Two Sensitive Fluorescent BOPIM Probes with Tunable TICT Character for Low-Level Water Detection in Organic Solvents

Article abstract of DOI:10.1007/s10895-015-1722-y

Two novel Boron-fluorine derivatives bearing dimethylamino moieties, BOPIM-1 and BOPIM-2, were proposed as sensitive fluorescent sensors for low-level water quantification in organic solvents. Two BOPIMs exhibit typical phenomenon for an emission from a twisted intra-molecular charge transfer (TICT) state, the emission red shift and intensity weakening with solvent polarity. Introduction of trace amount of water to solvent resulted in fluorescent quenching, accompanied by the red shift of the emission, which was attributed to the formation of TICT excitation of BOPIMs by hydrolysis. A quantification method to detect water content was developed, described by a linear equation lg I I 0 lg frac{I}{I-0} vs. lg φ _w in the range of φ _w (volume fraction of water) 0.001~0.01, 0.01~0.1, respectively. The experiment results of determination of water in real 1, 4-dioxane (Diox) samples proved that this method can be used in practical application.

Full text of DOI:10.1007/s10895-015-1722-y

J Fluoresc
DOI 10.1007/s10895-015-1722-y
ORIGINAL ARTICLE
Two Sensitive Fluorescent BOPIM Probes with Tunable
TICT Character for Low-Level Water Detection in Organic
Solvents
Ping Shen1 & Min Li1 & Chunlin Liu1 & Wei Yang1 & Sen Liu1 & Changying Yang1
Received: 25 August 2015 /Accepted: 2 November 2015
#
Springer Science+Business Media New York 2015
Abstract Two novel Boron-fluorine derivatives bearing
dimethylamino moieties, BOPIM-1 and BOPIM-2, were pro-
posed as sensitive fluorescent sensors for low-level water
quantification in organic solvents. Two BOPIMs exhibit typ-
ical phenomenon for an emission from a twisted intra-
molecular charge transfer (TICT) state, the emission red shift
and intensity weakening with solvent polarity. Introduction of
trace amount of water to solvent resulted in fluorescent
quenching, accompanied by the red shift of the emission,
which was attributed to the formation of TICT excitation of
BOPIMs by hydrolysis. A quantification method to detect
water content was developed, described by a linear equation
manufacturing, food processing, biomedical and environmen-
tal monitoring, since water is generally considered as the most
common impurity in many organic solvents. The Karl-Fischer
method is most frequently used because of its absolute mea-
surement, low capital cost, high sensitivity, applicability to
both liquid and solid samples, as well as large dynamic range
[1, 2]. Unfortunately, this method is less sensitive in aprotic
and non-alcoholic solvents, and can hardly be performed to
continuously monitor any occurrence of water in the sample
media [3, 4].
It is urgently needed to develop effective techniques for
ultra sensitive water content determination. Many advanced
techniques, for example, NMR, IR, UV/Vis and also fluores-
cence spectrometry have been proposed [5–8]. Among them,
fluorescent probes such as electron sensors and optical sensors
have demonstrated the advantages of high sensitivity, easy
operation and low cost, flexibility in readout, possibility of
remote and in situ monitoring [9]. A novel approach to the
quantification of relatively small amounts of water present in
low polarity has been proposed by Kłucińska, using 4-
methylumbelliferone as a fluorescence probe [10]. Optical-
fiber chemical sensor based on a fluorescence dye, 10-
allylacridine orange [11], a series of anthracene–boronic acid
ester [12–14], 4-(2-dimethylaminoethyloxy)-N-octadecyl-1,
8-naphthalimide (DON) with AIEE [15] were also developed
for determining water content in organic solvents.
I
lg I vs. lgφw in the range of φw (volume fraction of water)
0
0.001~0.01, 0.01~0.1, respectively. The experiment results of
determination of water in real 1, 4-dioxane (Diox) samples
proved that this method can be used in practical application.
.
Keywords BOPIMs Twisted intra-molecular charge transfer
.
.
.
(TICT) Fluorescent sensor Water determination Solvent
Introduction
Determination of water contents in organic solvents is essen-
tial for many practical applications, such as pharmaceutical
Many organic dyes possessing both electro donor and ac-
ceptor groups exhibit positive solvatochromism caused by an
intra-molecular charge transfer (ICT) or twisted ICT (TICT)
process [16, 17]. Their fluorescence characteristics in organic
solvent could be affected by the increase of water content
because of the change in the polarity of the solvent mixture
[18], suggesting a new kind of chemosensor for trace water
detection in solvent. Recently, we reported a BOPIM derivate,
BOPIM-1 [19] (Scheme 1), which exhibited extremely weak
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-015-1722-y) contains supplementary material,
which is available to authorized users.
