Photochromic transformations in solutions of 8,8′-diquinolyl disulfide and di(mercaptoquinolinato)nickel(II)

Article abstract of DOI:10.1007/s11172-006-0124-3

The nature of intermediate species and their reactions were studied by laser pulse photolysis for a photochromic system consisting of 8,8′-diquinolyl disulfide (RSSR) and a planar Ni^II complex di(mercaptoquinolinato)nickel(II) (Ni(SR)₂) in toluene and benzene solutions. Under exposure to laser radiation, disulfide RSSR dissociates to two RS^? radicals, whose spectrum has an intense absorption band with a maximum at λ = 400 nm (ε = 8400 L mol^-1 cm ^-1). The radicals disappear by recombination (2k_rec = 4.6 · 10⁹ L mol^-1 s^-1). In the presence of the Ni(SR)₂ complex, coordination of the radical (k_coord = 4.4 · 10⁹ L mol^-1 s^-1) competes with recombination to form a radical complex RS^?Ni(SR)₂ having an intense absorption band with a maximum at 460 nm (ε = 16 600 L mol^-1 cm^-1). This species decays in the second-order reaction (2k = 4.6 · 10⁴ L mol^-1 s^-1). Since the photochromic system returns to the initial state, the reaction of two radical complexes is assumed to produce radical recombination and reduction of the disulfide and Ni(SR)₂ complex. Analysis of the kinetic data showed that some RS^? radicals decay in the microsecond time interval due to the reaction with the RS^?Ni(SR)₂ radical complex (k = 3.1 · 10⁹ L mol^-1 s ^-1).

Full text of DOI:10.1007/s11172-006-0124-3

2364
Russian Chemical Bulletin, International Edition, Vol. 54, No. 10, pp. 2364—2373, October, 2005
Photochromic transformations in solutions of 8,8´ꢀdiquinolyl disulfide
and di(mercaptoquinolinato)nickel(II)
a
aꢀ
a
a
b
c
D. Yu. Vorobyev, V. F. Plyusnin, Yu. V. Ivanov, V. P. Grivin, S. V. Larionov, and H. Lemmetyinen
aInstitute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of Sciences,
3
ul. Institutskaya, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383 3) 30 7350. Eꢀmail: plyusnin@ns.kinetics.nsc.ru
Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences,
b
3
prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383 3) 30 9489
Institute of Materials Chemistry of Tampere University of Technology,
c
8
Korkeakoulunkatu, FINꢀ33101 Tampere, Finland.
Fax: +3 (58 03) 3115 2108
The nature of intermediate species and their reactions were studied by laser pulse photolysis
for a photochromic system consisting of 8,8´ꢀdiquinolyl disulfide (RSSR) and a planar NiII
complex di(mercaptoquinolinato)nickel(II) (Ni(SR) ) in toluene and benzene solutions. Unꢀ
2
•
der exposure to laser radiation, disulfide RSSR dissociates to two RS radicals, whose spectrum
has an intense absorption band with a maximum at λ = 400 nm (ε = 8400 L mol cm ). The
radicals disappear by recombination (2krec = 4.6•10 L mol
–
1
–1
9
–1 –1
s
9
). In the presence of the
–1 –1
Ni(SR) complex, coordination of the radical (k
= 4.4•10 L mol
s ) competes with
2
coord
•
recombination to form a radical complex RS Ni(SR) having an intense absorption band with
a maximum at 460 nm (ε = 16 600 L mol cm ). This species decays in the secondꢀorder
2
–
1
–1
4
–1 –1
reaction (2k = 4.6•10 L mol
s ). Since the photochromic system returns to the initial state,
the reaction of two radical complexes is assumed to produce radical recombination and reducꢀ
•
tion of the disulfide and Ni(SR) complex. Analysis of the kinetic data showed that some RS
2
•
radicals decay in the microsecond time interval due to the reaction with the RS Ni(SR)
2
9
–1 –1
radical complex (k = 3.1•10 L mol
s ).
Key words: photochemistry, nickel(II) complexes, sulfurꢀcontaining radicals, laser pulse
photolysis, optical spectra.
Photodissociation of disulfides affords sulfurꢀcontainꢀ
ing radicals (S radicals), which recombine rapidly (rate
radical escape from the coordination sphere in the
microꢀ and millisecond time ranges. In the solvent bulk,
the radical can disappear due to recombination to form
disulfide. However, a low concentration of the radiꢀ
cals increases the probability of repeated coordinaꢀ
tion with the initial complex. For this reason, the lifetime
of the radical complexes can elongate by two—three
orders of magnitude. In the case of the dithiophosꢀ
1
—4
constant is close to the diffusion limit).
