Synthesis, structure, and nonlinear optical properties of cross-conjugated perphenylated iso-polydiacetylenes

Article abstract of DOI:10.1002/chem.200400822

Monodisperse, cross-conjugated perphenylated iso-polydiacetylene (iso-PDA) oligomers, ranging from monomer 15 to pentadecamer 25, have been synthesized by using a palladium-catalyzed cross-coupling protocol. Structural characteristics elucidated by X-ray

Full text of DOI:10.1002/chem.200400822

FULL PAPER
Synthesis, Structure, and Nonlinear Optical Properties of Cross-Conjugated
Perphenylated iso-Polydiacetylenes
Yuming Zhao,[a] Aaron D. Slepkov,[b] Clement Osei Akoto,[c] Robert McDonald,[d]
Frank A. Hegmann,*[b] and Rik R. Tykwinski*[c]
Abstract: Monodisperse, cross-conju-
gated perphenylated iso-polydiacety-
lene (iso-PDA) oligomers, ranging
from monomer 15 to pentadecamer 25,
have been synthesized by using a palla-
dium-catalyzed cross-coupling protocol.
Structural characteristics elucidated by
X-ray crystallographic analysis demon-
strate a non-planar backbone confor-
mation for the oligomers due to the
steric interactions between alkylidene
phenyl groups.The electronic absorp-
tion spectra of the oligomers show a
slight red-shift of the maximum absorp-
tion wavelength as the chain length in-
creases from dimer 17b to pentade-
camer 25, a trend that has saturated by
the stage of nonamer 22.Fluorescence
spectroscopy confirms that the pendent
phenyl groups present on the oligomer
framework enhance emission, and the
relative emission intensity consistently
increases as a function of chain length
n.The molecular third-order nonlinear-
ities, g, for this oligomer series have
been measured via differential optical
Kerr effect (DOKE) detection and
show a superlinear increase as a func-
tion of the oligomer chain length n.
Molecular modeling and spectroscopic
studies suggest that iso-PDA oligomers
(n>7) adopt a coiled, helical confor-
mation in solution.
Keywords: cross
conjugation
·
enynes foldamers
·
·
nonlinear
optics · optical Kerr effect · poly-
diacetylenes
Introduction
cal signal processing/switching devices.[2] The realization of
such devices could substantially improve the capability of
modern computing and telecommunications technologies.In
the past several decades, enormous efforts have focused on
molecules with extensive p-delocalization that often afford
large nonlinear optical responses, that is, large molecular
first and second hyperpolarizabilities (b and g).[1,3] Conjugat-
ed oligomers are also attractive as nonlinear optical materi-
als due to their extended p-electron systems.[4] For example,
Organic compounds continue to be desirable materials can-
didates for optical and opto-electronic applications,[1] and or-
ganic NLO materials with large molecular hyperpolarizabili-
ty would be particularly useful for the fabrication of all-opti-
[a] Prof.Y.Zhao
Department of Chemistry
Memorial University of Newfoundland
St.Johnꢀs, NL A1B 3X7 (Canada)
polyynes,[5]
polyacetylenes
(PAs),[6]
polydiacetylenes
(PDAs),[7] polytriacetylenes (PTAs),[8] as well as many
others have been explored.[9] One disadvantage of linearly
conjugated oligomers, however, is the dramatic red-shift of
the maximum electronic absorption wavelength (lmax) to-
wards the visible region of the spectrum as the oligomer
chain length is increased.This results in longer oligomers
with considerably decreased transparency in the visible
region of the spectrum.[10] To circumvent this “transparency-
nonlinearity trade-off”, alternatives to linear conjugation
have been explored,[3] such as two-dimensional conjuga-
tion[11] and the substitution of functional groups in the
oligomer backbones.[12]
[b] A.D.Slepkov, Prof.F.A.Hegmann
Department of Physics, University of Alberta
Edmonton, AB T6G 2J1 (Canada)
Fax : (+1)780-492-0714
E-mail: hegmann@phys.ualberta.ca
[c] C.O.Akoto, Prof.Dr.R.R.Tykwinski
Department of Chemistry, University of Alberta
Edmonton, AB T6G 2G2 (Canada)
Fax : (+1)780-492-8231
E-mail: rik.tykwinski@ualberta.ca
[d] Dr.R.McDonald
X-Ray Crystallography Laboratory
Department of Chemistry
University of Alberta
In this context, cross-conjugated enyne oligomers[13,14]
have been examined with the following motives in mind:
1) The red-shift of lmax can be mediated, on account of the
Edmonton, AB T6G 2G2 (Canada)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2005, 11, 321 – 329
DOI: 10.1002/chem.200400822
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321

attenuated p-electron delocalization via a cross-conjugated
framework, yielding electronic transparency, even for ex-
tended oligomers, and 2) cross-conjugated oligomers remain
rich in p-electron density, which is essential for providing
large third-order molecular and bulk nonlinear optical sus-
ceptibilities.
