Acidochromism of stilbenoid chromophores with a p-aminoaniline centre

Article abstract of DOI:10.1002/poc.1091

Oligo(phenylenevinylene)s (OPV) with a 2,5-diaminobenzene centre were prepared via twofold Horner olefinations of substituted terephthalaldehydes. Whereas variations of the environment only slightly alter the electronic excitation spectra, the fluorescenc

Full text of DOI:10.1002/poc.1091

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 603–607
Published online 17 July 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1091
Acidochromism of stilbenoid chromophores
with a p-aminoaniline centrey,z
*
Heiner Detert and Volker Schmitt
Institut fu¨ r Organische Chemie, Duesbergweg 10–14, D-55099 Mainz, Germany
Received 10 October 2005; revised 6 April 2006; accepted 7 April 2006
ABSTRACT: Oligo(phenylenevinylene)s (OPV) with a 2,5-diaminobenzene centre were prepared via twofold Horner
olefinations of substituted terephthalaldehydes. Whereas variations of the environment only slightly alter the
electronic excitation spectra, the fluorescence spectra appear to be highly responsive. Besides a positive solvato-
chromism, the emission is very sensitive towards protonation. Quenching or appearance of new emitting species
depends on the substitution pattern and is controlled by the concentration of the acid. Very large Stokes shifts indicate
extensive structural changes in the excited states. Copyright # 2006 John Wiley & Sons, Ltd.
KEYWORDS: fluorescence; solvatochromism; acidochromism; halochromism; conjugated oligomers
INTRODUCTION
central phenylene-1,4-diamine unit and the impact of
interaction with protons on their optical spectra.
The substitution of conjugated polymers and oligomers
with side chains of different types has been widely used to
improve solubility and mechanical behaviour of these
materials.1,2 Another important aspect is the directed
tuning of their electrical and optical properties for
applications in optoelectronical devices. The combination
of electron-pair-donating (EPD) and electron-pair-
accepting (EPA) substituents can lead to materials for
non-linear optical applications such as second harmonic
generation or two-photon absorption.3–5 In this context,
amino groups, being efficient and typical donor groups,
are often combined with strong acceptors like nitro,
cyano, or sulfone groups. The donating ability of amines
is inverted upon protonation or quaternisation, particu-
larly the pyridinium ion is a common acceptor unit,6,7 but
ammonium ions have also received considerable atten-
tion.8 The capability of amino groups to act as a donor or
acceptor depending on environmental conditions can be
used to change the optical properties of a chromophore
directly attached to the basic site. Complexation with
Lewis acids or protonation has been shown to cause
significant changes of the UV/Vis-absorption and emis-
sion spectra of p-conjugated chromophores with the basic
site on the terminal positions.9–12 Herein, we report the
synthesis of oligo(phenylenevinylene)s (OPVs) with a
SYNTHESIS
The condensation and in situ oxidation of primary amines
with dimethyl cyclohexane-2,5-dionedicarboxylate 1
allows a rapid entry to 2,5-diaminoterephthalic esters
2,3.13 Further groups attached to the amine can be
introduced via acylation or, in case of arenes, via Ullmann
coupling14 to yield 4–6. LiAlH4 reduces the esters and
amides to give the 1,4-diaminobenzenedimethanols,
which, in a twofold Swern oxidation, were converted
to the intensively coloured terephthalaldehydes 7–9.
These dialdehydes were used as bifunctional units for
Horner olefinations with 1-hexyloxybenzylphosphonate
(10) and 1-hexyloxystilbenylmethylphosphonate (11).
Recrystallisation gave pure 12–17 without detectable
traces of cis-isomers15 (Scheme 1).
ELECTRONIC SPECTRA
The solid distyrylbenzenes (DSBs) 12–14 are light yellow
crystalline compounds which are good soluble in solvents
like toluene or dichloromethane. Extension of the p-
system to five phenyl rings (OPV-5) reduces the solubility
and shifts the colour to orange. Only 17, carrying two
diphenylamino groups, forms yellow crystals. Solutions
of these compounds in dichloromethane are yellow (12–
14), yellowish-orange (16, 17) or orange (15) and all are
fluorescent.
*Correspondence to: Dr. H. Detert, Johannes Gutenberg-Universitat,
Duesbergweg 10–14, 55099 Mainz, Germany.
E-mail: detert@mail.uni-mainz.de
yDedicated to Prof. Dr. R. C. Schulz, Mainz, on the occasion of his 85th
birthday
zSelected paper presented at the 10th European Symposium on Organic
Reactivity, 25–30 July 2005, Rome, Italy.
The UV/Vis-absorption spectra of these diamino-OPVs
are dominated by a low energy band with a maximum
Contract/grant sponsor: Deutsche Forschungsgemeinschaft; contract/
grant number: DE 515/5-1.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 603–607

