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Optically excited heme proteins release their energy into the hydration water in approximately 7 ps, subsequent to internal vibrational redistribution. Adapted from an image created by S.Duce. 

Elucidating the nature of intramolecular vibrational energy redistribution (IVR) can guide the design of molecular wires. The ability to steer these processes through a mechanistic understanding of IVR is assessed by utilizing two-dimensional infrared (2D IR) spectroscopy. 2D IR spectroscopy allows for the direct investigation of timescales of energy transfer within three aromatic molecular scaffolds: 4′-azido-[1,1′-biphenyl]-4-carbonitrile (PAB), 2′-azido-[1,1′-biphenyl]-4-carbonitrile (OAB), and 4′-(azidomethyl)-[1,1′-biphenyl]-4-carbonitrile (PAMB). Energy transfer pathways between azido (N3)- and cyano (CN)-vibrational reporters uncover the importance of Fermi resonances, anharmonic coupling, and specific structural components in directing energy flow. Among these systems, PAB exhibits the fastest energy transfer (22 ps), facilitated by its co-planar biphenyl structure, enabling strong π–π stacking interactions to optimize vibrational coupling. In contrast, OAB demonstrates a moderate IVR timescale (38 ps) due to an orthogonal molecular plane and steric hindrance, which disrupts coupling pathways. PAMB, with a para-methylene group, introduces a structural bottleneck that significantly impedes energy flow, slowing down the energy transfer to 84 ps. The observed IVR rates align with computational predictions, highlighting intermediate ring modes in PAB as efficient energy transfer bridges, a mechanism that is less pronounced in OAB and PAMB. This study demonstrates that IVR is dictated not only by anharmonic coupling strengths but also by the extended alignment of vibrational modes across molecular planes and their delocalization within aromatic scaffolds. By modulating structural features, such as steric constraints and π–π interactions, we provide a framework for tailoring energy flow in conjugated molecular systems. These findings offer new insights into IVR dynamics for applications in molecular electronics. 

Determining rates of energy transfer across non-covalent contacts for different states of a protein can provide information about dynamic and associated entropy changes during transitions between states. We investigate the relationship between rates of energy transfer across polar and nonpolar contacts and contact dynamics for the β 2 -adrenergic receptor, a rhodopsin-like G-protein coupled receptor, in an antagonist-bound inactive state and agonist-bound active state. From structures sampled during molecular dynamics (MD) simulations, we find the active state to have, on average, a lower packing density, corresponding to generally more flexibility and greater entropy than the inactive state. Energy exchange networks (EENs) are computed for the inactive and active states from the results of the MD simulations. From the EENs, changes in the rates of energy transfer across polar and nonpolar contacts are found for contacts that remain largely intact during activation. Change in dynamics of the contact, and entropy associated with the dynamics, can be estimated from the change in rates of energy transfer across the contacts. Measurement of change in the rates of energy transfer before and after the transition between states thereby provides information about dynamic contributions to activation and allostery.