Excited State Emission Research Articles

Selective Dual-Channel Imaging on Cyanostyryl-Modified Azulene Systems with Unimolecularly Tunable Visible-Near Infrared Luminescence.

Although organic light-emitting molecules have received a growing attention and applicability in modern bioimaging science, the design and control of complex photoluminescent properties in unimolecularly selective imaging remains a challenging topic. Considering that tunable multipathway imaging can be advantagedly connected with treatment processes in therapy, the integration of an azulene and a cyanostyryl moiety into one skeleton is carried out for the generation of in situ stimuli-responsive luminescent materials, with the aim to achieve tunable and effective emissions in distinct channels through smart molecular design on a single-molecular platform. This strategy takes advantage of 1) the Z/E isomerization of the cyanostyryl unit that can vary the push-pull effect of the substitution on azulene, accompanied by altering absorption and emission of individual excited states, and 2) an optimized excited-state regulation for opening a near infrared emissive channel and making up for a controllable dual-pathway luminescent system together with the utilization of visible emission. As exemplified by a demonstration of manipulating the luminescence at the cell level, the materials exhibit a superior application potential for unimolecularly selective imaging, labeling and probing events.

Luminescent Eu3+ and Tb3+ complexes of 4-aminophenyl terpyridine (ptpy): Photophysical aspects, DNA and serum protein binding properties

Two lanthanide complexes namely [Eu(ptpy)2Cl2]Cl (1) and [Tb(ptpy)Cl2]Cl (2) where ptpy is 4′-(4-aminophenyl)-2,2′:6′,2′′-terpyridine were synthesized, and photophysical aspects, speciation in solution, excited state properties, and binding interactions with DNA and protein were studied. Here ptpy acts as light-harvesting photosensitizing antenna moiety for Ln3+ luminescence. The bright luminescence in 1 and 2 suggests favorable photo-induced energy transfer happens from the ptpy antenna to the Ln3+ to fill long-lived emissive excited states of Eu3+ and Tb3+ and respective f→f transitions. The time-gated luminescence spectral profile of complexes 1 and 2 show characteristic sharp multiple emission bands assigned to the 5D0→7FJ and 5D4→7FJf→f transitions originating from Eu3+ and Tb3+ ions respectively. Inner-sphere hydration number (q) in 1 and 2 was determined from the excited states lifetime measurements in H2O and D2O confirming possible ligand-displacement reactions with H2O in solution (q=1.35 (1), 1.86 (2)). The studies on binding of the compounds with DNA suggesting partial intercalation with planar ptpy ligands present in 1 and 2 with the planar DNA base pairs (Kb~105M−1). Complexes also showed strong interaction to the serum albumin proteins (BSA and HSA) (K~105M−1) through tryptophan fluorescence quenching studies due to perturbation in secondary structure of protein.

Scheme for quantum state manipulation in coupled cavities

By controlling the parameters of the system, the effective interaction between different atoms is achieved in different cavities. Based on the interaction, scheme to generate three-atom Greenberger–Horne–Zeilinger (GHZ) is proposed in coupled cavities. Spontaneous emission of excited states and decay of cavity modes can be suppressed efficiently. In addition, the scheme is robust against the variation of hopping rate between cavities.

Red-Emitting Ruthenium(II) and Iridium(III) Complexes as Phosphorescent Probes for Methylglyoxal in Vitro and in Vivo.

Transition-metal complexes, ruthenium(II) and iridium(III) complexes in particular, with fascinating triplet emissions are rapidly emerging as important phosphorescent dyes for application in the sensing and imaging of biological makers in live cells and organisms. In this contribution, two red-emitting transition-metal complexes, [Ru(bpy)2(DA-phen)](PF6)2 and [Ir(ppy)2(DA-phen)](PF6) (bpy = 2,2'-bipyridine, DA-phen = 4,5-diamino-1,10-phenanthroline, and ppy = 2-phenylpyridine), were designed and synthesized as phosphorescent probes for the highly sensitive and selective detection of methylglyoxal (MGO), an essential biomarker in the etiopathogenesis of several diseases. Both probes showed weak emissions in aqueous media because of the existence of an effective photoinduced-electron-transfer process, while their emissions could be remarkably enhanced upon the addition of MGO. The photophysical and electrochemical properties, as well as phosphorescent responses of the probes toward MGO, were examined. The ground- and excited-state properties of the probes and their reaction products with MGO, [Ru(bpy)2(MP-phen)](PF6)2 and [Ir(ppy)2(MP-phen)](PF6) (MP-phen = 2-methylpyrazino-1,10-phenanthroline), the sensing mechanism, and several important experimental facts were investigated and validated using density functional theory (DFT)/time-dependent DFT computations. The results indicated that the phosphorescence switch-ON is due to the elimination of electron transfer and followed the reestablishment of emissive triplet excited states. To evaluate the feasibility of [Ru(bpy)2(DA-phen)](PF6)2 and [Ir(ppy)2(DA-phen)](PF6) as bioprobes, their cytotoxicity was examined, and their applicability for visualizing intracellular and in vivo MGO was demonstrated.

