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Luminescence Stimulated by Electron Transfer Fluorescent DonorAcceptor-Substituted Stilbenes Containing Pyrenoid and Heteroaromatic Subunits.

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COMMUNICATIONS
M. G . Davidson, A. J. Edwards, M. A. Paver, P. R. Raithhy, C. A. Russell, A.
I
Chem. Soc. Chem. Commun. 1995,
Steiner, K. L. Verhorevoort, D. S. Wright, .
1989.
M . A. Matchett. M. Y Chiang, W. E. Buhro, fnorg. Chem. 1994, 33, 1109.
S. C . Goel. M. Y. Chiang, D. J. Rauscher, W. E. Buhr0.J Am. Chem. Soc. 1993,
115, 160.
B. L. Benac, A. H. Cowley, R. A. Jones, C. M. Nunn. T. C. Wright, J. Am.
Chem. Soc. 1989, ill, 4986.
A. Eichofer, J. Eisenmann, D . Fenske. F. Simon, Z . Anorg. Allg. Chem. 1993,
619, 1360.
K. Issleih, G. Doll, Chem. Ber. 1963, Y6, 1544.
Crystal data for I : C,,,H,,,Cd,Li,08P,,~ 5.5THF. M = 3288.58, triclinic.
space group Pi, u = 17.261(6), b = 17.996(6), c = 29.470(13) A, a = 88.89(3),
fi = 89.14(2), y = 61.85(2)”, V = 8070(5)A’, Z = 2, p,,,, =1.353 M ~ I I - ~ ,
E. = 0.71073 A, T = 1 5 3 K, p(Mo,,) = 0.679 mm-’. Data was collected on a
Siemens-Stoe AED diffractometer using an oil-coated, rapidly cooled crystal
of dimensions 0.4 x 0.3 x 0.3 mm mounted directly from solution 1131; S/w
method (5” 5 20 5 45”). The data were corrected for absorption by a semiempirical method based upon P-scan data with maximum and minimum transmission of 0.908 and 0.775, respectively. Of 21 897 reflections collected, 21 131
were independent. The structure was solved by direct methods (SHELXTL
PLUS) and refined by full matrix least squares on F 2 ;R1 [F > 4a(F)] and wR2
(all data) to 0.0729 and 0.1656, respectively (SHELXL-93, Gottingen, 1993).
All the non-hydrogen atoms in the anion of 1 were refined with anisotropic
displacement parameters. One of the [Li(thf),]+ cations and 3.5 of the 5.5
independent noncoordinated T H F molecules are disordered on two positions,
halfof them on the inversion center. All five non-hydrogen atoms ofeach T H F
molecule in the unit cell were refined with the same isotropic parameters and
distance restraints. Hydrogen atoms were set geometrically. Maximum and
minimum residual electron density in the final difference map. 2.018 and
- 1.103 e k 3 . respectively. Further details of the crystal structure investigation may he obtained from the Director of the Cambridge Crystallographic
Data Centre, 12 Union Road. GB-Cambridge CB2 1EZ (UK), on quoting the
full journal citation.
T. Kottke, D . Stalke, J. Appl. Crystullogr. 1993, 26, 615.
P. A. W. Dean, J. J. Vittal, N. C. Payne, Inorg. Chem. 1987, 26, 1683.
J. L. Hencher. M. A. Khan, F. F. Said, D . G. Tuck, Polyhedron 1985, 4, 1263.
P. A. W. Dean, J. J. Vittal, Y Wu, Cun. J Chem. 1992, 70, 779.
P. A. W Dean, N. C. Payne, J. J. Vittal, Y.Wu, Inorg. Chem. 1993, 32, 4632.
H. Burger. W. Sawdony. U. Wannagat. J. Organomet. Chem. 1965, 3, 133.
R. Richter, J. Kaiser, J. Sieler, H. Hartung, C. Peter, Actu Crysrallogr. Sect. B
1977.33, 1887.
