Synthesis, structure, and nonlinear optical properties of diarylpolyynes

Article abstract of DOI:10.1021/ol047931q

(Chemical Equation Presented) A series of α,ω-diarylpolyynes has been synthesized. In addition to the synthesis of three hexaynes (3a-c), a notably improved synthesis of 1,16-diphenylhexadecaoctayne (5) is described. The third-order nonlinear optical char

Full text of DOI:10.1021/ol047931q

ORGANIC
LETTERS
2005
Vol. 7, No. 1
51-54
Synthesis, Structure, and Nonlinear
Optical Properties of Diarylpolyynes
Thanh Luu,† Erin Elliott,† Aaron D. Slepkov,‡ Sara Eisler,† Robert McDonald,†
Frank A. Hegmann,‡ and Rik R. Tykwinski*,†
Department of Chemistry, UniVersity of Alberta,
Edmonton, Alberta, T6G 2G2 Canada, and Department of Physics,
UniVersity of Alberta, Edmonton, Alberta, T6G 2J1 Canada
rik.tykwinski@ualberta.ca
Received October 7, 2004
ABSTRACT
A series of
r,
ω
-diarylpolyynes has been synthesized. In addition to the synthesis of three hexaynes (3a−c), a notably improved synthesis of
1,16-diphenylhexadecaoctayne (5) is described. The third-order nonlinear optical characteristics for these molecules have been studied and
show a substantial increase in molecular hyperpolarizability (
compounds 3a and 3b are reported.
γ
) as a function of increasing length. The unusual solid-state structures of
It is easy to appreciate the allure of conjugated polyynes,
from their structural simplicity to their potential to function
as linear molecular wires. A number of classical syntheses
have been developed for polyynes during the middle of the
past century,1 many of which have recently been applied to
the synthesis of more structurally diverse derivatives.2 A
common characteristic of these syntheses is the use of an
oxidative homocoupling reaction as the final step, reactions
that require terminal alkynes as starting materials.3 Such
homocoupling reactions under Hay,4 Glaser,5 or Eglinton-
Galbraith6 conditions can be quite efficient for the formation
of shorter polyyne molecules. Instability of the terminal
alkyne starting materials can, however, pose a problem in
the formation of longer derivatives. This often results in low
yields and difficult purification, which has historically limited
the ability to study the unique properties of these molecules.
We report herein the successful synthesis of a series of
aryl end-capped polyynes that have been constructed in order
to explore their structural and nonlinear optical (NLO)
properties. In several cases, traditional homocoupling meth-
ods were effective for formation of the desired polyynes. In
other cases, an alternative method was exploited based on a
modified Fritsch-Buttenberg-Wiechell (FBW) rearrange-
ment.2a A comparison of the two methods is briefly described.
The first two molecules in the series, diphenyl butadiyne
(1) and octatetrayne (2), were synthesized on the basis of
an adaptation of a known route.7,8 The synthesis of hexaynes
† Department of Chemistry.
‡ Department of Physics.
(1) Hopf, H., Classics in Hydrocarbon Synthesis; Wiley-VCH: Wein-
heim, 2000; Chapter 8.
(2) (a) Eisler, S.; Chahal, N.; McDonald, R.; Tykwinski, R. R. Chem.
Eur. J. 2003, 9, 2542-2550. (b) Mohr, W.; Stahl, J.; Hampel, F.; Gladysz,
J. A. Chem. Eur. J. 2003, 9, 3324-3340. (c) Stahl, J.; Bohling, J. C.; Bauer,
E. B.; Peters, T. B.; Mohr, W.; Martin-Alvarez, J. M.; Hampel, F.; Gladysz,
J. A. Angew. Chem., Int. Ed. 2002, 41, 1872-1876. (d) Gibtner, T.; Hampel,
F.; Gisselbrecht, J. P.; Hirsch, A. Chem. Eur. J. 2002, 8, 408-432. (e)
Schermann, G.; Gro¨sser, T.; Hampel, F.; Hirsch, A. Chem. Eur. J. 1997, 3,
1105-1112.
(5) Glaser, C. Ann. Chem. Pharm. 1870, 154, 137-171.
