Photoprocesses in styryl dyes and their pseudorotaxane complexes with cucurbit[7]uril

Article abstract of DOI:10.1016/j.jphotochem.2012.12.023

The formation of inclusion complexes between styryl dyes containing a dimethylamino group or azacrown-ether residue and cucurbit[7]uril (CB[7]) in aqueous solution was studied by ¹H NMR spectroscopy and quantum chemistry. The complexes are characterized by 1:1 composition and the pseudorotaxane structure, which was confirmed by quantum chemical calculations. The free dyes and dye@CB[7] complexes exhibit both prompt fluorescence and an ability to intersystem crossing. The triplet-triplet absorption spectra and the triplet lifetime of the free dyes and dye@CB[7] were measured by ns-laser photolysis.

Full text of DOI:10.1016/j.jphotochem.2012.12.023

Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photoprocesses in styryl dyes and their pseudorotaxane complexes with
cucurbit[7]uril
Levon S. Atabekyan, Artem I. Vedernikov, Vitaly G. Avakyan, Natalia A. Lobova,
Sergey P. Gromov∗, Alexander K. Chibisov∗
Photochemistry Center, Russian Academy of Sciences, 7A-1 Novatorov str., 119421 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
The formation of inclusion complexes between styryl dyes containing a dimethylamino group or
azacrown-ether residue and cucurbit[7]uril (CB[7]) in aqueous solution was studied by 1H NMR
spectroscopy and quantum chemistry. The complexes are characterized by 1:1 composition and the
pseudorotaxane structure, which was confirmed by quantum chemical calculations. The free dyes and
dye@CB[7] complexes exhibit both prompt fluorescence and an ability to intersystem crossing. The
triplet–triplet absorption spectra and the triplet lifetime of the free dyes and dye@CB[7] were measured
by ns-laser photolysis.
Received 14 August 2012
Received in revised form
10 December 2012
Accepted 15 December 2012
Available online 6 January 2013
Keywords:
Styryl dyes
© 2013 Elsevier B.V. All rights reserved.
Cucurbit[7]uril
Inclusion complexes
Stability constants
Triplet states
Fluorescence
1. Introduction
[23]. It was shown that the fluorescence intensity of CSB is
enhanced with increasing of HP-␤-CD concentration that clearly
Styryl dyes of the general formula R Het+ CH CH Ar X− can
undergo E–Z isomerization, electrocyclization and [2 + 2] cycload-
dition photoreactions owing to the presence of ethylene bonds in
the dye molecules [1–5]. The photochemical properties of styryl
dyes were studied in a number of publications [6–11]; however,
several aspects of the mechanism of the primary photoreactions
remained unclear. Data on the short-lived intermediates are miss-
ing either. Lately, the complexation of dyes of various classes with
macroheterocyclic cavitands, cucurbit[n]urils (CB[n]), has been vig-
orously studied [12–14]. It was found [11,15–17] that the formation
of inclusion complexes of styryl dyes with CB[6] and CB[7] affects
the spectral and luminescence properties of the dyes, in particular,
results in substantial enhancement of fluorescence. The latter was
also observed in the formation of inclusion complexes of styryl dyes
with other cavitand molecules, for example, cyclodextrins [18–27].
In particular, we studied the influence of 2-hydroxypropyl-␤-
cyclodextrin (HP-␤-CD) on spectral and luminescent properties
of neutral crown-containing 2-styrylbenzothiazole (CSB) in water
demonstrates the formation of 1:1 inclusion complex CSB@HP-␤-
CD. The quantum yield of CSB fluorescence is five-times increased
in the presence of HP-␤-CD. The stoichiometry of inclusion com-
plex was found to be 1:1 for concentration of HP-␤-CD varied up to
0.004 M. The value of stability constant for this complex is changed
from logK = 3.64 to logK = 3.47 in the 10–40 ◦C temperature range
[23].
Recently we found that 4-pyridine styryl dyes with a dimethy-
lamino group ((E)-1) or an aza-18-crown-6-ether residue ((E)-2)
demonstrate an ability to intersystem crossing. Along with radia-
tionless decay of the triplet state the phosphorescence and delayed
fluorescence also make a contribution in the overall triplet decay. It
was pointed out that the vibration relaxation of the Franck–Condon
excited state resulted in the planar intramolecular charge trans-
fer (PICT). The following pathways of PICT deactivation are the
intersystem crossing, fluorescence and transition to the twisted
intramolecular charge transfer (TICT) [28]. To our best knowledge
no data are available in the literature on the influence of CB[n]
on the efficiency of intersystem crossing of styryl dye molecules.
