-
1 Aleksandr Ivanovich Ponyaev Dr. Sci. (Chem), professor
Chemical Technology of Organic Dyes and Phototropic Compounds
Department, e-mail: [email protected]
A.I. Ponyaev1
INVESTIGATION OF SPECTRAL–KINETIC PARAMETERS OF PHOTOINDUCED
FORMS OF NAPHTHOTHIAZINE, ACRIDINE AND PHENANTHRIDINE SPIROPYRANS
BY FLASH PHOTOLYSIS St. Petersburg State Institute of Technology
(Technical Uni-versity), Moskovskii pr. 26, St. Petersburg, 190013
Russia Abstract—General relations have been revealed in the
reversible photo-chemical coloration–decoloration of
naphthothiazine and acridine spiropy-rans that are characterized by
favorable (from the viewpoint of possible practical applications)
combination of photostability, color depth, and pho-tocoloration
quantum yield. Photoinduced merocyanine forms of compounds of the
naphtho[1,8-de]thiazine series exhibit positive solvatochromism.
The lifetime of the photoinduced form shorters in the series
indoline > phenan-thridine > naphthothiazine > acridine
derivatives. Photocoloration of acri-dine spiropyrans involves the
singlet excited state. Naphthothiazine spiropy-rans in alcohol
solution are converted into the colored form through the singlet
state, and in toluene, partly through the triplet state. The
absorption spectra of photoinduced forms of acridine spiropyrans
are displaced toward longer wavelengths as compared to analogous
indoline derivatives. Intro-duction of a fluorine atom into the
3′-position of phenanthridine spiropyrans increases the rate
constant for the reverse decoloration reaction by 2 to 7 orders of
magnitude. Factors favoring enhanced fatigue resistance of
spiro-pyrans have been determined, in particular the absence of a
nitro group in molecule, singlet path of photocoloration, and
quinoid structure and short lifetime of the photoinduced form. Key
word: spiropyran, photochemistry, photochrome, flash photolysis,
naphthothiazine, acridine, singlet, excited state, rate constant,
fatigue, quinoid structure, fluorine atom, photoinduced form
The Dye Department is one of the oldest departments of
the Institute of Technology. Since 1968 it is called Depart-ment
of Chemical Technology of Organic Dyes and Photo-tropic Compounds.
Dyes have long been known as substances endowing various materials
with different colors, whereas phototropic compounds, i.e., those
capable of reversibly change their properties under irradiation,
have come into practice relatively recently. This wonderful ability
of substanc-es to change their properties in reversible manner by
the action of absorbed light is widely used in light-controlled
sys-tems. Among phototropic substances, the most interesting are
organic compounds that undergo light-induced structural
rearrangement and thus change their optical properties. Compounds
capable of reversibly change their color by the action of absorbed
light are called photochromic. Photo-chromic phenomena occur in
organic, inorganic, complex, and biological systems, in gaseous,
liquid, and solid phases, in polymeric matrices, glasses, gels,
melts, and biological mem-branes. Photochromism forms the basis of
natural photosyn-thesis and visual process. Photochromic
transformations of dyes-photocatalysts mediate photoinduced
decomposition of water to hydrogen in solar energy accumulation
systems [1–6].
Photochromism implies that molecules A on exposure to light with
a definite spectral composition are converted into state B whose
absorption spectrum differs from that of A. The reverse
transformation is induced by irradiation with a different spectral
composition, or it occurs spontaneously (dark reaction), and
molecules revert to the initial state:
The above scheme of photochromic process may be regarded as
ideal; in a real case it is much more complex. Reversible reactions
can be accompanied by irreversible photochemical and thermal
transformations of both initial and photoinduced species, which may
be illustrated by the following scheme:
Many research teams in different countries display sus-
tained interest in various aspects of photochromism. Much
attention to photochromic materials is given due to wide pro-
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I. CHEMISTRY AND CHEMICAL TECHNOLOGY • TECHNOLOGY OF ORGANIC
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spects in practical usage of light-controlled system. There is a
view that the XXIst century will be the century of photonic
technologies [7]. First of all, these are data recording and
processing systems [8], nonlinear optical materials [9], mo-lecular
machines [10], photo computers [11], variable density filters [12],
optical switches [11], sensors (including those for biological
targets) [13], Photo hem for photodynamic therapy of cancer [14],
etc. Particular application imposes specific requirements on
photochromic materials. For example, eye protection requires strong
absorption in the visible and UV regions [15], whereas data
processing systems need materi-als absorbing in the range of
irradiation of semiconductor lasers [11, 16], i.e., generally in
the IR region. Compatibility of photochromic materials with
polymeric matrix is necessary for the design of optical computer
components [17]; light-switchable systems utilize photochromes
possessing a high thermal stability [18]; rapidly relaxing
photochromes are used in data processing devices [19], whereas data
storage re-quires those capable of retaining photoinduced color for
many years [20]. Photochromic sensors should be selectively
sensi-tive to particular parameters, such as pH value, the presence
of metals or charged species, etc. [21]. Solar energy storage
systems based on strain energy of metastable but kinetically stable
structures require materials that maximally utilize the solar
radiation spectrum, while the photoinduced form should be colorless
[3]. By contrast, the initial state of photochromic compounds for
eyeglasses and antidazzle glasses must be colorless, and they
should darken under oncoming traffic light.
