Showing papers on "Excited state published in 1999"
TL;DR: In this article, the radiative rate constants for depopulation of the excited states of closely-related series of anionic, neutral and cationic europium, terbium and ytterbium complexes have been measured in H2O and D2O.
Abstract: The radiative rate constants for depopulation of the excited states of closely-related series of anionic, neutral and cationic europium, terbium and ytterbium complexes have been measured in H2O and D2O. With the aid of selective ligand deuteriation, the relative contributions of OH, NH (both amide and amine) and CH oscillators have been measured and critically assessed. Quenching of the Eu 5D0 excited state by amine NH oscillators is more than twice as efficient as OH quenching. The importance of the distance between the excited Ln ion and the XH oscillator is described with recourse to published crystallographic information. The general equation, q = A′(ΔkH2O–kD2O)corr is presented and revised values of A′ for Eu (1.2 ms), Tb (5 ms) and Yb (1 µs) given, which allow for the quenching contribution of closely diffusing OH oscillators. The relevance of such studies to the hydration state of certain gadolinium complexes is described and clear evidence provided for a break in hydration at gadolinium.
1,233 citations
TL;DR: The observation of de Broglie wave interference of C60 molecules by diffraction at a material absorption grating is reported, which is the most massive and complex object in which wave behaviour has been observed.
Abstract: Quantum superposition lies at the heart of quantum mechanics and gives rise to many of its paradoxes. Superposition of de Broglie matter waves1 has been observed for massive particles such as electrons2, atoms and dimers3, small van der Waals clusters4, and neutrons5. But matter wave interferometry with larger objects has remained experimentally challenging, despite the development of powerful atom interferometric techniques for experiments in fundamental quantum mechanics, metrology and lithography6. Here we report the observation of de Broglie wave interference of C60 molecules by diffraction at a material absorption grating. This molecule is the most massive and complex object in which wave behaviour has been observed. Of particular interest is the fact that C60 is almost a classical body, because of its many excited internal degrees of freedom and their possible couplings to the environment. Such couplings are essential for the appearance of decoherence7,8, suggesting that interference experiments with large molecules should facilitate detailed studies of this process.
1,042 citations
TL;DR: In this article, the authors describe nanocrystal photophysics and make comparisons between inorganic solid-state materials and organic dye molecules, showing that in the absence of defects, internal or surface, nanocrystals should exhibit near unity fluorescence quantum yield, and partial protection from quenching.
Abstract: Introduction Sp3-hybridized semiconductors (including InP, GaAs, CdSe, and Si) are remarkable from the perspective of physical chemistry. A single electron created by HOMOLUMO promotion moves rapidly in response to an applied electric field, because there is little lattice distortion (i.e., small Franck-Condon factors) accompanying its creation. According to Marcus-Hush electron transfer theory, electron motion is resonant in the limit of vanishing reorganization energy. Franck-Condon factors are also small around an electron-hole pair, which is an electronically excited state. As a consequence, radiationless internal conversion (unimolecular decay converting electronic energy into heat) is extremely slow. Excited states decay radiatively in a defect-free, direct gap semiconductor such as CdSe. These simple spectroscopic facts have important practical consequences. Semiconductor lightemitting diodes have narrow emission bands and can show near 30% efficiency in converting electrical power into light. Semiconductor lasers and diodes also show excellent long-term stability against photochemical and current-induced degradation, when compared with many organic materials. All these properties reflect the strong chemical bonding, and the extremely delocalized nature of the electronic wave functions. Semiconductor nanocrystals lie between the traditional regimes of chemistry and solid-state physics.1 Nanocrystal research was initially motivated by an effort to understand the evolution of bulk structural and electronic properties from the molecular scale.2 Presently, technological interest in nanocrystals stems from the prospect of creating novel materials with distinct physical properties. Nanocrystals act like molecules as they interact with light via their electronic transition dipoles. Yet, their delocalized solid-state parentage causes them to display unusual photophysics relative to molecules. In many molecules vibronic interaction in the excited state is strong as the wave function is localized on just one or a few bonds. The molecular excited state has a different structure which promotes fast nonradiative deactivation into the ground state. Emission quantum yields can be low, and often sensitive to quenching by the local environment. The situation is different in nanocrystals. In a 23 A diameter nanocrystal, for example, the wave function is delocalized over ∼100 unit cells with little probability density at the surface. This suggests that, in the absence of defects, internal or surface, a nanocrystal should exhibit near unity fluorescence quantum yield, and partial protection from quenching. The emission spectrum should be sharp as the Franck-Condon factors are small. At room temperature nanocrystals can be better photoemitters than bulk semiconductors because in nanocrystals the electron and hole remain superimposed due to quantum confinement. Nanocrystals have the potential to serve as ideal chromophores if their surface chemistry can be understood and controlled. In this Account we describe nanocrystal photophysics and make comparisons between inorganic solid-state materials and organic dye molecules.
