Picosecond laser photolysis studies of DMA–DMPP in solution

Picosecond transient absorption spectra of: 4-(4′-N,N-dimethylaminophenyl)–3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′-e]-pyridine (DMA–DMPP), 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′-e]-pyridine (BPP) and 3,5-dimethyl-1,4,7-triphenyl-bis-pyrazolo-[3,4-b;4′,3′-e]-pyridine (H-DMPP) were measured in solvents of different polarity. The results revealed the previously postulated change of the character of the fluorescing state from a primary excited, low polar state in non-polar solvents to a CT state in highly polar ones. Transient absorption spectra in the equilibrium fluorescent state of DMA–DMPP in polar solvents comprised the band ascribable to the cation radical of the DMA moiety. The charge transfer process is fastest in methanol and acetonitrile; in the former it is not controlled by a dynamic Stokes shift.