Synthesis and properties of bipolar derivatives of 1,3,5-triazine and carbazole

Article abstract of DOI:10.1016/j.dyepig.2015.11.001

Three new bipolar star-shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine containing carbazolyl groups were designed, synthesized and characterized. All the materials possess high thermal stability and high glass transition temperatures ranging 97-226 °C. Photophysical study of the dilute solutions and neat films of the synthesized compounds was performed. Lippert-Mataga plots revealed linear dependence of Stokes shifts on the orientation polarizability for all the compounds. The dilute solutions of the triazine derivatives exhibited high photoluminescence quantum yields reaching 0.85, while for the neat films photoluminescence efficiency of 0.20-0.33 was observed. Ionization potentials of the solid layers of carbazole-triazine adducts estimated by photoelectron spectroscopy were found to be in the range of 5.49-5.97 eV. Hole drift mobility of the materials well exceeded the magnitude of 10^-3 cm² V^-1 s^-1 at an electric field of 6.4 · 10⁵ V/cm. The selected compounds were tested as light emitting materials in organic light emitting diodes based on host-guest systems.

Full text of DOI:10.1016/j.dyepig.2015.11.001

Accepted Manuscript
Synthesis and properties of bipolar derivatives of 1,3,5-triazine and carbazole
T. Matulaitis, N. Kostiv, J.V. Grazulevicius, L. Peciulyte, J. Simokaitiene, V.
Jankauskas, B. Luszczynska, J. Ulanski
PII:
S0143-7208(15)00428-3
10.1016/j.dyepig.2015.11.001
DYPI 4992
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 July 2015
Revised Date: 30 October 2015
Accepted Date: 2 November 2015
Please cite this article as: Matulaitis T, Kostiv N, Grazulevicius JV, Peciulyte L, Simokaitiene J,
Jankauskas V, Luszczynska B, Ulanski J, Synthesis and properties of bipolar derivatives of 1,3,5-triazine
and carbazole, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.11.001.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Synthesis and properties of bipolar derivatives of 1,3,5-triazine and
ACCEPTED MANUSCRIPT
carbazole
a
a
a*
a
a
b
T. Matulaitis , N. Kostiv , J. V. Grazulevicius , L. Peciulyte , J. Simokaitiene , V. Jankauskas , B.
c
c
Luszczynska , J. Ulanski
a
Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19,
LT-50254, Kaunas, Lithuania, Fax: +37037 300152; Tel: +37037 300193
b
Department of Solid State Electronics, Vilnius University, Sauletekio al. 9, LT-10222, Vilnius, Lithuania
c
Department of Molecular Physics, Lodz University of Technology, Lodz, Poland
Abstract
Three new bipolar star-shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine containing carbazolyl
groups were designed, synthesized and characterized. All the materials possess high thermal stability and
high glass transition temperatures ranging 97-226 °C. Photophysical study of the dilute solutions and neat
films of the synthesized compounds was performed. Lippert-Mataga plots revealed linear dependence of
Stokes shifts on the orientation polarizability for all the compounds. The dilute solutions of the triazine
derivatives exhibited high photoluminescence quantum yields reaching 0.85, while for the neat films
photoluminescence efficiency of 0.20-0.33 was observed. Ionization potentials of the solid layers of
carbazole-triazine adducts estimated by photoelectron spectroscopy were found to be in the range of 5.49-
-3
2
-1 -1
5
.97 eV. Hole drift mobility of the materials well exceeded the magnitude of 10 cm V s at an electric
5
field of 6.4⋅10 V/cm. The selected compounds were tested as light emitting materials in organic light
emitting diodes based on host-guest systems.
Keywords
Triazine;
Carbazole;
*
Corresponding author e-mail: juozas.grazulevicius@ktu.lt
1

Photoluminescence;
Solvatochromism;
Hole mobility.
ACCEPTED MANUSCRIPT
Highlights
•
•
•
•
•
Three carbazolyl containing 1,3,5-triazine derivatives were synthesized.
The compounds exhibited superior thermal characteristics.
The solvatochromic behavior was demonstrated.
High photoluminescence quantum yields were observed for solutions and films.
Blue and green OLEDs were fabricated.
1. Introduction
Organic conjugated materials have received great attention in recent decades in optoelectronic and
electronic applications such as organic light−emitting diodes (OLEDs), organic solar cells, organic field
effect transistors because of their potentially low cost and possibility of easy fabrication of the devices [1–
4]. Recently, growing interest has been focused on the search of the materials with bipolar
charge−transporting properties for the application in OLEDs [5,6]. Using bipolar materials for the
preparation of emitting layers, enhanced performances and operational stability were demonstrated [7]. The
utilization of bipolar materials also offers the possibility to achieve efficient and stable single−layer OLEDs,
which are highly desirable because of simplification of the manufacturing process and reduction of the
production costs [8].
The most widely adopted strategy to obtain bipolar materials is to incorporate D−A combinations into
the same molecule, facilitating injection and transport of both holes and electrons [7,9,10]. Incorporation of
electron−donating and electron−withdrawing moieties into a light−emitting material may promote the
emission efficiency because of an extended π−conjugation. However, this might bring about a huge
bathochromic effect. Therefore, one of the major challenges in developing bipolar blue emitters with D−A
2

structures is preventing the red−shift in the emission while maintaining or even increasing their
ACCEPTED MANUSCRIPT
photoluminescence quantum yield.
Compared to other donors, carbazole is useful in the generation of short−wavelength absorption and
emission, as it is weaker π−donor than aromatic amines [11]. Bipolar hosts with carbazole as the donor
moiety usually possess higher triplet energy than their counterparts with triphenylamino group. The triplet
energy of carbazole (T = 3.19 eV) is higher than that of triphenylamine (T = 3.04 eV) [12]. Moreover,
1
1
carbazole is a fully aromatic moiety providing high thermal, chemical and environmental stability [13]. It
can be easily substituted with a wide variety of functional groups [14–16].
Triazine has an electron affinity larger than those of other typical electron−deficient heteroaromatic
compounds (e.g., pyridine, pyrimidine) [17–19]. Due to this reason, triazine has been frequently
incorporated into the backbone of conjugated compounds to improve their electron−injection and
electron−transportation abilities [20–22]. In addition, derivatives of triazine exhibit good thermal stability
[
19]. Rigid skeleton of 2,4,6-triphenyl-1,3,5-triazine ensures high symmetry. Compounds containing triazine
unit as electron-accepting center and carbazolyl or diphenylamino pheripheral units linked through
π−conjugated bridges showed aggregation induced emission and two-photon absorption properties [23].
In this paper we report on the synthesis of new star-shaped compounds containing both donor
(carbazole) and acceptor (2,4,6-triphenyl-1,3,5-triazine) moieties linked through various linking bridges. We
describe the results of optical, photophysical, electrochemical and computational studies which were carried
out in order to study structure−properties relationship of these hybrid materials.
2. Experimental
2.1. Materials
Trifluoromethanesulphonic acid, copper iodide, bis-(triphenylphosphin)-palladium(II) dichloride
Pd(PPh ) Cl ), triphenylphosphine (PPh ), trimethylsilylacethylene, triethylamine (Et N),
(
3
2
2
3
3
tetrabutilammoniumhydrosulfate (TBAHS), 1-bromohexane, 1M tetrabutylammoniumfluoride (n-Bu NF)
4
solution in THF, 4-iodobenzonitrile, aluminium trichloride (anhydrous), 18-crown-6, copper, sodium
3

sulphate, 9H-carbazole, potassium iodide, potassium iodate, 2-chloro-2-methylpropane, acetic acid, sodium
ACCEPTED MANUSCRIPT
hydroxide, potassium carbonate were purchased from Sigma-Aldrich and used as received. Poly(N-vinyl
carbazole) (PVK) and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) were also obtained
from Aldrich. The solvents, i.e. toluene, chloroform, ethyl acetate, n-hexane, diethyl ether, methanol,
acetone, acetonitrile, (Penta), dichloromethane (Poch), o-dichlorobenzene (Sigma−Aldrich) were dried and
distilled according the conventional procedures [24].
2.2. Instrumentation
Nuclear magnetic resonance spectra of deuterated chloroform solutions of the synthesized compounds
1
13
were recorded with a “Varian Gemini−2000” (300 MHz ( H), 75.4 MHz ( C)) spectrometer. All the data
are given as chemical shifts in δ (ppm), multiplicity, integration down field from (CH ) Si as the internal
3
4
standard. Mass spectra (MS) were obtained on “Waters ZQ 2000”. Elemental analysis data were obtained on
a EuroEA Elemental Analyzer. Infrared (IR) spectra were recorded using “Perkin Elmer Spectrum GX II
FT−IR System”. The spectra of the solid compounds were recorded in the form of KBr pellets.
-4
UV−Vis spectra of 10 M solutions of the compounds were recorded in quartz cells using Perkin
-5
Elmer Lambda 35 spectrometer. Photoluminescence (PL) spectra of 10 M solutions of the compounds were
recorded using Edinburgh Instruments’ FLS980 Fluorescence Spectrometer. Thin solid films for recording
of UV−VIS and PL spectra were prepared by drop casting 2 mg/ml solutions of the compounds in toluene on
the pre−cleaned quartz substrates. Fluorescence quantum yields (η) of the solutions and of the solid films
were estimated using the integrated sphere method [25]. An integrating sphere (Edinburgh Instruments)
coupled to the FLS980 spectrometer was calibrated with two standards: quinine sulfate in 0.1 M H SO and
2
4
rhodamine 6G in ethanol. Each quantum yield measurement was repeated 5 times and the error corridor was
estimated. Fluorescence decay curves of the samples were recorded using a time−correlated single photon
counting technique utilizing the nF920 Nanosecond Flashlamp as an excitation source. The phosphorescence
spectra were recorded at 77 K for the solid solutions of the compounds (1 wt %) in Zeonex polymer matrix
using nanosecond gated luminescence measurements (from 400 ps to 1 s) using a high energy pulsed
Nd:YAG laser emitting at 355 nm (EKSPLA). A model liquid nitrogen cryostat (Janis Research) was used
4

