The Effect of Imide Substituents on the Excited State Properties of Perylene Diimide Derivatives

Abstract

Solid state optical properties of fluorescent materials are important for many photonic devices such as organic light emitting diodes, frequency down-converters or luminescent solar concentrators. Perylene diimides (PDIs) represent one of the most popular organic semiconductors which find application in such photonic device applications. In this study, photophysical properties of two dibrominated PDI (DiBrPDIs), one of which contains a branched alkyl chain (2-ethylhexyl, 2-EH) and the other with an aromatic substituent (diisopropylphenyl, DIA) at the imide positions are comparatively studied. We report their absorption and photoluminescence, lifetime and photoluminescence quantum yield (PLQY), as well as photoinduced absorption properties (PIA) examined by fs-transient absorption spectroscopy. Having the same π conjugated system, DiBrPDIDIA and DiBrPDI-2EH exhibited identical absorption and photoluminescence (PL) spectra in chloroform (λabs:527 nm and λPL:552 nm). However, in film phase, DiBrPDI-DIA (λPL-DIA:596 nm; PLQY:73.4%) presented a shorter PL wavelength with a higher PLQY than that of DiBrPDI-2EH (λPL-2EH:649 nm; PLQY:36.7%). Bond lengths and core bending angles of PDI derivatives were calculated using Chem3D pro software. It was determined that the 2,6-diisopropylphenyl group in DiBrPDI(DIA) extends a distance of about 6.8 Å out from the imide positions, providing more effective steric protection from aggregation than the smaller 2EH group.