Effects of additional linkers in biphenyl-4,4′-dinitrene on the low-lying singlet-triplet energy gap and zero-field splitting

Article abstract of DOI:10.1021/jp9629710

Perturbation effects of additional linkers on the spin-spin coupling in biphenyl-4,4′-dinitrene (1) were examined by introducing a linking group between 2- and 2′-positions of 1. Five different doubly linked systems showed triplet ESR spectra corresponding to quinonoid dinitrenes. Curie law analyses suggested that all those triplet states were thermally excited triplet states. In addition, the singlet-triplet energy gaps, which were determined by the Curie law analyses, were well correlated with their corresponding zero-field-splitting (zfs) D values. The result could be explained by the stability of dinitrene character which is estimated from the resonance energy of the intervening π-system. Our semiempirical molecular orbital calculations supported the experimental correlation between the singlet-triplet energy gap and the zfs D value.

Full text of DOI:10.1021/jp9629710

J. Phys. Chem. A 1997, 101, 2083-2088
2083
Effects of Additional Linkers in Biphenyl-4,4′-dinitrene on the Low-Lying Singlet-Triplet
Energy Gap and Zero-Field Splitting
Shigeaki Nimura,† Osamu Kikuchi,† Tsuguyori Ohana,‡ Akira Yabe,†,‡ Shigeo Kondo,‡ and
Masahiro Kaise*,‡
Department of Chemistry, UniVersity of Tsukuba, Tsukuba, Ibaraki 305, and National Institute of Materials and
Chemical Research, Tsukuba, Ibaraki 305, Japan
ReceiVed: September 26, 1996; In Final Form: December 13, 1996X
Perturbation effects of additional linkers on the spin-spin coupling in biphenyl-4,4′-dinitrene (1) were examined
by introducing a linking group between 2- and 2′-positions of 1. Five different doubly linked systems showed
triplet ESR spectra corresponding to quinonoid dinitrenes. Curie law analyses suggested that all those triplet
states were thermally excited triplet states. In addition, the singlet-triplet energy gaps, which were determined
by the Curie law analyses, were well correlated with their corresponding zero-field-splitting (zfs) D values.
The result could be explained by the stability of dinitrene character which is estimated from the resonance
energy of the intervening π-system. Our semiempirical molecular orbital calculations supported the
experimental correlation between the singlet-triplet energy gap and the zfs D value.
Introduction
Along with the studies on the high-spin organic molecules,
investigation of intramolecular spin-spin interactions is one of
the most exciting research areas.1-8 In the course of our
research in aromatic dinitrenes, we have studied spin-spin
interactions between two nitreno groups.7,8 Dinitrenes with
quinonoid structures such as biphenyl-4,4′-dinitrene (1) have
The higher field signal grew, and subsequently the lower field
two localized σ-spins which are coupled with the π-system by
ones grew. In addition, 2-azidofluorene (A1) and 2-azidofluo-
a one-center interaction. Their interesting features have been
renone (A2) were irradiated by the same procedure. Each of
the ESR spectra after the photolyses shows only one mononi-
reported by several groups as follows.
trene resonance, which has the zfs D value 0.889 cm-1 (A1) or
0.881 cm-1(A2), corresponding to the higher field signal of the
dinitrene. These results show that the lower field resonances
arise from secondary products such as 2-amino-7-nitrenofluo-
rene, which is produced by the hydrogen abstraction of dinitrene
from the matrix.
Firstly, the experimental zero-field-splitting (zfs) D values
were very large compared to the D values that are estimated
theoretically by a point-dipole approximation.2-7
Secondly, we recently reported that the singlet-triplet energy
gap and zfs D value have an interesting correlation in four types
of quinonoid dinitrenes that have different distance between two
localized σ-spins.8 For the electronic structures of quinonoid
dinitrenes, a contribution of dinitrene character, which has two
unpaired electrons in the π-system in addition to two localized
σ-spins, is expected in the total wave function. We showed
that the difference of the contribution explains the correlation.
In this work, we introduced additional linkers to 1 (Scheme
1) to perturb the π-system, keeping the distance of two localized
σ-spins. The effects of perturbation of these additional linkers
on the contribution of the dinitrene character and the spin-
spin interaction in 1 were examined by ESR measurements and
molecular orbital calculations.
