Thiophene-Inserted Aryl-Dicyanovinyl Compounds: The Second Generation of Fluorescent Molecular Rotors with Significantly Redshifted Emission and Large Stokes Shift

Fluorescent molecular rotors can be used as molecular sensors for the viscosity of a microenvironment. However, these molecular rotors are limited to 9-(dicyanovinyl) julolidine (DCVJ) and a few derivatives. Furthermore, these traditional rotors show short absorption/emission wavelengths and small Stokes shifts. To address these drawbacks, we have developed a small library of new molecular rotors for viscosity sensing, prepared by incorporating a thiophene unit into the conventional fluorescent molecular rotors with the aim of accessing molecular rotors with redshifted excitation/emission wavelengths and larger Stokes shifts compared with the known rotors. The new rotors show substantially improved photophysical properties. For example, rotor 4 shows absorption/emission wavelengths of 559/697 nm, respectively, and a very large Stokes shift of 138 nm compared with the absorption/emission wavelengths (465/503 nm) and very small Stokes shift (38 nm) of the traditional fluorescent molecular rotor DCVJ. The photophysical properties of the rotors were rationalized by DFT calculations.