for the experiment. The blue-edge highest energy peak in the phosphorescence spectrum was taken for the
ACCEPTED MANUSCRIPT
1
0
T →S transition.
Differential scanning calorimetry (DSC) measurements were carried out with a TA Instruments “DSC
Q100” calorimeter. The samples were heated at a scan rate of 10 °C/min under nitrogen atmosphere.
Thermogravimetric analysis (TGA) was performed on a “Mettler TGA/SDTA851e/LF/1100”. The samples
were heated at a rate of 20 °C/min.
EP
Ionization potentials (IP ) of the films of the synthesized compounds were measured by electron
photoemission in air by the earlier reported procedure [26]. The materials were dissolved in chloroform and
coated onto Al plates pre−coated with ~0.5 µm thick methylmethacrylate and methacrylic acid copolymer
(MKM) adhesive layer. The function of MKM layer was not only to improve adhesion, but also to eliminate
the electron photoemission from Al layer. In addition, this layer is conductive enough to avoid charge
accumulation on it during the measurements. The measurement method was, in principle, similar to that
described in literature [27]. The samples were illuminated with monochromatic light from the quartz
monochromator. The negative voltage of -300 V was applied to the sample substrate. The counter−electrode
with a slit for illumination was placed at ~5 mm distance from the sample surface. The counter−electrode
0
.5
was connected to the input of electrometer for the photocurrent measurement. The I =f(hν) dependence
was plotted. The linear part of this dependence was extrapolated to the hν axis and the value was determined
as the photon energy at the interception point. We evaluate the ionization potential measurement error as
0.03 eV.
Cyclic voltammetry (CV) measurements were carried out with a glassy carbon working electrode in a
three−electrode cell using a µ−Autolab Type III (EcoChemie, Netherlands) potentiostat. Platinum wire and
Ag/AgNO (0.01 mol/l in acetonitrile) were used as counter and reference electrodes, respectively, and
3
Bu NBF in dichloromethane (0.1 M) was used as electrolyte. The data were collected using GPES (General
4
6
Purpose Electrochemical System) software. Electrochemical measurements were conducted at room
temperature at a potential rate of 100 mV/s. The reference electrode was calibrated versus
CV
ferrocene/ferrocenium redox couple. The solid state ionization potential energy (Ip ) was estimated from
CV
the onset oxidation potential by using the relationship Ip = 4.8 + E , where the potential is related to that
ox
5

SS
of ferrocenium/ferrocene. The electron affinity (EA ) values were obtained from the reduction potential
ACCEPTED MANUSCRIPT
CV
using the approximation EA = 4.8 + E .
red
Charge drift mobility measurements were performed by a xerographic time−of−flight (XTOF) method
[
28,29]. The samples for the charge mobility measurements were prepared as described earlier [30]. The
samples for the measurements were prepared by drop−casting 10 mg/ml solutions of the compounds in
o
CHCl onto cleaned ITO coated glass substrate. After casting the cells were heated at 70 C for 5 minutes.
3
The charge carriers were generated at the layer surface by illumination with pulses of nitrogen laser (pulse
duration was 1 ns, wavelength 337 nm). After photo excitation of the sample with a short light impulse, the
rate of potential discharge in the XTOF measurements has a plateau region. The transit time was determined
from the kink point in the transient photocurrent curves. The transit time t with the surface potential (U ) at
tr
0
the moment of illumination indicates the passage of holes through the entire thickness of the films (d) and
2
enables determination of the hole mobility as µ=d /U ·t . The experimental setup consists of a delay
0
tr
generator Stanford Research DG 535 and a digital storage oscilloscope Tektronix TDS754C.
The theoretical calculations were carried out using the Gaussian 09 quantum chemical package [31].
Full geometry optimizations of the compounds in their electronic ground state were performed with DFT
using the B3LYP functional consisting of Becke’s three parameter hybrid exchange functional combined
with the Lee–Yang–Parr correlation functional with the 6-31G(d) basis set in vacuum. The energies of the
highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals were obtained from
single point calculations in the framework of DFT B3LYP/6-311G(d,p) approach for the CH Cl solution.
2
2
Absorption spectra were simulated from the oscillator strengths of singlet transitions calculated by the TD-
DFT B3LYP/6-31G(d) method in vacuum.
2.3. Synthesis
2
,4,6-Tris(p-iodophenyl-)1,3,5-triazine (1) was prepared by the electrophilic cyclization reaction as
described in the literature [32]. 3,6-Di-tert-butylcarbazole (2) and 3-tert-butylcarbazole (3) were obtained by
the Friedel−Crafts alkylation according to the literature source [33]. The usage of iodination method of
6

Tucker yielded 3-iodocarbazole (4) [34]. 3-Iodo-9-hexylcarbazole (5), 3-(trimethylsilyl)ethynyl-9-
ACCEPTED MANUSCRIPT
hexylcarbazole (6) and 3-ethynyl-9-hexylcarbazole (7) were synthesized according the reported procedures
[
35].
2,4,6-Tris(4-(3-tert-butyl-carbazol-9-yl)phenyl)-1,3,5-triazine (TR1)
2,4,6-Tris(p-iodophenyl)1,3,5-triazine (1) (0.20 g, 0.29 mmol), 3-tert-buthylcarbazole (3) (0.39 g, 1.75
mmol) and 18-crown-6 (0.02 g, 0.03 mmol) were dissolved in o-dichlorobenzene (10 mL) and the resulting
solution was heated to reflux under nitrogen atmosphere. Then, K CO (0.41 g, 0.99 mmol) and Cu (0.11 g,
2
3
0.58 mmol) were added. After being stirred for 24 h (TLC control), the reaction mixture was cooled down to
the room temperature and filtered. The residue was carefully washed with chloroform and the solvent was
evaporated under vacuum. The product was purified by silica gel column chromatography, using the mixture
of hexane and dichloromethane in the volume ratio of 4:1 as an eluent, and recrystallized from the eluent
o
o
mixture of solvents to afford the yellowish crystals (0.24 g, 43% yield). m.p. = 346−348 C (DSC 344 C).
-1
IR (KBr), ν/cm : 3042, 2955, 2901, 2864, 1604, 1588, 1572, 1508, 1487, 1455, 1412, 1367, 1327, 1294,
1
1
6
256, 1229, 1172, 1150, 1015, 813, 767, 744, 730, 636, 517. H NMR (300 MHz, CDCl , δ ppm): 9.13 (d,
3
H, J = 8.63 Hz, Ar); 8.25 (d, 3H, J = 4.17 Hz, Ar); 8.24 (d, 3H, J = 1.37 Hz, Ar); 7.92 (d, 6H, J = 8.64 Hz,
Ar); 7.66 (d, 3H, J = 8.13 Hz, Ar); 7.60 (d, 6H, J = 1.04 Hz, Ar); 7.50 (td, 3H, J = 7.17 Hz, J = 1.17 Hz, Ar);
1
3
7
.38 (td, 3H, J = 7.95 Hz, J = 0.86 Hz, Ar); 1.54 (s, 27H, CH -). C NMR (75.5 MHz, CDCl , δ ppm):
3
3
171.4, 143.9, 142.4, 140.8, 138.7, 134.6, 130.9, 126.7, 126.3, 124.4, 123.8, 120.6, 116.8, 110.2, 109.7, 35.1,
3
2.2. Elemental analysis found: C, 85.18; H, 6.20; N, 8.62%; molecular formula C H N requires: C,
69
60
6
+
+
8
5.15; H, 6.21; C, 8.63%. MS (APCI , 20 V), found: [M+H] m/z: 974; molecular formula C H N
69 60
6
requires: M = 973.29 g/mol.
2,4,6-Tris(4-(3,6-di-tert-butyl-carbazol-9-yl)phenyl)-1,3,5-triazine (TR2)
7

