Systematic modulation of hydrogen bond donors in aminoazobenzene derivatives provides further evidence for the concerted inversion photoisomerization pathway

Article abstract of DOI:10.1002/ejoc.201300525

A series of aminoazobenzene derivatives structurally related to AzoAMP-1 have been prepared and characterized by using a variety of analytical techniques. AzoAMP-1 is based on 2,2′-diaminoazobenzene with N-methylpyridine groups. The new derivatives all co

Full text of DOI:10.1002/ejoc.201300525

FULL PAPER
DOI: 10.1002/ejoc.201300525
Systematic Modulation of Hydrogen Bond Donors in Aminoazobenzene
Derivatives Provides Further Evidence for the Concerted Inversion
Photoisomerization Pathway
H. M. Dhammika Bandara,[a,b] Prem N. Basa,[a] Jingjing Yan,[a] Casey E. Camire,[b]
John C. MacDonald,[a] Randy K. Jackson,[b] and Shawn C. Burdette*[a]
Keywords: Diazo compounds / Photochemistry / Isomerization / Molecular devices / Hydrogen bonds
A series of aminoazobenzene derivatives structurally related
to AzoAMP-1 have been prepared and characterized by
using a variety of analytical techniques. AzoAMP-1 is based
on 2,2Ј-diaminoazobenzene with N-methylpyridine groups.
The new derivatives all contain a hydrogen bond between
the aniline hydrogen atom and the azo group, as well as a
separate pendant functional group that could contribute an
additional hydrogen-bond acceptor to an intramolecular net-
work. A combination of photoisomerization studies and NMR
spectroscopic and X-ray crystallographic investigations sug-
gest that AzoAMP-1 possesses a unique structure that pre-
vents isomerization through the concerted inversion path-
way, which cannot be reproduced with other types or arran-
gements of substituents. Only AzoAMQ, which contains a
similar quinolone heterocycle in place of the pyridine group
of AzoAMP-1, displayed somewhat similar photochemistry.
Introduction
Azobenzene (AB) undergoes reversible trans↔cis photo-
isomerization when irradiated with light (Scheme 1).[1–4]
Changes in geometry and electronic structure upon isomer-
ization enable AB to be used as a light-triggered switch.
Azobenzene photoswitches have been used to tune the bind-
ing affinity of metal-chelators,[5,6] modulate the activity of
biological probes,[7,8] influence catalysis,[9,10] design molecu-
lar machines,[11,12] cause structural changes in photorespon-
sive polymeric materials,[13,14] and alter the photochemical
properties of liquid crystals,[15,16] holographic data storage
devices,[17,18] and optical gratings.[19,20]
Despite the prevalence of AB derivatives in a broad spec-
trum of chemical applications, light is the sole input for
controlling isomerization. Developing secondary switches
for controlling the trans↔cis isomerization would be of
considerable value for expanding the scope of possible AB
applications. Computational experiments suggest that me-
chanical stress can induce AB isomerization;[21] in addition,
electrons from the probe tip of a scanning tunneling micro-
scope can isomerize AB molecules adsorbed on a gold sur-
face.[22] Using structure and chemistry to control AB isom-
Scheme 1. Isomerization pathway in azobenzene and AzoAMP-1.
erization is more appealing; however, very few studies have
examined AB structure–photochemistry relationships
(SPRs) systematically.
Several systems that control AB photoisomerization be-
yond single wavelength radiation have been reported. By
introducing a redox active metal ion such as iron or cobalt
coordinated onto an AB-functionalized ligand, metal oxi-
dation state changes shift the AB absorption maximum
(λmax), which causes both isomers to absorb strongly at the
same wavelength.[23–25] Various methods for suppressing
AB transǞcis photoisomerization have also been reported.
Formation of ion pairs between AB carboxylates and bulky
quaternary ammonium salts block photoisomerization
through steric interactions.[26] In 2-hydroxy-ABs, intramo-
[a] Department of Chemistry and Biochemistry, Worcester
Polytechnic Institute,
100 Institute Road, Worcester, Massachusetts 01609-2280, USA
E-mail: scburdette@wpi.edu
Homepage: http://www.wpi.edu/academics/cbc/
[b] Department of Chemistry, University of Connecticut,
55 North Eagleville Road U-3060, Storrs, Connecticut 06269,
USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300525.
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Aminoazobenzene Derivative Photoisomerization
lecular hydrogen-bonding between the azo-nitrogen atom for the formation of the cis-isomer. Further analysis showed
and the hydroxyl group locks the molecule in the trans con- that this conical seam is absent in AzoAMP-6, allowing it
formation, resulting in minimal transǞcis photoisomeri- to reach the S1/S0 crossover point and isomerize.
zation and fast thermal relaxation.[27,28] Alternately, an AB
Our investigation on AzoAMP compounds demon-
functionality may be incorporated into a metal binding li- strated that H-bonding and electronic effects can suppress
gand motif such as a crown ether[29,30] or a catechol.[31]
AB photoactivity; however, developing reversible light-
Previously we reported the unusual photochemistry of orthogonal switching mechanisms will be required to im-
the substituted aminoazobenzene AzoAMP-1 (1; Figure 1), plement these concepts in practical applications. Inspired
which exhibited minimal transǞcis photoisomerization and by the work on AzoAMP compounds, we investigated the
fast cisǞtrans thermal isomerization.[32] AzoAMP-1 con- extent to which H-bonding can influence AB photoactivity.
sists of an AB core bearing two aminomethylpyridine To address this goal, we prepared a library of AzoAMP
(AMP) moieties at the arene ring ortho position. Computa- analogues to study SPRs by using X-ray crystallographic,
tional and X-ray crystallographic data indicated intramo- UV/Vis and NMR spectroscopic analyses.
lecular H-bonds between the anilino-hydrogen atom and
azo- and pyridyl-nitrogen atoms are responsible for block-
ing the concerted inversion pathway in AzoAMP-1 by lock-
ing the molecule in the trans conformation (Scheme 1).
