Triptycene scaffolds: Synthesis and properties of triptycene-derived Schiff base compounds for the selective and sensitive detection of CN? and Cu2+

Article abstract of DOI:10.1016/j.tet.2017.04.005

A series of triptycene-derived Schiff bases were synthesized by condensation between amino triptycenes with an appropriate aldehyde and were isolated in good to excellent (85–90%) yields. Amongst these, a triptycene-hydroxybenzaldehyde Schiff base compound proved to be a selective sensor for cyanide. It exhibited a “turn-on” fluorescence response at 490?nm to CN^? facilitated by the nucleophilic addition of CN^? to the aldehyde group, accompanied by a visible color change from orange to yellow. Likewise, a triptycene salicylaldehyde adduct was shown to be highly sensitive towards the detection of the CN^? ion with a detection limit of 0.9?μM. On the other hand a triptycene-BODIPY Schiff base compound could be used for the detection of Cu²⁺ ions over other competing, biologically relevant metal ions in acetonitrile. Photophysical studies revealed a 1:1 binding model for the triptycene-BODIPY compound.

Full text of DOI:10.1016/j.tet.2017.04.005

Accepted Manuscript
Triptycene scaffolds: Synthesis and properties of triptycene-derived Schiff base
−
2+
compounds for the selective and sensitive detection of CN and Cu
Ganapathi Emandi, Michelle P. Browne, Michael E. Lyons, Caroline Prior, Mathias O.
Senge
PII:
S0040-4020(17)30368-X
10.1016/j.tet.2017.04.005
TET 28606
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 11 December 2016
Revised Date: 21 March 2017
Accepted Date: 3 April 2017
Please cite this article as: Emandi G, Browne MP, Lyons ME, Prior C, Senge MO, Triptycene scaffolds:
Synthesis and properties of triptycene-derived Schiff base compounds for the selective and sensitive
−
2+
detection of CN and Cu , Tetrahedron (2017), doi: 10.1016/j.tet.2017.04.005.
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Graphical Abstract
Triptycene scaffolds: Synthesis and
Leave this area blank for abstract info.
properties of triptycene-derived Schiff base
compounds for the selective and sensitive
-
2+
detection of CN and Cu
a
b
b
a
c,
Ganapathi Emandi , Michelle P. Browne , Michael E. Lyons , Caroline Prior and Mathias O. Senge ∗
a
School of Chemistry, SFI Tetrapyrrole Laboratory, Trinity Biomedical Sciences Institute, 152-160 Pearse
Street, Trinity College Dublin, the University of Dublin, Dublin 2, Ireland
b
Physical and Materials Electrochemistry Laboratory, School of Chemistry, Trinity College Dublin, the
University of Dublin, Dublin 2, Ireland
c
Medicinal Chemistry, Institute of Molecular Medicine, Trinity Centre for Health Sciences, Trinity College
Dublin, the University of Dublin, St. James’s Hospital, Dublin 8, Ireland

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Triptycene scaffolds: Synthesis and properties of triptycene-
derived Schiff base compounds for the selective and sensitive
-
2+
detection of CN and Cu
a
b
b
a
Ganapathi Emandi , Michelle P. Browne , Michael E. Lyons , Caroline Prior and
c,
Mathias O. Senge *
a
School of Chemistry, SFI Tetrapyrrole Laboratory, Trinity Biomedical Sciences
Institute, 152-160 Pearse Street, Trinity College Dublin, the University of Dublin, Dublin
2
, Ireland
b
Physical and Materials Electrochemistry Laboratory, School of Chemistry, Trinity
College Dublin, the University of Dublin, Dublin 2, Ireland
c
Medicinal Chemistry, Trinity Translational Medicine Institute, Trinity Centre for Health
Sciences, Trinity College Dublin, the University of Dublin, St. James’s Hospital, Dublin
8
, Ireland
∗
Corresponding author. Tel.: +353-1-896-8537; fax: +353-1-896-8536; e-mail:
sengem@tcd.ie
A series of triptycene-derived Schiff bases were synthesized by condensation between
amino triptycenes with an appropriate aldehyde and were isolated in good to excellent
(85-90 %) yields. Amongst these, a triptycene-hydroxybenzaldehyde Schiff base
compound proved to be a selective sensor for cyanide. It exhibited a “turn-on”
-
-
fluorescence response at 490 nm to CN facilitated by the nucleophilic addition of CN to
the aldehyde group, accompanied by a visible color change from orange to yellow.
Likewise, a triptycene salicylaldehyde adduct was shown to be highly sensitive towards
-
the detection of the CN ion with a detection limit of 0.9 µM. On the other hand a
2
+
triptycene-BODIPY Schiff base compound could be used for the detection of Cu ions
over other competing, biologically relevant metal ions in acetonitrile. Photophysical
studies revealed a 1:1 binding model for the triptycene-BODIPY compound.
Key words: Triptycene; Schiff base; Sensor; Cyanide; Copper ions
1

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1
. Introduction
1
Triptycene 1, as the first representative of the iptycene family, has been known
2
since the early 1940s and is the parent compound in this unique family of
homoconjugated aromatic compounds with D3h symmetry. The three-dimensional rigid
framework offers a void space in the clefts between the rings to create a well-defined
3
spatial arrangement and substantial internal free volume (IFV). The arene subunits of
triptycene are amenable to further functionalization and it has found use in a wide range
4
5
6
of applications such as in electron transfer systems, anti-cancer, sensing, molecular
7
6a,7c,8
6,9
machines, material and polymer science,
in host–guest chemistry or as building
1
0
blocks for functional and supramolecular systems.
Our long term interest in this area is to use the triptycene scaffold for the
presentation of multiple and different functional units to prepare multi-modal systems
which may find use in sensing and theranostics. After reporting on triptycene dye
4
b,9d,10b
systems
and recently on organometallic hexafunctionalization reactions for
1
0c
triptycene we have now turned our attention to simple sensing systems prior to the
investigation of unsymmetrical triptycenes with multiple and diverse binding and
reporting units (Fig. 1).
Figure 1. Illustration of unsymmetrically substituted triptycenes with different effector
units.
We were interested in Schiff-base derivatives, which have been used for a long time
1
1,12
for the formation of various (metal) complexes or as bidendate ligands.
For initial
studies we chose mono- and trifunctionalized Schiff base targets of triptycene as these
might be more easily accessible from available triptycene amino precursor compounds.
As coupling partner we used commercially available salicylaldehyde/salicyldialdehyde
and fluorescent molecules having hydroxyl groups suitable for sensing applications.
As possible analytes for sensing evaluations we chose cyanide and copper(II).
Cyanide is one of the most toxic anions deleterious to human health and the environment
and has both natural and anthropogenic origin. It still finds use in many products and
processes. Examples are resins, fibers, dyes, gold extraction and precious metal mining
and plating, plastics and fertilizer plants. It is also used as a chelating ligand for
1
3
pharmaceuticals and new sensors thereof are actively sought after. Copper is an
essential metal ion for humans and is involved in various physiological reactions such as
the formation of hemoglobin and iron utilization. Copper deficiency can increase the risk
of developing coronary heart disease, while excess copper can promote the production of
2