TR2 was prepared by the Ullmann coupling reaction from 1 (0.38 g, 0.55 mmol) and 2 (0.93 g, 3.32
ACCEPTED MANUSCRIPT
mmol) using the same procedure as for the synthesis of TR1. The product was purified by silica gel column
chromatography, using the mixture of hexane and dichloromethane in the volume ratio of 6:1 as an eluent,
and recrystallized from the eluent mixture of solvents to afford the yellowish crystals (0.31 g, 49% yield).
o
o
-1
m.p. = 451−452 C (DSC 448 C). IR (KBr), ν/cm : 3048, 2960, 2903, 2866, 1604, 1590, 1570, 1509, 1489,
1
1
471, 1410, 1367, 1324, 1297, 1261, 1232, 1175, 1150, 1035, 1016, 878, 841, 810, 612. H NMR (300
MHz, CDCl , δ ppm): 9.11 (d, 6H, J = 8.71 Hz, Ar); 8.21 (dd, 6H, J = 1.64 Hz, J = 0.85 Hz, Ar); 7.89 (d,
3
6
1
1
8
2
H, J = 8.72 Hz, Ar); 7.60 (d, 1H, J = 0.74 Hz, Ar); 7.57 (dd, 10H, J = 1.86 Hz, J = 1.09 Hz, Ar); 7.53 (d,
1
3
H, J = 1.75 Hz, Ar); 1.52 (s, 54H, CH -). C NMR (75.5 MHz, CDCl , δ ppm): 171.4, 143.7, 142.6, 140.7,
3
3
38.9, 134.38, 130.9, 126.5, 124.1, 124.0, 120.6, 116.6, 109.7, 35.1, 32.3. Elemental analysis found: C,
+
5.21; H, 7.44; N, 7.35%; molecular formula C H N requires: C, 85.22; H, 7.42; N, 7.36%. MS (APCI ,
8
1
84
6
+
0 V), found: [M+H] m/z: 1142; molecular formula C H N requires: M = 1141.61 g/mol.
8
1
84
6
2,4,6-Tris(4-((9-hexyl-carbazol-3-yl)ethynyl)phenyl)-1,3,5-triazine (TR3)
Compound 1 (0.80 g, 1.16 mmol) was suspended in Et N (20 mL) in a dry 100 mL two−necked flask
3
and the resulting mixture was heated to reflux under nitrogen atmosphere. Then, bis-(triphenylphosphin)-
palladium (II) dichloride (0.05 g, 0.07 mmol), CuI (8.00 mg, 0.04 mmol) and 3-ethynyl-9-hexylcarbazole (7)
o
(
1.44 g, 5.24 mmol) were added. The reaction mixture was stirred for 24 hours at 90 C. Then the solvent
was evaporated under vacuum and the residue was extracted with chloroform. The organic phase was dried
over anhydrous Na SO . After evaporation of the solvents under reduced pressure the product was purified
2
4
by silica gel column chromatography, using the mixture of hexane and chloroform in the volume ratio of 9:1
-
as an eluent, and precipitated into methanol to afford the yellowish solid (0.44 g, 34% yield). IR (KBr, cm
1
1
)
: ν = 3048, 2924, 2853, 2203, 1595, 1568, 1505, 1477, 1369, 1352, 1175, 1124, 813, 744, 726. H NMR
(
300 MHz, CDCl , δ ppm): 8.77 (d, 6H, J = 8.53 Hz, Ar); 8.38 (d, 3H, J = 1.35 Hz, Ar); 8.14 (d, 3H, J =
3
7
.55 Hz, Ar); 7.79 (d, 6H, J = 8.55 Hz, Ar); 7.73 (dd, 3H, J = 8.48 Hz, J = 1.56 Hz, Ar); 7.53 (t, 3H, J =
.04 Hz, Ar); 7.44 (d, 3H, J = 8.20 Hz, Ar); 7.39 (d, 3H, J = 8.62 Hz, Ar) 7.30 (t, 3H, J = 7.02 Hz, Ar); 4.29
7
8

(
t, 6H, J = 7.08 Hz, -N-CH -); 1.89 (quin, 6H, J = 6.23 Hz, -CH -); 1.29-1.45 (m, 18H, -CH -); 0.92 (t, 9H, J
2
2
2
ACCEPTED MANUSCRIPT
1
3
=
6.86 Hz, -CH ). C NMR (75.5 MHz, CDCl , δ ppm): 171.2, 141.1, 140.5, 135.4, 131.8, 129.7, 129.1,
3
3
128.5, 126.4, 124.5, 123.1, 122.7, 120.8, 119.6, 113.1, 109.2, 109.0, 94.4 (-C≡C-), 88.1 (-C≡C-), 43.4 (-N-
C-), 31.8, 29.2, 27.2, 22.8, 14.3. Elemental analysis found: C, 86.14; H, 6.41; N, 7.45%; molecular formula
+
+
C H N requires: C, 86.13; H, 6.43; N, 7.44%. MS (APCI , 20 V), found: [M+H] m/z: 1130; molecular
81
72
6
formula C H N requires: M = 1129.52 g/mol.
8
1
72
6
2.4. Device fabrication
Electroluminescent devices were with the following configuration were fabricated: ITO/ poly 3,4-
ethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS)/PVK:PBD (40 wt %): TR2 (3 wt %)/LiF/Al
(device I), ITO/PEDOT:PSS/PVK:PBD (40 wt %):TR3(3 wt %)/LiF/Al (device II). PEDOT:PSS was used
for hole transport layer (HTL) (Fig. S14). The ITO−coated− glass substrates were cleaned by ultrasonic
treatment in the de−ionized water, acetone and isopropanol step by step during the 15 min for each solution.
The layer of PEDOT:PSS was obtained on the glass substrate with ITO sublayer by spin coating of the 1:2
solution with methanol at the speed of substrate rotation of ca. 2000 rpm. The fabricated layer of
o
PEDOT:PSS was dried at 200 C for 10 min. The active light emitting layers PVK:PBD (60/40 wt %) doped
by 3% of TR2 or TR3 with the thickness of 70 nm were spin coated from chlorobenzene solution on the top
of PEDOT:PSS . The speed of substrate rotation was 2000 rpm and after 10 s was increased to 3000 for 10 s.
o
After drying active the layers at 90 C for 30 min, the top electrode LiF/Al was deposited through the shadow
-5
mask by means of thermal vacuum evaporation at the pressure of ca. 10 Torr. The thicknesses of the layers
were measured by profilometer (Dektak XT, Brucker).
The current density−voltage characteristics of the fabricated OLEDs were recorded by Keithley 2400
Source Meter. The luminance and color of the light emission of the fabricated devices were measured with
the Minolta CS−200 camera. The electroluminescence spectra were recorded by MicroHR spectrometer and
a CCD camera 3500 (Horriba Jobin Yvon). Photoluminescence spectra (PL) of the active layers containing
TR2 and TR3 and without them were recovered by FLS980 fluorescence spectrometer with TMS300
monochromators and a red cooled detector (Hamamatsu R928P). The standard light source for measuring of
9

PL spectra was a 450 W xenon arc lamp. The absorption (UV) spectra of the films PVK:PBD,
ACCEPTED MANUSCRIPT
PVK:PBD:TR2, PVK:PBD:TR3 were recorded with the Cary 5000 UV−Vis−NIR spectrophotometer. All
the measurements were carried out at the room temperature.
3. Results and discussion
3.1. Synthesis
Star−shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine and carbazole were synthesized by the
synthetic routes shown in Scheme 1. TR1 and TR2 were obtained by the three−step synthesis. At the first
step, 4-iodobenzonitrile was subjected to cyclization with the help of trifluoromethansulphonic acid to
furnish 2,4,6-tris(p-iodophenyl-)1,3,5-triazine (1). At the second step alkylation of carbazole in the presence
of 2-chloro-2-methylpropane and AlCl was carried out. The alkylation products, 3,6-di-tert-butylcarbazole
3
(2) and 3-tert-butylcarbazole (3), were isolated. At the final step, compound 1 and carbazole derivatives 2
and 3 were subjected to Ullmann coupling [36] to obtain the target compounds TR1 and TR2. The synthetic
route to compound TR3 included five stages. Compound 7 was synthesized following the established routes
and finally it was coupled with 1 by Hagihara−Sonogashira cross−coupling reaction [37] to afford the target
product.
The chemical structures of the target compounds were identified by mass spectrometry, as well as IR
and NMR spectroscopies and elemental analysis. The data were found to be in good agreement with the
1
3
proposed structures. All the compounds exhibited characteristic signals at 171.2-171.4 ppm in C NMR
spectra which can be attributed to the carbon atoms of the triazine heterocycle. For compounds TR1 and
TR2 the characteristic singlet signal corresponding to the protons of methyl group of the tert-butyl moiety
1
was found at 1.54-1.55 ppm in H NMR spectra. Meanwhile, the carbon atoms of the ethynyl bridge linking
13
the chromophores of TR3 were characterized by the signals at 94.38 and 88.04 ppm in the C NMR
-1
spectrum and by absorption band at 2203 cm in the IR spectrum. Compounds TR2 and TR3 were found to
be well soluble in common organic solvents, such as chloroform, THF, methylene chloride, while TR1 was
moderately soluble in the above mentioned solvents.
1
0

