Crystal engineering using hydrogen bonds : orientation of SHG chromophores using saccharide auxilliaries
by Chen Mei Fong
1 v. (various pagings) : ill. ; 30 cm
The ability to control tensor properties of organic crystals is a goal of crystal engineering. In organic crystals this implies organisation of the supra-molecular arrangement. In this work one aspect of crystal engineering is explored, namely whether hydrogen bonding can be used to overcome the natural tendency of molecules with large ground state dipole moments to align in an anti-parallel fashion....[ Read more ]
The ability to control tensor properties of organic crystals is a goal of crystal engineering. In organic crystals this implies organisation of the supra-molecular arrangement. In this work one aspect of crystal engineering is explored, namely whether hydrogen bonding can be used to overcome the natural tendency of molecules with large ground state dipole moments to align in an anti-parallel fashion.
A potential application of this is the favourable alignment of polar chromophoric groups in organic crystals for Second Harmonic Generation (SHG) of laser light. The testing of powder crystalline samples by the method of Kurtz and Perry also allows a rapid screening of materials with such favourable arrangements.
The use of chiral auxilliaries attached to the chromophores ensures that an SHG signal must be observed but cannot ensure pseudo-centrosymmetry is eliminated. By using sugars as the chiral auxilliaries and a variety of anilines as the chromophores, we have synthesised families of N-glycoside derivatives, which offer the prospect of a variety of packing arrangements based upon their unique H-bonding capabilities.
The glycoside products were tested for crystallinity and powder SHG efficiencies measured. Where possible structure-property relationships between the SHG efficiency and the orientation of the chromophores in the material were studied by single-crystal X-ray structure determination. Our findings can be divided into four chromophore groups, small π-chromophores based on PNA which are hydrophobic or hydrophilic and extended chromophores based on donor-acceptor substituted tolanes or stilbenes which are either hydrophobic or include a hydrophilic group.
The glycosides from small hydrophobic chromophores were generally highly crystalline and a wide dynamic range of SHG signals were found. Several compounds were efficient. (eg L-Fuc-20Me-PNA = 650 x sucrose). The hydrophilic analogues were, surprisingly, rather less crystalline but one high efficiency material (2-DeoxyGlc-2-OH-PNA 2900 x sucrose) was identified. The best materials were found to crystallise in monoclinic system and had their charge transfer axes oriented between 30-60 degrees to the crystllographic 2-fold.
The hydrophobic extended chromophores could not be successfully oriented by this approach. The compounds were either non-crystalline or in the case of the stilbene derivatives were always found in anti-parallel arrangement. When a hydrophilic substituent was attached to the tolane, both crystallinity and the maximum SHG efficiencies were greatly enhanced. The ability of hydrogen bonding in overcoming centrosymrnetry was also shown in the chromophore itself (CH2OH-Tol) which had a powder SHG of 1100 x sucrose.
Our studies show that these mostly ambiphilic molecules crystallise by molecular aggregations to form hydrophilic and hydrophobic regions. The hydrophilic regions may be either 1-D channels or form 2-D sheets. Analysis of the SHG results on our crystalline glycoside families, together with previous results from our group on PNA and MNA glycosides clearly indicate that optimal SHG materials are only ever seen for sugars with three hydroxyl groups, either pentoses or deoxy-hexoses. Hexose based glycosides which have four hydroxyl groups, or those from 2-Deoxyribose, which has only two hydroxyls invariably had only medium to weak SHG efficiencies.
In conclusion successful control of the degree of hydrogen bonding, and the aggregation to form supra-molecular arrays, can be achieved in certain cases, and is critical to obtaining optimal alignments of chromophore away from anti-parallel orientation.