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harmful reactive oxygen species and is implicated in oxidative stress and
1
4
neurodegenerative disorders.
-
2+15
Although many CN and Cu
selective sensors have been reported only a few
1
6,17
studies describe “turn-on” fluorescent sensors,
rather than those quenching
fluorescence. In the following we outline that a combination of salicylaldehyde or
1
8
19
salicyldialdehyde or BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) units with
the triptycene scaffold gives triptycene-derived Schiff base compounds which selectively
turn on fluorescence as sensors towards cyanide and copper(II) ions. Analyte binding was
accompanied by changes in the NMR and absorption spectra and visual color changes,
too.
2
2
. Results and Discussion
.1. Synthesis of triptycene Schiff bases
2
0
The essential starting materials of 2-aminotriptycene 4, 2,6,14-triaminotriptycene
and meso-salicylaldehyde substituted BODIPY 9, were synthesized over a series of
2
1
22
5
sequential steps from commercially available triptycene 1 and 4-tert-butylphenol 6
according to procedures previously reported (Scheme 1). First, triptycene was treated
with a mixture of nitric acid and glacial acetic acid under reflux conditions to give
mononitrotriptycene 2 in 58 % yield, while longer reaction times afforded the three-fold
functionalized 2,6,14-trinitrotriptycene 3 in 61 % yield. The mononitro 2 and trinitro 3
derivatives were reduced by Raney-Ni in the presence of hydrazine to yield the
corresponding monoaminotriptycene 4 and triaminotriptycene 5 in 68 % and 70 % yields,
respectively.
Scheme 1. Synthesis of triptycene and BODIPY monomers 7-13. Reagents and
conditions: a) Conc. HNO
rt; c) hexamine, TFA, rt; d) pyrrole, BF
3
, CH
3
COOH, 75 °C, 24h; b) Raney-Ni, NH
•OEt , rt; e) DDQ, CH Cl ; f) Et
2
NH
2
•H
2
O, THF,
, rt.
3
2
2
2
3
N, BF
3
•OEt
2
3

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On the other hand, treatment of 4-tert-butylphenol 6 with hexamine in the presence
of trifluoroacetic acid gave 5-tert-butyl-2-hydroxyisophthalaldehyde 7 in good yield.
Subsequently, 5-salicylaldehyde substituted BODIPY 9 was prepared by reaction of 5-
tert-butyl-2-hydroxyisophthalaldehyde 7 with an excess of pyrrole in the presence of
catalytic amounts of BF •OEt in dichloromethane to afford dipyrromethane 8. This in
3
2
turn was further oxidized with DDQ and neutralized with triethylamine, followed by
complexation with BF •OEt to yield the salicylaldehyde substituted BODIPY 9 as an
3
2
orange solid in 27 % yield.
Afterwards, two series of target compounds were prepared (Scheme 2). The
synthesis of the triptycene-derived Schiff base compounds 10-15 was achieved by using
aminotriptycene (4/5) with an appropriate aldehyde in methanol. The reaction of amino
triptycene (4/5) with salicylaldehyde was performed at room temperature for 12 h to give
a precipitate of the required compounds 10 and 13 in 92 % and 90 % yields, respectively.
Under the same reaction conditions, treatment of aminotriptycene (4/5) with either 5-tert-
butyl-2-hydroxyisophthalaldehyde 7 or meso-salicylaldehyde substituted BODIPY 9 gave
no products. However, addition of 3 mol-% TFA and heating the reaction mixture for 12
h resulted in product precipitation from the reaction medium and the target compounds
1
0-15 were isolated in yields of 85-91 %.
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Scheme 2. Synthesis of triptycene-derived Schiff base compounds 10-15.
The identity and purity of the new compounds was established by FT-IR, H, H- H
1
1
1
1
3
COSY and C NMR spectroscopy as well as high resolution mass spectrometry analysis.
-1
In the FT-IR spectra, characteristic stretching bands between 1603 to 1610 cm were
identified for the imine bond of the triptycene Schiff base compounds 10-15 and a band
-1
around 1680 cm corresponded to the salicylaldehyde moiety of compounds 11, 14 and
1
was not observed for the other compounds. Similarly, the H NMR gave characteristic
1
signals. Taking the H NMR spectrum of compound 12 as an example (Fig. S10 in
5

