S2 Emission Research Articles

Aggregation Dependent S1 and S2 Dual Emissions of Thiophene−Acrylonitrile−Carbazole Oligomer

Dual emissions from S1 (blue region) and S2 (UV region) states of a chromophore with donor−acceptor structure were observed at room temperature. The photophysical properties of the chromophore depend critically on the packing of the molecules. The molecule exhibits weak S1 emission and strong S2 emission; however, when it undergoes solid aggregation, the S1 emission is enhanced while the S2 emission is quenched. The H-packing (face-to-face packing) of the molecules is revealed as the key factor for this interesting aggregation-dependent dual emission.

Fluorescence of Amphotericin B-Deoxycholate (Fungizone) Monomers and Aggregates and the Effect of Heat-Treatment

Fluorescence excitation and emission spectra are reported for the polyene macrolide antifungal agent Amphotericin B formulated as micellar dispersion Fungizone (FZ) and its modified counterpart heat-treated Fungizone. The addition of sodium dodecyl sulfate or sodium deoxycholate surfactant to modulate the aggregation state of Amphotericin B confirms that the monomer and dimer states have different fluorescence spectra. Energy transfer from excited dimer to monomer is observed. Both FZ and heat-treated FZ (HTFZ) show expected S1 --> S0 fluorescence emission as well as anti-Kasha fluorescence emission from the S2 state. The excitation and S1 --> S0 emission spectra of HTFZ are similar to those of FZ, while the S2 --> S0 fluorescence differs in intensity between them. The variation in the rate constant for internal conversion from S2 to S1 as the surfactant concentration is increased differs for FZ and HTFZ; we propose that this may form a new basis for examining the super-aggregated character of AmB preparations. FZ and HTFZ have a similar stability to disaggregation by added sodium dodecyl sulfate surfactant. These findings provide the groundwork for future fluorescence characterization of FZ or HTFZ interactions with cell membranes.

Symmetry Control of Radiative Decay in Linear Polyenes: Low Barriers for Isomerization in the S1 State of Hexadecaheptaene

The room temperature absorption and emission spectra of the 4-cis and all-trans isomers of 2,4,6,8,10,12,14-hexadecaheptaene are almost identical, exhibiting the characteristic dual emissions S1-->S0 (21Ag- --> 11Ag-) and S2-->S0 (11Bu+ --> 11Ag-) noted in previous studies of intermediate length polyenes and carotenoids. The ratio of the S1-->S0 and S2-->S0 emission yields for the cis isomer increases by a factor of approximately 15 upon cooling to 77 K in n-pentadecane. In contrast, for the trans isomer this ratio shows a 2-fold decrease with decreasing temperature. These results suggest a low barrier for conversion between the 4-cis and all-trans isomers in the S1 state. At 77 K, the cis isomer cannot convert to the more stable all-trans isomer in the 21Ag- state, resulting in the striking increase in its S1-->S0 fluorescence. These experiments imply that the S1 states of longer polyenes have local energy minima, corresponding to a range of conformations and isomers, separated by relatively low (2-4 kcal) barriers. Steady state and time-resolved optical measurements on the S1 states in solution thus may sample a distribution of conformers and geometric isomers, even for samples represented by a single, dominant ground state structure. Complex S1 potential energy surfaces may help explain the complicated S2-->S1 relaxation kinetics of many carotenoids. The finding that fluorescence from linear polyenes is so strongly dependent on molecular symmetry requires a reevaluation of the literature on the radiative properties of all-trans polyenes and carotenoids.

