Structures and photophysical properties of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides

Article abstract of DOI:10.1016/j.molstruc.2017.06.048

Structural features of 3,4-diaryl-1H-pyrrol-2,5-diimines and their derivatives have been studied by molecular spectroscopy techniques, single-crystal X-ray diffraction, and DFT calculations. According to the theoretical calculations, the diimino tautomeri

Full text of DOI:10.1016/j.molstruc.2017.06.048

Accepted Manuscript
Structures and photophysical properties of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-
diarylmaleimides
Anastasiia M. Afanasenko, Dina V. Boyarskaya, Irina A. Boyarskaya, Tatiana G.
Chulkova, Yakov M. Grigoriev, Ilya E. Kolesnikov, Margarita S. Avdontceva, Taras L.
Panikorovskii, Andrej I. Panin, Anatoly N. Vereshchagin, Michail N. Elinson
PII:
S0022-2860(17)30829-3
10.1016/j.molstruc.2017.06.048
MOLSTR 23934
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 16 January 2017
Revised Date: 11 June 2017
Accepted Date: 12 June 2017
Please cite this article as: A.M. Afanasenko, D.V. Boyarskaya, I.A. Boyarskaya, T.G. Chulkova,
Y.M. Grigoriev, I.E. Kolesnikov, M.S. Avdontceva, T.L. Panikorovskii, A.I. Panin, A.N. Vereshchagin,
M.N. Elinson, Structures and photophysical properties of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-
diarylmaleimides, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.06.048.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Structures and photophysical properties of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-
diarylmaleimides
Anastasiia M. Afanasenkoa,b, Dina V. Boyarskayaa, Irina A. Boyarskayaa, Tatiana G. Chulkovaa, , Yakov M.
Grigorieva, Ilya E. Kolesnikova, Margarita S. Avdontcevaa, Taras L. Panikorovskiia, Andrej I. Panina, Anatoly
N. Vereshchaginc, Michail N. Elinsonc
aSaint Petersburg State University, 7/9 Universitetskaya Nab., Saint Petersburg, 199034 Russia;
bStratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The
Netherlands;
cN. D. Zelinsky Institute of Organic Chemistry, 47 Leninsky Prospect, Moscow, 119991 Russia
HIGHLIGHTS
Structural features of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides.
UV-vis absorption and fluorescence spectra investigation.
The electronic structures of the ground and excited states elucidation.
ABSTRACT
Structural features of 3,4-diaryl-1H-pyrrol-2,5-diimines and their derivatives have been studied by molecular
spectroscopy techniques, single-crystal X-ray diffraction, and DFT calculations. According to the theoretical
calculations, the diimino tautomeric form of 3,4-diaryl-1H-pyrrol-2,5-diimines is more stable in solution than
the imino-enamino form. We also found that the structurally related 2,3-diarylmaleimides exist in the solid state
in the dimeric diketo form. 3,4-Diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides exhibit fluorescence in
the blue region of the visible spectrum. The fluorescence spectra have large Stokes shifts. Aryl substituents at
the 3,4-positions of 1H-pyrrol-2,5-diimine do not significantly affect fluorescence properties. The insertion of
donor substituents into 2,3-diarylmaleimides leads to bathochromic shift of emission bands with hyperchromic
effect.
Keywords: 2,3-diarylmaleimides; 3,4-diaryl-1H-pyrrol-2,5-diimines; DFT and TD-DFT studies; fluorescence
1.
Introduction
In recent years, organic fluorescent compounds have attracted considerable attention as objects for the basic
research and commercial applications [1–3]. These substances are useful in organic light-emitting diodes
(OLEDs), fluorimetric sensors, surface coatings, inks, cosmetics, and textile industries. No reports on
fluorescence properties of pyrrol-2,5-diimines were published until this work, however, optical properties of
some of their derivatives were studied both experimentally and theoretically [4–6]. Most of the species in the
dicarboxylic acid imide family, such as phthalimide, naphthalimide, and perylenebisimide have been generated
for their UV-vis absorption or fluorescence property [7, 8]. Recently, Chen et al. reported the first bright and
efficient non-doped red organic emitting diode based on naphthylphenylamino-substituted N-methyl-2,3-
diphenylmaleimide as the host emitter, which is a deep red fluorophore with a relatively large Stokes shift both
in solution and the solid phase [9]. Chen et al. also reported on a new synthesis procedure for a series of 2,3-
diarylmaleimide based fluorophores, where the aryl substituent could be phenyl, 1-naphthyl, 2-naphthyl, etc.
carrying varied functionality [8]. It is known, that 2,3-diarylsubstituted maleimide compounds also behave as
fluorophores with various colors of fluorescence and exhibit larger Stokes shift in more polar solvents [10, 11].
In this work, we studied structures and photophysical properties of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-
diarylmaleimides. Tautomerizm, conformations, non-covalent intermolecular interactions and their influence on
optic properties of these compounds are in the focus of our investigation.
2.
Experimental
2.1.
