Ground and excited-state properties of 1,3-benzoselenazole derivatives: A combined theoretical and experimental photophysical investigation

Article abstract of DOI:10.1016/j.molstruc.2020.127817

A series of 1,3-benzoselenazole derivatives were prepared with different substituents by complementary methodologies starting from aldehydes or carboxylic acids, using sodium metabisulfite and tributylphosphine, respectively, according to the substituent

Full text of DOI:10.1016/j.molstruc.2020.127817

Journal of Molecular Structure 1207 (2020) 127817
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Ground and excited-state properties of 1,3-benzoselenazole
derivatives: A combined theoretical and experimental photophysical
investigation
Catia Schwartz Radatz a, Felipe Lange Coelho a, Eduarda Sangiogo Gil b,
Fabiano da Silveira Santos c, Juliana Maria Forain Miolo Schneider d,
Paulo Fernando Bruno Gonçalves b, Fabiano Severo Rodembusch c,
,
*
Paulo Henrique Schneider a
a Instituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), CP 15003, CEP 91501-970, Porto Alegre, Brazil
b
ꢀ
Grupo de Química Teorica, Universidade Federal do Rio Grande do Sul, Instituto de Química, CEP 91501-970, Porto Alegre, RS, Brazil
c
^
Grupo de Pesquisa em Fotoquímica Organica Aplicada, Universidade Federal do Rio Grande do Sul, Instituto de Química, CEP 91501-970, Porto Alegre, RS,
Brazil
d
^
^
Departamento de Farmacociencias, Universidade Federal de Ciencias da Saúde de Porto Alegre (UFCSPA). Rua Sarmento Leite, 245, CEP 90050-170, Porto
Alegre, RS, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 1,3-benzoselenazole derivatives were prepared with different substituents by complementary
methodologies starting from aldehydes or carboxylic acids, using sodium metabisulfite and tribu-
tylphosphine, respectively, according to the substituent pattern. The photophysical behaviour in solution
of the benzoselenazoles was studied using UVeVis absorption and steady-state fluorescence emission
spectroscopies. These compounds present absorption maxima located in the UV region due to spin and
symmetry, which allowed electronic transitions 1pp*. The absorption maxima location, as well as the
evidence of intramolecular charge transfer (ICT), was shown to be tailored by the position and electronic
character of the substituents. Quantum chemical calculations using DFT and TDDFT were performed in
order to investigate the electronic and photophysical features of the molecules. The calculations
confirmed the presence of ICT in the molecules which contained nitro group in their structure and in all
compounds where the allowed electronic transitions 1pp* were taking place. In addition, the compounds
showed significantly different fluorescence emissions depending on the solvent and their chemical
structures, where structured and non-structured spectra in the range of 300e600 nm, with ICT character,
were obtained.
Received 29 November 2019
Received in revised form
27 January 2020
Accepted 28 January 2020
Available online 29 January 2020
Keywords:
1,3-Benzoselenazole
Fluorescence
TD-DFT
Photophysics
UVeVis
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
as medicinal chemistry with the development of anti-oxidants,
microbicides, anti-tumours, and anti-inflammatories [16e20],
Organoselenium chemistry has undergone rapid growth in
recent decades, driven by many factors such as the discovery of
selenium elements in protein composition, its unique chemical
properties, biological activity and other applications as in materials
science [1e13]. Some classes, like selenium-containing heterocy-
cles, are of particular interest, since recent literature [14,15] has
revealed their versatility through broad use in different areas, such
among others, and also in materials science with the construction
of organic thin-film transistors (OTFTs) [21], organic photovoltaic
cells (OPVs) [22,23], organic light-emitting diodes (OLEDs) [24],
and fluorescent dyes [25e28]. In Fig. 1, we present some selenium-
containing heterocycle structures, highlighting their main
application.
In the same way, 1,3-benzoselenazole derivatives feature high
relevance as this moiety is present in many pharmaceutically active
compounds or optoelectronic devices. Like sulfur analogues, sele-
nazole derivatives have been explored in medicinal chemistry for
* Corresponding author.
E-mail address: paulos@iq.ufrgs.br (P.H. Schneider).
PET detection of amyloid plaques (Ab) in Alzheimer’s disease[29],
https://doi.org/10.1016/j.molstruc.2020.127817
0022-2860/© 2020 Elsevier B.V. All rights reserved.

