Light initiated E-Z and Z-E isomerization of isatinphenylsemicarbazones: Tautomeric equilibrium effect

Article abstract of DOI:10.1016/j.jphotochem.2014.05.004

The light and thermally initiated mutual E- and Z-isomer transformations of two efficient isatin N-phenylsemicarbazone colorimetric sensors for strongly basic anions differing in sensitivity, selectivity and sensing mechanism were investigated in solvents

Full text of DOI:10.1016/j.jphotochem.2014.05.004

Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Light initiated E–Z and Z–E isomerization of
isatinphenylsemicarbazones: Tautomeric equilibrium effect
Klaudia Jakusováa, Marek Cigánˇ a,∗, Jana Donovalováa, Martin Gáplovsky´ b,
Robert Sokolíka, Anton Gáplovsky´ a
a Faculty of Natural Sciences, Institute of Chemistry, Comenius University, Mlynská dolina CH-2, SK-842 15 Bratislava, Slovakia
b Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Comenius University, Odbojárov 10, SK-832 32 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
The light and thermally initiated mutual E- and Z-isomer transformations of two efficient isatin N-
phenylsemicarbazone colorimetric sensors for strongly basic anions differing in sensitivity, selectivity
and sensing mechanism were investigated in solvents of different polarity. The reaction mechanism of
photochemically and thermally initiated E–Z isomerization and the absence of back Z–E isomerization is
discussed in terms of self-aggregate formation, hydrazide-hydrazonol tautomeric equilibrium, the role of
intra- and inter-molecular hydrogen bonds, excited-state proton transfer, solvent polarity and concentra-
tion effects. The effect of the activation energies related to conformational and geometric changes of the
isomers and the possible analytical application were investigated. Theoretically calculated properties are
in good agreement with obtained experimental results and support the proposed reaction mechanism.
© 2014 Elsevier B.V. All rights reserved.
Received 7 February 2014
Received in revised form 7 May 2014
Accepted 10 May 2014
Available online 16 May 2014
Keywords:
Isatinphenylsemicarbazones
Light initiated E–Z and Z–E isomerization
Thermally initiated back reaction
Tautomeric equilibrium effect
1. Introduction
This advance was based on previous studies investigating exter-
nal stimuli influence on receptor conformer populations. Wu et al.
E–Z isomerization of organic compounds, including C C, C N or
N double bond isomerization, is among the most studied pho-
established that E–Z isomerization effectively competed with radia-
tion deactivation of excited molecules leading to rapid fluorescence
quenching [9].
Other types of “host–guest” intermolecular interactions can
also influence E–Z isomerization efficiency (quantum yield) or the
overall ratio of potential isomers. Signal molecules thus obtain
properties of a sensor (receptor). In that manner, the photoisomeri-
zation of azobenzenes, stilbenes, spiropyrans and rhodopsine was
used for enzyme activity monitoring [10–12].
Supramolecular chemistry has focused on the development of
selective colorimetric or fluorescent anion sensors for the last
twenty years [13–16], and researches including Wenzel, Chudzin-
ski, Bergamaschi, Jiménez, Martínez-Mán˜ez and Gale continue this
work [17–22].
N
tochemical and thermal reactions and thus continues to attract
scientific attention. Research in this area now focuses on practi-
cal applications of E–Z isomerization, particularly in the field of
organic electronics. Due to the photochromic properties of organic
compounds containing C C, C N or N N double bonds, photo-
initiated E–Z isomerization reaction mechanism is a powerful tool
in the development of photo-optical switches and optical memories
[1–7].
Reaction pathway control is essential for practical application
of E–Z isomerization. Therein, the study of reaction mechanism,
kinetics, structure–property and structure–environment relation-
ships, and the subsequent active control of E–Z isomerization play
key roles. For example, conformational or spatial E–Z isomerization
restriction of the C N bond, due to sensor–metal cation interaction
was used in the design of new metal cation sensing mechanisms to
provide simple and efficient fluorescence enhancement [8].
Anion
induced
tautomerism
of
isatin-3-4-
phenyl(semicarbazone) derivatives was investigated in our
recent−study (Scheme 1) [23]. The interaction of F−, AcO−,
anions with E- and Z-isomers of
−
H2PO4
,
Br− or HSO4
isatin-3-4-phenyl(semicarbazone)
I
and N-methylisatin-3-4-
phenyl(semicarbazone) II as sensors influenced the equilibrium
ratio of the individual sensor tautomeric forms in the liquid phase.
The E- and Z-sensor isomers differed in sensitivity, selectivity
and sensing mechanism. The equilibrium ratio of the individual
tautomeric forms, affected by (1) the inter- and intra-molecular
interaction modulation of isatinphenylsemicarbazone molecules
∗
Corresponding author. Tel.: +421 2 60296306; fax: +421 2 60296342.