All the spectra, except the spectrum of 2, show typical triplet
resonances due to the quinonoid dinitrenes, but the peak
positions are different from one another. Dinitrene 2 shows
only a weak triplet XY transition at 467 mT. In the spectra of
the quinonoid dinitrene in this work, the lower field XY
transitions tend to be smaller and broader than the higher one,
and thus the lower field XY transition of 2, which is expected
to appear at ca. 100 mT, is probably too weak to detect. In
addition, no |∆Ms| ) 2 line of 2 appeared because of the large
zfs D value.
The zfs parameters of all the quinonoid dinitrenes were
determined as listed in Table 2. The temperature dependence
of these triplet signal intensities was measured three times
independently for each dinitrene, as shown in Figure 3. To
determine the singlet-triplet energy gaps of quinonoid dini-
trenes, these plots were fitted by
Results
Typical ESR spectra obtained by the photolyses of diazide
precursors (DA2-DA6) are shown in Figures 1 and 2. The
strong peaks in the g ) 2.00 region are observed in all the cases.
They are ascribed to impurities of radicals formed under the
photochemical conditions used in this work.
I ) (C/T)[3 exp(-∆E/RT)/{1 + 3 exp(-∆E/RT)}] (1)
In the region 500-700 mT, all the spectra showed two
overlapped triplet XY transitions that are characteristic signals
of mononitrenes. These zfs parameters are listed in Table 1.
where I is the intensity, C is a constant, ∆E is the singlet-
triplet energy gap, R is the gas constant, and T is the absolute
temperature. The obtained energy gaps are listed in Table 2.
The largest energy gap of 2 suggests the smallest population of
triplet state, indicating the weak resonance intensity of the
quinonoid triplet.
† University of Tsukuba.
‡ National Institute of Materials and Chemical Research.
X Abstract published in AdVance ACS Abstracts, February 15, 1997.
S1089-5639(96)02971-4 CCC: $14.00 © 1997 American Chemical Society

2084 J. Phys. Chem. A, Vol. 101, No. 11, 1997
Nimura et al.
Figure 2. ESR spectra obtained after irradiation of (a) DA2 and (b)
DA6. Symbols TQ, T, D, and U denote the triplet quinonid dinitrene,
triplet mononitrene, doublet radicals, and unidentified signal, respec-
tively.
TABLE 1: zfs Parameters of Mononitrenes
zfs parametera/cm-1
precursor photolyzed
lower field
higher field
DA2
DA3
DA4
DA5
DA6
0.897
0.891
0.868
0.834
0.814
0.936
0.927
0.914
0.878
0.864
a Zfs E parameters are almost zero. Lower field signals are due to
byproducts.
Figure 1. ESR spectra obtained after irradiation of (a) DA3, (b) DA4,
and (c) DA5. Symbols TQ, T, and D denote the triplet quinonid
dinitrene, triplet mononitrene, and doublet radicals, respectively.
TABLE 2: Singlet-Triplet Energy Gaps and zfs Parameters
of Quinonoid Dinitrenes
SCHEME 1
compound
S-T gapa/kcal mol-1
zfs parameterb/cm-1
2
3
4
5
6
0.93 ( 0.05
0.88 ( 0.02
0.85 ( 0.01
0.78 ( 0.02
0.79 ( 0.01
0.286 (0.0)
0.236 (0.037)
0.205 (0.008)
0.188 (0.004)
0.174 (0.004)
a The values are average data from three plots. b Values in paren-
theses are zfs E values.
In the case of the photolyses of diazides DA2 and DA6, the
ESR spectra show additional strong signals in the lower regions
(Figure 2). One of the possible origins of these peaks is products
from side reactions. Those peaks could not be identified,
although the following results led to the conclusion that these
additional resonances were at least not ascribed to the quinonoid
dinitrene: (1) the temperature dependence of the signal intensi-
ties followed the Curie law, indicating that these signals were
resonances from ground state species, and (2) their irreversible
decay rates were different from the quinonoid triplet signals.
Then, why does the contribution of the dinitrene character
change in each quinonoid dinitrene? The magnitude of reso-
nance energies of intervening π-systems, such as fluorene in 5
connecting two nitreno groups, can be applied to this question.
