Benzodifuroxazinones, a new class of heteroacene molecules for possible applications in organic electronics: Synthesis, electronic properties and crystal structure

Article abstract of DOI:10.1016/j.dyepig.2012.03.033

A novel class of heteroacene molecules, benzodifuroxazin-4-ones, has been effectively synthesized by means of a thermally activated double cyclization reaction starting from amidic precursors. The new molecules were thermally and optically characterized, revealing an outstanding thermal stability to oxidation and an uncommon enhancement of fluorescent properties in solid state as compared to solution. As shown by single-crystal XRD analysis, the molecules crystallize in a face-to-face (π-stack) arrangement instead of the herringbone structure typical of the most acene derivatives. The electronic properties of both molecules and crystals have been investigated by means of a detailed Density Functional Theory computational analysis: very stable HOMO energies have been calculated and, from the band structure analysis, it is possible to suggest a preferential direction of charge transport along the π-stacking direction. All the reported properties indicate this new class of heteroacene derivatives as interesting candidates for a possible application in organic electronics.

Full text of DOI:10.1016/j.dyepig.2012.03.033

Dyes and Pigments 95 (2012) 116e125
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Benzodifuroxazinones, a new class of heteroacene molecules for possible
applications in organic electronics: Synthesis, electronic properties
and crystal structure
,
a
a
b
a
Antonio Carella a , Fabio Borbone , Antonio Roviello , Giuseppina Roviello , Angela Tuzi ,
*
Alexey Kravinsky c, Rafi Shikler c d, Giovanni Cantele e, Domenico Ninno e
,
a Dipartimento di Chimica, Università degli Studi di Napoli “Federico II”, Complesso Universitario Monte S. Angelo, Via Cintia snc, 80126 Napoli, Italy
b Dipartimento per le Tecnologie, Universita degli Studi di Napoli Parthenope, Centro Direzionale Isola C4, 80143 Napoli, Italy
c Department of Electrical Computer Engineering, Ben-Gurion University of the Negev, POB 653 Beer-Sheva 84105, Israel
d Ilse-Katz center for Nanotechnology, Ben-Gurion University of the Negev, POB 653 Beer-Sheva 84105, Israel
e CNR-SPIN and Dipartimento di Scienze Fisiche, Università degli Studi di Napoli “Federico II”, Complesso Universitario Monte S. Angelo, Via Cintia snc, 80126 Napoli, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel class of heteroacene molecules, benzodifuroxazin-4-ones, has been effectively synthesized by
means of a thermally activated double cyclization reaction starting from amidic precursors. The new
molecules were thermally and optically characterized, revealing an outstanding thermal stability to
oxidation and an uncommon enhancement of fluorescent properties in solid state as compared to
Received 31 October 2011
Received in revised form
28 March 2012
Accepted 31 March 2012
Available online 10 April 2012
solution. As shown by single-crystal XRD analysis, the molecules crystallize in a face-to-face (p-stack)
arrangement instead of the herringbone structure typical of the most acene derivatives. The electronic
properties of both molecules and crystals have been investigated by means of a detailed Density Func-
tional Theory computational analysis: very stable HOMO energies have been calculated and, from the
Keywords:
Solid phase synthesis
Heteroacenes
band structure analysis, it is possible to suggest a preferential direction of charge transport along the p-
stacking direction. All the reported properties indicate this new class of heteroacene derivatives as
interesting candidates for a possible application in organic electronics.
Ó 2012 Elsevier Ltd. All rights reserved.
Crystal structure determination
Density functional calc.
Organic electronics
Solid state fluorescence
1. Introduction
Most of the investigated organic semiconductors could be
roughly divided into two classes, conjugated polycyclic compounds
Organic semiconductors has received a great deal of attention
for their possible use as active layers in thin film electronic devices
such as transistors [1e3], light emitting devices (OLED) [4e6] or
solar cells [7e9]. As compared to the inorganic semiconductors
typically used in electronics, the unique mechanical properties of
organic molecules and polymers may allow the fabrication of
flexible electronic devices, while their easier processability might
open the way to low-cost manufacturing techniques such as spin-
coating, ink-jet printing, and roll-to-roll printing processes [10];
this technological aspect is particularly striking in a field like
electronics where manufacturing costs weight much more than the
cost of the material itself. Another important factor justifying the
huge interest in organic semiconductors is the possibility to design
with low molecular weight [11e15] and polyheterocycles [11,16,17].
As far as the former are concerned, oligoacenes are one of the most
intensively investigated class of materials in the literature: organic
transistors endowed with outstanding mobility have been fabri-
cated by using pentacene and its derivatives as active layer.
However, acene-type molecules show a number of drawbacks for
practical applications like for instance a high-energy-lying HOMO
and a narrow bandgap that make them extremely sensitive to
photo-oxidation [18]. A possible synthetic approach for improving
the oxidative stability is to insert, within the fused ring system,
more electronegative heteroatoms [19,20].