Oligomers with a cross-conjugated enyne framework, such
as iso-polydiacetylenes (iso-PDAs) 1 and 2[15] and iso-poly-
triacetylenes (iso-PTAs),[16,17] have been studied previous-
ly.[18] These studies show that electronic communication is
observed along the cross-conjugated enyne frameworks and
that the oligomers also demonstrate high transparency in
the visible region of the spectrum.[19] The pendant alkyl sub-
stituents (e.g., isopropylidene 1 and adamantylidene 2),
however, did not supply sufficient stability and solubility for
extensive studies of nonlinear optical properties.In a quest
for oligomers more suitable for study of third-order NLO
behavior, a series of perphenylated iso-PDA oligomers 3
was targeted.It was also envisioned that both the stability
and solubility (processabiltiy) could be improved.[15–17] The
synthesis, spectroscopic, nonlinear optical, and conforma-
tional properties of these new oligomers are detailed in this
paper.
Scheme 1.Synthesis of vinyl triflate building blocks 11–13.a) AlCl
CH2Cl2, 08C; b) triflic anhydride, 2,6-di(tert-butyl)-4-methylpyridine,
CH2Cl2, RT.
,
3
Scheme 2.Synthesis of perphenylated iso-PDA monomers 15 and 16.
60–658C (reflux), however, led to substantially accelerated
reaction rates and excellent yields of enediynes 15 and 16,
respectively.
Initial efforts to assemble perphenylated iso-PDA oligo-
mers were directed toward dimer 17a (Scheme 3).Monomer
16 was selectively desilylated and cross-coupled with triflate
13 by using [PdCl2(PPh3)2] as the catalyst, but this reaction
consistently gave almost exclusively the homocoupled prod-
uct 18a.Attempts to optimize the dimer formation were
then directed toward the formation of dimer 17b.The TMS
group of 15 was carefully removed with catalytic K2CO3 in
MeOH/THF, and the mono-deprotected product cross-cou-
pled with triflate 12.Using [PdCl 2(PPh3)2] as the catalyst,
this reaction also resulted in a substantial amount of dimer
Results and Discussion
Synthesis: Friedel–Crafts acylation[20] of silyl-protected ace-
tylenes 4–6 with diphenylacetyl chloride 7 gave ketones 8–
10, and triflation[21] under standard conditions gave the vinyl
triflate building blocks 11–13 (Scheme 1).In the synthesis of
ketone 8, a mixture of bis and mono-acylated products was
formed, and desilylation of ketone 8 under the reaction con-
ditions was also observed.Replacing one of acetylenic pro-
tecting groups with a more robust triethylsilyl (TES) or tri-
isopropylsilyl (TIPS) group, however, reduced these trouble-
some side-reactions during acylation and greatly facilitated
the purification of ketones 9 and 10.Since ketones 8–10
showed limited stability, even when stored under inert gas
and at low temperature (ꢀ208C), transformation into the cor-
responding vinyl triflates 11–13 was immediately effected.
