Phenanthroimidazole dimers: Structure and free radical reactivity; piezochromism, thermochromism, and photochromism

Article abstract of DOI:10.1139/v98-085

A series of 2-phenyl phenanthroimidazole dimers was synthesized by means of a two-phase preparative method, which allowed their isolation in pure form. Definitive proof of structure was obtained by means of ¹H and ¹³C NMR spectroscop

Full text of DOI:10.1139/v98-085

884
Phenanthroimidazole dimers: structure and free
radical reactivity; piezochromism,
thermochromism, and photochromism
Yuehui Zhou, W.E. Baker, P.M. Kazmaier, and E. Buncel
Abstract: A series of 2-phenyl phenanthroimidazole dimers was synthesized by means of a two-phase preparative method,
which allowed their isolation in pure form. Definitive proof of structure was obtained by means of 1H and 13C NMR
spectroscopy; five of the six possible structural isomers of the dimer could thus be eliminated. The dimers exhibited
thermochromic and piezochromic behavior, but not photochromic. The colored species were shown to be free radicals formed
on dissociation of the dimers; this was confirmed through ESR measurements. The electronic absorption spectra of the
radicals were influenced by substituents on the phenyl ring. The UV–VIS measurements allowed the evaluation of the
dimer–radical equilibrium constants, which were also found to be dependent on substituents on the phenyl ring.
Key words: phenanthroimidazole dimers, free radical equilibrium, piezochromism, thermochromism, photochromism.
Résumé : Faisant appel à une méthode de synthèse en deux phases qui permet d’isoler les produits à l’état pur, on a synthétisé
une série de dimères de la 2-phénylphénanthroimidazole. On a obtenu des preuves irréfutables de structure grâce à la
spectroscopie RMN du 1H et du 13C; on a ainsi pu éliminer cinq des six structures isomères possibles du dimère. Les dimères
présentent un comportement thermochromique et piézochromique, mais pas photochromique. Il a été démontré que les
espèces colorées proviennent de radicaux libres formés lors de la dissociation des dimères; cette conclusion a été confirmée
par des mesures RPE. Les spectres d’absorption électronique des radicaux sont influencés par les substituants présents sur les
noyaux phényles. Les mesures UV–VIS ont permis d’évaluer les constantes d’équilibre dimère–radical; on a trouvé que
celles-ci dépendent des substituants présents sur le noyau phényle.
Mots clés : dimères de la phénanthroimidazole, équilibre de radicaux libres, piézochromie, thermochromie, photochromie.
[Traduit par la rédaction]
Introduction
let in solution under UV irradiation. The authors suggested (3)
that the colored species was a free radical, and soon afterwards
EPR evidence for the existence of the radical was presented
(4).
Nitrogen-containing heterocyclic compounds play an impor-
tant role in many biological systems (1). It is known that cer-
tain dimeric nitrogen-containing heterocyclic compounds such
as dimeric NAD (N-benzyldihydronicotinamide) are involved
in some biological redox processes (2). On the other hand,
triphenylimidazole dimers have also attracted attention be-
cause of the sensitivity of these compounds to light and heat,
as well as to pressure. Hayashi and Maeda (3) observed that,
upon oxidation with potassium ferricyanide in ethanolic solu-
tion, triphenylimidazole (1) gave rise to a violet color; the
color then disappeared rapidly. The oxidation product was ob-
tained as a pale violet-colored powder, which turned deep vio-
It is now known that the oxidation product is a piezo-
chromic dimer, which transforms to the photochromic dimer
on dissolution in a solvent. In solution the photochromic dimer
exists in a photostationary equilibrium with its radical form.
Radical stability in these systems has been investigated by
a number of workers with respect to the influence of substi-
tuents (5). It was found that an electron-donating group on the
phenyl ring will shift the photostationary state to the radical
form. Interestingly, on replacing 1,4,5-triphenylimidazole by
phenanthro[9,10-d]imidazole, so that the conjugation system
is extended, the characteristics of the dimer changed dramati-
cally. The electronic absorption of the radical form of the
phenanthroimidazole dimers is shifted to longer wavelength
(~600 nm) so that the solution appears blue or green, whereas
the triphenylimidazole dimer solution is violet. Moreover, the
dissociation of the phenanthroimidazole dimers becomes a
thermal process and irradiation is no longer necessary. Various
papers and patents have described the application of
triphenylimidazole dimers in silver-free photographic proc-
esses (6); their application as initiators in polymerization has
also been investigated (7). However, relatively few papers
have elucidated problems inherent in the synthesis and char-
acterization of the phenanthroimidazole dimer system (8). We
Received January 8, 1998.
This paper is dedicated to Professor Erwin Buncel in
recognition of his contributions to Canadian chemistry.
Y. Zhou, W.E. Baker, and E. Buncel.1 Department of
Chemistry, Queen’s University, Kingston, ON K7L 3N6,
Canada.
P.M. Kazmaier. Xerox Research Center of Canada,
Mississauga, ON L5K 2L1, Canada.
1
Author to whom correspondence may be addressed.
Telephone: (613) 545-2616. Fax: (613) 545-6669. E-mail:
buncele@chem.queensu.ca
Can. J. Chem. 76: 884–895 (1998)
© 1998 NRC Canada

