Third-order optical nonlinearities of metallotetrabenzoporphyrins and a platinum poly-yne

Nonlinear Optical Materials for the Smart Filtering of Optical Radiation

The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the function of optical power limiting. All known mechanisms of optical limiting have been analyzed and discussed for the different types of materials.

Synthesis and optical properties of poly (aryl ether ketone)s-containing nonlinear optical chromophores

Novel functional poly (aryl ether ketone)s bonded nonlinear optical chromophores (meso-tetrakis (4-hydroxyphenyl) porphyrin, THPP) with different backbone structures were synthesized and characterized using proton nuclear magnetic resonance, Fourier transform infrared, and ultraviolet–visible absorption spectra. Their optical properties were evaluated by nonlinear optical analyses and optical limiting (OL). The results showed that these polymers possessed good OL properties and large nonlinear optical (NLO) susceptibilities. All polymer films-containing THPP presented excellent thermal stability (>460°C). Polymers bonded with –CH3 to the backbone exhibited better performances in OL and NLO properties than those bonded with –CF3; however, the thermal stability of the former was slightly inferior. Backbone bonded with electron donating groups was conducive to the OL and NLO properties.

Nonlinear absorbing cationic iridium(III) complexes bearing benzothiazolylfluorene motif on the bipyridine (N∧N) ligand: synthesis, photophysics and reverse saturable absorption

Four new heteroleptic cationic Ir(III) complexes bearing benzothiazolylfluorene motif on the bipyridine (N∧N) (1 and 2) and phenylpyridine (C∧N) (3 and 4) ligands are synthesized and characterized. The influence of the position of the substituent and the extent of π-conjugation on the photophysics of these complexes is systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The complexes exhibit ligand-centered (1)π,π* transitions with admixtures of (1)ILCT (π(benzothiazolylfluorene) → π*(bpy)) and (1)MLCT (metal-to-ligand charge transfer) characters below 475 nm, and very weak (1,3)MLCT and (1,3)LLCT (ligand-to-ligand charge transfer) transitions above 475 nm. The emission of these complexes at room temperature in CH2Cl2 solutions is ascribed to be predominantly from the (3)MLCT/(3)LLCT states for 1 and from the (3)π,π* state for 2, while the emitting state of 3 and 4 are assigned to be an admixture of (3)MLCT, (3)LLCT, and (3)π,π* characters. The variations of the photophysical properties of 1-4 are attributed to different degrees of π-conjugation in the bipyridine and phenylpyridine ligands induced by different positions of the benzothiazolylfluorenyl substituents on the bipyridine ligand and different extents of π-conjugation in the phenylpyridine ligands, which alters the energy and lifetime of the lowest singlet and triplet excited states. 1-4 all possess broadband transient absorption (TA) upon nanosecond laser excitation, which extends from the visible to the NIR region. Therefore, 1-4 all exhibit strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. However, the TA of complexes 1, 2, and 3 are much stronger than that of 4. This feature, combined with the difference in ground-state absorption and triplet excited-state quantum yield, result in the difference in RSA strength, which follows this trend: 1 ≈ 2 ≈ 3 > 4. Therefore, complexes 1-3 are strong reverse saturable absorbers at 532 nm and could potentially be used as broadband nonlinear absorbing materials.

Z-scan studies and quantum chemical calculations of meso-tetrakis(p-sulfonatophenyl)porphyrin and meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin and their Fe(III) and Mn(III) complexes

Optical self-defocusing, characterized by the non-linear refractive index n2, was investigated by the Z-scan technique in water solutions of two porphyrins (PPhs), negatively charged meso-tetrakis(p-sulfonatophenyl)porphyrin ( TPPS 4) and positively charged meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin ( TMPyP ), in their free base forms and as Fe (III) and Mn (III) complexes. Significant n2 values were observed only for the TMPyP metal complexes, while for the other porphyrins the n2 values were negligible. The effect is explained by the reorientation of the porphyrin molecule due to interaction of its permanent dipole moment perpendicular to the molecular ring plane with the electromagnetic field of the exciting light pulse. The permanent dipole moment is due to the shift of the metal atom out of the molecular plane. The electrostatic interaction between the metal atom and charged substituents increases (repulsion) or decreases (attraction) the shift of the metal atom and consequently affects the dipole moment value. Ligand binding to the metal atoms also increases the out-of-plane metal shift and hence the dipole moment and n2 value. pH changes were shown to modify the Fe (III)-ligand structure, thus changing n2. The experimental data correlate well with the metal shift and dipole moment values calculated for simplified PPh structures by the ZINDO method.

