In this review, we address the optical signatures of defects in two-dimensional transition metal dichalcogenides (2D TMDs), whether they occur unintentionally during growth or are deliberately introduced post-growth. We detail their primary responses as probed by photoluminescence (PL), magneto-PL, Raman, tip-enhanced PL and Raman, and nonlinear spectroscopies. Defects significantly impact the electronic, vibrational, magneto-optical, and nonlinear properties of TMDs, influencing outcomes based on application needs. This comprehensive overview highlights the distinctive optical fingerprints of various defects, providing guidance for their identification and characterization. Additionally, we discuss new optical phenomena induced by defects in TMD monolayers and future challenges in defect engineering for these materials. The insights from this review underscore the potential of TMDs for technological applications, with advancements in spectroscopy and defect engineering driving future innovations and enhancing our understanding of these materials.

The photophysical properties of two difluoroboron flavanone β-diketonates (DK1 and DK2) and their interaction with fullerene (C60) in toluene solution and spin-coated films were investigated using time-correlated single-photon counting, absorption spectroscopy, and steady-state fluorescence spectroscopy. In molecular crystal, the complexes exhibited red-shifted absorption and emission relative to their spectra in solution. Additionally, both complexes displayed bi-exponential decay behavior in time-resolved fluorescence measurements, indicating their capability to form both H and J types of aggregates in the solid state. The introduction of C60 resulted in significant fluorescence quenching and reduced excited-state lifetimes for both complexes. This quenching, observed in both solution and spin-coated films, was primarily driven by photo-induced electron transfer (PET) processes, underscoring the potential of these complexes as donors in fullerene-based heterojunction organic solar cells. To elucidate the process of aggregate formation and the impacts of different dimerization types within the crystalline structure of the complexes, first-principles calculations using Density Functional Theory (DFT) and time-dependent density functional theory (TD-DFT) were performed. We also employed DFT to explore various DK configurations on the fullerene surface, evaluating intermolecular distances and formation energies. These calculations highlighted the energetically favorable gap between the low-lying LUMO levels of the complexes and C60, confirming their suitability for such applications.