Study on photophysical properties and prototropic equilibria of trans-2-[4-(N,N-dimethylaminostyryl)]pyridine

• Photophysical properties of trans-2-[4-(N,N-dimethylaminostyryl)]pyridine.
• Density functional theory level calculations support experimental observations.
• Donor twisting coordinate is possible path for creation of charge transfer state.
• Ground and excited states acidity constants of various equilibria determined.
• Photophysical properties of dication resemble with that of planar state of dye.

Photophysical properties of trans-2-[4-(N,N-dimethylaminostyryl)]pyridine, a “push–pull” molecule, have been studied in different pure solvents. trans-2-[4-(N,N-dimethylaminostyryl)]pyridine exhibits fluorescence from a locally excited state in nonpolar solvents. However, in polar solvents, fluorescence occur from an intramolecular charge transfer state with high dipole moment obtained not only through the rapid relaxation of locally excited state upon a single excitation, but also by the excitation of ground intramolecular charge transfer state. The twisting of the donor moiety with respect to the acceptor moiety results in a complete charge transfer giving highly Stokes-shifted fluorescence in polar solvents. The potential energy surfaces along the donor and acceptor twist coordinates for the ground and first two singlet excited states have been evaluated at density functional theory and time dependent density functional theory levels, respectively. Monocation–neutral and dication–monocation equilibria of trans-2-[4-(N,N-dimethylaminostyryl)]pyridine at both ground and excited states have been studied.

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