Therefore, it appears important to study the effect of CB[n] on the
primary photoprocesses in styryl dye complexes Scheme 1.
In this paper, we present the results on a study of the photo-
physical properties of dyes (E)-1 and (E)-2 and their complexes
∗
Corresponding authors. Tel.: +7 495 9361204; fax: +7 495 9361255.
E-mail addresses: alexander.chibisov@gmail.com, chibisov@photonics.ru
(A.K. Chibisov).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.12.023

L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
53
_
ClO4
NMe2
+
Et
N
(E)-1
O
O
O
O
N
O
Me
Me
N
O
1. TsOEt
_
2. HClO4, EtOH
Ppy, MeOH
N
Et
3
+
ClO4
O
O
O
O
2'
3'
5'
_
2
6
3
5
b
ClO4
Et
N
+
N
a
6'
O
(E)-2
Scheme 1.
with CB[7] in aqueous solutions. The principal result of this work is
that complexes are characterized by 1:1 composition and the pseu-
dorotaxane structure which is confirmed by quantum chemical
calculations. The complexes dye@CB[7] show both prompt fluores-
cence and an ability to intersystem crossing and the triplet lifetime
of dye@CB[7] is close to that of free dye.
2.1.2. 1-Ethyl-4-{(E)-2-[4-(1,4,7,10,13-pentaoxa-16-
azacyclooctadecan-16-yl)phenyl]vinyl}pyridinium perchlorate
((E)-2)
A solution of a mixture of compound 3 (181 mg, 0.82 mmol), N-
(4-formylphenyl)aza-18-crown-6 ether (330 mg, 0.90 mmol) and
piperidine (40 ␮L) in MeOH (10 mL) was heated at 65 ◦C (oil bath)
for 20 h and then cooled to −10 ◦C. A dark-red glassy solid was sep-
arated by decantation, washed with hot benzene (3 × 20 mL), and
dried in vacuo. The yield of (E)-2 was 296 mg (63%), m.p. 64–66 ◦C.
1H NMR (500 MHz, DMSO-d6) 30 ◦C, ı: 1.50 (t, 3H, J = 7.3 Hz, Me);
3.53 (s, 8H, 4CH2O); 3.54 (m, 4H, 2CH2O); 3.55 (m, 4H, 2CH2O); 3.63
(m, 8H, (CH2)2N, 2CH2O); 4.44 (q, 2H, J = 7.3 Hz, CH2Me); 6.79 (d,
2H, J = 8.9 Hz, 3ꢀ-H, 5ꢀ-H); 7.14 (d, 1H, J = 16.2 Hz, CH = CHPy); 7.57
(d, 2H, J = 8.9 Hz, 2ꢀ-H, 6ꢀ-H); 7.89 (d, 1H, J = 16.2 Hz, CH = CHPy); 8.04
(d, 2H, J = 6.9 Hz, 3-H, 5-H); 8.76 (d, 2H, J = 6.9 Hz, 2-H, 6-H). 13C NMR
(125 MHz, DMSO-d6) 30 ◦C, ı: 15.96 (Me); 50.57 ((CH2)2N); 54.54
(CH2N); 67.74 ((OCH2CH2)2N); 69.82 (2CH2O); 69.89 (2CH2O);
69.95 (4CH2O); 111.57 (3ꢀ-C, 5ꢀ-C); 116.83 (CH = CHPy); 122.21
(1ꢀ-C); 122.28 (3-C, 5-C); 130.25 (2ꢀ-C, 6ꢀ-C); 141.83 (CH = CHPy);
143.07 (2-C, 6-C); 149.83 (4ꢀ-C); 153.57 (4-C). Anal. calcd. for
2. Experimental
2.1. Materials
The synthesis of dye (E)-1 was described earlier [4]. Ethyl
p-toluenesulfonate, 70% HClO4 (aq), N-(4-formylphenyl)aza-18-
crown-6 ether, CB[7]·13H2O, and fluorescein were used as received
(Aldrich). Distilled water (HPLC grade, Aldrich), acetonitrile (extra
high purity, Cryochrom), and Li2CO3 (Merck) were used to prepare
solutions.
C
27H39ClN2O9: C, 56.79; H, 6.88; N, 4.91; found: C, 56.71; H, 6.87;
2.1.1. 1-Ethyl-4-methylpyridinium perchlorate (3)
N, 4.88%.