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Supporting Information (SI)), a broad signal of the hydroxyl proton appears at 13.80 ppm
and the imine (-CH=N) proton appears as a singlet at 8.68 ppm. The BODIPY unit in
compound 12 containing six pyrrole protons appears as three sets of signals at δ = 7.91
(s), 6.91(d) and 6.50 (dd) ppm. The other signals at δ = 7.51 to 7.01 ppm can be assigned
as the aromatic protons of the triptycene and BODIPY units and the bridgehead protons
of triptycene appear as two singlet resonances at 5.45 and 5.44 ppm while the tert-butyl
1
3
group could be identified as a singlet at 1.37 ppm. In the C NMR spectrum of
compound 12, the resonance of the imine carbon nucleus signal is located at 162.6 ppm
and the bridgehead carbon nuclei give signals at 54.3 and 53.8 ppm. In addition,
compound 12 with a BODIPY unit exhibited two sets of octets at -143 and -147 ppm in
1
9
the F NMR spectrum (Fig. S14 in SI). This can be attributed to the presence of two non-
equivalent fluorine atoms in the BODIPY moiety, each fluorine coupled with boron to
2
3
give a quartet, which is further coupled with non-equivalent fluorine and appeared as an
1
1
octet. The B NMR spectrum of compound 12 showed a typical triplet at 0.3 ppm (Fig.
S13 in SI).
2
.2. Photophysical and electrochemical properties
The target compounds 10-15 were characterized further by absorption and emission
spectroscopy. For compounds 12 and 13 data were obtained in six different solvents of
varying polarity (see Fig. S43-S46 in SI), while a comparison of normalized absorption
and emission spectra of compounds 12, 13, 14 and 15 is presented in Figure 2. The
absorption spectra of compounds 13 and 15, which contain BODIPY units, show a
characteristic strong absorption band corresponding to the S →S transition at 500 nm
0
1
along with one vibronic component on the lower wavelength side as a shoulder at 475
nm. Furthermore, an ill-defined band in the 300-400 nm range corresponding to an
S →S transition is also observed, similar to other meso-aryl BODIPYs. Both compounds
0 2
exhibit very weak fluorescence compared to the individual BODIPY unit 9 (Φ=0.136)
with a quantum yield of Φ=0.004 for compound 13 and Φ=0.003 for compound 15. This
is due to a photo-induced electron transfer (PET) from the triptycene moiety to the
BODIPY unit causing an effective quenching of fluorescence.
6

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Figure 2. Comparison of normalized absorption (a) and emission (b) spectra of
compounds 11, 12, 14 and 15 recorded (λ =380 nm for 11, 14 and λ =488 nm for 12,
ex
ex
1
5) in CHCl3.
The redox potentials of compounds 12 and 15 were measured in chloroform by
−
1
using cyclic voltammetry at a scan rate of 100 mV s with tetrabutylammonium
perchlorate as the supporting electrolyte. The electrochemical redox behavior of
compounds 3 and 6 was similar to that observed for salicylaldehyde substituted BODIPY
2
2b
1
3. Thus, the cyclic voltammograms of the salicylaldehyde substituted BODIPY 9
showed one reversible reduction at -0.58 V, whereas the triptycene Schiff base
compounds 12 and 15 exhibited one reversible reduction at -0.846 V and -0.843 V,
respectively. The reduction peak of both triptycene Schiff base compounds 12 and 15
occurred at more negative potentials relative to that found for meso-aryl BODIPY
derivatives. The anodic shift in reduction potential was more pronounced (270 mV) than
for BODIPY 13. This indicates that the triptycene Schiff base compounds 3 and 6 are
more difficult to reduce compared to salicylaldehyde substituted BODIPY 13 (see Fig.
S47-S48).
2
.3. Anion sensing studies
The chemodosimetric approach involves the use of a specific chemical reaction
2
2b
initiated by the analyte coupled to a color or spectral variation. In our case, the aldehyde
7

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group can be converted irreversibly into a cyanohydrin group via nucleophilic addition of
cyanide. Therefore, compounds 11 and 14 are potential chemodosimetric sensors for the
−
presence of CN by monitoring the absorption, emission and/or NMR spectral changes.
First, we investigated color changes of compound 11 upon addition of different anions.
Compound 11 was weakly fluorescent under UV light, however, upon addition of
cyanide its fluorescence (UV lamp) was enhanced significantly to green and a color
change from orange to yellow was observable by naked eye, too (Fig. 3). In contrast,
-
-
-
-
-
-
-
-
addition of various other anions, such as F , Cl , Br , I , AcO , ClO , H PO , HSO (used
4
2
4
4
as their tetrabutylammonium salts) did not result in a color change. This indicated that
-
compound 11 has the potential to act as a selective turn-on chemodosimeter for CN ions.
Figure 3. Changes in the color of 11(10 µM) upon addition of 10 equiv. of different
anions in acetonitrile. Top: naked-eye; bottom: excitation at 360 nm.
-
This prompted further investigations into the selectivity of compound 11 for CN
using various spectrophotometric techniques. Fig. 4a shows the absorption spectra of 11
-
(
10 µM) upon addition of different anions in CH CN. Only in the presence of CN is a red
3
shift of about 100 nm compared to the original compound 11 observed. Additionally, the
fluorescence emission spectrum of compound 11 was found to be considerably altered in
-
the presence of CN (Fig. 4b). Note, that the emission maxima remained unchanged in the
presence of other anions.
8

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Figure 4. Changes in (a) absorption and (b) emission of compound 11 (10 µM) upon
addition of various anions (20 equiv.) in acetonitrile (λ =380 nm).
ex
-
Next, we studied the specific detection of CN by compound 11 by following the
concentration dependent changes in the absorption and fluorescence spectra (Fig. 5).
Changes in the absorption spectra of compound 11 upon addition of increasing amounts
-
of CN ion in CH CN are shown in Fig. 5a. With increasing amounts of cyanide, the
3
intensity of the absorption band at 374 nm decreased accompanied by the appearance and
concomitant increase of a new absorption band at 480 nm with a clear isosbestic point at
-
4
07 nm indicating the binding of CN ion to compound 11. Further evidence is derived
from the ratiometric plot, which shows the changes in the absorption bands at 374 nm and
85 nm versus the concentration of cyanide. The sigmoidal nature clearly indicates
analyte binding (inset in Fig. 5a).
4
9

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Figure 5. Changes in (a) absorption, (b) fluorescence spectra of compound 11 (10 µM)
-
upon titration with CN (0 to 12 equiv.) in acetonitrile (λ =380 nm). Insets show: (a) plot
ex
-
of absorbance vs [CN ]/[11] molar ratio for absorbance at 374 and 485 nm, respectively;
b) plot of relative intensity maxima at 490 nm as a function of [CN ]/[11] molar ratio.
-
(
Cyanide sensing was investigated using fluorescence studies (Fig 5b). With
-
increasing amounts of CN ions the fluorescence spectra of the sensor displayed a
continuous enhancement of the emission intensity of the band at 490 nm. Based on the
2
4
Stern-Volmer equation, the binding constant (K) for the binding of cyanide to
5
-1
compound 11 was determined as 7.41 (±0.32) × 10 M (Fig. S51). The Job’s plot
-
analysis of the CN ion titration with compound 11 also corroborated a 1:1 binding
-
stoichiometry (Fig. S52). The sensitivity of compound 11 for the CN ion was further
evaluated in fluorescence titration profiles, which demonstrated that compound 11 has a
-6
-
detection limit of 0.9 × 10 M for the CN anion, which is lower than the WHO guideline
2
5
of 1.9 µM cyanide (Fig. S53).
10

