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Picosecond laser spectroscopy of poly(methylphenylsilylene). Confirmation of previous assignment of the broad band
Volume 160, number 3
CHEMICAL
PHYSICS LETTERS
11 August 1989
PICOSECOND LASER SPECTROSCOPY OF POLY(METHYLPHENYLSILYLENE). CONFIRMATION OF PREVIOUS ASSIGNMENT OF THE BROAD BAND Masahide TERAZIMA, Osamu IT0 and Tohru AZUMI Depnriment ofChemistry, Faculty ofScience, Tohoku University, Sendai 980, Japan Received 3 March 1989; in final form 30 May I989
Photoluminescence of poly(methylphenylsilylene) is investtgated by picosecond laser spectroscopy. The sharp band at 350 nm decays with a lifetime of 7Ok 15 ps. The broad band in the 400-500 nm region first rises with a lifetime of 7Ok I5 ps and then decays with a lifetime of 4 600 ps. The agreement between the decay rate constant of the sharp band and the rise rate constant of the broad band unambiguously substantiates the previous assignment that the sharp and the broad bands are emissions from the ’ (a, o*) and ’ (0, K*) states, respectively.
1. Introduction Poly (methylphenylsilylene), referred hereafter to (MePhSi)., exhibits two kinds of photoluminescence: a relatively sharp band at 350 nm and a significantly broad and structureless band in the 400500 nm region [ 1,2]. The sharp band has been assigned as the emission from the ’ (a, a*) state delocalized over the silicon skeleton. The origin of the broad band, on the other hand, had been controversial for some time [ 1,2]. We have previously [ 3,4] proposed that the broad band in the long wavelength region is the emission from the ’ (6, x*) state, i.e. intramolecular skeleton o to pendant n* charge transfer state. This assignment was based on a number of experimental observations and theoretical considerations, but some alternative assignments have been suggested since then #‘. Especially, there has been a suggestion that the broad band might be better understood in terms of the emission between the ’ (ts, CT*) and ’ (0, n* ) states. In previous papers, we have not considered this possibility in view of the general understanding that the emission between the two excited states is rather unlikely to take place for large molecules. However, #’ Suggestions given personally to TA during the special symposium on o-conjugated system at March 1988 APS Meeting in New Orleans.
0 009-26 14/89/s 03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
those in favor of this assignment argue that the lack of an absorption spectrum corresponding to the reverse process of the broad emission band can only be understood in terms of this ’ (u, CT*)+ ’ (CT,x*) emission. (We did, however, observe a weak absorption which we have assigned as the absorption to the ‘(0, x*) state [4].) The two alternative assignments should be most clearly distinguished experimentally by the picosecond analysis of the decay and rise, if any, of the two emission bands. If the broad band is the emission from the ’ (IS, x*) state as we have previously proposed, the emission should, in the earlier stage after the pulse excitation to the ’ (0, CT*) state, exhibit a build-up with a rate constant identical to the decay rate constant of the sharp band. If, on the other hand, the broad band is due to the ‘(CT, @) to ’ (CT,rc*) emission, the decay of the broad band should be identical to the decay of the sharp band. That is, if the excitation is carried outto the ‘(a, CT*)state, there should be no build-up of the broad band in the detectable time scale. Based on these considerations, we have carried out the picosecond time-resolved measurements. As will be described in detail below, the broad band rises slowly with a rate constant identical to the decay rate constant of the sharp band. This observation conclusively substantiates the previous assignment.
B.V.
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Volume 160, number 3
CHEMICAL PHYSICS LETTERS
11 August 1989
2. Experimental The sample polymer was synthesized and purified as reported previously [ 31. The purified polymer was fractionated by a Shodex GPC-A-804 gel permeation chromatograph, and the portion of the molecular weights in the range of 9.1 x 103- 1.8 x 1O4was used for the measurements. The picosecond analysis was carried out for a dioxane solution with a concentration of 3x lo-“ M with respect to Si atom. The time profile (i.e. decay and rise) of the emission was measured at various temperatures between 10 and 85 K by means of a picosecond time-correlated single photon counting system. The temperature was controlled by using an Air Products displex closed-cycle refrigeration system.
