Specific Cu(II) detection using a novel tricarbazolyl-tristriazolotriazine based on photoinduced charge transfer

Article abstract of DOI:10.1039/c3ra46239k

Tricarbazolyl-tristriazolotriazine, a novel Cu(ii)-sensing and selective fluorescent sensor, was designed and synthesised on the basis of the mechanism of photoinduced charge transfer. The synthesised compound exhibits highly sensitive and significant fluorescence decline response towards Cu²⁺ over other metal ions, with a detection limit of 0.2 μM in THF-H₂O (9 : 1, v/v) at neutral pH. Its optical, thermal, electrochemical and morphological properties were studied. The results demonstrate that the compound has high relative fluorescence quantum yield (Φ_F = 0.99) in xylene solution, excellent thermal stability (T_d = 325 °C, T _g = 99.9 °C), wide energy gap (E_g = 4.09 eV), desirable triplet energy (E_T = 2.98 eV) and the highest occupied molecular orbital energy (-5.67 eV). Morphological investigation showed that the compound easily forms ribbon-like microstructures. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ra46239k

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Specific Cu(II) detection using a novel tricarbazolyl-
tristriazolotriazine based on photoinduced charge
transfer†
Cite this: RSC Adv., 2014, 4, 13161
*
*
He Zhao, Yongtao Wang, Zhiyong Liu and Bin Dai
Tricarbazolyl-tristriazolotriazine, a novel Cu(II)-sensing and selective fluorescent sensor, was designed and
synthesised on the basis of the mechanism of photoinduced charge transfer. The synthesised compound
exhibits highly sensitive and significant fluorescence decline response towards Cu2+ over other metal
ions, with a detection limit of 0.2 mM in THF–H2O (9 : 1, v/v) at neutral pH. Its optical, thermal,
electrochemical and morphological properties were studied. The results demonstrate that the
compound has high relative fluorescence quantum yield (FF ¼ 0.99) in xylene solution, excellent thermal
Received 30th October 2013
Accepted 24th February 2014
stability (Td ¼ 325 ꢀC, Tg ¼ 99.9 ꢀC), wide energy gap (Eg ¼ 4.09 eV), desirable triplet energy (ET
¼
DOI: 10.1039/c3ra46239k
2.98 eV) and the highest occupied molecular orbital energy (ꢁ5.67 eV). Morphological investigation
showed that the compound easily forms ribbon-like microstructures.
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including voltammetry,11 inductively coupled plasma mass
spectroscopy (ICP-MS),12 ICP-atomic emission spectrometry13
and atomic absorption spectrometry.14 However, these detec-
Introduction
Copper is important in various applications, particularly in
biological and environmental elds. Cu is an essential element
in the ecosphere, and is the third most abundant transition
element in our bodies and in many living organisms.1–4
Although the divalent copper cation [Cu(II)] participates in a
series of biological processes, at certain high concentrations it
is a toxic and harmful heavy metal element to humans and
some organisms. Elevated concentrations of Cu in the human
body are connected with neurodegenerative diseases, such as
Menkes syndrome and Wilson's diseases. High Cu concentra-
tion also causes infant liver damage.5–9 Moreover, Cu is a
common contaminant in agriculture and other industries. It
can be incorporated in the environment, drinking water and
food chain.10 Therefore, an efficient and selective method to
detect Cu ions is important for environmental protection and
human health.