anions. In the broadest sense, this structural principle is responsible for the basic process of artificial and natural photosynthesis (photochemically induced charge separation)[’I and also serves as a basis for developing model compounds.[31The energy
stored in radical anion/radical cation pairs can also lead to the
emission of electromagnetic radiation, a process that occurs in
solution as electrochemically generated luminescence (ECL) .r41
This process is at present under intensive investigation in view
of the application of polymeric and oligomeric compounds in
light-emitting devices.t51We report here on specially designed
donor/acceptor-substituted stilbenes. The “optoelectronic”
multifunctionality of these compounds is determined by the
electron transfer active donor/acceptor subunits and by the luminescent substructure. These stilbenes have been studied in
detail by using electrochemical and optoelectrochemical methods to clarify the relation of electron transfer, electronic structure, and molecular structure. The long-range goal is to develop
substances and materials for the conversion of electrochemical
energy into photonic energy.L6]
Stilbenes 1-4 contain the pyrene chromophore as an acceptor
group and phenothiazine, thianthrene, dibenzodioxine, or phenoxathiin building blocks as donor groups. These combined units
Me
Emission
rBu
tBu
Luminescence Stimulated by Electron Transfer :
Fluorescent Donor/Acceptor-Substituted
Stilbenes Containing Pyrenoid and
Heteroaromatic Subunits**
Andreas Knorr and Jorg D a u b *
Dedicated lo Professor Paul von Rag& Schleyer
on the occasion of his 65th birthday
The donor/acceptor concept has gained fundamental meaning for the chemistry and the physics of dyes because it describes
both the absorption and the nonlinear optical (NLO) effects
(e.g. molecular hyperpolarization) of compounds with auxochromic and antiauxochromic groups.[” Donor and acceptor
subunits not only lead to the polarization of conjugated n-electron systems, they can also participate in the exchange of electrons, resulting in the formation of radical cations and radical
[*I Prof. Dr. J. Daub, Dr. A. Knorr
lnstitut fur Organische Chemie der Universitit
Universititsstrasse 31, D-93040 Regensburg (Germany)
Telefax: Int. code + (941) 943-4984
e-mail: joerg.daubidchemie.uni-regensburg.de
[**I A. K. is grateful to the Stiftung Stipendien-Fonds des Verhandes der Chemischen Industrie, Frankfurt, for a doctoral fellowship. This project was supported by the Bundesministerium fur Forschung und Technologie (grant number
03N1004C6) and the Bayerische Staatsregierung (Sonderprogramm fur Infrastrukturmassnahmen).
2664
0 VCH
~rlagsgesellschuftmbH. 0-69451 Weinheim. 1995
X=S,Y=S 2
X=O,Y=O 3
X=S,Y=o 4
tBu
are expected to give rise to an adequate emission.[’] For comparison, pyrenyl vinyl anthraquinone 5 is included in the investigations..
Compounds 1-5 were synthesized by Wittig reactions of
)
were separated by consuitable precursors. The ( E ) / ( Z isomers
ventional column chromatography on silica
Cyclic voltammetry shows that it is not the heterocyclic but
rather the pyrene subunit that is responsible for the formation
of the radical cations of 2-4 by oxidation; the radical cations
are formed at a potential typical of pyrenes and the process is
irreversible. The oxidation of the anthraquinone derivatives
(Z)-5and (E)-5is also irreversible. In the case of the phenothiazine derivative 1, the reversible oxidations observed are typical
of phenothiazines. In compounds 1-4, the electron transfer in
the reduction region is determined by the pyrene substructure,
as expected. On the other hand, the anthraquinone derivatives
(Z)-5 and (E)-5 show three reversible reduction waves in the
cyclic voltammogram: the first two electron transfers can be
assigned to the anthraquinone and the third electron transfer to
the pyrene (Table 1 ) .
Generally, in all electrochemical processes of 1-5 one observes a shift of the I El values of the electron transfer processes to
0570-0833/95/3423-2664 $10.00+ ,2510
Angew. Chem. Int. Ed. Engl. 1995, 34, No. 23/24
COMMUNCATIONS
Table 1 . Selected electrochemical data for compounds 1 -5 obtained by cyclic voltammetry. FOC' = ferrocene.
o\
235
573
-2339
- 2570
223
569
-2306
-2585
546
- 1320
-1905
- 2466
530
- 1297
- 1844
- 2462
1
2
Red. 1
Red. 2
Or. 1
Ok.
ox.
