Electron Transfer versus Proton Transfer in Excited States of Bichromophoric Aniline/Olefin Systems

Abstract

Photolysis of 2-allylaniline (1a) and trans-2-cinnamylaniline (2a) produced mainly the five- or the six-membered ring products 3a or 9a, respectively. Compound 1b, the N-acetyl derivative of 1a, preferentially underwent photo-Fries rearrangement of the anilide moiety, while − in contrast − the analogous compound 2b, derived from 2a, displayed competition between photocyclisation and double-bond isomerisation. The latter process, characteristic of the styrene chromophore, largely predominated in the case of 2c, the N-trifluoroacetyl derivative of 2a, while the allyl analogue 1c was essentially unreactive. The photochemical behaviour of the cis-cinnamyl compounds 7a and 7b was analogous to that of their trans isomers 2a and 2b, although double bond isomerisation occurred to a smaller extent. Thus, the introduction of electron-withdrawing acyl groups decreased photocyclisation. The nature of the excited states involved in the photochemistry of 1a−c, 2a−c and 7a and 7b was studied by fluorescence measurements. The most remarkable observation was the formation of intramolecular charge-transfer exciplexes in the cases of 1a, 2a, 2b, 7a and 7b. The exciplex bands of the cinnamyl compounds 2a, 2b, 7a and 7b in acetonitrile were considerably red-shifted (maxima at ca. 500 nm). A satisfactory correlation of the photochemical and photophysical data could be achieved by considering that photocyclisation took place when clear exciplex emission was observed. Alltogether, the above data strongly supported the involvement of an excited state electron-transfer mechanism in the photocyclisation of aniline/olefin bichromophoric systems. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)