Results and Discussion
Whereas coplanarization of the aryl groups, which is a pre-
requisite to isomerization by inversion, cannot occur due to
the intramolecular H-bonds in AzoAMP-1 (Scheme 1), the
Photochemistry of Azobenzene, Aminoazobenzenes, and
AzoAMP-1
methylated AzoAMP derivative AzoAMP-2, which lacks
The two well-separated absorption bands of trans-AB at
the ability to form intramolecular H-bonds, undergoes pho- ca. 320 and 450 nm correspond to πǞπ* and nǞπ* transi-
toisomerization. AzoAMP-1, therefore, provides the first tions, respectively.[1] Upon continuous irradiation of the
example of photoisomerization of AB being blocked by hy- πǞπ* transition, AB reaches a photostationary state con-
drogen-bonding amine functionalities.
sisting of 1:9 trans/cis isomers.[34] Because cis-AB is not
thermodynamically stable, relaxation back to trans-AB oc-
curs in the dark. Thermal relaxation of cis-AB has a half-
life (t1/2) of several days.[35] In contrast to AB, aminoAB
derivatives containing ortho or para amine substituents ex-
hibit different behavior. In aminoABs, the πǞπ* transition
shifts to higher wavelengths and overlaps the nǞπ* transi-
tion.[36,37] Whereas, like AB, aminoABs undergo transǞcis
isomerization when irradiated, thermal cisǞtrans isomer-
ization occurs quickly within minutes or hours.[35] As a re-
sult, accurately determining the trans/cis ratio at the photo-
stationary state can be difficult. Both AB and aminoABs
have barely detectable emission at room temperature,[38,39]
but become weakly emissive when frozen in a solid matrix
at 77 K.[40]
Figure 1. Structure of AzoAMP compounds studied to date.
AzoAMP-1 exhibits minimal photoisomerization owing to intra-
molecular H-bonds, whereas AzoAMP-2 photoisomerizes in a sim-
ilar manner to other aminoABs. AzoAMP-4 and AzoAMP-5, with
electron-donating substituents, exhibit minimal photoisomerization
because of electronic effects, whereas AzoAMP-6, with the ester
group, photoisomerizes in a similar manner to other aminoABs.
AzoAMP-1 possesses absorption bands typical for ami-
noABs but exhibits remarkably different photochemistry.
Upon irradiation of AzoMAP-1, only minimal photoisom-
erization occurs, which corresponds to a small decrease
(Ͻ 5%) in integrated absorption. The thermal cisǞtrans
isomerization occurs within seconds. X-ray crystallographic
analysis shows that AzoAMP-1 adopts a completely planar
We also prepared the AzoAMP derivatives AzoAMP-4 structure in the solid state, despite the methylene carbon
and -5, which bear electron-donating hydroxy and methoxy atom and aniline nitrogen atom being formally sp3-hy-
groups, respectively, para to the azo moiety.[33] These bridized. Intramolecular H-bonds between the aniline-hy-
AzoAMP derivatives also exhibited minimal photoiosmer- drogen atoms and both the azo-nitrogen and pyridine-nitro-
ization and fast thermal isomerization; however, these com- gen atoms enforce the planarity. Computational experi-
pounds lack any intermolecular H-bonds. When the hy- ments suggest that the H-bonding makes photoisomeriza-
1
droxyl group was converted into an electron-withdrawing tion energetically unfavorable. The H NMR signal for the
ester group in AzoAMP-6, photoactivity was restored. Cal- aniline hydrogen atom of AzoAMP-1 appears significantly
culations showed the presence of a conical seam between S1 downfield at δ = 9.03 ppm compared with those of di-
and S0 states on the trans-side of the S1/S0 crossover point aminoazobenzene (6; DAAB; δ = 5.20 ppm), which pro-
in AzoAMP-4 and -5. This prevented these molecules from vides further evidence for the significance of H-bonding in
reaching the S1/S0 crossover point, which is a prerequisite the molecule.
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Design and Synthesis of Aminoazobenzene Derivatives
the aniline hydrogen atom and the azo nitrogen atom on
each half of the molecule.[41]
Based on the photochemistry of AzoAMP-1, we envisioned
The linkers and pendant H-bond acceptors were varied
the possibility of controlling the trans↔cis isomerization by to systematically change the degree of possible intramolecu-
modulating the degree of H-bonding between the aniline lar H-bonding in the corresponding AB derivative
hydrogen atom and a dangling H-bond acceptor. The ab- (Scheme 2). Replacing the methylene linker in AzoAMP-1
breviated names were derived from azobenzene (Azo); the with the ethylene group in AzoAEP, changes the five-mem-
linker between anilino nitrogen atom and the H-bond ac- bered ring formed by the H-bond to a potential six-mem-
ceptor, either 2-aminomethyl (AM), 2-aminoethyl (AE), or bered ring while retaining the properties of the acceptor
2-amido (A); combined with a one-letter abbreviation for group. The remaining derivatives retain the five-membered
the H-bond acceptor to give names corresponding to deriv- ring structure but, with the exception of AzoAMQ, vary
atives containing pyridine (AzoAMP, AzoAEP, AzoAP), the nature of H-bond acceptor. AzoAMF contains sp3-hy-
quinoline (AzoAMQ), furan (AzoAMF), benzene bridized oxygen atoms as the H-bond acceptors, which, by
(AzoAMB), methyl ester (AzoAME), or carboxylic acid virtue of the five-membered furan ring structure, orients the
(AzoAMC). Each compound contains the core structure of donor electrons at a slightly different angle to that in
DAAB, which exhibits an intramolecular H-bond between AzoAMP-1. Both AzoAMC and AzoAME provide H-
Scheme 2. Synthesis of new aminoazobenzene derivatives. Reagents and conditions: (a) 2-Pyridinecarbaldehyde, NaBH(OAc)3, CH2Cl2;[32]
(b) 2-vinylpyridine, AcOH, MeOH; (c) 1. picolinic acid, N-methylmorpholine, PyBOP, CH2Cl2/DMF; 2. picolinic acid chloride, K2CO3,
THF; (d) 2-quinolinecarbaldehyde, NaBH(OAc)3, CH2Cl2; (e) benzaldehyde, NaBH(OAc)3, CH2Cl2; (f) 2-furalaldehyde, NaBH(OAc)3,
CH2Cl2; (g) ethyl bromoacetate, N,N-diisopropylethylamine, NaI, CH3CN; (h) 1. 2 m NaOH, EtOH; 2. HCl.
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Aminoazobenzene Derivative Photoisomerization
bond acceptors through carbonyl oxygen atoms, which di-
rect the donor electrons in a very similar manner to that in
AzoAMP-1. Whereas the benzyl groups possess similar
steric requirements to pyridylmethyl functionality,
AzoAMB lacks the ability to form intramolecular H-bonds,
like AzoAMP-1.
All AB derivatives were prepared from the DAAB scaf-
fold (Scheme 2). AzoAMB, AzoAMF, and AzoAMQ were
synthesized by reductive amination between DAAB and the
appropriate aldehyde by a reaction analogous to the prepa-
ration of AzoAMP-1. AzoAEP was prepared by Michael
addition with 2-vinylpyridine. AzoAP was synthesized by a
two-step process using an amide coupling with picolinic
acid followed by reaction with picolinic acid chloride.
