Impact of non-symmetric 2,9,10-aryl substitution on charge transport and optical properties of anthracene derivatives

Article abstract of DOI:10.1016/j.dyepig.2015.06.010

A new methodology for the synthesis of non-symmetric 2,9,10-arylanthracenes is proposed. The anthracenes were intentionally decorated with non-symmetric aryls to improve charge transport properties. The impact of non-symmetric aryl substituents on optical and electrical properties was thoroughly studied. Non-symmetric 9,10-aryl substituents ensured blue emission in the range of 422-436 nm and fluorescence quantum efficiency of ca. 0.45, while the type of substituents had a minor impact on excited state deactivation due to their twisted configuration. Optical properties of the compounds obtained in dilute solution and in a rigid polymer matrix revealed the dominating non-radiative decay mechanism to be due to the intersystem crossing (ISC), which was mainly governed by 2-phenyl substitution. Favourable alignment of the singlet and triplet energy levels for efficient ISC was confirmed by density functional theory calculations. Some of the non-symmetrically substituted anthracene derivatives showed very high hole drift mobilities reaching 10^-2 cm²/Vs in solution-processed neat films.

Full text of DOI:10.1016/j.dyepig.2015.06.010

Accepted Manuscript
Impact of non-symmetric 2,9,10-aryl substitution on charge transport and optical
properties of anthracene derivatives
Tomas Serevičius, Povilas Adomėnas, Ona Adomėnienė, Karolis Karpavičius, Jonas
Bucevičius, Regimantas Komskis, Gediminas Kreiza, Vygintas Jankauskas, Karolis
Kazlauskas, Saulius Juršėnas
PII:
S0143-7208(15)00228-4
10.1016/j.dyepig.2015.06.010
DYPI 4812
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 4 May 2015
Revised Date: 6 June 2015
Accepted Date: 9 June 2015
Please cite this article as: Serevičius T, Adomėnas P, Adomėnienė O, Karpavičius K, Bucevičius J,
Komskis R, Kreiza G, Jankauskas V, Kazlauskas K, Juršėnas S, Impact of non-symmetric 2,9,10-aryl
substitution on charge transport and optical properties of anthracene derivatives, Dyes and Pigments
(2015), doi: 10.1016/j.dyepig.2015.06.010.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Impact of non-symmetric 2,9,10-aryl substitution on charge transport and
optical properties of anthracene derivatives
a
a
a
a
Tomas Serevičius , Povilas Adomėnas , Ona Adomėnienė , Karolis Karpavičius , Jonas
b
a
a
a
Bucevičius , Regimantas Komskis , Gediminas Kreiza , Vygintas Jankauskas , Karolis
a
a
Kazlauskas and Saulius Juršėnas
a
Institute of Applied Research, Vilnius University, Saulėtekio 9-III, LT-10222 Vilnius, Lithuania
b
Department of Organic Chemistry, Vilnius University, Naugarduko 24, LT-03225 Vilnius, Lithuania
Corresponding author: Tel.: +370 52 366028; fax: +370 52 366059.
E-mail address: tomas.serevicius@ff.stud.vu.lt
.
A new methodology for the synthesis of non-symmetric 2,9,10-arylanthracenes is
proposed. The anthracenes were intentionally decorated with non-symmetric aryls to
improve charge transport properties. The impact of non-symmetric aryl substituents
on optical and electrical properties was thoroughly studied. Non-symmetric 9,10-
aryl substituents ensured blue emission in the range of 422 - 436 nm and
fluorescence quantum efficiency of ca. 0.45, while the type of substituents had a
minor impact on excited state deactivation due to their twisted configuration. Optical
properties of the compounds obtained in dilute solution and in a rigid polymer
matrix revealed the dominating non-radiative decay mechanism to be due to the

ACCEPTED MANUSCRIPT
intersystem crossing (ISC), which was mainly governed by 2-phenyl substitution.
Favorable alignment of the singlet and triplet energy levels for efficient ISC was
confirmed by density functional theory calculations. Some of the non-symmetrically
substituted anthracene derivatives showed very high hole drift mobilities reaching
-
2
2
1
0 cm /Vs in solution-processed neat films.
Keywords: anthracene, non-symmetric substitution, fluorescence, hole mobility,
intersystem crossing,
1. Introduction
Anthracene is well-known aromatic compound used for drugs [1], anticancer agents [2],
chemosensors [3], and as scintillators for high energy particle detection [4]. Owing to the
extended π-electron system anthracene derivatives find a large variety of applications in organic
optoelectronic devices including thin-film transistors [5,6], solar cells [7,8], and light emitting
diodes (OLEDs) [9–14]. Easy modification, deep-blue emission and high photostability make
them particularly attractive for manufacture of blue emitters. Moreover, the unique feature of
anthracene compounds is its perfect energy level alignment for triplet-triplet annihilation (TTA)
enhanced emission, when fusion of two triplet excitons results in energy up-conversion and
boosting the fluorescent OLEDs efficiency with additional 37.5% [15]. Anthracene OLEDs
employing TTA overcome the theoretical 5% limit of quantum efficiency for conventional
OLEDs (with extraction efficiency of 0.2) and reaches values up to 11% [9,14,16–18].
Anthracene derivatives were also applied for engineering of thermally-activated delayed
2

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fluorescence emitters [19]. Low-power color-tunable and light upconversion devices is another
important example of the application of TTA, when the low energy photons are converted to
high energy ones applying various anthracene derivatives [20].
Owing to their flat and rigid backbone, unsubstituted anthracene, suffers from
crystallization induced fluorescence quenching and low fluorescence quantum yield (Φ =0.3)
F
th
th
[
21]. Substitution at the 9 and 10 position with phenyl groups (or other aryl groups [22–24])
remarkably decreases ISC rate and enhances fluorescence quantum yield up to the unity [24,25].
th
th
However the structural modifications with phenyl moieties at 9 and 10 positions are
insufficient to eliminate unwanted crystallization and fluorescence quenching in the films [10,26].
th
th
Further modifications with more bulky aryl groups at 9 and 10 positions [17,26–29] greatly
reduces the intermolecular interactions and enables to achieve amorphous thin films. The
nd
additional substitution at the 2 position also helps to ensure amorphous state and improves
charge transport properties of the films [10,30]. However the additional π-conjugation extension
towards the long axis of the molecule results in the reorganization of singlet and triplet energy
levels when the T level emerges below S leading to the efficient ISC recombination and low Φ
n
1
F
[26]. Despite the importance of the intersystem crossing, recent studies have suggested that the
vibrational relaxation may also be the important pathway for the non-radiative decay in
anthracene derivatives [31,32]. Modification of anthracene core with various alkyl groups allows
to obtain amorphous thin films, however those anthracene derivatives like 9,10-di(2-
-7
-5
2
h
naphthyl)anthracene posses low hole drift mobility (μ ) [33–35] of about 10 – 10 cm /Vs.
-3
2
Nevertheless, observations of remarkably higher μ , exceeding 10 cm /Vs were also reported
h
for a few anthracene compounds when symmetrically functionalized with arylamino [36] or
3