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-
To further explore the utility of compound 11 towards CN , we carried out
-
competition experiments. Compound 11 (10 µM) was treated with 12 equiv. CN in the
presence of different background anions (50 equiv.). All tested background anions
showed small or no interference with the detection of cyanide (Figure S64 in SI)
-
indicating that 11 could be used as a potential CN selective turn-on fluorescent sensor.
Similar observations were made in the case of the three-fold functionalized triptycene 14;
full details are given in the supporting information (Fig. S54-S55).
To gain further insight into the structural aspects of compound 11 upon binding with
-
1
CN a H NMR titration in CD CN was performed (Fig. 6). In native form compound 11
3
exhibited three singlet signals (a, b, c) at 10.57, 8.65 and 13.80 ppm, respectively. Upon
-
addition of one equiv. of CN ion to the solution the signal corresponding to the aldehyde
proton at 10.57 ppm disappeared and a new signal appeared at 4.49 ppm. This signal
corresponds to the formation of the cyanohydrin which was further confirmed by LC-
mass spectrometry. Here, a molecular ion peak was found at m/z = 483.3128 which
-
corresponds to the cyanohydrin form {11.CN } (Fig. S56).
1
−2
Figure 6. Partial H NMR spectra of compound 11 (3.64 × 10 M) alone and with one
equivalent of cyanide in CD CN.
3
2
.4. Metal ion sensing studies
Compounds 12 and 15 contain different binding sites and can easily form the
complexes with different metal ions, which can be monitored by absorption and
fluorescence techniques. To test the metal ion sensing property of compound 12
+
+
2+
2+
2+
2+
spectroscopic studies with different metal ions such as Na , K , Mg , Ca , Mn , Fe ,
2
+
2+
2+
2+
2+
2+
Co , Ni , Cu , Zn , Cd and Hg (as their perchlorate salts) in CH CN were
3
conducted. In all experiments using compound 12 (10 µM) with these metal ions, only
2
+
Cu affected the absorption and fluorescence spectra in a significant manner (Fig 7a and
11

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2
+
7
b). As evidenced in Fig. 7c increasing the concentrations of Cu (0 to 10 equiv.) is
accompanied by an increase in the absorbance at 415 nm and a decrease in the 350 nm
and 500 nm bands with four clear isosbestic points at 313, 375, 487 and 510 nm. This
2
+
clearly indicates conversion of free compound 12 to its Cu bound species with nitrogen
and oxygen donor atoms.
2
+
In order to further investigate the selective binding of Cu ions to compound 12
fluorescence spectra (10 µM) in the presence of the above mentioned metal perchlorates
2+
were recorded (Fig 7d). Here incremental addition of Cu ion to a solution of compound
2 (10 µM) led to significantly enhanced fluorescence at 520 nm, whereas addition of the
1
other metal ions under identical conditions resulted in either no or significantly weaker
changes in either fluorescence band or intensity. In order to establish the selectivity of
2
+
compound 12 as a selective fluorescence probe for Cu ion competitive fluorescence
binding titration studies with other competing metal ions were performed. The initial
fluorescence intensity of compound 12 did not change significantly upon mixing with
excess equivalents of alternative metal cations (Figure S65 in SI). Taken together these
2+
data confirm that compound 12 acts as a selective chemodosimeter for detection of Cu
over other biologically relevant metal ions.
Figure 7. Changes in (a) absorption and (b) emission of compound 12 (10 µM) upon
addition of various cations (50 equiv.) in acetonitrile. Systematic titration of compound
2
+
1
2 (10 µM) with Cu (0 to 10 equiv.) in acetonitrile: (c) absorption spectra, (d)
fluorescence spectra (λ =488 nm). Inset (d) shows plot of relative intensity maxima at
ex
12

