End injection of triplet electronic excitation energy into a polymer molecule using chromophores bound at chain ends

Abstract

Polystyrene molecules labeled at opposite ends with naphthalene and aromatic carbonyl (AC) groups were prepared and their triplet photophysical properties investigated. By use of excitation at 337 nm the aromatic carbonyl was photoexcited selectively and time-resolved phosphorescence spectra were recorded at delay times from 100{mu}s to 100 ms. The AC phosphorescence was inhomogeneously broadened at the shortest delay times but becomes narrow and more structured with increasing delay time at 77K. At ambient temperature the spectrum remains broad and structureless even at long delay times. Using dye laser excitation at 360 nm a narrow structured band is observed even at 150{mu}s after the excitation pulse. Energy transfer to the naphthalene group occurs efficiently and may take place either by direct transfer from excited AC groups or by an indirect process involving backbone phenyl groups. Electronically excited phenyl groups are thought to be produced by energy transfer from that set of triplet AC chromophores lying at the high-energy edge of the inhomogeneously broadened manifold. The resulting emission spectrum suggests the possibility of exciplex formation.