ACCEPTED MANUSCRIPT
pyridine [37] moieties. However, the presence of highly-polar substituents resulted in the
undesirable redshift of the compound’s emission wavelength.
Thus non-symmetric modification of anthracene derivatives by aryl substituents seems to
be a promising way of chemical engineering to improve the film-forming, charge-transfer and
emissive properties of the functional layers, however the impact of such modification on charge
transport and energy level alignment should be thoroughly addressed.
In this work the new synthesis methodology and properties of a series of 2-
th
th
phenylanthracene derivatives non-symmetrically modified at 9 and 10 positions are presented.
The impact of the non-symmetric substitution was evaluated by estimating thermal,
electrochemical, optical and charge-transport properties of the compounds. Remarkable
improvement of the charge transport properties for some of the non-symmetric 2,9,10-
anthracenes is validated. Disclosed energy level scheme, supplemented by analysis of excitation
relaxation pathways, enable us to distinguish the dominant role of intersystem crossing and the
impact of the introduced substituents on ISC rate variation.
2. Experimental methods
2.1. Instrumentation
Solvents were purified using standard procedure. Reactions were carried out under dry
argon atmosphere at magnetic stirring. All reactions and the purity of the synthesized compounds
were monitored by gas chromatography using Agilent Technologies 7890A GC System with
triple axis detector 5975C inert XL MSD and Agilent Technologies 6890N Network GC System,
4

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and thin layer chromatography using Silica gel 60 F254 aluminum plates (Merck). Purification of
synthesized compounds was performed by preparative column chromatography using silica gel
1
13
Kieselgel 60 0.06 – 0.2 mm. H and C NMR spectra were recorded with a Varian Unity
INOVA spectrometer (300 MHz) or Brucker ASCEND 400 (400 MHz) in chloroform-d or
3
benzene-d , using residual solvent signal as an internal standard. HRMS spectra were obtained
6
on a mass spectrometer Dual-ESI Q-TOF 6520 (Agilent Technologies). All melting points were
determined using Digital Melting Point Apparatus IA9000 Series. Differential scanning
calorimetry (DSC) measurements were carried out using DSC 8500 (PerkinElmer) thermal
analysis system at a heating/cooling rate of 10 C/min under nitrogen flow. Optical properties of
-6
the 2,9,10-arylanthracene derivatives were assessed in dilute 10 M tetrahydrofuran (THF)
-3
solutions and wet-casted thin films prepared from 5 × 10 M THF solutions. Absorption spectra
were recorded on a UV−Vis−NIR spectrophotometer Lambda 950 (Perkin-Elmer). Fluorescence
of the investigated compounds was excited by a 365 nm wavelength light from Xe lamp
(
(
FWHM<10 meV) and measured using a back-thinned CCD spectrometer PMA-11
Hamamatsu). Fluorescence transients were measured using a time-correlated single photon
counting system PicoHarp 300 (PicoQuant) utilizing a semiconductor diode laser (repetition rate
MHz, pulse duration 70 ps, emission wavelength 375 nm) as an excitation source.
1
Fluorescence quantum yields (Φ ) of the solutions were estimated using the integrated sphere
F
method [38]. An integrating sphere (Sphere Optics) coupled to the CCD spectrometer via optical
fiber was also employed to measure Φ of the thin films. Cyclic voltammetry (CV) experiments
F
were performed on the Edaq ER466 Integrated Potentiostat System. Pt/Ti wire, glassy carbon
disk [Ø 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)
5

ACCEPTED MANUSCRIPT
with tetrabutylammonium perchlorate – as the supporting electrolyte (0.1 M) under Ar flow;
-1
concentrations of compounds were 0.002 M. The scan rate was 50 mV s . Carrier drift mobility
of wet-casted neat films was measured by xerographic time of flight (XTOF) method [39–41].
The samples for the charge carrier mobility measurements were prepared as described earlier
[
42]. The film thickness was in the range of 2−6 µm. The ionization potentials (I ) of the
p
compound films were measured by electron photoemission in air method as described elsewhere
43].
[
2.2. Synthesis
Non-symmetric 2-phenylanthracene based derivatives with different type of aryl-
th
th
substituents at 9 and 10 positions are showed in Fig. 1.
10-bromo-2-phenylanthracene (M8). N-bromosuccinimide (4.2 g, 0.0236 mol) was
slowly added to a solution of 2-phenylanthracene (M7) (6.0 g, 0.0236 mol) in dry N,N-
dimethylformamide (200 ml) at 30 °C and the mixture was stirred for 30 min. The reaction
mixture was diluted with water (250 ml); a yellow solid was obtained and collected by filtration.
The solid was washed with hot water and crystallized from mixture of isopropanol and toluene
1
(
1:3) to provide fine yellow crystals of M8 (6.3 g, 80%): mp 144 – 146 ˚C; H NMR (400 MHz,
C D ) δ: 8.89 (t, J = 8.8 Hz, 1H), 8.57 (dd, J = 8.8, 0.8 Hz, 1H), 8.00 (s, 1H), 7.72 – 7.62 (m,
6
6
4
H), 7.55 (dd, J = 8.7 Hz, 1.6 Hz, 1H), 7.32 – 7.22 (m, 3H), 7.22 – 7.17 (m, 1H), 7.14 (t, J = 1,6
1
3
Hz, 1H). C NMR (101 MHz, C
6
D
6
) δ: 140,94, 139.94, 132.28, 131.44, 131.11, 130.95, 129.25,
6

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1
28.98, 128.53, 127.62, 127.59, 127.58, 127.19, 126.95, 125.73, 125.42, 125.29, 122.72. ppm.
+
HRMS (ES): M + H , found 333.0283. C H Br requires 333.028.
20
13
1
0-Aryl-2-phenylanthracenes (1a and 1b). 92 mg of 1 mol% Pd(PPh ) Cl was added to
3 2 2
the solution of 4.0 g (0.012 mol) of 10-bromo-2phenylantracene in 250 ml toluene in argon
atmosphere. Then boronic acid (0.0138 mol) was added to the heated mixture, after that 100 ml
2 3
of 2M K CO solvent in water was poured to the mixture and then boiled. After the reaction
completion, which was monitored by the gas chromatography, reaction mixture was quenched
with 200 ml of water extracted and water layer was washed with toluene (3 * 100 ml). Combined
organic layers were dried with Na SO , filtrated through silica gel. After the evaporation of
2
4
solvent under reduced pressure, the material was purified by recrystallization from isopropanol
and toluene mixture (1:5).
1
1
0-(naphthalen-2-yl)-2-phenylanthracene (1a). (3.8 g 85%): H NMR (400 MHz, C D )
6
6
δ 8.36 (s, 1H), 8.22 (t, J = 1.2 Hz, 1H), 7.99 (d, J = 8.8 Hz, 1H), 7.94 (d, J = 8.5 Hz, 1H), 7.81 –
7
3
1
1
1
5
.75 (m, 2H), 7.74 – 7.66 (m, 4H), 7.62 – 7.56 (m, 1H), 7.50 – 7.41 (m, 4H), 7.29 – 7.26 (m,
1
3
6 6
H), 7.15 – 7.10 (m, 1H), 7.04 – 6.98 (d, 2H). C NMR (101 MHz, C D ) δ 141.58, 138.65,
37.81, 136.69, 134.02, 133.30, 132.16, 131.56, 131.28, 131.25, 130.66, 129.79, 129.59, 129.12,
28.82, 128.54, 128.42, 127.94, 127.60, 127.47, 127.43, 127.04, 126.65, 126.45, 126.01, 125.78,
+
25.53, 124.86. ppm. MS (EI) (m/z): 380.2 [M] ; Anal. Calcd for C H (380.49): C, 94.70; H,
3
0
20
.30. Found: C, 94.55; H, 5.45.
1
1
6 6
0-(Biphenyl-4-yl)-2-phenylanthracene (1b). (4.6 g 94%): H NMR (400 MHz, C D ) δ
8
.34 (s, 1H), 8.31 – 8.28 (m, 1H), 7.97 (d, J = 8.8 Hz, 1H), 7.94 – 7.87 (m, 2H), 7.69 (dd, J =
7

