An MNDO and CNDO / S(S + DES CI) study on the structural and electronic 
properties of a model squaraine dye and related cyanine

Bigelow, Richard W.; Freund, Hans-Joachim

doi:10.1016/0301-0104(86)85001-7

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An MNDO and CNDO / S(S + DES CI) study on the structural and electronic properties of a model squaraine dye and related cyanine

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Bigelow, R. W., & Freund, H.-J. (1986). An MNDO and CNDO / S(S + DES CI) study on the structural and electronic properties of a model squaraine dye and related cyanine. Chemical Physics, 107(2-3), 159-174. doi:10.1016/0301-0104(86)85001-7.

Cite as: https://hdl.handle.net/21.11116/0000-000E-5897-5

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 S_n ← 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).