Volume Conserving Geometric Isomerization of Encapsulated Azobenzenes in Ground and Excited States and as Radical Ion

Article abstract of DOI:10.1021/acs.orglett.7b02963

To probe the role of the supramolecular steric effects and free volume on photoreactions, geometric isomerization of neutral azobenzenes (ABs) and their radical ions, generated by electron transfer with gold nanoparticles, included within an octa acid capsule, was investigated. A comparison of the isomerization of ABs that proceed by volume conserving pyramidalization and stilbene analogues that proceed by volume demanding one bond flip has indicated the differing influence of 4-alkyl groups on these two processes.

Full text of DOI:10.1021/acs.orglett.7b02963

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Volume Conserving Geometric Isomerization of Encapsulated
Azobenzenes in Ground and Excited States and as Radical Ion
A. Mohan Raj and V. Ramamurthy*
Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States
S
* Supporting Information
ABSTRACT: To probe the role of the supramolecular steric
effects and free volume on photoreactions, geometric isomer-
ization of neutral azobenzenes (ABs) and their radical ions,
generated by electron transfer with gold nanoparticles,
included within an octa acid capsule, was investigated. A
comparison of the isomerization of ABs that proceed by
volume conserving pyramidalization and stilbene analogues
that proceed by volume demanding one bond flip has indicated the differing influence of 4-alkyl groups on these two processes.
o establish the disparate, selective behavior of “contained
and constrained” molecules from the free ones, we have
explored the excited state behavior of organic molecules within a
bimolecular organic capsule of the cavitand octa acid (OA, Figure
1).1 Our exploration of geometric isomerization of stilbenes and
of AuNP-catalyzed geometric isomerization of cis-AB. The
current study afforded the examination of the geometric
isomerization of AB in the ground and excited states as well as
that of AB radical ion in the dark and the behavior of AB relative
to the volume demand, with one bond flip isomerization of
stilbene analogues. The results of the thermal isomerization of
AB and its radical ion and the photochemical isomerization of AB
are presented below.
T
Trans-azobenzenes 1−5 (Figure 1) were synthesized, purified,
and characterized by methods reported in the literature.11,12
Complexation of 1−5 with OA in aqueous solution was achieved
as outlined in the Supporting Information. Characteristic
changes in the 1H NMR signals of the host OA and the upfield
shifts of the signals due to para-alkyl groups (CH2 and CH3) of
the guest (note signals in the region of δ 5.5−8 and below 0 ppm,
respectively, in Figure 2) in the Na2B4O7−D2O solution
Figure 1. Structures of host and guest molecules.
GFP chromophores revealed that their excited state behavior
within a capsule could only be predicted by considering the
overall structure of a host−guest complex and not that of the free
guest.2−4 As geometric isomerization in these olefins proceeds,
via the volume demanding 180° bond flip, we examined the effect
of constraint on the alternate volume conserving geometric
isomerization process, namely, pyramidalization. Geometric
isomerization of azobenzene (AB) in the excited and ground
states, occurring through the pyramidalization mechanism, made
it an ideal choice to examine the influence of constraints and
containment.5,6 Radical anion of cis-AB generated by electron
transfer (eT) was also reported to isomerize swiftly via
pyramidalization and progress by a chain mechanism.7 Our
earlier studies establishing the feasibility of eT between a donor
and an acceptor separated by the OA capsular wall gave us the
confidence to generate the source cis-AB radical ion within a
capsule.8,9 In this study, gold nanoparticles (AuNPs) known to
act as an electron source and sink in the dark were employed to
generate AB radical ion.10 We hoped our studies using OA
capsule would determine the yet to be fully unraveled mechanism
Figure 2. 1H NMR (500 MHz) spectra of (i) OA only, (ii) 1@OA2, (iii)
2@OA2, (iv) 3@OA2, (v) 4@OA2, and (vi) 5@OA2 ([OA] = 1 mM) in
10 mM Na2B4O7 buffer/D2O [1−5] = 0.5 mM; the red dot represents
the residual D2O.