A mixture of 4-picoline (1.0 mL, 10.3 mmol) and ethyl p-
toluenesulfonate (3.07 g, 15.4 mmol) was heated at 120 ◦C (oil bath)
for 1 h. The resulting mass was dissolved in hot abs. EtOH (5 mL),
and 70% HClO4 (aq) (1.77 mL, 20.5 mmol) was added to the etha-
nolic solution. The mixture was cooled to −10 ◦C and a yellowish
glassy precipitate was separated by decantation, washed with hot
benzene (3 × 20 mL), and re-crystallized from abs. EtOH. The yield
of 3 was 0.68 g (30%), m.p. 56–58 ◦C.
2.2. Methods
Melting points were measured on a Mel-Temp II instru-
ment. Elemental analyses were carried out at the Microanalytical
Laboratory of the A.N. Nesmeyanov Institute of Organoelement
Compounds (Russian Academy of Sciences, Moscow).
1H NMR (500 MHz, DMSO-d6) 30 ◦C, ı: 1.51 (t, 3H, J = 7.5 Hz,
MeCH2); 2.60 (s, 3H, Me); 4.55 (q, 2H, J = 7.5 Hz, CH2N); 7.98 (d,
2H, J = 6.6 Hz, 3-H, 5-H); 8.91 (d, 2H, J = 6.6 Hz, 2-H, 6-H). 13C NMR
(125 MHz, DMSO-d6) 25 ◦C, ı: 16.09 (MeCH2); 21.21 (Me); 55.46
(CH2N); 128.27 (3-C, 5-C); 143.36 (2-C, 6-C); 158.61 (4-C). Anal.
calcd. for C8H12ClNO4: C, 43.35; H, 5.46; N, 6.32; found: C, 43.32;
H, 5.50; N, 6.25%.
2.2.1. NMR spectroscopy measurements and titration
The 1H and 13C NMR spectra were recorded on a Bruker DRX500
spectrometer in DMSO-d6 and a D2O–MeCN-d3 mixture (10:1, v/v)
at 25–30 ◦C with the solvent or the HOD signal as the internal
standard (ıH 2.50 or 4.70, respectively; ıC 39.43 for DMSO-d6).
In 1H NMR titration, the compositions and stability constants of

54
L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
the complexes of dyes (E)-1, (E)-2 with CB[7] were determined
by analyzing the changes in the positions of the H signals (ꢀıH)
of the dye depending on the concentration ratio of CB[7] and the
dye. The CB[7] concentration was varied in the range from 0 to
1 × 10−3 mol L−1, while the overall concentration of dye did not
change remaining equal to ∼5 × 10−4 mol L−1. The ꢀıH values were
measured to an accuracy of 0.001 ppm with correction for MeCN-d2
signal shift. The stability constants of the complexes were calcu-
lated using the HYPNMR program [29].
2.2.2. Quantum chemical calculations
The quantum chemical calculations of the structures of CB[7]
and dyes 1, 2 and the structures and the formation enthalpies of
complexes dye@CB[7] were carried out by DFT with the exchange-
correlation PBE functional [30] in a three-exponent basis set [31]
by a reported program [32] and with zero-point energy (ZPE) cor-
rection.
Fig. 1. 1H NMR spectra (aromatic proton region) of (a) free dye (E)-2 and (b) 1:1.5
mixture of (E)-2 and CB[7] (C2 = 5 × 10−4 mol L−1) in a D2O–MeCN-d3 mixture (10:1,
v/v) at 25 ◦C.
2.2.3. Optical spectroscopy measurements and titrations
The spectra of triplet–triplet absorption and the kinetics of
triplet decay of dyes (E)-1, (E)-2 and the complexes with CB[7] were
measured on a nanosecond laser photolysis setup [10]. The irra-
diation was performed with the Nd:YAG laser (Solar, ꢁ = 355 nm,
E ≤ 70 mJ, 15 ns) in quartz cells 1 × 1 × 4 cm3. The dissolved oxygen
was removed by bubbling with nitrogen. The absorption spec-
tra of dyes (E)-1, (E)-2 and their complexes with CB[7] in the
ground state were recorded on an Agilent 8453 as well as a Cary
4000 spectrophotometers. Fluorescence emission and fluorescence
excitation spectra were measured on a Varian Eclipse spectrofluo-
rimeter. All the spectra were recorded in deionized water at room
temperature. The fluorescence quantum yield was measured by
comparison [33] with the standard (fluorescein). The concentra-
tions of (E)-1 and (E)-2 were (0.4–3) × 10−5 mol L−1, and the CB[7]
molecule in the complex is mainly arranged above the positively
charged area of the dye molecule. With increasing distance from
the center of the Py+ CH CH moiety, the dye protons become less
shielded, and the signals of the benzene ring shift downfield, prob-
ably due to the deshielding influence of the carbonyl groups of the
host molecule portals. The signals of the most remote protons of the
azacrown-ether residue are little sensitive to complexation. This
pattern of variation of the magnitude and sign of ꢀıH attests to the
pseudorotaxane structure of complex (E)-2@CB[7] where one end
of the guest molecule is located inside the CB[7] cavity, while the
other end is outside the cavity. The changes in the 1H NMR spectra
of dyes (E)-1 and (E)-2 following the complex formation with CB[7]
proved to be similar. Complex (E)-1@CB[7] has a structure similar
to that of complex (E)-2@CB[7]; however, substantial line broad-
ening is observed in the spectra due to the slower exchange on the
1H NMR time scale as compared with the (E)-2/CB[7] system.