Materials and Methods
All solvents were dried and purified by conventional methods and were freshly distilled under argon shortly
before use. Other reagents were used without further purification. FTIR spectra were recorded on Shimadzu
1
FTIR-8400S (4000–400 cm–1) and IRAffinity-1 (4000–350 cm–1) spectrometers using KBr pellets. H NMR
measurements were performed on a Bruker-DPX 400 instrument at ambient temperature. Electrospray
ionization mass spectra were obtained on a Bruker micrOTOF spectrometer equipped with electrospray
ionization (ESI) source using MeOH as the solvent. The instrument was operated in both positive and negative
Corresponding author. E-mail: t.chulkova@spbu.ru
1

ACCEPTED MANUSCRIPT
ion modes using a m/z range of 50–3000. The capillary voltage of the ion source was set at –4500 V (ESI+–MS)
or 3500 V (ESI+–MS) and the capillary exit at ±(70–150) V. The nebulizer gas flow was 0.4 bar and drying gas
flow 4.0 L/min. The absorption spectra were recorded on a Perkin–Elmer precision spectrophotometer Lambda
1050. The emission spectra, excitation spectra, measurements of the lifetimes of excited states were measured
on a modular spectrofluorimeter Fluorolog-3 (Horiba Jobin Yvon). Fluorescence lifetime measurements are
based on time-correlated single photon counting (TCSPC). Device also includes an integrating sphere Quanta-φ
with fiber optics which enables direct measurement of quantum yields of luminescence.
2.2.
Synthetic procedures
3,4-Diaryl-1H-pyrrol-2,5-diimines 2a−f were obtained from the appropriate commercially available
arylacetonitriles by two step reaction. The synthetic scheme is shown in Fig. 1. Starting from a series of
arylacetonitriles, following Linstead’s procedure [12], yielded the nitriles of 2,3-diarylbutenedioic acids 1a−d.
2,3-Diarylfumaronitriles 1e and 1f were produced electrochemically from the appropriate arylacetonitriles [13].
Compounds 1a−f were converted to the corresponding 3,4-diaryl-1H-pyrrol-2,5-diimines 2a−f by bubbling of
dry ammonia into ethylene glycol solution [14].
Fig. 1. Preparation of 3,4-diaryl-1H-pyrrol-2,5-diimines 2a−f.
1
3,4-Diphenyl-1H-pyrrol-2,5-diimine (2a): Yield 0.480 g (89%). H NMR spectrum (300 MHz, CDCl3) δ,
ppm: 7.43–7.35 (m, 6Н, H−Ar), 7.27–7.19 (m, 4Н, H−Ar). 13C{1H} NMR spectrum (100 MHz, CDCl3) δ, ppm:
166.2 (C=N), 129.6, 129.4, 129.1, 128.9, 128.3. IR spectrum (KBr, selected bands, cm−1): 1659 s ν(C=N).
HRMS (ESI+), m/z: 248.1181 [M+H]+, 495.2234 [2M+H]+. C16H14N3. Calcd., m/z: 248.1188.
1
3,4-Di(4-methylphenyl)-1H-pyrrol-2,5-diimine (2b): Yield 0.476 g (91%). H NMR spectrum (400 MHz,
3
3
CDCl3) δ, ppm: 7.19 (d, J = 9.0 Hz, 4Н, H−Ar), 7.13 (d, J = 9.0 Hz, 4Н, H−Ar), 2.37 (s, 6H, CH3). 13C{1H}
NMR spectrum (100 MHz, CDCl3) δ, ppm: 166.6 (=C), 139.1 (Cp), 129.9, 129.6 (CH), 128.6 (CH), 127.0, 21.4
(CH3). IR spectrum (KBr, selected bands, cm−1): 1641 s ν(C=N), 1535 m ν(СCAr). HRMS (ESI+), m/z:
276.1504 [M+H]+. C18H18N3. Calcd., m/z: 276.1501.
1
3,4-Di(4-methoxyphenyl)-1H-pyrrol-2,5-diimine (2c) [15]: Yield 0.493 g (94%). H NMR spectrum (400
3
3
MHz, CDCl3) δ, ppm: 7.82 (d, J = 9.0 Hz, 4H, H−Ar), 7.04 (d, J = 9.0 Hz, 4H, H−Ar), 3.91 (s, 6H, CH3).
13C{1H} NMR spectrum (100 MHz, CDCl3) δ, ppm: 166.7, 160.1, 130.8, 122.1, 122.0, 114.5, 55.3 (OCH3). IR
spectrum (KBr, selected bands, cm−1): 1647 s ν(C=N), 1605 s, 1504 s ν(СCAr). HRMS (ESI+), m/z: 308.1402
[M+H]+. C18H18N3O2. Calcd., m/z: 308.1399.
