Radiative lifetimes of confined excitations in sigma-conjugated silane oligomers

Chemical Physics 157 (1991) 401-408 North-Holland

Radiative lifetimes of confined excitations in sigma-conjugated silane oligomers Jonathan R.G. Thome, Scott A. Williams, Robin M. Hochstrasser Department of Chemistry, University ofPennsylvania. 231 S. 34th St, Philadelphia, PA 19104. USA

and Paul J. Fagan Central Research and Development, E.I. DuPont de Nemours and Co., Wilmington, DE 19980. USA Received 28 May 199 I

Emission studies of polysilanes of well-defined length are used to deduce the chain length necessary to support delocalized excitations. The chemical conditions necessary for interruption of extended sigma-conjugation are also described.

1. Introduction Understanding of the optical properties of conjugated polymers and in particular the degree of localization of excitations has considerable importance for their photochemical and non-linear optical behavior. Sigma-conjugated polymers, amongst which the polysilanes, (RR’ Si),, are the most accessible, present a particular series of challenges in the field of photophysics of compounds with delocalized excited states. Evidence of sigma-conjugation for the polysilanes comes from several sources. The absorption frequency of the first electronic transition of silicon oligomers (RR’ Si ) ,, ( n = 2- 100) shows a dependence on chain length strongly indicative of delocalization of the excitation [ 1,2]. Absorption in the high polymers (n= 100-l 0000) in solution has been understood in terms of an inhomogeneous distribution of chain length segments possessing different degrees of excitation localization [ 31. Fluorescence is red shifted from absorption which results from energy transfer following excitation, populating low-energy states of the polymer. As a result of the Si-Si o-conjugation the extended chromophores have very short “superemissive” radiative lifetimes [ 4-61. Measurements of the poly-

mer radiative emission lifetimes ( z 700 ps) indicate a large transition dipole suggesting that the excitation in these materials could be delocalized over as many as 20-30 silicon sites [ 41. Complementary evidence of this extent comes from saturation of excited state absorption experiments: the maximum packing density of excitations is deduced to correspond to one excitation every 25 silicon atoms in the chain [ 7 1. The extent of delocalization in the high polymers is, however, insufficiently defined because the results rely on the assumed segmented or continuous disorder, which leads to excitation confinement. The deduced value of conjugation length must of necessity represent some average number for the inhomogeneous polymer. The source of the inhomogeneity is not yet fully understood and energy transfer complicates the analysis of the data. This situation is greatly clarified by studies of oligomeric systems, where the maximum chromophore length can be defined by the molecule length itself and excitation cannot have longer-range mobility. We report here on time-resolved fluorescence measurements on silane oligomers, with backbone chains of between 9 and 23 atoms, interrupted by differently substituted silicon, germanium or oxygen atoms, and describe the effects of these various linkages on the

0301-0104/91/S 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

J.R. G. Thorne et al. /Confined exciiations in a-conjugated silane oligomers

402

degree of conjugation.

confinement

or

interruption

of

o-

2. Experimental The materials used in this study are a series of oligomeric silanes and germanium or oxygen substituted silanes prepared by the methods of Fagan [ 8 1. The chemical formulae are listed in table 1: since they are lengthy we adopt the following abbreviation system throughout. The basic construction unit is a chain of five diphenyl substituted silicon atoms. The letters S, G, 0 are used to denote Si (in silane), Ge (in silagermane) and 0 (in siloxane): the numbers 1 and 3 to denote the number of 5-chain units in the compound. In the compounds SSl, GGl and SOS1 this chain is terminated on each end by methyl substituted units consisting of two silicon atoms, two germanium atoms and a siloxane linkage, respectively. In the compounds SS3, GG3 and SOS3 the basic 5silicon chain unit is repeated three times, both interspersed and terminated by the methyl substituted subunit. Thus the l-compounds are 9-atom chains: the 3-compounds are 23-atom chains. SB 1 is a 5-chain unit terminated by branching allsilicon substituted groups it has an unbranched 9atom chain, a linear chain of 13 atoms and 17 atoms in total. The quantum yield standards were polysilanes having chain lengths in excess of 1000. DNHS indicates poly (di-n-hexylsilane), PMS is poly (phenylmethylsilane ) . Fluorescence lifetime measurements were made using a time correlated single photon counting (TCSPC) apparatus described elsewhere [ 4,6]. All

