Functionalized thienoacridines: Synthesis, optoelectronic, and structural properties

Article abstract of DOI:10.1139/cjc-2014-0348

The preparation and characterization of hexylated, brominated, and 5-hexyl-2-thienyl-substituted tetrathienoacridine (TTAc) is described. Comparative electrochemical, optical, computational, and solid-state studies with their thienoanthracene (TTAn) analogs demonstrate that replacement of CH with nitrogen leads to an overall lowering of the frontier molecular energy levels and decrease in the HOMO-LUMO energy gap. While the brominated TTAc and TTAn derivatives are isostructural, as indicated by X-ray diffraction studies, the lower sublimation temperature and thermochromic behaviour exhibited by bromo-TTAc is attributed to an additional dipole-dipole interaction resulting from the dipole moment afforded by carbon/nitrogen substitution in the core aromatic ring. This work represents a fundamental study of heteroatom substitution in 2-D conjugated small molecules and investigates how inclusion of an electron deficient aromatic ring into the molecular framework (i.e., acridine versus anthracene) can influence the physical properties.

Full text of DOI:10.1139/cjc-2014-0348

1106
ARTICLE
Functionalized thienoacridines: synthesis, optoelectronic, and
structural properties
Sean F. Robertson, Alicea A. Leitch, Ilia Korobkov, Dmitriy V. Soldatov, and Jaclyn L. Brusso
Abstract: The preparation and characterization of hexylated, brominated, and 5-hexyl-2-thienyl-substituted tetrathienoacridine
(
(
TTAc) is described. Comparative electrochemical, optical, computational, and solid-state studies with their thienoanthracene
TTAn) analogs demonstrate that replacement of CH with nitrogen leads to an overall lowering of the frontier molecular energy
levels and decrease in the HOMO–LUMO energy gap. While the brominated TTAc and TTAn derivatives are isostructural, as
indicated by X-ray diffraction studies, the lower sublimation temperature and thermochromic behaviour exhibited by bromo-
TTAc is attributed to an additional dipole−dipole interaction resulting from the dipole moment afforded by carbon/nitrogen
substitution in the core aromatic ring. This work represents a fundamental study of heteroatom substitution in 2-D conjugated
small molecules and investigates how inclusion of an electron deficient aromatic ring into the molecular framework (i.e.,
acridine versus anthracene) can influence the physical properties.
Key words: ␲-conjugated small molecules, fused-ring systems, heterocyclic chemistry, dipole−dipole interactions, carbon/nitrogen
substitution, thermochromic behaviour.
Résumé : La préparation et la caractérisation de tétrathiénoacridine (TTAc) portant un substituant hexyle, bromure ou 5-hexyl-
2
-thiényl est présentée. Des études comparatives électrochimiques, optiques, computationnelles et a` l’état solide portant sur les
analogues thiénoanthracène (TTAn) de ces composés démontrent que la substitution d’une unité CH par azote produit un
abaissement global des niveaux d’énergie des orbitales moléculaires frontières et une réduction de la bande d’énergie interdite
entre les orbitales moléculaires HO et BV. Bien que les dérivés bromés TTAc et TTAn soient isostructurels, comme l’indiquent les
études de diffraction des rayons X, la température de sublimation inférieure et le comportement thermochromique observés
dans le cas du bromo-TTAc sont attribuables a` une interaction dipôle−dipôle supplémentaire résultant du moment dipolaire
occasionné par la substitution du carbone par l’azote dans le noyau aromatique central. Les présents travaux représentent une
étude fondamentale de la substitution d’hétéroatomes dans de petites molécules conjuguées en 2-D dans le cadre de laquelle on
a analysé comment l’inclusion d’un noyau aromatique pauvre en électrons dans une structure moléculaire (comme l’acridine
par comparaison avec l’anthracène) peut influer sur les propriétés physiques. [Traduit par la Rédaction]
Mots-clés : petites molécules ␲-conjuguées, systèmes cycliques fusionnés, chimie des hétérocycles, interactions dipôle−dipôle,
substitution carbone/azote, comportement thermochromique.
