Fluorescent substituted amidines of benzanthrone: Synthesis, spectroscopy and quantum chemical calculations

Article abstract of DOI:10.1016/j.saa.2012.09.104

Several new substituted amidine derivatives of benzanthrone were synthesized by a condensation reaction from 3-aminobenzo[de]anthracen-7-one and appropriate aromatic and aliphatic amides. The obtained derivatives have a bright yellow or orange fluorescence in organic solvents and in solid state. The novel benzanthrone derivatives were characterized by TLC analysis, ¹H NMR, IR, MS, UV/vis, and fluorescence spectroscopy. The solvent effect on photophysical behaviors of these dyes was investigated, and the results showed that the Stoke's shift increased, whereas quantum yield decreased with the growth of the solvent polarity. The structure of some dyes was confirmed by the X-ray single crystal structure analysis. AM1, ZINDO/S and ab initio calculations using Gaussian software were carried out to estimate the electron system of structures. The calculations show planar configurations for the aromatic core of these compounds and two possible orientations of amidine substituents. The calculation results correlate well with red-shifted absorption and emission spectra of compounds.

Full text of DOI:10.1016/j.saa.2012.09.104

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Fluorescent substituted amidines of benzanthrone: Synthesis, spectroscopy
and quantum chemical calculations
b
c
d
c
d
Svetlana Gonta a, , Maris Utinans , Georgii Kirilov , Sergey Belyakov , Irena Ivanova , Mendel Fleisher ,
⇑
Valerij Savenkov a, Elena Kirilova c
a Laboratory of Microbial Storage Product Research, Institute of Microbiology and Biotechnology, University of Latvia, Kronvalda blvd. 4, Riga LV-1586, Latvia
b Faculty of Materials Science and Applied Chemistry, Riga Technical University, Azenes str. 14, Riga LV-1048, Latvia
c Department of Chemistry and Geography, Faculty of Natural Sciences and Mathematics, Daugavpils University, Vienibas str. 13, Daugavpils LV5401, Latvia
d Latvian Institute of Organic Synthesis, Aizkraukles str. 21, Riga LV-1006, Latvia
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
"
A series of novel benzanthrone dyes
containing amidino group were
synthesized and characterized.
Spectroscopic properties of the new
molecules were examined by FT-IR,
NMR and UV techniques.
Solvatochromic effect was studied.
X-ray crystal structures were
determined.
HOMO and LUMO energies,
molecular electrostatic potential
distribution and dipole moments
were calculated.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 July 2012
Received in revised form 4 September 2012
Accepted 9 September 2012
Available online 12 October 2012
Several new substituted amidine derivatives of benzanthrone were synthesized by a condensation reac-
tion from 3-aminobenzo[de]anthracen-7-one and appropriate aromatic and aliphatic amides. The
obtained derivatives have a bright yellow or orange fluorescence in organic solvents and in solid state.
The novel benzanthrone derivatives were characterized by TLC analysis, 1H NMR, IR, MS, UV/vis, and fluo-
rescence spectroscopy. The solvent effect on photophysical behaviors of these dyes was investigated, and
the results showed that the Stoke’s shift increased, whereas quantum yield decreased with the growth of
the solvent polarity. The structure of some dyes was confirmed by the X-ray single crystal structure anal-
ysis. AM1, ZINDO/S and ab initio calculations using Gaussian software were carried out to estimate the
electron system of structures. The calculations show planar configurations for the aromatic core of these
compounds and two possible orientations of amidine substituents. The calculation results correlate well
with red-shifted absorption and emission spectra of compounds.
Keywords:
Benzanthrone dyes
Fluorescence solvatochromism
Substituted amidines
Crystal structure
Solid-state luminescence
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
and industry. Especially donor–acceptor p-conjugated organic
materials have attracted considerable interest owing to their
potentially wide application in the development of electro- and
photoactive materials [1,2]. Such fluorescent dyes have a very wide
range of applications in chemical and biological research.
Design and synthesis of new fluorescent molecules is of contin-
uing interest for understanding the fundamental electronic struc-
ture of molecules, as well as for many applications in research
Benzo[de]anthracen-7-one dyes in particular have found many
applications because they provide a whole range of colours among
which the most popular are red, orange, and yellow [2]. Many
⇑
Corresponding author. Tel.: +371 67034889.
E-mail address: svetago7@gmail.com (S. Gonta).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.104

326
S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
benzanthrone derivatives are strongly fluorescent and that is why
widely used as a laser dye [3], daylight fluorescent pigment [4],
and lipophilic fluorescent probe for biochemical and medicinal
investigations [5,6]. These probes are used to study microviscosity
and fluidity of biological membranes because benzanthrone deriv-
atives such as many fluorophores are characterized by fluorescence
properties that are dependent on their environment.
General procedure for the synthesis of substituted amidines
3-Aminobenzanthrone was prepared by nitration of benzan-
throne and the following reduction of the obtained 3-nitro-deriva-
tive according to the literature procedure [20,21].
