EPR Spectral Determination of Electronic Substituent Effects on the D Values of Hydrocarbon Polyradicals (Quintet and Septet Spin States) Composed of Localized 1,3-Cyclopentanediyl Spin-Carrying Units Linked by 1,3-Di- And 1,3,5-Trimethylenebenzene Ferromagnetic Couplers

Article abstract of DOI:10.1021/jo0012795

The parent and p-nitrophenyl-substituted diradicals D-3a,b (triplets), tetraradicals T-3a,b (quintets), and hexaradicals H-3a,b (septets) were photochemically generated in matrix-isolated form (toluene, 77 K) by successive denitrogenation of the trisazoalkanes 3a,b and EPR spectrally characterized. In these high-spin polyradicals the spin-spin interaction within the localized spin-carrying 1,3-cyclopentanediyl diradical unit is much stronger than within the cross-conjugated ferromagnetic coupling unit. Accordingly, a change of the electronic properties in the cyclopentanediyl unit affects decisively the D value of the whole polyradical. Therefore, the spin-accepting p-nitro group reduces the D value of the tetra- and hexaradical in the same amount as that of the diradical. Thus, irrespective of the spin multiplicity, the substituent stabilizes electronically the triplet (D-3a,b), quintet (T-3a,b), and septet (H-3a,b) species with equal efficacy.

Full text of DOI:10.1021/jo0012795

7650
J . Org. Chem. 2000, 65, 7650-7655
EP R Sp ectr a l Deter m in a tion of Electr on ic Su bstitu en t Effects on
th e D Va lu es of Hyd r oca r bon P olyr a d ica ls (Qu in tet a n d Sep tet
Sp in Sta tes) Com p osed of Loca lized 1,3-Cyclop en ta n ed iyl
Sp in -Ca r r yin g Un its Lin k ed by 1,3-Di- a n d
1,3,5-Tr im eth ylen eben zen e F er r om a gn etic Cou p ler s
Waldemar Adam* and Wiebke Maas
Institut fu¨r Organische Chemie, Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
adam@chemie.uni-wuerzburg.de
Received August 23, 2000
The parent and p-nitrophenyl-substituted diradicals D-3a ,b (triplets), tetraradicals T-3a ,b
(quintets), and hexaradicals H-3a ,b (septets) were photochemically generated in matrix-isolated
form (toluene, 77 K) by successive denitrogenation of the trisazoalkanes 3a ,b and EPR spectrally
characterized. In these high-spin polyradicals the spin-spin interaction within the localized spin-
carrying 1,3-cyclopentanediyl diradical unit is much stronger than within the cross-conjugated
ferromagnetic coupling unit. Accordingly, a change of the electronic properties in the cyclopen-
tanediyl unit affects decisively the D value of the whole polyradical. Therefore, the spin-accepting
p-nitro group reduces the D value of the tetra- and hexaradical in the same amount as that of the
diradical. Thus, irrespective of the spin multiplicity, the substituent stabilizes electronically the
triplet (D-3a ,b), quintet (T-3a ,b), and septet (H-3a ,b) species with equal efficacy.
In tr od u ction
We consider the 1,3-cyclopentanediyl diradical D-1 to
be a good spin carrier because of its triplet ground state,
its remarkable persistence (several months at 77 K), and
its convenient synthetic accessibility. These 1,3-cyclo-
pentadiyl triplet diradicals, classified as localized diradi-
cals since the two spins are not conjugated, are readily
generated from the corresponding azoalkane 1. Their
electronic and structural properties may be characterized
The development of organic high-spin polyradicals is
of current interest in basic research as well as in
materials science.1 Compared to metal-type magnets,
organic magnetic materials would open up the op-
portunity to combine magnetic with advantageous opti-
cal, electrical, and mechanical properties. The strategy
to design organic polyradicals is to link spin-carrying
fragments by ferromagnetic couplers, which induce a
parallel alignment of all the spins in the molecule. Such
spin alignment may be achieved by means of open-shell
organic molecules. An important prerequisite for efficient
ferromagnetic coupling is that the coupler and the spin
carrier are not twisted out of conjugation.2 The choice of
adequate ferromagnetic couplers is quite limited. Estab-
lished ones are the cross-conjugated non-Kekule´ struc-
tures 1,3-di-3 and 1,3,5-trimethylenebenzene,4 in which
the Hu¨ckel NBMOs are non-disjoint.5 However, in the
selection of an appropriate spin-carrying unit, there are
numerous options, since in principle the plethora of
persistent organic radicals apply.1a
* To whom correspondence should be addressed. Fax: +49 931/888-
4756; http://www-organik.chemie.uni-wuerzburg.de.
