Crown-containing styryl derivatives of naphthopyrans: Complexation with alkaline-earth metal cations and photochemistry

Article abstract of DOI:10.1134/S0023158412010144

The photochemistry and complexation with alkaline-earth metal cations of two crown-containing naphthopyrans and their crownless analogues are reported. Two types of photoreactions occur in the system, namely, the reversible formation of the open form, whi

Full text of DOI:10.1134/S0023158412010144

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2012, Vol. 53, No. 1, pp. 54–64. © Pleiades Publishing, Ltd., 2012.
Published in Russian in Kinetika i Kataliz, 2012, Vol. 53, No. 1, pp. 56–67.
CrownꢀContaining Styryl Derivatives of Naphthopyrans:
Complexation with AlkalineꢀEarth Metal Cations
and Photochemistry1
,
b
,
b
, b
A. B. Smolentseva, V. V. Koroleva , E. M. Glebova , V. P. Grivina ,
V. F. Plyusnina , A. I. Kruppaa , A. V. Chebun’kovac, S. V. Paramonovc,
,
b
, b
O. A. Fedorovac, V. Lokshind, and A. Samatd
a Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences,
Novosibirsk, 630090 Russia
b Novosibirsk State University, Novosibirsk, 630090 Russia
c Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, 119991 Russia
d Centre Interdisciplinaire de Nanoscience de Marseille (CINaM, CNRS), Marseille, 13288 France
eꢀmail: s_art@ngs.ru
Received January 25, 2011
Abstract—The photochemistry and complexation with alkalineꢀearth metal cations of two crownꢀcontaining
naphthopyrans and their crownless analogues are reported. Two types of photoreactions occur in the system,
namely, the reversible formation of the open form, which is responsible for the photochromism of these comꢀ
pounds, and the geometric isomerization of the closed form. The photochromic properties (UV spectrum and
lifetime) of the naphthopyrans do not change upon complexation with metal cations. This is explained by the
specific features of the structure of the open form and by the kinetic parameters of the processes.
DOI: 10.1134/S0023158412010144
Photochromism is a reversible photoinduced pheꢀ
nomenon in which a photosensitive compound transꢀ
forms to another isomer having a different absorption
spectrum. Photochromic compounds are promising
for creation of photosensitive systems: optical variꢀ
ableꢀtransmission materials, optical information storꢀ
age systems, and photoswitchers [1–4].
The naphthopyrans (chromenes) are one of the
most interesting families of organic photochromes
since many of them have a high fatigue resistance [3].
The photochromism of chromenes is caused by the
transformation of the colorless closed (
S) form to the
colored open merocyanine ( ) form (Scheme 1). The
M
change in color is due to the molecule changing its
geometry from spatial to planar [1–6].
h
ν
Δ
O
O
Closed (S) form
Open merocyanine (M) form
Scheme 1.
Incorporation of crownꢀether moieties into photoꢀ cations. The properties of crownꢀcontaining organic
chromic molecules opens up the possibility of controlꢀ
ling their properties via complex formation with metal
photochromic compounds have been actively investiꢀ
gated in the last two decades. The binding of the metal
cations by the crownꢀether moiety leads to changes in
the photochromic properties of azobenzenes [7, 8],
1
The article was translated by the authors.
54

CROWNꢀCONTAINING STYRYL DERIVATIVES OF NAPHTHOPYRANS
55
diarylethenes [9], styrylbenzothiazoles [10–12],
In this work, complexation with alkalineꢀearth
spiropyranes [13–15], spirooxazines [16–22] and metal cations and the effect of complexation on the
chromenes [23–25]. This approach opensup the posꢀ photochromic properties were studied for two types of
sibility of creating photocontrolled receptors. Comꢀ crownꢀcontaining naphthopyrans (compounds 1b and
plex formation with metal cations can affect the 2b on the Scheme 2). For comparison, similar experiꢀ
parameters of the S
↔
M reaction [5, 21]. For ments were carried out on cromenes that do not conꢀ
instance, the complexation of crownꢀcontaining tain a macrocyclic moiety (model compounds 1a and
spironaphthooxazine with Li+ cations extends the lifeꢀ 2a in Scheme 2). This work continues a series of studꢀ
time of the open form by two orders of magnitude [16]. ies initiated by Glebov et al. [26].
