Dibenzotetraphenylperiflanthene: Synthesis, photophysical properties, and electrogenerated chemiluminescence

Article abstract of DOI:10.1021/ja9537888

Fissure coupling of the fluoranthene adduct (7,12-diphenyl)benzo[k]fluoranthene (3) using AlCl₃/NaCl, CoF₃/TFA, or T1(OCOCF₃) gave the new polyaromatic hydrocarbon dibenzo{[f,f]-4,4',7,7'-tetraphenyl}diindeno[1,2,3-cd:1',2

Full text of DOI:10.1021/ja9537888

2374
J. Am. Chem. Soc. 1996, 118, 2374-2379
Dibenzotetraphenylperiflanthene: Synthesis, Photophysical
Properties, and Electrogenerated Chemiluminescence
Jeff D. Debad, Jonathan C. Morris, Vince Lynch, Philip Magnus,* and
Allen J. Bard*
Contribution from the Department of Chemistry and Biochemistry, The UniVersity of Texas at
Austin, Austin, Texas 78712
ReceiVed NoVember 10, 1995X
Abstract: Fissure coupling of the fluoranthene adduct (7,12-diphenyl)benzo[k]fluoranthene (3) using AlCl3/NaCl,
CoF3/TFA, or Tl(OCOCF3) gave the new polyaromatic hydrocarbon dibenzo{[f,f']-4,4',7,7'-tetraphenyl}diindeno-
[1,2,3-cd:1′,2′,3′-lm]perylene (4). Crystal data for 4: triclinic space group P1h, a ) 10.569(2) Å, b ) 11.565(4) Å,
c ) 13.001(3) Å, R ) 95.05(2)°, â ) 111.24(1)°, γ ) 100.53(1)°, Z ) 1, RF ) 0.075%. Compounds 3 and 4 are
both highly fluorescent in solution and display relative fluorescence quantum yields of φF ) 1.0 and 0.85, respectively.
The electrochemistry and electrogenerated chemiluminescence (ECL) of each compound has been investigated. The
cyclic voltammogram of 3 in benzene-acetonitrile (9:1) shows that the compound undergoes a reversible reduction
and an irreversible oxidation, whereas the cyclic voltammogram of 4 displays the reversible formation of both singly
and doubly charged cations and anions. Compounds 3 and 4 undergo ECL to yield blue and orange-red light,
respectively, with an ECL efficiency of ∼2% for 4. Emission from 4 is observed in the ECL of unstirred solutions
of 3. This indicates that 4 is produced at the electrode during the ECL experiment, presumably via an electrochemical
oxidative coupling process during the anodic potential steps.
Introduction
rubrene and diphenylanthracene being the most studied by far.4
Potential applications of ECL include uses such as in display
devices,5 in the analytical determination of PAHs in minute
concentrations,6 and as labels in immunoassay.7
Here we report the unique synthesis of the soluble, nonal-
ternant PAH, dibenzo{[f,f′]-4,4′,7,7′-tetraphenyl}diindeno[1,2,3-
cd:1′,2′,3′-lm]perylene (4), via the oxidative coupling of (7,12-
diphenyl)benzo[k]fluoranthene (3) (see Scheme 1). The
electrochemistry and photophysical properties of the starting
material and product compounds are also presented, along with
an investigation of their electrogenerated chemiluminescence.
The discovery of the fullerenes has spurred a renaissance of
interest in the synthesis of polycyclic aromatic hydrocarbons
(PAHs).1 Investigations in this area are stimulated not only by
the synthetic challenges involved in producing larger, more
complex molecules but also by the interesting and potentially
useful properties that PAHs display as a whole.
One of the most useful properties of PAHs is their intense
fluorescence emissions, a characteristic that has been applied
to their qualitative and quantitative analysis.2 In 1964, it was
shown that the emitting states of PAHs could also be produced
by electron-transfer reactions,3 and this discovery sparked
interest in what became known as electrogenerated chemilumi-
nescence (ECL). ECL is initiated by the generation at an
electrode of ions capable of undergoing energetic electron-
transfer reactions. The simplest scheme involves production
of radical anions and radical cations of a compound at an
electrode surface, with subsequent ion-annihilation reactions
generating excited states that are capable of luminescence (eq
1).
Experimental Section
Tetra-n-butyl ammonium hexafluorophosphate (TBAPF6) (SA-
CHEM, Inc.) was recrystallized from EtOH/H2O (4:1) three times and
dried at 100 °C before use. Rubrene (Aldrich) was used as received.
Diphenylanthracene (Aldrich) was recrystallized from absolute ethanol
before use. N,N,N′,N′-Tetramethyl-p-phenylenediamine (TMPD, East-
man Kodak) was sublimed before use. Benzene (Aldrich, ACS grade),
cyclohexane (Sigma spectrophotometric grade), and CH3CN (Burdick
and Jackson, UV grade) were used as received after being transported
unopened into an inert atmosphere drybox (Vacuum Atmospheres
Corp.). All UV, fluorescence, electrochemical, and ECL solutions were
prepared in a drybox and were sealed in air-tight vessels for measure-
ments.
