Optical excitations in thiophenic based polymeric heterostructures

29 April 1996

PHYSICS

ELSEVIER

LETTERS

A

Physics Letters A 2 13 ( 1996) 288-292

Optical excitations in thiophenic based polymeric heterostructures L. Rossi avh,C. Botta ‘, S. Destri ‘, S. Luzzati ‘, A. Borghesi a,d, R. lkbino a*e a Istituto Nazionale per la Fisica della Materia, Pavia, Italy h Dipartimento di Fisica “A. Volta”, Universitci degli Studi di Pavia, Pavia. Italy ’ Istituto di Chimica delle Macromolecole CNR, Milan, Italy d Dipartimenta di Fisica, Universitci degii Studi di Modena, Modena. Italy e Dipartimento di Fisica. Vniversitri di M&no, Milan, Italy Received 10 January 1996; accepted for publication 19 January 1996 Communicated by V.M. Agranovich

Abstract We present evidence for the localization effect of photogenerated carriers in a regular block copolymer made of alternating sequences of thiophene and phenylene rings due to the presence of energy barriers which confine the excitations. Photoinduced (both in the visible and in the infrared regions) and Raman spectra have been studied in dependence of the degree of polymerization of the chains. They indicate that in a long chain copolymer photoinduced species like bipolarons are more strongly confined. PACS: 78.66.Qn; 73.20.D~; 42.70.Nq; 63.20.Kr

In the last few years many efforts have been made to build new materials based on inorganic semiconductors with strong nonlinear optical properties and controlled optical and electrical properties for applications in optoelectronic devices [ 11. The enhancement of nonlinear optical properties of a bulk semiconductor can be achieved, for example, by confining the carriers in a potential well of a multiple quantum well structure, because carrier confinement increases the exciton binding energy [ 21. Hitherto many structures have been projected and realized, such as superlattices, quantum wires and quantum dots, where carriers are confined to moving in two, one and zero directions, respectively. Organic polyconjugated materials are natural quantum wires because of their quasi-one-dimensional

electronic structure due to the delocalization of Telectrons along the backbone. In a conjugated chain, the ordered sequence of segments with a different energy gap ( Eg) can add a further quantization to the electronic levels and the nonlinear optical properties can be enhanced, because of the sharpening of the absorption bands. In order to have a true alternation of low (well) and high (barrier) (Es) segments, the length of the segments must be large enough, but the synthesis of such a structure is not a trivial task. A regular alternating conjugated copolymer was produced by Jenekhe et al. [3] by alternating aromatic and quinoidal thiophene base structures. They reported very high second order hyperpolarizabilities for these materials [4,5], Copolymers of regularly alternating thienylenic

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L. Rossi er al./Physics

(T6) and phenylenic (B2) moieties were projected and realized recently by our group via a coupling reaction [ 61. This synthetic route gave controlled sequences of copolymers with a low degree of polymerization (-2). We expect that in these copolymers there is an alternation of low (thienylenic) and high (phenylenic) energy gap segments because the parent conjugated polymers polythiophene and polyparaphenylene have Eg N 2.1 and Eg N 4 eV, respectively. Previous works [ 7,8] showed that there is a real alternation of low and high energy gap segments and that photoexcitations like bipolarons were demonstrated to localize within the potential well. The aim of this work is to investigate the optical and photoinduced properties of a (ThBz),, copolymer with a high degree of polymerization obtained with a new synthetic route [9] in order to clarify the influence of the length of the chain on the position of the photoinduced peaks. The long (T6B2)” copolymer was synthesized following the synthetic route reported in Ref. [ 91. The copolymer, whose general formula is [ - (T6) -CH-N(B2) -CH-N-l,,, was dispersed in KBr and pressed in pellets. The absorption spectra in the visible and ultraviolet regions were recorded with a Cary 5E spectrometer. The Raman scattering was performed at room temperature on KBr pellets. Pre-resonant Raman spectra were recorded with a Bruker IF66, equipped with a FRAlOO spectrometer, and with a Nd-YAG laser as a pump ( A,,, = 1064 nm) . The photoinduced absorption (PA) measurements were carried out at 77 K by using a continuous wave (cw) photomodulation technique with a mechanically chopped cw Ar+ laser as pump (A,,, = 488 nm) and a tungsten-halogen lamp as a probe. Optical transmission was performed by a McPherson 218 monochromator in the 0.4-1.4 eV range with cooled PbS or InSb detectors and in the 1.4-2.5 eV range with a photomultiplier tube. The PA signal was detected with a Stanford SR850 DSP lock-in amplifier. To obtain the PA spectra ( -AT/T) we normalized the fractional changes of the sample transmission due to the laser excitation (AT) to the transmission (T) . The steady state PA spectra in the medium (MIR) and near (NIR) infrared were measured with a Fourier transform infrared (FTIR) Bruker IFS66 spectrometer covering a spectral range of 450-15000 cm-‘. The