604
H. DETERT AND V. SCHMITT
Scheme 1. Synthesis of diamino-OPVs
around l ¼ 360 nm for the DSBs and l ¼ 410 nm for the
OPV-5, respectively.15 Both series show an additional
shoulder on the red side of this band, which is further red-
shifted in the sequence di(1-propyl)amino—phenyl(1-
propyl)amino—diphenylamino. Compared with similar
three- and five-ring OPVs carrying only alkoxy donors on
the central and terminal positions (RO-OPVs),16 the
absorption maxima of 12–17 are strongly shifted to higher
energies (Dl ꢀ 30–40 nm). According to the molecule
symmetry, solvatochromism of the absorption is negli-
gible. The broad fluorescence spectra are separated from
the excitation bands by huge Stokes shifts. These
stabilisations of Dey ¼ 7700–9700 cmꢁ1 for the DSBs
12–14 and still more than Dey ¼ 6000 cmꢁ1 in the five-ring
series (15–17) indicate large structural changes in the
excited state. The related RO-OPVs and even acceptor-
donor-acceptor substituted OPVs17 show substantially
smaller Stokes shifts which are in the range of
Dey ¼ 2600–3400 cmꢁ1. Since the fluorescence of all
compounds is positive solvatochromic, an additional
mechanism to stabilise the first excited state is probably
an intramolecular charge transfer.18 Bathochromic shifts
of the emission extend to Dl ꢀ 30 nm upon changes of the
solvent from C6H12 [ET(30) ¼ 30.9] to CH2Cl2
[ET(30) ¼ 40.7] or acetonitrile [ET(30) ¼ 45.6] but inverts
in protic solvents to a negative solvatochromism
(Dl ꢀ 10 nm).19 In addition to the bathochromic shifts,
the efficiencies decrease by ca. 75% relative to the
efficiencies in C6H12, but in the protic solvent ethanol not
only the emission maxima are shifted to the blue, but also
the efficiencies recover the values found for solutions in
toluene. Whereas the fluorescence of the DSBs 12–14
appears as
a single, broadband centred around
lmax ¼ 510 nm, the main emission of higher homologues
is a broadband in the region of lmax ¼ 540 nm, but
accompanied by a weak (15, 16) or very weak (17),
structured emission between l ¼ 400 and 450 nm. This
high-energy emission of 15 is invisible in cyclohexane but
becomes more prominent in dichloromethane or ethanol,
those of 16, 17 are nearly unbiased by solvent polarity
(Table 1).
An addition of strong acids such as trifluoroacetic acid
(TFA) to solutions of 12–17 in dichloromethane results in
a protonation of the amino-OPVs, thus changing the
strongly electron-donating amino groups to weak
acceptors. Since (de)-protonation can occur in the ground
as well as in the excited state, absorption and emission
spectra can be altered independently.20 Whereas pheny-
lenevinylene chromophores are fairly stable towards light
or acid, UV-irradiation of solutions with traces of acids
initiated a fast and irreversible decomposition of these
materials.21 Chromophores with basic sites like 12–17
show a much higher photostability, even in very
acidic solution. Protonation was studied by UV–Vis
and fluorescence spectroscopy of the OPVs dissolved
in dichloromethane containing 10ꢁ6–1 molar concen-
trations of TFA. The impact of Brønsted acids on the
UV/Vis-absorption spectra of 12–17 is mainly a
reduction of the extinction coefficient and a loss of the
low-energy shoulder. Contrary to the behaviour of the
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 603–607

