UV-irradiated formation of diradicals of diphenylbutadiyne and some of its p,p′-disubstituted derivatives

Article abstract of DOI:10.1246/bcsj.78.1986

It was found for the first time that aromatic diacetylenes give strong ESR signals of stable carbon radical species in solution or in the solid state upon irradiation of UV light. The numbers of radicals reached to the order of 10 ²⁰ radicals p

Full text of DOI:10.1246/bcsj.78.1986

1
986 Bull. Chem. Soc. Jpn., 78, 1986–1993 (2005)
Ó 2005 The Chemical Society of Japan
UV-Irradiated Formation of Diradicals of Diphenylbutadiyne
0
and Some of Its p,p -Disubstituted Derivatives
1
ꢀ
Miriam F. Beristain, Sergei Fomine, Eduardo Mu n˜ oz, Roberto Salcedo, and Takeshi Ogawa
Instituto de Investigaciones en Materiales, Universidad Nacional Aut o´ noma de M e´ xico,
Apartado Postal 70-360, Ciudad Universitaria, M e´ xico DF 04510, M e´ xico
1Instituto de F ´ı sicaa, Universidad Nacional Aut o´ noma de M e´ xico,
Apartado Postal 70-360, Ciudad Universitaria, M e´ xico DF 04510, M e´ xico
Received September 21, 2004; E-mail: ogawa@servidor.unam.mx
It was found for the first time that aromatic diacetylenes give strong ESR signals of stable carbon radical species in
solution or in the solid state upon irradiation of UV light. The numbers of radicals reached to the order of 1020 radicals
per mole of diacetylene, and the solution became red colored due to the formation of oligomers by their recombination.
The radical species were stable almost indefinitely under an inert atmosphere. The unusual stability of these radicals is
attributed to a resonance effect of aromatic rings linked to the butadiyne. Very slow quenching in the dark seems to be
due to the recombination between them. Theoretical calculations indicated that monomeric diradicals on 1- and 4-
carbons with the sp configuration, are formed, which combine to form dimeric or higher species detected by ESR spec-
troscopy.
The photo-polymerization of diacetylenes (DAs) in the solid
aromatic DAs.7,8 The polymers obtained in the presence of
DPB do not contain any fragment of DPB, according to the ul-
traviolet absorption spectra of the polymers, indicating that
there is no bond between the DPB and the polymer radicals.
It seems that aromatic DAs simply stabilize the propagating
radicals by a certain interaction. When aromatic DAs, such
state, which is often called topochemical polymerization, is
well known, and the polymerization mechanism has been more
or less established as dicarbene propagation in the crystal lat-
1
tice. It is said that the butatriene diradical is first formed on
carbons 1 and 4 by irradiation or heat, which then adds to
the adjacent DA molecules to form dimeric and oligomeric di-
radicals, which then transform to the more stable dicarbenes
and continue further propagation. These intermediate diradi-
cals, or dicarbenes, have been detected by ESR spectroscopy
only for single crystals, or at low temperatures.2 This is be-
cause the DAs studied by many researchers, were hexa-2,4-
diyne derivatives, and their diradicals have no contribution
to resonance stabilization by aromatic rings; also, they have la-
bile propargyl hydrogen atoms, readily abstracted by diradicals
or dicarbenes. There have only been a few studies4 on the
topochemical polymerization of aromatic DAs, probably be-
cause the majority of them do not undergo solid-state polymer-
ization, and the product polymers are intractable and useless
0
as DPB and 4,4 -methoxycarbonyldiphenylbutadiyne, were
heated, intense ESR signals were observed showing the pres-
ence of stable radicals, and the numbers of radicals increased
in the presence of a stable radical such as diphenylpicrylhidra-
,3
9
zyl (DPPH). When DPB was added to a solution of DPPH, the
yellow color of the solution disappeared, and the amount of
DPPH consumed, increased with the temperature, however,
upon cooling the yellow color reappeared, suggesting that
the interaction is reversible. Such rather unusual behaviors of
aromatic DAs are intriguing, and it seems that there are inter-
esting undiscovered properties of aromatic DAs. When DPB
was irradiated with UV light in the solid state or in solution,
strong ESR signals were detected. Therefore, the UV-irradiat-
ed radical formation of some aromatic DAs in solution is
reported in this paper.