885
Zhou et al.
Ph
Ph
Ph
N
N
pressure
N
N
Ph
Ph
Ph
Ph
Ph
Piezochromic Dimer
N
N
[O]
.
Ph
Ph
Ph
Ph
N
N
H
Ph
Ph
Ph
Ph
N
N
N
hν
N
1
Ph
Ph
Photochromic Dimer
Scheme 1.
report here our results concerning the synthesis and charac-
terization of phenanthroimidazole dimers 2a–i, using the oxida-
tive synthesis method shown in Scheme 1. Compounds 2c, 2d,
2f, 2g, and 2i have not been reported previously. Some compari-
sons are also made with corresponding dimer systems derived
from triphenylimidazole (lophine), 1. The original purpose of this
work was to devise pressure- and sheer-sensitive dye molecules
that could be used as sensors in polymer processing (9a–d)];
this is an extension of our studies on thermochromic and photo-
chromic dye molecules and other photoactive pigments (9e–r).
due to decomposition. In our experience, only dimer 2a could
be synthesized in pure form by the method used in the synthe-
sis of triphenylimidazole dimers, probably because of its lower
solubility in organic solvents, thus retarding decomposition.
Coincidentally, this compound was the only one whose struc-
ture was fully elucidated by Sakaino et al. (8a).
Oxidation with potassium ferricyanide is of common use in
organic synthesis, especially in oxidative coupling of phenols
(12) and of amines (13), but, importantly, in both cases two-
phase reaction conditions were used: inorganic reactant dis-
solved in water and the organic reactant in methylene chloride.
A two-phase system (water–benzene) was used previously by
Cescon et al. (5a) in the synthesis of triarylimidazole dimers
and has been successfully applied to the synthesis of phenan-
throimidazole dimers in the present work. The dimer product
could be obtained in crystalline form from the organic phase
after most of the solvent was evaporated; these products gave
unambiguous NMR spectra. Dimer 2e is an exception, since
this gave the correct molecular peak corresponding to the di-
mer in the mass spectrum but failed to give a definitive NMR
Results and discussion
Synthesis
The synthesis of phenanthroimidazole dimers by the literature
method used in the synthesis of triphenylimidazole dimers (10,
11), i.e., addition of an aqueous solution of potassium ferricy-
anide to a cold solution of the imidazole in ethanolic potassium
hydroxide, proved to be unsuitable in the present work because
of precipitation of reactants and isolation of impure products
© 1998 NRC Canada

886
Can. J. Chem. Vol. 76, 1998
Fig. 1. 13C NMR spectra of representative phenanthroimidazole dimers 2a, 2g, and 2h.
spectrum, probably because of decomposition, as indicated by
the color change of the CDCl3 solution. The other dimers were
relatively more stable in CDCl3 and allowed definitive char-
acterization by NMR.
NMR signals: 16 for the protonated aromatic carbons and 15
for nonprotonated aromatic carbons in the case of para-substi-
tuted dimers 2b–d, 2h, and 2i. For the unsubstituted dimer 2a
the number of signals for protonated aromatic carbons is 18
and for nonprotonated aromatic carbons 13. However, due to
overlap only 17 distinct signals were observed. In the case of
the p-fluoro-substituted dimer 2g, due to coupling between 19F
and 13C (JCF = 249.1 Hz, 247.7 Hz; 1JCF = 22.0 Hz, 21.9 Hz;
Structure elucidation by NMR
Triarylimidazole dimers can, in principle, exist in six structur-
ally different isomeric forms (11). Since our work entailed a
modified literature method, the structure suggested by Sakaino
et al. (8a) for the phenanthroimidazole dimers in their work
cannot be taken for granted in the case of dimers 2a–i. Struc-
ture elucidation in the present work was based mainly on the
use of NMR spectroscopy. Structures 5 and 6 can readily be
excluded because of their symmetry, which would lead to a
smaller number of signals of aromatic carbons than actually
observed in the 13C NMR spectra. Structures 7 and 8 could also
be excluded, for the opposite reason, i.e., lower symmetry,
which should give rise to more signals of aromatic carbons
than actually observed in the 13C NMR spectra. Only structures
3 and 4 satisfy the requirement of the observed number of 13C
3
2JCF = 8.5 Hz, 8.3 Hz; JCF = 3.1 Hz, 3.0 Hz) the number of
signals for protonated aromatic carbons increased to 20 and for
nonprotonated aromatic carbons to 19 (Fig. 1). For 2f, on the
other hand, the three fluorines of the trifluoromethyl group
split the signals of the vicinal carbons of the phenyl ring to a
quartet (1JCF = 32.5 Hz, 32.3 Hz), increasing the signal number
of nonprotonated ring carbons to 21. Again, due to overlap
only 15 protonated and 20 nonprotonated carbon signals were
observed. The other eight signals corresponding to two quar-
tets (JCF = 272.3 Hz, 272.3 Hz) in the aromatic chemical shift
region stem from the two trifluoromethyl carbons due to the
deshielding and coupling with the three fluorines (Table 1).
© 1998 NRC Canada

887
Zhou et al.
1
To distinguish between structures 3 and 4, we have made
use of the reference compounds 9–14 reported in the literature
(8a, 14), which are good representatives of the half-dimer
subunits. The H NMR chemical shifts (in ppm) for the 2,6-
protons on the 2-phenyl ring are as follows: <8.0 in 9, 10; 8.47,
8.18 in 11; 7.50 in 12; 7.85 in 13; and 8.45 for 14.
© 1998 NRC Canada