2
C.S. Radatz et al. / Journal of Molecular Structure 1207 (2020) 127817
shift are given in parts per million (d) and are referenced from
tetramethylsilane (TMS) in 1H NMR spectra and from CDCl3 or
(CD3)2CO in 13C NMR spectra. ATR/FTIR spectra were recorded on a
Varian FT-IR-640 spectrometer. Low-resolution mass spectra were
obtained with a Shimadzu GC-MS-QP5050 mass spectrometer
interfaced with a Shimadzu GC-17 A gas chromatograph equipped
with a DB-17 MS capillary column. The reactions under microwave
irradiation were conducted using a CEM Discover mode operating
systems, working at 2.45 GHz, with a power programmable from 1
to 300 W. UVeVis absorption spectra were performed on a Shi-
madzu UV-2450 spectrophotometer. A slit width of 2.0 nm was
applied in the UVeVis measurements. Steady state fluorescence
spectroscopy was measured with a Shimadzu spectrofluorometer
model RF-5301PC, using emission and excitation monochromator
slit widths of 3.0 nm/5.0 nm. In all photophysical experiments, the
absorption maximum was used as the excitation wavelength for
fluorescence measurements. All experiments were performed at
Fig. 1. Chemical structure and respective application of selenium containing
heterocycles.
25 ꢀC in a concentration of 10ꢁ5 mol Lꢁ1
.
as well as having been explored as anti-cancer and anti-oxidant
agents [30,31]. Furthermore, squaraine dyes containing 1,3-
benzoselenazoles were explored as organic semiconductors, and
substituted 1,3-selenazoles reveal consistent photophysical and
electrochemical properties for application in electronic devices
[32]. In spite of restricted synthetic protocols for selenazole core
building in comparison with other chalcogen analogues (thiazole
and oxazole), novel approaches have emerged in recent years for
exploring selenium reactivity. In this context, our group recently
developed two synthetic protocols for this purpose, employing
amino-substituted diaryl diselenides and aldehydes or carboxylic
acid [33,34]. Some alternative protocols are reported, starting from
different selenium species, such as elemental selenium [35e37],
aryl selenocyanates [38] or woolin’s reagent [39], affording the
desired heterocycles in satisfactory yields. When considering this
potential application of selenium-based benzazoles as organic
conductors, semiconductors, and in the optoelectronics field, a
major photophysical investigation is required due to the lack in-
formation in this area for known derivatives. The presence of se-
lenium may affect emission intensity and quantum yield due to the
heavy atom effect, as this property is associated with its particular
reactivity and high polarizability, gives selenium fluorophores the
ability of recognize a range of analytes as they can selectively bind
or react with different oxidizing reagents, metals, ions, and orga-
nophosphurus, among others, giving different response depending
on chemical or electronic changes [40e43].
2.2. General procedure for the synthesis of substituted 2-
phenylbenzoselenazoles
Method A. The appropriate bis(2-aminoaryl) diselenide 1
(0.5 mmol), substituted aryl aldehyde 2 (1.0 mmol) and Na2S2O5
(1.0 mmol) were dissolved in anhydrous DMSO (3 mL). The
resulting reaction mixture was stirred under nitrogen atmosphere
at 120 ꢀC for 48 h. After this time, the mixture was cooled to room
temperature, diluted with saturated aq. NH4Cl (20 mL) and washed
with ethyl acetate (3 ꢂ 20 mL). The organic phase was separated,
dried over MgSO4, and concentrated under vacuum. The residue
was purified by flash chromatography on silica gel using hex-
ane:AcOEt (95:5) as eluent to afford the substituted 1,3-
benzoselenazoles 3a-i.
Method B: In a 10 mL glass vial equipped with a small magnetic
stirring bar under inert atmosphere, the bis(2-aminophenyl) dis-
elenide 1 (0.25 mmol) was dissolved in anhydrous toluene (1.5 mL)
and n-tributylphosphine (0.75 mmol) was added. The mixture was
allowed to stir at room temperature for about 5 min followed by
addition of the respective carboxylic acid 4 (0.25 mmol). The
mixture was then irradiated in a microwave reactor (CEM Explorer)
for 2 h at 100 ꢀC (temperature was measured with an IR sensor on
the outer surface of the reaction vial), using an irradiation power of
100 W (the ramp temperature rate was 3 min). After the reaction
was complete, the reaction mixture was diluted with saturated
solution of Na2CO3 (20 mL) and washed with dichloromethane
(3 ꢂ 20 mL). The organic phase was separated, dried over MgSO4,
and concentrated under vacuum. The residue was purified by flash
chromatography on silica gel using hexane:ethyl acetate (95:5) as
eluent, yielding the final products 3a-e and 3g.
In this context, we dedicated our efforts to the understanding of
the photophysics of 1,3-benzoselenazoles in the ground and excited
states by comparing the experimental techniques of UVeVis ab-
sorption and steady-state fluorescence emission and theoretical
(TD-DFT) properties, focused on the substituents of the hetero-
aromatic core.
