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Optical properties and structure of thiophene substituted copolymers
Solid State Communications, Vol. 99, No. 10, pp.707-712, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00 + .OO
Pergamon
PII: s0038-1098(%)00246-3
OPTICAL PROPERTIES
AND STRUCTURE
OF THIOPHENE
SUBSTITUTED
COPOLYMERS
G. Lanzani, ** A. Piaggi,?’ C. Botta,b M. Catellani,b E. Prevosti,b P.C. SteinC and R. Tubino*d ‘Istituto di Matematica e Fisica, Universita’ di Sassari, Via Vienna 2, 07100 Sassari, Italy bIstituto de Chimica delle Macromolecole (CNR), Via Bassini 15, 20133 Milano, Italy ‘Kemisk Institut, Odense Universitet, Campusvej 55, 5230 Odense, Denmark ‘Dipartimento di Fisica, Universita’ di Milano, Via Celoria 16, 20133 Milano, Italy (Received 17 February 1996; accepted 3 April 1996 by E. Molinari)
The optical properties and the structure of random copolymers consisting of mono- and disubstituted alkylthiophenes have been studied by optical and NMR spectroscopy. While the optical data are consistent with the presence of randomly distributed mono and disubstituted units, NMR spectra reveal that preferential enchainment between monosubstituted units takes place. Copyright 0 1996 Elsevier Science Ltd Keywords: A. polymers, D. optical properties, E. nuclear resonances.
1. INTRODUCTION Among electroactive organic materials, thiophenebased polymers are the subject of considerable interest due to their electronic properties and to their potential applications [l]. The chemical versatility of the thiophene ring has led to the synthesis of polymers with various structure and different physical and electronic properties, and which are designed specifically for particular applications in electronics, communication or information technologies [2-41. The modulation of the chemical architecture of the polymeric chain may strongly influence the environmental stability and the processability of the material, as well as its charge transport and optical properties. The synthesis of new tailored organic semiconductors has been achieved in altering both type and condition of polymerization, and copolymerization offers one of the most powerful ways for the preparation of materials for new technologies. Recently the copolymerization of different thiophene-based monomers was used to produce macromolecules with narrow band-gap [5], or materials stable in both the neutral
and doped state [6], or to prepare polymers for non linear optical applications [7]. We have synthesised a series of alkyl-substituted thiophene copolymers with the aim to obtain processable materials having electrochromic properties suitable for application in optical devices. The copolymer&&on of 3-butylthiophene and 3,4-dibutylthiophene monomers in different ratios leads to a series of materials (see Fig. 1) in which it is possible to modulate continuously the effective conjugation length and the energygap of the chain [8]. The presence of dialkyl substituted rings in a poly(3-alkylthiophene) produces a local deviation from planarity of the backbone. This interrupts or decreases the pr orbital overlap, thus affecting the mean conjugation length of the chain. As a result, the copolymer series shows a different energy gap and its colour tones off from dark red to transparent. The aim of the present work is twofold. First, we theoretically evaluate the optical and electronic properties of the copolymer as a function of its composition. We assume that for each growing chain site the probability of enchainment by the mono- (M) and disubstituted (D) thiophene units is the same. The theoretical results are compared with the experimental absorption spectra. Then, we study the enchainment regularity by NMR technique, by detecting the effects of the monoalkyl thiophene
* INFM, Unita di Milano. t INFM, Unita di Pavia. 707
708
OPTICAL PROPERTIES
AND STRUCTURE
n Fig. 1. Copolymer structure. x and y represent the fractions of the two monomers in the copolymer composition. asymmetry on the MD and MM enchainments and the possible deviation from the statistical behavior. It will be shown that, while the optical data indicate that those effects are relatively unimportant on a lengthscale determined by the conjugation length, the NMR spectroscopy is suggestive of a quite complex copolymerization scenario on shorter lengths.
2. EXPERIMENTAL The copolymer series has been prepared via chemical oxidation with FeCls following the literature [9]. The materials are soluble in chloroform and processable from solution. The UV-VIS-NIR study was performed with a Varian 2400 Instrument on both solution and film samples. NMR experiments were performed on a Varian UNITY-500 spectrometer on CDCls filtered solutions whose concentration is about 10e2 g ml-‘. Proton chemical shifts are referenced to TMS. The temperature was stabilized at 298K. Some ‘H spectra were performed with i3C decoupling of the CHC13 resonance.