Photochromism is a phenomenon combining a number of fundamental
problems of excited state physics and chemistry and “dark” organic
chemistry into a whole entity. Population of excited states
determines the physical photochromism of organic compounds, whereas
chemical bond breakage, isom-erization, redox processes,
tautomerism, dimerization, and other processes underlie the
chemical photochromism.
The efficiency of new technologies and their further
de-velopment are closely related to understanding of fundamen-tal
mechanisms of processes occurring in photochromic com-pounds. Of
radical importance is to elucidate how structural modification of a
molecule affects its photochromic proper-ties; in particular, the
relation between the structure of initial compound and spectral and
kinetic parameters of its photoin-duced form in the series of
complex heteroaromatic com-pounds is a quite significant problem.
Incident light often only initiates a process, while further
transformations include com-plex chemical rearrangements resulting
from two or more types of elementary reactions.
Among all known photochromic compounds, valence isomerization of
spiropyran derivatives has been studied best. Nevertheless, even
that class of photochromic substances is not free from some
significant gaps primarily related to fac-tors determining their
fatigue stability and synthesis of com-pounds absorbing at
maximally long wavelengths.
Spiropyrans constitute the most important and extensively
studied class of organic photochromes. Research work on these very
interesting compounds at the Chemical Technology of Organic Dyes
and Phototropic Compounds Department was initiated by Prof. L.S.
Efros and Dr. E.R. Zakhs. Spiropyrans undergo reversible color
change upon exposure to light in amorphous [22] and crystalline
states [23], aluminosilicate gels [24], clay interlayers [25],
porous glasses [26], liquid solutions [27–30], liquid crystalline
state [31], undercooled melts [32], and polymeric matrix [33–36] as
a result of trans-formation of closed (spiro) structure A into open
(merocya-nine) structure B and backward. There are experimental
proofs indicating that this transformation is mediated by
cis-cisoid isomer C [37–39].
O O
OO
Z
R
Z
R_
+
Z+ _
R
Z
R
hν, Δ
hν', Δ'
А С3 4
5
6
78
3
4
5 6
7
8B
Many spiropyrans also exhibit thermo- and electrochrom-
ism [40–42]. Variation of the molecular fragments linked
to-gether through a spiro carbon atom and introduction of vari-ous
substituent groups makes it possible to vary the spectral range of
photochromic transitions and lifetime of photoin-duced form over a
very broad range [1–5]. Examples of re-verse photochromism were
also reported [43, 44]. In this case, light induces transformation
of initial colored form into colorless. For instance, spiropyrans
of the coumarin series become colored on heating and lose color
under irradiation [45].
The structure–property relations in the spiropyran series were
extensively studied using both experimental and theo-retical
methods [1–5, 46–52]. The nature of the heteroring affects the
relative stability of the Spirocyclic and merocya-nine forms and
spectral–kinetic parameters of photoinduced forms [53–55]. The
stability of merocyanine isomer and its spectral parameters are
determined by the degree of planari-ty of the chromophore
conjugation chain, intramolecular hy-drogen bonding, and evenness
of π-bond orders [56].
Only three representatives of acridine spiropyrans (1–3) have
been described previously as model compounds for studying the
effect of heteroring nature on thermochromic properties of spiro
compounds [57]. Twenty years later, Fish-er [58] reported the
results of low-temperature flash photoly-sis study on photochromic
properties of one of these com-pounds (3). Deeper color of
merocyanine isomers prompted us to examine a larger series of
acridine spiropyrans (1–9), the more so that they have never been
studied in liquid solu-tion at room temperature.
Photochromic properties of acridine spiropyrans (1–9) were
studied by flash photolysis at room temperature.
