Synthesis, photophysical and electrochemical properties of novel carbazole-triazine based high triplet energy, solution-processable materials

Article abstract of DOI:10.1016/j.dyepig.2018.06.014

A series of molecules; tBuCz1SiTrz, tBuCz2SiTrz and tBuCz3SiTrz, which contain carbazole unit as hole-transporting group (donor-D) and triazine unit as electron transporting group (acceptor-A) were synthesized and characterized as high-triplet energy (>2.9 eV), solution-processable bipolar emitting materials. The conjugation between the D-A groups was interrupted by using bulky tetraphenylsilane groups as spacer aiming to obtain large bandgap and high-triplet energy. The photophysical behaviors of the molecules were investigated by UV-Vis absorption, photoluminescence, phosphorescence, photoluminescence quantum yield and lifetime measurements. Solvent polarity effects were investigated on the intramolecular charge transfer (ICT) behaviour and large solvatochromic effect was observed with the increasing solvent polarity. Electrochemical properties were determined by cyclic voltammetry. All molecules showed oxidation bands arise from the carbazole groups. Reduction bands were originated from the triazine groups and the intramolecular charge transfer between D-A groups. Photophysical, electrochemical and computational characterizations addressed that tBuCz2SiTrz has the weakest ICT character, highest photoluminescence quantum yield (PLQY) and charge balance.

Full text of DOI:10.1016/j.dyepig.2018.06.014

Accepted Manuscript
Synthesis, photophysical and electrochemical properties of novel carbazole-triazine
based high triplet energy, solution-processable materials
Saliha Oner, Murat Aydemir, Fatih Yesil, Cigdem Sahin, Canan Varlikli
PII:
S0143-7208(18)30659-4
10.1016/j.dyepig.2018.06.014
DYPI 6819
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 25 March 2018
Revised Date: 20 May 2018
Accepted Date: 8 June 2018
Please cite this article as: Oner S, Aydemir M, Yesil F, Sahin C, Varlikli C, Synthesis, photophysical and
electrochemical properties of novel carbazole-triazine based high triplet energy, solution-processable
materials, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.06.014.
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ACCEPTED MANUSCRIPT

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Synthesis, photophysical and electrochemical properties of novel carbazole-
triazine based high triplet energy, solution-processable materials
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Saliha Onera, b, Murat Aydemirc, Fatih Yesila, Cigdem Sahind, Canan Varliklie*
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aSolar Energy Institute, Ege University, Bornova, 35100 Izmir, Turkey
bBernard Dyo Boya Fab. San. Tic. A.Ş., AOSB Çiğli, 35620 Izmir, Turkey
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cDepartment of Basic Sciences, Erzurum Technical University, 25070 Erzurum, Turkey
dDepartment of Chemistry, Art&Science Faculty, Pamukkale University, Denizli, Turkey
eDepartment of Photonics, Izmir Institute of Technology, 35430 Urla-Izmir, Turkey
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*Corresponding author. Tel: +90 232 7507732;
E-mail: cananvarlikli@iyte.edu.tr
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Abstract
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A series of molecules; tBuCz1SiTrz, tBuCz2SiTrz and tBuCz3SiTrz, which contain carbazole
unit as hole-transporting group (donor-D) and triazine unit as electron transporting group
(acceptor-A) were synthesized and characterized as high-triplet energy (> 2.9 eV), solution-
processable bipolar emitting materials. The conjugation between the D-A groups was
interrupted by using bulky tetraphenylsilane groups as spacer aiming to obtain large bandgap
and high-triplet energy. The photophysical behaviors of the molecules were investigated by
UV-Vis absorption, photoluminescence, phosphoroscence, photoluminescence quantum yield
and lifetime measurements. Solvent polarity effects were investigated on the intramolecular
charge transfer (ICT) behavior and large solvatochromic effect was observed with the
increasing solvent polarity. Electrochemical properties were determined by cyclic
voltammetry. All molecules showed oxidation bands arise from the carbazole groups.
Reduction bands were originated from the triazine groups and the intramolecular charge
transfer between D-A groups. Photophysical, electrochemical and computational
characterizations addressed that tBuCz2SiTrz has the weakest ICT character, highest
photoluminescence quantum yield (PLQY) and charge balance.
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Keywords: Bipolar materials; charge transfer; carbazole; triazine
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1. Introduction
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In terms of organic optoelectronic research, obtaining efficient and stable blue color is still a
challenge. Up to date many different blue emitting and host materials including carbazole
derivatives have been reported [1-3]. High triplet energy (ET) values and high oxidative
potentials make carbazole derivatives attractive as hole transport materials as well [4-8].
Recently, bipolar emitting materials those of which provide both hole and electron
transporting groups in the same molecule and ensure balanced charge transfer for effective
radiative recombination started to take intensive attention [6,9-12]. Several electron accepting
groups such as phosphine oxide [13], diarylborane [14], benzimidazole [6] and triazine [15-
18] have been attached to carbazole through continuous or interrupted π conjugation. These
bipolar materials have been presented as both bipolar hosts available to be used with blue, red
or yellow-green-emitting materials and emitters by themselves. Within the electron accepting
groups, triazine attracted much attention. However in most of the triazine containing bipolar
material reports, high electron affinity of it could not be balanced and increasing the units of
electron donor carbazoles around the triazine core resulted in bathochromic shifts mainly
because of the extended π conjugation [16, 19-21]. Interrupted π conjugation allows
intramolecular charge transfer (ICT) mechanism [22, 23] and may provide balanced charge
transfer by preventing those shifts.
