ARTICLE pubs.acs.org/JPCC
Photochemical E(trans)Z(cis)E Isomerization of an Amphiphilic Cholest-5-en-3β-yl(E)-9-anthraceneprop-2-enoate on Solid Substrate Suthari Prashanthi,† P. Hemant Kumar,† D. Siva,† Srinivasa Rao Lanke,† V. Jayathirtha Rao,‡ Soumen Basak,§ and Prakriti Ranjan Bangal*,† †
Inorganic and Physical Chemistry Division, ‡Organic Chemistry Division-II, Indian Institute of Chemical Technology, Tarnaka, Hyderabad, -500607, India § Chemical Physics Division, Saha Institute of Nuclear Physics, Sector-I, AF Block, Bidhannagar, Kolkata, -00064, India
bS Supporting Information ABSTRACT: Surface morphology and photochemical isomerization properties of monolayers of anthrancene acrylic acid derivative with cholesterol (a new class of bistable compound), cholest-5-en-3β-yl(E)-9-anthraceneprop-2-enoate (CAE), transferred onto quartz substrates were studied. The spectroscopic and photochromic behavior of CAE on solid substrates and in solution are compared keeping in mind the possible application of CAE in constructing molecular electronic devices. Monolayers of the trans(E)-isomer of CAE transferred from the airwater interface onto quartz plates show regular distribution of “holes” in the film, whereas similar monolayers of the cis(Z)-isomer of CAE (∼96%) show very smooth surfaces, free from any definite structures. The surface pressurearea (πA) isotherms of both monolayers at the airwater interface are found to be irreversible, indicating formation of 2D/3D aggregates for both isomers. The surface potentialarea (ΔVA) isotherms of the two isomers predict the orientation of their molecular dipoles to be different. The fluorescence peak intensity of the E-isomer of CAE in transferred monolayers shows a sharp decrease upon irradiation with 405 nm light, indicating the successful E-to-Z isomerization in the monolayer. Fluorescence excitation and emission polarization studies on the solid substrate also confirm the change of molecular orientation resulting from the E-to-Z isomerization. The isomerization rate is found to be faster in solid substrates than that in the solution phase. Six alternate monolayers of E-CAE and triplet sensitizer (liphophilic porphyrin) film shows 5% efficiency of Z-to-E isomerization upon exciting on 550 nm, where porphyrin has substantial absorbance where as film of 24 monolayers of mixture solution of the E-isomer of CAE (1 mM) and liphophilic porphyrin (1 mM) in chloroform increases 5-fold efficiency of Z-to-E conversion. These results suggest that the E-CAE has the potential to be used in making optical data storage devices employing the transcistrans isomerization process.
’ INTRODUCTION Photochemical isomerization is a light-driven transformation of a photochromic compound1,2 between two isomeric forms where the two forms may have their own spectroscopic identity with different absorption and emission spectra. A photochromic compound is thus a bistable system that can be interconverted either directly or indirectly (via sensitizer) by optical radiation of two different wavelengths. Because of the difference in electron distribution in two isomers, their physical and chemical properties may differ in many ways, such as refractive index, dielectric constant, redox potential, chelation potential, absorption spectrum, fluorescence properties, and so on, and make them suitable for practical applications. When such photochromic compounds are embedded in a proper medium (e.g., polymer, liquid crystal, or a suitable solvent), their properties can be tuned by photoirradiation in accordance with the requirements. Examples include rewritable holographic systems,3,4 photosensitive optical waveguide components,5,6 reversible photoinduced phase transition of liquid crystals,7,8 and optoelectronic systems.9,10 Therefore, this type of system has attracted much interest recently due r 2011 American Chemical Society
to its potential application in information storage and construction of molecular switches.1114 Although a large number of photochromic compounds have been reported in the literature,2 only a few of them are fluorescent and allow modulation of their fluorescent properties.1518 Anthracene acrylic acid derivatives are diarylethelene type of photochromic compounds which can be used for constructing molecule-based optical devices exploiting features of optical switching, optical storage, and optical modulators.19 Their selective trans(E)-to-cis(Z) isomerization upon direct irradiation (λ > 400 nm) in organic solvents leads to the formation of a thermodynamically less stable Z-isomer (over 96% yield) and triplet-sensitized isomerization selectively produces a Z-to-E isomer (over 98% yield).20,21 Their quantum chain forward isomerization process, as well as remarkable back isomerization via triplet sensitization, could make them suitable for the preparation Received: May 27, 2011 Revised: August 17, 2011 Published: September 15, 2011 20682
