A push-pull 4a,4b-dihydrophenanthrene

Article abstract of DOI:10.1039/b104510p

Irradiation of (Z)-3-cyano-3′-aminostilbene in the absence of oxygen yields a push-pull substituted dihydrophenanthrene with substantial quinoidal character as evidenced by the pronounced solvent dependence of its absorption spectrum.

Full text of DOI:10.1039/b104510p

A push–pull 4a,4b-dihydrophenanthrene†
Frederick D. Lewis,*a Todd L. Kurtha and Rajdeep S. Kalgutkarab
a Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 USA.
E-mail: lewis@chem.northwestern.edu
b Science Research Center, 3M Company, St. Paul, MN 55144, USA
Received (in Corvallis, OR) 17th May 2001, Accepted 12th June 2001
First published as an Advance Article on the web 6th July 2001
Irradiation of (Z)-3-cyano-3A-aminostilbene in the absence of
oxygen yields a push–pull substituted dihydrophenanthrene
with substantial quinoidal character as evidenced by the
pronounced solvent dependence of its absorption spec-
trum.
tion of Ic, its N,N-dimethylamino analog Id, and (Z)-3-amino-
3A-cyanostilbene Ie.
Irradiation of the (Z)-3-aminostilbenes Ic, Id, and Ie in
freeze–pump–thaw degassed cyclohexane (C6H12) or acetoni-
trile (CH3CN) solution promptly followed by acquisition of the
absorption spectrum results in the observation of a broad
absorption band at long wavelengths.18 The deconvoluted
absorption spectra of IIc, IId, and IIe in acetonitrile solution are
shown in Fig. 1. These spectra are obtained by means of singular
value decomposition (SVD)-self modeling from a matrix of
absorption spectra obtained at different conversions (see:
supplementary information). This procedure also establishes the
presence of one major dihydrophenanthrene product along with
the (Z)- and (E)-stilbenes in each case. The major products are
assigned structures IIc–IIe on the basis of formation of the
corresponding 2-aminophenanthrenes IIIc–IIIe as the major
products obtained from the irradiation of aerated solutions. In
the case of Ie irradiation of an aerated solution results in the
formation of IIIe and IVe in a 4.3+1 ratio, but no detectable
4-amino-7-cyano isomer.19 The absence of evidence for the
presence of 4-aminodihydrophenanthrene products under an-
aerobic conditions may reflect more rapid thermal ring opening
of the 4-amino-substituted dihydrophenanthrenes.
The absorption band of IIc (Fig. 1) is rapidly bleached by
visible light but is stable in the dark for a period of several days
in CH3CN solution and undergoes repeated formation and
bleaching without degradation. This permits acquisition of the
1H NMR spectrum of IIc as a mixture with Ic.20 Salient features
of the spectrum include singlets at 3.23 and 3.27 ppm assigned
to the 4a,4b protons and unresolved multiplets between 5–7
ppm assigned to the vinyl protons.3 Optimum conversions of Ic
to IIc are obtained using monochromatic 276 nm irradiation as
observed for conversion of Ia to IIa by Muszkat and Fischer.21
The absorption bands of IId and IIe bleach rapidly in the dark
and thus no attempt was made to obtain their NMR spectra.
Absorption spectral data for the (Z)-stilbenes Ia–Ic, and Ie in
C6H12 and CH3CN are summarized in Table 1. The presence of
3-amino substituents in Ic and Ie results in a large red-shift in
their absorption bands compared to that of Ia. Similar red-shifts
are observed for the trans isomers of Ic and Id and are attributed
The photocyclization of diarylethenes is among the most
extensively studied of photochromic reactions.1–5 The proto-
type for this reaction is the photocyclization of the colorless (Z)-
stilbene Ia to the yellow 4a,4b-dihydrophenanthrene IIa, which
is rapidly converted to phenanthrene in the presence of oxygen
(Scheme 1). In the absence of oxygen IIa undergoes ring
opening to reform Ia both thermally and photochemically. Lack
of thermal stability and sensitivity to oxygen has limited the
utility of stilbene photocyclization in photochromic applica-
tions. The search for photochromic molecules which are
thermally stable even in the presence of oxygen has led to the
development of diarylethenes with heterocyclic aryl groups.5,6
The presence of an electron donating group on one heterocycle
and an electron accepting group on the other has provided
photochromic systems that absorb throughout the visible and
into the infrared. Curiously, the photochemical generation of
push–pull stabilized 4a,4b-dihydrophenanthrenes has not been
investigated.
The effects of single electron donating or accepting sub-
stituents on the quantum yield for phenanthrene formation (Fp)
from (Z)-stilbenes in the presence of oxygen was investigated
by Güsten and co-workers.7,8 They report higher values of Fp
for 3-cyanostilbene Ib and 3-aminostilbene Ic (0.012 and 0.28,
respectively) than for their 4-substituted isomers. Irradiation of
either Ib or Ic in the presence of oxidizing agents results in the
formation of both 2- and 4-substituted phenanthrenes (III and
IV), in a ca. 3.5+1 ratio. Muszkat et al.9 have investigated the
anaerobic irradiation of several monosubstituted (Z)-stilbenes
and report a higher quantum yield for formation of dihy-
drophenanthrene IIb from Ib (0.012) than for its 4-substituted
isomer. The photochemistry of a large number of push–pull
4,4A-disubstituted stilbenes including 4-cyano-4A-aminostilbene
has also been investigated.10–13 These stilbenes undergo
reversible trans–cis photoisomerization. However, to our
knowledge there have been no reports of their photocyclization.
As part of our study of the photochemical behavior of
aminostilbenes,14–17 we have investigated the anaerobic irradia-
Scheme 1
† Electronic supplementary information (ESI) available: electronic absorp-
tion spectral matrices for the irradiated aminostilbenes and SVD analyses (6
figures). See http://www.rsc.org/suppdata/cc/b1/b104510p/
Fig. 1 Deconvoluted absorption spectra of the aminodihydrophenanthrenes
IIc, IId, and IIe in acetonitrile solution.
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Chem. Commun., 2001, 1372–1373
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104510p

