Effects of conformation on the electronic and optical properties of aryleneethynylenes
Materials based upon the aryleneethynylene skeleton are currently of great interest. The fluorescent and electroluminescent properties of both molecular and polymeric systems has prompted speculation about the suitability of these materials as the emitting layer in electroluminescent devices, while the conjugated π-system has led to the development of molecular wire-like architectures, and materials which display negative differential resistance. The degree of conjugation in the π-system is dependent on the relative orientation of the aromatic rings. The barrier to rotation about the aryl-alkynyl single bond for phenyleneethynylenes is very low, leading to a continuum of rotational isomers when in solution. Engineering physical control over the conformation of aryleneethynylenes therefore presents a significant challenge. In this work, a review is presented on the uses, behaviour and synthesis of aryleneethynylenes, followed by an account of the practical work carried out. Initially various series of 1,4-bis(phenylethynyl)benzene (BPEB) analogues containing naphthalene and thiophene moieties were produced. Study of the photophysical behaviour and molecular modelling of these systems suggests that the choice of central ring in a three ring aryleneethynylene system has the greatest effect on the compounds’ characteristics. This can be attributed to the π-electrons of the central aromatic ring giving the largest contribution to the frontier orbitais of these compounds. Conformational control of aryleneethynylenes is then examined. Derivatives of BPEB and 9,10-bis(phenylethynyl)anthracene (BPEA) with bulky alkyl substituents have been synthesised, with the aim of using steric bulk to restrict rotation about the acetylenie bond. Initial attempts at this approach resulted in the additional steric bulk only giving slight conformational restriction in the solid state as well as only having a small effect on the observed photophysics. Subsequently, compounds were successfully synthesised where, in the ground state, the two outer rings were orthogonal to the central ring system, switching off their contribution to the frontier orbitais, and thus leading to significant changes in photophysical behaviour being observed relative to BPEB and BPEA.