The stability constants of complexes dye@CB[7] were deter-
mined by 1H NMR titration. The dependence of ꢀıH of the dye
protons on the change in the cucurbituril and dye concentration
ratio was described by a model that took into account a single
equilibrium:
concentration was (0.4–2) × 10−4 mol L−1
.
The spectrophotometric and fluorescent titrations were per-
formed in water–MeCN (10:1, v/v) containing 0.01 M Li2CO3 at
room temperature. The compositions and stability constants of the
complexes of dyes (E)-1, (E)-2 with CB[7] were determined by ana-
lyzing the changes in the absorption and fluorescence spectra of
the dye depending on the concentration ratio of CB[7] and the dye.
In spectrophotometric titration, the CB[7] concentration was var-
ied in the range from 0 to 1.3 × 10−4 mol L−1, while the overall
concentration of dye was 2 × 10−5 mol L−1. In fluorescent titra-
tion, the CB[7] concentration was changed in the range from 0 to
1.8 × 10−4 mol L−1, and the overall concentration of dye remained
equal to 1 × 10−5 mol L−1. The stoichiometry and stability constants
of complexes were calculated using HypSpec software (Hyperquad)
[34].
K
dye + CB[7]ꢀdye@CB[7]
where K is the stability constant of complex dye@CB[7] [L mol−1].
The stability constants of complexes (E)-1@CB[7] and (E)-
2@CB[7] turned out to be similar: logK = 4.3 0.1 and 4.4 0.1,
respectively. Previously [15,16], by the example of a styryl dye with
two methoxy groups in the benzene ring, it was shown that replace-
ment of a D2O–MeCN-d3 mixture (10:1, v/v) by water increases the
stability constant of the dye complex with CB[7] by 1.9 logarithmic
units. Probably, the increase in the stability constant of the complex
is caused by increase in the polarity of the medium on passing from
aqueous acetonitrile to neat water, which promotes the location of
the hydrophobic guest molecule in the cavitand cavity. Presumably,
the stability of complexes (E)-1@CB[7] and (E)-2@CB[7] in water
will also be 1–2 orders of magnitude higher that their stability in a
water–acetonitrile mixture (10:1, v/v).
3. Results and discussion
3.1. 1H NMR spectroscopy studies
A
1H NMR study of the complexation of dyes (E)-1, (E)-2 with
CB[7] provides data on the structures and thermodynamic stabil-
ity of supramolecular complexes. To increase the dye solubility to
the millimolar range, a D2O–MeCN-d3 mixture (10:1, v/v) was used
[15]. Fig. 1 shows the 1H NMR spectra of free dye (E)-2 and its mix-
ture with excess CB[7]. It can be seen that signals of the (E)-2/CB[7]
mixture differ significantly in the direction and the magnitude of
their shifts relative to similar signals of free (E)-2. Thus the sig-
nals of the N-ethylpyridinium residue and the ethylene bond shift
upfield (ꢀıH of up to −0.89 ppm), while the signals of the benzene
ring protons, conversely, shift downfield (ꢀıH of up to 0.25 ppm).
This means that the CB[7] molecule shields to the highest extent
the protons of the Et Py+ CH CH moiety and, thus, the cavitand
3.2. Quantum chemical simulation
The quantum chemical calculations of the structures 1 and 2 in
the ground state indicate the possible existence of two dye isomers,
namely E and Z forms. Fig. 2 shows the calculated structures and

L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
55
Fig. 2. Structures of (a) E and (b) Z isomers of dye 1 calculated by DFT/PBE method.
