Adding a substituent to a molecule can affect its excitation energy in various ways, depending on the nature of the substituent and its position on the molecule. Excitation energy is the energy required to promote a molecule from its ground state to an excited state. The effect of a substituent on excitation energy can be analyzed using quantum chemical calculations.To compare the excitation energies of a set of closely related molecules, one with a substituent and one without, we can use computational chemistry software like Gaussian or ORCA to perform quantum chemical calculations. These calculations involve solving the Schrödinger equation for the molecules and obtaining their electronic structure.Here's a general outline of the process:1. Choose a set of closely related molecules: For example, consider benzene C6H6 as the parent molecule and toluene C6H5CH3 as the molecule with a substituent methyl group .2. Perform geometry optimization: Use a computational chemistry software to optimize the geometry of both molecules. This step ensures that the molecules are in their lowest energy conformation.3. Choose an appropriate method and basis set: Select a suitable quantum chemical method e.g., Time-Dependent Density Functional Theory or TD-DFT and basis set e.g., 6-31G d,p for the calculations. The choice of method and basis set will depend on the size of the molecules and the desired accuracy of the results.4. Calculate excitation energies: Perform the quantum chemical calculations to obtain the excitation energies of the molecules. The software will provide a list of excited states and their corresponding excitation energies.5. Compare the excitation energies: Analyze the results to determine the effect of the substituent on the excitation energies of the molecules. For example, if the excitation energy of toluene is lower than that of benzene, it indicates that the presence of the methyl group has stabilized the excited state, making it easier to excite the molecule.The effect of the substituent on the excitation energy can be attributed to various factors, such as electron-donating or electron-withdrawing properties, steric effects, and conjugation. Understanding these effects can help in designing molecules with specific photophysical properties, such as fluorescence or photochromism, which are important in various applications like sensors, imaging, and photovoltaics.