,5
6
for any application. Iwase et al. recently reported on the ther-
mal polymerization of 1,4-bis(4-benzylpyridinium)butadiyne
triflate, and detected ESR signals when its DMF solution
ꢁ
was heated to 80 C; they simply assumed that the signal
was of the dicarbene species.
Experimental
DAs. Aromatic DAs were synthesized by the usual oxidative
coupling reaction of respective aromatic acetylenes. Dipheylbuta-
diyne, DPB was prepared from phenyl acetylene, supplied by
Apart from topochemical polymerization, aromatic DAs are
very interesting compounds in various fields of chemistry. It
was previously found that aromatic DAs, such as diphenylbu-
ꢁ
Farchan (GFS Chemicals), [mp 82–84 C, elemental analysis:
Calcd: C, 95.05; H, 4.95%. Found: C, 95.20; H, 4.80%]. In
0
tadiyne (DPB) and butadiynylene-p,p -(N,N-di-n-butylbenz-
amide) (BBA), interact with transient free radicals, such as
propagating methacrylate radicals, without forming a chemical
bond, and intense ESR signals of the propagating methacrylate
and acrylate radicals were observed at polymerization temper-
0
0
0
the cases of N,N -dimethyl-N,N -di-n-butyl-p,p -butadiynylene-
ꢁ
dibenzamide (MBA), [mp 92–94 C, elemental analysis: Calcd:
C, 78.50; H, 7.53; N, 6.54%. Found: C, 78.22; H, 7.61; N,
0
0
0
6.50%], and N,N,N ,N -tetra-n-butyl-p,p -butadiynylenedibenz-
8
ꢁ
ꢁ
atures above 50 C for systems containing small amounts of
amide (BBA), [mp 112–113 C, elemental analysis: Calcd: C,
Published on the web October 31, 2005; DOI 10.1246/bcsj.78.1986

M. F. Beristain et al.
9.65; H, 8.65; N, 5.46%. Found: C, 80.78; H, 8.49; N, 5.39%],
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 1987
7
ature using a JEOL ESR spectrometer (Model RE3X). The sam-
ples were placed in quartz tubes with an inner diameter of 3
mm, supplied by Wilmad LabGlass, and after degassing the sys-
tem by a freeze-and-thaw process, the tubes were sealed off in a
vacuum. In the case of photoreaction, the tubes were irradiated
with a 450 W medium-pressure mercury lamp supplied by Ace
Glass, at a distance of 20 cm between the lamp and the tubes.
Of total energy radiated, approximately 40–48% was in the ultra-
violet portion of the spectrum, 40–43% in the visible, the balance
in the infrared, according the specification of the lamp.
