Pyrene-dihydrophenazine bis(radical cation) in a singlet ground state

Article abstract of DOI:10.1021/ol9009927

A new pyrene-dihydrophenazine dyad was prepared. Oxidation of the neutral species produced a bis(radical cation) species, which was characterized by the absorptions of their component radical cations In the visible region. A thermally accessible triplet state was observed In the ESR measurement In frozen n-PrCN. The energy gap between the singlet and triplet states was determined to be 2J/k_B = -36 ± 3 K.

Full text of DOI:10.1021/ol9009927

ORGANIC
LETTERS
2009
Vol. 11, No. 13
2816-2818
Pyrene-Dihydrophenazine Bis(Radical
Cation) in a Singlet Ground State
Shuichi Suzuki, Takuma Takeda, Masato Kuratsu, Masatoshi Kozaki,
Kazunobu Sato, Daisuke Shiomi, Takeji Takui, and Keiji Okada*
Departments of Chemistry and Materials Science, Graduate School of Science,
Osaka City UniVersity, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
okadak@sci.osaka-cu.ac.jp
Received May 6, 2009
ABSTRACT
A new pyrene-dihydrophenazine dyad was prepared. Oxidation of the neutral species produced a bis(radical cation) species, which was
characterized by the absorptions of their component radical cations in the visible region. A thermally accessible triplet state was observed in
the ESR measurement in frozen n-PrCN. The energy gap between the singlet and triplet states was determined to be 2J/kB ) -36 ( 3 K.
Pyrene (1) is a unique alternant hydrocarbon with a nodal
plane σ passing through the C2 and C7 carbon atoms in the
HOMO (highest occupied molecular orbital) and LUMO
(lowest unoccupied molecular orbital). These carbon sites
in the pyrene radical ions are occupied by negative spin
densities (Figure 1). Because of this nature, 2,2′-bipyrene
a small S-T (singlet-triplet) energy gap.1a The cationic
version of pyrene derivatives has been theoretically inves-
tigated in previous studies. Karabunarliev and Baumgarten
reported that 1,2′-bipyrene and 2-(anthracen-9-yl)pyrene
bis(radical cation) possess triplet ground states using AM1-
ROHF calculations with extended CI (configuration inter-
action).1b The calculated S-T gap was rather small, +97 cal/
mol (+49 K) for 2-(anthracen-9-yl)pyrene bis(radical cation).
There have been no experimental reports on bis(radical
cation) of pyrene derivatives, probably because of the high
oxidation potential of pyrene and the poor availability of
2-substituted pyrene derivatives.
In connection with our studies on the development of ionic
high-spin molecules2 as spin-building blocks of molecular
magnets,3 we decided to investigate bis(radical cation) of
pyrene derivatives. In this paper, we report the synthesis and
Figure 1. Structures of 1 and 2 with pictures of (a) KS
(1) (a) Kreyenschmidt, M.; Baumgarten, M.; Tyutyulkov, N.; Mu¨llen,
K. Angew. Chem., Int. Ed. Engl. 1994, 33, 1957–1959. (b) Karabunarliev,
S.; Baumgarten, M. Chem. Phys. 2000, 254, 239–247.
(Kohn-Sham)-HOMO of 1 and (b) spin density distribution of 1•+
(blue and green colors denote positive and negative spin densities,
respectively).
(2) (a) Masuda, Y.; Kuratsu, M.; Suzuki, S.; Kozaki, M.; Shiomi, D.;
Sato, K.; Takui, T.; Hosokoshi, Y.; Lan, X.-Z.; Miyazaki, Y.; Inaba, A.;
Okada, K. J. Am. Chem. Soc. 2009, 131, 4670–4673. (b) Hiraoka, S.;
Okamoto, T.; Kozaki, M.; Shiomi, D.; Sato, K.; Takui, T.; Okada, K. J. Am.
Chem. Soc. 2004, 126, 58–59. (c) Terada, E.; Okamoto, T.; Kozaki, M.;
Masaki, M. E.; Shiomi, D.; Sato, K.; Takui, T.; Okada, K. J. Org. Chem.
2005, 70, 10073–10081.
bis(radical anion) has been experimentally shown to have a
relatively stable triplet state with a singlet ground state with
10.1021/ol9009927 CCC: $40.75
Published on Web 06/11/2009
 2009 American Chemical Society

