Solvent-dependent fluorescence of donor-substituted (E)-1,2-bis(stilbenyl-1,3,4-oxadiazolyl)ethenes

Article abstract of DOI:10.1002/poc.523

The Huisgen reaction of aryltetrazoles and fumaryl chloride leads to symmetrically aryl-substituted 1,2-bis(1,3,4-oxadiazolyl)ethenes. Molecules with extended conjugated systems are accessible using stilbenyltetrazoles or higher homologues. The substitution with solubility-permitting alkoxy side-chains results in molecules of C_2h symmetry, consisting of a central electron-accepting segment and two terminal electron density-releasing units. The solvatochromism of the absorption spectra is negligible, while in the emission spectra a strong positive solvatochromism connected with a dramatic decrease of the quantum yield is observed, indicating intramolecular charge transfer. The influences of different alkoxy substitution patterns on the luminescence properties are discussed. Copyright

Full text of DOI:10.1002/poc.523

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2002; 15: 638–641
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.523
Solvent-dependent ¯uorescence of donor-substituted (E)-
1,2-bis(stilbenyl-1,3,4-oxadiazolyl)ethenes†
Heiner Detert,* Erli Sugiono and Gabriele Kruse
Institut fuÈ r Organische Chemie, Duesbergweg 10±14, D-55099 Mainz, Germany
Received 4 September 2001; revised 2 March 2002; accepted 15 March 2002
ABSTRACT: The Huisgen reaction of aryltetrazoles and fumaryl chloride leads to symmetrically aryl-substituted
1,2-bis(1,3,4-oxadiazolyl)ethenes. Molecules with extended conjugated systems are accessible using stilbenyltetra-
zoles or higher homologues. The substitution with solubility-permitting alkoxy side-chains results in molecules of C2h
symmetry, consisting of a central electron-accepting segment and two terminal electron density-releasing units. The
solvatochromism of the absorption spectra is negligible, while in the emission spectra a strong positive
solvatochromism connected with a dramatic decrease of the quantum yield is observed, indicating intramolecular
charge transfer. The influences of different alkoxy substitution patterns on the luminescence properties are discussed.
Copyright  2002 John Wiley & Sons, Ltd.
KEYWORDS: fluorescence; solvatochromism; heterocycles
INTRODUCTION
chains on the lateral rings improve the solubility
tremendously. However, this substitution pattern leads
to p-systems with a pronounced donor–acceptor–donor
structure.
Since the late 1980s, organic molecules with extended
conjugated systems such as poly(phenylenevinylene)s or
polythiophenes have received considerable attention
thanks to their semiconducting, luminescent and non-
linear optical properties.1–4 Oligomers with well-defined
conjugated systems are interesting as model compounds
for the polymers and are also used as electronic
materials.5 The low electron affinity of (alkoxylated)
oligo- and poly(phenylenevinylene)s is disadvantageous
for using these materials in light-emitting diodes owing
to high barriers for electron injection from aluminium
cathodes reducing the efficiencies of these devices.6,7
Additional layers of conjugated materials with a high
electron affinity, based on heterocycles such as 1,3,4-
oxadiazole,8,9 1,3,5-triazine,10,11 or quinoxaline,12,13
were used to improve the electron injection process.
Alternatively, the electron affinity of the luminescent
materials can be enhanced by substitution with electron-
accepting nitriles14,15 or sulfonyl groups.16,17 Here we
report our work on soluble oligo(phenylenevinylene)s
with additional 1,3,4-oxadiazoles in the main chain. Two
stilbenes are connected to the centre of these compounds,
a heterocyclic analogue of stilbene. Flexible alkoxy side-
SYNTHESIS
The synthesis of aryl-substituted (E)-1,2-bis(1,3,4-oxa-
diazolyl)ethenes from 5-aryltetrazoles and fumaryl
chloride via acylation of the heterocycles and thermal
ring transformation (Huisgen reaction) has recently been
reported.18 Stilbenyltetrazoles 4a–e with solubilizing
side-chains were prepared by PO-activated olefinations
of alkoxy-substituted benzaldehydes 1a–e with p-cyano-
benzyl phosphonate 2 and subsequent addition of azide to
yield the tetrazoles 4a–e. (Scheme 1) Using LiN3 in
methylglycol or DMF, excellent yields (91–97%) of
tetrazoles were obtained from simple benzonitriles and
cyanostilbene 3a, whereas the additions of azide to
alkoxylated cyanostilbenes 3b–e were slow and accom-
panied by partial cleavage of the alkyl ethers. Tetrazoles
4b–e with up to three alkoxy groups on the stilbene were
isolated in moderate to good yields when the addition of
azide was performed in toluene and in the presence of
triethylammonium ions. The title molecules 6a–e (Table
1) were assembled by acylation of the tetrazoles 4a–e
with fumaryl chloride in anisole containing stoichio-
metric amounts of pyridine and subsequent thermolysis
of the tetrazolide (80–120°C). The amine reduces the
temperature for the thermal ring transformation, but it
also induces a decomposition of fumaryl chloride 5.
*Correspondence to: H. Detert, Institut fu¨r Organische Chemie,
Duesbergweg 10–14, D-55099 Mainz, Germany.
E-mail: detert@mail.uni-mainz.de
†Presented at ESOR VIII, European Symposium on Organic
Reactivity, Dubrovnik, Croatia, September 2001.
Contract/grant sponsor: Deutsche Forschungsgemeinschaft; Contract/
grant number: DE 515/2-1.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 638–641