The number of free radicals was calculated by double numeri-
cal integration of the first derivative of the resonance curve from
the equation12
p-bromobenzoic acid (Aldrich) was converted to the respective
amides, which were then reacted with trimethylsilyl acetylene to
obtain p-ethynylbenzamides by the Sonogashira reaction.10 The
ethynylbenzamides were then converted to the respective DAs
0
by the oxidative coupling reaction. Di(p,p -methoxyphenyl)buta-
diyne (MPB) was prepared by an oxidative coupling reaction of
p-methoxyethynylbenzene, synthesized from iodoanisole and tri-
0
1
methylsilyl acetylene by the Sonogashira reaction [mp 140–
ꢁ
1
41 C, elemental analysis: Calcd: C, 82.42; H, 5.38%. Found:
0
C, 81.35; H, 5.11%]. p,p -Dinitrodiphenyldiacetylene (DNP) was
prepared by an oxidative coupling reaction of p-ethynylnitroben-
zene [mp no melting, elemental analysis: Calcd: C, 65.76; H, 2.76;
N, 9.59%. Found: C, 65.56; H, 2.50; N, 9.39%]. An asymmetric
Z
Z
HB
H
0
0
0
diacetylene, p-pentoxy-p -nitrodiphenylbutadiyne (PNB) was syn-
A ¼
dH
dH SðH Þ;
ð1Þ
HA
HA
thesized by an oxidative coupling reaction of p-pentoxyethynyl-
benzene and p-nitroethynylbenzene, instead of the usual Cadiot
coupling, and the three products were separated by extraction
where HA and HB are the initial and final parts of the resonance
0
curve, respectively, and SðH Þ is the value of the absorption at
0
and recrystallization using the difference in solubility [mp 145–
ꢁ
field H . The free radical concentrations were measured by a com-
þþ
1
55 C, elemental analysis: Calcd: C, 75.66; H, 5.74; N, 4.20%.
Found: C, 75.81; H, 5.63; N, 4.10%]. The yield was about 30%.
parison with a NaCl:Mn crystal calibrated by atomic absorption
spectroscopy, taking both spectra under the same ESR spectrom-
eter conditions.
0
m,m -Di(N-n-butylcarbonylamino)diphenylbutadiyne (CAB), was
synthesized by the reaction of m-ethynylaniline (Aldrich) with
1
2
3
4
valeryl chloride (Aldrich), followed by an oxidative coupling
5
6
8
O N
O
2
reaction [no mp, elemental analysis: Calcd: C, 77.97; H, 7.05;
N, 6.99%. Found: C, 77.78; H, 7.11; N, 6.87%]. Because PNB
9
5
7
1
is a new compound, its H NMR spectrum is shown in Fig. 1. In-
tegration of the peaks exactly corresponded to the respective pro-
ton quantities (peak 1: 2.03, peak 2: 2.02, peak 3: 2.03, peak 4:
2
3
.06, peak 5: 2.07, peak 6: 2.06, peak 7–8: 4.015, and peak 9:
.00). The chemical structures of these aromatic diacetylenes
are shown in Scheme 1. Characterizations of some of the products
were made by FTIR (Nicolet 510 P) and NMR (Bruker Avance
8
.2
7.6 7.4 7.2 7.0 ppm 4.0
2.0 1.8 1.6 1.4 1.2 1.0 0.8
1
0
4
00 MHz) spectroscopy.
ESR Measurement. ESR spectra were taken at room temper-
Fig. 1. H NMR spectrum of p-pentoxy-p -nitrodiphenyl-
butadiyne (PNB).
O
N
O
N
C
C
C
C
C
C
C
C
C
C
H C
C H
4 9
9
4
H C
C H
9
4
4
9
DPB
BBA
O
O
O
O
C
C
C
C
C
C
C
C
C
O
C
C
C
C
C
O
H15C7
C H
H C
CH3
7
15
3
HEPT
MPB
O
O
N
C
C C
O N
C
C
C
NO2
H C
N
CH3
2
3
H C
C H
9
4
4
9
DNP
MBA
O
H
C
N
n-C H9
4
C
C C C
O N
C
C
C
C
O
2
n-C H
NH
5
11
O
C
PNB
n-C H9
4
CAB
Scheme 1. Chemical structures of aromatic diacetylenes which appear in this article.

1
988 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
Stable Diradicals of Aromatic Diacetylenes
g = 2.0043
g = 2.0023
g = 2.0026
Power: 4mW
Power: 0.1mW
Modulation: 0.4mT
Gain: 40
Modulation: 0.079mT
Gain: 100
C. Field: 325mT
Mic. Freq.: 9.17GHz
C. Field: 325mT
Mic. Freq.: 9.17GHz
g = 2.0032
g = 2.0034
Power: 1mW
Power: 1mW
Modulation: 0.1mT
Gain: 200
Modulation: 0.1mT
Gain: 200
C. Field: 326mT
Mic. Freq.: 9.1676GHz
C. Field: 326mT
Mic. Freq.: 9.1676GHz
Fig. 2. ESR spectra of THF solutions of DPB, BBA, and MBA irradiated with UV light.
