5-Ureido-3 , 3-diphenyl-3 H-naphtho [ 2 , 1-b ] pyrans : Photoswitchable self-assembling architectures

5-Ureido-3,3-diphenyl-3H-naphtho[2,1-b]pyrans have been synthesized and their photochromic properties have been quantified using UV-Visible spectroscopy. The urea pattern is responsive for the selfassembling properties under both colourless and coloured forms whereas the corresponding amino derivatives do not show any aggregative properties. The geometries of the different states have been assigned using 1H and 19F NMR spectroscopies, and thereafter their respective kind of aggregates could be proposed through the different interand intramolecular interactions. Upon UV irradiation, the pyranic ring is opened and the resulting photogenerated carbonyl group is competing with the urea in the self-assembling process and consequently strongly affects the supramolecular assemblies. The interand intramolecular contributions have been quantified through 1D and 2D NMR experiments along with the variation in the concentration.


INTRODUCTION
The design and the successful synthesis of nanostructured molecular architectures through the "bottomup approach" offer a wide range of possibilities [1].The control by external stimuli of the aggregation process is of paramount importance and may pave the way to new functional supramolecular device [2,3].A promising approach toward such tunable assemblies is the introduction of an addressable function onto the supramolecular building blocks.Photochromic molecules could offer the possibility to strongly modify the self-assembly process of the individual molecules and also the resulting nanostructurated aggregates by means of light, as inherently they possess bistability and can be photochemically interconverted between two states [4,5].Among the organic photochromic molecules, 2H-chromenes and more especially 3,3diphenyl-3H-naphtho [2,1-b]pyrans have attracted considerable attention because of their potential applications in ophthalmic lenses, information storage and imaging devices [6] and their relative easy synthetic access.
Herein, we report the design of photoswitchable assemblies consisting of naphthopyrans covalently linked to recognition units such as urea function and we aim to explore their photoswitchability as a means of reversible light modulation of properties for chemical assemblies.The urea moiety has been selected as it corresponds to one of the most powerful selfassembling unit.Indeed, each urea group is involved in four H-bonds and has been shown to efficiently promote the formation of large aggregates such as in the case of low molecular mass organogelators [7,8].† E-mail: jl.pozzo@lcoo.u-bordeaux1.fr

