Electron-transporting naphthalimide-substituted derivatives of fluorene

Article abstract of DOI:10.1016/j.dyepig.2013.07.016

Naphthalimide-substituted derivatives of fluorene were synthesized by Suzuki-Miyaura coupling reactions. Most of the synthesized materials were found to constitute glasses with glass transition temperatures ranging from 30 to 76 C as characterized by differential scanning calorimetry. Their initial weight loss temperatures range from 315 to 466 C. Dilute solutions of the naphthalimide-substituted derivatives of fluorene in nonpolar solvents were found to emit in the blue region with high quantum yield ranging from 0.47 to 0.69, while in the solid state fluorescence quantum yields were found to be in the range of 0.06-0.25. Fluorenes possessing two naphthalimide moieties exhibited higher values of the fluorescence quantum yield due to enhanced oscillator strength of the transition. The derivatives of fluorene containing naphthalimide moieties exhibited pronounced positive solvatochromic behaviour in polar solvents indicating on charge-transfer character of the lowest excited states. Fluorescence red shift in dimethylsulfoxide is found of 94 nm, while the fluorescence quantum yield is increasing with polarity from 0.47 in cyclohexane to 0.84 in dimethylsulfoxide. Such untypical enhancement of fluorescence quantum yield is accounted to reduced intersystem crossing in polar solvents, which is proved by almost 10 fold increase in the values of nonradiative decay rate for compounds dissolved in polar solvents. Time-of-flight electron drift mobilities of the layer of 2,7-di((N-octyl-1,8-naphthalimide)-4-yl)-9,9-dioctyl- 9H-fluorene in air at 25 C appproach 1.2 × 10^-3 cm² V^-1 s^-1 at an eletric field of 6.4 × 10⁵ Vcm^-1.

Full text of DOI:10.1016/j.dyepig.2013.07.016

Dyes and Pigments 99 (2013) 895e902
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Electron-transporting naphthalimide-substituted derivatives
of fluorene
,
a
a
D. Gudeika a, R.R. Reghu a, J.V. Grazulevicius a , G. Buika , J. Simokaitiene ,
*
A. Miasojedovas b, S. Jursenas b, V. Jankauskas c
a Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania
b Institute of Applied Research, Vilnius University, Sauletekio aleja 9, LT-10222 Vilnius, Lithuania
c Department of Solid State Electronics, 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:
Naphthalimide-substituted derivatives of fluorene were synthesized by SuzukieMiyaura coupling re-
actions. Most of the synthesized materials were found to constitute glasses with glass transition tem-
peratures ranging from 30 to 76 ꢀC as characterized by differential scanning calorimetry. Their initial
weight loss temperatures range from 315 to 466 ꢀC. Dilute solutions of the naphthalimide-substituted
derivatives of fluorene in nonpolar solvents were found to emit in the blue region with high quantum
yield ranging from 0.47 to 0.69, while in the solid state fluorescence quantum yields were found to be in
the range of 0.06e0.25. Fluorenes possessing two naphthalimide moieties exhibited higher values of the
fluorescence quantum yield due to enhanced oscillator strength of the transition. The derivatives of
fluorene containing naphthalimide moieties exhibited pronounced positive solvatochromic behaviour in
polar solvents indicating on charge-transfer character of the lowest excited states. Fluorescence red shift
in dimethylsulfoxide is found of 94 nm, while the fluorescence quantum yield is increasing with polarity
from 0.47 in cyclohexane to 0.84 in dimethylsulfoxide. Such untypical enhancement of fluorescence
quantum yield is accounted to reduced intersystem crossing in polar solvents, which is proved by almost
10 fold increase in the values of nonradiative decay rate for compounds dissolved in polar solvents. Time-
of-flight electron drift mobilities of the layer of 2,7-di((N-octyl-1,8-naphthalimide)-4-yl)-9,9-dioctyl-9H-
Received 8 May 2013
Received in revised form
12 July 2013
Accepted 16 July 2013
Available online 25 July 2013
Keywords:
Naphthalimide
Fluorene
Electron-transporting
Glass formation
Photophysical properties
Fluorescence quantum yield
fluorene in air at 25 ꢀC appproach 1.2 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 at an eletric field of 6.4 ꢁ 105 Vcmꢂ1
.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Naphthalimides have been of interest in organic electronics
research for many years as n-type semiconductors due to their
capability to be reduced into stable radical anions [3,9]. The de-
rivatives of naphthalimide are also known as photochromic mate-
rials [10,11]. Moreover wide possibility of functionalization via
imide moiety and carbocyclic core using non chromophore or
chromophore substituents in order to improve the processability or
electronic properties have also driven research interest extensively
on naphthalimides and their derivatives [9,12]. The substitutions
may change glass transition temperatures, shift photoluminescence
(PL) wavelengths, change PL quantum yields, shift redox potentials
and/or improve photoelectrical characteristics [13e15]. It is known
that absorption, emission, electrochemical and/or photoelectrical
properties of naphthalimides and their analogues can be varied by
Low-molar-mass organic semiconductors with electron-
transporting property have received great deal of research inter-
est in recent years due to their potential applications in the field of
(opto)electronics [1e3]. Solution processable amorphous molecular
materials with appealing photoelectrical and operational stability
are particularly interesting in the standpoint of possible applica-
tions in cost-effective large-area electronic devices [4e6]. However,
practical devices based on organic semiconductors possessing air-
stable electron-transporting properties are limited due to the lack
of ambient stability of charge carriers, i.e. electrons [7,8]. Therefore,
the development of soluble electron-transporting materials that
can operate in air is of considerable interest for the device
applications.
extending the p-conjugation at the carbocyclic core using electron-
donor chromophores which affect the electronic structure of the
derivatives [9,16e18]. However, the substitutions at the imide po-
sition have little or no influence on the electronic properties of the
derivatives [9,16e19].
* Corresponding author.
