Benzothiadiazole-Substituted Aza-BODIPY Dyes: Two-Photon Absorption Enhancement for Improved Optical Limiting Performances in the Short-Wave IR Range

Aza-boron dipyrromethenes (aza-BODIPYs) presenting a benzothiadiazole substitution on upper positions are described. The strong electron-withdrawing effect of the benzothiadiazole moiety permits enhancement of the accepting strength and improves the delocalization of the aza-BODIPY core to attain a significant degree of electronic communication between the lower donating groups and the upper accepting groups. The nature of the intramolecular charge transfer is studied both experimentally and theoretically. Linear spectroscopy highlighted the strongly redshifted absorption and emission of the synthesized molecules with recorded fluorescence spectra over 1000 nm. Nonlinear optical properties were also investigated. Strong enhancement of the two-photon absorption of the substituted dyes compared with the unsubstituted one (up to 4520 GM at 1300 nm) results in an approximately 15–20 % improvement of the optical power limiting performances. These dyes are therefore a good starting point for further improvement of optical power limiting in the short-wave IR range.     

Electronic effect on the optical properties and sensing ability of AIEgens with ESIPT process based on salicylaldehyde azine

Two novel AIE-active salicylaldehyde azine(SAA) derivatives with a typical excited-state intramolecular proton transfer(ESIPT) process are prepared by introducing electron-withdrawing and donating groups at para-position of phenolic hydroxyl group(CN-SAA and TPA-SAA). The effect of the proton activity in SAA framework on their optical behaviors is investigated spectroscopically. The results from NMR and solvation measurements show that the proton of phenolic hydroxyl group has higher activity when there are electron-withdrawing groups, and the absorption and fluorescence spectra in buffers with different pH also provide the same results. After inviting F. as a nucleophilic probe, this proton activity difference in CN-SAA and TPA-SAA becomes more obvious. The potential application of both molecules is investigated. TPA-SAA exhibits good quantitative sensing ability towards F. with a fluorescence "turn-on" mode, whereas the aggregates of TPA-SAA can selectively and sensitively detect Cu2+ in aqueous solution. From these results, a structure-property relationship is established: the occurrence of ESIPT process will become much easier when linking electron-withdrawing groups at the para-position of phenolic hydroxyl group(e.g., CN-SAA),and it is better to introduce electron-donating groups to enhance the sensing ability towards ions(e.g., TPA-SAA). This work will provide guidance for further design and preparation of AIE-active luminogens with ESIPT process for sensing applications.     

Molecular packing of lysozyme,fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces. While lysozyme molecules adsorbed on silica surfaces yield relatively similar SFG spectra, regardless of the surface concentration, SFG spectra of fibrinogen and BSA adsorbed on silica surfaces exhibit concentration-dependent signal intensities and peak shapes. Quantitative SFG data analysis reveals that methyl groups in lysozyme adsorbed on hydrophilic surfaces show a concentration-independent orientation. However, methyl groups in BSA and fibrinogen become less tilted with respect to the surface normal with increasing protein concentration at the surface. On hydrophobic polystyrene surfaces, all proteins yield similar SFG spectra, which are different from those on hydrophilic surfaces. Although more protein molecules are present on hydrophobic surfaces, lower SFG signal intensity is observed, indicating that methyl groups in adsorbed proteins are more randomly oriented as compared to those on hydrophilic surfaces. SFG data also shows that the orientation and ordering of phenyl rings in the polystyrene surface is affected by protein adsorption, depending on the amount and type of proteins.     

