Photoinitiated processes in molecules of Schiff bases having an NH...N intramolecular hydrogen bond

Article abstract of DOI:10.1023/A:1020809814260

Study of the absorption and luminescence spectra of Schiff bases derived from o-(p-tolylsulfonyl-amino)benzaldehyde has shown that these systems, like those derived from salicylaldehyde, are characterized by intramolecular proton transfer in the excited a

Full text of DOI:10.1023/A:1020809814260

Russian Journal of Organic Chemistry, Vol. 38, No. 7, 2002, pp. 1018 1022. Translated from Zhurnal Organicheskoi Khimii, Vol. 38, No. 7, 2002,
pp. 1060 1065.
Original Russian Text Copyright
2002 by T. Ivakhnenko, Makarova, Burlov, E. Ivakhnenko, Knyazhanskii, Metelitsa, Garnovskii.
Photoinitiated Processes in Molecules of Schiff Bases
Having an NH N Intramolecular Hydrogen Bond*
T. E. Ivakhnenko, N. I. Makarova, A. S. Burlov, E. P. Ivakhnenko,
M. I. Knyazhanskii, A. V. Metelitsa, and A. D. Garnovskii
Research Institute of Physical and Organic Chemistry, Rostov State University,
pr. Stachki 194/2, Rostov-on-Don, 344090 Russia
Received January 16, 2002
Abstract Study of the absorption and luminescence spectra of Schiff bases derived from o-(p-tolylsulfonyl-
amino)benzaldehyde has shown that these systems, like those derived from salicylaldehyde, are characterized
by intramolecular proton transfer in the excited and ground electronic states. This process is responsible for
the appearance at 77 K of fluorescence with an anomalous Stokes shift and formation of ketone structures.
Unlike o-[alkyl(or aryl)iminomethyl]phenols, o-[alkyl(or aryl)iminomethyl]anilines do not give rise to
acoplanarization of the ketone fragment, which could follow intramolecular proton transfer in the excited state
and is responsible for fluorescence with an anomalous Stokes shift and formation of colored structures.
Intramolecular proton transfer (IPT) in excited state
(ESIPT) is one of the most common and extensively
studied adiabatic processes [1]. Excited state intra-
molecular proton transfer in structurally nonrigid
molecules may be the first step of a series of adiabatic
transformations leading to quenching of fluorescence
with an anomalous Stokes shift (which is typical of
ESIPT) [2 5] and/or formation of metastable colored
structures [6, 7] (photochromism). A characteristic
phototransformation of the latter type is associated
with photochromic properties of Schiff bases derived
from substituted anilines and o-hydroxybenzalde-
hydes. These properties were formerly explained in
terms of IPT OH NH in the excited state [8] and
were subsequently studied in a number of works (see,
e.g., reviews [9 14]). Only in the recent time photo-
chromic properties of Schiff bases were reasonably
related to structural transformations which follow IPT
in the excited state [7]. However, an important prob-
lem remains unsolved, namely that concerning the
effect of intramolecular hydrogen bond, specifically
of the nature of proton-donor center, on the feasibility
of ESIPT and subsequent structural transformations
responsible for fluorescent and photochromic pro-
Previously, Chernova et al. were the only authors
who studied [10] a series of compounds having
an NH N intramolecular hydrogen bond and model
structures on the basis of p-tolylsulfonyl derivatives of
Schiff bases. The observed fluorescence with an ano-
malous Stokes shift was reasonably assigned to IPT
in the excited state.
The goal of the present work was to further study
the nature of photoinitiated processes in molecules
possessing an NH N intramolecular hydrogen bond.
For this purpose, we examined the absorption and
luminescence spectra and photochemical properties of
some p-tolylsulfonyl- and phenylamino derivatives of
Schiff bases (compounds I VI) and compared them
with those of the corresponding derivatives of salicyl-
aldehyde, o-(methyliminomethyl)phenol (VII) and
o-(phenyliminomethyl)phenol (VIII).
The main parameters characterizing intramolecular
hydrogen bond and also electron absorption and
perties of Schiff bases.
