The electronic properties of a molecule or material play a crucial role in determining its behavior under light irradiation. When light interacts with a molecule or material, it can be absorbed, reflected, or transmitted. The absorption of light can lead to various photochemical reactions and photophysical processes, which are governed by the electronic properties of the system.1. Absorption of light: When a molecule or material absorbs light, its electrons can be excited to higher energy levels. The probability of this transition depends on the electronic structure of the molecule and the energy of the incident light. Molecules with conjugated systems or extended -orbitals have a higher probability of absorbing light due to the delocalization of electrons.2. Photochemical reactions: Photochemical reactions occur when the absorbed light energy causes a chemical change in the molecule or material. This can involve bond breaking, bond formation, or electron transfer processes. The electronic properties of the system determine the types of photochemical reactions that can occur and their efficiency. For example, molecules with low-lying excited states are more likely to undergo photochemical reactions, as they can easily access the required energy levels.3. Photophysical processes: Photophysical processes involve the relaxation of the excited state without any chemical change. These processes include fluorescence, phosphorescence, and internal conversion. The electronic properties of the molecule or material, such as the energy gap between the ground and excited states, determine the likelihood of these processes occurring. For instance, a small energy gap can lead to efficient fluorescence, while a large gap can result in phosphorescence.Quantum chemistry is a powerful tool for studying the electronic properties of molecules and materials, as well as the mechanisms underlying photochemical reactions and photophysical processes. By solving the Schrödinger equation for a given system, quantum chemistry can provide information about the energy levels, electron distribution, and transition probabilities. This information can be used to predict the behavior of molecules and materials under light irradiation and to design new materials with desired photochemical and photophysical properties.