ACCEPTED MANUSCRIPT
8
3
1
1
.8, 1.7 Hz, 1H), 7.61 – 7.50 (m, 6H), 7.41 – 7.37 (m, 2H), 7.31 – 7.17 (m, 5H), 7.13 – 7,04(m,
1
3
H). C NMR (101 MHz, C D ) δ 141.75, 141.09, 140.68, 138.70, 138.18, 137.62, 132.17,
6
6
31.41, 131.28, 131.15, 129.60, 129.17, 129.08, 128.85, 128.18, 127.95, 127.71, 127.63, 127.54,
+
27.49, 127.39, 127.04, 125.97, 125.81, 125.54, 124.86. ppm. MS (EI) (m/z): 406.2 [M] ; Anal.
Calcd for C H (406.17): C, 94.55; H, 5.45. Found: C, 94.62; H, 5.38.
3
2
22
9-Bromo-10-aryl-2-fenilantracenes (M11 and M19). N-bromosuccinimide (1.13 g, 6.35
mmol) was slowly added to a solution of 10-aryl-2-fenilantracene (1a or 1b) (6. 20 mmol) in dry
N,N-dimethylformamide (100 ml) at 50 °C and allowed to stir for 30 min. Then the reaction
mixture was diluted with water (125 ml) and a yellow solid was obtained and collected by
filtration. Solid was washed with hot water and recrystallized from mixture of isopropanol and
toluene (1:4) to provide fine crystals.
1
1
0-(naphthalen-2-yl)-2-phenylanthracene (M11). (2.1 g 74%): mp 184 – 186 ˚C; H
NMR (400 MHz, C
6
D
6
) δ 8.82 (d, J = 9.2 Hz, 1H), 8.75 (d, J = 8.9 Hz, 1H), 8.13 (d, J = 1.4 Hz,
1
2
H), 7.72 – 7,68 (m, 5H), 7.59 (dd, J = 6.1, 3.3 Hz, 1H), 7.43 – 7.37 (m, 2H), 7.37 – 7.30 (m,
1
3
H), 7.30 – 7.23 (m, 3H), 7.08 – 6.99 (m, 1H), 6.98 – 6.91 (m, 3H). C NMR (101 MHz, C D )
6
6
δ 141.75, 141.09, 140.68, 138.70, 138.18, 137.62, 132.17, 131.41, 131.28, 131.15, 129.60,
1
1
29.17, 129.08, 128.85, 128.18, 127.95, 127.92, 127.89, 127.71, 127.63, 127.54, 127.49, 127.39,
+
27.04, 125.97, 125.81, 125.54, 124.86. ppm. MS (EI) (m/z): 458.1 [M] ; Anal. Calcd for
C H Br (459.39): C, 78.44; H, 4.17; Br, 17.39. Found: C, 78.62; H, 4.29.
30
19
8

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10-(Biphenyl-4-yl)-9-bromo-2-phenylanthracene (M19). (2.6 g 85%): mp 221 – 226 ˚C;
1
H NMR (400 MHz, C D ) δ 8.80(d, J = 9.6 Hz, 1H), 8.74 (d, J = 8.8 Hz, 1H), 8.19 (d, J = 1.3
6
6
Hz, 1H), 7.79 (d, J = 8.7 Hz, 1H), 7.71 (dd, J = 9.2, 1.8 Hz, 1H), 7.61 – 7.55 (m, 2H), 7.54 –
7
4
1
1
.46 (m, 4H), 7.34 – 7.27 (m, 3H), 7.27 – 7.21 (m, 2H), 7.20 – 7.16 (m, 1H), 7.12 – 7.01 (m,
1
3
H). C NMR (101 MHz, C D ) δ 140.92, 140.89, 138.71, 138.22, 137.71, 132.06, 132.03,
6
6
31.88, 130.97, 130.21, 129.22, 129.18, 129.13, 128.30, 128.18, 128.06, 127.94, 127.82, 127.77,
+
27.62, 127.50, 127.47, 127.36, 126.12, 125.07, 123.20. ppm. MS (EI) (m/z): 484.1 [M] ; Anal.
Calcd for C32H21Br (485.42): C, 79.18; H, 4.36; Br, 16.46. Found: C, 79.32; H, 4.28.
1
0-(naphthalen-2-yl)-9-aryl-2-phenylanthracenes (2a – 5a). Pd(PPh ) Cl (13 mg; 5
3 2 2
mol%) was added to the 160 mg (0.347 mmol) of 10-(naphthalen-2-yl)-9-bromo-2-
phenylanthracene dissolved in 50 ml toluene in argon atmosphere. After that the boronic acid
(
0.434 mmol) was added to the heated mixture, then 30 ml of 2M K CO solvent in water was
2 3
poured to the mixture and then boiled. After the completion of the reaction was monitored by
TLC chromatography, reaction mixture was quenched with 100 ml of water, extracted and water
2 4
layer was washed with toluene (3 * 100 ml). Combined organic layers were dried with Na SO ,
filtrated through silica gel. After the evaporation of solvent under reduced pressure, the material
was purified by preparative column chromatography (hexane – diethyl ether 10:1), then
crystallized from isopropanol and toluene mixture (1:5).
1
1
0-(naphthalen-2-yl)-2,9-diphenylanthracene (2a). (85 mg 54%): H NMR (400 MHz,
6 6
C D ) δ 8.31 (d, J = 1.3 Hz, 1H), 8.03 (d, J = 9.1 Hz, 1H), 7.97 – 7.91 (m, 1H), 7.89 – 7.82 (m,
2
H), 7.78 – 7.69 (m, 2H), 7.64 – 7.44 (m, 8H), 7.41 – 7.24 (m, 5H), 7.15 – 7.07 (m, 1H), 7.01 –
9

ACCEPTED MANUSCRIPT
1
3
6
1
1
1
5
6 6
.93 (m, 3H). C NMR (101 MHz, C D ) δ 141.37, 139.76, 138.27, 137.96, 137.76, 137.05,
34.07, 133.34, 132.10, 131.74, 131.73, 131.33, 131.06, 130.81, 130.77, 129.90, 129.23, 129.14,
28.89, 128.56, 128.52, 128.18, 127.94, 127.53, 127.51, 127.48, 126.71, 126.49, 125.79, 125.71,
25.63, 124.95. ppm. Anal. Calcd for C H (456.59): C, 94.70; H, 5.30. Found: C, 94.87; H,
36
24
.23.
1
9
-(Biphenyl-4-yl)-10-(naphthalen-2-yl)-2-phenylanthracene (3a). (162 mg 87%): H
NMR (400 MHz, C ) δ 8.35 (d, J = 1.4 Hz, 1H), 8.12 (d, J = 9.1 Hz, 1H), 8.03 (d, J = 8.2 Hz,
H), 7.92 (s, 1H), 7.89 (d, J = 8.3 Hz, 1H), 7.78 (d, J = 8.4 Hz, 1H), 7.72 – 7.68 (m, 3H), 7.67 –
.61 (m, 3H), 7.61 – 7.55 (m, 2H), 7.50 – 7.43 (m, 4H), 7.36 – 7.27 (m, 3H), 7.25 – 7.18 (m,
6
D
6
1
7
4
1
1
1
1
3
H), 7.15 – 7.09 (m, 1H), 7.00 – 6.96 (m, 2H). C NMR (101 MHz, C D ) δ 141.77, 141.37,
6
6
41.29, 140.91, 138.68, 138.35, 138.04, 137.47, 137.05, 134.09, 133.36, 132.25, 131.38, 131.11,
30.88, 130.79, 129.97, 129.91, 129.25, 129.16, 129.05, 128.57, 128.40, 128.18, 127.94, 127.67,
27.55, 127.53, 127.51, 127.47, 126.74, 126.52, 125.87, 125.78, 125.71, 125.01. ppm. Anal.
Calcd for C42H28 (532,69): C, 94.70; H, 5.30. Found: C, 94.84; H, 5.26.
9-(benzo[a]anthracen-4-yl)-10-(naphthalen-2-yl)-2-phenylanthracene (4a). (94 mg
1
4
1
7
6
1
5%): H NMR (400 MHz, C D ) δ 9.22 (s, 1H), 8.86 (d, J = 8.3 Hz, 1H), 8.40 (d, J = 1.3 Hz,
6 6
H), 8.09 – 8.02 (m, 2H), 8.01 – 7.92 (m, 3H), 7.88 (d, J = 9.1 Hz, 1H), 7.85 – 7.78 (m, 3H),
.77 – 7.61 (m, 5H), 7.50 (dd, J = 9.1, 1.8 Hz, 1H), 7.47 – 7.30 (m, 7H), 7.14 – 7.05 (m, 2H),
1
3
.98 – 6.94 (m, 3H). C NMR (101 MHz, C
6 6
D ) δ 140.96, 138.18, 138.01, 137.99, 137.72,
36.65, 136.63, 135.65, 135.64, 133.74, 133.02, 132.34, 132.33, 132.26, 132.24, 131.33, 131.20,
10

