Site-selective fluorescence studies on polysilylenes

Chemical Physics 150 ( 199 1) 8 l-9 1 North-Holland

Site-selective fluorescence studies on polysilylenes A. Elschner, R.F. Malwt, L. Pautmeier, H. BIssler Fachbereich Physikalische Chemie und Zentrumj2r Materialwissenschaften der Philipps-Vniversittit, D-3550 Marburg, Germany

M. Stolka and K. McGrane Xerox Webster Research Center, Webster, NY 14580, USA Received 26 July 1990

Low temperature site-selective fluorescence spectroscopy was performed on poly( di-n-hexylsilylene) and poly (methyl- phenylsilylene), both present as solid film and matrix-isolated in a 2MTHF glass. The experiments confirm that absorption and emission are exciton processes controlled by disorder arising from the length distribution of ordered chain segments that depends on the morphology of the sample. Upon exciting into the center portion of the density of states (DOS), excitations relax to a common occupational density of states mapped by the inhomogeneously broadened fluorescence spectrum. Upon tail state excitation, resonant emission shifting linearly with the excitation energy becomes progressively important. Coupling to both phonons and vibrations of the chain is very weak. The experiments are in quantitative accord with the results of Monte Carlo computer simulations for the random walk of excitations in an array of hopping sites featuring a Gaussian distribution of site energies. The existing analogy between exciton and charge transport suggests that site-selective fluorescence studies are a useful tool also for delineating the key features of charge carrier motion in polymeric systems.

1. Introduction

The synthesis of silicon-based polymers, the polysilylenes, opened a new field of polymer spectroscopy and technology. Due to the lower excitation energy of silicon relative to carbon based polymers, their lowest excited states are those of the chain rather than those of the pendant groups. Studying their spectroscopic properties will, therefore, provide a handle on the dynamics of the excitations of a o-bonded chain, unaffected by rapid localization of excitations at the pendant groups. The reduction of the excitation energy relative to that of a o-bonded carbon chain has also practical advantages: The relevant spectroscopic range is conveniently accessible and the number of accidental low molecular weight impurities acting as potential trapping sites is reduced. It is known from the literature that polysilylenes are strongly fluorescent [ 1,2], the fluorescence decay time being of the order of several hundred ps [ 31. The fluorescence spectrum is a more or less structureless band several hundred cm-’ wide, and its peak 0301-0104/91/$03.50

position depends on the morphology of the chain. For instance, stretching the chain in the course of crystallization shifts the emission towards lower energies consistent with the notion of an exciton being delocalized over an increased number of monomer units [3,41. This paper is focused on the dynamics of optical (singlet) excitations of the polysilylene chain and the origin of spectral broadening, both in absorption and emission [ 5 1. A simple explanation for the latter is based on the disorder present in a polymer. It should give rise to a distribution of effective conjugation lengths [ 1,3] and/or of van der Waals interaction energies of an excitation with its environment. This results in inhomogeneous line broadening, analogous to what has been observed with pendant group polymers such as polyvinylcarbazole [ 6 ] or x-conjugated polymers like poly ( phenylenevinylenes ) [ 7 1. An alternative explanation invokes structural relaxation of a 1D chain in the vicinity of either an kxciton or a charge carrier [ 8,9 1. The classic polaron description [lo] rests on the notion that photon absorption cre-

0 1991 - Elsevier Science Publishers B.V. (North-Holland)

