Semiempirical (ZINDO-PCM) approach to predict the radiative and nonradiative decay rates of a molecule close to metal particles

In this work, we present an extension of a model previously proposed (Andreussi et al. J. Chem. Phys. 2004, 121, 10190) to treat the effect of a metal particle on the optical properties of a molecule in solution (close to such a particle) in the framework of the polarizable continuum model (PCM). This extension concerns the combination of such a model with the semiempirical method Zerner's intermediate neglect of differential overlap (ZINDO), which allows us to treat large size molecular systems, as the ones normally used in the experiments. A refinement of the model is also introduced to take into account the effect of the metal specimen on the absorption process of the molecular system, which affects the probability that a molecule reaches the excited state. Numerical tests are presented to validate the reliability of the ZINDO results with respect to quantum-mechanical DFT methods. Comparisons with experimental results on two different large molecular systems are reported, and the effect of the metal on the absorption process is discussed.     

Radiative and nonradiative rate fluctuations of single colloidal semiconductor nanocrystals

Spectrally and time-resolved single-molecule fluorescence spectroscopy was used to investigate fluctuations of the photophysical characteristics of different types of semiconductor nanocrystals (NCs) at room temperature. Correlation of photoluminescence (PL) emission maxima, decay time, and intensity of individual NCs with millisecond time resolution reveals new sources of intensity fluctuations and photophysical properties. In particular, we demonstrate that independent of quenched states spectral diffusion is associated with changes of the radiative rate constant k(r) by means of the quantum-confined Stark effect. Correlation of the different photophysical parameters revealed an intrinsic nonradiative rate and enabled the disentangling of intrinsic and extrinsic nonradiative rate constants. Moreover, it allowed us to assess the PL quantum yield of single NCs. Finally, the presented technique was successfully applied to demonstrate that the addition of antiblinking reagents such as mercaptoethylamine accelerates the observed fluctuations between different photophysical states.     

How the central torsion angle affects the rates of nonradiative decay in some geometrically restricted p-quaterphenyls

A small series of p-quaterphenyl derivatives has been prepared in which the dihedral angle (phi) for the two central rings is constrained by dialkoxy spacers of varying length. The photophysical properties of these compounds remain comparable, but there is a clear correlation between the rate constants for nonradiative decay of both singlet and triplet excited states and phi in fluid solution. The rates tend toward a minimum as phi approaches 90 degrees . These effects are attributed to the general phenomenon of extended delocalization and can be traced to a combination of changes in the Huang-Rhys factor and the electron-vibrational coupling matrix element, both relating to displacement of the relevant potential energy surfaces and to the medium-frequency vibronic mode coupled to decay. The latter effect arises because of different levels of conjugation in the ground-state molecule. Such findings might have important implications for the design of improved light-emitting diodes. A similar angle dependence is noted for the yield of the pi-radical cation formed on photoionization in a polar solvent, but here, the effect is due to variations in the respective energy gaps between the relevant excited states.     

Effective polarizability of a molecule physisorbed on a spherical metal particle: nonlocal effects

The electromagnetic interactions between a molecule and a spherical metal particle affect the linear response of the molecule. We present a general method to calculate the effective dynamic polarizability of such physisorbed systems by including the nonlocal character of the electron response in the sphere. Our model takes into account several long-range mechanisms connected to the dispersion, induction and local field effects. When the sphere reduces to a single atom, we recover results already known for a pair of atoms.     

Multimode Jahn-Teller and pseudo-Jahn-Teller interactions in the cyclopropane radical cation: complex vibronic spectra and nonradiative decay dynamics

The complex vibronic spectra and the nonradiative decay dynamics of the cyclopropane radical cation (CP+) are simulated theoretically with the aid of a time-dependent wave packet propagation approach using the multireference time-dependent Hartree scheme. The theoretical results are compared with the experimental photoelectron spectrum of cyclopropane. The ground and first excited electronic states of CP+ are of X2E' and A2E' type, respectively. Each of these degenerate electronic states undergoes Jahn-Teller (JT) splitting when the radical cation is distorted along the degenerate vibrational modes of e' symmetry. The JT split components of these two electronic states can also undergo pseudo-Jahn-Teller (PJT)-type crossings via the vibrational modes of e', a1' and a2' symmetries. These lead to the possibility of multiple multidimensional conical intersections and highly nonadiabatic nuclear motions in these coupled manifolds of electronic states. In a previous publication [J. Phys. Chem. A 2004, 108, 2256], we investigated the JT interactions alone in the X2E' ground electronic manifold of CP+. In the present work, the JT interactions in the A2E' electronic manifold are treated, and our previous work is extended by considering the coupling between the X2E' and A2E' electronic states of CP+. The nuclear dynamics in this coupled manifold of two JT split doubly degenerate electronic states is simulated by considering fourteen active and most relevant vibrational degrees of freedom. The vibronic level spectra and the ultrafast nonradiative decay of the excited cationic states are examined and are related to the highly complex entanglement of electronic and nuclear degrees of freedom in this prototypical molecular system.     