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Chart 1. Structure of trans-CAE (a) and cis-CAE (b)
of molecular devices (e.g., optical data storage device, molecular fluorescence switch, etc). In preparing useful devices based on such photoactive compounds, it is important to know what changes in the isomerization processes may take place upon immobilizing the molecules on solid substrates. In particular, the reversible photoinduced E-Z-E isomerization needs a “free volume” around the acrylic group, the availability of which in the solid state must form an essential criterion for the feasibility of the process. Hence, to make the compound more amphiphilic and suitable for transferring LangmuirBlodgett (LB) film onto the solid substrate, a cholesterol group has been attached. We have chosen the LangmuirBlodgett (LB) technique22 to construct a film of the photoactive molecules on a quartz substrate. The LB technique has distinct advantages over solvent-cast or self-assembly techniques. For example, it allows one to prepare nanoscale devices with improved control without destroying the anisotropic properties of the molecule.23 To our knowledge, no attempt aimed at understanding the transcistrans (EZE) isomerization process of diarylethelenes on solid substrates (such as LB films) has been reported. In this paper we focus on comparing the characteristics of the EZ isomerization and ZE isomerization process via triplet sensitization of the anthracene acrylic acid derivative with cholesterol (Chart 1) in the solid phase (in the form of a LB film) and the bulk liquid phase, with a view to explore the possibility of using this molecule to design a bistable molecular device.
’ EXPERIMENTAL SECTION The synthesis of the cholesterol derivative (cholest-5-en-3β-yl(E)-9-anthraceneprop-2-enoate) has been previously described.20,21 Spectroscopic grade chloroform with 99.8% purity (Aldrich, U.S.A.) was used as the solvent for preparing the monolayers. The triplet sensitizer was chosen to be lypophilic porphyrin, tetrakis (4-eikosyl oxymethyl phenyl) porphyrin (4-EoTPP) and was obtained from Porphyrin Systems GbR, Germany. The subphase was triple-distilled water deionized in a Milli-Q plus water purification system from Millipore (U.S.A.). The pH and resistivity of the distilled water were 6.8 and 18.2 MΩ cm, respectively. The measurements of the surface pressure (π)-area (A) and surface potential (ΔV) area (A) isotherms and deposition of the monolayers onto quartz surface were carried out using a KSV Mini trough System 2 (Finland). A 200 μL aliquot of a 1 mM solution of CAE in chloroform was spread onto the water subphase (initial area = 243 cm2) at room temperature. After a delay of about 30 min to allow solvent evaporation, the monolayer was compressed by moving the barrier with a constant speed of about 5 mm/min. The monolayer was transferred onto a
quartz plate by the LangmuirBlodgett technique at constant surface pressure of 30 mN/m with a transfer speed of 3 mm/min. The transfer coefficient was ∼0.95 for surface pressures between 15 to 30 mN/m. The fluorescence of the LB films (excitation and emission spectra) was measured on a fluorolog-3 spectrofluorometer (Horiba Jobin-Yvon, U.K.). The degree of polarization P was measured using the following equation.24 P ¼
IVV GI VH IVV þ GI VH
where IVV is fluorescence intensity with both excitation and emission polarizers mounted vertically, IVH is fluorescence intensity with excitation polarizer mounted vertically and emission polarizer mounted horizontally, IHV is fluorescence intensity with excitation polarizer mounted horizontally and emission polarizer mounted vertically, and IHH is fluorescence intensity with both excitation and emission polarizer mounted horizontally. G = IHV/IHH is the instrumental correction factor or grating factor, where G corrects the wavelength response of polarization of the emission optics and detectors. Absorption spectra were recorded using a Cintra 10e UVvis spectrophotometer (GBC) under normal incidence. The alternate monolayers of E-CAE and 4-EoTPP on the quartz plate or monolayer of mixture solution of E-CAE and 4-EoTPP onto quartz plate were irradiated with 405 nm light with a 250 W xenon lamp of the spectrofluorometer (slit width = 23 nm) and when required to convert the E-isomer to Z-isomer in LB film or in solution, respectively. Similarly, 550 nm light was used for the conversion of Z-CAE to E-CAE. Scanning electron microscopy was performed on a Hitachi S-3000N scanning electron microscope operating at 5 kV. The LB films of respective compounds on a quartz plate for SEM imaging were coated with gold to avoid the charging effect.