Tuning excited-state-intramolecular-proton-transfer (ESIPT) process and emission by cocrystal formation: a combined experimental and theoretical study.

The formation of two-component molecular cocrystals can lead to the tunable excited state intramolecular proton transfer (ESIPT) process and emission, as first confirmed by both experimental and computational studies.

Fluorescence quenching of polycyclic aromatic hydrocarbons within deep eutectic solvents and their aqueous mixtures

Two common and popular deep eutectic solvents (DESs) composed of the salt choline chloride and H-bond donors glycerol and urea in 1:2mol ratio named glyceline and reline, respectively, are investigated for the analysis of polycyclic aromatic hydrocarbons (PAHs) using quenching of both steady-state and time-resolved fluorescence of ten different PAHs by nitromethane at 30°C. Based on their quenching efficiencies, the PAHs are divided into two groups – group 1 is constituted of the five PAHs whose fluorescence are quenched less effectively by nitromethane whereas the other five exhibiting high quenching efficiency are associated to group 2. Quenching of steady-state fluorescence of group 1 PAHs by nitromethane, albeit not very significant, follow a simple Stern-Volmer behavior. The excited-state emission intensity decay of these PAHs, in both absence and presence of nitromethane, fit best to a single exponential model with small but monotonic decrease in lifetimes. The decrease in lifetime also follows Stern-Volmer behavior, however, the quenching constants (KD) are lower than those obtained from steady-state fluorescence (KSV). This is ascribed to the possible formation of charge-transfer complex between the PAH and the nitromethane. Steady-state fluorescence quenching of group 2 PAHs exhibit distinct upward curvature from linear Stern-Volmer behavior implying highly efficient quenching. The intensity decay fits best to a double exponential decay model with longer of the decay times following simple Stern-Volmer behavior. Formation of a complex or the presence of nitromethane within the quenching sphere of action of the PAH having short decay time is proposed. Quenching behavior was found to be similar irrespective of the identity of the DES. A representative group 2 PAH, pyrene, is employed to investigate diffusion dynamics within aqueous mixtures of the two DESs. The bimolecular quenching rate constant (kq) is found to increase linearly with the mole fraction of water (xw) in aqueous mixtures of both the DESs. Stokes-Einstein relationship is not obeyed within the DES aqueous mixtures and is attributed to be due to the inherent heterogeneity of the mixtures. Diffusion dynamics of pyrene-nitromethane pair is compared with that of a tracer fluorescein isothiocyanate obtained using fluorescence correlation spectroscopy. Good correlation between the two highlights generality of diffusion dynamics with aqueous DES mixtures as far as solutes are concerned.

Tunneling in the Delayed Luminescence of Colloidal CdSe, Cu+-Doped CdSe, and CuInS2 Semiconductor Nanocrystals and Relationship to Blinking

The photoluminescence decay dynamics of colloidal CdSe, Cu+:CdSe, and CuInS2 nanocrystals have been examined as a function of temperature and magnetic field. All three materials show photoluminescence decay on time scales significantly longer than the intrinsic lifetimes of their luminescent excited states, i.e., delayed luminescence, involving formation of a metastable trapped excited state followed by detrapping to re-form the emissive excited state. Surprisingly, the delayed luminescence decay kinetics are nearly identical for these three very different materials, suggesting they reflect universal properties of the delayed luminescence phenomenon in semiconductor nanocrystals. By measuring luminescence decay over 8 decades in time and 6 decades in intensity, we observe for the first time a clear deviation from power-law dynamics in delayed luminescence. Furthermore, for all three materials, the delayed luminescence decay dynamics are observed to be nearly independent of temperature between 20 K and roo...

Butterfly Deformation Modes in a Photoexcited Pyrazolate-Bridged Pt Complex Measured by Time-Resolved X-Ray Scattering in Solution.

