Intramolecular Energy Transfer Rates Research Articles

Quantum–Chemical Study of the Photophysical Behavior of Mesogenic Europium(III) Complexes with β‐Diketones and Lewis Bases

AbstractMetal‐containing liquid crystals such as Ln(III) complexes possess unique structural, liquid‐crystalline (LC), optical, and magnetic properties and represent modern cutting‐edge multifunctional materials. Application of mesogenic Ln(III) complexes is hindered by their nontrivial structure–property relationships. These relationships, in turn, depend on various factors, including insufficiently studied physicochemical processes. Although a proper Ln(III) element and the respective ligand environment are selected prior to synthesis of such complexes, the structural features of the coordination polyhedra, especially upon photoexcitation, are not uniquely defined. Therefore, this work focuses on the development of theoretical approaches to creating multifunctional materials represented by highly luminescent mesogenic Eu(III) complexes with β‐diketones and Lewis bases. The relationships between their structure, the parameters of Voronoi–Dirichlet polyhedra, the luminescence efficiency, and the LC properties were considered. The calculated excited states and intramolecular energy transfer rates were used to determine intramolecular energy transfer channels. The LC behavior of the studied materials was shown to mainly depend on the ligand environment, whereas their optical properties were found to be mostly governed by the coordination polyhedra.

The effect of the outer-sphere cations on the photophysical and magnetic properties of rare earth complexes with 2,2,2-trichloro-N-(diphenylphosphoryl)acetamide

Two series of rare earth coordination compounds have been synthesized (Na[RE(L4)] (labeled as 1RE) and PPh4[RE(L4)] (labeled as 2RE), where RE = Y3+, Eu3+, Tb3+, L = 2,2,2-trichloro-N-(diphenylphosphoryl)acetamide) to determine the possibility of modifying their photophysical and magnetical properties by changing the counterion. The crystal structure of 1RE was determined based on the single-crystal X-ray diffraction and the crystal structure of 2RE was determined based on quantum chemistry computational procedures. Characterization of the physicochemical properties of the compounds was carried out using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), 1H and 31P NMR spectroscopy as well as FT-IR, absorption and luminescence spectroscopy in the temperature range of 300 - 77 K. The rate constants of radiative (Arad) and non-radiative (Anrad) transitions, intrinsic (QLnLn) and overall (QLnL) emission quantum yields, sensitization efficiency (ηsens), the experimental and theoretical intensity parameters (Ωλ), the forward (WS, WT) and backward (WbS, WbT) intramolecular energy transfer (IET) rates were determined. Magnetic properties of Tb3+ compounds were studied in a constant field of 0.5 T in the temperature range of 1.8–300 K. Dynamic AC magnetic susceptibility measurements were carried out as a function of temperature and frequency for 1Tb and 2Tb. Only in the case of 2Tb, in the presence of an external DC field, a slight temperature dependence of in-phase χ′ and out-of-phase χ'' susceptibility was recorded. Based on experimental and theoretical results, the significant effect of counterion on the photophysical and magnetic properties of RE chelates has been demonstrated and clarified, providing valuable guidance for the design of bifunctional electromagnetic radiation converters.

Luminescent Ln3+-based silsesquioxanes with a β-diketonate antenna ligand: toward the design of efficient temperature sensors.

We report the synthesis and single-crystal X-ray diffraction, magnetic, and luminescence measurements of a novel family of luminescent cage-like tetranuclear silsesquioxanes (PhSiO1.5)8(LnO1.5)4(O)(C5H8O2)6(EtOH)2(CH3CN)2⋅2CH3CN (where Ln = Tb, 1; Tb/Eu, 2; and Gd, 3), featuring seven-coordinated lanthanide ions arranged in a one-capped trigonal prism geometry. Compounds 1 and 2 exhibit characteristic Tb3+ and Tb3+/Eu3+-related emissions, respectively, sensitized by the chelating antenna acetylacetonate (acac) ligands upon excitation in the UV and visible spectral regions. Compound 3 is used to assess the energies of the triplet states of the acac ligand. For compound 1, theoretical calculations on the intramolecular energy transfer and multiphonon rates indicate a thermal balance between the 5D4 Stark components, while the mixed Tb3+/Eu3+ analog 2, with a Tb:Eu ratio of 3:1, showcases intra-cluster Tb3+-to-Eu3+ energy transfer, calculated theoretically as a function of temperature. By utilizing the intensity ratio between the 5D4→7F5 (Tb3+) and 5D0→7F2 (Eu3+) transitions in the range 11-373K, we demonstrate the realization of a ratiometric luminescent thermometer with compound 2, operating in the range 11-373K with a maximum relative sensitivity of 2.0% K-1 at 373K. These findings highlight the potential of cage-like silsesquioxanes as versatile materials for optical sensing-enabled applications.

