The Role of Side-Arms for Supramolecular Affinity Materials Based on 9,9′-Spirobifluorenes

Article abstract of DOI:10.1002/ejoc.201700758

An eightfold functionalized D_2d-symmetric 9,9′-spirobifluorene was condensed with a collection of diketones with elaborated structural features to form three-dimensional supramolecular architectures with active surfaces. Gas-sorption measuremen

Full text of DOI:10.1002/ejoc.201700758

A Journal of
Accepted Article
Title: The Role of Side-arms for Supramolecular Affinity Materials
based on 9,9'-Spirobifluorenes
Authors: Siegfried R Waldvogel, Isabella Pyka, Joachim Nikl, and
Dieter Schollmeyer
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700758
Link to VoR: http://dx.doi.org/10.1002/ejoc.201700758
Supported by

10.1002/ejoc.201700758
European Journal of Organic Chemistry
COMMUNICATION
The Role of Side-arms for Supramolecular Affinity Materials
based on 9,9’-Spirobifluorenes
Isabella Pyka,[a] Joachim Nikl,[a] Dieter Schollmeyer[a] and Siegfried R. Waldvogel*[a]
An eightfold functionalized D2d-symmetric 9,9’-spirobifluorene was
condensed with a collection of diketones with elaborated structural
features to form three-dimensional supramolecular architectures with
active surfaces. Gas sorption measurements by quartz crystal
microbalances revealed remarkable indications about the molecular
interactions for the application as affinity materials for the detection of
volatile organic compounds. Single-crystal X-ray structure analysis
further gave insight by packing motifs and for potential host-guest
interactions.
molecule, interacting with active surfaces that enable strong
attractions, leading the way to efficient affinity materials.[15]
An attractive entity for the construction of those architectures is
9,9’-spirobifluorene. The orthogonally oriented fluorene moieties
guarantee the desired rigidity. Moreover, the 9,9’-spirobifluorene
core offers versatile functionalization opportunities for a rich
variety of three-dimensional architectures.[16] By careful selection
of end groups, a major contribution for a deeper insight into host
and guest interaction is expected.
The design of supramolecular architectures with active surfaces
and their specific applications are widely investigated.[1] Therefore
complex and distinct rigid building blocks like triptycenes,[2]
propellanes,[3]
tribenzotriquinacenes,[4]
or
polyphenylene
dendrimers[5] are of particular interest. Those structural motifs
enable the generation of shape-persistent voids inside the
molecule. Their stiff conformation along with the existence of
cavities are the prerequisite for their use as molecular capsules,[6]
container molecules,[7] cage compounds,[8] and porous organic
materials.[9] McKeown and co-workers summarized the past
studies into three design criteria for an increasing porosity: the
necessity for molecular rigidity, bulky end groups, and three-
dimensionality.[10] The resulting structures following these
guidelines displayed a rich porosity with various applications in
[12]
[13]
gas separation,[11] energy storage,
luminescence.[14]
catalysis,
or
A particular research topic is the design and evaluation of porous-
like molecules as supramolecular affinity materials. An easily
accessible proof for those porous-like characteristics is the
gravimetric detection of volatile organic compounds (VOCs) by
quartz crystal microbalances (QCMs). The investigation by QCM
gives an indication of the preferred interactions between affinity
material and the particular analyte, displayed in a high affinity. We
could show, that this affinity is essentially affected by hydrogen
bonding, ⋅⋅⋅stacking, CH⋅⋅⋅ interactions, or dipole-dipole
interactions.[20] Therefore, the affinity materials have to fulfill
several crucial criteria. The existence of binding sites is an
essential requirement, along with their sufficient accessibility,
which is chiefly controlled by a conformational less flexible host
structure. The rigidity ensures the pre-organization of the guest
[a]
I. Pyka, J. Nikl, Dr. D. Schollmeyer, Prof. Dr. S. R. Waldvogel
Institut für Organische Chemie, Johannes Gutenberg-Universität
Mainz
Duesbergweg 10-14, 55128 Mainz, Germany
E-mail: waldvogel@uni-mainz.de
http://www.chemie.uni-mainz.de/OC/AK-Waldvogel
Scheme 1. Synthesis of the 9,9’-spirobifluorene-based condensation products
containing the individual structural motifs for each diketone. Reaction
conditions: 4 eq. NaOAc, THF, 100 °C, 48 h, sealed tube.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201700758
European Journal of Organic Chemistry
COMMUNICATION
In our ongoing research on suitable affinity materials for the
detection of VOCs, we demonstrated that the rigid core building
block 9,9’-spirobifluorene is more efficient for the generation of
supramolecular affinity materials than the rather flexible
tetraphenylmethane congener.[15] In our current study we
investigated the influence of the end group onto the affinity
behavior and the ability to affect the intermolecular interactions.
Therefore, a series of 9,9’-spirobifluorene-based compounds 3a–
h with varying end groups was observed by QCMs (Scheme 1).
The gravimetric sensing method employs high fundamental
quartz crystal microbalances (HFF-QCMs) with a fundamental
frequency of 195 MHz, enabling detections up to the picogram
