Dendritic triphenylmethylium and tetraphenylmethane compounds

Article abstract of DOI:10.1055/s-0028-1083374

Stilbenoid dendrimers possessing a methanol or methane core are prepared using Wittig-Horner reactions. The dendritic tris(stilbenyl)carbinols 3a-c are converted into the corresponding methylium salts 3′ a-c on treatment with trifluoroacetic or tetrafluo-

Full text of DOI:10.1055/s-0028-1083374

848
PAPER
Dendritic Triphenylmethylium and Tetraphenylmethane Compounds
Dendritic
T
ripheny
e
lmethylium
r
and Te
b
traphenylme
e
thane
C
om
p
r
ounds t Meier,* Soungkyoo Kim, Annette Oehlhof
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
Fax +49(6131)39225396; E-mail: hmeier@mail.uni-mainz.de
Received 23 September 2008; revised 10 November 2008
R1
O
H
Abstract: Stilbenoid dendrimers possessing a methanol or methane
core are prepared using Wittig–Horner reactions. The dendritic
tris(stilbenyl)carbinols 3a–c are converted into the corresponding
methylium salts 3¢a–c on treatment with trifluoroacetic or tetrafluo-
roboric acid. The extended cross-conjugation in the dendritic meth-
PO(OC2H5)2
R2
+
HC
3
R1
1
2a–c
ylium ion 3¢c results in a long-wavelength absorption (lmax = 697
nm) whereas the conjugated model systems 3¢a and 3¢b are red-
air
α
O
R1
shifted (lmax = 772 nm and 748 nm, respectively). Charge delocal-
KOt-Bu
β
ization in the cations is evident from the low-field shifts of the 13
C
C
52–75%
R2
H
NMR signals. In contrast to the dark-blue dyes 3¢a–c, the tris(stilbe-
nyl)carbinols and tetrakis(stilbenyl)methane derivatives 3a–c and 7
exhibit long-wavelength absorption bands in the UV region.
R1
3a–c
3
Key words: stilbene, charge delocalization, Wittig–Horner reac-
tion, olefination, dendrimers, dyes
a
b
c
OC6H13
OC6H13
OC6H13
R1
R2
OC12H25
OC12H25
H
From photophysical and photochemical viewpoints, stil-
bene-containing molecules are a well-studied class of
compounds.1 Therefore, dendrimers possessing stilbenoid
building blocks represent an attractive area of research.2 A
number of dendritic structures, in which the dendrons (but
not the core) consist of stilbene building blocks, have been
described.3
OC12H25
H
PO(OEt)2
OMe
C
O
H
+
OC6H13
C
H
We report here on novel stilbenoid dendrimers containing
triphenylmethylium or tetraphenylmethane cores. The
dendrons of these compounds represent extended p-sys-
tems in which full conjugation is blocked by a saturated
center (sp3–C) or is ‘opened’ by a carbenium center (sp2–
C+). Peripheral alkoxy groups serve as solubilizing
groups, and moreover, as electron-donor groups which
impart the methylium systems with acceptor–donor char-
acter (AD3).4
OMe
PO(OEt)2
5
4
OC6H13
OC6H13
O
C
KOt-Bu
HCl
1)
2)
H
2d
Preparation of the target dendrimers 3a–c and 7 was start-
ed from the triphosphonate 15 or the tetraphosphonate 6,6
(Scheme 1). Wittig–Horner reactions of 1 and aldehyde
2c, and 6 with aldehyde 2d, yielded the desired products
3c (52%) and 7 (68%), respectively. The star-shaped mod-
el compounds 3a (70%) and 3b (75%) were prepared via
analogous reactions of triphosphonate 1 with aldehydes
2a and 2b, respectively. The substrate aldehydes 2a,7,8
2b,7,9 2c10,11 and 2d contain long, flexible alkoxy chains.