923 citations
TL;DR: The results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets, in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons.
Abstract: Some conjugated polymers have luminescence properties that are potentially useful for applications such as light-emitting diodes, whose performance is ultimately limited by the maximum quantum efficiency theoretically attainable for electroluminescence1, 2,. If the lowest-energy excited states are strongly bound excitons (electron–hole pairs in singlet or triplet spin states), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the π*-band and a hole (or missing electron) in the π-band can form a triplet with spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively3. But if the electron–hole binding energy is sufficiently weak, the ratio of the maximum quantum efficiencies for electroluminescence and photoluminescence can theoretically approach unity. Here we report a value of ∼50% for the ratio of these efficiencies (electroluminescence:photoluminescence) in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons. This value significantly exceeds the theoretical limit for strongly bound singlet and triplet excitons, assuming they comprise the lowest-energy excited states. Our results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets.
761 citations
TL;DR: In this paper, the authors reported time-resolved pump-probe laser spectroscopy measurements that reveal the occurrence of fast resonant intermolecular transfer of OH-stretch excitations over many water molecules before the excitation energy is dissipated.
Abstract: Many biological, chemical and physical processes involve the transfer of energy. In the case of electronic excitations, transfer between molecules is rapid, whereas for vibrations in the condensed phase, resonant energy transfer is an unlikely process because the typical timescale of vibrational relaxation (a few picoseconds) is much shorter than that of resonant intermolecular vibrational energy transfer1,2. For the OH-stretch vibration in liquid water, which is of particular importance due to its coupling to the hydrogen bond, extensive investigations have shown that vibrational relaxation takes place with a time constant of 740 ± 25 femtoseconds (ref. 7). So for resonant intermolecular energy transfer to occur in liquid water, the interaction between the OH-stretch modes of different water molecules needs to be extremely strong. Here we report time-resolved pump-probe laser spectroscopy measurements that reveal the occurrence of fast resonant intermolecular transfer of OH-stretch excitations over many water molecules before the excitation energy is dissipated. We find that the transfer process is mediated by dipole–dipole interactions (the Forster transfer mechanism9) and additional mechanisms that are possibly based on intermolecular anharmonic interactions involving hydrogen bonds. Our findings suggest that liquid water may play an important role in transporting vibrational energy between OH groups located on either different biomolecules or along extended biological structures. OH groups in a hydrophobic environment should accordingly be able to remain in a vibrationally excited state longer than OH groups in a hydrophilic environment.
534 citations
TL;DR: In this article, a comparative study on the electronic spectra of a number of unsaturated organic molecules, using on the one hand density functional linear response theory and on the other multiconfigurational second-order perturbation theory, was performed, in order to establish the accuracy that the density functional based methods can give for excitation energies and energy surfaces for excited states.
