Synthesis, Spectroscopy, and Morphology of Tetrastilbenoidmethanes

Full text of DOI:10.1021/ja974209x

J. Am. Chem. Soc. 1998, 120, 2987-2988
2987
Synthesis, Spectroscopy, and Morphology of
Tetrastilbenoidmethanes
Scheme 1. Products Obtained from the Reaction of 1 with
Different Stilbenoid Compounds in the Presence of Pd(OAc)
NBu Br/DMF/K CO
2
/
4
2
3
†
Warren J. Oldham, Jr., Rene J. Lachicotte, and
Guillermo C. Bazan*
Department of Chemistry, UniVersity of Rochester
Rochester, New York 14627
ReceiVed December 12, 1997
Low band gap molecular and polymeric organic materials are
currently being considered for the active component in a variety
of optoelectronic devices, most notably as the emissive layer in
light-emitting diodes.1 It is generally believed that the morphol-
ogy adopted by these materials plays a fundamental role in de-
fining their bulk performance. For example, completely amor-
phous and ultrapure thin films appear to provide the longest device
lifetimes, combined with the highest efficiency and emissive
brightness.2 While sufficiently pure films of low molecular weight
fluorescent compounds can be obtained by sublimation under high
vacuum, these tend to recrystallize over time, thus precipitating
device failure.3 Luminescent polymeric materials can be designed
to remain amorphous, even at elevated temperatures, but they are
more difficult to obtain in high purity.4,5 Furthermore, the close
association of polymer chains in the solid state can lead to
crystalline domains.6 Ordered polymer regions appear to reduce
emission efficiency by promoting low-energy non- or weakly
The parent compound in this series, tetrastilbenylmethane (2),
is obtained via palladium-catalyzed Heck coupling of tetrakis(4-
iodophenyl)methane12 (1) with styrene (see Scheme 1). Reac-
7
emissive excimer or aggregate states. Ultimately, because of
facile energy migration, these low-energy sites can dominate the
optical properties, even when present in small concentration.8
In this paper, we describe a strategy to obtain precisely defined
and readily purified luminescent materials of intermediate mo-
lecular size with amorphous morphology. For this purpose, new
molecules containing a tetrahedral array of four stilbenoid units
13
14
tions carried out under phase-transfer conditions were found to
give the highest yields (86%), and this procedure was used
exclusively in subsequent coupling reactions. Slightly reduced
15
yields (70%) were obtained using the Herrmann catalyst, and
only traces of 2 were observed using the traditional catalyst
3
coupled to a central sp -hybridized carbon atom have been
16
system composed of a mixture of Pd(OAc)
2
and P(o-tol)
3
.
prepared.9 The rigid tetrahedral framework orients the chromo-
phores such that the possibility of intramolecular π-stacking is
minimized. These molecules are highly symmetrical and might
be expected to favor a crystalline morphology, as observed for
most structures which incorporate a tetraphenylmethane core.10
However, we show that when the stilbenoid units are sufficiently
long, crystalline packing becomes unfavorable and a stable
amorphous phase is obtained.11
Incomplete reaction was observed, even under optimized condi-
11
tions, if tetrakis(4-bromophenyl)methane was substituted for 1.
Reactions carried out with pentafluorostyrene gave tetrakis-
(
pentafluorostilbenyl)methane (3), which is only slightly soluble
17
in aromatic or chlorinated solvents.
Longer stilbenoid arms were obtained by coupling 4,4′-tert-
18
butylvinylstilbene with 1 to yield the pale yellow and freely
19
soluble tetrakis(4-tert-butylstyrylstilbenyl)methane (4). Com-
pounds 3 and 4 were isolated in yields of 31 and 17%,
respectively. The low yields for these two reactions are attributed
to the insolubility of partially coupled intermediates, which were
observed to precipitate from solution during early reaction times.
Appropriate NMR and analytical data were obtained for each
†
Current address: FED Corporation, Hopewell Junction, NY 12533.
(
1) Forrest, S. R.; Burrows, P. E.; Thompson, M. E. Laser Focus World
1
995, 31, 99-101.
(
2) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.;
Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884-888.
(
(
3) Naito, K. Chem. Mater. 1994, 6, 2343-2350.
4) Dendritic conjugated polymers show glassy properties: Deb, S. K.;
1
compound. Importantly, H NMR signals attributed to unreacted
Maddux, T. M.; Yu, L. J. Am. Chem. Soc. 1997, 119, 9079-9080.
4-iodophenyl groups were not observed.
(
5) Surface morphology of conjugated polymers can be modified by
The absorption and emission spectra for 2 and 4 dissolved in
CHCl are shown in Figure 1. The respective absorbance and
3
constructing star-shaped structures: Wang, F.; Rauh, R. D.; Rose, T. L. J.
