N,N-dimethylamino-phenylene-acetylene scaffolding: Structure-property relationships

Article abstract of DOI:10.1016/j.dyepig.2014.04.018

Seven model donor-substituted phenyleneethynylene molecules with up to three 1,4-phenylene and two acetylenic units were synthesized by Suzuki-Miyaura and Sonogashira cross-coupling reactions and further studied by absorption and emission spectra and theo

Full text of DOI:10.1016/j.dyepig.2014.04.018

Dyes and Pigments 108 (2014) 50e56
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N,N-dimethylamino-phenyleneeacetylene scaffolding:
Structureeproperty relationships
,
a
b
ꢀa
ꢀ
ꢀa
ꢀ
Numan Almonasy a , Filip Bures , Milos Nepras , Hana Prichystalová , Günter Grampp
*
a Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice CZ-532 10,
Czech Republic
b Institute of Physical and Theoretical Chemistry, TU Graz, Stremayergasse 9/I, Graz A-8010, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven model donor-substituted phenyleneethynylene molecules with up to three 1,4-phenylene and two
acetylenic units were synthesized by SuzukieMiyaura and Sonogashira cross-coupling reactions and
Received 8 March 2014
Received in revised form
3 April 2014
Accepted 14 April 2014
Available online 24 April 2014
further studied by absorption and emission spectra and theoretical calculations. The
p-system between
the N,N-dimethylamino group and terminal acetylene was systematically elongated which allowed
elucidation of the fundamental structureeproperty relationships. Structural factors such as molecular
length and chromophore planarity proved to be crucial for the observed spectroscopic behavior.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Acetylene
Phenylene
p-System
Absorption
Emission
Donor
1. Introduction
transfer (ICT) [15,20]. Such arrangement assures molecule polariza-
tion and accounts for their unique properties. We have recently
Extended and functionalized organic
p
-conjugated systems are
synthesized donor 4,5-disubstituted pyrazine-2,3-dicarbonitrile
pushepull chromophores with systematically extended -system
which comprised of 1,4-phenylene and acetylenic subunits [21]. In
these De eA molecules, the ICT has been investigated and its impact
on the molecular properties has been evaluated. Although several
structureeproperty relationships studies on De eA system exist to
date [20e24], less attention has been paid to the influence of the
donor-substituted part of the molecule (De ) [25e28]. Hence, in this
currently attracting great interest by materials scientists [1e3].
Amongothers, cyclicand linearhydrocarbons, benzeneand acetylene
represent probably the most essential building blocks for the con-
struction of large scaffolds, systems and poly(p-phenyl-
eneethynylene) derivatives (PPE) [4e7]. These two units were widely
p
p
p
employed as a part of the p-conjugated systems of various materials
for organic photovoltaics (OPVCs) [8,9], organic light emitting diodes
(OLEDs) [10,11], dye sensitizing solar cells (DSSCs) [12], sensors and
chelating ligands [13,14], acceptor units [15], and active biomolecules
[16,17]. The main reason of their popularity in material science can be
attributed to direct availability, well-known chemistry and reactivity,
p
work we will focus on N,N-dimethylamino-substituted molecules
comprising 1,4-phenylene and acetylenic units with systematically
extended p-system length.
and relative stability. Electron donor/acceptor (D/A) substituted
systems based on (hetero)aromates in combination with multiple
p-
2. Experimental
bonds known as pushepull chromophores are presently one of the
most widely designed, prepared and investigated class of organic p-
2.1. Materials and methods
conjugated materials [15,18,19]. These molecules often contain 1,4-
phenylene and acetylenic units connecting donor and acceptor
parts and allow direct DeA interaction and intramolecular charge
The absorption spectra were measured on an UV/Vis Perkine
Elmer Lambda 35 spectrophotometer at room temperature. The
steady-state fluorescence spectra were measured on a Perkine
Elmer LS 55 spectrophotometer. The instrument provides corrected
excitation spectra directly; the fluorescence spectra were corrected
for the characteristics of the emission monochromator and for the
* Corresponding author.
E-mail address: numan.almonasy@upce.cz (N. Almonasy).
http://dx.doi.org/10.1016/j.dyepig.2014.04.018
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

N. Almonasy et al. / Dyes and Pigments 108 (2014) 50e56
51
photomultiplier response. For fluorescence measurements, very
weakly absorbing solutions (optical density w 0.05 at the exciting
wavelength in 1-cm cell) were used. Following spectral-grade sol-
vents were used for the measurements: dibutylether (DBE), ethyl-
acetate (EtAc), acetonitrile (MeCN), and 1,4-dioxane (DO).