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Herein, series of bipolar materials are reported by increasing the number of electron donating
(D) carbazole units around the electron accepting (A) triazine. Tetraphenylsilane is used for
the interruption of the π conjugation between D and A and to ensure a high ET with the δ -Si
structure and to increase the steric hindrance [24, 25]. Variation of charge transfer
mechanisms depending on the number of D units is reported.
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2. Experimental section
2.1. Materials and instruments
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All the reagents and solvents used were purchased from Aldrich and used without further
purification. The solvents used for the purification steps were technical grade and distilled
before using. THF was dried over Na/benzophenone and freshly used for the reactions.
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Toluene was dried over Na wire. H-NMR and C-NMR spectra were recorded on a Bruker
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spectrometer (1H at 400 MHz, C at 100 MHz) using tetramethylsilane (TMS) as an internal
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standard. Mass spectra were obtained using Bruker Daltonics – Autoflex III smartbeam
MALDI TOF/TOF MS. UV-Vis absorption spectra were recorded by using Analytik Jena S
600 UV-Vis spectrophotometer. Photoluminescence (PL), lifetime and photoluminescence
quantum yield (PLQY) studies were performed by using Edinburgh Instruments FLS920P
spectrophotometer. Absolute PLQY values of the solution and films were determined with the
integrating sphere attachement. Molecular geometries of molecules were optimized by using
Hyperchem Package Version 8.0 computational software. Semi empirical calculations were
performed using AM1 method [26, 27]. Electrochemical studies were carried out with a CH
Instrument 660 B Model Electrochemical Workstation by using a three-electrode
electrochemical cell configuration. Glassy carbon as the working electrode, Pt wire as the
auxiliary electrode and Ag wire as the reference electrode were used. Ferrocene/ferrocenium
(CP2Fe/CP2Fe+) redox couple was used as the internal standard and 0.1 M TBAP in DMF was
used as the supporting electrolyte. Cyclic voltammograms were obtained at a scan rate of 100
mV s−1 and the oxidation potentials (Eox) were determined from the onsets of the oxidation
peaks. Current density-Voltage curves of hole and electron only devices were obtained using
a Keithley 2400 source measurement unit.
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2.2. Synthesis
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Synthesis of 3,6-di-tert-butyl-9H-carbazole (1): Carbazole (5.01 g, 30 mmol) and AlCl3
(3.99 g, 30 mmol) were weighed into a 250-mL 3-necked round-bottom flask.
Dichloromethane (DCM, 100 mL) was added to the flask and the solution was cooled to 0 °C
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with an ice-water bath. Tert-butyl chloride (t-BuCl, 66 mmol, 7.3 mL) was dissolved in
CH2Cl2 (20 mL) and added to the flask dropwise at 0 °C. The resultant solution was left to stir
at room temperature (RT) overnight. 100 g of cold water was added to the reaction mixture
and the organic phase was extracted. The water phase was washed with CH2Cl2 (50 mL) and
the organic phases were combined and dried over MgSO4. After removal of the solvent, the
residue was purified by recrystallization from CH2Cl2/petroleum ether mixture and pale white
solid was obtained. Yield: 54%. 1H-NMR (CDCl3, 400 MHz): δH (ppm) 8.07 (2H, s, Ar), 7.66
(1H, s, N–H), 7.47–7.42 (2H, d, J=8.4 Hz, Ar), 7.25–7.22 (2H, d, J=8.8 Hz, Ar), 1.44 (18H,
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s). C-NMR (CDCl3, 100 MHz): δC (ppm) 142.49, 138.29, 123.76, 123.56, 116.40, 110.30,
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34.96, 32.31.
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Synthesis of bis(4-bromophenyl)(diphenyl)silane (2): 1,4-dibromobenzene (12 mmol, 2.83
g) was weighed into a 250-mL, 2 necked flask and dissolved in THF (50 mL). The solution
was cooled to –78 °C by using a dry ice/acetone mixture. N-butyllithium (n-BuLi, 2.5 M in
hexane, 14.40 mmol, 5.76 mL) was added dropwise to the flask using a syringe and the
solution was stirred at –78 °C for 1 h. After that, dichloro(diphenyl)silane (5 mmol, 1.05 mL)
was added dropwise and the resultant solution was stirred at RT overnight. Water (50 mL)
was added to the reaction mixture and the THF phase was extracted. The water phase was
washed with CH2Cl2 (2x50 mL), and the organic phases were collected and dried over
MgSO4. After the removal of the solvents, a white solid was obtained and purified by
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recrystallization from the DCM/EtOH mixture. Yield: 51%. H-NMR (CDCl3, 400 MHz): δH
(ppm) 7.60–7.52 (8H, m, Ar), 7.47–7.35 (10H, m, Ar). 13C-NMR (100 MHz, CDCl3): δC
(ppm) 138.07, 136.47, 133.27, 132.93, 131.47, 130.25, 128.36, 125.23.