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A. Elschner et al. / SSF studies on polysilylenes

ates a pair of decoupled charge carriers, both distorting their local environment and undergoing subsequent radiative recombination thereby emitting fluorescence associated with a large Stokes shift. Spectral broadening would reflect the strength of electron-phonon coupling and, thus, would be essentially homogeneous in nature. Formation of an excitonic polaron would manifest itself in a similar way. Site-selective fluorescence spectroscopy (SSF) [ 111 provides an easy tool for distinguishing among both possibilities. In the case of a set of identical absorbers coupled strongly to the lattice one expects to see broad luminescence spectra invariant with the excitation wavelength, because structural ielaxation erases the excitation memory. The other extreme is a set of electronically coupled chromophores whose excitation energies are subject to a distribution reflecting the built-in disorder. Excitation at an arbitrary site will, on average, be followed by energy transfer lo lower-lying acceptor sites, experimentally revealed by spectral diffusion, unless initial excitation occurs into the tail states of the distribution of states (DOS) where the dilution of acceptor states prevents energy transfer on the time-scale of the excited state lifetime [ 121. Tuning a spectrally narrow excitation laser across the inhomogeneously broadened absorption profile will, in this case, generate fluorescence spectra that are spectrally invariant and inhomogeneously broadened upon excitation above a certain localisation threshold vlocyet vary resonantly with excitation and be homogeneously broadened for v, < vloc. A previous letter reported [ 13 ] first SSF results on poly (octylmethylsilylene ) . They supported the disorder model and demonstrated that the true Stokes shift between absorption and emission must be less than the spectral resolution in that experiment ( = 20 cm- ’ ) . In this paper we present a full analysis of lowtemperature fluorescence spectra of poly (di-n-hexylsilylene ) (PDHS) and poly (methylphenylsilylene) (PMPS), both matrix-isolated in a methyltetrahydrofuran (MTHF) glass or as a film, employing siteselection techniques. We will attempt to establish the applicability of the random walk concept for describing transfer of oo* singlet excitations in polysilylenes and the absence of polaronic effects, and will also point to the close relationship that exists between energy and charge transport in these systems.

2. Experimental Poly(di-n-hexylsilylene) (PDHS) was synthesized by Wurtz coupling reaction of 0.4 mol dichlorodi-n-hexylsilane with a twofold molar excess of molten sodium at about 110” C in 800 ml of a 85 / 15 toluene/n-octane mixture, basically according to Zeigler [ 141. In this procedure, the dispersion of sodium in a parafflnic oil was added steadily in approximately 20 min to the solution of monomer in the solvent mixture. The total reaction time was 4 h. Towards the end of the reaction, additional 300 ml of the solvent mixture was added to reduce the viscosity of the reaction slurry. The hot mixture was then filtered on glass fiber filters to separate the insolubles (sodium chloride, residual sodium and traces of crosslinked polymer) from the polymers in solution. The polymer was then isolated from the clear colorless solution by precipitation in isopropanol and further purified by repeated reprecipitation from heptane into isopropanol. PMPS was prepared in an analogous way from the methylphenyl silicon dichloride. Average molecular weights typically exceeded 200000. Films were prepared by solution coating on either an oxidized aluminum substrate (PDHS ) or on quartz (PMPS ) . For measuring fluorescence as well as absorption spectra of matrix-isolated samples, the polymer was dissolved in 2-MTHF at a concentration ranging between 10m5and 10m6mol of polymer repeat units per liter. A 2 mm suprasil cuvette containing the solution was attached to the cold finger of a helium flow cryostat using an indium wire for thermal coupling. The sample was typically cooled at a rate of 2 K s-l. For recording film spectra, the glass or metal support carrying the film was mounted onto the cryostat finger. The sample temperature was between 5 and 10 K. A tunable pulsed dye laser, frequency doubled for producing photon energies above 29000 cm-‘, with spectral band widths of less than 1 cm-’ was used for site-selective excitation. Fluorescence was recorded with a 1 m Jobin-Yvon double monochromator employing box-car averaging techniques. Choosing a gate width of 5 ns ensured that only short-lived fluorescence is detected. Filters blocking the laser light were removed once the reading of the motor-driven monochromator was off-set from the laser by 20 to

