External Heavy Atom Research Articles

Photoexcited triplet states of new UV absorbers, cinnamic acid 2-methylphenyl esters

Phosphorescence spectra of nonphosphorescent or very weakly phosphorescent new UV absorbers, 2-methylphenyl cinnamate (MePC), 2-methylphenyl 4-methoxycinnamate (MePMC) and 2-methylphenyl 4-ethoxycinnamate (MePEC) have been observed by using external heavy atom effects of ethyl iodide in ethanol at 77 K. The lowest excited triplet (T(1)) energies of these new UV absorbers are lower than those of a widely used UV-A absorber, 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM), in both keto and enol forms. The intermolecular triplet-triplet energy transfer from photolabile BM-DBM to MePMC was observed by measuring the time-resolved phosphorescence spectra. Electron paramagnetic resonance spectra have been observed for the T(1) states of these new UV absorbers in ethanol at 77 K by using benzophenone as a triplet sensitizer. The observed T(1) lifetimes, zero-field splitting (ZFS) parameters and molecular orbital calculations of the ZFS parameters suggest that T(1) states of these new UV absorbers posses mainly (3)ππ* character. The deactivation processes of the lowest excited singlet (S(1)) states are predominantly fluorescence and internal conversion to the ground (G) states in MePMC and MePEC, while the main deactivation process of the S(1) state of MePC is internal conversion to the G state. The molar absorption coefficients of MePMC and MePEC in the UV-A and UV-B regions are larger than that of most widely used UV-B absorber, octyl methoxycinnamate.

An alternative way to use the triplet energy of fluorescent dyes in organic light-emitting devices via an external iodide

An unusual heavy atom effect has been identified in an organic light emitting device (OLED) containing polyvinylcarbazole (PVK) as the host, the red fluorescent dye 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)-vinyl]-pyran-4-ylidene}-malononitrile (DCM2) as the emitter, and non-emitting 1,8-diiodooctane (RI) as a heavy atom source instead of a rare metal. The intensity of electroluminescence (EL) of DCM2 changes with the concentration of RI, with a maximum EL intensity obtained for DCM2 at a concentration of 0.25% of RI. Photoluminescence (PL) spectra of PVK–DCM2 films show increased singlet emission from DCM2 in the presence of iodide at 12 K. The enhanced fluorescence induced by iodide is caused by energy transfer from both the singlet and triplet states of PVK to the singlet states of DCM2. These results suggest an alternative way to use the triplet energy of fluorescent materials with external heavy atoms rather than conventional phosphorescent dyes containing rare heavy metal atoms.

Mechanistic Insights into Oxidation of 2-Methyl-1-naphthol with Dioxygen: Autoxidation or a Spin-Forbidden Reaction?

Oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen proceeds efficiently under mild reaction conditions (3 atm O(2), 60-80 °C) in the absence of any catalyst or sensitizer and produces 2-methyl-1,4-naphthoquinone (MNQ, menadione, or vitamin K(3)) with selectivity up to 80% in nonpolar solvents. (1)H NMR and (1)H,(1)H-COSY studies revealed the formation of 2-methyl-4-hydroperoxynaphthalene-1(4H)-one (HP) during the reaction course. Several mechanistic hypotheses, including conventional radical autoxidation, electron transfer mechanisms, photooxygenation, and thermal intersystem crossing (ISC), have been evaluated using spectroscopic, mass-spectrometric, spin-trapping, (18)O(2) labeling, kinetic, and computational techniques. Several facts collectively implicate that ISC contributes significantly into MNL oxidation with O(2) at elevated pressure: (i) the reaction rate is unaffected by light; (ii) C-C-coupling dimers are practically absent; (iii) the reaction is first order in both MNL and O(2); (iv) the observed activation parameters (ΔH(‡) = 8.1 kcal mol(-1) and ΔS(‡) = -50 eu) are similar to those found for the spin-forbidden oxidation of helianthrene with (3)O(2) (Seip, M.; Brauer, H.-D. J. Am. Chem. Soc.1992, 114, 4486); and (v) the external heavy atom effect (2-fold increase of the reaction rate in iodobenzene) points to spin inversion in the rate-limiting step.

