Origin of the strong effect of protic solvents on the emission spectra, quantum yield of fluorescence and fluorescence lifetime of 4-aminophthalimide: Role of hydrogen bonds in deactivation of S1-4-aminophthalimide

The paper presents results of the spectral (absorption and emission) and photophysical study of 4-aminophthalimide (4-AP) in 1-chloro-n-alkanes (RCl) with which 4-AP interacts only nonspecifically, in hexafluoroisopropanol (HFIP) and dimethyl sulfoxide (DMSO) in which 4-AP forms only one type of hydrogen bond, and in water and deuterated water in which 4-AP forms both accepting and donating hydrogen bonds. Based on our solvatochromic data and independent literature data used for the determination of the energy of hydrogen bonds in the S0 state, the energies of hydrogen bonds in the S1-LE and S1-ICT states of 4-AP were estimated. The results confirmed that, apart from 4-AP, there can be S1-exciplexes (S1-exc) formed as a result of strong hydrogen-bond formation between 4-AP molecules in the S1-ICT state and the solvent molecules. In strongly protic solvents (e.g. HFIP) there can also be S0-complexes (S0-comp) in the ground state. The existence of solvent-dependent species of different character in the excited state forms the basis for explaining the origin of the large effects of protic solvents on the emission spectra, quantum yield of fluorescence, and fluorescence lifetime of 4-AP.