New derivatives of triphenylamine and naphthalimide as ambipolar organic semiconductors: Experimental and theoretical approach

Article abstract of DOI:10.1016/j.dyepig.2014.02.023

Four new derivatives of triphenylamine containing different number of naphthalimide moieties were designed and synthesized by Suzuki condensation and their properties were studied by the experimental and theoretical tools. The compounds obtained are capable to form molecular glasses with glass transition temperatures ranging from 45 °C to 84°C. They exhibit very high thermal stabilities with 5% weight loss temperatures ranging from 429°C to 483°C. Fluorescence quantum yields of the dilute solutions in nonpolar solvents of the synthesized materials range from 0.63 to 0.78. Due to the pronounced electron donor-acceptor character, the compounds show dramatic solvatochromic red shifts of fluorescence (up to 250 nm) in polar solvents. The ionization potentials of the solid samples of the compounds established by electron photoemission spectrometry in air ranged from 5.57 to 6.01 eV. 4-(4′-(Di-(4″-methoxyphenyl)amino)phenyl)-N-(2-ethylhexyl)-1, 8-naphthalimide (5) was found to show ambipolar charge transport in air with the mobilities of charges exceeding 10^-4 cm² V^-1 s^-1 at high electric fields. The electron mobility of the compounds containing no methoxy groups were found to exceed the hole mobility by 2-3 orders of magnitude. The special role of methoxy groups in the ambipolar charge transport character of compound 5 is discussed in the frame of hopping Marcus theory, by applying a static theoretical analysis followed by a qualitative discussion of the positional disorder in some of these materials.

Full text of DOI:10.1016/j.dyepig.2014.02.023

Dyes and Pigments 106 (2014) 58e70
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New derivatives of triphenylamine and naphthalimide as ambipolar
organic semiconductors: Experimental and theoretical approach
,
b
,
a
Dalius Gudeika a, Juozas Vidas Grazulevicius a , Gjergji Sini
, Audrius Bucinskas ,
*
**
Vygintas Jankauskas c, Arunas Miasojedovas d, Saulius Jursenas d
a Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania
b Laboratoire de Physicochimie des Polymères et des Interfaces, EA 2528 Université de Cergy-Pontoise, 5 mail Gay-Lussac, 95031 Cergy-Pontoise Cedex,
France
c Department of Solid State Electronics, Vilnius University, Sauletekio aleja 9, LT-10222 Vilnius, Lithuania
d Institute of Applied Research, Vilnius University, Sauletekio aleja 9, LT-10222 Vilnius, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new derivatives of triphenylamine containing different number of naphthalimide moieties were
designed and synthesized by Suzuki condensation and their properties were studied by the experimental
and theoretical tools. The compounds obtained are capable to form molecular glasses with glass tran-
sition temperatures ranging from 45 ꢀC to 84 ꢀC. They exhibit very high thermal stabilities with 5%
weight loss temperatures ranging from 429 ꢀC to 483 ꢀC. Fluorescence quantum yields of the dilute
solutions in nonpolar solvents of the synthesized materials range from 0.63 to 0.78. Due to the pro-
nounced electron donoreacceptor character, the compounds show dramatic solvatochromic red shifts of
fluorescence (up to 250 nm) in polar solvents. The ionization potentials of the solid samples of the
compounds established by electron photoemission spectrometry in air ranged from 5.57 to 6.01 eV. 4-(40-
(Di-(400-methoxyphenyl)amino)phenyl)-N-(2-ethylhexyl)-1,8-naphthalimide (5) was found to show
ambipolar charge transport in air with the mobilities of charges exceeding 10ꢁ4 cm2 Vꢁ1 sꢁ1 at high
electric fields. The electron mobility of the compounds containing no methoxy groups were found to
exceed the hole mobility by 2e3 orders of magnitude. The special role of methoxy groups in the
ambipolar charge transport character of compound 5 is discussed in the frame of hopping Marcus theory,
by applying a static theoretical analysis followed by a qualitative discussion of the positional disorder in
some of these materials.
Received 30 December 2013
Received in revised form
21 February 2014
Accepted 25 February 2014
Available online 6 March 2014
Keywords:
Donoreacceptor
Suzuki condensation
Triphenylamine
1,8-Naphthalimide
DFT
Ambipolar charge transfer
Ó 2014 Published by Elsevier Ltd.
1. Introduction
particular in electrophosphorescent devices [7a]. In the continual
effort to search for ideal materials for OLEDs, small molecules with
In the last decades many kinds of organic hole-transporting
amorphous molecular materials were reported [1]. Lesser assort-
ment is available of glass-forming electron-transporting organic
molecular materials especially those capable of working in air [2,3].
Even smaller amount of amorphous organic molecular materials
capable of effectively transporting both holes and electrons were
reported, however such materials recently attract much attention
[4,5]. Ambipolar charge-transporting materials are of interest for
applications in organic light emitting diodes (OLED) [6] and in
ambipolar charge transporting character are extremely attractive as
they offer the possibility to achieve efficient and stable OLEDs even
in a simple single-layer configuration. Organic ambipolar semi-
conductors are divided into two categories [4]. One category is
represented by materials the molecules of which consist both of
donor and acceptor moieties. Such materials are widely studied as
dyes for dye sensitized solar cells [7b]. The materials composed of
the molecules having no donoreacceptor structure are assigned to
another category. The molecules of the latter category usually have
extended systems of conjugated p-electrons. Most of the ambipolar
organic semiconductors are capable of transporting both holes and
electrons when protected from air. Much less information is re-
ported on the materials capable of transporting both negative and
positive charges at ambient conditions [8,9].
* Corresponding author. Tel.: þ370 37 300193; fax: þ370 37 300152.
** Corresponding author.
E-mail addresses: juozas.grazulevicius@ktu.lt (J.V. Grazulevicius), gjergji.sini@u-
cergy.fr (G. Sini).
http://dx.doi.org/10.1016/j.dyepig.2014.02.023
0143-7208/Ó 2014 Published by Elsevier Ltd.