solutions were 30 mg/Q in toluene with the exception of DNHS which was the same concentration in hexane. Experiments were carried out in flowing solution to minimize photodamage, in a 0.2 cm cell under a flow of dry nitrogen. Many of the compounds are not very photostable (particularly SB 1 and GG3 ) . The quantum yields were assessed by comparison with known yields for two long-chain polysilane poly (di-n-hexylsilane ) DNHS and polymers, poly( phenylmethylsilane), PMS, absorbing at 3 1500 cm- ’ and 29200 cm- ‘, respectively. Fluorescence was also recorded from a methyl-tetrahydrofuran glass for compound SS 1 in a Dewar at 77 K. Emission spectra were recorded both with a fluorimeter for continuous excitation, and using the TCSPC instrument which provided time-gated spectra by giving the amplitude of the various lifetime contributions at selected wavelengths. Time-resolved fluorescence anisotropy, r(t), equal to I, -I, / (I,, + 21, ), was measured by alternately recording fluorescence polarized parallel (I,, ) and perpendicular (I, ) to the exciting laser beam.

3. Results The emissive behavior of the compounds is summarized in table 2 where they have been separated into two classes. Type I (which absorbs at a wavelength similar to DNHS which is thus used as a quantum yield standard, Qr~O.27, 310 nm [ 3,4,9,10]) have very low quantum yields and broad fluorescence: the materials are SS 1, GG 1, SOS 1 and SOS3. Their fluorescence maxima, +, are red shifted from their absorption maxima, v,, by vs>2500 cm-‘, as exemplified by the SS 1 spectra in fig. 1a. Type II, ab-

Table 1 Materials SSI

ss3 GGI GG3 SOS1 SOS3 SBI DNHS PMS

C1(SiMe2)2(SiPh,),(SiMe2)2C1 CI(SiMe2)2(SiPhZ)S(SiMeZ)2(SiPh2),(SiMe2)*(SiPh*)~(SiMe*)*Cl CI(GeMe2)2(SiPh2)5(GeMe2)2Cl CI(GeMe2)2(SiPh2)5(GeMe2)Z(SiPh2),(GeMe,)2(SiPh2),(GeMe2)2CI Ph(SiMe2)0(SiMe2)(SiPh,),(SiMe2)O(SiMe2)Ph PhSiMezOSiMez( SiPh,),SiMe20SiMe2 ( SiPh2),SiMe20SiMe2 ( SiPh2),SiMe20SiMe2Ph (SiMe3)$i(SiMe2)2(SiPh2)5(SiMe2)2Si(SiMes)3 (Si(C 6H 132” ) ) (SiPhMe),

403

J.R.G. Thorne et al. /Confined excitations in a-conjugated silane oligomers Table 2 Absorptive and emissive behavior Lifetime

Compound VA (cm-‘)

v,

ts

yield

Type-I compounds SSI 4800 GGl 5800 SOS1 2800 SOS3 2000

31200 31000 31900 31500

28600 27000 29400 29400

2600 4000 2500 2500

0.003 0.003 0.002 0.003

Type-11 compounds DNHS 10500 PMS 4000 ss3 9600 GG3 8800 SBl 4800

31500 29200 28000 27900 29500

29200 28200 26900 26700 26900

2300

0.42

1000

0.1I

1100 1200 2700

0.14 0.03 0.05

0)

30000 I

350

300 b)

300

30000

I

350

20000 cm-’ 1

25000 I

400

450

25000

20000 cm-’

I

400

500 nm

450

500nm

Fig. I. Absorption and emission spectra for representative type-1 and type-II compounds. (a) SS 1 fluorescence excited at 3 10 nm. (b) SS3 fluorescence excited at 340 nm.

sorbing to longer wavelength, have yields comparable to the polysilane PMS reference ( @f= 0.11, 340 nm [ 3,4,10] ), and relatively narrow fluorescence