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1
1
1
4
31.04, 130.78, 130.66, 130.50, 130.49, 130.30, 129.55, 129.33, 128.77, 128.54, 128.24, 128.13,
28.10, 127.81, 127.57, 127.31, 127.15, 126.60, 126.45, 126.41, 126.19, 125.80, 125.70, 125.67,
25.06, 125.01, 124.72, 123.18, 122.10. ppm. Anal. Calcd for C H (606.77): C, 95.02; H,
4
8
30
.98. Found: C, 95.13; H, 4.87.
9-(4-(9H-carbazole-9-yl)phenyl)-10-(naphthalen-2-yl)-2-phenylanthracene (5a). (155
1
mg 72%): H NMR (400 MHz, C
6
D
6
) δ 8.38 (d, J = 1.3 Hz, 1H), 8.17 – 8.09 (m, 3H), 8.04 (d, J
=
8.8 Hz, 1H), 7.95 – 7.88 (m, 3H), 7.78 (d, J = 8.3 Hz, 1H), 7.75 – 7.68 (m, 2H), 7.66 – 7.62
(m, 1H), 7.59 (d, J = 8.1 Hz, 2H), 7.56 – 7.49 (m, 4H), 7.45 – 7.39 (m, 4H), 7.36 – 7.23 (m, 5H),
1
3
7
1
1
1
.14 – 7.09 (m, 1H), 7.03 – 6.96 (m, 3H). C NMR (101 MHz, C D ) δ 141.44, 141.23, 138.51,
6 6
38.43, 137.62, 136.89, 136.76, 134.07, 133.38, 133.17, 133.13, 131.38, 131.13, 130.82, 130.77,
29.95, 129.80, 129.33, 129.23, 128.63, 128.57, 128.18, 127.94, 127.63, 127.51, 127.38, 127.30,
26.82, 126.60, 126.46, 126.00, 125.95, 125.93, 125.09, 124.28, 120.87, 120.66, 110.41. ppm.
Anal. Calcd for C H N (621,78): C, 92.72; H, 5.03; N, 2.25. Found: C, 92.83; H, 5.07.
4
8
31
1
9
-(4-ethoxyphenyl)-10-(naphthalen-2-yl)-2-phenylanthracene (6a). (140 mg 81%): H
NMR (400 MHz, C D ) δ 8.33 (d, J = 1.5 Hz, 1H), 8.17 (d, J = 9.1 Hz, 1H), 8.08 (d, J = 8.6 Hz,
6
6
1
7
7
H), 7.90 (s, 1H), 7.88 (d, J = 8.6 Hz, 1H), 7.77 (d, J = 8.3 Hz, 1H), 7.74 – 7.70 (m, 1H), 7.67 –
.60 (m, 2H), 7.57 (dd, J = 8.3, 1.6 Hz, 1H), 7.51 – 7.40 (m, 4H), 7.34 – 7.25 (m, 2H), 7.21 –
.18 (m, 2H), 7.14 (d, J = 1.3 Hz, 1H), 7.08 – 7.02 (m, 2H), 6.99 – 6.95 (m, 3H), 3.77 (q, J = 7.0
1
3
Hz, 2H), 1.24 (t, J = 7.0 Hz, 3H). C NMR (101 MHz, C D ) δ 159.21, 141.43, 138.25, 137.81,
6
6
1
1
37.75, 137.15, 134.09, 133.34, 132.82, 131.42, 131.23, 131.14, 130.80, 130.32, 129.96, 129.14,
28.57, 128.55, 128.52, 127.94, 127.71, 127.56, 127.53, 127.46, 126.70, 126.47, 125.79, 125.60,
11

ACCEPTED MANUSCRIPT
1
25.53, 124.99, 115.01, 114.99, 63.50, 14.96. ppm. Anal. Calcd for C38H28O (500,64): C, 91.17;
H, 5.64; O, 3.20. Found: C, 91.32; H, 5.55.
1
0-(Biphenyl-4-yl)-9-aryl-2-phenylanthracenes. Pd(PPh ) Cl (22 mg; 5 mol%) was
3 2 2
added to the 600 mg (0.618 mmol) of 10-(Biphenyl-4-yl)-9-bromo-2-phenylanthracene dissolved
in 100 ml toluene in argon atmosphere. Then boronic acid (0.773 mmol) was added to the heated
2 3
mixture, after that 80 ml of 2M K CO solvent in water was poured to the mixture and then
boiled. After the finish of the reaction was monitored by TLC chromatography, reaction mixture
was quenched with 100 ml of water, extracted and water layer was washed with toluene (3 * 100
ml). Combined organic layers were dried with Na SO , filtrated through silica gel. After the
2
4
evaporation of solvent under reduced pressure, the material was purified by preparative column
chromatography (hexane – diethyl ether 10:1), then recrystallized from isopropanol and toluene
mixture (1:5).
1
9
-Biphenyl-4-yl-10-phenylanthracene (2b) (0.46 g, 38%): H NMR (400 MHz, CDCl
3
) δ
1
3
7
.39-7.49 (m, 5H), 7.54-7.67 (m, 9H), 7.76-7.91 (m, 8H. C NMR (101 MHz, CDCl
3
) δ 141.1,
1
40.5, 139.3, 138.3, 137.4, 137.0, 132.0, 131.6, 130.1, 129.2, 128.7, 127.7(2), 127.4(2), 127.3,
27.2, 125.3(2)25.81, 125.76, 124.97. ppm. Anal. Calcd for C H (406.52): C, 94.55; H, 5.45.
1
3
2
22
Found: C, 94.63; H, 5.37.
1
9
,10-Di(biphenyl-4-yl)-2-phenylanthracene (3b). (92 mg 27%): H NMR (400 MHz,
) δ 8.43 (d, J = 1.4 Hz, 1H), 8.04 – 7.97 (m, 2H), 7.91 (d, J = 8.8 Hz, 2H), 7.83 – 7.78 (m,
H), 7.70 – 7.63 (m, 3H), 7.61 – 7.52 (m, 9H), 7.39 – 7.34 (m, 2H), 7.29 – 7.24 (m, 2H), 7.22 –
6 6
C D
2
12

