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Photoluminescence Properties of a Molecular Organic Switching System €rg Hallmann and Simone Techert* Jo Max Planck Institute for Biophysical Chemistry, IFG Structural Dynamics of (Bio)chemical Systems, 37070 G€ ottingen, Germany
ABSTRACT In R-styrylpyrylium trifluoromethanesulfonate (R-styrylpyrylium TFMS), the holographic storage mechanism is based on a molecular switching process in the bulk crystalline material. We have studied the stationary and timeresolved optical emission properties of this material with respect to its optical response function and during the molecular switching process. Typical for the emission spectrum of R-styrylpyrylium TFMS is its double band structure with two maxima at 560 and 680 nm (quantum yield: 3%). Fluorescence excitation spectra reveal the monomer absorption properties. The monomer fluorescence lifetime has been determined as 684 ps. It has been found that the emission properties of R-styrylpyrylium TFMS are sensitive to the switching mechanism, which allows one to use them for monitoring of the storage process. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter
O
ptical data storage systems and holographic devices open various application possibilities in optical information technology.1-3 Compared to techniques based on magnetic storage devices, changes of the chemical or physical properties, induced by photoirradiation can be alternatively used for information storage. Photons do not only initiate the writing process, but they can also be used as a reading tool. Due to the efficiency and speed of the readout process, changes in the optical absorption characteristics as well as changes of the luminescence properties are ideal mechanisms on which such device functionalities can be built.4 There are various read-write and write-read techniques for the various materials used in such devices. In dye systems, which show a bleaching upon optical writing, the decrease of the absorption band can be used as an indicator for the information to be read out.5 Other compounds show an absorption increase or even an additional appearance of emission bands which can selectively be used for reading out with wavelengths different from the writing wavelength.6 In this Letter, we add another characteristic of the optical properties of R-styrylpyrylium trifluoromethanesulfonate (R-styrylpyrylium TFMS) with respect to its switching mechanism which might potentially be used for writing-reading purposes. In particular, the ability of using R-styrylpyrylium TFMS for two-wavelength applications and the use of the emission properties of R-styrylpyrylium TFMS for online monitoring during the switching process have been studied. R-Styrylpyrylium TFMS and derivatives are discussed in the literature for holographic storage.7 The principle of the photoinduced writing process of R-styrylpyrylium TFMS relies on a molecular switching mechanism which is a reversible [2 þ 2] photodimerization reaction (Scheme 1).8,9 The ring-closing and -opening reactions of R-styrylpyrylium TFMS follow strictly the Woodward-Hoffmann rules.10
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Absorption of optical light hν leads to a breaking of the CdC double bond in the styrylpyrylium cation and the formation of the cyclobutyl dimer. The reaction (ring-opening reaction) back to the monomer molecules is induced by thermal energy (ΔT > 80 C). During the ring-opening and -closing reactions, the optical and structural properties of the material change remarkably, which can be used for optical storage. Below 350 K, the reaction from the monomer to the dimer is irreversible, which allows the use of R-styrylpyrylium TFMS as a long-term storage material. At temperatures higher than 350 K, the system returns to the monomer state, which leads to an annihilation of the optically stored information. By building devices in which R-styrylpyrylium TFMS is used at higher temperatures, it is possible to store optical information in an intermediate state with transformation kinetics faster than 30 ps.12 The structural features of the dimerization reaction of R-styrylpyrylium TFMS have intensively been studied by applying various techniques like infrared spectroscopy or photocrystallography. The structural change during the photoreaction yields a change of color of R-styrylpyrylium TFMS from red (monomer absorption band maximum at 444 nm) to yellow (dimer absorption band maximum 390 nm) during the dimerization reaction. However, so far, the optical photoluminescence properties of R-styrylpyrylium TFMS have not been investigated in great detail, and it is not clear whether that branch of the photoreaction can also be of potential use for monitoring the switching process (or for storage purposes). In Figure 1, the photoluminescence spectra of the R-styrylpyrylium TFMS monomer are shown. On the left side of Figure 1, the Received Date: January 28, 2010 Accepted Date: February 23, 2010 Published on Web Date: February 26, 2010
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DOI: 10.1021/jz100115k |J. Phys. Chem. Lett. 2010, 1, 959–961
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Figure 1. Photoluminescence spectra of the R-styrylpyrylium TFMS monomer. Left hand side: excitation spectra for 660 nm emission (dotted line). Right hand side: Emission spectra with pronounced double band structure (transition bands 1 and 2). Excitation wavelengths: 440 (dashed line) and 470 nm (solid line).