Letters A 213 (1996) 288-292

289

3sCII -Long (ThBJn ]

0,s. 400

200

600

Wavelength

XIX)

1000

(nm)

Fig. 1. Absorption spectra (T = 300 K) of (A) short (TbB2)” synthesized by the coupling reaction (dashed line) and (B) long (ThB2 ),, synthesized by the polycondensation method (solid line).

IJOO

1450

1500

1550

1600

I650

1700

Raman shift (cm-‘) Fig. 2. Raman scattering of (A) short and (B) long (T6B2L ( kxC = 1064 nm, T = 300 K).

-AT/T spectra were taken at 80 K by exciting with a cw Ar+ laser incident on the sample for 10 s. Successive cycles were accumulated and the signal averaged until a satisfactory signal to noise ratio was achieved. In Fig. 1 the absorption spectra of the short copolymer (A) obtained by the coupling method [6] and of the long copolymer (B) obtained by the polycondensation method [ 91 are reported. The product B shows a strong redshift in the r-q* transition with respect

290

Fig.

L. Rmsi et aI./ Physics Letters A 213 (1996) 288-292

3.

Photoinduced

absorption

spectrum

of

long

( kxC = 488 nm, the laser intensity is 70 mW/cm*, frequency is 13 Hz, T = 77

K)

1100

700

(TeB?)”

the chopper

1500

1900

Wave number (cm-‘) Fig.

4.

(&

= 488

Photoinduced

absorption

spectrum

of

nm, the laser intensity is 40 mW/cm’,

long

(ThBz)”

T = 77 K) in

the MIR region.

to the polymer A, indicating a more extended conjugation length, in agreement with an increase in the mean molecular weight. From a line profile analysis of an in-chain X-ray diffraction peak [9] we deduce an approximate length of about four repeating units, which is significantly longer than that derived for the copolymer obtained from the coupling reaction. In Fig. 2 the Raman spectra of the A and B copolymers are shown. The frequency position of the thiophenic stretching mode shifts from 1463 cm-’ (A) to 1459 cm-’ (B) indicating an increase of the conjugation length in copolymer B in agreement with the absorption spectra. The other differences in the Raman spectra are consistent with the two different synthetic

methods. In the following we discuss the PA spectrum of the B copolymer in comparison with that reported in Ref. [ 81 for the A copolymers, in order to understand the influence of the chain length on the photoexcitation properties. In Fig. 3 the PA spectrum of the long (ThB2)* is shown. It has been taken out of phase with respect to the laser because the sample was strongly photoluminescent. Two peaks at 0.68 eV and at 1.42 eV are observed. The bleaching of the r-r* transition is covered by a strong modulation up to 2 eV. The dependence on the intensity of the 0.68 eV PA band on the laser power and on the chopper frequency indicates a bimolecular recombination dynamics and a life-time of the photoinduced species lower than 10m3 s. The behavior of the 1.42 eV peak could not be studied because at this energy there was a strong photoluminescence signal. A confirmation of the presence of the same peaks was obtained with the FTIR spectrometer modulation technique. We can compare the photoinduced absorption properties of (T&Z),, with those reported by Vardeny et al. [ lo] for polythiophene (PT), and with those of thiophenic based conjugated polymers [ 111 and of the a-sexithyenil oligomer [ 121. In PT the two peaks at 0.45 and at 1.25 eV in the photoinduced absorption spectrum were assigned to charged species like bipolarons. In the (T6B2) n we can see two similar peaks at 0.68 eV and at 1.42 eV. Their positions in energy are consistent with the higher energy gap of the (TeBz),, with respect to PT. To have a deeper comprehension of the origin of the PA peaks we studied the photoinduced IR properties of long (TeBz),, to check if there were IR active vibrational (IRA%‘) bands as expected for charged photoinduced species (see for example Ref. [ 131) . In Fig. 4 the PA spectrum in the IR region is reported. There are strong IRAV bands at 1046,1136,1208, and 1335 cm-‘. The IRAV modes of the long (T&2),, shown in Fig. 4 are peculiar to the bipolaronic states of the thiophene based polymers [ 11,141. The four bands at 1046,1136,1208, and 1335 cm-’ are recognized as the translational modes Tt , T2, T3, and T4, respectively. Weaker features are observed at 730, 893, and 985 cm-‘, which can be assigned to ring modes. Two additional features are observed at 1178 and 1383 cm -‘. These bands are probably related to the local-