In the presꢀ
II
ence of planar Ni dithiolate complexes, some S radicals
reversibly coordinate to the metal atom.5—7 Reversibility
of coordination provides good photochromic properties
of these systems, which sustain many transformation cycles
without degradation. The S radicals resemble some hetꢀ
erocyclic extraligands, for example, pyridine, by the abilꢀ
i
•
phinate radical (Bu PS ) and planar complex with
2
2
8
—14
i
ity to reversible coordination with nickel ions.
dithiophosphinate ligands Ni(S PBu ) , the biradical
2 2 2
•
•
i
•
i
For instance, dithiocarbamate (dtc
=
•
S CNAlk )
complex (Bu PS ) Ni(S PBu ) is formed due to the
2
2
2 2 2 2 2 2
•
and perfluorothionaphthyl ( SNF = SC F ) radiꢀ
coordination of the second radical with the primary radiꢀ
1
0 7
i
•
i
7
cals generated by the photodissociation of disulfides
cal complex (Bu PS )Ni(S PBu ) . In most cases, the
2 2 2 2 2
1
5,16
(
S CNAlk ) and (SC F ) ,
respectively, react rapꢀ
system returns to the initial state due to the recombinaꢀ
tion of S radicals.5
2
2 2
10 7 2
—7,15
idly and reversibly with the planar dithiocarbamate,
xanthogenate, and dithiophosphate complexes of bivaꢀ
lent nickel (Ni(dtc) , Ni(xan) , and Ni(dtp) , respecꢀ
In the present work, we studied photochemical proꢀ
cesses that occur in solutions of 8,8´ꢀdiquinolyl disulfide
(RSSR) and di(mercaptoquinolinato)nickel(II) (Ni(SR)2)
in toluene and benzene. The photodissociation of RSSR
generates S radicals having two heteroatoms (S and N)
2
2
2
tively) to form the intermediate radical complexes
•
5,15
•
•
(
dtc )Ni(dtc) ,
( SNF)Ni(dtc) , ( SNF)Ni(xan) ,
2 2
2
•
6
and ( SNF)Ni(dtp) . These complexes dissociate with
2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2291—2300, October, 2005.
066ꢀ5285/05/5410ꢀ2364 © 2005 Springer Science+Business Media, Inc.
1

Photochromism of RSSR and Ni(SR) solutions
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2365
2
capable of coordinating with a metal atom. This coordiꢀ
nation can increase considerably the lifetime of the radiꢀ
cal complex, which makes it possible to organize a photoꢀ
chromic system in which the forward and backward reacꢀ
tions are photoinitiated. The choice of the complex is
caused by the capability of reversible adding extraligands
of the type of pyridine and amines and by the presence of
Results and Discussion
Photochemistry of solutions of 8,8´ꢀdiquinolyl disulꢀ
fide. The Ni—S and Ni—N distances in the Ni(SR) comꢀ
plex are 2.40 and 2.06 Å, respectively, and the bond lengths
in the quinoline ring vary from 1.33 to 1.47 Å. The
2
21
optical absorption spectrum of the Ni(SR) complex in
2
an intense absorption band in the visible spectral reꢀ
16
benzene contains chargeꢀtransfer bands with maxima at
gion.1
6,17
This band allows one to monitor transformaꢀ
5
54, 396, 336, and 308 nm (Fig. 1) and molar absorption
coefficients of 900, 7240, 12270, and 11970 L mol cm
tions of the complex upon laser pulse photolysis.
–1
–1
,
respectively. The spectrum of a solution of the disulfide
in benzene has an absorption band with a maximum
–
1
–1
at 324 nm (ε = 11490 L mol cm ) and ε =
9
–
1
–1
400 L mol cm in the wavelength region of XeCl laꢀ
ser radiation (308 nm).
The energy of the S—S bond in disulfides is
–
1
~
100 kJ mol and, hence, these molecules dissociate to
two sulfurꢀcontaining radicals in a high quantum yield
upon absorption of photons from a XeCl laser with an
–
1 2,22—25
energy of 400 kJ mol
.
For a benzene solution
of 8,8´ꢀdiquinolyl disulfide after a laser pulse, an interꢀ
mediate absorption was detected, whose spectrum conꢀ
tains bands with maxima at ~364 (shoulder), 384,
4
<
04, 560, and 605 nm (Fig. 2, a, curve 1). At wavelengths
345 nm, the absorbance decreases (bleaching) after a
Experimental
laser pulse, which is caused by the disappearance of the
starting disulfide that absorbs in this spectral region
(see Fig. 1).
Laser pulse photolysis was carried out on a setup with a XeCl
excimer laser (308 nm, 15 ns, 30 mJ, surface area of the laser
pulse cross section on a sample 10 mm ). Exciting and probing
light pulses were directed to a 2—10ꢀmm cell at a small angle
2
18
The absorption, whose spectrum is shown in Fig. 2, а,
•
belongs to the 8ꢀmercaptoquinoline radical (RS ). The
(
~2°) to each other. All experiments were conducted at 25 °C.
disappearance of the intermediate absorption obeys the
To study slow processes in the millisecond and second time
3
intervals, special quartz microcells were used (volume 50 mm ,
2
surface area 10 mm , thickness 5 mm) in which the solution
ε•10–3/L mol cm–1
volume was transmitted with a laser beam and the photoreaction
kinetics was not violated by convective flows. When microcells
were used at long measurement times, we applied a less powerful
halogen incandescent lamp, whose emission was passed through
narrowꢀband interferential filters, instead of the powerful xenon
lamp to decrease the effect of the probing light. Selected experiꢀ
12
10
ments were carried out on a similar laser pulse photolysis equipꢀ
ment19 with the perpendicular geometry of the exciting and
8
2
probing light beams.
