Synthesis, photophysical, and electrochemical properties of wide band gap tetraphenylsilane-carbazole derivatives: Effect of the substitution position and naphthalene side chain

Article abstract of DOI:10.1134/S0036024416130124

Four tetraphenylsilane-carbazole derivatives with wide bandgaps (3.38–3.55 eV) were synthesized. The effects of the substitution position and of the presence of naphthalene groups on the photophysical, electrochemical and thermal properties were investigated. The derivatives exhibited maximum absorption peaks ranging from 293 to 304 nm and maximum emission peaks ranging from 347 to 386 nm. Changing the carbazole substitution position on the tetraphenylsilane did not significantly change the photophysical and electrochemical properties. However, p-substituted compounds exhibited higher glass transition temperatures than m-substituted compounds. Naphthalene groups with bulky structures had extended the conjugation lengths that red-shifted both the absorption and emission spectra. The LUMO level was decreased, which reduced the optical bandgap and triplet energy level. However, the naphthalene groups significantly improved the thermal stability by increasing the glass transition temperature of the compounds.

Full text of DOI:10.1134/S0036024416130124

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 13, pp. 2590–2599. © Pleiades Publishing, Ltd., 2016.
STRUCTURE OF MATTER
AND QUANTUM CHEMISTRY
Synthesis, Photophysical, and Electrochemical Properties
of Wide Band Gap Tetraphenylsilane-Carbazole Derivatives:
Effect of the Substitution Position and Naphthalene Side Chain1
Kar Wei Ho* and A. Ariffin
Department of Chemistry, University Malaya, 50603 Kuala Lumpur, Malaysia
*e-mail: karwei0808@hotmail.com
Received October 26, 2015
Abstract—Four tetraphenylsilane-carbazole derivatives with wide bandgaps (3.38–3.55 eV) were synthesized.
The effects of the substitution position and of the presence of naphthalene groups on the photophysical, elec-
trochemical and thermal properties were investigated. The derivatives exhibited maximum absorption peaks
ranging from 293 to 304 nm and maximum emission peaks ranging from 347 to 386 nm. Changing the carba-
zole substitution position on the tetraphenylsilane did not significantly change the photophysical and elec-
trochemical properties. However, p-substituted compounds exhibited higher glass transition temperatures
than m-substituted compounds. Naphthalene groups with bulky structures had extended the conjugation
lengths that red-shifted both the absorption and emission spectra. The LUMO level was decreased, which
reduced the optical bandgap and triplet energy level. However, the naphthalene groups significantly improved
the thermal stability by increasing the glass transition temperature of the compounds.
Keywords: phosphorescence, tetraphenylsilane, carbazole, naphthalene, organic light emitting diode
DOI: 10.1134/S0036024416130124
INTRODUCTION
The HOMO and LUMO levels for previously reported
tetraphenylsilane derivatives were approximately 6.2–
6.4 and 2.0 eV, respectively [12, 35]. Previous studies
have reported that derivatives based on tetraphenylsi-
lane deposited onto a substrate crystallized due to their
glass transition temperature being near room tempera-
ture [8]. The poor thermal stability of tetraphenylsi-
lane derivatives has led to various efforts towards
designing an ideal host material based on tetraphenyl-
silane with improved film forming morphological and
thermal stability. Methyl, triindole, phosphine oxide
and carbazole moieties have been used to increase the
glass transition temperature and lower the carrier
injection barriers [6, 10, 11, 26, 50]. Carbazole deriva-
tives have been widely studied for their good hole
transporting capabilities [3, 7, 12, 31]. The tetraphe-
nylsilane bandgap and carrier injection can also be
adjusted using electron-donating carbazole group [1,
6, 10–12, 20, 28, 50]. Electron-rich naphthalene
groups have also been extensively studied for light
emitting applications [13, 15, 29, 30, 40, 49]. They are
used to increase the free volume and thus the solubil-
ity, which allows these materials to be solution pro-
cessed. Bulky naphthalene groups are also good at
preserving thermal stability [9, 18, 36]. The energy lev-
els (HOMO and LUMO) for the host materials can
also be adjusted using naphthalene groups [15, 18].
Phosphorescent organic light emitting diodes
(PhOLEDs) are currently the focus of intense
research. Theoretically, PhOLEDs can generate light
from both triplet and singlet excitons, which allows the
internal quantum efficiency to reach almost 100% [6,
12, 37, 42, 48]. PhOLEDs have potential for use in
large screen displays and for general solid-state light-
ing. However, efficient host materials with wide band-
gaps and triplet energies are still limited and in high
demand. Achieving a balance between a high triplet
energy host and efficient carrier injection is difficult.