2-phenylbenzo[d][1,3]selenazole (3a). Yellow solid. M. p:
114e116 ꢀC. 1H NMR (300 MHz, CDCl3)
d
8.03 (d, 1H, J 8.1 Hz);
7.91e7.94 (m, 2H); 7.84 (d, 1H, J 7.9 Hz); 7.40e7.42 (m, 4H); 7.22 (t,
1H, J 7.5 Hz). 13C NMR (75.5 MHz, CDCl3)
173.2, 165.4, 156.4, 138.9,
/cmꢁ1 1587,
2. Experimental
d
2.1. Materials and methods
136.8, 131.7, 129.7, 128.6, 127.0, 125.9, 125.5. IR (KBr) n
1552, 1510, 1479, 1430, 1299, 1216, 939, 763. MS m/z: 259 m/z (Mþ).
Anal. calc. for C13H9NSe (258.99): C 60.48, H 3.51, N 5.43; found: C
60.09, H 3.07, N 5.35.
The tributylphosphine was purchased from Aldrich and sodium
metabisulfite (Na2S2O5) was purchased from Reagen® and both
used without further purifications. Toluene and dimethylsulfoxide
(DMSO) were obtained from Sigma Aldrich and dry through clas-
sical method [44]. The Sodium Metabisulfite (Na2S2O5) was pur-
chased from Reagen® and used without further purifications. The
reactions were monitored by thin-layer chromatography (TLC) and
column chromatography was performed using Merck Silica Gel
(230e400 mesh). Nuclear magnetic resonance spectra were ob-
tained on Varian Inova 300 and Bruker 400 spectrometer. Chemical
2-p-tolylbenzo[d][1,3]selenazole (3b). Yellow solid. M. p:
78e80 ꢀC. 1H NMR (300 MHz, CDCl3)
d
8.13 (d, 1H, J 8.1 Hz);
7.93e7.97 (m, 3H); 7.51 (t, 1H, J 7.3 Hz); 7.29e7.35 (m, 3H); 2.44 (s,
3H). 13C NMR (75.5 MHz, CDCl3)
173.2, 156.5, 142.2, 138.8, 134.2,
/cmꢁ1 1604,
d
130.4, 126.9, 125.7, 125.4, 125.3, 128.6, 22.2. IR (KBr)
n
1490, 1432, 1305, 1220, 937, 815, 754. MS m/z: 273 m/z (Mþ). Anal.
calc. for C14H11NSe (273.01): C 61.77, H 4.07, N 5.15; found: C 61.64,
H 4.00, N 5.06.

C.S. Radatz et al. / Journal of Molecular Structure 1207 (2020) 127817
3
2-(4-bromophenyl)benzo[d][1,3]selenazole (3c). White solid.
M. p: 124e126 ꢀC. 1H NMR (300 MHz, CDCl3)
8.01 (d, 1H, J 8.1 Hz);
Functional Theory) methodologies in order to evaluate the elec-
tronic and photophysical proprieties of the molecules 3a-i. The
geometry optimizations were performed by DFT with the CAM-
B3LYP functional and cc-pVDZ basis set. In addition, frequency
calculations of the optimized structures were performed at the
same level to confirm if the optimized structures are stationary
points. TDDFT was performed in order to calculate the theoretical
vertical transitions for the first 10 excited states with CAM-B3LYP/
jun-cc-pVDZ level of theory. We chose to use jun-based base
functions because it presents diffuse functions, which are funda-
mental for a suitable description of the excited states [45]. The
CAM-B3LYP functional was used in all the calculations since it
presents the attributes of the B3LYP functional and long-range
correction. Long-range corrections are crucial for a good descrip-
tion of excited states, once they are construct to describe better
asymptotic regions of molecular systems, wherein the electron
densities decay exponentially [46]. The solvent effect was consid-
ered in all calculations with PCM (Polarizable Continuum Model)
and the solvents considered were acetonitrile, dichloromethane
and 1,4-dioxane [47]. All the calculations described above were
carried out with the Gaussian 16 package [48]. In order to verify if
the molecules undergo charge transfer, the charge transfer
d
7.84 (d, 1H, J 8.0 Hz); 7.78 (d, 2H, J 8.2 Hz); 7.52 (d, 2H, J 8.3 Hz); 7.41
(t, 1H, J 7.2 Hz); 7.24 (t, 1H, J 7.3 Hz). 13C NMR (75.5 MHz, CDCl3)
d
171.6, 156.3, 139.0, 135.7, 132.9, 130.0, 127.2, 126.2, 126.1, 125.6,
125.5. IR (KBr) n
/cmꢁ1 1510, 1475, 1432, 1392, 1303, 1216, 1064, 937,
825, 752. MS m/z: 337 m/z (Mþ). Anal. calc. for C13H8BrNSe
(336.90): C 46.32, H 2.39, N 4.16; found: C 46.68, H 2.30, N 5.65.
4-(benzo[d][1,3]selenazol-2-yl)phenol (3d). Dark orange solid.
