The effects of incorporation into a solid matrix on the photophysical properties of a nonlinear material have been of interest for some time in our group. It is well known in the literature that for a nonlinear absorbing dye to be the most effective, high concentrations are generally needed. Understanding how the larger concentration and placement into a solid matrix affects their photophysical properties is the key of this study. Here we look at two metallated substituted tetrakis(cumylphenoxy) phthalocyanines with either Pb or In as the central metal. A detailed study of their photophysical properties based on concentration allows for a better understanding of the constraints this environment has to a given material.
It is well known in the literature that for a two photon nonlinear absorbing dye to be the most effective, high concentrations are needed. The problem is that most photophysical studies in solution are done at low concentration and in a solution. These low concentration studies are important for understanding inherent materials properties but it is also important to understand what happens in a material at high concentration. In addition to this, efforts have been made to study the effects of incorporating a dye into a solid matrix environment to better understand the constraints this environment has to a given material. Preliminary results for an epoxy system reveal the formation of excimers (excited state dimers) with an increase in concentration. Excimers are forming from the triplet excited state of the E1-BTF. A rate constant for this formation is 2.6 × 105 M-1 s-1. While rather slow, at greater than 50 mM concentration the excimer is readily formed with <90% efficiency. This must be considered when making nonlinear absorption measurements since the excimer will certainly contribute to the overall nonlinearity.
Change of permanent electric dipole moment in the lower-energy charge transfer transitions for a series of symmetrical and non-symmetrical ferrocene-phenyleneethynylene oligomers were studied by measuring the corresponding femtosecond two-photon absorption cross section spectra, and were determined to be in the range Δμ = 3 – 10 D. Quantum-chemical calculations of Δμ for the non-symmetrical oligomers show good quantitative agreement with the experimental results, thus validating two-photon absorption spectroscopy as a viable experimental approach to study electrostatic properties of organometallics and other charge transfer systems.
In order to understand electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a model series of chromophores trans-Pt(PBu3)2(CCPhenyl-X)2, where X = NH2, OCH3, diphenylamino, t-Bu, methyl, H, F, benzothiazole, trifluoromethyl, CN and nitro. We collected linear spectra, including ground state absorption, phosphorescence and phosphorescence excitation spectra. We also performed DFT and TDDFT calculations on the ground and excited state properties of these compounds. The calculations and experimental data show the excited state properties are a function of the electronic properties of the substituents and the molecular conformation.
Synthesis of several C60-(antenna)x conjugates was performed to demonstrate high flexibility in the design of organic nonlinear optical (NLO) nanostructures showing broadband characteristics with capability to absorb light over a wide range of wavelengths. It was achieved by covalent attachment of a hybrid combination of two types of light-harvesting fluorescent antenna chromophores on a C60 cage. Ultrafast photoresponsive intramolecular Föster resonance energy-transfer among antenna units and shared excited energy-accepting C60 cage is proposed as a plausible mechanism to enhance the broadband NLO ability. Characterization of the branched triad C60(>DPAF-C18)(>CPAF-C2M) and the tetrad C60(>DPAF-C18)(>CPAF-C2M)2 was carried out by various spectroscopic techniques. These compounds showed approximately equal extinction coefficients of optical absorption over 400‒550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780‒1100 nm. These nanomaterials may be utilized in NLO coatings for achieving efficient light-transmittance reduction at the same NIR wavelengths.
We have been studying exciplex formation in nonlinear optical materials containing a high concentration of 2PA chromophores (AFX dyes) as a means to enhance the nonlinear optical properties. A number of dipolar AFX dyes having various electron-accepting moieties (π-excess and π-deficient examples) and three bisimide compounds having substituents with varying electron-withdrawing power were synthesized to study exciplex formation in their solid state blends. In substrate supported thin films of various equimolar blends containing an AFX dye and bisimide, the dye monomer emission was severely quenched, and a new emission, red-shifted by up to 110 nm, appeared. The emission energies were consistent with the charge recombination energy calculated from the energy levels of the donor and acceptor present in the blend, which confirmed the emission was from an exciplex. Time resolved emission measurements also indicated the presence of much longer lived transients in the blends, consistent with exciplex formation. Spectroelectrochemistry confirmed that the radical cations of these dyes had strong absorption in the NIR region, so exciplex formation is a means to enhance nonlinear optical absorption of the dyes in this spectral region.