scale within seconds.[17] The electrodes were individually coated
with 3a–h by using a well-established electrospray protocol.[18]
This method allows the deposition of an accurately defined
amount of affinity material (10.4 ng, 50 kHz) onto the electrode of
the QCM, without affecting the properties of the deposited
material.[19]
To combine 9,9’-spirobifluorene with the diketones 2a–h, we
initially transformed 9,9’-spirobifluorene into its octaamino
hydrochloride salt 1.[15] The subsequent condensation reaction of
1 with the diketones 2a–h provide the supramolecular hosts 3a–
h. After chromatography, 3a–h were further purified by
precipitation from methanol to provide 3a–d, 3f, and 3h as a
single product and 3e as well as 3g as a mixture of isomers,
respectively.
higher substituted analytes (Figure 1). 3a, 3g and 3h constantly
exhibit higher affinities compared to the other 9,9’-spirobifluorene
derivatives, whereas the high affinity of 3g (dark blue bars) is
striking. By contrast, 3b–d and in particular 3f show the opposite
behavior with obviously lower affinity to the analytes, respectively.
3e exhibits a slightly enhanced affinity for the analytes o-xylene,
mesitylene and pseudocumene.
3a
3b
3c
3d
3e
1,4
3f
3g
1,2
3h
1,0
0,8
0,6
0,4
0,2
0,0
e
e
e
e
e
e
d
e
n
e
n
n
e
u
n
n
n
n
e
l
y
t
e
z
e
e
e
l
y
l
y
l
y
-x
p
l
m
cu
n
o
t
e
-x
-x
si
b
o
e
m
o
m
u
se
p
With this collection of diketones in hand, a sufficient scope of end
groups should provide the structural features for a knowledge-
driven design. The diketone building blocks range from small to
large, bicyclic, symmetric as well as asymmetric, aliphatic, and
aromatic substructures. These were combined with the 9,9’-
spirobifluorene core 1 by condensation reaction. The generated
affinity materials are supposed to vary in their active surface,
accessibility, type of binding moieties and their periphery.
To quantify the analyte interactions of 3a–h, concentration
profiles of the supramolecular guest molecules were measured.
Therefore, a continuous flow of nitrogen, enriched with a defined
partial pressure of the analyte was mixed and directed over the
coated QCMs. The sensor responses were evaluated to different
analyte concentrations at equilibrium conditions. A subsequent
fitting of the sensor data using the Langmuir equation, provide the
calculated affinities of the individual affinity material. For a direct
comparison of the affinities with each 9,9’-spirobifluorene
derivative, hence relative affinities were calculated and depicted.
More details about the coating process, experimental setup and
determination of the affinities are given within the Supporting
Information.
The analytes used to characterize the adsorption behavior are
nonpolar aromatics and a selection of hazardous, illicit and
common chemicals or ubiquitous compounds. Considering that
the analytes differ greatly in their boiling points (and therefore in
vapor pressure) only affinities to an individual analyte can be
compared. Accordingly, the affinities are grouped by analyte in
the graphical presentation. We compare the compound affinities
of single isomers with isomeric mixtures as no significant affinity
deviation between isomers and their mixture was found in the
past.[15]
Figure 1. Relative affinities of 9,9’-spirobifluorenes 3a-h to a variety of BTX
analytes (normalized to 3a; for absolute data, see the Supporting Information).
3b–d and 3f have relative small end groups with less pronounced
binding sites and few possibilities for guest interactions, leading
to obviously lower affinities for the aromatic analytes, compared
to the affinity properties of 3a, 3e, 3g, and 3h. The partly
increased affinities of 3e could be attributed to the increased steric
demand of the bornane moiety as end group. This bicyclic moiety
exhibits three terminal methyl groups, which partially define the
binding sides. This affects the size of the active surface and the
pre-organization of the guest, particularly for higher substituted
analytes such as mesitylene and pseudocumene. 3a, 3g and 3h
have the most pronounced binding sites because of the extensive
and sophisticated diketones condensed. Therefore, more and
stronger interactions between the supramolecular host and the
analyte are possible, resulting in a higher affinity. The comparison
of the different 9,9’-spirobifluorene derivatives 3a–h clearly
reveals an influence onto the affinity behavior on QCM. A small
side-arm leads only to a low affinity, whereas significantly higher
affinity with a steric demanding side-arm can be achieved.
With the exception of cyclohexane, the analytes depicted in
Figure 2 are distinctly polar compounds, in which 3a–f display a
similar trend as observed in Figure 1 for the BTX analytes. Here,
a variety of previous studies already proved the successful
application by QCM for tracing these analytes, such as triacetone
triperoxide (TATP),[21] phenol,[20a] γ-butyrolactone (GBL),[20c] or
The eight 9,9’-spirobifluorene derivatives 3a–h show a strongly
differing affinity behavior for benzene, toluene, xylenes (BTX) and
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10.1002/ejoc.201700758
European Journal of Organic Chemistry
COMMUNICATION
acetone.[20b] The impressive affinity enhancement of 3a, 3h, and
