Structure-property relationship of blue solid state emissive phenanthroimidazole derivatives

Article abstract of DOI:10.1039/c7cp02248d

Seven new derivatives of phenanthro[9,10-d]imidazole having differenet substituents at the 1st and the 2nd positions of the phenanthroimidazole moiety were synthesized and characterized. The comparative study of their properties was performed employing thermal, optical, electrochemical and photoelectrical measurements. The properties of the newly synthesized compounds were compared with those of earlier reported derivatives of phenanthroimidazole and several interesting new findings were disclosed. Density functional theory calculations accompanied by optical spectroscopy measurements have shown the possibility of tuning the emission properties (excited-stated decay rate, fluorescence quantum yield, etc.) of phenanthro[9,10-d]imidazole derivatives via attachment of different substituents to the 1st and the 2nd positions. The most polar and bulky substituents linked to the 2nd position were found to have the greatest impact on the emissive properties of compounds causing (i) fluorescence quantum yield enhancement of dilute liquid and solid solutions (up to 97%), (ii) suppression of intramolecular torsion-induced nonradiative excited-state relaxation in rigid polymer films as well as (iii) inhibition of aggregation-promoted emission quenching in the neat films. Most of the studied compunds exhibited ambipolar charge transport character with comparable drift mobilities of holes and electrons. The highest hole and electron mobilities approaching 10^-4 cm² V^-1 s^-1 were observed for the derivative having a triphenylamino group at the 1st position of the imidazole ring and the phenyl group at the 2nd position. The estimated triplet energies of phenanthro[9,10-d]imidazole compounds were found to be in the range of 2.4-2.6 eV, which is sufficiently high to ensure effective energy transfer to yellow/red emitters.

Full text of DOI:10.1039/c7cp02248d

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Structure-properties relationship of blue solid state emissive
phenanthroimidazole derivatives
Agne Ivanauskaite,a Ramunas Lygaitis,a Steponas Raisys,b Karolis Kazlauskas,b Gediminas Kreiza,b
Received 00th January 20xx,
Accepted 00th January 20xx
Dmytro Volyniuk,a Dalius Gudeika,a Saulius Jursenasb and Juozas V. Grazuleviciusa,†
Seven new derivatives of phenanthro[9,10-d]imidazole having differenet substituents at the 1st and the 2nd positions of
phenanthroimidazole moiety were synthesized and characterized. The comparative study of their properties was performed
employing thermal, optical, electrochemical and photoelectrical measurements. Properties of the newly synthesized
compounds were compared withose of earlier reported derivatives of phenanthroimidazole and several interesting new
findings were disclosed. Density functional theory calculations accompanied by optical spectroscopy measurements have
shown the possibility to tune the emission properties (excited-stated decay rate, fluorescence quantum yield etc) of
phenanthro[9,10-d]imidazole derivatives via attachment of different substituents to the 1st and the 2nd positions. The most
polar and bulky substituents linked to the 2nd position were found to have the greatest impact on the emissive properties of
compounds causing i) fluorescence quantum yield enhancement of dilute liquid and solid solutions (up to 97%), ii)
suppression of intramolecular torsion-induced nonradiative excited-state relaxation in rigid polymer films as well as iii)
inhibition of aggregation-promoted emission quenching in the neat films. Most of the studied compunds exhibited ambipolar
charge transport character with comparable drift mobilities of holes and electrons. The highest hole and electron mobilities
approaching 10-4 cm2/V∙s were observed for the derivative having triphenylamino group at the 1st position of imidazole ring
and phenyl group and the 2nd position The estimated triplet energies of phenanthro[9,10-d]imidazole compounds were found
to be in the range of 2.4-2.6 eV which is sufficiently high to ensure effective energy transfer to yellow/red emitters.
DOI: 10.1039/x0xx00000x
www.rsc.org/
electron transporting (acceptor) moieties and to exhibit high
photoluminescence quantum yields (ΦF) in the solid state; (ii)
INTRODUCTION
Organic light-emitting devices (OLEDs) are of great interest
because of their present and potential applications in next
generation full-color flat-panel displays and solid-state light
sources.1,2 Deep blue emitting emitters with the balanced
mobilities of holes and electrons and with high triplet energies
may be employed as hosts for green (G), yellow (Y) and red (R)
phosphorescent emitters.3 Achieving of high performance R, Y,
G and blue electroluminescence (EL) based on simple materials
and simplification of the structures of OLEDs are the important
issues. For the preparation of the multi-functional electroactive
materials, which can be employed not only as blue emitters, but
also as hosts for efficient G, Y and R OLEDs, we propose the
following design strategy of molecular materials: (i) the
compounds have to contain hole transporting (donor) and
acceptor and donor moieties should be able to form electron
and hole transporting channels in the solid layers of the
materials (iii) to obtain deep blue emission, the charge transfer
property of donor-acceptor molecules has to be relatively week
since strong donor-acceptor interaction can cause red shift of
emission; (iv) the difference between singlet and triplet
energies (ΔEST) has to be small to ensure that triplet excited
state energy is high enough to excite a dopant.3,4
Color quality and stability of blue phosphorescent OLEDs are
often not sufficient.5 The preparation of blue multifunctional
compounds exhibiting high luminescence quantum yields,
balanced charge transport properties and high thermal and
morphological stability remains one of the challenging topics.6
Usually, electron-transporting properties of organic
semiconductors are inferior relative to hole-transporting
properties, especially for blue emitting compounds.7 One of the
basic requirements for blue emitters is their enhanced electron
affinities which enable to achive balanced charge injection and
transport.8,9 Compounds having heteroaromatic moieties, such
as pyridine,10 quinolone11 and oxidazole12 were reported to
show good electron injection and transport properties,
however these materials showed low luminescence quantum
yields. 1,3,5-Tris(N-phenylbenzimidazol-2-yl)benzene (TPBi),
a.Department of Polymer Chemistry and Technology, Kaunas University of
Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania Fax: +37037 300152;
Tel: +37037 300193.
b.Institute of Applied Research, Vilnius University, Sauletekio 3, LT-10257 Vilnius,
Lithuania.