Thus, most synthetic effort and structure-property
studies have focused on designing ideal and potential
materials for high efficiency OLEDs [5, 19, 38, 39, 42,
47, 52, 56]. Derivatives based on tetraphenylsilane
were widely investigated materials because of their
wide bandgaps (4.5–5.0 eV). Various studies reported
the application of tetraphenylsilane derivatives (either
polymers or small molecules) in organic light emitting
diodes [1, 4, 6, 8, 10, 11, 16, 18, 21–24, 26, 28, 32–34,
37, 41, 44, 46, 53–55]. However, host materials based
on tetraphenylsilane exhibit a high driving voltage due
to their deep HOMO levels, which creates a high hole
injection barrier and low glass transition temperatures.
1
The article is published in the original.
2590

SYNTHESIS, PHOTOPHYSICAL, AND ELECTROCHEMICAL PROPERTIES
2591
onset
Previous studies have shown that the position and
‒(
+ 4.8). The energy levels of the lowest unoc-
Eox
structure of substituent groups affect the photophysi-
cal properties and performance [17, 18, 38, 43, 45].
Thus, this paper investigates the effect the carbazole
substitution position has on the photophysical, elec-
trochemical and thermal properties of the host mate-
rials. Carbazole groups were attached at p-position
and m-position on the two phenyl groups in the tetrap-
henylsilane core to obtain bis(4-(9H-carbazol-9-
yl)phenyl)diphenylsilane (p-CzSi) and bis(3-(9H-car-
bazol-9-yl)phenyl)diphenylsilane (m-CzSi), respec-
tively. Bis(4-(3,6-di(naphthalen-1-yl)-9H-carbazol-
cupied molecular orbitals (LUMOs) were calculated
using the HOMO value and optical band gap (Eg)
obtained from the onset of UV–Vis absorption spec-
trum. Reagents and solvents were available commer-
cially and used as received. Tetrahydrofuran (THF) is
distilled over sodium benzophenone before use.
Theoretical Calculation Studies
The geometrical and electronic calculations were
performed using Gaussian 09 program. Optimizations
and calculations were carried out using density func-
tional theory (DFT) calculations at the restricted
Becke’s three parameterized Lee-Yang-Parr exchange
functional (B3LYP) using 6-311G(d, p) basis set.
9-yl)phenyl)diphenylsilane
(p-NapCzSi)
and
bis(3-(3,6-di(naphthalen-1-yl)-9H-carbazol-9-yl)phe-
nyl)diphenylsilane (m-NapCzSi) were also synthesized
to investigate the effect naphthalene has on the photo-
physical, electrochemical and thermal properties of
the synthesized derivatives were then characterized.
The experimental results will be compared to theoret-
ical calculations performed using Gaussian 09.
Synthesis
Bis(4-bromophenyl)diphenylsilane (1). Compound 1
was prepared according to modified procedure from
the literature [11]. 1,4-Dibromobenzene (4.20 g,
17.8 mmol) was dissolved in 15 mL dry THF under
nitrogen atmosphere. The solution was cooled to
‒78°C. 9.6 mL of n-BuLi (2.0 M in cyclohexane) was
added dropwise and stirred for 1 h. Dichlorodiphenyl-
silane (1.68 mL, 8 mmol) was added slowly at –78°C.
The mixture was stirred overnight while allowing the
temperature to rise to room temperature. The reaction
was then quenched with water and extracted with
chloroform. The organic layer was dried over anhy-
drous sodium sulfate. After the solvent was evapo-
rated, the crude product was purified by recrystalliza-
tion from ethanol to afford white solid (2.54 g, 64.3%).
1HNMR (CDCl3, 400 MHz): δ (ppm) = 7.57–7.52
EXPERIMENTAL
General
Proton and carbon nuclear magnetic resonance
13
spectroscopy (1H NMR and C NMR spectra) were
obtained on a Bruker AVANCE III spectrometer using
deuterated chloroform as the solvent with tetramethyl-
silane (TMS) as internal standard. Matrix-assisted
laser desorption/ionization time of flight (MALDI-
TOF) measurements were carried out with a Bruker
Microflex series spectrometer in order to measure the
molecular masses. 2,5-Dihydroxybenzoic acid (DHB)
was used as matrix. Differential scanning calorimetry
(DSC) measurements were performed on TA Instru-
ment DSC Q20 under dry nitrogen flow, in the tem-
(m, 8H), 7.48–7.38 (m, 10H). 13C NMR (CDCl3,
perature range from 25 to 350°C with a heating rate of 100 MHz): δ (ppm) = 137.93, 136.32, 134.98, 133.36,
10°C/min, followed by cooling to room temperature 131.14, 129.88, 128.02, 124.82.
