12 December 1997 Ž . Chemical Physics Letters 280 1997 551–555 Molecular first hyperpolarizabilities of retinal and its derivatives Elmar Schmalzlin a , Klaus Meerholz a , Stefan Stadler a , Christoph Brauchle a , ¨ ¨ Heiko Patzelt b , Dieter Oesterhelt b a Institute of Physical Chemistry, UniÕersity of Munich, Sophienstrasse 11, 80333 Munich, Germany b Max Planck Institute for Biochemistry, Am Klopferspitz, 82152 Martinsried, Germany Received 4 June 1997; in final form 6 October 1997 Abstract Ž . The hyper-Rayleigh scattering technique HRS has been used to determine the molecular first hyperpolarizabilities b of retinal, retinoic acid, vitamin A acetate and a retinal Schiff base in its protonated and unprotonated form. The former is responsible for the linear and non-linear optical properties of bacteriorhodopsin. A previous examination of these compounds by HRS at 1064 nm yielded surprisingly high b-values. Here, we report HRS measurements performed at 1064, 1300 and 1500 nm. The derived hyperpolarizabilities are self-consistent with the two-state model for all three wavelengths, but they are one order of magnitude lower than those reported previously. q 1997 Published by Elsevier Science B.V. 1. Introduction The light-energy transducing protein bacterio- Ž . rhodopsin BR , a component of the purple mem- wx brane of Halobacterium salinarum 1 , has been investigated for its optical properties for many years. Upon irradiation with light, BR undergoes a photo- cycle, of which two states are particularly interesting Ž. for possible non-linear optical applications: i the Ž . violet B-state l s 570 nm which contains the max all-trans retinal Schiff base in its N-protonated form Ž q . RSB and which exhibits a high first molecular wx Ž. hyperpolarizability b 2; ii the yellow M-state Ž . l s 410 nm which contains the retinal Schiff max Ž . base in its unprotonated form RSB and gives a rather small b-value compared to that of the B-state wx 3 . Since one can populate the two states by irradia- Ž tion with light of the appropriate wavelength i.e. . 410 and 570 nm, respectively at any ratio, the development of an all-optical switching device seems possible. Although incorporation of the chro- mophores in the protein pocket of BR changes their Ž . Ž non-linear optical properties drastically e.g. the absorption maximum of the free protonated RSB in acetonitrile is 449 nm compared to 570 nm in the . protein , the non-linear optical properties of the free chromophores are nevertheless of fundamental inter- est. The chromophores have been investigated previ- ously by second-harmonic generation experiments on Ž . wx Langmuir–Blodgett LB films 4 and by the Ž . wx hyper-Rayleigh scattering HRS technique 5 . The measurements on the LB films were carried out at a fundamental wavelength of 532 nm. To evaluate the second-harmonic signal generated from a monolayer it was calibrated against the second-harmonic inten- sity of an x-cut quartz plate. The HRS experiments were carried out at the standard fundamental wave- length of 1064 nm. Thus, in both cases, fluorescence at the wavelength of the second harmonic or reso- Ž . nance dispersion enhancement can seriously affect 0009-2614r97r$17.00 q 1997 Published by Elsevier Science B.V. All rights reserved. Ž . PII S0009-2614 97 01168-8