Length N en

N

r,

rs,

34 38 32 30

I1 12 15 10

18 18 22 29

9 9 9 23

l-2 l-2 1-2 2-3

155 73 127 34 70

0.4 0.7 0.9 1.1 1.4

7 20 12 12 23

2100 7000 23 23 17

19 28 13 11 16

rr

bands and includes the materials SS3, GG3 and SBl. An example is that of SS3 in fig. 1b, where the Stokes shift is very small in comparison with the type-1 compounds. Fluorescence spectra were independent of excitation wavelength. The fluorimeter spectra of type-1 compounds are dominated by impurity fluorescence. Time-resolved decays for these compounds show two-component decays (fig. 2a). The short one ( = 30 ps) is attributed to the compound itself, the long one ( z 5 ns) which was observed to grow with time, is ascribed to impurity. The TCSPC instrument was thus used to produce the emission spectra of the short component (shown in fig. la for SSl ) and it is the amplitude of this emission that is used to calculate the quantum yields in table 2. The apparent shoulder at 400 nm of the main emission peak in fig. la, we attribute to a very short lived charge transfer state (see section 4.6) and this contribution is not included in the quantum yield estimate: its presence makes the type-1 estimates somewhat uncertain ( z + 50°~ ). All emission detectable from type-II compounds, on the other hand, appears to be intrinsic and have a single exponential decay lifetime ( z 30- 100 ps). Measured lifetimes are given in table 2 and the measured quantum yields are used to calculate the radiative lifetimes. The absorption oscillator strength per silicon or germanium bond is used to predict a radiative lifetime (rsi), according to the procedure of refs. [ 11,12 ] :

J.R.G. Thorne et al. /Confined excitations in a-conjugated silane oligomers

404

a)

0

4. Discussion

r

4.1. Fluorescence lifetimes

0

CJ -3

L

i

n

b)

TIME

(ns)

04-+A ii

_

F % 5

_ 02-

Q

-

oli.l-lJ 0

0.2 TIME

04.

06

00

(ns)

Fig. 2. (a) Fitted fluorescence decay SSI together with instrumental response (shown on logarithmic scale). (b) Fluorescence anisotropy SS3, excited at 360 nm, together with fluorescence, 375 nm. on linear scale.

l/rsi=2.9x =7x

10-9n2/(~~3>~~

10-9V;emaXV*whmlVA,

I

e(v,)

dv, (1)

where VA, VE and Vfw,,,,,are the absorption, emission and full width at half maximum of the absorption in wavenumbers, n is the refractive index and c is the extinction coefficient. The ratio of the predicted to observed radiative lifetimes is given in the final column: this is an estimate of the chromophore length, N,, in units of Si-Si bonds. The time decay of the fluorescence anisotropy for SS3 excited at 27800 cm-’ is shown in fig. 2b. The fitted anisotropy decays from its theoretical maximum value of 0.4 for parallel absorbing and emitting dipoles in a time of 12 ps to a value z 0.32 and then has a slow decay component with x2 ns time constant. The depolarization rates show very similar behavior for GG3, SSl and SBl with rapid relaxation times of 8, 13 and 12 ps, respectively.

The radiative lifetime (7,) and the extended chromophore length (New) are both indicators of the spatial extent of the absorbing and emitting chromophore in the series of oligomers. Type-II compounds are polysilane like, have shortened superemissive lifetimes of about 1 ns and chromophore lengths, N,, a large fraction of the molecular length of 23 silicon atoms, comparable to those observed in DNHS and PMS high polymers. In contrast, type-1 quantum yields are very low. The emission is broad and structured. The radiative lifetimes calculated are typical of isolated organic chromophores ( x lo-30 ns): they show much reduced conjugation ( Nefl<< N). We interpret the large red shifts seen in type-1 compounds compared to type-II as evidence that the former excitations are strongly bound to the lattice. In the latter, however, the increased length of chain avai,lable to the excited state and high mobility of the carrier means that in effect, the excitation moves before the lattice has time to relax around it. The 9- and 23-atom chains then correspond to the Toyozawa selftrapped (S) and free (F) cases, where the parameterization is in terms of the bandwidth (B) and the lattice relaxation energy ELR (equal to half the Stokes shift) [ 13,141. Type I contains all the 9-atom oligomers and the 23-atom siloxane SOS3. Type II includes the other 23-atom compounds. A differently substituted Si-Si atom pair as found in SS3 or a Ge-Ge pair as in GG3 does not appear to interrupt conjugation. However, an ether linkage in SOS3 completely interrupts conjugation and the molecule behaves as isolated SOS 1 units. Interestingly, the extended conjugation applies to the branched material SB 1 (a straight chain of only 9 silicons but a total of 17 Si atoms). 4.2. Molecular orbital models We use the Sandorfy model [ 151 to understand the molecular orbitals of oligosilanes at the simplest level. Alternating resonance integrals /I2 (vicinal) and /I1 (geminal) describe the overlap of sp3 hybrid orbitals on neighboring and on the same silicon atom, respec-

J.R.G. Thorne et al. /Confined excitations in a-conjugated silane oiigomers

tively. The optical gap, for a chain of length Natoms, or difference in energy between the HOMO and LUMO is then given analytically by [ 16 ] E,(N)=2[P:+P:-228,82COS(n/N)]“2.