and subsequent heating at the same rate. UV–Vis
Bis(3-bromophenyl)diphenylsilane (2). The proce-
absorption spectra were recorded on a 1650 PC spec-
dure for compound 1 was followed to prepare com-
trophotometer. The photoluminescence (PL) spectra
pound 2 from 1,3-dibromobenzene (2.66 mL,
were measured using a Varian Cary 50 Scan UV–Vis
22 mmol) and dichlorodiphenylsilane (2.10 mL,
spectrophotometer. Phosphorescence spectra at low
10 mmol). The crude product was purified by column
temperature (77 K) were also measured to calculate
chromatography using dichloromethane and hexane
the triplet energy. Cyclic voltammetry (CV) measure-
1
as eluent to give white powder (1.30 g, 26.2%). H
ments were carried out in a conventional three elec-
NMR (CDCl3, 400 MHz): δ (ppm) = 7.56 (s, 2H),
trode cell using glassy carbon as a working electrode, a
7.51–7.30 (m, 14H), 7.20–7.17 (m, 2H). 13C NMR
platinum wire counter electrode and a Ag/AgCl refer-
(CDCl3, 100 MHz): δ (ppm) = 138.60, 136.92, 136.29,
ence electrode on a computer controlled potentio-
134.75, 132.99, 132.53, 130.15, 129.80, 128.19, 123.15.
stat–galvanostat PGSTAT101 at room temperature.
CV Measurements of the compounds were carried out
Bis(4-(9H-carbazol-9-yl)phenyl)diphenylsilane (p-
in dimethylformamide containing tetrabutylammo- CzSi). Compound p-CzSi was prepared according to
nium hexafluorophosphate (TBAPF6) (0.1 M) as a procedure from the literature [10, 11]. Compound 1
(0.71 g, 1.43 mmol), carbazole (0.61 g, 3.65 mmol),
copper powder (0.40 g, 6.3 mmol), and 18-crown-6
(0.07 g, 0.26 mmol) and K2CO3 (1.57 g, 11.4 mmol)
supporting electrolyte. The energy levels were calcu-
lated using ferrocene as internal standard with value of
–4.8 eV below the vacuum level. The HOMO molecu-
lar orbital energies were calculated from the onset of were dissolved in 3 mL of anhydrous o-dichloroben-
oxidation potential using the equation HOMO = zene under nitrogen atmosphere. The reaction mix-
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2592
KAR WEI HO, ARIFFIN
ture was refluxed for 72 h at 180°C. After cooling to allowed to cool to room temperature. The crude prod-
room temperature, the mixture was diluted in uct separated by filtration and recrystallized from
dichloromethane and filtered through silica gel. The dichloromethane to give brown solid (13.97 g, 66.7%).
filtrate was evaporated under reduced pressure to give 1H NMR (CDCl3, 400 MHz): δ (ppm) = 8.36 (s, 2H),
crude product, which was then purified by column
chromatography to give white powdery product
8.14 (br, s, 1H), 7.71 (dd, 2H, J = 8.50, 1.65 Hz), 7.24
13
(d, 2H, J = 8.56 Hz). C NMR (CDCl3, 100 MHz):
1
(0.355 g, 37.2%). H NMR (CDCl3, 600 MHz):
δ (ppm) = 138.53, 134.83, 129.40, 124.58, 112.70,
δ (ppm) = 8.18 (d, 4H, J = 7.7 Hz), 7.88–7.95 (m,
4H), 7.73–7.79 (m, 4H), 7.70 (d, 4H, J = 8.25 Hz),
7.50–7.59 (m, 10H), 7.41–7.49 (m, 4H), 7.30–7.37
82.44.
3,6-Di(naphthalen-1-yl)carbazole (5). Compound 3
(0.89 g, 5.2 mmol) and compound 4 (1.09 g,
2.6 mmol) were dissolved in 25 mL toluene in a round
bottom flask under nitrogen. A solution of potassium
carbonate (2 M, 6.5 mL) was added to the reaction
mixture. Tetrakis(triphenylphosphine)palladium(0)
(0.12 g, 0.10 mmol) was added. The reaction mixture
was reflux for 24 h under nitrogen. After cooling to
room temperature, the reaction mixture was evapo-
rated to remove solvent and extracted with chloroform
and then washed with water. The organic layer was
evaporated using a rotary evaporator, and the residue
was subjected to a column chromatography using hex-
ane/ethyl acetate as the eluent. White powdery prod-
13
(m, 4H). C NMR (CDCl3, 100 MHz): δ (ppm) =
140.59, 139.26, 137.92, 136.46, 133.58, 133.20, 130.05,
128.21, 126.28, 125.97, 123.58, 120.34, 120.14, 109.91.
MALDI-TOF MS (m/z): calcd. for C48H34N2Si
666.25, found 666.986.
