Photoisomerization and thermal isomerization of shuttlecock- and bowl-equipped (phenylazo)pyridines

Article abstract of DOI:10.1021/jp909922y

The photoisomerization and thermal isomerization of sterically hindered (phenylazo)pyridine derivatives with a shuttlecock-shaped (TbetNNPy) or a bowl-shaped framework (BmtNNPy as well as the azoxy counterpart, BmtNN(O)Py) have been investigated. The crystal structures of these compounds revealed a planar conformation of the (phenylazo)pyridine moiety for TbetNNPy and severely distorted conformations for BmtNNPy and BmtNN(O)Py. The quantum yields of the trans-to-cis photoisomerization of TbetNNPy and BmtNNPy is lower than those of unsubstituted (phenylazo)pyridines. The low quantum yields may, to a large part, be attributed to electronic factors rather than steric factors. While the shuttlecock framework in TbetNNPy does not affect the thermal cis-to-trans isomerization, as the activation parameters for TbetNNPy are quite similar to those of azobenzene and azopyridine derivatives, the bowl framework in BmtNNPy renders the thermal isomerization process slower by lowering the frequency factor to an extent that more than compensates for the lowered activation energy. The process is characterized with a large negative activation entropy and a small activation enthalpy, implying that the isomerization proceeds through a limited range of intermediates stabilized by the presence of the bowl framework.

Full text of DOI:10.1021/jp909922y

884
J. Phys. Chem. A 2010, 114, 884–890
Photoisomerization and Thermal Isomerization of Shuttlecock- and Bowl-Equipped
(Phenylazo)pyridines
Kazuya Suwa,† Joe Otsuki,*,† and Kei Goto‡
College of Science and Technology, Nihon UniVersity, 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308,
Japan, and InteractiVe Research Center of Science, Graduate School of Science and Engineering, Tokyo
Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
ReceiVed: October 16, 2009; ReVised Manuscript ReceiVed: NoVember 27, 2009
The photoisomerization and thermal isomerization of sterically hindered (phenylazo)pyridine derivatives with
a shuttlecock-shaped (TbetNNPy) or a bowl-shaped framework (BmtNNPy as well as the azoxy counterpart,
BmtNN(O)Py) have been investigated. The crystal structures of these compounds revealed a planar
conformation of the (phenylazo)pyridine moiety for TbetNNPy and severely distorted conformations for
BmtNNPy and BmtNN(O)Py. The quantum yields of the trans-to-cis photoisomerization of TbetNNPy and
BmtNNPy is lower than those of unsubstituted (phenylazo)pyridines. The low quantum yields may, to a large
part, be attributed to electronic factors rather than steric factors. While the shuttlecock framework in TbetNNPy
does not affect the thermal cis-to-trans isomerization, as the activation parameters for TbetNNPy are quite
similar to those of azobenzene and azopyridine derivatives, the bowl framework in BmtNNPy renders the
thermal isomerization process slower by lowering the frequency factor to an extent that more than compensates
for the lowered activation energy. The process is characterized with a large negative activation entropy and
a small activation enthalpy, implying that the isomerization proceeds through a limited range of intermediates
stabilized by the presence of the bowl framework.
Introduction
CHART 1: 4-(Phenylazo)pyridine Derivatives Equipped
with a Shuttlecock or Bowl-Shaped Framework
Azobenzene is a representative photochromic compound,
which shows reversible trans/cis photoisomerization.1 Its deriva-
tives have been widely studied to be applied to molecular
switches and devices taking advantage of differences in proper-
ties between the two isomers, such as structures, absorption
spectra and dipole moments.2-5 (Phenylazo)pyridine, which
bears a pyridine ring in place of one of the phenyl rings of
azobenzene, has been extensively utilized in the field of
coordination chemistry owing to its coordination ability to a
metal ion.6,7 For (phenylazo)pyridine derivatives whose trans
and cis isomers experience steric hindrance to different extents,
the coordination ability of the pyridine moiety to metal-
porphyrin can be changed by the photoisomerization. Light-
driven on-off switching systems for fluorescence were built
up making use of these affinity changes.8,9 Fluorescence is
switched on when one of the isomers of the azopyridine
derivative with the lower affinity dissociates from the zinc-
porphyrin and is switched off when the other isomer with the
higher affinity, which is produced by photoirradiation, binds to
the zinc-porphyrin. Chemical reactions catalyzed by an
aluminum-porphyrin such as carbon dioxide fixation were
controlled by photoirradiation with the same principle,10 using
stilbazole derivatives as a closely related class of compounds.
