- Email: [email protected]
Exciton transfer and dephasing in polysilanes at low temperature
-
JOIJRNALOF-
-
Journ cil of Lu nit nescenec ~3 (I 992) 1 71)— 1 / 4 North-Holland
Exciton transfer and dephasing in polysilanes at low temperature J.R.G. Thorne A. Tilgncr c, Y.R. Kim a, J.M. Zeigler ~, H.P. Trommsdorff C and R.M. Hochstrasser Department of (.7remistry, Un,rernt~’of Pennsylrania, Philadelphia, 101 19104, USA Si/diem Inc., 2208 Lester Dr., Albuquerque, NM 87112, USA Laboratoire de Spectronietrie I’hvsiqite a,ssodle an C NRS. Unit Crate .hrseph I’oimru’c Grenoble, bramice
We report on time—correlated single photon counting studies ol the sigma eonjtigated polvsi lane, polv) di—n—hexvlsilane 1. at 2 K. ‘ihe emission processes of Raman and dephased fluorescence are distinguished. It is shos~n that the risetinie ot fluorescence and the decay time of the Raman polarization are stint lar and that e tic rgv transfer occurs on it te is 01 picosecond timescale. Simultaneous depolarization of the emission suggests energy transfer along a twisted chain. The energy transfer time is several times longer than the ( T~) dephasing time derived from hole burning studies of the polymer.
1. Introduction
posed that disorder can he introduced in the chain by random introduction of trans-gauche
The polysilane high polymers, (RR Si)1. are a class of saturated sigma-conjugated materials with a number of important photophysical and photochemical properties (see ref. [1] for a review). The need to understand spatial and spectral diffusion of low-dimensional excitations has led to studies of fluorescence depolarization [2—4],excited state absorption depolarization [5], exciton—exciton annihilation [6] and hole burning [7—11]. Poly(di-n-hexylsilane) is the most extensively studied of the polymers. The absorption spectrum in low temperature 3-methyl-pentane glass is shown in fig. 1 [It)]. The emission spectrum upon line-narrowed excitation is also shown in the figure. The absorption spectrum has been understood in terms of an inhomogeneous distribution of states having a range of excitation energies. Emission originates from the lower energy states following energy transfer. The solution ahsorption spectrum has been numerically modelled [121 under the assumption of segmentation using Huckel linear chain theory and it has been pro
links in an otherwise perfect all-trans chain. It was shown [8—Ill that the spectrum can he moddied by introducing a small random Gaussian energy disorder at every site in a linear chain. These alternative views of polysilane excitations are termed “segmented chain”/ “intermittent disorder”, and “random disorder” respectively. The random disorder model explains the fluorescence and absorption spectral shapes and Franck—Condon factors, and the effects of energy -
..
. -
I
a R
L
I
_______
350 C orrespondence to: Dr. R.M. 1 lochstrasser, Department of Chemistry. University of Pennsylvania, Philadelphia, PA 19104. USA. 0022-2313/92/$O5.U))
1992
—
WAVELENGTH
________
360 (nm)
Fig. 1. Poly(di-n-hexylsilane) in 3-methylpentane glass at 2 K. absorption and emission (351 nm excitation) spectra.
Elsevier Science Publishers B.V. All rights reserved
J.R.G. Thorne eta!.
/ Exciton
transfer and dephasing in polysilanes at low temperature
171
transfer prior to photochemistry on the hole
~l5Ops
to observed spectra leads to the conclusion that Ops
the lowest energy states of the chain have extents of severalspectrum burning tens of silicon [10]. Application atoms in agreement of this model with results from radiative lifetimes. Time-resolved, frequency-selective emission spectroscopy is the method of choice for observing spectral energy diffusion. The process of spatial energy transfer along a twisted chain can be followed by the technique of time-resolved fluorescence depolarization. Previously these studies were performed in room temperature solution [2,4,5]. In the present work the samples are at liquid helium temperature so that the dephasing rates may be compared with those obtained from hole burning experiments. 2. Experimental Poly(di-n-hexylsilane) samples of molecular weight 420000, having an average chain length of 2100 silicons, were prepared by the methods of Zeigler et al. [13]. Solutions in 3-methylpentane were prepared by the techniques described by Tilgner et al. [101and rigorously excluded oxygen. The solution was 1 x i0~ M on a per silicon basis giving an optical density of 0.5 at the absorption peak in the 0.5 mm cell used to prepare the low temperature glass. Fluorescence was excited by the frequency doubled output of a mode-locked YAG pumped
t~5Ops t~0 Ps t~5Ops
363 WAVELENGTH(nm)
Fig. 2. Time-resolved spectra, 351 nm excitation (50—150 ps). Steady-state spectrum from fig. I is shown for comparison.