Bis(3-(9H-carbazol-9-yl)phenyl)diphenylsilane (m-
CzSi). Compound m-CzSi was prepared according to
the procedure reported for p-CzSi by using compound
2 (0.50 g, 1.01 mmol) and carbazole (0.38 g,
2.27 mmol). The crude product was purified by col-
umn chromatography to give white powder product
1
(0.304 g, 45.1%). H NMR (CDCl3, 600 MHz):
1
uct was obtained (0.67 g, 30.7%). H NMR (CDCl3,
δ (ppm) = 8.13 (d, 4H, J = 7.34 Hz), 7.85 (s, 2H),
600 MHz): δ (ppm) = 8.30 (s, 1H), 8.22 (s, 2H), 8.03
(d, 2H, J = 8.53 Hz), 7.94 (d, 2H, J = 8.12 Hz), 7.89
(dd, 2H, J = 7.36, 1.44 Hz), 7.62 (d, 4H, J = 1.10 Hz),
7.54–7.60 (m, 4H), 7.51 (ddd, 2H, J = 7.98, 6.88,
1.10 Hz), 7.44 (ddd, 2H, J = 8.32, 6.95, 1.24 Hz). 13C
NMR (CDCl3, 100 MHz): δ (ppm) = 140.93, 139.26,
133.90, 132.35, 132.18, 128.41, 128.24, 127.36, 127.26,
126.34, 125.93, 125.67, 125.42, 123.49, 121.85, 110.39.
MALDI-TOF MS (m/z): calcd. for C32H21N 419.17,
found 418.905.
7.73–7.64 (m, 10H), 7.49–7.43 (m, 6H), 7.35–7.25
13
(m, 12H). C NMR (CDCl3, 100 MHz): δ (ppm) =
140.64, 137.52, 136.43, 136.30, 135.06, 134.47, 133.01,
130.11, 129.59, 128.27, 128.22, 125.90, 123.38, 120.23,
119.94, 109.67. MALDI-TOF MS (m/z): calcd. for
C48H34N2Si 666.25, found 666.415.
1-Naphthaleneboronic acid (3). 1-Bromonaphtha-
lene (2.1 mL, 15 mmol) was prepared in 20 mL dry tet-
rahydrofuran under nitrogen and cooled to –78°C in
dry ice bath. n-BuLi (2.0 M, 7.5 mL, 15 mmol) was
added to the solution dropwise via a syringe. After
stirred for 1 h, trimethyl borate (5 mL, 3.0 equiv) was
added at –78°C. The reaction was warmed to room
temperature and stirred overnight. The reaction mix-
ture was then poured into a mixture of ice and water.
The mixture was acidified with 2 N HCl and
extracted with chloroform. The organic layer was
washed three times with water, dried over sodium sul-
fate (Na2SO4). The crude product was purified by
chromatography using hexane/ethyl acetate as the elu-
ent to obtain white powder (1.338 g, 51.9%). 1H NMR
(CDCl3, 400 MHz): δ (ppm) = 9.31 (d, 1H, J =
8.44 Hz), 8,70 (dd, 1H, J = 6.91, 1.28 Hz), 8.14 (d,
1H, J = 8.07 Hz), 7.89–8.07 (m, 1H), 7.68–7.72 (m,
2H), 7.61 (ddd, 1H, J = 8.04, 6.88, 1.10 Hz). 13C NMR
(CDCl3, 100 MHz): δ (ppm) = 137.74, 133.60, 132.29,
130 89, 128.83, 128.11, 126.89, 126.57, 125.17.
Bis(4-(3,6-di(naphthalen-1-yl)-9H-carbazol-9-
yl)phenyl)diphenylsilane (p-NapCzSi). Compound
p-NapCzSi was prepared according to the procedure
reported for p-CzSi from compound 1 (0.17 g,
0.35 mmol) and compound 5 (0.44 g, 0.77 mmol).
The crude product was subjected to column chroma-
tography using hexane/ethyl acetate as eluent to give
1
white powder product (0.17 g, 41.5%). H NMR
(CDCl3, 600 MHz): δ (ppm) = 8.30 (d, 4H, J =
1.10 Hz), 8.04 (d, 4H), 8.04 (d, 4H, J = 8.44 Hz), 7.94
(d, 4H, J = 8.07 Hz), 7.88–7.91 (m, 4H), 7.86 (d, 4H,
J = 8.44 Hz), 7.82 (dd, 4H, J = 7.70, 1.65 Hz), 7.73 (d,
4H, J = 8.25 Hz), 7.63 (dd, 4H, J = 8.44, 1.65 Hz),
7.55–7.60 (m, 14H), 7.51 (t, 4H, J = 7.47 Hz), 7.44
(ddd, 4H, J = 8.34, 6.88, 1.28 Hz). 13C NMR (CDCl3,
100 MHz): δ (ppm) = 140.75, 140.36, 139.33, 138.11,
136.51, 133.92, 133.54, 133.47, 133.09, 132.15, 130.14,
128.53, 128.28, 127.35, 126.30, 125.98, 125.71, 125.43,
123.72, 121.87, 109.79. MALDI-TOF MS (m/z):
calcd. for C88H58N2Si 1171.44, found 1171.938.