Procedure A (for amidines 3, 4, 8–10)
For several years our research group has been working on ben-
zanhrone dyes with D-p-A architecture, appeared to be particu-
To the solution of 0.50 g (2.0 mmol) amine I in 2–3 ml of appro-
priate amides, the phosphorus oxychloride (0.2 ml, 2.1 mmol) was
added dropwise under stirring. The resulting mixture was heated
for 3 h at 90–100 °C. After cooling to the ambient temperature,
the crude product that precipitated on pouring into 50 ml of 2%
NaOH water solution was filtered, washed with water, and dried.
larly interesting because these dyes ultimately lead to
prospective luminescent materials. In previous works, a series of
benzanthrone amines and amidines were prepared [7–9]. These
solvatochromic fluorescent substances have a large extinction
coefficient and a significant Stokes shift. The obtained results tes-
tify that the fluorescence of synthesized amidino derivatives is sen-
sitive to the change in polarity of surrounding, and fluorescence in
the red region (above 600 nm) of spectrum contributes to a high
analytical sensitivity of the method using these fluorophores [10].
Bearing this in mind, and in connection with our interest in syn-
thesis, characterization, and application of a fluorescent probe, we
have extended our preliminary investigations related to the fluo-
rescent derivatisation of the benzanthrone core by including new
substituents in the chromophoric system in order to find fluores-
cent dyes with prospective applications.
Procedure B (for amidines 2, 5, 6, 7)
To the solution of 0.50 g (2.0 mmol) amine and 2.5 mmol of
appropriate amides in 5 ml toluene, the phosphorus oxychloride
(0.2 ml, 2.1 mmol) was added dropwise under stirring. The result-
ing mixture was heated for 3 h at 90–100 °C. After cooling to the
ambient temperature, toluene evaporated in vacuum and the mix-
ture was neutralized by 50 ml of 2% NaOH water solution, the
crude product was filtered, washed with water, and dried.
Spectroscopic measurements
Compounds containing an amidine group have played impor-
tant roles as ligands for various complexes of s-, p-, d- and f-block
metals in organometallic chemistry [11]. It is known that substi-
tuted N-aryl amidines display an intense luminescence in solutions
[9,10,12]. Moreover, the use of amidinate-ligated iridium com-
plexes for fabrication of high efficiency phosphorescent organic
light-emitting devices has been recently demonstrated [13]. There-
fore it was of interest to prepare new luminescent dyes containing
an electron-donating amidine group and an electron-accepting car-
bonyl group linked by an aromatic spacer. Herein, we present the
facile synthesis and characterization of N-substituted amidino
derivatives based on benzo[de]anthracen-7-one. The optical prop-
erties and crystal structure of novel synthesized compounds is
studied in the present work. Quantum chemical calculations are
also presented in order to demonstrate the electronic structures
and properties of synthesized dyes.
Spectral properties of the investigated compounds were mea-
sured in chloroform and ethanol solutions with concentrations
10ꢁ5 M at an ambient temperature in 10 mm quartz cuvettes. All
solvents were of p.a. or analytical grade. The absorption spectra
were obtained using the UV–visible spectrophotometer ‘‘Specord’s
UV/vis’’. The fluorescence emission spectra were recorded on a
FLSP920 (Edinburgh Instruments Ltd.) spectrofluorimeter using
Rhodamine 6G (U0 = 0.88) as a standard.
X-ray crystallography study
Diffraction data were collected on a Bruker-Nonius KappaCCD
diffractometer using graphite monochromated Mo K
a radiation
(k = 0.71073 Å). The crystal structures were solved by direct meth-
ods and refined by full-matrix least squares. For further details, see
crystallographic data for these compounds deposited with the
Cambridge Crystallographic Data Centre as Supplementary Publi-
cations. Copies of the data can be obtained, free of charge, on appli-
cation to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK.
Experimental
General
All reagents were of analytical grade (Aldrich Chemical
Company) and were used as received. The progress of chemical
reactions and purity of products were monitored by a thin-layer
chromatography (TLC) on silica gel plates, Silufol UV254, 15 ꢀ 15,
0.2 mm, using the solvent system benzene/acetonitrile (3:1) as elu-
ent. Column chromatography on silica gel was carried out on the
Merck Kieselgel (230–240 mesh) with benzene as eluent. Melting
points were measured on a Kofler apparatus and were not cor-
rected. IR spectra were recorded on the SHIMADZU Prestige-21FT
spectrometer in KBr pellets. 1H NMR spectra were recorded on
the Bruker AVANCE300 spectrometer operating at 300 and
75 MHz in DMSO-d6 or CDCl3 (with TMS as an internal standard)
at an ambient temperature. The chromatomass spectroscopic stud-
ies were carried out using the Shimadzu QP2010 chromatograph
with EI ionization, 70 eV, the mass range 39–400 m/z. The thermal
gravimetric analyses (TG-DSC) were carried out with an Ex-
star6000 TG/DTA 6300 thermal analyzer with a heating rate of
10 K minꢁ1 in the temperature interval 30–400 °C. The quantum
chemical calculations were performed using the AM1 [14,15], ZIN-
DO/S [16–18] and ab initio (using DFT approach [19]) methods.