In a previous report,11 we designed and synthesized 4-(phe-
nylazo)pyridine (4-PhNNPy) derivatives (Chart 1) bearing a
shuttlecock-shaped framework, a tetrakis(tert-butylethynyl)-1,1′:
3′,1′′-terphenyl group (TbetNNPy) or a bowl-shaped framework,
a 4-tert-butyl-2,6-bis[(2,2′′,6,6′′-tetramethyl-m-terphenyl-2′-yl-
)methyl]phenyl group (BmtNNPy). The bowl-shaped framework
has been developed by the Goto group.12,13 Incorporation of a
highly reactive chemical species in the bowl-shaped framework
suppresses reactions that would occur rapidly without the
framework, which makes it possible to reveal the structure and
properties of the reactive species. We expected that incorporation
of a (phenylazo)pyridine moiety into the shuttlecock or the bowl-
shaped framework would widen the difference in coordination
ability between the trans and cis isomers if a molecule can be
so designed that the pyridine-N of the cis isomer is covered by
the framework while that of the trans isomer protrudes out of
the framework (Figure 1). In fact, it was found that the affinities
to zinc-tetraphenylporphyrin of the trans and cis isomers of
TbetNNPy and BmtNNPy depend heavily on whether they are
* Corresponding author. E-mail: otsuki@chem.cst.nihon-u.ac.jp.
† Nihon University.
‡ Tokyo Institute of Technology.
10.1021/jp909922y  2010 American Chemical Society
Published on Web 12/17/2009

Shuttlecock- and Bowl-Equipped (Phenylazo)pyridines
J. Phys. Chem. A, Vol. 114, No. 2, 2010 885
Figure 1. Supramolecular photoswitch using 4-(phenylazo)pyridine
derivatives bearing a shuttlecock- or bowl-shaped framework.
in the trans configuration or in the cis configuration, as published
in the previous report.11
To further develop photoresponsive molecular switches based
on the above-mentioned principle using (phenylazo)pyridine
derivatives with bulky substituents, it is important to reveal the
effects of the bulky substituents on the photochromic behavior.
However, it is not well understood how photochromic properties
and thermal isomerization of azobenzene and azopyridine
derivatives are influenced by bulky groups around the azo
moiety.6,8,14-20 In this work, we have obtained the crystal
structures of TbetNNPy and BmtNNPy and its azoxy congener,
BmtNN(O)Py (Chart 1), which allow us to discuss the effect
of bulky substituents on the photochromic properties and thermal
isomerization on a solid basis. Photochromic and thermal
isomerization processes were studied by means of UV-vis and
1H NMR spectroscopies and are discussed in terms of the impact
exerted by the bulky substituents.
Results
Crystal Structures. Single crystals of the trans isomers of
TbetNNPy, BmtNNPy, and BmtNN(O)Py suitable for X-ray
analysis were obtained from CH2Cl2-CH3CN solutions by
allowing the solvent to evaporate slowly. TbetNNPy crystallized
in orthorhombic space group Pbca with eight molecules per
unit cell, while BmtNNPy and BmtNN(O)Py crystallized in
j
triclinic space group P1 with two and four molecules per unit
cell, respectively. The crystal of BmtNN(O)Py includes two
independent molecules in the asymmetric unit. The ORTEP plots
are shown in Figure 2 and the crystallography data and
representative bond lengths and angles are listed in Tables 1
and 2, respectively.
Figure 2. ORTEP plots (50% probability) of (a) TbetNNPy, (b)
BmtNNPy, and (c) BmtNN(O)Py (one of the two independent mol-
ecules).
The lengths of the NdN bond in TbetNNPy and BmtNNPy
are 1.250(4) and 1.2527(17) Å, respectively. These lengths are
similar to those in azobenzene (1.247(2) Å),21 4,4′-azobispyri-
dine (1.2513(15) Å),22 and some 4-(phenylazo)pyridine deriva-
tives (1.260-1.261 Å).23 The lengths of the NdN and NfO
bonds of BmtNN(O)Py are 1.28 (average) and 1.26 (average)
Å, respectively, which are similar to those of 4-(phenylazoxy-
)pyridine N-oxide (1.274(2) (average) and 1.265(2) (average)
Å, respectively),24 indicating that the lengths of the NdN and
NfO bonds in these compounds are not influenced by the
presence of these bulky substituents.
the crystal structures that TbetNNPy has rather an open structure
with the divergent substituents, while BmtNNPy has a more
compact structure with the substituents wrapping around the
central azobenzene moiety. As we could not obtain crystals of
cis isomers, molecular mechanics (MM3) calculations were
carried out to predict the structures of the cis isomers of
TbetNNPy and BmtNNPy (Figure 3). The pyridine moieties of
these cis isomers appear to be covered by the shuttlecock- and
bowl frameworks. It is particularly noteworthy that the pyridyl
groups of BmtNNPy are embedded in the cleft made by the
two terphenyl groups.
