Synthesis of phthalimides, isoindolin-1-ones and isoindolines bearing aminobenzoic acids as a new fluorescent compounds

Article abstract of DOI:10.1016/j.jphotochem.2021.113185

Both experimental and theoretical methods were used in order to study the fluorescent properties of nine new compounds based on phthalimides, isoindolin-1-ones and isoindolines bearing aminobenzoic acids (2-aminobenzoic acid, 3-aminobenzoic acid and 4-aminobenzoic acid), which were obtained under mild reaction conditions. The photophysical properties of all the compounds were studied by electronic absorption and fluorescence spectroscopy in methanol solutions. All compounds exhibited fluorescence emission and high quantum yields. Additionally, it was found that the intramolecular charge in these donor-acceptor systems is significantly depending on electron-withdrawing substituents at the carboxylic acid position.

Full text of DOI:10.1016/j.jphotochem.2021.113185

Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis of phthalimides, isoindolin-1-ones and isoindolines bearing
aminobenzoic acids as a new fluorescent compounds
a
a,
b
a
´˜
´
´
Melchor Solis-Santos , Mario Ordonez *, Adrian Ochoa-Teran , Rodrigo Morales-Cueto ,
a
´
Victoria Labastida-Galvan
a
´
Centro de Investigaciones Químicas–IICBA, Universidad Autonoma del Estado de Morelos, Av. Universidad 1001, 62209, Cuernavaca, Morelos, Mexico
b
´
´
´
´
Centro de Graduados e Investigacion en Química, Tecnologico Nacional de Mexico/Instituto Tecnologico de Tijuana, Tijuana, B.C., Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
Both experimental and theoretical methods were used in order to study the fluorescent properties of nine new
compounds based on phthalimides, isoindolin-1-ones and isoindolines bearing aminobenzoic acids (2-amino-
benzoic acid, 3-aminobenzoic acid and 4-aminobenzoic acid), which were obtained under mild reaction con-
ditions. The photophysical properties of all the compounds were studied by electronic absorption and
fluorescence spectroscopy in methanol solutions. All compounds exhibited fluorescence emission and high
quantum yields. Additionally, it was found that the intramolecular charge in these donor-acceptor systems is
significantly depending on electron-withdrawing substituents at the carboxylic acid position.
Phthalimides
isoindolin-1-ones
Isoindolines
Fluorescence
Absorption and emission
1. Introduction
different natural and synthetic products [18]. Among these compounds,
the phthalimide, isoindolin-1-one and isoindoline cores are of great
In the last two decades, the compounds with fluorescence properties
have played a key role as analytical and diagnostic tools in organic and
medicinal chemistry [[1,2]]. These compounds also called as chemo-
sensors and fluorescent biomarkers [3–6], have been used to detect
diseases, sense metal ions or imprint cellular marking to follow up the
mechanisms that occur within them, consequently this area have
received considerable attention due to the significant applications in
several research fields such as medicine, molecular and cellular biology,
biophysics, biotechnology and environmental sciences [7–9]. Despite
the efforts made to develop fluorescent compounds with higher quan-
tum yield and specificity, most of the obtained compounds show a high
structural complexity and synthesis. One of the challenges resides on the
creation of a non-toxic sensor that brings high effectiveness, sensitive-
ness and an easy homogeneous association with substrates of different
nature. Donor-acceptor (DA) compounds [10] have recently attracted
technological and academic research, since they are increasingly finding
applications in molecular electronics and optoelectronics, including
organic light-emitting diodes (OLEDs) [11,12], electrogenerated chem-
iluminescence (ECL) [13,14], photovoltaic devices [15], biochemical
fluorescent technology [16] and nonlinear optics [17].
importance as drugs and building blocks [19–21] in the preparation of
compounds with a wide range of pharmacological activities, acting as
antiviral [22], antimicrobial [23], antipsychotic [24] and antitumoral
agents [25].
On the other hand, the phthalimides and isoindolin-1-ones are
compounds with interesting photophysical properties [26–29]. Their
versatility depends on the chromophore groups and auxochromes that
these compounds possess, by favoring their different applications in the
absorption of multi-photons, organic field effect transistors (OFET),
organic light emitting diodes (OLED) and organic photovoltaic energy
(OVP) [30,31]. The electronic and photonic properties of these type of
molecules behaving as DA system are a function of the HOMO-LUMO
energy [32]. Some of them have the potential as new optical mate-
rials, due to their delayed photoluminescence (DLP) behavior [33]. They
may have solvatochromic effects [34,35], in addition to their impor-
tance for the detection of diseases [36].
Due to the biological importance and the multiple applications in
different areas of phthalimides, isoindolin-1-ones and isoindolines
including their interesting chemical and photophysical properties, in
connection with our current research interest in the synthesis of 2,3-
disubstituted isoindolin-1-ones [37,38], herein we describe the synthe-
sis of three phthalimides, three isoindolin-1-ones and three isoindolines
Additionally, heterocyclic compounds containing heteroatoms such
as nitrogen are considered as the most abundant molecular structures in
* Corresponding author.