Computational Details. All geometry optimizations were car-
ried out with the Jaguar 5.0 suite of programs,13 using the B3LYP
hybrid functional with the 6-31(d,p) basis. An unrestricted method
was used to treat open shell systems, while for closed systems a
restricted formalism was applied. All geometry optimizations
were carried out without symmetry restrictions. Time-dependent
B3LYP in combination with the 6-31G(d,p) basis set (TD-
B3LYP) implemented in the Gaussian 03 suite of programs was
used to model the UV spectra.14
3
.5
2
2
1
0
.5
1
Results and Discussion
.5
Diradical Formation. When these aromatic DAs were ir-
radiated with UV light at room temperature in tetrahydrofuran
0
2
70
290
310
330
350
370
(THF) or dioxane, colorless solutions gradually developed a
Wavelength (nm)
reddish color; intense ESR signals (Fig. 2), appeared, and
the signal intensity increased with the irradiation time. Such
phenomenon had not been observed before, and aliphatic
DAs did not show any ESR signal on irradiation in solution.
The splitting in the spectra of BBA and MBA is due to the in-
teraction of radicals with the nucleus of amide nitrogen. The
UV spectra of some aromatic DAs in THF solution are shown
in Fig. 3. The ꢀ–ꢀ transitions of butadiyne groups lie in the
range of 290–390 nm, and the substituted groups decrease the
transition energy, showing red shift of the peaks. During irra-
MPA
DPB
BBA
Fig. 3. UV absorption spectra of aromatic DAs in a THF
solution. Concentrations: ꢂ10ꢃ5 M.
diation with UV light, the direct formation of a triplet state is
forbidden. However, there is a possibility of spin inversion in
the S1 state to produce T0, which corresponds to the energy of
visible region. According to TD-B3LYP calculations, a verti-
cal transition to the S1 state of the diphenyldiacetylenes is
ꢀ

M. F. Beristain et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 1989
highly allowed (oscillator strength 0.89), and lies 3.62 eV
(
has two main contributions, HOMO–LUMO and HOMO–1–
LUMO+1 excitations, and this energy corresponds to the
near-UV region. Since the T0 state of DPB lies below S1
electrons on pure p orbitals. Therefore, the radical species are
2
ꢁ ¼ 342 nm) above the S0 state. The S0 ! S1 transition
more likely the triplet sp diradicals, rather than the sp dirad-
icals, and they are not dicarbenes. Figure 5 shows the unpaired
electron density distribution for triplet state of DPB diradicals.
As can be seen the unpaired electrons are located mostly on Pz
orbitals of the C1 and C4. The C2 and C3 carbons, ortho and
para positions of phenyl rings (Pz orbitals), also present an un-
paired electron density due to delocalization. Phenyl carbons
linked to sp-carbons present a negative spin density due to spin
polarization.
Stability of Diradicals. With respect to the stability of the
diradicals, the monomeric diradicals may not be stable enough
to be detected by ESR spectroscopy at room temperature or
higher. Therefore, the ESR signals can be attributed to dimeric
diradicals and oligomeric diradicals formed by the combina-
tion of monomeric diradicals. No emission was observed dur-
ing irradiation, or when the irradiation was stopped. However,
the nonradiative S1 ! S0 and the slow T0 ! S0 quenching
processes are possible. Such coupling of monomeric diradicals
(2.35 eV above S0 state), T0 can be formed. The theoretical
structure of previously radicals was calculated by geometry
optimizations (Fig. 4). According to a natural bond orbital
analysis, four sp hybrid atoms of the DA unit in the S0 state
remained as sp hybrids in the T0 state with a total spin density
of 0.76 on the sp carbons linked to the benzene rings. The most
appropriate valence structure for the description of the elec-
tronic state of the T0 state is butatriene structure with unpaired
Fig. 4. B3LYP/6-31(d,p) optimized structure of DPB at the
S0 and T0 states. The numbers are the bond lengths.