two-electron (2e) oxidation of a novel pyrene-dihydrophena-
zine dyad 2. The bis(radical cation) 22(·+) is shown to be in
a singlet ground state with an S-T gap of 2J/kB ) -36 K, H
) -2JS1·S2.
Compound 2 was prepared from 2-nitropyrene (3),4 as
shown in Scheme 1. Bromination (bromine, 2.7 equiv,
Scheme 1. Syntheses of 2 and 8
Figure 2. Crystal structure of 2; (a) top and (b) side views. The
hydrogen atoms and the incorporated solvents (2CH2Cl2) were
omitted for clarity.
to the dihydrophenazine plane in the neutral state. A model
compound, 2-mesitylpyrene (8) was also prepared.
In the cyclic voltammogram of 2, two reversible oxidation
waves were observed at Eox1 ) -0.27 and Eox2 ) -0.07 V
vs Fc/Fc+ (Figure S2 and Table S2, Supporting Information).
These values are comparable to those of reference com-
pounds 8 (Eox ) -0.22 V vs Fc/Fc+) and 5,10-diphenyl-
5,10-dihydrophenazine (9) (Eox ) -0.24 V vs Fc/Fc+),
suggesting that Eox1 and Eox2 of 2 are attributed to the
oxidation of the dihydrophenazine and pyrene moieties,
respectively. The considerable shift of Eox2 in the positive
direction compared to Eox ) -0.22 V of 8 is attributed to
the inductive effect of the dihydrophenazine radical cation
moiety in 2·+. Electrochemical oxidation was carried out in
a stepwise manner in CH2Cl2 at suitable applied voltages
(-0.06 and +0.14 V vs Fc/Fc+, Figure S3, Supporting
Information, for the spectral change). The final spectrum in
the stationary state for the 2e oxidation state is denoted by
a black line in Figure 3. The molar absorptivity is an
approximate value assuming complete conversion. Figure 3
also shows the electrochemical oxidation of the model
compounds (the red line for 8·+, the blue line for 9·+). The
observed spectrum (262, 350, 437, 470, and 550-800 nm)
for the 2e oxidation state of 2 is almost superimposed with
the spectra of the components (348, 444, and 500-700 nm
for 1e oxidation of 8 and 262, 373, 444, 472, and 600-800
nm for 1e oxidation of 9). This observation reflects the node
of pyrene in the HOMO and the perpendicular geometry that
is probably maintained in the 2e oxidation state. On the other
hand, an entirely new absorption may be expected if the
orbitals of the two radical cations are directly overlapped
yielding, for instance, a quinoid type electronic structure.
90-120 °C for 2 h) of 3 in nitrobenzene gave 4 in moderate
yield. The cross-coupling of 4 with pyrrolidine was achieved
by using Pd2(dba)3 and BINAP (2,2′-bis(diphenylphosphino)-
1,1′-binaphthalene) as catalysts,5 giving 5 in good yield.
Reduction of the nitro group 5 to an amino group followed
by a Sandmeyer reaction with CuBr afforded bromide 7.
Compound 7 was successfully transformed into the target
compound 2 following our procedure for the synthesis of
unsymmetric 5,10-diaryldihydrophenazine derivatives through
a Pd(0)-mediated cross coupling.6 The structure of 2 was
confirmed by NMR and HRMS.7 Figure 2 shows the
molecular structure of 2 determined by X-ray analysis (Figure
2, Table S1, Supporting Information).8 Although the R-value
is considerably large because of the disorder in the pyrro-
lidine moieties and the incorporated CH2Cl2 molecules, it is
quite apparent that the pyrene plane was almost perpendicular
(3) (a) Magnetic Properties of Organic Materials; Lahti, P. M., Ed.;
Marcel Dekker: New York, 1999. (b) Molecular Magnetism; Itoh, K.,
Kinoshita, M., Eds.; Kodansha, and Gordon and Breach Science Publishers:
Tokyo, 2000. (c) Magnetism: Molecules to Materials; Miller, J. S.; Drillon,
M., Eds.; Wiley-VCH: New York, 2001. (d) Hicks, R. G. Org. Biomol.
Chem. 2007, 5, 1321–1338.
(4) Miller, D. W.; Herreno-Saenz, D.; Huang, K. H.; Heinze, T. M.;
Fu, P. P. J. Org. Chem. 1992, 57, 3746–3748.
(5) Witulski, B.; Senft, S.; Thum, A. Synlett 1998, 504–506.
(6) Okamoto, T.; Terada, E.; Kozaki, M.; Uchida, M.; Kikukawa, S.;
Okada, K. Org. Lett. 2003, 5, 373–376.
(7) Selected physical data of 2: mp 279 °C (dec); 1H NMR (400 MHz,
DMSO-d6) δ 8.24 (d, J ) 9.28 Hz, 2H), 7.89 (s, 2H), 7.75 (d, J ) 9.28 Hz,
2H), 7.73 (t, J ) 7.8 Hz, 2H), 7.56 (t, J ) 7.58 Hz, 1H), 7.47 (d, J ) 7.08
Hz, 2H), 7.09 (s, 1H), 6.28-6.19 (m, 4H), 5.55-5.52 (m, 4H), 3.58 (br,
8H), 2.03 (br, 8H) ppm; 13C NMR (100 MHz, C6D6) δ (ppm) 147.35, 141.45,
138.56, 137.93, 137.25, 136.20, 131.79, 131.34, 128.91, 128.50, 126.70,
125.36, 124.96, 122.32, 121.60, 121.20, 118.04, 113.77, 113.11, 104,18,
53.19, 25.42 ppm; HRMS (FAB+) m/z Calcd. for C42H36N4: 596.2940.
Found: 596.2950.
(8) Crystallographic data for 2·(CH2Cl2)2: monoclinic; space group P21/m
(#11); a ) 12.2153(17) Å, b ) 12.1148(14) Å, c ) 12.4071(17) Å, ꢀ )
101.044(7)°; V ) 1802.1(4) Å3; Z ) 2; Fcalcd ) 1.413 g cm-3; T ) 93 K;
R ) 0.0900, Rw ) 0.1079, GOF ) 1.467. The crystallographic data is given
in the Supporting Information and has been deposited with the Cambridge
Crystallographic Data Centre (CCDC 730413).
Org. Lett., Vol. 11, No. 13, 2009
2817