(6) Eglinton, G.; Galbraith, A. R. Chem. Ind. (London) 1956, 737-738.
(7) Armitage et al. have previously reported compounds 2, 3a, and 5;
see: Armitage, J. B.; Entwistle, N.; Jones, E. R. H.; Whiting, M. C. J.
Chem. Soc. 1954, 147-154.
(3) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed.
2000, 39, 2633-2657.
(4) Hay, A. S. J. Org. Chem. 1962, 27, 3320-3321.
10.1021/ol047931q CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/13/2004

Scheme 1. Synthesis of Phenyl Hexaynes 3a-ca
Scheme 2. Synthesis of Hexayne 3a and Octayne 5a
a Reagents and conditions: (a) K2CO3, MeOH/THF, rt; (b) CuCl,
TMEDA, O2, CH2Cl2, rt.
3a-c required triyne precursors 4a-c (Scheme 1), which
were formed in good yields using a recently developed
adaptation of the FBW rearrangement.2a,8 Protiodesilylation
of 4a-c afforded the unstable terminal triynes,9 which were
immediately carried on, following workup, to the subsequent
homocoupling reaction under Hay conditions.4 The yields
of hexaynes 3a-c were surprisingly good in comparison to
that previously reported for 3a.10,11
Although successful, the formation of hexaynes via this
route was not ideal because of the instability of the
deprotected triyne intermediates. For example, Armitage and
co-workers reported that attempted isolation of the phenyl-
1,3,5-hexatriyne afforded a solid that exploded at 0 °C in
the absence of air.7 Furthermore, attempts by the same group
to extend this methodology to the next longer homologue,
octayne 5, afforded only 0.5% yield of an unstable orange
solid. In an effort to avoid unstable polyyne intermediates
and provide sufficient amounts of octayne 5 for study, an
alternate approach was explored (Scheme 2). The known
dibromoolefins 6a,b2a were desilylated and then subjected
to Hay coupling conditions4 at low temperature12 with an
excess of the catalyst (2 equiv) in an effort to decrease
reaction times. Despite their highly unsaturated structures,
products 7a and 7b were isolated in excellent yields as
relatively stable solids and survived for months under
refrigeration.
a Reagents and conditions: (a) K2CO3, MeOH/THF, rt; (b) CuCl,
TMEDA, O2, CH2Cl2, 0 °C; (c) BuLi, PhCH3, -78 °C (3a) or -100
°C (5).
the best solubility, leading as well to the best yield of
polyynes 3a and 5. Thus, 2.4 equiv of BuLi was added to
the respective tetrabromide in toluene at low temperature,
and the reaction was warmed to approximately -20 °C over
a period of 30 min, and then quenched. As the least polar
product of the reaction, crude polyynes 3a and 5 could be
readily purified by flash chromatography. Although the yield
of 3a was somewhat lower for the final step of the FBW
reaction than for the Hay homocoupling, the FBW route
remains quite attractive because of the stability of the
immediate precursors, facile purification, and its success for
longer derivatives such as 5.
Hexaynes 3a-c were isolated as kinetically stable orange
solids, whereas octayne 5 was typically a less stable dark
orange/red solid. Surprisingly, when 5 was crystallized from
THF, the resulting rust colored solid showed substantially
increased kinetic stability, surviving for over a year if kept
under refrigeration. It is unclear whether the improved
stability derives from a more stable polymorph or from
increased purity in comparison to other samples.13
Crystals of 3a suitable for X-ray crystallography were
obtained by vapor diffusion of hexanes into a solution of 3a
in CH2Cl2/hexanes (1:1).14 The molecular geometry and
solid-state packing as viewed down the crystallographic
a-axis are shown in Figure 1. The structure of 3a is nearly
linear, with all sp-sp carbon bond angles between 177° and
180°. Molecules of 3a are aligned in a parallel fashion as
pairs of symmetry related dimers. Although intermolecular
distances between sp-hybridized carbons are sufficiently short
(e.g., C(2)-C(12′) ) 3.5 Å and C(1)-C(5′) ) 3.7 Å) to
support topochemical polymerization, the arrangement and
stacking angle suggest that an ordered polymerization would
Successful conversion of bromides 7a and 7b to the desired
polyynes via a FBW rearrangement necessitated the use of
a nonpolar solvent, typically hexanes.2a Both 7a and 7b were,
however, nearly insoluble in hexanes. After exploring a
number of possible media, toluene was determined to provide
(8) Synthetic and characterization details are provided as Supporting
Information.
