Three mono-fluorinated derivatives of the flavin core system 10-methyl-isoalloxazine (MIA) were synthesized. Aqueous solutions of these compounds were characterized by steady-state and time-resolved spectroscopy. The positions for the fluorination (6, 7 and 8) were motivated by the nodal structure of the frontier orbitals of MIA. In comparison with MIA, the fluorination results in bathochromic (6F- and 7F-MIA) and hypsochromic (8F-MIA) shifts of the adiabatic excitation energy of the lowest allowed transition. Shifts of up to ~500 cm were observed. These spectroscopic shifts go along with changes in fluorescence quantum yields and lifetimes. In addition, triplet yields are affected. For 7F-MIA, a 50% increase in the fluorescence quantum yield as well as a 50% decrease in triplet yield is observed rendering the compound interesting for fluorescence applications. The measured effects are discussed in terms of qualitative perturbation theory.
The interaction of psoralens with DNA has been used for therapeutic and research purposes for decades. Still the photoinduced behavior of psoralens in DNA has never been observed directly. Femtosecond transient absorption spectroscopy is used here to gain direct insight into the photophysics of a DNA-intercalated psoralen (4'-aminomethyl-4,5',8-trimethyl-psoralen (AMT)). Intercalation reduces the excited singlet lifetime of AMT to 4 ps compared with 1400 ps for AMT in water. This singlet quenching prohibits the population of the triplet state that is accessed in free AMT. Instead, a DNA to AMT electron transfer takes place. The resulting radical pair decays primarily via charge recombination with a time constant of 30 ps. The efficient electron transfer observed here reveals a completely new aspect of the psoralen-DNA interaction.
An acridone derivative (N-methyl-difluoro-acridone, NMA-dF) is characterized with respect to its utility as an emitter in organic light emitting diodes (OLEDs). Using steadystate and time-resolved spectroscopy as well as quantum chemistry, its ability to convert singlet and triplet excitons into light was scrutinized. NMA-dF emits in the deep blue range of the visible spectrum. Its fluorescence emission occurs with quantum yields close to 1 and a radiative rate constant of ≈5 × 10 8 s −1 . So, it processes singlet excitons very efficiently. Using 1,4-dichlorobenzene as a sensitizer, it is shown that NMA-dF also converts triplet excitons into light. With the aid of quantum chemistry, this is related to a reverse intersystem crossing starting from a higher triplet state (HIGHrISC).
The photophysics of N-methylphthalimide (MP) in solution (cyclohexane, ethanol, acetonitrile, and water) was characterized by steady state as well as time resolved fluorescence and absorption spectroscopy.
First order processes are of paramount importance for chemical kinetics. In a well-established demonstration experiment, the flow of water out of a vertical glass tube through a capillary simulates a chemical first order process. Here, a digital version of this experiment for lecture hall demonstrations is presented. To this end, water flowing out of the capillary is collected in a beaker which stands on an electronic scale interfaced with a computer. The computer generates a plot of the data in real time which is projected during the lecture. Hereby, with proper explanations, a very intuitive grasp of the essence of first order processes is obtained: The rate (represented by the water flow) is proportional to the concentration of the reactant (height of the water column). With a modified setup, consecutive first order kinetics as well as the concept of a rate-limiting step are illustrated.
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