The two benzene rings are exactly coplanar in azobenzene
in the crystal.21 The pyridine and benzene rings in the (pheny-
lazo)pyridine moiety in TbetNNPy are also nearly coplanar with
the dihedral angle of 2.30°, whereas the corresponding angles
in BmtNNPy and BmtNN(O)Py are approximately 60°. The
large twist angle for the latter indicates a severe steric demand
imposed by the bowl-shaped Bmt framework. It is seen from
Absorption Spectra and Photochromism. The absorption
spectrum of 4-PhNNPy is characterized by a large ππ* band in
the UV region (309 nm) and a small nπ* band in the visible
region (453 nm).14 For trans-TbetNNPy, the ππ* and nπ* band
appear at ∼330 (6300 M-1 cm-1) and ∼450 nm (220 M-1
cm-1), respectively, in toluene. For trans-BmtNNPy, these bands
appear at 348 (2700 M-1 cm-1) and ∼470 nm (110 M-1 cm-1),

886 J. Phys. Chem. A, Vol. 114, No. 2, 2010
Suwa et al.
TABLE 1: Crystallographic Data for TbetNNPy, BmtNNPy, and BmtNN(O)Py
compound
TbetNNPy
BmtNNPy
BmtNN(O)Py
C61H61N3O
formula
fw
C47H49N3
C61H61N3
655.89
836.13
852.13
crystal size/mm
crystal system
space group
a/Å
b/Å
c/Å
0.30 × 0.20 × 0.15
0.45 × 0.20 × 0.15
0.50 × 0.40 × 0.40
orthorhombic
triclinic
triclinic
j
j
Pbca
11.408(2)
25.189(5)
28.093(5)
90
90
90
8073(3)
P1
P1
11.4828(14)
12.8037(16)
17.557(2)
103.9375(17)
103.8370(8)
98.1698(14)
2377.4(5)
2
13.967(8)
17.661(9)
20.091(11)
95.541(9)
99.166(9)
96.727(9)
4825(4)
R/deg
ꢀ/deg
γ/deg
V/Å3
Z
8
4
Dcalcd/mg m-3
temp/K
1.079
123(2)
1.168
123(2)
1.173
120(2)
refinement method
radiation
monochromator
2θmax/deg
no. of rflns collcd
no. of indep rflns
no. of rflns obsd (>2σ)
R1 (I > 2σ(I))
wR2 (I > 2σ(I))
R1 (all data)
wR2 (all data)
no. of params
GOF
RIGAKU MercuryCCD
RIGAKU MercuryCCD
Mo KR (λ ) 0.710 75 Å)
graphite
50.00
15955
8139 [R(int) ) 0.0307]
6273
0.0522
0.1261
0.0627
0.1353
588
0.923
RIGAKU MercuryCCD
Mo KR (λ ) 0.710 70 Å)
graphite
50.00
31001
16492 [R(int) ) 0.0195]
12530
0.0660
0.1563
0.0875
0.1747
1228
1.116
Mo KR (λ ) 0.710 75 Å)
graphite
50.00
51443
7079 [R(int) ) 0.0947]
5352
0.0990
0.1860
0.1313
0.2174
475
1.171
max and min peaks in final
0.456; -0.422
0.249; -0.353
0.310; -0.243
diff map/e-1 Å-3
TABLE 2: Summary of Representative Bond Lengths and Angles for Azopyridine Derivatives
bond length/Å
bond angle/ deg
C(Ph)-N(azo)-N(azo)
dihedral angle/ deg
compound
N(azo)-N(azo)
C(Ph)-N(azo)
C(Py)-N(azo)
N(azo)-O
Py-Pha
TbetNNPy
BmtNNPy
BmtNN(O)Py
1.250(4)
1.2527(13)
1.285(3)
1.278(3)
1.247(2)
1.435(5)
1.4269(18)
1.425(3)
1.424(3)
1.428(2)
1.438(5)
1.4469(19)
1.469(3)
1.461(3)
114.0(3)
2.30
59.88
64.84
58.80
0
117.14(11)
115.23(18)
116.73(19)
114.1(1)
1.254(2)
1.261(3)
azobenzeneb
a Dihedral angle defined by the mean planes of the pyridyl (phenyl for azobenzene) ring and the phenyl ring. b Reference 21.
respectively. For trans-BmtNN(O)Py, the ππ* band appears at
361 nm (2500 M-1 cm-1), while the nπ* band is absent, which
is common to azoxybenzene derivatives.25-28
Gaussian program29 with B3LYP30 and cc-PVDZ31 as a
functional and a basis set, respectively, on the structures taken
from the crystal structures without further optimization, except
for 4-PhNNPy, for which the optimized structure was employed.