The dihydroisoindole approach to linearly annelated π-extended porphyrins

The dihydroisoindole strategy for the synthesis of linearly annelated π-extended porphyrins is a common approach to tetrabenzo-, tetranaphtho-, and tetraanthraporphyrins with variable functionality and substitution patterns. This method implies the use of a common type of precursors involving the 4,7-dihydroisoindole moiety, which are the closest possible stable relatives of the unstable isoindole, benzoisoindole and naphthoisoindole. A key feature of this strategy is the unprecedented mildness of the final oxidative aromatization step, which accounts for synthetic versatility, better functional group tolerance and high purity of the products. Many new linearly annelated π-extended porphyrins of the tetrabenzo-, tetranaphtho-, and tetraanthraporphyrin families were produced and characterized for the first time, including soluble planar and highly emissive 5,15-diaryl derivatives. The double bond of the dihydroisoindole fragment can also be useful for further modification of the porphyrin by means of addition of cycloaddition reactions, leading to new, previously inaccessible porphyins bearing multiple halide, hydroxy, or acetoxy groups at the periphery.

Nonlinear Optical Properties of Tetrabenzporphyrins

Optical limiting in a solution of tetrabenzporphyrin (TBP) dissolved in tetrahydrofuran (THF) was measured in an f/5 optical focusing arrangement using 532 nm wavelength laser light with a 9 ns pulse duration. The performance of this solution was compared to a variety of other materials. In order to better understand the nonlinear behavior of this material, apertured and unapertured Z-scans were performed. From these measurements it was determined that the excited-state cross section and its ratio to ground-state cross section were large. The nonlinear refraction of this material is made up of contributions from thermal density change and population redistribution. Limiter performance was modeled in an f/64 limiter. The limiter behavior of a slightly modified compound and an additional solvent were measured.

Two-Photon Absorption in Polybenzidine

Polymers of Benzidine were synthesized by hydrogen peroxide reaction catalyzed by horseradish peroxidase enzyme. The polymerization reaction was carried out at room temperature in a monophasic organic solvent with a small amount of water at pH 7.5.

The technique of degenerate four-wave mixing (DFWM) with nanosecond and picosecond pulses was employed to measure the third-order nonlinear optical susceptibility χ. The samples were studied in solution in Dimethyl sulfoxide:Methanol in volume ratio 4:1. The observed values are of order 10−9 to 10−8esu. Measurements on a thin film agree approximately with the extrapolated values fron solution measurements. Picosecond time resolved measurements indicate a pulse-width limited response followed by a small slow component. Investigation of the total energy transmission as a function of the incident intensity and fluence at 532 nm for pico- and nanosecond pulses indicates reverse saturable absorption. As we observe the nanosecond and picosecond curves to be superimposed for the intensity plot but not for fluence, we conclude that the nonlinearity is predominantly due to two-photon absorption. Numerical analysis of the data yields a value of 12.25 cm/GW for the two-photon absorption coefficient β. The imaginary component of χ obtained is 5 × 10−9esu. The material appears to be a good candidate for applications in optical power limiting and switching.

