Solid State Phenomena Vols. 97-98 (2004) pp. 207-214 online at http://www.scientific.net © 2004 Scitec Publications, Switzerland
Reaction Dynamics and Applications in Patterning of Bisthienylcyclopentene-Based Photochromic Switches P.R. Hania1, A. Pugžlys1, L.N. Lucas2, J.J.D. de Jong2, J.H. van Esch2, B.L. Feringa2 and K. Duppen1 1
Ultrafast Laser and Spectroscopy Laboratory and 2Organic Chemistry Laboratory, Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
[email protected] Keywords: photochromic switches, aggregation, holographic gratings, diffusion.
Abstract. The structural and optical properties of the gel-forming photochromic switch 1,2-bis(2’methyl-5’-{{((R) phenylethyl)amino}carbonyl}thienyl-3’-yl)cyclopentene are studied by means of the linear absorption and holographic grating techniques. The use of diffractive optics enables recording of holographic gratings with high long-term phase stability. The diffraction efficiency of the recorded holographic gratings approach values of 30% for low writing beam energies when diffusion is the rate determining factor. At higher writing pulse energies the competition between the diffusion and photodecomposition processes causes lower diffraction efficiencies. At irradiation doses above 10 mJ the spatial profile of the recorded gratings is strongly influenced by saturation effects. Because of the well-determined grating profile the holographic grating technique is potentially applicable for the quantitative characterisation of the diffusion process in photochromic gels. Introduction Organic photochromic compounds are of considerable interest because of their potential applications in information processing, data storage and nonlinear optical devices. Bisthienylethenes constitute a new class of thermally irreversible photochromic switches [1]. Apart from high thermal and photostability these switches feature remarkable switching sensitivity (high quantum yields) as well as rapid response [1,2]. Photochromism is defined as a reversible photo-induced chemical reaction during which optical, dielectric and conformational properties of molecules change. In the case of bisthienylethenes, photochromism is achieved by means of a reversible photoinduced ring-closure reaction. The ground-state interconversion between the isomers is prohibited but upon UV irradiation the open form (1A) converts to the closed form (1B) (see Fig. 1). In the open form there is no conjugation between the thiophene units. On the contrary due to the arrangement of olefinic bonds in the closed form π-conjugation spreads throughout the molecule resulting in the appearance of a new red shifted absorption band situated in the visible spectral region. Fig. 1. Chemical structure of the open Upon excitation of the closed form this conjugation is and closed isomers of the broken again by opening of the ring, which yields the bisthienylethene photochromic switch. initial open firm. The typical absorption spectra of hν1 and hν2 are the energies of UV and open and closed forms are presented in Fig. 2. VIS photons, respectively. Below the Due to their excellent switching properties, chemical structure of the (R)-N(1bisthienylethene-based photochromic switches can phenylethyl)amide substituent, which fulfill a variety of roles in larger (supra)molecular leads to the formation of a gel, is shown. structures. The most obvious applications currently
208
Self-Formation Theory and Applications
under investigation are in electron transfer and energy transfer systems [3,4]. However, also structural properties change upon switching between the flexible open and rigid closed forms, which can be used in several ways. Fig. 1 shows the molecular structure of 1,2-bis(2’-methyl-5’{{((R)-1-phenylethyl) amino} carbonyl}thienyl3’-yl)cyclopentene, the photo-switchable gelator under investigation [5]. The basic photochromic unit (top), which has been used in several other photoswitchable molecular systems [3,4,6], is functionalised with hydrogen-bond forming (R)1-phenylethylamine derived amides (bottom). Intermolecular hydrogen bonds are formed between the amide protons and oxygens, which provides a 1-dimensional growth scheme leading to aggregation into large helical fibres. This in turn results in gelation of the solution above a Fig. 2. Linear absorption spectra of gels of certain minimum concentration, which depends open-ring (dashed line) isomer of 1,2-bis(2’on the solvent and temperature. Since solid-state methyl-5’-{{((R) phenylethyl) amino} materials are valued for practical application, the carbonyl}thienyl-3’-yl) cyclopentene and of discovery of photochromic materials in the gelphotostationary state (solid line). state opens new perspectives in the design of novel optical memory storage devices. In this paper we discuss the structural and optical properties of the 1,2-bis(2’-methyl-5’-{{((R) phenylethyl)amino}carbonyl}thienyl-3’-yl)cyclopentene photochromic switch and how they affect the optical recoding process. The holographic grating technique is applied for the studies. We show that by recording the diffraction efficiency of the holographic grating during its creation the progression of the ring closure reaction can be investigated. Experimental Sample preparation. The open form of 1,2-bis(2’-methyl-5’-{{((R)-1phenylethyl)amino}carbonyl}thienyl-3’-yl)cyclopentene (1A in Fig. 1.) was synthesized as described in [5]. The molecules were dissolved at elevated temperatures (ca. 70 °C) in toluene at a concentration of 2.0 mM and transferred to a fused silica cell of 1 mm pathlength. Slow cooling of the sample to room temperature then yielded the gel. Experimental setup. The schematics of the experimental setup is shown in Fig. 3. UV light of tunable wavelength was generated by using a 1 kHz Ti:sapphire laser system (Hurricane, Spectra Physics) and an optical parametric amplifier (NOPA). The laser system produces 120-fs, 800-µJ pulses at 1 kHz, centered at 800 nm. About 250 µJ of that energy is used to pump a NOPA (Topas White, Light Conversion LTD), which generates 30 fs pulses tunable in the spectral region 500750 nm. The sample was irradiated by the second harmonics of the NOPA output, which was generated in a 0.2 mm thick BBO crystal (SHG). Experiments were performed with writing pulse energies varying from 180 nJ to 0.7 nJ and centered Fig. 3. Schematic representation of the at 330 nm. Polarization of the light was kept experimental setup.