E-mail address: juozas.grazulevicius@ktu.lt (J.V. Grazulevicius).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.07.016

896
D. Gudeika et al. / Dyes and Pigments 99 (2013) 895e902
Fluorene and its derivatives are proven to be useful building
J2 ¼ 13.7 Hz, CH3, ꢂHaliphatic). IR (KBr,
(aliphatic CeH) 2958, 2927, 2871, 2852; (imide C]O) 1699; (Ar C]
C) 1585, 1512, 1458; (imide CeN) 1351, 1234; (CeBr) 665, 560. 13
NMR spectrum (75.5 MHz, CDCl3, , ppm): 163.61, 133.21, 132.01,
131.21, 131.09, 130.62, 130.19, 129.02, 128.08, 123.16, 122.30, 40.65,
31.83, 29.29, 28.10, 27.14, 22.66, 14.12. Anal. Calcd. for C20H22BrNO2:
C, 61.86; H, 5.71; Br, 20.58; N, 3.61; O, 8.24%. Found: C, 61.78; H,
5.77; N, 3.59%. MS (APCIþ, 20 V), m/z: 389 ([M þ H]þ).
y
cmꢂ1): (arene CeH) 3068;
blocks for the preparation of luminescent and charge-transporting
materials [20e22]. They are applied extensively in the design and
synthesis of polymers, low-molar-mass compounds, and den-
drimers for electronic and optoelectronic applications [20e24]. The
possibility of functionalization at 2, 7 and 9 positions, of fluorene
moiety not only improves the solubility of its derivatives but also
influences the molecular properties like glass transition, absorption
or emission.
C
d
Formation of donor-acceptor (D-A) combinations represents an
important methodology for the preparation of semiconducting
materials with the tunable properties [25]. Low-band-gap co-
polymers containing electron-accepting naphthalene bisimide
moieties and fluorene derivatives as electron-donor moieties for
organic solar cell applications have been reported [26]. We recently
reported low-molar-mass D-A derivatives exhibiting hole-
transporting and glass forming abilities in which naphthalimide-
acceptor moieties are functionalized with triphenylamino donor
chromophores via hydrazine linkages or ethenyl spacers [14,15].
In the present study, we report on the synthesis and thermal,
photophysical, electrochemical, and charge-transporting proper-
ties of naphthalimide-substituted fluorene derivatives containing
different alkyl substituents.
2.2. General procedure for SuzukieMiyaura reactions
4-bromo-N-(2-ethylhexyl)-1,8-naphthalimide or 4-bromo-N-
octyl-1,8-naphthalimide (300 mg, 0.77 mmol) and 9,9-dialkyl-2-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9H-fluorene
(0.85 mmol) or 9,9-dialkyl-2,7-di(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane-2-yl)-9H-fluorene (0.38 mmol) were taken in a
100 mL Schlenk flask. A solvent mixture of 20 mL of tetrahydro-
furan (THF) and 2 mL of water and powdered potassium carbonate
(2.7 mmol) was added. The reaction mixture was purged with ni-
trogen for 5 min. The reaction flask was degassed and then again
purged with nitrogen for 2 min. Bis(triphenylphosphine)palladiu-
m(II) dichloride (0.04 mmol) was added into it and stirred for 12e
16 h at 80 ꢀC under nitrogen. The reaction mixture was allowed to
cool down to the room temperature, diluted with water and
extracted using ethyl acetate. The organic layers were dried over
sodium sulfate and evaporated at reduced pressure. The crude
products were purified by silica gel column chromatography (the
details are given below).
2. Experimental section
2.1. Materials
The starting compounds, i.e. 2-bromofluorene, 2,7-dibromo-
fluorene, 4-bromo-1,8-naphthalic anhydride, 9,9-(2-ethylhexyl)-
9H-fluoren-2-yl-boronic acid (2b), [9,9-bis(2-ethylhexyl)-9H-fluo-
rene-2,7-diyl]bisboronic acid (3b) were purchased from Sigma
Aldrich and used as received. The reagents and the required ma-
terials, i.e. iodoethane, 1-bromo 2-ethylhexane, 1-bromooctane, 2-
ethylhexylamine, n-octylamine, n-butyllithium, potassium tert-
butoxide, tetrabutylammonium hydrogen sulfate, sodium sulfate,
potassium carbonate, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-diox-
aborolane, bis(triphenylphosphine)palladium(II) dichloride and
tetrabutylammonium hexafluoroborate (Bu4NBF6) were also pur-
chased from Sigma Aldrich and used as received.
4-Bromo-N-(2-ethylhexyl)-1,8-naphthalimide (1a) (lit. [15]
m.p.: ¼ 82e83 ꢀC), 9,9-diethylfluorene-2-boronic acid (2a) (lit.
[27] m.p.: ¼ 118e120 ꢀC), 9,9-dioctylfluorene-2,7-diboronic acid
(3c) (m.p.: ¼ > 400 ꢀC, lit. [28] > 400 ꢀC), 9,9-dioctylfluorene-2-
boronic acid (2c) (m.p.: ¼ 72e74 ꢀC, lit. [29] 70.1e77.3 ꢀC were
prepared according to the published procedures.
2.2.1. 2-((N-(2-ethylhexyl)-1,8-naphthalimide)-4-yl)-9,9-diethyl-
9H-fluorene (1)
The crude product was purified by column chromatography
using an eluent mixture of hexane and ethyl acetate in a volume
ratio of 9:1. Yield ¼ 73% (370 mg); Yellow crystals (FW ¼ 529.73 g/
mol); m.p.: 133e134 ꢀC. 1H NMR (300 MHz, CDCl3,
d, ppm): 8.69 (t,
J ¼ 5.3 Hz, 2H), 8.38 (d, J ¼ 3.2 Hz, 1H), 7.92 (d, J ¼ 2.6 Hz, 1H), 7.85e
7.72 (m, 3H), 7.53e7.50 (m, 2H), 7.46e7.40 (m, 3H), 4.26e4.14 (m,
2H, NCH2), 2.13 (q, J ¼ 7.3 Hz, 4H, CH2Fluorene), 2.04e1.97 (m, H,
CHNaphthalimide), 1.49e1.32 (m, 8H, CH2Naphthalimide), 1.01e0.90 (m,
6H, CH3Naphthalimide), 0.45 (t, J ¼ 4.9 Hz, 6H, CH3Fluorene). 13C NMR
(75.5 MHz, CDCl3, d, ppm): 165.0, 164.8, 150.7, 150.4, 147.7, 142.2,
140.9, 137.7, 132.8, 131.5, 131.1, 130.5, 129.1, 128.2, 127.9, 127.3, 127.0,
124.7, 123.3, 121.8, 120.2, 120.0, 56.5, 44.4, 38.2, 32.9, 31.0, 29.0,
24.3, 23.3, 14.4, 10.9, 8.9. IR (KBr,
y
cmꢂ1): (arene CeH) 3063;
(aliphatic CeH) 2961, 2929, 2853; (imide C]O) 1698; (Ar C]C)
1659, 1588; (imide CeN) 1353, 1231. Anal. Calc. for C37H39NO2: C,
83.89; H, 7.42; N, 2.64; O, 6.04%. Found: C, 83.82; H 7.36; N, 2.69%.