____________
*
I, R1 = Ts, R2 = iso-C3H7; II, R1 = Ts, R2 = tert-C4H9;
III, R1 = Ts, R2 = (CH2)6CH3; IV, R1 = Ts, R2 = Ph; V,
This study was financially supported by the Russian Founda-
tion for Basic Research (project nos. 99-03-32485 and 00-
15-97320) and by the Ministry of Education of the Russian
Federation (project no. 2000-5-121).
R1 = Ts, R2 = 2,6-(iso-C3H7)2C6H3; VI, R1 = Ph, R2
2,6-(iso-C3H7)2C6H3; VII, R3 = CH3; VIII, R = Ph.
=
1070-4280/02/3807-1018$27.00 2002 MAIK Nauka/Interperiodica

PHOTOINITIATED PROCESSES IN MOLECULES OF SCHIFF BASES
1019
fluorescence spectra of compounds I III and IV VI
[11 13] and N-salicylidenealkylamines (which were
studied in detail previously), including Schiff bases
VII and VIII [14 16], are generally similar. This
suggests similarity in the mechanisms of photo-
initiated processes in their molecules; in particular,
it seems to be possible to use the results of previously
performed quantum-chemical calculations to interpret
on a qualitative level the absorption and fluorescence
spectra of the compounds under study. Like salicyl-
aldehyde derivatives [14], compounds I III in non-
polar solvents at room temperature exist in the enol
form (E) with a strong intramolecular hydrogen bond.
This bond weakens to some extent on introduction
of bulky alkyl groups to the N2 atom [11, 12]. The
position and intensity of the long-wave absorption
replacement of R1 = Ts by R1 = Ph (compound VI)
weakens the intramolecular hydrogen bond so that
the S0 S1 transition is localized mainly over the
N2 Ar fragment (in contrast to IV and V). The result
is that a strong red shift of the absorption band is
observed, the dipole moment does not change on
excitation, and the corresponding shift of the absorp-
tion band occurs on variation of the solvent polarity
(Table 1). Unlike molecules I III, significant reduc-
tion of electron density on the C N2 group induced
by delocalization of n-electrons on N2 due to nN
2
Ar
conjugation in IV VI (as well as in the corresponding
derivatives like VIII [7, 14]) does not favor GSIPT
and formation of structure K even in polar solvents
at room temperature. Therefore, the AK band typical
of structure K (
400 415 nm) is not observed in
max
band AE (
315 nm) do not depend on the size
the spectra of compounds IV VI (Table 1).
of the alkylmraaxdical (Table 1) and are almost similar
Lowering the temperature to 77 K, both in nonpolar
and glassy polar solvents, leads to a sharp shift of
the equilibrium of compounds I III (in contrast to
IV VI) toward structure K due to IPT in the ground
state, and a strong AK absorption band appears with
400 415 nm (Table 1). This unusual pattern,
wmhaixch is also observed with N-salicylidenealkyl-
amines, may be explained by specific spatial inter-
actions between nonrigid molecular structures and
glassy solvent matrix [14].
Excitation of compounds I VI at the long-wave
absorption maxima of either tautomeric structure in
liquid solution in various solvents at room tempera-
ture does not give rise to fluorescence (by analogy
to compounds VII and VIII); however, unlike the
latter, efficient photodecomposition of I VI occurs.