Table 1 Absorption spectral data and calculated dipole moments for (Z)-
stilbenes and dihydrophenanthrenes
contributions of quinoidal resonance structures to both the
ground and excited states [eqn. (3)]. These values are larger than
lmax/nm
lmax/nm
(CH3CN)
Compd.
(C6H12
)
mg/Da
meFC/Db
(3)
Ia
276
277
315
317
450
467
470
482
530
276
those for the corresponding 4- or 4,4A-substituted (E)-stilbenes,
even though the distance between the donor and acceptor
substituents is smaller for the dihydrophenanthrenes. Thus the
extent of charge separation must be significantly larger for the
dihydrophenanthrenes. Photocyclization of push–pull 4,4A-
substituted stilbenes would result in the formation of 3,3A-
substituted dihydrophenanthrenes for which quinoidal reso-
nance is not possible. This may explain the absence of reports of
photocyclization of these stilbenes. In addition to providing the
first example of a simple push–pull stabilized dihydrophenan-
threne, these results provide a design principle which may prove
useful in the development of new photochromic systems. While
the dihydrophenanthrene IIe reacts with oxygen, its stability in
the absence of oxygen, ease of synthesis and compact molecular
structure may provide advantages for Ie when compared to
more complex photochromic molecules such as If.5,6
Ibc
Icd
Ie
320
328
450
IIae
IIbe
IIcd
IId
IIe
0.7
0.7
482
494
578
2.4
2.1
7.2
11.4
11.7
16.0
a Calculated ground state dipole moment. b Dipole moment of the Franck–
Condon singlet state calculated using eqn. (2). c Data from ref. 8. d Data
from ref. 17. e Data from ref. 3.
to splitting of the long-wavelength band which results from
configuration interaction.15,16 Much smaller shifts are observed
for the 3-cyano substituents in Ib (vs. Ia) and Ie (vs. Ic).9 Also
reported in Table 1 are the absorption maxima for the
dihydrophenanthrenes IIa–IIe. The substituent-induced red-
shifts are larger than those observed for the corresponding (Z)-
stilbenes and similar shifts are observed for the 3-amino and
3-cyano substituents in IIb and IIc. The shift for IId is larger
than that for IIc, as previously observed for (E)-3-dimethyl-
amino- vs. 3-aminostilbenes.16 The aminodihydrophenan-
threnes IIc and IId display large solvent-induced red-shifts in
CH3CN vs. C6H12 solution and an exceptionally large red-shift
is observed for IIe (1600 cm21). This shift is nearly as large as
that reported for IIf, the ring-closed form of the more elaborate
diarylethene If (eqn. (1), Dn ≈ 2000 cm21 in DMSO vs.
hexane).6
Funding for this project was provided by NSF grants CHE-
9734941 and CHE-0100596. We thank Dr Wilfried Weigel for
providing a sample of Id.
Notes and references
1 J. Saltiel and J. L. Charlton, in Rearrangements in Ground and Excited
States, ed. P. de Mayo, Academic Press, New York, 1980.
2 J. Saltiel and Y.-P. Sun, in Photochromism, Molecules and Systems, ed.
H. Durr and H. Bouas-Laurent, Elsevier, Amsterdam, 1990.
3 K. A. Muszkat, Top. Curr. Chem., 1980, 88, 89.
4 F. B. Mallory and C. W. Mallory, in Organic Reactions, John Wiley and
Sons, Inc., New York, 1984.
5 M. Irie, Chem. Rev., 2000, 100, 1685.
6 S. L. Gilat, S. H. Kawai and J.-M. Lehn, Chem. Eur. J., 1995, 1, 275.
7 H. Güsten and L. Klasinc, Tetrahedron, 1968, 24, 5499.