E-mail addresses: jakusova@fns.uniba.sk (K. Jakusová), cigan@fns.uniba.sk
(M. Cigánˇ), donovalova@fns.uniba.sk (J. Donovalová), gaplovsky@fpharm.uniba.sk
(M. Gáplovsky´), sokolik@fns.uniba.sk (R. Sokolík), gaplovsky@fns.uniba.sk
(A. Gáplovsky´).
http://dx.doi.org/10.1016/j.jphotochem.2014.05.004
1010-6030/© 2014 Elsevier B.V. All rights reserved.

K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
61
Scheme 1. Molecular structure of the studied isatinphenylsemicarbazones.
by anion induced change in receptor molecule solvation shells and
(2) the sensor–anion interaction with urea hydrogens, was easily
determined by UV–vis spectroscopy. The appropriate selection of
experimental conditions led to a high degree of sensor selectivity
for some investigated anions. Sensors Ia–IIb provided an excellent
signal to noise ratio and also a wide detection range. Detection of F−
or CH3COO− anions at high weakly-basic anion excess was also pos-
sible. Furthermore, due to excellent E-isomer sensitivity in organic
media, these isomers can be used for F− or CH3COO− sensing in
semi-aqueous media.
fluorescence measurements were performed by the FSP 920
(Edinburgh Instruments, UK) spectrofluorimeter. The dimethyl-
formamide (DMF), methanol (MeOH), acetonitrile (MeCN), benzene
and dichloromethane (CH2Cl2) were all UV-spectroscopy or HPLC
grade (Merck, Darmstadt, Germany; Fisher Slovakia, Levocˇa, Slo-
vakia). All photochemical measurements were performed at 25 ◦C
in the dark, with only 405 nm (or 341 nm) LED diodes Thorlabs
as light sources with optical power of P = 6 mW and P = 0.33 mW,
respectively.
Our quoted study [24] showed that the aggregate formation
in weakly interacting non-polar solvents at higher isatin-
phenylsemicarbazone Ia and IIa concentrations prevents their
room temperature E–Z isomerization to more stable Z-isomers. In
contrast to the situation in non-polar solvents, E–Z isomerization
from the monomeric form of phenylsemicarbazone E-isomers Ia
and IIa occurs in highly interactive polar solvents only at temper-
atures above 70 ◦C. The aggregate formation at higher Ia and IIa
concentrations and the presence of a new hydrazonol tautomeric
form with a high degree of conjugation at low Ia and IIa concentra-
tions prevent the room temperature E–Z isomerization of Ia and IIa
in highly interactive polar solvents. In addition, no formation of the
corresponding E-isomers was observed during thermally initiated
back Z–E isomerization.
Herein, the tautomeric equilibrium effect on light initiated E–Z
isomerization quantum yield of isatinphenylsemicarbazones I and
II is reported; including comprehensive investigation of the ther-
mally initiated back Z–E isomerization. This research is necessary
for practical application of these compounds as anion sensors and
also for better understanding of the photophysical properties of
their subsequent derivatives for effective development and design
of novel anion sensors. This investigation can also be useful in eval-
uation of the applicability of this type of compounds in chemical
actinometry or molecular switching.
2.3. Light initiated E–Z and Z–E isomerization
2.3.1. General procedure
Photochemical measurements were performed using the appa-
ratus described elsewhere (Fig. 7 in [26] – without ultrasonic horn
H and lens L1; using Ocean Optics SD 2000 diode array spectropho-
tometer). The light sources were four 405 nm LED diodes Thorlabs
with overall incident photon flux I0 = 5.6 0.1 × 10−4 mol s−1 dm−3
,
or three 341 nm LED diodes Thorlabs. The actual concentration of
compounds studied in air-saturated solutions during irradiation
in a 1 cm quartz fluorescence cuvette was measured spectropho-
tometrically in right-angle arrangement (HP 8452A). The LED
light sources were turned off during the concentration measure-
ments. The incident concentrations of the studied isomer were
c0 = 1 × 10−4 mol dm−3 and 3 × 10−6 mol dm−3. The presence of the
second isomer during the irradiation of the corresponding E- or
Z-isomer was confirmed/negated by HPLC chromatography, as
described in Section 2.4.1.
2.3.2. Quantum yield determination
The E–Z isomerization quantum yield (˚E–Z
phenylsemicarbazone E-isomers Ia and IIa in solution was
) of isatin-
determined according to Eqs. (1) and (2):
ꢀ
c
t
dc
ꢀc
c
0
˚
=
=
(1)
(2)
2. Experimental and theoretical methods
ꢀ
ꢀ
E−Z
t Iadt
t Iadt
0
0
2.1. Synthesis
ꢀAꢁ
ꢀc =
εꢁZ − εꢁE
Synthesis of the studied compounds was published by Jakusová
et al. [25].
where ꢀc is the concentration change in E- or Z-isomer, Ia is the
absorbed photon flux by E-isomer at the irradiation wavelength ꢁ
using a monochromatic light source, ꢀAꢁ is the absorbance change
at irradiation wavelength ꢁ using a monochromatic light source,
εꢁE is the molar extinction coefficient of E-isomer at irradiation
wavelength ꢁ using a monochromatic light source, εꢁZ is the molar
2.2. Spectroscopic measurements
Electronic absorption spectra were obtained on a HP 8452A
(Hewlett Packard, USA) diode array spectrophotometer, and

62
K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
extinction coefficient of Z-isomer at irradiation wavelength ꢁ using
a monochromatic light source and t is irradiation time.