888
Can. J. Chem. Vol. 76, 1998
Table 1. Summary of 13C NMR chemical shifts of ring carbons in phenanthroimidazole dimers (solvent CDCl3, CDCl3 +
C6D6 (3:1 v/v) for 2g, TMS as internal reference).
Protonated carbons
Nonprotonated carbons
2a
133.55
128.61
127.01
125.19
122.81
133.39
128.50
126.18
123.89
133.35
127.92
126.22
123.86
134.65
129.10
126.36
124.38
134.21
127.13
125.11
123.27
133.79
128.41d
125.38
123.02
133.98
128.62
126.79
124.14
134.02
129.01
126.77
124.15
129.47
128.43
126.83
124.52
128.98
127.94
126.22
123.89
128.66
127.55
125.59
122.96
161.47
135.55
128.76
116.21
154.79
134.89
127.13
137.97
129.28
125.55
135.71
128.92
123.69
2b
2c
2d
2f
129.36
128.23
125.51
122.91
129.39
127.08
125.50
122.85
132.19
128.55
126.22
123.37
129.88
126.60
124.59
122.92
131.43c
127.14
124.88
122.91
130.80
127.85
125.88
123.18
131.63
128.68
125.90
123.20
129.11
127.09
125.22
122.84
128.615
127.03
125.19
122.81
131.33
127.25
125.60
122.98
128.99
126.04
124.56
122.75
128.82
126.97
124.10
115.44e
128.94
127.65
125.48
122.90
131.04
127.89
125.51
122.90
128.72
126.80
124.43
122.79
128.56
126.80
124.40
122.79
130.05
127.05
125.39
122.63
128.51
125.55
124.17
161.20
137.94
129.21
125.82
161.17
137.92
129.39
125.74
162.71
138.32
128.90
118.27
162.32
138.27
129.26
123.19
162.94g
138.26
129.48
125.50
161.83
134.97
129.23
125.32
161.86
134.76
128.87
123.31
154.93
134.84
128.86
123,82
154.95
134.81
128.85
123.80
152.78
135.07
126.74
117.91
153.04
135.02
128.90
115.57
162.69h
134.96
129.21
123.74
153.50
134.39
129.11
123.42
153.74
134.61
126.95
122.75
138.87
132.82
128.76
116.22
145.13
132.91
128.75
116.25
140.88
129.39
124.93
115.32
139.72
131.20a
126.92
138.01
132.45
127.20
144.18
132.53
127.17
139.97
129.29
122.94
139.31
129.85b
124.99
115.57
153.97
131.57j
127.24
2g
2h
2i
128.52
125.92
123.28
114.61f
128.67
127.02
124.92
122.75
130.78
127.05
124.95
122.73
161.61
131.94i
128.99
115.88
138.13
134.22
128.86
115.67
138.12
129.20
125.30
155.71
135.03
134.66
126.97
134.98
129.14
123.37
a Quartet, J = 32.5 Hz.
b Quartet, J = 32.3 Hz.
c Doublet, J = 8.3 Hz.
d Doublet, J = 8.5 Hz.
e Doublet, J = 22.0 Hz.
f Doublet, J = 21.9 Hz.
g Doublet, J = 249.1 Hz.
h Doublet, J = 247.7 Hz.
i Doublet, J = 3.0 Hz.
j Doublet, J = 3.1 Hz.
The reference compounds 9–14 fall into two structurally
similar sets: the triaryl imidazole series 9–11 and the phenan-
throimidazole series 12–14. These structural parallels are re-
flected in parallel trends in the 1H NMR chemical shifts of the
2-phenyl 2,6-protons. The COSY and 1H NMR spectra of the
synthesized dimers indicate that the resonance of the 2-phenyl
2,6-protons was lower than 8.00 ppm, the chemical shift val-
ues being <8.00 (2a); 7.31, 7.0 1(2b); 7.34, 7.05 (2c); 7.51,
7.19 (2d); 7.41, 7.25 (2f); 7.35, 7.06 (2g); 7.35, 7.05 (2h); and
7.28, 7.15 (2i), respectively (Table 2). This shows that the di-
mer structure must be made of two moieties that resemble the
reference compounds 12 and 13. Hence generic structure 3 is
assigned to the synthesized dimers, which is consistent with
the conclusions of the earlier work (8a). Moreover, by analysis
of the COSY spectra on the dimers and the NOE results, we
could make unambiguous assignment of the 1H NMR spectra
of the dimers (in the case of 2a, only assignment for the phen-
anthryl rings was made) as shown in Table 2.
The assignment is made on the following basis: (i) the
chemical shift values of protons in ring A bonded to the imi-
dazole ring via an sp2 carbon will be higher (further downfield)
than those in the ring B bonded to the imidazole ring via an
sp3 carbon, because of more facile transmission of electronic
effects in the former case. (ii) The protons on the asymmetric
phenanthroimidazole ring at positions 4 and 11 have the same
splitting pattern and integral values, but the higher chemical
shift is assigned to position 11 since molecular models have
shown that the proton at this position is the more “crowded.”
© 1998 NRC Canada

