Engineering TADF, mechanochromism, and second harmonic up-conversion properties in regioisomeric substitution space

Abstract

This research article explores the distinct TADF efficiency of three donor-acceptor based regio-isomers: DPAOCN (<i>ortho</i>-isomer), DPAMCN (<i>meta</i>-isomer), and DPAPCN (<i>para</i>-isomer). DPAPCN exhibits maximum TADF efficiency in both solution and solid-state with an impressive reverse inter-system crossing (RISC) rate of ∼10⁶ s^-1; the underlying cause being the minimum singlet-triplet splitting energy or Δ<i>E</i>_ST and maximum SOC (spin-orbit coupling) between the S₁ & T₁ states. Apart from TADF, differences in crystal packing of the regio-isomers result in intriguing bulk phase properties. DPAOCN, with its non-centrosymmetric <i>P</i>2₁2₁2₁ space group and substantial crystal void volume, exhibits reversible tri-color mechanochromic luminescence behavior, while the <i>meta</i> and <i>para</i> isomers, due to their centrosymmetric packing and diminished crystal void volume, remain inert to mechanical pressure. Expanding the horizon of possibilities, the non-centrosymmetric nature of <i>ortho</i>-isomer further renders it an excellent SHG material, with a <i>χ</i>⁽²⁾ value of 0.19 pm V^-1 at 1220 nm and a laser-induced damage threshold (LIDT) value of 13.27 GW cm^-2. Overall, a comprehensive investigation into the regio-isomers has been carried out, encompassing their TADF, SHG, and mechanochromic luminescent properties.