Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases

Article abstract of DOI:10.1016/j.saa.2020.118416

The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV–Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (K_T in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol^?1 of 4 and 0.63 kcal mol^?1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.

Full text of DOI:10.1016/j.saa.2020.118416

Journal Pre-proof
Indirect solvent assisted tautomerism in 4-substituted phthalimide
2
-hydroxy-Schiff bases
Dancho Yordanov, Vera Deneva, Anton Georgiev, Aurelien
Crochet, Katharina M. Fromm, Liudmil Antonov
PII:
S1386-1425(20)30394-2
https://doi.org/10.1016/j.saa.2020.118416
SAA 118416
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 8 February 2020
Revised date:
21 April 2020
5 April 2020
Accepted
date:
2
Please cite this article as: D. Yordanov, V. Deneva, A. Georgiev, et al., Indirect solvent
assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases, Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy (2018), https://doi.org/10.1016/
j.saa.2020.118416
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Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-
Schiff bases
a
a
b,c,*
d
d
Dancho Yordanov , Vera Deneva , Anton Georgiev
, Aurelien Crochet , Katharina M. Fromm ,
a,**
Liudmil Antonov
a
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Science,
Acad. G. Bonchev avenue, bldg. 9, Sofia 1113, Bulgaria.
b
Department of Organic Chemistry, University of Chemical Technology and Metallurgy, 1756
Sofia, 8 St. Kliment Ohridski avenue, Bulgaria.
c
Institute of Optical Materials and Technologies, Bulgarian Academy of Science, 1113 Sofia, Acad.
G. Bonchev avenue, bldg. 109, Bulgaria.
d
Department of Chemistry and Fribourg Center for Nanomaterials FriMat, University of Fribourg,
CH-1700 Fribourg, Switzerland
*
Corresponding author: e-mail: antonchem@abv.bg
**
Corresponding author: e-mail: lantonov@orgchm.bas.bg
Abstract
The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-
hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural
differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour,
depending on the solvent environment were studied by absorption UV-Vis spectroscopy.
Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (K in toluene =
T
0
.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state
theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier
-
1
-1
between enol/keto tautomer 5.6 kcal mol of 4 and 0.63 kcal mol of 5, which confirms the
experimentally observed impossibility of the tautomeric equilibrium in the former. The
experimentally observed specific solvent effect in methanol is modelled via explicit solvation.
The excited state intramolecular proton transfer (ESIPT) was investigated by steady state
fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto
tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-
propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane,
1