R
•- + R•+ f R* + R f R + R + hν
(1)
Other methods of producing ECL include the use of coreactants
or other electrogenerated species capable of reacting with either
the anion or cation alone to produce the desired excited state.
Many PAH systems have been shown to undergo ECL, with
(4) For reviews on ECL see: (a) Knight, A. W.; Greenway, G. M. Analyst
1994, 119, 879. (b) Faulkner, L. R.; Bard, A. J. Electroanalytical Chemistry;
Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10, p 1.
(5) (a) Manakata, H., Japanese Patent JP86-259286 A2 (861117), 1986.
(b) Kojima, H.; Takagi, Y.; Teramoto, H. Electrochim. Acta 1988, 33, 1789.
(c) Laser, D.; Bard, A. J. J. Electrochem. Soc. 1975, 122, 632.
(6) (a) Fleet, B.; Keliher, P. N.; Kirkbright, G. F.; Pickford, C. J. Analyst
1969, 94, 847. (b) Sato, M.; Yamada, T. Kagaku Zokan (Kyoto) 1978, 78,
63. (c) Herejk, J.; Holzbecker, Z. Chem. Listy 1984, 78, 1254. (d) Greenway,
G. M. Trends Anal. Chem. 1990, 9, 200. (e) Rozhitskii, N. N. J. Anal.
Chem. USSR 1992, 47, 1288.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) The fullerenes have recently been reviewed: (a) Acc. Chem. Res.
1992, 25(3). (b) Hirsch, A. The Chemistry of the Fullerenes; Thieme: New
York, 1995.
(2) (a) Chemical Analysis of Polycyclic Aromatic Compounds; Vo-Dihn,
T., Ed.; John Wiley and Sons: New York, 1989. (b) Polycyclic Aromatic
Compounds, Garrigres, P., Lamotte, M., Eds.; Gordon and Breach Science
Publishers: Langhorne, PA, 1993.
(7) (a) Ege, D.; Becker, W. G.; Bard, A. J. Anal. Chem. 1984, 56, 2413.
(b) Bard, A. J.; Whitesides, G. M. U.S. Patent No. 5,221,605, 1993; U.S.
Patent No. 5,238,808, 1993. (c) Yang, H.; Leland, J. K.; Yost, D.; Massey,
R. J. Bio/Technol. 1994, 12, 193.
(3) (a) Hercules, D. M. Science 1964, 145, 808. (b) Visco, R. E.;
Chandross, E. A. J. Am. Chem. Soc. 1964, 86, 5350. (c) Santhanam, K. S.
V.; Bard, A. J. J. Am. Chem. Soc. 1965, 87, 139.
0002-7863/96/1518-2374$12.00/0 © 1996 American Chemical Society

Dibenzotetraphenylperiflanthene
J. Am. Chem. Soc., Vol. 118, No. 10, 1996 2375
a
Table 1. Crystallographic Data for Compound 4‚3C7H8
Scheme 1
formula
fw
C85H60
1081.34
F
calc, g/cm3
no. of reflcns measd 12553
1.25
a, Å
b, Å
c, Å
R, deg
â, deg
γ,deg
V, Å3
Z
10.569(2) no. of unique reflcns 8339
11.565(4) decay correction
0.997-1.02
13.001(3)
R
int (F2)
0.041
0.71
95.05(2) µ, cm-1
111.24(1) crystal size, mm
100.53(1) Rw(F2)b
1435.5(7) R(F)c
0.13 × 0.24 × 1.4
0.211
0.075
1
goodness of fit, Sd
parameters
1.027
F(000)
1140
401
crystal system triclinic
Max |∆/σ|
<1.9 (Uij’s of some
atoms of the
disordered toluene)
-0.3, 0.4
space group
T, °C
P1h
-100
2θ range (deg) 4-60
Min, max peaks
(e-/Å3)
a Data were collected at -100 °C on a Siemens P4 diffractometer
equipped with a Nicolet LT-2 low-temperature device and using
monochromatized Mo KR radiation (λ ) 0.71073 Å). Data were
collected using ω scans with a 1° scan range in ω. Lattice parameters
were obtained from the least-squares refinement of 20 reflections with
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 unless
otherwise indicated. Exact mass determinations were obtained on a
VG analytical ZAB2-E instrument.
