Regulating the Optical Processes and Charge Carrier Transport Behaviour in Molecular Solids by Structural and Supramolecular Engineering

Ghosh, Tapan (2022) Regulating the Optical Processes and Charge Carrier Transport Behaviour in Molecular Solids by Structural and Supramolecular Engineering. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Tapan Ghosh (16RS036)) 16RS036.pdf - Submitted Version Restricted to Repository staff only Download (12MB)

Abstract

The primary objective of the thesis is to understand the photophysical and charge carrier transport mechanism in some small molecular weight organic semiconductors. It is known that the optical properties and charge transport behaviour are highly dependable on the precise arrangement of the semiconductor molecule in their bulk states. The basic molecular level interaction, especially the intramolecular and intermolecular interactions, is the main controlling factor in the molecular arrangement, which was thoroughly investigated on a few sets of diverse designed and synthesized small organic semiconducting molecules based on experimental and computational simulations. Chapter 1 describes the scope of organic molecules in the field of modern electronics. The speciality of small molecular weight organic semiconductors in that field and the fundamental parameters which control the performances of the molecules, along with the details discussion of the monitoring factors that regulate the parameters also. Besides, we discussed the fundamental theory and model that describe qualitatively and quantitatively the basic parameter, especially the charge transfers and photo-luminescence properties of the targeted molecules. Chapter 2 deals with two different sets of di-substituted anthracene molecules in which one set of molecules substituted with electron-withdrawing (-CF₃) groups to yield four positional isomers (FAs) with similar optical features in their monomeric states. Another set of derivatives (FPAs) also with similar positional substitution introduces an additional phenyl ring along with --CF₃ (3,5-trifluoromethylphenyl). It was revealed that the solid-state spectral characteristics of these molecules were regulated by the kind of exciton coupling. In connection with this, the optical and charge carrier transport features have been entirely different across the different derivatives and were solely controlled by the percentage of aromatic ring overlap of the anthracene unit. Additionally, it documented that the enhancement of π-conjugation due to the addition of an extra two phenyl groups in core anthracene enhances the PL efficiency (higher PLQY) compared to FAs derivatives. Chapter 3 highlights a novel molecular arrangement (Magic angle arrangement) in an as-designed molecule (CF3DPT) with high luminescence mediated by the null coulombic and negligible CT couplings. The M-aggregate molecular packing with a very small orientation factor (K²) (close to zero) assisted the negligible intermolecular transition dipole resonance and Chapter 4, places of interest the visible range luminescence enhancement through the minute change of torsional orientation with external stimuli of terphenyl derivatives (CF₃TP and MeTP). A minute alteration of the torsional angle of the peripheral phenyl rings of CF₃TP was found to be the cause of such fluorescence escalation in which the supramolecular factors, especially the H…F and π-π interactions, drove the stabilization of the polymorphic forms. Planarization persuaded aromatic energy gain, causing the amplification of the blue range fluorescence to be accompanied by subordinate H…F noncovalent interactions absent in the model compound MeTP. Therefore, in spite of having a similar molecular structure, it does not display the similar optical gain seen in the case of CF₃TP. In Chapter 5, we discussed a series of donor-acceptor-type molecules synthesized by combining rhodanine group with different derivatives of thiophene and anthracene. This chapter describes that a similar molecular arrangement with additional methyl groups turns on the solid-state luminescence by restricting the molecular motions. A small substitutional change in chemical structure was found to alter the solid-state fluorescence behaviour enormously which was attributed to the non-radiative loss in the case of non-radiative molecules due to the solid-state c-c bond flip, which can be seized at low temperatures. The second part of this chapter highlights the controls over the molecular luminescence and charge transport behaviour by controlling the nature of crystallization to yield two different polymorphs of 25TR. The distinct dimeric states of this polymorph are comprised of diverse optical features; the single-crystal comprehended of C-dimer was found to be non-fluorescent, whereas the crystal formed from S-dimer was highly emissive along with the charge carrier transport behaviour revealed that the S-dimer was ambipolar while the C-dimer was majorly n-type.

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