Ketocoumarins: A new class of triplet sensitizers

Several derivatives of 3-ketocoumarins were prepared and are shown to have many of the photophysical criteria required for efficient triplet sensitizers. These compounds include 3-aroylcoumarins (1) and 3,3′- carbonylbiscoumarins (2). The aryl groups in 1 are either phenyl and substituted phenyl derivatives or heterocyclic groups such as thienyl and benzofuryl. The substituents on the coumarin moiety in 1 and 2, if any, are alkoxy or dialkylamino. These compounds, with absorption maxima between 330 and 450 nm, have extinction coefficients in the range of 10⁴ to almost 10⁵, which is an important criterion for efficient sensitization of thin films of polymers as those used in photoresists and lithography. The singlet-triplet intersystem crossing (isc) efficiencies of several derivatives approach unity. In others, however, a radiationless decay process competes with the isc. The decay process is particularly dominant in the asymmetrically substituted derivatives of 2, but seems to be considerably suppressed in polymeric matrices. The triplet energies of these compounds range from ca. 48 to 60 kcal/mol. Some of these ketocoumarins show phosphorescence spectra that suggest the presence of “frozen-in” rotamers.