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2
+
5
20 nm as a function of [Cu ]/[12] molar ratio and the color change of compound 12
2
+
solution upon addition of Cu .
A Job’s plot based on the fluorescence data agrees well with a 1:1 complex (Fig.
S58) and based on the Stern-Volmer equation, the binding constant (K) for the binding of
2
+
4
-1
Cu to compound 12 was determined as 3.07 (±0.62) × 10 M (Fig. S59). This value
3
12
2+
2+
was within the range (10 –10 ) of reported Cu chemosensors. The sensing of Cu by
compound 12 is further demonstrated by observing the visual fluorescence under UV
lamp illumination in the presence of different metal ions (Fig. S60). Similar results were
obtained with compound 15 and are given in the supporting information (Fig. S62-S63).
2
.5. Triptycene boranil complexes
Boron complexes of Schiff base compounds are yet another type of four-coordinate
boron complex that has gained interest due to their greater stability than tri-coordinate
boron compounds. E.g., Ziessel and co-workers reported the first synthesis and
1
2b
photophysical properties of boranil complexes. Later, Lee and co-workers reported
boron complexes of stackable pseudo triphenylenes, which showed interesting enhanced
1
2d
fluorescence upon aggregation in solution. In view of the potential applications of
these boranil complexes, we synthesized the triptycene-derived boranil complexes 16 and
1
7. Compounds 10 and 13 were deprotonated by using base (DIEA) in dry 1,2-
dichloroethane in an argon atmosphere, followed by reaction with excess amount of
BF •OEt to afford the boranil complexes 16 and 17 in moderate yields (Scheme 3).
3
2
Boranil formation was evidenced by the disappearance of the signal for the phenolic
1
proton at 13.80 ppm in the H NMR spectrum of 16. This was supported by the required
11
19
signals in the B (triplet at 0.8 ppm) and F NMR (quartet at -133.9 ppm) spectra.
Further evidence is provided in the Supporting Information as are data for compound 16.
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Scheme 3. Synthesis of boranil complexes 16 and 17.
The absorption and emission profiles of both boranil complexes (16 and 17) were
recorded in chloroform and are shown in Fig 8. Compounds 16 and 17 exhibit absorption
maxima at 395 and 399 nm, respectively while the maximum absorption of the molecules
showed fluorescence emission at 468 and 469 nm, respectively.
Figure 8. Comparison of normalized absorption and emission spectra of 16 (blue line)
and 17 (red line) recorded in chloroform. The excitation wavelength was 400 nm.
3
. Conclusions
In conclusion, we have synthesized a series of new triptycene-derived Schiff base
compounds (10-15) by condensation of either mono- or triaminotriptycene with an
-
appropriate aldehyde in methanol. The nucleophilic addition of CN to triptycene-
hydroxybenzaldehyde Schiff-base compound 11 was monitored by absorption and
1
emission spectral changes as well as “naked-eye” detection. Additionally, H NMR
studies clearly indicated the formation of the cyanohydrin confirming its utility as a
fluorescence “turn-on” cyanide sensor. The triptycene-BODIPY Schiff base compound
2+
1
2 can be used for detection of Cu over other metal ions. Furthermore, to test the
reactivity of these triptycene-derived Schiff base compounds, we synthesized triptycene
boranil complexes and studied their properties. Overall, these novel triptycene scaffolds
-
2+
have been used for the detection of CN and Cu ions illustrating that the triptycene
scaffold is a convenient scaffold system for dyes and analytical residues.
4
4
. Experimental
.1. General methods
Chemicals, such as triptycene, boron trifluoride diethyl etherate, and 2,3-dichloro-
,6-dicyano-1,4-benzoquinone (DDQ) were used as obtained from Aldrich. All other
5
chemicals used for the synthesis were reagent grade and solvents were dried by routine
procedures immediately before use. Thin layer chromatography was performed with
silica gel 60 (fluorescence indicator F254; Merck) or pre-coated aluminium sheets and
visualized with UV irradiation. Column chromatography was performed on silica gel (60
1
13
11
19
Å, 230-400 mesh). H, C, B and F NMR spectra were recorded with an Agilent 400
1
13
MHz (400 MHz for H; 100.6 MHz for C) and Bruker Advance 400 and 600
spectrometers (400 MHz for H; 100.6 or 150 MHz for C; 128.4 MHz for B and 376.5
1
13
11
14

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1
9
MHz for F). NMR spectra were recorded at room temperature with the deuterated
solvent indicated in each case. UV-Vis absorption measurements were performed with a
Specord 250 spectrophotometer. Fluorescence spectra were obtained using a Varian
Cary-Eclipse instrument. Infrared spectra were recorded on a Mattson Genesis II FTIR
spectrophotometer equipped with a Gateway 2000 4DX2-66 workstation. HRMS spectra
were measured with a MALDI-Q-TOF Premier Micromass and Micromass/WatersCorp.
USA liquid chromatography time-of-flight spectrometer equipped with an electrospray
ionization source (ESI). Melting points were acquired with a Stuart SMP-10 melting
point apparatus.
4
.2. Binding studies
The binding constant of the complexes formed in solution were estimated by using
the standard Stern-Volmer equation, viz., I /I = 1 + K [A]; where I is the intensity of the
0 0
compounds 11 and 12 before addition of anion/cation, I is the intensity in the presence of
anion/cation [A] and K is the association constant of the complex formed. Solutions of
-2
Bu
containing compounds 11 and 12 was placed in a quartz cell (1 cm width), and Bu
or Cu(ClO ) solutions were added in an incremental fashion. The corresponding UV-Vis
4
N salts and metal perchlorates were prepared in CH
3
CN (1 × 10 M). The solution
4
NCN
4
2
1
and fluorescence spectra were recorded at 298 K. For H NMR titration experiments, the
spectra were measured on a Bruker Advance 400 MHz NMR spectrometer. A solution of
1 in CD CN was prepared (3.64 × 10 M), and a 0.4 mL portion of this solution was
−
2
1
3
transferred to a 5 mm NMR tube. Small aliquots of Bu NCN in CD CN were added in an
4
3
incremental fashion, and the corresponding spectra were recorded.
4
.3. Electrochemistry
All electrochemical experiments were undertaken in a standard three-electrode cell
using a high performance digital potentiostat (CH model 1760 D Bi-potentiostat system
monitored using CH1760D electrochemical workstation beta software). The working
electrodes consisted of a glassy carbon electrode; a Pt wire was employed as a counter-
electrode and a saturated calomel electrode (SCE) was used as reference standard.
Electrochemical measurements were taken at a constant temperature of 25 °C, using a
thermal bath with the temperature maintained by a thermostat. All solutions were
degassed with N for 10 minutes before commencing any analysis to eliminate any
2
dissolved oxygen present in the electrolyte. Cyclic voltammetry (CV) experiments were
-1
conducted in chloroform with 0.1 M TBAP electrolyte at 100 mV s between the limits
of 0 V and 1.2 V vs. SCE. Differential Pulse Voltammetry (DPV) was measured between
the limits of 1.5 V and -1.5 V vs. SCE.
4
.4. Syntheses
.4.1. General procedure I for the synthesis of triptycene-Schiff bases. The
4
corresponding aldehydes were added to a stirred solution of aminotriptycene (50 mg) in
dry methanol. The reaction mixture was stirred for 12 h at room temperature and the
15