Solid State Phenomena Vols. 97-98
209
horizontal. All the experiments were performed at room temperature. The UV light passes a diffractive optics element (DO), which is a phase grating specially designed to diffract into the +1 and –1 orders about 30% of the input energy. Since only transmission optics is used after the DO element, no phase distortions are introduced between the two interacting beams. In addition the use of diffractive optics ensures optimum spatial overlap of the two writing pulses in the sample [7]. The imaging system based on two lenses with focal strength f1 and f2 allows control of the incidence angle α between the writing beams and so, of the holographic grating period Λ:
λ in Eq. (1) is the wavelength of irradiation and d is the spacing of the phase grating (DO). Different approaches are applied for characterization of thin and volume gratings. The separation between the two types of gratings can be made by evaluating the parameter Q = 2πλl / nΛ2 [8] where next to the parameters mentioned above n denotes the refraction index of the medium while l is the thickness of the sample. In this contribution we discuss the properties of holographic gratings having a period of 13.4µm which were recorded in a 1-mm thick fused silica cell. By taking into account the refraction index of toluene of 1.496, and probe wavelength 650 nm, Q adopts a value of about 14. This indicates that we deal with volume gratings [8]. In the case of volume gratings diffraction proceeds only at the Bragg diffraction angle θ B = arcsin(λ / 2nΛ), which in the present case is estimated to be about 0.93°. The micrograms of the recorded gratings were analyzed with a microscope (BX60, Olympus) supplied with a CCD camera. The illumination wavelength was selected by interference filters having a bandwith of about 10 nm FWHM. Deepness resolution in the case of microscope objective (LCPlanFL, 40X, n.a. = 0.6) was determined to be in the order of 80 µm. Results and Discussion Real time dynamics. Formation of the holographic gratings in photochromic gels is initiated by modulation of the complex refractive index during either the ring opening or the ring-closure reaction. Knowledge of the dynamics of this modulation is important in predicting the properties of the holographic gratings. As it was demonstrated in our previous work [2,9] the real time dynamics of the photochromic reaction can be traced by irradiating the sample in the UV spectral region where both the openring and the closed-ring isomers absorb while probing the dynamics by visible light within the absorption band of the closed-ring isomer. In the case when the dynamics is described by a set of first order differential equations the growth of the measured curve at initial times is mainly determined by the rate constants of the ring-closure and ring-opening reactions while the decay is dominated by the limited
Fig. 4. Real time dynamics probed at 650 nm while iradiating the sample at 330 nm with pulses having energies of 1.4 nJ (solid line), 14 nJ (dashed line) and180 nJ (dotted line). In the insert the initial part of the curves is zoomed.
