Molecular rotors based on styryl dyes. Viscosity dependence of rotation of molecular fragments

Article abstract of DOI:10.1007/s11172-014-0660-1

Three donor-acceptor styryl dyes R-Het⁺-CH=CH-C₆H₄-NR'R′ClO₄ ^- (Het is pyridyl) were synthesized. Their spectral-luminescence behavior allows their assignment to a class of molecular rotors. The influ

Full text of DOI:10.1007/s11172-014-0660-1

Russian Chemical Bulletin, International Edition, Vol. 63, No. 8, pp. 1728—1733, August, 2014
1728
Molecular rotors based on styryl dyes.
Viscosity dependence of rotation of molecular fragments
V. V. Volchkov,a M. N. Khimich,a L. D. Uzhinova,a B. M. Uzhinov,a† M. Ya. Mel´nikov,a
S. P. Gromov,a,b A. I. Vedernikov,b S. K. Sazonov,b and M. V. Alfimovb
aChemistry Department, M. V. Lomonosov Moscow State University,
Building 3, 1 Leninskie Gory, 119991 Moscow, Russian Federation.
Fax: +7 (495) 932 8846. Eꢀmail: volchkov_vv@mail.ru
bPhotochemistry Center, Russian Academy of Sciences,
7Aꢀ1 ul. Novatorov, 119421 Moscow, Russian Federation.
Fax: +7 (495) 936 1255. Eꢀmail: spgromov@mail.ru
–
Three donor—acceptor styryl dyes R—Het+—CH=CH—C6H4—NR´RClO4 (Het is
pyridyl) were synthesized. Their spectralꢀluminescence behavior allows their assignment to
a class of molecular rotors. The influence of the viscosity, polarity, and temperature of the
medium on their absorption and fluorescence properties was studied. A pronounced enhanceꢀ
ment of dye fluorescence accompanied by a shortꢀwavelength shift of the fluorescence maxiꢀ
mum is observed with an increase in the viscosity of the medium and temperature decrease. The
measurements of the fluorescence spectra of the dyes in the poly(methyl methacrylate) films at
293 and 77 K confirmed the effect of the medium viscosity on the rotation ability of molecular
fragments upon photoexcitation. According to the quantum chemical calculations, in excited
molecules of the styryl dyes in nonpolar solvents, molecular fragments rotate mainly about the
...
central ethylene bond (HC—CH). In polar solvents, the rotation barriers around the ordinary
bonds of the ethylene fragment decrease.
Key words: styryl dyes, fluorescence, viscosity, molecular rotors, quantum chemical calcuꢀ
lations; MCSCF, MCQDPT, and PCM methods.
Serious interest in the preparation of fluorophores,
which are molecular rotors, is due to the dependence of
their fluorescence ability on the viscosity of the environꢀ
ment with the prospect of their use as sensors of local
viscosity in chemical and biological systems. Molecular
rotors are applied as sensors of local viscosity in chemical
systems and living cells,1 viscous flow of fluids,2 dynamic
processes accompanying polymerization,3 and conformaꢀ
tional changes in protein molecules.4
The question about bond determination in a styryl dye
molecule about which the excited fragments rotate remain
unanswered. Molecular rotors based on the aniline fragꢀ
ment differ from the julolidine molecular rotors by a greater
number of bonds about which rotation and formation of
TICT states (TICT is twisted intermolecular charge transꢀ
fer) with different energies are possible. The possibility of
rotation about three carbon—carbon bonds of the ethylꢀ
ene fragment of the styryl dye (two ordinary bonds and
one double bond) has previously been assumed.10 However,
it was revealed later that rotation about the double bond is
impossible, and the formation of the TICT state was exꢀ
plained by the mutual turn of the fragments about one of
the ordinary bonds.11
Problems of the viscosity dependence of internal rotaꢀ
tion in molecular rotors are widely discussed in literature.
Many molecular rotors are classified as TICT molecules
in which the efficiency of intramolecular charge photoꢀ
transfer (i.e., LE (locally excited)  CT (charge transfer))
depends substantially on the environment parameters.
A decrease in the intensity ratio of the longꢀ and shortꢀ
wavelength fluorescence bands (ICT/ILE) of 4ꢀ[(N,Nꢀ
dimethylamino)benzoyl]oxydodecyl methacrylate is symꢀ
Diverse classes of molecular rotors are presently known.5
A common feature of molecular rotors is the presence of
electronꢀdonor and electronꢀacceptor groups and the conꢀ
jugation chain between them in the structure of the
molecule. These properties are characteristic, in particuꢀ
lar, of styryl dyes R1—Het+—CH=CH—Ar—NR2 X–.
2
Compounds of this class were successfully applied6 as moꢀ
lecular rotors for the determination of local viscosity in
micelles of triblockcopolymer Pluronic F 127. Styryl dyes
with a long hydrocarbon Nꢀsubstituent in the heterocyclic
residue are easily soluble in the intracellular medium and
serve as viscosity sensors in cells in vivo.7—9
† Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1728—1733, August, 2014.
1066ꢀ5285/14/6308ꢀ1728 © 2014 Springer Science+Business Media, Inc.

