[2+2] Photocycloaddition of Styryl Dyes in the Cucurbit[8]uril Cavity and Its Ultrafast Dynamics

Article abstract of DOI:10.1134/S0018143919030147

Abstract: [2+2] Photocycloaddition of a series of styryl dyes (a, b, and c, see text) in the cavity of cucurbit[8]uril (CB8) in aqueous solutions has been studied by optical spectroscopy. The complex formation constants were calculated for the 1 : 1 and 2 : 1 complexes of the styryl dyes with CB8. The optimal CB8 to dye molar ratios that correspond to the highest concentrations of the 2 : 1 complexes in the solution have been determined as 0.5, 0.3, and 1 for dyes a, b, and c, respectively. The quantum yields of photocycloaddition have been calculated from the results of dye photolysis in the presence of CB8: 0.06, 0.02, and 0.04 for a, b, and c, respectively. The fluorescence decay kinetics has been studied on the picosecond timescale. The lifetimes found are 1–2 ps, which correspond to the characteristic time of the solvation shell effect on the redistribution of the dye charge in the excited state. The long component of the fluorescence decay on the order of tens of picosecond is also observed. This component decreases in the presence of CB8, indicating the formation of dimeric state.

Full text of DOI:10.1134/S0018143919030147

ISSN 0018-1439, High Energy Chemistry, 2019, Vol. 53, No. 3, pp. 204–210. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Khimiya Vysokikh Energii, 2019, Vol. 53, No. 3, pp. 199–205.
PHOTOCHEMISTRY
[2+2] Photocycloaddition of Styryl Dyes in the Cucurbit[8]uril
Cavity and Its Ultrafast Dynamics
A. D. Sviridaa, b, *, D. A. Ivanova, I. V. Kryukova, Yu. A. Shandarova, N. Kh. Petrova, b, M. V. Alfimova, b,
N. A. Aleksandrovaa, S. K. Sazonova, A. I. Vedernikova, and S. P. Gromova, b
aPhotochemistry Center of the “Crystallography and Photonics” Federal Research Center, Russian Academy of Sciences,
Moscow, 119421 Russia
bMoscow Institute of Physics and Technology (State University), Dolgoprudnyi, Moscow oblast, 141700 Russia
*e-mail: svirida.anton@yandex.ru
Received November 30, 2018; revised December 26, 2018; accepted December 28, 2018
Abstract― [2+2] Photocycloaddition of a series of styryl dyes (a, b, and c, see text) in the cavity of cucur-
bit[8]uril (CB8) in aqueous solutions has been studied by optical spectroscopy. The complex formation con-
stants were calculated for the 1 : 1 and 2 : 1 complexes of the styryl dyes with CB8. The optimal CB8 to dye
molar ratios that correspond to the highest concentrations of the 2 : 1 complexes in the solution have been
determined as 0.5, 0.3, and 1 for dyes a, b, and c, respectively. The quantum yields of photocycloaddition have
been calculated from the results of dye photolysis in the presence of CB8: 0.06, 0.02, and 0.04 for a, b, and c,
respectively. The fluorescence decay kinetics has been studied on the picosecond timescale. The lifetimes
found are 1–2 ps, which correspond to the characteristic time of the solvation shell effect on the redistribu-
tion of the dye charge in the excited state. The long component of the fluorescence decay on the order of tens
of picosecond is also observed. This component decreases in the presence of CB8, indicating the formation
of dimeric state.
Keywords: styryl dyes, cucurbit[8]uril, [2+2] photocycloaddition
DOI: 10.1134/S0018143919030147
INTRODUCTION
(guest) : 1 (host) composition with various organic
molecules. The guest preorganization of this kind in
the CB8 cavity can be successfully used for controlling
photochemical processes and even allows using СВ8
as a supramolecular catalyst [8].
