Solid state optical properties of fluorescent materials are important for many optical and electronic 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 those phonic 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 has an aromatic substituent (diisopropylphenyl, DIA) at the imide positions are comparatively studied. Besides their absorption and photoluminescence, lifetime and photoluminescence quantum yield (PLQY), photoinduced absorption properties (PIA) were also examined by fs-transient absorption spectroscopy. Due to their the same π conjugated system, DiBrPDI-DIA and DiBrPDI-2EH exhibited identical absorption and photoluminescence (PL) spectra in solution phase (λabs:527 nm and λPL:552 nm). However, in their film phases which were prepared at the same conditions, 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%). 3-D investigations performed by using Chem3D pro software addressed the higher intermolecular distance between the perylene rings induced by the bulky DIA groups, as the main reason of this difference.
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