Photophysics and photochemistry of diiodomethane and hexabromoiridate - paradigm molecules for organic and inorganic chemistry - studied with sub-50-fs broadband pump-probe spectroscopy

The present work is the detailed study of photophysical and photochemical processes in molecules by means of femtosecond (fs) time-resolved transient absorption spectroscopy both in the gas and solution phases. This knowledge is important for an understanding of many light-driven mechanisms in photosensitive systems – building blocks of light-triggered molecular devices. Many light-driven reactions occur on a time scale comparable to that of vibration motion, and therefore, can only be “seen” by using advanced methods such as the fs transient absorption. This method provides ultrahigh temporal resolution on a vibrational period timescale, which is superior to other time-resolved methods and, importantly, can be utilized to study the dynamics of molecular systems. Specifically, ultrafast transient absorption spectroscopy was used to investigate radiationless relaxation dynamics of two paradigm transition metal complexes in octahedral (IrBr62 ) and tetrahedral (CuCl42-) environment. Following excitation at 2000-nm, both systems undergo internal conversion to the ground electronic state with significant difference in lifetimes (55-fs and 360 -ps for CuCl42- and IrBr62-, respectively). The difference was explained by the presence of conical intersection between the excited and ground electronic states in the Cu2+ system due to strong Jahn Teller effect and the strong spin-orbit coupling in the Ir4+ complex.Upon visible irradiation, IrBr62- was found to undergo cascade-like relaxation through the long-lived (360 ps) metal-centered 2Ug\u27(T2g) excited state. Transient absorption studies upon 330 nm excitation evidence ultrafast intersystem crossing in the initially-excited state to form an excited state in quartet multiplicity. This state decays via a series of internal conversion steps into the lowest-energy quartet excited state. We found that the electron transfer within the t2g metal orbital is found to be the rate-limited step for non-radiative relaxation of IrBr62-. For the first time, the photochemical reaction pathway was documented in the UV photochemistry of CH2I2 molecule. It was shown that CH2I2 excited into the S1 state undergoes direct ~50-fs isomerization through a conical intersection forming the isomeric species with the iodine-iodine bond. Methodologically, this work demonstrated the synergy between fs transient absorption spectroscopy and surface hopping dynamics simulations in revealing the evolution of complicated ultrafast photochemical reactions in small polyatomic molecules