Table 2. Assignment of 1H NMR chemical shifts in structure 3 (J in Hz).
3a
3b
3c
3d
3f
3g
3h
3i
4
8.89 (d, 7.5, 1H)
7.71 (t, 7.4, 1H)
7.64 (d, 6.9, 1H)
8.69 (d, 8.1, 1H)
8.70 (d, 8.2, 1H)
7.49 (t, 7.8, 1H)
7.41 (t, 7.6, 1H)
9.37 (d, 8.4, 1H)
8.06 (d, 8.1, 2H)
7.60 (t, 7.4, 2H)
7.34 (t, 7.5, 2H)
8.03 (d, 7.7, 2H)
8.85 (d, 7.9, 1H)
7.67 (t, 7.0, 1H)
7.61 (t, 7.6, 1H)
8.66 (d, 8.2, 1H)
8.67 (d, 7.9, 1H)
7.46 (t, 7.6. 1H)
7.39 (t, 7.7, 1H)
9.38 (d, 8.3, 1H)
8.09 (d, 8.1, 2H)
7.61 (t, 7.6, 2H)
7.35 (t, 7.5, 2H)
8.05 (d, 7.2, 2H)
7.31 (d, 7.9, 2H)
6.90 (d, 7.8, 2H)
7.01 (d, 8.3, 2H)
6.81 (d, 8.2, 2H)
2.11 (s, CH3)
8.85 (d, 7.9, 1H)
7.65 (t, 7.4, 1H)
7.59 (t, 9.0, 1H)
8.65 (d, 7.9, 1H)
8.65 (d, 7.9, 1H)
7.44 (t, 7.5, 1H)
7.36 (t, 7.6, 1H)
9.32 (d, 8.4, 1H)
8.04 (d, 8.5, 2H)
7.30 (d, 8.0, 2H)
7.57 (d, 8.3, 2H)
8.02 (d, 8.0, 2H)
7.34 (d, 8.0, 2H)
6.90 (d, 7.7, 2H)
7.05 (d, 8.2, 2H)
6.83 (d, 8.1, 2H)
2.41 (q, 8.0, CH2)
2.39 (q, 8.0, CH2)
1.02 (t, 7.2, CH3)
0.99 (t, 7.3, CH3)
8.77 (d, 7.8, 1H)
7.70 (t, 7.5, 1H)
7.65 (d, 7.6, 1H)
8.66 (d, 7.4, 1H)
8.68 (d, 7.6, 1H)
7.51 (t, 8.2, 1H)
7.41 (t, 8.5, 1H)
9.28 (d, 8.3, 1H)
8.16 (d, 8.1, 2H)
7.72 (t, 7.7, 2H)
7.45 (t, 7.5, 2H)
8.00 (d, 7.6, 2H)
7.51 (d, 8.1, 2H)
7.40 (d, 8.4, 2H)
7.19 (d, 8.6, 2H)
7.30 (d, 8.6, 2H)
8.81 (d, 7.9, 1H)
7.70 (t, 7.3, 1H)
7.64 (t, 7.3, 1H)
8.67 (d, 8.2, 1H)
8.69 (d, 7.9, 1H)
7.50 (t, 7.6, 1H)
7.40 (t, 7.9, 1H)
9.30 (d, 8.3, 1H)
8.12 (d, 8.1, 2H)
7.67 (t, 7.7, 2H)
7.37 (t, 7.5, 2H)
7.96 (d, 7.5, 2H)
7.41 (d, 8.2, 2H)
7.55 (d, 7.9, 2H)
7.25 (d, 8.6, 2H)
7.31 (d, 8.6, 2H)
8.93 (d, 8.0, 1H)
7.64 (t, 7.5, 1H)
7.54 (t, 7.7, 1H)
8.59 (d, 8.1, 1H)
8.61 (d, 8.3, 1H)
7.41 (t, 7.9, 1H)
7.41 (t, 7.9, 1H)
9.39 (d, 8.3, 1H)
8.02 (d, 7.7, 2H)
7.21 (t, 7.5, 2H)
7.42 (t, 7.9, 2H)
7.87 (d, 8.1, 2H)
7.35 (q, 8.6, 2H)a
6.69 (t, 8.7, 2H)b
7.06 (q, 8.9, 2H)c
6.57 (t, 8.6, 2H)d
8.81 (d, 7.9, 1H)
7.69 (t, 7.3, 1H)
7.62 (t, 8.0, 1H)
8.66 (d, 8.2, 1H)
8.68 (d, 8.1, 1H)
7.49 (t, 7.6, 1H)
7.41 (t, 7.5, 1H)
9.35 (d, 8.4, 1H)
8.11 (d, 8.1, 2H)
7.65 (t, 8.2, 2H)
7.41 (t, 7.5, 2H)
8.04 (d, 7.7, 2H)
7.35 (d, 8.4, 2H)
7.10 (d, 8.5, 2H)
7.05 (d, 8.8, 2H)
6.99 (d, 8.8, 2H)
8.80 (d, 7.9, 1H)
7.69 (t, 7.1, 1H)
7.63 (t, 7.7, 1H)
8.67 (d, 8.6, 1H)
8.69 (d, 8.7, 1H)
7.51 (t, 7.4, 1H)
7.42 (t, 7.8, 1H)
9.34 (d, 8.3, 1H)
8.14 (d, 8.1, 2H)
7.68 (t, 7.9, 2H)
7.44 (t, 7.7, 2H)
8.04 (d, 7.7, 2H)
7.28 (d, 8.7, 2H)
7.25 (d, 8.6, 2H)
7.15 (d, 8.6, 2H)
6.98 (d, 8.6, 2H)
5
6
7
8
9
10
11
4′(11′)
5′(10′)
6′(9′)
7′(8′′)
2′′(6′′)
3′′(5′′)
2′′′(6′′′)
3′′′(5′′′)
2.09 (s, CH3)
a JHF(meta) = 5.4 Hz.
b JHF(ortho) = 8.7 Hz.
c JHF(meta) = 5.1 Hz.
d JHF(ortho) = 8.6 Hz.

890
Can. J. Chem. Vol. 76, 1998
Fig. 2. (a) Spectra of dimer 2h in benzene (8.55 × 10–5 M) after UV
irradiation for 20 s at 25°C, solid line: before irradiation; dotted
line: after irradiation. (b) Spectra of lophine dimer in benzene (9.15
× 10–5 M) after UV irradiation for 20 s at 25°C, solid line: before
irradiation; dotted line, after irradiation.
Fig. 3. (a) Spectra of dimer 2h in benzene (8.67 × 10–5 M) on
heating from 25°C (solid line) to 60°C (dotted line). (b) Spectra of
lophine dimer in benzene (9.15 × 10–5 M) on heating from 25°C
(solid line) to 60°C (dotted line).
Photochromism
The synthesized phenanthroimidazole dimers are not photo-
sensitive at room temperature. Thus the absorbance of the di-
mer solution in benzene remains constant under irradiation
(Fig. 2a); in contrast the absorbance of the solution of lophine
dimer in benzene increased 2–3 fold under identical irradiation
conditions (Fig. 2b). Figure 2a shows the constant absorbance
of the solution of dimer 2h in benzene at 25°C after UV irra-
diation for 20 s. All other synthesized phenanthroimidazole
dimers behave similarly to dimer 2h toward UV irradiation.
This crowding is also found in the optimized structure of dimer
2e by the AM1 computational method.2 Other cases have been
recorded in the literature where “crowded” protons in aromatic
compounds were found to have abnormally high chemical
shifts (15). (iii) The protons at positions 4′(11′) and 7′(8′) on
the symmetric phenanthroimidazole ring have the same split-
ting pattern and integral value, and the higher chemical shift
value is assigned to position 4′(11′) due to proximity of the
nitrogen in the imidazole ring. (4). Other assignments are
based on results of COSY experiments with each dimer.
Features (i) and (ii) have been confirmed by means of an
NOE experiment with dimer 2b, the results of which show a
strong Overhauser effect between protons having chemical
shifts 9.38 and 7.01. These two values have been assigned to
the protons at positions 11 and 6′′′(2′′′). The results of the AM1
optimization on dimer 2b also reveal that the spatial distance
between positions 11 and 6′′′(2′′′) is 2.81 D,2 which falls in the
effective range of Overhauser effects (16). In the work of
Sakaino et al. (8a), a chemical shift of 9.3 was also assigned
to the proton at position 11 of dimer 2a, though definitive proof
for the assignment was not given.3
Thermochromism
In contrast to the lack of an effect of UV irradiation on spectra
in the phenanthroimidazole dimer series, the absorbance of the
dimer solutions in benzene increased 2–3 fold when the tem-
perature of the solutions was raised from 25°C to 60°C
(Fig. 3a); on cooling the color faded again. The process of
color development on heating and the subsequent fading of
color on cooling could be repeated a number of times. How-
ever, a gradual decomposition took place on standing at a
given temperature. This color fading followed a first-order
process, but it was not possible to identify the products of
decomposition. The triphenylimidazole dimers, in contrast,
show little or no change in their UV–VIS spectra on heating
(Fig. 3b), even though literature reports claim these com-
pounds to also be thermochromic (10, 17).
2
S. Chen, Y. Zhou, and E. Buncel. Unpublished results.
It has been pointed out by a referee that one can distinguish
3
between structures 3 and 4 on the basis of symmetry. Structure 4
has a stereogenic centre at the dimer linkage to the top imidazole
ring. Hence the lower phenanthroimidazole ring system does not
Piezochromism
We have found that all synthesized phenanthroimidazole di-
mers are pressure sensitive: the originally brown or yellow
crystals turned green or bluish green upon grinding. The col-
ored ground dimers could be pressed to a transparent disc with
potassium bromide and the absorption in the visible range
have symmetry and will have the same carbon count in the 13
C
NMR as the lower ring in 7, which leaves the generic structure 3
as the only possible one. We thank the referee for this
observation, which complements the structural assignment made
on the basis of the experimental NMR evidence.
© 1998 NRC Canada