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acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in
MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto
ones, which explains the origin of the experimentally observed keto form emission.
Keywords: ESIPT, phthalimides, 2-hydroxy-Schiff bases, enol/keto tautomerism,
phototautomerization, DFT, UV-Vis spectroscopy, crystallography
1
. Introduction
The aromatic 2-hydroxy substituted Schiff bases (SB’s), able to undergo solvent-assisted or
photoinduced enol/keto tautomerization via excited state intramolecular proton transfer (ESIPT)
[
1–3], are of substantial scientific interest. The ESIPT phenomenon finds potential applicability in
the development of metal ions sensors, proton based photoswitches, light driven molecular
motors, molecular logic gates, OLED’s and solar cell fabrication, labeling cell image biology and
others [4–13]. There are a large number of synthesized and investigated 2-hydroxy substituted
SB’s and other derivatives by intramolecular hydrogen H-O- bonding with multifarious structures
–
from 2-hydroxyazo and azomethine chromophores to differently substituted pyridines,
pyrazoles, coumarins, quinolones, isoquinolines, etc [1,3].
Depending on the electronic effects of 2-hydroxyaryl substituents, the tautomerization –O–
…
…
……
H N≤
=O HN< in SB’s could be triggered by solvents or photoexcitation. Several factors
influence the process: (i) the –O-H acidity of 2-hydroxyaryl substituents enable hydrogen bonding
with azomethine nitrogen resulting in enol/keto equilibrium. The conjugated keto base is
stabilized by π-π delocalization with the aryl moiety by increasing the number of rings or by
introducing the electron withdrawing (EW) substituents resulting in a shifted equilibrium toward
the keto form [3,14–18]; (ii) the basicity (nucleophilicity) of :N≤ azomethine by a nonbonding
2
2
electron pair to form a conjugate acid. The key factor here is the Ar–CH=N−Ar
Ar=CH−HN−Ar
rearrangement by proton transfer reaction, where the π-motion is favored by the stability of keto
2
form and by donor-acceptor interactions in –NH–Ar moiety. The electron donating (ED) groups
2
increase the basicity of nitrogen atom able to form a conjugated –NH-Ar acid. On the other
hand, EW groups cause n-π conjugation, which is increased after the protonation of nitrogen; (iii)
the solvent polarity is critically important for the tautomerization. Most aprotic solvents shift the
equilibrium toward the keto tautomer by increasing the polarity. The polar protic solvents can
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form intermolecular hydrogen bonds resulting in a stabilized keto form [19–22]; (iv) the planarity
of 2-hydroxy species is essential to intramolecular hydrogen bonding and proton transfer
reactions [23–25]; (v) last but not least, is the temperature influences on tautomerization and
ESIPT efficiency. Several useful papers provide a very insightful investigation of low temperature
spectral behavior [26–30]. McDonald and co-authors have investigated a non-fluorescent Schiff
base (2′-Hydroxybenzylidene hydrazine carboxylate) in aprotic solvents such as acetonitrile
(AcCN) and tetrahydrofuran (THF) at room temperature; however, upon addition of water, an
emission at 453 nm related to the ESIPT appeared. By low temperature fluorescence study at -
o
1
90 C in AcCN, a strong emission was registered at 521 nm related to the keto emission [28].
Hence, tautomerization and ESIPT display cumulative behavior by the abovementioned factors,
where by variation of the condition, control over solvent assisted- and photoinduced
tautomerization between two switched states could be achieved.
One of the useful features of ESIPT is the possibility to produce both enol (N*) and keto (K*)
emission based on the N* K* equilibrium in the excited state. Usually, the keto emission is
characterized by a large Stokes shift [31–33]. A very useful and seminal review by Kwon and Park
[
11], devoted to ESIPT and its application in materials science, considers different types of
molecules and the factors influencing the fundamental photophysical properties. Due to
quenching and predominating non-radiative relaxation to the ground state, most of the
experimental data indicate that in (polar or nonpolar) solution the quantitative manifestation of
the process is characterized by a low fluorescence quantum yield. In contrast, in the solid state,
due to J- or H-aggregation, the efficiency can be increased by up to 50-70 %, which is one of the
reasons for successfully device fabrication such as OLEDs [8,34–36]. Up to now, there are some
examples for ESIPT and tautomerization study of the compounds containing fluorophore units
with very good performance as sensors and molecular logic gates [10,37,38]. One of the effective
fluorophore classes is the 4-aminophthalimide and its aminoalkylated derivatives. They show
strong sensitivity upon different solvent environments and are used as fluorescence probes in
cell biology, protein labeling as well as fluorescent polymers for flexible OLED’s devices. For
example, 4-aminophthalimide, having an electron-donating amino group, exhibited a large
quantum yield in THF (Φ = 0.70), dichloromethane (DCM) (Φ = 0.76) and acetone (Φ = 0.68),
fl
fl
fl
which forms a base to find applicability in materials science [39–41].
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Until now, based on analysis of the existing literature, there is scarce information for the
synthesis and tautomerism of phthalimide containing 2-hydroxy SB’s [42,43]. Therefore, the aim
of the current study is to describe the synthesis of two novel 4-substituted phthalimide 2-
hydroxy SB’s containing salicylic and 2-hydroxy-1-naphthyl parts and to investigate their ground
state tautomerism and ESIPT behavior. This is achieved by a combined approach using optical
(absorbance and emission) spectroscopy, advanced data processing, DFT calculations and
crystallographic analysis. The obtained information could be useful in the design of new
tautomeric systems with desired photochemical properties.
2
. Experimental
2
.1. Materials used
Phthalimide, 2-hydroxy-1-naphthaldehyde, salicylaldehyde, ethyl bromide (EtBr), and
trifluoroacetic acid (TFA) were purchased from Sigma-Aldrich and used without further
purification. Nitric acid, sulphuric acid, potassium carbonate, N,N-dimethylformamide (DMF), and
methanol (MeOH) were delivered from local supplier Valerus Ltd. For spectroscopy
measurements, all solvents were of spectroscopy grade purity.
2
.2. Synthesis
The synthetic pathway of the compounds is presented in Scheme 1. To synthesize 1 (4-
nitrophthalimide), 2 (4-nitro-N-ethylphtalimide) and 3 (4-amino-N-ethylphthalimide), we have
1
13
followed the methodology of previously published papers [44,45]. The H- and C-NMR spectra
of 3 used as an intermediate compound to synthesize 2-hydroxy-Schiff bases are presented in the
Supporting information.
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Scheme 1. Synthetic pathway of the 4-substituted phthalimide 2-hydroxy-Schiff bases.
2
.3. General procedure for the condensation reaction between 4-amino-N-ethylphthalimide and
aromatic aldehydes to 2-hydroxy-Schiff bases
N-ethyl-4-aminophthalimide 3 (2.628 mmol, 1.0 equiv) and 2-hydroxy-aromatic aldehydes (2.628
mmol, 1.0 equiv) were suspended in absolute ethanol (EtOH) (25 mL). Subsequently, a catalytic
amount of TFA was added (3 drops) and the mixture heated to reflux without stirring for 3-4
hours. After cooling to room temperature, the products precipitate and then were filtered off
and recrystallized in a minimal amount of EtOH to yield the pure Schiff bases, TLC (CHCl :EtOAc =
3
1
13
3
:2) (Scheme 1). The H- and C-NMR spectra are presented in the Supporting information.
2
.3.1. Synthesis of the (E)-2-ethyl-5-((2-hydroxybenzylidene)amino)isoindoline-1,3-dione (4)
The compound was synthesized as described in the general procedure above from 0.5 g (2.628
mmol) 4-amino-N-ethylphthalimide and 0.321 g / 0.275 mL (2.628 mmol) salicylaldehyde, yield
-
1
6
8 % as yellow -orange solids, m.p. = 185 °C. ATR-FTIR cm : 3443 (ν -OH), 3045 (ν Ar-H), as 2968,
s 2930 (ν -CH and >CH ), s 1762, as 1695 (ν >C=O), 1625 (ν –CH=N-), 1603, 1570, 1492 (ν Ar C=C),
3
2
1
as 1400, s 1371 (δ -CH and >CH ), 1351, 1335 (C-N-C stretching vibration of imide ring). H-NMR
3
2
(
(
(
DMSO-d ) ppm: 12.45 (1H, s), 9.07 (1H, s), 8.01-7.82 (2H, m), 7.74 (2H, dd, J = 18.4, 7.9 Hz), 7.46
6
1
3
1H, t, J = 7.8 Hz), 6.99 (2H, p, J = 8.3, 7.9), 3.61 (2H, q, J = 7.0 Hz), 1.17 (3H, t, J = 7.2 Hz). C-NMR
DMSO-d ) ppm: 167.80, 167.75, 166.23, 160.77, 134.69, 134.02, 133.16, 128.35, 124.81, 119.89,
6
5