The first stabilizing factor of the dinitrene character is the
preference of benzenoid structure. Thus the dinitrene character
may depend on the resonance energy of the intervening
π-system. Therefore, it is expected that as the resonance
energies of these intervening π-systems increases, the contribu-
tions of dinitrene character increase. These resonance energies
were calculated on the basis of the Hu¨ckel molecular orbital
theory, as listed in Table 3. The order of these resonance
energies is in accord with that of the D values and energy gaps.
Accordingly, the stability of benzenoid structure affects the
contribution of the dinitrene character in the total wave function,
and then the correlation between the D values and energy gaps
appears.
Discussion
Figure 4 shows a correlation between the obtained singlet-
triplet energy gaps and D values of the quinonoid dinitrene
including 1; that is, the energy gap increases as the D value
gets larger. The correlation is comparable with that observed
in stilbene-4,4′-dinitrene and azobenzene-4,4′-dinitrene.8 There-
fore the additional paths affect the total π-systems and then
change the contribution of the dinitrene character in the total
wave function.
Now let us look at the D values of mononitrene (Table 1).
The order of these values is consistent with the order of the

Effects of Additional Linkers in Biphenyl-4,4′-dinitrene
J. Phys. Chem. A, Vol. 101, No. 11, 1997 2085
Figure 3. ESR signal intensities vs temperature of (a) 2, (b) 3, (c) 4, (d) 5, and (e) 6 with fitted curves. One of the three plots is shown for each
compound.
the above discussion of the relation between the D values and
energy gaps.11 In addition, CdN bond lengths are expected to
be long as the contribution of the dinitrene character becomes
large. The calculated lengths of the triplet states are almost
the same, but the trend appears.
Finally, let us consider biphenylene-2,7-dinitrene (10), which
has not yet been examined experimentally. Biphenylene, which
is the intervening π-system of 10, has the smallest resonance
energy, as shown in Table 3, and thus a small contribution of
the dinitrene character is expected in 10. The PM3-CI calcula-
tion of 10 shows that the energy gap and the contribution of C
are the smallest in these dinitrenes (Figure 6). This result is
consistent with the smallest resonance energy of biphenylene.
Figure 4. Relationship between the energy gap and zfs D value.
resonance energy of intervening π-systems. The large D values
of these mononitrenes are mainly attributable to the resonance
structure 7 (Scheme 3) because of a one-center interaction.10
The contribution of structure 7, which has a benzenoid structure,
is dependent on the resonance energy. Therefore, in analogy
with the quinonoid dinitnrene, the contribution of benzenoid
Conclusions
structure affects the zfs D values.
Despite the same distance of two localized σ-spins, the
singlet-triplet energy gaps and zfs D values were changed and
interrelated by the additional linkers in 1. The correlation
between the energy gaps and zfs D values was explained by
the contribution of dinitrene character in the total wave function.
Moreover, it is shown that the contribution of dinitrene character
is attributable to the resonance energy of the intervening
π-system. It is concluded that the crucial factor in the singlet-
triplet energy gaps and D values in these systems is the stability
of the dinitrene character.
Next, semiempirical molecular orbital calculations were
carried out to clarify more detailed features of the electronic
structures of the quinonoid dinitrenes. The principal configura-
tions of the triplet state in the CI calculations are shown in
Figure 5. The configurations C are related to the dinitrene
character and thus prerequisite configurations to the large D
values. Against the contributions of C, the calculated singlet-
triplet energy gaps are plotted in Figure 6. Figure 6 shows a
similar relationship as seen in Figure 4. These results support

2086 J. Phys. Chem. A, Vol. 101, No. 11, 1997
Nimura et al.
SCHEME 2
TABLE 3: Calculated Resonance Energies of Intervening
π-System Skeletons
compound
resonance energy/â
phenanthrene
dibenzothiophene
dibenzofuran
biphenyl
fluorene
fluorenone
0.546a
0.539
0.513
0.502a
0.475
0.427
0.123a
biphenylene
a These data are taken from ref 9.
Figure 6. Relationship between the calculated singlet-triplet energy
gap and the contribution of configuration C.
1
226 °C); H NMR (CDCl3, 200 MHz) δ 3.07 (s, 4H), 7.94 (d,
J ) 8.5 Hz, 2H), 8.18-8.25 (m, 4H); 13C NMR (CDCl3, 50
MHz) δ 28.35, 122.58, 123.49, 125.57, 138.53, 139.39, 147.80.