AzoAME was prepared by reacting DAAB with ethyl
bromoacetate, and AzoAMC was prepared by the subse-
quent hydrolysis of ester groups. All compounds were iso-
lated as the thermodynamically more stable trans-isomer,
1
which was confirmed by H NMR analysis.
Evidence for H-Bonding in the Solid State
Single-crystal X-ray structure determinations were car-
ried out to investigate the extent of H-bonding present and
the effect of intramolecular forces on photochemistry (Fig-
ure 2). All the AB derivatives crystallized in the thermody-
namically stable trans conformation, typical of solid-state
AB structures. The anilino nitrogen atoms in all the AB
derivatives appear to be planar, despite being formally sp3
hybridized, which enforces a planarity on the core of all the
aminoAB derivatives. The orientation of the anilino hydro-
gen atom toward the azo nitrogen atoms and the structural
planarity of the azo core suggest the presence of an intra-
molecular H-bond between each of the anilino hydrogen
atoms and one of the azo nitrogen atoms.
The length of the NH···N=N hydrogen bond varies be-
tween 1.980 to 2.357 Å, with DAAB (2.330 Å),[41] AzoAP
(2.208 Å), and AzoAEP (2.357 Å) as the only derivatives
with bond lengths exceeding 2.07 Å (Table 1). The parent
DAAB lacks an alkyl substituent on the aniline group,
which accounts for the bond lengthening, but the behavior
of AzoAEP and AzoAP appears to be more complex. Upon
examination of the neighboring molecules in the crystal lat-
tice, it is clear that the pyridyl groups of each AzoAEP com-
Figure 2. ORTEP diagram of aminoAB derivatives showing 50%
thermal ellipsoids and selected atom labels. Hydrogen atoms on
carbon atoms are omitted for clarity. The structures of AzoAMP-
1[32] and DAAB[41] were previously determined.
Table 1. Hydrogen bond lengths [Å] in new aminoazobenzene de-
rivatives.
Compound
NH···N=N
NH···X[a]
DAAB
AzoAMP-1
AzoAP
AzoAEP
AzoAMQ
AzoAMF
AzoAME
AzoAMC
AzoAMB
2.330(2)
2.046(1)
2.208(1)
2.467(2)
2.066(6)
2.023(1)
2.012(1)
2.046(2)
1.980(1)
n.a.
2.219(2)
2.191(1)
n.a.
2.305(5)
n.a.
2.338(2)
2.355(3)
n.a.
[a] n.a.: not applicable. Estimated standard deviations in the last
digit(s). Atom labels are provided in Figure 2. Pendant H-bond ac-
ceptor AzoAMP-1, AzoAP, AzoAMQ:
AzoAMC: X = O.
X = N; AzoAME,
pound engage in an intermolecular H-bond at 2.296(2) Å ino hydrogen atom. This H-bond was hypothesized to be
with anilino hydrogen atoms of two adjacent AzoAEP mo- responsible for reduced transǞcis photoisomerization.[32]
lecules. Similarly, the two anilino hydrogen atoms engage in AzoAMB lacks any H-bond acceptors; in addition, no in-
intermolecular H-bonds with the pyridine groups of neigh- teraction was observed in AzoAMF or AzoAEP, which do
boring AzoAEP molecules, so each AzoAEP unit H-bonds possess heteroatoms that could engage in a second H-bond-
independently to four additional AzoAEP molecules to ing interaction. The oxygen atoms in the furanyl rings ori-
make a hydrogen-bonded network. The intermolecular in- ent away from the azo core in AzoAMF and twist by
teractions can account for the longer intramolecular roughly 45° with respect to the parent diaminoAB plane.
NH···N=N H-bond in the solid state. AzoAP lacks inter- Unlike in AzoAEP, for which intermolecular H-bonding
molecular H-bonding between the heteroatoms, so the can account for the lack of a heteroatom-anilino hydrogen
longer intramolecular NH···N=N H-bond can be attributed atom interaction, AzoAMF does not engage in intermo-
to the presence of the amide carbonyl group.
lecular H-bonds.
AzoAMP-1 exhibits an apparent 2.219 Å H-bonding in-
AzoAME (2.338 Å), AzoAMC (2.355 Å), AzoAMQ
teraction between the dangling pyridine group and the anil- (2.305 Å), and AzoAP (2.191 Å) all appear to contain H-
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S. C. Burdette et al.
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bonds, based on examination of the orientation of the do-
The downfield signal (δ = 12.0 ppm) for the anilino hy-
nor group toward the anilino hydrogen atom, but only drogen atoms along with the X-ray structure provides evi-
AzoAP has a H-bond length that is close to the length mea- dence that AzoAP could possess H-bonding between the
sured in AzoAMP-1, which suggests the interaction will be pyridine nitrogen atom and the anilino hydrogen atom in
weaker in these derivatives. Whereas the NH···pyridine H- both solution and the solid state. For comparison, the anil-
1
bond is shorter in AzoAP than in AzoAMP-1, NH···N=N ino H NMR signal of N-phenyl-2-pyridinecarboxamide[42]
is longer and may be more indicative of the structural appears at δ = 10.03 ppm,[43] 12.75 ppm in N-(2-benzo-
changes associated with the amide group than the strength ylphenyl)picolinamide, and 10.2 ppm for 4-(picolinoyl-
of the respective H-bonds.
amino)acetophenone,[44] which all contain similar structural
features to AzoAP. The anilino H NMR signal of AzoAP
1
appears downfield closest to that of N-(2-benzoylphenyl)-
picolinamide, which contains a H-bond accepting carbonyl
oxygen in the same position as the azo group as well as the
picolinamide group. When the structure lacks an H-bond
acceptor, as in N-phenyl-2-pyridinecarboxamide, or the
electron-withdrawing ketone cannot H-bond, as in 4-(picol-
Degree of H-Bonding in Solution
1
The anilino H NMR signals of DAAB appear at δ =
5.02 ppm, whereas those of AzoAMP-1 appear at δ =
9.03 ppm (Table 2). The crystal structure of both DAAB
and AzoAMP-1 show H-bonding between the azo group
and the aniline hydrogen atom, so the additional downfield
1
inoylamino)acetophenone, the anilino H NMR signal ap-
1
pears shifted more upfield. The combination of solution
and X-ray data suggest an intramolecular H-bonding net-
work in AzoAP that is similar to AzoAMP-1.
