Synthesis, crystal structures, and photophysical properties of electron-accepting diethynylindenofluorenediones

Article abstract of DOI:10.1021/ol200525g

A series of 6,12-bis[(trialkylsilyl)ethynyl]indeno[1,2-b]fluorene-5,11- diones has been synthesized. X-ray crystallographic analysis of these compounds reveals that triisopropylsilyl (TIPS) substitution on the alkyne terminus affords the largest number of

Full text of DOI:10.1021/ol200525g

ORGANIC
LETTERS
2
011
Vol. 13, No. 8
106–2109
Synthesis, Crystal Structures,
and Photophysical Properties
of Electron-Accepting
2
Diethynylindenofluorenediones
Bradley D. Rose, Daniel T. Chase, Christopher D. Weber, Lev N. Zakharov,
Mark C. Lonergan, and Michael M. Haley*
Department of Chemistry & Materials Science Institute, University of Oregon, Eugene,
Oregon 97403-1253, United States
haley@uoregon.edu
Received February 25, 2011
ABSTRACT
A series of 6,12-bis[(trialkylsilyl)ethynyl]indeno[1,2-b]fluorene-5,11-diones has been synthesized. X-ray crystallographic analysis of these
compounds reveals that triisopropylsilyl (TIPS) substitution on the alkyne terminus affords the largest number of intermolecular πꢀπ interactions
in the solid state. Conversely, use of trialkylsilyl groups smaller or larger than TIPS furnishes a variety of crystal-packing motifs that contain fewer
πꢀπ interactions. Electrochemical and photophysical data suggest that these molecules are excellent electron-accepting materials.
3
d
Conjugated hydrocarbons with extended polycyclic frame-
works possess a rich history because of the fundamental,
inherent properties (e.g., aromaticity, color, fluorescence)
currently difficult topredictusingtheoretical modeling. It
is well-known that the mobilities of holes (for p-type) or
electrons (for n-type) in solid-state organic materials are
increased when both the intermolecular overlap of the π
1
attributable to such structures. More recently, these mol-
ecules as well as their heteroatom-containing analogues
3
c
orbitals is maximized and those orbitals are in phase.
2
have been explored as organic materials for applications in
electronic devices such as photovoltaics, field-effect tran-
In recent years, there has been considerable interest in
the indenofluorene (IF) framework as an n-type material
because of the planarity of the IF skeleton and its ability to
3
sistors, and light-emitting diodes. An important indicator
4
of potential device performance when using organic mole-
cules is the solid-state ordering of the material, which is
reversibly accept electrons. Our laboratory has been ex-
amining alkynylated, fully conjugated indeno[1,2-b]-
5
fluorenes (e.g., 1), which are derived from indenofluor-
6
ene-5,11-diones (IF-diones) such as 2. Prior work by
(
1) (a) Balaban, A. T.; Banciu, M.; Ciorba, V. Annulenes, Benzo-,
Hetero-, Homo- Derivatives and their Valence Isomers; CRC Press: Boca
Raton, 1987. (b) Hopf, H. Classics in Hydrocarbon Chemistry; Wiley-VCH:
Weinheim, 2000.
(4) Inter alia: (a) Jacob, J.; Sax, S.; Piok, T.; List, E. J. W.; Grimsdale,
A. C.; M u€ llen, K. J. Am. Chem. Soc. 2004, 126, 6987–6995. (b) Hadizad,
T.; Zhang, J.; Wang, Z. Y.; Gorjanc, T. C.; Py, C. Org. Lett. 2005, 7, 795–
797. (c) Miyata, Y.; Minari, T.; Nemoto, T.; Isoda, S.; Komatsu, K. Org.
Biomol. Chem. 2007, 5, 2592–2598. (d) Nakagawa, T.; Kumaki, D.;
Nishida, J.-i.; Tokito, S.; Yamashita, Y. Chem. Mater. 2008, 20, 2615–
2617. (e) Usta, H.; Risko, C.; Wang, Z.; Huang, H.; Deliomeroglu,
M. K.; Zhukhovitskiy, A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc.
2009, 131, 5586–5608.
(5) Chase, D. T.; Rose, B. D.; McClintock, S. P.; Zakharov, L. N.;
Haley, M. M. Angew. Chem., Int. Ed. 2011, 50, 1127–1130.
(6) Zhou, Q.; Carroll, P. J.; Swager, T. M. J. Org. Chem. 1994, 59,
1294–1301.
(
2) (a) Functional Organic Materials; M €u ller, T. J. J., Bunz, U. H. F.,
Eds.; Wiley-VCH: Weinheim, 2007. (b) Organic Light Emitting Devices:
Synthesis, Properties and Applications; M €u llen, K., Scherf, U., Eds.;
Wiley-VCH: Weinheim, 2006. (c) Carbon-Rich Compounds: From
Molecules to Materials; Haley, M. M., Tykwinski, R. R., Eds.; Wiley-VCH:
Weinheim, 2006.
(
3) (a) Horowitz, G. Adv. Mater. 1998, 10, 365–377. (b) Dong, H.;
Wang, C.; Hu, W. Chem. Commun. 2010, 5211–5213. (c) Anthony, J. E.;
Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. Adv. Mater. 2010, 22,
3876–3892. (d) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier,
Y.; Silbey, R.; Br ꢀe das, J.-L. Chem. Rev. 2007, 107, 926–952. (e) Facchetti, A.
Mater. Today 2007, 10, 28–37.
1
0.1021/ol200525g r 2011 American Chemical Society
Published on Web 03/23/2011

