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Four benzothiazolium crown ether-containing styryl dyes were prepared through an optimized synthetic procedure. Two of the dyes (4b and 4d) having substituents in the 5-position of the benzothiazole ring are newly synthesized compounds. They demonstrated a higher degree of trans–cis photoisomerization and a longer life time of the higher energy forms in comparison with the known analogs. The chemical structures of all dyes in the series were characterized by NMR, UV–vis, IR spectroscopy and elemental analysis. The steady-state photophysical properties of the dyes were elucidated. The stability constants of metal complexes were determined and are in good agreement with the literature data for reference dyes. The temporal evolution of trans-to-cis isomerization was observed in a real-time regime. The dyes demonstrated a low intrinsic fluorescence of their Ba2+ complexes and high yield of E/Z photoisomerization with lifetimes of the higher energy form longer than 500 seconds. Density functional theory (DFT) calculations at the B3LYP/6-31+G(d,p) level were performed in order to predict the enthalpies (H) of the cis and trans isomers and the storage energies (ΔH) for the systems studied.
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1096
Precious metal-free molecular machines for solar thermal
energy storage
Meglena I. Kandinska1, Snejana M. Kitova2, Vladimira S. Videva1,2,
Stanimir S. Stoyanov1, Stanislava B. Yordanova1, Stanislav B. Baluschev3,
Silvia E. Angelova*4 and Aleksey A. Vasilev*1
Full Research Paper Open Access
Address:
1Faculty of Chemistry and Pharmacy, Sofia University “St. Kliment
Ohridski”, 1164 Sofia, Bulgaria, 2Institute for Optical Materials and
Technologies “Acad. J. Malinowski”, Bulgarian Academy of Sciences,
1113 Sofia, Bulgaria, 3Max Planck Institute for Polymer Research,
Ackermannweg 10, 55128 Mainz, Germany, and 4Institute of Organic
Chemistry with Centre of Phytochemistry, Bulgarian Academy of
Sciences, 1113 Sofia, Bulgaria
Email:
Silvia E. Angelova* - sea@orgchm.bas.bg; Aleksey A. Vasilev* -
ohtavv@chem.uni-sofia.bg
* Corresponding author
Keywords:
aza-15-crown-5 ether; benzothiazolium crown ether-containing styryl
dyes; E/Z photoisomerization; molecular solar thermal system
Beilstein J. Org. Chem. 2019, 15, 1096–1106.
doi:10.3762/bjoc.15.106
Received: 25 February 2019
Accepted: 29 April 2019
Published: 14 May 2019
This article is part of the thematic issue "Novel macrocycles – and old
ones doing new tricks".
Guest Editor: W. Jiang
© 2019 Kandinska et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Four benzothiazolium crown ether-containing styryl dyes were prepared through an optimized synthetic procedure. Two of the dyes
(4b and 4d) having substituents in the 5-position of the benzothiazole ring are newly synthesized compounds. They demonstrated a
higher degree of trans–cis photoisomerization and a longer life time of the higher energy forms in comparison with the known
analogs. The chemical structures of all dyes in the series were characterized by NMR, UV–vis, IR spectroscopy and elemental anal-
ysis. The steady-state photophysical properties of the dyes were elucidated. The stability constants of metal complexes were deter-
mined and are in good agreement with the literature data for reference dyes. The temporal evolution of trans-to-cis isomerization
was observed in a real-time regime. The dyes demonstrated a low intrinsic fluorescence of their Ba2+ complexes and high yield of
E/Z photoisomerization with lifetimes of the higher energy form longer than 500 seconds. Density functional theory (DFT) calcula-
tions at the B3LYP/6-31+G(d,p) level were performed in order to predict the enthalpies (H) of the cis and trans isomers and the
storage energies (ΔH) for the systems studied.
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Introduction
Molecular photoswitches permanently attract considerable
interest because they hold potential for application in molecular
electronic and photonic devices [1-5]. Photoswitches are a class
of switches that can alternate between the thermodynamically
stable forms by application of light (or change of the light inten-
sity) as an external stimulus. If the stable forms (isomers) are
Beilstein J. Org. Chem. 2019, 15, 1096–1106.
