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Radical anions of π-conjugated oligomers and polymers: Delocalization and self trapping
Synthetic Metals 69 (1995) 633-636
RADICAL ANIONS OF x-CONJUGATED OLIGOMERS AND POLYMERS: DELOCALKATION AND SELF TRAPPING
Martin Baumgarten, Lileta Gherghel, Stoyan Karabunarliev Max-Planck-lnstitut fur Polymerforschung Ackermannweg 10, D- 55128 Mainz, Germany Abstract:
We have studied radical anions of oligomeric series of differently linked conjugated x-systems by visible absorption and EPRIENDOR spectroscopy, supported by quantum chemical calculations. The results obtained show very different features depending on the topology and geometry of the x-skeleton, the specific chain ends, and the counter ion effects. Upon chain length extension both spectroscopic methods yield complementary data allowing for a much deeper insight into characteristic molecular properties as limit of conjugation and structural reorganization upon charging. Three major topics for influencing the conjugation are summarized i) a comparison between linear and ladder type x-systems , ii) the topological control of conjugation as in para-, meta-, and orthophenylenevinylenes, and iii) the geometric hindrance in bi- and oligoarylenes. 1. Introduction: Homologous series of soluble conjugated oligomers have gained considerable interest in the last years, since. they enable a clean characterization and study of geometric and electronic structures of the extended x-systems [l]. Many optical and electronic properties of conjugated polymers closely correspond to those of oligomers containing only a few numbers of repeat units [2], and by following these properties as fbnction of chain length [ 1,3,4], some problems often faced in studying conducting polymers as i) the polydispersi~, ii) the limited solubility, and iii) structural inhomogeneity could be overcome. Major concerns within the description of conjugated x-chains are the conjugative interaction of the subunits and the characteristic charge distribution on a chain (effective conjugation length). We have used the parallel approach of studying the radical ions by optical absorption and EPRIENDOR spectroscopy allowing a reliable characterization of the mono and higher charged states [ 1,5]. Two principles for enhanced solubility have been used in majority [5]: i) solubilizing alkyl/alkoxy groups at both ends of the conjugated chain (type l), enhancing solubility only to some extent, but retaining the original x-overlap between the subunits and ii) alkyl/alkoxy groups on the repeat units (type 2) leading to higher solubility, such that even longer chains are still soluble, but sometimes inducing additional geometric distortion lowering the x-overlap. type1
type2
R*R
gqq-@ n
0379-6779/95/$09.50
0
1995 Elsevier Science S.A. All rights reserved
SSDI 0379-6779(94)02597-R
2. Results: 2.1. Comparison of linear and ladder type z-chains In linear type x-chains as e.g. polyenes 1 a considerable degree of rotational freedom exists which may be overcome in ladder type x-systems as e.g. rylenes 2. The radical anions of the series of polyenes la-c show a nice decrease of the largest hypertime couplings with increasing chain length measured by ENTIOR spectroscopy in solution and a first crude assignment based on HMO/McL,achlan calculations places the position of highest spin density at the two outermost carbon centers. A very similar situation is found from the ENDOR spectra of the radicalanions of oligorylenes 2a-d-’[6,7].
/*&p&N/ -N la-c, N=03 While the optical absorptions exhibit a bathochromic shift with increasing chain length for both series, the energies of the optical transitions of Za-d and Za-d-- and Za-d2- are nearly half the size as for l,P’, and 12- (Fig1 ). This holds also for the extrapolation towards the corresponding polymers from a plot of E versus I/n, where n is the number of repeat units, yielding optical band gaps of 1.6 eV and 0.8 f 0.1 eV, for 1 and 2, respectively. A closer inspection of the two largest hypertine couplings (hfc’s) with increasing chain length in a l/n plot (Fig. 2) reveals, that for 2’ the curves are nearly linear, while two different curvatures are found in polyenes, which cut/hit already for the octaene 1~‘. This phenomenon suggests a spin density redistribution and finds support from calculations involving geometry optimization by the semiempirical AM1 and PM3 methods and consecutive open-shell configuration interaction to account for spin polarization.
et al. I Synthetic Metak
M. Baumgaflen
634
04
0,oo
I
0.05
.
0,lO
I
I
.