ACCEPTED MANUSCRIPT
1
3
7
1
1
1
1
6 6
.17 (m, 2H), 7.11 – 7.05 (m, 2H), 7.02 (d, J = 7.2 Hz, 1H). C NMR (101 MHz, C D ) δ
41.54, 141.11, 140.77, 138.54, 138.37, 137.86, 137.64, 137.15, 134.15, 133.42, 132.32, 132.31,
31.19, 131.01, 130.95, 130.71, 130.11, 129.88, 129.21, 129.13, 128.59, 128.55, 128.46, 128.18,
27.94, 127.68, 127.63, 127.57, 127.52, 1237,36, 126.78, 126.56, 125.85, 125.81, 125.76,
24.97. ppm. Anal. Calcd for C H (558.72): C, 94.59; H, 5.41. Found: C, 94.63; H, 5.37.
4
4
30
9-(benzo[a]anthracen-4-yl)-10-(biphenyl-4-yl)-2-phenylanthracene (4b). (110 mg
1
(
28%): H NMR (400 MHz, C
6
D
6
) δ 9.32 (s, 1H), 8.64 (d, J = 8.4 Hz, 1H), 8.36 (d, J = 1.7 Hz,
1
7
3
1
1
1
1
5
H), 8.14 – 8.08 (m, 2H), 8.06 – 7.84 (m, 4H), 7.80 (d, J = 8.8 Hz, 1H), 7.79 – 7.75 (m, 3H),
.63 – 7.54 (m, 6H), 7.53 (m, 1H), 7.48 – 7.25 (m, 7H), 7.12 – 7.08 (m, 2H), 6.89 – 6.86 (m,
1
3
H). C NMR (101 MHz, C D ) δ 146.54, 142.17, 138.12, 137.88, 137.73, 136.66, 136.62,
6
6
35.67, 135.62, 135.07, 134.16, 133.88, 133.12, 132.54, 132.28, 132.27, 132.22, 131.51, 131.26,
31.13, 130.96, 130.74, 130.35, 130.34, 130.30, 129.44, 129.34, 128.96, 128.67, 128.32, 128.21,
28.09, 127.93, 127.81, 127.55, 127.32, 126.76, 126.39, 126.14, 126.03, 125.81, 125.60, 125.55,
25.14, 125.10, 124.41, 124.07, 123.52. ppm. Anal. Calcd for C50
.10. Found: C, 95.03; H, 4.97.
H32 (632.81): C, 94.90; H,
9-(4-(9H-carbazole-9-yl)phenyl)-10-(biphenyl-4-yl)-2-phenylanthracene (5b). (284 mg
1
7
1%): H NMR (400 MHz, C D ) δ 8.44 (d, J = 1.2 Hz, 1H), 8.14 (d, J = 7.6 Hz, 2H), 8.10 (d, J
6 6
=
9.1 Hz, 1H), 8.03 (d, J = 9.1 Hz, 2H), 7.70 (dd, J = 9.1, 1.7 Hz, 1H), 7.66 (d, J = 8.1 Hz, 2H),
1
3
7
.63 – 7.47 (m, 10H), 7.45 – 7.38 (m, 4H), 7.36 – 7.18 (m, 7H), 7.13 – 6.99 (m, 3H). C NMR
101 MHz, C ) δ 141.42, 141.40, 141.04, 140.86, 138.49, 138.47, 138.37, 138.20, 137.61,
36.74, 133.09, 132.25, 131.23, 131.00, 130.84, 129.97, 129.33, 129.28, 129.15, 128.57, 128.48,
(
6 6
D
1
13

ACCEPTED MANUSCRIPT
1
1
28.15, 127.70, 127.66, 127.63, 127.52, 127.33, 126.46, 126.04, 125.95, 125.91, 124.27, 120.87,
+
20.66, 110.41. ppm. HRMS (ES): [M] , found 647.2607. C H N requires 647.2608; Anal.
5
0
33
Calcd for C H N (647.82): C, 92.70; H, 5.13; N, 2.16. Found: C, 92.51; H, 4.99.
5
0
33
1
1
0-(Biphenyl-4-yl)-9-(4-ethoxyphenyl)-2-phenylanthracene (6b). (62 mg 19%): H
NMR (400 MHz, C D ) δ 8.48 (d, J = 1.8 Hz, 1H), 8.24 (d, J = 9.2 Hz, 1H), 8.14 (d, J = 9.4 Hz,
6
6
1
7
7
H), 8.05 (s, 1H), 7.92 (d, J = 9.4 Hz, 1H), 7.75 (d, J = 8.8 Hz, 1H), 7.73 – 7.69 (m, 2H), 7.68 –
.59 (m, 2H), 7.58 (dd, J = 8.4, 1.8 Hz, 1H), 7.56 – 7.36 (m, 5H), 7.35 – 7.26 (m, 2H), 7.25 –
.17 (m, 2H), 7.12 (d, J = 1.8 Hz, 1H), 7.10 – 7.03 (m, 2H), 7.00 – 6.93 (m, 3H), 3.79 (q, J = 7.2
1
3
Hz, 2H), 1.25 (t, J = 7.2 Hz, 3H). C NMR (101 MHz, C D ) δ 144.26, 142.45, 138.75, 138.12,
6
6
1
1
1
37.93, 137.47, 134.17, 133.85, 132.14, 131.81, 131.55, 131.42, 131.31, 130.77, 130.52, 129.82,
29.37, 128.66, 128.65, 128.45, 127.99, 127.79, 127.82, 127.63, 127.36, 127.12, 126.90, 126.50,
25.81, 125.60, 125.45, 124.26, 121.49, 118.65, 63.60, 14.81. ppm. Anal. Calcd for C H O
4
0
30
(526,68): C, 91.22; H, 5.74; O, 3.04. Found: C, 91.33; H, 5.65.
2.3. Computational methods
Quantum chemical calculations of the anthracene derivatives were performed using density
functional theory B3LYP method implemented in the Gaussian09 software package [44].
Ground-state geometries of molecular structures were optimized in a 6-31G(d) basis set
including solvent (THF) effects by using polarizable continuum model (PCM). 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
14

ACCEPTED MANUSCRIPT
using time-dependent density functional theory (TDDFT) (for singlets only and for triplets only,
respectively).
3
. Results and discussion
.1. Synthesis
3
th
th
Substitution of 2-phenylanthracene by different aryl groups in 9 and 10 positions were
performed using Suzuki coupling reaction (see Fig. 2). A selective monobromination is very
important step for synthesis of unsymmetrical 2,9,10-triarylanthracenes. According to the
th
th
numerous published data, halide atoms simultaneously enter the 9 and 10 positions of
anthracene during the halogenation reactions [10,12,27,28]. However in this work we present a
new methodology for preparation of monohalogenated anthracenes, which enables synthesis of
non-symmetrical 2,9,10-triarylanthracenes. Firstly we chose 2-phenylanthracene (M7) as a
starting material. Due to influence of the electron donating 2-phenyl group, the 10th position of
2
-phenylanthracene was selectively brominated by N-bromosuccinimide. The position of
bromine atom in 2-phenylanthracene molecule was determined by a NOE (Nuclear Overhauser
Effect) technique of NMR spectroscopy. Namely, when the sample of 2-phenylanthracene was
irradiated by frequency, corresponding to the signal of H-9, an integral intensity of H-1 atom
signal (dd) increased by 10%, thus indicating, that H-1 proton has a neighbor proton, which
interacts with H-1 through space. This effect would be absent if C-9 atom would be brominated.
15