SOLVATOCHROMISM OF BIS(STILBENYLOXADIAZOLYL)ETHENES
639
Scheme 1
Preferentially, Huisgen reactions with 5 were performed
in the absence of a base using 1,2-dichlorobenzene as a
solvent (6b, d and e). All compounds were characterized
tion spectra of 6b–e are nearly unbiased by the solvent.
Compared with solutions in cyclohexane, only small
hypsochromic shifts (Dlmax = 2–9 nm) were recorded
with ethanol as a solvent [electronic spectra were
recorded at ambient temperature, using a MCS320/340
UV/Vis spectrometer (Zeiss) (c ꢀ 10À5 mol lÀ1) for
absorption and an LS 50B spectrometer (Perkin-Elmer)
for fluorescence spectra (c ꢀ 10À5–10À8 mol lÀ1)]. So-
lutions of 6a–e in cyclohexane [" = 2.023,
ET(30) = 30.9]19 show an intense fluorescence band
with a well resolved vibrational structure. This structure
is blurred in solvents with a slightly higher dielectric
constant, such as 1,4-dioxane [" = 2.209, ET(30) = 36.0]
or toluene [" = 2.379, ET(30) = 33.9]. The loss of
structure is accompanied by bathochromic shifts and
reduced fluorescence intensities of the alkoxylated
compounds 6b–e, whereas the fluorescence intensity
of the basic chromophore 6a is increased. In dichloro-
1
by IR, mass and H and 13C NMR spectra and gave
satisfactory elemental analyses.
ELECTRONIC SPECTRA
(E)-1,2-Bis(stilbenyl-1,3,4-oxadiazolyl)ethene 6a, re-
presenting the basic chromophore of the title com-
pounds, is an almost colourless but nearly insoluble
material which emits blue–green light upon UV
irradiation. Substitution on the terminal rings with
flexible alkoxy side-chains results in highly soluble,
yellow–orange solids, but causes a gradual (6b) to
complete loss of fluorescence for crystalline materials
6c–e. Compared with solutions of the reference mol-
ecule 6a, both the absorption and fluorescence spectra of
6b–e are shifted to lower energies, owing to the
auxochromic effect of the alkoxy groups. The absorp-
methane,
a
moderately polar solvent [" = 8.93,
ET(30) = 40.7], the fluorescence intensities of the
alkoxylated title compounds 6b–e are strongly reduced.
Table 1. Substitution pattern of (E)-1,2-bis(stilbenyl-1,3,4-oxadiazolyl)ethenes 6a±e
Compound
R1
R2
R3
CH3
R4
Yield (%)
M.p. (°C)
6a
92
87
68
86
77
368
170
165
225
189
6b
6ca
6da
6e
OC8H17
OC8H17
C(CH3)3
OC8H17
CH3
OCH3
OC6H13
OC8H17
OC6H13
OC6H13
a
OC8H17 = 2-ethylhexyloxy.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 638–641

