Molecular tuning in highly fluorescent dithieno[3,2-b:2′,3′-d] pyrrole-based oligomers: Effects of N-functionalization and terminal aryl unit

Article abstract of DOI:10.1039/c2cp40161d

A series of eight conjugated oligomers consisting of central dithieno[3,2-b:2′,3′-d]pyrroles (DTPs) end-capped with either thienyl or phenyl groups have been prepared from N-alkyl-, N-aryl-, and N-acyl-dithieno[3,2-b:2′,3′-d]pyrroles via Stille and Suzuki cross-coupling. The DTP-based quaterthiophene, N-phenyl-2,6-bis(2-thienyl) dithieno-[3,2-b:2′,3′-d]pyrrole was characterized via X-ray crystallography and was found to crystallize in the orthorhombic space group Pna2₁ with a = 10.8666(3) A, b = 22.8858(6) A, c = 7.4246(2) A, and Z = 4. The full oligomeric series was thoroughly investigated via photophysical, electrochemical, and DFT calculations in order to correlate the cumulative effects of both aryl end-groups and N-functionalization on the resulting optical and electronic properties. Through such molecular tuning, it was found to be possible to modulate the HOMO energy by as much as 0.32 V and to generate highly fluorescent oligomers with solution fluorescence efficiencies as high as 92%. This journal is the Owner Societies 2012.

Full text of DOI:10.1039/c2cp40161d

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PAPER
Molecular tuning in highly fluorescent dithieno[3,2-b:20,30-d]pyrrole-based
oligomers: effects of N-functionalization and terminal aryl unitw
Sean J. Evenson,a Ted M. Pappenfus,b M. Carmen Ruiz Delgado,c
´
Karla R. Radke-Wohlers, J. T. Lopez Navarrete and Seth C. Rasmussen*
a
c
a
Received 16th January 2012, Accepted 1st March 2012
DOI: 10.1039/c2cp40161d
A series of eight conjugated oligomers consisting of central dithieno[3,2-b:20,30-d]pyrroles (DTPs)
end-capped with either thienyl or phenyl groups have been prepared from N-alkyl-, N-aryl-, and
N-acyl-dithieno[3,2-b:20,30-d]pyrroles via Stille and Suzuki cross-coupling. The DTP-based
quaterthiophene, N-phenyl-2,6-bis(2-thienyl)dithieno-[3,2-b:20,30-d]pyrrole was characterized via
X-ray crystallography and was found to crystallize in the orthorhombic space group Pna21 with
a = 10.8666(3) A, b = 22.8858(6) A, c = 7.4246(2) A, and Z = 4. The full oligomeric series was
thoroughly investigated via photophysical, electrochemical, and DFT calculations in order to
correlate the cumulative effects of both aryl end-groups and N-functionalization on the resulting
optical and electronic properties. Through such molecular tuning, it was found to be possible to
modulate the HOMO energy by as much as 0.32 V and to generate highly fluorescent oligomers
with solution fluorescence efficiencies as high as 92%.
typically more soluble than their polymeric analogues and
Introduction
often can be vaporized at reduced pressure. As such, oligomer
thin films can be deposited through a variety of methods
including solution casting and vacuum sublimation.7 This ease
of processing can make such oligomeric materials useful not
only as model compounds, but also as potential active materials
for a number of device applications.
Conjugated organic materials continue to be a topic of
considerable fundamental and technological interest as they
combine the electronic and optical properties of inorganic
semiconductors with a number of distinct advantages typical
of organic plastics. Such benefits include light-weight and
mechanically-flexible materials, low production costs, and
the ability to tune key material properties at a molecular level
via synthetic modification.1–4 Within the larger class of conjugated
materials, oligomers have played an important role as models of
polymeric materials, allowing greater insight into their respective
electronic and optical properties. Such oligomeric models
allow for strong correlation between observed properties and
overall chemical structure, which can be more rigidly controlled
in oligomers, particularly in terms of a defined conjugation
length.4–17 Low molecular weight oligomeric materials are also
Oligothiophenes are thought to be one of the more dynamic
classes of conjugated organic oligomers because of their
environmental stability, as well as the ease in which they can
be synthetically modified.5 Thiophene chemistry is well established
and modification of the core backbone is nearly limitless, allowing
extensive tuning of the electronic properties. In addition, the
planar structure and strong p interactions of oligothiophenes
enhance favourable stacking on substrates and in the bulk, while
the polarizability of sulfur leads to excellent charge transport
properties.5 One approach to the synthetic modification of
oligothiophenes has been the introduction of fused aromatic
units into the conjugated backbone, resulting in materials
exhibiting enhanced carrier mobilities and lowered band gaps. Such
fused-ring thiophene building blocks include 4H-cyclopenta-
[2,1-b:3,4-b0]bithiophene (1, CPBT), dithieno[3,2-b:20,30-d]thiophene
(2, DTT), and N-alkyl- or N-aryl-dithieno[3,2-b:20,30-d]pyrroles
(3, DTPs) as shown in Fig. 1.18–25
a Department of Chemistry and Biochemistry, North Dakota State
University, NDSU Dept. 2735, P.O. Box 6050, Fargo, ND 58108,
United States. E-mail: seth.rasmussen@ndsu.edu;
Fax: +1 701 231-8831; Tel: +1 701 231-8747
b Division of Science and Mathematics, University of Minnesota,
Morris, MN 56267, United States.
E-mail: pappe001@morris.umn.edu; Fax: +1 320 589-6371;
Tel: +1 320 589-6340
c Department of Physical Chemistry, University of Ma´laga, Campus
In particular, DTPs have found recent popularity and their
application to oligomeric systems has resulted in increased
conjugation, reasonable hole mobilities, and significantly enhanced
solution and solid-state fluorescence properties.20–25 For example,
the fluorescence quantum yield of DTP-based quaterthiophenes
have been reported to be as high as 66%, constituting one of
´
de Teatinos s/n, Malaga 29071, Spain. E-mail: carmenrd@uma.es
w Electronic supplementary information (ESI) available: NMR
spectra for all compounds, X-ray data for 3b, 7b, and 15a, and full
computational details and results. CCDC reference numbers
863469–863471. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cp40161d
c
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Results and discussion
Synthesis
As previously reported, both first and second generation
N-functionalized dithieno[3,2-b:20,30-d]pyrroles (3) can be
readily synthesized from 3,30-dibromo-2,20-bithiophene.19,26
N-Functionalized DTPs can then be incorporated into oligomer
backbones via either Stille or Suzuki coupling methods as
shown in Scheme 1. The distannyl intermediates (14) of either
N-alkyl- or N-arylDTPs were generated from a previously
reported procedure in nearly quantitative yields.26 Reaction
with the appropriate arylbromide via Stille cross-coupling27
allowed production of oligomers 7 and 12 in moderate to good
yield (41–81%).
