Intramolecular phototransfer of protons and the quenching of fluorescence of derivatives of 2-(coumarinyl-3)-5-(ortho-hydroxyphenyl)-1,3,4-oxadiazole

Article abstract of DOI:10.1023/A:1011621025169

The spectral-luminescence properties of ortho-hydroxy derivatives of 2-(coumarinyl-3)-5-phenyl-1,3,4-oxadiazole have been studied. It is shown that the basic reason for the decreased quantum yield of emission for the compounds studied is the high-speed phototransfer of a proton (estimated as ～10⁹ s^-1). Fluorescence of the products of this reaction (phototautomers) was not observed. It was confirmed by quantum-chemical calculations that the increase in efficiency of nonradiative dissipation of the electron excitation energy in phototautomeric forms of ortho-hydroxycoumarinyloxadiazoles is explained by an increase in intramolecular donor-acceptor interaction on introduction of the coumarin unit into the molecule. As a result of the high efficiency of nonradiative deactivation of the excited state, the ortho-hydroxyderivatives studied have promise as UV photostabilizers in polymeric materials.

Full text of DOI:10.1023/A:1011621025169

Chemistry of Heterocyclic Compounds, Vol. 37, No. 5, 2001
INTRAMOLECULAR PHOTOTRANSFER
OF PROTONS AND THE QUENCHING OF
FLUORESCENCE OF DERIVATIVES OF
ortho
2-(COUMARINYL-3)-5-(
-
HYDROXYPHENYL)-1,3,4-OXADIAZOLE
A. O. Doroshenko1, E. A. Posokhov1, K. M. Sytnik2, and S. N. Kovalenko2
The spectral-luminescence properties of ortho-hydroxy derivatives of 2-(coumarinyl-3)-5-phenyl-1,3,4-
oxadiazole have been studied. It is shown that the basic reason for the decreased quantum yield of
emission for the compounds studied is the high-speed phototransfer of a proton (estimated as ~109 s-1).
Fluorescence of the products of this reaction (phototautomers) was not observed. It was confirmed by
quantum-chemical calculations that the increase in efficiency of nonradiative dissipation of the electron
excitation energy in phototautomeric forms of ortho-hydroxycoumarinyloxadiazoles is explained by an
increase in intramolecular donor-acceptor interaction on introduction of the coumarin unit into the
molecule. As a result of the high efficiency of nonradiative deactivation of the excited state, the
ortho-hydroxyderivatives studied have promise as UV photostabilizers in polymeric materials.
Keywords: 2-(coumarinyl-3)-5-(ortho-hydroxyphenyl)-1,3,4-oxadiazole, intramolecular proton transfer,
extinction of fluorescence.
Intramolecular phototransfer of protons (IMPTP) is one of the most thoroughly studied adiabatic
photochemical processes [1-3], interest in which has scarcely decreased in more than 40 years [4-8]. This results
not only from the importance of this process in the chemistry of the excited state and photobiology, but also
from the range of technical applications in which IMPTP can be used (we mention only photochromic materials
based on IMPTP molecules [9], organic scintillators [10, 11], photostabilisation of polymeric materials by
protecting them from the ultraviolet radiation [12, 13], components of the active media of lasers in organic
luminophores [14-16], concentrators of solar energy [17], etc.).
The principal structural factor necessary for IMPTP is the presence in an organic molecule of
conjugated electron donor and electron acceptor groups, linked by an intramolecular hydrogen bond. If on
transition to the excited state the acidity of the proton donor group and the basicity of the electron acceptor
group are increased, conditions are created for phototransfer of a proton between them. The product of the
IMPTP reaction (a phototautomeric form) is characterized by the existence of a more longwave emission
spectrum and an anomalously large Stokes shift of the fluorescence in comparison with the intrinsic initial
"normal" (or enolic) form. As a rule IMPTP is accompanied by efficient nonradiative dispersion of the electron
excitation energy emanating principally in the phototautomeric form [3-5, 18-20]. As a result quantum yields for
__________________________________________________________________________________________
1
2
V. N. Karazin Kharkov National University, Kharkov 61077, Ukraine. Ukraine Pharmaceutical
Academy, Kharkov 61001, Ukraine; e-mail: andrey.o.doroshenko@univer.Kharkov.ua. Translated from
Khimiya Geterotsiklicheskikh Soedinenii, No.5, 686-698, May, 2001. Original article submitted May 28, 1999.
