Sebastian Jezowski Inelastic Neutron Scattering Study of Molecular Vibrations in a Novel Light-Activated Mechanophore The aim of this proposal is a study of molecular vibrations involved in the transformation of the novel photoactive mechanophore pair, BAn (9,9’:10,10’-diethano9,9’:10,10’-bi-9,10-dihydroanthracene) into its photoisomer (BAn2) during illumination as well as during the back reaction (BAn2-to-BAn) with the goal to better understand the dynamics underlying the formation of the intermediate(s) in an intramolecular reaction in a crystal. Compared to already known mechanophores [1], BAn is a fairly simple molecule [2], based solely on carbon and hydrogen atoms (C32H24). When illuminated with ambient light, BAn efficiently undergoes the classic [4+4] cycloaddition reaction in a crystal phase which can then be reversed with high temperature [2] or with deep-UV photon illumination with the release of heat as a byproduct. We have recently demonstrated yet another path for the intramolecular BAn2-BAn back reaction. It turns out that even moderate pressures, on the orders of several kilobars, greatly enhance the BAn2-to-BAn dissociation process. This reversible back-reaction involves carbon-to-carbon covalent bond breaking in BAn2 at two symmetrical positions and the reformation of two anthracenes held face-to-face by two ethylene bridges (BAn). In contrast, similar photochromic dimers [3] are stabilized by high pressure, showing no pressure-induced enhancement of the dissociation rate. This particular light-harvesting (BAn-BAn2) pair of mechanophores, when subjected to several kilobars of pressure, releases up to 90kJ/mol of stored solar energy (DFT-D/ 631/+G(d,p)). The photoinduced cycloaddition and the pressure-induced back reaction can be cycled numerous times without signs of aging in the mechanophore. Systems such as BAn demonstrate enough properties to potentially find applications as both reversible informationand sustainable solar energy storage materials as well as stress sensors or self-healing systems. BAn (ANTMEU03) and BAn2 (ANTMET05) both are monoclinic, with space group P21/c. BAn: a=10.266, b=12.773, c=8.446, ß=112.86°. BAn2: a=9.846, b=12.997, c=8.532, ß=111.89° [4]. Some significant changes in crystal structure have been observed with X-ray diffraction studies at room temperature and ambient pressure during the illumination process. At 40% of the BAn-to-BAn2 isomerization reaction, the cell volume becomes the largest after which it continues to decrease consistently until all BAn turns into its photoproduct. The authors were able to establish that the molecular volume of BAn2 overall decreased by ~ 8 Å3 when compared to BAn. This indicates that the higher pressure favors the formation of a higher-volume product, BAn. It seems that the transition state(s) involved are stabilized by higher pressure but the true reason for this counterintuitive process that we observed is not known at this moment. We have determined the difference in volume of activation (- 7 Å3) through the pressure dependence of the rate of the back reaction. This particular cycloaddition reaction thus must proceed through an even smaller volume intermediate. We have found that the energy barrier height for the BAn2-to-BAn process is significantly lowered at higher pressures, ranging from around 80 kJ/mol at ambient pressure to around 25 kJ/mol at 9 kbar. Figure below shows the kinetics of the pressure-induced covalent C-C bond cleavage in BAn2 (studied by optical spectroscopy at room temperature) being enhanced by pressure (here up to 11.2 kbar). 1 Experimental details The forward reaction (BAn-to-BAn2) will be initiated with light and the back reaction with temperature (an alternative to pressure). The data will be collected as powder diffraction data of both the protonated BAn and its deuterated form (see results expected). Approximately 8 hours will be needed for a single inelastic spectrum at each temperature point with FDS. We plan to collect data at four temperature points (T= 10 - 320 K) for each of the two types of BAn samples (at ambient pressure) during illumination to monitor the formation of BAn2 in situ in a quartz sample compartment (~ 65 hours). We also plan to monitor the back-reaction (BAn2-toBAn) in the dark at around 40°C for both BAn2 and its deuterated form (~16 hours). We estimate that around 5 days on the Filter Difference Spectrometer (FDS) will be required to complete the set of requested experiments. Samples will be available in sufficient quantities for the FDS scans and no safety issues nor technical difficulties are expected. Results expected We expect that the isotopic hydrogen substitution exclusively in the two ethylene bridges in BAn will allow us to properly assign the low-frequency vibrational modes. The weak in nature, low energy (~ 2 kcal/mol) C-H/π hydrogen bond [4] between the bridge (H15 at C15; donor) and the center of the middle ring in the anthracene system of the neighboring molecule (acceptor) could play an important role in the formation and subsequent stabilization of the reaction's intermediate. Once the intermediate is formed (induced either by pressure or by temperature) and the subtle C-H/π hydrogen bonding network is at least partially restored, this event could be responsible for triggering the formation of the BAn (as opposed to reformation of BAn2) as a product of the back-reaction (BAn2-BAn). Even though only small movements of atoms can take place during reactions in a solid, once formed, the molecules of a product eventually adopt satisfactory positions in a crystal lattice, causing significant changes in the cell parameters as confirmed with X-ray diffraction in the BAn/BAn2 crystals. References [1] Douglas A. Davis et al., Force-induced activation of covalent conds in mechanoresponsive polymeric materials, Nature, (2009), 459, 68-72 [2] J. H. Golden, Bi(anthracene-9,10-dimethylene) (Tetrabenzo-[2,2]-paracyclophane], J. Chem. Soc., (1961), 3741-3748 [3] Otto Berg et al., s-Dipentacene: Structure, Spectroscopy, and Temperature- and PressureDependent Photochemistry, J. Phys. Chem. A, (1999), 103, 2451-2459 [4] Elzbieta Trzop et al., Monitoring structural transformations in crystals. 12. Course of an intramolecular [4+4] photocycloaddition in a crystal, Acta Cryst., (2008), B64, 375-382 2
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