Synthesis and properties of fully conjugated indacenediselenophene and diindenoselenophene derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.12.096

The synthesis and characterization of indacenediselenophene (IDS) and diindenoselenophene (DIS), the selenophene-containing analogues of indacenedithiophene (IDT) and diindenothiophene (DIT), respectively, are described. Cyclic voltammetry reveals that ID

Full text of DOI:10.1016/j.tetlet.2014.12.096

Accepted Manuscript
Synthesis and properties of fully conjugated indacenediselenophene and diin-
denoselenophene derivatives
Jonathan L. Marshall, Gabriel E. Rudebusch, Chris L. Vonnegut, Lev N.
Zakharov, Michael M. Haley
PII:
S0040-4039(14)02173-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.12.096
TETL 45620
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
26 November 2014
15 December 2014
17 December 2014
Please cite this article as: Marshall, J.L., Rudebusch, G.E., Vonnegut, C.L., Zakharov, L.N., Haley, M.M., Synthesis
and properties of fully conjugated indacenediselenophene and diindenoselenophene derivatives, Tetrahedron
Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.12.096
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Graphical abstract:
Synthesis and properties of fully conjugated indacenediselenophene and
diindenoselenophene derivatives
Jonathan L. Marshall, Gabriel E. Rudebusch, Chris L. Vonnegut, Lev N. Zakharov, Michael M.
Haley*

Synthesis and properties of fully conjugated indacenediselenophene and
diindenoselenophene derivatives
Jonathan L. Marshall, Gabriel E. Rudebusch, Chris L. Vonnegut, Lev N. Zakharov, Michael M.
Haley*
Department of Chemistry & Biochemistry and the Materials Science Institute, University of
Oregon, Eugene, OR 97403-1253, USA
* Corresponding author. Tel.: +1 541 346 0456; fax: +1 541 346 0487.
E-mail address: haley@uoregon.edu (M.M. Haley).
Requiescat in pace Harry H. Wasserman
ABSTRACT
The synthesis and characterization of indacenediselenophene (IDS) and diindenoselenophene
(DIS), the selenophene-containing analogues of indacenedithiophene (IDT) and
diindenothiophene (DIT), respectively, are described. Cyclic voltammetry reveals that IDS and
DIS both undergo a two-electron reduction and a one-electron oxidation and have narrower
HOMO/LUMO energy gaps when compared to their thiophene-containing counterparts. The
UV-Vis spectrum of IDS exhibits a red-shifted absorbance maximum with respect to the S-
containing IDT, and single crystal XRD shows close C–Se contacts of 3.407 Å (IDS) and close
C–C contacts of 3.358 Å (DIS).
Keywords: Selenophene, indenofluorene, quinodimethane, polycyclic aromatics, organic
electronics
2

Introduction
The study of highly conjugated polycyclic hydrocarbons has attracted great interest in the
last two decades due to their interesting photophysical and electronic properties, as well as the
practical applicability of these compounds.1-9 Polycyclic aromatic hydrocarbons (PAHs) such as
pentacene (1) and related polycyclic aromatics (PAs) such as anthradithiophene (2) have been
used with great success in device applications from field effect transistors to light emitting
diodes (Fig. 1).5-7,10-13 Unfortunately, acenes are often susceptible to oxidative and photolytic
degradation, and as a result, heteroatom inclusion has been explored both to increase stability
and to tune the physical and electronic properties of the resultant PAs.13-19 Thieno-fusion has been
a particularly attractive option due to increased stability, processability and modified electronic
properties.20-24 In addition to its inclusion within small molecules, thiophene has long been an
important building block of oligomers and polymers because of the resultant polymer’s low
oxidation potential, high conductivity, and highly tunable electronic properties.14,25-27
Figure 1. Pentacene (1) and anthradithiophene (2).
In the past five years, selenium has emerged as a promising heteroatom for use in
electronic organics.11,28-37 Typically Se incorporation results in a narrower HOMO/LUMO energy
gap as well as stronger intermolecular interactions due to selenium’s easily polarized nature,
larger van der Waals radius, and typically lower oxidation and reduction potentials than
sulfur.31,34 Indeed, one of the best performing thin-film transistors to date is a selenophene-
containing polymer with an average hole mobility of 9.8 cm2 V–1 s–1.38
3

Since 2010 our lab has been studying an underexplored class of highly conjugated
pentacyclic hydrocarbons called indenofluorenes (IFs).39 Structurally similar to pentacene, the
inclusion of the two five-membered rings in indenofluorenes such as 340-42 and 443 (Figure 2)
imparts an intrinsic ability of this motif to readily accept electrons.44 In addition to purely
hydrocarbon structures, we recently disclosed the synthesis and characterization of two
thiophene-containing IFs variants based on indacenedithiophene (IDT, 5)45 and
diindenothiophene (DIT 6).46 Given the promising attributes observed in other Se-containing
compounds, we were interested in exploring the effects of selenophene substitution into our
previously reported thiophene-containing indenofluorene analogues. We report herein the
synthesis of indacenediselenophene (IDS, 7) and diindenoselenophene (DIS, 8) and describe
their respective optical, electrochemical and structural data.
Figure 2. Indenofluorenes 3 and 4 with their thiophene- and selenophene-containing counterparts IDT 5,
DIT 6, IDS 7 and DIS 8.
Results and discussion
The syntheses of IDS 7 (Scheme 1) and DIS 8 (Scheme 2) follow the typical pathway
utilized to generate IFs, IDTs and DITs—addition of a nucleophile to the corresponding dione (9
4