With the pure vinyl triflates in hand, the cross-conjugated
perphenylated iso-PDA oligomers were prepared via an iter-
ative cross-coupling strategy.[22,23] Whereas the synthesis of
alkyl substituted enediynes 1 and 2 was accomplished under
Pd-catalyzed cross-coupling conditions at room tempera-
ture,[15] the analogous reactions of triflates 12–13 with trime-
thylsilylacetylene 14 proceeded at an unbearably slow rate,
likely due to the increased steric demands of the phenyl sub-
stituents (Scheme 2).Increasing the reaction temperature to
18b.Switching to [Pd(PPh )4] as the catalyst, however, sub-
3
stantially reduced the yield of 18b (ca.3%) and afforded an
excellent yield of 17b.To date, the optimized cross-coupling
conditions for these and subsequent reactions used
[Pd(PPh3)4] as the catalyst, CuI as the co-catalyst, iPr2NH as
the base in THF under positive Ar pressure, at reflux for at
least 12 h.
Exhaustive deprotection of 15 with tetrabutylammonium
fluoride (TBAF) gave the bisterminal alkyne, which was
cross-coupled with triflate 12 to afford trimer 19 as a yellow
solid (Scheme 4).Repetition of the protiodeprotection and
cross-coupling sequence then afforded pentamer 20 and
heptamer 21 in respectable yields.Further elaboration of
heptamer 21 afforded nonamer 22, and while 22 shows
decent solubility in organic solvents, purification via column
chromatography (silica gel or alumina) was problematic be-
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Polydiacetylenes
FULL PAPER
uct(s) were also detected in subsequent reactions, the com-
pounds were extremely difficult to isolate pure because of
the small quantity of material.
Characterization: Electrospray mass spectrometry (ESIMS)
proved to be a particularly powerful tool for characteriza-
tion of the oligomers.For all compounds, a clear signal for
[M+X]+ (X = Na and/or Ag) was observed in analysis
(positive mode) of MeOH/toluene solutions with added
AgOTf.For example, analysis of the tridecamer 24 shows a
signal at m/z 2991 as expected for [M+Ag]+, and that of
pentadecamer 25 shows m/z 3309 and 3392, consistent with
[M+Na]+ and [M+Ag]+, respectively.
Scheme 3.Synthesis of dimeric perphenylated iso-PDAs.
cause of significant aggregation of the various products.
Size-exclusion chromatography (SEC) thus proved to be in-
valuable in the purification of nonamer 22, as well as longer
oligomers, by using CH2Cl2 as an eluent.This general proto-
col led to good yields of undecamer 23, tridecamer 24 and
pentadecamer 25.Compound 25 is the longest monodisperse
iso-PDA oligomer synthesized to date, and it consists of a
cross-conjugated enyne backbone composed of 62 sp and sp2
hybridized carbons.
In the 13C NMR spectra of the iso-PDA oligomers, the
resonances of the alkylidene carbons outside of the main
chain (Ph2C=C) are the most easily identified, resonating
quite downfield in a range of 155–158 ppm.In the spectrum
of heptamer 21, these four carbons remain discernible at d
154.9, 155.1, 155.4, and 156.9. For the longer oligomers, how-
ever, chemical shift degeneracy of these carbons exists, that
is, only five of seven are distinguishable for tridecamer 24,
and only four of eight can be seen for 25.The in-chain alkyl-
idene carbons (Ph2C=C) fall
It was only in the transformation of 15 to 19 where an
isolable amount of a homocoupled by-product 26 was also
isolated (8%).Although small amounts of such by-prod-
farther upfield, in the chemical
shift range of the acetylenic
carbons.For the longer
oligomers, significant chemical
shift degeneracy is also ob-
served for resonances of sp
and sp2 carbons of the enyne
framework (even when mea-
sured at 125 MHz).For in-
stance, already in the spectrum
of pentamer 20, only 11 of 12
unique sp and sp2 carbon reso-
nances (excluding the phenyl
carbons) are discernible.
All iso-PDA oligomers show
reasonable thermal stability.