M. p: 212e213 ꢀC. 1H NMR (400 MHz, (CD3)2CO)
d 9.04 (s, 1H); 7.93
(d, 1H, J 8.0 Hz); 7.86 (d, 1H, J 8.0 Hz); 7.81 (d, 2H, J 8.6 Hz); 7.35 (t,
1H, J 7.3 Hz); 7.17 (t, 1H, J 7.2 Hz); 6.86 (d, 2H, J 8.6 Hz). 13C NMR
(100 MHz, (CD3)2CO)
125.7, 125.5, 124.7, 116.6. IR (KBr)
1292, 1218, 1172, 833, 756. MS m/z: 275 m/z (Mþ). Anal. calc. for
13H9NOSe (274.98): C 56.95, H 3.31, N 5.11; found: C 56.60, H 2.80,
N 5.05.
2-(4-methoxyphenyl)benzo[d][1,3]selenazole (3e). Orange
solid. M. p: 122e124 ꢀC. 1H NMR (300 MHz, CDCl3)
7.97 (d, 1H, J
d
172.4, 161.1, 156.7, 138.4, 130.2, 128.6, 126.9,
n
/cmꢁ1 3434, 1604, 1484, 1432,
C
d
8.1 Hz); 7.81e7.89 (m, 3H); 7.38 (t, 1H, J 7.3 Hz); 7.19 (t, 1H, J 7.5 Hz);
6.90 (d, 2H, J 8.5 Hz); 3.80 (s, 3H). 13C NMR (75.5 MHz, CDCl3)
d
172.7, 162.6, 156.6, 138.7, 130.2, 129.7, 126.9, 125.4, 125.0, 115.0,
56.2. IR (KBr)
n
/cmꢁ1 1600, 1496, 1434, 1259, 1168, 1024, 831, 765.
descriptor Dr index was carried out with the Multiwfn program
[49,50]. The molecular orbitals were rendered with the Avogadro
software [51].
MS m/z: 289 m/z (Mþ). Anal. calc. for C14H11NOSe (289.00): C 58.34,
H 3.85, N 4.86; found: C 58.12, H 3.48, N 4.67.
2-(4-nitrophenyl)benzo[d][1,3]selenazole (3f). Yellow solid.
M. p: 147e148 ꢀC. 1H NMR (300 MHz, CDCl3)
d
8.27 (d, 2H, J 8.7 Hz);
8.10 (d, 3H, J 8.7 Hz); 7.92 (d, 1H, J 8.0); 7.47 (t, 1H, J 7.5 Hz); 7.32 (t,
1H, J 7.6 Hz). 13C NMR (75.5 MHz, CDCl3)
169.9, 156.2, 149.6, 142.1,
139.6, 129.3, 127.6, 126.9, 126.2, 125.7, 125.0. IR (KBr)
/cmꢁ1 1592,
3. Results and discussion
d
3.1. Synthesis and spectroscopic characterization
n
1521, 1342, 1211, 1141, 943, 854, 769. MS m/z: 304 m/z (Mþ). Anal.
calc. for C13H8N2O2Se (303.98): C 51.50, H 2.66, N 9.24; found: C
51.97; H 2.63; N 9.30.
In recent years, our research group has developed two efficient
protocols for the direct synthesis of 1,3-benzoselenazole in high
yields. Both methodologies involve reactions between bis(2-
aminophenyl) diselenides prepared according to the literature
[52], and different aldehydes or carboxylic acids (Scheme 1) [33,34].
In case of condensation with aldehydes, the reaction is promoted by
the inorganic reducing agent sodium metabisulfite (Na2S2O5) in
DMSO under an inert atmosphere, at 120 ꢀC for 48 h. The versatility
of the protocol tolerates a range of substituents in which aryl al-
dehydes containing withdrawing groups obtain slightly better
yields in comparison with donating ones. A limitation was observed
for aliphatic aldehydes, and therefore, the reaction could be per-
formed under microwave irradiation leading to shorter reaction
with maintenance or increasing of yields.
On the other hand, the second protocol starts from equimolar
amounts of diselenides and carboxylic acids (Scheme 1), requiring
the use of 3 equivalents of tributylphosphine to activate starting
materials. It is conducted in toluene for 2 h at 100 ꢀC. The main
advantage is that this condition well works with aliphatic and
sterically hindered carboxylic acids, along with protected amino-
acids furnishing the desired products in yields ranging from good
to excellent. The influence of electronic effects follows a similar
profile in which withdrawing substituents better activate the
carboxyl group. In the same way as other methodologies, the re-
action is performed under microwave irradiation, and a wide range
of 1,3-benzoselenazoles examples is obtained in similar yields.