* Changying Yang
changying.yang@ctgu.edu.cn
1
College of Biology and Pharmaceutical Science, Three Gorges
University, Yichang 443002, People’s Republic of China

J Fluoresc
400 NMR spectrometers. Chemical shifts are reported in ppm
with TMS as reference. Fluorescence spectra were obtained
using a Hitachi F-4500 fluorescence spectrometer.
Synthesis of two BOPIMs
Scheme 1 Structure of BOPIMs
The BOPIM-1 was synthesized and characterized according
to literature procedure [19].
The BOPIM-2 was synthesized as Scheme 2. First, dimeth-
yl amine acetophenone (1.3 g), 1, 4-dioxane (10 mL), water
(0.5 mL), selenium dioxide (1.11 g) was added in a round
bottom flask, heated to reflux for 3 h, filtered. Then water
(20 mL) was added into the organic phase, refluxed for 3 h,
cooling and crystallization, 1 was obtained. 1 (6.2 mmol,
1.1 g) in 32 mL methanol was mixed with 2- aldehyde pyri-
dine (6.2 mmol, 0.66 g) and ammonium acetate (2.30 g,
30.0 mmol) in 30 mL methanol, stirred at room temperature
overnight. The crude product was subjected to column chro-
matography. Then the resulting ligand 2 was mixed with Et3N
(5 mL) in anhydrous CH2Cl2 (25 mL) and stirred, and BF3·
OEt2 (7.0 mL) was added dropwise at 0 °C. After the addition
of BF3·OEt2, the reaction mixture was allowed to warm to
room temperature and stirred at room temperature overnight
to obtain a orange solid, which was purified by silica gel
fluorescence in polar solvent due to the TICT mechanism.
Meanwhile, we noticed that BOPIM-1 is an efficient Boff–
on^ switcher for pH between 4.0 and 2.4 ascribed to the inhi-
bition of TICT upon protonation [20]. Based on the above
mentioned considerations, we employed another BOPIM der-
ivate, BOPIM-2 (Scheme 1), which possessed the same parent
structure with BOPIM-1, but contained only one electron do-
nor group -Ph-N (CH3)2. In this work, our investigation found
that two BOPIMs (BOPIM-1 and BOPIM-2) both showed
TICT characteristic, verified by their positive
solvatochromism of fluorescence spectra and frontier MOs
of at the equilibrium geometries. They can act as fluorescence
sensors for monitoring the water content in organic solvent,
for their sensitive response in fluorescence intensities and
wavelengths. Compared with Karl-Fischer method, our fluo-
rescence sensor shows more sensitive and visible for measure-
ment application, especially for medium polarity organic sol-
vents. Five organic solvents, 1,4-dioxane (Diox), tetrahydro-
furan (THF), acetonitrile (MeCN), iso-propanol, acetone, as
representatives, were investigated in this paper.
1
column chromatography to afford product. Yield: 20 %. H
NMR (CDCl3, 400 MHz): δ 8.41 (d, J=5.6 Hz, 1H), 8.13 (t,
J=1.20 Hz, 1H), 7.96 (d, J=8.00 Hz, 1H), 7.71 (d, J=2.00 Hz,
2H), 7.46 (s, 1H), 7.41(m, 1H), 6.78 (d, J=2.20 Hz, 2H), 3.00
(s, 6H). 13C NMR (CDCl3, 100 MHz): δ 149.30, 143.28,
140.02, 130.80, 126.91, 125.92, 120.97, 117.06, 116.33,
11.57, 39.36. The NMR spectra were shown in supplementary
information (Fig. S1).
Experimental
Materials and Apparatus
General Measurements
Solvents were purchased from Aldrich Chemical, purified by
normal procedures and dried over freshly activated molecular
sieves (MS 4A). All starting materials used to synthesize
BOPIMs were obtained from commercial supplies and used
as received. 1H-NMR and 13C-NMR were recorded on Bruker
The stock solution of BOPIMs was prepared in various anhy-
drous solvents (5.0 mmol) and working solutions were freshly
prepared from the stock solutions. The fluorescence spectra
were scanned 10 min later after working solutions prepared.