R =
(1a);
(1b);
OMe
OMe
O
O
O
O
O
O
O
O
R
(
2a);
(2b).
NEt2
N
1а, 1b
;
2а, 2b
O
O
Scheme 2.
EXPERIMENTAL
Isomers of the closed form of 1b
The synthesis, physical properties, and NMR–
spectra of 1a (5ꢀ[2ꢀ(3,4ꢀdimethoxyphenyl)ethenyl]ꢀ
3,3ꢀdiphenylꢀ3Hꢀbenzo[f]chromene), 1b (5ꢀ[(E)ꢀ2ꢀ
(2,3,5,6,8,9,11,12ꢀoctahydroꢀ1,4,7,10,13ꢀbenzopenꢀ
taoxacyclopentadecanꢀ15ꢀyl)ethenyl]ꢀ3,3ꢀdiphenylꢀ
3Hbenzo[f]chromene), 2a (4ꢀ[2ꢀ(3,3ꢀdiphenylꢀ3Hꢀ
O
O
benzo[f]chromenꢀ5ꢀyl)ethenyl]ꢀ
nylꢀ3Hꢀbenzo[f]chromenꢀ5ꢀyl)ethenyl]ꢀN,N
N
,
N
ꢀ4ꢀ[2ꢀ(3,3ꢀdipheꢀ
ꢀdiethyꢀ
O
O
laniline), and 2b (5ꢀ{2ꢀ[4ꢀ(1,4,7,10ꢀtetraoxaꢀ13ꢀazaꢀ
cyclopentadecanꢀ13ꢀyl)phenyl]ethenyl}ꢀ3,3ꢀdiphenylꢀ
3Hꢀbenzo[f]chromene) have been reported in [26].
According to NMR data, all of the synthesized comꢀ
pounds are a mixture of trans and cis isomers (Scheme 3).
The cis isomers of chromene derivatives 1a and 2a are
stable. This allowed the pure trans and cis isomers to be
isolated chromatographically. In the case of crownꢀ
containing compounds 1b and 2b, we failed to isolate
the individual isomers. According to NMR data,
naphthopyrans 1b and 2b are a mixture of cis and trans
isomers in a ratio of 5 : 2 and 5 : 1, respectively.
O
O
O
O
O
O
O
O
Scheme 3.
UV absorption spectra and kinetic curves on the
second and minute time scales were recorded on an
Agilent HP–8453 spectrophotometer (Agilent Techꢀ
nologies) witha characteristic spectrum recording
time of about 2 s. A highꢀpressure mercury lamp with
a set of glass filters was used as the light source for staꢀ
Mg(ClO4)2 and Ba(ClO4)2 (Aldrich) were used as
the sources of Mg2+ and Ba2+ ions. Spectrophotometꢀ tionary photolysis. To measure the rate constants of
ricꢀgrade acetonitrile (Aldrich) was used as the solꢀ the thermal reaction of naphthopyrans (M
→
S), samꢀ
vent. ples were irradiated in the cuvette box of the spectroꢀ
KINETICS AND CATALYSIS Vol. 53
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2012

56
SMOLENTSEV et al.
performed in standard quartz cuvettes (optical path
A
length of 1 cm) at room temperature. If necessary, oxyꢀ
gen was removed from the samples by blowing argon
for 15 min.
1.5
5
(a)
The composition of the complexes of crownꢀconꢀ
taining naphthopyrans with metal cations was deterꢀ
mined using Job’s method [29]. For construction of
Job’s plot, measurements were carried out on a series
of solutions in which the sum of metal and ligand conꢀ
1
1.0
0.5
0
1
5
centrations (CL
CM C0 ratio was varied. Let us assume that the metal
ion M and the ligand L form only the complex МpLq
M + МpLq. (1)
+ CM = C0) was constant, while the x =
:
:
p
qL
↔
300
350
λ, nm
400
(b)
The largest amount of the complex МpLq forms
when the components are combined in the proportion
A360
–
ε
1bС1b
equal to the ratio of the stoichiometric coefficients
and , i.e. when CМ CL
CL M = (1 –
Job’s plot has an extremum at
р
0
q
:
= p : q,
:
C
xmax)/xmax
=
=
q
:
p.
