Excitonically Coupled States in Crystalline Coordination Networks

Article abstract of DOI:10.1002/chem.201702968

When chromophores are brought into close proximity, noncovalent interactions (π–π/CH–π) can lead to the formation of excitonically coupled states, which bestow new photophysical properties upon the aggregates. Because the properties of the new states not only depend on the strength of intermolecular interactions, but also on the relative orientation, supramolecular assemblies, where these parameters can be varied in a deliberate fashion, provide novel possibilities for the control of photophysical properties. This work reports that core-substituted naphthalene diimides (cNDIs) can be incorporated into surface-mounted metal– organic structures/frameworks (SURMOFs) to yield optical properties strikingly different from conventional aggregates of such molecules, for example, formed in solution or by crystallization. Organic linkers are used, based on cNDIs, well-known organic chromophores with numerous applications in different optoelectronic devices, to fabricate MOF thin films on transparent substrates. A thorough characterization of the properties of these highly ordered chromophoric assemblies reveals the presence of non-emissive excited states in the crystalline material. Structural modulations provide further insights into the nature of the coupling that gives rise to an excited-state energy level in the periodic structure.

Full text of DOI:10.1002/chem.201702968

A Journal of
Accepted Article
Title: Excitonically coupled states in crystalline coordination networks
Authors: Ritesh Haldar, Antoine Mazel, Reetu Joseph, Michael Adams,
Ian Howard, Bryce Richards, Manuel Tsotsalas, Engelbert
Redel, Stephane Diring, Fabrice Odobel, and Christof Wöll
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201702968
Link to VoR: http://dx.doi.org/10.1002/chem.201702968
Supported by

10.1002/chem.201702968
Chemistry - A European Journal
COMMUNICATION
Excitonically coupled states in crystalline coordination networks
Ritesh Haldar,[a] Antoine Mazel,[b] Reetu Joseph,[c] Michael Adams,[c] Ian A. Howard,[c][d] Bryce S.
Richards,[c][d] Manuel Tsotsalas,[a] Engelbert Redel,[a] Stéphane Diring,*[b] Fabrice Odobel,*[b] Christof
Wöll*[a]
Abstract: When chromophores are brought into close proximity,
noncovalent interactions (π-π/CH-π) can lead to the formation of
excitonically coupled states, which bestow new photophysical
properties upon the aggregates. Since the properties of the new
states not only depend on the strength of intermolecular interactions,
but also on the relative orientation, supramolecular assemblies,
where these parameters can be varied in a deliberate fashion,
provide novel possibilities for the control of photophysical properties.
Here, we demonstrate that core-substituted naphthalene diimides
(cNDIs) can be incorporated into surface mounted metal-organic
structures/frameworks (SURMOFs) to yield optical properties
strikingly different from conventional aggregates of such molecules,
Scheme 1. Zn-SURMOF-2 structure of cNDI-1. The 2D layers constructed by
e.g. formed in solution or by crystallization. We use organic linkers
Zn-paddlewheel and cNDI-1 linker stack over each other to allow an offset π
based on cNDIs, well-known organic chromophores with numerous
stacked arrangement of the cNDI cores; the transition dipoles align in a
applications in different optoelectronic devices, to fabricate MOF thin
films on transparent substrates. A thorough characterization of the
properties of these highly ordered chromophoric assemblies reveals
the presence of non-emissive excited states in the crystalline
material. Structural modulations provide further insights into the
nature of the coupling that gives rise to excited state energy level in
the periodic structure.
parallel fashion.
crystal structure is achieved remains a major challenge.[3]
Alternatively an external bias - such as temperature, pH or
solvation medium - has also been found to be effective to control
the aggregation behaviour and consequently the properties.[4] An
interesting, more rational approach to yield structures with
predictable properties is to modify the chromophores with
functionalities, so that they are suited as linkers to form metal-
organic frameworks (MOFs).[5] The use of metal-coordination
driven assembly exhibits a number of advantages; in addition to
the more easily predictable structures, MOFs exhibit a high
degree of crystallinity and also porosity.[6] The latter is of high
interest with regard to the incorporation of optically active guests
in the chromophoric host lattice. Furthermore, a number of
approaches have been developed to grow well-defined thin films
on optically transparent, conductive substrates, an important
prerequisite for optoelectronic applications.[7] Here, we
demonstrate the potential of chromophoric coordination
polymers for the case of naphthalene diimides (NDIs), one
important class of optically active compounds. There have been
seminal works on the cooperative properties in self-assembled
organic structures of these particularly interesting dyes for
photoinduced energy and electron transfers, and in organic
electronics.[8] Substitution at the imide –N with carboxylate
appended groups is a suitable choice for construction of metal-
coordination driven polymeric structures, better known as
MOFs.[9] In recent years, a few reports on NDI based MOFs
already demonstrated their potential as electro/photochromic
materials, specifically exploiting the redox active features.[9]
Nevertheless, insights into the specific absorption and emission
properties of the metal-bound NDIs π-stacks are not available.
