Intramolecular Energy Transfer Research Articles

Luminescent Properties of Europium(III), Gadolinium(III), and Terbium(III) Complexes with Alkyloxybenzoic Acids

Luminescent properties of europium(III), gadolinium(III), and terbium(III) complexes with 4-dodecyl- and 4-octadecyloxybenzoic acids were studied. Energies of excited triplet states of the acid anions (T1) were determined from phosphorescence spectra of the Gd(III) complexes measured at 77 K. The luminescence mechanism found that intramolecular energy transfer from the organic ligand to the lanthanide ion was effective for the Eu(III) complexes. Complexes of composition LnL3Phen, where Phen is 1,10-phenanthroline, were prepared to increase the luminescence intensity of the Tb(III) complexes. Experimental spectra showed that the integrated luminescence intensity of Tb3+ in TbL3Phen increased as compared with TbL3.

Boosting the efficiency of organic persistent room-temperature phosphorescence by intramolecular triplet-triplet energy transfer

Persistent luminescence is a fascinating phenomenon with exceptional applications. However, the development of organic materials capable of persistent luminescence, such as organic persistent room-temperature phosphorescence, lags behind for their normally low efficiency. Moreover, enhancing the phosphorescence efficiency of organic luminophores often results in short lifetime, which sets an irreconcilable obstacle. Here we report a strategy to boost the efficiency of phosphorescence by intramolecular triplet-triplet energy transfer. Incorpotation of (bromo)dibenzofuran or (bromo)dibenzothiophene to carbazole has boosted the intersystem crossing and provided an intramolecular triplet-state bridge to offer a near quantitative exothermic triplet–triplet energy transfer to repopulate the lowest triplet-state of carbazole. All these factors work together to contribute the efficient phosphorescence. The generation and transfer of triplet excitons within a single molecule is revealed by low-temperature spectra, energy level and lifetime investigations. The strategy developed here will enable the development of efficient phosphorescent materials for potential high-tech applications.

Synthesis, structure, Hirshfeld surface analysis and photophysical studies of red emitting europium acetylacetonate complex incorporating a phenanthroline derivative

Two new octa-coordinated red emitting complexes of the type [Ln(acac)3(Br2-phen)] [acac: acetylacetonate, Br2-phen: 4,7-dibromo-1,10-phenanthroline, Ln: Sm(III) and Eu(III)] have been synthesized and characterized. Single crystal X-ray diffraction (SCXD) of [Eu(acac)3(Br2-phen)] and [Eu(acac)3(H2O)2](H2O) revealed distorted square antiprism structure for both complexes. 2D fingerprint plots and Hirshfeld surface analyses signify important interactions in crystal packing [H⋯H (72.2%), O⋯H/H⋯O (18.9%) and C⋯H/H⋯C (6.7%) contacts for [Eu(acac)3(H2O)2](H2O), while [Eu(acac)3(Br2-phen)] showed H⋯H (40.5%), Br⋯H/H⋯Br (12.9%) and O⋯H/H⋯O (6.5%)] contacts]. Photophysical measurements gave quantum yield (QLnL) of 1.1 and 13% with corresponding luminescence lifetime (τobs) of 38.76 ± 0.61 μs and 790 ± 1.53 μs for [Sm(acac)3(Br2-phen)] and [Eu(acac)3(Br2-phen)], respectively. We have further evaluated photophysical properties of [Eu(acac)3(Br2-phen)] theoretically from the optimized sparkle structure and steady-state emission spectrum of Eu(III) complex. The intramolecular energy transfer (WET) and back-energy transfer (WBT) rate predicted from the calculated singlet (S1) and triplet (T1) levels to the emissive 5D1 and 5D0 states for Eu(III) follow the path S0 → S1 → T1 → 5D1 → 5D0 → 7F0,4. The Eu(III) complex showed large stimulated emission cross-section (σ(0→2) = 1.27 × 10−20 cm2) similar to Nd-glass laser (1.6–4.5 × 10−20 cm2) and offers promise for high-power laser medium.