3,6-Diiodocarbazole (4). Carbazole (8.36 g,
50 mmol) was dissolved in boiling acetic acid
(200 mL). To the solution, potassium iodide (10.8 g,
50 mmol) was added. The solution was cooled and
potassium iodate (21.5 g, 50 mmol) was added. The yl)phenyl)diphenylsilane (m-NapCzSi). Compound m-
mixture was refluxed at 80°C for 1 h. The solution was NapCzSi was prepared according to the procedure for
decanted from undissolved potassium iodate and p-CzSi from compound 2 (0.13 g, 0.26 mmol) and
Bis(3-(3,6-di(naphthalen-1-yl)-9H-carbazol-9-
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SYNTHESIS, PHOTOPHYSICAL, AND ELECTROCHEMICAL PROPERTIES
2593
with trimethyl borate. Compound 5 was obtained by
reacting compound 3 with iodinated carbazole (com-
pound 4) via a Suzuki coupling reaction catalyzed
with Pd(PPh3)4. A white solid powder was obtained
as the product with a 30.7% yield. Lastly,
compound 5 was reacted with compounds 1 and 2 via
an Ullmann coupling reaction to obtain p-NapCzSi
(41.6% yield) and m-NapCzSi (48.5% yield), respec-
tively. The compounds were fully characterized by
1H, 13C NMR and MALDI-TOF mass spectroscopy.
The results were agreed well with the proposed struc-
tures. The final products are readily soluble in com-
mon solvents such as dichloromethane, chloroform,
tetrahydrofurane, and dimethylformamide, which
indicates the possibility for solution processing [47].
Normalized absorption intensity, a.u.
1.0
p-CzSi
m-CzSi
0.8
p-NapCzSi
m-NapCzSi
0.6
0.4
0.2
0
300
350
400
450
Wavelength, nm
Fig. 1. UV-visible absorption for p-CzSi, m-CzSi, p-Nap-
CzSi and m-NapCzSi in dilute dimethylformamide solu-
tion.
Photophysical Properties
The photophysical properties of the compounds
were analyzed using UV–Vis and fluorescence spec-
trophotometry. Figure 1 shows the UV-vis electronic
absorption spectra for p-CzSi, m-CzSi, p-NapCzSi,
and m-NapCzSi in a dilute dimethylformamide solu-
tion. Table 1 summarizes the photophysical proper-
ties of these compounds. Figure 1 shows that the
absorption spectra of p-CzSi and m-CzSi were simi-
lar. The tetraphenylsilane compounds containing
naphthalene and carbazole moieties (p-NapCzSi and
m-NapCzSi) exhibited similar absorption spectra due
to their similar structures. The spectra exhibited a
compound 5 (0.31 g, 0.54 mmol). The crude product
was subjected to column chromatography using hex-
ane/ethyl acetate as eluent to give white powder
1
(0.148 g, 48.5%). H NMR (CDCl3, 600 MHz):
δ (ppm) = 8.21 (d, 4H, J = 1.28 Hz), 8.05 (s, 2H), 7.96
(d, 4H, J = 8.44 Hz), 7.91 (d, 4H, J = 8.25 Hz), 7.83–
7.86 (d, 8H), 7.80 (dd, 4H, J = 7.43, 1.74 Hz), 7.75–
7.78 (m, 2H), 7.41–7.57 (m, 26H), 7.38 (td, 4H, J =
7.70, 1.10 Hz). 13C NMR (CDCl3, 100 MHz): δ (ppm) =
140.68, 140.34, 137.61, 136.71, 136.40, 135.19, 134.42, broad absorption band ranging from 260 to 360 nm.
133.85, 132.98, 132.08, 130.24, 129.79, 128.45, 128.31, Both compounds p-CzSi and m-CzSi exhibited a pri-
128.22, 128.18, 127.30, 127.28, 126.25, 125.92, 125.65, mary maximum main absorption peak at 293 nm
125.39, 123.53, 121.77, 109.52. MALDI-TOF MS which was attributed to the π–π* transitions in the
(m/z): calcd. for C88H58N2Si 1171.44, found 1171.927.
tetraphenylsilane core [10, 11, 26, 28]. Shoulder
peaks were observed for p-CzSi at 325 and 339 nm.
For m-CzSi, the shoulder peaks were located at 327.5
and 340 nm. These shoulder peak were attributed to
the characteristic π–π* transitions of the carbazole
unit [3]. The maximum absorption peaks for p-Nap-
CzSi and m-NapCzSi were red-shifted to 303 and
304 nm, respectively. A small shoulder peak was
observed at 354 nm for both p-NapCzSi and m-Nap-
CzSi. These results indicate the naphthalene groups
red-shifted the absorption peak due to their electron-
opt
RESULTS AND DISCUSSION
Synthesis
The synthetic routes including intermediates and
final products are presented in Scheme 1 and
Scheme 2. The synthesis for compounds p-CzSi and
m-CzSi consists of two parts. This synthesis began by
reacting dichlorodiphenylsilane with lithiated
1,4-dibromobenzene and 1,3-dibromobenzene to
obtain compounds 1 and 2, respectively. The bromi-
nated tetraphenylsilane (compounds 1 and 2) was then
reacted with carbazole via an Ullmann coupling reac-
tion. Copper powder and 18-crown-6 were used to
catalyze the reaction under basic conditions in
o-dichlorobenzene. The mixture was then refluxed
under nitrogen for 72 h to obtain compounds p-CzSi
and m-CzSi in 37.2 and 45.2% yield, respectively. The
carbazole was attached at the p- and m-positions in
p-CzSi and m-CzSi, respectively. Compounds p-Nap-
donating character [47]. The optical bandgaps (
)
Eg
determined from the absorption spectra onset are
shown in Table 1. The optical bandgaps calculated
for p-CzSi and m-CzSi were 3.55 and 3.54 eV, respec-
tively [6, 10, 11, 28]. The calculated optical bandgaps
for compounds p-NapCzSi and m-NapCzSi signifi-
cantly decreased to 3.38 eV. The naphthalene groups
reduced the optical bandgap by approximately
0.17 eV.
