Non-symmetric 9,10-diphenylanthracene-based deep-blue emitters with enhanced charge transport properties

Article abstract of DOI:10.1039/c4cp00236a

Realization of efficient deep-blue anthracene-based emitters with superior film-forming and charge transport properties is challenging. A series of non-symmetric 9,10-diphenylanthracenes (DPA) with phenyl and pentyl moieties at the 2nd position and alkyl groups at para positions of the 9,10-phenyls were synthesized and investigated. The non-symmetric substitution at the 2nd position enabled to improve film forming properties as compared to those of the unsubstituted DPA and resulted in glass transition temperatures of up to 92 °C. Small-sized and poorly conjugated substituents allowed to preserve emission in the deep blue range (<450 nm). Substitution at the 2nd position enabled to achieve high fluorescence quantum yields (up to 0.7 in solution, and up to 0.9 in the polymer host), although it caused an up to 10-fold increase in the intersystem crossing rate as compared to that of the unsubstituted DPA. Further optimization of the film forming properties achieved by varying the length of the alkyl groups attached at the 9,10-phenyls enabled to attain very high hole drift mobilities (～5 × 10^-3-1 × 10 ^-2 cm² V^-1 s^-1) in the solution-processed amorphous films of the DPA compounds.

Full text of DOI:10.1039/c4cp00236a

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Non-symmetric 9,10-diphenylanthracene-based
deep-blue emitters with enhanced charge
transport properties†
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 7089
Tomas Serevicius,*a Regimantas Komskis,a Povilas Adomenas,b Ona Adomeniene,b
ˇ
˙
˙
˙
˙
Vygintas Jankauskas,c Alytis Gruodis,d Karolis Kazlauskasa and Saulius Jursenasa
ˇ
Realization of efficient deep-blue anthracene-based emitters with superior film-forming and charge
transport properties is challenging. A series of non-symmetric 9,10-diphenylanthracenes (DPA) with
phenyl and pentyl moieties at the 2nd position and alkyl groups at para positions of the 9,10-phenyls
were synthesized and investigated. The non-symmetric substitution at the 2nd position enabled to
improve film forming properties as compared to those of the unsubstituted DPA and resulted in glass
transition temperatures of up to 92 1C. Small-sized and poorly conjugated substituents allowed to
preserve emission in the deep blue range (o450 nm). Substitution at the 2nd position enabled to
achieve high fluorescence quantum yields (up to 0.7 in solution, and up to 0.9 in the polymer host),
although it caused an up to 10-fold increase in the intersystem crossing rate as compared to that of the
unsubstituted DPA. Further optimization of the film forming properties achieved by varying the length
of the alkyl groups attached at the 9,10-phenyls enabled to attain very high hole drift mobilities
Received 16th January 2014,
Accepted 14th February 2014
DOI: 10.1039/c4cp00236a
(B5 Â 10À3–1 Â 10À2 cm2 VÀ1
s
À1) in the solution-processed amorphous films of the DPA compounds.
www.rsc.org/pccp
capabilities via utilization of triplet–triplet annihilation
Introduction
(or triplet fusion)12,14 and thermally-activated delayed fluores-
Anthracene derivatives nowadays are used in a large variety cence.15 The latter phenomenon enabled to achieve a very
of chemical, photophysical and medical applications. The high external quantum efficiency (16.5%) in anthracene-based
most important applications encompass fluorescence probes OLED.
and chemosensors,1 anticancer agents,2 efficient organic
Generally, the unsubstituted anthracene is of less practical
scintillators3 as well as numerous optoelectronic devices including use in optoelectronics as it suffers from low fluorescence
anthracene-based thin-film transistors,4–6 solar cells,7,8 and quantum yield (30%) in solution and ready crystallization in
organic light-emitting diodes (OLEDs).9–15 The liquid anthra- the films.1 To overcome these shortcomings structural modifi-
cenes are believed to have prospects for future applications cations of anthracene are commonly used. The most popular
such as low-power color-tunable and light upconversion way to evade a high intersystem crossing rate is an introduction
devices.16–18 Owing to the high stability and high-efficiency of various substituents into the 9th and 10th positions of
deep-blue fluorescence of anthracene compounds they were molecular anthracene. This substitution enhances the strength
found to be particularly attractive for OLED applications.19 of an allowed optical transition S0 - S1 with the transition
Recently, the interest in anthracenes has risen even more dipole moment lying along the short axis of the anthracene
because of the demonstration of OLED efficiency boosting molecule.20,21 A well-known 9,10-diphenylanthracene (DPA)
serves as a nice example of such enhancement boosting
fluorescence quantum efficiency up to 95%.22 Although out-
a
˙
Institute of Applied Research, Vilnius University, Sauletekio 9-III,
of-plane twisted 9,10-phenyl moieties cause only a minimal
decrease in the emission energy, which is important for pre-
serving emission in the deep-blue range, they are insufficient
to prevent crystallization of DPA films. The crystallization
is detrimental as it severely limits the efficiency of OLED
devices.23 It was demonstrated that film forming properties
of DPA can be improved either by introducing more bulky
aryl-based substituents at the 9th and 10th positions9,11 or by
LT-10222 Vilnius, Lithuania. E-mail: tomas.serevicius@ff.stud.vu.lt;
Fax: +370 5 2366059; Tel: +370 5 2366028
b Fine Synthesis Ltd, Kalvariju˛ g. 201E, LT-03225 Vilnius, Lithuania
c
˙
Department of Solid State Electronics, Vilnius University, Sauletekio 9-III,
LT-10222 Vilnius, Lithuania
d Department of General Physics and Spectroscopy, Vilnius University,
˙
Sauletekio 9-III, LT-10222 Vilnius, Lithuania
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cp00236a
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incorporating additional moieties at the 2nd and 6th posi- Experimental methods
tions.24,25 Unfortunately, most of the DPA derivatives subjected
Instrumentation
to such modifications and exhibiting a glassy state showed poor
H1, C13 NMR spectra were measured using BRUKER ASCEND
400 (400 MHz) and Varian Unity Inova 300 (300 MHz) spectro-
meters. Purity of the synthesized materials was analyzed using
Agilent Technologies 6890N Network GC System and Agilent
Technologies 7890C GC systems gas chromatographs. Mass
spectra were set using the Agilent Technologies 5975C gas
chromatograph/mass selective detector (GC/MSD) system with
the triple-axis detector. HRMS mass spectrometry analyses were
carried out on a quadrupole, time-of-flight mass spectrometer
(micrOTOF-Q II, Bruker Daltonik GmbH). Melting points of the
materials were determined using the Thermo Scientific 9100
apparatus. Differential scanning calorimetry (DSC) measurements
were carried out using the DSC 8500 (PerkinElmer) thermal
analysis system at a heating–cooling rate of 10 1C minÀ1 under
nitrogen flow. Optical properties of the DPA derivatives were
assessed in dilute 10À6 M tetrahydrofuran (THF) solutions and
wet-casted films prepared from 5 Â 10À3 M THF solutions.