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+
1
2
19.80, 117.23, 115.82, 32.96, 14.12. MS (ESI+) calculated for C H N O , [M ]: 294.1004; found
1
7 14 2 3
94.0996.
2
.3.2. Synthesis of the (E)-2-ethyl-5-(((2-hydroxynaphthalen-1-yl)methylene)amino)isoindoline-
,3-dione (5)
1
The compound was synthesized as describied in the general procedure above from 0.5 g (2.628
mmol) 4-amino-N-ethylphthalimide and 0.452 g (2.628 mmol) 2-hydroxy-1-naphthaldehyde, yield
-
1
7
2
1 % as red-orange solids, m.p. = 218 °C. ATR-FTIR cm : 3447 (ν -OH), 3061 (ν Ar-H), as 2987, s
942 (ν -CH and >CH ), s 1762, as 1697 (ν >C=O), 1628 (ν –CH=N-), 1599, 1572, 1479 (ν Ar C=C),
3
2
1
as 1470, s 1395 (δ -CH and >CH ), 1321, 1302 (C-N-C stretching vibration of imide ring). H-NMR
3
2
(
(
(
DMSO-d ) ppm: 15.37 (1H, s); 9.71 (1H, s); 8.54 (1H, d, J = 8.4 Hz), 8.19 (1H, d, J = 1.7 Hz), 7.93
6
1H, d, J = 9.2 Hz), 7.84-7.70 (3H, m), 7.54 (1H, dd, J = 8.4, 8.6 Hz); 7.35 (1H, t, J = 7.4 Hz); 6.98
1
3
1H, d, J = 9.1 Hz); 3.59 (2H, q, J = 7.2 Hz), 1..17 (3H, t, J = 7.2 Hz). C-NMR (DMSO-d ) ppm:
6
1
1
71.47, 167.65, 167.62, 157.43, 149.92, 138.35, 134.15, 133.51, 129.50, 128.78, 128.72, 127.31,
27.13, 124.75, 124.36, 122.41, 121.35, 114.76, 109.54, 32.92, 14.14. MS (ESI+) calculated for
+
C H N O , [M ]: 344.1161; found: 344.1154.
2
1 16 2 3
2
.4. Spectral measurements
The NMR spectra were recorded on a Bruker Avance II+ spectrometer operating with frequency
1
13
6
00 MHz for H and 125 MHz for C in DMSO-d . ATR-FTIR spectra of the compounds were
6
-
1
recorded on a Bruker Tensor 27 FTIR spectrophotometer in the range of 4400-600 cm а with
-
1
resolution of 2 cm at room temperature. The external reflection diamond crystal was used and
the samples were scanned 128 times. The molecular mass of the compounds was confirmed by a
high resolution mass spectrometer Thermo DFS. The UV-VIS spectra were measured on a Jasco V-
-5
5
70 UV–vis–NIR spectrophotometer in spectral grade solvents in the concentration range ~ 10
-1
mol L . The quantitative analysis of the observed enol keto equilibria for 5 was performed by
using a Fishing-Net algorithm as described elsewhere [46,47]. The steady-state fluorescence
spectra were recorded with a FluoroLog 3-22 (HORIBA) spectrofluorometer in the range 200-800
nm with a resolution of 0.5 nm and double-grating monochromators using as excitation
-
6
wavelength a value near the absorption maxima of the dyes with concentrations of ~ 6×10 mol
-
1
L .
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2
.5. Quantum chemical calculations
Quantum-chemical calculations were performed using the Gaussian 09 D.01 program suite [48].
The M06-2X functional [49,50] with the TZVP [51] basis set was used for the calculations in the
ground and excited state. This fitted hybrid meta-GGA functional with 54% HF exchange was
especially developed to describe main-group thermochemistry and non-covalent interactions.
The M06-2X functional was selected by a benchmarking procedure, which has shown that there
is good empirical relations between the relative stability of the tautomers, defined as E, and the
o
experimentally determined G values of set of tautomeric dyes in cycloxexane [52,53]. This
qualitative approach shows very good results in predicting the position of tautomeric equilibria
for compounds with intramolecular hydrogen bonds as well as describing ground and excited
o
state proton transfer mechanisms [54–58]. Although the computationally G values should be
preferable, their accuracy is under question even at high level calculations for compoiunds of the
size used in the current study [59,60].
All ground state structures were optimized without restrictions, using tight optimization criteria
and an ultrafine grid in the computation of two-electron integrals and their derivatives. The true
minima were verified by performing frequency calculations in the corresponding environment.
The implicit solvation was described using the Polarizable Continuum Model (the integral
equation formalism variant, IEFPCM, as implemented in Gaussian 09) [61]. The specific effect of
methanol was modeled by explicit solvation, keeping the solvent implicitly as well.
The TD-DFT method [62–64] was used for singlet excited state optimizations again without
restrictions applying tight optimization criteria and an ultrafine grid in the computation of two-
electron integrals and their derivatives.
The transition states were estimated using the STQN method [65] and again verified by
performing frequency calculations in the corresponding environment.
2
.6
Crystallographic analysis
Single crystals of 4 and 5 were obtained by crystallization from ethanol and acetone, respectively.
Suitable crystals were selected and mounted on a MiTeGen holder in oil on
a StadiVari diffractometer. Crystals were kept at 200(2) K during data collection. Using Olex2
7

