Efficient Nonradiative Decay Research Articles

Dynamics of exciton energy renormalization in monolayer transition metal disulfides

Fundamental understandings on the dynamics of charge carriers and excitonic quasiparticles in semiconductors are of central importance for both many-body physics and promising optoelectronic and photonic applications. Here, we investigated the carrier dynamics and many-body interactions in two-dimensional (2D) transition metal dichalcogenides (TMDs), using monolayer WS2 as an example, by employing femtosecond broadband pump-probe spectroscopy. Three time regimes for the exciton energy renormalization are unambiguously revealed with a distinct red-blue-red shift upon above-bandgap optical excitations. We attribute the dominant physical process in the three typical regimes to free carrier screening effect, Coulombic exciton–exciton interactions and Auger photocarrier generation, respectively, which show distinct dependence on the optical excitation wavelength, pump fluences and/or lattice temperature. An intrinsic exciton radiative lifetime of about 1.2 picoseconds (ps) in monolayer WS2 is unraveled at low temperature, and surprisingly the efficient Auger nonradiative decay of both bright and dark excitons puts the system in a nonequilibrium state at the nanosecond timescale. In addition, the dynamics of trions at low temperature is observed to be significantly different from that of excitons, e.g., a long radiative lifetime of ~ 108.7 ps at low excitation densities and the evolution of trion energy as a function of delay times. Our findings elucidate the dynamics of excitonic quasiparticles and the intricate many-body physics in 2D semiconductors, underpinning the future development of photonics, valleytronics and optoelectronics based on 2D semiconductors.

Excited State Lifetimes of Sulfur-Substituted DNA and RNA Monomers Probed Using the Femtosecond Fluorescence Up-Conversion Technique.

Sulfur-substituted DNA and RNA nucleobase derivatives (a.k.a., thiobases) are an important family of biomolecules. They are used as prodrugs and as chemotherapeutic agents in medical settings, and as photocrosslinker molecules in structural-biology applications. Remarkably, excitation of thiobases with ultraviolet to near-visible light results in the population of long-lived and reactive triplet states on a time scale of hundreds of femtoseconds and with near-unity yields. This efficient nonradiative decay pathway explains the vanishingly small fluorescence yields reported for the thiobases and the scarcity of fluorescence lifetimes in the literature. In this study, we report fluorescence lifetimes for twelve thiobase derivatives, both in aqueous solution at physiological pH and in acetonitrile. Excitation is performed at 267 and 362 nm, while fluorescence emission is detected at 380, 425, 450, 525, or 532 nm. All the investigated thiobases reveal fluorescence lifetimes that decay in a few hundreds of femtoseconds and with magnitudes that depend and are sensitive to the position and degree of sulfur-atom substitution and on the solvent environment. Interestingly, however, three thiopyrimidine derivatives (i.e., 2-thiocytidine, 2-thiouridine, and 4-thiothymidine) also exhibit a small amplitude fluorescence component of a few picoseconds in aqueous solution. Furthermore, the N-glycosylation of thiobases to form DNA or RNA nucleoside analogues is demonstrated as affecting their fluorescence lifetimes. In aqueous solution, the fluorescence decay signals exciting at 267 nm are equal or slower than those collected exciting at 362 nm. In acetonitrile, however, the fluorescence decay signals recorded upon 267 nm excitation are, in all cases, faster than those measured exciting at 362 nm. A comparison to the literature values show that, while both the DNA and RNA nucleobase and thiobase derivatives exhibit sub-picosecond fluorescence lifetimes, the 1ππ* excited-state population in the nucleobase monomers primarily decay back to the ground state, whereas it predominantly populates long-lived and reactive triplet states in thiobase monomers.

Nitration of Tyrosine Channels Photoenergy through a Conical Intersection in Water.

