Star-shaped push-pull compounds having 1,3,5-triazine cores

Article abstract of DOI:10.1002/ejoc.200600066

The 1,3,5-triazines 9-11 having OPV chains in the 2-, 4- and 6-position represent star-shaped compounds which exhibit strong push-pull effects. Their long-wavelength absorption S₀ → S₁ is characterized by an intramolecular charge tra

Full text of DOI:10.1002/ejoc.200600066

FULL PAPER
DOI: 10.1002/ejoc.200600066
Star-Shaped Push-Pull Compounds Having 1,3,5-Triazine Cores
Herbert Meier,*[a] Elena Karpuk,[a] and Hans Christof Holst[a]
Keywords: Absorption / Conjugation / Protonation / Push-pull effects
The 1,3,5-triazines 9–11 having OPV chains in the 2-, 4- and
6-position represent star-shaped compounds which exhibit
strong push-pull effects. Their long-wavelength absorption
S0 Ǟ S1 is characterized by an intramolecular charge transfer
(ICT) from the dialkylamino group as electron donor D via
the conjugated chain to the 1,3,5-triazine core as electron ac-
ceptor A. Protonation of the dialkylamino group removes the
donor capability, whereas protonation of the 1,3,5-triazine
ring enhances the acceptor strength. Thus, the prevailing
through a minimum for the second generation 10 in the elec-
troneutral series 9–11; on the contrary they pass through a
maximum for 10b in the protonated series 9a, 10b, 11b. These
unexpected results are explained by the superposition of dif-
ferent effects, namely the extension of the conjugation from
9 to 11, the decreasing ICT for growing distances of D and
A, the aggregation, and the positions of the protonation in
the presence of increasing amounts of CF3COOH.
protonation site has a crucial influence on the push-pull ef- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
fect and the ICT. The transition energies ∆E (S0 Ǟ S1) pass
Germany, 2006)
minal dialkylamino groups. This structure concept conveys
the compounds an octupolar character with a strong push-
pull effect in each of the three arms. Such systems promise
outstanding properties in nonlinear optics (NLO), two-pho-
ton absorption (TPA) and two-photon excited fluorescence
(TPEF).[11]
Introduction
Conjugated oligomers with donor–acceptor substitution
attract a great deal of attention because they represent
interesting materials for various applications in materials
science.[1–5] The push-pull effect can be realized in conju-
gated linear chains by terminal substitution with a donor
group D and an acceptor group A. A further structure con-
cept is based on star-shaped compounds whose arms bear
(alternatingly) donor and acceptor functionalities.[1,2] Ben-
zene ring systems with three donor groups D and three ac-
ceptor groups A, which are linked directly or through a
conjugated spacer to the core, are typical examples.[6,7] The
acceptor functions can be discerned by nitrogen atoms as
well, when a 1,3,5-triazine core is used (Scheme 1).
Results and Discussion
The synthetic strategy for the preparation of 1,3,5-tri-
azines with three OPV chains in 2-, 4- and 6-position is
based on threefold condensation reactions of 2,4,6-
trimethyl-1,3,5-triazine and corresponding aldehydes.
Vilsmeier formylation of N,N-dihexylaniline (1) gives 4-(di-
hexylamino)benzaldehyde (2) in high yields (Scheme 2).
Subsequent Wittig–Horner reaction of 2 and diethyl 4-bro-
mobenzylphosphonate (3) affords the (E)-stilbene 4. The
bromo substituent in 4 can be replaced by a formyl group
by applying a Bouveault process with n-BuLi/DMF. Thus,
the aldehyde 5 is obtained from 1 in three steps with an
overall yield of 60%. The phosphonate 6 serves then for the
extension of the conjugated chain. Wittig–Horner reaction
of 5 and 6 yields directly the aldehyde 7, because the pro-
tecting 1,3-dioxane group in 6 is cleaved in the work-up of
the Wittig–Horner process. Diethyl 4-[1,3-dioxan-2-yl]ben-
zyl phosphonate (6) is a very useful reagent to extend the
conjugated chain of OPV aldehydes. The reaction of 2 and
6 leads in a one-step process to 5; however, the overall yield
for 1Ǟ2Ǟ5 is somewhat lower than 60%. The (E) selectiv-
ity of the Horner variant is generally high in the stilbenoid
series so that (Z) configurations can not be detected in the
Scheme 1. Push-pull-substituted benzene systems with D3h sym-
metry, corresponding donor-substituted 1,3,5-triazines and their
tautomeric 1,3,5-triazinium ions showing a fast automerization.
Some time ago we reported on the preparation and the
properties of 2,4,6-tristyryl-1,3,5-triazines and their higher
homologues having oligo(1,4-phenylenevinylene) [OPV]
chains.[8–10] We are studying now star-shaped systems with
a 1,3,5-triazine core and three OPV chains which bear ter-
[a] Institut für Organische Chemie der Johannes Gutenberg Uni-
versität,
Duesbergweg 10–14, 55099 Mainz, Germany
Fax: +49-6131-3925396
E-mail: hmeier@mail.uni-mainz.de
1
solid products 5 and 7. The detection limit in H and 13C
NMR spectroscopy amounts to about 3%.
Eur. J. Org. Chem. 2006, 2609–2617
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2609

H. Meier, E. Karpuk, H. C. Holst
FULL PAPER
Table 1. UV/Vis absorption of the compounds 9–11 measured in
diluted solutions (c Յ 10–5 ) in CH2Cl2.
