Exploring the Integration of TADF and RTP Materials in Organic Polymorphic Crystals for the Development of Efficient Photonic Integrated Circuits

Pattanayak, Pradip (2025) Exploring the Integration of TADF and RTP Materials in Organic Polymorphic Crystals for the Development of Efficient Photonic Integrated Circuits. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Pradip Pattanayak (19RS051)) 19RS051.pdf - Submitted Version Restricted to Repository staff only Download (25MB)

Abstract

Even though there is a vast library of self-assembled organic molecules offering practically infinite possibilities to produce thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP), obtaining both from a single candidate is a formidable challenge due to the difficulty in controlling the excited-state dynamics. Polymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. This dissertation demonstrates how a single polymorphic molecule with a donor–acceptor–donor (D–A–D) architecture can be used to regulate both TADF and RTP with the help of polymorph engineering. Thermodynamically controlled macrocrystals show TADF due to intermolecular charge transfer (inter-CT), which reduces the singlet–triplet energy gap (ΔEST), enhances reverse intersystem crossing (RISC), and boosts the delayed singlet radiative decay. The energy gap in kinetically controlled self-assembled microcrystals is greater due to larger intermolecular distance and hence weaker inter-CT. On the other hand, the presence of stronger intramolecular charge transfer (intra-CT), in addition to restricted RISC in the microcrystals, stabilizes the triplet excitons to favour RTP. Considering that regulation of thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) is necessary for a fluorophore to survive and properly show its characteristic features, it is successfully shown herein that a carefully designed novel pyrene derivative can exhibit remarkable aggregation-induced emission (AIE) overcoming the characteristic fast excitonic decay of pyrene compounds. Moreover, it is established that J-aggregation in the red-emitting molecular crystals of the compound reduces ΔEST, allowing TADF with a very high photoluminescence quantum yield (PLQY) of 72%. Whereas, anisotropic grinding of the same crystals generates well-defined microcrystals that show RTP due to change in the nature of aggregation. The crystals show near-infra-red (NIR) emission with a very high piezochromic luminescence sensitivity of 46.2 nm GPa−1 under isotropic hydrostatic pressure. The results have established that formation of stable H-aggregates in the microcrystals is responsible for the ultra-long RTP. A highly sophisticated full-spectrum Mueller-matrix analysis demonstrate the details of the effect of perturbation (pressure) on molecular conformation. In this thesis, two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5′-(4-(diphenylamino)phenyl)-[2,2′-bithiophene]-5-carbaldehyde (TPA-CHO) have been showcased. GY exhibits TADF and OR shows RTP. Additionally, both of them display mechanical flexibility and optical waveguiding capability. Leveraging the AFM-tip-based mechanophotonics technique, the GY optical waveguide could be positioned at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals has been established in this work, paving the way for advancements in polymorphic crystal-based photonic circuits.

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