The lowest-unoccupied molecular orbital (LUMO) of these
compounds is commonly the azo π* orbital on the (phenyla-
zo)pyridine moiety, which has a node between the azo nitrogen
atoms (Figure S1, Supporting Information). The highest oc-
cupied molecular orbital (HOMO) of 4-PhNNPy is the azo n
orbital. The HOMOs of TbetNNPy and BmtNNPy also contain
the azo n orbital as a major component but are substantially
mixed with orbitals delocalized on different parts of the
molecule. The HOMO of BmtNN(O)Py is an azo π orbital,
instead of the n orbital, which lies at a lower energy. The azo
π orbital of 4-PhNNPy and TbetNNPy are HOMO-1 and
HOMO-5, respectively. The π orbitals of BmtNNPy (HO-
MO-11) and BmtNN(O)Py mix with their n orbital because
of the nonplanarity of the (phenylazo)pyridine moiety.
TbetNNPy and BmtNNPy showed trans/cis photoisomeriza-
tion (Figure 4). As shown in Table 3, the quantum yields of
trans-to-cis photoisomerization for TbetNNPy upon ππ* and
nπ* excitation are φππ* ) 0.022 and φnπ* ) 0.013, respectively.
The corresponding yields for BmtNNPy are φππ* ) 0.076 and
φnπ* ) 0.056, respectively. These values are smaller than those
of azobenzene and 4-PhNNPy (see Table 3). It is also noted
that the generally observed trend that nπ* excitation gives a
higher quantum yield is not applied to TbetNNPy and Bmt-
NNPy. The cis/trans ratios of TbetNNPy and BmtNNPy in C6D6
in the photostational state were determined from the integral
values of 1H NMR spectra as 71/29 (λex ) 330 nm) and 76/24
(λex ) 348 nm).
BmtNN(O)Py showed only a slight sign of photoisomerization
even though azoxybenzene derivatives generally show photoi-
somerization just like azobenzene derivatives.26-28 This result
indicates that the presence of the bowl framework somehow
hinders the rotation of the azoxy group, which is supported by
1H NMR data as described later.
Time-dependent DFT (TD-DFT) calculations were carried out
to assign the absorption spectra. The obtained energies and
oscillator strengths of transitions are drawn with vertical bars
in Figure 5. A list of main transitions by TD-DFT is shown in
Table S1 (Supporting Information). The TD-DFT calculations
correctly reproduced the gross trend of the absorption features.
DFT Calculations. DFT calculations were carried out for
4-PhNNPy, TbetNNPy, BmtNNPy, and BmtNN(O)Py using the

Shuttlecock- and Bowl-Equipped (Phenylazo)pyridines
J. Phys. Chem. A, Vol. 114, No. 2, 2010 887
Figure 3. Calculated structures of cis isomers with molecular mechanics (MM3): (a) TbetNNPy; (b) BmtNNPy.
predicted absorption lower in energy than experimentally
observed by ∼3000 cm-1. The calculated ππ* (HOMO-1 f
LUMO) and nπ* (HOMO f LUMO) transitions of 4-PhNNPy
appear at 323 and 501 nm, respectively. The nπ* transition has
zero oscillator strength in the calculation because of its forbidden
nature of transition for a planar molecule. The calculated ππ*
(HOMO-5 f LUMO) transitions of TbetNNPy are distributed
in a range of 302-342 nm, which includes significant charge-
transfer components from orbitals on the Tbet moiety to the
localized π* orbital of the azo group (LUMO). The calculated
nπ* transition (HOMO fLUMO) of TbetNNPy appears at 502
nm, which has a nonzero oscillator strength because of the
nonplanarity between the phenyl and the pyridyl rings bonded
to the azo group as found in the crystal structure. The calculated
ππ* (HOMO-11 f LUMO) transitions of BmtNNPy are
distributed in a range of 300-313 nm, which include significant
charge-transfer components from orbitals on the Bmt moiety
to the localized π* orbital of the azo group (LUMO). The
calculated nπ* transition (HOMO f LUMO) of BmtNNPy
appears at 504 nm. The calculated ππ* transition (HOMO f
LUMO) of BmtNN(O)Py appears at 420 nm.
1H NMR Studies. The 1H NMR chemical shifts of the
protons ortho and meta to the nitrogen of the pyridyl ring of
trans-TbetNNPy are 8.65 and 7.27 ppm, respectively, which
are shifted upfield by 0.17 and 0.44 ppm, respectively, from
those of trans-4-PhNNPy in CDCl3 (Table 4). Similarly, those
of trans-BmtNNPy are 8.79 and 7.27 ppm, respectively, which
are shifted upfield by 0.03 and 0.45 ppm, respectively. These
protons are shielded by the ring-current effect of the outside
phenyl rings of the m-terphenyl groups.
1H NMR spectra before and after irradiation in C6D6 are
shown in Figure S2 (Supporting Information). The resonances
of the cis isomer of azobenzene derivatives generally appear at
upper fields compared to those of the trans isomer because of
the ring-current effect by the phenyl rings more or less in a
parallel configuration with each other. The changes in the
chemical shifts of pyridyl protons in TbetNNPy and BmtNNPy
accompanying the trans-to-cis isomerization are mostly similar
Figure 4. Photoisomerization of (a) 0.1 mM TbetNNPy and (b) 0.1
mM BmtNNPy in toluene at 25 °C. Spectra (1) before irradiation, (2)
after irradiation with UV light for 10 min, and (3) after further
irradiation with visible light for 5 min.