Table 1.Spectral and kinetic parameters of photoinduced forms of
spiropy-
rans in toluene (20ºC, c0 = 1.5·10–5 M, E = 125 J, d = 20 cm)
Comp.
No R λmax, nm (D)а kt, s-1
1 H 470 (0.03), 620 (0.29) 220 2 8'-OCH3
530 (0.07), 620 (0.10) 360
3 5',6'-benzo 585 (1.15) 1400 4 6'-ОСН3 610 (0.37) 570 5 6'-Br
660 (0.05), 700 (0.07) 74 6 6',8'-Br2
550 (0.008), 725 (0.02) 17
7 6'-Cl-8'-OCH3 655 (0.14) 320 8 6'-NO2 >750 (1.2)а 45 9
6'-NO2-8'-OCH3 >750 (1.3)а 50
10 Ind. 6'-NO2б 595 (1.90) 0.062 11 Ind. 5',6'-benzoб 550 (0.15)
22
а - D is the optical density at the absorption maximum (λ 750 nm
for compounds 8 and 9). б - Indoline heteroring
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I. CHEMISTRY AND CHEMICAL TECHNOLOGY • TECHNOLOGY OF ORGANIC
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O
NMe R
RNMe
O
1
2 3
4
5
67
8
3 4
5
6
78
'
' ''
''
3
4
'
'
hν , Δ hν ', Δ '
Pulsed photoexcitation of colorless spiropyrans 1–9 in po-
lar and nonpolar solvents gave rise to short-lived merocyanine
isomers B which absorbed in the visible region (Fig. 1). The
absorption spectrum of the photoinduced form depended on the
substituent in the chromene fragment. As might be ex-pected,
acridine merocyanines absorb at longer wavelengths than the
corresponding indoline analogs (Table 1). The ab-sorption region of
merocyanines generated from nitro deriva-tives 8 and 9 extends
beyond 750 nm (Fig. 1). We failed to determine the exact positions
of their absorption maxima because of limitations of the flash
photolysis setup in use. Deep color is generally typical of
acridine dyes belonging to different classes. For example, the
absorption maximum of 9-(p-dimethylaminostyryl)acridinium in
alcohol is located at λ 614 nm [59, 60], and acridine monomethine
cyanine dye absorbs at λmax 671 nm [61].
Fig. 1. Electronic absorption spectra of photoinduced forms of
(1–3) acri-
dine and (4) indoline spiropyrans in benzene at 22°C, c0 =
1.5·10–5 M, E = 125 J; (1)
10-methylspiro[acridine-9,2′-[2H]chromene] (1), (2) 10-
methylspiro[acridine-9,3′-[3H]benzo[f]chromene] (3), (3)
10-methyl-6′-nitrospiro[acridine-9,2′-[2H]chromene] (8), (4)
1′,3′,3′-trimethyl-6-nitro-
1′H,3′H-spiro[chromene-2,2′-indole] (10)
The decay of the colored form of all the examined spiro-pyrans
in nonpolar (benzene, toluene) and polar (ethanol) solvents follows
first-order kinetics. Electron-withdrawing substituents stabilize
the open isomer, thus slowing down the ring closure. By contrast,
electron-donating groups shorten the lifetime of the colored form,
which is consistent with the generally observed substituent effects
on the rate of bleach-ing of spiropyrans of other classes [51].
Table 1 also contains photochromic parameters of model
1′,3′,3′-trimethyl-6-nitro-1′H,3′H-spiro[chromene-2,2′-indole] (10)
determined under analogous conditions. It is seen that the
bleaching rate con-
stant of 10 is lower by three orders of magnitude than that
found for nitro-substituted acridine spiropyran 8. This differ-ence
may be rationalized by change of the heteroring nature and steric
hindrances intrinsic to acridine spiropyrans due to hydrogen atoms
in positions 1 and 8. The merocyanine form of
acridine–benzochromene derivative 3 turned colorless at a rate
exceeding the rate of decoloration of its indoline analog 11 by two
orders of magnitude. Thus acridine spiropyrans are characterized by
deeper color and faster decoloration of their merocyanine isomers
as compared to extensively studied indoline analogs.
Photochromic parameters of benzo derivative 3 were measured in
ethanol and benzene. The absorption maximum of the merocyanine
isomer shifts from λ 580 to 640 nm in going from benzene to
ethanol, indicating positive solvato-chromism and hence dominant
contribution of the quinoid structure to the photoinduced form. The
bleaching rate con-stant in alcohol was lower than in benzene only
by 25%, which suggests the absence of hydrogen bonding between the
merocyanine isomer and solvent, in contrast to indoline spiropyrans
[62].