Absorption spectra were recorded on a UV-Vis-NIR Lambda 950
spectrophotometer (Perkin-Elmer). Fluorescence of the investi-
gated compounds was excited by a 365 nm wavelength light
from a Xe lamp (FWHM o 10 meV) and measured using a
back-thinned CCD spectrometer PMA-11 (Hamamatsu) at room
temperature. Fluorescence transients were measured using a
time-correlated single photon counting system PicoHarp 300
(PicoQuant) utilizing a semiconductor diode laser (repetition
rate 1 MHz, pulse duration 70 ps, emission wavelength 375 nm)
as an excitation source. Fluorescence quantum yields (FF)
of the solutions were estimated using the integrated sphere
method.31 An integrating sphere (Sphere Optics) coupled to
the CCD spectrometer via optical fiber was also employed to
measure FF of the films. Fluorescence concentration quench-
ing effects of the DPA derivatives were analyzed by dispersing
molecules in an inert polymer (polystyrene, PS) matrix and
estimating FF dynamics vs. molecule concentration in the
range of 0.1–100 wt%. The DPA derivatives dispersed in poly-
styrene films were prepared by dissolving the DPA derivatives
and PS at appropriate ratios in THF solutions and then
wet-casting the solutions on quartz substrates. Cyclic voltam-
metry experiments were performed on the Edaq ER466 Inte-
grated Potentiostat System. Platinum wire, glassy carbon disk
[+ 1.6 mm, + 3.0 mm], and Ag/AgCl were used as counter,
working, and reference electrodes, respectively. In all cases, CV
experiments were performed in DMF (N,N-dymethylformamide)
with tetrabutylammonium perchlorate – as the supporting
electrolyte (0.1 M) under Ar flow; concentrations of compounds
were 0.002 M. The scan rate was 50 mV sÀ1. Carrier drift
mobility in the wet-casted neat films was measured by the
xerographic time of flight (XTOF) method.32–34 The samples for
the charge carrier mobility measurements were prepared as
described earlier.35 The film thickness was in the range of
3–6 mm. The ionization potentials (Ip) of the compound films
were measured by the electron photoemission in air method as
described elsewhere.36
hole drift mobility (mh), in the order of 10À7 cm VÀ1 sÀ1 26,27
.
Nevertheless, observations of remarkably higher mh for a few
DPA derivatives were also reported.28 For instance, DPA func-
tionalized with arylamino29 or pyridine30 moieties resulted in
carrier drift mobilities exceeding 10À3 cm2 VÀ1 À1. However,
s
the downside of the bulky aromatic substituents was the
extension of the p-conjugated electron system, which shifted
the compound emission to longer wavelengths (4450 nm).
Therefore, one of the challenges from the optoelectronic appli-
cation point of view could be the realization of anthracene-
based deep-blue emitters featuring fluorescence efficiencies
close to DPA, though, in contrast to DPA, demonstrating good
film forming properties and high carrier drift mobilities. Excel-
lent emission and charge transport properties realized in the
same functional layer accompanied with solution processability
could make the anthracene emitters promising for simplified
architecture deep-blue OLEDs.
To this end, a series of structurally modified non-symmetric
9,10-diphenylanthracene derivatives (Fig. 1) were synthesized
and investigated. To enhance film forming properties of the
DPA derivatives they were non-symmetrically substituted at
the 2nd position by using either phenyl or pentyl moiety.
The impact of different length alkyl groups attached at the
para position of 9,10-phenyls on the fluorescence efficiency,
concentration quenching and carrier drift mobility was
also studied. The influence of various substituents on the
electron density distribution in molecular orbitals, the energy
spectrum of singlet and triplet manifolds, and oscillator
strengths of the studied DPA derivatives was evaluated by
density functional theory calculations, which were compared
with experimental data.
Fig. 1 Chemical structures of the DPA derivatives studied in this work.
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After this the mixture was cooled down to +140 1C and poured
into 5 L of hot water and left for a night to cool down. The
mixture was filtered, the solid was placed into a 10% aqueous
potassium hydroxide solution, the mixture boiled for 0.5 h,
filtered, the solid washed with water and filtered again. The
solid was extracted with 1.3 L of toluene, concentrated; the
remainder was distilled under vacuum, b.p. 220–275 1C/1 Torr.
126 g (0.443 mol) 2-phenylanthraquinone was obtained, yield
53%. M.p. 162–163 1C.
2-Pentylanthraquinone. 2-Pentylanthraquinone was prepared
in a similar way; just the cyclization of 2-(4-pentylbenzoyl)benzene-
carboxylic acid was carried out not in polyphosphoric acid, but in
5% oleum at 80 1C. M.p. 85 1C.
2-Pentyl-9,10-bis(4-methylphenyl)anthracene. A round 250 mL
glass bulb was charged with 3.19 g (0.131 mol) magnesium
powder and 100 mL dry tetrahydrofuran. 21.4 g (0.125 mol)
4-bromotoluene was added dropwise with constant stirring,
then refluxed for 3 h till most of the magnesium dissolved.
A solution of 3.5 g (0.0125 mol) 2-pentylanthraquinone in
30 mL tetrahydrofuran was added to this mixture dropwise at
room temperature, then the resulting mixture was refluxed
for 4 h. The mixture was cooled down to room temperature
and poured into a saturated aqueous solution of ammonium
Fig. 2 The synthesis scheme of the DPA derivatives 1–6.
Materials
The DPA derivatives 1–6 (see Fig. 1) were prepared according to chloride. The mixture was extracted using dichloromethane,
the scheme presented in Fig. 2. A substituted benzene was then an organic layer separated and was evaporated.
acylated using phthalic anhydride, the resulting substituted
To the residue 17 g (0.102 mol) potassium iodide, 14.7 g
benzoylbenzoic acid underwent an intramolecular cyclization (0.167 mol) sodium dihydrophosphite and 50 mL anhydrous
upon the action of an inorganic acid, which led to the formation acetic acid were added and the whole mixture was heated at
of 2-substituted anthraquinone. It reacted with arylmagnesium reflux for 3 h. The mixture was poured into water, extracted by
bromide; the resulting 9,10-diaryl-9,10-dihydroxydihydro- dichloromethane and purified by column chromatography on
anthracene was reduced, thus forming 2,9,10-trisubstituted silica gel, toluene was used as the eluent. The collected product
anthracene.
was crystallized from acetone. 0.7 g (1.6 mmol) of a light yellow
(4-Phenylbenzoyl)-2-benzenecarboxylic acid. A 2 L glass bulb powder, m.p. 78–80 1C, was obtained, in 13% yield from
equipped with mechanical stirrer, thermometer and reflux 2-pentylanthraquinone. GC-MS: m/z 428.3. C33H32, calculated:
1
condenser was placed in an ice bath and charged with 1.2 L FW 428.61. H-NMR (400 MHz, CDCl3): d 7.744–7.675 (m, 3H),
dichloroethane, then cooled down to 0–5 1C. While stirring 7.493–7.383 (m, 9H), 7.329–7.285 (m, 2H), 7.234–7.208 (dd,
139 g (0.9 mol) biphenyl, 120 g (0.9 mol) aluminium trichloride J1 = 8.8 Hz, J2 = 1.6 Hz, 1H), 2.658 (t, J = 8 Hz, 2H), 2.575
and 133 g (0.9 mol) phthalic anhydride were added at such a (m, 6H), 1.607 (m, 3H), 1.318 (m, 3H), 0.890 (t, 3H). 13C-NMR
rate that the temperature remained within the 0 to +5 1C range. (400 MHz, CDCl3): d 139.36, 136.95, 136.86, 136.82, 136.20(2),
After all the reagents were introduced, the mixture was stirred 131.27, 131.22, 130.20, 129.57, 129.10, 129.07, 128.89, 127.02,
for 0.5 h, then the ice bath was changed to a water bath and 126.98, 126.93(2), 124.72, 124.69, 124.43, 36.27, 31.53, 30.81,
heated up to +40 1C. 120 g (0.9 mol) of aluminium chloride was 22.53, 21.45, 21.42, 14.04.
added portionwise and the temperature kept in the 40–45 1C
According to the same procedure the rest of the 2,9,10-
range. The hydrogen chloride released through the reflux trisubstituted anthracenes were obtained.
condenser was absorbed in the sodium hydroxide solution.
