Structural and Solvent Effects on Excited State Dynamics of Rhenium(I) Tricarbonyl α-Diimine Complexes: A Time-Resolved Spectroscopic and Computational Study

Abstract

The nature of the lowest excited state in rhenium(I) complex with 1,10-phenathroline as ligand was first reported by Mark Wrighton and David. L. Morse. Since then many rhenium(I) complexes with different ligands having α-diimine (N,N) functionality have been extensively studied. The present work deals with understanding of the structural variations and solvent effect on the excited state properties and dynamics in few rhenium(I) tricarbonyl α-diimine (fac-[Re(N,N)(CO)3L]0/+) complexes. The fac-[Re(N,N)(CO)3L]0/+ complexes possess different excited states like metal-to-ligand charge transfer (MLCT), metal-ligand-to-ligand charge transfer (MLLCT), ligand-to-ligand charge transfer (LLCT) or intraligand (IL) states which are energetically close to each other. These states can be tuned by structural variations in the ligands or polarity of the surrounding solvent medium. The complexes reported in this thesis were studied in detail by resonance Raman (RR), TCSPC, time-resolved absorption (TRA) and time-resolved infrared spectroscopy (TRIR), and computational methods. In one of the chapters, the differences in the excited state properties of fac-[Re(bpy)(CO)3(3ampy)](PF)6 (Re3ampy) and fac-[Re(bpy)(CO)3(4ampy)](PF)6 (Re4ampy) complexes as a function of subtle structural variation of the axial ligand and solvent polarity has been studied. It was hypothesized that, by using the isomers of aminopyridine as axial ligand, the excited state decay mechanism in Re3ampy and Re4ampy were altered. In the next chapter, the influence of hydrogen bonding on fac-[Re(phendione)(CO)3Br] (RepdBr) complex was studied. The RR spectroscopic results showed differences in the excited state structures in ACN and MeOH solvents, upon photoexcitation. The lowest triplet states were found to possess 3MLCT and IL 3ππ* character in ACN and MeOH, respectively. Finally, the excited state dynamics of two rhenium(I) Schiff base complexes – fac-[Re(pyan)(CO)3Br] (Repyan) and fac-[Re(pydian)(CO)3Br] (Repydian) – were explored. It was observed from RR spectroscopy that an MLCT(pyridine) state was formed upon photoexcitation in both the complexes. The TRIR results revealed that the decay mechanisms of the complexes were different due to the hindrance to rotation of ligand in the excited state of Repydian in comparison to Repyan complex. Overall, a comprehensive study of different Rhenium(I) tricarbonyl α-diimine complexes was carried out using a host of different spectroscopic techniques and it was found that structural variations in the ancillary and α-diimine ligands, and the solvent environment affects the excited state dynamics of these complexes.