Synthesis and optical properties of the isomeric pyrimidine and carbazole derivatives: Effects of polar substituents and linking topology

Article abstract of DOI:10.1016/j.dyepig.2015.03.008

Abstract A series of nonlinear oligoarylenes with a pyrimidine unit as central core and carbazole containing aryl branches in positions 2, 4 and 4, 6 of the pyrimidine ring were successfully synthesized by sequential Suzuki coupling of the corresponding 2,4- or 4,6-dichloropyrimidines. Optical properties of the compounds were thoroughly investigated in the media of different polarity. Owing to pronounced electron-accepting properties of pyrimidine core, the compounds with donating carbazole groups showed expressed intramolecular charge transfer (ICT) character of the excited states, which was proved by solvatochromic dynamics and supported by DFT calculations. ICT resulted in the dramatic enhancement of excited state lifetime (up to 5 times) with increased solvent polarity. The competition of constantly decreasing radiative and nonradiative relaxation rates resulted in non-monotonous variation of fluorescence quantum yield from 24% to 74%. The tailoring of fluorescence quantum yield of the pyrimidine derivatives via altering linking topology and polarity of the substituents is discussed.

Full text of DOI:10.1016/j.dyepig.2015.03.008

Dyes and Pigments 118 (2015) 118e128
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and optical properties of the isomeric pyrimidine
and carbazole derivatives: Effects of polar substituents and linking
topology
Lina Skardziute a, Jelena Dodonova b, Andzejus Voitechovicius b, Justina Jovaisaite a,
Regimantas Komskis a, Asta Voitechoviciute b, Jonas Bucevicius b, Karolis Kazlauskas a,
,
*
Saulius Jursenas a, Sigitas Tumkevicius b
a
_
Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania
b Department of Organic Chemistry, Faculty of Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of nonlinear oligoarylenes with a pyrimidine unit as central core and carbazole containing aryl
branches in positions 2, 4 and 4, 6 of the pyrimidine ring were successfully synthesized by sequential
Suzuki coupling of the corresponding 2,4- or 4,6-dichloropyrimidines. Optical properties of the com-
pounds were thoroughly investigated in the media of different polarity. Owing to pronounced electron-
accepting properties of pyrimidine core, the compounds with donating carbazole groups showed
expressed intramolecular charge transfer (ICT) character of the excited states, which was proved by
solvatochromic dynamics and supported by DFT calculations. ICT resulted in the dramatic enhancement
of excited state lifetime (up to 5 times) with increased solvent polarity. The competition of constantly
decreasing radiative and nonradiative relaxation rates resulted in non-monotonous variation of fluo-
rescence quantum yield from 24% to 74%. The tailoring of fluorescence quantum yield of the pyrimidine
derivatives via altering linking topology and polarity of the substituents is discussed.
Received 25 November 2014
Received in revised form
5 March 2015
Accepted 7 March 2015
Available online 17 March 2015
Keywords:
Pyrimidine oligoarylenes
Carbazole derivatives
Photophysical properties
Solvatochromism
DFT calculations
Dyes
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
p-Conjugated pyrimidine derivatives have been recently utilized in
the fabrication of OLEDs [4], sensors [5], supramolecular assemblies
[6], liquid crystals [7], and dyes for solar cells [8]. Pyrimidine ring is
frequently used in pushepull structures as strong electron-
withdrawing unit and together with electron-donating moieties
results in intramolecular charge transfer character, which plays a
key role in most of the important applications.
Over recent years, organic molecules with a
p-conjugated
backbone have received much attention in both academic and in-
dustry due to their applications in a wide range of electronic and
optoelectronic devices [1]. Introduction of heteroaryl moieties into
the backbone of
p-extended systems considerably influence mo-
lecular orbitals, stereochemical structure and linking topology of
the substituents. In particular, azaheterocyclic compounds are
widely used in organic electronic devices, such as organic light
emitting devices (OLEDs), solar cells, sensors [2] etc. Owing to
On the other hand, carbazole is a well-known donor featuring
good hole transporting properties [9], intense blue luminescence
and electroluminescence [10], rigid structure and enhanced ther-
mal stability [11]. Thus, it has been widely used as functional
building block in the fabrication of various optoelectronic devices
[12]. Recently, carbazole derivatives [13]a,b and bipolar 1,3,4-
oxadiazole and pyrimidine derivatives bearing carbazole based
arms have been found to be useful as host materials for phospho-
rescent OLEDs [1g,13cef].
strong aromaticity, significant
p-deficiency, presence of n-p elec-
tronic states, pH sensitivity, and ability of its nitrogen atoms to take
part in the chelation processes, pyrimidine, among other azahe-
terocycles is a desired functional moiety to be incorporated in more
complex organic structures targeted for numerous applications [3].
Herein we report on the synthesis and photophysical properties
of novel donor-acceptor-type chromophores based on a pyrimidine
core and carbazolylphenyl branches at the periphery. Multifrag-
ment pyrimidine systems were analysed with an emphasis on
* Corresponding author. Tel.: þ370 (5) 219 3197; fax: þ370 (5) 233 0987.
E-mail address: sigitas.tumkevicius@chf.vu.lt (S. Tumkevicius).
http://dx.doi.org/10.1016/j.dyepig.2015.03.008
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
119
emission properties induced by the polarity and linking topology of
various aryl substituents.
2.2.1. 2-Chloro-4-(4-methoxyphenyl)-6-methylpyrimidine (2a)
The general procedure was followed using
4-
methoxyphenylboronic acid (0.33 g, 2.2 mmol). Reaction time
6.5 h. Yield 0.173 g (40%), mp 66e67 ꢁC. 1H NMR (CDCl3, 300 MHz)
2. Experimental section
d
(ppm): 2.59 (s, 3H, CH3), 3.91 (s, 3H, CH3), 7.02 (d, J ¼ 8.7 Hz, 2H,
CH), 7.45 (s, 1H, CH), 8.08 (d, J ¼ 8.7 Hz, 2H, CH). 13C NMR (CDCl3,
2.1. General information
75 MHz)
d (ppm): 24.4, 55.7, 113.9, 114.6, 127.9, 129.3, 161.5, 162.8,
166.5, 170.6. Anal. Calcd. for C12H11N2ClO: C, 61.41; H, 4.72%; found:
C, 61.49; H, 4.77%. HRMS-ESI: m/z calcd. for MHþ (C12H12N2ClO):
257.0452, found: 257.0457.