The relative fluorescence quantum yields were calculated ac-
cording to the relations qF ¼ n2$P$K/A, where K ¼ qFs$As/ns2$Ps; n
denotes the reflective index of a solvent, P denotes the fluorescence
area, A denotes the absorbance at excitation wavelength, subscript s
denotes the values for the used fluorescence standard.
The fluorescence quantum yields of the studied compounds
were measured using quinine sulfate (qF ¼ 0.54 in 0.5 mol/L H2SO4)
[29] as the standard. Deaeration of the samples by bubbling with N2
did not affect the spectra and qF; therefore, the data presented here
correspond to aerated solutions. By using anthracene as standard
for some of the studied compounds we obtained practically iden-
tical qF as for the quinine sulfate standard.
Filtrations through a plug were carried out with silica gel 60
(particle size 0.040e0.063 mm, 230e400 mesh; Merck) and
commercially available solvents. Thin-layer chromatography (TLC)
was conducted on aluminum sheets coated with silica gel 60 F254,
obtained from Merck, with visualization by a UV lamp (254 or
through the solution for 10 min, whereupon [PdCl2(PPh3)2] (35 mg;
0.05 mmol) and Na2CO3 (116 mg; 1.1 mmol) were added, and the
reaction mixture was stirred under argon at 65 ꢀC for 5 h. The re-
action was diluted with water and extracted with CH2Cl2
(3 ꢁ 100 mL). The combined organic extracts were dried (Na2SO4)
and the solvents were evaporated in vacuo. The crude product was
purified by filtration through a plug (SiO2; CH2Cl2/hexane 1:1).
2.3. General method for Sonogashira cross-coupling
(compounds 5 and 7)
Terminal acetylenes
2
or
3
(1.0 mmol) and 4-
iodophenylethynyltrimethylsilane 8 (300 mg; 1.0 mmol) were
dissolved in triethylamine (15 mL) and dry THF (100 mL). Argon
was bubbled through the solution for 10 min, whereupon
[PdCl2(PPh3)2] (35 mg; 0.05 mmol) and CuI (19 mg; 0.1 mmol) were
added, and the reaction mixture was stirred under argon at 50 ꢀC
for 5 h. The solvents were evaporated in vacuo and the crude
product was purified by filtration through a plug (SiO2; CH2Cl2/
hexane 1:1).
}
360 nm). Melting points (m.p.) were measured with a Buchi B-540
2.4. General method for TMS-deprotection
melting-point apparatus in open capillaries and were uncorrected.
1H and 13C NMR spectra were recorded at 400 and 100 MHz,
respectively, with a Bruker AVANCE 400 instrument at 25 ꢀC.
Chemical shifts are reported in ppm relative to the signal of Me4Si.
The residual solvent signal in the 1H and 13C NMR spectra was used
as an internal reference (CDCl3 7.25 and 77.23 ppm). IR spectra were
recorded with a PerkineElmer FTIR Spectrum BX spectrometer.
Mass spectra were measured with an LCeMS Micromass Quattro
Micro API (Waters) instrument with a direct input (ESI, CH3OH,
mass range 200e1000 Da). Elemental analyses were performed
with an EA 1108 Fisons instrument.
TMS-protected acetylene 4e7 (0.5 mmol) was dissolved in THF
(50 mL), cooled to 0 ꢀC and tetrabutylammonium fluoride trihy-
drate TBAF (50e100 mg) was added and the reaction mixture was
followed by TLC (SiO2; CH2Cl2/hexane 1:1) and stirred for 2e3 h.
The reaction was diluted with water (50 mL), extracted with CH2Cl2
(3 ꢁ 100 mL), the combined organic extracts were dried (Na2SO4)
and the solvents were evaporated in vacuo. The crude product was
purified by filtration through a plug (SiO2; CH2Cl2/hexane 1:1).
Compounds 1e3 were synthesized earlier [30], the synthesis of
compounds 4e7 is shown in Scheme 1 [21], less-substituted com-
plementary analogs 1a, 2aeb, 3aeb, 4aeb and 7aeb are known
compounds.