ACIDOCHROMISM OF STILBENOID CHROMOPHORES
Table 1. Solvatochromism of the diamino-OPVs 12–14
605
l
max/nm DCM
Dey/cmꢁ1 DCM
lFmax/nm CH
lFmax/nm Tol
lFmax/nm DCM
lFmax/nm AN
lFmax/nm EtOH
12
13
14
15
16
17
352 (398)
365 (417)
366 (422)
394 (416)
408 (445)
399 (450)
9683
8166
7789
6117
6427
6336
505
488
484
534
534
510
526
501
497
543
540
526
534
520
510
558
553
534
534
520
511
565
549
537
526
508
499
544
542
530
DCM, dichloromethane; CH, cyclohexane; Tol, toluene; AN, acetonitrile; EtOH, ethanol.
dipropylamino-OPVs (12, 15), interaction of protons with
propylphenylamino- (13, 16) and diphenylamino-OPVs
(14, 17) results in the build-up of a new maximum around
lmax ¼ 340 nm and, at very high concentrations of TFA
(0.1–1 M), of a broad but weak long-wavelength band
around lmax ¼ 540 nm (13, 14) or 560 nm (16, 17)
(Table 2).
The emission spectra of these chromophores are
strongly influenced by the presence of TFA. A gradual
quenching of the emission of DSBs with phenylamino
side groups (13, 14) occurs upon the addition of acid.
Compared to neutral solutions, the fluorescence intensity
is reduced to ca. 60% in 10ꢁ3 M TFA and vanishes at
higher TFA concentrations. Chromophore 12, carrying
the more basic (in the ground state) dipropylamino groups
shows an entirely different behaviour. In the presence of
traces of TFA (10ꢁ6 M) the long-wavelength band is
retained but a prominent shoulder at lmax ¼ 478 nm
arises-resulting in an increase of the fluorescence about
35%. This shoulder becomes the sole maximum (relative
intensity equal to neutral solution) in slightly higher
concentrations of TFA (10ꢁ5 M), the initial emission band
turns to be now invisible. A further increase of the amount
of TFA results in a gradual decrease of this band to a
residual intensity of ca. 8% in highly acidic solutions
(0.1–1 M TFA). Figure 1 shows the titration curves of 12,
observed in the absorption and in the fluorescence at two
different wavelengths each.
A related behaviour was found in the homologous
series 15–17. The emissions of 16, 17 are nearly
unaffected by TFA up to 10ꢁ3 M. At higher concen-
trations, the fluorescence of 17 is shifted to the red and
becomes less efficient and is finally completely quenched
(10ꢁ1 M) (Fig. 2).
A 10ꢁ2 M TFA solution is sufficient to quench the
l ¼ 530 nm emission of 16 but does not alter the weak
emission in the l ¼ 430 nm region. This band is reduced
to ca. 50% in 10ꢁ1 M TFA and disappears only in acid as
strong as 1 M TFA. The emission of dipropylamino-OPV
15 in dichloromethane has a broad maximum at
lmax ¼ 558 nm with an efficiency of only 60% relative
to the propylphenyl analogue 16. The addition of traces of
TFA (10ꢁ5 M) changes the fluorescence properties
fundamentally. The long-wavelength maximum vanishes
and a strong band centred at lmax ¼ 480 appears, the
intensity increases by a factor of 6. Apart from a
Table 2. Acidochromism of the diamino-OPVs 12–14
DCM
10ꢁ6 M TFA
10ꢁ5 M TFA
10ꢁ4 M TFA
10ꢁ3 M TFA
10ꢁ2 M TFA
10ꢁ1 M TFA
max/nm
1 M TFA
max/nm
l
max/nm
lmax/nm
lmax/nm
l
max/nm
lmax/nm
lmax/nm
l
l
12
13
14
15
16
17
352 398
365 417
366 422
394 416
408 445
399 450
354
358
357
364
365
408
406
402
353
365
364
408
407
398
366
328
328
417
404
398
363
362
325 528
329 534
418
329 558
328 559
318 523
399 523
408
328 552
328 555
416
408
lFmax/nm
lFmax/nm
lFmax/nm
lFmax/nm
lFmax/nm
lFmax/nm
lFmax/nm
lFmax/nm
12
13
14
15
16
17
534 (1)
530 (1.34)
478 (0.99)
475 (0.68)
519 (1.15)
511 (1.53)
483 (1.80)
538 (0.45)
536 (0.80)
472 (0.35)
517 (0.79)
510 (0.92)
480 (1.60)
534 (0.47)
533 (0.91)
467 (0.16)
n.f.
498 (0.13)
486 (1.52)
433 (0.18)
506 (0.46)
478 (0.08)
n.f.
n.f.
531 (0.57)
438 (0.08)
n.f.
479 (0.08)
n.f.
n.f.
533 (0.17)
n.f.
n.f.
520 (1.25)
510 (1.69)
558 (0.29)
537 (0.52)
534 (0.75)
492 (0.70)
488 (1.77)
TFA: trifluoroacetic acid in dichloromethane, italics: second maximum, values in brackets: relative fluorescence intensities, normalized to solutions of equal
optical density at ¼ 345 nm and referenced to 12 in dichloromethane ¼ 1.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 603–607