All reagents and solvents were purchased from commercial
sources and dried by using standard procedures before use. Melting
points were determined in open capillaries with a digital melting
point IA9100 series apparatus (ThermoFischer Scientific). 1H and
13C NMR spectra were recorded on a Varian INOVA spectrometer
(300 MHz and 75 MHz respectively) using residual solvent peaks as
an internal standard.
2.2.2. 4-(1,10-Biphenyl-4-yl)-2-chloro-6-methylpyrimidine (2b)
The general procedure was followed using 4-biphenylboronic
acid (0.44 g, 2.2 mmol). Reaction time 10 h. Yield 0.171 g (33%),
mp 121e122 ꢁC. 1H NMR (CDCl3, 300 MHz)
d (ppm): 2.64 (s, 3H,
Elemental analysis was performed with a ThermoFischer Sci-
entific Flash 2000 elemental analyzer.
CH3), 7.58e7.44 (m, 4H, CH), 7.67e7.71 (m, 2H, CH), 7.77 (d,
J ¼ 8.7 Hz, 2H, CH), 8.19 (d, J ¼ 8.7 Hz, 2H, CH). 13C NMR (CDCl3,
High Resolution Mass Spectrometry (HRMS) analyses were
carried out on a quadrupole, time-of-flight mass spectrometer
(microTOF-Q II, Bruker Daltonik GmbH, Bremen, Germany).
Column chromatography was performed using Silica gel 60
(0.040e0.063 mm) (Merck). All reactions and purity of the syn-
thesized compounds were monitored by TLC using Silica gel 60 F254
aluminium plates (Merck). Visualization was accomplished by UV
light.
75 MHz) d (ppm): 24.5, 114.8, 127.4, 127.9, 128.1, 128.4, 129.2, 134.3,
140.2, 144.7, 161.7, 166.6, 171.1. Anal. Calcd. for C17H13N2Cl: C, 72.73;
H, 4.67%; found: C, 72.89; H, 4.88%. HRMS-ESI: m/z calcd. for MHþ
(C17H14N2Cl): 303.0659, found: 303.0662.
2.2.3. 2-Chloro-4-[4-(9H-carbazol-9-yl)phenyl]-6-
methylpyrimidine (2c)
The absorption spectra were recorded on a PerkineElmer UV-
vis-NIR spectrophotometer Lambda 950. Fluorescence of the
sample solutions was excited by 320 nm wavelength light-
emitting diode and measured using back-thinned CCD spec-
trometer (Hamamatsu PMA-11). The fluorescence quantum yield
The general procedure was followed using 4-(9-carbazolyl)
phenylboronic acid (0.63 g, 2.2 mmol). Reaction time 6 h. Yield
0.40 g (59%), mp 159e161 ꢁC. 1H NMR (CDCl3, 300 MHz)
d (ppm):
2.68 (s, 3H, CH3), 7.30e7.39 (m, 2H, CH), 7.47e7.52 (m, 4H, CH), 7.63
(s,1H, CH), 7.78 (d, J ¼ 8.7 Hz, 2H, CH), 8.19 (d, J ¼ 9 Hz, 2H, CH), 8.36
(
FF) of the solutions was estimated by comparing wavelength-
(d, J ¼ 9 Hz, 2H, CH). 13C NMR (CDCl3, 75 MHz)
d (ppm): 24.6, 110.0,
integrated fluorescence intensity of the solution with that of
the reference. Quinine sulfate in 0.1 M H2SO4 was used as a
reference. Optical densities of the reference and the sample so-
lutions were ensured to be below 0.05 to avoid reabsorption
effects. Estimated quantum yield was verified by using an alter-
native method of an integrating sphere (Sphere Optics), which
was coupled to the CCD spectrometer by an optical fiber. Fluo-
rescence transients of the sample solutions were measured using
time-correlated single photon counting system (PicoQuant
PicoHarp 300).
114.9, 120.7, 120.8, 124.0, 126.4, 127.4, 129.3, 134.2, 140.6, 141.2,
161.8, 166.1, 171.4. Anal. Calcd. for C23H16N3Cl: C, 74.69; H, 4.36%;
found: C, 74.83; H, 4.59%. HRMS-ESI: m/z calcd. for MHþ
(C23H17N3Cl): 392.0925, found: 392.0924.
2.3. General procedure for the synthesis of 2,4-diaryl-6-
methylpyrimidines (3aed)
To a solution of 4-aryl-2-chloro-6-methylpyrimidine (2aec)
(0.36 mmol) in anhydrous dioxane (10 ml) Pd(OAc)2 (1.6 mg,
Cyclic voltammetry experiments were performed on the Edaq
ER466 Integrated Potentiostat System. Pt/Ti wire, glassy carbon
disk [Ø 3.0 mm] and Ag/AgCl were used as counter, working, and
reference electrodes, respectively. In all cases, CV experiments were
performed in DMF (N,N e dimethylformamide) with tetrabuty-
lammonium perchlorate - as supporting electrolyte (0.1 M) under
Ar flow; concentrations of compounds were 0.002 M. The scan rate
0.0072 mmol,
2 mol%) and 2-biphenyldicyclohexylphosphine
(5.05 mg, 0.014 mmol, 4 mol%) were added and the reaction
mixture was stirred for 15 min. Then the corresponding arylboronic
acid (0.43 mmol) and K3PO4 (0.18 g, 0.85 mmol) were added and
the reaction mixture was refluxed for 5.5e18 h under argon at-
mosphere. The solvent was evaporated under reduced pressure to
dryness, the residue was dissolved in water and the solution was
extracted with CH2Cl2. The organic layer was dried with Na2SO4,
filtered and evaporated to dryness. The obtained solid was purified
by column chromatography using chloroform as an eluent.
was 50 mV sꢀ1
.
2.2. General procedure for the synthesis of 4-aryl-2-chloro-6-
methylpyrimidines (2aec)
2.3.1. 2-[4-(9H-Carbazol-9-yl)phenyl]-6-methyl-4-(4-
methoxyphenyl)pyrimidine (3a)
To a solution of 2,4-dichloro-6-methylpyrimidine (1) (0.3 g,
1.84 mmol) in dioxane (10 mL) Pd(OAc)2 (20.6 mg, 0.092 mmol,
5 mol%) and PPh3 (48.3 mg, 0.184 mmol, 10 mol%) were added and
the reaction mixture was stirred for 15 min. Then the corre-
sponding arylboronic acid (2.2 mmol) and saturated solution of
sodium carbonate (1.33 ml) were added and the reaction mixture
was refluxed for 6e10 h under argon atmosphere. The solvents
were evaporated under reduced pressure to dryness, the residue
was dissolved in water and the solution was extracted with CH2Cl2.