Abstract: A comparative study has been performed on the electronic spectra of a number of unsaturated organic molecules, using on the one hand density functional linear response theory and on the other multiconfigurational second-order perturbation theory, in order to establish the accuracy that the density functional based methods can give for excitation energies and energy surfaces for excited states. The following molecules are included in the study: tetrazine; the five-membered ring systems cyclopentadiene, furan, pyrrole, and thiophene; acetone; and a dipeptide. The results show that DFT valence excited states have errors that vary between 0 and 1 eV, while Rydberg states are accurate to about 0.2eV in most cases. The use of an asymptotically corrected exchange-correlation potential was essential for the latter result. However, transitions which involve a considerable charge transfer have much larger errors. The results show in some cases a surprisingly strong interaction between valence and Rydberg excited st...
531 citations
TL;DR: In this article, the authors proposed a scheme to create distant entangled atomic states based on driving two or more atoms with a weak laser pulse, so that the probability that two atoms are excited is negligible.
Abstract: We propose a scheme to create distant entangled atomic states. It is based on driving two (or more) atoms with a weak laser pulse, so that the probability that two atoms are excited is negligible. If the subsequent spontaneous emission is detected, the entangled state is created. We have developed a model to analyze the fidelity of the resulting state as a function of the dimensions and location of the detector, and the motional properties of the atoms.
520 citations
TL;DR: In this paper, a spin-unrestricted time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies for the first several excited states of four diatomic radicals and methyl, nitromethyl, benzyl, anilino, and phenoxyl radicals.
423 citations
TL;DR: In this paper, strong anisotropies and nonlinearities of the longitudinal resistivity ρxx were observed at the ground state or first excited Landau level but begin abruptly in the third level.
Abstract: Magnetotransport experiments on high mobility two-dimensional electron gases in GaAs/AlGaAs heterostructures have revealed striking anomalies near half filling of several spin-resolved, yet highly excited, Landau levels. These anomalies include strong anisotropies and nonlinearities of the longitudinal resistivity ρxx which commence only below about 150 mK. These phenomena are not seen in the ground state or first excited Landau level but begin abruptly in the third level. Although their origin remains unclear, we speculate that they reflect the spontaneous development of a generic anisotropic many-electron state.
414 citations
TL;DR: In this article, a realistic model Hamiltonian including all 24 vibrational modes of the pyrazine molecule was presented, which determined the potential energy surfaces of the lowest two excited states, S1 and S2, which are strongly coupled to each other.
Abstract: The molecular dynamics of pyrazine after excitation to the S2 electronic state is investigated using the S2 absorption spectrum as a benchmark. We first present a realistic model Hamiltonian including all 24 vibrational modes of the pyrazine molecule. Using this model, we determined the potential energy surfaces of the lowest two excited states, S1 and S2, which are strongly coupled to each other. We then treated the nuclear motion of all 24 vibrational modes using the multiconfiguration time-dependent Hartree (MCTDH) wave packet propagation method. This method obtains results of good accuracy with acceptable computational effort for such a large system. The calculated spectrum is in good agreement with the experimental one. Furthermore, our results shed light on the role of the 20 modes which are only weakly coupled to the system, and demonstrate that essential physical features, such as symmetries, have to be considered when one wants to treat the molecular dynamics of pyrazine realistically.
408 citations
TL;DR: In this paper, the photophysics and electron injection dynamics of Ru(dcbpy)2(NCS)2 [DCbpy = (4,4‘-dicarboxy-2,2, 2,2'bipyridine)] (or Ru N3) in solution and adsorbed on nanocrystalline Al2O3 and TiO2 thin films were studied by femtosecond mid-IR spectroscopy.