The E-isomer absorbed photon flux Ia at particular irradia-
tion times during E–Z isomerization at irradiation wavelength
ꢁ = 405 nm and incident E-isomer concentration c0 was calculated
by Eq. (3) [27]:
ε
E,405(c0 − ꢀc)
Ia,405E = I0
ε
E,405(c0 − ꢀc) + εE,405ꢀc
× [1 − 10−[ε
(c −ꢀc)+ε
E,405
ꢀc]
E,405
0
]
(3)
In the case of photochemically initiated Z-isomer degradation
appearance during Ib (or IIb) solution irradiation at 405 nm, the
values of parameters ꢀc and Ia were estimated as the initial values
at low E–Z isomerization conversion. Here, the rate of the competi-
tive photochemical degradation was always less than the rate of Ia
and IIa light initiated Z-isomer degradation.
The incident photon flux I0 was determined by 2-
nitrobenzaldehyde (2-NB) as chemical actinometer, according
to Eq. (4) [28]:
k
I0
=
(4)
2.303 log ε2-NB,405 · ˚2-NB,405 · l
where ε2-NB,405 is the molar extinction coefficient of 2-NB at irra-
diation wavelength 405 nm, ˚2-NB,405 is the quantum yield of 2-NB
photodegradation at irradiation wavelength, l is the path length and
k is the slope of the 2-NB first-order photodegradation plot under
low-light-absorbing conditions (Fig. S1):
ꢁ
ꢂ
[2-NB]t
[2-NB]0
ln
= −kt
(5)
2.4. Thermally initiated E–Z and Z–E isomerization
Fig. 1. (A) UV–vis spectra of E- and Z-isomers I and II in MeOH (c = 10−4 mol dm−3
;
T = 25 ◦C) and (B) dependence of UV–vis absorption spectra of Ia in MeOH on the Ia
concentration (T = 25 ◦C).
2.4.1. General procedure
Ten volumetric flasks with the particular E- or Z-isomer solu-
tion were thermostated at the corresponding temperature in a
Julabo HE thermostat oil bath (Fisher Scientific, Czech Republic).
One volumetric flask was removed form the oil bath at selected
time intervals, cooled to 25 ◦C room temperature and analyzed by
HPLC chromatography.
HPLC chromatography was performed using the Agilent Tech-
nologies chromatographic system. This consisted of 1100 series
quarternary pump, column compartment, diode array detector
(VWDG1314A) and manual injector (Rheodyne model 7725i) with
20 ␮M sample loop and degasser (g1379A). Column ZORBAX-
SB-Phenyl (150 mm × 4.6 mm I.D.) was used in all experiments.
Methanol was the mobile phase in analyses of the unmethylated
semicarbazone I isomerization. For analyses of methylated semi-
carbazone II isomerization, mobile phase A was methanol (Merck,
Lichrosolv) and phase B acetonitrile (Merck, Lichrosolv). Analysis
of the reaction mixture: isocratic (for II isomerization, the ratio A/B
was 1:1), instituted at 0.8 mL/min flow rate at 25 ◦C, with detection
at 236 nm. The injection volume was 20 ␮L.
3. Results and discussion
As described in our previous papers [24,25], the polarity of the
solvent and concentration of isatin-3-4-phenyl(semicarbazones) I
and II have a major impact on their UV–vis spectra. Depending
on isatin-3-4-phenyl(semicarbazones) concentration, the UV–vis
spectra of the E-isomers Ia and IIa in strongly interacting polar
solvents have one or two main absorption maxima (ꢁA) at approx-
imately 330 nm and 410 nm (Fig. 1).
The presence of the absorption band at 410 nm for both E-
isomers Ia and IIa, and its increased intensity with decreasing
concentration of these isomers, is explained by the existence of
the hydrazide–hydrazonol chemical equilibrium (Scheme 2).
A
decrease in concentration of E-isomers shifts the tau-
tomeric hydrazide–hydrazonol equilibrium to the right, resulting
in decreased associate formation. An increase in intensity of the
hydrazonol C absorption band at 410 nm in strongly interacting
polar solvents is accompanied by a decrease in the intensity of
the absorption band for hydrazide B (and aggregate A) at approxi-
mately 330 nm. Compared to the hydrazide B form, the aggregate A
form has almost the same shape, intensity and position of absorp-
tion maximum at approximately 330 nm. In weakly interacting
solvents, such as benzene, CHCl3 and MeCN, phenylsemicarbazone
concentrations above 1 × 10−5 mol dm−3 again result in formation
of dimers or higher aggregates of E-isomers Ia and IIa. However, as
expected, this occurs without the formation of hydrazonol C.