Real-Time Mapping of Ultratrace Singlet Oxygen in Rat during Acute and Chronic Inflammations via a Chemiluminescent Nanosensor.

Sensing nonradiation-induced singlet oxygen (1 O2 ) in whole-animal is deemed as one of the most challenging tasks in noninvasive techniques due to the µs level lifetime of 1 O2 and quenching by numerous reductants in tissues. Here a distinct chemiluminescent (CL) nanosensor (NTPE-PH) that boasts ultrahigh concentrated CL units in one nanoparticle is reported. Taking advantage of the intramolecular energy transfer mechanism that promises high energy transfer efficiency and the aggregation-induced emission behavior that guarantees high CL amplification, the NTPE-PH sensor is sensitive to a nm level 1 O2 . Experiments demonstrate that the NTPE-PH yields a highly selective CL response toward 1 O2 among common reactive oxygen species. With proved low cytotoxicity and good animal compatibility, real-time mapping of ultratrace 1 O2 in whole-animal during acute and chronic inflammations is first achieved. It is anticipated that the NTPE-PH sensor can be a useful tool for monitoring 1 O2 variation during immune response and pathological processes corresponding to different stimuli, even with drug treatment included.

Photoinduced non-adiabatic energy transfer pathways in dendrimer building blocks.

The efficiency of the intramolecular energy transfer in light harvesting dendrimers is determined by their well-defined architecture with high degree of order. After photoexcitation, through-space and through-bond energy transfer mechanisms can take place, involving vectorial exciton migration among different chromophores within dendrimer highly branched structures. Their inherent intramolecular energy gradient depends on how the multiple chromophoric units have been assembled, subject to their inter-connects, spatial distances, and orientations. Herein, we compare the photoinduced nonadiabatic molecular dynamics simulations performed on a set of different combinations of a chain of linked dendrimer building blocks composed of two-, three-, and four-ring linear polyphenylene chromophoric units. The calculations are performed with the recently developed ab initio multiple cloning-time dependent diabatic basis implementation of the Multiconfigurational Ehrenfest (MCE) approach. Despite differences in short time relaxation pathways and different initial exciton localization, at longer time scales, electronic relaxation rates and exciton final redistributions are very similar for all combinations. Unlike the systems composed of two building blocks, considered previously, for the larger 3 block systems here we observe that bifurcation of the wave function accounted by cloning is important. In all the systems considered in this work, at the time scale of few hundreds of femtoseconds, cloning enhances the electronic energy relaxation by ∼13% compared to that of the MCE method without cloning. Thus, accurate description of quantum effects is essential for understanding of the energy exchange in dendrimers both at short and long time scales.

PREPARATION AND PHOTOLUMINESCENT PROPERTIES OF COMPLEXES OF TERBIUM, EUROPIUM AND THEIR COPRECIPITATES WITH METHYL ETHER 2-HYDROXYBENZOIC ACID

The amorphous coordination compounds of terbium (III) and europium (III) with methyl ester of 2-hydroxybenzoic acid, as well as their mixtures (coprecipitate), in the composition of TbxEu(1-x)L2OH·H2O, are isolated in a solid state by the method of coprecipitation from a weakly basic aqueous-ethanolic medium. According to elemental analysis and pH potentiometric studies, neutralization of cations’ charges is carried out by two molecules of dissociating ligand and one hydroxyl group. The method of thermogravimetric analysis has established that the water molecule that is part of the complexes is coordinated. According to the IR spectroscopy, the coordination of the ligand is carried out with the help of the oxygen atoms of the dissociating hydroxyl and the carbonyl group of the ester fragment. The obtained compounds are not volatile and when the mass spectra are registered in the BBA mode, they are completely destroyed. The complex of terbium (x = 1) in solid state exhibits an effective sensitized luminescence of green color, which is due to the intramolecular transfer of excitation energy from the ligand to the Tb3+ ion. The europium containing complex (x =0), which is isomorphic to terbium one, when excited by UV light, practically does not radiate due to the discrepancy between the energies of the triplet level of the ligand and the emission level of the Eu3+ ion. Sensitized f-f luminescence of the Eu3+ of red color be realized in solid precipitates of the complex mixture (x <1) due to the intermolecular transfer of the excitation energy from the terbium complex (III) to the compound of europium (III) (columinescence). The intensity of the glow of europium (III) depends on the ratio of ion emitters and reaches the maximum value at the values of Tb: Eu = 7: 3. At the same time there is a corresponding decrease in the intensity of the glow of terbium (III).