640
H. DETERT, E. SUGIONO AND G. KRUSE
Table 2. Solvent-dependent absorption and emission spectra of 6a±ea
lmax
lmax
lmax
lf,max
(CH)
lf,max
(Dio)
lf,max
(Tol)
lf,max
(DCM)
If, rel
(Af, rel
lf,max
(EtOH)
Compound (CH) (DCM) (loge) (EtOH)
ΦF
)
6a
6b
6c
6d
6e
376 (CHCl3)
388 387 (4.87)
400 398 (4.80)
392 399 (4.92)
389 387 (4.94)
412 (683)
436 (601)
422 (962)
447 (792)
440 (560)
467 (414)
0.77 442 (948) 422 (948)
442 (825)
473 (619)
511 (213)
455 (4)
1.1 (0.7) 443 (45)
4.5 (2.4) 490 (2)
384
398
387
380
0.88 449 (771) 456 (714)
0.54 478 (456) 457 (522)
135 (78)
484 (5)
428.5 (207) 0.44 473.5 (185) 454.5 (151) 531 (0.4)
454.5 (167)
429.5 (688) 0.43 475 (619) 435.5 (319) 455 (0.5) 1280 (599) 453 (0.4)
452.5 (638) 466 (438)
460 (250) 517 (1)
a
CH = cyclohexane; DCM = dichloromethane; EtOH = ethanol (96%); Dio = 1,4-dioxane; tol = Toluene; lmax = absorption maximum (nm); lf,max = emission
maxima (nm) and intensities (a.u.); If, rel = ratio of fluorescence intensities at lf,max of solutions of a compounds in cyclohexane and in CH2Cl2; Af, rel = relative
intensities of the fluorescence bands in these solvents.
Depending on the number and position of the peripheral
auxochromic groups, the intensities at lf,max are reduced
to values of 20% (6b) to less than 1% (6e) compared
with solutions of these compounds in cyclohexane.
Considering the broadening of the fluorescence band,
the residual fluorescence is in the range of 40% (6b) to
2% (6e). In contrast to the alkoxylated compounds, the
fluorescence band of the basic chromophore 6a in
CH2Cl2 is about 50% over that recorded from solutions
in cyclohexane [Af (CH)/Af (DCM) = 0.7] In ethanol
[" = 24.55, ET(30) = 51.9], the fluorescence of all 1,2-
bis(1,3,4-oxadiazolyl)ethenes is barely detectable; even
the emission of 6a is reduced to about 7%. Details of the
spectra are given in Table 2.
The fluorescence quantum yields were calculated from
fluorescence intensities of solutions of 6a–e in cyclohex-
ane and compared with the intensity of quinine sulphate
(Φf = 0.577) in aqueous 0.1 M H2SO4 and corrected for
the refractive indices.20 Intense fluorescence is observed
with solutions of the basic chromophore 6a (Φf ꢀ 0.8),
substitution of 6a with two alkoxy groups results in a
further increase in the fluorescence quantum yield (6b:
Φf ꢀ 0.9), whereas the introduction of additional auxo-
chromes substantially reduces the fluorescence (6c–e:
Φf ꢀ 0.4–0.5).
In addition, the introduction of several peripheral
alkoxy groups limits the photostability. The absorption
and fluorescence of those compounds showing the
highest response on solvent polarity and the lowest
fluorescence quantum yields, 6d and e, slowly decrease
upon continued exposure to UV radiation (366 nm,
cyclohexane).
DISCUSSION
Conjugated systems composed of an electron-deficient
heterocyclic analogue of stilbene and two lateral stilbene
units were investigated. The alkoxylated (E)-1,2-bis-
(stilbenyl-1,3,4-oxadiazolyl)ethenes are molecules with a
distinct donor–acceptor–donor structure. The influence of
the dielectric constant of the solvent on the ground state
of these molecules is only small, but the pronounced
solvatochromism of the fluorescence indicates severe
interaction of the excited state with the solvent mol-
ecules. These shifts of the emission to lower energies can
Figure 1. Solvent-dependent electronic spectra of 6b and c
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 638–641