2
2
4
23.0 < 2θ < 24.7°. b Rw ) {∑w(|Fo| - |Fc| )2/∑w(|Fo| }1/2, where
2
the weight, w, is defined as follows: w ) 1/{σ2(|Fo| ) + (a*P)2 +
2
2
b*P}; P ) [1/3 (maximum of (0 or |Fo| ) + 2/3|Fc| ]. The parameters a
and b were suggested during refinement and are 0.0918 and 0.3572,
respectively. c The conventional R index based on F where the 4418
2
2
observed reflections have Fo > 4(σ(Fo)). d S ) [∑w(|Fo| - |Fc| )2/(n
- p)]1/2, where n is the number of reflections and p is the number of
refined parameters.
to remove nonpolar impurities followed by dichloromethane to give 4
(277 mg, 56%) as a purple-black crystalline solid. Mp ca. 550 °C
(differential scanning calorimetry). 1H NMR (300 MHz, CDCl3) δ 6.51
(4H, d, J ) 7.7 Hz), 7.36-7.39 (4H, m), 7.46-7.48 (8H, m), 7.59-
7.62 (4H, m), 7.62-7.64 (12H, m), 7.74 (4H, d, J ) 7.7 Hz). 13C
NMR (75.2 MHz, CDCl3) δ 121.5 (CH), 122.9 (CH), 125.3 (C), 125.8
(CH), 126.8 (CH), 127.9 (CH), 129.2 (2CH), 129.9 (C), 130.0 (2CH),
132.8 (C), 134.6 (C), 135.0 (C), 136.0 (C), 136.2 (C), 138.4 (C). HRMS
(CI) calcd for C64H37 (M+ + 1) 805.2895. Found 805.2898.
The relative fluorescence efficiency of 3 was measured8 in cyclo-
hexane using diphenylanthracene as a standard (λex ) 380 nm, φDPA
)
0.90 in cyclohexane8), and that of 4 was measured in benzene relative
to rubrene (λex ) 500 nm, φRubrene ) 0.98 in benzene9 ). Fluorescence
spectra were recorded on an SLM Aminco SPF-500 spectrofluorometer,
and UV spectra were recorded on a Milton Roy Spectronic 3000 array
spectrophotometer (5 µM solutions). Electrochemical measurements
were performed on a Bioanalytical Systems 100A electrochemical
analyzer or a Princeton Applied Research Model 173/175 potentiostat/
universal programmer and Omnigraphic 2000 X-Y recorder (Bausch
and Lomb). The working electrode for cyclic voltammetric measure-
ments consisted of an inlaid platinum disk (1.5-mm diameter) that was
polished on a felt pad with 0.05-µm alumina (Buehler, Ltd.) prior to
each experiment. A platinum wire served as a counter electrode and
a silver wire was utilized as a quasi-reference electrode. All potentials
are reported versus SCE and were calibrated by the addition of ferrocene
as an internal standard using E°(Fc/Fc+) ) 0.424 V vs SCE. Potentials
are reported as half-wave values unless otherwise indicated.
X-ray Experimental for C64H36‚3C7H8. Crystals grew as long, thin
red needles from toluene. The data crystal was a needle of approximate
dimensions 0.13 × 0.24 × 1.4 mm. The data were collected at 173 K
on a Siemens P4 diffractometer, equipped with a Nicolet LT-2 low-
temperature device and using a graphite monochromator with Mo KR
radiation (λ ) 0.71073 Å). Details of crystal data, data collection,
and structure refinement are listed in Table 1. Three reflections (0,2,2;
2,5,-4; 4,-2,0) were remeasured every 97 reflections to monitor
instrument and crystal stability. A smoothed curve of the intensities
of these check reflections was used to scale the data. The scaling factor
ranged from 0.997 to 1.02. The data were corrected for Lp effects but
not absorption. Data reduction and decay correction were performed
using the SHELXTL/PC software package.13 The structure was solved
by direct methods and refined by full-matrix least-squares13 on F14 with
anisotropic thermal parameters for the non-H atoms. The hydrogen
atoms were calculated in idealized positions (C-H 0.96 Å) with
isotropic temperature factors set to 1.2 × Ueq of the attached atom.
The polyaromatic molecule lies around an inversion center and
crystallizes with 3 molecules of toluene solvate in the unit cell. One
molecule of toluene lies in disorder about an inversion center. The
geometry for this molecule, C1b-C7b, was idealized with C-C bond
Chronoamperometric data were recorded on the BAS 100A system
using a platinum microelectrode (25-µm diameter), and the data were
analyzed as previously described.10 The ECL efficiency of 4 was
measured in benzene-acetonitrile (9:1) solution, using Ru(bpy)3(ClO4)2
in the same solvent mixture as a standard and assuming φECL(Ru(bpy)-
2+) ) 0.050.11 Apparatus and methodologies for the measurement of
3
ECL spectra and ECL efficiencies have been previously reported.12
Synthesis of Dibenzo{[f,f′]-4,4′,7,7′-tetraphenyl}diindeno[1,2,3-
cd:1′,2′,3′-lm]perylene or Dibenzotetraphenylperiflanthene (4). A
mixture of (7,12-diphenyl)benzo[k]fluoranthene (3, 500 mg, 1.24 mmol)
and cobalt trifluoride (758 mg, 6.53 mmol) in anhydrous trifluoroacetic
acid (20 mL) was heated at reflux for 36 h. Upon cooling, the solution
was diluted with water and extracted with dichloromethane (3 × 100
mL). After washing the dichloromethane extracts with water and
saturated brine solution, the extracts were dried (MgSO4) and evaporated
in vacuo to give a black solid. The solid was purified by chromatog-
raphy over neutral alumina eluting with 10% dichloromethane/hexanes
2
2
lengths fixed at 1.395 Å. The function, ∑w(|Fo| - |Fc| )2, was
minimized, where w ) 1/[(σ(Fo))2 + (0.0918*P)2 + (0.3572P)] and P
2
2
) (|Fo| + 2|Fc| )/3. The data were corrected for secondary extinction
effects. The correction takes the following form: Fcorr ) kFc/[1 +
[8(2) × 10-6]Fc2λ3/sin(2θ)]0.25, where k is the overall scale factor.
