Donor/acceptor effects on the linear and nonlinear optical properties of geminal diethynylethenes (g-DEEs)

Article abstract of DOI:10.1002/hlca.200790092

A series of geminal diethynylethenes (g-DEEs) with electron-donating and/or electron-accepting (D/A) groups were synthesized via a Pd-catalyzed cross-coupling sequence. The UV/VIS spectra for donor-acceptor (D-A) functionalized g-DEEs 5, 8, and 11 show di

Full text of DOI:10.1002/hlca.200790092

Helvetica Chimica Acta – Vol. 90 (2007)
909
Donor/Acceptor Effects on the Linear and Nonlinear Optical Properties of
Geminal Diethynylethenes (g-DEEs)
by Yuming Zhao*a), Ningzhang Zhoua), Aaron D. Slepkovb), Sorin C. Ciuleic), Robert McDonaldd),
Frank A. Hegmann*b), and Rik R. Tykwinski*c)
a) Department of Chemistry, Memorial University of Newfoundland, St. Johnꢀs, NLA1B3X7, Canada
(e-mail: yuming@mun.ca)
b) Department of Physics, University of Alberta, Edmonton, AB T6G2J1, Canada
(fax: (þ1)780-492-0714; e-mail: hegmann@phys.ualberta.ca)
c) Department of Chemistry, University of Alberta, Edmonton, AB T6G2G2, Canada
(fax: (þ1)780-492-8231; e-mail: rik.tykwinski@ualberta.ca)
d) X-Ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton,
AB T6G2G2, Canada
A series of geminal diethynylethenes (g-DEEs) with electron-donating and/or electron-accepting
(D/A) groups were synthesized via a Pd-catalyzed cross-coupling sequence. The UV/VIS spectra for
donor– acceptor (D – A) functionalized g-DEEs 5, 8, and 11 show distinctive absorption trends
attributable to intramolecular charge-transfer (ICT). The bond-length-alternation (BLA) index for the
cross-conjugated enediyne framework varies slightly with different terminal substituents as determined
by density-functional theory (DFT) calculations and single-crystal X-ray analysis. Ultrafast third-order
optical nonlinearities for the g-DEEs were measured by the differential optical Kerr effect (DOKE)
technique and show that terminal donor– acceptor substitution of g-DEEs enhances molecular second
hyperpolarizabilities (g) in comparison to donor or acceptor g-DEEs. A small increase in the two-
photon-absorption cross-section (s(2)) is observed in the series 9 – 11 as a result of increased
functionalization. The effects of donor/acceptor substitution on electron delocalization along the
cross-conjugated enediyne structure are evaluated on the basis of natural-bond-orbital (NBO) analysis.
Solid-state structures of the four derivatives 3b, 4b, 7 and 8 were characterized by single-crystal X-ray
structural analysis and show an asymmetric unit cell for one derivative, D – A g-DEE 8.
1. Introduction. – Conjugated molecules with highly delocalized and polarizable p-
electron systems are prospects for third-order nonlinear optical (NLO) materials, and
could form the basis for achieving ultrafast all-optical-based computing and tele-
communications technology [1][2]. To find ideal and applicable organic NLO
materials, a large number of organic chromophores have been extensively explored,
with many efforts focused on linearly conjugated aromatic and/or nonaromatic p-
conjugated systems [2 – 5]. In assessing the suitability of an organic material for NLO
purposes, its efficiency/transparency trade-off at a specific wavelength is an important
factor to be considered, and maintaining transparency in the visible region of the
spectrum is a significant challenge [6]. Many linearly p-conjugated chromophores tend
to perform poorly in this respect, and the pursuit of alternative molecular designs has
become more and more appealing and important. In particular, the use of cross-
ꢁ 2007 Verlag Helvetica Chimica Acta AG, Zürich

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conjugated p-systems, the topological isomers of linearly conjugated molecules, has
been explored [7 – 10]. One promising aspect of using cross-conjugation in place of
linear conjugation lies in the satisfactory electronic transparency arising from the
moderated conjugative p-pathway.
We have reported the synthesis and optical properties of cross-conjugated iso-
poly(diacetylene) oligomers (iso-PDAs) with pendent methyl and phenyl substituents
(see 1 and 2) [10][11]. The more stable and soluble series 2 displays a superlinear, albeit
modest increase in second hyperpolarizability (g) as a function of chain length (up to
n ¼ 15), while electronic transparency (absorptivity in the visible region of the
spectrum) is maintained at a satisfactory level [10]. With the structure – property
relationships as a function of chain length established, the task of revealing the impact
of p-electron-donating (D) and -accepting groups (A) appended to cross-conjugated
enynes has now been targeted [12]. Such ꢂpush-pullꢀ functionalization is a well-
established mode for dictating the electronic characteristics and molecular hyper-
polarizabilities for various linearly conjugated molecules [5]. Our work [13], and that
of others [7][14][15], has suggested that D/A-substitution can cause notable changes in
the electronic-absorption behavior of cross-conjugated oligomers as well.
In this paper, we report on a series of electron-donor 4-(dimethylamino)phenyl and
ferrocenyl- and/or electron-acceptor (4-nitrophenyl)-substituted geminal diethynyl-
ethenes (g-DEEs; see 3 – 11) that have been synthesized via a sequence of Pd-catalyzed
cross-coupling reactions. The electronic-absorption properties of these D/A- and D –
A-substituted g-DEEs are characterized by UV/VIS spectroscopy, and the structural
properties of several derivatives in the solid state are studied by X-ray crystallography.
Second-order molecular hyperpolarizabilities (g) and two-photon-absorption cross-
sections (s(2)) are determined by the differential optical Kerr effect (DOKE) detection
technique. Finally, the electron-delocalization characteristics throughout the geminal
enediyne p-bridge have been investigated by means of density-functional theory
(DFT) calculations and natural-bond-orbital (NBO) analysis.
Results and Discussion. – 2.1. Synthesis of D/A Substituted g-DEEs. D/A-
Substituted g-DEE molecules 3 – 11 were prepared by using one of two routes
(Scheme)1). In the first case, vinyl triflate 12a [16] or 12b [10] was used as the essential
building block, to which the appropriate acceptor or donor group could be attached via
Pd-catalyzed Sonogashira coupling with the respective terminal alkynes, to give 3a, 4a,
6, and 10. Mono-substituted g-DEEs 6 or 10 were then desilylated and cross-coupled to
1
)
The synthesis of compounds 3b, 4b, and 5 has been previously communicated, see [13].