Am. Chem. Soc. 1997, 119, 11106-11107.
(
6) Prosa, T. J.; Winokur, M. J.; McCollough, R. D. Macromolecules 1996,
2
9, 3654-3656.
(11) See also: Wilson, L. M.; Griffin, A. C. J. Mater. Chem. 1993, 3,
991-994.
(12) Su, D.; Menger, F. M. Tetrahedron Lett. 1997, 38, 1485-1488.
(
7) Conwell, E. Trends Polym. Sci. 1997, 5, 218-222 and references
therein.
1
(
8) Guillet, J. E. Polymer Photophysics and Photochemistry; Cambridge
2 2
(13) Data for 2: H NMR (CD Cl ) 7.52 (d, J ) 7.4 Hz, 2 H), 7.47, 7.31
University Press: Cambridge, U.K. 1985.
(AA′BB′ pattern, J ) 8.5 Hz, 2 H each), 7.36 (t, J ) 7.4 Hz, 2 H), 7.26 (t,
(
9) Cubane and adamantane frameworks have been used in the formation
J ) 7.4 Hz, 1 H), 7.13 (s, 2 H, vinylene). Anal. Calcd for C57H44: C, 93.9;
of stable organic glasses containing mesogenic groups. See: Chen, S. H.;
H, 6.1. Found: C, 94.00; H, 5.99.
Shi, H.; Conger, B. M.; Mastrangelo, J. C.; Tsutsui, T. AdV. Mater. 1996, 8,
(14) (a) Jeffery, T. J. Chem. Soc., Chem. Commun. 1984, 1287-1289. (b)
9
98-1001. For alternative approaches, see: Shirota, Y.; Kobata, T.; Noma,
Jeffery, T. AdV. Metal-Organic Chem. 1996, 5, 153-260.
N. Chem. Lett. 1989, 1145-1148. Ishikawa, W.; Inada, H.; Nakano, Y. Shirota,
(15) Herrmann, W. A.; Brossmer, C.; O¨ fele, K.; Reisinger, C.-P.; Priermeier,
T.; Beller, M.; Fischer, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 1844-1847.
Y. Chem. Lett. 1991, 1731-1734. Bettenhausen, J.; Strohriegl, P. AdV. Mater.
1
3
996, 8, 507-510. Salbeck, J.; Bauer, J.; Weissortel, F. Polym. Prepr. 1997,
8, 349.
(16) Heck, R. F. Org. React. 1982, 27, 345-389.
1
(17) Data for 3: H NMR (C
D
6 6
) 7.42, 7.26 (AA′BB′ pattern, J ) 8.4 Hz,
(
10) For example, see: (a) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc.
2 H each), 7.29, 6.82 (AB pattern, J ) 16.4 Hz, 1 H each). Anal. Calcd for
20: C, 62.9; H, 2.22; F, 34.90. Found: C, 63.17; H, 2.07; F, 34.00.
(18) Oldham, W. J., Jr.; Miao, Y.-J.; Lachicotte, R. J.; Bazan G. C. J. Am.
Chem. Soc. 1998, 120, 419-420.
1
990, 112, 1546-1554. (b) Simard, M.; Su, D.; Wuest, J. D. J. Am. Chem.
57 24
C H F
Soc. 1991, 113, 4696-4698. (c) Reddy, D. S.; Craig, D. C.; Desiraju, G. R.
J. Am. Chem. Soc. 1996, 118, 4090-4093.
S0002-7863(97)04209-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/17/1998

2988 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Communications to the Editor
Figure 1. Absorption and emission data for 2 and 4 in CHCl
3
. Excitation
Figure 3. DSC curves for compounds 2-4.
frequencies correspond to the absorption maxima.
Analysis by differential scanning calorimetry (DSC) provides
important information concerning solid-state morphology (Figure
23
3). Both 2 and 3 are cystalline compounds and accordingly
show distinct melting endotherms at 274 and 316 °C, respectively.
However, a similar phase transition was not observed for 4 up to
400 °C, at which point the onset of thermal decomposition was
evident. Careful inspection of the heating cycle for 4 (Figure 3)
shows a step transition at 175 °C which we assign to a glass
transition.24 The absolute change in heat capacity (∆C
) at the
glass transition is proportional to the fraction of amorphous
material in a sample. In addition, ∆C for 4 was found to be
p
p
independent of thermal history, which suggests that this material
has few if any crystalline domains. Thus, samples which were
annealed at 300 °C and then rapidly air cooled to 100 °C gave
DSC heating curves very similar to those of samples which had
been annealed at 300 °C and then slowly cooled to 100 °C (∆T
)
1 °C/min). In the latter experiment, if crystalline regions had
been formed during the slow cooling cycle, then ∆C at T would
p g
have been reduced.25
A comparison of the thermal behavior of 4 against that of 2,5-
dimethyl-4′-tert-butyldistyrylbenzene (5) highlights how bringing
together four molecular fragments at a tetrahedral junction changes
the morphology of the bulk material. Instead of a glass transition,
2
6
compound 5 displays a sharp melting endotherm at 133 °C.