Results and discussion
Synthesis and characterization
A number of different methods are available for the synthesis of
substituted amidines. A traditional preparation method involves
condensation of secondary amine and amide in presence of
phosphorus oxychloride [22]. The target dyes were synthesized
in high yields by condensation of 3-amino-benzanthrone (1) with
R
N
NH2
NR1R2
POCl3
O
R
+
NR1R2
O
O
1
2-10
Fig. 1. Synthesis of benzanthrone amidines 2–10.

S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
327
Table 1
330–400 °C, the thermogravimetric curves of these compounds
show a major loss in weight, with decomposition temperatures
338, 379, 402, and 367 °C for derivatives 5–8, respectively. These
results confirm that the prepared dyes are thermally stable
compounds.
Substituents, yields and identification data for prepared dyes 2–10.
Compound
R
R1
R2
Yield
(%)
M.p.
(°C)
Rf (benzene:
acetonitrile 3:1)
2
3
4
5
6
7
8
9
Me
H
H
Me
H
H
H
Me
H
H
H
Et
H
H
Ph
Me
Et
4-MeC6H4
Ph
52
67
90
82
65
92
80
87
85
244.2
243.5
159.4
250.0
247.1
226.5
250.8
202.1
218.2
0.68
0.35
0.71
0.57
0.66
0.75
0.37
0.06
0.36
Spectroscopic features
Me Ph
Photophysical properties of dyes 2–8 consisting of donor (ami-
dine) and acceptor (carbonyl) units were investigated in various
organic solvents with different dielectric constants. Positions of
maxima of the absorption and emission bands of the studied dye
solutions are presented in Tables 2 and 3. Absorption and emission
spectra of amidines 2–8 was recorded in eight different organic
solvents with a wide range of polarities (see absorption and emis-
sion spectra for derivative 4 in Fig. 2). Generally speaking, it could
be seen that absorption spectra do not show significant variation
with solvents.
H
H
H
H
Me
Et
10
appropriate amides using phosphorus oxychloride as a condensa-
tion agent. This synthetic route is outlined in Fig. 1.
In this paper, development of two synthetic procedures (A and
B) is described. Method A was used for liquid amides, with its ex-
cess as reaction media. Method B was employed for solid amides,
using toluene as a solvent for reacting compounds. Target products
were readily obtained by recrystallization from chloroform or ben-
zene in good yields (Table 1).
The electronic absorption spectra of the prepared dyes (Fig. 3)
show bands around 210–230 nm, 250–280 nm, and a broad long-
wave band around 415–490 nm (log
e
= 3.68–4.43), which has a
charge transfer character, due to
p
?
pꢂ electron transfer during
The obtained amidine derivatives are crystalline compounds
colored from yellow to deep red. The reaction and purity of prod-
ucts were monitored by a thin-layer chromatography. IR, 1H
NMR, and mass spectroscopic studies confirmed the chemical
structure of the new dyes 2–10. The structures of prepared deriv-
atives were determined by NMR spectroscopy, based on the analy-
sis of HAH coupling constants and chemical shifts. In the 1H NMR
spectra of the dyes, the signals of appropriate alkyl group and mul-
tiplet signals (from d 6.50 to 9.00 ppm) of aromatic protons of ben-
zanthrone ring were found (see Supplementary Data). The
synthesized derivatives were characterized by their melting point,
TLC Rf value, absorption, and fluorescence maxima.
In the present work, the mass spectra of synthesized amidines
were studied. Almost all spectra have a peak of the 1-phenyl-naph-
thalene (M = 201) ion, which is usual for benzanthrone derivatives
losing C@O and amidine side chain [23]. Besides, similarly to other
N-containing derivatives of benzanthrone [24], the following peaks
the S0 ? S1 transition. The charge transfer in benzanthrone dyes
occurs from the electron donor–acceptor interaction between the
electron-donating substituents at a C-3 position and an electron-
accepting carbonyl group of the chromophoric system [3]. The
long-wave absorption maximum of the investigated derivatives
in chloroform solution is similar to that of initial 3-aminobenzan-
throne (470 nm), the absorption band of the amidines in ethanol
solution is hipsochromically shifted by 50–100 nm, if compared
to that of amine 1 (518 nm), due to the weaker interaction between
the amidine group and the
p-electron system of benzanthrone in
contrast to interaction between the amino group and the aromatic
core. It is interesting that amidines 5 in dimethylsulfoxide solution
has two long-absorption bands with the same extinction at 448
and 466 nm.
