White Light Emission from Vegetable Extracts
A mixture of extracts from two common vegetables, red pomegranate and turmeric, when photoexcited at 380 nm, produced almost pure white light emission (WLE) with Commission Internationale d’Eclairage (CIE) chromaticity index (0.35, 0.33) in acidic ethanol. It was also possible to obtain WLE in polyvinyl alcohol film (0.32, 0.25), and in gelatin gel (0.26, 0.33) using the same extract mixture. The colour temperature of the WLE was conveniently tunable by simply adjusting the concentrations of the component emitters. The primary emitting pigments responsible for contributing to WLE were polyphenols and anthocyanins from pomegranate, and curcumin from turmeric. It was observed that a cascade of Forster resonance energy transfer involving polyphenolics, curcumin and anthocyanins played a crucial role in obtaining a CIE index close to pure white light. The optimized methods of extraction of the two primary emitting pigments from their corresponding plant sources are simple, cheap and fairly green.
White light emitting materials have attracted significant attention in recent years as key components in display and lighting devices based on LEDs1. LEDs accounts for almost 20% of the total worldwide energy consumption and have wide applications in backlights, displays, lasers and indicators2, 3, 4. White light emission has also been used for sensing5. There has been a high level of interest in recent years in looking for white light emitting organic and inorganic molecules and materials, when photoexcited at near UV wavelengths6. A variety of photophysical principles have been used for achieving emission containing the three essential red-green-blue components for effective white light perception. These include FRET7, 8, Inter- and intra- molecular charge transfer9, excited state intramolecular proton transfer (ESIPT)10, hydrogen bonding mediated J- aggregation11 and the mixing of monomer and excimer fluorescence12 etc. Although single-molecule WLE is conceptually attractive, it is difficult to adjust the CIE index and colour temperature of such molecules conveniently. In this regard multi component WLE comprising of a mixture of different molecules can, in principle, make it easy to tune the colour temperature by simply adjusting the composition. Not surprisingly, there is significant recent interest for generation of mixed emitter WLE. Such systems comprise of inorganic, organic and hybrid systems like nanomaterials/quantum dots13, 14, 15, polymer16, 17, metal-organic framework18, 19, 20, inorganic-organic hybrids21, metal complexes22 and lanthanide doped systems23, 24. Various systems tried by using mixtures of organic fluorophores include, (i) a simple mixing of three emitting dyes at RGB region25, (ii) donar-acceptor conjugated pairs26, (iii) organic liquids such as π- conjugated oligo (p-phenylenevinylene) (OPV)27, (iv) controlled donor self assembly based organogels28, (v) and a peptide link based two component system29.
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