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Synthesis of 9-{4-[(4-bromophenyl)(diphenyl)silyl]phenyl}-3,6-di-tert-butyl-9H-carbazole
(3): 100 mL, 2-necked round-bottom flask equipped with a reflux condenser was dried by
using heat-gun.Bis(4-bromophenyl)(diphenyl)silane (7.0 mmol, 3.46 g), 3,6-di-tert-butyl-9H-
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carbazole (5.0 mmol, 1.40 g), CuI (1.0 mmol, 0.19 g) and K3PO4 (10.0 mmol, 2.12 g) were
weighed and transferred into the reaction flask. Three times vacum - gas (Ar) was applied to
remove the O2 from the reaction flask. 1,4-dioxane (25 mL) was added to the reaction flask
and stirred for about 10 minutes. After that, trans-1,2-diaminocyclohexane (2 mmol, 0.24 mL)
was added and the resultant solution was stirred at 100 oC for overnight. EtOAc was added to
the mixture and unsoluble material was filtered through short silica column. The crude
product was purified by column chromatography using hexane/CH2Cl2 (10/1) solvent system.
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Yield: 48%. H-NMR (CDCl3, 400 MHz): δH (ppm) 8.08-8.04 (2H, d, Ar), 7.70-7.64 (2H, d,
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Ar), 7.58-7.46 (8H, m, Ar), 7.44-7.32 (12H, m, Ar), 1.42-1.36 (18H, s). 13C-NMR (CDCl3,
100 MHz): δC (ppm) 143.34, 140.02, 139.16, 138.21, 137.94, 136.59, 133.66, 133.32, 132.40,
131.50, 130.25, 128.38, 126.05, 125.20, 123.84, 116.59, 116.41, 109.60, 34.97, 32.17.
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Synthesis of {4-[[4-(3,6-di-tert-butyl-9H-carbazole-9-yl)phenyl](diphenyl)silyl]phenyl}
boronic acid (4): 50 mL, 2-necked round-bottom flask was dried by using heat-gun. 9-{4-[(4-
bromophenyl)(diphenyl)silyl]phenyl}-3,6-di-tert-butyl-9H-carbazole (2.90 mmol, 2.00 g) was
added to the reaction flask. Three times vacum - gas (Ar) was applied to remove the O2 from
the reaction flask. Dry THF (30 mL) was added to the reaction flask and the solution was
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cooled down to -78 C by using dry ice/acetone mixture. After cooling, n-BuLi (5.80 mmol,
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2.32 mL) was added dropwise to the flask. The resultant solution was stirred at -78 C for 1
hours. After that, B(OMe)3 (11.6 mmol, 1.29 mL) was added dropwise and the solution was
left for stirring at RT for overnight. The reaction was stopped with the addition of MeOH (10-
15 mL) and solvents were evaporated on rotary evaporator. The crude product was purified by
column chromatography using hexane/EtOAc (2/1) solvent system. Yield: 43%. This product
is directly used for the next step.
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Synthesis of 3,6-di-tert-butyl-9-{4-[[4-(4,6-dipheyl-1,3,5-triazine-2-yl)phenyl](diphenyl)
silyl] phenyl}-9H-carbazole (tBuCz1SiTrz) (5): 25 mL, 2-necked round-bottom flask was
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dried by using heat-gun. {4-[[4-(3,6-di-tert-butyl-9H-carbazole-9-yl)phenyl](diphenyl)
silyl]phenyl} boronic acid (0.27 mmol, 177.7 mg), 2-chloro-4,6-diphenyl-1,3,5-triazine (0.27
mmol, 72.4 mg) and Cs2CO3 (0.54 mmol, 176.0 mg) were added to the reaction flask. Three
times vacum - gas (Ar) was applied to remove the O2 from the reaction flask.1,4-dioxane (5
mL) was added to the flask and Argon gas was bubbled through the solution for about 30
minutes. Pd(PPh3)4 (0.027 mmol, 31.3 mg) was added to the flask and the resultant mixture
was stirred at 100 oC for overnight. 1,4-dioxane was removed by using rotary evaporator and
the crude product was purified by column chromatography using hexane/CH2Cl2 (10/1)
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solvent system. Yield: 40%. H-NMR (CDCl3, 400 MHz): δH (ppm) 8.85-8.79 (6H, m, Ar),
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8.17 (2H, s, Ar), 7.92-7.89 (2H, d, J=8.0 Hz, Ar), 7.87-7.85 (2H, d, J=8.0 Hz, Ar), 7.75-7.72
(4H, d, J=6.0 Hz, Ar), 7.67-7.57 (9H, m, Ar), 7.53-7.47 (9H, m, Ar), 1.50 (s, 18H). 13C-NMR
(CDCl3, 100 MHz): δC (ppm) 171.74, 143.06, 139.75, 139.60, 138.95, 137.80, 137.53, 136.76,
136.45, 136.20, 133.56, 132.54, 132.35, 129.97, 128.98, 128.63, 128.18, 128.14, 125.83,
123.61, 123.58, 116.24,109.38, 34.74, 32.01, 29.70. MALDI-TOF MS, Found: [M]+ m/z:
845.2498; 'molecular formula C59H52N4Si' requires [M]+ 845.1586.
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Synthesis of 3,6-di-tert-butyl-9-(4-{diphenyl[4-(tributylstannyl)phenyl]silyl}phenyl)-9H-
carbazole (6): 250 mL, 2-necked round-bottom flask was dried by using heat-gun. 9-{4-[(4-
bromophenyl)(diphenyl)silyl]phenyl}-3,6-di-tert-butyl-9H-carbazole (6.64 mmol, 4.60 g) was
added to the reaction flask. Three times vacum - gas (Ar) was applied to remove the O2 from
the reaction flask. Dry THF (100 mL) was added to the reaction flask and the solution was
cooled down to -78 oC by using dry ice/acetone mixture. After cooling, n-BuLi (8.0 mmol, 3.2
mL) was added dropwise to the flask. The resultant solution was stirred at -78 oC for 1 hours.