Dynamics of the Energy Transfer Process in Eu(III) Complexes Containing Polydentate Ligands Based on Pyridine, Quinoline, and Isoquinoline as Chromophoric Antennae.

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H2O)2]+ (where L = bpcd2– = N,N′-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; bQcd2– = N,N′-bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; and bisoQcd2– = N,N′-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate), while the neutral complexes present the Eu(L)(H2O)2 formula (where L = PyC3A3– = N-picolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; QC3A3– = N-quinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A3– = N-isoquinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (RL), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd–Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (ηsens) in the different complexes were determined. The spectroscopically measured ϕovl values are in good agreement with the ones obtained theoretically in this work.

Enhancing the Luminescence of Eu(III) Complexes with the Ruthenocene Organometallic Unit as Ancillary Ligand.

Five novel Eu(III)-β-diketonate complexes containing ruthenocene ancillary ligands (1,1'-bis(diphenylphosphoryl)ruthenocene─RcBPO) were synthesized and characterized. The coordination compounds presented the general formula [Eu(β-dik)3(RcBPO)], where β-dik stands for 2-thenoyltrifluoroacetonate (tta), 3-benzoyl-1,1,1-trifluoroacetone (btf), 2-dibenzoylmethanate (dbm), 2-acetyl-1,3-indandionate (aind), and 2-benzoyl-1,3-indandionate (bind), and RcBPO stands for 1,1'-bis(diphenylphosphoryl)ruthenocene. The [Eu(aind)3(RcBPO)] complex crystallizes in a monoclinic Cc non-centrosymmetric space group with the europium site environment, assuming a bicapped trigonal prism coordination polyhedron with the symmetry point group close to C2v. Photoluminescent properties for the solid-state samples were described in terms of excitation, emission, lifetime decay curves, and intrinsic and overall quantum yields. The replacement of the two coordinated H2O molecules by the RcBPO ancillary ligand leads to great enhancements of the overall quantum yields (QEuL), with the minimum increment by a factor of 5 for the case of [Eu(btf)3(RcBPO)] and the maximum enhancement of 270 times for the case of the [Eu(dbm)3(RcBPO)] complex. In addition, theoretical calculations were carried out to model the spectroscopic properties of the investigated compounds. To obtain theoretical Judd-Ofelt parameters (Ωλ, λ = 2, 4, and 6) and intramolecular energy transfer rates, the JOYSpectra web platform was employed using the structure obtained from density functional theory calculations. Hence, a rate equation model provided theoretical overall quantum yields, which are in great agreement with measured data.

Spectroscopic aspects for the Yb3+ coordination compound with a large energy gap between the ligand and Yb3+ excited states

We present the experimental and theoretical results that made it possible to propose the energy transfer mechanism for a Yb complex with a large energy gap between the ligand and Yb excited states using a theoretical model and experimental data. Absorption and emission spectroscopy in the 300–4 K range is used for the study of the Yb3+ compound with N-phosphorylated sulfonamide (Na[YbL4]), which, despite the large energy gap, is characterized by high emission sensitization efficiency (ηsens = 40%) and relatively long Yb3+ emission lifetime (27 μs). The crystal structure of Na[YbL4], radiative lifetime (930 μs), refractive index (1.46), intrinsic (3.0%), and overall (1.3%) emission quantum yield were determined. To obtain the electronic properties of the Na[YbL4], a time-dependent density functional theory (TD-DFT) was performed. The intramolecular energy transfer (IET) rates from the excited states S1 and T1 to the Yb3+ ion as well as between the ligand and the ligand-to-metal charge transfer (LMCT) states were calculated. Once the intersystem crossing S1 → T1 is not so effective due to a large energy gap between S1 and T1 (≈10000 cm−1), it has been shown that the LMCT state acts as an additional channel to feed the T1 state. Then, the T1 can transfer energy to the Yb3+ 2F5/2 energy level (WT), where WT is dominated by the exchange mechanism. Based on IET and a rate equation model, the overall emission quantum yield QLLn was simulated with and without the LMCT, this also confirmed that the pathway S1 → LMCT → T1 → Yb3+ is more likely than the S1 → T1 → Yb3+ one.

