Organic nanocrystals are of significant interest due to their enhanced performance in devices and unique functionalities not seen in bulk materials. Controlling the crystallization process and crystal size is crucial for designing nanocrystals with specific properties. However, understanding the crystallization dynamics of organic compounds, particularly in the early stages and polymorphic behavior during growth, remains challenging. Techniques like electron microscopy, IR, and Raman spectroscopy have been employed, but *in situ* real-time observation of the early stages is difficult. Luminescence changes during crystallization offer a powerful *in situ* observation method, but most organic materials suffer from aggregation-caused quenching (ACQ), where aggregation reduces luminescence. Aggregation-induced emission (AIE) materials, which exhibit enhanced luminescence upon aggregation, offer a solution. Some Au(I) complexes, due to aurophilic interactions (non-covalent Au-Au bonds), exhibit AIE activity, increasing luminescence through aggregation. This study hypothesizes that introducing multiple aurophilic interactions can enhance AIE and allow visualization of the crystallization process, with the luminescence properties being sensitive to the aggregate structure. The research focuses on synthesizing three Au complexes with slightly different alkyl side chains to explore the relationship between crystal structure and luminescence properties and subsequently investigate the crystallization process from solution.

The existing literature extensively explores the size-dependent properties of organic nanocrystals and the challenges in understanding organic crystallization processes. While techniques like electron microscopy, IR, and Raman spectroscopy have been used to study crystallization, real-time in situ observation, particularly in the early stages, has been limited. Aggregation-caused quenching (ACQ) is a common issue in organic luminescent molecules, hindering direct visualization of the crystallization process. The emergence of aggregation-induced emission (AIE) materials, which exhibit enhanced luminescence upon aggregation, has opened new avenues for studying crystallization. Previous research on Au(I) complexes has highlighted their AIE properties due to aurophilic interactions. Studies have demonstrated the luminescence sensitivity of Au complexes to aggregate structure. The researchers draw upon this background knowledge to investigate the use of trinuclear Au complexes with modified alkyl chains for in situ visualization of crystallization dynamics.

Three trinuclear Au(I) complexes (DT4-DT6) with varying alkyl chain lengths were synthesized. The photoluminescence of the resulting crystals was observed using UV light (254 nm) at room temperature. The photoluminescence and excitation spectra were measured, and photophysical parameters (maximum RTP wavelength (λmaxlum), quantum yield (Φ), and lifetime (τ)) were determined. Single-crystal X-ray diffraction was performed to determine the crystal structures and analyze intermolecular interactions (Au-Au and Au-π interactions). Time-dependent density functional theory (TD-DFT) calculations were conducted on dimers extracted from the crystal structures to investigate electronic transitions and understand the origin of different RTP colors. Nanocrystals were prepared using a reprecipitation method (THF/water) and their luminescence properties were characterized. The effect of crystal size on RTP was investigated by controlling the crystal size through solvent combinations (CHCl3/methanol). Powder X-ray diffraction (XRD) analysis was used to compare crystal structures of nano- and microcrystals with bulk crystals. To further investigate size-dependent RTP, micro- and nanocrystals were also prepared by ball milling bulk crystals and subsequent filtration to separate crystals by size. Dynamic light scattering was used to determine the crystal size distributions.

The study found that the synthesized trinuclear Au(I) complexes exhibited efficient room-temperature phosphorescence (RTP) in crystalline form, even in air, but the RTP was highly sensitive to the crystal structure. Slight changes in the alkyl side chain length caused significant variations in crystal packing and resulting RTP color (yellow for DT4, purple for DT5, and red for DT6). Single-crystal X-ray analysis revealed different space groups and the formation of supramolecular polymers. Intermolecular interactions (Au-Au and Au-π) were identified and their influence on RTP color was discussed. TD-DFT calculations supported the experimental findings, showing the impact of dimer formation on electronic transitions. Nanocrystals of all complexes exhibited red emission, unlike the bulk crystals, indicating size-dependent crystal structure. The XRD patterns of DT4 and DT5 nanocrystals differed from their bulk counterparts, confirming crystal structure changes. In microcrystals, reversible RTP color changes were observed during crystal growth, indicating phase transitions between polymorphs. The size threshold for this polymorphism was estimated to be around 10 µm. Mechanical pulverization of bulk crystals also induced a size-dependent change in RTP color, supporting the role of crystal size in determining the crystal structure and hence the RTP behavior. The most stable dimer, DT6, showed consistent RTP behavior across different crystal sizes. This was explained by considering the balance between surface and bulk free energy during crystal growth. The lower energy barrier for nucleation of the metastable polymorph allowed this structure to form in the nanocrystals. Subsequent growth led to a phase transition to the more stable polymorph only when crystal size exceeded the threshold of ~10 µm.

The study successfully demonstrated a strong correlation between crystal structure and RTP color in trinuclear Au(I) complexes. The sensitivity of RTP to subtle changes in alkyl chain length and crystal size highlights the importance of considering crystal packing effects in designing luminescent materials. The observation of crystalline polymorphism in DT4 and DT5, with a size-dependent phase transition, provides valuable insights into the early stages of crystallization and the impact of confinement on crystal structure. The findings suggest that materials can exhibit different behavior in nano- to micrometer-sized systems compared to their bulk counterparts, highlighting the importance of considering size effects in device applications. The utilization of RTP as an in situ probe for studying crystallization offers a promising approach for understanding these complex processes.

This research revealed the significant influence of crystal structure and size on the room-temperature phosphorescence of trinuclear Au(I) complexes. The high sensitivity of RTP to subtle structural changes provides a powerful tool for investigating crystallization dynamics. The observed polymorphism in DT4 and DT5 and the size-dependent phase transitions contribute to a deeper understanding of crystallization mechanisms. Future research could explore a wider range of Au(I) complexes with diverse structural modifications, investigate different crystallization methods, and develop advanced in situ characterization techniques to further elucidate crystallization processes at the molecular level.

The study focused on a limited set of Au(I) complexes, potentially limiting the generalizability of the findings to other systems. While the TD-DFT calculations provided valuable insights, they are approximations and might not perfectly capture all aspects of the electronic transitions. The size determination methods employed had inherent uncertainties, potentially affecting the precise determination of the size thresholds for polymorphism.