(2)
1 : 1 complex
x
xmax. In our case,
–0.05
–0.10
–0.15
the experimental parameter that is linear with respect
to the complex concentration is the absorbance of the
complex at a given wavelength,
A( ), which is equal to
λ
the difference between the absorbance of the sample at
the given concentrations of chromene CL and metal
CM (Аexp) and the maximum possible absorbance of
chromene at this concentration.
The proper wavelength was chosen soas to maxiꢀ
mize the change in absorbance due to complexation
0
0.2
0.4
0.6
2+
СMg /(CMg + C1b)
0.8
1.0
2+
(i.e. the
M/(CL
mum depends on the stoichiometry of the complex: if
the extremum is observed at M/(CL CM = 0.5, 0.33
A(
λ
)
value). The dependence of
A(
λ
) on
C
+ CM) was plotted. The position of the extreꢀ
Fig. 1. (a) Changes in the UV spectrum of 1b as a result of
2+
complexation with Mg cations; the 1b concentration in
–5
C
+
)
acetonitrile is 4.55
tration is ( ) 0, ( 3.0
and ( 6.0
×
10 mol/l; the Mg(ClO ) concenꢀ
4 2
× × ×
–5
–5
–4
1 2
)
10 , (
3) 6.0 10 , (4) 1.2 10 ,
or 0.67, this means that the stoichiometry is 1 : 1, 1 : 2,
or 2 : 1.
–4
2+
5
)
×
10 mol/l. (b) Job’s plot; the sum of the
chromene and Mg concentrations is 9.0
the proper wavelength is 360 nm.
–5
×
10 mol/l;
The dependences of absorbance at the chosen
wavelength
the absence of metal cations) on the relative concenꢀ
tration of the reactants, CM CL, were used to estiꢀ
mate the stability constants of the 1 : 1 complexes. As
the metal concentration is increased, tends to the
limiting value Amax, which corresponds to the situaꢀ
tion in which the entire ligand is in the metal complex.
For each value of M at a fixed L, we determined the
value of (%) = Amax 100%. If the metal and
Δ
A = A(z) – A0 (where A0 is absorbance in
photometer (optical path length of 1 cm). Irradiation
was performed until the equilibrium between the
closed and open forms was established (equilibration
was monitored as the absorption of the open form in
the blue spectral region). After the establishment of
the equilibrium, irradiation was ceased and kinetic
curves characterizing the return of the system to the
initial closed state were recorded.
z
=
/
Δ
A
Δ
C
A
C
y
Δ
/
Δ
×
chromene form only the complex 1 : 1, then
The pure cis isomer of naphthopyran 1a was used to
measure the quantum yield of geometric isomerizaꢀ
tion. Measurements were performed in the initial segꢀ
ments of the kinetic curves of photolysis, where the
absorption of the trans isomer is negligible. A modified
ferrioxalate actinometer was used to measure the
intensity of the mercury lamp [27].
y(z)
(3)
2
⎡
⎤
= 5 0 (1 + z + 1 K1CL) − (1 + z + 1 K1CL) − 4 z ,
⎣
⎦
where K1 = [ML]/[M][L]. The optimal value of the
stability constant K1 was determined by fitting the
Laser flash photolysis experiments were carried out
using a setup with YAG : Nd3+ laser (355 nm, pulse
duration of 5 ns, pulse energy of 0.5–2 mJ) similar to
experimental
y(z) dependence to Eq. (3).
In the case of stepwise complex formation, the
that described previously [28]. All experiments were dependences of absorbance at different wavelengths,
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CROWNꢀCONTAINING STYRYL DERIVATIVES OF NAPHTHOPYRANS
57
A
1.5
1
5
1.0
1
5
0.5
0
250
300
350
400
450
, nm
λ
2+
Fig. 2. Changes in the UV spectrum of 1b as a result of complexation with Ba cations; the 1b concentration in acetonitrile is
–5
–6
–6
–5
–5
4.36
×
10 mol/l; the Ba(ClO ) concentration is (
4 2
1) 0, (
2) 5.4
×
10 , (
3) 7.3
×
10 , (
4) 1.1
×
10 , and (
5
) 4.4 10 mol/l.
×
A(
λ
), on the total metal cation content of the solution wavelengths coincided within the experimental error,
(
CBa) at a constant total naphthopyran content (CL and this was evidence of the adequacy of the algoꢀ
)
were analyzed to estimate the stability constants of rithm.
complexes with Ba2+ cations. The iterative procedure
for estimating the stability constants is detailed in [30].