The organic layer was dried with Na2SO4, filtered and evaporated to
dryness. The obtained solid was purified by column chromatog-
raphy using chloroform as an eluent.
The general procedure was followed using 4-(9-carbazolyl)
phenylboronic acid (0.12 g, 0.43 mmol). Reaction time 5.5 h. Yield
0.11 g (68%), mp 163e164 ꢁC. 1H NMR (CDCl3, 300 MHz)
d (ppm):
2.71 (s, 3H, CH3), 3.95 (s, 3H, OCH3), 7.1 (d, J ¼ 8.4 Hz, 2H, CH),
7.35e7.57 (m, 7H, CH), 7.76 (d, J ¼ 9 Hz, 2H, CH), 8.21 (d, J ¼ 7.5 Hz,
2H, CH), 8.28 (d, J ¼ 9 Hz, 2H, CH), 8.84 (d, J ¼ 9 Hz, 2H, CH). 13C NMR
(CDCl3, 75 MHz) d (ppm): 24.9, 55.7, 110.2, 113.5, 114.5, 120.3, 120.6,
123.8, 126.3, 127.5, 129.0, 130.2, 131.2, 137.5, 139.8, 140.9, 162.2,
163.6, 163.8, 167.9. Anal. Calcd. for C30H23N3O: C, 81.61; H, 5.25%;
found: C, 81.79; H, 5.42%. HRMS-ESI: m/z calcd. for MHþ
(C30H24N3O): 442.1914, found: 442.1922.

120
L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
2.3.2. 4-[4-(9H-Carbazol-9-yl)phenyl]-6-methyl-2-(4-
85.52; H, 5.01%. HRMS-ESI: m/z calcd. for MHþ (C41H29N4):
methoxyphenyl)pyrimidine (3b)
577.2387, found: 577.2396.
The
general
procedure
was
followed
using
4-
methoxyphenylboronic acid (0.065 g, 0.43 mmol). Reaction time
2.5. 4,6-Bis[4-(9H-carbazol-9-yl)phenyl]-2-methylpyrimidine (5a)
18 h. Yield 0.129 g (81%), mp 110e112 ꢁC. 1H NMR (CDCl3, 300 MHz)
d
(ppm): 2.72 (s, 3H, CH3), 3.95 (s, 3H, OCH3), 7.08 (d, J ¼ 8.7 Hz, 2H,
A mixture of 4,6-dichloro-2-methylpyrimidine (4a) [14] (0.64 g,
3.95 mmol), 4-(9-carbazolyl)phenyl boronic acid (2.84 g,
9.88 mmol), Pd(PPh)3Cl2 (69.3 mg, 0.099 mmol, 2.5 mol%), and
K3PO4 (8.1 g, 38 mmol) in anhydrous dioxane (60 ml) was refluxed
under argon atmosphere for 4 h. After cooling to room temperature
the reaction mixture was poured into water, the water solution was
stirred for 30 min and then extracted with dichloromethane. The
extract was filtered through a layer of silica gel and dichloro-
methane was evaporated to dryness. The residue was recrystallized
to give 1.25 g (55%) of compound 5a, mp 326e330 ꢁC (from
CH), 7.35e7.41 (m, 2H, CH), 7.48e7.49 (m, 4H, CH), 7.54 (s, 1H, CH),
7.78e7.81 (d, J ¼ 8.7 Hz, 2H, CH), 8.21 (d, J ¼ 13.5 Hz, 2H, CH), 8.48
(d, J ¼ 8.7 Hz, 2H, CH), 8.63 (d, J ¼ 9 Hz, 2H, CH). 13C NMR (CDCl3,
75 MHz)
d (ppm): 24.9, 55.7, 110.1, 113.5, 114.1, 120.5, 120.7, 123.9,
126.4, 127.4, 128.6, 129.1, 130.3, 136.5, 140.2, 140.8, 162.1, 163.0,
164.4,168.1. Anal. Calcd. for C30H23N3O: C, 81.61; H, 5.25%; found: C,
81.41; H, 5.47%. HRMS-ESI: m/z calcd. for MHþ (C30H24N3O):
442.1914, found: 442.1920.
toluene:2-propanol 8:2). 1H NMR (CDCl3, 300 MHz)
d (ppm): 3.01
2.3.3. 4-[4-(1,10-Biphenyl)-4-yl]-2-[4-(9H-carbazol-9-yl)phenyl]-6-
methylpyrimidine (3c)
(s, 3H, CH3), 7.37 (d, J ¼ 7.35 Hz, 4H, CH), 7.50 (d, J ¼ 8.3 Hz, 4H, CH),
7.56 (d, J ¼ 8.4 Hz, 4H, CH), 7.83 (d, J ¼ 8.7 Hz, CH), 8.12 (s, 1H, CH),
8.21 (d, J ¼ 7.8 Hz, 4H, CH), 8.46 (d, J ¼ 8.4 Hz, 4H, CH). 13C NMR
The general procedure was followed using 4-(9-carbazolyl)
phenylboronic acid (0.12 g, 0.43 mmol). Reaction time 16.5 h. Yield
(CDCl3, 75 MHz)
d (ppm): 26.81, 110.05, 110.28, 120.63, 120.73,
0.135 g (77%), mp 160e162 ꢁC. 1H NMR (CDCl3, 300 MHz)
d (ppm):
123.93, 126.41, 127.55, 129.23, 136.33, 140.42, 140.75, 164.44, 169.17.
Anal. Calcd. for C41H28 N4: C, 85.39; H, 4.89; N, 9.72%; found: C,
85.66; H, 5.13; N, 9.55%. HRMS-ESI: m/z calcd. for MHþ (C41H29N4):
577.2387, found: 577.2397.
2.76 (s, 3H, CH3), 7.36e7.39 (m, 2H, AreH), 7.48e7.56 (m, 4H,
AreH), 7.62e7.84 (m, 10H, AreH), 8.21 (d, J ¼ 8.4 Hz, 2H, AreH),
8.38 (d, J ¼ 8.7 Hz, 2H, AreH), 8.87 (d, J ¼ 8.4 Hz, 2H, AreH). 13
C
NMR (CDCl3, 75 MHz) d (ppm): 24.9, 110.2, 114.3, 120.4, 120.6, 123.8,
126.3,127.1,127.5,127.8,127.9,128.2,129.2,130.2,136.2,137.4,139.9,
140.6, 140.9, 143.9, 163.7, 163.9, 168.3. Anal. Calcd. for C35H25N3: C,
86.21; H, 5.17%; found: C, 86.53; H, 5.63%. HRMS-ESI: m/z calcd. for
MHþ (C35H26N3): 488.2121, found: 488.2132.
