Synthesis and photophysical properties of a new carbazole-based acidochromic molecular switch

Article abstract of DOI:10.1016/j.jphotochem.2019.03.028

A reversible color–changing and fluorescence molecular switch was designed and synthesized based on carbazole fluorophores and amino group receptors. The crab shape molecule 1 is not soluble in most organic solvents and showslow solubility in acetic acid and DMSO (1 mg/ml in DMSO). The solution of 1 in DMSO performed as single on/off switch triggered by acidic protons. The colorless solution in basic and neutral conditions turned to yellow upon addition of a small amount of acid together with quench in fluorescence emission. The solid state of 1 has a dual-mode fluorescence switching property. The creamy white powder of 1has an emission λ_max at 398 nm but when exposed to acidic vapors or suspended into acidic mediathe color of the powder changed to yellow and its emission λ_max appeared at 516 nm. Monte-Carlo simulation has been performed to search the conformational space of the neutral and acidic forms. The frontier molecular orbitals of the most stable conformers has been calculated using DFT/B3LYP method using 6–31 g (d) basis set in attempt to explain the difference in electronic transition of 1 and its porotonated form.

Full text of DOI:10.1016/j.jphotochem.2019.03.028

Accepted Manuscript
Title: Synthesis and Photophysical Properties of a
NewCarbazole-basedAcidochromicMolecular Switch
Authors: Zahra Heydari, Parviz Rashidi-Ranjbar
PII:
S1010-6030(19)30065-6
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.03.028
JPC 11765
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
13 January 2019
3 March 2019
18 March 2019
Please cite this article as: Heydari Z, Rashidi-Ranjbar P, Synthesis and
Photophysical Properties of a NewCarbazole-basedAcidochromicMolecular Switch,
Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.03.028
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Synthesis and Photophysical Properties of a NewCarbazole-basedAcidochromicMolecular Switch
Zahra Heydaria, Parviz Rashidi-Ranjbara,*
aSchool Of Chemistry, College of Science, University of Tehran, Tehran, Iran.
E-mail: parvizrashidi2@ut.ac.ir
Graphical Abstract
A crab shape reversible color–changing and fluorescence molecular switch was designed and synthesized
based on carbazole fluorophores and amino group receptors.
Highlights
-
-
-
-
A fluorescence and colorimetric switch molecule was designed and synthesized.
The switch responds to vapor or solution of acids in both solution and solid state.
In solution, the response is a simple On/Off switching process.
The solid state shows a dual mode switching with a large bathochromic shift.
Abstract: A reversible color–changing and fluorescence molecular switch was designed and synthesized
based on carbazole fluorophores and amino group receptors. The crab shape molecule 1 is not soluble in
most organic solvents and showslow solubility in acetic acid and DMSO (1mg/ml in DMSO). The
solution of 1 in DMSO performed as single on/off switch triggered by acidic protons. The colorless
solution in basic and nuetral conditions turned to yellow upon addition of a small amount of acid together
with quench in fluorescence emission. The solid state of 1 has a dual-mode fluorescence switching
property. The creamy white powder of 1has an emission λmax at 398 nm but when exposed to acidic
vapors or suspended into acidic mediathe color of the powder changed to yellow and its emission λmax
appeared at 516 nm. Monte-Carlo simulation has been performed to search theconformational space of
the nuetral and acidic forms. The frontier molecular orbitals of the most stable conformers has been
calculated usind DFT/B3LYP method using 6-31g (d) basis set in attempt to explain the difference in
electronic transition of 1 and its porotonated form.