MS (APCIþ, 20 V), m/z: 530 [M þ H]þ.
2.1.1. 4-Bromo-N-octyl-1,8-naphthalimide (1b)
A
solution of 4-bromo-1,8-naphthalic anhydride (1 g,
3.61 mmol) in 25 ml of dimethylformamyde (DMF) was added to a
100 mL three neck round bottom flask equipped with a reflux
condenser and a magnetic stirrer. Then n-octylamine (0.60 g,
3.61 mmol) was added drop-wise and the reaction mixture was
heated up to 120 ꢀC and stirred under nitrogen for 24 h. The re-
action mixture was concentrated using rotary evaporator and then
the product was precipitated out into 1M HCl solution, filtered off
and washed with 1M HCl solution. The crude product was purified
by silica gel column chromatography using hexane/ethylacetate,
8:1 as an eluent. Yield: 1.03 g (74%) of white crystals; m.p.: ¼ 74e
2.2.2. 2-((N-(2-ethylhexyl)-1,8-naphthalimide)-4-yl)-9,9-di(2-
ethylhexyl)-9H-fluorene (2)
The product was purified by column chromatography using an
eluent mixture of hexane and ethyl acetate in a volume ratio of 8:1.
Yield ¼ 78% (180 mg); Yellow crystals (FW ¼ 698.03 g/mol); m.p.:
55e56 ꢀC. 1H NMR (300 MHz, CDCl3,
d, ppm): 8.74e8.66 (m, 2H),
8.45 (d, J ¼ 7.9 Hz, 1H), 8.37 (dd, J1 ¼ 1.1 Hz, J2 ¼ 8.5 Hz, 1H), 7.94e
7.88 (m, 1H), 7.85e7.81 (m, 2H), 7.79e7.72 (m, 1H), 7.54e7.49 (m,
2H), 7.47e7.40 (m, 2H), 4.26e4.13 (m, 2H, NCH2), 3.56e3.47 (m, 1H,
CH), 2.18e2.07 (m, 4H, CH2Fluorene), 2.05e1.93 (m, 2H, CHNaph-
thalimide), 1.55e1.27 (m, 24H, CH2Naphthalimide, CH2Fluorene), 1.07e0.80
(m, 12H, CH3Naphtahlimide, CH3Fluorene), 0.47 (t, J ¼ 7.3 Hz, 6H,
75 ꢀC. 1H NMR spectrum (300 MHz, CDCl3,
d, ppm): 8.67 (d, 1H,
J ¼ 8.4 Hz, 5-HNaphthalene), 8.57 (d,1H, J ¼ 7.5 Hz, 7-HNaphthalene), 8.43
(d, 1H, J ¼ 7.9 Hz, 2-HNaphthalene), 8.05 (d, 1H, J ¼ 7.9 Hz, 3-
CH3Fluorene). 13C NMR (75.5 MHz, CDCl3,
d, ppm): 164.9, 164.3, 150.8,
H
Naphthalene), 7.87 (tr, 1H, J1 ¼ 7.3 Hz, J2 ¼ 15.8 Hz, 6-HNaphthalene),
150.4,147.6, 142.2,141.0, 137.8, 133.4, 132.9,132.3,131.5, 131.2, 130.4,
129.1, 128.1, 127.9, 127.3, 127.1, 124.7, 123.3, 121.8, 120.2, 119.8, 56.5,
44.5, 44.4, 38.2, 38.2, 33.0, 31.1, 29.0, 24.4, 23.5, 23.3, 14.4, 11.0, 10.9,
4.21e4.13 (m, 2H, ꢂCH2, ꢂHaliphatic), 1.80e1.69 (m, 2H,
CH2, ꢂHaliphatic), 1.51e1.18 (m, 10H, CH2), 0.89 (tr, 3H, J1 ¼ 6.7 Hz,

D. Gudeika et al. / Dyes and Pigments 99 (2013) 895e902
897
8.9. IR (KBr,
y
cmꢂ1): (arene CeH) 3063; (aliphatic CeH) 2960, 2927,
2.2.6. 2,7-di((N-octyl-1,8-naphthalimide)-4-yl)-9,9-dioctyl-9H-
fluorene (6)
2855; (imide C]O) 1699; (Ar C]C) 1658, 1588; (imide CeN) 1354,
1231. Anal. Calc. for C49H63NO2: C, 84.31; H, 9.10; N, 2.01; O, 4.58%.
Found: C, 84.37; H 9.14; N, 2.03%. MS (APCIþ, 20 V), m/z: 698
[M þ H]þ.
The product was purified by column chromatography using
silica gel as stationary phase and an eluent mixture of hexane and
ethyl acetate in a volume ratio of 8:1. Yield ¼ 65% (112 mg); Yellow
crystals (FW ¼ 1005.42 g/mol); m.p.: 161e162 ꢀC. 1H NMR
2.2.3. 2-((N-(2-octyl)-1,8-naphthalimide)-4-yl)-9,9-dioctyl-9H-
fluorene (3)
(300 MHz, CDCl3,
d
ppm): 8.78-8.66 (m, 4H), 8.38 (dd, J1 ¼ 1.0 Hz,
J2 ¼ 8.5 Hz, 2H), 8.00 (d, J ¼ 7.5 Hz, 2H), 7.85 (d, J ¼ 7.5 Hz, 2H), 7.75
(dd, J1 ¼ 7.3 Hz, J2 ¼ 8.5 Hz, 2H), 7.63e7.52 (m, 4H, CH2Fluorene),
4.32e4.20 (m, 4H, NCH2), 2.18e2.02 (m, 4H, CH2Fluorene), 1.86e1.74
The product was purified by column chromatography using an
eluent mixture of hexane and ethyl acetate in a volume ratio of 8:1
to yield viscous solid. Yield ¼ 71% (134 mg); (FW ¼ 698.03 g/mol).