606
H. DETERT AND V. SCHMITT
fluorescence intensity. This holds for aprotic solvents, in
ethanol the fluorescence spectra and intensities are similar
to those from solutions in toluene. Enormous Stokes shifts
separate the fluorescence maxima from the excitation
maxima. Compared to the related RO-OPVs (15), the
bathochromic shifts of the emission of 12–17 were
expected since the amine is a powerful auxochrome.
Contrary to the fluorescence, the absorption is shifted to
higher energies compared to RO-OPVs. Though the
amines are potent auxochromes, distortions of the p-
system due to steric interaction of the substituents on the
amines and the ortho-vinylene moieties predominate.
Dihedral angles of 33–508 between the planes of the
central benzene ring and the adjacent vinylene groups of
12–14 were calculated using AM1. The lone pair of the
nitrogen is twisted out of the conjugation with the
benzene ring by 448 (12) to 678 (14). Comprehensive
changes of the geometry—and therefore of the conju-
gation within the OPV and of the OPV with the
auxochromes—can occur in the excited state resulting
Figure 1. Titration curve of 12
successive red shift of Dl ¼ 8 nm and variations of the
relative intensity (ꢂ10%), this emission characterises
solutions containing 10ꢁ5 to 10ꢁ3 M TFA. The same band
still dominates the emission of 15 in 10ꢁ2 M TFA. Here, a
shoulder emerges on the low-energy flank (530 nm) and
in Stokes shifts of Dey ¼ 6000–9700 cmꢁ1
.
The relative fluorescence intensities of the DSBs 12–14
are significantly higher than of the homologous OPV-5
15–17, a similar behaviour has been observed with RO-
OPVs. Phenylamino-OPVs are stronger fluorescent
(14 > 13; 17 > 16) than those with propylamino groups
(12, 15), this may be attributed to the increasing energy
gap between excited states and ground states and to the
stiffness of the phenyl rings compared to propyl chains.
In general, the acidochromism of the excitation spectra
is comparatively small, the main absorption is reduced in
intensity and shifted to the blue, both resulting from the
reduced auxochromic effect of the diaminophenylene
when protonated. The appearance of a weak band at much
longer wavelengths (lmax ¼ 540–560 nm) upon protona-
tion of 13, 14 and 16, 17 is unique for the N-phenyl-
substituted chromophores and indicates a significant
electronic interaction of the lateral aniline moieties and
the phenylenevinylene strand. Indeed, AM1 calculations
show a participation of the orbitals of the phenyl side
develops to be the absolute maximum in TFA of 10ꢁ1
–
1 M strength. 15 shows two distinct alterations of the
fluorescence due to the presence of TFA. The transitions
between the three different emitting species occur at 10ꢁ6
and 10ꢁ2 M TFA, spectra obtained from these solutions
are superimposed spectra of 15 in dichloromethane and
10ꢁ5 M TFA, and 10ꢁ3 and 10ꢁ1 M TFA, respectively.
DISCUSSION
OPVs with a 2,5-diaminobenzene centre and terminal
alkoxy groups can be regarded as quadrupolar chromo-
phores with two weak and two strong donor groups. Like
other quadrupolar substituted OPVs, solvatochromism of
the UV/Vis-absorption spectra is very small, but the
influence of a polar environment is visible as a positive
solvatochromism of the fluorescence and reduced
Figure 2. Emission spectra of 12 and 17 in dichloromethane containing TFA. Excitation at l ¼ 345 nm
Copyright # 2006 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2006; 19: 603–607

ACIDOCHROMISM OF STILBENOID CHROMOPHORES
607
chains in the wave functions of HOMO, LUMO and
energetically close orbitals of the neutral and protonated
chromophores. Regarding the emission, a successive
quenching of the fluorescence of 13, 14, 16 and 17 with
increasing concentrations of acid results. Protonation is
the most likely additional process and might be
responsible for the quenching. Since the degree of
fluorescence quenching is higher than the change of the
absorption spectrum, it can be concluded, that the basicity
of the excited diamino-OPVs is higher than the basicity of
the ground-state molecules. This is in contrast with the
observations that the basicity of aniline-type bases is
reduced about several orders of magnitude upon
excitation. The dipropylamino-OPVs 12 and 15 are the
most interesting in this series. The fluorescence of 12
shows a new band at very low concentrations of TFA,
already at 10ꢁ6 M this band appears as a strong shoulder
and reaches maximum intensity at ca 10ꢁ5 M TFA.
Further interaction with protons gives a successive
quenching of this band.
UV/Vis-absorption spectra. Contrary to other aniline-type
bases, the basicities of the excited states of these
systems appear to be higher than of their ground states.
Since concentrations as low as 10ꢁ6 M TFA change the
emission behaviour of 12 and 15 significantly, these dyes
are potent fluorescent sensors for changes in the local
environments.
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The fluorescence of phenylenevinylene chromophores
with a central p-aminoaniline moiety is much more
sensitive towards changes of the environment than the
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 603–607