3.2. Thermal properties
ACCEPTED MANUSCRIPT
The temperature of the onset of thermal degradation and glass transition temperature (T ) are important
g
parameters of glass−forming organic semiconductors, since they provide information on the thermal and
morphological stabilities of the active layers of the optoelectronic devices. The thermal properties of the
synthesized star−shaped derivatives were investigated by the combination of DSC and TGA. All the target
o
compounds exhibited very high thermal stability with the 5% weight loss temperatures exceeding 460 C
(see Fig. S1). Compounds TR1 and TR2 in which carbazole moiety contains branched tert-butyl groups
o
o
exhibited superior thermal stability (Td TR1 = 528 C and T
= 514 C) with respect of compound TR3 in
d TR2
o
which carbazole moiety contains long, linear hexyl group at N-9 position (Td TR3 = 467 C). The values of
the glass transition temperatures (T ), melting temperatures (T ), crystallization temperatures (T ) and the
g
m
cr
temperatures, at which initial loss of mass (5%) was observed (T ) are summarized in Table 1.
d
DSC scans of TR1-TR3 are presented in Figs. S2-S4. It was observed by DSC that both TR1 and
TR2, having one and two tert-butyl groups per carbazole moiety, respectively, can be transformed into the
glassy state by cooling from the melt. They showed melting in the first heating scans but did not crystallize
upon cooling. Compounds TR1 and TR2 exhibited glass transitions in the second DSC heating scans at 190
o
and 226 C, respectively. Compound TR3 was isolated after the synthesis as an amorphous substance. It did
not show any melting or crystallization during the DSC experiment and exhibited only glass transition at 97
o
C.
3.3. Optical and photophysical properties
UV−Vis absorption and fluorescence spectra of the dilute hexane solutions, as well as the spectra of
the thin films of compounds TR1-TR3 are presented in Fig. 1. The details of the optical and photophysical
properties of the compounds are summarized in Table 2. The shape of absorption spectra of compounds
TR1 and TR2 are similar, however the low−energy absorption maximum and the absorption edge of TR2
exhibit small bathochromic shift with respect to those of TR1. This observation can apparently be explained
by the larger amount of electron−donor tert-butyl groups in TR2. UV spectra of compounds TR1 and TR2
contain two well−expressed bands, while the spectra of compound TR3 reflect three well−pronounced
1
1

transitions. A notable difference in the absorption spectra of TR3 compared to those of TR1 and TR2 is that
ACCEPTED MANUSCRIPT
the oscillator strength of the low−energy absorption band of TR3 is larger than those of TR1 and TR2 (see
1
chapter 3.6.). The larger oscillator strength suggests that there is a higher probability for the S → CT
o
transition in TR3. The highest−energy absorption bands of the studied derivatives can be associated with
π→π* transitions. For compounds TR1 and TR2 absorption shoulders located at 261 and 262 nm,
respectively, correspond to the absorption of triphenyltriazine moiety [38]. For TR3 this shoulder was
observed only in the theoretical spectrum (Fig. S18). Absorption bands located in the range of 280–300 nm
in the UV spectra of the investigated compounds can be attributed to a local excitation of the carbazole
moiety [39]. The lowest−energy absorption bands located in the region of 340-430 nm can be assigned to the
1
intramolecular charge transfer (ICT) complex transition S → CT. The similar tendencies were observed for
o
the UV spectra of thin solid films. The shape of the absorption bands of the solid films of the derivatives
was found to be similar to those of the solutions. However, the red shifts (up to 8 nm) and negligible
broadening of the bands were observed indicating enhanced intermolecular interactions in the solid state.
UV and photoluminescence (PL) spectra of the dilute solutions of the studied compounds at room
temperature revealed their solvent−dependant absorption and emission properties. In order to investigate
solvatochromic effect UV and PL spectra of the dilute solutions of the compounds in the solvents of
different polarity, i.e. hexane (ε = 1.88), toluene (ε = 2.3), chloroform (ε = 4.81), tetrahydrofuran (ε = 7.6),
dichloromethane (ε = 8.93), acetone (ε = 20.7) and acetonitrile (ε = 37.5), were recorded. The measurements
performed for the solutions in the solvents of different polarity revealed small hypsochromic shifts of the
lowest−energy absorption bands with the increase of solvent polarity for all the compounds investigated (see
Fig. S5). We attribute these observations in the absorption spectra to the changes in the solvent refractive
index n and the solute dipole moment [40].
Phosphorescence spectra of the solid solutions of compounds TR1-TR3 in Zeonex polymer matrices
were recorded at 77 K temperature (Fig. 1). All the phosphorescence spectra possess well-defined vibronic
structures and are located in blue-green and green spectral regions. Since tert-butyl moiety is relatively weak
donor, no significant differences between phosphorescence spectra of TR1 and TR2 were observed. The
phosphorescence spectrum of TR3 was found to be red-shifted, as compared to those of TR1 and TR2. The
1
2

estimated values of triplet energy of TR1-TR3 are presented in Table 2. They were found to be comparable
ACCEPTED MANUSCRIPT
for TR1 and TR2 (2.65-2.68 eV) and lower for TR3 (2.38 eV).
Fig. 2 presents PL spectra of the solutions of TR1 in various solvents (results for other compounds are
given in Fig. S6). PL spectra of the solutions of all the compounds in hexane exhibited vibronic structures,
consisting of two well−expressed maxima, which disappeared in the spectra of the solutions in more polar
solvents. Disappearance of vibronic structure in polar solvents indicates that the photoexcited intramolecular
charge transfer (ICT) from carbazolyl− to triazine moiety was enhanced when the polar solvents were used.
Solvatochromism is caused by the differential solvation of the ground state and the first excited state of
the radiation−absorbing molecules [41]. With the increase of solvent polarity, the PL emission spectra of the
studied compounds became broader and exhibited bathochromic shifts of 36-182 nm. This observation
indicates that the dipole moments of all the compounds investigated are larger in the excited state than in the
ground state (i.e. µ > µ ). The differential solvation of the two states by solvents of varying polarity gives
e
g
rise to an increase in the Stokes shift with increasing solvent polarity [42]. All the compounds exhibited
positive solvatochromism. More precisely, the solvent relaxation process of the ICT fluorophore can be
described by the Lippert–Mataga equation (1) [43,44]:
ꢁ
ꢂꢃ
ꢆꢇꢂꢃ
∆
ꢀ =
−
(1),
ꢄ∙ꢁꢅꢃ
ꢄ∙ꢆꢅꢃ
where ∆f is defined as orientation polarizability, ε stands for a solvent dielectric constant, n corresponds to
the refractive index of a solvent. The gap between the maximum of the first absorption band and the
maximum of the corresponding fluorescence band is called the Stokes shift, and is usually expressed in
wavenumbers as ∆ṽ = ṽ − ṽ . Fig. 3b shows linear relationship of Stokes shift of compound TR1 on the
a
f
orientation polarizability, ∆f. This linearity indicates that the general solvent effects are dominant in the
spectral shifts for all the compounds. All the studied compounds showed linear Stokes shift dependance on
∆f, suggesting that they all are strong ICT fluorophores with highly localized excited state charge separation.
-1
-1
In the case of TR1 and TR3, the Stokes shift increased from 578 cm and 377 cm for the solutions in
-1
-1
hexane to 8776 cm and 8925 cm for the solutions in in acetonitrile, respectively. The largest Stokes shift
-1
-1
was exhibited by TR2, varying from 616 cm for the solution in hexane to 9843 cm for the solution in
1
3