The typical crystalline structure presented by heteroacene
compounds is characterized by a herringbone pattern, in which the
molecules are packed edge-to-face in 2D layers (with the highest
charge mobility in this plane) [21]. However, theoretical works have
shown that charge mobility in organic semiconductors is intrinsi-
or modify
p-conjugated chemical structures in ways that could
directly impact the properties of the material.
cally related to the overlap of
conjugated molecules [22]: in this respect, the herringbone
p molecular orbitals of adjacent
* Corresponding author. Tel.: þ39081674446; fax: þ39081674367.
E-mail address: antonio.carella@unina.it (A. Carella).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.03.033

A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
117
structure is not the most suitable one and it could be argued that, in
principle, higher mobility could be obtained by designing new
(TA SDT 2960, air, 20 K/min). 1H and 13C NMR spectra were recorded
with a Varian XL 200-MHz apparatus. UV/Vis absorption and
fluorescence spectra of the prepared chromophores, both in solu-
tion and as thin films, were recorded respectively with a Jasco V560
spectrophotometer and a Jasco spectrofluorimeter FP750. PL-
efficiency measurements on thin film (thickness of about 50 nm)
were conducted using a conventional setup that consists of
a 376 nm solid state laser as a light source and an integrating sphere
coupled to photospectrometer (Black Comet) by Stellarnet. The
experiment was controlled by an LABVIEW script that also calcu-
lated the efficiency. The setup and the calculation algorithm is the
same as the one outlined in de Mello paper [29]. Maldi mass spectra
were recorded in positive ion mode on a Maldi Voyager STR Applied
Biosystems.
molecules that crystallize in a face-to-face (
Indeed, proper synthetic modifications of the molecules can turn the
crystal packing from herringbone to -stacked, for example through
p-stack) arrangement.
p
functionalization of the acene core with halogens [23,24] or bulky
substituents in the peripositions of the oligoacenes [24,25].
In a recent paper dealing with the amino-benzodifurane reac-
tivity, we hypothesized the formation of the B-OXA compound (see
Fig. 1) based on an unprecedented penta-atomic fused heterocycle,
a benzodifuroxazinone prepared through a cyclization reaction
thermally initiated in the bulk [26]. While in the mentioned paper
only preliminary results were reported, a later single-crystal X-Ray
Diffraction (XRD) analysis confirmed the structure of the B-OXA.
Here, we report (for the first time) the optimized synthesis of B-
OXA and of two new fluorinated derivatives, FB-OXA and DFB-
OXA. The molecular structure of these molecules, presented in
Fig. 1, shows several elements of interest: the core structure is
composed of five fused rings, as in pentacene, and should allow
a high degree of planarity that, in turn, could lead to high electrical
performance. Moreover, because of the presence of more electro-
negative heteroatoms, it can be argued a higher stability of ben-
zodifuroxazinone derivatives against oxidation, compared to that of
pentacene. Finally, the versatility of the synthetic strategy to
prepare this class of compounds could easily give access to a wide
number of derivatives with tailored features.
The chemical physical properties of the three benzodifurox-
azinones derivatives and their intermediates have been fully char-
acterized. A peculiar feature of all three compounds is revealed in
fluorescence experiments: they are good green emitters in solid
state while they show very weak fluorescence when excited in
solution. This phenomenon (also referred to in the literature as
“aggregate induced emission, AIE” [27,28]) makes them interesting
candidates in OLED technology.
The structures of FB-OXA and DFB-OXA have been resolved by
means of single-crystal XRD. The obtained crystal structures and
crystallographic coordinates have been used as the basis for
a detailed computational analysis that will be extensively described
in the next sections.
Single crystals of B-OXA, FB-OXA and DFB-OXA suitable for XRD
analysis were obtained by sublimation. One crystal of B-OXA
(red, 0.60
ꢀ
0.25
ꢀ
0.10), one crystal of FB-OXA (yellow,
0.50 ꢀ 0.05 ꢀ 0.01 mm) and one crystal of DFB-OXA (yellow,
0.60 ꢀ 0.20 ꢀ 0.01 mm) were mounted at 296 K on a Bruker-Nonius
KappaCCD diffractometer equipped with
a graphite mono-
chromated MoKa radiation (
l
¼ 0.71073 Å, CCD rotation images,
thick slices, 4 and scans to fill asymmetric unit). Semiempirical
u
absorption correction (SADABS) was applied. The structures were
solved by direct methods (SIR97 package [30]) and refined by the
full matrix least-squares method on F2 against all independent
measured reflections (SHELXL program of SHELX97 package [31]).
All non-hydrogen atoms were anisotropically refined. The
hydrogen atoms were introduced in calculated positions and
refined according to a riding model, except for H atoms on ben-
zodifurane ring system that were found in difference Fourier maps.
The final refinement converged to R1 ¼ 0.0891, wR2 ¼ 0.224 for B-
OXA, R1 ¼ 0.0613, wR2 ¼ 0.0875 for FB-OXA and R1 ¼ 0.0501,
wR2 ¼ 0.0950 for DFB-OXA. Evidences of twinning by 180ꢁ rotation
around the a axis (twin law 1 0 0 0 ꢂ1 0 0 0 ꢂ1) were found in B-
OXA, data were not merged and refined using HKLF 5 instruction of
SHELXL program. In DFB-OXA the Fourier maps showed a static
disorder of fluorine F2 atom in two positions related by 180ꢁ
rotation around C4eC7 bond (occupancy factor refined to 0.63).