Compound λmax [nm]
λ0.1 [nm][a]
εmax [cm2 mmol–1]
9
10
11
442
457
442
488
530
522
131870
435385
138990[b]
[a] Wavelength, where ε = 1/10 εmax. [b] Very broad band.
The charge-transfer character of the band becomes evi-
dent by the absorption in the visible region. 2,4,6-Tris[(E)-
styryl]-1,3,5-triazine has a λmax value in the UV at
327 nm.[8] However, the intramolecular charge transfer
(ICT) depends strongly on the distance of donor D and
acceptor A. The transition energy ∆E (S0 Ǟ S1) of push-
pull oligomers is lowered by the extension of the conjuga-
tion and by the ICT. Both effects are superimposed and can
lead to four different functions λmax(n), where n represents
the number of repeat units in a conjugated oligomer
series:[1,5]
Scheme 2. Preparation of the aldehydes 2, 5 and 7.
a) ∆E (n + 1) Ͻ ∆E (n) monotonous bathochromic shift
b) ∆E (n + 1) Ͼ ∆E (n) monotonous hypsochromic shift
c) ∆E (n + 1) Ϸ ∆E (n) borderline case between a) and b)
d) ∆E goes through a minimum for a certain nЈ.
The fifth thinkable case, in which ∆E passes a maximum
for a special nЈ, is still unknown.
The present series 9–11 belongs to category d). The de-
crease of the ICT with increasing n provokes a hypso-
chromic effect, which can not be compensated for n = 2 by
the bathochromic effect caused by the extension of the con-
jugation (growing values n). This result is in contrast to the
analogous star-shaped oligomers with a 1,3,5-triazine core
and three OPV arms with terminal alkoxy groups which
belong to category a).[8] Alkoxy groups are weaker electron
donors than dialkylamino groups; therefore the extension
of the chromophores (increasing numbers n) can overcom-
pensate the decrease of the ICT.
The three “homologous” aldehydes 2, 5 and 7 are then
condensed in an alkaline medium with 2,4,6-tri-
methyl-1,3,5-triazine (8). The yield is high for the smallest
system 9, moderate for 10 with longer side chains, and poor
for 11, which has the longest side chains. 1H NMR reaction
spectra show that the first two condensation processes work
reasonably well; however, the third process proved to be dif-
ficult – in particular for the longer aldehydes 5 and 7. The
hexyl chains on the amino groups guarantee a good solubil-
ity of the target compounds[12] (see Scheme 3).
The absorption of 9–11 depends unusually strong on the
concentration. Figure 1 demonstrates this effect for the
range between c1 = 1.29·10–5 and c5 = 2.26·10–4  solutions
of 11 in CH2Cl2. Doubling of the concentration in each of
the four steps from c1 to c5 does not result in a doubling of
the Amax values. Moreover, λmax changes from 441 to
486 nm. From c3 = 5.15·10–5  on, the effect on Amax
and λmax becomes very pronounced. Obviously Lambert–
Beer’s law is not valid. The change of the band shape gives
the impression, that for increasing concentrations more and
more aggregates absorb, which have the character of J ag-
gregates with higher λmax values but lower Amax (εmax) val-
ues. The compounds 9 and 10 exhibit the same behavior.
The εmax values, listed in Table 1, refer to concentrations of
Scheme 3. Preparation of the star-shaped compounds 9–11 having
a 1,3,5-triazine core and three OPV chains with dihexylamino
groups as electron-donor groups in the terminal positions.
The long-wavelength absorption data of 9–11 are listed 10–5 mol·L–1 or less. Further dilution changes the εmax val-
in Table 1. As the band does not have a fine structure, which ues to a minor extent; the εmax value of 11 varies for exam-
allows the determination of the 0 Ǟ 0 transition, the λ0.1 ple from 128020 to 138990 cm2 mmol–1 on going from
values are taken with ε (λ0.1) = 0.1ε (λmax) at the long-wave- 1.028·10–5  to 2.57·10–6  solutions in CH2Cl2. One can
length foot of the band. Both values λmax as well as λ0.1 imagine, that the disc-like molecules 9–11 form aggregates,
show the same behavior in the series 9–11, namely an in- in which the 1,3,5-triazine core of the molecule is superim-
crease of the values from 9 to 10 and then a decrease to 11. posed by an amino group of the next molecule, etc. Thus,
2610
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2609–2617

Star-Shaped Push-Pull Compounds Having 1,3,5-Triazine Cores
FULL PAPER
Figure 1. Long-wavelength absorption of compound 11 at increasing concentrations in CH2Cl2: c1 = 1.29·10–5, c2 = 2.58·10–5, c3
5.15·10–5, c4 = 1.03·10–4, and c5 = 2.26·10–4 mol·L–1.
=
additional to the intramolecular push-pull effect, an inter- dition of CF3COOH to a solution of 9 in CD2Cl2.The as-
molecular charge transfer can occur. However, a very regu- signment of the signals is based on HMQC and HMBC
lar self-organization, which provokes discotic mesophases, measurements. Scheme 5 contains the descriptors for the
could not be observed in the DSC.
carbon atoms in 9–11.
An important point concerns now the influence of pro-
tonating media. Protonation of the amino groups should
lead to the disappearance of their donor character, whereas
protonation of the 1,3,5-triazine ring should enhance the
acceptor capability. Thus, protonation can weaken or
strengthen the push-pull effect – depending on the preferred
side. Scheme 4 depicts the equilibria of the protonated com-
pounds 9–11.
Scheme 5. Carbon atoms in 9–11, selected for the determination of
the protonation side.