2,9,10-Triphenylanthracene. A light yellow powder, crystallized
After all the aluminium chloride had been introduced, the twice from acetic acid and once from hexane was obtained. Yield
reaction mixture was stirred at the same 40–45 1C for 2 h, then 35%. M.p. 175–177 1C. GC-MS: m/z 406.3. C32H22, calculated: FW
poured into an ice and 0.2 L hydrochloric acid mixture. The 406.52. 1H-NMR (300 MHz, CDCl3): d 7.37–7.46 (m, 5H), 7.57–7.88
solid product was filtered off, washed three times with water (m, 16H), 7.98 (s, 1H). 13C-NMR (300 MHz, CDCl3): d 124.8,
and dried at 100 1C. 253 g of 2-(4-phenylbenzoyl)benzene- 125.3, 125.4, 127.2, 127.3, 127.5, 127.6, 127.8(2), 127.9, 128.7,
carboxylic acid was obtained, which was used in the next step 129.1, 129.3, 130.3, 130.4, 130.6, 131.6, 131.7, 137.3, 137.6,
without additional purification.
137.7, 139.2, 139.3, 141.4.
2-Phenylanthraquinone. A 2 L glass bulb was charged with
2-Phenyl-9,10-bis(4-methylphenyl)anthracene. A light yellow
800 g of polyphosphoric acid and heated up to 160 1C, then in powder, crystallized from acetic acid and three times from hexane
20 min, 253 g (0.836 mol) of 2-(4-phenylbenzoyl)benzene- was obtained. Yield 22%. M.p. 209–211 1C. GC-MS: m/z 434.3.
1
carboxylic acid was added and stirred for 4.5 h at 160–166 1C. C34H26, calculated: FW 434.57. H-NMR (300 MHz, CDCl3): d 7.98
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(s, 1H), 7.87–7.29 (m, 19H), 2.58–2.59 (s, 6H). 13C-NMR
(300 MHz, CDCl3): d 141.5, 137.7, 137.5, 137.4, 137.3, 139.3,
136.2, 136.1, 131.4, 130.4, 129.4(2), 129.0, 127.9, 127.6, 127.4,
127.3, 125.2, 125.1, 124.9, 21.6.
2-Phenyl-9,10-bis(4-n-hexylphenyl)anthracene. A light yellow
powder, crystallized from acetic acid and twice from hexane
was obtained. Yield 26%. M.p. 105–107 1C.
1
GC-MS: m/z 574.5. C44H46, calculated: FW 574.84. H-NMR
(300 MHz, CDCl3): d 7.98 (s, 1H), 7.85–7.60 (m, 6H), 7.46–7.29
(m, 13H), 2.87–2.81 (t, J = 1.2 Hz, 4H), 1.83 (m, 4H), 1.53–1.43
(m, 12H), 1.00 (t, J = 2.4 Hz, 6H). 13C-NMR (300 MHz, CDCl3):
d 142.4, 142.3, 141.5, 137.8, 137.3, 137.2, 136.3, 136.2, 131.4,
130.7, 130.4, 129.4, 129.0, 128.7(2), 128.0, 127.6, 127.4, 127.3,
125.2, 125.1, 124.9, 36.2(2). 32.1, 31.8, 31.6, 29.4(2), 22.9, 14.4.
2-Pentyl-9,10-diphenylanthracene. A light yellow powder, crystal-
lized from acetone, was obtained. Yield 28%. M.p. 87–89 1C. GC-MS:
m/z 400.3. C31H28, calculated: FW 400.55. 1H-NMR (400 MHz,
CDCl3): d 7.725–7.572 (m, 9H), 7.522–7.518 (d, J = 1.6 Hz, 4H),
7.501 (s, 1H), 7.450–7.215 (m, 3H), 1.612–1.580 (m, 4H), 1.326–1.289
(m, 4H), 0.881 (t, 3H). 13C-NMR (400 MHz, CDCl3): d 139.50, 139.30,
139.26, 136.88, 136.23, 131.39, 131.33, 130.06, 130.04, 129.42,
128.75, 128.39, 128.36, 127.39, 127.34, 127.07, 126.94, 126.89,
126.84, 124.85, 124.59, 124.57, 36.24, 31.48, 30.71, 22.50, 14.02.
2-Pentyl-9,10-bis(4-n-hexylphenyl)anthracene. A light yellow
powder, crystallized twice from acetone was obtained. Yield
23%. M.p. 64–65 1C. GC-MS: m/z 568.5. C43H52, calculated: FW
568.87. 1H-NMR (400 MHz, CDCl3): d 7.744–7.675 (m, 3H),
7.482–7.394 (m, 9H), 7.330–7.286 (m, 2H), 7.234–7.208
(m, 1H), 2.852–2.797 (q, J = 8 Hz, 4H), 2.687–2.649 (t, J =
7.2 Hz, 2H), 1.851–1.795 (m, 4H), 1.639–1.575 (m, 2H), 1.511–1.298
(m, 17H), 0.995–0.906 (2t, 9H). 13C-NMR (400 MHz, CDCl3): d 142.02,
141.91, 139.26, 136.89, 136.38, 136.36, 136.25, 131.22, 131.18,
130.20(2), 129.58, 128.91, 128.36, 128.34, 127.06, 127.00, 126.96,
126.91, 124.69, 124.41, 36.21, 35.95, 31.83, 31.54, 31.48, 30.66,
29.21, 29.19, 22.69, 22.53, 14.15, 14.04.
Computational methods
Fig. 3 HOMO and LUMO of the DPA compounds 1–6 calculated using
the B3LYP/6-31G basis set. The positions of the short (denoted as S0–S1)
and long (denoted as S0–S2) molecular axes of anthracene molecule are
denoted in the top of the picture.
Quantum chemical calculations of the DPA derivatives were
performed using the density functional theory B3LYP method
implemented in the Gaussian09 software package.37 Ground-state
geometries of molecular structures were optimized in a 6/31G basis
set. Electronic excitation energies, oscillator strengths of the singlet
and triplet transitions and spatial distributions of electron density
for HOMO and LUMO for ‘‘freezed’’ structures were calculated
using the semiempirical ZINDO procedure (for singlets only and for
triplets only, respectively).
anthracene core, whereas in the studied non-symmetrically
substituted DPA compounds 1–6 this angle in some cases
reduces to about 801. The 2-phenyl group in the compounds
1–3 is twisted with respect to the core by about 551. Generally,
the spatial distribution of electron density in the ground and
excited states resembles that of the unsubstituted anthracene.38
Only a small fraction of molecular orbitals extends towards the
substituents. This extension towards the 2-phenyl group is
somewhat more noticeable than towards the 9,10-phenyls.