Abstract: The photophysics and electron injection dynamics of Ru(dcbpy)2(NCS)2 [dcbpy = (4,4‘-dicarboxy-2,2‘-bipyridine)] (or Ru N3) in solution and adsorbed on nanocrystalline Al2O3 and TiO2 thin films were studied by femtosecond mid-IR spectroscopy. For Ru N3 in ethanol after 400 nm excitation, the long-lived metal-to-ligand charge transfer (3MLCT) excited state with CN stretching bands at 2040 cm-1 was formed in less than 100 fs. No further decay of the excited-state absorption was observed within 1 ns consistent with the previously known 59 ns lifetime. For Ru N3 absorbed on Al2O3, an insulating substrate, the 3MLCT state was also formed in less than 100 fs. In contrast to Ru N3 in ethanol, this excited state decayed by 50% within 1 ns via multiple exponential decay while no ground-state recovery was observed. This decay is attributed to electron transfer to surface states in the band gap of Al2O3 nanoparticles. For Ru N3 adsorbed onto the surface of TiO2, the transient mid-IR signal was dominated by the IR abs...
TL;DR: In this paper, the authors used the method of adiabatic following to prepare a single molecule in its fluorescing excited state, and showed that up to 74% of the sweeps lead to the emission of a single photon.
Abstract: We use the method of adiabatic following to prepare a single molecule in its fluorescing excited state. Spontaneous emission from this state gives rise to a single photon. With our current experimental conditions, up to 74% of the sweeps lead to the emission of a single photon. Since the adiabatic passage is done on command, the molecule performs as a high rate source of triggered photons. The experimental results are in quantitative agreement with quantum Monte Carlo simulations.
TL;DR: In this paper, the authors showed that in all members of the series the lowest excited states in the spectra do not correspond to ligand field excitations, as has been accepted in the past, but instead correspond to charge transfer (CT) states.
Abstract: Relativistic time dependent density functional calculations have been performed on the excited states of the M(CO)6 (M = Cr, Mo, W) series. Our results, in agreement with previous density functional1 and ab initio2 calculations on Cr(CO)6, indicate that in all members of the series the lowest excited states in the spectra do not correspond to ligand field (LF) excitations, as has been accepted in the past. Instead they correspond to charge transfer (CT) states. The LF excitations are calculated at much higher energy than suggested by the original assignment by Beach and Gray3 and at different energy along the M(CO)6 series, being much higher in the heavier carbonyls than in Cr(CO)6. These results lead to a definitive reassessment of the role of the LF states in the photochemical dissociation of the metal−CO bonds in the M(CO)6 series, suggesting that the experimentally observed photodissociation of the M−CO bond upon irradiation into the lowest energy bands occurs in the heavier carbonyls, as it does in C...
TL;DR: In this paper, a far-infrared magnetospectroscopy in self-assembled InAs quantum dots was used to investigate the electronic transitions from the ground $s$ levels to the excited $p$ levels.
Abstract: Using far-infrared magnetospectroscopy in self-assembled InAs quantum dots, we have investigated the electronic transitions from the ground $s$ levels to the excited $p$ levels. The experiments consist of monitoring, by means of Zeeman tuning of the excited level, a resonant interaction between the discrete ( $p,0$ LO phonon) state and the continuum of either ( $s,1$ LO phonon) or ( $s,2$ LO phonons). We show that the electrons and the LO phonons are always in a strong coupling regime and form an everlasting mixed electron-phonon mode.
TL;DR: In this article, the green fluorescent protein (GFP) of the bioluminescent jellyfish Aequorea and its mutants have gained widespread usage as an indicator of structure and function within cells.
Abstract: The green fluorescent protein (GFP) of the bioluminescent jellyfish Aequorea and its mutants have gained widespread usage as an indicator of structure and function within cells. Proton transfer has been implicated in the complex photophysics of the wild-type molecule, exhibiting a protonated A species excited at 400 nm, and two deprotonated excited-state species I* and B* with red-shifted excitation ∼475 nm. Photochromicity between the protonated and deprotonated species has been reported upon 400 nm excitation. Using precise time-resolved spectroscopy, we have been able to distinguish the fluorescence lifetimes of the I and B species (∼3.3 and ∼2.8 ns, respectively) and show that the irreversible photochromicity which we observe is due to formation in the excited state of the B species, which cannot return to other species in the ground state. The ground state A and I species are in thermal equilibrium. Anisotropy measurements indicate that the chromophore lies rigidly in the molecule with a rotational c...