1-
Rcd. 1
Rcd. 2
ox.
Rcd 1
Red. 2
Red. 3
ox.
Red. 1
Red. 2
Red. 3
0h
Red. 1
Rcd. 2
Uk.
-2176
- 2450
595
-2215
-2510. -2600
605
Red. I
Red. 2
- 2235
- 2605
O h
Red. I
Red. 2
326
686
-2241
-2525.
287
686
-2238
-2530.
840
-1250
- 1826
- 2408
755
-1236
-1783
-2381
705
-2113
-2347
680
-2154
-2530.
685
-2176
-2535.
-710,
280
630
-2290
-695
255
625
-2270
-481
~
~
- 1285
-1865
-2435
~
- 1265
-1815
- 2420
~
-
2145
particular, the longest wavelength absorptions of 1 -5 are significantly red-shifted with respect to those of the unsubstituted
subunits (Table 2). This indicates that the optical properties are
not determined by substructures but by the whole chromophore. However, in case of (2)-5 similarities with the unsubstituted anthraquinone are apparent, indicating a localized anthraquinone/semiquinone spectrum, which can be explained by
a nonplanar structure and pronounced twisting between the
substructures.
The cis-stilbenes (Z)-1 and (Z)-5show a photochemical "oneway" isomerization:"' irradiation of the ( Z )compounds in solution (cyclohexane and acetonitrile) leads exclusively to the ( E )
isomers, as proven by a comparison with the spectra of the pure
( E ) isomers. In contrast, irradiation of the (E)isomers has no
effect on the UV spectra. The spectroelectrocheinical studies of
these cisoid stilbenes indicate that the oxidation of (Z)-l is accompanied by a cisitrans isomerization at the stage of the radical cation. After one oxidationireduction cycle the UV spectrum
of the neutral form of (E)-1 is obtained (Scheme 1). The two-
~
~
-2185
-495
~
- 2205
-2440.
-505
[a] Half-wave potentials E l , , [mV] of the parent compounds: pyrene: Red. -2535.
Ox. ( E J 910; N-methylphenothiazine: Ox. 325; 9,10-anthraquinone: Red. 1
- 1320. Red. 2 - 1920; thianthrene: Ox. 835; dioxine: Ox. 995; phenoxathiin: Ox.
800.
(z)-5*-
lower potentials in comparison to those of the unsubstituted
electrophores. This indicates that the energy of the highest occupied molecular orbital increases and/or the energy of the lowest
unoccupied molecular orbitals decreases in comparison to those
of the parent structures (Table I ) .
This observation is again confirmed by spectroelectrochemical studies. Reversible processes on the time scale of electrochemistry are found for the oxidation of ( E ) - 1 to its radical
cation as well as for the reduction of 2-5 to their radical anions.
In all cases both the wavelength of the absorption bands and the
structure of the spectra are remarkably different from those
from the spectra of the unsubstituted electrophores (Table 2). In
Table 2. UV;Vis:NIR data of the radical ions and UV;Vis data of the neutral
compounds from spectroelectrochemical experiments. Isosb. pt. = isosbestic point.
~
~~
Process [a]
SN
+
An -An.(2)-5-(2)-5'(E)-S - ( E j - Y
P + P.2-2'3-3'-
398 (4.5). 358 (sh), 287 (4.4).
236 (4.6)
425
275
436
27X
Scheme 1. Isomerization of (Z)-1and (Z)-S during irradiation and electron transfer.
step reduction/reoxidation of (Z)-5via the dianion leads to the
UV spectrum of (E)-5, indicating ( Z ) / ( E )isomerization of the
dianion 5'-. At the end of the spectroelectrochemical experiment, the UV absorptions of (Z)-5 are found. In contrast, the
radical anion of (23-5 is configurationally stable. Thus, one can
conclude that the "one-way" isomerization may be effected not
only photochemically but also by both reductive and oxidative
electron transfer processes.