(1) For reviews, see: (a) Dougherty, D. A. Acc. Chem. Res. 1991,
24, 88-94. (b) Iwamura, H.; Koga, N. Acc. Chem. Res. 1993, 26, 346-
351. (c) Rajca, A. Chem. Rev. 1994, 94, 871-893. (d) Miller, J . S.;
Epstein, A. J . Angew. Chem., Int. Ed. Engl. 1994, 33, 399-432. (e)
Baumgarten, M.; Mu¨llen, K. Top. Curr. Chem. 1994, 169, 1-104.
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through the EPR zero-field-splitting parameters D and
E, which derive from the dipole-dipole interaction
between the two uncoupled spins.6
10.1021/jo0012795 CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/04/2000

EPR Determination of Electronic Substituent Effects
J . Org. Chem., Vol. 65, No. 22, 2000 7651
The tetraradical T-2 represents the first polyradical
with a quintet-spin ground state, in which two localized
1,3-cyclopentanediyl diradicals have been linked ferro-
magnetically by m-phenylene.7 When three 1,3-cyclo-
pentanediyl diradical units are linked by 1,3,5-trimeth-
ylenebenzene the hexaradical H-3 results, in which the
six unpaired electron may acquire a septet spin state.
Not only would the array of parallel spins be extended,
but the two-dimensional structure would allow the design
of dendrimeric polyradicals.1c Indeed, the 1,3,5-trimeth-
ylenebenzene unit has been used successfully to connect
simple radicals8 and carbenes9 to form such high-spin
polyradicals. Similarly, we have recently demonstrated
that the septet hexaradical H-3 (the first example in
which localized 1,3 diradicals are ferromagnetically
coupled by 1,3,5-trimethylenebenzene) possesses a septet-
spin ground state.10
Resu lts
Syn th esis. The precursor for the hexaradicals H-3a
and H-3b, namely the trisazoalkanes 3a and 3b, were
prepared in a seven-step sequence, in which the key step
entailed amine-catalyzed trimerization12 of the acetylene
functionality of the dione 613 to generate the central
benzene ring of the hexaketone 7 (Scheme 1). After
generation of the trisisopyrazole 8 by reaction of the
hexaketone 7 with hydrazine hydrate (90% yield for 8a
and 95% for 8b), the photolabile trisazoalkanes 9 were
obtained by 3-fold cycloaddition with cyclopentadiene
(60% yield for 9a and 71% for 9b). In the case of the nitro
derivative 9b, instead of the Pd-catalyzed hydrogenation
used for the unsubstituted case 9a (95% yield) which also
reduces the nitro group, the double bond was saturated
with diimide14 (44% yield). On photolysis, the trisazoal-
kane 9a gave cleanly the trishousane 10a (94% yield) as
a mixture of diastereomers. In analogy to the synthesis
of the p-nitro-substituted monohousane,15 the trishousane
10b was obtained by thermolysis. The moderate yield of
40% is due to the formation of undesirable side products
during the long reaction time. The bisazoalkane 2 was
synthesized as described earlier.7
For the aryl-substituted 1,3-cyclopentanediyl triplet
diradical D-1, we have previously shown that the D
parameter (eq 1) is a valuable probe to assess electronic
substituent effects, because the D value is a linear
EP R Sp ectr a l Da ta . Toluene solutions (ca. 8 µM) of
the particular trisazoalkane were frozen at 77 K, and
photochemical extrusion of dinitrogen was effected by
irradiation with the 333-nm line of the argon-ion laser.16
Short-time photolysis (ca. 3 min) of the trisazoalkanes
3a produced an EPR spectrum, which was virtually
superimposable to that of the irradiated bisazoalkane 2
(Figure 1). Since the irradiated bisazoalkane 2 gave a
mixture of the triplet diradical D-2 and quintet tet-
raradical T-2 (Figure 1a),7 we anticipated that on short
irradiation of the trisazoalkane 3a also a mixture of the
diradical D-3a and tetraradical T-3a would be formed.
