**Abstract**

In this chapter we explored the fluorescence properties of the title compound 1–10 phenanthroline hydrate (phh), {(C12N2H8)H2O}. The structure of phh is stabilized by strong as well as weak intermolecular interactions in the crystal. These interactions O▬H⋯O, O▬H⋯N, C▬H⋯O and C▬H⋯N hold the crystal structure in a three-dimensional network. Optical analysis (fluorescence) was performed on the test compound. The measurements in solvents of different polarities were carried out at ambient temperature (298 K). These results prompted us to investigate some photoluminescence applications for heterocyclic compounds as the sensing of blue-light luminescent materials. The time-dependent density functional theory (TD-DFT) calculations were performed on this compound, with the purpose to identify the origin of absorption and emission band, the nature of the electronic transitions. The atoms in molecules (AIM) theory and orbital analysis and molecular electrostatic potential (MESP) were applied to analyze the electron densities, their properties and the energy diagram of the molecular orbitals. The AIM and MESP analysis have been applied for part B of phh to demonstrate that the O1W▬H11W⋯N1B type of interaction has the strongest hydrogen bond.

**Keywords:** aromatic molecule, X-ray diffraction, fluorescence, QTAIM/TD-DFT/MESP theoretical analysis