1097
not isoenergetic, they have the ability to capture and store
imported (solar) energy [6]. As the stored energy is further re-
leased as thermal energy, such materials are called molecular
solar thermal systems (MOST) [7]. For a pure MOST system
the maximal solar energy conversion efficiency was estimated
to be 10.6% [8]. It was demonstrated experimentally that by
merging a MOST-system [9] with a triplet–triplet annihilation
photon energy upconversion system (TTA-UC) [10,11] an
effective utilization of sub-bandgap photons is possible [12,13].
Nevertheless, the identification of a precious metal-free MOST
system utilizing the visible part of the sun spectrum remains a
considerable challenge.
The stated requirements for such systems can be summarized as
follows [7]:
(i) the potential barrier between the higher energy form and the
lower energy form of the energy carriers should be as high as
possible;
(ii) the higher energy form must be stable for a long time;
(iii) the higher energy form has to be with quite small molar
absorptivity in comparison to the lower one;
(iv) the quantum yield of the photoisomerization has to be as
high as possible, which requires the design of MOST systems
with completely suppressed or minimal fluorescence, intramo-
lecular charge transfer or other processes, quenching the photo-
isomerization;
(v) the energy storing MOST materials have to utilize light in
the visible range of the spectrum;
(vi) it will be a crucial benefit for the MOST technology,
if toxic and precious metals are avoided. Focusing on
environmentally friendly MOST system is an unavoidable
requirement;
(vii) the MOST materials should be of low cost and easily
accessible; their synthesis and purification should be straight-
forward, reliable, fast, inexpensive and ideally environmentally
benign.
Therefore, only MOST systems fulfilling all requirements
(i)–(vii) simultaneously can be regarded as realistic technologi-
cal systems for long-term solar energy storage.
However, the design and preparation of molecules matching all
the above-mentioned criteria is quite difficult. Thus, the inven-
tion of new materials possessing at least parts of these require-
ments provides the base for further improvements and brings
the scientists nearer to the identification of the “perfect” MOST
system. In this connection we identified crown ether-containing
styryl dyes [14,15] as promising substances due to their ability
to undergo trans-to-cis photoisomerization (E/Z) with very high
quantum yields. In addition, their structures predispose to an
easy functionalization and low cost synthesis. Another advan-
tage of these dyes is the very low molar absorptivity [16,17] of
the higher energy cis isomer, which is a prerequisite for its
photostability. The cis isomers of styryl dyes linked to crown
ethers have been identified to form complexes with alkaline-
earth metal cations with higher stability constant than the
respective trans isomer forms [14,15,18]. Lednev et al. pro-
posed as an explanation of the difference in the determined
stability constants values for the complexes with cis/trans
isomers the formation of an additional intramolecular coordina-
tion bond between the “crowned” cation and the alkylsulfonate
anchoring group which is only possible for the cis form [19].
The further studies by the same group of authors indicated that
benzothiazolium styryl monoazacrown ether dyes have several
main advantages. First of all, the azacrown nitrogen atom is
linked directly to the dye and it is part of the chromophore
system, responsible for the “push–pull” effect and the photo-
physical properties of the dye. This provides control over the
“push–pull” effect in the chromophore by switching on and off
states (i.e., metal-in/metal-out from the crown ligand) [20].
Further, it was found that the azacrown benzothiazolium styryl
dyes 4a and 4c exhibit ion-sensitivity in the thermal cistrans
photoisomerization [18,21], a feature, which may be used in the
development of thermoreversible photoionic molecular devices
[19]. Crown ether-containing styryl dyes are used as sensors for
dications such as Ba2+, Ca2+ and Mg2+, or as materials for
optical data storage [19]. Thus, to the best of our knowledge
this kind of dyes has not been specified as potential MOST ma-
terial yet.
The aim of the present study is to reveal the potential of the aza-
15-crown-5-containing styryl dyes as efficient material for the
TTA-UC accelerated MOST process.
