E. V. Sitzmann

Polarity-dependent barriers and the photoisomerization dynamics of molecules in solution

Abstract The dynamics of molecular isomerizations that involve major charge redistributions are studied using picosecond lasers. The usual assumptions that the isomerization barrier is independent of temperature and constant within a solvent series are found to be incorrect due to solvent polarity effects. Polarity and hydrogen bonding effects on isomerizations involving large dipole moment changes ( p -dimethylaminobenzonitrile) and those involving a polar intermediate ( t -stilbene) are discussed.

Study of chemical reactions by surface second harmonic generation: p‐Nitrophenol at the air–water interface

Chemical reaction at the interface of a liquid and air has been studied using the technique of surface second harmonic generation. The reaction investigated was the acid–base equilibrium of p‐nitrophenol at the air–water interface.

Singlet−triplet interconversion of diphenylmethylene, energetics, dynamics and reactivities of different spin states

Abstract A combination of picosecond and nanosecond laser spectroscopy measurements, chemical quenching experiments and triplet sensitization experiments has allowed the determination of the rapid singlet to triplet and slower triplet to singlet intersystem crossing rates for diphenylmethylene in fluid solution at room temperature. It is shown that under the conditions of the kinetic measurements, singlet and triplet diphenylmethylene ( 1 DPM and 3 DPM, respectively) are in rapid equilibrium relative to reactions, so that knowledge of the values of k ST and k TS allows determination of the equilibrium constant and change in free energy for the 1 DPM 〈 3 DPM process. The absolute reactivity of 1 DPM toward a series of alcohols has been determined and is discussed in terms of other current investigations of carbene reactivity.

Dynamics of intermolecular electronic energy transfer at an air/liquid interface

Electronic energy transfer dynamics at the air/liquid interface have been studied by surface second harmonic techniques. The effects of dimensionality on the dynamics are discussed.

Picosecond laser studies of the charge-transfer reaction of excited triplet diphenylcarbene with electron donors

Abstract Evidence of a one-electron transfer process in a carbene reaction has been observed for the first time. The example is the quenching of the photoexcited triplet state of diphenylcarbene ( 3 *DPC) by electron donors. Measurement of the fluorescence lifetime as a function of donor concentration yielded the bimolecular rate constant, 3 * k . An explanation is offered as to why 3 * and 1 DPC react efficiently with amines as well as alcohols, whereas the ground triplet, 3 DPC, does not.

Effects of solvent polarity and structure on intersystem crossing in diphenylcarbenes. A picosecond laser study on dimesitylcarbene

Abstract The singlet to triplet intersystem crossing rate (kST) for a series of diphenylcarbene derivatives is found to be a sensitive function of the energy splitting (ΔEST), which in turn depends on the solvent polarity and on the structure of the carbene. A turn-over in the dependence of kST on ΔEST is found for the case of dimesitylcarbene (DMC), where kST is a decreasing function of ΔEST. The results for DMC provide the first example of intersystem crossing in the large gap limit for a carbene.

Picosecond laser studies of the effects of reactants on intramolecular energy relaxation of diphenylcarbene: Reaction of diphenylcarbene with alcohols

Abstract Picosecond laser induced fluorescence measurements provide for the first time the direct measurement of the intramolecular and intermolecular energy decay dynamics of singlet diphenylcarbene ( 1 DPC) in the presence of reactive molecules. As exemplified by the reaction of 1 DPC with alcohols it is found that reactive molecules provide 1 DPC with not only a chemical decay channel but also an intramolecular decay channel which is due to a solvent polarity effect. These chemical and physical effects can act in opposite directions leading to novel results such as a significant increase in the singlet state lifetime upon addition of reacting molecules. The absolute reaction rate constants of 1 DPC with alcohols, in different solvents, obtained by direct measurements are also reported.

Picosecond kinetics of state-selective singlet oxygen producing photochemistry of aromatic endoperoxides

Abstract Picosecond measurements show that the photo-fragmentation of anthracene endoperoxides leading to the anthracene moiety and singlet oxygen does not occur in a single step, but involves a short-lived intermediate. The upper excited state dual bond dissociation kinetics of the endoperoxides of 9,10-diphenylanthracene (9,10-DPA-O2) and of 9-phenylanthracene (9,10-PA-O2) were measured. For 9,10-DPA-O2 the ground state aromatic appears in 95 ± 10 ps, whereas for 9,10-PA-O2 it appears in 60 ± 7 ps. The results are consistent with our earlier proposed mechanism for the dissociation from the upper excited singlet states of isomers of the anthracene endoperoxides.

Inverse deuterium isotope effect in the intersystem crossing of diphenylcarbene

The singlet-to-triplet intersystem crossing rate k/sub ST/ of diphenylcarbene (DPC) is found to exhibit an inverse isotope effect in various solvents. An off-resonance coupling model between the initial singlet state and a sparse triplet vibronic manifold accounts for k/sub ST/ showing both an inverse isotope effect in a given solvent as well as an inverse energy gap effect in a solvent series.