The S0 - S1 optical transition dipole moment of the
Results and discussion
Theoretical calculations
Fig. 3 demonstrates spatial electron density distribution in the
HOMO and LUMO of the DPA derivatives 1–6 with optimized anthracene lies along the short axis of the molecule, while
geometries. Phenyl groups at the 9th and 10th positions in the the transition dipole moment of S0 - S2 lies along the long
unsubstituted DPA are known to form a right angle with the axis. Therefore, introduction of various substituents at the
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Table 1 Calculated transition energies for S0 - S1 and S0 - T1/T2, and
oscillator strengths for S0 - S1 and S0 - S2
Transition Transition
energy of
energy of
Oscillator
Oscillator
S0 - S1
Comp. (eV)
S0 - T1/T2 strength of strength of
(eV)
S0 - S1
S0 - S2
2-Phenyl
2-Pentyl
1
2
3
3.091
3.069
3.076
0.981/1.956 0.326
0.09
0.093
0.093
0.98/1.96
0.98/1.959
0.356
0.361
4
5
6
DPA
3.124
3.115
3.12
0.994/2.037 0.361
0.996/2.032 0.385
0.995/2.027 0.388
0.995/2.053 0.376
0.067
0.067
0.068
0.072
3.14
9th and 10th positions increases the oscillator strength of the
S0 - S1 optical transitions, whereas the modifications at 2, 3,
6 and 7 positions mainly affect the S0 - S2 transition. Table 1
displays calculated oscillator strengths and energies of the
lowest singlet and triplet transitions of the DPA derivatives
1–6 and of the unsubstituted DPA. The S0 - S1 transition
energy of the DPA is estimated to be 3.14 eV. The energy
reduces down to 3.115–3.124 eV with the introduction of the
2-pentyl substituent (compounds 4–6) and slightly more down
to 3.069–3.091 eV with the introduction of the more conjugated
2-phenyl substituent (compounds 1–3). Calculated energies of
the two lowest triplet states T1 and T2 of the DPA derivatives
Fig. 4 (a) Molecular structures of the compounds 3 and 6. b, the dihedral
angle between the planes of anthracene and 4-hexylphenyl substituents.
correlated well with the first singlet transition energies and Bottom graphs: the calculated total energy of the ground state S0 and the
first excited state S1 (upper picture) and oscillator strength of the S0 - S1
were found to depend on the substitution at the 2nd position.
For the compounds 4–6 possessing the 2-pentyl substituent the
triplet energies were rather similar to those of the unsubsti-
transition (lower picture) as a function of the dihedral angle b for the
compound 3 (b) and compound 6 (c). Vertical dashed lines mark energy
minima of the S1 state.
tuted DPA (0.995/2.05 eV for T1/T2), whereas for the compounds
1–3 the T1/T2 energies were somewhat lower (0.98/1.9605 eV for
T1/T2) due to the more conjugated 2-phenyl group. Although the angle b (between the anthracene core and the phenyl substi-
differences in singlet and triplet energies of the compounds 1–3 tuents at 9th and 10th positions) are shown in Fig. 4(b). Since
and 4–6 are not significant, they can notably affect the rate of the twist angle of the labile phenyl group at the 2nd position of
intersystem crossing thereby severely impacting photophysical anthracene was previously shown to have a small effect on the
and photoelectrical properties of the compounds. The oscillator transition energies and oscillator strengths,39 the angle b served
strength of the dominating singlet transitions S0 - S1 was as the main reaction coordinate in the geometry optimization of
found to be at least 4 times as large as that of the nearest the DPA compounds. The optimization of the total energies
S0 - S2 transitions. The oscillator strength of the S0 - S1 indicated flat energy minima of the S0 state with respect to the
transition of the DPA derivatives 1–6 showed variations depend- angle b persisting from about 701 to 1101, whereas the S1 state
ing on the nature of substituents. The highest oscillator exhibited two slightly non-symmetrical energy minima at the
strength was obtained for the unsubstituted DPA (0.376) and angles b of 701 and 1101. The barrier height between the
for 2-pentyl-substituted compounds (0.361–0.388), meanwhile minima was about 40 meV, which is close to the thermal energy
2-phenyl-substituted DPA derivatives exhibited a slightly lower (25 meV) at room temperature. The asymmetry in energy
oscillator strength (0.326–0.361). The oscillator strength was minima is obviously caused by the non-symmetrical substi-
slightly larger for the DPA compounds with longer alkyl groups tution of the DPA compounds. The oscillator strengths had
attached at the para position of 9,10-phenyls due to the minima at the right angles b, though a rapid increase in the
increased dipole moment of the S0 - S1 transition. In contrast, strengths was clearly observed with a larger deviation from
the largest oscillator strength of the S0 - S2 transition (having this angle.
the dipole moment oriented along the longer molecular axis)
was found for the 2-phenyl-substituted derivatives as compared 2nd position of DPA by phenyl or pentyl moieties weakly
to 2-pentyl-substituted analogues. perturbs electronic transitions, while the 2,9,10-substitution
Thus theoretical calculations show that substitution at the
Fig. 4 illustrates the optimized geometry as well as the total ensures a complex geometry of the molecules, which can
energy and oscillator strength of the compounds 3 and 6. The significantly influence film-forming and thereby charge trans-
total energy and oscillator strength as a function of the dihedral port properties.
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Table 2 Thermal properties of DPA derivatives 1–6
a
b
Comp.
Tm (1C)
Tg (1C)
1
2
3
4
5
6
209
213
101
86
63
60
71
92
7
À1
À10
À24
a
b
The melting temperature. The glass transition temperature.
Thermal properties
Thermal properties of the non-symmetric DPA derivatives were
assessed by utilizing DSC. Melting (Tm) and glass transition (Tg)
temperatures of the derivatives are summarized in Table 2. The
typical DSC curves for the compounds 2 and 5 are presented in
Fig. 5 and the rest of the thermograms are presented in Fig. S1
in the ESI.†
All the DPA compounds 1–6 exhibited similar DSC thermo-
grams by demonstrating a melting signal only in the first
heating scan with peaks centered at 101–209 1C and 60–86 1C
for compounds 1–3 and 4–6, respectively. The additional low
intensity melting signals at 174 1C and 106 1C were observed for
compounds 2 and 3, respectively, which are possibly due to the
presence of a different crystalline phase. An absence of crystal-
lization peaks during the cooling indicated that the compounds
were transformed into an amorphous phase. The second heat-
ing scan revealed glass transition temperatures of 7–92 1C for
compounds 1–3 and À24–(À1) 1C for compounds 4–6. The
introduction of the 2-pentyl substituent remarkably lowered
the Tm and Tg. The continuous decrease of Tm and Tg was also
observed with the increasing length of the alkyl groups
attached at the 9,10-phenyls. It should be noted, that fluores-
cence measurements above the Tg temperature for samples
3–6 had just a minor impact on its properties (see Fig. S2 in
the ESI†).
