A nitrophenyl-carbazole based push-pull linker as a building block for non-linear optical active coordination polymers: A structural and photophysical study

Article abstract of DOI:10.1016/j.dyepig.2020.109012

Non-linear optical effects (NLO) such as multi-photon absorption, second harmonic generation (SHG) etc. have a wide range of applications. Nevertheless, the performance of many NLO-active organic dyes is limited by their thermal stability and photobleaching. These problems can be overcome by integrating the dyes into coordination polymers or metal-organic frameworks. Here, we present a structural and photophysical study of dipropyl-9-(4-nitrophenyl)-carbazole-3,6-dicarboxylate, a new “push-pull” organic dye molecule designed as a chromophore linker for NLO-active coordination polymers. Structure determination of a single-crystal showed that it crystallizes in a monoclinic crystal system P 2₁/c. The solvated chromophore exhibits two aromatic absorption bands at 250 nm and 275 nm as well as broad long wavelength band at 350 nm, which we assign to an intramolecular charge transfer state. Photoluminescence measurements in solvents of different polarities revealed two main effects: In nonpolar solvents, the spectrum shows an emission band at 360 nm, whereas in solvents with a higher polarity, the emission maximum broadens and redshifts. Solid-state emission measurement of sample powder exhibits an emission band at 520 nm which is redshifted compared to the measurements in solution, due to excimer formation in the solid-state. The optical as well as solvation-related properties of the investigated pigment render it to be a versatile ligand in coordination polymers.

Full text of DOI:10.1016/j.dyepig.2020.109012

Dyes and Pigments xxx (xxxx) xxx
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
A nitrophenyl-carbazole based push-pull linker as a building block for
non-linear optical active coordination polymers: A structural and
photophysical study
a
a
b
b
Sebastian J. Weish a¨ upl , David C. Mayer , Erling Thyrhaug , Jürgen Hauer ,
Alexander P o¨ thiga , Roland A. Fischer
,
**
a,*
a
Chair of Inorganic and Metalꢀ Organic Chemistry, Department of Chemistry & Catalysis Research Center, Technical University of Munich, Lichtenbergstraße 4, 85748,
Garching, Germany
b
Professorship of Dynamic Spectroscopy, Department of Chemistry & Catalysis Research Center, Technical University of Munich, Lichtenbergstraße 4, 85748, Garching,
Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
Non-linear optical effects (NLO) such as multi-photon absorption, second harmonic generation (SHG) etc. have a
wide range of applications. Nevertheless, the performance of many NLO-active organic dyes is limited by their
thermal stability and photobleaching. These problems can be overcome by integrating the dyes into coordination
polymers or metal-organic frameworks. Here, we present a structural and photophysical study of dipropyl-9-(4-
nitrophenyl)-carbazole-3,6-dicarboxylate, a new “push-pull” organic dye molecule designed as a chromophore
linker for NLO-active coordination polymers. Structure determination of a single-crystal showed that it crys-
Chromophore
Emission
Metal-organic frameworks
Non-linear optics
1
tallizes in a monoclinic crystal system P 2 /c. The solvated chromophore exhibits two aromatic absorption bands
at 250 nm and 275 nm as well as broad long wavelength band at 350 nm, which we assign to an intramolecular
charge transfer state. Photoluminescence measurements in solvents of different polarities revealed two main
effects: In nonpolar solvents, the spectrum shows an emission band at 360 nm, whereas in solvents with a higher
polarity, the emission maximum broadens and redshifts. Solid-state emission measurement of sample powder
exhibits an emission band at 520 nm which is redshifted compared to the measurements in solution, due to
excimer formation in the solid-state. The optical as well as solvation-related properties of the investigated
pigment render it to be a versatile ligand in coordination polymers.
1
. Introduction
Coordination polymers (CPs) and metal-organic frameworks (MOFs)
surpassing often found drawbacks with classical NLO materials (e.g.
limited performance, toxicity, photostability, non-polar arrangements
etc.) [6,7]. Such an approach, however, requires comparatively small
NLO-active chromophore linker molecules, with the desired optical
properties and good crystallization tendencies.
featuring chromophore linkers as building blocks have been recognized
as a promising solid-state material class for non-linear optical (NLO)
applications, as their high chemical tunability, among other effects,
enables to organize chromophore molecules in a defined oriented
fashion [1].
NLO properties of chromophores have systematically been investi-
gated within the electronic push-pull concept [8], which reveals a strong
correlation between both the NLO-activity and the intramolecular
charge transfer (ICT) characteristics and the size or structure of the
charge transfer network. The NLO response was shown to be tailored by
(i) connecting electron donors and acceptors of various electronic na-
ture, (ii) assuring efficient donor-acceptor interaction (iii) extent,
To date, NLO studies on CPs and MOFs have been focusing on sec-
ond- and third-harmonic generation (SHG, THG) as well as multi-photon
absorption (MPA) properties [2–5]. These studies follow a common
strategy, to assemble small NLO active organic building blocks in
achieving highly functional crystalline solid-state materials, meanwhile
composition and steric arrangement of -electron bridges (iv) reduction
π
*
Corresponding author.
* Corresponding author.