’ RESULTS AND DISCUSSION Solution Phase Behavior. CAE is a particularly interesting molecule (Chart 1) who’s sensitive and specific responses in the EZ isomerization process in the solution phase makes it a good candidate for the study in the solid/film state. Because its typical isomerization process and energy-flow pathways have been the subject of much interest, it is helpful first to review briefly the essential observations and interpretations in the solution phase. The E-isomer of CAE has a strong, minimally structured, broad, and solvent-independent absorption band peaking at ∼388 nm, whereas the Z-isomer has a very weak, relatively narrow, wellstructured absorption band with well-resolved 00 transition at 20683
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Figure 1. Absorption and emission spectra of trans-CAE and cis-CAE in chloroform solution (A). Change of fluorescence in terms of intensity and peak position as a function of irradiation time with 405 nm light. Inset shows the intensity vs time plot at 460 nm (B).
Figure 2. Surface pressure (π) versus mean molecular area (A) isotherm and three compressions and decompression cycles of E-CAE (A). Surface pressure (π) vs mean molecular area (A) isotherm and three compressions and decompression cycles of Z-CAE (B).
385 nm (Figure 1A). Above 400 nm, the absorbance of the E-isomer is much higher (5 times at 400 nm) than that of Z-isomer, which enables us to perform selective photoexcitation of E-isomer. Likewise, E-isomer shows strong fluorescence peaking at 498 nm in chloroform solution as well as solvent polarity dependent bathochromic shift resulting from charge transfer or polar singlet excited state. Z-Isomer has weak fluorescence peaking at around at 492 nm with very little solvatochromic effect (Figure 1A). Upon direct excitation by >400 nm light, the E-isomer of CAE undergoes E-to-Z isomerization. Triplet sensitization by exciting at 550, 515, or 525 nm using external triplet sensitizers (rose bengal, eosin or lypophilic porphyrin, respectively) produces only Z-to-E isomer.20,21 Triplet induced isomerization yields higher quantum efficiency leading to the involvement of quantum chain isomerization process. The detailed mechanism of photochemical EZ isomerization process in solution phase has been discussed earlier.19 Monolayer at the AirWater Interface. Surface pressure area (πA) isotherm: Monolayer behavior of E- and Z-isomers of CAE was examined separately once the organic solvent had been deemed to have evaporated (30 min after spreading). The barriers were then slowly closed at 515 mN/m and then reopened. Monolayer behavior was then plotted as surface pressure (π) versus mean molecular area (A) isotherm by reducing the area available to the material at a rate of 5 mm/ min until a compressed state was achieved at surface pressure of about 3040 mN/m. Figure 2A shows typical surface pressure (π)-area (A) isotherms corresponding to three successive compressiondecompression cycles of a monolayer of E-isomer of CAE. They were reproducible and showed only minor changes with temperature, concentration and amount of stock solutions
spread on the trough. The stability of the compressed monolayer as a function of time was checked by monitoring the percentage decrease of its area over a particular time span and was found to be quite good. The isotherm representing the first cycle (marked “1” in Figure 2A) shows an initial increase in surface pressure at a molecular area corresponding to ∼15 Å2. The surface pressure rises at a moderate rate with decreasing area until it attains a value of ∼7 mN/m, at which point the rate of increase in pressure with barrier compression decreases a bit. This slightly ascending plateau region of the isotherm continues until a surface pressure ∼17 mN/m is reached. Further barrier compression leads to a much faster rate of increase of surface pressure until evidence of film collapse appears beyond surface pressures of 40 mN/m. Three distinct