Fig. 6 Absorption (dashed line) and fluorescence (solid line) spectra of the
DPA derivatives 1 (a), 2 (b), 3 (c), 4 (d), 5 (e) and 6 (f) in dilute 10À6 M THF
solutions and neat films (thin solid lines).
unsubstituted DPA, which served as the reference compound,
are summarized in Table 3. Note that for the compounds 3–6
optical measurements were performed at temperatures exceed-
ing their glass transition temperatures. Absorption spectra of the
DPA compounds 1–6 contain clearly resolved vibronic structures
with the dominant 1st vibronic replica similarly to that observed
in the spectra of unsubstituted DPA (see Fig. S3 in the ESI†) or
Absorption and fluorescence spectra
Absorption and emission spectra of the dilute solutions and
neat films of the DPA derivatives 1–6 are displayed in Fig. 6. The
detailed optical properties of all the derivatives and also of the
Table 3 Absorption and fluorescence data of the DPA derivatives 1–6 and
unsubstituted DPA in dilute (B10À6 M) THF solutions and neat films
Dilute solution
Neat film
max a
abs
max c
FL
max d
Comp.
l
(nm)
eb (l molÀ1 cmÀ1
)
l
(nm)
l
FL
(nm)
1
2
3
4
5
6
DPA
363, 383, 404 13 000
365, 384, 405 12 500
365, 384, 405 10 700
352, 370, 390 10 050
353, 371, 391 11 500
353, 372, 391 13 100
355, 374, 394 13 430
422
425
426
416
419
421
410
453
456
454
441
444
450
441
a
b
Absorption band maximum. Molar absorption coefficient of the 0th
c
vibronic transition maximum. Fluorescence band maximum of dilute
THF solution. Fluorescence band maximum of the neat film.
d
Fig. 5 DSC curves of the DPA compounds 2 (a) and 5 (b).
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Table 4 Fluorescence decay time constants, quantum yields, radiative and non-radiative decay time constants of the DPA derivatives 1–6 in 10À6 M THF
solutions, neat films and 0.1 wt% in PS films
Dilute solution
Neat film
PS matrix (0.1 wt% in PS)
a
b
c
d
e
d
a
b
c
d
Comp.
tF (ns)
tr (ns)
tnr (ns)
FF
tF (ns)
FF
tF (ns)
tr (ns)
tnr (ns)
FF
2-Phenyl
1
5.2
11.6
9.5
9.7
0.45
0.48
0.49
0.68
0.68
0.71
0.45 [70%]
0.95 [26%]
4.09 [4%]
0.24 [57%]
1.07 [30%]
3.48 [14%]
0.83 [55%]
3.41 [45%]
0.06
0.03
0.06
0.27
0.3
8
14.5
10.9
10.7
11
17.8
15.3
19.9
80.8
41.7
51.3
0.55
0.59
0.65
0.88
0.79
0.84
2
3
4
5
6
5.1
5
10.6
10.2
9.1
6.45
6.96
9.69
8.75
8.36
9.8
2-Pentyl
6.2
5.8
5.6
19.4
18
2.98 [82%]
1.03 [13%]
12.48 [6%]
0.66 [9%]
3.05 [76%]
8.55 [15%]
1.15 [7%]
8.5
11.1
10
7.9
19.3
0.43
5.86 [76%]
16.56 [18%]
DPA
6
6.4
100
0.94
0.64 [17%]
2.66 [44%]
7.62 [39%]
0.26
8.1
8.3
405
0.98
a
b
c
d
e
Fluorescence decay time. Radiative decay time. Non-radiative decay time. Fluorescence quantum yield. Fluorescence decay time of neat films.
other various DPA derivatives due to the presence of the rigid of the DPA derivatives 1–4 points out dense molecular packing
anthracene core.40–42 Non-symmetric substitution at the 2nd in the films, whereas nearly structureless shapes observed for
position obviously increases lability of the anthracene structure the compounds 5–6 imply more random molecular arrangement,
thus making vibronic replica less pronounced.40 The lowest energy which is typical for amorphous films. A weak long-wavelength tail
vibronic bands in the absorption spectrum of the 2-phenyl observed in the fluorescence spectra of compounds 1–3, which is
substituted DPA compounds 1–3 peak at 405 nm, whereas especially well pronounced in the compound 2 showing a small
the bands of the 2-pentyl substituted counterparts 4–6 are shoulder at 580 nm, is likely caused by excimer states formed as
shifted more in the UV (at about 391 nm) due to the worse a result of denser molecule packing of the 2-phenyl-substituted
p-conjugation of the substitutent. The molar extinction coeffi- DPA. Excimer bands typically lower the fluorescence quantum
cient of the derivatives does not exceed that of the unsubsti- yield (FF), which is exactly the case of compounds 1–3, yielding
tuted DPA (e = 13 400 L molÀ1 cmÀ1) and varies between 10 000 5 to 10 times lower FF values as compared to that of 4–6
and 13 000 L molÀ1 cmÀ1
.
compounds bearing 2-pentyl substituents (see Table 4). This
Emission spectra of the DPA compounds 1–6 in the dilute also adversely affects color purity of the films by shifting color
solution also express vibronic bands; however a lack of mirror coordinates closer to the white point of the chromaticity
images between absorption and emission implies flexibility of diagram.
the molecules and geometrical transformations occurring in
Thus, in spite of enhanced electron–vibronic interaction,
the excited state in agreement with DFT calculation results. introduction of poorly-conjugated substituents at the 2nd posi-
In accordance with the absorption spectra, fluorescence bands tion weakly affects electronic transition energies ensuring
of more conjugated 2-phenyl-substituted DPA derivatives 1–3 deep-blue emission in both non-interacting molecules and
are located at 422–426 nm, and thus are slightly redshifted as aggregate states. The small differences in transition energies,
compared to those of the less conjugated 2-pentyl-substituted however might result in dramatic changes of excited state
DPA counterparts 4–6 peaking at 416–421 nm. To compare, the relaxation pathways, as discussed below.
0th vibronic replica of the unsubstituted DPA in a solution was
Excited state relaxation
observed at 409 nm.