We demonstrate a new material composed of isotropic liquid crystal (ILC) blended with semiconductor nanoparticles,
which could result in a novel high-speed, multiple-notch broadband passive optical switch to selectively discriminate
bands of electromagnetic radiation in intelligence, surveillance, or reconnaissance systems. The new material has been
demonstrated high nonlinear 3rd order optical Kerr coefficients (light-induced refractive index change, n2) exceeding 100
times of classic nonlinear material CS2 with n2 = 1.2E-11 esu. Details of fabrication and experimental results are
presented.
To develop a structure-spectroscopic property relationship in platinum acetylides having poly(aromatic hydrocarbon)
ligands, we synthesized a series of chromophores with systematic variation in the number of fused aromatic rings(nFAR)
and ligand topology(polyacene(L), polyphenanthrene(Z) or compact(C)). We measured ground state absorption,
fluorescence and phosphorescence spectra. We also performed nanosecond and picosecond flash photolysis
experiments. To extend the range of compounds in the structure-property relationship, we did DFT calculations on an
expanded series of chromophores to calculate the S1 and T1 state energies. In both the DFT results and experiment, the
ground state and phosphorescence spectra are a function of both nFAR and ligand topology. In the L chromophores, the
S1 and T1 state energies decrease linearly with nFAR. In contrast the S1 and T1 state energies of the Z chromophores
oscillate with increasing nFAR. The C chromophores have behavior intermediate between the L and Z chromophores.
The picosecond transient spectra show complex behavior, having spectra reflecting intersystem crossing, vibrational
cooling and solvent relaxation processes. The nanosecond transient spectra result from the T1 - Tn transition. The timeresolved
spectra show no systematic variation with structure, showing more complex behavior than previously studied
platinum acetylides having phenylene ethynylene ligands.
There has been much interest in the development of two-photon absorbing materials and many efforts to understand the
nonlinear absorption properties of these dyes. We have recently explored a new type of two photon absorbing dye
containing a platinum center with ligands that vary in length that contain electron withdrawing benzothiazole. With
increased π-π* conjugation we expect to observe a red shift in the absorption properties of the material. We have
investigated the photophysical properties of the platinum chromophores using a variety of experimental techniques.
Previously we determined that the singlet and triplet excited states are responsible for nearly all of the nonlinearity in the
nanosecond regime accept the two photon mechanism that is primarily used for excitation. Therefore we would like to
tune the photophysical properties of both the singlet and triplet excited state in these materials. To our surprise we found
there is quite a bit of red shifting due to a metal-to-ligand charge transfer from the platinum to the ligand rather than the
expected shifting due to increased π-π* conjugation. However, with increased ligand length the chromophore does take
on more π-π* character.
To learn about excited state geometry in biphenyl-containing platinum acetylides, we synthesized a series of
compounds that have biphenyl ligands. The ligands consisted of biphenyl(I), the hindered 2'-methyl biphenyl(III) and
planar fluorenyl(IV) groups. We also synthesized a "half" complex(II) consisting of one ligand attached to the platinum
atom. The optical properties of these compounds were measured by ground state absorption, phosphorescence, ultrafast
transient absorption and nanosecond transient absorption spectroscopy. DFT calculations were performed to determine
the ground state and triplet state geometries and the lowest triplet energy. TDDFT calculations were performed to
determine singlet excited state energies. Compared to the reference compound I, ground state spectra show a blue shift
in II and III and red shift in IV, showing the singlet energy is sensitive to conjugation and biphenyl twist angle.
Comparison of the phosphorescence spectra of I and II shows the triplet exciton is confined to one ligand. The time
behavior of the ultrafast excited state absorption spectrum of I shows a red shift within 1 ps from the initial spectrum.