RESULTS AND DISCUSSION
The experimental
Eqs. (4) and (5):
A( ) dependences were fitted to
λ
Complexation of the Closed Form of Naphthopyrans
with Metal Cations
CL(εL + ε1K1[Ba2+] + ε2K1K2[Ba2+]2)
1 + K1[Ba2+] + 2K1K2[Ba2+]2
Addition of alkaliꢀearth metal cations to the soluꢀ
tions of crownless naphthopyrans does not affect their
UV absorption spectra. For crownꢀcontaining comꢀ
pounds, the addition of cations was found to change in
their UV spectra. Figure 1a demonstrates the spectral
changes in the course of complex formation between
chromene 1b and Mg2+ cations. The persistence of
isosbestic points in the UV spectra (at 325, 336 and
347 nm) indicates that, in the concentration range
examined, only two forms of chromene coexist in
solution. This makes it possible to determine the comꢀ
position of the complex by Job’s method [29].
(4)
(5)
A(λ) =
l,
K1K2[Ba2+]3 + (K1 + 2CLK1K2 − K1K2CBa)[Ba2+]2
+ (1 + K1CL − K1CBa) [Ba2+] − CBa = 0,
where K2 = [ML2]/[ML][L]; εL, ε1, and ε2 are the
extinction coefficients of the initial naphthopyran and
the 1 : 1 and 2 : 1 complexes, respectively; and
optical path length.
l
is the
The sought parameters (K1
,
K2,
ε1(
λ
), ε2
(
λ
)
) were
In the case of Mg2+ cations, the minimum of Job’s
estimated by fitting the experimental dependences. plot for compounds 1b and 2b was observed at
These procedures were carried out at several waveꢀ CM C0= 0.5 (Fig. 1b). This indicates the formation
lengths. The stability constants determined at different of a 1 : 1 complex (complex in Scheme 4). This comꢀ
x
=
/
A
KINETICS AND CATALYSIS Vol. 53
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58
SMOLENTSEV et al.
position of the complex corresponds to the location of solution (Fig. 2), which indicated the formation of
the metal cation inside the crown ether cavity. The complexes with different stoichiometries. The Ba2+
ionic radius of the Mg2+ cation is 0.72 Å [31], while the
cation, whose ionic radius is 1.36 Å [31], forms two
complexes with chromene 1b, in which [Ba2+] : [1b] =
radius of the 15ꢀcrownꢀ5–ether cavity is 0.85 Å [32]
and the radius of the cavity in azaꢀ15ꢀcrownꢀ5ꢀether
falls in the 0.85–1.1 Å range [33].
1 : 1 and 1 : 2. The latter ratio corresponds to the sandꢀ
wich structure of the complex (Scheme 4). Geometriꢀ
For the 1b–Ba2+ system, no wellꢀdefined isosbestic cally, the sandwich complex can have both syn and anti
points were observed while adding metal cations to the structures (complexes
B
and
C
in Scheme 4).
Complexes of the closed form
form of crownꢀcontaining chromenes with metal cations
O
A
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Б
В
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Scheme 4.
In contrast to compound 1b, naphthopyran 2b
,
K
1 = 4
×
103 l/mol (Fig. 3, curve
1
). Curves
2
and
3
3
in
which contains an azaꢀcrown moiety, forms only a 1 : 1 Fig. 2 correspond to equilibrium constants of
×
103
complex with the Ba2+ cation. Generally, sandwich and
5
×
103 l/mol. In the determination of the stepwise
complex formation is not typical of azaꢀcrownꢀconꢀ stability constants for the 1b–Ba2+ system, it was
taining compounds [23]. This is probably explained by found that experimental curves at different waveꢀ
the steric hindrance caused by the nonplanar structure lengths can be satisfactorily fitted to Eqs. (4) and (5)
of the molecules. In particular, the syn structure, with similar stability constants (table).
which seems to be energetically more favorable
because of the stacking interaction between chroꢀ
mophores [5], cannot be realized.