Protonation of 9 does not change the C3 symmetry axis
valid in the NMR spectra; that means a fast exchange of
protons has to be assumed. A comparison of parts a and b
of Figure 2 reveals very small low-field shifts for aЈЈ, aЈ, bЈ,
and dЈ, the carbon atoms on the donor side, and big shifts
for a (∆δ = –4.4 ppm), b (∆δ = –7.1 ppm) and c (∆δ =
+7.2 ppm) on the acceptor side. In particular the polariza-
tion of the olefinic double bond is strongly enhanced. The
difference δ(c) – δ(b) increases from 20.4 to 34.7 ppm. These
findings demonstrate that the initial protonation, obtained
for the molar ratio 9/CF3COOH = 1:2.8, preferentially
takes place on the 1,3,5-triazine ring. Normally tertiary
amines are much more basic than 1,3,5-triazines. The strong
push-pull effect in 9 reverts obviously the situation.
Scheme 4. Equilibria of the protonated compounds of 9–11 in
CH2Cl2/CF3COOH.
Further protonation to 9c (molar ratio 9/CF3COOH =
The 13C NMR spectra provide a reliable tool for the ob- 1:86) occurs then on the amino nitrogen atoms. Consider-
servation of partial charges and their changes by proton- able down-field shifts are observed (see part c of Figure 2)
ation. Figure 2 shows the alteration of the signals by ad- for the carbon atoms aЈЈ (∆δ = +8.0 ppm), bЈ (∆δ =
Eur. J. Org. Chem. 2006, 2609–2617
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2611

H. Meier, E. Karpuk, H. C. Holst
FULL PAPER
Figure 2. 13C NMR signals of 9 and their shift caused by protonation: a) 3.24·10–2  solution of 9 in 0.75 mL CD2Cl2; b) addition of
5 µL (6.73·10–5 mol) CF3COOH; c) addition of 155 µL (2.09·10–3 mol) CF3COOH.
+10.1 ppm), dЈ (∆δ = +13.1 ppm) and b (∆δ = +8.0 ppm). ferent. Addition of a small portion of CF3COOH (molar
The polarization of the olefinic double bond in 9c [δ(c) – ratio 10/CF3COOH = 1:1.4) provokes strong down-field
δ(b) = 27.0 ppm] is between that of 9a and the unproton- shifts of aЈЈ (∆δ = +6.9 ppm) and bЈ (∆δ = +9.5 ppm) and
ated compound 9. All other carbon signals with the excep- an up-field shift of aЈ (∆δ = –5.0 ppm). Accordingly, pro-
tion of aЈ are less affected by the additional protonation. tonation preferentially takes place at the donor end; struc-
A comparison of parts a and c of Figure 2 demonstrates ture 10b predominates in the equilibrium.
unambiguously that both positions are protonated in 9c.
The decrease of the electron density by changed mesomeric
effects in the position bЈ and c becomes evident by strong
down-field shifts. A proton exchange mechanism, which is
fast in the sense of the NMR time scale, keeps the D3h sym- the push-pull effect and changes consequently the electron
metry of 9 intact in the protonated species 9a,c. density to such an extent, that the amine basicity prevails –
The larger distance D–A in 10 (compared to 9) reduces
Figure 3 demonstrates the protonation of 10. The same as a priori expected for the whole series. Further proton-
carbon atoms were selected as for 9 (Scheme 5), but the ation to 10c (molar ratio 10/CF3COOH = 1:10.5) occurs
observed signal shifts caused by protonation are widely dif- than on the 1,3,5-triazine nitrogen atoms. A strong down-
Figure 3. 13C NMR signals of 10 and their shift caused by protonation: a) 1.29·10–2  solution of 10 in 0.75 mL CD2Cl2; b) addition of
1 µL (1.35·10–5 mol) CF3COOH; c) addition of 7.5 µL (10.10·10–5 mol) CF3COOH.
2612
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2609–2617

Star-Shaped Push-Pull Compounds Having 1,3,5-Triazine Cores
FULL PAPER
field shift (Figure 3, c) of signal c (∆δ = +7.1 ppm) and a
strong up-field shift of b (∆δ = –8.6 ppm) are good indica-
tions for this effect.
Compound 11 has an even larger distance D–A than 10
and consequently an even weaker push-pull character. The
protonation effects on the 13C chemical shifts are similar to
those observed for 10. Initial protonation shifts aЈЈ and bЈ
down-field and aЈ up-field.
Figure 4. Color change of a 2.44·10–5  solution of 9 in CH2Cl2 by
Further protonation to 11c occurs than on the nitrogen
atoms of the 1,3,5-triazine ring, indicated for example by
the up-field shift of carbon a by 4.4 ppm. The exchange
mechanism, which establishes the D3h symmetry is in 11b,c
somewhat slower than in 10b,c, so that the NMR signals
are broader in 11b,c.
addition of CF3COOH.