2-(3,4-difluorophenyl)benzo[d][1,3]selenazole (3g). Yellow
solid. M. p: 94e97 ꢀC. 1H NMR (300 MHz, CDCl3)
d
8.14 (d, 1H, J
8.0 Hz); 7.91e7.98 (m, 2H); 7.73e7.78 (m, 1H); 7.54 (t, 1H, J 7.8 Hz);
7.30e7.40 (m, 2H). 13C NMR (75.5 MHz, CDCl3)
171,7, 156.1, 154.5
d
(d, JC-F ¼ 12.9 Hz), 153.0 (d, JC-F ¼ 13.2 Hz), 151.1 (d, JC-F ¼ 12.9 Hz),
149.7 (d, JC-F ¼ 12.9 Hz), 145.2, 139.1, 130.4 (d, JC-F ¼ 77.4 Hz), 126.9
(d, JC-F ¼ 76.6 Hz), 125.6 (d, JC-F ¼ 10.9 Hz), 118.6 (d, JC-F ¼ 18.0 Hz),
117.3 (d, JC-F ¼ 19.0 Hz). IR (KBr)
n
/cmꢁ11606,1500,1427,1309,1268,
1106, 977, 757, 634. MS m/z: 295 m/z (Mþ). Anal. calc. for
13H7F2NSe (294.97): C 53.08, H 2.40, N 4.76; found: C 55.07, H 2.48,
N 4.13.
5-chloro-2-phenylbenzo[d][1,3]selenazole (3h). Yellow solid.
M. p: 138e140 ꢀC. 1H NMR (300 MHz, CDCl3)
8.09 (d, 1H, J 2.0 Hz);
7.98e8.01 (m, 2H), 7.83 (d, 1H, J 8.4 Hz), 7.45e7.52 (m, 3H), 7.29 (dd,
1H, J 8.4 Hz, 2.1 Hz). 13C NMR (75.5 MHz, CDCl3)
175.2,164.4, 157.4,
/cmꢁ1
C
d
d
136.9, 136.5, 133.1, 132.1, 129.8, 128.7, 126.2, 125.3. IR (KBr)
n
1625, 1477, 1427, 1255, 1062, 883, 759, 686. MS m/z: 293 m/z (Mþ).
Anal. calc. for C13H8ClNSe (292.95): C 53.36, H 2.76, N 4.79; found: C
53.22, H 2.19, N 4.80.
5-chloro-2-(4-nitrophenyl)benzo[d][1,3]selenazole (3i). Yel-
low solid. M. p: 158e160 ꢀC. 1H NMR (300 MHz, CDCl3)
d
8.26 (d, 2H,
J 8.8 Hz); 8.08 (d, 2H, J 8.3 Hz), 8.06 (s,1H); 7.81 (d,1H, J 8.5 Hz); 7.28
(dd, 1H, J 8.4 Hz, 2.1 Hz). 13C NMR (75.5 MHz, CDCl3)
171.3, 156.7,
d
149.4, 141.3, 137.2, 133.2, 128.9, 126.8, 125.9, 125.4, 124.6. IR (KBr) n/
cmꢁ1 1598, 1515, 1340, 1068, 848, 611. MS m/z: 338 m/z (Mþ). Anal.
calc. for C13H7ClN2O2Se (337.94): C 46.25, H 2.09, N 8.30; found: C
50.34, H 2.76, N 6.70.
3.2. Photophysical characterization
The photophysical study of the 1,3-benzoselenazole derivatives
3a-i was performed in solution using different organic solvents
(10ꢁ5 M) with a wide range of dielectrics. The relevant data from
this investigation is summarized in Table 1. The studied com-
pounds, despite similarity in the benzoselenazole core, present
significant differences in UVeVis shape and location, with maxima
2.3. Theoretical calculations
Quantum chemical calculations were carried out using DFT
(Density Functional Theory) and TDDFT (Time Dependent Density

4
C.S. Radatz et al. / Journal of Molecular Structure 1207 (2020) 127817
Scheme 1. Synthetic routes for substituted 1,3-benzoselenazoles. Reagents and conditions: (i) Bu3P, toluene, MW, 100 ꢀC, 2 h and (ii) Na2S2O5, DMSO, 120 ꢀC, 48 h, N2.
Table 1
Photophysical data of 1,3-benzoselenazole derivatives, where ε is the molar absorptivity (104 Mꢁ1 cmꢁ1), labs and lem are the absorption and emission maxima (nm),
respectively, fe is the calculated oscillator strength, k0e is the calculated radiative rate constant (108 sꢁ1), 0 is the calculated pure radiative lifetime (ns) and DlST is the Stokes
t
shift (cmꢁ1).