Figure 2. (a) Packing diagram of the unit cell as viewed down the c
axis. (b) 50% ORTEP diagram, with H atoms omitted for clarity.
emisson maxima are noted at 322 and 379 nm for 2, compared
to 368 and 427 nm for 4.20 Interestingly, the spectra for both 2
In summary, the results described herein provide a versatile
strategy for controlling the morphology of organic materials. This
approach should be useful in designing suitable materials for
application in optoelectronic devices by combining the advantages
of small molecules (e.g., high purity and volatility) with those of
polymers (e.g., amorphous morphology). A detailed study of the
photophysical properties of the new stilbenoid compounds and
the effect of structural diversity on their morphology is underway.
and 4 are red shifted with respect to stilbene and distyrylbenzene,
respectively (ca. 26 nm for 2 and ca. 16 nm for 4). This
3
observation indicates delocalization through the sp -hybridized
carbon center. The absorption and emission data of 3 are nearly
identical to that shown for 2 in Figure 1, indicating that the
HOMO-LUMO gap is largely unaffected by fluorine substitution.
Crystals of 2 suitable for X-ray diffraction studies were obtained
from a MeOH/benzene solution.2 As shown in Figure 2, two
1
Acknowledgment. The NSF (DMR 9500627) and the Office of Naval
Research (N00014-1-0643) are gratefully acknowledged for partial support
of this work. The authors thank Andy Hunt for assistance with the powder
diffraction experiments.
1
independent molecules lie on special positions (0, 0, 0 and /
2
,
1
2
/ , 0) in the tetragonal unit cell (P 4h ). The results of this study
reveal a molecular structure with normal intramolecular metrical
parameters as compared to those of trans-stilbene.22 All inter-
molecular C‚‚‚C interactions were greater than the sum of their
van der Waals radii. The closest contact is ca. 3.54 Å between
C4 and C15 on a symmetry-related molecule (see Figure 2). In
sharp contrast to 2, the solid samples of 4 were consistently
obtained as amorphous powders as detailed below.
Supporting Information Available: Complete experimental details
for the preparation of all new compounds and for the X-ray crystal-
lographic determination of 2 (9 pages, print/PDF). See any current
masthead page for ordering information and Web access instructions.
JA974209X
(
23) Data shown in Figure 3 were obtained as follows. Samples were
(
19) Data for 4: 1H NMR (CDCl
pattern, J ) 8.5 Hz, 2 H each, arene), 7.41, 7.26 (m, 2 H each, arene), 7.10,
3
) 7.48 (s, 4 H, arene), 7.45, 7.37 (AA′BB′
annealed by heating to 300-350 °C (depending on T ) and then cooled to
m
100 °C (∆T ) 10 °C/min). Subsequent heating cycles (shown in figure) were
measured at rates of 10 °C/min for 2 and 3 and at 20 °C/min for 4.
(24) Low molecular weight organics can exhibit glass transition phenomena.
See: Eisenberg, A. Physical Properties of Polymers; American Chemical
Society: Washington, DC, 1993.
7
.04 (AB pattern, J ) 16.3 Hz, 1 H each, vinylene), 7.09 (br s, 2 H, vinylene);
1
3
1
C{ H} NMR (CDCl
3
) 150.8, 146.08, 146.04, 136.9, 136.5, 135.1, 134.5,
1
3
31.3, 128.6, 128.4, 127.9, 127.5, 126.8, 126.7, 126.3, 125.8, 125.6, 34.6,
1.3. Anal. Calcd for C105
H100: C, 92.6; H, 7.4. Found: C, 92.50; H, 7.19.
(
20) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399-1420.
(25) Powder diffraction experiments show no peaks for 4, confirming its
amorphous nature. A pattern is obtained for 2 which matches the reflections
calculated from the single-crystal data. Bazan, G. C.; Lachicotte, R. J. Work
in progress.
(26) 5 was available from a previous study. A similar sharp endotherm
was measured for trans-stilbene at 126 °C, consistent with literature values.
(
21) Crystal data for C57 colorless needles, tetragonal, (P 4h ), a )
44
H :
3
1
)
7.5046(5), c ) 7.1921(4) Å, V ) 2203.1(2) Å , Z ) 2, R ) 10.10%, GOF
1.389.
22) Hoekstra, A.; Meertens, P.; Vos, A. Acta Crystallogr. 1975, B31,
813-2817.
(
2