Anharmonically Coupled Vibrational Scaffolds for Energy Dissipation in a Terpyridine-Aldehyde Ligand

The role of intramolecular vibrational energy redistribution is critical when considering the thermal properties of coordination complexes. The interactions and redistribution of vibrational energy can be uncovered through the use of two-dimensional infrared (2D IR) spectroscopy by observing the cross-peaks between different vibrational signatures. In this study, the energetic landscape consisting of the vibrations of [6,2':6',2″-terpyridine]-2-carbaldehyde (terpyCHO) is studied directly in the region between 1400 and 1740 cm^-1. Information on the anharmonic coupling between different groups of vibrations estimates the delocalization in the detected normal modes. The delocalization of the aldehyde stretch is directly comparable to that calculated from density functional theory (DFT) calculations and local mode analysis via the LModeA program, a freeware that can characterize the normal modes in terms of a nonredundant set of local modes. Tracking of the relative cross-peak intensities as a function of the waiting time reveals a complex network of energy exchange pathways responsible for energy dissipation and vibrational dephasing. These dynamics reveal the aldehyde to be an efficient inlet and outlet of vibrational energy flow and the C-H bends as energetic dumps with the pyridine ring stretches acting as bridging modes between them. Finally, signatures of vibrational coherences are observed in the diagonal peak dynamics of the 2D IR data, further revealing a network of couplings to lower energy modes that mediate this energy exchange. These findings provide new insights into the energetic landscape of organic molecules potentially useful as ligands in single molecule magnets.