Energy transfer in and fluorescence studies of the Eu-Tb doped BPA-phen system

ABSTRACTTo enhance the red emission efficiency of Eu3+ complexes, [Eu/Tbx(BPA)3phen] (BPA = bisphenol A, phen = 1,10-phenanthroline) is designed and synthesized. The complexes are characterized fully and their luminescence properties are evaluated. Co-fluorescence is detected in the Eu/Tbx(BPA)3phen complexes, and the existence of Tb3+ enhances the luminescence intensity of the central Eu3+ because of the intramolecular energy transfer from the 5D4 level of Tb3+ to the 5D0 level of Eu3+. The luminescence intensity of europium ions at 615 nm is the highest for Eu/Tb1(BPA)3phen. To improve the thermal stabilities and mechanical properties of pure complexes, Eu/Tb1(BPA)3phen/PAN (PAN = polyacrylonitrile) is used to fabricate fibres by electrospinning. Compared with Eu/Tb1(BPA)3phen (τ = 1.1087 ms), the fibres have a longer fluorescence lifetime of 1.533 ms. The fibres also retain a high quantum yield of 47.16%. Thus, the flexible luminescent fibres have potential applications in many fields.

Conductivity relaxation and photocurrent generation in reduced graphene oxide-poly(9,9′-dioctyl-fluorene-co-bithiophene) composite with application in temperature sensing

The electrical transport properties and photocurrent generation in a reduced graphene oxide-poly(9,9′-dioctyl-fluorene-co-bithiophene) (RGO-F8T2) composite were investigated. The semiconducting nature of the RGO-F8T2 composite was jointly demonstrated by dc and ac conductivity measurements. The dc conductivity obtained from both dc and ac measurements follows the Arrhenius relationship with the activation energy of the order of 80 meV. The RGO-F8T2 composite also showed excellent temperature sensing properties. The temperature coefficient of resistance was compared to commercially available Platinum, Polysilicon, and Germanium temperature sensor. The conductivity relaxation mechanism in the RGO-F8T2 composite depicted the mechanism behind ac conduction. This was due to phonon assisted tunneling between the defect states. The density of states at the Fermi level increases by one order of magnitude for the temperature change of 301 to 433 K. The scaling of conductivity isotherms established the occurrence of intramolecular energy transfer from disordered to ordered chain segments or both in the composite. The photocurrent generation in the RGO-F8T2 composite thin film under simulated solar light illumination was also studied. Here, a linear variation of the photosensitivity with the variation of the incident light intensity was observed.

Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions

AbstractInvitedInvited for the cover of this issue are AnneàKathrine Junker and ThomasàJustàSørensen from University of Copenhagen, Denmark. The cover image shows the fireworks of lanthanide luminescence and illustrates that intramolecular excited state energy transfer happens with a rate constant similar to that of the intermolecular process.

Front Cover: Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions (Eur. J. Inorg. Chem. 9/2019)

Front Cover: Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions (Eur. J. Inorg. Chem. 9/2019)

Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions

Bright lanthanide‐based probes rely on sensitised lanthanide luminescence. The outcome of the excited state energy transfer cascade – from sensitiser chromophore to lanthanide(III) ion – should be the population of the lanthanide excited state with an efficiency equal to unity. This is rarely the case, and this is a particular problem for europium(III) due to the low energy of the Eu(II)/Eu(III) redox couple. Thus, photoinduced electron transfer quenching can be an efficient drain for the excited state energy, reducing the brightness of a probe. Here, the effect of intramolecular PeT in a lanthanide‐based probe is investigated by contrasting intramolecular processes in coumarin‐appended 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetate (DO3A) complexes with intermolecular processes induced through collisional quenching. We find that the excited energy transfer processes involving terbium(III) and europium(III) are slow in the complex when compared to collisional quenching. Further, we show that the quenching event may lead to lanthanide luminescence and suggest that the fate of a PeT quenching event may not be non‐radiative loss of the excitation energy.

