Cyanine molecular modeling

This model has been successfully applied to J aggregates of cyanine dyes in a brick stonework arrangement [47,48]. However, this model cannot explain the spectral shift of chromophores having transition moments in two or more directions as shown in Figure 8 for long-axis and short-axis transition dipoles of carbazolyl chromophores, nor it can predict the orientation of chromophores with respect to the substrate. In order to explain such spectral shifts and molecular orientation of alloxazine and carbazolyl chromophores as mentioned above, we proposed a three-dimensional extended dipole model which takes a three-dimensional... 

Fig. 7. A plot of the upheld ring current shifts measured in porphyrins, and in phtalo-cyanine model complexes where aliphatic hydrocarbons were attached to the central metal ion (Caughey and Ibers (18) Esposito et al. (30)). The positions of the observed protons were derived within the accuracy indicated by the ellipses from the known molecular structures. Z and r are the distances from the ring center perpendicular to the plane, and in the plane of the ring, respectively. The dashed line indicates the van der Waal thickness of the porphyrin ring. (Reproduced from ref. (99))...

The visible spectra of the cyanine dyes is a traditional physical chemistry spectroscopy experiment. In this exercise, students are asked to interpret these electronic spectra in terms of three quantum mechanical models the electron in a box model, the perturbed electron in a box model, and the Hiickel molecular orbital model. The students use the numerical methods of an earlier computer laboratory for the first and second model, and their classroom notes and Mathcad for the third. In this latter case, they use Mathcad s symbolic processor (MAPLE) to find eigenvalues for the Huckel matrices that emerge. 

To understand the effects of additional states beyond the two-level model in centrosymmetric molecular materials, it is helpful to analyze symmetry conditions that dictate allowable transitions between states. Because the ground state (also called the lAg state) is of Ag symmetry, the lowest optically allowed one-photon transition is to the IBu state (of opposite ssmimetry), which is the lowest lying one-photon excited state in the linear absorption spectrum. The IBu state is not necessarily the lowest lying excited state, however. One fundamental difference between centrosymmetric polyenes and cyanines is the location of another excited state, the lowest lying two-photon state or 2Ag state. Studies show that the 2Ag state lies energetically below the linear absorption peak (IBu state) for polyenes, but above the peak for cyanines. Electron correlations that lower the... 

---