1
3,4-Di(4-fluorophenyl)-1H-pyrrol-2,5-diimine (2d): Yield 0.432 g (81%). H NMR spectrum (400 MHz,
CDCl3) δ, ppm: 7.23 (m, 4H, H−Ar), 7.11 (d, 3J = 8.0 Hz, 2H, H−Ar), 7.08 (d, 3J = 8.0 Hz, 2H, H−Ar). 13C{1H}
NMR spectrum (100 MHz, CDCl3) δ, ppm: 165.3 (1JCF = 189.0 Hz, Cp), 161.9 (=C), 131.9 (Cipso), 131.4 (3JCF
=
8.0 Hz, Co), 125.6 (C=C), 116.3 (2JCF = 22.0 Hz, Cm). IR spectrum (KBr, selected bands, cm−1): 1643 s ν(C=N),
1601 s, 1502 s ν(СCAr). HRMS (ESI+), m/z: 284.0998 [M+H]+. C16H12N3F2. Calcd., m/z: 284.0999.
1
3,4-Di(4-chlorophenyl)-1H-pyrrol-2,5-diimine (2e): Yield 0.424 g (81%). H NMR spectrum (400 MHz,
CDCl3) δ, ppm: 7.39 (d, 3J = 8.0 Hz, 4H, H−Ar), 7.18 (d, 3J = 8.0 Hz, 4H, H−Ar). 13C{1H} NMR spectrum (100
MHz, CDCl3) δ, ppm: 166.2 (C=N), 135.7, 130.7, 129.5 (CH), 128.0 (CH), 128.0. IR spectrum (KBr, selected
bands, cm−1): 1645 s ν(C=N), 1593, 1483 m ν(СCAr). HRMS (ESI+), m/z: 316.0391[M+H]+. C16H12N3Cl2.
Calcd., m/z:. 316.0408.
1
3,4-Di(4-bromophenyl)-1H-pyrrol-2,5-diimine (2f) [15]: Yield 0.438 g (83%). H NMR spectrum (400 MHz,
CDCl3) δ, ppm: 7.55 (d, 3J = 8.0 Hz, 4H, H−Ar), 7.11 (d, 3J = 8.0 Hz, 4H, H−Ar). 13C{1H} NMR spectrum (100
MHz, CDCl3) δ, ppm: 166.1 (C=N), 132.4, 131.9 (CH), 130.9 (CH), 128.4, 124.0. IR spectrum (KBr, selected
bands, cm−1): 1645 s ν(C=N), 1533, 1479 m ν(СCAr). HRMS (ESI+), m/z: 509.8365 [M+Ag]+. C16H11N3Br2Ag.
Calcd., m/z: 509.8371.
2

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2,3-Diarylmaleimides 3a−d were synthesized by hydrolysis of 3,4-diaryl-1H-pyrrol-2,5-diimines in aqueous
methanol [16]. These compounds were prepared with quantitative yields. Crystals of 3b−d suitable for X-ray
diffraction were grown from a concentrated chloroform solution.
1
2,3-Diphenylmaleimide (3a): H NMR spectrum of the compound is identical to previously published data
[17].
2,3-Di(4-methylphenyl)maleimide (3b): 1H NMR spectrum (400 MHz, CDCl3) δ, ppm: 7.86 (s, 2H, NH), 7.41
(d, J=8.00 Hz, 8H, Ar), 7.19 (d, J=8.00 Hz, 8H, Ar), 2.39 (s, 12H, CH3). 13C{1H} NMR spectrum (100 MHz,
CDCl3) δ, ppm: 170.8, 140.2, 136.4, 129.8, 129.3, 125.6, 21.5. IR (KBr, selected bands, cm−1): 1713 s ν(С=O),
1607, 1503 m ν(СCAr). HRMS (ESI+), m/z: 277.1341 [M+H]+ (calc. 277.1103), 300.1005 [M+Na]+ (calc.
300.1000), 577.2118 [2M+Na]+ (calc. 577.2103).
1
2,3-Di(4-methoxyphenyl)maleimide (3c): H NMR spectrum (400 MHz, CDCl3) δ, ppm: 7.40 (s, 2H, NH),
7.50 (d, J=8.00 Hz, 8H, Ar), 6.91 (d, J=8.00 Hz, 8H, Ar), 3.86 (s, 6H, CH3O). 13C{1H} NMR spectrum (100
MHz, CDCl3) δ, ppm: 170.7, 160.8, 135.0, 131.5, 121.1, 114.1, 55.3. IR spectrum (KBr, selected bands, cm−1):
1707 s ν(С=O), 1601 m, 1516 m ν(СCAr). HRMS (ESI+), m/z: 332.0888 [M+Na]+ (calc. 332.0899), 641.1889
[2M+Na]+ (calc. 641.1899).
2,3-Di(4-fluorophenyl)maleimide (3d): 1H NMR spectrum (400 MHz, CDCl3) δ, ppm: 7.88 (s, 2H, NH), 7.51
(d, J=8.00 Hz, 4H, Ar), 7.48(d, J=8.00 Hz, 4H, Ar), 7.105 (d, J=12.00 Hz, 4H, Ar), 7.075 (d, J=12.00 Hz, 4H,
Ar). 13C{1H} NMR spectrum (100 MHz, CDCl3) δ, ppm: 170.1, 164.9, 162.4, 135.8, 132.0, 131.9, 124.2, 116.2,
116.0. IR spectrum (KBr, selected bands, cm−1): 1710 s ν(С=O), 1599, 1503 m ν(СCAr). HRMS (ESI+), m/z:
308.0504 [M+Na]+ (calc. 308.0499), 555.5091 [2M+Na]+ (calc. 555.1132).
