Z.-L. Zhu et al. / Dyes and Pigments 146 (2017) 219e225
221
FLS980 spectrophotometer. Cyclic voltammetry (CV) was carried
out in nitrogen-purged CH2Cl2 (positive scan) at room temperature
using a CHI voltammetric analyzer. Tetrabutylammonium hexa-
fluorophosphate (TBAPF6) (0.1 M) was used as the supporting
electrolyte. The measurements were carried out at a scan rate of
100 mV sꢀ1 with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire
auxiliary electrode, and a saturated calomel reference electrode
calibrated with ferrocene/ferrocenium (Fc/Fcþ). For theoretical
calculation, geometrical properties were optimized at the B3LYP/6-
31 g(d, p) level using the Gaussian 09 program. The excited states
are then calculated at the TD-PBE0/6-31 g(d, p) level. The LUMO
energy level was estimated by subtracting the optical band gap
from the HOMO energy level.
d2)
d
8.78 (dd, J ¼ 21.4, 8.4 Hz, 6H), 7.81 (d, J ¼ 7.5 Hz, 6H), 7.70 (d,
J ¼ 8.3 Hz, 6H), 7.58 (d, J ¼ 9.2 Hz, 2H), 7.52 (dd, J ¼ 11.3, 8.0 Hz, 8H),
7.32 (t, J ¼ 7.7 Hz, 2H), 7.24 (d, J ¼ 8.2 Hz, 2H), 1.47 (s, 18H); MS (ESI)
m/z: [MþH]þ calcd for C68H50F4N4, 999.15; found, 1000.26 and
500.33.
3. Results and discussion
3.1. Theoretical calculations
To analyze ground state and excited state properties of 4FBTPI,
quantum chemistry calculation was carried out using the Gaussian
09 program. Fig. S1 shows the optimized geometry of its ground
state. Across the whole D-p-A-p-D system, twisting angles be-
Before device fabrication, pre-cleaned ITO-coated glass sub-
strates with a sheet resistance of 15
tween building blocks are relatively small. The dihedral angle be-
tween the PI unit and the phenyl bridge is 25.4ꢂ and that between
the acceptor and phenyl bridge is 41.6ꢂ. This gives good conjuga-
tion, electronic communication and state mixing. As shown in
Fig. 2, the “hole” of S1/S0 transition spreads over the whole
skeleton of the molecule, except the benzene ring connected to the
N1 position (nitrogen atom in position 1 of PI unit). On the other
hand, its “particle” mainly locates at the p-tetrafluorophenyl bridge.
Obviously, the transition contains both a LE component at the tet-
rafluorophenyl bridging group and a CT transition from PI units to
the terphenyl group. Besides, the very large oscillator strength of
S1/S0 transition (GS /S ¼2.1897), which is comparable to some LE
U
squareꢀ1 were subjected to
UV-ozone treatment for 20 min. All organic films were deposited by
thermal evaporation in a deposition chamber with a base vacuum
of 5 ꢁ 10ꢀ6 Torr. J-V characteristics were recorded with a Keithley
2400 Sourcemeter. Electroluminescence spectra and CIE color co-
ordinates were measured with a Spectrascan PR650 photometer.
2.2. Preparation of compounds
2.2.1. Synthesis of 2-(4-bromophenyl)-1-(4-(tert-butyl)phenyl)-1H-
phenanthro[9,10-d]imidazole (1) [21]
1
0
The 4-bromobenzaldehyde (1.86 g, 10 mmol), 9,10-
phenanthrenequinone (2.08 g, 10 mmol), 4-(tert-butyl)aniline
(1.49 g, 10 mmol), and ammonium acetate (4.62 g, 60.0 mmol) were
refluxed in an acetic acid solution under an argon atmosphere for
24 h. After cooling to room temperature, an orange-yellow mixture
was obtained and poured into methanol under stirring. The raw
product was separated by filtration and washed with methanol,
and then dried under vacuum. The product was purified by column
chromatography (petroleum ether: CH2Cl2, 1:2) on silica gel to give
a white powder, with a 92.1% yield.
molecules [31,32], suggests the LE component has a larger contri-
bution than the CT component in this HLCT state. Such a HLCT state
is a reasonable result as we insert a
p bridge between D and A
groups to weaken electronic push-pull effect and enlarge LE tran-
sition range. Other key calculation data including energy diagram
and transition of singlet and triplet states are shown in Fig. S2 and
Table S1 in the supporting information.
