Andreas Klein

Co-worker of AG Molecular Interactions and Energy transfer

	Experimental and theoretical investigations on aromatic peroxides
	Endoperoxides of ortho-fused benzenes like naphtalene and anthracene are of fundamental interest due to their photochemical behavior. When such molecules are excited lambda=270 nm cycloreversion to singlet oxygen and the aromat in its ground state occurs. Excitation at lambda > 435 nm induces the homolytic cleavage of the peroxide bridge (see figure). Until recently it was assumed that cycloreversion occurs from a higher excited state (S_n >= 2) whereas excitation into the S₁ state leads to cleavage of the peroxide bridge. This assignment contradicts Kasha's rule wich states that efficient photochemical reactions should occur from the first excited singlet state. Experimental and theoretical investigations show that at least 9,10-anthracenendoperoxid and 9,10-dimethylanthracenendoperoxide are no exceptions of Kasha's rule. The S₁ state is located at ca. 34000 cm^-1 (old assignment: S₂) and the S₂ state is located at ca. 23000 cm^-1 (old assignment: S₁).

	Experimental Investigations on 1:1 Contact Complexes (Dr. M. Kalb)
	Laser photolysis (275 nm) of anthracene endoperoxides in an argon matrix leads through cycloreversion to 1:1 contact complexes which consist of singlet oxygen and the aromat. The matrix cage prohibits the diffusion of the photofragments so that molecular interactions and energy transfer processes can be studied in a well definded spatial environment. The lifetime of singlet oxygen in 1:1 contact complexes after excitation of the aromat followed by energy transfer is significiantly shorter as in a corresponding experiment in statistical distributet matrices. This effect can be explained through efficient quenching processes between the aroamt and the oxygen molecule which are based on electron-to-electron (e-e) and electron-to-vibrational (e-v) energy transfer. This energy transfer depends on the spatial configuration and the concentration of the oxygen molecule.

	Experimental investigation of the photochemical induced energy transfer after photolysis of three-atomic molecules
	The mechanismn of the photochemical induced energy transfer could be observed for the first time at the CO₂ molecule. The aim of this project is to probe if this mechanism can be applied to other three atomic linear and non linear molecules. These experiments are currently carried out at the BESSY synchrotron in Berlin.