Helvetica Chimica Acta – Vol. 90 (2007)
911
give disubstituted g-DEEs 7, 8, and 11. Alternatively, enediyne 13a [17] or 13b [10]
could be selectively desilylated and cross-coupled to give g-DEEs 3b, 4b, and 9. Donor
substituted 3b was then carried on to D – A 5 via removal of the triisopropylsilyl group
with Bu4NF and cross coupling with 1-iodo-4-nitrobenzene. In all cases, the yields were
reasonable to good, and the stable products could be isolated pure by column
chromatography.
2.2. Electronic Absorption Properties. UV/VIS Spectroscopic analysis of g-DEEs
was performed in CHCl3 solutions at room temperature. The effects of donor
substitution in g-DEE 4b show an intense absorption band at lmax 325 nm, whereas
g-DEE 3b with an electron-withdrawing group shows a red-shifted lmax 354 nm (Fig. 1).
Empirically, D – A-disubstituted g-DEE 5 appears as a more highly colored orange
solid in comparison to 3a,b and 4a,b which are all yellowish. The deeper color of 5 is
suggestive of a D – A intramolecular charge-transfer (ICT) in the ground state, and the
UV/VIS spectrum of 5 shows an absorption tail extending out beyond 400 nm, in
addition to a strong absorption at lmax 325 nm (Fig. 1). The electronic behavior of this
series of g-DEEs is markedly different from that observed in linearly conjugated
enynes, for which the ꢂpush-pullꢀ effect of donors and acceptors gives rise to a more
substantial red-shift in lmax values [5][18][19]. Obviously, cross-conjugation reduces
the extent of electronic communication dramatically.
In the UV/VIS spectra of ferrocenyl-substituted g-DEEs 6 – 8 (Fig. 2), a signature
broad, weak absorption band spanning from 380 – 550 nm is observed due to the
ferrocenyl group, and compounds 6 – 8 show virtually superimposable UV/VIS-
absorption profiles in this range (380 – 500 nm). In the higher-energy region, donor-

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Helvetica Chimica Acta – Vol. 90 (2007)
Scheme. Synthesis of D/A g-DEEs
Fig. 1. UV/VIS Spectra (CHCl3) of compounds 3b, 4b and 5
substituted 7 shows a more significant absorption at 330 nm than 6, similar to that of 4b.
Since ferrocenyl is a p-donor group, the reddish-brown compound 8 is a D – A analogue
to compound 5, and 8 shows a low-energy ICT absorption band at lmax 352 nm.
The p-framework of g-DEEs 9 – 11 is longer than that of compounds 3 – 5 as a result
of the pendent phenyl rings. As such, the UV/VIS-spectroscopic features of 9 – 11
(Fig. 3) are dominated by a strong absorption in the range of 380 nm for each

Helvetica Chimica Acta – Vol. 90 (2007)
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Fig. 2. UV/VIS Spectra (CHCl3) of compounds 6, 7, and 8
Fig. 3. UV/VIS Spectra (CHCl3) of compounds 9, 10 and 11
compound, as found for iso-PDAs 2 [10]. The absorption profile of D – A 11 does,
however, show a distinctive shoulder band that appears at 425 nm and indicates ICT
between the donor and acceptor moieties via the cross-conjugated p-linkage.

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2.3. X-Ray Single-Crystal Structure Analyses. The solid-state structures of com-
pounds 3b, 4b, 7, and 8 were established by single-crystal X-ray analysis, and the
ORTEP plots are drawn in Fig. 4. The crystal structure of 3b (Fig. 4,a) shows a
predominantly planar conjugated framework, with the benzene ring rotated only
slightly out of the plane of the enediyne segment by 10.44(10)8. The strongly electron-
withdrawing nitrophenyl group, in conjugation with the enediyne backbone, exerts
little influence on the observed bond lengths. The alkyne bond lengths of C(1)ÀC(2)
and C(7)ÀC(8) are comparable to the other derivatives of this study (Table 1), as well
as to other cross-conjugated enediynes [10][17][20 – 23]. The alkylidene bond angle
C(2)ÀC(3)ÀC(7) of 117.80(14)8 is, however, one of the largest observed to date for g-
DEE structures. Also of note is a rather substantial bending found for the bond angle
SiÀC(1)ÀC(2) of 171.88(14)8, which demonstrates the inherent flexibility that alkynyl
bonds often show in the solid state [24].
Fig. 4. ORTEP Drawings of a) 3b, b) 4b (molecule A), c) 7, and d) 8 (all at 20% probability level)
Table 1. Selected Bond Lengths and Angles for 3b, 4b, 7, and 8
3b
1.211(2)
1.196(2)
1.439(2)
1.362(2)
4ba)
4bb)
7
8
Bond length [Š]: C(1)ÀC(2)
C(7)ÀC(8)
1.200(3)
1.200(3)
1.196(2)
1.195(3)
1.432(3)
1.348(2)
1.201(4)
1.187(4)
1.438(4)
1.355(4)
1.193(4)
1.433(4)
1.335(4)
1.182(4)
1.434(4)
1.339(4)
C(8)ÀC(21)
C(3)ÀC(4)
Bond angle [8]:
SiÀC(1)ÀC(2) or
171.88(14) 177.6(3)
175.7(3)
178.16(18) 175.9(3)
C(10)ÀC(1)ÀC(2)
C(1)ÀC(2)ÀC(3) 176.82(17) 178.3(3)
C(2)ÀC(3)ÀC(7) 117.80(14) 114.6(3)
C(3)ÀC(7)ÀC(8) 177.52(19) 178.4(3)
C(7)ÀC(8)ÀC(21) 176.42(19) 178.4(3)
178.5(3)
116.5(3)
179.3(3)
177.6(3)
176.10(18) 178.6(3)
114.05(15) 116.6(2)
176.07(18) 178.2(3)
177.58(17) 175.4(3)
a) Crystallographic molecule A. b) Crystallographic molecule B.

Helvetica Chimica Acta – Vol. 90 (2007)
915
Two crystallographically independent molecules are found in the unit cell of 4b, and
the structure of molecule A is shown in Fig. 4,b. The cross-conjugated enediyne
skeletons of molecules A and B both adopt a similar planar geometry, whereas the
dihedral angle between the benzene-ring plane and the enediyne plane differ
considerably between the two structures. In molecule A, the benzene-ring plane
rotates from the enediyne plane by 71.56(11)8, while this value is only 10.5(2)8 in
molecule B. The twisting likely results simply from crystal-packing forces. The olefinic
C(3)ÀC(4) and alkyne bond lengths show no significant variation between the two
structures. The enediyne alkylidene angles C(2)ÀC(3)ÀC(7) (114.6(3)8 for A and
116.5(3)8 for B) are in line with other reported geminal enediynes and the theoretical
calculations (vide infra).
Ferrocenyl compound 7 (Fig. 4,c) shows a nearly orthogonal twisting between the
cyclopentadiene (Cp) ring and the enediyne plane in the solid-state structure with a
torsional angle of 87.77(7)8. Also, the benzene ring slightly rotates out of the enediyne
plane by 20.70(10)8. Bond lengths and angles are, however, unremarkable.
Crystallographic analysis of 8 (Fig. 4,d) shows a remarkably planar configuration
for the conjugated framework, and the Cp, enediyne, and benzene moieties are all
virtually coplanar. Specifically, the benzene ring is twisted from the enediyne plane by
only 1.56(19)8 and the Cp ring deviates from the enediyne plane by 14.70(16)8. Such a
coplanar arrangement supports the prospect of maximum electron delocalization by
means of p-orbital overlap. Also interesting is the packing of 8, which assumes a
noncentrosymmetric geometry of the monoclinic P21 (No. 4) space group, with the
major dipole component aligned approximately along the b-axis, as shown in Fig. 5. It
has been shown that this anisotropic feature favors maximal bulk second-order
nonlinear optical susceptibility (c(2)) in the solid state [25].
2.4. Third-Order Nonlinear Optical (NLO) Measurements. The oligomers of cross-
conjugated iso-poly(diacetylenes) have shown interesting third-order NLO properties
as a function of length [10]. To complement these results, an understanding of the
impact of D/A functionalization on their third-order NLO behavior is needed,
especially in comparison to related molecules such as DEEs and tetraethynylethenes
(TEEs) [3][7]. The third-order NLO response of various D/A-functionalized g-DEEs
was measured by the differential optical Kerr effect (DOKE) technique [26] at 800 nm
(far from resonance). Samples of g-DEEs 3 – 11 were prepared in THF solution with
concentrations of 0.10 – 0.35m. Table 2 summarizes the second molecular hyperpolar-
izabilities, g, and selected two-photon-absorption (TPA) cross-sections, s(2), in
comparison to the unsubstituted g-DEE derivative 14, TEEs 15 and 16, and DEEs
17 [3]2).
Several trends can be seen from this data. First, it is evident that donor/acceptor-
substituted g-DEEs 3 – 11 show an increased NLO response compared to unsubstituted
g-DEE 14. This is not unexpected, as it has been well established that an increase in g
can be achieved through increasing the level of static polarization within a molecule.
2
)
It is, unfortunately, difficult to compare the absolute g-values for molecules measured by different
methods (i.e., THG (third-harmonic generation) vs. DOKE). We, therefore, concentrate our
discussion on general trends in the observed values.