2.6. 4,6-Di[4-(9-carbazolyl)phenyl]-2-methylthiopyrimidine (5c)
Compound 5c was synthesized from 4,6-dichloro-2-
methylthiopyrimidine (4c) [15] by the procedure described for 5a
using 4-(9-carbazolyl)phenyl boronic acid (2.84 g, 9.88 mmol).
Reaction time 11 h. Yield 1.23 g (51%), mp 257e260 ꢁC (dec.) (from
2.3.4. 2-[4-(1,10-Biphenyl)-4-yl]-4-[4-(9H-carbazol-9-yl)phenyl]-6-
methylpyrimidine (3d)
toluene). 1H NMR (CDCl3, 300 MHz)
d (ppm): 2.84 (s, 3H, CH3),
The general procedure was followed using 4-biphenylboronic
acid (0.085 g, 0.43 mmol). Reaction time 15 h. Yield 0.12 g (68%),
7.37e7.58 (m, 6H), 7.80 (d, J ¼ 7.8 Hz, 4H), 7.97 (s, 1H, CH), 8.20 (d,
J ¼ 7.8 Hz, 4H), 8.46 (d, J ¼ 8.1 Hz, 4H). 13C NMR (CDCl3, 75 MHz)
mp 187e188 ꢁC. 1H NMR (CDCl3, 300 MHz)
d (ppm): 2.76 (s, 3H,
d
(ppm): 14.74, 107.98, 110.07, 120.68, 120.73, 123.98, 126.43, 127.40,
CH3), 7.35e7.56 (m, 10H, CH), 7.74e7.83 (m, 6H, CH), 8.21 (d,
J ¼ 7.8 Hz, 2H, CH), 8.51 (d, J ¼ 8.4 Hz, 2H, CH), 8.74 (d, J ¼ 8.4 Hz, 2H,
129.18, 135.81, 140.65, 140.73, 164.20, 173.58. Anal. Calcd. for
C
41H28N4S: C, 80.89; H, 4.64%; found: C, 80.53; H, 4.52%. HRMS-ESI:
CH). 13C NMR (CDCl3, 75 MHz)
d (ppm): 25.0, 110.1, 114.2, 120.6,
m/z calcd. for MHþ (C41H29N4S): 609.2107, found: 609.2117.
Synthesis and characterization of compounds 5b,d is described
in Ref. [16].
120.7,123.9,126.4,127.4,127,5,127.6,127.9,129.1,129.2,136.4,137.2,
140.2, 140.8, 140.9, 143.5, 163.1, 164.5, 168.4. Anal. Calcd. for
C
35H25N3: C, 86.21; H, 5.17%; found: C, 86.25; H, 5.19%. HRMS-ESI:
m/z calcd. for MHþ (C35H26N3): 488.2121, found: 488.2129.
3. Results and discussion
2.4. 2,4-Bis[4-(9H-carbazol-9-yl)phenyl]-6-methylpirimidine (3e)
3.1. Materials synthesis
To a solution of 2,4-dichloro-6-methylpyrimidine (1) (0.15 g,
0.92 mmol) in anhydrous dioxane (10 ml) Pd(OAc)2 (4.1 mg,
Isomeric 2,4-diaryl-6-methylpyrimidines 3aed were synthe-
sized by sequential Suzuki cross-coupling reactions [17] of 2,4-
dichloro-6-methylpyrimidine (1) with the appropriate arylboronic
acids. Monoarylation reaction of compound 1 to give the corre-
sponding 4-aryl-2-chloropyrimidines (2aec) was carried out using
Pd(OAc)2/PPh3/Na2CO3 (aq.) as a catalyst system and a slight excess
(1.2 equiv.) of the corresponding arylboronic acid (Scheme 1). For
the second cross-coupling reaction catalyst system e Pd(OAc)2/(2-
biPh)Cy2P/K3PO4 [18] appeared to be more suitable. Using a slight
excess of arylboronic acids the corresponding 2,4-diaryl-6-
methylpyrimidines 3aed were obtained in 66e81% yields
(Scheme 1). It is worth mentioning that catalyst system - Pd(OAc)2/
(2-biPh)Cy2P/K3PO4 appeared to be unsuitable for the synthesis of
4-aryl-2-chloropyrimidines 2aec from dichloropyrimidine 1. For-
mation of diarylpyrimidines was observed even when equivalent
amounts of compound 1 and arylboronic acids were used in the
reaction. Thus, employing this catalyst system, the Suzuki reaction
of 2,4-dichloro-6-methylpyrimidine (1) with 4-(9-carbazolyl)phe-
nylboronic acid gave the corresponding pyrimidine 3e in 79% yield
(Scheme 1).
0.0184 mmol,
2 mol%) and 2-biphenyldicyclohexylphosphine
(13.3 mg, 0.014 mmol, 4 mol%) were added and the reaction
mixture was stirred for 15 min. Then 4-(9-carbazolyl)phenyl
boronic acid (0.63 g, 2.2 mmol) and K3PO4 (0.93 g, 4.41 mmol) were
added and the reaction mixture was refluxed for 8 h under argon
atmosphere. The solvent was evaporated under reduced pressure to
dryness, the residue was dissolved in water and the solution was
extracted with CH2Cl2. The organic layer was dried with Na2SO4,
filtered and evaporated to dryness. The obtained solid was purified
by column chromatography using chloroform as an eluent to give
0.42 g (79%) of compound 3e, mp 291e292 ꢁC. 1H NMR (CDCl3,
300 MHz)
d (ppm): 2.79 (s, 3H, CH3), 7.38e7.66 (m, 4H, CH),
7.49e7.59 (m, 8H, CH), 7.66 (s, 1H, CH), 7.79e7.85 (m, 4H, CH),
8.21e8.23 (m, 4H, CH), 8.54 (d, J ¼ 8.4 Hz, 2H, CH), 8.91 (d,
J ¼ 8.7 Hz, 2H, CH). 13C NMR (CDCl3, 75 MHz)
d (ppm): 25.0, 110.1,
110.2, 114.4, 120.4, 120.5, 120.6, 120.7, 123.8, 123.9, 126.3, 126.4,
127.1, 127.5, 129.1, 130.3, 136.2, 137.2, 140.1, 140.4, 140.8, 140.9, 163.2,
164.1, 168.6. Anal. Calcd. for C41H28N4: C, 85.39; H, 4.89%; found: C,

L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
121
Scheme 1. Reagents and conditions: (i) e 1.2 equiv. ArB(OH)2, 5 mol% Pd(OAc)2, 10 mol% PPh3, Na2CO3 (aq.), dioxane, tꢁ, Ar; (ii) - 1.2 equiv. ArB(OH)2, 2 mol% Pd(OAc)2, 4 mol% P(2-
biPh)Cy2, K3PO4, dioxane, tꢁ, Ar; (iii) - 2.4 equiv. 4-(9-carbazolyl)C6H4B(OH)2, 2 mol% Pd(OAc)2, 4 mol% P(2-biPh)Cy2, K3PO4, dioxane, tꢁ, Ar.