A. Elschner et al. / SSF studies on polysilylenes

30 cm-‘. The spectral resolution varied between 10 and 25 cm-‘, the latter value corresponding to the upper spectral range. Fluorescence spectra have not been corrected for instrumental sensitivity. There was no indication of photolytic decomposition of the sample at about 6 K. Repeated warming of the sample to room temperature and refreezing gave identical results. The experiments were complemented by Monte Carlo simulation aimed at revealing how an ideal hopping system would behave under analogous conditions. Following a well tested simulation routine [ 15 ] a cubic test sample consisting of sites with energies distributed according to a Gaussian DOS with standard deviation TV was set up by computer. Excitations were started at specified energies and their energetic relaxation in course of their random walk was followed as a function of time up to a time 104t0, to being the dwell time of an excitation in a hypothetical ordered counterpart hopping system. This corresponds to excitons with intrinsic lifetime of 10m9s

83

1

27.0

275

28.0

28.5

~,(lO~cm-') Fik 2.Variation of the energy Y,, of the main emission features of fig. I with excitation energy u,.

!‘A

*,...,

00 Wavenumber (cm-')

Fig. 3. Fluorescence spectra of a PDHS film parametric in excitation energy, indicates by arrows. 28

27

26

Energy (103~m“) Fig. 1. Fluorescence spectra of PDHS/MTHF (T= 5 K). Excitation energies are marked by arrows.

and an inter-site hopping time of 1O- *3 i and appears to be realistic for singlet excitations in polysilylenes. For details of the computations the reader is referred

A. Elschner et al, / SSF studies on polysilylenes

k

/

27.0L

;Y Y --_I

:-24OCrd vc vc -220cm-l

v-6 90cm-’

I

vc - 69Om

I

27.0 28

29

Fig. 5. Emission versus excitation energy for the fluorescence features of fig. 4.

, 28

27

Energy ( 1O%s1-’1 Fig. 4. (a), (b) Fluorescence spectra of PMPS/MTHF (T= 5 K, monochromator resolution 25 cm-‘). A, 8, C label the three different emission features. A” is a vibronic feature of emission A.

to earlier work [ 15,161. Suffice to mention that the exchange rate was chosen to be the product of a frequency factor, a wavefunction overlap factor and a Boltzmann factor. Although originally intended to describe exchange-coupled systems this turns out to be a good approximation for the FiSrster coupling case

290 vex(103cm-‘)

27

Energy ( 103cd’ )

I

28.0

29

L>l 28.5 Energy

28

27.5

27

C103cm-’ 1

Fig. 6. Temperature dependence of the fluorescence of PMPS/ MTHFexcitedat u,=29180 cm-‘.

as well, provided that the Fiirster radius is low enough to ensure that transfer is essentially restricted to nearest neighbors. In the course of the computations both the occupational density of states as we11as the number of new sites an excitation has visited was followed as a function of time, parametric in excitation energy. Averaging involving 3OCKl excitations started

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31

30

29

28

27

26 J

Energy (lOfca_’ 1

28

27

25

25

24

22

ENERGY (103m-‘1

Fig. 7. Absorption and fluorescence spectra (r&=33900 cm-‘) of a PMPS film. Crosses represent a Gaussian tit to the absorption tail (a=510 cm-‘). Open circles represent the inhomogeneous emission profile simulated for a DOS of Gaussian width 510 cm-‘. The separation between the center of the Gaussian DOS and the fluorescence peak is 2.25 a.

Fig. 9. Fhtorescence spectrum of a PMPS film (T= 10 K) recorded at a spectral resolution of 10 cm-’ upon exciting into tails statesoftheDGS (v,=27730 cm-‘).

1

0

aI -1 r k -2

-?I i 0

2 lg 111.

4

Fig. 10. Simulated relaxation of an ensemble of excitations as a function of time and initial start energy within a Gaussian DOS of width u (standard deviation). The parameters are u= 0.05 eV, T= 30 K, 2ya= 10. Averaging involves 3000 excitations.

consecutively in 30 different configurations.