Cucurbit[n]urils (CB[n]) are hollow macrocyclic
cavitands consisting of n glycoluril units (n = 5–8, 10)
linked by pairs of methylene bridges [1]. The trivial
name of these molecules is due to their shape and orig-
inates from the Latin word cucurbitus, which means
pumpkin. Cucurbiturils were synthesized for the first
time in 1905. [2]; however, their structure was
unknown until 1981 [3]. The carbonyl oxygen atoms
with partial negative charge are at the ends of mole-
cules of the cucurbituril series, framing cavity portals.
This feature promotes bonding between cucurbiturils
and relatively small organic cations to form host–guest
complexes. The rigid structure. as well as the possibil-
ity of the formation of host–guest complexes, makes
cucurbiturils attractive compounds for design of new
supramolecular systems [1, 4].
The 1 : 1 and 2 : 1 host–guest complexes between
dye a (Fig. 1) and CB8 were detected in aqueous
solutions by NMR spectroscopy and spectrophotom-
etry by Gromov et al. [9], and it was shown that the
photolysis of these solutions gives cyclobutane stereo-
isomer d (Fig. 1). It is worth noting that even an
amount of CB8 as low as 5 mol % is enough for the
almost complete dye conversion into the cyclobutane
derivative. This confirms the ability of the CB8 cav-
itand to be a supramolecular catalyst. Moreover, the
reaction proceeds stereospecifically; i.e., only one of
eleven theoretically possible cyclobutane isomers is
formed, for which strictly defined relative spatial ori-
entation of interacting molecules is required.
The concept of controlling photochemical pro-
cesses in homogeneous liquid solutions with the use of
preorganization into supramolecular ensembles,
including cucurbiturils host–guest complexes, has
become popular in recent years [5–7]. The cavitand
The above-described reaction between two dye a
molecules is an example of one of the most important
photochemical reactions, [2+2] photocycloaddition.
CB8 with a large enough (479 Å3 [1]) and relatively These reactions are quite extensively investigated due
rigid cavity is of a special interest. These features allow to their considerable practical applicability [10]. The
CB8 to selectively form host–guest complexes of the 2 studies of photochemical reactions in the cavity of var-
204

[2+2] PHOTOCYCLOADDITION OF STYRYL DYES
205
OMe
OMe
tions, the results of NMR study on the amount of
water in the cavity of dry CB8 were taken into account.
Styryl dyes a, b, and c, which structures are given in
Fig. 1, were synthesized according to the procedure
described elsewhere [9]. The dye concentration in all
samples was 10−5 mol/L. The CB8 concentration var-
ied from 0 to 9 × 10−5 mol/L. In order to avoid spon-
taneous photochemical reactions, all sample prepara-
tion operations and all measurements were carried out
in the dark at room temperature.
OMe
OMe
N+
Et
a
NH3+
b
N+
ClO–4
OMe
OMe
O
CH2
CH2
N
N
N
N
Absorption spectra were recorded in standard
quartz cells with an optical pathlength of 1 cm on a
Shimadzu UVmini 1240 spectrophotometer. Fluores-
cence spectra were recorded on a Fluorolog 3 Tau flu-
orimeter. The fluorescence spectra were measured at
the excitation wavelengths of 415 nm for a and c and
410 nm for b, because absorbance of all samples at
these wavelengths in corresponding titration series dif-
fers insignificantly, although these points are not isos-
bestic. Furthermore, these wavelengths are close to the
absorption maximums for all the samples. It should be
taken into account that photochemical reactions
actively proceed in the samples under the action of
light at 400 nm and the fluorescence measurement
itself affects the result (the fluorescence intensity at
maximum changes by 5% between two successive
measurements). In order to avoid the accumulation of
the measurement errors, a separate sample was pre-
pared for each point of the fluorescent titration. The
complexation constants were determined from the
–O3S
N+
n
c
O
CB[n]
R
OMe
OMe
N+
d
MeO
MeO
N+
R
Fig. 1. Structures of styryl dyes a, b, and c; cucurbit[8]uril;
and cyclobutane product of photochemical reaction d.
ious macrocyclic cavitands, such as cyclodextrins, results of fluorescent and spectrophotometric titra-
calixarenes, carbon nanotubes, and, of coarse, cucur- tions with the use of the program HypSpec of the
biturils, occupy significant part in the field [11]. In Hyperquad package [20].
particular, many examples of [2+2] photocycloaddi-
tion that efficiently proceeds in the cucurbit[8]uril
cavity can be found in the literature [12–17]. However,
the ultrafast kinetics of [2+2] photocycloaddition has
not yet been studied in sufficient detail. Hoyer et al.