regions/phases of the isotherm are observed during compression: gaseous, liquid, and solid. After reaching a pressure of 35 mN/m, the monolayer is decompressed to zero pressure at the same barrier speed (5 mm/min) and the same three phases are found to be retained, though with substantial hysteresis. The existence of such strong hysteresis between the compression and decompression arms of each cycle may be attributed to ππ interaction between anathracene groups, which can lead to the formation of irreversible 3D aggregates or condensed 2D phases with very slow dispersion rate during the plateau and subsequent ascent of the isotherms. The mean molecular area at the transition point from the liquid to the solid phase during compression (Ac0) and decompression (Ar0), obtained by extrapolating the steepest segments of the corresponding isotherms to zero pressure, was found to be ∼12.5 and 10 Å2, respectively. This value is closely correspond to the values observed for anthracene based systems.28 The low molecular area 20684
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Figure 3. πA isotherm and surface potential (ΔV); mean molecular area (A) isotherm of E and Z isomers of CAE. Plots 1 and 2 are πA isotherms and plots 3 and 4 are ΔVA isotherms of respective isomers.
in the condensed phase clearly indicates that molecules are vertically oriented on the water surface.25 Monolayer behavior of the cis(Z)-isomer (∼96%) of CAE was also studied in a similar manner. Figure 2B shows the πA isotherms for the first two compression/relaxation cycles. Three distinct phases could be barely resolved in the isotherm for the first compression cycle, but unlike for the E-isomer a clear plateau region was not found. Existence of hysteresis during relaxation of the film suggested formation of aggregates. The area per molecule (A) obtained by extrapolating the steepest region of the isotherms to zero surface pressure, which correspond to the hypothetical states of an uncompressed close-packed layer in which the molecules at the interface make the transition from the liquid to the condensed phase (during compression) or vice versa (during decompression), were found to be ∼12 and ∼9.3 Å2, respectively, which are somewhat smaller or of the same order with those of E-isomer, although the size of the Z-isomer is relatively big compared to that of the E-isomer (vide infra). This result indicates that Z-isomers are perhaps more closely packed in their condensed phase than E-isomers. Surface PressureSurface Potential (πΔV) Isotherm. Figure 3 shows typical surface pressure and surface potential isotherms for monolayers of E- and Z-isomers (96%) spread onto the water subphase. For the E-isomer, the surface potential increases steeply during the initial phase of compression but starts to flatten out at around the gas-to-liquid phase transition point (mean molecular area = 10.3 Å2), becoming almost constant in the condensed phase of the isotherm. The maximum surface potential for the E-isomer is found to be 215 mV. In contrast, the potential of the film of Z-isomers does not show any jump, but increases slowly with compression to reach a maximum value of 30 mV. The normal component of the dipole moment of the molecule can be calculated using the following equation.26 μ^ ¼ ΔVA2:65 102 where ΔV is the change in surface potential, A is the mean molecular area in Å2, and 2.65 102 is the conversion factor of the dipole moment in Debye unit (1D = 3.33564 1030 C.m.). The normal component of the dipole moment of E and Z forms of CAE are thus calculated to be 53 mD and 0.8 mD, respectively. This large difference between the normal components of dipole moment clearly indicates that the molecular orientations of the two isomers in the monolayers are drastically different. Geometry optimization studies using DFT/B3LYP (3-21 g level predict the different orientations of anthracene and cholesterol group
Figure 4. DFT/B3LYP3-21g level optimized geometry of trans-CAE (A) and cis-CAE (B).