(INVITED) JOYSpectra: A web platform for luminescence of lanthanides

JOYSpectra is a free-of-charge web platform to perform online calculations of spectroscopic properties of lanthanide-based compounds and materials. It has several features and functionalities to provide a detailed analysis of the intensity parameters. For instance, their dependence on the chemical environment of the lanthanide ion (Ln3+) such as structure and nature of ligating atoms, can be performed automatically and consider covalency and thermal effects. The program can also calculate the intramolecular energy transfer rates from excited donor states up to 310 transitions involving 12 Ln3+ ions. The web platform was designed to be user-friendly for experimentalists and theoreticians interested in calculations and analyses of the photophysical behavior of lanthanides. Its use requires only the structure of the compound and the choices of a few calculation options. To show its easiness and usefulness, a detailed step-by-step calculation and analysis are performed for the [Ln(tta)3(H2O)2] (Ln: Eu and Tb) complexes to explain and quantify their distinct luminescence properties.

Theoretical Evidence of the Singlet Predominance in the Intramolecular Energy Transfer in Ruhemann's Purple Tb(III) Complexes

AbstractThis work presents a theoretical study on the geometries and intramolecular energy transfer (IET) process of Tb3+‐complexes based on the Ruhemman's purple (RP) as ligands. Density functional theory and its time‐dependent extension are performed to examine the coordination energies and excited states using the ωB97X‐D3/MWB54/def2‐TZVP/polarizable continuum model level of theory. The inclusion of solvent effect causes a blueshift in all excitation energies, which is crucial for a better description of the electronic situations of RP isomers and coordination compounds. The IET rates are assessed for 18 Tb‐RP different compounds, each one with 44 IET pathways, also, it is obtained that the main energy transfer channel comes from the singlet state, in complete agreement with previous experimental data. The energy transfer from the singlet state is mainly composed of the nonradiative absorptions 7F6→5G6 and 7F5→5G5, representing together 97% of the total IET rate. As far as it is known, this is the first time that the solvent effect is included in the IET rates calculation, registering a step toward the development of IET analysis without any experimental or phenomenological input data.

How minor structural changes generate major consequences in photophysical properties of RE coordination compounds; resonance effect, LMCT state

Lanthanide coordination compounds of the formula Na[Ln(L)4] (1Ln), where Ln = La3+, Eu3+, Gd3+, Tb3+, L = [L]− and HL = dimethyl(4-methylphenylsulfonyl)amidophosphate, were synthesized. Their structural and spectroscopic properties were discussed in detail based on X-ray diffraction measurements, IR spectroscopy, absorption and emission spectroscopy at 293 and 77 K and theoretical calculations of the intramolecular energy transfer (IET) rates. DFT calculations were used to investigate the 1Ln electronic properties required to calculate the transition rates. 30 and 22 pathways of intramolecular nonradiative energy transfer were examined in the case of 1Eu and 1Tb, respectively. It is shown that the main pathway for sensitization of the lanthanide emission is either the triplet (1Eu) or singlet (1Tb) transfer, occurring mainly through the exchange mechanism. The energy rates for energy transfer from S1 and T1 equal WS=1.53×105s−1 (1Eu), WT=5.14×106s−1 (1Eu) and WS=4.09×107s−1 (1Tb), WT=6.88×105s−1 (1Tb). The crucial role of the 7F5 level in the energy transfer process of 1Tb and the participation of the LMCT state in the depopulation of the ligand singlet state of 1Eu were demonstrated. The influence of the resonance effect on the splitting of the 7F1 level in 1Eu was analyzed. By comparing the properties of 1Ln with the properties of 2Ln coordination compounds, sharing the same ligand and crystallizing in the same crystallographic system (monoclinic), but with a different space group, it is demonstrated how slight structural changes can affect the photophysical properties of Ln compounds.

