Stokes shift calculations, electron-transfer

Semi-empirical calculations of the molecular orbitals reveal that both the ground and the excited states of squaraines exhibit donor-acceptor-donor (D-A-D) intramolecular charge transfer [67]. This class of compounds possesses a resonance-stabilized zwitterionic structure. Typical squaraines have a four-membered central electron-deficient ring and two-electron donor groups. As monomers in solution, these compounds strongly absorb at wavelengths above 600 nm with high molar absorptions (e > 10 1 mol cm ) and intense fluorescent emission with a small Stokes shift moreover, squaraines are photostable [66, 67]. 

---