Transition dipole moment orientation

The polarization properties of single-molecule fluorescence excitation spectra have been explored and utilized to detennine botli tlie molecular transition dipole moment orientation and tlie deptli of single pentacene molecules in a /7-teriDhenyl crystal, taking into account tlie rotation of tlie polarization of tlie excitation light by tlie birefringent... 

UV-Vis absorption spectra show that the polythiophene films exhibit an absorption band at 535 nm, corresponding to the first 7t-7t transition, with a clear vibronic fine structure. This absorption can be attributed to a structure that mainly consists of stacks of nearly coplanar extended chains, with the transition dipole moment oriented along the polymer backbone <2005AM708>. 

If it is assumed that the TDDs are oriented preferentially along a <110> axis, with a C2V site symmetry, the piezospectroscopic results can be explained satisfactorily on the basis of the stress-induced line shifts and polarizations calculated by Kaplyanskii [73], which are discussed in the next section. This led to propose the C2V site symmetry for the TDDs in silicon [137]. In expression (8.15), the non-zero components of the piezospectroscopic tensor for C2v centres, labelled as orthorhombic (or rhombic) I, are Axx (A2), Ayy (A2), Azz Wi) and Axy = Ayx (A3). These orthorhombic I centres have a C2 symmetry axis in the <100> direction and the 1 s —> 2po transitions have their transition dipole moment oriented along this axis for a Is state constructed from a pair of valleys along this axis, while the Is —> 2p transitions have their transition dipole moment oriented in a plane perpendicular to this axis. In a cubic crystal, a C2V centre has a sixfold orientational degeneracy represented by the six diagonals of a cube (see Fig. 8.16a). 

Rod-shaped molecule with restricted motion For a rod-shaped molecule with its transition dipole moment oriented along the rod s long axis and whose motion is confined to a cone, with one end of the rod fixed at the cone s apex, the emission anisotropy is given by... 

By the analysis of the transition dipole moment orientation relative to the polarization of the incident radiation the authors evidence the tilting of the molecular axis from 30° for the basic system, to 40° for naphthalene and respectively to 45° for the anthracene side groups, as shown in the following Fig. 4.7. 

Both molecular and transition dipole moment orientation can be probed within the solid state samples, especially upon combining structural information with polarized absorption measurements. Small-area electron diffraction experiments are also effective since they allow the orientation of crystalline regions within polymer nanofibers to be probed. Most of these techniques are already well established from the study of polymer alignment in thin-films. Improved analysis methods, which make use of combined polarized Raman spectroscopy and UV-visible absorption data, are especially worthwhile to be mentioned as valuable tools to investigate the orientational properties of light-emitting polymer systems. We will come back in depth to optical properties of polymer nanofibers in Chapter 5. 

IR-spectroscopy only vibrations exhibiting transition dipole moments orientated normal to the surface can be detected. 

The result of all of the vibrational modes contributions to la (3 J-/3Ra) is a vector p-trans that is termed the vibrational "transition dipole" moment. This is a vector with components along, in principle, all three of the internal axes of the molecule. For each particular vibrational transition (i.e., each particular X and Xf) its orientation in space depends only on the orientation of the molecule it is thus said to be locked to the molecule s coordinate frame. As such, its orientation relative to the lab-fixed coordinates (which is needed to effect a derivation of rotational selection rules as was done earlier in this Chapter) can be described much as was done above for the vibrationally averaged dipole moment that arises in purely rotational transitions. There are, however, important differences in detail. In particular. 

Rh(CO)2 [16]. Such a dicarbonyl should possess two vibration modes. However, only the symmetric mode is observable in the IR spectrum. The asymmetric mode is inaccessible to an IR experiment on a metal surface due to the so-called metal surface selection rule, which prohibits the observation of dipole excitation if the transition dipole moment is oriented parallel to the surface. It should be noted that the observed frequencies fit well to values observed for Rh(CO)2 on technical Rh/Al203 catalysts [35-40] ( 2100 cm ) and Rh(CO)2 on planar TiO2(110) surfaces [41] (2112 cm ). 

We have seen that the transition dipole moment occurring upon excitation of a molecule has a distinct orientation with regard to the molecular axis. This orientation can be determined by measuring the absorption of polarized light (oscillating in only one plane) by oriented single crystals,... 

Another important linear parameter is the excitation anisotropy function, which is used to determine the spectral positions of the optical transitions and the relative orientation of the transition dipole moments. These measurements can be provided in most commercially available spectrofluorometers and require the use of viscous solvents and low concentrations (cM 1 pM) to avoid depolarization of the fluorescence due to molecular reorientations and reabsorption. The anisotropy value for a given excitation wavelength 1 can be calculated as... 

IR spectroscopy is a powerful and readily available orientation characterization technique. It offers a high chemical selectivity since most functional groups absorb at distinct wavelengths (typically in the 2.5-25 pm range (4,000 00 cm-1 range)), which often depend on their local environment. IR spectroscopy thus provides qualitative and quantitative information about the chemical nature of a sample, its structure, interactions, etc. The potential of IR spectroscopy for orientation characterization stems from the fact that absorption only occurs if the electric field vector of the incident radiation, E, has a component parallel to the transition dipole moment, M, of the absorbing entity. The absorbance, A, is given... 

The second problem of interest is to find normal vibrational frequencies and integral intensities for spectral lines that are active in infrared absorption spectra. In this instance, we can consider the molecular orientations, to be already specified. Further, it is of no significance whether the orientational structure eRj results from energy minimization for static dipole-dipole interactions or from the competition of any other interactions (e.g. adsorption potentials). For non-polar molecules (iij = 0), the vectors eRy describe dipole moment orientations for dipole transitions. 

The angular dependence of the polarized absorption spectra of LB films containing AMP deposited at lower (20 mN m1) and higher (43 or 50 mN m 1) surface pressures was studied to determine the molecular orientation of porphyrins as schematically shown in Figure 5. No polarization angle (a) dependence was observed at normal incidence. This indicates that the projections of the transition dipole moments of the porphyrins are statistically... 

Figure 1.20. (a) Angles 0 0y, and y, describing the relative orientation of the electronic transition dipole moments s between two dye molecules, (b) Relative orientations of the electronic transition dipole moments between two equal dye molecules in the channels of zeolite L. (c) Angular dependence of the orientation factor k2 under the anisotropic conditions (b) and averaged over y. 

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]. 

IR spectroscopy is not only useful for determining the chemical constitution of polymers. It additionally provides profound information on chain orientation and on the orientation of attached lateral substituents of polymers. In this case, polarized IR radiation is applied which is only absorbed by an IR-active bond if the plane in which the electrical field vector E of the IR beam oscillates is parallel to the transition dipole moment p of the vibration to be excited. If, on the other hand, the transition dipole moment p is perpendicular to the electrical field vector E of the IR beam no absorption is observed. Using this effect, the degree of orientation of a polymer sample (film, fiber) can be estimated by comparing the intensity at maximum /(11) and at minimum I ) absorption, i.e., the dichroic ratio. 

K,y depends on the angles 0 0, and 4), describing the relative orientation of the electronic transition dipole moments shown in Figure 1.20(a). 

---