Syntheses, Spectral and Electrochemical Investigation of Coordination Complexes of Octakis(benzylthio)tetraazaporphyrinmaganese(II)

Synthesis of [Mn(OBTTAP)], 1 and bichromophoric, di- and pentanuclear complexes with diimine-ruthenium(II) of the type [{Mn(OBTTAP)}{Ru(bpy)2}][PF6]2, 2, [{Mn(OBTTAP)}{Ru(phen)2}][PF6]2, 3, [{Mn(OBTTAP)}{RuCp(PPh3)}][PF6], 4, [{Mn(OBTTAP)}{Ru(bpy)2}4]- [PF6]8, 5, [{Mn(OBTTAP)}{Ru(phen)2}4][PF6]8, 6 and [{Mn(OBTTAP)}{RuCp(PPh3)}4][PF6]4, 7, (OBTTAP = octakis(benzylthio)tetraazaporphyrin) have been described. They were characterized using IR, 1H NMR, UV–visible, and mass spectral data. In the electronic absorption spectra the relative intensities and positions of the Soret and Q-bands, in the di- and pentanuclear complexes were observed shifted vis-à-vis that in the precursor complex 1. They all exhibit strong S2 emission. The emission intensity of the equimolar solutions of complexes 2, 3, 4 and 7 were found to be significantly higher than that of 1. The excitation-emission behaviors of the complexes are indicative of interchromophore energy transfer. Complexes 2–7 exhibited good electrode activity, particularly with multiple reversible redox waves in oxidative CV scans. The OBTTAP ring oxidations were observed as one or two reversible waves, depending upon number and nature of the peripheral metal units. Particularly, with four (bpy)2RuII units bonded to the [Mn(OBTTAP)] periphery, it was observed as two reversible, one electron oxidation waves at E 1/2 0.81 and 1.02 V vs. Ag/AgCl. Also the Ru(II)/Ru(III) oxidations were observed at significantly lower potential, in this complex, at E1/2 0.49 V vs Ag/AgCl due to weaker π-inteaction with dπ(S) orbitals.

Striving To Understand the Photophysics and Photochemistry of Thiophosgene: A Combined CASSCF and MR−CI Study

The potential energy surfaces for Cl(2)CS dissociation into ClCS + Cl in the five lowest electronic states have been determined with the combined complete active space self-consistent field (CASSCF) and MR-CI method. The wavelength-dependent photodissociation dynamics of Cl(2)CS have been characterized through computed potential energy surfaces, surface crossing points, and CASSCF molecular dynamics calculations. Irradiation of the Cl(2)CS molecules at 360-450 nm does not provide sufficient internal energy to overcome the barrier on S(1) dissociation, and the S(1)/T(2) intersection region is energetically inaccessible at this wavelength region; therefore, S(1) --> T(1) intersystem crossing is the dominant process, which is the main reason S(1)-S(0) fluorescence breaks off at excess energies of 3484-9284 cm(-1). Also, the S(1) --> T(2) intersystem crossing process can take place via the S(1)-T(2) vibronic interaction in this range of excess energies, which is mainly responsible for the quantum beats observed in the S(1) emission. Both S(2) direct dissociation and S(2) --> S(3) internal conversion are responsible for the abrupt breakoff of S(2)-S(0) fluorescence at higher excess energies. S(2) direct dissociation leads to the formation of the fragments of Cl(X(2)P) + ClCS(A(2)A' ') in excited electronic states, while S(2) --> S(3) internal conversion followed by direct internal conversion to the ground electronic state results in the fragments produced in the ground state.

Photophysical Studies of Porphyrins and Metalloporphyrins: Accurate Measurements of Fluorescence Spectra and Fluorescence Quantum Yields for Soret Band Excitation of Zinc Tetraphenylporphyrin

For the test system zinc tetraphenylporphyrin in ethanol, the S2−S0 and S1−S0 absorption and emission spectra and fluorescence quantum yields have been measured as a function of excitation wavelength within the Soret and Q-bands under conditions where self-absorption of emission and solute aggregation are either eliminated or properly compensated. Under these conditions, the smallest S2−S0 Stokes shift yet measured, 115 cm-1, and the largest S2−S0 absolute fluorescence quantum yield yet measured, 1.84 × 10-3, are obtained. Accurate measurements of the relative quantum yields of S2−S0 to S1−S0 emission as a function of excitation wavelength reveal that a fast radiationless process that bypasses S1 is operative among states accessed at excitation energies that span the Soret band. The data can be interpreted using the evidence of Yu, Baskin, and Zewail that a second excited state, S2‘, contributes a small fraction of the Soret band's integrated molar absorptivity and is responsible for an increasing fractio...