Here, we investigate ordered π-stacked structures formed by the
assembly of suitable MOFs. We focus on MOF-2, a framework
of stacked 2D layers with a square grid-like structure, which are
obtained by tethering metal paddle-wheel secondary building
Immobilization of organic moieties/chromophores into solids can
lead to new materials with exciting functionalities resulting from
cooperative effects, for example long-lived charge-separated
states, light harvesting antennae, electro/photochromism or
electronic conductivity.[1] In light harvesting, for example, the
formation of supramolecular, well-defined assemblies is crucial
for efficient charge separation and transport.[2] One important
aspect in this area is the architectural design of such assemblies,
the control of distances and relative orientations of optically
active molecules. The straightforward approach of simple
crystallization is rather limited in this respect and the
modification of the molecules in such a way that a particular
[a]
Dr. R. Haldar, Dr. E. Redel, Dr. M. Tsotsalas, Prof. Dr. C. Wöll
Karlsruhe Institute of Technology (KIT)
Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz
Platz-1, 76344 Eggenstein-Leopoldshafen (Germany)
E-mail:
[b]
[c]
[d]
A. Mazel, Dr. S. Diring, Prof. Dr. F. Odobel
Universite Lunam, Universite de Nantes, CNRS, Chimie et
Interdisciplinarite: Synthese, Analyse, Modelization (CEISAM), UMR
6230, 2 rue de la Houssiniere, 44322 Nantes cedex 3, France
R. Joseph, M. Adams, Dr. I. A. Howard, Prof. Dr. B. S. Richards
Karlsruhe Institute of Technology (KIT)
Institute of Microstructure Technology (IMT), Hermann-von-
Helmholtz Platz-1, 76344 Eggenstein-Leopoldshafen (Germany)
Dr. I. A. Howard, Prof. Dr. B. S. Richards
Karlsruhe Institute of Technology (KIT)
Light Technology Institute (LTI), Engesserstrasse 13, 76131
Karlsruhe (Germany)
This article is protected by copyright. All rights reserved.

10.1002/chem.201702968
Chemistry - A European Journal
COMMUNICATION
comparatively fewer studies on cNDIs than their parent
unsubstituted NDI chromophore.[14] Here, we have selected
three different substitutions; bromo(isopropylamine)-NDI (cNDI-
1) and bis(ethoxy)-NDI (cNDI-2) and bis(cyclohexylamine)-NDI
(cNDI-3) (Figure 1a). In this study, these changes in the
substitution modulate the absorption spectra of the linker as well
as the thin films. The synthesis of these new, ditopic cNDI
ligands is described in the supporting information section. The
pink colored thin films of {Zn2(cNDI-1)2} (1) have been grown on
various substrates using liquid phase epitaxy (LPE) in a layer-
by-layer (lbl) fashion using zinc acetate as metal source and
cNDI-1 as the organic linker. Such method of crystalline film
growth is advantageous over the bulk powder synthesis,
because it can be conducted at ambient temperature (~ 20-
25 °C) and it is compatible with highly diluted precursor
concentration. In addition to requiring only very small amounts of
material, the NDI linker precursors used in the present study
often aggregate for higher concentrations ( < 1 mM). Therefore,
we kept the concentration below 20 μM. Similar LPE
methodology has already been well explored by our group and
few others to construct crystalline oriented SURMOFs from a
number of other ligands. [15] The out-of-plane and in-plane XRD
patterns of 1 grown on silicon substrates are shown in Figure 1b.
The diffraction data reveal high crystallinity and are comparable
to similar oriented SURMOF-2 type structures.[11b] The lattice
dimensions perpendicular and parallel to the substrate surface
are similar (2θ = 3.35°) and this distance closely matches with
the simulated structure of 1. Referring to the previously
synthesized Zn-SURMOF-2 structure,[11] in the present case a
square grid like structure of cNDI-1 and Zn paddle wheel is
evident (Scheme 1, Figure 1c and see supporting information).
These 2D sheets are then arranged in AA stacking fashion, as
expected in a P4 symmetric SURMOF-2 structure (Scheme 1,
Figure 1d). The inter-sheet distance is found to be ~ 5.4 Å from
the in plane XRD measurements (Figure 1b). Further, the
infrared reflection-absorption spectroscopy (IRRAS) showed the
symmetric and asymmetric stretching frequencies of carboxylate
groups at 1577 and 1416 cm-1 suggesting formation of paddle
wheel units, the key constituents of the structure (Figure S3a).
Presence of bands at 1675 and 1719 cm-1 are attributed to the
symmetric and asymmetric stretching frequencies of imide –C=O
group of cNDI-1 linker. Scanning electron microscopy (SEM)
image of the cross section of 1 shows a thickness of ~ 300 nm
for 20 cycles of spray; thus each cycle forms ~15 nm thick film.
