White light emission from coumarin and rhodamine derivatives based on RGB multicomponent system

Abstract

The mixture of red, green, and blue (RGB) organic fluorophores in a single blend is becoming more relevant to produce metal-free, low cost and color tunable white light emission. Herein we describe the combination of pure organic molecules based on the coumarin and rhodamine frameworks that generated broad or multi-band emission in both solution and solid forms. The derivatives of coumarin (CB), furocoumarin (CG) and rhodamine (CR1 and CR2) were synthesized and characterized using 1H NMR, 13C NMR, FTIR and ESI-MS spectroscopic techniques. The “closed-form” of the synthesized rhodamines have been further elucidated with the single crystal X-ray analysis. Their photophysical, thermal stability and electrochemical properties have been investigated, accompanied by DFT calculations to assess the plausible mechanism responsible for white light along with their eminent characteristics. Single mixture of ethanol-solubilized CR2:CG:CB in the ratio of 3:10:3 showed extremely pure white light fluorescence with CIE coordinates of (0.32, 0.33) when excited at 360 nm while CR1:CG:CB revealed almost pure white light (0.27, 0.32) in the ratio of 1:2:1 when excited at a much longer wavelength (λex = 395 nm). The results obtained are very encouraging in such a way that only minimal proportions of the blue and green fluorophores are required in the single mixture for a highly efficient FRET system when combined with the newly synthesized rhodamine derivatives. The ratio differences can be down to 30–42 % as compared to white light mixture with the commercial red R640. Furthermore, all of the synthesized RGB components exhibit reasonably good thermal stability with onset of decomposition above 210 °C, endowing their suitability for general applications of optoelectronic devices. The HOMO-LUMO energy band gaps from the cyclic voltammetry and geometrical optimized structures of the ground state are generally in nice agreement. White light emission was also generated when the polymer mixtures containing each RGB emitters was coated on a commercial UV-LED, demonstrating potential low cost and purely organic device set-up.