Introduction
towards air oxidation due to the presence of electronegative ni-
trogen atoms that lowers the energy of the HOMO, (ii) the poten-
Organic semiconductors, both molecular and polymeric, have
seen significant development over the past two decades owing to
their potential use in next-generation electronic devices. In the
case of molecular materials, a particularly important requirement
for the realization of high-performance thin film optoelectronic
devices is tight ␲-stacking interactions, which provide a pathway
for charge transport. For example, field effect transistors based on
tial for structural control through S–N and N–HC interactions, and
iii) the possibility of strong orbital overlap between the larger
(
␲
-orbitals of sulfur leading to enhanced electronic communica-
3
–5
tion between molecules.
Taking advantage of these desirable
attributes, an abundance of S−N-containing heterocycles have
been reported. For example, due to the electron-deficient nature
of the framework, systems containing pyridyl thiadiazole (1) (Fig. 1)
have been employed as acceptor components in donor−acceptor
1
small molecules (e.g., pentacene) generally attain higher mobili-
ties compared with polymer-based devices owing to the crystallin-
ity and, hence, morphology of the organic semiconductor. To
enhance intermolecular interactions, extended ␲-frameworks are
often utilized; however, the stability of higher oligoacenes under
ambient conditions can prove to be problematic for device oper-
ation and durability and provides the impetus to develop stable
alternatives.2
3
,6
7
5,8,9
molecules and polymers used in light-harvesting materials.
1
0
Heterocycles such as thiazolothiazole (2), benzobis(thiadiazole)
4
,5,9
11
(3),
and dithiatetrazocines (4) have been explored as electron-
transporting (n-type) semiconductors for use in electronic appli-
cations (e.g., organic field effect transistors, photovoltaics, etc.)5
due to their low-energy LUMOs.
,12,13
In that regard, incorporating heteroatoms such as nitrogen and
sulfur into planar ␲-conjugated frameworks provides several ben-
efits over their all-carbon variants including (i) improved stability
In addition to incorporation of heteroatoms, enhanced stability
and intermolecular interactions may be achieved by increasing
the dimensionality of ␲-conjugation molecules through the use of
Received 30 July 2014. Accepted 5 September 2014.
S.F. Robertson, A.A. Leitch, and J.L. Brusso. Department of Chemistry, University of Ottawa, Ottawa, ON K1N 6N5, Canada; Centre for Catalysis
Research and Innovation, University of Ottawa, Ottawa, ON K1N 6N5, Canada.
I. Korobkov. Department of Chemistry, University of Ottawa, Ottawa, ON K1N 6N5, Canada.
D.V. Soldatov. Department of Chemistry, University of Guelph, Guelph, ON N1G 2W1, Canada.
Corresponding author: Jaclyn L. Brusso (e-mail: jbrusso@uottawa.ca).
Can. J. Chem. 92: 1106–1110 (2014) dx.doi.org/10.1139/cjc-2014-0348
Published at www.nrcresearchpress.com/cjc on 17 October 2014.

Robertson et al.
1107
Fig. 1. Examples of heterocyclic and star-shaped core moieties.
represents a fundamental study of heteroatom substitution in
2-D conjugated small molecules and investigates how a carbon/
nitrogen exchange in a fused heterocyclic compound can influ-
ence the physical properties.
Results and discussion
Synthesis
The thienoacridine framework can be realized via fourfold
Stille coupling of tetrabromopyridine with 2-tributylstannyl-5-
hexylthiophene (9b) followed by oxidative cyclodehydrogenation
1
9,30
with FeCl3,
as outlined in Scheme 1. While this route was
effective in the preparation of 8b, 8d was not accessible through
this synthetic procedure due to the electron-donating character
2
1,22,30,31
of the alkylated oligothiophenes.
The 5-hexyl-2-thienyl
functionalized thienoacridine framework can, however, be
achieved through fourfold Stille coupling of 2-tributylstannyl-5-
hexylthiophene (9b) with the brominated derivative 8c, the latter
prepared from treatment of unsubstituted tetrathienopyridine
(
10a) with N-bromosuccinimide followed by FeCl3 cyclization.