E-mail addresses: alexander.poethig@tum.de (A. P o¨ thig), roland.fischer@tum.de (R.A. Fischer).
*
https://doi.org/10.1016/j.dyepig.2020.109012
Received 28 September 2020; Received in revised form 13 November 2020; Accepted 14 November 2020
Available online 21 November 2020
0
143-7208/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Sebastian J. Weishäupl, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.109012

S.J. Weish a¨ upl et al.
Dyes and Pigments xxx (xxxx) xxx
of the bond-length alternation of the
π
-bridges and (v) planarization of
(LIFDI 700) from Linden CMS GmbH. Single-crystal X-ray diffraction
data was collected on a BRUKER D8 Venture system equipped with a Mo
TXS rotating anode (λ = 0.71073 Å) and a CMOS photon 100 detector.
UV/VIS spectra were recorded on a double beam Lambda 365 UV–Vis
spectrophotometer from PerkinElmer. Fluorescence measurements were
recorded on an FS5 spectrofluorometer from Edinburgh Instruments.
Cyclic voltammetry measurements were conducted on a BioLogic SP-
200. Melting point analysis was performed on a Büchi M-565.
the
π
-electron system [8,9].
In general, the photophysical (including NLO) properties of coordi-
nation polymers can be influenced by each of their components - both
the organic linkers as well as the metal nodes as well as the excitonic
coupling between the linker molecules [10]. For example, photoactive
ions capable of metal-centered transitions can be used, e.g. lanthanides,
steering the photophysical properties of the material solely by choice of
the metal [11]. Often the situation is more complex, exhibiting a delicate
interplay of (also photo-inactive) metal ions, especially transition metal
ions, and chromophore linkers. Hereby, metal-to-ligand charge transfer
2.2. Computational methods
(
MLCT) as well as ligand-to-metal charge transfer (LMCT) can introduce
All quantum mechanical calculations were performed using the
Gaussian09 software package [20]. The long-range corrected
coulomb-attenuated exchange-correlation functional cam-B3LYP [21]
and the basis set def2TZVP were employed as implemented in the soft-
ware package. Theoretical absorption spectra were calculated on the
level of time-dependent DFT using the above functional and basis set. No
symmetry or internal coordinate constraints were applied during the
optimization. The optimized geometry was verified as being a true
minimum by the absence of negative eigenfrequencies in the vibrational
frequency analysis.
new charge transfer bands, which possibly allow for a higher NLO
response of the material compared to the isolated linker. As stated
above, the emission properties of organic chromophores are well un-
derstood and can be specifically tailored towards desired applications.
Therefore, tailoring the organic chromophores within linker platforms is
also the method of choice for optimizing photoactive coordination
compounds and networks. Additionally, there are two further advan-
tages: First, the incorporation of linker molecules inside a framework
usually increases their emission efficiency, through limitation of radia-
tionless energy decays as a result of a restriction of conformational
changes in the coordinated chromophore [12]. Second, by arranging the
chromophores in a rigid framework a dense packing of the chromo-
phores is enabled, whereas fluorescence quenching via aggregation
2.3. Synthesis
3,6-Dibromocarbazole (2): A solution of N-bromosuccinimide
(22.35 g, 125 mmol) in 50 mL DMF was slowly added through a syringe
pump to a stirring solution of carbazole 1 (10 g, 59.80 mmol) in 20 mL
DMF in an ice bath. After 24 h of reaction time, the mixture was poured
into 600 mL ice water and then filtered through a suction filter to give a
dark grey powder. The crude product was recrystallized with Ethanol to
(
which is usually observed in highly concentrated solutions) is pre-
vented [13]. Based on this, we identified dicarboxy-functionalized car-
bazoles as highly promising donor building blocks within the push-pull
linker design of prospective NLO-photoactive coordination polymers or
MOFs [14–16].
Herein, we report the synthesis and complete characterization of a
new potential push-pull linker, comprising a carbazole donor and a
1
give a grey powder of 3,6-Dibromocarbazole 2 (18,68 g, 96%). H NMR
(400 MHz, 298K, DMSO‑d ) δ (ppm) = 7.47 (d, J = 8.6 Hz, 2H), 7.53 (dd,
6
1
3
–
NO
2
acceptor group. Both are linked via a phenylene bridge, mimicking
J = 2.0, 8.6 Hz, 2H), 8.43 (d, J = 1.9, 2H), 11.59 (s, 1H, N–H). C NMR
the well-studied showcase NLO chromophore p-nitroaniline (PNA) [17].
The carbazole moiety is equipped with carboxylic acid groups, which
should enable an incorporation into framework structures. 9-(4-nitro-
(101 MHz, 298K, DMSO‑d ) δ (ppm) = 110.96, 113.18, 123.35, 123.29,
6
128.70, 138.78.
Carbazole-3,6-Dicarbonitrile (3): 3,6-Dibromocarbazole 2 (9.75 g,
30.0 mmol) and dppf (100 mg, 0.18 mmol) were added to a 100 mL
Schlenk flask and solved in 30 mL DMF and 0.3 mL water. The sus-
pension was degassed via bubbling argon for 1 h through the mixture.