tion methods require complicated pretreatment procedures
and sophisticated instrumentations, making them unsuitable
for on-line or in-eld monitoring.15 In the past decade, design
and development of uorescent chemosensors with high
selectivity and sensitivity targeting Cu ions have been given
signicant attention because they allow nondestructive, low-
cost and prompt detection of metal cations, as well as easy-to-
realise in situ and real-time detection by a simple uorescence
enhancement or quenching response.5,8,16–20
Thus far, numerous luminescent organic compounds in
probe development for selective detection of Cu ions have been
developed.21–23 However, some of them weaken or are even
quenched at high concentrations. The effective ranges of their
practical application are limited.24 Developing uorescent
probes with high uorescence quantum yield, strong uores-
cent emission, large extinction coefficient and insensitivity to
solvents for Cu detection remains a challenge. Given their
excellent photophysical properties, internal charge transfer
(ICT) compounds are being given signicant interest in cation
sensing. Moreover, their inherently large Stokes shi addresses
the problem of self-reabsorption and makes them important
candidates for probe applications.25–27 The mechanism of ICT
compounds interacting with a cation is called photoinduced
charge transfer (PCT), which has been extensively used to
explain sensing mechanism.28–31 Heterocycle tris[1,2,4]triazolo-
[1,3,5]triazine reportedly exhibits good organic solvent
compatibility and strong uorescence in a solution.32 Never-
theless, carbazole derivatives are highly efficient blue uores-
cence materials that are widely used in Organic Light-Emitting
High-sensitivity detection of copper ions in various samples
has been performed using several traditional methods,
School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of
Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi 832003,
Xinjiang, P. R. China. E-mail: wyt_shzu@163.com; lzyongclin@sina.com; Fax: +86
993 2057270; Tel: +86 993 2057277
† Electronic supplementary information (ESI) available: Experimental details for
1H NMR, 13C NMR, EI-MS and HRMS of compound 5 and 1H NMR and LRMS
of compound 7. Fluorescence spectra of compound
concentrations of Cu2+. Calibration plots of relative F/F0 of the compound 5 and
against different concentrate of Cu2+
TGA and DSC thermograms of
7
with varying
7
.
compound 5 recorded under nitrogen atmosphere at the heating rate of 10 ꢀC
minꢁ1. Representative cyclic voltammogram of compound 5 measured. SEM
microphotographs of compound 5. See DOI: 10.1039/c3ra46239k
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Diodes (OLEDs).33–35 According to these inspirations, a combi- with carbon working, platinum wire auxiliary, and Ag/AgCl
nation of tris[1,2,4]triazolo[1,3,5]triazine and carbazole may be pseudo-reference electrodes.32
advantageous for application as uorescent probe for the
detection of Cu(II). To the best of our knowledge, the application
of tristriazolotriazine derivative in discotic liquid crystals has
Synthesis
been studied; however, it has never been reported as a uo-
Synthesis of 9-butyl-9H-carbazole.38 (1) To a 250 mL ask
rescent probe.
containing carbazole (16.7 g, 100 mmol), NaOH (6.0 g, 150
In this paper, based on PCT sense mechanism, a novel mmol), hexadecyltrimethyl ammonium bromide (0.15 g) and
disc-like uorescent chemosensor, namely tricarbazolyl-tris- acetone (100 mL). The mixture was stirred at 60 ꢀC for 2 h. Aer
triazolotriazine (compound 5), containing the electron- cooling to room temperature, 1-bromobutane (100 mmol) was
donating carbazole moieties and the electron-accepting tris- added dropwise to a stirred mixture solution. Aer the addition
[1,2,4]triazolo[1,3,5]triazine (TTT) core, was successfully the mixture was reuxed with stirring for 24 h. Then acetone
designed and synthesised. As non-planar conformation prod- was evaporated as far as possible. The residue mixture was
ucts, the disc-like compounds avoid p–p molecular stacking poured into a large number of ice water, and vigorous stirred.
leading to uorescence quenching at elevated concentration. In The white precipitates collected by ltration were recrystallized
addition, we determined the thermal, electrochemical and from absolute ethanol to give white acicular crystal (yield 92%):
ꢀ
morphological properties of these compounds, revealing that m.p. 57.5–57.7 C.
compound 5 has a potential extensive application.
Synthesis of 3-bromo-9-butyl-9H-carbazole.39 (2) 3.47 g (19.51
mmol) of N-bromosuccinimide (NBS) dissolved in dime-
thylformamide (DMF, 27 mL) was added dropwise at room
temperature to a stirred DMF (20 mL) solution of 4.167 g (18.58
mmol) 9-butyl-9H-carbazole, aer the addition the mixture was
stirred overnight. The reaction was quenched by addition of ice
water. The solids formed were extracted with ethyl acetate (3 ꢂ
50 mL), the combined ethereal extracted was dried over anhy-
drous Na2SO4 and evaporated to dryness to leave a colorless
residue, further purication by silica gel column chromatog-
raphy [petroleum ether (PE) : ethyl acetate (EA) ¼ 50 : 1, v/v] to
Experimental
Materials
All the reagents were obtained from commercial sources and
used without further purication. The organic solvents were of
analytical grade quality and all were dried by traditional
methods. In general, all the intermediates and nal compounds
were puried by column chromatography on silica gel (200–300
mesh), and crystallization from analytical grade solvents.