Since 8c is essentially insoluble, purification required high-
temperature vacuum sublimation at which point a colour change
from bright yellow to orange was noted at elevated temperatures,
a property not observed in the anthracene analog (supporting
information Fig. S1 (see Supplementary material section)). Inter-
estingly, not only was thermochromic behaviour detected but the
acridine derivative also sublimed at much lower temperatures
(
350 versus 420 °C for 7c). To probe the differences between the
“
star-shaped” aromatic cores.14–16 In general, for 2-D aromatic
brominated anthracene and acridine derivatives, single-crystal
X-ray diffraction analysis was carried out on suitable crystals ob-
tained from sublimation (vide infra). Although the insolubility of
17
molecules (e.g., trithienothiophene (5), dibenzothienotetrathio-
phene (6), tetrathienoanthracene (TTAn) (7)15,19–23), packing in
18
the solid state favours ␲···␲ interactions over the ␲···H–C herring-
8
c may be a cause of concern, coupling reactions with solubilizing
alkylthiophenes is possible, as fourfold Stille coupling of 8c with
b afforded 8d as a microcrystalline red-orange solid.
bone structures commonly observed in linear acenes,1
3,24
leading
to an enhancement in the degree of intermolecular communica-
tion.
9
1
4,15
Combining expansion of conjugation in 2-D with the inclusion
of heteroatoms, tetrathienoacridine (TTAc) (8) represents an ideal
central core unit for star-shaped conjugated materials. Not only
does TTAc possess a planar, delocalized ␲-electron system, but it is
also anticipated to form a highly organized 2-D thin film network
and exhibit high thermal stability in air similar to TTAn, its an-
Electrochemical, theoretical, and spectroscopic studies
To elucidate the electronic structure of the acridine framework
and explore the impact of nitrogen versus CH in the thienoacene
core, electrochemical, theoretical, and optical spectroscopic
studies were carried out on 8b and 8d and compared with their
anthracene analogs (7b and 7d, respectively); the results are sum-
marized in Table 1. The redox behaviour of 8b and 8d was probed
by cyclic voltammetry, revealing irreversible oxidation processes
corresponding to the formation of a radical cation (supporting
information Fig. S2) from which the HOMO levels of 8b and 8d
were estimated to be –5.50 and –5.16 eV, respectively. This corre-
sponds to a lower HOMO energy level for the acridine derivatives
compared with their anthracene analogs of 0.17 eV (7b ¡ 8b) and
1
9–22,25
thracene analog.
In regards to the acridine moiety, it has
been explored for a variety of applications (e.g., dyes for photovol-
2
6
27
taics and organic light-emitting diodes, dopants for C60, as a
2
8
component in metal-free magnetic materials, etc.); however, un-
til now, acridine has been absent from fused 2-D ␲-conjugated
frameworks such as 8. Inclusion of the electron-deficient pyridyl
ring in the core is expected to lower the energy levels of the
frontier molecular orbitals and, due to introduction of a dipole
moment along the center of the molecule, influence the solid-
state interactions. Recently, a report on phenazine-based systems
demonstrated such an impact on the frontier molecular orbitals
0.05 eV (7d ¡ 8d). This trend is consistent with DFT calculations
and can be attributed to inclusion of the electron-deficient pyridyl
3
2
ring in the ␲-conjugated framework.
To probe the photophysical properties of 8b and 8d, absorption
andphotoluminescencestudieswereperformedondichlorometh-
ane solutions; the results are presented in Fig. 2 and Table 1. The
UV-vis spectrum of 8b is comprised of two regions, one consisting
of intense bands at ϳ300–350 nm and another containing a clus-
ter of weak, broad bands at ϳ400–460 nm. The absorption profile
for 8d is similar to that of 8b; however, a much less defined
vibronic structure is observed, which can be attributed to a less
rigid structure resulting from flexibility of the peripheral thienyl
due to the increased electronegativity of the core (compared with
anthracene).29 However, unlike acridine, phenazine does not pos-
sess a dipole moment along the conjugated core. In this work, we
set out to investigate the effect replacement of CH with nitrogen
in the core moiety of star-shaped molecules and the subsequent
dipole moment that results have on the optoelectronic and solid-
state properties of thienoacenes. As such, we herein report the
preparation and characterization of the hexylated parent com-
pound 8b and the 5-hexyl-2-thienyl functionalized derivative 8d.
