Synthesis and Excited State Dynamics of Oligothiophene-Nitronyl Nitroxides Open Access

Abstract

Spin-electronics or spintronics are envisioned to have faster data processing and accessing capabilities and higher information storage density compared to traditional electronics due to their intrinsic spin properties. Currently, the field of spintronics is dominated by silicon-metal multilayer materials. Recently, organic-based materials, particularly pi-conjugated chromophores and persistent radicals, have been of interest due to their metal-like spin properties, photo-controllable spin states, structural flexibility, low-cost, and Eco-friendly manufacturing processibility. However, low charge carrier mobility, weak ferro-magnetism, and marginal understanding of the spin-dynamics in organic radicals have limited their applications. Oligothiophene-based radicals represent an attractive alternative and may potentially address these challenges because of the high carrier mobility observed in oligothiophenes (0.6 cm²V^-1s^-1). Furthermore, photoexcitation of oligothiophenes generate large population of long-living triplet state via intersystem crossing, where the spins are aligned, which infer potential ferromagnetic properties of oligothiophenes-radicals. This thesis presents and elaborates on the synthesis and spectroscopic studies of oligothiophene-nitronyl nitroxide (OT-NN) radicals. The crystal structure, packing pattern and steady-state magnetic properties of bithiophene-bis(nitronyl nitroxide) (1.4) are also studied. Electronic coupling as well as the presence of unpaired spins have completely changed the electronic structure. Computational and experimental approaches both prove that the existence of radical components in OTNN lower the bandgap through electronic coupling. Several new n-π* induced low-lying charge-transfer states are also present in the OTNN system. The possible post-photoexication spin transitions are analyzed using ultrafast transient absorption.

Table of Contents

1 Synthesis and Excited State Dynamics of Oligothiophene- Nitronyl Nitroxides 2

1.1 Introduction................................... 2

1.1.1 Spintronics............................... 2

1.1.2 Organic Radicals............................. 3

1.1.3 Oligo- and Poly-Thiophenes....................... 7

1.1.4 Oligothiophene-Radical.............................. 10

1.2 Experimental Methods............................. 13

1.2.1 General Procedure........................... 13

1.2.2 Synthesis................................. 15

1.3 Results and Discussion............................. 22

1.3.1 Synthesis................................. 22

1.3.2 Crystal Structure of 1.4......................... 26

1.3.3 EPR Spectra of 1.4............................ 28

1.3.4 Steady-State Absorption........................ 30

1.3.5 Steady-State Emission......................... 34

1.3.6 Excited-states Analysis........................ 37

1.3.7 Computation............................... 50

1.4 Conclusion.................................... 53

A Appendix........................................ 54

A.1 Appendix section................................. 54

Bibliography....................................... 57

About this Master's Thesis

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School	Laney Graduate School
Department	Chemistry
Degree	MS
Submission	Master's Thesis
Language	English
Research Field	Chemistry, Organic Engineering, Materials Science Chemistry, Physical
Keyword	Nitronyl Nitroxide Transient Absorption OligoThiophene derivative Organic radical
Committee Chair / Thesis Advisor	Egap, Eilaf , Emory University
Committee Members	Davies, Huw, Emory University Hill, Craig L, Emory University

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