When the molar ratio 9/CF3COOH reaches 1:55, the new
band reaches a λmax value of 564 nm. Further addition of
The NMR results are important to understand the UV/ CF3COOH does not change this value anymore, but the
Vis spectra of 9–11/a–c. The yellow solution of 9 in CH2Cl2 intensity of the band decreases continuously from then on
turns first red on addition of CF3COOH, then violet and and another new absorption band emerges at λmax
=
finally almost colorless (Figure 4). When the molar ratio 9/ 326 nm (molar ratio 9/CF3COOH = 1:276). An even larger
CF3COOH = 1:2.8, used for the NMR measurement shown excess of CF3COOH (molar ratio 1:1930) finally provokes
in part b of Figure 2, is kept constant for different concen- an almost complete disappearance of the band at 564 nm,
trations of 9, one recognizes, that the violet color is owing and a new maximum appears at 373 nm. Figure 6 demon-
to protonated aggregates of 9. Figure 5 shows that a new strates this effect which corresponds to the color change in
absorption band between 500 and 600 nm emerges, whose Figure 4.
relative intensity increases with increasing concentration of
The interpretation of this behavior takes advantage of
9. The preliminary aggregation model, mentioned before, is the NMR results. Protonation occurs first at the 1,3,5-tri-
in accordance with this result, since the intermolecular azine ring. The push-pull effect is enhanced and conse-
charge transfer between the dialkylamino group and the quently the bathochromic shift of the charge-transfer band
1,3,5-triazine ring should be even more favored, when the increased.[1] Further protonation occurs then on the nitro-
1,3,5-triazine ring is protonated.
gen atoms of the amino groups. In this situation, the overall
Figure 5. Long-wavelength absorption of 9 in CH2Cl2/CF3COOH for a constant molar ratio 9/CF3COOH = 1:2.8 and different concentra-
tions of 9: c1 = 2.55·10–6, c2 = 5.10·10–6, c3 = 1.02·10–5, c4 = 2.04·10–5, c5 = 4.08·10–5 mol· L–1.
Eur. J. Org. Chem. 2006, 2609–2617
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2613

H. Meier, E. Karpuk, H. C. Holst
FULL PAPER
Figure 6. Absorbance of a 2.44·10–5  solution of 9 in CH2Cl2 (10 mL): neutral, ---- with 1 µL (1.35·10–5 mol) CF3COOH, · · · with
5 µL (6.73·10–5 mol) CF3COOH, – · – · – with 35 µL (47.12·10–5 mmol) CF3COOH.
ICT is lost, and ions are generated with smaller charge stant; that means aggregation plays here a role, too. Contin-
transfers from the stilbene unit to the ammonium group uing protonation leads then to a bathochromic shift. A high
and also in the opposite direction from the stilbene unit to excess of CF3COOH gives rise to a strong band with a λmax
the triazinium core.
value of 451 nm, which is similar to the original band of
The higher “generation” 10 behaves totally different unprotonated 10. The molar ratios 10/CF3COOH amount
(Figure 7). The very first protonation causes already a hyp- to 1:–, 1:11.6, 1:155 and 1:774 for the measurements de-
sochromic shift in so far as the band with λmax = 457 nm picted in Figure 7.
decreases, and a new band with λmax = 394 nm emerges.
Whereas protonation of 9 leads first to a bathochromic
The relative height of the two bands depends on the concen- shift and then to a hypsohromic shift, 10 behaves vice versa,
tration, when the molar ratio 10/CF3COOH is kept con- initial protonation causes a hypsochromic shift, which is
Figure 7. Absorbance of a 1.74·10–5  solution of 10 in CH2Cl2 (10 mL): neutral, ---- with 0.15 µL (0.20·10–5 mmol) CF3COOH, · · ·
with 2 µL (2.69·10–5 mmol) CF3COOH, – · – · – with 10 µL (13.46·10–5 mmol) CF3COOH.
2614
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2609–2617

Star-Shaped Push-Pull Compounds Having 1,3,5-Triazine Cores
FULL PAPER
then followed by a bathochromic shift. The shifts observed belongs to the category d) mentioned in the beginning –
in the UV/Vis spectra of 10 are smaller than those of 9.
and not as a priori assumed to category a).[1] The initial
The third “generation” 11 resembles 10. Protonation of protonation seems to lead to the often searched but until
11 leads first to a hypsochromic effect, λmax decreases from now never found category in which ∆E passes with increas-
440 nm to 412 nm (Figure 8). Further protonation causes ing n through a maximum.[1,5] But in reality this statement
then a bathochromic effect of the long-wavelength band to is not correct, because the site of protonation changes
λmax = 485 nm. The difference to the spectra of 10 is due to within the series 9a, 10b, 11b. The series of final protonation
the fact, that a high excess of CF3COOH causes a stronger 9c, 10c, 11c belongs to category a), increasing numbers n
bathochromic effect, so that the band surpasses the original cause a monotonous decrease of ∆E.
maximum at 440 nm. The molar ratios of the measurements
depicted in Figure 8 amount to 11:CF3COOH = 1:–, 1:29.8,
1:89, 1:298.
The absorption curves depicted in the Figures 6, 7, and
8 were selected from many measurements with different
concentrations of 9–11 and of CF3COOH in order to make
the effects particularly clear. Each curve corresponds to the
sum of absorptions of all species which are present in the
equilibria. The exchange mechanisms, which were impor-
tant for the NMR interpretation, do not play a role here,
but the preferred protonaton sides are of course the same
in both spectroscopical measurements.