Tunable phosphorescent NIR oxygen indicators based on mixed benzo- and naphthoporphyrin complexes

A series of π-extended phosphorescent palladium(II) and platinum(II) porphyrin complexes were synthesized, in which additional benzene rings are fused radially onto at least one of the four peripheral benzo groups. The photophysical properties of the metalloporphyrins palladium(II)-meso-tetra-(4-fluorophenyl)mononaphthotribenzoporphyrin (Pd1NF), cis-palladium(II)-meso-tetra-(4-fluorophenyl)dibenzodinaphthoporphyrin (Pd2NF), and palladium(II)-meso-tetra-(4-fluorophenyl)monobenzotrinaphthoporphyrin (Pd3NF) and the corresponding platinum(II) compounds (Pt1NF, cis-Pt2NF, Pt3NF) were investigated. The compounds under investigation absorb intensively in the near-infrared region (628-691 nm) and emit at room temperature at 815-882 nm. Phosphorescence quantum yields of the platinum(II) porphyrins range from 25 to 53% with luminescence decay times of 21 to 44 μs in deoxygenated toluene solutions at room temperature. The corresponding palladium(II) complexes exhibit quantum yields in the range of 7 to 18% with lifetimes of 106 to 206 μs. Density functional theory (DFT) calculations revealed nonplanar geometries for all complexes and corroborate the absorption characteristics. The subsequent π extension of the porphyrin system leads to near-infrared absorbing oxygen indicators with tailor-made luminescence properties as well as tunable oxygen sensitivity.

Organometallic acetylides of Pt(II), Au(I) and Hg(II) as new generation optical power limiting materials

Within the scope of nonlinear optics, optical power limiting (OPL) materials are commonly regarded as an important class of compounds which can protect the delicate optical sensors or human eyes from sudden exposure to damaging intense laser beams. Recent efforts have been devoted to developing organometallic acetylide complexes, dendrimers and polymers as high performance OPL materials of the next generation which can favorably optimize the optical limiting/transparency trade-off issue. These metallated materials offer a new avenue towards a new family of highly transparent homo- and heterometallic optical limiters with good solution processability which outperform those of current state-of-the-art visible-light-absorbing competitors such as fullerenes, metalloporphyrins and metallophthalocyanines. This critical review aims to provide a detailed account on the recent advances of these novel OPL chromophores. Their OPL activity was shown to depend strongly on the electronic characters of the aryleneethynylene ligand and transition metal moieties as well as the conjugation chain length of the compounds. Strategies including copolymerization with other transition metals, change of structural geometry, use of a dendritic platform and variation of the type and content of transition metal ions would strongly govern their photophysical behavior and improve the resulting OPL responses. Special emphasis is placed on the structure-OPL response relationships of these organometallic acetylide materials. The research endeavors for realizing practical OPL devices based on these materials have also been presented. This article concludes with perspectives on the current status of the field, as well as opportunities that lie just beyond its frontier (106 references).

The crystal structure and optics of trans-bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine)-platinum(II): trans-[(nBu3P)2Pt(C≡CC6H4C≡CC6H5)2]

The crystal and molecular structure of trans-bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine) platinum(II), trans-[(nBu3P)2Pt(C≡CC6H4C≡CC6H5)2], C56H72P2Pt, has been determined at room temperature. The pale-yellow crystals are triclinic, space group P[unk] with unit cell dimensions a = 10.884(2) Å, b = 12.979(3) Å, c = 10.083(2) Å, a = 109.75(2)°, β = 95.17(2)°, γ = 101.94(2)°, Z = 1 and Dx = 1.288 Mg m−3. The structure was refined by a full-matrix least-squares procedure on F to final R = 0.036 using 3828 reflections with I ≥ 1.0σ(I). The Pt atom in the centrosymmetric structure forms Pt–P and Pt–C distances of 2.304(1) Å and 1.981 (6) Å, respectively. The optical properties of the title complex were also determined, i.e. α, β, and γ are 1.531, 1.615 and 1.947, respectively.