0009-3122/01/3705-0633$25.00©2001 Plenum Publishing Corporation
633

the fluorescence of IMPTP molecules seldom exceed 0.2, and in most cases they are at the level of a few
percents. However, for one of the important technical applications of intramolecular phototransfer of protons –
photostabilisation of polymeric materials – effective quenching of the excited state formed plays a determining
role.
The objectives of our preceding work were ortho-hydroxy derivatives of 2,5-diphenyloxazole [18, 19]
and 2,5-diphenyl-1,3,4-oxadiazole [20], for which the connection between the intramolecular redistribution of
electron density on transfer to the excited state and the efficiency of IMPTP was described, and also extinction
of fluorescence in the phototautomeric form.
In this paper ortho-hydroxy derivatives of 2-(coumarinyl-3)-5-phenyl-1,3,4-oxadiazole (1a-5a, R = H)
and the corresponding methoxy derivatives (1b-5b, R = Me) will be discussed as model compounds.
R
R
O
N
N
O
N
N
O
O
O
O
O
O
R
1
O
2
N
N
O
MeO
O
R
O
3
R
O
N
N
O
N
N
O
O
N
O
O
Et2N
O
O
5
4
Substitution of one of the benzene rings in ortho-hydroxydiphenyloxadiazole with an electron-acceptor
coumarin unit should lead not only to an increase in the size of the π-conjugated system of the molecule but also
to a notable increase in the strength of the intramolecular donor-acceptor interaction as, for example in the cases
we have described [21]. This, in its turn, produces an important change in the spectral characteristics which to a
considerable extent determine the IMPTP process and is connected with the subsequent nonradiative dissipation
of the electron excitation energy.
The hypothesis that the intramolecular donor-acceptor interaction is strengthened in the
coumarinyloxadiazole series is confirmed with the example of compound 1b for which the effect of the solvent
on the spectral characteristics and the dipole moment in the excited state have been estimated (Table 1). The
choice of this molecule (the methoxy derivative which is not substituted in the coumarin nucleus) and the group
of solvents used is explained by the desire to avoid the effect of specific intra- and intermolecular interactions,
therefore we excluded such polar proton-donor solvents as water, ethanol, formamide, etc. The molecular
parameters required to estimate the nature of the intramolecular electron density redistribution by
N. G. Bakshiev's modification of the method of spectral shifts [22, 23] (radius of the Onsager cavity (~7.2 Å)
and the ground state dipole moment (~3.9 D) were determined in the AM1 semiempirical quantum-chemical
approximation [24].
634

TABLE 1. Spectral-luminescent Characteristics* of 2-(Coumarinyl-3)-5-(2-
methoxyphenyl)-1,3,4-oxadiazole (1b) in Aprotic Solvents with Different
Polarities
Solvent
Dioxane
Benzene
Toluene
n
νa
νf
∆νST
ϕf
τf
kf
kd
ε
2.21 1.4224 29220 21950 7270
2.28 1.5011 28500 22110 6390
2.38 1.4961 28640 22180 6460
2.57 1.5055 28620 22020 6600
4.70 1.4459 29400 21980 7420
0.56
0.46
0.54
0.56
0.54
0.62
0.55
0.54
0.48
0.28
2.62 2.1·108 1.7·108
2.24 2.1·108 2.4·108
2.60 2.1·108 1.8·108
2.34 2.4·108 1.9·108
2.73 2.0·108 1.7·108
3.13 2.0·108 1.2·108
3.12 1.8·108 1.4·108
2.73 2.0·108 1.7·108
3.42 1.4·108 1.5·108
2.04 1.4·108 3.5·108
Xylene
Chloroform
Butyl acetate
Ethyl acetate
Methylene chloride
Acetonitrile
5.01
1.394 29040 21940 7100
6.02 1.3723 29080 21850 7230
8.90 1.4242 28620 21950 6670
36.2 1.3441 29280 21110 8170
Dimethylformamide 36.7 1.4303 29020 20400 8620
_______
* ε and
n are the dielectric permeability and refractive index of the solvent;
νa νf, and Δ
,
are the positions of the maxima in the absorption and
νST
fluroescence spectra, and the Stokes shift of the fluorescence (cm-1 f is the
), φ
quantum yield for fluorescence, τ
f is the fluroescence life time (ns), kf and kd
are the rate constants for the initial photophysical processes, emission of
fluorescence and nonradiative deactivation of the excited state (s-1),
= φf/τ
= (1 – φ )/τ
.
calculated from the relations kf
f and kd
f
f
It can be seen from the data cited in Table 1 that compound 1b has relatively high quantum yields for
luminescence which are not reduced much in solvents of high polarity. The extinction time for fluorescence is
also relatively large and is no less than 2 ns even in highly polar solvents. These facts indicate the absence of
π*-states, introduced into the system of terms of this molecule with
any serious deactivating influence of the n
the introduction of the coumarin unit, on the spectral-luminescent characteristics of the molecules studied.