Aldehyde 2d was obtained in 82% yield analogously to 2c
by reaction of diphosphonate 4, which contains a formyl
group protected as a dimethylacetal, with 4-(hexyl-
oxy)benzaldehyde (5).10
PO(OEt)2
+
2d
C
4
6
KOt-Bu
68%
OC6H13
OC6H13
C
4
7
Scheme 1 Preparation of the dendrimers 3c and 7 and the model
compounds 3a and 3b
The Wittig–Horner olefinations gave the expected prod-
ucts in cis/trans ratios of about 5:95. After purification by
crystallization, the amount of cis isomers still present was
SYNTHESIS 2009, No. 5, pp 0848–0852
Advanced online publication: 11.02.2009
DOI: 10.1055/s-0028-1083374; Art ID: T16608SS
x
x
.
x
x
.2
0
0
9
© Georg Thieme Verlag Stuttgart · New York

PAPER
Dendritic Triphenylmethylium and Tetraphenylmethane Compounds
849
OC6H13
OC6H13
OC6H13
H13C6O
OC6H13
H13C6O
OC6H13
OC6H13
+
α
C
OC6H13
OC6H13
+ CF3COOH
– H2O
β
H13C6O
H13C6O
3c
+ H2O
– CF3COOH
CF3COO–
OC6H13
OC6H13
H13C6O
OC6H13
OC6H13
H13C6O
3′c
Scheme 2 Formation and hydrolysis of the dendritic triphenylmethylium salt 3¢c
Table 1 Selected 13C NMR Data of Carbinols 3a–c in CDCl3 and
Their Corresponding Triphenylmethylium Ions 3¢a–c in CDCl3–
CF3COOD (7:3)
less than 2%. The detection limit for cis-configured dou-
ble bonds in our study was 2%, and hence we were unable
to detect any cis-configured dendrimers in our products.
This was found to be the case for the solid products 3a, 3c
and 7, as well as for the oily product 3b which was puri-
fied by column chromatography on silica gel.
Compound
Centrala C+
81.8
a-Ca
b-Ca
3a
d
128.3
140.3
12.0
136.9
150.9
14.0
Treatment of the carbinols 3a–c with trifluoroacetic acid
led to elimination of water and formation of the corre-
sponding triphenylmethylium trifluoroacetates 3¢a–c. In
order to shift the equilibrium to favour the salts 3¢a–c, we
used a large excess of trifluoroacetic acid (molar ratio of
3/acid = 1:200) (Scheme 2).
3¢a
d
191.0
109.2
81.8
Dd
d
3b
128.3
140.6
12.3
136.6
150.5
13.9
3¢b
d
192.9
111.1
82.3
Table 1 summarizes the key 13C NMR data for the trans-
formations of 3a–c into salts 3¢a–c. For comparison, we
have included the corresponding 13C NMR data for tri-
phenylcarbinol (Ph3COH) and its cation (Ph3C+).5 The 13C
chemical shifts are highly sensitive towards changes in the
partial charges. The formation of the triphenylmethylium
ions is apparent from the strong low-field shift of the sig-
nal due to the central carbon atom (C+). The Dd value of
Ph3COH/Ph3C+ is 131.7 ppm. The corresponding Dd val-
ues of 3a–c/3¢a–c decrease in the order 3c > 3b > 3a
(120.8, 111.1 and 109.2 ppm, respectively). This trend re-
sults from the increasing delocalization of the positive
charge. The meta-branching in dendrimer 3¢c is less suited
to charge delocalization than the linear conjugation
present in 3¢a,b. Therefore, dendrimers 3c/3¢c exhibit the
highest Dd value. Charge delocalization occurs over the
whole ion, but the major effect is apparent at the ortho-
Dd
d
3c
128.3
140.8
12.5
136.4
150.7
14.3
3¢c
d
203.1
120.8
81.9
Dd
d
Ph3COHb
Ph3C+ b
127.8
143.6
15.8
127.1
144.6
17.5
d
213.6
131.7
Dd
a The positions of C+, a-C and b-C are shown in Scheme 2.
b The corresponding 13C chemical shifts of Ph3COH and cation Ph3C+
are included for comparison.
and para-positions (a and b) of the benzene rings adjacent
to the central carbon atom (Scheme 2 and Table 1).