Simultaneous Determination of Plant Growth Regulators 1‐Naphthylacetic Acid and 2‐Naphthoxyacetic Acid in Fruit and Vegetable Samples by Room Temperature Phosphorescence

1-Naphthylacetic and 2-naphthoxyacetic acids belong to the synthetic branch of auxins. Auxins have attracted considerable interest as a subject of study by virtue of their biological and physiological significance. Their broad use as plant growth regulators has raised the need for simple, rapid, sensitive and selective analytical methods for their determination in real samples. The primary aim of this work was to develop an analytical method for the simultaneous determination of 1-naphthylacetic acid and 2-naphthoxyacetic acid in commercial technical formulations, tomato and various fruit types (apple, strawberry, orange and plum) by room temperature phosphorescence. Filtrated solutions of aqueous slurries from ecological fruit and tomato samples are acidified and then extracted with dichloromethane. Once the solvent is evaporated, the dried residue is dissolved in sodium dodecyl sulphate (a micellar agent), and supplied with thallium (I) nitrate as an external heavy atom source and sodium sulphite as deoxygenation agent to enhance the ensuing phosphorescence. The broad-band overlapping spectra for the two analytes were resolved by first- and second-derivative phosphorescence spectrometry. Zero-crossing measurements at 488.5 nm in the first-derivative spectrum and 469.5 nm in the second derivative spectrum exhibited a linear dependence on the 2-naphthoxyacetic acid and 1-naphthylacetic acid concentration, respectively. The detection limits as determined in accordance with the error propagation theory were 11.5 ng/mL for 1-naphthylacetic acid and 15.6 ng/mL for 2-naphthoxyacetic acid. The proposed method affords the determination of 1-naphthylacetic acid and 2-naphthoxyacetic acid in real samples with near-quantitative recoveries from agricultural products.

Systematic Investigation of the Metal-Structure–Photophysics Relationship of Emissive d10-Complexes of Group 11 Elements: The Prospect of Application in Organic Light Emitting Devices

A series of new emissive group 11 transition metal d(10)-complexes 1-8 bearing functionalized 2-pyridyl pyrrolide together with phosphine ancillary such as bis[2-(diphenylphosphino)phenyl] ether (POP) or PPh(3) are reported. The titled complexes are categorized into three classes, i.e. Cu(I) complexes (1-3), Ag(I) complexes (4 and 5), and Au(I) metal complexes (6-8). Via combination of experimental and theoretical approaches, the group 11 d(10)-metal ions versus their structural variation, stability, and corresponding photophysical properties have been investigated in a systematic and comprehensive manner. The results conclude that, along the same family, how much a metal d-orbital is involved in the electronic transition plays a more important role than how heavy the metal atom is, i.e. the atomic number, in enhancing the spin-orbit coupling. The metal ions with and without involvement of a d orbital in the lowest lying electronic transition are thus classified into internal and external heavy atoms, respectively. Cu(I) complexes 1-3 show an appreciable metal d contribution (i.e., MLCT) in the lowest lying transition, so that Cu(I) acts as an internal heavy atom. Despite its smallest atomic number among group 11 elements, Cu(I) complexes 1-3 exhibit a substantially larger rate of intersystem crossing (ISC) and phosphorescence radiative decay rate constant (k(r)(p)) than those of Ag(I) (4 and 5) and Au(I) (6-8) complexes possessing pure π → π* character in the lowest transition. Since Ag(I) and Au(I) act only as external heavy atoms in the titled complexes, the spin-orbit coupling is mainly governed by the atomic number, such that complexes associated with the heavier Au(I) (6-8) show faster ISC and larger k(r)(p) than the Ag(I) complexes (4 and 5). This trend of correlation should be universal and has been firmly supported by experimental data in combination with empirical derivation. Along this line, Cu(I) complex 1 exhibits intensive phosphorescence (Φ(p) = 0.35 in solid state) and has been successfully utilized for fabrication of OLEDs, attaining peak EL efficiencies of 6.6%, 20.0 cd/A, and 14.9 lm/W for the forward directions.

Long-Lived Photoinduced Charge Separation Due to the Inverted Region Effect in 1,6-Bis(phenylethynyl)pyrene−Phenothiazine Dyad

Photoinduced electron-transfer processes in a 1,6-bis(phenylethynyl)pyrene−phenothaiazine dyad, BPEP−PT, are examined using various techniques. The BPEP singlet excited state is quenched by electron transfer from PT leading to formation of BPEP radical anion and PT radical cation. Rate constants and quantum yields of the PET processes were determined from steady-state and time-resolved fluorescence experiments, and spectroscopic identification of the radical ion products was achieved using picosecond and nanosecond flash photolysis experiments. The charge-separated (CS) state was found to be long-lived but decayed to the BPEP triplet state under the influence of external heavy atom effect. The energy level diagram constructed on the basis of experimental data revealed the existence of the local triplet state below the CS state, yet the CS state did not exhibit any tendency to decay to this level. This showed that hyperfine interaction (HFI) or spin−orbit charge-transfer intersystem crossing (SOCT-ISC) mec...