2.3.
Crystallography
The crystals of 3b−d were obtained by a slow evaporation of solvent at room temperature. Crystals of
compounds 3b−d were immersed in cryo-oil, mounted in a nylon loop, and analysed at a temperature of 100 K.
The X-ray diffraction data were collected on an Agilent Technologies Excalibur Eos and Supernova Atlas
diffractometers. The temperature for all experiments was kept at 100 K. The structures have been solved by the
direct methods and refined by means of the SHELXL–97 [18] program incorporated in the OLEX2 program
package [19]. The carbon-bound H atoms were placed in calculated positions and were included in the
refinement in the ‘riding’ model approximation, with Uiso(H) set to 1.5Ueq(C) and C–H 0.96 Å for CH3 groups,
Uiso(H) set to 1.2Ueq(C) and C–H 0.93 Å for the CH groups, and Uiso(H) set to 1.2Ueq(N) and N–H 0.86 Å for
the NH groups. Empirical absorption correction was applied in CrysAlisPro program complex [20] using
spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.
The crystallographic details and refinement parameters are summarized in Table 1. The crystal structures and
crystallographic details are given in Figs. S1–S3 and Tables S1–S4 of the Supporting Information. Crystal data
have been deposited at the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers CCDC
1518210, CCDC 1518077, and CCDC 1518211 for 3b, 3c, and 3d, respectively.
Table 1
Crystallographic data and refinement parameters
3b dimer
C36H30N2O4
554.62
3c dimer
C36H30N2O8
618.63
3d dimer
C32H18N2O4F4
570.48
100(2)
Empirical formula
Molecular weight
Temp (K)
100(2)
100(2)
Radiation
CuKα
CuKα
MoKα
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
P-1
Triclinic
P-1
Monoclinic
P21/c
10.1015(6)
21.6426(11)
11.7949(7)
90
97.996(5)
90
2553.6(3)
4
10.8587(4)
11.8492(4)
12.7173(5)
69.065(4)
81.847(3)
72.045(3)
1452.93(9)
2
5.9948(3)
8.8120(6)
13.9547(9)
91.627(5)
95.486(5)
98.101(5)
725.82(8)
2
α
β
γ
(°)
(°)
(°)
V (Å3)
Z
ρ
µ
calc (mg/mm3)
1.268
0.663
1.520
1.277
1.484
0.118
(mm–1)
Total reflections
32384
6096
17389
Unique reflections
5496
2844
5727
3

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GOOF (F2)
Rint
Rσ
R1 (all data)
wR2 (all data)
R1 (|Fo| ≥ 4σF)
wR2 (|Fo| ≥ 4σF)
1.052
1.027
1.009
0.0934
0.0442
0.0659
0.1614
0.0512
0.1447
0.0298
0.0370
0.0549
0.1264
0.0421
0.1106
0.0370
0.0518
0.0769
0.1027
0.0478
0.0922
R1 = Σ||Fo| – |Fc||/Σ|Fo|; wR2 = {Σ[w(Fo – Fc2)2]/Σ[w(Fo ) ]}1/2; w =1/[σ2(Fo )+(aP)2 + bP], where P = (Fo
+
2
2 2
2
2
2Fc2)/3; s = {Σ[w(Fo – Fc2)]/(n – p)}1/2 where n is the number of reflections and p is the number of refinement
2
parameters.
2.4.
Computational details
The full geometry optimization, orbital views, orbital energies, total energies, electron transitions in the UV-vis
spectra have been carried out at the DFT hybrid level of theory using Becke’s three-parameter hybrid exchange
functional in combination with the gradient-corrected correlation functional of Lee, Yang, and Parr (B3LYP)
[21−24] and standard basis 6-31+G(d,p) in methanol using the Gaussian 03 [25] (if not explicitly stated
otherwise) and Gaussian 09 [26] program packages. The relative energies of pyrroldiimine (or maleimide)
dimerization were calculated per one molecule of pyrroldiimine (or maleimide).
3.
Results and discussion
Tautomeric forms of 1H-pyrrol-2,5-diimines and their derivatives (maleimides) were studied theoretically [27].
The existence of two tautomers of 2,3-diarylmaleimides is possible; the diketo tautomer (A1) and keto-enol
tautomer (B1) are represented in Fig. 2.
Fig. 2. Tautomers of 2,3-diarylmaleimides.
According to the theoretical calculations, diketo A1 tautomer of maleimide is the most stable both in the gas
phase and in solution [27]. The labile hydrogen atom is attached to the nitrogen atom (A1 in Fig. 2). Additional
stabilization of the diketo form is accomplished by hydrogen bonds between two neighboring molecules in the
solid phase [28]. We have studied the structures of 2,3-diarylmaleimides both theoretically and experimentally.