Reactions were monitored by using thin layer chromatography
(TLC) (Qingdao Jiyida silica gel reagent factory GF254).
ꢀ
give a white solid (yield 90%): m.p. 48.1–48.2 C.
Synthesis of 3-cyano-9-butyl-9H-carbazole.40 (3) A mixture of
the 3-bromo-9-butyl-9H-carbzaole (2.4 g, 8 mmol) and dried
copper cyanide (1.0 g, 12 mmol) in dried N-methyl-2-pyrrolidone
(NMP) 1 mL, the solution was stirred and heated at 140 ꢀC under
Characterization
1H NMR spectra was obtained with a Varian inova-400 MHz nitrogen atmosphere overnight. Then 2 N HCl (25 mL) and
instrument using tetramethylsilane (TMS) as the internal stan- CH2Cl2 were poured into the reaction mixture. The mixture was
dard. 13C NMR spectra was recorded on a Varian inova-100 MHz ltered, and then the solution was extracted and washed with
spectrometer. Glass transition temperature was determined by an aqueous saturated solution of NaCl. The organic layer is
¨
DSC measurements carried out utilizing Netzsch Geratebau dried with anhydrous Na2SO4. The mixture was ltered, the
GmbH R.O. Thermal stability was determined by thermogravi- solvents were evaporated furnishing the crude product which
metric analyzer (TGA, Netzsch STA449F3) over a temperature was puried by column chromatography on silica-gel (PE : EA ¼
ꢁ1
ꢀ
ꢀ
ꢀ
range of 25–1000 C at a heating rate of 10 C min under a 20 : 1, v/v) to give a white solid (yield 75%): m.p. 65.0–65.2 C.
nitrogen atmosphere. EI/MS spectra was obtained on an Agilent
1100 LC-MS. ESI/HRMS was obtained on a micrOTOF-Q II mass suspension of 1 g (4.03 mmol) of 3-cyano-9-butyl-9H-carbazole,
spectrometer (BrukerDaltonik, Germany). 1.0156 g (16.12 mmol) of sodium azide, and 0.08624 g (16.12
Synthesis of 5-(9-butyl-9H-carbazolyl)tetrazole.41 (4)
A
MALDI-TOF-MS data were obtained on a Bruker BIFLEX III mmol) of ammonium chloride in 3 mL of DMF were stirred at
TOF mass spectrometer. A MAPADA UV-3200PCS spectropho- 160 ꢀC under nitrogen atmosphere overnight. Aer cooling, the
tometer was used to record absorption spectra. Fluorescence solvent was concentrated in a rotary evaporator and the reaction
spectra was recorded on a Hitachi F-2500 and relative mixture was poured into 25 mL of ice/water and acidied with
quantum yield was obtained according to published method36 hydrochloric acid. The white precipitate was isolated by ltra-
using 2-phenyl-5-(4-bi-phenylyl)-1,3,4-oxadiazole (PBD) as tion, washed with ice water to give the crude product. Puri-
standard (FF ¼ 0.83).37 The electrochemical measurement was cation was by column chromatography on silica-gel
carried out with a ChenHua 660D potentiostat. Cyclic vol- (CH2Cl2 : CH3OH ¼ 50 : 1, v/v) to give the pure product as a pale
ꢀ
tammograms was recorded at scan rates of 50, 100, 200 and solid (yield 60%): m.p. 240.1–240.2 C.
500 mV sꢁ1 from a solution of compound 5 (10ꢁ3 mol dmꢁ3) in
Synthesis of tris-(3-9-butyl-9H-carbazolyl)tris[1,2,4]triazolo-
dry dichloromethane solution containing 0.1 mol dmꢁ3 tet- [1,3,5]triazines.32,42 (5) A mixture of corresponding tetrazole
rabutylammonium hexauorophosphate (TBAPF6), using a (9 mmol), cyanuric chloride (3 mmol), and anhydrous potas-
gastight single-compartment three-electrode cell equipped sium carbonate (36 mmol) in butanone (80 mL) was heated at
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temperature of reux under nitrogen atmosphere and vigorous characterised by standard spectroscopic methods, from which
stirring for 20 h. The still hot reaction mixture was ltered off satisfactory analysis data corresponding to their molecular
washing with dichloromethane. The solvents were evaporated structures were obtained. The nal product gives [M + Na]+
furnishing the crude product which was puried by column peaks at m/z 887.4025 (calculated for C54H48N12Na+ ¼ 887.4017)
chromatography on silica-gel (eluant dichloromethane) to give in the HRMS spectra for compound 5 (Fig. S2, ESI†).
