Synthesis and study of the dyeing properties of aurones and development of a visible light-mediated polyene cyclization methodology

Abstract

Aurones are primarily responsible for pigmentation, imparting yellow and orange hues to flowers and fruits. Despite their proven coloring role in nature, other applications of their pigmentation and photophysical properties have been limited and focused almost entirely on their fluorescent properties. To address this gap, two different color-related projects were explored in this dissertation. In one project, different dimethyl amino- and ferrocene-appended hydroxy and methoxy substituted aurone derivatives were screened for their dyeing potential using an environmentally sustainable deep eutectic solvent of choline chloride and urea (chapter II). Considering various factors, including solubility, accessibility of the aurones, and their color-imparting ability to different fabrics, only four hydroxy derivatives of dimethylamino-aurones 22-25 were selected for further study. Their dye exhaustion ability, color fastness, color strength, and fixation property on wool fabric were studied. Among the four aurones, 6-OH-DMAA (22) showed the highest dye exhaustion, whereas 6,7-diOH-DMAA (25) dyed wool showed the lowest value. In qualitative grey-scale tests, all four aurones showed excellent color fastness against rub, wash, and light exposure, revealing their promising dyeing ability. The color strength (K/S) were calculated using the reflectance values of the dyed fabrics. Fabrics dyed with aurone 22 exhibited high reflectance overall, showing low K/S value for all four tests. Aurones 23 and 25 dyed fabrics showed comparable K/S against rub, wash, and light tests. Wool fabric dyed with 24 (4,6-diOH-DMAA) showed consistently high K/S for the tests. The other study sought to employ the aurone scaffold as a versatile organic photocatalyst, looking initially at polyene cyclizations (chapter III). Polyene cyclizations allow easy excess to complex polycyclic molecules from acyclic precursors through stereo-controlled multiple C-C bond formation. While this transformation has been used frequently over the years, existing strategies largely demand the use of expensive and toxic transition metal complexes to accomplish the desired reaction. In a highly unusual result, it was discovered that the use of a blue LED enabled a completely catalyst-free cascade cyclization of geranyl phenols containing bromo- and chloro- groups at different positions of the phenolic ring. The desired cyclization products were obtained with high stereoselectivity in the hexafluoro-2-propanol reaction medium. This reaction condition is the simplest option yet reported and has potential for future extension to additional catalyst-free photochemical transformations.