Optical spectra of two families of symmetrical polymethine dyes, bearing a positive and a negative charge, are analyzed based on an essential-state model recently developed for quadrupolar dyes. The model accounts for molecular vibrations and polar solvation and reproduces the anomalous evolution with solvent polarity of experimental absorption band shapes. Fluorescence and excited-state absorption spectra are well-described within the same model, which also quantitatively reproduces the recent observation of an intense two-photon absorption toward the (two-photon forbidden) lowest excited state. An extensive analysis of optical spectra demonstrates that the essential-state model developed for quadrupolar dyes also applies to polymethine dyes and that long polymethine dyes offer the first experimental example of class III quadrupolar chromophores.

The multifunctional properties of carbon nanotubes (CNTs) make them a powerful platform for unprecedented innovations in a variety of practical applications. As a result of the surging growth of nanotechnology, nanotubes present a potential problem as an environmental pollutant, and as such, an efficient method for their rapid detection must be established. Here, we propose a novel type of ionic sensor complex for detecting CNTs – an organic dye that responds sensitively and selectively to CNTs with a photoluminescent signal. The complexes are formed through Coulomb attractions between dye molecules with uncompensated charges and CNTs covered with an ionic surfactant in water. We demonstrate that the photoluminescent excitation of the dye can be transferred to the nanotubes, resulting in selective and strong amplification (up to a factor of 6) of the light emission from the excitonic levels of CNTs in the near-infrared spectral range, as experimentally observed via excitation-emission photoluminescence (PL) mapping. The chirality of the nanotubes and the type of ionic surfactant used to disperse the nanotubes both strongly affect the amplification; thus, the complexation provides sensing selectivity towards specific CNTs. Additionally, neither similar uncharged dyes nor CNTs covered with neutral surfactant form such complexes. As model organic molecules, we use a family of polymethine dyes with an easily tailorable molecular structure and, consequently, tunable absorbance and PL characteristics. This provides us with a versatile tool for the controllable photonic and electronic engineering of an efficient probe for CNT detection.

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.

A detailed experimental and theoretical study of the linear and nonlinear absorption of a series of asymmetrical D-p-A cyanine dyes with the same trimethylindolin donor (D) and diethylaminocoumarin-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 p-conjugation in the investigated series of dyes leads to an enhancement of the excited-state absorption and two-photon absorption (2PA) cross-sections (d 2PA ). 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, S 0 / S 1 , 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 S 0 / S 1 band, resulting in a large intermediate state resonance enhancement and, therefore, a larger 2PA in acetronitrile (d 2PA z 10000 GM) compared to toluene (d 2PA z 4700 GM). Experimental methods and results Materials characterization and linear spectroscopic propertiesThe molecular structures of the dyes studied in this paper are shown in the inset of Fig. 1a-c. Their chemical names are:

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.

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Self-organization of organic molecules with carbon nanomaterials leads to formation of functionalized molecular nano-complexes with advanced features. We present a study of physical and chemical properties of carbon nanotube-surfactant-indocarbocyanine dye (astraphloxin) in water focusing on aggregation of the dye and resonant energy transfer from the dye to the nanotubes. Self-assembly of astraphloxin is evidenced in absorbance and photoluminescence depending dramatically on the concentrations of both the dye and surfactant in the mixtures. We observed an appearance of new photoluminescence peaks in visible range from the dye aggregates.The aggregates characterized with red shifted photoluminescence peaks at 595, 635 and 675 nm are formed mainly due to the presence of surfactant at the premicellar concentration. The energy transfer from the dye to the nanotubes amplifying near-infrared photoluminescence from the nanotubes is not affected by the aggregation of astraphloxin molecules providing important knowledge for further development of advanced molecular nano-complexes. The aggregation with the turned-on peaks and the energy transfer with amplified photoluminescence create powerful tools of visualization and/or detection of the nanotubes in visible and near-infrared spectral range, respectively, boosting its possible applications in sensors, energy generation/storage, and healthcare.

A series of meso-polymethine-substituted BODIPY compounds have been synthesized by the reaction of mesomethyl-3,5-diphenylboradipyrromethene with a number of hemicyanine derivatives. The dyes obtained exhibit intense long-wavelength absorption, but weak fluorescence. Upon protonation of the dyes the long-wavelength band disappears and the intensity of the short-wavelength band in- IntroductionBoradipyrromethene dyes (4,4-difluoro-4-bora-3a,4a-diazas-indacenes, BODIPY, BDP) have attracted considerable attention over the past two decades. The ever increasing interest in this type of compound is due to their excellent thermal, chemical, and photochemical stability, high molar absorptivity, high fluorescence quantum yields, insensitivity to solvent polarity and pH, long excited-state lifetimes, a large two-photon cross-section for multiphoton excitation, lack of ionic charge, and good solubility.[1] However, the absorption maxima of the majority of BODIPYs are below 600 nm. Long-wavelength dyes are important for both basic and applied research [2] and therefore many synthetic approaches exist for modifying the BODIPY core to give structures that absorb at longer wavelengths. One of the most promising approaches to the modification of BOD-IPY is its peripheral functionalization with conjugated chromophores. Scheme 1.[a] Institute of Organic Chemistry, National Academy For example, distyryl-substituted derivatives A exhibit a bathochromic shift of the absorption maximum of 79 nm compared with diphenyl-substituted analogues (see Scheme 1). The bathochromic shift of phenylethynyl analogues B is somewhat smaller, however, both A and B are typical BODIPYs and demonstrate high fluorescence quantum yields that are fairly insensitive to the nature of the solvent.[3] The introduction of additional 4-(dialkylamino) substituents into A (structure C [4] ) results in even more pronounced spectral changes, with bathochromic shifts of around 80 nm. The structures C exhibit very small solvatochromism and considerable positive solvatofluorochromism. The fluorescence intensity increases significantly with decreasing solvent polarity. Protonation of the dialkylamino groups in C leads to optical properties that are very similar to those of A. Although the charge-separated mesomeric form C2 is typical of merocyanine dyes, the properties of the structures C (e.g., easy protonation, weak solvatochromism, and relatively low molar absorptivity) as a whole do not correspond to those of merocyanines. Thus, the dialkylamino groups in structures C provide considerable spectral changes, but do not change the color of the V. P. Yakubovskyi, M. P. Shandura, Y. P. Kovtun FULL PAPER BODIPY core. Therefore in this case they cannot be referred to as true "end groups" as defined in the ideology of polymethine dyes.This statement is even more evident for structures D. [3a,4b,5] In this case the dialkylamino group leads to a small hypsochromic shift of the BODIPY absorption maximum. The fluorescence quantum yields are quenched in polar solvents but not i...