2.5. Compound 4
Yellowish solid, overall yield over two steps 81%, Rf ¼ 0.28 (SiO2;
CH2Cl2/hexane 1:1), m.p. 242 ꢀC (dec.). 1H NMR (400 MHz, CDCl3,
25 ꢀC):
d
¼ 7.65e7.53 (m, 10H; Ar), 6.81 (d, 3J(H,H) ¼ 8.8 Hz, 2H; Ar),
2.2. General method for SuzukieMiyaura cross-coupling
(compounds 4 and 6)
3.12 (s, 1H; CH), 3.00 ppm (s, 6H; N(CH3)2). 13C NMR (100 MHz,
CDCl3, 25 ꢀC):
d
¼ 150.3, 141.5, 140.9, 137.9, 132.8, 128.5, 127.8, 127.5,
126.9, 126.8, 120.9, 83.9, 77.9, 40.8 ppm. IR (neat):
n
¼ 1665, 1595,
Compound
9
or
10
(1.0
mmol)
and
4-
1488, 1353, 1198, 945, 809 cmꢂ1. MS (ESI): m/z (%): 298 [M þ 1]þ.
Elemental analysis: calcd (%) for C22H19N (297.39): C 88.85, H 6.44,
N 4.71; found C 88.55, H 6.37, N 4.68.
iodophenylethynyltrimethylsilane 8 (300 mg; 1.0 mmol) were
dissolved in THF (75 mL) and water (15 mL). Argon was bubbled
Scheme 1. Synthetic approach to NMe2-substituted extended p-phenyleneethynylenes 4e7.

52
N. Almonasy et al. / Dyes and Pigments 108 (2014) 50e56
Table 1
2.6. Compound 5
Molecular structures of the investigated compounds 1e7.
Yellowish solid, overall yield over two steps 86%, Rf ¼ 0.32 (SiO2;
CH2Cl2/hexane 1:1), m.p. 216e218 ꢀC. 1H NMR (400 MHz, CDCl3,
Comp. Substituents Central -linker
p
X
Y
25 ꢀC):
d
¼ 7.55e7.45 (m, 10H; Ar), 6.79 (d, 3J(H,H) ¼ 8.8 Hz, 2H; Ar),
3.17 (s, 1H; CH), 3.00 ppm (s, 6H; N(CH3)2). 13C NMR (100 MHz,
CDCl3, 25 ꢀC):
d
¼ 150.4, 141.5, 132.3, 132.2, 131.6, 128.1, 127.8, 126.2,
1
NMe2
H
C^CH
C^CH
C^CH
124.2, 121.8, 120.4, 112.9, 92.0, 89.2, 83.6, 79.0, 40.7 ppm. IR (neat):
n
¼ 1670, 1594, 1507, 1346, 1217, 948, 813 cmꢂ1. MS (ESI): m/z (%):
1a
2
322 [M þ 1]þ. Elemental analysis: calcd (%) for C24H19N (321.41): C
89.68, H 5.96, N 4.36; found C 89.19, H 5.88, N 4.29.
NMe2
2.7. Compound 6
2a
2b
H
H
C^CH
H
Yellowish solid, overall yield over two steps 82%, Rf ¼ 0.30 (SiO2;
CH2Cl2/hexane 1:1), m.p. 189e190 ꢀC. 1H NMR (400 MHz, CDCl3,
25 ꢀC):
d
¼ 7.60e7.53 (m, 8H; Ar), 7.42 (d, 3J(H,H) ¼ 8.8 Hz, 2H; Ar),
3
NMe2
C^CH
6.66 (d, 3J(H,H) ¼ 8.8 Hz, 2H; Ar), 3.14 (s, 1H; CH), 2.99 ppm (s, 6H;
N(CH3)2). 13C NMR (100 MHz, CDCl3, 25 ꢀC):
133.0, 132.8, 131.9, 127.0, 127.0, 123.9, 121.3, 112.0, 110.1, 92.0, 87.4,
d
¼ 150.3, 141.0, 139.2,
3a
3b
H
H
C^CH
H
83.7, 78.1, 40.4 ppm.; IR (neat):
n
¼ 1668,1595,1515,1353,1136, 944,
818, 776 cmꢂ1. MS (ESI): m/z (%): 322 [M þ 1]þ. Elemental analysis:
calcd (%) for C24H19N (321.41): C 89.68, H 5.96, N 4.36; found C
89.85, H 5.98, N 4.39.