The results of the determination of the stoichiomꢀ
etry and stability constants of the complexes are preꢀ
sented in the table. The stability constants for the
complexes of naphthopyrans with the Mg2+ cation
The experimental dependence of
100% on СBa
isfactorily to Eq. (3) with the equilibrium constant where a slightly different computational procedure
Δ
A
/
Δ
Amax
×
/
С2b for the 1 : 1 complex 2b–Ba2+ fits satꢀ coincide with the same constants reported in [26],
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CROWNꢀCONTAINING STYRYL DERIVATIVES OF NAPHTHOPYRANS
59
was used. Note that the stability constant for the 2b
–
Δ
A360
/
Δ
A380max, %
[Complex]/C2b, %
Mg2+ complex, in which the cation is located inside
the crown ether cavity, is two orders of magnitude
higher than that for the 2b–Ba2+ complex, in which
the cation is located outside the cavity.
100
100
3
1
2
Photochromism of the Naphthopyrans
Irradiation of all of the chromenes in the region of
the longꢀwavelength absorption band (355 nm) gives
rise to characteristic absorption at 450 nm (Fig. 4a),
which makes the colorless solution yellow. According
to the literature, the absorption band at 450 nm is due
to the open form of the chromene [3]. After the cessaꢀ
tion of irradiation, the open form transforms therꢀ
mally to the closed form, which results in the restoraꢀ
tion of the initial UV spectrum.
50
50
An example of a kinetic curve for the photochemiꢀ
cal formation of the open form and its thermal converꢀ
sion to the closed form (for compound 1b at room
temperature) is shown in Fig. 4b (the beginning of the
descent of the curve corresponds to the instant the
exciting irradiation was switched off). The kinetic
curves for the M
→
S thermal reactions are exponenꢀ
10
tial. The characteristic lifetime of the open form for all
of the naphthopyrans is several tens of seconds at room
temperature. The rate constants for the thermal
0
20
40
СВа
60
80
100
M
→
S reaction were determined at different temperꢀ
2+
/С2b
atures in the 270–300 K range. This allowed us to calꢀ
culate the activation energies for the reverse reactions
of naphthopyrans. The Arrhenius parameters of this
process were reported in [26]. The activation energy
for all of the chromenes was about 15 kcal/mol, which
is typical of spirocompounds [34].
Fig. 3. Dependence of the relative change of absorbance at
380 nm (dots, left scale) and the calculated percentage of
2+
the complex (lines, right scale) on the [Ba ]/[2b] ratio
–5
(chromene concentration of 5 × 10 mol/l): (1) results of
fitting the experimental data to Eq. (3) with a stability conꢀ
3
stant of
K
= 4000 l/mol/, (
2)
K
= 3 × 10 l/mol, and (3)
1
1
3
K
= 5 × 10 l/mol.
1
The degree of photodegradation per excitation–
thermal reaction cycle is small for all of the naphthoꢀ
pyranes. After 10 cycles, the absorbance of the initial
compound at 450 nm increased by 0.05 (Fig. 5). The
absorbance of the open form in the photostationary
state therewith did not changed significantly.
naphthopyran 1b leads to an increase in UV absorꢀ
bance at 300–400 nm.
By contrast, for naphthopyran 2b UV absorbance
at 300–400 nm decreases markedly upon irradiation
Photoinduced Geometric Isomerization of the Closed
Form of Naphthopyrans
Stoichiometry and stability constants of the complexes of
the crownꢀcontaining naphthopyrans (closed form) with
alkalineꢀearth metal cations
As it was mentioned in Experimental, the closed
form of crownꢀcontaining naphthopyrans in solution
is a mixture of trans and cis isomers. Prolonged irradiꢀ
ation of the solutions in the region of longꢀwavelength
absorption band (313 nm) results in changes in their
1H NMR spectra. An analysis of the initial and final
NMR spectra of 1b demonstrated that the signals from
the protons of the cis isomer of the closed form disapꢀ
peared entirely as a result of photolysis. At the same
time, a moderate increase in the intensity of signals
from the protons of the trans isomer was observed.
logK =
logK1 + logK2
logK1
logK2
Composiꢀ
Complex
tion
[l/mol]
1b–Mg2+ 1 : 1
2b–Mg2+ 1 : 1
1b–Ba2+ 1 : 1
4.7 0.2*
5.7 0.2*
–
–
–
–
4.7 0.4 5.6 0.4 10.3 0.8
and 2 : 1
2b–Ba2+ 1 : 1
3.6 0.2
–
–
Therefore, the observed reaction is the cis
isomerization of the closed form. As was shown in
[26], the cis trans isomerization of the closed form of
–trans
* Reported in [26].
