An oligopeptide doubly labelled with an azulene chromophore and a TEMPO radical. Azulene triplet generation by enhanced ISC from S-2

Azulene and a nitroxyl radical have been incorporated in the same helical peptide template in order to study the excited state quenching of the chromophore by the radical. This process has been investigated by time resolved EPR by exciting the molecule with visible or UV laser pulses. Spin polarised radical EPR signals have been observed only by using UV light, hence only exciting azulene to its S-2 excited state. When azulene and the nitroxyl radical are not linked together, no spin polarisation was observed. The results are accounted for by azulene triplet generation from the S-2, state by enhanced intersystem crossing due to the close proximity of the free radical.