Fig. 5. Electron density distribution for the triplet state of
DPB diradicals.
Table 1. Diradical Formation of Aromatic Diacetylenes in THF with UV Irradiation at Room Temperature
Irradiation
time
No. of radicals
/molDA
ꢄ10
—
2.18
7.82
11.2
14.8
16.2
11.8
11.2
[diradical]
ꢃ5
bÞ
DA]0
[
(
System
ꢄ10
g ꢅ 0:0006
ꢃ1
mol L
)
19
ꢃ1
(
min)
(mol L
)
0
5
—
—
1.4
4.9
6.9
9.2
10.1
7.3
6.9
2.0023
2.0023
2.0023
2.0023
2.0023
3
5
7
9
2
0
6
1
6
0.75
0.27
0.41
DPB-THF
aÞ
aÞ
days
1
6 days
0
7
5
9
6
5
—
—
—
—
—
2.0032
2.0032
2.0032
2.0032
no detectable
12.2
17.8
29.8
40.8
3
4
7
8
2.7
4.0
6.7
9.1
7.7
2.6
BBA-THF
aÞ
aÞ
6
days
6 days
34.3
11.6
1
0
1
6
3
9
4
0.41
24.7
44.5
71.2
91.9
101
0.1
8.4
2.0034
2.0034
2.0034
2.0034
2.0034
2.0034
1
1
3
5
7
15.1
24.2
31.3
34.4
19.1
19.4
MBA-THF
aÞ
aÞ
6
days
6 days
56.3
57.0
1
a) Without irradiation and kept in a dark place at room temperature. b) Initial concentration of DA in THF.

1
990 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
Stable Diradicals of Aromatic Diacetylenes
Fig. 6, the radical concentration decreased when the irradiation
was stopped, but after a certain time the radical concentration
remained unchanged in the dark for 10 days, suggesting that no
recombination took place. However, in the case of BBA, the
concentration decreased upon standing in the dark; the differ-
ence may be due to the steric effect of the dibutylamino
groups, which may decrease the rate of oligomerization. Thus
continues the slow recombination in the dark. The others form
oligomeric diradicals more rapidly than BBA.
In the case of the solid-state polymerization of DAs, ESR
signals of such diradicals or triplet dicarbenes in propagation
have been investigated in detail for single crystals.1 Howev-
er, they never give ESR signals in solution. When a solution of
topochemically polymerizable aliphatic DAs, such as 1,8-
6,17
Fig. 6. Relationships between the time and the numbers of
radicals formed per mole of DA.
0
N,N -dibutylaminocarboxyocta-3,5-diyne, was irradiated with
UV light, nothing happened, and no ESR signals were ob-
served. This is because the diradicals of aliphatic DA molecule
are too labile to be detected by ESR spectroscopy; or more
likely, they simply do not form diradicals as isolated mole-
cules. The fact that those aliphatic DAs undergo polymeriza-
tion in the solid state at moderate temperature, indicates that
several molecules should be situated at a very close position,
so that oligomers are formed in the beginning, and in a such
manner that the energy required for the formation of diradicals
is decreased. In contrast to these observations for topochemi-
cally polymerizable aliphatic DAs, the aromatic DAs in this
work show their ESR signals even at elevated temperatures,
because of the resonance stabilization by the aromatic groups
and because there is no labile hydrogen adjacent to the radi-
cals, such as propargyl hydrogen. On the other hand, butadiyn-
R
R
R
UV
.
.
.
R
R
R
.