(9) Caution: isolation of deprotected 1,3,5-hexatriynes should not be
attempted as they can explode even at low temperature and in the absence
of air; see ref 7.
(10) Selected spectral data. Compound 3a: 1H NMR (300 MHz, CDCl3)
δ 7.55 (d, J ) 7.5 Hz, 4H), 7.44 (t, J ) 7.5 Hz, 2H), 7.35 (t, J ) 7.5 Hz,
4H); 13C NMR (75.5 MHz, CDCl3, ATP) δ 133.5, 130.4, 128.7, 120.2,
77.5, 74.3, 67.3, 64.6, 63.6, 62.6. Compound 5: 1H NMR (300 MHz, THF-
d8) δ 7.32 (d, J ) 7.6 Hz, 4H), 7.20 (t, J ) 7.6 Hz, 2H), 7.11 (t, J ) 7.6
Hz, 4H); 13C NMR (75.5 MHz, THF-d8, ATP) δ 134.4, 131.8, 129.7, 120.3,
79.1, 74.1, 67.9, 64.6, 63.8, 63.75, 63.72, 63.4.
(13) It is worth noting that octayne 5 was previously reported to be stable
as a solid for only a period of hours when crystallized from ethyl acetate,
perhaps the result of impurities remaining in the sample.
(14) Crystal data for 3a: C24H10, M ) 298.32, monoclinic space group
P21/n (an alternate setting of P21/c [No. 14]), Dc ) 1.194 g cm-3, a )
7.7266(9), b ) 12.4682(14), c ) 17.2826(19) Å, â ) 94.797(2)°, V )
1659.1(3) Å3, Z ) 4, µ ) 0.068 mm-1. Final R(F) ) 0.0534, wR2(F2) )
(11) A 30% yield for 3c had been reported by Armitage and co-workers;
see ref 7.
(12) Low temperature was necessary to prevent side reactions that likely
arise from Castro-Stephens chemistry at the dibromoolefin groups:
Stephens, R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 3313-3315.
2
2
0.1305 for 217 variables and 3404 data with Fo g -3σ(Fo ) (1612
2
2
observations [Fo g 2σ(Fo )]); CCDC 249138.
52
Org. Lett., Vol. 7, No. 1, 2005

pochemical polymerization in a 1,6-manner.15 Several in-
termolecular close contacts exist along the polyyne chains
for both molecules A and B, and minima are observed for
C(4)-C(9) in both (3.84 and 3.68 Å, respectively). In both
cases, the stacking angle is 30° (optimal is ca. 27°) and the
packing distance d ) 8.2 Å (optimal is 7.4 Å). Similar to
3a, DSC analysis of 3b shows a narrow exotherm (Tonset
)
144 °C, Tmax ) 151 °C) consistent with that expected of a
topochemical polymerization process.19
The absorption spectra for compounds 1-3 and 5 are
shown in Figure 3 and summarized in Table 1. The polyynes
Figure 1. ORTEP drawing of compound 3a showing several
parallel-oriented symmetry-related molecules (20% probability
level; hydrogen atoms omitted for clarity).17
not be favored.15 Surprisingly, differential scanning calo-
rimetry (DSC) of 3a shows no melting point but a rather
sharp exotherm at 142 °C (Tonset ) 133 °C, Tmax ) 142 °C),
implying that a topochemical reaction may have resulted.16
Slow diffusion of hexanes into a CH2Cl2 solution of 3b
provided X-ray quality crystals.18 The unit cell shows two
crystallographically unique molecules, which face each other
about a pseudo center of inversion (Figure 2). Molecule B
Figure 3. UV-vis absorption spectra for 1-3 and 5 (in THF).