Molecular rotors based on styryl dyes
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 8, August, 2014
1729
CDCl3 using the solvent as an internal standard (residual proꢀ
tons  2.50 and 7.27, respectively;  39.43 for DMSOꢀd6).
bate with an increase in the size of the adjacent (to the diꢀ
methylaminobenzoyl chromophore) side polymer chain.12
This indicates the weakening of the solvation of the TICT
state in the polymer compared to the monomer, which is
confirmed by the data of timeꢀresolved fluorescence
spectroscopy.13 The fluorescence kinetics of the hemiꢀ
cyanine dyes in poly(methyl methacrylate) (PMMA) films
indicates that the rigid environment prevents the highꢀ
amplitude motion of the fragments, blocking light emisꢀ
sion from the relaxed state.14 Thus, the viscosity of the
environment exerts a substantial effect on the spectralꢀ
luminescence characteristics of molecular rotors.
The fluorescence properties of the crownꢀcontaining
styryl dyes have been studied earlier.15,16 The purpose of
the present work is to synthesize new molecular rotors
based on styryl dyes 1—3 and to establish the influence of
the viscosity, temperature, and polarity of the medium on
their photorelaxation. The difference in structures of the
studied compounds can be understood more deeply by the
study of the dependence of the spectralꢀluminescence charꢀ
acteristics on the size of the substituent at the acceptor
fragment and on the possibility of free rotation of the diꢀ
alkylamino group in the benzene ring. To determine bonds
about which rotation occurs, we performed the quantum
chemical calculations of the dependences of the potential
energy of the singletꢀ and tripletꢀexcited molecule of dye 1
on torsion about one of three carbon—carbon bonds of the
ethylene fragment.
H
C
Chemical shifts were measured with the 0.01 ppm, and spinꢀspin
coupling constants were measured with the accuracy to 0.1 Hz.
Heteronuclear correlation 2D 1H—13C (HSQC and HMBC)
spectra were used for assignment of signals from carbon atoms.
Elemental analyses were carried out at the Microanalysis Laboꢀ
ratory of the A. N. Nesmeyanov Institute of Organoelement
Compounds (Russian Academy of Sciences, Moscow). The samꢀ
ples for elemental analyses were dried in vacuo at 80 С.
N,NꢀDimethylꢀ4ꢀ[(E)ꢀ2ꢀpyridinꢀ4ꢀylvinyl]aniline and 1ꢀbenzꢀ
ylindolineꢀ5ꢀcarbaldehyde were obtained according to described
procedures.17,18 1ꢀBromooctadecane, NaClO4, 4ꢀpicoline, a 70%
solution of HClO4, and pyrrolidine (Aldrich) were used without
additional purification.
4ꢀ{(E)ꢀ2ꢀ[4ꢀ(Dimethylamino)phenyl]vinyl}ꢀ1ꢀethylpyridinium
perchlorate (1). The synthesis of dye 1 was described earlier.19
13C NMR (DMSOꢀd6, 27 C), : 16.02 (MeCH2); 39.59 (NMe2);
54.58 (CH2N); 111.86 (C(3´), C(5´)); 117.01 (CH=CHHet);