An important aspect of photoinduced transformations of
spiropyrans is the nature of excited state that undergoes ring
opening. The relative quantum yield for photocoloration of acidine
spiropyrans decreases in going from unsubstituted compound 1 to
bromo and dibromo derivatives 5 and 6 (1, 0.3, 0.1). Solutions of
these compounds with equal initial concentrations were subjected to
flash photolysis under simi-lar conditions. The relative quantum
yield was calculated as the ratio of the optical densities at the
absorption maxima of the photoinduced forms assuming equal molar
absorption coefficients at the respective wavelengths. This
assumption seems to be reasonable, for the molar absorption factors
of the photoinduced forms of phenanthridine spirochromenes with the
same substituents differ by a factor of no more than 1.7 [63].
Moreover, the absorbance of the dibromo derivative is even higher
than that of the mono bromo derivative.
Addition of bromobenzene to a benzene solution of spiro-pyran 2
(c0 = 1.96·10–5 M) did not change the absorption spectrum of the
colored form, but the optical density at λ 610 nm decreased almost
twofold as the concentration of bromo-benzene increased from 0 to
1.6 M. To avoid internal filter effect, the jacket of the
spectrophotometric cell and the beakers encompassing the flash
lamps were filled with pure bromobenzene so that the overall
bromobenzene layer thick-ness was about 1 cm. Therefore,
bromobenzene present in solution at a low concentration could not
absorb exciting light and act as internal filter. This means that
the observed drop of the optical density is the result of
competition between the photochemical ring opening reaction in the
singlet excited state and external heavy atom-facilitated
intersystem crossing to the triplet state. Atmospheric oxygen
almost does not af-fect the bleaching rate, and the optical density
of the colored form in air-saturated solution is lower by 6–15%
than in oxy-gen-free solution.
The above experimental results, namely reduction of the
photoisomerization quantum yield in the series 1 > 5 > 6 and
effects of external heavy atom and atmospheric oxygen, sug-gest
singlet nature of the reactive state of acridine spiropy-rans
having no nitro group. The effect of oxygen on the be-havior of
nitro-substituted spirochromenes 8 and 9 was more appreciable: the
optical density of their oxygen-free solutions was higher by 35–40%
as compared to air-saturated ones. However, the kinetics of the
bleaching process almost did not change. This pattern implies some
contribution of triplet states to the photocoloration in direct
photoexcitation of ni-tro-substituted spiropyrans.
The photocoloration process can be sensitized by appro-priate
triplet sensitizers. With benzophenone as triplet donor (Et = 69
kcal/mol), ring opening in acridine spiropyrans 3 and
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I. CHEMISTRY AND CHEMICAL TECHNOLOGY • TECHNOLOGY OF ORGANIC
SUBSTANCES
9 was observed in deoxygenated benzene. In this case, the
absorption spectrum and back reaction kinetics were identical to
those observed on direct photoexcitation. The only differ-ence was
a sharp reduction of the light fatigue resistance upon triplet
sensitization. Even the most photostable spiropy-ran 3 decomposed
to an appreciable extent after a few flash-es under flash
photolysis in benzene solution in the presence of benzophenone.
When the flash photolysis was carried out under simultaneous
continuous irradiation with a DRSh-1000 lamp, signal from the
photoinduced form disappeared in 15 min. In the absence of
benzophenone, the parameters of the photoinduced form did not
change over a period of more than 2 h. To prevent direct absorption
of the exciting light by spi-ropyran, the flash lamps were immersed
in a solution of the same spiropyran with a tenfold concentration.
Therefore, we concluded that the photodecomposition is mediated by
the triplet excited state which is populated via triplet–triplet
trans-fer from benzophenone. Addition of benzophenone to an
air-saturated benzene solution where triplet–triplet transfer is
ruled out by triplet quenching with oxygen did not lead to an
appreciable decomposition (Fig. 2).
Fig. 2. Relative variation of the photoinduced optical density
at λmax of
spiropyrans (at λ 750 nm for compound 9) versus the number of
flashes (N); benzene, c = 1.5·10–5 M, E = 125 J; (1–3) on exposure
to air; (4, 5)
oxygen-free solution; (1, 4)
10-methylspiro[acridine-9,3′-[3H]benzo[f]chromene] (3), (2, 3, 5)
8′-methoxy-10-methyl-6′-
nitrospiro[acridine-9,2′-[2H]chromene) (9); (1, 2) in the
absence of benzo-phenone; (3–5) in the presence of
benzophenone.