´˜
E-mail address: palacios@uaem.mx (M. Ordonez).
https://doi.org/10.1016/j.jphotochem.2021.113185
Received 2 June 2020; Received in revised form 6 January 2021; Accepted 1 February 2021
Available online 20 March 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
incorporating fragments of aminobenzoic acid under mild reaction
conditions as well as the evaluation of their photophysical properties as
donor-acceptor compounds and the correlation with their theoretical
predictions.
2.2.2.2. 2-(3-Methoxycarbonylphenyl)isoindolin-1-one 3b. The crude
was purified by column chromatography using a mixture of CH2Cl2/
AcOEt (80:20) as eluent, obtaining (0.38 g, 85 %) of 2b as white crystals,
m.p. 123ꢀ 125 ◦C. 1H NMR (400 MHz, CDCl3): δ = 3.94 (s, 3H, CH3O),
4.91 (s, 2H, CH2), 7.48–7.54 (m, 3H, Harom), 7.61 (ddd, 7.36, 1.07 Hz,
1H, 7.84 (d, J = 7.87, 1H, Harom), 7.92 (dd, 7.56, 1.12 Hz, 1H, Harom),
8.25–8.29 (m, 1H, Harom), 8.37–8.41 (m, 1H, Harom). 13C NMR
(100 MHz, CDCl3): δ = 50.8, 52.4, 119.6, 122.9, 124.1, 124.4, 125.5,
128.6, 129.5, 131.1, 132.5, 133.1, 139.9, 140.2, 166.9, 167.8. HRMS
(FAB+): calculated for C16H13NO3 [M+H]+, m/z 267.0895; found for
[M+H]+, m/z 268.0831.
2. Experimental section
2.1. Materials and instruments
All commercial materials were used as received noted otherwise.
Reactions were monitored using analytical TLC plates (Merk, silica get
F254, 0.25 mm). Flash chromatography was performed using 230–400
mesh Silica Flash 60® silica gel. 1H and 13C spectra were recorded on a
Bruker Avance DPX 200 and DPX 400 instrument operating at 400 MHz
(1H) and 100 MHz (13C). Chemical shifts are reported in ppm using TMS
as internal standard. Mass spectra were obtained on a Shimadzu GCMS-
QP5050 instrument. Melting Points were determined on a Büchi in-
strument and are uncorrected. Fluorescence emission spectra were ob-
tained using a Varian spectrofluorometer. UV–vis absorption spectra
were obtained using a Cary 300 spectrophotometer.
2.2.3. General procedure for the preparation of isoindolines 5a-c
To a stirred solution of α,αꞌ-dibromo-o-xyleno (0.25 g, 0.94 mmol) in
acetonitrile (5 mL), K2CO3 (0.23 g, 2.1 mmol) and the appropriate
methyl aminobenzoate (0.14 g, 0.94 mmol) was added. The reaction
mixture was heated at 50 ◦C for 3.0 h. After this time, the reaction
mixture was filtered and the solid was washed with CH2Cl2. The com-
bined organic solvent was evaporated and the residue was purified by
column chromatography using a mixture of CH2Cl2/hexane (85:15) as
eluent.
2.2. Synthesis section
2.2.3.1. 2-(2-Methoxycarbonylphenyl)isoindoline 5a. White crystals
(0.17 g, 69 %); m.p. 150ꢀ 151 ◦C. 1H NMR (400 MHz, CDCl3): δ = 3.84
(s, 4 H, CH2), 4.50 (s, 3H, CH3O), 6.61 (dd, J =8.03 Hz, 1H, Harom), 6.65
(d, J =8.53 Hz, 2H, Harom), 7.24–7.27 (m, 1H, Harom), 7.29–7.34 (m, 2H,
Harom), 7.38ꢀ 7,41 (m, 1H, Harom), 7.92 (dd, J = 7.99, 1.50 Hz, 1H,
2.2.1. General procedure for the preparation of phthalimides 1a-c
To a stirred solution of phthalic anhydride (0.25 g, 1.7 mmol) and the
appropriate methyl aminobenzoate (0.26 g, 1.7 mmol) in toluene 15 mL,
triethylamine (0.17 g, 1.7 mmol) was added. The reaction mixture was
heated at reflux with azeotropic removal of water assisted by Dean-Stark
trap. Upon completion of the reaction (12 h), it was allowed to room
temperature and the formation of a solid was observed, the solvent was
decanted, and the crude product was purified by column chromatog-
raphy using a mixture of CH2Cl2/AcOEt (85:15) as eluent. The 1H and
13C NMR spectroscopic data for the compounds 1a-c are identical with
those described in the literature [39].