The extension of the conjugation in the series of the neu-
tral compounds 9, 10, 11 is connected with a transition en-
ergy ∆E (S0 Ǟ S1), which proceeds through a minimum for
10, whereas ∆E for the initially protonated species 9a, 10b,
Figure 9. Energies ∆E of the electronic transition S0 Ǟ S1 of the
11b proceeds through a maximum for 10b. The finally pro-
compounds 9–11 and their protonated species (CH2Cl2/CF3COOH).
tonated species 9c–11c represent a bathochromic series
whose red shift is mainly due to the extension of the chro-
Apart from the linear optics, the 1,3,5-triazines 9–11 and
mophore.
their protonated species should show interesting nonlinear
The push-pull effect in each arm of 9–11 decreases with optics (NLO), in particular high β (–2ω;ω,ω) values (sec-
increasing length of the three arms. The same effect can be ond-order polarizabilities) can be expected. The NLO ac-
expected for the ICT. Figure 9 visualizes the energies for the tivity of such dipole-less compounds is due to large off-
electronic transitions ∆E (S0 Ǟ S1) in dependence of the diagonal components of the second-order polarizability
number of OPV repeat units. The neutral series 9, 10, 11 tensor.[13–16] Until now only 1,3,5-triazines with small NLO
Figure 8. Absorbance of a 2.26·10–5  solution of 11 in CH2Cl2 (10 mL): neutral, ---- with 0.5 µL (0.67·10–5 mmol) CF3COOH, · · · with
1.5 µL (2.02·10–5 mmol) CF3COOH, – · – · – with 5 µL (6.73·10–5 mmol) CF3COOH.
Eur. J. Org. Chem. 2006, 2609–2617
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2615

H. Meier, E. Karpuk, H. C. Holst
FULL PAPER
(s, 1 H, CHO) ppm. 13C NMR (CDCl3): δ = 14.0 (CH3), 22.6, 26.7,
27.1, 31.6 (CH2), 51.1 (NCH2), 110.6 (C-3), 124.5 (C-1), 132.6
(C-2), 152.6 (C-4), 189.8 (CHO) ppm. FD MS: m/z (%) = 289 (100)
[M+·]. C19H31NO (289.5): calcd. C 78.84, H 10.79, N 4.84; found
C 79.18, H 10.59, N 5.10.
chromophores were studied,[17,18] but even these have very
high β values. Since the β values in the appropriate three-
level model[17,18] increase with the product of the three tran-
sition dipoles and ∆E2, it will be highly interesting, how β
(n) behaves in the series 9–11 and their protonated species.
4-[(E)-2-(4-Bromophenyl)vinyl]-N,N-dihexylaniline (4): Aldehyde 2
(4.74 g, 16.9 mmol) and diethyl 4-bromobenzylphosphonate (3)
[19,20]
(5.19 g, 19.9 mmol) dissolved in dry DMF (150 mL) were de-
Conclusions
gassed and slowly added whilst stirring in Ar to KOC(CH3)3
The newly synthesized push-pull 1,3,5-triazines 9–11, (5.71 g, 51.0 mmol) in dry DMF (150 mL). After 2 h at room tem-
perature, the mixture was poured on crushed ice. The water layer
was extracted CHCl3 (3×50 mL) (eventually some NaCl was
added), and the unified organic phases were dried with Na2SO4.
The volatile parts were evaporated and the residue filtered through
SiO2 (10 × 5 cm) with petroleum, b.p. 40–70 °C/ethyl acetate, 95: 5.
The obtained orange solid (6.95 g, 93%) showed in the DSC a melt-
ing point of 76.4 °C. It was identical with an authentic sample.[21]
1H NMR (CDCl3): δ = 0.88 (t, 6 H, CH3), 1.30 (m, 12 H, CH2),
1.57 (m, 4 H, β-CH2), 3.26 (t, 4 H, α-CH2), 6.59/7.34 (AAЈMMЈ,
4 H, 2-H, 3-H, 5-H, 6-H), 6.78/6.99 (AB, 3J = 16.2 Hz, 2 H, olefin.
having OPV chains (n = 1–3) in 2-, 4- and 6-positon and
terminal dialkylamino groups exhibit an unusual absorp-
tion behavior (S0 Ǟ S1) in neutral and protonating media
(CH2Cl2/CF3COOH). We assumed previously,[1] that 9–11
(n = 1–3), as the corresponding compounds with OR in-
stead of NR2 groups,[8] show a bathochromic shift of their
long-wavelength absorption (S0 Ǟ S1) with increasing num-
bers n. However, it turned out that ∆E (S0 Ǟ S1) passes
through a minimum for n = 2. This result is explained by
the opposite effects, that ∆E (n) decreases with the exten- H), 7.31/7.40 (AAЈBBЈ, 4 H, aromat. H) ppm. 13C NMR (CDCl3):
δ = 14.0 (CH3), 22.6, 26.8, 27.3, 31.7 (CH2), 51.0 (NCH2), 111.6
(C-2), 119.9 (CqBr), 122.2, 129.6 (olefin. CH), 124.0 (C-4), 127.4,
131.6 (aromat. CH), 127.8 (C-3), 137.3 (aromat. Cq), 148.0 (C-1)
ppm.
sion of the conjugation (increasing n), but it increases with
the reduction of the intramolecular charge transfer (ITC),
when the distance of donor group (NR2) and acceptor
(1,3,5-triazine core) grows. Increasing protonation of 9 (n =
1) leads first to a strong red shift and then to a blue shift:
4-[(E)-2-(4-Dihexylaminophenyl)vinyl]benzaldehyde (5): A solution
the yellow solution in CH2Cl2 turns violet and then almost of n-BuLi (2.74 ) in hexane (8.4 mL, 23.0 mmol) was added with
a syringe under Ar to 4 (6.77 g, 15.3 mmol), dissolved in dry diethyl
ether (250 mL) at –10 °C. Stirring was continued for 30 min at this
temperature till dry DMF (2.9 mL, 2.75 g, 37.9 mmol) was added.
