Squaraines intramolecular charge transfer

As discussed above, the degree of intramolecular charge transfer is related to the magnitude of the 2PA cross section, and, as such, cyanines and squaraines could also have interesting 2PA properties. 

The intramolecular charge-transfer nature of the electronic transitions of squaraines makes their photophysical properties highly sensitive to the properties... 

The energies of the excitation and absorption spectra of the T dye components are complementary resulting in the 98 % intramolecular charge transfer efficiency from any component to the SQ moiety. The latter operated as both the light-harvesting and electron-withdrawing moiety. The use in DSSC of the dye T resulted in performance of DSSC superior with respect to that of the dye D and single squaraine dye SQ (Table 11). 

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]. 

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