Solid-State Luminescent Materials Containing Both Indole and Pyrimidine Moieties: Design, Synthesis, and Density Functional Theory Calculations

Abstract

Heterocyclic compounds with effective solid-state luminescence offer a wide range of uses. It has been observed that combining pyrimidine and indole moieties in a single molecule can enhance material behavior dramatically. Here, different heterocyclic compounds with indole and pyrimidine moieties have been synthesized effectively, and their structures have been validated using NMR, IR, and mass spectroscopy. The photoluminescence behavior of two substances was investigated in powder form and solutions of varying concentrations. After aggregation, one molecule displayed a redshifted luminescence spectrum, whereas another homolog showed a blueshift. Thus, density functional theory calculations were carried out to establish that introducing a terminal group allows modifying of the luminescence behavior by altering the molecular packing. Because of the non-planarity, intermolecular interactions, and tiny intermolecular distances within the dimers, the materials demonstrated a good emission quantum yield (Φ_em) in the solid state (ex. 25.6%). At high temperatures, the compounds also demonstrated a stable emission characteristic.

Scheme 1. Synthetic Route for Obtaining the Amino Carboxamide (10) and Pyrimidothienopyrimidine Compounds (11–13)

Scheme 2. Reactions of Indolylpyrmidothienopyrimidine Derivatives 12 and 13

Figure 1

(a) 11 and (b) 12 geometry of the ground state. (c) 11 and (d) 12 geometry of the most stable dimers optimized using B3LYP-D3BJ/6-31+G(d,p). For both dimers, specific intermolecular distances between some atoms and the center of the nearest ring (in Å) are presented. Color: blue = N; gray = C; yellow = S; red = O; green = Cl. (e) 11 and (f) 12 HOMO and LUMO and the energies calculated at the B3LYP-D3BJ/6-31+G(d,p) level of theory.

Figure 2

Key changes in the structural parameters calculated at the B3LYP-D3BJ/6-31+G(d,p) level of theory between the (a) monomer and dimer of molecule 11, (b) monomer and dimer of molecule 12, (c) excited and ground states of molecule 11, and (d) ground and excited states of molecule 12.

Figure 3

Photophysical behavior of the powders (dashed lines) and DMSO solutions (solid lines). Absorption (blue, 3 × 10^–6 mol L^–1) and excitation spectra (red, λ_em = 520 nm for 11 and 464 nm for 12) of (a) 11 and (b) 12. Emission spectra at λ_ex = 300 nm of (c) 11 and (d) 12 (violet: powder, blue: 1 × 10^–3 mol L^–1, black: 3 × 10^–6 mol L^–1, and red: 1 × 10^–7 mol L^–1). CIE diagrams of the emission colors of (e) 11 and (f) 12. Photos under UV irradiation at λ_ex = 360 nm of (g) 11 and (h) 12.

Figure 4

(a) 11 and (b) 12 emission decay profiles in the air at room temperature excited at 340 nm; green: decay, blue: IRF, and red: fitting.

Figure 5

Photoluminescence spectra of 12 at λ_ex = 300 nm under various temperatures: (a) first heating and (b) first cooling.