Fig. 1 Fused-ring bithienyl building blocks.
the highest reported for any oligothiophene.21 In addition, such
DTP-based oligomers exhibit measurable solid-state emission
and thus such oligomers could be promising materials for organic
light emitting diode (OLED) applications.21
More recently, successful OLED devices utilizing DTP-based
polymeric materials have been reported in which it was found that
the solution and solid-state emission of the DTP-based materials
were significantly enhanced via copolymerization with arylene
units.26 In order to model such DTP-arylene materials and to
strengthen our understanding of the structure-property relationships
of DTP-based oligomers, a series of 2,6-bis(2-thienyl)- and
2,6-bis(phenyl)-substituted DTPs (7,8 and 12,13 respectively,
as shown in Fig. 2) have been prepared in order to thoroughly
investigate the effect of the aryl endgroups on the oligomer
properties.
In the application of the N-acylDTPs, a different approach
is required due to the susceptibility of the acyl carbonyl to
nucleophilic attack, which eliminates the use of processes
requiring deprotonation via BuLi. However, N-acylDTPs
can be readily brominated via treatment with NBS giving the
dibromo intermediates (15) as stable species in high yield
(70–95%). This is in marked contrast to the N-alkylDTPs
for which the dibromides are extremely reactive and are prone to
readily decompose via polymerization.28 Intermediates 15 can
then be used to produce the corresponding N-acylDTP-based
oligomers 8 or 13 via either Stille or Suzuki29 cross-coupling in
moderate yields (32–47%).
Despite the popularity and success of DTP-based materials,
the high energy of the DTP HOMO has been viewed as a
limitation which can hinder stability and the effective application
of DTP-based materials to various devices. As a solution, a new
class of DTPs incorporating N-acyl groups have been recently
reported.19 The resulting N-acylDTPs exhibit stabilized
HOMO and LUMO energy levels, as well as a slight red-shift
in absorption energies. To further investigate the effect of
these ‘second generation’ DTP building blocks, oligomers
8 and 13 have been included in the current study in order to
fully investigate the molecular tuning effects of both aryl
endgroups (i.e. 7 vs. 12; 8 vs. 13) and N-functionalities
(i.e. 7 vs. 8; 12 vs. 13).
X-ray crystallography
There has been relatively little structural data reported for
oligothiophenes, particularly for species containing fused-ring units
such as DTT or DTP. While structures of a few DTT-based
oligomers have been reported,30–32 DTP-based structures have been
limited primarily to the DTP monomers.18,19 Only three structures
Fig. 2 Quaterthiophenes and phenyl-capped analogues.
6102 Phys. Chem. Chem. Phys., 2012, 14, 6101–6111
Scheme 1 Synthesis of DTP-based oligomers.
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Table 1 Selected Experimental Geometric Parameters of DTP-based
Quaterthiophenes 7b and 7c, and DTP 3b
Parameter
7b
7ca
3b
S(3)–C(7)
S(3)–C(22)
C(6)–C(7)
C(7)–C(8)
1.703(3)
1.699(3)
1.445(3)
1.43(1)
1.643(2)
1.583(3)
1.467(3)
1.643(2)
—
—
—
—
C(8)–C(9)
C(9)–C(22)
S(2)–C(5)
S(2)–C(6)
C(4)–C(5)
C(5)–C(11)
C(11)–C(21)
C(6)–C(21)
N(1)–C(11)
1.42(1)
1.605(3)
1.337(4)
1.720(2)
1.744(2)
1.407(3)
1.394(3)
1.419(3)
1.360(3)
1.400(2)
1.497(3)
95.35(15)
116.8(2)
116.8(2)
93.79(14)
120.98(17)
90.77(10)
112.83(16)
111.72(15)
111.97(18)
112.71(17)
106.54(15)
109.14(17)
107.42(17)
—
—
1.343(5)
1.726(3)
1.756(3)
1.415(3)
1.396(4)
1.407(3)
1.369(4)
1.391(3)
1.412(4)
92.59(15)
112.7(2)
113.5(5)
110.7(7)
121.8(2)
91.07(13)
112.56(18)
110.44(18)
111.3(2)
114.6(2)
106.7(2)
109.6(2)
106.8(2)
1.723(1)
1.737(1)
1.416(2)
1.392(2)
1.422(2)
1.365(2)
1.393(2)
1.420(1)
—
—
—
—
—
90.95(6)
113.6(1)
110.86(9)
110.5(1)
114.0(1)
106.68(9)
109.6(1)
107.0(1)
N(1)–C(13)
Fig. 3 Face (A) and edge (B) ellipsoid plots of oligomer 7b at the
C(7)-S(3)–C(22)
S(3)–C(22)–C(9)
C(8)–C(9)–C(22)
C(7)–C(8)–C(9)
S(3)–C(7)–C(6)
C(5)-S(2)–C(6)
S(2)–C(6)–C(21)
S(2)–C(5)–C(11)
C(6)–C(21)–C(11)
C(5)–C(11)–C(21)
C(11)-N(1)–C(19)
N(1)–C(11)–C(5)
C(4)–C(5)–C(11)
50% probability level.
of larger oligomeric species have been previously reported,
those of quaterthiophene 7c,21 a tetra(2,5-dimethylthien-3-yl)-
capped DTP25 and a bis[(diphenylamino)phenylene]-capped
DTP.22b Recently, however, oligomer 7b has been successfully
crystallized and its determined structure is shown in Fig. 3.
Selected bond distances and angles for 7b are given in Table 1,
along with those for 3b and 7c for comparison.
Oligomer 7b crystallizes in the orthorhombic space group
Pna21, with four molecules per unit cell. The central DTP unit
is fairly planar with a slight sigmoidal distortion (mean deviation
of 0.059 A) and its bond lengths and angles are in good
agreement with the analogous DTP monomer 3b. Both external
thiophene rings are rotated out of the plane of the central DTP
unit, with torsion angles of 23.81 and 21.01, respectively. This is
consistent with the structure of 7c and agrees well with current
(see supporting information) and previous21 DFT geometry
optimizations exhibiting torsion angles of B201 between the
terminal thiophenes and the DTP unit. In a similar manner, the
N-phenyl group is also out of the plane of the DTP unit, with a
torsion angle of 46.11. While this is greater than the analogous
torsion angle of 37.01 observed in DTP 3b, it does fall within the
range of angles exhibited by all known N-phenylDTP structures
(37.0–49.71)18,22b,33 and is in excellent agreement with the value
predicted by its DFT-optimized structure (44.11).