Optical absorption spectra were recorded on HP 8453 and
Shimadzu UVꢀ2501 spectrophotometers. Spectrally pure solꢀ
vents (Merck) were used to prepare solutions. The intensities of
laser pulses were measured by the tripletꢀtriplet absorption
of anthracene at 431 nm in an oxygenꢀfree benzene solution
6
4
1
(
quantum yield of the triplet state is 0.53, and the molar
absorption coefficient of the T—T absorption band is
–
1
–1 20
2
4
2 000 L mol cm ). A specially developed program based
on the Runge—Kutta method of the fourth order was used in
numerical calculations of the decay kinetics of the intermediate
optical absorption to solve differential equations. The Ni(SR)2
4
00
500
600 λ/nm
1
6
complex was synthesized according to a known procedure.
,8´ꢀDiquinolyl disulfide (Reagent corp., Russia) was used as
received.
8
Fig. 1. Optical absorption spectra of RSSR (1) and Ni(SR) (2)
2
in benzene.

2366
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Vorobyev et al.
∆
D
∆D
0
0
0
0
.4
.3
.2
.1
0
1
a
b
0
0
.2
.1
0
6
1
2
1
6
6
–
–
0.1
0.2
7
1
–
0.1
4
00
500
600 λ/nm
400
500
λ/nm
•
•
Fig. 2. Optical spectra of the RS radical (a) and RS Ni(SR) radical complex (b) detected by laser pulse photolysis: a, spectrum of a
2
solution of RSSR (1.4•10–4 mol L ) in benzene after a laser pulse (50 ns) (1); spectrum corrected for the absorption of disulfide (2);
b, spectra of a solution of RSSR (3.3•10 mol L ) and Ni(SR) (4.1•10 mol L ) in benzene after 0 (1), 0.8 (2), 1.6 (3), 3.2 (4),
.4 (5), and 44 µs (6); spectrum of the RS Ni(SR)2 radical complex obtained from the spectrum (6) with allowance for the
–1
–
4
–1
–5
–1
2
•
6
disappearance of absorption of the starting complex and disulfide (7) (shifted down by 0.3 for clarity).
•
second order kinetics and is associated with radical reꢀ
combination to form the starting disulfide.
sorption spectrum of the RS radical at 320—370 nm (see
Fig. 2, а, curve 2).
Appearance of intermediate absorption in solutions of
•
•
RSSR and Ni(SR) under laser irradiation. The absorpꢀ
tion of the RS radical appears in a solution of the disulꢀ
2
RS + RS
RSSR
(1)
•
fide RSSR and complex Ni(SR) after a laser pulse (see
2
The observed rate constant of radical decay at room
Fig. 2, b, curve 1). This absorption rapidly changes with
retention of two isosbestic points at 425 and 495 nm, and
a new absorption band with a maximum at 460 nm apꢀ
pears in the spectrum in 20 µs (see Fig. 2, b, curve 6). In
the region of 500—620 nm, the absorbance decreases
5
–1
temperature (298 K) is kobs = (2krec/ε)•(∆D/l) = 3•10 s
,
where ∆D is the absorbance of the radical after a pulse,
and l is the cell thickness.
To determine the recombination rate constant (2krec),
it is necessary to know the molar absorption coefficient of
the RS• radical. These parameters were determined in
(
bleaching) due to the disappearance of the longꢀwave
absorption band of the starting complex. The new specꢀ
trum disappears within several hundreds of milliseconds.
•
experiments using the reaction of the RS radical with
•
3
the stable nitroxyl radical (RNO ), which gives inꢀ
tense absorption bands in the UV (λ = 342 nm,
Coordination of the S radicals with the dithiolate comꢀ
max
5—7,15
plexes, which has been found earlier
for a number of
–
1
–1
ε = 14000 L mol cm ) and visible (λmax = 600 nm,
ε = 1000 L mol cm ) spectral regions. The reaction
systems, and the ability of the planar nickel complexes to
–
1
–1
8
—14
add extraligands
suggest that the new band at 460 nm
•
•
RS + RNO proceeds with the rate constant close to the
diffusion limit, which allows it to compete successfully
with the recombination of the RS• radicals. The molar
absorption coefficient of the absorption band of the RS•
radical with a maximum at 400 nm measured by this
•
belongs to the radical complex RS Ni(SR) , which is
formed due to the coordination of the RS• radical with
2
the Ni(SR) complex.
2
RS• + Ni(SR)2
•
RS Ni(SR)
(2)
2
–
1
–1
method is 8400±900 L mol cm . Using this value, we
calculated the recombination rate constant (2k
=
This reaction agrees with the disappearance of the
rec
9
–1 –1
4
.6•10 L mol
s
in benzene) and determined the abꢀ
longꢀwave absorption band of the starting Ni(SR) comꢀ
2

Photochromism of RSSR and Ni(SR) solutions
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2367
2
plex with a maximum at 560 nm (see Fig. 2, b). Only
reactions (1) and (2) occur at short times after a laser
pulse (τ < 1/(k •C ), where C is the initial concentration
absorption curve at 460 nm was processed by an equation
of the first order and determined the observed rate conꢀ
stant (kobs). Using Eq. (4), we find
2
0
0
of the Ni(SR) complex). Further other reactions involvꢀ
2
•
kobs(A∞/R0) = k2C0.
