One- and Two-Photon Stimulated Emission Depletion of a Sulfonyl-Containing Fluorene Derivative

One- and two-photon stimulated emission transitions were investigated by the fluorescence quenching of a sulfonyl-containing fluorene compound, 2,7-bis(4-(phenylsulfonyl)styryl)-9,9-didecyl-9H-fluorene (1), in solution at room temperature using a picosecond pump-probe technique. The nature of stimulated transitions under various fluorescence excitation and quenching conditions was analyzed theoretically, and good agreement with experimental data was demonstrated. Two-photon stimulated transitions S1f S0 were shown for 1 at λq) 1064 nm, representing the first report of two-photon stimulated emission depletion (STED) in a molecular system. The two-photon stimulated emission cross section, δ2PE(λq), of fluorene 1 was estimated to be ∼240-280 GM, suggesting that this compound may be a good candidate for use in two-photon STED microscopy. 1

Nature of the electronic transitions in thiacarbocyanines with a long polymethine chain

Abstract A detailed experimental investigation and quantum-chemical analysis of symmetric cyanines of different conjugation lengths have been performed with the goal of understanding the nature of the electronic transitions in molecules that possess a long chromophore. The nature of electronic transitions in cyanines with a relatively short chromophore (inside the cyanine limit) has already been investigated and many properties of these molecules are well understood. However, little is known about the nature of the transitions beyond the cyanine limit. Their unusual properties, which were proposed by Tolbert and Zhao to be connected with symmetry breaking, still remain unexplored. The analysis of the spectral data in various solvents and results of femtosecond pumpprobe saturable absorption measurements enable us to conclude that an increase in the length of the chain leads to a symmetry breaking and the appearance of two forms with symmetrical and asymmetrical distributions of the charge density in the ground state. For thiacarbocyanines, symmetry breaking is predicted and observed for a pentacarbocyanine dye. Quantum-chemical calculations provide additional proof of this hypothesis. The excited-state absorption properties of a pentacarbocyanine in the visible region are also reported. For the first time we have observed an excited-state cross-section that is larger (%3·) than the ground state cross-section at the peak spectral position

Singlet Oxygen Quantum Yield Determination For A Fluorene-Based Two-Photon Photosensitizer

The quantum yield, Φ Δ, of singlet oxygen generation under two-photon excitation has been determined for a fluorene derivative. A photochemical method was developed using 1,3-diphenylisobenzofuran (DPBF), a chemical quencher of 1O 2, and 2-(9,9-didecyl-7- nitrofluoren-2-yl)benzothiazole (1) as a two-photon photosensitizer (PS). The photochemical kinetics of the quencher was measured by two different fluorescence methods. Fluorene 1 exhibited relatively high singlet oxygen quantum yield, Φ Δ∈≈∈0.4∈±∈0. 1, and had a two-photon absorption cross-section of 28∈±∈5GM. Thus, 1 may have potential for use as a two-photon PS in the near-IR spectral region for biomedical applications. © 2006 Springer Science+Business Media, Inc

Picosecond Absorption Anisotropy Of Polymethine And Squarylium Dyes In Liquid And Polymeric Media

Time-resolved excitation-probe polarization measurements are performed for polymethine and squarylium dyes in ethanol and an elastopolymer of polyurethane acrylate (PUA). These molecules exhibit strong excited-state absorption in the visible, which results in reverse saturable absorption (RSA). In pump-probe experiments, we observe a strong angular dependence of the RSA decay kinetics upon variation of the angle between pump and probe polarizations. The difference in absorption anisotropy kinetics in ethanol and PUA is detected and analyzed. Anisotropy decay curves in ethanol follow a single exponential decay leading to complete depolarization of the excited state. We also observe complete depolarization in PUA, in which case the anisotropy decay follows a double exponential behavior. Possible rotations in the PUA polymeric matrix are connected with the existence of local microcavities of free volume. We believe that the fast decay component is connected with the rotation of molecular fragments and the slower decay component is connected with the rotation of entire molecules in local microcavities, which is possible because of the elasticity of the polymeric material. © 2001 Published by Elsevier Science B.V

Photophysical Characterization Of 2,9-Bis(7-Benzothiazole-9,9′-Didecylfluoren-2-Yl)Perylene Diimide: A New Standard For Steady-State Fluorescence Anisotropy

The absorption, fluorescence excitation and emission spectra have been obtained in solution for 2,9-bis(7-benzothiazole-9,9-didecyl-fluoren-2-yl) perylene diimide. Efficient resonance energy transfer from the fluorenyl group to the perylene ring center was observed. Interestingly, fluorescence emission was detected from the second excited electronic state of the perylene ring system. Fluorescence excitation anisotropy spectra obtained at room temperature exhibited a parallel orientation of the main absorption and emission band transition moments for the perylene-based dye in CH2Cl2. The value of excitation fluorescence anisotropy for the perylene dye in solution approached the theoretical maximum limit (r ≈ 0.4), and indicated that the rotational correlation time exceeded the lifetime of the first excited state. These results provide the basis for using this unique compound as an anisotropy reference standard. © 2002 Elsevier Science B.V. All rights reserved