Subsequently, Zn(CN)2 (4.21 g, 36 mmol), zinc powder (78 mg, 1.2
mmol), Zn(OAc)2 ⋅ 2 H O (0.26 g, 1.2 mmol) and Pd (dba) ⋅ dba (69.5
2 2
phenyl)-carbazole-3,6-dicarboxylic acid (H CbzNO , 8) was synthesized
in a multi-step procedure starting from carbazole. The molecule was
characterized by NMR spectroscopy, mass spectrometry and elemental
analysis. Single-crystal X-ray diffraction (SCXRD) analysis reveals an
antiparallel packing motif, minimizing the overall dipole-moment of the
unit cell, a behavior often found for polar molecules [18,19]. Finally, the
absorption and emission behavior were studied in solution as well as in
solid state. The spectroscopic findings were corroborated with the help
of electronic structure calculations at the level of time-dependent den-
sity functional theory (TD-DFT). The spectroscopic results and the
theoretical calculations show the formation of an ICT state, proving
2
2
3
mg, 0,06 mmol) were added under a positive pressure of argon. This
◦
mixture was heated to 110 C for 2 days. The suspension was subse-
quently cooled and then poured into a 100 mL mixture of H O/NH Cl/
2
4
NH3 (5/4/1) and filtered through a suction filter. The filter cake was
washed with the same volume of the above mixture, toluene (3 x 30 mL)
and MeOH (3 x 30 mL) to give a grey solid. The crude product was
1
H
2
CbzNO
2
as a highly prospective NLO ligand for future CP and MOF
recrystallized with DMF to give a white solid (3) (5.2 g, 81%). H NMR
synthesis.
(400 MHz, 298K, DMSO‑d ) δ (ppm) = 7.72 (d, J = 8.5 Hz, 2H), 7.85 (d,
6
1
3
J = 9.9 Hz, 2H), 8.80 (s, 2H), 12.38 (s, 1H, N–H). C NMR (101 MHz,
2
. Experimental
298K, DMSO‑d
6
) δ (ppm) = 101.74, 112.84, 120.10, 121.85, 126.8,
1
29.93, 142.32.
2
.1. Material and methods
Carbazole-3,6-dicarboxylic acid (4): Carbazole-3,6-Dicarbonitrile
3
(4.2 g, 19.3 mmol) was suspended in an aqueous NaOH solution
All purchased reagents were received from chemical suppliers and
(12.45 g in 150 mL). To this solution CuI (37,5 mg, 0.195 mmol) was
◦
used without any further purification if not otherwise stated. All re-
actions with air and moisture sensitive compounds were carried out
under standard Schlenk techniques using Argon 4.6 (Westfalen) or in a
glove box (UNIlab, M. Braun). Required glass ware was flame-dried in
vacuo prior to use. Elemental analysis was performed at the micro an-
alytic laboratory at the Technical University of Munich. Analysis of C, H
added and then quickly heated to 125 C for 2 days, until the starting
material was dissolved. Afterwards active carbon was added, and the
◦
mixture was again heated to 125 C for 2 h. After cooling, the suspension
was filtered through celite, which was pre-washed with aq. NaOH-
solution. The filtrate was acidified with 6 M HCl-solution to give a
white precipitate. The precipitate was filtered, washed with water and
then dried to give a withe solid (4) (4.0 g, 85%). 1H NMR (400 MHz,
◦
and N values was conducted by flash combustion method at 1800 C.
NMR spectra were recorded on a Bruker AV400 at room temperature at
298K, CDCl
3
) δ (ppm) = 7.60 (d, J = 8.5 Hz, 2H), 8.06 (d, J = 8.4 Hz,
+
13
4
00 MHz. ESI-MS was performed on a LCQ fleet and MS Q from Thermo
2H), 8.85 (s, 2H), 12.04 (s, 1H, N–H), 12.69 (bs, 2H, COOH). C NMR
(101 MHz, 298K, DMSO‑d
Fischer Scientific. LIFDI-MS spectra were measured at a Waters Micro-
6
) δ (ppm) = 111.13, 122.00, 122.27, 122.79,
mass LCT TOF mass spectrometer equipped with an LIFDI ion source
127.65, 143.12, 167.94.
2

S.J. Weish a¨ upl et al.
Dyes and Pigments xxx (xxxx) xxx
Dipropyl-carbazole-3,6-dicarboxylate
(5):
Carbazole-3,6-
literature known compound 9-(4-phenyl)-carbazole-3,6-dicarboxylic
acid [22] was synthesized in a saponifaction reaction, using the analo-
gous reaction conditions to that described for 8. 1H NMR (400 MHz,
dicarboxylic acid 4 (4.0 g, 15.64 mmol) was suspended in 100 mL 1-
propanol. To this suspension, conc. sulfuric acid (2 mL) was added
◦
and then refluxed at 110 C for 24 h. After cooling, the suspension was
298K, DMSO‑d
8.10 (dd, J = 1.6 Hz, 8.7 Hz, 2H), 8.98 (s, 2H), 12.82 (bs, 2H). C NMR
(101 MHz, 298K, DMSO‑d
6
) δ (ppm) = 7.43 (d, J = 8.7 Hz, 2H), 7.58–7.79 (m, 5H),
13
concentrated on a rotary evaporator and extracted with 200 mL
dichloromethane. The organic layer was washed with aq. NaHCO
mL) and then dried with MgSO . The solvent was evaporated to give a
yellowish solid (5) (4.5 g, 84%). H NMR (400 MHz, 298K, CDCl
3
(150
6
) δ (ppm) = 109.87, 122.46, 123.01, 123.36,
4
127.00, 128.27, 128.65, 130.40, 135.78, 143.49, 167.62.
