Orientation of xanthene adsorbate molecules at dielectric interfaces

CkiEMlCAL PHYSICS LETTERS

Volume 114, number 1

ORIENTATXON OF XANTHENE

ADSORBATE

15 February

1985

MOLECULES AT RIELECTRIC INTERFACES

P. DI LAZZARO, P. MATALONI and F. DE MARTINI Quantum@tics Laboratory. Physics Department, University of Rome, 00185 Rome. Italy Received 3 August 1984; in kinalform 27 November 1984

- The polarization dependence of the second harmonic signal from monolayers of xanthenc dyes allows us to develop a model for the orientation of the adsorbates and to locate the group responsible for adsorption. Possible extension of this method to other dyes of biologi4 and medical interest is discussed.

1. Introduction The determination of molecular orientation of monokyer adsorbates by optical second-harmonic generation (SHG) has been described in several places [l] _ The experimental apparatus for this kmd of experiment embodies a high-power, continuously tunable laser to generate the non-linear signal and to obtain spectroscopic data from the resonant behavior of the SHG. In this way it is possible to calculate the average orientation of the molecular adsorbates at resonance (svhenever the pump frequency w coincides with that of a transition [2))_ In &IS Letter we demonstrate that it rs possible to obtain the same results by means of a once-frequent NdYAG laser and very simple system of detection; moreover we have identified the carboxyphenyl substituent as the group which is important in the process of adsorption of rhodamine dyes [3].

2. Surface SHG It is well known that in any material possessing a center of inversion, the process of optical SHG is forbidden vvlthin the electric-dipole approximatron [4] _As a consequence, the second-harmonic (SH) radiation produced by the excitation of an interface between two centrosymmetric medra actually arises in the few atonuc or molecular layers of the interface, where the inversion Gwen is broken. Thus the SHG process exlnbrts an intrinsrc surface specificity on an atomic scale, a property not usually possessed by the conventional surface probes. We can calculate the intensity of the SH radiation generated from a thin. film in the forward or reflected direction by excitation of a plane wave of frequency w and polarization Z(a) (51I 1(2u)

= 32Gc,~~c-~

se~~O16(2w)~;~)

: e(o)

+J)[~I~(o)_

(1)

In &is equation B represents the angle of incrdence,l(o) is the pump intensity and d(20) is the detected polarization at the SH frequency. The tensor& (2l represents the second-order non-linear susceptibility of the surface, defimed (for sufficiently dilute systems) as a sum of molecular contributions: x S@I= Me&+,

1’21

where N is the surface density of the adsorbates, e (2) is the molecular second-order non-linear pohuizabihty and the angular brackets signify an avetige over molecular orientation of the adsorbed molecules. Let us make rough estimate of the SH output from an adsorbed layer of aligned molecules, assuming the 0 U~9-2614/85/~ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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typical value forN= 1014 CITI-* and (Y@) z 10W30 esu_ In our case, by a 2 ns pulse of laser radiation at 1.06 pm havrng an intensityI = 110 MW/crn2, for 0 = 45O, we can obtain from eqs_ (2) and (1) a SH output of =103 photons per pulse, wluch should be readiIy detectable.

3. Experimental The monolayer samples of xanthene dyes were prepared from a 3 X 10 4 M ethanolic solution placed on optically flat fused silica substrate by the spinning technique. The substrate was cleaned and mounted on a highspeed motor, a few drops of ethanolic solution of the dye placed on the substrate, and the sample spun at 2000 rpm for 2 min. The average surface density of the dye molecules was =r1014 cmB2_ The source used was a selfmjected neodymium laser, previously described in ref. [6] _ In fig 1 the time diagram for Q-switching, self-injection, cavity dumpmg operation, and the schematic of the optical resonator are shown. In our case the I .06 I.rm cavity dumped optical pulse was 2 ns long with an associated energy of 36 mJ. When necessary, the SH output may easily be increased using the same laser in the picosecond version [7]. In our experiment it was not necessary to use picosecond pulses, nor to focus the pump beam; no change of

c qp

I

I

Fig. 1 - Selfinjected laser cavity elements- PC is a Pock& celi, Pa poianzer. M1 and M2 are totally reflecting minors. DC is the dye cell (for picosecond version only). (a) Time diagrim of voltage applied to Pock& cell. ml Evolution of pulse bluedup tide the cavity. Cc) Single output dumped pulse.

