Morphological dependence of radiative and nonradiative relaxation energy balance in photoexcited aryl ether dendrimers as observed by fluorescent andthermal lens spectroscopies

Citation
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
Citations number
21
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF PHYSICAL CHEMISTRY B
ISSN journal
15206106 → ACNP
Volume
105
Issue
19
Year of publication
2001
Pages
4441 - 4445
Database
ISI
SICI code
1520-6106(20010517)105:19<4441:MDORAN>2.0.ZU;2-L
Abstract
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.