(2)

For an illustrative example we take the resonance integrals to have values 3.4 eV and 1.7 eV, in keeping with the ratio used by Soos [ 171. The optical gap (HOMO-LUMO) in chains of 9 and 23 silicon atoms then has values of 3.8 eV (320 nm) and 3.46 eV (360 nm), approximately as observed, and is equal to 2(&-/I, ) = 3.4 eV for an infinite chain. The bandwidth B=2P, has the same numerical values. We define the orbital participation ratio [ 18 ] for a particular state as the sum of the inverse fourth powers of the MO coefficients CiC;4p as a measure of the delocalization of that eigenstate. For the unperturbed 23-atom linear chain MOs this ratio is found to be about 16 atoms. This number is typical of the values found for N,e for the type-II compounds. In germanium-substituted chains we must consider different diagonal energies cy for Si and Ge orbitals in the chain. A suitable choice for oGe and (Ysi are the values of the ionization potentials of GeMe, and SiMe, of 9.2 eV and 9.9 eV [ 191. To a first approximation we retain the same off-diagonal couplings j?, and p2 for all interactions. Theoretical calculations [20] suggest that the band gap in the allgermanium chain is x0.5 eV narrower than the allsilicon chain. The energies of HOMO and LUMO are then obtained by diagonalization of the 2 (N- 1) sp3 orbital nearest-neighbor interactions in the chain. In general, we find for 6a c/3, the optical gap and delocalization extent of the states at the gap remains almost unchanged. Thus for the germanium-substituted molecule GG3 we expect delocalization to be unaffected. If 8a >> /I,, jIZ the HOMO and LUMO states are considerably altered. Excitation across the gap then corresponds to electron transfer to/from the Si chain unit from/to the bridging units. This is apparently the case when siloxane forms the bridge in SOS3. The result is that HOMO and LUMO become localized and have reduced participation ratios corresponding to the length of the bridging and the Si5 chain units. We regard a molecule like SOS3 as a series of three one-dimensional quantum wells. Excitons are thus confined within one well by the bridging oxygen which inhibits delocalization. Energy

405

transfer rates between wells remain to be investigated. 4.3. Polaron models Rice and Phillpot [ 16 1, using slightly reduced values of /3, and j?* in the Sandorfy model, suggest that a neutral polaron exciton will be the character of excitation in polysilanes. These authors use a value of the electron-phonon coupling (djI,/dr), = 2.8 eV/ A to predict a Stokes shift of 0.32 eV and a neutral polaron full width half maximum of 3-5 lattice spacings, which they conclude will be strongly bound. However, in the 22-bond SS3 and GG3 compounds the molecular length greatly exceeds this postulated polaron width yet motional narrowing is observed. The relaxation energy (half the Stokes shift) is x 500 cm-‘, about 2.5 times less than that seen in the typeI compounds. The excitation does not move (hop) as a dressed polaron as the bandwidth (B) exceeds the lattice relaxation ( ELR). We consider it likely that these authors underestimate the strength of the geminal coupling /I,. The polaron spatial width depends upon the ratio j?, /fiZ (j$ - /I, ) which for our parameters would have a value 1.5 times greater and be much more weakly bound. Because of the character of the fluorescence (narrow, small Stokes shift, superemissive lifetime) we believe excitations are weakly bound (F-type) in the polysilanes but the balance between localization and delocalization is near critical. A small polaron (S-type) is characterized by a polaron bandwidth [ 2 1 ] : B’ =BevS,

(3)

where S= ( 1/N) &, the yk values represent the ratios of the polaron binding energy to the quanta of vibrational energy and the sum is over the k vibrational modes for the N-atom chain. This bandwidth reduction reflects the overlap between absorption and emission spectra shown in fig. 1 on going from the 23-atom to the 9-atom chain. The effective coupling modes are likely to be optic phonons. The most active modes occur at 230 cm-’ (chain mode unassigned), at 370 cm-’ (Si-Si symmetric stretch) and at 700 cm- ’ (Si-C symmetric stretch) [ 18,221. Near-resonance Raman studies of the analogous polygermane compounds show that the latter mode is resonance enhanced [ 23 1. This is the same mode that Rice and Phillpot use in their pola-