Observation of the Direct S2 → S0 Two-Photon Fluorescence between 370 and 480 nm and the Hyperpolarizability of Crystal Violet (CV) from Spectrally Resolved Hyper-Rayleigh Scattering Measurement

We present the observation of the direct S2 → S0 two-photon fluorescence (TPF) of crystal violet (CV) in methanol solution. The S2 → S0 emission spectra of CV with excitation wavelengths from 730 to 900 nm are obtained for the first time. These spectra present three clear features: the broad S2 → S0 TPF (370−480 nm) peaked at 408 nm, the hyper-Rayleigh scattering (HRS) at half of the excitation wavelength, and the hyper-Raman scattering with a Stokes shift about 1415 cm-1 from the HRS wavelength, which can be attributed to the phenyl groups in the CV molecule. The two-photon excitation spectra measured from this S2 → S0 emission is peaked at 374 ± 2 nm. This TPF emission has the same depolarization ratio as the HRS signal, indicating a lifetime much shorter than the rotational relaxation lifetime. Subtraction of the TPF contribution leads to a hyperpolarizability value of CV at 800 nm, which is much smaller than that previously reported. Moreover, with the ability to directly measure HRS of the pure solvent, the hyperpolarizability value of CV also has much higher precision.

Spectroscopic behavior of hybrid materials obtained by the sol–gel technique

The need for new, chemically and physically stable luminescent materials operating with UV excitations has stimulated research on luminescence properties of doped sol–gel material. In this work we present technology of production of silica gels doped with organic molecules, lanthanide compounds and organic/inorganic composites. Optical properties of these materials as functions of temperature, concentration and excitation wavelength are presented. Dynamics of the excited states is discussed based on the decay times and emission efficiencies data. Mechanisms of ligand-to-metal energy transfer as well as other processes affecting emission efficiency are considered. Silica sol–gels doped with di-aminoacid derivatives of porphyrins: PP(Ser)2(Arg)2, PP(Ala)2(Arg)2, PP(Met)2(Arg)2, where Met, Arg and Ser denote methionine, serine and arginine aminoacids, respectively, and H2TTMePP {tetrakis[4-(trimethylammonio)phenyl]-21H,23H-porphine} have been obtained and spectroscopically studied. The samples emit only from the lowest excited singlet state (S1). Intensity of this emission depends on the concentration of the active molecule and time of the exposition to the excitation beam. The sample containing PP(Ser)2 (Arg)2 co-doped with Tb(III) ions exhibits only 5 D4 emission from the metal center. When co-doped with Pr(III) ions it displays only the S1 emission and it has been shown that the metal ions affect the S2 fi S1 internal conversion. These materials can find applications as phosphors or sensors of UV irradiation. The efforts have been undertaken also to obtain chiral anisotropic materials. � 2003 Elsevier B.V. All rights reserved.

Effect of peripheral substitution on the electronic absorption and fluorescence spectra of metal-free and zinc phthalocyanines.

The effect of substituents on the position and intensity of the electronic absorption and fluorescence spectra of phthalocyanines (Pcs) was examined for 35 Pc compounds. When electron-releasing groups are bound to four alpha-benzo positions of the Pc skeleton, the B and Q bands shift to longer wavelength. Relative to this shift, the effect of introducing the same electron-releasing groups at the other four alpha positions amounts to about 1.6-2.0. Although the effect is not always clearly seen, introduction of electron-releasing groups in the beta-benzo positions of the Pc skeleton generally shifts the Q band to shorter wavelength. The effect of electron-withdrawing groups is exactly the opposite with respect to the alpha and beta positions. These effects can be reasonably explained by considering the magnitude of the atomic orbital coefficients of the carbon atoms derived from molecular orbital (MO) calculations. In addition, the following intriguing phenomena were observed in the experiments, although not all were explained theoretically: 1) the splitting of the Q band of metal-free Pcs decreases with increasing wavelength of the Q band, 2) the ring currents of Pcs with Q bands at longer wavelength are generally smaller, and 3) the absorption coefficients of the Q band of Pc compounds with 16-electron-releasing substituents are larger than those of the corresponding tetra- and octasubstituted Pcs by several tens of percent. 4) Our PPP calculations suggested that the absorption coefficient of the Q band of Pcs with more strongly electron releasing substituents is larger. 5) The second HOMO of the Pcs with the Q band at longer wavelength has b(1u) symmetry, as opposed to the a(2u) symmetry of normal Pcs. 6) Pcs showing S1 emission maxima at wavelengths longer than about 740 nm generally have quantum yields of less than 0.1.