Synthesis 2009, No. 5, 848–852 © Thieme Stuttgart · New York

850
H. Meier et al.
PAPER
1
alkene H), 7.74 (s, 1 H, ArH), 7.81 (s, 2 H, ArH), 10.02 (s, 1 H,
CHO).
13C NMR (100 MHz, CDCl3): d = 14.0 (CH3), 22.6, 25.7, 29.2, 31.6
(CH2), 68.2 (OCH2), 114.9, 128.2 (CH, outer benzene rings), 124.8,
125.6, 129.4, 130.2 (Ar, alkene CH), 129.7, 137.2, 139.1, 159.4
(ArC), 192.3 (CHO).
Apart from small effects due to the medium, the H and
13C NMR data of dendrimer 7 are the same in CDCl3 and
CDCl3–CF3COOD (7:3).
The tris(stilbenyl)carbinols 3a–c and tetrakis(stilbe-
nyl)methane 7 show long-wavelength electron transitions
in the UV region at about 330 nm in CHCl3. Dendrimer 3c
has, for example, a lmax value of 329 nm (e = 112350 L
mol–1cm–1) and dendrimer 7 has a lmax value of 327 nm
(e = 151950 L mol–1cm–1).
MS (FD): m/z (%) = 510 (100) [M+].
Anal. Calcd for C35H42O3: C, 82.31; H, 8.29. Found: C, 82.03; H,
8.30.
The saturated sp3 center in 3a–c and 7 prevents the sys-
tems being fully conjugated whilst homoconjugation has
a comparably small effect. The obtained e values indicate
the presence of three and four more or less independant
chromophores in 3a–c and 7, respectively.
Wittig–Horner Reactions; General Procedure
Triphosphonate 15 (1.0 g, 1.44 mmol) or tetraphosphonate 66 (1.0 g,
1.09 mmol) and an excess amount of the corresponding aldehyde
2a–d7–11 (5.41–7.20 mmol) were dissolved in DMF (20–30 mL) and
added dropwise to a suspension of KOt-Bu (2.0 g, 17.86 mmol) in
DMF (30 mL) (KOH can be used as the base instead). The reaction
mixture was heated at 85 °C for about 30 min and stirring was con-
tinued at r.t. for 24 h. Next, crushed ice (100.0 g) was added and the
mixture extracted with CHCl3 (2 × 100 mL). The organic phase was
washed with H2O (100 mL), dried (MgSO4) and concentrated. The
solid products 3a, 3c and 7 were obtained by crystallization or pre-
cipitation from CHCl3 or EtOH. The oily product 3b was purified
by column chromatography (toluene) over silica gel.
Formation of the methylium dye 3c → 3¢c shifts the UV
band into the visible region (lmax = 697 nm) because the
central sp2 carbon atom allows cross-conjugation of the
dendrons. Interestingly, the bathochromic shift resulting
from formation of methylium ion 3¢a is larger (lmax = 772
nm). Apart from the ‘opened’ conjugation through the
center, each ‘arm’ of 3¢a has push–pull character. In prin-
ciple, dendrimer 3¢c represents an AD3 system,4 but due to
the meta-branching, the intramolecular charge transfer
(ICT) is much smaller in 3¢c since it is reduced to a mere
inductive or field effect.
Tris(4-{(E)-2-[4-(dodecyloxy)phenyl]ethenyl}phenyl)methanol
(3a)
Reaction of triphosphonate 15 (1.0 g, 1.44 mmol) and aldehyde 2a7,8
(1.57 g, 5.41 mmol) gave 3a; yield: 1.13 g (70%); mp 96 °C.