4,6-Bis[4-(9-carbazolyl)phenyl]-2-substituted
pyrimidines
Comparison of molecular orbitals of 4,6-di[4-(9-carbazolyl)
phenyl]pyrimidines 5aed, bearing various substituents at the po-
sition 2 of the pyrimidine ring, revealed that localization and nature
of LUMO are almost the same as in the case of compounds 3aed.
However, the nature of the substituent at position 2 of the pyrim-
idine ring had a great influence on HOMOs. For compounds 5a,b
HOMO is located on both electron-donating (carbazolyl)phenyl
chromophores, while for 5c, bearing methylthio group at position
2, it is located on a single (carbazolyl)phenyl branch. The incorpo-
ration of strongly electron-donating 4-dimethylaminophenyl group
in compound 5d abruptly changes the situation, since HOMO be-
comes localized mainly on this chromophore with a slight shift to
the pyrimidine ring.
5aed were synthesized by the Suzuki coupling of 4,6-dichloro-2-
substituted pyrimidines 4aed with 4-(9-carbazolyl)phenylboronic
acid in the presence of Pd(PPh3)2Cl2/K3PO4 according to the pro-
cedure described in Refs. [16,19] (Scheme 2).
3.2. Molecular orbital calculations
DFT calculations of compounds 3aee and 5aed were performed
using the Gaussian 09 program package [20]. To get more insight
into the effect of different substituents on the conformational
changes of the compounds, the geometries were optimized using
B3LYP exchange-correlation hybrid functional together with the 6-
311G** basis set. The optimized molecular geometries of all the
compounds are provided in Fig. 1 in the supporting information.
The calculations revealed that aryl substituents located at po-
sition 2 of the pyrimidine ring are coplanar, whereas the benzene
rings at the 4- and 6-positions are slightly out of plane with the rest
of the molecule with dihedral angles between 16ꢁ and 23ꢁ. More-
over, dihedral angle between phenyl and carbazolyl moieties is
about 54e55ꢁ. Comparison of the molecular orbitals of isomeric
2,4-diarylpyrimidines 3a,b and 3c,d revealed that the electron
density in the HOMOs shows the same character, i.e, it is localized
on electron-donating (carbazolyl)phenyl moieties regardless of
their positions at the pyrimidine ring (Fig. 1). The LUMO is mostly
located on the electron-withdrawing pyrimidine ring with a shift to
biphenyl moiety for compound 3c. The HOMO of 3e is positioned on
the (carbazolyl)phenyl moiety at the position 2 of the pyrimidine
ring.
This is in line with the calculated HOMO and LUMO energies of
the pyrimidine derivatives (Fig. 1). Strong electron-donating 4-
(dimethylamino)phenyl group connected to the pyrimidine ring led
to a decrease in the HOMO-LUMO gap (compound 5d, 3.38 eV),
which explains the largest bathochromic effect observed among all
the compounds (see Table
1 and discussion below). Strong
electron-attracting character of the pyrimidine ring along with
electron-donating nature of the substituents enables large intra-
molecular charge transfer between the donor and the central core,
which manifests itself in the fluorescence properties examined in
media of increasing polarity (see Table 2 and discussion below).
3.3. Optical properties
The optical properties of 3aee and 5aed were investigated by
UV/vis absorbance and photoluminescence (PL) spectroscopy in
dilute toluene solutions at room temperature and at low concen-
trations. The data obtained are summarized in Table 1.
The UV/vis spectra of the synthesized compounds 3aee and
5aed in non-polar toluene solution are characterized by several
absorption bands positioned in the UV region (labs ¼ 282e364 nm)
(Table 1, Fig. 2). Strong absorbance band at 292 nm is present in the
absorbance spectra of all compounds and is attributed to the
transitions involving carbazole moiety (calculated value for 3a is at
298 nm). Typically, carbazolyl moiety also shows a weaker long-
wavelength absorbance band at around 340 nm, which is also
evident in the absorbance spectra of all the compounds (calculated
value for 3a is at 336 nm) [21]. The lowest absorption bands, located
in the range from 350 nm to 370 nm (calculated value for 3a is at
367 nm), show pronounced intramolecular charge transfer char-
acter of the transitions and their intensity and position varies
depending on the substituent. Although absorption spectra of
Scheme 2. Reagents and conditions: (i)
e 2.5 equiv. 4-(9-carbazolyl)C6H4B(OH)2,
2.5 mol% Pd(PPh3)2Cl2, K3PO4, dioxane, tꢁ, Ar.

122
L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
Table 1
UV/vis and photoluminescence (PL) data of compounds 3aee and 5aed in toluene
solutions (c ¼ 10ꢀ5 M).
Entry
1
Compound
3a
labs, nm
lem, nm
Ff, %
t
, ns
tR, ns
3.56
tNR, ns
292
314
341
292
330
341
293
316
341
292
341
293
342
286
291
343
355
282
290
344
354
291
342
364
282
292
353
378
34
1.21
1.87
1.15
1.83
2
3
3b
3c
390
386
46
24
4.07
4.79
3.46
1.51
4
5
6
3d
3e
5a
392
398
402
52
64
65
2.27
2.44
2.05
4.37
3.81
3.15
4.73
6.78
5.86
7
5b
402
71
2.05
2.89
7.07
8
9
5c
410
498
45
7
1.45
7.85
3.22
2.64
8.44
5d
112.14
compounds containing one (carbazolyl)phenyl group (3aed)
resemble very much each other, some features in the absorption of
isomeric compounds 3a,b and 3c,d can be highlighted. Compounds
3a and 3c bearing electron-donating (carbazolyl)phenyl moieties at
the position 2 of the pyrimidine ring show an additional absor-
bance band peaking at 314e316 nm. Introduction of the second
(carbazolyl)phenyl group into position 4 of the pyrimidine ring
(compound 3e) manifests in almost two-fold increase of the
absorbance of the lowest-energy optical transitions.
