Vibronic excitons

Weak coupling (U AE, Aw U As) The interaction energy is much lower than the absorption bandwidth but larger than the width of an isolated vibronic level. The electronic excitation in this case is more localized than under strong coupling. Nevertheless, the vibronic excitation is still to be considered as delocalized so that the system can be described in terms of stationary vibronic exciton states. 

Linear response polarizability theory of spectral bandshapes was applied to the numerical analysis of the chiroptical spectra obtained for DNA-acridine orange complexes [85]. After analysis of various models of conformation, it was concluded that a dimer-pairs repeating sequence model was best able to account for the observed spectral trends. In another work, the CD induced in the same band system was studied at several ionic strengths [86]. The spectra were able to be interpreted in terms of the long-axis-polarized electronic transitions of the dyes, with the induced CD being attributed to intercalated and non-intercalated dye species superimposed by degenerate vibronic exciton interactions between these. 

The vibronic exciton approximation restricts H to a subspace corresponding to a given vibronic molecular state. In this subspace the degeneracy of the localized vibronic states is lifted by the interactions JnmB Bm. Using the translational invariance, the eigenstates of the crystal are seen to be the vibronic excitons, or vibrons ... 

The vibronic exciton is a collective oscillation of the crystal where vibronic molecular states (e.g. electronic and molecular excitations) stay on the same site. To the first order of the perturbations Jnm the vibron dispersion is given by... 

Absorption and fluorescence spectra of 7-aminocoumarin derivatives have been recorded and the photophysics of coumarin inclusion complexes with 0- and y-cyclodextrins investigated. Solvent effects on the photophysical behaviour of pyrrolocoumarin derivatives are similar to those on the psorolens . Vibronic exciton bands and the absorption spectra of Eosin Y dimers have been analysed theoretically and compared with experimental data. Picosecond spectroscopic studies have been made on the ionic photodissociation dynamics of malachite green leucocyanide . The lowest state produces ions within 0.1 to 0.5 ns whilst for higher states 6-13 ps are sufficient for the development of charge separation. The photophysics of crystal violet in a series of n-... 

Fluorescence-sensitization experiments have been made with molecular crystals containing a small amount of an impurity with lower excitation energy. The best known example is that of anthracene crystals with traces of naphthacene . These crystals emit the naphthacene fluorescence even at an impurity mole ratio of 10 with exclusive excitation of the host molecules. Here the multi-step nature of the transfer process is evident because of the weak coupling properties of the host material. It is further substantiated by the low concentration of the acceptor required and by the decrease in transfer efficiency at low temperature . The transfer is expected here to occur by the migration of vibronic excitons, which, after more or less efficient scattering, are finally trapped by an impurity molecule with its lower energy state. 

In polydiacetylene single crystals electroreflectance revealed the weak interband transitions, hidden under the stronger vibronic excitons (2). The data yield the band gap, the exciton binding energy and derive in case of DCHD a surprisingly small effective mass m = 0.05 m (3). Charge transfer excitons too respond to an external... 

A further clear establishment of the absoiption due to singlet excitons and the phonons coupled to them is the electroabsorption experiment reported in Ref. [18]. The main results are shown in Figure 9-14 the top panel shows the absorption spectrum of m-LPPP at 20 K. It becomes clear that the peaks at 2.7, 2.9, and 3.1 eV, representing A0, A i, and A2 (see Fig. 9-10) are not the only vibronic replicas. There are additional peaks between these dominant ones if the experiment is conducted at low temperature. The bottom panel in Figure 9-14 shows a so-called electroabsorption spectrum which is obtained as the modulation (or change) of the absorption under the application of an electric field. Below 3.2 eV the electroab-... 

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

The photoconductivity and absorption spectra of the multilayer polydiacetylene are shown in Fig. 22 [150]. The continuous and dotted line relate to the blue and red polymer forms respectively. Interpretation is given in terms of a valence to conduction band transition which is buried under the vibronic sidebands of the dominant exciton transition. The associated absorption coefficient follows a law which indicates either an indirect transition or a direct transition between non-parabolic bands. The gap energies are 2.5 eV and 2.6 eV for the two different forms. The transition is three dimensional indicating finite valence and conduction band dispersion in the direction perpendicular to the polymer chain. 

In summary, even though the implicit approximations used in the application of exciton CD to determine absolute configuration ignore important complications such as the exciton splitting order, d-orbital mixing, and vibronic couplings, it has been a remarkably successful method for determination of the absolute configuration. 

The PL from PPV is quite strong, with a quantum yield in the range from a couple of percent to several tens of percent [25,26], and there is a clear evidence of vibronic structure, with the zero-phonon transition at 515 nm. The efficiency of the photoluminescence is larger when the lifetime of the singlet exciton is... 

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