Crystal transition dipole

Rosell FI, Boxer SG (2003) Polarized absorption spectra of green fluorescent protein single crystals Transition dipole moment directions. Biochemistry 42 177-183... 

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... 

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,... 

Our method has evolved during many studies over the last two decades. These include studies on the effect of strong internal electric fields in crystals on optical transition dipole directions of nucleic acid bases [2, 3], QM-MM predictions of time-dependent solvatochromism on 3-methylindole (3MI) in water [4], and on tryptophan in several proteins [5-8]. More recently, the same techniques have been... 

Sreerama N, Woody RW, Callis PR (1994) Theoretical study of the crystal field effects on the transition dipole moments in methylated adenines. J Phys Chem 98 10397-10407... 

Depending on the size of an incorporated dye, the angle of the transition dipole moment to the c axis lies between 0° for long molecules and 72° for smaller ones. Therefore, if a small molecule is inserted into the channels of zeolite L, part of the emission will be parallel to the c axis. Due to the flat and parallel ends of appropriately prepared zeolite crystals, one can envisage to arrange crystals between two mirrors or to add a reflecting layer on individual crystals. This might lead to a microlaser with a plane-parallel resonator. Apart from experimental difficulties, the realization of a dye-loaded zeolite L microlaser appears to be feasible. 

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. 

With this set of identities, we may express the ath vector component of the vibronic crystal field transition dipole moment as given below ... 

FIGURE 3.20 (a) NSOM topography, (b) polarization modulation amplitude, and (c) polarization modulation phase images of rhodamine 110 microcrystals. The arrows appended to image c depict the transition dipole orientation in each crystal. 

For a maximum of simplification we restrict ourselves to a crystal with two dipoles per unit cell and to two transitions, characterized by the transition dipoles d (a is the cell index, while / indicates the transition). Then, the determinant of (1.33), with the explicit matter variables, factorizes, in each subspace of momentum K, into determinants of order 4 ... 

We must remark that the amplitude of these processes is generally weak compared to the direct exciton-photon amplitude, owing to the small libration amplitudes (of the order of 1 °) at low temperatures. It is still smaller when the incident light polarization is parallel to the molecular transition dipole. For instance, in anthracene-crystal excitation, we expect the exciton-photon-phonon contribution to be more important for the a than for the b polarization. On the contrary, these processes become much more important in nonresonant excitations, in Raman scattering for instance (cf. Section II.D). 

This expression shows also a nonanalytic dispersion at K = 0, but which is continuous The nonanalyticity of the 2D exciton has regressed by one order compared to the 3D exciton cf. Section I.B.l. Furthermore, as we are interested in effects around K = 0 for the b component of the anthracene crystal, it is legitimate to replace the whole unit cell by a transition dipole... 

In the specific case of optical transitions in a molecular crystal, the transition dipole takes the form d f B f + Blf) (1.10). Starting from thermally populated states, only the terms in B f will resonate during the excitation (the terms in Bnf will be active during the transition back to the initial state). Thus, the optical absorption takes the well-known form... 

Dyes for liquid crystalline media typically have (1) nonionic structures, (2) high purity, (3) solubility and compatibility with the medium, (4) a transition dipole that is parallel with the alignment axis of the molecular structure, and (5) good alignment with the liquid crystal molecule.40 Examples include the disazo and anthraquinone dyes in Fig. 13.28. 

We propose that following the onset of the phase transition the small Cu+ ions residing in the large K+ sites are shifted to an off-center position and produce a distortion in the neighboring unit cells. This imparts to each of these unit cells the ability to behave as a relaxing dipole. In a pure (copper-free) crystal these dipoles are closely interlaced with the complementary part of their respective unit cells and, hence, are unable to reorient. Thus the relaxation process that is linked with these dipoles is not observed in the pure KTN crystal. 

The alignment of the molecules is also clear from the visual inspection of pNA-loaded AIPO4-5 crystals under polarized light (Fig. 3) [30]. The crystals appear yellow only when the electric field vector E of the linearly polarized light oscillates in the direction of the transition dipole moment of the blue absorption of the pNA molecules (which is responsible for their yellow colour). When E and these dipole moments are perpendicular to each other, no absorption occurs in the visible region so that the/7NA/A1P04-5 crystals appear colourless [30]. 

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