† E-mail address: juozas.grazulevicius@ktu.lt (Juozas Vidas Grazulevicius).
Electronic Supplementary Information (ESI) available: [Experimental section, NMR,
IR, UV/FL spectra of compounds, TOF transients for hole and electron transport, TGA
and DSC curves, spatial distribution of frontier orbitals, cyclic voltammograms.]. See
DOI: 10.1039/x0xx00000x
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containing three 1-phenylbenzimidazole moieties linked by different substituents to packing specificity in the neat films is
View Article Online
DOI: 10.1039/C7CP02248D
benzene ring, is widely used in OLEDs as electron-injecting and estimated. We expect the rigid PI fragment to be beneficial for
hole-blocking compound.13 However, TPBi can not be used as the thermal and morphological stability without sacrificing good
emitter. It emits in the ultraviolet region with the intensity electron-transport ability and high triplet energy.
maximum at 368 nm. In addition, it has too large barrier for
injection of holes.
For the design of efficient fluorescent emitter a suitable extent
RESULTS AND DISCUSSION
Synthesis and characterization
of intramolecular charge transfer (ICT) has to be secured by
linking a donor (D) and acceptor (A) moieties.14 This approach
often gives the additional advantage, i.e., bipolar charge
transporting property, which is beneficial for the improvement
of characteristics of OLEDs.15 The problem that arises in the
design of deep-blue emitters is that the direct linkage between
D and A moieties unavoidably increases the conjugation length,
which can lead to red shift and broadening of the emission
band. Therefore, efficienct deep-blue OLEDs satisfying the
National Television System Committee standard color
coordinates are rather rare.16 To limit the ICT induced emission
into blue region, the strength of donors and acceptors along
with short π-conjugation linkage in organic luminogens are
important aspects.17 The longer conjugated bridge between
donor and acceptor units of emitters is also a reason for
bathochromic shifts in emission wavelength. Thus, the major
challenge in the design strategy D-A-D compounds is to limit
emission in deep blue region with short communication length
between donor and acceptor.18,19,20 Due to high luminescence
quantum yields, high thermal stability, and relatively high triplet
energies, phenanthroimidazole (PI) derivatives were reported
to be useful as emitters or host materials.21 Using PI moiety it is
possible to design and obtain sky-blue and deep blue
fluorescent emitters with fine color purities.4,22 PI can act as an
electron-acceptor when linked to electron-donor moiety, thus
forming bipolar materials.23 Deprotonation of NH group of
imidazole moiety allows attaching wide variety of substituents,
which can be capable to inhibit strong π-π stacking and
molecular interactions in the solid state.24 Introduction of
electron-deficient PI units is known to effectively enhance
electron injection and transport ability while allowing to adjust
ionization potentials of compounds.25
In this work, we employ the D-A strategy to develop efficient
deep-blue emitters by exploiting the merits of
phenanthro[9,10-d]imidazole chromophore. Acting as a π-
acceptor PI can be combined with an electron donor to form
bipolar compounds and at the same to retain only weak
intramolecular charge transfer. Bipolar molecules are expected
to feature ambipolar charge transfer, therefore, both electron
and hole transport can be realized. In addition, we evaluate
radiative and non-radiative relaxation processes of PI
derivatives with different polar substituents at 1st and 2nd
position in various media such as solution, polymer matrix with
low concentration of investigated compounds and in the neat
films. Also, the influence of radiative and non-radiative
processes on fluorescence quantum yield and absorption
intensity is assessed. Phosphorescence measurements allowed
estimating energy level of triplet state. As the bulk and sterically
hindered molecular configuration is expected to effectively
enhance photoluminescence quantum yield,26 the impact of
All ten derivatives of phenanthroimidazole were synthesized in
one-pot procedure as shown in Scheme 1. The synthesized
derivatives were identified by mass spectrometry, 1H, 13C
nuclear magnetic resonance (NMR) (see Supporting
Information), infrared (IR) spectroscopy and element analysis.
The materials were soluble in common organic solvents such us
chloroform, toluene, acetone etc. Transparent thin films of
these materials could be prepared by spin coating from
solutions or by vacuum evaporation.
Scheme 1. Synthesis of phenanthroimidazole derivatives 1-10
.
R1
N
R-CHO, R1-NH2
O
O
R
CH3COOH,CH3COONH4 , N2 , 120 oC
N
1-10
1
2
3
4
5
OCH3
OCH3
H3CO
OCH3
R
*
*
*
*
*
R1
H
6
H
7
*
*
*
8
9
10
CF3
OCH3
F3C
CF3
H3CO
OCH3
N
N
R
*
*
*
*
*
CF3
CF3
OCH3
R1
*
*
*
*
*
Thermal properties
Thermal properties of the synthesized compounds were studied
by thermal gravimetric analysis (TGA) and differential scanning
2 | J. Name., 2012, 00, 1-3
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calorimetry (DSC) under a nitrogen atmosphere. The data are
presented in Table 1.
View Article Online
DOI: 10.1039/C7CP02248D
Table 1. Thermal characteristics of compounds 1-10.
Compound
1
2
3
4
5
6
7
8
9
1027
273,
273c)
-
T
m [oC] (1st heat)
325
297, 297c)
206, 206c)
200
207
200
227
286
281
Tg [oC] (2nd heat)
Tcr [oC]
-
135
185b)
342
65
135b)
-
82
-
82
-
-
-
-
303a)
117a)
165a)
191a)
222a)
227a)
TID [oC]
330
344
350
380
381
337
315
430
420
a)
T
is crystallization temperature determined during the cooling scan, b) Tcr is crystallization temperature determined during the 2nd heating scan; Tm is melting
cr
point; c) Tm is melting point recorded during the 2nd heating scan; Tg is glass transition temperature, TID is 5% weight loss temperature, recorded at scan rate
20 °C/min, N2 atmosphere.
o
All the compounds (1-10) demonstrated high thermal stability. with glass transition temperature (Tg) of 82 C. Compound
6
The initial 5% weight loss temperature (TID) ranged from 315 to demonstrated the similar behavior during the DSC test. When
o
420 oC, as confirmed by TGA (Figure S1a, Supporting its crystalline sample was heated, Tm was observed at 200 C.
