Synthesis, Structures, and Photoinduced Electron Transfer Reaction in the 9,9‘-Spirobifluorene-Bridged Bipolar Systems

A series of 9,9‘-spirobifluorene-bridged bipolar compounds DnAm bearing various n:m ratios for
triarylamine (D) versus 1,3,4-oxadiazole-conjugated oligoaryl moiety (A) have been synthesized to
investigate the corresponding photoinduced electron transfer (PET) property. The excitation behaviors
were probed by steady-state absorption, emission, fluorescence solvatochromism, and femtosecond
fluorescence up-conversion spectroscopy. The overall reaction dynamics can be rationalized by the rate
of PET, in combination with solvent relaxation dynamics. It was found that the rate of PET is dependent
on the anchored D/A ratio. The rate of D1A1 and D2A1 was resolved to be ∼2.44 × 1012 and 2.32 ×
1012 s-1, respectively, while it is irresolvable in D1A2 and D2A2 (>6.6 × 1012 s-1). In another approach,
based on the comprehensive X-ray data, cyclic voltammetry, and absorption/emission spectra, the rate of
photoinduced electron transfer was also qualitatively estimated. Fair comparisons were made between
experimental and theoretical approaches to gain detailed insight into the PET for the titled systems.