Morphological dependence of radiative and nonradiative relaxation energy balance in photoexcited aryl ether dendrimers as observed by fluorescent andthermal lens spectroscopies
Y. Wakabayashi et al., Morphological dependence of radiative and nonradiative relaxation energy balance in photoexcited aryl ether dendrimers as observed by fluorescent andthermal lens spectroscopies, J PHYS CH B, 105(19), 2001, pp. 4441-4445
Radiative and nonradiative relaxation processes after excitation by ultravi
olet light were measured for o-, m-, and p-aryl ether dendrimers of fourth
generation (o-, m-, and p-Ar(L4)(2)) by using fluorescent and thermal lens
spectroscopies. Samples were dissolved in CH2Cl2 to provide concentrations
of a constant absorbance at excitation light wavelength (244 nm). Regarding
the nonradiative process, we investigated thermal lens signal dependence o
n the light modulation frequency to determine the nonradiative relaxation r
ate. When frequency ranged from 4 to 200 Hz, the thermal lens signal became
larger in the order p-, o-, m-Ar(L4)(2), while the fluorescent intensity b
ecame larger in the order p-, m-, o-Ar(L4)2. We transformed these results i
nto an energy balance of the radiative and nonradiative relaxation processe
s. Our analysis showed that 50% of the excitation energy was not released f
rom p-Ar(L4)2 for a 100-ms order. Next, the measured fluorescence decay tim
es of the three Ar(L4)2 were obtained as 1.7 ns which revealed that the ano
malous properties of dendrimers did not originate in long-lived electronic
excitation states, but in long-term storage of internal energy. To explain
this phenomenon, a novel mechanism for intramolecular energy storage with n
onergodic energy transfer should be considered. Last, we proposed that the
nonlinear conjugated oscillator model of Fermi-Pasta-Ulam theory would be s
uitable for the intramolecular energy storage.