Craig J. Eckhardt

Thermochromism in a polydiacetylene crystal

Specular reflection spectra are reported for a thermochromic polydiacetylene crystal, ETCD [substituent group – (CH2)4OCONHC2H5] over the temperature range 23–130°C. A reversible thermochromic phase change occurs at ∼120°C and is accompanied by a 2750 cm−1 blue shift in the reflection spectra. Evidence is presented which strongly suggests that the dramatic change in optical properties of ETCD at the phase transition is due to an acetylene to butatriene transformation in the bonding sequence of the polydiacetylene backbone.

A model for mechanochemical transformations: Applications to molecular hardness, instabilities, and shock initiation of reaction

A basic theoretical structure for mechanochemical transformations based on prior models for solid-state reactions and HOMO–LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) gap closing produces the concept of distortion-induced molecular electronic degeneracy (DIMED) of the highest occupied and lowest unoccupied molecular orbitals of an energetic molecule. Both intermolecular and intramolecular charge transfer are involved. The resulting distortion-induced local instability, a mechanochemical effect, leads to chemical transformations and can be analyzed by renormalization of the molecular hardness through the molecular deformation energy. Linear combinations of normal modes are shown to be useful for description of the mechanically induced reaction path. Numerical calculations for the RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) molecule are used to construct a path for initiation of a reaction by shock. They show the breaking of a single N–N bond as the primary step. DIMED is shown to be a kind of “inverse Jahn–Teller effect” leading to the general conclusion that distortion-induced instabilities and mechanically induced reactions require some, but not necessarily complete, HOMO–LUMO gap closure. This indicates that large local strains due to defects or cracks will contribute to DIMED. The DIMED concept, because of its generality, has wide applicability in solid-state chemistry.

The elastic constants and related properties of β-HMX determined by Brillouin scattering

By scattering from a variety of acoustic phonons, a complete stiffness tensor has been determined for crystalline beta-HMX. The results are compared with recent experimental and theoretical determinations of the elastic constants and bulk modulus. Reasons for disagreement are discussed in terms of experimental limitations and anharmonic effects. The observed ordering of stiffness constants, C(11) (18.4 GPa), C(22) (14.4 GPa), and C(33) (12.4 GPa), is qualitatively associated with physical phenomena including cleavage planes, patterns in crystal growth, and molecular packing. This interpretation is further corroborated by the linear compressibilities plotted in three crystallographic planes. The Voigt-Reuss-Hill bulk and shear moduli were found to be 9.9 and 3.7 GPa, respectively. The elasticity of beta-HMX is also discussed in relation to proposed mechanisms for the initiation of detonation.

The elastic constants and related properties of the energetic material cyclotrimethylene trinitramine (RDX) determined by Brillouin scattering.

The acoustic phonons of cyclotrimethylene trinitramine (RDX) have been studied using Brillouin scattering. The analysis of the acoustic-phonon velocities allowed determination of the complete stiffness tensor for this energetic material. The results are compared to other recent experimental and theoretical determinations of the RDX elastic constants, bulk moduli, and shear moduli. The observed ordering of elastic constants, C11>C22>C33, is qualitatively associated with a (001) cleavage plane and molecular packing. This interpretation is further corroborated by the linear compressibilities plotted in three crystallographic planes, and a comparison to recent theoretical and experimental hydrostatic compression studies on RDX. Finally, the elasticity of RDX is compared to a recently published report on the beta polymorph of cyclotetramethylene tetranitramines elasticity, and is related to several proposed mechanisms for detonation initiation.

Optical and electrical properties of a polydiacetylene crystal: poly(5,7-dodecadiyne-1,12-diol-bis phenylurethane)☆

Abstract The specular reflection spectra and photoconductivity are reported for a polydiacetylene crystal, TCDU, where the conjugation of the backbone is a repeating butatriene unit. The spectra are distinctly different from PTS crystals and the transition moment for the 18800 cm−1 band is polarized 4° off the polymer chain direction. Both the spectra and the photoconductivity data support a model for polydiacetylenes which explicitly includes excitons.

Optical properties of a polydiacetylene crystal: poly-[1,6-di(N-carbazolyl)-2,4-hexadiyne]

Abstract Normal incidence reflection spectra and Kramers-Kronig transforms are reported for a polydiacetylene crystal, DCHD (substituent group: N-carbazolymethyl). The lowest energy optical transition for the polymer backbone is found at 15 300 cm−1, the lowest value obtained thus far for a polydiacetylene. The polymer chain and the carbazolyl substituent group may be treated as isolated chromophores - the former dominating the visible portion of the spectra and the latter dominating the UV portion. Polarization studies for the (001) and (10 1 − ) faces of DCHD permit unambiguous assignment of the two lowest energy transitions in the carbazolyl group at 28 750 cm−1 and 33 250 cm−1 as short- and long-axis polarized, respectively.

Quasi-metallic reflection spectra of TCNQ single crystals

Abstract The optical constants of TCNQ single crystals in the range from 15 kK to 40 kK are obtained by polarized near normal incidence specular reflection for the (010), (001) and {110} faces. The absorption are obtained through Kramers-Kronig transforms. The direct absorption spectrum for (010) is also reported. The assignment of the parent molecular transition is a π* ← π singlet which is long axis polarized. The metal-like reflection spectra arise from very strong coupling and they are calculated by Drude-Lorentz theory and the theory of molecular polaritons. The Lyddane-Sachs-Teller relation is also applied. The system is found to be exemplary for the study of very strong coupling.

Influence of crystal fields on the quasimetallic reflection spectra of crystals: Optical spectra of polymorphs of a squarylium dye

The reflection and Kramers–Kronig absorption spectra have been obtained from the monoclinic and triclinic polymorphs of a squarylium dye, 2,4‐bis(4‐diethylamino‐2‐hydroxy phenyl) cyclobutadienediylium‐1,3‐diolate. The extremely different optical responses were found to arise from two molecular singlet transitions of essentially long axis polarization. Successful application of a four oscillator molecular exciton–polariton theory required the use of point charge densities rather than point dipoles in an extension of the theory which employed both interactions between the two singlets as well as a frequency dependent lattice damping. An intermolecular charge transfer transition is also assigned.

Thermal effects on the optical spectra of a polydiacetylene: Poly{1,2-bis [4-(isopropylcarbamoyloxy) - n -butyl]-1-buten-3-ynylene}

The reflection spectra of single crystals of the polydiacetylene, IPUDO [R= (CH2)4OCONHCH (CH3)2], were measured between 25° C and 160° C. A nearly reversible thermochromic change is observed together with a variation of hysteresis upon cycling. The latter is attributed to annealing that permits the pendant groups to adopt a conformation which favors the butatriene‐type configuration of the polymer at high temperatures. Intensities of the high and low temperature spectra obtained by Kramers–Kronig transforms are in excellent agreement with the spectral intensities of polydiacetylenes which are prototypes for the two configurations.

Stress Induced Change of Electronic Structure in a Polydiacetylene Crystal

A stress induced change of phase in a single crystal of the polydiacetylene, TCDU, is observed wherein a change of electronic configuration of the polymer from a butatriene to an acetylenic electro...