Strongly Fluorescent Heterocyclic Molecule: Crystallography, 3D Hydrogen-Bonded, Fluorescence Study and QTAIM/TD-DFT/MESP Theoretical Analysis

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.


Introduction
The study of the photochemical and photophysical properties of heterocyclic compounds has received a great deal of attention during the last decade.1,10phenanthroline hydrate is a heterocyclic organic compound, used as a ligand in coordination chemistry; it has been the object of numerous studies, owing to its excellent complexing properties on metal ions.The multitude of applications of this cation motivated large development in synthesis of phenanthroline [1].Various physico-chemical and biochemical techniques including UV/visible, fluorescence and viscometric titration, thermal denaturation and differential pulse voltammetry have been employed for 1,10-phenanthroline complexes to probe the details of DNA binding [2,3].The influence of the presence of nitrogen atoms on the fluorescence spectral maxima of aromatic molecules is our interest, including comparison of the spectra in polar and non-polar solvents.Such studies facilitate to understand the features of chemical bonding in molecules from the topological analysis of electron densities [4] and various electrostatic properties of molecules.These fundamental properties are directly related to the properties of materials [5].The present compound, viz.{(C 12 N 2 H 8 )ÁH 2 O}, has been characterized with X-ray crystallography, and we are interested in fluorescence properties.The fluorescence spectra of the present compound have been obtained in various conditions.The time-dependent density functional theory (TD-DFT)/quantum theory of atoms in molecules (QTAIM)/orbital analysis and molecular electrostatic potential (MESP) theoretical calculations have been applied too.

Structural commentary
The structure of {(C 12 N 2 H 8 )ÁH 2 O} is trigonal; space group P31, the molecular structure, is shown in Figure 1.The asymmetric unit is formed from three molecules of phenanthroline and three molecules of water.The crystal structure is built of successive rings formed by six molecules of phenanthroline; the centre of these

2
Electron Crystallography rings are the three molecules of water.Bond lengths and valence angles compare well with the average values from related phenanthroline structures [6,7].

Supramolecular features
The crystal structure is built of successive rings of phenanthroline and water molecules extending parallel to (001) plane that are connected by an extensive three-dimensional hydrogen-bonded network of the type O▬H⋯O, O▬H⋯N, C▬H⋯O and C▬H⋯N(Table 1 and Figure 2a).Aqua molecules and N groups are involved in hydrogen bonding and form a three-dimensional network of infinite chains and variable degrees of rings [C3 3 (7), R 2 1 (5), R 2 2 (9), R 5 6 (22) and R 4 6 (15)] which deploy along the two crystallographic axes a and b (Figure 2b and c) [8].

Luminescent properties
Photoluminescence spectra were measured using a Cary Eclipse (Agilent Technologies) fluorescence spectrophotometer in a quartz cell (1.1 cm cross section) equipped with a xenon lamp and a dual monochromator.The measurements in solvents of different polarities were carried out at ambient temperature (298 K) with the slit ex/em = 20/10 nm.The photoluminescence properties of {(C 12 N 2 H 8 )Á H 2 O} in any solution were investigated in the visible region.The emission spectra of phh (10 -4 M) in non-polar solvents, viz.benzene and toluene, show maximum at $420 nm at λ ex = 320 nm (Figure 3).Studies at higher concentrations of the probe were not possible as it is marginally soluble in non-polar solvents.In polar solvents, viz.methanol, ethanol and acetonitrile, the solution exhibit emission maximum at
$440 nm, which resembles with the emission spectrum in non-polar solvents.This also indicates the absence of any aggregates or other conformers in the ground state.
The studied compound was found to be fluorescently active in solution at room  temperature showing an intense blue fluorescence with high fluorescent quantum yield, independent of the excitation wavelength in different solvents.The spectra clearly reveal that the emission originate from an excited state-charge transfer state π-π* because of the presence of N atoms.Thus, the compound phh may be a candidate for blue-light luminescent materials.

Theoretical analysis
All geometry optimizations of the molecular structure of phh were performed within the Amsterdam Density Functional (ADF) software [9], and all of theoretical calculations were carried out by the density functional theory (DFT), using the Perdew, Burke and Ernzerhof's exchange functional along with generalized gradient approximations, exchange and correlation functional GGA (PBE) [10,11], employing the triple-zeta polarized (TZP) basis set.
The geometric optimization has been carried on the molecular structure of phh.The ground state geometry was adapted from the X-ray data.Calculated structural parameters reveal a good agreement with the original X-ray diffraction data, which indicates significant stability of both compounds (Table 2).

TD-DFT absorption spectra
The time-dependent density functional theory calculations were performed on this compound, with the aim to identify the nature of the electronic transitions.The absorption spectra of the studied compound are shown in Figure 1.The computed absorption bands, dominant transitions, characters and oscillator strengths (f) are given in Table 3.
The compound phh that absorbs in the region (λ = 165-280 nm) appears in Figure 4, and it shows three peaks with maxima at λ = 190, 225 and 268.Table 3 clearly indicates that the lowest energy and the intense peak is at λ = 268 nm (E = 4.62 eV), transition character as corresponding it results from a transition HÀ2 to L+1.From Figure 4 we observe two absorption peaks positioned at λ = 225 and 190 nm; the absorption bands correspond to the HÀ1 to L+3 and HÀ3 to L+2, respectively; and these transitions are affected to a π-π* transition.