(m, 4H, CH2Naphthalimide), 1.49e1.11 (m, 32H, CH2Naphthalimide
CH2Fluorene), 0.96e0.80 (m, 12H, CH3Fluorene, CH3Naphthalimide). 13C
NMR (75.5 MHz, CDCl3, ppm): 164.6, 164.4, 151.8, 147.4, 141.0,
,
1H NMR (300 MHz, CDCl3,
d ppm): 8.76e8.65 (m, 2H), 8.36 (dd,
J1 ¼ 1.1 Hz, J2 ¼ 8.52 Hz, 1H), 7.90 (d, J ¼ 8.5 Hz, 1H), 7.86e7.79 (m,
2H), 7.72 (m, 2H), 7.52 (dd, J1 ¼1.5 Hz, J2 ¼ 7.0 Hz, 2H), 7.46e7.39 (m,
2H), 4.30e4.22 (m, 2H, NCH2), 2.11e1.98 (m, 4H, CH2Fluorene), 1.87e
1.74 (m, 2H, NCH2CH2), 1.41e0.94 (m, 22H, CH2Naphthalimide
CH2Fluorene), 0.94e0.82 (m,11H, CH2, CH3Fluorene, CH3Naphthlimide). 13
NMR (75.5 MHz, CDCl3, ppm): 164.6, 164.4, 151.5, 151.3, 147.8,
141.8, 140.6, 137.6, 132.9, 131.4, 131.1, 130.5, 129.0, 128.1, 127.9, 127.3,
127.0, 124.8, 123.2, 121.9, 120.3, 120.1, 119.9, 55.6, 40.8, 40.5, 32.0,
30.3, 29.7, 29.5, 28.4, 27.5, 24.2, 22.9, 14.3. IR (KBr,
d
138.3, 132.9, 131.5, 131.1, 130.4, 129.4, 129.1, 128.2, 127.1, 124.9, 123.3,
122.0, 120.5, 55.8, 40.9, 40.5, 32.0, 30.2, 29.7, 29.5, 28.4, 27.5, 24.3,
,
C
22.9, 14.4. IR (KBr, y
cmꢂ1): (arene CeH) 3060; (aliphatic CeH)
2956, 2926, 2856; (imide C]O) 1701; (Ar C]C) 1658, 1589; (imide
CeN) 1354, 1232. Anal. Calc. for C69H84N2O4: C, 82.43; H, 8.42, N,
2.79, O, 6.37%. Found: C, 82.49; H, 8.46; N, 2.78%. MS (APCIþ, 20 V),
m/z: 1004 [M]þ.
d
y
cmꢂ1): (arene
CeH) 3061; (aliphatic CeH) 2958, 2923, 2845; (imide C]O) 1698;
(Ar C]C) 1655, 1585; (imide CeN) 1353, 1234. Anal. Calc. for
2.3. Instrumentation
C
49H63NO2: C, 84.31; H, 9.10; N, 2.01; O, 4.58%. Found: C, 84.35; H
Nuclear magnetic resonance spectra were obtained using
deuterated chloroform as a solvent with a Varian Unity Inova
9.15; N, 2.07%. MS (APCIþ, 20 V), m/z: 698 [M þ H]þ.
spectrometer operating at 300 MHz for 1H and at 75.5 MHz for 13
C
2.2.4. 2,7-di((N-2-ethylhexyl-1,8-naphthalimide)-4-yl)-9,9-diethyl-
9H-fluorene (4)
nuclei, respectively. All the datas are given as chemical shifts
(ppm) downfield from TMS. IR-spectroscopy measurements were
d
The product was purified by column chromatography using
silica gel as stationary phase and an eluent mixture of hexane and
ethyl acetate in a volume ratio of 9:1. Yield ¼ 85% (270 mg); Yellow
crystals (FW ¼ 837.12 g/mol); m.p.: 147e149 ꢀC. 1H NMR (300 MHz,
performed on a Perkin Elmer Spectrum GX spectrophotometer,
using KBr pellets. Mass spectra were recorded using a Waters ZQ
(Waters, Milford, MA) mass spectrometer. Elemental analysis was
performed with an Exeter Analytical CE-440 elemental analyzer.
Differential scanning calorimetry measurements were performed
on a PerkineElmer DSC-7 ꢂs1eries thermal analyzer at a heating/
CDCl3,
d
ppm): 8.73e8.68 (m, 4H), 8.40 (d, J ¼ 3.2 Hz, 2H), 8.01e7.92
(m, 2H), 7.86 (d, J ¼ 2.5 Hz, 2H), 7.80e7.72 (m, 2H), 7.60e7.54 (m,
4H), 4.27e4.14 (m, 4H, NCH2), 2.19 (q, J ¼ 7.2 Hz, 4H, CH2Fluorene),
2.06e1.95 (m, 2H, CHNaphthalimide), 1.49e1.31 (m, 16H, CH2Naph-
thalimide), 1.01e0.90 (m, 12H, CH3Naphthalimide), 0.57 (t, J ¼ 4.9 Hz, 6H,
cooling rate of 10 ꢀC min
under nitrogen atmosphere. Ther-
mogravimetric analysis was executed on a Mettler Toledo TGA/
SDTA 851e under nitrogen atmosphere at a heating rate of
20 ꢀC minꢂ1. Absorption spectra of the dilute solutions were
recorded on UVeViseNIR spectrophotometer Lambda 950 (Perkin
Elmer). Fluorescence (FL) of dilute solutions, and of the neat films
was excited at a 365 nm from 150 W xenon arc lamp light source
passed through a monochromator and measured using back-
thinned CCD spectrophotometer PMA-11 (Hamamatsu). For these
measurements, the dilute solutions of the investigated compounds
were prepared by dissolving them in a spectral grade solvents at
10ꢂ6 M concentration. The drop-casting of the solutions with the
concentration of 1 ꢁ 10ꢂ3 M was employed to prepare the neat
CH3Fluorene). 13C NMR (75.5 MHz, CDCl3,
d ppm): 164.9, 164.7, 151.1,
147.4, 141.4, 138.4, 132.7, 131.5, 131.1, 130.4, 129.4, 129.1, 128.2, 127.1,
124.9, 123.2, 122.0, 120.4, 56.8, 44.47, 38.2, 34.9, 32.9, 31.8, 31.0,
29.0, 27.1, 25.5, 24.3, 23.3, 22.9, 20.9, 14.4, 10.9, 9.1. IR (KBr, y
cmꢂ1):
(arene CeH) 3048; (aliphatic CeH) 2958, 2926, 2855; (imide C]O)
1700; (Ar C]C) 1656, 1588; (imide CeN) 1353, 1232. Anal. Calc. for
C
57H60N2O4: C, 81.78; H, 7.22, N, 3.35, O, 7.64%. Found: C, 81.81; H,
7.17; N, 3.36%. MS (APCIþ, 20 V), m/z: 837 [M]þ.
