Photoactive High Explosives: Substituents Effects on Tetrazine Photochemistry and Photophysics.

High explosives that are photoactive, i.e., can be initiated with light, offer significant advantages in reduced potential for accidental electrical initiation. We examined a series of structurally related tetrazine based photoactive high explosive materials to detail their photochemical and photophysical properties. Using photobleaching infrared absorption, we determined quantum yields of photochemistry for nanosecond pulsed excitation at 355 and 532 nm. Changes in mass spectrometry during laser irradiation in vacuum measured the evolution of gaseous products. Fluorescence spectra, quantum yields, and lifetimes were measured to observe radiative channels of energy decay that compete with photochemistry. For the 6 materials studied, quantum yields of photochemistry ranged from <10(-5) to 0.03 and quantum yield of fluorescence ranged from <10(-3) to 0.33. In all cases, the photoexcitation nonradiatively relaxed primarily to heat, appropriate for supporting photothermal initiation processes. The photochemistry observed was dominated by ring scission of the tetrazine, but there was evidence of more extensive multistep reactions as well.

Use of the Gerchberg-Saxton algorithm in optimal coherent anti-Stokes Raman spectroscopy.

We are utilizing recent advances in ultrafast laser technology and recent discoveries in optimal shaping of laser pulses to significantly enhance the stand-off detection of explosives via control of molecular processes at the quantum level. Optimal dynamic detection of explosives is a method whereby the selectivity and sensitivity of any of a number of nonlinear spectroscopic methods are enhanced using optimal shaping of ultrafast laser pulses. We have recently investigated the Gerchberg-Saxton algorithm as a method to very quickly estimate the optimal spectral phase for a given analyte from its spontaneous Raman spectrum and the ultrafast laser pulse spectrum. Results for obtaining selective coherent anti-Stokes Raman spectra (CARS) for an analyte in a mixture, while suppressing the CARS signals from the other mixture components, are compared for the Gerchberg-Saxton method versus previously obtained results from closed-loop machine-learning optimization using evolutionary strategies.

Molecular factors that determine Curie spin relaxation in dysprosium complexes.

Dysprosium complexes can serve as transverse relaxation (T(2)) agents for water protons through chemical exchange and the Curie spin relaxation mechanism. Using a pair of matched dysprosium(III) complexes, Dy-L1 (contains one inner-sphere water) and Dy-L2 (no inner-sphere water), it is shown that the transverse relaxation of bulk water is predominantly an inner-sphere effect. The kinetics of water exchange at Dy-L1 were determined by (17)O NMR. Proton transverse relaxation by Dy-L1 at high fields is governed primarily through a large chemical shift difference between free and bound water. Dy-L1 forms a noncovalent adduct with human serum albumin which dramatically lengthens the rotational correlation time, tau(R), causing the dipole-dipole component of the Curie spin mechanism to become significant and transverse relaxivity to increase by 3-8 times that of the unbound chelate. These findings aid in the design of new molecular species as efficient r(2) agents.

The Gd(3+) complex of a fatty acid analogue of DOTP binds to multiple albumin sites with variable water relaxivities.

The 20 MHz water relaxivity (r(1)) of gadolinium(III) complexes formed with two fatty acid analogues of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonate) were shown to increase substantially in the presence of albumin. The r(1) values of Gd(C(8)-DOTP)(5-) and Gd(C(11)-DOTP)(5-) in water were similar to that of the parent GdDOTP(5-), a q = 0 complex known to relax water very efficiently via an outer-sphere mechanism. Neither fatty acid analogue formed apparent aggregates or micelles in water up to 20 mM, but both showed dramatic increases in r(1) upon addition of albumin. Further ultrafiltration studies of Gd(C(11)-DOTP)(5-) in the presence of non-defatted HSA showed that the complex binds at a minimum of five high-affinity fatty acid sites with stepwise binding constants ranging from 1.27 x 10(5) to 2.7 x 10(3) M(-1). The 20 MHz relaxivity of Gd(C(11)-DOTP)(5-) in the presence of excess HSA was 23 mM(-1) s(-1) at 25 degrees C. The NMRD curve showed a broad maximum 20-30 MHz which fitted well to standard theory for a q = 0 complex with rapid outer-sphere water exchange. The r(1b) of Gd(C(11)-DOTP)(5-) bound at the tightest site on HSA was approximately 40 mM(-1) s(-1) at 5 degrees C, an extraordinarily high value for an outer-sphere complex. However, the r(1b) of Gd(C(11)-DOTP)(5-) bound at the weaker sites on HSA was considerably lower, approaching the relaxivity of the free complex in water. This suggests that the complex bound in the highest affinity fatty acid site is less mobile than the same complex bound at the weaker affinity fatty acid sites. This combined ultrafiltration and relaxivity study demonstrates that the common assumption of a single r(1b) value for a Gd(3+) complex bound at several protein sites is not a valid approximation.