Emission spectra of the studied dyes were recorded in solution
and in solid state upon excitation of the samples at the lowest en-
ergy absorption band. The obtained compounds are strongly fluo-
rescent in solutions (except the dyes 3 and 7 in hexane) in the
region of 505–540 nm (in hexane) to 560–665 nm (in ethanol)
and display large bathochromic shifts (up to 4000 cmꢁ1) from hex-
ane to ethanol solutions with regard to the amino derivatives for
which this shift is about 1000 cmꢁ1 [7,8]. In comparison with
amine 1 (627 nm), all studied amidines showed a hypsochromic
shift of emission maxima in chloroform solutions. But in ethanol
solutions, a bathochromic shift (for amidines 2, 5) or a hypsochro-
mic shift are observed in comparison with amine 1 (659 nm).
The effect of polarity of the medium on fluorescence is more
pronounced than on the absorption spectrum. This is because the
intramolecular charge transfer effect leads to a large dipole
are observed:
a peak of molecular ion of unsubstituted
benzanthrone (M = 230), a peak of ions [BA-N@C]+ (M = 256) and
[BA-N@CH2]+ (M = 258), a peak of molecular ion of 3-aminoben-
zanthrone (M = 245), and
a
peak of ion [BA-N@CHACH2]+
(M = 270). In addition, the mass spectrum of dyes 2, 5, 6 and 7
has a Ph+ peak (M = 77) in contrast to other derivatives that have
no phenyl group.
The thermal stability of dyes is one of the key requirements in
some practical applications. In order to gain more insight into
these dyes, 5, 6, 7 and 8 were subjected to the thermogravimetric
analysis to investigate their thermal stabilities. The thermal stabil-
ity studies were performed at a heating rate of 10 °C/min. Above
Table 2
Absorption maxima of prepared dyes in organic solvents (concentration 10ꢁ5 M).
Absorption kabs (loge) (nm)
2
3
4
5
6
7
8
9
10
Hexane
Benzene
CHCl3
429 (4.16)
445 (4.21)
448 (4.18)
448 (4.26)
457 (4.16)
466 (4.17)
464 (4.21)
468 (4.22)
–
443 (4.35)
461 (4.31)
460 (4.25)
465 (4.30)
474 (4.24)
476 (4.12)
484 (4.31)
491 (4.35)
430 (3.68)
446 (3.90)
448 (3.94)
448 (3.88)
448 (3.87)
464 (3.80)
465 (3.83)
448 (4.00)
466 (4.00)
426 (3.69)
444 (4.22)
448 (4.27)
448 (4.24)
458 (4.22)
462 (4.15)
468 (4.23)
474 (4.21)
434 (4.11)
448 (4.24)
460 (4.26)
449 (4.26)
461 (4.24)
465 (4.13)
470 (4.19)
475 (4.18)
–
432 (3.96)
447 (3.91)
448 (4.11)
449 (4.11)
449 (4.06)
448 (4.07)
467 (4.14)
473 (4.13)
427 (4.13)
443 (4.06)
448 (4.09)
448 (4.13)
447 (3.99)
448 (3.99)
472 (4.06)
472 (4.02)
418 (4.40)
414 (4.41)
420 (4.42)
420 (4.43)
420 (4.40)
427 (4.40)
430 (4.41)
418 (4.40)
414 (4.41)
420 (4.43)
420 (4.43)
420 (4.40)
427 (4.40)
430 (4.41)
EtOAc
Acetone
Ethanol
DMF
DMSO

328
S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
Table 3
Fluorescence maxima of prepared dyes in organic solvents (concentration 10ꢁ5 M).
Fluorescence kem (U0) (nm)
2
3
4
5
6
7
8
9
10
Hexane
Benzene
CHCl3
527 (0.28)
554 (0.37)
603 (0.29)
584 (0.30)
605 (0.18)
663 (0.16)
624 (0.15)
634 (0.16)
532 (0.70)
556 (0.74)
615 (0.72)
586 (0.72)
603 (0.65)
655 (0.70)
615 (0.41)
635 (0.28)
529 (0.67)
558 (0.81)
603 (0.73)
585 (0.59)
608 (0.54)
662 (0.56)
628 (0.47)
652 (0.23)
536 (0.62)
544 (0.59)
580 (0.58)
571 (0.57)
590 (0.45)
644 (0.29)
605 (0.23)
620 (0.21)
–
507
525 (0.59)
565 (0.63)
608 (0.52)
596 (0.51)
644 (0.46)
665 (0.15)
636 (0.12)
644 (0.14)
513 (0.29)
543 (0.32)
585 (0.28)
569 (0.23)
590 (0.18)
643 (0.08)
614 (0.09)
624 (0.11)
508 (0.43)
527 (0.39)
513 (0.37)
542 (0.28)
560 (0.33)
532 (0.29)
544 (0.22)
546 (0.84)
577 (0.78)
569 (0.77)
593 (0.79)
641 (0.83)
603 (0.53)
633 (0.31)
549 (0.79)
546 (0.78)
551 (0.75)
543 (0.68)
573 (0.63)
535 (0.49)
572 (0.37)
EtOAc
Acetone
Ethanol
DMF
DMSO
transition is almost the same and, consequently, a net change is not
observed in the absorption spectra recorded in each case. As seen
from Table 4, the Stokes shift was bigger in the case of more polar
solvents, indicating the intramolecular charge transfer character of
the excited state.