210
Self-Formation Theory and Applications
photostability of the sample. Furthermore, within the linear excitation limit the dynamics is independent on the flux of excitation energy. In Fig. 4 the real time dynamics of the gel-switch as probed at 650 nm while irradiating at 330 nm with pulses having energies of 1.4 nJ, 14 nJ and 180 nJ are plotted. The excitation beam during measurements was defocused in order to avoid nonlinear sample excitation. The probe wavelength of 650 nm, i.e. at the edge of the absorption spectrum (see Fig. 2), was chosen in order to minimize the influence of the probe beam to the real time dynamics by means of initiating the ring-opening reaction. As it comes into view from Fig. 4. the measured dynamics are evidently excitation flux dependent. First, the optical density of the sample approaches higher values for lower excitation flux. Second, with increasing excitation pulse energy formation of the equilibrium between the molecules in the open and closed-ring configuration slows down. In general the equilibrium is determined by the different rate constants of the ring-opening and closure processes as well as by the absorption constants of the two conformers at the irradiation wavelength. However, none of these constants is supposed to be influenced by the excitation density within the linear excitation limit. An explanation for the observed dependence of the real time dynamics on the excitation pulse energy can be given by taking into account a diffusion process. Because of the small binding energy of the aggregates which is evidenced by the fact that the gelation temperature at concentration of 2 mM lies only about 17 K above room temperature [5], a certain dynamic equilibrium between the number of aggregated and non-aggregated molecules is present. The non-aggregated molecules are capable of migrating within the relatively open fibre -like network of the aggregates, which causes diffusion. It is well known that in the closed form the switch molecules have a more rigid geometry [1], which causes stronger binding revealed by the higher gelation temperature of the closed-ring isomers [5] and, therefore, less pronounced diffusion. The justification is supported by the observation that under irradiation of the sample containing open-ring isomers by UV light the volume of the gel decreases within the sample as closed-ring isomers are formed. Since diffusion in a gel is supposed to be relatively slow, it proceeds in competition with the photodecomposition process, i.e. at low excitation flux diffusion dominates the real time dynamics at longer irradiation times while at high excitation flux the photodecomposition is the rate determining process. Diffraction efficiency. Because of the well-defined spatial structure of the holographic pattern the holographic grating technique is a more powerful method for studying the real time dynamics in the presence of diffusion. In general both the phase and amplitude modulations contribute to the diffracted signal. According to the coupled wave theory [8] the diffraction efficiency of a mixed volume grating is given by:
where α0 is the average absorption coefficient, l is the thickness of the grating, θ is the angle between the direction of the transmitted light and normal of the grating, ∆n1 is the modulation of the real part of the refractive index and ∆α is the modulation of the absorption coefficient.
Fig. 5. Micrographs of the created pattern, taken at illumination wavelength of 550 nm after different doses of irradiation (increasing from left to right). The bar corresponds to 20 µm. Below the corresponding Fourier spectra are presented.
Solid State Phenomena Vols. 97-98
The
values
of
the
absorption
211
coefficient
∆α=ODln10/l can be calculated from the measured
optical density (OD) curves (Fig. 4). The values of the real part of the refraction index though are not known. However if to take into account that the real and imaginary parts of the refractive index are interconnected via the Kramers-Kronig relations, the time development of the real part of the refraction index is expected to follow that of the imaginary part. Consequently, the real part of the refraction index in Eq. (2) can be expressed via the imaginary part: ∆n2 = λ∆α/4π, ∆n1 = x∆n2, where x is the scaling factor between the real (∆n1) and imaginary (∆n2) parts of the complex refractive index. The coupled wave theory [8], which we use here Fig. 6. Spatial profiles of the holographic to analyze the obtained data is applicable in the case gratings (right column) calculated for when the spatial modulation of the refractive index different irradiation doses (indicated in the and the absorption constant is sinusoidal. However panels) and corresponding Fourier spectra the evidently nonlinear character of the curves (left column). presented in Fig. 4 predicts pronounced saturation effects. In Fig. 5 the micrograms of the holographic gratings recorded in a 0.1 mm cell at different irradiation doses are shown. The thickness of the sample was chosen to match the microscope deepness resolution (for details see experimental section). Fourier analysis of the micrographs indicates that higher spatial frequencies start contributing to the diffraction at high irradiation doses. In order to determine the sensitivity of the shape of the holographic gratings to the irradiation dose we simulated the spatial profiles by using the real time dynamics curve measured at excitation pulse energy of 180 nJ (see Fig 4). As can be seen from Fig. 6, where the results of simulations are presented, the spatial profile of the grating remains sinusoidal below irradiation doses of 10 mJ of absorbed energy. As it comes into view from the insert of Fig. 4, the energy interval corresponds to a quasi-linear real time dynamics. With increasing irradiation dose the weight of the first diffraction order starts to decrease until it fully disappears while the contribution of higher diffraction orders (up to 9th in the case of 575 mJ of absorbed energy) gradually increases. The results suggest that the sinusoidal shape of the grating is retained up to 10 mJ of absorbed energy and that at these irradiation doses the coupled wave theory can be applied for the calculation of the diffraction efficiency.
Fig. 7. Measured (solid line) and calculated diffraction efficiencies in the case of pure amplitude (dotted line) and mixed (dashed line) gratings for different irradiation doses (indicated in the panels). For experimental details see text.