H
arom). 13C NMR (100 MHz, CDCl3): δ = 44.8, 51.7, 110.5, 111.9, 115.2,
127.9, 128.6, 131.8, 134.8, 136.3, 150.8, 169.2. HRMS (FAB+): calcu-
lated for C16H15NO2 [M+H]+, m/z 253.1103; found for [M+H]+, m/z
253.1024.
2.2.3.2. 2-(3-Methoxycarbonylphenyl)isoindoline
5b. White
solid
(0.20 g, 85 %); m.p. 123–123.5 ◦C. 1H NMR (200 MHz, CDCl3): δ = 3.90
(s, 3H, CH3O), 4.58 (s, 4H, CH2), 6.72–6.79 (m, 1H, Harom), 7.20–7.42
(m, 7H, Harom). 13C NMR (100 MHz, CDCl3): δ = 52.0, 53.7, 112.3,
115.8, 117.2, 122.6, 127.2, 129.2, 130.9, 137.5, 146.9, 167.6. HRMS
(FAB+): calculated for C16H15NO2 [M+H]+, m/z 253.1103; found for
[M+H]+, m/z 253.1176.
2.2.2. General procedure for the preparation of isoindolin-1-ones 3a-c
A solution of 2-formylbenzoic acid (0.25 g, 1.7 mmol) and phenyl-
boronic acid (20 mg, 0.16 mmol) in methanol (5 mL) was cooled at 0 ◦C
and the appropriate methyl aminobenzoate (0.25 g, 1.7 mmol) was
added. The reaction mixture was stirred at 0 ◦C for 15 min, followed by
the addition of sodium borohydride (0.75 g, 1.9 mmol). After the gas
evolution was ceased, the reaction mixture was heated at 50 ◦C for 2.0 h.
After this time, the solvent was evaporated under reduced pressure and
the residue was treated with aqueous solution of NH4Cl (2 mL) and
extracted with AcOEt (3 × 10 mL). The combined extracts were washed
with brine (5 mL), dried over anhydrous Na2SO4, filtered and evapo-
rated. The residue was purified by column chromatography to obtain the
corresponding isoindolin-1-ones 3a-c. The 1H and 13C NMR spectro-
scopic data for the compound 3c are identical with those described in
the literature [40].
2.2.3.3. 2-(4-Methoxycarbonylphenyl)isoindoline
5c. White
solid
(0.17 g, 70 %); m.p. 217ꢀ 219 ◦C. 1H NMR (200 MHz, CDCl3): δ = 3.85
(s, 3H, CH3O), 4.67 (s, 4H, CH2), 6.61 (system AA’BB’, J =8.97 Hz, 2H,
H
H
arom), 7.29–7.35 (m, 4H, Harom), 7.97 (system AA’BB’, J =8.99 Hz, 2H,
arom). 13C NMR (100 MHz, CDCl3): δ = 51.6, 53.8, 110.9, 117.5, 122.8,
127.6, 131.7, 137.2, 150.3, 167.6. HRMS (FAB+): calculated for
C
16H11NO3 [M+H]+, m/z 253.0739; found for [M+H]+, m/z 253.0840.
2.2.4. General procedure for the preparation of 2a-c, 4a-c and 6a-c
A solution of phthalimides 1a-c (0.25 g, 0.88 mmol) and isoindolin-
1-ones 3a-c (0.25 g, 0.93 mmol) in a mixture of THF/MeOH/H2O (3:2:2)
(5 mL), LiOH (0.89 g, 3.71 mmol) was added. The reaction mixture was
heated at 50 ◦C for 1.0 h. After this time, the solvent was evaporated and
the resulting residue was treated with 1.0 N HCl (2 mL). The precipitated
was filtered, dried and recrystallized from CH2Cl2/MeOH, obtaining the
desired compounds 2a-c and 4a-c. The saponification of the isoindolines
5a-c (0.25 g, 0.98 mmol) was carried out using NaOH (0.16 g,
3.92 mmol) under similar procedure, to give the isoindolines derivatives
6a-c. The 1H and 13C NMR spectroscopic data for the compounds 2a, c
[41], 4c [42] are identical with those described in the literature.
2.2.2.1. 2-(2-Methoxycarbonylphenyl)isoindolin-1-one 3a. The crude
was purified by column chromatography using a mixture of CH2Cl2/
AcOEt (95:5) as eluent, obtaining (0.43 g, 95 %) of 3a as white crystals,
m.p. 151ꢀ 152 ◦C. 1H NMR (400 MHz, CDCl3): δ = 3.86 (s, 3H, CH3O),
4.93 (s, 2H, CH2), 6.54 (dd, J = 8.54, 1.03 Hz, 1H, Harom), 6.59 (ddd,
J = 8.09, 7.08, 1.07 Hz, 1H, Harom), 7.26 (ddd, J = 8.68, 7.03, 1.72 Hz,
1H, Harom), 7.36 (ddd, J = 7.80, 7.01, 1.64 Hz, 1H, Harom), 7.48–7.52 (m,
1H, Harom), 7.54 (dd, J = 7.79, 1.04 Hz, 1H, Harom), 7.93 (dd, J = 8.04,
1.69 Hz, 1H, Harom), 8.15 (dd, J = 7.77, 1.39 Hz, 1H, Harom). 13C NMR
(100 MHz, CDCl3): δ = 45.8, 51.7, 110.5, 111.9, 115.1, 127.2, 127.6,
128.2, 131.8, 132.3, 133.6, 134.7, 142.3, 151.1, 169.3, 172.8. HRMS
(FAB+): calculated for C16H13NO3 [M+H]+, m/z 267.0895; found for
[M+H]+, m/z 268.0755.
