The excited state plays a crucial role in the mechanism of photochemical reactions in organic compounds. When a molecule absorbs light, it transitions from its ground state to an excited state. In this excited state, the molecule has higher energy and a different electronic configuration, which can lead to various chemical reactions that are not possible or have a low probability of occurring in the ground state.The impact of the excited state on the selectivity and efficiency of photochemical reactions can be explained as follows:1. Reactivity: In the excited state, molecules have higher energy and are more reactive, allowing them to undergo reactions that are otherwise energetically unfavorable in the ground state. This increased reactivity can lead to the formation of new products or intermediates, which can further react to form the final products.2. Selectivity: The excited state can alter the selectivity of a reaction by changing the electronic configuration of the molecule. This can lead to the preferential formation of certain products over others. For example, in the excited state, a molecule may undergo a reaction at a specific site that is not reactive in the ground state, leading to the formation of a specific product.3. Reaction pathways: The excited state can open up new reaction pathways that are not accessible in the ground state. This can lead to the formation of different products or intermediates, which can further impact the selectivity and efficiency of the reaction.4. Efficiency: The efficiency of a photochemical reaction depends on the ability of the molecule to absorb light and efficiently convert the absorbed energy into chemical energy. The excited state plays a crucial role in this process, as it determines the absorption properties of the molecule and the subsequent energy transfer processes.In summary, the excited state in photochemical reactions of organic compounds plays a vital role in determining the reactivity, selectivity, and efficiency of the reaction. By altering the electronic configuration and energy of the molecule, the excited state can lead to the formation of new products, intermediates, and reaction pathways that are not possible in the ground state.