acetonitrile. This observation provides the further evidence of the larger excited−state dipole moment of
ACCEPTED MANUSCRIPT
TR2 compared to those of TR1 and TR3. These results are in agreement with those reported for the other
bipolar triazine−based compounds [45–47].
Commission internationale de l'éclairage (CIE 1931) color coordinates (x, y) were calculated for the
emission of the solutions and thin films of TR1−TR3. It was found that the solutions of all the studied
compounds in non−polar hexane emit deep blue light (Table 2, Figs. S9−S11). With the increase of the
polarity of solvents the emission of the solutions of the compounds shifts from deep blue through green up
to yellow color. Estimation of chromaticity coordinates of the neat films of compounds TR1−TR3 revealed
that the emission of TR1 and TR2 remained blue, while the emission of TR3 appeared in the green region.
The different shift of the emission band of the film of TR3, as compared to those of TR1 and TR2, can be
explained by the different molecular packing in films. Compounds TR1 and TR2 contain bulky tert-butyl
moieties, which provide high sterical hinderance, and thus more chaotic packing in the films. Meanwhile, all
the aromatic part of TR3 lies in one plane (see Fig. S17), which can result in more favourable molecular
packing in the layer, leading to the stronger interaction (π−π stacking) between the molecules, which results
in more pronounced bathochromic shift of the emission color.
PL quantum yield (η) gives the information on the efficiency of the emission process. It is defined as
the ratio of the number of photons emitted to the number of photons absorbed [48]. PL quantum yields of the
solutions of the investigated compounds in the solvents of the different polarity as well as of thin solid films
were estimated using integrating sphere. Fig. 3b represents fluorescence quantum yield versus ∆f plot for
TR1 (the plots for TR2 and TR3 are given in Figs. S7b and S8b). Generally, the η of the solutions of
triazine derivatives TR1−TR3 decreased with the increase of the solvent polarity. The PL quantum yields of
TR1 demonstrated linear dependance on orientation polarizability. PL quantum yield of 83% was observed
for the solution in non−polar hexane, the lowest value of 23% was recorded for the solution in acetonitrile.
Compounds TR2 and TR3 demonstrated the highest values of η of 80% and 85% in toluene solutions, while
for the solutions in acetonitrile the values of η of 18% and 11% respectively were observed. With the
increase of the solvent orientation polarizability (∆f), more time for a typical ICT fluorophore is required to
reach its relaxed excited state, which enhances the non−radiative processess and leads to higher energy
1
4

losses. Therefore, the PL quantum yield is expected to decrease. PL efficiency of the solid films of TR1 and
ACCEPTED MANUSCRIPT
TR2 were found to be 33% and 29%, respectively, while that of the film of TR3 was 20%. The details of the
photophysical properties of the solutions of TR1 in different solvents are summarized in Table 3 (the data
for the solutions of TR2 and TR3 are given in Tables S1 and S2).
1
For more information on the dynamics of the CT→S emission, the PL decay curves of the dilute
o
solutions of compounds TR1−TR3 in the solvents of different polarity were recorded. The PL decays of the
dilute solutions of all the compounds studied could be well described by single−exponential fits (except the
2
solution of TR3 in acetonitrile). The accuracy of the fits was characterized by χ values of 1.00−1.08. With
the increase of the solvent polarity from hexane to dichloromethane the solutions of all the compounds
showed increasing values of fluorescence lifetime (τ), while τ of the solutions in acetone and acetonitrile
were found to be 1−3 ns shorter, compared to those observed for the solutions in dichloromethane. τ of the
solutions of TR3 was found to be 2.7−3.5 times shorter than those of TR1 and TR2. The shorter
fluorescence lifetimes and higher PL quantum yields of the solutions of TR3 as compared to those of the
solutions of TR1 and TR2 shows that the S →S transition is dominant in TR3. This is in agreement with
1
0
the significantly higher (by 2.7−3.0 times) oscillator strength of the transition S →S observed in the UV
0
1
spectrum of the solution of TR3 as compared with those of the solutions of TR1 and TR2.
To evaluate the contributions of radiative and nonradiative decay processes in the solutions of the
studied compounds, radiative and nonradiative decay rates, Γ and kNR, respectively, were estimated [49]. Γ
decreased exponentially with the increase of solvent polarity for the solutions of all the compounds (for TR1
see Fig. 3a, for TR2 and TR3 see Figs. S7a and S8a). For the solutions of TR3 the values of Γ were found
to be by ca. 3 times higher than those of the solutions TR1 and TR2. This observation is consistent with
larger oscillator strength of the S →S transition in the UV spectrum of TR3. The rate of kNR increased with
0
1
the increase of the solvent polarity, indicating the enhancement of the non−radiative processess. This
observation is consistent with the decrease of PL quantum yields with the increase of the solvent polarity.
Fluorescence decays of the neat films exhibited nonexponential behavior, pointing out several different
origins of the radiative transitions (Fig. 4). PL decays of the compounds investigated in the solid state were
best described by biexponential fits (see Table 2). The emission of the film of TR1 had dominant τ of 1.05
1
5

ns (67%) and also exhibited fluorescence of long-lived excited state species with a τ of 4.70 ns. The neat
ACCEPTED MANUSCRIPT
films of TR2 and TR3 demonstrated almost equal contribution of short-lived 1.29 ns and 1.38 ns for TR2
and TR3, respectively, and of long-lived excited state species with τ of 5.27 ns and 4.56 ns, respectively. To
our mind, the large difference between the dominant PL lifetimes of the all the compounds in solution and
solid state suggests complications due to intermolecular interactions or morphology, or can be a
consequence of different CT species in different solid-state conformations.
3
.4. Electrochemical properties
Electrochemical properties of TR1−TR3 were studied by CV in order to elucidate the electronic
energy levels which determine the energy and electron transfer processes and reversibility of redox
processes. Figure 5 illustrates the CV graphs of compounds TR1−TR3. Only TR2 showed reversible
oxidation at 1.10 V during CV experiment. Since compounds TR1 and TR3 contain unsubstituted active
C−6 positions of carbazole moieties, they showed irreversible oxidation processes at 0.89 V for TR1 and at
0.77 V for TR3. The similar observations were reported earlier [50,51]. For such compounds growing peak
of current during the CV experiment and the resulting formation and growth of thin films on the working
electrode demonstrate the occurrence of oxidation induced electropolymerization [52,53]. Compounds TR1
and TR2, in which the carbazole moiety is linked to triazine core through N−9 position of carbazole moiety,
demonstrated reversible reduction at -1.74 V and -1.77 V respectively. This observation shows that in these
conditions a stable radical anion is formed on the triphenyltriazine moiety [54]. TR3 did not exhibit any
notable reduction peak during the CV experiments.
Solid−state ionization potential is a useful parameter for organic semiconducting materials for the use
CV
CV
in optoelectronic applications. Solid state ionization potential (IP ) and electron affinity (EA ) values of
the synthesized compounds were estimated using ferrocene/ferrocenium as the standard redox system. The
CV
CV
half wave potentials and IP and EA values are summarized in Table 4. Compound TR2 was found to
CV
possess the highest IP value of 5.70 eV, while compound TR3 demonstrated the highest electron donating
CV
ability showing the value of IP of 5.37 eV.
1
6

3
.5. Photoelectric properties
ACCEPTED MANUSCRIPT
EP
The ionization potentials (IP ) of the solid samples of TR1−TR3 were also measured by the electron
photoemission in air method. The electron photoemission spectra of the layers of TR1−TR3 are shown in
EP
EP
Fig. S12. The values of IP are presented in Table 4. The IP values were found to be by 0.12 – 0.46 eV
CV
higher than the IP values of the corresponding compounds. However both the methods revealed the same
trend. The lowest IP values were recorded for TR3 while the highest values were observed for TR2. The
small differences between the values of IP obtained by electron photoemission spectrometry and by
electrochemical studies are apparently due to the difference in molecular interactions and molecular
arrangements in the solid layers and in dilute solutions of the derivatives.
3
.6. Theoretical calculations
The DFT approach [55] was applied for the interpretation of the structure−property relationship of
TR1−TR3. DFT calculations were performed using the Gaussian suite of programs (Gaussian 09W) [31].
The geometries were optimized in the ground state using basic restricted DFT B3LYP 6-31G(d) basis set in
vacuum. Our calculations show that the optimal neutral electronic ground state geometries are not planar for
the molecules of TR1 and TR2. The B3LYP ground state geometry shows a torsional angle of ca. 48°
between carbazole and phenylene fragments. The presence of ethynyl linker in the molecule of TR3 forbids
rotations around it. Therefore mutual orientation of lateral fragments with respect to the ethynyl linker is of
ca. 0.23°. The analysis of the optimized bond lengths in the electronic ground state obtained at the B3LYP
theoretical level for all three molecules revealed the similar bond lengths, with the smallest values observed
for TR3. The geometry optimizations for the molecules of TR1−TR3 can be found in Figure S17.
In order to estimate the role of 1,3,5-triazine−based star−shaped structure on the optical transitions, the
occupied and the unoccupied molecular orbitals contributing to the lowest energy electronic transitions were
studied using time−dependent version of DFT method (TD−DFT) [56]. Absorption spectra were simulated
from the oscillator strengths of singlet transitions calculated by the TD−DFT B3LYP/6-31G(d) method in
vacuum. Two dominant transitions are observed in the absorption spectra of compounds TR1 and TR2 (Fig.
S17), which can be attributed to the transition S →S , with the oscillator strengths of 0.55 for TR1 and of
0
1
1
7

