A method is described for obtaining ultrahigh time-resolution vibrational spectra of shocked polycrystalline materials. A microfabricated shock target array assembly is used, consisting of a polymer shock generation layer, a polymer buffer layer, and a thin sample layer. A near-IR pump pulse launches the shock. A pair of delayed visible probe pulses generate a coherent anti-Stokes Raman (CARS) spectrum of the sample. High-resolution Raman spectra of shocked crystalline anthracene are obtained. From the Raman shock shift, the shock pressure is determined to be 2.6 GPa. The rise time of shock loading is 400 ps. This rise time is limited by hydrodynamics of the shock generation layer. The shock velocity in the buffer layer is found to be 3.7 (±0.5) km/s, consistent with the observed shock pressure. As the shock propagates through a few μm of buffer material, the rise time and pressure can be monitored. The rise time decreases from ∼800 to ∼400 ps over the first 6 μm of travel, and the pressure begins to decline after about 12 μm of travel. The high-resolution CARS method permits detailed analysis of the vibrational line shape. Simulations of the CARS spectra show that when the shock front is in the crystal layer the spectral linewidths are inhomogeneously broadened by the distribution of pressures in the layers. When the crystal layer is behind the front, the spectral linewidth can be used to estimate the temperature. The increase of the spectral width from the ambient 4 to ∼6.5 cm−1 is consistent with the expected temperature increase of ∼200°.

The anthracene cyclophane bis-anthracene (BA) can undergo a [4 + 4] photocycloaddition reaction that results in a photodimer with two cyclobutane rings. We find that the subsequent dissociation of the dimer, which involves the rupture of two carbon-carbon bonds, is strongly accelerated by the application of mild pressures. The reaction kinetics of the dimer dissociation in a Zeonex (polycycloolefin) polymer matrix were measured at various pressures and temperatures. Biexponential reaction kinetics were observed for all pressures, consistent with the presence of two different isomers of bis(anthracene). One of the rates showed a strong dependence on pressure, yielding a negative activation volume for the dissociation reaction of ΔV(++) = -16 Å(3). The 93 kJ/mol activation energy for the dissociation reaction at ambient pressure is lowered by more than an order of magnitude from 93 to 7 kJ/mol with the application of modest pressure (0.9 GPa). Both observations are consistent with a transition state that is stabilized at higher pressures, and a mechanism for this is proposed in terms of a two-step process where a flattening of the anthracene rings precedes rupture of the cyclobutane rings. The ability to catalyze covalent bond breakage in isolated small molecules using compressive forces may present opportunities for the development of materials that can be activated by acoustic shock or stress.

Using optical flash photolysis and timeresolved Raman methods, we examined intermediates formed during the photocycle of bacteriorhodopsin (bR), as well as the bR color change, as a function of pH (in the 7.0-1.5 region) and as a function of the number of bound Ca2+ ions. It is found that at a pH just below 3 or with less than two bound Ca21 per bR, the deprotonation (the L550 -_ M412) step ceases, yet the K610 and L550 analogues are still formed as in native bR. The lack of deprotonation in the photocycle of both acid blue and deionized blue bR and the similarity of their Raman spectra as well as of their K610 and L550 analogues strongly suggest that both blue samples have nearly the same retinal active site. It is suggested that in both blue species, bound cations are removed via a proton-cation exchange equilibrium, either on the cation exchange column for the deionized sample or in solution for the acid blue sample. The proton-cation exchange equilibrium is found to quantitatively account for the pH dependence of the purple-to-blue color change. The different mechanisms responsible for the large reduction (=11 units) of the pKa value of the protonated Schiff base (PSB) during the photocycle are discussed. The absence of the L550 1M412 deprotonation process in both blue species is discussed in terms of the previously proposed cation model for the deprotonation of the PSB during the photocycle of native bR. The extent of the deprotonation and the blue-to-purple color change are found to follow the same dependence on either the pH or the amount of cations added to deionized blue bR. This observed correlation is briefly discussed.Bacteriorhodopsin (bR) is the only protein found in the purple membrane of Halabacterium halobium, a light-harvesting bacterium. It contains retinal as a chromophore, which is covalently bound via a protonated Schiff base linkage to the E-amino group of a lysine residue in the protein (1, 2). Upon absorbing a photon, it undergoes a photochemical cycle (3) The photocycle causes protons to be pumped across the cell membrane to the outside, establishing a pH gradient used by the organism for metabolic processes such as ATP synthesis (4-7). The protons are ejected from the cell at a rate comparable to that for the formation of the M412 intermediate (8,9). A good correlation has been found between the number ofprotons pumped and the amount of slow decaying (10) form of M412. This intermediate is the only one in which the Schiff base is unprotonated (11-16). Consequently, many studies have inferred that the protonated Schiff base (PSB) deprotonation is closely associated with the proton pump mechanism (17). Preceding M412 formation (or PSB deprotonation) is the formation of the early intermediates K610 and L550. The retinal in bR570 is in the all-trans form, while in K610 it has a distorted 13-cis conformation (18)(19)(20). In the L550 form, the isomerization is complete (18-20).The pKa value of the PSB is 13.3 in bR570 (21, 22), yet it deprotonates during the cycle, suggesting a lar...

Mid-infrared spectra of neat n-heptane at room temperature are presented over a pressure range from ambient to 70 kbar. The application of hydrostatic pressure induces frequency shifts, band splittings, and significant changes in the line shapes of internal vibrational modes both in liquid and in solid phases. The results are discussed in terms of the liquid-solid phase transition and changes of the population of molecular conformers. Evidence for a solid-solid phase transition near 30 kbar is also presented.

Symmetric dimers have the potential to optimize energy transfer and charge separation in optoelectronic devices. In this paper, a combination of optical spectroscopy (steady-state and time-resolved) and electronic structure theory is used to analyze the photophysics of sulfur-bridged terthiophene dimers. This class of dimers has the unique feature that the interchromophore (intradimer) electronic coupling can be modified by varying the oxidation state of the bridging sulfur from sulfide (S), to sulfoxide (SO), to sulfone (SO2). Photoexcitation leads to the formation of a delocalized charge resonance state (S1) that relaxes quickly (<10 ps) to a charge-transfer state (S1*). The amount of charge-transfer character in S1* can be enhanced by increasing the oxidation state of the bridging sulfur group as well as the solvent polarity. The S1* state has a decreased intersystem crossing rate when compared to monomeric terthiophene, leading to an enhanced photoluminescence quantum yield. Computational results indicate that electrostatic screening by the bridging sulfur electrons is the key parameter that controls the amount of charge-transfer character. Control of the sulfur bridge oxidation state provides the ability to tune interchromophore interactions in covalent assemblies without altering the molecular geometry or solvent polarity. This capability provides a new strategy for the design of functional supermolecules with applications in organic electronics.