Bathochromic or hypsochromic effects via the extension of conjugation: A study of stilbenoid squaraines

Article abstract of DOI:10.1039/a900888h

In contrast to normal conjugated oligomers, the stilbenoid squaraines 3a-d do not show a convergence of VIS/NIR absorption due to the extension of conjugation; the observed bathochromic effect in the beginning of the series is followed by a hypsochromic e

Full text of DOI:10.1039/a900888h

Bathochromic or hypsochromic effects via the extension of conjugation: a
study of stilbenoid squaraines
Herbert Meier,* Ralf Petermann and Jürgen Gerold
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany.
E-mail: hmeier@mail.uni-mainz.de
Received (in Liverpool, UK) 1st February 1999, Accepted 12th April 1999
O
In contrast to normal conjugated oligomers, the stilbenoid
i
PhNR2
NR2
squaraines 3a–d do not show a convergence of VIS/NIR
absorption due to the extension of conjugation; the observed
bathochromic effect in the beginning of the series is followed
by a hypsochromic effect; this result seems to be character-
istic for stilbenoid compounds with donor–acceptor sub-
stitution.
4
H
5b 92%
Br
ii
5b,c
NR2
Due to their special chromophore, squaraines (1,3-dicondensa-
tion products of squaric acid)1,2 not only attract theoretical
interest, they are also the molecular basis for numerous
applications in materials science. Electrophotography,1 optical
data storage,3 non linear optics4,5 and conversion of solar
energy6 should be mentioned in this context.
n
7c n = 2 88%
d n = 3 77%
Br
CH2PO(OEt)2
6
iii
MeO
O
Recently we succeeded in the preparation of symmetrical
squaraines which bear stilbenyl substituents in the 1- and
3-positions.7 Compared to 1,3-diarylsquaraines, the extended
conjugation causes a dramatic bathochromic shift of the p–p*
transition at long wavelengths.7 This finding raised the
question: is it possible to increase this effect by the stepwise
addition of further stilbene building blocks? For this purpose we
synthesized the squaraines 3a–d8 (Scheme 1) which are–with
the exception of 3d—reasonably soluble in CHCl3.
The condensation of the resorcinols 1a–d with squaric acid 2
was performed at the boiling point of an azeotropic mixture of
toluene and n-butanol (10:3). The stilbenoid components 1b–d
were generated by the Wittig–Horner reaction (Scheme 2). The
substituted aniline 4 was first transformed to the aldehyde 5b
which yielded the monostilbene 7c via reaction with phospho-
nate 6. Formylation led to aldehyde 5c which gave in an
analogous reaction sequence via 7d the higher aldehyde 5d. The
aldehydes 5b–d were subsequently transformed via reaction
with 8 to the stilbenoid compounds 9b–d. Deprotection with
BBr3 yielded the dihydroxy compounds 1b–d which served as
coupling components. The starting compound 1a was obtained
by N-alkylation and subsequent ether cleavage of 3,5-dime-
thoxyaniline.
H
NR2
CH2PO(OEt)2
n
8
MeO
5c n = 2 85%
d n = 3 95%
MeO
iv
v
1b n = 1 63%
c n = 2 33%
d n = 3 35%
5b–d
NR2
MeO
n
9b n = 1 87%
c n = 2 99%
d n = 3 68%
Scheme 2 Reagents and conditions: i, POCl3, DMF; ii, 6, ButOK, DMF; iii,
BuLi, DMF; iv, 8, ButOK, DMF; v, BBr3, H2O.
increasing number (n) of repeating units; we described recently
this convergence behavior with the aid of exponential func-
tions.9 The convergence is used for the evaluation of the
effective conjugation length (ECL). Surprisingly, the donor–
acceptor substituted systems 3a–d exhibit a totally different
behavior. Fig. 1 demonstrates that the extension of the
conjugation from 3a to 3b provokes a strong bathochromic
effect. The shift Dl of the absorption maximum amounts to
more than 250 nm and leads far into the NIR region. On going
from 3b to 3c and finally to 3d the lmax value decreases again
Many examples of conjugated oligomers show that the long
wavelength absorption approaches a limiting value with
O
O
OH
–H2O
R2N
+
HO
OH
2
n
OH
OH
1a–d
O–
2+
HO
HO
NR2
R2N
O–
OH
n
n
C6H13
3a n = 0 36%
b n = 1 13%
c n = 2 26%
d n = 3 11%
R = CH2 CH
C6H13
Fig. 1 Maxima of the long wavelength absorption of (Ω) 3a–d (in CHCl3),
(8 3a+–d+ (in CF3CO2H) and (2) 10a–d (in DMF).
Scheme 1
Chem. Commun., 1999, 977–978
977