Volunie114, number1

CHEMICAL

PHYSICS

15 February

LEITERS

1985

L 0

PD

0 \

I-I

T I

1

20

u F

P

F?g. 2. Experimental apparatus. L = laser, PD = detector, S = sample, F = colour filter, P = polarizer, PM = photomultiplier. 0 = oscilloscope.

PM

the output sign.al was detected, giving evidence that no laser-induced damage had taken place. This can be due to the fact in our case there rs no linear absorption at the pump frequency. For the same reason no background contnbution of luminescence was detected. The apparatus 1s shcwn in fig. 2. The SH signal, appropriately filtered and polarization analyzed, was detected by a XP 1010 Philips photomultiplier and a 7D20 Tektronix averager memory oscilloscope. The signal at frequency 2w, whrch amounted to ==5 X lo3 photons per pulse for the Rh6G, showed 2 quadratic dependence on the pump laser power as expected_ No appreciable conoibution to the SGH from the substrate was observed_ The output laser pulse was p-polanzed with respect to the horizontal plane of incidence. The spolarized excitation measurement was taken by rotating the incidence plane of 90°r-in this way. the SH signal was reflected in a vertical drrection. In either case, we detected both p and s polarization analyzed SH signals.

4. Results and anrdysis The SH intensity was independent of rotation of the sample about its surface normal: this rotational invariance is consistent with the expected isotropy of the adsorbed layer in the plane. The presence of 2 strong SH signal then implies that the molecules do not lie flat on the surface, but stand up with the molecular plane perpendicular to the surface plane [2]. The optical properties of the xanthene dyes have been widely studied [8] : they are molecules with planar structure, whose transition moment of the main absorption band is oriented parallel to the long axis of the chromophore. Transitions at shorter wavelength are oriented perpendicular to the long axis. In other words, the Se + Sr transition dipole moment is directed along the chromophore axis (say, the X’ direction) while the So + S2 moment is perpendicular to it (z’ axis), as illustrated in fig. 3a. A more realistic, simple model of adsorbate orientation * assumes that the molecular y’ axes (direction normal to the plane of each molecule) lie in random orientations in the plane of the surface (fig. 3b), so that to 2 first approximation, the sample possesses unia_xiai symmetry. It is then possible to define a “renormalized intensity” as the projection of the exciting electromagnetic field and of the SH electromagnetic field on the transition dipole moment squared. By simple trigonometry and taking advantage of the symmetry, we obtain the matrix [9] J (sin4cr>

4(sin2a

cos2cr)

3(sin4ru)

4(sirl%

co&1

4(m2,

l

Otha possible models have been rejected ref. [2] for details.

c&Y)

beaux

8(cos4a)

of symrnetxy

.

(3)

1 considerations

and the lack of agreement

H-&II experiment.

See

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Volume 114, number 1

CHEMICALPHYSICS

LE-ITERS

15 February 1985

,P 'OH

S.

18900

7-r

cni’

18700

cm-’

2fil

12% - OL. I=

0

SO

8OOCmi'

H-O f luorescein,acidic

20000

cm-’

12 OL - w_

I=

1100crli’

l-00

cd

I 20,

- w.-

I=0

a

fig. 3. (a) Energy-leveldiagramsand molecular structuresof tie dyes used iu the expedient. The energy levels correspond to the absorption hne centers of the dyes dissolvedin ethanol. The off-zesonant amount for each dye is indimtecl. @) Diagram of the an& P desm-iiingthe molecular axes x’y’z’ in relation to the XYZ coordinate system tixed to the surface. Each Ii,- (I j = X,Y,Z) is proportional to the SH intensity measured in the substrate coordinates; L*is the angle between the x’ axis of an adsorbed molecule and the Z surface normal, and the averages are extended over the distribution of all such angles for the adsorbates (see fig. 3b). In this way, the ratio of two independently measurqd SH intensities can be related to a function of the angle of attachment Q, without needing to be at the resonance. With a complete set of measurements it is possible to calculate various ratios of the matrix elements (3) and to compare the different values of the angles obtained (for a narrow distribution), testing the reliabiIity of the results. Table 1 lists the expenmental results for various xanthene dyes. 106