Thus the main path of singlet state deactivation of acri-
dine spiropyrans is photochemical pyran ring opening. The
contribution of intersystem crossing to the triplet state is small,
but it increases upon introduction of a nitro group, which is
reflected in increased induced absorbance of oxygen-free
solutions.
Rise in the concentration of photoinduced merocyanine and just
the possibility for triplet sensitization suggest that the
photochemical ring opening could involve the triplet state. On the
one hand, participation of the triplet state in the
pho-tocoloration process increases the overall yield of the colored
form, and on the other, sharply reduces the light fatigue
re-sistance of spiropyrans since irreversible photodegradation (as
follows from the obtained data; see Fig. 2) occurs mainly through
triplet states.
Photochromic parameters of spiropyrans are determined by the
nature of both heterocyclic fragments linked by a spiro atom. The
relative stabilities of colored merocyanine isomer B and colorless
spiropyran A strongly depend on the stereoelec-tronic parameters of
the constituent fragments and substitu-ents therein [64–66]. For
example, the presence of bulky substituents in positions
neighboring to the spiro carbon atom
is essential for enhanced stability of the closed isomer of
ben-zothiazoline and azine spiropyrans [67, 68].
ONR
R,
R,,
X
X = C(Alk)2, S, O, Se; R' = Alk, Ar, OAlk, OAr, SMe, SPh.
It is also known that spiropyrans of the perimidine series under
ambient conditions are more stable than isomeric mer-ocyanines
[69], whereas their azole analogs, benzimidazole derivatives, in
most cases are stable only in the merocyanine form [66, 70].
O
N
NMe
Me
RNCH3
+
N
OCH3-
We presumed that the presence of a 1,3-thiazine frag-
ment in the peri-naphthothiazine system should favor spiro
cyclization as well.
O
S
NMe
S
N
OMe
R +
_
Rα
β
hν
12А-15А 12B-15B 12 R=H, 13 R=5',6'-benzo, 14 R=8'-OMe, 15
R=7'-NEt2
Fig. 3. Electronic absorption spectra of (1)
3′-methyl-2′,3′-dihydrospiro[2H-
chromene-2,2′-naphtho[1,8-de][1,3]thiazine] (12) in ethanol and
(2, 3)
N,N-diethyl-3′-methyl-2′,3′-dihydrospiro[2H-chromene-2,2′-naphtho[1,8-
de][1,3]thiazin]-7-amine (15) in (2) heptane and (3)
ethanol.
Crystalline spiropyrans 12A–15A are colorless, and their
solutions in common organic solvents are also colorless. Only
compound 15A having a diethylamino group undergoes par-tial
isomerization upon dissolution in ethanol; its solution ac-quires
an intense pink color, and the long-wave absorption band features
two maxima with almost equal intensities at λ 526 and 547 nm
(apparent molar absorption coefficients ε =
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I. CHEMISTRY AND CHEMICAL TECHNOLOGY • TECHNOLOGY OF ORGANIC
SUBSTANCES
1200 and 1250 cm2/mmol; Fig. 3). To evaluate spectral
pa-rameters of isomer 15B we used styryl dye SD1 as model; the
absorption maximum of the latter almost coincides with that of
colored isomer 15B [71, 72]. Assuming that the molar absorption
coefficients of 15B and SD1 are similar, the equi-librium
concentration of 15B in alcohol was estimated at about 2.5%.
S
N NMe2CH3
+
ClO4_
SD1 Strong stabilization of colored structure B is achieved
by
introduction into the 7′-position of a strong electron-donating
substituent which, after opening of the pyran ring, appears in
direct polar conjugation with the cationic heteroatom. The thermal
equilibrium of diethylamino-substituted peri-naphthothiazine
derivative 15A at room temperature is dis-placed toward the
spirocyclic structure to a considerably greater extent as compared
to indoline and phenanthridine analogs 16 and 17. This is
consistent with the lower acidity of the peri-naphthothiazine
system relative to indoline and phenanthridine, which was estimated
by the deviations of λmax of the corresponding styryl dyes. The
minimal deviation was found for naphthothiazine dye SD1 (16%), a
larger deviation, for indoline derivative SD2 (24%), and the
maximal, for phe-nanthridine analog SD3 (~65%).
To calculate the relative deviations we used λmax for styryl
dyes SD1 and SD3 and previously reported values for indo-line
derivative SD2 (λmax 545 nm) [74], symmetric indoline
trimethinecyanine (λmax 548 nm) [75], Michler’s hydrol blue (λmax
607.5 nm) [76], and symmetric phenanthridine (λmax 610 nm) [77] and
peri-naphthothiazine trimethinecyanines (λmax 530 nm) [71].
