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Abstract:
The ground- and excited-state electronic structure of the highly polar squaraine dye bis(4-dimethylaminophenyl)squaraine, or 2,4-bis(4-dimethylaminophenyl)cyclobutadienediylium-1,3-diolate, is addressed using the MNDO and CNDO/S(S + DES CI) semi-empirical molecular orbital approximations. MNDO geometry optimizations indicate a distinct polyene-like single-double bondlength alternation. Such "quinoid" character is significantly enhanced when explicit solvent/solute interactions (complexation) were considered. The "quinoid" model yields Sn ← S0 energy and transition intensity profiles in essential accord with the detailed UV/VIS solution-phase absorption spectrum above 2500 Å. Properties of the model squaraine are compared and contrasted to those of the underlying or "parent" straight-chain polymethine cyanine (CH3)2 NCH(CH)10 N(CH3)2+. The intense long-wavelength transition of the squaraine is considerably less sensitive to correlation or configuration mixing than the corresponding cyanine transition. S1 ← S0 intramolecular charge-transfer character is discussed and referenced to solvent sensitive absorption properties of the model D+-A- molecule p-nitroaniline (PNA).
