900
J. Phys. Chem. B 2001, 105, 900-904
Electronic State of Radical Anions on Poly(methyl-n-propylsilane) Studied by Low Temperature Pulse Radiolysis S. Seki,* Y. Kunimi, K. Nishida, Y. Yoshida, and S. Tagawa* The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ReceiVed: March 30, 2000; In Final Form: October 16, 2000
The effect of Si skeleton structures of poly(methyl-n-propylsilane) (PMPrS) on the electron delocalization has been studied in polar liquid matrices by the low-temperature pulse radiolysis technique over a wide range of temperatures. PMPrS was revealed to have a rigid rodlike Si main chain in the polar matrix at low temperature ( 20. This suggests that the main chain of PMPrS consists only of segments with N > 20 below 193 K. Michl et al. and Kanemitsu et al. also suggested a relationship between the conjugation length (nconj) and the radiative lifetimes based on the absorption by one Si-Si bond (τabs) and fluorescence (τfl):16,17
τabs )
2
νmax
nconj )
1 fVB-ES
τabsΦfl τfl
(2)
where Φfl represents the fluorescence quantum yield, νmax, the wavenumber at maximum absorption. The values of Φfl and τfl were reported in the dilute solution of PMPrS as 0.76 and ∼0.2 ns, respectively.18,19 With eqs 1 and 2, we obtained an estimate of nconj as approximately 15 in PMPrS at 293 K. The value of nconj also obeys the relationship
nconj ∝ fVB-ES
(3)
which also gives nconj > 20 below 193 K. The absorption maximum slightly shifts toward shorter wavelength region upon cooling under ca. 130 K. The shift was also observed in the main chain chromophore of polysilanes with the rigid backbone conformation such as poly(methyl-(S)(2-methylbutyl)silane),14 poly(n-decyl-(S)-(2-methylbutyl)silane),14 and poly(hexylphenylsilane),20,21 which had been ascribed to the Si-Si bond elongation leading to an increase in the band gap energy associated with the higher 1D exciton level. The fluorescence spectra of PMPrS are displayed in Figure 2. A fluorescence peak at 344 nm shows no shift between 293 and 253 K. This indicates that there is no change in the conjugation length of the energetically most stable segment. The fluorescence peak shifts toward the longer wavelength region between 253 and 193 K with decreasing temperature, as summarized in Table 1, followed by a small blue shift below 190 K. Apparently the thermochromic behavior can be divided into three phases: (1) the gradual bathochromic shift of the absorption edge (>250 K), (2) the abrupt decrease in the energy of both the absorption edge and fluorescence (250 K > T >
902 J. Phys. Chem. B, Vol. 105, No. 4, 2001
Seki et al.
Figure 2. Fluorescence spectra of PMPrS in MTHF at (5.0-6.7) × 10-3 M/dm3 concentrated (base mol unit).
Figure 3. Kinetic traces of the transient absorption monitored at 355 nm. Solid, dashed, and dotted lines are the traces at 293, 223, and 193 K, respectively.
190 K),and (3) the blue shift of absorption edge associated with the fluorescence peak (