891
Zhou et al.
Fig. 4. Representative UV–vis spectra of dimers 2e and 2h in KBr
disc after grinding and pressurization.
Table 4. Equilibrium constants and molar extinction
coefficients calculated from the slope and intercept of
A versus [D]o/A plots of dimer solutions in benzene at
25°C, and the fraction of dissociation, α = K1/2
.
–log K
10-3ε
103α
2a
2b
2c
2g
2h
2i
5.99±0.06
5.19±0.04
5.35±0.04
5.88±0.07
5.88±0.02
5.84±0.04
8.87±0.64
10.10±0.06
11.54±0.41
13.43±1.11
10.72±0.24
8.95±0.49
1.01±0.07
2.58±0.08
2.13±0.09
1.16±0.10
1.16±0.03
1.21±0.06
Table 5. Influence of substituents (19) on λmax of free
radicals and on the dimer–radical dissociation constants in
benzene at 25°C.
Table 3. Spectral characteristics of
dimers in benzene solution and in KBr
disc.
σpara
λ
max (nm)
–log K
2e (OCH3)
2b (CH3)
2c (CH2CH3)
2a (H)
2g (F)
2h (Cl)
2i (Br)
2f (CF3)
2d (CN)
–0.27
–0.17
–0.15
0
648
612
612
584
582
600
604
570
552
λ
max (nm)
5.19
5.35
5.99
5.88
5.88
Benzene
KBr
2a
2b
2c
2d
2e
2f
2g
2h
2i
584
612
612
552
648
570
582
600
604
586
594
580
552
654
570
574
588
604
0.06
0.23
0.23
0.54
0.66
a
a
a K values too small to be measured accurately.
D U R + R
where D stands for the dimer and R for the radical formed from
the dimer on dissociation. The equilibrium constant is given by
recorded (Fig. 4). After several days the color formed through
grinding and pressurization faded. Interestingly, it was found
that spectra recorded in this way displayed a bathochromic
shift with electron-donating substituents such as OCH3 and
CH3, similar to the observation with the benzene solutions of
these dimers (Table 3).
[R]2
[R]2
K =
=
4[D]
4[D]0 − 2[R]
where [D] and [R] refer to the equilibrium concentrations
while [D]0 is the initial dimer concentration. If the Lam-
bert–Beer law is valid and the radical species is responsible for
the color formation, then [R] = A/εl, where A is absorbance, ε
the molar extinction coefficient, and l, the light pathlength,
equals 1. Hence
It seems plausible that the observed color development on
grinding and pressurization is due to dissociation of the dimers
to free radicals, although it is not clear whether the dissociation
is brought about by crystal deformation or rupture through
external pressure, or as a result of a thermal effect from me-
chanical grinding. Both factors have been reported to cause
color change through structural changes (18). In general, one
expects formation of two radicals from a dimer to be accom-
panied by an increase in molecular volume, but in the present
system the reverse situation may obtain since the radicals will
be essentially planar and capable of stacking, which is clearly
not the case with the dimer as shown by molecular models and
AM1 calculations. The experiments described below prove
conclusively that the colored species formed from the dimers
are indeed free radicals.
(A ⁄ ε)2
K =
4[D]0 − 2(A ⁄ ε)
[D]0
[D]0
A = 4Kε2
− 2Kε = a
− b
A
A
It follows that a plot of A against [D]0/A will be linear with
slope a(= 4Kε2) and intercept b(= 2Kε). The molar extinction
coefficient ε and equilibrium constant K can then be obtained:
ε = a/2b and K = b2/a. The degree of dissociation is given by
α = K1/2 (where α < < 1).
Dimer–radical equilibrium
In Fig. 5 is shown the relevant plot of A vs. [D]o/A for the
various substituted dimers and it is seen that satisfactory
linearity is obtained. The resulting values of K, ε, and α are
given in Table 4. Importantly, the results show that the fraction
of dissociation of the phenanthroimidazole dimers to radical is
of the order of 10–3. This is consistent with the fact that the
NMR spectra of the dimers appear to be free of paramagnetic
It was possible to investigate the dimer–radical thermal
equilibrium for the phenanthroimidazole dimers and to deter-
mine the molar extinction coefficient as well as the equilibrium
constant for formation of the radical species by means of the
following derivations.
Consider the dissociation of the dimer to its radical form:
© 1998 NRC Canada