Anthony et al. has explored the relationship between alky-
7
nylation and crystal packing for acene derivatives. We
recognized that, starting from 2, we could perform a similar
detailed study on IF-diones. To this end, we report herein
the synthesis and photophysical properties of alkynylated
IF-diones 3ꢀ8 as well as solid-state structures of 3, 6, and 8.
Compounds 3ꢀ8 were synthesized in low to moderate
yields by Sonogashira cross-coupling of the appropriate
(trialkylsilyl)acetylene with 2 (Scheme 1). One possible
explanation for the low isolated yields is the lability of
Scheme 1. Synthesis of Alkynylated Indenofluorenediones
Figure 1. Crystal packing of IF-diones illustrating (a) herring-
bone (3), (b) 1-D colums without πꢀπ interactions (8), and (c)
coplanar slip stacking (6). (d) A side view of 6 shows the
favorable brick and mortar packing. Thermal ellipsoids drawn
at the 30% probability level.
iodine atoms of 2. Solutions of 2 at elevated temperatures
turn pale violet, characteristic of iodine formation; how-
ever, the Sonogashira reaction proceeds sluggishly at
temperatures less than 50 °C.
Orange single crystals of 3 and 6 suitable for X-ray
diffraction were grown by slow cooling of hot hexanes
solutions of the IF-diones, whereas 8 was recrystallized by
slow evaporation from a binary combination of THF and
hexanes. Similar to conjugated systems such as acenes, the
IF-diones exhibit three types of packing in the solid state:
Figure 1c and d). This last arrangement, also known as
brick and mortar packing, was found only in 6 and
maximizes the πꢀπ interactions in two dimensions
˚
Figure 1d) with an interplanar distance of 3.40 A, in
(
˚
contrast to the interplanar distance of 3.77 A in 8. The
situation for 6 is reminicient of 6,13-bis[(triisopropylsilyl)-
ethynyl]pentacene, which showed coplanar slip stacking as
7
well. Anthony attributed this phenomenon to the dia-
meter of the triisopropylsilyl (TIPS) group being close to
(
i) herringbone packing (e.g., 3, Figure 1a), (ii) one-dimen-
sional columns without πꢀπ interactions (e.g., 8,
the stack spacing needed for intermolecular πꢀπinteractions
Figure 1b), and (iii) coplanar slip stacking (e.g., 6,
7
b
˚
3.4 A). Interestingly, previous crystal structures of IF-
(
diones have also displayed herringbone packing with slip-
(
7) (a) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am.
Chem. Soc. 2001, 123, 9482–9483. (b) Anthony, J. E.; Eaton, D. L.;
Parkin, S. R. Org. Lett. 2002, 4, 15–18. (c) Purushothaman, B.; Parkin,
S. R.; Anthony, J. E. Org. Lett. 2010, 12, 2060–2063.
4c
stacked one-dimensional πꢀπ interactions, lamellar one-
4
d
4e
dimensional stacks, or essentially no πꢀπ interactions.
Org. Lett., Vol. 13, No. 8, 2011
2107