Compounds 5aed possessing two electron-donating (carba-
zolyl)phenyl fragments in positions 4, 6 of the pyrimidine ring
show typical carbazole absorbance bands which are slightly
broader and red shifted due to extended
p-conjugation (Fig. 3).
Compound 5d, bearing strong electron-donating 4-(dimethyla-
mino)phenyl group in position 2 of the pyrimidine ring, showed
distinct changes in the absorbance spectral properties with almost
two-fold increase of the intensity of the lowest energy absorbance
band, peaking at 354 nm. This band corresponds to the transitions
associated mainly with dimethylamino group according to the DFT
modelling results.
All the compounds comprising (carbazolyl)phenyl moieties as
electron-donating fragments, exhibit strong UV-blue fluorescence
with the emission maxima located in a region of 378e410 nm
(Table 1, Figs. 2, 3). The dilute toluene solutions of pyrimidines
3aee demonstrated similar broad and structureless fluorescence
spectra, with only slight variation of the fluorescence maxima
from 378 nm for compound 3a to 398 nm for 3e. This is in line
with the quantum chemical calculations, which revealed the
LUMO localized mainly on the pyrimidine core for all the de-
rivatives with slight variation of extension to phenyl fragments
(Fig. 1). Dilute toluene solutions of the compounds 5aec showed
red shifted fluorescence spectra as compared to those of com-
pounds 3aee, which ranged from 402 nm to 410 nm (Table 1).
Compound 5d, bearing dimethylamino group as a donor, showed a
large bathochromic shift of almost 100 nm in the dilute toluene
solution as compared to other derivatives.
Fig. 1. Calculated spatial distributions of HOMO and LUMO of compounds 3 and 5.

L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
123
Table 2
UV/vis and photoluminescence (PL) data of compounds 3 and 5 in various solventsa (c ¼ 10ꢀ5 M).
Compound
3a
3b
3c
3d
3e
5a
5b
5c
5d
CHCl3
labs, nm
293
312
342
406
68
292
330
341
421
65
260
293
313
416
43
292
341
292
342
286
291
343
438
61
282
290
344
435
58
291
343
364
446
69
282
292
348
520
7
lem, nm
Ff, %
423
75
426
61
t, ns
2.97
3.91
3.19
3.93
4.11
3.86
3.57
3.24
8.37
19.49
138.55
10.43
291
tR, ns
tNR, ns
labs, nm
4.37
9.28
257
292
312
339
402
47
6.02
11.17
255
291
340
7.42
5.60
258
292
314
341
418
22
5.24
15.72
257
293
341
6.74
10.54
292
6.33
9.90
291
343
6.16
8.50
290
343
4.70
10.45
292
343
363
THF
341
345
lem, nm
Ff, %
418
47
422
62
423
59
423
61
436
60
447
65
576
3.5
t
, ns
2.56
5.45
4.83
292
315
340
432
66
3.76
8
3.36
16.5
4.65
293
317
4.02
6.48
10.58
292
341
4.26
7.22
10.39
292
342
4.1
3.72
6.2
9.3
282
290
344
482
74
3.22
4.95
9.2
291
343
364
497
57
9.18
262.3
9.51
283
292
348
tR, ns
tNR, ns
labs, nm
6.72
10.51
286
291
343
479
73
7.09
292
330
341
453
68
DMF
lem, nm
Ff, %
474
24
458
59
461
63
487
1.1
t
, ns
5.31
8.05
15.62
6.51
9.57
20.34
12.25
51.04
16.12
6.66
11.29
16.24
7.57
12.02
20.46
6.95
9.52
25.74
7.10
9.59
27.29
7.39
12.96
17.19
7.01
637.27
7.09
tR, ns
tNR, ns
a
Data in toluene are presented in Table 1.
The fluorescence quantum yield values for the synthesized
compounds 3, 5 markedly depend on the nature of aryl branches
and their position in the pyrimidine ring. FF values of dilute toluene
solutions of the compounds 3a and 3c, bearing (carbazolyl)phenyl
fragments at the position 2 of the pyrimidine ring, are 34% and 24%,
respectively. The attachment of the (carbazolyl)phenyl moieties at
the position 4 of the pyrimidine ring (compounds 3b and 3d)
enabled to increase FF up to 46% for 3b and 52% for 3d, respectively.
Incorporation of two (carbazolyl)phenyl fragments into positions 2
and 4 of the pyrimidine ring (compound 3e) resulted in further
enhancement of fluorescence quantum yield up to 64%.
examined by studying the luminescence transients of the mate-
rials. The decay transients of the synthesized compounds 3, 5
fitted to single exponential profiles are displayed in Fig. 4. The
estimated values of decay lifetimes in the dilute toluene solutions
are summarized in Table 1. Since the tR and tNR values are related
to the corresponding emission quantum yields and lifetimes by
Ff
¼
t/
tR and tꢀ1
¼
tꢀR 1 þ tꢀNR1, it is possible to calculate the values
of tR and tNR whenever quantum yield and lifetime data are
available. For compounds 3aee the estimated radiative decay
lifetimes (tR) show minor changes varying from 3.56 ns to 4.79 ns.
The variation in the non-radiative decay rates is much more
prominent and causes distinct changes in the fluorescence quan-
tum yields of differently linked isomers. Depending on the posi-
tion of the substituents in the isomeric compounds the non-
radiative decay lifetimes vary 2e3 times, from 1.83 ns (com-
pound 3a) to 3.46 (compound 3b) and from 1.51 ns (compound 3c)
3.4. Excitation recombination pathways
To get more insight into the variation of the fluorescence effi-
ciency the radiative and non-radiative decay pathways were
Fig. 2. UV/vis and normalized emission spectra of compounds 3aee in toluene.
Fig. 3. UV/vis and normalized emission spectra of compounds 5aed in toluene.