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Fig. 5. Crystal packing of compound 8
The sole anomaly to this trend is the electron-rich derivative 7, for which a relatively
low value of g ¼ 3 · 10À36 esu was obtained. While the origin of this low value is not
empirically obvious, theoretical analysis does potentially shed some light on this
behavior (vide infra). Second, the g-values of acceptor-substituted g-DEEs (see 3a and
9) are consistently higher than those of their donor-substituted g-DEEs analogues (see
4a and 10). This contrasts those observed for linearly conjugated TEEs 15a vs. 15b for
which the g-value of 15b is nearly three times that of 15a, and those found for linearly
conjugated DEEs, where bis-donor 17b (g ¼ 300 · 10À36 esu) shows a superior response
to bis-acceptor 17a (g ¼ 210 · 10À36 esu). Finally, D – A-type g-DEEs 5, 8, and 11 all
show greater g-values than either mono-D- or mono-A-type g-DEEs, demonstrating
that electronic interactions between D – A groups can affect the molecular second
hyperpolarizabilities through a ꢂpush-pullꢀ electronic structure. This contrasts the
behavior of g-TEEs 16a – c, where the D – A molecule 16c shows the lowest value of any
of the TEE derivatives. The difference in behavior between the TEE 16c vs. g-DEEs 5,
8, and 11 may derive from the fact that the TEE core itself is known to be a strong

Helvetica Chimica Acta – Vol. 90 (2007)
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Table 2. Molecular Second Hyperpolarizabilities (g) and Selected Two-Photon-Absorption Cross-
Sections (s(2)) for 3 – 17
Compound
g [10À36 esu]
s(2) [GM]
Type of D/A substitution
3a
3b
4a
5
6
7
8
9
10
23 Æ 2a)
16 Æ 2a)
14 Æ 2a)
28 Æ 3a)
15 Æ 12a)c)
3 Æ 8a)d)
35 Æ 9a)
29 Æ 2a)
15 Æ 1a)
37 Æ 3a)
9.2 Æ 0.4a)
95e)
acceptor– g-DEE
acceptor– g-DEE
donor– g-DEE
donor– g-DEE – acceptor
donor– g-DEE
donor– g-DEE – donor
donor– g-DEE – acceptor
acceptor– g-DEE
donor– g-DEE
donor– g-DEE – acceptor
not applicable
6 Æ 1b)
12 Æ 2b)
18 Æ 3b)
22 Æ 4b)
11
14
15a
15b
16a
16b
16c
17a
17b
17c
acceptor– TEE
donor– TEE
270e)
130e)
donor– g-TEE – donor
donor– g-TEE – donor
donor– g-TEE – acceptor
acceptor– DEE – acceptor
donor– DEE – donor
donor– DEE – acceptor
310e)
59e)
210e)
300e)
610d)
b
a) Referenced to THF (gTHF ¼ 5.2 · 10À37 esu), see [26]. ) Referenced to MPPBT (s(2) ¼ 380 GM), see
c
[27]. ) The sample displayed unexplained absorption and/or scattering during DOKE analysis resulting
in a larger level of uncertainty compared to other samples. d) The sample displayed incomplete solubility
at the concentration used for NLO measurements. Thus, the g-value represents a minimum value only
and contains a large level of uncertainty. e) Measured by THG at 1.9 mm, see [3].

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electron acceptor3), and this may moderate the D – A interaction of the pendent
substituents of 16c and result in a lower overall polarization. It can also be noted that
the largest g-value observed in the current study – that of D – A g-DEE 11 – is likely
higher than that of D – A g-DEEs 8 and 5 because 11 possesses a longer linearly-
conjugated p-delocalization, due to the pendent phenyl rings.
In addition to the measurement of g, the DOKE setup used in this study also allows
for the concurrent assessment of the two-photon-absorption (TPA) cross-section (s(2))
of the samples under study at 800 nm [30]. Several samples, i.e., 5 and 9 – 11, were
therefore examined, and, while modest, the TPA cross-section does show an increase in
s(2) from 6 to 12 GM upon changing from an acceptor (see 9) to a donor g-DEE (see
10), and a further increase in s (2) to 22 GM is found for the D – A g-DEE 11. The more
highly conjugated D – A g-DEE 11 also shows a superior s(2) to that of D – A g-DEE 5.
2.5. Theoretical Evaluation of Electron Delocalization along the Cross-Conjugated
g-DEE Backbone. Electronic communication has been shown experimentally as well as
theoretically to be present in cross-conjugated oligomers with g-DEEs as the repeat
unit [7 – 10]. Theoretically, the g-DEE structure represents a challenge to classical
resonance theory that predicts that electron delocalization should not be permitted
between the two cross-conjugated moieties [31]. Such discrepancies with our
experimental results are due to the omission of the weak orbital interactions of p-
and s-bonding and antibonding orbitals in simplified theoretical models. The use of ab
initio and density-functional-theory (DFT) calculations, however, can help to solve this
problem by taking into account these weak interactions. This is especially true using a
combination of DFTand natural-bond-orbital (NBO) analysis [32], which are reported
to be effective in assessing the electron-delocalization properties of linearly conjugated
and cross-conjugated geminal enediynes [33]. In general, the localized (bonding)
structure of g-DEEs can be expressed by three terms: the two sets of orthogonal p-
orbitals, denoted p and p (Fig. 6), as well as the in-plane s-orbitals, denoted s . As for
?
jj
jj
the electron delocalization, the corresponding antibonding orbitals, p*?, p*jj, and s*jj,
are thus regarded as the three most significant participants (acceptors). By applying the
NBO analysis, first proposed by Weinhold [32], it is possible to evaluate the
contribution of each set of orbitals to the total electron-delocalization energy.
Fig. 6. Schematic illustration of orbitals in g-DEEs. a) Out-of-plane (vertical) p-orbitals (denoted p ), b)
?
in-plane p-orbitals (denoted p ) and s-bonds (highlighted in bold, denoted s ).
jj
jj
DFT Calculations on g-DEE molecules by Lüthi and co-workers have indicated
that the stability of an unsubstituted g-DEE is primarily controlled by the vertical p-
3
)
For a theoretical analysis of the electron-deficient nature of the DEE backbone, see [28], and for
experimental studies, see [29].