Keywords: Fluorescence switch, Acidochromic, Solid State, Dual mode, pH sensor
1. Introduction
Fluorescent switching materials have attracted unprecedented attentions owing to their potential
applications in optoelectronics,1 logic gates,2 sensors,3 memory storage materials,4 etc. These
molecules have the ability to respond to external stimulants as a change in optical outputs. There
are different kinds of stimuli that have been used to trigger switching process such as chemicals,5
mechanical forces,6 heat,7 light8 and acid/base.9
Application of smart molecules in device fabrication requires them to have efficient fluorescence
in solid forms like thin films and powders.10 Reversible switching processes have been widely
studied in solutions,11 but solid-state switches are quite rare. Molecular properties highly depends
on their structure and subsequently on their aggregation forms.12 In most cases, aggregation
promotes self-quenching phenomena due to intermolecular interactions leading to non-radiative
relaxation.13 Inclusion of bulky group in structure of molecules,14 J-aggregation15 and changing the
packing mode would help to overcome aggregation caused quenching (ACQ). Intermolecular
interactions such as π-π interactions and hydrogen bonding would have high impacts on the
molecular electronic environment, leading to conformation or frontier molecular orbital change.
These incidents cause change in color and/or luminescence emission of compound.16
Another important issue in the application of sensors and switches is the ease of detection of
changes between two states of the switch molecule. Naked eyes detection of the color change is
the easiest and cheapest rapid way that needs no instrumentation.17 Vapochromic substances
change color upon exposure to specific vapors. Mostly, these compounds are also
vapoluminescence, which means that their photoluminescence properties alters due to vapor
exposure.18 Recently, a variety of reversible solid-state acid/base vapochromic and
vapoluminescence has been reported.19 Different kinds of materials including simple organic
molecules,20 organometallic complexes,21 co-crystals, polymers,22 hydrogels,23 etc. has been used
for this purpose.
A numerous amount of fluorescence chemosensors has the fluorophore-spacer-receptor
architecture.24 Upon bond formation between the receptor and the guest, energy and/or intensity of
fluorescence signal will change. This might be due to the formation of excimer/exciplex, photo-
induced energy transfer, Förster resonance energy transfer, charge transfer, dissociation or charge
induced polarity changes.25 With this knowledge in mind, in this study we used two
carbazolegroups as fluorophore and great π-donors (which are connected to each other by a
phenyl spacer) to design a crab shape molecule bearing amino groups with high tendency toward
acidic protons and simple methylene groups as spacers, to be able to sense acidic vapors and give
the output signal as changes in both color and photoluminescence properties. (Scheme 1) The
single molecular switch has the advantage of less complexity compared with mixed multi-
chromophore systems requirement for energy matching.12

Scheme 1 The structure of the molecular crab (1) (Single
Column)
2. Experimental section
2.1.Materials and methods
All chemicals were purchased from Merck and were used without further purification. All
solvents were lab grade except DMSO, which was spectroscopy grade.
1H-NMR and13C-NMR spectra were recorded in DMSO-d6by a Varian 500 MHz spectrometer at
1
room temperature except the H-NMR of (CN)4C2P which was recorded at 60oC. FTIR-ATR
spectra were measured with a Bruker TENSOR 27 instrument. Solution phase UV/Vis
spectroscopy has been studied using a PerkinElmer Lambda 850 UV/Vis spectrophotometer.
Solid-state UV–Vis absorption spectra were recorded with a Shimadzu MPC-2200
spectrophotometer. Solid state and solution photoluminescence spectroscopy analysis were
carried out with an Agilent G9800A fluorescence spectrophotometer with a Xenon lamp
excitation source. Excitation and emission slits width were 5 nm. Mass spectroscopy data was
obtained from an Agilent 5975C VL MSD with Tripe-Axis Detector. A Zag Chemie PP-775
portable pH meter with pH glass combination electrode was used to prepare the required
solutions. Double rings filter paper 102 was used to make sensor sheets. Spartan 10 was employed
to perform Monte Carlo simulation for conformational space search also DFT/B3LYP method
using 6-31g (d) basis set to calculate the energy of the frontier orbitals.
2.2.Synthetic Procedure
Synthesis of 1,4-bis[(3,6-dicyanocarbazol-9-yl)methyl]benzene ((CN)4C2P). 9H-carbazol-3,6-
dicarbonitrile (0.46 g, 2.1 mmol), 1,4-bis(bromomethyl)benzene (0.26 g, 1mmol) and sodium hydride
(0.14 g, 5.8 mmol) were poured into a round-bottom flask. Dry THF (10 mL) was added to the mixture
under Ar atmosphere. The reaction mixture was stirred at room temp. for 5h. Water was added to the
mixture and the precipitate was filtered and washed with water excessively. The residue was washed with
boiling ethanol several times. 0.49 g of a white solid compound was obtained. (Yield: 91%).mp> 360oC.
1H-NMR (500 MHz, DMSO-d6) δ 8.82 (s, 4H), 7.85 (ABq, 8H, ΔνAB=13.4 Hz, JAB=8.6 Hz), 7.08 (s, 4H),
5.73 (s, 4H). ATR-FTIR (cm-1) ν 2223 (CNstretching)
Synthesis of 1,4-Bis[(3,6-di(aminomethyl)carbazol-9-yl)methyl]benzene ((NH2)4C2P)(1).
(CN)4C2P (0.27 g, 0.5 mmol) and LiAlH4 (0.23 g, 6 mmol) were poured into a small round-bottom flask
and the flask was purged by Ar. Dry THF (10 mL) was added and the mixture was stirred at room temp.
for 20h. The reaction was then quenched very carefully with slow addition of crushed ice. When the
mixture turned completely from red to greenish yellow, water (100 mL) was added and the precipitate
was filtered off and washed with water until the pH of the washings was neutral. The dried precipitate
was dissolved in boiling acetic acid. The solid impurities were filtered off. The filtrate was concentrated
and its pH was raised up to 6 through slow addition of NaOH. The resulting precipitate was filtered off