Typical of all DTPs, the fused bond between the hetero-
cycles (i.e. C(5)–C(11) = 1.396 A, C(4)–C(19) = 1.391 A) is
elongated compared to thiophene (1.370 A) and is more
consistent with pyrrole (1.382 A).34 Due to the presence of
the pyrrole ring, the C(4)–C(5) distance (1.415 A) of 7b is
slightly shorter than the corresponding central interannular
bond of quaterthiophene (4), although it is fairly consistent
with the analogous bond of pyrrole. The corresponding inter-
annular bond lengths (1.46 and 1.45 A) between the central
DTP and the terminal thiophenes, however, are in good
agreement with the interannular bonds of 4 (1.45 A).35
The external thiophenes exhibit some deviations from typical
thiophene geometry due to positional disorder resulting from a
portion of these rings that are rotated 1801 around the interannular
bond, resulting in some mixing of the S3/C8 (ca. 9.64%)
and S4/C12 (ca. 12.98%) positions. Such disorder has been
previously observed for dialkylsexithiophenes,36 as well as
for 7c.21 Other deviations include shortened external a,b-CꢀC
a
Ref. 21.
bonds (i.e. C(9)–C(22) = 1.335 A) in comparison to the parent
thiophene (1.370 A).34 However, such shortening of external
a,b-CꢀC bonds is typical of a-oligothiophenes35 and has been
observed in previous DTP-based structures as well.18,19,21
The packing of 7b consists of herringbone edge-to-face
packed columns separated by the N-phenyl moieties (Fig. 4).
The molecules are oriented with an interplanar angle of B751
and each edge-to-face contact consists of two short Sꢁ ꢁ ꢁp
contacts of 3.390 A between the two central DTP units, as
well as two additional C–Hꢁ ꢁ ꢁp contacts. The shorter C–Hꢁ ꢁ ꢁp
contact (2.721 A) occurs between a hydrogen on an external
thiophene and the central DTP unit, while the longer contact
(2.795 A) is between two external thiophenes. In addition, each
unit exhibits a third C–Hꢁ ꢁ ꢁp contact (2.868 A) between
the N-phenyl group and the DTP unit of a third 7b molecule.
Fig. 4 Herringbone packing of 7b.
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Table 2 Collected solution and solid-state photophysical data
Solutiona
Thin Film
abs
em
abs
l
(nm)
e (Mꢀ1 cmꢀ1
)
l
(nm)
FF
l
(nm)
Compound
max
max
max
3ab
3b
310
310
305
320
390
408f
387j
400
396
399
398
376f
389f
369f,g
381
378
394f
379h
380
380
26 100
32 000
15 400
11 000
34 700
324
7.78 ꢂ 10ꢀ4
3cc
3dc
4d
474e
0.16
331, (385), 450
340
344, 356
404, (434)
409, (439)
417, (444)
409, (430), (460)
440
5f
459, 486, (522)g,h
432, 455, (486)k
440, 470, (506)
444, (464)
0.24g,h
6i
7a
56 000
45 100
54 000
44 000
35 500f
0.32
0.70
0.68
0.73
0.29f
0.27h
7b
8a
446, 468, (504)
440, (467)
8b
9l
433, 455, (490)f
435h
10m
11
413, 432n
12a
12b
12co
12dp
13a
13b
61 000
58 400
48 800
422, 442, (473)
421, 440, (476)
0.53
0.87
393, (420)
354, (414)
361
400
406, 430
381, (419)
423, 444h
424, 439, (466)
424, (443)
0.57h
0.62
0.92
43 200
45 500
a
b
c
d
e
f
g
In CH3CN, values in parentheses represent distinct shoulders. Ref. 18. Ref. 19. Ref. 37. Ref. 38. In CHCl3. Ref. 39. In THF.
h
i
j
k
Ref. 40. In CH2Cl2. In C6H12
l
m
n
o
p
. Ref. 41. Ref. 42. Ref. 43. Ref. 23b. Ref. 22b.
Along the b axis, adjacent herringbone columns are interconnected
by two additional types of C–Hꢁ ꢁ ꢁp contacts, a DTP-terminal
thiophene contact of 2.786 A, as well as a phenyl-terminal
thiophene contact of 2.889 A.
The most dramatic trend within the oligomers investigated
is the stark energetic difference in the absorbance of the
thiophene-capped oligomers in comparison to their phenyl-
capped analogues. As shown in Fig. 5, the phenyl-capped
oligomers exhibit a B20 nm blue-shift in comparison to the
corresponding thiophene-capped analogues. This trend is
consistent with the difference in the parent oligomers 4 and 9
and can be at least partially attributed to increased steric
interactions between the phenyl group and the bithienyl core.
This is in line with the more distorted geometry calculated for
the phenyl-capped DTPs in comparison to their thienyl-
substituted analogues (i.e., DFT modeling of thiophene- and
phenyl-capped DTPs reveal that the torsional angles between
the terminal external rings and the DTP unit are B71 greater
for phenyl end-capped than for their thienyl end-capped
analogues, see supporting information). Another additional
factor here is the increased electron confinement in the more
aromatic benzene in comparison to that of thiophene.44
In terms of N-functionalization effects, previous studies of
DTP monomers have shown that N-alkyl- and N-aryl-DTPs
exhibit similar lmax values for the lowest energy transition.
However, the additional conjugation of the N-aryl group
results in broadening of this transition and thus a slightly
red-shifted onset of absorption (B5–10 nm).18,28 Comparing
N-alkyl- to N-acyl-DTPs again show significant broadening of
the low energy transitions of the N-acylDTPs and an even
greater red-shift for the absorption onset (B20 nm). It
has been proposed that the lowest energy transition of the
N-acylDTPs exhibits some charge transfer (CT) character due
to the fact that the N-acyl group does not contribute to the
DTP HOMO, but does contribute to the LUMO.19 This
CT nature thus accounts for the observed broadening and
red-shifted absorption. The combination of aryl and acyl
functionality in N-benzoyl side chains resulted in the greatest
observed red-shifts.
Absorption properties
Photophysical data for all of the DTP-based oligomers are
given in Table 2 and representative UV-vis spectra of 7a, 8a,
12, and 13a are shown in Fig. 5. For comparison, data for
the DTPs 3a–d, the parent oligomers quaterthiophene (4) and
5,5-diphenyl-2,20-bithiophene (9) and the fused-ring CPBT
and DTT analogues (5, 6, 10, and 11) are also included in
Table 2.37–43 Previous studies on oligothiophenes have shown
that an increase in oligomer size, and thus conjugation length,
results in a bathochromic shift in the absorption spectrum as
well as an increase in the molar absorptivity.37 As such, it is
not surprising that all of the oligomers in Table 2 exhibit lower
energy absorptions than their fused-ring bithienyl precursors
(i.e. DTP, CTBT, or DTT).