Nitration of tyrosine occurs under oxidative stress in vivo. The product, 3-nitrotyrosine (3NY), has a dramatically decreased quantum yield and can be used as a molecular ruler. In this study, femtosecond transient absorption spectroscopy and quantum calculations were implemented to elucidate the photoinduced relaxation processes of anionic 3NY in water. Upon 400 nm excitation into an excited electronic state with notable charge-transfer (CT) character, a barrierless nitro-twisting motion rapidly (<100 fs) guides the chromophore into an adjacent twisted intramolecular CT state, therein reaching a sloped S1/S0 conical intersection on the ∼100 fs time scale. Once in the hot ground state, excess energy is further released through vibrational cooling with biexponential time constants of ∼140 and 680 fs in water. Nitro back-twisting occurs on longer time scales (∼1.1 and 9 ps in water), returning the system to original ground state. Systematic evaluations of excited-state potential energies of anionic 3NY were performed by density functional theory (DFT) and time-dependent DFT calculations, showing that intersystem crossing (ISC) from the first singlet state (S1) to the first or second triplet state (T1 or T2) is unlikely. Inclusion of an explicit water molecule in calculations leads to improved mapping of the excited-state energy ordering of the second singlet state (S2) and T2, further diminishing ISC probability from S1 and favoring an ultrafast internal conversion to S0. These results provide deep insights into the highly efficient nonradiative decay of anionic 3NY in aqueous solution, with nitro-site-specific information that can help infer the characterization and potential optogenetic control of 3NY in protein environment.

Theoretical Rationalization of the Dual Photophysical Behavior of C60+

Interest in fullerenes has been renewed recently in astrophysics as a consequence of their detection in circumstellar environments. In particular, C60+ was detected in the diffuse interstellar medium and its presence has been related to some diffuse interstellar bands (DIBs) whose origin was previously unknown. A single recent laboratory experiment ( J. Phys. Chem. A 2017, 121, 7356-7361) shows that upon laser excitation at 785 nm, C60+ in neon matrixes exhibits a radiative decay at 965 nm, while UV photoexcitation does not lead to any significant luminescence. To rationalize this original dual photophysical behavior, we have performed time-dependent density functional theory (TD-DFT) calculations on C60+ to investigate the potential energy surfaces of the relevant electronic states, completed by the simulations of vibrationally resolved absorption and emission spectra. The proposed photophysical pathways shed light on the experimental measurements: The near-IR laser excitation populates the 11th doublet excited state (D11) that decays to the lowest first bright excited state D5, from which photoluminescence is predicted. Indeed, D5 is largely separated from the lower electronic states (D0-D4). Thus, D5 behaves effectively as the first excited state, while the D0-D4 set of states act as the electronic ground state. In addition, there are no low-lying conical intersections between D5 and lower excited states energetically accessible upon near-IR excitation that can provide efficient nonradiative decay channels for this state, leaving radiative decay as the most likely deactivation pathway. However, a sloped conical intersection between D5 and D4 was located around 2.9 eV above D0. While it is too high in energy to be accessible upon near-IR excitation, it provides a funnel for efficient nonradiative decay down to the ground state (D0) accessible upon UV light excitation. Thus, the photophysics of C60+ is controlled by the ability to access this funnel: Upon near-IR excitation, the system fluoresces because the funnel for nonradiative decay cannot be reached, while UV irradiation provides a different route by opening up a radiationless decay channel via this funnel, accounting for the absence of fluorescence.

Ga-Polar (In,Ga)N/GaN Quantum Wells Versus N-Polar (In,Ga)N Quantum Disks in GaN Nanowires: A Comparative Analysis of Carrier Recombination, Diffusion, and Radiative Efficiency