122.34 (C(3), C(5), C(1´)); 130.07 (C(2´), C(6´)); 141.96
(CH=CHHet); 143.14 (C(2), C(6)); 151.82 (C(4´)); 153.59 (C(4)).
4ꢀ{(E)ꢀ2ꢀ[4ꢀ(Dimethylamino)phenyl]vinyl}ꢀ1ꢀoctadecylpyrꢀ
idinium perchlorate (2). A mixture of N,Nꢀdimethylꢀ4ꢀ[(E)ꢀ2ꢀ
pyridinꢀ4ꢀylvinyl]aniline (112 mg, 0.50 mmol) and 1ꢀbromoꢀ
octadecane (167 mg, 0.50 mmol) was heated for 4 h at 140 C
(oil bath). The obtained mixture was dissolved on heating in
EtOH (5 mL) and then cooled to 5 C. The formed precipitate
was filtered off, washed with EtOH (2×1 mL), and dried in air.
Bromide salt of the dye was obtained as a red powder in a yield of
230 mg (82%). The bromide salt (130 mg, 0.23 mmol) was disꢀ
solved on heating in EtOH (5 mL), NaClO4 (42 mg, 0.34 mmol)
was added, and the obtained mixture was cooled to 5 C. The
formed precipitate was filtered off, washed with EtOH (2×1 mL)
and then with Et2O (2×2 mL), and dried in air. Dye 2 was
obtained as a dark red powder in a yield of 110 mg (83%),
m.p. 193—194 C. Found (%): C, 68.99; H, 9.52; N, 4.98.
C33H53ClN2O4. Calculated (%): C, 68.66; H, 9.26; N, 4.85.
1H NMR (CDCl3, 25 C), : 0.89 (t, 3 H, MeCH2, J = 6.9 Hz);
1.21—1.39 (m, 30 H, (CH2)15); 1.96 (m, 2 H, CH2CH2N); 3.12
(s, 6 H, NMe2); 4.48 (t, 2 H, CH2N, J = 7.1 Hz); 6.98 (d, 1 H,
CH=CHHet, J = 15.8 Hz); 7.08 (d, 2 H, H(3´), H(5´), J = 8.3 Hz);
7.58 (d, 2 H, H(2´), H(6´), J = 8.3 Hz); 7.62 (d, 1 H, CH=CHHet,
J = 15.8 Hz); 7.92 (d, 2 H, H(3), H(5), J = 5.6 Hz); 8.52 (d, 2 H,
H(2), H(6), J = 5.6 Hz). 1H NMR (DMSOꢀd6, 25 C): 0.84 (t, 3 H,
MeCH2, J = 6.9 Hz); 1.18—1.29 (m, 30 H, (CH2)15); 1.87 (m, 2 H,
CH2CH2N); 3.02 (s, 6 H, NMe2); 4.40 (t, 2 H, CH2N, J = 7.3 Hz);
6.78 (d, 2 H, H(3´), H(5´), J = 8.9 Hz); 7.17 (d, 1 H, CH=CHHet,
J = 16.1 Hz); 7.59 (d, 2 H, H(2´), H(6´), J = 8.9 Hz); 7.92 (d, 1 H,
CH=CHHet, J = 16.1 Hz); 8.05 (d, 2 H, H(3), H(5), J = 6.8 Hz);
8.76 (d, 2 H, H(2), H(6), J = 6.8 Hz). 13C NMR (DMSOꢀd6,
25 C), : 13.81 (MeCH2); 21.96 (CH2); 25.28 (CH2); 28.24 (CH2);
28.57 (CH2); 28.63 (CH2); 28.75 (CH2); 28.90 (8 CH2); 30.31
(CH2CH2N); 31.16 (CH2); 40.33 (NMe2); 59.08 (CH2N); 111.84
(C(3´), C(5´)); 116.97 (CH=CHHet); 122.28 (C(3), C(5));
122.36 (C(1´)); 130.07 (C(2´), C(6´)); 142.09 (CH=CHHet);
143.33 (C(2), C(6)); 151.85 (C(4´)); 153.65 (C(4)).
Experimental
4ꢀMethylꢀ1ꢀoctadecylpyridinium perchlorate (4). A mixture
of 4ꢀpicoline (1.0 mL, 0.01 mol) and 1ꢀbromooctadecane (3.33 g,
0.01 mol) was heated for 1.5 h at 160 C (oil bath). The reaction
mixture was dissolved on heating in EtOH (6 mL), and a 70%
solution of HClO4 (1.0 mL, 0.012 mol) was added. The mixture
Melting points (uncorrected) were measured in capillaries
1
on a MelꢀTemp II instrument. Н and 13C NMR spectra were
recorded on a Bruker DRX500 spectrometer (500.13 MHz for
protons and 125.76 MHz for carbon atoms) in DMSOꢀd6 and