Pyrazolate-bridged dinuclear Pt(II) complexes represent a series of molecules with tunable absorption and emission properties that can be directly modulated by structural factors, such as the Pt-Pt distance. However, direct experimental information regarding the structure of the emissive triplet excited state has remained scarce. Using time-resolved wide-angle X-ray scattering (WAXS), the excited triplet state molecular structure of [Pt(ppy)(μ-t-Bu2pz)]2 (ppy = 2-phenylpyridine; t-Bu2pz = 3,5-di-tert-butylpyrazolate), complex 1, was obtained in a dilute (0.5 mM) toluene solution utilizing the monochromatic X-ray pulses at Beamline 11IDD of the Advanced Photon Source. The excited-state structural analysis of 1 was performed based on the results from both transient WAXS measurements and density functional theory calculations to shed light on the primary structural changes in its triplet metal-metal-to-ligand charge-transfer (MMLCT) state, in particular, the Pt-Pt distance and ligand rotation. We found a pronounced Pt-Pt distance contraction accompanied by rotational motions of ppy ligands toward one another in the MMLCT state of 1. Our results suggest that the contraction is larger than what has previously been reported, but they are in good agreement with recent theoretical efforts and suggest the ppy moieties as targets for rational synthesis aimed at tuning the excited-state structure and properties.

NLO active CuII/RuII and CdII/RuII coordination-organometallic complexes: synthesis, structural characterization and photoluminescence properties

CuII/RuII and CdII/RuII hybrid complexes [Cu(L1–3)(NC5H4C≡CRu(dppe)2Cl)] (1a-3a) and [Cd(L1-3)(NC5H4C≡CRu(dppe)2Cl)] (1b–3b) have been prepared by reaction of trans-[RuCl(dppe)2(C≡C-py-3)] (1) with copper or cadmium acetate in the presence of Schiff base ligands LH1–3 (where LH = 2-(pyrrole-2-yl-methylidine)aminophenol (LH1), 5-bromo-2-(pyrrole-2-yl-methylidine)aminophenol (LH2) and 5-nitro-2-(pyrrole-2-yl-methylidine)aminophenol (LH3)). The hybrid materials were characterized on the basis of elemental analyses, TEM, IR, UV–visible, 1H NMR, and 31P NMR spectral studies. TEM overview observations revealed well-dispersed spherical nanoparticles of ~60 nm are formed. Quasireversible redox behavior is observed for CuII/RuII complexes corresponding to CuI/CuII and RuII/RuIII couples. All the complexes exhibit blue-green emission as a result of fluorescence from the intraligand (π → π*) emission excited state with good quantum yield. The second-order nonlinear optical (NLO) properties of CuII/RuII and CdII/RuII complexes have been investigated by the Kurtz-powder method. The second harmonic generation efficiency of these complexes show that these complexes are NLO active and display good second-order nonlinear optical activity.

Assessing One- and Two-Photon Optical Properties of Boron Containing Arenes

Linear and nonlinear optical properties of a series of bis(E-dimesitylborylethenyl)-substituted arenes have been modeled by high-level computational protocols. The former compounds show a remarkable interest as infrared two-photon absorbers and hence may be used in the field of optical active and smart materials or for energy storage purposes. Excited state topologies, absorption and emission spectra, excited state metrics, natural transition orbitals and two-photon absorption cross-section of a series of chromophores have been computed by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). An extended benchmark test on the performance of different functionals had been performed. Dynamic and vibronic effects on absorption and emission spectra have been taken into account by sampling the conformational space by means of Wigner distribution and the former have been evidenced as rather important in order to recover absorption maxima and spectral band shape. Important infrared two photon...

Are Very Small Emission Quantum Yields Characteristic of Pure Metal-to-Ligand Charge-Transfer Excited States of Ruthenium(II)-(Acceptor Ligand) Chromophores?

Metal to ligand charge-transfer (MLCT) excited state emission quantum yields, ϕem, are reported in 77 K glasses for a series of pentaammine and tetraammine ruthenium(II) complexes with monodentate aromatic acceptor ligands (Ru-MDA) such as pyridine and pyrazine. These quantum yields are only about 0.2-1% of those found for their Ru-bpy (bpy = 2,2'-bipyridine) analogs in similar excited state energy ranges (hνem). The excited state energy dependencies of the emission intensity are characterized by mean radiative decay rate constants, kRAD, resolved from ϕem/τobs = kRAD (τobs = the observed emission decay lifetime; τobs(-1) = kRAD + kNRD; kNRD = nonradiative decay rate constant). Except for the Ru-pz chromophores in alcohol glasses, the values of kNRD for the Ru-MDA chromophores are slightly smaller, and their dependences on excited state energies are very similar to those of related Ru-bpy chromophores. In principle, one expects kRAD to be proportional to the product of (hνem)(3) and the square of the transition dipole moment (Me,g).(2) However, from experimental studies of Ru-bpy chromophores, an additional hνem dependence has been found that originates in an intensity stealing from a higher energy excited state with a much larger value of Me,g. This additional hνem dependence is not present in the kRAD energy dependence for Ru-MDA chromophores in the same energy regime. Intensity stealing in the phosphorescence of these complexes is necessary since the triplet-to-singlet transition is only allowed through spin-orbit coupling and since the density functional theory modeling implicates configurational mixing between states in the triplet spin manifold; this is treated by setting Me,g equal to the product of a mixing coefficient and the difference between the molecular dipole moments of the states involved, which implicates an experimental first order dependence of kRAD on hνem. The failure to observe intensity stealing for the Ru-MDA complexes suggests that their weak emissions are more typical of "pure" (or unmixed) (3)MLCT excited states.