&E)
=
C+Y, j
and optical response
In this section we will analyse the evolution of the optical response of the random copolymers on the basis of a simple statistical description. This way, we are able to reproduce the absorption spectra as a function of copolymer composition and to compare them with the measured ones. First of all, we observe that the average conjugation length in substituted polythiophene is determined by the competition between the steric hindrance of the side groups that leads to a departure from planarity, and the r-electron delocalization that favors planar geometry. As a consequence the valence band width and the energy of the first optical allowed T--~F* transition depend on the chain configuration. In
Vol. 99, No. 10
mixed copolymers the chain twisting is directly related to the unit enchainment, i.e. the kind of adjacent rings. In particular, large deviations from planarity are associated to disubstituted thiophene rings. The value of the torsion inter-ring angle which results from steric repulsion of side chains can be roughly estimated considering the case of a pure disubstituted polythiophene chain and using the cosine law [lo] for 7r--7r overlap. The absorption maximum at 3.7eV would correspond to an average inter-ring torsional angle of 70”, producing a reduction in conjugation of about 66 % with respect to a planar chain. As a first approximation we can then assume that disubstituted rings cause a complete break of conjugation, while adjacent M-type monomers give rise to an almost planar conjugated sequence (see the NMR section for a less simplified model of the copolymer structure). A polymer chain is thus a sequence of different lengthconjugated segments alternated with non-conjugated defect segments. If we further assume a fully statistical enchainment of mono- and disubstituted rings we can use a simple statistical approach to describe the average r-r* transition energy as a function of the defect concentration. The absorption spectrum can be reproduced as a weighted integral on a distribution of different segments possessing different conjugation lengths and energy gaps E0 between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). E,, is described by the standard law: E,,(N) = A + B/N, where N is the number of conjugated (planar) rings and A and B are fixed parameters to be determined. A linear increase of the oscillator strength with N is assumed up to a saturation for N = 10. The contribution aN(E) to the absorption for a certain length is
3. RESULTS AND DISCUSSION 3.1 Statistical description of the copolymers
OF THIOPHENE
E - Ej(NI,
.
(1)
where g is the Gaussian lineshape function centered in Ej = E,,(N) + jhw and having a width 7, and S is the Huang-Rhys factor. The sum (1) is truncated atj = 2. The absorption of the real sample comes from the superposition of the contributions of all the possible conjugation lengths, weighted by a probability factor. We write Iv-1
a =
c
K=O
P(NM - K)Q~~-~(E).
(2)
In order to evaluate the probability factors we can count the number of different ways a certain length (NM -K) can be obtained keeping ftxed the total
Vol. 99, No. 10
OPTICAL PROPERTIES
AND STRUCTURE
1.0 h
5
0.8
4 m V
0.6
$
0.4
ii e 0.2 s n a
0.0 350
300 tot
,
.
400
,
.
450
,
.
,
500 .
,
600
550 .
,
.
,
11. 9
300
350
400
450
500
550
600
Wavelength (nm) Fig. 2. Top panel: Experimental absorption spectra at various concentrations of di-substituted thiophene units (a) c = 0.75, (b) c = 0.5, (c) c = 0.25. Bottom panel: Simulated curves. number of M-type or D-type monomers in the chain. This is simply done for K = 0. There are III,(O) = No + 1 ways to obtain a distinguishable sequence of NY conjugated monomers and No randomly placed defects. This is the maximum conjugation length. For K = 1 we count the ways to have NM - I conjugated rings like II,_,(K)
= (N;+,2)!
- 2!I-I&
M-
where we have subtracted combinations which have already been taken into account previously. By iteration we have (N,+K+
Kvnr-KW> =
l)!
NM!K!
-2l-I(K-
1)-v..
iy$qO)
-
where 0 I K 5 NM - 1; P(K) is the number of times a sequence of (NM-K) conjugated rings appears in all the possible distinguishable sequences. The probability of having (NM - K) conjugated rings is then WM
-
K) =
WK) NM_l
c
K=O
.
Jw)
(5)
OF THIOPHENE
709
Results obtained using equation (2) for different defect concentrations c (where c = NM/(NM + No)) are shown in Fig. 2. The best parameters reproducing the experimental spectra are: B = 1 eV, A = 2.74eV, y = 0.9, S = 0.7. In Fig. 2 the comparison between simulated and measured curves is reported. The agreement is reasonable and the trend upon changing c is well reproduced showing a red shift of the absorption maximum for smaller D-type concentrations. The high energy side of the absorption band, due to disubstituted sequences, is not included in the simulation; this accounts for the discrepancy between calculated and experimental optical density at shorter wavelengths. We conclude that for what it deals with optical density a fully statistical model is valid, indicating that if non-stochastic linking between different monomers exists (see the NMR section) it leads to negligible changes in the absorption spectrum. We like to point out that equation (2) does not represent a particular sequence generated at random, but takes into account the whole statistical population. This simple approach accounts for the evolution of the absorption lineshape showing the maximum full-width-half-maximum (FWHM) for c = 0.5. Nevertheless the simulation is rough for a number of reasons: (i) Disubstituted rings do not fully break the conjugation; (ii) non-statistical nature of the real copolymer is confirmed by NMR measurements (see below); (iii) MD sequence is assumed to always break the conjugation regardless of the substituent position on the M ring. 3.2 Tight-binding calculation of the electronic structure A more sophisticated approach is followed in this section to get information about the optical response through the analysis of the electronic density of states as a function of the copolymer composition. We adopt a simple semiempirical tight-binding (TB) model which, for the case of regular copolymers, has been proven to give a good semiquantitative physical picture of the electronic levels (and the associated wavefunctions) with a reduced computational workload [l 11. We consider conjugated chains made by randomly distributed units of low-gap and high-gap polymers M and D, respectively. The carbon atoms are taken to be collinear, and the TB scheme is used to describe the 7relectron states. Only nearest-neighbour interactions are taken into account, so that calculations depend only on the transfer integral between adjacent atoms. In our model-system, every unit cell of M and D is made by two atoms. The transfer integrals for the single and the double bonds of M (D) are indicated by /3, and & (7, and ^(2).We assume that M and D are connected by a long bond, since this is the expected
OPTICAL PROPERTIES
:’ 1
/
I..
.
I
2
1.5
*
.
.
AND STRUCTURE
.