4
NMe2
C^CH
4a
4b
H
H
C^CH
H
2.8. Compound 7
Yellowish solid, overall yield over two steps 83%, Rf ¼ 0.50 (SiO2;
CH2Cl2/hexane 1:1), m.p. 238e239 ꢀC. 1H NMR (400 MHz, CDCl3,
5
6
7
NMe2
C^CH
25 ꢀC):
d
¼ 7.46 (br s, 8H; Ar), 7.40 (d, 3J(H,H) ¼ 8.8 Hz, 2H; Ar), 6.65
C^CH NMe2
(d, 3J(H,H) ¼ 8.8 Hz, 2H; Ar), 3.17 (s, 1H; CH), 2.99 ppm (s, 6H;
N(CH3)2). 13C NMR (100 MHz, CDCl3, 25 ꢀC):
131.7, 131.7, 131.4, 124.6, 123.9, 122.1, 121.9, 112.0, 109.8, 93.2, 91.6,
d
¼ 150.5, 133.0, 132.3,
NMe2
C^CH
90.5, 87.5, 83.5, 79.2, 40.4 ppm. IR (neat):
n
¼ 1606,1590, 1520, 1347,
1120, 939, 836, 819 cmꢂ1. MS (ESI): m/z (%): 346 [M þ 1]þ.
Elemental analysis: calcd (%) for C26H19N (345.44): C 90.40, H 5.54,
N 4.05; found C 90.77, H 5.60, N 4.09.
7a
7b
H
H
C^CH
H
3. Results and discussion
terminal acetylene carbon of the optimized molecule geometry)
seems to be the principal structural factor affecting the position of
the absorption maxima, the following observations and compari-
sons must also be taken into account:
3.1. Synthesis
Molecular structure of the studied molecules 1e7 is shown in
Table 1. Compounds 1e3 were synthesized by the methods
described in our earlier Ref. [30], the synthesis of extended mole-
cules 4e7 is outlined in Scheme 1 and follows protocols shown in
Ref. [21] and the experimental part. Two step synthesis involves
SuzukieMiyaura and Sonogashira cross-coupling reactions of
boronic acid pinacol esters 9e10 [30] and terminal acetylenes 2e3
with 4-iodophenylethynyltrimethylsilane 8 [21]. Subsequent tri-
methylsilyl group removal with tetrabutylammonium fluoride
(TBAF) provided desired compounds 4e7 in the overall yields of
81e86%. In this way, N,N-dimethylamino- and ethynyl-terminated
molecules with terphenyl (4), biphenylethynylphenyl (5), phenyl-
ethynylbiphenyl (6), and phenylethynylphenylethynylphenyl (7)
ꢃ
lmax values of compounds 2 and 4 are practically the same even
though 4 is longer by one benzene ring; the same relation was
found for the compounds 3 and 7;
lmax values of 5 and 6 are situated at shorter wavelengths than
for 3 even though 5 and 6 are larger;
ꢃ
ꢃ by going from 3 to 4, a strong hypsochromic shift was found
whereas compound 4 is longer.
Hence, the observed spectral behavior of 1e7 must be eluci-
dated in a more complex manner. As can be seen from Fig. 2 (left),
the measured lmax values split into two series (1, 2, and 3) and (1, 4,
5, 6, and 7) as a dependence on the molecular length. However,
structural genesis of these series is not clear especially by going
from compound 2 to 3 and from 4 to 7. The only feature of these
series is the molecular length and the number of phenyl rings (two
rings for 2 and 3; three rings for 4, 5, 6, and 7). On the contrary,
when considering the structural arrangement of 1e7, they can be
divided into two series A and B (Fig. 2). Whereas non-planar
chromophores 2 (biphenyl) and 4 (terphenyl) generate series A
(practically identical lmax values), planar compounds 1 (phenyl), 3
(phenylethynylphenyl), and 7 (phenylethynylphenylethynylphenyl)
belong to series B (strong bathochromic shift of the absorption
systematically evaluated central p-linker were synthesized.