Information). TID of compounds 3-6 increased with the When the melt sample was cooled down, it formed glass with
increasing number of methoxy groups in the molecules. This Tg of 82 oC.
observation can apparently be explained by the enhancement Compounds
1
,
4
,
7
,
8
,
9
and 10 demonstrated different behavior
are
of intermolecular interaction due to hydrogen bonding. TID of during the DSC experiments. Thermograms of compound
7
8
containing trifluoromethyl substituents is lower by shown in Figure S1b as an example. When the crystalline sample
o
approximately 30 C compared to that of compound
3 having was heated, an endothermic peak due to melting was observed
no substituents at para positions of phenyl substituents. This at 273 oC. When the melt sample was cooled down, its
difference may be attributed to the plasticizing effect of crystallization was observed at 191 oC to form the same crystals,
trifluoromethyl groups. Compounds
atom at the 1st position of phenanthroimidazole moiety have derivatives
lower TID as compared to that of compound possessing phenyl during the DSC tests (Table 1).
group at this position. This observation can be explained by the Compound (Figure S1c, Supporting Information)
higher molecular weight of compound , which predetermines demonstrated different behavior during the DSC experiments.
stronger intermolecular interaction. Compounds
1
and
2
bearing hydrogen which were obtained by crystallization from solution. The
1, 4, 7, 9
and 10 demonstrated the similar behavior
3
3
3
9
and 10 have When the crystalline sample was heated, endothermic peak due
o
the highest TID (430 and 420 oC, respectively) compared to to Tm was observed at 206 C. When the molten sample was
compounds 1-8. This observation again can be explained by the cooled down, it formed glass with Tg of 65 oC followed by
higher molecular weight of compounds
All the synthesized compounds (1-10) were obtained as behavior during the DSC test.
crystalline materials as confirmed by DSC, however some of the Tg of compound having unsubstituted NH group is higher
materials could be converted to amorphous phase by cooling compared to those of compounds and , having phenyl groups
9
and 10
.
crystallization at 135 oC. Compound
2 demonstrated the similar
2
3
5
the melted samples. DSC thermograms of
confirm this statement.
5
shown in Figure 1 at the 1st position of phenanthroimidazole moiety. This effect is
apparently determined by the enhanced intermolecular
interaction in the sample of
NH groups. Compound
group showed higher Tg by 17 C compared to
2nd position of phenanthroimidazole has phenyl group.
2 due to hydrogen bonds formed by
5
containing 3,4,5-trimethoxyphenyl
o
Tg = 82 oC
3
, which at the
0
1st heating
cooling
Density functional theory calculations
2nd heating
-10
Geometry optimization of compounds 1-10 in the gas phase was
performed using density functional theory (DFT) calculations at
the B3LYP/6-31G* level as implemented in the Gaussian 09
software package.28 Transition energies and oscillator strengths
of the compounds were calculated using time-dependent DFT
(TD-DFT) approach. The parameters involving the lowest singlet
and triplet states are summarized in Table 2.
-20
0
50
100
150
200
250
Temperature [oC]
Figure 1. DSC curves of compound 5.
When a crystalline sample of
temperature (Tm) was recorded at 207 C. When the molten
sample was cooled down, it formed an amorphous material
5
was heated, melting point
o
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Table 2. Calculated transition energies, wavelengths and oscillator strengths for the 1st
spin-allowed and spin-forbidden transition of compounds 1-10.
1
View Article Online
DOI: 10.1039/C7CP02248D
S0 → S1
S0 → T1
Ea) (eV) b) (nm)
Compound Ea) (eV)
λ
b) (nm)
fc)
λ
1
2
3
4
5
6
7
3.666
3.649
3.723
3.666
3.649
3.675
3.382
338.25
339.77
333.02
338.17
339.81
337.39
366.56
0.400
2.653
2.654
2.727
2.721
2.699
2.705
2.658
467.29
467.24
454.59
455.65
459.33
458.39
466.54
0.462
0.303
0.032
0.127
0.214
0.053
3
9
8
9
3.425
3.346
361.97
370.59
0.082
0.929
2.646
2.577
468.67
481.22
10
3.564
347.87
0.106
2.724
455.10
a) Transition energy. b) Transition wavelength. c) Oscillator strength.
The compounds bearing substituents at the 2nd position only (
and ) exhibited planar molecular structure (Figure 2), which
1
2
can result in remarkable fluorescence quenching at high
compound concentrations, e.g. in the neat films. The additional
substitution of the phenanthroimidazole moiety by phenyl
group at the 1st position (
molecular geometry causing 75
3
-
10) induced large distortions to the
and 30 twist angles of the
°
°
Figure 2. HOMO and LUMO for compounds 1, 3 and 9 as calculated by DFT using B3LYP/6-
31G* basis set. Frontier molecular orbitals of the rest compounds can be found in Figure
S2 in Supporting Information.
phenyl groups linked to the 1st and 2nd positions, respectively,
in regard to phenanthroimidazole core (Figure S2, Supporting
Information). The twisted molecular geometry is likely to
prevent close packing of the molecules at increased
concentrations, and thus can suppress fluorescence quenching
in the neat films.
Optical and photophysical characterization
The impact of various substituents introduced at the 1st and 2nd
positions of PI has been assessed by evaluation of their optical
properties. The properties of the PI derivatives observed for
dilute solutions or for dispersions in rigid polymer matrices at
low compound concentration allowed to reveal the influence of
intramolecular mechanisms, whereas the results acquired for
the neat films enabled to evaluate the contribution of
intermolecular interactions, e.g., molecular packing and exciton
migration induced excitation relaxation. Obvious differences in
the polarity and steric properties of the substituents were
shown to readily affect radiative and nonradiative relaxation
processes of the PI compounds implying a possibility to tune
their optical properties.