2 2.5
Energy (eV) Fig. 3. Tight-binding total density of conduction electronic states as a function of energy E for different low-gap polymer content. ground state configuration. Only one Coulomb integral (Y(which is set equal to zero, so that the Fermi energy results at E = OeV) is considered for the two monomers. This assumption is justified on the basis of very recent calculations of the valence band offset for the interface between two different polymers [12]. The transfer integral c at the M/D interface is fixed according to [ll]. The TB electronic states are obtained by diagonalization of a matrix M conceptually similar to that reported in [l l] for copolymers (M-J),),, made by regular alternation of x and y units of M and D. In the present case the non-zero matrix elements depend on the specific statistical arrangements of M and D segments. Moreover, the matrix dimension is now determined by the number of unit cells composing the chain since no boundary conditions are applicable. The wavevector k is no longer a good quantum number so that M has a simple tridiagonal form. We have simulated statistical copolymers by using a random generator to place the M and D units within the chain, according to their relative concentration. In order to reproduce a truly statistical sample, chains consisting of a large number of monomeric units have to be considered. For the present calculations, we have found that 1000 units are widely sufficient to get consistent results. In the following we investigate the electronic properties by analysing the total density of states dJ?) as a function of the energy E. The final spectra have been obtained through further averaging on different randomly generated distributions. Figure 3 presents the evolution of p(E) as a function of the low-gap polymer content. Only energies higher than the Fermi level EF are reported, p(E) being symmetrical around EF. The low-energy singularity in the density of states of the homopolymers become smeared out in the copolymers due to the
0
OF THIOPHENE
*.*'*'."'*'*'*1*..? 0.2 0.4
Vol. 99, No. 10
0.6
0.8
1
Concentrationof monosubsitutedrings Fig. 4. Experimental (open triangles) and calculated (solid triangles) copolymer absorption maxima as a function of concentrations of monosubstituted rings. The experimental (open circles) and calculated (solid circles) HOMO-LUMO energy gaps are also reported. statistical disorder, so that the onset of p(E) becomes clearly separated from the peak. One aspect of interest in the present calculation is the analysis of the HOMO-LUMO energy gap Eo. The energy position of these levels is directly obtained from the onset of the density of states spectra. As reported in Fig. 4, E. exhibits a quite complex behavior. Addition of small amounts of low gap constituent induces a strong reduction of Eo, while by increasing the M fraction from about 0.7 to 1 no further substantial lowering is attained. In the same figure, the experimental data for the absorption onset are reported. A behavior similar to the theoretical one is followed. Similar results have been described by dos Santos et al. [ 131for statistical copolymers containing segments of poly(p-phenylene vinylene) and its dimethoxy-substituted derivative. The considerably more sophisticated Valence Effective Hamiltonian technique was used. In order to explain the non linear dependence of E. on the copolymer composition, these authors concluded that occupation of the HOMO levels is concentrated on to the low-gap sequences, whereas the unoccupied levels are smoothly delocalized over the high-gap segments. As a consequence the HOMO-LUMO separation for the copolymer corresponds to a transition from the top of the valence band of the low-gap polymer to the bottom of the conduction band of the high-gap polymer, and this accounts for the non linear dependence of the gap experimentally observed. On the other hand, Fig. 4 shows that the energy of the experimental absorption maxima is almost linearly dependent on the copolymer composition. Calculated peaks follow the same behavior but in
Vol. 99, No. 10
OPTICAL PROPERTIES
AND STRUCTURE
JLL
LLJ
ULL
JLJ
LLL
UJJ
Err-HH
TT-HH
TT-HT
HT-HT
ET-HT
4?J
@I
Fig. 5. Scheme of the four possible enchainments between M monomers (a) and two different enchainments for the DMM sequence (b). the figure they have been shifted by 0.62eV toward high energies in order to be directly compared with the experimental data. It is reasonable to suppose that this additional blue shift of the absorption maxima is mainly caused by a further broadening of the density of states due to structural (inhomogeneous, non statistical) disorder of the samples. This implies that the additional 0.62eV blue shift does not depend on the copolymer composition, as actually found.
ii .I Y I a
The number and type of defects in the enchainments of the M and D units can be studied in detail with the NMR spectroscopy [14]. Neglecting defects due to enchainment in the 2,4 instead of the 25 position, defects in the monosubstituted chain are TT-HT (Tail-Tail, Head-Tail), HT-HH (Head-Tail, HeadHead) and TT-HH (Tail-Tail, Head-Head) enchainments while the regular one is HT-HT (Head-Tail, Head-Tail) as shown in the scheme of Fig. 5(a). In the previous section we did not distinguish between the DM enchainments for which the alkyl substituent is in the 3 or 4 position of the M monomer, as shown in Fig. 5(b). Indeed, due to the steric hindrance of the alkyl side chain, a very different torsion angle is expected in the two different MD enchainments which correspond to a TT-HT (see later) defect (the substituent of the M monomer is close to the D monomer) or to the regular HT-HT enchainment (the hydrogen of the M monomer is close to the D monomer). Moreover, in the previous approach we have neglected the defects in the chain segments of M monomers. A more detailed approach should take into account the asymmetry of the M monomer which produces defects in the DM and MM
251
100
P
711
3.3 NMR results
LU M D TT-HT
OF THIOPHENE
80 60
0
0.6
0.8
1
TT-HI-I . ...".,.",...,'.. 0 0.2 0.4 0.6
0.8
1
0.2
0.4
S-
Orn.
