Molecular recognition at growing crystal faces revealed by phase sensitive second harmonic microscopy

Abstract

The absolute orientation of the local average polarization in relation to the main crystal growth direction has been investigated for channel-type inclusions of perhydrotriphenylene (PHTP) and five guests: N,N-dimethyl-3-nitroaniline (DMNA), 1-(4-nitrophenyl)-piperazine (NPP), 4-iodo-4′-nitrobiphenyl (INBP), 4-amino-4′-nitrostilbene (ANS) and N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA). For this we have used phase sensitive second harmonic generation microscopy (PH-SHM) and a crystal of 2-cyclooctylamino-5-nitropyridine (COANP) as a NLO reference material for which we know the absolute polarity from X-ray anomalous scattering. Bi-polar states of all five inclusion compounds have been confirmed. Application of PS-SHM revealed that the nitro groups preferably point into the direction of growth. The attributed molecular orientation is further on confirmed (PHTP/DNAA and PHTP/INBP) by scanning pyroelectric microscopy. Results from Monte Carlo (MC) simulations allow us to conclude that the fundamental energy parameter ΔE_f driving stochastic polarity formation is larger than zero.