Results and Discussion
Synthesis
The synthetic pathway starts with the quaternization of
2-methylbenzothiazoles 1a and 1b with 1,3-propanesultone
(1c) or 1,4-butansultone (1b) in a sealed tube at 145 °C without
any solvent, as it was described in the literature [17]
(Scheme 1).
A four-step synthetic pathway was used for the synthesis of
4-(aza-15-crown-5)benzocarbaldehyde 3 (Scheme 2) following
the procedure described in [22].
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Scheme 1: Quaternization of 2-methylbenzothiazoles with alkane sultones.
Scheme 2: Synthesis of 4-(aza-15-crown-5)benzocarbaldehyde (3) [22].
Scheme 3: Synthesis of dyes 4a–d.
A series of the target dyes 4a–d was prepared using a modified
literature procedure (Scheme 3) [22]. The modification consists
in the addition of a 10% molar excess of 4-(aza-15-crown-
5)benzocarbaldehyde (3, Scheme 3) and usage of ethyl acetate
as an additional solvent for product precipitation. Because of its
better solubility in ethyl acetate the unreacted excess of crown
ether 3 was easily removed. In general the zwitterionic salts
2a–d reacted to complete depletion in ethanol and in the pres-
ence of piperidine as catalyst with the crown ether benzalde-
hyde 3 (TLC monitoring, ethyl acetate/ethanol 4.5:0.5). After
the addition of ethyl acetate the precipitated target products
4a–d were isolated by filtration.
It must be mentioned explicitly, that the modification of
the procedure helped us to avoid cost-intensive and time-
consuming purification procedures which makes the synthesis
suitable for large material quantities. In this way only one
further precipitation from ethanol/ethyl acetate 1:3 was needed
to obtain analytically pure target dyes 4a–d.
The dyes 4a and 4c were previously described [18,21] and were
used as reference compounds. To the best of our knowledge
dyes 4b and 4d are new compounds. The chemical structures of
all dyes from the series were proved by NMR spectroscopy,
elemental analysis, IR and UV–vis spectroscopy.
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Figure 1: a–c) Dependence of the absorption spectra of the dyes 4b, 4c and 4d, respectively (cL = 1.0 × 105 M in ACN) on the concentration of
Ba(ClO4)2, ranging from 1 × 104 M up to 5 × 101 M. d) Absorption of dyes 4a–d at λ = 520 nm as a function of Ba(ClO4)2 concentration. Experimen-
tal conditions: rt, sample thickness d = 10 mm.
Photophysical properties
Steady-state absorption and emission spectroscopy
Next we elucidated the photophysical properties of the chosen
compounds and determined their suitability to be used as
MOST systems. First, we defined their photophysical behavior
in neat acetonitrile (ACN) solution and in the presence of
barium cations. Figure 1 shows the absorption spectra of dyes
4a–d measured at different concentrations (cM) of Ba(ClO4)2.
All dye solutions have similar absorption profiles with a broad
long wavelength band in the vis region (Δλ = 400–600 nm) with
long wavelength maxima at λmax = 520 nm indicating that their
spectral properties are determined principally by the core
chromophore structure. The addition of Ba2+ ions to the ACN
solution of all dyes induced a decrease in the absorption
maximum intensity (at about 520 nm) and to a substantial
hypsochromic shift of the maximum absorption with peak at
around λmax = 440 nm. The changes in the absorption spectra of
the dyes upon the addition of Ba(ClO4)2 in ACN solution are
characteristic for an ion complexation by the azacrown ether
group of the chromoionophore and the spectra in Figure 1 were
assigned to the formation of the trans-dye–Ba2+ complex [19].
For all dye solutions, a distinct isosbestic point upon titration
was observed, indicating only one kind of complex formation
even at the highest Ba2+ concentration.
To determine the optimal Ba2+ concentration for our measure-
ments it was necessary to define the stability constants for each
dye–Ba2+ complex. The dependencies of the absorption A
(Equation 1) of the dyes 4a–d at a fixed wavelength λ = 520 nm
on the Ba2+ concentration in ACN is shown in Figure 1d. The
curves were approximated by Equation 1, which is true for the
simplest form of complexation [18]:
(1)
(2)
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Table 1: Absorption maxima (λmax,abs), fluorescence emission maxima (λmax,f), shift of the absorption maxima (Δλmax), and quantum yields of fluores-
cence (Φf) in % (at 488 nm and 440 nm excitation) for the trans isomers of dyes 4a–d in acetonitrile solution at different Ba2+ concentrations.