2.2.5. 2,7-di((N-2-ethylhexyl-1,8-naphthalimide)-4-yl)-9,9-di(2-
ethylhexyl)-9H-fluorene (5)
films of the compounds. Fluorescence quantum yields (h) of the
The product was purified by silica gel column chromatography
using an eluent mixture of hexane and ethyl acetate in a volume
ratio of 8:1. Yield ¼ 72% (145 mg); Yellow crystals (FW ¼ 1005.42 g/
solutions were estimated by using integrated sphere method [30].
Integrating sphere (Sphere Optics) coupled to the CCD spectrom-
eter via optical fiber was also employed to measure
h of the neat
mol); m.p.: 118e119 ꢀC. 1H NMR (300 MHz, CDCl3,
d
ppm): 8.75e
films. Fluorescence transients of the samples were recorded using a
time-correlated single photon counting system PicoHarp 300
(PicoQuant) utilizing semiconductor diode laser (repetition rate
1 MHz, pulse duration 70 ps, emission wavelength 375 nm) as an
excitation source. Cyclic voltametry measurements were carried
out with a glassy carbon working electrode in a three-electrode cell
8.67 (m, 4H), 8.40e8.30 (m, 2H), 8.00 (d, J ¼ 7.7 Hz, 2H), 7.80e7.71
(m, 4H), 7.63e7.53 (m, 4H), 4.28e4.14 (m, 4H, NCH2), 2.19e2.09 (q,
J ¼ 7.1 Hz, 4H, CH2Fluorene), 2.08e1.93 (m, 4H, CHNaphthalimide), 1.51e
1.31 (m, 16H, CH2Naphthalimide), 1.07e0.76 (m, 28H, CH2Naphthalimide
,
CH3Naphthalimide), 0.75e0.58 (m, 12H, CH3Fluorene). 13C NMR
(75.5 MHz, CDCl3, ppm): 165.1, 164.8, 151.6, 151.6, 151.5, 147.5,
d
using a m-Autolab Type III (EcoChemie, Netherlands) potentiostat.
141.2,137.9, 132.8,131.7, 131.4,130.6,129.4,128.0,127.0,125.9,123.3,
122.1, 120.5, 55.9, 44.9, 44.5, 38.2, 35.2, 34.3, 31.1, 29.0, 28.9, 27.3,
24.4, 23.2, 14.3, 11.0, 10.5. IR (KBr,
(aliphatic CeH) 2957, 2925, 2856; (imide C]O) 1702; (Ar C]C)
1660, 1588; (imide CeN) 1353, 1233. Anal. Calc. for C69H84N2O4: C,
82.43; H, 8.42, N, 2.79, O, 6.37%. Found: C, 82.51; H, 8.49; N, 2.71%.
MS (APCIþ, 20 V), m/z: 1004 [M]þ.
Platinum wire and Ag/AgNO3 (0.01 mol Lꢂ1 in acetonitrile) were
used, respectively, as counter and reference electrodes and Bu4NBF6
in dichloromethane (0.1 M) was used as electrolyte. The datas were
collected using GPES (General Purpose Electrochemical System)
software. Electrochemical measurements were conducted at room
temperature at a potential rate of 100 mV/s. The reference electrode
was calibrated versus ferrocene/ferrocenium redox couple.
y
cmꢂ1): (arene CeH) 3062;

898
D. Gudeika et al. / Dyes and Pigments 99 (2013) 895e902
O
N
R
O
B
O
O
O
N
O
R1
R1
R1
R1
1, 2, 3
R
2a-c
+
Br
O
O
O
O
B
B
N
R
1a-b
O
O
N
R
R1
R1
R1 R1
O
O
3a-c
4, 5, 6
R
R1
R
R1
C4H9(C2H5)CHCH2
C8H17
C2H5
C4H9(C2H5)CHCH2
C8H17
C2H5
C4H9(C2H5)CHCH2
C8H17
C4H9(C2H5)CHCH2
C4H9(C2H5)CHCH2
C8H17
1a
1b
2a, 3a
2b, 3b
2c, 3c
1, 4
2, 5
3, 6
Scheme 1. Synthesis of compounds 1e6. Reagents and conditions: Pd(Ph3)2Cl2, K2CO3, THF/H2O, 80 ꢀC, 12e16 h.