Fig. 5 Representative UV-vis spectra in CH3CN.
c
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Table 3 Results of TDDFT//B3LYP/6-31G** calculations for the DTP-based oligomers
Oligomer
Emax (eV)
f
Description
7a
7b
8a
8b
12a
12b
13a
13b
2.96
2.97
2.87
2.73
3.21
3.23
3.11
2.87
1.29
1.24
1.08
0.37
1.31
1.24
1.03
0.22
HOMO - LUMO (99%)
HOMO - LUMO (99%)
HOMO - LUMO (98%)
[HOMO - LUMO] (77%) + [HOMO - LUMO + 1] (20%)
HOMO - LUMO (98%)
HOMO - LUMO (98%
HOMO - LUMO (98%)
[HOMO - LUMO] (77%) + [HOMO - LUMO + 1] (15%)
It could be expected that the same trends in N-functionalization
effects would carry over to the corresponding DTP-based
oligomers. However, as illustrated by the solution absorption
data in Table 2 and Fig. 5, this does not seem to be the case.
All of the investigated oligomers exhibit spectra with nearly
identical absorption profiles (lmax shifts of B4 nm or less and
nearly identical absorption onsets), with the only deviation
being that the N-acyl-functionalized oligomers exhibited an
additional higher energy transition of low intensity near 300 nm.
To make these observations even more perplexing, the side chains
that gave red-shifts in the DTP monomers actually produced
slight blue-shifts in the corresponding oligomers, and thus the
N-alkylDTP oligomers gave the lowest energy lmax values.
To investigate the optical properties of the DTP-based
oligomers, the lowest-energy electronic excited states were
calculated using the time-dependent DFT (TDDFT) approach
(see Table 3). The calculated transition energies, although slightly
red-shifted, are in good agreement with the experimental values.
For all the DTP-based oligomers, the most intense lowest-
energy electronic excitation is mainly attributed to a one-
electron excitation from the HOMO to the LUMO. As previously
seen for N-alkyl-18 and N-acyl-DTPs19 and the DTP-based
oligomer 7c,21 both the HOMO and LUMO are of p nature
and mainly spread over the entire conjugated backbone with no
contribution from the N-alkyl or N-aryl groups (see Fig. 6),
although significant electron density is found on the N-acyl
group. The observed blue-shift in absorption for the phenyl-
capped oligomers is well reproduced by the theoretical calcula-
tions (i.e., the excitation energy is calculated to increase by 0.25 eV
on going from 7a and 12a). However, theoretical calculations
predict a red-shift of the lowest energy absorption upon replacing
N-alkyl or N-aryl groups with N-acyl (i.e., 3.21 eV in 8a and
3.11 eV 13a), which is in line with the experimental trend
previously found for the DTP monomers.
The deviation of the solution absorption data from the trend
previously observed from the monomeric DTPs could be explained
by different degrees of rotational disorder among the various
DTP-based oligomers investigated. It is understood that
conjugated oligomers and polymers adopt a nonplanar, or coiled,
conformation in solution,45–47 providing that interannular
rotations between units are possible. It is theorized that the
extent of this disorder (or deviation from planarity) reflects a
competition between solvation energy, largely influenced by
the polarizability of the conjugated species, and the energy of
the rotational conformation.45 The extent of conjugation
influences the polarizability of the backbone, and thus the
solvation energy. As such, the solvent-oligomer interactions
determine the optimal backbone conformation and therefore the
extent of conjugation in solution. While changes in typical side
chains should have little effect on polarizability, the addition of
the N-acyl groups directly affect the electronic nature of the
oligomer and could thus contribute to changes in polarizability.
As a result, the solution conformation of the N-acyl oligomers
could be very different than the N-alkyl analogues, thus resulting
in attenuation of the full conjugation length which would account
for the lack of red-shift in the spectra as initially expected.
In an attempt to verify this theory, the absorption properties
of the DTP-based oligomers were also examined as spin-cast
thin films. As solid-state films, the solvent interactions are
removed and the oligomers would be expected to adopt more
planar conformations with maximized conjugation lengths. As
shown in Fig. 7, the solid-state spectra now follow the trends
experimentally observed in their parent monomers and those
as predicted by theoretical calculations. It can therefore be
shown that for conjugated backbones beyond the monomers,
alkyl vs. aryl side chains have little effect on the absorption
properties, while acyl and benzoyl groups both give respective
shifts to lower energy.
Fig. 6 Electronic density contours, calculated at B3LYP/6-31G**
level, for the HOMO and LUMO frontier molecular orbitals of
7a and 8a.
Fig. 7 Solid-state absorption spectra of DTP-based quaterthiophenes.
Phys. Chem. Chem. Phys., 2012, 14, 6101–6111 6105
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The lack of correlation between the solution data and the
predicted structure-function relationships for these oligomers
is an excellent example of the complexity of determining such
relationships by utilizing solution data alone. The greater
understanding of the competing effects involved in determining
the solution absorption properties of these DTP-based oligomers
now accounts for some of the previously unexplained incon-
sistencies found in comparing solution data of seemingly similar
oligomers.21,23a
of DTP in comparison to other fused-ring units such as
CPBT.21 The application of saturated alkyl side chains such
as the N-octyl groups in 7a and 12a add high frequency modes
which can result in a greater number of deactivation pathways
via internal conversion, contributing to reductions in FF.10,52
These modes increase with the number of methylene units on
the alkyl chain and it has been previously shown that the
application of smaller alkyl groups results in significantly higher
efficiencies (FF = 0.65 for 7c in CH3CN).21 The lack of such
high frequency modes in the N-phenyl derivatives also accounts
for the large increase in FF for 7b and 12b.
Solution emission properties
Somewhat unexpectedly, the application of N-acyl groups
also results in large increases in fluorescence efficiency. The
N-octanoyl groups of 8a and 13a do have one less methylene
unit in comparison to the N-octyl groups of 7a and 12a, but
this is not enough to account for the large difference in FF. The
addition of carbonyl groups to oligothiophenes has previously
been shown to reduce non-radiative decay and increase
FF.53,54 In these cases, it was proposed that this was due to
an increase in conjugation, which is known to decrease inter-
system crossing and thus reduce non-radiative decay via
vibrational relaxation from the triplet state.37 Nevertheless, a
doubling of FF has been shown to be possible by application
of the acyl group to DTP-based oligomers. The gains in
efficiency are greatest in oligomers initially exhibiting weaker
fluorescence (7a to 8a), although smaller increases are still seen
even for stronger emitters (12a to 13a). Although the N-benzoyl
oligomers 8b and 13b both exhibit strong emission, particular
care had to be made to remove an unknown trace impurity that,
when present, quenched the fluorescence in these species.