Single-Particle Photoluminescence Spectra, Blinking, and Delayed Luminescence of Colloidal CuInS2 Nanocrystals

Single-nanocrystal and ensemble photoluminescence measurements on CuInS2 semiconductor nanocrystals reveal luminescence bandshapes that are broad compared to those typical of individual II–VI or related semiconductor nanocrystals. This finding is consistent with the hypothesis of strong electron–phonon coupling in the emissive excited state of these CuInS2 semiconductor nanocrystals. Blinking is observed that resembles that of other semiconductor nanocrystals. Ensemble luminescence measurements also reveal the existence of a remarkably long-lived excited state in these nanocrystals that continues to emit photons over several orders of magnitude in time following the excitation pulse. The delayed luminescence overlaps in time and shows similar distributed kinetics to the blinking “off” times of the same nanocrystal sample, supporting the proposal that these two phenomena arise from the same microscopic carrier-trapping and -detrapping processes. Excitation power dependence measurements illustrate that the delayed luminescence saturates at very low emission intensities under the excitation power densities used in the single-nanocrystal measurements, consistent with this metastable charge-trapped state being the “off” state of the luminescence blinking cycle.

Evidence for Cation-Controlled Excited-State Localization in a Ruthenium Polypyridyl Compound.

The visible absorption and photoluminescence (PL) properties of the four neutral ruthenium diimine compounds [Ru(bpy)2(dcb)] (B2B), [Ru(dtb)2(dcb)] (D2B), [Ru(bpy)2(dcbq)] (B2Q), and [Ru(dtb)2(dcbq)] (D2Q), where bpy is 2,2'-bipyridine, dcb is 4,4'-(CO2(-))2-bpy, dtb is 4,4'-(tert-butyl)2-bpy, and dcbq is 4,4'-(CO2(-))2-2,2'-biquinoline, are reported in the presence of Lewis acidic cations present in fluid solutions at room temperature. In methanol solutions, the measured spectra were insensitive to the presence of these cations, while in acetonitrile a significant red shift in the PL spectra (≤1400 cm(-1)) was observed consistent with stabilization of the metal-to-ligand charge transfer (MLCT) excited state through Lewis acid-base adduct formation. No significant spectral changes were observed in control experiments with the tetrabutylammonium cation. Titration data with Li(+), Na(+), Mg(2+), Ca(2+), Zn(2+), Al(3+), Y(3+), and La(3+) showed that the extent of stabilization saturated at high cation concentration with magnitudes that scaled roughly with the cation charge-to-size ratio. The visible absorption spectra of D2Q was particularly informative due to the presence of two well-resolved MLCT absorption bands: (1) Ru → bpy, λmax ≈ 450 nm; and (2) Ru → dcbq, λmax ≈ 540 nm. The higher-energy band blue-shifted and the lower-energy band red-shifted upon cation addition. The PL intensity and lifetime of the excited state of B2B first increased with cation addition without significant shifts in the measured spectra, behavior attributed to a cation-induced change in the localization of the emissive excited state from bpy to dcb. The importance of excited-state localization and stabilization for solar energy conversion is discussed.

Energy exchange between modes in a multimode two-color quantum dot laser with optical feedback.

We investigate experimentally and theoretically the multimode dynamics of a two-color quantum dot laser subject to time-delayed optical feedback. We unveil energy exchanges between the longitudinal modes of the excited state triggered by variations of the feedback phase, and observe that the modal competition between longitudinal modes appears independently within the ground state and excited state emission. These features are accurately reproduced with a quantum dot laser model extended to take into account multiple modes for both ground and excited states. Finally, we discuss the significant impact of such behavior on feedback-based control of two-color quantum dot lasers.