TL;DR: A small proportion of atomic nuclei can form highly excited metastable states, or isomers, and these isomers are among the longest-lived and have the potential to reach the highest energies as mentioned in this paper.
Abstract: A small proportion of atomic nuclei can form highly excited metastable states, or isomers. Of particular interest is a class of isomers found in deformed axially symmetric nuclei; these isomers are among the longest-lived and have the potential to reach the highest energies. By probing their properties, insights into nuclear structure have been gained. The possibility of stimulated isomer decay may ultimately lead to new forms of energy storage and γ-ray lasers.
TL;DR: In this article, the authors consider systems of static nuclei and electrons coupled to the quantized radiation field and prove that for sufficiently small values of the fine structure constant α, the interacting system has a ground state corresponding to the bottom of its energy spectrum.
Abstract: We consider systems of static nuclei and electrons – atoms and molecules – coupled to the quantized radiation field. The interactions between electrons and the soft modes of the quantized electromagnetic field are described by minimal coupling, p→p−e
A (x), where A(x) is the electromagnetic vector potential with an ultraviolet cutoff. If the interactions between the electrons and the quantized radiation field are turned off, the atom or molecule is assumed to have at least one bound state. We prove that, for sufficiently small values of the fine structure constant α, the interacting system has a ground state corresponding to the bottom of its energy spectrum. For an atom, we prove that its excited states above the ground state turn into metastable states whose life-times we estimate. Furthermore the energy spectrum is absolutely continuous, except, perhaps,
in a small interval above the ground state energy and around the threshold energies of the atom or molecule.
TL;DR: In this article, local spectra of self-affine clusters of silver colloid particles recorded with subwavelength resolution by near-field spectroscopy are reported, which consist of several resonances with highly location-dependent frequencies.
Abstract: Local spectra of self-affine clusters of silver colloid particles recorded with subwavelength resolution by near-field spectroscopy are reported. Spectra were also simulated computationally. The observed and calculated near-field spectra consist of several resonances with highly location-dependent frequencies. The most highly resolved of these resonances correspond to individual surface plasmon (SP) normal modes. All of these features are only observable in the near field. Both theory and experiment also show that when excited by light in the SP region of the spectrum, the field-intensity distribution in the near field is very heterogeneous with most of the excitation concentrated in ``hot spots'' on the cluster surface that are strongly excitation-wavelength dependent. This field-intensity localization provides a rationale for recently reported surface-enhanced Raman enhancements in excess of ${10}^{10}.$
TL;DR: In this article, a refined treatment of H, He I, and He II recombination in the early universe is presented, which allows excited atomic level populations to depart from an equilibrium distribution, replacing the total recombination coefficient with recombination to and photoionization from each level directly at each redshift step.
Abstract: We present a refined treatment of H, He I, and He II recombination in the early Universe. The difference from previous calculations is that we use multi-level atoms and evolve the population of each level with redshift by including all bound-bound and bound-free transitions. In this framework we follow several hundred atomic energy levels for H, He I, and He II combined. The main improvements of this method over previous recombination calculations are: (1) allowing excited atomic level populations to depart from an equilibrium distribution; (2) replacing the total recombination coefficient with recombination to and photoionization from each level directly at each redshift step; and (3) correct treatment of the He I atom, including the triplet and singlet states. We find that the ionization fraction x_e = n_e/n_H is approximately 10% smaller at redshifts <~800 than in previous calculations, due to the non-equilibrium of the excited states of H, which is caused by the strong but cool radiation field at those redshifts. In addition we find that He I recombination is delayed compared with previous calculations, and occurs only just before H recombination. These changes in turn can affect the predicted power spectrum of microwave anisotropies at the few percent level. Other improvements such as including molecular and ionic species of H, including complete heating and cooling terms for the evolution of the matter temperature, including collisional rates, and including feedback of the secondary spectral distortions on the radiation field, produce negligible change to x_e. The lower x_e at low z found in this work affects the abundances of H molecular and ionic species by 10-25%. However this difference is probably not larger than other uncertainties in the reaction rates.