R
R
R
R
.
R
R
.
R
R
5
ylene dibenzamides, which are topochemically polymerizable
Scheme 2. Diradicals formed by the irradiation of UV light.
aromatic DAs, gave ESR signals at room temperature when
irradiated in the solid state with UV light, suggesting that their
propagating diradicals or dicarbenes are stable enough to be
detected by ESR spectroscopy. Figure 7a shows ESR spectrum
of another topochemically polymerizable aromatic DA
(CAB) irradiated with UV light in the solid state for 170
min at room temperature. CAB rapidly polymerized to give
deep-blue crystals of polydiacetylene, which is a totally in-
tractable solid. According to an extensive work done by Sixl,18
the propagating species of the solid-state topochemical poly-
merization of aliphatic DAs are dicarbenes. Therefore, al-
though a detailed analysis of spectrum is difficult, the ESR sig-
nal of CAB, which undergoes solid-state polymerization, may
be that of the dicarbenes in propagation according to Sixl.
However, because CAB is an aromatic DA, the signal could
be from diradicals. Figure 7b shows the ESR signal of CAB
irradiated in THF solution, which is red colored due to much
less conjugation than poly-CAB obtained in the solid state.
The signal of the solution is thought to be that of the sp-dirad-
icals of a mixture of oligomeric species. However, the signals
of the two difference systems resemble each other, since both
are highly conjugated species, and although it is difficult to
determine whether they are dicarbenes or diradicals, it can
be said that the signals in solution are likely to be those of
the sp-diradicals.
is evident from the fact that when solutions of linear polymers
containing aromatic DA groups in their main chains were irra-
diated by UV light, the solutions became gel as cross-linking
1
5
took place. Coloring of the systems, due to an increase in
conjugation, also indicates the formation of oligomeric species
by recombination.
The changes in the numbers of diradicals with the irradia-
tion time are shown in Table 1, and plots are shown in
Fig. 6. The numbers of radicals increased with the irradiation
time, and after about 90 min 1–10 diradicals are formed per
4
1
0 DA molecules, which was calculated ignoring the con-
sumption by the coupling between them. The increase slowed
down with lapse of time, indicating that slow recombination
between the diradicals proceeds as their concentrations in-
creases, as shown in Scheme 2. The numbers of radicals
formed per mole of DA are greater for the substituted DAs
than DPB, and this is probably because of the resonance con-
tribution of the carbonyl groups. The diradicals of DPB, which
are smaller than the others, can couple more efficiently, but the
differences between MBA and the others are too large to be
explained. The diradicals may undergo other reactions apart
from the coupling, which is discussed in a later section. It is
worth mentioning that the red color of the solutions, which de-
veloped rapidly when irradiated, did not change appreciably
with the irradiation time, indicating that a high-molecular-
weight species was not being formed. As can be seen in
Polymerization by Diradical Coupling. It can be seen
from the Table 1 that the concentration of the diradicals is in
ꢃ5{ꢃ4
the order of 10
mol/L, which is considered to be high

M. F. Beristain et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 1991
g = 2.0060
g = 2.0058
g = 2.0015
g = 2.0018
g = 2.0008
g = 2.0006
a
b
Microwave Frequency: 9.172 GHz
Gain: 500, Modulation: 0.1 mT
Microwave Frequency: 9.1614 GHz
Gain: 400, Modulation: 0.1 mT
: 325.0 mT ∆H : ±10.0 mT
H
0
: 325.0 mT, ∆H : ±10.0 mT
H
0
Fig. 7. ESR spectra of a topochemically polymerizable aromatic DA (CAB) in the solid state (a) and in THF solution (b) irradiated
with UV light.
N2
R'
.
.
R'
UV
.
R
.
.
R
N2
R'
R
R'
.
UV
R
R'
2
R
.
