Activation of fluorescence of lactone forms of rhodamine dyes by photodehydrogenation of aryl(hetaryl)pyrazolines

Article abstract of DOI:10.1007/s11172-016-1365-4

Aryl(hetaryl)pyrazolines are effective photogenerators of acid providing an irreversible photochemical activation of fluorescence of rhodamine group dyes. The effect of solvent on two consecutive reactions, photodehydrogenation of pyrazoline and lactone ring opening of the dye was studied. An increase in solvent polarity leads to an increase in the rate of both reactions by different degrees. The systems under study are believed to be promising for the formation of recording media for multilayer optical file type disks of ultrahigh data storage capacity with fluorescent readout.

Full text of DOI:10.1007/s11172-016-1365-4

Russian Chemical Bulletin, International Edition, Vol. 65, No. 3, pp. 735—740, March, 2016
735
Activation of fluorescence of lactone forms of rhodamine dyes
by photodehydrogenation of aryl(hetaryl)pyrazolines
V. F. Traven, S. M. Dolotov, and I. V. Ivanov
D. I. Mendeleev University of Chemical Technology of Russia,
9 Miusskaya pl., 125047 Moscow, Russian Federation.
Eꢀmail: valerii.traven@gmail.com
Aryl(hetaryl)pyrazolines are effective photogenerators of acid providing an irreversible
photochemical activation of fluorescence of rhodamine group dyes. The effect of solvent on
two consecutive reactions, photodehydrogenation of pyrazoline and lactone ring opening of the
dye was studied. An increase in solvent polarity leads to an increase in the rate of both reactions
by different degrees. The systems under study are believed to be promising for the formation of
recording media for multilayer optical file type disks of ultrahigh data storage capacity with
fluorescent readout.
Key words: pyrazoline, rhodamine lactone, fluorescence.
Rapid development of computer and information techꢀ
nologies requires higher performance data recording sysꢀ
tems, capable of storage, readout and processing of huge
amounts of data using organic light sensitive media of
various types.1—7 One of the most promising ways of inꢀ
creasing the amount of recorded information is the develꢀ
opment of multilayer optical disks, which provide an inꢀ
crease in data capacity by several orders of magnitude
compared to modern optical data carriers through the use
of a twoꢀphoton process of data recording.
In particular, a promising direction is the development
of recording media for multilayer optical file type disks, in
which a single twoꢀphoton recording of data with laser
radiation is carried out in the form of the soꢀcalled fluorꢀ
escent pits of information. The pits are formed as a result
of photochemical transformation of nonfluorescent orꢀ
ganic precursors to compounds, which are characterized
by strong fluorescence (a reverse process of fluorescence
decay is also possible as part of a photochemical reacꢀ
tion).8—12 The recorded fluorescent form of the material,
which makes up the pits, must possess a high fluorescence
quantum yield, whereas the precursor and the photoprodꢀ
uct must be thermally stable.13—15 Coumarins, lactones,
and lactams of xanthene rhodamine group dyes, thioindiꢀ
goid dyes, stilbene derivatives, substituted anthracenes,
anthraquinones, and other compounds were studied as
fluorescent precursors suitable for use in systems of optical
data recording.16—19 Some of the precursors listed above
are able to activate fluorescence in the absence of any
additives merely upon influence of radiation. However,
the UV radiation required for this is quite strong and may
cause destruction of the dye.
Irradiation of the precursor in the presence of an acid
photogenerator (APG) is less destructive. For this proꢀ
cess, the role of the APG consists in the transformation of
the fluorescent precursor molecule: elimination of a proꢀ
tecting group,20,21 lactone/lactam ring opening in the
leukobase,17,22—24 protonation of the amino group for the
purpose of deactivation of fluorescence or the shift of the
emission maximum to the long wavelength region. A light
sensitive thermally stable compound, which under excitaꢀ
tion by light of an appropriate wavelength undergoes
a photochemical transformation with the formation of an
acid, is used as the APG. Typical acid photogenerators,
which are suggested for use in systems of file type data
recording, are triarylsulfonium and triaryliodonium salts
of some organic and inorganic acids, sulfonic acid derivaꢀ
tives, as well as nitrobenzaldehydes and nitronaphthꢀ
aldehydes.14
By studying new photoregistering media for file type
data recording, we showed that the role of the APG can
also be carried out by some halogenꢀcontaining comꢀ
pounds.25 However, their photochemical properties reꢀ
quire the use of short wavelength UV radiation (λ = 254 nm),
which leads to partial photodestruction of the precursor.