π*-type, localized on the carbonyl group of the coumarin lie at considerably
Evidently the triplet levels of n
greater energy than the lower singlet excited sate of molecules 1-5 (a,b) and intersystem conversion cannot
occur concurrently with fluorescence at room temperature. It is possible that the efficiency of intersystem
conversion in conditions of thermal activation would be increased, leading to a decrease in quantum yield and
mission life time, however detailed study of this is outside the realms of the current work.
The Stokes shifts of the fluorescence of compound 1b is quite large even in nonpolar solvents which
may be interpreted as an indication of a change in conformation of this molecule in the excited state. However,
the reason might also be redistribution of the intensities of the individual vibrational components in the emission
spectrum, explained analogously in our previous discussion [25] with respect to the increased Stokes shift of
unsubstituted 2,5-diphenyl-1,3,4-oxadiazole in comparison with their oxazole analogs.
excited state μe
The estimates of the values of the dipole moment in the
increase in the polarity of compound 1b in the excited state: the dipole moment increases from 3.9 D (S0 G) to
*), while the vector difference Δμ = μ – μ
show the existence of an
, μ
is 7.9 D. This corresponds approximately to a shift of
9.4 D (S1
e
G
0.2-0.25 e, if one takes into account that pyrone ring is basically an electron acceptor while the methoxy
substituent on the benzene ring is an electron donor system (the distance between the poles of the vector Δμ are
averaged over the four possible conformers of compound 1b). For comparison, the absolute value of the vector
Δμ for
para-OCH3 substituted 2,5-diphenyl-1,3,4-oxadiazole is 3.8 D and it only reaches a value of 7.6 D for the
corresponding para-N(CH3)2 derivative [26].
635

Thus the introduction of the coumarin ring in the molecule of ortho-methoxy (and in the ortho-hydroxy)
derivative of 2,5-diphenyl-1,3,4-oxadiazole in fact leads to a notable increase in the intramolecular donor-
acceptor interaction, which, in agreement with earlier conclusions [8-20], should have a notable effect on the
spectral characteristics of the ortho-derivatives studied. In this way the possible negative consequences (from
the point of view of the efficiency of luminescence) as a result of introduction (in the system of energy states of
π*-type, localized in the CO group of the coumarin unit, will not appear.
molecules 1-5) of levels of the n
The basic spectral characteristics and the parameters of the initial photophysical processes occurring in
the excited states of the compounds studied are given in Table 2. The absence of longwave emission in the case
of the tautomeric forms was somewhat unexpected. We expected it for the ortho-hydroxy derivatives 1a-5a: the
position and even the shape of the bands in their emission spectra differed little from those characteristic of the
model ortho-methoxy derivatives 1b-5b. The definite asymmetry of the spectral bands was similar for both
groups of compounds. In our view this causes the absence from the spectrum of even one low intensity band of
the fluorescence product of the IMPTP reaction and the appearance of vibrational structure only. Attempts to
observe emission from phototautomeric forms, which might be formed in the case of compounds 1a-5a, and
their time-resolved fluorescence spectra, were unsuccessful.