Comparative studies with their anthracene analogs (7b and 7d,
respectively) using cyclic voltammetry, UV-vis, and photolumines-
cence spectroscopy along with computational methods demon-
strate that the acridine framework leads to lower HOMO–LUMO
energy gaps. The crystal structure of the brominated derivative 8c
was determined by single-crystal X-ray diffraction and revealed
slipped ␲-stacks with close intermolecular interactions. This work
3
0
substituents. Furthermore, the spectrum of 8d is bathochromi-
cally shifted by ϳ50–100 nm, as is expected due to the increased
conjugation afforded by the additional thiophene moieties.2
Based on the onset of UV-vis absorption, the extended ␲-conjugated
framework of 8d leads to a decrease in the optical energy gap of
0.23 eV (cf. 8b). Compared with their anthracene analogs, replace-
ment of CH with nitrogen in the aromatic core leads to a decrease
1,22
Published by NRC Research Press

1108
Can. J. Chem. Vol. 92, 2014
Scheme 1. Synthesis of functionalized thienoacridines. Reagents
and conditions: (a) 2,3,5,6-tetrabromopyridine, PdCl (PPh ) , DMF,
Fig. 2. (a) Solution and (b) TD DFT calculated absorption spectra of
7b (red broken line), 7d (blue broken line), 8b (red solid line), and
8d (blue solid line). (c) Molecular orbital energy diagram of 7b, 7d,
8b, and 8d. H = HOMO, L = LUMO. Colour in online version only.
2
3 2
1
30 °C; (b) N-bromosuccinimide, THF, 0 °C; (c) FeCl , MeNO /DCM or
3 2
PhCl, RT; (d) 9b, PdCl (PPh ) , DMF, 130 °C.
2
3 2
Table 1. Electrochemical,a theoretical,b and photophysical proper-
c
ties for 7b, 7d, 8b, and 8d.
7
bd
7dd
0.94
−5.11
−5.12
2.95
475
8b
1.33i
−5.50
−5.50
3.32
450
8d
0.98i
−5.16
−5.28
2.83
500
E1/2ox (V)
1.15
−5.33
−5.21
3.35
431
EHOMO (eV)e
exp
calcd
EHOMO
Egap
(eV)
calcd
(eV)
abs
␭max (nm)
abs
␭edge (nm)
440
493
483
530
Egapopt (eV)f
2.83
416
2.52
501
2.57
436
2.34
528
Calculated ␭max (nm)g
abs
abs
g
Calculated ␭edge (nm)
430
527
452
550
opt
g
Calculated E
(eV)
3.303
435
2.833
490
3.300
482
2.796
490
gap
PL
h
␭max (nm)
Stokes shift (eV)
0.03
0.08
0.06
0.08
a
4 6
Dichloromethane solutions with 0.1 mol/L n-Bu NPF as supporting electro-
+
33
lyte referenced to the Fc/Fc couple of ferrocene at +0.48 V versus SCE.
b
DFT/B3LYP/6-311+G(d,p) level of theory where R = Me.
c
Measurements performed in dichloromethane.
d
21
Data taken from Leitch et al. and provided here for comparison purposes.
e
ox
Fc/Fc+
34
Derived using the following expression: EHOMO = –e(E – E
+ 4.8).
f
Calculated from ␭edge
.
g
From TD DFT calculations on optimized geometries.
In dichloromethane, upon excitation at 370 nm for 7b, 320 nm for 7d, 340
h
nm for 8b, and 400 nm for 8d.
iIrreversible behaviour; Epa value quoted.
in the optical energy gap of 0.26 eV for 7b ¡ 8b and 0.18 eV for
7d ¡ 8d, which is consistent with the DFT calculations.
The optical transitions were investigated further using time-
dependent DFT (TD DFT) calculations performed on optimized
geometries of 8b and 8d (where R = Me) along with their an-
thracene analogs. These calculations are often employed to gain
3
5
insight into the optical transitions observed for organic and
3
6
inorganic molecules. The theoretical spectra and frontier mo-
lecular orbitals are presented in Fig. 2. The 20 vertical excitation
energies calculated for each compound are listed in the support-
ing information. Overall, the calculated absorption profiles are
consistent with those experimentally obtained; the calculated
longest wavelength absorptions and small bathochromic shifts
associated with the introduction of nitrogen atoms into the aro-
matic core (i.e., acridine versus anthracene) match the experimen-
tal behaviour well.