After 1 h at room temperature, H2O (100 mL) was added. The or-
ganic layer was separated, extracted 2 times with the equivalent
volume of H2O, and dried with Na2SO4. Column filtration (8 ×
13 cm SiO2, petroleum b.p. 40–70 °C/ethyl acetate, 2:1) yielded
4.43 g (74%) of an orange solid, which melted at 75 °C. The prod-
uct was identical with an authentic sample.[22] 1H NMR (CDCl3):
δ = 0.89 (t, 6 H, CH3), 1.31 (m, 12 H, CH2), 1.58 (m, 4 H, β-CH2),
colorless. The homologues compounds 10 and 11 (n = 2, 3)
behave opposite. Initial protonation provokes a blue shift
and further protonation a red shift. The explanation for this
unexpected result makes use of different protonation sites.
Whereas 9, having the strongest push-pull effect, is first pro-
tonated on the triazine ring (NMR proof), 10 and 11 are
first protonated on an NR2 group. Protonation on the tri-
azine ring enhances the push-pull effect and the ICT, pro-
tonation on the NR2 group decreases them. When both
sides are protonated, the ICT becomes less important and 3.27 (t, 4 H, α-CH2), 6.60/7.38 (AAЈMMЈ, 4 H, aromat. H), 6.87/
3
7.17 (AB, J = 16.4 Hz, 2 H, olefin. H), 7.56/7.80 (AAЈBBЈ, 4 H,
2-H, 3-H, 5-H, 6-H), 9.93 (s, 1 H, CHO) ppm. 13C NMR (CDCl3):
δ = 14.0 (CH3), 22.7, 26.8, 27.3, 31.7 (CH2), 51.0 (NCH2), 111.5,
126.1 (aromat. CH), 121.9, 132.6 (olefin. CH), 123.8, 134.3, 144.8,
148.4 (aromat. Cq), 128.4 (C-3), 130.2 (C-2), 191.6 (CHO) ppm.
MS ESI: m/z (%) = 392 (100) [M+H]+.
the extension of the conjugation causes a bathochromic
shift.
Experimental Section
General Remarks: UV/Vis: Zeiss MCS 320/340; CHCl3 or CH2Cl2
4-((E)-2-{4-[(E)-2-(4-Dihexylaminophenyl)vinyl]phenyl}vinyl)benzal-
dehyde (7): KOC(CH3)3 (0.72 g, 6.4 mmol) dissolved in dry DMF
(50 mL) was added dropwise to a mixture of 5 (1.00 g, 2.56 mmol)
and diethyl 4-(1,3-dioxane-2-yl)benzylphosphonate (6)[23] (0.788 g,
2.50 mmol) in dry DMF (50 mL). TLC control (SiO2, petroleum
b.p. 40–70 °C/ethyl acetate, 2:1) indicated the end of the reaction
after about 3 h stirring at room temperature. The mixture was
poured on crushed ice and the aqueous layer was extracted with
CH2Cl2 (3×50 mL). To the concentrated organic phase (ca. 25 mL)
was added CF3COOH (3.0 mL, 0.039 mol) in H2O (10 mL). After
2 d stirring at ambient temperature, the mixture was neutralized
with NaHCO3, washed with a saturated NaCl solution and H2O
and dried with Na2SO4. Evaporation of the solvent yielded 1.00 g
(81%) of an orange solid, which melted at 171 °C. 1H NMR
(CDCl3): δ = 0.89 (t, 6 H, CH3), 1.30 (m, 12 H, CH2), 1.57 (m, 4
H, β-CH2), 3.27 (t, 4 H, α-CH2), 6.60/7.36 (AAЈMMЈ, 4 H, aromat.
1
as solvents. H and 13C NMR: Bruker Avance 600, AMX 400 and
AC 300. CDCl3/TMS as internal standard. MS: Finnigan MAT 95
(FD; accelerating voltage 5 kV), Micromass QTOF Ultima-3 (ESI,
reference: CsI – NaI standard solution), DSC: Perkin – Elmer DSC
7. Melting points: Stuart Scientific SMP/3; uncorrected.
4-(Dihexylamino)benzaldehyde (2): POCl3 (7.33 mL, 12.28 g,
0.08 mol) was slowly added to N,N-dihexylaniline (21.0 g, 0.08 mol)
in DMF (18.5 mL, 17.52 g, 0.24 mol). The reaction mixture turned
green on stirring at room temperature and was then heated to 80 °C
for 3 h. After having poured it on crashed ice, the mixture was
extracted with diethyl ether (3×50 mL). The unified organic phases
were washed with saturated aqueous NaHCO3 and H2O and dried
with Na2SO4. Evaporation of the solvent led to an oil (19.5 g,
87%), which could be purified in small portions by column
1
chromatography (20 × 3 cm SiO2, CH2Cl2). H NMR (CDCl3): δ
3
3
= 0.87 (t, 6 H, CH3), 1.30 (m, 12 H, CH2), 1.58 (m, 4 H, β-CH2), H), 6.85/7.10 (AB, J = 16.2 Hz, 2 H, olefin. H), 7.06/7.24 (AB, J
3.13 (t, 4 H, α-CH2), 6.61/7.67 (AAЈMMЈ, 4 H, aromat. H), 9.67 = 16.5 Hz, 2 H, olefin. H), 7.45/7.48 (AAЈBBЈ, 4 H, aromat. H),
2616
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2609–2617

Star-Shaped Push-Pull Compounds Having 1,3,5-Triazine Cores
FULL PAPER
7.63/7.84 (AAЈBBЈ, 4 H, aromat. H) ppm. 13C NMR (CDCl3): δ =
14.0 (CH3), 22.7, 26.8, 27.3, 31.7 (CH2), 51.1 (NCH2), 111.6, 126.2,
126.8, 127.2, 127.9, 130.2 (aromat. CH), 122.8, 126.3, 129.6, 132.1
(olefin. CH), 124.3, 134.6, 135.0, 138.8, 143.7, 148.0 (aromat. Cq),
191.6 (CHO) ppm. FD MS: m/z (%) = 493 (100) [M+·]. HRMS
(ESI): calcd. for [C35H43NO+H+] m/z = 494.3423; found 494.3412.