Surface polarity of cellulose derivates observed by coumarin 151 and 153 as solvatochromic and fluorochromic probes

Solvent‐dependent ultraviolet–visible (UV–vis) absorption and Stokes shifts including strong hydrogen‐bond‐donating (HBD) solvents such as 2,2,2‐trifluoroethanol and 1,1,1,3,3,3‐hexafluoro‐2‐propanol of two coumarine dyes (Co 151 and Co 153) were analyzed with multiple‐square analyses of linear solvation energy relationships and the Kamlet–Taft solvent parameter set to α (HBD capacity), β (hydrogen‐bond‐accepting capacity), and π* (dipolarity/polarizability). The UV–vis absorption and emission spectra of Co 151 and Co 153 were measured when adsorbed on various polysaccharides such as different cellulose batches, carboxymethylcelluloses with different degrees of substitution, and chitine. As a result of this evaluation, Co 153 is recommended as an alternative UV–vis probe for evaluating the dipolarity/polarizability of cellulose and cellulose derivates. Multiple adsorption of Co 153 on Linters cellulose took place indicating a wide‐surface polarity distribution, which makes the determination of a rigid polarity parameter questionable. Thus, fluorescence measurements of adsorbed Co 153 are suitable to detect inhomogenities on a surface but not for the determination of empirical polarity parameters. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1210–1218, 2003     

Fluoride Adsorption on Goethite in Relation to Different Types of Surface Sites

Metal (hydr)oxides have different types of surface groups. Fluoride ions have been used as a probe to assess the number of surface sites. We have studied the F(-) adsorption on goethite by measuring the F(-) and H(+) interaction and F(-) adsorption isotherms. Fluoride ions exchange against singly coordinated surface hydroxyls at low F(-) concentrations. At higher concentrations also the doubly coordinated OH groups are involved. The replacement of a surface OH(-) by F(-) suggests that all F charge (-1) is located at the surface in contrast to oxyanions which have a charge distribution in the interface due to the binding structure in which the anion only partially coordinates with the surface. Analysis of our F(-) data with the CD-MUSIC approach shows that the formation of the fluoride surface complex is accompanied by a redistribution of charge. This is supposed to be due to a net switch in the H bonding as a result of the change of the type of surface complex from donating (FeOH, FeOH(2)) to proton accepting (FeF). The modeled redistribution of charge is approximately equivalent with the change of a donating H bond into an accepting H bond. At high F(-) concentrations precipitation of F(-), as for instance FeF(3)(s), may occur. The rate of formation is catalyzed by the presence of high electrolyte concentrations. Copyright 2000 Academic Press.     

Quantitative study of solvent and surface effects on analyte ionization in desorption ionization on silicon (DIOS) mass spectrometry

Deuterated solvents and DIOS surfaces derivatized with different functional groups are used to investigate impacts of local chemical environment on analyte ionization. Both solvent molecules and surface functional groups are found to directly participate in analyte protonation in the condensed phase. The corresponding protonation effectiveness is quantitatively estimated based on the relative MS peak intensities of [M+2]+/[M+1]+. A direct correlation between ionization of triethylamine and the relative acidities of the surface and the solvent is evident. In addition, the proton donating effectiveness of a solvent is found to be related to its vapor pressure. Improved MS detection of small molecules via proper surface treatment and solvent selection is demonstrated.     

Monitoring the Thiol/Thiophenol Molecule-Modulated Plasmon-Mediated Silver Oxidation with Dark-Field Optical Microscopy

Surface plasmon can trigger or accelerate many photochemical reactions, especially useful in energy and environmental industries. Recently, molecular adsorption has proven effective in modulating plasmon-mediated photochemistry, however the realized chemical reactions are limited and the underlying mechanism is still unclear. Herein, by using in situ dark-field optical microscopy, the plasmon-mediated oxidative etching of silver nanoparticles (Ag NPs), a typical hot-hole-driven reaction, is monitored continuously and quantitatively. The presence of thiol or thiophenol molecules is found essential in the silver oxidation. In addition, the rate of silver oxidation is modulated by the choice of different thiol or thiophenol molecules. Compared with the molecules having electron donating groups, the ones having electron accepting groups accelerate the silver oxidation dramatically. The thiol/thiophenol modulation is attributed to the modulation of the charge separation between the Ag NPs and the adsorbed thiol or thiophenol molecules. This work demonstrates the great potential of molecular adsorption in modulating the plasmon-mediated photochemistry, which will pave a new way for developing highly efficient plasmonic photocatalysts.     