Benzoselenazole
Solvent
labs
ε
fe
ke0
t0
lem
DlST
3a
1,4-Dioxane
Dichloromethane
Acetonitrile
1,4-Dioxane
Dichloromethane
Acetonitrile
1,4-Dioxane
Dichloromethane
Acetonitrile
1,4-Dioxane
Dichloromethane
Acetonitrile
1,4-Dioxane
Dichloromethane
Acetonitrile
1,4-Dioxane
Dichloromethane
Acetonitrile
1,4-Dioxane
Dichloromethane
Acetonitrile
1,4-Dioxane
Dichloromethane
Acetonitrile
277
296
304
300
306
300
281
304
302
332
338
332
312
313
311
335
340
335
298
299
297
293
294
291
328
328
322
8.87
1.35
0.95
2.16
1.50
1.87
1.96
0.94
1.54
0.35
0.44
1.55
0.98
1.75
0.92
1.22
1.30
1.14
0.63
2.14
0.64
0.68
6.15
0.55
0.54
0.53
0.41
0.39
0.31
0.24
0.54
0.29
0.45
0.33
0.55
0.86
0.31
0.44
0.67
0.63
0.65
0.54
0.35
0.81
0.23
0.24
0.19
0.24
0.72
0.43
0.89
0.44
0.87
0.89
5.09
3.57
2.60
5.95
3.11
5.01
4.28
5.98
9.43
4.16
3.85
6.07
5.57
6.68
5.56
6.54
7.05
2.02
2.67
2.09
2.73
8.44
4.98
10.5
4.07
8.09
8.60
1.96
2.80
3.85
1.68
3.21
2.00
2.34
1.67
1.06
2.34
2.60
1.65
1.79
1.50
1.80
1.53
1.42
4.95
3.74
4.19
3.66
1.19
2.01
0.96
2.46
1.24
1.16
385
400
350
387
362
363
352/416
352/456
437
407
406
395
367
367
388/432
374
436
411/514
379
352
344
325/485
354
396
400
477
415
10,127
7895
5212
7494
5709
3b
3c
3d
3e
3f
5132
7178/11,549
4703/11,183
10,011
5550
5490
4269
4803
4906
6176/8801
3113
6915
5081/9957
7172
5261
4375
3360/13,511
6116
8761
5488
10,092
6391
3g
3h
3i
1,4-Dioxane
Dichloromethane
Acetonitrile
between 277 and 340 nm. The solvent also seems to play a signif-
icant role on the maxima position, but without any clear tendency.
It could also be observed that both chromophoric and auxochromic
groups presented similar behaviour, since compounds 3d, 3f and 3i
presented redshifted absorption maxima in spite of the other
compounds. In addition, taking the absorption maxima location of
the compound with no substituents 3a into account, it can be
observed that the substituents methyl (3b), bromo (3c), methoxy
(3d), fluoro (3g) and chloro (3h) do not affect the benzoselenazole
electronic structure in the ground state Fig. 2.
ð
fez4:3x10ꢁ9 εdv;
(1)
where it is possible to achieve the experimental oscillator strength
(fe) from the integral under the absorption curve from a plot of the
molar absorptivity coefficient ε vs wavenumber. In this relation-
ship, the oscillator strength is defined as a quantity that measures
the probability of an electronic transition induced by the interac-
tion of electrons and matter with the electromagnetic field of a light
wave. It represents the ratio between the intensity of absorbed
radiation by a molecule compared to a single electron behaving as a
perfect harmonic oscillator bound to a molecule [54]. In addition,
In this study, the relationship between absorption intensity and
fluorescence lifetime of these fluorophores was also obtained by
applying Equation (1) [53]:

C.S. Radatz et al. / Journal of Molecular Structure 1207 (2020) 127817
5
Fig. 2. Normalized UVeVis absorption spectra in solution (10ꢁ5 M) of (a) 3a-i in acetonitrile, (b) 3c and (c) 3f in different organic solvents.
the rate constant (k0e) for fluorescence emission can also be related
to the extinction coefficient using Equation (2) [54]:
300e650 nm range, depending on the substituents (Fig. 3a). These
results indicate that the organic groups bounded to the benzose-
lenazole core play a fundamental role in the excited state radio-
active deactivation in these compounds. The benzoselenazoles 3c,
3g, and 3h presented smaller Stokes shift values, related to locally
excited species. In addition, it could be observed that on these
compounds, there is a low, intense, and redshifted emission band,
which in turn is probably related to an ICT character. On the other
hand, compounds 3a-b and 3d-e presented intermediate values of
the Stokes shift, but are associated with broader emission curves.
Finally, compounds 3f and 3i showed longer Stokes shift values,
probably associated to ICT mechanism. In this way, taking the
maxima location and shape of the emission curves into account, it
can be observed that in some way, all the studied compounds
showed to a lesser or greater extent an ICT character in the excited
state. From Fig. 3b and c, it can be observed that the solvent also
plays a fundamental role on their photophysics, presenting a better
solvation of the excited state (Fig. 3b in dichloromethane), or fa-
voring ICT in compound 3f. In a general way, it could be observed
that the fluorophoric benzoselenazole core, although it is the main
factor responsible for the fluorescence emission, is significantly
affected by the media and the substituent groups attached to it. In
this way, this class of compounds in shown to be promising for
sensing, since their excited-state photophysical properties can be
tailored by the solvent and its chemical structure.
