Polymers for Second-Order Nonlinear Optics - American Chemical

Chapter 6

Experimental and Theoretical Investigation of the Second-Order Optical Properties of the Chromophore Retinal and Its Derivatives Downloaded by CORNELL UNIV on September 14, 2016 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch006

Modeling the Bacteriorhodopsin Binding Pocket 1

2

1

2

1

E. Hendrickx , C. Dehu , K. Clays , J. L. Brédas , and A. Persoons 1

Center for Research on Molecular Electronics and Photonics, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Center for Research on Molecular Electronics and Photonics, Université de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium

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In this study we describe the second-order optical properties of retinal and related molecules. Hyper-Rayleigh scattering was used to measure the first hyperpolarizability of several retinal derivatives and to determine the solvent dependence of the first hyperpolarizability of retinal. This technique is also shown to be sensitive to fluorescence induced by multiphoton absorption, which precluded the measurement of trans retinol. By comparing the experimental results with the theoretical predictions obtained with the semi-empirical INDO/SCI/SOS method we show the validity of the two-state model for β. The two-state model is extended to explain the large solvent dependence of the first hyperpolarizability of retinal, that is also related to the degree of bond-length alternation. Retinal is the light-absorbing chromophore of the protein Bacteriorhodopsin (bR), a constituent of the purple membrane of the bacterium Halobacterium Halobium. The chromophore is covalently bonded to the protein backbone by a protonated Schiff base linkage (7). The exact nature of the retinal binding pocket has been the subject of extensive investigations and in literature only modest agreement exists. Photoexcitation of the chromophore starts a photocycle during which a proton is pumped from the cytoplasmic side to the extracellular side of the membrane (2,5). The proton gradient thus created is used by the bacterium to synthesize ATP when the concentration of oxygen in the environment is too low to generate ATP via oxidative phosphorylation. This unusual behaviour and the extraordinary thermal and photochemical stability make this protein an outstanding candidate for optical applications (4). The uncertainty that still exists on the exact mechanism of the photocycle has not prevented the use of the protein in devices such as light detectors (5), holographic media (6) and, recently, three-dimensional optical memories (7). Genetic engineering allows for modification of the protein according to the requirements of each application. 0097-6156/95/0601-0082$12.00A) © 1995 American Chemical Society Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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HENDRICKX ET AL.

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Second-Order Optical Properties of Retinal

We have been able to do a direct measurement of the first hyperpolarizability P of the protein by using the hyper Rayleigh scattering technique (HRS) (8). The retrieved value of 2100xl0" esu at 1064 nm is in good agreement with previous estimates based on two-photon absorption (9) and second harmonic generation from a thin poled polyvinyl alcohol) film (10). This technique also was shown to be sensitive to the extent of solubilization of the protein, which resulted in a decay by a factor of ten in the apparent hyperpolarizability. A value of 2100xl0' was only obtained after 100 hours in an acetate buffer (0.1 M, pH 5.0) with Triton X-100 (5% Vol.), which corresponds to monomelic bR (77). This value for P is to be compared with a value of 450xl0" esu for dimethylaminonitrostilbene (DANS), a typical chromophore for non-linear optics. To understand this high hyperpolarizability we started a thorough experimental and theoretical study of thefirsthyperpolarizability of the chromophore retinal and some of its typical derivatives, which are shown in Figure 1. Only for trans retinol the first hyperpolarizability could not be determined due to fluorescence induced by multiphoton absorption. Both the theoretical and experimental results are in excellent mutual agreement for the effects of cis-to-trans isomerization of a retinal double bond and the dependence of P on the position of the absorption band maximum. The two-state model is shown to predict the correct trends for p. By extending this two-state model and taking into account the change in molecular polarizability upon excitation, the large solvent dependence can be explained in a simple manner. 30

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Downloaded by CORNELL UNIV on September 14, 2016 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch006

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The Hyper-Rayleigh Scattering Technique An HRS measurement is performed by measuring the intensity of the second-order scattered light on focusing an intense laser beam on an isotropic solution (72,75). For a detailed description of the experimental set-up the interested reader is referred to reference 14. On the molecular level, the intensity of the incoherently scattered second harmonic can be connected to the molecular first hyperpolarizability in the series expansion of the induced molecular dipole moment in the amplitude of the applied field: p. = a.E + P:E.E+...