Dimer 17b, trimer 19, penta-
mer 20, and heptamer 21 show
well defined melting points at
67–69, 60–61, 85–86, and 124–
1268C, respectively.Nonamer
22, undecamer 23, tridecamer
24, and pentadecamer 25 de-
compose at 134–135, 140–142,
148–150, and 155–1598C, re-
Scheme 4.Iterative synthesis of perphenylated iso-PDAs 19–25.a) TBAF, wet THF, RT; b) 12, [Pd(PPh3)4],
CuI, iPr2NH, THF, reflux, 12 h.
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F.A.Hegmann, R.R.Tykwinski et al.
spectively.Differential scanning calorimetric (DSC) analysis
of these longer oligomers confirmed the decomposition.In
comparison to the alkyl substituted iso-PDAs (i.e., oligo-
mers 1 and 2),[15] the perphenylated oligomers exhibited sub-
stantially improved kinetic stability and no significant de-
composition could be detected by either TLC or NMR spec-
troscopic analysis for samples exposed to ambient conditions
for several weeks.
Solid-state structural properties: Crystallographic analyses
of monomer 15, dimer 17b, trimer 19, and homocoupled
dimer 18a were performed to examine the solid-state struc-
tural characteristics of the iso-PDA oligomers.While disor-
der, particularly in the ethyl groups, hampered the refine-
ment of the data for 15 (Figure 1), the resulting structure is
sufficient for an empirical analysis and as a point of compar-
ison to that of the other oligomers.The alkylidene angle C2-
C3-C5 at 114.3(3)8 is comparable to the that of its isopropyl-
idene analogue at 112.9(4)8.[15] Both SiCꢁC bond angles, at
172.2(3) and 174.9(4)8, are distorted from linearity, presuma-
bly due to crystal packing effects.Steric interactions be-
tween the two alkylidene phenyl groups prohibit co-planari-
ty and force the phenyl rings out of the enediyne plane by
47.7(6) and 43.0(6)8.
Figure 2.a) ORTEP drawing (20% probability level) of 17b.Selected
bond angles [8]: Si1-C1-C2 175.4(3), C1-C2-C3 175.4(4), C2-C3-C5
113.9(3), C3-C5-C6 176.5(4), C5-C6-C7 171.6(3), C6-C7-C9 113.6(3), C7-
C9-C10 176.8(4), Si2-C10-C9 172.7(3); b) crystal packing of 17b.
Figure 1.ORTEP drawing (20% probability level) of 15.Selected bond
angles [8]: Si1-C1-C2 172.2(3), C1-C2-C3 176.9(4), C2-C3-C5 114.3(3),
C3-C5-C6 177.5(4), C21-C4-C31 116.8(3), Si2-C6-C5 174.9(4).
Crystallographic analysis of dimer 17b shows a pseudo
s-trans orientation of the two olefins about the central acet-
the phenyl groups of neighboring alkylidene moieties.Nota-
bly, trimer 19 shows a pseudo-cis-trans orientation rather
than an all s-trans orientation; the only iso-PDA to date that
displays a cisoid orientation for neighboring alkylidene
groups in the solid state (Figure 3).The intramolecular dis-
tance between H(55) and H(56) to phenyl plane 4 (C61,
C62, C63, C64, C65, C66) is observed at 3.20 and 3.37 Š, re-
spectively, with an angle between the two phenyl planes is
798.This orientation suggests p-stacking of the “face-to-
edge” type that stabilizes the pseudo-cis orientation in the
solid state.