3. Results 3.1. PDHS / 26.5

27.5

26.5

25.5

Energy (lO%e-t 1 Fig. 8. Fluorescence spectra of a PMPS film (T= 10 K, spectral resolution 25 cm-‘) parametric in u,, indicated by arrows.

Fig. 1 portrays a series of fluorescence spectra of PDHS/MTHF glass, parametric in excitation energy Y,. As long as V, 2 27800 cm-’ the emission profile is a broad featureless band with maximum at 27800 cm- ’ carrying only a weak low energy shoulder off-

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A. Elschner et al. / SSF studies on polysilylenes

Fig. 12. Number of near sites an excitation has visited after time t as a function of its start energy (Start energies (top to bottom) are relative to the center of DOS.) Table 1 Maxima of the inhomogeneously broadened fluorescence bands and associated widths of the density of singlet states. Labels A, B and C refer to fip. 4

Fig. 11. Occupational density of states attained at time 1O%,after generatingthe excitations at various positions (v-) within a DDS Gaussian (dotted curve). (a) Absolute number of excitations per computational energy slice of width 0.1 u plotted on a logarithmic ordinate scale. The plot shows that the inhomogeneously broadened low energy portions of the occupational DOS superimpose for variable v,. (b) Linear plot of the occupational DOS as a function of v, and normalixed to the maximum. It shows growth and spectral shift of the resonant features as a function of v,.

set from the maximum by x 670 cm-l. As V, is lowered beyond 27780 cm-‘, the high energy wing of the fluorescence spectrum becomes steeper and is finally resolution limited. Additional features, off-set from the laser by constant energy shift of 240,720 and 1400

Sample

v0 (cm-‘)

u (cm-‘)

PMPS fti PMS/MTHF

28300 27200 29940 27800 27340

510 2380 450 = 300 2140

PDHS film PDHS/MTHF

26800 27800

(weak) (A) (B) (C)

230 350

cm- 1 respectively (fig. 2), become noticeable. Zero vibronic peaks, if at all occurring would be obscured by the laser line. The film spectra, shown in fig. 3 are similar to the glass spectra as far as the broad emission band is concerned. Excitation above x26800 cm-’ gives rise to more or less featureless emission bands whose full width at half maximum is about 270 cm-‘. The emission peak is located at 26800 cm-‘, independent of vex. As v, is moved beyond 26800 cm-’ the emission spectrum degenerates to a low energy tail only. Superimposed is a weak feature at vex- 690 cm-’ shifting linearly with V, for arbitrary vu. Upon lowering vu, a second sharp feature appears, off-set for the laser by 288 cm-l. Both grow in relative intensity as v,, decreases and will be assigned to Baman signals.

A. Elschner et al. / SSF studies on polysilylenes

3.2. PMPS

The situation is more complex for PMPS. Fig. 4 compiles a series of fluorescence spectra of the PMPS/ MTHF glass parametric in v,,. Three groups of features, designated A, 3, C can be distinguished, each behaving similarly to the emission feature seen in the PHDS/MTHF glass. Upon high energy excitation (v,, > 29000 cm- ’ ) broad emission bands A and B with maxima at v”“(A)=28940 cm-’ and vmax(B ) = 27800 cm- ’ , respectively, are observed, independent of v,. As v, is lowered, emission band A degenerates to a tail with superimposed weak vibronic features shifting linearly with v,, and off-set from the laser by 240 and 540 cm-’ respectively (fig. 5 ) . The second main feature (B ) grows in intensity as v,, is lowered yet remains spectrally invariant until v,, passes the maximum of emission band B (27800 cm-‘). Finally it shrinks to a tail onto which a vibronic feature (B’) is superimposed, shifting linearly with the laser and maintaining an energy separation of 690 cm-’ (fig. 5). The third emission feature (C) becomes dominant at low v,,< 27600 cm-‘. It consists of a main band centered at 27300 cm- ’ and vibronic features with vibrational energies of 220 and 690 cm-‘, respectively. The band system C is independent of vex.An attempt to detect the states corresponding to emissions B and C in the absorption spectrum of the glass failed because of the poor. signal to noise ratio in the in-situ absorption measurement. Varying the temperature between 5 and 113 K has little effect on the inhomogeneously broadened emission bands A and B (fig. 6). This argues against associating emission B with a defect populated via thermally activated energy transfer. Warming up the sample to room temperature leads to spectral broadening and a bathochromic shift of the (A) maximum by about 650 cm-‘. A typical fluorescence spectrum of a thin PMPS film, recorded upon excitation at v, = 33900 cm- ’ is plotted in fig. 7 together with the absorption spectrum. The maximum extinction of this sample was = 2. The main emission feature peaks at 28300 cm- ’ with a satellite at 27200 cm-‘. The overall absorption and emission pattern agrees with the spectra of polysilylenes recorded under broad band excitations [ 5 1. The low energy satellite is absent in the fluores-

