Alexey D. Kachkovski

Efficient Two-Photon Absorbing Acceptor-π-Acceptor Polymethine Dyes

We present an experimental and theoretical investigation of the linear and nonlinear optical properties of a series of acceptor-pi-acceptor symmetrical anionic polymethine dyes with diethylamino-coumarin-dioxaborine terminal groups and different conjugation lengths. Two-photon absorption (2PA) cross sections (delta(2PA)) are enhanced with an increase of pi-conjugation length in the investigated series of dyes. 2PA spectra for all dyes consist of two well-separated bands. The first band, located within the telecommunications window, occurs upon two-photon excitation into the vibrational levels of the main S(0) --> S(1) transition, reaching a large delta(2PA) = 2200 GM (1 GM = 1 x 10(-50) cm(4) s/photon) at 1600 nm for the longest conjugated dye. The position of the second, and strongest, 2PA band for all anionic molecules corresponds to the second-excited final state, which is confirmed by quantum-chemical calculations and excitation anisotropy measurements. Large delta(2PA) values up to 17,000 GM at 1100 nm are explained by the combination of the large ground- and excited-state transition dipole moments. The three shortest dyes show good photochemical stability and surprisingly large fluorescence quantum yields of approximately 0.90, approximately 0.66, and approximately 0.18 at the red to near-IR region of approximately 640, approximately 730, and approximately 840 nm, respectively. The excited-state absorption spectra for all samples are also studied and exhibit intense bands throughout the visible wavelength region with peak cross section close to 5 x 10(-16) cm(2) with a corresponding red shift with increasing conjugation lengths.

Nonlinear absorption in a series of Donor–π–Acceptor cyanines with different conjugation lengths

A detailed experimental and theoretical study of the linear and nonlinear absorption of a series of asymmetrical D–π–A cyanine dyes with the same trimethylindolin donor (D) and diethylamino-coumarin-dioxaborine acceptor (A) terminal groups and different conjugation lengths, is presented. Strong solvatochromic behavior affecting the fluorescence quantum yields, lifetimes, and the linear and nonlinear absorption properties is observed due to the presence of permanent ground state dipole moments. Detailed experimental studies of lifetime dynamics are performed by direct time-correlated single photon counting and pump–probe techniques. We find that an increase in π-conjugation in the investigated series of dyes leads to an enhancement of the excited-state absorption and two-photon absorption (2PA) cross-sections (δ2PA). The 2PA spectra for all of the investigated dyes consist of two well-separated bands. The first band occurs at two-photon excitation into the vibrational levels and not into the absorption peak of the main transition, S0 → S1, which is more typical of that observed for symmetrical cyanines. The position of the second 2PA band for all the molecules remains unchanged in solvents of different polarity contrary to the large solvatochromic shift of the S0 → S1 band, resulting in a large intermediate state resonance enhancement and, therefore, a larger 2PA in acetronitrile (δ2PA ≈ 10000 GM) compared to toluene (δ2PA ≈ 4700 GM).

Two-Photon Absorption in Near-IR Conjugated Molecules: Design Strategy and Structure-Property Relations

In the past few years, applications built around two-photon absorption (2PA) have emerged, which require new materials to be designed and characterized in order to discover new applications and to advance the existing ones. This chapter describes the nonlinear optical processes and characterization techniques along with design strategies and structure-property relations of cyanine and cyanine- like molecular structures with the goal of enhancing 2PA in the near-IR for multiphoton fluorescence sensing applications. Specifically, a detailed analysis of the linear and nonlinear optical properties of several classes of polymethine dyes, which include symmetrical and asymmetrical combinations of p-conjugated bridges with electron donating (D) or electron accepting (A) terminal groups, are presented. These structures are: D-p-D, A-p-A, D-p-A, and a quadrupolar type arrangement of D-p-A-p-D. The results of this research combined with the growing literature on structure- property relations in organic materials is moving us closer to the ultimate goal of developing a predictive capability for the nonlinear optical properties of molecules.

Enhanced Intersystem Crossing Rate in Polymethine-Like Molecules: Sulfur-Containing Squaraines versus Oxygen-Containing Analogues

Two different approaches to increase intersystem crossing rates in polymethine-like molecules are presented: traditional heavy-atom substitution and molecular levels engineering. Linear and nonlinear optical properties of a series of polymethine dyes with Br- and Se-atom substitution, and a series of new squaraine molecules, where one or two oxygen atoms in a squaraine bridge are replaced with sulfur atoms, are investigated. A consequence of the oxygen-to-sulfur substitution in squaraines is the inversion of their lowest-lying ππ* and nπ* states leading to a significant reduction of singlet-triplet energy difference and opening of an additional intersystem channel of relaxation. Experimental studies show that triplet quantum yields for polymethine dyes with heavy-atom substitutions are small (not more than 10%), while for sulfur-containing squaraines these values reach almost unity. Linear spectroscopic characterization includes absorption, fluorescence, quantum yield, anisotropy, and singlet oxygen generation measurements. Nonlinear characterization, performed by picosecond and femtosecond laser systems (pump-probe and Z-scan measurements), includes measurements of the triplet quantum yields, excited state absorption, two-photon absorption, and singlet and triplet state lifetimes. Experimental results are in agreement with density functional theory calculations allowing determination of the energy positions, spin-orbital coupling, and electronic configurations of the lowest electronic transitions.