3. Kinetic analysis Kinetic analysis of the time profile of the luminescence is given below based on the previously suggested mechanism. The analysis based on the alternative assignment is certainly possible, but is not given below since, as mentioned above, the existence of the slow build-up of the broad band unambiguously rules out the alternative (i.e. ’ (0, c*)+ ’ (0, n*) emission) mechanism. The time dependence of the populations of the three relevant excited states, ’ ((3, o*), ‘(IT, rc*), and ’ (6, x*), are expressed by the following rate equations:
n1 (t) I[ 1 n2(t)
,
where n,(t), nz(t), and n3(t), respectively, refer to the populations of the ’ (0, o* ), ’ (n, IT*), and ’ (0, a*) states at time t after the pulse excitation. The rate constants denoted by k, through k, are defined in fig. 1. Of these k, and kb are the radiative rate constants and all the others are the nonradiative rate constants. Further, K,=k,+k,,
K,=k,+k,. (2)
We assume that both the ’ (CT,o*) and ’ (n, n*) states are created by light absorption, and further that the 320
Fig. I. Schematic energy diagram of (MePhSi). and various decay rate constants. The solid and wavy arrows, respectively, indicate the radiative and nonradiative decays.
direct excitation to the ’ (CT,n*) state is negligibly small as is experimentally found [ 41. The time dependence of the intensity of the sharp band and the broad band is then expressed as follows:
n1(t)=[n~(0)+n~(O)k~l(~~-K,)lex~(--K,t) - [n~(Okl(fG -fG)lex~(--K~t) , n,(t)=--A,
(3)
exp(-K,t)-&exp(-K,t)
+A3 exp( -Gt)
,
(4)
where
A, =n,(O)ksl(K,
-nr(O)kkl(K,
-K3) -Kx) (K-K,)
h=[n,(O)l(&-K,)l
>
[k,-k,k,l(&-K,)l,
A, =A, +A2 .
(5)
(11
n3(t)
K,=k,+k,+k,,
IT*
Provided only the cited, n2 (0) in eqs. (3 The time dependence then simply expressed
’ (ts, CT*) state is initially ex)- (5) may be equated to zero. of the two excited states are as follows:
n,(t)lnl(0)=exp(--Klt),
(‘3)
n3(t)lnl(0)=WtK~
-K3)
x[-exp(--K,t)+exp(-K,t)].
(7)
Volume 160, number 3
CHEMICAL
PHYSICS LETTERS
4. Results and discussion 4.1. Confirmation of the previous assignments for
the sharp and the broad bands The time profile of the sharp and the broad bands obtained at the 3 18.5 nm excitation for a 77 K pdioxane solution is analyzed in figs. 2 and 3. At this excitation condition only the ’ (CT,cs*) state is excited
Fig. 2. Time profile of the sharp emission of (MePhSi), in dioxane measured at 77 K. Excitation is carried out at 318.5 nm and emission is monitored at 355 nm. The dotted, solid and broken lines represent the fluorescence, simulated and instrument-response curves, respectively. The dotted curve represented in the upper part indicates the weighted residual. The convolution analysis is made in terms ofeq. (6) with K, = 1.4 x 10’Os-‘.
11 August 1989
and thus the time profile should be analyzed in terms of eqs. (6) and (7). The convolution analysis of the decay of the sharp band-shown in fig. 2 indicates that the sharp band rises within the time resolution of the instrument ( z 10 ps) and decays with a single exponential, with a lifetime of 70+ 15 ps. Deviations of 6 15 ps observed in the repeated measurements is probably due to the inhomogeneous distribution of various conformers of the polymer in the solid phase. The decay feature of the sharp band is essentially identical to that observed by Kim et al. [ 5 ] at room temperature. The analysis of the time profile of the broad band is shown in fig. 3. The most important observation is that there exists a slow build-up. The initial portion of the time profile can be simulated by a single exponential rise with a rise time of 70+ 15 ps, that is identical to the decay lifetime of the sharp band **. The existence of the slow build-up of the broad band and the agreement between the decay time of The decay of the broad band is, in fact, nonexponcntial if the time range is taken sufficiently long (say, O-5 ns). The gradient of the decay at around 5 ns corresponds 10about a 24 ns hfetime. Fig. 3 presents the decay only up to 500 ps and in this time range the decay is approximately represented by a single exponential function. The relatively large deviation of the residual observed in fig. 3 may probably be due to the approximation of single exponential decay. The nonexponential decay feature will be discussed elsewhere.
