Synthesis, characterization and semiconducting properties of dioctyl substituted vanadyl phthalocyanines

Abstract

Solution processible organic semiconductors have attracted great attention in recent years due to their applications in printable electronics, such as organic thin-film transistor (OTFT). This type of device requires semiconductors characterized by low reorganization energies and strong intermolecular electronic interaction. Phthalocyanines are 18 π -electron disc-like aromatic macrocycles with two-dimensional (2D) π -electron delocalization over the whole molecule, and are featured with low reorganization energies and close π - π stacking. Moreover, tight 2D cofacial π - π stacking, which is beneficial for charge transport in the channel of OTFTs, can be realized by incorporating axially substituted metals or large metals in the core of phthalocyanines. Introducing alkyl substituents can endow phthalocyanines good solubility in organic solvents. However, the number and positions of alkyl substituents have great influence on the packing structures thereby OTFT performance of phthalocyanines. In the current paper, four dioctyl substituted vanadyl phthalocyanines, i.e. 2,3-dioctyl vanadyl phthalocyanine (2,3-C8OVPc), 2,16(17)-dioctyl vanadyl phthalocyanine ( dp - C8OVPc), 1,15-dioctyl vanadyl phthalocyanine (1,15-C8OVPc) and 1,18-dioctyl vanadyl phthalocyanine ( dp -C8OVPc), were synthesized, and the effect of the positions of octyl groups on their photophysical properties, OTFT performance was studied in detail. All four compounds are soluble in organic solvents such as chloroform and exhibit excellent thermal stability with the decomposition temperature ( T d) beyond 400°C. In dilute solution, the positions of octyl groups have negligible influence on their absorption spectra and frontier molecular orbital energy levels. Their absorption maxima ( λ max) are all at ca. 700 nm and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels are - 5.2 and - 3.8 eV, respectively. From solution to film, their absorption spectra exhibited a remarkable bathochromic shift. In film state, four compounds all can form ordered thin films, in which they arrange in an edge-on way. Bottom gate and top contact OTFTs based on thermally annealed films of these four compounds were fabricated via solution spin-casting. All compounds showed p-type transport behavior with field-effect mobility above 0.1 cm2/(V s) 2,3-C8OVPc displayed the best OTFT device performance. A mobility of 0.19 cm2/(V s) has been demonstrated when the films were annealed at 150°C.