Therefore, a pressing concern is the search for new acid
photogenerators, which are capable of activating the fluoꢀ
rescence of precursors under longer wavelength irradiaꢀ
tion, which is consequently less destructive.
Results and Discussion
Earlier we reported on the photodehydrogenation reꢀ
action of 5ꢀ(4ꢀanisyl)ꢀ3ꢀ(4ꢀhydroxycoumarinꢀ3ꢀyl)ꢀ1ꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0735—00740, March, 2016.
1066ꢀ5285/16/6503ꢀ0735 © 2016 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 65, No. 3, March, 2016
Traven et al.
A (arb. units)
1.0
phenylpyrazoline (1) under irradiation treatment in the
presence of carbon tetrachloride.26—28 As it turns out, phoꢀ
todehydrogenation is not limited to this pyrazoline among
derivatives of coumarin and its analogues. About 20 pyrꢀ
azolines containing at positions 1, 3, and 5 fragments of
coumarin, its analogs, and benzene derivatives underwent
photodehydrogenation. A scheme for transformations ocꢀ
curring during the photodehydrogenation process is sugꢀ
gested (Scheme 1).28
1
0.8
0.6
0.4
0.2
10
350
400
450
500
550 λ/nm
Fig. 1. Changes in the absorption spectrum of a solution of
pyrazoline 1 (c = 24 μmol L–1) in toluene in the presence of
hexachloroethane (c = 8.2 mmol L–1) before (1) and after (2—10)
irradiation with UV light with a wavelength of 420 nm.
(λ = 405 nm, toluene)
abstract a hydrogen atom from the corresponding subꢀ
strate; in turn, the chloride ion can act as a proton acꢀ
ceptor.30,31
In contrast to authors of work,29 discussing the photoꢀ
dehydrogenation mechanism of Hantzsch dihydropyꢀ
ridines under similar conditions, we do not assume a diꢀ
rect transfer of energy from the pyrazoline molecule to
a CCl4 molecule. During the first stage, pyrazoline apparꢀ
ently is transferred into an excited state, after which an
electron is transferred from the excited pyrazoline moleꢀ
Continuing the study of the photodehydrogenation reꢀ
action of aryl(hetaryl)pyrazolines, we investigated their
behavior as APG in various media. It was found that the
indicated reaction can take place not only in carbon tetraꢀ
chloride, but also in other solvents in the presence of subꢀ
strates, which contain trihalomethyl fragments. As it turns
out, this reaction proceeds smoothly, for example, in toluꢀ
ene in the presence of hexachloroethane. The change in
the absorption spectrum of pyrazoline 1 upon of irradiaꢀ
tion its solution in toluene is shown in Fig. 1.
The particularity of the transformation (see Scheme 1)
is the abstraction of a proton, i.e., the generation of acidity
during the process of irradiation of aryl(hetaryl)coumarin.
We studied the possibility of using aryl(hetaryl)pyrazolines
as APG for lactone form ring opening and activation of
fluorescence of rhodamine dyes, using lactone forms of
rhodamine B and rhodamine 19 as examples.
.–
cule to the CCl4. The CCl4 radical anion is extremely
unstable and consequently quickly dissociates into a triꢀ
chloromethyl radical and a chloride ion. The formed pyrꢀ
azoline radical cation splits off a proton and reacts
with the trichloromethyl radical to form pyrazole.