1730
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 8, August, 2014
Volchkov et al.
was diluted with Et2O (10 mL) and cooled to –5 C. The formed
precipitate was filtered off, washed with Et2O (2×10 mL), and
dried in air. Salt 4 was obtained as a white powder in a yield
of 3.03 g (68%), m.p. 66—68 C. Found (%): C, 64.25; H, 10.20;
N, 3.13. C24H44ClNO4. Calculated (%): C, 64.62; H, 9.94;
N, 3.14. 1H NMR (DMSOꢀd6, 22 C), : 0.79 (t, 3 H, MeCH2,
J = 6.9 Hz); 1.15—1.28 (m, 28 H, (CH2)14); 1.29 (m, 2 H,
CH2CH2CH2N); 1.92 (m, 2 H, CH2CH2N); 2.60 (s, 3 H,
MeHet); 4.84 (m, 2 H, CH2N); 7.75 (d, 2 H, H(3), H(5),
J = 6.4 Hz); 9.16 (d, 2 H, H(2), H(6), J = 6.4 Hz).
Quantum chemical calculations were performed using the
Firefly (version 8) program23 on the MGU Chebyshev computaꢀ
tional complex at the Research Computational Center of the
M. V. Lomonosov Moscow State University. For conformationꢀ
al analysis in the ground and excited electron states, the geomeꢀ
try of the molecule was optimized by the MCSCF method (averꢀ
aging over the first seven states with equal weights was used) in
the 6ꢀ31 (1p, 1d) basis set at the fixed value of the corresponding
torsion angle. The total energy of the molecule was determined
by the MCQDPT2 method.24 The solvation energies in ethanol
and heptane were estimated by the MCSCF method in terms of
PCM (Polarized Continuum Model). The values of dipole moꢀ
ments were determined by the xMCQDPT method.
4ꢀ[(E)ꢀ2ꢀ(1ꢀBenzylꢀ2,3ꢀdihydroꢀ1Hꢀindolꢀ5ꢀyl)vinyl]ꢀ1ꢀoctaꢀ
decylpyridinium perchlorate (3). A mixture of salt 4 (135 mg,
0.30 mmol), 1ꢀbenzylindolineꢀ5ꢀcarbaldehyde (71 mg, 0.30 mmol),
pyrrolidine (1 droplet), and anhydrous EtOH (10 mL) was heatꢀ
ed for 25 h at 80 C (oil bath). The reaction mixture was cooled
to 5 C, and the formed precipitate was filtered off, washed with
EtOH (2×1 mL) and then with Et2O (2 mL), and dried in air.
Dye 3 was obtained as a red powder in a yield of 180 mg (89%),
m.p. 138—139 C. Found (%): C, 72.28; H, 8.56; N, 4.07.
C40H57ClN2O4. Calculated (%): C, 72.21; H, 8.64; N, 4.21.
1H NMR (DMSOꢀd6, 25 C), : 0.85 (t, 3 H, Me, J = 7.0 Hz);
1.19—1.30 (m, 30 H, (CH2)15); 1.86 (m, 2 H, CH2CH2CH2N);
3.03 (t, 2 H, NCH2CH2Ar, J = 8.5 Hz); 3.51 (t, 2 H, NCH2CH2Ar,
J = 8.5 Hz); 4.39 (t, 2 H, CH2N, J = 7.3 Hz); 4.45 (s, 2 H,
NCH2Ar); 6.66 (d, 1 H, H(5´), J = 8.6 Hz); 7.13 (d, 1 H,
CH=CHHet, J = 16.2 Hz); 7.29 (t, 1 H, H(4), J = 7.1 Hz);
7.31—7.40 (m, 5 H, H(6´), H(2), H(3), H(5), H(6)); 7.50
(br.s, 1 H, H(2´)); 7.88 (d, 1 H, CH=CHHet, J = 16.2 Hz); 8.01
(d, 2 H, H(3), H(5), J = 6.8 Hz); 8.73 (d, 2 H, H(2), H(6),
J = 6.8 Hz). 13C NMR (DMSOꢀd6, 25 C), : 13.80 (Me);
21.95 (CH2); 25.26 (CH2); 26.90 (NCH2CH2Ar); 28.22 (CH2);
28.56 (CH2); 28.62 (CH2); 28.74 (CH2); 28.89 (9 CH2);