ylenic bond and
a nonplanar the enyne framework
(Figure 2).The dihedral angle between the two alkylidene
least-squares planes, namely plane 1 (C2, C3, C4, and C5)
and plane 2 (C6, C7, C8, and C9), is 33.70(16)8.The two
enediyne alkylidene angles at 113.9(3) and 113.6(3)8 are
similar to that of monomer 15.The packing diagram of 17b
shows that the distance between H(32) and phenyl plane
(C41’, C42’, C43, C44’, C45’, C46’) is 3.26 Š, which is an in-
dication of the intermolecular p-stacking in a “face-to-edge”
manner or “T-stacking” (Figure 2b).[24,25]
The cross-conjugated enyne framework of 18a (Figure 4)
shows a transoid, centrosymmetric, and nearly planar con-
formation (C2h symmetry), with a maximum deviation of
0.078(2) Š from the least-squares plane (excluding the
Crystallographic analysis of trimer 19 shows that the
cross-conjugated oligoenyne framework does not assume a
planar conformation due to the steric interactions between
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Polydiacetylenes
FULL PAPER
Figure 5.Crystal packing diagram of 18a viewed along b axis.
Figure 3.ORTEP drawing (20% probability level) of 19.Selected bond
angles [8]: Si1-C1-C2 168.9(3), C1-C2-C3 176.5(3), C2-C3-C5 114.4(3),
C3-C5-C6 176.8(3), C5-C6-C7 175.1(3), C6-C7-C9 114.2(3), C7-C9-C10
174.4(4), C9-C10-C11 170.4(3), C10-C11-C13 113.0(3), C11-C13-C14
177.5(4), Si2-C14-C13 172.2(3).
were investigated by UV/Vis spectroscopy (Figure 6).The
most obvious aspect of the UV/Vis spectra is the steadily in-
creasing molar absorptivity as the chain length is extended,
which ultimately reaches e=160000 for iso-PDA 25.The
spectrum of dimer 17b shows two distinct absorptions at 373
and 323 nm.In the absence of a significant contribution
from cross-conjugation, the major absorptions for 17b, as
well as the other iso-PDAs, are expected to arise from the
longest linearly conjugated framework, ene-yne-ene segment
shown in bold in Scheme 3.[27] Previous work with cyclic iso-
PDAs suggests that the lower energy absorption (373 nm)
results from the HOMO!LUMO transition for both the
cisoid and transoid conformers of this ene-yne-ene segment,
with the cisoid absorption occurring at slightly higher
energy than the transoid.It is predicted that absorption at
323 nm arises from the HOMO!LUMO + 1 transition of
the cisoid conformer, a transition that is formally symmetry
forbidden for the transoid conformer.[15] The spectrum of
trimer 19 shows a similar absorption profile to that of 17b,
with distinctive absorptions at 375 and 324 nm.In the spec-
trum of pentamer 20, the low-energy absorption at 377 nm
becomes the only notable characteristic, as the higher-
energy absorption at 324 nm merges into a barely distin-
guishable shoulder.From heptamer 21 to pentadecamer 25,
the absorption profiles show a broad and featureless absorp-
tion at lmax =378 nm.Overall, a slight lowering of the lmax
Figure 4.ORTEP drawing (20% probability level) of 18a.Selected bond
angles [8]: Si1-C1-C2 178.2(2), C1-C2-C3 176.2(3), C2-C3-C5 115.13(19),
C3-C5-C6 174.4(3), C5-C6-C6’ 178.7(3).
phenyl and silyl moieties).It is clear that the central buta-
diynyl linkage sufficiently distances the alkylidene moieties
such that steric interactions are relieved, resulting in a fully
planar enyne conformation in the solid state.
Compound 18a shows a parallel stacking arrangement in
the solid state (Figure 5), with solid-state packing parame-
value versus chain length is observed dimer 17b (lmax
=
ters for the central butadiynyl moieties of F=358, R1,4
=
373 nm) to pentadecamer 25 (lmax =378 nm).This effect has,
however, already reached saturation by the length of about
n=7–9, similar to that of iso-PDAs 1.[15]
6.1 Š, d=8.6 Š. When compared to the optimal parameters
for topochemical polymerization of butadiynes in a 1,4-addi-
tion fashion, F=458, R1,4 <4.0 Š, d=5.0 Š, however,[26]
there is little possibility for reaction, since the distance be-
tween neighboring molecules is too great.In the packing of
18a, it is interesting to note that there are no intermolecular
and/or intramolecular p-stacking interactions observed be-
tween proximal phenyl groups.