Due to existing intra-molecular hydrogen bonding, the corre-
sponding Z-isomers Ib and IIb have only one bathochromically
shifted ꢁA at approximately 340 nm, and the longwavelength
2.4.2. Theoretical calculations
The relative stabilities of isatin-phenylsemicarbazone isomers
were investigated using quantum-chemical calculations. Structural
geometries were optimized at the B3LYP 6-31+G(dp) level. Sta-
tionary points were characterized as minima by computations
of harmonic vibration frequencies at the same theoretical level
as the geometric optimization. Single point energies were calcu-
lated at the MP2 6-311++G(d,p) level. Zero-point vibration energies
and thermal corrections to free energies were determined using
unscaled B3LYP 6-31+G(dp) frequencies. All calculations were per-
formed by the Gaussian 09 program package [29].

K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
63
Scheme 2. Aggregate A/hydrazide B/hydrazonol C chemical equilibrium of the studied isatinphenylsemicarbazones.
absorption band at 410 nm was not observed in their UV–vis spectra
in strongly interacting polar solvents.
3.1. Light initiated E–Z isomerization
To avoid ˚E–Z distortion due to possible back Z–E photo-
isomerization (Z-isomers have higher molar extinctions in the
long-wavelength absorption range of 320–380 nm – Fig. 1A [25]),
and also molecule degradation using short-wavelength irradiation
(ꢁirr < 320 nm), 405 nm was chosen as the irradiation wavelength.
At this wavelength, molar extinction coefficients of E-isomers are
higher than for Z-isomers in strongly interacting polar solvents, and
comparable to them in weakly interacting solvents (Figs. 1A and 2).
Furthermore, the hydrazonol tautomeric form with ꢁA at
410 nm in strongly interacting polar solvents is responsible
for the dominant absorption of E-isomers at low isatin-3-
4-phenyl(semicarbazone) concentration (Figs. 1B and 3), thus
allowing us to estimate the role of hydrazonol in light initiated E–Z
isomerization of the studied compounds.
Fig. 3. Dependence of UV–vis absorption spectra of IIa in DMF on the IIa concen-
tration (T = 25 ◦C).
Both E-isomers quite rapidly isomerize to the corresponding Z-
isomers under these experimental conditions (Figs. 4 and 5).
Fig. 4. Kinetic changes in the absorption spectra of Ia in CH2Cl2 during irradiation
of Ia at 405 nm (cIa = 1 × 10−4 mol dm−3; I0 = 5.6 0.1 × 10−4 mol s−1 dm−3).
Fig. 2. UV–vis spectra of E- and Z-isomers I and II in CHCl3 (c = 10−4 mol dm−3
T = 25 ◦C).
;

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K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
Table 1
The E–Z isomerization quantum yield for isatinphenylsemicarbazone Ia in various solvents.
c [mol dm−3
]
˚
E–Z
(Ia) × 10−3
DMF
MeOH
MeCN
Benzene
CH2Cl2
a
1 × 10−4
3 × 10−6
6.96 0.13
3.77 0.18
3.20 0.10
2.55 0.37
–
3.93 0.52
2.48 0.45
1.53 0.12
0.66 0.08
0.37 0.05
˚
– the E–Z isomerization quantum yield.
Not soluble at this concentration.
E–Z
a
Table 2
The E–Z isomerization quantum yield for isatinphenylsemicarbazone IIa in various solvents.
c [mol dm−3
]
˚
E–Z
(IIa) × 10−3
DMF
MeOH
MeCN
Benzene
CH2Cl2
1 × 10−4
3 × 10−6
7.97 0.39
4.61 1.80
4.43 0.01
1.34 0.15
0.16 0.05
0.07 0.06
0.36 0.04
0.13 0.02
0.24 0.05
0.10 0.03
˚
E–Z
– the E–Z isomerization quantum yield.
The E–Z isomerization quantum yield (˚E–Z) depends on solvent
deactivation pathways of this state. In contrast to the other solvents
used, MeOH and DMF have the highest Gutmann donor and accep-
tor numbers [30] and therefore effectively interact with hydrogen
donor or acceptor parts of molecules Ia and IIa in their ground
electronic state. These interactions can weaken the competitive
intermolecular hydrogen bonding responsible for self-association
of Ia and IIa (Scheme 2 – form A), resulting in dissociation of self-
aggregates and an altered associate/hydrazide/hydrazonol ratio in
solution. Tables 1 and 2 show that the ˚E–Z values for Ia and IIa
in the remaining three solvents do not correlate with Gutmann
acceptor and donor values.
type, E-isomer concentration and the presence/absence of the –CH3
substituent in position 1 of the isatin skeleton (Tables 1 and 2).
Tables 1 and 2 illustrate that the highest ˚E–Z for these
compounds at both 1 × 10−4 mol dm−3 and 3 × 10−6 mol dm−3 con-
centrations was in DMF. Depending on solvent type, the ˚E–Z
value for isatinphenylsemicarbazone E-isomer Ia decreases at the
higher 1 × 10−4 mol dm−3 concentration in the following order:
DMF > benzene > MeOH > CH2Cl2 > MeCN. It should be noted that
˚
is one magnitude higher in DMF, benzene and MeOH com-
E–Z
pared to CH2Cl2 and MeCN.
