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Kenji Okada, Masahide Takahashi et al.
Infrared crystallography for framework and linker
orientation in metal–organic framework films

Volume 12
Number 27
21 July 2021
Pages 9249–9562

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Infrared crystallography for framework and linker
orientation in metal–organic framework ﬁlms†
Bettina Baumgartner, a Ken Ikigaki,a Kenji Okada
and Masahide Takahashi *a*abPore alignment and linker orientation inﬂuence diﬀusion and guest molecule interactions in metal–organic
frameworks (MOFs) and play a pivotal role for successful utilization of MOFs. The crystallographic
orientation and the degree of orientation of MOF ﬁlms are generally determined using X-ray diﬀraction.
However, diﬀraction methods reach their limit when it comes to very thin ﬁlms, identiﬁcation ofchemical connectivity or the orientation of organic functional groups in MOFs. Cu-based 2D MOF and
3D MOF ﬁlms prepared via layer-by-layer method and from aligned Cu(OH)2 substrates were studied
with polarization-dependent Fourier-transform infrared (FTIR) spectroscopy in transmission andReceived 28th April 2021
Accepted 7th June 2021

attenuated total reﬂection conﬁguration. Thereby, the degrees for in-plane and out-of-plane orientation,
the aromatic linker orientation and the initial alignment during layer-by-layer MOF growth, which is
impossible to investigate by laboratory XRD equipment, was determined. Experimental IR spectra

DOI: 10.1039/d1sc02370e

correlate with theoretical explanations, paving the way to expand the principle of IR crystallography to

rsc.li/chemical-scienceoriented, organic–inorganic hybrid ﬁlms beyond MOFs.

Introduction
In metal–organic frameworks (MOFs), organic linkers andmetal-containing units are combined, forming porous materials with great variety and multiplicity regarding constituents'
geometry, pore size and functionality.1 Their elds of applications are equally manifold and include photonics,2 energyrelated applications such as solar or fuel cells,3,4 homogeneous and heterogeneous catalysis,5,6 gas and fuel storage,7,8 or(bio-)sensing.9–11 While MOFs are available in a great variety ofshapes, such as nanocrystals, nanospheres, or hierarchicalmonoliths, MOF thin lms hold tremendous potential totranslate the structural properties of individual crystals tocontinuous, scalable materials. Scalability and processability
are essential for the application of MOFs in numerous elds12,13
such as optics,9 photonics,2 catalytic coatings, sensing, solar
cells or for batteries.14 The majority of these applications rely on
the precise crystallographic orientation and uniform porosity of
the MOF to allow for e.g. electron- or photoconduction. Hence,
controlling and determining the crystallinity and orientation,
including framework and linker orientation, of the MOF lmwith respect to the substrate is of prime importance forsuccessful implementations.
aDepartment of Materials Science, Graduate School of Engineering, Osaka Prefecture
University, Sakai, Osaka, 599-8531, Japanb

JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan. E-mail:k_okada@mtr.osakafu-u.ac.jp; masa@mtr.osakafu-u.ac.jp† Electronic supplementary
10.1039/d1sc02370e

information

9298 | Chem. Sci., 2021, 12, 9298–9308(ESI)

available.

See

DOI:

Typically, oriented MOF thin lms are synthesized by thelayer-by-layer (LbL) approach on substrates modied with selfassembled monolayers (SAM).15 Thereby, MOF lms
composed of 2D sheets (2D MOF) and coordinatively connected
2D sheets by pillar molecules (3D MOFs) with a multitude of
diﬀerent organic and metallic building blocks have been obtained.16–18 Although, this method allows to control the orientation of the lattice plane parallel to the substrate (out-of-planeorientation) by changing the terminated functional group of theSAM, these MOF lms lack in-plane orientation.19 Recently, ourgroup reported a synthetic method to obtain threedimensionally oriented 2D and 3D Cu-based MOF lms by
epitaxial growth on copper hydroxide,20,21 yielding large scalelms with controlled pore alignment.
The standard method to determine the crystallographic
orientation, and thus the pore alignment, of MOF lms is X-ray
diﬀraction (XRD) in out-of-plane (OOP) and in-plane (IP)conguration, i.e. perpendicular and horizontal relative to thesubstrate. The degree of in-plane orientation of lms can bequantied using azimuthal angle dependent intensity proles(4 scan) in the in-plane XRD conguration, i.e. rotating thesample around the axis perpendicular to the substrate center (4axis) at constant X-ray source and detector position.20–22 Besides
dedicated diﬀractometer equipment and rather long measurement times of several hours, this method requires for considerably thick MOF lms (typically >40 layers of LbL MOF lms) toachieve suﬃcient signal-to-noise ratios.
With respect to high surface sensitivity, infrared reection
absorption spectroscopy (IRRAS) has proven as powerful tool to© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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determine the structure and orientation of molecular layers onplanar surfaces,23,24 to study the initial growth of MOF lms,25 ordetermine structural defects within MOF lms.26,27 Polarized IR
radiation is reected from a thin lm deposited on metallic
surfaces under a grazing angle of incidence. In this conguration, parallel-polarized light is solely absorbed by moleculeswith chemical bands having their transition dipole moment
perpendicular to the substrate's surface.28,29 Thus, IRRAS
provides structural, chemical information on the axis perpendicular to the surface. This fact has been exploited by Terfort
and co-works to diﬀerentiate between surface-attached MOF(SURMOF) lms synthesized in diﬀerent crystallographic
orientations.19,30 The carboxylate bands originating from thecoordination of the organic linker with the metal nodes are
ideally suited to study the MOF orientation: four carboxylatebonds in their bridging conguration are oriented in a Cupaddle wheel that directs the structure of all MOFs withbimetal building units (e.g. MOFs based on M ¼ Cu, Cr, Mo, etc.,
compare Fig. 1).31 The perpendicular orientation of the
carboxylate bonds to each other and also to the surface allowed
Terfort and co-works to determine the out-of-plane orientationof MOF lms with diﬀerent preferred orientation synthesized by
LbL synthesis on gold surfaces functionalized with SAMs.19,30
Although IRRAS is well suited to study MOF lms prepared from
SAMs, the technique is restricted to oriented lms deposited on
metal surfaces. Furthermore, information on the orientation ofchemical bonds apart from perpendicular orientation, i.e. outof-plane, is not accessible.
Polarization-dependent IR spectroscopy in transmission andattenuated total reection (ATR) geometry is highly sensitive tostructural changes and atomic interactions in x, y and x, y, zdirection (i.e. in-plane and out-of-plane, compare Fig. 1),

