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J. Phys. Chem. 1996, 100, 446-448
Orientation of Toluene and Effect of the Photochromic Fulgide (E)-[r-(2,5-Dimethyl-3-furyl)ethylidene](dicyclopropylmethylene)-2,5-furandione at the Air/Toluene Interface D. T. Cramb, S. C. Martin, and S. C. Wallace* Department of Chemistry, 80 St. George Street, the UniVersity of Toronto, Toronto, Ontario, M5S 1A1 Canada ReceiVed: July 19, 1995; In Final Form: NoVember 6, 1995X
The properties of the air/liquid interface of toluene have been examined using second harmonic generation of laser light. By selection of the appropriate input laser polarization and output second harmonic polarization, the molecular plane of toluene was found to be inclined by 42° to the surface normal. The effect of the photochromic fulgide (E)-[R-(2,5-dimethyl-3-furyl)ethylidene](dicyclopropylmethylene)-2,5-furandione dissolved in toluene on the air/toluene interface was also studied. The presence of the fulgide was found to lower the intensity of second harmonic, and upon apparent monolayer formation, its molecular plane was found to be inclined to the surface normal.
Introduction Recently, nonlinear optical studies such as second harmonic generation (SHG) and sum frequency generation (SFG) have yielded much information about the structure of neat liquid/air interfaces1 and insoluble molecular monolayers at the liquid/ air interface.2-4 The exploration of the structure of a neat liquid/ air interface is of interest because the surface properties will depend on the orientation of its constituent molecules.5 The surface structure of liquids made almost entirely of nonpolar molecules could provide insight into the instantaneous structure of the bulk liquid. Molecular dynamics simulations of the properties of molecules at interfaces and in bulk have been used to understand industrially relevant features of various solvents. For example, a molecular dynamics simulation of bulk liquid toluene was undertaken by Kavassalis and co-workers6 in order to determine diffusion coefficients for neat toluene and mixed solutions of toluene and tetrahydrofuran. Since toluene is a fairly universal solvent of aromatic molecules, it would make an interesting case study for a surface SHG experiment. The use of toluene in the surface coating industry is extensive as it is used as a carrier enabling the coating material to be applied thinly and evenly. The solvent must then be easily removed, through surface evaporation, so as to leave the coating in the desired smooth state. Given this function, it is also of interest to attempt to characterise how solutes dissolved in toluene affect the structure of the liquid/air interface. To this end, the photochromic fulgide (E)-[R-(2,5-dimethyl3-furyl)ethylidene](dicyclopropylmethylene)-2,5-furandione (DCPF) is a suitable solute probe because it dissolves readily in toluene and has nonfluorescent electronic states.7 Photochromic fulgides are molecules whose structure is based on furan derivatives of dimethyl succinic anhydride. In the case of DCPF, there are two stable forms with which we will be concerned. The E form is uncolored which upon illumination with light of wavelength 315-390 nm undergoes a reversible photochromic reaction to the C (colored) form.8 The reverse reaction is driven by light of wavelength 575-450 nm. The structures of these two forms of DCPF are depicted in Figure 1. Neither form has appreciable absorption of radiation whose wavelength is greater than 610 nm. In this report, we discuss the results of studies where we have employed surface second harmonic generation from laser light X
Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0446$12.00/0
Figure 1. Structures of the E form (uncolored or bleached) and C form (colored) of (E)-[R-(2,5-dimethyl-3-furyl)ethylidene](dicyclopropylmethylene)-2,5-furandione. Interconversion between the two forms can be accomplished by exposure to light of appropriate wavelength.