The Stellar Cusp around the Supermassive Black Hole in the Galactic Center

We analyze deep near-IR adaptive optics imaging (taken with NAOS/CONICA on the Very Large Telescope at the European Southern Observatory, Chile), as well as new proper-motion data of the nuclear star cluster of the Milky Way. The surface density distribution of faint (H ≤ 20, Ks ≤ 19) stars peaks within 02 of the black hole candidate Sgr A*. The radial density distribution of this stellar follows a power law of exponent α ~ 1.3-1.4. The K-band luminosity function of the overall nuclear stellar cluster (within 9'' of Sgr A*) resembles that of the large-scale Galactic bulge but shows an excess of stars at Ks≤ 14. It fits population synthesis models of an old, metal-rich stellar population with a contribution from young, early, and late-type stars at the bright end. In contrast, the cusp within ≤15 of Sgr A* appears to have a featureless luminosity function, suggesting that old, low-mass, horizontal-branch/red-clump stars are lacking. Likewise, there appear to be fewer late-type giants. The innermost cusp also contains a group of moderately bright, early-type stars that are tightly bound to the black hole. We interpret these results as evidence that the stellar properties change significantly from the outer cluster (≥a few arcseconds) to the dense innermost region around the black hole. We find that most of the massive early-type stars at distances of 1''-10'' from Sgr A* are located in two rotating and geometrically thin disks. These disks are inclined at large angles and counterrotate with respect to each other. Their stellar content is essentially the same, indicating that they formed at the same time. We conclude that of the possible formation scenarios for these massive stars the most probable one is that 5-8 million years ago two clouds fell into the center, collided, were shock compressed, and then formed two rotating (accretion) disks orbiting the central black hole. For the OB stars in the central arcsecond, on the other hand, a stellar merger is the most appealing explanation. These stars may thus be super-blue stragglers, formed and rejuvenated through mergers of lower mass stars in the very dense (≥108 M☉ pc-3) environment of the cusp. The collider model also accounts for the lack of giants within the central few arcseconds. The star closest to Sgr A* in 2002, S2, exhibits a 3.8 μm excess. We propose that the mid-IR emission comes either from the accretion flow around the black hole itself or from dust in the accretion flow that is heated by the ultraviolet emission of S2.

Ultrafast Relaxation Dynamics from the S2 State of Malachite Green Studied with Femtosecond Upconversion Spectroscopy

We have studied the relaxation dynamics in a triphenyl methane (TPM) dye, malachite green (MG), following S2-state excitation using fluorescence up-conversion measurements in ethanol and ethylene glycol solutions. Information on the mechanisms and dynamics of radiationless transitions from higher excited states in TPM dyes is interesting from the torsional dynamic aspects of the phenyl rings and for the applications of TPM dyes involving knowledge of the excited-state relaxation channels. Kinetic measurements at different wavelengths suggested a cascade population relaxation along the S2 state with a time constant of ∼130 fs that is almost independent of solvent viscosity in the two solvents used, contrary to the relaxation dynamics of the S1 state. Wavelength-dependent time-resolved anisotropy measurements along the S2 and S1 emission bands displayed successive reductions in the anisotropy values, indicating a continuous evolution to the S1 surface. A detailed analysis led us to propose a relaxation path...

