Exploration of the Excited States of Organic Molecules and Metal Complexes Using Ultrafast Laser Spectroscopy

The excited states of organic molecules and inorganic complexes were examined using ultrafast time resolved spectroscopies. Femtosecond transient absorption was used to unveil the early-time dynamics of organic molecules and inorganic complexes. Picosecond time correlated single photon counting was used to observe emission lifetimes. Nanosecond transient absorption spectroscopy was used to observe the molecules and complexes on the nanosecond to microsecond time scale. The nature of the lowest excited symmetry state was studied in 1,4-diphenyl-1,3-butadiene and 1,1,4,4-tetraphenyl-1,3-butadiene and was determined to have mixed character that was highly dependent on solvent. The inorganic complexes studied here have application as photodynamic therapy (PDT) agents. [Ru(bpy)2(BN)2]2+ complexes, where BN is methylbenzonitrile or methoxybenzonitrile, undergo ligand exchange in water. Upon photoaquation, the complexes may bind to DNA and cause cell death. The process of ligand exchange was studied on the femtosecond time scale. It was found that ligand exchange occurs in concert with intersystem crossing to the triplet state within the time duration of the laser pulse (300 fs). Finally, the PDT agents [Ru(bpy)2(dppn)]2+ and [Os(bpy) 2(dppn)]2+ were studied to unveil the effects of the low-lying LC3pipi* state on the excited-state decay processes. From the initial excited state in [Ru(bpy)2(dppn)]2+ and [Os(bpy)2(dppn)]2+ deactivation to the 3pipi* state and the 3MLCT state occurs within 300 fs.