Breakdown of the first hyperpolarizability/bond-length alternation parameter relationship

Abstract

We have investigated the dependence of the static first hyperpolarizability on the bond-length alternation (BLA) parameter. Our analysis indicates that the validity of the first hyperpolarizability/BLA parameter relationship is restricted to the no-field, vacuum, limit, while it successively breaks down along with increasing polarity of a surrounding medium, becoming invalid, for instance, in an aqueous solution. This contention is based on a series of TD-DFT, TD-DFT/PCM and hybrid TD-DFT/MM calculations of the first hyperpolarizability for a set of molecular configurations generated from Car-Parrinello hybrid QM/MM simulations of the stilbazolium merocyanine chromophore in chloroform and water solvents, and on a rationalization by means of the two-state model for the first hyperpolarizability. The BLA dependence of the three parameters entering the two-state model is shown to be qualitatively different in vacuum and in solvents. Particularly, in the vacuum case, the difference between ground and excited state dipole moments goes to zero for a vanishing BLA, which is not true in the presence of an aqueous medium. In the aqueous medium, an opposing behavior of the parameters involved in the two-state model results in an almost constant first hyperpolarizability with varying BLA parameter.bond-length alternation | hybrid QMMM modeling | nonlinear optical materials S implifying thumb rules and structure-property relations are of utmost significance in rational design of new materials with prespecified properties. Computational modeling plays an important role in that context because it can provide accurate number prediction of the property in question and can elucidate the merits and limitations of the simplifying thumb rules and so make their utility more precise (1). We find good evidence of this in the research of nonlinear optical (NLO) materials, where several structure-property relations are highlighted (2-12) and where computational modeling has advanced quite far (13). One such important structure-property relation for NLO materials is the relation between the first hyperpolarizability and the bond-length alternation (BLA) parameter for molecules that can exist in resonance between the two, neutral or charge-separated (zwitterionic) forms (2-12). The BLA parameter has been defined as the difference between the average single and double bond lengths in the conjugation pathway (14). By convention, it has been defined to be positive for the neutral form of a molecule while it takes negative values for the charge-separated form. Many theoretical studies have been performed with the purpose of investigating the effect of electric fields and solvents in altering BLA (9, 14-16). It has been shown that with increasing field strength and increasing solvent polarity, the BLA parameter decreases and in some cases changes sign with a cyanine-like structure corresponding to a BLA parameter equal to zero (14, 17). The BLA parameter has therefore been proposed to be a structural parameter to be tuned for optimizing nonlin...

Distinguishing the Effects of Bond-Length Alternation versus Bond-Order Alternation on the Nonlinear Optical Properties of π-Conjugated Chromophores

“…This is consistent with the previously observed weak correlation between BLA and NLO properties in geometries extracted from molecular dynamics simulations. 13 In this case, the focus should be on BOA and not BLA since changes in BOA will be a …”

“…13 To provide a comprehensive understanding of this question, we elucidate here the limits of when the commonly used relationships between BLA and the molecular NLO properties are applicable. In particular, we demonstrate that BLA and BOA are well correlated only when the molecular geometry is optimized in the environment of interest.…”

Theoretical investigation of the two-photon absorption properties of 3,6-bis(4-vinylpyridinium) carbazole derivatives—new biological fluorescent probes

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