Second harmonic generation and crystal structure of self-assembled Boc-p-nitro-L-phenylalanyl-p-nitro-L-phenylalanine dipeptide

Abstract

(Boc=tert-butoxycarbonyl)-p-nitro-L-phenylalanyl-p-nitro-L-phenylalanine, a dipeptide characterised by acentric symmetry, self-assembles into microtapes. This study explores its thermal, structural, and nonlinear optical properties. Thermogravimetric analysis reveals an onset degradation temperature of 190 ºC, with primary and secondary peaks at 202 ºC and 220 ºC, respectively. The crystal structure of the dipeptide was determined through single crystal X-ray diffraction at 100 K, confirming its crystallisation in a monoclinic crystal lattice, space group P2 with two molecules per unit cell. Additionally, optical second harmonic generation polarimetry indicates a nonlinear optical response, with an effective coefficient (deff) estimated to be at least 0.67 pm/V. This value is only four times lower than that of a state-of-the-art phase-matched oriented β-barium borate nonlinear crystal. The crystal is unusually robust against optical damage for an organic crystal, underlining the potential of this dipeptide in nonlinear optical applications.