Temperature effect on 4-aminophtalimide fluorescence in n-alcohols

The compound 4-aminophthalimide (4-AP) is a well-known dye used as an environment polarity sensitive probe e.g. in solvation studies. This paper presents the effect of temperature on 4-aminophthalimides steady-state and time-resolved fluorescence in five n-alcohols. It is shown that the hydrogen bonding ability of n-alcohols affects the shifts of steady-state absorption and fluorescence spectra of 4-aminophthalimide at room temperature, and the shifts of fluorescence also at temperatures from the range 180 to 323 K. Temperature is shown to affect the change in hydrogen bond energy that follows 4-AP excitation, in a way dependent on the n-alcohol alkyl chain length. On the other hand, time-resolved results indicate that the temperature dependence of 4-AP deactivation follows mainly from the energy-gap dependent non-radiative deactivation rate. Fluorescence transition dipole moments at room temperature have been found to be slightly dependent on the solvent, but nothing proves that these changes are connected to different hydrogen bonding character of each n-alcohol. Therefore, while the steady-state results provide clear evidence of hydrogen bonding between 4-AP and n-alcohols, the time-resolved results do not show any evident sign of hydrogen bonding, besides the influence of the position of fluorescence emission on the radiative and non-radiative rates.