(6)
ing the radical complex RS Ni(SR) can occur. For inꢀ
2
stance, the complex can add the second radical to form a
biradical complex. In terms of the scheme of reactions (1)
and (2), the following expressions can be written for the
time plots of the concentrations of the radical and radical
complex (R(t) and A(t), respectively):
The concentrations A and R can be calculated from the
∞
0
7
absorbances at 460 and 400 nm, because the molar abꢀ
sorption coefficients of the absorption bands of the radiꢀ
cal and radical complex were determined above. The linꢀ
ear plot of kobs(A /R ) vs. C , whose slope ratio deterꢀ
∞
0
0
9
–1 –1
–k2C0t
/[1 + R (2k /k C )(1 – e–k2C0t)]},
mines the rate constant k = (4.4±0.1)•10 L mol
s ,
R(t) = R {e
(3)
2
0
0
1
2 0
is presented in Fig. 4.
A(t) = R (k C /2k R )ln[1 + (2k R /k C )(1 – e–k2C0t)], (4)
0
2
0
1
0
1
0
2
0
After the molar absorption coefficients and rate conꢀ
stants of recombination and radical coordination were
determined, we can consider a possibility of participation
of the radical and radical complex in other reactions. The
experimental kinetic curves at 400, 460, and 560 nm at
where R is the initial concentration of the radical after a
0
laser pulse. These equations make it possible to determine
the ratio of the absorbance of the radical complex at long
times (∆D ) to the initial absorbance (∆D ) of the radical
•
∞
0
maxima of the absorption bands of the RS radical,
after a laser pulse (in the band maximum at 400 nm)
•
RS Ni(SR) radical complex, and Ni(SR) complex are
2
2
presented in Fig. 5. Curve 4 (see Fig. 5) is the calculation
for 460 nm in terms of reactions (1) and (2), which agrees
with the experimental curve only in the initial region
∆
D /∆D = [(ε – ε )/ε ](k C /2k R )•
(5)
∞
0
A
C
R
2
0
1 0
•
ln[1 + (2k R /k C )]
(εA – ε )/ε ,
C R
1
0
2
0
(
0—3 µs). At long times, the experimental kinetic curve
where ε and ε are the molar absorption coefficients of
the radical complex and initial complex, respectively, at
A
C
runs much lower than the calculated one. Thus, a considꢀ
erable amount of the radical complexes decays in the
parallel process.
The yield of the radical complex at different concenꢀ
trations of the starting complex, depending on the initial
radical concentration (variation of the laser pulse intenꢀ
sity), is shown in Fig. 6 (points). Line 1 corresponds to
the hypothetical case of coordination of all radicals
the wavelength of measurements. The plot of the ∆D /∆D0
∞
ratio vs. initial concentration R measured at 460 nm is
0
shown in Fig. 3. It is seen that ratio (5) at R →0 tends to
0
the (ε – ε )/ε value, which allows one to determine the
A
C
R
molar absorption coefficient of the absorption band
of the radical complex at 460 nm (ε = 16 600±500
A
L mol–1 cm ).
–1
(
no recombination and no parallel processes). Smooth
The rate constant of coordination of the radical (k2)
was determined at its low initial concentrations when the
recombination is suppressed and other possible reactions
are absent. The initial region (1—2 µs) of the kinetic
kobs•A •R 1•10–5/s–1
–
∞
0
6
4
2
∆
D /∆D
∞
0
1
1
1
.8
.5
.2
[RS ] •10–5/mol L–1
•
2
4
6
8
0
0
5
–1
5
10
[Ni(SR) ] •10 /mol L
2 0
Fig. 3. Ratio of the absorbance of the radical complex at 460 nm
∆D , time 30—50 µs) to the initial absorbance of the radical at
(
Fig. 4. Parameter kobs(A /R ) vs. initial concentration of the
∞
0
∞
4
00 nm (∆D , immediately after a laser pulse) vs. initial concenꢀ
complex ([Ni(SR) ] ) (k and A were determined at low conꢀ
2 0 obs
∞
0
•
–4
–5
–1
tration of the radical ([RS ] ); solution of RSSR (2.6•10
mol L ) and Ni(SR) (14.8•10 mol L ) in benzene.
centrations of the radical (R < 10 mol L )); solution of
0
0
–
1
–5
–1
–4
–1
RSSR (1.7•10 mol L ) and Ni(SR) in benzene.
2
2

2368
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Vorobyev et al.
∆
D
∆D
1
2
3
4
2
5
0
0
.6
.4
6
4
0
0
0
.6
7
0.2
0
.4
.2
0
•
5
–1
2
4
6
[RS ] •10 /mol L
0
•
Fig. 6. Absorption of the RS Ni(SR) radical complex at 460 nm
2
•
(30—50 µs after a pulse) vs. initial concentration of the RS
radical (absorption at 400 nm immediately after a pulse):
, calculation for coordination of all radicals with the complex
(no recombination and reaction (7)); 2—4, calculation for ocꢀ
1
1
3
5
currence of reactions (1) and (2) only at [Ni(SR) ]•10
=
2
4.8 (2), 7.5 (3), and 2.2 mol L– (4); 5—7, calculation for the
1
1
same concentrations of the complex for occurrence of reacꢀ
tions (1), (2), and (7). Points are experimental values at the
indicated concentrations. For parameters of calculation, see
Tables 1 and 2.