TABLE 2. Spectral-luminescent Characteristics of Derivatives of
Coumarinylphenyloxadiazoles*
Com-
Solvent
kf
kd
νa
νf
∆νST
ϕf
τf
pound
Toluene
28120 23100
28620 22210
28960 21820
28840 21600
28640 22180
29280 21110
29020 20400
25340 21080
25660 21360
25500 21240
25660 21200
26140 21390
26080 21020
26960 22040
27220 22320
27220 22250
27380 21620
27860 22420
27500 22090
23280 20720
22850 20380
22760 20000
23760 20730
23240 20370
23120 20100
5020
6410
7150
7240
6460
8170
8620
4260
4300
4260
4460
4750
5060
4920
4900
4970
5760
5440
5410
2560
2470
2760
3030
2870
3020
0.12
0.009
0.003
0.0003
0.54
0.48
0.28
0.11
0.07
0.03
0.53
0.74
0.55
0.13
0.09
0.05
0.58
0.74
0.78
0.48
0.42
0.31
0.49
0.53
0.44
0.44
—
2.7·108 2.0·109
1a
Dioxane
—
—
—
—
—
—
Acetonotrile
—
Dimethylformamide
—
Toluene
2.60
3.42
2.04
0.74
0.37
0.42
3.43
3.85
3.60
0.86
0.36
0.24
2.48
2.59
2.49
2.34
1.58
1.48
2.33
2.08
1.82
2.64
3.36
3.66
2.1·108 1.8·108
1.4·108 1.5·108
1.4·108 3.5·108
1b
2a
2b
3a
3b
4a
4b
5a
5b
Acetonotrile
Dimethylformamide
Toluene
1.5·108 1.2·109
1.9·108 2.5·109
1.0·108 2.3·109
Acetonotrile
Dimethylformamide
Toluene
1.5·108 1.4·108
1.9·108 0.7·108
1.5·108 1.3·108
Acetonotrile
Dimethylformamide
Toluene
1.5·108 1.0·109
2.5·108 2.5·109
2.1·108 4.0·109
Acetonotrile
Dimethylformamide
Toluene
2.3·108 1.7·108
2.9·108 1.0·108
3.1·108 0.8·108
Acetonotrile
Dimethylformamide
Toluene
2.1·108 2.2·108
2.7·108 3.7·108
2.1·108 4.7·108
Acetonotrile
Dimethylformamide
Toluene
2.1·108 2.2·108
2.5·108 2.3·108
2.4·108 3.1·108
Acetonotrile
Dimethylformamide
Toluene
22480 20300
21830 19700
21780 19460
22920 20460
22220 19890
22120 19650
2180
2130
2320
2460
2330
2470
0.42
0.55
0.46
0.43
0.52
0.48
1.6·108 2.2·108
1.6·108 1.4·108
1.3·108 1.5·108
Acetonotrile
Dimethylformamide
Toluene
2.57
3.14
3.02
1.7·108 2.2·108
1.7·108 1.5·108
1.6·108 1.7·108
Acetonotrile
Dimethylformamide
_______
* Symbols as for Table 1.
636

A considerable decrease in the quantum yield for fluorescence was also observed for compounds 1a-3a
in comparison with their methoxy analogs, which was effectively not observed for the dimethylamino and
quinolizinyl derivatives 4a and 5a. The similarity of the spectral characteristics of the systems 4a,b and 5a,b in
solvents of varying polarity should also be noted. This indicates an extremely small effect on their fluorescent
properties of possibly formated non-luminescent TICT states, which are traditionally discussed as the basic
cause for the decreased efficiency of luminescence of aromatic dialkylamino derivatives in polar solvents [27,
28]. Thus one can promote the suggestion that the decrease in emission intensity for compounds 1a-3a is
basically due to IMPTP and the subsequent effective intramolecular extinction of fluorescence of the
phototautomeric forms produced. Unfortunately only indirect methods are available to determine the efficiency
of proton phototransfer in the series of compounds studied because the more or less reliable direct estimation of
the rate of this process from the kinetics of fluorescence is only possible in those case when emission of the
normal and phototautomeric forms is observed.
Fluorescence (rate constant kfM) and the various nonradiative transitions, for example intra- and
M
intersystem conversion (rate constant kd ) are ascribed to the first photoprocess which leads to "consumption"
excitation of the methoxy derivatives. That the quantum yields of fluorescence for compounds 1b-5b is notably
less than unity shows that the second process takes place; the lifetime of the excited state of the methoxy
derivatives depends on the rate constants of the initial photophysical processes:
M
1/
= kfM + kd .
(1)
τM
For the hydroxy derivatives there is another initial photochemical process, the proton phototransfer reaction
(rate constant kEIPT) which leads to dissipation of the excitation energy of the normal form. An analogous
relation can be obtained for the extinction time for the fluorescence of the excited normal form:
1/
= kfOH + kdOH + kESIPT
.
(2)
τOH
If it is assumed that the rate constants for the initial photoprocesses for the series of hydroxy and
OH
M
methoxy derivatives are sufficiently close to one another (kfOH ~ kfM and correspondingly kd ~ kd ), then the
rate of the proton phototransfer can be estimated from a combination of equations (1) and (2):
kESIPT = (
/
– 1)/
.