Received: September 21, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02963
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Organic Letters
Letter
containing 1−5 and OA confirmed the inclusion of the former
within OA.13,14 The continuous change in the 1H NMR spectra
with addition of the guest to the OA solution that ended with 0.5
equiv of the guest suggested ABs formed stable 1:2 complexes
with OA (guest@OA2) (Figures S1−S5). Absence of any signals
due to the uncomplexed guest and appearance of guest peaks at
the same position independent of the host/guest ratio suggested
that the complex is stable in the NMR time scale. DOSY
experiments measuring the diffusion constants (Table S1) of the
complexes confirmed the 1:2 guest/host ratio.13 The lack of
exchange between complexed and free guests was evident from
the absence of signals due to the latter during titration. Despite
being insoluble in water, 1−5 formed a clear transparent solution
as complexes of OA.
Table 1. Photostationary State Composition of Azobenzenes
1−5 and Stilbene 6 in Solution with and without OA
photostationary state in toluene photostationary state inside OA
ab
,
ab
,
compound
(cis/trans)
(cis/trans)
1
2
3
4
5
86:14
90:10
88:12
95:5
83:17
77:23
97:3
68:32
70:30
91:9
c
6
76:18
20:80
a
b
c
1
See Figure 4 for irradiation details. Estimated by H NMR. From
ref 2.
The difference in the influence of the capsule on the
Absorption spectra of all trans-AB@OA2 complexes had two
maxima in the UV (350 10 nm, ππ*) and another in the visible
region (440 10 nm, nπ*) (Figure S6)15 along with absorption
due to OA being below 300 nm. OA complexes of 1−5 in
Na2B4O7 buffer solutions were irradiated with UV light (350
20 nm, Luzchem reactor) for 30 min, and the progress of the
isomerization was monitored by their UV−vis absorption and 1H
NMR spectra. To reverse the isomerization following the
generation of the cis isomer, the samples were irradiated with
isomerization process is revealed upon comparison of the
photobehavior of 4 and 6 within OA both containing 4,4′-
dimethyl phenyl substituents. Unlike the cis isomer enrichment
in isotropic solution in both cases, the photostationary mixture
with 6@OA2 was enriched in trans2 and in the cis isomer with 4@
OA2. We believe nearly the same 1H NMR chemical shift of the
methyl groups of 6@OA2 and 4@OA2 (Figure 4), suggesting
1
visible light (420 20 nm, Luzchem reactor). The H NMR
spectra of 3 irradiated with UV and then visible light are
presented in Figure 3 and those of the others in Figures S7−S10.
1
Figure 4. Partial HNMR (500 MHz) spectra (a) (i) 6@OA2 before
Figure 3. Photoisomerization of 3@OA2 monitored by 1H NMR ([OA]
= 1 mM in D2O borate buffer [3] = 0.5 mM); NMR recorded after UV
(350 20 nm) and visible (420 20 nm) irradiation; each irradiation is
for 30 min.
irradiation; (ii) 6@OA2 after 30 min UV (>280 nm); (b) (i) 4@OA2
before irradiation; (ii) 4@OA2 after 30 min UV (350 20 nm); [OA] =
1 mM [4 and 6] = 0.5 mM.
their placement in a similar environment within OA to be from
them interacting with the four aryl rings of the narrow top and
bottom ends of the OA capsule. Weak CH−π interaction
between the dimethyl group and the capsular wall and lack of free
space around the aryl ring likely restricted the isomerization of
trans-4,4′-dimethylstilbene to the cis within OA via one-bond flip,
a process requiring a large sweeping motion (Figure 5).2 Despite
the two molecules’ similar positioning within OA, photo-
isomerization of trans-4 to cis-4 proceeded within OA, resulting
in 66% of the latter isomer in the photostationary state mixture.
We trace the above variance to the mechanistic difference of the
isomerization in the excited state of the two chromophores.
While CC can isomerize only by 180° rotation,16,17 the NN
can do so by either a 180° rotation or by the volume conserving
pyramidalization process.5,6,18 As illustrated in Figure 5, the two
distinct pathways require different amounts of free space around
the chromophore: trans to cis conversion of 6 requires the (Ar)−
CC−(Ar) bond to sweep a larger cone through rotational
Following UV irradiation of the complexes, the observed
stronger and weaker absorption in the visible and UV regions
were consistent with the new signals in the 1H NMR spectra due
to the cis isomers. These signals in the upfield (<δ 0) region in the
1H NMR spectra confirmed the cis isomer’s presence within the
OA capsule. The photostationary state composition of cis and
trans isomers under UV irradiation conditions consisted of major
amounts of the cis (68−97%) and minor amounts of the trans
isomers (Table 1). A photostationary state isomer ratio
comparison under identical irradiation conditions in the
presence and absence of OA (aqueous solution and toluene,
respectively) (Table 1) suggested the effect of the substituent on
the 4-position of AB to be much smaller than in the case of
stilbenes (Table 1). Irradiation of the above solutions of host−
guest complexes containing a photostationary state mixture of
trans and cis isomers with visible light resulted in the reversal of
the cis to the trans isomers (Figure 3 and Figures S7−S10).