SOLVATOCHROMISM OF BIS(STILBENYLOXADIAZOLYL)ETHENES
641
be attributed to a stabilization of a charge-transfer state
by reorientation of the solvent molecules.19 As the
structure of the fluorescence bands of 6a–e changes
completely with solvent polarity, no meaningful correla-
tion of Dlf,max and a solvent parameter can be obtained.21
In addition to the influence on the energy of the emitted
light, solvent polarity has a strong impact on the
fluorescence quantum yield. The intense light emission
from solutions in non-polar solvents (cyclohexane,
dioxane) is reduced to negligible values in the polar
and protic ethanol. Using cyclohexane and the moder-
ately polar dichloromethane as reference solvents, the
ratios of the fluorescence intensities [If (CH)/If (DCM); Af
(CH)/Af (DCM)] discriminate 6a–e according to the extent
of the increasing donor–acceptor–donor structure of these
compounds. Whereas Φf of the basic chromophore 6a
remains nearly constant in these solvents, progressive
introduction of donor groups gradually reduces the
individual quantum yield. The fluorescence intensity of
6e in CH2Cl2 is reduced to about 1% of its intensity from
solutions in cyclohexane. This behaviour is closely
related to the positive solvatochromism and reduction
of fluorescence intensities found for cyano-substituted
oligo(phenylenevinylene)s with a pronounced acceptor–
donor–acceptor structure. In this series, a minimum
fluorescence intensity of 5% was preserved even in
acetonitrile.15
The substitution of a fluorescent p-system with
auxochromes is known to increase the fluorescence,22
and this also holds for the first set of alkoxy groups
comparing 6a (Φf = 0.8) with 6b (Φf = 0.9). However,
additional ether groups reduce the fluorescence quantum
yield to about half the value (6c–e: Φf = 0.4–0.5). The
impact of polarity on Φf parallels the fluorescence
behaviour of these compounds in different solvents: an
enlarged donor–acceptor–donor character (6b–e) and an
increased solvent polarity favour a charge transfer in the
excited state. An enhanced radiationless decay results
from this stabilization.
stitution with solubilizing alkoxy side-chains. This
favours a charge transfer in the excited state, resulting
in solvatochromism of the emission and a marked decline
in the fluorescence quantum yield in polar solvents.
Acknowledgement
This work was supported by the Deutsche Forschungs-
gemeinschaft, grant No DE 515/2-1.
REFERENCES
1. Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K,
Friend RH, Burn PL, Holmes AB. Nature (London) 1990; 347:
539–541.
2. Jenekhe SA, Wynne KJ (eds). Photonic and Optoelectronic
Polymers. ACS Symposium Series, vol. 672. American Chemical
Society: Washington, DC, 1997.
3. Kraft A, Grimsdale AC, Holmes AB. Angew. Chem. 1998; 110:
416–443; Angew. Chem., Int. Ed. Engl. 1998; 37: 402–408.
4. Gurge RM, Hickl M, Krause G, Lahti PM, Hu B, Yang Z, Karasz
FE. Polym. Adv. Technol. 1998; 9: 504–510.
5. Mu¨llen K, Wegner G. Electronic Materials: the Oligomer
Approach. Wiley-VCH: Weinheim, 1998.
6. Cacialli F, Li X-C, Friend RH, Moratti SC, Holmes AB. Synth.
Met. 1995; 75: 161–168.
7. Cornil J, Beljonne D, Dos Santos DA, Bre´das JL. Chem. Eur. J.
1997; 3: 287–293.
8. Adachi C, Tsutsui T, Saito S. Appl. Phys. Lett. 1990; 57: 531–533.
9. Berggren M, Granstro¨m M, Ingana¨s O, Andersson M. Adv. Mater.
1995; 7: 901–903.
10. Fink R, Frenz C, Thelakkat M, Schmidt H-W. Polym. Prepr. 1997;
38: 323–327.
11. Po¨sch P, Fink R, Thelakkat M, Schmidt H-W. Acta Polym. 1988;
49: 487.
12. Bettenhausen J, Greczmiel M, Jandke M, Strohriegl P. Synth. Met.
1997; 91: 223–225.
13. Yamamoto T, Inoue T, Kanbara T. Jpn. J. Appl. Phys. 1994; 33:
L250–252.
14. Greenham NC, Moratti SC, Bradley DDC, Friend RH, Holmes AB.
Nature (London) 1993; 365: 628–630.
15. Detert H, Schollmeyer D, Sugiono E. Eur. J. Org. Chem. 2001;
2927–2938.
16. Do¨ttinger SE, Hanack M, Tompert A, Oelkrug D. Adv. Mater.
1997; 9: 233–236.
17. Detert H, Sugiono E. Synth. M. 2001; 122: 15–17.
18. Detert H, Schollmeyer D. Synthesis 1999; 999–1004.
19. Reichardt C. Solvents and Solvent Effects in Organic Chemistry,
2nd edn. VCH: Weinheim, 1988; 286–311.
CONCLUSION
20. Lackowicz JR. Principles of Fluorescence Spectroscopy, 2nd edn.
Kluwer: New York, 1999.
21. Suppan P, Ghoneim N. Solvatochromism. Royal Society of
Chemistry: Cambridge, 1997; 96–143.
22. Fo¨rster Th. Fluoreszenz Organischer Verbindungen. Vandenhoek
& Ruprecht: Go¨ttingen, 1951; 94–122.
1,2-Bis(1,3,4-oxadiazolyl)ethenes with extended conju-
gated systems were prepared via Huisgen reaction of
stilbenyltetrazoles. Chromophores with a pronounced
donor–acceptor–donor structure are obtained upon sub-
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 638–641