Table 1 Long wavelength absorption of 3a–d in CHCl3
3b with its components 1b and 2 (the correlation of the signals
1
is based on normal and long-range H,–13C COSY measure-
3
nmax/cm21
log e
S/cm mol21
˜
ments).
The Dd values in Table 2 prove the low-field shift in the
stilbene unit caused by bonding to the squaric acid. Besides the
substitution position c, the alternating sequence d, f, h, j and l of
carbon atoms is mainly affected. The central squaric acid unit
induces positive partial charges in these positions. The
protonation (deuteriation) on the nitrogen atom cancels the
push-pull effect and causes strong low-field shifts, especially in
a
b
c
15221
11099
12255
13587
5.602
5.301
2.42 3 108
3.47 3 108
5.041
3.62 3 108
a
a
d
a The extremely low solubility of 3d in CHCl3 at ambient temperature does
not permit an exact evaluation of e and S; the limit of error for S of 3a–c is
±5%.
A
the aniline building block. The Dd values indicate the
differences in the chemical shifts between 3b and 3b+.
In conclusion, the extension of the conjugation of chromo-
phores does not always induce a bathochromic shift of the
absorption. In a series of push-pull substituted systems like 3a–
d the initial strong red-shift is replaced for higher systems by a
blue-shift. The distance between the donor and the acceptor
groups has a decisive influence on the charge transfer
transition.16 We explain this using quinoid resonance structures,
which become energetically unfavorable with growing number
n, since more and more benzene rings are displaced by p-
quinone rings (Scheme 3). Obviously this effect is contrary to
the normal effect due to extension of conjugation; thus, a
maximum of the bathochromic shift is observed in the series
3a–d. The quantum chemical treatment17 [MNDO and CNDO/
S (S + DES CI)] of the electron transitions S0 ? S1 in
squaraines localizes the charge transfer almost completely in the
four-membered ring. The strong influence of the stilbene
building blocks proves that this result is not valid; nevertheless,
the squaraine chromophore remains a challenge for theo-
reticians.
O–
HO
d
e
a
f
g
b
c
j
k
2+
R
R
h
i
l
N
O– HO
n
3
O–
HO
R
R
+
N
O
HO
n
Scheme 3
Table 2 13C NMR data of 3b compared to its components 1b and 2 and
compared to the deuteriated form 3+ (d values in CDCl3 and in CF3CO2D,
respectively)
C Atom dC (1b/2)
dC (3b)
Dd
27.5
dC (3b+)
DdA
+1.6
+8.9
+2.3
a
b
c
d
e
f
g
h
i
j
k
l
189.5
189.5
101.4
156.8
105.7
141.2
123.0
129.8
124.1
127.7
112.6
148.2
56.8
182.0
167.1
110.2
162.5
107.0
154.1
121.9
137.8
123.2
129.8
112.6
150.0
56.7
183.6
176.0
112.5
164.5
110.6
141.8
132.0
136.1
138.5
131.7
123.9
156.1
67.5
Notes and references
1 K. Y. Law, Chem. Rev., 1993, 93, 449 and references therein.
2 A. H. Schmidt, in Oxocarbons, ed. R. West, Academic Press, New
York, 1980, p. 185.
3 M. Emmelius, G. Pawlowski and H. W. Vollmann, Angew. Chem.,
1989, 101, 1475; Angew. Chem., Int. Ed. Engl., 1989, 28, 1445.
4 C.-T. Chen, S. R. Marder and L. T. Cheng, J. Chem. Soc., Chem.
Commun., 1994, 259; J. Am. Chem. Soc., 1994, 116, 3117.
5 G. J. Ashwell, G. Gefferies, D. G. Hamilton, D. E. Lynch, M. P. S.
Roberts, G. S. Bahra and C. R. Brown, Nature, 1995, 375, 385.
6 R. O. Loutfy, C. K. Hsiao and P. M. Kazmaier, Photogr. Sci. Eng., 1983,