406

J.R.G. Thorne et al. /Confined excitations in a-conjugated silane oligomers

ron description and it corresponds to a bond lengthening without change of backbone length. We believe the 230 cm-’ mode is also enhanced [ 181. There may, however, be many contributions to the relaxation from side chain vibrations analogous to the solvent reorganization contribution to the Stokes shift seen for molecules in solution. The lack of structure seen in absorption and emission suggests this is the case. As the molecular length is decreased by a factor of x 2.5, the value of S rises linearly by the same factor as does the Stokes shift. The polaron bandwidth is, at the same time, reduced exponentially until the inverse bandwidth, or residence time of the exciton, becomes larger than the dephasing time and the excitation is dephased before transfer (self-trapped) and no longer super-emissive at room temperature. The Rice and Phillpot static polaron model then has some validity for the 9-atom chain and also apparently the isolated segments in SOS3, where the Stokes shifts are indeed of the order of 0.3 eV, chromophore lengths N,,are one or two silicon bonds and excitons are strongly bound. It is not clear as yet whether the transition from large to small polaron, or F- to S-character, occurs at some critical value of chain length. Theoretical predictions for localization depend upon the nature of the coupling modes, whether optic or acoustic [ 13,141. Study of this experimentally will require silicon chains of all lengths. It is not possible from these data to completely rule out the possibility that in the shorter chains the emission originates from a different excited state than oo*, having a much weaker absorption and hence a significantly longer lifetime. In the high polymers motional narrowing may be made even more extensive than in the 23-atom chains by energy transfer between the extended chromophores of the long chain. Studies of DNHS at low temperature for the optic-phonon Raman modes give Franck-Condon factors in emission of only 5Ohand no measurable zero-phonon line shift [ 18 1. This very small lattice relaxation corresponds to that of an initially created delocalized excitation. Even at low temperature, the coherence time, as measured from hole-burning studies, in polysilanes is z 10 ps or shorter [ 18 1. We note that even though the fluorescence lifetime exceeds the coherence decay time, super-radiance is not destroyed [ 241, anal-

ogous to the situation observed in molecular aggregates [ 25 1. When the band width is large compared to kT, scattering populates only a small subset of the non-zero k states of the band. This effect is also seen in the relatively temperature independent nature of the lifetime in the polysilanes. Because of this observation we ascribe the limitation of conjugation length to Neff= 20-30 in the high polymers to structural defects rather than phonon dephasing. This is consistent with conclusions drawn from fitting the spectral shapes to a continuous disorder model [ 181. 4.4. Branched silanes Partial doping of the high polysilanes with branched silicon atoms of the type that is found in the network polysilyne, ( RSi),, high polymers has been shown to give rise to broad emission at z 450 nm [ 26 1. As the number of branching points increases to a level greater than z 5% of the total silicon atoms present, the normal fluorescence is quenched. The authors have interpreted this result as excitation trapping at the branch point. We do not see this behavior in the doubly branched SBl compound. The Si( SiMeS), terminating groups are not acting as excitation traps in this molecule. Molecular orbital models for the branched chain compound suggest that HOMO and LUMO states are almost fully delocalized over the whole 17-atom molecule. The value obtained for the chromophore length is 16 Si bonds, even larger than that obtained for SS3 and GG3. There are indications that there may be contributions to the oscillator strength from more than one excited state: the lower symmetry of this compound may have something to do with this. There may well be a charge-transfer component to the emission also (section 4.6). The effects of excitation confinement have been investigated in CuCl microcrystals [ 271. The superradiant three-dimensional exciton has a radius of 7 A and indeed shows an effective linear dependence of the inverse radiative lifetime upon the crystal volume, N,, down to radii of z 15 A. However, the effect of further confinement below this radius could not be investigated.The result shows that the oscillators are cooperative in more than one dimension which may be relevant to the material SB 1.