30.29 (CH2CH2CH2N); 31.15 (CH2); 50.39 (NCH2Ar); 51.69
(NCH2CH2Ar); 58.96 (CH2N); 105.61 (C(5´)); 116.40
(CH=CHHet); 122.07 (C(3), C(5)); 123.43 (C(2´)); 123.84
(C(1´)); 127.09 (C(4)); 127.65 (C(2), C(6)); 128.40 (C(3),
C(5)); 130.56 (C(3´)); 131.25 (C(6´)); 137.27 (C(1)); 142.35
(CH=CHHet); 143.24 (C(2), C(6)); 153.65 (C(4)); 154.43 (C(4´)).
Absorption and fluorescence spectra were recorded on
a UVꢀ3100 spectrophotometer (Shimadzu) and an Elyuminꢀ2M
spectrofluorimeter. A controlled flow of liquid nitrogen vapor
through the Dewar flask was used to record the fluorescence
spectra at lowered temperatures. The fluorescence quantum
Results and Discussion
The synthesis of dye 1 was described earlier.19 Quaterꢀ
nary salt 4 and new dyes 2 and 3 were synthesized in high
yields according to Scheme 1. The structures of compounds
1—4 were determined using the data of 1H and 13C NMR
spectroscopy and spectrophotometry and confirmed by
elemental analysis. All dyes were obtained as Eꢀisoꢀ
3
mers (spinꢀspin coupling constants JtransꢀCH=CH
=
= 15.8—16.2 Hz).
Scheme 1
yields ( ) in the range 293—77 K were calculated by comparing
f
the areas under the corrected fluorescence spectra of the fluoroꢀ
phores and standards. The standards were solutions of quinine
sulfate in 1 N H2SO4 ( = 0.546),20 Coumarin 522 in MeCN
f
( = 1),21 and Rhodamine B in ethanol ( = 0.7).22 A correcꢀ
f
f
tion to temperature compression of the solvents was applied when
The absorption spectra of compounds 1—3 at 293 K
undergo as a whole (except for dye 1 in diethyl ether)
a slight shortꢀwavelength shift on going from polar to lowꢀ
ly polar solvents (Table 1). Similar shifts are characteristic
of the fluorescence spectra of these compounds. The Stokes
shifts of the fluorescence spectra (a—f) of the dyes are
anomalously high (up to 145 nm) and depend weakly on
the size of substituents in the donor and acceptor fragꢀ
ments. The highest values of a—f are observed in polar
aprotic butyronitrile. The fluorescence quantum yields of
compounds 1—3 at 293 K are low (a little higher for 2 and 3
calculating  . Samples of the fluorophores in the PMMA films
f
were obtained from solutions 1—3/acetone/PMMA by the evapꢀ
oration of acetone in the Petri dish following by film drying in
air. The quartz Dewar flask, which made it possible to fix the
position of the film in the cell compartment of the spectroꢀ
fluorimeter, was used to record the fluorescence spectra of the
films at 293 and 77 K. PMMA synthesized by radiation polymerꢀ
ization was used for the preparation of the films. Spectrally pure
ethanol was dehydrated by distillation above CaH2. Spectrally
pure PrCN, Et2O, and acetone were used without additional
purification. Spectrally pure AcOEt was liberated from acid traces
by distillation above potash.