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[
66], structures were solved with the ShelXT [67] structure solution program using Intrinsic
Phasing and refined with the ShelXL [68] refinement package using least squares minimisation.
Crystal Data for 4: C H N O (M =294.30 g/mol): monoclinic, space group P2 /c (no. 14), a =
1
7
14
2
3
1
3
9
2
≤
.4560(5) Å, b = 5.0043(2) Å, c = 29.9445(16) Å, β = 98.079(4)°, V = 1402.93(12) Å , Z = 4, T =
-
1
3
00(2) K, μ(CuKα) = 0.797 mm , Dcalc = 1.393 g/cm , 14720 reflections measured (10.442° ≤ 2Θ
114.396°), 1879 unique (R = 0.0895, R = 0.0514) which were used in all calculations. The
int
sigma
final R was 0.1072 (I > 2σ(I)) and wR was 0.2903 (all data). 5: C H N O (M =344.36 g/mol):
1
2
21 16 2 3
monoclinic, space group P2 /c (no. 14), a = 6.8909(4) Å, b = 14.1083(11) Å, c = 16.8448(10) Å, β =
1
3
-1
3
9
2
0
0
2.268(5)°, V = 1636.35(19) Å , Z = 4, T = 200(2) K, μ(CuKα) = 0.772 mm , Dcalc = 1.398 g/cm ,
097 reflections measured (10.512° ≤ 2Θ ≤ 113.8°), 2097 unique (R = NA, R = 0.0250, BASF:
int
sigma
.244(11)) which were used in all calculations. The final R was 0.0963 (I > 2σ(I)) and wR was
1
2
.2857 (all data).
Accession Codes CCDC 1978200-1978201 contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
3
. Results and Discussion
3
.3. Ground state tautomerism
Two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic and 2-hydroxy-1-
naphthyl moieties, respectively, were synthesized via a four steps synthetic procedure (Scheme
1
). A classical nitration of phthalimide to 4-nitrophthalimide (1) was performed as a first step,
followed by a nucleophilic substitution of NH imide by ethyl bromide in DMF in the presence of
K CO to N-ethyl-4-nitrophthalimide (2) [69]. The ethyl group was introduced in order to
2
3
increases the solubility and to avoid NH deprotonation. Then catalytic hydrogenation of −NO to
2
–
NH by a Pd/C catalyst in MeOH led to N-ethyl-4-aminophthalimide (3) and finally, condensation
2
of 3 with salicylic aldehyde or 2-hydroxy-1-naphthaldehyde by refluxing in absolute EtOH in the
presence of catalytic amount of TFA yielded 4-substituted phthalimide 2-hydroxy-Schiff bases (4
and 5).
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Figure 1. Absorption spectra of 4 (A) and 5 (B) in toluene, AcCN, EtOH and MeOH. Absorption
spectra of 5 (C1) in toluene/MeOH binary solvent mixture from 100 % toluene (red dashes) to
1
00 % MeOH (blue dashes) along with the individual spectra of enol (red solid line) and keto (blue
solid line) forms. Absorption spectra of 5 (C2) in AcCN/EtOH binary solvent mixture from 100 %
AcCN (red dashes) to 100% EtOH (blue dashes) along with the individual spectra of enol (red solid
line) and keto (blue solid line) forms.
Both compounds are potentially tautomeric with a proton transfer proceeding through six-
membered intramolecular hydrogen bonding. Hence, from a structural point of view, this
bonding and the difference in the 2-hydroxyaryl substituents should influence their tautomerism.
As seen from Figure 1A, the absorption spectra of 4 in nonpolar and polar solvents consist of a
single band at ~ 360 nm and no tautomerism related spectral changes are observed upon
changing the solvent. This is expected, because the lack of ground state tautomerism in phenyl
9