According to this scheme, the aryl(hetaryl)pyrazoline molꢀ
ecule acts as a destruction sensitizer of CCl4 under the
effect of radiation. Earlier it was shown that the CCl4
molecule can act as an acceptor of electrons. Accepting
an electron, this molecule forms an unstable radical anion,
which very quickly dissociates liberating the chloride ion
and forming a trichloromethyl radical, which can then
Scheme 1

Fluorescence of rhodamine dyes
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 3, March, 2016
737
A (arb. units)
0.7
1
0.6
12
0.5
0.4
0.3
Rhodamine B
(λ fl = 587 nm)
max
0.2
0.1
12
1
350 400 450 500 550 600 650
λ/nm
Fig. 2. Changes in the absorption spectrum of a solution of pyrꢀ
azoline 1 (c = 48 μmol L–1) and rhodamine B (c = 50 μmol L–1
)
)
in toluene in the presence of hexachloroethane (c = 840 mкmol L–1
before (1) and after (2—12) irradiation with UV light with
a wavelength of 420 nm.
Rhodamine 19
(λ fl = 560 nm)
max
an appearance of a fluorescence band with a maximum at
587 nm that indicates the accumulation of the open form
of rhodamine B in the solution (Fig. 2). The irradiation of
a solution of the lactone form of rhodamine B not conꢀ
taining pyrazoline 1 under the same conditions does not
result in any spectral changes.
The appearance of color of rhodamine B under the
indicated conditions is caused by two following reactions
occurring in the solution (Scheme 2): the photodehydroꢀ
genation of pyrazoline with the proton abstraction and the
dye lactone ring opening upon treatment with this proton.
We measured the rates of the indicated reactions in
various solvents. First of all, it was determined that the
presence of the lactone form of the rhodamine dye in the
solution does not significantly affect the rate of pyrazoline
photodehydrogenation. The pseudoꢀfirst order reaction
rate constants in the absence and in the presence of
the lactone form of the dye were equal to 0.0402 and
0.0391 mol L–1 s–1, respectively.
fl
(λmax = 385 nm, toluene)
(λ
= 360 nm, toluene)
max
For the activation of fluorescence of the lactone forms
of these dyes, pyrazolines 1—3 were used.
As it turns out, aryl(hetaryl)pyrazolines 1 and 2 are
effective activators of fluorescence of rhodamine dyes
under irradiation of the corresponding solutions in the
pyrazoline absorption maxima in both carbon tetrachloride
and toluene with the addition of 2—5% CCl4 or hexachloroꢀ
ethane. For example, irradiation of a solution of pyrꢀ
azoline 1 and the lactone form of rhodamine B in toluene
in the presence of hexachloroethane with filtered light
(light filter ZhSꢀ10) leads to the solution changing its
color to pink (absorption band with λmax = 560 nm) and
Using the change of the absorption band intensity of
pyrazoline 1 (420 nm) and rhodamine B (560 nm) during
Scheme 2

738
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 3, March, 2016
Traven et al.
of this H+ꢀcatalyzed reaction. The reaction of photodehyꢀ
drogenation of pyrazoline is less sensitive to solvent polarꢀ
ity due apparently to a radical character of some of its
steps (see Scheme 1). The results of experiments carried
out in the presence of inhibitors of radical reactions such
as trialkylphenol 4, nitroxyl radical 5, acetophenone, and
benzophenone, indicate that the character of this reaction
is at least partially radical (Table 2).
lnC
–9
1
2
–10
–11
–12
–13
–14
5
10
15 τ/s
Fig. 3. Semilogarithmic plots of the dependence of the substrate
concentration (C) on the duration of irradiation with UV light
with a wavelength of 420 nm for the photodehydrogenation of
pyrazoline 1 (1) and opening the lactone form of rhodamine B (2).
All studied inhibitors decrease the rate of photodeꢀ
hydrogenation reaction of pyrazoline 1, and the most
effective inhibitor turned out to be the nitroxyl radical 5.
A comparably low inhibition effect is probably explained by
fact that the step of the pyrazolinyl radical formation
(see Scheme 1) is not rateꢀlimiting. Similar changes were
also observed in absorption spectra of pyrazolines 2 and 3
upon irradiation of their solutions under the conditions
described above in the presence of the lactone form of
rhodamine dyes.
At the same time, the changes in the absorption specꢀ
tra under irradiation of solutions of pyrazoline 1 and lacꢀ
tone form of rhodamine dye in DMF (Fig. 4) is noticeably
different from the results, obtained in toluene (see Fig. 2).