Energy-Transfer and Charge-Transfer Dynamics in Highly Fluorescent Naphthalimide–BODIPY Dyads: Effect of BODIPY Orientation

We report evidence for photoinduced energy transfer from the naphthalimide (NI) to the BODIPY in NI–BODIPY dyads. Three dyads (1a, 1b, and 1c) were investigated, characterized by different positions of attachment of the BODIPY to the conjugated bridge (meso-, β-, and α-). The time-resolved spectroscopic results showed that the intramolecular energy-transfer rate is enhanced for the dyads characterized by the unusual β- (1b) and α- (1c) with respect to the largely employed meso- (1a) substitution. All of the dyads were strongly fluorescent. Fluorescence quantum yields and lifetimes were interestingly enhanced in the dyads with respect to the monomers (NI and BODIPY). These properties together with their two-photon absorption ability make these bichromophoric systems excellent candidates as fluorescent probes for bioimaging. In the β-substituted dyad, intramolecular charge transfer takes place following energy transfer. The concerted energy- and charge-transfer dynamics is appealing for solar energy convers...

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.

Synthesis and photo-physics of red emitting europium complexes: An estimation of the role of ancillary ligand by chemical partition of radiative decay rate

Three highly luminescent Eu(III) complexes of the type [Eu(β-diket.)3(tm-phen)] where β-diket. = hexafluoroacetylacetone (hfaa), btfa; 4,4,4-trifluoro-1-phenyl-1,3-butanedione (btfa), 2-thenoyltrifluoroacetone (tta) and tm-phen = 3,4,7,8-tetramethyl-1,10-phenanthroline have been synthesized and characterized. The detailed photophysical properties of the complexes were analyzed both theoretically and experimentally. The theoretical photophysical properties calculated using the LUMPAC program [1] are in excellent agreement with experimental results. The Judd-Ofelt (J-O) parameters (Ω2 and Ω4), radiative (Arad) and non-radiative (Anrad) decay rates were calculated and discussed. Chemical partition of the radiative decay rates (Arad) of electric-dipole transitions revealed that β-diketone contributes 77 to 84% while tm-phen contributes 22 to 16% towards the Arad. The intramolecular energy transfer (WET) and back-energy transfer (WBT) rate are predicted from the singlet (S1) and triplet (T1) levels to the emissive 5D1 and 5D0 states for Eu(III) and follow the path S0 → S1 → T1 → 5D1 → 5D0 → 7F0,4. The complexes emit typical Eu(III) red emission with long luminescence lifetime (0.75–0.88 ms), show high intrinsic quantum yield (ϕLn) (66–70%) and may be used as one of the red components in light emitting devices.

Synthesis and photoluminescence study of two new complexes [Sm(hfaa)3(impy)2] and [Eu(hfaa)3(impy)2] and their PMMA based hybrid films

The synthesis, characterization and photoluminescence studies of two new highly photoluminescent complexes, [Sm(hfaa)3(impy)2] and [Eu(hfaa)3(impy)2] (impy is 2-(1H-imidazol-2-yl)pyridine and hfaa is the anion of hexafluoroacetylacetone) are reported. The photoluminescence of these complexes were studied in solid, solution and PMMA thin films. These complexes were incorporated independently into polymer matrix of PMMA with varying amounts. The calculated quantum yield (φ) and experimental life time (τ) in solution for (i) Sm complex are φ = 0.013, τ = 0.036 ms and (ii) Eu complex are φ = 0.36, τ = 0.98 ms. The Judd–Ofelt intensity parameters, radiative parameters, intramolecular energy transfer rates and theoretical quantum efficiency are calculated and discussed. The intramolecular energy transfer rates, in the case of europium complex, suggest that the major energy transfer channels are T1 → 5D1 and T1 → 5D0 and these contribute 55.33% and 44.65%, respectively, to the overall energy transfer process. The emission colour changes with a change in concentration of the complexes in PMMA substrate. This feature may be employed in fabricating 'colour indicator diodes'. The thin hybrid films of these structurally ten-coordinate complexes in PMMA matrix exhibit outstanding photostability upon UV irradiation that defines their robust nature. The efficient photophysical properties of PMMA hybrids of these complexes, may possibly find their applications in optoelectronic devices.