or 10, respectively) followed by a SnCl2-mediated reductive dearomatization reaction. Although
neither dione has been reported in the literature, the preparation of both IDS-dione 9 and DIS-
dione 10 proved relatively simple and followed a similar methodology to the synthesis employed
for the respective diones of 5 and 6.45,46
Our previous investigation of IDTs indicated that the steric bulk of a mesityl group was
necessary to kinetically stabilize the indacene core; thus, we elected to use mesityl lithium for the
preparation of 7. A Stille cross-coupling of dibromide 1147 with tri-n-butyl(selenophen-2-yl)tin48
provided key diester 12. Ester saponification followed by Friedel-Crafts acylation provided gram
quantities of IDS-dione 9 in excellent yield.49 Addition of mesityllithium gave the crude diol,
which in turn was reacted with SnCl2 to afford stable, deep purple crystals of IDS 7 in moderate
yield after recrystallization.
Scheme 1. Synthesis of indacenediselenophene 7 via IDS-dione 9.
Conversely, our studies into the DITs revealed no need for kinetic stabilization with a
bulky group immediately next to the fully conjugated core. Typically, as substitution with silyl-
protected acetylenes leads to more favorable packing in the solid state (important to device
performance), we elected to use lithium tri(isopropylsilyl)acetylide (LiCCSi(i-Pr)3) as the
5

nucleophile (Scheme 2). Suzuki cross-coupling of 3,4-dibromoselenophene (13)36 with 2-
ethoxycarbonylbenzeneboronic acid yielded 14. Saponification of the diester followed by
Friedel-Crafts acylation of the resultant diacid furnished selene-dione 10 in good yield.
Treatment of 10 with LiCCSi(i-Pr)3 provided the crude diol which, when subjected to the SnCl2-
mediated reductive dearomatization, gratifyingly furnished DIS 8 as stable dark green crystals in
modest overall yield.
Scheme 2. Synthesis of diindenoselenophene 8 via DIS-dione 10.
Figure 3 shows the electronic absorption spectra for IDT 5 and IDS 7 (left) and DIT 6 and
DIS 8 (right). These data, along with the experimentally determined Eredox values, HOMO and
LUMO energies and energy gaps are summarized in Table 1. The spectrum of 7 displays a
maximum absorbance from 310 to 380 nm along with a lower energy absorption with a
of
max
577 nm. This value is 21 nm red-shifted from the max of 5 and is expected due to the typically
smaller band gap observed in selenophene-containing compounds compared to their thiophene
counterparts.31,34 The spectrum of 8 displays acene-like vibronic features in the 350-500 nm
6

region and a low energy absorption reaching into the NIR (520-825 nm). DIT 6 and DIS 8 show
almost identical absorbance profiles with 6 and 8 having max of 676 and 675 nm, respectively.
Similar to the IDTs and DITs, both 7 and 8 are non-emissive, a trait inherent to all
indenofluorenes and related structures.50
Figure 3. Electronic absorption spectra of IDT 5 and IDS 7 (left) and DIT 6 and DIS 8 (right) in CH2Cl2.
Table 1. Electrochemical and optical data for compounds 5–8
Electrochemicala
Opticalb
1
2
1
Ered
(V)
Ered
(V)
Eox
(V)
EHOMO
(eV)
ELUMO
(eV)
Egap
(eV)
Egap
(nmmax)
556
compd
(eV)
2.06
–1.69c
–0.92
0.93
–5.57
–5.63
–5.38
–5.71
–3.72
–3.84
–3.69
–3.89
1.85
1.93
1.69
1.82
5
6
7
8
–1.22c
–1.74c
–1.17c
–0.80
–0.95
–0.76
0.99
0.74
1.07
676
577
675
1.52
2.02
1.52
a CVs were recorded using 1-5 mM of analyte in 0.1 M Bu4NBF4/CH2Cl2 at a scan rate of 50 mV s–1 with a glassy
carbon working electrode, a Pt coil counter electrode, and a Ag wire pseudo-reference. Values reported as the half-
wave potential (vs. SCE) using the Fc/Fc+ couple (0.46 V) as an internal standard. HOMO and LUMO energy levels
in eV were approximated using SCE = –4.68 eV vs. vacuum(see ref. 51) and E1/2 values for reversible processes or
b
Ep values for irreversible processes. Spectra were obtained in CH2Cl2. The optical HOMO/LUMO energy gap was
determined as the intersection of the x-axis and a tangent line passing through the inflection point of the lowest
energy absorption. c Reported as V at peak current, not half-wave potential.
7