The calculated highest occupied molecular orbital (HOMO) was
found to be spread over the entire molecule (except for the
highly twisted phenyl moiety at the 1st position) for all the
studied PI compounds, whereas the lowest unoccupied
molecular orbital (LUMO) for majority of the compounds was
more shifted towards the substituents (Figure 2 and Figure S2,
Supporting Information). This was particularly well observed in
the case of compounds having trifluoromethylphenyl
substituents (7, 8) due to their strong electron-accepting
character. The quite opposite behavior, i.e. shift of the LUMO
towards phenanthroimidazole core, was observed for
triphenylamino-substituted derivative
9 as a result of the
pronounced electron-donating property of this substituent.
Generally, rather good overlap of electron wave functions in
HOMO and LUMO revealed by DFT predicts fairly high oscillator
strengths, which translate to high fluorescence quantum yields,
as well as large singlet-triplet energy splittings of 0.7–1.0 eV
Figure S3 (Supporting Information) shows near-band-edge
absorption spectra of dilute dimethylformamide (DMF)
solutions of the PI derivatives. Compounds
1-8 exhibit similar
absorption spectra with the 0th vibronic peak of the lowest
energy band ranging from 360 to 366 nm (see Table 3). The
slight variation can be attributed to a different polar and steric
character of the substituents affecting extent of the electron
delocalization. Generally, the absorption spectra of the PI
compounds are red shifted by ca. 20 nm as compared to that of
phenanthrene29 due to the enhanced conjugation ensured by 2-
phenylimidazole group. The absorbance of the lowest energy
band of the phenyl- or methoxyphenyl-substituted PI
(Table 2). Somewhat reduced f for compounds 4, 7 and 8 can be
justified by less overlapped HOMO and LUMO due to the
emergence of the additional orbital transitions with charge
transfer character.
HOMO
LUMO
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compounds
1
-
6
is found to be in the range of (7.4–11.5)×103 enhancement is accompanied by the significant broadening of
View Article Online
mol-1 cm-1. The increasing polarity of the substituents is the absorption spectrum, which is particularly noticeable in the
⋅
DOI: 10.1039/C7CP02248D
L
⋅
observed to enhance the absorbance, i.e., 1.5-fold compounds
enhancement is achieved for carbazole and –CF3 substituents increasing electron donating ability of the substituents is typical
(compounds 10 ), whereas 3-fold enhancement is attained of D-A type molecules and is due to the increased transition
in the case of diphenylamino group (compound ). The dipole moment.
9 and 10. The increase of absorbance with
, 7, 8
9
Table 3. Optical properties of 10-5 M DMF solutions, 0.5 wt% molecular dispersions in PS and of the neat films of PI derivatives. Phosphorescence properties were
evaluated for the films of 2 wt% molecular dispersions in PMMA additionally doped with 25 wt% of benzophenone.
Compound
1
2
3
4
5
6
7
8
9
10
DMF solution
a
ΦF )
%
L∙mol-1cm-1
nm
57
11540
364
379
392
51
9330
366
384
392
59
9900
361
374
390
56
7360
363
387
392
401
6.7
65
8500
363
380
392
66
8550
363
380
392
78
15880
360
67
15090
360
81
29800
365
61
13460
370
Absb)
Absmax
c
)
d
Flmax
)
nm
408
421
424
403
τe)
ns
ns
ns
6.8
11.9
15.8
6.7
13.1
13.7
6.9
11.7
16.8
5.8
8.9
16.6
6
9.1
17.6
2.1
2.7
9.5
2
3
6.1
1.8
2.2
9.5
4.3
7
11
τrf)
12
15.2
f
τnr
)
0.5 wt% PS film
a
ΦF )
%
63
55
379
396
7
12.7
15.5
64
370
389
6.9
10.7
19
55
84
376
393
6.1
7.2
37.9
71
376
394
6.1
8.6
21
90
84
97
408
423
1.5
1.5
49.3
80
384
403
4
5
20
376
394
7.1
11.3
19.3
377
394
7.1
12.9
15.8
d
Flmax
)
nm
ns
ns
395
2.6
2.8
403
2.4
2.8
τe)
τrf)
f
τnr
)
ns
25.6
14.7
Neat film
a
ΦF )
%
nm
9
4
365
12
364
377
8
366
385
399
22
365
22
365
26
363
12
368
23
357
17
375
c
Absmax
)
367
402
423
d
Flmax
)
nm
402
395
405
403
431
436
433
413
576
594
0.45
[36%]
1.34
0.19
[56%]
1.11
[32%]
6.13
0.06
[17%]
1.17
[42%]
3.47
[41%]
1.9
0.23
[21%]
0.91
[50%]
2.04
[29%]
1.1
0.69
[20%]
2.70
[46%]
7.66
[34%]
4
1.40
[12%]
2.87
0.25
[58%]
1.28
0.25
[28%]
0.87
0.36 [6%]
2.61
[65%]
7.51
[29%]
3.9
τg)
ns
ns
2.78
2.8
[56%]
[79%]
[37%]
[66%]
5.45 [8%]
6.39 [9%] 3.53 [5%] 3.29 [6%]
[12%]
1.2
h
τa )
1.3
3
0.8
0.8
2 wt% PMMA film with 25 wt% of BP
i
Phos max
)
nm
eV
eV
-
-
-
481
2.58
0.69
495
2.51
0.85
493
2.52
0.77
507
2.45
0.85
506
2.45
0.85
516
2.40
0.74
520
2.38
0.69
490
2.53
0.51
510
2.43
0.80
j
ET )
k
ΔEST
)
a) Fluorescence quantum yield. b) Absorption intensity at lowest energy absorption band maximum. c) Lowest energy absorption band maximum. d) Fluorescence
band maxima at vibronic peaks. e) Fluorescence lifetime at the highest vibronic peak of fluorescence band. f) τr – radiative decay time, τnr non-radiative decay
g)
time. Fluorescence lifetime measured at fluorescence band maximum. Fractional contribution (f) to the total fluorescence intensity is given in the
parentheses. h) Average fluorescence lifetime calculated by τa =Σ τi × fi. i) Phosphorescence maximum at 0th vibronic replica. j) Energy level of the triplet state
derived from Phos max.
k) Singlet-triplet energy splitting.