2.2.4.1. 2-(3-Carboxyphenyl)phthalimide 2b. White crystals (0.19 g, 80
%); m.p. 289ꢀ 290 ◦C. 1H NMR (400 MHz, CDCl3): δ = 7.46 (dd, J
=7.92 Hz, 1H, Harom), 7.56–7.61 (m, 2H, Harom), 7.68 (ddd, J = 7.48,
1.36 Hz, 1H, Harom), 7.80 (d, J =7.77 Hz, 1H, Harom), 7.91 (ddd,
2

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
Scheme 1. Synthesis of phthalimides 2a-c.
J = 8.14, 1.11 Hz, 1H, Harom), 8.04 (dd, J = 7.80, 1.25 Hz, 1H, Harom),
8.31ꢀ 8-32 (m, 1H, Harom). 13C NMR (100 MHz, CDCl3): δ = 113.9,
117.1, 117.6, 119.9, 121.0, 121.8, 122.6, 124.5, 131.5, 160.6, 162.4.
HRMS (FAB+): calculated for C15H9NO4 [M+H]+, m/z 267.0532; found
for [M+H]+, m/z 267.0498.
2.2.4.6. 2-(4-Carboxyphenyl)isoindoline 6c. White solid (0.15 g, 62 %);
m.p. 204ꢀ 206 ◦C. 1H NMR (400 MHz, CDCl3): δ = 4.69 (s, 4H, CH2),
6.70 (system A’ABB’, J =8.54 Hz, 2H, Harom), 7.32–7.34 (m, 2H, Harom),
7.40–7.43 (m, 2H, Harom), 7.84 (system A’ABB’, J =8.57 Hz, 2H, Harom).
13C NMR (100 MHz, CDCl3): δ = 53.7, 111.4, 118.0, 123.2, 127.8, 131.7,
137.5, 150.4, 168.1. HRMS (FAB+): calculated for C15H13NO2 [M+H]+,
m/z 239.0946; found for [M+H]+, m/z 239.0963.
2.2.4.2. 2-(2-Carboxyphenyl)isoindolin-1-one 4a. White solid (0.22 g,
95 %); m.p. 176ꢀ 177 ◦C. 1H NMR (400 MHz, CDCl3): δ = 4.85 (s, 2H,
CH2), 6.50–6.64 (m, 2H, CH2), 7.20–7.37 (m, 2H, Harom), 7.39–7.54 (m,
2H, Harom), 7.95 (dd, J = 8.24, 1.72 Hz, 1H, Harom), 8.02 (ddd, J = 7.37,
1.32, 0.63 Hz, 1H, Harom). 13C NMR (100 MHz, CDCl3): δ = 45.4, 110.2,
111.6, 112.2, 114.7, 126.9, 128.2, 129.2, 131.4, 132.4, 134.6, 141.1,
151.2, 169.9, 171.2. HRMS (FAB+): calculated for C15H11NO3 [M+H]+,
m/z 253.0739; found for [M+H]+, m/z 253.0730.
2.3. Quantum yield calculation
The quantum yields (ɸF) were determined following the Eq. 1.
ɸF = ɸR × Int AR n2/ Int A n2
(1)
R
Where ɸF is the sample quantum yield, Int is the area under the emission
peak (at a given wavelength scale), A is the absorbance at the excitation
wavelength, and n is the refractive index of the sample. The subscript R
denotes the respective values of the reference substance. The reference
substance was anthracene.
2.2.4.3. 2-(3-Carboxyphenyl)isoindolin-1-one 4b. White crystals (0.20 g,
86 %); m.p. 235ꢀ 270 ◦C. 1H NMR (400 MHz, CDCl3): δ = 5.11 (s, 2H,
CH2), 7.52–7.63 (m, 2H, CH2), 7.67–7.84 (m, 4H, Harom), 8.21 (d, J
=8.0 Hz, 1H, Harom), 8.54 (s, 1H, Harom). RMN 13C (100 MHz, CDCl3):
δ = 50.8, 120.2, 123.6, 123.7, 123.8, 125.1, 128.6, 129.7, 131.9, 132.6,
132.9, 140.1, 141.5, 167.3, 167.5. HRMS (FAB+): calculated for
Fluorescence quantum yield was calculated by preparing ten
anthracene samples in ethanol used as standard. The nine compounds
were dissolved in methanol, found that the samples present absorbances
between 0.01ꢀ 0.1. The fluorescence spectra of the same samples were
obtained with a slit 2.5:2.5 and the integral of the fluorescence intensity
(that is, the area of the fluorescence spectrum) was calculated. Finally,
the magnitude of the integrated fluorescence intensity was plotted
against the obtained absorbance. The result is a straight line with
gradient proportional to the quantum yield. The absolute values were
calculated using standard samples (anthracene) that have fixed with
fluorescence quantum yield reported.