One- and two-photon absorption of highly conjugated multiporphyrin systems in the two-photon Soret transition region

This study presents a detailed investigation of near-infrared one- and two-photon absorption (TPA) in a series of highly conjugated (porphinato)zinc(II) compounds. The chromophores interrogated include meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZn(n) species), (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes, PZn(n) structures featuring terminal electron-releasing and -withdrawing substituents, related conjugated arrays in which electron-rich and -poor PZn units alternate, and benchmark PZn monomers. Broadband TPA cross-section measurements were performed ratiometrically using fluorescein as a reference. Superficially, the measurements indicate very large TPA cross-sections (up to approximately 10(4) GM; 1 GM = 1x10(-50) cm(4) s photon(-1)) in the two-photon Soret (or B-band) resonance region. However, a more careful analysis of fluorescence as a function of incident photon flux suggests that significant one-photon absorption is present in the same spectral region for all compounds in the series. TPA cross-sections are extracted for the first time for some of these compounds using a model that includes both one-photon absorption and TPA contributions. Resultant TPA cross-sections are approximately 10 GM. The findings suggest that large TPA cross-sections reported in the Soret resonance region of similar compounds might contain significant contributions from one-photon absorption processes.

Effects of structural deformations on optical properties of tetrabenzoporphyrins: free-bases and Pd complexes

A recently developed method of synthesis of pi-extended porphyrins made it possible to prepare a series of tetrabenzoporphyrins (TBP) with different numbers of meso-aryl substituents. The photophysical parameters of free-bases and Pd complexes of meso-unsubstituted TBP's, 5,15-diaryl-TBP's (Ar2TBP's) and 5,10,15,20-tetraaryl-TBP's (Ar4TBP's) were measured. For comparison, similarly meso-arylsubstituted porphyrins fused with nonaromatic cyclohexeno-rings, i.e. Ar(n)-tetracyclohexenoporphyrins (Ar(n)TCHP's, n = 0, 2, 4), were also synthesized and studied. Structural information was obtained by ab initio (DFT) calculations and X-ray crystallography. It was found that: 1) Free-base Ar4TBP's are strongly distorted out-of-plane (saddled), possess broadened, red-shifted spectra, short excited-state lifetimes and low fluorescence quantum yields (tau(fl) = 2-3 ns, phi(fl) = 0.02-0.03). These features are characteristic of other nonplanar free-base porphyrins, including Ar4TCHP's. 2) Ar2TBP free-bases possess completely planar geometries, although with significant in-plane deformations. These deformations have practically no effect on the singlet excited-state properties of Ar2TBP's as compared to planar meso-unsubstituted TBP's. Both types of porphyrins retain strong fluorescence (tau(fl) = 10-12 ns, phi(fl) = 0.3-0.4), and their radiative rate constants (k(r)) are 3-4 times higher than those of planar H2TCHP's. 3) Nonplanar deformations dramatically enhance nonradiative decay of triplet states of regular Pd porphyrins. For example, planar PdTCHP phosphoresces with high quantum yield (phi(phos) = 0.45, tau(phos) = 1118 micros), while saddled PdPh4TCHP is practically nonemissive. In contrast, both ruffled and saddled PdAr(n)TBP's retain strong phosphorescence at ambient temperatures (PdPh2TBP: tau(phos) = 496 micros, phi(phos) = 0.15; PdPh4TBP: tau(phos) = 258 micros, phi(phos) = 0.08). It appears that pi-extension is capable of counterbalancing deleterious effects of nonplanar deformations on triplet emissivity of Pd porphyrins.

Synthesis of 5,15-diaryltetrabenzoporphyrins

A general method of synthesis of 5,15-diaryltetrabenzoporphyrins (Ar 2TBPs) has been developed, based on 2 + 2 condensation of dipyrromethanes followed by oxidative aromatization. Two pathways to Ar 2TBPs were investigated: the tetrahydroisoindole pathway and the dihydroisoindole pathway. In the tetrahydroisoindole pathway, precursor 5,15-diaryltetracyclohexenoporphyrins (5,15-Ar 2TCHPs) were assembled from cyclohexeno-fused meso-unsubstituted dipyrromethanes and aromatic aldehydes or, alternatively, by way of the classical MacDonald synthesis. In the first case, scrambling was observed. Aromatization by tetracyclone was more effective than aromatization by DDQ but failed in the cases of porphyrins with electron-withdrawing substituents in the meso-aryl rings. The dihydroisoindole pathway was found to be much superior to the tetrahydroisoindole pathway, and it was developed into a general preparative method, consisting of (1) the synthesis of 4,7-dihydroisoindole and its transformation into meso-unsubstituted dipyrromethanes, (2) the synthesis of 5,15-diaryloctahydrotetrabenzoporphyrins (5,15-Ar 2OHTBPs), and (3) their subsequent aromatization by DDQ. Ar 2TBP free bases exhibit optical absorption spectra similar to those of meso-unsubstituted tetrabenzoporphyrins and fluoresce with high quantum yields. Pd complex of Ph 2TBP was found to be highly phosphorescent at room temperature.