Oligomers containing DTP cores have been previously shown
to exhibit quite strong fluorescence in comparison to typical
oligothiophenes20,21 and thus one of the primary goals of the
current study was to further understand how to enhance
fluorescence in these systems via molecular tuning. The emission
energies and quantum yields (FF) for the full series are collected in
Table 2 and representative emission spectra of the DTP-based
oligomers 7a, 8a, 12a, and 13a are shown in Fig. 8. For all
oligomers, the observed emission spectra are essentially identical
to the parents 437 or 941 in both energy and structure. The
observed structured emission are characteristic of a planar excited
state and is consistent with the planar quinoid-like structure
proposed for the singlet excited state of oligothiophenes.48 The
vibrational spacings are B1100–1500 cmꢀ1, which correspond
well with the ring-breathing modes of thiophene and pyrrole.49
As with the absorption data discussed above, the reduced
conjugation of the phenyl-capped oligomers causes a blue-shift
of B20 nm in the emission in comparison to the thiophene-
capped analogues. However, this increase in energy is offset by a
significant increase in quantum efficiency. This enhanced emission
is consistent with other phenyl-containing oligothiophenes41 and
can be attributed to the fact that reduced thiophene content
lessens spin–orbit coupling as a consequence of the heavy atom
affect. As a result, lower triplet quantum yields reduce loss via
non-radiative processes from the triplet state.21
Electrochemistry
In order to determine the effect of molecular structure on the
HOMO, the electrochemistry of the DTP-based oligomer
series was investigated by cyclic voltammetry (CV). The
representative voltammogram of 7a is shown in Fig. 9 and
the CV data for the full series is collected in Table 4. All
oligomers exhibit an initial well-defined, quasi-reversible redox
couple followed by a second irreversible oxidation. This
electrochemical behaviour agrees well with that previously
reported for the analogous phenyl-capped species 12c23b and
12d,22b although the second oxidation was not observed in the
case of 12c.
While not that surprising for the phenyl-capped oligomers,55
the reversibility of the initial oxidation is somewhat unusual for
an oligothiophene. For typical oligothiophenes, oxidation results
in the formation of a radical cation with significant radical density
on the a-positions of the external thiophenes. As such, these
oxidations are typically irreversible due to rapid coupling of the
In terms of N-functionalization effects, oligomers containing
simple alkyl side chains (7a and 12a) exhibit the lowest quantum
yields within the DTP-based series. However, these lower efficiencies
are still higher than any of the other analogous oligomers without
DTP cores. The enhanced emission of DTP-based oligomers has
been previously attributed to its ring fusion, which reduces low
frequency, interannular torsional vibrations responsible for
non-radiative deactivation,50,51 and the photochemical stability
Fig. 8 Representative emission spectra in CH3CN.
6106 Phys. Chem. Chem. Phys., 2012, 14, 6101–6111
Fig. 9 Cyclic voltammogram of 7a.
c
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Table 4 Cyclic voltammetry data for DTP oligomersa
(V) EHOMO (eV)b
0/+1
+1/+2
Oligomer E1/2
(V) DE (mV) Epa
7a
7b
0.30
100
80
80
80
0.93
ꢀ5.29
ꢀ5.35
ꢀ5.37
ꢀ5.45
ꢀ5.40d
ꢀ5.43
ꢀ5.43
ꢀ5.46
ꢀ5.56
ꢀ5.60
0.36
0.38
0.47
0.36
0.49
0.47
0.51f
0.58
0.62
1.06
1.05
1.09
12a
12b
12cc
12de
8a
8b
13a
13b
180
100
1.31
0.86
1.09
1.13
1.13
60
80
a
Potential vs. Ag/Ag+ measured in millimolar argon-sparged CH3CN
b
solutions with 0.10 M TBAPF6 as supporting electrolyte. EHOMO
=
Fig. 10 DFT-calculated HOMO levels (in absolute values) and
vertical ionization potentials (IPs) for DTP-based oligomers. The
experimental potential values for the first oxidations are included for
comparison.
c
ꢀ(E[onset,ox vs. Fc+/Fc] + 5.1)(eV). Ref. 23b, in CH2Cl2, second oxidation
d
not observed. Determined from E1/2, rather than onset. Ref. 22b,
e
f
in CH2Cl2. Irreversible oxidation, Epa is reported.
the large HOMO stabilization experimentally observed upon
N-acyl substitution; for instance, the IP value increase by
0.25 eV on going from N-alkyl-substituted 7a to the analogous
N-acyl-substituted 8a. The combined experimental and
computational results clearly show that the high lying HOMO
thought to be a limitation in some DTP-based materials can be
overcome through application of the new N-acylDTP building
blocks.
radical cations resulting in larger oligomeric or polymeric
species.56 For the DTP-based oligothiophenes here, however,
it is possible to cycle repeatedly through the first redox couple
with no change in electrochemistry and no detectable
coupling.
In order to further investigate this unusual behaviour, the
electron spin density was calculated for the radical cation of 7a
(see supporting information). While the calculated spin-density
contour was delocalized over the full oligomer backbone, the
density profile revealed that 62% of the unpaired spin was
localized on the central DTP unit, with only 14% of the spin on
each of the a-positions of the external thiophenes. Thus,
coupling of the radical cation may be possible at either lower
scan rates or if the potential was held after the initial oxidation,
but was not observed at a scan rate of 100 mV sꢀ1. Coupling
and the generation of polymeric material was observed to occur
fairly rapidly if cycling was carried out to potentials beyond the
second oxidation. The fact that coupling occurs after the second
oxidation suggests that this process generates a diradical
dication, rather than a simple dication.
Experimental methods
Unless noted, all materials were reagent grade and used without
further purification. N-OctylDTP 3a,26 N-acylDTPs 3c,d,17
oligomer 7a,21 and 2-(tributylstannyl)thiophene58 were prepared
according to previously reported literature procedures. Dry
THF and toluene were obtained via distillation over sodium/
benzophenone. All glassware was oven-dried, assembled hot,
and cooled under a dry nitrogen stream before use. Transfer of
liquids was carried out using standard syringe techniques and all
reactions were performed under dry nitrogen. Chromatographic
separations were performed using standard column chromato-
graphy methods with silica gel (230–400 mesh). Melting points
were determined using a digital thermocouple with 0.1 1C
resolution. 1H and 13C NMR spectra were recorded in CDCl3
on a 400 MHz spectrometer. All NMR data are referenced to
the chloroform signal and peak multiplicity reported as follows:
s = singlet, d = doublet, t = triplet, q = quartet, p = pentet,
m = multiplet and br = broad. HRMS (ESI) was performed
in house.