J.R.G. Thorne et al. /Confined excitations in o-conjugatedsilane oligomers

4.5. Fluorescence anisotropy The oligomer fluorescence spectra are independent of excitation wavelength, unlike the case of the high polymers. Under the assumption that the anisotropy at t = 0 is the maximum 0.4 and proceeds to a plateau level of z 0.32 (corresponding to an order parameter of 0.8) with a single rate constant, we are able to extract values of this constant from the parallel to perpendicular fluorescence counts within the excitation pulse envelope. The values obtained are 10 + / - 5 ps for all the materials studied, but these should be regarded as upper values because of the difficulties inherent in extracting anisotropy decay times shorter than the instrument function [ 71 .The polarization decay in the oligomers is attributed to molecular reorientation rather than excitation transfer as was the case with the high polymers. The polarization decay characteristics are similar to those observed in long-chain polysilanes [4,6] but in the polymer the initial anisotropy maximum is attained only on long-wavelength excitation, when spatial energy transfer does not occur. The form of the decay indicates that a rapid dipole reorientation occurs upon excitation within a time of x 10 ps and would correspond to an average angle of transition dipole reorientation of z 20”. It has been suggested [ 91 that the driving force for torsional motion in the excited state is the greater stabilization energy associated with a planar all-trans structure in the LUMO compared to the HOMO. This results from consideration of l4 Si-Si interactions not incorporated in the nearestneighbor Sandorfy C treatment in section 4.2. The nanosecond decay probably corresponds to overall molecular rotation: this is faster in the smaller SBl compared to SS3 and GG3. An excitation confined to a silicon chain segment would conformationally relax in z 10 ps. The magnitude of the relaxation energy is determined by the segment length: when the number of atoms is small as in the 9-chain type-l materials, the excitation is strongly bound: in the 23-chain compounds it is weakly bound. In the high polymer energy transfer readily takes place within 10 ps, causing extensive depolarization, but little conformational change because of the effect of motional narrowing. The coupling to phonon modes seen in emission is very weak at early times after excitation. If eventually such an

407

excitation is trapped it is then subject to the forces of the lattice upon the region of its confinement. The narrow structured Raman spectrum of the polymer evolves to the broadened relaxed fluorescence spectrum. 4.6. Photochemistry There are consequences for photochemistry of the localization of excitations in short-chain silane oligomers. If indeed the extent of the small static exciton-polaron is only one or two lattice spacings, then it seems very likely that such a localized oo* excitation could lead to bond scission and photochemistry. The very low quantum yield for fluorescence that is seen, together with the observation of a photoproduct are likely evidence for the weakening of a Si-Si bond. This apparently does not occur so readily in the delocalized polymers. We have investigated the possibility of chargetransfer excited state formation in the 9-oligomers. In aryldisilanes [ 281 and arylethynyldisilanes [ 291 silicon-to-aryl electron transfer is understood to occur. At 77 K these charge transfer states are stable and have fluorescence lifetimes of several nanoseconds: at room temperature their lifetime is very short and bond scission occurs. We have investigated the behavior of SSl at 77 K and find that a very similar broad emission with a highly non-exponential decay in the nanosecond time regime appears at = 400 nm in a methyltetrahydrofuran glass. We therefore ascribe the component of the emission near 400 nm in fig. 1 at room temperature to weak charge transfer fluorescence that develops a much longer lifetime at low temperature. We consider the charge transfer path the likely photochemical route at room temperature. The possibility exists that higher energy states of polysilanes are small-polaron-like in contrast to the large-polaron-like first excited state. The quantum yield for fluorescence in the high polymers is observed to be excitation wavelength dependent [9]. For example, in DNHS excited at energies higher than 3 1000 cm- ’ the quantum yield of fluorescence is constant at 0.27, but to lower energy it increases to a value x 0.5. Thus excitation above this energy would initially create localized states having a high yield of photochemistry and a smaller probability of popula-

J.R.G. Thorne et al. /Conjined excitations in a-conjugated silane oligomers

408

tion of lower energy delocalized

fluorescent

states.

5. Conclusions In this communication we have sought the conditions that need to be satisfied for excitation delocalization in o-conjugated systems. We find that a chain of at least 10 silicon atoms is required for large polaron (free exciton) formation. In longer chains motional narrowing is seen and the exciton bandwidth is large such that substitution of silicon by germanium does not affect the spatial extend of the excitation. In shorter chains the effective conjugation length is reduced below the molecular length as the excitation becomes strongly bound to the lattice (self-trapped exciton): photochemical decomposition through a charge transfer intermediate is suggested to occur as a result of this localization. Fluorescence depolarization results suggest that confined excitations undergo conformational relaxation within 10 ps. Since the submission of this paper we have learned of recent calculations by Balaji and Michl [ 301 that relate to the caveat mentioned earlier concerning the possibility of there being nearby states in short-chain silanes. Their work [ 301 suggests a decreasing separation between err* and oo* as the chain length shortens. The resulting level crossing or level mixing should be taken into account in a complete description of the localization and concomitant lifetime lengthening introduced here. We are indebted to Prof. Michl for useful discussions of this point.

Acknowledgement This work was supported

by NSF-DMR-85

19059.

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