.
.
.
Fraction of D monomers
0
Fraction ofD mononwr~
Fig. 6. Relative fractions of the HT-HT (solid circles), TT-HT (open circles), HT-HH (solid triangle) and TTHH (open square) NMR peaks. The same fractions as expected for a statistical enchainment are also reported (solid lines).
712
OPTICAL PROPERTIES
AND STRUCTURE
enchainments. In fact, these latter defects also reduce the degree of planarity of the backbone and shorten the conjugation length of the polymer. In order to test the importance of these defects in the series of copolymers here studied, we have performed an NMR analysis of the aromatic proton resonances. The data presented in this paper are obtained by integrating the resonances of the aromatic protons at 6.97, 7.00, 7.02 and 7.05ppm, assigned to the HT-HT, HT-HH, TT-HT and HH-TT environments, respectively [ 141. In Fig. 6 the fractions of the four NMR signals are reported as a function of the fractionfof dibutylsubstituted monomers. For the monosubstituted polymer cf = 0) the fraction r of the resonance assigned to the regular enchainment (HT-HT) is 0.58, in agreement with the values obtained for other alkylthiophenes synthesized with the same polymerization method [15]. As the fraction f of D monomers increases, the observed decrease of r (increase of TTHT defect) is smaller with respect to that expected for a statistical enchainment (curves in Fig. 6). The polymerization mechanism of these materials leads thus to enchainments which do not follow a pure statistical distribution of defects along the copolymer chain. Indeed we have shown [16] that the same polymerization mechanism used for the synthesis of these copolymers favours the pairing of the HH and TT defects (defects in enchainment are preferentially correlated) in monosubstituted polyalkylthiophenes. The preferential correlation of defects in the enchainment of M monomers increases the length of the defect free segments [16]. In Fig. 6 we report the fractions of the NMR signals evaluated for a statistical distribution of defects in the enchainment by assuming that along the segments formed by M units the same defects as observed for f = 0 are present. With this assumption, from the NMR experimental data it follows that the correlation of the defect increases as f increases yielding an average length of the defect free segments (segments of M monomers with HT-HT enchainment) higher than the statistical average length. 4. CONCLUSIONS A novel class of conjugated systems, namely statistical copolymers consisting of mono and disubstituted thiophene units, have been recently synthesized. These materials exhibit an absorption maximum, and therefore a colour, which can be continuously modulated from 320 nm to 450 nm by increasing the disubstituted content. The emission spectrum is also sensitive to the copolymer composition, thus making
OF THIOPHENE
Vol. 99, No. 10
these materials potentially interesting for obtaining LED devices of controlled colour. We have been able to show that the changes of the optical spectra as a function of the copolymer composition are satisfactorily accounted for in terms of a random distribution of the mono and disubstituted units. However, NMR spectroscopy, which is sensitive to the environment of the individual thiophene units, demonstrates the presence of a non-statistical enchainment of the M and D monomers, mainly due to the polymerization method. The NMR spectra can be interpreted by assuming that a preferential grouping of the M monomers along the copolymer chains is introduced during the polymerization. These local effects appear to be unimportant in determining the electronic properties of the copolymers.
REFERENCES 1. 2.
Roncali, J. Chem. Rev., 92 (1992), 711. Arbizzani, C., Mastragostino, M., Panero, S. Prosperi, P. and Scrosati, B., Synth. Metal. 28 (1989), C663; Kawai, T., Kuwabara, T., Wang, S. and Yoshino, K. J. Electrochem. Sot., 137 (1990), 3793 3. Roncali, J., Garreau, R., Yassar, A., Marque, P. Garnier, F. and Lamaire, M. J. Phys. Chem. 91 (1987) 6706; Gustafsson, J.C., Inganas, 0. and Andersson, A.M. Synth. Metal. 62 (1994), 17. 4. Chao, S. and Wrighton, MS. J. Am. Chem. Sot. 109, (1987) 2197. 5. Lorcy, D. and Cava, M.P. Adv. Mater. 4 (1992), 562; Musmanni, S. and Ferraris, J.P. J. Chem. Sot., Chem. Commun. (1993), 172. 6. Pei, Q., Inganas, O., Osterholm, J.E. and Laakso, J. Polymer 34 (1993), 247. 7. Spangler, C.W., Liu, P.K., Hall, T., Polis, D.W., Sapochak, L.S. and Dalton, L.R. Polymer 33 (1992), 3937. 8. Catellani, M., Botta, C., Stein, P.C., Luzzati, S. and Consonni, R. Synth. Metals 69 (1995), 375. 9. Sagimoto, R., Takeda, S., Gu, H.B. and Yoshino, K. Chem. Express 1 (1986), 635. 10. Themans, B., Salaneck, W.R., Bredas, J.L. Synth. Met. 28 (1989), ~359. 11. Piaggi, A., Tubino, R. and Colombo, L. Phyx Rev. B51 (1995), 1624. 12. Massidda, S. and Piaggi, A. Private communication. 13. dos Santos, D.A., Quattrocchi, C., Friend, R.H. and Bredas, J.L. J. Chem. Phys. 100 (1994), 3301. 14. Sato, M. and Morii, H. Macromol. 24 (1991), 1196. 15. Barbarella, G., Bongini, A. and Zambianchi, M. Macromol. 27 (1994), 3039. 16. Botta, C., Stein, P.C., Bolognesi, A., Catellani, M. and Geng, Z. J. Chem. Phys. 99 (1995), 3331.