Cu-paddle wheel structure within an oriented MOF ﬁlm.
Transition dipole moments of the IR vibrations of the carboxylate andthe aromatic ring present in the MOF are indicated with arrows.
Schematic of IR spectroscopy in transmission and ATR conﬁguration
for thin ﬁlm analysis and accessible transition dipole moments withrespect to the surface.

Fig. 1© 2021 The Author(s). Published by the Royal Society of Chemistry

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respectively. Researchers mainly in the elds of macromolecular chemistry and structural biology have taken advantage ofthese sensitivity to study the degree of crystallinity and theorientation in (semi-)crystalline polymers32–34 or the conformational changes in biomolecules.35–37 In this context, the term
infrared crystallography is commonly used, even though themethod only provides information on the orientation of molecules and functional groups but not the crystal's periodicity.36
In this contribution, as outlined in Fig. 1, we employedpolarization-dependent IR spectroscopy to study the in-planeand out-of-plane orientation of Cu-based 2D and 3D MOF
lms on Si substrates. Besides conrming the lm orientationand comparing favorably with results from XRD measurements
just obtained at shorter time scales, additional structural
information was retrieved: the orientation of the aromatic
linker in the 3D MOF Cu2(BDC)2DABCO (BDC: 1,4-benzenedicarboxylate, DABCO: 1,4-diazabicyclo[2.2.2]octane), to dateinaccessible with conventional techniques such as XRD and
IRRAS, and highly essential for the accessibility of the pores,
was determined to be parallel to the 2D MOF sheets andperpendicular to the bridging carboxylate plane. Furthermore,
the initial orientation of MOF lms in the LbL synthesis,
otherwise only feasible with synchrotron techniques due to the
low amount of material, could be investigated.

Results and discussion
Degree of in-plane orientation from polarization-dependent
IR spectroscopy in transmission
Oriented CuBDC and Cu2(1,4-NDC)2DABCO (1,4-NDC: 1,4naphthalenedicarboxylate) lms were grown from Cu(OH)2
nanobelt lms via a heteroepitaxial growth approach.20,21 MOF
lms with high and low degrees of in-plane orientation wereprepared on silicon substrates from aligned and randomlyaligned Cu(OH)2 nanobelt lms (see ESI† for Cu(OH)2 preparation and characterization). The corresponding SEM images
and XRD patterns are given in Fig. 2. SEM observation indicatesthat MOF crystals grown from oriented Cu(OH)2 nanobelts areoriented in three dimensions (x, y and z direction), while
randomly oriented Cu(OH)2 nanobelts yield MOF crystals onlyoriented perpendicular to the surface (z-direction) without inplane orientation. This conclusion drawn from MOF morphologies in the SEM images was further conrmed with XRD. For
MOF thin lms prepared from randomly oriented Cu(OH)2,
there is a clear diﬀerence between out-of-plane and in-plane
XRD patterns, but no diﬀerence is observed between the two
types of in-plane XRD patterns, conrming that the CuBDC andCu2(1,4-NDC)2DABCO MOF lms have only out-of-orientation
as reported previously.38,39 In contrast, MOF lms preparedfrom oriented Cu(OH)2 nanobelt lms show strong dependenceon X-ray incidence angle as visible in the IP XRD patterns,
conrming the fabrication of three-dimensionally oriented
MOF lms. Collectively, in MOF thin lms synthesized fromoriented Cu(OH)2, the c axis in CuBDC and b axis in Cu2(1,4NDC)2DABCO are along the z-direction and the b axis in CuBDC
and c axis in Cu2(1,4-NDC)2DABCO are parallel to the longitudinal direction (y-direction) of the underlying Cu(OH)2

