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Traditionally, fluorescence is associated with aromatic groups. The delocalisation of electron pairs within the ring, combined with the presence of the excited π* bond state, enables electron transitions that result in energy emission within the visible spectrum upon excitation with higher energy photons. Thus, it was generally accepted that compounds without aromatic groups, e.g. cellulose, could not fluoresce and that any observed fluorescence must come from a contamination, such as lignin.
In the early 2000s a new group of compounds were reported that exhibited fluorescence upon aggregation. This was a groundbreaking discovery because typical fluorophores are quenched upon aggregation. It transpired that these compounds contained aromatic groups that could rotate, or vibrate, in solution. This meant that any energy absorbed by the compound was dissipated in the infrared spectrum rather than the visible spectrum. Upon aggregation, the rotation, or vibration, was inhibited enabling fluorescence to occur. This phenomenon was termed ‘aggregation induced emission’ and enabled new fluorophores to be developed. Whilst this did not explain why cellulose could fluoresce, it did open a new field of research that led to the discovery of the ‘cluster triggered emission’ phenomenon.
In the 2010s, papers started being published that reported on the aggregation-induced fluorescence of non-aromatic dendrimers. These compounds typically contained atoms with free electron pairs, such as oxygen and nitrogen, within chemical groups containing π bonds, such as carboxyls and amides. It was theorized that the free electron pairs were able to interact with the π bonds, much in the same way as within an aromatic group. Aggregation, or clustering, resulted in the formation of intermolecular bonds that stabilized these interactions, enabling fluorescence to occur. It was also theorized that dipole-dipole interactions, such as observed in hydrogen bonding, or through-space interactions between free electron pairs could also induce fluorescence. This explained why compounds that do not contain π bonds, such as cellulose, can fluoresce. This form of fluorescence is known as ‘cluster triggered luminescence’.