shifting of the AzoAMP-1 H NMR signal derives from a
combination of contributions from the pendant alkyl group
and additional H-bonding to the pyridine nitrogen atom. In
contrast to AzoAMP-1, the AzoAMB phenyl group cannot
contribute an additional H-bond to the anilino hydrogen
atom that appears at δ = 8.52 ppm. With the exception of
Photoisomerization and Thermal Relaxation in Solution
1
AzoAMQ and AzoAP, the anilino H NMR signal of the
diaminoAB derivatives falls between 8.26–8.52 ppm, which
To assess photoisomerization in the aminoABs, a
suggests minimal H-bonding occurs with the pendent H- 3.00 mL aliquot of a 25 μm solution of each derivative in
bond acceptors. Consistent with the structural similarity to 1:1 EtOH/Et2O was placed in a quartz cuvette and irradi-
AzoAMP-1, AzoAMQ has a nearly identical chemical shift ated with a 1000 W light source. AzoAP experiments were
for the anilino hydrogen atoms.
conducted at 100 μm due to the weaker extinction coeffi-
1
The upfield H NMR signals for the aniline hydrogen cient of the derivative. Upon irradiation, a decrease in ab-
atoms in AzoAME and AzoAMC in solution suggest a lack sorbance consistent with transǞcis isomerization was ob-
of a H-bond between the carbonyl oxygen atom and anilino served for all aminoABs (Table 2); however, the rate of ther-
hydrogen atom, despite the X-ray structure indicating one mal cisǞtrans isomerization precludes accurate measure-
in the solid state. The spectrum of AzoAMC was acquired ment of the trans/cis isomer ratio using standard steady-
in [D6]dimethyl sulfoxide for solubility reasons, which could state techniques for all the new compounds except AzoAP.
disrupt H-bonding; however, the data for AzoAME was ob- Because the cis-AB isomers absorb weaker than the trans
1
tained in CDCl3 like the other derivatives. The anilino H isomer at λmax, the decrease in absorbance corresponds to
NMR signal in AzoAME is also sharper than that observed disappearance of the trans isomer. As an indirect indicator
in the other aminoAB compounds, so some fluctional H- of the degree of photoisomerization, the integrated absorp-
bonding probably occurs. The difference between the solu- tion was calculated and compared with that of DAAB,
tion-state and solid-state data in AzoAMC and AzoAME AzoAP, and AzoAMP-2, which have photostationary states
suggest that crystal packing contributes significantly to the containing approximately 3:1 trans/cis isomer and that of
H-bond observed in the X-ray structure.
AB, which reaches a 1:9 ratio.
Table 2. Photophysical properties of aminoazobenzene derivatives.
Compound
AB
λmax [nm]
ε [m–1 cm–1]
Abs. change [%][a]
ΦtransǞcis
δ aniline H [ppm] Half-life of cis-isomer
320
450
469
490
456
500
410
499
499
490
483
486
22000
400
11000
10800
9000
13000
8500
9200
15200
13200
11200
11200
Ͼ 90
0.10–0.20,
0.25–0.35
n.a.
n.a.
Ͼ 2 days
DAAB
30
11
35
19
29
20
27
29
28
n.a.
5.20
9.03
n.a.
8.29
12.5
9.25
8.52
8.30
8.68
8.26
n.a.
Ͻ 5 s
25 min
18 s
4 min
13 s
25 s
35 s
60 s
n.a.
AzoAMP-1
AzoAMP-2
AzoAEP
AzoAP
AzoAMQ
AzoAMB
AzoAMF
AzoAME
AzoAMC
n.a.
0.19
n.a.
0.20
n.a.
n.a.
n.a.
n.a.
n.a.
[a] Change in integrated absorbance.
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Aminoazobenzene Derivative Photoisomerization
In DAAB and AzoAMB, which only contain an intra- nances in the 1H NMR spectrum. The trans/cis isomer ratio
molecular H-bond between the anilino hydrogen atom and was determined from integrated peak areas of anilino hy-
the azo group, an approximate 30% decrease in integrated drogen atoms, and ΦtransǞcis was calculated. Irradiation
absorbance was observed following irradiation. In converted 25% of AzoAP into the cis-isomer with ΦtransǞcis
AzoAMP-2, which has no intramolecular H-bonds, a 35% = 20%, which is typical of azobenzenes bearing amide func-
decrease in integrated absorbance was observed following tionalities.[29,47] AzoAP photochemistry reflects the impact
irradiation. The decreased absorbance change in DAAB of the electronic effects of amide groups, which dominate
and AzoAMB compared with AB reflects decreased any effect introduced by intramolecular hydrogen bonds.
transǞcis conversion of this class of ABs, as typified by
AzoAMP-2. A similar decrease in absorbance was observed
in AzoAMF, AzoAME, and AzoAMC, which bear oxygen-
derived H-bond acceptors. The modest, but measurable
photoisomerization in these molecules provides additional
proof that the dangling ligand does not contribute signifi-
cantly to intramolecular H-bonds and is consistent with the
NMR studies. In contrast, the integrated absorption for
AzoAMQ changes by 20%, indicating slightly decreased in-
tramolecular H-bonds in AzoAMQ compared with
AzoAMP-1, which prevent photoisomerization by in-
hibiting the concerted inversion pathway. AzoAEP exhibits
anomalous behavior. Although spectroscopic evidence sug-
gests that minimal photoisomerization occurs, the NMR
analysis does not indicate any H-bonding through the pyr-
idine group; however, this does not preclude intermolecular
phenomena like those observed in the X-ray analysis, which
could prevent isomerization. AzoAEP and several structur-
ally related compounds are the subject of an ongoing re-
search effort involving metal complexation.
When left in the dark, the absorbance of all aminoABs
except that of AzoAMC recovered completely, due to
cisǞtrans thermal relaxation. Irradiation of AzoAMC re-
sulted in irreversible changes in the absorption spectrum,
indicating photodegradation of the compound. The rate of
thermal isomerization was generally faster in AzoAMP-1
and AzoAMQ, for which the heterocyclic nitrogen atom
can engage in H-bonding. Minimal photoisomerization and
fast thermal isomerization in these derivatives indicate that
the intramolecular hydrogen bonds inhibit the concerted in-
version pathway as well as increase the thermodynamic sta-
bility of the trans-isomer.
Figure 3. Absorption changes in AzoAP. Irradiation of trans-
AzoAP leads to isomerization, with a steady state 3:1 trans/cis iso-
mer ratio being reached after 1 min (top). In the dark, AzoAP re-
turns to the original all-trans state in 20 min (bottom).