8 H. Jungmann, H. Güsten and D. Schulte-Frohlinde, Chem. Ber., 1968,
101, 2690.
(1)
9 K. A. Muszkat, H. Kessel and S. Sharafi-Ozeri, Israel J. Chem., 1977,
10, 765.
10 J.-F. Letard, R. Lapouyade and W. Rettig, J. Am. Chem. Soc., 1993, 115,
2441.
11 J.-F. Letard, R. Lapouyade and W. Rettig, Chem. Phys. Lett., 1994, 222,
209.
12 V. Il’ichev, W. Kuehnle and K. A. Zachariasse, Chem. Phys., 1996, 211,
441.
13 Y. V. Il’ichev and K. A. Zachariasse, Ber. Bunsenges. Phys. Chem.,
1997, 101, 625.
The solvent-induced red-shift for the absorption maximum
IIe is exceptionally large for an uncharged molecule, indicative
of both a large ground state dipole moment and a change in the
dipole moment upon electronic excitation. Ground state dipole
moments calculated using the INDO/S-SCF-CI (ZINDO)
algorithm22 for IIa and IIc–IIe are reported in Table 1. The
values calculated for IIc and IId are larger than that for (E)-
4-dimethylaminostilbene (mg = 1.8 D) and the value calculated
for IIe is larger than that for (E)-4-dimethylamino-4A-cyano-
stilbene (mg = 6.6 D).16 The dipole moment of the Franck–
Condon excited state (meFC) may be estimated by using the
Lippert equation [eqn. (2)]
14 F. D. Lewis and J.-S. Yang, J. Am. Chem. Soc., 1997, 119, 3834.
15 F. D. Lewis, R. S. Kalgutkar and J.-S. Yang, J. Am. Chem. Soc., 1999,
121, 12045.
16 F. D. Lewis and W. Weigel, J. Phys. Chem. A, 2000, 104, 8146.
17 F. D. Lewis and R. S. Kalgutkar, J. Phys. Chem. A, 2001, 105, 285.
18 Typical experiments used 50 mM–5 mM solutions that were degassed by
five freeze–pump–thaw cycles at < 0.1 mTorr followed by flame sealing
of the cuvet. Irradiation was carried out in a Rayonet RPR-100
photochemical reactor fitted with RPR-3000A bulbs. All UV–Vis
spectra were promptly acquired on a Hewlett-Packard HP8452A diode
array spectrometer.
e -1
n2 -1
2mg(me - mg )
(2)
na = -
Df; Df =
-
4pe0hca3
2e +1 2n2 +1
19 1H NMR of IIIe (500 MHz, CDCl3) d: 7.08 (d, 2.3 Hz), 7.11 (dd, 8.8,
2.3 Hz), 7.63 (2H, s), 7.74 (dd, 8.6, 1.6 Hz), 8.14 (d, 1.6 Hz), 8.46 (d,
8.6), 8.56 (d, 8.8). IVe: 7.08 (d, 2.5 Hz), 7.12 (dd, 8.5, 2.5 Hz), 7.49 (dd,
7.1, 7.6 Hz), 7.61 (2H, s), 7.98 (dd, 7.1, 1.7 Hz), 8.01 (dd, 7.6, 1.7 Hz),
9.59 (d, 8.5 Hz).
20 1H NMR of IIc (300 MHz, CD3CN) d: 3.23 (s), 3.27 (s), 5.38 (s), 5.67
(m), 5.77 (m), 5.98 (m), 6.25 (m). Signals of IIc downfield from 6.5 ppm
were unresolved from those of the cis and trans isomers of Ic.
21 K. A. Muszkat and E. Fischer, J. Chem. Soc., 1967, 662.
22 Cache Release 3.5 reference, Oxford Molecular Group, Inc., Campbell,
CA 95008. (800) 876-9994. http://www.oxmol.com.
23 E. Z. Lippert, Z. Naturforsch., Teil A, 1955, 10, 541.
where n¯a is the absorption maximum, h is Planck’s constant, e0
is the vacuum permittivity, c is the speed of light, a is the radius
of the solvent cavity, e is the relative permittivity, and n is the
solvent refractive index.23 Calculated values of meFC for IIc and
IId (Table 1) are larger than that for (E)-4-dimethylamino-
stilbene (me = 7.7 D)16 and the calculated value for IIe is
FC
much larger than that for (E)-4-dimethylamino-4A-cyanos-
tilbene (me = 9.0 D).12
FC
The relatively large ground and excited state dipole moments
of the aminodihydrophenanthrenes IIc–IIe reflect the important
Chem. Commun., 2001, 1372–1373
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