Reduced photobleaching of chromophores close to a metal surface

The photobleaching of chromophores in front of a metal film is measured by recording the emitted fluorescence intensity from an ensemble of chromophores as a function of time. A strong dependence of the photostability on the distance from the metal surface is found. The experimental data are well described in a classical electromagnetic model with the additional assumption that photobleaching occurs at a constant rate from the excited state. The metal interface influences the photostability of the chromophores in two ways, first by altering the excitation rate by local enhancement of the electromagnetic field and second by altering the electromagnetic decay rate.     

Radiative and radationless decay of exciplexes of 1,12-benzperylene

Many molecules in their excited states react with other species having suitable electron donor or acceptor properties to form complexes (exciplexes) stable only in the excited state. This letter reports a study of the modes of decay of a series of exciplexes in which the donor molecule (1,2-benzperylene) formed exciplexes with a series of dimethylaniline derivatives. By measuring the flourescence and intersystem crossing quantum yields, together with the fluorescence lifetimes of the exciplexes, it was possible to derive the rate constants for fluorescence, radationless decay to the ground state, and intersystem crossing. The first two decay processes were found to show a marked sensitivity to the exciplex energy, while the intersystem crossing rate constant was affected only by the presence of heavy atoms.     

Radiative and nonradiative lifetimes in excited states of Ar,Kr and Xe atoms in a neon matrix

Synchrotron radiation with its intense continuum and its excellent time structure has been exploited for time resolved luminescence spectroscopy in the solid state. By selective excitation of n = 1, n′ = 2 exciton states of Xe, Kr and Ar atoms in a neon matrix we were able to identify the emitting states involved. Lifetimes within the cascade of radiative and radiationless relaxation between excited states as well as the radiative lifetimes for transitions to the ground state have been derived from the decay curves. Energy positions and radiative lifetimes of the emitting states correspond quite well with those of the free atoms. Radiative and radiationless relaxation processes take place within the manifold of excited states of the guest atoms. The rate constants for radiationless decay confirm an energy gap law. The order of the radiationless processes reaches in some cases extremely high values. Selection rules for spin and angular momentum are essential to understand the observed radiationless transition rates.     

Equivalence between the mechanical model and energy-transfer theory for the classical decay rates of molecules near a spherical particle

In the classical modeling of decay rates for molecules interacting with a nontrivial environment, it is well known that two alternate approaches exist which include: (1) a mechanical model treating the system as a damped harmonic oscillator driven by the reflected fields from the environment; and (2) a model based on the radiative and nonradiative energy transfers from the excited molecular system to the environment. While the exact equivalence of the two methods is not trivial and has been explicitly demonstrated only for planar geometry, it has been widely taken for granted and applied to other geometries such as in the interaction of the molecule with a spherical particle. Here we provide a rigorous proof of such equivalence for the molecule-sphere problem via a direct calculation of the decay rates adopting each of the two different approaches.     

Substituent effect on the nonradiative decay rates from 3MLCT excited state of ruthenium(II) complexes: A quantum chemical treatment

The radiationless transition rates from metal-to-ligand charge-transfer excited states of [Ru(phen)₃]²⁺ (phen = 1,10-phnanthroline) and its analogues were investigated in terms of the Huang–Rhys factors (S) using luminescence measurements and quantum chemical calculations. From the luminescence spectral fitting, it was found that not only S_M for medium frequency mode but also S_L for low frequency mode was significant for the nonradiative process. Huang–Rhys factors predicted by the conventional quantum chemical method were in agreement with the experimental values, S_M and S_L.     

Radiative decay channel assessment to understand the sensing mechanism of a fluorescent turn-on Al3+ chemosensor

The turn-on luminescent chemosensor [2-Hydroxy-1-naphthaldehyde-(2-pyridyl) hydrazone] (L), selective to Al³⁺ ions, was studied by means of density functional theory (DFT) and time-dependent-DFT quantum mechanics calculations. The UV-Vis absorption and the radiative channel from the adiabatic S₁ excited state were assessed in order to elucidate the selective sensing mechanism of L to Al³⁺ ions. We found that twisted intramolecular charge transfer (TICT) and photoelectron transfer (PET), which alter the emissive state, are responsible for the luminescence quenching in L. After coordination with Al³⁺, the TICT is blocked, and PET is no longer possible. So, the emission of the coordination complex is activated, and a fluorescence effect enhanced by chelation is observed. For compounds with Zn²⁺ and Cd²⁺, the luminescence quenching is caused by PET, while for Ni²⁺, ligand to metal charge transfer is the prominent mechanism. To go into more detail, the metal-ligand interaction was analyzed via the Morokuma-Ziegler energy decomposition scheme and the natural orbital of chemical valence.     