Helvetica Chimica Acta – Vol. 90 (2007)
919
delocalization, while the s-delocalization (i.e., hyperconjugation) has a relatively
insignificant role [33]. For D/A-substituted g-DEEs, both p*- and s*-antibonding
orbitals are likely subject to substantial changes according to the substituent groups. To
further understand the D/A effects, the delocalization energy (Edel) of the g-DEE
framework was calculated as a semi-quantitative measure to assess the degree of
electron delocalization. For clarity, we herein ascribe the D/A substituents as the
external perturbations, and only the delocalization energies contributed by the
antibonding orbitals on the g-DEE structure were calculated by using the method
reported by Lüthi and co-workers [33]. A series of compounds 18a – e representing D-
and/or A-functionalized g-DEEs were chosen for the investigation. The molecular
structures of 18a – e were first optimized by using DFT methods at the B3LYP/6-31G*
level. Frontier-molecular-orbital energies and dipole moments are reported in Table 3.
NBO Analysis was subsequently performed based on the optimized molecular
structures at the same level. For detailed analysis, the total delocalization energy
(Edel) due to the enediyne backbone is broken down into three components: a) the in-
plane p*-contribution, Ep*jj, b) the s*-contribution, Es*jj, and c) the out-of-plane
(vertical) p*-contribution, Ep*?
.
Table 3. Frontier-Molecular-Orbital Energies and Dipole Moments for 18a – e, Calculated at the B3LYP/
6 – 31G* Level
HOMO [Hartree]
LUMO [Hartree]
Band gap [Hartree]
Dipole moment [D]
18a
18b
18c
18d
18e
À 0.238
À 0.192
À 0.182
À 0.240
À 0.199
À 0.100
À 0.044
À 0.042
À 0.109
À 0.095
0.138
0.148
0.140
0.131
0.104
5.06
3.29
3.61
5.16
7.68
Table 4 lists the different contributions to delocalization energies due to the three
aforementioned components, as well as the total delocalization energies (Edel
,
calculated by deletion of all antibonding and Rydberg orbitals) for the series of the
g-DEEs 18a – e calculated by NBO analysis at the B3LYP/6-31G* level. Overall, the in-
plane s*-orbitals contribute more significantly to the total delocalization energy than
do the in-plane and out-of-plane p*-orbitals. This suggests that hyperconjugative

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effects through s-bonds play a larger role in stabilizing the g-DEE structure than p-
electron interactions such as homoconjugation that have been suggested previously
[34]. The remarkably large delocalization energies for D – A-type 18e supply evidence
for enhanced D – A electronic interactions via the enediyne backbone, which is in
accordance with the conclusions of the UV/VIS analysis (vide supra). The Ep*jj values
for monosubstituted g-DEEs 18a and 18b are comparable to that of an unsubstituted g-
DEE reported by Lüthi and co-workers [33], while the Ep*? values are greater than that
of the unsubstituted g-DEE. For the disubstituted g-DEEs 18c – e, each delocalization
contributor as well as the total delocalization energy increases although the total
delocalization energy for D – D-type 18c shows only a slight increase relative to mono-
D-type 18b. Such a low total delocalization energy could disfavor charge separation in
the excited state, and the change of dipole moments (Dm) between the ground and
excited states could be small. Given that Dm is considered an important factor
governing the third-order NLO susceptibility, this may explain the observation that D –
D g-DEE gives the smallest second-order hyperpolarizability g (vide supra).
Table 4. Delocalization Energies Contributed by In- and Out-of-Plane Orbitals on D/A Substituted g-
DEEs 18a – e
Ep*jj [kcal/mol]
Es*jj [kcal/mol]
Ep*? [kcal/mol]
total Edela) [kcal/mol]
D/A type
18a
18b
18c
18d
18e
18fb)
15.537
10.652
20.973
21.189
21.185
10.49
133.031
136.691
141.828
141.037
141.454
159.29
62.886
55.754
87.656
79.880
84.368
48.43
1062.238
763.286
1251.422
1905.480
1552.758
not available
mono-A
mono-D
D – D
A – A
D – A
a) Edel is the total delocalization energy calculated by deleting all antibonding orbitals, including those of
the aryl substituent(s). b) Values from [33].
Nevertheless, to assess the influence of D/A groups on electron delocalization along
the enediyne backbone, it seems more meaningful to look at the delocalization-energy
(Edel) differences between each pair of D/A compounds as shown in Table 5. The
contribution of vertical p*-orbitals (p*?) is consistently larger than those of the in-
plane contributors (p*jj), indicating that these p*-bonds are more sensitive to the
influence of terminal groups.
Table 5. Energy Differences between D/A Substituted g-DEEs
DEp*jj [kcal/mol]
DEsjj [kcal/mol]
DEp*? [kcal/mol]
18d – 18a (AA – A)
18c – 18b (DD – D)
18e – 18b (DA – D)
18e – 18a (DA – A)
5.652
10.321
10.533
5.648
8.006
5.137
4.763
8.428
16.994
31.902
28.614
21.482
Another approach to evaluate the delocalization properties of g-DEEs is to assess
the bond-length-alternation (BLA) index, dR. In general, the BLA index for the cross-
conjugated enynes is defined by Eqn. 1, where the CDÀCT, CArÀCT, C¼C, and CꢀC

Helvetica Chimica Acta – Vol. 90 (2007)
921
refer to the average bond distances for each type of single or multiple bond,
respectively [8]. The dR values for each type of g-DEE (Table 6) obtained from DFT
calculations show that dR decreases monotonically from 18a to 18e and suggest that the
degree of delocalization in the enediyne backbone tends to increase as more D/A
chromophores are appended. Experimental results from the X-ray structural analysis
also show a consistent trend where dR values decrease as the degree of substitution is
increased, with the exception of compound 3d. Given the range of geometries found in
the solid state, however, the observed trend is also quite possibly fortuitous.
dR ¼ (CDÀCT þ CArÀCT)/2 À (C¼C þ CꢀC)/2
(1)
Table 6. BLA Indexes dR for g-DEEs
dR [Š]
theoreticala)
experimentalb)
18a
18b
18c
18d
18e
0.145
0.144
0.142
0.141
0.140
0.158 (from 3d)
0.173 (from 3b)
not available
0.161 (from 4c)
0.159 (from 4b)
a) Based on DFT calculations. b) Based on X-ray crystallographic analysis.
3. Conclusion. – A series of D/A functionalized g-DEEs was prepared by Pd-
catalyzed coupling reactions and their solid-state structural properties investigated by
single-crystal X-ray analysis. Electronic absorption spectroscopic analysis suggests an
ICT band in the spectra of D – A-functionalized g-DEEs 5, 8, and 11, consistent with
electronic communication through the geminal enediyne framework. Ultrafast third-
order NLO properties for this g-DEEs series were evaluated by the DOKE detection
technique in THF solutions. The donor– acceptor motifs for 5, 8, and 11 yield the
largest second hyperpolarizability (g) values of the series, whereas donor-substituted g-
DEEs appear to show the weakest response. A theoretical study using NBO analysis
shows that the contributions of p*- and s*-bonds to the electron delocalization along
the geminal enediyne backbone vary with the nature of substituents. The vertical p*-
orbitals are most susceptible to influences that arise from pendent functionalization.
Overall, the joint experimental and theoretical studies may serve as a useful guide for
the chemical functionalization of molecular materials in efforts to tune electronic and
nonlinear optical properties.
This research was supported by the University of Alberta, the Natural Sciences and Engineering
Research Council of Canada (NSERC), ASRA, CIPI, iCORE, and Petro-Canada (Young Innovator
Award to R. R. T.). We thank Dr. Hans Peter Lüthi and Peter Limacher for helpful discussions for the
NBO analysis.