Figure 3
.
UV-vis spectra of the electrochemically generated 22(·+)
(black line), 8·+ (red line), and 9·+·ClO4 (blue line) in CH2Cl2.
-
Thus, the spectrum observed for 2e oxidation of 2 is assigned
to an open shell structure for 22(·+)
.
The 2e oxidation state 22(·+) was also accessible by
chemical oxidation with tris(p-bromophenyl)ammonium per-
-
chlorate. The radical ion salt 22(·+)·2ClO4 was obtained as
a green powder. The powder showed an absorption spectrum
similar to that with the electrochemical oxidation. Figure 4a
shows the ESR spectrum of the powder sample 22(·+) in a
butyronitrile matrix at 123 K. Fine-structured triplet-state
ESR signals (∆Ms ) ( 1) with an axial symmetry were
distinctly observed in addition to five intense central lines
due to monoradical impurities with a dihydrophenazine
radical cation structure. The strong signals from the impuri-
ties may be a reflection of sharper lines of the monoradicals
and thermally accessible triplet state (Vide infra). The spectral
simulation yields spin-Hamiltonian parameters as S ) 1, gx
) 2.0043, gy ) 2.0023, gz ) 2.0023, |D|/hc ) 0.00342 cm-1,
and |E|/hc ≈ 0 cm-1. The distance between two spin systems
was calculated to be ca. 7.2 Å from the D value using the
point dipole approximation. The calculated distance was
slightly longer than the distance between the centers of
the dihydrophenazine moiety and the pyrene moiety (6.4 Å).
The longer distance is probably due to the presence of
pyrrolidine substitutents. Figure 4b shows the temperature
dependence of the signal intensity of the forbidden transition
signals (∆Ms ) (2). The temperature dependence curve was
convex. Simulation of the temperature-dependent signal
intensities with the S-T model indicates a singlet ground state
with 2J/kB ) -36 ( 3 K.
Figure 4. (a) Observed ESR spectrum (solid line) of chemically
generated 22(·+) in PrCN at 123 K and its simulated spectrum (red
dashed line). (b) Temperature dependence of the intesties of the
forbidden transition of 22(·+) (9). (Inset) ESR spectrum of the
forbidden trasition at 40 K. Red solid line: calculated value using
the S-T model with 2J/kB ) -36 ( 3 K.
shell species would have a much higher energy. Studies on
pyrene-donor systems in which the π-systems have small
torsion angles are being carried out. These studies will reveal
the general aspects of this subject.
Acknowledgment. S.S. acknowledges the financial sup-
port provided by Izumi Science and Technology Foundation,
and The Circle for the Promotion of Science and Engineering.
Supporting Information Available: Detailed synthetic
procedures with compound data, X-ray crystallographic data
(Table 1), maps showing the HOMO and the spin density
for 1,3-di(pyrrolidine-1-yl)pyrene (Figure S1), a chart of
cyclic voltammogram of 2 (Figure S2) and redox data of 2,
8, and 9 (Table S2), and UV-vis spectral change during
the electrochemical oxidation of 2 and the model compounds
(Figure S3). This material is available free of charge via the
Internet at http://pubs.acs.org.
This study elucidates the exchange interaction of the highly
twisted pyrene-dihydrophenazine bis(radical cation), which
is in the singlet ground state with 2J/kB ) -36 ( 3 K. The
electronic absorption spectrum is characterized as a spectral
summation of the component radical cations, consistent with
an open shell structure. The conceivable quinoid-type closed
OL9009927
2818
Org. Lett., Vol. 11, No. 13, 2009