Dye cM/cLλmax,abs, nm Δλabs λmax,f, nm Φf %
λexc = 488 nm
Φf %
λexc = 440 nm
4a 0 520 597.6 5.9 1.9
1000 439 81 595 0.29 0.29
10000 439 81 595 0.34 0.37
4b 0 523 594 6.42 0.83
2000 438 85 588 1.9 0.38
10000 438 85 580.6 0.91 0.44
4c 0 522 593.8 0.23 0.4
1000 442 80 596 0.18 0.25
10000 442 80 596 0.14 0.13
4d 0 521 599 0.56 0.77
1000 441 80 596 0.31 0.32
10000 441 80 594 0.49 0.58
where A0 and A are the absorptions of the chromoionophore at
zero and infinite concentration of the metal ion, respectively; A
is the absorption at the concentration cM of the metal ion; K is
the stability constant of complex formation, and L and LM are
the ligand and the metal ion complex, respectively. A and K
were found by approximation of Equation 1.
The stability constant for the complex formation was estimated
to be K = 100 ± 15 M1, 49.4 ± 7.6 M1 and 44.7 ± 10 M1 for
dyes 4b, 4c and 4d, respectively. For dye 4a the stability con-
stant was found to be K = 70 ± 15 M1 which is in good correla-
tion with the published data [22]. These results prompted us to
identify the optimal Ba2+ concentration, necessary for a
maximum degree of dye–Ba2+ inclusion complex formation.
Obviously for a better complexation it is necessary to work with
high Ba2+ concentrations. The explanation is related to the size
of the barium cation which does not intercalate completely in
the crown ether cavity. This assumption is confirmed by the
slight downfield shift of the signals in the 1H NMR spectrum of
dye 4b in the presence of Ba2+ cations compared to that in neat
CD3CN (Supporting Information File 1, Figure S9 and Figure
S10).
A common behavior demonstrated in Figure 2 was observed:
the fluorescence quantum yield of all dyes (Table 1) decreases
if the concentration of the Ba2+ ions increases, i.e., the forma-
tion of the trans-dye–Ba2+ complexes causes the observed fluo-
rescence decrease.
The optical parameters of all 4 dyes, including absorption
maxima (λmax,abs), fluorescence emission maxima (λmax,f),
hypsochromic shift of the absorption maxima (Δλmax), and fluo-
Figure 2: Dependence of the fluorescence of 4b on the concentration
of the Ba2+ ions. Excitation wavelength λexc = 488 nm.
rescence quantum yields (Φf) for different excitation wave-
lengths, obtained for the dyes in ACN solution at different Ba2+
concentrations are summarized in Table 1. The emission
maximum of all dyes is at about λmax,f = 595 nm and does not
change significantly upon Ba2+ ion complexation. No changes
in the shape of the fluorescence curves were observed, when
the excitation wavelength was tuned within the region of
λexc = 400–550 nm.
Generally one of the advantages of the presented Ba2+–crown
ether containing styryl dye complexes as a MOST material is
the extremely low intrinsic fluorescence which can be a precon-
dition for higher quantum yields of the cis–trans photoisomer-
ization reactions.
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1101
Figure 3: a) Dependence of absorption spectra of dye 4b (cL = 1.0 × 104 M) with Ba2+ (cM = 1 M) on the irradiation time. b) Temporal absorption
evolution, measured at λabs = 500 nm, upon irradiation. Experimental conditions: Excitation wavelength λexc = 488 nm; excitation intensity
14 mW cm2, solvent, ACN; pulse duration t = 180 s.