124
L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
Fig. 4. UV-vis, normalized PL spectra (left) and fluorescence transients (right) of compounds 3aee in toluene, THF, chloroform, and DMF solutions (c ¼ 10ꢀ5 M).
to 4.73 ns (compound 3d) (Table 1, entries 1e4). In accordance
with quantum chemical calculations, the absence of steric inter-
action between pyrimidine and 2-phenyl moieties stabilizes
planar conformation of the phenyl fragments. Whereas, 4-phenyl
moiety is already twisted and prone to low-frequency torsions
causing enhanced non-radiative deactivation of the excited state
[22]. The attachment of more bulky carbazole group to the 4-
phenyl moiety suppresses torsional motions thereby justifying
reduced non-radiative decay rate and enhanced FF for compounds
3b,d. More rigid structure of the compound 3e, bearing two
electron-donating (carbazolyl)phenyl moieties, demonstrated the
highest quantum yield (64%) and the lowest non-radiative decay
rate due to higher suppression of the parasitic intramolecular
torsions by more bulky substituents (Table 1, entry 5).
Comparison of the isomeric compounds 3e and 5a revealed
similar emission maxima and fluorescence quantum yields (FF: 64%
for 3e and FF: 65% for 5a) (Table 1, entries 5, 6). The attachment of
the 4-ethylphenyl group at the position 2 in the case of compound
5b resulted in a slightly higher fluorescence efficiency of 71% and
longer non-radiative decay lifetime (Table 1, entry 7). A more pro-
nounced (almost two-fold) increase of the non-radiative decay rate
resulted in lowering of FF for compound 5c bearing an electron-
donating methylthio group at the position 2 of the pyrimidine
core (Table 1, entry 8). Compound 5d bearing a strong electron-
donating 4-(dimethylamino)phenyl group at position 2 of the py-
rimidine ring showed notable decrease in fluorescence quantum
yield to 7% and the longest radiative decay lifetime (112.14 ns)
among all studied compounds (Table 1, entry 9). Evidently, the
presence of a strongly electron-donating group at the position 2 of
pyrimidine ring adversely affects fluorescence quantum yield.
Although UV absorption of the compound 5d was enhanced by a
factor of 2, the radiative lifetime increased by more than 20 times.
Obviously, the radiative processes in compound 5d were unable to
compete with the non-radiative ones resulting in significantly
lowered FF. tNR varied slightly with the type of substituent, and so
did tR, except for the compound 5d, for which the lifetime
increased by more than 20 times (Table 1). The weakly dependent
non-radiative decay rate implied minor differences in the inter-
system crossing rate concerning different substituent. Conversely,
dramatic enhancement of tR for compound 5d suggested ICT as the
prime mechanism influencing excited state relaxation dynamics
[23].
3.5. Solvatochromic behavior
Since the pyrimidine compounds exhibited pronounced intra-
molecular charge transfer character, as revealed by the DFT
modelling, we investigated the solvatochromic behavior of

L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
125
Fig. 5. UV-vis, normalized PL spectra (left) and fluorescence transients (right) of compounds 5aed in toluene, THF, chloroform, and DMF solutions (c ¼ 10ꢀ5 M).
absorbance and emission of compounds 3aee and 5aed in solvents
of different polarity (Toluene, THF, CHCl3, and DMF). The corre-
sponding spectroscopic data are collected in Table 2.
diarylpyrimidines can be rationalized by analyzing the solvatic
behavior of the radiative and non-radiative fluorescence decay time
constants (Fig. 6b and c). The dynamics of the radiative decay
lifetimes with increasing solvent polarity is in good agreement with
the observed redshift of the emission spectra (Fig. 6a and b) and is
in accordance with the charge transfer nature of the excited states.
Concomitantly, the non-radiative decay lifetimes also increase with
increasing solvent polarity (Fig. 6c). Since the viscosity of the sol-
vents is similar, the decreased non-radiative decay rate cannot be
attributed to the hindrance of the intramolecular vibrations. One of
the plausible reasons for the decrease of non-radiative decay rates
with increasing solvent polarity could be the decrease of the
intersystem crossing rate to triplets, which could be altered by
changing separation of singlet and triplet energy levels due to the
solvent effects [25].
Pyrimidine compounds 3aee and 5aec showed fluorescence
quantum yields varying from 24% to 75% and non-monotonously
depending on the polarity of surrounding media. The competition
between the radiative and non-radiative processes results in
observed variation of the obtained quantum yields. While the
polarity-induced dynamics of the radiative and non-radiative decay
lifetimes displays similar increasing tendency (Fig. 6b), the later
was found to strongly depend on the linking topology of the sub-
stituents (Fig. 6c). The largest change in the non-radiative decay
time with increasing solvent polarity was observed for compound
3c, while the lowest - for compound 3e. The highest FF determined
by the longest non-radiative decay and only modest charge transfer
character of the excited states were revealed in 4,6-
The absorption spectra showed minor changes with increasing
polarity of the solvents revealing an insignificant intramolecular
interaction between donor and acceptor groups in the ground state.
In contrast, the emission spectra exhibit distinct solvent
dependence. Broad structureless emissions and larger Stokes shifts
were observed on increasing solvent polarity, which is typical for
solvent stabilized charge transfer states. Correlation of the emission
maxima with the solvent-dependent Dimroth-Reichardt polarity
parameter, DET(30) [24], is represented in Fig. 6.
The common tendency for the studied pyrimidine derivatives is
a bathochromic shift of the emission peaks from 50 nm to 90 nm
with increasing solvent polarity. This is typical solvatochromic
behavior for compounds that undergo an intramolecular charge
transfer upon excitation, leading to a highly polar, charge-separated
emitting state stabilized by polar solvents. The most pronounced
red shift was observed for compound 3c (lem ¼ 386 nm in toluene
and 474 nm in DMF) and compound 5c (lem ¼ 410 nm in toluene
and 497 nm in DMF). In contrast to the compounds 3aee and 5aec,
in compound 5d HOMO is located on 4-dimethylaminophenyl
group resulting in
a distinct solvatochromic behavior. Such
behavior is typical for compounds possessing strongly electron-
donating dimethylamino group [23].
Increasing solvent polarity induces dramatic increase of the
fluorescence lifetime (up to 5 times) for all of the compounds
except 5d (Figs. 4 and 5). The lifetime dynamics of the 2,4-

126
L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
Fig. 7. Cyclic voltammograms of the dilute solutions of compounds 3a (a) and 3b (b) in
N,N e dimethylformamide at 50 mV/s scan rate. Concentrations of compounds were
0.002 M with tetrabutylammonium perchlorate (0.1 M) used as the electrolyte and
ferrocene (0.002 M) couple as calibration material.
