Spontaneous orientation polarization of flavonoids

Abstract

Spontaneous orientation polarization (SOP) is macroscopic electric polarization that is attributed to a constant orientational degree of dipole moments of polar molecules on average. The phenomenon has been found in small molecules like H₂O at low temperatures and π-conjugated molecules employed in organic light-emitting diodes. In this study, we demonstrate that a thin film of baicalein, a flavonoid compound found in natural products, exhibits SOP and resultant giant surface potential (GSP) exceeding 5500 mV at a film thickness of 100 nm. Vacuum-deposition of baicalein under high vacuum results in smooth and amorphous films, which enables the generation of GSP with a slope of 57 mV/nm in air, a value comparable to the representative of an organic semiconductor showing GSP, tris(8-hydroxyquinoline)aluminum(III) (Alq₃). We also found the superior photostability of a baicalein film compared to an Alq₃ film. These findings highlight the potential of baicalein in new applications to organic electronics.

Figure 1

Molecular structures of baicalein and DHF in xy (top) and xz (middle) planes. White, gray, and red balls denote hydrogen, carbon, and oxygen atoms, respectively. Labels I–III denote three aromatic rings in a baicalein molecule. The fragment encircled by a dashed line indicates 4H-chromen-4-one skeleton. Arrows indicate vectors of the permanent dipoles. The dipole moments were calculated by molecular orbital calculations at the B3LYP/6-311G(d,p) level (see Experimental Section). Electron density maps of baicalein and DHF (isovalue = 0.001 e Å⁻³) in xy plane are also shown at the bottom for visual assistance to recognize the direction of the permanent dipoles.

Figure 2

FTIR-RAS spectra of vacuum-deposited baicalein films of 100 nm on ITO and Au substrates. FTIR spectrum of powdery baicalein is also shown for comparison.

Figure 3

The evolution of surface potential ϕ_s for ITO substrates as a function of thicknesses of baicalein and DHF. The dashed lines indicate linear fitting of the measured ϕ_s. The inset illustrates the definition of ϕ_s. The sign of ϕ_s becomes positive when work function Φ decreases (ΔΦ < 0). E_F denotes the Fermi level of an ITO substrate. The HOMO and LUMO means the highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively.

Figure 4

(a) XRD profiles of baicalein evaporated film of 100 nm and bare SiO_x substrate. Inset schematically illustrates molecular arrangement of baicalein in the evaporated film. (b) AFM image of 60-nm-thick baicalein film prepared on an ITO substrate.

Figure 5

Schematic illustration of molecular orientation in an evaporated film of baicalein.

Figure 6

(a) UV–Vis spectra of evaporated films of baicalein and Alq₃ prepared on quartz substrates. Thicknesses of both films are 100 nm. Onsets of optical absorptions were determined by the cross points of tangents and baselines indicated by dashed lines representatively for the spectrum of the baicalein film. Relative spectral irradiance of the simulated solar light is also plotted. The contribution of the photons below 320 nm is negligible. (b) Photostability tests of GSP for the baicalein and Alq₃ films of 100 nm under the illumination of simulated solar light at 1 sun in air. The inset shows the evolution of ϕ_s within 40 s.

Figure 7

The PL decay profiles of Alq₃ and baicalein films of 100 nm. The films were excited at 342 nm and PLs at 460 and 500 nm were detected for the baicalein and Alq₃ films, respectively. The steady-state PL spectra are shown in Fig. S3 of Supplementary Information. The gray curve shows an instrumental response function. The decays of the Alq₃ and baicalein films were fitted with an exponential and power law, respectively.