PH Dependence of Absorption and Emission Spectra of Ru(phen)2(phenOH)2+(PF6)2 Complex

A ruthenium complex of the 4-hydroxy- 1,10-phenanthroline ligand was synthesized, and the variation of its absorption and emission intensity and litetime with pH characterized. Excited state lifetime, luminescence intensity, and emission properties were determined. The complex exhibits a maximum at 460nm, and a small red shift at higher pH. The spectra show a well defined isobestic point. Luminescence intensity exhibited a sigmoidal relationship with pH, a behaviour that is similar to those of other ruthenium complexes carrying protonable functional groups. This characteristic is suggestive of the suitability of this complex for pH sensor design for medical practices as well as industrial processes. pH dependence is more evident in the emission than the absorption spectra, a behaviour characteristic of higher pH dependence on the excited than the ground state of the complex. Emission lifetimes of 165.4ns and 3.08ns for the protonated and deprotonated states respectively, were determined and the pK * a value calculated as 3.68.

Ru(Me4phen)2dppz](2+), a Light Switch for DNA Mismatches.

[Ru(Me4phen)2dppz](2+) serves as a luminescent "light switch" for single base mismatches in DNA. The preferential luminescence enhancement observed with mismatches results from two factors: (i) the complex possesses a 26-fold higher binding affinity toward the mismatch compared to well-matched base pairs, and (ii) the excited state emission lifetime of the ruthenium bound to the DNA mismatch is 160 ns versus 35 ns when bound to a matched site. Results indicate that the complex binds to the mismatch through a metalloinsertion binding mode. Cu(phen)2(2+) quenching experiments show that the complex binds to the mismatch from the minor groove, characteristic of metalloinsertion. Additionally, the luminescence intensity of the complex with DNA containing single base mismatches correlates with the thermodynamic destabilization of the mismatch, also consistent with binding through metalloinsertion. This complex represents a potentially new early cancer diagnostic for detecting deficiencies in mismatch repair.

Long‐Lived, Emissive Excited States in Direct and Amide‐Linked Thienyl‐Substituted Ru II Complexes

The excited state behavior of a new series of homoleptic and heteroleptic RuII complexes bearing thienyl groups appended to a 2,2′-bipyridine chelating ligand via direct, secondary and tertiary amide linkages is examined. The results of nanosecond transient absorption spectroscopy, emission lifetime measurements and bimolecular quenching experiments are correlated to determine that although the amide linkage does not act as a conjugated bridge to the peripheral substituents, it does not preclude possible electron transfer processes. Complexes bearing directly bound thienyl and bithienyl substituents exhibit long excited state and emission lifetimes (τem = 2 and 15 µs), with high emission quantum yields in solution (Φ = 0.35) and slow rates of non-radiative decay.

Cover Picture: Long‐Lived, Emissive Excited States in Direct and Amide‐Linked Thienyl‐Substituted Ru II Complexes (Eur. J. Inorg. Chem. 10/2016)

Cover Picture: Long‐Lived, Emissive Excited States in Direct and Amide‐Linked Thienyl‐Substituted Ru ^II Complexes (Eur. J. Inorg. Chem. 10/2016)

P-State Luminescence in CdSe Nanoplatelets: Role of Lateral Confinement and a Longitudinal Optical Phonon Bottleneck.

We evidence excited state emission from p states well below ground state saturation in CdSe nanoplatelets. Size-dependent exciton ground and excited state energies and population dynamics are determined by four independent methods: time-resolved PL, time-integrated PL, rate equation modeling, and Hartree renormalized k·p calculations-all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Depending on its detuning to the LO phonon energy, the PL decay of CdSe platelets is governed by a size tunable LO phonon bottleneck, related to the low exciton-phonon coupling, very large oscillator strength, and energy spacing of both states. This is, for instance, ideal to tune lasing properties. CdSe platelets are perfectly suited to control the exciton-phonon interaction by changing their lateral size while the optical transition energy is determined by their thickness.

Excited-state intramolecular single and double proton transfer emission of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

In this work, excited-state intramolecular single proton transfer emission (S-PTE) and double proton transfer emission (D-PTE) are presented by employing 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (1) as a prototype. 1 has been strategically designed and synthesized in an aim to explore single proton transfer and double proton transfer in the excited state. In solution 1 exhibits the lowest lying S0 → S1 absorption at ∼395 nm. Upon excitation, two large Stokes shifted emission bands maximized at 475 nm and 550 nm are resolved, which are ascribed to the tautomer emission resulting from single proton transfer and double proton transfer isomers, respectively. It is found that solvents have greatly influenced on excited-state intramolecular proton transfer emission.