Fig. 6. Fluorescence band maxima (a), radiative (b) and non-radiative (c) fluorescence
lifetimes of compounds 3, 5 as a function of Dimroth-Reichardt polarity parameter,
DET(30). Lines serve as guides for the eye.
diarylpyrimidines 5a (FF ¼ 73%) and 5b (FF ¼ 74%) bearing non-
polar substituents at the position 2 of the pyrimidine ring. On the
contrary, compound 5d with markedly stronger charge transfer
character in the excited state, showed systematic increase of radi-
ative decay lifetime up to 637 ns, which along with the short non-
radiative decay lifetime of 7 ns resulted in a poor fluorescence
efficiency.
380e410 nm) with up to 71% emission quantum yield in non-polar
solvents. Emission properties were strongly affected by the linking
topology of the aryl substituents resulting in up to 2-fold enhanced
FF for the pyrimidine derivatives with (carbazolyl)phenyl moiety
linked to the 4th position as compared to that attached to the 2nd
position. Importantly, the highest FF were attained in the de-
rivatives di-substituted by the electron-donating (carbazolyl)phe-
nyls at the 4th and 6th position and with non-polar moiety at the
2nd position.
3.6. Electrochemical properties
Spectroscopic data obtained in media with increasing polarity
combined with DFT calculations revealed intramolecular charge
transfer character in the studied pyrimidine compounds (3aee,
5aec) with HOMO primarily localized on (carbazolyl)phenyl and
LUMO on pyrimidine(phenyl). An exception was compound 5d with
HOMO primarily localized on much stronger 4-(dimethylamino)
phenyl electron-donating group linked to the 2nd position of the
pyrimidine, and thus, expressing the largest ICTand lowest FF. Most
of the compounds showed positive solvatochromic effect resulting
in moderate bathochromic shifts of the fluorescence spectra from
50 to 90 nm concomitantly with dramatic enhancement of excited
state lifetimes (by a factor of ca. 5). The studies of the compounds
photophysical properties in solvents of increasing polarity unveiled
non-monotonous variation of FF determined by two competing
excitation deactivation mechanisms, i.e. the decrease of radiative
relaxation rate due to enhanced ICT and the decrease of non-
radiative relaxation rate most likely induced by the reduced inter-
system crossing to triplets.
Cyclic voltammetry was employed to elucidate the energies of
HOMO and LUMO of the pyrimidine derivatives 3aee and 5aed
(see Fig. 7 and Table 3). All the compounds demonstrated non-
reversible oxidation-reduction processes by applying low scan
rate (50 mV/s) in the range of positive potentials (HOMO level). A
quasi-reversible oxidation e reduction process in the range of
negative potentials was observed for compounds 3aed. The rest of
the structures exhibited non-reversible oxidation e reduction
process at a LUMO level. For all the compounds strong adsorption of
species on a working electrode surface was observed, thus the
measured values of LUMO potential were overestimated. Electro-
chemical analysis revealed fixed HOMO state while LUMO level is
highly dependent on type of substitution in pyrimidine core which
is in a good agreement with theoretical data (see Fig. 1).
4. Conclusions
Synthesis of the pyrimidine derivatives bearing various conju-
gated aryl branches at 2,4- and 4,6-positions of the pyrimidine ring
was accomplished by Suzuki coupling of 2,4-dichloro- or 4,6-
dichloropyrimidines with the appropriate arylboronic acids. Most
of the compounds exhibited UV-blue fluorescence (peaking at
Multi-fragment pyrimidine and carbazole intramolecular
charge transfer compounds show potential for applications as blue
emitters in OLED technologies, while the sensitivity of their emis-
sion properties to the polarity of surrounding media implies po-
tential applications for biomaterial labeling.

L. Skardziute et al. / Dyes and Pigments 118 (2015) 118e128
127
Table 3
Electrochemical properties (oxidation and reduction potentials, energies of HOMO and LUMO levels, electrochemical and optical bandgaps) of the pyrimidine derivatives 3aee
and 5aed.
Eox onset (eV)a
Ered
Eg
EHOMO (eV)d
ELUMO (eV)e
Eg
el
opt.
Compound
onset (eV)b
(eV)c
(eV)f
3a
3b
3c
3d
3e
5a
5b
5c
5d
1.27
1.28
1.31
1.28
1.26
1.28
1.27
1.27
1.23
ꢀ2.09
ꢀ1.94
ꢀ1.87
ꢀ1.91
ꢀ1.91
ꢀ1.79
ꢀ1.76
ꢀ1.71
ꢀ1.81
3.36
3.22
3.18
3.19
3.17
3.07
3.03
2.98
3.04
ꢀ5.605
ꢀ5.615
ꢀ5.645
ꢀ5.615
ꢀ5.595
ꢀ5.615
ꢀ5.605
ꢀ5.605
ꢀ5.565
ꢀ2.245
ꢀ2.395
ꢀ2.465
ꢀ2.425
ꢀ2.425
ꢀ2.545
ꢀ2.575
ꢀ2.625
ꢀ2.525
3.48
3.38
3.42
3.33
3.34
3.22
3.20
3.08
3.20
a
On-set of oxidation potential for HOMO level.
On-set of reduction potential vs. Ag/AgCl for LUMO level.
b
c
Electrochemical bandgap, jEg elj ¼ jELUMOj-jEHOMOj.
HOMO and LUMO energy, EHOMO/ELUMO ¼ ꢀ(4.8 + E1ox=2/E1=2 ꢀ E1=2).
d
FC
red
HOMO and LUMO energy, EHOMO/ELUMO ¼ ꢀ(4.8 + E1ox=2/E1=2 ꢀ E1=2).
e
f
FC
red
Optical bandgap, estimated from the onset of the absorption band in DMF.
(b) Mati SS, Chall S, Konar S, Rakchit S, Bhattacharya SC. Pyrimidine-based
fluorescent zinc sensor: photo-physicalcharacteristics, quantum chemical
interpretation and applicationin real samples. Sens Actuators B 2014;201:
204e12;
(c) Weng J, Mei Q, Ling Q, Fan Q, Huang W. A new colorimetric and fluorescent
ratiometric sensor for Hg2þ based on 4-pyren-1-yl-pyrimidine. Tetrahedron
2012;68:3129e34.
Acknowledgments
This research was supported by the European Social Fund under
the Global Grant measure(grant no.: VP1-3.1-SMM-07-K-02-012).
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switching and SUBSTRATE binding in ion-controlled nanomechanical molec-
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.03.008.
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