D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
59
In this article we report on the synthesis and properties of
organic semiconductors consisting of 1,8-naphthalimide and tri-
phenylamine (TPA) moieties capable of effectively transporting
both holes and electrons in air. 1,8-Naphthalimide derivatives
represent an interesting group of electron-deficient organic mate-
rials with promising electron-transporting properties [10].
Electron-rich TPA derivatives have been typically used as hole-
transporting materials and/or blue light emitting materials [11]
due to the easy oxidizability of the nitrogen centre and the ability
to transport positive charges via the radical-cation species. These
properties are related to the presence of nitrogen atom linked to
three electron rich phenyl groups in a three-dimensional propeller-
like shape [12]. The photoinduced electron transfer was recently
observed in the solutions of the derivatives of 1,8-naphthalimide
and TPA [13]. Hydrazones containing 1,8-naphthalimide and tri-
phenylamino moieties were found to be capable of transporting
only positive charges in air [14]. To our knowledge no studies
demonstrating ambipolar charge transport in the derivatives of 1,8-
naphthalimide and TPA were yet reported.
The role of different substituents on the ionization potentials
and other parameters of TPA-based compounds has been already
considered [15e18]. It is well established that methoxy groups
decrease the ionization potentials of the TPA-based compounds
[15,16], influencing consequently the hole-injection barrier in the
devices. However, lower charge mobility was recorded for instance
in the case of p-methoxy and p-buthoxy substituted N,N0-bis(m-
tolyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine (TPD) as compared
to the non-substituted TPD, which was in part explained by the role
of the disorder on the dipole moments of these molecules [17].
Interestingly, the hole transporting properties of TPA-based com-
pounds were recently found to be enhanced by the presence of
methoxy groups in para-positions of the phenyl moieties [19],
which was partly explained by the hydrogen-bonding capacity of
the methoxy groups. As it will be shown in the following, among
the newly synthesized compounds, the ambipolar charge transport
properties of one of them containing methoxy groups are superior,
as compared to the compounds containing no methoxy groups. It is
consequently very intriguing to understand the reasons for the
higher electron-versus hole mobilities on the one hand, and for the
higher hole mobility of the methoxy-substituted compound (by
three orders of magnitude) as compared to the non-substituted
ones.
(POCH), ethyl acetate and n-hexane (Penta) were purified and
dried using the standard procedures [24]. 4-Bromo-N-(2-
ethylhexyl)-1,8-naphthalimide (1) [14], (4-bromo-phenyl)-di-
(4-methoxyphenyl)-amine (2b) [25], bis(4-bromophenyl)phe-
nylamine (2c) [26], bis(4-(4,4,5,5-tetramethyl-(1,3,2)dioxabor-
olan-2-yl)-phenyl)phenylamine (3c) [27], 4-(4,4,5,5-tetramethyl-
(1,3,2)dioxaborolan-2-yl)-phenyl)-di-(4-methoxyphenyl)-amine
(3b) [28], tris(4-(4,4,5,5-tetramethyl-(1,3,2)dioxaborolan-2-yl)-
phenyl)phenylamine (3d) [29] were prepared according to the
published procedures.
2.1.1. 4-(40-Diphenylaminophenyl)-N-ethylhexyl-1,8-naphthalimide
(4)
The solution of 4-bromo-1,8-naphthalimide (1) (0.5 g,
1.29 mmol) and Pd(PPh3)2Cl2 (0.03 g, 0.04 mmol) in THF (15 mL)
was purged with nitrogen, and the solution of 4-(diphenylamino)
phenylboronic acid (0.37 g, 1.28 mmol) in THF (3 mL) and aqueous
K2CO3 solution (1.70 g, 12.32 mmol) in H2O (2 mL) were added with
a syringe. The reaction mixture was stirred at 80 ꢀC for 24 h. After
cooling down, the product was extracted with CH2Cl2, washed with
water and dried over MgSO4. The solvent was evaporated to afford a
crude product. After column chromatography on silica gel with the
eluent mixture of ethyl acetate and hexane (1:8, V:V), compound 4
was obtained as yellow powder with the yield of 0.55 g (78%). M.p.
127e128 ꢀC; Rf ¼ 0.54; 1H NMR (300 MHz, CDCl3):
d 8.67 (d, 1H,
J ¼ 7.32 Hz, HNaphthalene), 8.66 (d, 1H, J ¼ 8.52 Hz, HNaphthalene), 8.46
(d, 1H, J ¼ 1.16 Hz, HNaphthalene), 8.43 (d, 1H, J ¼ 1.15 Hz, HNaphthalene),
7.75 (t, 1H, J ¼ 7.56 Hz, HNaphthalene), 7.44e7.31 (m, 6H, eAr), 7.27e
7.09 (m, 8H, eAr), 4.26e4.11 (m, 2H, eCH2, eHaliphatic), 2.06e1.95
(m, 1H, eCH, eHaliphatic), 1.48e1.31 (m, 8H, 4 ꢂ CH2, eHaliphatic),
1.01e0.89 (m, 6H, 2 ꢂ CH3, eHaliphatic). 13C NMR (75.4 MHz, CDCl3):
d
167.43, 147.76, 146.75, 146.14, 132.85, 131.90, 131.48, 131.29, 130.31,
130.05, 129.31, 128.42, 127.93, 126.73, 125.78, 124.77, 123.83, 121.64,
43.87, 38.14, 31.73, 28.88, 24.59, 23.47, 14.44, 10.82. IR (KBr,
cmꢁ1):
y
3060 (CHar), 2954, 2925, 2858 (CHaliphatic), 1697 (C]Oanhydride),
1656, 1584, 1505, 1486 (C]Car), 1350, 1279 (CeN), 784, 759, 695
(CHar); Anal. Calcd. for C38H36N2O2: C, 82.58; H, 6.57; N, 5.07; O,
5.79. Found: C, 82.63; H, 6.60; N, 5.08. MS (APCIþ, 20 V), m/z: 553
([M þ H]þ)
2.1.2. 4-(40-(Di-(400-methoxyphenyl)amino)phenyl)-N-(2-
ethylhexyl)-1,8-naphthalimide (5)
By applying a joint experimental and theoretical approaches, the
aim of this study is twofold: (i) report on the synthesis of four new
derivatives of TPA containing direct linkages with a different
number of naphthalimide moieties (ii) characterization of the four
new compounds for better understanding of the structureeprop-
erty relationships. The hole-transport properties of these amor-
phous compounds are discussed in the frame of Marcus theory
[20e23].
4-(40-(Di-(400-methoxyphenyl)amino)phenyl)-N-(2-ethylhexyl)-
1,8-naphthalimide (5) was prepared by the similar procedure as 4
using 3b (0.58 g, 1.42 mmol), 1 (0.5 g, 1.29 mmol), Pd(PPh3)2Cl2
(0.03 g, 0.039 mmol), K2CO3 (1.78 g, 12.89 mmol). The crude
product was purified by silica gel column chromatography using
the mixture of ethyl acetate and hexane (1:8, V:V) as an eluent to
obtain 5 as amorphous material with the yield of 0.59 g (75%);
Rf ¼ 0.51; 1H NMR (300 MHz, CDCl3):
d
8.66 (d, 1H, J ¼ 7.37 Hz,
H
Naphthalene), 8.64 (d, 1H, J ¼ 7.44 Hz, HNaphthalene), 8.45 (d, 1H,
2. Experimental
J ¼ 1.14 Hz, HNaphthalene), 7.73 (t, 2H, J ¼ 8.19 Hz, HNaphthalene), 7.35 (d,
2H, J ¼ 8.81 Hz, eAr), 7.20 (d, 4H, J ¼ 9.05 Hz, eAr), 7.08 (d, 2H,
J ¼ 8.78 Hz, eAr), 6.92 (d, 4H, J ¼ 9.05 Hz, eAr), 4.24e4.12 (m, 2H, e
CH2, eHaliphatic), 3.85 (s, 6H, 2 ꢂ OCH3), 2.04e1.94 (m, 1H, eCH, e
2.1. Synthesis
Materials. 4-Bromo-1,8-naphtalic anhydride, 4-bromoaniline
and 2-ethylhexylamine purchased from TCI, TPA, 1-iodo-4-
methoxybenzene, tris(4-bromophenyl)amine, 4-(diphenyla-
mino)phenylboronic acid, n-BuLi (2.5 mol Lꢁ1 in hexane), 2-
H
aliphatic), 1.48e1.30 (m, 8H, 4 ꢂ CH2, eHaliphatic), 1.01e0.90 (m, 6H,
2 ꢂ CH3, eHaliphatic). 13C NMR (75.4 MHz, CDCl3):
d 164.95, 156.78,
149.49, 147.29, 140.44, 133.20, 131.33, 130.87, 130.09, 127.87, 127.71,
127.32, 126.84, 123.12, 121.28, 119.63, 115.12, 55.72, 44.33, 38.15,
isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
bis(-
30.97, 28.96, 24.38, 23.33,14.36,10.89. IR (KBr, y
cmꢁ1): 3037 (CHar),
triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2), N-
bromosuccinimide (NBS) and 1,10-phenanthroline purchased
from Aldrich were used as received. Dimethylformamide (DMF,
Lachema) was dried by distillation over CaH2. THF was dried and
distilled over sodium and benzophenone. Dichloromethane
2955, 2927, 2856 (CHaliphatic), 1698 (C]Oanhydride), 1657, 1586, 1504,
1463 (C]Car), 1440, 1425 (OCH3), 1354, 1282 (CeN), 784, 758, 656
(CHar). Anal. Calcd. for C40H40N2O4: C, 78.40; H, 6.58; N, 4.57; O,
10.44. Found: C, 78.45; H, 6.65; N, 4.52. MS (APCIþ, 20 V), m/z: 613
([M þ H]þ)