Figure 2. Fluorescence spectroscopy can also be used for monitoring the recording process of R-styrylpyrylium TFMS. Inset: monomer fluorescence before illumination (solid line) and after 2 h (dotted line). Monomer concentration refinement with two Gauss functions. Graph: change of the monomer concentration upon photoirradiation. For the used ratio of No. (chromophores): No. (photons), a biexponential decay law has been found with Æτd1æ = (1.2 ( 0.4) min and Æτd2æ = (24.1 (13.5) min. Squares: integral fluorescence intensity decrease of emission band 1; circles: integral fluorescence intensity decrease of emission band 2.
Scheme 1. [2 þ 2] Cycloaddition Reaction of R-Styrylpyrylium TFMS and the Fluorescence Channel of the Monomera
a By light (þhν1), either the dimerization can be initiated or the molecules return back to the ground-state fluorescence (-hν2).
Figure 3. Single-photon counting fluorescence measurement of the R-styrylpyrylium TFMS monomer (excitation wavelength: 355 nm). Refined decay time: τf = 684 ps.
excitation spectra of the monomer are plotted as dotted lines recorded at an emission wavelength of 660 nm. The excitation spectrum closely resembles the R-styrylpyrylium TFMS monomer absorption spectrum with the maximum at 444 nm. On the right side of Figure 1, the emission spectra of R-styrylpyrylium TFMS monomer are shown with a pronounced double band structure with the emission maxima at λem1 = 560 nm and λem2 = 685 nm. Excitation in the monomer maximum at 440 nm yields the dashed line monomer emission with a 560 nm intensity maximum (emission band 1). Excitation in the red wing of the monomer absorption at 470 nm yields the emission with a 685 nm emission maximum (emission band 2). The fluorescence quantum yield of the R-styrylpyrylium TFMS monomer was determined at the 444 nm excitation wavelength to be 3%. It was determined by refinement of two Gauss functions through the double band emission. Analogously to absorption spectroscopy,8 the decrease of the fluorescence quantum yield can be used to monitor the dimerization process in R-styrylpyrylium TFMS crystals. The results of these investigations are summarized in Figure 2. For the used ratio of No. (chromophore)/No. (photons), the relative change of the monomer concentration upon photoirradiation (444 nm excitation wavelength) can be refined as a biexponential time decay law with Æτd1æ = (1.2 ( 0.6) min and Æτd2æ = (24.1 (13.5) min. Again, the integral intensity was determined
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by the refinement of two Gauss functions through the double band emission (inset of Figure 2). If both emission bands with maxima at 560 (emission band 1) and 685 nm (emission band 2) are treated separately, as indicated by the squares and dots in Figure 2, the refined time decay laws vary within the error bars of the refinement. Since evidently both emissions decrease with nearly identical time laws during the photodimerization, we assign both of them to the monomer emission. Due to the fact that the dimerization and the fluorescence are concurrence reactions (Scheme 1), the response function of the photoluminescence can be used as a tool for monitoring the dimerization process without destroying the structurally stored optical information. By the use of the appropriate wavelength or moderate excitation intensities, the interference to the dimerization as a potential storage mechanism is negligible. If using these optical features, finally, the response time of the readout device or the control unit needs to be adjusted to the fluorescence lifetime of the monomer emission. The lifetime of the excited monomer state in R-styrylpyrylium TFMS microcrystallites was measured using a single-photon counting apparatus. The recorded decay time is shown in Figure 3. The data were analyzed using a deconvolution technique and least-squares fit employing the LevenbergMarquardt algorithm to mono/multiexponential decay law
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P
i Ai exp(-t/τι). The accuracy was 30 ps, corresponding to a relative value of 5% at 600 ps. The decay kinetics could monoexponentially be refined to an observed decay time of τem = 684 ps (with χ2 =1.1). Biexponential kinetic analysis with the decay times of (τem1 = 0.694 ns, τem2 = 1.877 ns) yielded a low-quality fit with χ2 >2.8. Previous results show that the molecular switching kinetics of the dimerization is faster than 50 ps.8 Consequently, the photodimerization reaction occurs in less than 10% of the monomer fluorescence lifetime. It is a very effective reaction channel explaining the low fluorescence quantum yield of the R-styrylpyrylium TFMS monomer in the crystalline phase. In summary, it can be stated that though the intensity properties of the monomer fluorescence of R-styrylpyrylium TFMS bulk are extremely weak (quantum yield of 3%); they can be used to monitor the recording mechanism based on the molecular switching process in R-styrylpyrylium TFMS crystals. This is of particular advantage since it is difficult to monitor absorption properties of bulk colored crystals with high accuracy. R-Styrylpyrylium TFMS exhibits two pronounced emission bands at 560 and 680 nm. As concurrent reactions, their fluorescence intensity is sensitive to the photodimerization process and therefore can be used as an indirect tool for monitoring the switching process. The fluorescence lifetime of the R-styrylpyrylium TFMS monomer in the crystalline phase has been determined to be τem = 684 ps, which is at least a factor of 10 slower than the time scale in which the photodimerization reaction takes places.