We investigate the radiative and nonradiative recombination processes in planar (In,Ga)N/GaN(0001) quantum wells and (In,Ga)N quantum disks embedded in GaN$(000\bar{1})$ nanowires using photoluminescence spectroscopy under both continuous-wave and pulsed excitation. The photoluminescence intensities of these two samples quench only slightly between 10 and 300 K, which is commonly taken as evidence for high internal quantum efficiencies. However, a side-by-side comparison shows that the absolute intensity of the Ga-polar quantum wells is two orders of magnitude higher than that of the N-polar quantum disks. A similar difference is observed for the initial decay time of photoluminescence transients obtained by time-resolved measurements, indicating the presence of a highly efficient nonradiative decay channel for the quantum disks. In apparent contradiction to this conjecture, the decay of both samples is observed to slow down dramatically after the initial rapid decay. Independent of temperature, the transients approach a power law for longer decay times, reflecting that recombination occurs between individual electrons and holes with varying spatial separation. Employing a coupled system of stochastic integro-differential equations taking into account both radiative and nonradiative Shockley-Read-Hall recombination of spatially separate electrons and holes as well as their diffusion, we obtain simulated transients matching the experimentally obtained ones. The results reveal that even dominant nonradiative recombination conserves the power law decay for (In,Ga)N/GaN{0001} quantum wells and disks.

Photoisomerization of ethyl ferulate: A solution phase transient absorption study

Ethyl ferulate (ethyl 4-hydroxy-3-methoxycinnamate) is currently used as a sunscreening agent in commercial sunscreen blends. Recent time-resolved gas-phase measurements have demonstrated that it possesses long-lived (>ns) electronic excited states, counterintuitive to what one might anticipate for an effective sunscreening agent. In the present work, the photodynamics of ethyl ferulate in cyclohexane, are explored using time-resolved transient electronic absorption spectroscopy, upon photoexcitation to the 11ππ∗ and 21ππ∗ states. We demonstrate that ethyl ferulate undergoes efficient non-radiative decay to repopulate the electronic ground state, mediated by trans-cis isomerization. These results strongly suggest that even mild perturbations induced by a non-polar solvent, as may be found in a closer-to-market sunscreen blend, may contribute to our understanding of ethyl ferulate’s role as a sunscreening agent.

Azaindole modified imine moiety as fluorescent probe for highly sensitive detection of Fe3+ ions

An azaindole based schiff base AzIm was synthesized that serve as a luminescent turn “off” switch for the detection of Fe3+ ions. The photoluminescence intensity of AzIm was quenched significantly in the presence of 30μM Fe3+ ions. The sensing response of the probe was tested at different ranges of pH and at different cation concentrations between 2 and 25μM. Analytical characteristics were evaluated using standard Benesi-Hildebrand equation and the system displayed micromolar sensitivity for these ions. The specific diminished emission signal upon Fe3+ binding was interpreted in terms of the enhanced photoelectron transfer from azaindole group, which induces very fast and efficient non-radiative decay of the excited states of the sensor. In addition, practical utilization of luminescent AzIm was further supported by significant ion selective stability against the presence of extraneous charged species that makes this system compatible for quantitative analysis of environmental samples.

A BODIPY-based 'turn-on' fluorescent probe for hypoxic cell imaging.

A boron-dipyrromethene (BODIPY) dye containing a styryl substituent with a hydroxyl and nitro group, 1a, exhibits a 'turn-on' fluorescence response for hypoxic cells. The presence of the electron-withdrawing nitro group results in a highly efficient nonradiative decay of the S1 state, and hence a recovery of fluorescence intensity when nitro reduction occurs under hypoxic conditions.

Luminescent phosphine ruthenium(II) complexes with 8-hydroxyquinoline derivative ligands

Ruthenium(II) complexes with formulas [RuH(CO)(PPh3)2(L)] (where L=5-chloro-8-hydroxy-2-methylquinoline-7-carboxylic acid (L1), 8-hydroxy-2,5-dimethylquinoline-7-carboxylic acid (L2), 2-methyl-8-quinolinol (L3) and 8-hydroxy-2-methyl-5,7-dinitroquinoline (L4)) were synthesized from precursor complex [RuHCl(CO)(PPh3)3] and characterized by IR, NMR, UV–vis spectroscopy and X-ray crystallography. The emission properties of the complexes were studied in solutions and solid state at ambient temperature. The quantum yields of fluorescence, the lifetimes and the nature of excited states were described on the basis of molecular properties. To elucidate the structural, spectroscopic and bonding properties of the obtained complexes, calculations at the DFT level were undertaken. The complexes display green to yellow–orange emission depending mostly on the impact of substituents of the quinoline moiety. The emission spectra of the ligands were prepared in order to discuss fluorescence resonance energy transfer (FRET) between the metal center and the ligands. Excitation spectra of the complexes display a good agreement with the emission bands of the ligands. Thus, the ligand centered (LC) excited state, which interacts with metal to ligand charge transfer (MLCT) excited state plays a role in emission of complexes. Comparing to the ligands, decreasing of decay times and quantum yields in the received complexes are visible, which is connected with efficient non-radiative decay from the 3MLCT states in the complexes.

