Synthesis, mesomorphic behaviour and optoelectronic properties of phosphorus-based thermotropic liquid crystalline dendrimers

Abstract

A new series of thermotropic phosphorus-based liquid crystalline (LC) dendrimers based on a thiophosphoryl-phenoxymethyl(methylhydrazono) core (thiophosphoryl-PMMH) up to the fifth generation has been synthesised by solution condensation of aldehyde groups, surface-functionalised thiophosphoryl-PMMH dendritic substrates of generation numbers G_0.5 to G_5.5, with the appropriate molar equivalents of the pro-mesogenic n-hexadecylaniline mono-functional building block. Their chemical composition has been confirmed by ¹H/¹³C/³¹P nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, elemental analysis. Optical properties have been studied by ultraviolet-visible absorption, photoluminescence spectroscopy and polarised optical microscopy, and thermal characteristics by differential scanning calorimetry. Electrical studies have been made using the current-voltage characteristics of organic light-emitting diodes consisting of multi-layered indium tin oxide/dendrimer/aluminium tris(8-hydroxyquinoline)Al architecture. It has been demonstrated that the molecular engineering approach adopted can successfully lead to phosphorus-containing dendritic organic semiconductors (OSCs) which show tunable mesomorphic behaviour (extension of the observed smectic mesophase) and (opto) electronic properties, owing to their peripheral decoration with a tunable number of azomethine-based optically active chromophoric units. This rare combination of ‘tunable by design’ properties makes this series of thermostable thiophosphoryl-PMMH-based LC dendrimers a particularly appealing class of OSCs for use in optically and/or electronically active layers of (opto)electronic devices such as light-emitting diodes, field-effect transistors, solar cells and lasers.

Keywords:

• phosphorus dendrimers
• liquid crystalline dendrimers
• azomethines
• hole transporting materials

Acknowledgements

One of the authors (AI) is grateful to the Chemistry Department of CNRS for the funding of her one-year post-doctoral fellowship within the UMR5819–SPrAM (CEA/CNRS/Univ. J. FOURIER-Grenoble I) laboratory. The authors also wish to thank P.-A. Bayle and M. Bardet of the Laboratoire de Résonance Magnétique of INAC/CEA-Grenoble for their assistance with the ³¹P NMR spectroscopic characterisation of the thiophosphoryl-PMMH-based LC dendritic organic semiconductors.