The excitation in this and the experiments in figs. 2 and 3 was at 351 nm (laser wavelength marked L in fig. 1). Figure 2 shows time-resolved spectra detected in the frequency range 359—363 nm, at or near the position of the 702 cm vibrational frequency associated with the symmetric Si—C stretching mode [14], marked R in fig. 1. The spectra were derived from analysis of the temporal decay curves obtained at different emission frequencies. Magic angle lifetime data were fitted to a model of one rising and one or two decaying
‘~
_________________ .
(a)
II
z
dye0.5of laser, dumped at MHz, operating tion near mersed A 700 minmonochromator nm acavity 0.2 pumped (LDS nm with 698 liquid 200 dye). gave helium p~mslits. a5Samples frequency Dewar Time-correwere at 2imresoluK. lated single photon counting detection [2] produced an instrumental temporal response of 45 ps FWHM, determined from the Rayleigh scattering by the solvent away from the polymer resonance at 375 nm. ‘~
I— 0.4 z
~b) I I
>0~
H 0
0.2
if, ‘. ..~‘
0
3. Results Figure 1 shows the absorption and line-narrowed emission spectra of the polymer at 2 K.
0
~
0.5
1
TIME Ins) Fig. 3. Emission anisotropy: (a) parallel and perpendicular
emission decays (together with instrument function 45 ps..); (b) convoluted anisotropy decay.
JR. U. I borne
I 72
t’t
(11/ Ii.viiton tra,i ster and
components. The risetime (1’) ol fluorescence excited at 351 nm (and detected to longer wavelengths near R) was observed to he 51) ±I)) ps: the decay time of fluorescence detected at 351 nm (and excited at shorter wavelengths) was found to have a similar value. Time resolved emission at 360 nm, which is dominated by Raman scattering, is shown in fig. 3 as the emission was polarized parallel and perpendicular to the laser beam, together with the ratio r( 1) (1 / )/ (21 + I~) whtch del tnes the anisotropy, for the instrument convoluted data. It has a theoretical maximum of 0.4 for parallel absorbing and emitting dipole moments. The deconvoluted anisotropy was ohtained by fitting both ‘H and I~simultaneously and for this excitation gave a value for the (1/c) decay tinie (TT) of 25 ps, from a maximum value of 1)27. =
—.
4. Discussion The fluorescence spectrum of fig. I is cornposed of two parts: on the one hand, the Rayleigh and Raman sharp bands due to the initially cxcited wavepacket. with no rise time, and on the other hand, a broad background that grows with time and is composed of the relaxed dephased fluorescence emanating from states populated by energy transfer. The latter dominates the steadystate fluorescence spectrum at all wavelengths except the Rayleigh line, and the emission at all detection wavelengths (including at the Raman line R) has a rise time because of this hackground. We assume initially that states of the polymer absorb and emit at a particular wavelength W and can transfer energy with a single characteristic rate constant. Because it was not possible to perform experiments excited and detected at identical wavelengths, a summary of the deduced energy transfer rates is given in fig. 4, as determined from four different experiments: i) the decay of fluorescence detected at wavelength W (excited at shorter wavelengths); ii) the rise time of longer wavelength fluorescence (near R) when excited at wavelength W:
deplia.si,ii~in pols’.silancs
at low temnpcm’atioe
1
H
f
004
05
0
~002
0
3~O
360
WAVELENGTH mm) ..