in particular for 3g can be found in Figure 2, too. Also the lower
affinities of 3b–f are depicted. Unlike in Figure 1, no increased
affinity of 3e can be found in Figure 2. This may indicate a size
independence, whereby the more bulky analytes, such as
mesitylene and pseudocumene fit better into the pronounced
binding sites than the analytes of Figure 2.
chloroform and water molecules are implemented. We would like
to point out the different orientation of the cyclododecane
mioeties. One fluorene plane displays a syn orientation, whereas
in the other fluorene plane the side-arms are oriented anti to each
other. The syn-arranged fluorene plane generates with one side
of the anti-arranged fluorene
a pocket-like void, which
accommodates the solvent molecules. The considerably larger
and more rigid molecule 3h shows in crystal a structure, where a
significant high amount of toluene is incorporated, as well as
water molecules (see the Supporting Information, Figure S9 and
S10, S20 and S21). Apparently, a type of co-crystallization with
toluene is given. Furthermore, the structure is stabilized through
the hydrogen bonding network of the water molecules to the
nitrogen atoms and to each other. The potential to accommodate
analyte molecules at designed binding sites can be reflected in
our QCM investigations (Figure 1 and 2) as well. Looking on the
single-crystal X-ray structures of 3c and 3f (see the Supporting
Information, Figure S1–S4, S10–S13), an incorporation of
cyclohexane molecules occurs here, too. Furthermore, hydrogen
bonds between contained water molecules and the heterocyclic
nitrogen atoms can be observed in the smaller derivative 3f. But
in contrast to the molecular structures of 3b, 3d and 3h, the
aromatic fluorene moieties in 3c and 3f aren’t limited by the
bulkiness of the side-arms. Therefore, less pronounced void-like
interaction platforms were generated for 3c and 3f. The
incorporation of solvent molecules occurs here in a random
fashion.
3a
3b
3c
1,4
3d
3e
3f
3g
3h
1,2
1,0
0,8
0,6
0,4
0,2
0,0
e
e
e
n
o
P
AT
T
n
n
BL
e
o
t
a
G
t
x
e
ce
ce
a
phenol
h
a
o
l
y
cl
cy
n
h
p
Figure 2. Relative affinities of functionalized 9,9’-spirobifluorenes 3a-h to
hazardous, illicit, and common chemicals or ubiquitous analytes (normalized to
3a; for absolute data see the Supporting Information).
For a more detailed understanding of the molecular interactions,
X-ray structure analyses of suitable single-crystals of 3b–3d, 3f
and 3h were performed (see the Supporting Information, Figure
S1–S8, S10–S17 and Figure S9–S10, S20–S21). We are aware
that an observation made by crystal structure analyses cannot
necessarily be used to fully describe the dynamic interactions on
the surface of the QCM. With the observation of X-ray analyzed
structures, we introduce an additional view on those specific host-
guest systems on molecular level, which to the best of our
knowledge support the investigations made by QCM. The
medium-sized compound 3d is quite rigid and shows a relative
organized packing in crystal, which is mainly attributed to the
bicyclic side-arms. Figure 3 displays the molecular structure of 3d
in the crystal, whereby each 9,9’-spirobifluorene moiety
accommodates a disordered cyclohexane guest, which was used
as co-solvent for crystallization. For clarity, one possible
cyclohexane molecule is removed and the preferred binding site
between the two 9,9’-spirobifluorene moieties is highlighted.
Noteworthy are the aromatic planes as interaction areas, limited
by the bulkiness of the end groups. This is a strong argument for
McKeown guidelines, that bulkier end groups show a higher
porosity and therefore, influence the intermolecular interactions.
As well, the molecular structure of 3b (see the Supporting
Information, Figure S5 and S6, S14 and S15) shows an
accommodation of the co-solvent used to crystallize. Here,
Figure 3. Molecular structure of 3d by single-crystal X-ray analysis. Top:
Representation of two molecules in capped-stick and space-filling models with
incorporated cyclohexane, highlighted in red. Bottom: Packing viewed along the
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10.1002/ejoc.201700758
European Journal of Organic Chemistry
COMMUNICATION
crystallographic c* axis showing molecules of 3d and molecules of cyclohexane,
highlighted in red, with removed hydrogen atoms.
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Keywords: 9,9’-spirobifluorene • affinity materials • molecular
recognition • quartz crystal microbalances • supramolecular
chemistry
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Give me four at the end! The
selection of appropriately bulky end
groups condensed at the 9,9’-
I. Pyka, J. Nikl, D. Schollmeyer, S. R.
Waldvogel*
Page No. – Page No.
spirobifluorene backbone influences
essentially the three-dimensional
architecture. The potent host-guest
interactions were studied by quartz
crystal microbalance experiments.
Together with X-ray structure analysis
of suitable crystals their tremendous
influence onto the binding site
The Role of Side-arms for
Supramolecular Affinity Materials
based on 9,9’-Spirobifluorenes
definition was outlined creating a
consistent picture for their application
as affinity materials.
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