0
5
10
15
20
t/µs
cause the shape of the intermediate spectrum remains
unchanged (see isosbestic points in Fig. 2, b). All aboveꢀ
listed conditions correspond to the following reaction,
which also agrees with the photochromic properties of the
system:
Fig. 5. Kinetic curves of a change in the intermediate absorption
during laser pulse photolysis of a solution of RSSR (1.3•10
mol L ) and Ni(SR) (5.1•10 mol L ) in benzene: experiꢀ
ment (1—3, lines) and calculation (1—3, points) at 400 (1),
–
4
–
1
–5
–1
2
4
60 (2), and 560 nm (3) (calculation was performed for reacꢀ
tions (1), (2), and (7), for molar absorption coefficients and rate
constants, see Tables 1 and 2); the kinetic curve was calculated
with allowance for reactions (1) and (2) at 460 nm (4).
•
•
RS + RS Ni(SR)
RSSR + Ni(SR)2.
(7)
2
This reaction produces no new species that could transꢀ
form the optical spectrum of the radical complex. At high
radical concentrations, reaction (7) decreases substanꢀ
tially the concentration of the radical complex. After the
radical decay, this reaction is completed, and the "surꢀ
vived" radical complexes can disappear rather slowly. Reꢀ
action (7) can include the step of coordination of the
second radical to form a biradical complex. However, its
lifetime should be sufficiently short (must not exceed sevꢀ
eral hundreds of nanoseconds) to have no effect on the
rate and spectrum of the intermediate absorption.
curves 2—4 show the yield calculated by the above obꢀ
tained rate constant with allowance for reactions (1)
and (2). Points (5—7) present the experimental results. At
–
5
–1
R < 10 mol L , all calculated curves and experimental
0
data approach to line 1. However, a substantial diverꢀ
–
5
–1
gence begins at R > 10 mol L . Inconsistency beꢀ
0
tween the calculated and experimental yields of the radiꢀ
cal complex increases considerably with a decrease in the
concentration of the starting Ni(SR) complex. Thus, at
2
high initial concentrations of the radical, an additional
process develops in the system and results in the disapꢀ
pearance of the radical complex. This process ceases toꢀ
gether with the radical decay, because the absorption of
the radical complex formed within 30—50 µs further reꢀ
mains unchanged. The decay of the latter is much slower
Reactions (1), (2), and (7) make it possible to calcuꢀ
late the kinetic curves, which agree well with the experiꢀ
mental data (see Fig. 5), with the single varied parameter
9
–1 –1
k = (3.1±0.1)•10 L mol
s . Smooth curves 5—7 (see
3
Fig. 6) represent the calculated absorption of the radical
complex (460 nm) using the determined k value, which
3
(
during several hundreds of milliseconds) without changꢀ
coincides with the experimental points for different initial
ing the shape of the spectrum. Therefore, we can exclude
concentrations of the Ni(SR) complex. The molar abꢀ
2
•
the monomolecular decay of RS Ni(SR) to the free radiꢀ
sorption coefficients of species and the reaction rate conꢀ
stants are given in Tables 1 and 2. Thus, reactions (1), (2),
and (7) considered as a whole provide the complete exꢀ
2
cal and starting complex.5
—7,15
No coordination of the
second radical occurs to form a biradical complex, beꢀ
7

Photochromism of RSSR and Ni(SR) solutions
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2369
2
Table 1. Molar absorption coefficients (ε/L mol–1 cm–1) for the
initial and intermediate species under study in benzene
∆D•102
1
/∆D
RS•
RS Ni(SR)
•
λ/nm
Ni(SR)2
2
4
4
5
00
60
60
6630
1620
5620
8400±900
800±100
1500±200
8800±300
16600±500
800±100
8
6
4
2
0
100
Table 2. Rate constants of the reactions under study (k —k ) in
benzene and toluene
1
4
50
0
2k •10–9
k2•10–9
k3•10–9
2k •10
–4
Solꢀ
vent
1
4
L mol–1 s–1
4
8
t/s
Benzene 4.6±0.1
Toluene 4.5±0.7
4.4±0.1
4.7±0.6
3.10±0.1
2.8±0.8
—
4.6±0.1
planation of all kinetic transformations for solutions of
the disulfide and nickel complex that occur in the microꢀ
second time interval.
•
Decay of radical complex RS Ni(SR) . At room temꢀ
2
•
perature, the RS Ni(SR) radical complex decays in the
2
millisecond time interval accompanied by the appearance
of the absorption bands of the initial complex Ni(SR)2
and disulfide RSSR. Thus, this system possesses photoꢀ
chromic properties, which allow it to repeat multiply
phototransformation cycles. In the case of several similar
2
4
6
8
10
12 t/s
•
Fig. 7. Absorption decay kinetics of the RS Ni(SR) radical
2
complex at 460 nm under laser pulse photolysis of a solution of
5
—7,15
systems,
the complexes of sulfurꢀcontaining radiꢀ
–4
–1
–5
–1
RSSR (1.0•10 mol L ) and Ni(SR) (3.5•10 mol L ) in
2
cals with the dithiolate NiL complexes decay through
2
toluene. The anamorphosis of the secondꢀorder kinetics is shown
in insert.
monomolecular dissociation with radical escape from the
coordination sphere. The lifetime of these species lies in
the microꢀ or millisecond time intervals. The repeated
coordination of the radical elongates the effective lifetime
of the radical complex by two—three orders of magniꢀ
linear character of the curve that passes through the origin
indicates that the radical complex decays only due to
reactions of the second order. The decay rate constants
5
—7,15
tude.