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cence spectra of an optically thick film (equivalent to a sample thickness of > 10 pm). Spectra parametric in v, have the same general characteristic observed with a PDHS film ( ftg. 8). Upon decreasing v, beyond an energy corresponding to the emission maximum, the spectrum degenerates to an emission tail whose intensity keeps decreasing. No zero vibronic emission component can be distinguished from the stray light of the laser. However, a group of vibronic bands, off-set from the laser by 1440,157O and 1830 cm- ’ respectively, are clearly resolved. Above 100 K their intensity decreases sharply, which is incompatible with an assignment to Raman modes. A fluorescence spectrum excited at v,=27777 cm-’ (360 mm) and recorded at a spectral resolution of 10 cm-’ over a spectral range of 7500 cm-’ is shown in fig. 9. 3.3. Simulation results The relaxation of excitations whose intrinsic lifetime is lo4 times their hopping time to in a hopping system devoid of any disorder has been studied as a function of the start energy within a Gaussian DOS having a Gaussian width o= 600 cm- ’ (0.05 eV) . For computational reasons a system temperature of 30 K was chosen, which is equivalent to a disorder parameter u/ kT= 19.3. From previous work we know that this value is sufficient for simulating the hopping behavior of a random system in the T-+0 limit on a time scale to < t < 104t0. The wavefunction overlap parameter 2ya, y being the inverse wavefunction decay radius and a the distance of nearest neighbor sites, was set equal to 10, ensuring that on the time scale of the computation only nearest neighbor jumps are important. Fig. 10 shows the temporal decay of the average energy of an ensemble of excitations parametric in energy ( vo) of the site at which they have been generated. It confirms that the lower is vex, the longer it takes for an excitation to jump to a site of still lower energy [ 121. It also shows that for long times the mean energy, (E), becomes independent of v,,. Of particular importance for analysis of fluorescence spectra measured as a function of v, is the occupational density of states (DOS-). In the absence of additional relaxation channels, such as coupling to phonons and vibrations, the luminescence spectra should, in a good approximation, map the energetic

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A. Elschner et al. / SSF studies on polysilylenes

distribution of the excitations at the lifetime-limited end of their random walk. Fig. 11 portrays a family of DOS”’ spectra plotted on both a logarithmic and a linear scale. They suggest that the emission spectrum be the superposition of a homogeneous peak shifting resonantly with the laser and an inhomogeneous band reflecting those excitations that had a chance to leave their parent site during their lifetime. The former grows in intensity as v, decreases. As long as v,, R v. --a, v,, denoting the maximum of the DGS, the inhomogeneous band has a maximum at v0- 2a, independent of v,,. The band shape is slightly asymmetric, the full width at half maximum being 2: 1.2tr. Plotting the occupational DOS on a linear scale (fig. 11b) clearly reveals the transition from an entirely inhomogeneous emission spectrum observed for vexk v. --d to a spectrum dominated by a resonant peak carrying an inhomogeneously broadened tail only. Remarkably, the number of jumps required by the excitations to establish the inhomogeneous portion of the occupational DOS is less than 5 (fig. 12). The relaxation of excitations executing a random walk in an energetically disordered 1D system has been simulated earlier [ 16 ]. Qualitatively, the results are similar. Because of fewer acceptor sites, to which an excitation can jump, the center of the inhomogeneous broadened emission band is off-set from the center of the DOS by only -0.750, its width being ~0.7%.