˚
respectively, i.e., they changed little as compared with ꢀE0 for (E)-
1@CB[7] and (E)-2@CB[7]. The decrease in ꢀE0 by 5.6 kcal mol−1 for
2@CB[7] on going from (E)-2 to (Z)-2 is apparently due to its more
pronounced steric repulsion from the cavitand molecule caused by
more bulky crown-ether residue. This is consistent with the fact
that (Z)-1 is inserted much more deeply than (Z)-2 into the CB[7]
the bond lengths (in A) for (E)-1 and (Z)-1. Nearly the same bond
lengths were found for the chromophore parts of (E)-2 and (Z)-2,
which are therefore not given here.
The Z forms of both dyes are non-planar due to the proximity of
the hydrogen atoms of the pyridine and benzene rings of the dyes
(see Fig. 2b). The local energy minima characterized by the absence
of imaginary frequencies correspond to structures (Z)-1 and (Z)-
˚
˚
cavity. The ꢀl value for the former was 1.78 A (∼0.2 A shorter than
2 with the dihedral angles ꢂCC
of 19.1 and 19.6◦, respectively.
˚
˚
ꢀl(E)-1), whereas for the latter ꢀl was 4.02 A (more than ∼2.0 A
longer than ꢀl(E)-2).
CC
Furthermore, the E forms of dyes 1 and 2 are thermodynamically
more favorable than the Z forms by ꢀH = 8.3 and 8.2 kcal mol−1
,
respectively.
3.3. Absorption and fluorescence spectra
Quantum chemical calculations of the structure and the energy
of formation of complexes 1@CB[7] and 2@CB[7] were carried out.
Each dye and CB[7] molecules were located in three different start-
ing positions. The following dye moieties were placed inside the
host cavity: (a) pyridinium residue, (b) aniline residue, (c) cen-
tral ethylene bond. The starting structures of the complexes were
obtained by the semiempirical PM3 method with standard param-
eters [35]. The calculation showed that the optimal structure of
complexes dye@CB[7] corresponded to the structures presented in
Fig. 3.
The measurements of the absorption spectra revealed two peaks
at 265 and 450 nm in the spectrum of (E)-1 in water (Fig. 4). The
absorption spectrum of (E)-2 nearly coincides with the spectrum
of (E)-1. Dyes 1 and 2 exhibit weak fluorescence at ꢁmax ∼ 600 nm
(Fig. 5).
The fluorescence excitation spectrum of 1 (ꢁmax = 480 nm) dif-
fers from the absorption spectrum (ꢁmax = 450 nm); the same is true
for 2. The difference between the absorption and excitation spectra
can be attributed to the presence of two dye isomers one of which
does not fluoresce. Indeed, the quantum chemical calculations of
structures 1 and 2 indicate the existence of two isomeric forms of
the dye, E and Z forms (see Fig. 2).
The low fluorescence intensity is assumed to be due to the for-
mation of a twisted intramolecular charge transfer (TICT) state
upon photoexcitation, similar to that formed for styryl dyes
whose structures resemble 1 [17,36–39]. A common feature of
the molecules giving rise to the TICT state is the presence of
electron-donating and electron-withdrawing groups and a conju-
gation chain, which facilitates the shift of electron density from
the donor to the acceptor upon photoexcitation [39]. The aniline
nitrogen atom usually serves as the electron donor whereas nitrile
and ester groups or heterocyclic residues as electron acceptors. The
structure of 1 fully meets these conditions. The high formation rate
The absence of imaginary frequencies confirmed that the struc-
tures found by calculations corresponded to energy minima. It
can be seen from Fig. 3 that in both complexes, the vinylpyri-
dinium moiety of the dye is located inside the cavity. This result
is in good agreement with 1H NMR data and with the view that
cucurbituril hosts form more stable inclusion complexes with pos-
itively charged molecules than with related neutral compounds.
The energies of insertion of (E)-1 and (E)-2 into the CB[7] cavity,
ꢀE0, turned out to be similar (−39.5 and −39.0 kcal mol−1), as was
to be expected, because both dyes are inserted in the cavitand with
the same moiety. The distances between the C C bond midpoint
˚
and the center of gravity of CB[7], ꢀl, were 1.99 and 1.96 A for
(E)-1@CB[7] and (E)-2@CB[7], respectively. The insertion energies
for (Z)-1@CB[7] and (Z)-2@CB[7] were −37.7 and −32.1 kcal mol−1
,

56
L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
Fig. 3. DFT-calculated structures of complexes (a) (E)-1@CB[7] (ꢀE0 = −39.5 kcal mol−1) and (b) (E)-2@CB[7] (ꢀE0 = −39.0 kcal mol−1).
of the TICT state results in a decrease in the photostationary con-
centration of the singlet excited E isomers and, as a consequence,
a decrease in the fluorescence. An increase in the fluorescence of 2
can be the reason of lower efficiency of the formation of the TICT
state as compared with 1.