The repeated coordination exerts an especially
coincide for the Ni(SR) complex concentrations differꢀ
2
pronounced effect on an increase in the lifetime of the
radical complex with an increase in the concentration of
the NiL complex.
The curve of changing the concentration of the radical
complex (absorption at 460 nm), which corresponds to
the reaction of the second order, is presented in Fig. 7
ing by an order of magnitude, which makes it possible to
exclude the monomolecular decomposition of the radical
complex and repeated coordination of the radical. Since
the system returns to the initial state, the reaction prodꢀ
ucts are the starting disulfide and complex. All these data
suggest that the radical complex disappears through the
reaction
5
2
(
corresponding anamorphosis is shown in insert). Nuꢀ
merical simulation shows that kinetic curves of the secꢀ
ond order can be obtained in different reaction schemes.
For instance, for reversible coordination of a radical, the
disappearance of the radical complex occurs finally due
to radical recombination, which usually results in the kiꢀ
netics similar to the second order.
•
•
RS Ni(SR) + RS Ni(SR)
RSSR + 2 Ni(SR)2. (8)
2
2
Since the molar absorption coefficient of the absorpꢀ
tion band of the radical complex was determined above
(see Table 1), the data presented in Fig. 8 make it posꢀ
•
To reveal the mechanism of decay of the RS Ni(SR)
sible to calculate the second order rate constant (2k =
2
4
4
–1 –1
radical complex, we studied the dependence of the obꢀ
served rate constant on the amplitude of the signal (variaꢀ
tion of the laser pulse intensity) in a wide range of conꢀ
(4.6±0.1)•10 L mol
s ).
Reaction (8) can proceed in either one step or via a
complicated mechanism. The latter can include, for exꢀ
ample, the step of radical transfer from the coordination
centrations of the starting Ni(SR) complex (Fig. 8). The
2

2370
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Vorobyev et al.
kobs/s–1
ln(k/L mol–1 s–1
)
1
0
0
0
0
.0
.8
.6
.4
.2
2
2
2
1
0
1
2
3
1
2
3
4
2
1
2
8
4
T –1•10 /K
3
–1
3
.0
3.5
4.0
0
•
6
–1
5
10
15 [RS Ni(SR) ]•10 /mol L
Fig. 9. Temperature plots of the rate constants of recombinaꢀ
tion (1), coordination of the RS radical with the initial comꢀ
plex (2) and the radical complex (3), and the reaction of two
radical complexes (4).
2
•
Fig. 8. Observed rate constant (kobs) of the absorption decay of
the RS Ni(SR)2 radical complex at 460 nm vs. its concenꢀ
tration: [Ni(SR) ]•10 = 0.67 (1), 1.53 (2), 3.58 (3), and
.1 mol L (4); [RSSR] = 1.7•10 mol L .
•
5
2
–
1
–4
–1
7
tained kinetic curves made it possible to determine the
temperature plots for the rate constants of all reactions
sphere of one complex to the sphere of another complex
(
Fig. 9). The calculated activation energies and preꢀexpoꢀ
•
to form a biradical complex (RS ) Ni(SR) .
2
2
nential factors are given in Table 3. These parameters
have similar values for reactions (1) and (2) (radical reꢀ
combination and coordination, respectively) and reacꢀ
tion (7). The rate constants of the listed reactions are
•
•
RS Ni(SR) + RS Ni(SR)
Ni(SR)2
RSSR + 2 Ni(SR)2
+
(9)
2
2
•
+
(RS ) Ni(SR)
2
2
If the first step of radical transfer is rateꢀdetermining,
then the whole reaction would obey the second order
kinetics. The question about the mechanism of reaction (8)
coincides with the same problem for reaction (7). In both
cases, the biradical complex can be an intermediate speꢀ
cies. Since no formation of this species was detected in
the microsecond time region, it can rapidly decompose,
indeed, in the nanosecond range.
lower than the diffusion limit by 2—3 times (k
=
diff
1
0
–1 –1
8RT/3000η = 1.1•10 L mol
s ), most likely, due to
the steric factor. In these cases, the activation energies are
similar to the activation energy of diffusion motion deterꢀ
mined by the temperature dependence of the solvent visꢀ
cosity.
The activation energy of reaction (8) is substantially
higher than that for the other reactions and comparable
with the activation energy of monomolecular dissociation
Temperature plot of the rate constants of reactions
involving radical and radical complex. Laser pulse phoꢀ
•
5—7
of RS NiL radical complexes.