in order to probe the structure of the neat toluene/air interface. Additionally, we have used SHG to assay the effect of the photochromic fulgide DCPF dissolved in toluene on the liquid/ air interface. Experimental Procedures The light source for these experiments was an amplified picosecond laser system. The details of this systems have been published previously9 and will be outlined only briefly here. The doubled output of a Nd:YLF (Coherent Antares 76-YLF) pumped dye laser (Spectra Physics SP-3500) was operated in the wavelength region of 650-680 nm (DCM). Nearly Fourier transform limited pulses of approximately 5 ps, as measured by autocorrelation, were produced. These pulses were amplified in a Nd:YAG (Spectra Physics TFR) pumped dye (DCM) jet amplifier to give an increase of 103 in pulse intensity. The resultant pulses have an energy of 8 µJ and a repetition rate of 1000 Hz. The amplified laser beam was directed through a Fresnel rhomb onto the liquid surface at 60° from normal incidence. A 7.5 mm focal length achromatic lens was used to produce a spot of 50 µm diameter on the surface. The reflected light was directed through a polarizer and a series of optical and spatial filters to remove the fundamental light and any fluorescence. The second harmonic light was detected with a photomultiplier tube (Hamamatsu R2801), the output of which was then amplified 100× and sent to a gated discriminator (EG&G ORTEC) for single-event counting. The dark background signal could be virtually eliminated by setting a narrow (∼30 ns) gate on the discriminator. Nevertheless, the signal intensity was low enough to warrant averaging for 40 s. © 1996 American Chemical Society
Letters
J. Phys. Chem., Vol. 100, No. 2, 1996 447
All experiments were performed at a controlled ambient room temperature of 18 °C. Results and Discussion Neat Toluene/Air Interface. For the neat toluene surface, measurements of the SHG intensity were taken for both s-polarized and p-polarized excitation. The output of the SHG was observed to be primarily p-polarized for both cases. The intensity of the second harmonic light coming from the surface can be modeled by considering light of frequency ω incident on the surface at angle θ from normal. If the incoming light has polarization jeω, then the reflected second harmonic light, of polarization je2ω, is
I2ω )
32π3ω2 sec θ |ej2ω‚χCs(2)(2ω):ejωjeω|2Iω2 c3
(1)
where Iω is the intensity of the exciting radiation. χCs(2) is a third-rank tensor representing the second-order surface nonlinear susceptibility. It can be approximated as the sum of the orientationally averaged second-order nonlinear polarizabilities, 〈R(2) jkl 〉i, of the molecules at the surface. If one assumes that the toluene molecules are arranged randomly with respect to their distribution in the XY plane of the surface, with Z being the surface normal, but have some preferred orientation of the nonlinear transition moment, µ1µ2, versus the surface normal, then the ratio of SHG for p-polarized to s-polarized, for our experimental incidence angle of 60° excitation, is10
R)
[
3〈cos3 θ′〉
2〈sin2 θ′ cos θ′〉
-
]
1 4
2
(2)
θ′ is the angle between the dominant component of R(2) jkl and the surface normal, Z. Our measured value of R ) 2.67 suggests an average angle of 42° to the surface normal for pure toluene. It has been observed that in the case of a neat methanol/air interface, the angle of the O-C bond with the surface normal spans a wide distribution1 of (30°. It is assumed that a similarly wide distribution about the average angle also exists for toluene. Because the resonant absorption for toluene lies outside our 2 photon wavelength, λabs e 300 nm, R(2) jkl will not be dominated by the first excited electronic state which is π,π*. This means that the polarization moment for toluene SHG at this wavelength lies within the molecular plane. The angle of 42° between Z and 〈R(2) jkl 〉 suggests that the toluene molecules are not laying flat on the surface but rather are oriented somewhere between parallel and perpendicular to it. This tilting of the molecular plane away from parallel to the surface may be rationalized as a balance between the tendencies toward a maximization of van der Waals bonding with the surface (i.e., the molecule is laying flat on the surface) and the randomness of the bulk structure driving a minimization of packing density at the surface. Similar tilting behavior has been observed experimentally for a neat methanol/air interface1 and in a molecular dynamics simulation of a Langmuir-Blodgett film of long-chain alkanes11 where a tilting angle of 30° was found. To our knowledge, this is the first determination of orientational anisotropy at the neat liquid/air interface for an aromatic molecule. DCPF at the Toluene/Air Interface. The response of the SHG intensity to increasing concentration of DCPF is plotted in Figure 2. The data points consist of an average of 10 scans, collected for 40 s each. The average diffusion time out of the
Figure 2. Intensity of second harmonic generation of laser light from a toluene/air interface as a function of concentration of the photochromic fulgide (E)-[R-(2,5-dimethyl-3-furyl)ethylidene](dicyclopropylmethylene)2,5-furandione for (a) p-polarized excitation light and p-polarized detection of SHG, (b) s-polarized excitation light and p-polarized detection of SHG. Each data point is an average of 10 samples. The error bars represent 1 standard deviation of the averaged data.