Dual fluorescence of ellipticine: excited state proton transfer from solvent versus solvent mediated intramolecular proton transfer

Photophysical properties of a natural plant alkaloid, ellipticine (5,11-dimethyl-6H-pyrido[4,3-b]carbazole), which comprises both proton donating and accepting sites, have been studied in different solvents using steady state and time-resolved fluorescence techniques primarily to understand the origin of dual fluorescence that this molecule exhibits in some specific alcoholic solvents. Ground and excited state calculations based on density functional theory have also been carried out to help interpretation of the experimental data. It is shown that the long-wavelength emission of the molecule is dependent on the hydrogen bond donating ability of the solvent, and in methanol, this emission band arises solely from an excited state reaction. However, in ethylene glycol, both ground and excited state reactions contribute to the long wavelength emission. The time-resolved fluorescence data of the system in methanol and ethylene glycol indicates the presence of two different hydrogen bonded species of ellipticine of which only one participates in the excited state reaction. The rate constant of the excited state reaction in these solvents is estimated to be around 4.2-8.0 × 10(8) s(-1). It appears that the present results are better understood in terms of solvent-mediated excited state intramolecular proton transfer reaction from the pyrrole nitrogen to the pyridine nitrogen leading to the formation of the tautomeric form of the molecule rather than excited state proton transfer from the solvents leading to the formation of the protonated form of ellipticine.     

Raman spectroscopic studies of terthiophenes for molecular electronics

The effect of thiol and selenol functionalization on the vibrational spectra and photochemical stability of terthiophene based molecular wires was investigated using surface-enhanced Raman scattering (SERS). The molecules were found to exhibit markedly different properties at the silver surface of the SERS substrate, despite having almost identical Raman spectra in solution and in the solid state. In contrast to terthiophene (3T), the bisthiolterthiophene (T3) and biselenol-terthiophene (Se3) molecules were stable against photoinduced structural changes when adsorbed to the metal surface at low concentrations. This indicates that the strong bonds to the silver surface, via S or Se terminal atoms, leads to a rapid decay of photoexcited states. Comparison with ab initio calculations shows that both T3 and Se3 bind with only one of the functional groups to the Ag surface.     

Fluorescence spectra and chemical species of fluorescein molecules adsorbed on a calcinated porous Vycor glass

Properties of fluorescence and the excitation spectra of fluorescein molecules adsorbed onto a calcinated (773 K) porous Vycor glass have been investigated as a function of the amount of adsorbed dye (the surface coverage, θ = 0.00051 and 0.0098). The fluorescence and fluorescence-excitation spectra of fluorescein adsorbed onto Vycor glass showed the spectrum only due to cation species at θ = 0.00051. On the other hand, the spectra observed at θ = 0.0098 suggested the presence of cation, anion, and dianion species on the surface of Vycor glass. These results indicated the existence of at least two different types of adsorption sites involving the Brønsted acid site on the surfaces of Vycor glass.     

Eosin y triplet state as a probe of spatial heterogeneity in microcrystalline cellulose

The photophysical behavior of eosin Y adsorbed onto microcrystalline cellulose was evaluated by reflectance spectroscopy, steady-state fluorescence spectroscopy and laser induced time-resolved luminescence. On increasing the concentration of the dye, small changes in absorption spectra, fluorescence redshifts and fluorescence quenching are observed. Changes in absorption spectra point to the occurrence of weak exciton interactions among close-lying dye molecules, whereas fluorescence is affected by reabsorption and excitation energy trapping. Phosphorescence decays are concentration independent as a result of the negligible exciton interaction of dye pairs in the triplet state. Lifetime distribution and bilinear regression analyses of time-resolved phosphorescence and delayed fluorescence spectra reveal the existence of two different environments: long-lived, more energetic triplet states arise from dyes tightly entrapped within the cellulose chains, while short-lived, less-energetic states result from dyes in more flexible environments. Stronger hydrogen bond interactions between the dye and cellulose hydroxyl groups lead in the latter case to a lower triplet energy and faster radiationless decay. These effects, observed also at low temperatures, are similar to those encountered in several amorphous systems, but rather than being originated in changes in the environment during the triplet lifetime, they are ascribed in this case to spatial heterogeneity.     

