Electronic excited states in quasi- one- dimensional organic solids with strong coupling of Frenkel and charge-transfer excitons

This work offers a concept to predict and comprehend the electronic excited states in regular aggregates formed of quasi-one-dimensional organic materials. The tight face-to-face stacking of the molecules justifies the idealization of the crystals and clusters as weakly interacting stacks with leading effects taking place within the columnar sub-structures. Thus, the concept of the small radius exciton theory in linear molecular chains was adopted to examine the excitonic states. The excited states are composed of molecular excitations and nearest neighbor charge transfer (CT) excitations. We analyzed the structure and properties of the excited states which result from the coupling of Frenkel and CT excitons of arbitrary strength in finite chains with idealized free ends. With the help of a partially analytical approach to determine the excitonic states of mixed Frenkel CT character by introducing a complex wave vector, two main types of states can be distinguished. The majority of states are bulk states with purely imaginary wavevector. The dispersion relation of these state matches exactly the dispersion relation known from the infinite chain. The internal structure of the excitons in infinite chains is directly transferred to the bulk states in finite chains. TAMM-like surface states belong to the second class of states. Owing to the damping mediated by a a non-vanishing real part of the wavevector, the wave function of the surface states is localized at the outermost molecules. The corresponding decay length is exclusively determined by the parameterization of the coupling and is independent of the system size. It can therefore be assigned as a characteristic quantum length which plays a vital role for the understanding of system-dependent behavior of the states. The number and type of surface states occurring is predicted for any arbitrary coupling situation. The different nature of bulk and surface states leads to distinct quantum confinement effects. Two regimes are distinguished. The first regime, the case of weak confinement, is realized if the chain length is larger than the intrinsic length. Both kinds of states arrange with the system size according to their nature. Derived from the excitonic states of the infinite chain, the bulk states preserve their quasi-particle character in these large systems. Considered as a quasi-particle confined in box, they change their energy with the system size according to the particle-in-a-box picture. The surface states do not react to a change of the chain length at all, since effectively only the outermost molecules contribute to the wavefunction. The second regime holds if the states are strongly confined, i.e., the system is smaller than the intrinsic length. Both types of states give up their typical behavior and adopt similar properties.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Technische Universität Dresden

Year

2003

Author dc:creator

• Schmidt, Karin

Contributors dc:contributor

• Leo, Karl
• Petelenz, Pjotr
• Agranovich, Vladimir M.

• Aggregat
• Charge-Transfer
• Exzitonen
• Frenkel
• Oberflächenzustand
• Quantum Confinement
• endlich
• organisch
• quasi-eindimensional
• aggregate
• charge transfer
• exciton
• finite
• organic
• surface state