I._,,.,,..,.‘.., :..:.:::.:.:: :.-..,. 1 1 q ..,..__
Fig. 3. Time profile of the broad emission of (MePhSl). in dioxane measured at 77 K. Excitation is carried out at 3 18.5 nm, and emission is monitored at 430 nm. The representation of the curves are the same as those of fig. 2. The convolution analysis is made intermsofeq.(7)withK,=1.4X10’0s-’andK,=I.7X1OPs-‘.
Fig. 4. Time profile of the broad emission of ( MePhSi), in dioxane measured at 77 K. Excitation is carried out at 285 nm, and the emission is monitored at 430 nm. The representation of the curves are the same as those of fig. 2. The convolution analysis is made in terms of eq. (8) with K,=1.4X IO” s-’ and KS= I .8x IO9 s-l.
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the sharp band and the rise time of the broad band was observed also at various temperatures between 10 and 85 K. This observation unambiguously substantiates the previous assignment that the sharp band and the broad bands are, respectively, the emissions from the ’ (0, o*) and ’ (0, n*) states. 4.2. Excitation
11 August 1989
Table 1 Excitation wavelength luminescence a)
dependence
Excitation wavelength
&IA, (nm)
318.5 307.0 300.0 287.7 285.0
wavelength dependence of the rise
of rise curve of the broad
0 0 0.11 0.33 0.67
When the ’ (E, n* ) state is initially excited by light absorption, the emission from the ’ (0, o*) and ’ ( CF,K*) states both should have an additional buildup due to the nonradiative transition from the ‘(x, x*) state. This effect will be examined in more detail below. Since the absorption to the ’ (CT,rr*) state is spread in relatively wide spectral range (due to the large dispersion of o and o* orbitals [ 6 ] ) one cannot excite only the ’ (II, x* ) state. One can instead enrich the relative contribution of the ‘(K, x*) excitation by moving the excitation wavelength to shorter wavelength. That is, as the excitation wavelength is shifted to shorter wavelength, the n2( 0) ln, (0) population ratio tends to increase. The effect of the I (xc, x*) excitation should be revealed in the rise CUNeS. We shall first focus attention on the sharp fluorescence. In view of eq. (3), the sharp fluorescence is expected to grow with a rate constant of &. Experimentally, however, whatever the wavelength of excitation is selected, even a trace of the build-up process was not detected; the observed time profile was simulated in terms of a simple decay followed by an instantaneous rise. This observation indicates that the expected rise time l/K, is shorter than the time resolution (= 10 ps) of the instrument. We will next discuss on the behavior of the broad band. In view of the abovementioned observation that the time resolution of the instrument is longer than I/&, exp( -l&t) in eq. (4) may be equated to zero. Then, the time depetidence of the broad band in the earlier stage should be expressed as follows
The work presented above conclusively substantiates the previous assignment that the broad and structureless luminescence of (PhMeSi) n observed in the 400-500 nm region is due to the emission from the ’ (0, n*) state to the ground state. From the picosecond analysis the various rate constants are obtained as follows:
n3(t)=--A,
K,=k,+kz+k3=1.4x1010s-‘,
exp(-K,t)+(A,+&)exp(-K3t). (8)
The observed time profile was analyzed in terms of eq. (8) with variable parameters ofAJA, and 4, K, being set to 1.4X 10” s-l. The experimental data obtained at various exciting wavelength were satis322
‘1 Based on eq. (8) of the text.
factorily simulated with K3 = 1.7 x 10’ s- ’ and A1 /A, values shown in table 1, From eq. ( 5 ), the parameter AJA, is related to the population ratio n,(O)/n, (0) by
[n2(0)lnl (0) la
Az’A1= l-[n~(o)/n,(O)]~’ where kc&
-K3)
,
P=k,/(&-K,).
Since (Yand p are both positive, A2fAI is expected to increase as tz2(0) /n, (0) increases. The results shown in table 1 are indeed in accord with this expectation. This agreement further supports the adequacy of the previously suggested mechanism.