Pergamon
PII: s0038-1098(%)00246-3
OPTICAL PROPERTIES
AND STRUCTURE
OF THIOPHENE
SUBSTITUTED
COPOLYMERS
G. Lanzani, ** A. Piaggi,?’ C. Botta,b M. Catellani,b E. Prevosti,b P.C. SteinC and R. Tubino*d ‘Istituto di Matematica e Fisica, Universita’ di Sassari, Via Vienna 2, 07100 Sassari, Italy bIstituto de Chimica delle Macromolecole (CNR), Via Bassini 15, 20133 Milano, Italy ‘Kemisk Institut, Odense Universitet, Campusvej 55, 5230 Odense, Denmark ‘Dipartimento di Fisica, Universita’ di Milano, Via Celoria 16, 20133 Milano, Italy (Received 17 February 1996; accepted 3 April 1996 by E. Molinari)
The optical properties and the structure of random copolymers consisting of mono- and disubstituted alkylthiophenes have been studied by optical and NMR spectroscopy. While the optical data are consistent with the presence of randomly distributed mono and disubstituted units, NMR spectra reveal that preferential enchainment between monosubstituted units takes place. Copyright 0 1996 Elsevier Science Ltd Keywords: A. polymers, D. optical properties, E. nuclear resonances.
1. INTRODUCTION Among electroactive organic materials, thiophenebased polymers are the subject of considerable interest due to their electronic properties and to their potential applications [l]. The chemical versatility of the thiophene ring has led to the synthesis of polymers with various structure and different physical and electronic properties, and which are designed specifically for particular applications in electronics, communication or information technologies [2-41. The modulation of the chemical architecture of the polymeric chain may strongly influence the environmental stability and the processability of the material, as well as its charge transport and optical properties. The synthesis of new tailored organic semiconductors has been achieved in altering both type and condition of polymerization, and copolymerization offers one of the most powerful ways for the preparation of materials for new technologies. Recently the copolymerization of different thiophene-based monomers was used to produce macromolecules with narrow band-gap [5], or materials stable in both the neutral
and doped state [6], or to prepare polymers for non linear optical applications [7]. We have synthesised a series of alkyl-substituted thiophene copolymers with the aim to obtain processable materials having electrochromic properties suitable for application in optical devices. The copolymer&&on of 3-butylthiophene and 3,4-dibutylthiophene monomers in different ratios leads to a series of materials (see Fig. 1) in which it is possible to modulate continuously the effective conjugation length and the energygap of the chain [8]. The presence of dialkyl substituted rings in a poly(3-alkylthiophene) produces a local deviation from planarity of the backbone. This interrupts or decreases the pr orbital overlap, thus affecting the mean conjugation length of the chain. As a result, the copolymer series shows a different energy gap and its colour tones off from dark red to transparent. The aim of the present work is twofold. First, we theoretically evaluate the optical and electronic properties of the copolymer as a function of its composition. We assume that for each growing chain site the probability of enchainment by the mono- (M) and disubstituted (D) thiophene units is the same. The theoretical results are compared with the experimental absorption spectra. Then, we study the enchainment regularity by NMR technique, by detecting the effects of the monoalkyl thiophene
* INFM, Unita di Milano. t INFM, Unita di Pavia. 707
708
OPTICAL PROPERTIES
AND STRUCTURE
n Fig. 1. Copolymer structure. x and y represent the fractions of the two monomers in the copolymer composition. asymmetry on the MD and MM enchainments and the possible deviation from the statistical behavior. It will be shown that, while the optical data indicate that those effects are relatively unimportant on a lengthscale determined by the conjugation length, the NMR spectroscopy is suggestive of a quite complex copolymerization scenario on shorter lengths.
2. EXPERIMENTAL The copolymer series has been prepared via chemical oxidation with FeCls following the literature [9]. The materials are soluble in chloroform and processable from solution. The UV-VIS-NIR study was performed with a Varian 2400 Instrument on both solution and film samples. NMR experiments were performed on a Varian UNITY-500 spectrometer on CDCls filtered solutions whose concentration is about 10e2 g ml-‘. Proton chemical shifts are referenced to TMS. The temperature was stabilized at 298K. Some ‘H spectra were performed with i3C decoupling of the CHC13 resonance.