TL;DR: In this article, it was shown that the low frequency acoustic breathing modes of Au particles are impulsively excited by the rapid heating of the particle lattice that occurs after laser excitation.
Abstract: Ultrafast laser spectroscopy has been used to characterize the low frequency acoustic breathing modes of Au particles, with diameters between 8 and 120 nm. It is shown that these modes are impulsively excited by the rapid heating of the particle lattice that occurs after laser excitation. This excitation mechanism is a two step process; the pump laser deposits energy into the electron distribution, and this energy is subsequently transferred to the lattice via electron–phonon coupling. The measured frequencies of the acoustic modes are inversely proportional to the particle radius; a fit to the data for the different sized particles yields vR=0.47cl/Rc, where R is the particle radius, cl is the longitudinal speed of sound in Au, and c is the speed of light. This functional relationship exactly matches the prediction of classical mechanics calculations for the lowest frequency radial (breathing) mode of a free, spherical particle. The inverse dependence of the frequency on the radius means that the modula...
TL;DR: In this paper, a detailed investigation on the excitation mechanisms of rare-earth (RE) ions introduced in Si nanocrystals (nc) is reported, and the exact mechanism of energy transfer has been studied in detail by excitation spectroscopy measurements and time-resolved photoluminescence.
Abstract: A detailed investigation on the excitation mechanisms of rare-earth (RE) ions introduced in Si nanocrystals (nc) is reported. Silicon nanocrystals were produced by high-dose 80-keV Si implantation in thermally grown SiO2 followed by 1100 °C annealing for 1 h. Subsequently some of the samples were implanted by 300-keV Er, Yb, Nd, or Tm at doses in the range 2×1012–3×1015 /cm2. The energy was chosen in such a way to locate the RE ions at the same depth where nanocrystals are. Finally an annealing at 900 °C for 5 min was performed in order to eliminate the implantation damage. These samples show intense room-temperature luminescence due to internal 4f shell transitions within the RE ions. For instance, luminescence at 1.54 μm and 0.98 μm is observed in Er-doped nc, at 0.98 μm in Yb-doped nc, at 0.92 μm in nc and two lines at 0.78 μm and 1.65 μm in Tm-doped nc. Furthermore, these signals are much more intense than those observed when RE ions are introduced in pure SiO2 in the absence of nanocrystals, demonstrating the important role of nanocrystals in efficiently exciting the REs. It is shown that the intense nc-related luminescence at around 0.85 μm decreases with increasing RE concentration and the energy is preferentially transferred from excitons in the nc to the RE ions which, subsequently, emit radiatively. The exact mechanism of energy transfer has been studied in detail by excitation spectroscopy measurements and time-resolved photoluminescence. On the basis of the obtained results a plausible phenomenological model for the energy transfer mechanism emerges. The pumping laser generates excitons within the Si nanocrystals. Excitons confined in the nc can either give their energy to an intrinsic luminescent center emitting at around 0.85 μm nor pass this energy to the RE 4f shell, thus exciting the ion. The shape of the luminescence spectra suggests that excited rare-earth ions are not incorporated within the nanocrystals and the energy is transferred at a distance while they are embedded within SiO2. Rare-earth excitation can quantitatively be described by an effective cross section σeff taking into account all the intermediate steps leading to excitation. We have directly measured σeff for Er in Si nc obtaining a value of ≈2×10−17 cm2. This value is much higher than the cross section for excitation through direct photon absorption (8×10−21 cm2) demonstrating that this process is extremely efficient. Furthermore, the non-radiative decay processes typically limiting rare-earth luminescence in Si (namely back-transfer and Auger) are demonstrated to be absent in Si nc further improving the overall efficiency of the process. These data are reported and their implications.