The photoluminescence (PL) spectra of the com-
CzSi and m-NapCzSi were synthesized in three stages. pounds in dilute solutions are shown in Fig. 2. Upon
First, 1-naphthaleneboronic acid (compound 3) was excitation, the emission spectra were similar between
obtained by reacting lithiated 1-bromonaphthalene p-CzSi and m-CzSi, while the spectra for p-NapCzSi
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2594
KAR WEI HO, ARIFFIN
Cl
Si
a
Br
Si
Br
Cl
(1)
Br
Br
Cl
Si
b
Si
Cl
(2)
H
N
c
(1) +
N
Si
N
Carbazole
p-CzSiCz
H
N
N
N
d
(2) +
Si
Carbazole
m-CzSiCz
Scheme 1. Synthetic routes to compounds p-CzSi and m-CzSi. Reagents and conditions: a, n-BuLi, dry THF, –78°C, and 1,4-
dibromobenzene; b, n-BuLi, dry THF, –78°C, and 1,3-dibromobenzene; and c, copper, 18-crown-6, K CO , o-dichloroben-
2
3
zene (o-DCB), and 180°C.
and m-NapCzSi were also similar due to similarity of exhibited an 82 nm Stokes shift, which is 27 nm higher
than for p-CzSi and m-CzSi.
their structure. The maximum emission peaks for
p-CzSi and m-CzSi were located at 347 and 349 nm,
respectively. The maximum emission peaks for p-Nap-
CzSi and m-NapCzSi were red-shifted to 386 and
385 nm, respectively. Shoulder peak were observed for
p-CzSi and m-CzSi at 364 nm. The naphthalene group
may have extended the conjugated backbone,
increased the electron density and thus red-shifted the
emission peak [47]. The calculated Stokes shift calcu-
lated for p-CzSi and m-CzSi were 54 and 56 nm,
The triplet energy (ET) is an essential require-
ment for an ideal host. This energy should be greater
than that of the phosphorescent guest to prevent
back transferring from the guest to the host. The low
temperature phosphorescent spectrum was used to
measure the triplet energy. The triplet energy calcu-
lated from the p-CzSi and m-CzSi from triplet emis-
sion peaks were 2.94 and 2.91 eV, respectively [10,
11, 28]. The triplet energy for p-NapCzSi and
m-NapCzSi were both 2.21 eV. The triplet energy
respectively. Compounds p-NapCzSi and m-NapCzSi was significantly reduced upon inclusion of the
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SYNTHESIS, PHOTOPHYSICAL, AND ELECTROCHEMICAL PROPERTIES
2595
Br
B(OH)2
a
1-Bromonaphthalene
(3)
H
N
H
N
b
I
I
(4)
Carbazole
H
N
B(OH)2
H
N
c
+
I
I
(3)
(4)
(5)
Br
Si
Br
Br
Br
Si
d
d
(1)
(2)
N
Si
N
N
N
Si
p-NapCzSi
m-NapCzSi
Scheme 2. Synthetic routes to compounds p-NapCzSi and m-NapCzSi. Reagents and conditions: (a) n-BuLi, dry THF, –78°C,
and B(OMe) ; (b) KI, KIO , acetic acid, reflux, and 2 h; (c) Pd(PPh ) , K CO 2 M, toluene, 90°C, and overnight; and (d) cop-
3
3
3 4
2
3
per, 18-crown-6, K CO , o-DCB, and 180°C.
2
3
naphthalene groups in the derivatives. The substitu-
ent position on the tetraphenylsilane core did not
significantly change the spectral features. This may
be because the substituents position does not dras-
tically change the vibrational structure and config-
uration structural changes between the ground and
excited states [25, 51].