Molecular rotors based on styryl dyes
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 8, August, 2014
1731
Table 1. Spectralꢀluminescence characteristics of dyes 1—3 in different solvents at 293 and 77 K
max
Dye

max,*

*
S**,

f
a
f
Et2O/EtOAc,
PrCN/EtOH,
293 K
Et2O/EtOAc,
PrCN/EtOH,
293 K
PrCN,
77 K
Et2O/EtOAc,
PrCN/EtOH,
293 K
Et2O/EtOAc,
PrCN/EtOH,
293 K
PrCN,
77 K
1
2
3
482/462
478/481
535
534
561
592/604
621/607
100/142
143/126
0.02/0.04
0.02/0.04
0.63
0.95
0.72
472/466
481/484
589/607
623/611
117/141
142/127
0.09/0.05
0.02/0.05
476/482
499/500
594/625
644/636
118/143
145/136
0.14/0.03
0.02/0.03
* In nm.
** Here and in Table 2, S is the Stokes shift (nm).
in diethyl ether). The fixation of the position of the N atom
of the amino group in the cycle (compound 3) to prevent
its rotation weakly affects the fluorescence spectra and
fluorescence quantum yields compared to the unfixed
amino group (compound 2). This suggests that the possible
rotation of the free dialkylamino group in molecules of the
styryl dyes makes no substantial contribution to the specꢀ
tralꢀfluorescence properties of these fluorophores.
On going from liquid butyronitrile (293 K) to the glassy
one (77 K), the fluorescence spectra of the dyes experiꢀ
ence the shortꢀwavelength shift by 83—89 nm accompaꢀ
nied by a considerable increase in f (see Table 1). The
Stokes shifts of fluorescence of compounds 1, 2, and 3 in
PrCN at 77 K are low: 57, 53, and 62 nm, respectively. As
the temperature consecutively decreases from 293 to 77 K,
the fluorescence spectra of the dyes in EtOH also demonꢀ
strate the shortꢀwavelength shift accompanied by an inꢀ
crease in the intensity (Fig. 1).
The intramolecular charge transfer from the electronꢀ
donor group NR1R2 to the electronꢀacceptor residue of
Nꢀalkylpyridinium with the formation of the TICT state
occurs upon the photoexcitation of studied compounds.
This is experimentally proved by anomalously high Stokes
shifts in polar solvents. The TICT state is characterized by
efficient nonradiative deactivation leading to low or even
zero values of f. The overall process consists of the intrinꢀ
sic charge transfer and mutual rotation of the donor and
acceptor fragments, whose efficiency should be determined
by the viscosity of the medium.
It is known25 that an increase in the environment visꢀ
cosity prevents the turn of large molecular fragments. In
addition, the time of orientation relaxation of the solvent
increases with the temperature decrease, which makes
worse the solvation of the TICT state and decreases the
probability of its formation. The character of changing the
fluorescence spectra of compounds 1—3 in EtOН and
PrCN in the range 77—293 K is completely consistent
with these observations. In particular, Fig. 1 shows that
the main shortꢀwavelength shift and the increase in f fall
on the region of maximum viscosity increase (at the left
from the glassy transition point of ethanol 159 K). In the
range 150—170 K, the value of fmax of dye 1 in EtOH
shifts by 20 nm against 5 nm in PrCN (see Fig. 1, inset),
which is explained by a flatter temperature dependence of
the increase in the viscosity of PrCN.
The spectralꢀluminescence properties of the studied
fluorophores depend substantially on the viscosity and temꢀ
perature of the medium. The influence of the viscosity can
be isolated by measuring the fluorescence spectra of the
dyes in the PMMA films at 293 K and in ethyl acetate,
which is close to PMMA in polarity. The fluorescence
spectra of dyes 1—3 in PMMA exhibit the hypsochromic
shift by 20—30 nm relative to the fluorescence spectra
in AcOEt. The corresponding fluorescence maxima in
I (arb. units)
f

max
25
20
15
10
5
2
1
625
600
575
550
8
100
200
300 T/К
7
4
6
3
2
5
0
1
500
550
600
650
700
/nm
Fig. 1. Fluorescence spectra of dye 1 in ethanol at 293 (1),
273 (2), 250 (3), 208 (4), 190 (5), 169 (6), 150 (7), and 77 K (8);

= 478 nm. Inset: the temperature dependence of the posiꢀ
exc
tion of the fluorescence maximum of dye 1 in (1) ethanol and
(2) butyronitrile.