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Chemical Science(A) Schematic representation of a MOF ﬁlm on Si substrate studied in transmission conﬁguration using polarized IR light. Orientation of thelong axis of the Cu(OH)2 nanobelts is indicated with white lines. (B) and (C) Cu-paddle wheel units in CuBDC (B) and Cu2(1,4-NDC)2DABCO (C)with respect to the polarization of the incident light. Transition dipole of symmetric and asymmetric carboxylate vibrations are highlighted inyellow and orange, respectively. Atom color code: C: black, H: white, Cu: blue, N: lilac, O: red. CuBDC and Cu2(1,4-NDC)2DABCO ﬁlms obtained
from random and aligned Cu(OH)2 nanobelts and corresponding SEM images, and polarized IR transmission spectra. nasymm(COO) bands arehighlighted in orange, while the bands corresponding to the nsymm(COO) band are highlighted in yellow. XRD patterns in out-of-plane (blacklines) and in-plane conﬁgurations (gray and orange lines, where the X-ray incident angle is parallel and perpendicular to the longitudinal directionof the nanobelts at 2q ¼ 0 ): reﬂections originated from MOFs are indicated. 4 scans were recorded for the 100 reﬂection for CuBDC and for the
001 reﬂection for Cu2(1,4-NDC)2DABCO. For comparability, 4-scans for random and aligned MOF ﬁlms are given for the same scale on the y-axis
for each MOF pair.
Fig. 2

nanobelts (see Fig. S4† for visualization of MOF lm structures).
This alignment was further quantied by 4 scan measurements,
which are given in Fig. 2.
From these proles, the degree of in-plane orientation wasdetermined using the full width at half maximum (FWHM) of
the peaks and eqn (1):40
FXRD ¼

180  FWHM
180

(1)

yielding FXRD ¼ 0.80 and 0.82 for aligned CuBDC and Cu2(1,4NDC)2DABCO lms, respectively. MOF lms from randomly

9300 | Chem. Sci., 2021, 12, 9298–9308

aligned Cu(OH)2 nanobelts show only broad signals in the 4
scan as expected.
FTIR spectra recorded with s- and p-polarized light of thesame set of lms with diﬀerent degrees of orientation wereacquired in transmission using blank silicon as backgroundand are given in Fig. 2 The bands at 1570 cm1 and 1616 cm1stem from the asymmetric carboxylate vibration, and the bands
at 1390 cm1 and 1423 cm1 are assigned to the symmetriccarboxylate vibration, which is consistent with previous
reports.39,41 The bands at 1465 and 1370 cm1 in the 3D MOF
stem from C–H vibrations of DABCO with a transition dipole© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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moment perpendicular to the N–N axis.42 No spectral diﬀerences between the polarized IR spectra were observed for the
MOF lms obtained from random Cu(OH)2 nanobelts, whilea strong polarization-dependence is visible for threedimensionally oriented MOF lms.
The reason for this becomes clear if we set the orientation of
the Cu-paddle wheel of CuBDC and Cu2(1,4-NDC)2DABCO in
relation to the Si substrate as given in Fig. 2A: the transition
dipole of the nsymm(COO) band (yellow arrow) is oriented in xdirection, thus, this band can only interact with p-polarized
light. Vice versa holds true for the asymmetric vibration thatcan only interact with s-polarized light. This polarization
dependence is visible in the spectra of both aligned lms.
Furthermore, the transition dipole moments of the C–H vibrations of DABCO present in the 3D MOFs are oriented in xdirection and hence show the same polarization-dependence as
the equally oriented dipole of the nsymm(COO) band. Note that,even if DABCO can rotate freely around the N–N axis, this would
not change the fact that it C–H groups only interacts with ppolarized light. For a perfectly aligned lm, no bands for thesymmetric and asymmetric vibration would be visible for p- ands-polarized light, respectively. However, the investigated MOF
lms show a degree of in-plane orientation of 0.80 and 0.82
determined from XRD data, respectively, hence, misaligned
crystallites are present. This misalignment leads to vibrations of
nsymm(COO) in the s-polarized spectrum and nasymm(COO) in
the p-polarized spectrum. This fact allows to use the linear
dichroism, i.e. the ratio of absorbance for s- and p-polarization,of each COO vibration to calculate the degree of in-planeorientation FIR from IR transmission spectra. The respective
band areas A were integrated, and FIR was retrieved using thefollowing equations:
FIR nasym ðCOO Þ ¼

Anasymm ðCOO Þ;kAnasymm ðCOO Þ;t þ Anaysmm ðCOO Þ;k

(2)



FIR nsym ðCOO Þ ¼

Ansymm ðCOO Þ;tAnsymm ðCOO Þ;t þ Ansymm ðCOO Þ;k(3)

Thereby, FIR(nasymm(COO)) values of 0.80 and 0.80 wereobtained from the spectra of both aligned MOF lms in Fig. 2,
which is in excellent agreement with the degree of orientation
obtained from XRD experiment. FIR(nsymm(COO)) ¼ 0.68 for
CuBDC, which has less correlation with the corresponding FXRD
value. We attribute this deviation of FIR to FXRD to crystallite
tilting and a diﬀerent structure, as we will discuss in the lastsection.
To demonstrate that this method is valid over the entire
range of in-plane orientation values, we prepared CuBDC andCu2(1,4-NDC)2DABCO lms from Cu(OH)2 lms with diﬀerent
degrees of orientation. The in-plane orientation of all lms wasdetermined with 4-scans in XRD and FTIR transmission spectroscopy. The results are compared in Fig. 3 and show similarvalues for FIR values >0.5. A lower correlation was found for thensymm(COO) band for CuBDC. Note that FIR ¼ 0.5 corresponds
to completely unoriented lms, while FIR ¼ 0 results only fromlms perfectly aligned perpendicular to lms with FIR ¼ 1 (see© 2021 The Author(s). Published by the Royal Society of Chemistry