It is known that 3a crystalizes in two polymorphic forms, viz. α and β forms [28]. According to the XRD data
[28], α-3a crystallizes in a triclinic crystal system, whereas packing of β-3a is monoclinic. The asymmetric unit
of α-3a contains two molecules, whereas that of β-3a contains one molecule. The two N––H O hydrogen
bonds link the molecules pair-wise in both polymorphs of 3a, thus forming the centrosymmetric dimers. The
hydrogen bond distances in α-3a are 2.02(3) and 1.99(3) Å, while in β-3a, the hydrogen bond is shorter (1.91(4)
Å).
The prepared 3a crystallizes in the triclinic space group P-1, which corresponds to α-3a.
Although, the crystals of 3b–d were obtained analogously, 3b and 3c crystallize in the triclinic space group
P-1, whereas 3d crystallizes in the monoclinic space group P21/c (Table 1). In 3b–d, two molecules are
connected to each other by two hydrogen bonds (Figs. S1–S3 of Supporting Information). In the dimeric
structure 3b, the hydrogen bond lengths are 2.104(2) and 2.069(2) Å (Table 2S of Supporting Information). In
the dimer 3c, the two hydrogen bonds exhibit the same lengths (2.013(2) Å) (Table 3S of Supporting
Information). The hydrogen bond lengths in the dimer 3d are 2.153(2) and 1.965(2) Å (Table 4S of Supporting
Information). In 3d, the additional weak C(O) F interaction links two dimers. Owing to this intermolecular
interaction, the orientations of dimers in two planes are observed and the crystallization in a monoclinic crystal
system becomes more preferable.
4

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The bond lengths C(1)=O(1), C(4)=O(2) (~1.20–1.23 Ǻ), C(1)–N(1), C(4)–N(1) (~1.37–1.39 Ǻ) are typical
for the corresponding bonds in imides [28]. The C=O bond lengths lie in the double bond region. The C–N bond
lengths are larger than the length of C=N double bond (~1.28 Ǻ). The O(1)−C(1)−N(1), O(2)−C(4)−N(1), and
C(1)−N(1)−C(4) angles in 3b−d (125.0−126.2°, 124.1−125.6°, and 110.7−111.2°, respectively) are comparable
to the appropriate previously published angles in 3a [28]. According to the XRD data given above, 2,3-
diarylmaleimides exist in the diketo tautomeric form in the solid state.
Accordingly to the DFT calculations, the diketo tautomer is stabilized by the dimer formation. The diketo
tautomers of 3a–d in the dimeric form have a comparatively lower relative energy than the single molecules of
3a−d (∼14 kJ mol–1). Therefore, the diketo form is stabilizing by the dimerization in the solid state. Also, the
stabilization of the diketo tautomer by the dimerization is possible in low-polar solvents. In the polar solvents,
the stabilization of the diketo tautomer occurs by forming of hydrogen bond between the hydrogen atom
H(N)imide and the basic atom of the solvent molecule. Such interaction is observed in the solvate of 3a and 1-
methylpyrrolidin-2-one [29].
The bond lengths and angles in the optimized structures of the dimers of 3a–d are comparable with the XRD
data. Larger differences are observed in the position of phenyls regarding to the maleimide ring. In all
molecules, the rotation of the two phenyl rings with respect to the maleimide ring is due to steric interactions of
the ortho-hydrogen atoms. In the calculated structures, the two phenyl rings are turned regarding the maleimide
ring on the similar angle (~60°). In the solid state, the different angles between the corresponding ring planes
(for example, 21 and 51° in 3b) are because of the packing effects.
3,4-Diaryl-1H-pyrrol-2,5-diimines may exist in two tautomeric forms; the diimino form (A2) and imino-
enamino form (B2) are represented in Fig. 3.
Fig. 3. Tautomers of 3,4-diaryl-1H-pyrrol-2,5-diimines.
In 1H-pyrrol-2,5-diimines, the diimino form is also more stable than the imino-enamino form, however, the
difference between the energies of tautomers is not so significant as in the case of maleimides [27]. Therefore
coexistence of the diimino and imino-enamino tautomers in solution is possible. It is known that two 5-imino-
3,4-diphenyl-1H-pyrrol-2-one molecules form centrosymmetric dimers via two Npyrrole–H Nimino hydrogen
bonds in the solid state [30]. The relative energy of 5-imino-3,4-diphenyl-1H-pyrrol-2-one in the dimeric form is
lower than the energy of single molecule (∆E =–18.5 kJ mol–1) (pages S65, S69 of Supporting Information). In
addition, we calculated the energy of dimer, where the two 5-imino-3,4-diphenyl-1H-pyrrol-2-one molecules
link together by two Npyrrole–H O hydrogen bonds. The relative energy of this dimer is higher than the energy
of dimer, where two molecules link together via two Npyrrole–H Nimino hydrogen bonds (4.4 kJ mol–1) (pages
S69, S72 of Supporting Information). Insofar as 5-imino-3,4-diphenyl-1H-pyrrol-2-one is structurally related to
3,4-diaryl-1H-pyrrol-2,5-diimines, we suggested that diimino tautomers of 3,4-diaryl-1H-pyrrol-2,5-diimines
can be stabilized by hydrogen bonding in the dimeric structure (Fig. 4).