Electrically driven flow near a colloidal particle close to an electrode with a Faradaic current

To elucidate the nature of processes involved in electrically driven particle aggregation in steady fields, flows near a charged spherical colloidal particle next to an electrode were studied. Electrical body forces in diffuse layers near the electrode and the particle surface drive an axisymmetric flow with two components. One is electroosmotic flow (EOF) driven by the action of the applied field on the equilibrium diffuse charge layer near the particle. The other is electrohydrodynamic (EHD) flow arising from the action of the applied field on charge induced in the electrode polarization layer. The EOF component is proportional to the current density and the particle surface (zeta) potential, whereas our scaling analysis shows that the EHD component scales as the product of the current density and applied potential. Under certain conditions, both flows are directed toward the particle, and a superposition of flows from two nearby particles provides a mechanism for aggregation. Analytical calculations of the two flow fields in the limits of infinitesimal double layers and slowly varying current indicate that the EOF and EHD flow are of comparable magnitude near the particle whereas in the far field the EHD flow along the electrode is predominant. Moreover, the dependence of EHD flow on the applied potential provides a possible explanation for the increased variability in aggregation velocities observed at higher field strengths.     

Four-level optical line shape of a single molecule coupled to a single tunneling two-level system

We propose a density-matrix theory for the four-level system consisting of a single optical two-level system (OTLS) coupled to a single two-level system tunneling along a vibrational coordinate (VTLS). Phonons induce jumping rates of the VTLS, but coherent tunneling has to be considered explicitly as well, because the Born-Oppenheimer potential of the tunnel variable may change upon optical excitation. The OTLS is subject to spontaneous emission and driven by a laser wave with arbitrary strength. Numerical simulations for various coupling cases reproduce limiting behaviors previously discussed separately in the literature, such as motional narrowing, cross transitions, optical saturation and pumping, and nonlinear effects. Our model also perfectly fits recent measurements of the spectra of a single molecule coupled to a single tunneling system in a disordered crystal.     

On the Born—Oppenheimer separation and the calculation of nonradiative transition rates

In this paper we discuss the dependence of calculated nonradiative transition rates on the vibronic wavefunctions chosen to describe excited molecular states. We comment on several fallacies and ambiguities in the literature and suggest a simple resolution to the problem.     

Investigation of polymer-protected noble metal nanoparticles by transmission electron microscopy: control of particle morphology and shape

 Noble metal nanoparticles were prepared by the in situ reduction of the respective metal salt precursors in the presence of various protective polymers. Transmission electron microscopy (TEM) has been used to determine the particle shapes and morphologies. These are strongly influenced by the reduction methods and conditions chosen, but the choice of the protective polymer is equally important for controlling the particle morphologies and for the stabilization of the colloids. A whole spectrum of nanoparticle morphologies and shapes was obtained, ranging from nanoagglomerates which are nevertheless well-defined and well-stabilized to nanosized single crystals with triangular shape. Received: 2 February 1998 Accepted: 29 May 1998     

Ultrafast excited-state dynamics of adenine and monomethylated adenines in solution: implications for the nonradiative decay mechanism