Correlation of degree of orientation F obtained from XRD and
FTIR spectroscopy in transmission for CuBDC and Cu2(1,4-NDC)2DABCO ﬁlms for nsymm(COO) and nasymm(COO) bands.
Fig. 3Fig. S6 in ESI† for graphical illustration). In contrast, FXRD is anarbitrarily dened ratio that allows comparison between
oriented lms.40 In addition, peak tting and determining the
FWHM becomes increasingly prone to errors for FXRD < 0.5 andyields N for lms without preferential orientation and at 4scans. For these reasons, a correlation between FIR and FXRD is
physically not valid and shall only be used to compare lms
during material development for F > 0.5. Nevertheless, thesendings show that the degree of in-plane orientation of 2D and
3D MOF lms in the given boarders can be derived from simple
FTIR transmission spectra on Si substrates, which allows forfast screening without the need of dedicated diﬀraction
instrumentation or substrates with metallic coatings.
Crystallographic orientation of MOF lms in all three axes
studied with ATR spectroscopy
In contrast to spectra acquired in transmission, ATR spectroscopy provides two advantages: (i) higher interaction pathlengths, and hence higher sensitivity, can be achieved withmultibounce ATR crystals, as used in this study; and (ii) ATR
spectroscopy gives access to all three axes of the studied materials. This is due to the fact that the evanescent wave generated
upon total reection at the ATR crystal/sample interface has
fractions of the eld amplitude in all directions, namely Ex, Ey,
Ez, which are polarization-dependent: s-polarized light only
generates an evanescent wave in y-direction, while p-polarized
light induces an evanescent wave in x and z-direction(compare Fig. 4). Ex, Ey, Ez are not equal in amplitude anddepend on the refractive index of sample and ATR crystal
material, n2 and n1, respectively, and the angle of incidence q.
The fractions of the eld amplitudes at the ATR crystal's (z ¼ 0)
surface can be determined using eqn (4)–(6):43
2i cos q
Ey;0 ¼ 
1
1 2
1
3
11
2
1
sin q C
B
3
C
Ex;0 ¼ 2 cosðqÞB
@1 cos2 q A
2
sin q  þ
3
32

(4)

0

2

(5)

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with 3 ¼ (n1/n2)2. The polarization dependent absorbance Ak and
At is proportional to the evanescent wave, the transition dipole
vector of the vibration in question and the angle between both
vectors (compare Fig. 4):44,45
At f (myEy)2 ¼ (m0 sin a sin bEy)2(7a)

Ak f (mxEx)2 + (mzEz)2 ¼ (m0 sin a cos bEx)2 + (m0 cos aEz)2 (7b)(A) Linearly polarized light entering a trapezoidal Si ATR crystal
coated with a MOF ﬁlm at an angle of incidence q. Total reﬂection of spolarized light causes an evanescent wave amplitude Ey, while p-polarized
light generates Ex and Ez. (B) Fractions of Ex2, Ey2 and Ez2 as function of therefractive index of the MOF ﬁlm for Si ATR crystals with q ¼ 45 . Eﬀective
refractive indices n2,eﬀective of CuBDC and Cu2(1,4-NDC)2DABCO ﬁlms areindicated by gray and black dashed lines, respectively.
Fig. 4

Ez;0 ¼

2i sin qcos q1
1 cos2 q 2
2
3 sin q  þ
3
32

(6)

The simplied eqn (7a) and (7b) hold true for MOFs with themolecule axis aligned along the coordination system of the ATR
crystal, as in the case for the studied MOF lms (see ref. 44 and45 for more complex systems), and only for aligned dipoles, notfor dipole distributions. The nsymm(COO) and nasymm(COO)bands in the lms investigated have dipole moments with a andß being either 0 or 90 , which simplies interpretation ofpolarization dependent spectra. However, the trigonometric
relationship allows also to study bands under a specic angle.

Fig. 5 Cu2(BDC)2DABCO and Cu2(1,4-NDC)2DABCO in diﬀerent crystallographic orientation with respect to the Si ATR crystal's surface: IR
spectra (p-polarized spectrum ¼ blue, s-polarized spectrum ¼ black) of MOF ﬁlms with zk[001] (A row) show polarization-dependence for thenasymm(COO) band, while equal absorbance is visible for the nsymm(COO) band (compare D). IR spectra of MOF ﬁlms with zk[010] show
polarization-dependent nsymm(COO) bands for MOFs only aligned in z-direction (B row), while both COO bands are aﬀected for MOF ﬁlms
oriented in x, y, z direction (C row and compare E). Background of IR spectra ¼ blank silicon ATR crystal.