922
Helvetica Chimica Acta – Vol. 90 (2007)
Experimental Part
General. All chemicals were purchased from Aldrich and used without further purification. THF was
dried over sodium/benzophenone and distilled under N2 prior to use. Column chromatography (CC):
silica gel 60 (230 – 400 mesh) from General Intermediates of Canada. Thin layer chromatography (TLC):
aluminum sheet coated with silica gel 60 F254 from Whatman; visualization by UV light or KMnO4 stain.
Melting point: Fisher– Johns or Gallenkamp apparatus; uncorrected. UV/VIS Spectra: Varian Cary 400
at r.t.; l in nm (e in l molÀ1 cmÀ1). IR Spectra: Nicolet Magna-IR 750 (neat) or Nic-Plan IR Microscope
(solids); in cmÀ1 1H- and 13C-NMR Spectra: Varian Gemini-300, -400, or -500 and Bruker AM-300
.
instruments; at r.t. in CDCl3; d in ppm with solvent peaks (d 7.24 for 1H and 77.0 for 13C) as reference, J in
Hz; for simplicity, the coupling constants of aromatic protons of 4-substituted phenyl groups are reported
as pseudo-first-order, even though they are second-order spin (AA’BB’) systems. EI-MS: Kratos MS-50
instrument in m/z; low-resolution data are provided in cases when Mþ is not the base peak; otherwise,
only high-resolution data is provided. Elemental analyses were performed by the Microanalytical
Service, Department of Chemistry, University of Alberta.
Cross-Coupling: General Procedure (G. P.). A mixture of the appropriate (trimethylsilyl)- or
(triisopropylsilyl)-protected alkyne and K2CO3 (ca. 0.2 equiv.) or Bu4NF (1.1 equiv.) in wet THF/MeOH
10 :1 (20 ml) or THF (20 ml), respectively, was stirred at r.t. until TLC analysis showed that desilylation
was complete (typically < 2 h). Et2O and sat. aq. NH4Cl soln. were added, the org. phase was washed
with sat. aq. NH4Cl soln. (2 Â 50 ml), dried, concentrated to ca. 3 ml, and added to a deoxygenated soln.
of the vinyl triflate or aryl halide coupling partner in THF or DMF (20 ml). [Pd(PPh3)4] or
[PdCl2(PPh3)2] (ca. 0.05 equiv.) and iPr2NH or Et2NH were sequentially added, the soln. was stirred for
5 min, CuI (ca. 0.15 equiv.) was added, and the soln. was stirred until TLC analysis no longer showed the
presence of the deprotected alkyne starting material. Et2O and H2O were added, the org. phase was
separated, washed with sat. aq. NH4Cl (2 Â 50 ml), dried, and concentrated. Flash CC gave the desired
product.
4-Methyl-3-[(4-nitrophenyl)ethynyl]-1-(trimethylsilyl)pent-3-en-1-yne (3a). According to the G. P.,
with 1-nitro-4-[(trimethylsilyl)ethynyl]benzene (100 mg, 0.502 mmol) and K2CO3 (60 mg, 0.4 mmol) in
wet THF (5 ml) and MeOH (25 ml), then with vinyl triflate 12a [16] (150 mg, 0.500 mmol),
deoxygenated THF (30 ml), iPr2NH (5 ml), [Pd(PPh3)4] (18 mg, 0.016 mmol), and CuI (9 mg,
0.047 mmol) for 1 h. CC (silica gel, hexane/Et2O 3 :1) afforded 3a (144 mg, 97%). Bright yellow solid.
M.p. 66 – 678. Rf 0.72 (hexane/Et2O 3 :1). IR (microscope): 3079, 2957, 2204, 2150, 1511, 1341. 1H-NMR
(400 MHz, CDCl3): 8.15 (d, J ¼ 9.2, 2 H); 7.56 (d, J ¼ 8.8, 2 H); 2.09 (s, 6 H); 0.21 (s, 9 H). 13C-NMR
(100 MHz, APT, CDCl3): 158.1; 146.8; 132.0; 130.4; 123.5; 101.2; 100.7; 97.0; 91.6; 89.6; 23.0; 22.9; À 0.06.
EI-MS: 297.1 (94, Mþ), 282.1 (100, [M À Me]þ). HR-MS: 297.1191 (C17H19NO2Siþ; calc. 297.1185).
4-Methyl-3-[(4-nitrophenyl)ethynyl]-1-(triisopropylsilyl)pent-3-en-1-yne (3b). According to the
G. P., with 13a (256 mg, 0.770 mmol) [17] and K2CO3 (58 mg, 0.42 mmol) in wet THF (5 ml) and
MeOH (25 ml), then with 1-iodo-4-nitrobenzene (187 mg, 0.751 mmol), deoxygenated Et3N (50 ml),
[PdCl2(PPh3)2] (21 mg, 0.03 mmol), and CuI (6 mg, 0.030 mmol) for 19 h. CC (silica gel, hexane/CH2Cl2
3 :1) afforded 3b (240 mg, 82%). Bright yellow solid. M.p. 70 – 728. Rf 0.32 (hexane/CH2Cl2 3 :1). UV/
VIS (CHCl3): 268 (16200), 351 (15600). IR (CH2Cl2, cast): 2942, 2865, 2206, 2148, 1592, 1520, 1343.
1H-NMR (300 MHz, CDCl3): 8.16 (d, J ¼ 8.8, 2 H); 7.54 (d, J ¼ 8.8, 2 H); 2.11 (s, 3 H); 2.10 (s, 3 H); 1.10
(s, 21 H). 13C-NMR (75.5 MHz, CDCl3): 157.2; 146.9; 132.0; 130.6; 123.6; 102.8; 101.7; 93.6; 92.1; 89.5;
i
23.1; 23.0; 18.7; 11.4. EI-MS: 381.2 (27, Mþ), 338.2 (100, [M À Pr]þ). HR-MS: 381.2115 (C23H31NO2Siþ;
calc. 381.2124). Anal. calc. for C23H31NO2Si: C 72.39, H 8.19, N 3.67; found: C 72.20, H 8.20, N 3.63.
3-{[4-(Dimethylamino)phenyl]ethynyl}-4-methyl-1-(trimethylsilyl)pent-3-en-1-yne (¼ N,N-Dimeth-
yl-4-{4-methyl-3-[(trimethylsilyl)ethynyl]pent-3-en-1-ynyl}benzenamine; 4a). According to the G. P.,
with 4-ethynyl-N,N-dimethylbenzenamine (66 mg, 0.46 mmol), vinyl triflate 12a [16] (215 mg,
0.683 mmol), deoxygenated THF (10 ml), Et3N (10 ml), [PdCl2(PPh3)2] (16 mg, 0.023 mmol), and CuI
(9 mg, 0.047 mmol) overnight. CC (silica gel, hexane/CH2Cl2 4 :1) afforded 4a (72 mg, 53%). Colorless
oil. Rf 0.62 (hexanes/CH2Cl2 1:1). UV/VIS (CH3Cl): 266 (15100), 294 (sh, 14100), 324 (19200). IR
(KBr): 2962, 2925, 2902, 2849, 2198, 2144, 1609, 1523, 1446. 1H-NMR (500 MHz, CDCl3): 7.34 (d, J ¼ 8.5,
2 H); 6.62 (d, J ¼ 8.5, 2 H); 2.97 (s, 6 H); 2.08 (s, 3 H); 2.07 (s, 3 H); 0.22 (s, 9 H). 13C-NMR (125 MHz,