Conclusions
AzoAP does not behave like aminoAB because the anil-
ino lone pair can engage in resonance with the carbonyl
The photochemical properties of AminoABs differ from
oxygen rather than the AB ring system. The change in the those of the parent AB chromophore, but both classes of
nature of the substituent is also reflected in red-shifting molecules undergo transǞcis isomerization upon irradia-
of the absorption wavelengths, which is typical of tion with light. Intramolecular H-bonds prevent isomeriza-
amidoABs.[37,45,46] Upon irradiation, the integrated absorp- tion in AzoAMP-1 by blocking the concerted inversion
tion decreases 29% and reaches an apparent photostation- pathway; however, after investigating at a series of related
ary state within 1 min (Figure 3). A small increase in ab- aminoAB derivatives with differing H-bond acceptors re-
sorbance was observed at wavelengths greater than 480 nm, placing the pyridine groups it is clear that this photochemis-
1
as expected for ABs bearing amide substituents.[45,46] The try remains unique. Whereas H NMR and X-ray studies
slow AzoAP thermal cisǞtrans isometization allowed the provide evidence for a degree of H-bonding in solution and
photoisomerization quantum yield (ΦtransǞcis) to be quanti- in the solid state, none of the intramolecular forces appear
fied by 1H NMR analysis. To determine ΦtransǞcis, a 3.4 mm to be strong enough to block concerted inversion as ef-
solution of trans-AzoAP in CDCl3 was placed in an NMR ficiently as in AzoAMP-1. AzoAEP exhibits some anoma-
1
tube and the H NMR spectrum was recorded. The NMR lous properties that are under further investigation along
tube and its contents were then irradiated and the 1H NMR with a series of related derivatives.
spectrum was measured again. Irradiation caused the for-
Introducing a secondary switch to block photoisomeriza-
mation of cis-AzoAP, resulting in the growth of new reso- tion remains a long-term goal of this research. Presently,
Eur. J. Org. Chem. 2013, 4794–4803
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4799

S. C. Burdette et al.
FULL PAPER
12 h. The reaction mixture was diluted with EtOAc (10 mL) and
washed with 2 m aqueous HCl (10 mL) and saturated aqueous
NaCl (10 mL). The organic layer was dried with MgSO4 and the
solvent was removed. Column chromatography on silica (EtOAc/
hexanes, 4:6) gave the monoamide as a red solid. The monoamide
(100 mg, 0.32 mmol), picolinic acid chloride (300 mg, 2.1 mmol),
and K2CO3 (440 mg, 3.2 mmol) were combined in THF (10 mL)
and stirred at 23 °C for 24 h. The mixture was filtered, the filtrate
was collected, and the solvent was removed under vacuum. The
crude residue was purified by column chromatography on alumina
there is no effective way to disrupt the H-bonding in
AzoAMP-1 to restore transǞcis isomerization, and no
means of effectively preventing the isomerization without a
pendant pyridyl group. Whereas the simplicity of H-bond-
ing provides an attractive strategy for blocking isomeriza-
tion, tuning the strength of the interaction has proven to be
more difficult than originally hypothesized; however, these
studies support the concerted inversion mechanism of AB
photoisomerization. To inhibit photoisomerization, the
concerted inversion pathway must be energetically unfavor- (hexanes/EtOAc, 5:1) to give 6 (80 mg, 60%) as a light-orange so-
lid; m.p. Ͼ 260 °C; Rf = 0.25 (alumina; EtOAc/hexanes, 1:4). 1H
NMR (400 MHz, CDCl3): δ = 8.93 (t, J = 8.3 Hz, 2 H), 8.68 (s, 2
H), 8.34 (d, J = 7.8 Hz, 2 H), 8.16 (d, J = 8.9 Hz, 2 H), 7.94 (t, J
able and only the H-bonding in AzoAMP-1 can prevent the
aryl group distortions that are a prerequisite to isomeriza-
tion by inversion. Alternative means to toggle between a
photoactive and photoinactive states in AB derivatives are
= 8.2 Hz, 2 H), 7.6 (t, J = 7.8 Hz, 2 H), 7.56 (s, 2 H), 7.29 (s, 2
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 162.2, 150.4, 148.5,
being investigated by our group.
140.8, 137.8, 137.5, 133.4, 126.8, 124.0, 122.7, 120.58, 117.57 ppm.
IR (neat): ν = 3300.2, 2920.7, 2850.9, 1682.5, 1586.3, 1568.1,
˜
1464.7, 1448.7, 1427.2, 1311.32, 1280.7, 1259.8, 1238.5, 1155.0,
1087.2, 1018.8, 997.2, 950.6, 902.1, 869.6, 790.7, 764.3, 741.2,
Experimental Section
+
700.9, 684.5 cm–1. HRMS (+ESI): m/z calcd. for C24H19N6O2
General Procedures: All reagents were purchased and used without
further purification. 2,2Ј-Diaminoazobenzene (6; DAAB) was pre-
pared according to literature procedures.[48] Dichloromethane
(CH2Cl2) and toluene were sparged with argon and dried by pass-
age through alumina-based drying columns. All chromatography
and thin-layer chromatography (TLC) were performed on silica
(200–400 mesh). TLCs were developed by using EtOAc/hexanes
mixtures. 1H and 13C NMR spectra were recorded with a 400 MHz
NMR instrument. Chemical shifts are reported in ppm relative to
tetramethylsilane (TMS). IR spectra were recorded with an FTIR
instrument and samples were analyzed neat. High-resolution mass
spectra were recorded in positive ion mode (+ESI).
423.1569; found 423.1560.