The solvatochromic effect in emission is clearly observed under
a UV lamp irradiation, as it is depicted in Fig. 4, where solutions of
dye 4 in n-hexane, benzene, chloroform, acetone, and dimethyl-
sulfoxide are shown. A net red shift can be observed in Fig. 4 show-
ing the change from n-hexane through to dimethylsulfoxide in
accordance with the increasing polarity of the solvents.
As seen from Table 2, with an increase of solvent polarity, the
fluorescence quantum yields of the studied dyes decreased with
a remarkable red shift. In a polar solvent with a relatively high
hydrophilicity, the conformation of the dye molecule is probably
twisted, and the twisted intramolecular charge transfer state
(TICT) emerges, which red shifts and weakens the emission of fluo-
rophore [1,25]. Therefore, the red shift and weakening of intensity
with growing solvent polarity might be caused by TICT processes.
Interesting results were received from photophysical properties
of the prepared dyes in the solid state. Crystals of amidines 2, 3, 4, 5
and 10 were obtained by recrystallization from benzene solutions.
Compounds 7 and 8, however, were precipitated in an amorphous
state after recrystallization from various solutions (chloroform,
benzene, ethanol, and acetone solutions). Amidine 9 forms crystals
as solvate with benzene, but these transform into amorphous pow-
der after quickly losing molecules of the solvent. Interestingly,
derivatives 3 and 10 also form solvate but with water molecules,
catching it during precipitation from the reacting mixture. It was
found that dyes 3 and 4 do not emit in crystals, but have a weak
luminescence in amorphous powder. Compounds 2, 5, 6, 7, 9 and
10 exhibit a solid-state luminescence in both crystalline and amor-
phous states. Dyes 5 and 9 are characterized by an especially high
solid-state luminescence (see Fig. 5). Amidines 2, 5, 6, 7, 9 and 10
exhibit a bright yellow–orange or red luminescence upon excita-
tion at both 254 and 365 nm. Maxima of solid-state luminescence
of dyes 2, 5, 6, 7, 9 and 10 are observed at 602, 632, 589, 645, 614,
and 653 nm, respectively.
Fig. 2. The UV–vis (a) and fluorescence emission (b) spectra of amidine 4 (1 ꢀ 10–
5 mol lꢁ1) in hexane (1), benzene (2), ethyl acetate (3), aceton (4), chloroform (5),
DMSO (6) and ethanol (7), respectively.
To elucidate the effect of crystal packing on solid-state lumines-
cence properties, we performed an X-ray crystallographic analysis.
Crystal structure analysis
moment in the excited state. Therefore the studied dyes are more
polar in the excited than ground state. The different behavior in
absorption and emission is related to the magnitude of the solvent
effect on the energy of states during electronic transition. There, it
can be observed that the lack of effect on the absorption solvato-
chromism might be due to the fact that both states, ground and ex-
cited, are not equally stabilized by the different solvents under
study (and so no significant variations occur in the energy of both
states) or, on the other hand, the same degree of stabilization could
occur for both. Therefore, for these two possibilities, the energy of
The structures of 2–5 and 10 were established by the X-ray dif-
fraction. The main crystallographic data and refinement parame-
ters of the crystal structures are listed in Table 5.
The molecular structures of the investigated compounds and
atomic thermal ellipsoid are shown in Fig. 6. The molecular struc-
ture of 2 is characterized by two plane fragments: one of them is a
benzanthrone system and the second is a phenylacetamidine frag-
ment. The dihedral angle between the planes of these fragments is
equal to 63.8°. In the crystal structure, there are intermolecular

S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
329
Fig. 3. Absorption spectra of amidines 2–8 in ethanol solution.
hydrogen bonds of NHꢃ ꢃ ꢃO type with length 2.910(3) Å
(Hꢃ ꢃ ꢃO = 1.89 Å, NAHꢃ ꢃ ꢃO = 167°). The chains along crystallo-
graphic axis a are formed in the crystal structure by means of these
H-bonds. The graph set of these chains is C (10) in accordance with
the classification of hydrogen-bond motifs in crystals [31]. There
Table 4
Stokes shift of prepared dyes in organic solvents (concentration 10ꢁ5 M).
Stock shift (cmꢁ1
)
2
3
4
5
6
7
8
9
10
are also
p–p stacking interactions in the structure: two benzan-
Hexane
4335
3776 4352 4817
–
4100 3926
Benzene 4421 4238 3706 4501 4141 4006 5708 4672 4157
CHCl3
EtOAc
Acetone 5353 5360 4513 5874 4885 4829 5394 6744 5422
Ethanol
DMF
throne systems connected by the center of inversion partially over-
lap; the distance between parallel benzanthrone planes is 3.414 Å.