Figure 5. SEM image of the surface of a single Z-type monolayer (A), three Y-type monolayers (B) and six Y-type monolayers (C) of the Eisomer of CAE transferred onto quartz substrates and SEM image of a film of 6 monolayers of the Z-isomer (96%) onto quartz plate (D). Scale bar indicates for 2 μm.
around the acrylic group in trans and cis forms of CAE and it is shown in Figure 4. The molecular diameter is found to be higher for the Z-isomer then that of the E-isomer, and they are 19.54 and 19.03 Å, respectively. The calculated total dipole moment of the E-isomer is much higher (4.8829 D) than that of the Z-isomer (1.9125 D). These results qualitatively support the observed surface potential difference between two isomers and the difference in the degree of fluorescence polarization (vide infra). Surface Morphology of Monolayers Transferred onto Quartz Plates. Monolayers of both of the isomers of CAE were transferred onto solid (quartz) substrates as multilayer Langmuir Blodgett (LB) films. It is of interest to see whether the observed differences in the monolayer characteristics, especially the huge difference in surface potential for two isomers, are preserved in 20685
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The Journal of Physical Chemistry C the transfer process. To make the comparison straightforward, all monolayers were transferred at a surface pressure of 30 mN/m. Several layers of the stable monolayers of E- and Z-isomers could easily be transferred as Y-type films (transfer during both up- and downstrokes) with near unity transfer ratio (TR). The transfer ratios for the upstrokes were close to unity, while those for the downstrokes were a bit poor (TR 0.8). The surface textures of the transferred films were characterized by SEM. Figure 5A shows a typical SEM image of surface morphology of a single Z-type monolayer of E-isomer on a quartz substrate, where a regular distribution of “holes” in the film of ∼400500 nm diameter was observed. Figure 5B and C show SEM images of the top surface of films consisting of three and six Y-type transferred layers, respectively. The surface morphology of the three-layer film is more irregular than that of the single layer (Figure 5A), which could be the result of space-filling arrangement of hydrophilic and hydrophobic group heads. A relatively smoother surface is observed with much less number of “holes” for the film made up of six Y-type layers. These surface morphologies and the low mean molecular area value in the condensed phase of the πA isotherms suggest aggregation of the E-CAE molecules, with their hydrophilic carboxylic tail groups going into the water subphase and the hydrophobic anthracene and cholesterol headgroups staying up in air keeping an angle between them to make a “V-shaped” structure (Figure 6A). As shown in Figure 4A, the optimized geometry of E-isomer is found to be “V-shaped”. Assuming this geometry for the E-isomer, a 2D gas phase like that shown in Figure 6A could form when the molecules are spread on the water subphase. At the surface pressure approaching the condensed phase, ππ interaction between anthracene groups can lead the molecules to arrange themselves in circular domains to minimize the surface energy. These arguments allow us to propose the possible schematic model shown in Figure 6B for the arrangement of the E-isomers in Z-type and X-type monolayers. This is a tentative model to explain the observed surface morphology and further investigation is needed to explore the detailed structure. Surprisingly, the surface morphology of both a single monolayer as well as of several monolayers of the Z-isomer was observed to be very smooth. Figure 5D shows the SEM image of a film of six monolayers of the Z-isomer, which is very compact. This result is in accordance with the πA isotherms (Figure 2B) that predicted the Z-isomers to be more closely packed than the E-isomers. Because the Z-isomer is created from an E-isomer by photoexcitation, the Z-isomer always contains 34% E-isomer (photostationary state composition, ∼96:4) and it does not allow for (πA) isotherm, (πV) isotherm, and surface morphology for pure Z-isomer. It is also important to mention here that the surface potential of Z-isomer was found to be much lower than that of E-isomer, indicating very different molecular orientations of the two species at the airwater interface. An X-ray diffraction study of multilayer LB films of the Z-isomer, which can reveal the packing structure of the molecules, is out of the scope of the present study. Spectroscopic Properties of Monolayers Transferred onto Quartz Plates. LB films of the two isomers of CAE were produced by transferring monolayers from the water subphase onto quartz substrates at a fixed surface pressure of 30 mN/m. Absorption, excitation, emission, emission polarization, and excitation polarization spectra of the two isomers in LB films were recorded. EZ isomerization process in the film was carried out by exposing it to 404 nm light (Figure 7). The absorption spectrum
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Figure 6. 2D gas phase of trans-CAE in airwater interface (A) and cartoon diagram of proposed aggregate forming a circle-type structure onto a quartz substrate (B).
