ARAMENDIA Pedro Francisco
congresos y reuniones científicas
Chirality change upon photoisomerization in twisted nematic liquid crystals
Cubatao, San Pablo, Brasil
Congreso; 9º Encuentro Latinoamericano de Fotoquímica y Fotofísica; 2008
<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.9pt 70.9pt 70.9pt 70.9pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> The use of bistable chemical systems has been the base of extensive studies aiming at the construction of optical memories for data and image recording devices. All light write-read systems must relay on reading conditions that do not cause photochemical erasing of the information. For this reason optical probing of the state of the system based on refractive index changes is an interesting possibility. Photochromic compounds have been extensively used to modify the state of liquid crystals (LC). In this approach, the compatibility change of the two stable forms of the photochromic compound in the susceptible liquid crystalline phase is taken advantage of. The photoisomerization induces perturbations in the LC that can be probed by measuring changes in transmitted light that is not absorbed by the system. In this work, we use the photoisomerization of azobenzenes, a phenanthrospirooxazine, and a fulgide in a twisted nematic liquid crystalline phase to change the chiral power of the system. The changes are probed by the rotatory power of linearly polarized light. Time resolved and steady state experiments are carried out. The chiral change and the photoisomerization process have similar characteristic recovery times and activation energy, thus probing that the change is induced by the modification in the chemical composition of the photochromic dopant system. The amplitude of the light twisting power change correlates with the order change in the LC but not with the modification in the absorption characteristics of the system. This indicates that the driving force of the chiral change is the microscopic order modification in the LC phase that affects the helical pitch of the phase.