2,7-Dinitrophenanthrene. A mixture of 0.4 g (1.5 mmol)
of 2,7-dinitro-9,10-dihydrophenanthrene, 0.26 g (1.5 mmol) of
N-bromosuccinimide, a trace of dibenzoyl peroxide, and 10 mL
of dry carbon tetrachloride was boiled under reflux for 3 h. The
solid obtained on removal of the solvent from the filtered
reaction mixture was crystallized from glacial acetic acid to give
0.2 g (50%) of 2,7-dinitrophenanthrene as bright yellow
Figure 5. Schematic principal electron configurations of the triplet
state in the CI calculations.
SCHEME 3
1
crystals: mp 337 °C (lit.12 333-336 °C); H NMR (CDCl3,
200 MHz) δ 8.04 (s, 2H), 8.53 (dd, J ) 2.4 Hz, J ) 8.9 Hz,
2H), 8.87 (d, J ) 8.9 Hz, 2H), 8.88 (d, J ) 2.4 Hz, 2H).
2,7-Diaminophenanthrene. A hot solution of 3.2 g (14
mmol) of stannous chloride dihydrate in 9 mL of concentrated
hydrochloric acid was added during 1 min to a hot solution of
0.15 g (0.6 mmol) of 2,7-dinitrophenanthrene in 25 mL of acetic
acid. After 3 min the solution was evaporated in vacuo. The
residual oil was dissolved into water, and excess 50% aqueous
sodium hydroxide was added. The liberated base was filtered,
washed with alkali and then with water, and dried. The yield
was 0.09 g (72%) of 2,7-diaminophenanthrene as a light yellow
solid: 1H NMR (CDCl3, 200 MHz) δ 3.81 (br s, 4H), 7.00-
7.04 (m, 4H), 7.47 (s, 2H), 8.31-8.36 (m, 2H).
Experimental Section
General. 1H and 13C NMR spectra were recorded on a
Bruker AC-200 spectrometer. IR spectra were obtained on a
Shimadzu FTIR-8500. Mass spectra (MS) were recorded on a
Shimadzu GCMS-QP1000EX gas chromatograph-mass spec-
trometer. Melting points were taken on a Yanagimoto Sei-
sakusyo micro melting point apparatus and are uncorrected.
High-resolution mass spectra (HRMS) were performed in the
analytical center of the National Institute of Materials and
Chemical Research. 2-Methyltetrahydrofuran was distilled from
sodium.
2,7-Dinitro-9,10-dihydrophenanthrene. In a 100 mL Er-
lenmeyer flask was added 6 mL of fuming nitric acid with
stirring over 3 h to a solution of 2.0 g (10 mmol) of
9,10-dihydrophenanthrene in 6 mL of glacial acetic acid, and
the mixture was maintained at 45-50 °C. Some solid begins
to separate after 1 h. The mixture was stirred for a further 3 h
before cooling, and the resulting solid was washed thoroughly
with water and dried. The obtained solid was crystallized from
acetic acid to give 0.77 g (29%) of 2,7-dinitro-9,10-dihydro-
phenanthrene as pale yellow crystals: mp 222-223 °C (lit.12
2,7-Diazidophenanthrene (DA2). In a 100 mL Erlenmeyer
flask were placed 25 mL of water, 7 mL of concentrated
hydrochloric acid, and 0.07 g (0.34 mmol) of 2,7-diami-
nophenanthrene. The flask was placed in an ice-bath and treated
dropwise with a solution of 0.1 g (1.4 mmol) of sodium nitrite
in 10 mL of water. The mixture was stirred in the ice-bath for
1.5 h and then treated with a solution of 0.1 g (1.6 mmol) of
sodium azide in 10 mL of water. The mixture was stirred for
an additional 1 h and extracted with dichloromethane. The
organic layer was dried over magnesium sulfate. The solvent
was evaporated and recrystallized from ethanol to give 0.07 g
(79%) of 2,7-diazidophenanthrene as light orange crystals: mp
1
131 °C (dec); IR (KBr, cm-1) 2140; H NMR (CDCl3, 200
MHz) δ 7.32 (dd, J ) 2.4 Hz, J ) 8.8 Hz, 2H), 7.49 (d, J )
2.4 Hz, 2H), 7.69 (s, 2H), 8.57 (d, J ) 8.8 Hz, 2H); 13C NMR
(CDCl3, 50 MHz) δ 117.49, 119.21, 124.67, 127.61, 132.94,
138.55; HRMS calcd for C14H8N6: 260.0810, found 260.0771.