3
1H NMR (400 MHz, CDCl3): d = 0.87 (t, J = 6.8 Hz, 9 H, CH3),
The dark blue dyes 3¢a–c could be isolated in pure solid
state as tetrafluoroborates, but they are very hygroscopic,
becoming sticky as they begin to hydrolyze and fade in
color. Although the reaction center is sterically shielded in
3¢c, it did not exhibit higher stability towards hydration in
comparison to 3¢a,b.
1.21–1.37 (m, 48 H, CH2), 1.40–1.44 (m, 6 H, CH2), 1.70–1.80 (m,
6 H, CH2), 3.95 (t, 3J = 6.6 Hz, 6 H, OCH2), 6.86 and 7.41 (AA¢BB¢,
12 H, ArH, outer benzene rings), 6.94 and 7.04 (AB, 3J = 16.2 Hz,
6 H, alkene H), 7.26 and 7.41 (AA¢BB¢, 12 H, ArH, inner benzene
rings).
13C NMR (100 MHz, CDCl3): d = 14.0 (CH3), 22.6, 26.0, 29.3, 29.4,
29.4, 29.5, 29.5, 29.6, 29.6, 31.9 (CH2), 68.2 (OCH2), 81.8 (central
C), 114.9, 125.8, 127.7, 128.3 (ArCH), 126.0, 128.7 (alkene CH),
131.8, 136.9, 145.7, 159.1 (ArC).
Melting points were measured with a Büchi melting point apparatus
and are uncorrected. UV/Vis spectra were obtained with a Zeiss
MS (FD): m/z (%) = 1119 (100) [M+].
1
MCS 320/340 diode array spectrometer. H and 13C NMR spectra
Anal. Calcd for C79H106O4: C, 84.74; H, 9.54. Found: C, 84.69; H,
9.48.
were recorded with a Bruker AMX 400 spectrometer at 400 MHz
and 100 MHz, respectively. CDCl3 served as solvent and TMS as
internal standard, unless otherwise noted. Field desorption (FD)
mass spectra were obtained with a Finnigan MAT 95 spectrometer.
Elemental analyses were performed in the Microanalytical Labora-
tory of the Institute of Organic Chemistry of the University of
Mainz. Column chromatography was carried out on silica gel (60
M, mesh 230–400, Machery-Nagel).
Tris(4-{(E)-2-[3,4,5-tri(dodecyloxy)phenyl]ethenyl}phe-
nyl)methanol (3b)
Reaction of triphosphonate 15 (1.0 g, 1.44 mmol) and aldehyde 2b7,9
(4.71 g, 7.15 mmol) gave 3b as a yellowish oil; yield: 2.40 g (75%).
1H NMR (400 MHz, CDCl3): d = 0.84–0.95 (m, 27 H, CH3), 1.22–
1.38 (m, 144 H, CH2), 1.38–1.51 (m, 18 H, CH2), 1.69–1.75 (m, 6
H, CH2), 1.75–1.83 (m, 12 H, CH2), 3.95 (t, 3J = 6.5 Hz, 6 H, OCH2,
p-OC12H25), 4.00 (t, 3J = 6.5 Hz, 12 H, OCH2, m-OC12H25), 6.69 (s,
6 H, ArH, outer benzene rings), 6.94 and 7.00 (AB, 3J = 16.2 Hz, 6
H, alkene H), 7.28 and 7.43 (AA¢BB¢, 12 H, ArH, inner benzene
rings).
13C NMR (100 MHz, CDCl3): d = 14.0 (CH3), 22.7, 26.1, 29.3, 29.4,
29.5, 29.7, 29.7, 30.4, 31.9 (CH2, partly superimposed), 69.3
(OCH2, m-OC12H25), 73.6 (OCH2, p-OC12H25), 81.8 (central C),
105.6, 126.0, 128.3 (ArCH), 127.1, 129.4 (alkene CH), 132.5,
136.6, 138.8, 145.9, 153.4 (ArC).