Note the abnormally great Stokes shift of the fluorescence spec-
trum (130 nm) (Fig. 5) observed for 1 and 2. This type of shift noted
previously [6,17,40,41] for similar compounds may be due to the
strong solvatochromic effect. The fluorescence and fluorescence
excitation spectra for 1 and 2 resemble each other in pairs, as was
to be expected since excitation occurs in the chromophore part of
the molecule (Table 1).
The formation of complexes dye@CB[7] induces changes in the
absorption spectra with respect to those of free dyes. System
(E)-1/CB[7] typically shows a bathochromic shift of the long-
wavelength absorption band and a new band at 330 nm (Fig. 4).
The spectral changes in the case of (E)-1/CB[7] mixture were
more pronounced than those of (E)-2/CB[7]. The absorption with
a maximum at 330 nm is due to the complex formed by the pro-
tonated form (dye·H)+@CB[7]. This is supported by the absorption
spectra of (E)-1 recorded in the presence of HClO4 (see inset in
Fig. 4).
The absorption and fluorescence spectra of (E)-2 measured
in the presence and in the absence of barium perchlorate are

L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
57
Fig. 6. Fluorescence spectra of dye 2 (C2 = 4 × 10−6 mol L−1): (1) free dye and (2) in
the presence of CB[7] (CCB[7] = 2 × 10−4 mol L−1) in aqueous solution.
Fig. 4. Absorption spectra of dye (E)-1 (C1 = 2 × 10−5 mol L−1) in the absence (1)
and presence (2) of CB[7] (CCB[7] = 2 × 10−4 mol L−1) in aqueous solution. Inset: the
absorption spectrum of dye (E)-1 in the presence of HClO4 (pH = 2).
Fig. 7. Absorption spectra of dye (E)-2 (C = 2 × 10−5 mol L−1) in the mixture of
water–MeCN (10:1, v/v) with 0.01 M Li2CO3 measured for different concentration
of CB[7] (0–1.25 × 10−4 mol L−1): dotted curve-evaluated spectrum of (E)-2@CB[7].
Fig. 5. Fluorescence excitation (1 and 3) and fluorescence emission (2 and 4) spectra
of dye 1 (1 and 2) and dye 2 (3 and 4) (Cdye = 4 × 10−6 mol L−1) in aqueous solution.
Fig. 8. Fluorescence spectra of dye 2 (C = 1 × 10−5 mol L−1) in the mixture of water–MeCN (10:1, v/v) with 0.01 M Li2CO3 measured for different concentration of CB[7]
(0–1.80 × 10−4 mol L−1); curve 1-fluorescence spectrum of dye 2, curve 2-evaluated fluorescence spectrum of 2@CB[7].

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L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
identical, which means that no complex (E)-2@Ba(ClO4)2 is formed
in aqueous solution.
It is noteworthy that in the presence of CB[7], the long-
wavelength absorption maximum of (E)-1 and (E)-2 coincides with
the maximum of the fluorescence excitation spectrum measured
for both dyes in the absence of CB[7] (Fig. 5). Apparently, the for-
mation of the host–guest complex stabilizes the planar E isomer
of the dye, which is responsible for fluorescence. Fig. 6 shows the
fluorescence spectra of 2 and its mixture with CB[7], and Table 1
summarizes the fluorescence and fluorescence excitation maxima
and the fluorescence quantum yields measured for 1, 2, and their
mixtures with CB[7].
Mixtures 1/CB[7] and 2/CB[7] show a 8- and 30-fold increase
in the fluorescence intensity, respectively, as compared with free
dyes (Fig. 6, Table 1), and as also was found for dyes of various types
[11,16,42–47]. Previously it has been shown that the inclusion com-
plex of CSB with HP-␤-CD demonstrated a five-fold increase of
fluorescence, however, the stability constant appeared to be notice-
ably smaller (logK = 3.5) [23]. One might suppose that the using of
CB[7] as a host molecule results in the formation of more strong
complex for dyes with the structure close to dyes (E)-1 and (E)-2.
Thus, the formation of the TICT state, which competes with the
radiative deactivation of the excited state of 1 and 2, is considerably
hindered in the presence of CB[7] due to stabilization of the pla-
nar conformation of the (E)-dye in the host cavity, and this results
in considerable increase in the fluorescence quantum yield of the
complexes.
