Photophysical control of aggregation in Perylene Bisimide derivatives by molecular design

Abstract

This systematic study reports on the photophysical properties of perylene bisimide (PBI) and its derivatives substituted with pyridine (PDP) and cholesterol (PDC) groups using UV–Vis, fluorescence spectroscopy methods, alongside density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. UV–Vis absorption and fluorescence spectroscopy show that PDP and PDC exhibit near mirror-image absorption and emission spectra with Stokes shifts of 29 and 19 nm, respectively. In contrast, PBI displays a negligible Stokes shift and lacks mirror symmetry due to strong excimer formation facilitated by its highly planar bisimide core. TDDFT computations accurately reproduce the main π → π* electronic transitions in high-energy (298–300 nm) and low-energy (430–454 nm) regions, in agreement with experimental absorption data. However, theoretical emission spectra indicate greater overlap between absorption and fluorescence than observed experimentally, reflecting limitations of the Franck–Condon approximation in accounting for excited-state relaxation. Dimer models reveal pronounced cofacial H-aggregate-like with shift characteristics, with PDC exhibiting the highest stability (binding energy of −6.46 eV), followed by PDP (−6.36 eV) and PBI (−5.48 eV). Despite its lower dimer stability, PBI shows the broadest excimer emission, attributed to its increased dipole moment and extended π-π interactions. Red-shifted electronic transitions in the dimers confirm excitonic coupling and reduced oscillator strengths typical of J-aggregates. The computational models, which omit the bulky cholesterol substituents, permit excimer stabilization in PDC. In contrast, in the experimental systems, steric hindrance imposed by the cholesterol groups restricts close π–π stacking and thus limits aggregation. This steric effect is further validated by calculations on the cholesterol-substituted PDC model, which exhibits a slip-stacked dimer configuration. During the dimer formation, the charge separation occurs, where one monomer act as Donor and other one as acceptor, where the hole migration dynamics are explored using the frozen nuclei treatment. This combined experimental and theoretical investigation highlights how molecular planarity, steric effects, and excitonic interactions govern the balance between excimer and J-aggregate emission in perylene derivatives. These insights will inform the rational design of organic semiconductors and fluorescence sensors employing PBI-based architectures in optoelectronic applications.