In addition to the electrochemical and spectroelectrochemical
behavior of these compounds, their fluorescent properties are of
interest. (Experimental optical data: Table 3; for conditions see
Experimental Procedure.)
~
Neutral cmpd.
L [nm] (log E ) [b]
SN"
(€)-I +(Ej-l.'
4-4'-
-
~~
A
c
(E)-5'-
(3.6). 343 (4.3). 327 (sh),
(shj. 265 (4.6). 241 (4.7)
(4.4). 370 (4.6). 295 (sh),
(shj, 235 (4.8)
375 (4.4). 283 (4.3). 235 (4.6)
375 (4.6), 286 (4.4). 233 (4.7)
375 (4.6). 285 (4.4), 233 (4.8)
Radical ion
Isosh. pt.
2 [nm] [c]
2. [nm] [cl
Table 3. Data from emission and ECL measurements; see also Fig. 1
846
1115
331
443
L [nml
Emission
Excimer [a]
ECL
(Ej-1
980
1025
337
444
593 [b]
455. 555 (shj
433,447
439 (sh). 456, 525 (sh)
620
550
545
541
600
525
510
523
1111
504
1030
1305
1325
1345
339
418
416
420
3
4
[a] For comparison. the data of the unsubstituted electrophores are also given:
SN = N-methylphenothiazine. An = 9.10-anthraquinone. P = pyrene. [h] Measurements in acetonitrile: sh = shoulder: not listed in the table: (Z)-I UV/Vis: i
[nm] [log I:] 386 (sh). 342 (4 2j, 326 (sh), 274 (4.4). 267 (4.4). 242 (4.5). [c] Only the
long wavelength absorptions and the isosbestic points of the radical ions are listed.
A n p r . C'lirm In: Ed. E i i ~ l 1995.
.
34. N o . 23/24
2
[a] The values given are approximate. [b] Uncorrected; the corrected emission
maximum is located slightly above 600 nm.
In almost all of the concentration-dependent measurements
made in various solvents (cyclohexane, methylene chloride, and
acetonitrile) an excimer emission could be observed in addition
to the normal emission band, a phenomenon not uncommon for
pyrenoid compounds.["]
'("
V C H ~ ' i ~ r / u ~ s ~ @ s e / l sinbH.
c h u J tD-69451 Weinherin, 1995
0570-0X33:Y5/3423-2665X 10.00+ .2.W
2665
:I,,
, ,
,",
500
i/nm
-
I
10
700
800
l 2 lo00
0
,
, , ,
,
-
-1000
EimV
-2000
,
I
,
,
-3000
I
-6-
-5
-
-4 -
-3 -2 -
400
Mx)
hlnm
:
800
/IuA
4
5
:
I000
It is also remarkable that in all cases except 5,which is nonemissive in acetonitrile, the generation of radical cation/radical
anion pairs electrochemically is followed by the formation of an
electronically excited state and subsequent emission. However,
whereas the ECL curve of (E)-1 is compatible with the photoemission spectrum, this is not the case for the compounds 2-4.
A comparison of the electrochemical and photochemical experiments suggests that the electrochemically generated emission is
essentially based on the formation of an excimer (Table 3,
Fig. I ) , especially since the energy brought into the system electrochemically would not be sufficient to populate directly the S,
state of the monomeric species. Furthermore the encounter
complex preceding an electron transfer is certainly structurally
similar to that of an excimer.
Experimental Procedure
Cyclic voltammetry: Solvent acetonitrile; potentials in [mV] versus ferrocene
(FOC); reversible half-wave potential E,,,, anodic and cathodic peak potentials E,.
and ED..Experimental conditions: room temperature. scan rate: 250 mVs-', working electrode: platinum disc electrode. pseudo reference electrode: AgiAgCI.
counter electrode: platinum spiral electrode; supporting electrolyte: tetrabutylatnmonium hexafluorophosphate (TBAHFP).