Electron-drift mobility values were estimated by xerographic time-
of-flight method [31,32]. The samples were prepared by drop
casting of the solutions of naphthalimide derivatives in tetrahy-
drofuran on aluminium coated glass plates [33]. After keeping for
1hr in a saturated atmosphere of THF at room temperature, the
samples were heated at 70 ꢀC for 1 h in a hot air oven. The thickness
tetramethyl-1,3,2-dioxaborolane-2-yl)-9H-fluorene were syn-
thesised by the n-butyllithium mediated reaction of the respective
bromo derivatives of fluorene with 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane at ꢂ78 ꢀC as reported elsewhere
[34,35]. 4-bromo-N-octyl-1,8-naphthalimide and 4-bromo-N-(2-
ethylhexyl)-1,8-naphthalimide were prepared from 4-bromo-1,8-
naphthalic anhydride by heating it with the respective amines at
120 ꢀC in DMF [14]. The synthesized compounds were purified by
silica gel column chromatography and characterized by 1H NMR,
13C NMR and IR spectroscopy and mass spectrometry, as well as by
elemental analysis. The spectral and elemental analysis datas are in
good agreement with their chemical structures.
of layers was from 3 to 7
mm. The electric field was created by
negative corona charging. The charge carriers were generated at
the layer surface by illumination with pulses of nitrogen laser
(pulse duration was 1 ns, wavelength 337 nm). As a result of pulse
illumination the layer surface potential decreases up to 2e5% of
initial potential. The capacitance probe connected to the wide fre-
quency band electrometer measured the speed of the surface po-
tential decrease dU/dt. The transit time tt for the samples with the
transporting material was determined by the kink on the curve of
the dU/dt transient in logelog scale. The drift mobility was calcu-
3.2. Thermal properties
The behaviour under heating of compounds 1e6 was studied
by DSC and TGA under a nitrogen atmosphere. The values of glass
transition temperatures (Tg), melting points (Tm), crystallization
temperature (Tcr) and the temperatures at which initial loss of
mass (TID) was observed are summarized in Table 1. All the de-
rivatives demonstrate high thermal stability. The initial mass
loss occured at the temperatures higher than 315 ꢀC, as
confirmed by TGA with a heating rate of 20 ꢀC/min. For the
disubstituted fluorene derivatives (4e6) the thermal stability was
found to be higher in comparison to that of monosubstituted
compounds 1e3.
lated by using the formula
ness, and U0 the surface potential at the moment of illumination.
m
¼ d2/U0tt, where d is the layer thick-
3. Results and discussion
3.1. Synthesis
Compounds 1e6 were synthesized by SuzukieMiyaura coupling
reactions as shown in Scheme 1. For the preparation of simple D-A
compounds (1e3) containing naphthalimide acceptor and fluorene
donor, mono-1,3,2-dioxaborolanyl derivatives of fluorene were
used. For the synthesis of donor-acceptor-donor (D-A-D) hybrid
compounds (4e6), 2,7-di-1,3,2-dioxaborolanyl derivatives of fluo-
rene were employed.
All fluorene derivatives synthesized in this work except com-
pound 3 were isolated after the synthesis as crystalline materials.
DSC thermograms of compound 2 are shown in Fig. 1. When the
The key intermediates, 9,9-dialkyl-2-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane-2-yl)-9H-fluorene or 9,9-dialkyl-2,7-di(4,4,5,5-
Table 1
Thermal characteristics of compounds 1e6.a
Compound
Tg/ꢀCb
Tm/ꢀC
Tcr/ꢀC
TID/ꢀCc
1
2
3
4
5
6
30
50
143
61
e
e
e
e
e
84
392
304
290
457
422
441
ꢂ19
e
76
145
49
59
124
169, 167b
a
Determined by DSC, scan rate 10 ꢀC/min, N2 atmosphere.
2nd heating. Tg e glass transition temperature, Tm e temperature at melting, Tcr
b
e crystallization temperature.
c
TID e 5% weight loss temperatures.
Fig. 1. DSC curves of compound 2 (heating/cooling rate 10 ꢀC/min).

D. Gudeika et al. / Dyes and Pigments 99 (2013) 895e902
899
Fig. 2. Absorption (thin solid line) and fluorescence spectra of the solutions (10ꢂ6 M) of 1e6 in cyclohexane (thick solid line) and of neat films (dashed grey line). Fluorescence
quantum yields are indicated.
crystalline sample of 2 was heated, the endothermic peak due to
melting was observed at 61 ꢀC. When the melted sample was
cooled down and heated again, the glass transition was observed at
51 ꢀC. On further heating, no peaks due to crystallization and
melting appeared. The crystalline samples of compounds of 1, 4 and
5 demonstrated the similar behaviour. They melted on the first
heating scan at 143 ꢀC (for 1), 145 (for 4) and 142 ꢀC (for 5), and
formed glasses upon cooling at 30 ꢀC, 76 ꢀC and 49 ꢀC, respectively.
Compound 6 demonstrated different behaviour in the DSC exper-
iments. The crystalline sample of 6 melted at 169 ꢀC on the first
heating and formed glass upon cooling. When the amorphous
sample was heated again, the glass transition was observed at
59 ꢀC. On further heating, an exothermic peak due to crystallization
was observed at 84 ꢀC to give the crystals which melted at 167 ꢀC.
Compound 3 having linear octyl chains was isolated after the
synthesis as viscous liquid. In the second DSC heating scan it
showed glass transition at ꢂ19 ꢀC. The values of Tg of
naphthalimide-substituted derivatives of fluorene can be modified
by attachment of the different alkyl chains to the fluorene and
naphthalimide moieties.
3.3. Photophysical properties
UVeVis absorption spectra and fluorescence spectra of dilute
solutions in nonpolar cyclohexane and of neat films of the syn-
thesized naphthalimide mono- and disubstituted fluorenes are
shown in Fig. 2. The details of the optical and photophysical
properties of the compounds are summarized in Table 2. The
lowest-energy absorption bands of the mono- as well as of
disubstituted compounds are located at 355e374 nm. These
bands are shifted to the longer wavelength in respect to the
absorbance band of the naphthalimide moiety (344 nm) [15,36]
implying extended conjugation. The less pronounced absor-
bance band peaked at 306 nm can be attributed to fluorene
moiety [37]. Oscillator strength of the lowest-energy transitions
of the disubstituted compounds 4e6 is nearly twice as large as
that of the monosubstituted compounds 1e3 as a result of
extended
p-conjugation on two naphthalimide moieties. Ab-
sorption spectra of the neat films of the investigated compounds
were broader and red-shifted by ca. 17e37 nm indicating on
excitonic coupling.
Table 2
Optical and photophysical properties of the dilute cyclohexane solutions and neat films of compounds 1e6.