Effects of Additional Linkers in Biphenyl-4,4′-dinitrene
J. Phys. Chem. A, Vol. 101, No. 11, 1997 2087
3,7-Dinitrodibenzothiophene 5-oxide. A solution of 0.2 g
(1 mmol) of dibenzothiophene-5-oxide in 7 mL of concentrated
sulfuric acid was cooled to 12 °C and 4 mL of fuming nitric
acid was added slowly so that the reaction temperature remained
at 10-15 °C. The mixture was left at this temperature for 30
min. and then allowed to come to room temperature over 30
min. The precipitate obtained by pouring the nitration mixture
on ice, was collected on a buchner, washed with water, and air
dried to give 0.23 g (79 %) of 3,7-dinitrodibenzothiophene-5-
oxide as pale yellow crystals: mp 258-259 °C (lit.13 mp 257-
recrystallized from hexane to give 0.22 g (44%) of 3,7-
diazidodibenzofuran as a yellow solid: mp 127-128 °C; IR
1
(KBr, cm-1) 2140; H NMR (CDCl3, 200 MHz) δ 7.03 (dd, J
) 2.0 Hz, J ) 8.0 Hz, 2H), 7.22 (d, J ) 2.0 Hz, 2H), 7.84 (d,
J ) 8.0 Hz, 2H); 13C NMR (CDCl3, 50 MHz) δ 102.56, 114.57,
121.04, 121.16, 139.28, 157.32; HRMS calcd for C12H6N6O
250.0603, found 250.0590.
2,7-Diazidofluorene (DA5). In a 50 mL Erlenmeyer flask
were placed 4 mL of water, 2 mL of concentrated hydrochloric
acid, and 0.2 g (1 mmol) of 2,7-diaminofluorene. The flask
was placed in an ice-bath and treated dropwise with a solution
of 0.2 g (3 mmol) of sodium nitrite in 2 mL of water. The
mixture was stirred in the ice-bath for 1 h and then treated with
a solution of 0.2 g (3 mmol) of sodium azide in 2 mL of water.
The mixture was stirred for an additional 15 min and extracted
with ether. The organic layer was dried over magnesium sulfate.
The solvent was evaporated and recrystallized from a mixture
of ether and hexane to give 0.1 g (40%) of 2,7-diazidofluorene
1
258 °C); H NMR (CDCl3, 200 MHz) δ 8.13 (d, J ) 8.6 Hz,
2H), 8.60 (dd, J ) 2.1 Hz, J ) 8.6 Hz, 2H), 8.94 (d, J ) 2.1
Hz, 2H).
3,7-Diaminodibenzothiophene. A solution of 2 g (9 mmol)
of stannous chloride dihydrate in 3 mL of concentrated
hydrochloric acid was added to a hot solution of 0.23 g (0.8
mmol) of 3,7-dinitrodibenzothiophene 5-oxide in 4 mL of acetic
acid and stirred 2 h at room temperature. Then the white
suspension was heated at 100 °C and stirred for 1 h. After
cooling to room temperature, the reaction mixture was filtrated
and washed with brine. The solid was dispersed into water,
and excess 40% aqueous sodium hydroxide was added. The
mixture was filtered, washed with water, and dried to give 0.14
g (81%) of 3,7-diaminodibenzothiophene as a pale yellow
solid: mp 169-170 °C (lit.13 mp 169-170 °C); 1H NMR
(CDCl3, 200 MHz) δ 3.77 (br s, 4H), 6.76 (dd, J ) 2.1 Hz, J
) 8.4 Hz, 2H), 7.05 (d, J ) 2.1 Hz, 2H), 7.74 (d, J ) 8.4 Hz,
2H).
1
as orange crystals: mp 127; IR (KBr, cm-1) 2120; H NMR
(CDCl3, 200 MHz) δ 3.85 (s, 2H), 7.03 (d, J ) 8.0 Hz, 2H),
7.16 (s, 2H), 7.66 (d, J ) 8.0 Hz, 2H); 13C NMR (CDCl3, 50
MHz) δ 36.76, 115.74, 118.0, 120.64, 138.16, 138.43, 144.79;
HRMS calcd for C13H8N6 248.0810, found 248.0815.