ð
k0e z2:88x10ꢁ9v20 εdv
(2)
In this equation, the pure radiative lifetime
t
0 is defined as 1/k0e.
According to the Strickler-Berg relation, when the electron is
considered to be a perfect harmonic oscillator, fe ¼ 1 and the limit
values to the molar absorptivity coefficient, as well as calculated
radiative rate constant in this case, corresponds to ε ~105 Mꢁ1 cmꢁ1
and k0e ~109 sꢁ1 [54]. The molar absorptivity coefficient ε values
range from 0.35e8.87 ꢂ 104 Mꢁ1 cmꢁ1, as well as the calculated
radiative rate constant (k0e) between 2.02e10.5 ꢂ 108 sꢁ1 for all
compounds which indicates that spin and symmetry allowed for
electronic transitions, which could be related to 1pp* transitions.
Moreover, quite similar radiative lifetimes could be observed for
these compounds, indicating that after the radiation, absorption
the 1,3-benzoselenazole derivatives populate the same excited
state.
The normalized fluorescence emission spectra of the studied
compounds are presented in Fig. 3. The emission curves were ob-
tained by exciting the compounds at the absorption maxima
wavelength. The relevant data from fluorescence emissions are
summarized in Table 1.
It can be observed that the 1,3-benzoselenazole derivatives
present, at the same solvent, a fluorescence emission located in the

6
C.S. Radatz et al. / Journal of Molecular Structure 1207 (2020) 127817
Fig. 3. Fluorescence emission spectra in solution of (a) 3a-i in acetonitrile, (b) 3c and (c) 3f in different organic solvents [ca. 10ꢁ5 M].
3.3. Theoretical calculations
transition are HOMO and LUMO.
In order to investigate whether the molecules 3a-i present ICT,
In this section, the electronic and photophysics features were
investigated by DFT and TD-DFT calculations. Fig. 4 shows the
optimized geometries for the ground and first excited-states of
benzenoselenazoles 3a-i in dichloromethane. Similar geometries
were obtained in acetonitrile and 1,4-dioxane.
D
r index calculations were performed in order to measure charge-
transfer length during electron excitation. The r index values in
dichloromethane for the first transition for molecules 3a-i are
presented in Table 2. The r index is based on the charge centroids
D
D
of the orbitals involved in the excitations, and it can be interpreted
It can be observed that the geometries of the ground state are
very similar of those in the first excited state in all calculated
compounds, indicating that these compounds do not exhibit
considerable nuclear motion after irradiation. In addition, the S1
state presented the highest oscillator strength among the 10
excited states considered for molecules 3a-i, indicating that the
state populated after irradiation is S1. The orbitals which participate
in S0/S1 transitions for these compounds are presented in Fig. 5 in
dichloromethane. Once again, similar results were obtained in
acetonitrile and 1,4-dioxane and are presented in the supporting
information.
Molecules 3f and 3i presented more then four contributions in
the first excitation, hindering their qualitative description. How-
ever, the one with the highest weight is between HOMO and LUMO
for both molecules. Due to these multiple contributions, Natural
Transition Orbitals (NTO) were calculated for molecules 3c and 3h.
NTOs drastically simplify the situation by providing a compact
representation of the transition density matrix [55]. In addition, in
all the structures, the orbitals which participate in the first
in terms of the holeeelectron distance.
These
the nitro group in their structures, stand out from the other mol-
ecules with high r index values, confirming that these molecules
have ICT. The r index values for the other molecules were quite
Dr index values for the molecules 3f and 3i, which preset
D
D
small, indicating that these molecules do not have ICT in the excited
state. Furthermore, when analyzing the orbital coefficients, it is
possible to confirm that
p/ p* transitions are taking place in all
the structures.
In addition, Tables 3 and 4 present the vertical transition anal-
ysis results of molecules 3a-i, where similar values were observed
for the theoretical absorption and emission maxima when
compared to the experimental ones. In this sense, it can be argued
that the CAM-B3LYP functional describes this system accurately. On
the other hand, unlike the experimentally observed which in the
solvent also seems to play a significant role in the maxima position,
but without any clear tendency in the theoretical results, the sol-
vent also plays a significant role in the excitation energy, but now a
clear tendency is observed. The excitation energy in polar media,

C.S. Radatz et al. / Journal of Molecular Structure 1207 (2020) 127817
7
Fig. 5. Orbitals involved in the first transition of molecules 3a-i computed at the CAM-
B3LYP/jun-cc-pVDZ level in dichloromethane.
Fig. 4. Geometries of the ground and first excited states of molecules 3a-i, calculated at
the CAM-B3LYP/cc-pVDZ level in dichloromethane.