L. Rossi et al./ Physics Letters A 213 (1996) 288-292

ized modes AT and A; associated to the T modes, as predicted by Hicks and Mele for PT [ 151. The oscillator strength of these A modes is predicted to increase with the confinement of the charged defects. In PT these modes are just weak features increasing their intensity in alkyl-substituted PT. In the (TsB2) ,t copolymer their oscillator strength is considerably higher. A stronger confinement due to the heterostructure may possibly explain this behavior. The modulation up to 2 eV (Fig. 3) could probably be due to an electromodulation effect of the band edge caused by the presence of charged photoinduced species. Comparing the PA spectra of the short and long (T~Bz)~ we can infer that both samples can host the bipolaronic states typical of PT. In the long (TGB~),, the bipolaronic peaks at low and high energy are shifted to lower and higher energy, respectively, in spite of the redshift of the r-r* transition. This behavior can be qualitatively explained by a stronger confinement of the bipolaronic states in the long (ThBz),,. In fact, the energies of the optical transitions associated to the low-energy bipolaronic state ( kiwi) and to the high-energy bipolaronic state (&2) can be related to the gap parameter A0 = iEg and to the confinement parameter y through the equations riwi =Ao(l

-a),

h~,,=Ao(l+Jt).

where d, is the extrinsic contribuwith y = &/Lta, tion to the gap parameter and A is the electron-phonon coupling parameter. These relations were obtained, within a Su-Schrieffer-Heeger model Hamiltonian, by Fesser et al. [ 161, neglecting the correlation effects. If A0 is lower than in short (ThBl),, the redshift of the kiwi bipolaronic transition and the blueshift of the F&Q bipolaronic transition can be explained only by supposing that the confinement of the photoinduced species became stronger. The better quality of band engineering in long (TGB~)~ can account for this behavior. Moreover, in the long ( T6B2)n there is no evidence for the presence of the 1.95 eV peak seen in the PA spectrum of the short sample by Piaggi and coworkers [ 81. The origin of this peak was not clear, though it could possibly be related to a long lived (triplet) exciton-polaron state or to an interface state trapped between the thienylenic and phenylenic segments of the copolymer. With the help of the present results on

291

the long (T6B2) n we conclude that this peak was due to a triplet state, because in a longer copolymer the transition associated to an interface state should be stronger. The presence of a strong triplet state transition in the more disordered A copolymer is justified by the observation of an intense triplet exciton transition in the PA spectra of thyophene-based polymers where 7r electron localization is induced by disorder [ 171. We presented new photoinduced absorption data on a (ThBz), sample with a high degree of polymerisation. We compared them with the PA data for a short ( T~Bz)~, for polythiophene, for thiophene based polymers, and for a-sexythienyl. We confirm that in (ThBz),, the potential well is large enough to host the photoexcitations typical of PT like bipolarons. The present study shows that by increasing the length of the copolymer chain a stronger confinement of the photoexcited species in the potential well was attained. Moreover, we assign the 1.95 eV PA observed in the short chain (TbB2) n to a triplet-triplet transition. The authors are grateful to Marco Moscardini the technical assistance.

for

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