5. Conclusions
K,=k,+k,>
10” s-’ 3
K3=k6+k,z1.7x109s-‘, where KJ refers to the rate constant,only in the earlier stage of the decay (see footnote 2). Further, if
Volume 160, number
3
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PHYSICS LETTERS
the fluorescence quantum yield of 0.11 determined by Han-ah and Zeigler [ 71 is taken into account, the following value of the radiative rate constant for the ’ (CT,7c*) fluorescence is obtained:
I I August 1989
cussion. The present paper was supported in part by a Grant-in-Aid for Scientific Research (No. 62430001 and No. 622 13003) from the Ministry of Education, Science and Culture.
References Acknowledgement
We wish to thank Professor I. Yamazaki and Dr. N. Tamai of the Institute for Molecular Science (IMS) for allowing us to use the picosecond single photon counting system. We are also indebted to Professor Y. Maruyama of IMS for permitting us to use the temperature control system. We also wish to thank Dr. S. Ito of Kyoto University who kindly performed gel permeation fractionation. Further, we acknowledge Professor H. Sakurai of this Department and Dr. N. Matsumoto of NTT for invaluable dis-
[ 1 ] T. Kagawa, M. Fujino, K. Takeda and N. Matsumoto, Solid State Commun. 57 (1986) 635. [ 2 ] L.A. Harrah and J.M. Zeigler, J. Polym. Sci. C 25 ( 1987) 205. [ 31 T. Azumi, 0. Ito, M. Terazima, N. Matsumoto, K. Takeda and M. Fujino, Bull. Am. Phys. Sot. 33 (1988) 540. [ 41 0. Ito, M. Terazima, N. Matsumoto, K. Takeda, M. Fujino and T. Azumi, Macromolecules 22 (1989) 1718. [5] Y.R. Kim. M. Lee, J.R.G. Thorne and.R.M. Hochstrasser, Chem. Phys. Letters 145 ( 1988) 75. [6] K. Takeda, H. Teramae and N. Matsumoto, J. Am. Chem. Sot. 108 (1986) 8186. [7] L.A. Harrah and J.M. Zeigler, Macromolecules 20 ( 1987) 601.
323
CHEMICAL
PHYSICS LETTERS
11 August 1989
PICOSECOND LASER SPECTROSCOPY OF POLY(METHYLPHENYLSILYLENE). CONFIRMATION OF PREVIOUS ASSIGNMENT OF THE BROAD BAND Masahide TERAZIMA, Osamu IT0 and Tohru AZUMI Depnriment ofChemistry, Faculty ofScience, Tohoku University, Sendai 980, Japan Received 3 March 1989; in final form 30 May I989
Photoluminescence of poly(methylphenylsilylene) is investtgated by picosecond laser spectroscopy. The sharp band at 350 nm decays with a lifetime of 7Ok 15 ps. The broad band in the 400-500 nm region first rises with a lifetime of 7Ok I5 ps and then decays with a lifetime of 4 600 ps. The agreement between the decay rate constant of the sharp band and the rise rate constant of the broad band unambiguously substantiates the previous assignment that the sharp and the broad bands are emissions from the ’ (a, o*) and ’ (0, K*) states, respectively.
1. Introduction Poly (methylphenylsilylene), referred hereafter to (MePhSi)., exhibits two kinds of photoluminescence: a relatively sharp band at 350 nm and a significantly broad and structureless band in the 400500 nm region [ 1,2]. The sharp band has been assigned as the emission from the ’ (a, a*) state delocalized over the silicon skeleton. The origin of the broad band, on the other hand, had been controversial for some time [ 1,2]. We have previously [ 3,4] proposed that the broad band in the long wavelength region is the emission from the ’ (6, x*) state, i.e. intramolecular skeleton o to pendant n* charge transfer state. This assignment was based on a number of experimental observations and theoretical considerations, but some alternative assignments have been suggested since then #‘. Especially, there has been a suggestion that the broad band might be better understood in terms of the emission between the ’ (ts, CT*) and ’ (0, n* ) states. In previous papers, we have not considered this possibility in view of the general understanding that the emission between the two excited states is rather unlikely to take place for large molecules. However, #’ Suggestions given personally to TA during the special symposium on o-conjugated system at March 1988 APS Meeting in New Orleans.