Enhancing the electrochemical properties of the p-type conductive polymer on the surface of the new synthesized 2-(pyridin-3-ylmethylene) hydrazine-1-carbothioamide-modified electrode: computational and electrochemical study

Herein, 2-(pyridin-3-ylmethylene)hydrazine-1-carbothioamide(3PHCT)was first fabricated through a chemical method, and subsequently, polyorthoaminophenol (POAP)/3PHCT films were prepared using electropolymerization of POAP, carried out in the presence of the as-prepared 3PHCT to serve as the active electrode for electrochemical application. Geometry optimization and calculations of structural and electronic/vibrational properties of the 3PHCT system are studied (based on the quantum atom-in-molecule theory, QTAIM). In addition, the local intra-molecular electron density, and its Laplacian, and, thus, intra-molecular virial forces, of this molecule system are calculated. These results show that the oxygen and sulfur atoms play domain role in intra-molecular charge and energy transfer in this molecular system. In addition, based on the QTAIM, it is predicted that a single electrochemical molecular system can be grouped into p-type-like and n-type-like sections, and would be good candidate to reduce intra-molecular repulsion of the bipolaron during electrochemical polymerization of POAP. Galvanostatic charge–discharge experiments, cyclic voltammetry and electrochemical impedance spectroscopy, were conducted to characterize the prepared composite films and investigate the performance of the system, respectively. In this study, novel POAP/3PHCT are presented for electrochemical redox capacitors possessing the merits of high active surface area, stability in an aqueous electrolyte and ease of synthesis.

Managing intramolecular energy transfer in well-defined polyfluorenes grafting one/two orange emissive groups on central or terminal fluorene unit

Three sets of polyfluorenes grafting one/two orange emissive groups on central or terminal fluorene unit have been synthesized successfully through the catalyst-transfer polymerization. The molecular weights linearly increase with increase in the feed ratios of [monomer]/[catalyst] in a certain molecular weight range. The initiators with bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) (IPr) as the ancillary ligand show better catalytic activity, leading to higher molecular weight of up to 126.3 kDa. All polymers in toluene have the similar absorption spectra to polyfluorene homopolymer, while the weak absorption bands of 420–500 nm are observed only in the polymers with low molecular weight. Photoluminescence spectra display a dominant blue emission from the polyfluorene backbone and a weak orange emission (508–661 nm) from the orange group. The clearly enhanced orange emission in the polymers with low molecular weight is mainly ascribed to the intramolecular Förster energy transfer from the polyfluorene segment to the orange group, which is further confirmed by the electroluminescent behavior of the polymers.

Systematic Control of the Overlapping Energy Region for an Efficient Intramolecular Energy Transfer: Functionalized Salen-Al/Triphenylamine Guest-Host Assemblies.

A series of triphenylamine (TPA)-containing salen-Al assembly dyads, [salen(3- tBu-5-R)2Al(OC6H4- p-N(C6H5)2)] [salen = N, N'-bis(salicylidene)ethylenediamine; R = H (D1), tBu (D2), Ph (D3), OMe (D4), and NMe2 (D5)], were prepared in good yield (50-80%) and fully characterized by NMR spectroscopy and elemental analysis. Both the UV/vis absorption and photoluminescence (PL) spectra of D1-D4, except for D5, in a tetrahydrofuran solution exhibited dual patterns, which are assignable to the salen-Al-centered π-π* transition (low-energy region) and the TPA-centered π-π* transition (high-energy region). In particular, the emission spectra of the dyads displayed interesting dual-emissive patterns via a significant intramolecular energy transfer (IET) process between the salen-Al moiety and TPA group. Notably, this IET process was systematically tuned by varying the substituents and dominantly observed in the rigid state. More interestingly, compared to the salen-Al complexes (A1-A4) without the TPA group, D1-D4 exhibited enhanced quantum efficiencies. Time-dependent density functional theory calculations on the S1-optimized structures of D1-D5 further supported these experimental results by indicating the existence of independent transition states between the salen-Al moiety and TPA group in the assembly dyads. The present study reports the first example of salen-Al complexes bearing electron-rich TPA moieties.