This similarity of actiꢀ
2
tolysis of solutions of RSSR and Ni(SR) in a wide temꢀ
perature range (230—350 K) and processing of the obꢀ
vation energies can serve as an argument in favor of a
complicated mechanism including radical transfer (reacꢀ
2
Table 3. Activation energies (E ) and preꢀexponential factors (logk ) of the reaction rate
a
0
constants for the RSSR + Ni(SR) system in toluene
2
E /kJ mol–
1
log(k /L mol s–1
–1
)
Reaction
a
0
RS + RS• → RSSR (1)
•
7.7±0.4
11.0±0.1
11.2±0.1
11.6±0.1
10.2±0.1
•
•
RS + Ni(SR) → RS Ni(SR) (2)
8.7±0.3
12.3±0.7
31.4±0.6
2
2
•
•
RS + RS Ni(SR) → RSSR + Ni(SR) (7)
2
2
•
2
RS Ni(SR) → RSSR + 2 Ni(SR) (9)
2 2

Photochromism of RSSR and Ni(SR) solutions
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
D
2371
2
•
tion (9)). In this case, the RS —Ni bond in one of the
complexes should be cleaved, and the energy of this bond
can determine the energetics of the whole process. For the
2.0
1.5
1.0
0.5
0
planar NiL dithiolate complexes, the energy of homolytic
2
–
1 26
cleavage of the Ni—S bond is about 200 kJ mol
and,
hence, the energy of radical coordination is much lower
than the bonding energy of the dithiolate ligand. It is
known27 than many planar nickel complexes reversibly
add nitrogenꢀcontaining compounds (amines, pyridine,
and picoline) as extraligands, which are coordinated by
the nitrogen atom at the fifth coordination site. The actiꢀ
vation energies of decomposition of these extra complexes
3
×10
2
–
1 27,14
lie in a range of 30—40 kJ mol
,
which agrees well
1
with the dissociation energy of the complexes of sulfurꢀ
containing radicals with nickel(II) dithiolate complexes
∼ 30 kJ mol–1).
5
(
–
0.5
Rather high activation energy induces a substantial
4
decrease in the rate constant of reaction (8) and an
increase in the lifetime of the radical complex. For
instance, at 230 K and the initial concentration
×
10
–1.0
•
–5
–1
[
RS Ni(SR) ] ≈ 10 mol L , the time of decay of the
2
radical complex is ~100 s, which allows one to detect
steadyꢀstate optical spectra. The spectra of a solution of
3
00
400
500
600 λ/nm
–
4
Fig. 10. Steadyꢀstate photolysis of a solution of RSSR (1.3•10
mol L– ) and Ni(SR) (5.1•10 mol L ) in cooled (230 K)
toluene (1ꢀcm cell): optical spectra before (1) and after (2)
irradiation (10 pulses of a XeCl laser), the difference of spectra 1
and 2 (3), and the optical spectrum of the RS Ni(SR) radical
RSSR and Ni(SR) in cooled toluene at 230 K before and
2
1
–5
–1
after irradiation with pulses of a XeCl laser (λ = 308 nm)
are shown in Fig. 10. The spectra were recorded on an
HP 8453 spectrophotometer with a diode array (time of
recording the full spectrum is 5 s). After irradiation, the
intensity of the band of the starting complex decreases
with the simultaneous appearance of a new band with a
maximum at 460 nm. The differential spectrum coincides
with that observed in pulse photolysis (see Fig. 2, b).
Taking into account the disappearance of absorption of
2
•
2
complex (4) (obtained with allowance for the absorption decay
of the Ni(SR) complex and RSSR disulfide).
2
species in water (at pH 5.2, the content is 96%) has a band
–
1
–1
with a maximum at 446 nm (ε ≈ 2000 L mol cm ). In
polar solvents without a hydroxyl group, the content of
the zwitterionic form decreases, and the maximum of its
longꢀwave absorption band shifts toward 550—605 nm,
which imparts violet color to solutions of 8ꢀmercaptoꢀ
quinoline.
The spectra of the 8ꢀmercaptoquinoline complexes of
metal ions with the filled dꢀshell (Zn , Cd , etc.) exꢀ
hibit two absorption bands at 260 and 380—420 nm.3
The position and intensity of these bands are similar to
the absorption parameters of the free ligand RS . Thus,
the bands in the spectra of these complexes correspond,
most likely, to intraligand transitions. Coordination with
transition metal ions (Ni , Cu ) complicates the specꢀ
trum giving a series of overlapped bands and additional
bands in a lowerꢀfrequency region of the spectrum. The
greater intensity of these bands makes it possible to atꢀ
the starting Ni(SR) complex and disulfide RSSR, one
2
•
can plot the optical spectrum of the RS Ni(SR) radical
2
complex (see Fig. 10, curve 4). Thus, the spectrum of this
species has two intense absorption bands with maxima at
3
1—33
4
58 and 353 nm, being, in fact, the longꢀlived species,
2
+
2+
whose lifetime in cooled solutions can achieve several
tens and hundreds of seconds.
1
Structure and nature of optical spectrum of radical
complex. Depending on the pH value, free 8ꢀmercaptoꢀ
quinoline in aqueous solutions can exist as protonated
–
2
+
±
(
(
RSH ), thiol (RSH), zwitterionic (RSH ), and ionized
–
2+
2+
RS ) species. Each of them has different optical absorpꢀ
tion spectra.
2
8—33
In nonpolar organic solvents, 8ꢀmerꢀ
captoquinoline usually exists in the thiol form, which is
characterized (in benzene) by absorption bands at 250
–
1
–1
16,17
and 324 nm (ε = 22000 and 4500 L mol cm , respecꢀ
tively). Coordination compounds contain the ionized form
as a ligand, whose free state in aqueous solutions exhibits
absorption bands with maxima at 260 and 367 nm (ε =
tribute them to metal—ligand chargeꢀtransfer bands.