Large third-order optical nonlinearity of self-assembled porphyrin oligomers

Third-order nonlinear optical properties of two series of self-assembled porphyrin wires, one being terminated by zinc porphyrin and the other by free base porphyrin, were measured by femtosecond time-resolved optical Kerr effect. The hyperpolarizability values of the latter series were extremely large ranging from 10-30 to 10-29 esu, 10 times larger than the former. The behavior is accounted for by the contribution of terminal free base porphyrin to enhance the molecular polarization by acceptor nature toward central metalloporphyrin array.

Changes in porphyrin nonlinear absorption owing to interaction with bovine serum albumin

The changes in the nonlinear absorption of meso-tetrakis (p-sulfonatophenyl) porphyrin (TPPS(4)) caused by its binding to bovine serum albumin (BSA) were investigated in aqueous solutions. The nonlinear absorption depended on the excitation wavelength and pH. It is noteworthy that, for excitation at 640 nm, saturated absorption was observed in a TPPS(4) solution (pH 4.0), whereas reverse saturated absorption occurred for all pH values from 4.0 to 7.0 when BSA was added to TPPS(4). The presence of BSA also initiates an increase in the saturation intensity and a reduction of the nonlinear absorption coefficient. The interpretation of the results based on the binding of TPPS(4) to BSA is corroborated by linear absorption, fluorescence, light scattering, and flash-photolysis data.

Exploitation of the Z-scan technique as a method to optically probe pK(a) in organic materials: application to porphyrin derivatives

We demonstrate the use of the Z-scan technique as a method to measure a chemical parameter, the pK(a) value, that characterizes the equilibrium constant in acid-base reactions. The measurements were performed with picosecond pulses at 532 nm in Fe(III)-meso-tetrakis(4-N-methyl-pyridyl) porphyrin. The results were compared with linear absorption measurements and electron paramagnetic resonance, the usual techniques employed to determine pK(a) values in aqueous solutions, and showed excellent agreement.

Facile Synthesis and Nonlinear Optical Properties of Push-Pull 5,15-Diphenylporphyrins

A series of 5,15-diphenylporphyrinatonickel(II) derivatives containing electron-releasing (Me(2)NC(6)H(4)C&tbd1;C-) and -withdrawing groups [-CHO, -CH=C(CN)(2), -CH=C(CO(2)Et)(2), -CH=CHCHO] in opposite meso positions has been synthesized through the classical formylation, halogenation, Knoevenagel condensation, and palladium-catalyzed cross-coupling reaction of porphyrins. The molecular first hyperpolarizabilities (beta) of two of these push-pull porphyrins, namely 5-(2',2'-dicyanoethenyl)-15-[[4"-(N,N-dimethylamino)phenyl]ethynyl]-10,20-diphenylporphyrinatonickel(II) (7) and 5-formyl-15-[[4'-(N,N-dimethylamino)phenyl]ethynyl]-10,20-diphenylporphyrinatonickel(II) (8) have been determined experimentally by electric-field-induced second-harmonic generation (EFISH) at 1907 nm and computationally by semiempirical methods (ZINDO sum-over-states). The values of beta(&mgr;) (124 and 66 x 10(-)(30) cm(5) esu(-)(1), for 7 and 8, respectively) are moderate because the acceptor group is significantly tilted with respect to the porphyrin core in these complexes, which decreases the electronic coupling across the system. This nonplanar geometry has been confirmed by X-ray structure analyses and molecular modeling studies.