Fluorescence and absorption spectra of oligophenylenevinylenes: Vibronic coupling, band shapes, and solvatochromism

Fluorescence emission and excitation spectra of para-phenylene vinylenes nPV with n=1–4 styryl units are investigated experimentally and theoretically as a function of the temperature and the polarizability of the solvent. At low temperatures, the vibronic structures of the S0↔S1 emission and excitation bands are mirror symmetrical with negligible 0–0 energy gaps. The frequencies of the prominent vibrational modes are assigned to the second longitudinal acoustic phonon modes of the entire molecules and to localized carbon–carbon stretching vibrations. The complete vibronic structures of the spectra are calculated at the ab initio Hartree–Fock (HF/6-311G*) and restricted configuration interaction singles (RCIS/6-311G*) levels of theory assuming planar C2h molecular symmetry. The theoretically predicted spectra are in good agreement with the experiments. At room temperature, a 0–0 energy gap between the first band maxima opens, and the mirror symmetry between absorption and emission is lost. The vibronic band shapes and 0–0 band gaps are successfully simulated with a combination of Gaussian and exponential broadening of the low temperature spectra. The exponential term reflects the differences in thermal population of the phenyl-vinyl torsional modes in the S0 and S1 electronic states. Spectral shifts upon changes in temperature and solvents are quantitatively explained by changes in the refractive index of the environment. From extrapolation of the experimental data the vertical and adiabatic transition energies of the oligomers in vacuo are obtained and compared to RCIS and semiempirical quantum chemical calculations, respectively.

Spectroscopic and Photochemical Properties of Open-Chain Carotenoids

The spectroscopic properties of open-chain, all-trans-C30 carotenoids having seven, eight and nine π-electron conjugated carbon−carbon double bonds were studied using steady-state absorption, fluorescence, fluorescence excitation and time-resolved absorption spectroscopy. These diapocarotenes were purified by high performance liquid chromatography (HPLC) prior to the spectroscopic experiments. The fluorescence data show a systematic crossover from dominant S1 → S0 (21Ag → 11Ag) emission to dominant S2 → S0 (11Bu → 11Ag) with increasing extent of conjugation. The low temperatures facilitated the determination of the spectral origins of the S1 → S0 (21Ag → 11Ag) emissions, which were assigned by Gaussian deconvolution of the experimental line shapes. The lifetimes of the S1 states of the molecules were measured by transient absorption spectroscopy and were found to decrease as the conjugated chain length increases. The energy gap law for radiationless transitions is used to correlate the S1 energies with the dynamics. These molecules provide a systematic series for understanding the structural features that control the photochemical properties of open-chain, diapocarotenoids. The implications of these results on the roles of carotenoids in photosynthetic organisms are discussed.

Steady-State Spectroscopic and Fluorescence Lifetime Measurements of New Two-Photon Absorbing Fluorene Derivatives

Steady-state excitation anisotropy, lifetimes, and time-resolved emission spectra of new 2-photon absorbing fluorene derivatives were measured in aprotic solvents at room temperature. Excitation anisotropy spectra in viscous silicon oil allowed the determination of the spectral position of three electronic transitions S 0 φι S1 ,S 0 φι S2 ,S 0 φι S3 (Si ,i 5 1, 2, 3 are the singlet electronic states) and the angles (’ 308) between absorption S0 φι S1 and emission S1 φι S0 dipole moments for the first electronic transition. Solvate relaxation processes in the first excited state of the investigated fluorene molecules affect the lifetimes of these states, t1, so that experimental values of t1 do not correspond to those calculated by Strickler and Berg theory. The influence of the molecular concentration on the fluorescence quantum yields and t1 have been investigated.

Intramolecular excited charge-transfer states in donor–acceptor derivatives of naphthalene and azanaphthalenes

Electronic structure and molecular conformation of selected donor–acceptor isomeric derivatives of naphthalene, quinoline and quinazoline as well as a family of isomeric triazanaphthalenes containing N,N-dimethylaniline as an electron donor in the excited charge transfer singlet (1CT) states have been investigated. The mechanism of the 1CT←S0 absorption and 1CT → S0 emission is discussed in terms of the Mulliken–Murrell model of the CT complexes and the Marcus formalism of radiative charge transfer. The determination of the electronic transition dipole moments corresponding to the CT absorption and fluorescence and the band-shape analysis of the corresponding spectra lead to the quantities relevant for the 1CT → S0 charge recombination in the Marcus inverted region. It is shown that some of the photophysical properties of the compounds can be predicted in terms of a simple LCAO MO description from the properties of individual chromophores.

