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ABSTRACT: We report the intrinsic absorption and photoluminescence spectra of rubrene single crystals, deriving them from a series of experiments performed at different excitation wavelengths and in different experimental geometries. We describe the absorption spectra for all three light polarizations in the crystal, and discuss how anisotropic wavelength-dependent absorption and emission affect the characteristics of observed photoluminescence spectra. We identify vibronic progressions both in absorption and emission and discuss their parameters and the main vibrational modes that are responsible for them. We propose that the most commonly measured absorption and emission in rubrene, the one with light polarization perpendicular to the c axis of the crystal, is caused by vibronic-induced depolarization of the c-polarized electronic transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
Physical Review B 01/2012; 86. · 3.69 Impact Factor
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ABSTRACT: We present linear and nonlinear optical properties of small donor-acceptor substituted molecules for third-order nonlinear optics and of their dense assemblies created by molecular beam deposition in high vacuum. The molecules are variations on the DDMEBT molecule, obtained by varying the structure of the electron donating groups around a compact conjugated core derived from a tetracyanobutadiene (TCBD) group. We discuss the optical properties, lifetime, losses, and robustness of vapor-deposited DDMEBT thin films and the influence of the different end-groups on both the linear and nonlinear optical properties of the molecules and the resulting supramolecular assemblies. © 2012 Optical Society of America OCIS codes: (190.4400) Nonlinear optics, materials; (190.4710) Optical nonlinearities in or-ganic materials., "A high optical quality supramolecular assembly for third-order integrated nonlinear optics,, "Efficient third-order optical nonlinearities in donor-substituted cyanoethynylethene molecules," Opt. Lett. 30, 3057–3059 (2005). 3. J. C. May, I. Biaggio, F. Bures, and F. Diederich, "Extended conjugation and donor-acceptor substitution to improve the third-order optical nonlinearity of small molecules,, "A new class of organic donor–acceptor molecules with large third-order optical nonlinearities," Chem. Commun., "Donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs): New chromophores with efficient intramolecular charge-transfer interactions by atom-economic synthesis," Chem.-Eur. J., "All-optical high-speed signal processing with silicon-organic hybrid slot waveguides,, "Silicon organic hybrid technology -a platform for practical nonlinear optics,, "All-optical FFT signal processing of a 10.8 Tb/s single channel OFDM signal,, "Comparison of cc triple and double bonds as spacers in pushpull chromophores," Eur. J. Org. Chem. 2011, 4307–4317 (2011). 13. B. Breiten, "Acetylenic scaffolding. nonplanar push-pull chromophores for opto-electronic applications," Phd dissertation (ETH, Zürich, 2011). DOI:10.3929/ethz-a-006724916. 14., "Donor-substituted octacyano[4]dendralenes: a new class of cyano-rich nonplanar organic acceptors,Switching the regioselectivity in cycloaddition-retro-electrocyclizations between donor-activated alkynes and the electron accepting olefins tcne and tcnq,, "Donor-substituted cyanoethynylethenes: π-conjugation and band-gap tuning in strong charge-transfer chromophores," Chem. Eur. J. 11, 3325–3341 (2005). 17. S. S. Andrews, "Using rotational averaging to calculate the bulk response of isotropic and anisotropic samples from molecular parameters," J. Chem, "Intrinsic hyperpolarizabilities as a figure of merit for electro-optic molecules," J., "Crystalline thin film of a donor-substituted cyanoethynylethene for nanoscale data recording through inter-molecular charge-transfer interactions,, "All-optical wavelength conversion of 56 Gbit/s NRZ-DQPSK signals in silicon-organic hybrid strip waveguides, third-harmonic generation of fused silica in gas atmosphere: Absolute value of the third-order nonlinear optical susceptibility χ (3) ,-mismatched degenerate four-wave mixing: complex third-order susceptibility tensor elements of C 6 0 at 768 nm,
Optical Materials Express. 01/2012; 2(3):294-303.
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ABSTRACT: A formal [2 + 2] cycloaddition-cycloreversion (CA-CR) between N,N'-dicyanoquinone diimides (DCNQIs) and electron-rich alkynes was explored, providing a new class of π-conjugated donor-acceptor chromophores. These DCNQI adducts exist in the solid state as single diastereoisomers, but as two interconverting diastereoisomers in solution. Solid- and solution-state evidence for intramolecular charge transfer (CT) was obtained; additionally, the DCNQI adducts exhibit positive solvatochromism and significant solution-state third-order polarizabilities.
