Efficient Nonradiative Decay Research Articles

Optical properties of Er 3+ in fullerenes and in β-PbF 2 single-crystals

With the aim of providing a thorough description of the optical properties of Er 3+-doped endohedral fullerenes, we studied their characteristics in the light of those of well-known Er 3+-doped β-PbF 2 single-crystals. Various Er 3+-doped endohedral fullerenes were considered: Er 2C 2@C 82, where the Er 2C 2 group is encapsulated inside a cage of 82 carbon atoms and the Er 3− x Sc x N@C 80 ( x = 0, 1 and 2) family, where the Er 3N, Er 2ScN and ErSc 2N clusters are trapped in a 80 carbon atom cage. In this article, we discuss the absorption and photoluminescence of trivalent erbium ions in fullerenes and in β-PbF 2 crystals. The extinction coefficient of Er 3N@C 80 was found to be 4.8 (±0.5) × 10 3 mol/l −1 cm −1 at 540 nm, due to the C 80 cage absorbance. Even in a saturated fullerene solution, the absorption of Er 3+ encapsulated inside a C 80 cage cannot be observed at room temperature. We suggest that this is due to an insufficient number of Er 3+ ions in the solution and their low absorption cross-section. Low temperature photoluminescence measurements show that the line width of Er 3+ in a carbon cage, dissolved in a polycrystalline solvent, is similar to the one of Er 3+ in β-PbF 2 single-crystals. The quantum efficiency of Er 3+ at 1.5 μm in fullerenes is four orders of magnitude lower than that for Er 3+ in crystals, due to very efficient non-radiative decay processes. Molecular vibrations of the cage might be responsible for those rapid non-radiative de-excitations.

DNA Excited-State Dynamics: From Single Bases to the Double Helix

Ultraviolet light is strongly absorbed by DNA, producing excited electronic states that sometimes initiate damaging photochemical reactions. Fully mapping the reactive and nonreactive decay pathways available to excited electronic states in DNA is a decades-old quest. Progress toward this goal has accelerated rapidly in recent years, in large measure because of ultrafast laser experiments. Here we review recent discoveries and controversies concerning the nature and dynamics of excited states in DNA model systems in solution. Nonradiative decay by single, solvated nucleotides occurs primarily on the subpicosecond timescale. Surprisingly, excess electronic energy relaxes one or two orders of magnitude more slowly in DNA oligo- and polynucleotides. Highly efficient nonradiative decay pathways guarantee that most excited states do not lead to deleterious reactions but instead relax back to the electronic ground state. Understanding how the spatial organization of the bases controls the relaxation of excess electronic energy in the double helix and in alternative structures is currently one of the most exciting challenges in the field.

Variable-Band-Gap Poly(arylene ethynylene) Conjugated Polyelectrolytes Adsorbed on Nanocrystalline TiO2: Photocurrent Efficiency as a Function of the Band Gap

A series of poly(arylene ethynylene) conjugated polyelectrolytes (CPEs) substituted with carboxylic acid side groups have been synthesized and characterized. The polymers feature a backbone consisting of a carboxylated dialkoxyphenylene-1,4-ethynylene unit alternating with a second arylene ethynylene moiety of variable electron demand. The HOMO-LUMO gap is varied across the series, giving rise to a set of four polymers that have absorption maxima ranging from 404 to 495 nm. The CPEs adsorb effectively from solution onto nanostructured TiO(2) films, giving rise to TiO(2)/CPE films that absorb approximately 90% of the incident light at the absorption band maximum. The photocurrent generation efficiency of the TiO(2)/CPE films was examined in a solar cell configuration using an I(3)(-)/I(-) propylene carbonate electrolyte and a Pt/fluorine-doped tin oxide counter electrode. Most of the films exhibit good photocurrent generation efficiency with a peak quantum efficiency of approximately 50% at wavelengths corresponding to the polymers' absorption band maximum. Interestingly, the photocurrent generation efficiency for the lowest-band-gap polymer is substantially lower compared to the other three systems. This effect is attributed to efficient nonradiative decay of excitons at trap sites arising from interchain contacts distal from the TiO(2)/CPE interface.