I iy 4 Ins crsc LncrLs Ii insfci tirncs is i Iunctii)n ol ibsoi p tion frequency. The solid curve is the absorption band having optical densities given by the right-hand scale. Ii) decay times: In)
S
• Ironi rise times: (iii)
Ironi anisotropy decay: (iv) ~
‘--
from S
- - -
from hole width assuming i~
pioress.
iii) the decay of the Raman line R anisotropy excited at W: iv) the width of the hole burnt at wavelength W. The lifetimes of states vary as the detection wavelength is shifted across the inhomogeneous line (hut they are virtually independent of excitation wavelength). For detection wavelengths longer than 355 nni. the lifetime becomes equal to equal to 150 ps, the value previously determined for non-transferring states [4]. ‘l’he results indicate that phonon assisted energy transfer takes place on a time scale of several tens of picoseconds. through energy loss processes leading to red-shifted dephased fluorescence Similar values are obtained for rise and decay times of the associated fluorescences l’or states near 351 nm shown in fig. 4, suggesting that the energy transfer involves very few steps (perhaps only one). Multi-step transfer would tend to produce longer rise times than decay times. We may approximate the energy transfer time indicated by the 25 ps (T’) decay time of the total emission anisotropy using the fluorescence to Raman integrated ratio in the unpolarized eniission spectrum of fig. I. Because the dephased fluorescence ultimately dominates, the observed decay is faster than the Raman dephasing time. With the assumption that the Raman line is polarized and energy transfer leads to complete depolarization
J.R.G. Thorne et a!.
/ Exciton
transfer and dephasing in poly.silanes at low temperature
of fluorescence we calculate a decay time (T) for the Raman dephasing of 40 ps which is consistent with the energy transfer rise and decay times. Figure 3 shows that almost total loss of emission polarization observed on the Raman line occurs on this 40—50 Ps timescale, which indicates that the states involved in the transfer are spatially separated by a distance large compared to the scale on which the polymer is twisted (the persistence length). This observation differs greatly from the situation found for the polymer in room temperature solution where considerable residual anisotropy persists following energy transfer, for a period of up to 2 ns [2]. It also differs from our previous findings [4] for glassy solutions prepared without rigorous exclusion of oxygen. We conclude that in these media, the polymer has less extended wavefunctions (as a consequence of greater disorder) and excitations are trapped or travel much shorter distances be‘~
‘~
fore trapping, with consequently less depolarization. The temperature dependence of the hole width [10] (non-zero and constant below 10 K) is indicative of a single phonon emission, T1 type, decay mechanism. A two-phonon, T2 type, process should have a zero value at 0 K. The narrowest hole burnt (at 351 nm) had a width of 0.44 cm_i corresponding to a T~time of 24 ps. The increase in hole width with excitation energy parallels the behavior of the lifetime in fig. 4. The energy transfer times derived from the hole burning studies are however several times shorter than we observe here. Two explanations are possible. Spectral diffusion processes on the timescale of the hole burning experiment (minutes) could account for these wider holes, but appears unlikely in view of the temperature independent widths below 10 K [11]. A distribution of energy transfer rates to low lying states would be expected for an inhomogeneously disordered material and would cause distributed non-exponential kinetics to be observed. Furthermore, there are indications from the maximum value of anisotropy observed (only 0.27) that there is an unresolved (< 10 ps) contribution to the kinetics, responsible for the broadening. We consider it likely that the data of fig. 4 represent the spread of these rates for a ‘~
173
particular set of excited states at fixed wavelength.
5. Conclusion We conclude that excitation transfer in polysilanes at low temperature proceeds by phonon assisted (emission) relaxations in a time of tens of picoseconds following excitation, populating low energy states of the disordered polymer and resuiting in wavelength dependent emission lifetimes. The energy transfer leads to almost total loss of emission polarization indicating transfer occurs over distances exceeding the persistence length of the chain, in keeping with the delocalized exciton model of a randomly disordered polymer. It is likely the energy transfer mechanism is responsible for dephasing of excitons in the polymer at low temperature although the T1 time measured from hole burning studies is observed to be a few times shorter than the fluorescence decay times.
Acknowledgements This research was supported by NSF-DMR. RMH and HPT are grateful for a NATO grant.
References [I] RD. Miller and J. Miehl, Chem. Rev. 89(1989)1359. [21YR. Kim,Zeigler, M. Lee, J.R.G. Thorne, R.M. Hochstrasser and J.M. Chem. Phys. Lett. 145 (1988) 75. .