ChemInform Abstract: Fluorescence Quenching of Protonated Aza-arenes by External Heavy-Atom Perturbers (Methyl Iodide and Silver Perchlorate).

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Primary photoprocesses in molecules of carbocyanine dyes in binary solvent mixtures

The effect of the composition of the dimethyl sulfoxide (DMSO)-toluene mixture on the photophysical processes of thia-, indo-, and imidacarbocyanine dyes was studied. In the mixtures with the DMSO content of more than 20 vol %, the dyes representing solvated cations are characterized by high efficiency of trans → cis photoisomerization and fluorescence, in contrast to the extremely low efficiency of intersystem crossing to the triplet state. With growing the toluene content in the mixture, ion pairs between the dye cation and Cl−, Br−, I−, or BF4− anion are formed. In the cases when the dye counterions are Br− or I−, a sharp increase in the yield of the triplet molecules and a decrease in their lifetime take place. The results are discussed in terms of “the external heavy atom effect” in ion pairs.

Distorted forms of metalloporphyrins probed by methods of highly resolved spectroscopy

The spectral manifestations of the distorted forms of sterically unconstrained metallocomplexes of porphyrin in rare gas and Shpol'skii matrices at cryogenic temperatures were detected. The planar and two kinds of distorted conformations of the Mg - and Zn -porphyrins in rare gas matrices have been measured simultaneously in the fluorescence and phosphorescence spectra due to external heavy atom effect. The distinctly different frequencies of the vibronic transitions have been revealed for planar and distorted forms. In the phosphorescence spectra of the Pd -porphyrin the manifestation of the two forms in the ground state has been proven also, with one of them being planar and the other being saddle-type distorted. The ratio of the planar and non-planar forms was shown to depend on the deuteration of both meso-positions of the porphyrin and n-alkane matrix. The appearance of the out-of-plane modes in the phosphorescence spectra of the Pd - and Pt -porphyrin has been demonstrated.

The specific feature of photochemical processes in molecules of 3,3′-dialkylthiacarbocyanines in binary solvent mixtures

The effect of the composition of the dimethyl sulfoxide (DMSO)-toluene mixture on the photochemical processes of 3,3′-diethylthiacarbocyanine (DETCC) iodide and 3,3′-dimethylthiacarbocyanine (DMTCC) chloride was studied. In the mixtures with the DMSO content more than 20 vol %, DETCC iodide and DMTCC chloride molecules are characterized by a high efficiency of both trans → cis photoisomerization and fluorescence, in contrast to intersystem crossing to the triplet state. With decreasing the DMSO content to 3 vol %, the relative quantum yield of DETCC iodide intersystem crossing increases by two orders of magnitude, which is accompanied by a decrease in the lifetime of DETCC iodide triplet molecules from 1.1 × 10−4 to 2.4 × 10−7 s and a decrease in the yield of the cis-isomers. To explain the results, the concepts of “external heavy atom (iodide) effect” and hyperfine coupling (HFC mechanism) in radical pairs that could be formed via electron transfer between the iodide and an excited dye molecule were used.

Uncertainties associated with signal measurements from solutions and from solid substrates using luminescence based methods

A careful study was performed to evaluate and compare uncertainties associated with the measurement of luminescence from chrysene by using fluorimetry in solution and by using solid surface room-temperature phosphorimetry (SSRTP). Chrysene was chosen as the luminescent analyte (measurand) for this study because it is a substance of environmental interest and also because it presents strong natural fluorescence and phosphorescence that is easily induced by the external heavy atom effect. The most common approach for uncertainty calculation is the one indicated in the Guide to the Expression of Uncertainty in Measurement which is recognized worldwide. The most relevant sources of uncertainty were identified. The expanded uncertainty (k = 2; 95%) for SSRTP was 3.27 × 10−6 mol L−1 (3.7 ng of chrysene) with relevant contributions from the internal reproducibility, preparation of solutions and from the analytical curve. This value is equivalent to 28% of the reference analyte value. For fluorimetry, an acceptable value for the expanded uncertainty was achieved by preparing solutions by weighing masses of analyte and solvent. In this case, the huge contribution from the preparation of solutions by adjusting volume is minimized and the uncertainty magnitude was 2.12 × 10−9 mol L−1 (21.2% of the chrysene reference concentration). In this case, relevant contributions were from the repeatability, the internal reproducibility and from the analytical curve. The preparation of solutions by weighing does not have a relevant impact in improving the quality of the signal measurements by SSRTP.