Neutral atom scattering factors and values used to calculate the linear
absorption coefficient are from the International Tables for X-ray
Crystallography (1990).15 Other computer programs used in this work
(8) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107.
(9) Stevens, B.; Algar, B. E. J. Phys. Chem. 1968, 72, 2582.
(10) Denuault, G.; Mirkin, M. V.; Bard, A. J. J. Electroanal. Chem. 1991,
308, 27.
(11) (a) Tokel-Takvoryan, N. E.; Hemingway, R. E.; Bard, A. J. J. Am.
Chem. Soc. 1973, 95, 6582. (b) Itoh, K.; Honda, K. Chem. Lett. 1979, 99.
(c) Wallace, W. L.; Bard, A. J. J. Phys. Chem. 1979, 83, 1350. (d) Glass,
R. S.; Faulkner, L. R. J. Phys. Chem. 1981, 85, 1159.
(12) McCord, P.; Bard, A. J. J. Electroanal. Chem. 1991, 318, 91.
(13) Sheldrick, G. M. (1994). SHELXTL/PC (Version 5.03). Siemens
Analytical X-ray Instruments, Inc., Madison, WI.
(14) Flack, H. D. Acta Crystallogr. 1983, A39, 876-881.
(15) International Tables for X-ray Crystallography; Wilson, A. J. C.,
Ed.; Kluwer Academic Press: Boston, 1992; Vol. C, Tables 4.2.6.8 and
6.1.1.4.

2376 J. Am. Chem. Soc., Vol. 118, No. 10, 1996
Debad et al.
are listed elsewhere.16 All figures were generated using SHELXTL/
PC.13 Tables of positional and thermal parameters, bond lengths, angles,
and torsion angles and figures are located in the supporting information.
Table 2. Bond Lengths (Å) and Angles (deg) for the
Non-Hydrogen Atoms of 4
atom 1
atom 2
atom 3
C19
atom 1-2
atom 1-2-3
C2
C19
C3
C4
C5
C1
C1
C2
C3
C4
C4
1.398(2)
1.427(3)
1.406(3)
1.381(4)
1.474(4)
118.1(2)
Results and Discussion
C1
C2
C20
C3
123.0(2)
119.4(2)
106.1(2)
135.9(2)
117.9(2)
121.3(2)
130.3(2)
108.3(2)
121.5(2)
118.6(2)
119.9(2)
119.0(2)
120.8(2)
120.2(2)
121.3(2)
120.4(2)
120.1(3)
121.5(2)
120.3(2)
122.1(2)
117.6(2)
120.6(2)
117.9(2)
121.4(2)
106.9(2)
132.0(2)
121.1(2)
117.3(2)
136.0(2)
106.6(2)
119.4(2)
123.4(2)
117.8(2)
117.7(2)
117.6(2)
124.7(2)
111.8(2)
123.7(2)
124.5(2)
118.9(2)
119.8(2)
121.2(2)
120.4(3)
120.3(3)
Synthesis and Solid-State Structure of 4. The known (7,12-
diphenyl)benzo[k]fluoranthene (3) was synthesized by the route
outlined in Scheme 1 in a yield greater than 90%.17 Remark-
ably, no investigations into the dehydrogenative coupling
chemistry of this compound have been reported. We considered
that the strategy shown in Scheme 1 would offer a very short
and flexible route to a variety of non-benzenoid aromatics
capable of dehydrogenation to more highly fused systems via
core, fjord, or fissure couplings. The intramolecular fjord
coupling would be particularly interesting as it would afford
tribenzo[a,d,g]corannulene 5.18 Upon treatment of 3 with either
AlCl3/NaCl (Scholl reaction),19 Tl(OCOCF3)3,20 or CoF3/TFA,20
a purple-black crystalline compound was formed whose structure
was shown be the fissure-coupling dimer 4 (56%).21 In benzene
solution, this compound exhibits a deep red fluorescent color.
The results of an X-ray crystallographic analysis of 4 are
recorded in Table 1. Bond lengths and angles are listed in Table
2, and Figure 1 contains an ORTEP drawing of the compound
and the numbering scheme.