Multicolor upconversion luminescence of dye-coordinated Er3+ at the interface of Er2O3 and CaF2 nanoparticles

ABSTRACT Multicolor upconversion luminescence of Er3+ was successfully enhanced by optimizing the interface in dye-coordinated nanoparticles with a core/shell structure. Red and green upconversion emissions of Er3+ were obtained at the interface of oxide nanoparticles via the intramolecular energy transfer from the coordinating squaraine dye with high light-absorption ability, which was more efficient than emissions through the energy transfer from metal ions such as Yb3+. Additionally, CaF2 nanoparticles as a core material minimized the energy loss with nonradiative downward relaxations in Er3+, resulting in the observation of unusual blue upconversion emissions from the upper energy level of Er3+ by nonlaser excitation using a continuous-wave (CW) Xe lamp at an excitation power of 1.2 mW/cm2.

Effect of Molecular Conformation Restriction on the Photophysical Properties of N^N Platinum(II) Bis(ethynylnaphthalimide) Complexes Showing Close-Lying 3MLCT and 3LE Excited States.

Using naphthalimide (NI), complexes (Pt-PhNI and Pt-PhMeNI) based on the N^N platinum(II) bis(phenylacetylide) coordination framework were prepared, in which there are two close-lying triplet states, i.e., the metal-to-ligand-charge-transfer (3MLCT) and the NI localized emissive state (3LE). Pt-PhNI has better electronic communication between the Pt coordination center and the NI moiety, whereas in Pt-PhMeNI, they are more isolated by orthogonal geometry. For Pt-PhMeNI, the S0 → 1MLCT and S0 → 1LE absorption bands are separated by 5655 cm-1, while they are more overlapped in Pt-PhNI. The 3MLCT → S0 and 3LE → S0 dual phosphorescence emissions were observed for both Pt-PhNI (in toluene) and Pt-PhMeNI (in benzonitrile). The molecular conformation tunes the 3MLCT/3LE state population ratio, and the orthogonal geometry makes the 3LE state in Pt-PhMeNI basically a dark state (in toluene). Switching of the relative energy levels of the 3MLCT/3LE states by variation of the solvent polarity and temperature was achieved. For Pt-PhMeNI, the energy level of 3MLCT state is higher in a polar solvent; thus, the 3MLCT emission decreases, while the phosphorescence lifetime is prolonged from 9.5 μs (in toluene) to 58 μs (in benzonitrile) because of the different equilibria with the nonemissive 3LE state. Conversely, increasing the temperature enhances the upward transition from the nonemissive 3LE state to the emissive 3MLCT state; as such, the phosphorescence of Pt-PhMeNI was intensified at higher temperature (which is unusual), and the phosphorescence lifetime decreased from 58 μs (298 K) to ca. 5 μs (348 K). The ultrafast intersystem crossing (ca. 0.5 ps) and intramolecular triplet-triplet energy transfer (3-11 ps) were studied by femtosecond transient absorption spectroscopy. These results are useful for an in-depth understanding of the photophysics of multichromophore transition-metal complexes and for the design of external stimuli-responsive sensing materials, for instance, temperature or microenvironment sensing materials.

Investigation of ultrafast energy transfer mechanism in BODIPY–Porphyrin dyad system

Novel β-fused BODIPY-Porphyrin compounds that contain free base porphyrin (TPP2BDP) and its Ni(II) complex (NiTPP2BDP) were synthesized to investigate intramolecular energy transfer mechanisms of β-fused BODIPY-porphyrin dyads and effect of the unfilled d shell metal ion on energy transfer mechanism. Fluorescence spectra of compounds exhibited that BODIPY emission was diminished upon excitation of the BODIPY unit because of the energy transfer from BODIPY to porphyrin unit. Femtosecond pump-probe spectroscopy measurements revealed that energy transfer of investigated compounds are faster than previous studies in literature. Rapid energy transfer (about 500 fs) from BODIPY to porphyrin was observed for both compounds when BODIPY unit is excited due to direct linkage of BODIPY to porphyrin unit. Intersystem crossing mechanism was also observed for the compound that contains free base porphyrin (TPP2BDP), whereas d-d transition was observed for the compound that contains metalloporphyrin (NiTPP2BDP) due to unfilled d orbital of Ni(II) ion.