After that, Bu3SnCl (9.3 mmol, 2.53 mL) was added dropwise and the solution was left for
stirring at RT for overnight. The reaction was stopped with the addition of H2O (100 mL) and
EtOAc (100 mL) was added. Organic phase was extracted and water phase was washed with
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2x50 mL EtOAc. Organic phases were combined and dried over MgSO4. Solvents were
evaporated on rotary evaporator. The crude product was purified by column chromatography
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using cyclohexane/CH2Cl2 (10/1) solvent system. Yield: 50%. H-NMR (CDCl3, 400 MHz):
δH (ppm) 8.18 (2H, s, Ar), 7.84-7.82 (2H, d, J=8.0 Hz, Ar), 7.71-7.69 (4H, d, J=8.0 Hz, Ar),
7.64-7.62 (4H, m, Ar), 7.59-7.57 (2H, d, J=8.0 Hz, Ar), 7.54-7.45 (10H, m, Ar), 1.64-1.57
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(16H, m), 1.51 (18H, s), 1.42-1.35 (6H, m), 1.14-1.10 (6H, m), 0.96-0.93 (9H, t). C-NMR
(CDCl3, 100 MHz): δC (ppm) 144.51, 142.98, 139.45, 138.96, 137.82, 136.48, 136.14, 135.65,
134.16, 129.75, 128.00, 125.70, 123.60, 123.51, 116.25, 109.40, 34.77, 32.05, 29.15, 27.45,
13.74, 9.62.
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Synthesis of 2,4-dichloro-6-phenyl-1,3,5-triazine (7): 250 mL 2-necked round-bottom flask
was dried by using heat-gun. 2,4,6-trichloro-1,3,5-triazine (36 mmol, 6.64 g) was added to the
reaction flask. Three times vacum - gas (Ar) was applied to remove the O2 from the reaction
flask. Dry toluene (60 mL) was added to the reaction flask and the solution was cooled down
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to 0 C by using ice/water bath. Phenylmagnesium bromide (PhMgBr, 3M/Et2O, 30 mmol,
4.81 mL) was added dropwise and the solution was left for stirring at RT for overnight. The
reaction was stopped with the addition of water (60 mL) and organic phase was separated.
Water phase was washed with CH2Cl2 (60 mL), organic phase were separated and dried over
MgSO4. Solvents were evaporated on rotary evaporator and the crude product was purified by
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crystallization from hexzane/CH2Cl2 solvent system. Yield: 65%. H-NMR (CDCl3, 400
MHz,): δH (ppm) 8.55-8.52 (2H, d, J=8.0 Hz, Ar); 7.70-7.67 (1H, t, Ar); 7.58-7.55 (2H, t, Ar).
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Synthesis of 9,9'-{(6-phenyl-1,3,5-triazine-2,4-diyl)bis[4,1-phenylene(diphenylsilanediyl)-
4,1-phenylene]}bis(3,6-di-tert-butyl-9H-carbazole) (tBuCz2SiTrz) (8): 50 mL, 2-necked
round-bottom flask was dried by using heat-gun. 3,6-di-tert-butyl-9-(4-{diphenyl[4-
(tributylstannyl)phenyl]silyl} phenyl)-9H-carbazole (1 mmol, 0.90 g) and 2,4-dichloro-6-
phenyl-1,3,5-triazine (0.45 mmol, 0.10 g) were added to the reaction flask. Three times vacum
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- gas (Ar) was applied to remove the O2 from the reaction flask. Toluene (20 mL) was added
to the flask and Argon gas was bubbled through the solution for about 30 minutes. Pd(PPh3)4
(0.10 mmol, 0.12 g) was added to the flask and the resultant mixture was stirred at 110 oC for
overnight. Toluene was removed by using rotary evaporator and the crude product was
purified by column chromatography using hexane/CH2Cl2 (5/1) solvent system. Yield: 20 %.
1H-NMR (CDCl3, 400 MHz): δH (ppm) 8.62-8.58 (4H, d, J=8.4 Hz, Ar), 8.15 (4H, s, Ar),
7.85-7.77 (8H, m, Ar), 7.71-7.67 (8H, m, Ar), 7.65-7.59 (5H, m, Ar), 7.54-7.42 (24H, m, Ar),
1.48 (36H, s). 13C-NMR (CDCl3, 100 MHz): δC (ppm) 174.23, 174.13, 171.82, 153.71,
143.06, 143.05, 140.37, 139.77, 139.71, 138.94, 138.91, 137.76, 136.70, 136.52, 136.41,
135.26, 133.85, 133.40, 133.01, 132.54, 132.15, 131.54, 129.99, 129.92, 129.16, 128.64,
128.31, 128.14, 128.09, 125.81, 123.60, 123.57, 121.48, 116.23, 109.36, 34.73, 32.00, 29.71.
MALDI-TOF MS, Found: [M+2H]+ m/z: 1382.8058; 'molecular formula C97H89N5Si2'
requires [M+2H]+ 1382.9685.
