Transition dipole moment chromophores

To calculate absorption spectra of multielectronic states, the transition dipole moment squares are needed for all the electronic states involved in the relevant optical transition. For this purpose, we next discuss the configurations of the transition dipole moments of the chromophores. 

The Anisotropy Between the Transition Dipole Moments of the Chromophores in the Dimer Model... 

We have tried to express the results of Weigang s treatment in pictorial form (Scheme 6), applying the language of the exciton chirality rulesld to the coupling of the chromophore transition dipole moments with those induced in the nearby bonds. These are regarded... 

Linear dichroism data with DNA oriented by an electric field [53, 54] or a linear flow [55, 56], under linearly polarised light, lead to determinations of the angle between the absorbing transition dipole moment of the chromophore in the molecule and the DNA helix axis conclusions concerning intercalation may thus be drawn from this technique. Finally, with chiral compounds, circular dichroism is also an attractive method to determine the enantioselectivity in the binding of the molecule [48, 57,58]. 

For non-coplanar electric transition dipole moments pi0a and pj0a of the two chromophores at Rjj interchromophoric distance the exciton chirality is nonzero and defined by ... 

In this expansion the dipole-dipole term is the most prominent if donor-acceptor distance R is not too small. The dipole-dipole term represents the interaction between the transition dipole moments Md and MA of donor and acceptor molecules, respectively. The square of these transition dipoles is proportional to the oscillator strengths fy> and fA for radiative transitions in the individual donor and acceptor molecules (equation 3.73). Higher order terms such as electric dipole-electric quadrupole, electric-dipole-magnetic dipole, become important at close approach and may be effective in crystals and highly ordered array of chromophores. 

The retinal Schiff base chromophore is embedded in rhodopsin with its transition dipole moment parallel to the plane of the discs, i.e., perpendicular to the direction of travel of the incoming photons. Absorption of a photon leads to a sequence of readily detectable spectral changes.37,46113,499,500 The relaxation times indicated in Eq. 23-37 are for 20°C. 

Movement of an electron from the ground electronic state of a molecule to an excited state creates a momentary dipole, called an electric transition dipole. Thus, associated with each electric transition is a polarization (electric transition dipole moment) that has both direction and intensity which vary according to the nature of the chromophore and the particular excitation. When two or more chromophores lie sufficiently close together, their electric transition dipoles may interact through dipole-dipole (or exciton) coupling. Exciton coupling arises from the interaction of two (or more) chromophores through... 

The value of reduced LD depends on the degree of orientation (G) of the enzyme molecules in the gel slab and angle (a) between the direction of the transition dipole moment in the chromophore ring and the axis of orientation of the enzyme molecules in the gel67 7,) ... 

Figure 38. (a) Absolute configuration of (55,125)-(+)-dimethyl-5,12-dihydro-5,12[l, 2 ]-benzonaphthacene-l,15-dicarboxylate (b), (c) and (d) orientations of pairs of electric transition dipole moments from the chromophores 1 and 3 from the methylbenzoate chromophores 2 from the naphthalene. The helicities are shown to the right. The 1,3 couplet is predicted to be zero since the dipoles are parallel. The 1,2 and 2,3 couplets have (+) chirality. The net chirality is predicted to be (+). (e) CD Cotton effects for the intense 233 nm transition of the benzotriptycene showing a (+) exciton chirality. 

For transitions in the visible region, an excitation with f = 1 wUl arise from a transition dipole moment of magnitude 2.5 Debye. An oscillator strength of f = 1 is the order of magititude seen for an organic dye molecule. For a transition at 500 mn the radiative lifetime of such an excited state would be 4ns, leading to a Lorentzian linewidth of 40 MHz or 1.3 x 10 cm. These values are appropriate to measurements in vacuo and need to be scaled by the dielectric constant of the medium in which the chromophore is embedded, in order to correct for the actual field experienced by the chromophore. 

Electronic transitions occur when Ught interacts with a chromophore via its transition dipole moment. The latter... 

The transition dipole moments of a chromophore return different amplitudes of interaction when responding to light polarized in different directions with respect to the chromophore. In a chromophore with no symmetry, the direction of the transition moment associated with each excitation may take a different direction. When there is a two-fold axis, spectra that are taken with light that is polarized either parallel or perpendicular to the symmetry direction are unique. When there is a three-fold or higher axis of symmetry, the transition moment is independent of the direction of polarization in the symmetry plane. 

The absorption of light by a chromophore is a directional process. Absorption is possible provided that the transition dipole of the chromophore and the plane of polarisation of the light are not at right angles to one another, and it is at a maximum when the two are parallel. Figure 7-12 shows the orientation of the transition dipole moment of adenine. Fluorescence emission is also a directional process, occurring in a plane parallel to the dipole moment. 

The arrow depicts the direction of the transition dipole moment for the 260 nm absorption band of the adenine chromophore. 

This deformation can be followed by UV-vis spectroscopy directly at the air/water interface. Upon compression of the monolayer, the absorbance changes not very much around the point where the first increase in surface pressure is recorded (see Figure 6.17), This is caused by the disappearance of the voids between the domains. Above the kink in the isotherm, howevei the K-n band (-323 nm) decreases while the band at 248 nm increases. This increase is due to the increase in chromophore concentration upon compression of the monolayer. The decrease of the 7t-ic band is caused by a preferential orientation of the chromophorra perpendicular to the surface. Chromophores oriented perpendicularly are not detected because the transition dipole of the n-n band is parallel to the probing light. The band at 248 nm, however is not sensitive to the orientation of the chromophore, because the transition dipole moment has a component perpendicular to the long axis of the molecule (see Figure 6.17). 

Hi) UV/visible spectroscopy. In comparison with IR and fluorescence spectroscopic techniques, UV/visible spectroscopy is only occasionally used for characterizing monolayers. It can be applied if the monolayer contains molecules with 7i-electron systems of which the electron transitions are in the UV/visible part of the spectrum. By measuring polarized transmission spectra or reflection-absorption spectra at different angles of incidence, the second order parameter of the absorption transition dipole moment in the chromophoric groups caii be determined. In the case of a reflection-absorption configuration, the underlying theory is similar to that of IRRAS, i.e. based upon calculation of the reflection and transmission coefficients in a stratified-layer system and extended to account for the anisotropic nature of monolayers ). 

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