C
15H11NO3 [M+H]+, m/z 253.0739; found for [M+H]+, m/z 253.0744.
2.2.4.4. 2-(2-Carboxyphenyl)isoindoline 6a. Colorless oil (0.17 g, 70 %).
1H NMR (400 MHz, CDCl3): δ = 5.48 (s, 4H, CH2), 6.52–6.67 (m, 1H,
H
arom), 7.19–7.30 (m, 2H, Harom), 7.34–7.43 (m, 2H, Harom), 7.48–7.55
(m, 2H, Harom), 7.87 (d, J =8.03 Hz, 1H, Harom). 13C NMR (100 MHz,
CDCl3): δ = 63.8, 110.6, 116.4, 116.8, 128.8, 129.9, 131.4, 134.4,
135.0, 150.8, 167.8. HRMS (FAB+): calculated for C15H13NO2 [M+H]+,
m/z 239.0946; found for [M+H]+, m/z 253.0932.
2.4. Free energy calculations
2.2.4.5. 2-(3-Carboxyphenyl)isoindoline 6b. Yellow solid (0.18 g, 78 %);
m.p. 176ꢀ 178 ◦C. 1H NMR (400 MHz, CDCl3): δ = 4.64 (s, 4H, CH2),
6.87 (d, J =7.55 Hz, 1H, CH2), 7.28–7.33 (m, 4H, Harom), 7.36 (d, J
=7.83 Hz, 1H, Harom), 7.39–7.42 (m, 2H, Harom). 13C NMR (100 MHz,
CDCl3): δ = 53.8, 112.5, 116.4, 117.3, 123.2, 127.7, 129.8, 132.1,
137.9, 147.4, 168.4. HRMS (FAB+): calculated for C15H13NO2 [M+H]+,
m/z 239.0946; found for [M+H]+, m/z 253.0902.
All the calculations described were performed with the GAUSSIAN
09 quantum chemistry package [43]. The ground and excited states
geometries of the nine compounds have been optimized at
PCM/B3LYP/TDDFT/6ꢀ 311++G(d,p) level of theory. We have used the
popular B3LYP hybrid functional in all cases, which mixes the Lee, Yang
and Parr functional for the correlation part and Becke’s three-parameter
3

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
Scheme 2. Synthesis of N-substituted isoindolin-1-ones 4a-c.
Scheme 3. Synthesis of isoindolines 6a-c.
functional for the exchange [44,45]. The geometry optimization was
of THF:MeOH:H2O at 80 ◦C, followed by the treatment with 1.0 N HCl,
afforded the phthalimides 2a-c in 62–80% yield (Scheme 1).
performed in methanol using 6ꢀ 311++G(d,p) basis set. The influence of
solvent
(methanol)
was
simulated
within
the
On the other hand, for the synthesis of the target N-substituted
isoindolin-1-ones 4a-c, the 2-formylbenzoic acid was reacted with the
appropriate methyl aminobenzoates in the presence of catalytic amounts
of phenylboronic acid as catalyst [20,49] in methanol at 0 ◦C, followed
by the addition of NaBH4 and subsequent heating at 80 ◦C for 3.0 h, to
obtain the isoindolin-1-ones derivatives 3a-c in 85–95% yield. Saponi-
fication of methyl ester in 3a-c with LiOH in a mixture of THF:MeOH:
H2O at 80 ◦C and subsequent treatment with 1.0 N HCl, afforded the
N-substituted isoindolin-1-ones 4a-c in 86–95% yield (Scheme 2).
Finally, for the synthesis of isoindolines 6a-c, initially we carried out
integral-equation-formalism version of the polarizable continuum
model (IEFPCM) developed by Tomasi et al. [46,47]. The wavelengths
corresponding to vertical transitions from S0 to a singlet excited state S1,
and oscillator strengths of each S0-S1 were calculated.
3. Results and discussion
3.1. Design and preparation
For the synthesis of the target phthalimides 2a-c, initially we carried
out the condensation of phthalic anhydride with the corresponding
methyl aminobenzoates in the presence of Et3N in toluene at reflux
under azeotropic removal of water [48], obtaining the phthalimides
1a-c in excellent yields, which by saponification with LiOH in a mixture
the reaction of α,αꞌ-dibromo-o-xylene with the corresponding methyl
aminobenzoates and K2CO3 in acetonitrile at 50 ◦C for 3.0 h, to obtain
the isoindolines 5a-c in 69–85% yield, which by saponification with
NaOH in a mixture of THF:MeOH:H2O at 80 ◦C and subsequent treat-
ment with 1.0 N HCl, produced the N-substituted isoindolines 6a-c in
4

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
Fig. 1. Absorbance spectra for: (a) phthalimides 2, (b) isoindolin-1-ones 4 and (c) isoindolines 6 in MeOH at 2.5 × 10ꢀ 5 M.