CCDC-830737 (B-OXA), CCDC-830738 (FB-OXA) and CCDC-
830739 (DFB-OXA) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (internet.) þ44 1223/336 033.
2. Materials and methods
2.1. Chemical and characterization
All solvents and reagent were purchased by Aldrich and used
without further purification. The thermal behaviour of the
compounds was studied by differential scanning calorimetry
(PerkineElmer Pyris 1, nitrogen atmosphere, scanning rate 10 K/
min), temperature controlled polarizing microscopy (Zeiss micro-
scope, Mettler FP5 microfurnace) and thermo-gravimetric analysis
2.2. Synthesis
2,6-Diamino-benzo[1,2-b:4,5-b0]difuran-3,7-dicarboxylic acid,
dibutyl ester (C4) was prepared as previously described. Amide
precursors were prepared by reacting C4 with the correspondent
acyl chlorides with the same procedure and the details of synthesis
are reported only for the synthesis of C4-DFB. The benzodifurox-
azinone chromophores were obtained by means of a double
thermal cyclization with the reaction parameters (time and
temperature) suggested by
gravimetric analysis.
a previously performed thermo-
2.2.1. Dibutyl 2,6-bis(2,4-difluorobenzamido)-4,8-
dihydrobenzofuro[5,6-b]furan-3,7-dicarboxylate (C4-DFB)
C4 (0.775 g, 2 mmol) were suspended in 10 mL of dry pyridine
and the system was kept under stirring at room temperature; then,
2,4-difluorobenzoylchloride (1.045 g, 5.92 mmol) was added
dropwise and the previous suspension turned to a dark solution.
The solution was then warmed and kept at a gentle boiling for
45 min until the formation of some precipitate was observed.
Fig. 1. Molecular structure of the prepared chromophores.

118
A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
Absolute ethanol (1 mL) and after 2 min distilled water (0.2 mL)
were added and the system slowly cooled down to room temper-
ature and filtered. The yellow product was then washed in 30 mL of
absolute ethanol and filtered again. The product was recrystallized
by CHCl3/ethanol. The yield was 55%. Mp: 285 ꢁC. 1H NMR (CDCl3,
2.2.6. 2,8-Bis-(2,4-difluorophenyl)-4,10-dihydro[1,3]oxazino[5000,
4000,400, 500]furo[300,200:40,50]-benzo [4,5]furo-[2,3-d][1,3]oxazine-4,10-
dione (DFB-OXA)
The same procedure used for the synthesis of B-OXA was
applied, except that the cyclization time in the furnace was 30 min.
The overall yield was 42%. 1H NMR (1,1-2,2-tetrachloroethane d2,
200 MHz)
d
(ppm): 1.04 (t, 6H, J ¼ 7.4 Hz, eCH3), 1.51e1.62 (m, 4H,
eCH2), 1.82e1.91 (m, 4H, eCH2), 4.45 (t, 4H, J ¼ 6.6 Hz, OeCH2),
7.56e7.64 (m, 4H, AreH), 7.96 (s, 2H, AreH), 8.07 (dd, 2H,
200 MHz) d (ppm): 7.07e7.17 (m, 4H, AreH), 8.24 (s, 2H, AreH),
8.27e8.34 (m, 4H, AreH). IR (KBr): 1787 (C]O stretching
ketone). MS: 520.98 (C26H8F4N2O6, calc. 520.03). Elemental anal-
ysis: (calc.) C 60.01 H 1.55 N 5.38, (exp.) C 60.14, H 1.57, N 5.37.
J1 ¼8.0 Hz, J2 ¼ 2.0 Hz, AreH), 11.34 (d, 2H, J ¼ 13.8 Hz, COeNH). 13
C
NMR (CDCl3, 50 MHz):
d 13.96, 19.55, 31.07, 65.26, 93.73, 103.46,
104.50, 105.05, 105.56, 113.07, 113.50, 116.87, 121.41, 134.60, 134.75,
148.96, 156.81, 158.40, 165.92. IR (KBr): 3265 (stretching NeH);
2873e2957 (stretching CeH aliphatic); 1699 (stretching C]O
ester); 1601 (stretching C]O amide).
2.3. Computational details
Theoretical analysis of B-OXA, FB-OXA and DFB-OXA were
performed in the framework of density functional theory (DFT).
The structural and electronic properties of the isolated mole-
cules were computed using the NWCHEM package [32]. Geometry
optimization, ground-state (GS) Density of States (DOS) and elec-
tronic properties were calculated using the B3LYP exchange-
correlations functional combined with the 6-311G* basis set.