R'
R
Scheme 3. Photo-dissociation of the dimeric dicarbene pro-
posed by Iwamura et al.20
.
for free radical reaction systems. In the free radical polymer-
ization of vinyl monomers, the concentration of the propagat-
R =
Octyl
O
N
C
R' =
CH OH
2
H
ꢃ7
ꢃ9
ing radicals in the steady state, is in the range of 10 –10
Scheme 4. Biscyclopropenyl compound from the trimeric
diradical.22
1
9
mol/L, and termination takes place by the collision of two
radicals. There is no doubt that these diradicals combine to
form oligomers as the numbers of radicals decrease in the dark,
as shown in Table 1. However, it can be concluded that these
diradicals are rather too stable to combine with each other to
form poly-DAs by recombination due to the steric hindrance
of aromatic rings as well as their resonance stabilization.
Iwamura and co-workers have reported dicarbenes of the
phenyldiacetylene dimer,20 in which they proposed photo-
dissociation of the dimeric dicarbene to phenyldiacetylene
of low temperature topochemical polymerization of aliphatic
DAs, the terminal carbene is said to add to the adjacent mono-
mer DA to form a cyclopropenyl chain end.21 However, in this
work the diradicals of aromatic DA are stable and in a dilute
solution the probability of addition to monomer is unlikely.
Sugawara and co-workers22 heated a liquid crystalline-forming
ꢁ
diacetylene at its nematic temperature (130 C), and isolated a
trimeric intermediate. They suggested that the trimer was a
biscyclopropenyl compound, rather than a butatriene-type tri-
meric diradical, as shown in Scheme 4. In this work, the reac-
tion conditions were somewhat different from those mentioned
above (low temperature and dilute solution), but the formation
of the cyclopropenyl compounds during the irradiation cannot
be excluded. However, the cyclopropenyl derivatives do not
give ESR signals.
(Scheme 3). They could not detect ESR spectra of either dicar-
bene or diradical, but obtained phenyldiacetylene in 70%
yield. It seems that the supposed dicarbene rapidly changes
to a more stable diradical, followed by rapid dissociation to
form phenyldiacetylene, which is no longer a radical species,
but a stable compound. The 1,4-diradicals of phenyldiacety-
lene will not be stable enough to be detected by ESR. In this
work, however, if such photo-dissociation of dimeric or trimer-
ic diradicals takes place, the products are also diradicals, but
less stable due to less conjugation. They can couple to each
other to return to a dimer or trimer again. Therefore, this is
not the reason why no polymer can be obtained. In the case
In order to see if an electrostatic attraction between the di-
radicals with different electronic densities might accelerate the
0
recombination, dioxane solutions of 4,4 -dimethoxydiphenyl-
0
butadiyne (MPB), 4,4 -dinitrodiphenylbutadiyne (DNP), and
their mixture (MPB/DNP) were irradiated at room tempera-

1
992 Bull. Chem. Soc. Jpn., 78, No. 11 (2005)
Stable Diradicals of Aromatic Diacetylenes
g = 2.0031
g = 2.0149
g = 2.0080
g = 2.0015
g = 2.0005
MPB
g = 2.0138
g = 2.0074
g = 2.0009
MPB + DNP
DNP
0
0
Fig. 8. ESR spectra of 4,4 -dimethoxydiphenylbutadiyne (MPB), 4,4 -dinitrodiphenylbutadiyne (DNP), and their mixture
(MPB þ DNP). Power: 1 mW, Cent. Field: 336.2 mT, Mod. Freq.: 100 kHz, Mic. Freq.: 9.44 GHz.
NO2
g = 2.0056
g = 2.0139
.
O N
g = 2.0009
2
.
OMe
MeO
NO2
OMe
Fig. 9. ESR spectrum of asymmetric DA (PNB) irradiated
for 275 min at room temperature. Concentration of PNB
in THF ¼ 0:138 M.
.
O N
2
.