Size Distribution and Dissociation Patterns of Iron-Sulfur Binary Cluster Ions from Sputter Ion Source.

The size distribution of iron-sulfur binary cluster ions, FenSm+ (n=1-36, m=1-23), was investigated using secondary ion mass spectrometry (SIMS). The dissociation patterns, n=2-26, m=1-17, were also investigated by varying the acceleration voltage of secondary ions step by step. Iron-sulfur binary cluster ions were produced by 10 keV Xe ion bombardment on a pyrite (FeS2) sheet and were mass-analyzed by a sector type mass spectrometer. Judging from the size distribution observed in mass spectra, the composition ratio n/m of the cluster ions gradually tended to increase as the cluster size became larger. The dissociation patterns of small size tended to yield the FenSn+ (n=2, 4, 6) and FenSn-1+ (n=7, 8, 9) which might be more stable structure. As the cluster size became larger, the dissociation mainly occurred by emitting Fe or S2. Dissociation patterns of FenSm+ suddenly changed at FenSM+ which had maximum peak intensity in the series of FenSm+ for the fixed n. In case of m≤M, Fe emission occurred and S2 emission for m>M

Observation of the S2-Emission from 1,4-Anthraquinone

Abstract Emission, excitation, and absorption spectra of 1,4-anthraquinone (1,4-AQ) have been measured in a degassed CCl4 fluid solution at different temperatures. It is shown that 1,4-AQ in CCl4 exhibits weak emission which can be attributed to the S2(&#960, &#960*) fluorescence at comparatively high temperature. The S2(&#960, &#960*) state is considered to be in thermodynamic equilibrium with the S1(n, &#960*) state as well as with the T2(n, &#960*) and T1(&#960, &#960*) states. The present observation provides the first example of the S2 emission from the medium-size molecules containing C=O groups.

Laser-induced fluorescence studies of S2+ in solid argon

The laser-induced fluorescence of S2+ is studied in solid argon at 7.5 K. The S2+ molecular ions were generated under 266 nm photolysis of a H2S2/Ar matrix. The A2Πu–X2Πg emission of S2+ with a lifetime of 805 ns was detected while the matrix was excited in the 330–455 nm region. The assignment is based on the comparison of the measured spectroscopic parameters with the gas-phase values. The possible charge-transfer mechanism of the photogeneration of S2+ is discussed. Our results indicate that S2+ molecular ions constitute a minor channel in 266 nm photolysis of H2S2 in an Ar matrix.

Decay Time of 2.09 eV Luminescence of S2--Vacancy Centers and Their Optical and Thermal Stability in NaCl:S2 Crystals

An investigation was carried out on decay time of the 2.09 eV emission of S2–vacancy pair centers in NaCl crystals. When the crystal was excited by a pulsed 337 nm light from an N2 laser, the 2.09 eV yellow luminescence principally decayed with the lifetime of 14.2 μs at low temperatures, and exhibited a weak thermal quenching with an activation energy of 51.3 meV above about 150 K. Such S2--vacancy pair centers responsible for the 2.09 eV luminescence were thermally stable at room temperature, and not bleached even when exposed to ultraviolet lights below about 5 eV. Thus, the 2.09 eV emission center possibly can be used as a solid-state laser active center working at room temperature.

Recombination kinetics of S1 and S2 bands in ZnSełZnTe superlattices

The origin of the S1 and S2 emission bands observed in ZnSełZnTe superlattices is discussed on the basis of time-resolved photoluminescence measurements. We show that the decay characteristics of the S1 and S2 band are described each by a single time constant, irrespective of observation energy. This indicates that the S1 and S2 bands are homogeneously broadened due to electron—phonon coupling, supporting the idea that the recombination of self-trapped excitons is a relevant mechanism for these emissions.