Stokes shifted implying enhanced solvation effect induced by
The solutions of phenyl- or methoxyphenyl-substituted PI the presence of more polar substituents (Figure S3, Supporting
compounds in DMF exhibit fluorescence in blue and Information). The compounds also exhibit somewhat higher
1-6
ultraviolet (UV) spectral region with emission maxima ranging fluorescence quantum yields of 78, 67 and 81%, respectively,
from 374 nm to 401 nm and with fluorescence quantum yields what nicely correlates with increased absorbance (see Table 3).
of 51–66%. Fluorescence spectra express noticeable vibronic
progression indicating rigid molecular backbone of PI.
Conversely, the fluorescence spectra of the solutions of
compounds 7, 8 and 9 contain no vibronic replicas and are more
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yet less than 2-fold decrease in τnr
. Thereby, for cVoiemwpAortiuclne dOnslin7e,
DOI: 10.1039/C7CP02248D
8
and τr was reduced down to ~2.6 ns, whereas τnr only down
9
to ~8.4 ns.
Figure 3. Fluorescence transients of the solutions of PI derivatives 1, 5, 7, 9, and 10
in 10-5 M DMF with fluorescence lifetimes of each compound indicated. The
intensity of transients is scaled and arbitrarily shifted for clarity.
Fluorescence transient measurements for DMF solutions of the
phenyl- and methoxyphenyl-substituted compounds
1-4
revealed mono-exponential decays with the fluorescence
lifetime (τ) of ~6.8 ns (Figure 3). Trimethoxyphenyl-substituted
compounds
whereas
trifluoromethylphenyl
5
the
and
6
expressed slightly shorter τ of ~5.9 ns,
compounds with carbazole 10),
and triphenylamino
(
(7
,
8
)
(9)
substituents showed even more rapid decay with τ of 4.3, 2.1,
2.0 and 1.8 ns, respectively.
Figure 4. Normalized fluorescence spectra of 10-5 M DMF solutions and of 0.5 wt%
molecular dispersions in PS of PI derivatives 1 (a), 6 (b), 7 (c) and 8 (d). Fluorescence
quantum yields are indicated.
To estimate the relative contribution of radiative and
nonradiative relaxation processes in the excited state
deactivation and determine the governing mechanism, the
radiative (τr) and nonradiative (τnr) decay time constants were
calculated. The calculated constants (τr = τ/ΦF, τnr = τ/(1-ΦF))
revealed similar τr = ~12.2 ns and τnr = ~15.4 ns for the
It is known that compounds bearing relatively small and flexible
groups, such as phenyl groups linked to the 1st and 2nd position
of phenanthroimidazole moiety, are prone to nonradiative
deactivation due to the effective low-frequency vibronic mode
(twist and torsion) coupling to the ground state.30,31 While being
efficient in the non-viscous media, e.g. solutions, the
intramolecular torsions are highly suppressed in the
environments of high viscosity, e.g. rigid polymer matrices.32,33
Therefore, to elucidate the influence of intramolecular torsions
of the various substituents on the optical properties of the
compounds they were embedded into rigid polystyrene (PS)
matrix at low concentration. Estimated fluorescence quantum
compounds
1, 2, 3 and 4. The increased polarity of the
substituents was found to strongly affect τr of the PI derivatives
while influencing τn at a much lesser degree. Particularly, for
r
trimethoxyphenyl-substituted compounds
(5, 6) τr was
shortened by 25% (to 9.0 ns), whereas τnr increased by only 10%
(up to ~17.0 ns). Introduction of even more polar
trifluoromethylphenyl and triphenylamino substituents
(compounds
7, 8 and 9) resulted in the remarkable 3-fold
enhancement of radiative decay rate, i.e. 3-fold reduction in τr,
6 | J. Name., 2012, 00, 1-3
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yield of all the compounds embedded into PS matrix showed 10- substitution of the 2-phenyl moiety with meVtihewoxAyrt-icplehOennlinyel
DOI: 10.1039/C7CP02248D
20% enhancement as compared to that observed for DMF (compound 4) did not prevent formation of excimers either,
solutions (Table 3). The increase of ΦF is attributed to the conversely, caused excimer band boost in intensity in respect to
restriction of intramolecular torsions and suppression of molecular band. The excimer band of 4 was observed to shift to
torsion-induced nonradiative excited state deactivation.34 The 594 nm also delivering shortened fluorescence lifetime of ~60
small increase in ΦF well correlates with the slightly longer ns. The incorporation of three methoxy groups into the meta-
fluorescence lifetimes estimated for the compounds dispersed and para-positions of the 2-phenyl moiety (5) effectively
in PS matrix. Importantly, although radiative τr of the inhibited formation of eximers, and nearly tripled ΦF (up to
dispersions in PS remains almost unchanged, τnr is enlarged 22%). This result indicates that at least three methoxy groups
significantly as compared to that for solutions. A particularly are required to ensure effective intermolecular separation. The
marked increase of τnr (by a factor of 2.5-5) is attained for the incorporation of the fourth methoxy group at the para-position
PI compounds bearing highly polar trifluoromethylphenyl and of the 1-phenyl moiety (
triphenylamino substituents (compounds and ). The optical properties (Table 3). Emission intensities for the neat
considerably suppressed nonradiative decay pathway can be films of both compounds and were found to peak at ~404
unambiguously attributed to increased restriction of nm with ΦF 22%. Generally, the substitution of
intramolecular torsions upon incorporation of the compounds phenanthroimidazoles at the 1st position resulted in a twisted
into rigid PS matrices. molecular structure, which suppressed aggregation induced
Fluorescence spectra of the films of dispersions of compounds fluorescence quenching, and thus enhanced ΦF of the neat films
and 10 in PS were found to be similar to those observed (Table 3) as predicted by DFT calculations.