We have also fabricated isostructural {Zn2(cNDI-2)2} (2) and
{Zn2(cNDI-3)2} (3), as yellow and blue colored thin films (Figure
1d), using the respective linkers. Both 2 and 3 have been
characterized using XRD, and IRRAS, which suggest that a Zn-
SURMOF-2 type structure has been formed (Figure 1b, and
Supporting Information). Changing the substituent does not
modulate the basic structure; however, the spatial orientation of
the chromophores might differ. A rotational flexibility along the
longer axis of the cNDI chromophore can allow for such different
spatial arrangement. Such a subtle change is not possible to be
characterized through XRD techniques, unless a single crystal
structure is obtained. Hence, we focus on the photophysics of
these SURMOFs to elucidate on their spatial arrangements.
1 grown on quartz substrate has been used for all the
photophysical studies. UV-Vis spectrum of the film (measured in
air, transmission mode) shows broader spectral feature in
comparison to that of the ligand in solution. The spectrum is
bathochromically shifted by ~ 12 nm for the lowest energy (S0-
Figure 1. (a) cNDI linkers used for Zn-SURMOF-2 construction, (b) calculated
XRD patterns of 1 (i), experimental out of plane (ii) and in plane (iii) XRD of 1,
(010) plane shown in the in plane XRD pattern 1 (iv), (c) 2D square grid like
structure of {Zn(cNDI-1)2} (1, (d) stacking of the cNDI-1 among the 2D layers;
for clarity only one part of structure is shown, (e) images of the thin films of 1
(pink), 2 (yellow) and 3 (blue) grown on quartz; (f) SEM image of the cross
section of 1 grown on silicon.
units via organic linkers. (Scheme 1).[10] Within these stacked
layers, the organic linkers are arranged in a periodic fashion with
a well-defined distance between the chromophores. A special
method, layer-by-layer synthesis, is employed to fabricate MOF
thin films, which are particularly well suited for optical
applications.[7c] In this particular case, the lbl method yields a
variant of the MOF-2 structure referred to as SURMOF-2
(Scheme 1).[11] Epitaxially grown surface mounted metal-organic
structures/frameworks (SURMOFs) have grown popular in the
recent years owing to their suitability for electronic and optical
applications.[12] Oriented crystalline SURMOFs can pave the way
to the fabrication of hybrid material displaying valuable
photophysical properties such as directional energy transfer,
electron transfer, photon upconversion and photovoltaic
effects.[12] The Zn-SURMOF-2 thus can be considered to be a
potential platform to study the above-mentioned effects of
chromophore packing on optical functionality. Here, we have
investigated the construction of core substituted NDI (cNDI)
based Zn-SURMOF-2 type oriented crystalline structures in a
strictly periodic fashion (Scheme 1). The growth of such oriented
cNDI based structures in the form of thin film represents an
important step towards the development of photofunctional
materials.
Structural
modulation
and
time-resolved
spectroscopic studies are employed to comprehend the metal-
coordination driven aggregation and its optical properties.
The cNDIs offer band gap tunability by changing the substituents
at the bay positions of the NDI core and are thus of high
significance to modify the electronic properties of the
chromophores.[13] By doing so, the absorption spectrum of cNDI
can be tuned over a fairly large regime, and the NDI can either
act as an electron acceptor or an electron donor unit. There are
This article is protected by copyright. All rights reserved.

10.1002/chem.201702968
Chemistry - A European Journal
COMMUNICATION
provide
any
insight
of
their
aggregation/stacking behavior and related
emission or absorption properties.[9e]
In order to rule out that the pronounced
photophysical changes observed for 1 result
from the presence of Zn(II), we have studied
the solution state aggregation behaviour of
cNDI-1 and Zn(II). We have monitored the
changes in UV-Vis and emission by titration of
the cNDI-1 solution with a solution of metal
acetate. Addition of several equivalents of
[Zn(OAc)2]·2H2O decreases the oscillator
strength (extinction coefficient), shifts the S0-
S1 transition to higher wavelength and
changes the ratio of the vibronic peaks. The
saturated absorption spectrum upon titration
features similar bands to that of 1, however,
time-resolved fluorescence measurement
reveals a lower quenching magnitude (~ 19 ±
0.01%) than that recorded in 1. The red
shifted
emission
spectrum
upon
[Zn(OAc)2]·2H2O addition also resembles to
that of 1. These observations point out the
existence of interacting states in the thin film,
which markedly differ from the solution. Upon
standing for couple of hours, the titrated
cNDI-1 solution with [Zn(OAc)2]·2H2O led to
the formation of a precipitate in the form of a
fine powder, which has been found to be
Figure 2. (a) UV-Vis spectra of 1, cNDI-1 in ethanol/DMF (90/10 v/v) with different equivalent of
Zn(OAc)2 added; (b) Emission spectra of 1 (blue), cNDI-1 in ethanol/DMF (90/10 v/v) (green) and
cNDI-1 with 10 eq of Zn(OAc)2 (wine), spectra are taken by integrating over their corresponding
fluorescence life-time range; (c) (upper panel) Fluorescence decay profiles of 1 (blue), cNDI-1 in
ethanol/DMF (90/10 v/v) (green) and cNDI-1 with 10 eq of Zn(OAc)2 (red), (lower panel) Transient
absorption decay (shorter delay time) profiles of 1 (pink), cNDI-1 in ethanol/DMF (90/10 v/v)
(black), cNDI-1 solid (gray), black solid line represents fitted data; (d) Transient absorption (longer
delay time) profile of 1.
