Photochromic molecular switches: Thienyl-based polyphenylenes and the π-delocalisation effect on photochemical properties

Abstract

Information storage is at the forefront of materials science due to an exponential increase in demand for faster, more efficient, higher density data storage units. Light-driven photochromic switches form one of the most active areas of research due to their application in devices which operate at molecular and supramolecular scales. Upon irradiation by different wavelengths of light, photochromic compounds are capable of undergoing clean interconversions between two perceptively different states through photocyclisation. Dithienylethene-based compounds have become a mainstay in this area of materials chemistry. Herein, a series of dithienylethene based switches with appended polyaromatic ligands of increasing conjugation, have been designed and synthesised to address the need for highly red-shifted photocyclisation absorption wavelengths, increased fatigue resistance and enhanced fluorescence emission intensity. Polycyclic aromatic hydrocarbons (PAHs), such as graphene based materials and their heteroatom analogues, have desirable chemical and physical properties such as low weight and high strength, efficient heat and electrical conductivity. Developing graphene-type aromatic molecules using lab-scale synthetic routes this investigation assesses the influence of incorporating PAH-like functionalities into novel DTE-switching systems. These delocalised platforms were synthesized using an ethynyl-spacer group to allow for facile communication between the appendages and the thienyl-core. EXternsive conjugation was seen to have a polarizing effect of the photophysics of the switch, with increased emission intensity and unique absorption spectra. A series of thienyl-based polyphenylenes were also synthesised, to analyse the effect of substitution of a thiophene moiety into a hexaphenylbenzene (HPB) system. The synthetic routes explored here allowed for synthesis of mono-, di- and tri-substituted HPB?s, substituted both directly to the central benzene ring and as an aromatic group on the periphery Some highly phosphorescent Ir(III) complexes were also synthesized and characterised to a high degree using a variety of NMR techniques, solid-state data. The nature of these emissive states and their effect on the photocyclisation of the switching core has also been assessed, with particular emphasis on the overall photophysical absorption and emission patterns. Complexes with dithienylethene-based switching ligands were found to have interesting but less efficient switching natures due to the transfer to the emissive 3MLCT.