Real-time E/Z-photoisomerization of dyes 4a–d and
their complexes
The photoisomerization of free dyes 4a–d and dye–Ba2+ inclu-
sion complexes were investigated in real time mode upon irradi-
ation with visible light (λ = 488 nm) close to their absorption
maxima. Figure 3a illustrates the characteristic changes in the
absorption spectra of dye 4b–Ba2+ complex under optical exci-
tation with λ = 488 nm and intensity of 14 mW cm2. The
temporal evolution of the absorption at specific wavelength of
λabs = 500 nm is demonstrated in Figure 3b: during the first
time interval from t1 = 10 s up to t2 = 180 s the absorption is de-
creasing monotonically, as a result of π π transition in the
trans isomer of the free dye and its Ba2+ complex. During the
next time interval for t3 > 180 s, the optical excitation was
terminated and constant increase of the optical absorption was
observed.
The degree (R) of trans-to-cis photoisomerization at the photo-
stationary state was evaluated from Equation 3:
(3)
where A0 is the absorption before irradiation and A is the
absorption at the photostationary state. It was found that the rate
of the photoisomerization process and the degree of conversion
trans-to-cis isomer depends strongly on the Ba2+ concentration.
The cis isomers formed upon irradiation are thermally unstable
and revert to the trans isomers in the dark [23]. The rates of
trans-to-cis isomerization were determined for several Ba2+
concentrations in the range of 2 × 103 M up to 1 M at a fixed
dye concentration (1 × 104 M) for all dyes. The kinetic data
were found to fit well to a single exponential function (Equa-
tion 1), giving a rate constant (k) corresponding to a lifetime
(1/k) [18]:
(4)
where A0 and A, are the initial and final absorptions, A is the
absorption at 500 nm at a time t after termination of the irradia-
tion. In all cases, after irradiation ceased, complete reversion
toward the initial absorption spectrum was observed. As a rule,
the reversion from the cis to the trans isomer leads to a gradual
increase in the intensity of the absorption maximum. The photo-
isomerization data for all 4 dyes are summarized in Table 2.
Generally, the addition of Ba2+ ions was found to increase sub-
stantially the lifetime of the cis isomers. Under the abovemen-
tioned conditions the cis-4b–Ba2+ complex was detected to be
most stable, while dye 4d formed stable cis-4d–Ba2+ complex
only at a higher concentration of Ba2+ ions (1 M).
Figure 4 is an additional demonstration of the ability of dye 4b
to undergo trans-to-cis photoisomerization in its free form and
in complex with Ba2+. The absorption band corresponding to
the π π transition in the cis isomer (at around 280 nm and
320 nm, shown in Figure 4), as for the free dye 4b, increased
with the irradiation time, suggesting that isomerization from
trans to a cis form of the free dye or its Ba2+ complex
proceeded until a photostationary state was reached. As can be
seen from Figure 4 the trans-to-cis isomerization takes place to
a higher extent in the free form of the dye than in the complex.
However, the lifetimes of the free cis form is extremely short in
comparison to that of the complex. The degree of photoisomer-
ization of the new dye 4b is apparently higher than that of the
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Table 2: Degree (R) of trans-to-cis photoisomerization at the photostationary state, rate constants (k) and lifetime of trans-to-cis isomerization.
Dye
M
Ba(ClO4)2
M
RLifetime
s
k
s1
4a
1.0 × 104
0 0.19 31 32.6 × 103
2.0 × 1030.53 69 14.4 × 103
2.0 × 1020.52 264 3.8 × 103
2.0 × 1010.56 284 3.5 × 103
1 0.53 342 2.9 × 103
4b
1.0 × 104
0 0.35 148 6.0 × 103
2.0 × 1030.82 292 3.4 × 103
2.0 × 1020.74 279 3.6 × 103
2.0 × 1010.54 481 2.1 × 103
1 0.39 514 1.9 × 103
4c
1.0 × 104
0 0.00 0 0
2.0 × 1030.00 0 0
2.0 × 1020.08 0 0
2.0 × 1010.52 330 3.0 × 103
1 0.44 390 2.6 × 103
4d
1.0 × 104
0 0.21 18 55.0 × 103
2.0 × 1030.23 20.9 47 × 103
2.0 × 1020.22 30.7 32 × 103
2.0 × 1010.58 137 7.3 × 103
1 0.26 301 3.3 × 103
Figure 4: UV–vis absorption spectra of dye 4b (1.0 × 104 M) free and
in complex with Ba2+ (1 M) before and after the end of the exposure at
λ = 488 nm.
known structure (4a). From one side it can be supposed that dye
4a aggregated much faster than its methyl-substituted analogue
4b. From another hand the substituent in the 5-position of the
benzothiazole heterocycle sterically hinders the rotation of the
alkylsulfo-anchoring group and thus plays the role of a
controller with regard to its direction towards the crown ether.