1
3
) δ
ppm) = 1.09 (t, J = 7.4 Hz, 6H), 1.87 (h, J = 7.2 Hz, 4H), 4.36 (t, J =
(
3. Results and discussion
6
2
1
.7 Hz, 4H), 7.47 (d, J = 8.5 Hz, 2H), 8.18 (dd, J = 1.5 Hz, 2H), 8.86 (s,
13
H). C NMR (101 MHz, 298K, CDCl
3
) δ (ppm) = 10.77, 22.40, 66.67,
3.1. Synthesis
10.64, 122.89, 123.19, 128.28, 142.85, 167.35.
Dipropyl-9-(4-nitrophenyl)-carbazole-3,6-dicarboxylate
The dipolar push-pull chromophore dipropyl-9-(4-nitrophenyl)-
(
Pr
2
CbzNO
2
) (6): Dipropyl-carbazole-3,6-dicarbox-ylate 5 (0.87 g,
2 2
carbazole-3,6-dicarboxylate (Pr CbzNO ) 6 was synthesized in a six
2
.56 mmol), 1-iodo-4-nitrobenzene (0.638 g, 2.56 mmol), K
3
PO
4
(2.18
step synthesis procedure starting from carbazole 1 (Fig. 1). Bromination
of 1 yielded 3,6-dibromocarbazole 2, which was subsequently reacted to
carbazole-3,6-dicarbonitrile 3 in a modified Negishi coupling reaction
[23,24]. Alkaline hydrolysis of 3 followed by acidic precipitation gave 3,
6-carbazole-dicarboxylic acid 4. It turned out that the low solubility and
acidic behavior of 4 hampered N-hetero cross-coupling reactions of
Buchwald-Hartwig or Ullman type. Consequently, to enhance the solu-
bility of the carbazole donor precursor 4, the corresponding propyl ester
5 was synthesized in an acid-catalyzed esterification reaction. Further-
more, different Ullmann and Buchwald-Hartwig coupling reaction at-
tempts were screened under varying reactions conditions finally
yielding the desired product 6 (compare the experimental part for a
detailed procedure). A modified Ullmann reaction procedure following
work by Eddaoudi et al. results in high yield and excellent purity [25]. As
the compound should be used as a building block for CP synthesis in
pursuing work, 6 was hydrolyzed by alkaline esterification finally giving
the corresponding carboxylic acid 8. Please note that all optical char-
acterization methods discussed in the following paragraphs were con-
ducted on 6 due to the higher solubility of the latter in differing organic
solvents. The effects of the propyl ester group of 6 on optical properties
are marginal, as the absorption and emission behavior are determined
by the push-pull system, as will be shown in the following paragraphs.
′
g, 10.3 mmol), N,N -dimethylethylenediamine (0.18 mL, 1.67 mmol)
and CuI (73.15 mg, 0.39 mmol) were dissolved in 20 mL dry toluene in a
◦
5
0 mL Schlenk flask and heated to 115 C for 2 days. After cooling, the
suspension was dissolved in 100 mL aq. NH
4
Cl, extracted with EtOAc (3
x 50 mL). The organic phase was combined and then dried with MgSO
4
.
The solvent was evaporated on a rotary evaporator to give an orange
solid. The crude product was then subjected to column chromatography
(
100% dichloromethane, R
f
= 0.75) to give a yellow powder (843 mg,
2%). mp 227 C; H NMR (400 MHz, 298K, CDCl
) δ (ppm) = 1.09 (t, J
7.4 Hz, 6H), 1.88 (h, J = 6.7 Hz, 4H), 4.37 (t, J = 6.7 Hz, 4H), 7.46 (d,
◦
1
7
3
=
J = 8.7 Hz, 2H), 7.81 (d, 8.9 Hz, 2H), 8.20 (dd, J = 1.4 Hz, 2H), 8.54 (d, J
=
=
8.9 Hz, 2H), 8.92 (s, 2H). 13C NMR (101 MHz, 298K, CDCl
1.16, 10.75, 22.37, 66.86, 109.65, 123.30, 123.90, 124.39, 125.94,
3
) δ (ppm)
•+
1
27.45, 128.76, 143.35, 166.86. LIFDI-MS: m/z [M] : calculated for
: 460.16; found: 459.55. EA: calculated for C26 : C,
7.82; H, 5.25; N, 6.08; found: C, 67.35; H, 5.27; N, 5.87.