–
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60
SMOLENTSEV et al.
A
A450
1.5
1.0
0.5
0
1
(a)
–
–
under irradiation
after the thermal reaction
0.8
6
1
5
0.6
6
300
420
500
, nm
600
A430
λ
0.8
0.4
0.2
0
(b)
0.6
0.4
0.2
0
10
20
30
, s
40
50
60
0
2
4
6
8
10
t
Cycles
Fig. 4. (a) Changes in the UV spectrum as a result of the
thermal reaction M S of 1b in acetonitrile in the presꢀ
Fig. 5. Photostability of naphthopyran 1a. The cycle is
sample irradiation at 365 nm until the establishment of the
photostationary state and the subsequent dark stage until
→
2+
–5
ence of Ba cations;
C
=
6
×
10 mol/l;
=
Ba(ClO4)2
C
1b
the end of the reaction M
centration in acetonitrile is
→
S. The naphthopyran conꢀ
10 mol/l.
0.01 mol/l; time elapsed after the cessation of irradiation,
s: ( ) 0 (UV spectrum after 10ꢀsꢀlong irradiation at
365 nm), ( ) 2, ( ) 5, ( ) 10, ( ) 20, and ( ) spectrum of
naphthopyrane 1b before irradiation. (b) Kinetics of the
thermal reaction S M (430 nm, 298 K).
–5
4
×
1
2
3
4
5
6
↔
plot of absorbance at 350 nm versus irradiation time, we
determined the quantum yield for the cis trans isomerꢀ
ization of the closed form of 1a to be 0.05 0.01
–
.
(Fig. 6). This is accompanied by an increase of the sigꢀ
nals from cis isomer protons and by a decrease of the
signals from trans–isomer protons. However, the sigꢀ
nals from trans isomer protons dot not disappear comꢀ
pletely. Therefore, the photolysis of naphthopyran 2b
The open form of the naphthopyrans can result from
both the trans isomer and cis isomer of the closed form.
This clearly demonstrates that the geometric isomerizaꢀ
tion of the closed form and the appearance of the open
form are parallel photochemical reactions. No effect of
the isomer composition of the closed form on the specꢀ
trum and lifetime of the open form was experimentally
observed.
at 313 nm results in trans–cis isomerization, which
leads to a new equilibrium distribution of the isomers
of the closed form. The composition of the equilibꢀ
rium mixture is determined by the photolysis quantum
yields and molar absorption coefficients of the trans
and cis forms at the irradiation wavelength.
Primary Processes in Naphthopyran
Photochemistry
For compound 1a, the pure trans and cis isomers of
the closed form were separated, which allowed us to
determine the quantum yield for geometric isomerizaꢀ
tion. The spectral changes in the course of the photolyꢀ
Primary photochemical processes for naphthopyrꢀ
anes were studied by means of laser flash photolysis
(
λex = 355 nm) with 50 ns time resolution. For 1a and
1b, only an instantaneous change in absorbance was
isomerization occurs. From the initial portion of the observed in the time interval examined (50 ns–10 ms).
sis (313 nm) of the cis isomer testify that cis–trans
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CROWNꢀCONTAINING STYRYL DERIVATIVES OF NAPHTHOPYRANS
61
The spectrum of this instantaneous change (with a form of naphthopyran (Scheme 5), proposed in [6], is
maximum at 450 nm) coincides with the spectra of the usually confirmed by femtosecond experiments.
open form of the naphthopyrans. Therefore, all proꢀ
cesses leading to the appearance of the open form come
to completion within <50 ns at room temperature.
According to [6], C–O bond cleavage occurs under
the action of light, leading to the primary intermediate
А
, which displays the properties of a zwitterion. The
characteristic time of zwitterion formation is several
hundreds of femtoseconds. Next, the intermediate
transforms into the intermediate having a cisoid
orthoꢀquinodal structure. The characteristic time of
this process is about 1 ps. The lifetime of the interꢀ
mediate is determined by the rate of mutual rotation
For the compounds examined, the dependences of
the appearing absorbance of the open form on the laser
pulse energy were obtained. All of these dependencies
were linear. Therefore, the photochemical reaction is
a oneꢀquantum process.
