As excimer models the [3.3)-and [4.4)pyrenophanes 1 and 2 were synthesized. As the key precursor for the synthesis of 1 the octahydro-dithia[4.4)pyrenophane 9 was obtained by cyciisation of 4 and 8, the syntheses of which are described. Disulfone 10, derived from 9, on vapour-phase pyrolysis yielded 11 which by dehydrogenation was converted into 1. In an analogous route 2 was obtained via 16, 17, and 18 starting from 4 and IS (prepared in the reaction sequence 12 -13 -14 -IS). -The molecular structures of 1 and 2, determined by X-ray structure analyses, are discussed with regard to transanular distances and deviations from planarity of the pyrene units. The first and most typical example of an 'excimer' formation has been observed for pyrene 1). On photoexcitation a complex is formed by an excited pyrene molecule with a second pyrene which is in the electronic ground state: Ar· + Ar ¢ (Arz)* (excited dimer = 'excimer'). Whereas absorption spectra are not affected, excimer formation results in a concentration dependent change in the fluorescence emission which, as compared to monomer emission, is strongly red-shifted, broad, and structureless. In spite of the undisputed importance of excimers for the photophysics and photochemistry of aromatic systems, so far only very limited experimental evidence has been

The four‐membered ring in kinetically stabilized tetra‐tert‐butylcyclobutadiene has sides of almost equal length and is also folded. Perhaps even cyclobutadiene itself is “stabilized” by ring folding.—Derivatives previously studied have two long and two short sides.

sterisch unbehinderte Tetrarnethyltetrahedran und sein Radikalkation jeweils Schnitte durch die Energiehyperflachen entlang der ,,Isornerisierungstaler" berechnet (Abb. 2).Die spontane Isomerisierung des C4-Clusters bei Elektronenentnahme (Abb. 2) laBt sich anhand der berechneten Ladungsverteilungen['I erlautern:Wie ersichtlich kann die positive Ladung unter Ubergang in das Cyclobutadien-n-System besser verteilt werden; fur eine groBere Stabilisierung des M-O-Grundzustandes spricht auch die relativ zu Tetrahedran urn 1.15 eV auf 6.35 eV erniedrigte Ioni~ierungsenergie~'~. Wahrend der neutrale C4-Cluster substituenten-unterstiitzt existenzfahig ist, bewirkt Oxidation also wie bei S4Ni@ seine Einebnung. Eingegangen am 22. Oktober 1979 [Z 404al 39. Mitteilung uber Radikalionen. Teil der Diplomarbeit B. Rorh, Universitat

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