Fig. 10. Transient absorption spectra of dye 1 (C1 = 3 × 10−5 mol L−1) in aqueous
solution at 30 ␮s after the laser pulse in the presence of HClO4 at (1) pH = 2 and
(2) pH = 4.2.
fluorescent titrations in a mixture of water–MeCN (10:1, v/v). In
the course of titration, the absorbance was decreased at 455 nm
and increased at 332 nm with increasing of CB[7] concentration.
Evidently the remaining amount of acid in CB[7] promotes the pro-
tonation of the dye molecule at aniline nitrogen atom that makes
difficult to determine the stability constant. In order to exclude the
processes caused by the protonation of dyes 1 and 2 the titrations
were performed in the presence of 0.01 M Li2CO3. In the course of
spectrophotometric titration, a bathochromic shift of the maximum
of the long-wavelength absorption band of dye and isosbestic point
were observed (see, for example, Fig. 7). With increase of CCB[7]/Cdye
ratio a strong enhancement of fluorescence was found (Fig. 8) along
3.4. Spectrophotometric and fluorescent titrations
The stability constants of complexes (E)-1@CB[7] and (E)-
2@CB[7] were also determined by spectrophotometric and
Fig. 9. Transient absorption spectra of (a and b) 1 and (c and d) 2 (Cdye = 2 × 10−5 mol L−1) in aqueous solution in the absence (a and c) and presence (b and d) of CB[7]
(CCB[7] = 2 × 10−4 mol L−1) at different time (s) after the laser pulse.

L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
59
Table 1
Spectral properties of dyes (E)-1, (E)-2 and their mixtures with excess CB[7] in water.
abs
fl.ex
max
Compound
ꢁ
(nm)
ꢁ
(nm)
ꢁflmax (nm)
ϕfl
max
(E)-1
(E)-2
445
455
475
485
475
490
485
490
600
590
595
585
0.005
0.01
0.04
0.3
(E)-1/CB[7]a
(E)-2/CB[7]a
a
CCB[7]/Cdye is 10 for absorption spectra and 50 for fluorescent spectra.
with blue shift of a maximum which reached 7 and 16 nm for dyes
1 and 2, respectively.
The HypSpec program was used to analyze dependencies of
measured absorption and fluorescence spectra upon the changes in
Fig. 12. Kinetics of transient absorption of dye 1 (C1 = 2 × 10−5 mol L−1) (1) in
air-saturated and (2) oxygen-free aqueous solution in the presence of CB[7]
(CCB[7] = 2 × 10−4 mol L−1) at (a) 440, (b) 500, and (c) 670 nm.
concentration of CB[7] with the following evaluation of the spectra
and stability constants of complexes dye@CB[7]. The standard devi-
ation, ꢃD or ꢃI, was used as the criteria of the validity. The results
are compiled in Table 2.
In all cases the value ꢃD is lower than 0.001 (in absorbance
units) and ꢃI is lower than 0.5% of observed maximal fluorescence
intensity (Imax), which indicates that the 1:1 complexation model
assumed for dyes (E)-1 and (E)-2 is valid. The values of stability
constants measured with both methods are similar for dyes 1 and
2 (logK = 3.94–4.11). A moderate decrease in stability constant val-
ues with respect to that found from 1H NMR titration (logK = 4.3 and
4.4) is probably due to different conditions of the titration experi-
ments. The main differences are the use of a buffer (0.01 M Li2CO3)
Fig. 11. Kinetics of transient absorption of dye
2
(C2 = 2 × 10−5 mol L−1
) in
oxygen-free aqueous solution (1) in the absence and (2) presence of CB[7]
(CCB[7] = 2 × 10−4 mol L−1) upon laser pulse at (a) 430, (b) 530 and (c) 670 nm.

60
L.S. Atabekyan et al. / Journal of Photochemistry and Photobiology A: Chemistry 253 (2013) 52–61
Table 2
Spectral properties of dyes (E)-1, (E)-2, complexes (E)-1@CB[7], (E)-2@CB[7] and stability constants.a
Spectrophotometric titration
Compound
Dye
ꢁmax (nm)
Dye@CB[7]
ꢁmax (nm)
εmax (L mol−1 cm−1
)
εmax, (L mol−1 cm−1
)
ꢀꢁmax (nm)b
LogKc
ꢃD
d
(E)-1
(E)-2
455
459
27100
35700
476
484
23400
34500
21
25
4.02
3.94
0.0003
0.0003
Fluorescent titratione
Compound
Dye
Dye@CB[7]
Imax, a.u.