Indeed, the EPR spectrum measured for the trisazoal-
kane 3b after 3-min irradiation was similar to that
observed for the bisazoalkane 2, with the only difference
that the EPR lines of the irradiated 3b are closer spaced
(Figure 1c). This EPR spectrum was assigned to the
diradical D-3b and tetraradical T-3b, whose D values are
significantly smaller than those for the corresponding
polyradicals D-3a and T-3a (Table 1, entries 2 and 3).17
function of the spin density at the R position of the cumyl-
radical fragment.6 Thus, the higher the spin-accepting
propensity of the aryl substituent, the more effectively
delocalized is the unpaired electron into the aromatic ring
and, consequently, the lower the spin density at the
cumyl-radical site as manifested by the lower D value.
Such electronic substituent effects have only been exam-
ined so far in high-spin (S > 1) polycarbenes.11 Accord-
ingly, the incentive of the present work was to explore
the change of the spin density caused by substituents on
the aryl ring of the 1,3-cyclopentanediyl spin carrier in
the high-spin (S > 1) polyradicals T and H. For this
purpose, the unsubstituted trisazoalkane 3a and the
p-nitro-substituted trisazoalkane 3b were synthesized
and photodenitrogenated to the corresponding tetraradi-
cals T-3a and T-3b as well as hexaradicals H-3a and
H-3b, and their D values were determined by EPR
spectroscopy under matrix isolation at 77 K. Herein we
present the results of this investigation.
On further irradiation of the trisazoalkane 3a , new
signals appeared which intensified, while the T-3a
signals stopped growth (Figure 2a). The peaks marked
with H in the time profile are attributed to the hexaradi-
cal H-3, which is formed from the tetraradical T-3a by
photodenitrogenation. Since prolonged irradiation did not
change the ratio of the signal intensities for the diradical,
tetraradical, and hexaradical, the D and E value of the
hexaradical H-3a had to be extrapolated from the com-
posite spectrum. For this purpose, computer simulation
(6) Adam, W.; Harrer, H. M.; Kita, F.; Nau, W. M. Adv. Photochem.
1998, 24, 205-254.
(7) Adam, W.; van Barneveld, C.; Bottle, S. E.; Engert, H.; Hanson,
G. R.; Harrer, H. M.; Heim, C.; Nau, W. M.; Wang, D. J . Am. Chem.
Soc. 1996, 118, 3974-3975.
(8) (a) Kothe, G.; Wilker, W.; Zimmermann, H. Chem. Ber. 1975,
108, 2124-2136. (b) Veciana, J .; Rovira, C.; Ventosa, N.; Crespo, M.
I.; Palacio, F. J . Am. Chem. Soc. 1993, 115, 57-64.
(9) (a) Takui, T.; Itoh, K.; Chem. Phys. Lett. 1973, 19, 120-124. (b)
Nakazawa, S.; Sato, K.; Kinoshita, T.; Takui, T.; Itoh, K.; Fukuyo, M.;
Higuchi, T.-I.; Hirotsu, K. Mol. Cryst. Liq. Cryst. 1995, 271, 163-171.
(c) Matsuda, K.; Nakamura, N.; Inoue, K.; Koga, N.; Iwamura, H.
Chem. Eur. J . 1996, 2, 259-264.
(12) (a) Balasubramanian, K.; Selvaraj, S.; Venkataramani, P. S.
Synthesis 1980, 29-30. (b) Matsuda, K.; Nakamura, N.; Iwamura, H.