0 009-26 14/89/s 03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
those in favor of this assignment argue that the lack of an absorption spectrum corresponding to the reverse process of the broad emission band can only be understood in terms of this ’ (u, CT*)+ ’ (CT,x*) emission. (We did, however, observe a weak absorption which we have assigned as the absorption to the ‘(0, x*) state [4].) The two alternative assignments should be most clearly distinguished experimentally by the picosecond analysis of the decay and rise, if any, of the two emission bands. If the broad band is the emission from the ’ (IS, x*) state as we have previously proposed, the emission should, in the earlier stage after the pulse excitation to the ’ (0, CT*) state, exhibit a build-up with a rate constant identical to the decay rate constant of the sharp band. If, on the other hand, the broad band is due to the ‘(CT, @) to ’ (CT,rc*) emission, the decay of the broad band should be identical to the decay of the sharp band. That is, if the excitation is carried outto the ‘(a, CT*)state, there should be no build-up of the broad band in the detectable time scale. Based on these considerations, we have carried out the picosecond time-resolved measurements. As will be described in detail below, the broad band rises slowly with a rate constant identical to the decay rate constant of the sharp band. This observation conclusively substantiates the previous assignment.
B.V.
319
Volume 160, number 3
CHEMICAL PHYSICS LETTERS
11 August 1989
2. Experimental The sample polymer was synthesized and purified as reported previously [ 31. The purified polymer was fractionated by a Shodex GPC-A-804 gel permeation chromatograph, and the portion of the molecular weights in the range of 9.1 x 103- 1.8 x 1O4was used for the measurements. The picosecond analysis was carried out for a dioxane solution with a concentration of 3x lo-“ M with respect to Si atom. The time profile (i.e. decay and rise) of the emission was measured at various temperatures between 10 and 85 K by means of a picosecond time-correlated single photon counting system. The temperature was controlled by using an Air Products displex closed-cycle refrigeration system.
3. Kinetic analysis Kinetic analysis of the time profile of the luminescence is given below based on the previously suggested mechanism. The analysis based on the alternative assignment is certainly possible, but is not given below since, as mentioned above, the existence of the slow build-up of the broad band unambiguously rules out the alternative (i.e. ’ (0, c*)+ ’ (0, n*) emission) mechanism. The time dependence of the populations of the three relevant excited states, ’ ((3, o*), ‘(IT, rc*), and ’ (6, x*), are expressed by the following rate equations:
n1 (t) I[ 1 n2(t)
,
where n,(t), nz(t), and n3(t), respectively, refer to the populations of the ’ (0, o* ), ’ (n, IT*), and ’ (0, a*) states at time t after the pulse excitation. The rate constants denoted by k, through k, are defined in fig. 1. Of these k, and kb are the radiative rate constants and all the others are the nonradiative rate constants. Further, K,=k,+k,,
K,=k,+k,. (2)
We assume that both the ’ (CT,o*) and ’ (n, n*) states are created by light absorption, and further that the 320
Fig. I. Schematic energy diagram of (MePhSi). and various decay rate constants. The solid and wavy arrows, respectively, indicate the radiative and nonradiative decays.
direct excitation to the ’ (CT,n*) state is negligibly small as is experimentally found [ 41. The time dependence of the intensity of the sharp band and the broad band is then expressed as follows:
n1(t)=[n~(0)+n~(O)k~l(~~-K,)lex~(--K,t) - [n~(Okl(fG -fG)lex~(--K~t) , n,(t)=--A,
(3)
exp(-K,t)-&exp(-K,t)
+A3 exp( -Gt)
,
(4)
where
A, =n,(O)ksl(K,
-nr(O)kkl(K,
-K3) -Kx) (K-K,)
h=[n,(O)l(&-K,)l
>
[k,-k,k,l(&-K,)l,
A, =A, +A2 .
(5)
(11
n3(t)
K,=k,+k,+k,,
IT*
Provided only the cited, n2 (0) in eqs. (3 The time dependence then simply expressed
’ (ts, CT*) state is initially ex)- (5) may be equated to zero. of the two excited states are as follows:
n,(t)lnl(0)=exp(--Klt),
(‘3)
n3(t)lnl(0)=WtK~
-K3)
x[-exp(--K,t)+exp(-K,t)].
