Donor-acceptor tt conjugated molecules

SOLVATOCHROMISM AND NONLINEAR OPTICAL PROPERTIES OF DONOR-ACCEPTOR tt-CONJUGATED MOLECULES... 

It has been shown in a few theoretical studies drat the TPA cross section S of the donor-acceptor TT-conjugated molecules can be correctly described, similarly as the molecular (hyper)polarizabilities by a simple two-state model, involving only the ground and the CT excited state [111-114]. Hence, the solvent effect on S can be discussed within this approximation. It leads to the better understanding of the theoretical results cited in the above papers. Including only the ground and the CT excited state in the Eq. (8), one obtains the two-state approximate equation for dominant component along the molecular axis (in this case chosen as x axis) ... 

This relation is restricted for (a) the positively solvatochromic donor-acceptor TT-conjugated molecules, (b) the two-photon absorption from the ground state to the CT excited state, and (c) the static molecular (hyper)polarizabilities. The above ordering was confirmed for the extended variety of the donor-acceptor rr-conjugated molecules exhibiting the monotonic behavior as a function of the solvent polarity [40, 113, 116]. However, this trend dose not hold in some cases (see [40]). 

The Oudar-Chemla equation has been tested for many donor-acceptor TT-conjugated organic molecules. The two-state model works quite well in most cases. However, it should not be left unmentioned that Are values of are usually overestimated in comparison to the more advanced quantum chemical calculations [3, 31, 38—40]. Hence, the two-state model should be treated only as a rather rough approximation to the SOS method. On the other hand, for the most applications, the relative values of the first-order hyperpolarizabilies are of the higher importance. The two-state model allows to establish the structure-NLO properties relationship in terms of relatively simple spectroscopic parameters. 

As it was mentioned in the Introduction, in contrast to the extensive investigations on the solvent influence on the molecular (hyper)polarizabilites, there are only few theoretical works on the TPA cross section (S) of the donor-acceptor TT-conjugated organic molecules in which TPA from the ground to the CT excited state is considered. Below, we give short review of these works. 

Layer-by-layer deposition techniques have been elaborated to precisely control the placement of charged molecules and the thickness of the active layer. Applying this technique to tt-conjugated molecules, the iterative dipping of a solid substrate first into a solution of polyanions and then to a solution of polycations affords the ability to prepare donor-acceptor films with relatively precise control (at the molecular level) over the structure and energetics of the active layers in photovoltaic... 

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