As can be seen in Table 4, it is possible to tune the energy of
the HOMO by over 300 mV through a combination of the choice
of aryl end-cap and N-functionalization. Not surprisingly, the
lower conjugation of the phenyl-capped oligomers results in
stabilization of the HOMO by B80–110 mV in comparison to
the analogous thienyl-capped oligomers. However, the choice of
N-functionalization also results in significant modulation of the
HOMO energy, with stabilization increasing in the following
order: N-alkyl o N-aryl o N-acyl o N-benzoyl. Thus the
combination of N-benzoyl groups with external phenyl groups
gives the most stabilized HOMOs, while N-alkyl groups and
thiophene end-caps give the most destabilized HOMOs.
N-Phenyldithieno[3,2-b:20,30-d]pyrrole (3b)
DTP 3b was prepared using previously reported methods,59
substituting tBu3P with BINAP. The crude product was
purified via column chromatography in hexanes, producing
a white solid (87%). mp 125.4–126.1 1C; 1H NMR d: 7.58
(m, 4H), 7.50 (m, 3H), 7.31 (m, 2H); 13C NMR d: 144.3, 141.2,
137.8, 129.9, 123.6, 122.8, 116.9, 112.6; HRMS: m/z 256.0242
[M+] (calcd for C14H9NS2 256.0249).
Ionization potentials (IPs) are calculated at both Koopmans’
theorem57 (KT) and self-consistent-field (DSCF) levels (see Fig. 10
and supporting information). It is found that the trends in the
vertical IPs and the calculated HOMO values are very similar;
that is, Koopmans’ theorem (where IP = ꢀEHOMO) applies well.
In agreement with the higher oxidation potentials obtained for
phenyl end-capped oligomers in comparison to their thienyl
analogues, larger IP values and more stabilized HOMO levels
are found (i.e., the vertical IP is estimated to be 6.01 eV in 7a and
6.11 eV in 12a). DFT-calculated IP values also nicely predict
General procedure for synthesis of 2,6-distannylDTPs
The desired DTP (2.08 mmol) was combined with dry
hexane (60 mL) and the solution cooled to 0 1C. TMEDA
c
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(0.89 mL, 6.0 mmol) was added, followed by BuLi (2.5 M in
hexanes, 2.4 mL, 6.0 mmol) and the mixture stirred for 2 h.
Me3SnCl (1.0 M in THF, 6.0 mL, 6.0 mmol) was then added
and the solution stirred overnight. This was then poured over
Et3N-treated silica gel, filtered, and rinsed with hexane. The
filtrate was concentrated via rotary evaporation and then
pumped overnight. The product was stored under N2 in the
freezer until further use.
123.2, 116.8, 108.3; HRMS: m/z 407.0807 [M+] (calcd for
C26H17NS2 407.0797).
General procedure for the bromination of N-acylDTPs
In a 3-necked flask, the desired N-acylDTP (2.31 mmol) was
dissolved in 20 mL of dry CHCl3. The flask was equipped with
an addition funnel and wrapped in Al foil in order to reduce
exposure to light. A solution of NBS (0.90 g, 5.08 mmol) in
20 mL MeCN was cooled to 0 1C and then added dropwise
via the addition funnel. The reaction was allowed to stir
overnight, after which water was added and the mixture was
extracted with CHCl3. The combined organic layers were dried
with MgSO4, filtered, and concentrated via rotary evaporation.
The crude solid was then placed in boiling hexanes, filtered hot,
and allowed to recrystallize producing a light brown solid.
N-Octyl-2,6-bis(trimethylstannyl)dithieno[3,2-b:20,30-d]pyr-role
(14a)
Isolated as a pale yellow oil (99% yield). 1H NMR: d 7.00
(s, 2H), 4.18 (t, J = 7.2 Hz, 2H), 1.88 (p, J = 7.2 Hz, 2H), 1.35
(m, 2H), 1.27 (m, 8H), 0.87 (t, J = 7.2 Hz, 3H), 0.40 (s, 18H);
13C NMR: d 148.1, 131.4, 120.8, 118.2, 47.7, 32.1, 30.7, 29.4,
28.7, 27.3, 22.9, 14.3, ꢀ7.9.
2,6-Dibromo-N-octanoyldithieno[3,2-b:20,30-d]pyrrole (15a)
N-phenyl-2,6-bis(trimethylstannyl)dithieno[3,2-b:20,30-d]-
85%; mp 98.9–99.3 1C; 1H NMR: d 7.52 (br, 2H), 2.93 (t, J =
7.2 Hz, 2H), 1.85 (p, J = 7.6 Hz, 2H), 1.37 (m, 8H), 0.90
(t, J = 7.0 Hz, 3H). 13C NMR: d 169.4, 142.1, 137.5, 119.8,
111.9, 36.5, 31.9, 29.4, 24.3, 22.9, 14.3; HRMS: m/z 462.9101
[M+] (calcd for C16H17NOS2Br2 462.9092).
pyrrole (14b)
Isolated as an orange solid (99% yield). mp 131.8 1C (dec);
1H NMR: d 7.63 (m, 2H), 7.54 (m, 2H), 7.33 (m, 1H), 7.17
(s, 2H), 0.40 (s, 18H); 13C NMR: d 147.1, 140.3, 136.9, 130.0,
125.9, 123.0, 122.8, 119.3, ꢀ7.9.
N-Benzoyl-2,6-dibromodithieno[3,2-b:20,30-d]pyrrole (15b)
General procedure for synthesis of N-alkyl- and N-aryl-DTP-
70%; mp 184.8–186.2 1C; 1H NMR: d 7.70 (m, 4H), 7.58
(tt, J = 7.2, 0.4 Hz, 1H), 6.88 (s, 2H); 13C NMR: d 167.0,
143.1, 134.5, 132.6, 129.1, 129.3, 121.8, 119.3, 111.7.
based oligomers
To a 60 mL Shlenk tube was added the desired intermediate 14
(0.50 mmol), Pd2dba3 (0.01 g, 0.01 mmol), and P(o-tolyl)3
(0.01 g, 0.04 mmol). The Schlenk tube was evacuated and
backfilled with N2 three times. Degassed toluene (10 mL) was
then added followed by either bromobenzene or 2-bromothio-
phene (1.1 mmol). The reaction was heated with stirring at
95 1C until completion (ca. 24 h). Water was added and the
mixture extracted with CHCl3. The combined organic layers
were dried with MgSO4 and concentrated via rotary evapora-
tion to give the crude product which was purified via column
chromatography (2% EtOAc in hexane).
General procedure for synthesis of N-acylDTP-based oligomers
via Stille cross-coupling
To a 60 mL Shlenk tube was added the desired intermediate 15
(1.00 mmol), 2-(tributylstannyl)thiophene (0.82 g, 2.20 mmol),
Pd2dba3 (0.02 g, 0.02 mmol), and P(o-tolyl)3 (0.02 g, 0.08 mmol).
