**5. Conclusion**

they behave as an insulator, while at higher temperature the semiconducting nature

**Complex Ea1 (eV) Ea2 (eV) Egd (eV)**

] (**2**) 0.48 1.18 3.45

)2].2H2O (**8**) 0.59 4.2 2.52

)(OAc)] (**9**) 0.54 — 2.75

)(OAc)].H2O (**10**) 0.97 0.76 2.37

)(OAc)].H2O (**11**) 0.34 2.14 1.58

**(Lower temp) (Higher temp)**

Optical absorption spectra was taken by using a UV–VIS spectrophotometer (Perkin Elmer Lambda 2S/45 Double Beam) and measured as function of wave-

The energy band gaps and the nature of the optical transitions involved in the metal complex framework systems have been practically determined by the fundamental absorption edge analysis of the recorded optical transitions using the theory of Mott and Davis [39]. It is also observed that the semiconducting behavior of a material increases with rise in temperature which may also damage the actual molecular structure of the material. Hence, Tauc method is used to calculate the

Utilizing the relation between the optical linear absorption coefficient (α) with photon energy (hν), the energy band gap (Eg) between the top of the valence band and bottom of the conduction band can be determined using equation (Eq. (2)):

<sup>n</sup> (2)

<sup>1</sup>*=*<sup>2</sup> (3)

<sup>2</sup> (4)

1/2 against f(hν) and then by the extrapolation of the

αhν ¼ A hν � Eg

where A is a constant characteristic parameter of the respective transition

αhν ¼ Ad hν � Egd

αhν ¼ Ai hν � Egi

To calculate the direct and indirect energy band gap, we need to plot a curve of

The satisfactory graphs were obtained for the metal complexes (**2**) and (**8**)–(**11**)

<sup>2</sup> against f(hν). Therefore, the energy gaps determined correspond

where Egd and Egi are direct and indirect energy gaps, respectively.

The values of n depend on the kind of optical transitions. For directly allowed, directly forbidden, indirectly allowed, and indirectly forbidden transitions, the values of n are ½, 3/2, 2, and 3, respectively. Thus the energy band gap for directly allowed (Egd) and indirectly allowed (Egi) transitions can be determined by

of complexes is observed.

*Activation energies and direct band gap values.*

*Stability and Applications of Coordination Compounds*

[VOL<sup>2</sup>

[Zn(L3

[Ni(L<sup>3</sup>

[Co(L<sup>3</sup>

[Cu(L3

**Table 3.**

**4.5 Optical properties**

independent of ν.

and

(αhν)

**16**

by plotting (αhν)

relating Eq. (2) as follows:

<sup>2</sup> against f(hν) and (αhν)

most linear part of the curve to zero.

length in the wavelength range 190–1100 nm.

energy band gap through optical absorption properties [40].

In this review, the synthesis, crystal structure, and solid-state properties of three Schiff base ligands derived from diacetylmonoxime with diethylenetriamine, 1,3 diaminopropane-2-ol, and morpholine N-thiohydrazide and their metal complexes have been vividly discussed. A zwitterionic nitrogen–sulfur heterocyclic compound with nonbonded SS interaction has also been reported to be formed by the reaction of diacetylmonoxime with morpholine N-thiohydrazide under long refluxing (16 h) condition in ethanol. The single X-ray crystal structures have shown many beautiful weak force interactions including a CH3O trifurcated interface communication. Wherever the single-crystal structures could not be grown, the PXRD study has enlightened their structural features. The electrical and optical properties also explored the semiconducting nature of some of the metal complexes. It is also observed that the electron transport process gets influenced by the supramolecular frameworks of the metal complexes.