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The ratio of absorbances Ak and Anormal measured for ppolarized and s-polarized light, respectively, is called thedichroic ratio R:
R¼

Ak
Ex 2 þ Ez 2
¼
At
Ey 2

(8)

Experimentally, R can be determined by comparing theabsorbance of a specic band in both polarizations. R ¼ 2 for anangle of incidence of 45 (as used in this study) and isotropic
bulk samples such as water (see Fig. S9† for spectra). Note that R
is unity for isotropic media investigated in transmission. The
calculated eld amplitudes for varying refractive indices of thesample are depicted in Fig. 4 (see ESI† for further information).
The fraction of Ey remains constant, while Ex and Ez are aﬀectedby the refractive index of the sample (see ESI† for further
information on the determination of the refractive index of thelms). With the relative intensities of the evanescent wave ineach direction of Ex2 ¼ 0.30, Ey2 ¼ 0.33, Ez2 ¼ 0.37 at hand, theorientation of MOF lms can be determined. 3D MOFs with
diﬀerent crystallographic orientation were prepared on Si ATR
crystals and the SEM images of the lms and corresponding
ATR-IR spectra of Cu2(BDC)2DABCO and Cu2(1,4-NDC)2DABCO
lms are given in Fig. 5. Their crystallographic orientations
were conrmed by XRD (see Fig. S10 in ESI†) and the orientation of the MOFs in x, y and z-direction is shown schematicallyon the le in Fig. 5.
The Cu–Cu axis of Cu-paddle wheel unit and the N–N axis of
the DABCO pillar are oriented perpendicular to the substrate in
3D MOF lms with [001] preferred OOP orientation (compare
Fig. 5D). Both MOF lms with [001] preferred OOP orientation
show no in-plane orientation (Fig. 5A). This gives rise to transition dipole moments of the nasymm(COO) bands oriented in zdirection, while the dipole moments of the nsymm(COO) band
are aligned in the x–y plane, which is consistent with therecorded IR spectra: nasymm(COO) is only visible in the ppolarized spectrum and only contributions of MOF crystallites
deviating from the z-axis alignment contribute to this band in
the s-polarized spectrum (compare Table 1 for R values). As
MOF lms with [001] preferred OOP orientation rotate freely

Theoretically and experimentally determined dichroic ratios
for all MOF ﬁlms given in Fig. 5

Table 1

x: randomy: randomzk[001]
x: randomy: randomzk[010]xk[100]yk[001]
zk[010]R (nasymm)
predicted

R (nasymm)
foundaR (nsymm)
predicted

R (nasymm)
founda

N

5.0
3.7

0.9

1.1
1.1

0.9

1.0
0.9

2

2.0
2.0

0

0.2
0.3

N

5.5
5.0

a
Top and bottom values correspond to Cu2(BDC)2DABCO and Cu2(1,4NDC)2DABCO, respectively.

© 2021 The Author(s). Published by the Royal Society of Chemistryaround the z-axis and due to the fact that both nsymm(COO) andnsymm(C–H) are oriented in the x–y plane, these bands show thesame absorbance for both polarizations. Two types of MOFs
lms with [010] preferred OOP orientation (compare Fig. 5E)
were prepared from random and aligned Cu(OH)2 nanobelts,
yielding MOFs lms with and without in-plane orientation (Fig.
5B and C). For 3D MOFs prepared from randomly oriented
Cu(OH)2, the transition dipole moment of the nasymm(COO)
rotates freely around the z-axis, characterized by identicalabsorbance in p- and s-polarized spectra. For the transition
dipole moment of the nsymm(COO) and of n(C–H) bands
determining the dichroic ratio is more complex as contributions in all three axes have to be considered (see ESI† for full
explanation).
A dichroic ratio of 2 was derived for this case as found in the
spectra of both 3D MOFs. Lastly, 3D MOF lms prepared fromoriented Cu(OH)2 nanobelts show a high degree of IP and OOP
orientation. The transition dipole moment of the nasymm(COO)band is oriented in y-direction, thus yielding absorbance in thes-polarized spectra. The dipole moments of nsymm(COO) andnsymm(C–H) bands are oriented in x- and z-direction and therefore interact with p-polarized light. Analogous to the previous
section, 4 scans were recorded and FXRD was determined to be
0.83 and 0.82 for Cu2(BDC)2DABCO and Cu2(1,4-NDC)2DABCO,
respectively. FIR was determined from the ATR spectra (usingeqn (2) and (3)) to be 0.84 and 0.83 for nsymm(COO) band. Foraligned lms, FIR(nasymm(COO)) ¼ FIR,OOP and was determined
to be 0.83 and 0.82 for the Cu2(BDC)2DABCO lm and Cu2(1,4NDC)2DABCO lm, respectively.
Note that also transmission IR spectra show distinct diﬀerences that can be used to determine the MOF orientation(compare ESI† for explanation). This set of 3D MOF lms
demonstrates that diﬀerent crystallographic orientations can beeasily distinguished and the degree of in- and out-of-planeorientation can be determined using ATR-IR spectroscopy.
Initial orientation of MOFs prepared via layer-by-layer
deposition
So far, MOF lms obtained from Cu(OH)2 have been discussed,
which have lm thicknesses of 300–1200 nm (see Table S1 in
ESI† for all lms). The high thickness of the lms allowed
analysis in transmission and showed great absorbance values of
up to 1 A.U. for spectra obtained in ATR conguration. The highsensitivity of the multibounce ATR crystals allows studying eventhinner lms as obtained by LbL synthesis. These lms typicallyrequire a high number of deposited layers, typically >40 layers,
to reach suﬃcient material enabling the characterization withlaboratory XRD equipment. We studied the initial orientation of
CuBDC, Cu2(BDC)2DABCO and Cu2(1,4-NDC)2DABCO lms
prepared on Si ATR crystals via LbL deposition methods (seeESI† for synthesis). In contrast to standard LbL deposition on
SAM-modied Au surfaces, which would impede IR spectroscopy due to the high absorbance of Au, we followed a recent
report on LbL on blank Si surfaces.46 The LbL method yields
MOF lm oriented in z-direction but without in-plane orientation. Therefore, the obtained XRD patterns of all lms for >10
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Chemical Sciencelayers for 3D MOFs and >40 layers for CuBDC were in accordance with the XRD patterns obtained from lms grown fromrandomly oriented Cu(OH)2 nanobelts (see Fig. S12 in ESI† for
XRD data) and with patterns reported in literature.30,47
Polarization-dependent ATR spectra of CuBDC, Cu2(BDC)2DABCO and Cu2(1,4-NDC)2DABCO with 3 and 10 layers,respectively, are given in Fig. 6. While due to the low lm
thickness of the lms with three layers, no reections in the
XRD patterns were detected using standard integration times,
the polarization dependence of the nasymm(COO) is clearlyvisible for the 3D MOFs lms. Spectra of CuBDC aer 3 and 10
layer deposition are identical with the spectra of CuBDC lms
from random Cu(OH)2. Cu2(BDC)2DABCO ts the structure ofthe lm with [001] preferred OOP orientation without in-planeorientation: the nasymm(COO) band shows strong polarization
dependence, as its transition dipole moment is aligned in zdirection, thus, this band is only visible in the p-polarized
spectrum. Free rotation around the z-direction yield identicalabsorbance values for the nsymm(COO) band. The nasymm(COO) bands allow to determine the degree of out-of-planeorientation (FIR(nasymm(COO)) ¼ FIR,OOP) according to eqn (2)to be 0.95 and 0.97 for 3 and 10 layers, respectively. A similar
behavior was found for Cu2(1,4-NDC)2DABCO for three deposited layers. However, the polarization-dependent spectra of thelm with 10 layers show an increased absorbance for the p-