In accordance with X-ray analysis data of 3 hydrate, there are
two independent molecules in the asymmetric unit. Two mole-
cules of 3 and two water molecules bond by means of OHꢃ ꢃ ꢃO
and NHꢃ ꢃ ꢃO types intermolecular hydrogen bonds and form
pseudocentrosymmetrical rings with the graph set R24ð24Þ in accor-
dance with [31]. The lengths of the OHꢃ ꢃ ꢃO type hydrogen bonds
5737 5180 5479 5737 5080 4408 5840 5874 5227
5198 4317 4440 5227 4808 4697 5661 5493 4746
6376 5953 5741 6446 6117 5904 6358 7283 6769
5526 4622 4401 5581 4839 4693 4727 5690 4899
5595 4874 4619 6984 4968 5255 5773 5614 5160
6122
DMSO
Fig. 4. Fluorescence solvatochromism of dye 4 in organic solvents. From left to right: n-hexane, benzene, chloroform, acetone and dimethylsulfoxide.

330
S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
Fig. 5. Solid-state fluorescence of dyes 5 and 9 upon UV excitation at 365 nm.
Table 5
Crystal data for the studied compounds.
2
3
4
5
10
Brutto-formula
Formula weight
Crystal system
a (Å)
b (Å)
c (Å)
C
25H18N2O
C19H14N2OꢃH2O
304.34
C22H20N2O
328.42
C26H20N2O
376.46
C20H16N2OꢃH2O
318.36
362.43
Monoclinic
6.8530(5)
22.691(2)
11.7584(9)
98.117(4)
1810.1(2)
P 21/c
Orthorhombic
8.6174(2)
13.6136(3)
25.5817(6)
90.0
3001.1(1)
P bc21
8
Monoclinic
8.1933(3)
17.5476(6)
11.7909(5)
96.874(1)
1683.0(1)
P 21/n
Monoclinic
13.1472(4)
7.2323(2)
20.0270(5)
90.628(2)
1904.14(9)
P 21/n
Monoclinic
13.1890(5)
8.6849(4)
13.5801(5)
93.920(3)
1551.9(1)
P 21/c
b (°)
V (Å3)
Space group
Z
4
4
4
4
l
(mmꢁ1
)
0.082
1.330
57.0
0.089
1.347
60.0
0.080
1.296
60.0
0.080
1.313
60.0
0.089
1.363
54.0
Density (calc.) (g/cm3)
2hmax for data (°)
Reflection collected
6579
8053
7699
8222
6244
Independent reflections 4551 (Rint = 0.046)
4437 (Rint = 0.047)
3066 (n = 2)
0.057
4767 (Rint = 0.034)
2951 (n = 3)
0.061
5349 (Rint = 0.034)
3118 (n = 3)
0.047
3670 (Rint = 0.069)
2194 (n = 2)
0.096
Reflections with I > n
r
(I) 2383 (n = 3)
0.064
Final R-factor
wR2 index for all data
Temperature (°C)
Using programs
0.231
ꢁ100
0.169
ꢁ100
0.232
ꢁ100
0.282
ꢁ100
0.347
ꢁ80
SIR97 [26], maXus [27] SHELXS97 [28], SHELXL97 [28] SIR92 [29], maXus [27] DETMAX [30], maXus [27] SHELXS97 [28], SHELXL97 [28]
CCDC deposition number CCDC 884478
CCDC 884884
CCDC 884480
CCDC 884481
CCDC 884482
are 2.918(4) and 2.921(4) Å; these values for the NHꢃ ꢃ ꢃO bonds are
2.961(4) and 2.962(4) Å. Furthermore, OHꢃ ꢃ ꢃN type intermolecular
H-bonds are found. The lengths of these hydrogen bonds are
2.875(4) and 2.897(4) Å. These bonds form new motifs in the crys-
tal structure, namely chains of C(6). The H-bonds in this structure
create the molecular layers which are parallel to the crystallo-
graphic plane (001). Fig. 7 illustrates a view of the layer along
the crystallographic axis c. There are weak stacking interactions
between benzanthrone systems of molecules, with distances corre-
sponding to the sum of van der Waals radii.
An intermolecular contact between amidine carbon and car-
bonyl oxygen (3.241(3) Å) should be noted in crystal structures
of 4; this contact can be considered as a weak CHꢃ ꢃ ꢃO type hydro-
gen bond (Hꢃ ꢃ ꢃO = 2.57 Å, NAHꢃ ꢃ ꢃO = 124°). The stacking interac-
tion is practically absent (the distance between parallel
benzanthrone systems in the crystal is equal to 3.728 Å).
shown in Fig. 9. These peculiarities of crystal structure 5 perhaps
are grounds of/might make the basis for luminescent properties
of 5.