1
the pure product as a white solid (yield 37%). H NMR (CDCl3,
Combination of typical electron-donating carbazole and
400 MHz): d (ppm) 8.99 (m, 3H), 8.33 (dd, J2 ¼ 8.61 Hz, J1 ¼ 1.72 electron-accepting TTT groups offers ready ICT process in
Hz, 3H), 8.19 (d, J ¼ 7.59 Hz, 3H), 7.59 (d, J ¼ 8.55 Hz, 3H), 7.51 compound 5. Given that photophysical characterisations of the
(m, 6H), 7.29 (m, 3H), 4.34 (t, J ¼ 7.16 Hz, 6H), 1.92 (m, 6H), 1.46 donor–p–acceptor (D–p–A) conjugated molecules in solutions
(m, 6H), 0.98 (t, J ¼ 7.38 Hz, 9H). LC-MS (EI) m/z 864.4 [M]+. are strongly dependent on the solvent polarity,43 we rst tested
13C NMR (CDCl3) d (ppm) 13.9, 20.6, 31.1, 43.1, 108.6, 109.0, the absorption and emission properties of target compounds in
120.9, 122.8, 123.1, 126.3, 127.6, 140.7, 141.0, 142.2, 152.0.
varying solvents. As shown in Fig. 1a, the UV-vis absorption
Synthesis of 5-phenyl-2H-tetrazole.41 (6) This prepared and spectra of compound 5 in various organic solvents exhibit two
aer treated in the same manner as with cꢀompound 4 to obtain characteristic absorption bands. A strong absorption peak is
white solid (yield 60%): m.p. 188.1–188.7 C.
observed at approximately 295 nm to 305 nm, which is attrib-
Synthesis of 3,7,11-triphenyltris[1,2,4]triazolo[1,3,5]triazine.32,42 uted to the p–p* transitions in the heteroaromatic portion of
(7) Compound 7 was obtained from cyanuric chloride and 6. The the molecule caused by the high molar absorption coefficient
crude product was puried by column chromatography on silica- (3 ¼ 0.83 ꢂ 104 to 1.92 ꢂ 104 Mꢁ1 cmꢁ1). A weak shoulder near
gel (PE : DCM : EA ¼ 5 : 2 : 1) to give the pure product as a white 327 nm to 332 nm is assigned to the donor–acceptor charge
1
powder (yield 50%): m.p. 298.7–299.0 ꢀC. H NMR (CDCl3, 400 transfer (CT) between the carbazole groups and TTT core. In
MHz): d (ppm) 8.13 (dd, J2 ¼ 7.60 Hz, J1 ¼ 0.8 Hz, 6H), 7.62 (m, general, the UV-vis absorption spectra from p–p* transitions
9H). MALDI-TOF MS: m/z 430.2 [M + H]+.
exhibit redshiing with the increase in solvent polarity.
However, the absorption maxima of compound 5 shows an
obvious bathochromic shi from 295 nm in acetonitrile to 305
nm in benzene. This phenomenon may be due to the p–p*
interactions between the TTT core with the benzene ring,
leading to the redshiing of the maximum absorption wave-
length. In addition, the intensity ratio of shoulder absorption to
main absorption decreases signicantly in the polar solvents,
such as dimethyformamide and acetonitrile, which is ascribed
to hydrogen-bonding interactions between the solute and the
solvent. The hydrogen-bonding interactions increase the energy
gap between ground and excited states, which cause bath-
ochromic shi of shoulder absorption. By contrast, the
increasing energy gap reduces the chance of electronic transi-
tion, thereby causing signicant decrease in shoulder absorp-
tion peak.