[31J. Michl, J.W. Downing, T. Karatsu, K.A. Klingensmith, G.M. Wallraff and R.D. Miller, Inorganic and Organo-
[4] [5] [6] [7] [8]
metallic Polymers ACS Symposium Series 360. M. Zeldin. K.J. Wynne and H.R. Alleock (ACS, Washington DC, 1988) chap. 5, p. 61. J.R.G. Thorne, R.M. Hochstrasser and J.M. Zeigler J. Phys. Chem. 92 (1988) 4275 J.R.G. Thorne, ST. Repinee. S.A. Abrash, J M. Zeigler and R.M. Hochstrasser, Chem. Phys 146 (1990) 315. R.G. Kepler and J.M. Zeigler, Advances in Silicon Based Polymer Science, ACS Workshop Hawaii (1987). H.P. Trommsdorff, J.M. Zeigler and R.M. Hoehstrasser, J. Phys. 89 Trommsdorff, 1988 4440. A.Chem. Tilgner, H.P. J.M. Zeigler and R.M. Hochstrasser, J. Lumin. 45 (1990) 373.
I 74
.1. R. U. ihorne et al / Lvcitomi [ranter amid dep/ia,sin,i~in
[9] A. ‘filgner. J.P. I’ique. 11.1’. Trommsdorti ..l.M. Zeigler and R.M. Iloehstrassei’, Polymer Prepi’iiits 31(199))) 244. [I))] A. Tilgiier. lIP. Trommsdorff. J.M. Zeigler and R.M. i-Iochstrasser. .1. Inorganic and Organometallic Polymers. in press. [II] A. Tilgncr, J.M. Zcigler, R.M. Hochstrasser and (1.1’. Trommsdorff. I. C ‘hcni. Phys., subn,itted. [121 L.A. Flarrah and J.M. Zeigler. ACS Symposium Series. eds. (‘.E. llovle and .I.M. Torkelson. Am. (‘heni. Soc. 358 (1987) 482.
113)
po/v,sda,ies at low reniperatore
J.M. Zeigler. l’olyni. Prepr. (Am. (‘hem. Soc., Div. Polym. (‘hem.) 27 (1986) 109: .J.M. L.A. Ilarrah and A.W Johnson, PoInt. 1repr.Zeigler. (Ant. (‘hem. Soc.. Div. Polym. Chem.) 28 (1987) l 424.
[14] Il. Kuzmany, i.E. Rabolt, 13.1.. Farmer and RI) Miller, .1 (‘hem. Phys. $5 I 986) 7413.
JOIJRNALOF-
-
Journ cil of Lu nit nescenec ~3 (I 992) 1 71)— 1 / 4 North-Holland
Exciton transfer and dephasing in polysilanes at low temperature J.R.G. Thorne A. Tilgncr c, Y.R. Kim a, J.M. Zeigler ~, H.P. Trommsdorff C and R.M. Hochstrasser Department of (.7remistry, Un,rernt~’of Pennsylrania, Philadelphia, 101 19104, USA Si/diem Inc., 2208 Lester Dr., Albuquerque, NM 87112, USA Laboratoire de Spectronietrie I’hvsiqite a,ssodle an C NRS. Unit Crate .hrseph I’oimru’c Grenoble, bramice
We report on time—correlated single photon counting studies ol the sigma eonjtigated polvsi lane, polv) di—n—hexvlsilane 1. at 2 K. ‘ihe emission processes of Raman and dephased fluorescence are distinguished. It is shos~n that the risetinie ot fluorescence and the decay time of the Raman polarization are stint lar and that e tic rgv transfer occurs on it te is 01 picosecond timescale. Simultaneous depolarization of the emission suggests energy transfer along a twisted chain. The energy transfer time is several times longer than the ( T~) dephasing time derived from hole burning studies of the polymer.
1. Introduction
posed that disorder can he introduced in the chain by random introduction of trans-gauche
The polysilane high polymers, (RR Si)1. are a class of saturated sigma-conjugated materials with a number of important photophysical and photochemical properties (see ref. [1] for a review). The need to understand spatial and spectral diffusion of low-dimensional excitations has led to studies of fluorescence depolarization [2—4],excited state absorption depolarization [5], exciton—exciton annihilation [6] and hole burning [7—11]. Poly(di-n-hexylsilane) is the most extensively studied of the polymers. The absorption spectrum in low temperature 3-methyl-pentane glass is shown in fig. 1 [It)]. The emission spectrum upon line-narrowed excitation is also shown in the figure. The absorption spectrum has been understood in terms of an inhomogeneous distribution of states having a range of excitation energies. Emission originates from the lower energy states following energy transfer. The solution ahsorption spectrum has been numerically modelled [121 under the assumption of segmentation using Huckel linear chain theory and it has been pro
links in an otherwise perfect all-trans chain. It was shown [8—Ill that the spectrum can he moddied by introducing a small random Gaussian energy disorder at every site in a linear chain. These alternative views of polysilane excitations are termed “segmented chain”/ “intermittent disorder”, and “random disorder” respectively. The random disorder model explains the fluorescence and absorption spectral shapes and Franck—Condon factors, and the effects of energy -
..