Fig. 6 Polarization-dependent ATR-IR spectra of MOF ﬁlms preparedwith the layer-by-layer method with 3 and 10 layers. The nasymm(COO) band of both 3D MOFs, Cu2(BDC)2DABCO and Cu2(1,4NDC)2DABCO, shows strong polarization dependence, which allows
to calculate the degree of out-of-plane orientation from the bandareas. Background ¼ blank Si ATR crystal.

9304 | Chem. Sci., 2021, 12, 9298–9308

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polarized spectrum for all bands (FIR,OOP ¼ 0.70 and 0.57 for 3
and 10 layers, respectively). This indicates that the preferential
order in z-direction of the Cu2(1,4-NDC)2DABCO MOF decreases
with increasing the number of deposition cycles. Although
being less ordered, a dichroic ratio of R ¼ 2 (for isotropic
materials) is not observed. We attribute the deviation from R ¼
2 to the low lm thickness that is known to decrease the eld
amplitude of Ez and thus the dichroic ratio for thin lms
converges to 1 for a lm thickness of 0.48 Given these results,
polarization-dependent ATR spectroscopy allows for quick
inspection and in-depth investigations of the orientation of
MOF ultrathin lms that are otherwise only accessible with longintegration times or in synchrotron facilities.
Orientation of aromatic linkers in MOF lms
So far, the carboxylate vibrations have been used to study theorientation of the MOF lm. By analyzing the vibrationalmodes of the aromatic rings in the MOF linkers, their orientation can be investigated as well. The skeletal ring vibrationsof aromatic compounds are located between 1400 cm1 and1600 cm1 with a transition dipole moment in-plane of thering and perpendicular to the 1,4-C–C-axis (compare Fig. 7).
Furthermore, the out-of-plane vibration of 1,4-substituted
benzenes is found at 805 cm1.49 We determined the directionof the transition dipole moments from a model compoundusing DFT calculations (see ESI† for details and Fig. 7A fororientation). The skeletal ring vibration is located at1506 cm1 for BDC. The lm without in-plane orientationshows no polarization dependence because of the free rotationof the MOF lm around the z-axis. In contrast, a strong
polarization dependence is observed for three-dimensionallyoriented Cu2(BDC)2DABCO lms (see Fig. 5). The dichroic
ratio Ak/At of the band at 1506 cm1 was calculated for aligned
and random MOFs lms with [010] preferred OOP orientationand yielded 2 and 7.3, respectively. These ratios are almost
identical to the Ak/At values of 2.1 and 6.4 found for thensymm(COO) band that has its transition dipole moment in
the same direction. Based on this fact, we conclude that thearomatic ring is oriented perpendicular to its carboxylate
plane (a–b lattice plane) as shown in Fig. 7B.
This nding was further veried in polarization dependent
IR transmission spectra of the out-of-plane vibration at
798 cm1, which has its dipole moment oriented in y-direction
and perpendicular to the band at 1506 cm1. Strong absorbanceof the Si-ATR crystal at wavenumber <1000 cm1 prevents
measurements in ATR conguration. With a single paththrough the substrate in transmission conguration, however,the vibrational modes around 800 cm1 can be analyzed (seeFig. S14 in ESI† for spectra). The band at 798 cm1 shows highabsorbance in the s-polarized spectrum, which is in line withthe dipole assignment for an aromatic ring oriented perpendicular to the carboxylate plane.
Here, IR spectroscopy provides for the rst time informationon the linker orientation in Cu2(BDC)2DABCO, which is notaccessible with XRD measurements as the rotation of thearomatic linker causes no change in the MOF patterns (compare© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Fig. 7 (A) Illustration of the transition dipole moments for the aromatic