and more NaOH was added to the filtrate. The precipitate was filtered and washed excessively with water.
0.18 g of a cream solid compound was collected. (Yield: 65%).mp> 360oC (decomp. at 310oC).1H-NMR
(500 MHz, DMSO-d6) δ 8.03 (s, 4H), 7.44 (d, 4H, J=8.3 Hz), 7.32 (d, 4H, J=8.3 Hz), 6.98 (s, 4H), 5.51
(s, 4H), 3.84 (s, 8H). 13C-NMR (126 MHz, DMSO-d6) δ 139.24, 136.91, 134.83, 126.79, 125.32, 122.06,
118.36, 109.04, 45.91, 45.23. ATR-FTIR (cm-1) ν 2600-3600 (hydrogen bonded NH2 stretching)
3. Results and Discussion
The molecular switch 1can be synthesized in three steps from commercially available chemicals.
The synthetic route is demonstrated in Scheme 2. 9H-carbazol-3,6-dicarbonitrile26 and 1,4-
bis(bromomethyl)benzene27, were prepared using a modified literature procedure. (CN)4C2P was
synthesized through a simple substitution reaction. The product had very low solubility only in
hot DMSO and pyridine, probably due to strong intermolecular π-π interactions.1HNMR spectrum
was acquired at 60oCafter dissolving the small amount of compound in hot DMSO-d6.13C-NMR
spectrum could not be obtaineddue to low concentration, also very rapid precipitation at 60
oC.Compound
1 was synthesized by simultaneously reduction of all four CN groups by lithium
aluminum hydride. ATR-FTIR spectra showed a broad signal at 2600-3600 cm-1representing the
existence of a highly hydrogen bonded assembly.28 The absence of the specific C
frequency around 2200 cm-1 indicated complete reduction of CN groups.
N vibrational
Scheme 2 i: CuCN, NMP, 200oC, ii: NaH, Dry-THF, R.T., iii: LiAlH4, Dry-THF, R.T.
(1.5-Column)
The product was soluble in acetic acid and had low solubility in DMSO(1mg/ml in DMSO with
heating) probably due to strong intermolecular hydrogen bond and π-π stacking interactions.29
Protonation of amino groups in acetic acid diminishes intermolecular hydrogen bonding, making
the compound more soluble due to the repulsion of ammonium group positive charges.1H-NMR
spectrum of
1 showed a downfield shift for all signals upon protonation. Amino group protons has
been observed as a boad signal at 3.25 ppm (Fig. S5), which disappeared upon protonation. The
closer the proton to the amino groups, the more change in chemical shift (0.27 ppm for the
methylene groups connected to the amino groups vs 0.02 ppm for the central benzene ring).
Figure 11HNMR spectra of 1in DMSO-d6 before and after addition of one drop of
CD3CO2D.(Should be printed colorful, Single column)