892
Can. J. Chem. Vol. 76, 1998
Fig. 5. Plot of A versus [D0]/A for dimer solutions in benzene at 25°C (correlation r2: 2a l, l; 2b 0.9999, 0.9992; 2c 0.9999, 0.9995; 2g 0.9999,
0.9997; 2h 0.9999, 0.9998; 2i 0.9999, 0.9998).
Fig. 6. (a) Hyperfine ESR spectrum of dimer 2b in benzene (1.873 × 10–4 M) at room temperature, scale: 1.52 G/cm, scan range 25 G.
(b) Hyperfine ESR spectrum of 2-p-methylphenyl-4,5-diphenylimadazole dimer in benzene (9.15 × 10–5 M) at room temperature, scale: 3.05
G/cm, scan range 50 G.
interference. In that regard, it is interesting that Maeda and
Hayashi (10) reported a degree of dissociation, α = 0.3, for
triphenylimidazole dimers, although one might have expected
α to be smaller for triphenylimidazole dimers than for phen-
anthroimidazole dimers.
The results in Table 5 indicate that the dimer–radical equi-
librium constants increase as the para substituents on the 2-
phenyl ring of the dimers become more electron donating
(Table 5). This is consistent with the trend that electron-donating
substituents stabilize the radicals, as was also found for disso-
ciation of triphenylimidazole dimers (5). The halogenated
phenanthroimidazole dimers do not show a clear trend, given
the experimental error in measurement of K, though the λmax
values exhibit a reverse trend relative to the electron-donating
substituents. The strong electron-withdrawing trifluoromethyl
and cyano substituents both exhibit a hypsochromic effect on
λmax and decreased dissociation constants, which were in fact
too small to be measured accurately.
© 1998 NRC Canada

893
Zhou et al.
Fig. 7. ESR spectra of lophine dimer in benzene (8.53 × 10–5 M) at room temperature, scale: 1.22 G/cm, scan range 20 G; (a) before
irradiation; (b) after irradiation.
Fig. 8. ESR signals of dimer 2b in benzene (1.173 × 10–4 M) at
room temperature (left, signal gain 5 × 104) and at 65°C (right,
signal gain 2 × 103), scale: 19.53 G/cm, scan range 500 G).
Fig. 9. ESR signals of dimer 2b solid sample (weak) and its ground
powder (strong) at room temperature, scale: 15.62 G/cm, scan
range 400 G.
the radical anion derived from lophine (22). It was found that
the free radical of the lophine dimer has spin density distrib-
uted in all three phenyl rings even though these are twisted out
of the plane of the imidazole ring and also with respect to one
another (21). One would expect that in the free radical derived
from the phenanthroimidazole dimer the spin density would be
more delocalized because of the extended conjugation, leading
to more complex ESR hyperfine structure. We have, indeed,
obtained a much more complex ESR hyperfine structure from
dimer 2b than from the corresponding 2-p-methylphenyl-4,5-
diphenyl imidazole dimer (Fig. 6).
While the ESR signal of the lophine dimer in benzene is
enhanced following UV irradiation (Fig. 7), the ESR signal of
dimer 2b in benzene becomes stronger on heating (Fig. 8). It
is interesting to note that the ESR signal of dimer 2b in the
solid state is also enhanced on grinding (Fig. 9). These experi-
ments confirm that the colored species obtained on UV
ESR spectra
Several studies have reported on the ESR hyperfine structure
of the free radical derived from the lophine dimer (20, 21) and
© 1998 NRC Canada

894
Can. J. Chem. Vol. 76, 1998
Fig. 10. Aromatic region of 1H NMR spectrum of 2b in CDCl3.
Table 6. MS data of dimers 2.
Melting point (lit.)
MS (m/z)
2a
2b
2c
2d
2e
2f
2g
2h
2i
200–203°C (8a)
170–173°C (8a)
(CI) 587.5(M+), 295.3, 194.1
(EI) 614.1(M+), 308.1, 189.9, 163.0, 153.2, 82.8
(EI) 642.2(M+), 320.2, 189.8, 162.9, 116.8
(EI) 636.1(M+), 319.1, 189.8, 175.8, 162.6, 127.2, 116.8
(EI) 646.4(M+), 428.4, 324.3, 279.2, 235.2, 190.1, 176.0, 165.0, 149.0, 111.0
(EI) 722.1(M+), 362.1, 275.0, 198.7, 176.0, 162.6, 136.7, 128.6, 116.7
(EI) 622.1(M+), 312.1, 249.4, 235.9, 189.7, 175.8, 162.5,155.6, 130.8, 122.8, 116.7
(EI) 654.1(M+), 328.2, 189.9, 162.8, 136.9, 110.8
172–180°C (8a)
(CI): 745.2(M+), 667.3, 587.5, 373.3, 295.3, 194.1, 163.0
The UV–VIS spectroscopic measurements on the dimer so-
irradiation of the triphenylimidazole dimers, and also on heat-
ing the solutions of the phenanthroimidazole dimers or on
grinding the phenanthroimidazole dimer, are free radical spe-
cies. These qualitative observations on solutions of phenan-
throimidazole dimers further validate the measurement of the
molar extinction coefficient of the colored species as described
above.
lution in benzene were carried out by dissolution of ca. 20 mg
of a dimer in 50 mL of benzene (BDH) freshly distilled from
calcium hydride, followed by repetitive dilution to half con-
centration. UV–VIS spectra were recorded on an HP 8452A
diode array spectrophotometer supported by HP89531A MS-
DOS – UV–VIS operating software.The temperature of solu-
tions in the cuvette (1 cm) was equilibrated to ± 0.1°C by
means of a thermostatted water bath, RTE-210 (Neslab Instru-
ments. Inc., Newington, NH 03801 U.S.A.). The UV light
source for the irradiation was Spectroline, CX-Series (Spec-
tronics Corporation, Westbury, New York).
NMR spectra were taken on a Bruker AM-400 NMR spec-
trometer operating at 400.134 MHz for 1H and at
100.614 MHz for 13C. The NMR solvents were purchased
from CDN. The ESR spectra were recorded on a Varian E 104
ESR spectrometer and the UV light source for ESR measure-
ment was a 1000-W Xe/Hg arc lamp. Sharp melting points
could not be obtained; instead the sample in the capillary
turned to a straw-colored resin-like material when the tempera-
ture was raised slowly (10°C/min). On raising the temperature
quickly (>20°C/min) a transient blue or green color charac-
teristic of the radical was observed; then the compound
changed to the resin-like material. Purity was judged by the 1H
NMR spectra, which showed the absence of any extraneous
peaks except for solvent (CH2Cl2 or EtOAc) of crystallization,
which was difficult or impossible to remove, rendering
Experimental
The starting phenanthroimidazole compounds were synthe-
sized according to the modification by Davidson et al. (23) of
Radziszewski’s imidazole synthesis (24). The dimers were
synthesized by means of a two-phase reaction as described
below.
To a solution of 1 g of the imidazole derivative in
250–350 mL methylene chloride was added a solution of po-
tassium ferricyanide (5 g) and sodium hydroxide (5 g) in
100 mL water while cooling to 0°C and with vigorous stirring.
The blue- or green-colored solution was stirred for 5 min at
0°C. The aqueous phase was separated and the organic phase
was dried over sodium sulfate, and evaporated under reduced
pressure to ca. 3 mL. After addition of ca. 3 mL ethyl acetate,
the solution was placed in a refrigerator until crystals formed;
these were filtered and washed with diethyl ether, yield over
90%. The MS data of the synthesized dimers are summarized
in Table 6.
© 1998 NRC Canada