The decrease in concentration of both isatinphenylsemicar-
bazones Ia and IIa to 3 × 10−6 mol dm−3 leads to a 2- or 3-fold
decrease in E–Z isomerization quantum yield in all solvents. At
this low concentration, the ˚E–Z of Ia decreases in the following
order: DMF > benzene ∼ MeOH > CH2Cl2 > MeCN. Almost the same
Although strongly interacting polar solvents reduce the inter-
molecular hydrogen bonding in the Ia and IIa aggregates, increased
aggregate formation with increasing isatinphenylsemicarbazone
concentration is evident in 1H NMR and UV–vis spectroscopy mea-
surements [24,25]. The concentration dependent shifts in emission
maxima support aggregate formation in both strongly interacting
polar solvents and weakly interacting ones – Figs. S2 and S3. We
consider that dependence of the emission maxima on the excita-
tion wavelength is associated with the different level and type of
the formed Ia or IIa aggregates.
DMF and MeOH solvents have a high ability to interact with
Ia and IIa, and we assume that the isatinphenylsemicarbazone
associated form A, the non-associated hydrazide form B and the
hydrazonol form C are all present in solution at both concentra-
tions. Similar ˚E–Z values for Ia and IIa in these solvents indicate
similar associate/hydrazide/hydrazonol ratio for these E-isomers
in solution. Because the ˚E–Z values in MeOH and DMF vary with
initial Ia and IIa concentration, forms A, B, C should differ in
˚
E–Z
decreased order at both low and high concentration was
observed for E-isomer IIa; with benzene providing the only excep-
tion.
These ˚E–Z orders for both E-isomers illustrate the complex
effects of solvents on ˚E–Z. In the ground electronic state, the
extent of the solvent’s solvation capacity can stabilize or destabi-
lize self-association of isatinpehnylsemicarbazone E-isomers and
determine the hydrazide/hydrazonol ratio in solution (Scheme 2).
In the excited state, solvent affects the energy and the competitive
˚
E–Z
.
As previously mentioned, decreased E-isomer concentration
shifts the tautomeric hydrazide-hydrazonol equilibrium in the
strongly interacting DMF and MeOH polar solvents to the right,
resulting in hydrazonol formation. The increase in ˚E–Z for both
E-isomers with increasing isatinphenylsemicarbazone concentra-
tion in highly interacting polar solvents thus clearly excludes the
dominant contribution of hydrazonol form C to E–Z isomerization
in these solvents. In contrast, the hydrazonol excited state is deacti-
vated through competitive deactivation pathways and hydrazonol
C excitation decreases E–Z isomerization efficiency by hydrazonol
absorption of photochemically active light. Therefore, Ia and IIa Z-
isomers originate from phototransformation of the associated form
A and/or hydrazide form B. Although the increase in ˚E–Z with
increasing isatinphenylsemicarbazone concentration in strongly
interacting polar solvents may be expressly the consequence of
hydrazonol C reduction in solution, and Z-isomers could thus orig-
inate from phototransformation of hydrazide B rather than the
Fig. 5. Kinetic changes in the absorption spectra of Ia in DMF during irradiation of
Ia at 405 nm (cIa = 3 × 10−6 mol dm−3; I0 = 5.6 0.1 × 10−4 mol s−1 dm−3).

K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
65
Scheme 3. Assumed aggregation type difference for studied IIa and Ia E-isomers.
self-aggregate A, we consider that the Z-isomers are produced by
both excited A and B forms. This conclusion is supported by the ˚E–Z
values for Ia and IIa in weakly interacting solvents such as benzene,
CH2Cl2 and MeCN. Despite hydrazonol C absence in these solvents,
˚
E–Z
again increases with increasing isatinphenylsemicarbazone
concentration. Compared to strongly interacting polar solvents, a
higher amount of aggregates is present in solution at low isatin-
phenylsemicarbazone concentrations. Therefore lower ˚E–Z for Ia,
and particularly for IIa, in weakly interacting solvents is most likely
the consequence of improved Z-isomer excited state stabilization in
strongly interacting polar solvents which decreases the rate of back
Z-isomer → aggregate A and/or Z-isomer → hydrazide B reaction in
their excited states. The increasing E–Z isomerization rate after sen-
sitization with benzophenone in MeOH indicates the important role
of intersystem crossing in the E–Z isomerization mechanism (triplet
state generation in the excited hydrazide molecule – Fig. S4).
Although E-isomers Ia and IIa form associates in both strongly
and weakly interacting solvents, the methyl substituent in position
1− of isatin cycle IIa limits hydrazide associate variation in solu-
tion. The methyl derivative IIa with its cyclic hydrogen bonding
most commonly forms the type D dimers (Scheme 3). These dimers
can be formed from the IIa molecule in varying conformational
arrangements.
Scheme 4. Assumed excited state proton transfer from strong intramolecular Z-
isomer hydrogen bonding.
their absorption maxima. Instead of E-isomers formation at both
irradiation wavelengths, only a very slow photodegradation path-
way was observed in weakly interacting polar solvents, no changes
were noted under irradiation in the more strongly interacting ones.
