4.2. Electronic spectra
The TD-DFT method has become one of the most popular and widely used approaches for the calculation of properties, such as excitation energies, oscillator strengths and excited state geometries of medium to large molecular systems [56]. TD-DFT has been widely used to find out the low lying excited states of molecules on the basis of fully optimized ground-state structure [57]. The vertical excitation energies, absorption wavelengths and oscillator strengths (f) of TMZ molecule along with their assignments are given in Table 1. The pictorial representation of HOMO‒LUMO orbitals and energy gap of TMZ are shown in Fig. 2 and the density of state spectrum is shown in Fig. 2S. ( supplementary data). The HOMO–LUMO energy gap characterizes the chemical activity, optical polarizibility and chemical hardness‒softness of the molecule [58]. The frontier molecular orbital energy gap is found to be 4.45 eV. The energy gap of (HOMO‒1)–(LUMO+1) has been calculated to be 5.69 eV. A large HOMO–LUMO energy gap is an indication of high stability of TMZ. It can be seen from the HOMO–LUMO plots that HOMO is spread over the entire molecule except the NH2 group. The LUMO is also spread over the entire molecule except CH3 group. The experimental and simulated electronic spectra of TMZ are shown in Fig. 3. The TD-DFT calculations in ethanol solution show three intense bands at 315, 252 and 207 nm having the oscillator strengths as 0.334, 0.1077 and 0.2545 respectively. These absorptions are found to be in good agreement with the experiment values. The maximum absorption wavelength, 317 nm, corresponds to HOMO→LUMO(94%) electronic transition. The absorption wavelengths, 245 and 206 nm, are assigned to HOMO→LUMO+1(83%) and HOMO→LUMO+2(50%) transitions respectively. The assignments of other transitions have also been shown in Table 1.