First- and Second-Order Polarizabilities of Simple Merocyanines. An Experimental and Theoretical Reassessment of the Two-Level Model

Taking four merocyanines [(CH3)2N-(CHdCH)n-C(CH3)O; n ) 1-4] (Mc1-4) as test D-A systems, weperformed a close experimental and theoretical examination of the two level model with reference to itsability to provide correct predictions of both absolute values and dependence on the conjugation path lengthof first- and second-order molecular polarizabilities. By 1H NMR spectroscopy merocyanines Mc1-4 werefound to be ∼1:1 mixtures of two planar conformers with cis and trans arrangements of the sC(CH3)Oelectron-acceptor group and all trans structure of the polyene like fragment. The degree of bond lengthalternancy (BLA) in the -(CHdCH)n- fragment, was quantified by extensive full geometry optimizations atboth semiempirical and ab initio level. DFT (6-31G**/B3LYP) optimized geometries were considered to bemost reliable and were used for calculations of the excited-state properties. The applicability of the two levelmodel, reducing the general sum-over-states (SOS) expansion to only one term involving the ground state (g)and the lowest-lying 1(ππ*) CT state (e), was checked by analysis of fluorescence and near UV absorptionspectra. Measurements of the basic two-level model quantities (Ege, μge and Δμeg), by which the dominantcomponents of r and tensors are expressed (RXX, XXX, X ≡ long molecular axis), were designed to giveapproximate free-molecule values. It is proposed, in particular, an adjustment of the solvatochromic methodfor the determination of Δμeg, based on accurate measurements of absorption spectral shifts in n-hexane/diethyl ether mixtures with small diethyl ether volume fractions. Such an approach led to Mc1-4 XXX’smatching well in both value and n-dependence with EFISH data reported in the literature for similarmerocyanines. For the fluorescent Mc4, the results were qualitatively well reproduced by an approach, whichcombines absorption and fluorescence solvent shifts. All the measured quantities were calculated for bothtrans and cis Mc1-4 by three semiempirical INDO-based approaches aiming at evaluating the performancesof different integral parametrizations and CI extensions: ZINDO/S, CS INDO SCI, CS INDO SDCI. In allcases, RXX and XXX were found to rise proportionally to about n1.3 and n2, respectively, in qualitatively goodagreement with the experimental values. As to the absolute values, however, experimental RXX’s and XXX’swere best reproduced by CS INDO SDCI combining Ohno-Klopman parametrization and CI including bothsingle and double excitations. The validity of the two-level model was checked by comparison with convergedSOS calculations for the longest chain merocyanine (Mc4) and finite field calculations of linear polarizabilitiesfor all of the four dyes (Mc1-4).