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Volchkov et al.
Table 2. Spectralꢀluminescence characteristics of dyes 1—3 in
the PMMA films at 293 and 77 K
The quantum chemical calculations of the depenꢀ
dences of the potential energy in the S0, S1, and T1 states
on the dihedral angles at ethylene bonds 1—3 of the
Het+—CH—CH—Ar fragment were performed for the catꢀ
ion of dye 1. The barrier is surmounted in the S1 state for
the mutual turn of the aromatic rings by 180 about any
indicated bond (Fig. 3). Planar configurations are preferaꢀ
ble for rotamers formed by turns about the ordinary bonds
max
max *,

*
S

1
2
3
Dye

a
f
293 K
293 K/77 K
293 K 77 K
293 K 77 K
1
2
3
473
476
494
577
583
595
560
566
580
104/87
107/90
101/86
0.18
0.62
0.71
0.28
0.72
0.75
* In nm.
E/kJ mol–1
250
a
S1
PMMA and in less polar diethyl ether differ insignificantꢀ
ly. At the same time, a substantial increase in f in the
polymer is observed (Table 2). It follows from this that it is
the viscosity increase that results in a decrease in the nonꢀ
radiative deactivation constant, which is related to a deꢀ
crease in the efficiency of TICT state formation in highly
viscous media. At 77 K, the Stokes shifts of fluorescence
of dyes 1—3 in PMMA decrease (see Table 2, Fig. 2) but
do not attain the values obtained in the lowꢀtemperature
glasses of PrCN. This confirms that the rotation of the
molecular fragments depends on both the viscosity and
polarity of the medium.
An elongation of the Nꢀsubstituent in the acceptor
moiety of the molecule results in an increase in f of dyes 2
and 3 at 293 K in diethyl ether and at 293 and 77 K in
PMMA compared to dye 1 (see Tables 1 and 2). Indeed,
the viscous environment more efficiently blocks the turn
of larger fragments, decreasing the rate constant of nonꢀ
radiative deactivation. The fluorescence spectra of dyes
1—3 in PMMA at 77 K exhibit the shortꢀwavelength
shift (by ~15 nm) compared to the spectra at 293 K. This
means that the mutual rotation of the fluorophore fragꢀ
ments is not completely frozen even in the solid PMMA
film at 293 K.
T1
200
150
100
50
S0
30 60 90 120 150 180 /deg
E/kJ mol–1
b
250
200
150
100
50
S1
S0
T1
30 60 90 120 150 180 /deg
E/kJ mol–1
I (arb. units)
c
15
S1
1´
250
200
150
100
50
2´
3´
2
T1
1
10
5
3
S0
0
30 60 90 120 150
/deg
500
550
600
650
700
/nm
Fig. 3. Relative total energies (E) of the cation of dye 1 vs diꢀ
hedral angle  between the planes of the pyridine and ethylene
fragments (a), fragments HetCH and CHAr (b), and ethylene
and benzene fragments (c) in the S0, S1, and T1 states.
Fig. 2. Fluorescence spectra of dyes 1—3 in the PMMA films at
293 (1—3) and 77 K (1´—3´). All spectra were normalized to the
equal fluorescence intensity at 293 K.

Molecular rotors based on styryl dyes
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 8, August, 2014
1733
Table 3. Potential barriers (Ea) and enthalpies of torꢀ
sion* (HTICT) for the planar conformers and the diꢀ
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R1—Het+—CH=CH—C6H4—NR1R2ClO4 on the visꢀ
–
cosity of the medium reflect rotations of the molecular
fragments in them and make it possible to consider these
compounds as molecular rotors. According to the quanꢀ
tum chemical calculations, for the barrier photorelaxation
of cations of the dyes, twisting occurs about the bonds of
the ethylene bridge to form weakly fluorescent TICT states.
The dependences of the efficiency of the formation of the
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This work was financially supported by the Rusꢀ
sian Foundation for Basic Research (Project Nos 11ꢀ03ꢀ
01026 and 13ꢀ03ꢀ12423) and the Russian Academy of
Sciences.
Received January 21, 2014;
in revised form June 2, 2014