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Upon irradiation of dicoumarins with ultraviolet light, intramolecular cycloadducts with the syn configuration are formed. The structure of the isomers was proved unequivocally by nmr spectral study and dipole moment measurements. Upon irradiation of 7,7'-polymethylenedioxycoumarins with a chain length exceeding four units, the syn head-to-tail isomer is the most abundant regioisomer. On irradiation of the polymethylenedicarboxylic acid (7-coumarinyl) diesters, the syn head-to-head isomer is formed predominantly. In the former class neither the chain length nor the solvent polarity seem to exert an important influence on the ratio between the two regioisomers. In the latter this ratio is more affected by length of the link between the two chromophores. Upon increase of the concentration of the dioxycoumarins, the amount of intermolecular photoproducts increases. Upon sensitization, intermolecular reaction prevails. Steric factors such as methyl substitution on the photoreactive double bond were investigated. The photocycloadditions are reversible upon irradiation at ~300 nm.During recent years some examples of intramolecular photopolymerization between suitable chromophores, which are linked by a flexible chain, have been reported.5 In all cases mentioned except the biscinnamates,5h the 1,7 dimer50•9 ****and the A^-ZV'-alkylene bismaleimides,6r the intramolecular reaction is limited to systems in which the two functions are separated by three methylene units.In view of the hypothesis put forward in the photocyclomerization of jV",2V'-alkylene bismaleimides51 and in view of the assumption that an excimer could be an intermediate in the coumarin dimerization,6 the photochemistry of some symmetric and asymmetric di-coumarins7 was studied. We wish here to report some synthetic aspects of this study.Although the dimerization of coumarins is a longknown reaction,8 only a limited number of photoreac-(1) Abstracted in part from the Ph.D. thesis of L. H. Leenders. Presented in part at the 6th International Photochemistry Conference, Bordeaux, France, Sept 1971. (2) Predoctoral Fellow, N. F. W. 0. (aspirant, Belgisch National Fonds voor Wetenachappelijk Onderzoek).(3) Predoctoral Fellow, I. W. O. N. L. (Instituut tot Aanmoediging van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw).(4) All inquiries should be addressed to this author.(5) (a) F. D.

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Over-oxidative degradation of polythiophenes, which breaks the conjugation and destroys the electronic conductivity of the polymer, is well documented as a liability in these materials. We use this 'weakness', via controlled electrochemical over-oxidation, in a novel subtractive patterning technique compatible with high-speed reel-to-reel printing technology. We demonstrate the use of electrochemical over-oxidation to pattern PEDOT:PSS films via an x-y plotter, silk-screen and high-resolution photolithographic techniques, resulting in patterning down to a resolution of 2 µm and a conduction contrast between unpatterned and patterned areas of up to 10 8 .