Helvetica Chimica Acta – Vol. 90 (2007)
923
CDCl3): 153.8; 150.0; 132.5; 111.8; 110.5; 102.2; 101.9; 95.6; 92.4; 84.0; 40.2; 22.72; 22.68; 0.09. HR-EI-
MS: 295.1753 (C19H25NSiþ, Mþ; calc. 295.1756).
3-{[4-(Dimethylamino)phenyl]ethynyl}-4-methyl-1-(triisopropylsilyl)pent-3-en-1-yne (¼ N,N-Di-
methyl-4-{4-methyl-3-[(triisopropylsilyl)ethynyl]pent-3-en-1-ynyl}benzenamine; 4b). According to the
G. P., with 13a (325 mg, 0.979 mmol) [17] and K2CO3 (50 mg, 0.36 mmol) in wet THF (5 ml) and MeOH
(25 ml), then with 4-iodo-N,N-dimethylbenzenamine (250 mg, 1.01 mmol), deoxygenated Et3N (50 ml),
[PdCl2(PPh3)2] (59 mg, 0.084 mmol), and CuI (31 mg, 0.16 mmol) for 20 h. CC (silica gel H, hexane/
CH2Cl2 3 :1) afforded 4b (229 mg, 63%). Pale yellow solid. M.p. 45 – 468. Rf 0.31 (hexane/CH2Cl2 2 :1).
UV/VIS (CHCl3): 268 (16500), 302 (sh, 15000), 325 (21000). IR (neat): 2942, 2865, 2202, 2150, 1607, 1521,
1
1366. H-NMR (300 MHz, CDCl3): 7.31 (d, J ¼ 9.0, 2 H); 6.61 (d, J ¼ 9.0, 2 H); 2.95 (s, 6 H); 2.063 (s,
3 H); 2.059 (s, 3 H); 1.09 (s, 21 H). 13C-NMR (75.5 MHz, CDCl3): 152.8; 150.0; 132.5; 111.9; 110.7; 104.2;
102.5; 92.3; 92.0; 84.5; 40.3; 22.8; 22.7; 18.8; 11.4. EI-MS: 379 (100, Mþ). HR-MS: 379.2700 (C25H37NSiþ;
calc. 379.2695).
3-{[4-(Dimethylamino)phenyl]ethynyl}-4-methyl-1-(4-nitrophenyl)pent-3-en-1-yne (¼ N,N-Dimeth-
yl-4-{4-methyl-3-[(4-nitrophenyl)ethynyl]pent-3-en-1-ynyl}benzenamine; 5). According to the G. P., with
4b (70 mg, 0.18 mmol) and 1m Bu4NF in THF (0.7 ml) in wet THF (20 ml), then with 1-iodo-4-
nitrobenzene (48 mg, 0.19 mmol), deoxygenated Et3N (20 ml), THF (20 ml), [PdCl2(PPh3)2] (5 mg,
0.007 mmol), and CuI (3 mg, 0.02 mmol) for 12 h. CC (silica gel H, hexane/CH2Cl2 1:1) afforded 5
(44 mg, 70%). Orange solid. M.p. 159 – 1628. Rf 0.45 (hexane/AcOEt 3 :1). UV/VIS (CHCl3): 302 (sh,
26300), 325 (34000). IR (CH2Cl2, cast): 2903, 2198, 1607, 1518, 1341. 1H-NMR (300 MHz, CDCl3): 8.16
(d, J ¼ 9.0, 2 H); 7.59 (d, J ¼ 9.0, 2 H); 7.35 (d, J ¼ 8.9, 2 H); 6.63 (d, J ¼ 8.9, 2 H); 2.97 (s, 6 H); 2.143 (s,
3 H); 2.137 (s, 3 H). 13C-NMR (75.5 MHz, CDCl3): 154.9; 150.2; 146.8; 132.6; 132.1; 130.7; 123.6; 111.9;
110.0; 101.5; 93.1; 92.6; 89.2; 83.6; 40.3; 23.03; 23.01. HR-EI-MS: 344.1522 (C22H20N2O2, Mþ; calc.
344.1525).
4-Methyl-3-(ferrocenylethynyl)-1-(trimethylsilyl)pent-3-en-1-yne (¼1-{4-Methyl-3-[(trimethylsilyl)-
ethynyl]pent-3-en-1-ynyl}ferrocene; 6). According to the G. P., with 1-ethynylferrocene (135 mg,
0.643 mmol), triflate 12a (193 mg, 0.643 mmol), degassed THF (20 ml), [PdCl2(PPh3)2] (22 mg,
0.031 mmol), iPr2NH (2 ml), and CuI (11 mg, 0.058 mmol) for 1.5 h. CC (hexane/CH2Cl2 2 :1) afforded
6 (178 mg, 77%). Yellow solid. Rf 0.4 (hexane/CH2Cl2 2 :1). M.p. 66 – 678. UV/VIS (CHCl3): 261 (17400),
299 (8400), 341 (1200), ca. 447 (500). IR (microscope): 3095, 2960, 2903, 2207, 2146, 1247. 1H-NMR
(300 MHz, CDCl3): 4.45 (br. s, 2 H); 4.22 (br. s, 7 H); 2.04 (s, 3 H); 2.03 (s, 3 H); 0.20 (s, 9 H). 13C-NMR
(75.5 MHz, CDCl3, APT): 154.2; 102.2; 102.0; 95.8; 90.2; 82.4; 71.4; 69.9; 68.7; 65.5; 22.8; 0.1 (one
coincident signal not observed). HR-EI-MS: 360.0994 (C21H24FeSiþ, Mþ; calc. 360.0997).
3-{[4-(Dimethylamino)phenyl]ethynyl}-1-ferrocenyl-4-methylpent-3-en-1-yne (¼1-{3-{[4-(Dimethyl-
amino)phenyl]ethynyl}-4-methylpent-3-en-1-ynyl}ferrocene; 7). According to the G. P., with 6 (102 mg,
0.28 mmol) and K2CO3 (10 mg, 0.14 mmol) in wet THF (5 ml) and MeOH (25 ml), then with 4-iodo-N,N-
dimethylbenzenamine (74 mg, 0.30 mmol), degassed THF (20 ml), [PdCl2(PPh3)2] (10 mg, 0.014 mmol),
iPr2NH (2 ml), and CuI (5 mg, 0.03 mmol) for 1.5 h. CC (hexane/Et2O 5 :1) afforded 7 (60 mg, 52%) as a
yellow solid. Rf 0.4 (hexane/Et2O 5 :1). M.p. 174 – 1758. IR (CHCl3, cast): 3084, 2925, 2197, 1612, 1526,
1441, 1369. UV/VIS (CHCl3): 305 (25400), 330 (22600), ca. 445 (500). 1H-NMR (300 MHz, CDCl3): 7.34
(d, J ¼ 8.4, 2 H); 6.62 (d, J ¼ 8.4, 2 H); 4.45 (t, J ¼ 1.8, 2 H); 4.21 (s, 5 H); 4.18 (t, J ¼ 1.8, 2 H); 2.96 (s,
6 H); 2.08 (s, 6 H). 13C-NMR (75.5 MHz, CDCl3, APT): 151.1; 150.0; 132.5; 111.9; 110.6; 102.2; 92.0;
89.6; 84.7; 83.1; 71.4; 69.9; 68.6; 65.8; 40.3; 22.8; 22.7. HR-EI-MS: 407.1350 (C26H25FeNþ, Mþ; calc.
407.1336).
1-Ferrocenyl-4-methyl-3-[(4-nitrophenyl)ethynyl]pent-3-en-1-yne (¼1-{4-Methyl-3-[(4-nitrophenyl]-
ethynyl]pent-3-en-1-ynyl}ferrocene; 8). According to the G. P., with 6 (52 mg, 0.14 mmol) and K2CO3
(6 mg, 0.04 mmol) in wet THF (5 ml) and MeOH (25 ml), then with 1-iodo-4-nitrobenzene (54 mg,
0.22 mmol), degassed THF (20 ml), [PdCl2(PPh3)2] (10 mg, 0.014 mmol), iPr2NH (2 ml), and CuI (5 mg,
0.03 mmol) for 1.5 h. CC (hexane/Et2O 5 :1) afforded 8 (47 mg, 80%). Brown solid. Rf 0.5 (hexane/Et2O
5 :1). M.p. 113 – 1148. UV/VIS (CHCl3): 302 (19200), 352 (14700). IR (CH2Cl2, cast): 3102, 2905, 2845,
2202, 1591, 1516, 1342. 1H-NMR (300 MHz, CDCl3): 8.17 (d, J ¼ 9.0, 2 H); 7.58 (d, J ¼ 9.0, 2 H); 4.49 (s,
2 H); 4.24 (s, 7 H); 2.132 (s, 3 H); 2.127 (s, 3 H). 13C-NMR (75.5 MHz, CDCl3, APT): 155.5; 146.9; 132.0;