&E)
=
C+Y, j
and optical response
In this section we will analyse the evolution of the optical response of the random copolymers on the basis of a simple statistical description. This way, we are able to reproduce the absorption spectra as a function of copolymer composition and to compare them with the measured ones. First of all, we observe that the average conjugation length in substituted polythiophene is determined by the competition between the steric hindrance of the side groups that leads to a departure from planarity, and the r-electron delocalization that favors planar geometry. As a consequence the valence band width and the energy of the first optical allowed T--~F* transition depend on the chain configuration. In
Vol. 99, No. 10
mixed copolymers the chain twisting is directly related to the unit enchainment, i.e. the kind of adjacent rings. In particular, large deviations from planarity are associated to disubstituted thiophene rings. The value of the torsion inter-ring angle which results from steric repulsion of side chains can be roughly estimated considering the case of a pure disubstituted polythiophene chain and using the cosine law [lo] for 7r--7r overlap. The absorption maximum at 3.7eV would correspond to an average inter-ring torsional angle of 70”, producing a reduction in conjugation of about 66 % with respect to a planar chain. As a first approximation we can then assume that disubstituted rings cause a complete break of conjugation, while adjacent M-type monomers give rise to an almost planar conjugated sequence (see the NMR section for a less simplified model of the copolymer structure). A polymer chain is thus a sequence of different lengthconjugated segments alternated with non-conjugated defect segments. If we further assume a fully statistical enchainment of mono- and disubstituted rings we can use a simple statistical approach to describe the average r-r* transition energy as a function of the defect concentration. The absorption spectrum can be reproduced as a weighted integral on a distribution of different segments possessing different conjugation lengths and energy gaps E0 between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). E,, is described by the standard law: E,,(N) = A + B/N, where N is the number of conjugated (planar) rings and A and B are fixed parameters to be determined. A linear increase of the oscillator strength with N is assumed up to a saturation for N = 10. The contribution aN(E) to the absorption for a certain length is
3. RESULTS AND DISCUSSION 3.1 Statistical description of the copolymers
OF THIOPHENE
E - Ej(NI,
.
(1)
where g is the Gaussian lineshape function centered in Ej = E,,(N) + jhw and having a width 7, and S is the Huang-Rhys factor. The sum (1) is truncated atj = 2. The absorption of the real sample comes from the superposition of the contributions of all the possible conjugation lengths, weighted by a probability factor. We write Iv-1
a =
c
K=O
P(NM - K)Q~~-~(E).
(2)
In order to evaluate the probability factors we can count the number of different ways a certain length (NM -K) can be obtained keeping ftxed the total
Vol. 99, No. 10
OPTICAL PROPERTIES
AND STRUCTURE
1.0 h
5
0.8
4 m V
0.6
$
0.4
ii e 0.2 s n a
0.0 350
300 tot
,
.
400
,
.
450
,
.
,
500 .
,
600
550 .
,
.
,
11. 9
300
350
400
450
500
550
600
Wavelength (nm) Fig. 2. Top panel: Experimental absorption spectra at various concentrations of di-substituted thiophene units (a) c = 0.75, (b) c = 0.5, (c) c = 0.25. Bottom panel: Simulated curves. number of M-type or D-type monomers in the chain. This is simply done for K = 0. There are III,(O) = No + 1 ways to obtain a distinguishable sequence of NY conjugated monomers and No randomly placed defects. This is the maximum conjugation length. For K = 1 we count the ways to have NM - I conjugated rings like II,_,(K)
= (N;+,2)!
- 2!I-I&
M-
where we have subtracted combinations which have already been taken into account previously. By iteration we have (N,+K+
Kvnr-KW> =
l)!
NM!K!
-2l-I(K-
1)-v..
iy$qO)
-
where 0 I K 5 NM - 1; P(K) is the number of times a sequence of (NM-K) conjugated rings appears in all the possible distinguishable sequences. The probability of having (NM - K) conjugated rings is then WM
-
K) =
WK) NM_l
c
K=O
.
Jw)
(5)
OF THIOPHENE
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Results obtained using equation (2) for different defect concentrations c (where c = NM/(NM + No)) are shown in Fig. 2. The best parameters reproducing the experimental spectra are: B = 1 eV, A = 2.74eV, y = 0.9, S = 0.7. In Fig. 2 the comparison between simulated and measured curves is reported. The agreement is reasonable and the trend upon changing c is well reproduced showing a red shift of the absorption maximum for smaller D-type concentrations. The high energy side of the absorption band, due to disubstituted sequences, is not included in the simulation; this accounts for the discrepancy between calculated and experimental optical density at shorter wavelengths. We conclude that for what it deals with optical density a fully statistical model is valid, indicating that if non-stochastic linking between different monomers exists (see the NMR section) it leads to negligible changes in the absorption spectrum. We like to point out that equation (2) does not represent a particular sequence generated at random, but takes into account the whole statistical population. This simple approach accounts for the evolution of the absorption lineshape showing the maximum full-width-half-maximum (FWHM) for c = 0.5. Nevertheless the simulation is rough for a number of reasons: (i) Disubstituted rings do not fully break the conjugation; (ii) non-statistical nature of the real copolymer is confirmed by NMR measurements (see below); (iii) MD sequence is assumed to always break the conjugation regardless of the substituent position on the M ring. 3.2 Tight-binding calculation of the electronic structure A more sophisticated approach is followed in this section to get information about the optical response through the analysis of the electronic density of states as a function of the copolymer composition. We adopt a simple semiempirical tight-binding (TB) model which, for the case of regular copolymers, has been proven to give a good semiquantitative physical picture of the electronic levels (and the associated wavefunctions) with a reduced computational workload [l 11. We consider conjugated chains made by randomly distributed units of low-gap and high-gap polymers M and D, respectively. The carbon atoms are taken to be collinear, and the TB scheme is used to describe the 7relectron states. Only nearest-neighbour interactions are taken into account, so that calculations depend only on the transfer integral between adjacent atoms. In our model-system, every unit cell of M and D is made by two atoms. The transfer integrals for the single and the double bonds of M (D) are indicated by /3, and & (7, and ^(2).We assume that M and D are connected by a long bond, since this is the expected
OPTICAL PROPERTIES
:’ 1
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.
I
2
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*
.