A
B
B
The primary processes in the photochemistry of around the C(3)–C(4) bond. This rotation leads to the
chromenes were studied in [6, 35–38] by means of formation of the metastable isomer of the open form
femtosecond pump–probe spectroscopy. In all cases,
it was shown that the overall time of the formation of spectrum coincides with the spectra observed in stationꢀ
the metastable isomer of the open form did not exceed ary photolysis. Therefore, the
form is the experimenꢀ
(C). This isomer has a transoid geometry. Its absorption
C
10 ps. The mechanism of the formation of the open tally observable open form of naphthopyran.
Mechanism of the formation of the open form
4
4
3
3
+
O
O•
O
hν
100 fs
1 ps
1
B
R
R
R
A
10 ps
O
C
Scheme 5.
R
For several chromenes, the formation of a triplet In the case of 1a and 1b, the removal of oxygen does
state of the initial molecule was observed, with the not result in such an effect.
triplet state quantum yield being less than 10% of the
total quantum yield of photolysis [6, 38]. In our experꢀ
iments on laser flash photolysis of oxygenꢀfree soluꢀ
tions of nitrogenꢀcontaining naphthopyrans (2а and
2b), we observed the appearance of shortꢀlived tranꢀ
sient absorption (Fig. 7). This absorption was not
observed upon the photolysis of oxygenꢀcontaining
Figure 7a demonstrates the transient absorption
spectra of an oxygenꢀfree solution of chromene 2b
obtained 0.1 and 50 s after the laser pulse. The kinetic
μ
curves at all the wavelengths (Fig. 7b) are satisfactorily
fitted to a monoexponential function with a characterꢀ
istic time of 3.5 0.5
μs. When experiments were perꢀ
formed at different laser pulse energies, a fourfold
solutions. This testifies that this transient absorption is increase in the initial amplitude of the transient
most likely due to the triplet state of the closed form. absorption signal did not change its characteristic
KINETICS AND CATALYSIS Vol. 53
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2012

62
SMOLENTSEV et al.
А
Δ
A
0.09
0.06
0.03
0
1.0
0.5
0
4
(a)
(a)
1
1
2
–0.03
250
300
350
400
, nm
450
500
400
450
500
550
λ
, nm
600
650
700
λ
А
Δ
A
(b)
0.6
(b)
520 nm
0.04
0.02
0
1
0.3
0
400 nm
10
6
–0.02
300
350
400
450
0
5
15
20
λ
, nm
t, μs
Fig. 6. Changes in the UV spectra of naphthopyrans during
the photoinduced cis trans isomerisation. (a) Photolysis
of chromene 1b 4.5
irradiation time (313 nm) is (
9 min. (b) Photolysis of chromene 2b
acetonitrile. The irradiation time (313 nm) is (
) 3, ( ) 6, ( ) 10, and ( ) 15 min.
–
Fig. 7. Laser flash photolysis (355 nm) of naphthopyran 2b
×
–5
–5
(
×
10 mol/l) in acetonitrile. The
) 0, ( ) 1, ( ) 6, amd (
(
6
10 mol/l) in acetonitrile in the absence of oxygen.
(a) Transient absorption spectra recorded ( ) 0.1 and (
50 s after a laser pulse. (b) Kinetic curves recorded at 520
1
2
3
4)
1
2)
–5
(4.5
×
10 mol/l) in
) 0, ( ) 1,
µ
1
2
and 400 nm (experimental curves and their fits to monoexꢀ
ponential functions).
(3
4
5
6
decay time. This means that secondꢀorder reactions
do not make any measurable contribution to the tranꢀ
sient absorption decay kinetics. The transient absorpꢀ
Effect of Metal Cations on the Photochromic
Properties of the Naphthopyrans
Based on the literature data on the photochemistry
of chromenes, one could expect a sufficient effect of
metal cations on the parameters of the electrocyclic
tion spectrum of the oxygenꢀfree sample 20 s after
μ
the laser pulse coincides with the spectrum of the open
form.
reaction M
chromenes studied in [23], the rate constant of the
reverse reaction M
S at [Ca2+] : [Chr] = 1000 was
→
S [5, 16, 21, 23]. For example, for
According to [6], one might hypothesize that the
observed reaction is the transition of the triplet state of
the closed form to the open form. If this were the case,
the quantum yield of the appearance of the open form
in oxygen free solutions would be higher than in soluꢀ
tions with the natural oxygen content. In our experiꢀ
ments, the relative quantum yield of the appearance of
the open form (derived from the final absorbance at
the band maximum) was not oxygenꢀdependent.
