60th Annual Report on Research 2015

The ultimate goal of this project is to develop a fundamental approach to increase and control selectivity of photochemical processes using spatially selective excitation of photoreactive compounds in liquid crystalline (LC) host media.  We use the nematic LC media to impose a unidirectional orientation on an array of dissolved photoreactive guest molecules.  Upon reaching the uniform orientation of the guest molecules, the LC solution is irradiated with plane polarized light in order to selectively excite a particular electronic transition in the guest molecule, which is expected to lead to an increased selectivity of their photochemical transformations.  In the first year of the project, we focused on two main tasks.  First, we designed a photochemical setup.  It consists of a variable intensity 200W mercury lamp with a shutter with digital exposure controller, wire-grid UV/vis polarizer with variable-angle mounting, and a custom-made liquid nitrogen cooled quartz sample holder which allows irradiation of an LC solution sample placed in a custom-made 2-inch diameter LC cell.  This setup allows irradiating an oriented LC sample kept either at the temperatures within 0 to 25 °C, or at the cryogenic temperature, using a plane-polarized UV light with changeable polarization angle.  This photochemical setup is now completely operational. 

The second task was to optimize preparation of oriented nematic LC samples, and carry out photochemical studies.  We chose to use compound 1 as a good model since this rigid molecule (which should align well in LC media) possesses two photochemically active fragments which react independently from each other.  Reaction of the conjugated dienone moiety in ring A occurs via type A enone (“lumiketone”) rearrangement to give a “lumiketone” derivative 2.  A competing reaction of ketone at C20 affords product 3 via Norrish type I photocleavage.  Because these two groups absorb at almost the same wavelengths, irradiating a solution of 1 at ~310 nm results in the formation of both photoproducts 2 and 3, and therefore this compound could be an excellent model to test the possibility to control selectivity of photoreaction by spatially selective excitation.  First, we investigated solution photochemistry of the compound 1, and found that irradiating with a mercury lamp in a quartz reactor indeed produces both products 2 and 3.  We separated and purified each individual product, and characterized them by ¹H and ¹⁹F NMR spectroscopy so we could use the obtained characteristic in further quantitative analysis of the products in bulk LC samples.  Then, we studied the possibility to align compound 1 in nematic LC phase.  We tested a number of LCs, and found that 1 is reasonably soluble in 4-pentyl-4′-cyanobiphenyl (5CB) nematic LC.  The prepared 1% solutions of 1 in 5CB were found stable at room temperature, and did not disrupt the nematic phase.  Thus, we proceeded to aligning the 1/5CB solutions in a custom-made LC cell.  The cell was made using two 2-inch quartz disks, spin coated from one side with poly(pyromellitic dianhydride-co-4,4′-oxydianiline) followed by baking at 100 °C to form polyimide layers.  The polyimide-treated disks were unidirectionally rubbed using velvet cloth wrapped around a rubber cylinder to form microaligning layers.  The two disks were then sandwiched between a 20 micron thick kapton separator and placed in a specially designed holder.  The 1% LC solution of 1 was then added in the cell, and the cell was repeatedly heated to 50 °C and slowly cooled to room temperature, to achieve better LC alignment.

The aligned LC solution of 1 was then irradiated with plane polarized light with polarization angles varying in 10° increments from 0 to 90° relative to the LC director.  The cell temperature was strictly controlled to ensure that LC remained in nematic phase.  The product composition was analyzed by ¹⁹F NMR.  Since ¹⁹F NMR peaks corresponding to both products 2 and 3 could be easily seen even at relatively low conversions, and without interference from 5CB matrix, this allowed reliable quantification of the results directly from the LC solution, without need to carry out tedious separations.  We carried out multiple runs, with varying irradiation times, and incident UV light intensity. In a most typical run, we carried the reaction to conversion of ~ 10%.  In a good agreement with the prediction, we found that the ratio between the photoproducts 2 and 3 was indeed dependent on the polarization angle.  Product 3 was practically the only product at polarization angles within 10 to 20° relative to the LC director, whereas a mixture of the products was obtained at most other angles.  Although the obtained results were highly promising, we encountered a range of problems, mostly related to reproducibility of the data.  We link most of these problems to irreproducible alignment of compound 1 by nematic LC 5CB, as well as insufficient ability of the nematic LC phase to “hold” the molecule 1 in a fixed position.  We tried to improve this by rapid freezing an aligned LC solution in a cell to cryogenic (liquid nitrogen) temperature.  Such a freezing was expected to provide sufficient robustness of the LC host matrix to withstand possible morphological reorganization upon guest isomerization, however we found no photochemical conversion occurred at such cryogenic conditions. 

Thus, despite obtaining very promising preliminary data, much work remains to be done in the year 2.  In particular, we are preparing a new photochemically reactive guest molecule with better ability to align within nematic LC phase.  This should resolve the main problem – insufficient alignment and weak “holding” of the photoreactive guest by LC matrix.  This work will now be carried out by the graduate student who got substantial training in the first year of the project (in her first-year at graduate program), and is capable of being completely responsible for this project, which will become her primary PhD project. 

In conclusion, we obtained promising preliminary results which show that our proposed approach to control selectivity of photochemical reactions by spatially selective excitation may actually work.  We are currently optimizing the molecular structure of the host compounds as well as experimental conditions to accomplish ultimate goal of this project.