ACCEPTED MANUSCRIPT
The first coupling reaction between 10-bromo-2-phenylanthracene (M8) and 4-
biphenylboronic acid (dry dioxane, potassium phosphate and bis(triphenylphosphine)palladium
chloride)] took 52 hours, until M8 disappeared from the reaction mixture (see Fig. 3). However,
an analysis of the reaction mixture has shown, that the main product was not the cross-coupling
product, but de-brominated precursor M7, its content was 90%.
Seeking to find acceptable reaction conditions for preparation of 10-aryl-2-
phenylanthracenes, we carried out a series of cross-coupling reactions in different solvents, using
Pd(2+) either Pd(0) catalysts and different bases. All the rest conditions were standard: into a
flask with a magnetic stirrer under argon atmosphere 40 mg of 10-bromo-2-phenylanthracene
(M8) was dissolved in 50 ml of appropriate solvent. Then 5 mol% of catalyst was added, mixture
was stirred for 10 minutes, then 18 mg of phenylboronic acid was added. After it dissolved, an
appropriate base was added, and reaction mixture was refluxed till full conversion of M8. The
results of these experiments are shown in Table 1.
Bromine atom was reduced under anhydrous conditions with no traces of the desired
coupling product formation in trials 1 - 10. However, using toluene and water solution of base
with phase transfer catalyst - benzyltriethylammonium chloride, 76% of the desired product M9
was formed (Table 1, entry 11). Even better results were achieved for the reaction when no phase
transfer catalyst were used, reaching 96% formation of 2,10-diphenylanthracene. These reaction
conditions were applied in further synthesis steps of 9,10-diaryl-2-phenylanthracenes.
3.1.1. Synthesis of 10-aryl-2-phenylanthracenes
16

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For synthesis of unsymmetrical 9,10-diaryl-2-phenylanthracenes, 10-bromo-2-
phenylanthracene underwent cross-coupling reaction with arylboronic acid, using the above
reported optimized procedure and 10-aryl-2-phenylanthracenes (M9, 1a and 1b) were obtained.
A full conversion of the starting material was reached by use of excess boronic acid (see Fig. 4).
The reaction time varied from 15 minutes till 5 hours, while yields ranged from 85 to 94% for
different boronic acids. All compounds were easily purified by crystallization.
3.1.2. Synthesis of 9-Bromo-10-aryl-2-phenylanthracene
2,10-diarylanthracenes were brominated by NBS - similarly as it was in the synthesis of
1
0-bromo-2-phenylanthracene, just with a negligible excess of NBS (see Fig. 5) in
dimethylformamide. Reaction was carried out at a slightly higher temperature (50 °C) due to the
low solubility of the starting materials.
3.1.3. Synthesis of 9,10-diaryl-2phenylanthracenes
9,10-Diaryl-2-phenylanthraces were synthesized using adjusted Suzuki-Miyaura reaction
conditions (see Fig. 6). Longer reaction times were required for an introduction of second aryl
group (the reaction between M11 and arylboronic acid) than for the preparation of 10-aryl-2-
phenylanthracenes.
17

ACCEPTED MANUSCRIPT
A larger excess of boronic acid (see Fig. 6) didn’t improve the yield, so the a.m. 1.15 times
excess was used in most preparations. All 9,10-diaryl-2-phenylanthracenes were purified by
column chromatography; the impurities have shown a strong fluorescence
3.2.Quantum chemical calculations
Spatial electron density distribution in the HOMO and LUMO of the 2,9,10-arylanthracene
derivatives with optimized geometry at Frank-Condon state are shown in Fig S1 and Table S1 in
the Supplementary data. 2-phenyl group is twisted at about 48° for all 2,9,10-arylanthracene
th
th
compounds with the extended conjugation over the substituent. Aryl groups at 9 and 10
positions are twisted almost perpendicularly with angles of about 86.3 – 89.5° in respect of the
th
th
anthracene core. This perpendicular geometry of substituents at the 9 and 10 positions results
in the minor conjugation extension for all compounds except 5a/b. The majority of the electron
density is allocated over the anthracene backbone, where its distribution resembles that of the
unsubstituted anthracene [45]. The observed conjugation extension towards phenyl-carbazole
moiety for compounds 5a and b can be accounted for its donor-like behaviour.
Table 2 represents the calculated oscillator strength and energies of the lowest singlet and
triplet states in 2,9,10-arylanthracene derivatives (1-6)a and (1-6)b. Owing to almost
th
th
perpendicular orientation of substituents at 9 and 10 positions, singlet excited state energies
show only minor variation of about 3%. The extension of conjugation along the shorter
molecular axis (short and long molecular axis of bare anthracene are shown in the Figure S1 in
the Supporting Information) lowers S
0 1
→S transition energy from 3.07 eV to 3.01 eV for
compounds (1-3)a and from 3.16 eV to 3.09 eV for compounds (1-3)b while for the rest of
18

ACCEPTED MANUSCRIPT
derivatives this energy is almost constant, ranging at 3.08-3.09 eV. The energy of the first triplet
state is in range of 1.7-1.77 eV for all 2,9,10-arylanthracene derivatives, similarly as it was
observed for similar various anthracene compounds [9,46]. It is known that S -T transition is
1
1
th
th
symmetrically forbidden in anthracene derivatives symmetrically substituted at 9 and 10
positions [24] and the intersystem crossing rate is almost suppressed [25] if there is no higher
triplet levels below the S . ISC notably increases when higher triplet levels with appropriate
1
symmetry emerges below the S
fluorescence efficiency [24,25]. In our case, the T
minor changes of energy. The energy of the T state was calculated to be below S
a and 4b and almost equal for compound 1a while for the rest of compounds T was situated
1
state energy and causes the remarkable decrease in the
state was observed at about 2.72 eV with
in compounds
2
3
1
4
3
above the S state. The energy of T state was higher than S for all compounds except the
1
4
1
mentioned 4a and 4b compounds where T was situated below S . The calculated oscillator
4
1
strength of S →S transition was found to increase for compounds 1-4a/b (from 0.125 to 0.232
0
1
and from 0.097 to 0.229 for a compounds and b compounds, respectively) due to the conjugation
extension [32] while a bit lower values were observed for compounds 5a/b and 6a/b (~0.2 and
0
.16, respectively).
3.3.Thermal properties
Thermal properties of 2,9,10-arylanthracene derivatives were assessed by utilizing DSC.
The DSC curves are presented in Fig. 7.
Most of compounds showed similar thermograms when melting signals were observed in
°
the first heating scan peaking at 146-330 C, similarly as in the second heating scan. Melting
19