It is assumed that the absence of back light initiated Z–E isomeriza-
tion results from excited state proton transfer in Z-isomer’s strong
intramolecular hydrogen bonding ([25] and Scheme 4).
The proposed mutual E–Z isomer transformations of anion
sensors I and II, including tautomeric equilibrium and aggre-
gate formation, is illustrated in Scheme 5. We assume that the
relatively low E–Z isomerization efficiency of ˚E–Z < 0.01 in all
solvents for both Ia and IIa E-isomers is connected with rapid inter-
nal conversion due to the presence of intra- and inter-molecular
hydrogen bonding of the hydrazide molecules, and also with the
hydrazide–hydrazonol tautomeric equilibrium in strongly interac-
ting polar solvents.
In contrast to IIa, the Ia E-isomer can form several types of
aggregates with a large number of bound Ia molecules in the asso-
ciate (Scheme 3 – form E). In this case, both the urea NH–CO–NH
fragment and also the isatine NH fragment may be involved in the
formation of the cyclic associate form. This explanation is consis-
tent with the concentration dependent change in the Ia E-isomer’s
UV–vis spectrum shape compared to the almost concentration-
independent shape for IIa (Fig. S5). It is considered that the
apparently higher ˚E–Z for Ia than for IIa in weakly interacting
solvents, particularly at higher isatinphenylsemicarbazone concen-
trations, is associated with this aggregation difference in the two
E-isomers.
The one order of magnitude lower ˚E–Z for IIa in benzene
compared to ˚E–Z for Ia at low isatinphenylsemicarbazone con-
centrations may be associated with the ␲–␲ interactions between
isatin and the benzene ring which are suppressed in IIa due to a bulk
methyl substituent. However, the actual reason for Ia and IIa ꢂE–Z
difference in this aromatic solvent at low concentrations currently
remains unidentified.
3.3. Thermally initiated E–Z and Z–E isomerization
The study of photochemical E–Z isomerization is often com-
plicated by thermally initiated back Z–E isomerization. The ratio
of individual isomers in the equilibrium is affected by the ratio
of the reaction rates running in opposite directions. The poten-
tial practical application of E–Z isomerization in signal switching,
molecular memories and sensors therefore depends on the ther-
modynamic stability of the less stable isomer. This is typically
3.2. Light initiated Z–E isomerization
The excitation of Z-isomers Ib and IIb at 405 nm did not lead
to their conversion to the corresponding E-isomers Ia and IIa. The
light initiated back Z–E isomerization was not observed even at
341 nm irradiation wavelength; close to where both Z-isomers have

66
K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
Scheme 5. Mutual E–Z isomer transformation in the studied I and II isatinphenylsemicarbazone anion sensors.
the Z-isomer. As mentioned in the Introduction, although E–Z
isomerization of phenylsemicarbazone E-isomers Ia and IIa occurs
in weakly interacting non-polar solvents at room temperature and
low isatinphenylsemicarbazone Ia and IIa concentrations and in
highly interactive polar solvents at temperatures above 70 ◦C, no
formation of the corresponding E-isomers was observed during
thermally initiated back Z–E isomerization in any type of solvent
[24]. Increased temperature during the Ib and IIb solution heat-
ing leads only to isatin 3-hydrazone or 1-methylisatin 3-hydrazone
decomposition products at both initial Ib or IIb concentrations
(10−6 mol dm−3 and 10−4 mol dm−3), without formation of the
corresponding E-isomers (Fig. S6). Therefore, we performed a the-
oretical study of this problem to show the impossibility of thermal
back Z–E isomerization of Ib and IIb in solution.
Fig. 6 graphically illustrates the relative overall energy of E- and
Z-isomers of isatinphenylsemicarbazone I in conformations E1, E2,
Z1 and Z2 (Scheme 6), and the relative energy of the transition
states related to the conformational changes in the particular con-
former and the geometric change of one isomer to another.
Fig. 6. Calculated relative Gibbs free energies of E1, E2, Z1, Z2 hydrazide
conformers of isatinphenylsemicarbazone isomers Ia and Ib at the B3LYP 6-
31+G(dp)//MP2/6-311++G(d,p) level (T = 298.15 K; TS – transition state).