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15 February 1985

Table 1 Xanthene dye

Antie of attachment (de@

rhodamine 6G rhodamine B fluore.scein acldrc pyronin B

52+4 55 f 4 53 f 2 67 f 4

Our model (Jo’ axes randomly oriented in the plane of the surface) is compatrble with the assumption that the process of adsorption is determined by the binding either of one of the amino groups or of the add group [2] _ But having obtained the same angle of adsorption, within experimental accuracy, for the rhodamine dyes and for the fluorescein dye (which does not possess an ammo group, as shown in fig. 3), one could deduce that the acid group IS the one that is responsible for the adsorption. This statement is consistent with the result of the same experiment made on other xanthene r’yes which lack the carboxyphenyl substituent. For mstance the pyronin dye lies at an angle a which is different from the one shown in other cases (table 1).

5. Discussion In conclusion, we have measured the orientation of xanthene dyes monolayers adsorbates on fused silica by a SHG technique _ Selective measurements on rhodamine, fluorescem and pyronin dyes suggest that attachment is through the carboxylic group. The authors believe that the acidic group COOH (for RhB and Iluorescem) and the group COOEt (for Rh6G) is responsible for the molecular adsorption, in the first case probably through hydrogen bonding with the fused silica substrate. There is as yet no positive experimental evidence of hydrogen bonding between monoIayers of xanthene dyes and the substrate in our experiment. Research on this point is in progress in our laboratory. The angle of attachment a for rhodamine dyes is m agreement with the corresponding values of the angle 8 = 90 - a reported by Shen and co-workers using resonant SHG with the So + ~2 electronic transition [2]. We believe that our non-resonant SHG technique is simpler and easier of realization than the one used by Shen even if possibly of less general application. The only restraint in our model is that the low-lying absorption band must include the doubled pump frequency, or, in other words, the So + S1 transition dipole moment should be nonlinearly excited. We believe that our technique may easily be extended to relevant brolognzal and chemical applications, for instance, it could be a useful tool for understanding the nature of the attachment of acridine and other organrc dyes to cancerous tissue.

Acbowledgement The authors take pleasure in thanking Dr. F. Castelli for helpful discussions. Nazionale di Elettronica Quantiskica e Plasmi, CN.R., Italy.

This work was supported

by Gruppo

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114. number 1

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PHYSICS

LEZTERS

15 February 1985

References [l] (21 [3] [4] [S] [6] [7]

[B] [9]

T.F. Hems, H.W.K. Tom and Y.R. Shen, in: Surface studies wth lass, eds. F.R. Aussenegg, A. Leitner and ME. Lippitsch (Sprmger, Be&n, 1983) p. 79, and references therein. T.F. Heinz, C.K. Chen, D. Richard and Y.R. Shen, Phys. Rev. Letters 48 (1982) 478. P. Di Lazzaro. P hfataloni and F. De Martini. Study of the SGuctural Anangement of Xanthene Molecular Adsorbates at Interfaces by Optical Second Harmonic Generation, work presented at the Xl11 I Q EC ‘84. Anaheim, California (June l8-21,1984). N. Bloembergen, Nonlinear optics (Benjamin, New York, 1977). N. Bloembergen and P.S. Pershan, whys. Rev. 128 (1962) 606. C.H. Brito Crux, E. Palange and F. De Martini, Opt. Commun. 39 (1981) 331; E. Palange, CH. Brito 011z, P. Di Lazza~o and F. De Martini, Appl. Phys. Letters 41 (1982) 213. CH Brito (Iruz, F. De hlarbni. H.L. Fragmto and E. Palange, Opt. Commun. 40 (1982) 298; F. Armani, F. De Marbm and P. Mataloni, in: Picosecond phenomena, Vol. 3, eds. K.B. Eisenthal, R.M. Hochsbzsser, W. Kaiser and A. Lauberau (Springer, BerIm. 1982) p. 71. K.H. Drexage, in: Topics of applied physics. Dye laser, ed. FS. Schafer (Springer, %rlm, 1978) p. 144; JP HermaIln and J. Ducuing, Opt. Commun. 6 (1972) 101. C.R. Desper and I I(lmura, J. Appl. Phys. 38 (1967) 4225.

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