ACCEPTED MANUSCRIPT
°
°
temperature was continuously increasing from 185 C to 330 C for compounds 1-5a while it
°
decreased a bit for compound 6a (223-227 C). No continuous changes of T were observed for
m
compounds 1-6b. A major part of anthracene compounds showed crystallization peaking at 59-
0
0
2
60 C. Glass transition temperature T of 30 and 60 C was obtained for compounds 1b and 6b,
g
respectively. Although the rest of the compounds showed smooth wet casted films, due to the
weak signal T could not be resolved.
g
3.4. Optical properties
Absorption and emission spectra of the dilute solutions and neat films of the 2, 9, 10-
arylanthracenes (1-6)a and (1-6)b are displayed in Figure 8. The detailed spectral properties of
all the derivatives are summarized in Table 4.
Absorption spectra of all 2,9,10-arylanthracene derivatives resembles those of
unsubstituted anthracene [47] as well as its derivatives like 2-phenylanthracene [49] and 9,10-
diphenylanthracene (DPA) [26], suggesting that charge density is allocated over the anthracene
backbone what is in line with the results of DFT calculations (see Section 3.2). All spectra
st
contain clearly resolved vibronic structure with the dominant 1 vibronic replica due to the
presence of rigid anthracene core. The lowest energy vibronic bands in the absorption spectrum
are observed at 387-406 nm with a slight redshift after the introduction of aryl substituents at the
th
9
position. The variations of position of absorption band edge are in-line with the DFT
calculations. Absorption spectra of the compounds 4a,b and 5a,b possess some attributes of the
absorption by the benz[a]anthracene and carbazole moieties at about 340-350 nm. The molar
absorption coefficient of the 2,9,10-arylanthracene derivatives 1(1-6)a and (1-6)b is in the range
20

ACCEPTED MANUSCRIPT
-1
-1
-1
-1
of 10500-15000 L mol cm , similar to the one of the unmodified DPA (ε=13400 L mol cm )
and other similar anthracene derivatives [26]. The absorption spectra of 2,9,10-arylanthracene
compounds in solid state are redshifted for about 120-130 meV and slightly broadened in respect
to those of in dilute solutions. The reduced redshift implies the weakened excitonic coupling as
compared to the unsubstituted anthracene crystal [48] due to the loose molecular packaging
induce by aryl substituents.
Fluorescence spectra of dilute solutions of 2,9,10-arylanthracene derivatives (1-6)a and (1-
6
)b peaked at 422-436 nm with 0.17-0.29 eV redshift in respect of 2-phenylanthracene emission
[
49] with lineshape similar to other anthracene derivatives like 2-phenylanthracene or DPA
26,47]. Emission spectra showed vibronic structure with 0.17-0.28 eV Stokes shift. No mirror
[
image between absorption and emission spectra clearly indicates the existence of geometrical
transformations of the molecular core upon the photoexcitation. Compounds 5 and 6a,b with
polar ethoxy-phenyl and bulky benz[a]anthracene groups showed the less expressed vibronic
structure due to the more labile molecular backbone.
Emission spectra of neat films of 2,9,10-arylanthracene derivatives (1-6)a and (1-6)b
peaked at 442-461 nm with 0.08-0.24 eV redshift in respect of the emission in dilute solutions
due to the presence of intermolecular interactions in the solid state. This energy difference is
small for most of the compounds except for 1a,b and 4a,b. The small difference evidences
reduced intermolecular interaction due to disorder introduced by twisted non-symmetric aryl
substituents. Simpler 1a,b molecules exhibit excimer fluorescence which is suppressed by
th
further substitution at the 10 position. 4a,b compounds possess bulky, conjugated
benz[a]anthracene group inducing stronger intermolecular coupling, thus resulting in a band
broadening and stronger redshift.
21

ACCEPTED MANUSCRIPT
Fluorescence quantum yield of the dilute solutions of 2,9,10-arylanthracenes derivatives
(
1-6)a and (1-6)b ranges from 0.36 to 0.49. Similar values of Φ in the range of 0.45-0.49 were
F
also obtained for 2-phenyl substituted anthracene derivatives [5,10,26], showing the domination
of non-radiative recombination. The non-radiative deactivation of the excited state can proceed
through molecular torsions via enhanced electron-vibron coupling or through intersystem
crossing to the triplet states. The modification of the molecular backbone can alter both
mechanisms: the increase of its flexibility or the rearrangement of the energy levels (mostly with
the help of 2-phenyl group) when the energy of optically active T
n
level becomes lower that the
S , leading to the efficient ISC and low Φ [46,21]. Experimental results discerning the
1
F
dominating process will be presented in the following chapters.
Fluorescence quantum yield of neat films of anthracene derivatives ranges in 0.03-0.13
(see Table 5), down to the order of magnitude lower as compared to that of dilute solutions.
Close molecular packaging in thin films facilitates excitation migration between neighbouring
molecules and enhances the possibility of quenching at various defect sites, leading to the
decrease of Φ . Very similar values of Φ were observed for neat films of anthracene derivatives
F
F
modified with 2-phenyl group and with alkyl chains at para position of 9,10-phenyls [26]. On
the other hand, neat films of the same derivatives with 2-pentyl instead of 2-phenyl showed Φ of
F
0
.3-0.4 [26]. It suggests, that 2-phenyl substituent promotes very close molecular packing with
efficient excitation migration and quenching at defect sites. Close molecular packing will
endorse enhanced charge transport properties as it will be shown below.
3.5.Excited state relaxation
22

ACCEPTED MANUSCRIPT
Fluorescence decay transients of 2,9,10-triarylanthracenes are shown in Figure 9, the
extracted fluorescence time constants (τ ) are listed in Table 5. All fluorescence transients
F
exhibit very similar exponential decay in dilute solutions with time constants of 3.5-4 ns,
remarkably faster than observed for 2,9,10-arylanthracene compounds (5.3 ns [49]) or DPA and
nd
its modifications with various groups at the 2 position (5-6.2 ns [26]).
Fluorescence decay transients of neat films of 2,9,10-arylanthracene derivatives shows
non-exponential decay profile with very fast initial part due to efficient excitation migration via
exciton hopping through the localized states in disordered media and non-radiative
recombination at defect sites and slower later decay, which time constants are comparable to that
of single molecule in dilute solution.
The contribution of radiative and non-radiative recombination to the fluorescence
properties was estimated after calculating the rates of radiative (τ
times (see Table 5). The time constants were calculated using the following relations:
τ =τ /Φ , τ nr
τ F /(1−ΦF )
r
) and non-radiative (τnr) decay
(1)
=
,
r
F
F
where the term τ takes into account all the possible nonradiative decay pathways including
nr
vibrational relaxation and intersystem crossing to triplet states.
3.6.Impact of substituents on excited state deactivation
Since the 9,10 substituent induced variations of τ are significantly lower than those of τ ,
nr
r
the minor variations of Φ are related to variation in the radiative rate constant and thus the
F
oscillator strength of the lowest transition. The highest Φ
F
of 0.46-0.49 was obtained for
compounds with fastest τ (3a, 4a, and 6b) and highest molar absorption coefficient. Clear trend
r
23

ACCEPTED MANUSCRIPT
was observed for compounds 1a→2a→3a where Φ
F
r
was increasing consistently with τ due to
increasing oscillator strength when the conjugation extends along the shorter molecular axis. For
compounds 1b→2b→3b the changes of Φ were less noticeable.
F
The non-radiative deactivation of excited state dominates for all 2,9,10-arylanthracene
derivatives, resulting in Φ values below 0.5. The nonradiative decay constant τnr is barely
F
altered by aryl substituents introduced at 9, and 10 positions and is almost the same for all
anthracene derivatives (τ = 5.9-7.2 ns). This can be attributed to the almost perpendicularly
nr
twisted geometry of the 9,10-aryl substituents as it is evident from DFT modelling. On the other
hand, all compounds possess extended π-conjugation towards 2-phenyl substituent which can be
responsible for nonradiative deactivation. Both enhanced electron-vibronic coupling and the
excitation relaxation through the triplet states can be responsible for reduced fluorescence
quantum yield in 2-phenyl anthracene derivatives. Our recent studies of 2-phenyl and 2-pentyl
substituted DPA derivatives showed that remarkable (up to 10 times) reduction of the
nonradiative relaxation time constant of DPA derivatives due to enhancement of ISC rate [26].
On the other hand, few recent studies suggested significant role of the electron-vibron coupling
facilitating non-radiative deactivation in 9,10-chloro substituted [31] and 9,10-aryl substituted
[32] anthracenes.
3.7.Nonradiatve relaxation: electron-vibron coupling vs intersystem crossing
To verify the nature of non-radiative recombination, compounds were dispersed in
polystyrene matrix at low mass percentage (0.1 wt %) with intramolecular torsions supressed.
Thereafter fluorescence decay transients and Φ were measured and the rates of radiative and
F
24