Results and discussion
The synthetic routes for compounds 5 and 7 are shown in
Scheme 1. Compounds 1 to 4 and 6 were prepared according to
previous procedures.32,38–42 All of the compounds were
In terms of emission behavior (Fig. 1b), compound 5 shows
intense blue uorescence in different organic solvents with
maximum emission wavelengths at approximately 400 nm. The
relative photoluminescence quantum yield (PLQY) is up to 0.99
in xylene solution, according to a published method36 that uses
Fig.
1 Normalised (a) absorption and (b) emission spectra of
compound 5 in various organic solvents and phosphorescence spectra
Scheme 1 Synthesis and structure of compound 5 and 7.
in chloroform at 77 K.
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PBD as standard (FF ¼ 0.83).37 Such high relative PLQY is
To determine the uorescent response range of compound 5
ascribed to carbazole conjugated with TTT, which extends the towards Cu2+, uorescence emission spectral variation of
degree of conjugation. The emission peak undergoes negligible compound 5 (10 mM) in THF–H2O (9 : 1, v/v) was monitored
shis upon varying solvent polarity, indicating that the solvent with different Cu2+ concentrations from 0.04 mM to 20 mM. As
polarity has almost no effect on the excitation state of shown in Fig. 2b, the uorescence intensity of compound 5
compound 5. In addition, the uorescence spectra of decreased with increase in Cu2+ concentrations, and signicant
compound 5 remain unchanged when the excitation wave- uorescence quenching was observed when the concentration
length at 300 and 320 nm are chosen, respectively, which are of Cu2+ reached 2.00 mM. Similarly, a sensitivity test experiment
due to the existence of the energy transfer between p–p* tran- employing a series of Cu2+ concentrations was performed. The
sitions in the heteroaromatic portion of the molecule and the results are shown in Fig. 3. The uorescent response of
ICT transitions. The triplet energy (ET) of compound 5 is compound 5 towards the Cu2+ ions covered a dynamic range
calculated to be 2.98 eV by low-energy phosphorescence emis- from 0.2 mM to 2 mM (Fig. 3, inset), with a detection limit of
sions in frozen solution (CHCl3, 77 K).44
0.2 mM for Cu2+. Meanwhile, compound 5 exhibited a cyan
ICT compounds have been used as uorescent probes. uorescence upon excitation with the use of a UV lamp at 254
Therefore, the recognition proles of compound 5 towards nm, and its intensities decreased with the increase in Cu2+
various metal cations (Fig. 2a), such a, Hg2+, Al3+, Pb2+, Cd2+, concentration and could be observed by the naked eye (Fig. 3,
Ca2+, Mn2+, K+, Zn2+, Ni2+, Ba2+, Na+, Mg2+ and Cu2+, were photographs), making it feasible for instrument-free Cu2+
primarily investigated by uorescence spectroscopy in THF– detection. This result provides solid support to the specic
H2O (v : v ¼ 9 : 1). The stock solution of compound 5 was detection of Cu(II) by uorescent methodology.
prepared at 100 mM THF. The test solution of compound 5
For the D–p–A conjugated molecules, they undergo ICT from
(10 mM) in 2 mL aqueous tetrahydrofuran solution was prepared the donor to the acceptor upon excitation by light. Two possible
by placing 0.2 mL of compound 5 stock solution and 0.2 mL of binding modes for uorescent PCT cation sensors exist. If a
the aqueous sample solution in 1.6 mL THF. The mixture cation interacting with a group acted as an electron donor
solution was mixed and kept at room temperature for 10 min within the uorophore, then a blueshi of the absorption
before the spectra were recorded. As shown in Fig. 2a, when 20 spectrum is observed together with a decrease in the extinction
equiv. of Hg2+, Al3+, Pb2+, Cd2+, Ca2+, Mn2+, K+, Zn2+, Ni2+, Ba2+, coefficient. Given the resulting conjugation reduction, the
Na+ and Mg2+ were added to a THF solution of compound 5 (10 ground state is more stabilised by the cation than the excited
mM), these metal cations showed very weak effect on the uo- state. By contrast, if the acceptor group interacts with a cation,
rescence intensity of compound 5 (blank bar). By contrast, when then the electron-withdrawing character of this group is
20 equiv. of Cu2+ was added to a THF solution of compound enhanced. The ground state is then more strongly unstable
5 (10 mM), a signicant uorescence decline was found. More- because of the cation than the excited state. The absorption
over, we tested the competition experiments to verify the prac- spectrum is thus redshied and the molar absorption coeffi-
tical applicability of compound 5 for Cu2+ detection. The same cient is increased.28 To verify the Cu2+ sensing mechanism of
concentrations of interfering metal ions and Cu2+ (0.2 mM) compound 5, the UV-vis absorption spectra of compound 5 was
were added to a THF solution of compound 5, and the detection rst recorded. Fig. 4a shows that the molar absorption coeffi-
results showed almost unchanged uorescence intensity to cient increased with the increase in Cu2+ concentrations. The
Cu2+ before and aer the addition of other competing metal results preliminary suggest that Cu2+ interacts with the acceptor
ions [Fig. 2a (black bar)]. This result indicates that the designed group. To investigate the mechanism further, another tris-
probe has a signicant potential for selective response, which triazolotriazine without pendant carbazolyl (compound 7) was
can be used in various applications.