Fluorescence spectra of the neat films of the DPA derivatives Estimated fluorescence quantum yields (FF) of the DPA deriva-
1–6 are slightly redshifted as compared to the spectra of their tives in dilute solutions were found to range from 0.45 to
dilute solutions, which is due to the enhanced intermolecular 0.49 for the compounds 1–3 and from 0.68 to 0.71 for the
interactions in the solid state. The films of the compounds 1–3 compounds 4–6 (see Table 3). As compared to FF of the
and 4–6 emit in the deep blue region with maxima positioned unsubstituted DPA, which is close to 1.0, significantly lower
at 453–456 nm and 441–450 nm, respectively. CIE color coordi- quantum yields of the DPA derivatives indicate a moderately
nates of the emitters are displayed in Fig. S4 in the ESI.† high rate of the non-radiative recombination processes. It is
A noticeable vibronic structure in the fluorescence bandshapes known that in planar and rigid molecules like anthracene,
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the non-radiative decay is caused entirely by the intersystem
crossing to triplet states.21,43 Efficient intersystem crossing
results from the proximity of higher-lying triplet states (T2 or
higher) and the singlet state S1. For example, in the case of
crystalline anthracene, the S1 is located below the T1 level, and
thus FF is increased to almost 1.0. Simple alteration of the
anthracene core, like introduction of phenyl groups at 9th and
10th positions, as in 9,10-diphenylanthracene, can easily
realign singlet and triplet energies resulting in close to unity
quantum yields.30 However, conversion of the symmetric DPA
structure to non-symmetric, for instance, by additional substi-
tution at the 2nd position with the phenyl group (compounds
1–3) might not only affect energy alignment, but also enhance
the electron–vibronic coupling, which can efficiently deactivate
the excited state via non-radiative intramolecular torsions.44,45
Indeed, 2-phenyl-substituted DPA compounds 1–3, exhibit
somewhat lower fluorescence efficiencies as compared to those
of 2-pentyl-substituted analogues suggesting the presence of
the torsional motions. To discriminate between the two effects
additional experiments, where one of the effects is fully elimi-
nated, are needed. The results of these experiments will be
discussed below.
FF values of the neat films of the DPA derivatives were found
to be considerably lower as those of their dilute solutions. FF
values of only 0.03–0.06 were obtained in 2-phenyl-substituted
DPA compounds 1–3, whereas 0.27–0.42 values were estimated
for the 4–6 compounds with 2-pentyl groups. Intermolecular
interactions in the films, which are known to promote exciton
migration, and thus enhance the possibility for the excitons to
be captured by the quenching sites (lattice distortions, impurity
traps or other defects) are responsible for this FF lowering and
depend on intermolecular separation. Obviously, only a 2-fold
drop in FF observed for the neat films of compounds 4–6 as
compared to their solution in contrast to a roughly 10-fold drop
observed for the compound 1–3 neat films indicates that the
2-pentyl moieties more effectively suppress aggregate forma-
tion as the 2-phenyl groups. This result is well supported by the
long-wavelength tails observed in the fluorescence spectra of
the neat films of compounds 1–3 being attributed to the
excimer emission (see Fig. 6) and signifying a reduced average
intermolecular distance.
Excited state relaxations dynamics of the DPA derivatives
was assessed by measuring fluorescence decay transients. The
transients of the compound dilute solutions and neat films are
shown in Fig. 7, whereas extracted fluorescence lifetimes (tF) are
listed in Table 4. Fluorescence transients of the DPA derivatives 1–6
in dilute solutions follow a single-exponential decay profile with tF
of 5.0–6.2 ns similar to that of the unsubstituted DPA (tF = 6.0 ns).
In contrast, fluorescence transients of the neat DPA films exhibit
highly non-exponential decay profiles. The non-exponential
temporal profile accompanied by the redshifted fluorescence
bands of the solid films is a clear signature of energy transfer
Fig. 7 Fluorescence transients of DPA derivatives in THF solutions and
neat films. Lines indicate single or double exponential fits to the experi-
mental data. (a) Compounds 1–3. (b) Compounds 4–6. The excitation
wavelength is 375 nm (repetition rate 1 MHz, pulse duration 70 ps).
The fractional intensity of each component, which signifies
the actual contribution of the component to the overall excited-
state decay, is indicated in the brackets next to the tF value.
A considerably faster excited state decay of the compound neat
films as compared to that of solutions indicated enhanced
excitation migration and migration-assisted non-radiative relaxa-
tion at the quenching sites.
To evaluate the contribution of radiative and non-radiative
relaxation rates and to reveal the dominant mechanisms deter-
mining differences in the optical properties of the compounds
1–3 and 4–6, radiative (tr) and non-radiative (tnr) decay time
constants were calculated (see Table 4). The time constants
were calculated using the following relations:
tr = tF/FF, tnr = tF/(1 À FF),
(1)
where the term tnr takes into account all the possible non-
radiative decay pathways including intersystem crossing to
triplet states, which is considered to be very important in the
anthracene-based compounds.1,48
Alteration of the intersystem crossing rate
occurring via exciton hopping through the localized states in Despite the similar fluorescence lifetimes of the two series of
disordered or partly disordered media.46,47 The fluorescence decay DPA derivatives 1–3 and 4–6 in solution, the series show
profiles were fitted by using the three-component exponential different behavior of the radiative and non-radiative relaxation
decay model to reveal the major component (see Table 4). processes. Radiative decay times of the compounds are
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rather similar, i.e. the average tr is 10.8 ns and 8.5 ns for the the first singlet and higher-lying triplet states is larger making
compound series 1–3 and 4–6, respectively, whereas non- the intersystem crossing rate not so efficient. This results in
radiative time constants of these series differ by a factor of 2. longer tnr and thus higher FF.
tnr is found to be significantly shorter, and so the non-radiative
The importance of the non-symmetric substitution at the
relaxation rates are faster, for the 2-phenyl-substituted DPA 2nd position of the DPA derivatives to the optical properties can
compounds 1–3 (tnr E 9.7 ns) as compared to those bearing the be confirmed by comparing their radiative and non-radiative
2-pentyl moiety, compounds 4–6 (tnr E 18.9 ns). The 2-fold decay rates (or time constants) with those of the reference
enhancement of non-radiative decay rates in the compounds compound, the unsubstituted DPA. The radiative decay time
1–3 can be associated with the intensified intramolecular of the solutions of DPA derivatives bearing either 2-phenyl or
torsions induced by the twisted phenyl group attached at the 2-pentyl moieties differs by less than twice as compared to that
2nd position of the DPA core, although tnr enhancement may of the reference DPA. Meanwhile the difference in the non-
also be promoted by the enhanced intersystem crossing rate radiative decay time of all the derivatives with respect to the
due to the 2-phenyl substituent, which can invoke better triplet reference is dramatic, i.e. tnr is one order of magnitude shorter
and singlet energy alignment. To differentiate between the two for the compounds with 2-phenyl moieties (1–3) and 5 times
possible mechanisms, the DPA compounds were dispersed in a shorter for those with 2-pentyl moieties (4–6). An analogous
rigid polystyrene (PS) matrix at a low mass percentage (0.1 wt%). behavior of tr and tnr with respect to those of the reference DPA
Such low molecular concentration in the matrix prevented the is also observed for the DPA derivatives in the PS matrix. This
DPA molecules from interacting while simultaneously inhibiting result again verifies the importance of the type of substituent
their intramolecular motions. Estimated fluorescence lifetimes, linked to the 2nd position of the DPA in governing the non-
quantum yields, radiative and non-radiative decay time con- radiative relaxation processes via the intersystem crossing to
stants for the DPA derivatives 1–6 as well as for the unsubstituted triplet states.
DPA in PS matrices are summarized in Table 3 for comparison.