Chem. Lett. 1994, 1765-1768.
(13) Dess, D. B.; Martin, J . C. J . Org. Chem. 1983, 48, 4155-4156.
(14) Moriaty, R. M.; Vaid, R. K.; Duncan, M. P. Synth. Commun.
1987, 17, 703-708.
(15) Adam, W.; Harrer, H. M.; Kita, F.; Nau, W. M.; Zipf, R. J . J .
Org. Chem. 1996, 61, 7056-7065.
(16) A detailed study of the photochemistry on the trisazoalkane
by UV and EPR spectroscopy has been submitted for publication in J .
Org. Chem.
(10) Adam, W.; Baumgarten, M.; Maas, W. J . Am. Chem. Soc. 2000,
122, 6735-6738.
(11) Nakazawa, S.; Sato, K.; Kinoshita, T.; Takui, T.; Itoh, K.;
Fukuyo, M.; Higuchi, T.-I.; Hirotsu, K. Mol. Cryst. Liq. Cryst. 1995,
271, 163-171.
(17) The D values of the tetraradicals T-3a and T-3b were deter-
mined from the outermost characteristic quintet EPR signals, which
are separated by 6D.

7652 J . Org. Chem., Vol. 65, No. 22, 2000
Sch em e 1. Syn th esis of th e Tr isa zoa lk a n es 3a a n d 3ba
Adam and Maas
a
Reagents and conditions: (a) Et2O, ca. 20 °C, 16 h; (b) HCl, Et2O, ca. 20 °C, 4 h; (c) C2H5BrMg, HCtCH, THF; X ) H: 0 °C f ca. 20
°C, 18 h; X ) NO2: -60 °C, 6 h; (d) X ) H: CrO3, H2SO4, acetone, 0 °C f ca. 20 °C, 7 h; X ) NO2: Dess-Martin periodinane,13 CH2Cl2,
ca. 20 °C, 2 h; (e) HNEt2 (0.13 equiv.), o-xylol, reflux, 3 d; (f) N2H4×H2O (5 equiv), CHCl3; X)H: reflux, 16 h; X ) NO2: ca. 20 °C, 2 d;
(g) CF3COOH (1.5 equiv), cyclopentadiene, CH2Cl2, 0-5 °C, 3 d; (h) X ) H: Pd/C, H2, benzene, ca. 20 °C, 24 h; X ) NO2: N2H4‚H2O (12
equiv), PhI(OAc)2 (5 equiv), NaHCO3, CH2Cl2, ca. 20 °C, 3 h; (i) X ) H: (λ ) 333-364 nm, 1.3 W, ca. 10 min, benzene; X ) NO2: 80 °C,
d6-benzene, NaHCO3, 5 d.
and spectral subtraction was conducted; the best fit
between the experimental and simulated spectrum was
achieved for DH-3a ) 0.0091 cm-1 and EH-3a ) 0.0002
crystalline matrix of an axial-symmetric spin system (E
≈ 0), there are only two orientations of the molecular axes
with respect to the external magnetic field, which absorb
at distinct magnetic field strength.18 In the case of a
septet-spin system with axial symmetry, six ∆ms ) (1
magnetic dipole transitions occur for each of the two
orientations, such that 12 peaks are expected in the EPR
spectrum for a septet-spin polyradical. Each of the six
positive EPR signals has a negative counterpart, and the
separations of corresponding pairs of positive and nega-
tive peaks are given by 1D, 2D, 3D, 5D, 6D, and 10D,
which are the eigen-energies of the septet spin-state
Hamiltonian.
In view of the relatively high amount of the nitro-
substituted tetraradical T-3b in the EPR spectrum of the
irradiated trisazoalkane 3b, the 3D signals of the hexarad-
ical H-3b are less pronounced in the EPR spectrum
(Figure 2b) of the irradiated trisazoalkane 3b compared
to H-3a from 3a (Figure 2a), for which the 3D signals
cm-1 10
(The D and E parameters for the particular
.
polyradical have been coded by subscripts of the respec-
tive structure numbers).