0,lS
0,20
0,25
8
030
l/N (Nwumber of doubk bonds)
Fig. 1: Plot of optical transition energies versus l/N
69 (1995) 633-636
The study of the radical anions of distyrenebenzene isomers in solution by EPR/ENDOR spectroscopy, on the other hand, show a reversed order compared to vis-NIR measurements mentioned above, with the largest hyperfime coupling being smallest for the ortho isomer 4~’ (0.36 mT) and intermediate in the para isomer 3~’ (0.45 mT). Surprisingly, tbis same ordering of hfc’s from highly resolved ENTXR spectra is found for all higher oligomers, even though old data of htic’s for 3au’ [l l] have been remeasured and corrected, since the data included most probably protonated intermediates for the higher oligomers (Fig. 3). For the assignment of the proton couplings to molecular positions, additional de&ration experiments with exchange of protons of spedific ethylenic bonds have been performed.
1 hfc vers. l/N 1 %’ 4,5 c 4,0
largest hfc’s of monoanions *‘_,_ 1.. .t,._ L\
‘E 3,50 . xl 300 x 2,s I! . g 2.0-
= 1,5-
1,0-l . i
l/N (N number of double bonds)
1
2
.
1
3
r
4
r
5
.
r
6
’
7
’
a
s
n (n number of phenylenes)
Fig. 2: plot of the two largest hype&e couplings for la-c’ and 20-d versus l/N, where N is the number of double bonds.
Fig. 3: Largest hfc’s for orthe and para PV’s versus n, sorted for even and odd number of phenylenes
The computational results suggests that the position of highest spin density is moved from the outermost to the next inner double bond in lc’ while it remains in the outermost positions for quaterrylene 2c’.
In the case of 4b’, thereby, the two largest hfc’s could be assigned to the central stilbene unit,while the exchanged protons in the double bond next to the stilbene gave rise only the third largest hypertime coupling. This differs from Htlckel-type calculations [12] neglecting structural distortions upon doping and predicts the latter position to cany highest spin density.
2.2. Topological control of conjugation The control of conjugation by topology rather thau geometry can nicely be demonstrated in the para, ortho, and meta isomers of phenylenevinylenes (PVs) 3-S. The oligo-pare-PVs 3, as most important model for PPV, exhibit a pronounced bathochromic shift with increasing number of repeat units n for all oligomers, demonstrating highly efficient conjugation between the subunits [S]. While the ortho-PV’s 4 only exhibit m&rate changes of the optical absorption energies with increasing chain length, nearly no change of the optical transitions with increasing n is observed for the meta isomers 5 [9]. Similar chain length dependent evolution is found for the absorption spectra of the corresponding monoanion radicals. Thereby, two new transitions at lower energies are observed, which shift bathochromically with increasing n for 3’ and 4’ but not at all for 9’. Jn the latter case a third, new broad absorption at very low energies has been assigned to an intramolecular charge transfer transition [lo]. R
Scheme 1: para-PVS
CB
CB
3t
VB 3ab = para &*a-otulo 6ad = mata
oltbo PVS
VB
M. Baumgarten
et al. I Synthetic MetaLF 69 (1995) 633436
635
Best agreement between these experimental fmdings and [15]. Therefore the diauion forms a biradical, which is theretical derived spin densities was again found when applying transformed to the ground state singlet only at very low all single excited states in the PPP approach for semiempirically temperatures (T < 40 K). optimized structures. However, a principle contradiction in 1 comparing the experimental and theretical spin densities of the para and ortho isomers remained: While theory predicts larger spin densities for ortho PVs, experiments show the larger hfc’s 00 for the para PVs. A plausible reason for this deviation cau only be found when postulating additional electron-transfer processes in the ortho PVs, which are fast on the time scale of the EPR/ENDOR experiment ( 10m7 - 10mg s), but slow on the time scale of