ON
MeMe
MeNEt2
NMe
MeMe
NMe2+
I_
16 SD2
λmax (EtOH) nm, (lg ε): 545, (4.92)
O
NMe
NEt2
NMe
NMe2
+
MeSO4_
17 SD3
λmax (EtOH) nm, (lg ε): 513, (4.42)
Fig. 4. Electronic absorption spectra of the colored form of
N,N-diethyl-3′-methyl-2′,3′-dihydrospiro[2H-chromene-2,2′-naphtho[1,8-de][1,3]thiazin]-7-amine
(15) in deoxygenated solutions in (1) toluene, c = 4.1·10–5 M, 100
μs after flash start; (2) ethanol, c = 6.8·10–6 M, 100 μs after
flash start;
and (3) toluene, c = 1.7·10–6 M, 50 ms after flash start. Flash
energy 125 J.
We were the first to reveal photochromic properties of
spiropyrans 12–17 in liquid solution [78]. Pulsed
photoexcita-tion of solutions of 12A–15A in toluene or ethanol at
room temperature induced absorption in visible region, and the
absorption pattern did not change to an appreciable extent if air
was present in the system (Fig. 4, Table 2). The absorp-tion
maximum of the colored form in more polar solvent ap-peared at
longer wavelengths.
Unlike indoline [1, 47] and phenanthridine spiropyrans [79]
studied previously, colored isomers 13B–15B exhibit positive rather
than negative solvatochromism, indicating their predominantly
quinoid structure. The absorption maxima of the colored form of
unsubstituted derivative (12B) in etha-nol and toluene are almost
similar.
Table 2. Thermal bleaching rate constants of the photoinduced
forms of
spiropyrans 12–15
Comp. No
R
k, s-1 Published data for analogs of 12–15 with different
heterocycles (toluene)
Tolu-ene
96 % Etha-nol
Phe-nanthri
thri-dine
Indo-line
1,3-Dithi-ole
12 H 91 25 29 (0.96) —13 5',6'-benzo 2800 700 100 17 (3.3)a14
8'-OCH3 70 16 8,70 1,9 —15 7'-N(C2H5)2
130 34 220b 1.1b,c (0.8)a a In dioxane. b Determined in this
work. c In heptane
5,6-Benzo annulation of the chromene fragment and in-
troduction of a diethylamino group into its 7-position
consid-erably increase the optical density at λmax of the colored
forms (B) of compounds 13 and 15 in toluene and ethanol relative to
that observed for unsubstituted derivative 12, other condi-tions
(initial concentration, flash energy, spectral composition of the
exciting radiation, and excitation geometry) being equal. Provided
that the molar absorption coefficients of mer-ocyanines 12B–15B are
fairly similar, as was determined for phenanthridine derivatives
[63], the observed difference in the photoinduced optical densities
should be attributed to considerably higher photocoloration quantum
yield of 13A and 15A.
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I. CHEMISTRY AND CHEMICAL TECHNOLOGY • TECHNOLOGY OF ORGANIC
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As noted above, compound 15 in alcoholic solution exists
mainly as spiropyran 15A, while the fraction of merocyanine
isomer 15B is as small as ~2.5%. Pulsed UV radiation leads to
strong increase of absorbance in the visible region due to rise in
the concentration of 15B as a result of A → B pho-toisomerization.
In all cases, the photoinduced optical density is strictly linear
in the flash energy at the initial part, which implies a
single-quantum photocoloration process. When the initial
concentration of 15A in alcohol is low (c0 = (10–6 M), the induced
optical density tends to saturation as the flash energy increases,
though it does not reach a limiting value even at a flash energy of
400 J. The gain in the optical densi-ty per energy unit merely
decreases at high flash energies (200–400 J). Therefore, the
limiting optical density may be estimated only by extrapolation.
The limiting value corre-sponds to the maximum possible (under the
given excitation conditions) displacement of the A↔B equilibrium
toward merocyanine isomer. The minimal molar absorption coefficient
of the photoinduced isomer can be evaluated assuming that there are
no processes reducing the concentration of isomer B (e.g., back
photoreaction B→A) and that the concentration of merocyanine 15B in
the saturation region is equal to the initial concentration of 15
(i.e., 100% conversion A→B). Its values should be no less than
1.1·104 cm2/mmol at the ab-sorption maximum (λ 550 nm).
Correspondingly, the fraction of merocyanine 15B in alcohol
solution at room temperature should not exceed 10%. The difference
from the value de-termined with the use of model styryl dye SD1
(2.5%) is likely to reflect processes neglected in the simplified
ap-proach, which reduce the real concentration of 15B.