This finding is a resemblance to the results from reference [24]
where the 5-methoxy-substituted benzothiazole styryl-crown
ethers demonstrated higher quantum yields of trans-to-cis pho-
toisomerization.
Insight from electronic structure calculations
To rationalize the experimental findings, we performed density
functional theory (DFT) calculations at the B3LYP/6-31+G(d,p)
level. The first step in the molecular modelling investigation
was the optimization of the molecular structures of the cis and
trans isomers of dyes 4a–d (with the –(CH2)nSO3 (n = 3, 4)
tails oriented to be in proximity to the aza-15-crown-5 frag-
ments) in the gas phase (Figure 5). The thermochemical data for
these are calculated at 298.15 K and a pressure of 1 atm. In
Table 3 we report the enthalpies (H) for the cis and trans
isomers and the energy storage capacity (ΔH) calculated as the
difference in enthalpy between the cis and trans isomers.
B3LYP calculations in the gas phase predict the trans forms of
the dyes 4a–d to be more stable. The cis forms are higher in
energy by only 3.3 and 3.0 kJ mol1 for 4a and 4b, respectively.
The enthalpy differences calculated for dyes 4c and 4d are
much higher (69.3 kJ mol1 (4c) and 67.9 kJ mol1 (4d), re-
spectively).
Compounds 4a–d can bind metal species at both isomeric
forms. To determine the geometries of the 1:1 complexes with
Ba2+ cations the metal cations were placed in the crown ether’s
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1103
Figure 5: Optimized structures of the cis and trans isomers of dyes
4a–d.
cavity and allowed to relax. In the optimized dye–Ba2+ com-
plexes the metal cations are displaced “above” the crown ether’s
cavity. Figure 6 depicts the gas-phase geometries of the Ba2+
complexes of the cis and trans isomer of 4b.
The calculated enthalpies for the complex-formation reaction
dye + Ba2+ [dye–Ba]2+ in the gas phase, where dye = 4ad,
Table 3: Calculated enthalpies (H) for the cis and trans isomers and
storage energies (ΔH) for the systems studied in the gas phase and in
acetonitrile solution.
Compound ΔH, kJ mol1
gas phase acetonitrile
4a 3.3 39.7
4b 3.0 42.9
4c 69.3 32.3
4d 67.9 31.8
Figure 6: Optimized structures of the Ba2+-complexes of cis and trans
isomer of 4b.
with bare metal cations are listed in Table 4. The results ob-
tained demonstrate that all reactions in the gas phase are pre-
dicted to be favorable. The ∆∆H1 values calculated for Ba2+
complex formation with the trans isomers of 4a–d are more
negative than the values obtained for the respective cis isomers.
Table 4: Calculated enthalpies, ∆∆H1, for [dye–Ba]2+ complex forma-
tion in the gas phase (in kJ mol1).
Complex ∆∆H1
[cis-4a–Ba]2+ 793.3
[trans-4a–Ba]2+ 838.4
[cis-4b–Ba]2+ 790.2
[trans-4b–Ba]2+ 843.2
[cis-4c–Ba]2+ 759.0
[trans-4c–Ba]2+ 805.3
[cis-4d–Ba]2+ 762.0
[trans-4d–Ba]2+ 785.2
The complex formation processes between dyes with shorter
–(CH2)nSO3 tail (n = 3) are characterized by more negative
∆∆H1 values than those calculated for the dyes equipped with
longer tails (n = 4), a trend corresponding to the trends in the
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Table 5: TDDFT/PBE0 calculated absorption maxima (λmax), oscillator strength (f) HOMO and LUMO energies and energy difference (HOMO–LUMO
gap, HLG) for the trans isomers of compounds 4a–d in acetonitrile.