τM
(3)
τM τOH
The estimates obtained in this way for toluene solutions are given in Table 3. Both the highly basic
polar solvents and the proton donor solvents were excluded from the discussion as they may form
intermolecular hydrogen bonds with the molecules of the hydroxy derivatives. In such solvate complexes
intramolecular proton phototransfer becomes impossible [1-8], consequently it is necessary to discuss for
solvents forming intermolecular hydrogen bonds a more complex scheme of initial processes and also intra- and
intermolecular photochemical processes, the analysis of which is outside the realms of this paper.
An alternative indirect estimate of the rate of IMPTP is possible if it is assumed that the basis of the
nonradiative process leading to the consumption of the normal forms of compounds 1a-5a is proton transfer and
other possible nonradiative processes cannot occur concurrently [1-8, 29] (however, this suggestion is not
correct for 4a and 5a). Then
OH
kESIPT = 1/
– kf
.
(4)
τOH
Values of kfOH were estimated from the absorption spectra according to [30] (Table 4), kfM and kESIPT
were obtained from formulas (3) and (4). The rate constants for proton phototransfer for the coumarinyl
derivatives of diphenyloxadiazole appear to be somewhat lower than the analogous values for the oxazole and
637

TABLE 3. Rate Constants* of Intramolecular Proton Phototransfer (kESIPT
)
in ortho-Hydroxysubstituted Compounds 1a-5a in Toluene
ortho-Hydroxy derivatives
ortho-Methoxy derivatives
kESIPT (formula)
Com-
Com-
kfOH (ESP)
kfOH (ESP)
(3)
(4)
τfOH
τfM
pound
pound
0.44
0.74
0.86
2.34
2.64
2.85·108
2.60·108
2.62·108
2.31·108
2.28·108
2.60
3.43
2.48
2.33
2.57
2.80·108
2.02·108
3.37·108
2.45·108
2.33·108
1.8·109
1.1·109
7.7·108
—
2.0·109
1.1·109
1a
2a
3a
4a
5a
1b
2b
3b
4b
5b
9.1·108
(2.0·108 )
(1.5·108 )
—
_______
* kf (ESP) – rate constants for emission of fluorescence estimated from the
electronic absorption spectrum according to [30]. Figures in brackets are
estimates in which it was observed that the deductions from formula (4)
were not acceptable when the rate of IMPTP exceed the rates of the other
initial photoprocesses.
oxadiazole derivatives we have studied [18-20] and also those known from the literature [1-8]. On one hand, this
circumstance may arise from the decreased basicity of the proton accepting N atoms in the oxadiazole ring as a
result of introduction of the electron acceptor coumarin unit, which in its turn, lowers the probability of proton
phototransfer. However, it cannot be denied that the assumptions for the two equations (3) and (4) are
sufficiently crude although it may be suggested that it is doubtful that the errors in carrying out the estimates
even in the most unfavorable cases would exceed 50%. Nevertheless the tendency to decrease the rate of proton
phototransfer in the series of compounds 1a-5a with increasing electron donor character of the substituent in the
coumarin unit of the molecules studied follows sufficiently clearly from the data in Table 3.
To clarify the principle of the phenomena described and to confirm them quantitatively we carried out
quantum-chemical calculations using the PPP CI method with a special set of parameters [31], which showed
its usefulness and efficiency for describing the spectral properties and the character of the redistribution of
electron density in the excited state of molecular systems with IMPTP [18-20]. Special quantum-chemical
indexes – the localization number of the excited electron Li and the charge transfer (CT) number lij [32]
(Table 4) – were also calculated. Three structural fragments were provisionally separated in the molecules
1a-5a: coumarinyl, oxadiazolyl, and ortho-hydroxybenzene fragments. The diagrams of Table 4 show the
general localization of the excited electron over the corresponding units. The CT number, which characterizes
interfragment shift of the electron density within the fragments, the interfragment CT number is depicted
approximately by arrows which show the direction of the transfer of electron density on transition to the excited
state.
As has been noted, introduction of the coumarin nucleus into the diaryloxadiazole molecule changes the
nature of the intramolecular donor-acceptor interaction considerably. Because of its considerable size the
coumarin fragment plays a notable role in the formation of the excited states of compounds 1-5 – the general
localization of the excited electron on this unit is quite large for all of the molecules studied. In distinction form
the ortho-hydroxy derivatives of diphenyloxazole and oxadiazole, in which the heterocycle and the benzene ring
with the hydroxy group take the principle part in redistribution of the electron density on electron excitation, in
compounds 1-4 the role played by the coumarin nucleus is quite large in all cases. As in the case of the oxazole
and oxadiazole derivatives in the compounds studied there is a tendency for a decrease in CT from the
2-benzene ring to the other parts of the molecule with increasing electron donor character of the substituent
introduced into the opposite 5-position.