Organic Letters 11/2011; 14(1):54-7. · 5.86 Impact Factor
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ABSTRACT: We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.
Physical Review Letters 07/2011; 107(1):017402. · 7.37 Impact Factor
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ABSTRACT: Significant acceleration of the photoluminescence (PL) decay rate was observed in water solutions of two rare earth ions (REIs), Tb and Eu. We propose that the time-resolved PL spectroscopy data are explained by a fluorescence resonance energy transfer (FRET) between the REIs. FRET was directly confirmed by detecting the induced PL of the energy acceptor, Eu ion, under the PL excitation of the donor ion, Tb, with FRET efficiency reaching 7% in the most saturated solution, where the distance between the unlike REIs is the shortest. Using this as a calibration experiment, a comparable FRET was measured in the mixed solution of REIs with single-wall nanotubes (SWNTs) wrapped with DNA. From the FRET efficiency of 10% and 7% for Tb and Eu, respectively, the characteristic distance between the REI and SWNT/DNA was obtained as 15.9 ± 1.3 Å, suggesting that the complexes are formed because of Coulomb attraction between the REI and the ionized phosphate groups of the DNA.
ACS Nano 06/2011; 5(7):6052-9. · 10.77 Impact Factor
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Brian B. Frank,
Philip R. Laporta,
Benjamin Breiten,
Mark C. Kuzyk,
Peter D. Jarowski,
W. Bernd Schweizer,
Paul Seiler, Ivan Biaggio,
Corinne Boudon,
Jean-Paul Gisselbrecht,
François Diederich
Annalen der Chemie und Pharmacie 06/2011; 2011(23):4307 - 4317. · 3.10 Impact Factor
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ABSTRACT: We have developed a new kind of tellurite glass with composition 70.5TeO 2 –12ZnO–10Na 2 O–7.5Bi 2 O 3 by conventional melting–quenching techniques. A suspended-core fiber (SCF) with a triangular-shaped core (~ 2.9 μm) has been drawn from an extruded perform. Several nonlinear frequency conversion processes are being demonstrated, which point to the potential of such a fiber.
Optics Communications 05/2011; · 1.49 Impact Factor
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ABSTRACT: The decay of the photoluminescence excited in rubrene single crystals by picosecond pulses is measured over 7 orders of magnitude and more than 4 time decades. We identify the typical decay dynamics due to triplet-triplet interaction. We show that singlet exciton fission and triplet fusion quantum yields in rubrene are both very large, and we directly determine a triplet exciton lifetime of 100 ± 20 μs, which explains the delayed buildup of a large photocurrent that has been reported earlier for low excitation densities. Photoluminescence dynamics in organic molecular crystals can provide information on the nature of excitons and their evolution from the initial photoexcitation to interaction with defects and ultimately to dissociation or recombination. Excitons in molecular crystals are of fundamental interest and their possible dissociation into free carriers is of relevant practical importance for optoelectronic applications such as photovoltaics. In particular, the rubrene single crystal has an extraordinarily large carrier mobility observed in field effect transistors, 1–3 and it is characterized by a large photoconduc-tivity appearing with a rise time of 100 μs after impulsive excitation, 4,5 which implies that a significant fraction of the photoinduced excitons in rubrene ultimately leads to mobile charge carriers. 5 Even though the exciton dissociation mechanism has been associated both with oxygen-related defects 5,6 and the crystal's surface, 7 the path that leads from photon absorption to delayed carrier release in rubrene has not been fully described yet. The initially photoexcited species in molecular crystals are singlet excitons with a lifetime of the order of nanoseconds or less. Under pulsed illumination, their radiative recombination leads to a photoluminescence (PL) signal that decays during the singlet lifetime. But in addition to this, creation of triplet excitons 8,9 can lead to a long-lived PL signal because of their long lifetime and their capability of fusing together to regenerate a singlet state that can radiatively recombine. 10 Most of the work in this field has concentrated on aro-matic single crystals such as antracene, 8,9 tetracene, 10 and naphthalene. 11 For rubrene, we are only aware of studies of delayed PL dynamics that were performed on molecules in solution. 