3.2. Absorption spectra
The absorption spectra of the studied compounds are formed by
the non-structural broad bands appearing within the range 280e
400 nm (Fig. 1, Table 2). Although the changes of the absorption
spectra are relatively small, some relationships with the molecular
characteristics of studied compounds may be drawn. Despite the
molecular length (the distance between the amino nitrogen and the

N. Almonasy et al. / Dyes and Pigments 108 (2014) 50e56
53
Fig. 1. Absorption (A) and fluorescence (F) spectra of 1e7 in DBE.
maxima). This splitting is more evident by considering biphenyl
(2b) as starting compound for the series A. Both series show a clear
convergence of the lmax values with increasing n: the increment Dl
is 38 nm between 2 and 1 and only 3 nm between 4 and 2; Dl is
65 nm between 3 and 1 and only 10 nm between 7 and 3. Chro-
mophores 5 and 6 are structurally isomeric which is reflected by
their close lmax positions and represent a cross-section between
structurally homogeneous series A or B. As the absorption maxima
of chromophores 1e7 are practically independent on the solvent
polarity (Table 2), all the aforementioned relationships are valid
regardless the solvent used.
corresponds well with the molecular length and with the number
of nodal planes. Compound 4 with two nodal planes absorbs at the
same wavelength as shorter 2 with only one nodal plane. Middle-
sized planar chromophore 3 absorbs at longer wavelength than 4
(two nodal planes) and even 5 and 6 (one nodal plane). As expected,
compound 1 absorbs at the shortest wavelengths while the largest
and fully planar compound 7 absorbs at the longest wavelength.
Theoretical characteristics of the first two INDO/S electronic
singlet transitions for optimized geometries of 1e7 are listed in
Table 3. The intense absorption band of all studied compounds,
except 1, corresponds to “pure” HOMOeLUMO allowed transition
To better understand the relationships between the structure
and the electronic spectra of 1e7, their equilibrium conformation
and the electronic excited state characteristics have been calculated
using the semi-empirical AM1 procedure and INDO/S method with
modified NishimotoeMataga gamma integrals as implemented in
WinMOPAC 2.0 Package. For optimized geometry, the dihedral
and has
forbidden and consists from
localized on the phenylacetylene fragment. The second transition
has a medium high oscillator strength and is strongly mixed by the
HOMOeLUMO and
acetylenic group. Likewise for 2, the HOMOeLUMO transition ap-
pears as the second one, but in contrast to 1, shows high “purity”
and very high oscillator strength. The first transition of 2 consists
p
e
p
* nature. For compound 1, the first transition is strictly
* configurations with eMO
s
ep
s
ses* (26e32) transitions localized on the
angle C(CH3)eNeC(phenyl)-C(phenyl) (angle a, Fig. 2) was found
18ꢀ for all studied compounds. While are the hydrocarbon skele-
tons of compounds 1, 3, and 7 planar, the dihedral angles between
1,4-phenylene units of biphenyl and terphenyl linkers are 40 ꢄ 1ꢀ in
compounds 2, 4, 5, and 6. The position of the absorption maxima
from several
pep* configurations. For the remaining compounds
3e7, the first electronic transition consists solely from the HOMOe
LUMO configuration. The second transition for 3, 5, 6, and 7 is
formed by
diphenylacetylene fragment while the second transition of 4 is
formed by several * configurations.
The allowed singlet transitions with the main HOMOeLUMO
configuration are spectroscopically most important. The HOMO of
sep* configuration with seMO localized on the
Table 2
Absorption and fluorescence maxima (nm) and the Stokes shifts (Dn, 103 cmꢂ1) of
compounds 1e7.
pep
Comp.
DBE
EtAc
MeCN
all studied compounds is spread over the
p-linker and N,N-dime-
lA
lF
Dn
lA
lF
Dn
lA
lF
Dn
6.48
6.79
7.59
9.95
10.15
9.94
9.51
thylanilino moiety dominantly. Several nodal planes appeared be-
tween the phenyl rings and triple bonds. Whereas in 1 is the LUMO
localized mainly on the hydrocarbon skeleton, in 2e7 is spread over
1
2
3
4
5
6
7
285
323
350
326
339
346
360
349
374
390
409
417
415
424
6.43
4.22
2.93
6.26
5.52
4.81
4.19
286
326
350
331
345
349
362
350
398
428
447
461
466
480
6.39
5.55
5.21
7.94
7.29
7.19
6.79
288
328
352
333
348
351
362
353
422
478
498
538
540
552
a whole
Simultaneously, a strong
p
-system apart from the N,N-dimethylanilino moiety.
-bonding has been found between the
p
particular phenyl rings (2, 4e6) and between the phenyl and
acetylenic groups (1e7) (Fig. 3).

54
N. Almonasy et al. / Dyes and Pigments 108 (2014) 50e56
Fig. 2. Series A and B (above) and the dependence of the experimental (DBE, left) and theoretical (HOMOeLUMO configuration, right) absorption maxima [nm] on the molecular length.