[18] studied the dimerization of α-cyano-4-hydroxy-
cinnamic acid and sinapic acid in crystals by subpico-
second time-resolved fluorescent methods [18]. The
dimerization of α-cyano-4-hydroxycinnamic acid and
sinapic acid was observed on 10 ps and subpicosecond
timescales, respectively. Using picosecond X-ray dif-
fraction, Hallmann et al. [19] showed that α-sty-
rylpyridinium trifluoromethanesulfonate undergoes
ultrafast photoinduced transformation in crystalline
phase to give a dimeric product.
Photolysis of the samples was carried out as
described elsewhere [21]. The procedure of up-con-
version experiment on the measurement of ultrafast
fluorescence decay time is described in [22]. The
experimental data obtained on the up-conversion
setup were the time dependence of f luorescence inten-
sity at a given wavelength with femtosecond resolu-
tion. For more accurate description of the data, this
dependence can be described by a set of descending
and ascending exponential functions.
To obtain quantitative parameters of the exponen-
tials, the reconvolution of exponential functions and
the Gauss function that describes the excitation laser
pulse was used. The FluoFit (Picoquant) software was
used to reduce routine calculations.
The aim of this work is to study the ultrafast kinet-
ics of [2+2] photocycloaddition for a series of styryl
dyes in the cucurbit[8]uril cavity in aqueous solutions.
RESULTS AND DISCUSSION
Steady-State Study
Figure 2 shows absorption spectra for an aqueous
solution of styryl dye b in the presence of CB8. The
EXPERIMENTAL
Millipor Simplicity water was used for preparation main absorption maximum of the dye is at 388 nm. In
of solutions. Cucurbit[8]uril purchased from Sigma- the presence of CB8, a bathochromic shift of the
Aldrich was used without further purufication. In absorption maximum is observed, with the highest
weighing cucurbituril samples to prepare the solu- value of the shift being 15 nm and the intensity in the
HIGH ENERGY CHEMISTRY Vol. 53 No. 3 2019

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SVIRIDA et al.
0.3
0.2
0.1
0
300
350
400
450
500
250
Wavelength, nm
−5
Fig 2. Absorption spectra of aqueous solutions of dye b (10 mol/L) depending on the CB8 concentration in the range of
−5
[0; 9 × 10 ] mol/L.
maximum decreasing approximately by 10%. The dye molecules are bound into the 1 : 1 complexes, and
short-wavelength absorption maximum (257 nm) does the increase in the CB8 concentration does not change
not shift, but its intensity decreases also by 10%.
the fluorescence.
The changes in the absorption and fluorescence
spectra for dyes a and c in titration by CB8 are similar
to those for dye b. However, one can see in Fig. 3 that
the highest concentration of the 2 : 1 complexes for a
and c corresponds to the CB8 : dye molar ratios in the
solution of 0.5 and 1, respectively (Table 1). The fluo-
rescence intensity increases almost linearly with an
increase in the CB8 concentration in this range of
molar ratios and does not attain the limiting value for
dye c.
The complex formation constants calculated with
the use of HypSpec from the spectrophotometric and
fluorescent titration data in the approximation of the
absence of fluorescence for the 2 : 1 complex are listed
in Table 1.
The results of fluorescent titration in the form of
dependence of the fluorescence intensity at maximum
on the ratio of CB8 and dye molar concentrations are
given in Fig 3. The shape of the fluorescence spectrum
of dye b in the presence of CB8 almost does not
change over the entire range of CB8 concentration.