Two-Photon Absorption Study Of Polymethine And Squarylium Molecules

The degenerate two-photon absorption spectra for a series of symmetric and asymmetric polymethine and squarylium dyes were measured by femtosecond Z-scan and upconverted fluorescence methods. Structure-property relationships were revealed and explained with quantum-chemical analysis. © 2005 Optical Society of America

Steady-State Spectroscopic And Fluorescence Lifetime Measurements Of New Two-Photon Absorbing Fluorene Derivatives

Steady-state excitation anisotropy, lifetimes, and time-resolved emission spectra of new 2-photon absorbing fluorene derivatives were measured in aprotic solvents at room temperature. Excitation anisotropy spectra in viscous silicon oil allowed the determination of the spectral position cf three electronic transitions S0 → S1, S0 → S 2, S0 → S3 (Si i = 1, 2, 3 are the singlet electronic states) and the angles (∼ 30°) between absorption S0 → S1 and emission S1 → S 0 dipole moments for the first electronic transition. Solvate relaxation processes in the first excited state of the investigated fluorene molecules affect the lifetimes of these states, τ1, so that experimental values of τ1 do not correspond to those calculated by Strickler and Berg theory. The influence of the molecular concentration on the fluorescence quantum yields and τ1 have been investigated

Polymethine And Squarylium Molecules With Large Excited-State Absorption

We study nonlinear absorption in a series of ten polymethine dyes and two squarylium dyes using Z-scan, pump-probe and optical limiting experiments. Both picosecond and nanosecond characterization were performed at 532 nm, while picosecond measurements were performed using an optical parametric oscillator (OPO) from 440 to 650 nm. The photophysical parameters of these dyes including cross sections and excited-state lifetimes are presented both in solution in ethanol and in an elastopolymeric material, polyurethane acrylate (PUA). We determine that the dominant nonlinearity in all these dyes is large excited-state absorption (ESA), i.e. reverse saturable absorption. For several of the dyes we measure a relatively large ground-state absorption cross section, σ01, which effectively populates an excited state that possesses an extremely large ESA cross section, σ12. The ratios of σ12/σ01 are the largest we know of, up to 200 at 532 nm, and lead to very low thresholds for optical limiting. However, the lifetimes of the excited state are of the order of 1 ns in ethanol, which is increased to up to 3 ns in PUA. This lifetime is less than optimum for sensor protection applications for Q-switched inputs, and intersystem crossing times for these molecules are extremely long, so that triplet states are not populated. These parameters show a significant improvement over those of the first set of this class of dyes studied and indicate that further improvement of the photophysical parameters may be possible. From these measurements, correlations between molecular structure and nonlinear properties are made. We propose a five-level, all-singlet state model, which includes reorientation processes in the first excited state. This includes a trans-cis conformational change that leads to the formation of a new state with a new molecular configuration which is also absorbing but can undergo a light-induced degradation at high inputs

Nonlinear light absorption of polymethine dyes in liquid and solid media

We investigate nonlinear optical absorption of a series of polymethine dyes in liquid solution and in a solid poly(urethane acrylate) elastopolymer host. Depending on the ratio between the absorption cross section from the ground to the first excited state (σ01) and from the first to the higher excited states (σ12) and on the pump wavelength, they demonstrate absorption that either increases or decreases with increasing incident energy density. Our interest is directed at studying the optical limiting bahavior of the dyes for sensor protection applications, which requires that σ12 be greater than σ01. We performed a detailed investigastion of the dynamics of these processes, using Z-scan and pump-probe techniques at the single wavelength of 532 nm. We obtained σ12/σ01 ∼ 80 for one of the dyes, which is significantly higher than any previously reported value of which we are aware. The linear absorption remained significant, a requirement for good optical limiting. Howerer, at high incident energy density for most of the dyes the increasing absorption converted to a bleaching effect with repeated irradiation. This effect was reversed after several seconds in liquid solutions and several minutes in the polymer host, possibly because of replenishment of the dye by diffusion in both media. © 1998 Optical Society of America

The Nature Of Excited-State Absorption In Polymethine And Squarylium Molecules

Subpicosecond transient absorption measurements were performed for several polymethine and squarylium dyes in ethanol solution and a polymeric host over the spectral range 400-1500 nm. A variety of nonlinear effects including saturable absorption, reverse saturable absorption, and gain were observed and analyzed. We observe strong excited-state absorption (ESA) in all dyes in the range 450-600 nm. We also report the first prediction and observation of additional ESA bands in the near-infrared range. The predictions were based on quantum chemical calculations and the ESA experiments were performed with femtosecond pump-continuum probe techniques. For polymethine dye 2-[2-[3-[(1,3-dihydro-3,3-dimethyl-1-phenyl-2H-indol-2-yli-dene) ethylidene]-2-phenyl-1-cyclohexen-1-yl]ethenyl]-3,3- dimethyl-1-phenylindolium perchorate, an additional ESA band was detected near 1250 nm, and for squarylium dye 1,3-Bis-[(1,3-dihydro-1-butyl-3,3-dimetyl-2H-benzo[e]indol-2-ylidene)methyl]squa raine, two additional ESA bands were found around 870- and 1380-nm, respectively. To further study the nature of these transitions, the steady-state excitation anisotropy was also studied and compared with predictions. The relationship between ESA spectra of organic dyes and their molecular structure is discussed