Effect of the cyano group on colour-tunability of aryl-substituted buta-1,3-diene based solid-state emissive copolymers

Nowadays, colour-tunable solid-state emissive polymers have become promising materials because of tremendous applications in the optoelectronic fields. Owing to molecular aggregation, the emissive property is substantially diminished in the solid state, limiting their use in real technological applications. To overcome this challenge, rational molecular design is a key to develop the solid-state emissive polymers. Herein, three solid state emissive copolymers, namely P1, P2 and P3, having the (1Z,3Z)-1,4-diphenyl-1,3-butadiene unit as the donor core and cyano substituted stilbene as the acceptor in the copolymer backbone have been designed and developed. It has been found that all three copolymers are soluble in common organic solvents and are highly emissive in the solid state with green (lambda em = 534 nm), yellow (lambda em = 578 nm), and red (lambda em = 630 nm) fluorescence colour, respectively. The effect of cyano groups (-CN) on the photophysical properties of these three polymers has been investigated and the emission colours have been tuned with the number of electron-withdrawing cyano group present on the backbone of the copolymer chain corresponding to band gaps of 2.50 eV, 2.36 eV and 2.22 eV for P1, P2 and P3, respectively, and these band gap values are in good agreement with the theoretically calculated band gap values. Furthermore, copolymers P2 and P3 have been utilized as the emitter to fabricate solution processed non-doped PLED devices resulting in reduced efficiency roll off, a luminance of up to 903 cd m-2, and maximum electroluminescent efficiencies of 0.21 lm W-1 and 0.34 cd A-1 with the maximum external quantum efficiency of 0.12% having CIE coordinates of (0.44, 0.51) for P2, revealing the potential of copolymers in polymer light emitting diode (PLED) application. Solid-state emissive copolymers with varying cyano groups have been developed and are utilised for the PLED device applications to demonstrate the effect of acceptor strength.