The DNA base adenine and four monomethylated adenines were studied in solution at room temperature by femtosecond pump-probe spectroscopy. Transient absorption at visible probe wavelengths was used to directly observe relaxation of the lowest excited singlet state (S(1) state) populated by a UV pump pulse. In H(2)O, transient absorption signals from adenine decay biexponentially with lifetimes of 0.18 +/- 0.03 ps and 8.8 +/- 1.2 ps. In contrast, signals from monomethylated adenines decay monoexponentially. The S(1) lifetimes of 1-, 3-, and 9-methyladenine are similar to one another and are all below 300 fs, while 7-methyladenine has a significantly longer lifetime (tau = 4.23 +/- 0.13 ps). On this basis, the biexponential signal of adenine is assigned to an equilibrium mixture of the 7H- and 9H-amino tautomers. Excited-state absorption (ESA) by 9-methyladenine is 50% stronger than by 7-methyladenine. Assuming that ESA by the corresponding tautomers of adenine is unchanged, we estimate the population of 7H-adenine in H(2)O at room temperature to be 22 +/- 4% (estimated standard deviation). To understand how the environment affects nonradiative decay, we performed the first solvent-dependent study of nucleobase dynamics on the ultrafast time scale. In acetonitrile, both lowest energy tautomers of adenine are present in roughly similar proportions as in water. The lifetimes of the 9-substituted adenines depend somewhat more sensitively on the solvent than those of the 7-substituted adenines. Transient signals for adenine in H(2)O and D(2)O are identical. These solvent effects strongly suggest that excited-state tautomerization is not an important nonradiative decay pathway. Instead, the data are most consistent with electronic energy relaxation due to state crossings between the optically prepared (1)pipi* state and one or more (1)npi* states and the electronic ground state. The pattern of lifetimes measured for the monomethylated adenines suggests a special role for the (1)npi* state associated with the N7 electron lone pair.     

Infrared photodissociation of a water molecule from a flexible molecule-H2O complex: rates and conformational product yields following XH stretch excitation

Infrared-ultraviolet hole-burning and hole-filling spectroscopies have been used to study IR-induced dissociation of the tryptamine.H2O and tryptamine.D2O complexes. Upon complexation of a single water molecule, the seven conformational isomers of tryptamine collapse to a single structure that retains the same ethylamine side chain conformation present in the most highly populated conformer of tryptamine monomer. Infrared excitation of the tryptamine.H2O complex was carried out using a series of infrared absorptions spanning the range of 2470-3715 cm-1. The authors have determined the conformational product yield over this range and the dissociation rate near threshold, where it is slow enough to be measured by our methods. The observed threshold for dissociation occurred at 2872 cm-1 in tryptamine.H2O and at 2869 cm-1 in tryptamine.D2O, with no dissociation occurring on the time scale of the experiment (approximately 2 micros) at 2745 cm-1. The dissociation time constants varied from approximately 200 ns for the 2869 cm-1 band of tryptamine.D2O to approximately 25 ns for the 2872 cm-1 band of tryptamine.H2O. This large isotope dependence is associated with a zero-point energy effect that increases the binding energy of the deuterated complex by approximately 190 cm-1, thereby reducing the excess energy available at the same excitation energy. At all higher energies, the dissociation lifetime was shorter than the pulse duration of our lasers (8 ns). At all wavelengths, the observed products in the presence of collisions are dominated by conformers A and B of tryptamine monomer, with small contributions from the other minor conformers. In addition, right at threshold (2869 cm-1), tryptamine.D2O dissociates exclusively to conformer A in the absence of collisions with helium, while both A and B conformational products are observed in the presence of collisions with helium. Using resolution-of-identity approximation to second-order Moller-Plesset binding energies extrapolated to the complete basis set limit and harmonic vibrational frequencies and transition states calculated at the density functional limit B3LYP/6-31+G* level of theory, Rice-Ramsperger-Kassel-Marcus (RRKM) predictions for the dissociation, isomerization, and water shuttling rates as a function of energy are made. At threshold, the experimental dissociation rate is almost 10(3) faster than RRKM predictions. Reasons for this apparent non-RRKM behavior will be discussed.     

On the use of isovalued surfaces to determine molecule shape and reaction pathways

Summary Novel insights into local molecule structure and reactivity can be gained from viewing isovalued surfaces of the molecular electron density, electrostatic potential and molecular orbitals rendered as colored, 3-D objects. For example, drawing positive and negative electrostatic isopotential surfaces partitions the molecule into regions subject to nucleophilic or electrophilic attack. Similarly, coloring isodensity surfaces to indicate the magnitude of the gradient of the electron density maps the molecule surface into regions of high and low electronegativity.A basic understanding of reaction mechanisms can also come from viewing and manipulating isovalued surfaces. A theory of molecular interactions, based upon second-order perturbation theory, provides for the decomposition of the intermolecular interaction energy into steric, electrostatic and orbital interactions. Color figures illustrate the docking of reactant molecular densities, electrostatic potentials and orbitals on low-energy pathways. The figures are used to visualize the steric, electrostatic and orbital contributions to molecular interaction energy. The visualization not only identifies low-energy reaction pathways, but it frequently reveals local interactions which determine the magnitude of the total interaction energy. Similar insight is not easily obtained by simple evaluation of the total interaction energy. Approximate transition states, built from structures along low-energy approach pathways, are excellent starting points for transition state searches.CAChe^TM Technical Report No. 1, Tektronix Inc., Beaverton, OR.