(1)

For a liquid composed of non-centrosymmetric molecules the macroscopic polarization at frequency 2co will be equal to : P(2co) = B(-2co;co,(o):E(co)E(co)

(2)

with B(-2co;co,co) the first nonlinear macroscopic susceptibility tensor for frequency doubling. The intensity of the harmonic light is then proportional to : I(2co) « B (-2co;G),co)I 2 (co) 2

(3)

Neglecting dipolar orientational correlations, B(-2co;co,co) can be decomposed in molecular P components by a transformation from laboratory to molecular coordinates and by averaging over time and molecular orientations, finally giving: I(2co) = G I ^ c o J X N i t p 2 ^ i

Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

(4)

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POLYMERS FOR SECOND-ORDER NONLINEAR

OPTICS

G is a factor that includes local field factors, the scattering geometry and instrumental factors. The summation is performed over all the scattering species i. The expression for [P2]i is a sum of molecular tensor components squared, each multiplied by a factor resultingfromthe orientational average over the direction cosines of the transformationfromlaboratory to molecular coordinates (15,16). For a binary solution consisting only of solvent (S) and solute (s), equation 4 can be simplified to: 2

2

I(2a>) = GI (co)(N [p ] s

2

s

+ N [p ] ) s

s

(5)

At low solute concentration, N can be taken as a constant andfromthe intercept and the slope of a plot of I(2a>)/I(©) versus N , we can calculate [P2]s provided [P2]s is known or vice-versa. This approach, called the internal reference method, effectively eliminates the need for localfieldcorrection factors at optical frequency. Another approach is to determine the G factor from the slope of a plot of I(2co)/I(a>) versus Ns and the known EFISHG P value for a reference molecule, such as para-nitroaniline, in this solvent From the slope of a similar plot for the solute molecule in a similar solvent and the G factor, the hyperpolarizability of this molecule can be determined. By analyzing the data in this way, it is no longer necessary to record the intensity of the second harmonic for solvents that have a low nonlinear susceptibility, such as dioxane or methanol. Furthermore, the slope of a plot of I(2co)/I(©) versus Ns can be determined more precisely than the intercept and the [P ]s that is required for applying the internal reference method is also determined by using an EFISHG P value. s

2


s

2

2

2

Fluorescence Induced by Multiphoton Absorption. On comparing a number of results obtained with the HRS set-up to those obtained with the EFISHG set-up, we found that the HRS p values of a number of compounds were consistently higher than the EFISHG P values. The emission of visible lightfroma HRS sample being illuminated by an intense laser beam (1064 nm) had been noticed before. To investigate this phenomenon, the 532 nm interferencefilterthat is always used to block out any light different from the second harmonic was replaced by a monochromator to analyse the emitted radiation. A laser pulse (1064 nm, 10 ns, 30 mJ) was focused in afiltered10' M solution of trans retinol (Vitamin A) in methanol. The recorded spectrum is shown in Figure 2. Trans retinol was selected because of its similarity to other retinal derivatives, which do notfluoresceunder the same circumstances, and because the multiphoton absorption behaviour has been studied thoroughly (17,18). Since the angle between the ionone ring and the polyene chain is approximately 50° due to steric interactions between the 1,1' methyl groups and the hydrogen atom on carbon 7, all the excited states of trans retinol, which has Ci symmetry, have the same symmetry (A). This would mean that all the excited singlet states can be investigated by both one-photon and two-photon spectroscopy. These retinyl chromophores, however, arefrequentlytreated as belonging to the C2h point group, since their excited states are somewhat similar to those of linear polyenes. Due to the weak interaction between the terminal hydroxyl group and the polyene system, this approximation applies well to trans retinol. Two-photon spectroscopy on trans retinol showed that the first excited state has Ag symmetry, analogous to the linear polyenes. Due to the well-known parity rule for one and multiphoton transitions, this state can not be observed in one-photon spectroscopy. Even though the C2h 2

1


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HENDRICKX ET AL.