λ
max for region 2 is marked by an upward blue arrow, and λmax for
region 1 is marked by a downward red arrow. The highest molar
absorptivity for the octayne is displayed.
display significant molar absorptivities for transitions at
higher energy (region 2) and well-resolved vibronic structure
for absorptions at lower energy (region 1). Increasing the
electron density (3c) and conjugation length (5) affords
Figure 2. ORTEP drawing of compound 3b showing several
parallel-oriented molecules A and B (20% probability level;
hydrogen atoms omitted for clarity).
Table 1. Selected Optical Data for Polyynes 1-3 and 5a
λmax (nm)b
λmax (nm)c
ꢀ (L mol-1 cm-1
)
ꢀ (L mol-1 cm-1
)
γ (10-36 esu)
on average shows somewhat more bending of the polyyne
framework than molecule A, with average C-CtC bond
angles of 177.4° and 177.9°, respectively. Rather than
assuming a coplanar relationship, the terminal aryl groups
of each molecule are nearly orthogonal. Neighboring mol-
ecules are stacked into rows, and packing parameters for both
molecule A and B are near the range necessary for to-
compd
1
2
3a
3b
3c
5
328 (30 000)
288 (143 000)
337 (155 000)
342 (206 000)
351 (164 000)
344 (272 000)
12 ( 2
49 ( 18
217 ( 10
300 ( 30
n/ad
399 (22 000)
465 (8 000)
469 (10 900)
476 (13 300)
512 (3 600)
588 ( 36
a THF solutions. b λmax defined as the most intense absorption in region
c
2. λmax defined as the lowest energy absorption in region 1. d Sample 3c
(15) For an excellent review of solid-state characteristics of polyynes
and parameters concerning topochemical polymerization, see: Szafert, S.;
Gladysz, J. A. Chem. ReV. 2003, 103, 4175-4205.
was insufficiently soluble to reliably determine a γ-value.
(16) See Supporting Information for details.
(17) Primed atoms are related to unprimed ones via the crystallographic
inversion center (0, 0, 0); double-primed atoms are related to unprimed
ones via the inversion center (0, /2, 0).
bathochromic shifts for absorptions in both regions. It is
interesting to note that with the exception of 3b the molar
absorptivities at λmax in region 2 for 3 and 5 are somewhat
1
(18) Crystal data for 3b: C32H26, M ) 410.53, triclinic space group P1h
(No. 2), Dc ) 1.136 g cm-3, a ) 8.2179(9), b ) 16.7512(18), c ) 17.8407-
(19) Å, R ) 90.584(2)°, â ) 91.395(2)°, γ ) 102.0395(19)°, V ) 2400.9-
(4) Å3, Z ) 4, µ ) 0.064 mm-1. Final R(F) )0.0703, wR2(F2) ) 0.2027
2
2
for 577 variables and 9710 data with Fo g -3σ(Fo ) (4752 observations
(19) Characterization of the product(s) from the heating of 3a and 3b is
currently inconclusive, and further investigation is underway.
2
2
[Fo g 2σ(Fo )]); CCDC 249139.
Org. Lett., Vol. 7, No. 1, 2005
53

lower than those found for similar polyynes with aryl ether
end-capping groups,2d highlighting the effect of structure on
oscillator strength for extended polyynes.
Whereas the linear optical characteristics of polyynes have
been reported for a number of series of molecules, the third
order NLO properties of polyynes are virtually unknown.20,21
The molecular third-order NLO susceptibilities (γ) for
molecules 1-3 and 5 were evaluated using the DOKE
technique,22 and the γ-values are listed in Table 1 and shown
graphically in Figure 4 in comparison to their triisopropylsilyl
two significant features (Figure 4). First, the magnitude of
the γ-values for the phenylated polyynes is higher than that
of the silylated derivatives of equal length, which is perhaps
not surprising because of the increased π-electron density
afforded by the phenyl groups. Second, an analysis of
γ-values as a function of length affords a power-law
relationship γ ∼ nc with an exponent of c ) 3.79 ( 0.25,
(where n is the number of acetylene units). This exponent
value is slightly lower than that determined for TIPS
polyynes 8, which display γ ∼ n4.28(0.13. The slightly lower
exponent for 1-3a and 5 might result from more significant
end-group interaction between the polyyne framework and
the conjugating phenyl groups relative to the more electroni-
cally inert silyl groups of 8. If the influence of the end-groups
were to diminish with length, however, inclusion of γ-values
for longer members of this series could give a power-law
relationship more in line with that observed for polyynes 8.