Phosphorescence of conjugated poly-para-phenylene polymers sensitized by an organometallic complex with a heavy metal atom

We investigated phosphorescence of conjugated poly-para-phenylene polymers sensitized by a dimethylamino derivative of benzyl (DMAB) and a methyl derivative of tri(2-phenylpyridyl)iridium(III) [Ir(meppy)3]. We found that adding both compounds to the polymer quenches the fluorescence of the polymer and increases its phosphorescence. Although the effectiveness of fluorescence quenching of the polymer is the same, a higher intensity and decay rate of phosphorescence quenching are observed for the polymer activated by the organometallic complex. The results are interpreted in the framework of the model of a remote resonance-enhanced effect of an external heavy atom.

Complexation of Tolane by Fluorinated Organomercurials—Structures and Luminescence Properties of an Unusual Class of Supramolecular π-Coordination Polymers

The interaction of the Hg(II) derivatives bis(pentafluoro)phenyl mercury (1), (pentafluoro)phenyl mercury chloride (2) and trimeric perfluoro-ortho-phenylene mercury (3) with tolane (diphenylacetylene) in CH2Cl2 leads to the formation of [1·tolane], [2 2·tolane], and [3·tolane·CH2Cl2]. These adducts have been characterized by elemental analysis, X-ray crystallography, and luminescence spectroscopy. In the solid state of these adducts, the tolane molecules interact with the molecules of 1, 2 and 3 via secondary Hg–π interactions and arene–fluoroarene interactions. As a result of an external mercury heavy atom effect, adducts [1·tolane] and [2 2·tolane] are phosphorescent at room temperature.

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

This investigation explored whether there is a spin barrier to recombination of first- and second-row transition metal-centered radicals in a radical cage pair. To answer this question, the recombination efficiencies of photochemically generated radical cage pairs (denoted as FcP) were measured in the presence and absence of an external heavy atom probe. Two methods were employed for measuring the cage effect. The first method was femtosecond pump-probe transient absorption spectroscopy, which directly measured FcP from reaction kinetics, and the second method (referred to herein as the "steady-state" method) obtained FcP from quantum yields for the radical trapping reaction with CCl4 as a function of solvent viscosity. Both methods generated radical cage pairs by photolysis (lambda = 515 nm for the pump probe method and lambda = 546 nm for the steady-state method) of Cp'2Mo2(CO)6 (Cp' = eta(5)-C5H4CH3). In addition, radical cage pairs generated from Cp'2Fe2(CO)4 and Cp*2TiCl2 (Cp* = eta(5)-C5(CH3)5) were studied by the steady-state method. The pump-probe method used p-dichlorobenzene as the heavy atom perturber, whereas the steady-state method used iodobenzene. For both methods and for all the radical caged pairs investigated, there were no observable heavy atom effects, from which it is concluded there is no spin barrier to recombination.

Effect of heavy atoms in bioluminescent reactions

Bioluminescent reactions of luminous organisms are excellent models for studying the effects of heavy atoms on enzymatic processes. The effects of potassium halides with halide anions of different atomic weight were compared in bioluminescent reactions of the firefly (Luciola mingrelica), a marine coelenterate (Obelia longissima), and a marine bacterium (Photobacterium leiognathi). Two mechanisms of the effects of the halides were examined-the physicochemical effect of the external heavy atom, based on spin-orbit interactions in electron-excited structures, and the biochemical effect, i.e. interactions with the enzymes resulting in changes of enzymatic activity. The physicochemical effect was evaluated by using photoexcitation of model fluorescent compounds (flavin mononucleotide, firefly luciferin, and coelenteramide) of similar structure to the bioluminescence emitters. The bioluminescent and photoluminescent inhibition coefficients were calculated and compared for the luminous organisms to evaluate the relative contributions of the two mechanisms. The biochemical mechanism was found to be dominant. Hence, the bioluminescent reactions can be used as assays to monitor enzyme inhibition, in metabolic processes, by Br or I-containing compounds.

