Introduction. Tamm states

In most well-studied molecular crystals, such as anthracene, tetracene, and other crystals of the aromatic series, the intermolecular distances are quite large in all directions and, hence, the overlap of the wavefunctions of neighboring molecules is very small. In such crystals, the deviations from electroneutrality of the molecules in the lowest energy excitations in the majority of cases can be neglected. The Frenkel excitons and weak exciton-phonon interaction are typical for the lowest energy states of this crystals. 

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. 

It is well known that for ideal linear molecular chains of finite length without mixing of Frenkel with CT states, only the usual space quantization of exciton states inside the band appears. Nearest neighbor approximation (this model is often used for analysis of spectra of J-aggregates, see, e.g. (42)) yields the energy of Frenkel exciton states  

However, even in perfect ID structures, the mixing of Frenkel and CT excitons destroys this simple picture. Below, following (43), we show that this mixing is responsible for the appearance of new excitonic states, which are localized at the ends of a one-dimensional crystal chain and which are analogous to Tamm surface states of electrons. Their energy can be blue- or red-shifted in comparison with the bulk states. In the case of red-shift, these states can determine the fluorescence spectrum of a molecular chain. They can also play an important role in quantum confinement of the states in the molecular chain. For the description 

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