The influence of the alkyl group length attached at the para
Fluorescence decay transients of the DPA derivatives 1–6 dis- position of 9,10-phenyls on the optical properties of the DPA
persed in the polystyrene (PS) matrix are provided in Fig. S5 in derivatives in a dilute solution or the polymer matrix is insig-
the ESI.†
nificant. The alkyl groups become by far important in the film
It is worth noting that the fluorescence quantum yield of the formation, fluorescence concentration quenching and carrier
unsubstituted DPA in solution and the rigid PS matrix is nearly transport properties as will be discussed below.
the same and approaches 100%. This unambiguously indicates
Fluorescence concentration quenching
that phenyl moieties at the 9,10-positions of the DPA core are
not involved in torsional motions, and thus, do not contribute Fluorescence concentration quenching is a key factor, which
to the non-radiative deactivation of the excited state. This is determines the applicability of a compound for OLED applica-
explained by the perpendicular orientation of the 9,10-phenyls tion.49,50 Utilization of an emissive material at high concentra-
with respect to the core, which ensures their stiffness and tions (typically of more than a few percent) in a host material is
resistance to twisting. Consequently, only the phenyl group limited by the enhanced molecule interaction, and in severe
linked to the DPA core at the 2nd position could invoke non- cases, physical molecule agglomeration activating exciton migra-
radiative deactivation via the intramolecular torsions. A com- tion and migration-induced exciton quenching, and therefore
parison of the FF of the compounds 1–3 with the 2-phenyl is detrimental to the device performance.51,52 Concentration
moieties in dilute solution and the PS matrix shows a slight quenching in the DPA derivatives was evaluated by dispersing
increase of the FF in the rigid matrix (from B0.47 in solution to DPA molecules in a rigid and transparent polystyrene (PS)
B0.60 in the PS matrix) mainly due to the decreased non- host while measuring fluorescence quantum yield changes vs.
radiative decay time. However a similar increase of FF is also concentration in the broad range of concentrations 0.1–100 wt%
observed for 4–6 derivatives without these moieties (from (see Fig. 8). All the DPA derivatives 1–6 demonstrated negligible
B0.69 in solution to B0.84 in the PS matrix), where torsional fluorescence concentration quenching up to 8 wt% in PS. The rate
motions are absent. This result clearly rules out the intra- of concentration quenching for the compounds 1–3 featuring
molecular torsions as the key-mechanism responsible for the 2-phenyl substituent is very similar (Fig. 8a). FF quenches
enhanced non-radiative relaxation of the compounds 1–3 and just slightly faster as compared to that of the unsubstituted
suggests enhanced intersystem crossing to triplet states to play DPA. Conversely, concentration quenching of the compounds
the decisive role. As in the case of solutions, tr values of the 4–6 possessing the 2-pentyl substituent is found to be depen-
DPA derivatives 1–3 and 4–6 in the PS matrix are estimated to dent on the alkyl group length (Fig. 8b). The shorter alkyl
be rather similar, 12 ns and 10.7 ns, respectively, whereas tnr groups attached at the 9,10-phenyls of the DPA result in more
values of the compounds 1–3 are found to be up to 4 times rapid quenching as compared to the longer groups. This is
shorter than those of the compounds 4–6. Similarly to the evidently caused by the reduced intermolecular separation, and
compound solutions, enhanced non-radiative relaxation in PS thus enhanced intermolecular interaction, due to the smaller
matrices for compounds 1–3 can be attributed to the 2-phenyl alkyl spacers. Importantly, the quenching with increasing
moieties induced enhanced intersystem crossing to triplet states. the concentration of 2-pentyl-substituted DPA compounds
In the case of 2-pentyl groups, most likely the misalignment of 4–6 in the PS matrix is slower as compared to that of the
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Table 5 Electrochemical properties (oxidation and reduction potentials,
energies of HOMO and LUMO levels, electrochemical and optical band-
gaps) of the DPA derivatives 1–6
1/2 a
c
d
e
E
Eox onsetb Ered
(eV) (eV)
EHOMO
(eV)
ELUMO
(eV)
Eegl f Eogpt. g
(eV) (eV)
FC
Comp. (eV)
1
2
3
4
5
6
0.52 1.03
À1.71 À5.31
À1.73 À5.26
À1.73 À5.27
À1.84 À5.39
À1.88 À5.28
À1.88 À5.31
À2.58
À2.55
À2.56
À2.45
À2.4
2.72 2.94
2.74 2.94
2.72 2.93
2.94 3.07
2.88 3.04
2.91 3.04
0.98
0.99
1.11
1
1.03
À2.41
a
Half-wave potential vs. Ag/AgCl for reversible oxidation reduction of
b
c
ferrocene. Onset of oxidation potential for the HOMO level. Half-
d
wave potential vs. Ag/AgCl for the LUMO level. HOMO and LUMO
energy, EHOMO/ELUMO = À(4,8 + E1o/x2/E
À E1F/C2). HOMO and LUMO
1/2
red
e
energy, EHOMO/ELUMO = À(4,8 + E1o/x2/E
À E1F/C2). Electrochemical
1/2
red
f
bandgap, |Eegl| = |ELUMO| À |EHOMO|. Optical bandgap, estimated from
g
the onset of the absorption band.
a quasi-reversible oxidation–reduction potential was observed
after the scan rate was increased to 200 mV sÀ1, whereas in the
range of negative potentials quasi-reversible oxidation–reduction
potentials were observed for all the DPA compounds independent
of the scan rate.
1–3 and 4–6 series of the derivatives exhibited rather similar
Fig. 8 The normalized fluorescence quantum yield of the DPA com- HOMO energies, B5.28 eV and B5.32 eV, respectively, whereas
pounds 1–3 (a) and 4–6 (b) as a function of their concentration in the PS
matrix. Lines are guides to the eye. Absolute values of fluorescence
quantum yields are displayed in insets.
somewhat greater variation in LUMO energies between the
series was obtained. The compounds 1–3 showed lower LUMO
levels (B2.56 eV) as compared to the levels (B2.42 eV) estimated
for the compounds 4–6 (see Table 5). The decreased LUMO
energy observed for the compounds bearing the more conju-
gated 2-phenyl substituent is likely a cause of the lower FF due to
the enhanced intersystem crossing rate.
reference DPA, and moreover, slower than that of the 2-phenyl
substituted DPA 1–3. The obtained results highlight a signifi-
cance of the 2-pentyl moieties in more effective suppression of
intermolecular interaction resulting from looser molecular
packing than that caused by the 2-phenyl groups.
Charge transport properties and ionization potential
Achieving high carrier mobility in the amorphous films requires
a good balance of the film forming properties and charge carrier
Electrochemical properties
Cyclic voltammetry was employed to elucidate the energies of transfer rate. The impact of alkyl group length (C0, C1 and C6)
HOMO and LUMO of the DPA derivatives 1–6 (see Fig. 9 and on the charge transport properties of the two non-symmetrically-
Table 5). All the compounds demonstrated non-reversible oxidation– substituted DPA compound series bearing 2-phenyl (compounds
reduction processes by applying a low scan rate (50 mV sÀ1) in 1, 2, 3) and 2-pentyl moieties (compounds 4, 5, 6) was investi-
the range of positive potentials. For the compounds 2 and 3, gated. The wet-casted amorphous films of the DPA derivatives
Fig. 9 Cyclic voltammograms of the dilute solutions of compounds 1–3 (a) and 4–6 (b) in N,N-dimethylformamide at 50 mV sÀ1 scan rate. Compound
concentration was 0.002 M with tetrabutylammonium perchlorate (0.1 M) used as the electrolyte.