. -
I
a R
L
I
_______
350 C orrespondence to: Dr. R.M. 1 lochstrasser, Department of Chemistry. University of Pennsylvania, Philadelphia, PA 19104. USA. 0022-2313/92/$O5.U))
1992
—
WAVELENGTH
________
360 (nm)
Fig. 1. Poly(di-n-hexylsilane) in 3-methylpentane glass at 2 K. absorption and emission (351 nm excitation) spectra.
Elsevier Science Publishers B.V. All rights reserved
J.R.G. Thorne eta!.
/ Exciton
transfer and dephasing in polysilanes at low temperature
171
transfer prior to photochemistry on the hole
~l5Ops
to observed spectra leads to the conclusion that Ops
the lowest energy states of the chain have extents of severalspectrum burning tens of silicon [10]. Application atoms in agreement of this model with results from radiative lifetimes. Time-resolved, frequency-selective emission spectroscopy is the method of choice for observing spectral energy diffusion. The process of spatial energy transfer along a twisted chain can be followed by the technique of time-resolved fluorescence depolarization. Previously these studies were performed in room temperature solution [2,4,5]. In the present work the samples are at liquid helium temperature so that the dephasing rates may be compared with those obtained from hole burning experiments. 2. Experimental Poly(di-n-hexylsilane) samples of molecular weight 420000, having an average chain length of 2100 silicons, were prepared by the methods of Zeigler et al. [13]. Solutions in 3-methylpentane were prepared by the techniques described by Tilgner et al. [101and rigorously excluded oxygen. The solution was 1 x i0~ M on a per silicon basis giving an optical density of 0.5 at the absorption peak in the 0.5 mm cell used to prepare the low temperature glass. Fluorescence was excited by the frequency doubled output of a mode-locked YAG pumped
t~5Ops t~0 Ps t~5Ops
363 WAVELENGTH(nm)
Fig. 2. Time-resolved spectra, 351 nm excitation (50—150 ps). Steady-state spectrum from fig. I is shown for comparison.
The excitation in this and the experiments in figs. 2 and 3 was at 351 nm (laser wavelength marked L in fig. 1). Figure 2 shows time-resolved spectra detected in the frequency range 359—363 nm, at or near the position of the 702 cm vibrational frequency associated with the symmetric Si—C stretching mode [14], marked R in fig. 1. The spectra were derived from analysis of the temporal decay curves obtained at different emission frequencies. Magic angle lifetime data were fitted to a model of one rising and one or two decaying
‘~
_________________ .
(a)
II
z
dye0.5of laser, dumped at MHz, operating tion near mersed A 700 minmonochromator nm acavity 0.2 pumped (LDS nm with 698 liquid 200 dye). gave helium p~mslits. a5Samples frequency Dewar Time-correwere at 2imresoluK. lated single photon counting detection [2] produced an instrumental temporal response of 45 ps FWHM, determined from the Rayleigh scattering by the solvent away from the polymer resonance at 375 nm. ‘~
I— 0.4 z
~b) I I
>0~
H 0
0.2
if, ‘. ..~‘
0
3. Results Figure 1 shows the absorption and line-narrowed emission spectra of the polymer at 2 K.
0
~
0.5
1
TIME Ins) Fig. 3. Emission anisotropy: (a) parallel and perpendicular
emission decays (together with instrument function 45 ps..); (b) convoluted anisotropy decay.