Ultrafast intramolecular energy transfer in a nanostructured organosilicon luminophore based on p-terphenyl and 1,4-bis(5-phenyloxazol-2-yl)benzene

Förster theory allows quantitative description of the ultrafast intramolecular energy transfer observed experimentally within nanostructured organosilicon luminophores (NOLs).

Construction of a luminescent sensor based on a lanthanide complex for the highly efficient detection of methyl parathion.

A highly sensitive and selective luminescent sensor for the detection of methyl parathion (MP) pesticide was described in this study. The target molecule HL was synthesized by modifying the structure of 4-hydroxybenzlidene imidazolinone (HBI) with nitrogen-containing heterocyclic 1,10-phenanthroline. In the presence of Eu3+, a HL–Eu3+ complex was formed which could emit strong red fluorescence due to the removal of coordinated water molecules and an intramolecular energy transfer from HL to Eu3+. Addition of MP into the strongly fluorescent solution of HL–Eu3+ induced quenching of the complex's fluorescence, and this quenching behavior occurred because of the competition coordination of MP and HL for Eu3+. A calibration curve was developed that related the extent of fluorescence quenching to MP concentration, making the HL–Eu3+ system a sensitive and selective fluorescent sensor for MP. Under the experimental conditions, the detection limit for MP was down to 95 nM based on LOD = 3σ/S. Moreover, the fluorescence assay developed here allowed the detection of MP in two different types of real samples including pond water and pear juice, and satisfactory results demonstrate that this fluorescent sensor based on HL–Eu3+ has potential application in environment and food analysis.

Direct evidence of solvent polarity governing the intramolecular charge and energy transfer: ultrafast relaxation dynamics of push-pull fluorene derivatives.

Photoinduced charge and energy transfer are significant photophysical processes controlling the efficiency of photosynthesis and molecular electronics. Here, the influence of solvent polarity and excitation wavelength on the dynamics of excited state relaxation pathways of a push-pull chromophore (PXFCN), where phenoxazine and cyano fluorene acted as a donor and an acceptor, respectively, is investigated in detail by using steady state spectroscopy, nanosecond and femtosecond transient absorption spectroscopy and picosecond emission spectroscopy. In acetonitrile (ACN), the steady state emission spectra of PXFCN exhibited three maxima at around 330, 405 and 620 nm covering the complete continuum range (CIE coordinates of 0.32, 0.40) with an absolute quantum yield of ≈0.12. The aggregation induced emission with an increased quantum yield of ≈0.32 was observed in a tetrahydrofuran and water mixture due to the formation of nano-aggregates. Interestingly the steady state and time resolved emission spectra of PXFCN in ACN obtained upon excitation at different wavelengths revealed the presence of both intramolecular charge and energy transfer processes, whereas in cyclohexane (CHX) the emission originated mainly from the local excited state revealing efficient intramolecular energy transfer. The femtosecond transient absorption spectrum in the polar solvent, ACN, shows that the excited state relaxation pathway is controlled by solvent stabilized twisted intramolecular charge transfer dynamics limiting the formation of the triplet state. However in the case of CHX, the charge transfer state formed upon photoexcitation decayed to the triplet state by geminate charge recombination. The nanosecond transient absorption spectra manifest the dominant feature of the triplet state and the charge transfer state in CHX and ACN, respectively, and their complete dynamics were obtained. Thus based on the transient absorption and emission spectra, it is inferred that the intramolecular charge transfer occurring along with the energy transfer is controlled by the polarity of the solvent through conformational changes leading to a favourable position yielding the charge and energy transfer between the donor and acceptor moieties.