(Figure 1).
To study the solution phase absorption and emission characteristics, a saturated solution of
1
(2*10-5 M) in DMSO was prepared. Figure 2 shows the absorption and emission spectra of the
solution before and after addition of one drop of 37% HCl solution. The change in color of the
solution from colorless to yellow and the appearance of a new absorption band at 378 nm upon
addition of acid might be ascribed to a intermolecular charge transfer (CT)or intramolecular CT
from carbazole group of one side of the molecule to the newly formed ammonium groups on the
other side.30It should be considered that the methylene spacer is too short to enable the interaction
between ammonium groups and carbazole on the same side of the molecule.31 Similar charge
transfer band has been reported in the literature.32Electrostatic repulsion could be lessened
because of solvation compared with gas phase calculations.Unlike other switch molecules with
similar structure, PL spectra showed no photo-induced electron transfer quenching in basic form.
This observation is likely due to the electron rich nature of carbazole fluorophores, which has
been used as the electron donor to design PET based sensors.33The charge transfer resulted in
quenching the fluorescence emission of the molecule. This process was completely reversible.
Figure 2 Absorption (solid lines) and emission (dashed lines) spectra of the solution
ofcarbazole as the fluorophore and1 in DMSO before and after addition of HCl. The color
change in the DMSO solution before and after acid treatment in daylight (left up) and
under irradiation at 365 nm (left down)(Should be printed colorful, Single Column)
Since the structure of the molecule is very flexible, systematic calculation to get the stable
conformers did not seem to be practical, Monte Carlo simulation was used to search the
conformational spacein the gas phase using MMFF force field implemented in Spartan 10.Many
stable conformers with small energy differences have been obtain for this structure. The results
showed the hydrogen bonding between amino groups in the most stable conformer of 1. The
protonation of amino groups eliminates the possibility for hydrogen bond formation and the
electrostatic repulsive interactions between positively charged ammonium groups changed the
preferred structure drastically to an extendedform. The energy difference between the lowest
energy conformer of the protonated moiety and the next stable conformer was 3kJ/mol.The most
stable conformer of the parent molecule and its protonated form were used to calculate the energy
difference between the frontier orbitals. HOMO and LUMO of 1are located at two different side
of the molecule. HOMO is on the hydrogen bond donor side and LUMO on the acceptor side
having the energy difference of 4.4 eV. For the acidic form of the molecule, HOMO spread
throughout the whole structure. There are two degenerated LUMO,each one located on one side
of the molecule.The energy difference of HOMO and LUMO in the protonated form is 4.7 eV.
This could be the reason for the absorption and emission λmax of the two forms to be almost the
same. (Figure 3)

Figure 3 Calculated stable conformers by Monte Carlo using Molecular Mechanics, optimized with b3lyp/6-31g (d) and frontier orbital
distributions. a: basic form and b: acidic form. (Should be printed colorful, Two Column)
Solid-state UV/Vis spectrum of the powderof
vapour. The difference in the absorption could be seen by naked eyes as a change in color from
off-white to yellow. Fluorescence emission of is quite similar to that of carbazole as the
1 shifted bathochromically upon treatment with HCl
1
fluorophore. It seems to be just broadened.In order to investigate the effect of acid vapor on
photoluminescence properties of the switch molecule, its powder was put in contact with both
acetic acid and HCl vapors. Regardless of which acid vapor, the maximum emission shifted from
398 nm for
1 to 516 nm and the emission intensity decreased. The only difference between the
two acids was the response time, which could be related to the vapor pressure of acids. The red-
shifted emission of the compound upon acid treatment could be due to charge transfer and cation-
π interactions which is probably the most powerful interaction in acidic form of the
molecule.(Figure 4)
Figure 4 Solid-state absorption (solid lines) and emission (dashed lines) spectra
ofcarbazole as the fluorophore and1 before and after acid treatment upon excitation
at 330 nm, Slit: 5*5, 650 Volt.Solid state of 1before and after acid treatment in
daylight (upper left) and under irradiation at 365 nm (upper right). (Should be printed
colorful, Single Column)
Powder X-ray diffraction pattern of the compound
1 and its acidic form after 10 min of exposure
to HCl vapor is depicted in figure 5. The change in crystallinity of the powder is obviously
observable from elimination of the lines in the pattern of basic form and appearance of new more

intense lines after acid treatment. It seems that the powder gets more crystalline after being
acidified. This change in crystallinity might be another reason for red-shift in the emission spectra
of the solid state. The reason for the more crystalline structure after acidification could probably
be related to the much more possible conformations of the
1 as Monte Carlo conformational space
search has revealed, but when it is protonated the number of possible conformations drops
considerably.
Figure 5 The XRD pattern of 1 before and after exposure to HCl vapour. (Should be
printed colorful, Single Column)
To have a uniform fumed surface, we immersed a filter paper into a saturated solution of
1 in
acetic acid. The filter paper was then dried and immersed into a 1M solution of NaOH. At the end,
the filter paper was washed thoroughly with water and was dried. The change in color of paper in
daylight and under irradiation at 365 nm upon treatment with HCl vapor was the same as the color
change, whichwas observed for powder.Figure 6 represents emission spectra of
and absorbed on filter paper, before and after treatment with HCl vapor. There is no significant
difference between the spectra of both acidic and basic forms. The decrease in the intensity of
1 as both powder
1
absorbed on the paper with respect to powder form is due to the lower amount of material in the
optical path.
Figure 6 Photoluminescence spectra of 1as both powder and absorbed on
filter paper before (solid lines) and after treatment with acid (dashed lines)
upon excitation at 330 nm, Slit: 5*5, 650 Volt. Filter paper of 1before and
after acid treatment in daylight (upper left) and under irradiation at 365 nm
(upper right). (Should be printed colorful, Single column)