4. Discussion The spectra of glasses and films of PDHS and PMPS reveal a general pattern. Upon excitation to the central portion of the absorption band, the fluorescence band is broad, featureless, and invariant with the excitation energy. As vex is scanned towards the absorption tail, it degenerates to a tail, whose relative intensity, but not its slope, continuously decreases. In case of PMPS, the spectrum is a superposition of three emission features, each following the above principle. Superimposed onto the dominant features, attributed to S, + S, O-O transitions of the o-bonded silicon chain, are weak features shifting linearly with vQ paralIe1 to the shift of the main band of the PDHS/ MTHF, PMPS/MTHF and PMPS/film emissions,

respectively. In case of the PDHS film, the weak features shift linearly with the laser at any v,,, independent of the main fluorescence band. This general behavior is incompatible with the notion of polaron formation, yet is consistent with the concept of energy transfer in a system composed of energetically inequivalent chromophores (“sites”) as will be shown below. It has been argued before that the absorption line shape of polysilylenes is determined by a distribution of lengths of stretched segments of polymer chains over which an excitation, a Frenkel exciton, is delocalized [ 3,4,17 1. The best ordered material appears to be PDMS for which X-ray studies indicate crystalline packing on a microscopic sale [ 18 1. The absorption peak of a PDHS film coated onto a quartz slide occurs near 27000 cm- ’ at room temperature. The half width of the low energy absorption tail is about 400 cm- I. Upon heating the film above 40 ’ C an order-disorder transition occurs as evidenced by an endothermic DSC peak [ 191. It results in a hypsochromic shift of the absorption maximum to 32000 cm-’ accompanied by an increase of the widths of the low energy absorption tail by a factor of 6 indicating a drastic shortening of the average length of ordered chain segments and a concomitant increase of its variance. To translate the spectral shift into absolute numbers for the effective lengths of ordered segments, calibration with oligomers of specified segment length is required. This is not presently available. The absorption maximum of PHDS in a 6 K MTHF glass occurs at 28000 cm-‘, about 1000 cm-’ above the absorption peak of a 298 K film. Since cooling the solution below the glass transition temperature of MTHF ( z 90 K) took only about 7 min we consider chain aggregation unlikely and concur with Johnson and McGrane [ 1] that the polymer molecule is present in the form of an array of ordered chain sequences in all-trans configuration separated by chain defects. The hypsochromic shift relative to the film on a quartz support indicates that the average effective segment length is somewhat shorter. In PMPS the side groups are less susceptible to crystallization and the stabilizing effect they exert on the silicon backbone chain is, therefore, Educed as compared to PDHS. This accounts for the blue-shifted absorption spectrum of a PMPS film as well as its