The Shlenk tube was evacuated and backfilled with N2 three
times. Degassed toluene (12 mL) was then added and the reaction
heated with stirring at 95 1C until completion (ca. 24 h). Water
was added and the mixture extracted with CHCl3. The combined
organic layers were dried with MgSO4 and concentrated via
rotary evaporation to give the crude product which was purified
via column chromatography (5% EtOAc in hexane).
N-Phenyl-2,6-bis(2-thienyl)dithieno[3,2-b:2030-d]pyrrole (7b)
81%; mp 193.5 1C (dec); 1H NMR: d 7.60 (m, 4H), 7.39
(m, 1H), 7.21 (m, 6H), 7.03 (m, 2H); 13C NMR: d 144.9, 142.1,
138.7, 135.8, 130.2, 128.1, 126.7, 124.4, 123.5, 123.2, 116.2,
108.8; HRMS: m/z 418.9931 [M+] (calcd for C22H13NS4
418.9925).
N-Octanoyl-2,6-bis(2-thienyl)dithieno[3,2-b:2030-d]pyrrole (8a)
1
47%; mp 138.6–139.3 1C; H NMR: d 7.57 (br s, 2H), 7.25
(dd, J = 3.6, 1.2 Hz, 2H), 7.23 (dd, J = 3.6, 0.8 Hz, 2H), 7.05
(t, J = 3.6 Hz, 2H), 3.02 (t, J = 7.2 Hz, 2H), 1.90 (p, J = 7.6 Hz,
2H), 1.52 (m, 2H), 1.38 (m, 6H), 0.91 (t, J = 6.8 Hz, 3H); 13C
NMR: d 169.7, 144.7, 139.1, 135.1, 128.2, 125.0, 123.9, 114.1,
107.7, 36.7, 32.0, 29.5, 29.4, 24.4, 22.9, 14.3; HRMS: m/z
469.0651 [M+] (calcd for C24H23NOS2 469.0657).
N-Octyl-2,6-diphenyldithieno[3,2-b:20,30-d]pyrrole (12a)
1
66%; mp 102.0–102.6 1C; H NMR: d 7.66 (d, J = 7.5 Hz,
4H), 7.40 (t, J = 7.5 Hz, 4H), 7.28 (t, J = 7.5 Hz, 2H), 7.27
(s, 2H), 4.32 (t, J = 7.0 Hz, 2H), 1.93 (p, J = 7.0 Hz, 2H), 1.30
(m, 10H), 0.87 (t, J = 6.0 Hz, 3H); 13C NMR: d 144.9, 142.1,
136.0, 129.2, 127.4, 125.6, 119.1, 107.2, 47.7, 32.0, 30.6, 29.4,
29.3, 27.3, 22.8, 14.3; HRMS: m/z 443.1733 [M+] (calcd for
C28H29NS2 443.1736).
N-Benzoyl-2,6-bis(2-thienyl)dithieno[3,2-b:20,30-d]pyrrole (8b)
1
37%; mp 151.2–152.0 1C; H NMR: d 7.77 (d, J = 6.4 Hz,
2H), 7.71 (t, J = 5.8 Hz, 1H), 7.59 (t, J = 6.2 Hz, 2H), 7.21
(d, J = 4.0 Hz, 2H), 7.14 (d, J = 2.4 Hz, 2H), 7.01 (t, J =
3.4 Hz, 2H), 6.91 (s, 2H); 13C NMR d: 169.9, 145.2, 142.4,
138.1, 136.7, 134.3, 132.6, 129.1, 128.9, 128.2, 124.9, 123.8,
112.9; HRMS: m/z 446.9887 [M+] (calcd for C23H13NOS4
446.9874).
N-Phenyl-2,6-diphenyldithieno[3,2-b:20,30-d]pyrrole (12b)
41%; mp 212.6 1C (dec); 1H NMR: d 7.64 (m, 6H), 7.58
(m, 2H), 7.40 (s, 2H), 7.39 (m, 5H), 7.27 (m, 2H); 13C NMR: d
144.2, 142.9, 139.9, 135.6, 130.2, 129.2, 127.6, 126.6, 125.6,
c
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General procedure for synthesis of N-acylDTP-based oligomers
via Suzuki cross-coupling
2525 unique (Rint = 0.0292) which were used in all calcula-
tions. The final wR2 was 0.0809 (all data).
To a 60 mL Shlenk tube was added the desired intermediate 15
(0.412 mmol), phenyl boronic acid (0.110 g, 0.906 mmol), and
Pd(PPh3)2Cl2 (0.014 g, 0.021 mmol). The tube was then
evacuated and backfilled with N2 three times. Degassed
toluene (20 mL) was then added, followed by a degassed solution
of K2CO3 in water (1.0 mL, 1.6 M). The reaction was then heated
with stirring at 95 1C until completion (ca. 24 h). Water was
added and the mixture extracted with CHCl3. The combined
organic layers were dried with MgSO4 and concentrated via
rotary evaporation to give the crude product which was purified
via column chromatography (5% EtOAc in hexane).
Crystal data for 15a
C16H17Br2NOS2, M = 463.25, mono-clinic, a = 8.9068(6),
b = 21.4647(14), c = 9.5892(6) A, V = 1736.2(2) A3, T =
100 K, space group P21/c, Z = 4, 15 990 reflections measured,
4003 unique (Rint = 0.0229) which were used in all calcula-
tions. The final wR2 was 0.0429 (all data).
Theoretical methodology
Calculations were performed with the Gaussian 03 program.60
The molecular geometries of the neutral and radical-ion
states were calculated at the DFT level using the B3LYP
functional61,62 and the 6-31G** basis set.63–65 For all model
oligomers, alkyl groups were shortened to save computational
time. The time-dependent DFT (TDDFT) approach was used
for the evaluation of the energies of the lowest singlet excited
states.66 The ionization potentials (IPs) and electron affinities
(EAs) were calculated directly from the relevant points on
the potential energy surfaces using the standard procedure
detailed in the literature.67
N-Octanoyl-2,6-diphenyldithieno[3,20b:20,30-d]pyrrole (13a)
1
32%; H NMR: d 7.63 (d, J = 7.2 Hz, 2H), 7.54 (br s, 2H),
7.41 (t, J = 7.6 Hz, 4H), 7.31 (t, J = 7.4 Hz, 4H), 2.99 (t, J =
7.2 Hz, 2H), 1.88 (p, J = 7.2 Hz, 2H), 1.43 (m, 8H), 0.90
(t, J = 6.8 Hz, 3H); 13C NMR: d 169.9, 144.9, 142.2, 136.3,
129.3, 128.2, 125.8, 120.2, 108.3, 36.6, 31.9, 29.5, 29.4,
24.4, 22.9, 14.3; HRMS: m/z 480.1460 [M+] (calcd for
C28H27NOS2 480.1426).