60
D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
2.1.3. 4,40-(Di(N-(2-ethylhexyl)-1,8-naphthalimide-4-yl)phenyl)
benzenamine (6)
The vertical ionization potentials (Ip) were calculated at the
B3LYP/6-31G(d,p) level as energy difference between neutral and
radical cation species at the neutral state geometry (optimized
geometries for the adiabatic Ip).
The internal reorganization energy (li) values of model com-
pounds M4eM7 were calculated at the B3LYP/6-31G(d,p) level
according to the following equation [33]:
4,40-(Di(N-(2-ethylhexyl)-1,8-naphthalimide-4-yl)phenyl)ben-
zenamine (6) was prepared by the similar procedure as 4 using 3c
(0.28 g, 0.59 mmol), 1 (0.5 g, 1.29 mmol), (PPh3)2Cl2 (0.02 g,
0.035 mmol), K2CO3 (1.21 g, 8.79 mmol). The crude product was
purified by silica gel column chromatography using the mixture of
ethyl acetate and hexane (1:8, V:V) as an eluent to obtain 6 as a red
crystals with the yield of 0.32 g (64%). M.p. 131e132 ꢀC; Rf ¼ 0.48;
ꢀ
ꢁ
ꢀ
ꢁ
þ
Geom Mþ
ð
Þ
ð
Þ
ll
¼
l1l þl2l
¼
EMGeom M ꢁEMGeomðMÞ
þ
EMGeþomðMÞ ꢁEMþ
1H NMR (300 MHz, CDCl3):
d
8.68 (d, 4H, J ¼ 7.52 Hz, HNaphthalene),
8.46 (d, 2H, J ¼ 8.48 Hz, HNaphthalene), 7.84e7.71 (m, 4H, HNaphthalene),
7.56e7.34 (m, 11H, eAr), 7.27e7.20 (m, 1H, eAr), 4.25e4.12 (m, 4H,
2 ꢂ CH2, eHaliphatic), 2.05e1.97 (m, 2H, 2 ꢂ CH, eHaliphatic), 1.53e
1.29 (m, 16H, 8 ꢂ CH2, eHaliphatic), 1.06e0.85 (m, 12H, 4 ꢂ CH3, e
In this equation, the quantity EM for instance corresponds to the
energy of the neutral molecule (M) in the geometry of the cationic
species (Mþ).
H
aliphatic). 13C NMR (75.4 MHz, CDCl3):
d 164.79, 148.09, 147.24,
The geometries of some selected dimers formed by two identical
M4 and M5 molecules have been optimized by employing the
wB97X-D [34] functional at the 6-31G(d,p) basis set. Previous
studies have shown that this functional provides good results on
the description of weak interactions [35,36]. The solvent effect
(tetrahydrofuran in this case) was also taken into account during
the dimer geometry-optimizations by using the conductor-like
polarizable continuum model (CPCM) [37]. The electronic cou-
plings between the HOMO orbitals of two adjacent molecules were
calculated at the B3LYP/6-31G(d,p) level, according to the approach
described by Valeev et al. [38] with the corresponding matrix ele-
ments evaluated with Gaussian 09 [39]. Note that the coupling
values depend on the functional used and generally increase with
the increasing percentage of HartreeeFock exchange in the func-
tional [35]. The transfer integrals calculated by using two different
functionals are thus expected to change roughly by a constant
factor, but the trends obtained for both functionals should be the
same.
146.63, 133.27, 132.82, 131.50, 131.27, 130.24, 130.02, 129.08, 128.02,
127.02, 126.01, 124.57, 123.80, 123.28, 121.78, 44.45, 38.23, 31.05,
29.00, 24.35, 23.37, 14.39, 10.96. IR (KBr, y
cmꢁ1): 3032 (CHar), 2955,
2926, 2856 (CHaliphatic), 1701 (C]Oanhydride), 1659, 1587, 1504, 1464
(C]Car), 1353, 1284 (CeN), 783, 758, 697 (CHar). Anal. Calcd. for
C
58H57N3O4: C, 80.99; H, 6.68; N, 4.89; O, 7.44. Found: C, 81.06; H,
6.65; N, 4.92. MS (APCIþ, 20 V), m/z: 861 ([M þ H]þ)
2.1.4. 4,40,4000-(Tris(N-(2-ethylhexyl)-1,8-naphthalimide-4-yl)
phenyl)benzenamine (7)
4,40,4000-(Tris(N-(2-ethylhexyl)-1,8-naphthalimide-4-yl)phenyl)
benzenamine (7) was prepared by the similar procedure as 4 using
3d (0.23 g, 0.39 mmol), 1 (0.5 g, 1.29 mmol), (PPh3)2Cl2 (0.02 g,
0.035 mmol), K2CO3 (1.08 g, 7.81 mmol). The crude product was
purified by silica gel column chromatography using the eluent
mixture of ethyl acetate and hexane (1:8, V:V) to obtain 7 as a
yellow crystals with the yield of 0.15 g (34%). M.p. 133e134 ꢀC;
The interaction free energies of some optimized dimers were
calculated with respect to the isolated monomers at the wB97XD/
6-31G(d,p) level and were corrected for the basis set super-
position error (BSSE) by using the counterpoise correction method
of Boys and Bernardi [40].
Rf ¼ 0.36; 1H NMR (300 MHz, CDCl3):
d
8.71 (d, 3H, J ¼ 2.76 Hz,
H
Naphthalene), 8.69 (d, 3H, J ¼ 2.51 Hz, HNaphthalene), 8.47 (d, 3H,
J1 ¼ 8.64 Hz, HNaphthalene), 7.76 (t, 6H, J ¼ 7.70 Hz, HNaphthalene), 7.58
(d, 6H, J ¼ 8.68 Hz, eAr), 7.51 (d, 6H, J ¼ 8.69 Hz, eAr), 4.26-4.12 (m,
6H, 3 ꢂ CH2, eHaliphatic), 2.04e1.96 (m, 3H, 3 ꢂ CH, eHaliphatic),
1.48e1.32 (m, 24H, 12 ꢂ CH2, eHaliphatic), 1.00e0.91 (m, 18H,
6 ꢂ CH3, eHaliphatic). 13C NMR (75.4 MHz, CDCl3):
d 162.58, 145.32,
146.44, 133.32, 133.87, 132.56, 132.24, 129.76, 129.11, 127.88, 127.14,
125.98, 125.12, 123.78, 123.27, 121.88, 44.47, 38.33, 31.15, 28.95,
3. Results and discussions
24.23, 23.27, 14.32, 11.03. IR (KBr,
y
cmꢁ1): (CHar), 2956, 2927, 2857
3.1. Synthesis and characterization
(CHaliphatic), 1700 (C]Oanhydride), 1659, 1586, 1504, 1464 (C]Car),
1354,1281 (CeN), 783, 758, 696 (CHar). Anal. Calcd. for C78H78N4O6:
C, 80.24; H, 6.73; N, 4.80; O, 8.22. Found: C, 80.29; H, 6.79; N, 4.85.
MS (APCIþ, 20 V), m/z: 1168 ([M þ H]þ).
Scheme 1 shows the synthetic routes to compounds 4e7. The
first step was condensation of 4-bromo-1,8-naphtalic anhydride
(BRA) with 2-ethylhexylamine in DMF which gave 4-bromo-N-(2-
ethylhexyl)-1,8-naphthalimide (1). Compound 2b was obtained
by Ullmann coupling of 4-bromoaniline (1b) with 1-iodo-4-
methoxybenzene [41]. Compound 2c was prepared by bromina-
tion of TPA (1c) with N-bromosuccinimide. Compounds 3b-d were
obtained by the reactions of 2b, 2c and commercially available
tris(4-bromophenyl)amine (2d) with n-BuLi at ꢁ78 ꢀC and the
following quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
2.2. Computational details
The theoretical study was carried out in the frame of density
functional theory (DFT) [30] employing the B3LYP [31] functional in
conjunction with the 6-31G(d,p) basis set. The geometry optimi-
zations of the model compounds M4eM7 (containing methyl
substituents instead of the experimental ones) were performed in
absence of medium effects without symmetry constraints. All the
geometry optimizations were followed by frequency calculations in
order to verify if real minima were obtained. The geometry opti-
mizations of the cationic radical species were performed at the
unrestricted open shell level.
The spectroscopic properties of the molecules were analysed in
the frame of time dependent density functional theory method
(TDDFT) [32]. The theoretical absorption bands were obtained by
considering a band half-width of 0.2 eV at half-height (Gaussview 5
software).
dioxaborolane.
4-(40-Diphenylaminophenyl)-N-ethylhexyl-1,8-
naphthalimide (4) was synthesized by Suzuki coupling of 4-
bromo-N-(2-ethylhexyl)-1,8-naphthalimide (1) with 4-(dipheny-
lamino)phenylboronic acid (3a). Compounds 5e7, were also syn-
thesized by the Suzuki coupling reactions between compounds 3b-
d and compound 1 under nitrogen atmosphere. All the derivatives
were characterized by 1H and 13C NMR, mass spectrometries and
elemental analysis.
The target compounds (4e7) are soluble in common organic
solvents such as dichloromethane (DCM), chloroform, tetrahydro-
furan (THF), chlorobenzene and toluene.