Figure 7. Fluorescence excitation, emission spectra, and polarization spectral study of E-isomer and Z-isomer on a quartz plate. The E-isomer converted to a Z-isomer on excitation by 404 nm for 30 min.
of a monolayer of E-isomer almost replicates that of the compound in chloroform solution, with a little blue shift of the peak position. Upon excitation at 390 nm, the monolayer of the E-isomer shows an emission peak at 485 nm, which is blue-shifted by ∼10 nm from its emission peak in solution (Figure1A). Upon irradiation with 405 nm light, the fluorescence emission intensity of a six-layer LB film is reduced by >60% within the first 15 min, while the emission peak shows a gradual blue shift to reach a stationary value of 465 nm (Figure 1B). Both of these fluorescence parameters are indicative of formation of Z-isomer (Figure 1A). This result shows that CAE can be efficiently converted from E-to-Z-isomer by irradiation in the solid state in the form of LB monolayers. Moreover, the E-to-Z conversion is found to be faster in solid substrate than that in the solution phase, as observed before.20,21 Figure 1B shows the fluorescence excitation and emission polarization spectra along with excitation and emission spectra of E-isomer and Z-isomer monolayers. The Z-isomer was obtained in situ by conversion of the E-isomer on exposure to 405 nm light for 20 min. The value of polarization (P) for the E-isomer monolayer is positive, whereas it is negative for the converted 20686
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The Journal of Physical Chemistry C Z-isomer monolayer. This result indicates that the direction of the transition dipole moment changes when E-isomer converts to Z-isomer in the monolayer by photoexcitation. Thus, it can be concluded that the LB film of E-isomer provides enough free space and flexibility for the isomerization process to occur in the solid state.27 Such freedom of segmental motion could also favor the triplet sensitized Z-to-E isomerization process on the solid substrate. To check the feasibility of the triplet sensitized cis-totrans isomerization process in solid state, we first transferred six alternate monolayers of the E-isomer and 4-EoTPP, respectively. Then we excited the film with 405 nm light to convert the E-isomer to the Z-isomer, and it shows no difference in the E-to-Z conversion process in the presence of a sensitizer. After conversion to the Z-isomer form, the film was exposed to 550 nm light, where 4-EoTPP has substantial absorption, but the Z-isomer has absolutely no absorption. Upon irradiation of 550 nm light, fluorescence intensity tends to increase, but only 5% regain of fluorescence intensity could be monitored after 15 min of irradiation, which reflects the Z-to-E conversion efficiency to be 5%. To improve the triplet sensitized Z-to-E conversion efficiency, we prepared film of 24 monolayers by transferring the monolayer of the mixture solution of E-isomer (1.4 mM) and 4-EoTPP (0.25 mM) from the water subphase onto a quartz plate. Upon exposing to 405 nm light to the film, the E-isomer converts to the Z-isomer, as usual, by decreasing the fluorescence intensity accompanied by a blue shift of peak position. After that, excitation by a 550 nm light for 30 min, the fluorescence intensity regain by ∼25% (Figure 3S), which reveals the Z-to-E conversion efficiency to be about 25%. This method shows 5-fold improvement in Z-to-E conversion efficiency than that of a previous method, and five cycles of transcistrans isomerization processes have been tested within this 25% efficiency without any loss of compound. However, still these results lack efficient triplet sensitization in solid substrate than that in the solvent phase. It should be mentioned here that excitation in the visible range is to sensitize the dye such that triplet (dye) to triplet (Z-isomer) energy transfer occurs and converts Z-isomer to E-isomer. This phenomenon efficiently occurred in the solution phase20,21 but does not occur so efficiently in the solid film. Tentatively, we suggest that in solution triplettriplet energy transfer is induced by collision, and diffusion plays a major role in energy transfer phenomena in solution. As a result, we observed very efficient (>98%) Z-to-E conversion through triplet sensitization in solution. Absence of diffusion in the solid state (i.e., in LB films) eliminates the collision-induced energy transfer process. In solid state or in LB films, energy transfer depends solely on a predetermined distance and orientation of the donors and acceptors, and these imposed restrictions in solid film control the efficiency of the energy transfer process, resulting in only a 25% Z-to-E conversion for the present molecule. However, this result suggests that the potential use of this molecule in making molecular devices exploiting transcistrans isomerization could be achievable under suitable conditions. To address these issues, an attachment of preferable triplet sensitizer at the cholesterol end is being planned soon.