N-Benzoyl-2,6-diphenyldithieno[3,2-b:20,30-d]pyrrole (13b)
Preparation of thin films for solid-state spectroscopy
1
Oligomer thin films were prepared by spin-casting from
saturated oligomer solutions in CHCl3. The solutions were
then loaded onto clean glass substrates (B1 ꢂ 2 cm) and spun
at 1000 RPM for 60 s.
68%; mp 179.8–180.0 1C; H NMR: d 7.80 (d, J = 5.6 Hz,
2H), 7.72 (t, J = 5.8 Hz, 1H), 7.60 (t, J = 6.2 Hz, 2H), 7.52
(d, J = 6.0 Hz, 4H), 7.37 (t, J = 6.2 Hz, 4H), 7.28 (t, J =
6.4 Hz, 2H), 7.08 (s, 2H); 13C NMR d: 169.9, 144.3, 143.7,
142.9, 135.0, 134.7, 132.5, 129.3, 129.1, 129.0, 127.9, 125.7,
112.4; HRMS: m/z 435.0760 [M+] (calcd for C27H17NOS2
435.0746).
Absorption and emission spectroscopy
UV-vis spectroscopy was performed on a dual beam scanning
UV-vis-NIR spectrophotometer using samples prepared as
dilute CH3CN solutions in quartz cuvettes or as thin films on
glass slides. Spectroscopy solvents were dried over molecular
sieves prior to use. Solid-state absorption spectroscopy was
conducted on thin films on glass substrates.
X-ray diffraction
X-ray quality crystals of 3b and 7b were grown by the slow
evaporation of isopropanol solutions. Crystals of 15a were
grown from hexane solutions. The X-ray intensity data of the
crystals were measured at 100 K on a Bruker Kappa Apex II
Duo CCD-based X-ray diffractometer system equipped with a
Mo-target X-ray tube (l = 0.71073 A) operated at 2000 W of
power. The detector was placed at a distance of 5.000 cm from
the crystal and data collected via the Bruker APEX2 software
package. The frames were integrated with the Bruker SAINT
software package. The unit cell was determined and refined
by least-squares upon the refinement of XYZ-centeroids of
reflections above 20s(I). The structure was refined using the
Bruker SHELXTL (Version 5.1) Software Package.
Emission spectroscopy was performed using dilute, deoxygenated
CH3CN solutions at room temperature. Quantum yields were
determined using secondary methods with 9,10-diphenylanthracene
in deoxygenated cyclohexane as the reference.68 Prior to each
fluorescence measurement, the absorption spectrum was measured
to ensure that the maximum absorption of the solution was less
than 0.1. As the oligomer data was collected in CH3CN, corrections
for the difference in the solvent refractive indices were applied using
the following values: 1.426 for cyclohexane, and 1.350 for CH3CN.
Electrochemistry
Crystal data for 3b
All electrochemical methods were performed utilizing a three-
electrode cell consisting of platinum disc working electrode, a
platinum wire auxiliary electrode, and a Ag/Ag+ reference
electrode (0.251 V vs. SCE).69 Supporting electrolyte consisted
C14H9NS2, M = 255.34, monoclinic, a = 9.9552(11), b =
14.2894(16), c = 8.1057(9) A, V = 1128.8(2) A3, T = 100 K,
space group P21/c, Z = 4, 9952 reflections measured, 2707
unique (Rint = 0.0167) which were used in all calculations. The
final wR2 was 0.0721 (all data).
of 0.10
M
tetrabutylammonium hexafluorophosphate
(TBAPF6) in dry CH3CN. Solutions were deoxygenated by
sparging with argon prior to each scan and blanketed with
argon during the measurements. All measurements were collected
at a scan rate of 100 mV sꢀ1. EHOMO values were estimated from
the onset of oxidation in relation to ferrocene (50 mV vs. Ag/Ag+),
using the value of 5.1 eV vs. vacuum for ferrocene.70
Crystal data for 7b
C22H13NS4, M = 419.57, orthorhombic, a = 10.8666(3),
b = 22.8858(6), c = 7.4246(2) A, V = 1846.43(9) A3, T =
100 K, space group Pna2(1), Z = 4, 6775 reflections measured,
c
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Thiophene-based Materials, ed. I. F. Perepichka and D. F. Perepichka,
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6 T. Izumi, S. Kobashi, K. Takimiya, Y. Aso and T. Otsubo, J. Am.
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Conclusions
The molecular tuning effects of both aryl endgroups (thienyl
vs. phenyl) and N-functionalities (alkyl, aryl, acyl, benzoyl) on
DTP-based oligomers has been fully characterized via optical
spectroscopy, electrochemistry, and DFT calculations. As
previously seen in related materials, DTP oligomers end-capped
with thienyl groups gave longer wavelength absorption and
emission in comparison to their phenyl end-capped analogues.
In solution, no obvious effect was seen in the absorption spectra
from changes in N-functionalities, which was proposed to be
due to differences in solution conformation. As solid-state films,
however, side chain dependent red-shifts in absorption were
observed in the following order: alkylBaryl o acyl o benzoyl.
In addition, significant enhancements in fluorescence efficiencies
were observed for oligomers containing aryl end-groups or aryl/
acyl N-functionalities. These effects were found to be additive,
with the greatest enhancement from the combination of aryl
end-groups with acyl or aryl side chains. Lastly, it was found
that the HOMO energies can be stabilized either by the
application of aryl end-groups instead of thienyl end-groups
or by the choice of N-functionalization, with stabilization
increasing in the order N-alkyl o N-aryl o N-acyl o
N-benzoyl. Thus the combination of N-benzoyl with external
phenyl groups gives the most stabilized HOMOs. In conclusion,
the results presented herein strongly illustrate the ability to tune
the optical and electronic properties of DTP-based materials
not only via backbone modification, but through the choice of
N-functionalization of the DTP unit applied.
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Acknowledgements
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SCR thanks the ND EPSCoR Flexible Electronics program
(EPS-0814442) and North Dakota State University for support of
this research. We would also like to thank Dr Angel Ugrinov
(NDSU) for the collection of the X-ray crystal data and NSF-
CRIF (CHE-0946990) for the purchase of the departmental XRD
instrument. TMP acknowledges the University of Minnesota,
Morris (UMM) Faculty Research Enhancement Funds supported
by the University of Minnesota Office of the Vice President for
Research and the UMM Division of Science and Mathematics for
financial assistance, and The Supercomputing Institute of the
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the Ministerio de Ciencia e Innovacion (MICINN) through project
reference CTQ2009-10098 and by the Junta de Andalucıa through
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for a ‘‘Ramon y Cajal’’ Research contract. Thanks also to Brendan
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´
´
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