Elucidation of the intrinsic and diffusion-driven electron transfer dynamics in <i>N</i> , <i>N</i> -diarylaminostilbenes: an ultrafast spectroscopic perspective

We report a systematic investigation of the photophysical properties and bimolecular photoinduced electron transfer (PET) dynamics for a series of aminostilbene systems in various time regimes. 4-(Dimethylamino)stilbene (DAS), 4-styryl triphenylamine (STPA), and 4-bromo-4-[di(<i>p</i>-toly)amino]stilbene (BTAS) are chosen as electron donors, and nitromethane (NM) is chosen as an electron acceptor. The studies have been carried out by employing various spectroscopic methods, including steady-state, femtosecond fluorescence upconversion (fs-FU), fs-transient absorption spectroscopy (fs-TA), and computational analysis performed using density functional theory (DFT) and time-dependent DFT. In the presence of NM, all three molecules exhibit drastic quenching in their fluorescence. The fluorescence and temporal Stern-Volmer plots reveal that the quenching process is predominantly diffusion-controlled. The fs-FU and fs-TA measurements reveal that, upon excitation, all three molecules in the absence of NM exhibit tri-exponential kinetics with the longest time component of ∼100 ps for DAS, ∼1 ns for STPA, and ∼2 ns for BTAS. However, in the presence of NM, such long components are reduced to ∼5 ps for DAS, ∼15 ps for STPA, and ∼23 ps for BTAS. The observed significant amount of quenching is predominantly attributed to the diffusion-based PET process. While in the case of NM as a solvent, the PET process occurs at ultrafast timescales of ∼500 fs, a clear depiction of the intrinsic ET rate, which competes with the initial solvation timescales and the structural relaxations. These findings are well corroborated by computational analysis performed using DFT and TD-DFT methods.