D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
61
Scheme 1. The synthetic routes to 4e7.
3.2. Geometries and frontier orbitals
increase of the thermal stability of the derivatives. Molecules 6, 7
having two and three 1,8-naphthalimide moieties showed higher
5% weight loss temperatures than molecules 4 and 5 containing one
1,8-naphthalimide moiety.
The geometries of model compounds M4eM7, (containing
methyl groups instead of the experimental alkyl groups) are pre-
sented in Fig. 1, along with some relevant geometrical parameters.
The TPA N atom of each compound and the three appended
carbon atoms form a plane (reported hereafter as “N-plane”), due to
Compounds 4, 6 and 7 were obtained as crystalline materials.
Their first DSC heating scans revealed melting in the range of 134e
148 ꢀC. Their second DSC heating scans revealed the glass transi-
tions in the range of 45e93 ꢀC and no peaks due to crystallization
appeared. The values of Tg of molecular glasses of 4e7 are affected
by the number of naphthalimido moieties in the para positions of
triphenylamino moiety and increase in the order 4 < 6 < 7. The
lower Tg values of 4 and 5 can apparently be explained in terms of
their lower molecular weight and lower intermolecular interaction.
The DSC traces of 5 did not display transition associated with
melting even during repeated scans. The absence of melting con-
firms the amorphous character of 5. This observation can appar-
ently be attributed to the presence of methoxy groups at the
triphenylamino moiety which increase the disorder in the molecule
the p-conjugation between the phenyl groups and the lone pair of
the central N atom. In the case of M5, only one conformer is pre-
sented. Other conformers of similar energy can be assumed, with
the methoxy groups combined in different orientations. Compared
to the model compound M4, the geometry around the N(TPA) atom
in M5 has more quinoidal character, which is due to the
p-donor
effect of the methoxy groups. The same observation can be done
when comparing M5 with M6 and M7. The frontier orbitals for the
three molecules are given in Fig. 2.
3.3. Thermal properties
packing. Previously it has been suggested that CeH...
tacts can be established between the methoxy groups and the
phenyl groups [19]. CeH. short contacts of different interaction
energies may thus be present in the films of compound 5, which
might be responsible for the absence of a well-defined melting
temperature. The amorphous nature of compound 5 is also re-
flected in its enhanced solubility. Fig. 3 shows DSC thermograms of
compound 4. During the first heating scan of the sample of 4
endothermal melting signal at 134 ꢀC was observed. After re-
p short con-
The thermal properties of 4e7 were examined by DSC and TGA
under a nitrogen atmosphere. The values of glass transition tem-
peratures (Tg), melting points (Tm) and 5% weight loss temperatures
(TID) are summarized in Table 1. TGA revealed that all the target
compounds exhibit excellent thermal stabilities. Their TID range
from 429 to 483 ꢀC. These temperatures are close to 5% weight loss
temperatures of methoxy-substituted derivatives of TPA [19]. In-
crease of the number of 1,8-naphthalimide moieties leads to the
p

62
D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
Fig. 1. Optimized geometries of M4 (a), M5 (b), M6 (c), and M7 (d) model compounds, obtained at the B3LYP/6-31G(d,p) level. Some relevant bond-lengths (NeC bondlengths in TPA
ꢀ
core in A) and dihedral angles (absolute values in degrees) are shown. The CeO bond-lengths for compound M5 in the neutral state are also indicated, along with the same distances
in the cationic and anionic states (shorter and longer distances respectively in parentheses).
cooling, the following second heating scan revealed glass transition
at 47 ꢀC and no crystallization signal was observed. This observa-
tion shows that the material can exist in solid amorphous state and
can be considered as molecular glass.
moieties and the naphthalimide acceptor moieties. In the case of
compounds M6 and M7, a second CT excitation (S0 / S2) is present
in the CT band (Fig. S2 (SI)). The S0 / S1 and S0 / S2 excitations
correspond to HOMO / LUMO and HOMO / LUMO þ 1 electronic
transitions, both virtual orbitals being exclusively localized on the
naphthalimide arms (see Fig. S2 for the pictograms of the LUMO þ 1
orbitals of M6 and M7). In the case of compound M6, the oscillator
strength of S0 / S2 transition is smaller as compared to S0 / S1
(0.079 and 0.419 respectively). As for compound M7, the two CT
electronic transitions, HOMO / LUMO and HOMO / LUMO þ 1,
are degenerate, (identical oscillator strengths values of 0.373). The
increasing intensity of the charge transfer bands from 4 to 7 (Fig. 4
and S2 (SI)) is thus related to the presence of a second CT transition
of increasing intensity in the order M6 < M7.
The lowest energy absorption band of 5 is red shifted by ca.
20 nm in comparison with those of 4, 6 and 7. This effect is due to
the electron donating ability of methoxy groups in the tripheny-
lamino moiety of 5, (see also the anti-bonding contribution of the
methoxy groups in the shape of the HOMO orbital of M5, Fig. 2). By
using the onset wavelengths of the absorption bands it was
possible to roughly estimate the optical band gaps of the molecules
which ranged from 2.40 to 2.58 eV (Table 3). The optical band gap of
compound 5 containing methoxy groups in the para positions of
3.4. Optical and photophysical properties
UVeVIS absorption spectra and fluorescence spectra of dilute
solutions in nonpolar cyclohexane, of dilute solid solutions in
polystyrene (PS) matrixes, and of neat films of the investigated TPA
and naphthalimide derivatives are shown in Fig. 4 (see also Fig. S1
in the Supporting Information (SI)). The photophysical character-
istics of the compounds are summarized in Table 2.
The theoretical UVeVIS spectra for the model compounds M4e
M7 (Fig. S2 (SI)) suggest that the absorption bands peaking at about
320 nm and 350 nm (Fig. 4) correspond to local
p-p* electron
transitions of the triphenylamino moieties and naphthalimide
fragments [42], respectively. Absorption peaks which appear in the
range of 406e436 nm correspond to HOMO / LUMO electronic
transition, with HOMO and LUMO orbitals being almost exclusively
localized on the donor and acceptor moieties, respectively (Fig. 2).
These broad unstructured bands correspond consequently to the
intramolecular charge-transfer transitions between the TPA donor
Fig. 2. Sketch of frontier orbitals for the model compounds M4eM7. The encircled parts delimit the TPA moiety. The frontier orbitals for the three molecules (Fig. 2) look very
similar for all compounds, being localized almost entirely on the TPA core for the HOMO or naphthalimide moiety for the LUMO. The presence of the -donor methoxy groups in M5
p
is expected to destabilize the HOMO orbital [19] as compared to M4, which can also be deduced from the anti-bonding contribution of the methoxy groups in the shape of the
HOMO orbital of M5.