Effective phonon bottleneck in the carrier thermalization of InAs/GaAs quantum dots

Received 29 January 2008; revised manuscript received 20 June 2008; published 20 August 2008We present a detailed study of the dependence of the spectral characteristics of quasiresonantly excitedInAs/GaAs quantum dots on electronic structure, temperature, and hydrogen content. Multiphonon resonancesdominate the resonantly excited emission spectra. Such multiphonon resonances are attributed to the compe-tition between two different channels ruling carrier thermalization in an ensemble of quantum dots: decaybetween polaronic states and efficient nonradiative intradot decay. This supports an effective “phonon bottle-neck” in quantum dots, which is only partially relaxed by the presence of a fast polaron thermalization channel.DOI: 10.1103/PhysRevB.78.085313 PACS number s : 78.67.Hc, 73.21.La, 63.20.kd

Linear and nonlinear optical spectroscopic characterisation of triphenylamine and 1,2,3-tris(3-methylphenylphenylamino)benzene

The molecule triphenylamine (TPA) in tetrahydrofuran (THF) and the starburst triphenylamine oligomer 1,3,5-tris(3-methylphenylphenylamino)benzene (m-MTDAB) in THF and as neat film are characterized. Absorption and emission spectroscopic parameters are determined. The S 0–S 1 transition is found to be weak (nπ * transition) with small fluorescence quantum yield and efficient non-radiative decay (singlet–triplet intersystem crossing). The S 0–S 1 absorption cross-section spectra in the tails of the broad first absorption bands of the compounds are separated out by radiative lifetime determination and applying the mirror-image relation between absorption and emission. Excimer formation is observed for m-MTDAB neat films, its photo-dynamics is studied, and monomer and excimer stimulated emission cross-section spectra are extracted. Reverse saturable absorption is observed for TPA and m-MTDAB in picosecond laser nonlinear transmission measurements (laser duration 35 ps, laser wavelength 347.15 nm) and the responsible excited-state absorption cross-sections are determined. A new method is developed to calculate the excited-state absorption cross-section spectra of the samples in the fluorescence spectral region by amplification/attenuation of spontaneous emission measurements. The excited-state absorption is found to be larger than the stimulated emission for TPA in THF and for m-MTDAB films excluding their lasing ability. For m-MTDAB in THF the stimulated emission is found to be slightly larger than the excited-state absorption at the wavelength position of peak fluorescence emission.

The interplay between molecular orientation, film morphology and luminescence properties of tetracene thin films on epitaxial AlO x/Ni 3Al(1 1 1)

We report a systematic study of the interplay between molecular orientation, film morphology and luminescence properties of tetracene thin films on epitaxial alumina films on Ni 3Al(1 1 1), employing high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and photoluminescence spectroscopy (PL). If deposited at low temperatures, tetracene forms laterally disordered and compact films in which at least the first monolayer is oriented parallel to the substrate. For thicknesses in the range of 10 Å or below, these as-deposited films show no luminescence, while thicker films exhibit weak luminescence from higher layers. On annealing to 210 K, tetracene films dewet the AlO x /Ni 3Al(1 1 1) surface and transform into an island morphology. At the same time, molecules tend to re-orient into a more upright configuration. In this island configuration, even thin films show luminescence. We can thus conclude that in spite of the insulating nature of the surface, the interaction of flat-lying tetracene molecules with AlO x /Ni 3Al(1 1 1) is strong enough to provide at least one efficient non-radiative decay channel.

Photophysics of the blue fluorescent protein

The blue fluorescent protein (BFP) is a mutant of the green fluorescent protein, where the phenolic ring of the chromophore has been replaced by imidazole cycle of histidine residue. The usability of BFP as a fluorescent marker is hampered by its low fluorescence quantum yield at room temperature. The intensity of fluorescence increases by a factor of 4.5 when the temperature is decreased from 320 K down to 225 K. The fluorescence is also enhanced by hydrostatic pressure. Both effects have been explained by shift of the equilibrium between hydrogen nonbonded and hydrogen-bonded chromophores. Our semi-empirical quantum chemical calculations show that the fluorescence quantum yield of the BFP chromophore is low due to isomerization in the electronically excited state -twisting of the bridging bond by 90°. At this twisted geometry the potential energy surfaces of ground and excited states are situated close to each other facilitating efficient nonradiative decay.