Compound
Dilute solution in cyclohexane
Neat film
labs (nm) (ε,b L molꢂ1 cmꢂ1
)
lmF axc (nm)
h
s
(ns)
sRd (ns)
sNRd (ns)
labs (nm)
lmF axc (nm)
h
T (ns)
a
a
1
2
3
4
5
6
363 (20505)
355 (24709)
357 (18505)
374 (42867)
369 (47535)
373 (42974)
423
423
423
425
424
424
0.50 1.42
0.53 1.42
0.47 1.45
0.69 1.22
0.64 1.23
0.65 1.24
2.84
2.68
3.09
1.77
1.92
1.91
2.84
3.02
2.74
3.94
3.42
3.54
380
378
393
392
406
410
474
495
477
489
469
480
0.23 2.18 (51%), 0.55 (31%), 6.84 (18%)
0.17 2.03 (44%), 7.01 (29%), 0.54 (27%)
0.25 2.41 (55%), 0.64 (28%), 8.29 (17%)
0.14 1.95 (49%), 0.57 (41%), 8.42 (9%)
0.07 0.26 (49%), 0.91 (40%), 3.40 (11%)
0.25 0.42 (49%), 1.04 (44%), 4.05 (7%)
a
Peak wavelength of absorption bands.
Molar extinction coefficient.
Wavelength at fluorescence band maximum.
Radiative and non-radiative decay time constants calculated as s/h and s/(1-h), respectively.
b
c
d

900
D. Gudeika et al. / Dyes and Pigments 99 (2013) 895e902
cyclohexane
toluene
chloroform
DCM
acetone
acetonitrile
DMSO
400
450
500
550
600
Wavelength (nm)
Fig. 3. Fluorescence spectra of the dilute cyclohexane, toluene, chloroform, dichloro-
methane, acetone, acetonitrile and dimethylsulfoxide solutions (10ꢂ6 M) of compound
1.
Excitation at 365 nm of dilute cyclohexane solutions of the
synthesized compounds resulted in a highly efficient fluorescence
in the blue spectral region with bands peaked at 423e425 nm.
Photoluminescence (PL) spectra of the dilute solutions of 1e6 in
nonpolar solvents show vibronic structure. Monosubstituted
compounds exhibited more intense 0e1 vibronic transition (at
435 nm) in respect to the disubstituted derivatives. PL spectra of the
neat films of naphthalimide derivatives 1e6 are broadened and
shifted to the long wavelengths by 45e72 nm as compared to those
of dilute solutions. Fluorescence quantum yields of the dilute
cyclohexane solutions of the compounds were found to be 0.47e
0.53 for monosubstituted compounds (1e3), whereas a higher
Fig. 4. a) FL quantum yield and b) FL decay time (triangles), radiative (circles) and
nonradiative (squares) decay times dependence on solvent dielectric constant. Lines
are given as guides for eyes.
with the increase of solvent polarities. FL QY of compound 1 in-
creases linearly from 0.55 to 0.84 with solvent polarity (Fig. 4a).
Fluorescence enhancement was also reported for other naph-
thalimide compounds dissolved in protic solvents [15,16,38].
Enhancement of the fluorescence quantum yield was accounted to
the decrease in the intersystem crossing by the hydrogen bond
formation [14]. In our case increase in FL QY in nonprotic solvents
is observed. However similar impact of polarity on the energy of
the resonant singlet and triplet levels can be expected. Analysis of
variation of radiative and nonradiative decay pathways with sol-
vent polarity supports this assumption (see Fig. 4b). Nonradiative
decay rate decreases almost 10 times with increasing solvent
polarity, while the radiative decay rate increases less than 2 times.
This is in agreement with modulation of the intersystem crossing
rate by solvent polarity.
Reduction of the rate of nonradiative recombination rate of
such molecules comprising singly bonded two polar aromatic
moieties can be also explained by solvent polarity impact on the
intramolecular twisting [39,40]. However in such a case the
twisting rate should be highly sensitive to the solvent viscosity.
There is minor correlation of the nonradiative decay rate with
solvent viscosity. Compounds dissolved in a solid polystyrene
matrix show values of FL QY slightly lower than those obtained for
the solutions in nonpolar solvents. The fluorescence decay time
parameters of the solid molecular dispersions are similar to those
obtained for the liquid solutions in nonpolar solvents. Rigid ma-
trix restricts possible intramolecular twisting of the singly bonded
naphthalimide and fluorene moieties, thus the impact of twisting
effect is minor.
value of
(4e6). Higher values of
h
(0.64e0.69) was obtained for disubstituted compounds
obtained for the disubstituted compounds
h
are consistent with the enhanced oscillator strength of the lowest
electron transitions revealed by the absorption measurements.
Photoluminescence quantum yield (PL QY) of the neat films was
estimated to be in the range of 0.07e0.25.
It is known that photophysical properties of naphthalenimide
derivatives are strongly dependant on the solvent polarity [15].
Significant variation of the excited state properties on the solvent
polarity indicates on pronounced charge-transfer character of the
lowest excited states. The normalized fluorescence spectra of the
dilute solutions of compound 1 in the solvents of different po-
larities are shown in Fig. 3. The details of the photophysical
properties of the compound 1 observed using different solvents
are summarized in Table 3. Batochromic shift of 94 nm, broad-
ening and shape change of fluorescence (FL) spectra was observed
by increasing solvent polarity. At the same time solvatochromic
shift of the absorbance band was found to be only 13 nm. Such
behoaviour is typical for compounds possessing small dipole
moment in the ground state and significantly larger dipole
moment in the excited state. The most interesting solvation
property is enhancement of fluorescence quantum yield (FL QY)
Table 3
Photophysical properties of the dilute solutions of compound 1 in the solvents different polarity.
Solvent
Dielectric constant (ε)
labs (nm)
lmF ax (nm)
FWHM (nm)
FL QY
s
(ns)
sR (ns)
sNR (ns)
Cyclohexane
Toluene
Chloroform
Dichloromethane (DCM)
Acetone
2.02
2.4
4.8
9.08
20.7
37.5
46.7
363
369
375
374
369
368
376
423
444
477
481
490
506
517
66
70
78
82
90
0.50
0.44
0.62
0.63
0.68
0.76
0.84
1.42
1.33
2.25
2.71
3.36
4.07
4.32
2.84
3.02
3.63
4.30
4.94
5.35
5.14
2.84
2.38
5.92
7.32
10.5
16.96
27
Acetonitrile
Dimethylsulfoxide (DMSO)
97
102

D. Gudeika et al. / Dyes and Pigments 99 (2013) 895e902
901
Fig. 6. XTOF transients for the neat film of 5. Arrow mark indicate transit time of
electrons at the respective surface voltage.