Especially, a good agreement is recognized in terms of transition
energies, except for BmtNN(O)Py, for which the calculation

888 J. Phys. Chem. A, Vol. 114, No. 2, 2010
Suwa et al.
Figure 5. Absorption spectra (curved line, right axis) in toluene and oscillator strengths (vertical bar, left axis) obtained form TD-DFT calculations
for (a) 4-PhNNPy, (b) TbetNNPy, (c) BmtNNPy, and (d) BmtNN(O)Py. The asterisk on the wavelength scale indicates the position of the calculated
nπ* transition (HOMO f LUMO).
TABLE 3: Summary of Features Associated with Trans-to-Cis Photoisomerization and Cis-to-Trans Thermal Isomerization
a
a
a
a
λ
max/nm (ε/M-1 cm-1
)
isosbestic
point/nm
Ea/kJ
log
∆G*/kJ ∆S*/J K-1 ∆H*/kJ
kct/10-5 s-1 mol-1 (A/s-1
)
mol-1
mol-1
mol-1
a
a
compound
ππ* nπ*
Φππ*
Φnπ*
azobenzeneb
4-PhNNPyb
315 (25000) 448 (413)
309 (19700) 453 (320)
374
372
395
405
0.11 ( 0.01
0.10 ( 0.01
0.24 ( 0.02
0.21 ( 0.03
0.25
1.08
0.25
0.17
94.2
91.7
10.9
11.1
105.0
101.4
105.0
100.4
-44.6
-40.7
-45.6
-110
91.7
89.3
91.9
67.6
TbetNNPyc ∼330 (6340)d 450 (220)
0.022 ( 0.002 0.013 ( 0.001
0.076 ( 0.003 0.056 ( 0.005
93.9e 10.8
BmtNNPyc
348 (2700) 469 (110)
70.1f
7.5
a Values at 25 °C. b In heptane. Reference 14. c This work. In toluene. d Appears as a shoulder. e The estimated standard deviation for Ea is
(4.3 kJ mol-1 f The estimated standard deviation for Ea is (2.7 kJ mol-1
.
.
TABLE 4: Representative Chemical Shifts for Azopyridine Derivatives
δ/ppm
PhCH2
compound
TbetNNPy
isomer
trans
solvent
PyHo
PyHm
CH3
CDCl3
C6D6
C6D6
CDCl3
C6D6
C6D6
CDCl3
CDCl3
C6D6
8.65
8.46
8.04
8.79
8.77
8.32
8.86
8.82
8.25
7.82
7.27
7.06
5.77
7.27
7.31
5.51
7.80
7.72
7.04
5.76
cis
trans
BmtNNPy
3.72
4.01
1.85
1.96
2.09, 1.85
1.81 (br)
cis
trans
trans
3.28 (d), 2.82 (d)
3.04 (br)
BmtNN(O)Py
4-PhNNPy
cis
C6D6
to those of 4-PhNNPy, except for the proton meta to the nitrogen
of the pyridyl ring in BmtNNPy, which exhibits a 1.80 ppm
upfield shift, larger than that for 4-PhNNPy by as large as 0.51
ppm. This large upfield shift corroborates the conformation
optimized by the molecular mechanics calculation for the cis
isomer, in which the pyridyl moiety is embedded in the bowl
framework. The interaction between the bowl framework and
the (phenylazo)pyridyl moiety in cis-BmtNNPy is further

Shuttlecock- and Bowl-Equipped (Phenylazo)pyridines
J. Phys. Chem. A, Vol. 114, No. 2, 2010 889
1
evidenced by the H NMR spectrum in which the two benzyl
of the protons at 3, 5, 3′′, and 5′′ positions on the m-terphenyl
groups, and those in the benzyl and CH3 groups in trans-
BmtNN(O)Py are broadened, while all signals in trans-Bmt-
NNPy are sharp (Figure S2, Supporting Information). This
implies that the presence of the oxygen atom combined with
the bowl framework severely restricts the flexibility of the
molecule, which, in addition to the electronic factors described
above, may lead to the suppression of the photoisomerization.