It is concluded from our theoretical calculations that both E-
and Z-isomers in these conformational arrangements have almost
planar geometry. Z1 is the most stable conformation of isatin-
phenylsemicarbazone I due to the existence of intra-molecular
hydrogen bonding between the urea NH hydrogen and the isatine
carbonyl oxygen. The E1 and Z2 conformer energy is approximately
20 kJ mol−1 higher than that for Z1. The most energetically dis-
favored is the E2 conformer with its Y-hydrogen arrangement in
the urea part of the molecule. The E2 energy is approximately
60 kJ mol−1 higher than for conformer Z1. The E2 conformation
does not allow intra-molecular hydrogen bond formation. Calcu-
lations also showed that the E1E2TS and Z1Z2TS energies of the
transition states corresponding to the conformational changes are
relatively low compared to the energy of the individual confor-
mations. However, the E–Z and Z–E isomerization determining
steps are the activation energies Ea related to these geomet-
ric transformations (Ea1(E–Z) = E1Z1TS-E1 or Ea2(E–Z) = E2Z2TS-E2
and Ea1(Z–E) = E1Z1TS-Z1 or Ea2(Z–E) = E2Z2TS-Z2). Gibbs activa-
tion energies of the E–Z and back Z–E isomerization for the most
stable conformers E1 and Z1 are relatively large; and quite simi-
lar at Ea1(E–Z) ∼ 140 kJ mol−1 and Ea1(Z–E) ∼ 160 kJ mol−1. Based
only on the comparison of these two Ea values, it is impossible
to explain the observed Ia E–Z isomerization and the absence
of back thermally initiated Z–E isomerization during heating of
the Z-isomer Ib solution. The evident difference in reactivity of
Ia and Ib can be explained by the two-step mechanism of E–Z
isomerization. In the first step, the E-isomer conformer E2 is gen-
erated through the E1E2TS transition state. Here, attainment of
the equilibrium state between E1 and E2 conformers is tem-
perature dependent. In the second step, Z-isomer Z1 and Z2

K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
67
Scheme 6. Hydrazide conformers E1, E2, Z1 and Z2 of the studied isatinphenylsemicarbazone E- and Z-isomers.
isatinphenylsemicarbazone I E- and Z-isomers (Scheme 7), and
the relative energy of the transition states relating to the con-
formational changes of the particular hydrazide conformer to the
hydrazonol conformer and the geometric change of one hydrazonol
isomer to another; together with the relative overall energy of the
E1, E2, Z1 and Z2 hydrazide B conformers.
The overall stabilities of the individual E1-C, E2-C, Z1-C and
Z2-C hydrazonol C conformers are very similar, with energy dif-
ferences lying within 22 kJ mol−1. Since the formation of the E1-C
hydrazonol form from the E1 hydrazide B form must overcome the
surprisingly high activation barrier of approximately 150 kJ mol−1
(through E1E1-CTS transition state), it is very unlikely that hydra-
zonol E1-C is formed by this transformation pathway. Experimental
results also indicate a different E1-C formation pathway: hydra-
zonol form C (E1-C or E2-C) exists in strongly interacting polar
solvents at room temperature in equilibrium with the hydrazide
form (E1 or E2) and almost instantaneous settling of the new
equilibrium with higher contents of the hydrazonol C form can
result only from dilution of the solution. This rapid equilibrium
settlement after dilution indicates that the hydrazide B form
of E-isomer Ia exists in solution at higher isatinphenylsemi-
carbazone Ia concentration also in the associated A form. The
intermolecular hydrogen bonds in the associates create appro-
priate conditions for hydrogen transfer, and thus also for the
formation of the E1-C hydrazonol C conformer with a signifi-
cantly lower activation barrier to overcome the E1E1-CTS transition
state energy. Despite E1-C or E2-C hydrazonol C form presence
in strongly interacting polar solvents at room temperature, the
very high subsequent E–Z isomerization activation barriers of
185 kJ mol−1 for Ea1(E–Z) = (E2-C/Z1-CTS)-(E2-C) and 187 kJ mol−1
for Ea2(E–Z) = (E1-C/Z2-CTS)-(E1-C) exclude hydrazonol C contri-
bution to the thermal Ia E–Z isomerization.
UV–vis spectra of the Ib and IIb Z-isomers exclude the presence
of their hydrazonol C forms in strongly interacting polar sol-
vents. These results concur with the calculated Ea = (Z1Z1-CTS)-Z1
activation energy above 170 kJ mol−1 for the corresponding con-
formational change from the Z1 hydrazide to the Z1-C hydrazonol
conformer. In contrast to E-isomers Ia and IIa, isatinphenylsemi-
carbazone Z-isomers Ib and IIb do not form aggregates in solution
and thus the high activation energy for conformational change
from hydrazide to hydrazonol conformer remains unchanged even
at high Z-isomer concentration. The absence of back thermal Z–E
isomerization of Ib and IIb support these conclusions.
Fig. 7. Calculated relative Gibbs free energies of E1, E2, Z1, Z2 hydrazide and
E1 − C, E2 − C, Z1 − C, Z2 − C hydrazonol conformers of isatinphenylsemicar-
bazone isomers Ia and Ib at the B3LYP 6-31+G(dp)//MP2/6-311++G(d,p) level
(T = 298.15 K; TS – transition state).
conformers are formed from the E2 conformer via the E2Z2TS tran-
sition state. The high Ea1(E–Z) activation energy of 140 kJ mol−1
related to E–Z isomerization between the most stable conform-
ers E1 and Z1 is counteracted in the two easier energy steps.
The E2 formation from E1 consumes 44 kJ mol−1 (through acti-
vation barrier Ea(E1–E2) = E1E2TS-E1 = 53 kJ mol−1), thus reducing
the Ea2(E–Z) = E2Z2TS-E2 barrier for subsequent E–Z isomerization
to 111 kJ mol−1
.