In analogy to the unsubstituted trisazoalkanes 3a , on
further irradiation of the nitro derivative 3b, new signals
also appeared. These occur at almost the same positions
as the EPR signals for the H-3a species and were,
consequently, assigned to the hexaradical H-3b. Since
the symmetry of both hexaradicals H-3a and H-3b is the
same, the E value (0.0002 cm-1) of the unsubstituted
hexaradical H-3a was adopted for the nitro-substituted
H-3b; this considerably simplifies the simulation of the
EPR spectrum for the hexaradical H-3b. In this manner,
for the nitro-substituted hexaradical H-3b the D param-
eter was found to have the value DH-3b ) 0.0082 cm-1
(Figure 2c), which is appreciably lower than for the
unsubstituted H-3a case (see Table 1, entry 4).
The multitude of peaks in the simulated spectrum
(Figure 2c) arises for the following reasons: In a poly-
(18) Kirmse, R.; Stach, J . ESR Spektroskopie - Anwendungen in
der Chemie; Akademie Verlag: Berlin, 1985; pp 39-61.

EPR Determination of Electronic Substituent Effects
J . Org. Chem., Vol. 65, No. 22, 2000 7653
Ta ble 1. Exp er im en ta l Zer o-F ield P a r a m eter s D a n d E
of Su bstitu ted Di-, Tetr a -, a n d Hexa r a d ica ls
F igu r e 1. (a) EPR spectrum of the bisazoalkane 2 in toluene
matrix, assigned to a mixture of the triplet diradical D-2 and
the quintet tetraradical T-2. (b) EPR spectrum of the trisa-
zoalkane 3a in toluene matrix, assigned to a mixture of the
triplet diradical D-3a and the quintet tetraradical T-3a . (c)
EPR spectrum of the trisazoalkane 3b in toluene matrix,
assigned to a mixture of the triplet diradical D-3b and the
quintet tetraradical T-3b (doublet impurities are labeled with
an asterix); characteristic signals of the diradical and tet-
raradical are labeled with D and T.
a
Generated by photolysis of the corresponding azoalkane in a
toluene matrix at 77 K. b D/hc values of the polyradicals, estimated
error ( 0.0001 cm-1
.
c D value of p-nitro-substituted polyradical
d
divided by the one of the unsubstituted polyradical. Reference
7.
monoradical (doublets) impurities, the characteristic EPR
signals for the triplet diradical D-1b were also observed.
Unquestionably, the nitroaryl group served as chro-
mophore for the photochemical bond fission in the hou-
sane; presumably, the strained σ bond is hyperconjugated
with the π system of the aryl ring, which facilitates
fragmentation.22 When this photoactivity of the nitroaryl
chromophore, a potentially attractive photochemical ac-
cess to polyradicals, was attempted to be employed for
the generation of the hexaradical H-3b from the nitro-
substituted trishousane 10b, irradiation with the 351-
nm line of the argon-ion laser (0.64 W) afforded only the
triplet diradical D-10b; the tetraradical T-10b or hexarad-
ical H-3b was not observed even under a variety of
irradiating conditions. Warm-up of the matrix sample
revealed an intractable complex product mixture (mainly
unreacted trishousane 10b) upon 1H NMR spectral
analysis, and further efforts along these lines were
abandoned.
are about as intensive as the neighboring tetraradical
signals T (compare Figures 2b and 2a). This makes an
unequivocal assignment of the nitro-substituted hexarad-
ical H-3b more difficult; however, fortunately the outer-
most 10D signals are more pronounced and unambigu-
ously pertain to the hexaradical H-3b.
