Enhanced Operational Stability of the Up‐Conversion Fluorescence in Films of Palladium–Porphyrin End‐Capped Poly(pentaphenylene)

Abstract

The phenomenon of up-conversion, that is, the generation of higher-energy photons by simultaneous or sequential absorption of two or more photons, with lower energy, is mostly associated with the use of relatively high light intensities available from pulsed lasers. In organic systems, up-conversion fluorescence has also been described as a product of bimolecular processes in which the mechanism of triplet–triplet annihilation plays a prominent role. In our previous work, we demonstrated photon up-conversion in films containing two active components—metallated porphyrin macrocycles (MOEP) in a matrix of a polyfluorene (PF). The so-excited up-conversion fluorescence emission of the matrix polymer occurs at relatively low intensity and under continuous wave (cw) optical excitation. The method was successfully extended to a series of blue-emitting polymer matrices. A general advantage of the systems with more than one active component is the possibility to optimise each one of them independently. The major challenge is the optimisation and stability of the component distribution in the system. The up-conversion efficiency and operational stability of the emitting system is limited by both exand intrinsic factors. The extrinsic factors include the concentration of oxygen—either adsorbed in the film during the spin-coating process or permeated through the film surface over the entire period of optical testing of the sample. Oxygen decreases drastically the life time of the excited triplet state in the dopant molecule and hence the efficiency of the up-conversion process. Preparation and encapsulation of the sample in an inert atmosphere can control the concentration of oxygen. However, the control of the intrinsic parameters that decrease the up-conversion efficiency of the sample during operation is more challenging. In samples blended with dopant molecules, phase separation between the active polymer matrix and the dopant molecules occurs during the operation mostly because of the local in-