Table 2
D
r index values for the first transition for
such as acetonitrile, was smaller then in less polar media (1,4-
dioxane). Increasing the dielectric, the absorption wavelength de-
creases, and it is observed that the absorption wavelength is greater
(in ascending order) for acetonitrile, dichloromethane, and 1,4-
dioxane, respectively. With these results, we can infer that polar
media stabilizes the S1 state. All the compounds presented high
absorption and emission oscillator strengths, as is expected ac-
cording to the high fluorescence that was intensity experimentally
observed.
molecules 3a-i in dichloromethane.
Molecule
D
r
3a
3 b
3c
3 d
3e
3f
3 g
3 h
3i
0.97
0.39
0.61
0.43
0.51
4.96
1.00
1.29
5.25

8
C.S. Radatz et al. / Journal of Molecular Structure 1207 (2020) 127817
Table 3
4. Conclusions
Photophysical data obtained from the CAM-B3LYP/jun-cc-pVDZ calculation in
several solvents for molecules 3a-i, wherein labs is the absorption wavelength (nm),
rS0 is the ground state dipole moment (Debye) and Gabs_osc is the absorption oscil-
lator strength.
The methodologies are simple and metal-free, employing al-
dehydes with sodium bisulfite or carboxylic acids with tribu-
tylphosphine, affording
a wide range of 1,3-benzoselenazole
Molecules
Solvent
Transition
labs
Gabs_osc
rS0
derivatives in good to excellent yields. In addition, the protocols can
also be performed under microwave irradiation, increasing reaction
efficiency. The molar absorptivity coefficient values, as well as the
calculated radiative rate constant by applying the Stricker-Berg
relation, indicated that spin and symmetry allowed for electronic
3a
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
/p
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
301
299
293
304
302
296
307
305
299
310
307
300
312
310
302
336
333
322
301
300
294
304
302
297
335
332
323
0.8278
0.7718
0.5831
0.9795
0.9144
0.7281
1.0394
0.9752
0.7875
1.278
2.28
2.21
2.14
1.91
1.52
1.02
3.97
3.89
3.69
2.77
2.54
1.84
5.25
4.93
3.96
18
17.4
15.36
6.57
6.41
5.96
3.5
3.26
2.68
16.09
15.56
13.91
3b
3c
3d
3e
3f
transitions, which could be related to the 1p
p* transitions. The
ꢁ
1,3-benzoselenazole derivatives presented fluorescence emission
located in the 300e650 nm range, where all the studied com-
pounds showed to a lesser or greater extent an ICT character in the
excited state. The photophysics in the excited state showed that
although the fluorophoric benzoselenazole is the main factor
responsible for the fluorescence emission, their properties could be
tailored by the solvent and its substituents. The theoretical calcu-
0.9735
0.791
1.0787
1.0256
0.8448
1.0103
0.9615
0.7843
0.7643
0.7132
0.547
0.7155
0.6664
0.4783
0.8945
0.8387
0.6411
lations confirmed, by
Dr index analysis, a charge transfer in the
excited state for molecules 3f and 3i, containing the nitro group in
their structures. In addition, the calculations showed that the state
populated after irradiation is the S1 (S0/S1 transition) and the
orbitals involved in this transition are HOMO and LUMO for all
structures.
3g
3h
3i
Credit author statement
Acetonitrile
Dichloromethane
1,4-Dioxane
All authors have contributed equally for this paper.
Acknowledgements
The authors would like to acknowledge FAPERGS, CNPq and
Table 4
~
Coordenaçao de Aperfeiçoamento de Pessoal de Nível Superior -
Photophysical data obtained from the CAM-B3LYP/jun-cc-pVDZ calculation in
several solvents for molecules 3a-i, wherein lemis is the absorption wavelength
(nm), rS1 is the ground state dipole moment (Debye) and Gabs_osc is the absorption
oscillator strength.
Brasil (CAPES) - Finance Code 001 for the financial support.
Appendix A. Supplementary data
Molecules
Solvent
lemis
Gemis_osc
rS1
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2020.127817.
3a
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
Acetonitrile
Dichloromethane
1,4-Dioxane
386
381
364
390
384
367
394
389
372
391
386
368
394
388
371
437
427
398
385
379
363
383
378
362
431
422
397
1.0324
0.9805
0.7960
1.1305
1.0771
0.8984
1.2079
1.1563
0.9719
1.1397
1.0884
0.9129
1.1970
1.1453
0.9668
1.2469
1.1831
0.9753
1.0107
0.9612
0.7830
0.9905
0.9311
0.6970
1.1527
1.0803
0.7876
1.71
1.61
1.44
1.99
1.63
1.09
3.33
3.24
3.03
2.78
2.52
1.78
5.27
4.89
3.84
18.06
17.11
14.39
5.77
5.60
5.11
3.58
3.32
2.58
15.94
15.17
13.19
3b
3c
3d
3e
3f
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