P er sisten ce. The polyradical mixtures obtained from
the irradiated trisazoalkanes 3a and 3b stand out for
their remarkable persistence. For example, the nitro-
substituted polyradical mixture was stored for five
months at 77 K, and no significant decay of the triplet,
quintet, or septet EPR signals was observed! Since the
signals of the polyradical mixture from the trisazoalkane
3a still persist at 120 K in a MTHF as well as toluene
matrix,19 these polyradicals are less persistent than the
polybrominated 1,3,5-tris{2-[4-(phenylcarbeno)phenyl]-
ethynyl}benzene20 (survives even at 140 K in MTHF
matrix), but more so than the structurally related 1,3,5-
tris(phenylmethylene)benzene.9a The latter triscarbene
decomposes in rigid glasses already at 85 K.20,21
Discu ssion
The comparison of the absolute D values of the di-,
tetra-, and hexaradicals D-1, D-3, T-3, and H-3 (Table
1) reveals that DT-3 value of the tetraradical T-3 is much
smaller than DD-3 value of the corresponding diradical
D-3 (entries 2 and 3). In contrast, the difference between
the DT-3 and DH-3 values of the respective tetra- and
hexaradicals is marginal (entries 3 and 4). The reduction
of the DT-3 compared to the DD-3 values was expected
since it was already experimentally observed for other
Th e Nitr o Ch r om op h or e for P olyr a d ica l Gen er a -
tion . Irradiation of the nitro-substituted azoalkane 1b
generated the triplet diradical D-1b, which expectedly
cyclized on warm-up to the corresponding housane
(Scheme 2).15 When an independently synthesized nitro-
substituted housane sample was irradiated, apart from
(19) No significant change of the D value was detected when the
sample was measured in the temperature range between 4 and 120
K.
(20) Tomioka, H.; Hattori, M.; Hirai, K.; Sato, K.; Shiomi, D.; Takui,
T.; Itoh, K. J . Am. Chem. Soc. 1998, 120, 1106-1107.
(21) Takui, T. Ph.D. Thesis, Osaka University, 1973.
(22) Michl, J .; Bonacic-Koutecky Electronic Aspects of Organic
Photochemistry; Wiley and Sons: New York, 1990; pp 137-139, 292-
295.

7654 J . Org. Chem., Vol. 65, No. 22, 2000
Adam and Maas
leads to eq 3 for the tetraradical T-3, which consists of
two structurally identical cyclopentanediyl diradical units
with the same zero-field splitting tensor DD-1 and a
m-phenylene-type coupling unit with DFC. For necessity
and convenience we assume that the DFC value is about
the same as that for m-phenylene, i.e., Dm-phenylene
)
0.0110 cm-1 26
.
Since the latter is much smaller than the
DD-1 value (0.0507 cm-1) of the diradical D-1, the DFC
term may be neglected in eq 3, and consequently the DT-3
value of the tetraradical T-3 should be about one-third
of the DD-1 value for the diradical D-1. The experimental
data in Table 1 (entries 1 and 3) for the D parameter
substantiate this expected trend at least qualitatively.
Evidently, the weak spin-spin interaction in the cross-
conjugated m-phenylene diradical, as reflected by the
small Dm-phenylene value, is responsible for the much lower
D value of the tetraradical T-3 compared to the diradical
D-1 as well as D-3.
That the observed DH-3 parameter of the hexaradical
H-3a is a bit smaller than that for the tetraradical T-2
and T-3a (Table 1, entries 3 and 4) is in accord with
theoretical and experimental results for related di- and
triradical pairs (Table 2). In fact, on the basis of theoreti-
cal considerations, it was concluded that the ratio of the
D values for structurally related di- and triradical pairs
should be constant.8a,27 This constancy was confirmed for
a set of 1,3,5-trimethylenebenzene-coupled triradicals8a
and m-phenylene-coupled diradicals,28 for which ratios
of 0.69 and 0.71 were obtained (Table 2). For the pair of
the structurally akin tetraradical T-3 (contains the
m-phenylene-coupled diradical species) and the hexaradi-
cal H-3 (contains the 1,3,5-trimethylenebenzene-coupled
triradical species), the DH-3/DT-3 ratio is 0.78 (Table 2).
This is in excellent agreement with the ratios of above-
mentioned di- and triradicals and corroborates our EPR-
spectral assignment.