Therefore, the triplet states of the closed forms of 2a
and 2b do not turn into the open forms (as distinct
from the chromenes studied in [6]).
→
smaller by a factor of 5 for crownꢀcontaining comꢀ
pounds and by a factor of 2 for their crownless anaꢀ
logues. This effect is explained by complex formation
due to the Coulomb interaction between the partial
negative charge on the carbonyl oxygen atom and the
positive charge of the metal cation, which can be
located either in the solution bulk or in the crown
ether cavity (Scheme 6) [21, 23, 34].
However, in our case both the position of the
absorption band of the open form and its lifetime did
KINETICS AND CATALYSIS Vol. 53
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2012

CROWNꢀCONTAINING STYRYL DERIVATIVES OF NAPHTHOPYRANS
63
not change even at [Me2+] : [Chr] = 6000. This was is unlikely to appear. This can be due to the competiꢀ
true both for the crownꢀcontaining naphthopyrans
and for the crownless ones.
The absence of a metal cation effect on the lifeꢀ
times of the open forms of model compounds 1a and 2a
testifies that the complex, in which the metal cation is
tion between the two possible reactions of the merocyꢀ
anine form of naphthopyran, namely, the complexꢀ
ation of the metal cation and conversion into the
closed form. The characteristic time of complex forꢀ
mation seems to be much shorter than the lifetime of
coordinated to the carbonyl oxygen atom (Scheme 6a), the open form (about 30 s at room temperature).
Complexation between the open form
of the naphthopyrans and metal cations
O
O
O
O
O
O
O
O
O
O
O
O
(а)
(b)
Scheme 6.
For compounds 1b and 2b, the absence of the influꢀ cavity should not affect the properties of the open
ence of the metal cation (located inside the crown form.
ether cavity) on the properties of the open form is
Thus, the naphthopyrans examined are involved in
probably explicable in terms of molecular structure.
two parallel photochemical reactions, namely, the
The cause of the possible effect is the Coulomb interꢀ
geometric isomerization of the closed form and the
action between the charge of the cation and the partial
formation of the merocyanine (open) form. Geometꢀ
negative charge of an oxygen atom (Scheme 6b). In
ric isomerization does not affect the lifetime of the
δ
–
our case, the distance between the Mg2+and
O
catꢀ
open form. The naphthopyrans form complexes of difꢀ
ferent compositions with Mg2+ and Ba2+ cations.
Complex formation exerts an effect on the spectral
characteristics of crownꢀcontaining chromenes in
closed form but does not change the properties of the
open form. This fact indicates the independence of the
two photochromic parts of the molecule. In spite of
the conjugation of these molecular moieties, the metal
cations located in the styryl moiety cannot effect the
electrocyclic reaction of the naphthopyran moiety.
ions is ca.
8 Å,and the Coulomb interaction over this
distance is weak. When metal cations are taken in great
excess, the coordination of a second metal cation
seems to be possible (Scheme 6a). However, the
absence of observable results of this coordination indiꢀ
cates that, as in the case of the crownless naphthopyrꢀ
anes, the lifetime of the open form is insufficient for
the complex to appear.
An alternative explanation of the absence of a metal
cation effect on the properties of the open form of
crownꢀcontaining naphthopyrans is possible for the
case of cis isomers (Scheme 3). The open form of the
cis isomer is likely nonflat, and this decreases the conꢀ
ACKNOWLEDGMENTS
jugation of the
π
ꢀelectronic systems of the styryl and
The work was supported by the Russian Foundaꢀ
benzo–crown moieties of the molecule. In this case, tion for Basic Research (grant nos. 11–03–00268 and
the lpresence of a metal cation inside the crown ether 09–03–00283).
KINETICS AND CATALYSIS Vol. 53
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2012

64
SMOLENTSEV et al.
metti, R., Girling, R.B., Moore, J.N., and Hester, R.E.,
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