amorphous/order-less
supporting
experiments
in
information).
we conclude
nature
From
(See
these
the
that
unexpected strong quenching of cNDI in 1
must be due the different aggregated state of
S1) peak and also showed changes of the intensity ratio of the
the chromophores, leading to a much stronger excitonic coupling
within the linear stacks (Scheme 2). The fluorescence decay of
free cNDI-1 linker in solid state was also measured and found to
be even shorter lived than 1, suggesting the presence of closely
packed aggregates in the solid. (Figure S8).
two vibronic overtones (~ 540 nm), which is known to be a
reliable evidence of the aggregation state of this type of
chromophores (Figure 2a).[8c] To gain an insight of the
photoluminescence
behaviour,
time-resolved
photoluminescence studies on the pristine cNDI-1 in solution
and on 1 have been performed using a streak camera system.
Figure 2c shows the emission decay of cNDI-1 and 1 at room
temperature. The emission profile of 1 is red shifted by 12 nm
relative to that of cNDI-1. The emission profiles fitted with bi-
exponential lifetimes show a drastically reduced emission decay
for 1 (~ 90 ps) compared to cNDI-1 in solution (~ 3.5 ns). The
calculated dynamic fluorescence quenching is ~ 97 ± 0.01% in 1.
The photophysical properties of 1 are thus strikingly different
from those observed in solution. These changes must result
from the packing of the cNDI-1 chromophores in the MOF
deviating substantially from the aggregation of the monomers in
solution.[14b-14c] The changes in the vibronic overtones of 1 hint
towards a rather closely packed arrangement of the cNDIs, in
accordance to the literature reports.[14b-14c] The structure derived
from the XRD-data reveals staples of closely spaced, 2D layers.
The cNDIs thus form continuous π-stacks or aggregates along
the 010 crystallographic directions (Figure 1d). Note, that such
an arrangement with a proximal distance of around 5.4 Å
between the chromophores is rather unusual and differs
substantially from other types of aggregation investigated in
earlier works.[4a,14b-14c, 14e] Previously reported NDI based metal-
organic structures show inter NDI distances of 3–7 Å, but do not
The best way to gain further information on the photophysics of
the excited-state species causing this unexpected optical
property is to examine the transient absorption (TA). The
transient absorption decay integrated over the wavelength
region from 600-650 nm (see Figure 2c) starting 1 ps after
excitation (to avoid coherent artifacts) can be fit by a triple
exponential decay. Although this wavelength range corresponds
to the emission band of the material, a negative transient
absorption signal is observed; the material shows net induced
Figure 3. (a) Schematic of QPDC doping in 1 showing possible stacking
between dopant and cNDI-1; (b) Fluorescence decay profiles observed in the
emission range of 540-650 nm for 1, 1:30 % and 1:10 % SURMOFs.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702968
Chemistry - A European Journal
COMMUNICATION
not exponential (for example
the
recombination
is
controlled by
a
diffusion
limited process as in free
charge recombination or
triplet-triplet
annihilation).
The TA decay profiles of
free cNDI-1 linker in the
solid state and in solution
are also shown in figure 2c.
The longer-lived transient in
the solution state of the free
linker is expected and
suggests the presence of a
monomeric state. For the
solid
chromophores,
state
free
a
substantially faster decay is
observed, even faster than
in case of 1. This suggests
a strong excitonic coupling,
as expected for the closely
Figure 4. (a) UV-Vis spectra of 2, cNDI-2 in ethanol/DMF (95/5 v/v) with different equivalents of Zn(OAc)2 added; (b)
Emission spectra of 2 (blue), cNDI-2 (green) in ethanol/DMF (95/5 v/v) and cNDI-2 (red) with 10 eq of Zn(OAc)2,
spectra are taken from their corresponding fluorescence life-time range; (c) Fluorescence decay profiles of 2 (blue),
cNDI-2 (green) in ethanol/DMF (90/10 v/v) and cNDI-2 (red) with 10 eq of Zn(OAc)2, black solid line represents fitted
data; (d) UV-Vis spectra of 3, cNDI-3 in ethanol/DMF (95/5 v/v) with different equivalent of Zn(OAc)2 added; (e)
Emission spectra of 3 (blue), cNDI-3 (green) in ethanol/DMF (90/10 v/v) and cNDI-3 (red) with 10 eq of Zn(OAc)2,
spectra are taken from their corresponding decay profile time range; (f) Fluorescence decay profiles of 3 (blue), cNDI-3
(green) in ethanol/DMF (95/5 v/v) and cNDI-3 (red) with 10 eq of Zn(OAc)2, black solid line represents fitted data.