3,5-Bis{(E)-2-[4-(hexyloxy)phenyl]ethenyl}benzaldehyde (2d)
4-(Hexyloxy)benzaldehyde (5) (5.2 g, 25.2 mmol) was reacted with
tetraethyl
[5-(dimethoxymethyl)-1,3-phenylene]bis(methylene-
phosphonate) (4) (5.6 g, 12.4 mmol) and KOt-Bu (4.0 g, 37.0 mmol)
in THF (200 mL) according to a protocol for a related aldehyde.10
After 1 h at r.t., the mixture was heated at reflux for 1 h. Acid work-
up using HCl (2 M, 100 mL) cleaved the acetal group. The organic
layer (CHCl3, 200 mL) was separated, dried (MgSO4) and evaporat-
ed to afford a residue which was crystallized from EtOH to give 2d
as colorless crystals; yield: 5.19 g (82%); mp 132 °C.
1H NMR (400 MHz, CDCl3): d = 0.89–0.95 (m, 6 H, CH3), 1.30–
1.40 (m, 8 H, CH2), 1.43–1.50 (m, 4 H, CH2), 1.74–1.82 (m, 4 H,
CH2), 3.96 (t, 3J = 6.6 Hz, 4 H, OCH2), 6.89 and 7.45 (AA¢BB¢, 8 H,
MS (FD): m/z (%) = 2226 (29) [M+], 733 (100).
Anal. Calcd for C145H250O10: C, 81.49; H, 11.32. Found: C, 81.38;
H, 11.29.
3
ArH, outer benzene rings), 6.97 and 7.15 (AB, J = 16.2 Hz, 4 H,
Synthesis 2009, No. 5, 848–852 © Thieme Stuttgart · New York

PAPER
Dendritic Triphenylmethylium and Tetraphenylmethane Compounds
851
Tris{4-[(E)-2-(3,5-bis{(E)-2-[3,4,5-tri(hexyloxy)phenyl]ethe-
nyl}phenyl)ethenyl]phenyl}methanol (3c)
13C NMR [100 MHz, CDCl3:CF3COOD (7:3)]: d = 13.2 (CH3),
22.2, 25.7, 28.9, 29.1, 29.3, 29.4, 31.3 (CH2, partly superimposed),
69.8 (OCH2), 116.0, 124.5, 127.6, 129.5, 129.8, 140.3 (Ar, alkene
CH), 138.1, 138.8, 150.9, 160.4 (ArC), 191.0 (central C+).
Reaction of triphosphonate 15 (1.0 g, 1.44 mmol) and aldehyde
2c10,11 (6.56 g, 7.20 mmol) gave 3c; yield: 2.24 g (52%); mp 75 °C.
1H NMR (400 MHz, CDCl3): d = 0.84–0.97 (m, 54 H, CH3), 1.24–
UV/Vis: lmax = 772 nm (measurement of a 7.5 × 10–5 M solution of
1.40 (m, 72 H, CH2), 1.40–1.52 (m, 36 H, CH2), 1.71–1.78 (m, 12
3a and a 0.20 M solution of CF3COOH in CHCl3).12
3
H, CH2), 1.78–1.85 (m, 24 H, CH2), 3.96 (t, J = 6.5 Hz, 12 H,
3
OCH2, p-OC6H13), 4.02 (t, J = 6.5 Hz, 24 H, OCH2, m-OC6H13),
Tris(4-{(E)-2-[3,4,5-tri(dodecyloxy)phenyl]ethenyl}phe-
nyl)methylium Trifluoroacetate (3¢b)
6.73 (s, 12 H, ArH, outer benzene rings), 7.00 and 7.09 (AB, 3J =
16.1 Hz, 12 H, alkene H, outer double bonds), 7.14 and 7.20 (AB,
3J = 16.2 Hz, 6 H, alkene H, inner double bonds), 7.35 and 7.52
(AA¢BB¢, 12 H, ArH, inner benzene rings), 7.49–7.56 (m, 9 H, ArH,
central benzene rings).