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derivatives of hydroxyaryl Schiff bases has already been described [3,70]. The low stability of the
keto tautomer is attributed to the reduced aromaticity in the 2-hydroxyphenyl ring compared to
the enol form. Compound 5, containing 2-hydroxy-1-naphthyl moiety, exhibits a major
absorbance band around 400 nm in most of the used solvents (Fig. 1B), attributed to the enol
form in analogy with similar tautomeric Schiff bases [21]. The absorbance of the keto tautomer
appears as a shoulder around 470 nm, the intensity of which depends on the properties of the
solvent (Fig. 1B). The spectra of 5 in binary solvent mixtures are shown in Figure 1C1
(toluene/MeOH) and 1C2 (AcCN/EtOH). It is evident that the increase of the volume ratio of
alcohol leads to an increase of the amount of the keto tautomer. The data from Figure 1 were
processed applying the protocol, earlier developed by us, for quantitative analysis of tautomeric
mixtures [47], which yielded the spectra and the molar fractions of the tautomers. The individual
spectra are presented on the Figure 1C1 and 1C2, indicating absorbance at 400 nm for the enol
form and a structured band in the range 450-500 nm for the keto tautomer. The spectral shapes
are very similar to the spectra of 1-((phenylimino)methyl)naphthalen-2-ol (6), reported before
[
21]. The advanced chemometric analysis allows, in addition to the individual spectra, an
estimation of the molar fractions of the tautomers in each studied solvent and hence, calculation
of the tautomeric constants (Table 1).
o
Table 1. Tautomeric constants* and G values at room temperature of 5 and 6 in various
solvents.
Toluene
AcCN
MeOH
o
o
o
Comp.
G
G
G
KT
KT
KT
-
1
-1
-1
kcal mol
1.25
kcal mol
0.90
kcal mol
0.27
**
5
6
0.12
0.22
0.63
***,****
****
****
0.09
1.40
0.56
0.34
0.95
0.03
*
Defined as keto/enol ratio; ** 0.53 in ethanol; *** in cycloxehane; **** taken from [21,72]
As seen, the amount of the keto tautomer of 5 is fairly low in toluene (11%), while some increase
is evident for AcCN (18%) and 40% keto form is observed in MeOH. Bearing in mind that the
intramolecular hydrogen bonding in Schiff bases is strong enough not to be broken by the solvent
1
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[
71], the expected effect is related mainly to the polarity of the solvent. Consequently, the
tautomeric equilibrium could not be affected by proton acceptor solvents and could be affected
by proton donor ones since both tautomeric forms could interact with them. In this respect, the
substantial stabilization of the keto tautomer, as evident from Figure 1, in alcohols is surprising
and needs a deeper consideration. In order to estimate the effect of the phthalimide moiety, a
comparative analysis of the new Schiff base 5 and the tautomerism of previously reported 6,
which has been studied in detail [21,31], will be made below.
Table 2. Theoretical prediction of the relative stability of the tautomers of 4-6 and some of their
structural parameters in the ground state (see also Scheme 2) by using implicit solvation.
,
Toluene
MeOH** ****
Comp Form
E
R(OH) R(NH)
φ*
E
R(OH)
[Å]
R(NH)
φ*
-
1

-1

kcal mol
[Å]
[Å]
  kcal mol
[Å]
 