2,2Ј-Bis[N,NЈ-(2-quinoline)methyl]diaminoazobenzene (AzoAMQ,
9): DAAB (200 mg, 0.94 mmol), 2-quinolinecarbaldehyde (320 mg,
2.0 mmol), and NaBH(OAc)3 (466 mg, 2.2 mmol) were combined
in CH2Cl2 (15 mL) and stirred at 23 °C for 24 h. Water (10 mL)
was added and the organic layer was collected and dried with
MgSO4. Solvent was removed under vacuum and the residue was
purified by flash chromatography on silica (hexanes/EtOAc, 1:9) to
give 9 (260 mg, 56%) as a dark-red solid; m.p. 101–102 °C; Rf =
1
0.20 (silica; EtOAc/hexanes, 1:4). H NMR (400 MHz, CDCl3): δ
= 9.25 (t, J = 5.3 Hz, 2 H), 8.16 (d, J = 8 Hz, 2 H), 8.08 (d, J =
8 Hz, 2 H), 8.00 (dd, J = 5.9, 1.8 Hz, 2 H), 7.80 (d, J = 7.8 Hz, 2
H), 7.76 (td, J = 7.0, 1.5 Hz, 2 H), 7.56 (td, J = 7.7, 1.2 Hz, 2 H),
7.4 (d, J = 8.7 Hz, 2 H), 7.27–7.25 (m, 2 H), 6.87–6.83 (m, 4 H),
4.82 (d, J = 5.16 Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ
= 158.6, 143.2, 137.5, 136.9, 131.8, 129.9, 129.2, 127.98, 127.9,
2,2Ј-Bis[N,NЈ-(2-pyridyl)ethyl]diaminoazobenzene (AzoAEP, 7):
DAAB (300 mg, 1.4 mmol) in MeOH (35 mL) was combined with
acetic acid (170 μL, 3.8 mmol) and freshly distilled 2-vinylpyridine
(260 μL, 3.8 mmol). The mixture was stirred at 65 °C for 12 h and
allowed to cool to room temperature. After removing half of the
MeOH, ice (50 g) was added and the resulting homogeneous solu-
tion was made basic (pH 10) with 1 m KOH. The product was ex-
tracted with EtOAc (3ϫ 20 mL), the organic layers were combined,
dried with MgSO4, and the solvent was removed under vacuum.
Flash chromatography on silica (EtOAc/hexanes, 1:9) gave 7
(380 mg, 64%) as a dark-red solid; m.p. 152–153 °C; Rf = 0.20 (sil-
ica; EtOAc/hexanes, 1:5). 1H NMR (400 MHz, CDCl3): δ = 8.58
(d, J = 5.0 Hz, 2 H), 8.28 (s, 1 H), 7.59 (t, J = 6.8 Hz, 4 H), 7.28–
7.19 (m, 5 H), 6.87 (d, J = 8.6 Hz, 2 H), 6.75 (t, J = 8.0 Hz, 2 H),
3.73 (t, J = 6.7 Hz, 4 H), 3.21 (t, J = 6.7 Hz, 4 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 159.0, 150.4, 143.9, 137.1, 131.8, 127.5,
127.6, 126.5, 119.6, 116.5, 112.4, 49.5 ppm. IR (neat): ν = 3253.7,
˜
3070.1, 2833.1, 1604.2, 1552.2, 1497.0, 1438.5, 1421.0, 1386.8,
1347.2, 1310.9, 1247.7, 1194.7, 1151.4, 1130.5, 1109.8, 1082.8,
1046.3, 964.3, 938.4, 811.2, 770.1, 753.43, 738.3 cm–1. HRMS
+
(+ESI): m/z calcd. for C32H27N6 495.2297; found 495.2338.
2,2Ј-Bis[N,NЈ-(2-benzyl)methyl]diaminoazobenzene (AzoAMB, 10):
DAAB (100 mg, 0.47 mmol), benzaldehyde (96 μL, 0.94 mmol),
and NaBH(OAc)3 (250 mg, 1.2 mmol) were combined in CH2Cl2
(15 mL) and stirred at 23 °C for 24 h. The reaction was quenched
by adding water (10 mL), the organic layer was separated, dried
with MgSO4, and the solvent was removed under vacuum. Flash
chromatography on silica (EtOAc/hexanes, 1:19) gave 10 (140 mg,
76%) as a bright-orange solid; m.p. 133–135 °C; Rf = 0.35 (silica;
123.4, 116.5, 112.2, 43.0, 38.0 ppm. IR (neat): ν = 3323.9, 3070.2,
˜
3006.4, 2926.5, 1595.0, 1566.4, 1504.0, 1472.5, 1433.1, 1363.6,
1318.9, 1308.1, 1287.3, 1246.9, 1210.9, 1172.9, 1144.8, 1113.8,
1095.0, 1075.7, 1042.5, 1002.5, 994.0, 880.8, 856.6, 834.0, 794.2,
1
EtOAc/hexanes, 1:4). H NMR (400 MHz, CDCl3): δ = 8.57 (s, 2
H), 7.61 (dd, J = 8.0, 1.3 Hz, 2 H), 7.41–7.30 (m, 10 H), 7.23 (t, J
= 7.6 Hz, 2 H), 6.78–6.74 (m, 4 H), 4.49 (s, 4 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 143.3, 139.0, 136.7, 131.7, 128.9, 127.9,
+
768.8, 736.6 cm–1. HRMS (+ESI): m/z calcd. for C26H26N6
423.2297; found 423.2279.
127.5, 127.5, 116.5, 112.4, 47.4 ppm. IR (neat): ν = 3234.8, 3064.0,
˜
2,2Ј-Bis[N,NЈ-(2-picolinamide)]diaminoazobenzene (AzoAP, 8): A
suspension of picolinic acid (240 mg, 2.0 mmol) in CH2Cl2 (7 mL)
was cooled to 0 °C and N-methylmorpholine (220 μL, 2.0 mmol)
and benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluoro-
phosphate (PyBOP, 520 mg, 1.0 mmol) were added. The reaction
mixture was warmed to room temperature and stirred for an ad-
ditional 12 h. The reaction mixture was treated with a solution of
DAAB (210 mg, 1.0 mmol) in DMF (5 mL) and stirred for another
3034.1, 2874.9, 2839.9, 1886.4, 1680.7, 1606.8, 1558.2, 1503.5,
1458.6, 1449.2, 1419.7, 1363.1, 1327.8, 1315.1, 1303.0, 1239.4,
1221.1, 1201.2, 1152.1, 1124.8, 1082.2, 1066.0, 1043.3, 1028.2,
932.2, 908.0, 870.3, 844.3, 831.4, 778.8, 733.6, 696.4 cm–1. HRMS
+
(+ESI): m/z calcd. for C26H24N4 393.2079; found 393.2048.