156.06
ꢁflmax (nm)
604
Imax, a.u.
5.64
ꢁflmax (nm)
597
585
Qf
25.0
23.4
LogKc
3.94
4.11
ꢃI
d
(E)-1
(E)-2
0.43
1.51
601
26.71
676.74
a
0.01 M Li2CO3 in the mixture of water–MeCN (10:1, v/v) at room temperature.
b
c
d
e
f
ꢀꢁmax = ꢁmax (complex) − ꢁmax(free dye).
K = [dye@CB[7]]/{[dye]·[CB[7]]} (L mol−1); the error of the stability constant determination is 20%.
Standard deviation (in absorbance units for ꢃD and in arbitrary units for ꢃI) for modeling the absorption and fluorescence spectra, respectively.
Excitation of fluorescence is at 480 nm.
The ratio of values of integrated fluorescence intensity of dye@CB[7] and free dye in the range of 500–700 nm.
and lower concentration of reactants (10−5 to 10−4 mol L−1),
as well as the use of common solvents instead of deuterated
ones.
the efficiency of the quenching of the triplet state by oxygen is not
influenced by the presence of CB[7].
4. Conclusions
3.5. Laser pulse photolysis
The formation of host–guest pseudorotaxane complexes of
styryl dyes having dimethylamino group or azacrown-ether
residue with CB[7] was found to reduce the conformational mobil-
ity of the dye molecule and to result in its relative isolation by the
cavitand molecule. In turn, this induces an increase in both the dye
luminescence and the probability of the intersystem crossing.
The laser excitation of solutions of dyes 1 and 2 and their com-
plexes with CB[7] induces short-term reversible changes in the
absorption spectra, which are shown in Fig. 9. It follows from these
data that the difference spectra measured in the absence and in
the presence of CB[7] are similar. The differences are in the mag-
nitude of the optical density change (ꢀD) and in the slight shift
in the absorption maxima. In the difference absorption spectra,
three characteristic wavelengths can be distinguished: 440, 500
and 670 nm.
The absorption at 430–440 nm may be attributed to E–Z pho-
toisomerization, the dye triplet and protonation of the dye. The
addition of CB[7] to an aqueous solution is known to decrease the
pH [48]. Measurement of the acidity of a solution of (E)-1/CB[7]
mixture at CCB[7] = 2 × 10−4 mol L−1 showed the pH = 4.2, whereas
for a solution of (E)-1 this value is 6.1. The wide band at 600–800 nm
is caused by the triplet–triplet (T–T) absorption of the dye. The
effect of pH on the difference absorption spectrum of the dye
measured upon laser excitation was studied. Fig. 10 shows the dif-
ference absorption spectra of 1 in the presence of perchloric acid. A
decrease in the pH value entails an increase in ꢀD at 400–450 nm
and a decrease in ꢀD at 600–700 nm. That is, the formation of the
protonated form of the dye reduces the probability of intersystem
crossing.
Fig. 11 shows the kinetic curves for the oxygen-free solution of 2
and 2@CB[7]. The curves in Fig. 11a and c demonstrate fast growth
of ꢀD at 430 and 670 nm for triplet 2@CB[7] molecules as compared
with free 2; Fig. 11b shows a bleaching at 530 nm. The lifetimes, ꢂT,
of the triplet of dye 1 and complex 1@CB[7] are 110 and 140 ␮s,
respectively, and are about equal to ꢂT for 2 and 2@CB[7].
From the analysis of kinetic curves it follows an increase in
the efficiency of the intersystem crossing of the dye in the pres-
ence of CB[7]. The kinetic curves in Fig. 12 demonstrates the effect
of oxygen on the photochemical reactions that occur upon laser
excitation of the complex (E)-1@CB[7]. In the presence of oxy-
gen, a relatively long-lived intermediate is formed which can be
attributed to the Z isomer. Thus, isomerization occurs in the excited
singlet state of the dye. In the absence of oxygen, T–T absorp-
tion of the dye takes place and, hence, the yield of the Z isomer
decreases.
Acknowledgements
This work was supported by the Russian Foundation for Basic
Research (Project Nos. 12-03-00107 and 12-03-00491) and the Pre-
sidium of the Russian Academy of Sciences. The structural files for
quantum chemical calculations were prepared using the program
modules developed at the Photochemistry Center of the Russian
Academy of Sciences within the State Contract No. 02.523.11.3014.
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