F igu r e 2. (a) Superimposed EPR spectra for the irradiation
of the trisazoalkane 3a in toluene matrix as a function of time;
characteristic signals of the diradical D-3, the tetraradical T-3,
and the hexaradical H-3 are labeled by D, T, and H. (b) EPR
spectrum for the irradiation of the trisazoalkane 3b in toluene
matrix. (c) Simulation of the septet spin state for hexaradical
H-3b with DH-3b ) 0.0082 cm-1 and EH-3b ) 0.0002 cm-1
.
m-phenylene-coupled spin-carrying units (carbenes,23 cy-
clobutanediyl,24 and trimethylenemethane25 diradicals)
and also supported by theoretical considerations.23 Thus,
to assess the D parameter for such polyradical species,
Itoh proposed the simple additive relation in eq 2 for the
Hamiltonian H of quintet-spin systems,23 made up of two
The experimental data in Table 2 reveal that the
absolute D values of the polyradical T-3 and H-3 are
larger compared to those corresponding triphenylmethyl-
type di- and triradicals in Table 2. This is due to the
higher spin density at the radical sites of the cyclopen-
tanediyl-type versus the triphenylmethylene-type spin
carriers. In the latter, the spin is more effectively
delocalized over the three adjacent phenyl rings and,
expectedly, the D value should be lower since the D
parameter depends linearly on the spin density (eq 1).6
The interdependence between the experimental D
parameter and the theoretical spin density in eq 1
permits assess to electronic substituent effects in high-
spin-carrying units (SC) that weakly interact through a
ferromagnetic coupler (FC). Application of this formula
(23) (a) Itoh, K. Pure. Appl. Chem. 1978, 50, 1251-1259. (b)
Scaringe, R. P.; Hodgson, D. J .; Hattfield, W. E. Mol. Phys. 1978, 35,
701-713.
(26) Wright, B. B.; Platz M. S. J . Am. Chem. Soc. 1983, 105, 628-
630.
(27) Seidl, H.; Schwoerer, M.; Schmid, D. Z. Phys. 1965, 182, 398-
426.
(28) (a) Kothe, G.; Denkel, K.-H.; Su¨mmermann, W. Angew. Chem.,
Int. Ed. Engl. 1970, 9, 906-907. (b) Hinrichs, K.; Kurreck, H.;
Niemeier, W. Tetrahedron 1974, 30, 315-320.
(24) J acobs, S. J .; Dougherty, D. A. Angew. Chem., Int. Ed. Engl.
1994, 33, 1104-1106.
(25) J acobs, S. J .; Shultz, D. A.; J ain, R.; Novak, J .; Dougherty, D.
A. J . Am. Chem. Soc. 1993, 115, 1744-1753.

EPR Determination of Electronic Substituent Effects
J . Org. Chem., Vol. 65, No. 22, 2000 7655
Sch em e 2. Gen er a tion of th e Nitr o-Su bstitu ted Tr ip let Dir a d ica l D-1b Eith er by Ir r a d ia tion of th e
Azoa lk a n e or th e Hou sa n e w ith th e 351- or 364-n m Lin e of a n Ar gon -Ion La ser
Ta ble 2. Ra tio of D Va lu es for Str u ctu r a lly Rela ted
To interpret the p-nitro substituent effect on the D
value of the tetraradicals T-2 and T-3, eq 3 must again
be considered. Since the p-nitro substituent is placed on
the outer phenyl rings of the spin-carrying units, the
ferromagnetic coupler is the same for the tetraradicals
T-3a and T-3b. As already mentioned, the DFC value of
m-phenylene is much smaller than DD-1a and also DD-1b
and therewith negligible in eq 3. Consequently, the DT-3
value of the tetraradical T-3 is mainly determined by the
DD-1 value of the D-1 diradical unit. The DT-3b/DT-3a ratio
of the p-nitro-substituted (T-3b) and unsubstituted (T-
3a ) tetraradicals should be the same as the DD-1b/DD-1a
ratio of the corresponding D-1b and D-1a diradicals.
Indeed, the data in Table 1 confirm this expectation, since
the ratios are 0.90 and 0.92 (Table 1, entries 1 and 3).