’ CONCLUSION We have successfully transferred both E-isomer and Z-isomer monolayers from the airwater interface onto quartz substrates. Transferred monolayers of E-isomer show circle-like surface structures where a relatively smooth surface is observed of Z-isomer
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monolayers. Substantial hysteresis in their πA isotherms confirms the formation of condensed 2D or 3D aggregates of E-isomer. A sharp contrast between the surface potentials of the E-isomer and Z-isomer at the airwater interface proves to be a distinct difference in the orientation of molecular dipoles of the isomers. In situ fluorescence polarization study finally confirms the efficient E-to-Z conversion on a quartz substrate. Triplet-sensitized Z-to-E isomerization process is realized on solid substrate with 25% efficiency. These results have implications in the application of this compound making optical storage devices, exploiting the transcistrans isomerization process on solid substrates in the presence of suitable triplet sensitizers.
’ ASSOCIATED CONTENT
bS
Supporting Information. Synthesis procedure, NMR spectral data, structure and isotherm of triplet sensitizer, isotherm of the mixture of compound and triplet sensitizer, and fluorescence change due to transcistrans isomerization. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +91 40 27191431. Fax: +91 40 27160921. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by Grant No. 2006/34/26-BRNS/ 2805 from the Department of Atomic Energy, Government of India. A partial support of MLP-0013 of IICT is acknowledged. ’ REFERENCES (1) D€urr, H.; Bouas-Laurent, H. Photochromism Molecules and Systems; Elsevier: Amsterdam, 1990. (2) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and Thermochromic Compounds; Kluwer Academic, Plenum Publishers: New York, 1999. (3) Belfield, K. D.; Liu, Y.; Negres, R. A.; Fan, M.; Pan, G.; Hagan, D. J.; Hernandez, F. E. Chem. Mater. 2002, 14, 3663–3667. (4) Luo, S.; Chen, K.; Cao, L.; Liu, G.; He, Q.; Jin, G.; Zeng, D.; Chen, Y. Opt. Express 2005, 13, 3123–3128. (5) Tanio, N.; Irie, M. Jpn. J. Appl. Phys. 1994, 33, 1550–1553. (6) Biteau, J.; Chaput, F.; Lahlil, K.; Boilot, J. P.; Tsivgoulis, G. M.; Lehn, J.-M.; Darracq, B.; Marois, C.; Levy, Y. Chem. Mater. 1998, 10, 1945–1950. (7) Lemieux, R.; Schuster, G. J. Org. Chem. 1993, 58, 100–110. (8) Feringa, B.; Huck, N.; van Doren, H. J. Am. Chem. Soc. 1995, 117, 9929–9930. (9) Gilat, S.; Kawai, H.; Lehn, J. M. Chem.—Eur. J. 1995, 1, 275–284. (10) Marsella, M. J.; Wang, Z.-Q.; Mitchell, R. H. Org. Lett. 2000, 2, 2979–2982. (11) Irie, M. Chem. Rev. 2000, 100, 1685–1716. (12) Feringa, B. L. Molecular Switches; Wiley-VCH: Weinheim, 2001. (13) Dhanabalan, A.; Dos Santos, D. S., Jr.; Mendonc€ua, C. R.; Misoguti, L.; Balogh, D. T.; Giacometti, J. A.; Zilio, S. C.; Oliveira, O. N., Jr. Langmuir 1999, 15, 4560–4564. (14) Liang, Y.; Dvornikov, A. S.; Rentzepis, P. M. J. Mater. Chem. 2000, 10, 2477–2482. (15) Liang, Y.; Dvornikov, A. S.; Rentzepis, P. M. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 8109–8112. 20687
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