Electrochemical Properties
The redox behavior of these compounds in dimeth-
ylformamide was studied via cyclic voltammetry to
determine the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular
orbital (LUMO) energies. This information is crucial
for examining the charge injection barriers and device
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2596
KAR WEI HO, ARIFFIN
Table 1. Photophysical properties for p-CzSi, m-CzSi, p-NapCzSi, and m-NapCzSi
Photophysical properties
Compound
opt b (eV)
Eg
(eV)
λa
edge (nm)
a
a
λabs
λem
ETc
2.94
Stoke shift (nm)
(nm)
(nm)
p-CzSi
293, 325, 339
293, 327.5, 340
304
349.5
350.0
366.5
366.5
347, 362
349, 364
386
54
56
82
82
3.55
3.54
3.38
3.38
m-CzSi
2.91
2.21
2.21
p-NapCzSi
m-NapCzSi
303
385
a
b
c
Recorded in dimethylformamide at r. t.
Optical bandgaps were determined from the absorption edge of the normalized absorption spectra.
Triplet energy was calculated from the first emission peak in the low temperature PL spectra (77 K).
optimization. The cyclic voltammograms are shown in level determined for p-NapCzSi was –6.04 eV. No sig-
Fig. 3, and the data are summarized in Table 2. An nificant changes were observed in the HOMO regard-
irreversible, single-electron oxidation peak was less of the substituent position or presence of naphtha-
lene. The LUMO levels calculated for p-CzSi and
m-CzSi were –2.50 and –2.51 eV, respectively. These
LUMO levels decreased significantly in the presence
of naphthalene for p-NapCzSi (–2.65 eV) and m-Nap-
CzSi (–2.67 eV).
observed during the anodic sweeping shown in Fig. 3
in the range of 1.37 to 1.50 V. The irreversible oxida-
tion peak was attributed to unstable electrochemical
properties due to the formation of new species formed
during the anodic sweep [2, 51]. The oxidation peak
for p-CzSi occurred at 1.50 V, which is slightly higher
than for m-CzSi (1.40 V). The oxidation peaks for
p-NapCzSi and m-NapCzSi are observed at 1.37 V.
Assuming that the HOMO energy for the ferrocence
standard is 4.8 eV with respect to the zero vacuum
level, the HOMO and LUMO can be calculated based
Thermal Properties
The thermal properties for these compounds were
evaluated via differential scanning calorimetry (DSC)
under a nitrogen atmosphere. From Fig. 4 it can be
seen that all of the compounds were stable up to 350°C
with no melting point. These results imply that the
compounds were in amorphous state, as desired for
PhOLED devices. The glass transition temperature
(Tg) was observed for p-CzSi, m-CzSi, p-NapCzSi, and
m-NapCzSi at 120, 90, 205, and 172°C, respectively.
The glass transition temperature for p-CzSi agrees well
with the literature data [6, 10, 11]. The glass transition
temperature may increase with the increasing rigidity,
on the oxidation potentials onset ( onset ) and optical
Eox
bandgaps ( opt). The HOMO and LUMO energy lev-
Eg
els were estimated using the empirical formulas:
onset
HOMO = –(
+ 4.8) and LUMO = HOMO +
Eox
opt. The HOMO levels calculated for p-CzSi,
Eg
m-CzSi, and m-NapCzSi using the onset potential
shown in Table 1 were both –6.05 eV. The HOMO
Normalized PL intensity, a.u.
Current, mA
0.10
1.0
p-CzSi
p-CzSi
0.08
m-CzSi
0.8
m-CzSi
p-NapCzSi
m-NapCzSi
p-NapCzSi
m-NapCzSi
0.06
0.6
0.04
0.02
0
0.4
0.2
0
300
400
500
600
0
0.5
1.0
1.5
Wavelength, nm
Potential (V vs. Ag/AgCl)
Fig. 2. Photoluminescence spectra for p-CzSi, m-CzSi,
p-NapCzSi, and m-NapCzSi in dilute dimethylformamide
solution.
Fig. 3. Oxidation cyclic voltammograms of p-CzSi,
m-CzSi, p-NapCzSi, and m-NapCzSi in dimethylforma-
mide solution containing 0.1 M TBAPF .
6
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SYNTHESIS, PHOTOPHYSICAL, AND ELECTROCHEMICAL PROPERTIES
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(eV)
Table 2. Electrochemical properties of p-CzSi, m-CzSi, p-NapCzSi, and m-NapCzSi
Experimental
Theoretical
Compound
HOMOa LUMOb
HOMOd LUMOd
Egopt
3.55
Egcalcd
peak
ox
onset
(eV)
(V)
(V)
(eV)
E
Eox
ETc
2.94
ETd
(eV)
(eV)
(eV)
(eV)
(eV)
p-CzSi
m-CzSi
p-NapCzSi
m-NapCzSi
1.50
1.25
–6.05
–6.05
–6.04
–6.05
–2.50
–2.51
–2.65
–2.67
–5.62
–5.52
–5.49
–5.41
–1.23
–1.19
–1.33
–1.28
4.39
2.87
1.40
1.37
1.37
1.25
1.24
1.25
3.54
3.38
3.38
2.91
2.21
2.21
4.33
4.16
4.13
2.87
2.22
2.23
onset
a
Calculated from the oxidation potential onset using HOMO = –(
+ 4.8).
