Excitonic interaction

The mobility of excited states, imposed by intermolecular interactions (see Sec. 2.4), can lead to their collision with each other and/or with other types of excited states as well as trapped or free carriers generated in an organic solid. Such collision processes, realizing various excitonic interactions, may result in annihilation of the excitons and/or their transformation into another set of particles and quasi-particles. As different types of excitonic interactions show up in different optical and electrical phenomena, we divide them into two categories corresponding to the interaction between quasiparticles (exciton-exciton interactions) and to the interaction between quasi-particles and particles (exciton-charge carrier interactions). 

The singlet-singlet collision process is often referred to as singlet exciton fusion. The end result of such a fusion reaction 

Experimentally this process can be observed as a quadratic light intensity dependent photoconduction [177—180] and external photoemission of electrons [181-183] or fluorescence quenching [148,155,184-186]. 

This kinetic analysis of the singlet exciton fusion process is provided by the equation 

At low excitation intensities, the quadratic term in (68) can be neglected, and the PL efficiency 

---

The NDCPA seems to be a very reasonable way to treat the properties of both electrons and excitons interacting with phonons with dispersion. In principal, the NDCPA can be applied to a system of the Hamiltonian with the electron(exciton)-phonon coupling terms of arbitrary structure. The NDCPA results in an algorithm which can be effectively treated numerically (for example, iteratively). The application of the NDCPA is not restricted to the... 

For dilute solutions of essentially independent donor and acceptor molecules the Forster or resonance interaction is quite important in molecular aggregrates and in molecular crystals exciton interactions are likely to be important. When the interaction is strong the excitation is not localized on the donor or acceptor but is spread over both. If larger aggregrates are involved, the excitation can be spread over many molecules/33-3 This can easily be seen for the case of a dimer where the donor and acceptor are... 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

Figure 6.6. Energy level diagram for exciton interaction with different geometric arrangements of molecules in a linear polymer.

Salares VR, Young NM, Carey PR, and Bernstein HJ. 1977. Excited-state (exciton) interactions in polyene aggregates—Resonance Raman and absorption spectroscopic evidence. Journal of Raman Spectroscopy 6(6) 282-288. 

While excited-state properties of monomeric carotenoids in organic solvents have been the subject of numerous experimental and theoretical studies (Polfvka and Sundstrom 2004), considerably less is known about excited states of carotenoid aggregates. Most of the knowledge gathered so far stems from studies of aggregation-induced spectral shifts of absorption bands of carotenoid aggregates that are explained in terms of excitonic interaction between the molecules in the aggregate. 

A Ca2+-ion selective rigid -flexible - rigid type bichromophoric sensors based on the conformation liable bis-squaraine dyes 27 works on the principle of Cation-steered folding, which leads to dramatic perturbations in the optical properties as a result of exciton interactions [87],... 

Figure 8. Model of excitonic interactions for the special pair (P) and accessory bacteriochlorophylls (B).

Because we are concerned only with the analysis of the absorption spectra of P band and B band, we consider the excitonic interactions among P, BL, and BM shown in Fig. 8. Here (oti, ot2,0C3,014) represent the diagonal matrix elements, while (p, (314, P23, P34) represent the off-diagonal matrix elements [67]. As shown in Introduction, a main feature of the P band is that its absorption maximum shows a pronounced temperature shift [42,52], According to the displaced oscillator model, the absorption maximum is independent of T. Although the distortion effect of potential surfaces will introduce some temperature shift, the effect cannot be as large as that shown in Fig. 2. 

The symbolic representation (MM) shows that the excitation energy is delocalized over the two moieties (as in an excitonic interaction described in Section 4.6). 

The chl-chl coupling estimated seems to be somewhat at variance with the suggestion of very strong coupling in LHCII, leading to delocalized, coherent excitonic interactions [51, 168]. 

Optical Absorption Spectra and Electronic Structure The optical spectra of all the doubledeckers are listed in Table I, On first glance, Ce(0EP)2 has a "normal" spectrum (7), However, the spectrum shows extra bands and therefore should be called "hyper", A small band appears at 467 nm (maybe a ligand-to-metal charge transfer band), and broad features extend far into the near infrared (NIR), The latter absorption may be due to exciton interactions. Contrary to the known rare earth monoporphyrins (7), it has been shown for the closely related cerium(IV)... 

Absorption studies of the model compound in polar and nonpolar solvents support the finding that the dimer is present in a form which allows for close proximity between the two porphyrin rings. The absorption spectra of the dimer and the monomer are shown in Fig. 9. The spectra in methanol show a significant blue-shift of the Soret peak and a small red-shift in the visible bands for the diporphyrin, consistent with the spectral properties of previously synthesized cofacial diporphyrins(16,17) wherein exciton interactions can take place. ( 18) In methylene chloride, the Soret blue shift appears to be much less (<5 nm with reference to 4). 

Figure 12. Splitting the excited slate into two energy levels by exciton interactions between the two chromophores i and j. Reprinted with permission from N. Harada, K. Nakanishi, Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books, Mill Valley, California, 1983.

The resemblance of the photocurrent to the optical adsorption spectrum has suggested the involvement of molecular excited states in the creation of charge carriers. While this resemblance is by no means universally observed, the concept of carrier creation via exciton interactions at or very near the illuminated electrode has become increasingly favored. Many of the data leading to these conclusions have been obtained by the use of pulsed light techniques (6, 7,3). These methods are virtually independent of electrode effects and the subsequent analysis of the transient current has led to considerable advances in the theory of charge transfer in molecular crystals. 

D. Olness, M. Swicord, and R. C. Jarnagin Photogeneration of free carriers in organic crystals via exciton-exciton interactions. Phys. Rev. Letters 10,12 (1963). 

The situation for reactions in solids is much more complex and is treated in a separate section (4.7.4, p. 153). Physical diffusion of molecules can be neglected within the lifetimes of excited states, but exciton interactions can become important. These have no counterpart in dark reactions and can lead to unusual photochemical properties in crystals and polymers. 

Soon after the original development of exciton chirality method346,347 for steroidal diols, Koreeda, Harada and Nakanishi348 extended its application to exciton interaction between benzoate transition at 230 nm (e 14,000) and enone n —> n transition at 230-260 nm (e 7,000-15,000). The p-chlorobenzoate of 3/J-hydroxycholest-5-en-7-one (151, Figure 17) exemplifies the application of this method348. The 3/J-hydroxy-enone has a As typical of the s-trans enone chromophore, and the relative orientation (helicity) of the two interacting dipoles in p-chlorobenzoate 151 is shown in Figure 17. Such positive exciton chirality... 

The pure contribution from exciton coupling in testosterone 17/Mp-chlorobcnzoatc) (152) was estimated by subtracting the CD spectra of exciton interaction-free 17/J-hydroxy-4-en-3-one (153) and 4-en-3/3-ol-17/Mp-chlorobenzoate) (154) from the experimental CD spectrum of 152. The exciton CD curve (+16.2 (247), —12.8 (230)) obtained is much more symmetrical, as required by the theory349. Several more examples of benzoates and sorbates of steroidal 4-en-3-ones were treated in similar way, thus smoothing the imbalance of the exciton Cotton effects due to contributions of component chromophores. 

---