LANL proposal in PDF - Dr. Sebastian Jezowski

Sebastian Jezowski
Inelastic Neutron Scattering Study of Molecular Vibrations in a Novel Light-Activated
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).
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.
[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