Synthesis, Structure, and Properties of 4,8,12-Trioxa-12c-phospha-4,8,12,12c-tetrahydrodibenzo[cd,mn]pyrene, a Molecular Pyroelectric

Abstract

The title compound (4) was synthesized, and its crystalline structure was determined. The molecule has C 3 v point symmetry and crystallizes in the trigonal space group R3m. Crystal data for 4: a = 16.6710(13) Å, b = 16.6710(13) Å, c = 4.2590(3) Å, α = β = 90°, γ = 120°, Z = 3, R(F) = 0.0234. The material has a permanent polarization and consequent pyroelectric properties. The room temperature pyroelectric coefficient was found to be −3 ± 2 μC m-2 K-1, which is in accordance with a calculated value of −3.2 μC m-2 K-1. The molecular dipole moment was determined to be 3.3 ± 0.2 D, the direction of which was unambiguously assigned with respect to the molecular coordinates. The thermal expansivity was determined at temperatures in the range −93 to 200 °C. The relative dielectric permittivity tensor was obtained at optical frequencies (ε11 and ε22 = 3.16 and ε33 = 2.48) and in the microwave region at 35 GHz (ε11 and ε22 = 5.2 ± 0.6 and ε33 = 2.9 ± 0.2), and at low frequencies (120 Hz and 1 kHz), the isotropic permittivity was determined (ε120 Hz = 4.7 ± 0.8 and ε1 kHz = 4.7 ± 1.1). Finally, an estimate of the molecular heat capacity was calculated (C p = 900 J Kg-1 K-1) and the material was considered for potential use in infrared detection as its detectivity merit factor, M r, was determined (M r = 8.8 × 10-2 m2 C-1).

Evaluation of the Solid State Dipole Moment and Pyroelectric Coefficient of Phosphangulene by Multipolar Modeling of X-ray Structure Factors

“…In order to resolve the discrepancy we have re-determined the absolute morphology of three phosphangulene crystals: the original crystal and two new crystals of similar morphology. This was done using data collected on a Siemens SMART area detector diffractometer with Mo Ka radiation at 120 K. All three crystals were found to have the same absolute configuration as originally determined, but it was found that the morphology presented previously [4] was in fact in error. The morphology determined for the three instances was the same, namely with the polar c axis pointing towards the pointed end of the crystals, which is opposite to the direction previously reported.…”

“…[4] Phosphangulene, Figure 1, crystallizes in the changes linearly with temperature (V(T) aTb) and the molecular dipole moment is considered independent of temperature, the pyroelectric coefficient is given as [4] where Z is the number of molecules in the unit cell.…”

“…With the advent of area detector diffractometers for rapid data collection, a multipolar derived pyroelectric coefficient can therefore be obtained with a modest amount of extra experimental effort. Furthermore, it is noteworthy that the method, contrary to traditional methods, [4] can be applied to small crystals with volumes of a few thousandths of a mm 3 , the size of the single crystal normally used in an X-ray study of the crystal structure.…”

“…The method was carefully calibrated on LiNbO 3 crystals with known polarization and on every phosphangulene crystal tested, positive surface charges where found at the pointed end of the bullet-shaped crystal, ref. [4] and Figure 6. The determination of the absolute morphology of the crystal is dependent on the right interpretation of the diffractometer geometry in relation to the crystal geometry.…”

“…It has to be commented that this direction is opposite to that of the dipole moment published earlier. [4] The previous determination of the molecular dipole moment direction was based on relating the measured macroscopic polarization of phosphangulene crystals of known morphology to that of the absolute molecular configuration, which was found by analysis of Friedel-related reflections.…”