638

TABLE 4. Quantum-chemical Calculations of the Normal and
Phototautomer Forms of Compounds 1a-4a to Determine the Localization
Numbers of the Excited Electron and the Charge Transfer Numbers [32]
Normal form*
Scheme for the electron
density redistribution
Phototautomer form
Com-
pound
Scheme for the electron
νS0-S1
∆qO
νS0-S1
density redistribution
21.0
33.0
46.1
29080 0.086
36.0
33.8
30.2
19160
1a
39
11
7
4
O
N
N
O
N
N
8
O
14
O
17
11
29
23
–
O
O
O
O
5
27000 0.028
18810
19570
19410
2a
67.4
21.9
10.7
24.9
30.8
44.3
6
35
3
8
O
9
N
N
O
N
N
8
8
O
O
–
55
–
O
O
O
O
32
28760 0.065
3a
43.8
31.9
24.3
7
19.0
34.1
46.9
40
1
O
O
N
N
N
N
9
O
13
O
33
13
–
13
O
O
O
O
O
O
24
3
80.3
15.3
4.4
23630 0.004
4a
33.8
31.3
34.9
30
10
16
69
O
N
N
O
N
N
–
O
O
–
N
O
O
N
O
O
9
_______
*
νS0-S1 – relative energy of the excited state (cm-1
); Δ
qO – change in charge
on the O atom of the hydroxyl group in the normal form, reflecting the
increased acidity of the hydroxyl group on transfer to the excited state (the
change in basicity of the oxadiazole ring is approximately the same in all
cases). Commentary on the molecular diagram is given in the text of the
paper.
The notable decrease in CT in the end leads to a reduction in the rate of IMPTP [18-20]. In particular, in
the series of compounds studied this effect aggravates the decrease in localization of the excited electron on the
phenyl ring when a donor substituent is introduced into the coumarin fragment or when its π-system is widened
(e.g., as a result of annelation, compound 2). In the case of dialkylamine derivatives the effect becomes so much
stronger that it can be accepted that the benzene ring in compounds 4 and 5 effectively takes no part in the
formation of the excited state: the overall localization on this fragment ~4%. On the basis of the results obtained
639

TABLE 5. Some Physicochemical Characteristics of the Synthesized Derivatives of Coumarinylphenyloxadiazole
1H NMR spectra, chemical shifts, δ, ppm (assignment)
IR spectra,
Com-
Empirical formula
Yield,
%
Found N, %
ν, cm-1
——————
mp, °C
1H, s,
OH
6
1H, s,
pound (molecular mass)
Harom
8
other protons
9
Calculated N, %
(assignment)
4-H
7
1
2
3
4
5
10
79
C17H10N2O4
(306.28)
9.23
9.15
229-231
3155 (OH)
1744 (C=O)
1607 C=C)
10.15
9.01
7.04 (1H, t, 4'-H); 7.09 (1H, d, 3'-H)
7.40-7.49 (3H, m, 6-,7-,8-H)
7.74 (1H, t, 5'-H); 7.89 (1H, d, 6'-H)
7.96 (1H, d, 5-H)
7.17 (1H, t, 4'-H); 7.30 (1H, d, 3'-H)
7.46 (1H, t, 6-H); 7.50 (1H, d, 8-H)
7.64 (1H, t, 5'-H); 7.76 (1H, t, 7-H)
7.93 (1H, d, 6'-H); 8.00 (1H, d, 5-H)
—
3.95 (3H, s, OCH3)
—
1a
C18H12N2O4
(320.30)