Nonradiative Relaxation in Thiophene-S,S-dioxide Derivatives: The Role of the Environment

The intramolecular radiative and nonradiative relaxation processes of three thiophene-S,S-dioxide derivatives with different molecular rigidity are investigated in different solutions and in inert matrix. We show that the fluorescence quantum efficiency and the relaxation dynamics are strongly dependent on the environment viscosity, whereas they are almost independent of the environment polarity. We demonstrate that this strong dependence is due to an environment dependent nonradiative decay rate, whereas no relevant variations of the radiative decay rate are observed. We demonstrate that the dipole coupling with the solvent does not provide an efficient nonradiative decay channel and that the S(n) - S(1) vibrational relaxation is very efficient in all of the molecules and for all of the investigated environments. Moreover first-principles time-dependent density-functional theory calculations in the correct, i.e., excited-state, molecular conformation, suggest that significant contributions of intersystem crossing to the triplet manifold can be excluded. We then conclude that the main nonradiative process determining the fluorescence quantum efficiency of this class of molecules is S(1) - S(0) internal conversion (IC). An explanation for the IC rate dependence in terms of the environment viscosity, molecular rigidity, S(1) - S(0) energy-gap, and molecular volume is presented.

Alanyl Side Chain Folding in Phenylalanine: Conformational Assignments through Ultraviolet Rotational Band Contour Analysis

Partially resolved ultraviolet rotational band contours associated with the S1 ← S0 origin bands of the six most populated conformers of jet-cooled phenylalanine have been recorded via resonant two-photon ionization. The strong dependence of their transition moment orientation on the conformation of the alanyl side chain has facilitated their structural assignment through simulations based upon ab initio computation. The S1 lifetimes of all six conformers, measured through pump−probe delayed ionization, reveal an efficient nonradiative decay pathway in the most stable conformer, which is stabilized through a chain of intramolecular hydrogen bonds linking the side chain to the benzene ring.

The Pacman effect: a supramolecular strategy for controlling the excited-state dynamics of pillared cofacial bisporphyrins.

The molecular recognition properties of dizinc(II) bisporphyrin anchored by dibenzofuran (DPD), Zn2(DPD) (1), were evaluated as a strategy for utilizing the Pacman effect to control the excited-state properties of cofacial bisporphyrin motifs. Crystallographic studies establish that DPD furnishes a cofacial system with vertical flexibility and horizontal preorganization. The structure determination of a substrate-bound DPD species, Zn2(DPD)(2-aminopyrimidine) (2), completes a set of structurally homologous zinc(II) porphyrin host and host-guest complexes, which offer a direct structural comparison for the Pacman effect upon substrate complexation. Binding studies reveal that pyrimidine encapsulation by the DPD framework is accompanied by a markedly reduced entropic penalty (approximately 60 J mol(-1)K(-1)) with respect to traditional face-to-face bisporphyrin systems, giving rise to a smaller conformational energy cost upon substrate binding. Transient absorption spectroscopy reveals that substrate encapsulation within the DPD cleft dramatically affects excited-state dynamics of cofacial bisporphyrins. The emission lifetime of host-guest complex 2 increases by more than an order of magnitude compared to free host 1. In the absence of the guest, the excited-state dynamics are governed by torsional motion of the porphyrin rings about the aryl ring of the DPD pillar. Host-guest binding attenuates this conformational flexibility, thereby removing efficient nonradiative decay pathways. Taken together, these findings support the exceptional ability of the DPD system to structurally accommodate reaction intermediates during catalytic turnover and provide a novel supramolecular approach toward developing a reaction chemistry derived directly from the excited states of Pacman constructs.

Relaxation Dynamics In Photo-Excited Halogen-Bridged Pt-Complexes Studied By Femtosecond Luminescence Spectroscopy

The ultrafast dynamics of excited electronic states in a series of quasi-one-dimensional halogen-bridged platinum-complexes was investigated by using femtosecond luminescence spectroscopy. In Pt-Br system, the wave-packet oscillation of the local vibration of self-trapped excitons (STEs) was observed. The time evolutions of luminescence intensity in Pt-Br and Pt-Cl were successfully described by using a model based on a wave-packet motion on the interaction mode coordinate. In Pt-I, very fast decay was observed, suggesting the existence of efficient non-radiative decay channel directing to structural change.