1H NMR [400 MHz, CDCl3–CF3COOD (7: 3)]: d = 0.87–0.90 (m,
27 H, CH3), 1.25–1.55 (m, 162 H, CH2), 1.84–1.90 (m, 18 H, CH2),
4.16 (t, 3J = 6.5 Hz, 6 H, OCH2, m-OC12H25), 4.25 (t, 3J = 7.0 Hz, 12
H, OCH2, p-OC12H25), 6.97 (s, 6 H, ArH, outer benzene rings), 7.27
and 7.62 (AB, 3J = 16.2 Hz, 6 H, alkene H), 7.69 and 7.94 (AA¢BB¢,
12 H, ArH, inner benzene rings).
13C NMR (100 MHz, CDCl3): d = 13.9, 14.0 (CH3), 22.5, 22.6, 25.8,
25.8, 29.4, 30.3, 31.6, 31.7 (CH2), 69.4 (OCH2, m-OC6H13), 73.6
(OCH2, p-OC6H13), 82.3 (central C), 105.6, 123.7, 126.2, 127.3,
128.3, 129.6, 132.3, 136.4, 137.9, 138.2, 138.7, 146.1, 153.3 (Ar,
alkene CH and ArC, partly superimposed).
13C NMR [100 MHz, CDCl3–CF3COOD (7:3)]: d = 13.2 (CH3),
22.3, 25.5, 25.7, 28.9, 29.1, 29.2, 29.3, 29.3, 29.4, 29.4, 29.5, 31.7
(CH2, partly superimposed), 70.1 (OCH2, m-OC12H25), 75.6 (OCH2,
p-OC12H25), 106.9, 126.2, 127.8, 132.4, 138.3, 138.5, 138.7, 140.6,
150.5, 152.7 (alkene CH, ArCH, ArC), 192.9 (central C+).
MS (FD): m/z (%) = 2982 (100) [M+].
Anal. Calcd for C199H286O19: C, 80.14; H, 9.67. Found: C, 80.11; H,
9.36.
UV/Vis: lmax = 748 nm (measurement of a 13.8 × 10–5 M solution
of 3b and a 0.53 M solution of CF3COOH in CHCl3).12
Tetrakis{4-[(E)-2-(3,5-bis{(E)-2-[4-(hexyloxy)phenyl]ethe-
nyl}phenyl)ethenyl]phenyl}methane (7)
Tris{4-[(E)-2-(3,5-bis{(E)-2-[3,4,5-tri(hexyloxy)phenyl]ethe-
nyl}phenyl)ethenyl]phenyl}methylium Trifluoroacetate (3¢c)
1H NMR [400 MHz, CDCl3–CF3COOD (7:3)]: d = 0.84–0.92 (m,
54 H, CH3), 1.32–1.50 (m, 108 H, CH2), 1.79–1.85 (m, 36 H, CH2),
4.10–4.20 (m, 36 H, OCH2), 6.85 (s, 12 H, ArH, outer benzene
Reaction of tetraphosphonate 66 (1.0 g, 1.09 mmol) and aldehyde 2d
(3.34 g, 6.54 mmol) afforded 7 as a colorless solid; yield: 1.74 g
(68%); mp 173 °C.
1H NMR (400 MHz, CDCl3): d = 0.86–0.95 (m, 24 H, CH3), 1.25–
1.40 (m, 32 H, CH2), 1.40–1.50 (m, 16 H, CH2), 1.72–1.82 (m, 16
H, CH2), 3.97 (t, 3J = 6.5 Hz, 16 H, OCH2), 6.89 and 7.47 (AA¢BB¢,
32 H, ArH, outer benzene rings), 6.99 and 7.12 (AB, 3J = 16.4 Hz,
16 H, alkene H, outer double bonds), 7.13–7.55 (m, 36 H, inner alk-
ene CH, ArH, inner and central benzene rings).