(7)
Volume 160, number 3
CHEMICAL
PHYSICS LETTERS
4. Results and discussion 4.1. Confirmation of the previous assignments for
the sharp and the broad bands The time profile of the sharp and the broad bands obtained at the 3 18.5 nm excitation for a 77 K pdioxane solution is analyzed in figs. 2 and 3. At this excitation condition only the ’ (CT,cs*) state is excited
Fig. 2. Time profile of the sharp emission of (MePhSi), in dioxane measured at 77 K. Excitation is carried out at 318.5 nm and emission is monitored at 355 nm. The dotted, solid and broken lines represent the fluorescence, simulated and instrument-response curves, respectively. The dotted curve represented in the upper part indicates the weighted residual. The convolution analysis is made in terms ofeq. (6) with K, = 1.4 x 10’Os-‘.
11 August 1989
and thus the time profile should be analyzed in terms of eqs. (6) and (7). The convolution analysis of the decay of the sharp band-shown in fig. 2 indicates that the sharp band rises within the time resolution of the instrument ( z 10 ps) and decays with a single exponential, with a lifetime of 70+ 15 ps. Deviations of 6 15 ps observed in the repeated measurements is probably due to the inhomogeneous distribution of various conformers of the polymer in the solid phase. The decay feature of the sharp band is essentially identical to that observed by Kim et al. [ 5 ] at room temperature. The analysis of the time profile of the broad band is shown in fig. 3. The most important observation is that there exists a slow build-up. The initial portion of the time profile can be simulated by a single exponential rise with a rise time of 70+ 15 ps, that is identical to the decay lifetime of the sharp band **. The existence of the slow build-up of the broad band and the agreement between the decay time of The decay of the broad band is, in fact, nonexponcntial if the time range is taken sufficiently long (say, O-5 ns). The gradient of the decay at around 5 ns corresponds 10about a 24 ns hfetime. Fig. 3 presents the decay only up to 500 ps and in this time range the decay is approximately represented by a single exponential function. The relatively large deviation of the residual observed in fig. 3 may probably be due to the approximation of single exponential decay. The nonexponential decay feature will be discussed elsewhere.
I._,,.,,..,.‘.., :..:.:::.:.:: :.-..,. 1 1 q ..,..__
Fig. 3. Time profile of the broad emission of (MePhSl). in dioxane measured at 77 K. Excitation is carried out at 3 18.5 nm, and emission is monitored at 430 nm. The representation of the curves are the same as those of fig. 2. The convolution analysis is made intermsofeq.(7)withK,=1.4X10’0s-’andK,=I.7X1OPs-‘.
Fig. 4. Time profile of the broad emission of ( MePhSi), in dioxane measured at 77 K. Excitation is carried out at 285 nm, and the emission is monitored at 430 nm. The representation of the curves are the same as those of fig. 2. The convolution analysis is made in terms of eq. (8) with K,=1.4X IO” s-’ and KS= I .8x IO9 s-l.