A. Elschner et al. / SSF studies on polysilylenes

larger tail-width indicative of a larger degree of disorder. We note, however, the presence of a feature occurring at 27200 cm-’ in the fluorescence spectrum of a thin film that is absent in thicker films. It suggests some epitaxial growth of fully aligned chains in the vicinity of the surface. The occurrence of an emission component at 27340 cm-’ in the PMPS/ MTHF spectrum indicates the presence of a small degree of alignment in PMPS polymer, even in the glass. The variations of the fluorescence spectra of PDHS/MTHF, PDHS film as well as PMPS film are in full accord with the concept of excitation migration among a manifold of chain segments differing in excitation energy as evidenced by comparing the spectra of, for instance, fig. 8, with the prediction of simulation. Upon excitation into the central portion of the absorption band, a spectrally invariant inhomogeneously broadened asymmetric emission band is observed. As v,, is lowered, the high-energy tail is gradually lost. The expected resonant component is buried underneath the laser line and the only emission detectable at v < vex- 25 cm- ’ is the decreasing tail of the residual inhomogeneous emission band. The important implication is that coupling to phonons is weak enough not to cause the appearance of a phonon wing that extends into the spectral regions amenable to spectroscopic probing without the interference of light scattering near the laser line. This proves that the excitation does not generate a distortion of the polymer chain, i.e. no excitonic polaron is formed. This conclusion is substantiated by the occurrence of hole burning, reported by Trommsdorff et al. [ 17 1, yet is at variance with the theory of Rice and Phillpot [ 10 1. To document the quantitative agreement between experiment and simulation, we included in fig. 7 the simulated emission profile expected for a width of the DOS of o= 5 10 cm- I. The low energy tail of the absorption profile of the PMPS film can be fitted by a Gaussian envelope of the same width, whose center is blue-shifted relative to the emission by 1150 cm- *, i.e. 2.25 CLRecall that the simulation predicted 2.00 in case of three-dimensional topology. This means that experimental and simulated spectra differ by not more than 10% as far as the displacement between absorption and emission is concerned, although the entire absorption profile and, thus, the DOS is all but Gaussian. It illustrates that it is only the tail section

89

of the DOS that controls the rate of excited state relaxation. An analogous situation is found with amorphous inorganic semiconductors. The spectra of PDHS and PMPS films show no vibronic features except for the PMPS film upon tail excitation. This parallels the observation of negligible phonon coupling as manifested by the absence of phonon wings extending over more than a25 cm-‘. Delocalization of the excitation over large segments, estimated to consist of about 55 monomer units [ 3,4], dilutes the amplitude of the exciton at the individual Si-Si bonds and minimizes coupling to chain vibrations. Coupling to 240, 700 and 1400 cm- ’ modes is, however, observed in the PDHS/MTHF spectra where the length of the emitting segments is lower compared to the film, as evidenced by the 1000 cm-’ hypsochromic shift. Notice also, that for a matrix-isolated PDHS chain, the maximum of the inhomogeneously broadened emission band, which indicates the energetic position within the DOS at which excitations terminate their random walk to decay radiatively, is well within the absorption shoulder. This is a consequence of the onedimensional topology and is in accord with the prediction of simulation [ 16 1. It also proves the absence of chain aggregation in the glass. The PDHS film spectra display a few sharp features moving resonantly with the laser in spectral regimes where the inhomogeneously broadened emission band onto which they are superimposed is spectrally invariant. This fact as well as their monochromator-limited shape suggests the assignment to resonance Raman features superimposed onto fluorescence. Employing Raman spectroscopy, Kuzmany et al. [ 18 ] have identified the 690 cm- I feature as a dominant mode of the silicon backbone. The fluorescence spectrum of matrix-isolated PMPS can be understood by invoking the presence of three different chain configurations of which the lowest energy component (feature C in fig. 4b) is the equivalent to the low energy emitter in a thin PMPS film, possibly present as an aggregate. Two facts argue against the notion that all species occur within an individual chain and, rather, support the assignment to different conformations of isolated chains: (i ) The intensity of the lower emitting species increases upon excitation into the tail states of the dominant conformation giving rise to A emission. If B and C states

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A. Elschner et al. / SSF studies on polysilylenes

occurred within each chain, they ought to be populated by excitation migration which, however, is progressively blocked upon decreasing v,,. (ii) Energy transfer should become more efficient as the temperature is raised which is at variance with experiment. It appears likely that PMPS, present in less ordered (coil) form in liquid solution, tends to stretch upor cooling but gets trapped in the matrix before adopting a rod-like morphology. Some fraction of the polymer molecules do, however, acquire the partially stretched conformation (B) which may aggregate, thereby elongating further (feature C ) . To summarize this section we list in table 1 the maximum positions of the inhomogeneously broadened fluorescence bands of the various samples together with the Gaussian widths of the associated DOSS derived from the band widths.

