Tetra(4-methoxyphenyl)phosphonium Iodide: A Twinned Crystal Built Up with Three-Dimensional Octupolar Nonlinear Optical Chromophores

The tetra(4-methoxyphenyl)phosphonium iodide C28H28O4PI (MOPPI) is a crystal built
up with quasi octupolar tetrahedral chromophores having quadratic nonlinear optical (NLO)
properties. The calculated values of the highest tensor component βxyz for the hyperpolarizable
chromophore, in its most stable S4 geometry, vary from 4.23 to 9.23 × 10-30 esu, depending
on the methods of calculation. Given the linear absorption maximum at 255 nm, this reveals
a good hyperpolarizability−transparency tradeoff. The crystal is twinned by merohedry. The
crystal structure was solved until a reliability factor R = 0.048 using 2189 untwinned
reflections. The material crystallizes in the noncentrosymmetric orthorhombic space group
P212121 with Z = 4 and the unit cell parameters a = b = 10.131(3) Å, c = 25.682(6) Å. The
cation−anion arrangement corresponds to a NaCl-type model weakly distorted. Signal of
second harmonic of a crystalline powder at 1.06 μm is nearly equivalent to that of urea.