A new aza-BODIPY based NIR region colorimetric and fluorescent chemodosimeter for fluoride

The synthesis and characterization of a novel NIR region fluoride sensor, that makes use of the aza-boron-dipyrromethene (aza-BODIPY) fluorophore, is described. An arylmagnesium bromide was formed by reacting 4-bromophenol with a tert-butyldimethylsilyl protecting group and magnesium, which was then used to prepare the aza-BODIPY through a reaction with phthalonitrile. The unusually strong affinity between the fluoride anion and silicon is used to create a sensor dye, which exhibits a highly specific, rapid colorimetric and ‘turn-off’ fluorescence response for F− in solution and in living HeLa cells. With F−, there is enhanced intramolecular charge transfer within the S1 state and this results in efficient nonradiative decay and hence in a marked decrease in fluorescence emission intensity.

Effect of Isomerism and Chain Length on Electronic Structure, Photophysics, and Sensitizer Efficiency in Quadrupolar (Donor)2–Acceptor Systems for Application in Dye-Sensitized Solar Cells

We report on quadrupolar (donor)2-acceptor sensitizers for dye-sensitized solar cells (DSSCs). The acceptor units are based on dithieno[2,3-a:3',2'-c]phenazine and dithieno[3,2-a:2',3'-c]phenazine coupled to thiophene donors. The optoelectronic and photophysical properties of two sets of isomers reveal a rigid structure for linear isomers and an efficient nonradiative decay for branched isomers. These sensitizers were integrated into DSSCs, and the quadrupolar structure is an operational design, as the IPCE reached up to 38% from 400 nm to 600 nm. The lengthening of the donor chain increases the efficiency, demonstrating the appeal of these oligomeric dyes for DSSCs.

Communication: Non-radiative recombination via conical intersection at a semiconductor defect

Localization of electronic excitations at molecule-sized semiconductor defects often precedes non-radiative (NR) decay, and it is known that many molecules undergo NR decay via conical intersection. Herein, we report the direct simulation of fast and efficient NR decay via a conical intersection at a known semiconductor defect. It is suggested that this silicon epoxide defect may selectively quench photoluminescence (PL) in small silicon nanocrystals (band gap > ~2.8 eV), and thus influence both the observed PL yield and PL energy of oxidized silicon nanocrystals.

A BODIPY fluorescent probe with selective response for hypochlorous acid and its application in cell imaging

A boron-dipyrromethene (BODIPY) derivative with a hydroxymethyl group at the meso-position, 1, was synthesized, which exhibits a highly specific and rapid ‘turn-off’ response for HOCl during confocal fluorescence microscopy experiments under physiological conditions and in live cells. The formation of an electron-withdrawing formyl substituent upon oxidation results in highly efficient nonradiative decay of the S1 state and hence in a drastic decrease in fluorescence emission intensity.