Dipropyl-9-(4-phenyl)-carbazole-3,6-dicarboxylate (Pr
7): Dipropyl-carbazole-3,6-dicarboxylate (0.3 g, 0.88 mmol), iodo-
benzene (0.18 g, 0.88 mmol), K PO (0.750 g, 3.54 mmol), DMEDA
0.06 mL, 0,58 mmol) and CuI (25.5 mg, 0.13 mmol) were dissolved in
C
26
H
24
O
6
N
2
24 6 2
H O N
6
2
CbzH)
(
3
4
(
◦
1
0 mL dry toluene in a 50 mL Schlenk flask and heated to 115 C for 3
Cl-
solution, extracted with EtOAc (4 x 30 mL). The organic phase was
combined and then dried with MgSO . The solvent was evaporated on a
rotary evaporator, to give a brown solid. The crude product was then
subjected to column chromatography (100% dichloromethane, R
= 0.8)
to give a white powder (6) (295 mg, 80%). mp 133 C; H NMR (400
MHz, 298K, CDCl
days. After cooling, the suspension was dissolved in 60 mL aq. NH
4
3.2. Photophysical characterization in solution
4
3.2.1. UV/VIS spectroscopy
f
The photophysical properties of 6 were studied using UV/Vis spec-
troscopy in solvents of different polarity (Fig. 2a). In all solvents the
absorption spectra show two strong absorption bands centered at 250
nm and 275 nm, as well as weaker bands around 300 nm and a char-
acteristic broad absorption band centered at approximately 355 nm. We
pursue two strategies to assign the origin of these spectral features: i)
comparison with the spectra of the reference compound dipropyl-9-(4-
◦
1
3
) δ (ppm) = 1.09 (t, J = 7.4 Hz, 6H), 1.88 (h, J = 6.7
Hz, 4H), 4.37 (t, J = 6.7 Hz, 4H), 7.46 (d, J = 8.7 Hz, 2H), 7.81 (d, 8.9
Hz, 2H) 8.20 (dd, J = 1.4 Hz, 2H), 8.54 (d, J = 8.9 Hz, 2H), 8.92 (s, 2H).
1
3
C NMR (101 MHz, 298K, CDCl
3
) δ (ppm) = 1.16, 10.75, 22.37, 66.86,
1
1
09.65, 123.30, 123.90, 124.39, 125.94, 127.45, 128.76, 143.35,
66.86. EA: calculated for C26 N: C, 75.16; H, 6.07; N, 3.37; found:
H
25
O
4
phenyl)-carbazole-3,6-dicarboxylate (Pr CbzH) 7 (Fig. 2, yellow), and
2
C, 74.99; H, 6.07; N, 3.52.
-(4-nitrophenyl)-carbazole-3,6-dicarboxylic acid (H
8): Dipropyl-9-(4-nitrophenyl)-carbazole-3,6-dicarboxylate (500 mg,
.2 mmol) was dissolved in 50 mL THF, aq. NaOH-solution (0.25 g in 10
ii) comparison with quantum chemical calculations of the absorption
9
2
CbzNO
2
)
spectrum.
(
Compound 6 and the reference 7 differ in molecular structure only
by the p-nitro group (Fig. 2c), which implies that any spectral differences
report on the influence of the nitro acceptor group on the molecular
photophysics of the push-pull chromophore. The similarity of the short-
wavelength range of the spectra suggest that the features below 300 nm
1
mL water) and 7.5 mL MeOH. This mixture was refluxed for 12 h at
◦
9
0 C. After cooling, the solvent was evaporated and 100 mL water was
added. The suspension was filtered and subsequently acidified with 3 M
HCl-solution. The precipitate was filtered, washed thoroughly with
water and then dried, to give a yellow solid (310 mg, 84%). 1H NMR
can be assigned to
π
- * transitions located at the carbazole moiety. In the
π
long-wavelength range, however, we observe significant differences. In
(
400 MHz, 298K, DMSO‑d
J = 9.0 Hz, 2H), 8.11 (dd, J = 1.6 Hz, 8.7 Hz, 2H), 8.55 (d, J = 9.0 Hz,
H), 9.00 (s, 2H), 12.89 (bs, 2H, COOH). 13C NMR (101 MHz, 298K,
DMSO‑d
) δ (ppm) = 110.12, 123.07, 123.11, 124.18, 125.76, 127.85,
28.49, 141.69, 142.76, 146.44, 167.48. ESI-MS: m/z [M ꢀ H]: calcu-
lated for C20 : 375.313; found: 375.14. EA: calculated for
⋅ 0.75 H O: C, 61.62; H, 3.49; N, 7.19; found: C, 61.80; H,
.47; N, 7.02.
-(4-phenyl)-carbazole-3,6-dicarboxylic acid (H
6
) δ (ppm) = 7.59 (d, J = 9.0 Hz, 2H), 8.04 (d,
particular, the absorbance spectra of 7 have only minor bands red-
shifted with respect to the
π
- * transitions (Fig. 2). Thus, the long-
π
2
wavelength band at 6 is clearly induced by the electron accepting
6
–NO group. While introducing electron-withdrawing groups may result
2
1
in charge-transfer transitions, the modest solvatochromy of the band
H
12
O
6
N
2
shows that these transitions are associated with only a minor change in
permanent dipole moment.
C
20
H
12
O
6
N
2
2
3
To obtain further insight into the electronic structure of 6, and to
corroborate our assignments of the spectroscopic data, we perform a
9
2
CbzH) (9): The
3

S.J. Weish a¨ upl et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 1. Reaction scheme for the preparation of 8 (and 9) showing the different conducted steps from commercially available starting materials.