This behavior is not seen in IV. The different behavior suggests formation of the triplet state of I is accompanied by
conversion from a non-planar to a planar conformation while IV retains a planar conformation.
There has been much interest in the development of two-photon absorbing materials and many efforts to understand the
nonlinear absorption properties of these dyes but this area is still not well understood. A computational model has been
developed in our lab to understand the nanosecond nonlinear absorption properties that incorporate all of the measured
one-photon photophysical parameters of a class of materials called AFX. We have investigated the nonlinear and
photophysical properties of the AFX chromophores including the two-photon absorption cross-section, the excited state
cross-section, the intersystem crossing quantum yield, and the singlet and triplet excited state lifetimes using a variety of
experimental techniques that include UV-visible, fluorescence and phosphorescence spectroscopy, time correlated single
photon counting, ultrafast transient absorption, and nanosecond laser flash photolysis. The model accurately predicts the
nanosecond nonlinear transmittance data using experimentally measured parameters. Much of the strong nonlinear
absorption has been shown to be due to excited state absorption from both the singlet and triplet excited states. Based on
this understanding of the nonlinear absorption and the importance of singlet and triplet excited states we have begun to
develop new two-photon absorbing molecules within the AFX class as well as linked to other classes of nonlinear
absorbing molecules. This opens up the possibilities of new materials with unique and interesting properties.
Specifically we have been working on a new class of two-photon absorbing molecules linked to platinum poly-ynes. In
the platinum poly-yne chromophores the photophysics are more complicated and we have just started to understand what
drives both the linear and non-linear photophysical properties.
To develop novel nonlinear dyes for photonic applications, we synthesized a series of transition metal-containing phenylacetylene oligomers. Theoretical properties of these compounds were measured by UV/Vis absorption, photoluminescence, and nanosecond flash photolysis experiments. It was found that as the number of oligomer units increased ,the transition energies decreased without saturation. The low ground state absorption and UV absorption edge gives rise to solutions that are nearly water clear. A very broad triplet state absorption extending from the absorption edge to the limits of our spectrometer is demonstrated to also be intense. These results enhance the understanding of these materials when used for nonlinear absorption applications and enable the prediction properties for materials extending this class of dyes.
To understand the photophysics of nonlinear absorbers, we have investigated the photophysics of a series of di(2- thienyl-3,3',4,4'-butyl)polyenes. Spectroscopic measurements, including UV/Vis, fluorescence, fluorescence lifetimes, fluorescence quantum yields, triplet state lifetime, solvent effects and two-photon absorption coefficient were obtained as a function of the number of double bonds (n equals 1 - 5). Trends in the data reflected the ordering, energy gap between and mixing of 1Bu* and 1Ag* excited state configurations. We investigated the solvatochromism of a series of (alpha) ,(omega) -di(2- dithienyl 3,3',4,4'-butyl) polyenes. Absorption (n equals 1 - 5 double bonds) were collected in a series of aprotic solvents. The absorption energy dispersion effect sensitivity increased smoothly with n, reaching asymptotic behavior as n approached 5. The emission energy had less solvent sensitivity, giving evidence for a polar 1Bu* absorbing state and a nonpolar 1Ag* emitting state. We observed sensitivity of the absorbing and emitting states to solute-solvent dipole-dipole interactions, suggesting the dithienyl polyenes had a polar syn ground state conformation.
The spectroscopy and nonlinear absorption of bis(diphenylamino) diphenyl polyenes have been studied in octane and dichloromethane solvents. The amines exhibit high fluorescence quantum yield and two photon excited emission. Two photon absorption cross section, (sigma) 2, was measured by Z-scan experiments. Strong two photon absorption is indicated by high values for (sigma) 2. Solvent has strong influence in the measurement of (sigma) 2 values.
Porphyrins are attractive compounds for optical applications. We have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and alkylation either at the pyrrole position or the meso-aryl groups. The characterization of these chromophores includes measurement of UV/Vis, fluorescence and fluorescence lifetimes. Furthermore, we have investigated their nonlinear absorption, excitation dynamics. The significant factors influencing limiting behavior appear to be the heavy atom effect, electron donating and withdrawing substituents conformation distortion and changes in conjugation. Detailed understanding will be gained from measurements of photophysical parameters underlying limiting behavior.