TL;DR: In this article, the potential energy profiles along the minimum energy reaction path for intramolecular proton transfer in the 1ππ* excited state have been calculated for the title compounds.
Abstract: Potential-energy profiles along the minimum-energy reaction path for intramolecular proton transfer in the 1ππ* excited state have been calculated for the title compounds. The CASSCF and CIS electronic-structure methods have been employed for excited-state geometry optimization. Single-point energy calculations along the reaction path have been performed using the CASPT2 and TDDFT methods. The TDDFT method has been tested against accurate CASSCF and CASPT2 data for malonaldehyde. CASPT2 yields transition energies for photon absorption and emission which are in excellent agreement with experimental data (within 0.2 eV). The CASPT2 potential energy functions exhibit, however, artifactual kinks (on a scale of a single kcal mol-1) which reflect inherent limitations of the CASSCF-based perturbation approach. TDDFT yields potential-energy functions which are essentially parallel to the CASPT2 functions and free of artifacts. Transition energies for absorption and emission are systematically overestimated, however, by about 0.5 eV in TDDFT. For all three title compounds, a barrierless 1ππ* potential-energy function is predicted. The location of the 1ππ* minimum varies from near-enol in salicylic acid to near-keto in 7-hydroxy-1-indanone.
TL;DR: In this paper, the optical absorption, photoluminescence, and photocurrent action spectra of trans-Pt(PBu3n)2Cl2 with one equivalent of the diterminal alkynyl oligothiophenes H-C≡C-R-C-H in CH2Cl 2/iPr2NH at room temperature were reported.
Abstract: Soluble, rigid-rod organometallic polymers trans-[-Pt(PBu3n)2–C≡C–R–C≡C–]∞ (R=bithienyl 2, terthienyl 3) have been synthesized in good yields by the CuI-catalyzed dehydrohalogenation reaction of trans-[Pt(PBu3n)2Cl2] with one equivalent of the diterminal alkynyl oligothiophenes H–C≡C–R–C≡C–H in CH2Cl2/iPr2NH at room temperature. We report the thermal properties, and the optical absorption, photoluminescence, and photocurrent action spectra of 1 (trans-[–Pt(PBu3n)2–C≡C–R–C≡C–]∞, R=thienyl), 2 and 3 as a function of the number of thiophene rings within the bridging ligand. With increasing thiophene content, the optical gap is reduced and the vibronic structure of the singlet emission changes toward that typical for oligothiophenes. We also find the intersystem crossing from the singlet excited state to the triplet excited state to become reduced, while the singlet–triplet energy gap remains unaltered. The latter implies that, in these systems, the T1 triplet excited state is extended over several thiophene ...
TL;DR: In this article, the effect of polymer chain interactions on excited state relaxation was studied for dilute solutions and thin films of polyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl groups at the 9 position.
Abstract: Absorbance and fluorescence spectra of dilute solutions and thin films of polyfluorene rigid-rod polymers 1-3 bearing two hexyl, octyl, or dodecyl groups at the 9-position define the effect of polymer chain interactions on excited state relaxation. Film morphology is controlled by annealing of 250 nm thick films. Under these conditions, the degree of interchain interaction follows the degree of thermotropic liquid crystalline ordering which is, in turn, a function of the length of the attached alkyl substituents. Alkyl substituents also affect the solubility of these polymeric liquid crystals in organic solvents; low solubility favors strong ground state aggregation, as is evidenced by a red-shifted absorption band. In the annealed films, aggregate and excimer formation is evidenced by a broadening of the absorbance band, an increase in the intensity of the low energy emission, the appearance of new long-lived fluorescent species, and structure-dependent changes in observed fluorescence quantum yields.
TL;DR: In this article, the effects of photoinduced intramolecular charge transfer on the electronic and molecular properties of a series of differently twisted 4-N,N-dimethylamino-4'-cyanobiphenyls are investigated by steady-state and time-resolved fluorescence.