If we assume the same reaction pathway for the back Z–E
isomerization, the Z-isomer conformer Z2 can be easily gen-
erated through the transition state Z1Z2TS (activation energy
Ea(Z1–Z2) = 25 kJ mol−1). However, in the second step, it is neces-
sary to overcome the high activation barrier Ea2(Z–E) = E2Z2TS-Z2
of 150 kJ mol−1. This activation energy for Z–E isomerization, sim-
ilar to the activation barrier Ea1(Z–E) = E1Z1TS-Z1 of 157 kJ mol−1
,
is so large that that degradation of the Z-isomer occurs rather
than E-isomer formation. This is reflected in experimental
results.
Since Ib and IIb methylated isomer behavior was very simi-
lar to that of the Ia and IIa unmethylated isomers in all thermal
experiments, we assume that the quantum-chemical calculation
conclusions for Ia and IIa are also valid for the Ib and IIb unmethy-
lated isomers.
In addition to hydrazide B E1 and E2 conformers, hydrazonol
form C can also contribute to overall Z-isomer formation in strongly
interacting polar solvents such as MeOH, DMF and DMSO. Fig. 7
shows the calculated relative overall energy of the correspond-
ing E1-C, E2-C, Z1-C and Z2-C hydrazonol C conformers of the

68
K. Jakusová et al. / Journal of Photochemistry and Photobiology A: Chemistry 288 (2014) 60–69
Scheme 7. Hydrazonol conformers E1-C, E2-C, Z1-C and Z2-C of the studied isatinphenylsemicarbazone E- and Z-isomers.
4. Conclusion
quantitative determination of anions using Ia and IIa E-isomers
because of lower Z-isomer sensitivity. However, the photochem-
ical E–Z isomerization efficiency is relatively low at ˚E–Z < 0.01,
and this allows reliable detection of strongly basic anions using Ia
or IIa. In addition, the easy E-isomer transformation to the corre-
sponding Z-isomer and the utilization of both isomers significantly
enlarges the detection range for F− or CH3COO− anions valid for Ia
or IIa in organic media from 0.1–1 equiv. to 0.1–100 equiv. of isat-
inphenylsemicarbazone (from 10−5 mol dm−3 to 10−2 mol dm−3 of
F− or CH3COO−) without interference at high excess of weak basic
anions. Using the different path length, this range can be further
enlarged to 10−6–10−1 mol dm−3. The detection range is shifted
to higher anion concentrations in semi-aqueous media because of
water competition.
Although the zero efficiency of back photochemical Z–E isomer-
ization excludes the use of isatinphenylsemicarbazones I and II as
molecular switches, the absence of thermally initiated E–Z isomer-
ization and both photochemically and thermally initiated back Z–E
isomerizations in strongly interacting polar solvents can prove ben-
eficial for Ia and IIa E-isomer application in chemical actinometry.
This paper investigated light and thermally initiated E–Z and
Z–E isomerization of two efficient isatin N-phenylsemicarbazone
colorimetric sensors for strongly basic anions in different polarity
solvents.
The E–Z isomerization quantum yield (˚E–Z) depends on sol-
vent type, E-isomer concentration and presence or absence of the
–CH3 substituent in position 1 of the isatin skeleton. The increase
in ˚E–Z for both E-isomers with increasing isatinphenylsemicar-
bazone concentration in highly interacting polar solvents excludes
dominant contribution of hydrazonol form C to the light initiated
E–Z isomerization. In these solvents, the Z-isomers are produced
by both the excited associated A and the excited hydrazide B
forms. Despite hydrazonol C absence in weakly interacting sol-
vents such as benzene, CH2Cl2 and MeCN, the ˚E–Z again increases
with increasing isatinphenylsemicarbazone concentration and this
indicates the aggregated A form’s higher contribution to over-
all Z-isomer production. We assume that the relatively low E–Z
isomerization efficiency of ˚E–Z < 0.01 in both solvent types is
connected with rapid internal conversion due to the hydrazide
molecule’s intra- and inter-molecular hydrogen bonds, and also
linked to the hydrazide–hydrazonol tautomeric equilibrium in
strongly interacting polar solvents. The lower ˚E–Z in weakly inter-
acting solvents compared to that in strongly interacting polar
solvents is most likely the consequence of increased Z-isomer
excited-state stabilization in strongly interacting polar solvents
which decreases the rate of back Z-isomer → aggregate A and/or
Z-isomer → hydrazide B reaction in their excited states. The appar-
ently higher ˚E–Z for Ia than for IIa in weakly interacting solvents is
associated with aggregation type differences in the studied Ia and
IIa E-isomers.
Acknowledgment
The authors greatly appreciate the financial support provided
by the VEGA Grant Agency (Grant No. 1/1126/11).
Appendix A. Supplementary data
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
2014.05.004.
The excitation of Z-isomers did not lead to their conversion
to the corresponding Ia and IIa E-isomers. We assume that the
absence of back light initiated Z–E isomerization is the consequence
of excited state proton transfer resulting from strong Z-isomer
intramolecular hydrogen bonding.
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