5. Comparison with charge transport The charge transporting properties of PDHS and PMPS films have recently been investigated as a function of temperature and applied electric field [ 8,201. A consistent interpretation has been put forward based upon the concept of charge carrier motion in a distribution of localized states resembling a Gaussian DOS [21]. The temperature dependence yielded 0.04 and 0.09 eV, respectively, for a Gaussian width (0”) of the relevant portion of the DOS of PDHS and PMPS, respectively. The ratio of occ( PMPS) /F( PDHS) = 2.25 agrees favorably with the ratio of the widths of the exciton DOSS, aexc(PMPS) /aeXc(PDHS) = 2.3. Such a correlation should, of course, exist if it is the disorder that controls transport of both charge carriers and excitons, since the variation of the HOMO energies - relevant to hole transport - with the length of an ordered segment should scale with the variation of excited state energies. One would, therefore, expect occ to be proportional to aexc.Not only does the verification of this correspondence confirm the key role the energetic disorder plays in electronic transport, it also demonstrates that optical experiments can be used profitably to probe the charge transporting properties, even though optical transitions between valence and conduction bands are not amenable to direct spectroscopy. Such an analogy has been noted before for hop-

ping in a distribution of states caused by a distribution of the van der Waals interaction energy of a charge carrier and an exciton, respectively, sitting in a spatially random environment [ 22 1. Even the proportionality factor relating ccc and cFxcin polyvinylcarbazole turned out to be similar ( 1.5 ) . Site-selective spectroscopy provides, therefore, a versatile tool for estimating the contributions of disorder and polaron formation to the localization energy of a charge carrier and, hence to the activation energy, with which the charge mobility is associated in random media 1231. 6. Concluding remarks Site-selective fluorescence studies performed on polysilylenes in various modifications support the notion that absorption and emission are excitonic processes controlled by disorder. The disorder originates from a length distribution of ordered chain segments. Upon excitation into the central portion of the density of states the excitation is subject to a random walk in whose course it relaxes into tail states associated with longer segments from which radiative emission can occur. Upon scanning the excitation towards the tail states of the DOS energy migration is eliminated and resonant emission dominates. The absence of phonon side bands indicates very weak phonon coupling corresponding to a Debye-Waller factor close to unity. The results of spectral diffusion studies in polysilylene films can be explained in a quantitative fashion in terms of hopping among sites with Gaussian distribution of energies. The success of this arguably simple model illustrates its versatility. Although originally developed for point sites it can, without loosing any of its essential features, be applied to a system composed of sites that are extended in one dimension provided that the excitation is delocalized within the site. This is apparently the situation in the polysilylenes and, possibly, in n-conjugated polymers like polyphenylenevinylenes, whose fluorescent properties can also be rationalized in similar terms [ 241. The ordered segments of an individual polymer chain, differing in length and, concomitantly, in site energy, are separated by conformational defects, e.g. a bond rotation. Transport of an exciton or a charge carrier

A. Elschner et al. / SSF studieson polysilylenes

will, therefore, occur by hopping either between energetically inequivalent segments of neighboring polymer chains or across the bond defect of the same chain involving superexchange. The dominant interaction will be among the nearest neighbor sites accounted for in the simulation by choosing an appropriate electronic overlap parameter. A matrix-isolated polymer molecular consisting of rod-like segments, should, therefore, behave like a 1D system with reduced efficiency of spectral diffusion as has been observed.

Acknowledgement

Financial support by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged.

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