B
DOI: 10.1021/acs.orglett.7b02963
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Organic Letters
Letter
aryl ring of the AB. Whereas 1, 4, and 5 isomerized to the trans
isomers within a week, 2 and 3 took a significantly longer time.
The 4-propyl derivative 3 took 78 days (!) to quantitatively
return to the trans isomer. All five cis-ABs in toluene and in the
absence of OA quantitatively isomerized from the cis to the trans
isomer in less than 8 days (Figure 6 and Figure S14), indicating
the little effect that the nature of the alkyl group has on the cis−
trans pyramidalization barrier in the ground state of free AB.
Guided by the conclusion above that 4-ethyl and 4-propyl groups
have no effect on the excited state interconversion of cis and trans
isomers, we attribute the reduced cis to trans thermal isomer-
ization rate within OA in these two examples to OA capsule’s
ability to modify the ground state thermodynamic stability of the
included isomers. Based on the complexation behavior of
stilbenes described above, we believe that among the five ABs
1−5, the cis isomers of 2 and 3 would be relatively more stable
within OA than the corresponding trans isomers. We surmise
that the relative stability of cis-AB@OA2 would translate into
larger barrier and slower isomerization from cis to trans in these
two cases. Most likely, cis isomers of 2 and 3 fit better than the
trans within OA capsule. Such an unusual steric effect is specific
to the capsule and to the alkyl substituent. Such highly selective
behavior can only be observed in the case of reactions in an
enzyme.
We then examined the isomerization of the −NN− radical
ion within the confined spaces of an OA capsule using AuNPs as
an electron source/sink.10 Although the catalysis of the
isomerization of cis-AB by AuNPs has been reported, the factors
governing the process has been variously attributed to (a)
weakening of the −NN− bond upon adsorption on Au
surface, (b) eT from cis-AB to Au surface, and (c) eT from Au
surface to cis-AB.20−24 The confinement of cis-AB within OA
ruling out its direct contact with the surface of AuNPs leaves the
latter two options. Metastable and citrate-capped AuNPs were
prepared by published procedures and characterized by their
dynamic light scattering and absorption spectra (Figures S15−
S18).25−28 They had the characteristic plasmon band and
possessed a diameter of 6.8 2.2 nm (metastable) and 25.7 2.5
nm (citrate-capped).29,30
Figure 5. Mechanistic pathway for trans-azobenzene and trans-stilbene
isomerization.
motion, whereas the same process in 4 is achieved via
pyramidalization of the NN chromophore that sweeps a
smaller cone (Figure 5). Thus, the comparison of the above two
closely related molecules in OA advocates the importance of free
space within a confined reaction container and the value of such a
medium in unraveling the mechanism of photoreactions.
Due to the relative thermodynamic stability of the trans isomer
with respect to the cis counterpart and low activation barrier for
isomerization by pyramidalization, the cis-ABs are well-known to
isomerize from cis to trans isomers at ambient temperature in the
dark.15 The relative thermodynamic stability of the two isomers
of the free AB molecules while dictated by their inherent nature
would, within the OA capsule, depend on their fit within the
confined space and the nature of weak interactions between the
guest and the host. This is established to be the case in stilbenes.
The size of the alkyl group (methyl, ethyl, and propyl) of
stilbenes is found to influence the relative stability of the cis and
trans isomers within OA.19 Whereas a stronger complex was
formed by trans-4,4′-dimethylstilbene with OA rather than the cis
counterpart, it was the cis-4-ethyl/propylstilbenes rather than
their trans isomers to do so. Both isomers of 4-methylstilbene
showed equal preference. Expecting such a trend to extend to AB,
which could have a significant impact on the thermal isomer-
ization of cis-AB to trans-AB, we probed the role of these 4-alkyl
groups within OA.