The excitations were carried out at 365 nm while both
Scheme 2 Synthesis of BOPIM-
2

J Fluoresc
Fig. 1 Fluorescence spectra of BOPIM-1 (a), BOPIM-2 (b) (5.0×10−5
M) in different solvents at room temperature, from left to right: n-hexane,
Diox, THF, CHCl3, MeCN, CH3OH. The insets show the fluorescence
photographs of compounds in corresponding solvents
excitation and emission slit widths were 5 nm. All experi-
ments were carried out at 298 K.
photoinduced TICT formation in excited state in both
BOPIM-1 and BOPIM-2.
Elucidation of the molecular orbitals (HOMO and LUMO)
of BOPIMs has been accomplished by optimizing the geom-
etry of the molecule on the software package GAMESS plat-
form [21], using B3LYP hybrid functional and 6-311G (d, p)
basis set [22].
Spectral Changes Induced by Addition of Water
in Organic Solvents
Introduction of water to organic solvent results in the charac-
teristic emission bands effectively quenching along with
bathochromic shifts for polarity changes. As it can be seen
from Fig. 3, introduction of even relatively small amount of
water, reaching φw 0.01 (volume fraction of water) to pure
Diox, the characteristic emission intensity at 450 nm
(exCitation 360 nm) drop dramatically to 28 % of that from
the water-free solution, with 5 nm spectra red-shift for
BOPIM-1, and the emission at 500 nm (exCitation
wavelength 360 nm) drop to 55 %, with 23 nm red-shift for
BOPIM-2, respectively. The emission decrease and red shift
will proceed on with the increase of the amount of water
(Fig. 3a, b; Table 1). Furthermore, BOPIM-2 exhibited dra-
matic fluorescent color variations under illumination with
310 nm light: green to brown, with the increase of water
Results and Discussion
Fluorescence and TICT Properties of Two BOPIMs
The emission spectra of BOPIMs in different solvents are
shown in Fig. 1. The emission maximum of BOPIM-1 was
changed from 433 nm in n-hexane to 521 nm in methanol
while BOPIM-2 was changed from 408 nm in n-hexane to
508 nm in methanol, indicative of the charge transfer nature
of the emitting state. As the solvent polarity increased, there
was a decrease in transition energy and a positive
solvatochromism effect was observed. The emission red shift
and intensity weakening with solvent polarity were typical
phenomenon for an emission from a TICT state [23]. Both
BOPIM-1 and BOPIM-2 in various solvent exhibited dramat-
ic fluorescent color variations under illumination with 310 nm
light (insets), which were consistent with the fluorescence
spectra.
Theoretical calculation has also been shown to correlate
well to the experimental findings in the light of the TICT
model. As shown in Fig. 2, the highest occupied molecular
orbital (HOMO) is mainly localized on the substituent –Ph-
N(CH3)2, the electron donor, and the lowest unoccupied mo-
lecular orbital (LUMO) is localized predominantly on BOPIM
precursor, limited flexibility, the electron acceptor. What’s
more, a visible separated charge state can be observed from
HOMO-1 to LUMO + 1, that means this compound is prone
to be a twisted geometrical state afford a clear view for the
Fig. 2 Frontier molecular orbital contours of BOPIMs on the B3LYP 6-
311G (d, p) level

J Fluoresc
Fig. 3 Water-dependent fluorescence emission spectra of BOPIM-1 (a,
c) and BOPIM-2 (b, d) (5.0×10−5 M) in Diox with increasing amount of
water (a, b: φw=0–0.1; c, d: φw=0–0.01); inset: a BOPIM-1 and b
BOPIM-2 fluorescence photographs of Diox (3.0 mL) in pure Diox to
Diox containing water (φw, from left to right: 0, 0.005, 0.02, 0.05, and
0.1)
content, along with a significant decrease in the fluorescence
intensity (inset of Fig. 3b). The illumination of blue almost
quenched completely for BOPIM-1 when φw>0.02 (inset of
Fig. 3a). It was found that the red shift of BOPIMs occurred
with the introduction of water to pure Diox is so similar to
their emission change and the TICT states prominent with the
increasing of solvent polarity.