Abstract: This photophysical study addresses the general question of how electron transfer in bichromophoric molecules influences the conformational relaxation, which can be toward either more or less π-conjugation. The effects of photoinduced intramolecular charge transfer on the electronic and molecular properties of a series of differently twisted 4-N,N-dimethylamino-4‘-cyanobiphenyls are investigated by steady-state and time-resolved fluorescence. The dipole moments, radiative rates, and torsional relaxations in the excited state are analyzed by comparison with the absorption spectra and interannular twist angle (φ)-dependent CNDO/S calculations. Independent of the twist angle φ and solvent polarity, the first excited singlet state of these donor−acceptor (D−A) biphenyls (I−III) is an emissive intramolecular 1CT state of the 1La-type transferring charge from the dimethylaminobenzene (D) to the cyanobenzene (A) subunit. Similar to the planar restricted D−A fluorene I, the flexible D−A biphenyl II shows only a we...
TL;DR: In this article, the potential energy profile of the lowest π σ ∗ excited singlet state is found to be essentially repulsive, crossing the potential-energy functions of the 1 L b and 1 L a excited states of ππ ∗ character as well as those of the ground state.
TL;DR: In this article, the emission line shapes of single CdSe nanocrystallite quantum dots are studied and a strong dependence of single dot line widths on excitation intensity, wavelength, temperature, and integration time is found and is correlated with the number of times that the quantum dot is excited during the acquisition of a single spectrum.
Abstract: We study the emission line shapes of single CdSe nanocrystallite quantum dots. Single dot line shapes are found to result from rapid spectral shifting of the emission spectrum rather than the intrinsic physics of the quantum dot. A strong dependence of single dot line widths on excitation intensity, wavelength, temperature, and integration time is found and is correlated with the number of times that the quantum dot is excited during the acquisition of a single spectrum. The observed results are consistent with thermally assisted spectral diffusion, activated by the release of excess excitation energy.
TL;DR: In this article, the lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in nonpolar solvents cyclohexane and benzene.
Abstract: The spectroscopic properties and dynamic behavior of peridinin in several different solvents were studied by steady-state absorption, fluorescence, and transient optical spectroscopy. The lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in the nonpolar solvents cyclohexane and benzene. The lifetimes show no obvious correlation with solvent polarizability, and hydrogen bonding of the solvent molecules to peridinin is not an important factor in determining the dynamic behavior of the lowest excited singlet state. The wavelengths of emission maxima, the quantum yields of fluorescence, and the transient absorption spectra are also affected by the solvent environment. A model consistent with the data and supported by preliminary semiempirical calculations invokes the presence of a charge transfer state in the excited state manifold of peridinin to account for the observat...
TL;DR: In this article, the formation and decay of the optical hole (bleach) for 4 nm CdSe nanoparticles (NPs) with adsorbed electron acceptors (1,4-benzoquinone and 1,2-naphthoquinone) were investigated by femtosecond laser spectroscopy.
Abstract: The formation and decay of the optical hole (bleach) for 4 nm CdSe nanoparticles (NPs) with adsorbed electron acceptors (1,4-benzoquinone and 1,2-naphthoquinone) and the rise and decay of the reduced electron acceptors formed after interfacial electron transfer from the CdSe NPs were investigated by femtosecond laser spectroscopy. The ultrashort (200−400 fs) rise times of the bleach at the band-gap energy of the CdSe NP as well as of the acceptor radical anion are found to increase with increasing the excitation energy. This suggests that the electron transfer from the CdSe NP to the quinone electron acceptor occurs after thermalization of the excited hot electrons. The decay times of the transient absorption for the electron acceptor radical anions are found to be comparable to that of the CdSe NP bleach recovery time (3 ps). This suggests that the surface quinones shuttle the electron from the conduction band to the valence band of the excited NP. We contrast this behavior with the excited-state dynamic...