REFERENCES (1)
Cumpston, B. H.; Ananthavel, S. P.; Barlow, A.; Dyer, D. L.; Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I.-Y. S.; McCord-Maughon, D.; Qin, J.; R€ ockel, H.; Rumi, M.; Wu, X.-L.; Marder, S. R.; Perry, J. W. Two-Photon Polymerization Initiators for Three-Dimensional Optical Data Storage and Microfabrication. Nature 1999, 398, 51–54. (2) Strickler, J. H.; Webb, W. W. 3-Dimensional Optical Data Stoarge in Refractive Media by 2-Photon Point Excitation. Opt. Lett. 1991, 16, 1780–1782. (3) Hamilton, T. D.; Papaefstathiou, G. S.; MacGillivray, L. R. Template-Controlled Reactivitiy: Following Nature'S Way to Design and Construct Metal-Organic Polyhedra and Polygons. J. Solid State Chem. 2005, 178, 2409–2413. (4) Sarid, D; Schechtmann, B. H. A Roadmap for Optical Data Storage Applications. Opt. Photonics News 2007, 18, 32–37. (5) Tomasulo, M.; Raymo, F. M. Optical Writing and Reading with Bilayer Assemblies of Photosensitive and Fluorescent Films. J. Mater.Chem. 2005, 15, 4354–4360. (6) S€ ollner, M.; Vieth, U.; Hsu, K. Y.; Lin, S. H.; Gruber, M. Holographic Data Storage with a Planar-Integrated Optical Write-Read Head. Proc. SPIE 2009, 7358, 73580–73584. (7) Buchholz, V.; Enkelmann, V. Photochemical Single-Crystal-toSingle-Crystal Transformation. Mol. Cryst. Liq. Cryst. 1998, 313, 309–314. (8) Hallmann, J.; Morgenroth, W.; Paulmann, C.; Davaasambuu, J.; Kong, Q.; Wulff, M.; Techert, S. Time-Resolved X-ray Diffraction of the Photochromic R-Styrylpyrylium Trifuloromethanesulfonate Crystal Films Reveals Ultrafast Structural Switching. J. Am. Chem. Soc. 2009, 131, 15018–15025. (9) Gavezzoti, A.; Simonetta, M. Crystal Chemistry in Organic Solids. Chem. Rev. 1982, 82, 1–13. (10) Woodward, R. B.; Hoffmann, R. Stereochemistry of Electrocyclic Reactions. J. Am. Chem. Soc. 1965, 87, 395–397. (11) Hesse, K.; H€ unig, S. Multistep Reversible Redox Systems. 42. [2 þ 2]Photocycloadditions of R-Systylpyrylium Salts to 4,40 -(1,3-Cyclobutanediyl)bis(pyrylium) Salts and Their Thermal and Base-Induced Cycloreversions. Liebigs Ann. Chem. 1985, 105, 715–739. (12) Techert, S.; Wiessner, A.; Schmatz, S.; Staerk, H. TimeResolved Fluorescence and Solvatochromy of Directly Linked Pyrene-DNA Derivatives in Alcoholic Solution. J. Phys. Chem. B 2001, 105, 7579–7587. (13) Petrov, N. Kh.; Gulakov, M. N.; Alfimov, M. V.; Busse, G.; Frederichs, B.; Techert, S. Photophysical Properties of 3,30 Diethylthiacarbocyanine Iodide in Binary Mixtures. J. Phys. Chem. A 2003, 107, 6341–6344.
METHODS R-Styrylpyrylium TFMS was synthesized according to ref 11. For the measurements, the grained crystals were pressed into KBr pellets. The fluorescence spectra of R-styrylpyrylium TFMS were recorded with a Fluorolog 3-22 (JOBIN YVON-SPEX) spectrofluorometer. As an external excitation light source, a 450 W XBO lamp was used. The slits were adjusted to a 0.5 nm bandwidth for excitation and emission. The recorded spectra were corrected in accordance with the wavelength dependence on the lamp intensity, monochromator transmission, and photomultiplier response. The fluorescence quantum yield was determined according to the standard procedure, with rhodamin101 as the reference. Further details of the described setups and the developed methodology can be found in ref 12. The fluorescence lifetime of R-styrylpyrylium TFMS was determined by the single-photon counting technique (apparatus: Edinburgh instruments). As an excitation source, the third harmonic (λexc = 355 nm) of an externally connected, pulsed Nd:YAG laser was used. The detailed setup is explained in ref 13.
AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed.
ACKNOWLEDGMENT This work was supported by SFB 755 Nanoscale Photonic Imaging and SFB 602 Complex Structures in Condensed Matter. S.T. and J.H. thank the Advanced Study Group of the Max Planck Society for continuous support.
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