2P-011
トランス-ドデカペンタエン気体の光物理化学(広島大院・総合科学) 羅 文文，伊藤 隆夫

Photophysics of all-trans-2,4,6,8,10-dodecapataene vapor(Hiroshima Univ.) Wenwen LUO and Takao ITOH
S1 and S2 fluorescence and excitation spectraof all-trans-2,4,6,8,10-dodecapataene (P5)vapor have been measured at different N2 andO2 gas pressures from 0 to 800 Torr withdifferent excitation energies along with theabsorption spectra. Based on the relaxation
model, we were able to extracted estimates forthe rates that characterize the excited-state
dynamical behavior of P5 vapor.

Experimental
P5 was synthesized bymeans of Wittig reaction. The sample waschromatographed on silica gel columns andrecrystallized from hexane. Only one peak
was detected in HPLC measurements. The
absorption and fluorescence spectra of thepurified sample in solution agree well withthose reported previously. The corrected
excitation spectrum in hexane agreed wellwith the corresponding absorption spectrum.
Emission and excitation spectra weremeasured with a Spex Fluorolog-3
spectrophotometer operated in photoncounting mode.
Results and Discussion Fig. 1 shows
fluorescence spectra of pure P5 vapor at 40 °Cobtained
byexcitation
atdifferent
wavelengths. The red shift of the S 1
fluorescence upon increasing the excitationenergy indicates that the fluorescence occurs
from the unrelaxed high vibronic levels of S1.
Fig. 2 shows fluorescence excitation spectraof pure P5 vapor without buffer gas obtainedby monitoring the S1 and S2 fluorescencealong with the absorption spectrum. It is seenthat the intensities of the excitation spectra ofthe S1 and S2 fluorescence decrease withincreasing excitation energy. In the presenceof 800 Torr N2 gas, the excitation spectrum of
the S1 fluorescence agrees well with theabsorption spectrum, but that of S2
fluorescence is almost the same as theexcitation spectrum measured at low pressure.
These observations suggest the presence of thefast S2  S1 internal conversion.

Fig. 1 Corrected fluorescence spectra of pure P5vapor (0.1 Torr) at 40 °C obtained by excitation at
different wavelengths. All the spectra are normalized
at the highest intensity.

Fig. 2 Corrected fluorescence excitation spectra ofpure P5 vapor at 0.1 Torr obtained by monitoring theS1 and S2 fluorescence and the absorption spectrum.

In Fig. 3, we show the excitation energy
dependence of the S1 and S2 fluorescencequantum yields for pure P5 vapor along withthat of the yield ratio  F2/ F1. These yieldvalues are obtained by division of the intensityof corrected excitation spectra of pure P5
vapor by that of the absorption spectrum ateach wavelength. Although both the S1 and S2
fluorescence yields decrease with increasing
the excitation energy, the decrease is not
significant for the latter yield. In the case ofhigh pressure samples the S1 fluorescence
yield is almost constant regardless of theexcitation energy, although the S2 fluorescenceyield shows a decrease with increasing theexcitation energy. The quantum yield of the S1
fluorescence,  F1, was found to increase withincreasing N2 pressure, while that of the S 2
fluorescence, F2, is nearly constant overwhole range of the pressure.
Fig. 4 shows fluorescence spectra of P5
vapor in the presence of O2 and N2 mixed gas
with different O2 contents, where the totalpressures of the buffer gas were always kept at
about 800 Torr. The total pressures of thesamples were always kept about 800 Torr inorder to investigate the effects of added O2 gas
only on the molecules relaxed to lower
vibrational levels in S1. As is seen clearly in
Fig. 4, the relative intensity of the S 1
fluorescencedecreases
systematicallydepending on the content of added O2 gas.
This observation demonstrates that thefluorescence lifetime of the S1 state issignificantly longer than that of the S2 state.
Based on the spectroscopic data including
the pressure dependence of the fluorescencequantum yields, the relaxation process of P5
vapor can be accounted for by a kinetic shownin Fig. 5. On the basis of the relaxation model
illustrated in Fig. 5, the  F1 and  F2
fluorescence quantum yields obtained by theexcitation into the S2 state are expressed in theforms,
 F1 = kF1  k21/(kF2 + k21)  [1 + kCp/(kF1 +k100)]/(kF1 + k10v + kCp)
(1)
 F2 = kF2/(kF2 + k21),(2)
Under the collision free condition Eq. 1 isapproximated by, F1  kF1/(kF1 + k10v)(3)
At high total pressure in the presence of 800
Torr N2 gas, Eq. 1 is approximated by F1  kF1/(kF1 + k100)(4)
It follows from Eqs. 2 and 3 that
 F2 / F1 = (kF2/kF1)  (kF1 + k10v + kCp)/[1 +kCp/(kF1 + k100)]/k21
(5)
v
Since
kF1<<k10 ,undercollision-freeconditions Eq. 5 can be approximated by F2 / F1  kF2/kF1  k10v/k21(6)
Further, since kF1 << k100 and kF2 << k21, Eqs.
2 and 3 can be approximated, respectively, by F2  kF2/k21 and  F1  kF1k10v(7)
Hence, the values of  F2/ F1,  F2 and  F1
plotted as a function of excitation energy inFig. 3 correspond, respectively, to thevariation of k10v/k21, 1/k21 and 1/k10v against
the excitation energy. It is seen in Fig. 3 thatalthough the values for k10v and k21 increase
with increasing excitation energy, the increaseof k21 is not so significant as that of k10v.

Fig. 3 Excitation energy dependence of the relative S 1and S2 fluorescence yields for pure P5 vapor at 0.1
Torr along with that of the yield ratio F2/F1.

Fig. 4 Corrected fluorescence spectra of P5 vapor in
the presence of added buffer gases with different O2contents.

Fig. 5 Scheme showing the electronic relaxation
processes of P5 vapor.

Ref. T. Itoh, Mol. Phys. 107 (2009) 1705.