The shapes of the HOMO, LUMO, and excited states dipole mo-
ments correspond well with the expected moderate charge-transfer
(CT) character of the allowed electronic transitions accompanied by
a partial electron transfer from the donor N,N-dimethylamino group
(1) or from the N,N-dimethylanilino moiety (2e7) to the appended
AM1 description of their twisted biphenyl and terphenyl p-linkers.
Changing the torsion angle between two phenyl rings of 2 by ꢄ10ꢀ
from that of equilibrium structure (39ꢀ) affects the first allowed
transition (HOMOeLUMO) only by þ5 nm for
g
¼ 29ꢀ and ꢂ8 nm for
g
¼ 49ꢀ. The electronic transitions of 1 are strongly mixed by
pep*
p-linker (Fig. 3). Hence, a
p-linker, which consists of 1,4-phenylene
and sep* configurations. Hence, any assignment of the theoretical
and acetylenic units and possesses electronegative sp2/sp carbons,
can be considered as a weak acceptor. This observation is in
agreement with previous findings [15,20e22,24]. The agreement of
the theoretical electronic transitions, characterized by the high
oscillator strength and by the predominant HOMOeLUMO CI
configuration, with the experimental absorption maxima of 3, 5, 6,
and 7 is within the limit of ꢄ6 nm. The calculated maxima for 2 and
4 are shifted hypsochromically by 17/15 nm which can be caused by
values to the spectrum is somewhat problematic. The theoretical
value for this compound is shifted bathochromically compared to
the experimentally obtained one.
Fig. 2 (right) shows a dependence of the theoretical electronic
transitions of 1e7 on the molecular length. For a detailed analysis of
the observed splitting, theoretical La transitions of less-substituted
complementary compounds 1a, 2aeb, 3aeb, 4aeb, and 7aeb have
been included as well (for their structures see Table 1). It is obvious
that biphenyl (2b) would represent a fundamental skeleton for the
hydrocarbon series 2b, 2a, and 4a and subsequently also for the
sequence 2b, 2, and 4 (series A). On the other hand, ethynylbenzene
(1a) seems to be a fundamental system for the hydrocarbon
sequences 1a, 3b, and 7b and 1a, 3a, and 7a as well as for N,N-
dimethylamino-substituted series 1, 3, and 7 (series B). Similarly to
the experimental data (Fig. 2, left), compounds 5 and 6 do not fit in
the aforementioned sequences and represent rather a cross-section
between both series. However, both experimental and calculated
lmax values correlate well with the increasing molecular length and
showed the same trends.
Table 3
INDO/S theoretical spectral characteristics of S0eSi transitions of studied
compounds.
Comp. State ltheor [nm]
f
Main CI
configuration
DM (Dipole
moment)
(Debay)
l
exp [nm]
(in DBE)
Exc. st. G. st.
1
1
2
342
309
0.000 26e29 (0.61)
26e31 (0.53)
0.222 28e29 (0.64)
26e32 (0.55)
2.9
2.0
285
2
3
4
5
6
7
1
2
306
304
0.042 42e45 (0.61)
1.032 42e43 (0.95)
4. Fluorescence spectra and fluorescence quantum yields
7.1
5.7
2.2
2.4
323
350
In contrast to the absorption maxima, the fluorescence ones are
bathochromically shifted monotonously by going from 1 to 7
(Table 2, Fig. 4 left). While the positions of the absorption maxima
are influenced by the solvent polarity, the fluorescence maxima
(except for 1) show a strong bathochromic shift. A hint of conver-
gence of fluorescence maxima with molecular length is apparent.
Since there are more factors that affect the Stokes shift (SS), e.g.
a change of the molecular geometry during excited state life time,
the excited state dipole moment, the solvent polarity, an analysis of
the SS related to the molecular structure is rather complicated.
Nevertheless, when comparing pair of chromophores, e.g. 2 with 4
1
2
356
333
1.186 46e47 (0.92)
0.200 44e47 (0.91)
1
2
311
306
1.661 56e57 (0.95)
0.043 Strongly mixed
9.1
4.7
8.5
7.4
2.2
2.4
2.4
2.5
326
339
346
360
1
2
345
327
1.910 60e61 (0.94)
0.008 57e61 (0.84)
1
2
341
317
1.992 60e61 (0.95)
0.003 57e61 (0.75)
1
2
366
331
2.341 64e65 (0.94)
0.003 61e65 (0.65)

N. Almonasy et al. / Dyes and Pigments 108 (2014) 50e56
55
Fig. 3. The HOMO/LUMO visualizations and the electron displacement in the first electronic excited states (1. ES) for the compounds 3 and 4.
and 3 with 7, larger SS for 4 and 7 were found, this could be caused
by higher transition dipole moments for 4 and 7. This observation
can be rationalized by the higher excited state dipole moments of 4
and 7. The obvious difference in the SS between similar compounds
3 and 4 can be attributed to their very different molecular structure.