We have evaluated the relative stability of the diimino tautomer of 3,4-di(4-methylphenyl)-1H-pyrrol-2,5-
diimine, its dimer, and the imino-enamino tautomer. According to DFT calculations, the dimer of 3,4-di(4-
methylphenyl)-1H-pyrrol-2,5-diimine is more stable than the single molecule (∆E = −17.9 kJ mol−1). The
difference between the total energies of imino-enamino and diimino tautomers of 3,4-di(4-methylphenyl)-1H-
pyrrol-2,5-diimine is 7.8 kJ mol−1.
5

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Fig. 4. Centrosymmetric dimer of 3,4-diaryl-1H-pyrrol-2,5-diimine.
To address the tautomerism, different types of the hydrogen-bonding motifs were designed and investigated
recently [31−34]. For example, K. Epa et al. [32] have illustrated how relative stabilities of tautomers can
drastically change with a change of environment and how, by controlling the environment, it is possible to
deliberately isolate a desired tautomer. The metastable 1H-imidazo[4,5-b]pyridine tautomer can be selectively
isolated by using urea derivatives as cocrystallizing agents capable to forming three hydrogen bonds. The
average energy of the hydrogen bond C=O H−N is ~48 kJ mol–1. Also, the hydrogen bonds stabilize the
nitrogen-linked conjugate 2-methyltetrazole-saccharinate in the 2H-2-methyltetrazole aminosaccharin
tautomeric form [33−34]. Pairs of molecules are linked in dimers through intermolecular hydrogen bonds
involving the NH spacer group as proton donor and the nitrogen in position 4 of the tetrazole ring as acceptor.
The energy of the Namino−H Ntetrazole hydrogen bond is estimated as ~14 kJ mol–1 by DFT calculations [34]. The
hydrogen bonds in the 2,3-diarylmaleimide, 5-imino-3,4-diphenyl-1H-pyrrol-2-one, and 3,4-diaryl-1H-pyrrol-
2,5-diimine dimers are relatively strength and comparable with those for other known dimeric structures. These
intermolecular interactions stabilize the diketo tautomers of 2,3-diarylmaleimides, keto-imino tautomer of 5-
imino-3,4-diphenyl-1H-pyrrol-2-one, and diimino tautomers of 3,4-diaryl-1H-pyrrol-2,5-diimines.
Two stable conformers (C1 and C2) of 3,4-di(4-methylphenyl)-1H-pyrrol-2,5-diimine have been obtained by
the geometry optimization (Fig. 5).
Fig. 5. Stable conformers (C1 and C2) of 3,4-di(4-methylphenyl)-1H-pyrrol-2,5-diimine obtained by DFT
calculations and their relative potential energies are indicated in units of kJ mol−1.
The relative energies are 0 and 5.3 kJ mol−1 for C1 and C2, respectively.
The optimized structures of 3,4-di(4-methylphenyl)-1H-pyrrol-2,5-diimine in the diimino and imino-enamino
tautomeric
forms,
its
dimer,
5-imino-3,4-di(4-methylphenyl)-1H-pyrrol-2-one,
and
2,3-di(4-
methylphenyl)maleimide with the bond lengths and angles are shown in Fig. S4.
The structures of the four 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides (aryl = phenyl, 4-
methylphenyl, 4-methoxyphenyl, and 4-fluorophenyl) have been optimized. It was found that the substituent at
the 4th position of the phenyl ring slightly affects the bond lengths and the values of the valence angles in the
6

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pyrrole ring. Change of substituent has a little effect on the N−H bond order. Consequently, the acidity does not
depend on the nature of substituent.
The schematic representations of the frontier molecular orbitals for 2b and 3b are exhibited in Fig. 6. The
nature of the frontier MOs of compounds with different substituents (compounds 2a−d, 3a−d) at the 4th
positions of benzene rings is the same. The formation of dimers changes the HOMO and LUMO energies
insignificantly. Also, dimers of 3,4-diaryl-1H-pyrrol-2,5-diimines and tautomeric forms (imino-enamino) have
the same nature of the frontier MOs. The HOMO energies for 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-
diarylmaleimides are almost identical. While the LUMO energies for 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-
diarylmaleimides differ considerably (ESI). This is due to the contribution of heteroatom atomic orbital to
LUMO. The HOMO energies for 3,4-diaryl-1H-pyrrol-2,5-diimines are almost independent on the substituent in
the phenyl ring. However, the LUMO energies depend on the substituent stronger. The insertion of electron-
withdrawing substituents leads to decreasing of frontier orbital energy. The differences between the frontier
orbital energies ∆EHOMO−LUMO for 2,3-diarylmaleimides are correlated with the fluorescence band shift value
caused by the changing of substituent. This correlation was not found for 3,4-diaryl-1H-pyrrol-2,5-diimines due
to a small band shift in the fluorescence spectra (Fig. 6).