Fig. 2. a) Side-by-side comparison of the absorbances of 6 in different solvents with increasing polarity, from toluene to ethanol, as well as 7 in DCM. b) Zoom-in of
the outlined area (dashed box) in the left panel focusing on the absorption of the charge transfer state. c) Molecular structure of chromophores 6 and 7.
quantum chemical analysis of the compound’s absorption spectra. This
was done at the level of TD-DFT using the long-range corrected cam-
B3LYP functional and def2TZVP as basis set (50 excited states, Fig. 3).
The calculated UV/VIS spectrum is in good accordance with the
experimental absorbance spectrum and shows two main groupings of
transitions in the UV region centered at 180 nm and 240 nm. Qualitative
electron accepting nitro group upon photo excitation is clearly
illustrated.
3.2.2. Photoluminescence and excitation spectroscopy
To unravel the emission properties of this new organic chromophore,
we conducted a detailed photoluminescence (PL) spectroscopic inves-
tigation. Excitation-emission-matrices (EEM) of 6 were measured in
order to study the excitation-wavelength dependent fluorescence
behavior in different polar surroundings (Fig. 4). The collected data
point towards a subtle interplay of a locally excited (LE) state at the
carbazole moiety and a twisted intramolecular charge-transfer (TICT)
state involving the carbazole donor and nitro acceptor as a function of
solvent polarity (Fig. 5). The EEMs reveal the formation of two contri-
butions. In solvents of low polarity (toluene, Fig. 4a) the spectra show a
(
visual) comparison of the most contributing Kohn-Sham molecular
orbitals (KS-MOs) of these excitations show that these transitions are
mostly of * character and located at the carbazole moiety (not
π-π
shown). The band at approximately 310 nm however, involves sub-
stantial transfer of charge-density towards the nitrobenzene moiety, and
the S
Fig. 3b shows the KS-MOs most strongly involved in the S
where the transfer of electron density from the donating carbazole to the
0
→S
1
transition can thus be assigned to a charge-transfer excitation.
1
transition,
Fig. 3. a) Theoretical UV/VIS spectrum of 6 (50 excited states, gaussian broadening) b) Charge transfer excitation of 6 located from the carbazole moiety to the
phenyl ring (S1, 310 nm, oscillator strength f, transition dipole moment tdm [D], HOMO (left) → LUMO (right)).
4

S.J. Weish a¨ upl et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 4. Excitation-emission-matrices (EEM) of 6 in different solvents: a) toluene, b) THF, c) DCM, d) MeCN and e) EtOH as well as 7 in DCM for comparison, f). Note
the different scaling of the x/y axis for clarity reasons.
Fig. 5. A pictorial description of the interplay between LE and CT states in the push-pull chromophore 6 and how the solvent reaction field shapes the excited state
PES. a) In non-polar solvents, the lowest state will hold
π
- * character. b) In more polar solvents the TICT state will be lowered due to stabilization by the reaction
π
field, resulting in an equilibration with the LE state. c) Finally, at strong-polar solvents the TICT state gets close to the S
radiative relaxation, preventing its contribution to emission.
0
surface, resulting in a large increase in non-
single emissive state, with a PL maximum at ̃ 360 nm. By increasing the
solvent’s polarity, an additional emission band appears which redshifts
and broadens distinctively (̃ 430 nm in THF, Fig. 4b; ̃ 570 nm in DCM).
Finally, in very polar and protic solvents such as ethanol, the long-
wavelength contribution vanishes (Fig. 4d and e). The EEMs show no
long-wavelength contribution to the emission signal, and the spectra are
remarkably similar to the spectra in low-polar solvents. To gain further
insights, we also measured an EEM of 7, following a similar strategy as
presented in the UV/Vis spectroscopic studies with the aim to under-
Taken the above into consideration, the dual-emission behavior of 6
can be understood within a TICT-model. In less polar solvents the lowest
energy state will show LE character with a narrow PL band and a
maximum at approximately 360 nm. Via comparison of EEMs of 6 and 7
displayed in Fig. 4, we assign this LE state to be largely located at the
carbazole moiety. However, in more polar surroundings TICT states will
be stabilized, due to the increased dipole moment through twisting of
the chromophore alongside the C–N single bond. Consequently, the TICT
state will contribute stronger to the emission band - which is most
pronounced in DCM. Finally, in very polar and protic surroundings (here
acetonitrile and ethanol) the TICT state is potentially lowered to a strong
extent, with the typical reduction in transition moment due to the
stand the influence of the –NO
in DCM reveal two emission bands, with lowest-energy transitions at
~290 nm) and (~360 nm), respectively.
2
substituent. Interestingly, the EEM of 7
(
5

S.J. Weish a¨ upl et al.
Dyes and Pigments xxx (xxxx) xxx
extensive charge-separation, likely leading to essentially complete
quenching of the TICT emission. Instead, the EEMs clearly show that
only the LE emission remains in these cases. Fig. 5 displays the interplay
of LE and TICT states of 6 in different polar surroundings. Note that such
an approach of describing the excited state dynamics is commonly used
for push-pull chromophores [26–28]. In summary, the static excitation
and emission spectroscopic analysis of 6 reveals a highly interesting
photophysical behavior of the linker molecule, in particular showing a
strong dependency on the polarity of the chromophore environment.