Only a small (5 nm) shift of the fluorescence maxi-
mum to shorter wavelengths is observed. The fluores-
cence intensity at maximum first decreases with
increasing cucurbituril concentration and then
increases, reaching a saturation value (Fig. 3b). The
decrease in the fluorescence intensity is observed at a
CB8 to dye b concentration ratio of n < 0.3 and corre-
sponds to the concentration quenching of the dye in
the cavity for 2 : 1 complexes [21]. The further increase
in the CB8 concentration shifts the equilibrium
To rum the photolysis, the initial ratio of dye and
toward the 1 : 1 complexes, for with the fluorescence CB8 concentrations was chosen so that the concentra-
rise is more significant as compared with the free dye. tions of the 2 : 1 complexes would be maximum
At a CB8 to b concentration ratio of n > 10, all of the (Table 1). Figure 4 shows the absorption spectra of a
Table 1. CB8 to dye molar ratio n corresponding to the highest amount of the 2 : 1 complexes; formation constants of 1 : 1
(logK1) and 2 : 1 complexes (logβ) for dyes a, b, and c with CB8; and the quantum yield of photochemical reaction (ϕ)
Dye
a
b
c
n
0.5
0.3
0.1
logK1
logβ
ϕ
6.2
11.9 (log K2 = 5.7)
0.06
6.3
12.6 (logK2 = 6.3)
0.02
5.4
11.2 (logK2 = 5.8)
0.04
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[2+2] PHOTOCYCLOADDITION OF STYRYL DYES
207
6 × 106
5 × 106
4 × 106
3 × 106
2 × 106
1 × 106
(a)
(b)
(с)
0
1
2
3
4
5
n
6
7
8
9
10
Fig. 3. Fluorescence intensity at maximum for aqueous solutions of styryl dyes and CB8 as a function of CB8 to dye molar ratio
n. The black, red, and blue symbols refer to dyes a, b, and c, respectively.
dye b sample irradiated in the photolysis setup for significantly exceed the dye concentration. However,
283 min. The intensity of the main absorption maxi- application of this model significantly complicates the
calculation of the complexation constant; therefore,
the model taking into account only 1 : 1 and 2 : 1 com-
plexes was used, probably having lowered the accuracy
of calculation. The above assumptions explain the
findings that the photolysis of dye b with CB8 is much
less effective than that of the other dyes and the calcu-
lated quantum yield of this reaction is significantly
lower. The sample with dye a has the highest quantum
yield of photochemical reaction (Table 1). The quan-
tum yield of the reaction for dye c is lower, which is
due the lower binding constant for the 2 : 1 complex.
mum (390 nm for b) is decreased by irradiation, but
the absorbance increases in the short-wavelength
spectral range (near 230 nm), which corresponds to
the absorption of the cyclobutane derivative formed in
the photochemical reaction. The absorption spectra of
dyes a and c behave similarly in photolysis. To calcu-
late the quantum yield of the photochemical reaction,
plots of the absorbance at maximum depending on the
photolysis time were built (Fig. 5). The distinguishing
feature of dye b is that the absorbance of the main band
does not disappear completely after irradiation for five
hours. This means that the photochemical reaction
does not occur after a certain point of time. However,
the absorbance of a and c disappears completely
during the photolysis, indicating high performance of
СВ8 as a supramolecular catalyst in the course of this
photochemical reaction. The quantum yields of pho-
tochemical reactions were calculated from the photol-
ysis data according the procedure described in [21]
(Table 1).