1b
2a
8.71
8.75
157-159
246-248
1744 (C=O)
1606 (C=C)
—
8.91
9.61
57
82
C21H12N2O4
(356.34)
7.08 (1H, t, 4'-H); 7.12 (1H, d, 3'-H)
7.89
7.86
3180 (OH)
1744 (C=O)
10.18
7.44 (1H, t, 5'-H)
7.59-7.68 (2H, m, 7-, 10-H)
7.80 (1H, t, 6-H)
1623, 1563 (C=C)
8.01-8.10 (2H, m, 6'-, 5-H)
8.31 (1H, d, 8-H); 8.71 (1H, d, 9-H)
C22H14N2O4
(370.36)
7.15 (1H, t, 4'-H); 7.24 (1H, d, 3'-H)
7.56-7.69 (3H, m, 5'-,7-,10-H)
7.80 (1H, t, 6-H); 8.00 (1H, d, 6'-H)
8.07 (1H, d, 5-H); 8.29 (1H, d, 8-H)
8.64 (1H, d, 9-H)
6.96 (1H, d, 6-H); 7.03 (1H, s, 8-H)
7.07-7.14 (2H, m, 3'-,4'-H)
7.46 (1H, t, 5'-H); 7.84 (1H, d, 5-H)
7.89 (1H, d, 6'-H)
3.97(3H, s, OCH3)
2b
7.49
7.56
225-228
259-260
1735 (C=O)
—
9.54
8.90
78
69
1606, 1563 (C=C)
C19H14N2O5
(350.33)
1.44 (3H, t, OCH2CH3)
4.19 (2H, q, OCH2CH3)
3а
8.09
8.00
3198 (OH)
3058, 2988 (CH)
1743 (C=O)
10.11
1615 (C=C)

TABLE 5 (continued)
1
2
3
4
5
6
7
8
9
10
82
C20H16N2O5
(364.36)
7.74
7.69
199-200 3034, 2978 (CH)
1744 (C=O)
—
8.81
6.95 (1H, d, 6-H); 7.01 (1H, s, 8-H)
7.12 (1H, t, 4'-H); 7.21 (1H, d, 3'-H)
7.57 (1H, t, 5'-H); 7.84 (1H, d, 5-H)
7.91 (1H, d, 6'-H)
1.45 (3H, t, OCH2CH3)
3.96 (3H, s, OCH3)
3b
1604 (C=C)
4.20 (2H, q, OCH2CH3)
C21H19N3O4
(377.40)
11.07
11.13
223
3197 (OH)
2982, 2927 (CH)
1735 (C=O)
10.09
8.67
6.58 (1H, s, 8-H); 6.75 (1H, d, 6-H)
7.01-7.10 (2H, m, 3'-,4'-H)
7.44 (1H, t, 5'-H) ;7.62 (1H, d, 5-H)
7.89 (1H, d, 6'-H)
1.19 (6H, t,
77
4a
N(CH2CH3)2)
3.51 (4H, q, N(CH2CH3)2)
1623, 1591 (C=C)
C22H21N3O4
(391.42)
10.81
10.74
175-178 2972, 2932 (CH)
1726 (C=O)
—
8.62
8.51
6.61 (1H, s, 8-H); 6.82 (1H, d, 6-H)
7.15 (1H, t, 4'-H); 7.29 (1H, d, 3'-H)
7.63 (1H, t, 5'-H); 7.68 (1H, d, 5-H)
7.88 (1H, d, 6'-H)
7.02-7.13 (2H, m, 3'-,4'-H)
7.18 (1H, s, 5-H); 7.44 (1H, t, 5'-H)
7.87 (1H, d, 6'-H)
1.17 (6H, t, N(CH2CH3)2)
3.92 (3H, s, OCH3)
61
56
4b
5a
1620, 1590 (C=C)
3.50 (4H, q, N(CH2CH3)2)
C23H19N3O4
(401.42)
10.55
10.47
248-250
3230 (OH)
2944, 2840 (CH)
1719 (C=O)
10.12
1.95 (4H, m,
N(CH2CH2CH2)2)
2.80 (4H, m,
1617, 1578 (C=C)
N(CH2CH2CH2)2)
3.36 (4H, t,
N(CH2CH2CH2)2)
C24H21N3O4
(415.45)
10.26
10.11
210-212 2940, 2846 (CH)
1720 (C=O)
—
8.43
7.12 (1H, t, 4'-H); 7.17 (1H, s, 5-H)
7.21 (1H, d, 3'-H); 7.56 (1H, t, 5'-H)
7.87 (1H, d, 6'-H)
1.96 (4H, m,
68
5b
N(CH2CH2CH2)2)
2.80 (4H, m,
1623, 1592 (C=C)
N(CH2CH2CH2)2)
3.35 (4H, t,
N(CH2CH2CH2)2)
3.96 (3H, s, OCH3)

it may be concluded that proton phototransfer should be considerably retarded on the introduction of electron
donor substituents, until it is completely blocked in dimethylamino-, and even more in quinolizine-substituted
ortho-hydroxycoumarinylphenyloxadiazole.