Absorption and Emission. The absorption and emission
spectra for compounds 3 and 4 are shown in Figure 2. (7,12-
Diphenyl)benzo[k]fluoranthene (3) exhibits a blue fluorescence,
with emission maxima of 418 and 444 nm. The relative
fluorescence quantum yield for the compound is φF ) 1.0, which
was measured in cyclohexane with diphenylanthracene as a
standard.8
Solutions of 4 are also highly fluorescent, exhibiting a bright
orange-red fluorescence under ultraviolet irradiation and even
a pronounced fluorescence under ambient room lighting. The
absorption and emission spectra for the compound are shown
in Figure 2b. Absorption bands in the visible region account
for the compound’s deep red color and also explain its ability
to fluoresce under room lighting. The fluorescence spectrum
displays two maxima at 596 and 646 nm and mirrors the
absorption spectrum with a small Stokes shift of 7 nm. The
structured emission observed for both compounds is typical of
C5
C20
C6
C6
C4
C5
C5
C3
C14
C4
1.416(3)
1.376(3)
C14
C7
C7
C5
C6
C6
C4
C21
C5
1.446(3)
1.425(4)
C21
C8
C8
C6
C7
C7
C5
C12
C6
1.491(3)
1.423(3)
C12
C9
C7
C8
C9
C6
C7
C8
C9
C10
C7
1.429(3)
1.348(4)
1.406(4)
1.372(3)
1.418(4)
1.444(3)
C10
C11
C12
C13
C13
C7
C14
C14
C27
C15
C15
C5
C16
C16
C20
C17
C18
C19
C20
C20
C1
C10
C11
C12
C12
C12
C13
C13
C13
C14
C14
C14
C15
C15
C15
C16
C17
C18
C19
C19
C19
C20
C20
C20
C21
C21
C21
C22
C23
C11
C11
C27
C12
C12
C5
C13
C13
C20
C14
C14
C15
C16
C17
C1
C18
C18
C15
C19
C19
C26
C6
1.377(4)
1.491(3)
1.482(3)
1.385(3)
1.414(4)
1.410(3)
1.394(4)
1.430(3)
1.394(3)
C4
C4
C15
C22
C22
C26
C23
C24
1.396(4)
C6
C21
C22
1.395(4)
1.384(4)
1.382(4)
(16) Gadol, S. M.; Davis, R. E. Organometallics 1982, 1, 1607.
(17) (a) Adams, R.; Gold, M. H. J. Am. Chem. Soc. 1940, 62, 56. (b)
Bergman, E. J. Am. Chem. Soc. 1952, 74, 1075.
(18) For a short review see: (a) Siegel, J. S.; Seiders, T. J. Chem. Br.
1995, 313. (b) Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1966, 88,
380. (c) Barth, W. E.; Lawton, R. G. J. Am. Chem. Soc. 1971, 93, 1730.
(d) Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren, H. B. J. Am.
Chem. Soc. 1991, 113, 7082. (e) Scott, L. T.; Hashemi, M. M.; Bratcher,
M. S. J. Am. Chem. Soc. 1992, 114, 1920. (f) Borchardt, A.; Fuchicello,
A.; Kilway, K. V.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992,
114, 1921. (g) Rabideau, P. W.; Abdourazak, A. H.; Folsam, H. E.;
Marcinow, Z.; Sygula, A.; Sygula, R. J. Am. Chem. Soc. 1994, 116, 7891.
(h) Mehta, G.; Rao, K. V. Synlett 1995, 319. (i) Abdourazak, A. H.;
Marcinow, Z.; Sygula, A.; Sygula, R.; Rabideau, P. W. J. Am. Chem. Soc.
1995, 117, 6410.
(19) Fu, P. P.; Harvey, R. G. Chem. ReV. 1978, 78, 317.
(20) McKillop, A.; Turell, A. G.; Young, D. W.; Taylor, E, C. J. Am.
Chem. Soc. 1980, 102, 6504.
(21) The systematic name for 4 is dibenzo{[f,f′]-4,4′,7,7′-tetraphenyl}-
diindeno[1,2,3-cd:1′,2′,3′-lm]perylene. The name of the diindeno core
structure has been abbreviated to periflanthene. Periflanthene i, was first
described in 1937, Braun, J.; Manz, G., Ber. 1937, 70, 1603. Zinke, A.,
Mh. Chem.- Zh. 1962, 95, 1117. λmax (ꢀ) 540 (4.8), 503 (4.63), 471 (4.25)
and 440 (3.76) nm, in 1,2,4-trichlorobenzene.
Figure 1. View of compound 4 showing the atom labeling scheme.
Thermal ellipsoids are scaled to the 30% probability level. Hydrogen
atoms are drawn to an arbitrary scale. The molecule lies around a
crystallographic inversion center. Atoms marked by a prime are related
by 2 - x, 1 - y, 1 - z.
planar, ridged PAHs.22 A relative fluorescence quantum yield
of φF ) 0.85 was measured for compound 4 in benzene using

Dibenzotetraphenylperiflanthene
J. Am. Chem. Soc., Vol. 118, No. 10, 1996 2377
Figure 3. Cyclic voltammogram of (a) 1.5 mM 3 and (b) 0.31 mM 4
in benzene-acetonitrile (9:1), scan rate 100 mV/s. Supporting elec-
trolyte 0.1 M TBAPF6.