= 14.1 (CH3), 22.7, 26.8, 27.3, 31.8 (CH2), 51.0 (NCH2), 111.6,
126.2, 126.9, 127.0, 127.8, 128.6 (aromat. CH), 123.0, 125.8, 127.1,
129.1, 129.7, 139.1 (olefin. CH), 124.3, 134.5, 135.9, 138.2, 141.1,
147.8 (aromat. Cq), 171.2 (C-2) ppm. FD MS: m/z (%) = 1550 (100)
[M+·]. HRMS (ESI): calcd. for [C111H132N6 +H+] 1550.0592; found
1550.0618.
2,4,6-Tris{(E)-2-[4-(dihexylamino)phenyl]vinyl}-1,3,5-triazine
(9):
Aldehyde 2 (924 mg, 3.30 mmol) dissolved in dry THF (15 mL) was
added dropwise under Ar to 2,4,6-trimethyl-1,3,5-triazine (8)[24]
(123 mg, 1.0 mmol) and KOC(CH3)3 (370 mg, 3.3 mmol) in dry
THF (15 mL). Stirring at 0 °C was continued for further 30 min,
before the temperature was raised to 70 °C. After several days (TLC
control: SiO2, CH2Cl2/ethyl acetate, 10:1) CH2Cl2 (100 mL) was
added, and the mixture was carefully washed with saturated NaCl
solution and H2O. After drying with Na2SO4, the volatile parts
were evaporated in vacuo and the residue purified by column
chromatography (3 × 30 cm SiO2, CH2Cl2). A red-violet wax
(738 mg, 79%) was obtained. 1H NMR (CDCl3): δ = 0.89 (t, 18 H,
CH3), 1.31 (m, 36 H, CH2), 1.59 (m, 12 H, β-CH2), 3.29 (t, 12 H,
Acknowledgments
We are grateful to the Deutsche Forschungsgemeintschaft, the
Fonds der Chemischen Industrie and the Center of Materials Sci-
ence of the University Mainz for financial support.
[1] a) H. Meier, Angew. Chem. 2005, 117, 2536–2561; Angew.
Chem. Int. Ed. 2005, 44, 2482–2506 and references cited
therein.
[2] J. J. Wolff, R. Wortmann, Adv. Phys. Org. Chem. 1999, 32, 121–
217.
[3] R. E. Martin, F. Diederich, Angew. Chem. 1999, 111, 1440–
1468; Angew. Chem. Int. Ed. 1999, 38, 1350–1377.
[4] Electronic Materials: The Oligomer Approach (Eds.: K. Müllen,
G. Wegner), Wiley-VCH, Weinheim, 1998.
[5] H. Meier, in Carbon-rich Compounds: Molecules to Materials
(Eds.: M. M. Haley, R. R. Tykwinski), Wiley-VCH, Weinheim,
2006.
3
α-CH2), 6.61/7.52 (AAЈMMЈ, 12 H, aromat. H), 6.88/8.14 (AB, J
= 15.7 Hz, 6 H, olefin. H) ppm. 13C NMR (CDCl3): δ = 14.0
(CH3), 22.7, 26.8, 27.3, 31.7 (CH2), 51.0 (NCH2), 111.3, 129.9 (aro-
mat. CH), 120.9, 141.3 (olefin. CH), 122.7, 149.3 (aromat. Cq),
171.3 (C-2) ppm. FD MS: m/z (%) = 937 (100) [M+·]. HRMS (ESI):
calcd. for [C63H96N6 +H+] m/z = 937.7775; found 937.7737.
[6] B. R. Cho, K. Chajara, H. J. Oh, K. H. Son, S.-J. Jeon, Org.
Lett. 2002, 4, 1703–1706.
all-(E)-2,4,6-Tris[2-(4-{2-[4-(dihexylamino)phenyl]vinyl}phenyl)-
vinyl]-1,3,5-triazine (10): Aldehyde 5 (489 mg, 1.25 mmol) dissolved
in dry THF (10 mL) was added dropwise under Ar to 8 (48 mg,
0.39 mmol) and KOC(CH3)3 (146 mg, 1.9 mmol) in dry THF
(20 mL). The reaction mixture was vigorously stirred for at least 1
d at room temperature. Treatment with cold CH3OH led to the
precipitation of the product (343 mg), which contained 2,4-bis[2-
(4-{2-[4-(dihexylamino)phenyl]vinyl}phenyl)vinyl]-6-methyl-1,3,5-
triazine and the target compound in a ratio of about 1:1. Column
chromatography (2 × 30 cm SiO2, toluene) afforded 165 mg (34%)
of 10 as a red solid, which melted at 166–167 °C. 1H NMR
(CDCl3): δ = 0.90 (t, 18 H, CH3), 1.31 (m, 36 H, CH2), 1.57 (m,
12 H, β-CH2), 3.27 (t, 12 H, α-CH2), 6.62/7.38 (AAЈMMЈ, 12 H,
aromat. H), 6.88/7.10 (AB, 3J = 16.2 Hz, 6 H, olefin. H), 7.13/8.24
[7] B. Traber, J. J. Wollf, F. Rominger, T. Oeser, R. Gleiter, M. Goe-
bel, R. Wortmann, Chem. Eur. J. 2004, 10, 1227–1238.