Compound λmax, nm fHOMO, eV LUMO, eV HLG, eV
4a 477 1.58 5.71 2.98 2.73
4b 481 1.63 5.64 2.93 2.71
4c 482 1.63 5.58 2.90 2.69
4d 483 1.65 5.56 2.87 2.69
Figure 7: Simulated spectra with spectral lifetime broadening TDPBE0 spectra in ACN for dye 4b and its Ba2+ complex.
experimentally derived stability constants. Conventional sol-
vent treatment by methods like the polarizable continuum
model (PCM) did not provide a good quantitative agreement
with the experimental stability constants [23] so the data for
∆∆H in acetonitrile are not provided.
Time-dependent density functional theory (TDDFT) calcula-
tions were used to probe the electronic reorganizations upon ex-
citation. TDPBE0 calculations with the 6–31+G(d,p) basis set
for all atoms (except for Ba) and with the Stuttgart-Dresden
SDD effective core potential (ECP) basis set for Ba predict one
intensive band for all compounds in the range of 400–500 nm.
The calculated optical parameters such as the absorption
maximum (λmax), oscillator strength (f) and frontier orbital
energy levels for the trans isomers of 4a–d are listed in Table 5.
The positions and intensities of the bands are consistent with the
experimentally observed ones. The first excited states are deter-
mined by HOMO (highest occupied molecular orbital)
LUMO (lowest unoccupied molecular orbital) transitions.
The simulated spectra with spectral lifetime broadening
(Gaussian function) with a full width at half-maximum
(FWHM) of 0.15 eV and a height proportional to the oscillator
strength for each transition spectrum for dye 4b and its
Ba2+ complex (Figure 7) are consistent with the experimental
ones. The experimentally measured substantial hypo- and
hypsochromic shift in the absorption spectra upon Ba2+ addi-
tion are also observed in the simulated spectra of the theoreti-
cally modeled structures of the dyes and the respective com-
plexes.
The simulated spectra of the metal-free and Ba2+ complexed cis
form of compound 4b are also presented in Figure 7. The oscil-
lator strengths of the cis forms, calculated at the same computa-
tional level, are found to be significantly lower than those
calculated for the respective trans forms (metal-free compound
trans-4b and trans-4b–Ba complex). These results correspond
to the experimentally observed gradual increase in the intensity
of the absorption maximum upon cis-to-trans reversion.
Conclusion
Four benzothiazolium crown ether-containing styryl dyes (two
known and two novel compounds) were synthesized through an
optimized synthetic procedure. The photophysical properties of
the new dyes were investigated in the absence and presence of
Ba2+ cations and compared with those of the known dyes. The
optimal conditions for the trans-to-cis photoisomerization of the
styryl-crown ether containing dyes were identified. The dyes 4b
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1105
and 4d with substituents in 5-position of the benzothiazole
moiety demonstrated much better photophysical properties as
molecular switches and MOST materials in comparison with
the unsubstituted known analogs. The calculated thermo-
dynamic changes associated with metal-ion complexation in the
gas phase match the trends in the experimental stability con-
stants.
Experimental
General
All solvents used in the present work were commercially avail-
able (HPLC grade). The starting materials 1a, 1b, 2a, and 2b
were commercially available and were used as supplied.
Melting points were determined on a Kofler apparatus and are
uncorrected. NMR spectra were obtained on a Bruker Avance
III 500 DRX 600 MHz spectrometer in DMSO-d6. The
MALDI–TOF/TOF spectra were measured at Bruker “RapifeX”
at MPIP, Germany. The stepwise experimental procedures for
the synthesis of compounds 2–4 and characterization data are
given in Supporting Information File 1. 4-(Aza-15-crown-
5)benzocarbaldehyde (3) was synthesized using a slightly modi-
fied procedure [16,17].
UV–vis spectra were measured on a Unicam 530 UV–vis spec-
trophotometer in conventional quartz cells of 1 cm path length.