Figure 2. Absorption and emission spectra of (a) 3 (λex ) 380 nm)
and (b) 4 (λex ) 500 nm) in benzene.
rubrene as a standard.8,9 Note the difference in emission
properties of this compound and that of its parent core molecule.
Periflanthene21 is reported to possess a very low luminescence
quantum yield (φF ) 0.015), and its highest energy emission is
at 540 nm.23 Thus, the addition of terminal benzo groups and
phenyl ligands has the expected effect of lowering the energy
of the transition, but also increases the compound’s fluorescence
efficiency dramatically.
cesses.25 As judged by the symmetry of the voltammetric
waves, the redox processes appear reversible and indicate that
all of the charged species are quite stable under the experimental
conditions employed.
Studies of the redox behavior of many PAHs have been
reported;26 however, such investigations on the larger members
of this class of molecules are rare, likely due to their low
solubility in common electrochemical solvents. Typically, stable
anions and dianions of PAH’s can be generated in aprotic
solvents,26c with more stable ions being formed for the
compounds with larger, more extended π systems. For example,
the cyclic voltammogram in DMF of periflanthene,21 the parent
core structure of 4, indicates reversible formation of the radical
anion and dianion,26b and both charged species have been
synthesized chemically by alkali metal reduction.27 Radical
cations of PAHs tend to be much less stable than their anions.
Those that have been produced electrochemically usually display
an increase in reversibility at higher scan rates,26d with reactions
such as deprotonation and polymerization occurring at slower
scan rates. Reports of stable dications of PAHs are scarce. An
example is rubrene, which exhibits a reversible first and second
oxidation in benzonitrile at a reasonably slow scan rate of 100
Electrochemistry. The redox behavior of compounds 3 and
4 was probed by solution electrochemistry, and the resulting
cyclic voltammograms are shown in Figure 3. Both species
display limited solubility in acetonitrile, and thus a solvent
mixture of benzene and acetonitrile (9:1) was employed.
Solutions of up to approximately 5 mM of 3 and 0.4 mM of 4
were prepared in this solvent combination and proved adequate
for electrochemical and ECL measurements. Compound 3
displays a reversible reduction at -1.90 V vs SCE to form the
radical anion and an irreversible oxidation at Epa ∼ 1.6 V vs
SCE (Figure 3a), with the peak current for the oxidation being
noticeably larger than that for the reduction. The unstable
radical cation of 3 reacts to form products that are themselves
oxidized at this potential, and thus an increase in the peak current
is observed due to the transfer of more than one electron for
each molecule of 3 oxidized at the electrode (vide infra).
As can be seen in the cyclic voltammogram of 4 (Figure 3b),
two oxidation and two reduction features are observed for the
(24) Immediately after recording the cyclic voltammogram in Figure 2,
ferrocene was added and another voltammogram recorded. Peak separations
in this voltammogram were 81 mV for the 42+/41+ couple, 105 mV for
compound within the solvent electrochemical window.
4
4/41+, 81 mV for Fc/Fc1+, 91 mV for 4/41-, and 83 mV for 41-/42-
.
undergoes reduction at -1.20 V and again at -1.45 V vs SCE
to generate radical anions and dianions, respectively. Oxidation
of the neutral compound to the radical cation and dication occurs
at potentials of 0.946 and 1.29 V vs SCE. All peak separations
compare reasonably well to that of added ferrocene under the
same conditions,24 and chronoamperometric measurements
confirm that all four waves correspond to one-electron pro-
(25) Solutions used for chronoamperometry were similar to those used
for cyclic voltammetry, and measurements were accomplished as described
in ref 10. Diffusion coefficients and n values measured for the various redox
processes are as follows: 4/41+, D ) 4.5 × 10-6 cm2/s, n ) 1.4; 41+/42+
,
D ) 1.1 × 10-5 cm2/s, n ) 1.1; 4/41-, D ) 5.9 × 10-6 cm2/s, n ) 1.1;
4
1-/42-, D ) 1.4 × 10-5 cm2/s, n ) 0.9.
(26) (a) Perichen, J. In Encyclopedia of the Electrochemistry of the
Elements; Bard, A. J., Lund, H., Eds.; Marcel Dekker: New York, 1978;
Vol. 11, p 71 and references therein. (b) Saji, T.; Aoyagui, S. J. Electroanal.
Chem. 1983, 144, 143. (c) Jensen, B. S.; Parker, V. D. J. Am. Chem. Soc.
1975, 97, 5211. (d) Peover, M. E.; White, B. S. J. Electroanal. Chem.
Interfacial Electrochem. 1967, 13, 93.
(27) (a) Minsky, A.; Rabinovitz, M. Tetrahedron. Lett. 1981, 52, 5341.
(b) Kubozono, Y.; Ata, M.; Gondo, Y. Spectrochim. Acta 1990, 46A, 57.
(22) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Molecules, 2nd ed.; Academic Press Inc.: New York, 1971.
(23) Schael, F.; Lo¨hmannsro¨ben, H.-G. J. Photochem. Photobiol. A:
Chem. 1992, 69, 27.

2378 J. Am. Chem. Soc., Vol. 118, No. 10, 1996
Debad et al.