Optical spectra: If not indicated otherwise, the emission spectra are true spectra. All
measurements were carried out in acetonitrile at concentrations between 5 x 10-'
M, excitation wavelength: (E)-l: 398 nm; 2-4: 375 nm (these excitaand 5 x
tion wavelengths were also used for recording the excimer emission). Due to low
solubility only an approximate wavelength of the excimer emission can be given,
since this band appears as a shoulder of the normal emission. The determination was
made by means o f measurements at different concentrations.
ECL: The ECL measurements were carried out with an unstirred solution (c = 1 x
10- to 1 x
M : supporting electrolyte: 0.1 M TBHFP) in an electrochemical cell
specially constructed for this purpose [I 11. The applied potential was switched with
alternation between the oxidation and reduction potentials determined by cyclic
voltammetry at time intervals of 20 my.
Received: May 12, 1995
Revised version: August 22, 1995 [Z 7979 IE]
German version: A n g w . Chmi. 1995. 107. 2925-2927
2666
R?
VCH ~ r l a g s ~ e . ~ i ~ l l s crnhH,
h u / t 0-69451 Wi,inheiiii, 1995
0
- 1000
EimV ___t
-3000
Fig. 1, Left: Comparison of the ECL
and emission spectra of (E)-1 and 2;
for conditions see Experimental Procedure. Right: Corresponding cyclic
voltammograms with the switching
potentials indicated by arrows
Keywords: electron transfer . isomerizations . luminescence
pyrenes . stilbenoids
[I] H . Zollinger. Color Chemistry - Syntheses, Properties und Applicurions o / O r gunk Dyes und Pigments. 2nd ed.. VCH, Weinheim, 1991.
[2] Primary Events in Photosynthesis (Isr. J. Chem. 1992. 32, 369-518).
[3] M. R. Wasielewski. M. P. ONeil, D. Gosztola. M. P. Niemczyk, W. A. Svec,
Pure Appl. Chem. 1992, 64. 1319-1325; D. Gust. T. A. Moore, Adi'. Photochem 1991. 16, 1-65; L. Sung, I. von Gersdorff, D . Niethammer, P. Tian. H.
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1994. 106, 2396-2399; Angen. Chem. I n t . Ed. Engl.
Kurreck, A ~ g r w Cheni.
1994.33, 2318-2320.
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1975-1976 1975. 9. 213-263; F. Pragst. Z . Chem. 1978. 18. 41 -50; L. R.
Faulkner. A. J. Bard, Electrounul. Chem. 1977. 10, 1-95.
[ S ] J. H. Burroughes. D. D. C. Bradley, A. R. Brown. R. N . Marks. K . Mackay.
R. H. Friend. P. L. Burns, A. B. Holmes, Nuturr 1990. 347. 539-541.
161 J. Pommerehne, H . Vestweber, W. Guss, R. F. Mahrt, H. Biissler, M. Porsch, J.
Daub. Ade. Muter. 1995. 7, 551 -554.
[7] B. M. Krasovitskii. B. M. Bolotin. Orgunic Lumme.went Muteriuls, VCH.
Weinheim. 1988.
[8] A. Knorr. Dissertation. Universitiit Regensburg. 1995.
[9] G . Bartocci. U. Mazzucato, A. Spalletti, G . Orlandi. G . Poggi, J. Chem. Soc.
Fciruduy 7kins. 1992. 88. 3139-3144; T. Arai. T. Karatsu, H. Misawa, Y Kuriyama. H. Okamoto. T. Hiresaki. H . Furuuchi. H . Zeng, H . Sakuragi, K.
Tokumaru. Pure Appl. Chem. 1988, 60, 989-998.
[lo] T. Forster, Angew. Cli~m.1969. 8f, 364-374; Angeii. Cheni. I n t . Ed. Engl.
1969. 8 , 333; T. J. Maloy, A. J. Bard, J. Am. Cliem. SOL..
1971, 93, 5968-5981.
[I 11 S . Hien, Dissertation. Universitdt Regensburg, 1995.
0570-0X33/9513423-2666 $ 10.0Oi .25/0
Angew. Chem. Inr. Ed. Engl.
1995, 34, N o . 23/24
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luminescence, containing, pyrenoid, fluorescence, heteroaromatic, transfer, subunit, electro, stimulate, substituted, donoracceptor, stilbene
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