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Crystallisation-enhanced Bulk Hole Mobility in Phenothiazine-Based Organic Semiconductors

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Crystallisation-enhanced Bulk Hole Mobility in Phenothiazine-Based Organic Semiconductors

D B Shinde et al. Sci Rep.

Abstract

A series of three novel donor-acceptor systems based on C(3)-malononitrile-substituted phenothiazines was synthesised in good overall yields and their thermal, spectroscopic, and electrochemical properties were characterised. The compounds were prepared through a sequence of Ullmann-coupling, Vilsmeier-Haack formylation and Knoevenagel-condensation, followed by Suzuki-coupling reactions for introduction of aryl substitutents at C(7) position of the phenothiazine. The introduction of a donor unit at the C(7) position exhibited a weak impact on the optical and electrochemical characteristics of the compounds and led to amorphous films with bulk hole mobilities in the typical range reported for phenothiazines, despite the higher charge delocalisation as attested by computational studies. In contrast, highly ordered films were formed when using the C(7)-unsubstituted 3-malononitrile phenothiazine, exhibiting an outstanding mobility of 1 × 10-3 cm2 V-1 s-1, the highest reported for this class of compounds. Computational conformational analysis of the new phenothizanes suggested that free rotation of the substitutents at the C(7) position suppresses the ordering of the system, thereby hampering suitable packing of the new materials needed for high charge carrier mobility.

Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Chemical structures of the phenothiazine derivatives O-1, O-2, and O-3.
Figure 2
Figure 2. Synthesis of push-pull organic semiconductors O-1, O-2 and O-3.
Reagents: (i) 4-Iodoanisole, Cu, K2CO3, TEGDME, 180 °C; (ii) POCl3, DMF, C2H4Cl2, 80 °C; (iii and (vii) CH2(CN)2, Piperidine, CHCl3, reflux; (iv) N-Bromosuccinimide, CHCl3, rt; (v) 4-methoxyphenyl boronic acid, Pd(PPh3)4, K2CO3 (2M), THF; (vi) 2-(6-butoxynaphthalen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane, Pd(PPh3)4, K2CO3 (2M), THF; (viii) 1-bromobutane, KOH, DMSO, rt; (ix) bis(pinacolato)diboron, Pd(dppf)Cl2, KOAc, 1,4-dioxane, 80 °C.
Figure 3
Figure 3
(a) molar absorptivity (ε) of O-1, O-2 and O-3 in chloroform solution; (b) absorption spectra of freshly prepared films of the target compounds.
Figure 4
Figure 4
Normalised absorption spectra of O-1 (a), O-2 (b) and O-3 (c) films, spin-coated from chlorobenzene solutions. Spectra of as-cast freshly deposited films, of as-cast films after 10 h at room temperature, and of thermally annealed films are shown.
Figure 5
Figure 5
ImZ as a function of frequency for hole-only devices made of O-1 (a), O-2 (b) and O-3 (c) films at different values of the dc voltage (the arrows indicate increasing voltage). Film thickness: 920 nm, 575 nm and 470 nm for O-1, O-2 and O-3, respectively.
Figure 6
Figure 6. Bulk hole mobility of O-1, O-2 and O-3 as a function of the square root of electric field.
For O-2 and O-3 the lines indicate the linear fit to the experimental data.
Figure 7
Figure 7. XRD patterns of powder samples (p) and film samples (f).
All patterns are subtracted for the background of the substrates in order to enhance the presence of halos due to the amorphous component. The recorded patterns are reported in Figure S14. Film samples were prepared and thermally treated in the same conditions used for the preparation of hole-only devices.
Figure 8
Figure 8. Energy levels and electron distribution for frontier molecular orbitals of O-1, O-2 and O-3(Me) calculated with DFT at PBE1PBE/6-31G** level of theory (isosurface value = 0.04).

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