packed
(~3.5-4
Å
interchromophore distance)
solid. It is important to note
that the fluorescence decay
in the solid is clearly faster
than in the chromophore
assembly 1. This is fully
consistent with the fact that
absorption in this region rather than stimulated emission. This
observation, coupled with the shorter lifetime of the transient
absorption signal compared to the emission decay (see Figure
2c), indicates that non-emissive excited states play an important
role in the excited-state population decay (in addition to the
emissive excited-states seen in the time-resolved emission). The
importance of dark states in the population decay is similar to
that seen for H-type aggregates.[16] A more detailed analysis
reveals that the lifetime of the transition into the dark states is
not monoexponential. There are three fast components (1 = 1.2
ps, 2 = 10 ps, 3 = 90 ps) on the timescale of picoseconds in the
TA, followed by a signal that decays much more slowly
(appearing as a constant offset in Figure 2c). We attribute this
observation to the presence of different decay channels. We
speculate that the shorter time constant (1) is associated with
the transition from the higher (S1H) to the lower excited state
(S1L) while the remaining two time constants correspond to the
decay to the ground state (See also the hypothesized energy
scheme displayed in scheme 2). The S1L state is long-lived (> 3
ns), and gives rise to the constant offset in the TAS data. To
selectively observe the decay of the longer-living species we
have measured transient absorption on an extended timescale,
from 3 ns to 9 μs. These transient absorption data of the long-
lived species are shown for the same wavelength region in
Figure 2d (now normalized to the signal after 3 ns). The decay of
the transient absorption cannot be fit with a bi-exponential
function. A stretched exponential with the form of: 푦(푡) =
퐴푒푥푝(−(푡/휏)^훽 ) with a stretching factor 훽 of 0.16 indicating a
very wide distribution of exponential decays would be needed to
fit these data. This indicates that the recombination of the long-
lived species is not monoexponential, either due to a broad
distribution of rates controlling the process (for example a broad
distribution of de-trapping rates) or that the decay is inherently
in the SURMOF structure cNDI-1 chromophores are separated
by larger interchromophore distance (~5.4 Å). In summary, the
time-resolved photoluminescence and absorption spectroscopy
suggest that packing of the cNDI linkers in the crystalline MOF
lattice affects the excitonic coupling between the cNDI linkers
and thus substantially alters the photophysical properties of the
chromophores. In the ordered cNDI structure the coupling to the
dark states is strongly increased relative to the randomly
aggregated state of the chromophores.
In order to further demonstrate the importance of the packing on
the observed excitonic coupling between the cNDI linkers, we
have introduced disorder in a deliberate fashion by replacing
some of the cNDI-linkers with an optically inert (i.e. aggregation
induced optical effect) linker of about the same length,
quadraphenyl dicarboxylate (QPDC) (Figure 3a). We could
successfully prepare two different defective SURMOFs on quartz
substrates by using mixtures of these two linkers (1:10% and
1:30%). As a result of the partial linker replacement, the out of
plane XRD patterns of both the doped SURMOFs showed small
changes; the (001) diffraction peaks at 2θ = 3.35° gets
broadened due to presence of diffraction peak at 2θ = 3.66°
corresponding to the QPDC linker based structure (Supporting
Information). The fluorescence lifetime of 1:10% and 1:30%
measured in the range of 550-700 nm shows distinct increment
(~ 335 and 365 ps, respectively) from that of pristine 1 (Figure
3b). Calculated fluorescence quenching values are 90% and
88%, significantly smaller than for pristine 1 (97%). This
observation clearly supports that in the SURMOF-2 type
structure, extensive excitonic coupling is caused by the special
packing of the chromophores. Altering this packing by
incorporating optically inactive linkers efficiently reduces the
excitonic coupling along the 1-D stacks of the chromophores.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702968
Chemistry - A European Journal
COMMUNICATION
lived states. The formation of these coupled excitonic states
depends strongly on the structural order within the 1D
chromophore staples, as demonstrated by substitution
experiments where the cNDI units were replaced by optically
inert linkers of the same length. This study thus demonstrates
the huge potential of MOFs with regard to realizing unusual
chromophore packings, which cannot be realized either in
solution or in the normal bulk structure of these materials. The
new packing with unusual distances and relative special
orientations between the transition dipole moments governing
the electronic excitations lead to new and unexpected
photophysical properties. The architecturing of chromophore
assemblies using the MOF-approach carries a huge potential as
regards tailoring exciton diffusion processes and creating
superstructures of fluorescent
J
aggregates with high
fluorescence life time and quantum yield. Future studies on such
chromophoric frameworks will focus on their redox and electron
Scheme 2. Schematic of the possible excited states formed due to different
aggregation of cNDI linkers in the SURMOF-2 structure and in solution after
metal ion addition.
acceptor
properties
to
explore
their
potential
in
photo/electrochromic and photovoltaic applications.