Combined Theoretical and Experimental Study of the Photophysics of Asulam

The photophysics of the neutral molecular form of the herbicide asulam has been described in a joint experimental and theoretical, at the CASPT2 level, study. The unique π → π* aromatic electronic transition (f, ca. 0.5) shows a weak red-shift as the polarity of the solvent is increased, whereas the fluorescence band undergoes larger red-shifts. Solvatochromic data point to higher dipole moment in the excited state than in the ground state (μ(g) < μ(e)). The observed increase in pKa in the excited state (pKa* - pKa, ca. 3) is consistent with the results of the Kamlet-Abboud-Taft and Catalán et al. multiparametric approaches. Fluorescence quantum yield varies with the solvent, higher in water (ϕ(f) = 0.16) and lower in methanol and 1-propanol (approx. 0.02). Room temperature fluorescence lifetime in aqueous solution is (1.0 ± 0.2) ns, whereas the phosphorescence lifetime in glassy EtOH at 77 K and the corresponding quantum yield are (1.1 ± 0.1) s and 0.36, respectively. The lack of mirror image symmetry between modified absorption and fluorescence spectra reflects different nuclear configurations in the absorbing and emitting states. The low value measured for the fluorescence quantum yield is justified by an efficient nonradiative decay channel, related with the presence of an easily accessible conical intersection between the initially populated singlet bright (1)(L(a) ππ*) state and the ground state (gs/ππ*)(CI). Along the main decay path of the (1)(L(a) ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright (1)(L(a) ππ*) to the dark (1)(L(b) ππ*) state, which is responsible for the fluorescence. Additionally, singlet-triplet crossing regions have been found, a fact that can explain the phosphorescent emission detected. An intersystem crossing region between the phosphorescent state (3)(L(a) ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy.

Photoswitchable fluorescent diarylethene derivatives with short alkyl chain substituents

The sulfone derivatives of 1,2-bis(2-alkyl-6-phenyl-1-benzothiophen-3-yl)perfluorocyclopentene having various short alkyl chain substituents at reactive carbons were prepared and the effect of alkyl substitution on the fluorescence property of the closed-ring isomers was studied. Upon irradiation with ultraviolet (UV) light the derivatives exhibit a brilliant green fluorescence under irradiation with visible (> 400 nm) light, while the fluorescence disappears upon irradiation with visible (> 400 nm) light alone. The fluorescence quantum yield of the methyl substituted derivative (1b) dramatically decreases from 0.84 to 0.15 when the solvent is changed from hexane to acetonitrile, while the changes of ethyl, n-propyl and n-butyl substituted derivatives (2b)–(4b) are moderate. The quantum yields of (2b)–(4b) are kept to values close to 0.7 even in polar acetonitrile. The fluorescence lifetime measurement revealed that efficient non-radiative decay processes took place in (1b) in polar solvent, while their contribution to the deactivation was not so large in (2b)–(4b). The neighboring short alkyl chains at the connecting carbons are considered to defend the sulfone units against the attack of polar solvent molecules and weaken the solvent polarity effect.

Spectroscopic properties of 2-phenylbenzothiazole and 1-phenylpyrazole luminophores metalated with platinum metals

The effect of metalation of 2-phenylbenzothiazole and 1-phenylpyrazole with Pd(II), Pt(II), and Rh(III) on the structure and optical properties of luminophore complexes is studied by 1H NMR spectroscopy, IR spectroscopy, and electronic absorption and emission spectroscopy. It is shown that metalation of luminophores leads to the formation of five-membered {M(C∧N)} fragments in the composition of square planar and octahedral complexes, which exhibit a long-wavelength charge-transfer band. The luminescence properties of the complexes are characterized by quenching of the fluorescence and by enhancement of the phosphorescence from the mainly intraligand excited state. Fluorescence quenching of complexes is attributed to the thermally activated energy transfer to metal-centered states and their efficient nonradiative decay.

Effect of Axially Projected Oligothiophene Pendants and Nitro-Functionalized Diimine Ligands on the Lowest Excited State in Cationic Ir(III) bis-Cyclometalates