Thermal Cis-to-Trans Isomerization. The shuttlecock frame-
work does not seem to affect the thermal cis-to-trans isomer-
ization, as the activation parameters for TbetNNPy are quite
similar to those of azobenzene and azopyridine derivatives
(Table 3). On the other hand, the kinetics of the thermal cis-
to-trans isomerization of BmtNNPy is rather unique, which is
characterized by a low frequency factor more than compensating
for a low activation energy. Thermodynamic analysis revealed
that the origin for the unusual behavior comes from a low
enthalpy of activation (∆H*) and a large negative entropy of
activation (∆S*). These characteristics for ∆H* and ∆S* imply
that the bowl moiety imposes a limited range of conformations
on the reaction coordinate from the cis isomer to the trans
isomer, which is stabilized in terms of enthalpy compared to
the transition state for 4-PhNNPy.
groups as well as the eight methyl groups are respectively not
equivalent, while those of the trans isomer are. These results
suggest that the pyridyl group in the cis isomer leans toward a
side of the bowl-framework to suppress the molecular motion;
that is, the rotation about the C-N bond between the Bmt and
azo groups, which becomes much slower than the 1H NMR time
scale.
Thermal Cis-to-Trans Isomerization. The rate constants of
cis-to-trans thermal isomerization (kct) of TbetNNPy and Bmt-
NNPy were found to be 0.25 × 10-5 and 0.17 × 10-5 s-1
,
respectively, in toluene at 25 °C (Table 3). The Arrhenius plots
in the range 20-50 °C gave linear relationships, from which
the activation energies for TbetNNPy and BmtNNPy were
obtained as Ea ) 93.9 ( 4.3 and 70.1 ( 2.7 kJ mol-1
,
respectively. The former is similar to those of azobenzene and
4-PhNNPy while the latter is significantly smaller. The values
of ∆G* and ∆S* were obtained through the relationships, ∆G*
) Ea s RT - T∆S* and ∆S* ) [ln A s 1 s ln(kBT/h)]R, which
are included in Table 3 along with values for related azo
compounds. Particularly noteworthy result is that ∆S* (negative)
and ∆H* (positive) for BmtNNPy are much smaller than for
other compounds.
Bunce and co-workers investigated the kinetics of thermal
cis-to-trans isomerization for sterically hindered azobenzene
derivatives that bear methyl, ethyl, and tert-butyl groups on the
four ortho positions, demonstrating that the activation parameters
all lie in narrow ranges (Ea ) 87-97 kJ mol-1 and log A )
10-11),17 which are common to azobenzene and azopyridine
derivatives sterically unhindered.14 Thus, it seems that the way
the bowl framework in BmtNNPy affects the cis-to-trans
isomerization is rather unusual. The isomerization of BmtNNPy
starts from the sterically demanding cis isomer as indicated by
Discussion
Photochromism. The quantum yields of photoisomerization
for TbetNNPy and BmtNNPy are smaller than those for
sterically unhindered azo compounds, such as 4-PhNNPy. It
cannot be assumed a priori, however, that the bulky substituents
exert a detrimental effect on the photoisomerization. Some
azobenzene derivatives surrounded by alkyl substituents, such
as 2,2′,4,4′,6,6′-hexaisopropylazobenzene, show efficient trans-
to-cis photoisomerization.16 However, sterically similar but
electronically different 2,2′,4,4′,6,6′-hexakis(phenylazo)benzene
exhibits a low quantum yield for the isomerization.16 These
observations infer that the low quantum yields of photoisomer-
ization for TbetNNPy and BmtNNPy, with aromatic substituents,
may be due to, to a large extent, electronic reasons rather than
steric ones. Otherwise, the even lower yields for TbetNNPy than
those for more sterically demanding BmtNNPy could not be
rationalized. In fact, DFT calculations showed that the azo n
and π orbital are perturbed significantly by being intricately
mixed with orbitals of the other portions in TbetNNPy and
BmtNNPy (Figure S1, Table S1, Supporting Information).
The quantum yields of photoisomerization for sterically
hindered azobenzenes, 2,2′,4,4′,6,6′-hexa(isopropylazo)benzene
(ππ*: 0.18, nπ*: 0.19) and 2,2′,4,4′,6,6′-hexakis(phenylazo)ben-
zene (ππ*: 0.04, nπ*: 0.05), do not show the wavelength
dependence commonly seen for azobenzene derivatives,16 in
which nπ* excitation gives a higher quantum yield than ππ*
excitation. Even azobenzene (ππ*: 0.11, nπ*: 0.24),32 when
encapsulated in R- or ꢀ-cyclodextrin,33 loses its wavelength
dependence (ππ*: 0.11, nπ*: 0.13 for azobenzene-R-CD
complex; ππ*: 0.13, nπ*: 0.13 for azobenzene-ꢀ-CD complex).
Thus, the dependence of the quantum yields on the excitation
wavelength tends to be lost by steric congestion around the azo
group, in addition to electronic factors. The lack of the
wavelength dependence for BmtNNPy fits into this tendency.
Even a reverse dependence was found for TbetNNPy, in which
ππ* excitation gives a higher quantum yields of isomerization
than nπ* excitation.
1
the molecular mechanics calculation and by the H NMR, as
described above, to another sterically demanding trans isomer
characterized by the 60° twist of the pyridyl and the azophenyl
planes as revealed by the crystal structure. These unique features
in BmtNNPy with the bowl-shaped framework may be respon-
sible for the unusual reaction pathway.