The results obtained for the two other, isostructural SURMOFs 2
and 3 are in full accordance with these conclusions. In the UV-
vis data of 2 (Figure 4) the bands appearing at ~ 470 nm are
shifted by ~ 6 nm compared to that of cNDI-2 in solution (Figure
4a). As in case of 1, the ratio of the vibronic peak intensities is
also found to be altered. Time resolved emission spectrum
collected the streak camera system reveals a shorter decay
profile (~ 0.3 ns) and rather large bathochromic shift; the
maximum of the emission band is shifted by ~ 70 nm compared
to that of cNDI-2 in solution (Figure 4b-4c). The strong
shortening of the fluorescence lifetime unequivocally
demonstrates an aggregation effect of the NDI core, similar to 1.
The relatively large stokes shift can originate from subtle
changes in their proximity or orientation. Further, titration of the
linker cNDI-2 in solution with [Zn(OAc)2]·2H2O suggested a
similar change of UV-Vis and emission profiles (Figure 4a).
However, the fluorescence decay is longer (0.7 ns) than found in
case of 2 which attributes to dissimilar aggregated structures
(Figure 4b). In case of 3, which bears bulkier cyclohexylamine
groups the same trend is observed. The UV-Vis spectrum of 3
shows a red shift of ~ 8 nm compared to the cNDI-3 in solution
(Figure 4d). In addition the change in vibronic peak ratio is also
observed. Unlike in 2, the emission spectrum of 3 shows a
smaller bathochromic shift of ~ 10 nm (Figure 4e). However, the
fluorescence decay profile suggests a striking quenching of
lifetime to ~ 220 ps (Figure 4f). The solution state titration of
cNDI-3 with [Zn(OAc)2]·2H2O revealed consistent changes in
UV-Vis and emission profiles(Figure 4d-4f). However, the
fluorescence decay is longer (~ 1.3 ns) in case of solution state
aggregated state than in 3. Considering the spectral features
observed in all three thin films and the corresponding
amorphous aggregated state formed in solution, corresponding
amorphous aggregated state formed in solution, the effect of
crystallinity/periodicity is evident.
Experimental Section
Zn(cNDI-1), Zn(cNDI-2), and Zn(cNDI-3) syntheses: All the Zn-
SURMOFs are fabricated by simple hand spray technique in a
layer-by-layer fashion on Au coated silicon, silicon and quartz
substrates. Au surfaces were functionalized by MHDA SAM,
while the silicon and quartz substrates were treated with plasma
under O2 (30 min) to remove the impurities and generate a
surface with hydroxyl groups. Synthetic details of the cNDI
linkers are described in the supporting information. 1 mM zinc
acetate ethanolic solution and 20 μM cNDI-1/2/3 solutions (5%
DMF + 95 % Ethanol) were sequentially deposited onto the
substrates for 15 s and 25 s in a layer-by-layer fashion. After
waiting for 35 s to allow for further coordination, the samples
were rinsed with ethanol to remove unreacted or by-products
from the surface. The thickness of the samples was controlled
by the number of deposition cycles. For the measurements in
streak camera and for transient absorption spectroscopy (TAS),
thin films grown on quartz have been used.
Keywords: Naphthalenediimide • Metal-organic thin films •
Liquid-phase epitaxy • Aggregation • Fluorescence quenching
Acknowledgements: R.H acknowledges
fellowship from Alexander von Humboldt (AvH) foundation; R.J
acknowledges fellowship from Deutscher Akademischer
a
postdoctoral
a
Austausch Dienst (DAAD); A.M, S.D and F.O acknowledge
Région des Pays de la Loire via the program LUMOMAT for the
financial support of this research with the project LumoMOF.
[1]
a) V. Stavila, A. A. Talin, M. D. Allendorf, Chem. Soc. Rev. 2014, 43,
5994-6010; b) M. C. So, G. P. Wiederrecht, J. T. Hupp, O. K. Farha,
Chem. Commun. 2015, 51, 3501-3510; c) C. A. Kent, D. M. Liu, L. Q.
Ma, J. M. Papanikolas, T. J. Meyer, W. B. Lin, J. Am. Chem. Soc. 2011,
133, 12940-12943; d) J. T. Joyce, F. R. Laffir, C. Silien, J. Phys. Chem.
C. 2013, 117, 12502-12509; e) J. Liu, W. Zhou, J. Liu, Y. Fujimori, T.
Higashino, H. Imahori, X. Jiang, X. Zhao, T. Sakurai, Y. Hattori, W.
Matsuda, S. Seki, S. Garlapati, S. Dasgupta, E. Redel, L. Sun, C. Wöll,
J. Mater. Chem. A. 2016, 4, 12739-12747; f) V. M. Martinez, S.
Furukawa, Y. Takashima, I. L. Arbeloa, S. Kitagawa, J. Phys. Chem. C.
