Design, Synthesis and Properties of ‘Photochromic Torsional Switches’ (PTS)

Sforazzini G

Primo

Project Administration

2015-01-01

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Abstract

The geometrical arrangement of the p-orbitals in organic semiconductors plays a pivotal role for the optoelectronic properties of the resulting bulk materials.[1] Control over the π-bond geometry, e.g. the planarity, of an extended conjugated system offers the possibility to modulate the effective conjugation length of a π-system, thus, allowing for the tuning of optical and electronic properties.[1,2] A promising way to reversibly modulate the orientation of the p-orbitals in a conjugated strucrure is to incorporate photochromic segments onto the ‘backbone’ of the π-system. Attempts to use photochromic molecules as monomer units in a polymer chain have shown that the photo-reversibility efficiency decreases inversely with the enhancement of the π-conjugation.[3] In the present work we report on a novel molecular architecture, referred to as a ‘photochromic torsional switch’ (PTS), which can overcome the limits of todays photochromic dyes towards their incorporation into extended π-system. The aforementioned molecular structure consists of a polymerizable bithiophene unit able to mechanically change its π-system planarity in response to a photochromic isomerization of a laterally attached azobenzene unit. In the dark and upon exposure of visible light, the azobenzene moiety assumes its extended trans conformation, thus, forcing the bithiophene backbone to twist out of coplanarity (dihedral angle from 50° to 68°). By contrast, exposure to UV light results in isomerization of the azobenzene unit to the cis conformation, which allows the bithiophene fragment to assume a planar, π-conjugated conformation (dihedral angles from 150° to 168°). The PTS architectures, proposed in this work, represent a new generation of photochromic dyes that can allow for the preparation of ‘conjugated photochromic polymers’, and help to gain deeper understanding of the correlation between molecular conformation and optoelectronic properties of π-conjugated macromolecules.