HOMO
LUMO
Fig. 6. Isosurfaces of MOs (HOMO and LUMO) for 3,4-di(4-methylphenyl)-1H-pyrrol-2,5-diimine (2b) and
2,3-di(4-methylphenyl)maleimide (3b).
For a deep insight into the optical properties of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides,
UV−vis absorption and fluorescence emission spectra of these compounds in chloroform were calculated.
7

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3.1.
UV-vis absorption spectra and TD-DFT computational studies
The absorption spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides in chloroform are shown
in Fig. 7, and the corresponding spectroscopic data are listed in Table 2.
All pyrroldiimines display very similar absorption spectra with one or two intensive absorption bands in the near
UV region. However, the absorption band of 2a is shifted in the field of higher frequencies, compared with other
pyrroldiimines. Use of methanol as a solvent causes a slight blue shift of the band at 278−311 nm accompanied
with a decrease of its intensity.
The absorption spectra of 2,3-diarylmaleimides exhibit 4−6 bands (Table 2). The extinction coefficients of the
bands at 240−253 nm are 0.9−1.1 104 M−1ꢀcm−1.
DFT and TD-DFT computational studies were performed to elucidate the electronic structures of the ground
and excited states of 2a−f and 3a−d. The calculated UV-vis spectra for 2b and 3b are represented in Fig. S5.
The calculated spectra are similar to the experimental spectra for these compounds. The ten transitions were
calculated. The nature of vertical transitions for 3,4-di(4-methylphenyl)-1H-pyrrol-2,5-diimine is analyzed in
Table 3.
The comparison of UV-vis spectra calculated for two conformations (C1 and C2) of 3,4-diaryl-1H-pyrrol-2,5-
diimines shows that the nature of the transitions does not depend on conformation. However, the positions and
intensities of the bands are very sensitive to the molecular conformation.
Table 2
The corresponding data of UV-vis absorption spectra in chloroform
Compound
λabs, nm (ε, M–1ꢀcm–1)
278 (17 000)
2a
305 (18 000), 375*
2b
311 (11 000), 385*
2c
303 (13 000), 350*
2d
308 (14 000), 367*
2e
311 (12 000), 367*
2f
242 (11 000), 251 (10 000), 269 (7 000), 304*, 319*, 357 (6 000)
253 (11 000), 262 (10 300), 280 (6 000), 313*, 325*, 380 (8 000)
240 (9 000), 257 (8 000), 340 (3 000), 409 (5 000)
252 (11 000), 273 (7 000), 306*, 318*, 366 (7 000)
3a
3b
3c
3d
* Shoulder.
Most long-wavelength band has a high-frequency shift of 20 cm–1 and the simultaneous decrease in the
relative intensity of the bands 1 and 2 in the spectrum of C2 in comparison with that of C1. This is true for any
type of substituent in the benzene ring.
a)
b)
Fig. 7. The UV-vis absorption spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines (a) and 2,3-diarylmaleimides (b) (C
= 2.5ꢀ10–5 M) in chloroform.
8

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On the other hand, the change of the substituent on the benzene ring has a little effect on the spectrum. The
insertion of electron-withdrawing substituent (fluorine) leads to a slight shift (20 nm) of the long-wave band to
the high frequency region.
According to the DFT-TD calculations of the long-wave band of the medium intensity (f = 0,17) for 2c is due
to the typical HOMO–LUMO transition, the second band is caused by the HOMO−1 − LUMO transition (f =
0.1).
The bathochromic shift of these transitions is due to a destabilizing contribution of oxygen p-orbital of
methoxy group to the energies of HOMO and HOMO−1 orbitals (Table 3).
Table 3
The transitions in the UV-vis spectrum of 3b
№
1
МО*
73 → 74
Transitions
HOMO → LUMO
Assignment
π(C(2)C(3)) − ꢀꢁꢂ(Nimino) + π(Ph,b1) → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
)
2
3
71 → 74 HOMO−2 → LUMO
70 → 74 HOMO−3 → LUMO
π(Ph,b1) − σ(CCpyrrole) − π(CH3) → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
π(Ph,a2) + n−(Nimino) → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
ꢀꢁꢂ(Nimino) − pπ(Npyrrole) → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
π(Ph,a2) + n+(Nimino) → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
n−(Nimino) - σ−(CCpyrrole) → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
)
)
4
5
72 → 74 HOMO−1 → LUMO
69 → 74 HOMO−4 → LUMO
68 → 74 HOMO−5 → LUMO
66 → 74 HOMO−7 → LUMO
73 → 75 HOMO → LUMO+1
67 → 74 HOMO−8 → LUMO
73 → 78 HOMO → LUMO+4
)
)
6
)
n+(Nim) + σ+(pyrrole ring) → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
π(C(2)C(3)) − ꢀꢁꢂ(Nimino) + π(Ph,b1) → π*(Ph,a2)
)
7
8
π−(C(2)C(1)=N)pyrrole → π*(C(1)C(2)) − ꢀꢁꢃ(Nimino
π(C(2)C(3)) − ꢀꢁꢂ(Nimino) + π(Ph,b1) → π*(Ph,b1)
)
9
10
* MO is participated in the vertical transition (only orbitals with the highest coefficient in the linear combination
are given for the transitions involving several MOs).