designed and synthesised. The UV-vis absorption and uores-
cence emission spectra of compound 7 were determined in the
Fig. 2 (a) Fluorescence response of compound 5 (10 mM) in THF–H2O
(9 : 1, v/v) in the presence of 20 equiv. of other metal ions (blank bar) Fig. 3 Calibration plot of relative F/F0 of compound 5 against different
and to the mixture of 20 equiv. of other metal ions with 2.0 mM (black Cu2+ concentrations. F0 and F stand for the fluorescent intensities in
bar); (b) fluorescence spectra of compound 5 with varying Cu2+ the absence and presence of Cu2+. (Inside) images of compound 5
concentrations (from 0.04 to 20 mM); lex ¼ 287 nm.
solution (10 mM) upon the addition of 0, 0.2, 2, 20 and 200 mM Cu2+
.
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sensitivity and selectivity. The test showed a wide dynamic
range from 0.2 mM to 2 mM, with a detection limit of 0.2 mM.
The compound also exhibits excellent selectivity in the presence
of other common cations, such as Hg2+, Al3+, Pb2+, Cd2+, Ca2+,
Mn2+, K+, Zn2+ and Ni2+, which could meet the selective
requirements for practical applications. Moreover, the syn-
thesised compound possesses wide energy gap (Eg ¼ 4.09 eV),
excellent quantum efficiency (FF ¼ 0.99), high triplet energy
ꢀ
(ET ¼ 2.98 eV), good thermal stability (Td ¼ 325 C) and glass-
transition temperature of approximately 100 ꢀC. These superior
performances suggest that the molecule can be used in various
applications.
Fig. 4 Absorption spectra of compounds 5 (a) and 7 (b) (10 mM) upon
addition of 0, 0.2, 2.0, 20 and 200 mM Cu2+ in THF–H2O (9 : 1, v/v).
presence of different Cu2+ concentrations. As shown in Fig. 4b,
Acknowledgements
the absorption wavelength of compound
7 signicantly
The authors greatly acknowledge the nancial support by
the Special Research Fund of High-Level Talents (project
Rczx201017) and Research and Development project (project
gxjs2010-zdgg02-05) in Shihezi University.
redshied and the molar absorption coefficient increased in the
presence of Cu2+. Compared with compound 7, the maximum
absorption wavelength of compound 5 showed no obvious
redshi due to the distances and weak interaction between
uorophores of compound 5 and Cu2+.45 Thus, the absorption
wavelength of compound 5 was almost unchanged in the
presence of Cu2+. Meanwhile, the emission spectra (Fig. S3†) of
compound 7 indicated that its uorescence intensity decreased
with the increase in Cu2+ concentrations. In addition, obvious
uorescence quenching was observed when the concentration
of Cu2+ reached 1.2 mM. These data suggest that Cu2+ interacts
with the electron-accepting TTT groups of compound 5, thereby
causing a change in the uorescence intensity of compound 5.
In addition, Cu2+ is a paramagnetic ion with an unlled d
orbital that could lead to strong uorescence quenching of
nearby uorophores via electron or energy transfer.46–48 Mean-
while, we compared the trend of uorescence intensity of
compounds 5 and 7 with different Cu2+ concentrations; their
decreased tendency of uorescence intensity were similar
(Fig. S4†). However, the detection range of compound 5 was a
little wider than that of compound 7. This difference further
conrms the farther distances and the weaker interaction
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