0
.60 for TR2, and π→π* transitions with the oscillator strengths of 0.43 for TR1 and of 0.48 for TR2. The
ACCEPTED MANUSCRIPT
theoretical simulation of the vertical transitions of TR3 revealed that there is larger contribution of π→π*
transitions during the excitation of this compound, as compared to the other two compounds. The highest
intensity absorption band appears from the S →S transition with the oscillator strength of 1.65.
0
1
Figure 6 shows the frontier orbitals of the investigated molecules. In the case of TR1 and TR2, the
HOMO orbitals are delocalized over all three phenylene−carbazole fragments and the lobes are
perpendicularly oriented to C-N bonds linked to these chromophores. The similar tendencies were observed
for TR3. The HOMO orbitals of TR3 are also delocalized over all three carbazole−acetylene−phenylene
fragments. In all three molecules the LUMO orbitals are localized mostly over the central triphenyltriazine
part. The lobes of these orbitals show the inter−ring bonding character. The HOMO orbitals also indicate
that the electron distraction from the molecule will be connected with the electronic structure changes on the
terminal carbazole fragments. On the other hand, the electron reduction, affects mostly the 2,4,6-triphenyl-
1,3,5-triazine moiety.
3
.7. Charge−transporting properties
Xerographic time−of−flight measurements were used to characterize the charge−transporting
properties of the synthesized compounds. Investigations were carried out for the layers of TR1−TR3 coated
on aluminum plated glasses by solution processing technique. The photogeneration quantum efficiency of
holes was estimated from the magnitude of extracted charge in XTOF experiment after excitation of samples
with short nitrogen laser light pulse (λ = 337 nm, duration – 1 ns). Figure 7 shows the electric field
dependencies of photogeneration quantum efficiency holes for the layers of investigated compounds. For all
the compounds photogeneration quantum efficiency is strongly field dependent. These dependencies may be
n
approximated by a power law function η ~ E , where factor n = 3.2 or 3.1 for TR1 and TR2, and n = 2.1 for
TR3. Such dependence can be explained by Onsager photogeneration mechanisms, and big values of factor
n indicate that thermalization distance in photogeneration process is less than the Onsager radius [57].
Compound TR3 is distinguished with the smallest factor n and the highest values of quantum efficiency,
with reaches 0.36 at strong electric field.
1
8

Dispersive hole−transport was observed in the layers of TR1−TR3 coated by solution processing
ACCEPTED MANUSCRIPT
technique (Fig 8a). The hole−transit times (t ) needed for the estimation of hole mobilities were established
t
from the intersection points of two asymptotes of the double−logarithmic plots. The representative electric
field dependency of hole−drift mobility for the neat film of TR2 is shown in Fig. 8b and is Poolee−Frenkel
type characteristic. Such dependence is commonly observed for organic charge transporting materials in
amorphous state due to a disorder. The data of the hole mobility of the neat films of compounds TR1−TR3
are presented in Table 5. The highest hole mobility value is observed for the compound TR2. It approached
-2
2
-1 -1
the magnitude of 10 cm V s at high electric fields.
3
.8. Electroluminescent properties
Compounds TR2 and TR3 were tested as light emitting materials in the OLED structures based on
host−guest system. The blend of poly N-vinyl carbazole (PVK) and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-
1
,3,4-oxadiazole (PBD) (Fig. S13) was used as host, and TR2 and TR3 were employed as emitting guest
materials. PVK is widely known as hole transporting material. In addition, it possess a large exciton lifetime
58,59]. In order to enable the host system to transport both types of charge carriers i.e. electrons and holes,
[
PBD was mixed with PVK [60,61]. The HOMO and LUMO levels of PVK and PBD are suitable for good
energy transfer to the light emitting dopants TR2 and TR3 in it (Fig.9). UV and PL spectra of the layers of
the molecular mixtures PVK:PBD, PVK:PBD:TR2 and PVK:PBD:TR3 are given in Figure S15. The shape
of absorption spectra of the films containing light emitting dopants TR2 and TR3 remained similar to those
of PVK:PBD film, what means that the TR2 and TR3 have no significant influence on the absorbance of
active layers. Doping with the guest materials, however, was found to have influence on the PL spectra. The
PL maxima of the films of PVK:PBD, PVK:PBD:TR2 and PVK:PBD:TR3 were observed at 422nm, 450
nm and 485 nm, respectively.
The electroluminescence (EL) spectra of the fabricated devices I and II are presented in Figure 10.
The EL intensity maxima of the devices I and II were observed at 463 nm and 515 nm, respectively. Since
pure PVK:PBD based devices showed emission maxima at 433 nm [62], and EL emission maxima of the
devices I and II were observed at the same wavelengths as PL emission maxima of the films of TR2 or TR3
1
9

(
Fig. 10), we can conclude that effective excitation energy transfer from the host matrices to the guests takes
ACCEPTED MANUSCRIPT
place in the devices.
Figure 11 shows the current density−voltage characteristics and luminance−voltage characteristics of
the devices I and II. The characteristis OLEDs are summarized in the Table 6. The turn−on voltage (V ) of
on
2
device I was observed at 8 V for electroluminescence of 5.24 cd/m . In the case of device II, the V was
on
2
2
observed at 9.5 V for EL of 1.5 cd/m . The brightness maxima of 1342 and 1221 cd/m were observed at 18
V for devices I and II, respectively, with the maxima of current efficiency in the range of 0.1-0.55 cd/A. CIE
chromaticity coordinates of devices I and II were found to be (0.15, 0.13) and (0.21, 0.33), respectively (Fig.
S16). The increasing of the bias led to the reduction of the device brightness, followed by the structural
degradation. It is known, that V , brightness and the current efficiency of OLEDs are determined by the
on
thickness of the active layer, injection barriers, balance of charge carriers in the device [63–66]. The
thickness of active layer in our OLEDs was 70 nm, which is typical for such kind of devices. OLEDs
fabricated in this work do not contain additional hole−transporting or electron transporting layers in addition
to PEDOT:PSS. Such additional layers could improve characteristics of the devices and reduce V . In
on
addition, charge carrier balance in the polymer host−guest matrix devices depends on the amount of light
emitting dopant in the matrix blend [67–69]. The optimization of the concentration of TR2 and TR3 in
polymer matrix and the application of the additional hole− and/or electron−transporting layers would
hopefully provide the possibility of improving the device performance.
4. Conclusions
Star−shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine and carbazole with the different linking
topologies of the chromophores were synthesized by a combination of classical and modern synthetic
methodologies including Ullmann coupling reaction and Sonogashira cross−coupling reaction. The
compounds possess high thermal stability with the temperatures of the onsets of thermal degradation
exceeding 460 °C. They form molecular glasses with glass transition temperatures ranging from 97 to 226
°
C. Solvatochromic behavior of the compounds was investigated studying their solutions in the solvents of
different polarity. All the compounds exhibited the positive solvatochromism. Emission color of the
solutions could be tuned from deep blue to the yellow with the increase of solvent polarity. The dilute
2
0

solutions of the synthesized compounds showed fluorescence quantum yields reaching 0.85 and high Stokes
ACCEPTED MANUSCRIPT
-1
shifts (377-9842 cm ). All the compounds show linear Stokes shift dependancies on orientation
polarizability, suggesting that they all are strong intramolecular charge transfer fluorophores with highly
localized excited state charge separation. The PL decays of the dilute solutions of the compounds were well
described by single−exponential fits. Fluorescence quantum yields of the solid films of the compounds were
found to range from 0.2 to 0.33. Ionization potentials of the layers of the compounds estimated by
photoelectron spectroscopy range 5.49 eV − 5.97 eV. The hole drift mobility of one compound approached
-2
2
-1 -1
the magnitude of 10 cm V s at high electric fields. The selected compounds were tested as light
emitting materials in OLEDs based on host−guest system. The brightness maxima of 1342 and 1221 cd/m
were observed at 18 V for devices, with the maxima of current efficiency in the range 0.1 − 0.55 cd/A.
2
Acknowledgements
This work has been financially supported by the Taiwan-Latvia-Lithuania cooperation project
"Synthesis and studies of organic electroactive materials for effective and reliable optoelectronic devices"
(grant No TAPLLT1/13). Dr. H.K. Bisoyi is thanked for the valuable advices concerning the synthesis of the
compounds. L. Skardziute and S. Jursenas are thanked for the results of the basic photophysical
characterization of the compounds. Prof. A. P. Monkman is thanked for providing the conditions for
phosphorescence measurements.
2
1