.
AND STRUCTURE
.
2 2.5
Energy (eV) Fig. 3. Tight-binding total density of conduction electronic states as a function of energy E for different low-gap polymer content. ground state configuration. Only one Coulomb integral (Y(which is set equal to zero, so that the Fermi energy results at E = OeV) is considered for the two monomers. This assumption is justified on the basis of very recent calculations of the valence band offset for the interface between two different polymers [12]. The transfer integral c at the M/D interface is fixed according to [ll]. The TB electronic states are obtained by diagonalization of a matrix M conceptually similar to that reported in [l l] for copolymers (M-J),),, made by regular alternation of x and y units of M and D. In the present case the non-zero matrix elements depend on the specific statistical arrangements of M and D segments. Moreover, the matrix dimension is now determined by the number of unit cells composing the chain since no boundary conditions are applicable. The wavevector k is no longer a good quantum number so that M has a simple tridiagonal form. We have simulated statistical copolymers by using a random generator to place the M and D units within the chain, according to their relative concentration. In order to reproduce a truly statistical sample, chains consisting of a large number of monomeric units have to be considered. For the present calculations, we have found that 1000 units are widely sufficient to get consistent results. In the following we investigate the electronic properties by analysing the total density of states dJ?) as a function of the energy E. The final spectra have been obtained through further averaging on different randomly generated distributions. Figure 3 presents the evolution of p(E) as a function of the low-gap polymer content. Only energies higher than the Fermi level EF are reported, p(E) being symmetrical around EF. The low-energy singularity in the density of states of the homopolymers become smeared out in the copolymers due to the
0
OF THIOPHENE
*.*'*'."'*'*'*1*..? 0.2 0.4
Vol. 99, No. 10
0.6
0.8
1
Concentrationof monosubsitutedrings Fig. 4. Experimental (open triangles) and calculated (solid triangles) copolymer absorption maxima as a function of concentrations of monosubstituted rings. The experimental (open circles) and calculated (solid circles) HOMO-LUMO energy gaps are also reported. statistical disorder, so that the onset of p(E) becomes clearly separated from the peak. One aspect of interest in the present calculation is the analysis of the HOMO-LUMO energy gap Eo. The energy position of these levels is directly obtained from the onset of the density of states spectra. As reported in Fig. 4, E. exhibits a quite complex behavior. Addition of small amounts of low gap constituent induces a strong reduction of Eo, while by increasing the M fraction from about 0.7 to 1 no further substantial lowering is attained. In the same figure, the experimental data for the absorption onset are reported. A behavior similar to the theoretical one is followed. Similar results have been described by dos Santos et al. [ 131for statistical copolymers containing segments of poly(p-phenylene vinylene) and its dimethoxy-substituted derivative. The considerably more sophisticated Valence Effective Hamiltonian technique was used. In order to explain the non linear dependence of E. on the copolymer composition, these authors concluded that occupation of the HOMO levels is concentrated on to the low-gap sequences, whereas the unoccupied levels are smoothly delocalized over the high-gap segments. As a consequence the HOMO-LUMO separation for the copolymer corresponds to a transition from the top of the valence band of the low-gap polymer to the bottom of the conduction band of the high-gap polymer, and this accounts for the non linear dependence of the gap experimentally observed. On the other hand, Fig. 4 shows that the energy of the experimental absorption maxima is almost linearly dependent on the copolymer composition. Calculated peaks follow the same behavior but in
Vol. 99, No. 10
OPTICAL PROPERTIES
AND STRUCTURE
JLL
LLJ
ULL
JLJ
LLL
UJJ
Err-HH
TT-HH
TT-HT
HT-HT
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Fig. 5. Scheme of the four possible enchainments between M monomers (a) and two different enchainments for the DMM sequence (b). the figure they have been shifted by 0.62eV toward high energies in order to be directly compared with the experimental data. It is reasonable to suppose that this additional blue shift of the absorption maxima is mainly caused by a further broadening of the density of states due to structural (inhomogeneous, non statistical) disorder of the samples. This implies that the additional 0.62eV blue shift does not depend on the copolymer composition, as actually found.
ii .I Y I a
The number and type of defects in the enchainments of the M and D units can be studied in detail with the NMR spectroscopy [14]. Neglecting defects due to enchainment in the 2,4 instead of the 25 position, defects in the monosubstituted chain are TT-HT (Tail-Tail, Head-Tail), HT-HH (Head-Tail, HeadHead) and TT-HH (Tail-Tail, Head-Head) enchainments while the regular one is HT-HT (Head-Tail, Head-Tail) as shown in the scheme of Fig. 5(a). In the previous section we did not distinguish between the DM enchainments for which the alkyl substituent is in the 3 or 4 position of the M monomer, as shown in Fig. 5(b). Indeed, due to the steric hindrance of the alkyl side chain, a very different torsion angle is expected in the two different MD enchainments which correspond to a TT-HT (see later) defect (the substituent of the M monomer is close to the D monomer) or to the regular HT-HT enchainment (the hydrogen of the M monomer is close to the D monomer). Moreover, in the previous approach we have neglected the defects in the chain segments of M monomers. A more detailed approach should take into account the asymmetry of the M monomer which produces defects in the DM and MM
251
100
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711
3.3 NMR results
LU M D TT-HT
OF THIOPHENE
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0.8
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.
.
.