While 3 is fully planar, 4 has two nodal planes and approximately
twice greater excited state dipole moment. Compound 1 exhibited
the highest SS (6430 cmꢂ1) in non-polar DBE. This compound has
practically the same dipole moment in the ground and excited
states. Moreover, no influence of the solvent polarity on its ab-
sorption and fluorescence maxima was found. The graphical rep-
resentation of the dependence of the SS on the molecular length
and the solvent polarity (Fig. 4, right) provides an interesting pic-
ture. The data are split again into two series: 1, 2, and 3 and H, 4, 5,
6, and 7 where H represents a hypothetical molecule with the
consist solely from HOMOeLUMO (pep*) configurations and their
qF is relatively high ranging from 0.41 to 0.90 (DBE), 0.20e0.58
(EtAc), 0.12e0.55 (MeCN), and 0.30e0.62 (DO). Similar to the
absorption maxima, nearly linear dependence of the qF on the
molecular length for the series of planar compounds 1, 3, and 7 was
found. While qF increases with the molecule elongation, it de-
creases with increasing the solvent polarity. For instance, the qF
values of 3 and 7 decreases within the range of 0.41/0.20/0.12
and 0.75/0.46/0.20 respectively when going from DBE-
/EtAc/MeCN. This may be caused by an increase of a radia-
tionless decay rate constant within the fluorescence bathochromic
shift (stabilization of emitting excited state) regulated by the
classical “energy gap law” [31]. A nonradiative process, probably
internal S1eS0 conversion, could be responsible for the fluores-
cence quenching. However, no relationships between the structure,
qF, and solvent polarity were found for non-planar systems having
one (2, 5, and 6) or two (4) nodal planes. On the contrary to planar
chromophores 3 and 7, twisted molecules 2 and 4e6 showed
generally higher qF with a diminished influence of the solvent
polarity. An interplay of the energy, geometry, and electronic
structure of the fluorescent state plays evidently most important
role in its deactivation mechanism, consequently in fluorescence
quenching.
length similar to 1 that features much higher SS (10e11 ꢁ103 cmꢂ1
)
and absorption and fluorescence independent on the solvent po-
larity. However, despite our attempts to find such molecule, the
structure of H remained unknown.
Table 4 shows the measured fluorescence quantum yields (qF) of
chromophores 1e7. It is obvious that qF of 1 is exceptionally low
which can be due to the strong mixing of
pep* and ses* config-
urations in the fluorescent state. The fluorescent states of 2e7
Fig. 4. Dependence of the fluorescence maxima (left) and the Stokes shift (right) on the molecular length of 1e7 measured in DBE (a), EtAc (b) and MeCN (c). H denotes a hy-
pothetical compound.

56
N. Almonasy et al. / Dyes and Pigments 108 (2014) 50e56
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qF
DBE
EtAc
MeCN
DO
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1
2
3
4
5
6
7
0.09
0.90
0.41
0.69
0.82
0.70
0.75
0.054
0.58
0.20
0.44
0.36
0.48
0.46
0.034
0.55
0.12
0.41
0.30
0.46
0.20
0.07
0.59
0.30
0.52
0.57
0.62
0.56
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5. Conclusion
Seven N,N-dimethylamino-substituted phenyleneethynylene
oligomers with systematically extended and varied -conjugated
systems were prepared and further studied by absorption and
fluorescence spectra as well as calculations. The -linker sepa-
rating N,N-dimethylamino donor and terminal acetylene moiety
consists of a combination of one to three 1,4-phenylene and none to
two acetylenic subunits. These molecules represent a compre-
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p
hensive series of model De
p compounds as a part of the more
common De eA pushepull chromophores and are intended to
p
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study fundamental structureeproperty relationships. Measured
and calculated properties were correlated with structural features
of 1e7 and following conclusions can be made:
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ꢃ the molecular length was proved to be the fundamental struc-
tural factor affecting spectral properties of linear compounds
such as 1e7;
ꢃ beside the molecular length, the spectral properties were also
significantly influenced by the molecule planarity;
ꢃ the most bathochromically shifted bands of 1e7 have CT char-
acter and correspond to HOMO/LUMO transition;
ꢀ
[18] Bures F, Schweizer WB, Boudon C, Gisselbrecht JP, Gross M, Diederich F. New
pushepull chromophores featuring TCAQ (11,11,12,12-tetracyano-9,10-
anthraquinodimethane) and other dicyanovinyl acceptors. Eur J Org Chem;
2008:994e1004.
