K. Röllig

Bio: K. Röllig is an academic researcher. The author has contributed to research in topics: Quenching (fluorescence) & Charge-transfer complex. The author has an hindex of 1, co-authored 1 publications receiving 158 citations.

Kinetics of Fluorescence Quenching by Electron and H‐Atom Transfer

Abstract: Fluorescence quenching rate constants, kq, ranging from 106 to 2 × 1010 M−1 sec−1, of more than 60 typical electron donor-acceptor systems have been measured in de-oxygenated acetonitrile and are shown to be correlated with the free enthalpy change, ΔG23, involved in the actual electron transfer process in the encounter complex and varying between + 5 and −60 kcal/mole. The correlation which is based on the mechanism of adiabatic outer-sphere electron transfer requires ΔG≠23, the activation free enthalpy of this process to be a monotonous function of ΔG23 and allows the calculation of rate constants of electron transfer quenching from spectroscopic and electrochemical data. A detailed study of some systems where the calculated quenching constants differ from the experimental ones by several orders of magnitude revealed that the quenching mechanism operative in these cases was hydrogen-atom rather than electron transfer. The conditions under which these different mechanisms apply and their consequences are discussed.

Excitons in nanoscale systems

Abstract: Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.

Invited review solvatochromic shifts: The influence of the medium on the energy of electronic states

Abstract: The displacement of electronic absorption and luminescence spectra (solvatochromic shifts) are related to the solute—medium interactions. These interactions can be non-specific (dielectric interactions) when they depend only on multiple and polarizability properties of the solute and solvent molecules; but specific associations such as hydrogen bonding can also be important. A number of examples of solvatochromic shifts are shown and discussed according to the various solute—medium interactions. The properties of solvent mixtures and those of rigid media are considered, as well as the “thermochromic shifts” which result from the change in the temperature of the medium. The use of solvatochromic shifts for the determination of the dipole moment and of the polarizability of electronically excited molecules has been important for an understanding of electron distribution changes in such states; examples of such determinations are given, together with references to the original literature. In the final section some limitations of the theories of solvent shifts and possible improvements are discussed.

Dynamics of Bimolecular Photoinduced Electron-Transfer Reactions

Abstract: From the accumulated results of several research groups over the last 25 years, it is clear that photoinduced electron-transfer reactions have significantly broadened the scope of organic photochemistry.1 The fundamental mechanistic principle is that when quenching of an excited state via electron transfer is sufficiently exothermic, the reaction occurs at or close to the diffusion-controlled limit (kdiff). In polar solvents, where most reactions are carried out, the primary intermediate is a geminate radical-ion pair, A•-/D•+ (eq 1).3 Return electron transfer within the

Multistep Photochemical Charge Separation in Rod-like Molecules Based on Aromatic Imides and Diimides

Abstract: A series of intramolecular triads with linear, rod-like structures has been developed that undergo very efficient two-step electron transfer following direct excitation of a chromophore possessing a charge transfer (CT) excited state. The CT state of 4-aminonaphthalene-1,8-imide (ANI), produced by direct excitation of the chromophore, has about 70% of a negative charge transferred from the amine to the imide. Attachment of aniline (An) and p-methoxyaniline (MeOAn) donors to ANI by means of a piperazine bridge results in linear dyads, An-ANI and MeOAn-ANI, that undergo rapid electron transfer in about 10-11 s to give a >99% yield of the ion pairs, An+-ANI- and MeOAn+-ANI-, in which the charges are separated by 7.7 A. The formation and decay of these ion pairs can be monitored directly by transient absorption spectroscopy. Further attachment of a 1,8:4,5-naphthalenediimide (NI) electron acceptor to the imide group of ANI using a 2,5-dimethylphenyl spacer results in triads An-ANI-NI and MeOAn-ANI-NI. Excitat...