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were investigated by using the XTOF method. The reference measurements impossible. Samples 3 and 6 were not suitable
DPA was found to form polycrystalline neat films with low for XTOF measurements because of high plasticity of their neat
photosensitivity, which made XTOF measurements practically film. The injected carriers were found to distribute unevenly
impossible. Introduction of the additional phenyl or pentyl causing film deformation by electrostatic forces in the sites
moieties at the 2nd position and the methyl groups at the para with high carrier density, and thus making carrier drift mobility
positions of 9,10-phenyls (compounds 1, 2, 4, 5) completely unattainable. The continuous increase of mh with the applied
suppressed DPA crystallization and ensured formation of amor- electric field for all the DPA derivatives implies the governing
phous neat films. It is worth emphasizing that these modified role of energetic disorder53 in the charge carrier transport of
non-symmetric DPA derivatives expressed very high hole drift the wet-casted amorphous films. The high drift mobilities
mobilities (mh) well exceeding 10À3 cm2 VÀ1 sÀ1 at 1 MV cmÀ1 obtained for the non-symmetric DPA derivatives 1, 2, 4, 5 are
electric field. The obtained drift mobility values varied from well above usually reported values estimated in other DPA
4.6 Â 10À3 cm2 VÀ1 sÀ1 in the compound 1 up to almost compounds.26,27 However remarkably high mh values (up to
1 Â 10À2 cm2 VÀ1 sÀ1 in the compound 4 at an electric field 10À2 cm2 VÀ1
s
À1) have also been achieved by introducing
of 1 MV cmÀ1 (see Fig. 10 and Table 6). XTOF transients electron donating 9,10-arylamino units into the anthracene
measured at different voltages are displayed in Fig. S6 in the core.29 However, the arylamino substitution enlarged conjuga-
ESI.† Although alkyl groups at the para positions of 9,10-phenyls tion of the DPA compound resulted in an undesirable shift of
negligibly affected optical properties of the DPA compounds, the emission color to longer wavelengths (4450 nm).
their length severely altered the molecular packing, and thus
Ionization potentials (Ip) of the DPA derivatives 1–6 showed
charge carrier transport properties. Introduction of the methyl very similar values 5.74–5.9 eV comparable to those reported for
group (compound 2) slightly enhanced the hole drift mobility 2-phenyl-substituted anthracene.39
and increased the glass transition temperature, whereas analo-
gous substitution for the compounds possessing the 2-pentyl
moiety, worsened the drift mobility and the glass transition
temperature. This can be attributed to the better steric hindrance
Conclusions
effect of the twisted 2-phenyl substituent. Further extension of Realization of anthracene-based deep-blue emitters exhibiting
the alkyl group length up to C6 atoms (compounds 3 and 6) fluorescence efficiency close to that of 9,10-diphenylanthracene
strongly impaired film forming properties and made XTOF (DPA), though, in contrast to DPA, demonstrating good film
forming properties and high carrier drift mobilities was
attempted. To this end, a series of structurally modified non-
symmetric DPA derivatives with the phenyl and pentyl moieties
at the 2nd position and the alkyl groups at the 9,10-phenyls
were synthesized and investigated. Relatively small and poorly
conjugated phenyl and pentyl substituents enabled to improve
film forming properties by resulting in glass transition tem-
peratures of up to 92 1C (for the 2-phenyl moiety), meanwhile
only up to À1 1C (the for 2-pentyl moiety). The poorly con-
jugated 2-phenyl and 2-pentyl substituents weakly impacted the
transition energy with respect to the unsubstituted DPA resulting
in the deep-blue emission with emission maxima below 450 nm.
Absorption and fluorescence measurements supported by DFT
calculations revealed relatively small shifts in singlet and triplet
transition energies (o100 meV), which however, caused drastic
Fig. 10 Hole drift mobility as a function of the applied electric field in the
neat films of the DPA compounds 1–6. Lines are guides to the eye. The
average film thickness is indicated.
changes in the intersystem crossing rate. Fluorescence measure-
ments in dilute solution and the rigid polymer matrix enabled to
distinguish between the two possible pathways of non-radiative
recombination, i.e. intramolecular torsions and intersystem
crossing, and demonstrated the dominant role of the latter
process. Roughly a 10-fold and 5-fold increase in the intersystem
crossing rate was obtained in the 2-phenyl- and 2-pentyl-substituted
DPA compounds, respectively. Despite the large enhancement of
the intersystem crossing rate the DPA compounds showed relatively
high fluorescence quantum yields, up to 0.7 in a solution and
up to 0.9 in a polymer host. The DPA derivatives exhibited
almost no concentration quenching of fluorescence up to
8 wt% in the PS matrix. Substitution at the 2nd position by
the pentyl moiety resulted in less pronounced concentration
Table 6 Hole drift mobilities and ionization potentials of the amorphous
neat films of DPA derivatives 1–6
b
Comp.
mh (cm2 VÀ1
a
s
À1) @ 1 MV cmÀ1
Ip (eV)
1
2
3
4
5
6
4.6 Â 10À3
6.5 Â 10À3
—
5.82
5.74
5.77
5.9
5.83
5.81
9.9 Â 10À3
6.5 Â 10À3
—
a
Hole drift mobility of ambient atmosphere processed neat films.
Ionization potential of ambient atmosphere processed neat films.
b
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15 K. Nasu, T. Nakagawa, H. Nomura, C.-J. Lin, C.-H. Cheng,
M.-R. Tseng, T. Yasuda and C. Adachi, Chem. Commun.,
2013, 49, 10385–10387.
16 S. S. Babu, M. J. Hollamby, J. Aimi, H. Ozawa, A. Saeki,
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H. Mohwald and T. Nakanishi, Nat. Commun., 2013, 4,
1969.
17 S. H. Lee, J. R. Lott, Y. C. Simon and Ch. Weder, J. Mater.
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18 P. Duan, N. Yanai and N. Kimizuka, J. Am. Chem. Soc., 2013,
135, 19056–19059.
quenching as compared to the substitution by the 2-phenyl
group and also with respect to the unsubstituted DPA. The
influence of the alkyl groups attached at the para position of
9,10-phenyls on the optical properties was found to be insigni-
ficant, whereas they strongly affected film forming and charge
transport properties. It is worth noting that the non-symmetric
DPA derivatives expressed very high hole drift mobilities of up
to 4.6 Â 10À3 cm2 VÀ1 sÀ1 for 2-phenyl-substituted compounds
and up to almost 1 Â 10À2 cm2 VÀ1 sÀ1 for 2-pentyl-substituted
analogues at an electric field of 1 MV cmÀ1
.
Further optimization of the DPA at the 2nd position by
introducing non-conjugated and more branched or bulky sub-
stituents should enable to achieve an even higher glass transi-
tion temperature and carrier drift mobility while preserving the
high fluorescence efficiency.
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Chem., 2012, 22, 123–129.
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Wiley-VCH, Weinheim, New York, 2002.
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Acknowledgements
The research was funded by a grant (No. 31V-28) from the
Science, Innovation and Technology Agency. The public access
supercomputer from the High Performance Computing Center
(HPCC) of the Lithuanian National Center of Physical and
Technology Sciences (NCPTS) at Physics Faculty of Vilnius
˙
University (HPC Sauletekis) was used for DFT calculations.
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