The GS crystal properties (band structure and DOS, in particular)
where obtained using the Quantum-ESPRESSO package [33], suit-
able for dealing with periodic systems. We used pseudopotentials to
represent the atomic cores and plane waves to expand both the
electronic wave functions and charge density. The ground state
properties were computed using ultrasoft pseudopotentials [34,35],
with a 30 Ry and 360 Ry cut-offs for the plane-wave basis set and
charge density, respectively. The GGA approximation for the
exchange-correlation (XC) functional with the PBE parametrization
[36] was used. The input cell parameters and atomic internal coor-
dinates were provided by the experimental XRD analysis, described
in a next section. Only the atomic internal coordinates were opti-
mized, by computing the HelmanneFeynmann forces until each
Cartesian component of the force acting on each atom was less than
0.026 eV/Å and the total energy difference between consecutive
steps less than 10ꢂ4 eV. Indeed, it is well known that DFT with local
or semilocal XC functionals yields a poor description of interlayer
interactions dominated by van der Waals (vdW) forces; yet, the
weak interlayer bonding usually gives a small contribution to most
of the computed properties. Using the input cell parameters usually
overcomes the above cited DFT shortcoming, with good agreement
with experimental data [37], with the advantage of a reasonable
computational cost, that would be lost using nonlocal XC func-
tionals, that correctly account for vdW interactions [38].
2.2.2. Dibutyl 2,6-bis(benzamido)-4,8-dihydrobenzofuro[5,6-b]
furan-3,7-dicarboxylate (C4-B)
The synthesis was performed following the same procedure as
for C4-DFB except that benzoyl chloride was used instead of 2,4-
difluorobenzoylchloride. Yield after recrystallization was 65%. Mp:
248 ꢁC. 1H NMR (CDCl3, 200 MHz)
d
(ppm): 1.04 (t, 6H, J ¼ 7.4 Hz,
eCH3),1.50e1.61 (m, 4H, eCH2),1.85e1.92 (m, 4H, eCH2), 4.45 (t, 4H,
6.6 Hz, OeCH2), 7.56e7.64 (m, 6H, AreH), 7.97 (s, 2H, AreH), 8.06 (d,
4H, J ¼8.2 Hz, AreH),11.17(s, 2H, COeNH).13CNMR (CDCl3, 50MHz):
d
13.94, 19.55, 31.05, 65.23, 92.78, 103.34, 121.14, 127.94, 129.29,
132.82, 133.29, 148.87, 157.79, 162.88, 166.63. IR (KBr): 3268
(stretching NeH); 2874e2965 (stretching CeH aliphatic); 1706
(stretching C]O ester); 1626 (stretching C]O amide).
2.2.3. Dibutyl 2,6-bis(benzamido)-4,8-dihydrobenzofuro[5,6-b]
furan-3,7-dicarboxylate (C4-FB)
The synthesis was performed following the same procedure as
for C4-DFB except that 4-fluorobenzoyl chloride was used instead
of 2,4-difluorobenzoylchloride. The yield was 60%. Mp: 275 ꢁC. 1H
NMR (CDCl3, 200 MHz)
d
(ppm): 1.05 (t, 6H, J ¼ 7.4 Hz, eCH3),
1.50e1.62 (m, 4H, eCH2), 1.83e1.92 (m, 4H, eCH2), 4.45 (t, 4H,
6.8 Hz, OeCH2), 7.19e7.27 (m, 4H, AreH), 7.93 (s, 2H, AreH),
8.04e8.10 (m, 4H, AreH), 11.12 (s, 2H, COeNH). 13C NMR (CDCl3,
50 MHz):
d 13.97, 19.54, 31.05, 65.33, 92.89, 103.37, 116.29, 116.73,
121.13,129.07,130.45,130.63,148.90,157.71,161.75,166.74. IR (KBr):
3281 (stretching NeH); 2876e2955 (stretching CeH aliphatic);
1712 (stretching C]O ester); 1609 (stretching C]O amide).
2.2.4. 2,8-Diphenyl-4,10-dihydro[1,3]oxazino[5000,4000,400, 500]furo
[300,200:40,50]benzo-[4,5]furo-[2,3-d][1,3]oxazine-4,10-dione (B-OXA)
C4-B (200 mg) was placed in a petri dish in a furnace at 300 ꢁC
for 20 min. The obtained cyclized compound was then purified by
sublimation. 98 mg of product was obtained for an overall yield
3. Results and discussion
3.1. Synthesis and chemical physical characterization
(cyclization
tetrachloroethane d2, 200 MHz)
þ
sublimation) of 65%. 1H NMR (1,1-2,2-
(ppm): 7.58e7.68 (m, 6H,
d
The synthetic strategy for the preparation of the benzodifur-
oxazinones compounds is reported in Fig. 2. The first step is the
Craven reaction [39] between benzoquinone and an alkyl ester of
cyanoacetic acid using NH3 as basic catalyst to obtain a diamino-
benzodifuran derivative (Cn). The latter can be reacted with
benzoyl chloride, 4-fluorobenzoyl and 2,4-difluorobenzoylchloride
to prepare the correspondent diamides. From the diamides it was
possible to obtain the B-OXA, FB-OXA or DFB-OXA molecules by
means of a thermally activated cyclization reaction that takes place
through elimination of two molecules of alcohol.