The photophysics of a series (alpha) ,(omega) -dithienyl polyenes was studied. Both one photon excited fluorescence and fluorescence originating from two photon absorption states were investigated. There is strong evidence of two photon absorption from fluorescence studies in agreement with our earlier studies involving nonlinear absorption.
KEYWORDS: Absorption, Optical limiting, Metals, Molecules, Pulsed laser operation, Chemical species, Picosecond phenomena, Chemical elements, Time resolved spectroscopy, Data modeling
We study one free base and seven metallo- octabromotetraphenylporphyrins by several techniques. In a pico-second pump-probe experiment, we monitor the transient transmission of each sample up to 11 ns after it is irradiated by an intense laser pulse. Combined with the results from time-resolved fluorescence spectroscopy, we propose a simple model to interpret the transmission data. We attribute the reduction in the transmission to triplet state absorption and extract the triplet state absorption cross-sections, as well as the lifetimes from the time dependent transmission data. In a separate experiment where the transmission of a nanosecond laser pulse is measured with various input energies, our measurement in the cross- sections predicts the correct optical limiting behavior. We assess the overall optical limiting performance of all 8 samples by direct comparison with C60 at the same ground state transmission.
Oriented films of poly((gamma) -benzyl-L-glutamate), PBLG, were prepared by evaporation of solvent from solutions held in electric fields applied parallel to the film surface. Films thus aligned showed non-linear optical (NLO) activity, i.e., second harmonic generation (SHG). Order parameters calculated from FTIR spectra were correlated with second order susceptibility, and the effects of field strength, solution concentration, and molecular weight on these parameters were investigated. Alignment was observed at field strengths as low as 0.8 kV/cm, as indicated by polarized IR spectra. However, the extent of alignment as measured by IR spectra and values of the second order nonlinear susceptibility, X(2), depend not only on field strength but also on solution concentration and the polymer molecular weight. Optimum conditions for alignment appear to be with a molecular weight of 118,000 D and a solution concentration of 2.5% (w/w). Dried films show the typical banded structures reported for polymer films oriented by mechanical shearing stresses. We have shown that these structures extend through the dried film as laminae perpendicular to both the film surface and to the field direction.
Synthetic polypeptides containing Disperse Red 1 (4-[N-ethyl, N- hydroxyethylamino]-4'-nitroazobenzene) nonlinear optical side chains were synthesized. The extent of side chain modification was ca. 10% and higher modifications were expected to produce crystallization of the polar NLO molecule and polymer insolubility. These polymers displayed a lyotropic liquid crystalline phase above a concentration of 15 - 20% (w/w) in dichloromethane. Corona poling (1.0 (mu) A/-5 kV) was used to align the Disperse Red 1 molecules of modified poly[L-glutamic acid] or modified poly[(gamma) -methyl-L-glutamate] thin films (1 - 2 micrometers ) that were produced by spin coating onto ITO covered glass substrates. Decay of the second harmonic (SH) signal was the slowest for films aligned by corona-onset poling at elevated temperatures (COPET) and the fastest for films aligned at room temperature. The SH intensities at 532 nm were comparable to the 1 mm thick, y-cut quartz standard and the (Is2(omega)/Iq2(omega))1/2 values were measured as a function of the film temperature during corona poling. The biexponential decay model was used to describe the trends in SH decay.
In order to develop polypeptide-based second order nonlinear optical materials, we have derivatized succinylated poly(l-lysine) with a spiropyran. Spin coated thin films (2 (mu) thickness) of this material on indium tin oxide-coated glass were corona-poled (6 X 105 V/m) under conditions of variable temperature. The degree of alignment and nonlinear optical properties were monitored by measurement of the intensity of second harmonic signal resulting from irradiation at 1064 nm. The dark-adapted polypeptide was in the spiropyran form. The second harmonic signal intensity was 20 times greater than that of the dark-adapted sample. The results demonstrate the feasibility of development of a light- activated nonlinear optical material.
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