Understanding Pathway, Structure and Property Correlations in Perylene bisimide Self-assembly

Samanta, Samaresh (2020) Understanding Pathway, Structure and Property Correlations in Perylene bisimide Self-assembly. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Samaresh Samanta (14RS004)) 14RS004.pdf - Submitted Version Restricted to Repository staff only Download (8MB)

Abstract

Under suitable conditions spontaneous organization of molecules to form ordered aggregates is referred to as molecular self-assembly. Since the recent past, multichromophoric self-assemblies have achieved tremendous attention due to its importance in the field of energy harvesting and organic semiconductors. The work reported in this thesis is concentrated on one of the most studied molecular chromophores, perylene bisimide (PBI), which shows n-type semiconducting properties and unique competence to self-assemble into functional materials. The thesis is divided into four chapters. Chapter 1: This chapter deals with preliminary information about perylene bisimide and literature survey. Chapter 2: The ability to reversibly alter molecular structures using an electrochemical stimulus is an area of great contemporary interest, where molecular design plays a key role in determining the efficiency and speed of the process. Although the choice of the redox-active center decides the threshold potential and energy efficiency of the process, selecting a backbone structure with a sufficiently low barrier to deformation ensures a fast response. This chapter describes a flexible perylene bisimide dimer that fulfills these criteria remarkably well and exhibits a rapid and very large conformational change at a much lower applied voltage than similar systems. A detailed investigation using cyclic voltammetry, spectroelectrochemistry, and EPR spectroscopy elucidates the mechanism of the actuation process. Chapter 3: Long-lived excitons in H-aggregates hold great promise for efficient transport of excitation energy, provided they are not scavenged by structurally relaxed excimers. We report solution self-assembly of a perylene bisimide (PBI) folda-dimer that exhibits two distinct kinetic stages: firstly an initial fast assembly leads to metastable aggregates with large excimer contribution that is followed by a sluggish growth of stable, extended H-aggregates free of excimers. Mechanistic investigations reveal the interplay of two competing aggregation pathways, where suppression of excimers is directly linked to the crossover from an isodesmic to cooperative aggregation. Furthermore, the competition between two self-assembly pathways is influenced by the conformational flexibility of the folda-dimer is also discussed. Chapter 4: Self-assembly provides an attractive way to functional organic materials, properties and performance depend on the organization of the molecular building blocks. The molecular arrangement is a direct outgrowth of the pathways involved in the supramolecular assembly process, which is more prone to detailed study when using one-dimensional systems. This chapter demonstrates H-bonding-incorporated supramolecular polymerization of a Phenylalanine substituted perylene diimide (1PBI) building block. 1PBI containing a carboxylic acid group exhibits H-aggregation in a non-polar methylcyclohexane solvent while in chloroform it remains in the monomer form. Temperature-dependent UV/Vis spectroscopy study corroborates a cooperative pathway is involved both in heating and cooling curves. The elongation temperature difference is observed between the heating and cooling curve which is known as the hysteresis loop. The most interesting aspect of this loop is that the monomer state (1PBI) is kinetically trapped and inactivated within the temperature window between Te and Te′. This presents a base initiated supramolecular polymerization of phenylalanine (Phe) appended PBI molecule. Moreover, the ability of this simple molecule to exist in an inactive state allows one to control the nucleation and elongation stages of growth with a careful choice of an external initiator molecule. Chapter 5: The fate of PBI H-aggregates as energy-harvesting materials depends on the ability to circumvent an extremely deleterious but efficient self-trapping process that scavenges the long-lived excitons to form deep excimeric traps. We present the first-ever report of an ambient-stable, bright, steady-state photoluminescence (PL) from the long-lived exciton of an H-aggregated PBI crystal. The crystal structure reveals a rotationally displaced H-aggregated arrangement of PBI chromophores, in which transition from the lowest energy exciton state is partially allowed. Polarized absorption spectroscopy on single microcrystals confirms an unusually large exciton splitting of ~1265 cm⁻¹ that stabilizes the lower exciton state and inhibits excimer formation. A PL Mueller matrix study shows an increase in the excited state polarization anisotropy, indicating a strong localization of the nascent exciton, which further safeguards it from the self-trapping process. Finally, the possibility of achieving excimer-free excitonic PL in solution self-assembly is also demonstrated.

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