To find the optimal temperature of cyclization we preliminarily
performed an optical analysis with a thermally controlled polarized
microscope: at a temperature of around 285 ꢁC, in the molten
isotropic phase of the amides, the formation of some crystalline
germs occurred and at 300 ꢁC a very fast crystallization process
(assigned to the cyclization reaction), involving all the bulk, was
AreH), 8.25 (s, 2H, AreH), 8.45 (d, 4H, J ¼ 8.4 Hz, AreH). IR (KBr):
1782 (C]O stretching ketone). MS: 448.03 (C26H12N2O6 calc.
448.07). Elemental analysis: (calc.) C 69.65 H 2.70 N 6.25, (exp.) C
69.70 H 2.64, N 6.26.
2.2.5. 2,8-Bis-(4-fluorophenyl)-4,10-dihydro[1,3]oxazino[5000, 4000,400,
500]furo[300,200:40,50]benzo-[4,5]furo-[2,3-d][1,3]oxazine-4,10-dione
(FB-OXA)
The same procedure used for the synthesis of B-OXA was
applied, except that the cyclization time in the furnace was 10 min.
The overall yield was 70%. 1H NMR (1,1-2,2-tetrachloroethane d2,
200 MHz)
d (ppm): 7.25e7.37 (m, 4H, AreH), 8.24 (s, 2H, AreH),
8.44e8.51 (m, 4H, AreH). IR (KBr): 1773 (C]O stretching ketone).
MS: 484.06 (C26H10F2N2O6, calc. 484.05). Elemental analysis: (calc.)
C 64.47 H 2.08 N 5.78, (exp.) C 64.34 H 2.11 N 5.86.

A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
119
Fig. 2. Synthesis of the B-OXA, FB-OXA and DFB-OXA compounds.
observed. This process took place independently of the length of
the alkyl chains of the intermediate compounds and at the same
temperature: anyway, in particular in the case of the synthesis of
DFB-OXA, the use of an intermediate amide bearing an octyl chain
slowed down the crystallization process probably because of an
increased solubility of DFB-OXA in the molten C8-DFB and
a consequent reduced number of crystalline germs. To avoid long
time treatment of an overheated liquid organic molecule (that
could lead to undesired secondary reactions) we chose as starting
amides for the synthesis of all the chromophores the butyl func-
tionalized ones: for these derivatives in fact, the cyclization process
occurred at temperatures only slightly above the melting temper-
ature of the amides.
After this preliminary qualitative screening, we obtained more
quantitative information by means of thermo-gravimetric analysis
(TGA): in the thermograms of Fig. 3, the weight loss shown, as
a function of time, by C4-B, C4-FB and C4-DFB, when placed at
a constant temperature of 300 ꢁC, is reported. In the case of the C4-
B and C4-DFB compounds, the weight loss reached a plateau after,
respectively, 20 and 10 min, that is consistent with the loss of two
molecules of butyl alcohol and therefore with the cyclization
process; for C4-DFB a slight negative slope was still observed even
after 30 min of thermal treatment and the weight loss was higher
than expected: this behaviour arises from a higher tendency to
sublime of DFB-OXA as compared to the other two benzodifurox-
azinone derivatives.
Fig. 3. Thermal cyclization of C4-B, C4-FB and C4-DFB at 300 ꢁC monitored by TGA
analysis. For sake of clarity C4-B and C4-DFB thermograms are translated along the Y
axis (respectively þ10% and 10%).
Fig. 4. FTIR spectra of C4-B and B-OXA.

120
A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
Fig. 5. Thermal stability of the B-OXA, FB-OXA and DFB-OXA compounds. For sake of
clarity B-OXA and DFB-OXA thermograms are translated along the
(respectively þ10% and 10%).
Y axis
Fig. 7. Emission spectra of B-OXA, FB-OXA and DFB-OXA compounds.
the aliphatic chains), w1700 cmꢂ1 (carbonyl stretching in ester),
w1620 cmꢂ1 (carbonyl stretching in amides). These signals dis-
appeared in the spectra of the cyclized compound B-OXA while
a new intense band at w1780 cmꢂ1 came out, assigned to carbonyl
stretching in the formed cyclic ketone. In Fig. 4, we report the
spectra of C4-B and B-OXA as example.
Driven by the information obtained from the TGA, we quanti-
tatively prepared the desired B-OXA, FB-OXA and DFB-OXA
compounds by placing C4-B, C4-FB and C4-DFB in a furnace, pre-
heated at 300 ꢁC, for, respectively, 20, 10 and 30 min. The weight
change measured after the thermal treatment of the amides C4-B,
C4-FB and C4-DFB is, respectively, 25.0%, 23.2% and 23.7%: this
weight losses are very close to the theoretical values of24.8%, 23.4%
and 22.2% expected for the formation of benzodifuroxazinone
derivatives (except C4-DFB/DFB-OXA system in which the weight
loss is higher because of the previously mentioned sublimation
problems).
