The Photophysics and Electrochemistry of
Carboranes

Abstract

This thesis describes research carried out on compounds containing icosahedral carborane moieties (ortho/meta/para-CB10H10C-), investigating their photophysical and electrochemical properties. The ortho carborane cage is shown to play an active role in the fluorescence and electrochemical reduction properties. Meta and para carborane groups, however, chiefly act as spectators. Chapter one gives an overview of carboranes in the literature, highlighting the syntheses, photophysical and electrochemical studies done previously on C-aryl-carboranes. Chapter two describes the syntheses of naphthyl carboranes. Ortho di-naphthyl carborane displays emission with a large Stokes shift, while the meta and para isomers emit with a small Stokes shift. Ortho monosubstituted naphthyl carborane, by contrast, displays two distinct fluorescent emission bands, with large and small Stokes shifts. TD-DFT calculations reveal that the two emission bands can be attributed to two different S1 excited states formed on excitation. The low energy band is due to a geometry in which the cage C-C length is elongated. The high energy band is due to a closed cage conformation in which the cage C-C bond length is smaller. In chapter three the syntheses and fluorescence emission of carboranyl triarylamines are investigated. Meta and para carboranyl triarylamines are found to emit in DCM solution,
giving higher fluorescence quantum yields than the non-cluster triarylamines. Emission of ortho carboranyl triarylamine, with a large Stokes shift, is observed in 1:99 THF:H2O solution, due to aggregation induced emission. The S1 excited state geometry is calculated to have an
extended cage C-C length. Chapter four investigates the syntheses and fluorescence of some bridged ortho carborane compounds of the type (1-R-1,2-C2B10H10)2X, where X=1,1’-
(C6H4)2/C4SH2/C6H4 or C6F4. These compounds are found to give a broad range of emission maxima, from 355 to 763 nm in THF:H2O solutions. Chapter five looks at the electrochemistry of some simple carboranes, naphthyl
carboranes, carboranyl triarylamines and bridged carboranes. Calculations of the cage C-C
bond lengths are shown to be successful at predicting trends in the reduction potentials of simple carboranes. On reduction to a dianion, the cage C-C bond length is elongated as the carborane cage opens up to accommodate the additional electrons. The greater the cage C-C bond length in the ground state, the greater the ease with which the reduction occurs. Dinapthyl ortho carborane displays formation of a radical anion before further reduction to the
dianion, as reported for di-phenyl ortho carborane. Bridged carboranes where X=C6H4/C6F4/C4SH2 display communication between carborane cages and suggest considerable bridge involvement.