vibrations found in the BDC linker. (B) Crystal structure of Cu2(BDC)2DABCO with the proposed aromatic linker orientation perpendicular to the carboxylate plane.

calculated patterns for crystal structures with diﬀerent linker
orientation in Fig. S16 in ESI†).
In contrast to Cu2(BDC)2DABCO, the aromatic linker vibration at 1515 cm1 of Cu2(1,4-NDC)2DABCO shows no polarization dependence in the ATR spectra (compare Fig. 5). However,a strong polarization dependence, identical to Cu2(BDC)2DABCO, is found for the out-of-plane vibrations in the region
around of 800 cm1 for the aligned MOF lm (compare spectrain Fig. S17 in ESI†). Although this conrms a preferential
orientation of the aromatic linkers with similar linker orientation as found in the Cu2(BDC)2DABCO, the polarization
dependence is less pronounced. This is most likely due to thefact that the 1,4-NDC linker can either rotate freely as it has
been suggested for coordination polymers,50 or because thearomatic ring is not oriented parallel or perpendicular to theCu-paddle wheel plane and is oriented under an angle of 48 asfound in the reported crystal structure.51

Structure of CuBDC
The in-plane orientation of CuBDC lms synthesized from theoriented Cu(OH)2 nanobelts was conrmed by XRD patterns
and transmission IR spectra, as shown in Fig. 1. Although thenasymm(COO) band showed good correlation with 4 scan dataof XRD investigations, FIR(nsymm(COO)) showed deviations
from FXRD. In order to investigate this cause in more detail,polarization-dependent ATR spectra of CuBDC lms werecollected to investigate the three-dimensional molecularorientation. While the spectra of the three-dimensionallyoriented CuBDC lm only allow for retrieving the degree oforientation, information of the molecular orientation can be© 2021 The Author(s). Published by the Royal Society of Chemistry

Chemical ScienceFig. 8 (Top) Polarization-dependent ATR spectra of CuBDC preparedfrom random and aligned Cu(OH)2. Background spectrum ¼ blank Si
ATR crystal. (Bottom) Cu-paddle wheel subunit found in 2D MOFs and
BDC linker alignment in a layered coordination polymer. The transition
dipole moment of the nsymm(COO) bands are indicated by yellow
arrows.

gained from the CuBDC lm without in-plane orientation
prepared from random Cu(OH)2 (compare Fig. 8): equal absorbance values for p- and s-polarized light were expected for thenasymm(COO) band and were found in the spectra (R ¼ 0.3/0.33¼ 0.9, found: 1.2). Although the nsymm(COO) band was expected to exhibit a dichroic ratio of 2, R ¼ 1.15 was observed.
The dichroic ratio from IR spectra in ATR and transmission
conguration of the nsymm(COO) band suggests that the BDC
linkers are to a great extent oriented parallel to the surface;unlike in the Cu paddle wheel structure with 50% of the
carboxylate bands in perpendicular orientation. The XRD
pattern of CuBDC shown in Fig. 1 can be attributed to thestructure consisting of Cu paddle wheels linked by four BDC
with P4 symmetry, as found in previous reports.38 However, ingeneral, there is less certainty in identifying the structure when
there are few reections in XRD patterns. It is also diﬃcult to
identify the local coordination structure in MOF by XRD. Our IR
results indicate a diﬀerent CuBDC structure constituting of
diﬀerent coordination bonds between Cu and carboxylic acids,
similar to the structures observed in copper(II) terephthalate
and copper(II) hydroxy-terephthalate coordination polymers(see Fig. S18 and S19 in ESI†).52–55
We considered these structures and tested them against the
IR spectra: for linkers parallel to the surface (g ¼ 0 ), bothcarboxylate vibrations would show no contribution in z-direction. Thus, the transmission spectra and the ATR spectra would
be identical. However, the band ratio nsymm(COO)/nasymm(COO) of 0.77 for transmission spectra of random CuBDC
compared to 0.92 for the same lm in ATR congurationsuggests that the nsymm(COO) band has a component in zdirection, which is not accessible in transmission. From thisChem. Sci., 2021, 12, 9298–9308 | 9305