Fraction of D monomers
0
Fraction ofD mononwr~
Fig. 6. Relative fractions of the HT-HT (solid circles), TT-HT (open circles), HT-HH (solid triangle) and TTHH (open square) NMR peaks. The same fractions as expected for a statistical enchainment are also reported (solid lines).
712
OPTICAL PROPERTIES
AND STRUCTURE
enchainments. In fact, these latter defects also reduce the degree of planarity of the backbone and shorten the conjugation length of the polymer. In order to test the importance of these defects in the series of copolymers here studied, we have performed an NMR analysis of the aromatic proton resonances. The data presented in this paper are obtained by integrating the resonances of the aromatic protons at 6.97, 7.00, 7.02 and 7.05ppm, assigned to the HT-HT, HT-HH, TT-HT and HH-TT environments, respectively [ 141. In Fig. 6 the fractions of the four NMR signals are reported as a function of the fractionfof dibutylsubstituted monomers. For the monosubstituted polymer cf = 0) the fraction r of the resonance assigned to the regular enchainment (HT-HT) is 0.58, in agreement with the values obtained for other alkylthiophenes synthesized with the same polymerization method [15]. As the fraction f of D monomers increases, the observed decrease of r (increase of TTHT defect) is smaller with respect to that expected for a statistical enchainment (curves in Fig. 6). The polymerization mechanism of these materials leads thus to enchainments which do not follow a pure statistical distribution of defects along the copolymer chain. Indeed we have shown [16] that the same polymerization mechanism used for the synthesis of these copolymers favours the pairing of the HH and TT defects (defects in enchainment are preferentially correlated) in monosubstituted polyalkylthiophenes. The preferential correlation of defects in the enchainment of M monomers increases the length of the defect free segments [16]. In Fig. 6 we report the fractions of the NMR signals evaluated for a statistical distribution of defects in the enchainment by assuming that along the segments formed by M units the same defects as observed for f = 0 are present. With this assumption, from the NMR experimental data it follows that the correlation of the defect increases as f increases yielding an average length of the defect free segments (segments of M monomers with HT-HT enchainment) higher than the statistical average length. 4. CONCLUSIONS A novel class of conjugated systems, namely statistical copolymers consisting of mono and disubstituted thiophene units, have been recently synthesized. These materials exhibit an absorption maximum, and therefore a colour, which can be continuously modulated from 320 nm to 450 nm by increasing the disubstituted content. The emission spectrum is also sensitive to the copolymer composition, thus making
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these materials potentially interesting for obtaining LED devices of controlled colour. We have been able to show that the changes of the optical spectra as a function of the copolymer composition are satisfactorily accounted for in terms of a random distribution of the mono and disubstituted units. However, NMR spectroscopy, which is sensitive to the environment of the individual thiophene units, demonstrates the presence of a non-statistical enchainment of the M and D monomers, mainly due to the polymerization method. The NMR spectra can be interpreted by assuming that a preferential grouping of the M monomers along the copolymer chains is introduced during the polymerization. These local effects appear to be unimportant in determining the electronic properties of the copolymers.
REFERENCES 1. 2.
Roncali, J. Chem. Rev., 92 (1992), 711. Arbizzani, C., Mastragostino, M., Panero, S. Prosperi, P. and Scrosati, B., Synth. Metal. 28 (1989), C663; Kawai, T., Kuwabara, T., Wang, S. and Yoshino, K. J. Electrochem. Sot., 137 (1990), 3793 3. Roncali, J., Garreau, R., Yassar, A., Marque, P. Garnier, F. and Lamaire, M. J. Phys. Chem. 91 (1987) 6706; Gustafsson, J.C., Inganas, 0. and Andersson, A.M. Synth. Metal. 62 (1994), 17. 4. Chao, S. and Wrighton, MS. J. Am. Chem. Sot. 109, (1987) 2197. 5. Lorcy, D. and Cava, M.P. Adv. Mater. 4 (1992), 562; Musmanni, S. and Ferraris, J.P. J. Chem. Sot., Chem. Commun. (1993), 172. 6. Pei, Q., Inganas, O., Osterholm, J.E. and Laakso, J. Polymer 34 (1993), 247. 7. Spangler, C.W., Liu, P.K., Hall, T., Polis, D.W., Sapochak, L.S. and Dalton, L.R. Polymer 33 (1992), 3937. 8. Catellani, M., Botta, C., Stein, P.C., Luzzati, S. and Consonni, R. Synth. Metals 69 (1995), 375. 9. Sagimoto, R., Takeda, S., Gu, H.B. and Yoshino, K. Chem. Express 1 (1986), 635. 10. Themans, B., Salaneck, W.R., Bredas, J.L. Synth. Met. 28 (1989), ~359. 11. Piaggi, A., Tubino, R. and Colombo, L. Phyx Rev. B51 (1995), 1624. 12. Massidda, S. and Piaggi, A. Private communication. 13. dos Santos, D.A., Quattrocchi, C., Friend, R.H. and Bredas, J.L. J. Chem. Phys. 100 (1994), 3301. 14. Sato, M. and Morii, H. Macromol. 24 (1991), 1196. 15. Barbarella, G., Bongini, A. and Zambianchi, M. Macromol. 27 (1994), 3039. 16. Botta, C., Stein, P.C., Bolognesi, A., Catellani, M. and Geng, Z. J. Chem. Phys. 99 (1995), 3331.