895
Zhou et al.
1
S.-R. Keum, E. Buncel, and R.P. Lemieux. Liq. Cryst. 24, 341
(1998); (i) S.-R. Keum,. M.J. Lee, S. Swansburg, E. Buncel, and
R.P. Lemieux. Dyes Pigm. In press; (j) K.-S Cheon, R.A. Cox,
S.-R. Keum, and E. Buncel. J. Chem. Soc. Perkin Trans. 2, 1231
(1998); (k) E. Buncel and K. -S Cheon. J. Chem. Soc. Perkin
Trans. 2, 1241 (1998); (l) S.-R. Keum, P.M. Kazmaier, K.-S.
Cheon. R.A. Manderville, and E. Buncel. Bull. Korean Chem.
Soc. 17, 391 (1996); (m) S.-R. Keum, S.S. Lim, B.H. Min, P.M.
Kazmaier, and E. Buncel. Dyes Pigm. 30, 225 (1996); (n) A. J.
McKerrow, E. Buncel, and P.M. Kazmaier. Can. J. Chem. 73,
1605 (1995); (o) E. Buncel, A.J. MacKerrow, and P.M.
Kazmaier. J. Chem. Soc. Chem. Commun. 1242 (1992); (p) A.J.
MacKerrow, E. Buncel, and P.M. Kazmaier. Can. J. Chem. 71,
390 (1992); (q) P.M. Kazmaier, A.J. McKerrow, and E. Buncel.
J. Imaging Sci. 36, 373 (1992); (r) E. Buncel amd S. Rajagopal.
Acc. Chem. Res. 23, 226 (1990).
elemental analysis a poor criterion of purity. A typical H
NMR spectrum is shown in Fig. 10.
Acknowledgments
This research was supported by a grant from the Ontario Cen-
ter for Materials Research. The authors are also grateful to Dr.
L. Johnston, Dr. J. Lusztyk, and Dr. D. Shukla (National Re-
search Council of Canada, Steacie Institute for Molecular Sci-
ence) for assistance with the ESR measurements. We thank
Prof. B.K. Hunter and Ms. S. Blake for helpful discussion on
NMR data analysis.
References
1. H.C. van der Plas, M. Simonyi, F.C. Alderweireldt, and J.A.
Lepoivre (Editors). Bio-organic heterocycles 1986. Elsevier,
Amsterdam. 1986.
10. K. Maeda and T. Hayashi. Bull. Chem. Soc. Jpn. 43, 429 (1970).
11. D.M. White and J. Sonnenberg. J. Am. Chem. Soc. 88, 3825
(1966).
2. M. Patz, Y. Kuwahara, T. Suenobu, and S. Fukuzumi. Chem.
Lett. 567 (1997).
12. W.I. Talor and A.R. Battersby. Oxidative coupling of phenols.
Marcel Dekker, Inc. New York. 1967.
3. T. Hayashi and K. Maeda. Bull. Chem. Soc. Jpn. 33, 565 (1960).
4. T. Hayashi, K. Maeda, S. Shida, and K. Nakada. J. Chem. Phys.
32, 1568 (1960).
13. D. Taub, C.H. Kuo, and N.L. Wendler. J. Org. Chem. 28, 2752
(1963).
14. H. Tanino, T. Kondo, K. Okada, and T. Goto. Bull. Chem. Soc.
Jpn. 45, 1474 (1972).
15. J.D. Memory and N.K. Wilson. NMR of aromatic compounds.
5. (a) L.A. Cescon, G.R. Coraor, R. Dessauer, E.F. Silversmith, and
E.J. Urban. J. Org. Chem. 36, 2262 (1971); (b) B.S. Tanaseichuk,
K.V. Stanovkina, A.N. Sunin, and L.G. Rezepova. Zh.Org.
Khim. 5, 2054 (1969); (c) B.S. Tanaseichuk and L.G. Rezepova.
Zh. Org. Khim. 6, 1065 (1970); (d) B.S. Tanaseichuk, A. A.
Bardina, and V. A. Makskov. Zh. Org. Khim. 7, 1508 (1971).
(e) B.S. Tanaseichuk, A.A. Bardina, and A.A. Khomenko.
Khim. Geterotsikl. Soedin. 7, 1255 (1971); (f) B.S. Tanaseichuk
and S.V. Yartseva. Zh. Org. Khim. 7, 1260 (1971); (g) Yu.A.
Rozin, V.E. Blokhin, and Z.V. Pushreva. Khim. Geterotsikl.
Soedin. 9, 1105 (1973); (h) V.N. Shishkin, B.S. Tanashechuk,
L.G. Tikhonova, and A.A. Bardina. Khim. Geterotsikl. Soedin.
9, 387 (1973); (i) Yu.A. Rozin, V.E. Blokhin, Z.V. Pushreva, and
V.I. Elin. Khim. Geterotsikl. Soedin. 10, 990 (1974); (j) H.
Baumgaertel and H. Zimmermann. Z. Naturforsch. B: Anorg.
Chem. Org. Chem. Biochem. Biophys. Biol. 18B, 406 (1961);
(k) Chem. Ber. 99, 843 (1966); (l) W. Suemmermann and H.
Baumgaertel. Collect. Czech. Chem. Commun. 36, 575 (1971);