ACCEPTED MANUSCRIPT
Manuscript for Tetrahedron
resulting precipitate was collected by suction filtration, washed several times with
methanol and dried in vacuo to give the desired product.
4
.4.2. General procedure II for the synthesis of triptycene-Schiff bases. The
corresponding aldehydes were added to a stirred solution of aminotriptycene (50 mg) in
dry methanol, followed by addition of TFA (3 mol-% in CH Cl ). The reaction mixture
2
2
was stirred for 12 h under heating to reflux and the resulting precipitate was collected by
suction filtration, washed several times with methanol and dried in vacuum to give the
desired product.
4.4.3. General procedure III for the synthesis of triptycene boranil complexes.
Samples of triptycene-derived Schiff base compound (50 mg) and N,N-
diisopropylethylamine (DIEA) were dissolved in anhydrous dichloroethane (5 mL) in a
5
0 mL Schlenk flask under argon atmosphere and refluxed for 10 min. To this reaction
mixture BF •OEt was added and heating under reflux continued for 3 h while monitoring
3
2
the reaction by TLC analysis. After cooling to rt, the reaction mixture was washed with a
saturated solution of NaHCO and extracted with dichloromethane. Organic layers were
3
dried over MgSO and the solvent evaporated under vacuum. The product was purified
4
by silica gel column chromatography by using ethyl acetate and hexane as an eluent.
4
.4.4.
10). Following general procedure I, monoaminotriptycene 4 (50 mg, 0.186 mmol) and
salicylaldehyde (23 mg, 20 ꢀL, 0.186 mmol) were dissolved in methanol (10 mL). After
2-(((9,10-Dihydro-9,10-[1,2]benzenoanthracen-2-yl)imino)methyl)phenol
(
1
2
1
7
2 h, compound 10 was obtained as a pale yellow solid (63 mg, 0.169 mmol, 92 %). Mp:
1
42 °C; R =0.67 (CH Cl /C H , 1:1, v/v); H NMR (400 MHz, CDCl ): δ =13.27 (bs,
f
2
2
6
14
3
H
H, OH), 8.56 (s, 1H, CH, imine), 7.45-7.41 (m, 5H, Ar-H), 7.37-7.35 (m, 3H, Ar-H),
.06-7.02 (m, 5H, Ar-H), 6.92-6.90 (m, 2H, Ar-H), 5.47 (s, 1H, bridgehead-H), 5.46 ppm
1
3
(s, 1H, bridgehead-H); C NMR (150 MHz, CDCl ): δ =162.2, 161.2, 146.9, 145.9,
3 C
1
1
1
45.1, 145.0, 144.4, 133.1, 132.3, 125.6, 125.5, 124.5, 123.9, 123.8, 119.4, 119.1, 118.1,
17.4, 116.9, 54.3, 53.8 ppm; IR (ATR): νmax (cm )- 3320, 1604, 1570, 1469, 1412,
-1
278, 1188, 1150, 1087, 1047, 970, 945, 880, 748; HRMS (ESI): m/z calcd. for
+
[C
27
H
20NO] (M+H) 374.1545; found 374.1550.
4
.4.5.
yl)imino)methyl)-2-hydroxybenzaldehyde (11). Prepared following general procedure II,
monoaminotriptycene (50 mg, 0.186 mmol) and 5-tert-butyl-2-
5-(tert-Butyl)-3-(((9,10-dihydro-9,10-[1,2]benzenoanthracen-2-
4
hydroxyisophthalaldehyde 7 (38 mg, 0.186 mmol) were dissolved in methanol (10 mL).
After 12 h, the title compound 11 was obtained as a light orange solid (77 mg, 0.168
1
mmol, 91 %). Mp: 258 °C; R
f
=54 (CH
2
Cl
2
/C
6
H
14, 1:1, v/v); H NMR (400 MHz, CDCl
3
):
δ
1
H
=13.80 (bs, 1H, OH), 10.57 (s, 1H, CHO), 8.65 (s, 1H, CH imine), 7.95 (d, J=6.5 Hz,
H, Ar-H), 7.62 (s, 1H, Ar-H), 7.46-7.40 (m, 6H, Ar-H), 7.08-7.01 (m, 5H, Ar-H), 5.47
13
(s, 1H, bridgehead-H), 5.46 (s, 1H, bridgehead-H), 1.34 ppm (s, 9H, CH ); C NMR
3
(
1
3
150 MHz, CDCl ): δ =196.7, 145.0, 144.9, 144.3, 141.9, 135.3, 130.3, 125.6, 125.5,
24.4, 123.9, 123.8, 118.2, 116.6, 113.1, 54.3, 54.2, 31.4 ppm; IR (ATR): νmax (cm )-
340, 1670, 1610, 1468, 1277, 1186, 1150, 1087, 1047, 946, 881, 743; UV-Vis (CHCl3):
3
C
-1
λmax (lg
ε)=374 nm (4.3); fluorescence: λ =470 nm with excitation at 380 nm; HRMS
em
+
(ESI): m/z calcd. for [C H NO ] (M+H) 458.2120; found 458.2119.
3
2
28
2
16

