Ultrafast Spectroscopic Studies on Excited State Dynamics of Different Alkyl Substituted Perylene Monoimides in Solution and Thin Films

Abstract

Perylene monoimides (PMI) are a unique class of polyaromatic molecules, which are known to undergo interesting excited state processes, making them attractive for the applications in optoelectronic devices. In this paper a comprehensive investigation is conducted on the photophysical properties and excited state dynamics of a series of PMI derivatives (PMI-C18, PMI-CE6, PMI-C8, and PMI-C6), with varying side chains substituted at the imide position. These studies involved utilization of both steady state and time resolved spectroscopic techniques. Femtosecond transient absorption (TA) spectroscopic studies demonstrated that PMI derivatives exhibit similar fluorescence characteristic in solution, irrespective of the various side chains substituted at the imide position. However, in thin films the various side chains led to prominent differences in their packing arrangement, which leads to difference in the TA spectra as compared to solution. The difference in the packing arrangement in PMI thin films is studied using X-ray diffraction (XRD) spectroscopy. It is notable that the PMI-CE6 thin films exhibited the existence of triplet state. The distinctive behavior of PMI-CE6 is attributed to the distinct packing arrangement, providing role of molecular structure in influencing the excited state dynamics. The triplet state formation in PMI thin films renders them well-suited for utilization in optoelectronic devices.