AIM topological analysis
Bader and Essen have shown that that the negative value of ∇ 2 (ρ) indicates that there is an electronic charge concentration at the BCP, which implies that the nature of the bond is covalent.For intermolecular interactions, is less weak and ∇ > 0, which implies the presence of a hydrogen-bonding interaction [16,17].
The atoms in molecules (AIM) theory for part B (C1B to C12B/O1W) was applied to analyze the electron densities and their properties; details obtained from this approach are shown in Table 4. Figure 5 shows the existence of a bond critical point between all of each two bonded atoms in part B.
We notice from Table 4, the values ρ > 0.2 and Δ < 0 indicate that covalent bonds are present.And for the bond between N1B and H11W, we notice that ρ = 0.034 is very weak and Δ = 0.091 > 0 (see Table 4) which indicate the presence of a strong hydrogen-bonding interaction.These results are in good agreement with those obtained by experimental refinement.

Orbital analysis and molecular electrostatic potential
The energy diagram of the molecular orbitals obtained by DFT approach for part B of compound phh is shown in Figure 6.
The HOMO and LUMO are important orbitals in a molecule; the gap between the occupied and vacant orbitals (HOMO/LUMO) is remarkable in the compound (2.92 eV; see Figure 6) and predicts its stability.
The highest occupied molecular orbital (HOMO) is mainly delocalized on the oxygen atom (93%), while it is found that the carbons of the cycles contribute strongly in the composition of the LUMO orbital with a participation rate of 88%.
The molecular electrostatic potential is a very important element for the illustration and visualization of the charge region variability and allows identifying the electrophilic and nucleophilic attack sites as well as the hydrogen-bonding interactions [18][19][20][21].
The blue region represents positive MESP, and the red region represents negative ESP, while the green region refers to the neutral region.From the MESP surface diagram, it can be seen that negative regions (red) are mainly localized over the oxygen and the nitrogen atom, while the positive regions (blue) are distributed over the H atoms. Figure 7 shows the presence of a hydrogen-bonding interaction between N1B and H11W.

Crystallization
On a Perkin-Elmer spectrometer, infrared spectra were recorded at room temperature in an interval of 500-4000 cm À1 .
The compound was crystallized by dissolving the 1-10 phenanthroline hydrate (1 g) in mixture solution of ethanol-water (V/V = 1:1).The solution was preserved at room temperature under agitation during 4 h.After a slow evaporation in the interior of 2 days, transparent prism crystals were obtained.H atoms) and refined in riding mode with U iso (H) = 1.2U eq (C).The O-bound H atoms were located in a Fourier map and refined with O▬H restraint of 0.85 (1)Å (U iso (H) = 1.5U eq (O)).

Conclusion
The experimental and theoretical charge-density analysis has been performed to understand the topological and electrostatic properties of the 1-10 phenanthroline hydrate molecule.The crystal structure reveals the molecule forming O▬H⋯O, O▬H⋯N, C▬H⋯O and C▬H⋯N types of interactions with the neighboring molecules.Furthermore, photoluminescence studies indicate that the hpp is fluorescently active in solution at room temperature and reveal that the emission originate from an excited state-charge transfer state π-π* which is proven by TD-DFT analysis.
The AIM analysis shows the existence of a bond critical point (BCP) between N1B and H11W.We notice that ρ = 0.034 is very weak and Δ = 0.091 > 0 which confirms that this interaction is the strongest interaction in the molecule (hydrogen-bonding interaction).These results are in good agreement with those obtained by experimental refinement.
The structural, topological and electrostatic properties of the 1-10 phenanthroline hydrate molecule obtained from the X-ray diffraction method may be useful to design a new candidate of blue-light luminescent materials.

Figure 1 .
Figure 1.View of the hpp molecule, with the atom-numbering scheme.Displacement ellipsoids are drawn at the 50% probability level, and H atoms are shown as circles of arbitrary radii.

Figure 2 .
Figure 2. (a) Partial view of the packing in the phh, showing the O▬H⋯O, O▬H⋯N, C▬H⋯O and C▬H⋯N hydrogen bonds.(b) Partial view of the packing in the phh, showing the formation of the infinite chain C 3 3 (7).(c) Graph set R 2 1 (5), R 2 2 (9), R 5 6 (22) and R 4 6 (15) motifs.Atoms are represented as circles of different radii.Hydrogen bonds are shown as dashed lines.

Figure 3 .
Figure 3.Fluorescence spectra of phh in polar and non-polar solvent.

N1A▬C1A
The calculated optical transition energies and their corresponding oscillator strengths (f) for phh.

Figure 6 .
Figure 6.Frontier molecular orbital diagram of part B.

Figure 7 .
Figure 7. MEP surface diagram of part B.

Table 4 .
Results of atoms in molecule approach for part B of phh molecule.