924
Helvetica Chimica Acta – Vol. 90 (2007)
130.6; 123.6; 101.4; 92.3; 90.9; 89.2; 81.8; 71.8; 70.4; 69.4; 65.7; 23.1; 23.0. HR-EI-MS: 409.0768
(C24H19FeNOþ2 , Mþ; calc. 409.0765).
3-(Diphenylethylidene)-1-(4-nitrophenyl)-5-(triisopropylsilyl)penta-1,4-diyne (9). According to the
G. P., with 13b [10] (41 mg, 0.090 mmol) and K2CO3 (6 mg, 0.04 mmol) in wet THF (0.5 ml) and MeOH
(3 ml), then with 1-iodo-4-nitrobenzene (25 mg, 0.10 mmol), deoxygenated Et3N (5 ml), [PdCl2(PPh3)2]
(4 mg, 0.006 mmol), and CuI (2 mg, 0.01 mmol) for 24 h. CC (silica gel H, hexane/CH2Cl2 3 :1) afforded 9
(33 mg, 73%). Yellow solid. M.p. 62 – 658. Rf 0.32 (hexane/CH2Cl2 2 :1). UV/VIS (CHCl3): 304 (18200),
382 (20400). IR (CHCl3, cast): 2941, 2864, 2194, 2147, 1593, 1519, 1342. 1H-NMR (300 MHz, CDCl3): 8.11
(d, J ¼ 8.8, 2 H); 7.55 – 7.29 (m, 12 H); 1.03 (s, 21 H). 13C-NMR (75.5 MHz, CDCl3): 158.2; 146.9; 140.3;
139.8; 132.0; 130.4 (2Â); 128.92; 128.89; 127.84; 127.82; 123.6; 104.3; 101.8; 95.9; 94.8; 89.6; 18.7; 11.4
(one coincident signal not observed). HR-EI-MS: 505.2444 (C33H35NO2Siþ, Mþ; calc. 505.2437).
1-[4-(Dimethylamino)phenyl]-3-(diphenylethylidene)-1-(triethylsilyl)penta-1,4-diyne (¼4-[3-(Di-
phenylethylidene)-5-(triethylsilyl)penta-1,4-diynyl]-N,N-dimethylbenzenamine; 10). According to the
G. P., with N,N-dimethyl-4-[(trimethylsilyl)ethynyl]benzenamine (100 mg, 0.460 mmol) and K2CO3
(20 mg, 0.15 mmol) in wet THF (10 ml) and MeOH (10 ml), then with 12b (180 mg, 0.386 mmol),
deoxygenated DMF (125 ml), [Pd(PPh3)4] (24 mg, 0.021 mmol), Et2NH (3 ml), and CuI (13 mg,
0.068 mmol) for 17 h. CC (silica gel, AcOEt/hexanes 1:6 followed by CH2Cl2/hexanes 1:3) afforded 10
(80 mg, 45%). Yellow semisolid. Rf 0.3 (hexane/CH2Cl2 2 :1). UV/VIS (CHCl3): 292 (20900), 385
(19100). IR (CH2Cl2, cast): 3052, 2954, 2924, 2187, 2147, 1608, 1519, 1362. 1H-NMR (300 MHz, CDCl3):
7.52 – 7.43 (m, 4 H); 7.34 – 7.24 (m, 6 H); 7.16 (d, J ¼ 9.0, 2 H); 6.58 (d, J ¼ 9.0, 2 H); 2.95 (s, 6 H); 0.92 (t,
J ¼ 8.0, 9 H); 0.55 (q, J ¼ 8.0, 6 H). 13C-NMR (75.5 MHz, CDCl3): 154.4; 150.0; 140.7; 140.5; 132.6;
130.43; 130.39; 128.1; 128.0; 127.51; 127.48; 111.7; 110.2; 104.9; 102.8; 95.0; 93.4; 87.2; 40.2; 7.4; 4.3. HR-
EI-MS: 461.2545 (C32H35NSiþ, Mþ; calc. 461.2539).
1-[4-(Dimethylamino)phenyl]-3-(diphenylethylidene)-1-(4-nitrophenyl)penta-1,4-diyne (¼4-[3-(Di-
phenylethylidene)-5-(4-nitrophenyl)penta-1,4-diynyl]-N,N-dimethylbenzenamine; 11). According to the
G. P., with 10 (65 mg, 0.14 mmol) and 1m Bu4NF in THF (0.3 ml) in wet THF (20 ml), then with 1-iodo-4-
nitrobenzene (42 mg, 0.17 mmol), deoxygenated Et3N (20 ml), THF (20 ml), [PdCl2(PPh3)2] (6 mg,
0.009 mmol), and CuI (4 mg, 0.02 mmol) for 17 h. CC (silica gel H, hexane/CH2Cl2 3 :1) afforded 11
(50 mg, 76%). Red solid. M.p. 108 – 1108. Rf 0.56 (hexane/CH2Cl2 1:2). UV/VIS (CHCl3): 272 (sh,
21000), 291 (24300), 367 (31300). IR (CH2Cl2, cast): 2925, 2185, 1607, 1516, 1340. 1H-NMR (300 MHz,
CDCl3): 8.16 (d, J ¼ 9.0, 2 H); 7.63 – 7.58 (m, 2 H); 7.56 – 7.50 (m, 2 H); 7.48 – 7.36 (m, 8 H); 7.27 (d, J ¼ 8.8,
2 H); 6.67 (d, J ¼ 8.8, 2 H); 3.02 (s, 6 H). 13C-NMR (75.5 MHz, CDCl3, APT): 156.0; 150.2; 146.8; 140.7;
140.2; 132.8; 132.1; 130.6 (2Â); 130.3; 128.7; 128.6; 127.8; 127.7; 123.5; 111.9; 102.0; 94.9; 94.1; 89.4; 86.3;
40.2 (one coincident signal not observed). HR-EI-MS: 468.1833 (C32H24N2Oþ2 , Mþ; calc. 468.1838).
X-Ray Single-Crystal Structure Analyses. Single crystals of 3b, 4b, 7, and 8 suitable for X-ray-analysis
were obtained at À 48 from MeOH/CH2Cl2. Crystal diffraction data were collected with a Bruker-
SMART-1000-CCD area detector used in combination with a P4 diffractometer/rotating-anode
generator (3b and 4b) or with a Platform diffractometer/sealed-tube generator (7 and 8). Structures
were solved with the program SHELXS-86 [35] (3b, 4b, and 8) or the DIRDIF-96 [36] program system (7).
Refinement was completed with the program SHELXL-934) [37]. CCDC-611083 (3b), CCDC-611086
(4b), CCDC-611085 (7), and CCDC-611084 (8) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif from the
Cambridge Crystallographic Data Centre. Details of the structure determinations are given in Table 7.
DOKE Experiments. The molecular second hyperpolarizabilities g were determined by a differential
optical Kerr effect (DOKE) experimental setup as reported elsewhere [26]. A multi-pass Ti/sapphire
laser amplifier was used, with output wavelength 800 nm, pulse width 90 fs, and a repetition rate of ca.
1 kHz. The laser beam was split into pump and probe beams, at a greater than 10 :1 power ratio. When
Refinement on Fo2 for all reflections (all of these having Fo2 ꢁÀ 3s(Fo2)). Weighted R-factors wR2
and all goodnesses of fit S are based on Fo2; conventional R-factors R1 are based on Fo, with Fo set to
zero for negative Fo2. The observed criterion of Fo2 > 2s(Fo2) is used only for calculating R1, and is
not relevant to the choice of reflections for refinement. R-Factors based on Fo2 are statistically about
twice as large as those based on Fo, and R-factors based on all data will be even larger.
4
)