62–78% yield (Scheme 3).
3.2. Spectral measurements
With the compounds 2a-c, 4a-c and 6a-c in hand, we carried out the
studies of electronic absorption and fluorescence emission, which were
performed in methanol HPLC grade at a concentration of 2.5 × 10ꢀ 5 M.
These compounds exhibit strong absorption band between 275–380 nm,
corresponding to
π
-
π* and
η
-
π
* transitions of the corresponding singlet-
singlet transitions (Fig. 1). The
η-π
* transitions are given by the delo-
calization of N electron pair to the carboxylic acid in the DA system.
Also, it can be observed that the maximum absorbance wavelength is
affected by the position of the carboxylic acid in the aromatic ring. The
pattern ortho > meta > para is followed by phthalimides 2 and iso-
indolines 6, but a sequence ortho > para > meta is observed in the
isoindolin-1-ones 4. As it is well known, the band position is related with
the HOMO-LUMO energy involved in the transition.
Fig. 2. Excitation (solid line) and absorbance (dashed line) spectra for phtha-
limide 2a, isoindolin-1-one 4a and isoindoline 6a in methanol at 2.5 × 10ꢀ 5 M.
The excitation spectra were acquired and compared with the
absorbance spectra, where the transitions of the basal state are the same
as in the excited state (Fig. 2). Highlighting the bands of minimum en-
ergy around 275–350 nm corresponding to the η-π* transitions. Bands at
250 nm were also observed, corresponding to the absorbance spectrum,
which include the * transitions.
π-π
5

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
Fig. 3. Fluorescence spectra for (a) phthalimides 2, (b) isoindolin-1-ones 4 and (c) isoindolines 6 in methanol at 1 × 10ꢀ 4 M.
Fig. 4. Solutions of compounds 2, 4 and 6 irradiated under UV light at 365 nm.
Table 1
Photophysical data for the phthalimides 2, isoindolin-1-ones 4 and isoindolines 6.
Photophysical data
Compound
λabs (nm)
λem (nm)
ΦF
Optical gap (eV)
HOMO-LUMO s0-s1 (eV)
Optical gap (eV)
LUMO-HOMO s1-s0 (eV)
SS
2a
2b
2c
4a
4b
4c
6a
6b
6c
298
292
280
351
281
303
344
341
310
410
431
346
422
418
351
414
459
356
0.19 (±0.04)
0.05 (±0.03)
0.073 (±0.04)
0.56 (±0.02)
0.45 (±0.02)
0.19 (±0.03)
0.58 (±0.01)
0.508 (±0.03)
0.34 (±0.04)
4.16
4.24
4.42
3.53
4.41
4.09
3.60
3.63
3.99
4.74
4.58
4.53
5.41
4.71
4.52
5.05
4.01
4.36
3.02
2.87
3.58
2.93
2.96
3.53
2.99
2.70
3.48
3.04
3.00
3.04
3.42
4.02
3.69
3.48
3.41
4.10
112
139
66
71
137
48
70
118
46
6

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
The fluorescence emission for the compounds 2, 4 and 6 in methanol
at room temperature (Fig. 3) are similar to those observed in previously
reported analogues [28,50,51]. The emission wavelengths obtained are
between 350 and 450 nm. With an intensity trend of ortho > para > meta
in case of phthalimides
2 and ortho > meta > para for the
isoindolin-2-ones 4 and isoindolines 6. For the compounds with sub-
stituents in para position, the emission bands were displaced to areas of
greater energy. Whereas for the phthalimides 2 and isoindolines 6 with
the carboxylic acid in meta position- the emission bands are at lower
energy, and for isoindolin-1-ones 4 with the carboxylic acid in ortho
position is most displaced to red. The isoindolin-1-one 4b, although its
emission is not so red with a band at 418 nm, has the highest Stokes shift
(137 nm). This trend is maintained regarding the displacements for the
rest of the target families meta > ortho > para.
Solutions of compounds 2, 4 and 6 irradiated under UV light at
365 nm are shown in Fig. 4.
Fig. 5. Excitation and fluorescence spectra for phthalimides 2a, isoindolin-1-
one 4a and isoindoline 6a in methanol at 1 × 10ꢀ 4 M.
The obtained quantum yields give important information about the
fluorescent properties of these compounds, such as their efficiency as
fluorophores, generating information on their possible applications.
