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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.
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Inhibition of vibrational energy flow within an aromatic scaffold via heavy atom effect
The regulation of intramolecular vibrational energy redistribution (IVR) to influence energy flow within molecular scaffolds provides a way to steer fundamental processes of chemistry, such as chemical reactivity in proteins and design of molecular diodes. Using two-dimensional infrared (2D IR) spectroscopy, changes in the intensity of vibrational cross-peaks are often used to evaluate different energy transfer pathways present in small molecules. Previous 2D IR studies of para-azidobenzonitrile (PAB) demonstrated that several possible energy pathways from the N3 to the cyano-vibrational reporters were modulated by Fermi resonance, followed by energy relaxation into the solvent [Schmitz et al., J. Phys. Chem. A 123, 10571 (2019)]. In this work, the mechanisms of IVR were hindered via the introduction of a heavy atom, selenium, into the molecular scaffold. This effectively eliminated the energy transfer pathway and resulted in the dissipation of the energy into the bath and direct dipole–dipole coupling between the two vibrational reporters. Several structural variations of the aforementioned molecular scaffold were employed to assess how each interrupted the energy transfer pathways, and the evolution of 2D IR cross-peaks was measured to assess the changes in the energy flow. By eliminating the energy transfer pathways through isolation of specific vibrational transitions, through-space vibrational coupling between an azido (N3) and a selenocyanato (SeCN) probe is facilitated and observed for the first time. Thus, the rectification of this molecular circuitry is accomplished through the inhibition of energy flow using heavy atoms to suppress the anharmonic coupling and, instead, favor a vibrational coupling pathway.
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- Award ID(s):
- 2102275
- PAR ID:
- 10436744
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 158
- Issue:
- 22
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0153760