JR. U. I borne
I 72
t’t
(11/ Ii.viiton tra,i ster and
components. The risetime (1’) ol fluorescence excited at 351 nm (and detected to longer wavelengths near R) was observed to he 51) ±I)) ps: the decay time of fluorescence detected at 351 nm (and excited at shorter wavelengths) was found to have a similar value. Time resolved emission at 360 nm, which is dominated by Raman scattering, is shown in fig. 3 as the emission was polarized parallel and perpendicular to the laser beam, together with the ratio r( 1) (1 / )/ (21 + I~) whtch del tnes the anisotropy, for the instrument convoluted data. It has a theoretical maximum of 0.4 for parallel absorbing and emitting dipole moments. The deconvoluted anisotropy was ohtained by fitting both ‘H and I~simultaneously and for this excitation gave a value for the (1/c) decay tinie (TT) of 25 ps, from a maximum value of 1)27. =
—.
4. Discussion The fluorescence spectrum of fig. I is cornposed of two parts: on the one hand, the Rayleigh and Raman sharp bands due to the initially cxcited wavepacket. with no rise time, and on the other hand, a broad background that grows with time and is composed of the relaxed dephased fluorescence emanating from states populated by energy transfer. The latter dominates the steadystate fluorescence spectrum at all wavelengths except the Rayleigh line, and the emission at all detection wavelengths (including at the Raman line R) has a rise time because of this hackground. We assume initially that states of the polymer absorb and emit at a particular wavelength W and can transfer energy with a single characteristic rate constant. Because it was not possible to perform experiments excited and detected at identical wavelengths, a summary of the deduced energy transfer rates is given in fig. 4, as determined from four different experiments: i) the decay of fluorescence detected at wavelength W (excited at shorter wavelengths); ii) the rise time of longer wavelength fluorescence (near R) when excited at wavelength W:
deplia.si,ii~in pols’.silancs
at low temnpcm’atioe
1
H
f
004
05
0
~002
0
3~O
360
WAVELENGTH mm) ..
I iy 4 Ins crsc LncrLs Ii insfci tirncs is i Iunctii)n ol ibsoi p tion frequency. The solid curve is the absorption band having optical densities given by the right-hand scale. Ii) decay times: In)
S
• Ironi rise times: (iii)
Ironi anisotropy decay: (iv) ~
‘--
from S
- - -
from hole width assuming i~
pioress.
iii) the decay of the Raman line R anisotropy excited at W: iv) the width of the hole burnt at wavelength W. The lifetimes of states vary as the detection wavelength is shifted across the inhomogeneous line (hut they are virtually independent of excitation wavelength). For detection wavelengths longer than 355 nni. the lifetime becomes equal to equal to 150 ps, the value previously determined for non-transferring states [4]. ‘l’he results indicate that phonon assisted energy transfer takes place on a time scale of several tens of picoseconds. through energy loss processes leading to red-shifted dephased fluorescence Similar values are obtained for rise and decay times of the associated fluorescences l’or states near 351 nm shown in fig. 4, suggesting that the energy transfer involves very few steps (perhaps only one). Multi-step transfer would tend to produce longer rise times than decay times. We may approximate the energy transfer time indicated by the 25 ps (T’) decay time of the total emission anisotropy using the fluorescence to Raman integrated ratio in the unpolarized eniission spectrum of fig. I. Because the dephased fluorescence ultimately dominates, the observed decay is faster than the Raman dephasing time. With the assumption that the Raman line is polarized and energy transfer leads to complete depolarization
J.R.G. Thorne et a!.
/ Exciton
transfer and dephasing in poly.silanes at low temperature
of fluorescence we calculate a decay time (T) for the Raman dephasing of 40 ps which is consistent with the energy transfer rise and decay times. Figure 3 shows that almost total loss of emission polarization observed on the Raman line occurs on this 40—50 Ps timescale, which indicates that the states involved in the transfer are spatially separated by a distance large compared to the scale on which the polymer is twisted (the persistence length). This observation differs greatly from the situation found for the polymer in room temperature solution where considerable residual anisotropy persists following energy transfer, for a period of up to 2 ns [2]. It also differs from our previous findings [4] for glassy solutions prepared without rigorous exclusion of oxygen. We conclude that in these media, the polymer has less extended wavefunctions (as a consequence of greater disorder) and excitations are trapped or travel much shorter distances be‘~
‘~
fore trapping, with consequently less depolarization. The temperature dependence of the hole width [10] (non-zero and constant below 10 K) is indicative of a single phonon emission, T1 type, decay mechanism. A two-phonon, T2 type, process should have a zero value at 0 K. The narrowest hole burnt (at 351 nm) had a width of 0.44 cm_i corresponding to a T~time of 24 ps. The increase in hole width with excitation energy parallels the behavior of the lifetime in fig. 4. The energy transfer times derived from the hole burning studies are however several times shorter than we observe here. Two explanations are possible. Spectral diffusion processes on the timescale of the hole burning experiment (minutes) could account for these wider holes, but appears unlikely in view of the temperature independent widths below 10 K [11]. A distribution of energy transfer rates to low lying states would be expected for an inhomogeneously disordered material and would cause distributed non-exponential kinetics to be observed. Furthermore, there are indications from the maximum value of anisotropy observed (only 0.27) that there is an unresolved (< 10 ps) contribution to the kinetics, responsible for the broadening. We consider it likely that the data of fig. 4 represent the spread of these rates for a ‘~
173
particular set of excited states at fixed wavelength.