The absorption maximum of pyrazoline 1 in DMF is
located at 380 nm, however, under irradiation it rapidly
shifts to a longer wavelength region to 420 nm, further
changes being similar to those shown in Fig. 2. We suggest
that the spectral changes in the initial stage of phototransꢀ
formation (see Fig. 4) are caused by tautomeric transforꢀ
mations of pyrazoline 1. This suggestion is in agreement
with the changes in the absorption spectrum of compound 1,
which were observed upon the change of the solvent comꢀ
position on going from toluene to DMF (Fig. 5) as a result
of the transformation of a keto form of pyrazoline 1 to
a hydroxy form (Scheme 3).
irradiation of the solution, we calculated the rate conꢀ
stants of the reaction of dehydrogenation of pyrazoline 1
and H+ꢀcatalyzed opening of the lactone form of rhodꢀ
amine B. As it can be determined from the linear depenꢀ
dencies lnC/τ, where τ is the duration of irradiation, both
reaction adhere to first order kinetics (Fig. 3). At that, the
photodehydrogenation reaction occurs somewhat slower
than lactone ring opening: the rate constants are equal to
0.039 0.001 and 0.122 0.006 mol L–1 s–1, respectively.
The noted ratio of the two reaction rates was observed in
other solvents as well, and, with an increase of solvent polarꢀ
ity, the rate of lactone ring opening increases to a greater
degree than the rate of the photodehydrogenation reaction.
Using the results of irradiation of solutions in different
solvents and with different concentration ratios of pyrazolꢀ
ine 1 and rhodamine B, we plotted linear dependencies of
the concentration as a function of the duration of irradiꢀ
ation (lnC/τ), which were used to calculate the rate conꢀ
stants of the photodehydrogenation reaction of pyrazoline 1
and lactone ring opening in rhodamine B (Table 1).
A greater sensitivity of lactone ring opening to the solꢀ
vent polarity is probably explained by the ionic character
Table 1. Rate constants (k) of the photodehydrogeꢀ
nation reaction of pyrazoline 1 (I) and lactone ring
opening of rhodamine B (II) in solvents of various
polarity
Table 2. Rate constants (k) of the photodehyꢀ
drogenation reaction of pyrazoline 1 in toluꢀ
ene with hexachloroethane in the presence of
some radical inhibitors
Solvent
k/mol L–1 s–1
I
II
Inhibitor
–k/mol L–1 s–1
Toluene
Ethyl acetate
Acetone
MeCN
–0.0391
–0.0625
–0.0931
–0.0589
–0.1670
0.1057
0.1333
0.9966
1.0894
1.5407
Without inhibitor
Compound 4
Compound 5
Acetophenone
Benzophenone
0.0931
0.0394
0.0227
0.0409
0.0363
DMF

Fluorescence of rhodamine dyes
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 3, March, 2016
739
A (arb. units)
A (arb. units)
3.0
Ifl (arb. units)
2.5
9
8000
6000
4000
2000
2.0
1.5
2.5
2.0
1.5
1.0
0.5
1.0
2
4
1
0.5
9
1
350
400
450
500
550
600 λ/nm
3
Fig. 4. Changes in the absorption spectrum of a solution of pyrꢀ
azoline 1 (c = 46 μmol L–1) and lactone form of rhodamine B
(c = 50 μmol L–1) in the presence of hexachloroethane in DMF.
Duration of irradiation is 2.2 s.
1
400
500
600
λ/nm
Fig. 6. Absorption (1, 2) and fluorescence (3, 4) spectra of pyrꢀ
azoline 2 (21 mmol L–1)—rhodamine B (2 mmol L–1)—hexaꢀ
chloroethane (0.18 mol L–1) system in a poly(methyl methaꢀ
crylate) film before (1, 3) and after (2, 4) irradiation with UV
light through a light filter UFSꢀ1 (radiation in the 330—400 nm
wavelength region).
