Modifying Vibrational Energy Flow in Aromatic Molecules: Effects of Ortho Substitution

Abstract

Ultrafast infrared (IR) Raman spectroscopy was used to measure vibrational energy transfer between nitrobenzene nitro and phenyl groups, in the liquid state at ambient temperature, when ortho substituents (-CH3, -F) were introduced. Quantum chemical calculations were used to assign the vibrations of these molecules to three classes, phenyl, nitro, or global. Combining transient anti-Stokes and Stokes Raman spectra determined the energies of multiple molecular vibrational modes, which were summed to determine the aggregate energies in the phenyl, nitro, or global modes. In a previous study (Pein, B. C.; Sun, Y.; Dlott, D. D., J. Phys. Chem. A 2013, 117, 6066-6072) it was shown that, in nitrobenzene, there was no energy transfer from nitro to phenyl or from nitro to global modes, but there was some transfer from phenyl to nitro and phenyl to global. The ortho substituents activated energy flow from nitro-to-phenyl and nitro-to-global and reduced phenyl-to-nitro flow. The -CH3 substituent entirely shut down the phenyl-to-nitro pathway, presumably by efficiently directing some of the phenyl energy into methyl bending excitations. There is (inefficient) unidirectional vibrational energy flow in nitrobenzene only in the nitro-to-phenyl direction, whereas in o-nitrotoluene, vibrational energy flows only in the nitro-to-phenyl direction.

Two-level system in spin baths: Non-adiabatic dynamics and heat transport

“…It transports heat more effectively in one direction of the temperature bias than in the reversed direction [41]. This effect has been investigated in detail experimentally and theoretically at the macro-micro scale [42,43], the nanoscale [44], and more recently in the molecular realm [45][46][47], with proposals relying on phononic, electronic, and photonic heat conduction [41]. It was also recently demonstrated that Josephson tunnel junctions (Cooper-pair condensates electrodes) can act as strong phase-tunable thermal diodes [48].…”

Quantum bottlenecks and unidirectional energy flow in molecules

“…We have carried out this analysis on one of the substituted benzenes, toluene, that was the subject of recent ultrafast studies by Dlott and coworkers [43][44][45], which reveal a preference in some substituted benzenes for energy to flow along one direction. We have shown that such vibrational energy rectification can arise from quantum bottlenecks to energy relaxation from the initially excited state, in the case of toluene from the CH fundamental of the phenyl group, which generates a preference for energy to flow in the methyl-to-phenyl direction during the time of relaxation into the surroundings.…”

“…Because it is inevitably somewhat arbitrary we avoid classifying modes as local or global, as done to describe the modes of the substituted benzenes in which energy flow was monitored experimentally [43][44][45]. To address the possibility of asymmetric energy flow we instead follow the flow of energy into the environment from each region of the molecule, following local excitation by about 3000 cm −1 in each region.…”

“…Recently, Dlott and coworkers carried out timeresolved IR-Raman studies on a series of substituted benzenes in liquids [43][44][45]. They examined point-topoint energy transport in these molecules and observed an interesting asymmetry: Breaking up the molecule, such as nitrobenzene, into two regions, phenyl and substituent, they found that by exciting one region, say phenyl, with a 3000 cm −1 IR pulse the excess energy flows into the other region more easily than energy flows into the phenyl if the substituent is initially excited with a similar amount of energy, at least over times long enough for the energy to relax into the liquid (ca.…”

“…This effect may play a role in the quasi-unidirectional flow observed in some substituted benzenes, since even though they are in liquids the rate of intramolecular energy flow may be quite slow despite coupling to the surroundings [50]. However, energy flow to other vibrational states is observed within the relatively short time of relaxation to the surroundings [43][44][45]. Even without quantum localization, bottlenecks due to a low local density of states, i.e., a sparsity of vibrational states resonantly coupled to the initially excited state, or to the first few tiers of states into which energy flows at early times, can give rise, prior to energy relaxation into the liquid, to a preference for energy to flow in one direction along the molecule.…”