Eox
LUMOs were calculated from the HOMO and bandgap ( opt ).
b
c
d
Eg
The triplet energy was calculated from the first emission peak for the low temperature PL spectra (77 K).
Calculated from the theoretical simulations.
intermolecular interactions and molecular backbone molecule. Density functional theory (DFT) calcula-
symmetry. The glass transition temperatures for the tions were performed using Gaussian 09 program with
p-substituted compounds were higher than for the a restricted Becke’s three parameterized Lee-Yang-
m-substituted ones. This may be explained by stronger
intermolecular interactions in p-substituted com-
pounds [9]. The presence of naphthalene moieties also
significantly improved the glass transition temperature
and thermal stability (>70%). The naphthalene group
in p-NapCzSi and m-NapCzSi increased the bulkiness
and molecular interactions and thus increase the glass
transition temperature [18, 36]. A glass transition tem-
perature was only barely seen for p-NapCzSi and m-
NapCzSi due to their small heat capacity and crystal-
linity [27].
Parr exchange functional (B3LYP) using a 6-311G
basis set. Figure 5 shows the optimized molecular
structures for these compounds. The molecules exhib-
ited a non-coplanar twisted conformation due to the
large torsional angle between the carbazole and tetrap-
henylsilane core. Such a non-planar structural feature
is beneficial to PhOLED applications by preventing
excessive intermolecular interactions against quench-
ing and suppressing crystallization to establish a uni-
form amorphous film in the device [14, 27].
The calculated HOMO and LUMO energy levels
are listed in Table 2. Figure 6 summarizes the experi-
mental and theoretical HOMO and LUMO energy
levels for these compounds and their corresponding
molecular orbital distribution. Comparing the experi-
mental and theoretical results shows that the calcu-
Theoretical Calculations
Theoretical molecular orbital calculations were
performed for the studied compounds to characterize
their optimized geometries, orbital energies, and elec- lated HOMO and LUMO levels are higher than the
tron densities for HOMO and LUMO states of each experimental results. The theoretical results indicate
(a)
(c)
(b)
(d)
p-NapCzSi
m-NapCzSi
p-CzSi
172 °C
m-CzSi
205 °C
90 °C
120 °C
50
100
150
200
250
300
Temperature, °C
Fig. 5. Optimized three-dimensional geometry for (a)
p-CzSi, (b) m-CzSi, (c) p-NapCzSi, and (d) m-NapCzSi via
DFT calculations using a B3LYP model with a 6-311G as
the basis set.
Fig. 4. DSC curves for p-CzSi, m-CzSi, p-NapCzSi and m-
NapCzSi measured at a heating rate of 10°C/min under
nitrogen.
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2598
KAR WEI HO, ARIFFIN
0
−1
−2
−3
−4
−5
−6
−7
−1.19
−2.51
−1.23
−2.5
−1.23
−2.67
−1.33
−2.05
LUMO
HOMO
−5.41
−6.05
−5.49
−6.04
−5.52
−6.05
−5.62
−6.05
Theoretical
calculation
Experimental
p-CzSi
m-CzSi
p-NapCzSi m-NapCzSi
Fig. 6. HOMO and LUMO energy levels for p-CzSi, m-CzSi, p-NapCzSi, and m-NapCzSi and their corresponding molecular
orbital distributions.
the HOMO and LUMO levels for the m-substituted nescence bands. The optical bandgap decreased as the
compounds shift slightly upwards compare to p-sub- LUMO levels decreased. However, no significant
stituted compounds. In contrast, the experimental changes were observed in the HOMO level. Theoreti-
results yielded no significant change in energy levels cal molecular orbital calculations for these com-
upon changing the substituent position. The theoreti- pounds yielded calculated HOMO and LUMO energy
cal data predicted the HOMO and LUMO energy lev- levels that were slightly higher than the experimental
els would narrow in the presence of a naphthalene results. However, the calculated triplet energies were
group. The theoretical calculations predicted much nearly identical to the experimental results. The elec-
higher bandgaps than those determined experimen- troluminescence properties of these derivatives will be
tally. However, the theoretical triplet energies were investigated to determine their potential for hole-
similar to the experimental results. The difference in transporting or host materials in PhOLEDs.
experimental and theoretical results may be attributed
to simulated orbital of the HOMO and LUMO ener-
gies being an estimate of the transition energy [47].
ACKNOWLEDGMENTS
The projected is supported by High Impact
Research of University Malaya (Grant no.
UM.C/625/1/HIR/208) and University Malaya Post-
graduate Research Fund PPP (grant no. PG181-
2014B).
Figure 6 shows the HOMO electrons populations
were uniformly dispersed over the carbazole unit, and
carbazole is well known to be an electron-donating
group [1]. Upon excitation, electron population for
the LUMO level was localized over the phenyl unit in
the tetraphenylsilane core. Thus, the π–π* transitions
for both tetraphenylsilane and carbazole are thought
yield the absorption and emission properties.
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