As follows from the data in Table 2, the bleaching rate constant
for all the examined compounds decreases in going from toluene to
96% ethanol. Such relation between the bleaching rate constant and
solvent polarity is generally typi-cal for negative solvatochromism
of the colored isomer (e.g., for indoline [42] and phenanthridine
spiropyrans [80]) rather than positive as in our case. Despite
predominating quinoid structure of the merocyanine isomer, it
remains more polar than the initial spiropyran, and lower bleaching
rate in more polar solvent is determined by better stabilization of
more polar merocyanine B compared to less polar spiropyran A [81].
Nevertheless, in the series of heterocyclic spiropyrans there are
examples of the reverse bleaching rate constant–solvent polarity
relation. For instance, naphthofuran [82] and 2-oxaindan
spiropyrans [83] display increase in the bleaching rate constant
with rise in solvent polarity. These compounds, as well as
naphthothiazine spiropyrans 13–15, are character-ized by positive
solvatochromism. The above data indicate an important role of the
heteroring nature in the bleaching pro-cess. The cyclization to
colorless spiropyran with almost or-thogonal orientation of
molecular fragments is preceded by isomerization and charge
transfer from the heteroatom to the carbon atom in position 2. This
charge transfer is more facile for less basic heterocycle [84],
while the ease of isomerization in solution is determined mainly by
the orders of bonds in-volved in that isomerization. Therefore,
provided that the cyclization is the rate-limiting step of
bleaching of spiro-chromenes, the nature of the other heteroring
becomes cru-cial, for it determines the magnitude of local charges
on the oxygen and carbon atoms. As already noted, naphthothiazine
ring is less basic than indoline (according to Brooker [73]); in
addition, quinoid structure of merocyanine isomer implies a lower
order of the C3′–C4′ bond; these factors should facilitate the
isomerization. Thus, the main reasons favoring the cy-clization of
merocyanines 12B–15B to the corresponding spiropyrans apply better
to naphthothiazine spiropyrans as compared to indoline and
phenanthridine derivatives. From the kinetic viewpoints, this is
reflected in shortening of the lifetime of the colored merocyanine
isomer in the series indo-line > phenanthridine >
naphthothiazine derivatives (Table
2). The lifetime of the colored form of differently substituted
naphthothiazine spiropyrans, determined under unimolecular
bleaching conditions, changes in approximately the same order of
substituents as that observed for indoline, phenan-thridine, and
dithiole spiropyrans. Benzo derivative 13 turned out to be most
kinetically labile at room temperature, though, according to [85],
5,6-benzo annulation of the chromene ring should enhance
thermodynamic stability of the colored (open) form compared to
spirocyclic structure. Presumably, the acti-vation barrier to the
back cyclization of 13B is lower than those for compounds with
different substituents, which is responsible for the shorter
lifetime of the merocyanine iso-mer.
Thus, a new class of photochromic spiropyrans belonging to the
naphthotiazine series has been revealed and character-ized by
spectral–kinetic method.
We tried to control the rate of bleaching in the series of
phenanthridine spiropyrans via introduction of a substituent into
the 3-position of the chromene moiety. Substituted
3-fluoro-5-methyl-5′,6′-dihydrospiro[2H-chromene-2,6′-phenanthridines]
18–22 were subjected to flash photolysis in benzene at room
temperature, and parameters characterizing their photochromic
behavior were compared with those for fluorine-free analogs.
O
NM e
F
R R = H (18), 8'-ОСН3 (19),
6'-Cl-8'-ОСН3 (20), 7'-N(C2H5)2 (21), 6'- NO2 (22)
Unlike fluorine-free analogs, the colored forms of fluorine-
containing spiropyrans 18–22 displayed more uniform ab-sorption
pattern in a broad range (λ 420–750 nm). The ther-mal bleaching
reaction conformed to first-order kinetic equa-tion at least within
the flash energy range from 20–500 J and in the flash duration
range from 50 μs to complete decolora-tion. By contrast, the
initial part of the kinetic curves for fluo-rine-free spiropyrans
indicated formation of a short-lived iso-mer in a low
concentration. The rate constants for bleaching of the colored
isomers of 18–20 were higher by two orders of magnitude than those
found for the corresponding fluo-rine-free analogs, and the
bleaching rate constant for nitro derivative 22 was higher even by
seven orders of magnitude (Table 3).