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Synthesis of 9,9',9''-{1,3,5-triazine-2,4,6-triyl-tris[biphenyl-4',4-diyl(diphenylsilanediyl)-
4,1-phenylene]}tris(3,6-di-tert-butyl-9H-carbazole) (tBuCz3SiTrz) (9): 50 mL, 2-necked
round-bottom flask was dried by using heat-gun. 3,6-di-tert-butyl-9-(4-{diphenyl[4-
(tributylstannyl)phenyl]silyl} phenyl)-9H-carbazole (2.21 mmol, 2.00 g) and 2,4,6-trichloro-
1,3,5-triazine (0.70 mmol, 0.13 g) were added to the reaction flask. Three times vacum - gas
(Ar) was applied to remove the O2 from the reaction flask. Toluene (20 mL) was added to the
flask and Argon gas was bubbled through the solution for about 30 minutes. Pd(PPh3)4 (0.22
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mmol, 0.26 g) was added to the flask and the resultant mixture was stirred at 110 C for
overnight. Toluene was removed by using rotary evaporator and the crude product was
purified by column chromatography using hexane/CH2Cl2 (5/1) solvent system. Yield: 23%.
1H-NMR (CDCl3, 400 MHz,): δH (ppm) 8.86-8.85 (6H, d, J=8.0 Hz, Ar), 8.18 (6H, s, Ar),
7.92-7.90 (6H, d, J=8.0 Hz, Ar), 7.87-7.85 (6H, d, J=8.0 Hz, Ar), 7.75-7.73 (12H, d, J=8.0
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Hz, Ar), 7.67-7.65 (6H, d, J=8.4 Hz, Ar), 7.57-7.48 (30H, m), 1.50 (54H, s). 13C-NMR
(CDCl3, 100 MHz): δC (ppm) 171.93, 143.06, 139.82, 139.76, 138.95, 137.80, 137.39, 136.79,
136.45, 133.53, 132.30, 129.99, 128.15, 125.84, 123.61, 123.59, 116.24, 109.37, 34.74, 32.02.
MALDI-TOF MS, Found: [M]+ m/z: 1916.7589; 'molecular formula C135H126N6Si3' requires
[M]+ 1916.7465.
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2.3. Preparation of hole and electron only devices
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Prior to organic layer deposition, the ITO substrates were cleaned with acetone, isopropanol
under ultrasonic for 15 min. and subjected to oxygen plasma treatment for 5 min.
PEDOT:PSS (Al4083) was spin coated at 2500 rpm and the layer was annealed in a vacuum
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oven at 120 C for 30 min. Following this, the molecules were dissolved in toluene at a total
concentration of 10 mg mL-1 and were spin-coated onto the PEDOT:PSS layer and baked at
100 oC for 30 min. Finally, for the hole only devices, Au (100 nm) was deposited by thermal
evaporation under a vacuum of 1 x 10-6 mbar. The active areas of the devices were 4 mm2.
For the electron only devices, ITO and Au layers of the above given device is replaced with
Al layer of 100 nm.
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3. Results and discussion
3.1. Synthesis
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The synthetic route followed for the bipolar host materials is shown in Fig. 1. 9H-carbazole
was alkylated by using the Friedel–Crafts alkylation method and 3,6-di-tert-butyl-9H-
carbazole (1) was obtained. Bis(4-bromophenyl)(diphenyl) silane (2) was synthesized by Li-
halogen exchange reaction. The Ullmann Coupling reaction of 3,6-di-tert-butyl-9H-carbazole
with molecule 2 allowed to obtain molecule 3. This -Br derivative was converted to boronic
acid derivative (4) and used for the synthesis of bipolar host tBuCz1SiTrz (5) with Suzuki
Coupling reaction.
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Fig.1. Synthetic route for the synthesized molecules.
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Molecule 3 was also converted to tributyl stannyl (6) derivative. 2,4,6-trichloro-1,3,5-triazine
and 2-chloro-4,6-diphenyl-1,3,5-triazine were purchased and 2,4-dichloro-6-phenyl-1,3,5-
triazine (7) was synthesized. tBuCz2SiTrz (8) and tBuCz3SiTrz (9) were prepared through
Stille Coupling reactions. The synthesized molecules were characterized by 1H and 13C NMR
spectroscopy. Details of the structural characterizations are provided at Figure S1a-h. The
5
6
7
1
8
ratios of aromatic resonance peaks of tBuCz1-3SiTrz in the H NMR spectra prove the
9
presence of carbazole, triazine and phenyl units. The singlet signal at 1.50 ppm indicates the
presence of the methyl substituents on phenyl units. In the 13C NMR spectra of tBuCz1-
10
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1
2
3
3SiTrz, the characteristic signals of the carbon atoms of the triazine units are observed at
171.7-174.2 ppm [20]. The formation of tBuCz1-3SiTrz were further supported by MALDI-
TOF spectra (Fig. S2a-c).
4
3.2. Photophysical properties
5
6
The UV-Vis absorption and photoluminescence (PL) properties of the compounds were
studied in cyclohexane (CH), toluene, CHCl3 and THF (Fig. 2). The characteristic absorption
peak at around 298 nm is identical for all compounds and assigned to the π–π* transition of
the carbazole moieties. The longer wavelength ranging from 310 nm to 360 nm generates
from the π-π* transition of the entire conjugated backbone. The intensity of 298 nm band
increased with the increasing number of carbazole units and so did the calculated molar
absorption constants (ε) in toluene; the ε values are in the range of 6.2 × 104 to 13.7 × 104
L/mol.cm. All materials presented characteristic PL bands of non-conjugated carbazole group
at ca. 354 nm and 370 nm [28]. In CH, well-structured locally exited (LE) state emission
coming from the carbazole group is observed [29]. However, as the polarity of solvent was
increased, gradual loss of the resolved fluorescence band, which is followed by appearance of
unstructured and red-shifted Gaussian shaped emission originating from a newly formed state.
