Rotaxanes––novel photonic molecules
Introduction
At present there is a great interest in the design and synthesis of new, addressable, functional organic molecules for use in various types of practical applications. Mechanically interlocked hydrogen bond assembled rotaxanes (cf. Fig. 1) are organic systems which offer unique architectural and structural properties [1], [2]. They are a class of mechanically interlocked molecules where a macrocyclic ring is locked onto a thread by two bulky stoppers [3] (cf. Fig. 1). They have attracted great attention as promising candidates for the development of prototypical structural units for device applications because macrocycles can rotate and translate back and forward along the chain. Rotaxane architectures are thus particularly attractive because the components of the molecule are held together by a dynamic mechanical bond which can be controlled at the molecular level by applying an external stimuli [4]. For these reasons they have been proposed as nanoscale devices such as switchable molecular brakes [5], shuttles [6], ratches [7] and electronically configurable logic gates [8]. Moreover the absence of chromophores or redox sensitive groups make rotaxanes an ideal backbone onto which additional electro-optically active subunits can be assembled to form a new type of interlocked artificial functional materials for useful electronics or optical properties.
Dynamics of the mechanical bonds linking the ring, the thread and the stoppers components of the rotaxane compounds occupy a special place due to the specific degrees of freedom they have. Indeed, rotaxanes exhibit a large rotational mobility and can be poled by the applied electric field, similarly as functionalized polymers.
These degrees of freedom, not available in other materials, can be used to switch the molecular properties through the adequate changes in the local environment, chemical stimuli and external optical or electrical applied fields. Indeed, using the electro-optic Kerr effect we have shown that the macrocycle rotates around the thread with frequency depending on the strength of external electric field and on the temperature [9]. These results are confirmed by independent NMR measurements and by theoretical modelling [9].
In this paper we describe linear and nonlinear optical properties of a serie of rotaxanes. In particular we show that rotaxanes can be processed into good optical quality thin films by vacuum evaporation. The linear optical properties of thin films, obtained by vacuum evaporation can be controlled by an appropriate functionalization of macrocycle. The films can be poled, with poling and depoling kinetics very similar to that of functionalized polymers. In solution the macrocycle rotates, as we have shown it by electro-optic Kerr effect and confirmed by NMR measurements. The speed of rotation can be controlled with applied electric field.
Section snippets
Molecule synthesis and thin film preparation
Rotaxanes were synthesized by the “clipping” methodology, consisting on the simultaneous slow addition of solutions of isophthaloyl dichloride and p-xylylene diamine in chlorinated solvents in the presence of the respective threads, with triethylamine as base. The thread provides the template information to form the benzylic amide macrocycle around itself via intermolecular hydrogen bond interactions (efficiency was more than 97% in the formation reaction, indicating close complementarity
Linear optical properties
The refractive indices of rotaxane thin films were measured in the visible and near infrared, at 623.8, 830 and 1314 nm, by means of m-lines spectroscopy. This technique allows to measure both the refractive index and the thickness when the films support more than one guided mode. It allows also to measure the optical birefringence by choosing the adequate polarization of the coupled light into thin film. Fig. 2 shows the measured and fitted with Sellmeier equation the wavelength dependence of
Poling
The films were poled using the experimental setup described elsewhere [10]. The amount of polar order was measured by in situ second harmonic generation. Fig. 3 shows the build up of SHG signal as function of poling time and at different temperatures. It is seen that this behavior is very similar to what is observed in functionalized polymers. With increasing temperature the mobility is increasing and the polar ordering time constant is decreasing. There is an optimum poling temperature,
Quadratic electro-optic Kerr effect mesurements
Using the experimental setup described elsewhere [11] we have studied the frequency dependence of the quadratic electro-optic Kerr constant in solution in an apolar solvent-dioxane and at room temperature. Fig. 5 shows the frequency variation of the Kerr constant for pure fumrot at different applied electric fields. First of all a resonance enhancement is observed, whose position depends on the applied electric field. When increasing the field strength the resonance frequency shifts towards
Conclusions
Rotaxanes appear to be interesting, functional materials, whose properties can be controlled by external stimuli, like electric field and light. The order of vacuum deposited films depends on the functionalization of macrocycle. Pure fumrot gives anisotropic films whereas di-substituted with NO2 group forms isotropic ones.
The films can be poled with a static external field. Rotaxanes behave very similar to functionalized polymers. Although the measured value of d coefficient for fumrot is
Acknowledgements
This work was supported by the European community under TMR contract: ERB4061PL95-0968 (ENBAC, A European Network on Benzylic Amide Catenanes).
References (15)
- et al.
Chem. Soc. Rev.
(1999) - et al.
J. Am. Chem. Soc.
(1999) - et al.
Pure Appl. Chem.
(1996) - et al.
J. Am. Chem. Soc.
(1998) - et al.
Bioorg. Chem. Frontiers
(1991) - et al.
Nature
(1994) - et al.
J. Org. Chem.
(1998)
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