Dye 10 is a hydrate compound; there is an intermolecular
H-bond system between water molecules and 10, which is similar
to 3. There are also weak stacking interactions between benzan-
throne systems of 10.
It is known that strong intermolecular p–p interactions and/or
continuous intermolecular hydrogen bonding between neighbor-
ing molecules is a principal factor of luminescence quenching in
the solid state [32]. But according to our data, most efficient lumi-
nescence is observed in the dye characterized by a stronger
p–p
stacking interactions, dye 5 in our case. Such packing of crystal 5
likely promotes the exciton–phonon interactions which are intensi-
fied by shortened contacts in the crystal structure. Effective lumines-
cence in solid state is also attributed to restricted intramolecular
motions [1]. A more detailed study with a larger number of
samples is needed for better understanding the correlations be-
tween solid-state luminescence and molecular packing structure
of benzanthrone dyes.
Fig. 8 shows projections of crystal structure 5 along the mono-
clinic axis. Similar to structure 2, there are intermolecular H-bonds
of the NHꢃ ꢃ ꢃO type (Nꢃ ꢃ ꢃO = 2.955(2) Å, Hꢃ ꢃ ꢃO = 2.05 Å,
NAHꢃ ꢃ ꢃO = 174°) with the graph set C(10). In the crystal structure
by means of these H-bonds the chains are formed along crystallo-
ꢀ
graphic direction ½101ꢄ. The crystal structure is characterized by
Quantum chemical calculations
strong p–p stacking interactions: benzanthrone systems lie paral-
lel to the crystallographic plane ð121Þ, and the distance between
the planes is 3.348 Å (the shortest contact between two carbons
is 3.351 Å). The perspective view of the stack of molecules 5 is
To better understand the nature of electronic spectra of benzan-
thrones 2–10, the quantum chemical calculations were carried out.
For each compound, the geometry optimization for two possible

S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
331
Fig. 6. ORTEP diagrams of the X-ray structure of 2 (a), 3 (b), 4 (c), 5 (d) and 10 (e).
isomers was performed by the AM1 method. As the calculated
electronic energies of isomers A and B (Fig. 10) do not show a big
difference, and rotation barrier of the amidine group is also low,
the preferred structures are those with a different orientation of
the amidine group in each case (crystallographic data, though,
prove that structures of type A have priority).
The ab initio calculations using DFT give the same geometries
(the differences are insignificant) and almost the same distribution
of charges (differences are also small), but HOMO energies calcu-
lated with this method (ꢅꢁ5.2 eV) look too high. It is obvious that
the first ionization potential must be greater (about (ꢁ) 7–8 eV) for
this class of compounds.
The substituents R, R1 and R2 do not show a great influence on
HOMO and LUMO energies calculated by any of the used method,
although their influence on HOMO is more remarkable than on
LUMO. An insignificant influence of substituents R, R1 and R2 on
charges of the nearest carbon atoms is noticed in benzanthrone
system. The substituents mainly have an effect on the carbon atom
of amidine group, where charges vary from ꢅ0.08 to 0.19 e. On the
other hand, the substituents in the amidine fragment affect notably
on the molecules dipole moments. The dipole moments vary,
depending on substituents, from 3.9 D to 7.7 D (AM1 method) or
from 6.0D to 10.4D (ZINDO/S method) (Table 6). The dipole
moments grow if the molecules in the amidine fragment have elec-
tron-donoring groups (Me, Et). Also, molecules with the structure
of type A show greater dipole moments than structures of type B
(structures 5 and 8, where amidine substituent is less or more
shifted out from the benzanthrone plane, make an exception).
The electronic spectra of benzanthrones were calculated using
the ZINDO/S method. Calculations show an intensive (oscillator
strength is about 0.6–0.9), long-wave absorption band at 370–
380 nm. This absorption is generally an electron shift from HOMO
to LUMO. Both orbitals are mainly localized in the benzanthrone
fragment (Fig. 11), whereas the amidine group investment in this
shift is small.
Long-wave absorption causes a notable increase of the system
dipole moment by about 3–4 D (Table 6). So, the excited state
dipole moments reach 10–14.7 D. As the molecules are strongly
polar compounds, it is not surprising that in real spectra, the
long-wave absorption band is shifted by about 60–70 nm due to
solvation effect (Table 2). This shift is caused by intermolecular
interactions or interactions of molecules with the solvent, and it
confirms the large solvatochromic effect. The next notable absorp-
tion band for amidines 2–10 is at about 280–300 nm. These bands

332
S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
Fig. 7. A molecular layer in the crystal structure of dye 3.
Fig. 8. Projection of the crystal structure of dye 5 on crystallographic plane (010).

S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
333
Fig. 9. A stack of molecules in the crystal structure of dye 5.
NR R
R
1
2
N
N
NR R
R
1
2
O
O
type A
type B
Fig. 10. Possible benzanthrone amidine isomers.