50 µm interaction region is estimated to be 5 s assuming a onedimensional random walk along the radius. It is evident that DCPF lowers the intensity of SH from the liquid/air interface with increasing concentration. The effect levels out at a concentration of approximately 10-3 M. A simple model which would rationalize this effect is the formation of a monolayer of DCPF at the air/toluene interface. There was no spectral evidence for the formation of dimers in the bulk at concentrations greater than 10-2 M. Using arguments similar to those of the preceding subsection, the orientation of the SHG transition moment can be determined with respect to the surface normal. In this situation, at a concentration where a monolayer can be considered established, 10-2 M, the ratio of the SHG from p-polarized to s-polarized excitation can be determined. The value of R ) 10 indicates an average angle of 34° versus the surface normal. In contrast to toluene, DCPF possesses an electronic state which falls into resonance with the two-photon energy available in our experiment. However, since the SHG intensity actually decreases, it would appear that a two-photon transition from the ground electronic state to the first excited state of the E form of DCPF is very weak. There is considerable debate as to the nature of the transition which is the first step in the photochromic transformation of fulgides from the E to the C form. Heller et al.12 have proposed that the initial excitation is that of a n-π* transition of one of the carbonyl chromophores in the molecule. Lenoble and Becker13 suggest that a π-π* transition is more plausible. Although results of MNDO molecular orbital calculations by Yoshioka et al.14 agree with the π-π* scenario, there is as yet no conclusive proof to eliminate either possibility. If one considers the carbonyl chromophore in DCPF in the n-π* excitation, then the system is of Cs symmetry containing only a mirror plane. The transition then must transform as an out of plane moment of A′′ symmetry. A two-photon transition must be driven by one photon of Γ(Tz) ) A′′ symmetry followed by one of Γ(Tx) ) Γ(Ty) ) A′′ symmetry, or vice versa. In the alternate case of a π,π* system, the excitation is of the hexatriene chromophore in the fulgide. Here, the transition moment for the first excited state is B1 and the chromophore is considered to possess C2V symmetry. For a two-photon transition, the sequence must be Γ(Tz) ) A1, Γ(Tx) ) B1 or vice versa. The alternative is more plausible in light of the fact that the E to C transition follows Woodward-Hoffman rules.15 In either case, the polarization data suggest that the plane of DCPF is inclined to the surface normal. If the transition is n-π*, then the plane of the molecule is 90° - 34° ) 56°
448 J. Phys. Chem., Vol. 100, No. 2, 1996 inclined to the surface normal. If the transition is π-π*, then the plane is also inclined at 56°. The decline of SHG, as a function of DCPF concentration, despite the near two-photon enhancement in DCPF indicates that either 〈R(2) jkl 〉 is smaller for DCPF than for toluene or is of similar magnitude and the difference is contained in the change in orientation. We propose a scenario whereby DCPF is situated at the interface such that the transition dipole alternates between pointing into and out of the liquid. This would serve to cause a net minimization of the macroscopic polarizability due to the cancelation individual terms in the polarizability tensor. Alternatively, since there is little difference in SHG production between the bleached (E) and colored (C) forms of DCPF, R(2) jkl must be small. This is consistent with efforts in our lab to drive the photochromic response with two-photon resonant excitation. The one-photon quantum efficiency is fairly large, ∼0.23, indicating that the