Figure 4. ECL spectra in benzene-acetonitrile (9:1) and 0.1 M
TBAPF6 of (s) 0.2 mM 4 (4+/4- reaction) and (- - -) 0.2 mM 4 and
0.2 mM TMPD (TMPD+/4- reaction).
Figure 5. Cyclic voltammogram of 0.40 mM TMPD and 0.40 mM 4
in benzene-acetonitrile (9:1) (0.1 M TBAPF6). The peak in the positive
region is due to TMPD oxidation, while the peak in the negative region
is due to the reduction of 4.
mV/s.28 The extended π system of compound 4 presumably
stabilizes both the cation and dication toward further decom-
position reactions. In fact, the oxidation of 4 to its dication is
reversible at scan rates as slow as 10 mV/s.
Since the ECL spectrum is of similar energy to the fluores-
cence emission, the singlet is likely the emitting state in each
case. The total free energy available in the 4+/4- annihilation
reaction, ∆Gann, is 2.15 eV based on the difference between
the half-wave potentials of the first oxidation and first reduction
peaks in the cyclic voltammogram. The energy (∆Hs) needed
to populate the singlet state, as calculated from the lowest energy
absorption peak of 596 nm, is 2.10 eV. The equation governing
the relation between reaction energetics and singlet energy for
a reaction that produces a singlet (an S-route system) is4a
Electrogenerated Chemiluminescence. Compound 4 is a
prime candidate for electrogenerated chemiluminescence be-
cause it possesses stable ions and a high fluorescence efficiency.
Indeed, light is produced at an electrode surface in solutions of
4 if the electrode potential is pulsed between the first reduction
potential and the first oxidation potential of the compound. For
these ECL experiments, solutions similar to those used for the
electrochemical experiments mentioned above were utilized.
This ECL is a result of annihilation reactions between the
electrogenerated anions and cations, which leave molecules of
4 in excited states (eq 1). These molecules then relax from the
excited singlet state by photon emission or by other non-radiative
pathways.
∆Gann g ∆Hs - T∆S ≈ ∆Hs + ∼0.1 eV
(2)
i.e., the free energy available from the electron transfer must
be larger than that needed to produce the excited singlet state.
Thus the energy available from our ion-annihilation reaction is
close to that required for direct population of the singlet state.
Alternatively, the singlet can be produced via the T-route, where
the annihilation reaction (eq 1) produces a triplet, with the
luminescent singlet state being formed via triplet-triplet an-
nihilation (TTA). Rubrene, for example, undergoes ECL via
both S- and T-routes.28,29
To determine whether the TTA mechanism was operative in
this system, the annihilation reaction between the radical anion
of 4 and the radical cation of a donor molecule whose oxidation
potential was much lower than that of 4 was carried out. Since
the free energy of the electron-transfer reaction in this case is
not sufficient to populate the singlet state of 4 but presumably
able to produce triplet states, any singlet emission observed
would be a result of triplet-triplet annihilations.4b
The cyclic voltammogram of an equimolar solution of 4 and
the donor molecule TMPD is shown in Figure 5. The energy
available in the TMPD+/4- annihilation reaction (∆Gann ) 1.31
eV) is not enough to directly populate the singlet state of 4.
However, ECL is observed from this system by alternately
generating these two ions at an electrode surface, and the
emission is similar to that produced via the 4+/4- annihilation
(Figure 4). The electron transfer between TMPD radical cations
and the radical anions of 4 thus presumably populates the triplet
state of 4, which then undergo triplet-triplet annihilation
reactions to form singlet states. The relatively low ECL
efficiency of 4 compared to its fluorescence quantum yield is
in part due to the involvement of this inefficient TTA mecha-
nism.
The efficiency of the ECL process for 4 has been determined
by comparing the photons of light produced per coulomb passed
with a standard whose ECL efficiency is known. Using
Ru(bpy)32+ as a standard, we obtained an efficiency of φECL
)
0.021 for complex 4. That is, for each 100 anions or cations
formed by reduction or oxidation at the working electrode,
approximately 2 photons are produced by subsequent reactions.
This efficiency is about half that of Ru(bpy)32+ (φECL ) 0.050).11
The light produced from a 0.2 mM solution of 4 is easily visible
in a darkened room and appears orange-red. The ECL spectrum
as measured with a spectrometer coupled to a CCD camera is
shown in Figure 4. The emission is centered around 660 nm.
Typically, such spectra are identical in energy to the fluorescence
spectrum of the compound, since the same singlet state is
responsible for the emission in each instance. The ECL
spectrum in Figure 4 does not display the structure exhibited
by the fluorescence spectrum (Figure 2b), due to the lower
resolution of the spectrometer used to measure the former.
However, the spectrum does overlap the fluorescence emission
as expected, and the slight red shift and change in relative peak
heights of the ECL spectrum can be explained by self absorption
of the higher energy wavelengths (an inner filter effect), as
solutions used for ECL measurements were much more con-
centrated than those employed for fluorescence measurements.