The compounds, which show a very poor solubility in organic
solvents, were further purified by means of sublimation at atmo-
spheric pressure in good (B-OXA and FB-OXA) or moderate (DFB-
OXA) yield. The sublimation process, performed for sake of
convenience on about 100 mg of compound, can be easily scale up
to higher amount, being the sublimation performed at atmospheric
pressure. The elemental analysis performed on the three
compounds prepared in this way is consistent with the proposed
structures shown in Fig. 1.
All the intermediates and the final products have been thermally
characterized by means of Differential Scanning Calorimetry (DSC)
and TGA (only the final compounds). For what concerns DSC
analysis on the final chromophores, neither melting or solidesolid
phase transitions were observed. As previously described, TGA gave
useful guidelines for the quantitative preparation of the benzodi-
furoxazinone chromophores. Moreover, the TGA analysis provided
information about the thermal stability of the synthesized chro-
mophores. In Fig. 5 we show the thermograms relative to weight
change vs temperature of the three cyclized compounds: for B-
OXA, FB-OXA and DFB-OXA an outstanding decomposition
temperature Td (defined as the temperature at which the weight
loss is 5%) in air, respectively of 390, 402 ꢁC and 384 ꢁC, is observed.
Due to their very poor solubility, only a qualitative optical
analysis has been carried out for B-OXA, FB-OXA and DFB-OXA
compounds in 1,1-2,2-tetrachloroethane (TCE) solution. The
UVeVis spectra, reported in Fig. 6 (black line), show for all the
chromophores an absorption maximum at around 400 nm and
a clear vibronic structure.
Both precursor amides and benzodifuroxazinone chromophores
were analysed by means of FTIR spectroscopy. In the spectrum of
the amides diagnostic peak are those at w3200 cmꢂ1 (NeH
stretching of the amide), around w2980 cmꢂ1 (CeH stretching of
Chromophores thin films were evaporated on quartz substrates
ad their UVeVis spectra are reported in Fig. 6 (red line). Also in this
case a vibronic structure (with the appearing of a new feature at
higher wavelength) is observed with a general broadening and blue
shift of the absorption, if compared to the behaviour shown in
solution. The hypsochromic shift of the absorption in the solid state
can be associated to the face-to-face packing of the molecules,
characterized, as shown in the next paragraph, by a small molecular
slippage along the long axis of the molecules (H aggregate).
Table 1
Optical properties of the synthesized chromophores.
Fsol b
l
em
Ffilm
sol
a
sol
a
film
Chromophores
l
(nm) lfiablms (nm)
l
(nm)
abs
em
(nm)
B-OXA
FB-OXA
DFB-OXA
425/399/379 440/408/384/370 439/471/498 0.020 525 0.14
426/400/380 438/406/382/366 438/470/498 0.023 521 0.15
421/395/379 437/403/379/364 439/474/495 0.011 535 0.10
a
Fig. 6. UVeVis spectra of chromophores in tetrachloroethane solution (black line) and
as thin film (red line). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
The spectra were recorded in 1,1-2,2-tetrachloroethane solution.
Measurement performed in 1,1-2,2-tetrachloroethane solution using quinine
b
sulphate as reference (Fquinine sulphate ¼ 0.546 in 1 N H2SO4, according to Ref. [43]).

A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
121
Fig. 8. Ortep view (from the top) of B-OXA, FB-OXA and DFB-OXA. Ellipsoids are drawn at 50% probability level (i ¼ ꢂxþ1, ꢂyþ1, ꢂz for B-OXA; ii ¼ ꢂx, ꢂy, ꢂz for FB-OXA;
iii ¼ ꢂxþ2, ꢂy, ꢂz for DFB-OXA).
Fig. 9. Crystal packing of FB-OXA. H atoms are not shown for clarity.

122
A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
Table 2
emission spectra are reported in Fig. 7), with F respectively of 14, 15
and 10%. As compared to solution, a consistent enhancement, up to
90 times, of fluorescence is observed in the solid state.
Pitch (:P) and roll (:R) angles and distances for B-OXA, FB-OXA and DFB-OXA.
:P (deg)a
:R (deg)b
d (Å)c
dP
dR
d
e
This noteworthy behaviour is rarely observed in organic
compounds (where the aggregate-induced quenching of the
emission is more common) and is referred to in the literature as
aggregation induced emission (AIE [27,28]). In many cases, AIE is
associated to a restricted motion of groups in the structure of the
molecule due to the constraints of the crystalline packing: the
mobility of these groups when the molecules are in diluted solution
(where they can be considered as isolated molecules) are respon-
sible for the thermal relaxation of the optical excitation and
therefore the quenching of the fluorescence. In this specific case it is
possible to argue that the rotation along the single bond connecting
the benzodifuroxazinone core and the terminal phenyl rings,
hindered in the crystalline state, is responsible for fluorescence
quenching observed in solution.
B-OXA
FB-OXA
DFB-OXA
59.9
40.9
59.5
25.6
2.3
25.6
3.38
3.40
3.38
5.84
2.94
5.74
1.61
0.13
1.62
a
:P is the pitch angle defined as in Ref. [22].
:R is the roll angle defined as in Ref. [22].
b
c
p-stacking distance.
d
e
Displacement distance along the long molecular axis.