This behaviour is attributed to formation ICT state between D and A groups [30-31].
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
In principle, as a result of photoexcitation, the LE state undergoes ICT from D to A (or vice
versa) which is often accompanied by structural changes (i.e., relative folding of the Dδ+ and
Aδ- moieties) to form a new stabilized dipolar ICT state [30]. Structurally, if the linkage
between the decoupled moieties is a formally single bond, the spatial folding between Dδ+ and
Aδ- moieties is precluded due to having a very limited degree of freedom in space, therefore,
the internal rotational relaxations around the central bond are promoted and Dδ+ moiety takes
an out of plane positions with respect to the Aδ- moiety, which significantly change the
electron density distribution to form a relaxed excited state. If the twist angle is around 90°
12

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1
2
between the corresponding (aromatic) molecular moieties, the stabilization of charge transfer
process significantly enhanced due to large amplitude torsional motion between Dδ+ to Aδ-
units. In such an orthogonal conformation, the electronic coupling is significantly favoured
due to the reduced (nearly zero) overlap between the orbitals involved, and concomitantly
intramolecular charge transfer state is formed between LE states. The prominent property of
ICT state is to observe dual fluorescence, arising from LE and ICT states [32]. However, it
must be noted that observation of dual fluorescence strongly depends on the competitions
between radiative and non-radiative decay processes involved in different polarity/viscosity
solvents [30,33]. Therefore, it is likely to see a broadened emission in polar solvents, arising
from highly polarized ICT state. In all the molecules, the HOMO is localized onto the
carbazole moiety, and the LUMO is confined to the triazine and the adjacent phenyl ring (Fig.
3). When electron-hole transport takes place between HOMO and LUMO orbitals, the charge
carriers follow spatially separated pathways even when they happen to meet on one molecule
(Fig. S3). Considering this point, the calculated emissive excited states involve mainly the
transition from the carbazole-based HOMO to a triazine based LUMO, which have strong CT
character (it carries no oscillator strength). The next emissive higher energy excited state
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
1
(1LE) is calculated to be ~0.20 eV above the CT state and has oscillator strength for
tBuCz1SiTrz (0.77), tBuCz2SiTrz (1.05) and tBuCz3SiTrz (1.28) (Table S1). We further
1
consider that the broad and red-shifted fluorescence arises from this CT state due to the
geometric relaxation in the excited state and concomitant planarization gives rise to slightly
enhanced wave function overlap.
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23
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2
3
Fig. 2. UV-Vis absorbance and PL spectra of the molecules (c=10-6 M).
4
5
6
7
Fig. 3. Calculated localization of boundary molecular orbitals (HOMO and LUMO,
semiempirical method AM1) participated in formation of the lowest singlet excited states of
the tBuCz1-3SiTrz compounds. HOMO-LUMO energy gaps are given in eV.
14

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1
2
3
Fig. 4.UV-Vis absorbance and PL spectra of the drop casted films (prepared in CHCl3 and
toluene, c=10-3 M, ).
4
5
That is the case in here, in CH, only one fluorescence band appears in all the molecules, as the
viscosity and polarity are not sufficient to stabilize this twisted configuration [32]. However,
once the polarity of solvent was slightly increased, it results in stabilization of an ICT state
and concomitantly a further long-wavelength emission band grows in relative intensity
(compared to LE emission), while the intensity of structured LE emission decreases with
6
7
8
1
9
increasing solvent polarity (Fig. 2). In CHCl3, the emission from LE state was totally
10
11
12
13
14
15
16
17
18
19
20
vanished and the observation of red-shifted and broadened fluorescence are the indicatives of
further relaxation of the excited-state geometric distortion and enhanced charge transfer
strength. In general, the strength of an ICT state can be associated with solvatochromic shifts
of the materials, and typically the largest positive solvatochromic shift is an indicative of the
strongest ICT character. According to the findings [32], the molecules showing strong ICT
character have reduced PLQY values, where monomolecular recombination of the geminately
bound electron-hole pairs results in active radiative decay channels. Addressing this point, we
measured the PL and PLQY values in toluene solution and drop casted films (Fig. 4), where
the PLQY values in toluene were 47.2%, 75.7%, 59.4% and in films were 8.2%, % 36.4%, %
9.7% for tBuCz1SiTrz, tBuCz2SiTrz, and tBuCz3SiTrz, respectively. Structurally, if the
electron coupling between D-A units is strong (dihedral angle is close to 0°), the CT character
15

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1
2
3
4
5
6
7
8
of the state is weak. Indeed, the orientations of D-A units play significant role on determining
the singlet-triplet splitting energies (∆ES-T) with the molecules having planar geometry show
very large ∆ES-T value (> 0.7 eV), however, the ∆ES-T is significantly decreased (< 0.01 eV)
with the molecules having > 800 dihedral angle [34]. The results indicate that tBuCz2SiTrz
molecule has the weakest ICT character (considering relatively narrow solvatochromic shift),
localized energetically below the LE state which presumably results from reduced dihedral
angle (118° Fig. S4), comparing with tBuCz1SiTrz (133°) and tBuCz3SiTrz (125°). This
conformational difference gives rise to increased PLQY values in tBuCz2SiTrz.
