Nonlinear Optical and Structural Properties of Langmuir−Blodgett Films of Thiohelicenebisquinones

Abstract

We provide a detailed investigation of the second-order nonlinear optical and structural properties of Langmuir-Blodgett (LB) films of nonracemic thiohelicenebisquinone (THBQ). We prepare both X- and Y-type films of different thicknesses and characterize them using optical second-harmonic generation and atomic-force microscopy (AFM). We find that the overall nonlinear properties of the samples are essentially independent of the film thickness and the deposition type and arise from susceptibility tensor components associated with chirality. Both X- and Y-type films can be described by D2 symmetry, which is a higher symmetry than the previously assumed C2 of LB films of THBQ and a similar helicenebisquinone (HBQ). However, the two types of films are shown to differ significantly with respect to the orientation of the in-plane axis. For Y type, the axis follows the direction of vertical sample deposition, but for X type, the direction of the axis varies randomly and significantly between different samples. The Y-type samples are therefore more ordered than the X-type samples. This was confirmed by AFM measurements in which the Y type exhibits uniform ordering into columnar structures. Similar structures in X type, on the other hand, are shorter and more randomly oriented, like those earlier observed for racemic samples of HBQ [Verbiest, T., et al. Science 1998, 282, 913]. The common nonlinear properties and different high-level ordering observed here for two different types of nonracemic samples reinforces that the nonlinearity of THBQ (and probably HBQ, as well) originates from the low-level columnar aggregation of the molecules with the higher-level structures playing a lesser role. In addition, within the columns, the molecules likely assume fairly random azimuthal orientations so that the columns themselves exhibit approximate Dinfinity symmetry.

Nonlinear chiral imaging of subwavelength-sized twisted-cross gold nanodimers [Invited]

“…SHG has been found to be extremely sensitive and highly versatile probe to investigate the chirality of molecules, solutions and thin films [33][34][35][36][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52], and more recently also in the study of metal nanostructures [18][19][20][21][22][23][24][25][26][27][28]. The sensitivity of SHG to chirality originates from its intrinsic sensitivity to symmetry properties.…”

“…However, if the sample has in-plane anisotropy, these techniques are compromised because the orientation of the anisotropy axis can make the experiment chiral, which can also give rise to a SHG-CD response [37]. Discrimination between anisotropy and chirality can be achieved by a technique, which involves azimuthal rotation of the sample about its surface normal [38]. It is evident that this approach is not suitable for nonlinear microscopy.…”

“…This problem arises because the orientation of the anisotropic sample can make the experimental setup chiral. Separation of chirality and anisotropy is thus a significant problem [38] and a particular concern for metamaterials, where the unintended structural features may lead to anisotropy of chiral samples and vice versa. The separation problem can be overcome by increasing the symmetry of the experimental geometry.…”

Chirality‐Controlled Self‐Assembly of Amphiphilic Dibenzo[6]helicenes into Langmuir–Blodgett Thin Films

“…[7] The monolayers/thin films of racemic/enantiopure helicenes are usually preparedb ys pin coating, drop casting,u ltra-high vacuum (UHV) deposition or throught he formation of self-assembled monolayers (SAM) on the surface of solidso rat the air-water interface. Surprisingly,t he relativelys imple Langmuir-Blodgett (LB) technique has so far not been systematically explored in the contexto fh elicene chemistry,e ven though such assemblies can exhibit unique chiroptical and NLO properties as reported by Katz et al [3,8] (see above)a nd Siltanen et al [9] In these studies,helicene units are equipped with multiple long alkyl chains to support the Langmuir monolayer formation. Yamaguchi et al incorporated the conformationally locked [ 4]helicene units into macrocycloamides to form optically activeL Bf ilms based on helical chirality for the first time.…”

Spectral and optoelectronic studies on 7,12,17-trioxa and 7,12,17-trithia [11] helicenes: a DFT view

“…These molecules forms helical columnar motifs which makes them potential applicant for nonlinear optical and circularly polarised light emitting material. Due to extended p-electron cloud these compounds display circular dichroism, exhibiting large second order nonlinear optical response (NLO) and can be used in liquid crystal displays [15][16][17][18][19].…”