optical absorption measurements (IO-l1 - IO-l3 s) [I]. Combined with a closer distance between the double bonds in 4 \ and a more flexible skeleton of o-PV’s, these findings suggest, for the firs time, that a deeper trap (stronger localization) of a charged “defect” in ortho PVs moves faster than a shallow trap 7e 6a-f 6a-e (better delocalization) in para PV’s, as outlined in Scheme 1. /--\ /=I For the oligo+neti-phenylenevinylenes 5a-d a conjugation ___qpqp___ barrier is found such that an injected charge is localized on one stilbene unit (stilbene polaron) although the steric demands are comparable to the one in 3a, b.[9] This difference can be 9a,b attributed to the meta-bridging which leads to non-Kekule structures in the extended rr-system and permits extension of the The anthrylenes in 8a-f are aligned nearly orthogonal such polaron-related resonance structure within one stilbene unit only. that the electrophores are uncoupled, and each unit can be charged independently leading to tri- and tetraradicalauions in 2.3. Conjugation in Bi- and Oligoarylenes the trimer 8b aud tetramer 8c, respectively [13, 141. The interring conjugation in bi- and oligoarylenes cau be tuned by the nature of the x-system and the bridgehead position Scheme 2: Charging of Polyanthrylene of linkage. This becomes immediately clear upon comparing biphenyl with biarylenes as l,l’-binaphthylene 6a, 3,3’biperylene 7a,[1,6,7] 9,9’-bianthrylene 8a [13,14],or 2,2’bipyrenylene 9~ [ 151 and their corresponding higher homologues. In 6a the angle of torsion is already 70’ leading to small xoverlap. From EPR-spectroscopic studies it has been found that this angle of torsion is reduced to about 50’ in the radical monoanion of 6a leading to complete delocalization of the unpaired electron. The study of higher oligomers revealed [ 161, on the other hand that i) the effective conjugation length is already reached in the pentanaphthylene 6&’ and ii) the spin density is largest for the central binaphthylene unit in the Even in the corresponding poymer, polyanion formation leads tetmmer already, as found from deuterium exchange of the to high spin multiplicities with triplet, quartet, and quintet states protons in the outer naphthylene units. (Scheme 2). Since the highly charged material becomes insoluble While in biperylene 7~ the geometric demands are identical in THF, it is not clear yet, whether even higher spin states are to those in 6a, the MO-coefficient at the bridgehead position is excluded by non charged or doubly charged anthrylenes. The reduced and only a vauishingly small bathochromic shift (6 nm) temperature dependent EPR studies of 8aE2’show i) a maximum is observed when compared to perylene. The radical anion 6aY of signal intensity at 20 K corresponding to a singlet-triplet exhibits a b&table state, where temperature variation enables a seperation of 60 c.al/mol and ii) au increase of deviation thorn switch from complete delocalization (RT) to localization (T < 160 orthogonal&y by turther splitting of the x and y zero-field K) in the timescale of the EPR experiment ( 10m7 s) [7]. The splitting components upon lowering of temperature. This rhombic dianion formation, however, leads to a diamagnetic state. zfs pattern at very low temperautre is in agreement with x-ray When coupling pyrenyls through the 2-position as in 9a,b, on results of the neutral compound, which yield au angle of torsion 8 the nodal plane of the HOMO and LUMO, the steric situation in of 75O between the anthracenes in the trimer. While calculations 9a is close to the one in biphenyl, but the lack of interaction for of the spin-spin interaction predict a ferromagnetic coupling for 6 the frontier MO leads to effectively uncoupled chromophores > 83O, turther synthetic work is directed towards stronger steric
3 :
3 :
3 :
3 >
\\ &o
o+ 3 : :
636
hindrance
M. Bamgmten
et al. / Synthetic Met&
by additional alkyl substitutents in the 1 and 4
pOStiOnS.