5. Conclusion We conclude that excitation transfer in polysilanes at low temperature proceeds by phonon assisted (emission) relaxations in a time of tens of picoseconds following excitation, populating low energy states of the disordered polymer and resuiting in wavelength dependent emission lifetimes. The energy transfer leads to almost total loss of emission polarization indicating transfer occurs over distances exceeding the persistence length of the chain, in keeping with the delocalized exciton model of a randomly disordered polymer. It is likely the energy transfer mechanism is responsible for dephasing of excitons in the polymer at low temperature although the T1 time measured from hole burning studies is observed to be a few times shorter than the fluorescence decay times.
Acknowledgements This research was supported by NSF-DMR. RMH and HPT are grateful for a NATO grant.
References [I] RD. Miller and J. Miehl, Chem. Rev. 89(1989)1359. [21YR. Kim,Zeigler, M. Lee, J.R.G. Thorne, R.M. Hochstrasser and J.M. Chem. Phys. Lett. 145 (1988) 75. .
[31J. Michl, J.W. Downing, T. Karatsu, K.A. Klingensmith, G.M. Wallraff and R.D. Miller, Inorganic and Organo-
[4] [5] [6] [7] [8]
metallic Polymers ACS Symposium Series 360. M. Zeldin. K.J. Wynne and H.R. Alleock (ACS, Washington DC, 1988) chap. 5, p. 61. J.R.G. Thorne, R.M. Hochstrasser and J.M. Zeigler J. Phys. Chem. 92 (1988) 4275 J.R.G. Thorne, ST. Repinee. S.A. Abrash, J M. Zeigler and R.M. Hochstrasser, Chem. Phys 146 (1990) 315. R.G. Kepler and J.M. Zeigler, Advances in Silicon Based Polymer Science, ACS Workshop Hawaii (1987). H.P. Trommsdorff, J.M. Zeigler and R.M. Hoehstrasser, J. Phys. 89 Trommsdorff, 1988 4440. A.Chem. Tilgner, H.P. J.M. Zeigler and R.M. Hochstrasser, J. Lumin. 45 (1990) 373.
I 74
.1. R. U. ihorne et al / Lvcitomi [ranter amid dep/ia,sin,i~in
[9] A. ‘filgner. J.P. I’ique. 11.1’. Trommsdorti ..l.M. Zeigler and R.M. Iloehstrassei’, Polymer Prepi’iiits 31(199))) 244. [I))] A. Tilgiier. lIP. Trommsdorff. J.M. Zeigler and R.M. i-Iochstrasser. .1. Inorganic and Organometallic Polymers. in press. [II] A. Tilgncr, J.M. Zcigler, R.M. Hochstrasser and (1.1’. Trommsdorff. I. C ‘hcni. Phys., subn,itted. [121 L.A. Flarrah and J.M. Zeigler. ACS Symposium Series. eds. (‘.E. llovle and .I.M. Torkelson. Am. (‘heni. Soc. 358 (1987) 482.
113)
po/v,sda,ies at low reniperatore
J.M. Zeigler. l’olyni. Prepr. (Am. (‘hem. Soc., Div. Polym. (‘hem.) 27 (1986) 109: .J.M. L.A. Ilarrah and A.W Johnson, PoInt. 1repr.Zeigler. (Ant. (‘hem. Soc.. Div. Polym. Chem.) 28 (1987) l 424.
[14] Il. Kuzmany, i.E. Rabolt, 13.1.. Farmer and RI) Miller, .1 (‘hem. Phys. $5 I 986) 7413.