A (arb. units)
1.0
6
0.8
0.6
(i) the irradiation of a solution of the lactone form of
rhodamine B in the presence of hexachloroethane in DMF
does not lead to noticeable changes in the absorption specꢀ
trum; (ii) the irradiation of the solution of pyrazolines 2
and 3 together with the lactone form of rhodamine B in
the presence of hexachloroethane in DMF is not accomꢀ
panied by the changes similar to those observed for pyrꢀ
azoline 1, since pyrazolines 2 and 3 cannot undergo tautoꢀ
meric transformations.
The irradiation of aryl(hetaryl)pyrazolines in polymeric
films in the presence of hexachloroethane and the lactone
form of rhodamine dyes is also accompanied by activation
of fluorescence of the dyes. Figure 6 gives the changes in
the absorption and fluorescence spectra of poly(methyl
methacrylate) film containing pyrazoline 2, hexachloroꢀ
ethane, and lactone of rhodamine B upon their irradiation
with light with a wavelength of 360 nm (see Fig. 6). The
obtained results indicate that the studied media are promꢀ
ising candidates for file type data recording with fluoresꢀ
cent readout.32,33 Similar to solutions, the irradiation of
the lactone form of rhodamine B in a poly(methyl acrylꢀ
1
0.4
0.2
350
400
450
500
550
600 λ/nm
Fig. 5. Electron absorption spectra of pyrazoline 1 in DMF (1),
in the mixtures of DMF—toluene in ratios 8 : 2 (2), 6 : 4 (3), 4 : 6 (4),
2 : 8 (5) and in toluene (6).
To sum up, under irradiation of a solution of pyrazoꢀ
line 1 and the lactone form of rhodamine B in the presꢀ
ence of hexachloroethane, initially a tautomeric transiꢀ
tion of pyrazoline 1 to a hydroxy form was observed that
led to a shift of the absorption maximum of pyrazoline to
a longer wavelength region. Further irradiation leads to
a gradual photodehydrogenation of pyrazoline 1 and openꢀ
ing of the lactone form of rhodamine B. The following
facts are in agreement with the suggested explanation:
Scheme 3
Hydroxy form (in toluene)
Keto form (in DMF)

740
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 3, March, 2016
Traven et al.
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ate) film in the absence of pyrazoline does not lead to any
spectral changes.
Aryl(hetaryl)pyrazolines are effective photogenerators
of acid, providing an irreversible photochemical activaꢀ
tion of fluorescence of dyes in the rhodamine group at the
irradiation of the corresponding solution in the presence
of CCl4 or C2Cl6. An increase of the polarity of the solvent
leads to an increase in both the rates of pyrazoline photoꢀ
dehydrogenation and the dye lactone form opening. Both
reactions proceed smoothly in polymeric films, which is
interesting for the development of new media for optical
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Experimental
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Irradiation was accomplished with a HAMAMAТZU lamp
(xenon lamp L 5283) with light filter UFSꢀ1 (spectral transmisꢀ
sion region 300—400 nm) and ZhSꢀ10 (spectral transmission
region >380 nm). Absorption spectra were recorded on a Cary 50
spectrophotometer, and fluorescence spectra were recorded on
a Varian Cary Eclipse spectrofluorimeter.
The synthesis and identification of pyrazolines 1 and 2 were
described earlier;28 pyrazoline 3 is a commercially available comꢀ
pound (high purity grade, Aldrich). As precursors of fluorescent
dyes, the lactone form of rhodamine B and rhodamine 19 were
used (high purity grade, Aldrich). Hexachloroethane was used as
a halogenꢀcontaining additive (high purity grade, Aldrich).
Polymeric films were prepared using the pouring method.
A solution containing poly(methyl methacrylate), the lactone form
of rhodamine B or rhodamine 19, pyrazolines 1 or 2, as well as
a halogen derivative in a mixture of toluene—ethyl acetate (1 : 1),
was poured onto a horizontally placed Petri dish, afterwards the
solvent was evaporated. The films were removed from the subꢀ
strate before undergoing irradiation. Films of methyl methaꢀ
crylate copolymer with 2,2,2ꢀtrichloroethyl methacrylate conꢀ
taining the lactone form of rhodamine B or rhodamine 19 were
prepared in a similar manner. The thickness of the film was
90—100 μm.
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Received March 20, 2015;
in revised form May 14, 2015