0
.989
-39.0
(-35.8)
-25.0
E
0
0.986
-
-39.3
0
(
0.988)
1.341
4
5
***
TS
K
8.1
7.3
1.361 1.148 -23.1
7.0
1.161
1.035
5
.8
-18.2
-
1.038 -15.2
-
(
5.6)
(1.034) (14.3)
1
.000
-37.6
E
TS
K
0
0.996
-
-38.0
0
-
(
0.998)
1.258
(-29.6)
***
4.6
2.1
1.268 1.201 -24.8
3.9
1.211
1.026
-26.4
-14.7
1
.08
-
1.028 -14.9
-39.9
-
(
0.63)
(1.024) (-12.7)
E
TS
K
0
0.999
-
0
1.003
1.253
-
-
-38.1
-29.5
-23.4
6
4.2
1.268 1.203 -30.2
1.027 -23.6
3.5
1.216
1.026
1.70
-
0.55
*
non-planarity of the phthalimide (in 4 and 5) or phenyl (in 6) ring, defined as dihedral angle
C(H)-N-C-C; ** the results for acetonitrile are the same; *** as each of the tautomers of 4 and 5
exists as two possible isomers with respect to the phthalimide part by rotation around the single
N-C bond, the most stable isomers (see also Scheme 2) are presented. However, it should be
taken into account that the differences in the energies between the isomers of a given tautomer
-
1
are within 0.3 kcal mol ; **** the values using explicit solvation (Figure 2) are given in brackets.
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Scheme 2. Sketch of the ground state potential energy surface of 4 and 5 in MeOH using implicit
solvation model. The results for explicit solvation in methanol (Figure 2) are given in red. All
-
1
values are in kcal mol units.
Figure 2. Model of the methanol – carbonyl group from phthalimide complexes of the enol (left)
and the keto (right) tautomers.
The theoretical prediction of the relative stability (Table 2 and Scheme 2) of the tautomers of 4-6
could be used to clarify the structural and solvent effects. It can immediately be seen that with
-
1
an energy difference exceeding 5 kcal mol , no tautomeric equilibrium is possible in 4 as it is
actually observed. Comparison of the relative stabilities of the tautomers of 5 and 6 and their
tautomeric constants leads to the conclusion that the replacement of the phenyl ring (6) with 4-
substituted phthalimide (5) leads to destabilization of the keto tautomer (increased relative
energies in Tables 1 and 2). It could be expected that phthalimide, which contains two carbonyl
groups, should act as a strong electron-withdrawing group and hence stabilizes preferentially the
keto tautomer. However, the Schiff bases are non-planar, as also seen in Table 2, and these
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effects are relatively weak. Analysis of the HOMO and LUMO molecular orbitals of both
compounds (Table S1) indicates that the phthalimide moiety provides better charge transfer
stabilization in the case of the enol form, which could be the explanation of the obtained relative
energies. In the case of 6, there is no substantial difference in the charge transfer in the
tautomers, which explains the lack of a straightforward substituent effect on the position of the
tautomeric equilibrium, as experimentally proven [21].
The obtained experimental data for the tautomerism of 5 and 6 (Table 1) allow to estimate the
reliability of the theoretical predictions. In toluene, the enol tautomer is dominating as both
theory and experiment suggest. In AcCN, the predicted stability of the enol tautomer in both 5
-
1
and 6 is overestimated by ~ 0.2 kcal mol , which indicates the reliability of the used theoretical
approach. As expected, the change from AcCN to MeOH, using implicit PCM solvation, does not
affect the geometry of the tautomers and their stability in terms of quantum-chemical
calculations. However, the experiment shows that in MeOH solution, a substantially stronger
relative stabilization of the keto tautomer is achieved in 5 in comparison with 6. In 6, a change in
o
-1
-1
the G values from 0.34 (AcCN) to 0.03 (MeOH) kcal mol (~ 0.3 kcal mol shift) is observed,
which is attributed to a better interaction of the carbonyl group of the keto tautomer with
MeOH, acting as a proton donor solvent in this particular case [71]. In the case of 5, the change
-
1
of the solvent leads to a shift of ~0.6 kcal mol , which is substantially large. Having in mind that
the tautomeric moiety is the same in 5 and 6, a similar effect of the MeOH should be expected.
However, in the case of 5, there are two additional carbonyl groups in the phthalimide ring that
can interact with methanol to provide a long-range indirect stabilization. A simplified model of
the complexes of the tautomers with two MeOH molecules is shown on Figure 2. This specific
-
1
interaction gives a relative energy of 0.63 kcal mol (Scheme 2, the red-colored values) which
o
corresponds, in the frame of the offset discussed above, to the experimental G values of 0.27
-
1
kcal mol . The formation of the complexes with the solvent leads to a decreasing of the
o
o
nonplanarity of the tautomers (-30 and -12.7 in enol and keto forms respectively, see Table 2
for comparison). This kind of indirect solvent assisted stabilization, however, is not able to
drastically change the equilibrium when this is not allowed for aromatic reasons, see Scheme 2 in
the case of 4. As seen from Figure 1B, the quantity of the keto tautomer of 5 in MeOH is slightly
higher comparing to the other alcohols. Although the alcohols are polar solvents with
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comparable proton donor and acceptor ability in this particular case (MeOH-tautomer
complexes), they can act only as proton donors. Considering the specific solvent parameters,
describing the used alcohols in the light of linear solvation energy relationship theory [73–75],
the proton donor parameter of MeOH is 0.93, while for the rest of the alcohols it is in the range
of 0.79-0.83. This explains reasonably well why the MeOH behaves different in Figure 1B.
The existence of 4 as a enol tautomer and the stabilization of the keto form in 5 are confirmed by
the structures obtained by single crystal X-ray data of both compounds, as shown in Figure 3. The
data clearly indicate the observed effects are native, at least in 4, and not a result of
intermolecular interactions in the crystal. The hydrogen bonds are formed between O9 and N11
with a distance of 2.620(8) Å and an O9-H27…N11 angle of 146.4° for compound 4 and between
O11 and N13 with 2.524(8) Å and an angle of 145.4° for compound 5. This corresponds to
moderate hydrogen bonding [76], explaining why it is so stable.
The non-planarity of 4 is confirmed by the angle formed by the two planes, one containing the
phthalimide group and the other one containing the hydroxybenzyl group, of ca. 34° . However,
the analogue angle in 5 is quasi planar with an angle of ~1.5°. In terms of packing and
intermolecular interactions, compound 4 presents some H-contacts between the hydroxyl group
and the carboxyl group of the phthalidime groups of neighboring molecules as well as carbon-
carbon interactions between the overlaying hydroxyphenyl groups of two molecules (distance
plane to plane: ~3.6 Å). In contrast to 4, strong interactions exist in compound 5, where
molecules stack along the a axis in a head-to-tail fashion and a distance between the planes
containing two parallel molecules of ca. 3.4 Å. In this motif, the naphthalene ring is taken in
sandwich between two benzene rings of the phthalimide groups.
Figure 3. ORTEP representation of 4 (left) and 5 (right), ellipsoids are drawn with 30 % of
probability and hydrogen bonds are represented by a blue dashed line.
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The observed results for 5 in solid state are not surprising. The crystal was obtained from
acetone, where the enol form is strongly dominating. The strong stacking leads to forced
planarization of the molecule and theoretically the sandwitch type aggregates could be
considered. According to the data from X-ray, two dimer motifs are possible, depending to the
mutual position of the N-ethyl groups (see Scheme S1). In both cases the aggregation stabilizes
