Intramolecular exciton

Fig. 2.1 Illustration of the two mechanisms for electron-hole capture discussed in the text. Electrons and holes are transported through their respective transport materials and accumulate at the heterojunction, a) Injection of one of the charges into the opposite polymer makes possible charge capture within the polymer bulk and formation of intramolecular excitons. b) Barrier-free electron-hole capture...

HMX or 1,3,5,7-tetranitro 1,3,5,7-tetrazacyclooctane exists in four polymorphic forms. These are molecular crystals. Both intramolecular excitons and intermolecular (charge transfer) excitons are predicted. Electronic charge transport depends on overlap of the molecular wave-functions and is therefore enhanced by the pressures (10 atm) in the shock wave during detonation. The mobilities of both types of excitons are also enhanced by pressure. 

Schematic formation for intramolecular exciton (A ), intermolecular exciplex (A B), and intermolecular charge-transfer complex (A+B-). A and B are two different molecules. The superscript represents excited state. The A+ and B- are positive and negative molecules.

As well as the intramolecular exciton component, excimers also have a charge-transfer component, corresponding to an electron on one of the molecules and a hole on the other. Representing the component with a hole on molecule m and an electron on molecule n as +)m —)n, an excimer state is written as... 

The effect of is to move charge from one chain to another. We consider the situation where the negative polaron is transferred from chain 2 to chain 1. Thus, the final electronic state, /), is an intramolecular exciton on chain 1 (denoted by EX)), leaving chain 2 in its ground electronic state. 

Fig. 9.12. A schematic representation of the initial and final electronic states. The initial state, i), is an electron and hole on neighbouring chains in a weakly bound intermolecular charge transfer state. The final state, /), is an electron and hole on chain 1 bound in an intramolecular exciton state, while chain 2 is in its electronic ground state.

Fig. 9.13. Electronic selection rules determine that intermolecular interconversion occurs from the n = 1, j = 1 intermolecular exciton to the n = 1, j = 1 intramolecular exciton.

Intermolecular interconversion is an isoenergetic process which occurs from the lowest pseudomomentum state of the charge-transfer manifold and the lowest vibrational levels of this state to the lowest pseudomomentum state of the intramolecular excitons at the same energy as the initial level. Thus, the vibrational levels in eqn (9.124) are i/p+ = 0 and j/p- = 0. However, the vibrational levels in eqn (9.125) are determined by the conservation of energy. 

Rolczynski BS, Szarko JM, Son HJ, Liang YY, Yu LP, Chen LX (2012) Ultrafast intramolecular exciton splitting d5mamics in isolated low-band-gap polymers and their implication in photovoltaic materials design. J Am Chem Soc 134 4142... 

Packard, B. Z., Toptygin, D. D., Komoriya, A. and Brand, L. (1996). Profluorescent protease substrates intramolecular dimers described by the exciton model. Proc. Natl. Acad. Sci. USA. 93, 11640-11645. 

Particularly useful in such measurements are 4-dimethylaminobenzoates160 and 4-dimethylaminocinnamates161 as their intramolecular charge-transfer bands at 309 nm and 362 nm, respectively, are well separated from shorter wavelength bands and the exciton amplitudes are higher, compared with other benzoates or cinnamates. 

Rigorously, ORD and CD spectra are related through the Kronig-Kramers theorem, a well-known general relationship between refraction and absorption, i.e. nL - nR is determined by eL - % for A from zero to infinity [128], (The analogous relationship between refraction and reflection applies to cholesteric liquid crystals.) Hence, in order to maximize ORD in the transparent region, Cotton effects, associated with exciton coupling (both intramolecular and intermolecular), have... 

A comparison of the UV-vVis spectra of the ball-type, its precursor, and mono Pc 28, 29, and 30 in (Fig. 8) in THF shows characteristic absorptions between 610 and 710 nm in the Q band region for metal-free Pcs. Because of the lower symmetry of the metal-free Pcs, the Q band splits into two intense bands in that region. An additional third band at 636 nm and a fourth band/shoulder at 610 nm are exciton coupling and charge transfer bands, respectively. The intensity of the third band of the compound 30 clearly indicates the presence of strong intramolecular interactions between two Pc macrocycles [45]. 

The ZnPc-Cgo dyad 110 has a low fluorescence quantum yield (f = 0.01) in toluene due to the intramolecular electron-transfer deactivation. Addition of 108 leads to a further reduction in the fluorescence intensity (4>f = 0.002) and a slight blue shift of the emission maximum, suggesting the formation of the ensemble 108-110, in which the two components have strong excitonic coupling. The addition of 108 (i.e. the hetero-association) also stabilizes the radical ion pair ZnPc+-C6o, increasing the lifetime from 130 ns (for 110) to 475 ns (for 108-110) (measured in tetrahydrofuran). 

When the two dipoles occupy symmetrical positions in the cell, as for instance in the case of the anthracene crystal, (1.70) may be further simplified by introducing symmetric and antisymmetric states for all directions of K, with respect to the assumed symmetry. Then (1.70) reduces to 2 x 2 determinants for the two (symmetric and antisymmetric) transitions. The solution of (1.70) leads to four values of co for each wave vector K, i.e. to four excitonic branches. In general, the crystal field is assumed weak compared to intramolecular forces, so that coupling between excitonic branches may be neglected. To a first approximation, each of the excitonic branches, symmetric and antisymmetric, is given by the equation... 

Thus the hamiltonian (2.15) couples the electronic excitations to the vibrations by linear terms in and by quadratic terms in A. The molecular eigenstates of (2.15) are the vibronic states they are different from tensorial products of electronic excitations and undressed vibrations. Even for this simple intramolecular effect, we cannot, when moving to the crystal, consider excitonic and vibrational motions as independent. 

First, we envisage the weak exciton-photon coupling (which allows an intuitive description of the phonon effects on the nature of the secondary emissions). Therefore we write the hamiltonian of the total system as sums of free photons (Hy), free excitons (He), and free phonons (Hp), with the appropriate interactions Hey (Section I) and Hep (see Sections II, A, B, C.), including intramolecular vibrations too. 

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