2012, 116, 26084-26090; g) L. Sun, M. G. Campbell, M. Dincă, Angew.
Chem. Int. Ed. 2016, 55, 3566-3579.
In conclusion, we have constructed cNDI based metal-organic
assemblies showing highly unusual photophysical properties.
Zn-SURMOF-2 type structures fabricated using lbl-approach
yield
crystalline
frameworks
containing
1D
stacked
chromophores, with all the transition dipoles arranged in a
parallel fashion. This special type of arrangement results in the
formation of coupled excitonic states that drastically alter the
lifetime of the singlet exciton state by a transfer to new, long-
This article is protected by copyright. All rights reserved.

10.1002/chem.201702968
Chemistry - A European Journal
COMMUNICATION
[2]
a) Q. Zhang, C. Zhang, L. Cao, Z. Wang, B. An, Z. Lin, R. Huang, Z.
Zhang, C. Wang, W. Lin, J. Am. Chem. Soc. 2016, 138, 5308-5315; b)
M. D. Ward, F. Barigelletti, Coord. Chem. Rev. 2001, 216-217, 127-
154; c) K. Leong, M. E. Foster, B. M. Wong, E. D. Spoerke, D. V.
Gough, J. C. Deaton, M. D. Allendorf, J. Mater. Chem. A. 2014, 2,
3389-3398.
3, 414-422; e) S. Basak, N. Nandi, K. Bhattacharyya, A. Datta, A.
Banerjee, Phys. Chem. Chem. Phys. 2015, 17, 30398-30403.
[15] a) D. Zacher, O. Shekhah, C. Wöll, R. A. Fischer, Chem. Soc. Rev.
2009, 38, 1418-1429; b) H. Gliemann, C. Wöll, Materials Today, 2015,
15, 110-116; c) P. Falcaro, K. Okada, T. Hara, K. Ikigaki, Y. Tokudome,
A. W. Thornton, A. J. Hill, T. Williams, C. Doonan, M. Takahashi, Nat.
Mater, 2017, 16, 342-348.
[3]
[4]
P. Z. Moghadam, A. Li, S. B. Wiggin, A. Tao, A. G. P. Maloney, A. P.
Wood, S. C. Ward, D. Fairen-Jimenez, Chem. Mater. 2017, 29, 2618-
2625.
[16] S. Verma, A. Ghosh, A. Das, H. N. Ghosh, J. Phys. Chem. B. 2010,
114, 8327-8334.
a) K. Bag, P. K. Sukul, D. C. Santra, A. Roy, S. Malik, RSC. Adv. 2016,
6, 34027-3403; b) S. Wang, G. C. Bazan, Chem. Commun. 2004, 2508-
2509; c) M. Busby, C. Blum, M. Tibben, S. Fibikar, G. Calzaferri, V.
Subramaniam, L. De Cola, J. Am. Chem. Soc. 2008, 130, 10970-10976.
a) S. Kitagawa, R. Kitaura, S.-I, Noro, Angew. Chem. Int. Ed. 2004, 43,
2334-2375; b) O. M. Yaghi, Nat. Mater. 2007, 6, 92-93; c) H. Li, M.
Eddaoudi, M. O’keeffe, O. M. Yaghi, Nature, 1999, 402, 276-279.
a) R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M.
O’Keeffe, O. M. Yaghi, Science. 2008, 319, 939-943; b) J. R. Li, R. J.
Kuppler, H.-C. Zhou, Chem. Soc. Rev. 2009, 38, 1477-1504; c) L. J.
Murray, M. Dincă, J. R. Long, Chem. Soc. Rev. 2009, 38, 1294-1314.
a) J. Liu, M. Paranidas, L. Heinke, M. Buck, C. Ocal, V. Mugnaini, C.
Wöll, ChemElectroChem, 2016, 3, 713-718; b) Z.-G. Gu, A. Pfriem, S.
Hamsch, H. Breitwieser, J. Wohlgemuth, L. Heinke, H. Gliemann, C.
Wöll, Microporous Mesoporous Mater. 2015, 211, 82-87; c) J. Liu, C.
Wöll, Chem. Soc. Rev. 2017, DOI: 10.1039/C7CS00315C; d) P.
Falcaro, R. Ricco, C. M. Doherty, K. Liang, A. J. Hill, M. J. Styles,
Chem. Soc. Rev. 2014, 43, 5513-5560.
[5]
[6]
[7]
[8]
a) S. V. Bhosale, C. H. Jani, S. J. Langford, Chem. Soc. Rev. 2008, 37,
331-342; b) S.-L. Suraru, F. Würthner, Angew. Chem. Int. Ed. 2014, 53,
7428-7448; c) F. Würthner, C. R. Saha-Möller, B. Fimmel, S. Ogi, P.