Environment‐Sensitive Fluorescent Probe: A Benzophosphole Oxide with an Electron‐Donating Substituent

Electron‐donating aryl groups were attached to electron‐accepting benzophosphole skeletons. Among several derivatives thus prepared, one benzophosphole oxide was particularly interesting, as it retained high fluorescence quantum yields even in polar and protic solvents. This phosphole‐based compound exhibited a drastic color change of its fluorescence spectrum as a function of the solvent polarity, while the absorption spectra remained virtually unchanged. Capitalizing on these features, this phosphole‐based compound was used to stain adipocytes, in which the polarity of subcellular compartments could then be discriminated on the basis of the color change of the fluorescence emission.     

Surface-enhanced Raman scattering and the adsorption behavior of (RS)-phenylsuccinic acid

The surface geometry of (RS)-phenylsuccinic acid molecule was studied by analysis of the SERS spectra of aromatic dicarboxylic acid adsorbed on silver colloid surfaces. For a reliable analysis of the SERS spectrum, we also performed density functional theoretical calculations. The SERS spectral features indicated that the RSPSA molecules should bound to the silver as dicarboxylate, with a strongly tilted orientation with respect to the normal to the surface. Such a tilted orientation was presumed to occur by the simultaneous sigma and pi-type coordination of carboxylate groups to silver surface caused by the steric hindrance and electrostatic repulsion between the two carboxylate groups, and thereby RSPSA on silver was easily displaced with aromatic carboxylic acids. A sigma-type coordination therefore seemed to be more important than a pi-type coordination for aromatic carboxylic acid derivatives to assemble on a silver surface. The large enhancement of in-plane bending, out of plane bending and ring breathing modes in the surface-enhanced Raman scattering spectrum indicates that the molecule is adsorbed on the silver surface in a 'at least vertical' configuration, with the ring perpendicular to the silver surface.     

2',7'-Difluorofluorescein excited-state proton reactions: correlation between time-resolved emission and steady-state fluorescence intensity

The presence of excited-state buffer-mediated proton exchange reactions influences the steady-state fluorescence signals from dyes in solution. Since biomolecules in general have some chemical groups that can act as proton acceptors/donors and are usually dissolved in buffer solutions which can also behave as appropriate proton acceptors/donors, the excited-state proton exchange reactions may result in distorted steady-state fluorescence signals. In a previous paper (J. Phys. Chem. A 2005, 109, 734-747), we evaluated kinetic and other pertinent parameters for the excited-state proton reactions of the prototropic forms of 2',7'-difluorofluorescein (Oregon Green 488, OG488), recording a fluorescence decay surface at different pH values and acetate buffer concentrations, analyzed by means of global compartmental analysis. In this article we use the rate constants and the corrected pre-exponential factors from the previously recorded fluorescence decay traces to simulate the decay times and associated pre-exponentials at different acetate buffer concentrations and constant pH and compare these theoretically calculated values with new experimental data. We also calculate the steady-state fluorescence intensity vs pH and vs acetate buffer concentration (at constant pH) and compare these calculated emission values with the experimental data previously published. The agreement between the experimental and simulated data is excellent.     

