Optical absorption spectra of linear and cyclic thiophenes—selection rules manifestation

We theoretically study the size-dependent relation between absorption spectra of thiophene-based oligomers and the corresponding cyclothiophenes. In our approach based on a Frenkel exciton Hamiltonian, we demonstrate that the geometry and selection rules determine the observed relations between the spectra.     

Charge transfer excitons: dynamic theory of vibronic spectra

The vibronic spectra of charge transfer excitons (CTE) in a molecular one-component or alternatingly ordered two-component chain are treated in the framework of a dynamic approach (neglecting thermal excitations of the intramolecular vibrations). The model introduces two mechanisms of coupling between CTEs and vibrational quanta: (1) shift of the equilibrium positions of the nuclei in the ionized donor or acceptor; (2) change of the vibrational frequency in the ionized molecule. This model allows to generalize the simple CTE Hamiltonian and the vibronic Hamiltonian of Frenkel excitons. The linear optical susceptibility is calculated in the vibronic region (one CTE and one vibrational quantum). The double splitting of vibronics of CTEs was analyzed: (1) the splitting connected with the location of the intramolecular vibration on the donors or on the acceptors; (2) the splitting connected with the symmetry of the vibronic spectra (in the degenerate case). The general structure of the vibronic spectra of CTEs is established. It contains structureless absorption lines, which correspond to two-particle bands (the phonon is excited on a neutral molecule neighboring the donor or the acceptor) and Lorentz-type lines of one-particle states, which correspond to the bound propagation of the CTE and the phonon.     

Physical grounds of high-sensitive room temperature detection of infrared radiation involving spherical excitons at impurities in crystals

The possibility of the IR-radiation detecting in crystals of direct-gap semiconductors, caused by effects of IR-quenching of probe visible-range radiation within the region of a crystal relative transparency, is studied both theoretically and experimentally. The comparison of some mechanisms investigated allows to conclude that the most probable explanation of the IR-quenching effect, experimentally observed in the CdS crystal, is the mechanism of probe radiation absorption with photon energy deficit with respect to exciton resonance, which is eliminated due to exchange interaction of a free exciton in the intermediate state with spherical excitons localized on manyelectron atoms of impurity.     

Excitonic detectors of infrared and submillimeter radiation

The theory of new type detectors based on the quenching of secondary emission in direct-gap semiconductors (lines of Raman light scattering due to interaction between free and bound excitons in the crystal, and also bands of edge radiation) caused by IR or submillimeter radiation is proposed. The results obtained are confirmed by the experiment performed for CdS crystal excited by ultraviolet radiation of mercury lamp, at liquid helium temperature.     

Movement and amplification of exciton condensed phase pulses and interaction between pulses in semiconductor quantum wells

Formation and movement of an exciton pulse in an inhomogeneous potential are studied. It is shown that the pulse does not blur and the maximum of the exciton density in the pulse remains constant during the exciton lifetime if the pulse is formed from the condensed phase. The path, traversed by the excitons, can be increased by imposing an additional laser pulse on the system. Thereby, such a system can be used for information transmission over the exciton condensed phase.     

Multi‐Pathway Excited State Relaxation of Adenine Oligomers in Aqueous Solution: A Joint Theoretical and Experimental Study

The singlet excited states of adenine oligomers, model systems widely used for the understanding of the interaction of ultraviolet radiation with DNA, are investigated by fluorescence spectroscopy and time‐dependent (TD) DFT calculations. Fluorescence decays, fluorescence anisotropy decays, and time‐resolved fluorescence spectra are recorded from the femtosecond to the nanosecond timescales for single strand (dA)₂₀ in aqueous solution. These experimental observations and, in particular, the comparison of the fluorescence behavior upon UVC and UVA excitation allow the identification of various types of electronic transitions with different energy and polarization. Calculations performed for up to five stacked 9‐methyladenines, taking into account the solvent, show that different excited states are responsible for the absorption in the UVC and UVA spectral domains. Independently of the number of bases, bright excitons may evolve toward two types of excited dimers having π–π* or charge‐transfer character, each one distinguished by its own geometry and spectroscopic signature. According to the picture arising from the joint experimental and theoretical investigation, UVC‐induced fluorescence contains contribution from 1) exciton states with a different degree of localization, decaying within a few ps, 2) “neutral” excited dimers decaying on the sub‐nanosecond timescale, being the dominant species, and 3) charge‐transfer states decaying on the nanosecond timescale. The majority of the photons emitted upon UVA excitation are related to charge‐transfer states.     

有机半导体非平衡态HOMO和LUMO能量位移规律与OLED中“热激子”形成的唯象理解

以能斯特方程为基础, 通过分析电流密度与氧化还原物种活度变化, 即载流子浓度变化的关系, 计算出有机半导体材料电极电势的变化, 从而建立起有机半导体前线轨道, 即最高占据分子轨道(HOMO)能级和最低未被占据分子轨道(LUMO)能级相对于热力学平衡态的能量位移随电流密度变化的数学关系. 进而依据能级能量位移引起的能隙变化, 提出了有机电致发光显示器（OLED）中“热激子”的产生机制.     

UV spectra and excitation delocalization in DNA: influence of the spectral width.

The singlet excited states of the model DNA duplex (dA)10.(dT)10 are studied. Calculations are performed in the exciton theory framework. Molecular dynamics calculations provide the duplex geometry. The dipolar coupling is determined using atomic transition charges. The monomer transition energies are simulated by Gaussian functions resembling the absorption bands of nucleosides in aqueous solutions. Most of the excited states are found to be delocalized over at least two bases and result from the mixing of different monomer states. Their properties are only weakly affected by conformational changes of the double helix. On average, the highest oscillator strength is carried by the upper eigenstates. The duplex absorption spectra are shifted a few nanometers to higher energies with respect to the spectra of noninteracting monomers. The states with larger spatial extent are located close to the maximum of the absorption spectrum.     

钛酸锶晶体的光致介电常数激增以及超顺电大极化子理论

钙钛矿类化合物钛酸锶具有明显的光致介电增长的性质,这是由于被光激发至Ti⁴⁺的3d能带上的电子具有良好的巡游特性,为了进一步揭示光致介电增长的微观机理,这里假设巡游电子同时与两类声子发生耦合作用,一方面,电子与A_1g模式的晶格呼吸子发生强相互作用,另一方面,电子还与T_1u模式的非简谐声子具有相对较弱的耦合.通过变分法计算可得,这种复杂的电声耦合作用在晶体中形成两种极化子:自陷极化子和超顺电大极化子.正是由于超顺电大极化子的形成,导致了光致介电激增的现象.