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 regi.

The first book devoted to a systematic consideration of electronic excitations and electronic energy transfer in organic crystalline multilayers and organics based nanostructures(quantum wells, quantum wires, quantum dots, microcavities). The ingenious combination of organic with inorganic materials in one and the same hybrid structure is shown to give qualitatively new opto-electronic phenomena, potentially important for applications in nonlinear optics, light emitting devices, photovoltaic cells, lasers and so on. The book will be useful not only for physicists but also for chemists and biologists.To help the nonspecialist reader, three Chapters which contain a tutorial and updated introduction to the physics of electronic excitations in organic and inorganic solids have been included.* hybrid Frenkel-Wannier-Mott excitons* microcavities with crystalline and disordered organics * electronic excitation at donor-acceptor interfaces * cold photoconductivity at donor-acceptor interface* cummulative photovoltage* Feorster transfer energy in microcavity* New concepts for LEDs

We present a theoretical and experimental study on the lowest electronically excited states in quasi-one-dimensional molecular crystals. The specific calculations and the experiments are performed for the model compounds MePTCDI (N-N'-dimethylperylene-3,4:9,10-dicarboximide) and TCDA(3,4:9,10-perylenetetracarboxylic dianhydride). The intermolecular interactions between nearest neighbors are quantum chemically analyzed on the basis of semi-empirical (ZINDO/S) Hartree-Fock calculations and a singly excited configuration interaction scheme. Supermolecular dimer states are projected onto a basis set of localized excitations. The nature of the lowest states is then completely explained as a superposition of molecular and low lying charge-transfer excitations. The CT excitations show a significant intrinsic transition dipole, which is oriented approximately parallel to the molecular planes and has a large component along the molecular M-axis. The exciton states in the one-dimensional stacks are described by a model Hamiltonian that includes interactions between three vibronic levels of the lowest molecular excitation and nearest-neighbor CT excitations. The three-dimensional crystal structure is considered by Frenkel exciton transfer between arbitrary molecules. This model is compared to polarized absorption spectra. With a small set of parameters, we can describe the key features of the absorption spectra, the polarization behavior, and the Davydov splitting. The variation of the polarization ratio for the various exciton states is analyzed as a direct qualitative proof for the mixing between Frenkel and charge-transfer excitons.

This book contains the proceedings of the Sixth International Conference of the Balkan Physical Union (BPU-6), held in Istanbul, Turkey, in August 2006. The BPU-6 Conference covers the most important results and trends in physics, including: Nuclear physics and nuclear energy, gravitation and cosmology, alternative sources of energy, econophysics, biophysics and medical physics, history of philosophy of physics, meteorology and instrumentation.

Excitons, as part of the InTech collection of international works on optics and optoelectronics, contains recent achievements of specialists from China, France, Japan, Switzerland, and Moldova jointly with Russia and the United States of America on properties and application of excitons in electronics. The growing number of countries participating in this endeavor and joint participation of the US, Moldova, Italy, and Russian scientists in investigations of excitons in the edition of this book testify to the unifying effect of science. An interested reader will find in the book the description of properties and possible applications of excitons, as well as the methods of fabrication and analysis of operation and the regions of application of modern excitonic devices.