The similar behavior was observed in acetone. Figure 4
presents emission spectra and luminescence photographs of
BOPIM-1 and BOPIM-2 in acetone, recorded for changing
water concentration in solvent. From pure to acetone contain-
ing water, there were also obvious decreases of fluorescence
intensity of BOPIMs, companied with red-shift in emissions
and dimming in photos. The emission intensity drop
dramatically to about 30 % when φw=0.01 for BOPIM-1,
48 % for BOPIM-2, respectively. Moreover, the fluorescence
emission spectra of two BOPIMs with the addition of water in
three other organic solvents: THF, MeCN, iso-propanol were
investigated (shown in Fig.S2-Fig.S4). Similarly the pres-
ences of low level water in organic solvents affect significant-
ly the emission intensity and wavelength except in iso-
propanol. The emissions of two BOPIMs in iso-propanol were
less obvious change with the water addition when φw below
0.01, complicating they were less sensitive of water in iso-
propanol.
Fluorescent quenching was observed with increasing water
content in various solvents, accompanied by the red shift of
the emission, which was attributed to the formation of TICT
Table 1 The fluorescence emission wavelength (λem, nm) changes in five organic solvents containing water (φw=0.1), excitation at 365 nm
Solvents
Diox
THF
MeCN
Acetone
iso-propanol
BOPIM-1
Pure solvent
455
472
495
543
465
483
462
502
500
514
548
555
493
501
536
543
467
472
547
546
Containing water
Pure solvent
BOPIM-2
Containing water

J Fluoresc
a
b
2500
2000
1500
1000
500
1400
1200
1000
800
600
400
200
0
c
d
w
0
w
0
0.01
0.01
0
400
400
450
500
550
600
650
450
500
550
600
650
700
Wavelength(nm)
Wavelength(nm)
Fig. 4 Fluorescence emission spectra of (a) BOPIM-1 and (b) BOPIM-2
(5.0×10−5 M) in acetone containing different amount of water (a, b: φw=
0–0.1; c, d: φw=0–0.01); inset: a BOPIM-1 and b BOPIM-2 fluorescence
photographs in acetone with water contents (φw, from left to right: 0,
0.005, 0.02, 0.05, and 0.1)
excitation of BOPIMs by hydrolysis. The introduction of wa-
ter to these organic solvents result in the suppression of local
emission and the formation of TICT state due to an non-
fluorescent twisted structure by hydrolysis of BOPIMs. Addi-
tion of water into solvent results in an increase in the solvent
polarity, which twists the luminogen conformation and moves
the equilibrium from local to the TICT state, leading to a
bathochromic shift in the emission with the dramatic decreas-
ing intensity. It is indicated that two compounds are two very
good water-sensitive indicators, and suitable for constructing
water sensors to monitor the concentration of water in organic
solvents.
BOPIMs Based Water Quantification
The fluorescence intensity of BOPIMs decreased dramatically
with the addition of water in organic solvents even below φw
0.01. For the purpose of establishing the quantitative
I
Fig. 5 Calibration curves of lg I
against lgφw for BOPIMs (5.0× 0
10−5 M) in water containing Diox.
a φw=0.001~0.01; b φw=
0.01~0.1

J Fluoresc
Table 2 The calibration equations and correlation coefficients
Solvent
Diox
Fluorescence indicator
BOPIM-1
The range of φw
φw<0.01
φw>0.01
φw<0.01
φw>0.01
φw<0.01
φw>0.01
φw<0.01
φw>0.01
φw<0.01
φw>0.01
φw<0.01
φw>0.01
φw<0.01
φw>0.01
φw<0.01
φw>0.01
φw>0.01
φw>0.01
Calibration equation
R2