The influence of solvent polarity on the electronic absorp-
tions was investigated using nonamer 22.In most cases, only
a slight change of lmax (ca.3 nm) was found.For example, in
CHCl3, hexanes, and benzene lmax =377 nm, whereas in
Et2O and THF lmax =374 nm.The absorption profiles are
also similar to each other.The UV/Vis spectrum of 22 in
acetonitrile, however, shows obvious differences when com-
pared with those from other solvents.In acetonitrile, the
lowest energy absorption has shifted toward higher energy
Electronic absorption and emission: The electronic absorp-
tion characteristics of dimer 17b through pentadecamer 25
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F.A.Hegmann, R.R.Tykwinski et al.
Figure 7.Solid state UV/Vis spectra for 17b and 19–25 as thin films cast
from CHCl3 onto quartz slides.
Figure 6.Electronic absorption spectra in CHCl 3 comparing the effects of
oligomer chain length between 17b, 19–25.
by about 12 nm (lmax =365 nm), and the high energy absorp-
tion at 323 nm, as observed in the spectra of dimer 17b and
trimer 19 in CHCl3, is also discernible.[28] This observed be-
havior in CH3CN is analogous to that observed for the fold-
ing of oligo(m-phenylene ethynylene) systems reported by
Moore and co-workers[29] and will be discussed in more
detail below.
UV/Vis spectra of iso-PDA oligomers 21 and 22 were re-
corded at varied concentrations (from 10ꢀ5 to 10ꢀ7 m) in
CHCl3 in order to probe the possibility of aggregation in so-
lution.These results showed consistent profiles and nearly
identical extinction coefficients, suggesting little or no aggre-
gation over the accessible range of concentrations.
The absorption spectra of iso-PDA oligomers as thin films
cast from CHCl3 onto quartz slides were measured
(Figure 7).The oligomers form reasonably good quality thin
films with absorption profiles that are similar to those mea-
sured in solution.Thus, no pronounced intermolecular aggre-
gation band could be detected.[30] The slight random change
in lmax values observed in the spectra of the films was pre-
sumably due to scattering effects and slight variances in the
films.
Emission spectra were measured in degassed CHCl3 with
an excitation wavelength of 380 nm (Figure 8), and the per-
phenylated iso-PDAs show enhanced emission when com-
pared to their alkylidene analogues 1 and 2.[31] All oligomers
show only one broad emission peak, and the relative intensi-
ty steadily increases with chain length.The maximum emis-
sion wavelength (lem) shifts toward lower-energy in the pro-
gression from dimer 17b (lem =468 nm) to heptamer 21
(lem =503 nm).From heptamer 21 to pentadecamer 25, lem
maintains an essentially constant value.This trend concurs
with that observed for the UV/Vis absorption data, suggest-
ing an effective conjugation length (ECL) of nECL ꢂ7.
Figure 8.Emission spectra in CHCl 3 for 17b and 19–25 (excitation at
380 nm); lem [nm] for each oligomer is shown in parentheses.
optical Kerr effect (DOKE) experiments.[14a,b] In these ex-
periments, the oligomer samples, from monomer 15 to pen-
tadecamer 25, were prepared as THF solutions of 0.035–
0.23m.The gs values were then obtained as previously de-
scribed,[14a,b] and all the gs values are referenced to that of
THF (gTHF).A summary of the gs values for the entire
oligomer series are listed in Table 1.
