Carborane-induced deformation of polycyclic aromatic hydrocarbons, and their intriguing emissive properties
File(s)
Author(s)
Marsh, Adam Patrick Vincent
Type
Thesis
Abstract
Carboranes, C2B10H12, are bulky, icosahedral cluster compounds, with unique molecular bonding and electronic properties. Since their discovery in the middle of the 19th century, they have been studied and used in a wide range of chemical fields, with the aims of exploiting their excellent thermal and chemical stability, electron-withdrawing capability, and, more recently, their ability to act as modifiers and promotors of photoluminescence (PL). The effects of carboranes on PL have now been widely reported in the literature, but questions remain about the underlying processes dictating these behaviours. This work is an attempt to answer some of those questions.
In this thesis, the three structural isomers of carborane (ortho, meta and para) have been incorporated into a range of linear and non-linear aromatic systems, forming highly sterically strained structures. This phenomenon correlates with reduced carborane rotation, as determined by DFT and NMR investigations. All carboranes studied are fluorescent, showing local emission (LE) from the aromatic moiety, however the ortho-carborane compounds also demonstrate multi-emissive behaviour, as the carborane introduces a delocalised emissive species, associated with intramolecular charge-transfer (ICT) and twisted-ICT (TICT) states, the latter of which occurs when the carboranyl C-C bond is perpendicular to the aromatic plane. By introducing phenyl groups to ortho-carborane, it was shown that the ground state could be tuned to favour TICT formation, through favourable π-π interactions between the aromatic substituents ‘locking’ the C-C bond in the perpendicular arrangement.
Chrysene and phenanthrene-based ortho-carborane compounds were found to exhibit a peculiar, concentration-dependent, low-energy emissive peak. Through detailed temperature-dependent optical studies, these emissive excited states were assigned to excimers, the first time such species, involving a carborane in a charge-transfer state, has been observed.
In aggregated states, formed in bad solvents, the PL intensity for the meta and para-carboranes is reduced, but for the ortho-carborane compounds the intensity increases enormously, as a result of aggregation-induced emission (AIE). AIE is determined to be as a result of TICT emissive species, where the excited state is localised inside ortho-carborane, reducing non-radiative decay routes and boosting PL quantum yield. The aggregates were directly imaged with a combination of scanning and transmission electron microscopy (SEM and TEM respectively) and were found to form spherical nanoparticles, with evidence of crystallinity.
In this thesis, the three structural isomers of carborane (ortho, meta and para) have been incorporated into a range of linear and non-linear aromatic systems, forming highly sterically strained structures. This phenomenon correlates with reduced carborane rotation, as determined by DFT and NMR investigations. All carboranes studied are fluorescent, showing local emission (LE) from the aromatic moiety, however the ortho-carborane compounds also demonstrate multi-emissive behaviour, as the carborane introduces a delocalised emissive species, associated with intramolecular charge-transfer (ICT) and twisted-ICT (TICT) states, the latter of which occurs when the carboranyl C-C bond is perpendicular to the aromatic plane. By introducing phenyl groups to ortho-carborane, it was shown that the ground state could be tuned to favour TICT formation, through favourable π-π interactions between the aromatic substituents ‘locking’ the C-C bond in the perpendicular arrangement.
Chrysene and phenanthrene-based ortho-carborane compounds were found to exhibit a peculiar, concentration-dependent, low-energy emissive peak. Through detailed temperature-dependent optical studies, these emissive excited states were assigned to excimers, the first time such species, involving a carborane in a charge-transfer state, has been observed.
In aggregated states, formed in bad solvents, the PL intensity for the meta and para-carboranes is reduced, but for the ortho-carborane compounds the intensity increases enormously, as a result of aggregation-induced emission (AIE). AIE is determined to be as a result of TICT emissive species, where the excited state is localised inside ortho-carborane, reducing non-radiative decay routes and boosting PL quantum yield. The aggregates were directly imaged with a combination of scanning and transmission electron microscopy (SEM and TEM respectively) and were found to form spherical nanoparticles, with evidence of crystallinity.
Version
Open Access
Date Issued
2019-07
Date Awarded
2019-11
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Heeney, Martin
Sponsor
Atomic Weapons Establishment (Great Britain)
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)