9
10
11
Fig. 5. Energy level diagram of the locally excited singlet (1LE), triplet (3LE) and charge
transfer state (1CT) of the molecules obtained from experimental data
12
13
14
15
16
17
The fluorescence lifetimes were measured under varying conditions (air saturated and
degassed) to understand the triplet contribution upon the fluorescence decays (Table 1). The
molecules in toluene solution were excited at 298 nm and the emission from the ICT state was
collected at 429 nm for tBuCz1SiTrz, 440 nm for tBuCz2SiTrz and 430 nm for tBuCz3SiTrz.
The ICT emission of molecules shows triexponential decay components, one long lasting (τ3)
and two fast decays (τ1 and τ2).
18
19
20
21
Table 1. Fluorescence lifetime decays in toluene [air saturated (A-Saturated) and degassed]
solution and drop cast films. In the table, τ represents lifetimes in nanosecond (ns) range, B
and f represent amplitudes of the components and fractional intensities (%), respectively
16

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Materials
Solution
(A-saturated)
B1=72395.3
f1=48.87
Solution
(Degassed)
B1=11164.08
f1=52.11
Drop-cast
Film
B1=4369.69
f1=10.20
tBuCz1SiTrz
τ1=2.37 ns
B2=5433.7
f2=36.41
τ1=2.6 ns
B2=616.2
f2=20.72
τ1=1.18 ns
B2=17.649
f2=51.72
τ2=2.37 ns
B3=385.8
f3=14.72
τ2=18.7 ns
B3=182.6
f3=27.18
τ2=14.79 ns
B3=306.54
f3=38.08
τ3=13.51 ns
τ3=82.82 ns
τ3=62.69 ns
χ2=1.26
χ2=1.12
χ2=1.12
B1=11806.38
f1=66.43
B1=10743.26
f1=39.83
B1=13391.92
f1=6.37
tBuCz2SiTrz
τ1=2.80 ns
B2=850.152
f2=27.5
τ1= 2.82ns
B2=660.45
f2=15.15
τ1=1.01 ns
B2=1213.68
f2=16.02
τ2=16.12 ns
B3=26.92
f3=6.07
τ2=17.46 ns
B3=364.16
f3=45.02
τ2=27.9 ns
B3=1290.59
f3=77.6
τ3=112.36 ns
τ3=94.09 ns
τ3=127.53 ns
χ2=1.11
χ2=1.09
χ2=1.3
B1=6737.77
f1=37.10
B1=11280.26
f1=44.54
B1=21655.61
f1=32.10
tBuCz3SiTrz
τ1= 2.55 ns
B2=5617.28
f2=30.93
τ1=2.54 ns
B2=932.88
f2=50.07
τ1=0.54 ns
B2=1283.23
f2=41.18
τ2=2.55 ns
B3=872.57
f3=31.97
τ2= 2.54ns
B3=13.74
f3=5.39
τ2=11.66 ns
B3=136.93
f3=26.73
τ3=16.96 ns
τ3=252.94 ns
τ3= 70.91ns
χ2=1.35
χ2=0.97
χ2=1.24
1
2
3
4
5
6
7
8
The large lifetime differences between the components indicate that they are of different
origin. One of the fast components (τ1) can be attributed to 1LE emission of molecules and τ2
may result from partially twisted geometry of the molecules giving rise to a fast non-radiative
decay channel. The long lasting component (τ3) shows strong oxygen dependency, where the
lifetimes increase significantly in tBuCz1SiTrz and tBuCz3SiTrz materials. However, in
tBuCz2SiTrz, the lifetime slightly decreases with oxygen (Table 1). The presumable reason is
that, the energy of ICT state is ~2.92 eV for tBuCz2SiTrz molecule, which is lower than the
17

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1
2
energy of locally excited triplet state (3LE ~ 2.98 eV, Fig. S5) and therefore, the ICT state of
tBuCz2SiTrz is energetically lowest state of the system. However, for the others, the lowest
3
3
energy of the system is LE state, where the ICT state of tBuCz1SiTrz and tBuCz3SiTrz
3
4
localizes ~0.12 eV and ~0.11 eV above the energy of LE state, respectively (Fig. 5).
5
Consequentially, long-lasting components (τ3) of the molecules indicates the lifetime of the
6
ICT states, where the triplets make significant contribution by forming a feeding mechanism
3
7
from LE state of tBuCz1SiTrz and tBuCz3SiTrz molecules. However, in tBuCz2SiTrz
3
8
molecule, the energy difference between ICT and LE state is ~0.06 eV, which is very close
9
for active triplet feeding mechanism, therefore, the τ3 component has the longest lifetime even
10
11
12
13
in air-saturated solution and drop-off slightly in degassed environment due to actively
3
operative non-radiative decay channels of LE state. Taken together, for all three molecules,
1
excitation at 298 nm accesses the LE state of carbazole, then in nanosecond range the ICT
state is directly populated by both 1LE and 3LE.
