Effect of viscosity on photoinduced electron transfer reaction: An observation of the Marcus inverted region in homogeneous solvents

Highlights: • Effect of viscosity on the Murcus inverted region. • Marcus inversion observed in the PET process in the highly viscous homogeneous solvent (12 cP and 16 cP). • No complex formation between the coumarin and DMA molecule. • Marcus inversion in highly viscous media does not depend on the fluorophore excited state lifetime. The viscosity effect of homogeneous solvents on the dynamics of photoinduced electron transfer (PET) reaction among the coumarins and N,N-dimethylaniline (DMA) is investigated using steady-state and time-resolved fluorescence spectroscopy. A bell shape Marcus inversion in the ET rates has been detected in the plot of ET rate constant (k_q) with free energy change (ΔG⁰) in viscous solvents decanol and EG, but it is not observed in DMSO like low viscous solvent. We have also reported that there is no complex formation between the coumarin dye and DMA molecule by using fluorescence correlation spectroscopy.