Direct Vibrational Energy Transfer in Monomeric Water Probed with Ultrafast Two Dimensional Infrared Spectroscopy

Vibrational relaxation dynamics of monomeric water molecule dissolved in d-chloroform solution were revisited using the two dimensional Infrared (2D IR) spectroscopy. The vibrational lifetime of OH bending in monomeric water shows a bi-exponential decay. The fast component (T1=(1.2±0.1) ps) is caused by the rapid population equilibration between the vibrational modes of the monomeric water molecule. The slow component (T2=(26.4±0.2) ps) is mainly caused by the vibrational population decay of OH bending mode. The reorientation of the OH bending in monomeric water is determined with a time constant of τ=(1.2±0.1) ps which is much faster than the rotational dynamics of water molecules in the bulk solution. Furthermore, we are able to reveal the direct vibrational energy transfer from OH stretching to OH bending in monomeric water dissolved in d-chloroform for the first time. The vibrational coupling and relative orientation of transition dipole moment between OH bending and stretching that effect their intra-molecular vibrational energy transfer rates are discussed in detail.

Theoretical study of vibrational energy transfer of free OH groups at the water-air interface.

Recent experimental studies have shown that the vibrational dynamics of free OH groups at the water-air interface is significantly different from that in bulk water. In this work, by performing molecular dynamics simulations and mixed quantum/classical calculations, we investigate different vibrational energy transfer pathways of free OH groups at the water-air interface. The calculated intramolecular vibrational energy transfer rate constant and the free OH bond reorientation time scale agree well with the experiment. It is also found that, due to the small intermolecular vibrational couplings, the intermolecular vibrational energy transfer pathway that is very important in bulk water plays a much less significant role in the vibrational energy relaxation of the free OH groups at the water-air interface.

Computing intramolecular charge and energy transfer rates using optimal modes.

In our recent work [X. Yang and E. R. Bittner, J. Phys. Chem. A 118, 5196 (2014)], we showed how to construct a reduced set of nuclear motions that capture the coupling between electronic and nuclear degrees of freedom over the course of an electronic transition. We construct these modes, referred to as "Lanczos modes," by applying a search algorithm to find linear combinations of vibrational normal modes that optimize the electronic/nuclear coupling operator. Here, we analyze the irreducible representations of the dominant contributions of these modes and find that for the cases considered here, these belong to totally symmetric irreducible representations of the donor and acceptor moieties. Upon investigating the molecular geometry changes following the transition, we propose that the electronic transition process can be broken into two steps, in the agreement of Born-Oppenheimer approximation: a fast excitation transfer occurs, facilitated by the "primary Lanczos mode," followed by slow nuclear relaxation on the final electronic diabatic surface.

Cation-induced switching on–off luminescence in an imidazole 4,5-dicarboxylate-bridged RuIIOsII bipyridine complex: A combined experimental and DFT/TD-DFT investigation

A heterobimetallic complex of the composition [(bpy)2Ru(Imdc)Os(bpy)2](ClO4), where Imdc3−=imidazole-4,5-dicarboxylate(3−) ion and bpy=2,2′-bipyridine, was synthesized and characterized using standard analytical and spectroscopic techniques, including X-ray crystallography. Steady state and time resolved luminescence data at room temperature showed that an efficient intramolecular electronic energy transfer takes place from the Ru center to the Os based component in the dyad. The cation binding properties of the complex were thoroughly investigated in solution using UV–Vis absorption, steady state and time-resolved emission spectroscopic techniques. It is interesting to note that the luminescence intensity, quantum yield as well as the lifetime of the complex were enhanced substantially in the presence of selective cations, showing cation-induced molecular switching behavior. The effect of different cations on the rate of photoinduced intramolecular energy transfer in the dyad was also explored. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies were utilized to investigate the structural and electronic properties of the dyad. The optimized geometrical parameters for the dyad, computed both in the gas phase and in solution, are reported and compared with the single crystal X-ray data. Computational (TD-DFT) studies also provide insight into the nature of the ground and excited states, with detailed assignments of the orbitals involved in the absorption and emission transitions. In particular, the blue-shifts of the absorption and emission bands in the presence of selective cations are reproduced by our calculations.