2,2Ј-Bis[N,NЈ-(2-furanyl)methyl]diaminoazobenzene (AzoAMF, 11):
DAAB (300 mg, 1.4 mmol), 2-furaldaldehyde (300 mg, 3.1 mmol),
4800
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 4794–4803

Aminoazobenzene Derivative Photoisomerization
and NaBH(OAc)3 (750 mg, 3.5 mmol) were combined in CH2Cl2
(25 mL). The mixture was stirred at 23 °C for 24 h. Water (10 mL)
was added and the organic layer was collected and dried with
MgSO4. Solvent was removed under vacuum and the residue was
purified by flash chromatography on silica (hexanes/EtOAc, 1:10)
solution was used. The solution was irradiated with a 1000 W radi-
ation source and spectra were recorded at 30 s intervals until no
further changes in absorption was observed. A 400 mL beaker con-
taining a saturated aqueous NaNO3 solution was placed in front of
the light source to filter off UV radiation (Ͻ 320 nm) and prevent
to give 11 (390 mg, 74%) as an orange-red solid; m.p. 115–116 °C; decomposition of the ABs. Upon completion of photoisomeriza-
Rf = 0.25 (silica; EtOAc/hexanes, 1:4). 1H NMR (400 MHz,
CDCl3): δ = 8.29 (s, 2 H), 7.67 (d, J = 8 Hz, 2 H), 7.41 (s, 2 H),
7.28 (t, J = 8 Hz, 2 H), 6.86 (d, J = 8 Hz, 2 H), 6.82 (t, J = 8 Hz,
2 H), 6.37 (s, 2 H), 6.30 (d, J = 3.2 Hz, 2 H), 4.50 (s, 4 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 152.5, 143.3, 142.2, 137.0, 127.1,
tion, the radiation source was turned off, and spectra were recorded
at 1 min interval until thermal isomerization was complete.
Percentage Change in Absorption: The percentage change in absorp-
tion was calculated by using Equation (1).
116.8, 112.1, 110.7, 107.2, 40.7 ppm. IR (neat): ν = 3237.9, 2923.0,
˜
2851.4, 1608.2, 1559.8, 1503.9, 1469.3, 1456.7, 1424.7, 1351.7,
1330.8, 1308.5, 1252.2, 1242.4, 1227.0, 1212.6, 1186.2, 1162.6,
1141.8, 1125.4, 1081.8, 1045.2, 1009.3, 982.4, 884.4, 798.3 cm–1.
(1)
+
HRMS (+ESI): m/z calcd. for C22H21N4O2 373.1665; found
373.1635.
͐
and ͐
are the integrated areas between 400 and
Abs final
Abs initial
2,2Ј-Bis[N,NЈ-(ethyl acetate)methyl]diaminoazobenzene (AzoAME,
12): DAAB (500 mg, 2.3 mmol), ethyl bromoacetate (770 mg,
5.0 mmol), N,N-diisopropylethylamine (750 mg, 5.8 mmol), and so-
dium iodide (520 mg, 3.4 mmol) were combined in CH3CN
(30 mL). The reaction mixture was heated to reflux at 70 °C for
5 h, allowed to cool to room temperature, and filtered. The residue
was washed with MeCN (20 mL), the filtrates were combined and
the solvent was removed under vacuum. Flash chromatography on
silica (EtOAc/hexanes, 1:10) gave 12 (520 mg, 58%) as a bright-
orange solid; m.p. 156–158 °C; Rf = 0.35 (silica; EtOAc/hexanes,
600 nm in the initial and final absorption spectra (before photo-
isomerization and after completion of photoisomerization), respec-
tively.
Half-Life of the cis-Isomer: A 25 μL AB solution (100 μm for
AzoAP) was irradiated with a 1000 W radiation source until no
further change in absorption was observed. A 400 mL beaker con-
taining a saturated aqueous NaNO3 solution was placed in front of
the light source to filter off UV radiation (Ͻ 320 nm) and prevent
decomposition of the ABs. Upon completion of photoisomeriza-
tion, the radiation source was turned off, and spectra were recorded
at 1 min interval until thermal isomerization was complete. Percen-
tage change in integrated absorption was calculated for each time
interval and plotted against time, from which half-life of the cis-
isomer (time required for percentage change in absorption to re-
cover by 50%) was determined.
1
1:4). H NMR (400 MHz, CDCl3): δ = 8.71 (s, 1 H), 7.94 (d, J =
8.2 Hz, 2 H), 7.29 (t, J = 6.1 Hz, 2 H), 6.88 (t, J = 7.8 Hz, 2 H),
6.66 (t, J = 8.1 Hz, 2 H), 4.34 (q, J = 7.2 Hz, 4 H), 4.13 (d, J =
3.9 Hz, 4 H), 1.36 (t, J = 7.2 Hz, 6 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 177.7, 141.8, 137.0, 132.2, 128.2, 111.7, 62.2, 45.9,
14.1 ppm. IR (neat): ν = 3209.0, 2989.1, 2993.8, 2838.1, 1738.7,
˜
1609.3, 1560.8, 1504.5, 1472.1, 1439.2, 1393.2, 1374.4, 1354.4,
1333.0, 1305.8, 1110.1, 1047.7, 1017.9, 1004.0, 940.9, 883.5, 855.4,
Quantum Yield of Photoisomerization: A sample of AzoAP (3.4 mg)
was dissolved in CDCl3 (0.60 mL) to achieve a final concentration
of 13.4 mm. The solution was placed in an NMR tube and the
1H NMR spectrum was recorded. The tube and its contents were
irradiated for a period of 1.0 min using a 1000 W radiation source,
and the 1H NMR spectrum was recorded. Growth of new peaks
was observed near the amide (NHCO) and aryl resonances. The
[cis-AzoAP]/[trans-AzoAP] ratio was calculated by using the inte-
grated peak areas of the NHCO resonance, from which the change
in AzoAP concentration (Δ[AzoAP]) was calculated. The quantum
yield of photoisomerization (Φ) of AzoAP was obtained by solving
Equation (2), where NA is the Avogadro number.
805.5, 747.7 cm–1. HRMS (+ESI): m/z calcd. for C20H24N4O4
+
385.1876; found 385.1856.