According to analysis of the spectra of several square plaꢀ
nar nickel complexes, the 4p ꢀorbital of the metal ion
and four πꢀorbitals of the coordinating atoms of the
ligands, form a bonding molecular πꢀorbital involved
in many transitions. For coordination of the nitrogenꢀ
3
4
z
–
1
–1
31
2
2 300 and 4300 L mol cm , respectively). The
zwitterionic form is characterized by an absorption band
in the visible part of the spectrum. The spectrum of this
containing extraligand along the axial axis, the 4p ꢀorbital
z

2372
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Vorobyev et al.
of the nickel ion stops interacting with the πꢀorbitals of
the equatorial ligands, and the chargeꢀtransfer bands
disappear.1 The spectrum of the adduct resembles the
spectra of the thioquinolate complexes with the filled
dꢀshell.
D
4
6
3
2
0
0
.10
.05
There is another explanation of a considerable transꢀ
formation of the spectrum of the Ni(SR) complex upon
2
1
the addition of nitrogenꢀcontaining compounds. A pair of
electrons of the nitrogen atoms of the extraligand is asꢀ
sumed31 to complete the dꢀshell of the nickel ion (3d ),
which changes the spectrum. In the framework of these
concepts, the optical spectrum of the complexes with the
unfilled dꢀshell is determined by a greater contribution of
the zwitterionic form of the ligand. On going to the filled
dꢀshell, the ligand structure becomes similar to the thiol
one.31 Despite different explanations of the nature of specꢀ
trum changing, it is shown in many studies that the apꢀ
pearance of an additional axial ligand results in the disapꢀ
pearance of the longꢀwave absorption band of the Ni(SR)2
complex at 560 nm and formation of a new band at
8
5
00
600
λ/nm
4
20—440 nm.
In the case of coordination of the RS• radical, the
Fig. 11. Change in the longꢀwave absorption band of the Ni(SR)2
complex during steadyꢀstate photolysis of a solution of RSSR
longꢀwave band of the starting complex also disappears
and a new band with a maximum at 460 nm appears. This
suggests that the radical is also coordinated at the axial
coordinate. At the moment of collision of the radical and
complex, the coordination occurs, most likely, due to the
sulfur atom on which a negative charge and a significant
spin density are usually localized in sulfurꢀcontaining radiꢀ
1.3•10–4 mol L ) and Ni(SR) (7.5•10 mol L ) in toluene
–1
–5
–1
(
(
2
2ꢀmm cell): 0 (1), 40 (2), 220 (3), and 460 pulses of a XeCl
laser (4).
formation of complexes in which the sulfur atoms of two
ligands are in the cisꢀposition to each other.
Thus, the primary process in photochromic transforꢀ
mations in solutions of 8,8´ꢀdiquinolyl disulfide and
di(mercaptoquinolinato)nickel(II) in toluene and benzene
is the photodissociation of the disulfide to two sulfurꢀ
containing radicals. In the presence of the nickel comꢀ
plex, the radicals decay in competing recombination and
coordination reactions, which proceed with the rate conꢀ
stants similar to the diffusion limit. The coordination of
4
•
cals. Quinoline (molecular analog of the RS radical
containing no sulfur atom but with the nitrogen atom)
exerts no effect on the spectrum of the Ni(SR) complex
2
–
1
–1
(
the quinoline concentration is 4•10 mol L , and that
–
5
–1
of the complex is 2•10 mol L ).
It cannot be excluded that after coordination with the
sulfur atom the radical is additionally bonded through the
nitrogen atom as well. The distortion of the planar strucꢀ
ture of the complex upon adduct formation can favor this
•
the radical produces the radical complex RS Ni(SR) ,
2
which is consumed in two reactions with considerably
different rate constants. At high initial concentrations of
the radicals (high intensities of a laser pulse) in the microꢀ
second time region, some amount of the radical complex
process.31 In this case, the RS Ni(SR) radical adduct
•
2
can be presented as an octahedral complex of the trivalent
nickel ion with three equivalent Ni(SR) ligands.
3
The radical nature of the adduct results in instability
of the radical complex as a whole, which finally decomꢀ
•
disappears due to the reaction with the RS radicals, which
have no time yet to recombine or disappear in the coordiꢀ
nation reaction. In a region of much longer times (interꢀ
vals of seconds), two radical complexes disproportionate
to regenerate disulfide molecules and the starting nickel
complex, thus completing the cycle of photochromic
transformations.
•
poses. For the RS Ni(SR) complex, the monomolecular
2
dissociation channel has, most likely, a low rate constant
(
absence of a cut on the ordinate axis gives an estimate of
–
2 –1
kdiss < 10
s
(see Fig. 8)), and this channel is blocked
by bimolecular reaction (8). If the radical is coordinated
due to the two atoms (S and N), the disulfide in reacꢀ
tion (8) can be formed from species that earlier were
ligands in the coordination sphere of the ion. A possibility
of this process is indicated by a small shift of the maxiꢀ
mum and an increase in the intensity of the longꢀwave
absorption band of the starting complex under prolong
irradiation (Fig. 11). These changes can be caused by the
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 02ꢀ03ꢀ32797,
05ꢀ03ꢀ32474, and 03ꢀ03ꢀ39008GFEN), the Ministry of
Education of the Russian Federation (Grant A04ꢀ2.11ꢀ
304), and the Program "Universities of Russia" (Grant
UR.05.01.002).

Photochromism of RSSR and Ni(SR) solutions
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2373
2
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