85

+

(a) X=0 (b) X=N-Bu (c) X=NH -Bu


^^^^

OH

Figure 1. Molecular structure of the compounds studied in this work : 1(a) trans retinal; 1(b) retinal Schiff base; 1(c) retinal protonated Schiff base; 2 retinoic acid; 3 vitamin A acetate; 4 trans retinol (vitamin A).

1000

350

400

450

500

550

600

650

700

750

Wavelength (ran)

Figure 2. Emission spectrum of trans retinol on excitation with 1064 nm.


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POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

symmetry is only an approximation, the selection rules for the absorption process still hold. The maximum of the one-photon absorption to the *Bu state of trans retinol is located at 331 nm in EPA at 77 K and the two photon transition is at 350 nm (16). The emission maximum, under the same circumstances, is at 461 nm. This emission was postulated to originate in the Ag state and not in the one-photon allowed *Bu state. Two bands can clearly be distinguished in the recorded emission spectrum. Gaussian curve fitting shows that the emission maxima are located at 560 nm and at 475 nm. A possible explanation for the observed pattern is that the emission from the Ag is pumped by three photon absorption (355 nm) into the Bu state, as well as two photon absorption (532 nm) into the *Ag state. This two-photon absorption process thus would stimulate emission below 532 nm (the second Gauss curve). The first Gauss curve has to be caused by three-photon absorption, since its energy lies well above the energy of two-photon absorption. The maximum of this Gauss curve also coincides with mat of the luminescence caused by one-photon absorption into the *Bu state. It should be noted that no peak is visible at 532 nm. This clearly indicates that the HRS signal is completely swamped by the much stronger fluorescence, making it impossible to determine a value for the hyperpolarizability with HRS at this wavelength with the current set-up. For other molecules, we are confident that a 3 nm bandwidth interference filter can eliminate fluorescence caused by two-photon absorption since this fluorescence is always red-shifted with respect to the two-photon excitation wavelength. A broad emission band triggered by three-photon absorption, however, will pass through the filter and artificially enhance the HRS P value. It can easily be verified whether such fluorescence is present by substituting the 532 nm interference filter for a 550 nm interference filter, or by substituting a 3 nm bandwidth interference filter at 532 nm for a 10 nm bandwidth interference filter at the same wavelength. The possibility of fluorescence stresses the importance of using an appropriate the interference filter. l


1

l

Theoretical Methodology The geometry of all the molecules was optimized using the semi-empirical HartreeFock Austin Model One (AMI) method (79). On the basis of the AMI geometries, the norm of the vector part of the first hyperpolarizability is calculated with the Intermediate Neglect of Differential Overlap/Sum-Over-States (INDO/SOS) technique based on the perturbation expansion of the Stark energy of the molecule (2021). This norm is defined as : (6) where, for example, component px is given by : Px = P

X X X

+

xyy

+ 2p

yyx

+P

xxz

+ 2p

zzx

3

(7)

The SOS method has the advantage of yielding easy access to the dynamic (i.e. frequency dependent) response in order to evaluate the dispersion in P of the present


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H E N D R I C K X E T AL.

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visual chromophores. In this work, we assume a damping factor of 0.1 eV for all the resonances, which is a good approximation for this kind of molecules (9). If we assume that the nonlinear optical response of a molecule is dominated by the ground state and a low-lying charge-transfer excited state, a simple expression can be derived for the second harmonic generation p component (22): xxx

(-2o; to, a»

-= 0,gas

+

a

E

0 rf

30

(13)

where |io,gas is the ground state dipole moment and ao is the ground state linear polarizability of the molecule in the ground state and in the solvent of choice. Since the reaction field is always in the same direction as the molecular dipole moment, which is also the direction of the main P tensor component for dipolar species such as para-nitroaniline and trans retinal, ao is the linear polarizability tensor component that is parallel to the dipole moment. A similar equation can be written for the excited state dipole moment: E

Hl,total = ^l.gas +

Polymers for Second-Order Nonlinear Optics - American Chemical

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