14
3.3. Electrochemical studies
15
16
17
18
19
20
21
22
23
24
25
The electrochemical behaviour of the synthesized molecules showed quasi reversible
oxidation (Eox) at around 1.3 eV, irreversible reduction (Ered1) between -0.70 and -0.48 eV and
reversible reduction (Ered2) between -1.30 and -1.42 eV (Figure 6a and Table 2). Eox generates
from the D whereas, Ered1 are assigned to the ICT state and the Ered2 belong to reduction of A
[16,31]. The HOMO and LUMO energy levels of the molecules are calculated from the onset
of Eox and Ered2 and found to be around -5.6 eV and -2.9 eV, respectively. Although the
number of electron donating carbazole groups around the electron accepting triazine was
increased, because of the interrupted conjugation, close running HOMO and LUMO energy
levels were obtained. The electrochemical stabilities of the synthesized molecules were
determined by consecutive cyclic measurements. Although no significant deviation was
expected at the Eox of all molecules as the C3 and C6 positions of carbazoles were protected20,
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2
only tBuCz2SiTrz ensured this expectation. tBuCz1SiTrz and tBuCz3SiTrz presented growing
current peaks at higher oxidative potentials (1.5 V), pointing out that the electron density on
the carbazole groups of these molecules were decreased. While the reduction of triazine was
preserved, a current increase at ICT state reduction potential of all molecules was obtained.
Growing of the currents suggests an electrochemically induced charge transfer with an
enhancement order of tBuCz2SiTrz < tBuCz3SiTrz < tBuCz1SiTrz (Fig. 6b-d). These
growing currents corresponds to the potentials of -0.92 V, -0.78 V and -0.89 V for the
increasing order of carbazole units. Lowest charge transfer reduction potential and current
enhancement obtained with tBuCz2SiTrz molecule, supports the discussions provided above
on low laying ICT state of this molecule.
3
4
5
6
7
8
9
10
11
Table 2. Electrochemical data obtained by the cyclic measurements.
Molecule
Eox
(V)
Ered1
(V)
Ered2
(V)
HOMO
(eV)
LUMO
(eV)
1.26 -0.70
1.27 -0.61
1.28 -0.48
-1.42
-5.59
-2.91
tBuCz1SiTrz
tBuCz2SiTrz
tBuCz3SiTrz
-1.34
-1.30
-5.55
-5.56
-2.94
-2.98
12
19

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1.0
1.5
1.0
tBuCz1SiTrz
tBuCz2SiTrz
tBuCz3SiTrz)
b)
a)
tBuCz1SiTrz
0.5
0.0
0.5
0.0
0.3
-0.5
-1.0
-1.5
-2.0
-2.5
-0.5
-1.0
-1.5
-2.0
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
0.0
-0.5 -1.0 -1.5 -2.0 -2.5
Potantial / V
2
1
0
-1
-2
2
1
0
-1
-2
Potential / V
Potential / V
1
1.0
0.5
1.0
0.5
d)
c)
tBuCz3SiTrz
tBuCz2SiTrz
0.0
0.0
-0.5
-1.0
-1.5
-2.0
-0.5
-1.0
-1.5
-2.0
2
1
0
-1
-2
2
1
0
-1
-2
Potential / V
Potential / V
2
3
4
Fig. 6. a) Cyclic voltammograms of the synthesized molecules (inset: zoomed reduction area)
and b-d) consecutive cyclic scanning of the molecules.
5
3.4. Studies of hole and electron only devices
6
7
In order to monitor the charge balance in the molecules, hole and electron only devices
with the structures of ITO/PEDOT:PSS (≈40 nm)/tBuCz1-3SiTrz (≈60 nm)/Au (100 nm) and
8
9
Al (200 nm)/tBuCz1-3SiTrz (≈60 nm)/Cs2CO3 (3 nm)/Al (200 nm), respectively were
prepared. The hole densities were increased with the increasing number of carbazole groups
while the electron currents were decreased. For tBuCz3SiTrz molecule, the hole density of the
device is higher than the electron density, the situation is the other way around for
tBuCz1SiTrz molecule and for tBuCz2SiTrz, current density-voltage curves address a charge
balance (Fig. 7).
10
11
12
13
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b)
a)
100
100
10
1
10
tBuCz1SiTz
tBuCz2SiTz
tBuCz3SiTz
tBuCz1SiTz
tBuCz2SiTz
tBuCz3SiTz
1
electron only
hole only
0.1
0.1
0
1
2
3
4
5
0
1
2
3
4
5
Potential [V]
Potential[V]
1
2
Fig. 7. Current density-Voltage curves of a) hole and b) electron only devices.
3
4. Conclusions
4
5
In conclusion, three novel, bipolar materials were synthesized by increasing the number of
electron donating (D) carbazole units around the electron accepting (A) triazine. The
conjugation between the D and A was interrupted by tetraphenylsilane groups and high-triplet
energy, solution-processable bipolar blue emitting materials were obtained. All presented
solvatochromic shifts. Photophysical and electrochemical characterizations showed the
potential of these molecules as blue emitters and bipolar host moterials for green emitting
OLED applications and that tBuCz2SiTrz has the weakest intramolecular charge transfer
character, highest PLQY and charge balance.
6
7
8
9
10
11
12
Acknowledgements
13
14
15
The authors would like to thank Ilker Oner and Daniel Voltz for fruitful discussions,
project support funds of TUBITAK (113Z253) and İYTE Biological Mass Spectrometry and
Proteomic Laboratory.
16
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ACCEPTED MANUSCRIPT
Highlights
• A series of carbazole and triazine containing bipolar materials were synthesized
• Tetraphenylsilane was used as spacer to obtain high-triplet energy
• Solvent polarity effects were investigated on the intramolecular charge transfer
• Two carbazole containing molecule has the weakest intramolecular charge transfer
• Hole and electron currents were balanced in two carbazole containing molecule