ACCEPTED MANUSCRIPT
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4.4.6.
4-(tert-Butyl)-2-(((9,10-dihydro-9,10-[1,2]benzenoanthracen-2-
yl)imino)methyl)-6-(BODIPY)-phenol (12). Following general procedure II,
monoaminotriptycene 4 (50 mg, 0.185 mmol) and meso-salicylaldehyde substituted
BODIPY 9 (68 mg, 0.185 mmol) were dissolved in methanol (20 mL). After 12 h, the
title compound 12 was obtained as an orange solid (101 mg, 0.163 mmol, 88 %). Mp: 233
1
°
C; R =0.62 (CH Cl /C H , 1:1, v/v); H NMR (400 MHz, CDCl ): δ =13.82 (bs, 1H,
f
2
2
6
14
3
H
OH), 8.68 (s, 1H, CH imine), 7.91 (s, 2H, Py-H), 7.51 (d, J=6.3 Hz, 1H, Ar-H), 7.45 (d,
J=6.8 Hz, 1H, Ar-H), 7.42-7.39 (m, 5H, Ar-H), 7.36 (d, J=7.1 Hz, 1H, Ar-H), 7.05-7.00
(
1
m, 5H, Ar-H), 6.91 (d, J=4.6 Hz, 2H, Py-H), 6.50 (dd, J=4.5, 2.8 Hz, 2H, Py-H), 5.47 (s,
1
3
H, bridgehead-H), 5.46 (s, 1H, bridgehead-H), 1.35 ppm (s, 9H, CH ); C NMR (150
3
MHz, CDCl ): δ =161.6, 156.9, 147.0, 145.0, 144.9, 144.1, 141.4, 132.7, 131.5, 130.9,
3
C
1
1
1
25.6, 125.5, 124.6, 123.9, 123.8, 121.7, 119.1, 118.4, 116.8, 54.3, 53.8, 31.4 ppm; B
NMR (128.4 MHz, CDCl ): δ =0.31 ppm (t, JB–F=28.9 MHz, 1B); F NMR (376.5
3 B
1
19
1
MHz, CDCl
3
): δ
max (cm )- 3340, 1608, 1458, 1411, 1383, 1257, 1088, 1047, 971, 879, 742; UV-Vis
CHCl ): λ (lg )=500 nm (4.9); fluorescence: λ =520 nm with excitation at 490 nm;
F
=−143.17 and -147.27 ppm (octate, JF–B=28.6 MHz, 2F); IR (ATR):
-1
ν
(
ε
3
max
em
+
HRMS (MALDI): m/z calcd. for [C H BN OF ] M 619.2606; found 619.2625.
4
0
32
3
2
4.4.7.
2,2',2''-(((9,10-Dihydro-9,10-[1,2]benzenoanthracene-2,6,14-
triyl)tris(azaneylylidene))tris(methaneylylidene)) triphenol (13). Synthesized following
general procedure I, triaminotriptycene 5 (50 mg, 0.167 mmol) and salicylaldehyde (68
mg, 59 ꢀL, 0.552 mmol) were dissolved in methanol (20 mL). After 12 h, compound 13
was obtained as a yellow solid (92 mg, 0.150 mmol, 90%). Mp: 248 °C; R =0.52
f
1
(
CH Cl /C H , 1:1, v/v); H NMR (400 MHz, CDCl ): δ =13.20 (bs, 3H, OH), 8.57 (s,
2
2
6
14
3
H
3
H, CH imine), 7.46 (dd, J=7.7, 3.9 Hz, 3H, Ar-H), 7.39-7.34 (m, 9H, Ar-H), 7.02 (d,
J=8.5 Hz, 3H, Ar-H), 6.97-6.91 (m, 6H, Ar-H), 5.53 (s, 1H, bridgehead-H), 5.52 ppm (s,
1
3
1
1
H, bridgehead-H); C NMR (150 MHz, CDCl ): δ =162.3, 161.0, 146.2, 146.1, 146.0,
3 C
43.3, 133.0, 132.1, 124.5, 124.4, 119.1, 119.0, 118.2, 118.0, 117.2, 116.9, 53.7, 53.2
ppm; IR (ATR): νmax (cm )- 3330, 1603, 1468, 1276, 1187, 1150, 1089, 1047, 946, 881,
49; LR-MS: calcd. for [C H N O ] 612.22; found 612.21; HRMS (ESI): m/z calcd. for
-1
7
4
1
30
3
3
+
[C
41
H
30
N
3
O
3
] (M+H) 612.2287; found 612.2284.
4.4.8. 3,3',3''-(((9,10-Dihydro-9,10-[1,2]benzenoanthracene-2,6,14-
triyl)tris(azaneylylidene))tris(methaneylylidene))tris(5-(tert-butyl)-2-
hydroxybenzaldehyde) (14). Prepared following general procedure II using
triaminotriptycene 5 (50 mg, 0.167 mmol) and 5-tert-butyl-2 hydroxyisophthalaldehyde 7
(
114 mg, 0.552 mmol) dissolved in methanol (20 mL). After 12 h, compound 14 was
obtained as a light orange solid (127 mg, 0.146 mmol, 88 %). Mp: 262 °C; R =0.45
=13.48 (bs, 3H, OH), 10.63 (s,
H, CHO), 8.69 (s, 3H, CH imine), 7.92 (d, J=6.9 Hz, 2H, Ar-H), 7.67 (d, J=7.3 Hz, 3H,
f
1
(
3
2 2 6 3 H
CH Cl /C H14, 1:1, v/v); H NMR (400 MHz, CDCl ): δ
Ar-H), 7.43-7.39 (m, 5H, Ar-H), 7.03-6.97 (m, 5H, Ar-H), 5.53 (s, 1H, bridgehead-H),
1
3
5
.52 (s, 1H, bridgehead-H), 1.36 ppm (s, 27H, CH
3 3
); C NMR (150 MHz, CDCl ):
δC=196.5, 162.5, 162.4, 162.0, 157.0, 156.6, 146.0, 142.3, 141.8, 141.2, 135.2, 132.3,
1
5
9
30.1, 129.0, 128.2, 125.7, 125.1, 124.5, 123.6, 120.1, 118.3, 116.9, 99.4, 98.9, 54.1,
4.0, 31.2 ppm; IR (ATR): νmax (cm )- 3302, 2961, 1467, 1363, 1269, 1207, 1109, 1052,
-1
78, 946, 887, 799, 726; UV-Vis (CHCl ): λ (lg )=361 nm (4.5); fluorescence:
ε
3
max
17