No photodecomposition but fluorescence is observed
in glassy solutions of I VI at 77 K. Compounds I III
in both polar and nonpolar solvents give rise to two
fluorescence bands excited at the absorption bands
of different tautomeric forms (Table 1). Tautomer K
is characterized by a strong fluorescence (FK) in the
to the corresponding parameters of compound VII
(
317 nm). Therefore, we can presume that the
max
electron transition S0 S1 in I III, as well as in VII
[14], originates from intramolecular charge transfer
(ICT) in structure E from the A ring (having a p-tolyl-
sulfonylamino group) to the C N moiety. Such ICT
reduces the positive charge on N2 and the dipole
moment on excitation, leading [17] to blue shift of
the long-wave absorption band in polar solvents
(Table 1). As with compound VII, in going to polar
solvents tautomeric equilibrium is displaced toward
more polar ketone structure K, and intramolecular
N1 N2 proton transfer in the ground state (GSIPT)
gives rise to long-wave absorption band AK
(
400 410 nm) (Table 1). The N2-aryl substituentmianx
IV VI substantially modifies the nature of the lowest
electron transition. It should be kept in mind that, by
analogy with VIII [14, 15], the modification involves
additional ICT from the N Ar acoplanar fragment
to the C N group. As a result, a red shift and struc-
turization of the absorption band of IV is observed,
1
presumably due to low-frequency ( 1000 cm )
region
470 480 nm. Excitation of structure E
max
bending vibrations of the C N Ph fragment. The
same factor leads to a blue shift and loss of fine struc-
ture of the absorption band of compound V because
of acoplanarization of the C N Ar fragment and
freezing of its bending vibrations, respectively,
which are caused by introduction of bulky substituent
to the N2 atom (Table 1).
induces FASS emission with an anomalous Stokes shift
1
(
> 11000 cm ). As with o-hydroxy Schiff bases,
such fluorescence is typical of dipolar structures in
the S1 state which are formed adiabatically as a result
of N1 N2 ESIPT on excitation of tautomer E of
compounds IV and V (Table 1). The situation caused
by replacement of proton-donor substituent in the
aldehyde ring is unusual for Schiff bases, and it may
be interpreted in terms of the lack of ESIPT because
of weakening (or rupture) of the N1H N2 intramole-
cular hydrogen bond in the ground state and/or un-
favorable localization of the lowest electron transition
(see above).
The above localization of the electron transition
ensures a considerable decrease of the dipole moment
on S0 S1 excitation (
< ground), and the absorp-
excit
tion band shifts to shorter wavelengths as the solvent
polarity increases (Table 1). Sharp reduction of the
electron-donor power of the R1N1H fragment on
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 7 2002

1020
IVAKHNENKO et al.
Table 1. Absorption and fluorescence of Schiff bases I VIII
Absorption, max(A), nm ( 10 , l mol 1 cm )
1
1
Compound
no.
Solvent
293 K
77 K
structure E (AE)
structure K (AK)
structure E (AE)
structure K (AK)
I
Isopentane
Ethanol
315 (4.4)
312
320
320
415 (0.14)a
410 (0.26)a
410 (0.18)a
411 (0.17)a
409 (0.19)a
II
Isopentane
Ethanol
315 (4.5)
311
312
320
400 (0.16)a
400 (0.25)a
III
IV
V
Isopentane
Ethanol
315 (4.5)
311
320
320
410 (0.14)a
410 (0.28)a
Isopentane
Ethanol
338 (11.2)b
340
340
336 (12.5)b
Isopentane
Ethanol
332 (7.2)
322 (7.35)
332
330
VI
VII
VIII
Isopentane
Ethanol
381 (11.0)
380 (10.5)
380
380
Isopentane
Ethanol
317
317
340 (6.25)b
320
320
342b
390 (10.0)a
390 (15.0)a
390 (2.5)a
Isopentane
Ethanol
430 (0.01)a
338
335
435 (0.05)a
Fluorescence (77 K)
Compound
no.
structure E (FASS
)
structure K (FK)
Solvent
Stokes shift
Stokes shift
fl
exc
fl
exc
max, nm
, nm
max, nm
, nm
max
1
1
max
, cm
, cm
I
Isopentane
Ethanol
510
510
320
320
11640
11640
475
470
415
400
3045
3725
II
III
IV
V
Isopentane
Ethanol
515
515
320
320
11830
11830
480
480
400
400
4170
4170
Isopentane
Ethanol
510
510
320
320
11640
11640
480
470
410
410
3560
3110
Isopentane
Ethanol
540
540
340
340
10890
10890
Isopentane
Ethanol
525
520
330
330
11255
11070
VI
Isopentane
Isopentane
Isopentane
440
500
512
380
320
340
3950
11250
9880
VII
VIII
465c
520
395
430
3800
4025
a
K
K
In parentheses is given the relative absorption intensity determined as D( max)/(c0 l), where D( max) is the optical density at the
absorption maximum of structure K, c0 is the initial concentration, and l is the cell thickness (1 cm).