By pulsing the working electrode between the second
oxidation and reduction potentials, higher currents and a much
brighter ECL are observed than are produced by pulsing between
the first oxidation and reduction potentials. This emission can
clearly be seen in a lighted room, and its spectrum is identical
to that shown in Figure 4.
(29) (a) Bezman, R.; Faulkner, L. R. J. Am. Chem. Soc. 1972, 94, 6324.
(b) Visco, R. E.; Chandross, E. A. Electrochim. Acta 1968, 13, 1187. (c)
Tachikawa, H.; Bard, A. J. Chem. Phys. Lett. 1974, 26, 246.
(28) Chang, J.; Hercules, D. M.; Roe, D. K. Electrochim. Acta 1968,
13, 1197.

Dibenzotetraphenylperiflanthene
J. Am. Chem. Soc., Vol. 118, No. 10, 1996 2379
systems. This coupling probably also occurs during electro-
chemical oxidation of 3, as suggested by the ECL experiments
discussed above.
The electrochemistry of both starting material and product
has been investigated. Complex 3 displays a reversible reduc-
tion; however, its oxidation is irreversible due to the reaction
of its radical cations, probably to eventually yield 4. This
coupling has been demonstrated by the observed emission from
4 during ion-annihilation ECL of its precursor, 3, under certain
experimental conditions. Compound 4 displays well-behaved
electrochemistry. The 42+, 41+, 41-, and 42- ions are all
accessible in a benzene-acetonitrile solvent system, and all
appear stable under the experimental conditions. The neutral
compound is an efficient fluorescer, with a relative fluorescence
quantum yield of 0.85. The electrogenerated chemilumines-
cence of 4 produced via an ion-annihilation route is similar in
energy to that of its fluorescence. However, the ECL efficiency
is relatively low given the high fluorescence efficiency and is
a result of indirect population of the singlet state during the
electron-transfer reaction (TTA mechanism).
Further synthetic studies are in progress to fabricate other
large PAH systems using this versatile oxidative coupling
reaction. In addition, attempts to synthesize derivatives of the
fluoranthene complex, 3, that favor fjord coupling to yield
corranulene-like products (like 5, Scheme 1) are planned. The
photophysical and electrochemical properties of these new
species will be investigated in light of the unique characteristics
of the compounds discussed here. The effect of molecular
structure on ECL performance and potential applications of such
systems will also be considered.
Figure 6. ECL spectrum of 3 in stirred (s) and unstirred (- - -)
benzene-acetonitrile (9:1) solutions (0.1 M TBAPF6).
Compound 3 also produces ECL, despite the fact that the
radical cation is unstable.30 Light is produced at the electrode
when the potential is pulsed between the oxidation and reduction
potentials of the compound; however, the spectrum of the
emitted light depends upon the experimental conditions em-
ployed. Figure 6 illustrates this point by showing the ECL
spectral emission from stirred and unstirred solutions of 3. The
energy of the light emitted from the electrode immersed in the
stirred solution matches that of the fluorescence spectrum for
the compound, as would be expected from the arguments above.
This blue light is quite bright and can be easily observed in a
lighted room. The ECL spectrum of the unstirred solution is
more complicated, however. We believe this is due to the
oxidative coupling of 3 to form 4 at the electrode surface during
the anodic pulse. Thus, the spectrum contains features attribut-
able to the ECL of 3 and 4, since the potentials employed are
sufficient to form anions and cations of both compounds.
Stirring the solution prevents the buildup of the red-emitting 4
at the electrode, thereby lowering the intensity of its emission
with respect to that in the unstirred solution. It is interesting
to observe the electrode during the unstirred ECL experiment.
The outer edges of the planar electrode glow bright blue, while
the central portion emits a red glow due to the ECL of 4 that
builds up in this area. Further experiments to investigate the
mechanism and kinetics of this coupling reaction are under way.
Acknowledgment. The support of this research by the Texas
Advanced Research Program (ARP-390 and ARP-231), the
National Science Foundation (CHE-9407977), and the Robert
A. Welch Foundation is gratefully acknowledged. J.D.D. thanks
the Natural Science and Engineering Research Council of
Canada for their support in the form of a Postdoctoral Fellow-
ship. We thank Professor G. C. Wilson of this department for
performing differential scanning calorimetry experiments.
Supporting Information Available: Tables of fractional
coordinates, equivalent isotropic and aniosotropic thermal
parameters, bond lengths, bond angles, torsion angles, and side-
on and unit cell packing ORTEP views for compound 4 (11
pages). This material is contained in many libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS, and
can be downloaded from the Internet; see any current masthead
page for ordering information and Internet access instructions.
Conclusions
The nonalternant hydrocarbon dibenzotetraphenylperiflan-
thene (4) has been synthesized by the oxidative coupling of
(7,12-diphenyl)benzo[k]fluoranthene (3). The reaction is high-
yielding and represents a simple route to large polyaromatic
(30) This has been observed in other ECL reaction studies4a and is based
on the very rapid reaction between R•- and R•+ competing with that of the
decomposition of R•+
.
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