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

Radiative and nonradiative relaxation processes after excitation by ultraviolet 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 on the light modulation frequency to determine the nonradiative relaxation rate. 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 became larger in the order p-, m-, o-Ar(L4)₂. We transformed these results into an energy balance of the radiative and nonradiative relaxation processes. Our analysis showed that 50% of the excitation energy was not released from p-Ar(L4)₂ for a 100-ms order. Next, the measured fluorescence decay times of the three Ar(L4)₂ were obtained as 1.7 ns which revealed that the anomalous 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 nonergodic energy transfer should be considered. Last, we proposed that the nonlinear conjugated oscillator model of Fermi-Pasta-Ulam theory would be suitable for the intramolecular energy storage.