3. conclusion In this work we describe the control of conjugative interaction in well detined soluble oligomeric and polymeric x-systems. It is shown that EPR/ENDOR spectroscopy in combination with parallel optical absorption measurements yield compkmentaty tiormation The well characterized conjugated oligomers, on the other hand, not only serve as model systems for the corresponding polymers, but have their own independent siguificance for materials science. [ 1,171 It has been shown, for instance, that oligophenylenes are very effkient in light emitting diodes to cover the blue spectral rauge, which otherwise is difficult to achieve.[l8] The different conjugated chains thus will allow to depict the requested optical properties which are mainly determined by their HOMO-LUMO energy difference or optical “band-gap”. Very recently, suggestions have been proposed for the fine tuning of the band-gap by use of different building blocks in a copolymerlike fashion leading immediately to novel applications. [ 191 The knowledge assembled for the charged states is important for their use in the design of novel building blocks for molecular electronics. References 1 M. Baumgarten, K.MUllen,Top. Cwr. Chem. ISs(l994) 1 2 H.H. H&hold, M. Helbig; Makromd. Chem. Macromd. Symp. 72 (1987) 229; K. MUllen, Pure SAppI Chem. 65 (1993) 89;
69 (1995) 633436
3
J.L. Bredas, R. Silbey, D.S. Boudreaux, R.R. Chance,
4 5
J. Am. Chem. Sot. 105 (1983) 6555. L.M. Tolbert; Act. Chem. Res. 25 (1992) 561 M. Baumgarten, W. Huber, K. Mullen, Adv. Phys. Org. Chem. 28 (1993) 1
6 7 8
M. Baumgatien,
U. Anton, L. Gherghel, K. Mullen,
Synth. Met. 5557 (1993) 4807 M. Baumgarten, K.-H. Koch, K. MUllen, J. Am. Chem. Sot. 1994 in press. R. Schenk, H. Gregorius, K. Mrlllen, Adv. Mat.
3(1991) 3198 H. Gregorius, M. Baumgarten, R. Reuter, N. Tyutyulkov, K. M6llen; Angew. Chem. 704(1992) 1621. IO S. Ksrabunarliev, M. Baumgarten, K. Mullen, N. Tyutyulkov, J. Phys. Chem. 1994 submitted 11 R. Schenk, M. Ehrenfreund, W. Huber, K. Mullen, JCS
9
Chem. Commun. 23 (1990) 1673 12 P. Brendel, A. Grupp, M. Mehring, R. Schenk, K. Mullen, W. Huber, Synth. Met. 45 (1991) 49 13 M. Baumgarten, U. MUller, A. Bohnen, K. Mullen, Angew. Chem. 104 (1992) 482 14 M. Baumgarten, U. Muller, Synth. Met. 5557(1993) 4755, U. Muller, M. Baumgarten, J. Am. Chem. Sot. submitted 15 M. Kreyenschmidt, M. Baumgarten, N. Tyutyulkov, K. Mullen, Aftgew. Chem. 1994 in press 16 M. Baumgarten, L. Gherghel, to be published 17 M. Baumgarten, U. Bunz, U. Scherf, K. Mullen, Nato Series of Adv. Stud. 1994 in press 18. G. Grem, U. leditzky, B. Ullrich, G. Leising, Adv. Mater.4 (1992) 36 19 F. Meyers. A.J. Heeger, J.L. Bredas, Synth. Met. 5557 (1993) 4308.
RADICAL ANIONS OF x-CONJUGATED OLIGOMERS AND POLYMERS: DELOCALKATION AND SELF TRAPPING
Martin Baumgarten, Lileta Gherghel, Stoyan Karabunarliev Max-Planck-lnstitut fur Polymerforschung Ackermannweg 10, D- 55128 Mainz, Germany Abstract:
We have studied radical anions of oligomeric series of differently linked conjugated x-systems by visible absorption and EPRIENDOR spectroscopy, supported by quantum chemical calculations. The results obtained show very different features depending on the topology and geometry of the x-skeleton, the specific chain ends, and the counter ion effects. Upon chain length extension both spectroscopic methods yield complementary data allowing for a much deeper insight into characteristic molecular properties as limit of conjugation and structural reorganization upon charging. Three major topics for influencing the conjugation are summarized i) a comparison between linear and ladder type x-systems , ii) the topological control of conjugation as in para-, meta-, and orthophenylenevinylenes, and iii) the geometric hindrance in bi- and oligoarylenes. 