References
ACCEPTED MANUSCRIPT
[
[
[
1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987;51:913.
doi:10.1063/1.98799.
2] Walzer K, Maennig B, Pfeiffer M, Leo K. Highly Efficient Organic Devices Based on Electrically
Doped Transport Layers. Chem Rev 2007;107:1233–71. doi:10.1021/cr050156n.
3] Kanibolotsky AL, Perepichka IF, Skabara PJ. Star-shaped π-conjugated oligomers and their
applications in organic electronics and photonics. Chem Soc Rev 2010;39:2695.
doi:10.1039/b918154g.
[
[
[
4] Tang J, Hua J, Wu W, Li J, Jin Z, Long Y, et al. New starburst sensitizer with carbazole antennas for
efficient and stable dye-sensitized solar cells. Energy Environ Sci 2010;3:1736.
doi:10.1039/c0ee00008f.
5] Reghu RR, Bisoyi HK, Grazulevicius J V., Anjukandi P, Gaidelis V, Jankauskas V. Air stable
electron-transporting and ambipolar bay substituted perylene bisimides. J Mater Chem 2011;21:7811.
doi:10.1039/c1jm11091h.
6] Son KS, Yahiro M, Imai T, Yoshizaki H, Adachi C. Analyzing Bipolar Carrier Transport
Characteristics of Diarylamino-Substituted Heterocyclic Compounds in Organic Light-Emitting
Diodes by Probing Electroluminescence Spectra. Chem Mater 2008;20:4439–46.
doi:10.1021/cm8004985.
[
[
[
7] Duan L, Qiao J, Sun Y, Qiu Y. Strategies to Design Bipolar Small Molecules for OLEDs: Donor-
Acceptor Structure and Non-Donor-Acceptor Structure. Adv Mater 2011;23:1137–44.
doi:10.1002/adma.201003816.
8] Zhu W, Hu M, Yao R, Tian H. A novel family of twisted molecular luminescent materials containing
carbazole unit for single-layer organic electroluminescent devices. J Photochem Photobiol A Chem
2
003;154:169–77. doi:10.1016/S1010-6030(02)00325-8.
9] Gong S, Fu Q, Wang Q, Yang C, Zhong C, Qin J, et al. Highly Efficient Deep-Blue
Electrophosphorescence Enabled by Solution-Processed Bipolar Tetraarylsilane Host with Both a
High Triplet Energy and a High-Lying HOMO Level. Adv Mater 2011;23:4956–9.
doi:10.1002/adma.201102758.
[
[
10] Gong S, Sun N, Luo J, Zhong C, Ma D, Qin J, et al. Highly Efficient Simple-Structure Blue and All-
Phosphor Warm-White Phosphorescent Organic Light-Emitting Diodes Enabled by Wide-Bandgap
Tetraarylsilane-Based Functional Materials. Adv Funct Mater 2014;24:5710–8.
doi:10.1002/adfm.201400149.
11] Moylan CR, Ermer S, Lovejoy SM, McComb I-H, Leung DS, Wortmann R, et al.
(Dicyanomethylene)pyran Derivatives with C2v Symmetry: An Unusual Class of Nonlinear Optical
2
2

Chromophores. J Am Chem Soc 1996;118:12950–5. doi:10.1021/ja962673g.
ACCEPTED MANUSCRIPT
[
12] Sasabe H, Takamatsu J, Motoyama T, Watanabe S, Wagenblast G, Langer N, et al. High-Efficiency
Blue and White Organic Light-Emitting Devices Incorporating a Blue Iridium Carbene Complex.
Adv Mater 2010;22:5003–7. doi:10.1002/adma.201002254.
[
[
[
[
13] Morin J-F, Leclerc M, Adès D, Siove A. Polycarbazoles: 25 Years of Progress. Macromol Rapid
Commun 2005;26:761–78. doi:10.1002/marc.200500096.
14] Michinobu T, Osako H, Shigehara K. Synthesis and Properties of 1,8-Carbazole-Based Conjugated
Copolymers. Polymers (Basel) 2010;2:159–73. doi:10.3390/polym2030159.
15] Blouin N, Leclerc M. Poly(2,7-carbazole)s: Structure−Property Relationships. Acc Chem Res
2
008;41:1110–9. doi:10.1021/ar800057k.
16] Tomkeviciene A, Grazulevicius JV, Kazlauskas K, Gruodis A, Jursenas S, Ke T-H, et al. Impact of
Linking Topology on the Properties of Carbazole Trimers and Dimers. J Phys Chem C
2
011;115:4887–97. doi:10.1021/jp111333v.
[
[
[
17] Nenner I. Temporary negative ions and electron affinities of benzene and N-heterocyclic molecules:
pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine. J Chem Phys 1975;62:1747.
doi:10.1063/1.430700.
18] Yamamoto T, Watanabe S, Fukumoto H, Sato M, Tanaka T. New Heteroaromatic Polymers
Consisting of Alkyl- and Amino-1,3,5-triazine Units. Their Basic Optical Properties andn-Type Time-
of-Flight Drift Mobility. Macromol Rapid Commun 2006;27:317–21. doi:10.1002/marc.200500773.
19] Fink R, Frenz C, Thelakkat M, Schmidt H-W. Aromatic polyethers with 1,3,5-triazine units as hole
blocking/electron transport materials in leds. Macromol Symp 1998;125:151–5.
doi:10.1002/masy.19981250113.
[
[
[
20] Zhong H, Xu E, Zeng D, Du J, Sun J, Ren S, et al. New Optoelectronic Materials Based on
Bitriazines: Synthesis and Properties. Org Lett 2008;10:709–12. doi:10.1021/ol702698r.
21] Kulkarni AP, Tonzola CJ, Babel A, Jenekhe SA. Electron Transport Materials for Organic Light-
Emitting Diodes. Chem Mater 2004;16:4556–73. doi:10.1021/cm049473l.
22] Chang C-H, Kuo M-C, Lin W-C, Chen Y-T, Wong K-T, Chou S-H, et al. A dicarbazole–triazine
hybrid bipolar host material for highly efficient green phosphorescent OLEDs. J Mater Chem
2
012;22:3832. doi:10.1039/c2jm14686j.
[
23] Huang W, Tang F, Li B, Su J, Tian H. Large cyano- and triazine-substituted D–π–A–π–D structures
as efficient AIEE solid emitters with large two-photon absorption cross sections. J Mater Chem C
2
014;2:1141. doi:10.1039/c3tc31913j.
2
3

[
[
[
24] Harwood LM, Moody CJ. Experimental organic chemistry: principles and practice. Oxford:
Blackwell scientific; 1990.
ACCEPTED MANUSCRIPT
25] de Mello JC, Wittmann HF, Friend RH. An improved experimental determination of external
photoluminescence quantum efficiency. Adv Mater 1997;9:230–2. doi:10.1002/adma.19970090308.
26] Malinauskas T, Daskeviciene M, Kazlauskas K, Su H-C, Grazulevicius JV, Jursenas S, et al.
Multifunctional red phosphorescent bis-cyclometallated iridium complexes based on 2-phenyl-1,2,3-
benzotriazole ligand and carbazolyl moieties. Tetrahedron 2011;67:1852–61.
doi:10.1016/j.tet.2011.01.026.
[
27] Miyamoto E, Yamaguchi Y, Yokoyama M. Ionization potential of organic pigment film by
atmospheric photoelectron emission analysis. Electrophotography 1989;28:364–70.
doi:http://doi.org/10.11370/isjepj.28.364.
[
[
[
28] Montrimas E, Gaidelis V, Pazera A. The discharge kinetics of negatively charged Se
electrophotographic layers. Lith J Phys 1966;6:569–76.
29] Vaezi-Nejad SM. Xerographic time of flight experiment for the determination of drift mobility in
high resistivity semiconductors. Int J Electron 1987;62:361–84. doi:10.1080/00207218708920988.
30] Reghu RR, Grazulevicius J V., Simokaitiene J, Miasojedovas A, Kazlauskas K, Jursenas S, et al.
Glass-Forming Carbazolyl and Phenothiazinyl Tetra Substituted Pyrene Derivatives: Photophysical,
Electrochemical, and Photoelectrical Properties. J Phys Chem C 2012;116:15878–87.
doi:10.1021/jp3019952.
[
[
31] Frisch MJ, Trucks GW, Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani,
G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M DJ. Gaussian 09,
Revision A.02. Gaussian 09, Revis A02 2009.
32] Hayami S, Inoue K. Structure and Magnetic Property of the Organic Triradical with Triazine
Skeleton; 2,4,6-Tris{p-(N-oxy-N-tert-butylamino)phenyl}triazine. Chem Lett 1999;28:545–6.
doi:10.1246/cl.1999.545.
[
[
[
33] Neugebauer FA, Fischer H. tert.-Butyl-substituierte Carbazole. Chem Ber 1972;105:2686–93.
doi:10.1002/cber.19721050829.
34] Tucker SH. LXXIV.-Iodination in the carbazole series. J Chem Soc 1926;129:546.
doi:10.1039/jr9262900546.
35] Beginn C, Gražulevičius J V., Strohriegl P, Simmerer J, Haarer D. Synthesis of poly(9-hexyl-3,6-
carbazolyleneethynylene) and its model compounds. Macromol Chem Phys 1994;195:2353–70.
doi:10.1002/macp.1994.021950706.
[
36] Weingarten H. Ullmann Condensation. J Org Chem 1964;29:977–8. doi:10.1021/jo01027a529.
2
4