2,7-Diazidofluorenone (DA6). A hot solution of 5.0 g (22
mmol) of stannous chloride dihydrate in 6 mL of concentrated
hydrochloric acid was added during 1 min to a hot solution of
1.0 g (3 mmol) of 2,7-dinitrofluorenone in 10 mL of acetic acid.
After 5 min the solution was evaporated in vacuo. The residual
oil was dissolved into water, and excess 50% alkali was added.
The liberated base was filtered, washed with alkali and then
with water, and dried, yielding 0.62 g (80%) of 2,7-diaminof-
luorenone as a brown solid: MS, m/z 210 (M+).
3,7-Diazidodibenzothiophene (DA3). In a 200 mL Erlen-
meyer flask were placed 50 mL of water, 15 mL of concentrated
hydrochloric acid, and 0.14 g (0.65 mmol) of 3,7-diamino-
dibenzothiophene. The flask was placed in an ice-bath and
treated dropwise with a solution of 0.15 g (2 mmol) of sodium
nitrite in 25 mL of water. The mixture was stirred in the ice-
bath for 1.5 h and then treated with a solution of 0.15 g (2
mmol) of sodium azide in 25 mL of water. The mixture was
stirred for an additional 1 h and extracted with chloroform. The
organic layer was dried over magnesium sulfate. The solvent
was evaporated and recrystallized from ethanol to give 0.14 g
(81%) of 3,7-diazidodibenzothiophene as a pale yellow solid:
mp 161-162 °C; IR (KBr, cm-1) 2130; 1H NMR (CDCl3, 200
MHz) δ 7.12 (dd, J ) 2.1 Hz, J ) 8.3 Hz, 2H), 7.46 (d, J )
2.1 Hz, 2H), 8.01 (d, J ) 8.3 Hz, 2H); 13C NMR (CDCl3, 50
MHz) δ 112.69, 116.42, 122.27, 132.16, 138.70, 140.72; HRMS
calcd for C12H8N6S 266.0374, found 266.0353.
In a 500 mL Erlenmeyer flask were placed 100 mL of water,
4 mL of concentrated hydrochloric acid, and 0.62 g (3 mmol)
of 2,7-diaminofluorenone. The flask was placed in an ice-bath
and treated dropwise with a solution of 0.5 g (7 mmol) of
sodium nitrite in 10 mL of water. The mixture was stirred in
the ice-bath for 2 h and then treated with a solution of 0.5 g (8
mmol) of sodium azide in 10 mL of water. The mixture was
stirred for an additional 1 h and extracted with chloroform. The
organic layer was dried over magnesium sulfate. The solvent
was evaporated to give 0.52 g (66%) of 2,7-diazidofluorenone
as an orange solid: mp 187 °C (dec); IR (KBr, cm-1) 2130; 1H
NMR (CDCl3, 200 MHz) δ 7.10 (dd, J ) 8.0 Hz, J ) 2.0 Hz,
2H), 7.33 (d, J ) 2.0 Hz, 2H), 7.44 (d, J ) 8.0 Hz, 2H); 13C
NMR (CDCl3, 50 MHz) δ 115.228, 121.36, 124.91, 135.82,
140.49, 141.11, 191.889; HRMS calcd for C13H6N6O 262.0603,
found 262.0613.
3,7-Diaminodibenzofuran. A mixture of 8 g (21 mmol) of
4,4′-diamino-2,2′-biphenyldisulfonic acid and 70 g of 50%
aqueous sodium hydroxide was heated at 280 °C (24-30 kg/
cm2) for 16 h in a 100 mL autoclave. After the mixture had
been cooled, 400 mL of water was added. The mixture was
filtrated, washed with water, and dried to give 0.85 g (20%) of
3,7-diaminodibenzofuran as a brown solid: mp 152-155 °C
(lit.14 152 °C); 1H NMR (CDCl3, 200 MHz) δ 3.81 (br s, 4H),
6.67 (dd, J ) 8.0 Hz, J ) 4.0 Hz, 2H), 6.82 (d, J ) 4.0 Hz,
2H), 7.54 (d, J ) 8.0 Hz, 2H); 13C NMR (CDCl3, 50 MHz) δ
98.10, 111.24, 116.70, 120.16, 145.35, 157.80.