12 In this work we investigate delayed PL dynamics in vapor transport grown, orthorhombic rubrene single crystals, varying from pristine as-grown samples to samples that have been exposed to air and light for a long time. The rubrene PL spectrum has prominent emission bands near 560 nm and 608 nm. 6,13 The amplitudes of smaller bands around 645 nm and beyond are more sample dependent and have been partially assigned to oxidation. 6,13,14 We first studied the PL dynamics using 1 ps long pulses obtained from a Light Conversion TOPAS (traveling-wave optical parametric amplifier system) pumped at 1 kHz by a Clark-MXR Ti:sapphire amplifier. We used pulses at a wavelength of 420 nm (absorption length of ∼3 μm) and with an energy of the order of 1 μJ focused to a spot-size of 40 μm on the ab surface of the crystals, paying attention to avoiding any surface imperfections. We detected the PL in a confocal geometry with a photomultiplier tube (PMT) connected to a 4 GHz LeCroy digital oscilloscope. Following excitation, we observed a fast PL component (inset in Fig. 1) that decays in less than 10 ns (limited by the time resolution of our equipment), followed by a slower component. The time dynamics of the slower component is shown in Fig. 1 for three wavelengths close to the most important bands in the rubrene PL spectrum. The wavelengths were selected by bandpass filters with a spectral width of 10 nm. The time-dependent data were obtained by combining several measurements taken at different time delays with a Hamamatsu gated PMT while keeping similar PMT output voltages to avoid saturation effects. The three decays are very similar to each other. In particular, the band at 645 nm, which has been argued to grow with oxidation, 6,13,14 decays in the same way as the other bands over multiple time decades. This behavior remained the same among various samples where the prominence of the 645 nm band was different.
Physical Review B 01/2011; · 3.69 Impact Factor
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ABSTRACT: We determine the refractive index dispersion of a quaternary tungsten-tellurite glass over a broad spectral range from 0.4 to 3 μ m from the wavelength dependence of the phase matching conditions for third-order nonlinear optical frequency conversion. We discuss the optimum phase-matching geometries for this task and obtain the wavelength-dependence of the refractive index to within three significant digits by a least-squares fit of a Sellmeier-type equation to the phase-matching data.
Applied Physics Letters 10/2010; · 3.84 Impact Factor
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Angewandte Chemie International Edition 08/2010; 49(35):6207-11. · 13.45 Impact Factor
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ABSTRACT: Excitons in rubrene single crystals dissociate into free charge carriers via two mechanisms whose relative importance depends on the illumination wavelength through the optical penetration depth into the crystal. The first mechanism is defect-induced dissociation in less than 10 ns after photoexcitation. For low photoexcitation densities, about 10% of the excitons that survive radiative recombination dissociate through this channel. The second mechanism, affecting the remaining 90% of the excitons, involves a previously reported state localized close to the surface of the crystal that leads to a delayed release of photocarriers a fraction of a millisecond after photoexcitation.
Applied Physics Letters 05/2010; 96(18):183302-183302-3. · 3.84 Impact Factor
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ABSTRACT: We discuss the potential functionalities of a mixed tellurite-chalcogenide glass where alternate oxygen atoms in a TeO(2) vitreous network are substituted with either sulfur or selenium atoms. We calculate the microscopic linear and nonlinear polarizabilities of chains with a finite number (n) of TeOX (X = S or Se) units from ab initio quantum chemistry computations and apply rotational averaging to the results. We find that the rotationally averaged linear polarizability, α(av), increases linearly with the number of units, while the second hyper-polarizability, γ(av), grows as a power law with an exponent that changes from 1.2 to 1.9 and 2.4 when going from pure tellurite to S substitution and Se substitution, respectively. We then estimate the density of the resulting glass and local field corrections to obtain the macroscopic linear and nonlinear susceptibilities for a glass made from chains of n = 5. We find that the extra Te-S (or Te-Se) bonds in the mixed tellurite-chalcogenide system enhance the linear refractive index n(0) and the third-order susceptibility χ((3)) by factors of ∼ 1.3 (or 1.4) and ∼ 8 (or 14), respectively, over the base tellurite glasses (X = O).