EXPERIMENTAL DETAILS
UV irradiation of the samples in the NMR tube was performed in a home-built apparatus developed by some of us.The emission spectrum of a 1000 W Xe-Hg highpressure filtered short-arc lamp (Oriel), was focused on the end of a silica light-pipe (length 6 cm, diameter 8 mm), leading the light to the spinning sample tube, inserted in a quartz dewar.The temperature of the sample was controlled with a variable temperature unit (B-VT1000-Bruker, 123 to 423 K, T range).The filter used was the Schott 011FG09: 259 < λ < 388 nm with λ max = 330 nm.
1D and 2D NMR spectra were obtained on a Bruker (DPX 300 or AC 300) NMR spectrometer equipped with a BBI probehead fitted with an actively shielded zgradient coil to deliver pulsed field gradients or with a QNP probehead (for 19 F NMR spectroscopy).
The synthesis of the target molecules was realised by reacting these aminoarenes 1 or 2 with commercially available isocyanate as depicted in Scheme 1.A dichloromethane solution of amino derivatives is added to five equivalent amounts of n-octylisocyanate and then is stirred for three days.Purification by flash chromatography using a dichloromethane-hexane mixture as eluent affords the desired compounds 3 and 4 respectively in 61% and 55%.All the described compounds gave satisfactory elemental analysis and spectroscopic data.The photochromic characteristics were first determined for both urea derivatives in both DMSO and toluene using a 366 nm irradiation lamp coupled to a UV-Visible spectrophotometer.The presence of the Nalkylurea directly linked on the 5-position of the 3,3diphenyl-3H-naphtho[2,1-b]pyran subunit does not alter the photochromic behavior.The actinic wavelengths were determined to be 335 and 357 nm for both compounds.The two fluorine atoms borne by the phenyl groups induce very tiny effects.Indeed, thermal fading rate constants are also very close ranging from 0.011 to 0.019 s −1 according to solvent polarity.For both compounds, the light induced open-forms exhibits an intense absorption band around 436 nm at 20 • C. The wavelength of the absorption spectra denoted hereafter λ max does not show strong dependence on solvent polarity as previously reported for unsubstituted parent molecule [9].The different λ max and fading rate constants k are collected in Table 1.Some of us have recently shown that NMR is a powerful tool to fully characterise all the open forms and follow their evolution during the thermal bleaching when UV irradiation is stopped [10].Within the naphtho[2,1-b]pyran series, it was undoubtedly reported that the TC-isomer is predominant, a TT-isomer being present from 0 to 15% according to substitution pattern [11].Within the reported series, the aminoarenes 1 and 2 exhibit the same behaviour.
As chromenic compounds usually possess many ethylenic or aromatic protons in the same range under their closed and open forms, it is difficult to distinguish easily between all photoproducts using 1 H NMR. Some of us investigated fluoro-substituted derivatives [10] in order to use this nucleus as an NMR molec- ular probe and overcome this complexity.Moreover, direct NMR monitoring of the individual concentrations of each photoproduct considerably improves kinetic studies.After UV irradiation several new signals were detected. 19F NMR experiments show that in contrast to the closed form, all the photoproducts possess unequivalent fluoro-phenyl groups.This effect is due to the sp 2 hybridisation of the previous spiro-carbon giving rise to two different isomers, the transoid-cis (TC) and the transoid-trans (TT) isomers being identified as major photoproducts in a 8 : 1 ratio (Figure 1).By recording 19 F NMR spectra at regular time intervals after irradiation, the kinetics of thermal bleaching could be determined, the decay following a firstorder kinetic.Figure 2 illustrates the evolution of the concentration of the various isomers during the thermal bleaching.The closed initial structure (FF) increases while transoid-cis isomer (TC) follows a monoexponential decay.Minor isomer namely TT seems to remain stable at this temperature but reverts totally back to the closed form after several hours.
As illustrated in the Figure 1, the 19 F NMR chemical shifts move along the recorded spectra, and more particularly, a pronounced broadening of one signal in TCisomer is observed which clearly indicates intermolecular interactions being characteristic of stacking phenomena.This observation is restricted to compounds 3 and 4 possessing the urea subunit as compounds 1 and 2 behave classically (vide supra).In order to study the aggregating process and its dependence on the photochromic reaction, we first perform a whole analysis using 1 H NMR and 19 F NMR on the colourless closed form 4.
2D NMR 1 H-1 H COSY and ROESY experiments and 1 H-13 C HSQC and HMBC experiments were performed before UV irradiation in toluene-d 8 at T = 293 K.All these experiments establish that strong dipolar contacts occur between the n-alkyl chain and the phenyl groups as indicated by the correlation found between protons atoms borne by the phenyl groups and the protons of the end-chain ethyl group.This short distance between these two groups could only be explained by intermolecular interaction between several molecules as the alkyl chain adopts usual zig-zag conformation.Furthermore proton H-6 is also affected by the urea group and self-recognition between molecules induces broadening of both NH.This is well corroborated by standard IR measurements which clearly exhibit typical shift for the urea groups through their amide I and amide II bands (respectively 1640 and 1580 cm −1 ).
Alternative evidence supporting self-aggregation is provided when the UV irradiation is stopped.Indeed, while the photomerocyanine TC is bleached, the concentration of the closed initial form increases.For these two compounds, a shift of some resonances is observed.In particular, the signals of both NH and H-6 move significantly, as illustrated in Figure 3.The different chemical shifts could be measured in each NMR spectrum along the thermal process (Figure 4).As shown in Figure 3, several chemical shifts such as those corresponding to NH-1' (7.7 ppm), NH-3' (4.7 ppm) and H-6 (9.5 ppm) in FF are shifting during measurements indicating that the increase of concentrations of closed form induces strong supramolecular interactions.All these data are consistent with the model for the molecular arrangement of 4 depicted in Scheme 2. In this architecture, the urea moieties form a hydrogen-bonded network, and the n-octyl chains and phenyl groups are connected through intermolecular interactions.
In the same way, a great shift of 1 H resonances (NH-1' and NH-3' respectively at 8.1 ppm and 4.4 ppm) is observed in TC isomer.Shifts can also be measured by the peak-picking of 19 F signals.In addition, the signal at −109.90 ppm, corresponding to fluorine atom borne by the phenyl group cis to carbonyl group is found to be markedly broadened.These effects can be interpreted as resulting from the formation of a dimer, and so from intermolecular contacts.The primary photoinduced ring-opening event should diminish the strength of H-bonds between the urea carbonyl and the two  nitrogen centres, and the photogenerated carbonyl group could be considered as an intermolecular competing Hydrogen-bonding acceptor group.A dimer also allows the formation of 4 H-bonds per molecules and could be stabilised through other weak bonds, and this intermolecular association strongly reduces the steric hindrance.From these NMR considerations, we can propose the association designed in Scheme 3. On the other hand, a less intense but existing shift is also observed for H-6 (9.1 ppm).Due to its weakness, we suggest that it results from an intramolecular Hydrogen-bond with the carbonyl group of the urea moiety.
The photochemically induced interconversion between closed and open forms has been repeated several times.No noticeable photodegradation has been substantiated as NMR spectra, fading rate constants and absorbance remain unchanged.Here, the photochromic reaction is accompanied by a large geometrical rearrangement at the molecular level but also at a supramolecular level.This opens new prospects for the introduction of organic photochromic molecules as a key switch between different kinds of array through specific recognition pattern.

CONCLUSION
In conclusion, we have demonstrated the photoswitchable self-assembly of ureido-2H-chromenes between an aggregate of closed form and a dimer of open form under its major TC-isomer.The ability of such systems for self-organizing under two different ways offers intriguing possibilities.This approach constitutes a new route toward nanosized architectures which could be reversibly influenced upon irradiation.The 2Hchromenes appear to be promising building-blocks for supramolecular purpose through the use of their photoinduced carbonyl group as a Hydrogen-bonding acceptor group.

Figure 1 . 19 F
Figure 1. 19F NMR spectra along thermal bleaching of 4 (T = 243 K, toluene-d 8 , experiments starting from bottom), FF corresponds to the closed form, TT and TC are isomers of the open form.

Figure 2 .
Figure 2. Time-evolution concentrations at 243 K after irradiation of 4.