13C NMR (100 MHz, CDCl3): d = 14.0 (CH3), 22.6, 25.8, 29.3, 31.6
(CH2), 64.5 (central C), 68.2 (OCH2), 114.8, 123.4, 126.0, 126.3,
127.8, 128.9, 130.0, 131.5, 138.3, 138.4, 146.2, 159.1 (Ar, alkene
CH, ArC, partly superimposed).
3
rings), 7.09 (A of AB, J = 16.1 Hz, 6 H, alkene H, outer double
bonds), 7.17 and 7.49 (AB, 3J = 16.1 Hz, 6 H, alkene H, inner dou-
ble bonds), 7.60–7.77 (m, 21 H, Ar and alkene H), 7.98 (m, 6 H,
ArH).
13C NMR [100 MHz, CDCl3–CF3COOD (7:3)]: d = 13.4 (CH3),
22.3, 22.4, 25.7, 29.2, 30.3, 31.4 (CH2, partly superimposed), 70.0
(OCH2, m-OC12H25), 75.5 (OCH2, p-OC12H25), 105.7, 125.2, 125.3,
127.8, 128.0, 129.6, 133.9, 136.0, 138.5, 138.6, 139.0, 140.8, 150.7,
152.4 (Ar, alkene CH and ArC), 203.1 (central C+).
UV/Vis: lmax = 697 nm (measurement of a 15.8 × 10–5 M solution
MS (FD): m/z (%) = 2346 (100) [M+].
of 3c and a 0.60 M solution of CF3COOH in CHCl3).12
Anal. Calcd for C169H188O8: C, 86.47; H, 8.07. Found: C, 86.54; H,
8.05.
Acknowledgment
Generation of the Triphenylmethylium Cations 3¢a–c
We are grateful to the Deutsche Forschungsgemeinschaft, the Fonds
der Chemischen Industrie and the Center of Materials Science of the
University of Mainz for financial support.
To carbinol 3a–c (0.02 mmol) dissolved in CHCl3 (0.7 mL) or
CDCl3 (0.7 mL), CF3COOH (0.3 mL) or CF3COOD (0.3 mL) was
added. (This corresponds to a molar ratio of 3/acid = 1:200). The
colorless to pale yellow solutions turned deep blue immediately.
References
Isolation of Triphenylmethylium Tetrafluoroborates
A saturated solution of 3a or 3c (0.05 mmol) in CH2Cl2 was treated
under argon with HBF4 (3 mL) and Ac2O (ca. 2 mL) until the cor-
responding dark blue salt precipitated. Filtration under argon was
followed by evaporation of the volatiles in vacuo (101–102 Pa). The
salts are highly hygroscopic and immediately become sticky and
start to hydrolyze in air.
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NMR and UV/Vis Characterization of Dyes 3¢a–c
Tris(4-{(E)-2-[4-(dodecyloxy)phenyl]ethenyl}phenyl)methyl-
ium Trifluoroacetate (3¢a)
1H NMR [400 MHz, CDCl3–CF3COOD (7:3)]: d = 0.87–0.91 (m, 9
H, CH3), 1.31–1.55 (m, 54 H, CH2), 1.84–1.91 (m, 6 H, CH2), 4.21
(t, 3J = 6.4 Hz, 6 H, OCH2), 7.08 and 7.68 (AA¢BB¢, 12 H, ArH, out-
er benzene rings), 7.27 and 7.68 (AB, 3J = 16.1 Hz, 6 H, alkene H),
7.68 and 7.91 (AA¢BB¢, 12 H, ArH, outer benzene rings).
Synthesis 2009, No. 5, 848–852 © Thieme Stuttgart · New York

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(12) Due to the equilibration, the intensity of the band depends on
the ratio of 3/CF3COOH. The lmax values are somewhat
blue-shifted by a high CF3COOH concentration. We assign
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increasing protonation of the alkoxy groups.
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Synthesis 2009, No. 5, 848–852 © Thieme Stuttgart · New York