As depicted by the values in Table 1, the iso-PDA series
shows an order of magnitude increase in gs from the dimer
17b to pentadecamer 25, although the gs values themselves
Table 1.DOKE results for the iso-PDA series.[a]
Entry
n
c [m]
gs (”10ꢀ36 esu)
gs/gTHF
15
17b
19
20
21
22
23
24
25
1
2
3
5
7
9
11
13
15
0.23
0.15
0.13
0.12
0.10
0.12
0.054
0.054
0.035
9.2ꢃ0.4
30.3ꢃ0.3
48ꢃ4
17.7ꢃ0.8
58.2ꢃ0.5
94ꢃ7
70ꢃ5
134ꢃ9
94ꢃ4
181ꢃ9
123ꢃ8
177ꢃ18
208ꢃ10
308ꢃ8
240ꢃ15
347ꢃ35
408ꢃ20
603ꢃ16
Nonlinear optical properties: One of the most appealing as-
pects of these new iso-PDA oligomers is their potential as
third-order NLO materials because of their significant opti-
cal transparency.The molecular second hyperpolarizabilities,
gs, for the iso-PDA samples were determined by differential
[a] gs and gTHF are the second hyperpolarizabilities of the oligomer sam-
ples and THF reference, respectively; n is the number of repeat units in
the oligomer.
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Polydiacetylenes
FULL PAPER
remain quite modest.On the basis of the absorption and
emission data, conventional wisdom dictates that limited
electronic communication is found across a cross-conjugated
bridge.Thus, increasing the number of repeat units would
simply increase the number of fixed-length linearly-conjugat-
ed paths (as shown in bold, Scheme 3).In the absence of sig-
nificant cross-conjugated p-delocalization, this would afford a
linear increase in gs values as a function of chain length.
Indeed, a linear trend for shorter iso-PDA oligomers (n=2–
7) was observed.[14a,b] Extending this investigation to longer
oligomers, however, revealed markedly different behavior as
shown in Figure 9a, where the oligomer nonlinearities clearly
increase superlinearly with respect to chain length.
A superlinear increase in g values as a function of chain
length is encountered in both theoretical and experimental
studies of linearly conjugated oligomers.These studies have
shown that optical nonlinearities typically increase with con-
jugated length according to a power-law relationship, g ~
nc, where c is the power law exponent.[32,33] Furthermore, a
fit of the data to expressions of the form g ~ nc is an indica-
tion of increased electronic communication along a conju-
gated backbone.While theories predict power law expo-
nents of c=2–4 for linearly conjugated systems, there are
currently no theoretical predictions for cross-conjugated sys-
tems.Figure 9b presents the data of iso-PDA series fit to a
power law function of the form g = a + bnc, where a and b
are constants.This analysis provides a superior fit to a
simple straight line, yielding a power of c=2.0ꢃ0.3. While
the data adequately fits a power-law, the correlation is not,
however, ideal.
A third possible relationship, consisting of two linearly in-
creasing regimes is considered as shown in Figure 9c.The gs
values show a good linear fit spanning n=2–7 and a second
linear fit with a steeper slope over the range of n=9–15.
Overall, the two-line analysis with a distinct change in slope
at nꢂ9, provides the best description of the data.
The analyses in Figures 9b and c would be explained by
two different phenomena.A power-law relationship be-
tween g and oligomer length (Figure 9b) would presumably
arise from increased p-electron delocalization, a surprising
prospect considering the UV/Vis absorption and emission
data found for the iso-PDAs.A superlinear increase in non-
linearities with a distinct change in slope, however, could
arise from an increased ordering of the sample in solution,
such as folding.[34] This prospect is discussed below.
Figure 9.Correlation of relative molecular second hyperpolarizabilities
(gs/gTHF) as a function of oligomer length n.a) Linear fit ( n=2–15).
b) Power-law fit of the form g=a + bnc (n=2–15).c) Two linear fits ( n=
2–7 and 9–15).
ometry adopts an all-cisoid conformation (n>7), the mini-
mized structure consistently converged to a helical confor-
mation with a “folded” enyne framework (Figure 10a).The
phenyl rings of neighboring alkylidene units each participate
in p-stacking, which contributes to stabilization of the
folded conformation (Figure 10b).The pitch angle for the
helix is calculated to be approximately 408.
