Chirality and hydrogen bonding in molecular crystals for phase-matched second-harmonic generation: N-(4-nitrophenyl)-(L)-prolinol (NPP)

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Abstract
The higher second‐harmonic efficiency, in powder, of a new organic molecular crystal N‐(4‐nitrophenyl)‐(L)‐prolinol (NPP) is reported. Electronic polarizability of NPP molecules relates to that of other para‐nitroaniline‐like structures such as that of previously reported N‐(2,4‐dinitrophenyl) methyl alaninate (MAP), and the increase in crystalline nonlinear efficiency by one order of magnitude above the latter is ascribed to an ‘‘optimized’’ crystalline structure rather than to minor changes of the molecular hyperpolarizability. The simultaneous chiral and hydrogen‐bonding character of the prolinol electron‐donating group leads to a quasioptimal angle, with respect to quadratic phase‐matched nonlinear interactions, between the molecular transition dipole moments and the twofold axis of the monoclinic P21 crystal structure. Based on a simple oriented gas description of the quasiplanar structure of the crystal, two possible mutually exclusive second‐harmonic phase‐matched configurations are evidenced and shown to promote the optimized nonlinear tensor coefficient.