the enol form as it is practically observed in solid state. It should be noted, however, that in the
concetration range of UV-Vis spectroscopy not aggregation has been detected in any solvent, but
upon evaporation of the solvent it is not excluded.
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3
.2. Excited state intramolecular proton transfer
Steady-state emissions of 4 display different sensitivity of ESIPT in aprotic solvents (toluene,
DCM, Acetone and AcCN), while in protic ones with decreasing polarity (MeOH > EtOH > 1-
Propanol > 1-Butanol > 1-Pentanol), the observed emissions are characterized by large Stokes
shifts of around 560 nm corresponding to the keto tautomer (Fig. 4A1, 4A2 and Table 3). The
emission spectra of 5 are characterized by large Stokes shifts that indicate ESIPT and emissions of
the keto tautomer. All emission bands in the five alcohols ~ 515-550 nm can be also assigned to
the keto emission (Fig. 4B1, 4B2 and Table 3).
Figure 4. Experimentally measured absorption and steady-state fluorescence spectra of 4 and 5
in various aprotic solvents (A1 and B1) and different polar alcohols (A2 and B2). The excitation
wavelength was set near to the absorption maxima.
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Table 3. Summarized results of absorption wavelengths (λ ), emission wavelengths (λ ), Stokes
abs
em
shifts (Δῡ) and efficiency of enol→keto conversion (η ) of the studied 2-hydroxy-Schiff bases in
T
different polarity solvents.
λabs
λem
[nm]
437
464
474
474
566
566
568
566
558
Stokes shifts, ῡ
ηT
[%]
Comp
Solvent
-
1
[
nm]
360
357
354
354
355
357
358
359
359
[cm ]*
4894
6459
7151
Toluene
DCM
Acetone
AcCN
MeOH
EtOH
-Propanol
-
-
-
-
-
-
-
-
-
7151
4
10501
10343
10327
10187
9934
1
1
-Butanol
1
-Pentanol
3
4
3
4
3
4
3
4
4
4
4
4
4
4
4
4
4
4
98
78
97
75
94
70
93
66
01
46
02
47
04
49
05
48
06
49
Toluene
DCM
505
507
503
492
548
542
514
532
533
5320
5460
5500
5120
6690
6430
5300
5890
5870
41
52
Acetone
AcCN
96
100
100
100
100
100
100
5
MeOH
EtOH
1
-Propanol
1
-Butanol
1
-Pentanol
ꢉ
ꢉ
*
ꢆꢃ
Determined by ꢀꢁꢂꢃꢄꢅ ꢇꢈ
ꢎꢃꢊ ꢑ using long wavelength absorbance band and
ꢊꢋꢌꢍ
ꢏꢐ
maximum of the emissions.
By setting the excitation wavelength near to the absorption of the keto tautomer, we have
observed the keto emissions (Fig. S8 and S9 in the Supporting information). In all alcohols, the
bands match with those obtained after excitation near the enol absorption, which confirms that
the observed emissions correspond to the keto tautomer. The reason is a specific solute-solvent
interaction as discussed in the previous section in terms of theoretical calculations, enabling to
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form intermolecular hydrogen bonding between the carbonyl oxygen of the imide ring and the
alcohol (Scheme 3A). This causes additional polarization of the solute, which favors the ESIPT.
Krystkowiak and co-authors have studied the fluorescence behaviour of 4-aminophthalimide in
various polar solvents [77]. They have observed a large sensitivity and a red-shifted emission in
MeOH and EtOH due to the specific solvation by intramolecular hydrogen bonding. The reason
for the specific behaviour of the studied SB’s is the stability of conjugated base after
tautomerization to the keto form, which mainly depends on the π-π delocalization between >C=O
and aromatic rings and the basicity (nucleophilicity) of –CH=N- nitrogen by the nonbonding
electron pair. Obviously, keto 5 has a bigger stability due to stronger π-π delocalization of the
aromatic naphthalene ring compared to the keto 4 with a salicylic moiety. The compounds
containing the phthalimide ring can produce aggregate induced emission (AIE) and this can lead
to masking of the ESIPT. In order to minimize this effect, we have worked with strongly diluted
-
6
-1
solutions C ~ 6×10 mol L . For comparison, the emissions of 5 at higher concentration C ~
M
M
-
5
-1
6
×10 mol L were measured (Fig. S10 in the Supporting information). At higher concentrations,
the red-shifted emissions are observed (506 ÷ 622 nm) in aprotic and protic solvents due to AIE
compared to the lower concentration.
Scheme 3. Specific dye-alcohol interactions with imide carbonyl oxygen by intermolecular
hydrogen bonding (А) and tautomerization triggered by a different way (B): photoinduced for 4
and both photoinduced and indirectly solvent assisted for 5.
The enol/keto tautomerization can be triggered by solvent or photoexcitation, which is essential
to the spatial and temporal control between the two switchable states (Scheme 3B). Moreover,
intramolecular photoinduced tautomerization ensures dynamic control of proton transfer
reaction in the excited state between both tautomers depending on the solvent environment.
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For qualitative (or near qualitative) evaluation of the proton transfer in the excited state, we
have used a previously described method by us, where the assumption is that in the excited
state, the enol form undergoes fast transformation to the excited keto form and the resulting
emission is characterized by large Stokes shifts compared to the enol one [31]. We have
estimated the relative fraction of the enol form by the excitation spectrum, which is converted
into the keto form through ESIPT. They express the contributions of both forms by the following
equations (Eq.1 and 2) [78]:
ꢛꢜꢝ
ꢒꢓꢔꢕꢈꢖꢑꢂꢗ ꢇꢙ ꢇꢈ
ꢑ
(1)
(2)
ꢘ
ꢚ
ꢛꢜꢝꢞꢛ ꢈꢘꢑ
ꢟ
ꢒꢓꢔꢕꢈꢠꢑꢂꢗ ꢇꢙꢚ
ꢚ
Where A and A are the absorbance maxima of the enol and keto forms; I (E) and I (K) are the
E
K
exc
exc
excitation intensities at the same wavelengths; k is ESIPT rate, k (E) is the decay rate and Q is
PT
d
k
the quantum yield of the keto form. If the k /k + k (E) is defined as the efficiency of conversion
PT PT
d
of enol into keto form η , from Eqs. (1) and (2) we can use follows expression (Eq. 3):
T
ꢣꢤꢥꢦꢈꢘꢑꢇꢧꢨ
ꢡꢢꢂꢣꢤꢥꢦꢈꢚꢑꢇꢧꢩꢇꢄꢅꢅ
(3)
Examples of absorbance and excitation spectra in nonpolar and polar aprotic as well as in polar
and less polar alcohols are depicted in the Fig. 5. Systemized values of η in all used solvents of 5
T
are available in Table 3. In most aprotic solvents, the compound displays excitation to keto
tautomer via ESIPT resulting in red-shifted emissions. The highest rate of conversion up to 100 %
is observed in all alcohols due to specific solute-solvent interactions.
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Fig. 5. Absorbance and excitation spectra of 5 in toluene and AcCN (A) as well as in MeOH and 1-
Pentanol (B).
The theoretical results strongly support this hypothesis (Scheme 4, Table 4). It can be seen that in
both compounds, the excited keto tautomer is substantially more stable. Although this is a
feature of the 2-hydroxy SB’s (strong donor-acceptor stabilization) [79], this could be to some
extent attributed also to the planarization of the molecules, which increases the effect of the
carbonyl groups from the phthalimide ring. The extent of stabilization of the keto tautomer of 4
is substantially larger, which, together with the higher energy of the same tautomer in the
ground state, explains the observed larger Stokes shifts.
Scheme 4. Sketch of the first singlet exited state potential energy surface of 4 and 5 in MeOH
using implicit solvation model. The results from explicit solvation in MeOH (Figure 3) are red-
-
1
colored. All values are in kcal mol units against the ground state enol tautomer (Scheme 2).
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Table 4. Theoretical prediction of the relative stability of the tautomers (with respect to the
ground state enol tautomer) of 4-6 and some of their structural parameters in the first singlet
excited state (see also Figure 2 and Scheme 3) by using implicit solvation.
Toluene
Methanol**,****
Comp Form
E
R(OH) R(NH)
φ*
E
R(OH)
[Å]
R(NH)
φ*
-
1