0.9975
0.9931
0.9840
0.9990
0.9941
0.9767
0.9992
0.9937
0.9847
0.9969
0.9863
0.9804
0.9847
0.9985
0.9859
0.9919
0.9947
0.9798
I
0
lg I ¼ −2:095−0:729lgφw
I
0
lg I ¼ −2:332−1:190lgφw
BOPIM-2
BOPIM-1
BOPIM-2
BOPIM-1
BOPIM-2
BOPIM-1
BOPIM-2
I
0
lg I ¼ −1:086−0:396lgφw
I
0
lg I ¼ −1:770−0:601lgφw
Acetone
I
0
lg I ¼ −1:431−0:471lgφw
I
0
lg I ¼ −1:356−0:467lgφw
I
0
lg I ¼ −1:844−0:665gφw
I
0
lg I ¼ −2:045−0:922lgφw
THF
I
0
lg I ¼ −1:269−0:409lgφw
I
0
lg I ¼ −1:904−0:638lgφw
I
0
lg I ¼ −0:450−0:132lgφw
I
0
lg I ¼ −0:387−0:095lgφw
MeCN
I
0
lg I ¼ −2:678−0:832lgφw
I
0
lg I ¼ −2:639−0:821lgφw
I
0
lg I ¼ −2:03−0:706lgφw
I
0
lg I ¼ −1:818−0:633lgφw
iso-propanol
BOPIM-1
BOPIM-2
I
0
lg I ¼ −0:6045−0:328lgφw
I
0
lg I ¼ −1:053−0:545lgφw
relationship between the fluorescence intensity (I) and water
calibration equations can serve as the quantitative basis for
the determination of water in organic solvents. As a conse-
quence, the quenching of the fluorescence intensity due to the
presence of water in solvents was described by linear equation
I
fraction in solvent (φw), the graph of lg (I0: the fluorescence
I0
intensity in pure solvent) as a function of lgφw was obtained in
the range of φw 0.001~0.01, 0.01~0.1, respectively. As it can
be seen from Fig. 5, for BOPIM-1 in Diox, the good linear
I
0
relationship can be developed: lg I ¼ −2:095−0:729lgφw
(R2=0.9975) when φw below 0.01, lg I ¼ −2:332−1:190lg
I
0
Table 3 Determination of water content in real organic solvent (Diox)
using the proposed method
φw (R2=0.9931) when φw higher than 0.01. Whereas the slope
Indicator
Sample
Added φw
Measured φw
Recovery %
of linear for BOPIM-2 are much smaller than BOPIM-1
I
(Table 2), and the equation is obtained as lg I ¼ −1:086−0:3
0
BOPIM-1
1
2
3
1
2
3
0.00210
0.0390
0.0780
0.0260
0.0650
0.0780
0.00205
0.0367
0.0724
0.0259
0.0595
0.0789
97.72
94.00
92.88
99.45
91.49
101.1
2
I
I0
96lgφw (R =0.9840) (φw<0.01), lg ¼ −1:770−0:601lgφw
(R2=0.9990) (φw>0.1), respectively. The response curves in
the other four different solvents showed a similar trend to that
in Diox (Fig. S5-S8), and the equation and the relation values
are summarized in Table 2. Many of the correlation coeffi-
cients (R2) reach up to 0.99, some even 0.999, thus these
BOPIM-2

J Fluoresc
6. Garrigues S, Gallignani M, dela Guardia M (1993) Flow-injection
determination of water in organic solvents by near-infrared spec-
trometry. Anal Chim Acta 281:259–264
I
0
of lg I vs. lgφw as measured for the solution of the indicator in
the organic solvent.
7. Sun H, Wang B, DiMagno SG (2008) A method for detecting water
in organic solvents. Org Lett 10:4413–4416
The foregoing procedure was applied to evaluate the values
of water content in real solvents using the proposed two fluo-
rescence indicators. Herein Diox without treatment was taken
for example to discuss the measurement recovery in real sol-
vent using standard addition method. The values of φw de-
rived from calibration equation according to measured fluo-
rescence intensity of BOPIMs, together with the recoveries in
three samples can be seen in Table 3. The tests showed con-
sistent results with the real water content in Diox samples, and
the reasonable recoveries were obtained both for BOPIM-1
and BOPIM-2. The results revealed that the present method
can workable in real organic solvent samples.