Conclusions
In this study, we have investigated the relationships between
the structures and the photoisomerization and thermal isomer-
ization processes for sterically hindered (phenylazo)pyridine
derivatives, with a shuttlecock-shaped (TbetNNPy) or a bowl-
shaped framework (BmtNNPy and BmtNN(O)Py). Regarding
the conformation of the (phenylazo)pyridine moieties in the trans
isomers of these compounds, the crystal structures revealed a
planar conformation for the former and a severely distorted
conformation for the latter. The quantum yields for the trans-
to-cis photoisomerization are lower for BmtNNPy compared
to unsubstituted 4-PhNNPy and are even much lower for
TbetNNPy. The primary reason for the low quantum yields may
be electronic rather than steric in origin. The common trend
for azobenzene derivatives that the quantum yields of trans-to-
cis photoisomerization is higher by nπ* excitation than by ππ*
excitation was not found for TbetNNPy or BmtNNPy (even the
reverse dependence was found in both TbetNNPy and Bmt-
NNPy). As for the thermal cis-to-trans isomerization, TbetNNPy
behaves as normal azopyridine or azobenzene derivatives, but
BmtNNPy behaves in a rather unusual manner. The bowl
framework in BmtNNPy appears to direct the course of the
reaction, resulting in an unusually large negative activation
entropy and a low activation enthalpy.
BmtNN(O)Py shows little photoisomerization although azox-
ybenzene derivatives generally show photoisomerization just like
the corresponding azobenzene derivatives.26-28 1H NMR signals

890 J. Phys. Chem. A, Vol. 114, No. 2, 2010
Suwa et al.
Experimental Section
References and Notes
(1) Suginome, H. CRC Handbook of Organic Photochemistry and
Photobiology, 2nd ed.; CRC Press: Boca Raton, FL, 2004; Chapter 94.
(2) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
2001.
(3) Muraoka, T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem.
Soc. 2003, 125, 5612–5613.
(4) Yagai, S.; Karatsu, T.; Kitamura, A. Chem.sEur. J. 2005, 11, 4054–
4063.
(5) Yagai, S.; Nakajima, T.; Karatsu, T.; Saitow, K; Kitamura, A. J. Am.
Chem. Soc. 2004, 126, 11500–11508.
(6) Cook, M. J.; Nygard, A.-M.; Wang, Z.; Russell, D. A. J. Chem.
Soc., Chem. Commun. 2002, 1056–1057.
(7) Busby, M.; Matousek, P.; Vlcˇek, A., Jr. Inorg. Chim. Acta 2007,
360, 888–896.
(8) Otsuki, J.; Narutaki, K. Bull. Chem. Soc. Jpn. 2004, 77, 1537–
1544.
(9) Otsuki, J.; Narutaki, K.; Bakke, J. M. Chem. Lett. 2004, 33, 356–
357.
(10) Sugimoto, H.; Kimura, T.; Inoue, S. J. Am. Chem. Soc. 1999, 121,
2325–2326.
(11) Suwa, K.; Otsuki, J.; Goto, K. Tetrahedron Lett. 2009, 50, 2106–
2108.
(12) Goto, K.; Okazaki, R. Liebigs Ann. Recl. 1997, 2393–2407.
(13) Goto, K.; Nagahama, M; Mizushuma, T.; Shimada, K.; Kawashima,
T.; Okazaki, R. Org. Lett. 2001, 3, 3569–3572.
(14) Brown, E. V.; Granneman, G. R. J. Am. Chem. Soc. 1975, 97, 621–
627.
(15) Nakagawa, M.; Rikukawa, M.; Watanabe, M.; Sanui, K.; Ogata,
N. Bull. Chem. Soc. Jpn. 1997, 70, 737–744.
(16) Rau, H.; Yu-Quan, S. J. Photochem. Photobiol. A 1988, 42, 321–
327.
(17) Bunce, N. J.; Ferguson, G.; Forber, C. L.; Stachnyk, G. J. J. Org.
Chem. 1987, 52, 394–398.
(18) Bisle, H.; Rau, H. Chem, Phys. Lett. 1975, 31, 264–266.
(19) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454–456.
(20) Graeber, E. J.; Morosin, B. Acta Crystallogr. 1974, B30, 310–317.
(21) Bouwstra, J. A.; Schouten, A.; Kroon, J. Acta Crystallogr. 1983,
C39, 1121–1123.
Materials. TbetNNPy, BmtNNPy, and BmtNN(O)Py were
prepared according to previously published procedures.11 Spec-
troscopic grade toluene was purchased from Wako Chemical.
