Polar-polarizable chromophores

From a theoretical perspective, since the designation of the lab-fixed axes is arbitrary, what is relevant is the relative orientation of the polarizations of the excitation and scattered light. Thus the line shape for excitation light polarized along axis p, and scattered light polarized along axis q (p or q denote X, Y, or Z axes in the lab frame) is called Ipq(co). When p = q this is lyy, and when p q this is IVH. Mixed quantum/classical formulae for Ipq(co) are identical to those for the IR spectmm, except mPi is replaced by apqP which is the pq tensor element of the transition polarizability for chromophore i. Thus we have, for example [6],... 

To begin to elucidate such issues and to create a theoretical framework for them, we have focused [4-9] on a model of a protonated Schiff base (PSB) in a nonequilibrium dielectric continuum solvent. A key feature for the Sj-Sq Cl in PSBs such as retinal which plays a key role in the chromophore s cis-trans isomerization is that a charge transfer is involved, implying a strong electrostatic coupling to a polar and polarizable environment. In particular, there is translocation of a positive charge [92], discussed further below. Charge transfer also characterizes the earliest events in the photoactive yellow protein photocycle, for example [93],... 

Since pNA and most of the chromophores of interest have large dipole moments an important feature of the continuum models is the introduction of the reaction field. The pNA molecule at the centre of the cavity in the continuum induces a polarization on the surface of the cavity, which produces the reaction field acting on the central molecule. This reaction field changes the dipole moment of the pNA molecule via the linear polarizability. A self consistent procedure is required in which the effects of the reaction field and also the effects of the applied macroscopic fields modified by the internal field factors are included in a self-consistent determination of the molecular response within a specified quantum mechanical model. 

The paper is organized as follows. Section 2 shortly introduces the exciton model and its approximations. Section 3 reviews calculations of ground state properties (mainly the polarization and polarizability) paying special attention to the mean-field approximation. Push-pull chromophores, the special family of polar and polarizable molecules studied in this contribution, are presented in Section 4, with a brief discussion of their properties in solution and of relevant models. In Section 5 we present a model for interacting push-pull chromophores that will be the basis for the discussion of collective and cooperative effects in relevant materials. Static susceptibilities of clusters of push-pull chromophores are discussed in Section 6, focusing attention on cooperative effects in tlie ground state. Excited state properties are addressed in Section 7, with special emphasis to systems where intermolecular interactions lead to extreme consequences. Section 8 finally summarizes main results. 

PUSH-PULL CHROMOPHORES AN INTERESTING FAMILY OF POLAR AND (HYPER-)POLARIZABLE MOLECULES... 

OGM, as discussed in Section 1, represents the simplest approach to NLO responses the response of the cluster is defined as the sum of the molecular responses, properly accounting for the chromophore orientation in the cluster. This approach is for sure very poor for clusters of pp chromophores, since it disregards the dependence of the molecular polarity, and hence of all molecular properties, on intermolecular interactions. A slightly more refined approach is a mf-based OGM. The mf gs describes a collection of uncorrelated molecules, each one in the local (mf) gs. Accordingly, the mf-OGM approach calculates the susceptibilities of the cluster as the sum of the susceptibilities of the (oriented) molecules in their local mf gs. The mf-OGM approach for sure improves over the crude OGM, since it accounts for the polarizability of the molecular units and hence for the dependence of the... 

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