2-Azidofluorene (A1). In a 100 mL Erlenmeyer flask were
placed 20 mL of water, 10 mL of concentrated hydrochloric
acid, and 0.4 g (2.2 mmol) of 2-aminofluorene. The flask was
placed in an ice-bath and treated dropwise with a solution of
0.2 g (3 mmol) of sodium nitrite in 5 mL of water. The mixture
was stirred in the ice-bath for 1.5 h and then treated with a
solution of 0.2 g (3 mmol) of sodium azide in 5 mL of water.
The mixture was stirred for an additional 2 h and extracted with
ether. The organic layer was dried over magnesium sulfate.
The solvent was evaporated and recrystallized from hexane to
give 0.4 g (86%) of 2-azidofluorene as a pale yellow solid: mp
3,7-Diazidodibenzofuran (DA4). In a 200 mL Erlenmeyer
flask were placed 100 mL of water, 10 mL of concentrated
hydrochloric acid, and 0.4 g (2 mmol) of 3,7-diaminodibenzo-
furan. The flask was placed in an ice-bath and treated dropwise
with a solution of 0.5 g (7 mmol) of sodium nitrite in 10 mL of
water. The mixture was stirred in the ice-bath for 1.5 h and
then treated with a solution of 0.6 g (9 mmol) of sodium azide
in 10 mL of water. The mixture was stirred for an additional
30 min and extracted with ether. The organic layer was dried
over magnesium sulfate. The solvent was evaporated and
1
105 °C (dec); IR (KBr, cm-1) 2110; H NMR (CDCl3, 200
MHz) δ 3.83 (s, 2H), 7.00 (dd, J ) 2.0 Hz, J ) 8.1 Hz, 1H),
7.16 (s, 1H), 7.22-7.39 (m, 2 H), 7.50 (d, J ) 7.3 Hz, 1H),
7.66-7.71 (m, 2H); 13C NMR (CDCl3, 50 MHz) δ 36.83,
115.71, 117.81, 119.63, 120.83, 125.01, 126.64, 126.91, 138.43,
138.92, 140.92, 142.89, 145.09; HRMS calcd for C13H9N3
207.0798, found 207.0765.

2088 J. Phys. Chem. A, Vol. 101, No. 11, 1997
Nimura et al.
2-Azidofluorenone (A2). In a 100 mL Erlenmeyer flask
were placed 10 mL of water, 1 mL of concentrated hydrochloric
acid, and 0.3 g (1.5 mmol) of 2-aminofluorenone. The flask
was placed in an ice-bath and treated dropwise with a solution
of 0.15 g (2 mmol) of sodium nitrite in 5 mL of water. The
mixture was stirred in the ice-bath for 1.5 h and then treated
with a solution of 0.13 g (2 mmol) of sodium azide in 5 mL of
water. The mixture was stirred for an additional 1 h and
extracted with ether. The organic layer was dried over
magnesium sulfate. The solvent was evaporated and recrystal-
lized from hexane to give 0.33 g (99%) of 2-azidofluorenone
the MOPAC ver. 6.0 program package.15 All calculations were
performed on IBM RS/6000 workstations or the Cray C90
supercomputer at RIPS of the Agency of Industrial Science and
Technology. In the semiempirical computations the geometry
of the triplet state of each molecule was optimized using a
restricted open-shell wave function employing the PM3 Hamil-
tonian. Then configuration interaction calculations were carried
out to compute the energy differences between the triplet and
singlet states at the optimized geometries. Each state was
assumed to be open-shell, with a five-orbital and six-electron
active space.
1
as a yellow solid: mp 117-119 °C; IR (KBr, cm-1) 2100; H
The resonance energies were also calculated by a program
written by Oka et al.16 on an NEC 9801 microcomputer. Hu¨ckel
parameters of the heteroatoms and the hyperconjugation of the
CH2 group are taken from ref 17.
NMR (CDCl3, 200 MHz) δ 7.06 (d, J ) 7.2 Hz, 1H), 7.30 (s,
2H), 7.43-7.52 (m, 3H), 7.63 (d, J ) 7.2, 1H); 13C NMR
(CDCl3, 50 MHz) δ 114.87, 120.11, 121.46, 124.52, 128.84,
134.00, 135.01, 135.84, 140.76, 141.17, 144.03, 192.79; HRMS
calcd for C13H7N3O 221.0589, found 221.0587.
ESR Measurement. ESR spectra were measured on a JEOL
JES-RE3X spectrometer equipped with an optical transmission
cavity and an Air Products LTD-3 liquid helium transfer system.