ACCEPTED MANUSCRIPT
[
[
[
37] Thorand S, Krause N. Improved Procedures for the Palladium-Catalyzed Coupling of Terminal
Alkynes with Aryl Bromides (Sonogashira Coupling) †. J Org Chem 1998;63:8551–3.
doi:10.1021/jo9808021.
38] Zhong H, Lai H, Fang Q. New Conjugated Triazine Based Molecular Materials for Application in
Optoelectronic Devices: Design, Synthesis, and Properties. J Phys Chem C 2011;115:2423–7.
doi:10.1021/jp109806m.
39] Tao Y-M, Li H-Y, Xu Q-L, Zhu Y-C, Kang L-C, Zheng Y-X, et al. Synthesis and characterization of
efficient luminescent materials based on 2,1,3-benzothiadiazole with carbazole moieties. Synth Met
2
011;161:718–23. doi:10.1016/j.synthmet.2011.01.020.
[
[
[
[
[
[
40] Reichardt C. Solvents and Solvent Effects in Organic Chemistry. Weinheim, FRG: Wiley-VCH
Verlag GmbH & Co. KGaA; 2002. doi:10.1002/3527601791.
41] Reichardt C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem Rev 1994;94:2319–58.
doi:10.1021/cr00032a005.
42] Valeur B. Molecular Fluorescence. Weinheim, FRG: Wiley-VCH Verlag GmbH; 2001.
doi:10.1002/3527600248.
43] Lippert E. Dipolmoment und Elektronenstruktur von angeregten Molekülen. Zeitschrift Für
Naturforsch A 1955;10:541–5. doi:10.1515/zna-1955-0707.
44] Mataga N, Kaifu Y, Koizumi M. Solvent Effects upon Fluorescence Spectra and the Dipolemoments
of Excited Molecules. Bull Chem Soc Jpn 1956;29:465–70. doi:10.1246/bcsj.29.465.
45] Kukhta NA, Simokaitiene J, Volyniuk D, Ostrauskaite J, Grazulevicius JV., Juska G, et al. Effect of
linking topology on the properties of star-shaped derivatives of triazine and fluorene. Synth Met
2
014;195:266–75. doi:10.1016/j.synthmet.2014.06.019.
[
[
46] Huang B, Yin Z, Ban X, Jiang W, Dai Y, Zhang J, et al. Thermally activated delayed fluorescence of
N-phenylcarbazole and triphenylamine functionalised tris(aryl)triazines. Dye Pigment 2015;117:141–
8
. doi:10.1016/j.dyepig.2015.02.014.
47] Wang Q, Wallace JU, Lee TY-H, Ou JJ, Tsai Y-T, Huang Y-H, et al. Evaluation of propylene-, meta-,
and para-linked triazine and tert-butyltriphenylamine as bipolar hosts for phosphorescent organic
light-emitting diodes. J Mater Chem C 2013;1:2224. doi:10.1039/c3tc00588g.
[
[
48] Braslavsky SE. Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations
2
006). Pure Appl Chem 2007;79:293–465. doi:10.1351/pac200779030293.
49] Lakowicz JR. Principles of Fluorescence Spectroscopy. Boston, MA: Springer US; 2006.
doi:10.1007/978-0-387-46312-4.
2
5

ACCEPTED MANUSCRIPT
[
[
[
50] Ambrose JF, Nelson RF. Anodic Oxidation Pathways of Carbazoles. J Electrochem Soc
968;115:1159. doi:10.1149/1.2410929.
1
51] Chiu S-K, Chung Y-C, Liou G-S, Su YO. Electrochemical and Spectral Characterizations of 9-
Phenylcarbazoles. J Chinese Chem Soc 2012;59:331–7. doi:10.1002/jccs.201100601.
52] Reghu RR, Grazulevicius JV., Simokaitiene J, Matulaitis T, Miasojedovas A, Kazlauskas K, et al.
Glass forming donor-substituted s-triazines: Photophysical and electrochemical properties. Dye
Pigment 2013;97:412–22. doi:10.1016/j.dyepig.2013.01.017.
[
[
[
[
53] Data P, Pander P, Lapkowski M, Swist A, Soloducho J, Reghu RR, et al. Unusual properties of
electropolymerized 2,7- and 3,6- carbazole derivatives. Electrochim Acta 2014;128:430–8.
doi:10.1016/j.electacta.2013.12.108.
54] Lukeš V, Rapta P, Idzik KR, Beckert R, Dunsch L. Charged States of 1,3,5-Triazine Molecules as
Models for Star-shaped Molecular Architecture: A DFT and Spectroelectrochemcial Study. J Phys
Chem B 2011;115:3344–53. doi:10.1021/jp111297y.
55] Calais J-L. Density-functional theory of atoms and molecules. R.G. Parr and W. Yang, Oxford
University Press, New York, Oxford, 1989. Int J Quantum Chem 1993;47:101–101.
doi:10.1002/qua.560470107.
56] Stratmann RE, Scuseria GE, Frisch MJ. An efficient implementation of time-dependent density-
functional theory for the calculation of excitation energies of large molecules. J Chem Phys
1
998;109:8218. doi:10.1063/1.477483.
[
[
57] Borsenberger PM, Weiss D. Organic Photoreceptors for Imaging Systems. New York: CRC Press;
993.
1
58] Bolink HJ, Santamaria SG, Sudhakar S, Zhen C, Sellinger A. Solution processable phosphorescent
dendrimers based on cyclic phosphazenes for use in organic light emitting diodes (OLEDs). Chem
Commun 2008:618–20. doi:10.1039/B715239F.
[
59] Yap CC, Yahaya M, Salleh MM. Influence of tetrabutylammonium hexafluorophosphate (TBAPF6)
doping level on the performance of organic light emitting diodes based on PVK:PBD blend films.
Curr Appl Phys 2009;9:722–6. doi:10.1016/j.cap.2008.06.013.
[
[
60] Huang J, Hou W-J, Li J-H, Li G, Yang Y. Improving the power efficiency of white light-emitting
diode by doping electron transport material. Appl Phys Lett 2006;89:133509. doi:10.1063/1.2357938.
61] Céspedes-Guirao FJ, García-Santamaría S, Fernández-Lázaro F, Sastre-Santos A, Bolink HJ.
Efficient electroluminescence from a perylenediimide fluorophore obtained from a simple solution
processed OLED. J Phys D Appl Phys 2009;42:105106. doi:10.1088/0022-3727/42/10/105106.
2
6

[
[
[
[
[
62] Luszczynska B, Dobruchowska E, Glowacki I, Ulanski J, Jaiser F, Yang X, et al. Poly(N-
vinylcarbazole) doped with a pyra oloq in line dye ee ue light-emitting composite for light-
z u o : A d p bl
ACCEPTED MANUSCRIPT
emitting diode applications. J Appl Phys 2006;99:024505. doi:10.1063/1.2162268.
63] Cherpak V, Stakhira P, Minaev B, Baryshnikov G, Stromylo E, Helzhynskyy I, et al. Mixing of
Phosphorescent and Exciplex Emission in Efficient Organic Electroluminescent Devices. ACS Appl
Mater Interfaces 2015;7:1219–25. doi:10.1021/am507050g.
64] Sun JW, Lee J-H, Moon C-K, Kim K-H, Shin H, Kim J-J. A Fluorescent Organic Light-Emitting
Diode with 30% External Quantum Efficiency. Adv Mater 2014;26:5684–8.
doi:10.1002/adma.201401407.
65] Stakhira P, Khomyak S, Cherpak V, Volyniuk D, Simokaitiene J, Tomkeviciene A, et al. Blue organic
light-emitting diodes based on pyrazoline phenyl derivative. Synth Met 2012;162:352–5.
doi:10.1016/j.synthmet.2011.12.017.
66] Seo JH, Kim HM, Choi EY, Choi DH, Park JH, Yoo HS, et al. Phosphorescent Organic Light-
Emitting Diodes with Simplified Device Architecture. Jpn J Appl Phys 2010;49:08JG04.
doi:10.1143/JJAP.49.08JG04.
[
[
[
67] Lee I-H, Gong M-S. New Fluorescent Blue OLED Host and Dopant Materials Based on the
Spirobenzofluorene. Bull Korean Chem Soc 2011;32:1475–82. doi:10.5012/bkcs.2011.32.5.1475.
68] Zeng L, Lee TY-H, Merkel PB, Chen SH. A new class of non-conjugated bipolar hybrid hosts for
phosphorescent organic light-emitting diodes. J Mater Chem 2009;19:8772. doi:10.1039/b909787b.
69] Zhou X, Qin DS, Pfeiffer M, Blochwitz-Nimoth J, Werner A, Drechsel J, et al. High-efficiency
electrophosphorescent organic light-emitting diodes with double light-emitting layers. Appl Phys Lett
2
002;81:4070. doi:10.1063/1.1522495.
2
7

ACCEPTED MANUSCRIPT
Table 1. Thermal characteristics of compounds TR1-TR3.
Table 2. Optical and photophysical properties of the dilute hexane solutions and neat films of
compounds TR1-TR3.
Table 3. Photophysical characteristics of the dilute solutions of compound TR1 in the solvents
of different polarity.
Table 4. Electrochemical and photoelectrical characteristics of compounds TR1-TR3.
Table 5. Hole mobility data of the layers of compounds TR1-TR3.
Table 6. EL performances of multi-layered devices with the synthesized emitters. Device:
ITO/PEDOT:PSS/PVK:PBD (40 wt %):guest(3 wt %)/LiF/Al.