[8] H. Meier, H. C. Holst, A. Oehlhof, Eur. J. Org. Chem. 2003,
4173–4180.
[9] H. C. Holst, T. Pakula, H. Meier, Tetrahedron 2004, 60, 6765–
6775.
[10] H. Meier, M. Lehmann, H. C. Holst, D. Schwöppe, Tetrahe-
dron 2004, 60, 6881–6888.
[11] Y.-Z. Cui, Q. Fang, Z.-L. Huang, G. Xue, G.-B. Xu, W.-T. Yu,
J. Mater. Chem. 2004, 14, 2443–2449.
[12] Dimethylamino groups are much less favorable; didode-
cylamino groups are comparably well solubilizing, but lead to
lower yields in the synthetic process according to Scheme 3.
[13] M. Joffre, D. Yaron, R. J. Silbey, J. Zyss, J. Chem. Phys. 1992,
97, 5607–5615.
[14] J.-L. Brédas, F. Meyers, B. M. Pierce, J. Zyss, J. Am. Chem.
Soc. 1992, 114, 4928–4929.
[15] J. Zyss, I. Ledoux, Chem. Rev. 1994, 94, 77–105.
[16] I. Ledoux, J. Zyss, J. Siegel, J. Brienne, J.-M. Lehn, Chem. Phys.
Lett. 1990, 172, 440–444.
[17] J. J. Wolff, D. Längle, D. H. Hillenbrand, R. Wortmann, R.
Matschiner, C. Glania, P. Krämer, Adv. Mater. 1997, 9, 138–
143.
3
(AB, J = 15.8 Hz, 6 H, olefin. H), 7.50/7.64 (AAЈBBЈ, 12 H, aro-
mat. H) ppm. 13C NMR (CDCl3): δ = 14.1 (CH3), 22.7, 26.8, 27.3,
31.7 (CH2), 51.0 (NCH2), 111.6, 126.3, 128.0, 128.6 (aromat. CH),
122.8, 125.3, 130.2, 141.3 (olefin. CH), 124.1, 133.7, 140.1, 148.0
(aromat. Cq), 171.3 (C-2) ppm. FD MS: m/z (%) = 1243 (100)
[M+·]. HRMS (ESI): calcd. for [C87H114N6 +H+]: 1243.9183; found
1243.9230.
[18] R. Wortmann, C. Glania, P. Krämer, R. Matschiner, J. J. Wolff,
S. Kraft, B. Treptow, E. Barbu, D. Längle, G. Görlitz, Chem.
Eur. J. 1997, 3, 1765–1773.
[19] R. Brettle, D. A. Dunmur, N. J. Hindley, C. M. Marson, J.
Chem. Soc., Chem. Commun. 1992, 410–411.
[20] H. E. Katz, S. F. Bent, W. L. Wilson, M. L. Schilling, S. B. Un-
gashe, J. Am. Chem. Soc. 1994, 116, 6631–6635.
[21] See also A. Hossner, D. Birnbaum, L. M. Loew, J. Org. Chem.
1984, 49, 2546–2551.
[22] X. Li, G. Zhang, H. Ma, D. Zhang, J. Li, D. Zhu, J. Am. Chem.
Soc. 2004, 126, 11543–11548.
[23] See E. Sugiono, H. Detert, T. Metzroth, Adv. Synth. Catal.
2001, 343, 351–359.
all-(E)-2,4,6-Tris(2-{4-[2-(4-{2-[4-(dihexylamino)phenyl]-
vinyl}phenyl)vinyl]phenyl}vinyl)-1,3,5-triazine (11): Aldehyde 7
(525 mg, 1.06 mmol), 8 (43 mg, 0.35 mmol) and KOC(CH3)3
(134 mg, 1.2 mmol) reacted as described for 10. The reaction time
was extended to 12 d at room temperature.[25] The red precipitate
(301 mg) obtained by the addition of CH3OH contained mainly the
1,3,5-triazine with two OPV arms and only a small amount of the
target compound. Column chromatography (2 × 30 cm SiO2, tolu-
ene) yielded 15 mg (3%) of 11 as a red solid, which melted at 97 °C.
1H NMR (CDCl3): δ = 0.89 (t, 18 H, CH3), 1.30 (m, 36 H, CH2),
1.55 (m, 12 H, β-CH2), 3.26 (t, 12 H, α-CH2), 6.60/7.36 (AAЈMMЈ,
12 H, aromat. H, outer ring), 6.85/7.03 (AB, 3J = 16.1 Hz, 6 H,
olefin. H, outer double bond), 7.12/7.15 (AB, 3J = 15.8 Hz, 6 H,
olefin. H, middle), 7.13/8.24 (AB, 3J = 16.0 Hz, 6 H, olefin. H,
inner double bond), 7.35–7.52 (m, 12 H, aromat. H), 7.54/7.66
(AAЈBBЈ, 12 H, aromat H, inner ring) ppm. 13C NMR (CDCl3): δ
[24] F. C. Schaefer, G. H. Peters, J. Org. Chem. 1961, 26, 2778–2784.
[25] Higher temperatures lead to impurities which are difficult to
separate.
Received: January 26, 2006
Published Online: March 31, 2006
Eur. J. Org. Chem. 2006, 2609–2617
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2617