Journal of Physics Condensed Matter 04/2010; 22(16):165903. · 2.55 Impact Factor
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Brian B. Frank,
Milan Kivala,
Berta Camafort Blanco,
Benjamin Breiten,
W. Bernd Schweizer,
Philip R. Laporta, Ivan Biaggio,
Eike Jahnke,
Rik R. Tykwinski,
Corinne Boudon,
Jean-Paul Gisselbrecht,
François Diederich
Annalen der Chemie und Pharmacie 03/2010; 2010(13):2487 - 2503. · 3.10 Impact Factor
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ABSTRACT: Donor-substituted cyanoethynylethenes (CEEs) are planar push-pull chromophores featuring intense intramolecular charge-transfer (CT) interactions and high third-order optical nonlinearities. Their thermal stability allows for the formation of crystalline thin films by vapor-phase deposition. On the other hand, high-quality amorphous thin films are preferred for opto-electronic applications and such films can be prepared using nonplanar push-pull chromophores with a less pronounced propensity to crystallize. By taking advantage of a versatile, atom-economic 'click-chemistry'-type transformation, involving a formal [2 + 2] cycloaddition of tetracyanoethene (TCNE) to electron-rich alkynes, followed by cycloreversion, stable donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) are obtained in high yield and large quantities. These nonplanar push-pull chromophores also feature intense intramolecular CT and, in many cases, high third-order optical nonlinearities. Some of these compounds form high-optical-quality amorphous thin films by vapor-phase deposition, and first applications in next-generation opto-electronic devices have already been demonstrated. Chiral derivatives display high helical twisting power and are efficient dopants to translate molecular into macroscopic chirality, by switching nematic into cholesteric liquid crystalline phases.
CHIMIA International Journal for Chemistry 01/2010; 64(6):409-13. · 1.21 Impact Factor
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Thomas Vallaitis,
Siegwart Bogatscher,
Luca Alloatti,
Pieter Dumon,
Roel Baets,
Michelle L Scimeca, Ivan Biaggio,
François Diederich,
Christian Koos,
Wolfgang Freude,
Juerg Leuthold
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ABSTRACT: Geometry, nonlinearity, dispersion and two-photon absorption figure of merit of three basic silicon-organic hybrid waveguide designs are compared. Four-wave mixing and heterodyne pump-probe measurements show that all designs achieve high nonlinearities. The fundamental limitation of two-photon absorption in silicon is overcome using silicon-organic hybrid integration, with a five-fold improvement for the figure of merit (FOM). The value of FOM = 2.19 measured for silicon-compatible nonlinear slot waveguides is the highest value published.
Optics Express 09/2009; 17(20):17357-68. · 3.59 Impact Factor
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Advanced Materials 10/2008; 20(23):4584 - 4587. · 13.88 Impact Factor
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ABSTRACT: A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)(2) acceptor moieties is systematically varied. All of the new push-pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X-ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual pi system, whereas the entire conjugated pi system in series b is nearly planar. Support for strong donor-acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge-transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two-dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push-pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D-A conjugation with increasing spacer length. Degenerate four-wave mixing experiments reveal high third-order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.
Chemistry 02/2007; 13(19):5378-87. · 5.93 Impact Factor
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ABSTRACT: By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.
Physical Review Letters 03/2006; 96(5):056604. · 7.37 Impact Factor
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ABSTRACT: We investigated the third-order nonlinear optical properties of several donor-substituted cyanoethynylethene molecules in the zero-frequency limit. We observed nonlinearities that are extraordinarily large relative to the small molecular mass of these molecules and that are within a factor of 50 from the fundamental limit. At a wavelength of 1.5 microm, the rotational average of the third-order molecular polarizability is 53 +/- 13 x 10(-48) m5 V(-2) (3.8 x 10(-33) esu) for the best molecule, which corresponds to 1.7 x 10(-48) m5 V(-2) per delocalized electron. The high nonlinear efficiency of these molecules is due to the compact two-dimensional conjugated system and the effective donor-acceptor substitution patterns.
Optics Letters 12/2005; 30(22):3057-9. · 3.40 Impact Factor
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ABSTRACT: By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are only created through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.
11/2005;