Helvetica Chimica Acta – Vol. 90 (2007)
Table 7. Crystallographic Experimental Details
925
4b
3b
7
8
Formula
Mr
C25H37NSi
379.65
C23H31NO2Si
381.58
C26H25FeN
407.32
C24H19FeNO2
409.25
Crystal size [mm]
T [8]
Crystal system
Space group
a [Š]
b [Š]
c [Š]
a [8]
b [8]
0.42 Â 0.22 Â 0.18 0.45 Â 0.22 Â 0.18 0.59 Â 0.38 Â 0.09 0.40 Â 0.19 Â 0.04
22
À 80
À 80
À 80
triclinic
P1 (No. 2)
triclinic
P1 (No. 2)
monoclinic
P21/c (No. 14)
19.3515 (17)
10.0900 (9)
10.4497 (9)
monoclinic
P21 (No. 4)
12.4819 (9)
6.0734 (5)
12.6720 (10)
¯
¯
8.8003 (6)
13.6654 (10)
20.8790 (14)
90.784 (1)
94.888 (1)
93.144 (1)
2497.6 (3)
4
7.7041 (5)
11.2582 (8)
13.1958 (9)
84.7476 (12)
82.7816 (16)
77.0706 (12)
1104.29 (13)
2
90.5693 (18)
94.7813 (14)
g [8]
V [Š3]
2040.3 (3)
4
957.29 (13)
2
Z
1calc. [g/cm3]
1.010
0.103
51.44
1.148
0.123
52.76
4474
1.326
0.750
52.82
4182
1.420
0.807
52.78
3883
m [mmÀ1
]
2q limit [0]
Independent reflections 9449
Observed reflections
3918
3415
3666
3526
(Fo2 ꢁ 2s(Fo2))
Goodness-of-fit (S)
(Fo2 ꢁÀ 3s( Fo2))
0.840
1.034
1.079
0.997
Final R1 (Fo2 ꢁ 2s(Fo2))
wR2 [Fo2 ꢁÀ 3s(Fo2)]
0.0543
0.1477
0.146, À 0.181
0.0460
0.1272
0.308, À 0.271
0.0321
0.0986
0.313, À 0.286
0.0330
0.0769
0.433, À 0.243
Largest diff. peak and
À3
hole [e Š
]
the two pulses simultaneously arrive at the sample, the maximum rotation in polarization due to the ultra-
I
fast electronic nonlinearity is given by Eqn. 2 where f and n2 are complex quantities that can be
I
I
I
expressed as f ¼ f’ þ if’’ and n2 ¼ n2 ’ þ in2 ’’. In theory, these quantities are related to third-order
nonlinear optical susceptibility c(3) and g. As established in previous work [26], the relative sample
second hyperpolarizability gs can be calculated by Eqn. 3, where the subscripts S and R correspond to the
sample and reference, respectively; L4 is the Lorentz local-field factor, and Nc is the molecular
concentration in cmÀ3. All samples were prepared as solns. in THF and then filtered into 1.0-mm quartz
cuvettes through Nylon syringe filters for measurement. By altering the polarization of the pump pulse,
DOKE detection can be used to measure nonlinear absorption (two-photon-absorption) dynamics. Two-
photon-absorption (TPA) cross-sections were obtained by iterative scans of samples of interest and a
3.0 mm standard soln. of MPPBT in DMSO [27]. The ultrafast TPA cross-section of the sample is then
calibrated to that of the reference sample (s(2)
¼ 380 GM; l ¼ 800 nm) according to [30]:
MPPBT
2pd
l
ꢀ ¼
nI2Ipump
(2)
(3)
(4)
ꢀ
ꢁ
Re cðR3Þ
ꢂ
ꢀS0
ꢃ
hgSi ¼
À 1
L4Nc
ꢀR0
ꢀ0S0
ꢀ0R0 Nc
N
cR
sðS2Þ
¼
sðR2Þ
S

926
Helvetica Chimica Acta – Vol. 90 (2007)
DFT and NBO Analyses. The full geometry optimizations of 18a – e were performed by using the
density-functional-theory (DFT) approach, in which Becke three-parameters Lee-Yang-Parr (B3LYP)
hybrid functional was employed together with the 6-31G* basis set [38]. The calculated geometry
parameters are in reasonable agreement with those determined in analogous crystal structures. All
optimized structures were confirmed to be pure minima by vibrational analysis calculated at the same
level. Delocalization energies were calculated by NBO analysis, in which three sets of antibonding
orbitals on the g-DEE skeleton (see Fig. 6), namely, out-of-plane p*-orbitals, in-plane p*-orbitals, and in-
plane s*-orbitals, were deleted separately. The energy increase after deletion of each set of antibonding
orbitals was thus deemed as the delocalization energy contributed by those orbitals. All the calculations
were performed with the Gaussian 03 package [39]. The NBO analysis was done by the program NBO
3.1 [40] as implemented in Gaussian 03.
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Received January 4, 2007