Displacement distance along the short molecular axis.
For what concerns the emission spectra, the molecules show
a very low fluorescence is in solution (spectra not shown), with
a maximum fluorescence quantum yield of 0.23%. On the other
side, in the solid state B-OXA, FB-OXA and DFB-OXA present
a significant emission in the green zone of the spectrum (thin film
F
Optical properties are summarized in Table 1.
Fig. 10. Molecules (from the top) of B-OXA, FB-OXA and DFB-OXA showing some short intermolecular distances within a
p-stack (left) and tilt of molecules in adjacent stack
(right). H atoms are not shown for clarity.

A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
123
Table 3
It turns out that the all-planar conformation of the molecules
allows a -stack structure with face-to-face molecules. This
The computed HOMO and LUMO energies for the four molecules under consider-
ation and pentacene.
p
arrangement is quite different from the herringbone packing mode
typical of acene-type organic semiconductors like pentacene [21]. It
is to be noted that the herringbone pattern was also found in
Molecule
HOMO (eV)
LUMO (eV)
B-OXA
FB-OXA
DFB-OXA-O
DFB-OXA-N
Pentacene
ꢂ6.05
ꢂ6.15
ꢂ6.21
ꢂ6.23
ꢂ4.77
ꢂ2.78
ꢂ2.89
ꢂ2.98
ꢂ3.00
ꢂ2.58
dibenzo[d,d]benzo[1,2-b:4,5-b0]difuran [20],
a compound very
similar to the benzodifuroxazinone derivatives studied here, owing
to the presence of the benzodifurane group and five condensed
rings in the all-planar and full-extended conformation. As
mentioned in the introduction, the face-to-face
OXA, FB-OXA and DFB-OXA could positively affect the charge
mobility of these compounds by allowing a better overlap of
p-stacking in B-
3.2. Crystal structure
p
molecular orbitals as compared to the classical herringbone
structure.
Single crystals of compounds B-OXA, FB-OXA and DFB-OXA
were easily obtained by sublimation and XRD studies were per-
formed to determine the solid state structure. The molecular
structures of B-OXA, FB-OXA and DFB-OXA are shown in Fig. 8,
Crystal data and refinement details and selected bond lengths and
angles are reported, respectively, in Table S1 and S2 in Supporting
information.
Such a kind of face-to-face
p-stacking arrangement in crystals
may be favoured by weak intermolecular interactions. As reported
previously in the literature, weak intermolecular hydrogen bonds
between aryl CeH and carbonyl oxygen atom may stabilize sheets
of molecules in the crystal [44]. Moreover, CeF/p, F/H and F/F
intermolecular interactions may play a role in driving the crystal
Each compound crystallizes in the monoclinic P21/c space group
with one half molecule (sitting on a crystallographic inversion
center) contained in the asymmetric unit. All bond lengths in the
three structures are normal [40]. Geometric parameters in the
oxazine-4-one group agree with similar structures [41,42] and in
particular the N1]C7 and C8]O3 double bonds are clearly
localized.
A central planar core, consisting of five condensed rings and by
two terminal phenyl rings, characterizes the molecular structure in
each compound. Each terminal phenyl ring bears one p-attached
fluorine atom in FB-OXA and two (o- and p-attached) fluorine
atoms in DFB-OXA. The terminal phenyl rings are quite in-plane
with the central benzodifuro-oxazin-4-one group, being stabi-
lized in this conformation by intramolecular CeH/O and CeH/N
hydrogen bonds. As a consequence, the overall shape of molecule is
completely planar and fully extended.
packing of fluoroorganic compounds [45] and may induce a stack-
ing of molecules with CeF bond lying parallel and above the
p ring
of the adjacent molecule.
These structural peculiarities are found in the crystal packing of
the three structures B-OXA, FB-OXA and DFB-OXA. In fact, sheets of
molecules are formed through intermolecular CeH/O]C weak
interactions (C2eH/O3i ¼ 0.93, 2.585 Å, :122.3ꢁ, i ¼ ꢂx, 1ꢂy, ꢂz
for B-OXA; C2eH/O3ii
¼
0.93, 2.494 Å, :169.2ꢁ
ii ¼ ꢂxꢂ1,ꢂyþ2,ꢂz for FB-OXA; C2eH/O3iii ¼ 0.93, 2.514 Å,
:160.1ꢁ iii ¼ 1ꢂx, 2ꢂy, ꢂz for DFB-OXA). The sheets of coplanar
molecules are then piled up in a slipped way to form endless
columns of molecules
distance between molecules in a
p
e
p
stacked along b axis. The interplanar
-stack is 3.38 Å for B-OXA, 3.40 Å
p
for FB-OXA and 3.38 Å for DFB-OXA (the stack distance is calcu-
lated between the mean planes of adjacent molecules using
Mercury program).
An interesting structural feature, common to all the three
The criteria for a good pep stacking can be defined by the pitch
compounds, concerns the crystal packing, where a pep stacking of
(:P) and roll (:R) angles, according to literature [22]. Small values
of these angles leave a good overlap between adjacent molecules.