The pH dependence of the response of the probe was investigated by soaking the paper in a series
of aqueous solutions with different pH values made by HCl and NaOH solutions. The emission
spectrum and the color of the paper did not change until the pH reached to 3 (Figure 7). It seems
that the change in color happens probably when all four amino groups are almost protonated.
Figure 7 The emission spectra of 1 absorbed on filter paper in solutions with different
pH upon irradiation at 330 nm, Slit: 5*5, 700 Volt. (Should be printed colorful, Single
column)
To peruse this hypothesis, titration in solution phase has been performed. A 3.5* 10-4 M solution
of compound
1 in DMSO was prepared and was titrated with 0.212 M solution of HCl. As it can
be observed from figure 8, there was no change in the fluorescence emission by increasing the
HCl concentration up to 175 mM (addition of 25 μL of 0.212 M solution of HCl) which shows
that the protonation of one of the amino group does not change the fluorescence intensity. The
emission quenches considerably when the HCl concentration reaches to 210 mM. The emission
also does not change by increasing the concentration of HCl more than 280 mM (addition of more
than 40 μL of 0.212 M solution of HCl). The need for excess of HCl to reach the full quenching
also suggests that all amino groups should be protonated.
Figure 8 Titration of the 3.5* 10-4 M solution of 1 in DMSO with 0.212 M
solution of HCl .(Should be printed colorful, single column)
Monte-Carlo analysis of conformational space has been performed for all of the possible
protonated forms of 1. The results showed that hydrogen bonding is possible in all protonated

forms of the compound except the fully protonated form and only this form adopts the extended
structure. (Figure 9)
Figure 9 The calculated stable conformers by Monte Carlo using Semi-empirical
RM1 optimized with b3lyp/6-31g (d). a: 1, b: 1-H+, c: 1-2H+ (protonation on one
carbazole group), d: 1-2H+ (protonation on two carbazole group), e: 1-3H+, f: 1-
4H+(Should be printed colorful, single column)
The important feature that a switch molecule is expected to have is the reversibility of switching
phenomenon. To evaluate this property, the filter paper was alternatively put in contact with HCl
vapor and after recording the emission, soaked in 1M NaOH solution, washed by water and again
the emission was recorded. This procedure was repeated for 9 cycles. The results showed that this
process could be repeated continuously for many times (Figure 10 and Figure S7 in the ESI). The
response time depends on the vapor pressure of the acid as mentioned previously. Fuming the
paper by placing it above 12M HCl solution resulted in color change in less than 1 second.
Fuming the same paper with the vapor pressure above a 4.5 M solution of HCl took 10 Min to
change the color of the paper.
Figure 10 Reversibility trend of the switching
process of 1(Single Column)
4. Conclusions

A crab shape molecular switch was designed and synthesized with fluorophore-spacer-receptor structure,
using carbazole as the fluorophore groups and amino groups as receptors. A downfield shift of all protons
has been observed in NMR spectrum of the switch molecule upon protonation. Thereversible solution
phase switching process included a color change from colorless to yellow and quenching of luminescence
upon acidification via charge transfer mechanism.Titration of the solution with HCl demonstrated that all
of the amino groups of the molecule should be protonated to show the quenching effect. Monte-Carlo
conformational space analysis also showed a considerable change in conformation upon protonation of all
amino groups. The solid-state form of the compound showed a reversible dual mode switching response
to acidic protons in gas phase and in liquids as both change in color-which was visible by naked eyes and
red-shifted fluorescence emission because of charge transfer and change in crystallinity of the powder.
The color change happens when the concentration of acidic protons reaches above 10-3M.
Competing interests
There is no competing interest to declare.
Acknowledgements
The financial support by the Research Council of University of Tehran is acknowledged. The authors are
grateful to Ehsan Hamzehpoor for useful help with DFT calculations.
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