*Corresponding author. Tel.: #81-22-217-7752; fax: #81-22-217-7746.

E-mail address: suto@surface.phys.tohoku.ac.jp (S. Suto)

Journal of Luminescence 87}89 (2000) 773}775

Excitation dynamics in p}n conjugatedsilylene}biphenylene copolymers

S. Suto!,*, R. Ono!, M. Shimizu!, T. Goto!, A. Watanabe",M.-C. Fang", M. Matsuda"

!Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan"Institute for Chemical Reaction Science, Tohoku University, Sendai 980-8577, Japan

Abstract

We have measured absorption spectra, luminescence spectra and the time response of luminescence intensity in p}nconjugated polymers of silylene}biphenylene copolymers in tetrahydrofuran solution at room temperature. In order toinvestigate the p-conjugated length dependence of optical properties, we have prepared the three copolymers which havedi!erent silylene chain lengths of m"1, 2 and 6. It is found that the excitations are strongly localized at biphenylene unitsfor m"1 and that a strong lattice relaxation occurs for m"2. For m"6, the charge transfer excitation from silyleneunits to biphenylene units is indicated. ( 2000 Elsevier Science B.V. All rights reserved.

Keywords: p}n conjugated polymers; Photo-excitation dynamics; One-dimensional system; Charge transfer; Lattice relaxation

Recent development in the synthesis of organometallicpolymers has led to quest for new functionality in novelpolymers. In particular, we are interested in the p}nconjugated polymers which consist of the p-conjugationunits of Si}Si chain and the n -conjugation units ofaromatic molecules because the structure is the same asthe one-dimensional quantum well structure. The p-con-jugation and n-conjugation units correspond to the welland the barrier structures, respectively. Moreover, thereare potential abilities of conducting, semiconducting andlight-emitting polymers. In order to investigate the op-tical properties of p}n conjugated polymers, Fang et al.synthesized the silylene}biphenylene copolymers(SBPCs) [1]. Fig. 1 shows the structure of SBPC, whichhas a regular alternating arrangement of the p-con-jugated unit of silylene chain and the n-conjugated unitof biphenylene. The number m indicates the silylene chainlength and the number n denotes the degree of polymeriz-ation. The chain lengths m"1, 2, 3, 4, 6 were prepared.

They measured the absorption and the luminescencespectra [2]. They found that the two spectra are in-#uenced by silylene chain length but the systematicmeasurements of SBPC have not been performed untilnow. In this paper, we have measured the absorptionspectra, the luminescence spectra and the time responseof luminescence intensity of SBPC for m"1, 2, 6.

SBPCs were synthesized by the procedure reported inRef. [2]. The molecular weights (M

8) were 7500, 8500

and 10,900 for m"1, 2, 6, respectively, and the distribu-tions (M

8/M

N) were 1.3, 1.7 and 1.6. The degree of

polymerization n is roughly estimated to be between 20and 35. The excitation source used in this work was thefrequency-doubled output of Rhodamine B or Rho-damine 6G dye lasers (Coherent 701-3) pumped by thedoubled output of a CW mode-locked Nd : YAG laser(Coherent Antares 76-s). The full-width at half-maximumof the doubled output was 6 ps and the repetition ratewas 76 MHz. The average output power was100}200 mW. The luminescence was analyzed witha CT-25CD(c) (Nihon Bunko Co.) subtractive doublemonochromator. The spectral resolution was 7.5 meV(61 cm~1). Luminescence spectra were detected bya photomultiplier (Hamamatsu R636). The time-resolvedluminescence spectra were obtained using a synchroscan

0022-2313/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 9 6 - 8

Fig. 1. Structure of silylene}biphenylene copolymers (SBPC).The number m shows the silylene chain length and the numbern indicates the degree of polymerization.

Fig. 2. Absorption spectra (solid lines) and the photo-lumines-cence spectra (dashed lines) of SBPC for m"1, 2, 6. The smallarrows indicate the excitation energy for the luminescencespectra.

Fig. 3. Time response of luminescence intensity of SBPC form"1, 2, 6. The dashed lines are the luminescence spectra.

streak camera (Hamamatsu C-1587) and a CCD camera(EG&G OMA Spec 4000). The time resolution, i.e. widthof the laser pulse measured by this system, was 20 ps. Allthe measurements were carried out for the tetrahyd-rofuran (THF) solution at room temperature. The con-centrations of SBPCs were 10~4 M/l.

Fig. 2 shows the absorption (thick lines) and lumines-cence (dashed lines) spectra for m"1, 2, 6. A broadabsorption peak is observed at 4.7 eV for m"1 andshifts to lower energy with increasing the number m, i.e.p conjugation length, as reported by Fang et al. [2]. Thepeak for m"6 is observed at 4.3 eV. This result indicatesthat the lowest transition is the p}p* transition at thep-conjugation unit for m"2, 6 and the n}n* transitionat biphenylene units for m"1 because the SBPC ofm"1 do not have the p-conjugation unit. The absorp-tion energy becomes smaller with increase in the p-conjugated length like a quantum well structure.