Time-Resolved Study
Ultrafast kinetics of fluorescence decay was stud-
ied for dyes a, b, and c and for their 2 : 1 complexes
with CB8. Figure 6 exemplifies fluorescence decay
curves on the timescale of 10 ps for free dye a and for
its 2 : 1 complex with CB8 at the highest concentration
of the 2 : 1 complexes, i.e., 0.5 equiv. of CB8. The sty-
ryl dyes under study have several characteristic decay
times in water (Table 2). A short time on the order of
1–2 ps is observed in the short-wavelength spectral
range. This is a characteristic time of the response of
the solvation shell on the charge redistribution in the
excited-state dye. Since water is the solvent for all the
samples, this exponential is similar for all the dyes. On
passing to the long-wavelength portion of the spec-
trum, the amplitude of this exponential decreases and
an ascending exponential with a time of 250–700 fs
appears in the right wing of the spectrum. The third
It can be assumed from the fluorescent titration
data that the formation of the 2 : 1 complexes of dye b
with cucurbit[8]uril is hindered as compared with dyes
a and c. This behavior of dye b is due to electrostatic
repulsion, since the molecule of b has the highest
charge (+2) among all the dyes under consideration.
At the same time, the binding of b with CB8 in the 1 : 1
complex, in contrast, is better than that for a and c
because of interaction with the carbonyl portals of
cucurbituril. Furthermore, dye b can form complexes
of other stoichiometry, for example 1 : 2; i.e., two CB8 exponential existing in all studied dyes is characterized
molecules per b molecule at CB8 concentrations that by a decay time of 20–75 ps depending on the sample.
HIGH ENERGY CHEMISTRY Vol. 53 No. 3 2019

208
SVIRIDA et al.
0.3
0.2
0.1
0
300
350
400
450
500
250
Wavelength, nm
−5
Fig. 4. Photolysis of dye b (10 mol/L) with CB8 at n = 0.3, i.e. at the minimum of the titration curve. Irradiation time is
283 min.
0.35
(a)
0.30
(b)
(с)
0.25
0.20
0.15
0.10
0.05
0
20 40 60 80 100120 140160180200220240260280300
Time, min
Fig. 5. Absorption intensity at maximum as a function of irradiation time. The black, red, and blue symbols respectively refer to
dyes a, b, and c.
Ultrafast exponents of 1–90 fs observed in the short- This implies that the charge and the structure of the
dye molecule have a significant effect on the photo-
chemical reaction of the dyes in the cucurbit[8]uril
cavity.
wavelength spectral range should also be noted, but
the experimental conditions and parameters of the
setup do not allow their exact characterization. The
addition of CB8 affects the third time; it decreases
twofold for dye a or by a factor of 1.5 for c and remains
almost unchanged for dye b. The addition of CB8 does
not affect the second time. The measurement results
correlate well with the photolysis data. The quantum
yield of photochemical reaction is the highest for dye a
among the dyes under consideration, and the highest
change of the second positive exponential in the fluo-
rescence decay is observed for this dye. The corre-
ACKNOWLEDGMENTS
The study was supported by the Russian Scientific
Foundation, project no. 14-13-00751, in part of time-
resolved measurements; by the Russian Foundation
for Basic Research, project no. 18-03-00214, in part of
the synthesis of styryl dyes; and by the Ministry of Sci-
ence and Higher Education under the State Program
sponding change is observed for the other dyes as well. of “Crystallography and Photonics” Federal Scien-
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[2+2] PHOTOCYCLOADDITION OF STYRYL DYES
209
4k
3k
2k
1k
a
a + CB8
0
3
2
4
6
8
Time, min
0
–3
Fig. 6. Fluorescence decay for dye a (red) and dye а in the 2 : 1 complex with CB8 (blue), λ 417 nm and λ 530 nm.
ex
fl
Table 2. Characteristic fluorescence decay times at 530 nm for dyes a, b, and c and their complexes with CB8. A1, A2, and
A3 are the amplitudes of corresponding components
Sample
T1, ps
A1
T2, ps
A2
T3, ps
A3
а
–0.18
–2650
1.43
1
4880
2200
3000
8800
2600
240
50
19
38
30
75
51
11000
2100
а + CB8
b
–0.32
–0.3
–3500
–13600
–1700
–184
2.1
1
17000
36000
7300
b + CB8
с
–0.28
–0.89
1.2
1
с + CB8
430
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Translated by T. Nekipelova
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