D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
63
Table 1
Thermal characteristics of compounds 4e7.
Compound
Tg [ꢀC] (2nd heating)
Tm [ꢀC]
TID [ꢀC]
4
5
6
7
47
45
76
84
134
e
437
429
448
483
136
141
Tm is melting point, Tg is glass transition temperature (both estimated by DSC), TID is
5% weight loss temperature estimated by TGA at a heating rate of 10 ꢀC/min in N2
atmosphere.
triphenylamino moiety is evaluated at 2.40 eV, which is by 0.15 eV
lower than that of the corresponding derivative (4) containing no
methoxy substituents (2.55 eV).
The absorption spectra of neat films of 4e7 and those of mo-
lecular dispersions in PS matrixes are presented in Fig. S1 (SI). The
low energy absorption bands of the films are broader and slightly
red shifted with respect to those of the dilute solutions. It is pre-
sumed that the red shifts of the absorption spectra of films as
compared to those of solutions may be due to smaller dihedral
angles in solid state, thus giving rise to more efficient
tion and decreased HOMO-LUMO gap, but can also be due to more
efficient -stacking and stronger intermolecular interactions in the
films than in dilute solutions [43]. Some degree of aggregation in
the ground state may also be suspected to influence the absorption
spectra of the films of these compounds. As a means to confirm this
assumption, we have optimized dimers of different geometries for
compounds 4 and 5 (Fig. 5).
p-conjuga-
p
Fig. 4. Absorption (dashed thin line) and normalized fluorescence spectra of the
10ꢁ6 M solutions of compounds 4e7 in cyclohexane (thick, solid line), of neat films
(dashed grey line) and of molecular dispersions in polystyrene matrix at 0.25 wt%
concentration (dotted line). Fluorescence quantum yields are indicated.
The theoretical UVeVIS spectra corresponding to some of these
dimers are presented in Figs. S3 and S4 (SI), showing the appear-
ance of new transitions in the proximity of the HOMO / LUMO
transition. Depending on the aggregation pattern, a blue or red shift
of roughly 20e30 nm (0.12 eV) can be observed between the band
maxima of dimer and monomer species (Figs. S3 and S4 (SI)), which
seem to be due to the contribution of these new slightly higher
energy transitions.
Fluorescence spectra of the dilute solutions of the synthesized
compounds in nonpolar cyclohexane (10ꢁ6 mol Lꢁ1), in polystyrene
(PS) and of neat films are shown in Fig. 4. Fluorescence spectra of
compounds 4, 6, 7 dissolved in nonpolar cyclohexane exhibit effi-
cient molecular emission with fluorescence quantum yields of
0.63e0.78. The spectra show vibronic progression and pronounced
Stokes-shift of ca. 55 nm. The Stokes-shift of compound 5 is of
72 nm. This observation indicates more polar nature of the excited
states, but can also be due to the additional relaxations in the Ce
OMe bonds in compound 5 as compared to 4. These bonds exhibit
anti-bonding and non-bonding contributions at the HOMO and
LUMO orbitals respectively (Fig. 2). After the HOMO / LUMO
transition, the anti-bonding character vanishes, resulting in
decreased CeO bond-lengths and non-negligible contribution in
the relaxation energy (see also CeOMe bond-lengths in the neutral,
cationic states, Fig. 1) [44]. The charge-transfer origin of the lowest
transitions is revealed by the spectral properties of the compounds
dissolved in solvents of various polarities. As it is evidenced in Fig. 6
the studied derivatives of TPA and 1,8-naphthalimide exhibit pro-
nounced positive solvatochromic behaviour.
In the case of the solution of compound 5 in acetonitrile, the FL
band is expected to be at about 850e950 nm and is undetectable.
Compound 4 is emitting at 470 nm in nonpolar cyclohexane and
shows remarkable batochromatic shift of the fluorescence in THF
(of 134 nm with respect to cyclohexane) and even larger red shift
(of 258 nm with respect to cyclohexane) in acetonitrile. The fluo-
rescence quantum yield of 4 is reduced down to 0.54 in THF and to
0.01 in acetonitrile. At the same time the absorbance spectra are
only weakly shifted (by ca. 6 nm). Such solvatochromic behaviour is
typical for compounds with significantly enhanced dipole moment
in the excited state. Solvation effect is even more pronounced for
compound 5 possessing polar methoxy groups in the donor tri-
phenylamino moiety. The batochromatic shift of the fluorescence
peak in THF solution is significantly larger (of 204 nm with respect
to cyclohexane), while the fluorescence quantum yield is found to
be below 0.01 (see Fig. 6(b)). The solvation-shift of fluorescence
bands decreases with increase of the number of naphthalimide side
arms, which is in line with reduced polarity of the excited state.
The dependence of the optical properties of the naphthalimide
derivatives on the solvent polarity has been reported [45e48]. The
fluorescence quenching in the most polar solvents was accounted
for exciplex or cluster formation [45]. Solvatochromic behaviour of
TPA and naphtahalimide derivatives seems to be different. It is very
weak in absorbance, while it is much stronger in fluorescence. Such
Fig. 3. DSC thermograms of compound 4 (scan rate of 10 ꢀC/min, N2 atmosphere).