1. Introduction: Homologous series of soluble conjugated oligomers have gained considerable interest in the last years, since. they enable a clean characterization and study of geometric and electronic structures of the extended x-systems [l]. Many optical and electronic properties of conjugated polymers closely correspond to those of oligomers containing only a few numbers of repeat units [2], and by following these properties as fbnction of chain length [ 1,3,4], some problems often faced in studying conducting polymers as i) the polydispersi~, ii) the limited solubility, and iii) structural inhomogeneity could be overcome. Major concerns within the description of conjugated x-chains are the conjugative interaction of the subunits and the characteristic charge distribution on a chain (effective conjugation length). We have used the parallel approach of studying the radical ions by optical absorption and EPRIENDOR spectroscopy allowing a reliable characterization of the mono and higher charged states [ 1,5]. Two principles for enhanced solubility have been used in majority [5]: i) solubilizing alkyl/alkoxy groups at both ends of the conjugated chain (type l), enhancing solubility only to some extent, but retaining the original x-overlap between the subunits and ii) alkyl/alkoxy groups on the repeat units (type 2) leading to higher solubility, such that even longer chains are still soluble, but sometimes inducing additional geometric distortion lowering the x-overlap. type1
type2
R*R
gqq-@ n
0379-6779/95/$09.50
0
1995 Elsevier Science S.A. All rights reserved
SSDI 0379-6779(94)02597-R
2. Results: 2.1. Comparison of linear and ladder type z-chains In linear type x-chains as e.g. polyenes 1 a considerable degree of rotational freedom exists which may be overcome in ladder type x-systems as e.g. rylenes 2. The radical anions of the series of polyenes la-c show a nice decrease of the largest hypertime couplings with increasing chain length measured by ENTIOR spectroscopy in solution and a first crude assignment based on HMO/McL,achlan calculations places the position of highest spin density at the two outermost carbon centers. A very similar situation is found from the ENDOR spectra of the radicalanions of oligorylenes 2a-d-’[6,7].
/*&p&N/ -N la-c, N=03 While the optical absorptions exhibit a bathochromic shift with increasing chain length for both series, the energies of the optical transitions of Za-d and Za-d-- and Za-d2- are nearly half the size as for l,P’, and 12- (Fig1 ). This holds also for the extrapolation towards the corresponding polymers from a plot of E versus I/n, where n is the number of repeat units, yielding optical band gaps of 1.6 eV and 0.8 f 0.1 eV, for 1 and 2, respectively. A closer inspection of the two largest hypertine couplings (hfc’s) with increasing chain length in a l/n plot (Fig. 2) reveals, that for 2’ the curves are nearly linear, while two different curvatures are found in polyenes, which cut/hit already for the octaene 1~‘. This phenomenon suggests a spin density redistribution and finds support from calculations involving geometry optimization by the semiempirical AM1 and PM3 methods and consecutive open-shell configuration interaction to account for spin polarization.
et al. I Synthetic Metak
M. Baumgaflen
634
04
0,oo
I
0.05
.
0,lO
I
I
.
0,lS
0,20
0,25
8
030
l/N (Nwumber of doubk bonds)
Fig. 1: Plot of optical transition energies versus l/N
69 (1995) 633-636
The study of the radical anions of distyrenebenzene isomers in solution by EPR/ENDOR spectroscopy, on the other hand, show a reversed order compared to vis-NIR measurements mentioned above, with the largest hyperfime coupling being smallest for the ortho isomer 4~’ (0.36 mT) and intermediate in the para isomer 3~’ (0.45 mT). Surprisingly, tbis same ordering of hfc’s from highly resolved ENTXR spectra is found for all higher oligomers, even though old data of htic’s for 3au’ [l l] have been remeasured and corrected, since the data included most probably protonated intermediates for the higher oligomers (Fig. 3). For the assignment of the proton couplings to molecular positions, additional de&ration experiments with exchange of protons of spedific ethylenic bonds have been performed.
1 hfc vers. l/N 1 %’ 4,5 c 4,0
largest hfc’s of monoanions *‘_,_ 1.. .t,._ L\
‘E 3,50 . xl 300 x 2,s I! . g 2.0-
= 1,5-
1,0-l . i
l/N (N number of double bonds)
1
2
.
1
3
r
4
r
5
.
r
6
’
7
’
a
s
n (n number of phenylenes)
Fig. 2: plot of the two largest hype&e couplings for la-c’ and 20-d versus l/N, where N is the number of double bonds.
Fig. 3: Largest hfc’s for orthe and para PV’s versus n, sorted for even and odd number of phenylenes
The computational results suggests that the position of highest spin density is moved from the outermost to the next inner double bond in lc’ while it remains in the outermost positions for quaterrylene 2c’.