Two-Photon Absorption Spectrum of a Single Crystal Cyanine-like Dye

The two-photon absorption (2PA) spectrum of an organic single crystal is reported. The crystal is grown by self-nucleation of a subsaturated hot solution of acetonitrile, and is composed of an asymmetrical donor-π-acceptor cyanine-like dye molecule. To our knowledge, this is the first report of the 2PA spectrum of single crystals made from a cyanine-like dye. The linear and nonlinear properties of the single crystalline material are investigated and compared with the molecular properties of a toluene solution of its monomeric form. The maximum polarization-dependent 2PA coefficient of the single crystal is 52 ± 9 cm/GW, which is more than twice as large as that for the inorganic semiconductor CdTe with a similar absorption edge. The optical properties, linear and nonlinear, are strongly dependent upon incident polarization due to anisotropic molecular packing. X-ray diffraction analysis shows π-stacking dimers formation in the crystal, similar to H-aggregates. Quantum chemical calculations demonstrate that this dimerization leads to the splitting of the energy bands and the appearance of new red-shifted 2PA bands when compared to the solution of monomers. This trend is opposite to the blue shift in the linear absorption spectra upon H-aggregation.

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-ylidene) 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]squ araine, 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.

Photophysics of dimethylamino-substituted polymethine dye in polymeric media

Abstract The photophysical mechanism responsible for the large Stokes shift in dimethylamino-substituted polymethine dye (DMA-PD) has been studied by steady-state and time-resolved fluorescence spectroscopy. Experiments in low-temperature ethanol glass and in polymer polyurethane acrylate (PUA) in glassy and elastic states allow us to conclude that the emission occurs mainly from two excited states with lifetimes of 2–3 and 6–7 ns, respectively. The excited-state dynamics includes fast subnanosecond rotation of the DMA group around the carbon–dimethylamino bond of the dye molecule and stabilization of this form due to environmental rearrangement. The large Stokes shift remains for this dye in solid media. This process is dynamic even in polymeric media, where it occurs in the subnanosecond–nanosecond time scale. The major spectral shift can be described as an intramolecular excited-state reaction affecting either or both emissive species. We suggest a model in which the twisting of the small dimethylamino group modulates the effective length of the polymethine chromophore, producing the spectral shift. In addition, time-resolved fluorescence anisotropy decays show a slow rotational motion (about 40 ns) of the dye inside the polymer microcavities. These interesting features of the spectral behavior of the PUA matrix doped by DMA-PD may find various applications.

Nature of the absorption bands of pyrylocyanines: heteroanalogues and isomers

Abstract The investigation of electron transition nature in pyrylocyanines and their heteroanalogues was carried out by spectral and quantum-chemical methods. The electron distribution diagrams have been used in order to determine the transition localization. The first transition is localized within the polymethine chain, and the higher ones are mainly localized within the end groups. It has been shown that the D 2h symmetry of the Huckel hamiltonian of γ-isomeric dyes had lead to quasi-generation of higher electron transitions, contrary to α-isomers being of C 2v symmetry.

Optimization of the Double Pump–Probe Technique: Decoupling the Triplet Yield and Cross Section

The double pump-probe technique (DPP), first introduced by Swatton et al. [Appl. Phys. Lett. 1997, 71, 10], is a variant of the standard pump-probe method but uses two pumps instead of one to create two sets of initial conditions for solving the rate equations, allowing a unique determination of singlet- and triplet-state absorption parameters and transition rates. We investigate the advantages and limitations of the DPP theoretically and experimentally and determine the influence of several experimental parameters on its accuracy. The accuracy with which the DPP determines the triplet-state parameters improves when the fraction of the population in the triplet state relative to the ground state is increased. To simplify the analysis of the DPP, an analytical model is presented, which is applicable to both the reverse saturable and the saturable absorption regimes. We show that the DPP is optimized by working in the saturable absorption regime. Although increased accuracy is in principle achievable by increasing the pump fluence in the reverse saturable absorption range, this can cause photoinduced decomposition in photochemically unstable molecules. Alternatively, we can tune the excitation wavelength to the spectral region of larger ground-state absorption, to achieve similar accuracy. This results in an accurate separation of triplet yield and excited-state absorption cross section. If the cross section at another wavelength is then desired, a second pump-probe experiment at that wavelength can be utilized given the previously measured triplet yield under the usually valid assumption that the triplet yield is independent of excitation wavelength.

Nature of the absorption bands of pyrylocyanines.2. Influence of t-Bu, Ph and Th ring substituents

Abstract The effects of t-Bu, Ph and Th substituents on the absorption spectra of α-and γ-pyrylocyanines and their heteroanalogues have been investigated using experimental and quantum-chemical methods. It has been found that changes in the position of the absorption band depend on the electronegativity of the ring heteroatom (O, S, N CH3), polymethine length, and the position of the bond between the chain and the end group. In the case of dye molecules, the effect of two substituents was additive, in contrast to the corresponding salts of the intial heterocycles. An explanation for the significant shifts in the absorption maximum of the thienylcyanines has been presented.