Second harmonic generation and crystal structure of a 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 °C, with primary and secondary peaks at 202 °C and 220 °C, 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 (d_eff) estimated to be at least 0.67 pm V⁻¹. 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.