AIE/ACQ Effects in Two DR/NIR Emitters: A Structural and DFT Comparative Analysis

Abstract

The effects of aggregation-induced emission (AIE) and of aggregation caused quenching (ACQ) were observed and discussed on two solid materials based on a phenylenevinylene (PV) and a dicyano-PV structure. The brightest emitter in solid films shows a high fluorescence quantum yield in the deep red/near IR (DR/NIR) region (75%). The spectroscopic properties of the two crystalline solids have been described and compared in terms of crystallographic data and time dependent DFT analysis. The influence of the cyano-substituents on AIE/ACQ mechanism activation was discussed.

Keywords: AIE/ACQ; DFT; DR/NIR emitter; PLQY.

Scheme 1

Chemical structure of the two fluorophores C1 and C2. The double Knoevenagel condensation between 4-nitrophenylacetic acid or 2-(4-nitrophenyl)acetonitrile respectively and the alkoxy-terephthalaldehyde [22,25] was a convenient symmetric pattern to synthesize the 4,4’-dinitrostilbene skeleton. The identification and purity were assessed by mass spectrometry and ¹H-NMR and by comparing melting points with the literature data [22]. The optical properties in solution were analyzed in three solvents with different polarity: dioxane < acetone<DMF (see Table 1).

Figure 1

Absorption (up) and emission (down) spectra of C1 (red line) and C2 (black line) in dioxane (left) and DMF (right) solution.

Figure 2

Fluorophores solutions (10%) in DMF (left) and dioxane (right) in natural (left column) and under 375 nm UV light (right column).

Figure 3

Samples of C1 and C2 deposed on quartz slides at different dopant percentages in natural (left column) and under 375 nm UV light (right column).

Figure 4

(a) Single rectangular cross-shaped tubular crystals of C1 on a glass slide; (b) magnification 4 × of one crystal; (c) view of the crystal mounted on the KappaCCD diffractometer (Bruker-Nonius B.V., The Netherlands) using the KappaCCD video microscope (Bruker-Nonius B.V., The Netherlands); (d) unit cell axis are reported on the crystal: a or a* in red, b or b* in green, c or c* in blue.

Figure 5

Up: Ortep view of C1 with thermal ellipsoids drawn at 30% probability level. Only major part of the disordered aliphatic group is reported for clarity. Down: perspective view in the edge of C9/C14 ring (H atoms and alkyl groups at O3 and O4 are not shown for clarity).

Figure 6

Partial packing showing the slipped stacking of molecules of C1 in the a axis direction. Shortest distances are drawn as green dashed lines.

Figure 7

Up: Ortep view of C2 with thermal ellipsoids drawn at 30% probability level. Only major part of disordered alifatic group attached at O4 is reported for clarity. Down: perspective view in the edge of C10/C15 ring (H atoms and alkyl groups at O3 and O4 are not shown for clarity).

Figure 8

Facing of aromatic rings in C2 packing. Shortest distances involving centroids of aromatic rings are drawn as green lines. Aliphatic groups bonded at O3 and O4 and H atoms are not reported for clarity.

Figure 9

Hirshfeld fingerprint plots for C1 (left) and C2 (right).

Figure 10

HOMO-1, HOMO, LUMO, and LUMO+1 orbitals for C1 (left) and C2 (right).