one-photon transition is not forbidden. Therefore, the weak second harmonic generation efficiency suggests that because of the lack of a large difference between the dipole moments of the states, the transition moment does not dominate and that R(2) jkl is contained in the plane of the molecule. This second possibility could help rationalize a small R(2) jkl but does not suggest why it would be significantly smaller than that of toluene. It is likely that in addition there is a selforganized alternating orientation of DCPF resulting in the decreased SHG efficiency. Second harmonic generation studies of DCPF in crystalline form are under way in order to help clarify the question of the direction and magnitude of the transition moment. Conclusions Using the technique of surface second harmonic generation of laser light, the interface between air and toluene has been examined. The toluene molecules were found to have a preferred orientation at the surface where the molecular plane is inclined to the surface normal by 42°. This tilting is explained by a balancing of the forces driving the surface system to attempt to maximize van der Waals bonding and lay flat on the surface, with the forces of the random and fluctuating bulk which drives a minimization of packing density. The effect of the photochromic fulgide (E)-[R-(2,5-dimethyl3-furyl)ethylidene](dicyclopropylmethylene)-2,5-furandione (DCPF) dissolved in toluene on the interface was also assayed. The DCPF was observed to lower the SHG efficiency despite
Letters the fact that it possesses an electronic state which is coincident with the two-photon energy. The dependence of the SHG on the input laser polarization infers that the plane of DCPF is most likely inclined by 34° to the surface normal. Since neat toluene displays a tendency to tilt at the liquid/air interface, it is not surprising that DCPF, which is a fairly rigid molecule, would also display this inclination. The experimental determination of the surface structure of toluene and molecules dissolved in toluene is useful for understanding the roles of solvent and solute in evaporative processes. The fact that orientation of DCPF at the air/toluene interface changes depending on surface concentration suggests that as toluene evaporates, a coating DCPF will display selforganization and reorient itself into the energetically most favorable macroscopic structure. Acknowledgment. Financial support from the Natural Sciences and Engineering Research Council of Canada and the Ontario Laser and Lightwave Research Centre is gratefully acknowledged. References and Notes (1) Superfine, R.; Huang, J. Y.; Shen, Y. R. Phys. ReV. Lett. 1991, 66, 1066. (2) Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys. ReV. 1983, 28, 883. (3) Raising, Th.; Shen, Y. R.; Kim, M. W.; Grubb, S. Phys. ReV. Lett. 1985, 55, 2903. (4) Heinz, T. F.; Chen, C. K.; Richard, D.; Shen, Y. R. Phys. ReV. Lett. 1982, 48, 478. (5) Fluid Interfacial Phenomena; Croxton, C. A., Ed.; Wiley: New York, 1986. (6) Bareman, J. P.; Reid, R. I.; Hryman, A. N.; Kavassalis, T. A. Mol. Simul. 1993, 11, 243. (7) Martin, S. C.; Singh, N.; Wallace, S. C., to be published. (8) Yokoyama, Y.; Goto, T.; Inoue, T.; Yokoyama, M.; Kurita, Y. Chem. Lett. 1988, 1049. (9) Demmer, D. R.; Leach, G. W.; Wallace, S. C. J. Phys. Chem. 1994, 98, 12834. (10) Mazely, T. L.; Hetherington, W. M. J. Chem. Phys. 1987, 86, 3640. (11) Bareman, J. P.; Klein, M. L. J. Phys. Chem. 1990, 94, 5202. (12) Heller, H. G.; Szewczyk, M. J. Chem. Soc., Perkin Trans. 1974, 1, 1487. (13) Lenoble, C.; Becker, R. S. J. Chem. Phys. 1986, 90, 2651. (14) Yoshioka, Y.; Tanaka, T.; Sawada, M.; Irie, M. Chem. Lett. 1989, 19. (15) Flurry, R. L. Symmetry Groups; Prentice-Hall: Englewood Cliffs, NJ, 1980.
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