Design, Synthesis and Optical Properties of Two novel Triphenylamine Donor Based D-A-A' type Luminogens

Abstract

The luminescent behavior of two newly synthesized pure organic luninogens containing a triphenylamine (TPA)-donating group and an electron withdrawing p-nitrophenyl group along with two different other acceptors (-COOEt (ester) and –COOH (acid)) in a common donor-acceptor-acceptor (D-A-A') framework is reported here. The density functional theory (DFT) studies reveal the separated electron density in the singlet excited state, suggesting higher CT character in the singlet state for both the molecules. In contrast, a significant overlap of electron densities for the triplet state (T1) in both compounds suggests that the triplet states have hybridized local and charge transfer state (HLCT) characters for both the compounds. We also observed very interesting photo-physical properties for both the compounds in various solvents as well as in the solid state. Both the compounds (TPA-E and TPA-A) exhibit dual emission peak in medium polar solvent. However, in low polar and high polar solvents, both the compounds show single peak. The two peaks are attributed to the emission from CT and HLCT (both LE and CT states) state based on the DFT studies and the solvatochromic behavior of the compounds. Interestingly, one of the luminogen (TPA-E) emits white light in THF solution as a result of the modulation of CT and HLCT peaks. Also, TPA-E (containing ester group, -COOEt) displays delayed fluorescence and phosphorescence along with prompt fluorescence in toluene. On the contrary, TPA-A (consisting of –COOH group) shows only prompt fluorescence and phosphorescence in the solution state. Notably, both the compounds in the solid state exhibit a red-shifted peak (along with the fluorescence peak) attributed to the phosphorescence emission.