64
D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
Table 2
Photophysical characteristics of the dilute cyclohexane solutions, neat films and 0.25 wt% solid solutions in PS matrixes of compounds 4e7.
Compound
Solution in cyclohexane
Neat film
In PS
a
d
d
e
a
a
labs (nm)
lmF axc
h
s
sR
sNR
lteor
labs
lmF ax c
h
savg
(ns)
labs
lmF ax c
h
savg
(ns)
(εb, L molꢁ1cmꢁ1
)
(nm)
(ns)
(ns)
(ns)
(nm)
(nm)
(nm)
(nm)
(nm)
4
5
6
7
413 (13,821)
434 (12,459)
410 (24,658)
407 (32,025)
470
506
465
461
0.78
0.68
0.68
0.63
3.2
4.7
2.6
2.2
4.1
6.9
3.8
3.5
14.5
14.7
8.1
488
532
482
470
425
448
426
425
557
630
569
570
0.11
0.10
0.16
0.23
13.1
10.8
9.2
430
452
426
426
501
541
498
493
0.58
0.66
0.64
0.58
5
7.2
2.8
3.8
5.9
13.3
a
Peak wavelength of absorption bands.
Molar extinction coefficient.
Wavelength at fluorescence band maximum.
Radiative and non-radiative decay time constants calculated as
The theoretical S0 / S1 values calculated at the TDB3LYP/6-31G(d,p) level.
b
c
d
e
s
/h
and
s/(1ꢁ
h
), respectively.
behaviour is typical for the compounds with enhanced polarity in
the excited state, what is in line with the DFT calculations [49].
The largest fluorescence quantum yield of 0.78 is found for the
monosubstituted derivative of TPA (4) in nonpolar surrounding
and it is steadily decreasing for compounds 6 and 7 down to 0.68
and 0.63, respectively. This is in contradiction with enhanced
oscillator strength of the lowest transition of the compounds
possessing one, two and three side arms (see Fig. 4) and thus less
pronounced charge transfer character of the lowest excited states.
Indeed, the radiative decay time systematically decreases in the
order 3.2 ns, 2.6 ns and 2.2 ns for compounds 4, 6, 7, respectively.
However increase in the number of the singly bonded naph-
thalimide arms results in almost threefold increase in the rate of
nonradiative recombination. Nonradiative decay time decreases in
the order 14.5 ns, 8.1 ns and 5.6 ns for compounds 4, 6, 7,
respectively. Incorporation of the polar methoxy groups for com-
pound 5 results in more expressed charge transfer character of the
excited states and, thus enhanced excited state lifetime of 4.7 ns
and decreased radiative decay rate with 6.9 ns radiative decay
constant. Meanwhile the nonradiative decay constant is almost the
same for both mono 1,8-naphthalimide-4-yl substituted com-
pounds 4 and 5.
Fluorescence spectra of the compounds molecularly dispersed
in a rigid PS matrix are similar to those observed for the dilute
solutions in nonpolar solvents. A systematic red shift of fluores-
cence spectra of about 20e30 nm is due to slightly different
polarity of the media. Since in the case of solid dispersion the
intramolecular motion is suppressed, few peculiarities of the
emission are evident. Fluorescence spectra are broader and non-
structured, the emission efficiency is slightly decreased and the
fluorescence transients are non-exponential. This observation can
be explained by “freezing” of the initial molecular geometry in
the ground state [50,51]. However, the impact of restriction of
intramolecular twisting is not strong. The largest decrease in
fluorescence quantum yield (from 0.78 to 0.58) is observed for
compound 4.
for enhanced oscillator strength and reduced intramolecular
vibrations.
3.5. Electrochemical and photoelectrical properties
In order to gain information on the charge injection capabilities,
the electrochemical behaviour of compounds 4e7 were estimated
by cyclic voltammetry at room temperature in dichloromethane
solutions, using a three electrode system i.e. platinum rod as a
counter electrode, glassy carbon as working electrode and Ag/
AgNO3 as the reference electrode. The measurements were carried
out using 0.1
M tetrabutylammonium hexafluorophosphate
(TBAHFP6) as supporting electrolyte, scan rate 50 mV sꢁ1. Table 3
outlines the onset oxidation and reduction potentials of 4e7.
Fig. 7 shows cyclic voltammograms (CVs) of the compounds.
Compounds 4e7 displayed one reversible reduction peak, which
might be due to the electron withdrawing nature of naphthalimide
moieties, as well as one reversible oxidation peak, which might be
attributed to the electron donating nature of TPA segment [54e56],
suggesting that these compounds possess excellent electro-
chemical stability. The reduction current of 6 and 7 is obviously
higher than that of the oxidation current in the cyclic voltammo-
grams, suggesting that 6 and 7 can be classified as electron-
accepting materials. The oxidative processes of 4e7 started at
0.54, 0.25, 0.57 and 0.58 V, respectively, and the reduction pro-
cesses started at ꢁ1.83, e184, e1.84 and ꢁ1.80 V, respectively. The
solid state ionization potentials (IPCV) and electron affinities (EASS
)
were also estimated by using the empirical formulas
ox
red
IPCV ¼ ðE
þ 4:8Þ [eV] and EASS ¼ ðE
þ 4:8Þ [57] (Table 3).
onset
onset
In the anodic scan regime of the cyclic voltammogram of 5,
reversible oxidation peak at 0.49 V is observed which can be
ascribed to oxidation of the electron-rich nitrogen atom in the TPA
core. The irreversible oxidation peak at 1.14 V can be attributed to
oxidation of methoxy groups, causing radical recombination and
formation of a quinoid structure [58] and this observation is
attributed to two-electron stepwise oxidation process.
Intramolecular interaction seems to have pronounced impact on
the optical properties, particularly on emission properties of the
solid films of compounds 4e7. Fluorescence spectra are broad,
unstructured and significantly red shifted. The Stokes shift is of
132 nm, 143 nm and 145 nm, and is weakly increasing with the
number of naphthalimide arms, for compounds 4, 6 and 7 respec-
tively. It is strongly dependent on the polarity of the excited state
and increases up to 182 nm for compound 5, possessing polar
methoxy groups. Such behaviour can be rationalized in the frame of
the model of self-trapped excitons [52,53]. Non-exponential fluo-
rescence decay and decreased fluorescence quantum yields indi-
cate on the impact of exciton migration. Interestingly, the
fluorescence quantum yield increases in the order 0.11, 0.16 and
0.23 for compounds 4, 6 and 7, respectively. This can be accounted
An important characteristic of electronically active compounds
intended for the application in optoelectronic devices is ionization
potential (IP), which characterizes the electron releasing work
under illumination. Except for the compound 5, the IPCV values
deduced from the onset redox potentials range in a very small
window (5.34e5.38 eV). The values of the ionization potentials
(IPEP) of the solid samples of compounds 4e7 were also estimated
by electron photoemission spectrometry (Fig. 8) and the results are
collected in Table 3.
The IPEP of amorphous layers of the compounds 4, 6, and 7 range
from 5.57 to 6.01 eV, indicating good air stability for these mate-
rials. Compounds 6 and 7 with two and three 1,8-naphthalimide
moieties, respectively, demonstrated a little higher Ip values with
respect to those of 4 and 5 with one 1,8-naphthalimide moiety.

D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
65
Table 3
Electrochemical, photoelectrical, and theoretical electronic characteristics of 4e7.
3.6. Charge transport properties
Egopt
(eV)c (eV)d
εH,
εL,
(eV)d
EI,
ox
red
Charge-transporting properties of the synthesized compounds
were estimated by xerographic time-of-flight technique. Fig. 9
shows electric field dependencies of hole and electron drift mo-
bilities for the molecular glasses of 4e7 and for the solid solution
(50%) of compound 7 in bisphenol Z polycarbonate ((PC-Z), 1:1).
Charge drift mobilities values are summarized in Table 4.
E
vs
E
vs IPCV
,
EASS
,
onset
onset
Fc/Va
Fc/Va
(eV)b (eV)b
(eV)e
4
5
6
7
0.54
0.25
0.57
0.58
ꢁ1.83
ꢁ1.84
ꢁ1.84
ꢁ1.80
5.34
5.05
5.37
5.38
ꢁ2.97 2.55
ꢁ5.28 ꢁ2.34 5.79 (6.39)
ꢁ4.96 ꢁ2.26 5.57 (5.97)
ꢁ5.48 ꢁ2.49 5.93 (6.45)
ꢁ5.68 ꢁ2.60 6.01 (6.51)
ꢁ2.96 2.40
ꢁ2.96 2.57
ꢁ3.00 2.58
a
ox
red
E
and E
are measured vs. ferrocene/ferrocenium.
All the synthesized compounds are capable of transporting both
holes and electrons. Their amorphous layers demonstrated hole
drift mobility (mh) values reaching 10ꢁ7e10ꢁ8 cm2/V s at an electric
field of 1 ꢂ 106 V/cm at room temperature and electron mobilities
reaching 10ꢁ4e10ꢁ5 cm2/V s at the same electric field. Compound 5
showed the best ambipolar charge-transporting properties. Its
amorphous layers showed the highest values of both hole and
electron mobilities (Table 4). We did not manage to estimate
electron mobilities in the layers of 7. In order to confirm the ability
of the compound to transport negative charge we estimated elec-
tron mobilities in the 50 wt% solid solutions of 7 in bisphenol Z. The
dU/dt transients of electrons for the layer of 5 are shown in poly-
carbonate (PC-Z) and observed relatively high mobilities reaching
4.2 ꢂ 10ꢁ5 cm2/V s at an electric field 1 ꢂ 106 V/cm (Fig. 10). It
exhibited dispersive electron transport, which, along with the
strong electric-field mobility dependence may suggest trap-
dominant charge transport in this material. The electron transit
times (tt) needed for the estimation of electron mobilities were
established from intersection points of two asymptotes from the
double logarithmic plots. Dispersive charge transport was also
observed for the other compounds studied in this work.
It is interesting to point out that, except for compound 5, the
electron mobilities are 2e3 orders of magnitude higher as
compared to hole mobilities. Related to this observation, the
ambipolar CT character of 5 is intriguing. Moreover, the larger hole
mobility of compound 5 as compared to that of the parent com-
pound 4 (by w3 orders of magnitude) seems quite different form
the results reported by Borsenberger et al. [59] and Maldonaldo
et al. [17], in which, the lower hole mobilities for the substituted
compounds had been related to the role of the increase (and the
disorder) of the dipole moments induced by the substitutions. The
calculated dipole moment for compounds 4 and 5 are 6.2 D and 6.5
onset
onset
b
Calculated with reference to ferrocene (4.8 eV). Ionization potentials and elec-
ox
tron
affinities
estimated
according
to
IPCV ¼ ðE
þ 4:8Þ ½eVꢃ.
onset
red
ox
red
EASS ¼ ꢁðE
þ 4:8Þ ½eVꢃ (where, E
and E
are the onset reduction and
of Fc/Fcþ measured in DCM
onset
onset
ox
onset
red
þ
oxidation potentials versus the Fc/Fc . E
and E
onset
onset
solution containing 0.1 M TBAHFP6 was 0.204 V vs. ferrocene/ferrocenium).
c
The optical band gap estimated from the onset wavelength of optical absorption
according to the formula: Eg ¼ 1240/ledge, in which the ledge is the onset value of
absorption spectrum in long wave direction.
d
HOMO and LUMO energies corresponding to the isolated model compounds
M4eM7 calculated at the B3LYP/6-31G(d,p) level.
e
Established from electron photoemission in air spectra. The values in paren-
theses correspond to the adiabatic IP of isolated model compounds M4eM7,
calculated at the B3LYP/6-31G(d,p) level.
While the range of the electrochemical IP values is smaller than
those estimated by the photoemission spectrometry, both methods
provide similar trends, with the IP values for 5 being smaller by
0.2e0.3 eV as compared to 4.
The trend in the theoretical IP values (Table 3) is in agreement
with the experimental ones. In the frame of Koopmans’ theorem
(relating in this case the first Ip values to the HOMO energies) these
trends can be explained by the similar evolution of the HOMO
energies in these compounds (Table 3). The evolution in the HOMO
energies in the order 4 > 6 > 7 is related to the electron withdrawing
nature of 1,8-naphthalimide arms, whereas the small experimental
and theoretical ranges of the IP values of compounds 4, 6, and 7 are
due to the similar nature of their HOMO orbitals (Fig. 2). As for
compound 5, the higher HOMO energy (and smaller IP) as
compared to that of 4 (by roughly 0.2e0.3 eV) is due to the strong
p
-donor effect of the methoxy groups, which can be observed in the
anti-bonding contribution of this group in the HOMO orbital
(Fig. 2).
Fig. 5. Dimers of different geometries optimized at the wB97XD/6-31G(d,p) level: a) and b) two different views of the same dimer for compound 4, reported here as Dim-M4-HH
(head-to-head); c) dimer for compound 5, reported here as Dim-M5-HH (head-to-head); d) dimer for compound 5, reported here as Dim-M5-HT (head-to-tail); e) dimer for
compound 5, reported here as Dim-M5-DA (donoreacceptor).