Long-wave maximum of the structurized band.
b
c
Fluorescence is also observed at 293 K.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 7 2002

PHOTOINITIATED PROCESSES IN MOLECULES OF SCHIFF BASES
1021
Table 2. Melting points, IR spectra, and elemental analyses of Schiff bases I VIa
1
Found, %
Calculated, %
IR spectrum, , cm
C N SO2
Comp.
no.
mp,
C
Formula
C
H
N
S
C
H
N
S
I
II
III
IV
V
77 78
114 115 65.35 6.82 8.60 9.65 C18H22N2O2S 65.42 6.70 8.47 9.70 1640 1340, 1150
68 69 65.86 7.47 7.62 8.70 C21H28N2O2S 65.75 7.52 7.52 8.60 1630 1345, 1160
107 108 68.41 5.03 8.23 9.12 C20H18N2O2S 68.55 5.18 7.99 9.15 1621 1342, 1160
164 165 71.75 7.00 6.35 7.28 C26H30N2O2S 71.80 6.91 6.45 7.18 1619 1340, 1160
64.96 6.47 8.15 10.23 C17H20N2O2S 64.56 6.37 8.85 10.14 1635 1334, 1154
VI
84.25 8.10 7.95
C25H28N2
84.22 7.91 7.85
a
Compounds I III are yellow, compound IV is light yellow, and compounds V and VI are colorless.
Unlike molecules VII and VIII and also their
in electronic properties of the N Ts substituent which
derivatives having bulky tert-butyl groups [9, 13 16],
even prolonged irradiation of glassy solutions of I V
does not give rise to reversible (on heating) accumula-
tion of metastable colored species, i.e., compounds
I V exhibit no photochromic properties despite IPT
in the excited state. Therefore, post-TICT structures K
which are responsible for coloration and precursor
structures K which are responsible for complete
quenching of fluorescence (with an anomalous Stokes
shift) of molecules VII and VIII and their tert-butyl
derivatives in liquid solution [9, 15, 16] are not
formed from compounds I V. Obviously, the reason
should be sought for not only in steric interactions,
as in tert-butyl derivatives of VII and VIII but also
hampers formation of twisted structures K. Then,
complete quenching of anomalous fluorescence of
compounds I V in liquid solutions does not result
from formation of TICT structures (as with derivatives
of VII and VIII) [7, 14, 18] but is caused by fast
concurrent photodecomposition processes in the S1
state, which are also responsible for fluorescence
quenching of each tautomer of compounds I VI.
The above data describing photoinitiated processes
in molecules I V in glassy solutions at 77 K may be
represented by Scheme 1. According to this scheme,
processes involving N1 N2 ground state intramole-
cular proton transfer occur only for N2-alkyl deriva-
tives I III. In keeping with the experimental data, the
main difference of Scheme 1 from analogous schemes
proposed for molecules VII and VIII [7, 14, 18] is the
absence of ESIPT-following twisting of the A ring,
which could give rise to formation of TICT and post-
TICT structures in the S1 and S0 state, respectively.
Here, it should be noted that weakening of intramole-
cular hydrogen bond and/or change of the localization
of the lowest electron transition on variation of the
R1 substituent in the proton-donor R1N1H fragment
(e.g., as in VI) could make the proposed scheme
invalid. Thus Scheme 1 is not general for all mole-
cules of Schiff bases in which N1H N2 intramole-
cular hydrogen bond is formed.
Scheme 1.
To conclude, we must emphasize a specific import-
ance of our comparative results which suggest that
photochromic properties of Schiff bases involving
intramolecular hydrogen bond are determined by fine
mechanisms of ESIPT-following structural trans-
formations. Presumably, such transformations are
typical only for Schiff bases derived from o-hydroxy-
benzaldehyde, in which O H N intramolecular
hydrogen bond is formed.
R = Alk, Ar.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 7 2002

1022
IVAKHNENKO et al.