3.2.
Emission spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines
3,4-Diaryl-1H-pyrrol-2,5-diimines give the highest fluorescence intensity in the visible (blue) region upon
excitation wavelength of 350 nm.
The normalized fluorescence spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines in chloroform are given in Fig. 8.
Excitation and emission band maxima and fluorescence quantum yields are presented in Table 4.
Fig. 8. Emission spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines 2a−f (C = 2.5ꢀ10−5 M) in chloroform.
9

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Table 4
Maximum excitation (λex) and emission (λem) wavelengths and FL quantum yields (Φ) for 2a−f in chloroform
Compound
2a
2b
2c
2d
2e
2f
Stokes shift, nm
λex, nm
340
341
295*, 348
352*, 370
343
λem, nm
478
478
494
493
Φ, %
3
16
17
7
138
137
146
123
145
145
488
493
3
3
348
*Shoulder.
The compounds 2a−c, e, f have the broad excitation spectra with the strongest band at 340−348 nm.
Only one type of bands is observed in the luminescence spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines in
chloroform. This indicates the presence of one luminescence form in the solution. The insertion of donor
substituents (CH3O, CH3) in the para-position of the benzene ring leads to increase of quantum yields of
luminescence. The position of the maximum of luminescence band is practically independent on the substituents
in the aromatic rings. All compounds are classified as fluorescent phosphors, because the short lifetimes of the
excited states (<< 0.1 ms) are observed. 2a−f display room temperature excited-state lifetimes between 2 and 13
ns.
The fluorescence spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines in methanol exhibit similar spectral properties.
The nature of the solvent does not affect the position and shape of the bands.
3.3.
Emission spectra of 2,3-diarylmaleimides
The fluorescence spectra of 2,3-diarylmaleimides in chloroform have one intensive broad structureless band
(Fig. 9).
Fig. 9. Emission spectra of 2,3-diarylmaleimides 3a−d (C = 6.25ꢀ10−6 M) in chloroform.
Table 5
Maximum excitation (λex) and emission (λem) wavelengths for 3a−d in chloroform
Compound
Stokes shift, nm
λex, nm
365
380
433
370
λem, nm
493
515
553
498
128
135
120
128
3a
3b
3c
3d
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The maxima of the spectra of H- and F-derivatives have the same position viz. 493−498 nm (Table 5). The
donor substituents (Me, MeO) cause the bathochromic shift (3b: λem =515 nm, red shift ∆λ = 22 nm; 3c: λem
553 nm, red shift ∆λ = 60 nm). The insertion of donor substituents leads to the increasing of the band intensity.
=
4.
Conclusion
The geometry and electronic structure of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides were
calculated by DFT B3LYP/6-31+G(d,p) method. We found that the substitution in 3,4-diaryl-1H-pyrrol-2,5-
diimines and 2,3-diarylmaleimides has no effect on the molecular geometry. The substitution has a little effect
on the electron density distribution and composition of frontier MOs. The UV-vis spectra of 3,4-diaryl-1H-
pyrrol-2,5-diimines and 2,3-diarylmaleimides were interpreted. Fluorescence of 3,4-diaryl-1H-pyrrol-2,5-
diimines and 2,3-diarylmaleimides were studied. The fluorescence spectra have large Stokes shifts. The
insertion of donor substituents causes bathochromic shift of emission bands with hyperchromic effect. The
effect of solvent on the emission spectra of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides is
insignificant.
5.
Supplementary information
Crystallographic data reported in this paper have been deposited with the Cambridge Crystallographic Data
Centre (3b, CCDC 1518210; 3c, CCDC 1518077; 3d, CCDC 1518211). The data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge, CB2 1EZ (e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgments
This research was supported by St. Petersburg State University (Grant 12.37.214.2016). We are grateful to Prof.
V. Yu. Kukushkin (Saint Petersburg State University) for helpful discussions and insightful comments. We also
thank the Centre for Optical and Laser Materials Research, Research Centre for X-ray Diffraction Studies,
Educational Resource Center of Chemistry, Centre for Chemical Analysis and Materials Research, and Centre
of Magnetic Resonance (all belong to Saint Petersburg State University) for physicochemical measurements.
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HIGHLIGHTS
Structural features of 3,4-diaryl-1H-pyrrol-2,5-diimines and 2,3-diarylmaleimides.
UV-vis absorption and fluorescence spectra investigation.
The electronic structures of the ground and excited states elucidation.