Presumably, these effects can be exploited further in sensor systems (e.
g., guest molecule detection) through controlled chromophore packing
in a coordination polymer.
fluorescence and excitation spectroscopy. Fig. 7a shows the emission
spectrum measured at different excitation wavelengths (280 nm and
425 nm, respectively) with an emission band located at 540 nm for both
excitations. This is different, compared to the photoluminescence mea-
surements in solution, where two emission bands can be seen: a LE band
at 360 nm, as well as a solvent dependent red-shifting ICT. Furthermore,
the solid-state spectrum shows two contributing transitions, as apparent
from the excitation spectrum (red line in Fig. 7a), which we assign to
carbazole based
π
- * transitions (280 nm) and the charge transfer state
π
(425 nm). To support our assignments, we also measured emission and
excitation spectra for powdered 7 and compared it to 6 (Fig. 7b).
Interestingly, 7 shows very similar emission - centered at 520 nm- but
the excitation spectrum reveals only one absorptive transition. Based on
these findings and the packing of the carbazole moieties into dimers
with a distance of ~3.4 Å in the crystal structure, the solid-state emis-
3
.3. Crystal structure analysis
sion of 6 (and 7) appears to originate from a -stacked sandwich excimer
π
Single crystals suitable for single crystal X-ray diffraction (SCXRD)
formation, which has been observed 2001 in photoluminescence studies
on carbazole dimers [29].
analysis were obtained by slow diffusion of pentane into a concentrated
solution of 6 in chloroform. Fig. 6a shows the molecular structure of 6
and its packing along the crystallographic a and b-axis (Fig. 6b and c).
For a future incorporation of the chromophore in a CP, a similar
effect can be expected, since a high chromophore density without
fluorescence quenching should be achievable. On top, an additional
advantage of these coordination materials compared to the crystallized
linker itself is, that the photophysical properties can be potentially
controlled via defined arrangement of the chromophores, whereas this is
intrinsically limited in an organic crystal. This leads to a remarkable
enlargement of the playfield of organic chromophores, deliberately
packed and arranged in solid-state materials, thereby steering the pho-
tophysical properties of the solid-state material.
1
The material crystallizes in the monoclinic space group P 2 /c with a
single molecule of 6 in the asymmetric unit and four molecules per unit
cell (Z = 4). The mean bond lengths found for the molecular structure
from SCXRD analysis lay in ranges as expected for organic molecules of
such types (Table S1). The carbazole unit and the nitrophenylene group
◦
hold a torsional angle of 52.50 . Interestingly, the –NO
2
group and the
◦
phenylene ring have a dihedral angle of almost 0 , proofing both
molecule fragments to lay in a single plane. As can nicely been seen in
Fig. 6b, the polar molecule crystallizes diametral in the ac-plane, mini-
mizing the overall dipole moment of the unit cell. Furthermore, the
molecule propagates along the crystallographic b-axis in a co-planar
fashion, with a mean center-to-center distance of ̃ 3.4 Å. Additional
intermolecular hydrogen bonds of the type Carom-H⋯O involving the
ester group and the nitrophenylene group with characteristic distances
ranging from 2.4 Å to 2.8 Å, contribute to the overall packing as found in
the crystal structure. In summary, the SCXRD analysis reveals that the
packing motif of 6 is mainly governed by electrostatic as well as inter-
4
. Conclusions
In conclusion, we have synthesized a novel dipolar “push-pull”
chromophore dipropyl-9-(4-nitrophenyl)-carbazole-3,6-dicarboxylate
via an modified Ullmann-coupling reaction, which should be used in
further studies for the synthesis of NLO active CPs and MOFs. The
1
13
chromophore was characterized by H NMR and C NMR spectroscopy,
LIFDI-MS and elemental analysis as well as SCXRD. The optical studies
of chromophore 6 in solution showed two aromatic absorption bands at
molecular interactions (minimization of dipole moments,
π-
π
in-
teractions of the carbazole moiety and Carom-H⋯O interactions).
2
50 nm and 275 nm as well as a broad long wavelength absorption band
at 350 nm with characteristics of an ICT transition. The PL measure-
ments in solvents of differing polarity, revealed an interesting interplay
between locally excited states, centered at the carbazole moiety, and a
twisted intramolecular charge transfer state between the carbazole
3
.4. Solid-state photoluminescence and excitation spectroscopy
In order to characterize the photophysical properties in the solid-
state, powder samples of were investigated with solid-state
6
Fig. 6. Hydrogen atoms are omitted for clarity. a) ORTEP representation of the single-crystal structure of 6 with thermal ellipsoids shown at the 50% probability
level. b) Alternating packing of 6 alongside the a/c plane. c) Co-planar packing of 6 alongside the b/c plane with a mean chromophore distance of 3.367 Å.
6

S.J. Weish a¨ upl et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 7. a) Solid-State emission (blue and yellow line, λex = 280 nm, 425 nm) and excitation spectrum (red line, λem = 600 nm) of 6. b) Solid-State emission (yellow
line, λex = 240 nm) and excitation spectrum (red line, λem = 520 nm) of 7. (For interpretation of the references to colour in this figure legend, the reader is referred to
the Web version of this article.)
donor and the nitro acceptor group. The solid-state emission measure-
[3] Medishetty R, Nemec L, Nalla V, Henke S, Samo ´c M, Reuter K, Fischer RA. Multi-
photon absorption in metal–organic frameworks. Angew Chem Int Ed 2017;56(46):
ments exhibited 6 as a greenish emitter with contributions from
1
4743–8.