Noteworthy, the number of carbonyl groups has an influence over the
quantum yields (Table 1). The isoindolines 6 present the higher fluo-
rescence quantum yields followed by isoindolin-1-ones 4, and finally the
phthalimides 2 (Fig. 5). The quantum yields of phthalimides 2 follows
the ortho > para > meta order and for the isoindolin-1-ones 4 and iso-
indolines 6 is ortho > meta > para, which are in agreement with the
fluorescence spectra presented in Fig. 3. These results may suggest a
predominant electronic effect in compounds 2 and a combination of
electronic and conformational in compounds 4 and 6.
Table 2
Different conformation of the molecule 4a.
θ
Hartree
Mol 4
a
ꢀ 20.3466
ꢀ 858.9214
The photophysical parameters of 2, 4 and 6 in methanol are listed in
Table 1.
b
19.6534
79.6534
ꢀ 858.9152
ꢀ 858.9248
Some studies carried out on the indoprofen and ketoprofen show that
a transition of a singlet to the triplet state is possible due to the presence
of the carbonyl groups in the compounds, where are attributed to
transitions η-π* [52]. The lower quantum yields in compounds 2 and 4
may be a consequence of a similar phenomenon in these compounds.
c
3.3. Molecular geometry optimization
To give us insight into the effect of base stacking on fluorescent
properties of phthalimides 2, isoindolin-1-ones 4 and isoindolines 6, we
used time-dependent density functional theory (TD-DFT) calculations to
address the excited-state energies of all compounds. Several groups have
used this method to address the spectroscopic results in Schiff bases
within organic solvents [53,54].
d
e
129.6532
139.6535
ꢀ 858.9212
ꢀ 858.9268
It was possible to use connections that are not symmetrical and the
mobility of the number of atoms during the optimization. The position of
benzoic acid was optimized with respect to isoindolin-1-one rotating at
certain angle the simple bond between the N and C atoms (Table 2). As a
result of this calculation, the XYZ coordinates of the optimized lower
energy structure were taken to improve the optimization of a ground
state (Table 2). Once the ground state optimization of the basic state has
been completed, the lower energy molecule “g” was chosen, with a
dihedral angle of -90.3464 and an energy of -858.9286. The found
conformer must be brought to an excited state (S1) of the calculation,
thus completing the S0-S1 transition, the state S1 is again optimized to
obtain the lower energy geometry, which simulates the non-luminescent
relaxation of the molecule in the excited state. In this calculation the
HOMO and LUMO orbitals responsible for the emission were obtained.
As can be seen in Fig. 6, the conformations of S0 and S1 singlet states
which simulates the energy absorption process promoting to an excited
state in the three ortho analogues (2a, 4a and 6a) keep the same sym-
metry with the same dihedral angle between the atoms 7, 8, 9 y 12. As
already observed in the experimental analysis, the presence of carbonyls
in the compound have an important role in the behavior of absorption
and emission, and according to the calculations as the number or car-
bonyls decreases, so it does the conformational restriction, then we can
f
179.6532
ꢀ 858.9181
ꢀ 858.9286
g
ꢀ 90.3464
7

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
Fig. 6. Phthalimide 2a, isoindolin-1-one 4a and isoindoline 6a dihedral angles.
see a change of the dihedral angle. In addition, as we can observe that
the decrease of this restriction allows an interaction between the ni-
trogen atom (N) of the ring with the carboxyl group, then the confor-
mation is now mediated by a hydrogen bond in the basal state in 4a and
6a structures.
for each of constitutional isomers of these compounds, so we can say that
absorption is affected directly by the position of the carboxylic acid in
the aromatic ring, being favored for the compounds substituted in ortho
position.
In order to visualize the main contributions for the S0-S1 and S0-S1*
differences, the relative energies of the HOMO-LUMO orbitals are pre-
sented (Fig. 8). Most of the main contributions belong to the single
HOMO-LUMO transition (the percentage for all the compounds may be
consulted in the Supporting Information). In Fig. 7 it can be observed
that the energy necessary for the transition in phthalimides 2 are similar,
while in the isoindolin-1-one 4 and isoindolines 6 have a small differ-
ence in the transition energies. When the transitions HOMO to LUMO
and LUMO to HOMO are compared, it is appreciated that the emission
process energy is lower than absorption process, which indicates that
calculations fit to the Stokes shift shown by experiments. Moreover, the
tendency of this Stokes shift for every group is well predicted by the
theory, e.g. 6c is blue shifted, 6a remains in the middle and 6b is red
shifted. For all the cases the S1 explore the excited state surface towards
planarity in the S1*.
The following process according to the Jablonski diagram, is the
molecule relaxation through non-radiative processes such as vibrational
relaxation, intramolecular shocks or internal conversion. This process
was simulated in the theoretical calculations and optimization of the
molecule on the excited state (S1) looking for the vibrational state of
lower energy, this theoretical state is known as S1*. The molecule
changed its dihedral angle, showing a conformation with a tendency to
planarity and lower energy, at this point the molecule may emit energy
as luminescence and return to the basal state.