ACCEPTED MANUSCRIPT
Manuscript for Tetrahedron
λ
54 3 6
em=510 nm with excitation at 380 nm; HRMS (ESI): m/z calcd. for [C56H N O ]
+
(M+H) 864.4013; found 864.4042.
4
.4.9.
triyl)tris(azaneylylidene))tris(methaneylylidene))tris(5-(tert-butyl))-tris(6-
BODIPY))triphenol (15). Following general procedure II, triaminotriptycene 5 (50 mg,
.167 mmol) and meso-salicylaldehyde substituted BODIPY 9 (203 mg, 0.552 mmol)
2,2',2''-(((9,10-Dihydro-9,10-[1,2]benzenoanthracene-2,6,14-
(
0
were dissolved in methanol (20 mL). After 12 h, compound 15 was obtained as an orange
1
solid (192 mg, 0.142 mmol, 85 %). Mp: 245 °C; R =0.53 (CH Cl /C H , 1:1, v/v); H
f
2
2
6
14
NMR (400 MHz, CDCl ): δ =13.70 (bs, 3H, OH), 8.69 (s, 3H, CH imine), 7.92 (s, 6H,
3
H
Py-H), 7.53 (d, J=6.9 Hz, 3H, Ar-H), 7.47 (d, J=6.7 Hz, 5H, Ar-H), 7.44 (d, J=7.2 Hz,
2
6
H, Ar-H), 7.39 (d, J=7.1 Hz, 3H, Ar-H), 6.98 (d, J=6.9, 2H, Ar-H), 6.91 (d, J=4.4 Hz,
H, Py-H), 6.51 (d, J=4.1 Hz, 6H, Py-H), 5.50 (s, 1H, bridgehead-H), 5.49 (s, 1H,
1
3
bridgehead-H), 1.35 ppm (s, 27H, CH ); C NMR (150 MHz, CDCl ): δ =162.0, 156.8,
3
3
C
1
1
46.3, 146.1, 145.6, 145.5, 144.2, 144.0, 143.8, 143.7, 141.5, 135.7, 132.9, 132.8, 131.5,
1
1
31.0, 124.8, 121.8, 119.1, 119.0, 118.5, 118.4, 117.2, 53.6, 53.4, 31.5 ppm; B NMR
1
19
(128.4 MHz, CDCl ): δ =0.31 ppm (t, J =28.7 MHz, 1B); F NMR (376.5 MHz,
3 B B–F
1
CDCl ): δ =−143.42 and −147.04 ppm (octate, JF–B=28.6 MHz, 2F); IR (ATR): νmax
3
F
-1
(cm )- 3342, 1607, 1465, 1385, 1259, 1167, 1072, 1046, 970, 880, 772; UV-Vis
(
CHCl )=501 nm (4.8); fluorescence: λem=520 nm with excitation at 490 nm;
3
): λmax (lg ε
+
68 3 9 3 6
HRMS (MALDI): m/z calcd. for [C80H B N O F ] M 1349.5628; found 1349.5643.
4.4.10.
3-(9,10-Dihydro-9,10-[1,2]benzenoanthracen-2-yl)-2,2-difluoro-
benzo[e][1,3,2]oxazaborinine (16). Compound 10 (50 mg, 0.133 mmol), N,N-
.
3 2
diisopropylethylamine (43 mg, 58 ꢀL, 0.330 mmol) and BF OEt (47 mg, 41 ꢀL, 0.330
mmol) were dissolved in dry dichloroethane (5 mL) and heated to reflux following
general procedure III to obtain compound 16 as a pale yellow solid in 38 % (21 mg,
1
0
.045 mmol) yield. Mp: 220 °C; R =0.72 (C H O /C H , 1:1, v/v); H NMR (400 MHz,
f
4
8
2
6
14
CDCl ): δ =8.25 (s, 1H, CH imine), 7.64-7.58 (m, 2H, Ar-H), 7.44-7.39 (m, 6H, Ar-H),
3
H
7
.13-7.08 (m, 2H, Ar-H), 7.05-7.02 (m, 4H, Ar-H), 6.98-6.96 (m, 1H, Ar-H), 5.51 (s, 1H,
1
3
bridgehead-H), 5.50 ppm (s, 1H, bridgehead-H); C NMR (150 MHz, CDCl ): δ =163.4,
3
C
1
1
59.7, 147.2, 146.9, 144.6, 144.5, 139.4, 139.2, 132.2, 125.7, 125.6, 124.5, 124.0, 123.9,
1
1
20.5, 120.2, 119.6, 119.3, 53.9, 53.7 ppm; B NMR (128.4 MHz, CDCl ): δ =0.77 ppm
3
B
1
19
1
(t, J =27.5 MHz, 1B); F NMR (376.5 MHz, CDCl ): δ =−133.8 ppm (quartet, J
B–F 3 F F–
-1
B=27.6 MHz, 2F); IR (ATR): νmax (cm )- 1609, 1560, 1468, 1411, 1385, 1259, 1169,
110, 1071, 1033, 947, 882, 828, 747; UV-Vis (CHCl )=395 nm (4.1);
flurorescence: λem=468 nm with excitation at 400 nm; HRMS (ESI): m/z calcd. for
1
3
): λmax (lg ε
+
[C
27
H
18BNOF
2
K] (M+K) 460.1087; found 460.1078.
4.4.11.
3,3',3''-(9,10-Dihydro-9,10-[1,2]benzenoanthracene-2,6,14-triyl)tris(2,2-
difluoro-benzo[e][1,3,2]oxazaborinine) (17). Following general procedure III, compound
3 (50 mg, 0.08 mmol), N,N-diisopropylethylamine (79 mg, 107 ꢀL, 0.6 mmol) and
1
BF
.
3
OEt
2
(87 mg, 76 ꢀL, 0.6 mmol) in dry dichloroethane (5 mL) were used to obtain
=0.64
C H O /C H , 1:1, v/v); H NMR (400 MHz, CDCl ): δ =8.38 (s, 3H, CH imine), 7.69-
compound 17 as a yellow solid in 32 % (20 mg, 0.025 mmol) yield. Mp: 226 °C; R
f
1
(
4
8
2
6
14
3
H
7
4
.64 (m, 6H, Ar-H), 7.52-7.46 (m, 6H, Ar-H), 7.19-7.17 (m, 2H, Ar-H), 7.15-7.12 (m,
H, Ar-H), 7.06-7.02 (m, 3H, Ar-H), 5.63 (s, 1H, bridgehead-H), 5.62 ppm (s, 1H,
18

ACCEPTED MANUSCRIPT
Manuscript for Tetrahedron
1
3
bridgehead-H); C NMR (150 MHz, CDCl
3 C
): δ =162.4, 1621.2, 146.3, 146.2, 143.5,
1
43.3, 133.2, 132.3, 124.7, 124.6, 119.3, 119.2, 118.4, 118.2, 117.3, 117.1, 53.9, 53.4
1
1
19
ppm; B NMR (128.4 MHz, CDCl ): δ =0.93 ppm (t, J=27.9 MHz, 1B); F NMR
3
B
-
1
(376.5 MHz, CDCl ): δ =−133.59 ppm (bs, J=28.3 MHz, 2F); IR (ATR): ν
(cm )-
3
F
max
1
606, 1561, 1468, 1385, 1253, 1166, 1109, 1071, 1032, 973, 947, 890, 829, 746; UV-Vis
(
CHCl ) λ (lg
ε
): 399 nm (4.3); fluorescence: λ =469 nm with excitation at 400 nm;
3
max
em
+
HRMS (MALDI): m/z calcd. for [C H B N O F Na] (M+Na) 778.2055; found
4
1
26
3
3
3 6
7
78.2090.
Acknowledgement
This work was supported by a grant from Science Foundation Ireland (SFI IvP
3/IA/1894 SFI).
1
Supplementary Material
Analytical spectra and photophysical characterizations for all compounds.
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