Photophysical relaxation mechanism of excited phtalimide compounds

Abstract

Fluorescent polymers are one of the most desired materials for making novel flexible electronic components or spectral converters. Favourable properties of polyimides (PIs) make them a potentially attractive class of polymers but, unfortunately, they generally show a very weak fluorescence. As a result, there has been a large amount of work directed towards finding the reasons behind the lack of fluorescence of PIs and making their new fluorescent variants. Amine-substituted phtalimides are heavily used as model compounds for fluorescent PIs. In this work we have examined the photophysical relaxation mechanism of such two phtalimides (3Pi and 3Pyr) that were previously discovered to have low quantum yields of fluorescence. Our aim was to find the reason behind this lack of fluorescence and to suggest ways to make these properties more attractive. We have investigated the photophysics of phtalimides by time resolved fluorescence and ultrafast transient absorption techniques in solvent mixtures of different viscosities. Our results demonstrate that intramolecular rotation is a crucial process that opens up an alternative relaxation pathway to fluorescence in phtalimides, which has to be suppressed for designing more fluorescent materials.