π
-stacked excimer formation. Taking all these photophysical properties
[
4] Mayer DC, Manzi A, Medishetty R, Winkler B, Schneider C, Kieslich G, P o¨ thig A,
Feldmann J, Fischer RA. Controlling multiphoton absorption efficiency by
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in account, the newly synthesized dipolar “push-pull” chromophore
presents itself as a highly interesting building block to study within a
structural-photophysical-property-relationship investigation on poten-
tial NLO active CPs or MOFs in the future.
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Author contributions
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7] He GS, Tan L-S, Zheng Q, Prasad PN. Multiphoton absorbing Materials: molecular
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Conceptualization and writing - original draft preparation, S.J.W.,
methodology, S.J.W., D.C.M., E.T. and A.P., writing – review and edit-
ing, D.C.M., E.T., J.H., A.P. and R.A.F., project administration, R.A.F. All
authors have given approval to the final version of the manuscript.
[9] Spangler Charles W. Recent development in the design of organic materials for
optical power limiting. J Mater Chem 1999;9(9):2013–20.
[
[
10] Medishetty R, Zareba JK, Mayer D, Samoc M, Fischer RA. Nonlinear optical
properties, upconversion and lasing in metal-organic frameworks. Chem Soc Rev
2
017;46(16):4976–5004.
Funding
11] Eliseeva SV, Pleshkov DN, Lyssenko KA, Lepnev LS, Bünzli J-CG, Kuzmina NP.
Highly luminescent and triboluminescent coordination polymers assembled from
lanthanide β-diketonates and aromatic bidentate O-donor ligands. Inorg Chem
The authors would like to thank the German Research Foundation
2
010;49(20):9300–11.
(
DFG) for funding within the frame of EXC 2089 Cluster of Excellence
[
[
12] Hu Z, Huang G, Lustig WP, Wang F, Wang H, Teat SJ, Banerjee D, Zhang D, Li J.
Achieving exceptionally high luminescence quantum efficiency by immobilizing an
AIE molecular chromophore into a metal–organic framework. Chem Commun
and the Priority Programme “COORNETs” (SPP 1928).
2
015;51(15):3045–8.
13] Jiang Y, Wang Y, Hua J, Tang J, Li B, Qian S, Tian H. Multibranched triarylamine
end-capped triazines with aggregation-induced emission and large two-photon
absorption cross-sections. Chem Commun 2010;46(26):4689–91.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[14] Venkateswararao A, Thomas KRJ, Lee C-P, Li C-T, Ho K-C. Organic dyes containing
carbazole as donor and -linker: optical, electrochemical, and photovoltaic
π
properties. ACS Appl Mater Interfaces 2014;6(4):2528–39.
[
[
[
15] Ledwon P. Recent advances of donor-acceptor type carbazole-based molecules for
light emitting applications. Org Electron 2019;75:105422.
16] Hongji J, Jian S, Jinlong Z. A review on synthesis of carbazole-based chromophores
as organic light-emitting materials. Curr Org Chem 2012;16(17):2014–25.
17] Nalwa HS, Watanabe T, Miyata S. A comparative study of 4-nitroaniline, 1,5-
diamino-2,4- dinitrobenzene and 1,3,5-triamino-2,4,6-trinitrobenzene and their
molecular engineering for second-order nonlinear optics. Opt Mater 1993;2(2):
Acknowledgments
The TUM is very greatly acknowledged for institutional funding. S.J.
W. and D.C.M. thank the TUM Graduate School for financial support.
The authors would like to thank Leibniz Supercomputing Centre of the
Bavarian Academy of Sciences and Humanities for provision of
computing time.
7
3–81.
[
[
18] Liao Y, Eichinger BE, Firestone KA, Haller M, Luo J, Kaminsky W, Benedict JB,
Reid PJ, Jen AKY, Dalton LR, Robinson BH. Systematic study of the
Structureꢀ Property relationship of a series of ferrocenyl nonlinear optical
chromophores. J Am Chem Soc 2005;127(8):2758–66.
19] Liao Y, Bhattacharjee S, Firestone KA, Eichinger BE, Paranji R, Anderson CA,
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Appendix A. Supplementary data
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21):6847–53.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.109012.
[20] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR,
Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV,
Bloino J, Janesko BG, Gomperts R, Mennucci B, Hratchian HP, Ortiz JV,
Izmaylov AF, Sonnenberg JL, Williams, Ding F, Lipparini F, Egidi F, Goings J,
Peng B, Petrone A, Henderson T, Ranasinghe D, Zakrzewski VG, Gao J, Rega N,
Zheng G, Liang W, Hada M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M,
Nakajima T, Honda Y, Kitao O, Nakai H, Vreven T, Throssell K, Montgomery Jr JA,
Peralta JE, Ogliaro F, Bearpark MJ, Heyd JJ, Brothers EN, Kudin KN,
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