2,2Ј-Bis[N,NЈ-(acetic acid)methyl]diaminoazobenzene (AzoAMC,
13): AzoAME (130 mg, 0.33 mmol) and 2 m NaOH (1.75 mL,
3.50 mmol) were combined in EtOH (10 mL) and heated to reflux
for 4 h. The solution was allowed to cool to room temperature and
half of the solvent was removed under vacuum. The mixture was
acidified (pH 2) with 2 m HCl and the resulting precipitate was col-
lected by filtration, washed with water (5 mL), and dried to give 13
(70 mg, 63%) as an orange solid; m.p. 171–173 °C; Rf = 0.20 (silica;
1
EtOAc/hexanes, 1:1). H NMR (400 MHz, [D6]DMSO): δ = 12.89
(s, 2 H), 8.26 (s, 2 H), 7.83 (d, J = 7.9 Hz, 2 H), 7.28 (t, J = 7.0 Hz,
2 H), 6.77 (q, J = 7.4 Hz, 4 H), 4.09 (s, 4 H) ppm. 13C NMR
(100 MHz, [D6]DMSO): δ = 172.8, 143.7, 136.9, 132.6, 125.6,
(2)
116.6, 111.9, 44.9 ppm. IR (neat): ν = 2875.8, 1721.4, 1611.6,
˜
1565.3, 1508.2, 1471.6, 1431.3, 1408.2, 1381.7, 1335.1, 1306.1,
1251.9, 1221.0, 1158.7, 1145.6, 1094.6, 1048.3, 1003.0, 895.4, 824.2,
760.2, 734.8, 676.5 cm–1. HRMS (+ESI): m/z calcd. for
Intensity of Source: To determine the intensity of the radiation
source, a 0.60 mL aliquot of a 6 mm potassium ferrioxalate solution
was placed in an NMR tube and irradiated with the 1000 W radia-
tion source for 20 s. This resulted in the reduction of FeIII oxalate
to FeII oxalate. The irradiated solution was combined with ferro-
zine (5.2 mg, 3 equiv.), resulting in the formation of a reddish-pur-
ple solution containing [Fe(ferrozine)32+], which has a molar ab-
+
C16H17N4O4 329.1250; found 329.1248.
General Spectroscopic Methods: All solutions were prepared in
spectrophotometric grade solvents. Absorption spectra were re-
corded at 25 °C in a 1-cm path length quartz cuvette.
Photoisomerization and Thermal Relaxation: A 3.00 mL aliquot of
a 1:1 EtOH/Et2O solution was placed in a quartz cuvette and the
background absorption spectrum was recorded. A 7.5 μL aliquot
of an AB stock solution (10 mm) was added to obtain a 25 μm
AB solution, and its spectrum was recorded. For AzoAP, a 100 μm
sorption coefficient (Φ) of 27,900 cm–1 –1 at 563 nm. A 100 μL ali-
m
quot of the resulting solution was diluted by a factor of 60, and its
absorbance was measured at 563 nm. The concentration of FeII
produced by the reduction of FeIII oxalate is given by Equation (3).
Eur. J. Org. Chem. 2013, 4794–4803
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4801

S. C. Burdette et al.
FULL PAPER
Table 3. Crystallographic parameters for aminoazobenzene derivatives.
AzoAEP (7) AzoAP (8)
AzoAMQ (9)
AzoAMB (10) AzoAMF (11) AzoAME (12) AzoAMC (13)
Formula
C26H26N6
422.53
Pbca
7.2668(7)
13.6245(14)
21.850(2)
C24H18N6O2 C32H26N6·C32H24N6 C26H24N4
C22H20N4O2
372.42
C2/c
20.6504(12)
12.1557(7)
7.6106(4)
C20H24N4O4
384.43
P21/c
10.111(4)
11.3557(4)
9.1630(4)
C16H16N4O4·2(C2H6OS)
484.60
P
Formula weight
Space group
a [Å]
b [Å]
c [Å]
422.44
987.16
392.49
P21/n
P21/c
P21/c
3.9237(4)
22.575(2)
11.3030(13)
27.013(6)
12.201(3)
7.5511(17)
11.3306(7)
12.5951(8)
7.8183(6)
5.043(5)
11.293(5)
11.529(5)
60.744(5)
80.664(5)
87.161(5)
564.9(7)
1
α [°]
β [°]
94.261 (6)
98.03
108.639 (4)
104.376(4)
109.846(3)
γ [°]
V [Å3]
2163.3(4)
4
1.297
0.08
998.41(18)
2
2464.3(10)
2
1057.23(12)
2
1850.59(18)
4
1.337
0.09
989.67(7)
2
1.290
0.09
Z
ρ calcd. [gcm–3]
Absorp. coeff. [cm–1]
Temp. [K]
1.405
0.09
1.330
0.08
1.233
0.07
1.424
0.28
100
100
299
100
100
100
200
Total number of data
Number of unique data
Obsd. data[a]
R [%][b]
14383
1869
1239
0.045
0.106
149
5552
1767
1122
0.021
0.082
145
13938
4327
1742
0.106
0.366
344
8036
1861
1200
0.028
0.092
144
11172
1633
1184
0.031
0.102
131
11629
1737
1244
0.033
0.134
128
3356
1974
1649
0.034
0.110
153
0.24/–0.40
wR2 [%][c]
Number of parameters
Max./min. peaks [e/Å]
0.13/–0.18
0.11/–0.14
0.49/–0.27
0.09/–0.13
0.11/–0.19
0.18/–0.14
2
2 2
[a] Observation criterion: I Ͼ 2σ(I). [b] R = Σ||Fo| – |Fc||/Σ|Fo|. [c] wR2 = {Σ[w(Fo – Fc ) ]/Σw(Fo2)2}1/2
.
Supporting Information (see footnote on the first page of this arti-
cle): Figures S1–S11 showing the photoisomerization and thermal
relaxation of AzoAMB, AzoAEP, AzoAMF, AzoAME, AzoAMQ.
1H and 13C NMR spectra for all new compounds synthesized.
Complete tables of X-ray data and fully labeled ORTEP diagrams.
(3)
The intensity of the radiation source is given by Equation (4),
where Φ = 1.25.
Acknowledgments
(4)
The work was funded by Worcester Polytechnic Institute. The au-
thors thank Professor Jim Dittami for insightful discussions.
Collection and Reduction of X-ray Data: Crystals were covered in
paratone oil on 100 μm polyimide micromounts or glued on the tip
of a glass fiber and mounted on a CCD diffractometer equipped
with a low temperature device. Diffraction data were collected at
room temperature or at 100(2) K by using graphite monochro-
mated Mo-Kα radiation (λ = 0.71073 Å) with the omega scan tech-
nique. Empirical absorption corrections were applied with the SA-
DABS program.[49] The unit cells and space groups were deter-
mined with the SAINT+ program.[49] The structures were solved
by direct methods and refined by full-matrix least-squares using
the SHELXTL program.[49] Refinement was based on F2 using all
reflections. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms on carbon atoms were all located in the difference
maps and subsequently placed at idealized positions and given iso-
tropic U values 1.2 times that of the carbon atom to which they
were bonded. Hydrogen atoms bonded to oxygen atoms were lo-
cated and refined with isotropic thermal parameters. Mercury 1.4.2
software was used to examine the molecular structure.[50] The crys-
tallographic data and refinement parameters are shown in Table 3,
selected hydrogen bond lengths are shown in Table 2 and the 50%
thermal ellipsoid plots are shown in Figure 2.
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Published Online: June 21, 2013
Eur. J. Org. Chem. 2013, 4794–4803
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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