212
Self-Formation Theory and Applications
In Fig. 7 the diffraction efficiencies as measured in a 1 mm cell containing a gel for different writing beam energies in the case of a 650 nm probe beam are shown. The experimentally obtained diffraction efficiency η is calculated as a ratio between the intensity of the diffracted beam with the sum of intensities of diffracted and transmitted beams: η=ID/(ID+IT). The diffraction efficiencies, calculated by using Eq. (2) and the real time dynamics curves presented in Fig. 4 are plotted in Fig. 7 for comparison. The calculated curves are scaled by matching the experimental and theoretical data at the irradiation doses below 10 mJ, where the spatial profile of the grating retains a sinusoidal shape. This reveals a scaling factor x = 14 for all calculated curves. This means that even at 650 nm the phase holographic grating plays a dominant role. From Fig. 7 it is evident that with decreasing the energy of the writing beams the grating efficiency increases substantially. Specifically, in the case of a writing pulse energy of 70 nJ the maximum diffraction efficiency is in the order of 3% while at ten times lower energy it approaches 20% and shows a tendency to increase. Finally, at a writing pulse energy of 0.7 nJ the diffraction efficiency exceeds 25% and features of saturation are observed. The higher diffraction efficiency at lower excitation flux, as it was discussed above, is caused by a diffusion process, which is determined not only by the mobility of the switch molecules but also by the rates of disaggregation and aggregation. Furthermore, as it is evident from the measured real time dynamics (see previous section), at low writing pulse energies diffusion from the nonirradiated to irradiated areas of the sample takes place. Explicit analysis of the diffusion-assisted aggregation in photochromic gels will be presented elsewhere [10]. At larger irradiation doses the experimental curves tend to level off from the calculated ones. The evident reason for that is a decreased contribution of the first and an enlarged contribution of higher diffraction orders. However the problem is much more complex because the changed spatial profile of the grating influences propagation of the writing beams as well. In order to trace this behaviour, comprehensive numerical simulations based on the differential Helmholtz equation should be performed. Finally, we would like to point out the utility of the DO element for recording holographic images. During the experiments at low irradiation flux the total irradiation time was in the order of 12 hours. The fact that neither the spatial profile of the grating nor the diffraction efficiency are influenced by such long irradiation time demonstrates the excellent phase stability of the experimental setup based on the DO element. Summary We have investigated the optical properties of a photochromic gel and how these influence the optical recording process. Because of diffusion of the open-form isomers from the non-irradiated to irradiated areas a rather high modulation of the refractive index can be achieved which results in a diffraction efficiency in the first diffraction order of ca. 30%. The main limiting factor in the diffraction efficiency is the saturation-induced modification of the grating profile. The saturation is caused by the evidently nonlinear approach of the equilibrium between the concentrations of the open-ring and closed ring isomers. At higher excitation flux the competition between the diffusion and photodecomposition of the switch molecules determines the diffraction efficiency. Acknowledgement This work was supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek” (NWO). Valuable discussions with Dr. A.N. Simonov are acknowledged.
Solid State Phenomena Vols. 97-98
213
References [1] M. Irie: Chem. Rev. Vol. 100 (2000), p. 1685; B.L. Feringa: Molecular Switches (Wiley VCH, Weinheim 2001). [2] P.R. Hania, R. Telesca, L.N. Lucas, A. Pugzlys, J.H. van Esch, B.L. Feringa, J.G. Snijders and K. Duppen: J. Phys. Chem. A Vol. 106 (2002), p. 8498. [3] J.M. Endtner, F. Effenberger, A. Hartschuh and H. Port: JACS Vol. 122 (2000), p. 3037. [4] D. Dulic, S.J. van der Molen, T. Kudernac, H.T. Jonkman, J.J.D. de Jong, J.H. van Esch, B.J. van Wees and B.L. Feringa: Phys. Rev. Lett. (accepted). [5] L.N. Lucas: Thesis, University of Groningen, 2001. [6] A. Mulder, A. Jukovic, L.N. Lucas, J.H. van Esch, B.L. Feringa, J. Huskens and D.N. Reinhoudt: Chem. Comm. (2002), p. 2734. [7] A.A. Maznev, T.F. Crimmins and K. A. Nelson: Optics Letters Vol. 23 (1998), p. 1378. [8] H. Kogelnik: Bell System Technical Journal Vol. 48 (1969), p. 2909. [9] J.J.D. de Jong, L.N. Lucas, P.R. Hania, A. Pugzlys, R.M. Kellogg, B.L. Feringa, K. Duppen and J.H. van Esch: Eur. J. Org. Chem. (2003), p. 1887. [10]P.R. Hania, A.Pugzlys, J.J.D. de Jong, J.H. van Esch, B.L. Feringa and K. Duppen: (in preparation).