The spectral bandwidth and the scan rate were 1 nm and
140 nm min1, respectively. Stock solutions of each compound
were prepared in spectroscopic grade acetonitrile (ACN) and all
experiments were carried out in red light and at room tempera-
ture. Complex formation of dyes with Ba(ClO4)2 was studied
by spectrophotometric titration. In the experiment aliquots of a
solution containing known concentrations of dyes and of
Ba(ClO4)2 were added to a solution of dyes alone at the same
concentration. So the absorption spectra were recorded for solu-
tions with identical total dye concentration (1 × 105 M) and
variable total Ba(ClO4)2 concentration ranging from 1 × 105 M
to 5 × 101 M in ACN.
Emission spectra were recorded on FluoroLog3-22, Horiba
Jobin Yvon spectrofluorometer with Quanta–φ accessory
having a large 150 mm integrating sphere for the quantum yield
measurements. All spectra we recorded using quartz cells of
1 cm path length. The solution concentrations were chosen to
give an absorbance 0.05 at the excitation wavelength of 440
and 488 nm.
The photoisomerization of the dyes and their complexes was
performed by irradiating the samples in quartz cells (1 cm) with
16 mW laser (Qioptiq iFLEX2000-P-2-488) at λ = 488 nm for
3 min, a time which was found to be long enough to reach a
photostationary state. The kinetics of the cistrans thermal
isomerization were studied by measuring the absorbance at a
fixed wavelength in the dark as a function of time after irradia-
tion stopped until the initial absorbance value before excitation
was reached. The absorbance was measured by Ocean Optic
HR2000+ high resolution USB fiber optic spectrometer fitted
with 500 nm interference filter in the incident beam (10 nm
bandwidth). During the irradiation and kinetic studies, the solu-
tions were intensively magnetically stirred. A cuvette holder for
fluorescent measurements was used allowing recording of the
absorbance spectra during the laser irradiation in the perpendic-
ular direction and immediately after the stop of irradiation.
Computational details
Equilibrium geometries and intermolecular interaction energies
for the host–guest assemblies between the dyes and metal
cations were obtained by density functional theory (DFT) calcu-
lations using the B3LYP functional [25,26] (the most often used
functional for organic molecules and complexes) and the
6-31+G(d,p) [27-29] basis set for the lighter atoms (C, O, S, N,
H) and SDD pseudopotential for Ba atoms as implemented in
the Gaussian 09 program package [30]. Frequency calculations
for each optimized structure were performed at the same level
of theory. No imaginary frequency was found for the lowest
energy configurations of any of the optimized structures. In
order to take into account the solvent effect induced by the
acetonitrile solvent environment, the equilibrium geometries of
the host–guest constituents and complexes were reoptimized
considering the PCM (polarizable continuum model) solvent
model [31]. The so-called basis set superposition error (BSSE)
was not taken into account for the geometry optimization
and intermolecular energy calculation. Time-dependent
density functional theory calculations (TDDFT) using
Perdew–Burke–Ernzerhof exchange-correlation functional
(PBE0) were performed to compute the 10 lowest excited states
of each structure [6–31+G(d,p) basis set for all atoms except
Ba]. PyMOL molecular graphics system was used for genera-
tion of the molecular graphics images [32].
Supporting Information
Supporting Information File 1
Experimental procedures for the synthesis of compounds
2–4 and characterization data of the new compounds.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-106-S1.pdf]
Acknowledgements
The financial support from Bulgarian National Science Fund
under the project “SunStore” (DFNI E 02/11 2014) and Materi-
als Networking (Twinning-692146) is gratefully acknowledged.
Beilstein J. Org. Chem. 2019, 15, 1096–1106.
1106
ORCID® iDs
Snejana M. Kitova - https://orcid.org/0000-0001-6086-655X
Stanimir S. Stoyanov - https://orcid.org/0000-0002-7830-1538
Stanislav B. Baluschev - https://orcid.org/0000-0002-0742-0687
Silvia E. Angelova - https://orcid.org/0000-0003-4717-8028
Aleksey A. Vasilev - https://orcid.org/0000-0003-2199-5644
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