Luminescence is a gift offered by nature and plays an important role in a wide range of fields. The observation of luminescence phenomenon and luminescent materials can be tracked back to the 16^th Century, when a Spanish physician reported a Mexican wood with blue emission to treat kidney and urinary diseases. Afterwards, the most famous luminescence in ancient reports, Bologna stone, was discovered and attracted wide attentions. After over three centuries of continuous research on luminescence phenomena and luminescent materials, a deeper understanding of the mechanism of luminescence has been achieved and researchers realized that the light of luminescent materials arises from the deactivation of the excited states of the luminescent materials that absorb light energy. Since then, lum...[ Read more ]

Luminescence is a gift offered by nature and plays an important role in a wide range of fields. The observation of luminescence phenomenon and luminescent materials can be tracked back to the 16^th Century, when a Spanish physician reported a Mexican wood with blue emission to treat kidney and urinary diseases. Afterwards, the most famous luminescence in ancient reports, Bologna stone, was discovered and attracted wide attentions. After over three centuries of continuous research on luminescence phenomena and luminescent materials, a deeper understanding of the mechanism of luminescence has been achieved and researchers realized that the light of luminescent materials arises from the deactivation of the excited states of the luminescent materials that absorb light energy. Since then, luminescence principles and laws were built up gradually, and with the development of techniques, luminescent materials were applied in life science, medical research, optoelectronics, and so on. Furthermore, the Nobel Prize of chemistry in 2008 and in 2014, was awarded to the discovery of green fluorescent protein and super-resolved fluorescence microscopy, respectively.

Most of the traditional luminophores show strong emission in solution, but weakly or even no emission in the aggregate and solid state, which was identified as aggregation-caused quenching (ACQ). Interestingly, some propeller-shaped molecules were found to be weakly emissive or non-emissive in the dilute solutions but highly emissive in the aggregate state in 2001. This effect was called aggregated-induced emission (AIE), an opposite photophysical property of ACQ. Not only does such discovery bring fundamental breakthrough, but also inspire immeasurable potential in applications. Followed by previous research of AIE, a series of mechanistic works and applications were carried out during my PhD research.

To date, AIE luminogens (AlEgens) have been widely utilized in cell imaging, cancer theranostics, organic optoelectronic devices (OLEDs), sensors, etc. However, the applications in forensic science are rare, even though AlEgens showed great potential. Hence, we first explored a powder dusting method based on AlEgens for latent fingerprint development. The method is applicable to latent fingerprints on both porous and non-porous materials, displaying excellent performance. Second, we demonstrated a facile approach for invisible bloodstain visualization based on the Click reaction between serum albumin and TPE-MI, an AIEgen. Compared to the widely adopted methods based on the harsh catalytic oxidation activity of hemoglobin, this working principle benefits from the specificity of the mild catalyst-free thiol-ene Click reaction that improves the reliability and resolution.

In addition to the traditional AIEgens with extended 2-conjugation, we designed a series of nonaromatic and nonconjugated small molecules with blue emission. This unusual phenomenon was coined as clusteroluminescence and the luminescence is attributed to intramolecular and intermolecular through-space interactions. We started with three nonaromatic and nonconjugated compounds as the model system to understand the origin of clusteroluminescence. Their photophysical property was well investigated and theoretical calculation was conducted for mechanism explanation.

Conventional clusteroluminescence comes from the through-space interaction between close-shell structures. This is because organic radicals with open-shell structures are non-luminescent, due to the favourable non-radiative decay after excitation. These radicals can function as luminescence quenchers even when they are attached to chromophores. Hence, the phenomenon of luminescent radical is rare and unusual. As yet, all reported luminescent radicals are categorized as π-radicals, featured with the presence of aromatic rings and extended conjugation, most of which also show the characteristic of ACQ. In the following-up work, the concept of nonconjugated and nonaromatic luminescent radical was demonstrated for the first time. The structure is distinctive from the common triphenylmethyl radical derivatives. Moreover, it presented the feature of aggregation-induced emission (AIE) and the luminescence comes from aggregation and molecular interactions, instead of single molecules.

The mechanism study was also carried out through theoretical calculation and experiments.

Conventional clusteroluminescence comes from the through-space interaction between close-shell structures. This is because organic radicals with open-shell structures are non-luminescent, due to the favourable non-radiative decay after excitation. These radicals can function as luminescence quenchers even when they are attached to chromophores. Hence, the phenomenon of luminescent radical is rare and unusual. As yet, all reported luminescent radicals are categorized as a-radicals, featured with the presence of aromatic rings and extended conjugation, most of which also show the characteristic of ACQ. In the following-up work, the concept of nonconjugated and nonaromatic luminescent radical was demonstrated for the first time. The structure is distinctive from the common triphenylmethyl radical derivatives. Moreover, it presented the feature of aggregation-induced emission (AIE) and the luminescence comes from aggregation and molecular interactions, instead of single molecules.

The mechanism study was also carried out through theoretical calculation and experiments.