The optimization process of states S0-S1 y S1-S1* was carried out in
the same way for all compounds, obtaining the molecular energies of
these in different states of the luminescent process, as well as the angles
and the HOMO and LUMO involved. As it can be seen in Fig. 7, phtha-
limides 2 need similar energies to change from HOMO to LUMO, while
for isoindolin-1-ones 4 and isoindolines 6 there is a marked difference
As can be seen in Fig. 8, the electron density on the behavior of the
8

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
Fig. 7. HOMO-LUMO energy gap transitions of TD and TDopt phthalimides 2, isoindolin-1-ones 4 and isoindolines 6 estimated by TDDFT calculations.
Fig. 8. Energy diagram of the orbitals HOMO-LUMO of TD and TDopt phthalimides 2, isoindolin-1-ones 4 and isoindolines 6 estimated by TDDFT calculations.
orbitals is equal to S0-S1 and S0-S1*, where the electron density is the
same for Fig. 8a and b. It is remarkable that the presence of O in the
molecule induces a stability, so it becomes less energy as can be seen in
phthalimide 2c. For isoindolin-1-one 4c it is seen how the attraction of
carboxylic acid begins to participate, so that the electron density is
distributed throughout the molecule in the ground state, but at the
moment of excitation this distribution moves towards the acid, Finally,
in the isoindoline 6c most of the electron density is concentrated
9

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
there was still data that did not match with the experimental results, so,
by doing a search in the literature, it was found that these compounds
can make banned transitions, with which they would be moving from a
single state to the triplet state [55,56]. For that reason, we calculated the
triplet state for our nine compounds. Due to the carbonyl groups present
and the known photochemistry primary processes that these groups can
carry out, in particular S1-Tn [57] this calculation give us insight into the
energies of the orbitals on the triplet state, where it might be possible a
change of multiplicity. El-Sayed’s rules state the three major reasons a
singlet-triplet intersystem crossing (ISC) may increase its probability
and kinetics. As intersystem crossing is forbidden by rules of conserva-
tion of energy and angular momentum, usually its kinetics is quite
slower than other photophysical processes. However, El-Sayed’s rules
predict an acceleration of the lowest singlet state to the triplet manifold
if the ISC transition involves a change of molecular orbital type,
homoenergetic S1-Tn states, and high density of receiving states.
In the case of phthalimides, some reports [58,59] state that these
compounds present triplet states formation after primary excited state
formation. As presented in Fig. 10, the presence of high density of iso-
energetic triplet orbitals nearby S1 singlet state, may promote a fast
transition. The presence of oxygen atom provides those energy levels
that could please this need. Besides the distribution of the orbitals in the
excited state seems to favor this transition and, in this way, the low
yields presented by this family of compounds could be explained.
Looking in detail the isoindolin-1-one isoenergetic states in Fig. 10
calculations this level of theory predicts the existence of upper triplet
states that enhance ISC. Moreover, the 4c molecule is quite like indo-
profen and there are reports [27,60] where they explain the transition
that this drug produces triplet states upon excitation. We can say that the
quantum yields of these compounds are low because some of them
convert into triplet states. For isoindolines 6a-c, the absence of car-
bonyls favors fluorescence, although these compounds do not have
Fig. 9. Absorbance and Fluorescence spectra of isoindolin-1-one 4c and theo-
retical wavelength.
towards the carboxylic acid, this tendency is observed from the HOMO
state.
As mentioned above, the calculations of linear response theory,
throw data on the spectrum information, for example, the wavelength
where it can be seen some of the bands, and this helps us to check if the
calculations are correct. In Fig. 9, it is observed the absorption and
emission spectrum of the compound 4c, in the lower part of the spec-
trum the signals that throw the calculations on the possible wavelengths
that would appear absorption and emission bands, being evident that
they are very similar.
In summary, the theoretical calculations help to understand the
behavior of the compounds during the luminescence process, however,
Fig. 10. Energy diagram of phthalimide 2c, isoindolin-1-one 4c and isoindoline 6c S0-S1, S0-S1* and S1-T1.
10

M. Solis-Santos et al.
Journal of Photochemistry & Photobiology, A: Chemistry 413 (2021) 113185
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Nine new fluorescent compounds were obtained in excellent yields
under mild, efficient and simple processes. According to the results of
the fluorescence study, the compounds behave like a donor-aceptor
system, identifying the electronic transitions of the fluorescence; it
also shown that the presence of carbonyl group in the compound plays
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phosphorescence. Besides the biological and industrial importance of
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probes. Furthermore, the dual luminescence is a desirable property as
additional spectroscopic features (increased time and bandwidth) and
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Author statement
The work in this manuscript was developed in the Universidad
´
Autonoma del Estado de Morelos (UAEM) and funded by the Consejo
´
Nacional de Ciencia y Tecnología (CONACYT) of Mexico via project
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286614. The authors declare that they have no conflict of interest be-
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Acknowledgements
The authors thank the CONACYT of Mexico, for their financial sup-
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