ꢀ
[19] Kulhánek J, Bures F. Imidazole as a parent
p-conjugated backbone in charge-
transfer chromophores. Beilstein J Org Chem 2012;8:25e49.
ꢃ a
p-conjugated system consisting of 1,4-phenylene and acety-
lenic units behave as weak electron acceptor due to the presence
ꢀ
[20] Bures F, Schweizer WB, May JC, Boudon C, Gisselbrecht JP, Gross M, et al.
Property tuning in charge-transfer chromophores by systematic modulation
of more electronegative sp2/sp carbons;
of the spacer between donor and acceptor. Chem Eur J 2007;13:5378e87.
ꢀ
ꢀ
[21] Bures F, Cermáková H, Kulhánek J, Ludwig M, Kuznik W, Kityk IV, et al.
Structureeproperty relationships and nonlinear optical effects in donor-
substituted dicyanopyrazine-derived pushepull chromophores with
ꢃ non-planar molecules showed the largest changes in the dipole
moment between the ground and excited states;
ꢃ the fluorescence maxima are shifted bathochromically with
increasing molecular length;
ꢃ for fully planar molecules 3 and 7 were the fluorescence quan-
tum yields significantly decreased with the increasing solvent
polarity;
enlarged and varied
p-linkers. Eur J Org Chem; 2012:529e38.
[22] Kirketerp MBS, Petersen MÅ, Wanko M, Zettergren H, Rubio A, Nielsen MB,
et al. Double-bond versus triple-bond bridges: does it matter for the charge-
transfer absorption by donoreacceptor chromophores? ChemPhysChem
2010;11:2495e8.
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photon absorbing molecules with rigid elongated pi-conjugation: synthesis,
spectroscopic properties and two-photon fluorescence cell imaging applica-
tions. J Fluoresc 2007;17:573e9.
ꢃ the influence of the solvent polarity on the qF is diminished for
non-planar molecules 2 and 4e6;
ꢀ
ꢂꢀ ꢀ
[24] Kulhánek J, Bures F, Pytela O, Mikysek T, Ludvík J, Ruzicka A. Pushepull
ꢃ the observed Stokes shifts mimic the size of the molecule.
molecules with a systematically extended -conjugated system featuring 4,5-
p
dicyanoimidazole. Dyes Pigm 2010;85:57e65.
In view of the current interest in novel organic
p
-conjugated
[25] Tykwinski RR, Gubler U, Martin RE, Diederich F, Bosshard C, Günter P.
Structureeproperty relationships in third-order nonlinear optical chromo-
phores. J Phys Chem B 1998;102:4451e65.
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lene-vinylene copolymers: synthesis and structureeproperty relationships.
Prog Polym Sci 2009;34:1023e67.
materials and their growing applications in optoelectronic devices,
we believe that this study would serve as useful guide to design
such organic molecules.
[27] Hlel A, Mabrouk A, Chemek M, Alimi K. Theoretical study of phenylenee
thiophene oligomers: structureeproperties relationship. Spectrochim Acta
Part A 2012;99:126e35.
Acknowledgment
[28] Ayachi S, Ghomrasni S, Bouachrine M, Hamidi M, Alimi K. Structureeproperty
relationships of soluble poly(2,5-dibutoxyethoxy-1,4-phenylene-alt-2,5-thieny-
lene) (PBuPT) for organiceoptoelectronic devices. J Mol Chem 2013;1036:7e18.
[29] Birks JB, Dyson DJ. The relations between the fluorescence and absorption
properties of organic molecules. Proc R Soc A 1963;275:135e48.
ꢀ
This research has been supported in part by the Czech Science
Foundation (13-01061S) and in part by the Czech Ministry of Edu-
cation, Youth and Sports (Aktion 63p20).
[30] Kulhánek J, Bures F, Ludwig M. Convenient methods for preparing
p-conju-
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