4
a,c,e,9
Another important consideration is the phase as well as
the amount of orbital overlap in the crystal lattice (vide
supra). As can be seen in Figure 2, the semiempirical single
potentials due to the low lying LUMO energy levels.
The first-reduction half-wave potential at ca. ꢀ0.81 V
(vs SCE) for 3ꢀ8 is indeed less negative than the unsub-
4
d
stituited parent IF-dione (ꢀ1.19 V), which can be attrib-
1
0
uted to the electron-withdrawing ethynyl moiety. In
comparison to other known IF-diones, the first reduction
half-wave potential of 3ꢀ8 is similar to 6,12-didodecyl-3,9-
9
dibromo-IF-dione (ꢀ0.77 V) and more negative than 3,9-
4
d,11
halogenated-IF-diones (ꢀ0.74 to ꢀ0.57 V)
and
4c,11
1
,2,3,4,7,8,9,10-octafluoro-6,12-diiodo-IF-dione (ꢀ0.45 V).
No oxidation process was observed, analogous to previous
IF-diones.
Figure 2. AM1-calculated (LUMO) interactions derived from
the crystal packing in the X-ray structure of 6.
point calculation based on the crystal structure of 6 shows
that the orbitals are indeed in phase with orbital overlap
between eight carbons between each molecule in one
direction and four carbons in the other. Hence, the crystal
packing of 6 possesses significant orbital interaction in one
direction and less in the other.
The B3LYP/6-311þG(d,p)-minimized structure of 3
gives the energy levels of the HOMO at ꢀ6.28 eV and
8
LUMO at ꢀ3.24 eV. Replacing the SiMe groups with H
3
Figure 4. UVꢀvis data for IF-diones 3-8 in CHCl ; inset mag-
3
atoms alters the calculated HOMO/LUMO values to
nified 12 times.
ꢀ
6.43 and ꢀ3.33 eV, respectively, illustrating the weak
influence of the silyl substituent. These calculated numbers
are in good agreement with the experimental cyclic vol-
tammetry (CV) data, shown in Figure 3 and compiled in
The UVꢀvis spectra of 3ꢀ8 (Figure 4) exhibit intense
absorbtions at approximately 312 and 330 nm due to the
πꢀπ* transitions. Very weak bands appear at ca. 500 and
5
25 nm and are attributed to the symmetry forbidden
4
e,12
nꢀπ* transition of the carbonyls.
The fluorescence
spectra of 3ꢀ8 (Figure S1 in Supporting Information)
show a single broad peak around 570 nm, with quantum
1
3
yields in the range of 8ꢀ10%.
In conclusion, a series of ethynylated IF-diones has been
synthesized, and the packing motifs elucidated from X-ray
diffraction of single crystals. The crystal packing and low
LUMO levels indicate that6 may be an ideal candidate as a
solution processable n-type semiconductor. The device
properties of 6 are the topic of current investigations.
(
8) All calculations were performed using Gaussian 09, Revision
A.02. Frisch, M. J. et al. Gaussian, Inc., Wallingford, CT, 2009. See
the Supporting Information for the full citation.
(
388.
9) Usta, H.; Facchetti, A.; Marks, T. J. Org. Lett. 2008, 10, 1385–
1
2
(10) Landgrebe, J. A.; Rynbrandt, R. H. J. Org. Chem. 1966, 31,
585–2593.
(
Fc/Fc to SCE, as described in: Connelly, N. G.; Geiger, W. E. Chem.
Rev. 1996, 96, 877–910.
11) For comparison, the potential references were changed from
Figure 3. Cyclic voltammetry of IF-diones 3ꢀ8; voltammogram
þ
1
currents are normalized to the to the Eanodic peak.
(
12) Oldridge, L.; Kastler, M.; M u€ llen, K. Chem. Commun. 2006,
85–887.
13) Porr ꢁe s, L.; Holland, A.; Palsson, L.; Monkman, A. P.; Kemp, C.;
Beeby, A. J. Fluoresc. 2006, 16, 267–272.
8
Table 1. In solution, the IF-dione scaffold is capable of
reversibly accepting up to two electrons, typically at low
(
2
108
Org. Lett., Vol. 13, No. 8, 2011

Table 1. Experimental Optical and Electrochemical Data
electrochemical
optical
1
a
2
a
b
c
d
e
compd
E
red (V)
E
red (V)
E
LUMO (eV)
EHOMO (eV)
λabs (nm)
Egap (eV)
λem (nm)
Φ
fluorescence
3
4
5
6
7
8
ꢀ0.80
ꢀ0.79
ꢀ0.81
ꢀ0.82
ꢀ0.84
ꢀ0.78
ꢀ1.21
ꢀ1.23
ꢀ1.25
ꢀ1.24
ꢀ1.26
ꢀ1.18
ꢀ3.89
ꢀ3.90
ꢀ3.87
ꢀ3.87
ꢀ3.85
ꢀ3.90
ꢀ6.26
ꢀ6.28
ꢀ6.23
ꢀ6.23
ꢀ6.22
ꢀ6.29
310, 330, 498, 524
311, 332, 498, 522
312, 331, 498, 526
313, 333, 500, 525
314, 333, 498, 524
312, 333, 496, 520
2.37
2.38
2.36
2.36
2.37
2.39
571
569
571
568
567
570
0.08
0.08
0.09
0.10
0.10
0.09
a
CV recorded using 1ꢀ5 mM of analyte in 0.1 M tetrabutylammonium trifluoromethanesulfonate/CH
2
2
Cl using a scan rate of 50 mV/s. The working
electrode was a glassy carbon electrode with a platinum coil counter electrode and silver wire pseudoreference. Values reported as the half-wave potential
þ
b
1
c
(
vs SCE) using the Fc/Fc couple (0.46 V) as an internal standard; see ref 11. Determined by ELUMO = ꢀ(4.44 þ Ered ); see ref 4e. Estimated by
d
subtracting the optical gap from the LUMO. Determined using the wavelength at the maximum absorption of the lowest energy n f π* transition from
e
the UVꢀvis spectrum. Determined via the integrating sphere method; see ref 13.
Acknowledgment. We thank the National Science
Foundation (CHE-1013032) for support of this re-
search as well as for support in the form of instrumenta-
tion grants (CHE-0639170 and CHE-0923589). The
electrochemical work was funded by the Division of
Chemical Sciences, Geosciences, and Biosciences, Of-
fice of Basic Energy Sciences of the U.S. Department
of Energy through Grant No. DE-FG02-07ER15907.
C.D.W. acknowledges the NSF for an IGERT fellow-
ship (DGE-0549503).
Supporting Information Available. Experimental de-
tails and spectroscopic data for all new compounds;
computational details for 3 and its desilylated analogue;
X-ray data for 3, 6, and 8 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 8, 2011
2109