To cis-1−5@OA2 (2 mL, 5 × 10−5 M) was added 200 mL of
AuNPs, and the absorption spectra of the solutions were
recorded over time (Figures S19−S22). In most cases, the cis
isomer was converted to trans within 15 min, except for cis-3@
OA2 requiring an hour. We believe that the faster isomerization
from cis to trans occurred from the radical ions of AB generated
via an eT process. These results illustrate AuNPs catalyzing the
geometric isomerization of encapsulated cis-AB from a distance
and without direct contact with the −NN− bond. To the best
of our knowledge, this is the first instance of an eT-induced
chemical transformation of a guest molecule fully enclosed
within a confined capsule. Of the five AB radical ions, the longer
time taken by 4-propyl-substituted AB (60 vs 15 min) to
isomerize to trans (Figures 6, S19, and S20), indicating greater
hindrance during pyramidalization within the capsule, is
consistent with the behavior of the corresponding neutral AB
during the thermal isomerization in the ground state.
UV−vis absorption spectra of cis-1-5@OA2 were recorded at
various time intervals by keeping the solution in the dark at room
temperature (Figure 6 and Figure S13). These figures reveal the
time taken for the cis-1-5@OA2 to isomerize to the
corresponding trans isomers varied with the substituent on the
The final point deals with the question of whether the cis-AB
serves as an electron donor or an acceptor with respect to AuNPs.
As noted below, we have not been able to resolve this question.
An early report on AuNP-catalyzed cis to trans isomerization of
AB suggested generation of cis-AB radical cation by eT from AB
to AuNP.20 However, an electrochemically and photochemically
generated AB radical anion has been demonstrated to isomerize
Figure 6. Absorption spectra of thermal isomerization of (a) cis-3 in
toluene; (b) cis-3@OA2; and (c) cis-3@OA2 + 200 μL of metastable
AuNP ([OA] = 1 × 10−4 M, [3] = 0.5 × 10−4 M).
C
DOI: 10.1021/acs.orglett.7b02963
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
from cis to trans very quickly.7 However, DFT calculations have
suggested a lower barrier for cis to trans conversion for both AB
radical anion and radical cation.21 To probe whether the AB
radical cation generated independently would lead to isomer-
ization of cis to trans, N-methylacridinium iodide was selectively
excited (Corning 7−60 band-pass filter, 320−400 nm) in the
presence of cis-3@OA2. Indeed, isomerization from cis to trans
occurred, suggesting that the AB radical cation also preferentially
isomerizes from cis to trans (Figure S23).31,32 Thus, based on
our’s and literature results, one can not unequivocally state
whether AB acts as the electron donor or acceptor with respect to
AuNP during thermal catalytic isomerization. Further work is
needed to narrow down the mechanism and identify the nature
of the radical ion. We contemplate a mechanism involving eT as
illustrated in Figure 7.
ORCID
V. Ramamurthy: 0000-0002-3168-2185
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
V.R. thanks the National Science Foundation (CHE-1411458)
for financial support.
■
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Figure 7. Cartoon representation of thermal isomerization of cis-ABs (a)
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The studies presented here on AB reinforce the utility of
confined space in controlling reactions including the excited and
ground state chemistry of neutral molecules as well as reactive
intermediates. Results presented above on thermal isomerization
of cis-azobenzenes@OA2 to trans (a) highlight the importance of
consideration of the structure of the host−guest complex as a
whole rather than the free guest’s alone in predicting the course
of a reaction in confined spaces, (b) stress how minor variations
in structure with no influence in isotropic solution affect the
chemistry in a confined space, and (c) emphasize that the
mechanism and the product formed within a supramolecular
assembly are governed by the environment instead of by the
intrinsic reactivity of the free molecule. The revealed possibility
of eT-mediated reactions within a capsule opens new avenues
which we plan to explore further.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02963.
■
S
1
Experimental procedure, H NMR spectra of the host−
guest complexes, absorption and 1H NMR spectra of the
UV and visible light irradiated samples, AuNP-catalyzed
samples and samples kept in the dark, characterization of
AuNPs (PDF)
(32) Lewis, F. D.; Kojima, M. J. Am. Chem. Soc. 1988, 110, 8664.
AUTHOR INFORMATION
Corresponding Author
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*E-mail: murthy1@miami.edu.
D
DOI: 10.1021/acs.orglett.7b02963
Org. Lett. XXXX, XXX, XXX−XXX