Characterization of acridine species adsorbed on (NH4)2SO4, SiO2, Al2O3, and MgO by steady-state and time-resolved fluorescence and diffuse reflectance techniques

The ground- and excited-state species of acridine adsorbed on (NH(4))(2)SO(4), SiO(2), Al(2)O(3), and MgO surfaces were investigated in order to determine the precursor species and electronic states responsible for acridine photodegradation on particles serving as models of atmospheric particulate matter. The species present on each solid surface were characterized by comparing the steady-state absorption and fluorescence spectra, time-resolved fluorescence, and absorption measurements on acridine in solution with those corresponding to adsorbed acridine. On silica, the ground-state species present were hydrogen-bonded, neutral, and protonated, while on alumina hydrogen-bonded and neutral species were identified. A comparison of the protonated acridine absorption and emission intensities on silica and alumina with those observed for acridine in acidic water demonstrated that the emission on the surfaces is higher than expected. This was interpreted as resulting from photoprotolytic reactions on silica and alumina. For acridine adsorbed on ammonium sulfate, protonated acridine was the only adsorbed species identified. Since, at a similar ground-state absorbance, the fluorescence intensity of acridine on ammonium sulfate was smaller than for acridine in acidic water, the quenching of the excited state or a rapid photochemical reaction with the surface was proposed. On magnesium oxide, the presence of neutral and hydrogen-bonded acridine species were characterized from the two-component analysis of the fluorescence, the triplet-triplet absorption decay curves, and the time-resolved emission spectra at different time delays. As demonstrated in these studies, acridine adsorbed species and their decay pathways depend on the acidic properties of these models of atmospheric particulate matter. In addition, a comparison of the photodegradation rates of acridine on the different solids tested is presented and discussed in terms of the nature of the species and their decay pathways.     

Molecular macrocluster formation on silica surfaces in phenol-cyclohexane mixtures

The adsorption of phenol, an aromatic compound with a hydrogen-bonding group, onto a silica surface in cyclohexane was investigated by colloidal probe atomic force microscopy (AFM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and adsorption isotherm measurements. ATR-FTIR measurements on the silica surface indicated the formation of surface macroclusters of phenol through hydrogen bonding. The ATR-FTIR spectra were also measured on the H-terminated silicon surface to observe the effect of the silanol groups on the phenol adsorption. The comparison of the ATR-FTIR spectra for both the silicon oxide and H-terminated silicon surfaces proved that the silanol groups are necessary for the formation of phenol clusters on the surface. The surface force measurement using colloidal probe AFM showed a long-range attraction between the two silica surfaces in phenol-cyclohexane mixtures. This long-range attraction resulted from the contact of the adsorbed phenol layers for the phenol concentrations below 0.6 mol %, at which no significant phenol clusters formed in the bulk solution. The attraction started to decrease at 0.6 mol % phenol due to the exchange of the phenol molecules between the clusters in the bulk phase and on the surface. The surface density of phenol in the adsorbed layer was calculated on the basis of the long-range attraction and found to be much smaller than the liquid phenol density. The plausible structure of the adsorbed phenol layer was drawn by referring to the crystal structure of the bulk phenol and orientation of the phenol molecules on the surface, estimated by the dichroic analysis of ATR-FTIR spectroscopy. The investigation of the phenol adsorption on the silica surface in a nonpolar solvent using this novel approach demonstrated the effect of the aromatic ring on the surface packing density.     

Charge separation in mesoporous aluminosilicates

EPR spectroscopy has been used to investigate spontaneous and/or photo-induced electron transfer between adsorbed organic molecules and the mesoporous aluminosilicate MCM-41 host. Spontaneous electron transfer occurs from the host to electron acceptor molecules with sufficiently favourable reduction potentials (TCNE, TCNQ, 1,4-benzoquinone, 1,4-naphthaquinone and 1,4-anthraquinone), provided the MCM-41 contains aluminium and the radical anion yield correlates with the aluminium content of the host. The semiquinone radical anions are interacting strongly with exposed Al³⁺ sites, whereas the TCNE and TCNQ radical anions are loosely bound and can be washed from the host. Radical cation formation is observed when electron donor molecules with favourable oxidation potentials are adsorbed in MCM-41 containing aluminium, and the radical cations formed interact with exposed Al³⁺ sites. This work shows that aluminium-containing MCM-41 contains both electron donating and electron accepting sites which may intervene in intra-molecular charge separation processes in adsorbed organic molecules.     