Table 3. Thermal bleaching rate constants for colored forms of
fluorine-
containing (kF) and fluorine-free (k) phenanthridine spiropyrans
in toluene Comp. No R k, s-1 (20º С) kF, s-1
18 Н 3·10 5,5.10319 8'-ОСН3 9 1,6.10320 6'-Cl-8'-ОСН3 5,1
1,5.10321 7'-N(C2H5)2 2,2.102 b 2,9.10222 6'- NO2 3,8.10-4 a
1,4.103
a In dioxane. b In heptane
The reason is that the presence of a fluorine atom in the α-
position with respect to the cyclization reaction center
increases the positive charge on the latter, thus favoring the
reverse reac-tion. Furthermore, larger size of fluorine atom
compared to hy-drogen causes the open photoinduced form to deviate
from pla-nar structure, which also reduces its stability.
Spiropyrans having no substituent in the 3-position of the
chromene moiety conform to an empirical rule according to which
electron-withdrawing substituents in the chromene frag-
-
I. CHEMISTRY AND CHEMICAL TECHNOLOGY • TECHNOLOGY OF ORGANIC
SUBSTANCES
ment reduce the rate of thermal bleaching while
electron-donating groups increase it. Although the dependence of
the bleaching rate constant upon Hammett substituent constant
qualitatively reflects this relation, it is far from being linear.
Espe-cially strong deviation is observed for 7-diethylamino
derivative 21. Deviations from linearity may be rationalized by
steric and conjugation effects in addition to electronic. In
particular, a strong electron-donating substituent in the
7-position appears, after pyran ring opening, to be conjugated with
the cationic het-eroatom via direct polar effect, which should
strongly stabilize the colored form (B) and slow down the
isomerization B→A.
On the other hand, electron-donor effect of the diethylamino
group should accelerate the same process due to increase of the
negative charge on the oxygen atom. Just these opposite effects
cause the Hammett dependence to deviate from linearity.
The relative light resistance of spiropyrans 12–22 belonging to
different classes was estimated in benzene using a combina-tion of
continuous irradiation with a DRSh-1000 lamp (λ 313 nm) and flash
photolysis. The optical density at the absorption maxi-mum of the
colored isomers (at λ 750 nm for acridine nitro deriv-atives)
generated by identical flashes was measured every 15 min under
continuous irradiation at λ 313 nm. Under these condi-tions,
5,6-benzo derivative 3 of the acridine series turned out to be the
most stable (it was superior to indoline analog 11), while 6-nitro
indoline spiropyran 10 was the least stable. Nitro substi-tution on
the chromene fragment in the series of acridine spiro-pyrans
reduces the light resistance as well, but even least stable (among
acridine spiropyrans) 6′-nitro derivative 8 displayed bet-ter
stability than known indoline spiropyran 10 (Fig. 5). Thus the
light resistance of spiropyrans increases in the following
heteror-ing series: indoline < phenanthridine <
phenanthridine (3-fluorochromene) < naphthothiazine <
acridine
Fig. 5. Relative variation of the optical density at λmax of the
colored forms
of spiropyrans 3, 7, and 9–11 in benzene solution upon
continuous irradia-tion with a DRSh-1000 lamp: (1)
10-methyl-10H-spiro[acridine-9,3′-[3H]benzo[f]chromene] (3), (2)
1′,3′,3′-trimethyl-1′H,3′H-spiro[3H-
benzo[f]chromene-3,2′-indole] (11), (3)
10-methyl-6′-chloro-8′-methoxy-10H-spiro[acridine-9,2′-[2H]chromene]
(7), (4)
8′-methoxy-10-methyl-6′-nitro-10H-spiro[acridine-9,2′-[2H]chromene]
(9) (λ 750 nm), (5) 1′,3′,3′-
trimethyl-6-nitro-1′H,3′H-spiro[2H-chromene-2,2'-indole]
(10).
To conclude, we have revealed experimentally the follow-ing
factors responsible for light resistance of spiropyrans:
predominately singlet path of the photocoloration process,
quinoid structure and short lifetime of the colored
photoin-duced form, and the absence of nitro substitution.
Obviously, spiropyrans of the acridine series are characterized by
favora-ble combination of light resistance, color depth, and
photo-coloration quantum yield, which makes them promising for
practical application.
The results presented in this article were obtained at the
Chemical Technology of Organic Dyes and Phototropic Com-pounds
Department in collaboration with E.R. Zakhs, V.P. Matrynova, R.P.
Polyakova, N.G. Leshenyuk, and L.A. Zvenigorodskaya.
The study was performed under financial support by the Russian
Foundation for Basic Research (project no. 13-08-1425-а).
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