NO2
ture of the diradical. The isolation of the products seems to be
technically impossible due to the low concentration of diradi-
cals; approximately being only 1 radical per 1000 molecules. It
is difficult to obtain a solution with high DNP concentration,
because of its very poor solubility in most solvents. Therefore,
an asymmetric DA (PNB) was irradiated in dioxane for 275
min; its ESR spectrum is shown in Fig. 9. The signal also in-
dicates that there are two different types of radicals, one adja-
cent to the alkoxyphenyl and the other to the nitrophenyl
groups, which greatly resembles that of the above mixed sys-
tem. A small amount of solid particles precipitated in the sys-
tem during irradiation in the ESR tube, and isolation of the
product was attempted; this was not successful, due to the min-
ute quantity of the product. It seems that even for the unsym-
metrical systems, the formation of recombination product is
MeO
O N
2
Scheme 5. Possible products of co-coupling of diradicals.
ture for 2 h; their ESR spectra are shown in Fig. 8. It is known
that DNP and MPB form a charge-transfer complex and co-
crystallize as a 1:1 adduct through their benzene rings.11 The
color of the solution of MPB is colorless, and that of DNP is
pale yellowish brown. A solution containing the both DAs
has a more intense color of pale brown. After irradiation, the
colors intensified, suggesting that more extended conjugation
systems were formed. The signal of the mixed system clearly
shows that there are two types of radicals. The mixture of DNP
and MPB is not equimolar due to the poor solubility of DNP in
the solvent. The species shown in Scheme 5 is a possible struc-

M. F. Beristain et al.
Bull. Chem. Soc. Jpn., 78, No. 11 (2005) 1993
very slow, indicating that the steric hindrance overcomes the
electrostatic attraction.
9
G. Canizal, G. Burillo, J. L. Boldu, E. Mu n˜ oz, and T.
Ogawa, Polym. J., 29, 230 (1997).
K. Sonogashira, Y. Tohda, and N. Hagihara, Tetrahedron
Lett., 50, 4467 (1975).
S. Fomine, L. Fomina, and T. Ogawa, J. Mol. Struct.:
THEOCHEM, 540, 123 (2001).
T. Chang and A. H. Vahn, ‘‘Electron Paramagnetic
Resonance Intensity Standard: SRM-2601; Description and
Use,’’ National Bureau of Standards Special Publication, National
Bureau of Standards, Washington, DC (1978), Pub. No. 269-59.
1
0
Conclusion
1
1
It was shown for the first time that aromatic DAs form sta-
ble diradicals by UV irradiation, of which intense ESR signals
are observed at room temperature. Based on theoretical calcu-
lations, the radical species are considered to be sp-diradicals,
1
2
2
rather than sp -diradicals or dicarbenes, but the observed
ESR signals are thought to be those of dimeric or higher spe-
cies formed by the recombination of the monomeric diradicals.
An estimation of the numbers of unpaired electrons formed af-
ter 1 h of irradiation (Table 1) indicated that one out of 40000
molecules exists as diradicals, and its concentration in the sys-
1
1
3
4
Jaguar 5.0, Schr o¨ dinger, LLC, Portland, Oregon (2003).
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millan, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez,
and J. A. Pople, ‘‘Gaussian 03, Revision B.04,’’ Gaussian Inc.,
Pittsburgh PA (2003).
ꢃ4
tems of this work, was on the order of 10 mol/L. The dirad-
icals were stable under an inert atmosphere. The coupling or
oligomeric diradicals to form high molecular diradicals is
extremely slow due to the steric hindrance, as well as to the
high stability by the resonance stabilization by the aromatic
rings. The formation of cyclopropenyl derivatives, or a cyclic
trimer or a tetramer, is possible, which this could be another
reason why a high polymer could not be obtained.
This work was part of a project supported by CONACYT
(
Consejo Nacional de Ciencias y Tecnolog ´ı a) under the
contract No. 33001U. Thanks are also due to Mr. Gerardo
Cedillo for his assistance in taking NMR spectra.
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