Study of the molecular first hyperpolarizabilities of two photoconducting merocarbocyanines

The dot products μ_gβ_CT of first hyperpolarizability and ground state dipole moment of two merocarbocyanines: 3-ethyl-5-[2-(3-ethyl-2-benzothiazolinylidene)ethylidene]rhodanine (dye I) and 3-cyano-5-[2-(3-ethyl-2-benzothiazolinylidene)- ethylidene]-4-phenyl-2(5H)-furanone (dye II) were measured experimentally with the Solvatochromism and compared to the values obtained by AM1/FF calculation. The comparable result was also obtained by the measurement of second harmonic generation for LB film of dye II. The μ_gβ_CT of dye II is larger than that of the famous NLO chromophore 4-N,N-dimethylamino-4-nitrostilbene (DANS). Through the calculation of the heat of formation of two dyes' various isomers, it is found that the most stable one is the (cis-,cis-) isomer which possesses the largest β value. The origin of large β values for dyes has been explained based on the two-level model from the investigation of the extended molecular conjugation length, change of charge distribution between the ground and first excited state and the analysis of the frontier molecular orbitals. In addition, Dye I is a quasi-two-dimensional chromophore while Dye II is a one-dimensional one, which can be explained by whether the electron-withdrawing group in dyes has mesoionic ring characteristic or not.