(m) Ber. Bunsen-Ges. Phys. Chem. 74, 19 (1970).
6. (a) R.L. Mitchell, W.J. Nebe and W.M. Hardam. J. Imaging Sci.
30, 215 (1986); (b) H.D. Hartzler (DuPont de Nemours & Co.).
US Pat. 4,017,313 (1977); (c) Th.E. Duebner, B.M. Monroe (Du-
Pont de Nemours & Co.). US Pat. 4,565,796 (1986); (d) H. Na-
gasaka (Mitsubishi Chem. Ind.). US Pat. 4,594,310 (1986).
7. L.A. Cescon, G.R. Coraor, R. Dessauer, A.S. Deutsch, H.L.
Jackson, A. MacLachlan, K. Marcali, E.M. Potraftke, R.E. Read,
E.F. Silversmith, and E.J. Urban. J. Org. Chem. 36, 2267 (1971).
8. (a) Y. Sakaino, H. Kakisawa, and T. Kusumi. J. Chem. Soc.
Perkin. Trans. 1, 2361 (1975); (b) J. Heterocycl. Chem. 12, 953
(1975).
John Wiley &Sons, New York. 1982. p. 45.
16. D.H. Williams and I. Fleming. Spectroscopic methods in organic
chemistry. 5th ed. McGraw–Hill Book Company, London. 1995.
p. 112.
17. (a) Y. Sakaino, T. Takizawa, Y. Inouye, and H. Kakisawa. J.
Chem. Soc. Perkin Trans. 2, 1623 (1986); (b) T. Hayashi, K.
Maeda, and M. Morinaga. Bull. Chem. Soc. Jpn. 37, 1563
(1964); (c) T. Hayashi, K. Maeda and T. Kanaji. Bull. Chem.
Soc. Jpn. 38, 857 (1965).
18. For pressure effects on organic compounds: (a) Z.A. Dreger and
H.G. Drickamer. Chem. Phys. Lett. 179, 199 (1991); (b) R.R.
Birge, C.T. Berge, L.L. Noble, and R.C. Neuman, Jr. J. Am.
Chem. Soc. 101, 5162 (1979); (c) D.G. Wilson and H.G.
Drickamer. J. Chem. Phys. 3649 (1975); (d) G. Kortuem and W.
Zoller. Chem. Ber. 100, 280 (1967); (e) S. Murphy. B. Sauer-
wein, H.G. Drickamer, and G.B. Schuster. J. Phys. Chem. 98,
13476 (1994); (f) L. Comeford and E. Grunwald. J. Phys. Chem.
94, 1105 (1990); (g) K. Song, R.D. Miller, and J.F. Rabolt. Mac-
romolecules, 26, 3232 (1993); (h) E. Orti, R. Crespo, and M.C.
Piqueras. Synth. Met. 41–43, 1575 (1991); (i) R.A. Nallicheri
and M.F. Rubner. Macromolecules, 24, 517 (1991). For pressure
effects on metal complexes: (j) S. Alshehri, J. Burgess, G.H.
Morgan, B. Patel, and M.S. Patel. Transition Met. Chem. (Lon-
don), 18, 619 (1993); (k) B. Scott and R.D. Willett. J. Am. Chem.
Soc. 113, 5253 (1991); (l) K.H. Drickamer and L. Bray. Acc.
Chem. Res. 23, 55 (1990); (m) M. Kotowski, R. van Edlik, R.B.
Ali, J. Burgess, and S. Radulovic. Inorg. Chim. Acta, 131, 225
(1987).
9. (a) B.M. Kosowski. CAMES sensor system, Plast. Technol. J.
March, 1995; (b) A.C. Cordo and B.M. Kosowski. Int. Annu.
Conf. ICT, 25th, 1994. p. 1; (c) D.C. Clark, W.E. Baker, A.C.
Cordo, and B.M. Kosowski. Int. Polym. Process. Submitted;
(d) D.C. Clark and W.E. Baker. Annu. Conf. Polym. Process.
Soc. Secaucus, N.J. Paper 6, June 1997; (e) S.-R. Keum, S.-S.
Hur, P.M. Kazmaier, and E. Buncel. Can. J. Chem. 69, 1940
(1991); (f) S.-R. Keum, K.-B. Lee, P.M. Kazmaier, R.A. Man-
derville, and E. Buncel. Magn. Reson. Chem. 30, 1128 (1992);
(g) S.-R. Keum, K.-B. Lee, P.M. Kazmaier, and E. Buncel. Tet-
rahedron Lett. 35, 1015 (1994); (h) S. Swansburg, Y.-K. Choi,
19. T.H. Lowry and K.S. Richardson. Mechanism and theory in or-
ganic chemistry. 3th ed. R.R. Donnelley & Sons, New York.
1987.
20. R.D. Allendoerfer and A.S. Pollock. Mol. Phys. 22, 661(1971).
21. N. Cyr, M.A. Wilks, and M.R. Willis. J. Chem. Soc. B: 404
(1971).
22. H. Ueda. J. Phys. Chem. 68, 1304 (1964).
23. D. Davidson, M. Weiss, and M. Jelling. J. Org. Chem. 2, 319
(1937).
24. B. Radziszewski. Ber. Dtsch. Chem. Ges. 10, 10 (1877).
© 1998 NRC Canada