Spectral properties of bichromophoric probes based on pyrene and benzothioxanthene in solution and in polymer matrices

Abstract The new initiator for controlled radical polymerization labeled with highly fluorescent chromophores was prepared. Highly fluorescent chromophores, such as pyrene and benzo[k,l]thioxanthene (BTX), were used for fluorescent labeling. The chromophores were combined with nitroxide on 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO), which is one of the best systems for mediating the radical polymerization of olefins. The spectral properties of a labeled initiator were measured in solution and in polymer matrix. In the absorption spectra of the bichromophoric initiator there is a broad band with a maximum in the range 450–485 nm depending on the medium. This band corresponds to the BTX moiety, whereas the band with vibrational structure at 320–345 nm arises from the pyrene structural unit. The emission spectra of the probes, recorded at an excitation of 480 nm in solution and in the polymer matrices exhibit a broad band in the range 490–525 nm and it is red-shifted in polar media. Excitation of the labeled initiator in the region of the maximum absorption of pyrene results exclusively in emission due to BTX which is caused by efficient transfer of the excitation energy from pyrene to the benzothioxanthene chromophore. Quenching of the fluorescence of the donor chromophore pyrene moiety of the labeled initiator was observed in solution and in the polymer matrices, and this process is primarily intra-molecular. The quantum yield of florescence of the labeled initiator is very high in all the studied environments and is between 0.5 and 1 depending on the polarity of medium. Both chromophores exhibit fluorescence, but the lifetimes of the fluorescence are very different. The lifetime is 100–400 ns for pyrene and is 4–8 ns for benzothioxanthene. The intermolecular quenching rate constant of 3.6 × 10 10 dm 3 mol −1 s −1 was determined from the Stern–Volmer plot for the quenching of the decay of pyrene with the BTXD model dye. Based on the intermolecular quenching rate of the pyrene–BTXD, the rate of intramolecular energy transfer from pyrene to the BTX chromophore in this bichromophoric initiator was estimated to be greater than 10 11 s −1 .

Quantum Chemical Calculations of Intramolecular Vibrational Redistribution and Energy Transfer of Dipeptides (GlyTyr and LeuTyr) and Applications to the RRKM Theory

AbstractRecently the quantum chemistry program can provide the anharmonic potentials for polyatomic molecules and clusters. It makes the quantum calculation of intramlecular vibrational redistribution (IVR) rates and intramolecular vibrational energy transfer possible. In the present paper, we apply the adiabatic approximation method to treat these two intramolecular energy redistribution processes. Using this method, we calculate the IVR rates of the dipeptides (glycyl tyrosine (GlyTyr) and leucyl tyrosine (LeuTyr)) and their cation radicals from the high frequency modes to the low frequency modes. Our theoretical results of the IVR lifetimes of the high frequency modes, like C‐H, N‐H, and O‐H stretching modes, for both GlyTyr and LeuTyr molecules and their cation radicals are about 1 ps, 10 ps, and 100 ps, respectively. It provides theoretical support for the claim that IVR takes place prior to dissociation and the IVR rates are determined by the functional group rather than the degrees of freedom of the system. Thus the Rice‐Ramsperger‐Kassel‐Marcus (RRKM) theory is suitable for investigating the photodissociation process of dipeptides. In addition, the calculated intramolecular energy transfer rate from O‐H stretching mode of the C‐termius of dipeptides to N‐H mode of N‐terminus is close to IVR rate.

Dependence of the Lifetime upon the Excitation Energy and Intramolecular Energy Transfer Rates: The 5D0 EuIII Emission Case

In many Eu(III)-based materials, the presence of an intermediate energy level, such as ligand-to-metal charge transfer (LMCT) states or defects, that mediates the energy transfer mechanisms can strongly affect the lifetime of the (5)D(0) state, mainly at near-resonance (large transfer rates). We present results for the dependence of the (5)D(0) lifetime on the excitation wavelength for a wide class of Eu(III)-based compounds: ionic salts, polyoxometalates (POMs), core/shell inorganic nanoparticles (NPs) and nanotubes, coordination polymers, β-diketonate complexes, organic-inorganic hybrids, macro-mesocellular foams, functionalized mesoporous silica, and layered double hydroxides (LDHs). This yet unexplained behavior is successfully modelled by a coupled set of rate equations with seven states, in which the wavelength dependence is simulated by varying the intramolecular energy transfer rates. In addition, the simulations of the rate equations for four- and three-level systems show a strong dependence of the emission lifetime upon the excitation wavelength if near-resonant non-radiative energy transfer processes are present, indicating that the proposed scheme can be generalized to other trivalent lanthanide ions, as observed for Tb(III)/Ce(III). Finally, the proper use of lifetime definition in the presence of energy transfer is emphasized.