N,N-Dimethyl-6-propionyl-2-naphthylamine, PRODAN, is a luminescent probe frequently used in biological systems to comment on the environment polarity. Indisputably, pulse laser photolysis and fluorescence spectroscopy data indicate that the luminescence of PRODAN originates from at least two emission states. This dual/multiple emission has been assigned to the first singlet excited state; the first (higher) energy level being a solvent-non-relaxed whereas the other is a solvent-relaxed state. The latter is called twisted internal charge transfer state, TICT, in which the dimethylamine and the propanoyl groups are rotated out of the naphthalene ring; this conformational change is attributed to the high charge separation. However, to what extent the PRODAN molecular confirmation changes upon excitation is still a matter of dispute1. Our work attempts to solve this dilemma by using temperature resolved fluorescence spectroscopy of PRODAN dissolved in ethanol. The figure below shows a small difference in the PRODAN emission spectra on cooling from room temperature to -96°C (where ethanol is still liquid) and large difference on cooling to -131°C (where ethanol is solid). The data obtained seems to support the hypothesis that the TCIT state is significantly twisted. The large spectral shift observed implies that PRODAN could be used as a luminescent probe for solid-liquid phase transitions. This hypothesis was tested on ethylene glycol (EG)-water mixture. From the temperature resolved spectra it was concluded that the EG-water phase diagram exhibits a single eutectic point at 65 w% EG; the corresponding temperature is -87.5°C. This corroborates our data presented at the 249 national ASC meeting2. References 1. C. C. Vequi-Suplicy, et al., J. Fluoresc. (2015) 25, 621–629 2. So Yeon Kim, et al., DSC and rheological study of supercooled ethylene glycol-water mixture (Phys-494), 249th ACS meeting, Denver, CO, March 22 -26, 2015

Temperature and confinement effects on the spectral properties of PRODAN indicate that it can be used as a luminoprobe for liquid–solid phase transitions

251st National Meeting (2016)

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