Folding of the oligomers is also supported by UV/Vis
spectroscopic analysis.Previous work on iso-PDAs provide
for the following assumptions: a) The HOMO!LUMO+1
transition at about 323 nm is symmetry allowed for the
cisoid conformation and forbidden for the transoid confor-
Solution state folding of iso-PDAs: The interesting results
from the NLO analyses suggested that the perphenylated
iso-PDA oligomers might prefer a specific, folded, confor-
mation in the solution state when n>7.This suggestion is
also supported by the crystallographic analysis of trimer 19,
where the pseudo-cis orientation between enediyne seg-
ments allows the neighboring phenyl groups to approach
close enough to effect p–p interactions, which could become
a dominant feature in the longer oligomers.
The geometry of iso-PDA oligomers of various length was
analyzed computationally.[35] When the initial oligomer ge-
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327

F.A.Hegmann, R.R.Tykwinski et al.
Figure 11.Plot of absorbance ratio A378/A323 vs chain length (n) in CHCl3
&
*
(
) and acetonitrile ( ). A378 denotes the absorbance intensity at 378 nm,
and A323 denotes the absorbance intensity at 323 nm.
guest molecule.These studies, unfortunately, did not give
conclusive evidence for (or against) solution-state folding, as
a biased twist sense could not be achieved.
Conclusion
An efficient synthesis for perphenylated cross-conjugated
iso-PDAs (n=1–15) has been developed by using an itera-
tive divergent route.The oligomers were synthesized and
purified in satisfactory yields as stable and soluble solids.
Solid-state properties of monomer 15 to trimer 19[38] were
analyzed by X-ray crystallography and show that these
oligomers prefer non-planar conformations due to the steric
demands from the phenyl groups.Their electronic absorp-
tion behavior indicates an effective conjugation length of
nECL =7–9. iso-PDA oligomers show steadily increasing fluo-
rescence intensity as a function of chain length, and the
emission wavelength shifts slightly from 468 nm (dimer 17b)
to 503 nm (heptamer 21).Beyond the length of the heptam-
er, the emission energy remains essentially constant.The
third-order NLO properties of the iso-PDAs in THF solu-
tions were studied by differential optical Kerr effect
(DOKE) experiments.Molecular second hyperpolarizabili-
ties, gS, give a super-linear increase as a function of chain
length.The best fit to the NLO data shows two linear re-
gimes, suggesting an increased order in the system, likely
folding.While definitive proof of a folded conformation for
extended iso-PDAs remains elusive, empirical evidence of
this conformational presence is provided by molecular mod-
eling studies as well as UV/Vis spectroscopy.Additional
studies of conformation, as well as the synthesis of chiral
and optically pure iso-PDA derivatives, are currently under-
way.
Figure 10.Minimized structure of tridecameric iso-PDA using MM3*
force field parameters.Only backbone carbons are displayed as space-fill-
ing representation for clarity: a) View from top; b) view from side.
mation,[15] and b) the HOMO!LUMO transition for the
cisoid ene-yne-ene chromophore occurs at slightly higher
energy than that of the transoid conformer (although both
are observed as a combined absorption band at about
378 nm in the present case).[15,36] The ratio of these two ab-
sorbances (denoted as A378/A323) could therefore be an indi-
cator of conformation change, and a plot of this indicator
versus chain length n is presented in Figure 11.In both
CHCl3 and acetonitrile, the ratio of A378/A323 steadily in-
creased from dimer 17b to heptamer 21, and then reached a
constant value for the longer oligomers (nꢄ7).In an effort
to provide additional support of a folded conformation, cir-
cular dichroism (CD) spectroscopy was also explored, as es-
tablished by Moore and co-workers,[37] including the use of
chiral solvents as well as attempts to incorporate a chiral
Acknowledgement
This research was supported by the University of Alberta, the Natural
Sciences and Engineering Research Council of Canada, Canada Founda-
tion for Innovation, IIPP, ASRA, CIPI, iCORE, and Petro-Canada
328
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www.chemeurj.org
Chem. Eur. J. 2005, 11, 321 – 329

Polydiacetylenes
FULL PAPER
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Received: August 10, 2004
Commun. 2000, 77–78.
Published online: November 18, 2004
Chem. Eur. J. 2005, 11, 321 – 329
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