Optical spectroscopy of hydrophobic sunscreen molecules adsorbed to dielectric nanospheres

Fluorescence and absorption spectra of hydrophobic sunscreens, weakly fluorescent octyl methoxycinnamate, moderately fluorescent octyl salicylate and highly fluorescent 2-ethylhexyl-4-(dimethylamino)benzoate (padimate O) adsorbed to dielectric microspheres in aqueous suspension, have been compared with spectra in organic solution. The fluorescence of adsorbed salicylate and padimate is enhanced compared with fluorescence in methanol: about a factor of 6 and 30 in terms of fluorescence yield per molecule of salicylate and padimate, respectively. Cinnamate, which has a low fluorescence yield, does not show a comparable fluorescence enhancement. The fluorescence amplification is independent of sphere diameter from 30 to 1500 nm, at least for salicylate. The enhancement, as well as the location of absorption spectral peaks, is consistent with a low-dielectric constant environment of the molecules, in spite of the presumed location near the interface between polystyrene (epsilon = 2.4-3.8) and water (epsilon = 78). The adsorbed state of these sunscreens represents a proposed improved in vitro model for the environment of sunscreens in vivo, as well as a general model for chromophores in heterogeneous environments.     

Ultrafast molecular dynamics of liquid aromatic molecules and the mixtures with CCl4

The ultrafast molecular dynamics of liquid aromatic molecules, benzene, toluene, ethylbenzene, cumene, and 1,3-diphenylpropane, and the mixtures with CCl(4) have been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The picosecond Kerr transients of benzene, toluene, ethylbenzene, and cumene and the mixtures with CCl(4) show a biexponential feature. 1,3-Diphenylpropane and the mixtures with CCl(4) show triexponential picosecond Kerr transients. The slow relaxation time constants of the aromatic molecules and the mixtures with CCl(4) are qualitatively described by the Stoke-Einstein-Debye hydrodynamic model. The ultrafast dynamics have been discussed based on the Kerr spectra in the frequency range of 0-800 cm(-1) obtained by the Fourier transform analysis of the Kerr transients. The line shapes of the low-frequency intermolecular spectra located at 0-180 cm(-1) frequency range have been analyzed by two Brownian oscillators ( approximately 11 cm(-1) and approximately 45 cm(-1) peaks) and an antisymmetric Gaussian function ( approximately 65 cm(-1) peak). The spectrum shape of 1,3-diphenylpropane is quite different from the spectrum shapes of the other aromatic molecules for the low magnitude of the low-frequency mode of 1,3-diphenylpropane and/or an intramolecular vibration. Although the concentration dependences of the low- and intermediate-frequency intermolecular modes (Brownian oscillators) do not show a significant trend, the width of high-frequency intermolecular mode (antisymmetric Gaussian) becomes narrower with the higher CCl(4) concentration for all the aromatics mixtures with CCl(4). The result indicates that the inhomogeneity of the intermolecular vibrational mode in aromatics/CCl(4) mixtures is decreasing with the lower concentration of aromatics. The intramolecular vibrational modes of the aromatic molecules observed in the Kerr spectra are also shown with the calculation results based on the density functional theory.     

Excitation spectrum of neutral molecules adsorbed on dielectric surfaces

It is shown that the excitation spectrum of neutral molecules physisorbed on a dielectric surface consists of two symmetric and two antisymmetric energy modes. The spectral functions of these modes are represented respectively by two lorentzian lines whose spectral widths are described by the radiative decay of the energy modes in question. Numerical results are derived for the energies of excitation and spectral widths for the rare-gas atoms adsorbed on graphite.