All of the molecules show the same sets of molecular orbitals,
since all of the molecules are based on the same ␲-conjugated
framework. Inclusion of nitrogen atoms does, however, influence
the energy and coefficients of the molecular orbitals. While the
LUMOs of the acridine and anthracene derivatives all have the
same orbital symmetry, the orbital symmetry of the HOMOs are
not all the same — the HOMO and HOMO–1 for 8d are switched
compared with the other analogs. This observation is consistent
with previously reported systems that replace CH for nitrogen in
the acene core;29 however, it should be noted that the energy
difference between these molecular orbitals is only 0.03 eV. In
regards to 8b, 7b, and 7d, the molecular orbitals that contribute
to the major absorption bands are the same due to the similarity
of the orbital symmetries around the frontier molecular orbitals.
For these compounds, the excitation attributed to the HOMO ¡
LUMO transition is associated with very low oscillator strength
(f ≤ 0.11) (see supporting information Table S2), whereas the highest
wavelength absorption has moderate oscillator strength (f < 0.5)
and results from contributions of the HOMO–1 ¡ LUMO and
HOMO ¡ LUMO+1 transitions. The band with the largest oscillator
strength (f > 1) corresponds to two transitions, one involving
excitation from the HOMO ¡ LUMO+1 and the other from the
HOMO–1 ¡ LUMO. In the case of 8d, the long-wavelength band
corresponds to transitions from the HOMO ¡ LUMO and HOMO ¡
Published by NRC Research Press

Robertson et al.
1109
Fig. 3. (a) Unit cell and (b) slipped ␲-stack drawings of 8c with close
intermolecular contacts highlighted with broken lines. Br–Br=
contacts are shown in brown, Br–S= in purple, S–S= in red, and S–C=
in green. Colour in online version only.
Table 2. Crystal data for 7c and 8c.
ca
7
8c
C H Br NS
4
719.14
Formula
Formula weight
Crystal system
Space group
a (Å)
C22H Br S
718.15
Monoclinic
6
4
4
21
5
4
Monoclinic
P2 /n
P2 /n
1
1
13.9674(6)
3.9096(2)
19.9845(9)
106.906(3)
1044.13(8)
2
2.284
298
8.119
13.9071(8)
3.9226(3)
20.0698(12)
107.672(3)
1043.18(12)
2
2.289
200
8.127
49.55
b (Å)
c (Å)
␤
(°)
3
V (Å )
Z
Dcalcd (g/cm3)
T (K)
–1
␮
(mm )
␪max (°)
No. of total reflections
2
56.62
10037
12355
No. of unique reflections 2536
Rint 0.0306
R , wR (on F )
1762
0.0460
2
0.0474, 0.1270 0.0397, 0.0965
1
2
b
␦
(Å)
3.557(1)
3.542(3)
64.56(5)
c
␶
(°)
65.48(2)
a
Taken from Leitch et al.21
b␦ is the mean interplanar separation between molecules along
the ␲-stack.
c
␶
is the tilt angle between the mean molecular plane and the
stacking axis.
that are aligned in herringbone arrays running along the z-direction.
The molecules are essentially planar, as the core atoms show min-
imal displacement from the mean plane (0.0183 Å versus 0.0211 Å
for 7c) formed by the 26-atom core. Close S–S= (3.701(2) Å), Br–Br=
(
3.692(1) Å), and S–Br= (3.728(1) Å) contacts exist laterally between
the herringbone ribbons, as shown in Fig. 3a. Additional S–C=
interactions (3.564(5) and 3.580(5) Å) exist along the slipped
␲-stacks (see Fig. 3b).
Since 8c is isostructural with 7c, the only significant difference
between the structures is the dipole moment resulting from re-
placement of CH with nitrogen along the middle of the molecular
framework. It is expected that the molecules in the slipped
LUMO+1. The band associated with the largest oscillator strength
(f = 1.579) corresponds to transitions involving excitation from the
HOMO–1 ¡ LUMO+1 and HOMO ¡ LUMO. Interestingly, enhanced
molar absorptivity is associated with inclusion of an electron-
deficient aromatic ring into the molecular framework (i.e., acri-
dine versus anthracene), which was observed experimentally (see
supporting information Table S4) and is evident in the calculated
spectra, with oscillator strengths more than doubled for the acri-
dine derivatives compared with their anthracene analogs for the
lower energy transitions.