for dilute solutions. Better resolved vibronic progressions for Compounds and possessing polar trifluoromethyl groups
6) caused no visible changes in the
7
,
8
9
5
6
=
1-6, 9
7
8
dispersions in PS were caused by the increased rigidity of the exhibited strongly Stokes-shifted fluorescence spectra with
surrounding environment (Figure 4a and 4b). Conversely, the emission bands peaking at 431 and 436 nm, respectively (Figure
fluorescence spectra of compounds
7 and 8 dispersed in PS S4, Supporting Information). As a result of the donor-acceptor
matrix were blue shifted up to 18 nm as compared to those type intramolecular interaction involving formation of charge
observed for solutions (Figure 4c and 4d). The blue shift is most transfer states, the vibronic modes were absent in the
likely caused by the change in the polarity of the surrounding fluorescence spectra of both compounds (Figure S4, Supporting
medium. Taking into account that PS is much less polar than Information). Compound
DMF, stabilization of the excited state in PS is relatively weaker, substituents linked at the para- position exhibited ΦF of 26% in
and thus, the spectrum appears at shorter wavelengths. the solid state, whereas compound with two trifluoromethyl
Absorption spectra of the neat films of phenanthroimidazoles groups linked at the meta- position showed 2-fold reduced ΦF
7 with two identical trifluoromethyl
8
.
(Figure S4, Supporting Information) were found to be very It is likely that the difference in ΦF in the solid state is provoked
similar to those of the dilute solutions with a spectral shift of by diverse molecular packing due to the different
the lowest energy band of only few nanometers. The absorption trifluoromethyl linking topology.
band of the films of compounds
substituents exhibited slightly larger redshift (up to 8 nm, see phenantroimidazole derivative (10) is enhanced as compared to
Table 3) likely due to increased intermolecular interactions in that of derivatives or , however out-of-plane twisted
the solid state facilitated by polar trifluoromethyl groups. carbazole moiety is incapable of avoiding close molecular
7 and 8 bearing trifluoromethyl It was found that ΦF of the carbazole-substituted
1
,
2
,
3
4
Although various substituents introduced at the 1st and 2nd packing and excimer formation in the neat films. Weak excimer
positions of phenanthroimidazole moiety had a rather minor band as a long-wavelength slope was observed in the
impact on the optical properties in dilute solutions, it strongly fluorescence spectrum of neat film of 10. The average lifetime
affected fluorescence properties of the solid samples (Figure S4, of this band was measured to be ~60 ns. It is worth noting that
Supporting Information). Fluorescence spectra of the films of introduction of triphenylamino substituent had minor influence
the compounds possessing phenyl and tert-butyl-phenyl groups on the fluorescence spectrum of the film as compared to that
(1 and 2) are located in the violet-deep blue region with the of solution and allowed to effectively suppress excimer
emission peak at 402 nm. The redshifted fluorescence spectra emission in the solid films with relatively high ΦF (23%).
along with the dramatically reduced fluorescence quantum
yield of the neat films of the compound (down to 4%) as
compared to those of the dilute solutions imply strong
aggregation induced quenching resulting from planar molecular
backbone.35 The introduction of the second phenyl group at the
1st position of phenanthroimidazole moiety (compound
3)
causes formation of excimer states, which are evident from the
broad, structureless, long living ( = 176 ns) fluorescence band
τ
at ~576 nm (Figure S4, Supporting Information). Although the
attachment of additional phenyl ring enhanced ΦF up to 12%,
the emergence of the broad excimer band at long wavelengths
is generally detrimental for the realization of blue emitters. The
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7 and 8 with trifluoromethyl groups
films of compounds
View Article Online
expressed prolonged τ as compared DtOoI: 1t0h.1o0s3e9/Cf7oCuPn0d224f8oDr
solutions. Taking into account considerably reduced ΦF of the
neat films, the prolonged τ can be caused by the enhanced
radiative decay channel as a result of polar groups induced
specific molecular packing.
Knowledge of the triplet energies of phenantroimidazole
compounds is essential for their possible application, e.g. as
host materials in light emitting devices. The triplet energies of
the compounds were estimated from their phosphorescence
spectra exhibiting relatively high intensity at room temperature
due to the triplet sensitization method employed.40 Triplet
sensitizer, benzophenone (BP), featuring 100% intersystem
crossing rate and high triplet energy (2.96 eV) was added to
poly(methyl methacrylate) (PMMA) films with the dispersed
phenantroimidazole compounds to foster singlet excitons
conversion to triplets as well as their transfer to the triplet
states of phenanthroimidazoles. PMMA films with 2 wt% of the
investigated compounds and 25 wt% of BP were used in the
phosphorescence measurements. The excitation wavelength
was tuned to the absorption of BP (330 nm). Additionally, to
avoid the interfering fluorescence signal in the background,
phosphorescence spectra were recorded with the delay of 1 µs
after the excitation pulse by using time-gated technique.
Phosphorescence spectra of the PI derivatives
presented in Figure 6. The spectra of the compounds
6
-
10 are
2-5 were
found to be almost identical to that of
show. Unfortunately, low phosphorescence quantum yield of
compound rendered its phosphorescence spectrum
6 and therefore are not
1
measurements impossible. All the compounds displayed
phosphorescence in the green spectral region with the band
maximum ranging from 490 to 550 nm. Phosphorescence
spectra have similar shape as compared to fluorescence of DMF
solution. The triplet energies of the compounds estimated from
the band maxima of phosphorescence spectra are listed in Table
3. Based on this data singlet-triplet energy splitting of the
phenanthroimidazoles was calculated from the fluorescence
and phosphorescence spectra. ΔEST was found to vary from 0.51
to 0.85 eV for the studied compounds (Table 3). Such large ΔEST
generally indicates a good overlap of the HOMO and LUMO
molecular orbitals as it was predicted by DFT calculations. This
observation also shows rather moderate influence of various
substituents on the distribution of molecular orbitals in
phenanthroimidazoles. Somewhat reduced ΔEST of 0.51 eV (due
to the blue shifted phosphorescence spectrum) obtained for
Figure 5. Fluorescence transients of the neat films of PI derivatives with average
fluorescence lifetimes indicated. The intensity of each transient was arbitrarily
shifted for clarity.