The luminescence peak is observed at 3.88 eV form"1, 2.98 eV for m"2, and 2.70 eV for m"6. Thepro"le of the spectrum for m"1 is the same as that of the

biphenylene molecules in cyclohexane solution [3]. Thepeak energy of SBPC is 400 meV lower than that of thebiphenylene molecule. The luminescence line shape form"2 is the Gaussian. The spectral feature for m"6 isbroad and structure less. The Stokes shift increases withincreasing the p-conjugated length.

Fig. 3 displays the time response of luminescenceintensity of SBPCs of m"1, 2, 6. The dashed lines are theluminescence spectra. For m"1, the decay is not a singleexponential and the two decay components of 300 ps and1.4 ns are observed between 3.6 and 4.1 eV. Below 3.6 eV,the rise time of 50 ps and the decay time of 900 ps areobserved. For m"2, the single decay component of1.0 ns is observed. For m"6, the decay is not a singleexponential. The two decay components depend on theobserved energy. The fast component changes from 150to 700 ps and the slow component from 850 ps to 4 s.

For m"1, the absorption and luminescence spectrashow that the optical properties are mainly attributed tothe n}n* transition at biphenylene units. The propertiesare similar to biphenylene molecules. The semiempiricalmolecular orbital calculation with the MNDO methodsupports this result. The lowest excitation, i.e. transitionbetween HOMO and LUMO, is strongly localized atbiphenylene units in SBPC for m"1. The polymeriz-ation e!ects are observed in the time response of theluminescence intensity. The luminescence of biphenylenemolecule decays single exponentially with a decay timeof 16 ns [3]. In contrast, the SBPC has two decay

774 S. Suto et al. / Journal of Luminescence 87}89 (2000) 773}775

Fig. 4. Energy diagram of the charge transfer model. The thickarrows denote the radiative transition and the arrow line indi-cates the nonradiative transition. First, the absorption occurs(1). Second, the excited electron at the p* orbital transfers to then * orbital (2). Finally, the electron at the n * orbital recombineswith the hole at the p orbital (3).

components much faster than that of biphenylene mol-ecule. Moreover, the rise and decay times observed below3.6 eV indicate the dynamics of excitations or excitons.The lower energy sites can be formed at some parts inSBPC chains. The excitation at biphenylene units moveto the lower energy site within 50 ps and decay radiative-ly with a time of 900 ps. For m"2, the luminescenceband has a Gaussian line shape and the single decay timedoes not depend on the observed energy. These resultsstrongly indicate that the luminescence occurs afterstrong lattice relaxation [4,5].

For m"6, we propose that the luminescence band isattributed to the charge transfer excitation. The dynam-ics of photo-excited electrons in SBPC is described bycharge transfer model as shown in Fig. 4. Since theoverlap of the wave functions between silylene units andbiphenylenunits is small [6], we consider the p and p*orbitals at silylene units and the n and n * orbitals atbiphenylene units. If the silylene chain length m is longerthan 2, the energy level of the n orbital is lower than thatof the p orbital as shown in Fig. 4 [2]. The thick arrowsindicate the radiative transition and the dashed arrowdenotes the nonradiative transition. First, absorptionoccurs at the p-conjugated units. Second, the excitedelectrons relax to the n* orbital at biphenylene units withconformational change between silylene and bipheny-lene. Finally, the electrons recombine with the holes to

emit the photons. Since the time response of lumines-cence intensity has two decay components, we considerthat there are two stable conformations between silyleneand biphenylene units and that the energies of the twoconformations are nearly equal. The polymer chains de-form easily in any other freedom than the two stableconformations. This #uctuation leads to the energy dis-tribution of the charge transfer states. There are twopossibilities to explain the larger decay components atthe lower energy. One is that the lifetime depends on thepolymer conformation, and the other is that the excita-tion is transfered from the higher energy state to thelower energy state.

In conclusion, we have studied the photo-excited statesin the p}n conjugated polymers of SBPC in the THFsolution at room temperature. We found the strong p-conjugation length dependence of absorption spectra,luminescence spectra and the time response of lumines-cence intensity. For m"1, the excitations are stronglylocalized at biphenylene units. For m"2, the absorptionoccurs at silylene units and the luminescence is observedafter strong lattice relaxation. For m"6, the chargetransfer model explains quite well. Absorption occurs atthe p-conjugated units and then the excited electronsrelax to the n* orbitals at biphenylene units. Finally, theelectrons recombine with the holes to emit light withconformational change between silylene and biphenyleneunits.

References

[1] M.-C. Fang, A. Watanabe, M. Matsuda, J. Organomet.Chem. 489 (1994) 15.

[2] M.-C. Fang, A. Watanabe, O. Ito, M. Matsuda: Macro-molecules 489 (1996) 15, and the references therein.

[3] I.B. Berlman, Handbook of Fluorescence Spectra of Aro-matic Molecules, Academic Press, New York, 1995.

[4] H.S. Plitt, V. Balaji, J. Michl, Chem. Phys. Lett. 213 (1993)158.

[5] H. Sumi, A. Sumi, J. Phys. Soc. Jpn. 63 (1994) 637.[6] H. Shizuka, Y. Sato, Y. Ueki, J. Chem. Soc. Faraday Trans.

180 (1984) 341.

S. Suto et al. / Journal of Luminescence 87}89 (2000) 773}775 775