The novel terthiophene (3T) oligomer 6 and a series of cationic Ir(III) bis-cyclometalates [Ir(C^N)(2)(N^N)]PF(6) 9-12 were prepared. The synthesis, characterization, electrochemical, and photophysical properties are reported. The cyclometalating ligands (C^N) are 2-phenylpyridinato (ppy) or the 3T oligomer (3T-ppy), asymmetrically capped in the 5 and 5" positions with the ppy and mesityl groups. The diimine ligands (N^N) are 2,2'-bipyridine (bpy) or 4-NO(2)-bipyridine (4-NO(2)-bpy). Hybrid metal-organic complexes 11 and 12 bear 3T-pendants ligated through the ppy cap, 10 and 12 contain NO(2) functionalized diimines, whereas 9 contains neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO(2) substituent and pseudo-octahedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing effect of the NO(2) substituent, providing an 850 mV shift toward lower potentials for the first diimine centered reduction of 10 and 12. Strong overlap of the intense π → π* absorptions of the 3T-pendants with Ir(III) charge transfer bands is evident in complexes of 11 and 12, precluding the possibility for selective excitation of either chromophore. Photoexcitation (λ(ex) = 400 nm) of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with φ(em) = 0.11. In stark contrast the NO(2) and 3T functionalized complexes 10-12 display near total quenching of luminescence. Computations of the ground and excited state electronic structure using density functional theory (DFT) and time-dependent DFT (TD-DFT) indicate that both the NO(2) and 3T substituents play an important role in excited state deactivation of complexes 10-12. A substantial electronic contribution of the NO(2) substituent results in stabilization of the diimine based molecular orbital (MO) and offers an efficient nonradiative decay pathway for the excited state. Spin-orbit coupling effects of the Ir(III) ion lead to efficient population of the low lying, nonluminescent, triplet states centered on the 3T-pendants.

Spectroscopic characterization of Tm 3+:TeO 2–K 2O–Nb 2O 5 glasses for 2-μm lasing applications

We have performed detailed experiments to investigate the spectroscopic properties of a new type of tellurite based host doped with thulium: Tm 2O 3:(0.70)TeO 2–(0.15)K 2O–(0.15)Nb 2O 5 having Tm 2O 3 concentrations of 0.125, 0.25, 0.5, and 1.0 mol%. By performing a Judd–Ofelt analysis of the absorption bands, we obtained average radiative lifetimes of 2.57±0.20 and 0.35±0.01 ms for the 3H 4 and 3F 4 levels, respectively. Furthermore, we also observed that an increase in the Tm 2O 3 concentration from 0.125 to 1.0 mol% results in a decrease of the measured fluorescence lifetime from 814 to 439 μs and from 258 to 47 μs for the 3H 4 and 3F 4 levels, respectively, due to efficient non-radiative decay. The highest quantum efficiency of 32% was obtained for the sample doped with 0.125 mol% Tm 2O 3 for the 3H 4 level. Results show that cross relaxation becomes important as the ion concentration is increased, leading to the quenching of the 1460-nm band and enhancement of the 1860-nm emission. The highest emission cross section of 6.85×10 −21 cm 2 measured for the 1860-nm band reveals the potential of this host for the development of 2-μm lasers in bulk glass as well as fiber media.

Picosecond deactivation of azo dye excited states in solution and in cellulose

Ultrafast time-resolved UV–visible absorption spectroscopy has been used to study four model azo dyes and five commercial azo dyes. All of the dyes have been found to show strong bleaching of their ground-state absorption bands on photoexcitation, with recovery lifetimes of 1–5ps in aqueous solution that are attributed to rapid and efficient non-radiative decay; two commercial azo dyes deposited in cellophane have been found to show an additional longer-lived component with a lifetime of >50ps that is attributed to the effects of intermolecular interactions with the cellulosic environment.

Excited-State Intermolecular Proton Transfer of Lumazine

Time-resolved and steady-state emission spectroscopies, as well as absorption spectroscopy, were employed to study the protolytic reactivity of lumazine in both the ground and the excited states. We found that the luminescence of lumazine consists of two emission bands with their respective band maxima at 375 and 465 nm. We attributed these emission bands to the protonated (375 nm) and the deprotonated (465 nm) forms. The protonated form fluoresces with a short decay time of 35 ps and fits the rise-time of the deprotonated form’s fluorescence. A study of the temperature dependence of the time-resolved emission of the protonated band shows that in the wide temperature range of 80−300 K the decay time is only slightly affected by the temperature. On the basis of the experimental results, we propose that an excited-state proton transfer process takes place in lumazine. The weak temperature dependence is explained by an efficient nonradiative decay rather than by a weak temperature dependence of the proton tr...