ACCEPTED MANUSCRIPT
F
non-radiative recombination were estimated (see Table 5). Generally, Φ values were almost the
same in a rigid matrix and in solution, indicating the minor role of vibronic relaxation, similar to
results obtained for DPA derivatives [26]. Analysis of radiative and nonradiative decay time
constants discloses some peculiarities of excitation relaxation in a rigid surrounding. Both
radiative and nonradiative decay rates were reduced by 20-30% in PS matrix, as compared to
those in solutions, resulting in minor changes in Φ values. Smaller τ values in solution as
F
r
compared to those in PS indicate on enhancement of corresponding transition dipole moment,
which can occur during excitation relaxation due to twisting of the 9,10-aryls towards more
planar geometry. While increased nonradiative time constant in PS can be caused by both
reduced torsional motion of aryl substituents and enhanced ISC due to favorable alignment of
singlet and triplet energies. In the case of dominating vibrational relaxation, introduction of
additional phenyl torsional groups (compounds 1a, 2a and 2b) should lead to enhanced
nonradiative relaxation [50], while introduction of more bulky substituents (compounds 2a, 3a
and 5a) should reduce vibrational motions of 9-phenyls [51], however the experimental results
show opposite trends. Introduction of addition phenyl rings increases the nonradiative decay
constant from 5.9 ns to 7.1 ns in solution, for compounds 1a and 2b, respectively. While
introduction of more bulky substituents reduces the nonradiative decay constant from 7.0 ns to
6
.52 ns in solution, for compounds 2a and 5b, respectively. Thus more realistic route of
nonradiative deactivation is mechanism of intersystem crossing, which rate is tuned by alignment
of S and T (most likely of T or T ) energy levels [23,24,26,46]. The introduction of 2-phenyl
1
n
2
3
moiety results in proper S and T level alignment (see Section 2.2) when effective relaxation
1
n
through triplet states occur what is evidenced reduced τnr values of compounds 1-6a and 1-6b.
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ACCEPTED MANUSCRIPT
Introduction of 10-naphtyl or 10-biphenyl substituents has a minor impact on the rate of
nonradiative relaxation of tested 2,9,10-antracene derivatives.
Thus we rule-out the enhanced electron-vibronic coupling introduced by 9,10-aryl
substitution and consider enhanced ISC due to 2-phenyl substitution as the main mechanism
limiting the fluorescence emission quantum yield in non-symmetrically 9,10-substituted 2-
phenylanthracenes.
3.8. Electrochemical properties
Cyclic voltammetry was employed to elucidate the energies of HOMO and LUMO for
several characteristic 2,9,10-phenylanthracenes, namely 3a and 3b and 5a,b (see Figure 10 and
Table 6). All compounds demonstrated non-reversible oxidation-reduction processes in positive
potential range (at HOMO level). Compound 3a showed strong adsorption on an electrode
surface with half – wave oxidation potential around 1 V. A second half – wave in positive
potential range implies to HOMO states. All compounds demonstrated quasi – reversible
oxidation – reduction process in negative potential range (at LUMO state).
The estimated energies of LUMO for 2,9,10-arylanthracene compounds 3a,b and 5a,b
were very similar ranging at about -2.55 eV. Slightly more noticeable changes were estimated for
the energy of HOMO. Interestingly, HOMO values were varying by less than 0.1 eV. The
electrochemical bandgap obtained by CV measurements correlate well with the optical bandgap
estimated from the onset of absorption spectra.
3.9.Charge transport properties and ionization potential
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ACCEPTED MANUSCRIPT
Charge transport is key factor determining the performance of organic optoelectronic
device, since it has a direct impact on charge balance and recombination efficiency. High carrier
drift mobility (μ ) of a molecule is achieved by balancing good film forming properties and close
h
molecular packing, resulting in efficient carrier transport. Crystallization should be avoided since
polycrystalline materials possess low μ like in DPA [26]. On the other hand, modification of
h
h .
anthracene with bulky alkyl groups diminishes charge transfer rate and μ [33] Incorporation of
non-symmetric steric groups introduces pronounced disorder and variety in molecular packing
configurations with hardly predictable structure. In some cases this ensures amorphous structure
of the wet casted films with pronounced charge transfer properties. In this series of 2,9,10-
-2
2
arylanthracene derivatives hole drift mobility shows very high values exceeding 10 cm /Vs at
the field strength of about 1 MV/cm (see Figure 11 and Table 7 and Figure S2 in the
-3
-2
2
Supplementary Data). μ for compounds 1a, 3a, 4a, 2b and 4b is of 1⋅10 - 1⋅10 cm /Vs in the
h
broad range of electric field strength (0.25-1.7 MV/cm). Compounds 5 and 6, possessing polar
substituents showed poor charge transport properties. The continuous increase of μ with the
h
applied electric field for all the 2,9,10-arylanthracene derivatives implies the governing role of
energetic disorder [52] in the charge carrier transport of the wet-casted thin films. The estimated
h
values of μ of 2,9,10-arylanthracene derivatives are higher than the most reported ones for
-3
simple anthracene derivatives [33–35] and comparable with few reported values exceeding 10
2
cm /Vs [26,37,53]. The exceptional charge transport properties of the studied 2,9,10-
arylanthracene derivatives make them promising for OLED technologies.
The ionization potential is important parameter while considering charge carrier injection.
Non-symmetric substitution of 2,9,10-arylanthracenes has a minor impact on estimated values of
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ACCEPTED MANUSCRIPT
ionization potentials ranging from 5.63 to 5.9 eV (Table 6). These values are typical for
anthracene derivatives [26,48].
4. Conclusions
Non-symmetric substitution of anthracenes at 2,9,10 positions by aromatic aryl moieties
shows potential for improvement of charge transport properties in the neat films. A new
synthesis methodology for preparation of monohalogenated anthracenes was demonstrated,
enabling efficient non-symmetric substitution. Non-symmetric, out of plane twisted 2,9,10-aryls
introduce structural disorder in the films resulting in most cases in homogeneous film
morphology. While small aryl substituents still enable close molecular packing of anthracene
core fragments ensuring pronounced wave function overlap needed to obtain very high values of
hole drift mobility for solution-processed thin films of compounds 1a, 3a, 6a, 2b and 4b ranging
-3
-2
2
from 1⋅10 to 1⋅10 cm /Vs (for electric field strength of 1 MV/cm). The exceptional charge
transport properties of the studied 2,9,10-arylanthracene derivatives make them promising for
OLED technologies.
Introduced 9, 10 aromatic substituents had a minor impact on excited state deactivation
pathways, ensuring blue emission in the range of 422-436 nm and fluorescence quantum
efficiency up to 0.49. Analysis of the radiative and nonradiative recombination pathways in
dilute solution and in a rigid surrounding proved the ISC to be the nonradiative relaxation
mechanism. It was found that ISC rate was mainly governed by 2-phenyl substitution.
Further improvement of the fluorescence efficiency of non-symmetric anthracenes is
nd
foreseen by incorporating small non-conjugated substituents at the 2 position while maintaining
28