In the case of 4b’, thereby, the two largest hfc’s could be assigned to the central stilbene unit,while the exchanged protons in the double bond next to the stilbene gave rise only the third largest hypertime coupling. This differs from Htlckel-type calculations [12] neglecting structural distortions upon doping and predicts the latter position to cany highest spin density.
2.2. Topological control of conjugation The control of conjugation by topology rather thau geometry can nicely be demonstrated in the para, ortho, and meta isomers of phenylenevinylenes (PVs) 3-S. The oligo-pare-PVs 3, as most important model for PPV, exhibit a pronounced bathochromic shift with increasing number of repeat units n for all oligomers, demonstrating highly efficient conjugation between the subunits [S]. While the ortho-PV’s 4 only exhibit m&rate changes of the optical absorption energies with increasing chain length, nearly no change of the optical transitions with increasing n is observed for the meta isomers 5 [9]. Similar chain length dependent evolution is found for the absorption spectra of the corresponding monoanion radicals. Thereby, two new transitions at lower energies are observed, which shift bathochromically with increasing n for 3’ and 4’ but not at all for 9’. Jn the latter case a third, new broad absorption at very low energies has been assigned to an intramolecular charge transfer transition [lo]. R
Scheme 1: para-PVS
CB
CB
3t
VB 3ab = para &*a-otulo 6ad = mata
oltbo PVS
VB
M. Baumgarten
et al. I Synthetic MetaLF 69 (1995) 633436
635
Best agreement between these experimental fmdings and [15]. Therefore the diauion forms a biradical, which is theretical derived spin densities was again found when applying transformed to the ground state singlet only at very low all single excited states in the PPP approach for semiempirically temperatures (T < 40 K). optimized structures. However, a principle contradiction in 1 comparing the experimental and theretical spin densities of the para and ortho isomers remained: While theory predicts larger spin densities for ortho PVs, experiments show the larger hfc’s 00 for the para PVs. A plausible reason for this deviation cau only be found when postulating additional electron-transfer processes in the ortho PVs, which are fast on the time scale of the EPR/ENDOR experiment ( 10m7 - 10mg s), but slow on the time scale of optical absorption measurements (IO-l1 - IO-l3 s) [I]. Combined with a closer distance between the double bonds in 4 \ and a more flexible skeleton of o-PV’s, these findings suggest, for the firs time, that a deeper trap (stronger localization) of a charged “defect” in ortho PVs moves faster than a shallow trap 7e 6a-f 6a-e (better delocalization) in para PV’s, as outlined in Scheme 1. /--\ /=I For the oligo+neti-phenylenevinylenes 5a-d a conjugation ___qpqp___ barrier is found such that an injected charge is localized on one stilbene unit (stilbene polaron) although the steric demands are comparable to the one in 3a, b.[9] This difference can be 9a,b attributed to the meta-bridging which leads to non-Kekule structures in the extended rr-system and permits extension of the The anthrylenes in 8a-f are aligned nearly orthogonal such polaron-related resonance structure within one stilbene unit only. that the electrophores are uncoupled, and each unit can be charged independently leading to tri- and tetraradicalauions in 2.3. Conjugation in Bi- and Oligoarylenes the trimer 8b aud tetramer 8c, respectively [13, 141. The interring conjugation in bi- and oligoarylenes cau be tuned by the nature of the x-system and the bridgehead position Scheme 2: Charging of Polyanthrylene of linkage. This becomes immediately clear upon comparing biphenyl with biarylenes as l,l’-binaphthylene 6a, 3,3’biperylene 7a,[1,6,7] 9,9’-bianthrylene 8a [13,14],or 2,2’bipyrenylene 9~ [ 151 and their corresponding higher homologues. In 6a the angle of torsion is already 70’ leading to small xoverlap. From EPR-spectroscopic studies it has been found that this angle of torsion is reduced to about 50’ in the radical monoanion of 6a leading to complete delocalization of the unpaired electron. The study of higher oligomers revealed [ 161, on the other hand that i) the effective conjugation length is already reached in the pentanaphthylene 6&’ and ii) the spin density is largest for the central binaphthylene unit in the Even in the corresponding poymer, polyanion formation leads tetmmer already, as found from deuterium exchange of the to high spin multiplicities with triplet, quartet, and quintet states protons in the outer naphthylene units. (Scheme 2). Since the highly charged material becomes insoluble While in biperylene 7~ the geometric demands are identical in THF, it is not clear yet, whether even higher spin states are to those in 6a, the MO-coefficient at the bridgehead position is excluded by non charged or doubly charged anthrylenes. The reduced and only a vauishingly small bathochromic shift (6 nm) temperature dependent EPR studies of 8aE2’show i) a maximum is observed when compared to perylene. The radical anion 6aY of signal intensity at 20 K corresponding to a singlet-triplet exhibits a b&table state, where temperature variation enables a seperation of 60 c.al/mol and ii) au increase of deviation thorn switch from complete delocalization (RT) to localization (T < 160 orthogonal&y by turther splitting of the x and y zero-field K) in the timescale of the EPR experiment ( 10m7 s) [7]. The splitting components upon lowering of temperature. This rhombic dianion formation, however, leads to a diamagnetic state. zfs pattern at very low temperautre is in agreement with x-ray When coupling pyrenyls through the 2-position as in 9a,b, on results of the neutral compound, which yield au angle of torsion 8 the nodal plane of the HOMO and LUMO, the steric situation in of 75O between the anthracenes in the trimer. While calculations 9a is close to the one in biphenyl, but the lack of interaction for of the spin-spin interaction predict a ferromagnetic coupling for 6 the frontier MO leads to effectively uncoupled chromophores > 83O, turther synthetic work is directed towards stronger steric
3 :
3 :
3 :
3 >
\\ &o
o+ 3 : :
636
hindrance
M. Bamgmten
et al. / Synthetic Met&
by additional alkyl substitutents in the 1 and 4
pOStiOnS.
3. conclusion In this work we describe the control of conjugative interaction in well detined soluble oligomeric and polymeric x-systems. It is shown that EPR/ENDOR spectroscopy in combination with parallel optical absorption measurements yield compkmentaty tiormation The well characterized conjugated oligomers, on the other hand, not only serve as model systems for the corresponding polymers, but have their own independent siguificance for materials science. [ 1,171 It has been shown, for instance, that oligophenylenes are very effkient in light emitting diodes to cover the blue spectral rauge, which otherwise is difficult to achieve.[l8] The different conjugated chains thus will allow to depict the requested optical properties which are mainly determined by their HOMO-LUMO energy difference or optical “band-gap”. Very recently, suggestions have been proposed for the fine tuning of the band-gap by use of different building blocks in a copolymerlike fashion leading immediately to novel applications. [ 191 The knowledge assembled for the charged states is important for their use in the design of novel building blocks for molecular electronics. References 1 M. Baumgarten, K.MUllen,Top. Cwr. Chem. ISs(l994) 1 2 H.H. H&hold, M. Helbig; Makromd. Chem. Macromd. Symp. 72 (1987) 229; K. MUllen, Pure SAppI Chem. 65 (1993) 89;
69 (1995) 633436
3
J.L. Bredas, R. Silbey, D.S. Boudreaux, R.R. Chance,
4 5
J. Am. Chem. Sot. 105 (1983) 6555. L.M. Tolbert; Act. Chem. Res. 25 (1992) 561 M. Baumgarten, W. Huber, K. Mullen, Adv. Phys. Org. Chem. 28 (1993) 1
6 7 8
M. Baumgatien,
U. Anton, L. Gherghel, K. Mullen,
Synth. Met. 5557 (1993) 4807 M. Baumgarten, K.-H. Koch, K. MUllen, J. Am. Chem. Sot. 1994 in press. R. Schenk, H. Gregorius, K. Mrlllen, Adv. Mat.
3(1991) 3198 H. Gregorius, M. Baumgarten, R. Reuter, N. Tyutyulkov, K. M6llen; Angew. Chem. 704(1992) 1621. IO S. Ksrabunarliev, M. Baumgarten, K. Mullen, N. Tyutyulkov, J. Phys. Chem. 1994 submitted 11 R. Schenk, M. Ehrenfreund, W. Huber, K. Mullen, JCS
9
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