Fig. 5. Cyclic voltammograms of compounds 1e6.
These values are in agreement with those of the previously re-
ported naphthalimide-containing hydrazones [14]. The values of
IPSS of disubstituted fluorenes (4e6) are lower than those of
monosubstituted compounds (1e3).
All monosubstituted fluorene derivatives (1e3) demonstrated
similar solvatation properties, while the disubstituted compounds
(4e6) exhibited less pronounced effect.
3.5. Charge-transporting properties
3.4. Electrochemical properties
Xerographic time-of-flight measurements were used to char-
acterize the charge-transporting properties of the selected com-
pounds. Dispersive electron-transport was observed in the
amorphous layers of compounds 1, 2, 5, 6 coated on aluminium
plated glass by solution processing technique. The representative
dU/dt transient for the neat film of 5 is shown in Fig. 6. It clearly
demonstrates dispersive electron-transport. The electron-transit
times (tt) needed for the estimation of electron mobilities were
established from the intersection points of two asymptotes of the
double-logarithmic plots.
The electric field dependencies of electron-drift mobilities of the
amorphous films of naphthalimide derivatives are shown in Fig. 7.
Compound 2 did not form good amorphous film. Therefore the
layer of its 1:1 molecular mixture with a polymer host bisphenol Z
polycarbonate (PC-Z) was prepared. For the amorphous films of
compounds 1, 5 and 6 and for the 1:1 blend of 2 with PC-Z, the
Electrochemical investigation of the solutions of compounds 1e
6 in dichloromethane were carried out in order to elucidate the
reduction redox processes and thereby electron affinities (EASS).
The cyclic voltametry (CV) profiles of compounds 1e6 are given in
Fig. 5. The reduction potentials, and the values of the solid state
ionization potentials (IPSS) and EASS are collected in Table 4. EASS
were calculated by referencing with ferrocene/ferrocenium (Fc/
Fcþ) redox couple according to the established relationship: EASS
(eV) ¼ ꢂ4.8 e En, where EASS is the electron affinity which can be
related to the LUMO and En is the first reduction potential onset
versus Fc/Fcþ. The IPSS were calculated according to the reltionship:
IPSS ¼ EASS e Egopt, where Egopt is the optical band gap calculated from
the solution absorption onset.
All the naphthalimide-substituted derivatives of fluorene
exhibited reversible reduction processes corresponding to the
reduction of naphthalimide moieties to radical anions (Fig. 5). This
reduction behaviour clearly demonstrates the electron deficient
nature of these derivatives. The reduction potentials range
between ꢂ1.58 V and ꢂ1.81 V. No substantial influence of the
number of naphthalimide moieties in the molecules on the
reduction potentials were observed. The EASS of compounds 1e6
range from ꢂ3.29 to ꢂ3.52 eV with respect to the vacuum level.
Table 4
Electrochemical characteristics of compounds 1e6.
Compound
Ered/Va
Eoxi/Vb
En/Vc
Egopt/eV
EASS/eV
IPSS/eV
1
2
3
4
5
6
ꢂ1.81
ꢂ1.79
ꢂ1.60
ꢂ1.79
ꢂ1.82
ꢂ1.58
ꢂ1.61
ꢂ1.70
ꢂ1.48
ꢂ1.61
ꢂ1.71
ꢂ1.47
ꢂ1.46
ꢂ1.37
ꢂ1.28
ꢂ1.43
ꢂ1.51
ꢂ1.28
3.04
3.04
3.03
2.98
3.00
3.00
ꢂ3.34
ꢂ3.43
ꢂ3.52
ꢂ3.37
ꢂ3.29
ꢂ3.52
6.38
6.47
6.55
6.35
6.29
6.52
a
Reduction potential vs Ag/Agþ
.
.
b
c
Oxidation potential vs Ag/Agþ
Fig. 7. Electric field dependencies of electron-drift mobilities of the amorphous layers
of 1, 5, 6 and of the molecular mixture of 2 with PC-Z (1:1).
First reduction potential onset vs Fc/Fcþ
.

902
D. Gudeika et al. / Dyes and Pigments 99 (2013) 895e902
linear dependencies of electron drift-mobility on the square root of
the electric field were observed at room temperature.
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In conclusion, we have synthesized a series of naphthalimide-
substituted derivatives of fluorene which exhibit high thermal
stability with the initial weight loss temperatures ranging from
315 to 466 ꢀC. Most of the synthesized compounds can be
attributed to molecular glasses with glass transition tempera-
tures ranging from 30 to 76 ꢀC. Dilute solutions of the fluorenyl
substituted naphthalimide compounds in nonpolar solvents were
found to emit in the blue region with high fluorescence quantum
yield ranging from 0.47 to 0.69. Fluorenes possessing two
naphthalimide moieties exhibited higher values of the fluores-
cence quantum yield due to enhanced oscillator strength of the
transition. Fluorenyl substituted naphthalimides exhibited pro-
nounced positive solvatochromic behaviour in polar solvents
indicating on charge-transfer character of the lowest excited
states. Fluorescence red shift of the solutions in dimethylsulf-
oxide was found of 94 nm, while the fluorescence quantum yield
increased linearly with polarity of the solvents from 0.47 for
cyclohexane to 0.84 for dimethylsulfoxide. Such untypical
enhancement of fluorescence quantum yield is accounted to
reduced intersystem crossing in polar solvents. The electron drift
mobilities of the layer of 2,7-di((N-octyl-1,8-naphthalimide)-4-
yl)-9,9-dioctyl-9H-fluorene reached 1.2 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 at
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.
Acknowledgements
This research was funded by the European Social Fund under the
Global Grant measure. This research work was also partially sup-
ported by FP-7 PEOPLE PROGRAMME, Marie Curie ActionsꢂITN
Grant No. 215884.
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