3.2. Photophysical properties
To acquire a better understanding of its excited state property,
photophysical properties of the material was investigated by
characterizing its UVeVis absorption and photoluminescence (PL)
spectra (Fig. 3) in dilute dichloromethane solution (10ꢀ5 M). In the
2.2.2. Synthesis of 1-(4-(tert-butyl)phenyl)-2-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-phenanthro[9,10-
d]imidazole (2) [30]
A 3-neck 100 mL round bottom flask was charged with a stir bar,
1 (4.04 g, 8 mmol), bis(pinacolato)diboron (B2pin2) (2.12 g,
8.4 mmol), anhydrous potassium acetate (KOAc) (2.14 g,
21.6 mmol), dichloro[1,1'-bis(diphenylphosphino)-ferrocene]palla-
dium(II) dichloromethane adduct (Pd(dppf)Cl2$CH2Cl2) (174 mg,
0.214 mmol) and degassed dioxane (40 mL). After the reaction
mixture was heated at 85 ꢂC for 16 h, the product was extracted
with CH2Cl2 and washed with distilled water. The organic extract
was dried over MgSO4, and solvent was removed under reduced
pressure to obtain a brown viscous oil. Purification with flash
chromatography (petroleum ether: CH2Cl2, 1: 4) yielded 3.38 g
(76.5%) of tacky white solid.
absorption spectrum, typical absorption of p-p* transition of ben-
zene ring and the PI unit at around 260 and 340e370 nm are
observed, which are similar to other PI derivatives [11,33], indi-
cating that the tetrafluorobenzene acceptor has relatively little in-
fluences on molecular ground state properties. The PL spectrum of
4FBTPI in solution peaks at 448 nm, while the PL spectrum of the
film shows a mildly bathchromatically shifted peak at 457 nm.
Encouragingly, impressive PLQY data were obtained in both solu-
tion and neat film of 4FBTPI (99% and 69%) (Table 1). In order to
gain a deeper blue emission, we doped the material into CBP. With
8
1 wt% of 4FBTPI as dopant, PL emission of the doped film ex-
hibits a significant hypochromatic shift of 29 nm compared to neat
film of 4FBTPI. Correspondingly, PLQY of the doped film is
improved to 88%. Such significant changes may not only origin from
eradicating of aggregation effect in solid state, but also influenced
by sensitive emitting nature toward circumstance of the CT state
and blue shifting effect of tetrafluorobenzene [24].
2.2.3. 2,2'-(20,30,50,6'-tetrafluoro-[1,1':40,100-terphenyl]-4,400-diyl)
bis(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole)
(4FBTPI)
1,2,4,5-tetrafluoro-3,6-diiodobenzene (0.80 g,
2 mmol), 2
(2.66 g, 4.8 mmol), Pd(PPh3)4 (0.23 g, 0.02 mmol), and K2CO3
aqueous (2 M, 6 mL) in toluene (60 mL) and ethanol (12 mL) was
heated to reflux in an argon atmosphere for 48 h. The solution was
cooled to room temperature and extracted with CH2Cl2. The extract
was dried with anhydrous Na2SO4 and concentrated by rotary
evaporation. The residue was purified by column chromatography
(ethyl acetate: CH2Cl2, 1: 100) to obtain the pure product as white
powder yield: 53% (0.74 g). 1H NMR (400 MHz, Methylene Chloride-
3.3. Solvatochromic effects
Systematic solvatochromic experiments using ten different
aprotic solvents were conducted to further study properties of both
ground state and excited state of 4FBTPI. From its UVeVis ab-
sorption spectra, similar absorption peaks and curves were
observed from low polarity hexane to high polarity acetonitrile.