Fluorescence excitation spectra of jet-cooled 4-(diisopropylamino)benzonitrile and related compounds

Fluorescence excitation spectra of 4-(diisopropylamino)benzonitrile (DIABN) and 4-(dimethylamino)benzonitrile (DMABN) in thermal vapour and seeded jet expansions are compared. The spectrum of jet-cooled DIABN shows an intense 0–0-transition at 31751.8 cm −1 . The spectrum collapses at excess excitation energies above 800 cm −1 , indicating the presence of an efficient non-radiative decay channel. In the gas phase, fluorescence emission of DIABN occurs from the intramolecular charge transfer (ICT) state. The non-radiative decay channel, therefore, is attributed to rapid ICT in the isolated molecule. The related compounds 4-(methylamino)-3,5-dimethylbenzonitrile (MHD), 4-(azetidinyl)-3,5-dimethylbenzonitrile (M4D), and 4-(dimethylamino)-3,5-dimethylbenzonitrile (MMD) in the jet show extremely weak and structureless emission.

Cooperative Substituent Effects on the Excited States of Copper Phenanthrolines.

Copper phenanthrolines are attractive as potential photosensitizers because of the ready availability of the metal, but efficient nonradiative decay including a solvent-induced quenching phenomenon ordinarily limits their utility. However, the present studies show that the addition of methyl substituents in the 3,8-positions of 1,10-phenanthroline can enhance the protective effect that bulky groups in the 2,9-positions have on the reactive charge-transfer excited state of a bis-ligand copper(I) derivative. Thus, the photoexcited Cu(dbtmp)(2)(+) complex has a lifetime of 920 ns in dichloromethane, whereas the parent complex without the methyl substituents has a lifetime of only 150 ns under the same conditions (dbtmp = 2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline). In dichloromethane, the complex with the 2,9-diphenyl-3,4,7,8-tetramethyl-1,10-phenanthroline ligand also exhibits a long lifetime (480 ns). Even more importantly, the latter combination of substituents appears to eliminate the problem of solvent-induced exciplex quenching.

Characterization of the indacene S0/S1 conical intersection: an MMVB and CASSCF study

A conical intersection between the S0 and S1 states of s-indacene has been located with the molecular mechanics with valence bond (MMVB) and complete active space SCF (CASSCF) methods. In both cases the intersection minimum is close in energy and geometry to the overall minimum on the S1 surface, and can be reached directly along the coordinate which leads from the Franck—Condon geometry to the minimum on S1. The existence of such an accessible S1/S0 surface crossing leading to efficient non-radiative decay can explain the short S1 excited state lifetime and lack of fluorescence for s-indacene. Based on a comparison with X-ray structures for a derivative of s-indacene, we argue that the CASSCF ordering of S0 and S1 states is the correct one, whereas MMVB inverts the two states. However, because the energy differences involved are a few kcal mol-1, a definitive prediction will probably require a multi-reference correlated level of theory with gradient optimization.

Characterization of the indacene S0/S1 conical intersection: An MMVB and CASSCF study

A conical intersection between the S0 and S1 states of s-indacene has been located with the molecular mechanics with valence bond (MMVB) and complete active space SCF (CASSCF) methods. In both cases the intersection minimum is close in energy and geometry to the overall minimum on the S1 surface, and can be reached directly along the coordinate which leads from the Franck—Condon geometry to the minimum on S1. The existence of such an accessible S1/S0 surface crossing leading to efficient non-radiative decay can explain the short S1 excited state lifetime and lack of fluorescence for s-indacene. Based on a comparison with X-ray structures for a derivative of s-indacene, we argue that the CASSCF ordering of S0 and S1 states is the correct one, whereas MMVB inverts the two states. However, because the energy differences involved are a few kcal mol-1, a definitive prediction will probably require a multi-reference correlated level of theory with gradient optimization.

Product Distributions from Molecular Mechanics−Valence Bond Dynamics: Modeling Photochemical [4 + 4] Cycloadditions

The purpose of this paper is to study the model [4 + 4] photocycloaddition of butadiene + butadiene by using direct dynamics calculationswith no geometric constraintsto describe motion along excited-state reaction paths and subsequent decay to the ground state. We use the molecular mechanics−valence bond (MMVB) potential, which is calibrated against previous CASSCF calculations for this system (Bearpark, M. J.; Deumal, M.; Robb, M. A.; Vreven, T.; Yamamoto, N.; Olivucci, M.; Bernardi, F. J. Am. Chem. Soc. 1997, 119, 709−718). Our dynamics calculations show that efficient nonradiative decay of butadiene + butadiene in the presence of two different S1/S0 conical intersections can account for the formation of many products. The major product predicted by MMVB is consistent with the limited experimental data available.