Introduction of a coumarin ring also markedly changes the character of the intramolecular donor-
acceptor interaction in the product of the IMPTP reaction, the phototautomer form. As a result of proton
phototransfer to the benzene ring a considerable excess of electron density arises which is then redistributed to
the other parts of the molecule. In one of our papers [20] a suggestion was made about the connection between
the efficiency of extinction of fluorescence in the phototautomer form and the intensity of intramolecular CT
between units in it. According to the calculations cited, introduction of the electron acceptor coumarin ring
considerably strengthens the expected redistribution of electron density, which is 1.5 to 2 times as great for the
compounds studied than for the derivatives of oxazole and oxadiazole studied previously [20]. In the case of the
dialkylamine derivatives the presence of such strong electron donor substituents in the opposite part of the
molecule can only decrease a similar redistribution of charge to a small extent. So on the basis of the results of
the calculations a considerable increase in the efficiency of intramolecular extinction of fluorescence of the
phototautomer forms of the compounds studied can be forecast.
The experimental results (Tables 2 and 3) confirm this proposal. For example, the absence of emission
bands for the phototautomer forms of 1a-3a is explained by the considerable increase in the efficiency of their
nonradiative deactivation, whereas the absence of IMPTP in compounds 4a and 5a reflects the localization of
the excited electron on their 7-dialkylamine substituted coumarin units. The increased efficiency of fluorescence
as a result of the increased electron donor power of the substituents introduced is indicated by the decrease in
the rate of proton phototransfer in the series H, OCH3, N(C2H5)2, quinolizine, leading to practically blockage in
the last two cases.
Taking into account the high efficiency of nonradiative deactivation of the excited states of molecules
1a-3a, compounds of this type have potential for use as UV protectors for polymeric materials.
EXPERIMENTAL
Derivatives of coumarinylphenyl-1,3,4-oxadiazole were synthesized by a previously described method
[33]. 2-Iminocoumarin-3-carboxamides and hydrazides of salicylic or 2-methoxybenzoic acids were used as
starting materials. Nitrogen elemental analysis results agreed with the calculated values (Table 5).
IR spectra of the compounds synthesized as 1% suspensions in KBr pellets were recorded with a
Specord M-80 spectrometer. A strong band of the C=O group of a lactone ring was observed at 1719-1744 cm-1,
while vibrations of the C=C bonds of the aromatic and heterocyclic rings were observed at 1578-1623 cm-1.
Broad weak vibrations of the associated O–H bond were observed in the 3155-3230 cm-1 for the ortho-hydroxy
derivatives.
1H NMR spectra of DMSO-d6 solutions with TMS as internal standard were recorded with a Varian
VXR-400 instrument. For the compounds synthesized signals for the aromatic protons were observed at
6.57-8.71 ppm, a singlet for the proton at position 4 of the coumarin ring was observed at 8.43-9.61 ppm, and
the signal for the OH proton was observed at 10.09-10.18 ppm. This signal was replaced by the singlet for the
methoxy group at 3.92-3.97 ppm in the corresponding methoxy derivatives.
Absorption spectra were measured with Hitachi U-3210 and Specord M-40 spectrophotometers, while
fluorescence spectra and quantum yields were measured with a Hitachi F4010 spectrofluorometer. A solution of
quinine hydrogen sulfate in 1 N sulfuric acid (φ
f = 0.546) [34] was used as the standard for determination of the
quantum yield of fluorescence. In all cases RMS corrections for the difference in refractive indexes of the
measured and standard solutions were introduced [35].
Kinetics and time-resolved fluorescence spectra were measured with an apparatus [36, 37] working in a
photon counting regime with a nanosecond range. The lifetime of the fluorescence was calculated with a
642

nonlinear least squares method [38, 39]. To estimate the dipole moment of the molecule of 1b in the ground
state quantum-chemical calculations were carried out in the semiempirical AM1 method with optimization of
the geometry of the different planar conformations of this compound, which differ in the mutual orientation of
the coumarin unit, oxadiazole ring, and the ortho-methoxysubstituted phenyl unit. The effective dipole moment
of structure 1b (~3.9 D) was estimated as the weighted mean of the geometric dipole moments of the four
possible conformers taken with statistical weights, with inverse squares of their heats of formation, with the
weight of the most energetically suitable conformer adjusted to unity. The Onsager radius of polarity was taken
as equal to the radius of the sphere in which the conformer having the greatest linear dimension (7.2 Å) can be
placed. The quantum-chemical calculations in the π-electron approximation was carried out by the PPP KV
method using 100 once excited configurations and using a set of semiempirical parameters [31].
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