Electronic spectroscopy of toluene–rare-gas clusters: The external heavy atom effect and vibrational predissociation

Toluene-X van der Waals clusters (where X = Ne, Ne2, Ar, Ar2, Kr, Xe) have been investigated by fluorescence excitation spectroscopy in the region of the S1-S0 transition. With the exception of Xe, for each rare-gas studied, we have assigned cluster transitions in the region of all the strong monomer vibrational bands up to 1000 cm(-1) above the origin band. We have further investigated the S1 relaxation dynamics for each vibrational level of each complex, via their fluorescence decay profiles. Clustering with neon has little appreciable effect on the vibrationless S1 lifetime. By contrast, the clusters with argon and krypton exhibit markedly shorter fluorescence lifetimes compared with the monomer. The effect is so severe in the case of toluene-Xe clusters that no fluorescence signals were observed. We interpret these results in terms of an external heavy atom effect in which the rate of intersystem crossing in toluene is influenced by the cluster partner. For clusters built upon excited S1 vibrational levels, the situation is potentially complicated by intramolecular vibrational redistribution and vibrational predissociation (VP). The majority of the fluorescence decay profiles were satisfactorily modeled using single exponential decays. The emission following pumping of the 37(1) level in the toluene-Kr cluster, however, is an exception. We have modeled the decay of this level with a simple kinetic scheme including VP and determined a predissociation rate of (1.04 +/- 0.54) x 10(7) s(-1).

The Influence of Polymer Matrix Parameters on Intersystem Crossing in Dopant Molecules of Aromatic Amines

ABSTRACT The luminescence spectra of carbazole and phenyl-substituted amines in toluene solution and wide set of polymer matrix were studied at 5 and 295 K, as well as their parameters were obtained. The intersystem crossing was shown to be important process influencing on the energy relaxation of excited singlet state. Its efficiency depends not only on the chemical nature of substituents and spatial conformation of molecule, but also is conditioned by interaction with ambient medium. Therefore, the rate of intersystem conversion increases upon influence of external heavy atom, and also when the polymer matrix is changed or pressure up to 3 kbar is applied.

Calculation of the phosphorescence of porphyrins by the density functional method

The absorption and phosphorescence spectra of porphin and its complexes with internal (Mg, Zn) and external heavy atoms (Ne, AR, Kr, and Xe) are calculated by the B3LYP density functional method with the 3-21G basis set and taking into account the linear and quadratic responses. A reasonable agreement with experiment is obtained. The low radiation ability of the phosphorescence of porphin is explained by the mutual compensation of contributions to the transition dipole moment. A considerable increase in the phosphorescence of the complex of porphin with xenon is explained by an increase in the matrix elements of the operator of the spin—orbit coupling between the ππ* and σπ* states due to the admixture of contributions from configurations with charge transfer.

Theoretical study of the external heavy atom effect on phosphorescence of free-base porphin molecule

The radiative lifetime of phosphorescence of free-base porphin (H 2P) molecule and its complexes with noble-gas atoms are calculated by time-dependent density functions theory (TD DFT) with quadratic response functions for account of spin–orbit coupling and electric dipole activity. The complexes with Ne, Ar, Kr, and Xe are used to simulate the external heavy atom (EHA) effect on phosphorescence of the H 2P molecule in the corresponding noble gas matrices. The B3LYP functional and small basis set (3–21G) are used throughout the study and comparison of all complexes but other basis sets are also utilized to support the chosen approach. A slow radiative rate constant of free-base porphin phosphorescence (about 10 −3 s −1) is obtained with all basis sets being in the order of magnitude agreement with experimental estimations. A strong enhancement of the H 2P phosphorescence rate (by 360 times) is calculated for Xe complex; while for Ne, Ar, and Kr complexes, the enhancement is equal to 1.1, 1.3, and 10.3 times, respectively. In these complexes, the noble gas atom is disposed at 3.6 Å above the center of the porphin ring. In spite of shortcomings of the chosen simple model, the TD DFT calculations explain the most important features of the EHA effect on phosphorescence of free-base porphin. Calculations of the hyperfine coupling tensors for all magnetic nuclei in the lowest triplet state of H 2P molecule and its complexes with noble-gas atoms indicate an appreciable penetration of the spin density to the EHA region. This can be connected with the enhancement of spin–orbit coupling in the H 2P molecule.

A combined experimental and computational investigation of the microscopic external heavy atom effect in van der Waals clusters.

We present a combined experimental and computational study of the external heavy atom effect in van der Waals clusters of para-difluorobenzene (pDFB) with rare-gas atoms. Experimentally, clustering with rare-gas atoms is observed to shorten significantly the S1 fluorescence lifetime compared with that of the pDFB monomer, an effect we interpret in terms of an enhancement of the S1-T1 intersystem crossing rate. In order to test the validity of this widely held assumption, we have calculated the S1-T1 spin-orbit coupling matrix elements in the X-pDFB complexes (X=Ne, Ar, Kr) using a multiconfigurational linear response approach.