X-ray Crystallography. Single crystals of trans isomers of
TbetNNPy, BmtNNPy, and BmtNN(O)Py suitable for X-ray
analysis were obtained from CH2Cl2-CH3CN solutions by
allowing the solvent to evaporate slowly. Crystallographic data
were collected with a RIGAKU MercuryCCD difractometer
using graphite-monochromated Mo KR radiation (λ ) 0.710 75
(TbetNNPy and BmtNNPy) and 0.710 70 (BmtNN(O)Py) Å).
All structures were solved by SHELXS-9734 and refined by full
matrix least-squares on F2 with SHELXL-97.35 The measure-
ment was carried out at 123 K for TbetNNPy and BmtNNPy
and 120 K for BmtNN(O)Py.
Instruments. 1H NMR spectra were measured with a JEOL
JNM-GX400 spectrometer. Absorption spectra were taken with
a Shimadzu UV-2400 PC spectrometer in a 1 × 1 cm quartz
optical cell maintained at 25 °C with a Peltier thermostat. The
light source of Shimadzu RF-5300PC fluorometer was used as
an excitation light in photochromic reactions, with a slit width
of 20 nm. The slit width of 5 nm was used in the determination
of quantum yields. The absorption spectra of cis isomers, which
were used for the determination of quantum yields, were
obtained by extrapolation of the absorption spectrum for a cis-
1
rich mixture for which the composition was known from H
NMR integration.
Quantum Yields. A well-stirred solution of trans-azo ligand
(0.1 mM) in toluene was irradiated at ππ* and nπ* bands and
the absorbance (ππ* band) was recorded every 10 s. The initial
trans-to-cis photoisomerization rates (V) were obtained from the
slope of the concentration of trans isomer against irradiated time.
The quantum yields (φ) were obtained by using the equation
(22) Theilmann, O.; Saak, W.; Hasse, D.; Beckhaus, B. Organometallics
2009, 28, 2799–2807.
(23) (a) Zenkina, O; Altman, M.; Leitus, G.; Shimon, L. J. W.; Cohen,
R.; van der Boom, M. E. Organometallics 2007, 26, 4528–4534. (b) Maury,
O; Guegan, J.-P.; Renouard, T.; Hilton, A.; Dupau, P.; Sandon, N.; Toupet,
L.; Le Bozec, H. New J. Chem. 2001, 25, 1553–1566.
(24) Buncel, E.; Keum, S. R.; Cygler, M.; Varughese, K. I.; Birnbaum,
G. I. Can. J. Chem. 1984, 62, 1628–1639.
V ) (φI0/V)(1 - 10-Abs
)
where I0 is the photon flux at the front of cell, V is the volume
of the solution, and Abs is the initial absorbance at the irradiation
wavelength. The value of I0 was obtained by using azobenzene
as the standard (φ ) 0.11 and 0.24 for ππ* and nπ*
excitations,32 respectively) under the same conditions.
Thermal Cis-to-Trans Isomerization. The thermal cis-to-
trans isomerization rates were obtained by monitoring absorption
changes intermittently for cis-rich solutions, which were pre-
pared by ππ* irradiation for 10 min, kept in the dark at a
constant temperature in the range from 20 to 50 °C.
DFT Calculations. DFT calculations were performed using
a Gaussian 03w program package29 on a personal computer.
We used the B3LYP30 functional and cc-PVDZ31 basis set on
the structures taken from the crystal structures without further
optimization except for 4-PhNNPy.
(25) Rhee, S.-B.; Jaffe, H. H. J. Am. Chem. Soc. 1973, 95, 5518–5521.
(26) Aronzon, D.; Levy, E. P.; Collings, P. J.; Chanishvili, A.; Chilaya,
G.; Petriashvili, G. Liq. Cryst. 2007, 37, 707–718.
(27) Otsuki, J.; Omokawa, N.; Yoshida, K.; Yoshikawa, I.; Akasaka,
T.; Suenbu, T.; Takido, T.; Araki, K.; Fukuzumi, S. Inorg. Chem. 2003,
42, 3057–3066.
(28) Weinstein, J.; Sousa, J. A.; Bluhm, A. L. J. Org. Chem. 1964, 29,
1586–1588.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03,
Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(30) Beck, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(31) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007–1023.
(32) Zimmerman, G.; Chow, L.; Paik, U. J. Am. Chem. Soc. 1958, 80,
3528–3531.
Acknowledgment. This work was supported by the High-
Tech Research Center Project for Private Universities (MEXT
and Nihon University) and the Nihon University Strategic
Projects for Academic Research.
Supporting Information Available: Crystallographic data
for TbetNNPy, BmtNNPy, and BmtNN(O)Py in CIF format.
Representative molecular orbitals (Figure S1), NMR spectra
(Figure S2), and a list of main transitions by TD-DFT calculation
(Table S1) for TbetNNPy, BmtNNPy, and BmtNN(O)Py. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(33) Bortolus, P.; Monti, S. J. Phys. Chem. 1987, 91, 5046–5050.
(34) Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(35) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
JP909922Y