␲-stacks pack in such a way that their dipoles alternate, which
should double the unit cell dimension along b. However, since CH
and nitrogen groups are isoelectronic, they are virtually indistin-
guishable and the model used involves disorder of the nitrogen
atom over two positions about the acridine core with 0.5 occu-
pancy in the unit cell. At the same time, in the real structure, the
dipoles of adjacent molecules in each ␲-stack are ordered and
alternate to minimize the lattice energy. This ordering of the
dipoles explains the thermochromic behaviour observed in 8c but
not in 7c. The colour of 8c crystals reversibly changes from yellow
to orange-red with increasing temperature gradually (differential
scanning calorimetry did not reveal a phase transition and the
colour change can be seen visually; see supporting information
Figs. S4 and S1, respectively). The temperature-induced red shift of
the corresponding absorption band indicates that the energy dif-
ference between the ground and excited states decreases with
Both acridine derivatives (8b and 8d) showed blue fluorescence
with the trends observed in the absorption profiles also noted in
the emission spectra — namely the bathochromic shift associated
with extension of the ␲-conjugation as well as nitrogen introduc-
tion to the core (compared with their anthracene analogs). Com-
parison of the Stokes shifts reveals a slightly larger energy for 8d,
which may also be explained by molecular flexibility; however,
the difference is minimal and, when compared with fused thieno-
acenes (e.g., 0.28 eV for pentathienoacene),37 is indicative of little
structural distortion upon excitation.
3
8
increasing temperature. Previous experimental and theoreti-
3
9
cal studies for pyridine revealed that excitation (singlet n−␲*
transition) reverses the direction of its dipole moment. The corre-
sponding transition in 8c will disrupt the dipole−dipole ordering
in the crystal and lead to an increased energy gap. At the same
time, the energy gap could decrease enough to reveal colour
changes as the molecules move away from each other due to
thermal motion at elevated temperatures. In other words, the
temperature-induced colour change exhibited by 8c may be at-
tributed to a red shift of the HOMO–LUMO absorption band due to
weaker dipole−dipole interactions in the solid state at elevated
temperatures.
X-ray diffraction studies
To probe the effect of nitrogen inclusion on the packing
arrangement and provide insight into the thermochromic behav-
iour, single crystals of the brominated derivative 8c were ana-
lyzed by X-ray diffraction and compared with its anthracene
analog 7c. Two views of the solid-state structure, that of the unit
cell and the slipped ␲-stacked motif, are shown in Fig. 3. Pertinent
2
1
crystallographic data for 8c and 7c are listed in Table 2. Similar
to the anthracene derivative, crystals of 8c belong to the mono-
clinic space group P2 /n and consist of slipped ␲-stacks (along y)
1
Published by NRC Research Press

1110
Can. J. Chem. Vol. 92, 2014
(
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To summarize, hexylated tetrathienoacridine (8b) along with
its brominated analog 8c and 5-hexyl-2-thienyl functionalized de-
rivative 8d have been prepared and isolated. Comparative electro-
chemical, optical, computational, and solid-state studies with
thienoanthracene-based systems 7b, 7c, and 7d were carried out
to probe the influence that inclusion of an electron-deficient moi-
ety into the conjugated core has on the optoelectronic and struc-
tural behaviour. Based on these studies, we can conclude that
replacement of CH with nitrogen leads to an overall lowering of
the frontier molecular energy levels and decrease in the energy
gap between the HOMO and LUMO as evidenced by the shifts in
the oxidation potentials and absorption profiles, respectively, and
is supported by DFT calculations. X-ray diffraction studies reveal
that 8c is isostructural with 7c, but the lower sublimation tem-
perature and thermochromic behaviour exhibited by 8c may be
attributed to an additional dipole−dipole interaction resulting
from the dipole moment afforded by carbon/nitrogen substitu-
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The authors would like to thank Philip Bulsink for assistance
with the TD DFT calculations and are grateful to the Natural Sci-
ences and Engineering Research Council of Canada for funding
this work.
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