The microwave frequency was measured with a Hewlett Packard
5340A frequency counter, and the resonance magnetic field
values of the signals were measured with the aid of an Echo
Electronics Co., Ltd., EFM-2000 NMR field meter.
Acknowledgment. We thank Dr. Takaaki Hanaoka, National
Institute of Materials and Chemical Research, for assistance in
the syntheses.
References and Notes
(1) Iwamura, H.; Miller, J. S., Eds. Proceedings of the Symposium on
Ferromagnetic and High Spin Molecular Based Materials, Mol. Cryst. Liq.
Cryst. 1993, 232, 233.
(2) Trozzolo, A. M.; Murray, R. W.; Smolinsky, G.; Yager, W. A.;
Wasserman, E. J. Am. Chem. Soc. 1963, 85, 2526.
(3) Singh, B.; Brinen, J. S. J. Am. Chem. Soc. 1971, 93, 540.
(4) Minato, M.; Lahti, P. M.; van Willigen, H. J. Am. Chem. Soc. 1993,
115, 4532.
(5) Ichimura,A. S.; Lahti, P. M. Mol. Cryst. Liq. Cryst. 1993, 233, 33.
(6) Minato, M.; Lahti, P. M. J. Phys. Org. Chem. 1993, 6, 483.
(7) Ohana, T.; Nimura, S.; Kikuchi, O.; Kaise, M.; Yabe, A. Chem.
Lett. 1993, 765.
(8) Nimura, S.; Kikuchi, O.; Ohana, T.; Yabe, A.; Kaise, M. Chem.
Lett. 1996, 125.
(9) Aihara, J. J. Am. Chem. Soc. 1976, 98, 2750.
(10) Coope, J. A. R.; Farmer, J. B.; Gardner, C. L.; McDowell, C. A. J.
Chem. Phys. 1965, 42, 54.
(11) For electronic structures of diradical and dicarbene intermediates,
more detail and extensive semiempirical molecular orbital calculations have
been performed by C. Kollmar. His work was much benefit to us in the
calculations: Kollmar, C. J. Chem. Phys. 1993, 98 (9), 7210.
(12) Hallas, G.; Wada, B. T. Chem. Ind. 1978, 630.
(13) Brown, R. K.; Nelson, N. A.; Wood, J. C. J. Am. Chem. Soc. 1952,
74, 1165.
(14) Wirth, H. O.; Waese, G.; Kern, W. Makromol. Chem. 1965, 86,
139.
(15) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209, 221.
(16) Oka, O.; Takashima, H.; Aihara, J. Kagaku 1985, 40 (1), Appendix
1 (in Japanese).
Sample temperature was controlled with gas flow rates and
a temperature controller. The temperature was monitored
continuously just below the sample position in the cryostat. In
addition, the temperature at the actual position was calibrated
by measuring temperature with a chromel-AuFe thermocouple
placed in the sample tube before each ESR experiment. The
temperature was controlled with an Oxford ITC 503 temperature
controller and an Advantest TR2114H temperature measurement
with a chromel-AuFe thermocouple.
Appropriate diazide precursors were dissolved in purified
2-methyltetrahydrofuran. The solution (ca. 10-3 mol dm-3) was
placed in a quartz 4 mm o.d. ESR sample tube and degassed
by freeze-pump-thaw cycles. The diazide precursors were
irradiated with an USHIO UI-501C xenon lamp through Toshiba
UV-D33S and UV-27 cutoff filters (270 nm < λ < 400 nm) to
generate the dinitrenes. Photolysis of the various diazide
precursors was carried out at various temperatures between 60
and 77 K and maintained at each temperature for at least 30
min before analyzing the ESR spectral behavior. The measure-
ments were carried out at an appropriate low microwave power
under each measurement condition to avoid the signal saturation
effect.
(17) (a) Hess, B. A., Jr.; Schaad, L. J.; Holyoke, C. W., Jr. Tetrahedron
1972, 28, 3657, 5299. (b) Hess, B. A., Jr.; Schaad, L. J. J. Am. Chem. Soc.
1973, 95, 3907. (c) Brauer, H.-D.; Reinsch, E.-A. Ber. Bunsen-Ges. 1973,
77 (5), 348.
Molecular Orbital Calculation. The molecular orbital
calculations were carried out by a semiempirical method with