Interaction of light with matter forms a foundation to provide a comprehensive understanding on the electronic properties and reactivity of thermalized chemical system. Our research focus on Density Functional Theory (DFT) and time-dependent Density Functional Theory (TDDFT) to explore the fundamentals of vibrational & electronic excited states and transition states; to further investigate the molecular reactivity. Besides, we also perform ab initio molecular dynamics (AIMD) calculations to model the dynamic evolution of gas phase ion chemistry and then evaluate the solvation structure, frequency domain spectra such (IR spectra, UV spectra, vibrational spectra) which provide insights toward the chemical reactivity. In combination with experimental investigations, eventually, we describe the reaction dynamics such as water splitting, electron transfer, radical isomerization, thermally assisted delayed fluorescence (TADF) and other exciting phenomena.