66
D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
the reorganization energy. This last parameter is the sum of two
terms: (i) ls, containing the contribution from the medium polari-
zation energy. (ii) li, representing the energetic effort due to the
intra-molecular geometric relaxations related to the charge trans-
fer between two adjacent molecules. In the following, some of
these molecular parameters will be discussed, while the influence
of morphological and other structural parameters will be ignored.
Based on equation (1), high charge-transfer rate-constants (kCT
)
need minimal values of the reorganization energy ( ) and large
l
values of the electronic coupling parameter (t). In order to compare
compounds 4e7, we have calculated some of these parameters and
collected them in Table 5.
The hole reorganization energies ðlhi Þ of model compounds M4,
M6, and M7 are almost identical, which is due to the identical
spatial distribution of the HOMO orbitals in these compounds
(Fig. 2) The electron reorganization energies ðlei Þ are also supposed
to remain similar for M4, M6, and M7 despite the increasing
number of naphthalimide arms. Indeed, the coupling between the
arms in M7 (w0.023 eV) is much smaller than the reorganization
energy of a single naphthalimide arm (0.399 eV for M4, Table 5),
suggesting localization of the LUMO orbital in M7 (and M6). Con-
cerning the compound M5, both values are larger than for the other
compounds. The much larger lhi value for M5 as compared to M4
could be due to the additional contribution inherent to the relax-
ation of the CeOMe bonds. Indeed, the local anti-bonding contri-
butions in the HOMO orbital at the CeOMe bonds (Fig. 2) are
released in the cationic state, as indicated by the bond-length
Fig. 6. Absorption and FL spectra of compounds 4 (a) and 5 (b) in dilute cyclohexane
(black solid line), toluene (red dashed line), THF (blue dotted line), and acetonitrile
(green short dashed line) solutions (10ꢁ6 M, lex ¼ 450 nm). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
reduction of w0.02 A. As for lei , much smaller difference between
ꢀ
D respectively, which, in line with the results of Borsenberger et al.
[59], suggests negligible role of this factor.
M5 and M4 is found, which is mostly due to the space separation
between the naphthalimide groups and the methoxy substituents
in M5.
In order to obtain some more insight in these questions, we
present a qualitative discussion of charge-transfer rate-constants
(kCT) based on the Marcus theory [20e23]. In the frame of Marcus’
“hopping” mechanism, the rate-constant of a charge-transfer re-
action between two adjacent molecules in amorphous materials
can be calculated by means of the following equation:
However, the main observation in Table 5 is that lei values are
more important than the hole analogues, suggesting that this
parameter cannot account for the larger electron mobilities found
experimentally. Similarly, both lei and lih values for M5 are larger
than for the other compounds, which is in opposite agreement with
the largest mobility values found for this compound. Consequently,
in order to obtain better understanding of the experimental ob-
servations, we focus on the influence of the electronic coupling
parameter (t). The calculation of the electronic couplings is based
on the assumption that, despite the irregular packing between
adjacent molecules in the amorphous materials, dimers of different
geometries can be established. The t values calculated for two
"
#
4
p2
1
ꢁð
D
4kBlT
Gꢀ þ
l
Þ2
2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
kCT
¼
t
exp
(1)
h
4
plkBT
In this equation, t is the electronic coupling between two
adjacent molecules,
reaction (approximated to zero in the case of charge hopping be-
tween identical molecules in the absence of electric field), and is
D
Gꢀ is the free energy of the charge-transfer
l
Fig. 7. Cyclic voltammograms of 4e7 (10ꢁ5 M solutions, scan rate of 50 mV sꢁ1 vs Ag/Agþ) in 0.1 M solution of TBAHFP6 in CH2Cl2.

D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
67
Table 4
Hole and electron mobility data for the layers of compounds 4e7 and for the solid
solution of 7 in PC-Z (50%).
Transport material,
host polymer
Electron mobility
Hole mobility
a ]
, [cm1/2 Vꢁ1/2
me, [cm2/V s]a
mh, [cm2/V s]a
Alþ4
2.3 ꢂ 10ꢁ4
7.5 ꢂ 10ꢁ4
4.7 ꢂ 10ꢁ5
e
3.7 ꢂ 10ꢁ7
1.1 ꢂ 10ꢁ4
4 ꢂ 10ꢁ7
2 ꢂ 10-8b
e
e
Alþ5
0.013c, 0.014d
Alþ6
w0.0068c, 0.0097d
w0.008d
Alþ7
Alþ[7þ(PC-Z), 1:1]
4.2 ꢂ 10ꢁ5
w0.0025c
a
Hole and electron drift mobility values at electric field 1 ꢂ 106 V/cm.
Hole drift mobility value at electric field 3.6 ꢂ 105 V/cm.
Pool-Frenkel parameter for electrons.
b
c
d
Pool-Frenkel parameter for holes.
suggesting much more sensitivity of the HOMOeHOMO overlap to
the displacements in this direction. A similar conclusion has been
reached also by Wetzelaer et al. [61] by calculating the LUMOe
LUMO and HOMOeHOMO couplings for different reciprocal posi-
tions between molecules containing naphthalene diimide and
bithiophene moieties. As for the transversal displacements (along
the short axes of Dim-M4-HH), both HOMO and LUMO orbitals
exhibit nodal surfaces, making the corresponding overlaps very
sensitive to the displacements. However, the transversal displace-
ments seem to be mostly limited by the steric hindrance between
the long alkyl chains of naphthalimide groups on the one hand, and
between the TPA phenyl groups on the other hand. One can thus
suppose that, due to this kind of displacements, the overall varia-
tion in the orbital overlap should be larger for the HOMO orbitals.
Larger disorder in the electronic couplings should be than expected
for the hole transfer, which, in terms of disorder models [62] would
result in smaller hole mobilities.
Fig. 8. Electron photoemission spectra of the neat layers of 4e7 recorded in air.
special dimers of compounds 4 and 5 (Dim-M4-HH and Dim-M4-
HH, Fig. 5(a)e(c)), are given in Table 6 (lines 1e2).
In both cases, simultaneous donor-to-donor and acceptor-to-
acceptor packing is assumed, which is similar to the packing re-
ported in polymers containing naphthalimide and bi-thiophene
alternated cores) [60]. The results given in Table 6 indicate larger
t values for the holes, suggesting faster hole transfer, which, again,
cannot account for the higher electron mobility found
experimentally.
However, the static description provided so far considers only
idealized dimer geometries, ignoring the displacements between
molecules in the real materials. We suspect that this last factor may
constitute a possible explanation for the above disagreements, as it
has been pointed out in a recent similar study [61]. In order to give
more insight in this respect, we firstly consider parallel displace-
ments between adjacent molecules in the model dimer Dim-M4-
HH shown in Fig. 5(b). A detailed analysis of the LUMO orbital of
compound 4 (Fig. 2), indicates absence of nodal planes along the
long axes, suggesting that the LUMOeLUMO overlap would be very
little affected by small displacements between adjacent molecules
in this direction. Similar analysis with respect to the HOMO orbital
indicates the presence of nodes in the longitudinal direction,
As for compound 5, additional coupling schemes can be sup-
posed, similar to those proposed previously [19]. Briefly, due to the
presence of the methoxy groups, different types of CeH.p(Ph) or
CeH.N,O hydrogen-bond interactions between TPA-OMe moieties
of adjacent molecules have been assumed [19], resulting in
strengthening of inter-molecular interactions. Stronger interactions
between the TPA-OMe groups can be thus expected in the case of
compound 5, as suggested by the stronger dissociation energy
found for Dim-M5-HH as compared to Dim-M4-HH
(15.7 kcal molꢁ1 and 13.9 kcal molꢁ1, Table 6). Consequently, in
the case of Dim-M5-HH, reduced geometry disorder and reduced
disorder in the HOMOeHOMO coupling would be expected as
compared to Dim-M4-HH, resulting in enhanced hole mobility.
Fig. 9. Electric field dependencies of hole and electron drift mobilities for the layers of
compounds 4e7.
Fig. 10. XTOF transients of electrons for a neat film of 5.