6. Ivakhnenko, T.E., Makarova, N.I., Ivakhnenko, E.P.,
EXPERIMENTAL
Minkin, V.I., and Knyazhansky, M.I., Int. J. Photo-
energy, 1999, vol. 1, pp. 166 169.
The IR spectra were recorded on a Specord 71-IR
spectrometer in mineral oil. Elemental analysis was
performed at the Microanalysis Laboratory (Reseacrh
Institute of Physical and Organic Chemistry, Rostov
State University). The electron absorption spectra
were measured on Specord M-40 and Varian Cary-100
spectrophotometers. The fluorescence spectra were
obtained on Shimadzu RF-5001 PC and Elyumin-2M
spectrofluorimeters equipped with an optical cryostat
for low-temperature (77 K) measurements. The sol-
vents used for recording absorption and fluorescence
spectra were purified by standard procedures and were
checked for impurities by luminescence spectra at
293 and 77 K.
7. Kletskii, M.E., Milov, A.A., Metelitsa, A.V., and
Knyazhansky, M.I., J. Photochem. Photobiol.,
Part A: Chem., 1997, vol. 110, pp. 267 270.
8. Cohen, M.D. and Schmidt, G.M.J., J. Phys. Chem.,
1962, vol. 66, pp. 2442 2445.
9. Hadjoudis, E., Photochromism. Molecules and
Systems, Durr, H. and Bounas-Laurent, Eds.,
Amsterdam: Elsevier, 1990, pp. 685 715.
10. Chernova, N.I., Bolotin, B.M., Nurmukhame-
tov, R.N., and Ryabokobylko, Yu.S., Reaktivy
i osobo chistye veshchestva (Chemicals and Ultra-
pure Substances), Moscow: IREA, 1973, no. 35,
pp. 57 63.
Schiff bases I VI were synthesized by condensa-
tion of 2-(p-tolylsulfonylamino)benzaldehyde [19] and
the corresponding amine, according to the following
procedure: a mixture of equimolar amounts of the
aldehyde and amine in dry benzene was refluxed with
a Dean Stark trap until water separated competely
(1 5 h) [11]. After cooling, the precipitate was filtered
off, recrystallized from methanol, and dried at 70
100 C under reduced pressure. Table 2 contains the
melting points, IR spectra, and analytical data of the
compounds prepared.
Model compounds, o-(methyliminomethyl)phenol
(VII) and o-(phenyliminomethyl)phenol (VIII), were
synthesized by condensation of equimolar amounts
of salicylaldehyde and methylamine or aniline in
methanol on heating on a water bath for 1 h under
reflux [20]. The properties of the products were con-
sistent with published data.
11. Burlov, A.S., Mistryukov, A.E., Garnovskii, A.D.,
and Sergienko, V.S., Zh. Strukt. Khim., 1996, vol. 37,
no. 3, pp. 545 549.
12. Obodovskaya, A.E., Starikova, Z.A., Trunov, V.K.,
and Bolotin, B.M., Zh. Strukt. Khim., 1981, vol. 22,
no. 1, pp. 104 108.
13. Obodovskaya, A.E., Starikova, Z.A., Trunov, V.K.,
and Bolotin, B.M., Zh. Strukt. Khim., 1981, vol. 22,
no. 1, pp. 109 113.
14. Knyazhansky, M.I., Metelitsa, A.V., Kletskii, M.E.,
Milov, A.A., and Besugliy, S.O., J. Mol. Struct.,
2000, vol. 526, pp. 65 79.
15. Knyazhanskii, M.I. and Metelitsa, A.V., Fotoinitsii-
rovanye protsessy v molekulakh azometinov i ikh
strukturnykh analogov (Photoinitiated Processes in
Molecules of Schiff Bases and Their Structural
Analogs), Rostov-on-Don: Rostov. Gos. Univ., 1992,
p. 180.
16. Knyazhansky, M.I., Metelitsa, A.V., Bushkov, A.Ja.,
and Aldoshin, S.M., J. Photochem. Photobiol.,
Part A: Chem., 1996, vol. 97, pp. 121 126.
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