The synthesis and study of these longer derivatives is
currently underway.
Although the power-law increase in γ-values for 1-3a
and 5 is slightly lower than that determined for the series of
polyynes 8, it is interesting to note that it is comparable to
or higher than that determined experimentally for other
conjugated oligomers, including polytriacetylenes (c ) 2.5),24
poly(arylene ethynylene)s (c ) 2.5),25 and polyenes (c )
3.6).26 Extended polyynes thus remain an intriguing prospect
for the realization of functional NLO materials.
Figure 4. Graphical representation of γ-values for diphenyl
polyynes 1-3a and 5 in comparison to TIPS end-capped derivatives
8. Curves are fit to a power-law of the form γ ) a + bnc, where
a and b are constants.20
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada (NSERC), iCORE, ASRA, CFI, IIPP, CIPI, and the
University of Alberta. Petro-Canada is gratefully acknowl-
edged for a Young Innovator Award (R.R.T.), NSERC for
a postgraduate scholarship (A.D.S.) and an undergraduate
research fellowship (E.E.), and Alberta Ingenuity for a
graduate scholarship (A.D.S.).
(TIPS) end-capped cousins 8.20,23 Hexayne 3b, an alkylated
derivative of 3a, shows a γ-value that is modestly higher
than the parent system 3a. The donor-donor system 3c,
unfortunately, did not yield reliable results because of poor
solubility in THF.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data for new compounds, DSC traces,
and crystallographic details for 3a and 3b. This material is
available free of charge via the Internet at http://pubs.acs.org.
A comparison of the γ-values determined for polyynes
1-3a and 5 to those of TIPS end-capped polyynes 8 shows
OL047931Q
(20) Only one prior study of third-order NLO properties of extended
polyynes has been reported; see: Slepkov, A. D.; Hegmann, F. A.; Eisler,
S.; Elliott, E.; Tykwinski, R. R. J. Chem. Phys. 2004, 120, 6807-6810.
(21) Polyynes have been widely studied theoretically, inter alia: (a)
Nalwa, H. S.; Mukai, J.; Kakuta, A. J. Phys. Chem. 1995, 99, 10766-
10774. (b) Archibong, E. F.; Thakkar, A. J. J. Chem. Phys. 1993, 98, 8324-
8329. (c) Buma, W. J.; Fanti, M.; Zerbetto, F. Chem. Phys. Lett. 1999,
313, 426-430. (d) Kuzyk, M. G. Phys. ReV. Lett. 2000, 85, 1218-1221.
(e) Kuzyk, M. G. Opt. Lett. 2000, 25, 1183-1185; corrigenda 2003, 28,
135.
(23) Eisler, S.; Slepkov, A. D.; Elliott, E.; Luu, T.; McDonald, R.;
Hegmann, F. A.; Tykwinski, R. R. J. Am. Chem. Soc., accepted for
publication.
(24) (a) Martin, R. E.; Gubler, U.; Boudon, C.; Gramlich, V.; Bosshard,
C.; Gisselbrecht, J. P.; Gunter, P.; Gross, M.; Diederich, F. Chem. Eur. J.
1997, 3, 1505-1512. (b) Gubler, U.; Bosshard, C.; Gunter, P.; Balakina,
M. Y.; Cornil, J.; Bre´das, J.-L.; Martin, R. E.; Diederich, F. Opt. Lett. 1999,
24, 1599-1601.
(25) Meier, H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar, A.;
Bubeck, C. Eur. J. Org. Chem. 2001, 4431-4443.
(26) Craig, G. S. W.; Cohen, R. E.; Schrock, R. R.; Silbey, R. J.; Puccetti,
G.; Ledoux, I.; Zyss, J. J. Am. Chem. Soc. 1993, 115, 860-867.
(22) Slepkov, A. D.; Hegmann, F. A.; Zhao, Y.; Tykwinski, R. R.;
Kamada, K. J. Chem. Phys. 2002, 116, 3834-3840.
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Org. Lett., Vol. 7, No. 1, 2005