Fig. 11. HOMO and LUMO localization for amidine 3A (ZINDO/S).
are characterized by a not so strong absorption (oscillator strength
is about 0.1), and these mostly consist of the electron shift between
different orbitals.
were characterized by NMR, FT-IR, UV–vis and X-ray crystallogra-
phy. The results of thermogravimetric analysis showed that these
dyes were thermally stable and therefore may have various appli-
cations. The synthesized dyes absorb at 410–490 nm with high
extinction coefficients, have relatively large Stokes’ shifts (up to
4800 cmꢁ1 in hexane and 7300 cmꢁ1 in ethanol), and emit at
505–665 nm. Some of the prepared dyes exhibit strong lumines-
cence in solid state. The relationships between the solid-state
Conclusion
Synthesis and spectral properties of novel benzanthrone
N-substituted amidine dyes were described. Novel dyes were syn-
thesized in good yields (52–92%) via the condensation reaction of
3-aminobenzanthrone with corresponding amides. Compounds
Table 6
Quantum chemical calculations of benzanthrone amidines 2–10.
Structure
AM1
ZINDO/S
ab initio B3LYP/6-31G(d)
kmax (nm) (oscillator strength)
lground (D)
lexcited (D)
HOMO (eV)
LUMO (eV)
HOMO (eV)
LUMO (eV)
HOMO (eV)
LUMO (eV)
2A
2B
3A
3B
4A
4B
5A
5B
6A
6B
7A
7B
8A
8B
9A
9B
10A
10B
ꢁ8.24
ꢁ8.37
ꢁ8.03
ꢁ8.29
ꢁ8.19
ꢁ8.23
ꢁ8.26
ꢁ8.33
ꢁ8.40
ꢁ8.28
ꢁ8.14
ꢁ8.27
ꢁ8.03
ꢁ8.20
ꢁ7.95
ꢁ7.86
ꢁ8.00
ꢁ8.26
ꢁ1.18
ꢁ1.27
ꢁ1.16
ꢁ1.23
ꢁ1.17
ꢁ1.13
ꢁ1.20
ꢁ1.23
ꢁ1.29
ꢁ1.27
ꢁ1.17
ꢁ1.22
ꢁ1.27
ꢁ1.20
ꢁ1.12
ꢁ1.21
ꢁ1.13
ꢁ1.20
ꢁ7.22
ꢁ7.29
ꢁ7.18
ꢁ7.30
ꢁ7.25
ꢁ7.25
ꢁ7.26
ꢁ7.27
ꢁ7.35
ꢁ7.31
ꢁ7.22
ꢁ7.28
ꢁ7.22
ꢁ7.30
ꢁ7.12
ꢁ7.07
ꢁ7.17
ꢁ7.29
ꢁ0.93
ꢁ0.95
ꢁ1.00
ꢁ0.97
ꢁ0.94
ꢁ0.94
ꢁ0.94
ꢁ0.93
ꢁ1.00
ꢁ0.99
ꢁ0.97
ꢁ0.96
ꢁ1.11
ꢁ1.02
ꢁ0.95
ꢁ1.03
ꢁ0.99
ꢁ0.96
ꢁ5.37
ꢁ5.29
ꢁ5.22
ꢁ5.28
ꢁ5.13
ꢁ5.20
ꢁ5.25
ꢁ5.22
ꢁ5.34
ꢁ5.40
ꢁ5.17
ꢁ5.27
ꢁ5.29
ꢁ5.32
ꢁ5.16
ꢁ5.16
ꢁ5.20
ꢁ5.19
ꢁ2.08
ꢁ2.10
ꢁ1.98
ꢁ1.99
ꢁ1.96
ꢁ1.98
ꢁ2.07
ꢁ1.98
ꢁ2.10
ꢁ2.16
ꢁ1.96
ꢁ2.04
ꢁ2.01
ꢁ2.04
ꢁ1.95
ꢁ1.94
ꢁ1.97
ꢁ1.96
371.8 (0.7062)
368.1 (0.6058)
382.6 (0.8733)
372.5 (0.6259)
374.9 (0.6199)
374.4 (0.6709)
369.7 (0.6238)
370.5 (0.7168)
370.6 (0.7254)
368.1 (0.6444)
374.1 (0.8148)
368.2 (0.6645)
388.0 (0.8173)
374.6 (0.7786)
383.7 (0.8988)
394.8 (0.8989)
382.9 (0.8720)
372.7 (0.6233)
9.3
6.1
10.0
6.2
8.1
7.6
7.0
8.1
6.8
6.0
9.3
6.5
6.6
8.8
10.4
7.6
10.1
6.4
13.6
9.9
14.0
10.0
12.5
11.7
10.8
12.2
10.6
9.9
13.3
10.2
10.4
12.7
14.7
11.8
14.1
10.1

334
S. Gonta et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 325–334
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