Fluorescence transients of the neat films of PI derivatives are
shown in Figure 5. Generally, the transients exhibit non-
exponential decay indicating dispersive exciton hopping
through the localized states in the disordered media.36,37 The
fluorescence lifetimes were extracted by fitting the transients
using multi-exponential decay model (Table 3). The average
fluorescence lifetimes are indicated in Figure 5. For the neat
films of most of compounds the average τ was significantly
shorter as that found for solutions. The shortened lifetime is
attributed to exciton migration (or energy transfer) in the
condensed phase and migration-induced enhanced exciton
quenching at non-radiative decay sites.38,39 Interestingly, the
compound
9 bearing electron-donating triphenylamino unit is
likely caused by the slightly reduced spatial overlap between
HOMO and LUMO in the donor-acceptor type molecule (Figure
2).
8 | J. Name., 2012, 00, 1-3
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Page 9 of 12
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found to be similar for all the studied compounds. During CV
View Article Online
experiments 1-10 showed only quasDi-OreI:v1e0r.1s0ib39le/C7oCxPi0d2a2t4io8Dn
couple and no reduction waves (Figure 7, Figure S5, Supporting
Information).
-5
2.0x10
-5
1.5x10
-5
1.0x10
-6
5.0x10
0.0
-5.0x10-6
-5
-1.0x10
1.0
1.1
1.2
1.3
1.4
1.5
Potential, V
Figure 7. Cyclic voltammogram of dilute solution of compound
8 in
dichloromethane at room temperature at sweep rate of 100 mV/s.
The oxidation potentials for the reversible oxidation are taken
as the average of the cathodic and anodic peak potentials. The
oxidation potentials for 1-10 were found to be 0.60-1.20 V.
The ionization potential values (IPCV) were determined from the
values of the first oxidation potential (Ep) with respect to
ferrocene (IPCV = Ep + 4.8 eV). IPCV values of the compounds
ranged from 5.20 to 5.80 eV. Compounds
9 and 10 having
electron donating triphenylamino and carbazolyl groups at the
2nd position of PI moiety showed the lowest values of IPCV. The
Figure 6. Phosphorescence spectra of the PI derivatives (6-10) at room
temperature. The intensity of each spectrum was arbitrarily shifted for clarity.
highest IPCV values were observed for compounds
7 and 8
containing trifluoromethyl substituents. The electron affinities
(EAcv) determined from the optical energy band gaps (Egopt) and
ionization energy values (EACV = −(|IPCV| − Egopt)) were in the
range of 2.19-3.37 eV. The calculated EACV ranged from 2.06 to
3.25 eV.
Electrochemical properties
Electrochemical properties of the synthesized PI derivatives
were investigated by cyclic voltammetry (CV). The results are
presented in Table 4. The shape of cyclic voltammograms was
Table 4. Electrochemical and photoelectrical data of PI derivatives 1-10.
Compound
Eox [V]a)
1
2
3
4
5
6
7
8
9
1027
0.69
0.49
3.23
5.29
5.21
2.06
0.69
0.49
2.19
5.29
5.40
3.25
0.98
0.78
2.19
5.58
5.43
3.24
0.97
0.77
3.34
5.57
5.60
2.23
0.85
0.65
3.25
5.45
5.54
2.20
0.85
0.65
3.31
5.45
5.42
2.14
0.81
0.61
3.31
5.41
5.43
2.10
1.14
0.94
3.28
5.74
-
1.20
1.00
3.24
5.80
-
0.60
0.40
3.03
5.20
5.21
2.17
Ep [V]b)
Egopt [eV]c)
IPCV [eV]d)
IPEP [eV]e)
EACV [eV]e)
2.46
2.56
a) onset oxidation potential vs. Ag/Ag+, b) Ep is onset oxidation potential vs. Fc/Fc+ (Ep = Eox – E1/2Fc/Fc+ (E1/2Fc/Fc+ = 0.20 V vs. Ag/Ag+)); c) optical band gap, estimated
from the edges of electronic absorption spectra (Egopt = 1240/λedge); d) IPCV = −(Ep+4.8); e) IPEP is ionization potential of thin solid layers; f) EACV = −(|IPCV| − Egopt).
Photoelectrical properties
values of the compounds were obtained at the points of
crossing straight lines with x-axis. Solid-state IPEP values ranged
Photoelectron emission measurements of the layers of the between 5.21-5.60 eV for compounds 1-6, 9, 10. These values
studied compounds were performed in order to get solid state are not far from the work function of indium tin oxide anode.
ionization potentials (IPEP). Photoelectron emission spectra for Therefore, good hole injection to functional layers in organic
solid layers of the compounds are plotted in Figure 8. The IPEP devices based on studied materials can be predicted. The IPEP
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values showed the similar tendencies as IPCV values. The lowest the molecules assist to hole and electron hopping between
View Article Online
DOI: 10.1039/C7CP02248D
9 and 10 having adjacent molecules resulting to the bipolar charge transport
IPEP values were observed for compounds
electron donating triphenylamino and carbazolyl groups at the properties in the compounds 4-6, 9 and 10. For example, the
2nd position of PI moiety. The IPEP values for trifluoromethyl HOMOs and LUMOs of
are localized on both donating and
substituted compounds and were not estimated, possibly, accepting moieties (Figure 2). This indicates that the adjacent
9
7
8
due to the limitation of photoelectron emission spectrometry molecules should facilely achieve mutual contact with each
performed in air (which is because of the absorbance of high other to obtain bipolar charge transport. In addition, C–H∙∙∙π, O,
energy UV radiation (with photon energy ≥6.0 eV) by oxygen in N hydrogen bonds induced by methoxy groups of compounds
air). The IPEP values for compounds 7 and 8 could be even higher 4-6 can have impact on the intermolecular interaction energies
than those estimated by CV measurements due to the increased resulting in different values of the charge mobility.43 Hole
intermolecular interactions in solid-state (Table 4). As a result, mobility values at electric fields of 0.5×105 – 5.5×105 V/cm for
the photoelectron emission spectra were not recorded hence the compounds range between ca. 10-7-10-4 cm2/Vs depending
the IPEP values were not estimated for these compounds.
on the substitutions (Figure 9). The highest hole mobility value
of 2.74×10-4 cm2/Vs at 4.23×105 V/cm was obtained for
5
1
compound
9 being substituted by a strongly donating
2
triphenylamino moiety. The hole transit times for the
compounds 3-10 were well seen on log-log plots of TOF
transients which displayed even low dispersity of hole transport
for compounds 5, 9 and 10 (Figure S6, Supporting Information).