The values calculated for the three structures are reported in
Table 1.
molecules is observed. Since a similarity in the crystal packing
mode of the three compounds was found, only a view of FB-OXA
crystal packing is reported in Fig. 9, as an example.
Fig. 11. The HOMO (left) and LUMO (right) wave function for the (a) B-OXA, (b) FB-OXA and (c) DFB-OXA molecules. Iso-surfaces are plotted for both positive (red) and negative
(blue) values at a value corresponding to 10% of maximum and minimum, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)

124
A. Carella et al. / Dyes and Pigments 95 (2012) 116e125
The relatively small values observed in the Table 2 are in keep
with a good overlap that is a necessary condition to ensure
3.3. Computational analysis
pep
a higher carrier mobility. In particular, the values very close to 40ꢁ
and 0ꢁ for the pitch and roll angles in FB-OXA indicate a parallel
superimposition of molecules with fluorine atom above the
centroid of the adjacent stacked fluoro-aryl group.
Some short intermolecular distances are shown in Fig. 10, where
it is also shown that the molecules stacked in adjacent columns are
tilted at 57.6ꢁ (B-OXA), 81.8ꢁ (FB-OXA) and 57.4ꢁ (DFB-OXA).
Density functional theory calculations have been performed to
get insight on the electronic properties of the molecular units
composing the B-OXA, FB-OXA, DFB-OXA crystals. It should be
noticed, concerning the DFB-OXA molecule, that two non-
equivalent configurations might be found, one (labelled as DFB-
OXA-O) with the two additional F atoms (“F2” position in Fig. 8)
facing two O atoms (“O1” positions in Fig. 8), the other (labelled as
DFB-OXA-N) with the two additional F atoms facing the two N
atoms (“N1” positions in Fig. 8).
It is interesting to have a look at the HOMO and LUMO values (at
DFT level) for all the considered molecules, as shown in Table 2. The
effect of fluorination stabilizes both HOMO and LUMO levels of the
molecule and thus, the chromophore bandgap changes only slightly
in passing from B-OXA to DFB-OXA. This behaviour is in good
agreement with some results reported in previous works on fluo-
rinated derivatives of pentacene [46] and sesquithiophene [47]
(Table 3).
Moreover, the calculated HOMO energies are much lower than
those calculated for pentacene and therefore a higher stability
against oxidation is expected for our chromophores as compared to
pentacene.
The contour plots of the HOMO and LUMO wave functions,
shown in Fig. 11, do also confirm the strong similarity of the three
molecules: small changes are induced by H / F substitution (B-
OXA / DFB-OXA).
Starting from the crystallographic analysis described in the
previous section, electronic structure calculations for the B-OXA,
FB-OXA, DFB-OXA periodic crystals have been performed using the
Quantum-ESPRESSO package, as described in the methodology
section. The band structure and density of states (DOS) of the B-
OXA, FB-OXA and DFB-OXA compounds are shown in Fig. 12.
Near the Fermi levels, the HOMO and LUMO complexes consist
each of two bands, as the unit cell contains two molecules. Such
bands show almost a dispersionless character, apart from Z-G and
B-D paths, where a 0.2e0.3 eV dispersion is found for all the
investigated crystals. Both directions correspond to 010 direction in
real space hence suggesting, as expected, higher charge mobility
along the
p-stack directions. Parabolic fits of the valence bands
near the Z point reveal band curvature corresponding to hole
effective masses in the range 5e10 me for all crystals under
consideration.
4. Conclusions
We reported on the synthesis and the properties of a new class
of heteroacene molecules based on a novel benzodifuroxazinone
core. The new molecules, prepared by means of a double thermal
cyclization starting from precursor amides, show an outstanding
thermal stability in air, with decomposition temperatures as high as
402 ꢁC. They are moreover characterized by an uncommon
enhancement of fluorescent properties in the solid state, with an
increase of photoluminescent quantum yield up to 90 times as
compared to solution. Single crystals suitable for XRD analysis were
obtained by sublimation and the crystal structure of the new
compounds was solved. It turned out that the all-planar confor-
mation of the molecules allows a
p-stack structure with face-to-
face molecules, stabilized by intermolecular CeH/O]C weak
interactions and different from the typical herringbone structure
found in most of the acene derivatives. The electronic properties of
the reported molecules and crystals were investigated by a detailed
computational analysis: an increasing stabilization of the HOMO
and LUMO energy of the molecules upon fluorination, was
demonstrated. It has to be pointed out that, for all molecules, the
Fig. 12. The band structure (left) and DOS (right) of (a) B-OXA, (b) FB-OXA and (c)
DFB-OXA crystal. The arrows mark the highest occupied and lowest unoccupied
electronic states. DOS have been computing with a 0.15 eV Gaussian smearing. Special
points in the Brilluoin zone (crystal units): Z(0 ½ 0);
B(0 0 ½); D(0 ½ ½); E(ꢂ½ ½ ½); C(½ ½ 0). The zero energy corresponds to the top
G
(0 0 0); Y(½ 0 0); A (ꢂ½ 0 ½);
valence band.

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