68
D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
Table 5
the hole and electron transport in these materials. Interestingly,
packing patterns between naphthalimide moieties similar to those
presented in Fig. 5(a)e(c) have been reported in polymers con-
taining naphthalimide and bi-thiophene alternated cores [61]
which comforts the validity of the above analysis.
Hole- and electron intramolecular reorganization energies for the model com-
pounds M4eM7 computed at the B3LYP/6-31G(d,p) level.
Compound
lhi ðeVÞ
lei ðeVÞ
M4
M5
M6
M7
0.139
0.258
0.138
0.136
0.399
0.422
a
a
4. Conclusions
a
Similar reorganization energy values as compared to M4 can be assumed for M6
and M7. See text for details.
A series of ambipolar materials containing electron accepting
1,8-naphthalimide moieties and electron-donating triphenylamino
groups were obtained via Suzuki cross-coupling reaction. The
synthesized compounds exhibit high thermal stability. Their 5%
weight loss temperatures range from 429 to 483 ꢀC. All the syn-
thesized compounds are capable of glass formation. Their glass
transition temperatures are in the range from 45 to 84 ꢀC. The dilute
solutions of the compounds in nonpolar solvents and in rigid
polystyrene solutions show fluorescence quantum yields from 0.58
to 0.78, while emission yields of the neat films are in the range of
0.10e0.23. Due to pronounced electron donoreacceptor character,
in polar solvents the compounds show dramatic solvatochromic
red shifts of fluorescence up to 250 nm, with significant reduction
of the emission yield. Compound 5, possessing polar methoxy
groups in the donor triphenylamino moiety shows the strongest
solvatochromic effect.
Results for other dimer geometries, based on M4 and M5, are also
shown in Table 6. One can observe that the HOMOeHOMO cou-
plings are generally larger for dimers containing no OMe groups,
which seem to contradict the experimental trend of hole mobilities
between 4 and 5. However, systematically stronger interactions are
found in the case of OMe-containing dimers, again suggesting that
the disorder in HOMOeHOMO couplings should be strongly
reduced for this compound as compared to 4. Concluding this
analysis, we suggest that the enhanced hole mobility and the
ambipolar CT character of 5 result from the stronger interactions
between adjacent molecules, making possible to maintain the hole
transport rate at a comparable level as the electron one.
Admittedly, this semi-quantitative analysis can not provide
thorough explanations, but aims only to point on some possible
mechanisms allowing to better understand the difference between
The ionization potentials of the layers of the synthesized com-
pounds range from 5.57 to 6.01 eV. The lowest ionization potentials
and the best charge-transporting properties were observed for the
amorphous layers of 4-(40-(di-(400-methoxyphenyl)amino)phenyl)-
N-(2-ethylhexyl)-1,8-naphthalimide (5). Electron mobilities of
7.5 ꢂ 10ꢁ4 cm2 Vꢁ1 sꢁ1 and hole mobilities of 1.1 ꢂ10ꢁ4 cm2 Vꢁ1 sꢁ1
were recorded for this compound at an electric field of 1 ꢂ 106 V/
cm. Compounds containing no methoxy groups exhibit electron
mobilities by 2e3 order of magnitude higher than the hole mo-
bilities. The comparison between compounds 4 and 5 shows that
the methoxy groups increase the hole mobility by w3 orders of
magnitude. Theoretical results in the frame of hopping Marcus
theory indicate that a static analysis, based on the comparison of
the intramolecular reorganization energies and the electronic
coupling parameter, predicts charge mobilities in opposite agree-
ment with the experimental result. A semi-quantitative discussion
of the positional disorder in some selected dimers from these
molecules suggests that the larger electron mobility for the com-
pounds containing no methoxy groups may be due to the smaller
influence of the disorder between adjacent molecules on the
LUMOeLUMO overlaps, as compared to the HOMOeHOMO over-
laps. In the case of 5, the presence of methoxy groups is shown to
allow stronger interactions between adjacent molecules, making
possible to maintain a good level of hole transfer for this com-
pound. Finally, our results suggest that theoretical static analysis
ignoring the effect of the disorder should be employed with
precautions.
Table 6
Electronic couplings (t) and dissociation energies (ED, corrected for BSSE and ZPE) for
some selected dimers of model compounds M4 and M5. The electronic couplings are
calculated at the B3LYP/6-31G(d,p)/CPCM/wB97XD/6-31G(d,p) level. The dissocia-
tion energies are calculated at the CPCM/wB97XD/6-31G(d,p) level. Negative
dissociation energies mean repulsive state (higher level calculations would slightly
modify these values, however, the trends are expected to remain unchanged).
Model dimer
t
h (eV)
t
e (eV)
ED (kcal molꢁ1
)
Dim-M4-HH
0.055 ꢁ0.032
0.045 ꢁ0.027
13.9
Dim-M5-HH
Dim-M4-HT
15.7
8.5
ꢁ0.007
0.052
Dim-M5-HT
Dim-M4-DA
ꢁ0.028 ꢁ0.001
0.014 ꢁ0.001
10.3
3.4
Acknowledgement
This research was funded by the European Social Fund under the
Global Grant measure. Habil. Dr. V. Gaidelis from the Department
of Solid State electronics, Vilnius University is thanked for the help
in the measurements of ionization potentials. A. Sakalyte from the
Department d’Enginyeria Química, Universitat Rovira i Virgili is
thanked for the help in TGA measurements. The computer center of
the University of Cergy-Pontoise, France is thanked for providing
the computing resources. Dr. V. Coropceanu from Georgia Institute
of Technology is thanked for stimulating discussions.
Dim-M5-DA
0.024 ꢁ0.004
3.9
1.8
Dim-M4-DD
Dim-M5-DD
0.048
0.001
0.001 ꢁ1.9
0.001

D. Gudeika et al. / Dyes and Pigments 106 (2014) 58e70
69
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