en e (4). 1,3-
Diphenylisobenzofuran (650 mg, 2.41 mmol) and 5-bro-
moacenaphthylene (555 mg, 2.40 mmol)9 in p-xylene (10 mL)
were heated at reflux for 18 h, whereupon the xylene was
removed by distillation. Anhydrous trifluoroacetic acid (1 mL)
and dichloromethane (10 mL) were added, and the resulting
mixture was heated at reflux for 18 h. Upon cooling, the
solution was evaporated to dryness under vacuum to afford a
brown solid. The solid was purified by chromatography over
silica eluting with ethyl acetate/hexanes (1:10) to afford 4 (930
mg, 80%) as a yellow crystalline solid: mp 245-247 °C; 1H
NMR (300 MHz, CDCl3) δ 6.37 (1H, d, J ) 7.7 Hz), 6.60 (1H,
d, J ) 7.1 Hz), 7.34-7.43 (3H, m), 7.51-7.55 (5H, m), 7.60-
7.68 (8H, m), 7.85 (1H, d, J ) 8.3 Hz); 13C NMR (75.2 MHz,
CDCl3) δ 121.1 (C), 122.8 (CH), 125.1 (CH), 126.0 (2CH), 126.8
(CH), 126.9 (CH), 128.1 (CH), 129.3 (2CH), 129.8 (C), 129.9
(CH), 131.2 (CH), 133.3 (C), 133.9 (C), 134.2 (C), 135.1 (C),
135.2 (C), 136.4 (C), 136.6 (C), 136.9 (C), 138.5 (C); MS (CI)
484 (100), 482 (94); HRMS (CI) calcd for C32H19Br (M+)
482.0670, found 482.0658.
(8) Denuault, G.; Mirkin, M. V.; Bard, A. J . J . Electroanal. Chem.
1991, 308, 27.
(9) Mitchell, R. H.; Chaudhary, M.; Williams, R. V.; Flyes, R.;
Gibson, J .; Ashwood-Smith, M. J .; Fry, A. J . Can. J . Chem. 1992, 70,
1015.
(5) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107.
(6) Morris, J . V.; Mahaney, M. A.; Huber, J . R. J . Phys. Chem. 1976,
80, 969.
(7) McCord, P.; Bard, A. J . J . Electroanal. Chem. 1991, 318, 91.