Electron transport for compounds 4-6, 9, 10 was detected by
TOF as the kink on photocurrent transients in the log–log for
these compounds was seen (marked by red lines, Figure S6,
Supporting Information). Electron mobility values for
3
4
5
6
9
4
3
2
1
0
10
fitting
compounds 4-6, 9 and 10 were found to be in the range from
3.7×10-7 to 8.8×10-5 cm2/Vs at electric fields of 0.5×105 - 5.5×105
V/cm (Figure 9). Electron mobilities for PI-based compounds 4-
5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4
6
, and 10 were obtained to be slightly higher than their hole
Photon energy (eV)
mobilities at the same electric fields. Only for compound hole
9
mobility was found to be approximately by one order higher
than its electron mobility apparently due to the presence of the
strong donor i.e. triphenylamino unit (Figure 9). Electron
transport for compounds 1-3, 7, 8 was not detected by the TOF
measurements as the transit times on the related photocurrent
transients were not seen for these compounds (Figure S6,
Supporting Information). Despite rather low hole and electron
mobilities of compounds 3-10 compared to those of organic
semiconductors published before,44 these values are
comparable with charge mobilities of recently published
effective OLED emitters.45
Figure 8. Photoelectron emission spectra of the solid state layers of compounds 1-
6, 9, 10.
To investigate influences of different substitutions of
compounds on their charge-transporting properties, the time of
flight (TOF) method was employed. Hole transport for the
compounds 3-10 substituted at the both 1st and 2nd positions of
PI were observed. Hole-transporting ability for compounds
1
and
2
substituted by phenyl group only at 2st position of PI was
not proved by TOF apparently due to the weak donating ability
of one phenyl moiety. Despite all studied compounds are based
on the accepting phenanthroimidazole unit, electron transport
was only observed for compounds 4-6, 9,10 substituted at both
1st and 2nd positions of PI by phenyl, methoxyphenyl, carbazolyl,
or triphenylamino moieties. Attachement of electron-accepting
3/ holes
4/ holes
5/ holes
6/ holes
7/ holes
8/ holes
10-4
10-5
9/ holes
10/ holes
fitting
trifluoromethyl groups to phenyl moieties of
induce electron-transporting ability to these compounds. In
contrast to methoxy substitution in compounds 4-6
trifluoromethyl substitution in compounds and apparently
7 and 8 did not
10-6
,
4/ electrones
5/ electrones
6/ electrones
9/ electrones
7
8
lead to the increase of electron hopping distance which
restricted electron transport in the layers of these
compounds.41 Consequently, bipolar charge transport
properties were observed for compounds 4-6, 9,10 substituted
at the 1st position of PI by methoxyphenyl, carbazolyl, or
triphenylamino moieties. Apparently, the different spatial
frontier orbital distributions resulting from the different
substitution of PI can be the reason for this observation.42
Appropriate orbital overlap between the HOMOs and LUMOs of
10-7
10/ electrones
0
200
400
600
800
1000
Electric field1/2 (V1/2/cm1/2)
Figure 9. Visualization of the experimental charge mobilities of compounds 3-10.
10 | J. Name., 2012, 00, 1-3
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8
9
M. S. Gong, H. S. Lee and Y. M. Jeon, J. Mater.VCiehweAmrti.c,le2O0n1lin0e,
20, 10735.
CONCLUSIONS
S. Tang, M. R. Liu, P. Lu, H. Xia, M. Li, DZO. QI: 1.0X.1ie03, 9F/.CZ7.CSPh0e22n4,8CD.
Gu, H. Wang, B. Yang and Y. G. Ma, Adv. Funct. Mater., 2007,
17, 2869.
In conclusion, we have synthesized and investigated structure-
property relationship of phenanthroimidazole derivatives by
studying their thermal, photophysical, electrochemical and
photoelectrical properties. Dilute liquid and solid solutions of
phenanthroimidazole derivatives were found to emit in a deep
blue spectral region (390-430 nm) with high fluorescence
quantum yield (up to 97%). Increase of the polarity of the
substituents linked to the 2nd position of phenanthroimidazole
moiety enhanced absorption and fluorescence quantum yields
of dilute solutions as a result of increased radiative decay rate.
Incorporation of phenanthroimidazole compounds into rigid PS
matrix revealed remarkable lengthening of nonradiative decay
time accompanied by enhancement of fluorescence quantum
yield due to the restriction of intramolecular torsions and
suppression of torsion-induced nonradiative excited-state
deactivation. This was particularly well pronounced for
10 N. Li, P. Wang, S. L. Lai, W. Liu, C. S. Lee, S. T. Lee and Z. Liu,
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phenanthroimidazoles substituted with polar groups.
A
significant drop of fluorescence quantum yield of the neat
phenanthroimidazole films (down to 4%) was obviously
facilitated by planar phenanthroimidazole core promoting
aggregation-induced emission quenching. However, the
attachment of phenyl moiety at the 1st position as well as other
bulky groups like methoxyphenyl or triphenylamino at the 2nd
position of phenanthroimidazole moiety conditioned up to 6-
fold boost in fluorescence quantum yield due to the sterical
hindrance effects effectively suppressing aggregation. The D-A
structure of the phenanthroimidazole compounds implied
ambipolar charged carrier transport in their films with charge
drift mobilities reaching 10-4 cm2/V∙s. The triplet energies of
phenanthroimidazoles revealed from phosphorescence
measurements were found to be 2.4-2.6 eV below singlet
manifold (in the green spectral region) ensuring effective
energy transfer to yellow and red dopants.
3, 7709.
3
4
3
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Acknowledgements
This work was supported by the Research Council of Lithuania.
(grant No TAP LLT-2/2017). D. Gudeika thanks for the support
from the Lithuanian Academy of Sciences.
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