Optical spectroscopy of and in

The configuration ions and have been incorporated in strontium fluorovanadate (SVAP) crystals grown using the Czochralski technique. Such crystals provide fourfold-coordinated sites for occupancy by the and ions, which are located near the centres of their respective tetrahedrally bonded molecular ions and . The optical absorption spectra of these ions are in the range 500 nm to 1100 nm and feature a number of overlapping bands related to the splittings of the and excited states by the distorted tetrahedral crystal field. In terms of point-ion crystal field theory strong- and weak-field descriptions are appropriate to and molecular ions respectively. This is confirmed for by photoluminescence spectra which reveal sharp zero-phonon lines attended by vibronic structure characteristic of coupling to molecular vibrations of both and the host ion. Luminescence from the ion is quenched by efficient nonradiative decay at all temperatures between 10 and 300 K. The energy level structure of these ions has been calculated using group theory assuming reductions in symmetry from to to . A comparison of the calculated level structure with the absorption and luminescence spectra shows that and occupy distorted tetrahedral sites in SVAP. Selection rules derived for all transitions assuming symmetry account for all the features in the absorption spectra of and and in the photoluminescence spectrum of .

Effect of metal films on the photoluminescence and electroluminescence of conjugated polymers

We report the modification of photoluminescence (PL) and electroluminescence (EL) from conjugated polymers due to the proximity of metal films. The presence of a metal film alters the radiative decay rate of an emitter via interference effects, and also opens up an efficient nonradiative decay channel via energy transfer to the metal film. We show that these effects lead to substantial changes in the PL and EL quantum efficiencies and the emission spectra of the polymers studied here [cyano derivatives of poly($p$-phenylenevinylene), PPV] as a function of the distance of the emitting dipoles from the metal film. We have measured the PL quantum efficiency directly using an integrating sphere, and found its distance dependence to be in good agreement with earlier theoretical predictions. Using the spectral dependence of the emission, we have been able to investigate the effect of interference on the radiative rate as a function of the wavelength and the distance between the emitter and the mirror. We compare our results with simulations of the radiative power of an oscillating dipole in a similar system. From our results we can determine the orientation of the dipoles in the polymer film, and the branching ratio that gives the fraction of absorbed photons leading to singlet excitons. We propose design rules for light-emitting diodes (LED's) and photovoltaic cells that optimize the effects of the metal film. By making optimum use of above effects we have substantially increased the EL quantum efficiencies of PPV/cyano-PPV double-layer LED's.

Modeling Photochemical [4 + 4] Cycloadditions: Conical Intersections Located with CASSCF for Butadiene + Butadiene

The [4 + 4] photocycloaddition of butadiene + butadiene has been studied at the CASSCF /4-31G level, as a prototype for a class of photochromic systems. For this model system, minima and transition structures are characterized by analytic frequency calculations, and conical intersections are located. Our results indicate that the standard model for the [4 + 4] addition (based on H4) needs to be revised. The reorganization of all 8π electrons is crucial (i.e., it is not always the same 4π electrons that are important). Efficient nonradiative decay of butadiene + butadiene can be explained by the presence of two distinct S1/S0 conical intersections. The first-the lowest-energy point on S1 overall-is preceded by a barrier for the formation of a new σ bond. The resulting structure is similar to those previously characterized for methyl migration in but-1-ene and the addition of ethylene to benzene. A higher-energy barrier leads to a second crossing which resembles the rhomboidal funnel for the [2 + 2] addition of ethylene + ethylene, but which involves only one double bond from each butadiene. Both reaction paths commence at a true pericyclic minimum, at which the (S0−S1) energy gap of ∼37 kcal mol-1 prohibits decay.

18]Annulene: a benzene-like S0/S1 conical intersection located with CASSCF + GVB

A further parallel between the planar [18]annulene and benzene has been demonstrated: the existence of a topologically equivalent S0/S1 conical intersection. The lowest energy point on the crossing has been optimized with the MMBV and CASSCF+GVB/4-31G methods. These calculations suggest that fluorescence might be observed from the S1 state of [18]annulene, and that this would disappear at higher vibrational excess energies as a route to efficient non-radiative decay via the conical intersection became accessible (the 'channel 3' mechanism for benzene).