Charge Transfer CT Excitons

Observation Information Quantities derived from the measurements Singlet or triplet Technique or measurement method 

Davydov splitting Sum of the interaction elements ( Resonance energy ) Slife S, T Absorption and emission spectroscopy 

Splitting of energy levels through aggregation, also exdton dimensionality Individual interaction matrix elements hi hi S, T Pair spectroscopy, high-resolution excitation spectroscopy, isotopic mixed crystals 

Band-band transitions including vibronic bands Density of states in the exciton band Band properties -widths of exciton bands S. T Hot band optical spectroscopy, absorption and emission 

To finish off this section, we have again collected in Table 6.2 the (static) interaction parameters which can be determined with the aid of optical spectroscopic methods. 

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In the analysis of the lowest electronic excitations in quasi-one-dimensional crystals, it is natural to take into account not only Frenkel excitons, but also one-dimensional charge-transfer (CT) excitons. We will show below that the spectrum of excited states in the molecular chain is strongly sensistive to the mixing of Frenkel and CT states. 

Fig. 6.13 Excitons with different radii diagram of a Frenkel exciton, a Wannier exciton, and a charge-transfer (CT) exciton.

This exciton diffuses to the donor/acceptor interface via an energy-transfer mechanism (i.e., no net transport of mass or charge occurs). (3) Charge-transfer quenching of the exciton at the D/A interface produces a charge- transfer (CT) state, in the form of a coulombically interacting donor/acceptor complex (D A ). The nomenclature used to describe this species has been relatively imprecise, and has... 

Fig. 4 Schematic illustration of the processes leading to photocurrent generation in organic solar cells, (a) Photon absorption in Step 1 leads to excitons that may diffuse in Step 2 to the donor/ acceptor (D/A) interface. Quenching of the exciton at the D/A interface in Step 3 leads to formation of the charge-transfer (CT) state. Note that processes analogous to Steps 1-3 may also occur in the acceptor material, (b) Charge separation in Step 4 leads to free polarons that are transported through the organic layers and collected at the electrodes in Steps 5 and 6, respectively, (c) The equilibria involved in Steps 1-4- strongly influence device efficiency...

Fig. 10 The three-dimensional potential energy surface describing the motion of protons between N6(A) and 04(T) and between N3(T) and N1(A) shows two critical points in the ground state. The deeper minimum corresponds to the amine/keto structure of AT and a shallow one to the imine/enol structure (A T ). Upon absorption of a UV photon the initaly delocalized excitonic states (1) undergo a rapid localization on f 10 ps timescale for single bases and 100 ps timescale for stacked base pairs to form a charge transfer (CT) states. The subsequent CT states passing through a conical intersection are carried back to the ground state.

Fig. 4 Energy correlation diagram between the gas phase (/G) and solvated (toluene) (/S) lowest excited states in a TFB F8BT model system calculated by TD-DFT (B3LYP/6-31G(d)). The eclipsed vs. staggered structures as shown in Fig. 3 are compared. The lowest-lying excitonic (XT) and charge transfer (CT) states are highlighted in red. Solvation effects tend to stabilize the CT state. Reprinted with permission from Ref. [41]. Copyright 2007, American Institute of Physics.

Fig. 6 Schematic illustration of the phonon-assisted exciton dissociation process. Due to the electronic state couplings, the photogenerated exciton (XT) wavepacket undergoes transitions to an interfacial charge transfer (CT) state, along with indirect XT — IS — CT transitions via an intermediate (IS) state (see panel (b)). In Ref. [52], the diabatic Hamiltonian of Eqs. (19)-(20) was parametrized for two relevant interface configurations (eclipsed (E) vs. staggered (S) as shown in panel (a)) which correspond to the configurations of Fig. 3.

Figure 4.11 Schematic representation of the Onsager-Braun model including generation of a charge transfer (CT) state from an exciton followed by vibrational relaxation, dissociation to and reformation from spatially separated...

If we consider only the HOMO and LUMO of the two molecules, a dimer will have two charge-transfer (CT) states in addition to the two exciton states that we have considered above. An electron can move from the HOMO of molecule 1 to the LUMO of molecule 2, as shown schematically in Fig. 8.6, or from the HOMO of 2 to the LUMO of 1. The two CT states can lie either above or below the corresponding exciton states of the dimer, depending mainly on the electrostatic interactions of the species with each other and with the surroundng medium. 

Charge transfer states (CT) are often found in molecular systems side by side with excitonic states. CT states describe polar nonconducting states bound by coulomb interaction of the electron-hole pairs. CT states may be ionized with localization of the charges on definite molecules. 

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