8. Pinheiro C, Lima JC, Parola AJ (2006) Using hydrogen bonding-
specific interactions to detect water in aprotic solvents at concen-
trations below 50 ppm. Sensors Actuators B 114:978–983
9. Niu HY, Huang DW, Niu CG (2014) Time-gated fluorescence sen-
sor for trace water content determination in organic solvents based
on covalently immobilized europium ternary complex. Sensors
Actuators B Chem 192:812–817
10. Kłucińska K, Jurczakowski R, Maksymiuk K, Michalska A (2015)
Ultrasensitive 4-methylumbelliferone fluorimetric determination of
water contents in aprotic solvents. Talanta 132:392–397
11. Yang X, Niu CG, Shang ZJ, Shen GLYRQ (2001) Optical-fiber
sensor for determining water content in organic solvents. Sensors
Actuators B Chem 75:43–47
12. Ooyama Y, Matsugasako A, Oka K, Nagano T, Sumomogi M,
Komaguchi K, Imae I, Harima Y (2011) Fluorescence PET
(photo-induced electron transfer) sensors for water based
onanthracene—boronic acid ester. Chem Commun 47:4448–4450
13. Ooyama Y, Uenaka K, Matsugasako A, Harima Y, Ohshita J (2013)
Molecular design and synthesis of fluorescence PET (photo-
induced electron transfer) sensors for detection of water in organic
solvents. RSC Adv 3:23255–23263
14. Ooyama Y, Matsugasako A, Hagiwara Y, Ohshita J, Harima Y
(2012) Highly sensitive fluorescence PET (photo-induced electron
transfer) sensor for water based on anthracene–bisboronic acid es-
ter. RSC Adv 2:7666–7668
15. Sun Y, Liang X, Wei S, Fan J, Yang XH (2012) Fluorescent turn-on
d e t e c t i o n a n d a s s a y o f w a t e r b a s e d o n 4 - ( 2 -
dimethylaminoethyloxy)-N-octadecyl-1,8-naphthalimide with
aggregation-induced emission enhancement. Spectrochim Acta A
Mol Biomol Spectrosc 97:352–358
16. Bangal PR, Panja S, Chakravorti S (2001) Excited state photody-
namics of 4-N, N-dimethylamino cinnamaldehyde: a solvent de-
pendent competition of TICT and intermolecular hydrogen bond-
ing. J Photochem Photobiol A Chem 139:5–16
Conclusions
It was found that two boron-fluorine derivatives with TICT
characteristic acts as highly sensitive fluorescence sensors for
the detection of a trace amount of water in organic solvents.
Introduction of low level water to organic solvents, especially,
for moderate polar solvents, resulted in obvious intensity de-
crease and bathochromic shift in fluorescence emission, which
was attributed to the formation of TICTexcitation of BOPIMs
by hydrolysis. Besides, the observable luminescence changes
were displayed in two fluorescence indicators. A quantifica-
tion method to detect water content was developed, described
I
0
by linear equations: lg I vs. lgφw. Real organic solvent sample
are also tested to support this method, it proves that two pro-
posed BOPIMs have good sensitivities and provide promising
strategies for the detection of trace water content in organic
solvents.
17. Ghosh R, Palit DK (2013) Dynamics of solvent controlled excited
state intramolecular proton transfer coupled charge transfer reac-
tions. Photochem Photobiol Sci 12:987–995
18. Ding L, Zhang ZY, Li X, Su JH (2013) Highly sensitive determi-
nation of low-level water content in organic solvents using novel
solvatochromic dyes based on thioxanthone. Chem Commun 49:
7319–7321
Acknowledgments We are grateful for the financial support from Na-
tional Natural Science Foundation of China (21473101).
19. Wang S, Xiao SZ, Chen XH, Zhang RH, Cao Q, Zou K (2013)
Crystalline solid responsive to mechanical and acidic stimuli:
Boron-fluorine derivative with TICT characteristic. Dyes
Pigments 99:543–547
20. Shen P, Xiao SZ, Zhan XQ, Zhang W, Chang KD, Yang CY (2013)
A new pH fluorescent molecular switch by modulation of twisted
intramolecular charge transfer with protonation. J Phys Org Chem
26:26858–26862
21. Schmidt MW, Baldridge KK, Boatz JA, Elbert ST, Gordon MS,
Jensen JH, Koseki S, Matsunaga N, Nguyen KA, Su SJ, Windus
TL, Dupuis M, Montgomery JA (1993) General atomic and molec-
ular electronic structure system. J Comput Chem 14:1347–1363
22. Lee C, Yang W, Parr RG (1988) Development of the Colle-Salvetti
correlation-energy formula into a functional of the electron density.
Phys Rev B 37:785–789
23. Grabowski ZR, Rotkiewicz K (2003) Structural changes accompa-
nying intramolecular electron transfer: focus on twisted intramolec-
ular charge-transfer states and structures. Chem Rev 103:3899–
4031
References
1. Liang YY (1990) Automation of Karl Fischer water titration by
flow injection sampling. Anal Chem 62:2504–2506
2. Ohira SI, Goto K, Toda K, Dasgupta PK (2012) A capacitance
sensor for water: trace moisture measurement in gases and organic
solvents. Anal Chem 848891–8897
3. MacLeod SK (1991) Moisture determination using Karl Fischer
titrations. Anal Chem 63:557A–566A
4. Margolis SA (1997) Sources of systematic bias in the measurement
of water by the coulometric and volumetric Karl Fischer methods.
Anal Chem 69:4864–4871
5. Langhals H (1990) A simple, quick, and precise procedure for the
determination of water in qrganic solvents. Anal Lett 23:2243–
2258