Cyclic voltammetry reveals that IDS 7 undergoes one partially reversible reduction in
solution with a second irreversible reduction, while DIS 8 undergoes two reversible reductions in
the solution state (Fig. 4).51 The oxidation of both 7 and 8 is irreversible. In both 7 and 8, a
smaller Egap (0.16 eV for 7 vs. 5 and 0.11 eV for 8 vs. 6) is observed, which is consistent with the
substitution of Se for S.28,29 Although there are some seemingly large variances between the
experimentally determined optical and electrochemical energy gaps (Table 1), such differences
are not uncommon.52
Figure 4. Cyclic voltammetry of IDS 7 and DIS 8.
Crystals suitable for x-ray diffraction were grown by slow diffusion of CH3CN into
CH2Cl2 (7) or by slow evaporation of CH2Cl2 (8). The structures of both molecules are shown in
Figure 5; select bond lengths for 7 and 8 as well as for thiophene counterparts 5 and 6 are given
in Table 2. Both 7 and 8 exhibit bond length alteration consistent with the proposed quinoidal
structure of the core. IDS 7 packs in a herringbone-like motif with the Se atoms pointing towards
the center of the five membered ring of an adjacent molecule (3.493 Å from the Se atom to the
centroid of the five membered ring). The Se atom participates in the closest intermolecular
interaction (van der Waals radius for C–Se is 3.60 Å) with two short contacts of 3.407 and 3.460
Å. The C4-Se1-C8 angle is 86.8° while the dihedral angle between average planes of the mesityl
8

Figure 5. Molecular structures of IDS 7 (a) and DIS 8 (b). Hydrogen atoms omitted for clarity. Ellipsoids
drawn at 50% probability level.
Figure 6. Solid-state packing of IDS 1 (a and b) and DIS 2 (c and d); H atoms omitted for clarity;
ellipsoids draws at 50% probability level.
9

Table 2. Selected bond lengths (Å) for compounds 5–8
IDT/IDS DIT/DIS
bonda
C1–C2
C1A–C3
C1–C5
C2–C3
C2–C6
C2–C7
C3–C4
C4–C5
C5–C6
C5–C7
C6–C7
C7–C8
C1–S/Se1
C4–S/Se1
C8–S/Se1
5
7
6
8
1.432(2) 1.432(5)
1.360(2) 1.356(5)
1.453(2) 1.449(5)
—
—
—
—
1.365(3) 1.356(5)
1.359(3) 1.367(5)
1.469(2) 1.460(5)
1.388(2) 1.386(5)
—
—
—
—
1.463(2) 1.470(5)
1.456(2) 1.453(5)
1.452(2) 1.443(5)
1.389(2) 1.383(5)
1.460(2) 1.461(5)
1.417(2) 1.424(5)
—
—
1.482(2) 1.493(5)
—
—
—
—
1.416(3) 1.418(5)
1.355(3) 1.354(6)
—
—
—
—
1.750(2) 1.898(3)
1.747(2) 1.893(4)
1.706(2) 1.865(4)
1.736(2) 1.884(4)
—
—
a Number scheme shown in Fig. 5.
group and the IDS core is 66.3°, a slightly smaller angle than for IDT 5 (68.2°). As with DIT 6,
DIS 8 arranges into pairwise slipped stacks in the solid state with an average distance between
planes of 3.357 Å within the close pairs and an average distance of 3.391 Å between planes of
far pairs. Lateral (short axis) slip of the cores is 1.004 Å for the close pairs and 5.877 Å for the
far pairs. The closest intermolecular interaction for DIS 8 is a C–C contact of 3.358 Å between
close pairs and 3.397 Å between far pairs. The C1-Se1-C4 angle for DIS 8 is 84.5°. Aside from
the longer C–Se bonds within the selenophene moiety, the core units within compounds 7 and 8
are essentially isostructural with their thieno-analogues 5 and 6.
10

Conclusion
In summary, we have developed the synthesis of two Se-containing indenofluorene
analogues, indacenediselenophene 7 and diindenoselenophene 8. Optical and electrochemical
characterization reveal that selenophene substitution results in a decrease in the HOMO/LUMO
energy gap compared to thiophene incorporation (1.69 vs. 1.85 eV for 7 vs. 5, and 1.82 vs. 1.93
for 8 vs. 6), while structural characterization shows shorter relative intermolecular contacts.
These data suggest that much like previously described indenofluorenes and related thiophene-
containing compounds, selenophene-containing IF conjoiners could make excellent candidates as
organic semiconductors for electronic applications.
Acknowledgments
We thank the National Science Foundation (CHE-1301485) for support of this research.
We also thank Prof. Mark Lonergan (University of Oregon) for use of his group’s potentiostat.
HRMS were obtained at the Mass Spectrometry Facilities and Services Core of the
Environmental Health Sciences Center, Oregon State University, supported by grant #P30-
ES00210, National Institute of Environmental Health Sciences, National Institutes of Health.
Supplementary data
1
Supplementary data (experimental procedures, characterization data and copies of H and
13C NMR spectra of all new compounds, UV-Vis spectra of diones 9 and 10, X-ray
crystallographic data) associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.201x.xxxx.
11

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15