Leowanawat, D. Schmidt, Chem. Rev. 2016, 116, 962-1052; d) Y. Liu,
L. Zhang, H. Lee, H.-W. Wang, A. Santala, F. Liu, Y. Diao, A. L.
Briseno, T. P. Russell, Adv. Energy Mater. 2015, 5, 1500195; e) E.
Ahmed, G. Ren, F. -S. Kim, E. C. Hollenbeck, S. A. Jenekhe, Chem.
Mater. 2011, 23, 4563-4577.
[9]
a) N. Sikdar, K. Jayaramulu, V. Kiran, K. V. Rao, S. Sampath, S. J.
George, T. K. Maji, Chem. Eur. -J. 2015, 21, 11701; b) T. Tao, Y.-H. Lei,
Y. –X. Peng, Y. Wang, W. Huang, Z.-X. Chen, X.-Z. You, Cryst. Growth.
Des. 2012, 4580-4587; c) K. L. Mulfort, O. K. Farha, C. D. Malliakas, M.
G. Kanatzidis, J. T. Hupp, Chem. Eur. J. 2010, 16, 276-281; d) Y.
Takashima, V. M. Martnez, S. Furukawa, M. Kondo, S. Shimomura, H.
Uehara, M. Nakahama, K. Sugimoto, S. Kitagawa, Nat. Commun. 2011,
2, 168-175; e) A. Mallik, B. Garai, M. Addicoat, P. S. Petkov, T. Heine,
R. Banerjee, Chem. Sci. 2015, 6, 1420-1425; f) B. Garai, A. Mallik, R.
Banerjee, Chem. Sci. 2016, 7, 2195-2200; g) Y.-X. Xie, W.-N. Zhao, G.-
C. Li, P.-F. Liu, L. Han, Inorg. Chem. 2016, 55, 549-551.
[10] H. Li, M. Eddaoudi, H. L. Li, O. M. Yaghi, J. Am. Chem. Soc. 1998, 120,
8571-8572.
[11] a) H. K. Arslan, O. Shekhah, D. C. Florian Wieland, M. Paulus, C.
Sternemann, M. A. Schroer, S. Tiemeyer, M. Tolan, R. A. Fischer, C.
Wöll, J. Am. Chem. Soc. 2011, 133, 8158-8161; b) J. Liu, B. Lukose, O.
Shekhah, H. K. Arslan, P. Weidler, H. Gliemann, S. Braese, S.
Grosjean, A. Godt, X. Feng, K. Müllen, I.-B. Magdau, T. Heine, C. Wöll,
Sci. Rep. 2012, 2, 921.
[12] a) H. J. Park, M. C. So, D. Gosztola, G. P. Weiderrecht, J. D. Emery, A.
B. F. Martinson, S. Er, C. E. Wilmer, N. A. Vermeulen, A. Aspuru-Guzik,
J. F. Stoddart, O. K. Farha, J. T. Hupp, ACS Appl. Mater. Interfaces.
2016, 8, 24983-24988; b) M. Oldenburg, A. Turshatov, D. Busko, S.
Wollgarten, M. Adams, N. Baroni, A. Welle, E. Redel, C. Wöll, B. S.
Richards, I. A. Howard, Adv. Mater. 2016, 28, 8477-8482; c) J. Liu, W.
Zhou, J. Liu, I. Howard, G. Kilibarda, S. Schlabach, D. Coupry, M.
Addicoat, S. Yoneda, Y. Tsutsui, T. Sakurai, S. Seki, Z. Wang, P.
Lindemann, E. Redel, T. Heine, C. Wöll, Angew. Chem. Int. Ed. 2015,
54, 7441-7445.
[13] N. Sakai, J. Mareda, E. Vauthey, S. Matile, Chem. Commun. 2010, 46,
4225-4237.
[14] a) O. Yushchenko, D. Villamaina, N. Sakai, S. Matile, E. Vauthey, J.
Phys. Chem. C. 2015, 119, 14999-1508; b) H. Kar, D. W. Gehrig, F.
Laquai, S. Ghosh, Nanoscale, 2015, 7, 6729-6736; c) H. Kar, S. Ghosh,
Chem. Commun. 2016, 52, 8818-8821; d) C. Thalacker, A. Miura, S.
De Feyter, F. C. De Schryver, F. Würthner, Org. Biomol. Chem. 2005,
This article is protected by copyright. All rights reserved.

10.1002/chem.201702968
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Periodically arranged core-substituted
naphthalene diimides in Zn-SURMOF-
2 structure exhibit excitonically
coupled states
Ritesh Haldar,[a] Antoine Mazel,[b] Reetu
Joseph,[c] Michael Adams,[c] Ian A.
Howard,[c][d] Bryce S. Richards,[c][d]
Manuel Tsotsalas,[a] Engelbert Redel,[a]
Stéphane Diring,*[b] Fabrice Odobel,*[b]
Christof Wöll*[a]
Page No. – Page No.
Title
This article is protected by copyright. All rights reserved.