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Chemical Science
ratio, a percentage of 19% of linkers aligned perpendicular tothe surface was derived. The fraction of CuBDC with linkers
oriented perpendicular to the surface causes a 100 reection in
the out-of-plane XRD pattern at 2q ¼ 8.2 , while the paralleloriented fraction leads to the same reection in the in-plane
pattern. Typically, the occurrence of this reection in both
XRD congurations is used to conrm the presence of the Cu
paddle wheel subunit connected by BDC linkers, and thus, the2D MOF structure. Since XRD patterns obtained from diﬀerent
congurations cannot be compared quantitatively, we cannot
assign the 100 reection in the OOP pattern unambiguously toeither the fraction of 19% of misaligned crystallites or thepresence of a 2D MOF structure. However, the usually low
porosity observed for CuBDC39 and the position of the COObands in the IR spectrum further point towards a more densely
packed layered structure. While in 3D MOFs the nasymm(COO)band is located between 1610–1640 cm1,30 this band is locatedat 1570 cm1 for CuBDC, which is the same position as reported
for layered structures.25 Therefore, a diﬀerent copper carboxylate connectivity, as reported for the layered structure, can beassumed in the CuBDC structure. Note that the methodology to
calculate the degree of in-plane orientation FIR based on IR
spectroscopy in transmission, as presented in the previous
section, is still applicable for this structure: FIR derived fromnasymm(COO) is not aﬀected by the tilting as its direction of thetransition dipole moment is still in x–y plane. In summary,polarization-dependent IR spectroscopy provided informationof the types of coordination bonds and their three-dimensional
orientations in the CuBDC structure, which are diﬃcult toreveal by XRD. It can be expected that the combination of XRD
and polarization-dependent IR spectroscopy will contributesignicantly to the complete clarication of the real structure ofother coordination polymers and MOFs with unknown
structures.

Conclusion
The orientation of 2D and 3D MOF lms was studied using
polarization dependent IR spectroscopy. MOF lms wereprepared on Si substrates and Si ATR crystals by layer-by-layer
synthesis and from aligned Cu(OH)2 precursor lms. Cu2(BDC)2DABCO and Cu2(1,4-NDC)2DABCO served as model
systems as their established synthesis allowed to easily adjust
their crystallographic orientation.21 However, we provide thetheoretical underpinnings to extend the principle to MOFs with
diﬀerent subunits (compare eqn (7a) and (7b)). A new method todetermine the degree of in-plane orientation of MOFs based on
straight-forward IR transmission measurements was presented
that bears comparison with conventional techniques based on
XRD. The method is not suitable for investigations of ultrathin
MOF lms, as FTIR spectroscopy in transmission has a low
sensitivity and the presented method thus relies on suﬃcientlm thickness. In contrast, the high sensitivity of multibounce
ATR crystals allowed us to study the orientation of the initial few
layers of MOF lms during LbL synthesis, usually requiring
synchrotron irradiations or extensive integration times. We
found a decrease in out-of-plane orientation with increasing

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number of layers only for Cu2(1,4-NDC)2DABCO, while Cu2(BDC)2DABCO remains oriented. For the rst time, experimental
evidence of the organic linker alignment perpendicular to thecarboxylate plane in Cu2(BDC)2DABCO lms was derived fromthe dichroic ratio of the ring vibrations of the aromatic linker.
Information on the linker orientation is essential for applications that rely on mass transport along the pores that might behindered by linkers extending into the pore channel. Finally,
inexplicable polarization dependent IR spectra of the 2D MOF
CuBDC made us revisit its structure. In contrast to data from
XRD, IR crystallography revealed that the metal ions are notconnected via a Cu-paddle wheel subunit as reported so far but
in a layered structure.
Future work will combine the experimental setup withadsorption studies of probe molecules, similar to ref. 56 and 57.
These would not just allow to investigate the orientation of the
MOF itself but the alignment of guest molecules during theadsorption process. Insights into the orientation and adsorption site of the probe molecules will deepen the understandingof interactions between adsorbent and adsorbate, which is key
to exploit the full potential of MOF lms.
The presented principles are not limited to MOF lms but
can be easily translated to other thin lms, e.g. inorganic,organic and inorganic–organic hybrid thin lms. For instance,
the orientation of nanocrystal lms based on heavy atoms canbe easily studied by X-ray and electron diﬀraction. However,these nanocrystals are typically stabilized with organic ligands,
which signicantly inuence the nanocrystal's properties. For
organic ligands and the orientation of functional groups,
diﬀraction techniques lack in sensitivity. In general, inorganic–organic composite materials account for the majority of nanomaterials with highly promising properties currently underintense investigations. Consequently, the potential of IR crystallography has yet to be fully exploited, allowing to ll theinformation gap le by diﬀraction techniques and giving accessto key information of hybrid material structures.

Author contributions
B. B.: Design of methodology, MOF synthesis, curation, writing– original dra; K. I.: MOF synthesis; K. O.: validation, writing –review & editing, resources; M. T.: validation, resources, writing– review & editing, funding acquisition.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
The present work is partially supported by Grand-in-Aids from
the Ministry of Education, Culture, Sports, Science and Technology (MEXT), administrated by Japan Society for the Promotion of Science (JSPS), and also by JST, PRESTO Grant Number
JPMJPR19I3, Japan. B. B. acknowledges the support via JSPS
Invitational Fellowships for Research in Japan (PE20025). K. O.
acknowledges nancial support from Izumi Science,© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Technology Foundation and the Cooperative Research Programof Institute for Catalysis, Hokkaido University (21B1019). The
authors thank Paolo Falcaro and Jakob Hayden for fruitful
discussions.

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