27, 5.
222.4
+8.8
+5.7
+1.3
+12.9
21.1
+8.0
20.9
+2.1
0
+2.0
+3.6
212.3
+10.1
21.7
+15.3
+1.9
+11.3
+6.1
+10.8
+1.8
20.1
NCH2
7 H. Meier and U. Dullweber, J. Org. Chem., 1997, 62, 4821.
8 Selected data for 3a: mp 92 °C; dH(CDCl3) 0.86 (t, 24 H, CH3), 1.24 (m,
80 H, CH2), 1.84 (m, 4 H, CH), 3.26 (d, 8 H, NCH2), 5.79 (s, 4 H, arom.
H), 10.94 (s, 4 H, OH). For 3b: mp 169 °C; dH(CDCl3) 0.86 (t, 24 H,
CH3), 1.24 (m, 80 H, CH2), 1.82 (m, 4 H, CH), 3.23 (d, 8 H, NCH2), 6.48
(s, 4 H, arom. H), 6.61/7.39 (AAABBA, 8 H, arom. H), 6.70/7.31 (AB, 3J
15.8, olefin H), 11.02 (s, 4 H, OH). For 3c: mp 220–225 °C; dH(CDCl2–
CDCl2, 315 K) 0.83 (t, 24 H, CH3), 1.20 (m, 80 H, CH2), 1.75 (m, 4 H,
CH), 3.10 (d, 8 H, NCH2), 6.42 (s, 4 H, arom. H), 6.54/7.23 (AAABBA,
by 85 and 80 nm, respectively. Obviously the extended
conjugation causes the hypsochromic effect. The intense
absorption bands broaden in the series from 3a (n = 0) to 3d (n
= 3). The absorption intensity S increases with growing
extension of the chromophore (Table 1).
In order to judge whether this behavior is general or not in the
series of push-pull substituted stilbenoid compounds, we made
a comparison with the known compounds 10a–d. Fig. 1 proves
the principally analogous behavior of this series. However, the
shifts Dl10 are considerably smaller than in the series 3a–d.
Probably the effect was not earlier detected due to the fact that
the compounds 10a–d were prepared by different groups.11–13
3
8 H, arom. H), 6.70/6.96 (AB, J 16.1, 4 H, olefin H), 6.74/7.18 (AB,
3J15.8, 4 H, olefin H), 7.34/7.38 (AAABBA, 8 H, arom. H), 10.88 (s, 4 H,
OH). For 3d: mp 245 °C (decomp.); dH(CDCl2–CDCl2, 313 K) 0.84 (t,
24 H, CH3), 1.20 (m, 80 H, CH2), 1.73 (m, 4 H, CH), 3.29 (d, 8 H,
NCH2), 6.47 (s, 4 H, arom. H), 6.51 (m, 4 H, arom. H), 6.79–7.25 (m,
12 H, olefin H), 7.33–7.60 (m, 20 H, arom. H), 10.95 (br s, 4 H,
OH).
9 H. Meier, U. Stalmach and H. Kolshorn, Acta Polym., 1997, 48, 379.
10 The Dn values are in both series of comparable size.
11 C. Laurence, J. Phys. Chem., 1994, 23, 5807.
12 H. Grün and H. Görner, J. Phys. Chem., 1989, 20, 7144.
13 G. Manecke and S. Lüttke, Chem. Ber., 1970, 103, 700.
14 See also W. T. Simpson, J. Am. Chem. Soc., 1951, 73, 5359 and
5326.
˜
10a n = 0
O2N
b n = 1
NMe2
c n = 2
d n = 3
n
The analysis of 3a–d in TFA leads to the result normally
observed for conjugated oligomers, namely to a monotonic
increase of lmax with growing number n. Protonation takes
place at the terminal amino groups as 13C NMR measurements
prove (Scheme 3). Thus the donor–acceptor–donor character of
3a–d is lost. The protonated series resembles the unsymmetrical
cyanine and the protonated merocyanine dyes.14,15
15 P. Rys and H. Zollinger, Farbstoffchemie, Verlag Chemie, 1982,
p. 91.
16 The enhanced polarity of the S1 state compared to the ground state S0 is
proved by a pronounced positive solvatochromic effect; 3b for example
has an absorption maximum in cyclohexane at 840 and in CH2Cl2 at 911
nm.
17 R. W. Bigelow and H.-J. Freund, Chem. Phys., 1986, 107, 159.
Conjugation with delocalisation of charge can also be
assessed via the 13C NMR data. Table 2 shows a comparison of
Communication 9/00888H
978
Chem. Commun., 1999, 977–978