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the sharp band and the rise time of the broad band was observed also at various temperatures between 10 and 85 K. This observation unambiguously substantiates the previous assignment that the sharp band and the broad bands are, respectively, the emissions from the ’ (0, o*) and ’ (0, n*) states. 4.2. Excitation
11 August 1989
Table 1 Excitation wavelength luminescence a)
dependence
Excitation wavelength
&IA, (nm)
318.5 307.0 300.0 287.7 285.0
wavelength dependence of the rise
of rise curve of the broad
0 0 0.11 0.33 0.67
When the ’ (E, n* ) state is initially excited by light absorption, the emission from the ’ (0, o*) and ’ ( CF,K*) states both should have an additional buildup due to the nonradiative transition from the ‘(x, x*) state. This effect will be examined in more detail below. Since the absorption to the ’ (CT,rr*) state is spread in relatively wide spectral range (due to the large dispersion of o and o* orbitals [ 6 ] ) one cannot excite only the ’ (II, x* ) state. One can instead enrich the relative contribution of the ‘(K, x*) excitation by moving the excitation wavelength to shorter wavelength. That is, as the excitation wavelength is shifted to shorter wavelength, the n2( 0) ln, (0) population ratio tends to increase. The effect of the I (xc, x*) excitation should be revealed in the rise CUNeS. We shall first focus attention on the sharp fluorescence. In view of eq. (3), the sharp fluorescence is expected to grow with a rate constant of &. Experimentally, however, whatever the wavelength of excitation is selected, even a trace of the build-up process was not detected; the observed time profile was simulated in terms of a simple decay followed by an instantaneous rise. This observation indicates that the expected rise time l/K, is shorter than the time resolution (= 10 ps) of the instrument. We will next discuss on the behavior of the broad band. In view of the abovementioned observation that the time resolution of the instrument is longer than I/&, exp( -l&t) in eq. (4) may be equated to zero. Then, the time depetidence of the broad band in the earlier stage should be expressed as follows
The work presented above conclusively substantiates the previous assignment that the broad and structureless luminescence of (PhMeSi) n observed in the 400-500 nm region is due to the emission from the ’ (0, n*) state to the ground state. From the picosecond analysis the various rate constants are obtained as follows:
n3(t)=--A,
K,=k,+kz+k3=1.4x1010s-‘,
exp(-K,t)+(A,+&)exp(-K3t). (8)
The observed time profile was analyzed in terms of eq. (8) with variable parameters ofAJA, and 4, K, being set to 1.4X 10” s-l. The experimental data obtained at various exciting wavelength were satis322
‘1 Based on eq. (8) of the text.
factorily simulated with K3 = 1.7 x 10’ s- ’ and A1 /A, values shown in table 1, From eq. ( 5 ), the parameter AJA, is related to the population ratio n,(O)/n, (0) by
[n2(0)lnl (0) la
Az’A1= l-[n~(o)/n,(O)]~’ where kc&
-K3)
,
P=k,/(&-K,).
Since (Yand p are both positive, A2fAI is expected to increase as tz2(0) /n, (0) increases. The results shown in table 1 are indeed in accord with this expectation. This agreement further supports the adequacy of the previously suggested mechanism.
5. Conclusions
K,=k,+k,>
10” s-’ 3
K3=k6+k,z1.7x109s-‘, where KJ refers to the rate constant,only in the earlier stage of the decay (see footnote 2). Further, if
Volume 160, number
3
CHEMICAL
PHYSICS LETTERS
the fluorescence quantum yield of 0.11 determined by Han-ah and Zeigler [ 71 is taken into account, the following value of the radiative rate constant for the ’ (CT,7c*) fluorescence is obtained:
I I August 1989
cussion. The present paper was supported in part by a Grant-in-Aid for Scientific Research (No. 62430001 and No. 622 13003) from the Ministry of Education, Science and Culture.
References Acknowledgement
We wish to thank Professor I. Yamazaki and Dr. N. Tamai of the Institute for Molecular Science (IMS) for allowing us to use the picosecond single photon counting system. We are also indebted to Professor Y. Maruyama of IMS for permitting us to use the temperature control system. We also wish to thank Dr. S. Ito of Kyoto University who kindly performed gel permeation fractionation. Further, we acknowledge Professor H. Sakurai of this Department and Dr. N. Matsumoto of NTT for invaluable dis-
[ 1 ] T. Kagawa, M. Fujino, K. Takeda and N. Matsumoto, Solid State Commun. 57 (1986) 635. [ 2 ] L.A. Harrah and J.M. Zeigler, J. Polym. Sci. C 25 ( 1987) 205. [ 31 T. Azumi, 0. Ito, M. Terazima, N. Matsumoto, K. Takeda and M. Fujino, Bull. Am. Phys. Sot. 33 (1988) 540. [ 41 0. Ito, M. Terazima, N. Matsumoto, K. Takeda, M. Fujino and T. Azumi, Macromolecules 22 (1989) 1718. [5] Y.R. Kim. M. Lee, J.R.G. Thorne and.R.M. Hochstrasser, Chem. Phys. Letters 145 ( 1988) 75. [6] K. Takeda, H. Teramae and N. Matsumoto, J. Am. Chem. Sot. 108 (1986) 8186. [7] L.A. Harrah and J.M. Zeigler, Macromolecules 20 ( 1987) 601.
323