-1

kcal mol
[Å]
[Å]
 
kcal mol
[Å]
 
7
6.1
1.029
(1.023)
1.149
-3.6
(-4.4)
0.5
E
79.1
79.2
73.2
1.054
-
-2.3
-
(74.7)
4
5
***
TS
K
1.121 1.391 0.0
76.6
1.348
1.030
7
0.0
69.2)
9.0
1.1
-
1.036 0.9
-
(
(1.031) (2.7)
6
1.034
(1.028)
1.191
-3.0
E
TS
K
72.3
73.0
70.4
1.038
-
-3.3
-
(
67.6)
69.6
(-4.7)
***
1.193 1.285 0.7
1.044 6.6
1.286
1.041
-0.2
1.4
6
6.8
-
-
(
65.8)
72.8
73.2
68.5
(1.039) (-4.1)
E
TS
K
1.036
1.157
-
-
6.7
2.1
6
1.327
1.034
-2.7
*
non-planarity of the phthalimide (in 4 and 5) or phenyl (in 6) ring, defined as dihedral angle
C(H)-N-C-C; ** the results for acetonitrile are the same; *** as each of the tautomers of 4 and 5
exists as two possible isomers with respect to the phthalimide part by rotation around the single
N-C bond, here the most stable isomers (see also Scheme 3) are presented; **** the values using
explicit solvation (Figure 2) are given in brackets.
4
. Conclusions
Two new 4-substituted phthalimide 2-hydroxy-Schiff bases (4 and 5) have been synthesized and
studied by molecular spectroscopy and theoretical calculations. It has been shown that their
ground state tautomerism depends on the existing intramolecular hydrogen bonding, the
implemented phthalimide moiety and the difference in the 2-hydroxyaryl substituents. In 4, no
tautomeric equilibrium has been observed. In 5, the tautomeric ratio is determined by the
solvent. Comparing spectral properties in acetonitrile and methanol, it has been found that
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additional indirect solvent stabilization of the keto tautomer is achieved in the latter. A simplified
theoretical model suggests that the interaction of the methanol molecules with the carbonyl
groups in the phthalimide moiety could be responsible for that. Both compounds exhibit excited
state intramolecular proton transfer, proven by the large Stokes shifts. The ESIPT barrier in the
excited state is fairly low, which leads to the absence of enol form emission at the expense of the
keto emission.
The comparative systematic study of 4 and 5 allow to make following conclusions in terms of
structure-property relationship: (i) 2-hydroxy-1-naphthyl moiety of 5 is responsible for indirect
solvent-assisted tautomerization in the most aprotic and all protic solvents in the ground state;
(ii) compound 4 containing a salicylic moiety does not display tautomerization in the ground
state; (iii) both compounds show a high rate of ESIPT in various protic and aprotic solvents; (iv) by
variation of the number of condensed phenyl rings in the 2-hydroxyaryl substituents, the control
over the ground state and photoinduced tautomerization between two switching states can be
achieved.
Conflict of interest
The authors declared that the article content has no conflicts of interest.
Acknowledgements
This work was financial supported by the Bulgarian National Science Fund under project ДН 18/5
“
Novel azo materials and application of their photophysical properties as reversible optical
optical storage devices”. Research equipment of distributed research infrastructure INFRAMAT
part of Bulgarian National roadmap for research infrastructures) supported by Bulgarian
(
Ministry of Education and Science under contract D01-284/17.12.2019 was used in this
investigation.
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Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered as
potential competing interests:
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