From eq 1 it is evident that the DD-1b/DD-1a ratio is equal
to the ratio of the corresponding spin densities, and
indeed the semiempirically (PM3) calculated value of
0.90830 is in excellent accord with the experimental ratio
of 0.90 (Table 1, entry 1) obtained from the respective D
values.
P olyr a d ica ls
For the hexaradicals H-3a ,b, the DH-3b/DH-3a ratio is
the same value as the DT-3b/DT-3a and DD-3b/DD-3a ratios
for the corresponding tetraradicals T-3a ,b and diradicals
D-3a ,b (Table 1, entries 3 and 4). Therefore, the spin-
accepting p-nitro group exercises the same electronic
influence on the D value of the hexaradical as on the
corresponding tetraradicals and diradicals. This cor-
respondence is expected from eq 3 because the D value
of the tetraradical T-3 and hexaradical H-3 is dominated
by the spin-carrying cyclopentanediyl unit and the influ-
ence of the weakly interacting 1,3-dimethylene- and 1,3,5-
trimethylenebenzene ferromagnetic couplers is negligible.
In summary, it has been demonstrated that the D
parameter of the tetraradicals T-3a ,b and hexaradicals
H-3a ,b is mainly determined by the spin density of the
localized 1,3-cyclopentanediyl spin carrier. Thus, the
contribution by the 1,3-dimethylene- as well as 1,3,5
trimethylenebenzene ferromagnetic coupler is essentially
negligible. Accordingly, the efficacy of spin delocalization
by the spin-accepting p-nitro group is about the same for
the herein examined polyradicals with triplet-, quintet-,
and septet-spin ground states.
spin species through EPR-spectral measurements, as we
have amply demonstrated for the triplet diradicals D-1
with the localized 1,3-cyclopentanediyl spin carriers.6
Similar substituent effects have been probed here; the
results for the p-nitro group are given in Table 1.
Unfortunately, the synthetic chore to prepare additional
aryl-substituted derivatives turned out to be too difficult
through the acetylation-trimerization route (Scheme 1)
to generate the required hexaketone starting materials.
Nevertheless, the p-nitro substituent is one of the stron-
gest radical-stabilizing spin acceptors29 to provide the
answer to our quest.
The results in Table 1 show that the D values of
p-nitro-substituted di-, tetra-, and hexaradicals D-1b,
D-3b, T-3b, and H-3b are significantly smaller than
those of the corresponding unsubstituted polyradicals
D-1a , D-3a , T-3a , and H-3a . Conspicuously, the relative
D values (Table 1, fifth column) for all the listed high-
spin systems are ca. 0.9. This influence of the p-nitro
group on the D parameter has already been reported15
for the simplest representative of high-spin polyradicals,
namely the 1,3-cyclopentanediyl triplet diradical pair
D-1a ,b, which has been explained in terms of the better
delocalization by the spin-accepting p-nitro substituent.
Consequently, compared to the unsubstituted diradicals
D-1a and D-3a , the p-nitro group in the diradicals D-1b
and D-3b diminishes the spin density of the benzyl
position and therewith the D value of the diradical is
correspondingly reduced (Table 1, entries 1 and 2).
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft, the Volkswagenstif-
tung and the Fonds der Chemischen Industrie.
Su p p or tin g In for m a tion Ava ila ble: Synthesis and pho-
tochemistry of the trisazoalkanes 3a ,b, and the EPR spectros-
copy of the polyradicals T-3a ,b and H-3a ,b. This material is
available free of charge in the Internet at http://pubs.acs.org.
J O0012795
(29) (a) Wayner, D. D.; Arnold, D. R Can. J . Chem. 1985, 63, 2378-
2383. (b) Creary, X.; Mehrsheikh-Mohammadi, M. E.; McDonald, S.
J . Org. Chem. 1987, 52, 3254-3263.
(30) Adam, W.; Harrer, H. M.; Kita, F.; Korth, H.-G.; Nau, W. M. J .
Org. Chem. 1997, 62, 1419-1426.